Here It Spent Nearly Four Months Until March 1834

Home , Andagua volcanic field, Belgrano Norte Line, Borsonia, Cerro Tres Picos, Chapadmalalan, Chon Aike Formation

Portrait with signature gifted by Charles Darwin to the Academia Nacional de Ciencias in 1878 acknowledging his nomination as an Honorary Member. Photograph taken by Julia Margaret Cameron in 1868. Unpublished original copy kept in the Academia (see Depetris this volume) 4 Revista de la Asociación Geológica Argentina 64 (1): 4 - 7 (2009)

DARWIN'S GEOLOGICAL RESEARCH IN ARGENTINA

On the occasion of the 200th. anniversary of Charles Darwin´s birth, the Asocia- ción Geológica Argentina decided to prepare a special issue devoted to the geological research undertaken by Dar- win in Argentina. As it is well known, during his journeys on board HMS Bea- gle under the command of Captain Ro- bert FitzRoy, he had the opportunity to survey overland different areas of South America. Darwin spent nearly three years - between August 1832 and April 1835 - visiting and studying different regions of our country, as illustrated in figure 1. The aim of this special issue is to analyze his important geological observations and to emphasize the validity of many of his ideas under a 21st Century perspective. In order to accomplish this aim, several key localities that Darwin examined from a geological point of view during his voyage were selected. Such an analysis was carried out by several geologists and paleontologists well acquainted with the diverse problems that Darwin faced along his journeys in Argentina. HMS Beagle's main mission was to survey the coasts of southern South Ame- rica, which it carried out during several journeys along the coast. The time needed to chart the coast in details gave Darwin several opportunities to spend considerable time on land. During the survey he also had many occasions to land at selected points along the Atlantic coast. The first of these fieldtrips was done between August and October 1832 and gave him the opportunity to visit Punta Alta near the fort of Bahía Blanca (Fig. 1). The second one was between No- vember 1832 and May 1833 to visit the Beagle Channel, the surrounding areas of Tierra del Fuego Island, and the Mal- vinas Islands. The third and more important fieldtrip was, after landing in Carmen de Pata- Figure 1: Journeys of HMS Beagle in southern South America (base map modified from Bowly gones in August 1833 (Fig. 1), to ride 1990). Dates of Darwin's rides overland are shown in bold while the trips on board HMS Beagle across the Pampas to Buenos Aires city are shown in italics, taken from Keynes (2001). Darwin in Argentina 5

and later on to Santa Fé and Bajada (to- Zorra is included as being part of a pyro- terial. Based on their observations, they day Paraná city) on the Río Paraná. clastic flow. Their petrographic studies answer Darwin's queries about the pro- The fourth trip began in Montevideo in confirm the previous description of Dar- cess that buried the forest in standing December 1833 and allowed Darwin to win on the presence of fine volcanic ma- position. It should be noted that these examine various points along the Pata- gonian coast such as Puerto Deseado and Puerto San Julián. Afterwards HMS Beagle sailed to Tierra del Fuego where it spent nearly four months until March 1834. Later on it visited again different areas of the Malvinas Islands. In April of 1834 it anchored in the mouth of Río Santa Cruz. It was during this trip that Darwin, FitzRoy and more than 20 crew members sailed upstream the Río Santa Cruz in three small whale boats, almost reaching its headwaters near the foothills of the Patagonian Andes. This fourth trip continued across the Magallanes and strait of along the Pacific coast landing in Valparaíso in July 1834, and spending several months surveying the Chilean coast. Darwin's last visit to Argentina was between March and April 1835 (Fig. 1), when he crossed the Andes between Santiago and Mendoza following two different cordilleran passes. The different contributions of this issue were organized based on stratigraphic record and location (Fig. 2). The sections of the Andes are the first to be analyzed, then Patagonia and Tierra del Fuego, and finally the Cenozoic of the Pampas, ending with the origin of Puente del Inca and Darwin's tectonic ideas on the formation of the Andes. Pedro J. Depetris describes the documents kept in the Academia Nacional de Ciencias in Córdoba related to the nomination of Darwin, first as corresponding member and then as honorary member of the Academy. This paper also presents Darwin's original letter where he ackno- wledges the nomination and the front page of the book on the Origin of the Species that Darwin dedicated to the academy. Stella Poma, Vanesa D. Litvak, Magda- lena Koukharsky, Beatriz Maisonnave and Sonia Quenardelle describe the rocks Figure 2: Approximate location of the different contributions dealing with Darwin's geological in which the Triassic forest at Agua de la research in Argentina indicated by an asterisk (compare with Figure 1). 6 Darwin in Argentina

pyroclastic flows were unknown at the still unsolved problem of Darwin's "Clay gravels described by Darwin which are time of Darwin's visit. Slate Formation" in the Fueguian Andes. now interpreted as indicators of a maxi- Mariana Brea, Analía E. Artabe and Luis This term coined for the Cretaceous suc- mum glacial expansion in Patagonia. A. Spalletti deal with the first in situ fo- cessions, presently includes a series of Those blocks together with some new rest discovered in South America by marine mudstones the stratigraphy of findings are ascribed by these authors to Darwin in his trip across the Andes. This which is based on scarce fossils, mainly a catastrophic event related to the out- forest was described in detail by Darwin inoceramids and ammonites. The present burst of a large lake. as monotypic and assigned to the Ter- use of trace fossils and distinctive ichno- Martín Iriondo and Daniela Kröhling co- tiary. Their study reveals that the paleo- fabrics, together with new ammonites, ver the visit of Darwin to Santa Fé with vegetation of Agua de La Zorra forest is are contributing to a better understan- special emphasis in those localities des- composed by conifers and corystos- ding of these mudstones, improving stra- cribed by him along the Río Paraná perms reaching up to 20-26 meters high tigraphic correlation among different banks. The authors stress the precise des- forming a two-storeyed arboreal strata units. criptions of Darwin and discuss the Late and an understorey of ferns. It was reas- Ana Parras and Miguel Griffin present Cenozoic age of these sediments. signed to the Triassic and reconstruc- the sedimentology and fossil contents of Marcelo Zárate and Alicia Folguera deal tions of the forest landscape are presen- a classic Miocene locality, first described with the geologic characteristics of the ted. by Darwin at the southern bank of Río Pampas south of Río Salado, a region Beatriz Aguirre-Urreta and Verónica Santa Cruz, along the Patagonian coast. that impressed Darwin on his ride from Vennari study a small collection of fossil This is the type locality of most of the Bahía Blanca to Buenos Aires. They dis- invertebrates from the Piuquenes Pass, a fossils found in his "Great Patagonian cuss the Pampean Formation of Darwin, locality discovered by Darwin during his Tertiary Formation" and later described by its present stratigraphic divisions, its age, crossing of the High Andes from San- Sowerby (1846). The rocks of this sec- and the different environments encom- tiago to Mendoza. They compared this tion are presently included in the Monte passed in this unit, from a modern pers- collection with the fossils that allowed León Formation. pective. Darwin to propose a Neocomian age for The contribution by Silvio Casadio and Mirta E. Quattrocchio, Cecilia M. Des- the bearing strata. This assignation is Miguel Griffin deals with the sedimento- champs, Carlos A. Zavala, Silvia C. Grill confirmed and the time span stretched to logy and paleontology of a classic loca- and Ana M. Borromei give an updated the Tithonian based on key ammonites lity at Puerto Deseado (Port Desire). vision of the scenery described by Dar- of that age. This locality was visited by Darwin in win when he visited Bahía Blanca and Laura Giambiagi, Maisa Tunik, Victor A. 1833 when he collected fossil mollusks, surrounding areas. Multidisciplinary re- Ramos and Estanislao Godoy compare and remained nearly unexplored by geo- sults were integrated by these authors in the observations of Darwin on the struc- logists and paleontologists ever since. a chronosequence chart. This diagram ture of the High Andes between the The authors assess this classic Miocene shows the suggested space-time correla- Piuquenes and Portillo passes with mo- locality and correlate it with several simi- tion of the recognized events in the Late dern structural knowledge. They remark lar sequences exposed along the Atlantic Miocene-Holocene history of the south- the awareness of Darwin on the episodic cliffs. western Pampas. uplift of the Andes with the rise of the Oscar A. Martínez, Jorge Rabassa and Juan Carlos Fernicola, Sergio F. Vizcaíno Principal Cordillera first and a later uplift Andrea Coronato discuss one of the and Gerardo De Iuliis provide a synthe- of the Frontal Cordillera, both in post- most intriguing lithostratigraphic units in sis of the present knowledge of the large Cretaceous times. the Late Cenozoic of Patagonia named fossil mammals collected by Darwin, Eduardo Zappettini and José Mendía by Darwin the "Patagonian Shingle Forma- mostly from the Pampean region of cen- introduce the first geological map of tion". They analyze the prevailing para- tral Argentina. These authors ponder on Patagonia ever compiled, publishing the digm of that time that assigned these the reasons that led Darwin to recognize first color hand-drafted copy. It was rocks to the "Great Universal Dèlluge" and only megatheres and mastodonts for the drawn by Darwin after his trip to South give a modern perspective to the origin large fossil remains and discuss the cu- America. These authors outline the main of the "Rodados Patagónicos". rrent taxonomic status of the taxa descri- geological units used in the map and vali- Jorge Strelin and Eduardo Malagnino bed or erected by Owen between 1837 date them with the present knowledge of analyze the observations of Darwin in and 1845. the geology of the region. his trip upstream the Río Santa Cruz in Sergio F. Vizcaíno, Richard A. Fariña and Eduardo B. Olivero, Francisco A. Me- 1834. Special emphasis is paid to the Juan Carlos Fernicola, in their contribu- dina and María I. López C. analyze the extension of the big erratic boulders and tion on the Pleistocene South American Darwin in Argentina 7

mammals, attempt to provide answers to Crampton (Institute of Geological and Retallack (University of Oregon, USA), Darwin's questions, such as what has Nuclear Sciences, New Zealand), A. P. Sruoga (SEGEMAR, Buenos Aires), J. exterminated so many living creatures? Crame (British Antarctic Survey, UK), D. Stilwell (Monash University, Australia), Their study on the palaeobiology of Croft (Case Western Reserve University, M. Suarez (SERNAGEOMIN, Chile), M. these organisms shed light on the un- USA), I.W.D. Dalziel (University of Toledo (Muséum National d'Histoire derstanding of their evolution intermin- Texas, USA), R. De Barrio (INREMI, La Naturelle, France) and G. Vucetich (Uni- gled with ecology and phylogeny. Plata), E. Derbyshire (University of Lon- versidad Nacional de La Plata). Victor A. Ramos searches on the origin don, UK), C. Deschamps (Universidad of Puente del Inca, a natural bridge des- Nacional de La Plata), E. Godoy (San- Beatriz Aguirre-Urreta. cribed by Darwin in the High Andes, tiago de Chile), P. Imbellone (Universi- Miguel Griffin. during his cross by the Uspallata o La dad Nacional de La Plata), C. Janis Victor A. Ramos. Cumbre Pass. Applying uniformitarism, (Brown University, USA), M. Kaplan Invited Editors the author interprets the origin of Puen- (Columbia University, USA), S.M. Kay te del Inca as initially formed as an ice (Cornell University, USA), H.A. Leanza bridge, latter on cemented by hot spring (MACN, Buenos Aires), E. Llambías WORKS CITED IN THE TEXT waters. Ramos also weighs Darwin's (CIG, La Plata), T. Manera (Universidad thoughts on mountain uplift based on Nacional del Sur, Bahía Blanca), S. Mar- Bowly, J. 1990. Charles Darwin, a new biography. the Mesozoic successions of the Puente tinez (Universidad de la República, Uru- Hutchinson Press, 511 p., London, Sydney, del Inca section. His old ideas on tecto- guay), G. McDonald (National Park Ser- Auckland and Johannesburg. nics anticipate in many decades some vice, USA), A. Mourgues (SERNAGEO- Keynes, R.D. 2001. Charles Darwin's Beagle Dia- concepts of the geosynclinal theory and MIN, Chile), C.A. Mpodozis (Antofa- ry. Cambridge Univesity Press. 464 p., Cam- also on the present fold and thrust belt gasta Minerals, Chile), D. Muhs (United bridge. mechanics. States Geological Survey, USA), S. Niel- Sowerby, G.B. II 1846. Descriptions of the Ter- We would wish to acknowledge the revie- sen (Christian-Albrechts-Universität, Ger- tiary fossil shells from South America. In wers of the present issue: S. Bargo (Uni- many), O. Orfeo (Centro de Ecología Darwin, C. Geological observations on South versidad Nacional de La Plata), M. Bond Aplicada del Litoral, Corrientes), E.G. Ame-rica. Being the third part of the geology (Universidad Nacional de La Plata), N. Ottone (Universidad de Buenos Aires), J. of the voyage of the Beagle, under the com- Carmona (CADIC, Ushuaia), C. Costa Panza (SEGEMAR, Buenos Aires), R.N. mand of Capt. Fitzroy, R.N. during the years (Universidad Nacional de San Luis), R. Page (SEGEMAR, Buenos Aires), G. 1832 to 1836. Smith Elder and Co. Ed., Charrier (Universidad de Chile), J.S. Pastorino (MACN, Buenos Aires), G.J. Appendix: 249-264, London. 8 Revista de la Asociación Geológica Argentina 64 (1): 8 - 12 (2009)

CHARLES ROBERT DARWIN AND ARGENTINA'S NATIONAL ACADEMY OF SCIENCES

Pedro José DEPETRIS

Academia Nacional de Ciencias, Avenida Vélez Sarsfield 229, 5000 Córdoba.

ABSTRACT Over 175 years ago Charles Robert Darwin arrived in Argentina to find a bare and boundless plain, the brave centaur called "gaucho", Quaternary fossils everywhere, and a society strikingly strange and aggressive to the British eyes of the young traveller. Although the voyage aboard HMS Beagle was the indispensable way towards increasing his stature as a biologist, Lyell's work awakened an inquisitive geological mind which allowed him to wonder at the splendour of the Andes. Forty-two years after having concluded his voyage on the Beagle, the National Academy of Sciences of Argentina appointed him as an Honorary Member. This must be interpreted as an early gesture of recognition -in the context of those times- to the magni- ficence of his scientific work. Keywords: Darwin, Argentina, National Academy of Sciences, geology, Sarmiento.

RESUMEN: Charles Robert Darwin y la Academia Nacional de Ciencias. Hace más de 175 años, Charles Robert Darwin llegaba a la Ar- gentina para descubrir una llanura desprovista de límites y de árboles, el valeroso centauro que era el gaucho, fósiles cuaternarios por doquier y una sociedad sorprendentemente extraña y agresiva a los británicos ojos del joven viajero. Aunque el viaje a bordo del Beagle fue el camino indispensable para incrementar su estatura como biólogo, la obra de Lyell despertó una mente inquisitivamente geológica que le permitió maravillarse ante la magnificencia de los Andes. Cuarenta y dos años des- pués de haber concluido su viaje en el Beagle, la Academia Nacional de Ciencias de Argentina lo designó Miembro Honorario, en lo que debe interpretarse como un temprano gesto de reconocimiento -en el contexto del momento- por la magnificencia de su obra científica.

Palabras clave: Darwin, Argentina, Academia Nacional de Ciencias, geología, Sarmiento.

INTRODUCTORY with the ex Spanish colonies and accura- nature. In his memoirs, he would simply COMMENTS te geographical information was urgently state that "the voyage of the Beagle has been by needed. far the most important event in my life and has On July 26, 1832, one day short of seven Charles Robert Darwin was only 22- determined my whole career". months after leaving Plymouth harbor, years-old when he saw the muddy waters Darwin had left unfinished his medical HMS. Beagle moored at Montevideo. It of the Río de la Plata for the first time; education at Edinburgh in 1827, and had was a Cherokee class 10-gun brig-sloop he had been recommended to FitzRoy as gone to Cambridge's Christ's College of the Royal Navy named after the Bea- a companion and naturalist without pay where he obtained his Bachelor of Arts gle, the famous British dog breed. The (Darwin's father covered all the expenses (BA) degree, without honors. He was an most important task entrusted by the involved in the long voyage) by John enthusiastic entomologist and an avid British Admiralty to 26-year-old Captain Stevens Henslow, clergyman, botanist, biological collector and his experience in Robert FitzRoy was about to begin - the and mineralogist, whom Darwin had met the Earth Sciences was limited to a field survey of the coasts south of the Río de in Cambridge. Young Darwin was expec- trip to northern Wales with the famous la Plata, down to the southern tip of ted to gather information on the natural Adam Sedgwick (1785-1873), Woodwar- Patagonia and Tierra del Fuego. There history and geology of many exotic re- dian Professor of Geology at Cambrid- were significant gaps in the geographical gions that he was going to visit during a ge. Notwithstanding, Darwin was suppo- knowledge that the British Admiralty had two-year long journey around the world. sed to become a clergyman and his father gathered through countless reports of The trip was initially extended to three- -Robert Waring Darwin- initially had sailors who had visited the region for years and then to five-years and it would strong objections when Charles was offe- many years. Now that George Canning be the most extraordinary experience in red the opportunity of boarding the had signed in 1825 a commercial treaty the life of Charles Darwin; an experience HMS Beagle for a journey to South with the newly independent federation of that would change dramatically the view America to "survey the S. extremity". How- Argentinean states, trade was flourishing that humankind had of itself and of ever, after Charles's uncle, Josiah Wedg- Charles Robert Darwin and Argentina's National Academy of Sciences 9

wood II, wrote an eloquent letter in which he stated that being young Charles a man of enlarged curiosity, "...it affords him such an opportunity of seeing men and things as happens to few". After an absence of five years and two days, on October 2nd, 1836, the Beagle anchored at Falmouth, and on the 4th Darwin returned home to Shrewsbury. As we know today, the voyage of the Beagle made him a scientific celebrity, as he produced several books and numerous articles, and profusely described and distributed many specimens that he brought home with him aboard the ship (Fig. 1). All these events rapidly led him to join the elite world of international science on an equal footing. The revolutionary ideas set into motion by this long voyage swirled through Darwin's later life until, precipitated by Alfred Russel Wallace, they suddenly broke through Figure 1: Portrait of into Victorian society in 1859, under the young (~31-year-old) form of a book that bore the provocati- Charles Darwin four years after he retur- ve title of On the Origin of Species by Means ned from the voyage of Natural Selection, or the Preservation of Fa- of the HMS. Beagle, voured Races in the Struggle for Life (Fig. 2). painted by George Richmond (1840). The best place to find a detailed and personal account of Darwin's rich experiences is his first book, entitled Journal of Researches into the Geology and Natural History of the Various Countries visited by H.M.S. Beagle, under the Command of Captain FitzRoy, R.N. from 1832 to 1836 (Fig. 3). The book was a success when first published, in 1839, and Darwin's fondness for this particular work was transparent when he wrote at the end of his life that "the success of this my first literary child always tickles my vanity more than that of any of my other books". Originally published in four volumes (Darwin's volume, plus FitzRoy's two and an extended appendix), Darwin describes not only the exciting travels, the exotic ports of call, and the fascinating inland expeditions (in fact, he spent much more time on land than he did at sea), but also his emotions, his intense feelings on first arriving in the tropics, his dangerous overland excur- sions with the gauchos (which he deeply Figure 2: Facsimile of the front page of Origin of Species. The volume was sent by the publisher to admired), and the awesome sight of the the National Academy of Sciences, in Córdoba, upon Darwin's request. 10 P.J. DEPETRIS

stars over the Cordillera de los Andes. Moreover, in his first work - later published as Voyage of the Beagle (e.g., Darwin 1989)- it is very clear that Darwin, at least, was never a detached observer and the book shows how he saw a totally foreign society through British eyes and strongly supported the social order and political structure as he understood it from his native country. Along this line, it is particularly interesting to read about his impressions and views on Argentine society in times of severe turmoil, when Juan Manuel de Rosas (whom he met personally) was growing as a political figure in the conflictive scenario that was at the time the Argentine Confederation. Darwin set foot for the first time on Argentina's soil in Patagonia when the Beagle arrived on August the 3rd off the mouth of the Río Negro. He traveled afterwards overland to Bahía Blanca, Buenos Aires, and Santa Fe. On Decem- ber 6th 1833 the Beagle set sail to Pata- gonia (Puerto Deseado) and then anchored within the mouth of the Río Santa Figure 3: Facsimile of Cruz, which was explored upstream for the front page of several weeks. On June 10th, 1834, the Darwin's Journal of Beagle "bade farewell for ever to Tierra del Researches, a.k.a. Voyage Fuego". Darwin was not particularly fond of the Beagle. of sailing rough seas and one can only the best-known texts in the field, particu- observed geological phenomena. Only imagine the feeling of relief that he larly Charles Lyell's famous Principles of his scheme explaining the structure and experienced when they left Cape Horn Geology (Lyell 1998), which was originally distribution of coral reefs has survived to for good. Darwin would enter again Ar- published in 1830. This shared interest in this day (Herbert 2005). gentina's territory when he crossed the geological sciences was probably a factor The British government sponsored pu- Andes and arrived in Mendoza during that drew FitzRoy and Darwin together blication of his research and numerous the 2nd half of March, 1835. The awe-ins- during their time on board. In fact, geologists, including Darwin's former piring sight of the Cordillera and the FitzRoy was approached by Lyell before teachers like Sedgwick, proved to be an possibility of seeing geological processes the Beagle sailed off Plymouth in order interested audience. Doubtlessly the ex- at such large scale made a very deep im- to ask that specific geological features be perience of the voyage of the Beagle had pression on the young scientist. recorded, such as erratic boulders of gla- been transformative: the methods and cial origin. Darwin gathered geological hypothesis of Victorian-era geology pro- DARWIN, THE GEOLOGIST specimens and took detailed notes on foundly shaped Darwin's mind and his geology during the circumnavigation of scientific methods as he worked toward a Since this article is published in a geolo- the globe. Contradicting widespread be- complete comprehension of evolution gical journal, it seems appropriate to lief, upon his return to Great Britain, it and natural selection. briefly consider the geological facet of was his geological findings that first pro- Charles Darwin. It is accepted now that moted enthusiastic scientific and public DARWIN AND ARGENTINA'S geology influenced Darwin and, conver- opinion. It must be kept in mind that, as NATIONAL ACADEMY OF sely, he influenced the science (Herbert a young scientist, Charles Darwin was SCIENCES 2005). Like FitzRoy, he had a special inte- eager to contribute with a simple theory rest in geology and he had read most of that would explain most, if not all, of the The Academy was founded on Septem- Charles Robert Darwin and Argentina's National Academy of Sciences 11

ber 11th 1869, by a bill passed on to Congress by Argentina's President Do- mingo Faustino Sarmiento (García Cas- tellanos 1987). The bill authorized Pre- sident Sarmiento to hire a significant number of European scientists in order to promote scientific research in the natural and exact sciences and foster higher education at the Universidad Nacional de Córdoba. The first to arrive in August 1871 was Prof. M. Siewert, from the Uni- versität Halle (known today as Martin- Luther-Universität Halle-Wittenberg), soon followed by Prof. P.G. Lorentz, a botanist from the Universität Munchen. Others, like the geologist Stelzner, the zoologist H. Weyenbergh, the astrono- mer K. Schultz-Sellack, arrived the following year (García Castellanos 1987). As academicians and naturalists from prominent European centers of higher lear- ning, all undoubtedly knew about the revolutionary ideas put forward in the Origin of Species and, although not all supported Darwin's views (G. Burmeister, a prominent biologist, organizer of the Academy, for example, opposed evolutionary ideas), there is ground to sustain that most were enthusiastic supporters of Darwin's work. Moreover, during his visit to Argentine territory Charles Dar- win made numerous and valuable observations that increased the knowledge on the natural history and geology of the young country. Hence, it should not come as a surprise that Darwin's magnificent work and his contact with Argentina led to his appointment as a Correspon- ding Member ("Miembro Corresponsal") of the National Academy of Sciences, which still has its seat in the city of Cór- doba, Argentina. On August 7th 1878, the President of the Academy, Dr. Hendrik Weyenbergh, wrote a letter to Argentina's Minister of Education Dr. Bonifacio Lastra requesting the appointment of several new members and the change of Charles Robert Darwin's status to Ho- norary Member ("Miembro Honorario") (Fig. 4), along with D. A. Grisebach. In Figure 4: Facsimile of the letter sent by the National Academy of Science's President, Dr. return, Darwin instructed his publisher Hendrik Weyenbergh to Argentina's Minister of Education, Dr. Bonifacio Lastra, requesting the to send a copy of the Origin of Species to appointment of Charles Darwin as an Honorary Member of the Academy. 12 P.J. DEPETRIS

the Academy (still in the Academy's collection of antique books), sent an autographed picture (see frontispiece of this volume), and wrote a grateful letter to the Academy's President. Darwin's letter dated March 18th 1879 to the Aca- demy's President, Dr. Hendrik Weyen- bergh, read: "Dear Sir, I beg leave to acknowledge the receipt of the very handsome Diploma of your Society, and to repeat my thanks for the honour conferred on me. According to your request I enclose my photograph, and I have directed my publisher to send a copy of my Origin of Species to the Society as I suppose that this is the best of my works. I have the honour to remain, Dear Sir, Yours faithfully, Charles Darwin" (Fig. 5). To this day, the volume presented by Darwin as a gift, as well as the photograph and the letter, are cherished in Argentina's Academy of Sciences as valuable icons of the development of world science. Darwin's approach to explain the evolu- Figure 5: Facsimile of the letter sent by Charles Darwin on March 18th 1879 to the Academy's President, Dr. Hendrik Weyenbergh, acknowledging his appointment as Honorary Member of tion of nature and, above all, of human- Argentina's National Academy of Sciences. kind rapidly rooted in Argentina's newly born but growing science. Perhaps perts. By contributing his own ideas on García Castellanos, T. 1987. Breve historia de la nothing better than Sarmiento's own natural selection to the evolutionism Academia Nacional de Ciencias de Córdoba, words to express the widespread feelings outlined by Erasmus Darwin -his grand- República Argentina. Academia Nacional de towards these revolutionary biological father-, Lamarck and other biologists, he Ciencias, Miscelánea 75, 39 p. ideas. In 1882, he expressively said "…Y raised the evolutionary hypothesis to a Herbert, S. 2005. Charles Darwin, geologist. Cor- yo, señores, adhiero a la doctrina de la evolución provable theory. nell University Press, 512 p., New York. así generalizada, como procedimiento del espíri- Lyell, C. 1998. Principles of Geology. Penguin tu, porque necesito reposar sobre un principio ACKNOWLEDGEMENTS Classics, 528 p., London. armonioso y bello a la vez, a fin de acallar la Sarmiento, D.F. 1951. Obras completas, Discur- duda que es el tormento del alma" (Sarmiento I wish to acknowledge the assistance of sos Populares. Ed. Luz del Día, 12, 382 p., 1951, p. 118). ("And I, gentlemen, adhe- my friend and fellow academician, Dr. Buenos Aires. re to the theory of evolution thus gene- Alfredo Cocucci, who supplied valuable ralized, as a procedure of the spirit, be- information for the preparation of this cause I need to rest upon a harmonious article. Gonzalo Biarnes and librarian as well as beautiful principle, in order to Sandra Ledesma, both members of the hush doubt, that torment of soul"). Academy's staff, kindly supplied the fac- Darwin died in 1882 after a long illness; similes herein included. Most important, it was not realized until after his death my wife Elizabeth kindly helped out in- that he had suffered from Chagas's dise- troducing needed corrections to the ori- ase, which he had contracted while visi- ginal manuscript. ting South America. Although he was not the only originator of the evolution WORKS CITED IN THE TEXT hypothesis, he certainly was the first man of science that gained for such theory a Darwin, C. 1989. Voyage of the Beagle. Penguin Recibido: 25 de agosto de 2008 wide acceptance among biological ex- Classics, 432 p., London. Aceptado: 2 de octubre de 2008 Revista de la Asociación Geológica Argentina 64 (1): 13 - 20 (2009) 13

DARWIN'S OBSERVATION IN SOUTH AMERICA: WHAT DID HE FIND AT AGUA DE LA ZORRA, MENDOZA PROVINCE?

Stella POMA*, Vanesa D. LITVAK *, Magdalena KOUKHARSKY*, E. Beatriz MAISONNAVE and Sonia QUENARDELLE*

Laboratorio de Petrología, Departamento de Ciencias Geológicas, FCEN (UBA) y *CONICET. Emails: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

ABSTRACT Scarcely 23 km from Uspallata, along the track of the old national highway 7, lies the district of Agua de la Zorra, in Mendoza province in western Argentina. Charles Darwin visited the area during his South American journeys in the 19th century and discovered a geological sequence that contained a paleoflora never described before. The flora includes an important number of species, particularly what is considered a small conifer forest with many silicified trunks still in life position. Darwin described and interpreted the sequence as sedimentary; his records show a very detailed level of observation. He also wondered about the processes that would cause the burial of the paleoflora, which he considered had happened in a marine sedimentary environment. In the modern geological framework and after a detailed study of the rocks containing the trunks, it is now interpreted that the conifer forest was buried by pyroclastic flows. Darwin accurately described the fine volcanic materials as an essential part of the deposit, but the key of the enigma about the origin of the deposits and the burial of the forest is the identification of the pyroclastic flow features; these were unknown process at the time of Darwin´s observations and interpretation. Keywords: Darwin Forest, Pyroclastic flow, Triassic, Agua de la Zorra, Mendoza province.

RESUMEN: Las observaciones de Darwin en Sudamérica: ¿Qué encontró en Agua de la Zorra, provincia de Mendoza? A escasos 23 km de Us- pallata, a lo largo de la antigua ruta nacional 7, se encuentra la localidad de Agua de la Zorra en la provincia de Mendoza al oeste de Argentina. La región fue explorada por Charles Darwin durante su viaje por Sudamérica en el siglo XIX. Allí descu- brió una secuencia geológica conteniendo una paleoflora que no había sido descripta hasta entonces. La flora incluye un importante número de especies y particularmente, un pequeño bosque de coníferas, los que se encuentran silicificados, y en posición de vida. Darwin describió e interpretó la secuencia como sedimentaria; sus notas y descripciones muestran un alto nivel de detalle en las observaciones. Darwin además se cuestionó acerca de los procesos que habrían causado el sepultamien- to de la paleoflora, la que de acuerdo a sus deducciones ocurrió en un ambiente sedimentario marino. En el contexto geoló- gico moderno y como consecuencia de un estudio detallado de las rocas que contienen los troncos, se interpreta que el bosque de coníferas fue sepultado por flujos piroclásticos. Darwin fue acertado en describir material volcánico como constitu- yente esencial del depósito, pero la clave del enigma acerca del origen de la secuencia y el soterramiento del bosque radica en la identificación de las características del depósito. Corresponden a acumulaciones producto del volcanismo fragmentario; que incluye un conjunto de procesos desconocidos al tiempo de las observaciones e interpretación de Darwin.

Palabras clave: Bosque de Darwin, Flujo piroclástico, Triásico, Agua de la Zorra, Provincia de Mendoza.

INTRODUCTION and Argentina (Fig. 1). During this jour- the modern geological framework. ney, Darwin recorded everything he Charles Darwin travelled around South found. In Chapter VII of one of his DARWIN´S OBSERVATIONS America between 1832 and 1835. While journals (Darwin 1846) he described the staying at Valparaiso - one of Chile's cordilleran geology, particularly the Agua First observations of Darwin in the most important seaports - during July de la Zorra region in Uspallata in Men- region of Agua de la Zorra in Uspallata 1834, HMS Beagle underwent repairs doza province. There he found a thick are mentioned in Chapter VII (Darwin and Captain FitzRoy faced some finan- sequence, which he considered sedimen- 1846), where he describes the area and cial difficulties. This situation provided tary, that contained some trunks never refers to the paleoflora found as: "… I Dar-win with some spare time and at his described before. The objective of this counted the stumps of fifty-two trees. They pro- expense took a six week land expedition work is to revise Darwin´s XIX century jected between two and five feet above the ground, to the east, i.e., the High Andes in Chile observations at Agua de la Zorra area in and stood at exactly right angles to the strata, 14 S. POMA, V. D. LITVAK, M. KOUKHARSKY, E. B. MAISONNAVE AND S. QUENARDELLE

not quite equal thickness with the superincumbent strata, and as these deposits are regularly stratified and fine-grained, not like the matter thrown up on a sea-beach, a previous upward movement, aided no doubt by the great accumulation of lavas and sediment, is also indicated". He interpreted a possible origin for the deposits taking into account the regional geological setting known then: "At first I imagined, that the strata with the trees might have been accumulated in a lake: but this seems highly improbable; for, first, a very deep lake was necessary to receive the matter below the trees, then it must have been drained for their growth, and afterwards re-formed and made profoundly deep, so as to receive a subsequent accumulation of matter several thousand feet in thickness. And all this must have taken place necessarily before the formation of the Uspallata range, and therefore on the margin of the wide level expanse of the Pampas! Hence I conclude that it is infinitely more probable that the strata were accumulated under the sea".

STRATIGRAPHIC SETTING OF DARWIN´S DISCOVERY

Darwin's discovery at Agua de la Zorra is located along the old National Highway 7, which links Caracoles de Villavicencio with the town of Uspallata (Fig. 1). The best exposures occur about 800 m to the east of Agua de la Zorra locality (32° Figure 1: a) Path followed by Darwin from Valparaiso, towards the north in Chile, and towards 28'39" S and 69°09'09" W). It was only in the east in Argentina, where he visited the Precordillera in Mendoza. b) Geographical location of 1910 that the deposits where Darwin's Darwin Forest in the Agua de la Zorra area, in Uspallata, Mendoza province. paleoflora was found were described which were here inclined at an angle of about each other; they are grouped in a clump within a again by Stappenbeck (1910) and later by 25° to the west". One of the aspects that space of about sixty yards across". "They all Harrington (1941, 1971). Figure 2 shows amazed him was that the trunks were in stand at about the same level. The longest stump a schematic geological map of the area. life position and well preserved. He not stood seven feet out of the ground: the roots, if The sequence containing the trunks, se- only described his finding but also asked they are still preserved, are buried and concealed. dimentary in origin as Darwin described, advice from specialists about the type of No one layer of the mudstone appeared much is partially covered by alkaline basalts and flora: "…Eleven of these trees were silicified darker than the others, as if it had formerly intruded by sills of similar composition. and well preserved: Mr. R. Brown has been so existed as soil". The abundant remains of conifers ren- kind as to examine the wood when sliced and Paragraphs ahead, he wondered about der it a classic site not only from the pale- polished; he says it is coniferous, partaking of the processes that may have explained ontological point of view, but also from the characters of the Araucarian tribe, with the origin of the deposits and the asso- a geological and historical perspective. some curious points of affinity with the Yew". ciated paleoflora: "Certainly the upright trees Thus, the name of Bosque de Darwin (Dar- His records show a very detailed level of have been buried under several thousand feet in win's Forest) for the site was formalized observation: "All the stumps have nearly the thickness of matter, accumulated under the sea. in the geological reports of Map 3369- same diameter, varying from one foot to eighteen As the land, moreover, on which the trees grew, 09, Uspallata (Brodtkorb et al. 1997, Cor- inches; some of them stand within a yard of is formed of subaqueous deposits, of nearly if tés et al. 1997). Darwin's observation in South America: what did he find ... 15

Figure 2: Schematic geologic map of the Agua de la Zorra region in Uspallata, Precordillera of Mendoza, Argentina. The geological record of the region was Kokogian et al. (1999) which is composed nic-rich shales. A noticeable feature of studied by several authors, who also agre- of coarse-grained conglomerates and this interval is the occurrence of abun- ed a sedimentary origin for the deposits breccias deposited in alluvial fan areas. dant pyroclastic levels covered by thick described by Darwin (Stipanicic 1972, These coarse-grained rocks are com- basaltic lava flows and sills included in 1979, Rolleri and Criado 1986, Strelkov monly found close to positive areas and the uppermost part of the synrift II and Alvarez 1984, Kokogian and Man- basinwards taper into fluvial sandy ephe- stage. cilla 1989, Ramos and Kay 1991, Morel meral braided deposits and finally pass Finally, during the Late Triassic a sag 1994, Cortés et al. 1997, Kokogian et al. into muddy playa-lake sequences in the phase sequence was defined by Ramos 1999). Stratigraphicaly, Cortés et al. central parts of the basin. Isolated levels and Kay (1991) and Kokogian et al. (1997) identified the deposits described of rhyolitic tuffs occur at some places. (1999), which characterized the final sta- by Darwin as part of the Triassic Pa- The second sequence, labelled as the ges of subsidence in the basin under li- ramillo Formation within the Uspallata synrift II stage (Kokogian et al. 1999), mited tectonic and magmatic activity. At Group. In a recent analysis of a section was very probably deposited during the this time prevailed sandstones and con- of the sequence, Poma et al. (2004) des- Middle Triassic and shows a wider areal glomerates sedimented by anastomosed cribed the volcaniclastic characteristics of distribution that the above considered and braided rivers that were covered by the rocks containing the trunks; these are synrift I. During this time prevailed diffe- transgressive mudstones, shales and tuffs still under study. rent types of high-sinuosity river depo- deposited in lacustrine environments. The area of Agua de la Zorra is part of sits composed of cross-bedded sandsto- the Cuyo Triassic extensional basin. Ac- nes, conglomerates and mudstones. La- GEOLOGY OF THE AGUA cording to Kokogian et al. (1999) the custrine and deltaic sediments also appe- DE LA ZORRA REGION Cuyo Basin evolved through three depo- ar in this section and indeed are domi- sitional sequences that characterized dif- nant at the uppermost level of this sec- Volcaniclastic deposits ferent stages of rift evolution. The lo- tion. Lake deposits are made up of inter- wermost deposits, Early Triassic in age, bedded sandstones-mudstones sequen- In the Agua de la Zorra region, as Dar- were included in the synrift I stage by ces with sporadic intercalations of orga- win described, there are a series of rocks 16 S. POMA, V. D. LITVAK, M. KOUKHARSKY, E. B. MAISONNAVE AND S. QUENARDELLE

containing tree trunks typical of coniferous - including Araucarioxylon sp. among them (Brea 1997, 2000) - and buried in life position. As shown in figure 3a, b, and c, Darwin´s paleoflora is contained in a sedimentary/volcaniclastic sequence covered by alkaline basalts, both of Triassic age. According to field and microscope observations the material interbedded with the fossils comprises two main deposit types that are repeated with different thickness along the sequence. One of them is a characteristic pyroclastic flow deposit with crystal fragments, glass shards, pumice lapilli, all of them magmatically fragmented juvenile clasts product of their explosive origin. The crystals are small and irregular quartz grains and tabular or irregular fragments of oligoclase-andesine plagioclase. The matrix contains abundant angular glass shards, most of which have cuspate shape (Fig. 4a, and b). Fine ash fills the interstices between the shards. The shards are characteristically undeformed and some of them retain original bubble wall shapes, suggesting that the deposition was at moderate temperature. The lapilli pumices are concentrated at the lower part of the deposits and are smaller than 16 mm (medium lapilli). Occa- sionally it is possible to find silicified plant sprigs. The rock is classified as a rhyolitic to dacitic medium lapilli tuff (White and Houghton 2006) with thin (decimetre scale) variably diffuse layering, thickness varying between 50 and 20 centimetres. They are defined by gradational grain size between pumice and lapilli. The set repeats itself several times along the sequence. Typically the diffuse stratification is subparallel with local gradual thinning. The rock commonly grades verti- cally into massive beds. Associated with the tuff and separated by a clearly noticeable surface there is a thin (centimetre to few decimetre scale) and well sorted deposit. It is poorly lithified and is inter- Figure 3: a) General view to the SW of the Triassic pyroclastic deposits covered by Triassic alka- preted as an air fall deposit that accom- line basalts; b) Closer view of the trunk, paleosoil, roots, and pyroclastic sequence including panied the emplacement of pyroclastic basal levels; c) Detailed view of the trunk, approximate diameter 40 cm. Darwin's observation in South America: what did he find ... 17

flows. It was preserved because a later ignimbritic flow covered it and protected it from erosion. The conspicuous features in these rocks are pyroclastic cuspate glass shards and pumice fragments. The shards and delicate spines of bubble walls were deposited undeformed, they indicate that the deposit is primary and has not been reworked from primary deposits (Fig. 4a, and b). The glassy shards are now trans- formed in zeolite and silica material. In the area, some massive lapilli tuff show normal vertical grading patterns defined by more or less gradual changes in grain size. Vertical grading typically occurs across a few decimetres. The record begins (Fig. 5) with a coarse lapilli tuff (White and Houghton 2006) (grain size average between 16 to 64 mm of pumice fragments), crystal fragments and lithics with minor amounts of glassy matrix (<10%), gradually changing to fine lapilli without or with only few pumice fragments. At the base these rocks show channel morphology probably indicative of some erosive behaviour by the flow. In this deposit, generally at lower levels, it is possible to notice small gas escape pipes. These structures are richer in pumice lapilli, lithics and poorer in fine ash than the enclosing tuff. This is characteristic of the removal of fine ash as a result of elutriation by gas Figure 4: Thin section pictures of Paramillo Formation tuffs. a) Rhyolitic vitric tuff with unde- escaping through the uncompacted formed shards (Parallel light); b) Rhyolitic vitric tuff with triaxon shards and rounded palagonite deposit. The pipes were formed simulta- fragments of basaltic origin (Cross polarized light). neously with the flow accumulation, with Triassic mafic rocks. These are sills palagonite lava. The size of the spheres is again illustrating the primary nature of and lava flows of basaltic composition variable but no larger than 5 cm. They the deposit. The pyroclastic rocks are (Massabie 1986, Ramos and Kay 1991). represent the interaction between lava intercalated with fluvial lenses of fine In the Agua de la Zorra region the olivi- flows and unconsolidated water soaked sandstones to conglomerate, interbedded ne-bearing basalts are easily noticeable material. Upwards there are peperites with shales. because of their dark colours, fine grain that result from the mingling between Geomorphology and patterns of thick- and scoriaceous or amygdaloidal textures. basalt, rhyolitic tuff and clastic sediments ness variation of the pyroclastic deposits The vesicles were partially filled with cal- in variables proportions. These textures suggest a possible western provenance cium carbonate and silica and zeolites in suggest that the basalts were produced even though there is no conclusive evi- subordinate amounts. These materials shortly after or simultaneously with the dence. are conspicuous in altered basalts and pyroclastic activity and sedimentation. sometimes are found in pyroclastic and There were not identified volcanological Tuff-basalts transition sedimentary rocks. structures that could relate the rhyolitic The lower part of the basalts are resting pyroclastic deposits with the basaltic lava The pyroclastic and sedimentary rocks over a poorly consolidate rhyolitic tuff flows, seeming the latter the result of fis- described are intercalated and covered that contains small spheres of basaltic sure eruptions. 18 S. POMA, V. D. LITVAK, M. KOUKHARSKY, E. B. MAISONNAVE AND S. QUENARDELLE

CHARACTER AND ORIGIN OF THE DEPOSITS

Just as Darwin described, the trunks are arranged vertical to the layers. They stand at approximately the same level, and although Darwin did not identify a normal paleosoil, he understood that these trees had to grow in a dry substrate. He mentioned (Darwin 1846): "As the trees obviously must once have grown on dry land, what an enormous amount of subsidence is thus indicated! Nevertheless, had it not been for the trees, there was no appearance which would have led any one even to have conjectured that these strata had subsided". Obviously, he was amazed at the great thickness of sediments that there are in the sequence, which hinders any simple interpretation on the origin of the set. Finally, Darwin makes up his mind and chooses the simplest interpretation, even though this is not satisfactory. "That it is infinitely more probable that the strata were accumulated under the sea". On the other hand he observes and describes that the fine material plastered around the trunks as a package preserves the mould with the external texture of the plants: "The bark round the trunks must have been circularly furrowed with irregular lines, for the mudstone round them is thus plainly marked". According to the observed and previously described characteristics of the deposits, such as their areal distribution, changing thickness, flow nature, type of structures and textures, we interpret that the deposit that encloses Darwin´s Flora in the area of Agua de la Zorra was originated as a consequence of a phreatomagmatic eruption (Poma et al. 2004). The key of the enigma about the origin of the deposits rests in the identification of the pyroclastic features of the rocks containing the trunks, particularly their Figure 5: Lithological stratigraphic section of phreatomagmatic origin; this was an unk- Paramillo Formation of Uspallata Group near nown process at the time of Darwin´s Darwin´s monument. observations and interpretation (Darwin ra, fauna and populations buried by this and women were able to describe the 1844). type of processes. Seldom were human catastrophic episode. Pompeii (79 BC), Along geologic time and humankind his- witnesses present while volcanic activity for example, was buried under four me- tory there are numerous examples of flo- was taking place. In those rare cases, men ters of ash and debris. Pliny the Younger, Darwin's observation in South America: what did he find ... 19

a Roman soldier, witnessed much of the eruption (Francis 1993). Pompeii was covered over with a hot cloud of volcanic ash, steam and some mud material. The pyroclastic cloud flowed down the side of the volcano at a speed of 30 meters per second (Francis 1993). It took less than four minutes for the pyroclastic flow to travel from Vesuvius to Pompeii a few kilometres away. In 1815 more than 100,000 people died when Mount Tam- bora erupted on the Indonesian island of Sumbawa (Sutawidjaja et al. 2006). Des- pite the presence of witnesses in Mount Tambora and the discovery of Pompeii, among other eruptions, in Darwin´s time advanced knowledge about the mecha- Figure 6: Schematic nism of this volcanic activity was limited. representation of a three stage evolu- It was only during the last century and tion for the after many eruptions that pyroclastic Darwin´s Forest flows were thoroughly studied and docu- burial. a) Inactive volcanic center mented by hundreds of volcanologists surrounded by the worldwide, such as the 1980 Saint Helens forest; b) Pyroclas- among others. With detailed monitoring tic eruption with associated volcanic and modeling of active volcanism, erup- plumes and pyro- tion styles, type of ejecta, flows and clastic flows rea- clouds were understood and only then ching Darwin´s Forest; c) Darwin´s past volcanic expressions could be pro- Forest is buried by perly interpreted. In the same way it was the pyroclastic only recently that science acknowledged flows and ashes. the significant impact of these types of Lahars recognized by Brodtkorb et al. phenomena on climatic change at a pla- (1997) are included netary scale. in the sketch. A phreatomagmatic eruption was also responsible for the burial of Darwin`s eruption, especially if the deposits are tified and the sequence ends with an ash forest in Agua de la Zorra. In continental distal or out of the lateral blast influence. fall, possibly related to the co-ignimbritic volcanic environments the interaction Volcanic activity was intense enough to cloud. There is no clear evidence of an between hot magma and water frequently eliminate most of the soft tissues of the extended ash fall deposit besides the co- produces this kind of volcanism, in flora such as leaves, which are more ignimbritic cloud deposits already men- which a voluminous explosive eruption abundant in the sedimentary levels. Bu- tioned. In that regard, the eruptive of magmatic ejects emulsionated with rial must have been in a short time, as a column is not considered to be too high. magmatic and external volatiles. Figure 6 consequence of one or more eruptions As indicated by Sparks and Wilson shows an interpretation of the eruption. but close in time, because there are no (1982), a high proportion of steam in The characteristics of the pyroclastic recorded breaks between the flows that phreatomagmatic eruptions results in sequence of Agua de la Zorra allow us to cover the trunks. shorter columns. Changing thickness and make another interpretation: The erup- Only one paleosoil was observed below a varying structures and textures seem to tion was not destructive (at least not in tree (Fig. 3b). Its thickness is less than 50 imply an unstable feeding system from that sector) as the trees remain in life po- cm. It is dark with root marks. It was the magma chamber to the eruptive co- sition; so it is inferred the eruption was covered by a thin and diluted flow with lumn. Instability could have been the not laterally directed and can be suppo- low particle/gas ratio; it possibly was a consequence of the variations in the sed to be vertical. However, there is no base surge above which a series of pyro- magma supply and in any case is respon- concluding evidence to disregard a lateral clastic flows accumulated. They are stra- sible for the recorded duality in the mas- 20 S. POMA, V. D. LITVAK, M. KOUKHARSKY, E. B. MAISONNAVE AND S. QUENARDELLE

sive form or diffusely stratified deposits. na 34: 485-496. Mendoza (Argentina) Parte I: geología, conte- This interpretation is consistent with a Brea, M. 2000. Paleoflora triásica de Agua de la nido paleoflorístico y cronoestratigrafía. first deposit in the area made up of initial Zorra, Uspallata, provincia de Mendoza, Ar- Ameghiniana 31: 161-176. base surge such as the ones locally obser- gentina: Lycophyta y Filicophyta. Ameghi- Poma, S., Quenardelle, S. and Koukharsky, M. ved overlying the paleosoil. Eruptions of niana 37: 199-204. 2004. Un flujo piroclástico triásico en la For- these characteristics are violent and also Cortés, J.M., González Bonorino, G., Koukhar- mación Paramillo, provincia de Mendoza. In of rapid deposition. This amount of ash sky, M., Pereyra, F. and Brodtkorb, A. 1997. Brodtkorb, M. K. De, Koukharsky, M., Que- had already surprised Darwin who, as we Hoja geológica 3369-09 Uspallata, provincia nardelle, S. and Montenegro, T. (eds.) Avances mentioned earlier, had interpreted it as de Mendoza. SEGEMAR, (unpublished re- en Mineralogía, Metalogenia y Petrología material of sedimentary origin. port) 165 p., Buenos Aires. 2004, 385-390, Río Cuarto. Darwin, C. 1846. Geological observations on Ramos, V.A. and Kay, S. 1991. Triassic rifting and CONCLUSIONS South America. Being the third part of the associated basalts in the Cuyo Basin, central geology of the voyage of the Beagle, under Argentina. In Harmon, R.S. and Rapela, C.W. The value of Darwin´s work is the accu- the command of Capt. Fitzroy, R.N. during (eds.) Andean Magmatism and its Tectonic racy of the descriptions and the detailed the years 1832 to 1836. Smith Elder and Co. Setting, Geological Society of America, Spe- degree of his observations, which are still 280 p., London. cial Paper 265: 79-91. valid nowadays. He was successful in the Darwin, C. 1844. Geological observations on the Rolleri, E.O. and Criado, R.P. 1968. La Cuenca interpretation that the forest was buried volcanic islands visited during the voyage of Triásica del norte de Mendoza. Terceras Jor- in life. Despite Darwin misinterpreted H.M.S. Beagle. Smith, Elder and co., 176 p., nadas Geológicas Argentinas (Comodoro Ri- the origin of the deposits as sedimentary, London. vadavia), Actas 1: 1-76. he was accurate in describing the fine Francis, P. 1993. Volcanoes, a planetary perspec- Sparks, R.S.J. and Wilson, L. 1982. Explosive vol- volcanic materials as an essential part of tive. Clarendon Press. 443 p., New York. canic eruptions- V. Observations of plume them, although he never considered the Harrington, H.J. 1941. Investigaciones geológicas dynamics during the 1979 Soufrière eruption, eruptive process as the one responsible en las sierras de Villavicencio y Mal País, pro- St. Vincent. Geophysical Journal Internacio- for the burial. The present interpretation vincia de Mendoza. Dirección Nacional de nal 69: 551-570. as a phreatomagmatic mechanism is now Geología y Minería, Boletín 49, 54 p., Buenos Stappenbeck, R. 1910. La Precordillera de San plausible because of the advances in vol- Aires. Juan y Mendoza. Ministerio de Agricultura de canological knowledge and understan- Harrington, H.J. 1971. Descripción geológica de la Nación, Sección Geología, Mineralogía y ding during the latter years of the la Hoja 22c, "Ramblón", provincias de Men- Minería, Anales 4(3): 3-187, Buenos Aires. Twentieth Century. doza y San Juan. Dirección Nacional de Geo- Stipanicic, P.N. 1972. La Cuenca Triásica de Ba- logía y Minería, Boletín 114, 81 p., Buenos rreal (provincial de San Juan). In Leanza, A.F. ACKNOWLEDGEMENTS Aires. (ed.) Geología Regional Argentina, Academia Kokogian, D.A. and Mancilla, G.H. 1989. Análisis Nacional de Ciencias: 537-566, Córdoba. This work was funded by University of estratigráfico y secuencial de la Cuenca Cu- Stipanicic, P.N. 1979. El Triásico del Valle del Río Buenos Aires grant UBACYT X207. The yana. In Chebli, G. and Spalletti, L. (eds.) Cuen- de Los Patos (provincia de San Juan). In Tur- authors are particularly grateful to S. Kay, cas Sedimentarias Argentinas. Facultad de Cien- ner, J.C.M. (ed.) Geología Regional Argentina, E. Llambías and P. Sruoga for their re- cias Naturales, Universidad Nacional de Tu- Academia Nacional de Ciencias 1: 695-744, view of the manuscript and to Miguel cumán, Serie Correlación Geológica 6: 169-201. Córdoba. Griffin for his valuable suggestions to Kokogian, D.A., Spalletti, L., Morel, E., Artabe, Strelkov, E.E. and Alvarez, L.A. 1984. Análisis improve the English version. A., Martínez, R.N., Alcober, O.A., Milana, J.P., Estratigráfico y Evolutivo de la Cuenca Triá- Zavattieri, A.M. and Papú, O.H. 1999. Los sica Mendocina - Sanjuanina. 9° Congreso Geo- WORKS CITED IN THE TEXT depósitos continentales triásicos. In Caminos, lógico Argentino (Bariloche), Actas 3: 115-130. R. (ed.) Geología Argentina, Instituto de Geo- Sutawidjaja, I.S., Sigurdsson, H. and Abrams, L. Brodtkorb, A., Cortés, J.M., González Bonorino, logía y Recursos Naturales, Anales 29(15): 2006. Characterization of volcanic deposits G. and Koukharsky, M. 1997. Sitios de interés 377-398, Buenos Aires. and geoarchaeological studies from the 1815 geológico correspondientes a la Hoja 3369- Massabie, A. 1986. Filón capa Paramillos de Us- eruption of Tambora volcano. Jurnal Geologi 09, Uspallata, provincia de Mendoza, SEGE- pallata, su caracterización geológica y edad, Indonesia 1(1): 49-57. MAR, unpublished, 40 p. Buenos Aires. Paramillo de Uspallata, Mendoza. Primeras White, J.D.L. and Houghton, B.F. 2006. Primary Brea, M. 1997. Una nueva especie fósil del géne- Jornadas sobre Geología de la Precordillera, volcaniclastic rocks. Geology 34: 677-680. ro Araucarioxylon Kraus 1870 emend. Mahesh- Asociación Geológica Argentina, Serie A, 2: wari 1972 del Triásico de Agua de la Zorra, 325-330. Recibido: 27 de agosto de 2008 Uspallata, Mendoza, Argentina. Ameghinia- Morel, E.M. 1994. El Triásico del cerro Cacheuta, Aceptado: 17 de octubre de 2008 Revista de la Asociación Geológica Argentina 64 (1): 21 - 31 (2009) 21

DARWIN FOREST AT AGUA DE LA ZORRA: THE FIRST IN SITU FOREST DISCOVERED IN SOUTH AMERICA BY DARWIN IN 1835

Mariana BREA1,4, Analía E. ARTABE2,4 and Luis A. SPALLETTI3,4

1Laboratorio de Paleobotánica, Centro de Investigaciones Científicas, Diamante. CICYTTP-Diamante, Consejo Nacional de Investigaciones Científicas y Técnicas, Diamante, Entre Ríos, Email: [email protected] 2División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, La Plata. Email: [email protected] 3Centro de Investigaciones Geológicas (CONICET-UNLP), La Plata. Email: [email protected] 4Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

ABSTRACT The Agua de la Zorra area (near Uspallata, Mendoza, Argentina) is one of the best renowned fossil localities of the country because of its spectacular in situ fossil forest. This forest was discovered by Charles Darwin in 1835, who described this forest as monotypic and assigned it a Tertiary age. Nowadays, this fossil locality is known as the Darwin Forest. Over a century and a half later it was reinterpreted as a mixed Middle Triassic forest and a new fossil monotypic palaeocommunity of horsetails was discovered. This palaeovegetation is included in the Paramillo Formation (i.e., lower section the Potrerillos Formation) of northwestern Cuyo Basin, Mendoza province (69°12' W and 32°30' S). The sediments were deposited in a sinuous fluvial system, in which channel-filling sand bodies were associated with mud-dominated floodplain deposits. The palaeoforest grew on an andisol soil that developed on volcaniclastic floodplain deposits. It had a density of 427 -759 trees per hectare, and was constituted by conifers and corystosperms distributed in two arboreal strata. The highest reached 20-26 m tall, and was dominated by corystosperms, but it also included the tallest conifers. The second stratum, mainly composed of conifers, ranged between 16-20 m tall. The forest has also emergent corystosperms, which reached 30 m tall. The understorey was composed of ferns. Growth ring anatomy suggests that conifers could have had an evergreen habit. Structure of vegetation, growth ring analyses and sedimentation suggest that the forest developed under dry, subtropical, and strongly seasonal conditions.

Keywords: Palaeovegetation, Palaeoecology, Palaeoenvironments, Middle Triassic, Southwestern Gondwana.

RESUMEN: El Bosque Darwin en Agua de la Zorra: El primer bosque in situ descubierto en América del Sur por Darwin en 1835. El área de Agua de la Zorra (cerca de Uspallata, Mendoza, Argentina) es uno de los sitios fósiles más espectaculares y renombrados del país porque aflora un bosque in situ. Este bosque descripto como monotípico y asignado al Terciario fue descubierto por Charles Darwin en 1835. Un siglo y medio más tarde, se reinterpretó como un bosque mixto del Triásico Medio y se recono- ció una nueva paleocomunidad monotípica de esfenófitas. La paleovegetación proviene de la Formación Paramillo (= sección inferior de la Formación Potrerillos) de la región noroeste de la Cuenca Cuyana, provincia de Mendoza (69°12'O y 32°30'S). Los sedimentos fueron depositados por un sistema fluvial de alta sinuosidad. El bosque fósil creció sobre suelos del tipo andisol que se desarrollaron sobre depósitos de planicies volcaniclásticas. El bosque Darwin tiene una densidad de 427-759 árbo- les por hectárea. Este está constituido por coníferas y corystospermas y tiene dos estratos arbóreos. El más alto se desarrolla entre los 20-26 m, y es dominado por corystospermas, pero también por las coníferas más altas. El segundo estrato arbóreo, principalmente compuesto de coníferas, con un rango entre 16-20 m. El bosque tiene también algunos emergentes de corystospermas las cuales alcanzan hasta 30 m de altura. El sotobosque estaba compuesto de helechos. La anatomía de los anillos de crecimiento sugiere que las coníferas podrían haber tenido hábito siempreverde. La estructura de la vegetación, el análisis de los anillos de crecimiento y la sedimentología sugieren que este bosque se desarrollo bajo condiciones climáticas subtropi- cales, secas y estacionales.

Palabras clave: Paleovegetación, Paleoecología, Paleoambientes, Triásico Medio, Sudoeste de Gondwana.

INTRODUCTION during the 18th and 19th centuries (Ottone logical observations of the Agua de la 2005). The British naturalist Charles Ro- Zorra area, Uspallata, near Villavicencio, The first fossil plants in South America bert Darwin (1809- 1882) was the first Mendoza province in Argentine territory were recorded by European naturalists who provided palaeontological and geo- (Darwin 1839a, 1839b, 1845, 1846). La- 22 M. BREA, A. E. ARTABE AND L. A. SPALLETTI

ter, this famous fossiliferous locality was On 30 March 1835 he collected the first fossil forests (Paramillo, Cortaderita, visited by other European naturalist- specimens of fossil wood from Agua de Ischigualasto, and Río Blanco Forma- explorers such as Burmeister 1861, Doe- la Zorra. The samples were sent to Lon- tions) and their occurrences have contri- ring 1882, Stelzner 1885, Avé-Lallemant don and were referred by Robert Brown buted significantly to our understanding 1891 (see Ottone 2005). to the genus Araucarites (Darwin 1846). of Triassic Gondwana ecosystems (Za- The expeditions of Charles Darwin -on Recently, one of the authors (M. B.) ca- muner 1992, Artabe et al. 2001, 2007a, board HMS Beagle- during his historic rried out three research field trips as part Brea et al. 2008). journery around the world between 1831 of work towards her PhD dissertation. The Darwin Forest was reinterpreted as a and 1836, contributed valuable informa- Brea (1995) characterized and studied the subtropical dry seasonal forest (Brea et al. tion and observations on the geology, Triassic units at Agua de la Zorra from a 2005, 2008). It grew on an andisol soil plant and animal fossils, and extant orga- palaeobotanical and sedimentological that developed on volcaniclastic flood- nisms of South America. Darwin also viewpoint. She defined two palaeocom- plain deposits. The volcanic detritus and collected a huge number of specimens munities in the Paramillo Formation, to- the rhythmic amalgamation of upper and samples, many of them new to scien- gether with their associated palaeoenvi- flow-regime tractional deposits overlying ce. roments (Fig. 2). The first paleocommu- the andisol indicate that the forest was Darwin arrived in Argentina in 1833 and nity, the in situ Darwin Forest (Fig. 2), buried rapidly by a subaerial, cool and visited the Río de La Plata area, La Ba- appears at four fossiliferous sites (Brea wet pyroclastic base surge flow (Poma et jada (now Paraná city, Entre Ríos), the 1995), while the second was found at al. 2004, Brea et al. 2008). Santa Cruz River and many other sites in only one locality and included only fossil The continental Triassic succession of Patagonia and Tierra del Fuego. Then, horsetails (Brea and Artabe 1999, Brea et southwestern Gondwana occurs in a se- after crossing the Cordillera de los Andes al. 2008). ries of narrow rifts produced as a result from Chile, he arrived in Mendoza pro- The Darwin Forest was first referred to of Triassic continental extension. These vince and discovered the first in situ fo- the Tertiary by Darwin (1839a, 1839b), rift basins are composed of a continental rest in South America, at Agua de la Zo- but new data now available suggest a late clastic infilling, and record complex rra, located about 25 km from the town Middle Triassic age (Spalletti et al. 1999, interactions between alluvial, fluvial, del- of Uspallata (Conwentz 1885, Rusconi Brea et al. 2008). The most important fea- taic and lacustrine depositional systems 1941, Harrington 1971, Brea 1995, Otto- ture of the Darwin Forest is that the with intercalations of volcaniclastic se- ne 2005). He recorded the presence of trees are still preserved at the sites on quences in most of these basins. The rich 52 fossil tree trunks measuring 90-152 which they grew (Fig. 2); it is the best floristic record allowed recognition of cm in diameter, buried in sandstones or renowned of its age in southwest Gond- several assemblages, biozones and stages volcanic sandstones and standing out as wana. Although petrified forests in characterized by floristic events (Spalletti columns several meters high (Darwin growth position are very scarce in the et al. 1999, 2003, Artabe et al. 2001, 2003, 1846). This discovery by Darwin is re- geological record, the Triassic fossil re- 2007b, Morel et al. 2003). membered by a monolith (Fig. 1). cord of SW Argentina shows four in situ Over the last two decades, most investi-

Figure 1: a. Charles Darwin's tribute plaque at Agua de la Zorra, near Uspallata, Mendoza, Argentina. This photo was taken in 1993. b. Panoramic view of Triassic sequences, the arrow shows the plaque. Darwin forest at Agua de la Zorra: the first in situ forest discovered ... 23

Figure 2: The Darwin Forest at Agua de la Zorra area (near Uspallata, Mendoza). a-b-c-e. Large fossil tree stumps in growth position in Triassic sequences; d-f-h. Petrified standing trees at fossiliferous sites C; g. Large fossil log exposed in Triassic sediments at fossiliferous site B. 24 M. BREA, A. E. ARTABE AND L. A. SPALLETTI

gations on the Argentinian Triassic have focused on the gross stratigraphy and taxonomy of fossil plants (Stipanicic 2001, Stipanicic and Marsicano 2002, Za- muner et al. 2001, and references therein). However, recently published studies (Spalletti et al. 1999, 2003, Artabe et al. 2001, 2003, 2007b, Morel et al. 2003) provided an increasing understanding of how the Triassic palaeofloras developed and changed over time as a response to variations in depositional systems, tectonism and climate. The most spectacular fossil sites of the Paramillo Formation- in the Agua de la Zorra area -, are located in the northern- western sector of the Cuyo Basin (between 32º and 36º SL). The Paramillo For- mation is composed of a 140 m thick succession of clast-supported conglomerates, pebbly sandstones, tuffaceous sandstones and mudstones (Figs. 3 and 4). These deposits have been previously described by Harrington (1971), Strelkov and Álvarez (1984), Kokogian and Mancilla (1989), Ramos and Kay (1991), Massabie (1985), Massabie et al. (1985), Linares and González (1990), Ramos (1993), Kokogian et al. (1993). These au- Figure 3: Location map showing Agua de la Zorra fossiliferous locality, Mendoza province, Argentina indicating the four fossiliferous sites: A, B, C, D respectively. thors all agreed in interpreting the Pa- ramillo Formation as deposited in highly dish ash fall tuffs (Fig. 4). The sedimen- ripple-laminated sandstones and interca- sinuous fluvial systems. tary record of the overlying Agua de la lations of laminated mudstones and sha- Previous palaeobotanical studies, com- Zorra Formation is dominated by bitu- les. Brea (1995) interpreted that these prising mostly lists of fossil plants, were minous shales and marls with subordina- sediments were deposited in a highly published by Darwin (1846), Conwentz te intercalations of yellowish fine-grai- sinuous (meandering) fluvial system, in (1885), Stappenbeck (1910), Kurtz ned sandstones and mudstones (Fig. 4). which channel-filling sand bodies are (1921), Du Toit (1927), Groeber (1939), Both units are intruded by several sills of associated with mud-dominated flood- Windhausen (1941), Harrington (1971) olivine diabase dated 235 + 5 Ma (K/Ar plain deposits. The cyclic arrangement of and Stipanicic et al. (1996), while modern whole age) by Ramos and Kay (1991). At well preserved soil horizons and sedi- systematic contributions were offered by that time Mendoza was a part of the vast mentary deposits lacking evidence of soil Brea (1995, 1997, 2000), Brea and Artabe supercontinent called Gondwana and it formation indicates alternation of pe- (1999), Artabe and Brea (2003) and Brea was placed approximately at the same riods with strong sediment aggradation et al. (2005, 2008). geographic latitude as it is nowadays. produced by non-channelized high-regi- Brea (1995) studied in more detail the me flows with periods characterized by THE PARAMILLO upper part of the Paramillo Formation very low accommodation rates that FORMATION and the lowermost Agua de la Zorra favoured the development of immature Formation and defined several lithofacies soil profiles (Brea et al. 2008). The volca- The Paramillo Formation is a volcani- and facies associations. The Paramillo nic nature of detrital components sug- clastic unit composed of yellowish lithic Formation consists of cross-bedded gests that the highly aggrading non-chan- sandstones, brownish and yellowish tuf- conglomerates, cross-bedded, plane-bed- nelized flows that produced the burial of faceous sandstones, dark gray and green ded and massive pebbly sandstones, the Darwin Forest could be related to shales and mudstones, and pink to red- cross-bedded, massive, plane bedded and pyroclastic events (Poma et al. 2004). The Darwin forest at Agua de la Zorra: the first in situ forest discovered ... 25

forest might have died as a result of a diluted, subaerial, cool and wet base surge pyroclastic flow (Poma et al. 2004, Brea et al. 2008). The Paramillo Formation, where the Darwin Forest emerges, has been correlated with the lower section of the Po- trerillos Formation (Spalletti et al. 1999, Morel et al. 2003) on the basis of Assem- blage Biozone chronology, analysis of main stratigraphic unconformities, and evolution of basin infill.

ECOLOGICAL RECONSTRUCTION OF THE DARWIN FOREST

The Triassic landscape at the Agua de la Zorra area was very different in comparison with the modern-day scenery. Sphe- nopsids dominated the flooded areas and conifers and corystosperms were the most important components of the arboreal vegetation. Four exposures of the Darwin Forest were found in the Paramillo Formation during fieldwork carried out in 1993- 1994 (Figs. 2 and 3). This unit was thoroughly examined at two localities: Dar- win and El Sauce (Figs. 3 and 4), where the lithology was logged bed-by-bed in vertical sections at a 1:100 scale (see Brea et al. 2008). Imperfect carbonization processes (Po- ma et al. 2004) preserved the stumps and in just over 2 km2 one hundred and twenty stumps in growth position and fallen logs were counted (Fig. 2). The fossiliferous levels (FL) with stumps were found at four localities belonging to the same in situ forest. Thus, FL IV (at A locality), FL V (at B and C localities) and FL VI (at D locality) belong to a single Figure 4: a. Lithostratigraphic section of the Paramillo and stratigraphic level (Figs. 2 and 4). Fossi- Agua de la Zorra Formations liferous levels FL I and FL III found at at locality A, showing the main locality A (Fig. 4) have ferns preserved as lithofacies and the position of the fossil forest level. b. impressions-compressions (Brea 2000). Lithostratigraphic section of The sedimentary sequences in which this the Paramillo and Agua de la forest is preserved consist of continental Zorra Formations at locality volcaniclastic units, resulting from de- C. PF: Paramillo Formation, AZF: Agua de la Zorra position on a highly sinuous fluvial sys- Formation. FL I - V: fossilife- tem associated with river flood-plains rous levels. 26 M. BREA, A. E. ARTABE AND L. A. SPALLETTI

(Fig. 4). develop under a climatic regime that shortage at the end of the growing sea- The palaeoecological reconstruction of includes an annual cycle with one season son, a sharp photoperiod indicating end the Darwin Forest was based on the in which water is unavailable to plants of growth season or abrupt leaf shed- quantitative data of the mapped forest because of lack of precipitation. The dry ding. Therefore, A. protoaraucana growth (mean separation of trees, basal area per season alternates with another in which rings suggest strongly seasonal condi- ha, species distribution) integrated with there is abundant water. In addition to tions. The average ring width in these the taxonomic and sedimentological in- structural data of the Darwin Forest that fossil woods is 1.49 mm (0.80-2.53 mm), formation (Brea et al. 2008). The mixed match those of an extant monsoon fo- the narrowest ring is 0.12 mm and the forest was composed by corystosperms rest, the polyxyly found in Cuneumxylon widest ring is 4.44 mm. Mean sensitivity (30-40%) and conifers (60-70%). The has been considered an important adap- (MS) values range between 0.14 and 0.38. corystosperms are a group of extinct tive wood character to avoid water stress. Average MS values (0.30) indicate that plants with mostly fern-like foliage with As suggested by the functional anatomy the growing environment was stressed ovules borne on modified leaves or cu- and distribution of living groups (Fahn and not uniform. Although available data pules. This group was the dominant seed 1990), the included phloem associated are limited, the complicated growth plant in Gondwana during the Triassic with great amounts of parenchyma could trends suggest that competition, distur- (Stewart 1983). All conifers are a diverse be a strategy of subtropical plants to bance events, or climatic stress influen- group of trees and shrubs that under- fight against water stress in arid regions ced the growth of trees in the Darwin went a major radiation during the Triassic during drought seasons. Forest. Furthermore, Falcon Lang's me- period, when the first occurrence and ra- During Permian-Triassic biosphere reor- thod (Falcon-Lang 2000a, 2000b) was diation of the eight conifer families oc- ganization, aridity of the Earth increased used to distinguish between evergreen curred (Willis and McElwain 2002). The and in the Triassic the Pangea was cha- and deciduous species (see Brea et al. conifers have a pyramidal arborescent racterized by strongly seasonal climates 2008). The quantitative growth ring ana- growth form, with cone-bearing seed in a warm-house period (Parrish et al. tomy analysis method (Falcon-Lang plants; the foliage is either needle-like or 1982, Dubiel et al. 1991, Parrish 1993, 2000a, 200b) indicates that the conifer scale-like. The plants are mainly small, Scotese et al. 1999). The spreading of fossil woods of the Darwin Forest pro- long, and thin arranged spirally and bea- continental climates caused the extension bably belong to an evergreen gymnos- ring the male and female reproductive of semiarid belts into middle latitudes perm. The understorey of the Darwin organs in separate cones on different and, partly, into high latitudes too Forest includes Cladophlebis mesozoica trees, or indifferent parts of the same. (Chumakov and Zharkov 2003). Recen- Frenguelli 1947 (Fig. 6, b), Cladophlebis The average density of the tree stumps in tly, general circulation models (GCMs) mendozaensis (Geinitz) Frenguelli 1947 this forest is 556 trees/ha. The spatial au- were developed by Sellowood and Valdes (Fig. 6, c), and Cladophlebis kurtzi Fren- tocorrelation for the species variable (2006) to simulate Mesozoic climatic pat- guelli 1947 (Fig. 6 a) (Brea 2000). Cla- indicates that in some places the corys- terns. In this scheme the Triassic South- dophlebis is an extinct genus of ferns cha- tosperms and conifers are intermingled western Gondwana is modelled as seaso- racterized by the presence of large steri- while in others the corystosperms and nal and winter-wet against the opinion of le bipinnate foliage and it was widely dis- conifers seem to be aggregated into co- Robinson (1973) who stated that it was tributed in southern Gondwana during hesive social groups (Fig. 5). The Darwin seasonal and summer-wet. the Mesozoic. Forest reveals two strata: the highest, Growth-ring analysis of coniferous Fossiliferous level FL II found at locality developed between 20-24 m, has a pre- wood - assigned by Brea (1997) to Arau- A (Figs. 3 and 4) records another mo- ponderance of corystosperms but also carioxylon protoaraucana - was used to eva- notypic palaeocommunity dominated by the tallest conifers; the second stratum, luate climatic conditions (Fritts 1976, sphenopsids (Brea and Artabe 1999).The mainly composed of conifers, ranges be- Holmes 1985). Conifer growth rings are sedimentary sequences in which this tween 16-20 m. The forest also has emer- narrow and subtly demarcated; they are community is preserved consist of conti- gent corystosperms which reach 30 m characterized by a relatively wide zone of nental clastic units composed of black tall (Brea et al. 2008). The corystosperms large, thin-walled early-wood cells termi- shales and mudstones resulting from were assigned by Artabe and Brea (2003) nated by ten to seventeen thick-walled deposition on the flood plains of a flu- to Cuneumxylon spallettii and constitute the late-wood cells. The presence of narrow vial system. This autochtonous taphoce- dominant species in the Darwin Foest. growth rings indicates poor conditions nosis is constituted by stems, nodal dia- This fossil forest presents values of bio- for cell division and expansion during phragms and reproductive structures mass and stand basal area comparable to growth season (Creber and Chaloner assigned to Neocalamites carrerei (Zeiller) those of the current subtropical seasonal 1984). In addition, a thin late-wood zone Halle 1908 (Fig. 6 d-e), aff. Nododendron forests (Brea et al. 2008). These forests within a ring can be the result of water suberosum Artabe and Zamuner 1991 (Fig. Darwin forest at Agua de la Zorra: the first in situ forest discovered ... 27

Figure 5: Reconstruction of the Triassic Darwin Forest landscape in a high sinuosity fluvial system, in which channel-filling sand bodies are associated with mud-dominated floodplain deposits. The canopy is integrated by two arboreal strata and emergent trees with conifers and corystosperms. The understorey is formed by ferns. Cinodonts are characteristic tetrapods during the Triassic of the Cuyo Basin (drawing by Jorge Gonzalez).

6 f), and Neocalamostachys arrondoi Brea Blanco (Late Triassic) formations (Za- forest community colonized stressed and Artabe 1999 (Fig. 6 g-h). The stem of muner 1992, Spalletti et al. 1999, Artabe et ecosystems (Artabe et al. 2007b). Neocalamites and nodal diaphragm of al. 2001, Artabe et al. 2007b). Presently, At very high latitudes, in the central Nododendron found in close association the only available structural data are from Transantarctic Mountains, another in situ with reproductive Neocalamostachys were the Late Triassic corystosperm forest corystosperm riparian forest was found probably based on parts of individuals of found at La Elcha Mine (Río Blanco For- and assigned to the Middle Triassic (Cú- a single taxon (Brea and Artabe 1999). mation, Cuyo Basin). This is an evergre- neo et al. 2003). The permineralized Because plants growing in wet habitats en and monotypic community composed stumps were described as Jeffersonioxylon are hygrophytes, this monotypic horsetail of 150 stumps in life position, and the Del Fueyo et al. (1995) and later assigned palaeocommunity conformed a water- trees colonized well-drained proximal to the Corystospermaceae by Cúneo et al. dependent, bamboo-like thicket with flood-plain areas, close to channel belts (2003). This monotypic forest is interpre- hygrophytic adaptations associated to the (Artabe et al. 2007b). The permineralized ted as deciduous and the tree density and flood plains of a fluvial environment stumps were described and assigned to the basal area are around 274 tree/ha and (Fig. 7). Elchaxylon zavattieriae (Corystosperma- 20.83 m2/ha respectively. The Antarctic ceae) by Artabe and Zamuner (2007). plant has been reconstructed as a tree of COMPARISON WITH Vegetation analysis of the La Elcha fo- ~20-30 m with coniferous-like habit OTHERS TRIASSIC rest shows that it has a clustered distribu- (Cúneo et al. 2003). PETRIFIED FORESTS tion pattern, with a forest density of 727- Petrified remains of a Late Triassic forest 1504 tree /ha. The deduced height of were preserved in the Paraná Basin, Three other in situ Triassic petrified fo- Elchaxylon and the distribution of class Brazil (Pires et al. 2005). This forest was rests dominated by corystosperms have diameters suggest that the canopy in the dominated by Sommerxylon spiralosus,a been discovered in Argentina, i.e., in the forest community would have had the morphotaxon with taxacean affinities Cortaderita (late Middle Triassic), Ischi- majority of specimens ranging 13-21 m. (Pires and Guerra Sommer 2004). gualasto (early Late Triassic) and Río Growth ring analysis indicates that the Growth ring analysis indicates that the 28 M. BREA, A. E. ARTABE AND L. A. SPALLETTI

Figure 6: a. Cladophlebis kurtzii Frenguelli (LPPB 12618); b. Cladophlebis mesozoica Frenguelli (LPPB 12614); c. Cladophlebis mendozaensis (Geinitz) Frenguelli (LPPB 12621); d-e. Neocalamites carrerei (Zeiller) Halle (LPPB 12584 and LPPB 12581); f. aff. Nododendron suberosum Artabe et Zamuner (LPPB 12591); g-h. Neocalamostachys arrondoi Brea et Artabe (LPPB 12593 and 12565). Scale bars, e =2 cm, f, g and h= 1cm. climate was not equable but rather with forest lies within the Chinle Formation nities were preserved in the Paramillo marked seasonal variations. The fossil and is mainly dominated by Woodworthia Formation, at Agua de la Zorra, north- wood shows a distinctive seasonal pat- arizonica and Schilderia adamanica (Creber western Cuyo Basin, in southwestern tern related to growth cycles, with exten- and Ash 2004). The fossil woods of this Gondwana, i.e., the Darwin Forest asso- sive favourable and restricted unfavoura- forest do not show annual growth rings ciated with banks of highly sinuous flu- ble growth periods. The external factors but contain irregular growth interrup- vial systems, and second including fossil that affected the cycles were mainly relations similar to those found in trees horsetails that grew on flood plains of ted to cyclic restrictions of water supply nowadays growing in humid tropics. fluvial systems. and irregular changes in environmental These interruptions could be due to en- The Darwin Forest was a mixed forest, conditions, probably linked to occasional dogenous hormonal effects or to occa- integrated by corystosperms (Cuneumxy- droughts during the growing season sional local variations in water supply lon spallettii) and conifers (Araucarioxylon (Pires et al. 2005). (Ash and Creber 1992). protoaraucana). Species distribution shows The Petrified Forest National Park in that in some places corystosperms and Arizona is a better known Late Triassic SUMMARY conifers are intermingled, whereas in fossil forest in the southwestern United others corystosperms and conifers appe- States (Ash and Creber 1992). This fossil Two late Middle Triassic palaeocommu- ar to be aggregated into cohesive mo- Darwin forest at Agua de la Zorra: the first in situ forest discovered ... 29

Figure 7: Reconstruction of Triassic horsetail landscape in the flood-plain of a fluvial environment. In the background are groups of conifers (drawing by Jorge Gonzalez). notypic groups. The canopy is formed by ACKNOWLEDGEMENTS Artabe, A.E., and Zamuner, A.E. 2007. Elcha- two arboreal strata with a small number xylon, a new corystosperm based on permine- of emergent; the understorey with ferns. The authors would like to express their ralized stems from the late Triassic of Argen- The included phloem and associated thanks to Guillermo Ottone, Greg Re- tina. Alcheringa 31: 85-96. parenchyma present in Cuneumxylon spa- tallack and one anonymous reviewer for Artabe, A.E., Morel, E.M. and Spalletti, L.A. llettii may be an important adaptative their critical and constructive comments 2001. Paleoecología de las floras triásicas ar- strategy to avoid water stress. Thus, po- on an earlier version of this paper. We gentinas. In Artabe, A.E., Morel, E.M. and lyxyly as a functional anatomy characters would like to thank the financial support Zamuner, A.E. (eds.) El Sistema Triásico en la correlates with a seasonal climatic regi- provided by the Consejo Nacional de Argentina. Fundación Museo de La Plata me. Morpho-functional analysis, structu- Investigaciones Científicas y Técnicas Fracisco P. Moreno: 199-225, La Plata. ral parameters, biomass and basal stand (Project PIP 5430) and Universidad Artabe, A.E., Morel, E.M. and Spalletti, L.A. area of the Darwin Forest allows classif- Nacional de La Plata (11/N535). The 2003. Caracterización de las provincias fitoge- ying it as a mainly dry evergreen subtro- charts of the Triassic vegetation recons- ográficas triásicas del Gondwana extratropi- pical seasonal forest. The anatomy of the truction were made by Jorge Gonzalez. cal. Ameghiniana 40: 387-405. growth rings of Araucarioxylon protoarau- Artabe, A.E., Morel, E.M and Ganuza, D.G. 2007 cana suggests strongly seasonal condi- WORKS CITED IN THE TEXT a. Las floras trásicas de La Argentina. In Ar- tions. Moreover, growth ring analysis in- changelsky, S., Sánchez, T. and Tonni, E.P. dicates that this species was an evergreen Artabe, A.E., and Brea, M. 2003. A New Appro- (eds.) Asociación Paleontológica Argentina, gymnosperm. ach to Corystospermales based on petrified Publicación Especial 11, Ameghiniana 50° The second palaeocommunity integrated stems from the Triassic of Argentina. Alche- aniversario: 75-86. by sphenopsids shows herbaceous-arbus- ringa 27: 209-229. Artabe, A.E., Spalletti, L.A., Brea, M., Iglesias, A., tive vegetation with hygrophytic adapta- Artabe, A.E., and Zamuner, A.E. 1991. Una nue- Morel, E.M. and Ganuza, D.G. 2007 b. tions. A fluvial flood-plain in an open va equisetal del Triásico de Cacheuta, Argen- Structure of a corystosperm fossil forest landscape was the environment in which tina, con estructura interna preservada. Ame- from the Late Triassic of Argentina. Palaeo- this fossil vegetation developed. ghiniana 28: 287-294. geography, Palaeoclimatology, Palaeoecology 30 M. BREA, A. E. ARTABE AND L. A. SPALLETTI

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Palaeontograhica, Supplement- Recibido: 8 de agosto de 2008 southernmost Parana Basin (Brazil): evidence bände 3, 29: 1-329. Aceptado: 28 de octubre de 2008 32 Revista de la Asociación Geológica Argentina 64 (1): 32 - 42 (2009)

ON DARWIN'S FOOTSTEPS ACROSS THE ANDES: TITHONIAN-NEOCOMIAN FOSSIL INVERTEBRATES FROM THE PIUQUENES PASS

Beatriz AGUIRRE-URRETA and Verónica VENNARI

Laboratorio de Bioestratigrafía de Alta Resolución, Departamento de Ciencias Geológicas, Universidad de Buenos Aires, Ciudad Universitaria, Buenos Aires. Email: [email protected]

ABSTRACT The aim of this work is to summarize the modern knowledge of the geology of the Piuquenes Pass, in the Main Andes of Argentina and Chile, and to describe a small fauna of Tithonian-Neocomian invertebrates mostly represented by ammonites. The present knowledge of the region is compared with Darwin's, as expressed in his famous book on the Geology of South America.

Keywords: Main Andes, Mesozoic, Ammonites, Piuquenes Pass, Darwin.

RESUMEN: Tras las huellas de Darwin a través de los Andes: invertebrados fósiles del Tithoniano-Neocomiano del paso de Piuquenes. El objetivo de este trabajo es presentar una breve revisión moderna de la geología del Paso de Piuquenes en los Andes Principales de Chile y Argentina y describir una pequeña fauna de invertebrados del Tithoniano-Neocomiano compuesta principalmente por amonites. Se compara también el conocimiento de esta región con la referida por Darwin en su famoso libro sobre la geología de América del Sur.

Palabras clave: Andes Principales, Mesozoico, Amonites, Paso de Piuquenes, Darwin.

INTRODUCTION scientific point of view as can be seen in thus: "I have just returned from Mendoza, the long and detailed letters he sent both having crossed the Cordilleras by two passes. During his 5 years' voyage around the to Professor Henslow and to his sister This trip has added much to my knowledge of world on board the HMS Beagle, Dar- Susan. The letter to Henslow begins the geology of the country. Some of the facts, of win spent more than 3 years on shore. This was most probably due to his interest in exploring new regions, but also to avoid the strong sea-sickness that anno- yed him when sailing. During the Beagle years and some time later Darwin was foremost a geologist, though we are used to think of him as a pure biologist. Thus, his enormous collection of fauna and flora came second to his interest as can be seen in the fact that he gave most of it to different specialists after his return to England. In March-April 1835, Darwin undertook a long ride to examine the geology of the Andes. He started the journey in Valpa- raiso on the Pacific coast, crossed the Andes along the Piuquenes and Portillo passes, reached Mendoza city and went back to Chile by the Cumbre or Uspallata Pass (Fig. 1). This trip was probably one of the most valuable to him from a Figure 1: Location of the Piuquenes Pass in the Main Andes of Argentina and Chile. On Darwin's footsteps across the Andes: Tithonian-Neocomian fossil ... 33

Figure 2: Geological map of the region of the Piuquenes Pass, Main Andes (from Pángaro et al. 1996). the truth of which I in my own mind feel fully dary between Argentina and Chile in the rhyolitic tuffs and lavas of Permian and convinced, will appear to you quite absurd & Main Central Andes (33°38'S, 69°52'W, Triassic age are unconformably overlying incredible", followed by several paragraphs Fig. 1). It reaches 4,030 m a.s.l., and the basement (Polanski 1964). The Me- devoted to the structure of the cordillera though it is the international divide, the sozoic sequences, which record a Pacific (Burkhardt and Smith 1985). highest pass is Portillo, located some 24 marine transgression, constitute different One hundred and sixty years later, in the km to the east, with 4,374 m a.s.l. stratigraphic cycles described by Groeber summer of 1995 one of us (BAU) follo- The geology of this part of the Main (1953), Malumián and Ramos (1984) and wed the steps of Darwin along the Piu- Andes was studied in detail by Polanski Riccardi (1983, 1988). quenes Pass, checking the stratigraphy (1964) as part of the mapping program These sedimentary successions are cove- and collecting fossils. Thus, the objective of the Geological Survey of Argentina. red by extensive Cenozoic volcanic and of this work is to summarize the modern It is characterized by a normal subduc- pyroclastic rocks, including some Recent knowledge of the geology and palaeon- tion zone, where the oceanic Nazca plate volcanic centers, such as San José and tology of the region of the Piuquenes subducts beneath South America at an Marmolejo volcanoes. Isolated patches Pass, in the Main Andes of Argentina angle of 30° (Jordan et al. 1983). As a of colluvial and alluvial deposits are wid- and Chile, comparing these views with result of that the Mesozoic sequence is ely distributed in the region (Fig. 2). Darwin's (1846), as expressed in his fa- heavily deformed in a series of thrust sli- mous book on the Geology of South ces (see Giambiagi et al. 2009). These THE GEOLOGY OF THE America. thrusts produced an imbricated sequen- PIUQUENES PASS ce, where the strata are repeated several REGIONAL GEOLOGICAL times, creating a complex structure. A geological map of the Piuquenes Pass SETTING The basement of the Andes at this latitu- region is shown in figure 2 (Pángaro et al. de is composed of metamorphic rocks of 1996), where Darwin's itinerary is mar- The Piuquenes (Darwin always spelled it Precambrian age. Carboniferous marine ked, and the rock succession according as Peuquenes) Pass runs across the boun- successions devoid of fossils and some to Darwin (1846) is also shown. He des- 34 B. AGUIRRE-URRETA AND V. VENNARI

cribed the following: the Auquilco Formation of Argentina nes and black shales with Spitidiscus riccar- "The ridge of Peuquenes, which divides the wa- (Río Colina Formation in Chile) of Ox- dii Leanza and Wiedmann, 1992 (Agui- ters flowing into the Pacific and Atlantic oceans, fordian-Kimmeridgian age, and T is rre-Urreta 2001), Protohemichenopus neu- extends in a nearly N.N.W. and S.S.E. line; again the Tordillo Formation. It is ob- quensis Camacho, 1953, Lucinidea indet., its strata dip eastward at an angle of between vious now that Darwin did not quite and callianassid crustaceans. 30° and 45°, but in the higher peaks bending understand the complex structure of the The fossils that Darwin collected in Q up and becoming almost vertical. Where the region, as can be seen in the geological and S (presently Lo Valdés Formation/ road crosses this range, the height is 13,210 feet map of figure 2, where a western back- Mendoza Group) were studied by d'Or- above the sea-level, and I estimated the neighbou- thrust puts the Auquilco Formation on bigny and cited by Darwin (1846, p. 181) ring pinnacles at from 14,000 to 15,000 feet. top of the Agrio Formation and a as follows: The lowest stratum visible in this ridge is a red second fore-thrust repeats again the stratified sandstone [P]; on it are superimposed Agrio and Chachao Formations (Pángaro "The fossils above alluded to in the black calca- two great masses [Q and S] of black, hard, et al. 1996, Giambiagi et al. 2009). reous shales are few in number, and are in an compact, even having a conchoidal fracture, cal- A section was measured on the Argenti- imperfect condition; they consist, as named for me careous, more or less laminated shale, passing nean slope (Fig. 3), along the northern by M. d'Orbigny, of into limestone: this rock contains organic re- bank of the Arroyo Piuquenes (Aguirre- 1. Ammonite, indeterminable, near to A. recti- mains, presently to be enumerated. The compac- Urreta 1996; see location in Fig. 2). The costatus, D'Orbig. Pal. Franc. (Neocomian ter varieties fuse easily in a white glass; and this top beds of the Tordillo Formation formation). I may add is a very general character with all the (Kimmeridgian) are sandstones interbed- 2. Gryphæa, near to G. Couloni, (Neoco- sedimentary beds in the Cordillera: although this ded with siltstones and green shales. The mian formations of France and Neufchatel). rock when broken is generally quite black, it Vaca Muerta Formation begins with a 3. Natica, indeterminable. everywhere weathers into an ash-grey tint. Be- few meters of limestone with algal lami- 4. Cyprina rostrata, D'Orbig. Pal. Franc. tween these two great masses [Q and S], a bed nation, followed in its lower part by black (Neocomian formation). [R] of gypsum is interposed, about 300 feet in papyraceous shales with abundant calca- 5. Rostellaria angulosa (?), D'Orbig. Pal. de thickness, and having the same characters as reous nodules with three fossiliferous l'Amer. Mer. heretofore described. I estimated the total thick- levels, from bottom to top with: Virga- 6. Terebratula ?" ness of these three beds [Q, R, S] at nearly tosphinctes aff. V. denseplicatus rotundus 3000 feet; and to this must be added, as will be Spath, 1931, Pseudolissoceras zitteli (Burck- However, it seems that d'Orbigny gave immediately seen, a great overlying mass of red hardt, 1903) and Aulacosphinctes proximus Darwin slightly different information sandstone. (Steuer, 1897). These shales grade up- (see fig. 4). According to his notes, the In descending the eastern slope of this great cen- wards to dark grey, thinly laminated li- specimens collected by Darwin in the tral range, the strata, which in the upper part dip mestones, reaching more than 110 me- "Cordillera Centrale du Chili" comprised: eastward at about an angle of 40°, become more tres (Fig. 3). The age of the Vaca Muerta and more curved, till they are nearly vertical; and Formation in the area is Early to Middle- "617/792. Gryphaea - Voisine du Grypha- a little further onwards there is seen on the fur- Late? Tithonian. The Chachao Forma- ea Couloni, du terrain Neocomien de France et ther side of a ravine, a thick mass of strata of tion - 40 metres thick - is composed by de Neuchatel bright red sandstone [T], with their upper extre- massive oyster coquinas interbedded 613. Natica (indeterminable) mities slightly curved, showing that they were with shales bearing Parodontoceras calistoides 619. Ammonite indeterminable, voisine de l'A. once conformably prolonged over the beds [S]: on (Behrendsen, 1891) of Late Tithonian recticostatus, d'Orb (Paleont. française) Du the southern and opposite side of the road, this age in its lower part. The Agrio Forma- terrain Neocomien red sandstone and the underlying black shaly tion is some 120 metres thick in the mea- 614. B. Cyprina rostrata, d'Orb. Paleont. rocks stand vertical, and in actual juxtaposi- sured section but is incomplete, as it is franç. Terrain Neocomien (non Lucina) tion" (Darwin 1846, p. 179-180, pl. 1, cut by a thrust that puts the Chachao C. Terebratula ? sketch 1, see Giambiagi et al. 2009). coquinas on top of its upper part. The A. Rostellaria angulosa, d'Orb. Paleont. de The red sandstones of P correspond to lower part is mostly composed by inter- l'Am. Mer. Pl. 18 fig. 4 ?" the Río Damas Formation in Chile calations of massive and finely laminated (Tordillo Formation in Argentina). Q and grey and yellowish limestones, bearing - d'Orbigny also listed : S of Darwin (1846) are the Lo Valdés in some levels- Steinmanella sp., Thalassi- "790. Arca peut être Arca gabrielis, d' Orb. Formation in Chile or the Mendoza noides isp. and poorly preserved flattened (Paleontologie francaise) du terrain Neocomien" Group in Argentina, where it is compo- ammonoids. In the upper part there are This fossil is not mentioned by Darwin sed by the Vaca Muerta, Chachao and non-fossiliferous olive green shales, li- from the Piuquenes Pass, and is almost Agrio Formations. R, the gypsum bed, is mestone with small oysters, silty limesto- certainly from Puente del Inca (Darwin On Darwin's footsteps across the Andes: Tithonian-Neocomian fossil ... 35

Darwin (Fig. 4). D'Orbigny assigned a bivalve to Cyprina rostrata d'Orbigny, 1853 pointing out that this is not Lucina Lamarck, 1799, which is interesting as here this fossil is referred to Lucinidae indet. (D. Lazo, oral comm.) (Fig. 5, p-q). He also added the figure of Rostellaria angulosa d'Orbigny, 1842 (d'Or- bigny 1842, p. 80, pl. 18, fig. 4) which was very helpful in the assignation of our specimens to Protohemichenopus neuquensis Camacho, 1953 (Fig. 5 o). It is worth mentioning here that, besides the ammonites that will be described in detail below, we have also found chelipeds of callianassid crustaceans in both the Vaca Muerta and Agrio Formations. Besides Darwin (1846) the other only published mention of fossils in the Piu- quenes range is that of Polanski (1964, p. 48) who listed the following Tithonian fossils: Corongoceras sp., Aulacosphinctes sp., Trigonia carricurensis Leanza, Lucina sp. and Exogyra sp.

SYSTEMATIC PALAEONTOLOGY

Family Haploceratidae Zittel, 1884 Genus Pseudolissoceras Spath, 1925

Pseudolissoceras zitteli (Burckhardt, 1903) Figs. 5 a-h

? 1897 Oppelia perlaevis Steuer, p. 73, pl. 6, figs. 7-9 1900 Oppelia aff. perlaevis Steuer; Burck- hardt, p. 46, pl. 26, figs. 5, 6; pl. 29, fig. 2 1903 Neumayria zitteli Burckhardt, p. 55, pl. 10, figs. 1-8 1907 Neumayria zitteli Burckhardt; Haupt, p. 200, pl. 7, figs. 3a, b, 4a-c ? 1921 Oppelia perlaevis Steuer, p. 102, pl. 6, figs. 7-9 ? 1921 Oppelia perglabra Steuer, p. 104, pl. 7, figs. 13-15 1925 Pseudolissoceras zitteli (Burckhardt); Spath, p. 113 (Gen. nov.) 1926 Pseudolissoceras zitteli Burckhardt; Figure 3: Stratigraphic section of the Mendoza Group in the Argentine slope of the Piuquenes Pass. Krantz, p. 436, pl. 17, figs. 4, 5 1846, p. 193), as this specimen and the Couloni)" have a note in the left margin 1928 Pseudolissoceras zitteli Burckhardt; "792 (Gryphaea - Voisine du Gryphaea stating Cumbre, probably handwritten by Krantz, p. 18, pl. 1, fig. 6; pl. 4, figs. 9a, b 36 B. AGUIRRE-URRETA AND V. VENNARI

1931 Pseudolissoceras zitteli Burckhardt; cimens figured as Oppelia perlaevis Steuer with only some minimum differences in Weaver, p. 401, pl. 43, fig. 291 (1897, p. 73, pl. 6, figs. 7-9; 1921, Spanish transversal section -which can be someti- 1942 Pseudolissoceras cf. Pseudolissoceras zit- translation of 1897 edition, p. 102, pl. 6, mes more inflated or depressed- or in teli (Burckhardt); Imlay, p. 1443, pl. 4, figs. 7-9) and Oppelia perglabra Steuer having a more or less acuminate or roun- figs. 1-4, 7, 8, 11, 12 (1921, p. 104, pl. 7, figs. 13-15) from the ded venter. In all cases, specimens are 1980 Pseudolissoceras zitteli (Burckhardt); lowermost Middle Tithonian of Men- found in association with a similar fauna Leanza, p. 17, pl. 1, figs. 1a, b, 2a, b doza, belong also in the same species. composed mainly of other haploceratid 2001 Pseudolissoceras zitteli (Burckhardt); This is due to the fact that there is a ammonoids, from which they can be Parent, p. 23, figs. 3a, b, 5a, b, 7a-f strong similarity in general form, orna- generally differentiated by the absence of Material: 10 specimens (CPBA 20552.1- mentation, and suture line between them a marked groove on the flanks, as occurs 20552.10) from the Piuquenes Pass, and Burckhardt's original form. Never- in Hildoglochiceras Spath, 1924, or its roun- Mendoza. theless, we can not confirm this until we ded venter and lesser degree of involu- have the chance of comparing our speci- tion than in Parastreblites Donze and Enay, Description: Well preserved, small to mens with Steuer's holotypes, although 1961. medium size specimens (largest speci- we can confidently assert that they do On the other hand, many authors (Haupt men is 62.2 mm diameter); body cham- not belong in Oppelia Waagen, 1869 1907, Krantz 1926, Weaver 1931, Leanza ber occupies at least half a whorl, usually because they show no trace of even a 1980) distinguish Pseudolissoceras pseudooli- laterally crushed. Shell discoidal, involute feeble keel preserved on the illustrated thicum (Haupt, 1907) recorded in the sa- (U c. 21% of diameter) with deep umbi- material. me beds that contain Pseudolissoceras zitte- licus, rounded umbilical border and an Specimens described and figured by li. That species never exhibits an umbili- abrupt umbilical slope on the outermost Haupt (1907, p. 200, pl. 7, fig. 3a, b, 4a- cal edge, it is only moderately involute, whorls while on the innermost it beco- c), Krantz (1926, p. 436, pl. 17, figs. 4, 5; and its flanks are more inflated with mes gently inclined. Subelliptic transver- 1928, p. 18, pl. 1, fig. 6; pl. 4, fig. 9a, b), whorls as high as wide in transverse sec- sal section with convex flanks converging Weaver (1931, p. 401, pl. 43, fig. 291) and tion. on a rounded and slightly acuminated Leanza (1980, p. 17, pl. 1, figs. 1a, b, 2a, venter. Whorls always higher than wide, b), were all found at Cerro Lotena, Neu- Ocurrence: Lowermost Middle Titho- with maximum width near the middle of quén, although some specimens have nian. Pseudolissoceras zitteli Zone. the flanks. Shell surface almost smooth. also been recovered from Picún Leufú Only where test is preserved is it possible (Weaver), Rodeo Viejo, Bardas Blancas, Family Perisphinctidae Steinmann, 1890 to distinguish extremely soft, falcoid ribs Arroyo Cienaguitas and from a locality Subfamily Virgatosphinctinae Spath, 1923 a densely packed, that start at the base of between Cajón del Burro and Río Choi- Genus Virgatosphinctes Uhlig, 1910 the umbilical slope. No dimorphic sexual ca, in Mendoza (Krantz 1926). Speci- structures observed. Suture line not com- mens studied by Parent (2001, p. 23, fig. Virgatosphinctes aff. Virgatosphinctes den- pletely preserved. 3a, b, 5a, b, 7a-f) are from Cañadón de los seplicatus rotundus Spath, 1931 Alazanes, a locality exposed at the Vaca Figs. 5 i-m Dimensions of specimens Muerta Range in Neuquén, while Imlay's Specimen Diameter Umbilicus U/D Whorl Height Whorl Width H/W ? 1890 Perisphinctes contiguus Catullo; Tou- (CPBA) (D) (U) (H) (W) cas, p. 581, pl. 14, fig. 4 20552.1 62.2 12.3 0.20 30.3 - - ? 1900 Perisphinctes contiguus Catullo; Burck- 20552.2 37.3* 7.4 0.20 17.9 9.2* 1.95 hardt, p. 45, pl. 24, fig. 1 20552.3 44.5 9.4 0.20 22.9 14.2 1.61 1903 Perisphinctes contiguus Catullo; Burck- 20552.4 43 9.1* 0.20 19.8 15.0 1.32 hardt, p. 38, pl. 4, figs., 7-10 20552.5 38.1 6.9 0.18 17.5 11.3 1.55 20552.6 28 6.1 0.22 13.0 7.8 1.67 ? 1931 Virgatosphinctes denseplicatus (waa- 20552.7 27.2 6.4 0.24 13.2 8.8 1.50 gen) var. rotunda Spath, p. 532, pl. 96, figs. 20552.8 20.5 4 0.20 10.3 8.1 1.27 3a-b; pl. 102, fig 4. 20552.9 32 6 0.19 12.6 - - 1954 Virgatosphinctes cf. denseplicatus (Wa- 20552.10 41.1* 9.7 0.24 16.4* - - agen) var. rotunda Spath; Indans, p. 106, * Indicates approximate measurement. pl. 21, fig. 1 Remarks: Specimens described above material (1942, p. 1443, pl. 4, figs. 1-4, 7, 1972 Virgatosphinctes denseplicatus Waagen; can certainly be assigned to Pseudolisso- 8, 11, 12) is from Cuba. In all cases spe- Fatmi, p. 346, pl. 8, figs. 5a, b ceras zitteli (Burckhardt 1903, p. 55, pl. 10, cimens are highly similar; there is a nota- 1980 Virgatosphinctes denseplicatus rotundus figs. 1-8), and it is also possible that spe- ble concordance in U/D and H/W ratios Spath; Leanza, p. 31, pl. 2, figs. 2, 3 On Darwin's footsteps across the Andes: Tithonian-Neocomian fossil ... 37

Material: 6 specimens (CPBA 20551.1- 20551.6) from the Piuquenes Pass, Men- doza.

Description: Medium size discoidal phragmacone (largest specimen is 45.9 mm in diameter), body chamber not preserved. Slightly involute (U c. 33% of diameter), with moderately deep umbilicus and umbilical border not very inclined. Venter slightly planar with gently curved flanks. Whorl section somewhat inflated, slightly wider than high, with maximum width at umbilical margin. Fine, sharp ribs, regularly distributed, beginning at the middle of the umbilical slope, with a slightly prorsiradiate tendency and then bifurcate at the external third of the flank, crossing the venter without interruption. A slight weakness occurs in the ribs in specimens that are less than 38 mm in diameter. At least two shallow constrictions are observed in the last whorl preserved, parallel to the rib pattern and bordered posteriorly by a simple asymmetrical rib. Suture line not observed. Figure 4 : List of fossils identified by d'Orbigny for Darwin from his collection of the Central Cordillera Dimensions of specimens of Chile (from www.darwin-online.org.uk). sembles the specimens described above, Specimen Diameter Umbilicus U/D Whorl Height Whorl WidthH/W (CPBA) (D) (U) (H) (W) with similar rib pattern and alternation of 20551.1 45.9 18.7* 0.41 16.1 17.2 0.94 triplicate ribs on one side with biplicate 20551.2 37.8 13.7 0.36 13.9 15.0 0.93 ribs on the other; there is also a slight 20551.3 35.5 14.7 0.41 12.2 12.7 0.96 attenuation of the ornamentation on the 20551.4 28.6 10.1 035 9.8 11.4 0.86 venter of some specimens. Toucas' mate- 20551.5 18.0 5.1 0.28 6.3 8.8 0.72 rial comes from the Lower Tithonian of 20551.6 10.5 1.9 0.18 4.9 - - the Carpathians, the Alps, the Apennines * Indicates approximate measurement. and from Andalucia, while Burkhardt's Remarks: Specific assignation of the cross the venter without any interrup- specimens are from the Lower Tithonian specimens to Virgatosphinctes denseplicatus tion. It is important to state here that the from Casa Pincheira in Mendoza. rotundus Spath (1931, p. 532, pl. 96, figs. gradual differentiation from biplicate Virgatosphinctes cf. denseplicatus (Waagen) 3a-b; pl. 102, fig. 4) is quite difficult due ribs, into triplicate, virgatotome, and fi- var. rotunda Spath (in Indans 1954, p. 106, to the fact that ornamentation suffers a nally fasciculate ribs that constitutes a pl. 21, fig. 1) from Cerro Alto in southern radical change in the last whorl, just as diagnostic character of Virgatosphinctes Mendoza also resembles the studied spe- illustrated by Waagen in Virgatosphinctes Uhlig, 1910 is not evident in Waagen's cimens, as well as the specimen from denseplicatus (1873, p. 201, pl. 46, fig. 3a, b; illustrated specimens from the Lower Ti- western Pakistan identified as Virgatos- pl.55, figs. 1a. b), where the innermost thonian of Kutch. phinctes denseplicatus Waagen by Fatmi fine ribs become stronger over the inner Perisphinctes contiguus Catullo (in Toucas (1972, p. 346, pl. 8, figs. 5a, b). However, third of the last whorl, and then differen- 1890, p. 581, pl. 14, fig. 4, Burckhardt there appears to be a difference in the tiate in the middle of the flank into a 1900, p. 45, pl. 24, fig. 1, Burckhardt ornamentation pattern with the Argen- bundle of 4 to 10 secondary ribs that 1903, p. 38, pl. 4, figs. 7-10) closely re- tinian specimens, as there are simple pos- 38 B. AGUIRRE-URRETA AND V. VENNARI

Figure 5: a-h) Pseudolissoceras zitteli (Burckhardt, 1903); a) lateral view CPBA 20552.1; b) lateral and ventral views CPBA 20552.5; c) lateral and ventral views CPBA 20552.4; d) lateral view CPBA 20552.3; e) lateral view CPBA 20552.2; f) lateral view CPBA 20552.6; g) apertural and lateral views CPBA 20552.7; h) lateral and ventral views CPBA 20552.8; i-m) Virgatosphinctes aff. Virgatosphinctes denseplicatus rotundus Spath, 1931; i) lateral view CPBA 20551.3; j) lateral view CPBA 20551.4; k) ventral and lateral views CPBA 20551.2; l) lateral view CPBA 20551.1; m) apertural and lateral views CPBA 20551.5; n) Spitidiscus riccardii Leanza and Wiedmann, 1992, lateral view CPBA 20557; o) Protohemichenopus neuquensis Camacho, 1953, apertural and lateral views CPBA 20556.1; p-q) Lucinidae indet. lateral views CPBA 20555.1-2; r-t) Aulacosphinctes proximus (Steuer, 1897); r) lateral view CPBA 20553.2; s) lateral and ventral views CPBA 20553.1; t) Parodontoceras calistoides (Behrendsen, 1891), lateral view CPBA 20554. All specimens from the Piuquenes Pass, Mendoza. All natural size. Specimens coated with ammonium chloride. On Darwin's footsteps across the Andes: Tithonian-Neocomian fossil ... 39

terior ribs bordering the constrictions, Weaver, p. 413, pl. 44, figs. 301-303 Remarks: Diagnostic characters obser- not joining the immediately anterior 1980 Aulacosphinctes proximus (Steuer); ved in the specimens studied, such as biplicate ribs. Leanza, p. 44, pl. 6, figs. 2a, b, 4a, b, 5a, b evolution degree, rib pattern and the pre- Finally, there is a strong resemblance bet- 1981 Aulacosphinctes proximus (Steuer); sence of a ventral furrow allow its assig- ween the specimens studied here and Leanza, p. 587, pl. 2, figs. 9, 10 nation to Aulacosphinctes proximus (Steuer Virgatosphinctes denseplicatus rotundus des- 1897, p. 34, pl. 8, figs. 7-11 as Reineckeia cribed and figured by Leanza from the Material: 2 specimens (CPBA 20553.1- proxima; 1921 Spanish translation, p. 61, Lower Tithonian of Cerro Lotena, Neu- 20553.2) from the Piuquenes Pass, Men- pl. 8, figs. 7-11). However, in the same quén (1980, p. 31, pl. 2, figs. 2, 3), though doza. publication Steuer describes another spe- his specimens show a more inclined um- cies, from Arroyo Cienaguitas in Men- bilical slope and a more subelliptical Description: Medium size planulate doza, and from contiguous beds. This whorl section. shell (largest specimen is 52.3 mm in dia- species, (Perisphinctes colubrinus (Reinecke) Virgatosphinctes denseplicatus rotundus is meter), body chamber partially preser- p. 62, pl. 15, fig. 11), is very similar to usually found in association with other ved, of at least 1/4 whorl. Strongly evo- Aulacosphinctes proximus (Steuer, 1897), Virgatosphinctes, like Virgatosphinctes ande- lute (U c. 45% of diameter), with shallow both in general form and ornamentation sensis (Burckhardt), from which it can be umbilicus, rounded umbilical border and pattern, but the ventral furrow is not visi- differentiated due to its whorl section gradually inclined umbilical slope. Sub- ble on the last whorl preserved, although only slightly wider than high, its softer quadrate transversal section with slightly it is present on previous ones. Neverthe- ornamentation and the lack of intercala- convex flanks and flat venter, whorls less the specimen illustrated is 83 mm in tory ribs and primary ribs describing an slightly higher than wide. Prominent, diameter, so this character may not be inflection on the middle of the flanks. fine, sharp and distant ribs start at the noticeable at that size, although it might base of the umbilical slope in a rursira- be quite important in smaller shells as Ocurrence: Basal section of Vaca Muer- diated mode, at the umbilical margin they can be clearly seen on the specimens stu- ta Formation, just above the biolamina- become prorsiradiated, crossing the died by Burckhardt (1900, p. 44, pl. 24, ted carbonate levels. Lower Tithonian, flanks in a prorsiradiated to rectiradiated figs. 5, 6; pl. 26, fig. 4) and Weaver (1931, Virgatosphinctes mendozanus Zone. pattern. Number of ribs decreases as dia- p. 413, pl. 44, figs. 301-303), from Men- meter increases; in a 52.3 mm diameter doza and Neuquén, respectively. Family Perisphinctidae Steinmann, 1890 specimen there are 25 ribs in the last Weaver (1931, p. 411, pl. 44, figs. 298, Subfamily Himalayitinae Spath, 1925 whorl and 20 in the preceding one. Ribs 299) also described a specimen that was Genus Aulacosphinctes Uhlig, 1910 on the innermost whorls are interrupted assigned to Aulacosphinctes proximus. How- over the venter by a narrow and shallow ever, his description does not mention Aulacosphinctes proximus (Steuer, 1897) furrow, which tends to disappear in outer any rib interruption over the venter at Figs. 5 r-t whorls although ornamentation there any shell size, an important character in may suffer an important weakening. the definition of the genus that casts 1897 Reineckeia proxima Steuer, p. 34, pl. 8, Simple primary ribs are irregularly inter- doubts on its assignation. figs. 7-11 calated with biplicate ribs that bifurcate Aulacosphinctes wanneri Krantz (1928, p. ? 1897 Perisphinctes colubrinus Reinecke; at the middle of the flanks without deve- 42, pl. 2, figs. 6a, b) from Arroyo La Steuer p. 62, pl. 15, fig. 11 loping any tubercle at the bifurcation Manga, Mendoza, closely resembles Au- 1900 Perisphinctes colubrinus Reinecke; point. Some intercalatory ribs are also lacosphinctes proximus. The development of Burckhardt, p. 44, pl. 24, figs. 5, 6; pl. 26, observed, not extending beyond the mid- a trifurcated rib is the only difference fig. 4 dle part of the flanks. Two wide but sha- between the two species, an unimportant 1907 Perisphinctes proximus Steuer; Haupt, llow constrictions occur near the shell character that may be considered as p. 192 end; they are posteriorly bounded by a intraspecific variation. 1921 Reineckeia proxima Steuer, p. 61, pl. 8, simple, acute and prominent primary rib. Finally, the studied specimens are com- figs. 7-11 Suture line not visible. parable with those studied by Leanza ? 1921 Perisphinctes colubrinus Reinecke; (1980, p. 44, pl. 6, figs. 2a, b, 4a, b, 5a, b; Steuer, p. 90, pl. 15, fig. 11 Dimensions of specimens 1928 Aulacosphinctes wanneri Krantz, p. 42, Specimen Diameter Umbilicus U/D Whorl Height Whorl Width H/W pl. 2, figs. 6a, b (CPBA) (D) (U) (H) (W) ? 1931 Aulacosphinctes proximus (Steuer); 220553.1 48.1 22.9 0.48 13.8 12.6 1.1 Weaver, p. 411, pl. 44, figs. 298, 299 20553.2 52.3 21.3 0.41 14.9 14.3 1.0 1931 Aulacosphinctes colubrinus (Reinecke); * Indicates approximate measurement. 40 B. AGUIRRE-URRETA AND V. VENNARI

1981, p. 587, pl. 2, figs., 9, 10) from the manner, then bend forwards at the umbi- bigny); Leanza and Wiedmann, p. 32, Middle Tithonian of Cerro Lotena in lical border describing an inflection, and figs. 6a-b Neuquén. then another inflection at the bifurcation 1995 Spitidiscus riccardii Leanza and Wied- point in the middle of the flanks. These mann; Aguirre-Urreta, p. 407, pl. 1, figs. Ocurrence: Middle Tithonian, Aulacos- biplicate ribs are intercalated with simple 1-23 phinctes proximus Zone. ribs and some intercalatory ones do not 1999 Spitidiscus riccardii Leanza and go beyond the external third of the flank. Wiedmann; Aguirre-Urreta et al., pl. 1, Family Neocomitidae Salfeld, 1921 All ribs are interrupted over the venter by fig. 4. Subfamily Berriasellinae Spath, 1922 a relatively wide and shallow furrow, 2005 Spitidiscus riccardii Leanza and Wied- Genus Parodontoceras Spath, 1923 a which becomes less visible as diameter mann; Aguirre-Urreta et al., figs. 7 e-f increases until it disappears completely. Parodontoceras calistoides (Behrendsen, Suture line is very badly preserved. Material: 1 specimen (CPBA 20557) and 1891) several fragments and impressions from Fig. 5 t Dimensions of specimen the Piuquenes Pass, Mendoza.

1891 Hoplites calistoides Behrendsen, p. Specimen Diameter Umbilicus U/D Whorl Height Whorl Width H/W (CPBA) (D) (U) (H) (W) 402, pl. 23, figs. 1a, b 20554 97.2 21.1 0.22 43.2 19.7* 2.19 1897 Odontoceras calistoides Behrendsen; * Indicates approximate measurement. Steuer, p. 41, pl. 17, figs. 13-16 1921 Odontoceras calistoides Behrendsen; Remarks: Specific assignation of the Description: Shell small, involute (ap- Steuer, p. 69, pl. 17, figs. 13-16 specimen to Parodontoceras calistoides (Beh- prox. diameter 30 mm), with rounded 1922 Hoplites calistoides Behrendsen, p. rendsen, 1891, p. 402, pl. 23, figs. 1a, b) is umbilical slope and slightly rounded 184, pl. 1, figs. 11a, b quite safe despite its rather poor preser- flanks converging towards a curved ven- 1923a Parodontoceras callistoides Behrend- vation, especially taking into account its ter. Ornament is composed by ribs and sen; Spath, p. 305 (Gen. nov.) eccentric shape, rib pattern and ventral constrictions. The ribs are fine, arise 1925 Berriasella calistoides Behrendsen; furrow. This species is known from many from the umbilical slope and bifurcate in Gerth, p. 88 localities in Neuquén and Mendoza, the lower third of the flank where they 1928 Berriasella (Parodontoceras) calistoides almost always in association with Subs- curve into a falcoid shape. They cross the (Behrendsen); Krantz, p. 24 teueroceras koeneni (Steuer, 1897) characte- venter without interruption bending 1945 Parodontoceras calistoides (Behrend- rized by a finer and denser rib pattern slightly towards the aperture. The cons- sen); Leanza, p. 41, pl. 5, figs. 5, 6 than Parodontoceras calistoides. Neverthe- trictions are less curved than the ribs and 1988 Paradontoceras calistoides (Behrend- less, they seem to be very close forms they dissect them. Suture line not preser- sen); Riccardi, pl. 3, figs. 3, 4 and only a future more detailed study will ved. prove if they are not just morphotypes of Material: 1 specimen (CPBA 20554) the same species. Remarks: Although the preservation of from the Piuquenes Pass, Mendoza. the specimens is rather poor, they can be Ocurrence: Uppermost Tithonian, Sub- assigned to Spitidiscus riccardii Leanza and Description: Medium to large phragma- steueroceras koeneni Zone. Wiedmann, 1992 due to the size, involu- cone (97.2 mm diameter), laterally crus- tion of the shell and ribbing pattern. The hed. Shell ellipticone, involute (U c. 22 % Family Holcodiscidae Spath, 1923 b studied specimens are comparable with of diameter), with poorly developed um- Genus Spitidiscus Kilian, 1910 those studied by Aguirre-Urreta (1995, p. bilical border and quite inclined umbilical 407, pl. 1, figs. 1-23) from Agua de la slope. Flanks are gently convex and ven- Spitidiscus riccardii Leanza and Wied- Mula and Agrio del Medio in Neuquén ter somewhat flattened. Whorls are hig- mann, 1992 where the species has been studied in her than wide, with the maximum width Fig. 5 n detail. probably located at the umbilical border. The whorl height increases considerably 1992 Spitidiscus riccardii Leanza and Wied- Occurrence: Upper Hauterivian. Spiti- with diameter, resulting in an eccentric mann, p. 33, figs. 4a-b discus riccardii Zone. shaped shell. Ornamentation consists of 1992 Spitidiscus aff. S. rotula (J. de C. Sow- moderately strong ribs, regularly spaced, erby); Leanza and Wiedmann, p. 32, figs. CONCLUDING REMARKS that begin at least from the middle part 3a-b of the umbilical slope in a rursiradiate 1992 Spitidiscus aff. S. gastaldianus (d'Or- When one compares the geological sec- On Darwin's footsteps across the Andes: Tithonian-Neocomian fossil ... 41

tion of the Piuquenes Pass by Darwin advice. This fauna was one of the first to Deutsche Geologische Gesellschaft 43 (1891): (1846, pl. 1, skecth 1) with the present be documented in the High Cordillera of 369-420; 44 (1892): 1-42. knowledge of the area (Pángaro et al. the Main Andes, which now we know Behrendsen, O. 1922. Contribución a la geología 1996, Fig. 2 herein), it can be seen that that also includes the Tithonian as seen de la pendiente oriental de la Cordillera Ar- Darwin was able to clearly identify the by the ammonites described above. gentina. Academia Nacional de Ciencias, succession of Meso-Cenozoic rocks and Actas 7: 157-227, Córdoba. some structural features. He only par- ACKNOWLEDGEMENTS Burckhardt, C. 1900. Profils géologiques trans- tially recognized the highly complex versaux de la Cordillère Argentino-Chilienne. structure of thrusts that affects the sedi- The authors wish to acknowledge Dario Museo de La Plata, Sección Geológica y Mi- mentary rocks, repeating parts of the Lazo for his information on the bivalves neralógica, Anales 2: 1-136. sequence. and Victor A. Ramos for his assistance in Burckhardt, C. 1903. Beitrage zur Kenntniss der Regarding the fossil content, it must be the field and his advice on the structure Jura und Kreide formation der Cordillere. understood that Darwin just rode along of the region. The reviewers Estanislao Palaeontographica 50: 1-144. the trail, suffering the effects of high alti- Godoy, Héctor Leanza and Amaro Burkhardt, F. and Smith, S. 1985. The Complete tude and that he was being hurried by the Mourgues improved the original manus- Correspondence of Charles Darwin. Vol. 1. "baqueanos" who were afraid of bad wea- cript with their comments. 1821-1836. Cambridge University Press, 752 ther. He wrote to Henslow: "It was late in p., Cambridge. the Season, & the situation particularly dange- WORKS CITED IN THE TEXT Camacho, H. H. 1953. Algunas consideraciones rous for Snow storms. I did not dare to delay, sobre los “Aporrhaidae” fósiles argentinos. otherwise a grand harvest might have been rea- Aguirre-Urreta, M.B. 1995. Spitidiscus riccardii Revista de la Asociaci´´on Geológica argenti- ped" (Burkhardt and Smith 1985). He was Leanza y Wiedmann (Ammonoidea) en el na 8: 183- 194. able to collect the most common fossil, Hauteriviano del Neuquén. Ameghiniana 32: Darwin, C. 1846. Geological observations on his big oyster Exogyra (Gryphacea) couloni 407-410. South America, being the third part of the (presently Aetostreon), other bivalves, so- Aguirre-Urreta, M.B. 1996. El Tithoniano marino geology of the voyage of the Beagle, under me gastropods and ammonites. It is diffi- de la vertiente argentina del paso de Piuque- the command of Capt. FirtzRoy, RN, during cult to know precisely from where he got nes, Mendoza. 13° Congreso Geológico Ar- the years 1832 to 1836. Smith, Elder and co., his specimens, though it seems he stop- gentino y 3° Congreso de Exploración de 268 p. London. ped several times along the trail collec- Hidrocarburos (Buenos Aires), Actas 2: 185. d'Orbigny, A. 1840-1841. Paléontologie Fran- ting loose material, a fact that would ex- Aguirre-Urreta, M.B. 2001. Marine Upper Juras- caise. Description des Mollusques et Rayon- plain why the fossils were poorly preser- sic-Lower Cretaceous Stratigraphy and Bios- nés Fossiles. Terrains Crétacés. Masson, 662 ved. tratigraphy of the Aconcagua-Neuquén p., Paris. D'Orbigny (manuscript, see Fig. 4, and Basin, Argentina and Chile. Journal of Iberian d'Orbigny, A. 1842. Voyage dans L'Amérique also Darwin 1846) regarded the assem- Geology 27: 71-90. Méridionale. Tome 3, 4° part: Paleontologie, blage as Neocomian age, based not only Aguirre-Urreta, M.B., Concheyro, A., Lorenzo, 188 p., Paris & Strasbourg. in the oysters but also in the presence of M., Ottone, E.G. and Rawson, P.F. 1999. Ad- Fatmi, A.N. 1972. Stratigraphy of the Jurassic an indeterminate ammonite resembling vances in the biostratigraphy of the Agrio and Lower Cretaceous rocks and Jurassic am- Ammonites recticostatus d'Orbigny, 1840. Formation (Lower Cretaceous) of the Neu- monites from northern areas of West Pakis- This species is presently assigned to the quén basin, Argentina: ammonites, palyno- tan. British Museum of Natural History 20: genus Macroscaphites Meek, 1876 of Ba- morphs, and calcareous nannofossils. Paleo- 297-380. rremian-Early Aptian age, and is comple- geography, Palaeoclimatology, Palaeoecology Gerth, E. 1925. La fauna Neocomiana de la tely unknown in the Neuquén Basin, 150: 33-47. Cordillera argentina, en la parte meridional de where the youngest ammonites are of Aguirre-Urreta, M.B., Rawson, P.F., Concheyro, la provincia de Mendoza. Academia Nacional Early Barremian age (Aguirre-Urreta et G.A., Bown, P.R. and Ottone, E.G. 2005. de Ciencias, Actas 9: 57-132, Córdoba. al. 2005). Most probably d'Orbigny con- Lower Cretaceous Biostratigraphy of the Giambiagi, L., Tunik, M., Ramo, V. A. and fused it with some Tithonian perisphinc- Neuquén Basin. In Veiga, G., Spalletti, L., Godoy, E. 2009. The High Andean Cordillera tids which are common in the area. Howell, J.A. and Schwarz, E. (eds.) The Neu- of Central Argentina and Chile along the In spite of the poor preservation of the quén Basin: A case study in sequence strati- Piuquenes Pass- Cordón del Portillo transect: specimens and the lack of knowledge of graphy and basin dynamics. The Geological Darwin´s pioueering observations compared faunas of this age in South America, it Society, Special Publication 252: 57-81, Lon- with modern geology. Revista de la Asocia- should be emphasized that Darwin was don. ción Geológica Argentina 64 (1): 44-54. correct in assigning the whole succession Behrendsen, O. 1891-1892. Zur Geologie des Groeber, P. 1953. Mesozoico. In Geografía de la to the Neocomian, following d'Orbigny's Ostabhanges der argentinischen Kordillere. República Argentina, Sociedad Argentina de 42 B. AGUIRRE-URRETA AND V.VENNARI

Estudios Geográficos GAEA 2(1): 9-541, 2: 559-597, Buenos Aires. monoidea of the Gault. Part 1. Palaeontogra- Buenos Aires. Leanza, H.A. and Wiedmann, J. 1992. Nuevos phical Society, Monographs 75 (353): 1-72. Haupt, O. 1907. Beiträge zur Fauna des oberen Holcodiscidae (Cephalopoda-Ammonoidea) Spath, L.F. 1925. Ammonites and Aptychi. In Malm und der unteren Kreide in der argenti- del Barremiano de la Cuenca Neuquina, Ar- Wyllie, B.N.K and Smellie, W.E. (eds.) The nischen Cordillere. Neues Jahrbuch für Mi- gentina y su significado estratigráfico. Neues collection of fossils and rocks from Soma- neralogie, Geologie und Paläontologie 23: Jahrbuch für Geologie und Paläontologie, liland. Hunterian Museum, Glasgow Univer- 187-236. Monatshefte 1992, 1: 24-38. sity, Monograph 7: 111-164. Imlay, R.W. 1942. Late Jurassic fossils from Cuba Malumián, N. and Ramos, V.A. 1984. Magmatic Spath, L.F. 1927-1933. Revision of the Jurassic and their economic significance. Geological intervals, transgression regression cycles and Cephalopod Fauna of Kachh (Cutch). Pa- Society of America, Bulletin 53: 1417-1478. oceanic events in the Cretaceous and Tertiary leontologia Indica, NS 9, 1 (1927): 1-72, 2 Indans, J. 1954. Eine Ammonitenfauna aus dem of Southern South America. Earth and (1928): 73-161, 3 (1928): 163-278, 4 (1931): Untertithon der argentinischen Kordillere in Planetary Science Letters 67: 228 237. 279-550, 5 (1931): 551-658, 6 (1933): 659-945. Süd-Mendoza. Palaeontographica 105 A: 96- Pángaro, F., Ramos, V.A. and Godoy, E. 1996. La Steinmann, G. 1890. Cephalopoda. In G. Stein- 132. Faja plegada y corrida de la Cordillera mann and L. Döderlein (eds.) Elemente der Jordan, T., Isacks, B., Ramos, V.A. and Allmen- Principal de Argentina y Chile a la latitud del Paläontologie, 344-475, W. Engelmann, Leip- dinger, R.W. 1983. Mountain building model: Cerro Palomares (33°35'S). 13° Congreso zig. The Central Andes. Episodes 1983(3): 20-26. Geológico Argentino y 3° Congreso Explo- Steuer, A. 1897. Argentinische Jura - Ablagerun- Kilian, W. 1910. Erste Abteilung: Unterkreide ración de Hidrocarburos (Buenos Aires), Ac- gen: Eine Beitrage zur Kenntniss der Geo- (Palaeocretacicum). Lieferung 2: Das bathyale tas 2: 315-324. logie und Paleontologie der argentinischen Palaeocretacicum im südostlichen Frankreich; Parent, H. 2001. The Middle Tithonian (Upper Anden. Palaeontologische Abhandlungen NS Valendis-Stufe; Hauterive-Stufe; Barreme- Jurassic) ammonoid fauna of Cañadón de los 7: 127-222, Jena. Stufe; Apt-Stufe. In Frech, F., Lethaea Geog- Alazanes, southern Neuquén-Mendoza Basin, Steuer, A. 1921. Estratos Jurásicos Argentinos. nostica. II. Das Mesozoicum, Band 3 (Kreide) Argentina. Boletín del Instituto de Fisiografía Contribución al conocimiento de la Geología (1907-1913), Schweizerbart, 169-288, pls. 1-8, y Geología 71: 19-38, Rosario. y Paleontología de los Andes Argentinos Stuttgart. Polanski, J. 1964. Descripción geológica de la entre el Río Grande y el Río Atuel. Academia Krantz, F. 1926. Die ammoniten des Mittel-und Hoja 25a Volcán San José, provincia de Nacional de Ciencias, Actas 7: 5-128, Cór- Obertithons. In Jaworski, E., Krantz, F. and Mendoza. Dirección Nacional de Geología y doba. Gerth, E. (eds.) Beiträge zur Paläontologie Minería, Boletín 98: 1-94, Buenos Aires. Toucas, A. 1890. Étude sur la fauna des couches und Stratigraphie des Lias, Doggers, Tithons Riccardi, A. 1983. The Jurassic of Argentina and tihoniques de l'Ardèche. Bulletin de la Société und der Unterkreide in der Kordillere im Chile. In Moullade, M. and Nairn, A.E.M. Géologique de France (Serie 3) 18: 560-629. Süden der Provinz Mendoza (Argentinien). (eds.) The Phanerozoic geology of the world Uhlig, V. 1910. The Fauna of the Spiti Shales. Geologische Rundschau 17a: 427-462. II, The Mesozoic B, Elsevier, 201-263, Palaeontologia Indica 15: 133-395. Krantz, F. 1928. La fauna del Titono superior y Amsterdam. Waagen, W. 1873-1875. Jurassic Fauna of Kutch. medio en la parte meridional de la provincia Riccardi, A.C. 1988. The Cretaceous System of Palaeontologia Indica 1 (1873): 1-22, 2 (1875): de Mendoza. Academia Nacional de Ciencias, southern South America. Geological Society 23-76, 3 (1875), 77-106, 4 (1875): 107-247. Actas 10: 1-57, Córdoba. of America, Memoir 168: 1-161. Weaver, C. 1931. Paleontology of the Jurassic and Leanza, A.F. 1945. Ammonites del Jurásico Salfeld, H. 1921. Kiel- und Furschenbildung auf Cretaceous of West Central Argentina. Superior y del Cretácico Inferior de la Sierra der Schalenaussenseite der Ammonoifrrn in Memoirs of the University of Washington 1: Azul, en la parte meridional de la provincia de ihrer Bedeutung für die Systematik und 1-496. Mendoza. Anales del Museo de La Plata NS 1: Festlegung von Biozonen. Zentralblatt für Zittel, K.A. 1884. Cephalopoda. In Zittel, K.A. 1-99. Mineralogie, Geologie und Paläontologie Handbuch der Palaeontologie, R.Oldenbourg, Leanza, H.A. 1980. The Lower and Middle Ti- 1921: 343-347. Band 1, Abteilung 2, Lieferung 3, 893 p., thonian Ammonite Fauna from Cerro Lotena, Spath, L.F. 1922. On Cretaceous Ammonoidea Munich and Leipzig.. Province of Neuquén, Argentina. Zitteliana 5: from Angola, collected by Professor J.W. Zittel, K.A. 1895. Grundzüge der Palaeontologie. 3-49. Gregory, D. Sc., F.R.S. Transactions of the R. Oldenbourg, viii + 971 p., 2.048 figs, Mu- Leanza, H.A. 1981. Faunas de ammonites del Ju- Royal Society of Edinburgh 53: 91-160. nich and Leipzig. rásico superior y del Cretácico inferior de Spath, L.F. 1923 a. On ammonites from New América del Sur, con especial consideración Zealand. In Trechmann, Ch.T. (ed.) The de la Argentina. In Volkheimer, W. and Mu- Jurassic Rocks of New Zealand. Geological sacchio, E.A. (eds.) Cuencas sedimentarias del Society of London, Quarterly Journal 79: Jurásico y Cretácico de América del Sur. 246-312. Recibido: 10 de septiembre de 2008 Comité Sudamericano del Jurásico y Cretácico Spath, L.F. 1923 b. A monograph of the Am- Aceptado: 4 de noviembre de 2008 Revista de la Asociación Geológica Argentina 64 (1): 43 - 54 (2009) 43

THE HIGH ANDEAN CORDILLERA OF CENTRAL ARGENTINA AND CHILE ALONG THE PIUQUENES PASS-CORDON DEL PORTILLO TRANSECT: DARWIN'S PIONEERING OBSERVATIONS COMPARED WITH MODERN GEOLOGY

Laura GIAMBIAGI1, Maisa TUNIK2, Victor A. RAMOS3, and Estanislao GODOY4

1 IANIGLA - CONICET, Mendoza. Email: [email protected] 2 CONICET-CIMAR - Universidad Nacional del Comahue. 3 Laboratorio de Tectónica Andina (FCEN) Universidad de Buenos Aires - CONICET. 4 Geólogo Consultor, Santiago de Chile.

ABSTRACT The geological observations made by Darwin in 1835 during his crossing of the Andes from Santiago to Mendoza via the Piuquenes Pass and Cordón del Portillo are compared with the present geological knowledge of the Cordillera Principal and Cordillera Frontal at 33°-34°S. The analysis of the complex stratigraphy of the Cordillera Principal, the imbricated structure of the Aconcagua fold and thrust belt, as well as the stratigraphy and structure of the inter mountain foreland Tunuyán Basin, allows to assess the pioneer observations of Darwin. He recognized the old metamorphic basement and the granitoids and volcanic sequences of late Paleozoic to Triassic age of the Cordillera Frontal, established the Cretaceous age of the marine successions cropping out along the eastern Cordillera Principal and studied the conglomeratic deposits associated with the uplift of the Cordillera in the Alto Tunuyán Basin. Based on the study of clast provenance of the synorogenic deposits of the Alto Tunuyán Basin, Darwin recognized that the Cordillera Frontal was uplifted later than the Cordillera Principal. The present knowledge of this sector of the Andean Cordillera confirms his pioneer observations and show that Darwin was one of the first scientists ever in realizing that in an orogenic system the sequence of uplift and deformation proceeds from hinterland towards foreland, according to a process that is exceptionally well-illustrated along the Piuquenes-Cordón del Portillo transect.

Keywords: Uplift, Provenance, Structure, Metamorphic rocks, Synorogenic deposits.

RESUMEN: La Alta Cordillera de los Andes del centro de Argentina y Chile a lo largo de la transecta del Paso Piuquenes-Cordón del Portillo: Las observaciones pioneras de Darwin comparadas con la geología moderna. Las observaciones geológicas efectuadas por Darwin en 1935 durante su cruce de la Cordillera de Los Andes entre Santiago y Mendoza realizado en 1835 a través de los pasos del Portillo y Piuquenes son examinadas y comparadas con el conocimiento actual existente de las Cordilleras Principal y Frontal entre los 33°-34°S. El análisis de la compleja estratigrafía de la Cordillera Principal, la estructura de las diferentes láminas imbricadas de la faja plegada y corrida del Aconcagua y la cuenca de antepaís intermonatana del Tunuyán, permiten ponderar las pioneras observaciones de Darwin, quien reconoció el basamento metamórfico y los granitoides y secuencias volcánicas neopaleozoi- cas a triásicas de la Cordillera Frontal, estableció la edad cretácica de las sucesiones marinas del sector oriental de la Cordillera Principal y describió e interpretó los depósitos conglomerádicos asociados al levantamiento de los Andes en el Alto Tunuyán. Sobre la base del estudio de la proveniencia de los clastos de los depósitos sinorogénicos de la cuenca del Alto Tunuyán pudo reconocer que la Cordillera Principal, fue alzada con anterioridad a la Cordillera Frontal, la que a través de varios pulsos adqui- rió su altura actual en épocas más recientes. Darwin fue uno de los primeros en reconocer que en un sistema orogénico, la secuencia de deformación asociada al levantamiento orogénico comienza en su región interna desde donde avanza progresi- vamente hacia el antepaís, como lo demuestran sus brillantes observaciones efectuadas ya hace más de 150 años.

Palabras clave: Levantamiento, Procedencia, Estructura, Rocas metamórficas, Depósitos sinorogénicos.

INTRODUCTION of himself as a geologist, crossed the The trip, which lasted from March 19th to Piuquenes and Portillo passes in 1835, March 24th, was a milestone in his kno- Charles Darwin, a naturalist who thought during his voyage on board HMS Beagle. wledge of Andean geology. It was in this 44 L. GIAMBIAGI, M. TUNIK, V. A. RAMOS AND E GODOY

sector of the Andes that Darwin suspec- ted that the giant mountains were uplifted as late as in the Cenozoic. The idea was a radical one, as well as the interpretation that the different ranges were uplifted one after the other, being the easternmost one the youngest. Later ge- nerations of geologists demonstrated that he was essentially right in both concepts. The Piuquenes and Portillo passes are situated in the southern Central Andes, at 33°40´S (Fig. 1). At these latitudes the Andes are formed by the Cordillera Principal and the Cordillera Frontal. The Piuquenes Pass is situated in the Cor- dillera Principal, at the border between Argentina and Chile, and the Portillo Pass is located in the high summits of the Cordillera Frontal to the east (Fig. 2). At the time Darwin crossed the Andes through the Piuquenes Pass, this sector of the Cordillera Principal was known as the Peuquenes range or western line, and the Cordillera Frontal, as the Portillo range. These two ranges are separated, by a wide valley filled by Neogene sediments (Depresión Intermontana del Alto Tunuyán, Figure 1: a) Location map of the High Andes with main geological provinces and lithological sequences Polanski 1957). The Cordillera Principal between Santiago (Chile) and Mendoza (Argentina); b) Block diagram showing the main structures of the can be subdivided into a western sector, Cordillera Principal and Cordillera Frontal with location of the Aconcagua fold and thrust belt. formed by a thick sequence of Oligocene and Molnar 1987). The distribution of narrower and formed only by the Cor- and Miocene volcanic rocks and intrusi- seismicity suggests that the Nazca Plate dillera Principal and Cordillera Frontal. ves, and an eastern zone corresponding is characterized by alternating flat and The study area (33°-34°S) is located to the strata of Mesozoic sedimentary normal segments of subduction (Baran- along the transition zone between the flat rocks deformed into the Aconcagua fold zangi and Isacks 1976, Cahill and Isacks and normal subduction segments (Fig. 3). and thrust belt (Ramos 1988, Cegarra 1992, Pardo et al. 2002). Several features and Ramos 1996, Ramos et al. 1996a). It of the Andes orogenic system appear to STRATIGRAPHY was observing these rocks that Darwin correlate with these changing subduction became astonished to see "shells which were geometries, such as volcanism and distri- The stratigraphy, examined in many pre- once crawling on the bottom of the sea, now stan- bution of morphostructural belts and vious studies such as those of by Po- ding nearly 14,000 feet above its level" tectonic style (Isacks et al. 1982, Jordan et lanski (1964), Thiele (1980), Charrier et (Darwin 1845, p. 320). al. 1983). North of 33°S, the flat subduc- al. (1997), Ramos et al. (2000), Jordan et tion segment lacks active arc magmatism al. (2001), Giambiagi et al. (2001, 2002), TECTONIC SETTING and underlies the Cordillera Principal, Tunik (2003), Tunik et al. (2004), Kay et Cordillera Frontal, Precordillera and al. (2005), Fock ( 2005) and Farías et al. The Andes of central Argentina and Sierras Pampeanas structural provinces (2008) includes seven main major tecto- Chile are a linear orogenic belt formed at while south of 34°S the "normal" sub- no-stratigraphic units (Fig. 4): (1) Prote- the convergent plate margin between the duction segment of south-central Argen- rozoic to Lower Triassic metamorphic, Nazca and South American plates. At tina and Chile shows the active volcanism volcanic and intrusive rocks; (2) Triassic 33°-34° S the Nazca plate subducts bene- of the Southern Volcanic zone (see Hil- and Jurassic rift sequences; (3) Titho- ath the South American plate at a relative dreth and Moorbath 1988, Stern et al. Neocomian marine strata; (4) Cretaceous velocity of nearly 10 cm/yr (Pardo-Casas 2007) and the orogenic system is much to Paleogene marine and non marine The High Andean Cordillera of central Argentina and Chile along ... 45

vailed more and more, with only a few quite small points of granite projecting through. Looking at the entire eastern face of the Portillo range, the red color far exceeds in area the black; yet it was scarcely possible to doubt, that the granite had once been almost wholly encased by the mica-schist" (Dar-win 1846, p. 184). Permian-Triassic intermediate and acid volcanic rocks unconformably overlie the previously deformed rocks. As Dar- win pointed out, these volcanics reach a thickness of more than 1,700 m. He described these rocks and assigned them a relative age based on their metamorphism and vein intrusion. Rifting was widespread in central Argentina and Chile during Triassic - Early Jurassic times. This was a consequence of extensional processes linked to the initial stages of the fragmentation of Gondwana and the opening of the South Atlantic (Uliana et al. 1989). A series of NNW trending rift systems was formed at this time along the western margin of Gond- Figure 2: a) Water divide bet- wana (Charrier 1979, 1984, Uliana et al. ween Chile and Argentina with 1989). In the Cordillera Principal, these indication of the Piuquenes rift basins, which at this latitude only Pass. The international landmark (hito) is located on crop out in the Chilean side of the Cor- Neocomian limestones; b) dillera Principal, were filled by Lower to Portillo Pass across the Middle Jurassic black shales (Godoy Cordillera Frontal. Glacial 1993, Alvarez et al. 1997, 1999). In the deposits are overlying late Paleozoic granites; c) Argentinean side, a thick layer of evapo- Approaching the international rites (Auquilco Formation) unconforma- boundary along Darwin's trail. bly rests upon Jurassic marine strata. This sedimentary rocks; (5) Eocene?-Oligo- range... I think it more probable, considering its was observed by Darwin who was stun- cene to Lower Miocene volcanics; (6) more perfect metamorphic character and its well- ned by its more than 600 m thickness. Upper Cenozoic foreland basin deposits; pronounced foliation, that it belongs to an ante- The evaporites grade into the red sands- and (7) Upper Cenozoic intrusives and rior epoch" (Darwin 1846, p. 184). These tones and claystones with subordinated volcanic-arc rocks. rocks are unconformably overlain by Up- conglomerates of the Kimmeridgian The oldest rocks correspond to Protero- per Paleozoic marine black shales and Tordillo Formation. zoic mica-schists cropping out at Cordón sandstones and are intruded by Carbo- The Jurassic units in the Cordillera Prin- del Portillo, in the Cordillera Frontal. niferous to Permian granitoids (Polanski cipal are capped by black shales, mudsto- These rocks were examined by Darwin at 1964). These granitoids were clearly nes, limestones and sandstones, deposi- the "base of the Portillo range, ...at a place identified by Darwin who stated that "the ted on a stable marine platform during called Mal Paso. The mica-schist here consists of red granite, from being divided by parallel joints, Titho Neocomian thermal times (Biro thick layers of quartz, with intervening folia of has weathered into sharp pinnacles, on some of 1964, Polanski 1964, Aguirre-Ureta finely-scaly mica, often passing into a substance which, even on some of the loftiest, little caps of 1996). These sedimentary rocks, now like black glossy clay-slate: in one spot, the layers mica-schist could be clearly seen: here and there grouped into the Mendoza Group, were of quartz having disappeared, the whole mass isolated patches of this rock adhered to the described in detailed by Darwin who became converted into glossy clay-slate. Where mountain-flanks, and these often corresponded assigned them a Neocomian age based the folia were best defined, they were inclined at in height and position on the opposite sides of on his collection of invertebrate fossils a high angle westward, that is, towards the the immense valleys. Lower down the schist pre- (see Aguirre-Urreta and Vennari, 2009). 46 L. GIAMBIAGI, M. TUNIK, V. A. RAMOS AND E GODOY (2003). Darwin, by The trail followed as the diffe- as well et al. agi Geologic map of Santiago between (Chile) and Mendoza (Argentina) the Cordillera Principal and Frontal based on Giambi rent camps, Darwin's (1845) description. following day by are indicated day Figure 3: The High Andean Cordillera of central Argentina and Chile along ... 47

Figure 4: Structural cross sections across the High Andes: a) Section of the Piuquenes pass and b) Portillo pass after Darwin (1846); c) Modern structural cross-section based on Giambiagi and Ramos (2002). Location of the section in figure 3. The Upper Cretaceous strata, se-parated region at that time. 2002, Farías et al. 2008). that crop out in from the Lower Cretaceous units by an After at least 10 Ma of magmatic quies- two north-trending belts made up of erosional unconformity, consist of con- cence, volcanic activity considerably basic lavas, tuffs and intermediate pyro- glomerates, sandstones and volcaniclastic increased in the western Cordillera Prin- clastic rocks, interbedded with sedimen- rocks. The presence of a continuous cipal during the Eocene?-early Miocene tary rocks totalling at least 2,000 m thick. interval of Maastrichtian to Da-nian (Godoy et al. 1996, Charrier et al. 1997, Eocene ages are reported only on the siltstones and limestones deposited Jordan et al. 2001, Kay et al. 2005, Fock western belt while, in general, volcanic during an Atlantic ingression that rea- 2005). This volcanic event corresponds rocks have a primitive isotopic signature ched up to the eastern slope of the pre- to the Abanico and Coya Machalí For- indicating crustal thinning and high pale- sent-day Cordillera Principal (Tunik mations of Late Eocene to Middle Mio- othermal gradient (Nystrom et al. 1993, 2003, Tunik et al. 2004) indicates a sub- cene age (Wyss et al. 1990, Flynn et al. Vergara et al. 1988, 1999) during the ac- dued relief close to sea level for this 1995, Gana and Wall 1997, Charrier et al. cumulation of these sequences, probably 48 L. GIAMBIAGI, M. TUNIK, V.A. RAMOS AND E GODOY

Figure 5: a) Tunuyán and Palomares rivers, looking southward. Cretaceous marine strata unconformably rest over the Paleozoic basement rocks of the Cordón del Portillo; b) Tunuyán Conglomerate in the Palomares river; c) Tuffs and conglomerates of the Palomares Formation unconformably resting over the Tunuyán Conglomerate; d) Butaló Formation.

in a volcano-tectonic extensional /trans- (SVZ) of the Andes (Hildreth and Moor- yán Conglomerate started around 18 to tensional basin (Charrier et al. 1997, 2002, bath 1988, Stern et al. 2007). 17 Ma, and continued to ~10 Ma. The Godoy et al. 1999, Jordan et al. 2001, Fock Along the eastern slope of the Cordillera intermediate Palomares Formation filled 2005). The Abanico and Coya Machalí Principal, in Argentina, the oldest Ceno- a gentle paleorelief in the Tunuyán Con- Formations are covered by 2,000 m of zoic volcanic unit is the Contreras For- glomerate between 8.5 and 7 Ma and Miocene calc-alkaline andesitic lava and mation, exposed at the base of a thick consists of 200 m of volcaniclastic and acid pyroclastic flows of the Farellones sequence of Neogene synorogenic depo- clastic sediments deposited in an alluvial- Formation from which radiometric ages sits, and consisting of basaltic lava flows fan setting (Fig. 5c). The Cordillera Fron- spanning from 20 Ma to 7 Ma have been and breccias. A sample from the middle tal was the main source area of this unit, reported (Rivano et al. 1990). This unit is sector of this unit yielded a whole rock as indicated by clast composition, paleo- intruded by several epizonal granitoid K-Ar age of 18.3 Ma (Giambiagi et al. current measurements and syntectonic plutons and stocks, some of them asso- 2001). Its geochemistry suggests a retro- geometries (Giambiagi et al. 2001). The ciated with the giant copper-porphyry arc setting in a thickening crust during its Butaló Formation records the final infi- copper deposits of Río Blanco and El extrusion (Ramos et al. 1996b). Late Ce- lling of the sedimentary trough, reaching Teniente with show radiometric ages nozoic foreland clastic deposits occur to a thickness of more than 300 m, and is constrained between 10 to 6 Ma (Cor- the east of the Cordillera Principal. made up of fluvial and lacustrine depo- nejo and Mahood 1997, Kurtz et al. 1997, Miocene sedimentary rocks filling the sits (Fig. 5d). It is unconformably over- Maksaev et al. 2004, Deckart et al. 2005). Alto Tunuyán Basin include three units. lain by andesitic volcanic rocks dated at During this journey across the Cordillera The oldest Tunuyán Conglomerate con- 5.9 Ma (whole rock K/Ar age, Ramos et Principal Darwin observed massive rocks sists of up to 1,400 m of conglomerates al. 2000). Darwin's description of the which "rarely contain any quartz" intruded and sandstones deposited in an alluvial- Conglomerate of Tenuyan (sic) indicated among the volcanic strata that probably fan setting (Fig. 5b). that "the included pebbles are either perfectly or correspond to sub volcanic dykes and The vertical variation in clast composi- only partially rounded: they consist of purplish sills emplaced in the volcanic sequences. tion and paleocurrent data reflect the sandstones, of various porphyries, of brownish Finally, the youngest geological units of progressive erosion and unroofing of the limestone, of black calcareous, compact shale the Cordillera Principal at this latitude Aconcagua fold and thrust belt (Giam- precisely like that in situ in the Peuquenes range, correspond to large composite Quater- biagi 1999). The synchronism of deposi- and containing some of the same fossil shells; nary stratovolcanoes such as the Tupun- tion of the upper 400 m of this unit with also very many pebbles of quartz, some of mica- gato, San Juan, Marmolejo and San José, the emplacement of thrusts within the ceous schist, and numerous, broken, rounded being these the northernmost volcanic basin is documented by syntectonic un- crystals of a reddish orthitic or potash feldspar centers of the Southern Volcanic Zone conformities. Deposition of the Tunu- (as determined by Professor Miller), and these The High Andean Cordillera of central Argentina and Chile along ... 49

Figure 6: a) Geological map of the Aconcagua fold-and-thrust belt in the Yeso and Palomares river (modified from Giambiagi et al. 2003) See location in figure 3; b) Structural cross-section A-B (Giambiagi and Ramos 2002); c) The Chacayal thrust in the Chilean slope of the Cordillera Principal, uplifting Late Jurassic red beds (Tordillo Formation) over Early Cretaceous continental and marine sequences (Colimapu Formation and Mendoza Group); d) Southern view of the Cerro Palomares. The Palomares fault system uplifts Cretaceous limestones over the Neogene synorogenic strata of the Tunuyán Conglomerate (a) and the Palomares Formation (b). 50 L. GIAMBIAGI, M. TUNIK, V. A. RAMOS AND E GODOY

from their size must have been derived from a In this sector of the belt it is common to the conglomerate have been thrown off at an coarse-grained rock, probably granite. From this observe thrusts cutting previously deve- angle of 45° by the red Portillo granite ..., we feldspar being orthitic, and even from its exter- loped structures, some of them showing may feel sure, that the greater part of the ... nal appearance, I venture positively to affirm younger-over-older relationships. This is upheaval of the already partially formed Portillo that it has not been derived from the rocks of the the reason why it is interpreted as evol- line, took place after the accumulation of the western ranges; but on the other hand it may well ved by a forward propagating thrust se- conglomerate, and long after the elevation of the have come, together with the quartz and meta- quence with periods of out-of-sequence Peuquenes ridge. So that the Portillo, the loftiest morphic schists, from the eastern or Portillo line, thrusting line in this part of the Cordillera, is not so old for this line mainly consists of coarse orthitic as the less lofty line of the Peuquenes. Evidence granite. The pebbles of the fossiliferous slate and The Alto Tunuyán Foreland Basin derived from an inclined stream of lava at the of the purple sandstone, certainly have been deri- eastern base of the Portillo, might be adduced to ved from the Peuquenes or western ranges." The Alto Tunuyán Basin is a Neogene show, that it owes part of its great height to ele- (Darwin 1845, p. 182). foreland basin located between the Cor- vations of a still later date. ... In most parts, dillera Principal and the Cordillera Fron- perhaps in all parts, of the Cordillera, it may be STRUCTURE tal, from 33°30´ to 34°S. The basin was concluded that each line has been formed by repe- generated by lithospheric flexure as res- ated upheavals" (Darwin 1845, p. 320). Aconcagua Fold and Thrust Belt ponse to thrust belt load of the Acon- These conglomerates are now part of the cagua fold and thrust belt and was filled Alto Tunuyán Basin (Polanski 1957), an During his ride through the Piuquenes with more than 1,800 m of continental intermontane basin studied by Giambiagi Pass Darwin felt admiration for the incli- synorogenic deposits of the Tunuyán (1999), who confirmed the uplift sequen- ned to overturned Mesozoic layers which Conglomerates and the Palomares and ce by means of precise provenance stu- had been formed undersea and later Butaló Formations. These strata were dies. The Alto Tunuyán Basin structure uplifted and deformed in an "extraordi- deformed as the Aconcagua fold-and- comprises thin-skinned fold-thrust she- nary manner", reaching an altitude of thrust belt deformation migrated eas- ets above a decóllement developed in Late more than 4,000 m (Figs. 4 a and b). tward and cannibalized the foreland ba- Cretaceous siltstones. Within this zone, Presently, the Argentine eastern sector of sin. One of Darwin's first geological ob- faults related to syntectonic unconformi- the Cordi-llera Principal is interpreted to servation was that the constitutions of ties recorded in the upper part of the form part of the Aconcagua fold-and- the two ranges are totally different and Tunuyán Conglomerate indicate sedi- thrust belt. This belt can be divided into that "one part of the double line of mountains mentation above an active thrust (Giam- two do-mains (Fig. 6b, c). (Cordilleras Principal and Frontal) is of biagi et al. 2001). Other faults indicate The structure of the western domain is an age long posterior to the other", which is activity after deposition of the Neogene dominated by open NNW trending folds nowadays known as the normal sequence units. The structure in this domain intri- and N-S thrust faults. The majority of of deformation and uplift of an orogenic gued Darwin, who observed that the these thrusts cut the previously develo- system, from the hinterland to the fore- newer coarse conglomerates dipped di- ped fold structures and show an eastward land. It is interesting to remark how Dar- rectly under the much older beds of gyp- vergence, with the exception of a belt of win, based on a provenance analysis of seous and limestone units. He stated that west verging back-thrusts located at the conglomeradic clasts, was able to infer the Mesozoic strata has been broken up border between Argentina and Chile (Fig. the order and the relative uplift age of by "several, distinct, parallel, unilinal lines of 4c). Although the basement does not each mountain chain across this section elevation" and estimated a vertical throw crop out, its involvement is deduced of the Andes. He noted that between the of over 600 m. These lines of elevation from the geometry of the structure and two main ranges "there rest beds of a conglo- represent the complex thrust system of significant thickness variations of the merate several thousand feet in thickness, which the Cerro Palomares (Fig. 6d). deformed Mesozoic successions (Giam- have been upheaved ... and dip at an angle of biagi et al. 2003). The eastern domain is 45° towards the Peuquenes line. I was astonis- Cordillera Frontal broad and narrows towards the south hed to find that this conglomerate was partly until it disappears south of Volcán Mar- composed of pebbles, derived from the rocks, The Cordillera Frontal consists of seve- molejo. This domain is characterized by a with their fossil shells, of the Peuquenes range; ral basement blocks uplifted east of the dense array of imbricate low-angle and partly of red potash granite, like that of the Aconcagua fold-and-thrust belt. During thrusts which exhibits flat and ramp geo- Portillo. Hence we must conclude, that both the the Late Cenozoic compression - at the metries, with flats corresponding to decó- Peuquenes and Portillo ranges were partially latitude of the Portillo Pass- it behaved llement levels located on the Late Jurassic upheaved and exposed to wear and tear, when as a rigid block disrupted by medium to evaporites and Early Cretaceous shales. the conglomerate was forming; but as the beds of high angle faults showing two regional The High Andean Cordillera of central Argentina and Chile along ... 51

trends of structures, i.e., NNE-trending Here we integrate recent structural and and the accumulation of the wedge of faults in the northern region and N to sedimentological studies carried out at proximal synorogenic deposits of the NNW-trending faults in the southern 33°-34° S (Godoy 1993, Godoy et al. Palomares and Butaló Formations. half, related to Late Paleozoic structures. 1996, 1999, Ramos et al. 1996b, 1997, The uplift of the Cordillera Frontal gene- A deep crustal detachment level is requi- Giambiagi et al. 2001, Giambiagi and rated a sticking point, preventing the pro- red in order to propagate shortening Ramos 2002, Giambiagi et al. 2003) as a pagation of the thrust belt towards the within the Cordillera Frontal. This de- way to understand what was envisaged by foreland. As a result, a series of out-of- tachment is inferred to be located betwe- Darwin's during his early observations. sequence thrusts developed in the eas- en 15 and 20 km deep, probably within a During the early Miocene - between 20? tern Cordillera Principal and the Alto Tu- brittle-ductile transition. and 15 Ma - shortening associated with nuyán foreland basin was partially canni- inversion of the Mesozoic and Early Ce- balized by the advancing thrust sheets. GEOMORPHOLOGY nozoic extensional systems started diach- After the emplacement of granitoids ronically in the Cordillera Principal. At with isotopic signatures indicating mag- Darwin stated "in the course of ages in this around 18 Ma deformation and uplift mas sources in a thickened crust (Kurtz et (referring to the Cachapoal) and other generated a western sediment source for al. 1997, Kay et al. 2005) in the western valleys, events may have occurred like between, the lower part of the foreland Tunuyán Cordillera Principal, magmatism shifted but even on a grander scale than, that described Conglomerate, which at 15 Ma shows an eastward to its present position along the by Molina, when a slip during the earthquake of abrupt change in clast composition indi- active volcanic front of the Tupungato- 1762, banked up for ten days the great river cated by the first appearance of Me- San José line. Thrusting along the Cordi- Lontué, which then bursting its barrier inunda- sozoic sedimentary debris (Giambiagi llera Principal ended by ~4 Ma and sub- ted the whole country and doubtless transported 1999, Irigoyen et al. 2000). This is consi- sequently the deformation front migra- many great fragments of rocks" (Darwin dered as evidence for a second phase of ted to the Precordillera - north of 33°S- 1846, p. 66-67). However, "not withstan- deformation, linked to E-W shortening and to the Cuyo Basin and the eastern ding this one case of difficulty" he then clai- along the eastern Cordillera Principal. Cordillera Frontal foothills south of med that "one of the most important conclu- Continued migration of deformation to- 33°S. Folding in the Cuyo Basin started sions to which my observations on the geology of ward the foreland is documented by the after the beginning of deposition of the South America have led me", is his lengthily occurrence of growth structures in the Mogotes Formation at ~ 3.0 Ma (Yri- exposed conviction that the "terrace-like upper section of the Tunuyán Conglo- goyen 1993, Chiaramonte et al. 2000). fringes, which are continuously united with the merate. At that time basement thrusts The last phase of deformation accom- basin-shaped plains at the foot of the Cordillera, transported eastwards earlier Cordillera modating ESE-WNW and ENE-WSW have been formed by the arrestment of river- Principal thrusts and folds, forming thus shortening concentrated in the Cuyo borne detritus at successive levels, in the same a hybrid thin and thick-skinned belt of Basin and in the La Carrera thrust system manner as we see now taking place at the heads deformation. in the Cordillera Frontal. Earthquake fo- of all those many, deep, winding fiords intersec- Overprinting relationships indicate that cal mechanisms and neotectonic activity ting the southern coasts." Due to this view the fold and thrust belt evolved by means indicate that the eastern border of the Darwin failed to recognize the Meson of a forward propagating thrust sequen- Precordillera and the Cuyo Basin are still Alto landslide and the plain in the Yeso ce including periods of out-of-sequence active (Cortés et al. 1999, Chiaramonte et valley (Embalse El Yeso in Fig. 6a) and thrusting. The denudation of the fold al. 2000). refers to it as "the one exception in which a and thrust belt provided the material for range was dislocated by a great and abrupt the uppermost levels of the Tunuyán CONCLUDING REMARKS fault" In page 178 this plain is described Conglomerate. The unconformity that as a "drained lake", yet once more no rela- separates the Palomares Formation from Darwin was the first geologist to descri- tion to a landslide is mentioned. The dam the Tunuyán Conglomerate, the changes be the stratigraphy of the Cordillera of this paleolake was later described as a in paleocurrents measured in the former Principal and the Cordillera Frontal of moraine and only recently has it been unit, and the occurrence of locally deri- central Chile and Argentina recognizing recognized as part of a big landslide derived conglomerates, provide evidence for the presence of an old metamorphic ba- ved from the granitic southern slopes of the beginning of a third phase of defor- sement - much older than its "red grani- the Yeso valley (Abele 1981). mation, associated this time with the on- tes" - presently considered to be late Pa- set of uplifting in the Cordillera Frontal. leozoic intrusives in the Cordillera Fron- STRUCTURAL EVOLUTION This is recorded by the regional influx of tal. He correctly assigned a Cretaceous OF THE ANDES AT 33°40´S thick coarse conglomerates, the genera- age to the marine sediments cropping tion of a broken foreland basin system, out along the eastern Cordillera Principal 52 L. GIAMBIAGI, M. TUNIK, V.A. RAMOS AND E GODOY

and also showed that the uplift of the lógica Argentina 64(1): 32-43. no (Antofagasta), Actas 1: 465-469. Andean range was much younger than Alvarez, P.P., Aguirre-Urreta, M.B., Godoy, E. Chiaramonte, L., Ramos, V.A. and Araujo, M. the marine sedimentation of the Creta- and Ramos V.A. 1997. Estratigrafía del Jurá- 2000. Estructura y sismotectónica del anticli- ceous deposits. By studying the composi- sico de la Cordillera Principal de Argentina y nal Barrancas, Cuenca Cuyana, provincia de tion of clasts included in Tertiary sedi- Chile (33°45´ - 34°00´ LS). 8° Congreso Geo- Mendoza. Revista Asociación Geológica ments of the Alto Tunuyán Basin he was lógico Chileno, Actas 1: 425-429, Santiago. Argentina 55: 309-336. able to establish the relative uplifting Alvarez, P.P., Godoy, E. and Giambiagi, L. 1999. Cornejo, P. and Mahood, G. 1997. Seeing past the times for the Cordillera Principal and Estratigrafía de la Alta Cordillera de Chile effects of re-equilibration to reconstruct mag- Cordillera Frontal conluding that the Central a la latitud del paso Piuquenes (33°35´ matic gradients in plutons: La Gloria pluton, Piuquenes range (Cordillera Principal) LS). 14° Congreso Geológico Argentino (Sal- central Chile. Contributions to Mineralogy was uplifted before the Portillo range ta), Actas 1: 55. and Petrology 127: 159-175. (Cordillera Frontal). To explain the drai- Barazangi, B.A. and Isacks, B.L. 1976. Spatial dis- Cortés, J.M., Vinciguerra, P., Yamín, M. and nage pattern of the Tunuyán-Cordón del tribution of earthquakes and subduction of Pasini, M.M. 1999. Tectónica cuaternaria de la Portillo region, he proposed a gradual the Nazca Plate beneath South America. region andina del Nuevo Cuyo (28°-38°LS). elevation of the Cordón del Portillo after Geology 4: 686-692. In Caminos, R. (ed.) Geología Argentina, the uplift of the Piuquenes range, which Biro, L. 1964. Límite Titónico-Neocomiano en Subsecretaría de Minería de la Nación, Ser- allowed the waters of the Tunuyán De- Lo Valdés. Memoria de título, Departamento vicio Geológico Minero Argentino, Anales 29: pression to cut a deep canyon through de Geología, Universidad de Chile, unpublis- 760-778. the latter as the range was been uplifted. hed, 118 p., Santiago. Darwin, C.R. 1845. Journal of researches into the As a tribute to his exceptional observa- Cahill, T. and Isacks, B.L. 1992. Seismicity and natural history and geology of the countries tional skills we should emphasize that the shape of the subducted Nazca Plate. Journal visited during the voyage of H.M.S. Beagle progressive advance of an orogenic de- Geophysical Research 97: 17503-17529. round the world, under the Command of formation front from hinterland towards Cegarra, M. and Ramos, V.A. 1996. La faja plega- Capt. Fitz Roy, R.N. John Murray 2d. edition, foreland, now considered a common da y corrida del Aconcagua. In Ramos, V.A. et 528 p., London. process in most mountain chains, was al. Geología de la región del Aconcagua, Pro- Darwin, C.R. 1846. Geological observations on early envisaged by Darwin more than 150 vincias de San Juan y Mendoza. Subsecretaría South America. Being the third part of the years ago during is historic 1835 journey de Minería de la Nación, Dirección Nacional geology of the voyage of the Beagle, under across the Andes. del Servicio Geológico, Anales 24: 387-422, the command of Capt. Fitzroy, R.N. during Bue-nos Aires. the years 1832 to 1836. Smith Elder and Co., ACKNOWLEDGEMENTS Charrier, R. 1979. El Triásico de Chile y regiones 280 p., London. adyacentes de Argentina: una reconstrucción Deckart, K., Clark, A., Aguilar, C., Vargas R., This work was found by the University of paleogeográfica y paleoclimática. Comunica- Bertens, A., Mortensen, J. and Fanning, M. 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THE FIRST GEOLOGICAL MAP OF PATAGONIA

Eduardo O. ZAPPETTINI and José MENDÍA

Servicio Geológico Minero Argentino (SEGEMAR) - Av. Julio A. Roca 651 (1322) Buenos Aires Emails: [email protected], [email protected]

ABSTRACT This contribution analyses the first geological map of Patagonia drawn by Darwin around 1840, and colour-painted by Darwin himself. It had remained unpublished and only a small version in black and white had been printed before. The different units mapped by Darwin are analysed from a modern perspective, and his ability to show a synthesis of the complex geological structure of Patagonia is stressed.

Keywords: Geological map, Patagonia, Patagonian Shingle, Darwin geologist.

RESUMEN: El primer mapa geológico de la Patagonia. La presente contribución analiza el primer mapa geológico de la Patagonia realizado por Darwin cerca de 1840, pintado en colores por el mismo Darwin, que ha permanecido inédito y del que sólo se cono- cía una versión de tamaño reducido en blanco y negro. Se analizan las diferentes unidades mapeadas por Darwin desde una perspectiva actual, destacándose su habilidad para mostrar en esa síntesis la compleja estructura de la Patagonia.

Palabras clave: Mapa geológico, Patagonia, Rodados Patagónicos, Darwin geólogo.

DARWIN AND THE VOYAGE OF HMS BEAGLE

At the time Charles Darwin set sail on board HMS Beagle on a journey that was to last two years and ended up lasting five, he was not more than an amateur naturalist that had quitted his medical courses and after that abandoned his intention of applying for a position in the Church of England, just to embrace the study of natural history. In those years Charles Lyell (1830-1833) published his theory of gradualism (the present is the key of the past) as a counterpart to catastrophism then prevailing in geological thought. During the five years (1831- 1835) that his journey on board HMS Beagle lasted - of which three years and one month were spent ashore - Darwin made countless observations on geology and biology and even on the communi- ties that he visited (Fig. 1). These altoge- ther paved the way towards his famous theory on natural selection and the origin of species and the influence that the environment - including its geological cons- Figure 1: Itinerary of HMS Beagle and landings and titution and evolution - has on the suc- journeys of Charles Darwin cess and adaptability of species. in the interior of South America. 56 E. O. ZAPPETTINI AND J. MENDÍA

DARWIN'S CONTRIBUTIONS on the topography produced by (1826- of gneiss and granitic slates that compri- TO GEOLOGICAL 1830) and published in London by John se the Cordillera presently bearing his KNOWLEDGE Murray. The watercolour is from the time name (Eastern Metamorphic Complex of of Darwin's voyage on the Beagle or Aysen and Magallanes in Chile and La- Darwin's contributions to the geological maybe just a few years later, circa 1840. pataia Formation in Argentina). Towards knowledge of South America were re- On the back of the map there is an anno- the northeast he pointed out the predo- corded in four main papers, i.e., On the tation reading "p 94", which suggests minance of gneiss and granites (Coast connection of certain volcanic phenomena in that it was drawn for inclusion as an illus- Batolith in Chile). Between Cabo Tres South America (1838), On the distribution of tration to one of the volumes of the Montes and the north of the Chonos erratic boulders and on the contemporaneous Geology of the voyage of H.M.S. Beagle Islands the predominant rock is a Mica stratified deposits of South America (1841), (1846c), but it was never printed. slate with organic matter (presently Canal An account of the fine dust which often falls on The only previous regional maps are tho- King Formation). vessels in the Atlantic Ocean (1846a,[Read 4 se published by d'Orbigny (1842) toge- June 1845), On the Geology of the Falkland ther with the geological sketch of the UNIT 2 (Trappean rock and porphyries). It Islands (1846b), and finally the book Geo- provinces of Misiones, Corrientes, Entre includes intrusive and extrusive rocks logical Observations on South America (1846c) Ríos, Santa Fe, and Buenos Aires, inclu- exposed in Hoste and Londonderry reprinted in 1876 as part of his Geological ding northern Patagonia down to Valdés Islands (Chile) and also Mesozoic ophio- observations on the volcanic islands and parts of Peninsula. litic rocks exposed in Wollaston Island. the South America visited during the voyage of It comprises Paleogene monzodiorites, H.M.S. Beagle. On the other hand, a first GEOLOGICAL MAP diorites and gabros and Mesozoic basal- unpublished essay of his geological ob- OF PATAGONIA tic lavas of the Hardy Formation, and the servations was written during his voyage Sarmiento and Tortugas ophiolitic com- on board H.M.S. Beagle ca. 1834 which is The observations by Darwin recorded on plexes in Chile. titled Reflection on reading my geological notes. the map are mainly referred to coastal In Wollaston Island Darwin described In this work Darwin put forward a the- areas surveyed by HMS. Beagle (Fig. 2) "greenstones" with pyrite and epidote with ory on the geological formation of South and involve seven geological units: evidence of metamorphism, and basaltic America and included his interpretations submarine volcanic rocks. In Hardy Pe- on the history of life in the continent 1) Granite, Mica slate, ninsula he recorded the pyrite-rich "trap- which, albeit not evolutionist is neverthe- 2) Trappean rock and porphyries, pean and basaltic rocks", trachytes with less sequential (cf. S. Herbert 1995). 3) Purple porphyries and infra metamorphics, columnar jointing, and diorites rocks. In these works there are numerous and 4) Clay slate, Darwin's precise drawing of the contact detailed descriptions of geological units. 5) Tertiary [newer - crossed out -] (Pliocene?), between this unit and Unit 1 is especially However, figures representing his obser- 6) Recent, remarkable when compared to modern vations are few and of limited interest. 7) Basaltic lava. maps. Despite of this, Darwin at some stage of About units 2 and 3 he states "These two his life as a scientist tried to synthesize in are perhaps very closely allied". UNIT 3 (Purple porphyries and infra meta- a map the main units that were subject of morphics). This unit was recognized by his work, although such a map was never UNIT 1 (Granite, Mica slate). It mainly Darwin on the coast of Santa Cruz. It published. involves the Cretaceous so-called Coast comprises the volcanic rocks of the The Library at Cambridge University Batolith that Darwin stated as exposed in Chon Aike Formation, which in this sec- keeps an original unpublished map pain- Chilean Patagonia south of the Strait of tor represents the easternmost exposures ted with water-colours by Darwin him- Magellan. It includes outcrops at the of the Deseado Massif. Darwin descri- self. This is the earliest known Regional notheastern corner of Hoste Island and bed these volcanics as trachytic porphy- Geological Map of the Patagonian re- at the northwestern sector of Navarino ries. The unit includes the tectonic silici- gion of Chile and Argentina (Fig. 2). A Island. In this unit are also included the fied breccias assigned to the Bahía Laura black and white version was published by Mica slates of the southernmost end of Group, and exposed at Cabo Blanco. Rhodes (1991, Fig. 3) who stated "the the Chonos Archipelago in the Taitao These were erroneously interpreted by magnificent structure of this map" in contrast and Tres Montes peninsulas. Darwin as siliceous sedimentary rocks with the other Darwin's unpublished In his description of this unit Darwin and compared them to those of the Mal- map of Patagonia which is rather simple recorded different lithologies, among vinas Islands. (Herbert 1991 and 2005, plate 6). The which he distinguished - west of the Off-limits he describes purple to pink, geological observations were represented Beagle Channel bifurcation - a complex and sometimes laminated, porphyric The first geological map of Patagonia 57

Figure 2: Geologic map of Patagonia (Darwin, circa 1840, unpublished). rocks, which are slightly vesicular with north and points out the presence of Navarino Island. He describes slates ex- feldspar and quartz crystals. He stated local chert beds. posed along the Beagle Channel between these as the dominant rocks in between its bifurcation and Lemaire Strait, consti- Port Santa Elena, Camarones and Ma- UNIT 4 (Clay slate). According to Dar- tuting Navarino Island to the south and laspina up to the "Paps of Pineda" (sic). win's description, it comprises siltstones, the eastern sector of Hoste Island and At Puerto Deseado he records the pre- feldspar volcanics, graywackes, scarce Hardy Peninsula. North of the Beagle sence of porphyries up to 40 km west of black limestones with Lower Cretaceous Channel this unit runs in a northwestern the coast, covered by Tertiary deposits. fossil remains, and metamorphic schists. direction towards Brunswick Peninsula He indicates that the composition is These are the core of the mountain ran- on both coasts of Admiralty Strait, con- similar to that of those described further ges of southern Tierra del Fuego and tinuing north along the eastern flank of 58 E. O. ZAPPETTINI AND J. MENDÍA

the Cordillera. Darwin collected fossils his description he indicates that not all ral inability to translate his images into visual from the limestone beds he recorded. the gravel beds are from recent times, representations". These indicate a Cretaceous age. particularly those of higher tablelands Lastly, it should be stressed that if it had Darwin stressed the change in the direc- (see discussion in Martínez et al. 2009). been published when originally planned, tion of the Cordillera, that north of the He recorded interesting observations on it would have constituted a singular in- Strait of Magellan runs with a N-S orien- the origin and transport mode of the gra- itial contribution to the geological carto- tation changing south of it to a E-W vels, noting the difficulty in explaining graphy of the South American conti- orientation. This unit includes the pre- the distribution covering the entire area nent. sent Lemaire and Yaghan Formations in from the Andes to the coast. He did Argentina and Cerro Toro, Yaghan and point out, however, that the present dis- WORKS CITED IN THE TEXT Punta Barrosa Formations in Chile, tribution of the gravels was due to mari- spanning the Late Jurassic-Cretaceous ne action. Casadío, S. and Griffing, M. 2009. Sedimentology (see Olivero et al. 2009). and Paleontology of a Miocene marine suc- Again, we must stress the precision with UNIT 7 (Basaltic lava). It represents the cession frst noticed by Darwin at Puerto De- which Darwin drew the contact between basaltic exposures along the middle and seado (Port Desire). Revista de la Asociación units 2 and 4 in Tierra del Fuego. upper course of the Río Santa Cruz and Geológica Argentina 64 (1): 83-89. the Meseta La Siberia. Darwin placed Darwin, C. 1838. On the connexion of certain UNIT 5 (Tertiary, Pliocene?). Darwin inclu- these outcrops as contemporaneous of volcanic phaenomena, and on the formation des here the Tertiary units exposed in the higher sections of the Tertiary for- of mountain-chains and volcanos, as the ef- extra-Andean Patagonia right across to mation. In fact, the basalts are of a Plio- fects of continental elevations. Proceedings the coast of Santa Cruz. He observes cene-Quaternary age (see Streling and of the Geological Society 2: 654-660, Lon- that these overlie porphyries of Unit 3. Malagnino 2009) and include the units don. In the map, he represents this unit in the presently called Meseta de las Vizcachas Darwin, C. circa 1840. Geological Map of Pa- hills of northern Tierra del Fuego and in Basalt and La Siberia Formation. tagonia image from the Cambridge University the northern areas of Brunswick Penin- In his observations he states that the out- Library (Manuscript MS.DAR.44:13). Availa- sula. Darwin's observations on this unit crops predominate on the Northern side ble at the Keeping time exhibition web-site: include the identification of several fau- of the Río Santa Cruz valley and that http://www.lib.cam.ac.uk/exhibitions/Keepi nal assemblages with abundant Turritella, they reach the foothills of the Andes. He ng_Time/images/large/darwin.jpeg Pecten and Ostrea. He called this complex recorded that the basalts were olivinic Darwin, C. 1841. On the distribution of erratic the "Patagonian Tertiary Formation" (see and that columnar disjunction was com- boulders and on the contemporaneous uns- Parra and Griffin 2009, Casadío and mon. He identifies at least three overl- tratified deposits of South America. Proce- Griffin 2009). ying beds. Acknowledging the great edings of the Geological Society 3(2): 425- From a lithologic point of view he recog- extension covered by the exposures he 430, London. nized the presence of beds with pumice- concludes that "This great deluge of lava is Darwin, C. 1845. Journal of researches into the ous siltstone with abundant gypsum, worthy, in its dimensions, of the great continent natural history and geology of the countries sandstones, and claystones with calcare- to which it belongs. The aggregate streams have visited during the voyage of H.M.S. Beagle ous concretions. At the headwaters of flowed from the Cordillera to a distance (unpa- round the world, under the Command of the Santa Cruz River he observed that ralleled, I believe, in any case yet known) of Capt. Fitz Roy, R.N. John Murray, 2d. edition, the sedimentary rocks are covered by about 100 geographical miles" (Darwin 1846, 519 p., London. basaltic lavas of unit 7. p. 116) Darwin, C. 1846a. An account of the fine dust which often falls on vessels in the Atlantic UNIT 6 (Recent). This unit represents CONCLUSIONS ocean. Quarterly Journal of the Geological Quaternary deposits that Darwin recog- Society 2: 26-30, London. nized at the southern end of Santa Cruz The brief description presented herein Darwin, C. 1846b. On the geology of the Fal- between the Río Gallegos and Cabo shows one of Darwin's less known fa- kland Islands. Quarterly Journal of the Vírgenes, in the northeastern coast of cets, i.e., his ability to represent his geo- Geological Society 2: 267-279, London. Tierra el Fuego, along both coasts of the logical observations in a map. This con- Darwin, C. 1846c. Geological observations on Strait of Magellan in its northeastern sec- trasts Stoddart (1995) who emphasized, South America. Being the third part of the tor, and in the sector between Useless when analyzing the geological results of geology of the voyage of the Beagle, under Bay and Bahía San Sebastián. He includes Darwin's voyage on the Beagle, "the con- the command of Capt. Fitzroy, R.N. during here the "Rodados Patagónicos", which he trast between Darwin's verbal facility in descri- the years 1832 to 1836. Smith Elder and Co. called "Gravel Formation of Patagonia". In bing landscapes and evoking mood, and his gene- 280 p., London. The first geological map of Patagonia 59

Darwin, C. 1876. Geological observations on the observations on the Patagonian Shingle Strelin, J. and Malagnino, E. 2009. Charles Dar- volcanic islands and parts of South America Formation ("Rodados Patagónicos"). Revista de win and the oldest glacial events/episodes in visited during the voyage of H.M.S. 'Beagle'. la Asociación Geológica Argentina 64(1): 90- Patagonia: the erratic blocks of the Río Santa 2d ed. Smith Elder & Co. 648 p., London. 100. Cruz Valley. Revista de la Asociación Geo- D'Orbigny, A. 1842. Voyage dans l'Amérique Mé- Parras, A. and Griffin, M. 2009. Darwin's Great lógica Argentina 64(1): 101-108. ridionale. Bertrand ed., 3(Géologie), 290 p., Tertiary Patagonian Formation at the mouth Paris. of the Río Santa Cruz: a reappraisal. Revista Herbert, S. 1991. Darwin as a prospective geolo- de la Asociación Geológica Argentina 64(1): gical author. British Journal for the History of 70-82. Science 24: 159-192. Olivero, E.O., Medina, F.A., and López C., M.I. Herbert, S. 1995. From Charles Darwin's Port- 2009. The stratigraphy of Cretaceous muds- folio: An early essay on South American tones in the Eastern Fuegian Andes: New Geology and species. Earth Sciences History data from body and trace fossils. Revista de la 14(1): 23-36. Asociación Geológica Argentina 64(1): 60-69. Herbert, S. 2005. Charles Darwin, Geologist. Rhodes, F.H.T. 1991. Darwin´s search for a the- Cornell University Press. 485 p., Ithaca. roy of the earth: symmetry, simplicity and Lyell, C. 1830-1833. Principles of Geology. John speculation. British Journal for the History of Murray 1(1830) 511 p.; 2(1832) 330 p.; Science 24: 193-229. 3(1833) 109 p., London. Stoddart, D. 1995. Darwin and the Seeing Eye: Martínez, O.A., Rabassa, J. and Coronato, A. Iconography and Meaning in the Beagle Recibido: 3 de octubre de 2008 2009. Charles Darwin and the first scientific Years. Earth Sciences History 14(1): 3-22. Aceptado: 4 de noviembre de 2008 60 Revista de la Asociación Geológica Argentina 64 (1): 60 - 69 (2009)

THE STRATIGRAPHY OF CRETACEOUS MUDSTONES IN THE EASTERN FUEGIAN ANDES: NEW DATA FROM BODY AND TRACE FOSSILS

Eduardo B. OLIVERO1, Francisco A. MEDINA2, and María I. LÓPEZ C.1

1 Centro Austral de Investigaciones Científicas (CADIC-CONICET), B.A. Houssay 200, 9410 Ushuaia, Tierra del Fuego. Email: [email protected] 2 Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, UBA, Intendente Güiraldes 2160, Ciudad Univer- sitaria, C1428EGA Buenos Aires.

ABSTRACT The stratigraphy of Cretaceous marine mudstones in the Fuegian Andes, roughly equivalent to Charles Darwin's clay-slate formation, remains a still unsolved problem. Previous records of Albian, Turonian-Coniacian, and Santonian-Campanian bivalves are combined with new findings of the Late Albian inoceramid Inoceramus anglicus Woods, and the Maastrichtian ammonites Diplomoceras sp., Anagaudryceras sp., Maorites densicostatus (Kilian and Reboul), Maorites sp., and Pachydiscus (Neodesmoceras) sp. to further constrain the Cretaceous stratigraphy of the eastern Fuegian Andes. In addition, new records of distinctive trace fossils and ichnofabric are meaningful for stratigraphic division and delineation of paleoenvironmental settings in these Cretaceous mudstones. The Lower Cretaceous ichnoassemblage of Chondrites targioni (Brongniart) and Zoophycos isp. is consistent with the inferred slope-volcaniclastic apron settings of the Yahgan Formation; Nereites missouriensis (Weller) reflects distal basin plain depositional settings for the Beauvoir Formation. In the Upper Cretaceous, the "Estratos de Buen Suceso" record the earliest extensively bioturbated horizons, reflecting prolonged well-oxygenated bottom conditions. In the Bahía Thetis Formation, organic-rich, channel margin or distal basin slaty mudstones record the last occurrence of inoceramid bivalves in the Austral Basin; the generalized absence of trace fossils is consistent with dysoxic bottom conditions. The thoroughly bioturbated Policarpo Formation, records a marked change in paleoceanographic conditions. The strong contrast in the intensity of bioturbation between the Upper Campanian-Maastrichtian Bahía Thetis Formation, almost devoid of trace fossils, and the highly bioturbated Maastrichtian-Danian Policarpo Formation reflects a change from dysoxic-anoxic to well ventilated conditions, probably associated with a cooling trend of bottom waters in the austral deep oceans.

Keywords: Clay-slate formation, Inoceramid, Ammonites, Cretaceous, Trace fossils, Fuegian Andes.

RESUMEN: Estratigrafía de las fangolitas del Cretácico en los Andes Fueguinos orientales: Nuevos datos de cuerpos y trazas fósiles. En los An- des Fueguinos la estratigrafía de las fangolitas del Cretácico marino (clay-slate formation de Charles Darwin), constituye un pro- blema no enteramente resuelto. Datos previos de bivalvos del Albiano, Turoniano-Coniaciano, y Santoniano-Campaniano, junto con nuevos hallazgos de inoceramidos del Albiano tardío, Inoceramus anglicus Woods, y amonites maastrichianos, Diplomoceras sp., Anagaudryceras sp., Maorites densicostatus (Kilian y Reboul), Maorites sp., y Pachydiscus (Neodesmoceras) sp. se utilizan para obtener una mejor resolución estratigráfica. Además, nuevas asociaciones distintivas de trazas fósiles permiten ajustar la interpretación paleoambiental. La asociación de Chondrites targioni (Brongniart) y Zoophycos isp. del Cretácico inferior es consistente con el modelo inferido de rampa volcaniclástica para la Formación Yahgan. En la Formación Beauvoir la presencia de Nereites missouriensis (Weller) refleja condiciones de depositación en planicies distales de la cuenca. En el Cretácico superior, los "Estratos de Buen Suceso" registran los primeros horizontes bioturbados asociados a fondos con prolongados periodos de condiciones de buena oxigenación. En la Formación Bahía Thetis, fangolitas pizarreñas ricas en materia orgánica depositadas en márgenes de canales o en planicies distales, registran la última aparición de bivalvos inocerámidos en la Cuenca Austral. La ausencia generalizada de trazas fósiles es consistente con condiciones de fondos anóxicos. La Formación Policarpo, densa- mente bioturbada, marca un profundo cambio en las condiciones paleoceanográficas. El fuerte contraste en la intensidad de la bioturbación entre las Formaciones Bahia Thetis, Campaniano-Maastrichtiano, con escasa presencia de trazas fósiles y Policarpo, Maastrichiano-Daniano, altamente bioturbada, refleja el cambio de condiciones de fondo disóxico-anóxico a óxico, probablemente asociado al comienzo del enfriamiento de las aguas profundas del océano austral.

Palabras clave: Clay-slate formation, Inoceramidos, Amonites, Trazas fósiles, Cretácico, Andes Fueguinos. The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes... 61

INTRODUCTION addition, new records of characteristic the consequent origination of the Rocas trace fossil assemblages with distinctive Verdes Marginal Basin. The earliest Late The stratigraphic division of Cretaceous ichnofabrics, restricted to particular sedi- Cretaceous ductile deformation, isoclinal marine mudstones in the Fuegian Andes mentary successions of the Yahgan For- folding, and low-grade regional meta- (Fig. 1) is a major and still unsolved pro- mation, Beauvoir Formation, Bahía The- morphism of these rocks indicate a com- blem. These mudstones correspond tis Formation, Policarpo Formation, and pressional tectonic regime that resulted roughly to the clay-slate formation of "Estratos de Buen Suceso", constitute in the closure of the marginal basin. The Charles Darwin (1846); a very extensive meaningful data for the stratigraphic Fuegian Andes were uplifted by the Late formation of folded, low-grade meta- division of the relatively homogeneous, Campanian; subsequent propagation of morphic rocks with scarce marine fossils. mudstone-rich Cretaceous deposits. the compressional deformation and sub- In the course of the voyage of HMS sidence by tectonic loading along the Beagle Darwin noted that "The great clay- STRATIGRAPHIC northern orogenic margin resulted in the slate formation of Tierra del Fuego being Cre- FRAMEWORK formation of the Austral and Malvinas taceous, is certainly a very interesting fact,--whe- foreland basins. The Turonian-Lower ther we consider the appearance of the country, The stratigraphy of southernmost South Campanian "Estratos de Buen Suceso" which, without the evidence afforded by the fos- America has been controlled since the represent the final stages of the closure sils, would form the analogy of most known dis- Mesozoic by a series of contrasting tec- of the marginal basin and/or the begin- tricts, probably have been considered as belonging tonic regimes, represented by Late Juras- ning of the foreland basins. Conglo- to the Palaeozoic series,…"(Darwin 1846, p. sic-Early Cretaceous extension, Late Cre- merate beds in the Bahía Thetis Forma- 152). taceous-Paleogene compression, and lation bear foliated clasts of Andean-deri- The evidence mentioned by Darwin in- test Paleogene-Neogene and present day ved rocks and thus they record uplifting cluded a few fossils from Mount Tarn, strike-parallel transcurrent motion (Katz and subaerial erosion of the Fuegian on the western side of the Strait of Ma- 1972, Dalziel et al. 1974, Suárez et al.1985, Andes. The youngest Cretaceous rocks gellan, Brunswick Peninsula in Chile (Fig. Kohn et al. 1995, To-rres Carbonell et al. are included in the Policarpo Formation, 2d), particularly the ammonites described 2008). Accordingly, Cretaceous rocks in which includes the Maastrichtian/Danian by Forbes (in Darwin 1846) as "Ancyloce- the Fuegian Andes are distributed in boundary at its top (Olivero et al. 2003). ras simplex" d'Orbigny, from Mount Tarn three tectonostratigraphic domains: 1) Figure 3 summarizes the paleontological (Fig. 2f), and "Hamites elatior" Sowerby, the magmatic arc, located along the axis information, including partly new body from the coastal area nearby Port Fami- of the southern archipelago, includes and trace fossil data, used in this study to ne. We now know that these fossils are Lower Cretaceous andesitic (Hardy For- differentiate particular horizons within not Albian hamitids; the "A. simplex" is a mation) and Upper Cretaceous plutonic these lithologically homogeneous strati- Maastrichtian kossmaticeratid, probably rocks (Beagle Channel Plutonic Group); graphic units. The stratigraphic implica- a deformed fragment of Maorites, and 2) the Rocas Verdes Marginal Basin in- tions of these data are described for each "H. elatior" is a Diplomoceras, also of Maas- cludes Lower Cretaceous, low-grade me- formation in the next section. trichtian age (Spath 1953, Lahsen and tamorphic rocks, mostly slates and sands- Charrier 1972). However, the correction tones of the Yahgan and Beauvoir for- LOWER CRETACEOUS in the age of these ammonite-bearing mations, and Upper Jurassic-Lower Cre- strata does not change the fact that in the taceous ophiolitic rocks; and 3) the On the main island of Tierra del Fuego, Fuegian Andes, Albian to Maastrichtian Austral and Malvinas foreland basins, Darwin (1846) recorded only Cenozoic fine-grained rocks have quite similar li- separated in the northern area by the fossils in the vicinity of Cape San Sebas- thological and structural features and Dungeness or Rio Chico Arch, include tian, including Nothofagus leaves and consequently they are very difficult to Upper Cretaceous rocks dominated by mollusks. The first fossils from the distinguish in the field (Fig. 2). slaty mudstones (Fig. 1). region, which are now accepted as upper- In this study, previous records of Albian, According to a recent review (see Oli- most Jurassic or Cretaceous in age, were Turonian-Coniacian, and Santonian- vero and Malumián 2008, and the biblio- recorded in 1839 near Nassau Bay (Na- Campanian bivalves, mostly inoceramids graphy cited therein), the Yahgan For- varino Island) by James Dwight Dana, a (Olivero and Martinioni 1996a, Olivero mation represents a volcaniclastic apron geologist of Charles Wilkes` United Sta- and Medina 2001) are combined with of mudstones and deep-marine andesite- tes Exploring Expedition (cf. Andersson new findings of latest Albian inocera- rich turbidites and the Beauvoir Forma- 1906, Kranck 1932). The first Cretaceous mids and Maastrichtian ammonites to tion records basin plain and slope muds- fossils from the main island of Tierra del further constrain the Cretaceous strati- tones. Both formations reflect Late Ju- Fuego were found in 1882 nearby graphy of the eastern Fuegian Andes. In rassic-Early Cretaceous extension and Ushuaia by Lovisato. They include trace 62 E. B. OLIVERO, F. A. MEDINA, AND M. I. LÓPEZ C.

Figure 1: Locality map, tectonic settings, and distribution of Lower and Upper Cretaceous rocks in the Rocas Verdes Marginal basin and Austral/ Malvinas Foreland basins. fossils (and calcareous algae?) described mudstones, and andesite-rich greywackes dominated by lithic andesitic fragments by Richter (1925), together with Creta- exposed in Mount Olivia and Ushuaia. and plagioclase, suggesting derivation ceous radiolaria from the Staten and The Formation is well exposed along the from the volcanic arc, located to the New Year islands (also collected by Lo- northern margin of the Beagle Channel, South in the Fuegian Archipelago (Winn visato) and several fossils, including be- from Ushuaia to Bahía Sloggett (Fig. 1); 1978, Suárez et al. 1985, Olivero and Mar- lemnites from the Hito XIX area, collec- and in Hoste, Navarino, Nueva, Lennox, tinioni 1996b). This is well documented ted by the Expedition of University of and Picton islands (Katz and Watters with paleocurrent data on the island of Buenos Aires to Tierra del Fuego (Doe- 1966, Dott et al. 1977, Winn 1978, Suárez South Georgia, now displaced along the llo Jurado 1922). et al. 1985). Dominant sedimentary facies North Scotia Ridge to the east (Dalziel et are: a) black mudstones, fine-grained, al. 1974, Macdonald 1986). Yahgan Formation thin-bedded turbidites (Fig. 2a) and tuff; Body and trace fossils are very scarce in b) classical turbidites; and c) massive to the Yahgan Formation (Fig. 3). On Nava- The Yahgan Formation was established graded sandstones. Petrographic compo- rino Island Tithonian-Neocomian am- by Kranck (1932) for the slates, slaty sition of the sandstones is uniform and monites and belemnites (Aguirre-Urreta The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes... 63

Figure 2: Structure, lithology, and paleontology of Cretaceous rocks. a) Thin-bedded turbidites, Yahgan Formation, Lower Cretaceous, Beagle Channel close to Ushuaia. b) Black mudstones, Beauvoir Formation, Lower Cretaceous (Albian), Knokeke Hill area. c) Black mudstones and interbedded light-gray tuffs, Estratos de Buen Suceso, Upper Cretaceous (Santonian-Lower Campanian). d) Upper Cretaceous (Maastrichtian) interbedded siltstones and fine- grained sandstones, Mount Tarn, Brunswick Peninsula, Chile. e) Highly folded, black mudstones and fine-grained sandstones, Bahía Thetis Formation, Thetis Bay. f) Crushed ?Maorites sp. from Mount Tarn. This is the original specimen, housed at the Natural History Museum, London (B.M. No. 2612), collected by Darwin and described by Forbes as "Ancyloceras simplex" d´Orbigny. Scale bar: 1 cm. and Suárez 1985); Aptian-Albian corals, ved due to the strong tectonic deforma- cribed by Richter (1925) from the vicinity gastropods, and bivalves (Dott et al. tion of the rocks (Kranck 1932, Olivero of Ushuaia. Biostratigraphically diagnos- 1977); and indeterminate inoceramids and Martinioni 1996b). The trace fossil tic Late Albian fossils are known from (Katz and Watters 1966) were reported. Chondrites targioni (Brongniart) and the Moat, where Actinoceramus concentricus Microfossils, particularly radiolaria, are presumed fossil calcareous algae "Litho- (Parkinson) and Inoceramus carsoni Mc Coy apparently common, but not well preser- caulon antarcticum" Bornemann were des- were recorded (Olivero and Martinioni 64 E. B. OLIVERO, F. A. MEDINA, AND M. I. LÓPEZ C.

just west of Good Success (Buen Suce- so) Bay (Figs. 1 and 2b). Locally, the black slates are rhythmically interbedded with thin, fine-grained sandstones. Stratification is only visible in the latter beds, whereas in the massive slates the bedding is defined only by the presence of thin tuff beds. The fossiliferous marlstones cropping out near Hito XIX in the Argentinean- Chilean border are provisionally included in the Beauvoir Formation. These marlstones contain abundant specimens of the Aptian-Albian bivalves Aucellina "radiotos- triata" Bonarelli, A. "andina" Feruglio (Macellari 1979), and A. striata Richter (Richter 1925). Additional fossils include serpulids, brachiopods, inoceramids, probably Inoceramus anglicus Woods or I. sutherlandi MacCoy (cf. Medina 2007) and abundant rostra of the belemnite Parahi- bolites fuegensis (Stolley) in interbedded sandstone beds (Richter 1925) and large, poorly preserved ammonites. The massive, dark slates of the Beauvoir Formation near Knokeke Hill and Buen Suceso Bay contain several horizons with abundant but ill-preserved inoceramids, which normally are concentrated in very thin beds. Part of this material includes the Albian Mytiloides cf. ipuanus (Well- man), of which the nominal species "Ino- ceramus" urius Wellman and "I. kapuus" Wellman are now considered as syno- Figure 3: Stratigraphic distribution of characte- nyms (see Crampton 2004). Associated, ristic body and trace fos- very thin beds include abundant well- sils in the Cretaceous of preserved, articulate shells of the Late the Fuegian Andes (explanation in the text). Albian Aucellina euglypha Woods (Fig. 5b) covered by a thin pyrite coating (Olivero 1996a). Fine-grained turbidites and mud- isp. (Fig. 3). A similar trace fossil assem- and Medina 2001). Rare, complete but stones nearby Ushuaia and adjoining blage, with the addition of Phycosiphon in- badly deformed echinoids are associated Mount Martial bear scarce and ill-preser- certum, was recorded in the Yahgan For- with the latter bivalves. Near the locality ved belemnites. mation by Winn (1978). of Knokeke Hill, black slates record rela- Trace fossils are rare and restricted to a tively well preserved and articulated few horizons; Chondrites targioni (Fig. 4b) Beauvoir Formation shells of Inoceramus anglicus (Fig. 5a) and and Zoophycos isp. are the most common delicate back-filled burrows of Nereites trace fossils and are locally abundant in The Beauvoir Formation (Camacho missouriensis (Weller) (Fig. 4a). particular beds. A few localities, e.g. on 1967) consists predominantly of homo- In the subsurface, the stratigraphically the coast of the Beagle Channel just east geneous black or bluish, massive to fain- equivalent mudstone-dominated rocks of from Ushuaia, bear a more diversified tly laminated dark slates and gray tuffs the Nueva Argentina Formation and Arro- ichnoassemblage, including Chondrites tar- mainly exposed in Sierra de Beauvoir, in yo Alfa Formation contain Early-Mid Al- gioni, Helminthopsis tenuis, Stelloglyphus isp., the belt stretching from the eastern part bian and Late Albian foraminiferal as- Ophiomorpha cf. annulata, and Zoophycos of Lake Fagnano to the Montes Negros, semblages, respectively (Flores et al. 1973). The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes... 65

Figure 4: Typical trace fossils and ichnofabrics of Cretaceous rocks in the Fuegian Andes. a) Nereites missouriensis (Weller) from Albian beds of the Beauvoir Formation near Knokeke Hill, CADIC PI 88. b) Dense ichnofabric of Chondrites targioni (Brongniart) and associated Ophiomorpha cf. annulata (Op), Lower Cretaceous Yahgan Formation, Beagle Channel near Ushuaia, field specimens. c) Localized horizons with a relatively dense ichnofabric in mudstones and isolated specimens of Rhizocorallium isp. (Rh), Estratos de Buen Suceso, Santonian-Lower Campanian, Buen Suceso Bay, field specimens. d) Typical ichnofabric affecting thick packages in the Maastrichtian Policarpo Formation, Zoophycos isp. (Zo) and Schaubcylindrichnus ("Terebellina") isp. (Sc) are the only recognizable structures in the intensely mottled background, field specimens. Scale bar: 1 cm. gascar and New Zealand, and to the differ from "I."? nukeus, which is almost UPPER CRETACEOUS Upper Turonian-Lower Coniacian in equivalve and much less inflated than T. Antarctica (cf. Olivero and Medina madagascariensis. "Estratos de Buen Suceso" 2001). The exact age and identity of ino- The upper part of the succession, expo- ceramids assigned to "Inoceramus" (=Te- sed along the northern margin of the These beds include a thick sedimentary thyoceramus) madagascariensis is now deba- bay, is dominated by dark slaty mudsto- succession of folded dark gray and black, ted. Walaszczyk et.al (2004) argue that the nes and silty sandstones, with some inter- slaty mudstones, marlstones, and sandy New Zealand material is not true "I." bedded light gray, thin tuff beds (Fig. 2c). siltstones with common quartz veins. madagascariensis and should be referred to The slaty mudstones contain occasional, The lower part of the succession, expo- "I." nukeus Wellman. However, Crampton large specimens (up to 50 cm high) of sed in the southern part of the bay, is (writ. comm. 2008) believes that some complete shells of Inoceramus (Platyce- dominated by marlstones, impure limes- New Zealand material is indistinguisha- ramus) sp. and relatively small, well-pre- tones, and fine-grained silty sandstones. ble from topotype T. madagascariensis (e.g., served specimens of Sphenoceramus sp. The slaty micritic limestones and marly compare Crampton 1996 pl. 16M and pl. The association described by Olivero and mudstone record well-preserved, articu- 17E, with Walaszczyk et al. 2004, Fig. 13B Medina (2001) was referred to the San- lated shells of Tethyoceramus madagascarien- and fig. 14E, respectively). The Tierra del tonian-Lower Campanian. sis (Heinz). This species has been refe- Fuego material is indistinguishable from The silty sandstones within the Santo- rred to the Middle Coniacian in Mada- the Antarctic specimens, and appears to nian-Lower Campanian beds record se- 66 E. B. OLIVERO, F. A. MEDINA, AND M. I. LÓPEZ C.

Figure 5: Cretaceous ammonites and bivalves from the Fuegian Andes. a) Inoceramus anglicus Woods, Upper Albian, Beauvoir Formation, right valve, CADIC PI 87; b) Black mudstones with pyritized shells of Aucellina euglypha Woods, Upper Albian, Beauvoir Formation, CADIC PI6 a; c) Fragment of the body chamber of Diplomoceras sp., Maastrichtian, Policarpo Formation, CADIC PI 85; d) Pachydiscus (Neodesmoceras) sp., Maastrichtian, Policarpo Formation, CADIC PI 84; e) and f) Fragments of the body chamber of crushed Maorites spp., Maastrichtian, Policarpo Formation; e) Maorites densicostatus (Kilian and Reboul) CADIC PI 86; f) Maorites sp. CADIC PI 89. Scale bar: 1 cm. veral fully bioturbated horizons. The died the Bahía Thetis Formation and re- of dysoxic environments and are charac- trace fossils Chondrites, Palaeophycus, Plano- cognized three packages, more than 250 terized by Rzehakina epigona (Rzehak), R. lites, Rhizocorallium (Fig. 4c), and Zoophycos m thick, of hard, highly deformed rocks lata Cushman and Jarvis and R. fissistoma- are recognized within a heavily bioturba- including: 1) dark, organic-rich, lamina- ta (Grzybowski) (Caramés and Malumián ted background. ted mudstones and tuffs with incipient 2006). In the second package, the con- cleavage; 2) resedimented conglomerates glomerates include large clasts of radiola- Bahía Thetis Formation and pebbly mudstones; and 3) turbidite rian-bearing slates and foliated rhyolites, sandstones and slaty mudstones. derived from the Beauvoir-Yahgan For- The dominant slaty mudstones and sand- The lowest organic-rich package bears mations and Jurassic volcanics, respecti- stones cropping out in the Thetis Bay abundant radiolaria and more restricted vely. In the third package, the turbidites area were first recognized by Furque and foraminifera. The latter comprise a low (Fig. 2e) bear scarce, ill-preserved ammo- Camacho (1949). Olivero et.al (2003) stu- diversity, agglutinated assemblage typical nites, including flattened fragments of The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes... 67

Diplomoceras sp. and kossmaticeratids. Relatively abundant invertebrate body a strong feeling of a Paleozoic age for Trace fossils are very rare, and only a few fossils are common in certain concretio- most of the clay-slate formation still specimens of Stelloglyphus were recovered. nary horizons, but they are very difficult grew. This was based mainly on compari- The absence of trace fossils is consistent to recover due to the extreme hardness sons of the regional features of the low- with the high organic matter content and of the rocks. Irregular echinoids are very grade metamorphic rocks with similar the preservation of delicate sedimentary common near the faulted contact with features in Paleozoic orogenic belts, par- banding in the slaty mudstones. Based on the Bahía Thetis Formation near Cabo ticularly the basement in the British Isles the recorded foraminifera and ammoni- San Vicente. Solitary corals, scaphopods, and in Alpine areas, which at that time tes the Bahía Thetis Formation was assig- gastropods, bivalves, and ammonites are was thought to be of Paleozoic age (see ned to the Late Campanian-?Early Maas- less abundant. One horizon at the con- Kranck 1932), and on the age interpreta- trichtian (Olivero et al. 2003, Caramés tact with the Río Bueno Formation near tions of the dubious fossils found by Lo- and Malumián 2006). Puesto Río Bueno, bears relatively well- visato in 1881 in Staten Island (see preserved specimens of Diplomoceras sp., Harrington 1943). However, the presu- Policarpo Formation Anagaudryceras sp., Maorites densicostatus med Cambrian quartzites and schists (Kilian and Reboul), Maorites sp. and large from Staten Island turned out to be The Policarpo Formation is very well ex- Pachydiscus (Neodesmoceras) sp. (Fig. 5c-f). Jurassic acidic tuffs and volcaniclastic, posed along the Atlantic shore of Penín- Near Puesto Donata, very well-preserved foliated rocks, and the dubious Paleozoic sula Mitre, between Policarpo Cove and specimens of the gastropod Stru-thioptera fossils were probably part of the fibrous San Vicente Cape (Furque and Camacho cf. gregaria Wilckens were recovered. structure of Cretaceous inoceramid 1949, Olivero et al. 2002, 2003). The ex- The foraminifera are dominated by shells (Richter 1925) or belemnite rostra tensive, but less well-exposed rocks for- agglutinated cosmopolitan forms that in- (Harrington 1943). ming the NW belt of strata in the Sierra clude the oldest record in the Austral Ba- As has been confirmed by additional fos- de Noguera and Sierra de Apen are also sin of Spiroplectammina spectabilis (Olivero sil discoveries in the clay-slate formation included in the Policarpo Formation et al. 2002, 2003, Olivero and Malumián of the main island of Tierra del Fuego, (Fig. 1). 2008). Dinocysts are scarce and not well- including the first locality visited by In the type area of Península Mitre the preserved, but two assemblages of Maas- Darwin on the island (Good Success Policarpo Formation includes a mini- trichtian and Danian age were tentatively Bay), we now know that all these rocks mum thickness of 350 m, and probably recognized (Olivero et al. 2003). are of Cretaceous age (Olivero and Mar- more than 700 m, of bioturbated, tuffa- tinioni 1996a, Olivero and Medina 2001). ceous, monotonous organic-rich, dark CONCLUDING REMARKS The relatively homogeneous lithology is gray, very hard sandy mudstones and silty a major obstacle to stratigraphic subdivi- sandstones. Fresh-pyroclastic material is The first fossils to be discovered in the sion; nonetheless, particular combina- relatively abundant, and some fine-grai- clay-slate formation were Cretaceous cri- tions of fossil content, ichnofabric, and ned tuffaceous sandstones are almost ex- noids, gastropods, bivalves, and ammoni- lithological features make it possible to clusively composed of zoned, euhedral tes found by Darwin in 1834 on Mount recognize characteristic sedimentary suc- plagioclase crystals and volcanic glass. Tarn (Darwin 1846). This discovery was cessions in the Lower and Upper Creta- Most part of the Policarpo Formation followed soon by new records of Cre- ceous rocks. consists of a crude alternation of tuffa- taceous invertebrates, notably among In the Lower Cretaceous, only Aptian- ceous mudstones and silty sandstones these were the findings of additional fos- Albian fossils have been recovered; but the stratification is not always appa- sil mollusks in Mount Tarn by Hombron however, the Yahgan Formation has rent due to the intense bioturbation. To- and Grange in 1837; belemnites in Nas- thick horizons with massive sandstones tally bioturbated thick packages occur sau Bay by Dana in 1839; and bivalves and sandy turbidites, a lithological asso- repeatedly, and they are commonly cha- and ammonites in 1887-1890 in the ciation that is not found in the homoge- racterized by a high abundance of sandy, islands of Saint Peter and Saint Paul, nous, black slaty mudstones and tuffs of agglutinated tubes of Schaubcylindrichnus Magellan Channel, reported by White the Beauvoir Formation. The Yahgan (previously assigned to "Terebellina"). In (see Andersson 1906, Bonarelli 1917, Formation also has localized horizons addition to a dense background mottling, Kranck 1932). Despite these discoveries dominated by the trace fossils Chondrites the trace fossils Tasselia, Rhizocorallium, of Cretaceous fossils in the clay-slate for- (Fig. 4b) and Zoophycos, an ichnoassem- Phycodes, Teichichnus and small Chondrites mation in distant localities, stretching blage that is consistent with the inferred are occasionally recorded (Fig. 4d). Some more than 500 km between Navarino setting of a low-oxygenated slope (cf. horizons bear a dense concentration of Island and Brunswick Peninsula south of Uchman 2007) in a volcaniclastic apron large Zoophycos burrows. Punta Arenas on the Strait of Magellan, environment (Olivero and Martinioni 68 E. B. OLIVERO, F. A. MEDINA, AND M. I. LÓPEZ C.

1996b, Olivero and Malumián 2008). The clastic material was uplifted Andean ACKNOWLEDGMENTS black, fine-grained rocks of the Beauvoir rocks, including foliated, Jurassic acidic Formation contain localized horizons volcanics and radiolarian-rich Cretaceous We thank N. Malumián (SEGEMAR- with Nereites missouriensis (Fig. 4a). This slates. These coarse-grained clastic rocks CONICET) for fruitful discussion over ichnofossil is a typical component of the probably reflect deposition in submarine the years on the geology of Tierra del Nereites ichnosubfacies, which is distribu- fans, with a source area in the hinterland Fuego. D. Martinioni (CADIC-CONI- ted in distal flysch facies and characteri- part of the growing Fuegian Andes. The CET) helped with part of the fieldwork zed by the dominance of deposit-feeding associated dark, organic-rich slaty muds- and analysis of information in German invertebrates, such as the tracemaker of tones (Fig. 2e) are thought to have origi- papers. We thank P. Torres Carbonell Nereites (cf. Seilacher 1974, Uchman nated in channel margin or more distant (CADIC-CONICET) for a critical re- 2007). Accordingly, these fine-grained basin settings. The foraminiferal content view of an early manuscript. M.B. Agui- deposits probably reflect distal basin and the generalized absence of trace fos- rre-Urreta (UBA-CONICET) provided plain settings, occupying an intermediate sils are consistent with the interpreted the pictures of the figured original mate- position between the Pacific volcaniclas- dysoxic bottom conditions (Olivero et al. rial collected by Darwin in Brunswick tic apron of the Yahgan Formation and 2003, Caramés and Malumián 2006). The Peninsula and housed in the Natural His- the typical, South-American cratonic slo- Bahía Thetis Formation probably re- tory Museum, London. We thank the repe settings of the Lower Cretaceous cords the last occurrence of inoceramid viewers A. Crame, J.S. Crampton, I.W.D. rocks in the Austral Basin (Wilson 1991). bivalves in the Austral Basin (Olivero et Dalziel, and M.D. Suárez for constructive In the Upper Cretaceous, four distinctive al. 2003, 2004). observations that improved the original successions are recognized: 1) the lower The Maastrichtian-Danian Policarpo For- manuscript. This study was financed by part (Turonian-Coniacian) of the "Estra- mation records a marked, regional chan- PIP 5100 CONICET and PICTO 36315 tos de Buen Suceso", 2) the upper part ge of paleoceanographic conditions in FONCYT. (Santonian-Lower Campanian) of the the basin as is evidenced by the tho- "Estratos de Buen Suceso"; 3) the Upper roughly bioturbated ichnofabric that cha- WORKS CITED IN THE TEXT Campanian-?Lower Maastrichtian Bahía racterizes thick sedimentary packages. Thetis Formation; and 4) the Maastri- This change in the ichnofabric occurs in Aguirre Urreta, M.B. and Suárez, M. 1985. Be- chtian-Danian Policarpo Formation (Oli- the Maastrichtian Policarpo Formation lemnites de una secuencia turbidítica volcano- vero and Medina 2001, Olivero et al. 2002, just above the inoceramid extinction clástica de la Formación Yahgan-Titoniano- 2003, Olivero and Malumián 2008). level in the Fuegian Andes. The inocera- Cretácico inferior del extremo sur de Chile. 4° The lower part of the "Estratos de Buen mids were apparently adapted to warm Congreso Geológico Chileno (Antofagasta), Suceso" is characterized by marlstones, and poorly oxygenated waters and their Actas 1: 1-16. impure limestones, and fine-grained silty global extinction pulses, during the mid- Andersson, J.G. 1906. Geological fragments from sandstones with T. madagascariensis, whe- Maastrichtian, were related to cooling Tierra del Fuego. Bulletin of the Geological reas the upper part is characterized by and enhanced bottom ventilation, pro- Institution, University of Upsala 8: 169-183. dark slaty mudstones, tuffs, and silty moted by circulation of deep Antarctic Bonarelli, G. 1917. Tierra del Fuego y sus turbe- sandstones. The latter beds appear to waters (MacLeod et al. 1996). The strong ras. Anales del Ministerio de Agricultura de la record the earliest successions with contrast in the intensity of bioturbation Nación, Sección Geología, Mineralogía y Mi- extensively bioturbated horizons in the between the Upper Campanian-Lower nería 12(3): 1-119, Buenos Aires. basin, probably reflecting for the first Maastrichtian Bahía Thetis Formation, Camacho, H.H. 1967. Las transgresiones del time prolonged periods with well-oxyge- almost devoid of trace fossils, and the Cretácico superior y Terciario de la Argentina. nated bottom conditions (Fig. 4c). These highly bioturbated Maastrichtian-Danian Revista de la Asociación Geológica Argentina beds alternate with those showing evi- Policarpo Formation reflects a change 22: 253-280. dence for more oxygen-deficient bottom from dysoxic-anoxic to well ventilated Caramés, A. and Malumián, N. 2006. La Familia conditions, lacking trace fossils and bottom conditions, probably associated Rzehakinidae (Foraminifera) en el Cretácico populated locally by Inoceramus with a cooling trend of bottom waters in superior-Paleógeno de la cuenca Austral y la (Platyceramus) sp. and Spheno-ceramus sp. the austral deep oceans (Olivero et al. plataforma continental atlántica adyacente, (Olivero and Medina 2001). 2003, 2004). In addition, the Policarpo Argentina. Ameghiniana 43: 649-668. The resedimented conglomerates, pebbly Formation also records a distinctive pe- Crampton, J.S. 1996. Inoceramid bivalves from mudstones, and sandy turbidites of the trographic composition, which is domi- the Late Cretaceous of New Zealand. Insti- Bahía Thetis Formation record a very nated by fresh volcaniclastic material tute of Geological and Nuclear Sciences Mo- distinctive petrographic composition, implying a coeval volcanic pulse in the nograph 14: 1-188. with clear evidence that the source of magmatic arc. Crampton, J.S. 2004. Shell composition, cryptic The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes... 69

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DARWIN'S GREAT PATAGONIAN TERTIARY FORMATION AT THE MOUTH OF THE RÍO SANTA CRUZ: A REAPPRAISAL

Ana PARRAS and Miguel GRIFFIN

Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa y CONICET, Santa Rosa, La Pampa. Emails: [email protected], [email protected]

ABSTRACT The classical section described by Darwin at the mouth of the Río Santa Cruz was a landmark in the later stratigraphy of the marine Cenozoic of Patagonia. Sedimentological and paleontological studies of rocks from the Monte León Formation at the Darwin and Mount Entrance sections - mouth of the Río Santa Cruz - suggest that they were deposited in an early Miocene inner shelf to subtidal - or even intertidal at the top - environment. The base of the sections is covered and the top is unconformably overlain by Quaternary deposits. The invertebrate fauna (mainly mollusks) mentioned herein - the earliest of which were collected by Darwin and described by Sowerby - were collected from the two sections with a detailed stratigraphic control. It includes brachiopods, echinoderms, scaphopods, bivalves, and gastropods. The similarity of this fauna with that one from the sections at Restinga Norte, Cabeza de León, Las Cuevas, Yegua Quemada and Monte Observación, all south of the study area, allow correlation with them. In addition to the equivalence of the faunas, sedimentological features at all these localities are also the same. Therefore, all the shell beds involved should be referred to the Punta Entrada Member of the Monte León Formation. The Monte Observación Member should be restricted to its original use and included as a member at the base of the overlying Santa Cruz Formation, outcrops at Cerro Monte León and Cerro Observación.

Keywords: Darwin Section, Mount Entrance Section, Mouth of Santa Cruz River, Paleoenvironments, Invertebrates, Patagonia.

RESUMEN: La Gran Formación Terciaria Patagónica de Darwin en la desembocadura del río Santa Cruz: una revaluación. La sección clásica descripta por Darwin en la desembocadura del río Santa Cruz fue un hito en el posterior desarrollo de la estratigrafía del Cenozoico marino de Patagonia. Los estudios sedimentológicos y paleontológicos realizados en sedimentitas de la Formación Monte León, en las secciones Darwin y Monte Entrada, en el área de la desembocadura del río Santa Cruz, sugieren que las mismas fueron depositadas en un ambiente de plataforma interna a submareal en la base, hasta intermareal en el techo, durante el Mioceno temprano. La base de las secciones se encuentra cubierta y su techo es cortado en forma discordante por depósitos del Cuaternario. La fauna de invertebrados (mayormente moluscos) mencionada en este trabajo, cuyos primeros representantes fueran coleccionados por Darwin y descriptos por Sowerby, fue colectada con un detallado control estratigráfico a lo largo de las dos secciones. La misma incluye braquiópodos, equinodermos, escafópodos, bivalvos y gasterópodos. La similitud en la composición taxonómica, como así también en las características sedimentológicas, entre estas secciones y aquellas ubicadas más al sur como son Restinga Norte, Cabeza de León, Las Cuevas, Yegua Quemada y Monte Observación, sugiere que las mismas se correlacionan lateralmente. Por lo tanto estos niveles deben ser referidos al Miembro Punta Entrada de la forma- ción Monte León, restringiéndose el Miembro Monte Observación a los niveles basales de la suprayacente Formación Santa Cruz, aflorantes en Cerro Monte León y Cerro Observación.

Palabras clave: Sección Darwin, Sección Monte Entrada, Desembocadura del río Santa Cruz, Paleoambientes, Invertebrados, Patagonia.

INTRODUCTION assembled a collection of invertebrate Charles Darwin (1846, p. 112-113) des- marine fossils later described by Sowerby cribed the geological section of marine On April 13th 1834, HMS Beagle ancho- (1846). Thus, the locality from where this Tertiary rocks, and discuss the possible red within the mouth of the Río Santa material was collected became the type paleoenvironmental conditions in which Cruz, a few miles upstream from the locality of most of the taxa described by it was deposited. This should aid in the Atlantic Ocean. During his numerous ex- Sowerby. The main purposes of this formulation of more precise correlations cursions ashore, Darwin made geological work are to pinpoint and re-describe as of the beds mentioned by Darwin with observations on the cliffs that rise along accurately as possible the locality at the others exposed in the area, and thus cla- the southern bank of the river. He also mouth of the Río Santa Cruz where rify the meaning of the formal names Darwin's Great Patagonian Tertiary Formation at the mouth of the Río Santa Cruz: ... 71

Figure 1: Location map of the Darwin Section and the Mount Entrance Section at the mouth of the Río Santa Cruz. proposed by later authors for the rocks In order to clarify the exact location of (Becker 1964, Bertels 1970, 1980, Di involved. Darwin (1839, 1846) himself Darwin's section, we surveyed the coast Paola and Marchese 1973, Olivera 1990, included these rocks in his "great Pata- along the southern bank of the river. del Río and Camacho 1998, Barreda and gonian tertiary formation" or "Patagonian Ter- Based on his writings (1846, p. 112-113) Palamarczuk 2000), but detailed descrip- tiary Formation", a unit subject to very dif- it became clear the section is the one tions that could enable correlation and ferent interpretations ever since. Accor- lying just east of the present port of paleoenvironmental reconstructions are ding to his description of the beds expo- Punta Quilla (S 50° 07' 39.9"; W 68° 24' still lacking. sed and after a survey of the area to try 10.04"), a point where the marine Study of these two stratigraphic sections to identify the exact location of his sec- Tertiary rocks are best exposed (Fig. 2), allowed recognition of sedimentary fa- tion, we were able to identify it on the and where they best meet the description cies, and hence the dominant deposition southern bank of the Río Santa Cruz, a and measurements he offers in his mechanisms could be interpreted. These short distance away from its mouth in "Geological Observations…". facies were grouped into three facies the Atlantic Ocean. As there has been some confusion as to associations and a depositional model The rocks exposed in this area are now the exact location of Darwin's locality, thus emerged. Detailed analysis of the included in the lithostratigraphic unit we also measured and described the sec- taxonomic composition of the fossil known as Monte León Formation (Ber- tion lying further east, approximately one assemblages revealed the presence of at tels 1970) and deemed to be of late kilometer away from Mount Entrance, at least 38 species of bivalves and 70 spe- Oligocene-early Miocene age (Malumián the very mouth of the river (S 50° 08' cies of gastropods, in addition to scapho- 1999, 2002, Barreda and Palalmarczuk 32.2"; W 68° 22' 23.6"). The two locali- pods, echinoderms, and brachiopods. 2000). This unit is exposed along the ties lie within the present Department of Diffe-rences among the different shell coast of Patagonia between Puerto San Corpen Aike, in the province of Santa beds suggest a complex history. The dif- Julián and Coy Inlet, at the mouth of the Cruz (Fig. 1). They both carry a rich ferent fossil concentrations were origina- Río Coyle (or Coig) (Fig. 1). It comprises fauna of exceptionally well preserved ted in different settings, ranging from mainly sandstones and siltstones with a mollusks that can be counted among the inner shelf to nearshore subtidal envi- high content of pyroclastic material, and best in southern South America. They ronments - and even intertidal, represen- carries numerous shell beds. have been mentioned in the literature ting deposition during the early Miocene 72 A. PARRAS AND M. GRIFFIN

Figure 2: Monte León Formation at the mouth of the Río Santa Cruz, Darwin Section, showing Darwin's strata and facies associations described herein. transgression in Patagonia. se units lie within the Austral Basin. overlying these are variably thick Qua- Further north, the rocks deposited du- ternary deposits of the Shingle Forma- GEOLOGICAL FRAMEWORK ring the Patagonian transgression are in- tion described by Darwin too (Darwin cluded in the Chenque and Gaiman for- 1839, 1846). The surface of Patagonia is covered mations, in the San Jorge Basin. In sou- The earliest record of marine rocks in mainly by marine rocks deposited during thern Chile, these rocks are known as Patagonia was by d'Orbigny (1842 a, b), the Atlantic transgressions that occurred Guadal Formation, and are exposed who explored this part of South America during the late Cretaceous, Paleogene south of Lago General Carrera. during the years 1826-1833. D'Orbigny and Neogene. These are intercalated with The basement rocks underlying the Ce- himself never went ashore further south non marine rocks and non deposition nozoic deposits are a Jurassic lava-pyro- than the coast of northern Patagonia. and erosion intervals (Malumián 1999). clastic complex known as Bahía Laura However, he named all the deposits that One of these transgressions is com- Group, including the Chon Aike Forma- constitute the cliffs along the coast bet- monly referred to as "Patagoniano" and tion (ignimbrites and scarce rhyolitic la- ween Carmen de Patagones (Río Negro) occurred during the high sea level period vas and tuffs) and the La Matilde For- and the Strait of Magellan as "Tertiaire that began in the latest Oligocene and mation (tuffs, claystones, siltstones and Patagonien". Although he did not inspect ended in the early middle Miocene. The coal beds). These two units are interdigi- the cliffs further south, he did have avai- final phases of this transgression are tated and unconformably overlain, in the lable the large oysters from San Julián, coincident with the Neogene climatic San Julián area, by the San Julián For- which he believed were the same ones as optimum. It covered the largest extent of mation. This late Oligocene unit includes he had collected in Río Negro and fur- land among the various Cenozoic trans- predominantly marine clastic -sandstone ther north in Entre Ríos. Darwin was gressions that occurred in Patagonia to siltstone - rocks (Parras and Casadío aware of this, and therefore used an equi- (Malumián 2002). This transgression 2005, Parras et al. 2008). valent name for these rocks. However, he deposited rocks containing the Paleoge- Deposition of the probably unconfor- also noticed that there were differences ne/Neogene boundary in Patagonia and mably overlying Monte León Formation between the faunas from the southern in the Cordillera sector of southern occurred during the late Oligocene and and northern areas in which his "Pata- Chile. They are presently included in the early Miocene. This unit includes siltsto- gonian Tertiary Formation" was exposed. San Julián and Monte León formations nes, sandstones and tuffs (Bertels 1970, After these initial surveys the area remai- along the coast of Santa Cruz and in the 1980), and is overlain by the early to mid- ned largely unexplored from a geological Centinela Formation in the Andean re- dle Miocene continental rocks of the point of view. A Chilean settlement near gion of this province. Exposures of the- Santa Cruz Formation. Unconformably the present town of Puerto Santa Cruz Darwin's Great Patagonian Tertiary Formation at the mouth of the Río Santa Cruz: ... 73

was headed by Vidal Gormaz, who During the same years, the area was also ned somewhat unstable. Accounts on the collected a few fossils from the cliffs visited by J. B. Hatcher, geologist in char- uses and controversies attached to such surrounding the settlement and sent ge of the Princeton Expedition to South- usage can be found in Camacho (1974, them to R.A. Philippi, then at the Museo ern Patagonia. Hatcher's observations 1979), Zinsmeister (1981), Legarreta and Nacional de Historia Natural in Santiago were published in the Reports and Geo- Uliana (1994), and Malumián (1999, de Chile. These fossils were described by logy of this expedition (Hatcher 1903) 2002). Philippi (1887), but hardly any comments and in a series of papers in the American At the mouth of the Río Santa Cruz, were made on the geology of the area, Journal of Science (Hatcher 1897, 1900). these rocks were formally named Monte except to note that the specimens came The large collection of Tertiary marine León Formation by Bertels (1970). The from the Tertiary marine beds there fossils that he assembled was described unit includes mainly yellowish-grey silts- exposed. It was not until the final years by Ortmann (1902). Hatcher's conclu- tones and fine sandstones with a high of the 19th Century that this region was sions on the geology of the area were pyroclastic content and intercalated shell again surveyed and its geology described different from those of Ameghino. beds. These rocks record a relative sea in more detail. This was due mainly to Hatcher did not recognize the existence level increase characterized by the grea- three early explorers who visited the of the "Superpatagónico", as the fauna des- test extent of the Patagonian Cenozoic mouth of the Río Santa Cruz and assem- cribed by Ortmann was essentially the sea, thus allowing input of corrosive bled the data that were then - and for same one as the one in the underlying Antarctic water onto the Argentine con- many years - used to formulate the first "Formación Patagónica". This initial contro- tinental shelf (Malumián 2002). Most stratigraphic divisions and the earliest versy pervaded all further work on these authors agree that in the study area, i.e. paleogeographic and paleobiogeographic units and has never been satisfactorily the mouth of the Río Santa Cruz, these interpretations about the rocks in the resolved. The reasons for this are varied rocks were deposited in a shallow marine area. and include imperfect knowledge on the environment, under littoral and neritic In the first place should be mentioned sedimentology and stratigraphy at the conditions (Panza et al. 1995, Barreda and Carlos Ameghino, brother of Florentino localities involved, inaccurate location of Palamarczuk 2000). Bertels (1980) sug- Ameghino- the famed Argentine paleon- some of these localities, outdated taxo- gested outer shelf conditions for the tologist. C. Ameghino surveyed the area nomic placement of some of the fossil lower part of this formation. Panza et al. and collected fossils that were subse- material on which the biostratigraphy (1995) stated that towards the top of the quently studied by his brother. Although was based, among other. unit conditions became progressively he did not publish his results extensively, At about the same time, C. Bicego was shallower, ending in a marshy environ- his keen and detailed geological observa- entrusted by H. von Ihering with the task ment just before the onset of the conti- tions were later used by Florentino to of assembling further collections at the nental environment in which the overl- formulate the earliest biostratigraphic localities initially visited by C. Ameghino. ying Santa Cruz Formation was deposi- scheme of the Tertiaire Patagonien or the His large collection of mollusks was des- ted. The abundant tuff beds and cineritic Great Patagonian Formation. Thus, based on cribed by von Ihering, but Bicego him- material in these rocks led Bertels (1970) the fossil content (mainly mollusks) of self did not provide further geological and Di Paola and Marchese (1973) to these marine beds at the mouth of the data beyond that mentioned by Ame- suggest they originated in coetaneous Río Santa Cruz and in the surroundings ghino. volcanic events in the Cordillera. The ash of Puerto San Julián, F. Ameghino (1898) Wichmann (1922), Windhausen (1931) was probably transported towards the subdivided the marine beds he had and Feruglio (1949) published detailed coast by the western winds and deposited named (Ameghino 1896) "Formación Pa- accounts on the geological features of in an environment that was protected tagónica" into two chronostratigraphic Patagonia, including the marine Ceno- from the action of marine currents. units he called "Juliense" and “Leonense”. zoic rocks. Their detailed sections and Although discrepancies still exist concer- Overlying these (Ameghino 1900-1902) geological observations are generally easy ning the age of the Monte León For- he recognized the continental "Formación to corroborate in the field. However, mation, most authors refer it to the late Santacruceña", at the base of which he interpretations largely reflect the ideas of Oligocene (Bertels 1970) or late Oligo- could distinguish a marine stage he called Ameghino and von Ihering. cene - early Miocene (Malumián 1999, "Superpatagónico". The faunas of marine After these initial studies, the use of "Pa- 2002). Yet, Náñez (1988) restricted it to invertebrates that were the basis of tagonian Formation" for the Cenozoic the Oligocene (middle section of P19 Ameghino's subdivisions were collected marine rocks in southern South America Zone and top of P21 Zone) while sug- by Carlos and described by Maurice Cos- was widespread, although the exact chro- gesting that the top may reach into the smann (1899) and Hermann von Ihering nostratigraphic and lithostratigraphic early Miocene. This was later confirmed (1897, 1899, 1902, 1907, 1914). meaning of this formational name remai- by 40Ar/39Ar ages published by Fleagle et 74 A. PARRAS AND M. GRIFFIN

al. (1995). Their sample came from the Punta Entrada Member of the Monte 0-1.2 m: 1.2 m Grey tuffaceous clayish top of the unit at Cerro Monte León and León Formation. This locality is precisely siltstones with abundant fossils and it yielded an age of 19.33 Ma (Burdiga- where our Mount Entrance section lies. numerous Ophiomorpha isp. Dominant lian, early Miocene). More recently Ba- According to her description (Bertels species is "Turritella" patagonica. It appears rreda and Palamarczuk (2000) considered 1980) the section measures there 103.8 in clump-geometry packets. Also recor- that in the study area the Monte León m, but this could not be confirmed in the ded are Nucula sp., Limopsis insolita, Formation should be restricted to the field by us, as the bottom of the section Cucullaea alta, Crassostrea? hatcheri, Fascicu- early Miocene on the basis of its palyno- is completely covered by modern foothill licardia patagonica, Pleuromeris cruzensis, Do- morph content, an age we agree with deposits and only 60 m are exposed. sinia laeviuscula, Ameghinomya darwini. Clo- herein. Therefore, the base of Bertels' section - se-packing within these clumps is den- as inferred from her description - must se/loose (25%), size-sorting is poor, METHODOLOGY have been measured at Darwin's section, orientation chaotic, disarticulation is to- where the exposure is complete, al- tal, fragmentation low in some clumps Just east of Punta Quilla (Darwin sec- though the top part is here extremely dif- and high in others, abrasion is nil. En- tion; S50°07'39.9"; W68°24'10.04") and ficult to reach. Riggi (1978, p. 166) mea- crusting and bioerosion is observed on at Mount Entrance (S50°08'32.2"; W68° sured 89 m but it remains unclear exactly the valves of C.? hatcheri. The taxa recor- 22'23.6") we measured detailed (1:10) where his section is (his Mount Entrance ded were en-crusting bryozoans, sponge stratigraphic-sedimentologic sections section lies "on the right bank of the Río borings (En-tobia isp.), polychaetes (Ma- (Fig. 3). We recorded bed geometry, li- Santa Cruz and its mouth in the ocean"). eandropolydora isp.), and bivalve borings thology, grain size, sedimentary structu- Barreda and Palamarczuk (2000) measu- such as Gastro-chaenolites isp. res, and color. Each bed was sampled and red 80 m south of the mouth of the 1.2-6 m: 4.8 m Grey tuffaceous clayish fossils and trace fossils were photogra- river, but most other published sections siltstones similar to the previous ones. phed and collected. Taphonomic obser- are compounds based on partial sections This bed carries three lenses, each ca. 0.2 vations were recorded too. Close-packing measured at various points ranging bet- m thick and dominated by "Turritella" and size-sorting were estimated follo- ween south of the mouth of the river to patagonica. These lenses are separated by wing the semi-quantitative categories for 30 km upstream along the right bank siltstones with very few clumps that carry coarse bioclastic fabrics of Kidwell and (Olivera 1990, del Río and Camacho almost exclusively "T". patagonica. Also Holland (1991). Terminology for geo- 1998). Other authors based their strati- present are, among many other taxa, metry of concentrations and orientation graphic framework on unpublished data Nucula (Lamellinucula) reticularis, Iheringinu- of bioclasts follows Kidwell et al. (1986). (Becker 1964) or previous work by others cula crassirugata, Scaeoleda? ortmanni, Neilo Sedimentary facies were distinguished (Di Paola and Marchese 1973). ornata, Arca patagonica, Cucullaea alta, Li- and grouped in facies associations for According to Darwin's description, the mopsis insolita, Glycymeris cuevensis, Atrina paleoenvironmental analysis. The inver- section mentioned by him is the one just magellanica, Neopanis quadrisulcata, Swifto- tebrate fauna collected was identified east of Punta Quilla (our Darwin Sec- pecten nodosoplicatus, Zygochlamys geminata, taxonomically. The different beds mention, Figs. 2 and 3). This is where the cliff Zygochlamys quemadensis, Reticulochlamys pro- tioned by Darwin (1846, p. 112-113) is best exposed and where the descrip- xima, Jorgechlamys centralis, Crassostrea? hat- were identified as was the exact prove- tion and measurements provided by cheri, Pteromyrtea crucialis, Pleuromeris cruzen- nance of the type material of the species Darwin himself best agree with field sis, Fasciculicardia patagonica, Spissatella lyelli, described by Sowerby (1846). observations. In order to refer the beds Hedecardium? ameghinoi, Hedecardium? puel- described by Darwin to the type section, chum, Trachycardium pisum, Cardium patago- SEDIMENTOLOGY we provide herein a description of both, nicum, Lahillia patagonica, Maorimactra indis- even if lithology is very similar - as is fos- tincta, Macoma? santacruzensis, Retrotapes Sections sil content. The Darwin Section is more striatolamellata, Dosinia laeviuscula, Ameghi- complete because the lower beds lie ex- nomya darwini, Ameghinomya meridionalis, The type section of the Monte León posed at low tide. However, access to the Caryocorbula hatcheri, Panopea quemadensis, Formation lies on the right bank of the topmost beds is extremely difficult. This Liotia scotti, Gibbula (Phorcus) margaritoides, Río Santa Cruz, one kilometer upstream is probably the reason why the section at Valdesia dalli, Solariella dautzenbergi, Ca- of its mouth into the Atlantic Ocean Mount Entrance (and nearby) has been lliostoma santacruzense, Calliostoma perara- (Bertels 1970, p. 499). However, Bertels better studied. tum, Calliostoma cossmanni, Calliostoma ga- (1970) did not describe or illustrate her rretti, "Turritella" ambulacrum, Struthioche- section until a decade later (Bertels 1980, a) Darwin Section (S50º07'39.9"; W68º nopus santacruzensis, Perissodonta ornata, Pe- p. 214-217), when she also defined the 24'10.04") rissodonta patagoniensis, Crepidula gregaria, Darwin's Great Patagonian Tertiary Formation at the mouth of the Río Santa Cruz: ... 75

Figure 3: Studied stratigraphic sections showing identified facies associations. Cerithioderma patagonica, Polinices santacru- nebra iheringi, Urosalpinx archipatagonica, Pe- drillia santacruzensis, Austrotoma cuevensis, zensis, Polinices ortmanni, Glossaulax vidali, onza torquata, "Cominella" annae, Penion sub- Fusiguraleus iheringi, Terebra quemadensis, Lunatia consimilis, Sassia bicegoi, Cerithiopsis rectus, Miomelon petersoni, Miomelon gracilior, Terebra santacruzensis, Odostomia synarthrota, juliana, Cirsotrema rugulosum, Cirsotrema que- Adelomelon pilsbryi, Neoimbricaria patagonica, Turbonilla cuevensis, Turbonilla observatonis, mandense, Trophon santacruzensis, Xymenella Dentimargo deuterolivella, Austroimbricaria Semiacteon argentinus, Cylichna juliana, Kaitoa dautzenbergi, Xymene cossmanni, Enthacant- quemadensis, Zeadmete ameghinoi, Zeadmete patagonica, "Odontostomia" euryope, Denta- hus monoceros, Crassilabrum hatcheri, cf. Oce- cruzialis, Antimelatoma quemadensis, Splen- lium sp. Within the lens close-packing is 76 A. PARRAS AND M. GRIFFIN

dense (55%), size-sorting poor - with at 6 m from the bottom of this bed. is dense (55%) at the base and loose juvenile and adult specimens alike - , 56-56.5 m: 0.2-0.5 m Very fine sandstone (15%) at the top, size-sorting is well sor- orientation is chaotic, disarticulation is with abundant specimens of Dosinia lae- ted, as complete specimens and frag- total, fragmentation very low, and abra- viuscula. This is a monospecific bed with ments show approximately the same size, sion nil. Encrusting and bioerosion oc- a wavy bioturbated contact at the bottom a few large whole bivalves are also obser- cur on the oysters. and a wavy one at the top. Close-packing ved, orientation is chaotic, disarticulation 6-27 m: 21 m Yellowish-brown tufface- is dense (55%), size-sorting is well sor- complete, fragmentation high, and many ous siltstones with at least two beds ted, orientation chaotic, disarticulation naticid and muricid boreholes are evident dominated by "Turritella" patagonica. They high, fragmentation is moderate to low, on the shells. This bed carries a very rich measure 1 and 2.5 m thick respectively. abrasion nil. At the base there are nume- fauna of well preserved mollusks, inclu- The top bed is the most conspicuous and rous tubes that reach down up to 0.7 m ding among others Nucula (Lamellinucula) shows sectors in which it is hardened into the underlying bed. This shell bed is reticularis, Iheringinucula crassirugata, Scaeo- into concretions that outstand in the sec- replaced laterally by light grey to white leda? ortmanni, Neilo ornata, Arca patagonica, tion. Faunal composition almost identical tuff of approximately the same thick- Cucullaria darwini, Cucullaea alta, Limopsis to that of the underlying bed. Contacts ness. insolita, Glycymeris cuevensis, Atrina magella- are wavy, close-packing is dense/loose 56.5-75.5 m: 19 m Interstratified siltstone, nica, Neopanis quadrisulcata, Swiftopecten no- (25%) at some places and dense (55%) at claystone and fine sandstone. At the bot- dosoplicatus, Zygochlamys geminata, Zygochla- others, size-sorting is poor (full size ran- tom it carries right valves of Crassostrea? mys quemadensis, Reticulochlamys proxima, ge of "T." patagonica), orientation is chao- hatcheri, in all cases convex-down. At 11 Jorgechlamys centralis, Crassostrea? hatcheri, tic, disarticulation high with very few m from the bottom the color changes Pteromyrtea crucialis, Pleuromeris cruzensis, specimens keeping conjoined valves, from brown to yellowish-green. Fasciculicardia patagonica, Spissatella lyelli, fragmentation is high in some sectors Spissatella kokeni, Hedecardium? ameghinoi, and low in others, abrasion nil. Between b) Mount Entrance Section (S50º08'32.2"; Hedecardium? pauciradiatum, Hedecardium? 15 and 18 m above the base of this bed W68º22'23.6") puelchum, Trachycardium pisum, Cardium pa- there are thin intercalated lenses of 0.05- 0-17 m: 17 m Grey and greenish tufface- tagonicum, Lahillia patagonica, Maorimactra 0.3 m thickness dominated also by "T." ous clayish siltstones with a few speci- indistincta, Serratina jeguaenis, Macoma? san- patagonica. At the very top there is mens of "Turritella" patagonica that may be tacruzensis, Retrotapes ortmanni, Retrotapes Dentalium in life position and Limopsis isolated or forming clumps. At 7 m from striatolamellata, Dosinia laeviuscula, Ameghi- insolita. the base there is a sandy lens dominated nomya darwini, Ameghinomya meridionalis, 27-30 m: 3 m Grey tuffaceous siltstones. by this species but also containing a few Caryocorbula hatcheri, Panopea quemadensis, Lithology similar to previous ones, but brachiopods and bivalves. At 14 m there Liotia scotti, Homalpoma philippii, Gibbula the color change from yellowish to grey are large "necklace" concretions of about (Phorcus) margaritoides, Valdesia dalli, So- is notable. 0.3 m diameter. Between 16 and 17 m the lariella dautzenbergi, Calliostoma santacruzen- 30-34 m: 4 m Grey tuffs. The first 0.4 m bed is strongly bioturbated (Ophiomorpha se, Calliostoma peraratum, Calliostoma coss- carry large Ophiomorpha isp. The rest of isp. and other small tubes in all direc- manni, Calliostoma garretti, "Turritella" am- the bed is massive, with only a few spar- tions). bulacrum, "Turritella" patagonica, Struthio- se fragments of shells. 17-27 m: 10 m Very bioturbated tufface- chenopus santacruzensis, Perissodonta ornata, 34-56 m: 22 m Yellow tuffaceous siltsto- ous sandstones. Perissodonta ameghinoi, Perissodonta patago- ne highly bioturbated, with numerous ve- 27-27.8m: 0.8 m Fine tuffaceous sandsto- niensis, Calyptraea eleata, Sigapatella america- ry large Thalassinoides isp. (up to 0.08 m nes, dark grey and very hard, with cross- na, Crepidula gregaria, Cerithioderma patago- diameter). Sparse body fossils include bedding. At the base it carries load struc- nica, Vermetus incertus, Polinices santacruzen- crabs, very small Lucinoidea in life posi- tures. These sandstones are very biotur- sis, Polinices ortmanni, Glossaulax vidali, Lu- tion and larger fragments of maybe the bated, with many tubes of about 0.01 m natia consimilis, Sconsia ovulum, Sassia bicegoi, same taxon. Preservation is gypsum re- diameter placed obliquely to the stratifi- Sassia morgani, Cerithiopsis juliana, Cirsotre- placement. The top 8 m carry very spar- cation planes. ma rugulosum, Cirsotrema qemandense, Tro- se valves of Cucullaea alta and Dosinia lae- 27.8-38.8 m: 11 m Tuffaceous bioturba- phon santacruzensis, Xymenella dautzenbergi, viuscula, in all cases disarticulate and con- ted sandstones similar to the underlying Xymene cossmanni, Enthacanthus monoceros, vex-up, some of them broken. Nume- bed. Crassilabrum hatcheri, cf. Ocenebra iheringi, rous concretions (as recorded by Dar- 38.8-39.4 m: 0.6 m Very fine sandstones Urosalpinx archipatagonica, Peonza torquata, win) of 0.8 to 1.4 m diameter and conjoi- with abundant fossils. This is a shell bed "Cominella" annae, Penion subrectus, Austro- ned in a "necklace" manner. An espe- with a wavy contact at the base and a gra- cominella cosmanni, Retizafra improvisa, Mio- cially conspicuous bed of concretions is dual transition at the top. Close-packing melon petersoni, Miomelon gracilior, Adelome- Darwin's Great Patagonian Tertiary Formation at the mouth of the Río Santa Cruz: ... 77

lon pilsbryi, Neoimbricaria patagonica, Den- diverse fauna of the Monte León For- ne conditions with normal salinity. Well timargo deuterolivella, Austroimbricaria que- mation (see description of Sections for a over half the taxa present in this fauna madensis, Zeadmete ameghinoi, Zeadmete cru- list of the most conspicuous taxa and are either very shallow infaunal (Turritella, zialis, Sveltia maior, Scalptia vidali, Anti- their stratigraphic distribution). Also pre- Cucullaea, Cardium, etc) or epifaunal (Zygo- melatoma quemadensis, Splendrillia santacru- sent are abundant bioturbations as small chlamys, Reticulochlamys, Swiftopecten, Trop- zensis, Austrotoma cuevensis, Fusiguraleus ihe- tubes running in all directions, and galle- hon, Austrotoma, etc.). Most trophic levels ringi, Eoturris santacruzensis, Borsonia patago- ries assigned to Ophiomorpha isp. Concre- are represented, including suspension fe- nica, Terebra quemadensis, Terebra santacru- tions of variable size and shape are abun- eders, deposit feeders and carnivores. zensis, Odostomia synarthrota, Odostomia sutu- dant, sometimes joined together forming Taxonomic composition of the assem- ralis, Turbonilla cuevensis, Turbonilla observa- "necklaces". This facies association mea- blage and the beds with articulate and tonis, Turbonilla iheringi, Semiacteon argenti- sures 30 m thick at Darwin's Section and highly bored oyster clusters suggest that nus, Cylichna juliana, Kaitoa patagonica, 17 m thick at Mount Entrance Section. the background sedimentation rate was "Odontostomia" euryope, Dentalium sp. Some of the skeletal concentrations oc- low. 39.4-59.4 m: 20 m Interstratified fine cur in laterally continuous beds, with The presence of thin consolidated lenses sandstone, siltstone and claystone. It wavy contacts and hardened concretio- with fragments of shells or specimens carries a few interstratified concretions nary sectors that form ledges in the sec- lying chaotically is interpreted as of de- with bivalves and echinoderms. At 12 m tion. There are a least three beds 1 - 2.5 posits formed by sporadic events recor- from the base of this bed there is a color m thick. The shell beds are densely to ding a sharp increase in current velocity. change from brown to yellowish green. loosely packed (25%) in some of the sec- The lenticular structure of some of these tors, and densely packed (55%) in others. skeletal concentrations and their tapho- Facies Analysis Size-sorting is poor; although by far the nomic features suggests they belong to numerically most abundant taxon is "T." the infilling of small scale tidal channels Vertical and lateral facies distribution at patagonica; it is represented by specimens in a subtidal to inner shelf environment. the two studied sections allows recogni- of all sizes. Orientation is chaotic, disar- tion of three facies associations (Figs. 2 ticulation is very high and only a few Facies Association 2 (FA2) and 3). Facies Association 1 (FA1): Silt- shells are conjoined. Fragmentation is stones and fine sandstones rich in pyro- low in some sectors and high in others, This facies association overlies FA1 and clastic material and clumps, lens and beds while abrasion is nil. Encrusting and is equivalent to Darwin's lower two stra- of skeletal fossils concentrations. Facies bioerosion are evident in valves of C.? ta of the upper part of the cliff. It com- Association 2 (FA2): Highly bioturbated hatcheri, which carry encrusting bryozo- prises massive and bioturbated tufface- siltstones and tuffs with concretions. ans, sponge borings (Entobia isp.), poly- ous siltstones and very fine sandstones, Facies Association 3 (FA3): Interstrati- chaetes (Maeandropolydora isp.) and bivalve and tuffs. These are grey at the base and fied siltstones, fine sandstones and clays- borings (Gastrochaenolites isp.). light yellow towards the top. The thick- tones, thinly laminated in some sectors. In addition to the aforementioned shell ness is approximately 27 m at Darwin's beds also common are lenses of between section and 22 m at the Mount Entrance Facies Association 1 (FA1) 0.05 and 0.3 m thickness. The fossil con- Section. The base carries large galleries tent of these lenses is the same as the assigned to Ophiomorpha isp. (Fig. 4b) and This facies association is roughly equiva- beds, as are all taphonomic attributes too. also few isolated shell fragments. lent to Darwin's (Darwin 1846, p. 112) The uppermost part of this facies asso- Towards the top there are frequent galle- lowermost stratum (i.e., "… to a thickness ciation shows a sharp decrease in fossil ries assigned to Thalassinoides isp. (up to 8 of fifty or sixty feet, consists of a more or less content, as the beds and lenses become cm diameter). It also carries very sparse hardened, darkish, muddy, or argillaceous much less frequent. However, a few dis- remains of crabs and small monospecific sandstone…"). It includes yellowish grey perse fossils such as a Dentalium in life clumps of a small bivalve possibly refera- siltstones and fine muddy sandstones position and Limopsis insolita can be ble to Pteromyrtea, and very few fragments with abundant pyroclastic material and observed. of larger bivalve shells. The uppermost 8 skeletal concentrations forming clumps, Facies Association 1 represents a transi- m carry a few isolated valves of Dosinia lens or beds, dominated by "Turritella" tion environment between the inner shelf laeviuscula. These are disarticulate, con- patagonica (Fig. 4a). Also intercalated are and the subtidal part of a tidal plain, be- vex-up, and mostly unbroken. Through- specimens of Crassostrea? hatcheri in life low fair weather wave base but above out this facies association are common position or slightly rotated but still with storm wave base. The mollusks contai- concretions of 0.8 - 1.4 m diameter that both valves articulated. This facies asso- ned in these beds are commonly conside- may be isolated or joined forming "nec- ciation also carries most of the rich and red to be indicative of shallow, fully mari- klaces" (Fig. 4c). Darwin had already 78 A. PARRAS AND M. GRIFFIN

Figure 4: a) Clumps of "Turritella" patagonica (FA1); b) Ophiomorpha isp. (FA2); c) Concretions (FA2); d) Skeletal concentration dominated by Dosinia laeviscula at the top of FA2. noticed them as a conspicuous feature in ning adult specimens of Dosinia laeviscula king is dense at the base and dispersed at this part of the section: "…Of the three (Fig. 4d). Contacts of this bed are wavy the top. Size-sorting is good, as the beds, the central one is the most compact, and and bioturbated, with many tubes rea- whole specimens and the shell fragments more like ordinary sandstone: it includes nume- ching up to 0.7 m down into the underl- are of the same size with only a few rous flattened spherical concretions, often united ying bed. Laterally it gives way to a white interspersed large bivalves and gastro- like a necklace, composed of hard calcareous tuff of equal thickness. Close-packing is pods. Orientation is chaotic, disarticula- sandstone, containing a few shells: some of these dense (55%), size-sorting good, orientation is complete, and fragmentation of concretions were four feet in diameter, and in a tion chaotic, disarticulation high, frag- large specimens is high but with a few horizontal line nine feet apart…"(Darwin mentation moderate to low, abrasion nil. whole specimens, while small specimens 1846, p. 113). At the Mount Entrance Section this are largely unbroken. Many specimens At the Mount Entrance Section there is a facies association ends with a skeletal show muricid and naticid boreholes. hard ledge of about 0.8 m thickness with concentration of about 0.6 m thickness, Facies association 2 represents a subtidal cross-bedding. It is highly bioturbated also with a fine sandstone matrix, but environment at the lowest part of a tidal with numerous tubes of about one cm with a polyspecific taxonomic composi- plain. The low diversity in the fauna diameter placed obliquely to stratification, carrying an abundant fauna of gas- throughout the facies association (Ptero- tion. tropods and bivalves (see description of myrtea and Dosinia), suggests a more mar- At the Darwin Section this facies associa- Sections for a list of the most conspi- ginal shallow environment than FA1. tion ends with a monospecific shell con- cuous taxa and their stratigraphic distri- The high degree of bioturbation and do- centration of 0.2 - 0.5 m thickness, with bution). The lower contact is wavy and minance of trace fossils assigned to Tha- a very fine sandstone matrix, and contai- the upper one transitional, as close-pac- lassinoides isp. and Ophiomorpha isp. sug- Darwin's Great Patagonian Tertiary Formation at the mouth of the Río Santa Cruz: ... 79

gests low sedimentation rate and an envi- her energy events. These were recorded species that is unique to the lowermost ronment with moderate to relatively low as beds and lenses of skeletal concentra- beds of the Santa Cruz Formation, i.e., energy conditions. Taphonomic features tions interpreted as having been deposi- Crasssotrea orbignyi. This oyster is quite dif- of the specimens in the skeletal concen- ted by channeled high energy unidirectio- ferent from Crassostrea? hatcheri, and is trations at the top, together with the pre- nal currents, perhaps tidal channels. widespread in Patagonia and always sence of apparently unselected material Towards the top of the sections (FA2) appears associated to the marginal mari- suggests that these concentrations were these lenses become sparser, while the ne rocks at the base of the Santa Cruz formed during a high energy event, per- bioturbation and pyroclastic content of Formation or its equivalent units. Se- haps representing a storm event. the deposit increase. The top carries condly, the Superpatagonian included finely laminated siltstones and sandsto- many taxa common to the "Leonense" Facies Association 3 (FA3) nes (FA3) that suggest a subtidal - or chronostratigrapic unit of Ameghino, even intertidal - environment. together with a suite of exclusive taxa This facies association comprises the not known from the underlying strata. uppermost bed of Darwin's "upper part of PALEONTOLOGY However, these were based on material the cliff" (Darwin 1846, p. 113). It includes coming from two different sources. In 19 - 20 m of interstratified siltstones, fine One of the most conspicuous features of the first place, it included material collec- sandstones and claystones, sometimes the Monte León Formation is the rich ted from the shell beds at the top of the with heterolithic lamination. At the base and excellently preserved mollusk fauna Punta Entrada member, at localities there are convex-down right valves of contained in the beds and lenses within where this unit is exposed along the coast Crassostrea? hatcheri. It carries a few con- the Punta Entrada Member; it occurs between the mouth of the Río Santa cretions with bivalves and echinoderms. mainly in FA1 and the most conspicuous Cruz and Monte Observación, just off At 11 m above the base color changes and common elements are mentioned limits to the south of Monte León abruptly from brown to yellowish green. above. This fauna has been known ever National Park. It also includes material Fine grain-size and sedimentary structu- since Darwin collected the first speci- collected from beds overlying the former res produced by variations between calm mens at the mouth of the Río Santa and containing Crassostrea orbignyi, expo- and turbulent water indicates deposition Cruz, and has been used over the years in sed a few kilometers further inland and by combined traction and decantation the formulation of diverse hypothesis on clearly at the base of the Santa Cruz For- processes. Such a process suggests that the biostratigraphic and paleobiogeogra- mation. The taphonomic attributes of this facies association developed in a sub- phic distributions of the taxa involved the material other than the autochtonous tidal to intertidal environment. (del Río 2004 and references therein). accumulations of C. orbignyi suggest that Darwin's initial collection of 16 species it is in all cases reworked material from Paleonvironments of mollusks has been increased to over the underlying shell beds. In most cases, 110 recognized from that locality nowa- taxonomic differences can be attributed Facies associations together with the days. Of these, virtually all are also to the differential preservation of the invertebrate fauna recorded in each bed known from the shell beds that were des- two suites of material. allow a detailed paleoenvironmental re- cribed later by Ameghino and Hatcher. construction of the rocks of the Monte The type locality of the species described MEANING OF THE GREAT León Formation in the Río Santa Cruz by Sowerby (1846) is the section descri- PATAGONIAN FORMATION area. These rocks can be included in the bed here as Darwin Section. Thus, all his AT THE MOUTH OF THE regressive stage of the high sea-level taxa come from beds lying within the RÍO SANTA CRUZ period that began at the end of the Punta Entrada Member. It is important Oligocene. The proposed deposition to note that taxa later described by von Upstream the Río Santa Cruz, the rocks model suggests that it took place in a Ihering (1897, 1907, 1914) and Ortmann described by Darwin at the mouth of the shallow sea, with variations that range (1902) came from further south. The river are gradually replaced by continen- from the inner shelf to subtidal or even beds bearing these taxa were then inclu- tal beds nowadays included in the Santa intertidal environments. ded in the Superpatagonian, although Cruz Formation (Darwin 1846, p. 113- At the base of the section (FA1) - i.e., the Hatcher and Ortmann could see no dif- 115). Although he described a section at inner shelf to subtidal environment - ference between the Patagonian and about 160 km west of Santa Cruz, Dar- there was a continuous deposition of Superpatagonian fauna. This is not sur- win was doubtful that the rocks could be siltstone with volcanic material (biotur- prising, as the Superpatagonian included included in "the great Patagonian tertiary for- bated) as result of decantation, with a two suites of taxa from entirely different mation", as the only fossils he could find few intercalated episodes recording hig- sources. In the first place it included the were very worn pieces of Ostrea patagoni- 80 A. PARRAS AND M. GRIFFIN

ca (i.e. Crassostrea? hatcheri or more likely at Cerro Monte León and Cerro Obser- by Di Paola and Marchese (1973) for the C. orbignyi). As Darwin never went asho- vación. He based such a subdivision on Monte Observación Member are quite re further south than Mount Entrance, the fossil content, a fact that was later clearly distinct from those observed (as he was unable to observe neither the disputed by other authors (Wilckens noted by Bertels herself; Bertels 1980, p. beds at Las Cuevas nor those within the 1905, Hatcher 1900, Ortmann 1902) who 216) in the shell beds at the top of the Monte Observación Member. Therefore, could see no difference in taxonomic sections at Monte León (Cabeza de his concept of "great Patagonian tertiary for- composition between the Superpatago- León) and Las Cuevas. The lithology in mation" or "Patagonian Tertiary Formation" nian beds and those appearing in the these shell beds is clearly the same as that only included those rocks exposed at typical Patagonian Formation. in the underlying beds attributed by her Santa Cruz (and other places further Di Paola and Marchese (1973) did not to her Punta Entrada Member. The north). recognize Bertels' introduction of a for- Superpatagonian of Ameghino originally At Mount Entrance Bertels (1980, p. mational status for Ameghino's Juliense included not only the shell beds at Monte 214) defined a lower member of the and Leonense (i.e., her San Julián and León and Las Cuevas, but also those at Monte León Formation that she named Monte León Formations). Instead they Yegua Quemada, Jack Harvey, and Mon- Punta Entrada Member and included in subdivided the Patagonia Formation te Observación all of them south of Las it all the marine beds exposed there. (sensu Ameghino 1894) into three mem- Cuevas. He also included the shell beds Darwin's Section is only 2.6 km away and bers based on lithological differences with Crassostrea orbignyi at Cañadón de los no major structural features alter the among them. The uppermost one, overl- Artilleros, inland from San Julián, also stratigraphic relations between the beds ying the Monte León Member, they underlying the continental Santa Cruz exposed at the two localities. Conse- named Monte Observación Member. Formation. Neither Ameghino nor any quently, we can confidently assume that They defined this unit as the rocks appe- author after him considered the fact that (as confirmed by descriptions above) the aring below the Santa Cruz Formation at the shell beds at these places are thicker, two sections are equivalent. About 40 km Cerro Observación and that showed fea- with a far more abundant and better pre- further south along the coast and within tures common to the Monte León Mem- served fauna than those at the mouth of the present Monte León National Park, ber at the base and to the Santa Cruz the Río Santa Cruz, the most likely rea- these rocks continue to form part of the Formation at the top. The bottom of this son for them supporting the difference coastal cliff and lithological features member is - according to them - an oys- between the Patagonian and Superpa- remain overall constant. However, a ter bed of Crassostrea orbignyi, and the top tagonian beds. Such a difference was short distance inland a second higher is the last bed of this same oyster before minimized by Ortmann (1902) and Hat- ledge is composed mainly by the conti- giving way completely to the continental cher (1900) based on the very abundant nental Santa Cruz Formation. The con- mammal-bearing rocks of the Santa Cruz fauna from Santa Cruz (possibly near tact between the two units has been sub- Formation (the contact here is transitio- Mount Entrance) that they had available. ject of heated debates over the years, nal). The total thickness measured by In fact, these authors were right in stating ever since the first subdivisions of the them between the two oyster beds was 27 that the different composition of the Patagonian Tertiary were made by Ame- m. When introducing her Punta Entrada fauna was due to local facies variations. ghino. The Patagonian Tertiary Forma- Member, Bertels (1980, p. 214) also ack- Moreover, the far more abundant fauna tion in Darwin´s sense was restricted by nowledged the difference between this at Las Cuevas is also surely the cause of Ameghino (1894) to the marine beds unit and the topmost beds of the Monte bias in this sense, as the chances of exposed along the coast between Chubut León Formation. Therefore she admitted collecting uncommon taxa are naturally and Monte Observación in Santa Cruz. the formal status given by Di Paola and much higher there. His subdivision of this unit (Ameghino Marchese (1973, p. 214) to the Monte Di Paola and Marchese (1973), while co- 1898) into "Juliense" and “Leonense” was Observación Member. However, she sta- rrectly defining their Monte Observación followed shortly afterwards (Ameghino ted that a) the Monte Observación Member, never mentioned the fact that 1900-1902) by his recognition that the Member included all the shell beds at the between the base of this unit (i.e., the overlying "Formación Santacruceña" inclu- top of the sections at Monte León (Ca- lowest oyster bed at Cerro Observación) ded a lower marine bed he called "Super- beza de León), Las Cuevas, and Monte and the top of the shell beds at Las patagoniano". In this he included the shell Observación (i.e., Cerro Observación, Cuevas there are ca. 20 m of rocks that beds at the top of the cliff exposed at just west of Las Cuevas), and b) it repla- are not exposed anywhere between Santa Las Cuevas, Yegua Quemada, Jack Har- ced Ameghino's chronostratigraphic Cruz and the last exposures with marine vey and Monte Observación, and also term "Piso Superpatagónico". However, the- shells at Puerto Coyle. As the lithology the oyster beds at the base of the Santa se assumptions appear to be unwarran- (and sedimentary structures as well) of Cruz Formation exposed further inland ted. The lithological features described the Monte Observación Member (other Darwin's Great Patagonian Tertiary Formation at the mouth of the Río Santa Cruz: ... 81

than the in situ beds with oysters in life tionally referred to the "Superpatago- Bertels, A. 1970. Sobre el "Piso Patagoniano" y la position) is identical with that of the nian", suggest that the Monte Observa- representación de la época del Oligoceno en Santa Cruz Formation and the beds with ción Member of the Monte León For- Patagonia austral, República Argentina. Revis- other marine shells included in this unit mation should be restricted to its original ta de la Asociación Geológica Argentina are clearly reworked material with taxa concept. As a consequence, the term 25(4): 495-501. that appear at Las Cuevas too, we suggest "Superpatagonian" becomes meanin- Bertels, A. 1980. Estratigrafía y foraminíferos that the Monte Observación Member gless, at least as a chronostratigraphic (Protozoa) bentónicos de la Formación Mon- should be restricted to the rocks descri- unit in the area studied. te León (Oligoceno) en su área tipo, provincia bed by Di Paola and Marchese (1973, p. de Santa Cruz, República Argentina. Actas 2º 214) and included in the Santa Cruz ACKNOWLEDGEMENTS Congreso Argentino de Paleontología y Bio- Formation. Likewise, the shell beds at the estratigrafía y 1º Congreso Latinoamericano top of the sections at Monte León We would like to thank Sergio Martínez de Paleontología (Buenos Aires, 1978) 2: 213- (Cabeza de León) and Las Cueva, Yegua (Montevideo, Uruguay) and Jeff Stilwell 273. Quemada, Jack Harvey and Monte Ob- (Melbourne, Australia) for their thought- Camacho, H. H. 1974. Bioestratigrafía de las for- servación should be definitely referred to ful comments that improved the original maciones marinas del Eoceno y Oligoceno de the Monte León Formation as they are version of this manuscript. Help in the la Patagonia. Anales de la Academia de Cien- equivalent to the shell beds lying within field by Juan Griffin is duly acknowled- cias Exactas, Físicas y Naturales 26: 39-57. the Punta Entrada Member at Mount ged too. The Facultad de Ciencias Exac- Camacho, H. H. 1979. Significados y usos de Entrance and Darwin's Section. There- tas y Naturales, Universidad Nacional de "Patagoniano", "Patagoniense", "Formación fore, the use of the term Superpatago- La Pampa supported part of the field- Patagonica", "Formación Patagonia" y otros nian becomes superfluos. work (Project PI188). CONICET finan- términos de la estratigrafía del Terciario mari- ced field and lab work (Project PIP1442 no argentino. Revista de la Asociación Geo- CONCLUSIONS /05). lógica Argentina 34(3): 235-242. Cossmann, M. 1899. Description des quelques As a result of this study we were able to WORKS CITED IN THE TEXT coquilles de la formation Santacruzienne en pinpoint the most probable location Patagonie. Journal de Conchiliologie 47(3): where Charles Darwin made his observa- Ameghino, F. 1894. Ennumération synoptique 223-241. tions in the Río Santa Cruz mouth. At des espèces des mammifères fossiles des for- Darwin, Ch. 1839. Narrative of the surveying this locality, i.e., our Darwin Section, we mations eocenes de Patagonie. Boletín de la voyages of His Majesty's Ships Adventure also identified the beds mentioned by Academia Nacional de Ciencias 13: 259-445, and Beagle, between the years 1826 and 1836, him, including the location of the beds Córdoba. describing their examination of the southern from where he collected the fossil mo- Ameghino, F. 1896. Notas sobre cuestiones de shores of South America, and the Beagle's llusks that were described by Sowerby. geología y paleontología argentina. Boletín del circumnavigation of the globe. Colburn, H. The strata recognized by him are equiva- Instituto Geográfico Argentino 17: 87-119. Ed., London, vol. 3, 615 p. lent to those described by us and of Ameghino, F. 1898. Sinopsis Geológico-Paleon- Darwin, Ch. 1846. Geological observations on which a detailed lithological description tológica. Segundo Censo de la República Ar- South America. Being the third part of the is provided. gentina 1: 111-225. geology of the voyage of the Beagle, under Rocks belonging to the Monte León Ameghino, F. 1900-1902. L'age des formations the command of Capt. Fitzroy, R. N. during Formation exposed at the mouth of the sédimentaires de Patagonie. Anales de la So- the years 1832 to 1836. Smith Elder and co Río Santa Cruz can be included in the ciedad Científica Argentina 50: 109-130, 145- Ed., London, 279 p. regressive stage of the Atlantic transgres- 165, 209-229 (1900); 51: 20-39, 65-91 (1901); del Río, C. J. 2004. Tertiary marine Molluscan sive-regressive cycle that flooded Patago- 52: 189-197, 244-250 (1901); 54: 161-180, Assemblages of Eastern Patagonia (Argen- nia during the late Oligocene-early Mio- 220-249, 283-342 (1902). tina): a biostratigraphic analysis. Journal of cene. The deposition model suggests that Barreda, V. and Palamarczuk, S. 2000. Palino- Paleontology 78(6):1097-1122. it took place in a shallow sea, with varia- morfos continentales y marinos de la For- del Río, C. J. and Camacho, H. H. 1998. Tertiary tions that range from the inner shelf to mación Monte León en su área tipo, provincia nuculoids and arcoids of eastern Patagonia subtidal or even intertidal environment. de Santa Cruz, Argentina. Ameghiniana 37(1): (Argentina). Palaeontographica Abt. A, 250: Analysis of sedimentological features of 3-12. 47-88. these rocks, together with the mollusk Becker, D. 1964. Micropaleontología del Super- Di Paola, E. C. and Marchese, H. G. 1973. fauna contained in them, and compari- patagoniense de las localidades Las Cuevas y Litoestratigrafía de la Formación Patagonia en son with those from nearby exposures Monte Entrance (provincia de Santa Cruz). el área tipo (Bajo de San Julián-desembocadu- further south along the coast and tradi- Ameghiniana 3(10): 319-351. ra del río Santa Cruz). Provincia de Santa 82 A. PARRAS AND M. GRIFFIN

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SEDIMENTOLOGY AND PALEONTOLOGY OF A MIOCENE MARINE SUCCESSION FIRST NOTICED BY DARWIN AT PUERTO DESEADO (PORT DESIRE)

Silvio CASADÍO and Miguel GRIFFIN

Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Santa Rosa, La Pampa. Email: [email protected]

ABSTRACT Rocks exposed just south of Puerto Deseado (Port Desire), Santa Cruz Province, were surveyed by Darwin during his journey on board HMS Beagle. The fossil mollusks collected there were studied later by Sowerby, who described four species based on the material from "Port Desire". Sedimentological and stratigraphical observations suggest that the marine rocks cropping out at Darwin's locality should be assigned to the Monte León Formation (early Miocene). The rocks were deposited in subtidal environments, most precisely in an offshore environment near fair weather wave base at the bottom to a lower shoreface at the top of the section. Keywords: Darwin, Early Miocene, Monte León Formation, Sedimentology, Paleontology.

RESUMEN: Sedimentología y paleontología de una sucesión marina miocena citada por primera vez por Darwin en Puerto Deseado. Las rocas ex- puestas al sur de Puerto Deseado, provincia de Santa Cruz, fueron relevadas por Darwin durante su viaje a bordo del HMS Beagle. Los moluscos fósiles que coleccionó fueron estudiados por Sowerby quien describió cuatro especies basadas en ejemplares procedentes de Puerto Deseado. Las observaciones sedimentológicas y estratigráficas sugieren que estos depósitos marinos, estudiados originalmente por Darwin, son asignables a la Formación Monte León (Mioceno temprano). En general, las rocas representan ambientes submareales que en el tramo inferior de la sucesión corresponden a un costa afuera, próximo al nivel de base de olas de buen tiempo, mientras que hacia el techo pasan a un shoreface inferior.

Palabras clave: Darwin, Mioceno temprano, Formación Monte León, Sedimentología, Paleontología.

INTRODUCTION coast, as he included all these exposed Paraná and Uruguay, in what he called rocks in his Tertiare Patagonien. the Great Patagonian Formation. Al- The Cenozoic marine deposits of Pata- A few years later, HMS Beagle sailed though this appears to be roughly equiva- gonia have been known since the early along the coast of Patagonia, and on this lent to d'Orbigny's Tertiaire Patagonien, 19th century and have been studied ever occasion the geology of the region was Darwin was aware that this succession since d'Orbigny (1842) first described his studied by Charles Darwin, who publis- included two distinct units, the lower one Tertiaire Patagonien. This author also des- hed his observations (Darwin 1846) to- marine, and the second one, continental cribed the fossils that he collected du- gether with descriptions of the fossil taxa with land vertebrate remains. Nowadays, ring his explorations in southern South he collected during this trip (Sowerby the four localities visited by Darwin America. All of the specimens he descri- 1846). In addition to the localities already include different formal units, deposited bed came from the marine rocks exposed mentioned by d'Orbigny, Darwin visited in different basins and environments. For in northern Patagonia - in the vicinity of four other places that became classic fos- example, the marine deposits at San José the mouth of the Río Negro, and in the siliferous localities over the years, and locality belong to the Puerto Madryn area surrounding the present city of that were crucial in establishing the basis Formation (late Miocene). The marine Paraná, in northeastern Argentina. Li- of Patagonian Cenozoic stratigraphy. rocks and fossils mentioned by Darwin at kewise, he did mention the presence of a These four localities are San José, Puerto San Julián belong either to the late large oyster similar to Crassostrea patagoni- Deseado, Puerto San Julián, and Santa Oligocene San Julián Formation (Bertels ca (d'Orbigny 1842) in Puerto San Julián, Cruz. San José lies on the northern coast 1970), or to the early Miocene Monte which had been given to him by a sailor. of Península Valdés in Chubut (northern León Formation (Bertels 1970). At the However, he never actually visited the Patagonia). The other three localities lie mouth of the Río Santa Cruz, the rocks coast further south of Río Negro. Du- much further south along the coast of described by Darwin belong to the ring his journey to Chile, Bolivia and the present province of Santa Cruz. Monte León Formation. Peru around Cape Horn, he must have Darwin included all the rocks with mari- Darwin's observations and fossil collec- seen from afar the cliffs rising along the ne fossils from these localities, and from tions drew the interest of later explorers, 84 S. CASADÍO AND M. GRIFFIN

geologists and paleontologists alike, especially during the second half of the 19th century and early decades of the 20th century. During this period, Ameghino (1896, 1898, 1900-1902) established the earliest stratigraphic subdivision of the Cenozoic rocks of Patagonia. This subdivision was mostly based on the fossil content of these richly fossiliferous beds. However, since Ameghino's subdivision, there has been a lot of controversies, not only because of the age of the rocks, but also because of the stratigraphic relationships and correlations of several outcrops that lie spread over a huge area in Patagonia, and that were attributed to the "Patagonian beds". Most of the rocks exposed there are now included in different formal units, although the exact relationships among them are still partially unresolved (Parras et al. 2008). Detailed history of these controversies can be found in Camacho (1974, 1979), Zins- Figure 1: Location map, showing Darwin's locality at Port Desire. meister (1981), Legarreta and Uliana (1994), and Malumián (1999, 2002). Ma- furthest upstream point reached by him, and in the Chenque Formation (in the ny of the rich faunas contained in the marine Cenozoic rocks rich in fossils be- San Jorge Basin). These rocks were depo- marine and continental Cenozoic rocks come exposed. sited during Atlantic transgressions that are crucial in the reconstruction of the These rocks remained unknown to Dar- covered large portions of Patagonia. evolutionary history of the South Ame- win. Nevertheless, among the fossils co- The Monte León Formation was for- rican biota. Therefore, a clear understan- llected by him and described by Sowerby mally described by Bertels (1970), and it ding of the stratigraphic relationships - (1846), there are several that come from includes those deposits at the mouth of especially for those type localities of taxa "Port Desire". Darwin mentioned as the the Río Santa Cruz, which are characteri- that are used with biostratigraphic and/ fossiliferous locality, the cliffs lying a few zed by their abundant and well preserved or paleobiogeographic purposes - is pre- miles south of the river mouth. This lo- fauna. These rocks and faunas were ex- requisite to any further inference invol- cality comprises two very small outcrops tensively studied throughout the years ving these rocks and the fossils that they that ever since remained almost unexplo- (Ihering 1897, 1899, 1907, 1914, Ort- contained. red by geologists or paleontologists, as mann 1902, Feruglio 1949, Di Paola and During its voyage around the world, the they are in a relatively inaccessible area Marchese 1973, Bertels 1980, Náñez HMS Beagle anchored in the Río Desea- and are not visible from the town. Four 1988, del Río 2004, del Río and Camacho do mouth. There, Darwin had the op- of the species of mollusks described by 1998, Crawford et al. 2008, and referen- portunity to explore the riverbanks ups- Sowerby come from this locality. Despite ces therein), but most of these studies tream and to provide detailed descrip- the fact that it is a type locality of taxa were carried out on the exposures found tions of the rocks he encountered. The later used in biostratigraphic and paleo- around the mouth of the Río Santa Cruz northern bank of the river - where the biogeograpic reconstructions, it has re- and in the San Julián area. In contrast, modern town of Puerto Deseado (Fig. 1) mained obscure and little known ever outcrops with similar rocks in the Puerto lies and along where the road and railro- since Darwin discovered it. Deseado area received far less attention ad run - is mainly composed of Jurassic The late Oligocene - early Miocene mari- (Frenguelli 1931, Feruglio 1949, del Río volcanic rocks for at least 30 km upstre- ne beds exposed along the Atlantic coast 2004, Giacosa et al. 1998). In this area, am. These rocks were well recorded by of Patagonia are presently included in the Neogene marine rocks overlie Juras- Darwin, who was aware of their lack of the San Julián (late Oligocene - early sic volcanic rocks of the Bahía Laura fossils. However, a few kilometers fur- Miocene) and Monte León (early Mio- Group, and are reduced to patchy thin ther upstream of "Darwin's Rock", the cene) formations (in the Austral Basin), exposures isolated one another. Giacosa Sedimentology and paleontology of a Miocene marine succession ... 85

et al. (1998) used the informal name "Patagoniano" to designate these outcrops and suggested that these were equivalent to the San Julián and Monte León formations. However, del Río (2004) included them in the Chenque Formation, a unit formally described for rocks exposed about 150 km to the northwest, in the vicinity of Comodoro Rivadavia city. Sedimentological and stratigraphical observations suggest that the Neogene marine rocks at Darwin's locality near Puerto Deseado should be assigned to the Monte León Formation. In this study we present a detailed description of these rocks and comments on its paleontological content. Institutional abbreviations used in this work are as follows: NHM-L (The Natu- ral History Museum, London, Depart- ment of Palaeontology); GHUNLPam (Colección Paleontológica del Departa- mento de Ciencias Naturales de la Uni- versidad Nacional de La Pampa).

GEOLOGICAL SETTING

South of Puerto Deseado, the Monte León Formation (early Miocene) is exposed between Punta Guanacos and Punta Norte. The lowermost portion of the section is exposed during low tide along the coast, with the remainder of the formation outcropping along the base of cliffs. At the studied locality (47º48'37''S; 65º52'32''W, see Fig. 1), the Monte León Formation is 10 m thick (Fig. 2) and unconformably overlies Jurassic volcanic Figure 2: Stratigraphic section at rocks of the Bahía Laura Group. The Darwin's locality. contact between the two units is only visible during low tide. The Monte León 3b). At the top of these concentrations isp.). Large burrow systems, with hori- Formation is unconformably overlain by there are specimens - mainly articulated - zontal and vertical components are fre- Quaternary conglomerates (Fig. 3a). of Crassostrea? hatcheri (Fig. 3c). Speci- quent in this facies association. These mens of Crassostrea? hatcheri in life posi- excavations are 5 cm in average diameter, Facies associations (FA) tion are also abundant. They are mainly and are assigned to Thalassinoi-des isp. The obvious du-ring low tide on the wide fill of these structures consists of alter- FA1: This association includes fine bio- abrasion platform, where some speci- nating snady and muddy laminae (Fig. turbated sandstone and mudstone, inter- mens are found forming clumps. 3d). calated with lenticular shell concentra- Commonly, the valves of this oyster are tions (less than one meter wide and 0.4 m bioeroded by sponge (Entobia isp.), poly- Interpretation thick) with trough-cross stratification, chaetes (Maeandropoly-dora isp.) and FA1 represents an environment lying dominated by "Turritella" patagonica (Fig. Pholadidea patagonica (Gastro-chaenolites above storm wave base and below nor- 86 S. CASADÍO AND M. GRIFFIN

Figure 3: a) Puerto Deseado section. The Monte León Formation is unconformably overlain by Quaternary conglomerates; b) Lenticular shell concentrations dominated by "Turritella" patagonica; c) Specimens - mostly articulated - of Crassostrea? hatcheri; d) Thalassinoides isp. Filled with alternating sandy and muddy laminae. mal wave base. The alternating sandy and very shallow infaunal group - suggests grained massive tuffaceous sandstones. muddy laminae filling the Thalassinoides that the surface sediments were most Bioturbation completely obliterated the burrows probably indicate tidal cyclicity. likely poorly consolidated. The concen- sedimentary structures. There are large The abundance of Thalassinoides isp. sug- trations of "Turritella" patagonica may re- burrow systems mainly with vertical gests that the benthos was well-oxygena- present the infilling of small tidal chan- components, with an average diameter of ted during deposition (Bottjer et al. 1986, nels. The beds with articulated and highly 5 cm, and more than 1.5 m long, bran- Bromley 1990) and the substrate was bored oyster clusters point towards ching points not observed, precluding probably firm. However, the abundance periods of lower sedimentation rates. ichnoespecific identification and therefo- of "Turritella" patagonica - member of a FA2: comprises mainly fine- to medium- re referred to Thalassinoides isp. The fill of Sedimentology and paleontology of a Miocene marine succession ... 87

these burrows is mainly composed by offshore environment near the fair wea- tagonica, Antimelatoma quemadensis, and sand and abundant fragments of "Turri- ther wave base at the bottom, to a lower Dentalium sp. All these taxa are also found tella" patagonica. Most of the fossils are shoreface at the top of the section. in the Punta Entrada Member of the disarticulate and the degree of fragmen- Monte León Formation, at the mouth of tation is low. In addition to semi-infaunal PALEONTOLOGY the Río Santa Cruz, the only difference is and epifaunal taxa such as Atrina magella- the relative abundance of taxa in the two nica, Neopanis quadrisulcata, Swiftopecten no- The most conspicuous feature of the areas. They are far less numerous at dosoplicatus, Zygochlamys geminata, Zygo- rocks described here is the rich and well Puerto Deseado, but this can be due to chlamys quemadensis, Reticulochlamys proxima, preserved fauna of mollusks contained the fact that the exposures are also very Jorgechlamys centralis among others, speci- in them. This fauna lies almost entirely much reduced there. Darwin's specimens mens of the deep burrowing bivalve Pa- within FA 2 and amongst the most abun- were described by Sowerby and four of nopea quemadensis and shallow infaunal Cu- dant elements are Nucula (Lamellinucula) the species described by him were based cullaea alta and Lahillia patagonica were reticularis, Neilo ornata, Cucullaea alta, Li- on material from "Port Desire". Three of found articulated and in life position. mopsis insolita, Glycymeris cuevensis, Neopanis these were bivalves and one is a gastro- quadrisulcata, Swiftopecten nodosoplicatus, Zy- pod. The latter is the most conspicuous Interpretation gochlamys geminata, Zygochlamys quemadensis, fossil at both localities: FA2 represents a lower shoreface. Bio- Reticulochlamys proxima, Jorgechlamys centra- Neilo ornata (Sowerby, 1846) (Fig. 4a). It turbation was reported for mixed sedi- lis, Crassostrea? hatcheri, Pteromyrtea crucialis, is not strange that Sowerby had only ments to a depth of as much as 1 m in Pleuromeris cruzensis, Fasciculicardia patagoni- "fragments" (see Griffin and Nielsen shallow marine environments (Smith and ca, Spissatella lyelli, Spissatella kokeni, Hede- 2008), as most of the specimens of this Nelson 2003) resulting in a homogeni- cardium? puelchum, Cardium patagonicum, La- species, although complete in the rock, zed, poorly-sorted layer (Fürsich and hillia patagonica, Dosinia laeviuscula, Ameghi- are very easily broken when collected, Pandey 2003). Mollusks suggest that nomya darwini, Panopea quemadensis, Valdesia possibly as a result of the shell structure background sedimentation rate was low. dalli, Solariella dautzenbergi, Calliostoma san- in this group. The species is quite com- Most of the taxa found in FA2 represent tacruzense, "Turritella" ambulacrum, "Turri- mon in Puerto Deseado, but it also oc- either very shallow infaunal (e.g. arcoids, tella" patagonica, Polinices santacruzensis, curs in the Santa Cruz area (sensu Darwin, cardiids) or epifaunal forms (e.g. pecti- Glossaulax vidali, Cirsotrema rugulosum, Tro- i.e., just upstream from the mouth of the nids, muricids), indicating that the subs- phon santacruzensis, Xymenella dautzenbergi, river in the Atlantic Ocean), where it is trate was stable for extended periods. Xymene cossmanni, Enthacanthus monoceros, generally found at the base of the sec- In general, the rocks exposed at Darwin's Crassilabrum hatcheri, cf. Ocenebra iheringi, tion. Other species of Neilo are known locality represent subtidal facies from an Urosalpinx archipatagonica, Neoimbricaria pa- from Paleogene and Neogene rocks ex-

Figure 4: a) Neilo ornata (Sowerby, 1846). Lectotype, NHM-L 27974; b) Cucuallaea alta Sowerby, 1846, GHUNLPam 16859, external view; c) Jorgechalmys centralis (Sowerby, 1846), GHUNLPam 16860, left valve; d) "Turritella" patagonica Sowerby, 1846, GHUNLPam 16861. 88 S. CASADÍO AND M. GRIFFIN

posed in different areas of Patagonia and beit in none of these it reaches the abun- sédimentaires de Patagonie. Anales de la So- Tierra del Fuego, but the relationships dance showed in the Monte León For- ciedad Científica Argentina 50: 109-130, 145- among them remain rather obscure, as in mation. 165, 209-229 (1900); 51: 20-39, 65-91 (1901); most cases the material consists of frag- 52: 189-197, 244-250 (1901); 54: 161-180, 220 mentary molds. CONCLUSIONS -249, 283-342 (1902). Cucullaea alta Sowerby, 1846 (Fig. 4b). Bertels, A. 1970. Sobre el "Piso Patagoniano" y la Also common at Monte León, this spe- South of Puerto Deseado the rocks ex- representación de la época del Oligoceno en cies appears sparsely in FA2, and is less posed along the cliffs show a lithological Patagonia austral, República Argentina. Re- abundant than at the localities further composition that is not distinguishable vista de la Asociación Geológica Argentina south. Darwin's original material - i.e., from those of the Monte León For- 25: 495-501. Sowerby´s syntypes - included three spe- mation at its type locality further south. Bertels, A. 1980. Estratigrafía y foraminíferos cimens, one of them a juvenile from The succession exposed in the Puerto (Protozoa) bentónicos de la Formación Mon- "Santa Cruz", and the other two (pos- Deseado area contains subtidal facies te León (Oligoceno) en su área tipo, provincia sibly the two separate valves of the same from an offshore environment near fair de Santa Cruz, República Argentina. 2º Con- specimen) are from "Port Desire". One weather wave base at the bottom to a greso Argentino de Paleontología y Bioestra- of the latter is a large well preserved spe- lower shoreface at the top of the section. tigrafía y 1º Congreso Latinoamericano de cimen, similar to those found by us at The type specimens of Neilo ornata, Cu- Paleontología (Buenos Aires, 1978), Actas 2: this locality. One of Darwin`s specimens cullaea alta, Jorgechlamys centralis and "Turri- 213-273. from Puerto Deseado is an adult, but the tella" patagonica described by Sowerby Bottjer, D.J., Arthur, M.A., Deanm, W.E., Hattin, two are fairly well preserved. This species (1846) come - in all cases - from the cliffs D.E. and Savrda, C.E. 1986. Rhythmic bed- also appears at other Patagonian locali- south of Puerto Deseado. The mollusks ding produced in Cretaceous pelagic carbona- ties in which rocks of similar age are collected by us from FA2 are also found te envirnments: sensitive recorders of clima- exposed, although it is never quite as in the Punta Entrada Member of the tic cycles. Paleoceanography 1: 467-481. abundant as it is at the exposures at the Monte León Formation at the mouth of Bromley, R.G. 1990. Trace Fossils. Unwin Hy- mouth of the Río Santa Cruz and envi- the Río Santa Cruz suggesting an early man, 280 p., London. rons. Miocene age for Darwin's Port Desire Camacho, H.H. 1974. Bioestratigrafía de las for- Jorgechlamys centralis (Sowerby, 1846) locality. maciones marinas del Eoceno y Oligoceno de (Fig. 4c). This large pectinid is known la Patagonia. Anales de la Academia de Cien- from rocks belonging to the Monte León ACKNOWLEDGMENTS cias Exactas, Físicas y Naturales 26: 39-57, Formation, or to its equivalents such as Buenos Aires. the Centinela and Chenque Formations. Prefectura Naval Argentina (Puerto De- Camacho, H.H. 1979. Significados y usos de "Pa- The type material comes from "Santa seado) was instrumental in carrying out tagoniano", "Patagoniense", "Formación Pa- Cruz" and "Port Desire", but lectotype field work at Darwin's locality. The help tagonica", "Formación Patagonia" y otros tér- designation by Griffin and Nielsen of Miguel Santillán is also acknowledged. minos de la estratigrafía del Terciario marino (2008) has restricted the type locality to J. Todd (The Natural History Museum, argentino. Revista de la Asociación Geológica Puerto Deseado. It is an important spe- London) provided photographs of Sow- Argentina 34: 235-242. cies as it has been used in biotratigraphic erby's type material. The manuscript Crawford, R.S., Casadío, S., Feldmann, R.M., zonations of Patagonian Cenozoic units benefited from thorough reviews by No- Griffin, M., Parras, A. and Schweitzer, C. (del Río 2004). elia Carmona, Sven Nielsen and Guido 2008. Mass mortality of fossil decapods with- "Turritella" patagonica Sowerby, 1846 Pastorino. Financial support of CONI- in the Monte León Formation (Early Mio- (Fig. 4d). This species is the most com- CET and Universidad Nacional de La cene), Southern Argentina: Victims of An- mon mollusk in the Monte León Pampa are also duly acknowledged. dean volcanism. Annals of the Carnegie Mu- Formation. It appears in great numbers seum of Natural History 77: 259-287. and constitutes a significant part of the WORK CITED IN THE TEXT Darwin, Ch. 1846. Geological observations on deposits in which it occurs. The exact South America. Being the third part of the generic placement of this species re- Ameghino, F. 1896. Notas sobre cuestiones de geology of the voyage of the Beagle, under mains as yet uncertain, as pointed out by geología y paleontología argentina. Boletín del the command of Capt. Fitzroy, R. N. during Griffin and Nielsen (2008). This taxon Instituto Geográfico Argentino 17: 87-119. the years 1832 to 1836. Smith, Elder and Co., occurs in many other localities where Ameghino, F. 1898. Sinopsis Geológico-Paleon- 279 p., London. marine rocks of this age are exposed. It tológica. Segundo Censo de la República Ar- del Río, C.J. 2004. Tertiary marine molluscan is very common in the Chenque, Cen- gentina 1: 111-225. assemblages of eastern Patagonia (Argenti- tinela, and Carmen Silva formations, al- Ameghino, F. 1900-1902. L'age des formations na): a biostratigraphic analysis. Journal of Pa- Sedimentology and paleontology of a Miocene marine succession ... 89

leontology 78: 1097-1122. Tertiaire et du Crétacé superieur de l'Argen- lation of marine beds based on Sr- and Ar- del Río, C.J. and Camacho, H.H. 1998. Tertiary tine. Anales del Museo Nacional de Buenos date determinations and faunal affinities nuculoids and arcoids of eastern Patagonia Aires, serie 3: 1-611. across the Paleogene/Neogene boundary in (Argentina). Palaeontographica Abt. A 250: Ihering von, H., 1914. Catalogo de molluscos southern Patagonia, Argentina. Journal of 47-88. Cretáceos e terciarios da collecçao do auctor. South American Earth Sciences 26: 204-216. Di Paola, E.C. and Marchese, H.G. 1973. Litoes- Notas do Museu Paulista 1: 1-113. Smith, A.M. and Nelson, C.S. 2003. Effects of tratigrafía de la Formación Patagonia en el Legarreta, L. and Uliana, M.A. 1994. Asociacio- early sea-floor processes on the taphonomy área tipo (Bajo de San Julián-desembocadura nes de fósiles y hiatos en el Supracretácico- of shelf skeletal carbonate deposits. Earth del río Santa Cruz). Provincia de Santa Cruz. Neógeno de Patagonia: una perspectiva estra- Science Reviews 63:1-31. República Argentina. 5º Congreso Geológico tigráfico-secuencial. Ameghiniana 31: 257-281. Sowerby, G.B. 1846. Descriptions of the Tertiary Argentino, Actas 3: 207-222. Malumián, N. 1999. La sedimentación y el volca- fossil shells from South America. In: Darwin, Feruglio, E. 1949. Descripción Geológica de La nismo terciarios en la Patagonia extraandina. Ch. Geological observations on South Ame- Patagonia. Ministerio de Industria y Comercio In Caminos, R. (ed.) Geología Argentina, rica. Being the third part of the geology of de La Nación, Dirección General de Yaci- Anales del Instituto de Geología y Recursos the voyage of the Beagle, under the command mientos Petrolíferos Fiscales 2: 1-349. Minerales 29(18): 557-612. of Capt. Fitzroy, R. N. during the years 1832 Frenguelli, J. 1931. Observaciones estratigráficas Malumián, N. 2002. El Terciario marino. Sus rela- to 1836. Smith, Elder and Co., Appendix: en Bahía Sanguinetti (Santa Cruz, Patagonia). ciones con el eustatismo. In Haller, M.J. (ed.) 249-264, London. Anales de la Sociedad Científica de Santa Fe 3: Geología y Recursos Naturales de Santa Cruz, Zinsmeister, W.J. 1981. Middle to late Eocene 237-283. Relatorio 15º Congreso Geológico Argentino invertebrate fauna from the San Julián For- Fürsich, F.T. and Pandey, D.K. 2003. Sequence I-15: 237-244. mation at Punta Casamayor, Santa Cruz pro- stratigraphic significance of sedimentary cy- Náñez, C. 1988. Foraminíferos y bioestratigrafía vince, Southern Argentina. Journal of Paleon- cles and shell concentrations in the Upper del Terciario Medio de Santa Cruz oriental. tology 55: 1083-1102. Jurassic and Lower Cretaceous of Kachchh, Revista de la Asociación Geológica Argentina western India. Palaeogeography, Palaeoclima- 43: 493-517. tology, Palaeoecology 193:285-309. Orbigny, A. d'. 1834-1847. Voyage dans l'Améri- Giacosa, R.E., Césari, O. and Genini, A. 1998. que méridionale (le Brésil, la République Hoja Geológica 4766-II y IV Puerto Deseado. orientale de l'Uruguay, la République Argen- Servicio Geológico Minero Argentino, Bole- tine, la Patagonie, la République du Chili, la tín 240, 72 p., Buenos Aires. République de Bolivia, la République du Pé- Griffin, M. and Nielsen, S.N. 2008. A revision of rou), exécuté pendant les années 1826, 1827, the type specimens of Tertiary molluscs from 1828, 1829, 1830, 1831, 1832 et 1833, 5 (3): Chile and Argentina described by d'Orbigny Mollusques: i-xliii, 1-758, láms. 1-85, vol. 3, (1842), Soweby (1846) and Hupé (1854). Jour- part. 3, 290 p., P. Bertrand. Paris, V. Levrault, nal of Systematic Palaeontology 6: 251-316. Strasbourg, Ihering von, H. 1897. Os Molluscos dos terrenos Ortmann, A.E. 1902. Tertiary Invertebrates. In terciarios da Patagonia. Revista del Museo Scott, W.B. (ed.) Reports of the Princeton Paulista 2: 217-382. University Expedition to Patagonia 1896- Ihering von, H. 1899. Die Conchylien der pataog- 1899, J. Pierpoint Morgan Publishing Founda- nischen Formation. Neues Jahrbuch für Mi- tion, volume 4, Paleontology I, Part 2: 45-332, neralogie, Geologie und Paläontologie 1899: Princeton. 1-46. Parras, A., Griffin, M., Feldmann, R., Casadío, S., Recibido: 14 de octubre de 2008 Ihering von, H. 1907. Les mollusques fossiles du Schweitzer, C. and Marenssi, S. 2008. Corre- Aceptado: 27 de noviembre de 2008 90 Revista de la Asociación Geológica Argentina 64 (1): 90 - 100 (2009)

CHARLES DARWIN AND THE FIRST SCIENTIFIC OBSERVATIONS ON THE PATAGONIAN SHINGLE FORMATION (RODADOS PATAGÓNICOS)

Oscar A. MARTÍNEZ1, Jorge RABASSA2,3, Andrea CORONATO2,3

1Universidad Nacional de la Patagonia-San Juan Bosco, Sede Esquel, Esquel, Chubut. Email: [email protected] 2CADIC, CONICET, C.C.92, 9410 Ushuaia, Tierra del Fuego. 3Universidad Nacional de la Patagonia-San Juan Bosco, Sede Ushuaia, Tierra del Fuego.

ABSTRACT The Rodados Patagónicos is one of the most intriguing lithostratigraphic units in the Late Cenozoic of Patagonia. Charles Darwin named these gravels as the "Patagonian Shingle Formation", when he discovered them during his trip to Patagonia on board HMS Beagle in 1832. According to the prevailing paradigm of the time, he assigned these deposits to a giant transgression during the Great Universal Déluge epoch, considering that their formation was related to wave action along the beach in ancient times. The name of Rodados Patagónicos, as they are generally known in the Argentine geological literature, is usually confusing since it has been applied to a wide number of geological units of multiple origin and age. Many authors have discussed the nature and origin of these gravels, considering them to have been formed by piedmont, alluvial, colluvial, glaciofluvial, and/or marine processes. Today, it is accepted that the term Rodados Patagónicos includes gravel deposits of varied nature and age, perhaps with a prevalence of piedmont genesis in northern Patagonia and glaciofluvial dynamics in southern Patagonia and Tierra del Fuego. Keywords: Charles Darwin, Patagonian Shingle Formation, Late Cenozoic, Patagonia, Tierra del Fuego.

RESUMEN: Charles Darwin y las primeras observaciones científicas sobre los Rodados Patagónicos. Los Rodados Patagónicos son algunas de las unidades litoestratigráficas más sorprendentes del Cenozoico tardío de Patagonia. Charles Darwin dio a estas gravas el nombre de Patagonian Shingle Formation, cuando las descubrió durante su viaje a Patagonia en el HMS Beagle en 1832. De acuerdo con los paradigmas dominantes de la época, asignó estos depósitos a una transgresión gigantesca durante el "Gran Diluvio Universal", considerando que su formación estaba relacionada a la acción del oleaje a lo largo de la playa en tiempos antiguos. El nombre de Rodados Patagónicos, como generalmente se los conoce en la literatura geológica argentina, es usualmente confu- so, ya que ha sido aplicado a un amplio número de unidades geológicas, de múltiple origen y edad. Muchos autores han dis- cutido la naturaleza y génesis de estas gravas, considerándolas como formadas por procesos diversos, ya sea pedemontanos, aluviales, coluviales, glaciofluviales, y/o marinos. En la actualidad, se acepta que el término Rodados Patagónicos incluye a depó- sitos de grava de naturaleza y edad muy variadas, quizás con una predominancia de aquellos de génesis pedemontana en Patagonia septentrional y debidos a la dinámica glaciofluvial en Patagonia austral y Tierra del Fuego.

Palabras clave: Charles Darwin, Rodados Patagónicos, Cenozoico tardío, Patagonia, Tierra del Fuego.

INTRODUCTION wledge that it can be considered an au- essentially, a succession of universal ca- thentic scientific revolution. However, taclysm that had dramatically modeled The outstanding work of Charles Dar- the strengthening of this new paradigm the surface of the Earth, generating mass win in the biological sciences has conce- on the origin and evolution of living extinctions and the rise of new species aled his significant contributions to geo- beings was accompanied, and mostly different to the previously existing ones. logy and other earth sciences. Perhaps complemented, by the formulation of Darwin's work, basically of a gradualist because of this reason, the great influen- new approaches to the great geological nature, fired catastrophism the final blow. ce of his findings in South American dilemmas of the times. By then, the para- Almost at the same time that Darwin was earth sciences is seldom appraised in the mount work of Charles Lyell (1830- traveling on board HMS Beagle, the last literature beyond his biological theories. 1833) represented the gradualist princi- steps towards the presentation of the The publication of The Origin of Species ples within the geological sciences, which Glacial Theory were being fulfilled in cen- (Darwin 1859) was the onset of a period appeared as a reaction and antipode posi- tral Europe (Louis Agassiz, in 1837; in which there were so many radical tion against the catastrophist theories, Agassiz 1840, in Imbrie and Imbrie changes in the structure of western kno- that postulated that natural history was, 1979), which would deeply modify the Charles Darwin and the first scientific observations 91

ideas about the origin and evolution of portunity in which Charles Darwin iden- ne (submarine) process interpretation. the landscape in the northern hemisphe- tified gravel deposits that are today Firstly, the great widening of the Río re and, as it happened later on, on the known as Rodados Patagónicos was in 1833, Santa Cruz valley nearby its sources at understanding of the global climate during his expedition to the surroun- Lago Argentino (Fig. 1), more than 300 system. This new theory did not adjust to dings of the present city of Bahía Blanca, km from the Atlantic coast, was wrongly Bible principles that underpinned the southern Buenos Aires province (Fig. 1). interpreted as an ancient estuary. This Great Universal Déluge as the main cause of There, he observed a layer that was less large landform is today known to have most of the present landscape features, than a meter thick, composed of small been generated due to recurrent Pleis- and strongly supported by the aforemen- pebbles, essentially porphyritic rocks, tocene glaciation and the action of gla- tioned catastrophist conception. The that were lying on top of the "Pampean ciofluvial streams (Mercer 1976, Clap- first volume of Lyell's Principles of Geo- beds" and that were the base over which perton 1993, Schellmann 1998, among logy was published in 1830, only one year the frequent large dunes in the area are others). It is also accepted today that the before Darwin set out on his 5-year voya- deposited (Darwin 1846). Starting here, building up of the extensive, step-like ge to the Southern Hemisphere. This vo- and later during different landings as they terraces and/or tablelands of the region lume, and the second one that he recei- sailed southwards, such as San Antonio, is also due to the same glaciofluvial proved when HMS Beagle was in Buenos the mouth of Río Chubut, Puerto De- cesses. However, Darwin linked these Aires in 1832, became the conceptual seado, San Jorge Gulf and the mouth of landforms to the impact of Atlantic platform from which Darwin made his Río Santa Cruz (Fig. 1), Darwin descri- Ocean transgressions that reached loca- observations and formulated his princi- bed the outcrops at the scarps of table- tions very close to the Andean Cordillera. pal hypotheses on the geological sciences lands and terraces stretching along the Besides, he also considered that it was in general and of South American geo- sea. At the same time, he began working very likely that Patagonia would have logy in particular. on the hypothesis that these gravels were been crossed by many sea passages in the It is frequently believed that Darwin's the product of alluvial accumulation at past, similar to the present Magellan main contributions to earth sciences are the foot of the Andean Cordillera and Straits (Fig. 1), which connected both his works on plutonic and metamorphic later spread out by wave action during a oceans. It should be considered that rocks and his ideas on the origin of the marine transgression. He verified the vast almost simultaneously with Darwin's pio- volcanic islands and reef barriers. In this continuity of these gravel beds he named neer scientific observations in Patagonia article, we want to emphasize the as the "Gravel Formation" or "Patagonian (1833-1834), the ideas that led Louis thoughts dedicated by this great scientist Shingle Formation", concluding that they Agassiz to postulate his Glacial Theory in to one of the more interesting and intri- represented one of the main physical fea- 1837, were growing steadily. For an guing geological units, not only of those tures of this region. The term "shingle" ample discussion of this epistemological days but even today, as are the so called referred to the gravels which are the re- process, see Imbrie and Imbrie (1979). Rodados Patagónicos. For a review of Dar- sult of wave action on the cliffs along Concerning Darwin's geological back- win's work as a Quaternary geologist and many sectors of the British coasts (Fig. 2). ground, it was probably not conceivable as a glaciologist see Rabassa (1995). To Darwin's eyes, the vast expanse of the that glaciers would have had in the recent The discussion of Darwin's process of Patagonian gravel beds was awesome and geological past a larger extent that today. identification, description and interpreta- astounding, in comparison to what he Even less conceivable was that areas tion of the Rodados Patagónicos, which are had seen in Europe before, to a point which are ice free today and very far ubiquitous over most of the surface of that he considered that these units were from the glacier boundaries could have Argentine Patagonia (Fig. 1), reveals once the largest ones of this kind in the entire been covered by large ice masses in the again his scientific talent and pioneer world. He assumed that a clear evidence past; a feature that has been shown was a activity in the area and also allows recog- of the marine origin (in fact, submarine distinctive characteristic of Patagonia nition of relevant aspects of the histori- for him) of these strata was the frequent (Caldenius 1932, Feruglio 1950, Clap- cal-scientific background in which such finding of Recent marine shells scattered perton 1993, Coronato et al. 2004, Ra- process occurred. on top and even within these terraces. bassa 2008). Darwin focused his geologi- Although several authors later discarded cal analysis accepting Lyell's statements CHARLES DARWIN AND this genetic interpretation, it was Feru- as a foundation, albeit in a critical man- THE "DISCOVERY" OF THE glio (1950) who confirmed that the shells ner, as Lyell was still supporting the hy- RODADOS PATAGÓNICOS had been accumulated by human action pothesis of a large flooding -a phenome- and they were actually archaeological non of Biblical roots- which had had a The voyage of the HMS Beagle took pla- sites. There are other elements that con- key role in the origin of many features of ce between 1831 and 1836. The first op- tributed to Darwin's choice of his mari- the Earth's surface. It is interesting to 92 O. A. MARTÍNEZ, J. RABASSA, A. CORONATO

Figure 1: Location map of Patagonia and Tierra del Fuego showing the areas covered by the Rodados Patagónicos (modified from Clapperton 1993). Charles Darwin and the first scientific observations 93

consider that in those years Lyell believed Figure 2: Beach gravel deposits that the erratic boulders, today accepted (shingle beach) in a classical locality along the English coast, reas essentially of strict glacial origin, had presenting the kind of deposits been transported through usually very that Darwin had seen before long distances by icebergs generated by starting his voyage on board HMS Beagle. Darwin's home such flooding, to be later abandoned on memories about these kind of land as the sea withdrew. This interpreta- deposits lead him to interpret tion, which Lyell abandoned a few years the gravel deposits that he later, was very influential on Darwin's found everywhere along the Patagonian coasts and rivers as intellectual work. The large boulders of "shingle formations", thus gi- foreign rocks (of Andean origin) that are ving support to the original, lying on or partially buried in the gravel historical name of these units. Reproduced with permission of beds along extra-Andean areas of sou- www.beenthere- donethat.org.uk, thernmost Patagonia (Darwin 1848), copyright © by Barry Samuels. were thus of marine origin for Darwin, becoming so another strong line of rea- as Rodados Tehuelches and assigned them a the tablelands and terraces of northwes- soning in favor of a similar or identical marine origin and a pre-Pliocene age. tern Patagonia represented alluvial baja- origin for the Shingle Formation. Hatcher (1897) also considered them of das built by fluvial streams coming from marine origin and attributed them to a the Andes, during the Pliocene and the THE SEDIMENTARY sea transgression that would have cove- Quaternary, in response to regional uplift MATERIALS THAT HAVE red all of Extra-Andean Patagonia du- events. Keidel was the first to note the BEEN NAMED AS ring the Pliocene, reinstating the Dar- unconformity between the gravels and RODADOS PATAGÓNICOS winian name of Shingle Formation. Nor- the underlying Late Tertiary marine and denskjöld (1897), who was strongly in- continental sedimentary rocks. Later, Previous works fluenced by the recently introduced Bonarelli and Nágera (1922) returned to Glacial Theory and his wide knowledge of the ideas about the marine origin of the After Darwin's early contributions there the glacial landscapes in Scandinavia and gravels and assumed that the so-called were several authors that documented northern Europe, correctly proposed a Rodados Tehuelches of the highest terraces the existence of these characteristic gra- glaciofluvial origin for the gravel deposits were at least of Pliocene age, which had vel and sand beds in Patagonia (Table 1). in southern Santa Cruz province and the been dispersed later by the action of Doering (1882) named them as Piso Magellan Straits, but he did not discuss marine waters pertaining to a transgres- Tehuelche and in a pioneer manner inter- the origin of similar units farther north. sion that reached the foothill of the preted them as of glaciofluvial origin in a Florentino Ameghino (1906) returned to Andes. These later were the source of moment in which the Glacial Theory was the topic from a regional perspective, the fluvial deposits of the lower terraces, well accepted by the scientific commu- insisting that it was not appropriate to carved after successive episodes of river nity. This author correlated them with assign a unique origin to all gravel depo- base drop. the lower section of the Pampean sedi- sits and that they could have a different Windhausen (1931) suggested that the ments, based on the occurrence of calca- genesis according to their location. higher beds were deposited in an alluvial reous duricrusts locally known as tosca, The first author to relate the Rodados Pa- manner over a rather flat relief with a and assigned them an Early Pliocene age. tagónicos to the development of the glacial very gentle slope, whereas the topogra- Carlos Ameghino (1890) was the first periods in the Patagonian Andes was Ro- phically lower, terraced gravels were the geologist to discard a single origin for vereto (1912), who recognized a link to consequence of glaciofluvial deposition these materials and he differentiated bet- four hypothetical glaciations according to in different stages of uplift that occurred ween the marine deposits forming the the Alpine scheme then in use. Accor- during the Quaternary. Based on the high terraces and the low terrace sedi- ding to him these glaciations were related ideas of Rovereto (1912), Frenguelli mentary beds, referring the first ones as to different marine terraces with a mo- (1931) distinguished the Tehuelchiano the Formación Araucanense, deposited in llusk fauna quite similar to the present beds, composed of three orders of mari- successive epochs since the Early Mio- one, as suggested by his studies along the ne terraces and other continental ones cene. Mercerat (1893) studied these accu- Atlantic coast. corresponding to the Post-Tehuelchiano, mulations in the southernmost part of Keidel (1917) disagreed with the hypo- formed by low terrace gravels, of post- Patagonia between the Río Santa Cruz theses of the previous workers, postula- glacial age. and the Magellan Straits. He named them ting that the gravels that cover much of Caldenius (1932, 1940) assigned a fluvial 94 O. A. MARTÍNEZ, J. RABASSA, A. CORONATO

TABLE 1: Summary table of the main contributions to the knowledge on the Rodados Patagónicos, since the first descriptions (Darwin 1848) up to the end of the 20th century. The great historical controversy -which still persists- may be identified, concerning the genesis and age of these units. Author Proposed denomination Geographical distribution Depositional environment Age Darwin (1846) "Gravel Formation" or Throughout Patagonia Fluvial, piedmont and marine Not specified "Shingle Formation" Moreno (1876) No specific new name given Throughout Patagonia Glacial and glaciofluvial Not specified Doering (1882) "Piso Tehuelche" Northern Patagonia Glaciofluvial Early Pliocene Ameghino (1890) "Formación Araucanense" Throughout Patagonia Marine Since Early Miocene Mercerat (1893) "Rodados Tehuelches" Southern Santa Cruz province Marine pre-Pliocene Hatcher (1897) "Shingle Formation" Throughout Patagonia Marine Pliocene Nordenskjöld (1897) No specific new name given Southern Santa Cruz province Glaciofluvial Quaternary glaciations Ameghino (1906) No specific new name given Throughout Patagonia Poligenetic Not specified Rovereto (1912) No specific new name given Northern Patagonia Glacial Quaternary glaciations Kiedel (1917, 1919) No specific new name given Northeastern Patagonia Fluvial, piedmont Not specified Bonarelli and Nágera (1922) No specific new name given Southern Patagonia Marine Not specified Windhausen (1931) No specific name given Throughout Patagonia Fluvial, piedmont and glaciofluvial Quaternary Frenguelli (1931) "Estrato Tehuelchiano" and Throughout Patagonia Marine and continental Post-glacial "Post-Tehuelchiano" Caldenius (1932, 1940) "Rodados Patagónicos", Throughout Patagonia Fluvial, glaciofluvial y solifluxión Pre-glacial and during "Patagonian Gravels" Quaternary glaciations Groeber (1936) "Rodados Patagónicos" Throughout Patagonia Fluvial, piedmont Not specified Feruglio (1949-1950) "Rodados Patagónicos" Central and southern Patagonia Glaciofluvial, fluvial Since the Pliocene Frenguelli ( 1957) "Rodados Patagónicos" Throughout Patagonia Glaciofluvial, fluvial Since the Pliocene Fidalgo and Riggi (1965, 1970) "Rodados Patagónicos" Northern and central Patagonia Fluvial, piedmont, glaciofluvial Pre-glacial and during Quaternary glaciations Mercer (1976) "Patagonian Gravels" Southern Patagonia Glaciofluvial Since the Miocene González Díaz and Malagnino "Rodados Patagónicos" Northern Patagonia Piedmont (the older ones), (1984) and Malagnino (1989) glaciofluvial (the younger ones) Quaternary glaciations Clapperton (1993) Lapido and "Rodados Patagónicos" Throughout Patagonia Fluvial, piedmont (northern Since the Miocene Pereyra (1999) Patagonia), glaciofluvial (central and southern Patagonia)

and glaciofluvial origin to the Rodados ted the mountain ice sheet of the Pa- plains that are found within the valleys or Tehuelches, originally deposited in the tagonian Andes since the Pliocene, and depressions around the mesetas and there- shape of piedmont glaciofluvial cones to a lesser extent, to phases of tectonic fore of younger age. According to Fi- and he suggested that these units had uplift. On these terraces Feruglio (1950) dalgo and Riggi (1965, 1970), all other undergone certain amount of reworking identified moraine deposits and glacio- deposits of more restricted extent as due to solifluction processes. Likewise, fluvial gravels of varied lithology, but those building up the flanking pediments he recognized the existence of higher mostly of eruptive rocks. Frenguelli should also be considered as Rodados level gravel beds and of an older age than (1957) agreed in general terms with Fe- Patagónicos, a proposal that Clapperton even the oldest glaciations, which he ruglio's (1950) interpretations. (1993) considered as of little value. named as Initioglacial. The first really rigorous systematic and The development of absolute dating and Groeber (1936) proposed a mixed allu- solid studies on the gravels were done by the consequent confirmation of the oc- vial and colluvial origin for these gravels. Fidalgo and Riggi (1965, 1970), who ba- currence of glaciations older than the Feruglio (1950) recognized the existing sed their interpretations upon geomor- Pleistocene in Santa Cruz province allo- relation among the fluvial terraces of the phological and sedimentological obser- wed Mercer (1976) to identify accumula- different fluvial systems of the southern- vations in the surroundings of Lago tions of glaciofluvial origin, referring most Patagonian meseta, in the valleys of Buenos Aires (Santa Cruz province; Fig. them to the Rodados Patagónicos, with an the Chubut, Deseado, Shehuen, Coyle, 1). In agreement with Caldenius (1932), age equivalent or even older than that of Santa Cruz and Gallegos rivers (Fig. 1). they classified these materials into two those of piedmont origin that had been The great dimensions of the terraces, the large groups: (a) those of fluvial and mentioned as the oldest by some authors. thickness of their alluvial mantles and piedmont origin (Rodados Patagónicos, sensu González Díaz and Malagnino (1984) the marked relief that separated them stricto), located at higher altitude and and Malagnino (1989) centered their ob- justified his interpretation linked to the covering the tablelands and pediments, servations in northern Patagonia and glacial and interglacial periods that affec- and (b) those that form the glaciofluvial they concurred in assigning a polygenetic Charles Darwin and the first scientific observations 95

character to the Rodados Patagónicos at and assigned them to the Cabo Vírgenes, generated at some time during the Late these latitudes, proposing an essentially Punta Delgada, Primera Angostura and Cenozoic. They may be forming diffe- glaciofluvial origin for the younger ones, Segunda Angostura glaciations, or the rent landforms or their relicts, such as and broadly a piedmont genesis, possibly Post-GGP I, II and III glaciations and inactive flood plains, alluvial terraces, associated to tectonic pulses for the older Last Glacial Maximum, according to alluvial fans, bajadas, pediment covers, ones. Clapperton (1993) and later Lapido Coronato et al. (2004) in the Magellan proglacial plains and structural plains and Pereyra (1999), reviving the essen- Straits, Skyring and Otway sounds ice covers (Fig. 5). Hence, the great diversity tials of Ameghino's (1906) hypothesis, lobes (Fig. 1). In high topographic posi- of the many variables that play a part in proposed classifying the deposits in (a) tions, Meglioli (1992) identified a thin the definition of these units (Table 2), i.e. those located in northern Patagonia, bet- gravel bed that is part of the Sierra de los (a) sedimentological / petrological (com- ween the Negro and Colorado rivers Frailes Drift, corresponding to the Great position, grain size, shape, selection, (Fig. 1), to which they assigned a domi- Patagonian Glaciation (GPG, according among other parameters), (b) spatial nantly piedmont origin and (b) the gra- to Coronato et al. 2004), whose age was (shape, elevation, slope, size, extent, vels of southern Patagonia, in the pro- established in ca. 1 Ma (Ton That et al. thickness of the beds), (c) chronological vinces of Chubut and Santa Cruz, which 1999, Rabassa 2008). Meglioli (1992) de- (tentatively between the Late Miocene were interpreted as of predominantly fined several units of rounded and and the Holocene) and (d) genetic (flu- glaciofluvial nature. During the second subrounded gravels of similar origin in vial, piedmont, glaciofluvial, periglacial, half of the 20th century the geological Tierra del Fuego and named them as among other possible environments). surveys of Extra-Andean Patagonia be- Rodados Fueguinos, thus recognizing that It is clear then that the concept of Ro- came more frequent and many authors this type of unit is also present in the dados Patagónicos is ample enough, and have proposed a series of lithostratigra- southernmost end of the continent. thus ambiguous, so as to hamper its use phic units corresponding to the Rodados Finally, the work of Panza (2002) provi- in a regional stratigraphic sense. How- Patagónicos. Among many others should ded an integrated view of the Cenozoic ever, it may have a useful practical appli- be mentioned the contributions of gravels within the province of Santa cation as a generic term in those cases - Volkheimer (1963, 1964, 1965 a and b, Cruz, whereas Martínez and Coronato not infrequent-, in which it would be 1973), Cortelezzi et al. (1965, 1968), Gon- (2008) extended this analysis to the rest impossible or unnecessary to establish zález (1971, 1978), Coira (1979), Fidalgo of Patagonia. the age and/or genesis of these gravel and Rabassa (1984), Page (1987), Cortés layers. As suggested by Lapido and Pe- (1987), González Díaz (1993a, b and c), CHARACTERIZATION reyra (1999) the lack of chronostratigra- Panza (1994a, 1994b), Panza and Irigo- OF THE RODADOS yen (1994) and more recently, Strelin et al. PATAGÓNICOS (1999), Caminos (2001), González Díaz and Tejedo (2002), Pereyra et al. (2002) The Rodados Patagónicos are accumulations and Leanza and Hugo (1997, 2005). of gravelly clasts (Figs. 3 and 4), cemen- Meglioli (1992) mapped as Patagonian ted or not, substantially rounded, with Gravels -without distinguishing about pebbles and cobbles as the dominant size their genesis- the plains located along the fractions, in a sandy or silty/clayish ma- southern margin of the Río Gallegos, the trix, of highly variable lithology, although Río Chico de Santa Cruz basin and seve- with a certain predominance of basic and ral basins in Tierra del Fuego Island (Fig. mesosilicic volcanics and acid plutonic 1). The slender relief of these gravelly rocks. They range between the Andean plains, undifferentiated from a genetic Cordillera and the Atlantic Ocean coast, point of view, is interrupted by the and from the northern flank of the Río Quaternary volcanic cones that form the Colorado valley to the island of Tierra Pali-Aike volcanic field. The glaciofluvial del Fuego (Fig. 1). They tend to form gravels from the Pleistocene glacial horizontal to subhorizontal mantles of advances are distributed according to the varied extension and thickness, which are moraine morphology, either in frontal or located in different topographical posi- marginal position. Although Meglioli tions, usually showing an east-west domi- (1992) did not present details of the loca- nant gradient, and the genesis of which Figure 3: A Rodados Patagónicos outcrop in the area of Lago Argentino, Río Santa Cruz valley, tion of each one of the glaciofluvial may be variable according to the conside- Province of Santa Cruz. Photograph by A. terraces, he defined their spatial setting red unit or geographical area. They were Coronato, 2006. 96 O. A. MARTÍNEZ, J. RABASSA, A. CORONATO

Figure 4: Outcrop of the Rodados Patagónicos in the tablelands of Central Chubut Province. Photograph by O.A. Martínez, 2007. phic studies and of absolute datings in Cenozoic tectonic and epeirogenic uplift tion of such extensive a region as Pa- the different Quaternary units of the re- (Strelin et al. 1999). Besides, it seems rele- tagonia (Lapido and Pereyra 1999). gion renders any predetermined time fra- vant to consider that major piedmont J.L. Panza (pers. comm., while acting as a mework and/or geographical location aggradation events should have followed reviewer of an earlier version of this pattern of these deposits only tentative and, in some cases, even coincided at the manuscript) did not agree with some of and incomplete. When the gravel mantles regional level with those of glaciofluvial our conclusions. He considered that are grouped more or less in a parallel nature, at least since the late Miocene most of, if not all, those deposits assig- manner with respect to the present drai- (Martínez and Coronato 2008). The ge- ned to the Rodados Patagónicos of ages nage networks, they might be genetically neral idea of advocating an older age for older than 1.2-1.0 Ma in the Province of related to fluvial valley processes. This the piedmont deposits in relation to Santa Cruz are not related to glaciofluvial possible genesis should be considered as those formed by glaciofluvial action proceses or genetically or timely associa- the result of both climatic fluctuations (Fidalgo and Riggi 1965, 1970) seems in- ted to the major Patagonian glaciations, (glacial and interglacial periods) and base convenient at least, considering the com- being much older than these. He unders- level modifications in response to Late plexity in the tectonic and climatic evolu- tands that there is no synchronism bet- Charles Darwin and the first scientific observations 97

Figure 5: Structural terraces and other landforms covered by the Rodados Patagónicos in southeastern Chubut Province, mapped on a Landsat satellite image (from Martínez and Coronato 2008). TABLE 2: Genetic classification and general sedimentological characterization of the Rodados Patagónicos (modified from Martínez and Coronato 2008). Genesis Areal Extent1 Latitudinal location Age distribution Lithology Slope of the original surface Elevation Fluvial valley processes Local, regional and Throughout Patagonia Since the Late Variable Highly variable, very gentle, Variable extra-regional Miocene according to local conditions Glaciofluvial processes Extra-regional and Throughout Patagonia, Glacial periods bet- Clasts of Andean Very gentle Variable, lowering regional predominant in southern ween Pliocene and volcanics and meta- with younger ages Patagonia Early Pleistocene morphic rocks Extra-regional With better development Miocene to Early Clasts of Andean Very gentle Variable, loweirng in Northern Patagonia Pleistocene volcanics and meta- with younger ages Piedmont processes morphic rocks Local, occasionally Throughout Patagonia Miocene to Early Variable, coming Moderate to strong Variable, lowering regional Pleistocene from local mountain with younger ages ranges "In situ"2, due to Local Cited in Central Patagonia Since the Pliocene Same as bedrock Horizontal to sub-horizontal Usually above the bedrock weathering present fluvial valleys 1 Three levels or cathegories are recognized: 1) those of local extent, when the depositis do not extend more than a few km from the source area, being this the mountain front (piedmont) or a moraine arc (glaciofluvial); 2) those of regional extent, considering for this purpose three main regions (or longitudinal stripes), a- western or Andean, b- central or Extra-Andean and c- eastern or coastal; 3) those of extra-regional extent, when the areal extent covers more than one region. 2 The authors of the present paper have considered appropriate to include this type corresponding to some gravel accumulations (González, 1978) in whose genesis superficial runoff would have not taken part of. 98 O. A. MARTÍNEZ, J. RABASSA, A. CORONATO

ween the main aggradational events and nent position in this discipline in Argen- WORKS CITED IN THE TEXT those of glaciofluvial nature, particularly tina, perhaps unintentionally, since his in Northern Patagonia. He also considers most insightful interests were in the Agassiz, L. 1840. Études sur les glaciers. Jent et inappropriate our discussion of the rela- fields of biology and anthropology. Ne- Gassmann Libraires, 346 p., Neuchâtel. tive ages of piedmont and glaciofluvial vertheless he is widely recognized in the Ameghino, C. 1890. Los plagiaulacídeos argenti- deposits. earth sciences particularly as a petrologist nos y sus relaciones zoológicas, geológicas y J.L. Panza's comments are very valuable (some of the first descriptions of pluto- geográficas. Boletín del Instituto Geográfico and worthy. However, we would like to nic and metamorphic rocks), sedimento- Argentino 11:143-201. state that we have never denied the flu- logist (pioneer reconnaissance of old se- Ameghino, F. 1906. Les formations sedimentaires vial/aggradational/piedmont origin for dimentary rocks and modern sediments), du Crétacé supérieur et du Tertiarie de Pa- some of the Rodados Patagónicos units. geomorphologist (identification and cha- tagonie. Anales del Museo Nacional Buenos Moreover, we have clearly maintained racterization of terraces, tablelands, du- Aires 15: 45-76. (see for instance Tables 1 and 2) that this nes, estuaries, moraines, erratic boulders, Bonarelli, G. and Nágera, J. 1922. Observaciones genesis is one of the possible major sour- etc.), stratigrapher (a visionary definition geológicas en las inmediaciones del Lago San ces for these units. Our intention has of the Pampean units), paleontologist Martín (Territorio de Santa Cruz). Dirección been just to make noticeable that some (transcendental discoveries of relevant General de Minas, Boletín 27B, 39 p., Buenos of the accumulations of Rodados Patagó- localities for Tertiary and Pleistocene Aires. nicos, and particularly those of Early fossil mammals in the Pampean region) Caldenius, C. 1932. Las glaciaciones cuaternarias Pleistocene and older ages (Rabassa et al. and glaciologist (innovative observations en la Patagonia y Tierra del Fuego. Dirección 2005), may have been generated by gla- of the Patagonian and Fuegian glaciers). General de Minas y Geología, Publicación 95, ciofluvial action during very ancient gla- Darwin was one of the most important 152 p., Buenos Aires. ciations, older than the Great Patagonian geologists and geomorphologists of the Caldenius, C. 1940. The Tehuelche or Patagonian Glaciation, even though these glacial 19th century, very far ahead of his time, shingle-formation. A contribution to the events were growing small, isolated ice and his forerunner ideas needed over a study of its origin. Geografiska Annaler 22: caps before the Patagonian Mountain Ice century to be revised, incorporated, con- 160-181. Sheet finally developed around ca. 1.2 firmed, or dismissed. Even today we Caminos, R. 2001. Hoja Geológica 4166-I, Val- Ma (Rabassa 2008). Though on-going continue revisiting his ideas and still cheta, Provincia de Río Negro. Servicio Geo- and future research will undoubtedly elu- work pursuing the search of valuable, lógico Minero Argentino, Boletín 310, 73 p., cidate this puzzle, the scale and comple- ground-breaking concepts which may Buenos Aires. xity of this problem has kept this discus- still be hidden within his unforgettable Clapperton, C. 1993. Quaternary Geology and sion open for over a century and writings. Geomorphology of South America. Elsevier obviously it will probably remain so for a Science Publishers B.V., 779 p., Amsterdam. long time. ACKNOWLEDGEMENTS Coira, B. 1979. Descripción Geológica de la Hoja 40d, Ingeniero Jacobacci, Provincia de Río FINAL REMARKS The authors would like to thank Beatriz Negro. Servicio Geológico Nacional, Boletín Aguirre-Urreta for her kind invitation to 168, 94 p., Buenos Aires. This article intends to give renewed contribute to this volume, as well as for Coronato, A., Martínez, O. and Rabassa, J. 2004. importance to the historic role that the her permanent help and collaboration Pleistocene Glaciations in Argentine Patago- work of Charles Darwin on the Rodados during the editorial process of the ma- nia, South America. In Ehlers, J. and Gibbard, Patagónicos had at his time, precisely in a nuscript. This paper has been largely im- P. (eds.) Quaternary Glaciations - Extent and profoundly revolutionary moment within proved by important observations and Chronology. Elsevier, Quaternary Book Ser- the earth sciences, when new ideas were criticism to earlier versions of this ma- ies, Part 3, 49-67, Amsterdam. thriving and new paradigms were precipi- nuscript by Raúl De Barrio, José Luis Cortés, J.M. 1987. Descripción geológica de la tously put forward. The Darwinian pro- Panza and an anonymous reviewer. We Hoja 42h, Puerto Lobos, Provincia del duction concerning the Rodados Patagó- are greatly indebted and deeply grateful Chubut. Servicio Geológico Nacional, Boletín nicos compels us to recognize the enor- to all of them for their thorough, meticu- 202, 68 p., Buenos Aires. mous merits of this author as an intuiti- lous and comprehensive comments. Cortelezzi, C., De Francesco, F. and De Salvo, O. ve geologist of great intellectual audacity Needless to say, inaccuracies and mista- 1968. Estudio de las gravas Tehuelches de la and who conceived science, as many kes that may still be present in this con- región comprendida entre el Río Colorado y other naturalists of those times, as an tribution are solely the authors' responsi- el Río Negro, desde la costa atlántica hasta la essentially integral and multidisciplinary bility. cordillera. 3º Jornadas Geológicas Argentinas, activity. Thus, Darwin achieved a promi- Actas 3: 123-145. Charles Darwin and the first scientific observations 99

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CHARLES DARWIN AND THE OLDEST GLACIAL EVENTS IN PATAGONIA: THE ERRATIC BLOCKS OF THE RÍO SANTA CRUZ VALLEY

Jorge STRELIN1 and Eduardo MALAGNINO2

1 Instituto Antártico Argentino, Universidad Nacional de Córdoba, Departamento de Geología Básica, Facultad de Ciencias Exactas Físicas y Naturales, Universidad Nacional de Córdoba, E-mail: [email protected] 2 Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires.

ABSTRACT Although the depositational environment assigned by Darwin to the large erratic blocks and gravels in the Río Santa Cruz valley has been reinterpreted, his geomorphological and stratigraphic observations are still in force. The large erratic blocks he described as crowning the Condor Cliff terrace and spread at the bottom of the valley just east of this locality (Sites 2 and 3), are now interpreted as indicators of the maximum glacial expansion in Patagonia. Similar blocks, though of a different lithology, accumulated over a lower terrace located up-valley (Site 4), are now linked to moraines and glacifluvial terraces of the Penultimate Glaciation. Finally, in addition to the erratic block discovered by Darwin in the lower Río Santa Cruz valley (Site 1), there are others - recently discovered - which probably account for a catastrophic event ascribed to a big glacier-lake outburst during the last interglacial. Keywords: Patagonia, Glaciations, Moraines, Erratic blocks, Darwin.

RESUMEN: Charles Darwin y las glaciaciones más antiguas de Patagonia: los bloques erráticos del alto valle del Río Santa Cruz. No obstante haber sido reinterpretado el ambiente depositacional asignado por Darwin a los grandes bloques erráticos y rodados del valle del río Santa Cruz, siguen vigentes sus observaciones geomorfológicas y estratigráficas. Los grandes bloques erráticos que describe coronando la terraza de Condor Cliff y dispersos en el fondo del valle inmediatamente al este de esta localidad (Sitios 2 y 3), son interpretados ahora como indicadores de la máxima expansión glaciaria de la Patagonia. Bloques similares, aunque de dispar litología, acumulados sobre una terraza más baja situada río arriba (Sitio 4), se vinculan actualmente a morenas y terrazas glacifluviales de la Penúltima Glaciación. Finalmente, al bloque errático descubierto por Darwin en el tramo inferior del valle del río Santa Cruz (Sitio 1), se le suman otros -de reciente descubrimiento- que probablemente den cuenta de un evento catastrófico atribuible al vaciamiento de un gran lago glaciar durante el último interglaciar.

Palabras clave: Patagonia, Glaciaciones, Morenas, Bloques erráticos, Darwin.

INTRODUCTION century later, with the help of modern with erratic blocks, occasionally connec- technology, that his deductions on the ted with till deposits, which he discove- Although Charles Darwin was a natura- great oceanic subsidence could be pro- red during the Beagle's itinerary along the list with a remarkably penetrating, inge- ved. This work, together with his one on coasts of Patagonia, Tierra del Fuego nious and broad mind in the field of bio- volcanoes (Darwin 1844) and South and Chiloé Island. His interest in these logy, the not so well-known geologist in American Geology (Darwin 1846), make deposits was not fortuitous. Charles Darwin -eclipsed by the former- did not up Darwin's most prolific geological Lyell, one of the foremost geologists at lag far behind. Many of the observations period - from 1841 to 1846. the time, had not only sent him a copy of that he made and the geological theories During his journey on board the HMS the first edition of his Principles of Geology he formulated when still a youth during Beagle, in his own words "one of the most but had also asked him to pay special his scientific voyage to South America important events" in his life, was when he attention to the presence of erratic and the Pacific Ocean are still current reached the valley of the Río Santa Cruz. blocks in these regions. and go beyond pure historical interest. It was here, in 1834 and when only 26 By the end of the 18th century, Cuvier's As an example of his innovative theories, years old, that he made observations of catastrophism theory had been dogmati- it is worth mentioning one which arose great scientific value. cally accepted by geologists. The first one from his published observations on coral Darwin (1842b) was the first one to des- to seriously oppose it was von Hoff reefs (Darwin 1842a), which made him cribe and discuss the origin of the exten- (1834). However, it was Charles Lyell famous among geologists. It was only a sive gravel and shingle layers scattered who, following Hoff's steps, introduced 102 J. STRELIN AND E. MALAGNINO

actualism in geology. In his Principles of During this journey, which took place a.s.l., i.e., 30 m above the river level) and Geology (Lyell 1830), together with his re- between April 3rd and May 8th, 1834, he 6 miles (11 km) away from Isla Pavón. volutionary contributions to this science focused his descriptions on four sites Our recent observations have allowed and loyal to his uniformitarian ideas, (Fig. 1). He established the geographic the discovery of numerous blocks at se- Lyell suggested that the big erratic blocks location of these sites by indicating their veral locations close to this site. One ex- found in the Northern European plains distance from the Atlantic Ocean and the ample are the large erratic blocks, par- had been transported there by icebergs in Andes Mountains in geographical miles. tially buried in alluvium, that can be shallow seas and had accumulated on the Herein we will see how, 166 years later, found on the northern side of the Río bottom jointly with non-stratified gravel Darwin's observations of those sites are Santa Cruz valley, on a terrace at an ele- of chaotic grain sizes which he called till. still in force and play a key role in the vation of 65 and 70 m a.s.l. (40 m above Just before Darwin's work on gravel beds interpretation of the oldest glacial events the river valley), 50 miles (91 km) away and erratic blocks in Patagonia was in Patagonia. from the Atlantic Ocean (19 miles - 35 published, Lyell (1840) discussed the ori- km - from Isla Pavón). The most outs- gin of this type of erratic blocks in DESCRIPTION OF THE SITES tanding are samples 6LA1 of andesitic Northern Europe in clear criticism of composition, striated, 1.70 m maximum the neocatastrophic theory proposed by Site 1 - The Lower Río Santa Cruz diameter (Fig. 2) and sample 6LA2 of Agassiz (1840), according to which ex- Valley basaltic composition, 0.80 m maximum tensive land-grounded ice sheets acted as diameter. There is also a series of blocks gatherers of erratic blocks. The first isolated erratic block which between 1.00 and 0.80 m diameter, partly After his journey to Patagonia and drew young Darwin's attention as he tra- disaggregated in situ, of rhyodacitic and strongly influenced by Lyell (1830-1833), veled up the Río Santa Cruz was partially basaltic composition. East-west oriented Darwin wrote to his cousin and friend buried in the alluvial plain of the river, 57 fluvial bars, ten to twenty meters long Darwin Fox (July 1835) from Lima, "I miles (105 km) away from the Atlantic and around 1 to 2 m high, can be obser- have become an enthusiast of Mr. Lyell's ideas Ocean and 110 miles (204 km) from the ved on the surface of this terrace. Seven and I have tried to explain them in my geologi- Cordillera (Site 1 - Fig. 1). However, as he erratic boulders up to 1.2 m long have cal work in South America". Twelve years could not find any other similar blocks been found to the east of this locality after Lyell's work, Darwin (1842b) also nearby, he did not consider it important (written communication from Mr. Sego- supported a glaciomarine origin for the and only described it as a feldspathic via), at the bottom of a depression which extensive gravel deposits and erratic rock of about 0.70 m diameter. lies partly below sea level. Another block blocks he had discovered in the southern This feature was pointed out again by was identified at the foot of a terrace part of South America. Moreno (1879) 45 years later. He also (elevation 40 m a.s.l.) located on the nor- Darwin obtained the most complete vi- mentioned erratic blocks in the lower thern margin of the river, near the point sion of the geomorphology and strati- valley of the Río Santa Cruz and particu- mentioned by Moreno (1879) (written graphy of gravel beds and erratic blocks larly described a 1m2 block partially bu- communication from Mr. Segovia). when, together with FitzRoy, he traveled ried in thin gravel, two thirds from the up the valley of the Río Santa Cruz. top of a terrace level (elevation 40 m

Figure 1: Satellite image of the Río Santa Cruz Valley showing the sites described by Darwin (1842b). Charles Darwin and the oldest glacial events in Patagonia:... 103

Site 2 - The Middle Río Santa Cruz Valley

Darwin (1842b) mentioned the reappea- rance of erratic blocks 100 miles (185 km) away from the Atlantic Ocean and 67 miles (124 km) away from the Cor- dillera (Site 2 - Fig. 1), a presence that is continuous and gradually increasing towards the Cordillera. Figure 2: Erratic block According to our observations, the erra- located 15 km downs- tic blocks reappear on the southern side tream of Site 1, at the top of a 65 m. a.s.l. of the valley, 7.8 miles (13 km) east of high terrace. the point indicated by Darwin. They are partly buried and partly lying on the flu- section of the river with the meridian of pebbles. Spread over this surface are vial terrace at an elevation of 200 m.a.s.l., 70º50' W, also provided by Darwin large blocks, in one case up to 20 m dia- approximately 100 m above the current (1842b). The distance that we measured meter and sticking out 1.8 m above the river bed. The erratic blocks, which can following the river track from this geode- gravel bed, a block of quartzose chlorite reach 0.90 m in diameter, are partially tic point to the Atlantic Ocean is 116 schist of 4.5 x 4.5 m and 1.5 m high, and weathered and mostly of basaltic and miles (215 km), only 4 miles longer than numerous blocks which range from 0.60 rhyodacitic composition, accompanied the distance measured by Darwin. to 1.20 square meters. Darwin (1842 b) by smaller blocks (0.20 - 0.25 m in diame- Darwin's (1842b) observations include a mentioned that the top of this sequence ter), some of which seem to be notice- stratigraphic section of the outcrop on is part of a high gravely terrace which ably faceted. the northern side of the valley, which reaches 425 m a.s.l. at Condor Cliff, clim- ends at the top of a terrace (at an eleva- bing up to 900 m a.s.l. towards the Cor- Site 3.- Condor Cliff tion of 425 m a.s.l.) covered by erratic dillera, which again does not exceed 2000 blocks (Fig. 3a). The covered base of this m a.s.l. Towards the Atlantic shore this It was here, at the narrowest point of the outcrop begins at an elevation of 90 m gravel layer, descends slowly down to an Río Santa Cruz valley, where Darwin a.s.l. with 180 m of small round pebbles elevation of only 245 m a.s.l. This low (1842b) described and sketched in more composed of clay-slate, feldspathic rock inclination of the terrace, the absence of detail (Fig. 3a) the morphologic and stra- and quartzose chlorite schist. The grain mounds and ridges, and the angularity of tigraphic characteristics of the gravel size decreases towards the upper half the boulders lead Darwin (1842b) to pos- beds and erratic blocks in Patagonia. where thin layers of a variety of colours tulate a glaciomarine (ice-drifted) origin Following Darwin's description, the site can be seen. Overlying these are 100 m for this accumulation. is located 112 miles (207 km) from the of basaltic lava which are in turn overlain One hundred and three years after Dar- Atlantic Ocean and 55 miles (102 Km) by 65 m of rounded, coarsely stratified win's observations, Mercer et al. (1975), from the Cordillera (Site 3 - Fig. 1). Site 3 gravel, similar in composition to the un- in a study of the oldest glaciation in the was plotted on figure 1 using the inter- derlying gravel but also including basaltic basins of Lago Argentino and Lago

Figure 3: Condor Cliff according to (a) Darwin's interpretation (1842b), (b) Mercer et al. (1975) and (c) the proposal in this paper. 1) Santa Cruz For- mation 2) San Fernando Terrace alluvium (Strelin et al. 1999) 3) Condor Cliff Basalts 4) Erratic blocks of the Estancia La Fructuosa Moraine (Stre- lin1995) 5) post Pliocene gravel strata (Mercer et al. 1975) 104 J. STRELIN AND E. MALAGNINO

Viedma, drew a new stratigraphic section demonstrate that the Condor Cliff ba- and an intermediate terrace (elevation at Condor Cliff, near the site described salts are not intercalated between the 240 m) are covered with erratic blocks of by Darwin (1842b). Comparing both sec- layers of gravel and that instead they flo- granitic, sienitic and conglomeratic com- tions, it is possible to observe that they wed down valleys incised in a terrace position, while no blocks of basaltic only differ in the position of the erratic level (elevation 500m) which we call La composition have been found. In his blocks. According to Mercer et al. (1975) Australasia terrace (Strelin et al. 1999). La description, Darwin (1842b) reveals that they are located right over the Pliocene Australasia terrace (Figs. 3c and 6) is car- there is an important difference in the basalts (Fig. 3b) while Darwin (1842b), as ved into the Santa Cruz Formation and composition of these erratic blocks and we have seen, described them as crow- crowned by gravel. Our conclusion dif- those which lie on the upper terrace at ning 65 m of gravel. In agreement with fers from Darwin's proposal (1842b) and Condor Cliff (San Fernando Terrace, Darwin (1842b), Mercer et al. (1975) Mercer et al. (1975) who considered that Strelin et al. 1999), which he considered intercalated the exposures of Pliocene the gravel was deposited on the basalts evidence of the occurrence of different basalt on the northern side of Condor (Fig. 3a and b). The mistake in the inter- glaciomarine episodes. Cliff within two layers of gravel. The 100 pretation is probably due to the presence Our study (Fig. 4) reveals that the inter- m thickness Darwin described for the of talus deposits formed by gravel which mediate terrace level that Darwin (1842b) basalt bed is probably an overestimation cover the contacts between the lava flows mentions when he described this section caused by basaltic landslides covering and the base of the Australasia terrace. of the valley matches the moraine and part of the northern side of the valley. The basalt channel over a lower terraced glacifluvial deposits of the Penultimate Mercer et al. (1975) indicated the presen- level called San Fernando Terrace (Strelin Glaciation termed Arroyo Verde (Strelin ce of calcinated gravels at the base of the et al. 1999), which at Condor Cliff can and Malagnino 1996). The moraine de- lava flows corresponding to an alluvial reach an elevation between 350 and 400 posits are buried by their own glacioflu- cycle (glaciofluvial) prior to the cycle m a.s.l. This terraced level is also carved vial deposits and the whole set reaches an which accumulated the 100 m of gravel into the Santa Cruz Formation and crow- elevation of 250 m a.s.l., 100 m above the which cover the same flows. By means of ned by gravel which appears to be calci- river. The inner slopes of these moraines radiometric dating they determined a nated when covered by basaltic flows. and glaciofluvial deposits, left behind by Pliocene age for such lava flows (2.92 ± Lava windows are common in this volca- ice during the glacial retreat, were modi- 0.07 Ma; 2.79 ± 0.15 Ma; and 2.66 ± 0.06 nic environment showing intact gravel fied by an ancient glacial lake (PLA, figu- Ma). According to these authors the outcrops with a maximum thickness of re 4). The most prominent lacustrine basal gravel bed reaches a thickness of 30 20 metres. In addition to gravel deposits, forms are the paleo-beach levels, bars, m and rests on the Santa Cruz For- the sections in the aggraded levels of the spits, lagoons and deltaic deposits. The mation. The 180 m of basal gravel asso- San Fernando Terrace show laminated lithology of the largest erratic blocks ciated with thinner stratified levels des- sand beds, tuffaceous and diamictitic which cover the moraine tops is more cribed by Darwin (1842b) are probably levels which include sometimes faceted varied than that of the oldest moraines, part of the fluvial facies and tuffaceous blocks of up to 0.40 m diameter (Fig. 5b) and as Darwin (1842b) indicated, blocks levels of the Santa Cruz Formation and of granitic composition appear and the psephitic layer described by Mercer et basaltic blocks are absent. This particular al. (1975). Site 4.- Upper Río Santa Cruz Valley feature is a result of the level of erosion Our observations (Strelin et al. 1999) of the Cordilleran valleys, which reached agree with Mercer et al.'s (1975) in the Upstream from Condor Cliff, the valley the granitic substratum (e.g. Fiordo Ma- sense that the erratic blocks, together expands into an extensive amphitheatre, yo, Cerro Murallón) in the Cordilleran with some moraine residue, rest directly which Darwin explained as an ancient source area. On the other hand, the de- on the basaltic flows. The allochthonous estuary of a post Pliocene sea and whose crease in the number of basaltic blocks is blocks found reach up to 3 m in diame- mouth opened towards the west, stres- related to the decrease of basaltic out- ter (Fig. 5a) and are mainly of an acidic sing the marine origin of this section of crops upstream. The continental sedi- volcanic nature (rhyodacitic), although the valley with the finding of marine mentary sequences of the Santa Cruz some are pelitic and occasionally striated. shells. The site Darwin (1842b) described Formation crop out along the southern Autochthonous blocks of basaltic com- is located along a section of the valley side of the valley, while along the nor- position are also common. However, we located between 127 and 137 miles (235 thern side - besides the continental depo- disagree with Darwin (1842b) and Mer- to 254 km) from the Atlantic coast and sits - there are outcrops of marine origin cer et al. (1975) regarding the stratigra- 30 - 40 miles (56 - 74 km) from the Cor- which belong to the Monte León For- phic relationship between the lava layers dillera (between sites 4W and 4E, Fig. 1). mation (Patagonian-Patagoniense), with and the gravel beds. We have been able to Both the valley floor (elevation 135 m) a high content of marine shells, espe- Charles Darwin and the oldest glacial events in Patagonia:... 105 Distribution of region (after Strelin and Malagnino 1996) Darwin's visited site location. the geomorphic units in the Upper Río Santa Cruz Valley Figure 4: 106 J. STRELIN AND E. MALAGNINO

Figure 5: a) Rhyodacitic erratic block located at the top of Condor Cliff; b) Diamictitic level underlying the Condor Cliff lava flows, top of San Fernando Terrace. cially oysters and pectinids. advances in the Río Santa Cruz valley. (Caldenius 1932) and at Condor Cliff na- The erratic blocks and moraine remnants rrow (Feruglio 1950). DISCUSSION AND deposited to the east and at the foot of Caldenius (1932), for example, conside- CONCLUSIONS Condor Cliff narrow (Sites 2 and 3 res- red the Per Dusen Moraines as the oldest pectively, Fig. 4) correspond to these ex- (initial glacial) and most expanded system Darwin (1842b) incorrectly interpreted tended glacial advances. in the Río Santa Cruz valley, with its clo- the depositional environment of the gra- Caldenius (1932) and Feruglio (1950) un- sure situated at 71º W. Feruglio (1950), vel and erratic blocks which cover the derestimated these observations and did on the other hand, referred -in a footno- Patagonian plateaus, largely influenced not take into account the location of the te- to the great erratic blocks at the clo- by Lyell's (1830) ideas. Nevertheless, his erratic blocks mentioned by Darwin sure of Cerro Fortaleza (Figs. 4 and 6) as data allowed the determination -with (1842b) in Site 2. Therefore, they situated indicators of the maximum glacial ex- considerable accuracy- of the extent of the maximum glacial expansion of the pansion in the Río Santa Cruz valley (in- the maximum expansion of the glacial Lago Argentino Basin further up-valley ternal moraines) but, unlike Caldenius, he Charles Darwin and the oldest glacial events in Patagonia:... 107

Figure 6: Relief model of the Condor Cliff narrow, showing the basaltic flows channeled on the San Fernando Terrace, between the relicts of the Australasian Terrace. did not consider them indicators of the 1995) and the glaciofluvial deposits asso- the referred basaltic volcanism took pla- oldest glaciation. ciated and defined as Pampa Alta Pro- ce at two different eruptive periods: i.e., Likewise, Mercer et al. (1975) thought the glacial (Strelin 1995). These morphologi- before and after the accumulation of the glacial advance reached the narrow of cal units make up the top of the plateau moraines which cover the terrace at Con- Condor Cliff twice. During the first ad- at Pampa Alta, located south of the up- dor Cliff (Strelin et al. 1999). The terrace vance of his Most Extensive Glaciation per valley of the Río Santa Cruz (Fig. 4). level on which the lava flows are channe- (Mercer et al. 1975) the glacier would Evidence of this first foothill glaciation led coincides with the ancient tributary have gone beyond the narrow depositing is also present at other localities in the valleys of the ancestral Río Santa Cruz, the big erratic blocks which lie on the Patagonian Andes. In the Lago Buenos the valley floors of which formed the basaltic cliffs (Figs. 3b and 5a). These Aires basin for example, Malagnino current San Fernando Terrace (Strelin et blocks would correspond to those men- (1995) identified the Chipanque Morai- al. 1999). The entrenchment of the tribu- tioned by Darwin (1842b) at the top of nes, as a set of three systems made up of tary valleys, adjusted to the old base level his section. During the next advance, the 27 belts located on the Guenguel Plateau of the Río Santa Cruz, continued until it glacier would have deposited the morai- at an elevation of 900 meters. These reached an elevation of 200 -250 m a.s.l. nes which lie over the western slope of were deposited by a glaciation (Chipan- (75 -135 m above the current level of Río Cerro Fortaleza, at the foot of the basal- que Glaciation, Malagnino 1995) which Santa Cruz). This can be verified right in tic cliffs (Mercer et al. 1975). had not been identified before in the front of Condor Cliff at the foot of We agree with Feruglio (1950) in consi- Lago Buenos Aires basin. With this dis- Cerro Fortaleza, along the southern mar- dering that the erratic blocks at Condor covery it was possible to raise to six the gin of the narrow, where 12 basaltic lava Cliff are not the signs of the oldest gla- number of glaciations in this area of the flows with a total thickness of 100 m lie ciation in this region (Strelin 1995 and Patagonian Andes and its foothills. (through pillow lavas and palagonite Strelin et al. 1999). Prior to the incision of After the Pampa Alta Glaciation and tuffs) over the layers of the Santa Cruz the valley of the Río Santa Cruz through before the maximum glacial expansion in Formation. which the second Patagonian glaciation the Río Santa Cruz valley (Estancia La To the west of the Condor Cliff narrow later channeled (Estancia La Fructuosa Fructuosa Glaciation, Strelin 1995) there the valley opens into an amphitheatre Glaciation, Strelin 1995 and Strelin et al. was a long interglacial period during which Darwin (1842b), as previously 1999), the Andean glaciers first irrupted which the course of the Río Santa Cruz mentioned, interpreted as a terraced pain ample lobes along the foothill belt of was formed, producing a deep canyon leo-estuary (intermediate terrace, eleva- the Cordilleran front leaving upon their which was later partially filled with basal- tion 240 m a.s.l.) opening towards the retreat the moraine arches corresponding tic lava flows during the Late Pliocene. Cordillera. He described it as abundant in to the Pampa Alta Glaciation (Strelin We have been able to demonstrate that erratic blocks with a different composi- 108 J. STRELIN AND E. MALAGNINO

tion from those on the upper terrace of is more likely that Darwin (1842b) found South America. Being the third part of the Condor Cliff and belonging to a diffe- reworked oyster and pectinid shells geology of the voyage of the Beagle, under rent glaciomarine event from that which coming from the coquina exposures of the command of Capt. Fitzroy, R.N. during deposited the blocks on the upper terra- the Monte Leon Formation located near the years 1832 to 1836. Smith Elder and Co. ce. Darwin (1842b) thought to confirm this site. It is worth mentioning that the 280 p., London. the marine environment with the finding surfacing of Monte Leon Formation in Feruglio, E. 1950. Descripción Geológica de la of shells on the valley floor (elevation this part of the valley is a feature which Patagonia. Yacimientos Petroliferos Fiscales 135 m a.s.l.). This interpretation strongly has yet not been clearly explained. 3, 431 p., Buenos Aires. contrasts with ours at first sight because Finally, the origin of the erratic blocks Hoff, K.E.A. von 1834. Geschichte der durch we consider that Darwin's intermediate (Fig. 2) found in the lower valley of the Ueberlieferung nachgewiesenen natürlichen terrace is part of the moraine and glacio- Río Santa Cruz (Site 1, Fig. 1) has not Veränderungen der Erdoberfläche, 1-4, Per- fluvial deposits formed by the glacial been elucidated yet. Darwin (1842b) was thes, 2086 p., Gotha. tongue which channeled through the Río sensitive to this enigma, which he tried to Lyell, C. 1830-33. Principles of Geology. John Santa Cruz valley during the Penultimate solve when he suggested that they could Mu-rray, Albemarle-Street, (1830) 1, 511 p.; Glaciation (Arroyo Verde Glaciation, have been accumulated after rafting over (1832) 2, 330 p.; (1833) 3, 109 p., London. Strelin and Malagnino 1996). However, if fluvial ice. At present we consider this Lyell, C. 1840. On the Boulder Formation, or we analyze the landscape features in de- feasible and furthermore that it could drift and associated freshwater deposits com- tail we will see that Darwin (1842b) did have been after the catastrophic draining posing the mud-cliffs of Eastern Norfolk. not let his imagination run away. On the of the ancient Arroyo Verde moraine- The London and Edinburgh Philosophical one hand, there is the fact -well-establis- dammed glacier-lake (Strelin and Mala- Magazine and Journal of Science, Third hed by Darwin- of a different glacial gnino 1996). Series 16(104): 345-380. event based on the different composition Malagnino, E.C. 1995. The discovery of the old- of the erratic blocks. On the other hand, ACKNOWLEDGMENTS est extra-Andean glaciation in the Lago Bue- we can justify his interpretation of the nos Aires Basin, Argentina. A.A. Balkema, existence of a paleo-estuary (paleo-fjord) We are especially grateful to Marcos Mo- Quaternary of South America and Antartic in the upper valley of Rio Santa Cruz if zetic, Eduardo Olivero, and Michael Ka- Peninsula 9: 69-83, Rotterdam. we consider the morphological similari- plan for their suggestions and for impro- Mercer, J.H., Fleck, R.J., Mankinen, E.A. and ties between this valley and other glacial ving the english of this paper. Also we Sander, W. 1975. Southern Patagonia: Glacial valleys, totally or partially flooded by the thank very much Beatriz Aguirre-Urreta Events Between 4 m.y. and 1 m.y. Ago. In sea, visited by Darwin during his voyage for inviting us to participate in this spe- Suggate, R.P. and Creswell, M.M. (eds.) Qua- to the southern seas (Strait of Magellan, cial issue dedicated to Charles Darwin. ternary Studies, The Royal Society of New Otway and Skyring Fjords, San Sebastián Zealand, 223-230, Wellington. Bay, Inútil (Useless) Bay, Beagle Channel, WORKS CITED IN THE TEXT Strelin, J.A. 1995. New evidences on the rela- etc). The morphological similarity with tionships between the oldest estra-andean gla- flooded valleys is even more noticeable if Agassiz, L. 1840. Études sur les glaciers. Jent et ciations in the the Santa Cruz River area. A.A. we consider that the section of the valley Gassmann Libraires, 346 p., Neuchâtel. Balkema, Quaternary of South America and Darwin (1842b) examined was reshaped Darwin, C. 1842a. The structure and distribution Antarctic Peninsula 9: 105-116, Rotterdam. by an ancient lake as shown by the lacus- of coral reefs. Being the first part of the geo- Strelin, J.A. and Malagnino, E.C. 1996. Glacia- trine paleo-forms (raised beaches, bars, logy of the voyage of the Beagle, under the ciones Pleistocenas del Lago Argentino y Alto spits, lagoons and deltas, in figure 4) han- command of Capt. Fitzroy, R.N. during the Valle del Río Santa Cruz. 13° Congreso Geo- ging at elevations of up to 250 m a.s.l. years 1832 to 1836. Smith Elder and Co. 214 lógico Argentino, Actas 4: 311-326. (100 m above the valley floor). This lake p., London. Strelin, J., Re, G., Keller, R. and Malagnino E. formed as a result of a moraine dam- Darwin, C. 1842b. On the distribution of erratic 1999. New evidences concerning the Plio- ming (Arroyo Verde moraines, Strelin boulders and on the contemporaneous uns- Pleistocene landscape evolution of southern and Malagnino 1996) during the glacial tratified deposits of South America. Transac- Santa Cruz region. Journal of South Ame- retreat which took place in the Paleo- tions Geological Society London 6: 415-431. rican Earth Sciences 12: 333-341. Lago Argentino Interglacial (Strelin and Darwin, C. 1844. Geological Observations on Malagnino 1996). As regards the marine The Volcanic Island, visited during the Voy- shells Darwin discovered in this section age of H. M. S. Beagle. The Geology of the of the valley, Feruglio (1950) suggests Voyage of the Beagle, Part 2, Smith Elder and that they may have been abandoned there Co. 172 p., London. Recibido: 19 de septiembre de 2008 by natives. We consider, however, that it Darwin, C. 1846. Geological observations on Aceptado: 29 de octubre de 2008 Revista de la Asociación Geológica Argentina 64 (1): 109 - 123 (2009) 109

FROM BUENOS AIRES TO SANTA FE: DARWIN'S OBSERVATIONS AND MODERN KNOWLEDGE

Martin IRIONDO and Daniela KRÖHLING

CONICET - Universidad Nacional del Litoral, CC 217 (3000) Santa Fe. E-mails [email protected], [email protected]

ABSTRACT During his historical voyage around the world, Darwin raided deeply in the South American interior, travelling over 600 kilometers from Buenos Aires to the north along the Río Paraná. During that journey, he crossed a vast plain characterized by aeolian sediments, something unfamiliar to a European naturalist. However, Darwin's acute observation powers and precise descriptions are noteworthy. After more than 170 years since his visit, modern geological knowledge identifies several sectors in the Buenos Aires-Santa Fe region. One of them (the Tertiary at La Bajada) he described admirably and others such as the Paraná flood plain were brilliantly abstracted in only two sentences. In short, Darwin traversed a first sector (Buenos Aires- Rosario) characterized by aeolian and paludal Early Pleistocene sediments. From Rosario to Santa Fe the plain is formed by Late Pleistocene aeolian and fluvial units. At La Bajada (presently Paraná city) lies exposed the marine Miocene and in SW Entre Ríos is a reconstructed loess-paleosol sequence generated at the Early/Middle Pleistocene transition. The Paraná flood plain and the littoral complex at the mouth (practically not observed by Darwin) underwent rather complex Holocene episodes. Keywords: Ch. Darwin, Pampa, South America, Quaternary, Tertiary.

RESUMEN: De Buenos Aires a Santa Fe: observaciones de Darwin y conocimiento actual. Durante su histórico viaje alrededor del mundo, Charles Darwin incursionó profundamente en el interior de Sudamérica recorriendo más de 600 kilómetros hacia el norte de Buenos Aires a lo largo del río Paraná. Durante ese viaje, él recorrió una gran planicie caracterizada por sedimentos principalmente eólicos, un caso poco familiar para un naturalista europeo. Sin embargo, sobresale su gran capacidad de observación y precisión en sus descripciones. Después de más de 170 años de la visita de Darwin, el conocimiento geológico moderno ha identificado varios sectores en la región Buenos Aires-Santa Fe, uno de los cuales (el Terciario de La Bajada) él describió admi- rablemente y otros, como la llanura aluvial del Paraná, reseñó magistralmente en solo dos frases breves. En resumen, Darwin recorrió un primer tramo (Buenos Aires-Rosario) caracterizado por sedimentos eólicos y palustres del Pleistoceno Inferior. Desde Rosario hasta Santa Fe la llanura está formada por unidades eólicas y fluviales de edad pleistocena superior. En La Bajada (hoy ciudad de Paraná) aflora el Mioceno marino y en el sudoeste de Entre Ríos se reconstruyó un sistema loess-paleosuelos generado a lo largo de la transición Pleistoceno Inferior-Pleistoceno Medio. La llanura aluvial del Paraná y el comple- jo litoral de su desembocadura (prácticamente no observados por Darwin) sufrieron cambios dinámicos bastante complejos durante el Holoceno.

Palabras clave: Ch. Darwin, Pampa, Sudamérica, Cuaternario, Terciario.

INTRODUCTION kilometers westwards. It is formed by a ments and Early Quaternary aeolian and slightly elevated block characterized by swampy formations. The last environ- Charles Darwin made the journey from Early Pleistocene formations and margi- ment Darwin recorded during his jour- Buenos Aires to Santa Fe by cart and nal Holocene littoral deposits. The ney is the Paraná river system, composed horse. After several days of work at La second sector covers the Rosario-Santa of a wide flood plain and a littoral com- Bajada (presently Paraná city) he retur- Fe road and reaches 300 km west to the plex at the mouth ("Paraná delta"), both ned to Buenos Airs by boat along the Río hills in Córdoba. This system is a relati- of them Holocene in age. Paraná. Such a trip allowed him (surely by vely sunken region dominated by Late In order to visualize Darwin's experien- chance) to cross four major geographic Quaternary loess bounded at the north ces, this article is organized in the same and Quaternary geological environments by the South American region of Chaco. sequence of geological terrains that he of southeastern South America (Fig.1). The third area he visited forms the found during his journey (Darwin 1846, The first one stretches from Buenos southwestern sector of Entre Ríos pro- 1945). Aires city (in fact, it begins further south) vince; it is represented by a hilly landsca- When reading his journal or his geologi- to Rosario and reaches several tens of pe carved into Miocene marine sedi- cal observations of the region, Darwin's 110 M. IRIONDO AND D. KRÖHLING

layers of bones¨ covering the bottom of some tributaries. Very probably such great drought was a short late occurrence of the Little Ice Age (LIA) climate, which was characterized by a marked dryness in the Pampas (Parra 1939, Iriondo and Kröhling 1995) and ended around 1800-1810. The Spa- nish naturalist Azara also described similar events in the late 18th Century. The Little Ice Age provoked advances of glaciers in the Cordillera and related processes in southern South America, but did not produce visible geological effects in the Pampas, probably owing to its relatively short duration and not so severe dryness. On the other hand, a longer dry period occurred between 3.5 and 1.4 ka BP and formed a thin aeolian mantle which covers the Pampas and surrounding regions, i.e., the San Guillermo Formation (Iriondo 1990, Fig. 2). It is an aeolian gray silt that generally tops the sedimentary sequence in the large interfluves. Figure 1: Map of the region showing the localities visited Most of the sediment is originated lo- by Darwin. cally, by deflation of the A-horizon of soils and the subsequent deposition of great observation power and exceptional THE GREAT DROUGHT dust. In Santa Fe province this mantle is ability for synthesis appear immediately formed by coarse silt with scarce propor- evident, notwithstanding the limitations An interesting environmental issue was tions of very fine sand and clay, brow- of geological theory at that time. For ins- described by Darwin in relation to the nish gray in color (10YR 5/1). It is fria- tance, the nature and origin of loess (the trip to Santa Fe. It was the "Gran Seca" ble, porous, permeable, and moderately dominant sediment in the Pampas) was (the great drought) that particularly af- structured. Typical thickness varies from discovered several decades later. It is evi- fected the north of Buenos Aires and 25 to 55 cm; ceramic shards and Indian dent that Darwin did not feel comforta- south of Santa Fe from 1827 to 1832. boleadora balls are included in the upper ble with such "muds", "clays" and similar. During those five years rain was excep- part of this deposit at some localities. In order to describe and interpret those tionally scarce and the plain underwent extensive deposits of the plain, he prefe- major changes: vegetation disappeared, BUENOS AIRES/ROSARIO rred to hold on to strata and concretions minor rivers dried up and the entire ELEVATED BLOCK of tosca (caliche), intercalated in the sec- country took the aspect of a dusty road, tions. His skill as a paleontologist in com- dust clouds continuously dominating the Darwin rode a 250 km long first journey parison with his abilities as a sedimento- air. The effect on animals was catastro- from Buenos Aires to Rosario in a logist is also clear. phic; cattle migrated massively to the straight northwestern direction. All that Besides, it is interesting to discover in south and assembled there in a giant time, the road borders the right banks of Darwin's writings the existence of quali- flock; droves with thousands of wild the Río de la Plata and Río Paraná and fied observers - i.e., people with some horses hurried furiously to the Paraná to runs atop a 10 to 20 m high cliff. This scientific knowledge - among the local drink and later were unable to climb back cliff is almost continuous, interrupted in population in those early years of Argen- up the slippery cliff and drowned. a few places by small Holocene estuaries tina. Some of them were British traders, Hundreds of thousands of animals floa- of minor tributaries (Luján, Areco, Arre- but others were local inhabitants that ted downstream to the Plata estuary. A cifes and others; Iriondo 2004). The cliff were great observers of Nature. few years later Darwin observed ¨true is tectonic in origin and exposes two From Buenos Aires to Santa Fe: Darwin's observations ... 111

insistent assertion of the French naturalist d'Orbigny on the general absence of stratification in the Pampean sediments. The academic point was then whether the Pampean System was diluvial or not: if stratified, not diluvial.

Baradero Section (NE Buenos Aires province)

The section on the Río Baradero north of the city of Baradero (100 km from Buenos Aires), is formed by the Punta Gorda Formation (5 m thick), conformably overlain by the Hernandarias For- mation (7 m thick, Iriondo 1980). From bottom to top: Figure 2: Map of the San Guillermo Formation, a 0 - 1 m Punta Gorda Formation. Bed of late Holocene environmen- clayish silt, dull brown in colour, modera- tal equivalent of Darwin´s tely consolidated. It contains numerous big drought (after Iriondo Fe-oxide segregations, CaCO 1990). 3 concretions of different sizes and large rhizo- Quaternary formations described by were transported by southwesterly winds concretions. It is interpreted as a paludal Darwin as "pale and red Pampean mud". from the Andes Cordillera. The Punta deposit. Both units were identified and studied Gorda Formation was deposited around 1 - 5 m Punta Gorda Formation. Bed of later in the Twentieth Century. 1 Ma BP during the most important a silty grain-size composition, pale brown González Bonorino (1965) refered the Quaternary glaciation occurring in South in colour, massive in general and in some sections outcropping in the Buenos Aires America (Mercer 1976). places with a weak medium to coarse area as the Pampeano Formation. Ac- The Red Pampean Mud/Hernandarias horizontal stratification. Medium to high cording to its mineralogy, the author dif- Formation is a large playa deposit depo- consolidation (highly calcareous); carbo- ferentiated two zones separated by a clear sited by the Río Uruguay at the Early- nate (tosca) concretions. Fine root moulds boundary: the upper zone characterized Middle Pleistocene transition. It forms covered by black films. The deposit has a by plagioclase and illite and the lower one the surface of most of Entre Ríos pro- general aeolian origin, with local rill dominated by quartz and montmorilloni- vince and below the surface reaches into reworking and krotovinas in some places. te. Fidalgo et al. (1973) later described the neighboring regions in Santa Fe and Remains of Glyptodon were found in the Pampiano Formation and Riggi et al. Buenos Aires to the southwest (Iriondo measured section. At the top, a marked (1986) discriminated the Ensenada For- 1980). The typical thickness of this for- horizontal paleosurface is recognized, mation and the Buenos Aires Formation mation varies from 20 to 40 m; it is com- locally joined to a residual material of in the same sections. Both units appear posed of loam and silty loam with mont- 0.60 - 0.80 m thick, with few local pedo- along all the distance to Rosario, but have morillonite as main clay mineral, and genic features. better outcrops on the left bank of the quartz and very fine sand containing gyp- 5 - 12 m Hernandarias Formation. It is Paraná, in Entre Ríos province. Here sum at the base. More than 90 % of the composed of clayish silt to a silty clay, they were defined as Punta Gorda For- clay minerals, i.e., montmorillonite, bei- olive gray in general colour and with va- mation (Iriondo and Kröhling 2008) and dellite and nontronite, show expan- riations to brown and olive; with abun- Hernandarias Formation (Iriondo 1980). sion/contraction properties. dant large and irregular carbonate con- Pale Pampean Mud/Punta Gorda For- The contact between both formations is cretions. Frequent Fe-Mn sesquioxide mation is an Early and Middle Pleisto- concordant - a feature correctly observed segregations appear. It is less consolida- cene loess-paleosol sequence. It is com- by Darwin, who stated: "I could clearly dis- ted than the underlying unit and is orga- posed of the Andean plagioclase-illite as- tinguish in this fine line of cliffs horizontal lines nized in poorly defined coarse strata, and sociation and has an origin similar to the of variation both in tint and compactness" interpreted as a paludal deposit. Nume- classical Last Glacial Maximum Pampean (Darwin 1846, p. 87). As remarked by rous dessication cracks appear along all loess (Kröhling 2001). Both minerals Darwin this observation contradicts the the section. The deposit includes me- 112 M. IRIONDO AND D. KRÖHLING

dium to coarse lenticular to tabular strata by the lixiviation of CaCO3 rhizoconcre- Ramallo is formed by the Punta Gorda of pale brown silt, with internal lamina- tions. Pedological features are: fine root Formation, represented by the loessic

tion and segregated CaCO3. Such lenses moulds covered by black films, argillocu- strata with frequent CaCO3 vertical platy are interpreted as aeolian, locally re- tans and ferriargillans. This paleosol co- concretions, 2.50 to 3 m thick. The unit transported material. In some sectors ap- rresponds to the unit G attributed by is covered by the paludal facies of the pear lenses of retransported botrioidal Imbellone and Cumba (2003) -Geosol El Hernandarias Formation, with frequent

CaCO3 concretions. Tala-Hisisa-. The Hernandarias Forma- CaCO3 concretions. The forms of erosion of both formation forms the rest of the exposed sections are very different and distinctive tion, with general paludal characteristics Arroyo Pavón and Arroyo Seco Sec- along the river cliffs. They were also and interbedded discontinuous soil hori- tions (Southern Santa Fe province) affected by several gullies with. zons in the middle sector of the unit. 12-15 Tezanos Pinto Formation. Yello- Numerous large krotovinas appear in this Entering the territory of Santa Fe pro- wish brown, friable, massive and calcare- formation. vince, Darwin described a cascade about ous silty deposits. It is the typical LGM twenty feet high in the Pavón creek, a Pampean loess. Vuelta de Obligado and Ramallo Sec- small tributary of the Paraná. He recor- Nabel et al. (1993) studied the magnetos- tions (NE Buenos Aires province) ded there two varieties of tosca-rock (li- tratigraphy of two sections at Baradero, mestone). finding reverse polarity (older than 0.78 The section of the cliff at Vuelta de Today the Pavón Cascade is located a few Ma) in the lower section of the cliff. Obligado is represented by the loessic hundred meters from Provincial High- Those authors identified two of the len- facies of the Punta Gorda Formation way 11. The cliff downstream the casca- ticular paleosols and named them El Tala (6.50 - 8 m thick), overlain by a calcrete de begins at the base with a brown sandy (Brunhes paleomagnetic chron) and Hi- (2.50 m thick). The unit includes a 1.30 m aeolian deposit (1.5 m thick) covered by

sisa Geosols (Matuyama chron > 780 ka). thick paleosol at the base, composed a loessic unit with segregated CaCO3 Nabel et al. (1993) discriminated eight from top to bottom as follows: (1.5-2 m thick). A paleosol (1 m thick) is litoestratigraphic units at Baradero sec- 0 - 0.35 m B2t horizon. Brown clayish silt recognized on top of it (it very probably tion: units I to V interpreted as loess and (7.5 YR 5/4), moderately to well structu- correlates with the soil exposed at the paleosols deposits and units VI to VIII red in very firm, fine to medium prisms, bottom of the Vuelta de Obligado sec- associated with limnic environments. bounded by fine fissures. Common clay tion). The section ends with the loessic The age of the deposit in the region was cutans and fine root moulds. Very fine to facies of the Punta Gorda Formation

attributed to the Ensenadense Mammal fine Fe-mottles and Fe-Mn oxides segre- with abundant CaCO3 concretions (2 m Age (Tonni et al. 1999). Later, Kemp et al. gations frequent. thick). Some kilometers to the north, the (2006) reconstructed the sequence of 0.35 - 0.57 m B3 horizon. Brown silt mo- Punta Gorda Formation forms the banks pedosedimentary processes in the same derately organized in very firm, fine to of the Arroyo Seco (9 m high at the tow- locality based on soil science methods medium prisms. Scarce argillocutans and n's camping grounds); locally differentia- and identified paleosols embedded in fine root moulds. ted is a 3 m thick brown silty strata, palu- swamp and aeolian sediments. Their ages 0.57 - 0.87 m BC horizon. Brown silt, dal in origin. as recorded by luminescence methods weakly structured in firm and fine blocky (OSL dates range from ca. 114 to 14 ka peds. Abundant very fine Fe-Mn mottles ROSARIO/SANTA FE BP) differ considerably from previous and fine root moulds. BLOCK paleomagnetic results of Baradero. 0.87 - 1.29 m C horizon. Light brown clayish silt with paludal characteristics (very From Rosario to Santa Fe and beyond to San Pedro Section (NE Buenos Aires fine resistant blocky peds, frequent Fe- the north and west, stretches another

province) mottles, common CaCO3 irregular and sector of the Pampas, dominated by platy concretions). Late Pleistocene geological formations. In a quarry excavated into the Río Paraná At that locality, a well developed 1.5 m The landscape is there shaped by two cliff at San Pedro the following section high notch appears at the base of the widespread aeolian units: Tezanos Pinto appears from bottom to top: a paleosol cliff; that feature indicates erosional ac- For-mation and San Guillermo For- developed on top of the Punta Gorda tion by waves during the Middle Holo- mation. However other units originated Formation and truncated by erosion cene marine ingression. Such a feature in fluvial environments have been des- (1.20 m thick) and constituted by brown also appears in the same position at Fray cribed. The stratigraphic sequence is the silt, weakly structured in peds. It contains Bentos, in the Banda Oriental (Uruguay). following: numerous large root moulds generated The Río Paraná cliff at the locality of From Buenos Aires to Santa Fe: Darwin's observations ... 113

Section of the Paraná cliff between exposed section was characterized by medium horizontal strata composed of Puerto Gaboto and Rosario (Santa Fe Darwin as follows, from the base to top: intraclasts in a sandy matrix or by inter- province) “it consists of a pale yellowish clay, abounding nally laminated strata. with concretionary cylinders of a ferruginous This unit, represented by the fluvial According to Kröhling (1999) the expo- sandstone”. It is interpreted as the Itu- channel sedimentary facies (paludal fa- sed stratigraphic column on the cliffs zaingó Formation. Darwin continues: cies is common too), outcrops at the base along the right bank of the Paraná and “The rest of the cliff at Gorondona, is formed of the cliff of the Río Paraná from its Coronda rivers, between the cities of of red Pampean mud, with, in the lower part, junction with the Río Coronda up to the Puerto Gaboto and Rosario, begins at the many concretions of tosca, some stalactiformed, city of Rosario, with variable thickness base with the Plio-Quaternary Ituzaingó and with only a few in the upper part… contai- between 0.5 m and 4.5 m. Locally the Formation, deposited by the Río Paraná. ning mammiferous remains close to its base” formation is covered by the Timbúes The sedimentary record of the unit is (Darwin 1846, p. 87). It corresponds to Formation separated by an erosional represented by mature and well selected the general features of the Puerto San unconformity, but in general the overl- sands, typical of channel facies of a flu- Martín Formation. ying unit is the Puerto San Martín vial system of high discharge. The condi- The main characteristics of the mentio- Formation. tions that favored the accumulation of ned formations exposed in the area visi- Main characteristics of the Ituzaingó the unit changed toward the top of the ted by Darwin were taken from Kröhling Formation in its type area (Corrientes formation to a fluvial regime of lower (1998) and are presented below: province) are presented in Georgieff et energy. Ituzaingó Formation (De Alba 1953, al. (2005). The Ituzaingó Formation is separated by Herbst 1971) Timbúes Formation (Kröhling 1998) an erosional unconformity from the It is composed of very fine quartz sand, It is composed of silty very fine to fine overlying Late Pleistocene Puerto San sandy silt to silty clay, grayish yellow in sand, opaque orange in colour (7.5 YR Martín Formation, formed by the dis- colour (2.5Y 6/2) with variations to olive 7/4), with visible micaceous minerals continuous accumulation of fine aeolian yellow (5Y 6/3). The sediment has abun- (frequency of 5-20%). It is organized in sediments. The accumulation environ- dant ferric segregations; also the postde- coarse to very coarse, internally lamina- ment was of semipermanent swamps, in- positional rubefication is in general dis- ted strata, lenticular to tabular in form, tercalated with periods of subaerial concordant to the stratification. The upper with irregular concordant to erosional ditions (loess). The existence of Ck-hori- part of the unit has abundant brown Fe- contacts among them. Deformational zons of truncated soils near the top of sesquioxides mottles (7.5 YR 5/8), loca- structures are common. There are Mn- the unit suggests more benign condi- lly concentrated in centimetric levels. The segregations, root moulds filled by sand tions. In some sites, between the two consistence of the sedimentary mass ran- and leaf moulds covered by Mn-films. mentioned formations there are interca- ges from friable to poorly consolidated Platty CaCO3 concretions appear at the lated several paleochannels infilled with non-calcareous. Segregations of CaCO3 contact between some strata, locally for- alluvial and paludal deposits. These were form irregular to botrioidal concretions. ming a calcareous net of phreatic origin. formed by successive avulsions of the Locally, in the lower part there are platty Large elliptic krotovinas are frequent. last segment of an important tributary in CaCO3 concretions forming an orthogo- At the Paraná cliff near the locality of the region - the Río Carcarañá (Timbúes nal net or ferruginous concretions of 30 Puerto San Martín, a B horizon of a pale- Formation). to 50 mm diameter. The unit is arranged osol appears at the erosional contact bet- The landscape developed over the Puer- into medium to coarse massive lenticular ween the Timbúes Formation and the to San Martín Formation was later cove- strata with 7-10 m of lateral extension, overlying Tezanos Pinto Formation. It is red and smoothed during the Last with variable concentrations of very fine 0.30 to 0.60 m thick, reddish brown in Glacial Maximum by the loess of the to medium Fe-mottles (up to 40%) and color and weakly to moderately structu- Tezanos Pinto Formation. The stratigra- Mn-segregations and mottles. Abundant red in medium angular prisms. phic sequence ends with the Late Ho- fine root moulds with black segregations. The Timbúes Formation (Late Pleisto- locene San Guillermo Formation, which In some sectors, the top of the unit is cene in age) outcrops at the cliff of the lies over the buried Holocene Optimum marked by a moderately developed B ho- Paraná and Coronda rivers, exhibiting a Climaticum soil developed on top of the rizon and by calcareous concretions thickness of 5 to 6.5 m and lateral exten- loess. bellow it. Locally, the truncated paleosol sions of tens of meters. The sedimentary Darwin described the Paraná cliff in the is replaced by medium cuneiform strata characteristics indicate a fluvial origin for area of Estancia Grondona, north of composed of fine sand, ochre-yellow in this unit. It is interpreted according to Rosario and near the last and more re- colour, that form a set with diagonal stra- mineralogical data as a deposit generated cent segment of the Río Carcarañá. The tification. Upwards they are replaced by by the Río Carcarañá and indicating the 114 M. IRIONDO AND D. KRÖHLING

position of its previous mouths in the sector, olive in colour, comprises the Ck sedimentary record of the lower basin Paraná. horizon of a truncated paleosol. The up- was reported by Kröhling (1999) and is Puerto San Martín Formation (Iriondo per sector is represented by a brownish explained below (Fig. 3). 1987) to reddish brown silt with diffuse crossed Carcarañá Formation (Kröhling 1999) The unit is practically continuous along stratification. It includes a lenticular fine This unit is composed of very fine to

the Río Paraná cliff from the mouth of bed formed by CaCO3 pebbles. A mode- fine silty sand, and silt with very fine the Río Coronda up to the city of San rately structured B horizon of a trunca- sand, dull orange in colour (7.5 YR 6/4). Lorenzo. Its thickness varies from 4 up ted soil tops this unit. It has very thick horizontal strata, not to 10 m. The unit is overlain by the Te- The sedimentary unit is interpreted as a well defined and generally with concor- zanos Pinto Formation, separated by an continuous sequence of strata represen- dant contacts. The sediment varies in unconformity that indicates an irregular ting a temporary swampy environment consistency between friable and consoli- paleosurface. alternated with typical aeolian facies that dated and it has a fine to medium blocky It is a yellowish brown silty deposit, orga- suggest an important accumulation of structure. In general, it is non-calcareous. nized in coarse to very coarse horizontal dust in the region. Taking into account It is affected locally by bioturbations, strata with poorly defined contacts. At the reference of Darwin about the pre- such as krotovinas (up to 0.75 m diame- Puerto San Martín - the type locality - the ponderant number of fresh-water spe- ter) and root casts (up to 0.60 m long and section is as follows from bottom to top: cies (Polygastrica and Phytolitharia) scraped 0.10 m diameter). In some places sedi- 0-0.60 m: bed composed of brownish from a tooth of one of the mastodons mentary structures are visible. These are olive silt, structured in peds, paludal in found in the red Pampean mud, we infer characterized by an irregular wavy pat- origin. 0.60-1.60 m: bed of yellowish that this corresponds to one of the cha- tern marked by differences in concentra- brown silt, massive, aeolian in origin. racteristics beds of paludal origin of the tions of colloidal materials, namely dissi- 1.60-2.10 m: bed formed by brownish Puerto San Martín Formation. pation structures. This unit forms the olive silt, organized in peds and accumu- Tezanos Pinto Formation (Iriondo 1987) lower sections of cliffs along the Río lated in a swampy environment. 2.10- This formation is the typical LGM loess Carcarañá, forming gentle to subvertical 3.30 m: yellowish brown loess. 3.30-4.80 of the northern Pampas that overlies the slopes, which reflect its resistance in m: bed composed by yellowish brown Puerto San Martín Formation, with comparison with the upper units. The silt, moderately structured, paludal in ori- thickness ranging between 1 and 4 m. maximum outcrop thickness is approxi- gin. 4.80-5.10 m: brownish olive silt, San Guillermo Formation (Iriondo 1987) mately 5.50 m. It is unconformably over- weakly structured, paludal in origin. 5.10- The brownish gray silty deposit (Late lain by the Tezanos Pinto Formation in 6.40 m: stratum of yellowish brown silt, Holocene in age) ends the sedimentary the interfluves and by the Lucio Lopez weakly structured in angular peds. sequence on a truncated soil developed Formation in the main fluvial valleys of

Locally a level with CaCO3 rhizoconcre- on top of the LGM loess. the area. tions appears immediately below the A representative section of the unit is ex- erosional unconformity at the top of the Sections of the lower Río Carcarañá posed along the cliffs of the Río Cañada unit indicating the existence of a Ck ho- (Santa Fe province) de Gómez, near the Río Carcarañá. From rizon of a truncated soils. bottom to top: At the Río Paraná cliff in front of the Darwin was right in making his observa- 0.00-2.00 m: Bed formed by very fine, city of San Lorenzo, two sectors can be tions along the banks of the lower Río slightly silty sand, bright reddish brown differentiated: the lower one is 4 m thick Carcarañá because its basin constitutes a in colour when moist, with dissipation and formed by olive sandy silt with com- favorable area for research on geomor- structures. The sedimentary mass is non- mon ferruginous mottles. The upper part phology and Quaternary stratigraphy of calcareous and contains medium to coar- carries calcareous precipitates of phreatic the northern Pampas plain (Kröhling se and hard Fe-Mn sesquioxide nodules.

origin. The upper sector is 7 m thick and 1999).The exposed sedimentary column There are rhizoconcretions of CaCO3 composed of massive, yellowish brown of the lower Carcarañá basin reveals and abundant very fine macropores. silt, including a pedogenic horizon mar- fluctuating environmental conditions, Concordant upper contact.

ked by CaCO3 concretions. characterized by dry intervals associated 2.00-4.50 m: Bed sedimentologically The Puerto San Martín Formation out- with episodes of accumulation and re- similar to the underlying one, without crops at sectors of the Río Carcarañá mobilization of aeolian sand or dust ac- sedimentary structures, and with less

cliff near the town of Oliveros. On the cumulation during the stages of Glacial resistance to erosion. Numerous CaCO3 left bank the unit is also differentiated Maximum. These alternate with humid rhizoconcretions appear at the contact into two sectors separated by a concor- intervals characterized by pedogenesis between the units. It includes krotovinas dant horizontal contact. The lower silty and the development of fluvial belts. The up to 0.50 m in diameter. Near the top From Buenos Aires to Santa Fe: Darwin's observations ... 115

another soil was developed. It is represented by a poorly developed B horizon, 0.40 m thick, dark brown in colour,

weakly structured and containing CaCO3 rhizoconcretions. The Carcarañá Formation is Late Pleisto- cene in age (OIS 3; a TL dating in the middle section indicates an age of 52.31 ± 1.2 ka BP). The mainly aeolian formation is the result of the reworking by erosion of a dune field generated during the OIS 4. Primary structures of the dunes appear in sections located immediately to the south of the area. Locally different paludal and alluvial facies of the Carca- rañá Formation are present. According to Darwin, “on the banks of the Carcarañá, a few miles distant (of the Paraná cliff at Grondona; next to the Villa La Ribera -Rosario-Santa Fe road-), the lowest bed visible was pale Pampean mud, with masses of tosca-rock, in one of which I found a much decayed tooth of the Mastodon: above this bed, there was a thin layer almost composed of small concretions of white tosca, out of which I extracted a well preserved, but slightly broken tooth of Toxodon Platensis: above this there was an unusual bed of very soft impure sandstone” (Dar- win 1846, p. 88). The section of the Río Carcarañá described by Darwin is interpreted as the Carcarañá Formation. Tezanos Pinto Formation (Iriondo 1980) It is the typical Late Quaternary unit of the Pampas plain, composed of aeolian silts (silt: 71-81%; clay: 13-29%; fine sand: 1-6%). At the base and the top, erosional unconformities connect these deposits with the Carcarañá Formation and with the San Guillermo Formation respectively (Kröhling 1999). Figure 3: General stratigraphic section of the Carcarañá river basin, typical of the Rosario-Santa Fe area (after Kröhling 1999). A primary loess facies or aeolian facies of the Tezanos Pinto Formation has the there is a level formed by a succession of coarse fissures, partly filled by CaCO3 greatest areal representation, with a typi- erosional geoforms, 0.50-0.80 m wide precipitates). The sediment is weakly cal- cal outcropping thickness of 2-4 m, and and 0.40-0.50 m high. careous to calcareous. There are nume- more developed on the interfluves (6-8 Relicts of a palaeosol appear disconti- rous epigenetic carbonate concretions m thick). It is a loose deposit, coarse silt nuously at the bottoms of gullies within produced by partial dissolution. CaCO3 with subordinate clay and fine sand, light the fluvial palaeovalleys of the region. concretions of phreatic origin occupy brown in colour (7.5 YR 6/4). It is a ho- Lateral variations are common in that coarse fissures between peds. Locally, mogeneous, porous and permeable de- soil: the structure changes from coarse to this level is represented by palaeogullies posit with a coarse granular to medium- very coarse blocky (strong) up to angular filled with retransported soil fragments. coarse blocky fabric. The sedimentary very coarse prismatic (strong, defined by At the top of the Carcarañá Formation mass is calcareous; it contains powdery 116 M. IRIONDO AND D. KRÖHLING

concentrations and hard concretions of sional unconformity. It is composed of a main rivers and at the bottom of the

CaCO3 (in a variable frequency; varied brownish grey (10 YR 5/1) coarse silt major fluvial palaeovalleys of the region. forms and centimetric sizes). The loess with scarce proportions of very fine sand A TL date in the upper section of the body is crossed by fine rhizoid ramified and clay. The unit is massive, friable in formation (a cineritic stratum) indicates canalicula. It is stable in steep walls, in general, porous, permeable, and modera- an age of 1.32 ± 0.12 ka BP. The soil parts altered by subcutaneous subfusion tely structured in very coarse firm prisms complex of the middle section was gene- and shaped by columnar disjunction. bounded by very fine fissures. This depo- rated during the Holocene Optimum Cli- TL datings gave ages of 31.69 ± 1.62 ka sit lacks nodules or mottles, but contains maticum. The formation was generated BP and 32.0 ka BP in samples of the aeo- numerous root moulds, abundant macro- during the Holocene and possibly even lian facies of the formation in the area, at pores, very fine and fine canalicula and the Late Pleistocene too. 4 m below the top of the unit (LGM, tubes generated by bioturbation. The se- This unit correlates with the Luján For- OIS 2). diment mass is non-calcareous. The unit mation defined by Fidalgo et al. (1973), Buried soil (Kröhling 1999) is partly the product of deflation of the representing the Late Quaternary conti- The top of Tezanos Pinto Formation is A horizon of the underlying soil and the nental infilling of the fluvial valleys of marked by a partially eroded soil, 0.30 - subsequent deposition of dust. The typi- NE Buenos Aires province. Toledo 1.10 m thick, typically represented by a cal thickness is 0.30 m, with a maximum (2005) presented a sequence stratigraphic Bt horizon. In a minor geomorphological of 0.55 m. model in the Lujan type section of this unit (fluvial valleys of the region) that The San Guillermo Formation generally unit, based on detailed stratigraphic ob- soil forms an accretionary pedocomplex forms the top of the sedimentary se- servations and C14 datings on mollusks. of argillic horizons separated by the ac- quence in the natural sections of the The author refered ages from > 40 ka BP cumulation of Andean volcanic ash, Pampas. It was deposited during a Late to >11 ka for the Guerrero Member (flu- mainly concentrated by alluvial proces- Holocene dry period that occurred bet- vial-paludal deposits or Lujanense) and ses. At the interfluves of the region, the ween 3.5 and 1.4 ka BP (Iriondo 1990). ages from 10.7 to 3.3 ka BP for the Río Bt-horizon is formed by a dark brown Lucio Lopez Formation (Kröhling 1996) Salado Member of the Luján Formation (7.5 YR 3/4) clayish silt, with intense illu- It constitutes a complex sequence diffe- (grayish to whitish silty facies or Platense). viation and formation of cutans. It has a rentiated into three sectors. It includes a Prieto et al. (2004) gave ages ranging strongly to moderately developed struc- clayish silt with scarce very fine sand (silt: from 11 to 3.5 ka BP for the Luján For- ture in medium angular prisms, strong 58-77%; clay: 19-37%; sand: 3-6%), orga- mation in the Río Luján. consistence, with very fine and fine nized in fine strata with variations in cracks between peds. It lacks nodules or colour from olive to grey; pedogenic LA BAJADA SECTION mottles, and the silt fraction is non-calca- horizons are intercalated in the middle reous. There are abundant root moulds. section (pedocomplex). Locally, it exhi- The town La Bajada is located across the The lower boundary is wavy in form and bits high proportions of biogenic mate- Río Paraná from Santa Fe. At present, well marked. The Bt horizon is compo- rial (mainly gastropods, ostracods, dia- that locality (nowadays Paraná) is the ca- sed of a dull brown (7.5 YR 5/4) slightly toms and abundant plant remains). The pital city of Entre Ríos province. Such clayish coarse silt. This horizon shows a Lucio Lopez Formation outcrops along area is crossed by an old transcurrent weakly to moderately well-developed the cliffs of the main rivers of the nor- fracture, the Tostado-Gualeguaychú fault, structure, formed by medium blocks, ve- thern Pampas region, with most typical which elevated its southern block some ry firm, limited by fine and very fine fis- characteristics and the greatest thickness 40 m in the Middle Pleistocene (Iriondo sures. Nodules or mottles are absent, but on the Río Carcarañá cliffs. The thick- 1989), thus exposing Tertiary marine fine macropores are present. The silt ness varies from 0.50 to 5.00 m. The for- strata. Darwin payed special attention to fraction is non-calcareous. The C hori- mation unconformably overlies the Car- those strata, collected an interesting fos- zon shows characteristics transitional to carañá Formation in sections located in- sil assemblage and made an excellent the underlying loess. side the fluvial valleys of the region. The stratigraphic description of the section. The buried soil was generated during the contact indicates a palaeotopography of Darwin quoted that "In Entre Ríos, the Holocene Optimum Climaticum and it is channels and gullies. In general the for- cliffs, estimated at between sixty and seventy feet a distinct pedostratigraphic marker in dif- mation is covered by contemporary or in height, expose an interesting section: the lower ferent areas of the Pampas plain. recent deposits of fluvial origin; locally half consists of Tertiary strata with marine San Guillermo Formation (Iriondo 1987) the unit forms the top of the section. shells, and the upper half of the Pampean for- A younger loessic formation overlies the The formation, with a clear paludal ori- mation. The lowest bed is obliquely laminated, buried soil on top of the Tezanos Pinto gin, constitutes the infilling of erosional blackish, indurated mud, with distinct traces of Formation, separated from it by an ero- landforms located in a recent belt of the vegetable remains. Above this there is a thick From Buenos Aires to Santa Fe: Darwin's observations ... 117

bed of yellowish sandy clay, with much crystalli- zed gypsum and many shells of Ostreae, Pectens, and Arcae: above this generally comes an arenaceous crystalline limestone, but there is sometimes interposed a bed, about twelve feet thick, of dark green, soapy clay, weathering into small angular fragments. The limestone, where purest, is white, highly crystalline, and full of cavities: it includes small pebbles of quartz, broken shells, teeth of sharks, and sometimes, as I was informed, large bones: it often contains so much sand as to pass into a calcareous sandstone, and in such parts the great Ostrea patagonica chiefly abounds. In the upper part, the limestone alternates with layers of fine white sand. The shells included in these beds have been named for me by M. d'Orbigny (a list of fossils follows) - M. d'Orbigny has given a detailed description of this section, but as he does not mention this lowest bed, it may have been concealed when he was there by the river. There is a considerable discrepancy between his description and mine, which I can only account for by the beds themselves varying considerably in short distances" (Darwin 1846, p. 88-89). Figure 4: The Paraná Formation outcropping in La In fact, the major discrepancies between Bajada. A: Type profile both naturalists appeared because Dar- (Iriondo 1973). B: Darwin´s win worked in the elevated block of the profile. fault and d'Orbigny researched in the Fig. 4): Obliquely laminated, blackish, indura- - Third bed - Darwin: 4 m. Dark green, downthrown block, where the marine ted mud, with distinct traces of plant remains. soapy clay weathering into small angular frag- beds appear only during low waters. Be- Type section: 3 m. Green quartz sand in ments. sides, as Darwin correctly stated, the stra- 30-50 cm thick strata, including a large Type section: 6 m. Sequence of interes- ta vary in short distances. However, Dar- proportion of clay intraclasts, most of tratified sand and clayish silt. The sand win's description fits considerably well them are platy angular fragments without bodies are short lenses with internal dia- with the type section of the Paraná For- rounding. Major axes of plates are para- gonal lamination, up to 1.60 m thick. mation, formally defined there 150 years llel to stratification planes. Mean grain Loose, quartzose and yellow in colour. later by Iriondo (1973). size around 0.2 mm, finer towards the Sandy bodies are generally composed of The marine beds of Bajada were rena- bottom. Transitional upper contact. 5-15 cm thick internal units with diago- med as Paraná Formation in the 20th - Second bed - Darwin: Yellowish sandy nal stratification. The fine sediments are Century. This formation represents the clay, with much arenaceous crystalline gypsum plastic, gray clayey silts, forming conti- last widespread marine transgression and Ostreae. nuous strata 1 to 30 cm thick and at least occurring in the interior of South Ame- Type section: 3 m. Massive green sand, 50-60 m long; internal laminae of sand rica (Yrigoyen 1969, Herbst 1971, Irion- without internal bedding, containing up are common. In parts with contorted in- do 1973, Marengo 2005) and the Bajada- to 10 cm long elliptic clay intraclasts. ternal lamination. Silt strata conformably Paraná area is considered the type loca- 20% of the sediment mass is composed cover the underlying sand lenses. Grain lity. Darwin's section, indeed, can clearly of chaotically distributed small platy size of the fines is 30 microns at the bot- be recognized in the type section of the intraclasts. Grain size around 0.25 mm tom, diminishing upwards to clay-sizes. formation (Iriondo 1973, Fig. 4): with variable dispersion, which grows The upper section of this bed changes to

- Both sections are composed of five upwards. Lightly cemented with CaCO3. white sandstone with dune structures beds and have similar thicknesses; a com- The detachment and fall of intraclasts containing well preserved bivalves. parison shows the following: produces numerous small hollows in out- Further on, the bed passes to a fine con- - The lowest bed - Darwin (profile B of crops. Neat upward contact. glomerate. 118 M. IRIONDO AND D. KRÖHLING

- Fourth bed - Darwin: Sandy crystalline was a shallow, platform branch of the Punta Gorda Section limestone. Where purest, it is white, highly Atlantic Ocean. Mean thickness of the crystalline, and full of cavities: it includes small formation is about 100 m; the sediment The Quaternary geologic column of the pebbles of quartz, broken shells, teeth of sharks is dominantly sandy in the type area (Pa- region (Kröhling 2001) records two well and sometimes large bones: it often contains so raná; La Bajada in Darwin times) and defined aeolian sedimentation cycles. In much sand as to pass into a calcareous sandsto- characterizes the upper half of the sec- both cases, the main source of sediment ne, and in such parts the great Ostrea patagoni- tion in all the basin. The lower half appe- has been the Andean region, that produ- ca abounds (Darwin 1846, p. 89). ars as composed of green sand. ced fine materials originated by nival pro- Type section: 1.5 m. White calcareous A research borehole drilled in the center cesses and volcanic eruptions. sandstone with dune structures and un- of the basin in the northern Pampas (San The older cycle, defined as the Punta dulose strata. Lateral and upward enrich- Guillermo locality; 30°15'S, 61°50'W) Gorda Group, is composed of three ment with shells of bivalves and scarce recorded 40.5 m of the Paraná Forma- units accumulated in paludal and sub- oysters. Bivalve shells underwent an ad- tion covered by 60.3 m of Quaternary aereal environments during the Early vanced degree of dissolution and re-pre- loessic formations (Kröhling and Iriondo Pleistocene. The second sedimentary cipitation, remaining often only as 2003). cycle occurred during the Late Pleisto- moulds. Oysters are well preserved. Following Darwin, "The upper half of the cene and Holocene (basically at the Last - Fifth (upper) bed - Darwin: The limesto- cliff in La Bajada, to a thickness of about Glacial Maximum) and is the Pampean ne alternates with layers of fine white sand. thirty feet, consists of Pampean mud, of which Aeolian System traversed by Darwin Type section: 5.5 m. Most of the outcrop the lower part is pale-coloured, and the upper from Buenos Aires to Santa Fe. It com- is covered by debris. The lower sector is part of a brighter red… Close above the mari- prises two formations accumulated in characterized by white sand, incipiently ne limestone, there is a thin stratum with a con- subaereal environments. A short period cemented by the carbonate provided by cretionary outline of white hard tosca-rock or of aeolian remobilization occurred the local dissolution of shells; the upper marl…" Three Quaternary formations during the Late Holocene. section is composed of fine quartz sand. can be recognized in that reference: The Several episodes of pedogenesis and lo- Green in colour. upper one is Tezanos Pinto Formation, cal erosional unconformities were recor- The general scenario suggested by the the main loessic deposit of the Last Gla- ded in both major sedimentation periods, section in both approaches (Darwin´s cial Maximum. The "lower mud" is the particularly in the older one. The long and modern) is one of a tidal environ- Hernandarias Formation, an Early Pleis- sedimentation hiatus, covering all the ment, with subtidal sands and gravels, tocene playa deposit of the Río Uruguay. Middle Pleistocene and a half of the Late intertidal muds, changing tidal channels The tosca-rock is a well developed level, Pleistocene constitutes a remarkable and coastal currents. Two different sedi- which thickens southwards up to 9 m and regional feature. mentary mechanisms can be deduced: a) is known as Puerto Alvear Formation. Important similarities were found betwe- Transport and accumulation of sand by en both sedimentary cycles: i) the same tractive currents, probably from nearby SOUTHWESTERN source of sediments; ii) the same long beaches if one considers the negative ENTRE RÍOS distance transport agent (wind); iii) stri- skewness of the sediment. b) Floccu- king similarity in grain-size distributions; lation of fines in calm environments. A key area of the region is located in the iv) scarce contribution of materials from The fossil assemblage also points to a lit- southwestern part of Entre Ríos provin- the Brazilian shield and associated areas toral/neritic position in a warm climate. ce and outcrops along the eastern cliff of in the north. A general setting of the Paraná Forma- the Paraná flood plain. Darwin did not The age of the older cycle can be located tion is as follows: The Paraná Formation study such outcrops, but navigated along between the Brunhes - Matuyama mag- covers an important sector of a major them in his journey back to Buenos Ai- netic polarity change (the Upper Matuya- geological event occurring in the Neo- res. However, he made a few comments ma Chron > 0.78 Ma.) and the Jaramillo gene of South America. Around the on "the Punta Gorda in Entre Ríos" (diffe- Subchron (1 Ma). This conclusion coin- Middle Miocene occurred a generalized rent from the other Punta Gorda in the cides with recent datings in the "Great marine transgression in large regions of Banda Oriental), which is an interesting Patagonic Glaciation" (Ton-That et al. the continent. The Paraná Formation sedimentary cycle that occurred around 1999), which rendered ages from 1 Ma to comprises the portion included in the one million years before present. That 1.17 Ma. The following sedimentary Chaco-Paranense Basin, which extends locality is placed 60 km south of Santa Fe units outcrop at Punta Gorda: from the central lowlands of Argentina and has the best stratigraphic section of Punta Gorda Group (Iriondo 1980): Sedi- at 36° lat. S northwards to the Bolivian the area (Fig. 5) in Entre Ríos. mentary cycle 1 and Paraguayan Chaco. The Miocene sea Puerto Alvear Formation (Iriondo 1980) From Buenos Aires to Santa Fe: Darwin's observations ... 119

Figure 5: Punta Gorda section, representative of SW Entre Ríos province (after Iriondo and Kröhling 2008).

This is a sedimentary body accumulated ness of 9 m. The most visible field featu- The clastic component of the unit is a as infilling of an abandoned belt of the re is a closed net formed by CaCO3 pla- silty clay to sandy silt with diffuse lamina- Río Paraná. The unit was a non-perma- tes of phreatic origin, 0.5 to 4 cm thick tion, light reddish brown in colour with nent swamp at the beginning of the with a dominant horizontal develop- olive patches. Abundant Mn- and Fe se- Pleistocene. The formation lies uncon- ment. Numerous vertical large and bo- gregations and nodules are conspicuous. formably on the Paraná Formation (ma- trioidal concretions indicate a post-depo- This formation is divided into two mem- rine Miocene), with a maximum thick- sitional remobilization of the carbonate. bers; the lower one is characterized by 120 M. IRIONDO AND D. KRÖHLING

thick continuous partition walls, with a calcic horizon formed by vertical cylin- The sedimentary sequence of Punta wavy trace. The upper member includes drical welded concretions 50 cm high. Gorda ends with an aeolian unit uncon- a similar horizontal carbonate net, The central section is formed by 4 m of formably accumulated over the eroded although formed by thinner and more homogeneuos light brown loess, with a soil. It is a brownish gray loess, 20 to 35

irregular platy precipitates. An internal vertical slope. A lattice pattern of CaCO3 cm thick, loamy to silty-loamy. This unit unconformity separates both members; precipitates appears in the exposures. is the result of deflation of the A-hori- in some places the top of the lower Three weak to moderate paleosols (B- zon of the soil on top of the Tezanos member are the Bw-and C horizons of a and Ck horizons) were recorded at the Pinto Loess and the subsequent deposi- weakly developed paleosol. That pedoge- typical site. These soils are laterally dis- tion of dust. Such an event occurred nic level is non-calcareous. continuous as a result of uneven erosio- from 3.5 to 1.4 ka BP. La Juanita Formation (Iriondo 1998) nal contact with the overlying loess. They This unit was formed in a paludal envi- are re-calcified from this material too. THE PARANÁ FLOOD ronment during the Early Pleistocene. It The upper section begins with a Ckm PLAIN lays unconformably on the Puerto Alvear horizon of another eroded paleosol, Formation, with a typical thickness of 2 covered by a one meter thick paludal Darwin travelled from Santa Fe back to to 3 m. The La Juanita Formation is deposit. Buenos Aires by boat along the Paraná composed of light brown to olive silt Tezanos Pinto Formation (Iriondo 1980): and made some remarks about the river, with scarce fine sand with horizontal dif- Sedimentary cycle 2 its banks and fauna. Particularly, he noted fuse bedding. Strata are 15 to 30 cm The unit constitutes the Peripheral Loess the large number of islands that are per- thick, with internal lamination. Fe- and Belt of the Pampean Aeolian System, manently eroded and reconstructed by Mn segregations and root moulds are lo- which was deposited during the OIS 2 the currents. cally abundant. The sediment is slightly (36-8 ka BP, Iriondo and Kröhling 1995). In fact, according to modern geomor- structured in firm blocks. Concretions, The aeolian dust was transported from phology the river flows there within a which form more than 10% of the sedi- Andean sources by S and SW winds. In large flood plain, approximately 600 km mentary mass, are rough vertical infi- SW Entre Ríos province the unit is yello- long, stretching from the Paraguay- llings of roots, 2 to 5 cm thick and up to wish brown loess, 2 to 3 m thick, compo- Paraná junction (27°25'S) to south of 20 cm long. The general tendency of the sed of powderish sandy silt, massive and Rosario (33°S). Downstream it passes

CaCO3 precipitates indicates infiltration friable. OSL ages of 32 and 24 ka BP transitionally to a well-developed Holo- in a non-permanent swamp environment were obtained for the loess in this region. cene littoral complex. The general direc- with a non-saturated bottom. There are The loess forms typical vertical slopes, tion of the flood plain is north-south, also infillings of horizontal fissures. Fre- with columnar disjunction. It contains crossing several tectonic blocks in a per-

quent Mn films cover the surface of con- small CaCO3 concretions and frequent pendicular or diagonal manner. This rela- cretions, indicating a younger age for the rhizoconcretions (15.48 ± 0.19 C14 ka tionship has generated local segments mobilization of such oxides. BP).The unit received contributions of within the plain, with slightly different Punta Gorda Formation (Iriondo and Kröhling fine sediments from the Paraná headwa- directions and slopes. The width of the 2008) ters, located to the northeast. floodplain varies from 13 to 45 km, with This is a brown loess characterized by The clay fraction of the loess is a com- typical values between 25 and 35 km. It is pedogenesis in several levels and fre- plex mixture of illite, interstratified illite- composed of several internal geomor- quent local internal unconformities. Ac- smectites and kaolinite. Illite is a pampe- phological units, the most modern of cording to palaeomagnetic analyses, the an mineral whereas smectites and kaoli- them is the “bar plain”, that is developing age of the unit is Early Pleistocene. Car- nite are derived from Brazil. A partially at present (Iriondo 2007). bonatic cement is widespread in the for- eroded soil on top of the loess is repre- The bar plain is a belt of large elliptic mation. Local second order variations are sented by well developed Bt-horizon and bars composed of very fine sand, that the rule in most outcrops. Three sections C horizons. The Bt horizon is 35 to 55 encloses the main channel along the were described in the unit at the type cm thick, clayey silt dark brown in colour, whole flood plain. This belt is perma- locality (Punta Gorda, Entre Ríos pro- structured in very firm prisms; argillocu- nently modified by erosion and sedimen- vince). The lower one was unconforma- tans are frequent. The buried soil was de- tation and has a width of 2-7 km; it is bly deposited over La Juanita Formation veloped under humid subtropical condi- formed by islands inside the main (navi- and begins with a paludal deposit com- tions during the Holocene Optimum gation) channel and sand bars juxtaposed posed of clayish silt, light brown in Climaticum. to the margins. The large bars are crossed colour, with Fe- and Mn segregations. San Guillermo Formation (Iriondo 1980): by minor active channels, most of them This is conformably overlain by a petro- Sedimentary cycle 2 with intense lateral migration, resulting in From Buenos Aires to Santa Fe: Darwin's observations ... 121

minor meander belts. The powerful brai- te period (from 3.5 to 1.4 ka BP, accor- tled, with numerous pores, tubes and ded mainstream - with a mean discharge ding to the latest datings). Water dischar- root moulds. Gley processes are fre- between 16,000 and 20,000 m3/sec - con- ges were lower than today in all the quent. c) Lake-and-swamp facies, com- tinuously reshapes islands and banks. system and tributaries did not reach the posed of dark gray silt with abundant This is the environment described by collector, depositing the sediment loads organic matter in different degrees of de- Darwin. in lateral positions inside the flood plain; composition. It is compact, with low po- Other important geomorphological units the Paraná River itself was also smaller rosity; sand and clay are scarce. A really of the flood plain are the hindered drai- then than it is today. That resulted in rela- modest proportion of the sediment volu- nage plain and the deltas of tributaries. tively short and wide delta-like sedimen- me is represented by a fourth facies, i.e., The hindered drainage plain was descri- tary bodies. Most of such deltas (Los aeolian dunes composed of very fine bed by Darwin as a labyrinth of small Amores, del Rey, Corrientes) are located sand, which appear in isolated spots in branches separated by low islands cove- north of Santa Fe, a region not visited by the northern half of the flood plain. red by forest, he observed this landscape Darwin owing to health problems. Te- when crossing the flood plain from Santa rraces also appear in the north, in Chaco THE LITTORAL COMPLEX Fe to La Bajada. This unit is characteri- and Corrientes provinces. AT THE RÍO PARANÁ zed by a morphology of flat areas with The sediments of the flood plain are MOUTH numerous shallow ponds, swamps and very homogeneous. Practically all the se- small adventitious channels. Such chan- dimentary mass is composed of only Darwin noted that "Some leagues downstre- nels are tortuous, active only at the begin- three sedimentary facies: a) Channel fa- am Rosario begins, at the western margin of the ning and end of floods, conveying water cies, which are composed of fine and Paraná, a line of perpendicular cliffs that into ponds during rising and back to the very fine quartz sand, generally in 10-40 extends beneath San Nicolás…The banks of main channel at low levels. The number cm thick horizontal strata with diagonal the river are formed by very soft earths; in conse- of ponds and shallow lakes is huge - internal structures. Such sand was origi- quence the waters are muddy…". The author about 5,000 according to Paira and nated in Mesozoic sandstones of the up- has entered the Paraná Delta, which in Drago (2007). This system was genera- per basin and comprises the major part fact is a large littoral complex of Holo- ted under a hydric regime different than of the buried sediments. b) Levee facies, cene age formed by several geomorpho- the present one. The deltas of tributaries which are characterized by silty-clayish logical and sedimentary units sequentially are areas of sedimentation formed inside very fine sand with intermediate plasti- generated. Four main phases have been the flood plain during a recent dry clima- city; ochre to greenish-gray in color, mot- identified there by Iriondo (2004, Fig. 6):

Figure 6: Map of the Holocene littoral complex at the Paraná mouth (after Iriondo 2004). 122 M. IRIONDO AND D. KRÖHLING

1) A fluvial period represented by river science and acute synthetic descriptions. Iriondo, M. 1973. Análisis ambiental de la For- flood deposits; 2) A marine ingression In order to make a comparison between mación Paraná en su área tipo. Boletín de la with development of a sand barrier, a the geological contributions of Darwin Asociación Geológica de Córdoba 2(1-2): 19- lagoon, minor tributary deltas and estua- and the present knowledge on the region, 23. ries and well-developed regression depo- the authors of this contribution necessa- Iriondo, M. 1980. El Cuaternario de Entre Ríos. sits; 3) An estuarine phase characterized rily follow the general structure of Revista de la Asociación de Ciencias Natu- by extensive tidal deposits in the central Darwin's books, a traveler's diary enri- rales del Litoral 11: 125-141, Santo Tomé. area; and 4) The present fluvial period, ched by digressions on Science, Politics Iriondo, M. l987. Geomorfología y Cuaternario with channel deposits and a deltaic unit and History. de la Provincia de Santa Fe. D'Orbignyana advancing into the Río de la Plata. The 4: 1-54, Corrientes. present dynamics of the deltaic complex WORKS CITED IN THE TEXT Iriondo, M. 1989. Major fractures of the Chaco- is dominated by the Paraná floods, the Pampa plain. Bulletin of the INQUA Neo- floods of the Uruguay and Gualeguay De Alba, E. 1953. Geología del Alto Paraná - tectonic Commission 12: 42-46, Stockholm. rivers, Atlantic tides, and floods produ- Relación con los trabajos de derrocamiento Iriondo, M. 1990. A Late Holocene dry period in ced by the southeasterly winds. entre Ituzaingó y Posadas. Revista de la Aso- the Argentine plains. Quaternary of South The southeasterly wind (Sudestada), inde- ciación Geológica Argentina 8:129-161. America and Antarctic Peninsula 7: 197-218. ed, is a major factor in the water dyna- Darwin, C. 1846. Geological observations on Iriondo, M. 1998. 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Geomorphology and Quater- surely, Darwin underwent such a meteo- edition), Buenos Aires. nary of the Flood-Plain. In Iriondo, M., rological phenomenon: "At night, the wind Fidalgo, F., De Francesco F. and Colado, U. Paggi, J. and Parma J. (eds.) Limnology of the is scantly favorable and we stop; the next day (1973). Geología superficial de las Hojas Middle Paraná River: A Subtropical Wetland, blows a strong wind…" Castelli, J.M. Cobo y Monasterio (provincia de Springer Verlag, 382 p., Berlin, Heidelberg, Buenos Aires). 5° Congreso Geológico Ar- London. COMMENTS AND gentino (Córdoba), Actas 4: 27-39. Iriondo, M. and Kröhling, D. 1995. El Sistema CONCLUSIONS Georgieff, S., Anis, K., Orfeo, O., and Rizo, G. Eólico Pampeano. Comunicaciones del Mu- 2005. Architecture of Ituzaingo Formation seo Provincial de Ciencias Naturales "Floren- Undoubtedly, the short incursion of the (Pliocene), a comparison with deposits of tino Ameghino" (NS) 5(1): 1-68, Santa Fe. great naturalist to the interior of the Paraná River, Argentina. University of Tech- Iriondo, M. and Kröhling, D. 2008. Cambios Argentine plains was a very modest por- nology, 8th. International Conference on ambientales en la cuenca del Uruguay (desde tion of his famous trip around the world, Fluvial Sedimentology, Abstracts: 107, Delft. el Presente hasta dos millones de años atrás). with plenty of discoveries, experiences González Bonorino, F. 1965. Mineralogía de las Colección Ciencia y Técnica, Ediciones UNL and amazing landscapes such as Tahiti or fracciones arcilla y limo del Pampeano en el 358 p., Santa Fe. Tierra del Fuego. The attention paid by área de la ciudad de Buenos Aires y su signifi- Kemp, R., Zárate, M, Toms, P. King, M., Sa- Darwin to these flat plains in his subse- cación estratigráfica y sedimentológica. Re- nabria, J. and Arguello, G. 2006. Late Quater- quent studies was necessarily far more vista de la Asociación Geológica Argentina nary paleosols, stratigraphy and landscape modest than those devoted to the Gala- 20: 67-148. evolution in the Northern Pampa, Argentina. pagos Islands or the Andes Cordillera. Herbst, R. 1971. Esquema estratigráfico de la Quaternary Research 66: 119-132. However, the few pages dedicated to the provincia de Corrientes. Revista de la Asocia- Kröhling, D. 1996. La Formación Lucio Lopez Buenos Aires-Santa Fe journey reveal the ción Geológica Argentina 26: 221-243. (Holoceno), noreste de la llanura pampeana. extraordinary abilities of Darwin for Imbellone, P.E. and Cumba, A. 2003. Una suce- 13° Congreso Geológico Argentino (Buenos keen observation and logical thinking, in sión con paleosuelos superpuestos del Pleis- Aires), Actas 4: 69. spite of the limitations of geological the- toceno medio tardío-Holoceno, zona sur de la Kröhling, D. 1998. Geomorfología y Geología ory at that time. No real errors can be Plata, provincia de Buenos Aires. Revista de la del Cuaternario de la cuenca del río Carca- noted in his geological observations Asociación Argentina de Sedimentología 10: rañá, desde la confluencia de los ríos Tercero recorded during this journey, rather good 3-22, La Plata. y Cuarto, provincias de Santa Fe y Córdoba. From Buenos Aires to Santa Fe: Darwin's observations ... 123

Doctoral Thesis, Universidad Nacional de Baradero, Provincia de Buenos Aires. Revista (Pleistocene) in a quarry near La Plata, Ar- Córdoba (unpublished), 224 p., Córdoba. de la Asociación Geológica Argentina 48: gentina. Journal of South American Earth Kröhling, D. 1999. Upper Quaternary of the 193-206. Sciences 12: 273-291. Lower Carcarañá Basin, North Pampa, Ar- Paira, A. and Drago, E. 2007. Origin, evolution Ton-That, T., Singer, B., Mörner, N. and Rabassa, gentina. In Partridge, T., Kershaw, P. and and types of flood plain water bodies. In J. 1999. Datación de lavas basálticas por Ar/ Iriondo, M. (eds.) Paleoclimates of the Iriondo, M., Paggi, J. and Parma, M. (eds.) The Ar y geología glacial de la región del lago Southern Hemisphere, Quaternary Interna- Middle Paraná: Limnology of a Subtropical Buenos Aires, prov. de Santa Cruz, Argentina. tional 57/58: 135-148, Oxford. Wetland, Springer 3: 53-82, Heidelberg. Revista de la Asociación Geológica Argentina Kröhling, D. 2001. Quaternary paleosol: loessic Prieto, A., Blassi, A., De Francesco, C. and Fer- 54: 333-352. sequences of southwestern Entre Ríos Pro- nández, C. 2004. Environmental history since Yrigoyen, M. 1969. Problemas estratigráficos del vince, Northern Pampa, Argentina. 6° Inter- 11,000 C14 yr. BP of the northeastern Pam- Terciario de Argentina. Ameghiniana 6: 315- national Symposium and Field Workshop on pas, Argentina, from alluvial sequences of the 328. Paleopedology in Mexico City (INQUA - Luján River. Quaternary Research 62: 146- ISSS), Proceedings: 27-28, México. 161. Kröhling, D. and Iriondo, M. 2003. El loess de la Parra, P. 1939. Diario y derrotero de sus viajes. Pampa Norte en el Bloque de San Guillermo. Editorial Solar, 251 p., Buenos Aires. Revista de la Asociación Argentina de Sedi- Riggi, J., Fidalgo, F., Martinez, O. and Porro, N. mentología 10(2): 137-150, La Plata. 1986. Geología de los "Sedimentos Pampea- Marengo, H. 2005. Micropaleontología y Estra- nos" en el Partido de La Plata. Revista de la tigrafía del Mioceno Marino de la Argentina: Asociación Geológica Argentina 41: 316-333. las transgresiones de Laguna Paiva y del ¨En- Toledo, M. 2005. Secuencias pleistocenas "luja- trerriense-Paranense¨. Doctoral Thesis, Uni- nenses" en su sección tipo: Primeras datacio- versidad de Buenos Aires (unpublished), 150 nes C14 e implicancias estratigráficas, arqueo- p., Buenos Aires. lógicas e históricas, Luján-Jáuregui, provincia Mercer, J. 1976. Glacial history of Southernmost de Buenos Aires. Revista de la Asociación South America. Quaternary Research 6: 125- Geológica Argentina, Comunicaciones 60(2): 166. 417-424. Nabel, P.E., Camilión, M.C., Machado, G.A., Tonni, E.P., Nabel, P., Cione, A.L., Etchichury, Spiegelman, A. and Mormeneo, L. 1993. M., Tófalo, R., Scillato Yané, G., San Cris- Magneto y litoestrafigrafía de los sedimentos tóbal, J., Carlini, A. and Vargas, D. 1999. The Recibido: 22 de agosto de 2008 pampeanos en los alrededores de la ciudad de Ensenada and Buenos Aires formations Aceptado: 27 de octubre de 2008 124 Revista de la Asociación Geológica Argentina 64 (1): 124 - 136 (2009)

ON THE FORMATIONS OF THE PAMPAS IN THE FOOTSTEPS OF DARWIN: SOUTH OF THE SALADO

Marcelo ZÁRATE1 and Alicia FOLGUERA2

1 CONICET- Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Santa Rosa, La Pampa. E-mail: [email protected] 2 Servicio Geológico Minero Argentino (SEGEMAR), Buenos Aires. Email: [email protected]

ABSTRACT In 1833 during his journey across the Buenos Aires Pampas, Charles Darwin made observations that reflected his thoughts on two major landscape units, Pampa interserrana and Pampa deprimida, later identified by other authors. Darwin grouped the Pampean sediments into a single unit, the Pampean Formation, based upon the lithological homogeneity and the large extension of the deposits; the unit was thought to be of estuarine-marine origin and attributed to the Recent Epoch considering the paleontological content (vertebrates and mollusks). At present, the Pampean sedimentary succession, which accumulated approximately during the last 11-12 Ma, is interpreted as a pedosedimentary sequence due to the ubiquity of pedogenetic features throughout the deposits. Four main subcycles of sedimentation are identified related to reactivations of the Pampean landscape. At a regional scale, the outcrop distribution of Pampean sediments of different ages suggests the dominance of more stable conditions since the late Miocene-Pliocene in a vast area of Pampa interserrana, documented by the formation of calcretes. However, sedimentation during the late Pliocene-Pleistocene was active within the domain of the Salado tectonic basin and Sierras de Tandil. The regional disparity shown by the Pampean stratigraphic record reveals the major morphostructural differences of its basement.

Keywords: Charles Darwin, Pampean sediments, Calcrete, Geomorphology, Buenos Aires.

RESUMEN: Sobre las formaciones de las Pampas en los pasos de Darwin: al sur del Salado. Durante su viaje por la Pampa bonaerense en 1833 Charles Darwin efectuó observaciones que reflejaban las dos grandes unidades de paisaje posteriormente reconocidas en la región, la Pampa interserrana y la Pampa deprimida. La homogeneidad litológica y la vasta extensión de los depósitos fueron los criterios básicos empleados para agruparlos en una única unidad, la Formación Pampeano; basado en criterios pale- ontológicos le atribuyó origen estuárico-marino y la asignó a la época Reciente. Si bien han existido otras propuestas estrati- gráficas, Formación Pampeano o simplemente Pampeano, son denominaciones empleadas informalmente y con un significado algo vago. Actualmente, sobre la base de la ubicuidad de los rasgos pedológicos de la sucesión sedimentaria se la interpreta como una secuencia pedosedimentaria acumulada durante los últimos 11-12 millones de años, se han reconocido 4 ciclos de sedimentación principales relacionados con reactivaciones del paisaje pampeano. Regionalmente, la distribución areal de niveles aflorantes de distintas edades sugiere una mayor estabilidad relativa en el sur, marcada por la formación de calcretos en la Pampa interserrana a partir del Mioceno tardío-Plioceno y mayor continuidad del proceso sedimentario en el ámbito de la cuenca tectónica del Salado y de las sierras de Tandil. Las variaciones regionales exhibidas por el registro estratigráfico del Cenozoico tardío constituyen una manifestación del comportamiento diferencial de las unidades morfoestructurales que inte- gran el basamento de la sucesión sedimentaria.

Palabras clave: Charles Darwin, Sedimentos pampeanos, Tosca, Geomorfología, Buenos Aires.

INTRODUCTION creation of a universal scheme of the continuing later to Santa Fe. Logistics for Earth history in which all the regions of the trip was provided by Juan Manuel de In the course of the winter of 1833, the world would be incorporated (Pod- Rosas - the Argentine administrator at Charles Darwin travelled across the Bue- gorny et al. 2008). the time - in the form of a passport to nos Aires Pampas. His observations, as In early August of 1833, Darwin disem- travel, horses, and a group of soldiers well as those by Alcide d´Orbigny (1842), barked from the Beagle at Patagones, a and gauchos to escort him. In his diary, the French naturalist who explored the town by the Rio Negro, in northern Pa- Darwin took notes on the Pampean sce- region some years before Darwin, were tagonia and began his journey heading nery, referring to the dominant vegeta- part of the initial expansion stage of north towards Bahía Blanca. From there tion, the available water sources, the to- geology as a science. The explorations of he travelled across the southern Pampas pographic features and the sediments both naturalists are framed within the to the Rio Salado area and Buenos Aires, and rocks he found along the journey. He On the formations of the Pampas in the footsteps of Darwin: ... 125

also provided a historical picture of Ar- gentina with descriptions and his own viewpoints on the people and the ongoing war with the aboriginal groups to expand the frontier of the southern terri- tories westward. The information gathered during the field trip was the raw material to write On the Formations of the Pam- pas, the fourth chapter of his Geological Observations on South America (1846). The vast extent, the disputed origin and the abundance of fossil mammal remains were the three main aspects underlined by Charles Darwin when he summarized the characteristics of the Pampean sediments. What was the meaning of the Pampean Formation as used by Darwin?; what is the meaning of this term at present?; what is our present understanding of the late Cenozoic Pampean geology?. This paper is particularly focused on the Buenos Aires Pampas, south of the Rio Salado, the region that Darwin explored during the winter of 1833; it includes comments on specific observations he made elsewhere to understand better his interpretations and inferences. Conse- quently, the main goal is to analyze and discuss his observations on the landscape and the sediments within the framework of our present understanding of the late Cenozoic geology of the southern Pam- Figure 1: Darwin´s itinerary across the Pampean plain of Buenos Aires province. S: Sauce Posta, T: Tapalqué, G: Guardia del Monte; 1833 frontier adapted from Rolleri et al. (2005) pean plain. With this purpose in mind, the present contribution brings in Dar- do to Buenos Aires following the road Darwin continued to the next military win´s comments and interpretations on that connected the military outposts outpost, "Sauce Posta", situated by the Rio the landscape along with the nature, (Postas) set up along the western frontier Sauce Grande, "…the distance was about 6 composition and areal distribution of the in 1833 during the ongoing war with the leagues from Sierra de la Ventana…" (Dar- Pampean sediments (terms, descriptions, aboriginal groups (Fig. 1). win's Beagle Diary 1831-1836). It is likely and special comments are quoted in ita- On the way to Bahía Blanca, once he that this Posta was situated in the vicinity lics and referred to his diary and the crossed the Río Colorado, Darwin noti- of the present village of Saldungaray. chapter on the Formations of the Pam- ced a major change between the Patago- From the Sauce Posta, Darwin headed to pas). These particular issues are then ana- nian and the Pampean environments Sierra de la Ventana (Ventania range). lyzed from our present geological pers- "...The country had a different appearance from There he stayed for around 3 days explo- pective, providing an updated geological that South of the Colorado: there were many ring the ranges which were described as review. different plants and grasses and not nearly so "…a desert mountain of pure quartz rock.Its many spiny bushes and these gradually became height is between 3 and 4000 feet …" (Dar- DARWIN´S ITINERARY AND less frequent; until a little to the North there is win's Beagle Diary (1831-1836). From HIS PERCEPTION OF THE not a bush..."(Darwin's Beagle Diary 1831- the notes reported in his diary he pro- PAMPEAN LANDSCAPE 1836). In southern Buenos Aires provin- bably examined the rock outcrops and ce he examined the neighboring areas of the landscape of the upper basin of Rio Charles Darwin travelled around 650 km Bahia Blanca and Punta Alta where he Sauce Grande and its surroundings, rea- (400 geographical miles) from Rio Colora- spent around a week. On August 27th ching the area of the highest peaks 126 M. ZÁRATE AND A. FOLGUERA

(Cerro Tres Picos). His observations include remarks on the vast extent of the plain surrounding Sierra de la Ventana "…the plain was like the ocean…", (Darwin's Beagle Diary 1831-1836); "...the high plain round this range sinks quite insensibly to the eye on all sides..."(Darwin 1846, p. 79). After surveying this sector of Sierra de la Ven- tana, he returned to the Sauce Posta. Se- veral days later he continued northward to Sierras de Tandil (Tandilia range). Pro- ceeding to the 3rd Posta, the landscape was perceived as "…a dry grassy plain and on our left hand at a greater or less distance were low hills.." (Darwin's Beagle Diary 1831- 1836); this description seems to depict the plain sector northward of Ventania (Fig. 2). From there he travelled around 160 km between the two mountain systems. Heading to the 4th Posta the landscape is reported as "…a low swampy country which extends for nearly 80 miles to the Sierra Tapalken…" (Darwin's Beagle Diary 1831 -1836). After crossing this large swampy area Darwin arrived to the 7th Posta in the proximity of Sierra Tapalken (spelled Ta- palqué nowadays) depicted as "…a low broken ridge of quartz rock, 2 or 300 feet high extending to the east to Cape Corrientes…" (Darwin's Beagle Diary 1831-1836). From this range Darwin headed to Bue- nos Aires along the basin of Rio Tapal- th ken (at present arroyo Tapalqué); the 8 Figure 2: Geographical location of the Laprida depression, fluvial streams and localities mentioned th and 9 Postas were situated along this in the text. 1: Río Quequén Grande, 2: Río Quequén Salado, 3: Río Sauce Grande, T: Tapalqué, S: stream. On the way to the Rio Salado he San Miguel del Monte. described the plain as "…partly swamp and Tandilia, he perceived the region as a the geomorphological expression of a partly good to the east of rio Tapalguen…" dominantly dry environment; northward, geologically complex structural block na- (Darwin's Beagle Diary, 1831-1836). He in the Rio Salado basin, Darwin referred med Positivo Bonaerense by Yrigoyen arrived at Guardia del Monte (the present to more fertile lands, the dominance of (1975). This positive structural unit sepa- town of San Miguel del Monte) on Sep- swampy environments and a general pau- rates the Salado and the Colorado tecto- tember 19th, 34 days after disembarking city of tosca exposures. The attributes of nic basins (Fig. 3). in Patagones, Rio Negro. the landscape and the environmental fea- The Pampa interserrana, characterized by tures reported by Darwin essentially re- Darwin as a dry plain of tosca higher than the THE SOUTHERN PAMPEAN flect the major geomorphological diffe- country rock (Darwin 1846, p. 79) is a lar- LANDSCAPE rences between the so-called Pampa depri- ge-scale plain extending across most of mida (depressed Pampa) and Pampa inter- southern Buenos Aires province and Swampy or dry areas, scarcity or more serrana (intermontane Pampa) (Frenguelli continuing southwestward into La Pam- frequency of tosca outcrops and rounded 1950), a still current subdivision of the pa Province, where it has been interpre- or flat topped hills were some of the fun- Buenos Aires Pampean plain (Fig.1). The ted as a structural plain (Calmels 1996). damental geographical attributes used by Tandilia and Ventania mountain systems The plain is dissected by fluvial systems Darwin to describe the landscape of the together with the Pampa interserrana, situa- that drain the central part of Pampa inter- Buenos Aires Pampas. Southward of ted in between the two ranges, constitute serrana and the neighboring ranges of On the formations of the Pampas in the footsteps of Darwin: ... 127

mountain ranges (Demoulin et al. 2005). A model of landscape evolution has been proposed hypothesizing that the highest erosion surfaces of both ranges formed prior to the break-up of Gond- wana and the rifting process which led to the opening of the South Atlantic Ocean. This long-lasting history of denudation included intervals of landscape stability and reactivation which are thought to be related to the evolution of the Colorado and Salado tectonic basins (Demoulin et al. 2005). In this context, one of Darwin´s observations in Sierra de la Ventana, becomes particularly reve- aling. He reported a rock outcrop, "…a few small patches of conglomerate and breccia firmly cemented by ferruginous matter at a height of 300 to 400 m…" (Darwin 1846, p. 79). This description is in agreement with the geomorphic setting and the general lithological characteristics of Brecha Cerro Co- lorado (Andreis et al. 1971) or Conglomerado Rojo (Harrington 1936), a deposit associated with one of the regional erosion Figure 3: Morphostructural units of Buenos Aires province, modified after Yrigoyen (1975). surfaces identified in Sierra de la Ventana (Keidel, 1916, Demoulin et al. 2005). Tandilia and Ventania. Fluvial terraces orly known. The environmental features Northward of Tandilia range, in the do- are present along the main river valleys of this area fit Darwin´s description of a main of the Salado tectonic basin, the generating a stepped topography. In the large swampy environment before he landscape can be subdivided into two upper basins of some of the main rivers reached Sierras de Guitru Gueyú (the pre- main geomorphic settings. Along the Sie- (Quequén Grande, Quequén Salado, Sau- sent group of low ranges at the north- rras de Tandil, Tricart (1973) pointed out ce Grande), as well as other streams drai- western part of Tandilia). the occurrence of extensive areas of flu- ning the Ventania range, the erosional Since Darwin´s observations on some vial discharge interpreted as alluvial fans retreat of the plain surface during the general morphological characteristics of that reached the Río Salado. Later, Fi- excavation of river valleys gave rise to a the southern ranges of Buenos Aires, the dalgo (1983) described a 20-km wide hilly and moderately undulating landsca- geomorphology of Tandilia and Venta- piedmont region and also identified allu- pe composed of flat-topped and smo- nia has been the focus of only a few stu- vial fan-like landforms exhibiting a distri- oth-shaped isolated hills (Fig.4a). dies, although these have documented butary drainage pattern. The area of The plain is capped by a calcareous duri- significant stages of landscape develop- arroyo Tapalqué visited by Darwin is lo- crust (Fig. 4b) or calcrete (Darwin´s tosca ment, some not yet fully understood. In cated in this piedmont setting, characteri- rock, see below), in turn covered by a lar- Sierra de la Ventana, Keidel (1916) poin- zed by a progressive altitudinal decrease ge apron of eolian sandy silts and silty ted out the presence of old erosion sur- northward, reflecting a gradual change to sands, around 1.5- 2 meters thick, making faces at different altitudes in the area of imperfect drainage conditions. A very up the parent material of the presently the upper basin of Rio Sauce Grande. In flat topography with gradients ranging cultivated Pampean soils. North-north- turn, Teruggi and Kilmurray (1980) men- from 0.1% to 0.01% typifies the central westward the landscape grades into a low tioned the presence of monadnocks and part of the Salado tectonic basin; nume- -gradient area characterized by very poor old peneplain surfaces in the Sierras de rous shallow lakes (lagunas) and swampy drainage conditions with numerous sha- Tandil. Recently, a stepped topography environments are present. The Río Sa- llow lakes and flooded environments, composed of erosion surfaces at diffe- lado floodplain, several kilometers wide known as Laprida depression (Fig. 2), an rent elevations was recognized and geo- in some areas, and including shallow la- endorheic sector (Dangavs 2005) still po- morphologically correlated at both kes interconnected during flooding 128 M. ZÁRATE AND A. FOLGUERA

slightly indurated argillaceous earth or mud, often but not always, including in horizontal lines concretions of marl, and frequently passing into a compact marly rock..." (Darwin 1846, p. 89). The reddish argillaceous earth was named "Pampean mud". Darwin emphasized the great extent of the Pampean For- mation as one of its most striking features. According to his own observations and the reports from dÓrbigny and other travelers he considered that the unit extended from the vicinity of Rio Colorado in the south to beyond Santa Fe in the north "…M. dÓrbigny traced it for 250 miles further north…"(Darwin 1846, p. 97); and from Banda Oriental (Uru- guay) in the east to Cordoba to perhaps Mendoza in the west. Finally he stated "… the area of the Pampean Formation, as remarked by dÓrbigny is at least equal to that of France and perhaps twice or three as great…" (Darwin 1846, p. 97). Thus Darwin correctly perceived what we now know to be the great geographic extent of the Pampean Formation. Darwin also stressed the lithological homogeneity of the sediments that he grouped into a single unit, the Pampean Formation. The concept of Formation (gebirgsformation), as a geological unit, was coined by Abraham Gottlob Werner (1749-1817), a German geologist from the School of Mines at Freiberg, with the purpose of defining a historical entity that included rock bodies formed during the same time interval; at a higher hierar- chical rank, the term terrain grouped se- Figure 4: Plain capped by the tosca rock (calcrete). a) Isolated hill formed by fluvial erosion of the veral formations. Both terms, formation Río Sauce Grande, south of Saldungaray; Sierra de la Ventana at the background; b) Calcrete crust and terrain, although representing units and resulting tableland aspect of the plain by the Río Sauce Chico valley, west of Bahia Blanca. of different stratigraphic hierarchy, were events, was crossed by Darwin who poin- 6). The hills represent lunettes generated later used as synonyms by other authors. ted out: "…12th to 13th to 14th posta: we had by eolian deflation of the topographic Nevertheless, during the decade of the to ride for a long distance in water above the depressions, presently occupied by sha- 1840s, the term formation took root in the horse knees..." (Darwin's Beagle Diary, llow water bodies, under drier conditions Anglo-Saxon community, while terrain, 1831-1836). After passing the Río Salado (Tricart 1973, Dangavs 2005). as a synonym of formation, was adopted and before arriving to Guardia del Mon- in the French geology literature (Pod- te, he stayed at a large estancia owned by Stratigraphy and lithology of Pam- gorny et al. 2008). Juan Manuel de Rosas. Several isolated pean sediments: the meaning of the Since Darwin´s visit, several other resear- hills (Los Cerrillos), occur in this sector. Pampean Formation chers have stressed the apparent litholo- These features are around 10 meters abo- gical homogeneity of the Pampean sedi- ve the surrounding plain and are located Darwin typified the sediments of the ments. At a regional scale of analysis the on the eastern side of shallow lakes (Fig. Pampean Formation as "...a dull reddish, deposits look rather alike, dominantly On the formations of the Pampas in the footsteps of Darwin: ... 129

composed of sandy silt and silts of light TABLE 1: Some of the best known stratigraphic schemes of the Pampean sediments. brown to reddish brown colors, common Those by Fidalgo et al. (1973) and Riggi et al. (1986) are currently used. presence of calcareous accumulations of Ameghino Kraglievich Frenguelli Fidalgo et al. Riggi et al. (1908) (1952) (1957) (1973) (1986) various morphologies and a general massive appearance. This perception of li- Post Platense Fm de Lobería Serie postpam- Fm La Postrera thological homogeneity was accentuated pampeano Lujanense peano /Fm Luján Fm Buenos Aires Boanerense Fm de Arroyo Seco Bonaerense by the nature of the exposures, scattered and discontinuous outcrops of relatively Ensenadense Fm de Miramar reduced thickness and mostly situated Fm de San Andrés Ensenadense along river banks, which constrain the Fm Fm Ensenada detailed analysis of the spatial and tem- Pampeano Fm de Vorohué Fm Pampiano poral relationships of the sediments. Ex- Fm de Barranca ceptions were the exposures along the de los Lobos Fm Chapalmalense Fm de Chapadmalal Chapadmalense Río Paraná and the Monte Hermoso sea- Araucano Monte- cliff, where Darwin was able to examine hermosense the deposits and their paleontological content in better exposures. To examine localities with exposures laterally conti- names after the locality where they were the Pampean subsurface he described the nuous for several km as the Mar del Plata described (among others, Epecuén For- lithology of some wells in the vicinity of sea-cliffs, the Río Paraná bank exposu- mation, Saldungaray Formation, Irene Buenos Aires and south of Bahia Blan- res and the sea-cliff at Monte Hermoso Formation, Arroyo Chasicó Formation; ca:"… in a well at the depth of seventy feet, ac- were subdivided on the basis of their pa- Fidalgo et al. 1975 and references the- cording to Ignatio Nunez, much tosca-rock was leontological content (among others rein). met with, and at several points, at 100 feet deep, Ameghino 1908, Frenguelli 1957, Krag- As time passed, in spite of the numerous beds of sand have been found..." (Darwin lievich 1952). At the end of the 19th cen- schemes proposed, Pampean Formation or 1846, p. 77). Also, 50 km SE Buenos Ai- tury, the stratigraphic analysis of the simply Pampeano developed into terms res, along the coast of Rio de la Plata at Pampean deposits was benefited signifi- that became deeply rooted in the Argen- Ensenada (Fig. 2), Darwin examined the cantly by deep excavations during the tine geological literature; both became exposures of a quarry "…specimens from construction of the harbours at Buenos synonyms to refer to the Pampean depo- near Ensenada, given me by Sir W. Parish, Aires and La Plata. These temporary ex- sits. At present, the term Pampeano is still where the rock is quarried just beneath the sur- posures provided sections several meters used informally with a rather loose sig- face of the plain..." (Darwin 1846, p. 78). thick that became type localities, not later nificance. However, general agreement Since the mid-XIX century, many contri- available, to characterize the uppermost exists to separate the surficial Late butions have been focused on the strati- interval of the Pampean deposits (Ame- Pleistocene-Holocene sedimentary cover graphy and several different classifica- ghino 1908). The resulting schemes beca- from the underlying Pampean deposits. tions were proposed. The purpose herein me the groundwork that guided subse- Following this line of reasoning, the pre- is not to make a review of the different quent stratigraphic studies. late Pleistocene deposits exposed in the stratigraphic classifications, but to stress Much later, the identification of units on Pampa interserrana and the Salado Basin the concepts and ideas on which the sub- the basis of the vertebrate fossil content were grouped again into a single unit, the division of the Pampean sediments is evolved into the concept of land-mam- Formación Pampeano (Pampean Formation) based. Up to the present, however, Pam- mal ages (Pascual et al. 1965), a funda- identified exclusively on the basis of li- pean stratigraphy has remained a contro- mental stratigraphic tool still widely thological attributes (Fidalgo et al. 1973). versial and debatable issue; various diffe- accepted and used. In recent years, these This proposal is somewhat close to the rent schemes are still used simultaneously biostratigraphic units have been redefi- original meaning given by Darwin to the (Table 1). ned and updated (Cione et al. 2007 and term Pampean Formation. Regardless of its The homogeneity of the deposits toge- references therein). The relative degree vertebrate fossil content, the unit defined ther with the discontinuity of the expo- of evolution inferred from the fossil as- is nevertheless useful for mapping pur- sures forced the search of a criteria other semblages was used in the stratigraphic poses and well descriptions. More recent than the lithology to separate the Pam- arrangement of the Pampean deposits. schemes tend to divide the upper part of pean sediments. Eventually this led to the Accordingly, and considering their pale- the Pampean deposits into two units utilization of the fossil vertebrate con- ontological content, several units were (Ensenada and Buenos Aires Forma- tent to separate the deposits into diffe- identified, and classified as lithostratigra- tions) distinguished on the basis of their rent stratigraphic units. Therefore, those phic units (formations) receiving their paleontological content (Table 1). In 130 M. ZÁRATE AND A. FOLGUERA

turn, the uppermost sedimentary mantle therein). Across the Pampean environ- Kemp and Zárate 2000). was subdivided into several lithostratigra- ment, primary loess facies have a low Yet not assessed, bioturbation structures phic units that represent lithofacies of preservation potential. The deposits are are an outstanding feature of the Pam- different geomorphic processes and en- locally reworked either by aqueous trans- pean sediments. Not only are there tra- vironmental settings such as eolian, flu- port agents (loess-like or loessoid depo- ces of invertebrates in the sediments, vial and littoral deposits (among others sits) or modified by pedogenesis (weathe- particularly those beds more intensely Fidalgo et al. 1973 a,b). red loess). Loess-like deposits exhibit a modified by pedogenesis, but also bur- weak and poorly expressed sedimentary rows, caves and galleries (krotovinas) ex- The Pampean sediments today: cu- structure consisting either of horizontal cavated by small rodents and larger mam- rrent ideas and interpretations bedding or cross bedding. Along the mals, mainly xenarthrans (Vizcaino et al. piedmont areas, channelized lithofacies 2001). The resulting structures are filled At present the Pampean sediments are representing fluvial paleoenvironments in by diamicton-like deposits, including lithologically characterized as light brown include rock fragments from bedrock of fragments of tosca, paleosol aggregates to reddish brown sandy silts or silty the Tandilia and Ventania ranges. At dis- and bones redeposited by water. Also, sands, moderately indurated with a gene- tal settings from the ranges, the coarse some burrows and caves are filled with ral massive appearance. Darwin´s inter- fluvial facies are made up of bones and eolian and/or fine grained laminated se- pretation was that the "...deposits were accu- tosca fragments embedded in a sandy diments. mulated in the former estuary of Rio de la Plata matrix. In the Pampean sediments, minor erosive and in the sea adjoining it…" (Darwin 1846, Paleosols are a very common feature episodes are recorded by both the partial p. 99); the source area of the Pampean with pedogenic traits (pedogenic structu- truncation of paleosol surfaces and the mud was thought to come from the wea- re, textural changes resulting from eluvia- local incision of fluvial paleochannels thering of igneous and metamorphic tion, carbonate leaching, evidences of (Ruocco 1989, Zárate and Fasano 1989). rocks of Brazil and Uruguay (Darwin bioturbation features from soil microfau- Three major episodes of landscape reac- 1846, p. 100). na and vegetation) distributed throug- tivation, inferred from stratigraphic The mineral composition of the Pam- hout the Pampean sediments. Welded pa- analysis and gemorphological evidence, pean sediments reflects their derivation leosols resulting from the superposition are documented by cycles of fluvial inci- from volcaniclastic material in turn mos- of pedogenetic process have been repor- sion. These generated erosional uncon- tly derived from Andean rocks, with mited in both Pliocene and Pleistocene stra- formities in the sedimentary record as nor contributions (Precambrian granitic tigraphic sections of the Pampean suc- well as terrace formations and different and metamorphic rocks and Paleozoic cession. Micromorphological studies al- aggradational surfaces in the Sierra de la quartzites) from the Tandilia and Venta- so point to the occurrence of intervals Ventana piedmont (Zárate 2005). nia ranges in the southern part. North- when sediments accumulated progressi- ward, the Pampean ranges of Córdoba vely on the surface while soil formation Carbonate accumulations of the Pam- and San Luis and the Brazilian shield we- was active, resulting in accretionary soil pean sediments: the Tosca-rock re secondary source areas (Teruggi 1957, surfaces. From a sedimentological pers- González Bonorino 1965, Zárate 2003 pective, the occurrence of numerous Darwin gave special attention to the cal- and references therein). A detailed analy- paleosol levels throughout the deposits careous accumulations in his general cha- sis of the deposits reveals grain size documents an episodic sedimentation racterization of the Pampean Formation. changes and variations in the density, co- process. Due to the ubiquity of pedoge- The duricrust (calcrete) that caps the se- lor, and mineralogical composition ac- nic features, the Pampean succession is diments in the Pampa interserrana area, and cording to the sediment age and the geo- interpreted as a pedosedimentary se- constitutes massive accumulations in so- graphic setting considered within the quence (i.e. Kemp and Zárate 2000, me other locations, was described as a Pampean plain. Zárate et al. 2002). The presence of wel- marly rock and named tosca-rock, after the The massive appearance and the rather ded paleosols and intervals of accretio- popular term given to the carbonate de- uniform lithology, among other attribu- nary pedogenesis suggest a variable posits by the local Pampean inhabitants tes, were usually taken as evidence of an balance of pedogenesis and sedimenta- (Darwin 1846). He reported the mor- eolian origin and the sediments have tion through time. At some sections, the phology (nodular and stalactiform concretions, been interpreted as loess. Nervertheless, occurrence of discrete paleosol levels, compact stratified tosca rock), the relation Teruggi (1957) pointed out that the dis- stratigraphically superposed, allow infe- with the Pampean mud, as well as the tribution of primary loess was quite limi- rring some sort of cyclicity in soil forma- geographical distribution and relative oc- ted, a statement reconfirmed by more re- tion that has been related to climatic currence (more frequent outcrops of cent studies (Zárate 2003 and references changes (among others Nabel et al. 2000, tosca in the area of the ranges and the On the formations of the Pampas in the footsteps of Darwin: ... 131

Pampa interserrana, general scarcity of tosca on those deposits. Carbonate nodules, at this latter place… are probably of subsequent exposures in the Rio Salado basin, al- concretions and massive accumulation of origin to the high tosca-plain round the Sierra most no tosca-rock in northeastern Bue- tosca are also very frequently found. Stu- Ventana..." (Darwin 1846, p. 87). nos Aires province:"… At Arroyo del Me- dies on specific accumulations suggest The Pampean succession is now inter- dio the bright red Pampean mud contains scar- different mechanisms of formation, such preted as the late Cenozoic (approxima- cely any tosca-rock…", "… At Rosario there is as precipitation from phreatic waters, tely the last 12 Ma) continental record of but little tosca-rock..." (Darwin 1846, p. 87). capillary rise, and pedogenic carbonate central Argentina. Sediments of the Pam- He also made comparisons of tosca sam- leaching (Bk horizons) (Tricart 1973, pean succession accumulated after a ples collected throughout the Pampas, Imbellone and Teruggi 1986). Miocene regression of the Paranense sea referring to their similar characteristics (Folguera and Zárate, submitted). Major (the absolute identity in his own words) of Age and origin of the Pampean sedi- differences are evident with regard to the specimens collected in the northern ments regional distribution of the deposits. Fi- Pampas, Tandilia (Tapalqué) and south of dalgo et al. (1975) mapped the exposures Bahia Blanca. In addition, thin sections On the basis of its lithological homoge- bearing Late Miocene and Pliocene ver- of tosca were petrologically analyzed by a neity (similar composition) and "...the ap- tebrate fossil assemblages, showing a dis- colleague "...Dr Carpenter has kindly exami- parent absolute specific identity of some of its tribution restricted to the Positivo Bona- ned under the microscope, sliced and polished mammiferous remains..." (Darwin 1846, p. erense area. Darwin examined an outcrop specimens of these concretions and of the solid 100), Darwin inferred that all over its on the bank of the Río Sauce Grande marl-rock collected in various places between the large area the Pampean Formation be- noting that "...there is an imperfect section Colorado and Sta Fe Bajada…." (Darwin longed to the same geological epoch. about 200 feet high displaying in the upper part 1846, p. 77). This age interpretation followed Charles tosca rock and in the lower part red pampean Despite the economic importance of cal- Lyell´s ideas, where the percentages of mud...". This likely corresponds to late careous crusts as a source of road buil- living and extinct mollusks was a funda- Miocene?-early Pliocene sandy siltstones. ding material and in agricultural develop- mental tool for assigning relative ages to Several other outcrops are disconti- ment across most of the southern Bue- the Tertiary strata. He collected some of nuously exposed along other neighboring nos Aires Pampean plain, no detailed and the mollusk shells from late Pleistocene- river systems as well as some railroad systematic studies of these materials ha- Holocene marine deposits exposed along cuts. Northward, in the Río Salado valley, ve been carried out. In the Positivo Bona- the Pampean coastal fringe. At Punta the relatively oldest exposures of Pam- erense area, the morphology and thickness Alta Darwin considered that the propor- pean sediments are much younger (Pleis- of calcretes vary according to the relative tion and specimens of shells found in the tocene sensu lato, see below). These depo- age of the geomorphic surfaces that they sediments was comparable to those spe- sits consist of compacted clayish silts cover. The oldest crust capping late Mio- cies living today in the bay. Therefore, and sandy silts, showing evidence of hy- cene-Pliocene deposits is up to 1.5-2 m taking into account the paleontological dromorphic conditions (mottles, gree- thick. The general morphology of calcre- content including living species of mo- nish colors, manganese nodules, massive tes suggests a high degree of relative de- llusks that he considered contemporane- carbonate accumulations). velopment close to morphological stages ous with the mammiferous remains found in Considering the stratigraphic relations, 5 and 6 as defined by Machette (1995). the sediments, Darwin attributed the numerical ages and relative ages inferred Thus, a typical calcrete section is usually Pampean Formation to the Recent Pe- from the vertebrate fossil assemblages, composed of three main parts from top riod or what is now called the Holocene. the Pampean succession of sedimenta- to bottom. The uppermost part is made From a stratigraphic viewpoint, Darwin tion has been recently subdivided into up of platy calcrete consisting of 5-10 recognized stages in the Pampean four stratigraphic intervals, interpreted as cm thick indurated layers, showing an Formation on the basis of both the dif- subcycles of sedimentation (Zárate 2005) irregular horizontal fracturing (Fig 5B). ferent altitudes shown by the outcrops (Table 2). Lamination, brecciation, and dissolution of Monte Hermoso, Punta Alta and The oldest sedimentary subcycle, is ex- are very common features suggesting a Sierra de la Ventana, and an observation posed in the Positivo Bonaerense and the very complex history of multiple episo- now regarded as a taphonomic attribute northern part of the Colorado tectonic des of carbonate precipitation and wea- of a fossil remnant: "...From the rolled frag- basin. It is composed of Late Miocene thering on the surface. The middle part ment of black bone and from the plain of Punta deposits (11-12 Ma to circa ?7-6 Ma) bea- consists of powdery and massive carbo- Alta being lower than that of Monte Hermoso, ring Chasicoan and Huayquerian fossil nate grading downward into the host I conclude that the course sub-littoral deposits of remains. These deposits have an average Pampean sediments or showing a very Punta Alta are of subsequent origin to the thickness of 150-200 m, and overlie the sharp lower contact (erosional surface) Pampean mud of Monte Hermoso; and the beds Precambrian and Paleozoic bedrock of 132 M. ZÁRATE AND A. FOLGUERA

Figure 5: Plain edge in the area of Bahía Blanca. a) Plain surface with a thin eolian cover and fragments of calcrete, Atlantic Ocean in the background; b) Calcrete section at a quarry showing typical platy structure in upper part, grading downward into Pliocene sediments. Note hammer for scale.

Pampa interserrana. The geographical dis- southwestward into La Pampa province morphologically, these late Miocene tribution of this unit is broad, extending (Folguera and Zárate, submitted). Geo- deposits, capped by the thickest calcrete On the formations of the Pampas in the footsteps of Darwin: ... 133

crust form the very extensive plain with- TABLE 2: Age, geographic distribution and fossil mammal remains of the subcycles of in which the present river valleys were sedimentation identified in the Pampean deposits (after Zárate 2005). excavated. Age of sedimentation Morphostructural Areal Fossil remains stage/age subcycles unit distribution Cione et al.(2007) The second sedimentary subcycle, represented by late Miocene?-early to late Plio- -Positivo Bonaerense -Eolian mantle covering the Platan cene deposits (?7-6-3.2 Ma) is disconti- Late Pleistocene- -Salado and Colorado entire region Holocene tectonic basins -Fluvial sequence of present Lujanian nuously exposed along the river banks of 0.040-0.030 Ma-present Pampean river systems the drainage system. The deposits inclu- late Pliocene-late -Positivo Bonaerense -Tandilia piedmont and Bonaerian de Huayquerian, Montehermosan and Pleistocene circa 3.2 Ma- intramountain valleys Ensenadan Chapadmalalan fossil remains. The out- 0.040-0.030 Ma -Salado tectonic -Fluvial terrace Río Sauce Marplatan crops are located in the Positivo Bonaerense basin Grande and distributed throughout the Sierra de -Lower reaches of Pampa la Ventana piedmont, the valleys of Que- interserrana fluvial valleys? Bedrock of fluvial valleys quén Salado, Quequén Grande as well as (Late Miocene?)-early to -Positivo Bonaerense Pampa interserrana, Venta- Chapadmalalan of minor streams of the southern part of late Pliocene nia piedmont. Tandilia pied- Montehermosan Pampa interserrana, and along the lower 7-6? to 3.2 Ma -Colorado tectonic mont. Chasicó depresión Huayquerian section of the Mar del Plata-Chapad- basin Colorado valley. malal sea-cliffs. The Monte Hermoso Late Miocene-(early -Positivo Bonaerense Pampa interserrana, Huayquerian stratigraphic section (Quattrocchio et al. Pliocene?) basin, Macachín tec- Piedmont of Ventania Chasicoan 2009) is included in this cycle of sedimen- 11-12 to 7-6? Ma tonic basin Structural plain of La Pampa -Bloque de province tation. Farther southward, deposits of Chadileuvú Chasicó depression comparable age crop out in the Colorado -Colorado tectonic tectonic basin. Exposures up to 15 me- basin ter thick and 200-300 m long can be found at various quarries in the surroun- rian fossil remains. They are distributed roadcuts, suggesting that they form an dings of Bahia Blanca. The sediments in the Salado tectonic basin, forming the extensive plain. The stratigraphic and consist of fluvial facies, including rock bedrock in which fluvial valleys are exca- geomorphologic relationship with older fragments from Sierra de la Ventana, cap- vated. The lower sections of deep qua- deposits is still unknown. The lithofacies ped by a 1-1-5 meter thick calcrete (Fol- rries sometimes penetrate levels contai- are similar to the Pleistocene exposures guera and Zárate, submitted). Darwin ning Ensenadan fossil mammal remains. of Mar del Plata and might be part of the explored the very top part of the exposu- Rabassa (1973) reported sediments bea- late Pliocene-late Pleistocene subcycle of res (Fig. 5a) "…At the settlement of Bahia ring Ensenadan fossil remains in the cen- sedimentation which should be confir- Blanca, the uppermost plain is composed of very tral part of Tandilia range. These depo- med in future studies. compact, stratified tosca rock containing rounded sits are also discontinuously exposed The most recent subcycle of sedimenta- grains of quartz distinguishable by the naked along the SW piedmont of Tandilia, tion (40-30 ka-to the present) is compo- eye...". Further eastward, deposits of this along the Atlantic coast between Mar del sed of a large eolian apron along with cycle still poorly known, crop out along Plata and Miramar. The late Pliocene-late fluvial deposits accumulated during the the lower reaches of the Quequén Salado Pleistocene subcycle unconformably over- last glacial cycle. Recent numerical ages and the Quequén Grande rivers; Huay- lies Pliocene siltstones along the Mar del obtained in fluvial deposits of northern querian, Montehermosan and Chapad- Plata sea cliff. Included in this subcycle Buenos Aires point to an older age for malalan fossil remains have been exhu- are the Sauce Grande fluvial terrace sedi- the beginning of this cycle of sedimenta- med (among others Verzi et al. 2008). ments with Bonaerian fossil remains re- tion (Blasi et al. 2008). The eolian lithofa- Along the Mar del Plata-Chapadmalal sea ported by Deschamps (2005). cies constituting the parent material of cliffs, this stratigraphic interval is compo- Along the Atlantic coast, between Mira- the present cultivated soils are composed sed of early to late Pliocene (3.2 to 4.5-5 mar and west of Necochea, the sea cliffs of sandy deposits towards the west, gra- Ma), massive siltstones with well develo- are composed of fine sandy facies including into sandy silts and silt east and nor- ped paleosols bearing Chapadmalalan ding diamicton layers with a high content theastward. In the Pampa interserrana area fossil remains. of calcareous nodules and capped by a the eolian mantle consists of loessial The third subcycle of sedimentation (late platy calcrete. Inland, at a distance of se- sands, sand mantles, and sandy loess (Zá- Pliocene-late Pleistocene circa 3.2 Ma- veral kilometers from the coastline, the rate and Blasi 1993, Bidart 1996), repre- 0.040/0.030 Ma) includes deposits bea- uppermost 2 meters of these deposits senting proximal eolian facies derived by ring Marplatan, Ensenadan and Bonae- are exposed in numerous quarries and deflation from the Río Colorado flood- 134 M. ZÁRATE AND A. FOLGUERA

geographical distribution of exposures of different ages suggests significant variations of the sedimentary dynamics throughout the region, and four main subcycles of sedimentation have been identified. The outcrops of the first two cycles recording the late Miocene and most of the Pliocene sedimentation are restricted to the Positivo Bonaerense (Sierra de la Ventana piedmont, part of Pampa interserrana and Sierras de Tandil). A remarkable geomorphological characteristic of this southern area is the occurrence of several intervals of landscape stability documented by the development of calcrete crusts (tosca-rock) on topographic surfaces of different ages. During the very late Pliocene-Pleistocene, sedimentation continued in the piedmont of Sierras de Tandil and the Salado tectonic basin. In the meantime, the Positivo Bo- naerense was geomorphologically much more stable with episodes of fluvial incision along the main river systems. Yet, no information is available on the Laprida depression, the swampy environment crossed by Darwin, and the Atlantic fringe where possible Pleistocene deposits are exposed. In both the Positivo Bonaeren- se and the Salado tectonic basin, the last subcycle of sedimentation is related to incision of the present fluvial valleys sometime circa 40 ka and the accumulation of an eolian mantle during the last glacial cycle. What was the primary control of sediment accumulation at a regional scale?. Why did sedimentation cease in the late Miocene over a vast area of Positivo Bona- erense?. Which factor triggered the subcycles of sedimentation now identified?. At Figure 6: The Río Salado Valley, south of San Miguel del Monte (Guardia del Monte). a) Hill on the present, research in progress is addres- northeastern-eastern side of the Río Salado floodplain; b) Quarry excavated at the hill showing a 3 sing these main questions. The studies m thick loess section; present soil developed on top. under way are focused on the potential plain. In the area of the Río Salado valley, after him emphasized the relatively uni- role that might have been played by both the eolian facies are made up of silty de- form lithological composition, the vast Andean tectonism and tectonic dynamics posits (loess) named La Postrera Forma- extent and the rich vertebrate fossil con- of the Colorado and Salado basins. With tion by Fidalgo et al. (1973) (Fig. 6b). tent of the Pampean sediments. His per- this purpose, future stages of current re- ception of the Pampean environment search will be mainly directed towards FINAL REMARKS illustrated not only the major landscape establishing improved chronological con- differences but also some key lithological trol of the deposits along with detailed Darwin as well as many other researchers features of the deposits. To date, the mapping and sedimentological analysis On the formations of the Pampas in the footsteps of Darwin: ... 135

of the areas, still poorly known (the nor- Variaciones ambientales y climáticas durante Fidalgo, F. 1983. Algunas características de los thern area of Pampa interserrana, the La- el Pleistoceno tardío en la cuenca media del sedimentos superficiales en la cuenca del río prida depression and the Atlantic sea cliff río Luján, Argentina. 12° Reunión Argentina Salado y en la Pampa ondulada. In Coloquio west of Mar del Plata. de Sedimentología, Resúmenes: 43, Buenos Internacional sobre Hidrología de Grandes Last but not least, the Geological Observat- Aires. Llanuras, 11-20 abril 1983, Olavarria, Argen- ions on South America was read by one of Calmels, P. 1996. Bosquejo geomorfológico de la tina. Comité Nacional para el Programa Hi- us (MZ) in the early 1980s. From the pre- provincia de la Pampa. Universidad Nacional drológico Internacional: 1045-1067, Buenos sent perspective, after almost three decade la Pampa, Facultad de Ciencias Exactas y Aires. des of exploring the geology of this vast Naturales, 110 p., Santa Rosa. Fidalgo, F, De Francesco, F. and Colado, U. 1973a. environment, Darwin´s careful and detai- Cione, A.L., Tonni, E.P., Bargo, M.S., Bond, M., Geología superficial en las Hojas Castelli, J.M. led descriptions and his interpretations, Candela, A.M., Carlini, A.A., Deschamps, C. Cobo y Monasterio (Pcia. de Buenos Aires). capture the attention of those of us who M., Dozo, M.T., Esteban, G., Goin, F.J., Mon- 5° Congreso Geológico Argentino, Actas 4: have devoted time and patience to talvo, C.I., Nasif, N., Noriega, J.I., Ortiz-Jau- 27-39. understanding the nature of the Pampas. reguizar, E., Pascual, R., Prado, J.L., Reguero, Fidalgo, F, Colado, U. and De Francesco, F. 1973 M.A., Scillato-Yané, G.J., Soibelzon, L., Verzi, b. Sobre ingresiones marinas cuaternarias en ACKNOWLEDGEMENTS D.H., Vieytes, E.C., Vizcaino, S.F. and Vuce- los partidos de Castelli, Chascomús y Mag- tich, M.G. 2007. Mamíferos continentales del dalena (Provincia de Buenos Aires). 5° Con- We thank Beatriz Aguirre-Urreta for in- Mioceno tardío a la actualidad en la Argen- greso Geológico Argentino, Actas 4: 225-240. viting us to participate in this special vo- tina: los últimos cincuenta años de investiga- Fidalgo, F., De Francesco, F. and Pascual, R. lume. The critical and helpful comments ciones. Ameghiniana 50º aniversario, Publica- 1975. Geología superficial de la llanura bonae- and suggestions on the text by the revie- ción especial 11: 257-278. rense. In Relatorio Geología de la Provincia wers Daniel Muhs, Edward Derbyshire Dangavs, N.V. 2005. Los ambientes acuáticos de de Buenos Aires, 6° Congreso Geológico Ar- and Cecilia Deschamps, improved and la provincia de Buenos Aires. In de Barrio, R., gentino: 103-138. smoothed the manuscript. We are grate- Etcheverry, R., Caballé M. and Llambías, E. Folguera, A. and Zárate, M.(submitted). La sedi- ful to Alejandra Pupio and Irina Pod- (eds.) Geología y Recursos Minerales de la mentación neógena continental en el sector gorny for helping us with literature and Provincia de Buenos Aires, Relatorio 16° extraandino, Argentina central. Revista de la information used in this paper. Vivi Mar- Congreso Geológico Argentino: 219-236, La Asociación Geológica Argentina. tinez helped with the illustrations. This Plata. Frenguelli, J. 1950. Rasgos generales de la morfo- paper was financially supported by Uni- Darwin's Beagle Diary 1831-1836. In The Com- logía y la geología de la provincia de Buenos versidad Nacional de la Pampa (Research plete Work of Charles Darwin. www.darwin- Aires. LEMIT, serie 2, 33: 1-72, La Plata. P.186-06), and is a contribution to PIP- online.org.uk,University of Cambridge, Cam- Frenguelli, J. 1957. 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GEOLOGY OF THE AREA OF BAHÍA BLANCA, DARWIN'S VIEW AND THE PRESENT KNOWLEDGE: A STORY OF 10 MILLION YEARS

Mirta E. QUATTROCCHIO1-2, Cecilia M. DESCHAMPS3, Carlos A. ZAVALA1-2, Silvia C. GRILL1 and Ana M. BORROMEI1-2

1 Departamento de Geología, Universidad Nacional del Sur. 2 CONICET 3 Museo de La Plata, CIC Emails: [email protected]; [email protected]; [email protected]; sgrill2criba.edu.ar; [email protected]

ABSTRACT The aim of this paper is to give an updated outlook of the scenery described by Charles Darwin when he visited Bahía Blanca and surrounding areas, following the itinerary during his voyage on board HMS Beagle. Such an outlook is a state of the art of the current understanding of the Late Miocene-Holocene history in the southwestern Pampas (Argentina). Multidisciplinary results were integrated in a chronosequence chart synthesizing the suggested space-time correlation of the recognized events. Some of the studied localities covering the whole time interval represented in the area were arranged in this chart in a hypothetical E-W line crossing the Río Sauce Grande basin and the surrounding highlands. This line is also approximately the one followed in part by Darwin when riding from Bahía Blanca to Tapalqué (Tapalguen) as he crossed the region toward the Río Sauce. Paleoenviromental and paleoclimatic inferences for the last 10m.y. are also given. Paleontological studies included vertebrates, ostracods and palynomorphs. Many of the results of these investigations are the answers to Darwin's question when he first visited the area. Keywords: Pampas, Tapalqué, Pampean Formation, Punta Alta, Red earth cliffs.

RESUMEN: Geología del área de Bahía Blanca, los comentarios de Darwin y el conocimiento actual: una historia de 10 millones de años. El objetivo de este trabajo es brindar una actualización del escenario descrito por Charles Darwin cuando visitó Bahía Blanca y sus alrededores, siguiendo el itinerario de su viaje alrededor del mundo a bordo del Beagle. Se trata de una puesta al día del conocimiento de la historia del sudoeste de la región pampeana (Argentina) desde el Mioceno tardío hasta el Holoceno y tiempos históricos. Las investigaciones multidisciplinarias llevadas a cabo por el grupo de investigación del Laboratorio de Palinología, UNS, Bahía Blanca, se integraron en una carta cronoestratigráfica secuencial, en la cual se sintetizaron las corre- laciones espacio-temporales de los eventos reconocidos. En esta carta, algunas de las localidades estudiadas que cubren todo el lapso representado en el área, se ordenaron en una línea hipotética E-O que corta la cuenca del río Sauce Grande y sus divi- sorias. En parte, esta línea también sigue aproximadamente el primer tramo de la cabalgata de Darwin desde Bahía Blanca hacia Tapalqué (Tapalguén), cuando cruzó la región hacia el Río Sauce. Asimismo, se brinda información paleoambiental y pale- oclimática de los últimos 10 ma para esta región. Los estudios paleontológicos incluyeron vertebrados, ostrácodos y palinomorfos. Muchos de los resultados de estas investigaciones son respuestas a comentarios que Darwin hizo en su visita.

Palabras clave: Pampas, Tapalqué, Formación Pampeano, Punta Alta, Acantilados de tierras rojas.

INTRODUCTION Miocene-Holocene history in the south- days in this area, from September 7 to west Pampas (Argentina) (Fig. 2). This October 20, 1832, and from August 17 to The aim of this paper is to give an upda- information was assembled by resear- September 8, 1833. The amount of geo- ted outlook of the scenery described by chers at the Palynology Laboratory of logical and biological information provi- Charles Darwin when he visited Bahía the UNS (Bahía Blanca) over almost 20 ded in his notes and further publications, Blanca and surrounding areas, following years of mapping, stratigraphic studies and the acute observations concerning the itinerary during his voyage on board and analyses of the fossil record. Pa- the genesis of the deposits, the past envi- the Beagle (Fig. 1). leontological studies included vertebra- ronments and organisms are overwhel- Most of the information is taken from tes, ostracods and palynomorphs. Many ming even today, more than one and a Quattrocchio et al. (2008) - and referen- of the results of these investigations are half century later. Only those issues that ces therein - who gave a state of the art the answers to Darwin's comments when have been revisited by the authors are account of the understanding of the late he first visited the area. Darwin spent 67 here discussed. 138 M. E. QUATTROCCHIO, C. M. DESCHAMPS, C. A. ZAVALA, S. C. GRILL AND A. M. BORROMEI

Figure 1: Itinerary of Charles Darwin, and below right, the Fortaleza Protectora Argentina. Taken from Dozo and Ginobili (1999).

Charles Darwin mentioned Bahía Blanca, Beagle anchored there on September 6, founding was named Fortaleza Protec- and described this area several times in 1832. "Baia (sic) Blanca has only been settled tora Argentina (Argentine Protective his reports. The writer Eduardo Mallea within the last six years: previous to which even Fortress, Fig. 1), intended to protect set- (Bahía Blanca, 1903-Buenos Aires, 1982) the existence of the bay was not known. It is tlers and their cattle from raids of native already underlined this visit when he designed as a frontier fort against the Indians & aborigines, and also to protect the coast wrote in his novel "Todo verdor perecerá" thus to connect Buenos Ayres to Rio Negro" from the Brazilian navy, which had lan- (Mallea 1941): "Cuántas civilizaciones huma- (Keynes 2001). He stayed in the area until ded in the area the previous year. The nas habían evolucionado y perecido cuando este October 19, visiting Punta Alta and fort was attacked by malones (incursions pedazo de desierto emergía de su sopor! En torno Monte Hermoso. Then the Beagle went of nomad aborigines on horseback) se- al fortín, valla opuesta al indio predatorio, co- back to Montevideo. On November 28 veral times, most notably in 1859 by menzó a crecer, hacia los ochocientos veintiocho, they sailed again south to Patagonia, and 3,000 Calfucurá warriors. la población militar, y cuatro años más tarde when returning to Buenos Aires, he left About the area, he wrote: "I have several Rosas y Darwin se paraban ante aquellos sali- the Beagle at Patagones and rode to Bue- times alluded to the surface of the ground being trales que después de los secos calores extendían nos Aires. On August 17, 1833 he wro- incrusted with salt. This phenomenon is quite en la bahía su ardiente sábana blanca." ["All te:"Bahia Blanca scarcely deserves the name of different from that of the salinas, and more ex- greenness will perish": "¡How many civilizations a village. A few houses and the barracks for the traordinary. In many parts of South America, had evolved and perished by the time this piece of troops are enclosed by a deep ditch and fortified wherever the climate is moderately dry, these desert was emerging from its drowsiness! Su- wall" (Darwin 1845, p. 76). incrustations occur; but I have nowhere seen rrounding the fortress, boundary for the preda- Bahía Blanca is located in the southeas- them so abundant as near Bahia Blanca. The tory aborigines, there started to grow, near 1828, tern part of the province of Buenos Ai- salt here, and in other parts of Patagonia, con- the military population, and four years later, res, Argentina, on the Atlantic Ocean. It sists chiefly of sulphate of soda with some com- Rosas and Darwin stood before those saltpeter has a population of 325,000 inhabitants mon salt. As long as the ground remains moist beds that, after dry heat waves, spread their fiery according to the 2005 census. The city in the salitrales (as the Spaniards improperly white sheet along the bay"]. was founded as a fort on 11th. April call them, mistaking this substance for saltpeter), 1828 by Colonel Ramón Estomba (Fig. nothing is to be seen but an extensive plain com- THE ARRIVAL 3) under the orders of Brigadier-General posed of a black, muddy soil, supporting scatte- and subsequent Governor of Buenos Ai- red tufts of succulent plants. On returning Darwin first saw Bahía Blanca when the res, Juan Manuel de Rosas. The original through one of these tracts, after a week's hot Geology of the area of Bahía Blanca, Darwin's view ... 139

Figure 2: Map of the studied area with the localities mentioned in the text. weather, one is surprised to see square miles of harbour. The plain, at the distance of a few mi- so (Zavala 1993) formations. These se- the plain white, as if from a slight fall of les from the coast, belongs to the great Pampean quences, stretch across South America snow..." (Darwin 1845, p. 78). formation, which consists in part of a reddish from 23ºS to 41ºS (Teruggi 1957, Bargo Bahía Blanca means White Bay. The na- clay, and in part of a highly calcareous marly and Deschamps 1996). They indicate cli- me is due to the typical colour of the salt rock" (Darwin 1845, p. 81). matic fluctuations, alternating between covering the soils surrounding its shores. This sedimentation, including both loess arid and cold (loess deposition) and "The wide expanse of water is choked up by nu- (sediments with a high content of volca- warm and humid (palaeosol develop- merous great mud-banks, which the inhabitants nic derived particles) and loessoid sediment) intervals. Further interpretation of call Cangrejales, or crabberies, from the number ments (reworked loess) (Zárate 2003), these sediments and the related tosca beds of small crabs..." (Darwin 1845, p. 80). was related to a phase of late Miocene are subject of another article in this volu- The bay (which is presently considered (ca. 10 Ma) orogeny in the Andes, which me (Zárate and Folguera 2009). an estuary, Perillo et al. 2004) was seen by acted as a barrier to moisture-laden Pa- The stratigraphic correlation among ex- Ferdinand Magellan in 1520 during the cific winds. This initiated ''the desertification posures of the Pampean sediments is quite first circumnavigation around the world of Patagonia caused by the rain shadow while difficult because of the lithologic unifor- on the orders of Charles I of Spain, as he precocious Pampa environment probably came mity, but they bear faunas of different searched for a pass connecting the Atlan- into prominence at about this time'' (Patterson ages (from the late Miocene to the Mid- tic and Pacific oceans. and Pascual 1972). dle Pleistocene) which greatly help in this Darwin's observations of the area are Traverse (1982) refers to this 10-million- correlation. The oldest exposures of the- arranged according to the age of the year period as the "Ultimogene". Its in- se Cenozoic sequences in the area are sediments he described, from the oldest ception is based on the presence of prac- those of Grünbein and Barrancas de Sar- to the youngest. tically 100% extant plant genera. By this miento near Bahía Blanca city (Fig. 2), in- time, also began the establishment of cluding loessoid sediments devoid of po- THE GREAT PAMPEAN steppe vegetation, dominated by grasses llen, but yielding vertebrate remains. The FORMATION and by shrubby composites and cheno- fauna-especially the octodontoid rodents pods in America and Eurasia. of the genera Xenodontomys and Phtora- "The Beagle arrived here on the 24th of August, In the southern Pampas (Argentina), at mys-suggests a late Miocene age (Verzi and a week afterwards sailed for the Plata. least the last 9-3 Ma record is described and Deschamps 1996, Deschamps et al. With Captain Fitz Roy's consent I was left be- as sequences composed of alternating 1998, Verzi et al. 2008). Other loessoid hind, to travel by land to Buenos Ayres. I will loess and palaeosol units known in gene- sediments exposed at Las Obscuras (Fig. here add some observations, which were made ral as Pampean sediments (Fidalgo et al. 2), in the middle basin of Río Sauce during this visit and on a previous occasion, 1975) and locally as the Saldungaray, La Grande, were also sterile in pollen, but when the Beagle was employed in surveying the Toma (Furque 1967), and Monte Hermo- the mammal remains - especially the oc- 140 M. E. QUATTROCCHIO, C. M. DESCHAMPS, C. A. ZAVALA, S. C. GRILL AND A. M. BORROMEI

nicola et al. 2009). molitas Estratificadas Member and Li- These red earth cliffs, today known as molitas Claras Member of Bonaparte Farola Monte Hermoso (Fig. 4) to distin- 1960) was deposited through a fluvial guish it from Monte Hermoso Beach, dynamics of high sinuousity muddy fine- were described by Darwin with more grained rivers, with associated floodplain detail in 1846: and meandering channel facies. The dis- "At Monte Hermoso there is a good section, continuity observed between the lower about one hundred feet in height, of four distinct section (Hermosense típico) and the strata, appearing to the eye horizontal, but thic- upper section (Limolitas Estratificadas kening a little towards the N.W. The uppermost and Limolitas Claras members) is here bed, about twenty feet in thickness, consists of interpreted as an internal erosional surfa- obliquely laminated, soft sandstone, including ce developed during the migration of a many pebbles of quartz, and falling at the sur- highly sinuous fluvial channel over flood face into loose sand. The second bed, only six plain deposits. The Puerto Belgrano For- inches thick, is a hard, dark-coloured sandstone. mation (arenas estratificadas of Bonaparte The third bed is pale-coloured Pampean mud; 1960) is interpreted as accumulated in an and the fourth is of the same nature, but darker aeolian environment with facies of low coloured, including in its lower part horizontal relief dunes, and associated dry interdu- layers and lines of concretions of not very com- nes. The cliff is topped by the Punta Te- Figure 3: Map of Bahía Blanca city. pact pinkish tosca-rock." Zavala (1993) and jada Formation, formed by sands and todontoid rodent Actenomys priscus - sug- Zavala and Navarro (1993), based on conglomerates with quartzite pebbles ac- gest an early Pliocene age. On this basis, facies analysis, gave a new interpretation cumulated in wadi and fluvial environ- Las Obscuras Formation may be correla- of the stratigraphy and paleoenvironments. ted with the base of the Monte Her- ments of this locality, worldwide known Several points of this interpretation moso Formation - Montehermosan Sta- by its fossil content since the visit of agree with that of Darwin: "It appears to ge/Age - exposed at Farola Monte Her- Ameghino (1887), being the basis for the me far more probable, that during the Pampean moso (Fig. 2) on the Atlantic coast. Montehermosan Age (see Pascual et al. period this whole area had commenced slowly 1966). They formally recognized three rising ..., and that tracts of land had thus been MONTE HERMOSO: THE litostratigraphic units from bottom to formed of Pampean sediment round the Ven- RED EARTH CLIFFS NEAR top, the Monte Hermoso, Puerto Bel- tana and the other primary ranges, on which the PUNTA ALTA grano and Punta Tejada formations (the several rodents and other quadrupeds lived, and first unit including the two lower strata that a stream (in which perhaps the extinct On October 19, 1832 he visited Monte described by Darwin as of Pampean mud). aquatic Hydrochœrus lived)... As the land con- Hermoso for the first time: "The Captain Darwin argued about the origin of these tinued to rise, it appears that this source of sedi- landed for half an hour at Monte Hermoso, (or strata at Monte Hermoso: "...were the stra- ment was cut off; and in its place sand and peb- Starvation point as we call it) to take observa- ta at Monte Hermoso depositing at the bottom bles were borne down by stronger currents, and tions" (Keynes 2001). "At the distance of of a great open sea, between 800 and 1000 feet conformably deposited over the Pampean strata" about thirty miles from Punta Alta, in a cliff of in depth? I much doubt this; for if so, the almost (Darwin 1846, p. 82). The record of ex- red earth, I found several fragments of bones, perfect carcasses of the several small rodents, the tinct aquatic capybaras as evidence for the some of large size. Among them were the teeth remains of which are so very numerous in so presence of water-related sediments was of a gnawer, equalling in size and closely resem- limited a space, must have been drifted to this analyzed in Deschamps et al. (2007). bling those of the Capybara (now Phugathe- spot from the distance of many hundred miles. It By this time grasslands were dominant at rium cataclisticum = Chapalmatherium pertur- appears to me far more probable, that during the high latitudes. They developed during a bidum, see Vucetich et al. 2005), whose ha- Pampean period this whole area had commenced global cooling event and marine trans- bits have been described; and therefore, probably, slowly rising ... As the land continued to rise, it gression (Janis 1993), this latter probably an aquatic animal. There was also part of the appears that this source of sediment was cut off; locally responsible for the cliffs of head of a Ctenomys; the species being different and in its place sand and pebbles were borne Grünbein and Barrancas de Sarmiento from the Tucutuco, but with a close general re- down by stronger currents, and conformably dep- (Zavala and Quattrocchio 2001). For the semblance (now Actenomys priscus, Des- osited over the Pampean strata". period between 5 and 3 Ma, Lambeck et champs 2005). The red earth, like that of the Zavala and Navarro (1993) interpreted al. (2002) proposed global warm condi- Pampas, in which these remains were embedded, that the Monte Hermoso Formation (in- tions in view of low values of δ18O in ...." (Darwin 1845, p. 82). (see also Fer- cluding the Hermosense típico, the Li- foraminifera of marine sediments, and Geology of the area of Bahía Blanca, Darwin's view ... 141

low range sea level oscillations. The environments might have been similar to the modern Chaqueña Phytogeographic Pro- vince with open xerophytic woodlands, but more humid, with seasonal differences in rainfall (Pascual and Ortiz Jau- reguizar 1990).

THE LATE PLIOCENE- EARLY PLEISTOCENE

"Even the view was insignificant;-a plain like the sea, but without its beautiful colour and defined outline." Sea level changes produced different geomorphic features in these homogeneous sediments, including erosion, differences in the equilibrium river profiles, regressive and transgressive events, and building of shell terraces during the re- Figure 4: The red earth cliffs near Punta Alta: Farola Monte Hermoso. gressive phases. During the late Pliocene-Early Pleisto- cene interval this area was probably affected by erosion processes (and no deposition; see Fig. 9). In the study area, a single late Pliocene event has been identified in the conglomerates of the La Delta Sequence within the valley of Río Sauce Grande at Dique Paso Piedras Figure 5: The original drawing of Punta Alta section. (Deschamps 2005). The complex terra- shadow, increasing the desertification of I employed myself in searching for fossil bones; ces in the Río Sauce Grande (see below) the western areas (Pascual 1984). The this point being a perfect catacomb for monsters were interpreted within an evolutionary caviomorph rodent fauna in the southe- of extinct races." (Darwin 1845, p. 80). Ver- model of valleys in arid-semiarid regions astern coast of the Buenos Aires tebrate fossils are referred to in another (Zavala and Quattrocchio 2001). In this Province, suggests a strong arid pulse at chapter of this volume (Fernicola et al. model, valleys were active only sporadi- the end of the late Pliocene that could be 2009). This area is today buried beneath cally and most of the time behaved as coeval with the global climatic deteriora- the naval base of Puerto Belgrano and geomorphologically depressed zones, tion at the Gauss-Matuyama boundary could not be revisited; he reported the hosting locally sourced gravitational and (Verzi 2001, Verzi and Quintana 2005). description and a drawing of the profile aeolian deposits. Geologic and sedimen- (Darwin 1846, p. 82-83): "Punta Alta is tological data suggest that La Delta Se- THE LATE PLEISTOCENE: situated about thirty miles higher up on the nor- quence could represent the second stage PUNTA ALTA AND THE thern side of this same bay: it consists of a of initial filling within a transport zone, MONSTERS OF EXTINCT small plain, between twenty and thirty feet in or zone 2 of the evolution of fluvial RACES height, cut off on the shore by a line of low cliffs valleys in arid and semiarid zones. This about a mile in length, represented in the dia- process is related to the beginning of a The Pleistocene-Holocene is recorded in gram with its vertical scale necessarily exaggera- transgressive cycle of an interglacial the coastal area as well as in the valleys of ted (Fig. 5). He described four beds (A- period. Semidesertic environments that the main rivers of the area. One of the D). "The lower bed (A) ... stratified gravel or began their development during the glo- most renowned Pleistocene sites visited conglomerate ... curvilinear, owing to the action bal climatic deterioration of the late Mio- by Darwin is Punta Alta. He spent here of currents, and dip in different directions; they cene were dominant during the Pliocene several days looking for fossils in Sep- include an extraordinary number of bones of in western Argentina. The final rise of tember and October, 1832 and August gigantic mammifers and many shells... The the Sierras Pampeanas acted as a wind 1833: "We passed the night in Punta Alta, and second bed (B) is about fifteen feet in thickness 142 M. E. QUATTROCCHIO, C. M. DESCHAMPS, C. A. ZAVALA, S. C. GRILL AND A. M. BORROMEI

... of red, tough clayey mud, with minute linear cavities... The bed (C) is of stratified gravel, like the lowest one... These three lower beds are covered by an unconformable mantle (D) of stratified sandy earth, including many pebbles of quartz, pumice and phonolite, land and sea- shells." The mammal bones found in this section "... were associated with twenty-three species of shells, of which thirteen are recent and four others very closely related to recent forms; whether the remaining ones are extinct or simply unknown, must be doubtful, as few collections of shells have been made on this coast. As, however, the recent species were embedded in nearly the same proportional numbers with those now living in the bay, I think there can be little doubt, that this accumulation belongs to a very late tertiary period" (Darwin 1845, p. 83). The 23 mentioned species are in fact 20 mollusk species, the other three are bar- nacles (Balanus), anthozoan corals (As- traea) and bryozoans (Flustra), all of them pertaining to the middle Holocene transgression recorded in the Bahía Blanca estuary (Farinati personal communication). Darwin made extensive observations on one pennatulaceous (Coelente- rata, Anthozoa) which he named Vir- gularia patagonica, but known today as Sty- latula darwini (Farinati 1989): At low water hundreds of these zoophytes might be seen, projecting like stubble, with the truncate end upwards, a few inches above the surface of the muddy sand" (Darwin 1845, p. 99). Especially noteworthy in this area, although not seen by Darwin, is the record of fossil trackways discovered by researchers of the UNS some kilometres east of Punta Alta (Fig. 2), in the paleoichno- logical site of Pehuén-Có (Fig. 6a). These Figure 6: a) Trackway of Megatherium on Pleistocene sediments of the Pehuén-Có ichnological site; b) Human footprint on Holocene sediments of the Monte Her-moso I archaeological site. trackways are quite rare in the fossil record, particularly in such amounts, and is that of early aborigines' footprints (Fig. which flow across Tertiary units. The Río have been visited by researches of many 6b) which were found at the Monte Her- Sauce Grande is the main river in the foreign countries. They belong to birds, moso1 site and were dated in 7,000 years southwestern Pampas, and Darwin cros- xenarthrans, camelids, macraucheniids, BP (Bayón and Politis 1996, Zavala et sed it ridding to Buenos Aires on Sep- and were dated in 12,000 + 100 years 14C al. 1992). tember 8th. 1833: "I hired a Gaucho to BP. They have been extensively studied accompany me on my ride to Buenos Ayres... by Drs. Manera and Aramayo (Aramayo THE RIO SAUCE The distance to Buenos Ayres is about four hun- and Manera de Bianco 1996, Quattro- dred miles, and nearly the whole way through an cchio and Borromei 1998). (Fig. 6a) The Late Pleistocene-Holocene is also uninhabited country. We started early in the Another outstanding record in this area recorded in the valleys of the main rivers morning; ascending a few hundred feet from the Geology of the area of Bahía Blanca, Darwin's view ... 143

basin of green turf on which Bahia Blanca stands, we entered on a wide desolate plain... Af- ter a long gallop, having changed horses twice, we reached the Rio Sauce: it is a deep, rapid, little stream, not above twenty-five feet wide. The second posta on the road to Buenos Ayres stands on its banks; ..." (Darwin 1845, p. 106; Figs. 7 and 8). Remains of three levels of ancient terraces assigned to La Delta (see above), San José and Agua Blanca sequences (dated as Late Pliocene, Middle Pleistocene and Late Pleistocene-Holocene respectively; Zavala and Quattrocchio 2001, Des- champs 2005), document different episodes of incision and valley infilling. Figure 7: The Río Sauce Grande and Sierra de la Ventana, with aborigines as drawn by E. Lassalle in The Middle Pleistocene was particularly 1827 during the voyage with the French naturalist Alcide d'Orbigny. studied at Bajo San José, represented by the Bajo San José Sequence, which was deposited by a braided river, typical of arid to semiarid environments. The characteristic longitudinal bars and channels infilling would have provided varied niches for the rich fauna found in these deposits (Deschamps and Borromei 1992, Deschamps 2003, 2005). No pollen was found, but mammals, especially the octodontoid rodent Ctenomys kraglievichi as well as murid rodents and Tayassuidae, suggest the strongest warm pulse so far recognized in the Middle Pleistocene (Bonaerian age) of southern South Ame- rica (Verzi et al. 2004). Figure 8: The Río Sauce Grande today. THE VEGETATION AT THAT PERIOD Bahia Blanca, is any sure guide that they for- analyses suggest arid conditions associa- merly were clothed with a luxuriant vegetation: I ted with more continental environments Darwin wondered about the vegetation have no doubt that the sterile country a little and related to lower sea level in the coexisting with the large Late Pleistocene southward, near the Rio Negro, with its scatte- Atlantic coast of Buenos Aires province mammals of Punta Alta: "What, it may red thorny trees, would support many and large (Quattrocchio et al. 1995, 1998), quite naturally be asked, was the character of the vege- quadrupeds" (Darwin 1845, p. 84-85). This different from "the luxuriant vegetation" tation at that period; was the country as wret- latter issue is developed in another article and close to a "sterile country" of Dar- chedly sterile as it now is? As so many of the co- of this volume (Vizcaíno et al. 2009), but win's comments. embedded shells are the same with those now pollen and ostracod analyses performed The pollen assemblage is dominated by living in the bay, I was at first inclined to think at several localities of the Río Sauce Chenopodiaceae-Amaranthaceae, Poaceae that the former vegetation was probably similar Grande (e.g. Puesto La Florida, Fig. 2), and Asteraceae. Pollen grains that reflect to the existing one; but this would have been an and Arroyo Napostá Grande, including the shrub and thorn scrub plant commu- erroneous inference, for some of these same shells one site at the outlet near Punta Alta nities include among others the families live on the luxuriant coast of Brazil... Never- (Bertels and Martínez 1990, Borromei Rhamnaceae (Condalia microphylla, or pi- theless, from the following considerations, I do 1992, 1995, 1998, Grill 1993, 1995, 1997, quillín), Papilionaceae (Geoffroeae decorticans, not believe that the simple fact of many gigantic Martínez 2002) provided information or chañar) and Mimosaceae (Prosopis or quadrupeds having lived on the plains round about the vegetation and climate. These algarrobo). The pollen assemblage repre- 144 M. E. QUATTROCCHIO, C. M. DESCHAMPS, C. A. ZAVALA, S. C. GRILL AND A. M. BORROMEI

Figure 9: Chronostratigraphic chart showing space-time correlation of the geologic events in the area. ABS. Agua Blanca Sequence; ChLBFm., Chacra La Blanqueada Formation; LDS, La Delta Sequence; MH-S-LT fms., Monte Hermoso-Saldungaray-La Toma formations; MSFm., Matadero Saldungaray Formation; SJS, San José Sequence. Thick black lines indicate the levels exposed at each locality.

sents the modern halophytic steppe and event is associated with the gramineous val represented in the area- were arran- psammophytic herbaceous steppe vege- steppe in the continent reflecting more ged in a hypothetical E-W line crossing tation, along with shrubby woodland ve- temperate or local humid conditions. The the Río Sauce Grande basin and the getation. Sporadic high abundances of relative sea level rise led to the flooding highlands. In part, this line is also appro- Cruciferae probably reflect habitats sub- of the riverbeds, producing deposition of ximately the one followed by Darwin ject to natural causes of disturbance such grey muddy facies. Approximately at when riding from Bahía Blanca to Ta- as intense eolian action under an arid cli- 3000 years BP the marine influence en- palqué (Tapalguen) as he crossed the re- mate (from Quattrocchio and Borromei ded in the area. After 2610 years BP, the gion toward the Río Sauce and stopped 1998). development of a psammophytic herba- at the second posta. ceous steppe suggests arid to semiarid THE RECENT PAST conditions, followed by an interval of ACKNOWLEDGEMENTS higher humidity inferred at approxima- The Late Pleistocene/Holocene transi- tely 2000 years BP, through the develop- The authors thank Dr. Beatriz Aguirre- tion is characterized by development of ment of gramineous steppe communi- Urreta and the Asociación Geológica Ar- palaeosols. During the early Holocene ties. A relative rise of temperature may gentina for the invitation to participate in sea level was still lower than today; pollen be inferred by the southward expansion this volume, and the reviewers Drs. M.G. records reflect the development of a of the Brazilian mammal fauna. Vucetich and M. Toledo for their fruitful vegetation community characteristic of These multidisciplinary research results comments which certainly improved the coastal dunes. Temperature and humidity are presented in a chronosequence chart manuscript. Dr. E. Farinati kindly provi- reached its maximum during the mid- synthesizing the suggested space-time ded information about Pleistocene shells. Holocene when the high diversity and correlation of the events recognized (Fig. abundance of marine dinocysts and acri- 9). In this chart, some of the studied tarchs indicate a transgression. This localities-covering the whole time inter- Geology of the area of Bahía Blanca, Darwin's view ... 145

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THE FOSSIL MAMMALS COLLECTED BY CHARLES DARWIN IN SOUTH AMERICA DURING HIS TRAVELS ON BOARD THE HMS BEAGLE

Juan Carlos FERNICOLA1, Sergio F. VIZCAÍNO2 and Gerardo DE IULIIS3

1 Sección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales "Bernardino Rivadavia". Buenos Aires, Argentina. Email: [email protected] 2 División Paleontología Vertebrados, Museo de La Plata, La Plata, Argentina. CONICET. Email: [email protected] 3 School of Nursing, George Brown College of Applied Arts and Technology, Toronto; and Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Canada. Email: [email protected]

ABSTRACT During the first two years of his voyage aboard HMS Beagle, Charles Darwin collected a considerable number of fossil mammals from various localities in Argentina and Uruguay. Among these remains are those of large mammals that Darwin informally assigned to Megatherium and Mastodon, the only large taxa then known for South America, and of small and medium- sized mammals that Darwin recognized as representing at least two rodents and a horse. The study of Darwin's collection was entrusted to Richard Owen, who described eleven taxa between 1837 and 1845, including the six following ones: Toxodon platensis, Macrauchenia patachonica, Equus curvidens, Scelidotherium leptocephalum, Mylodon darwini and Glossotherium sp. This contribution provides a synthesis of Darwin's preliminary assignments and evaluates the reasons that led him to recognize only megatheres and mastodonts for the large fossil remains. Also, it discusses the current taxonomic status of the taxa described or erected by Owen between 1837 and 1845 and the influence that Owen's taxonomic and phylogenetic conclusions had on the development of Darwin's ideas on evolution. Keywords: Darwin, Taxonomy, South America, Fossil mammals.

RESUMEN: Los mamíferos fósiles colectados por Charles Darwin en América del Sur durante su viaje a bordo del HMS Beagle. Durante los dos primeros años de su viaje a bordo del HMS Beagle, Charles Darwin colectó en distintas localidades de Argentina y Uruguay un considerable número de mamíferos fósiles. Entre estos se cuentan los grandes mamíferos que informalmente Darwin asig- nó a Megatherium y Mastodon, únicos grandes taxones conocidos hasta ese momento para América del sur y entre los pequeños y medianos mamíferos reconoció la presencia de al menos dos tipos de roedores y un caballo. El estudio posterior de todos los ejemplares colectados por Darwin fue llevado a cabo Richard Owen, quien entre 1837 y 1845 describió once taxones, entre los cuales, seis eran nuevos taxones: Toxodon platensis, Macrauchenia patachonica, Equus curvidens, Scelidotherium leptocephalum, Mylodon darwini y Glossotherium sp. En esta contribución se brinda una síntesis de las asignaciones preliminares efectuadas por Charles Darwin y se evalúan los motivos que lo habrían llevado a reconocer sólo megaterios y mastodontes entre los grandes mamí- feros fósiles por él colectados. También tiempo, se discute el estatus taxonómico de los taxones descritos o fundados por Richard Owen entre 1837 y 1845 y la influencia que habrían tenido las conclusiones taxonómicas y filogenéticas de Owen en el desarrollo de las ideas evolutivas de Darwin.

Palabras clave: Darwin, taxonomía, América del Sur, mamíferos fósiles.

INTRODUCTION (Buenos Aires Province, Argentina). Dar- from the Arroyo Sarandí near the city of win returned to Punta Alta between Mercedes (Soriano Department). He re- Charles Darwin collected fossil mammals August 29 - 31, 1833. Several weeks la- turned to Argentina and collected his last from various South American localities ter, around September 19, he collected specimens at Puerto San Julián (Santa during his voyage aboard HMS Beagle. in Guardia del Monte (Buenos Aires Pro- Cruz Province) during the first several He recovered his first fossil at Punta Alta vince); the Rio Carcarañá or Tercero months of 1834 (see Fig. 1). All the spe- (Buenos Aires Province, Argentina) on (Santa Fe Province, Argentina) on Octo- cimens, sent to his mentor John Stevens September 23, 1832, and continued co- ber 1; and the Bajada Santa Fe (Paraná, Henslow and later deposited in the Royal llecting intermittently at this locality until Entre Ríos Province, Argentina) on Oc- College of Surgeons (London, England), October 16. Three days later he collected tober 10. Moving to Uruguay, he collec- were studied by Richard Owen beginning several specimens from Monte Hermoso ted fossils during November 25 - 26 in 1836. 148 J. C. FERNICOLA, S. F. VIZCAÍNO AND G. DE IULIIS

in the earliest histories of America (e.g. Cieza de León 1553, Falkner 1774). The- se objects were interpreted as the remains of an ancient race of giant humans that inhabited this part of the world before being smitten by divine force. Cieza de León (1553, p. 183) provided the following vivid tale about huge bones found in Santa Elena, Ecuador: "Y así, dicen que, estando todos juntos envueltos en su maldita sodomía, vino fuego del cielo temero- so y muy espantable, haciendo gran ruido, del medio del cual salió un ángel resplandeciente, con una espada tajante y muy refulgente, con la cual de un solo golpe los mató a todos y el fuego los consumió, que no quedó sino algunos huesos y calaveras, que para memoria del castigo quiso Dios que quedasen sin ser consumidas por el fuego. Esto dicen de los gigantes; lo cual creemos que pasó, porque en esta parte que dicen se han hallado y se hallan huesos grandísimos. Y yo he oído a españoles que han visto pedazo de muela que juzgaban que a estar entera pesara más de media libra carnicera, y también que habían visto otro pedazo del hueso de una canilla, que es cosa admirable contar cuán grande era, lo cual hace testigo haber pasado…". This sort of general belief persisted, in South America at least, until the end of the 18th century, even though some of Figure 1: Map of the localities the remains were recognized as nonhu- where Charles Darwin collec- man, as indicated in the following passa- ted fossil mammals. ges by the English Jesuit Thomas Falkner Regrettably, the intense bombardment Surgeons retained only a single specimen, (1774, p. 54-55) "On the banks of the River suffered by this institution during April the remains of the Megatherium america- Carcarania, or Tercero, about three or four lea- 10 and 11, 1941, destroyed a large part of num specimen that apparently allowed gues before it enters into the Parana, are found its paleontological treasures. Cave's (1942) Owen (1840) to recognize the presence great numbers of bones, of an extraordinary catalogue of the surviving specimens of a fifth upper molariform in this taxon bigness, which seem human. There are some gre- revealed that some 95% of the collection (see appendix 1). ater and some less, as if they were of persons of had been lost - of 5,000 specimens, only The aim of this contribution is to provi- different ages. I have seen thigh-bones, ribs, bre- 175 remained. Among these, fortunately, de an update on the taxonomic status of ast-bones, and pieces of skulls. I have also seen are various of the specimens collected by the various taxa recognized or establis- teeth, and particularly some grinders which were Charles Darwin and described by Ri- hed by Owen (1837-45), and to consider three inches in diameter at the base. These bones chard Owen (1837-1845) as new taxa, in- the influence the specimens and their (as I have been informed) are likewise found on cluding Equus curvidens, Glossotherium sp; assignment had on the development of the banks of the Rivers Parana and Paraguay, Macrauchenia patachonica, Mylodon darwini, Darwin's ideas. as likewise in Peru. The Indian Historian, Scelidotherium leptocephalum and Toxodon Garcilasso de la Vega Inga, makes mention of platensis (Cave 1942). Beginning in 1946, HISTORICAL BACKGROUND these bones in Peru, and tells us that the Indians nearly all of Darwin's collection was have a tradition, that giants formerly inhabited transferred to the Natural History Mu- The first notices of South American those countries, and were destroyed by God for seum (London), where it is still housed "fossils" appear in the reports of early the crime of sodomy. (see appendix 1). The Royal College of Spanish explorers to South America and I myself found the shell of an animal, composed The fossil mammals collected by Charles Darwin in South America ... 149

of little hexagonal bones, each bone an inch in dillierès" and "mastodonte humboldien". Cu- ments that Owen (1837-1845) later mo- diameter at least; and the shell was near three vier (1823) later formally named them dified. Several of his assignments were yards over. It seemed in all respects, except it's Mastodon angustidens, Mastodon andium and considered taxonomic errors or misinter- [sic] size, to be the upper part of the shell of the Mastodon humboldti, respectively (see Ca- pretations (e.g., Sulloway 1982, Keynes armadillo; which, in these times, is not above a brera 1929). In this paper, Cuvier (1823, 2001), even though his inferences were in span in breadth." p. 179) included a handwritten communi- accordance with what was then known. It was during this period that the illus- cation by "don Dámaso Antonio Larrañaga, trious French naturalist George Cuvier a priest of Montevideo, to Mr. Augusto de DARWIN'S FOSSIL MAMMALS (1796) published the first scientific work Saint-Hilaire" that briefly described a on a South American fossil, which he group of fossil specimens, including a The following presents a synthesis of described and named Megatherium america- partial exoskeleton (or carapace), under Owen's initial taxonomic assignments num based on the specimen recovered by the name "Dasypus (Megatherium Cuv)." (1837-45) of the remains collected by Fray Manuel Torres in 1787 from Luján Cuvier (1823) thus acknowledged that Darwin in South America, including the in Buenos Aires Province, Argentina and this carapace probably belonged to Mega- preliminary assignments made by Dar- sent the following year by the Marquis of therium. Whatever the reasons that prom- win. The latter's assignments were glea- Loreto, Viceroy of Río de la Plata, to the pted Larrañaga's subgeneric assignment ned from field notes (Darwin 2002- Real Gabinete de Historia Natural in Ma- (that is, in recognizing the existence of 2008), personal correspondence (Darwin drid, Spain. Mones (1986) considered the armored megatheres), the fact that his 1985) and Charles Darwin's Beagle Diary erection of this species as the first Lin- opinion appeared in a work by Cuvier en- (in Keynes 2001), all material written by nean nomenclatorial act for a South sured its credibility, so that several in- Darwin before 1836, the year of his American fossil species. This specimen complete remains later sent to Europe return to England. The suprageneric had, however, previously been described were similarly recognized (e.g., as by classification used here follows McKen- and figured by Juan Bautista Bru de Ra- Weiss 1830, Clift 1835). It was not until na and Bell (1997). món, an artist and first taxidermist of the later that Owen (1839a) demonstrated Real Gabinete de Historia Natural. In this that megatheres were not armored and SYSTEMATIC regard, Garriga (1796, as quoted in Ló- that the remains belonged instead to the PALEONTOLOGY pez Piñeiro and Glick 1993, p. 126) noted group of mammals he termed glypto- that "… restaba superar la dificultad de que donts, a group closely related to armadi- Grandorder Ungulata Linnaeus, 1776 permitiese Bru que se diese a luz una obra que llos. Mirorder Meridiungulata McKenna, 1975 él había tenido en otro tiempo intento de publi- Knowledge of South American fossil Order Notoungulata Roth, 1903 car, y por varias circunstancias imprevistas mammals remained at this elementary le- Suborder Toxodontia Owen, 1853 sepultó en el olvido." López Piñeiro and vel for much of the early part of the 19th Family Toxodontidae Owen, 1845 Glick (1993, p. 66) maintained that the century, as is made clear from Owen's Subfamily Toxodontinae Owen, 1845 preparation of the monograph "… no fue (1838, p. 13) opening paragraph on his Genus Toxodon Owen, 1837 entonces editada, pero su preparación debía estar work on the fossil mammals in Darwin's tan adelantada que un tal Roume, representan- The Zoology of the Voyage of H.M.S. Beagle: Toxodon platensis Owen, 1837 te del gobierno de francés en Santo Domingo, "It may be expected that the description of the Fig. 2 consiguió un juego de pruebas de las planchas a osseous remains of extinct Mammalia, which su paso por Madrid en 1793. rank amongst the most interesting results of The specimens assigned to this taxon by Roume envió las pruebas de las planchas al Mr. Darwin's researches in South America, Owen (1837, 1838) were recovered by Institut de France, del que era miembro corres- should be preceded by some account of the fossil Darwin from various South American pondiente, acompañadas de una "corta descrip- mammiferous animals which have been pre- localities. On October 1 and 10, 1833, he ción" del esqueleto. La sección de ciencias del Ins- viously discovered in that Continent. The results collected at Río Carcaraña o Tercero titut encargó a George Cuvier un informe sobre of such a retrospect are, however, necessarily (Santa Fe Province) and at Bajada Santa el tema, que apareció publicado en 1796…" comprised in a very brief statement; for the Fe (Paraná, Entre Ríos Province). His Cuvier (1806) also studied fossil probos- South American relics of extinct Mammalia, field notes (Darwin 2002-2008) describe cidean remains found by Dombey in Pe- hitherto described, are limited, so far as I know, a fossil specimen as "…a large rotten tooth ru and by Humboldt in various localities to three species of Mastodon, and the gigantic & in the layer large cutting tooth", a descrip- in Bolivia, Chile, Colombia, and Ecua- Megatherium." tion that agrees with those included in dor, among which he recognized three It was against this systematic framework his Beagle Diary ("… a curious & large cut- morphotypes, designated informally as that Darwin began his efforts in collecting tooth"; in Keynes 2001, p. 193) and "mastodonte a dents étroites", "mastodonte Cor- tion and preliminary taxonomic assign- written to John Stevens Henslow ("In ye 150 J. C. FERNICOLA, S. F. VIZCAÍNO AND G. DE IULIIS

R. Carcarana I got a tooth, which puzzles even my conjectures, it looks like an enormous gnawing one"; Letter 229 in Darwin 1985). Darwin's (2002-2008) field notes for No- vember 26, 1833, state: "Started went round by a house to see large head & bones, washed out of Barranca & found after a flood. - pieces here also of Casca - Barranca" while his Bea- gle Diary (in Keynes 2001) records: "Be- gan my return in a direct line to M. Video; went by an Estancia where there was a part, very perfect, of the head of a Megatherium. I purcha- sed it for a few shillings". An alveolus of this specimen, recovered from Arroyo Saran- dí, Departament of Soriano, Uruguay, easily accommodated a molariform tooth found by Darwin in Carcaraña (Owen 1838).

Toxodon sp.

The mandible and isolated molariforms assigned by Owen (1837, 1838) to this genus were collected between September 23 - October 16, 1832, at Punta Alta (Buenos Aires Province, Argentina). On the molariforms, Darwin (1985, Letter Figure 2: Skull of Toxodon platensis in ventral view (from 192) noted that "some large molar teeth, Owen 1838). Scale bar = 10 which in some respects would seem to belong to cm. an enormous Rodentia". Burmeister (1866) based the species Toxodon darwini on the from the Rodentia, and the existing Pachyder- gulates of South America. The Notoun- mandible described by Owen (1838) mata, and it manifests an affinity to the Dino- gulata, a clade of the Mirorder Meridiun- Bond (1999) acknowledged the possible therium and the Cetaceous order. Mr. Owen, gulata, shares an ancestry with North synonymy of Toxodon darwini with Toxo- however, observed, that the development of the American condylarths (Cifelli 1993) In don platensis, although he recognized that nasal cavity and the presence of frontal sinuses, other words, Toxodon has no close phylo- a revision of the genus was required. renders it extremely improbable that the habits genetic relationships with the groups Owen (1837, 1838, p. 16) recognized To- of he Toxodon were so exclusively aquatic as mentioned by Owen (1838). xodon as "A gigantic extinct mammiferous ani- would result from the total absence of hinder mal, referrible to the Order Pachydermata, but extremities; and concludes, therefore, that it was Grandorder Ungulata Linnaeus, 1776 with affinities to the Rodentia, Edentata, and a quadruped, and not a Cetacean; and that it Mirorder Meridiungulata McKenna, 1975 Herbivorous Cetacea". Darwin (1839, p. manifested an additional step in the gradation of Order Liptopterna Ameghino, 1889 180) summarized the morphological ba- mammiferous forms leading from the Rodentia, Superfamily Macrauchenioidea Gervais, sis leading to this idea of multiple affini- through the Pachydermata to the Cetacea; a gra- 1855 ties: "Mr. Owen says, judging from the portion dation of which the water-hog of South Ame- Family Macraucheniidae Gervais, 1855 of the skeleton preserved, the Toxodon, as far rica (Hydrochærus capybara) already indi- Subfamily Macraucheniinae Gervais, 1855 as dental characters have weight, must be refe- cates the commencement amongst existing Ro- Genus Macrauchenia Owen, 1838 rred to the rodent order. But from that order it dentia, of which order it is interesting to observe deviates in the relative position of its supernu- this species is the largest, while at the same time Macrauchenia patachonica Owen, 1838 merary incisors, in the number and direction of it is peculiar to the continent in which the re- Fig. 3a the curvature of its molars, and in some other mains of the gigantic Toxodon were discovered". respects. It again deviates, in several parts of its The genus Toxodon is now considered In a letter from March, 1834, to John structure which Mr. Owen enumerated, both among the more derived native notoun- Stevens Henslow, Darwin (1985, Letter The fossil mammals collected by Charles Darwin in South America ... 151

238) recounted that "At Port St Julian I found some very perfect bones of some large animal, I fancy a Mastodon". On this specimen, the only one found by Darwin in Puerto San Julián, Owen (1838, p. 35) erected Macrauchenia patachonica, which he considered "A large extinct Mammiferous Figure 3: a) Cervical vertebra Animal, referrible to the Order Pachydermata; of Macrauchenia patachonica in but with affinities to the Ruminantia, and espe- lateral and ventral views (from cially to the Camelidae". The evolutionary Owen 1838). Scale bar = 5 cm; b) Tooth of Equus sp. in significance of this purported kinship occlusal and lateral views impressed Darwin (1839, p. 210): "The (from Owen 1840). Scale bar most important result of this discovery, is the = 1 cm. confirmation of the law that existing animals Diary (in Keynes 2001), or personal co- Currently Stegomastodon platensis is the only have a close relation in form with extinct species. rrespondence (Darwin 1985). Although proboscidean species recognized from As the guanaco [Camelidae] is the characteris- Owen (1840) did not attempt specific as- the areas yielding Darwin's specimens tic quadruped of Patagonia, and the vicuna of signation of these molars, he did note (Alberdi and Prado 1995). the snow-clad summits of the Cordillera, so in that the only difference between them bygone days, this gigantic species [Macrauche- and those of Equus caballus was the sma- Magnorder Xenarthra Cope, 1889 nia patachonica] of the same family must ller size of the former. Owen (1845) Order Pilosa Flower, 1883 have been conspicuous on the southern plains". erected Equus curvidens based on Darwin's Suborder Phyllophaga Owen, 1842 The genus Macrauchenia is now conside- specimens, a species which is considered Infraorder Mylodonta McKenna and red as among the more derived native synonymous with Equus (Amerhippus) neo- Bell, 1987 South American litopterns. As with the geus Lund by Prado and Alberdi (1994). Superfamily Mylodontoidea Gill, 1872 Notoungulata, the Litopterna comprises Family Scelidotheriidae Ameghino, 1889 a clade of the Mirorder Meridiungulata Grandorder Ungulata Linnaeus, 1776 Subfamily Scelidotheriinae Ameghino, and is closely related to North American Order Uranotheria McKenna and Bell, 1889 condylarthrans, rather than camelids, as 1987 Genus Scelidotherium Owen, 1839b erroneously proposed by Owen (1838). Suborder Tethytheria McKenna, 1975 Infraorder Behemota McKenna and Bell, Scelidotherium leptocephalum Owen, 1839b Grandorder Ungulata Linnaeus, 1776 1987 Fig. 4 Mirorder Altungulata Protero and Schoch, Parvorder Proboscidea Illiger, 1811 1989 Superfamily Elaphantoidea Gray, 1821 This taxon was based by Owen (1839b) Order Perissodactyla Owen, 1848 Family Gomphotheriidae Hay, 1922 on the only nearly complete skeleton Suborder Hippomorpha Word, 1937 Subfamily Rhynchoteriinae Hay, 1922 found by Darwin at Punta Alta (Buenos Family Equidae Gray, 1821 Tribe Cuvieroniini Cabrera, 1929 Aires Province). We are unable to find Genus Equus Linnaeus, 1758 Genus Mastodon Rafinesque, 1814 any preliminary assignment of this specimen by Darwin, although Sulloway Equus sp. Mastodon sp. (1982, p. 353) noted that Darwin consi- Fig. 3b dered the specimen as "allied to the Rhi- The specimens assigned to this taxon, noceros". Darwin made this conjecture on The remains assigned to this genus re- collected by Darwin at Bajada Santa Fe September 23, 1832 (in Keynes 2001) in present the first fossil horses found in (Entre Ríos Province) and Río Carcarañá the following Beagle Diary entry: "… I South America. The two molars from or Tercero (Santa Fe Province), were walked on to Punta Alta to look after fossils; & Argentina were recovered from Punta noted by Owen (1840, p. 180) as "…the to my great joy I found the head of some large Alta (Buenos Aires Province) and Bajada fragments of the teeth and portions of the skele- animal, imbedded in a soft rock. … It took me Santa Fe (Entre Rios Province). The spe- ton which reached England, are not sufficient to nearly 3 hours to get it out: As far as I am able cimen from Bajada Santa Fe was referred lead to a determination of the species; but suffi- to judge, it is allied to the Rhinoceros. … I to by Darwin as a horse (in Keynes ciently prove it to have been nearly allied, if not did not get it on board till some hours after it was 2001). We have not been able to find any identical, with the Mastodon angustidens of dark". However, the specimen assigned mention of the other specimen in his Cuvier, and unquestionably distinct from the by Owen (1839b) to Scelidotherium was co- field notes (Darwin 2002-2008), Beagle Mastodon giganteum of the United States". llected by Darwin in Punta Alta in Au- 152 J. C. FERNICOLA, S. F. VIZCAÍNO AND G. DE IULIIS

ther it was collected during his first or second sojourn to this area. In a letter sent to John Stevens Henslow in No- vember, 1832, Darwin (1985, Letter 192) noted a completed list of fossils collected in Bahia Blanca, among which he emphasized "… the upper jaw & head of some very large animal, with 4 square hollow molars….& the head greatly produced in front…. I at first thought it belonged either to the Me- galonyx or Megatherium…. In confirmation, of this, in the same formation I found a large surface of the osseous polygonal plates, which ``late observations'' (what are they?) show belong to the Megatherium". A footnote on this page, presumably inserted by the editors, indicates that this specimen was "Described in Fossil Mammalia, p. 63-73, by Richard Owen, who identified it as belonging to a distinct subgenus of Megatheroid Edentata, to which he gave the name Mylodon darwinii". However, we believe this to be incorrect - the specimen is not the one that Owen (1839b) later assigned to Mylodon darwini, as Dar-win indicated that the specimen was an upper jaw and skull, rather than the mandible on which Mylodon darwini is based. Darwin's indication of the presence of four molariforms possibly led to Figure 4: Partial skeleton of Scelidotherium leptocephalum in the assumption (i.e., in the footnote) that ventral view (from Owen, the specimen belonged to Mylodon darwi- 1838b). Scale bar = 10 cm. ni, given that five upper molariforms are gust, 1833, the time during which Darwin Suborder Phyllophaga Owen, 1842 present in Megatherium. However, the had returned to this locality (Keynes Infraorder Mylodonta McKenna and earliest descriptions of Megatherium indi- 2001, p. 178). Further confirmation of Bell, 1987 cate four upper molariforms. As noted this date appears in a letter from Darwin Superfamily Mylodontoidea Gill, 1872 above, it was Owen (1840, p. 102) who (1985, Letter 188), dated September 20, Family Mylodontidae Gill, 1872 recognized the presence of a fifth upper 1833, to Caroline S. Darwin, in which he Subfamily Mylodontinae Gill, 1872 molariform: "Upon clearing away the matrix recounts that "I likewise at Bahia Blanca Genus Mylodon Owen, 1839b from the palatal and alveolar surface of one of found some more bones more perfect than those I the cranial fragments of the Megatherium in formerly found, indeed one is nearly an entire Mylodon darwini Owen, 1839b Mr. Darwin's collection, I was gratified by the skeleton". As indicated below, Darwin's Fig. 5a detection of the crown of a fifth molar". "allied to the Rhinoceros" comment may The taxonomic history of Mylodon is more likely have been a reference to one The specimen assigned to this genus by among the more complex for the taxa of the cranial remains that Owen (1840) Owen (1839b, p. 69) consists of "…the erected by Owen (1838-1840), due main- later assigned to the genus Megatherium, lower jaw with the series of teeth entire on both ly to the ambiguity in the type species. particularly as Owen's generic assigna- sides: but the extremity of the symphysis, the Owen (1839b) erected Mylodon for two tions of the sloths are for the most part coronoid and condyloid processes, and the angu- species, Mylodon darwini and Mylodon harla- still considered correct. lar process of the left ramus, are wanting". ni. The former species was based on a This specimen was recovered by Darwin left dentary from Punta Alta (Buenos Ai- Magnorder Xenarthra Cope, 1889 at Punta Alta (Buenos Aires Province), res Province), whereas the second was Order Pilosa Flower, 1883 but we were unable to determine whe- based on a cast of a mandible from The fossil mammals collected by Charles Darwin in South America ... 153

North America that Harlan (1835) had assigned to Megalonyx laqueatus.Owen (1839b) was unclear about his choice of type species, given that in the title of his description of his new genus he referred to the fossil collected by Darwin, but in the text Owen noted that Mylodon darwini was the second species of the genus. This ambiguity would be trivial if the two species were congeneric, but they have been subsequently considered as belonging to different genera. Consequently, some later researchers, such as Reinhardt (1879), Lydekker (1887) and Brown (1903), considered Mylodon harlani as the Figure 5: a) Mandible of type of the genus, whereas other, such as Mylodon darwini in occlusal Leidy (1855), bestowed this status on view (from Owen, 1839b). Scale bar = 10 cm; b) Mylodon darwini. The issue was resolved Cranial fragment of the skull by Kraglievich (1928) in favor of Mylodon of Glossotherium in lateral darwini based on the following reasons: view (from Owen 1839b) Scale bar = 10 cm. 1) the title of the generic description refers to the mandible collected by Dar- Tribe Glossotheriini McKenna, 1987 Mylodon robustus, and 3) that the cranial win; Genus Glossotherium Owen, 1839b fragment originally assigned by Owen 2) the original material available to Owen (1839b) to Glossotherium was closely allied was the dentary from Punta Alta; Glossotherium sp. generically to Mylodon robustus. Not recog- 3) Owen (1842) recognized Mylodon harla- Fig. 5b nizing Mylodon darwini as type of the ni as the second species of the genus; genus, Reinhardt (1879) proposed the 4) in his "Conspectus familiarum, generum et This genus was erected by Owen (1839b) new genus Grypotherium, in which he pla- specierum", Owen (1842, p. 169) listed My- based on the posterior half of a skull ced the dentary assigned by Owen lodon darwini as the first species of the recovered from Arroyo Sarandí (Soriano (1839b) to Mylodon darwini and the speci- genus. Departament, Uruguay). In his Beagle men from Pergamino as the species Kraglievich's (1928) logic has been ac- Diary, Darwin (in Keynes 2001) noted Grypotherium darwini; and recognized My- cepted by the scientific community, so that "We heard of some giants bones, which as lodon robustus as the type species of My- that Mylodon darwini is currently conside- usual turned out to be those of the Megatherium lodon. Ameghino (1889) accepted the ge- red the type species of Mylodon, whereas - With much trouble extracted a few broken neric differences noted by Reinhardt Mylodon harlani is placed in the genus fragments". As with Mylodon darwini, the (1879), but considered, as Owen (1842) Paramylodon. The taxonomic solution pro- taxonomic history of Glossot-herium is had before, that the cranial fragment of vided by Kraglievich (1928) not only re- complex. Owen (1842) erected the new Glossotherium and the dentary of Mylodon solved the taxonomic problem with My- species Mylodon robustus on remains darwini belonged to the same species, and lodon, but also helped clarify the proble- incluiding a nearly complete skull; and so included these specimens - as well as matic nomenclature of the genus Glosso- assigned to Mylodon darwini (based on a the skull and mandible from Pergamino - therium (see below). dentary) the cranial fragment previously in Glossotherium darwini, given that in this described by Owen (1839b) as Glossothe- scenario Glossotherium has priority over Magnorder Xenarthra Cope, 1889 rium. Reinhardt's (1879) detailed descrip- Grypotherium. Smith-Woodward's (1900) Order Pilosa Flower, 1883 tion of a fossil sloth skull and mandible revision of Darwin's South American Suborder Phyllophaga Owen, 1842 from Pergamino (Buenos Aires Pro- fossil sloth collection concluded that the Infraorder Mylodonta McKenna and vince) recognized, 1) that the mandible cranial fragment originally assigned to Bell, 1987 was very similar to that described as Glossotherium was congeneric with the Superfamily Mylodontoidea Gill, 1872 Mylodon darwini by Owen (1839b), 2) that specimen assigned to Mylodon robustus by Family Mylodontidae Gill, 1872 its skull features were sufficiently distinct Owen (1842) and that the dentary of Subfamily Lestodontinae Ameghino, 1889 as to suggest generic separation from Mylodon darwini and the specimen from 154 J. C. FERNICOLA, S. F. VIZCAÍNO AND G. DE IULIIS

Pergamino, described by Reinhardt (1879), were conspecific. However, as Smith-Woodward did not recognize Mylodon darwini as type species of Mylo- don, he resurrected Reinhardt's (1879) Grypotherium. Kraglievich (1928) modified the taxonomy of these taxa based on a detailed revision of the group. This author held that the root of the problem was the rejection of Mylodon darwini as type species of Mylodon and the lack of agreement on the assignment of the cranial fragment assigned to Glossotherium by Figure 6: a) Parasagittal sec- Owen (1839b). Once Kraglievich (1928) tion through the maxilla of had established Mylodon darwini as type Megatherium cuvieri (= Megatherium americanum, see species of Mylodon, Grypotherium fell as a text) showing the five mola- synonym of Mylodon. Kraglievich (1928) riforms (from Owen 1840). agreed with Reinhardt (1879) and Smith- Scale bar = 10 cm; b) Woodward (1900) that the cranial frag- Mandible of Megalonyx jeffer- soni (= Mylodon darwini, see ment of Glossotherium was congeneric text) in occlusal view (from with Mylodon robustus but not conspecific Owen 1840). Scale bar = 10 with it. Consequently, and with the gene- cm. ral understanding that Mylodon robustus Suborder Phyllophaga Owen, 1842 above) from consideration, and leaving was generically distinct form Mylodon dar- Infraorder Megatheria McKenna and Megatherium as the only taxon to which he wini, Kraglievich (1928) revalidated Glos- Bell, 1987 assigned skull material from Punta Alta. sotherium, but with two species, i.e., Glos- Superfamily Megatheroidea Gray, 1821 Owen's (1840) assignment to the species sotherium robustus and Glossotherium urugua- Family Megatheriidae Gray, 1821 Megatherium cuvieri was based on acceptan- yense, the latter including Owen's (1839b) Subfamily Megatheriinae Gray, 1821 ce of the specific name change proposed cranial fragment. These nomenclatural Genus Megatherium Cuvier, 1796 by Desmarest (1822) for Megatherium ame- conclusions were accepted by Cabrera ricanum. However, this proposed nomen- (1936), although in this author's revision Megatherium cuvieri Desmarest, 1822 clatural alteration is unsupported and the of the species of Glossotherium, he recog- Fig. 6a name was considered a nomen illegiti- nized its two valid species as Glossotherium mum by Mones (1986). In this context, robustum and Glossotherium lettsomi (Owen), Owen (1840) assigned to this species va- the specimens assigned to Megatherium cu- with Glossotherium uruguayense a synonym rious cranial fragments collected by Dar- vieri by Owen (1840) correspond to Mega- of the latter (Glossotherium lettsomi was ori- win in Punta Alta (Buenos Aires Pro- therium americanum. ginally assigned to Pleurolestodon lettsomi by vince). Although Darwin referred most Gervais and Ameghino (1880), based on of the large remains he recovered to Magnorder Xenarthra Cope, 1889 observation of a skull exhibited in the Megatherium, as shown by his field notes, Order Pilosa Flower, 1883 Natural History Museum, London, that Beagle Diary, and personal correspon- Suborder Phyllophaga Owen, 1842 had been labeled by Owen as Mylodon lett- dence, Owen (1840) did likewise only for Infraorder Megatheria McKenna and somi (see Ameghino 1889). the specimens collected from Punta Alta. Bell, 1987 Esteban's (1996) review of Mylodonti- In discussing Darwin's (in Keynes 2001, Superfamily Megatheroidea Gray, 1821 nae considered Glossotherium lettsomi (sen- p. 107) "allied to the Rhinoceros" com- Family Megalonychidae Gervais, 1855 su Cabrera 1936) a synonym of Glossothe- ment, we noted that it did not pertain to Subfamily Megalonychinae Gervais, 1855 rium robustum, so that the cranial fragment the Scelidotherium skeleton but to various Tribe Megalonychini Gervais, 1855 collected by Darwin in Uruguay is skulls assigned by Owen (1840) to Mega- Subtribe Megalonychina Gervais, 1855 currently assigned to the latter species. therium. Indeed, Darwin's comment, da- Infratribe Megalonychi Gervais, 1855 ted September 23, 1832, could only refer Genus Megalonyx Harlan, 1825 Magnorder Xenarthra Cope, 1889 to those skulls collected up to this time, Order Pilosa Flower, 1883 effectively excluding Scelidotherium (see Megalonyx jeffersonii (Desmarest, 1822) The fossil mammals collected by Charles Darwin in South America ... 155

Fig. 6b rosettes, and so closely correspond in size and tober 19, 1832. Owen (1840, p. 10) com- pattern with the bony armour described by M. mented that "Mr. Darwin discovered the Owen (1840) assigned a mandible collec- Lund [1839], as characterizing his species, decomposed molar of a Rodent, equalling in ted by Darwin at Punta Alta (Buenos Hoplophorus euphractus, that I feel no size, and closely resembling in the disposition of Aires Province) to this species. A particu- hesitation in referring them to that animal". its oblique component laminae, the hinder molar lar feature of this specimen, figured by Burmeister (1870-1874) assigned the os- of the Capybara (Hydrochoerus.) The fossil Owen (1840, pl. 29), is that it preserves teoderms figured by Owen (1840, pl. 32, differs, however, in the greater relative breadth of only one of the molariforms of the right figs 4 and 5) to Hoplophorus ornatus (= the component laminae. dentary. This agrees with the description Neosclerocalyptus ornatus) and Hoplophorus ele- I have, lastly, to notice the head of a femur, and given by Darwin (in Keynes 2001, p. 109) gans, respectively. Ameghino (1889) syno- some fragments of pelvic bones from the same for a mandible collected October 8, nymized the latter with Lomaphorus elegans. formation which bear the same proportion to the 1832: "After breakfast I walked to Punta tooth above alluded to, as subsists between the Alta, the same place where I have before found Magnorder Xenarthra Cope, 1889 teeth and bones of the Capybara, and which are fossils… I obtained a jaw bone which contained Order Cingulata Illiger, 1811 sufficient to prove that there once has existed in a tooth: by this I found out that it belongs to the Superfamily Glyptodontoidea Gray, 1869 South America a species of the family Caviidæ, great ante-diluvial animal the Megatherium". Family Glyptodontidae Gray, 1869 as large as the present Capybara, but now appa- Leidy (1852) erected a new taxon, Gna- Subfamily Glyptodontinae Gray, 1869 rently extinct". thopsis oweni on this specimen, which Kra- Tribe Glyptodontini Gray, 1869 These elements were among those that glievich (1928) assigned to Mylodon darwini. Genus Glyptodon Owen, 1839c Darwin (in Keynes 2001, p. 110-111) encountered as follows: "The Captain landed Magnorder Xenarthra Cope, 1889 Glyptodon clavipes Owen, 1839c for half an hour at Monte Hermoso, (or Star- Order Cingulata Illiger, 1811 vation point as we call it) to take observations. - Superfamily Glyptodontoidea Gray, 1869 Owen (1840) tentatively assigned to this I went with him & had the good luck to obtain Family Glyptodontidae Gray, 1869 species distinct osteoderms collected by some well preserved fossil bones of two or three Subfamily Hoplophorinae Huxley, 1864 Darwin from the Uruguayan locality of sorts of Gnawing animals. - One of them must Tribe Hoplophorini Huxley, 1864 Arroyo Sarandí (Mercedes, of Soriano have much resembled the Agouti but it is sma- Genus Hoplophorus Lund, 1938 Departament, Uruguay); and the Argen- ller". Even so, it is not possible to deter- tinean localities of Bajada Santa Fe (Pa- mine which of the remains Darwin Hoplophorus euphractus Lund, 1938 raná, Entre Ríos Province) and Guardia thought similar to an agouti, though it is Fig. 7a del Monte (Buenos Aires Province). worth noting that what Darwin considered an agouti was a Patagonian hare In general, the carapace fragments and Magnorder Epitheria McKenna, 1975 (Darwin, in Keynes 2001, p. 103). osteoderms collected by Darwin in South Superorder Preptotheria McKenna, 1975 The following Hydrochoeridae are cu- America were assigned preliminarily to Grandorder Anagalida Slazay and Mc rrently recognized from the fossiliferous Megatherium. Certainly such assignments Kenna, 1971 horizons of Monte Hermoso: Phugathe- were due to Cuvier's (1823) tacit approval Mirorder Simplicidentata Weber, 1904 rium cataclisticum, Anchimysops villalobosi of Larrañaga's (in Cuvier 1823) sugges- Order Rodentia Bowdish, 1821 and Chapalmatherium perturbidum. Vucetich tion that megatheres were armored. Suborder Hystricognatha Woods, 1976 et al. (2005) considered the type speci- Owen (1840, p. 107) recognized the close Infraorder Hystricognathi Tullberg, 1899 mens of the first two species to represent relationship, still considered correct, bet- Parvorder Caviida Bryant and McKenna, juvenile individuals and that they might ween glyptodonts and living armadillos 1995 be conspecific with Chapalmatherium per- and remarked the following about Superfamily Cavioidea Fisher de Wald- turbidum. If this is correct, then the valid Hoplophorus euphractus: "The portions of the heim, 1817 name, as determined by Vucetich et al. tesselated bony dermal covering of a Dasypodoid Family Hydrochoeridae Gray, 1825 (2005), for the species would be Phu- quadruped, figured in Pl. XXXII. figs. 5 and Subfamily Hydrochoerinae Gray, 1825 gatherium cataclisticum. In this respect, the 4, of the natural size, were discovered folded Genus Hydrochoerus Brisson, 1762 specimen collected by Darwin might round the middle and ungueal phalanges, figs. 2 represent this latter species. and 3, at Punta Alta, in Bahia Blanca, in an Hydrochoerus sp. earthy bed interstratified with the conglomerate Magnorder Epitheria McKenna, 1975 containing the remains of the fossil Edentals. The fossils that Owen (1940) assigned to Superorder Preptotheria McKenna, 1975 In one of these fragments, measuring six inches this genus were collected by Darwin (in Grandorder Anagalida Slazay and Mc long by five broad, the tesserae are arranged in Keynes 2001) in Monte Hermoso on Oc- Kenna, 1971 156 J.C. FERNICOLA, S.F. VIZCAÍNO AND G. DE IULIIS

Mirorder Simplicidentata Weber, 1904 Order Rodentia Bowdish, 1821 Suborder Hystricognatha Woods, 1976 Figure 7: a) Osteoderms Infraorder Hystricognathi Tullberg, 1899 assigned by Owen (1840) to Parvorder Caviida Bryant and McKenna, Hoplophorus euphractus (= Neosclerocalyptus ornatus, see 1995 text) in dorsal and lateral Superfamily Octodontoidea Waterhouse, views (from Owen 1840). 1839 Scale bar = 1 cm; b) Maxilla and mandible assigned by Family Octodontidae Waterhouse, 1839 Owen (1840) to Ctenomys Subfamily Octodontinae Waterhouse, priscus (= Actenomys priscus, 1839 see text) (from Owen 1840). Genus Ctenomys Blainville, 1826 Scale bar = 5 mm. Owen (1840) but not assigned to a genus. representing the genus Hydrochoerus. Ctenomys priscus Owen, 1840 Owen (1840, p. 110) described "The por- Among the large mammals, Darwin re- Fig. 7b tion of the right hind-foot of the Rodent figured cognized an enormous rodent and four at fig. 12, includes the calcaneum, astragalus, genera: Rhinoceros, Megalonyx, Megatherium This species was erected by Owen (1840) cuboides, external and middle cuneiform bones, and Mastodon. The first two listed do not based on remains collected by Darwin (in and the metatarsals and proximal phalanges of appear in Cuvier's (1823) taxonomic list Keynes 2001) on October 11, 1832. the toes corresponding with the three middle toes of fossil South American mammals. The Owen (1840, p. 109) allocated to this spe- of five-toed quadrupeds. The metatarsals are assignment to Rhinoceros was the first cies a "…fragment of the upper jaw, figured in chiefly remarkable for the well-developed double- such attempt by Darwin (in Keynes 2001, Pl. XXXII. fig. 6" and a "...fragment of the trochlear articular surface, and intermediate rid- p. 107) for a South American fossil skull, lower jaw of the same fossil Rodent . figured at ge. These remains, as well as the jaws and teeth collected during his first visit to Punta fig. 10 and 11". It should be noted that of the Ctenomys, were discovered at Monte Alta (September, 1832). However, this Owen's (1840) figure 6 possibly does not Hermoso in Bahia Blanca". These pedal re- assignment does not appear in the taxo- represent an upper jaw, but a posterior mains are possibly those that Darwin nomic list of specimens collected from fragment of a mandible that may corres- (1985, Letter 192) described in a letter, Punta Alta and Monte Hermoso sent to pond to the posterior part of the mandi- dated November 12, 1832, to John Ste- John Stevens Henslow (Darwin 1985, ble figured by Owen (1840, figs. 10, 11) vens Henslow as "the Tarsi & Metatarsi Letter 192). Darwin may possibly have (D. Verzi, pers. comm.). very perfect of a Cavia" although it is not changed his first impression, but it is not Mones (1994) provided a detailed discus- possible to determine this with certainty. easily ascertainable with the available sion of the taxonomy of Ctenomys priscus, evidence. concluding that it might belong to the Mastodonts and armored megatheres The only mention of Megalonyx appears genus Dicoelophorus, erected by Ameghino in the previously noted list as "2nd the upper (1888). However, the generic name As noted in the preceding historical sum- jaw & head of some very large animal, with 4 currently in use for this species is Ac- maries, the large South American fossil square hollow molars.-& the head greatly pro- tenomys, erected by Burmeister (1888). mammals recognized by Cuvier (1823) duced in front.- I at first thought it belonged The reasons for maintaining Actenomys amounted to an armored megathere and either to the Megalonyx or Megatherium.- In were clearly set forth by Verzi (1994, p. three mastodonts. A detailed considera- confirmation, of this, in the same formation I 183), as follows "Dicoelophorus Ame- tion of Darwin's preliminary assign- found a large surface of the osseous polygonal ghino, 1888 es, por prioridad, un sinónimo "se- ments demonstrates that he was strongly plates, which “ate observations” (what are they?) nior" de Actenomys Burmeister… . Sin em- influenced by Cuvier's taxonomic sche- show belong to the Megatherium.- Immediately I bargo, luego de que Reig (1958) adoptara el me. In essence, the fossil specimens re- saw them I thought they must belong to an enor- nombre genérico Actenomys para todos los cognized by Darwin may be grouped as mous Armadillo, living species of which genus materiales referibles a las formaciones Monte large vs. medium and small mammals. are so abundant here…". In this as in many Hermoso y Chapadmalal, este nombre ha sido Among the latter is the first fossil of a other cases (see below) Darwin decided ampliamente usado hasta el presente por lo cual South American horse, obviously identi- in favor of Megatherium based on the pre- nosotros nos inclinamos a mantenerlo". fied as such by Darwin, and three ro- sence of osteoderms collected in the dents, two of which were assigned by same formation. Rodentia Darwin to Cavia and the Patagonian hare Owen (1838-1840) recognized the speci- (Dolichotis patachonica), whereas the third mens assigned by Darwin to Megatherium A third rodent was briefly described by was later considered by Owen (1840) as as glyptodonts, toxodonts, and large The fossil mammals collected by Charles Darwin in South America ... 157

ground sloths. In our estimation, the on- the Origin of Species and only two years af- erroneous taxonomic assignments, Sullo- ly factor that led Darwin to assign re- ter he had begun recording his thoughts way (1982, p. 353) recognized that "The mains of such a heterogeneous group of on transmutation, Darwin (1839, p. 209- general effect of these confusions during the Bea- forms to Megatherium was the presence of 210) had this to say about the South gle voyage was to minimize the evolutionary osteoderms or carapace fragments in the American paleofauna: "The most important implications of the diverse fossil forms". same horizon. In effect, all the specimens result of this discovery, is the confirmation of Paradoxically, many of Owen's (1838- that Darwin assigned to Megatherium were the law that existing animals have a close rela- 1840) proposed relationships that so recovered from the Arroyo Sarandí (Uru- tion in form with extinct species. As the guana- influenced Darwin (1839, 1845, 1859) in guay) and Punta Alta (Argentina), and co is the characteristic quadruped of Patagonia, the development of his transformatio- Darwin also reported carapace elements. and the vicuna of the snow-clad summits of the nist theory were later rejected. Indeed, Further evidence supporting our conten- Cordillera, so in bygone days, this gigantic species toxodonts and macrauchenids are highly tion appears in Darwin's (1985, Letter of the same family must have been conspicuous derived native South American ungulates 215) letter of September 20, 1833 sent to on the southern plains. We see this same relation distantly related phylogenetically to Caroline S. Darwin, in which he wrote of type between the existing and fossil Cte- rodents and guanacos, whereas the large "At the Guardia del Monte, I found some more nomys, between the Capybara (but less plain- glyptodonts are not the ancestors of of the armour of the giant Megatherium, ly, as shown by Mr. Owen) and the gigantic armadillos, but to the contrary, the latter which was to me very interesting, as connecting Toxodon; and lastly, between the living and are antecedent to the former. In this res- the Geology of the different parts of the Pam- extinct Edentata. At the present day, in South pect, we may suppose that the affinities pas". Indeed, Darwin used the presence America, there exist probably nineteen species of proposed by Owen (1838-1840) maximi- of osteoderms as a diagnostic feature of this order, distributed into several genera; while zed the evolutionary implications of the Megatherium. throughout the rest of the world there are but South American paleofauna. In this context, one might hypothesize five. If, then, there is a relation between the living that it was the absence of osteoderms in and the dead, we should expect that the ACKNOWLEDGMENTS the layers yielding the only Santa Cruz Edentata would be numerous in the fossil state. specimen, which led Darwin to regard I need only reply by enumerating the Megathe- We are grateful to the editors for the invi- this specimen as follows: "At Port St Ju- rium, and the three or four other great species, tation to participate in this special issue lian I found some very perfect bones of some discovered at Bahia Blanca; the remains of some of Revista de la Asociación Geológica Argen- large animal, I fancy a Mastodon" (Darwin of which are also abundant over the whole im- tina, and to Greg McDonald and Mariano 1985, Letter 238) and Darwin (1839) later mense territory of La Plata. I have already Bond for their thoughtful comments and admitted that "I had no idea at the time, to pointed out the singular relation between the suggestions that improved the manus- what kind of animal these remains belonged". armadilloes and their great prototypes, even in a cript. Andrew Currant, Curator of Fossil Certainly, this is the only mention of point apparently of so little importance as their Mammals Palaeontology, Natural Histo- Mastodon that was not later corroborated external covering". Similarly, Darwin (1845, ry Museum London, Simon Chaplin, Di- by Owen (1838-1840). 1859, p. 339) noted that "In South Ame- rector of Museums and Special Collec- Lastly, Toxodon was the only species the rica, a similar relationship is manifest, even to tions and Sarah Pearson, Assistant Cu- remains of which Darwin (1985, Letter an uneducated eye, in the gigantic pieces of ar- rator, The Royal College of Surgeons of 192, 2002-2008) assigned to different mour like those of the armadillo, found in seve- England, for information on Darwin's species. Darwin assigned a molar collec- ral parts of La Plata; and Professor Owen has fossil mammal collections, and Louise ted in Punta Alta to an enormous rodent shown in the most striking manner that most of King, Assistant Archivist of The Royal and a skull from Arroyo Sarandí to Megat- the fossil mammals, buried there in such num- College of Surgeons of England, for herium. The latter assignment makes bers, are related to South American types… … invaluable help on the bibliography. sense in that the horizon yielding the I was so much impressed with these facts that I skull also preserved osteoderms, whereas strongly insisted, in 1839 and 1845, on this WORKS CITED IN THE TEXT Darwin's inference on the molar is 'law of the succession of types,'-on 'this wonder- understandable given that the teeth of ful relationship in the same continent between the Alberdi, M.T. and Prado, J.L. 1995. Los Masto- Toxodon, as noted by Owen (1838), bear a dead and the living". dontes de América del Sur. In Alberdi, M. T., certain resemblance to those of rodents. As noted by Sulloway (1982), the role of Leone, G. and Tonni, E.P. (eds.) Evolución the naturalists who studied the South climática y biológica de la región Pampeana Implications of the "erroneous" as- American fauna was critical to the deve- durante los últimos cinco millones de años. signments of the youthful Darwin lopment of Darwin's transformationist Un ensayo de correlación con el Mediterráneo ideas in that they provided a systematic occidental, Consejo Superior de Investigacio- Twenty years before the publication of framework. In reference to Darwin's nes Científicas, Museo Nacional de Ciencias 158 J.C. FERNICOLA, S.F. VIZCAÍNO AND G. DE IULIIS

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roy, during the years 1832 to 1836. 1(1): 13-40. Grypotherium (Neomylodon) listai and associated and ex RCS2214); right astragalus (ex Owen, R. 1839a. Description of a tooth and part mammals from a cavern near Consuelo Cave, M16569 and ex RCS2216); middle cervi- of the skeleton of the Glyptodon clavipes, a large Last Hope Inlet, Patagonia. Proceeding of the cal vertebrae (ex NMH M16570 and ex quadruped of the Edentate order, to which Zoological Society 5: 64-79, London. RCS2208); fragments of pelvis (ex NMH belongs the tesselated bony armour described Sulloway, F.J. 1982. Darwin's Conversion: The M16571 and ex RCS2210); proximal end and figures by Mr. Clift in the former Volume Beagle Voyage and Its Aftermath. Journal of of fused radius and ulna (ex NMH of the Transactions of the Geological So- the History of Biology 15(3): 325-396. M16572 and ex RCS2212); metacarpals ciety; with a consideration of the question Verzi, D.H. 1994. Origen y Evolución de los Cte- and phalanges of the right manus (ex whether the Megatherium possessed an analo- nomyinae (Rodentia, Octodontidae): un análi- NMH M16573 and ex RCS2213); incom- gous dermal armour. Transactions of the sis de anatomía cráneo-dentaria. Tesis Doc- plete left scapula (ex NMH M16574 and Geological Society 2: 81-106, London. toral, Facultad de Ciencias Naturales y Museo, ex RCS2211); metatarsal (ex M16575 and Owen, R. 1839b. Fossil Mammalia. In Darwin, Universidad Nacional de La Plata, (unpublis- ex RCS2216). C.R. (ed.) Zoology of the Voyage of H.M.S hed), 227 p., La Plata. Megatherium americanum. NMH M16585, Beagle, under the command of Captain Fitz- Vucetich, M.G., Deschamps, C.M., Itatí Olivares, posterior part of cranium (ex RCS3445); Roy, during the years 1832 to 1836. 1(2-3): 41- A. and Dozo, M.T. 2005. Capibaras, size, sha- NMH M16588, fragment of left maxilla 80. pe, and time: A model kit. 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Description of the skeleton of ted by Charles Darwin and housed in the M16580, upper molariform (ex RCS an extinct gigantic sloth, Mylodon robustus, Natural History Museum, London, 3507); Holotype: NMH M16581, cervi- Owen, with observations on the osteology, England (provided by Andrew Currant, cal vertebrae (ex RCS3510); Holotype: natural affinities, and probable habits of the Curator of Fossil Mammals), and in the NMH M16582, parts of ribs (ex RCS megatherioid quadruped in general. R. and Royal College of Surgeons of England 3511); Holotype: NMH M16583, left J.E. Taylor, 176 p., London. (provided by Simon Chaplin, Director of astragalus (ex RCS3520); Holotype: Owen, R. 1845. Descriptive and illustrated catalo- Museums and Special Collections). Ab- M16584, right astragalus with distal end gue of the fossil organic remains of Mam- beviations. RCSHM/CO: Royal College of right tibia (ex RCS3519). malia and Aves contained in the Museum of of Surgeons of England Hunterian / Toxodon platensis. Holotype NMH the Royal College of Surgeons of England. R. College Museum. RCS: Royal College of M16560: incomplete cranium lacking and J.E. Taylor, 391 p., London. Surgeons of London. NHM: Natural teeth (ex RCS2223). Prado, J.L. and Alberdi, M.T. 1995. A quantitative History Museum, London. Toxodon platensis. NMH M16562 right review of the horses Equus from South Ame- M2 (ex RCS2224); NMH M16564 part rica. Palaeontology 37: 459-481. Equus sp. NMH M16557: left upper of left dentary with fragments of four Reig, O.A. 1958. Notas para una actualización del molar (ex RCS2012). NMH M16558: molars (ex RCS2226); NMH M16565 conocimiento de la fauna de la Formación left upper molar (ex RCS2013). right lower molar (ex RCS2227); NMH Chapadmalal. 1. Lista faunística preliminar. Glossotherium sp. NMH M16586 part of M16566 fragment of right dentary with Acta Geológica Lilloana 2: 241-253. cranium (ex RCS3491). incisor roots and most of the molars (ex Reinhardt, J.T. 1879. Beskrivelse af Hovedskallen Machrauchenia patachonica. Holotype NMH RCS2225); NMH M16567 left lower af et Kaempedovendyr, Grypotherium darwinii. M43402: vertebral fragments (ex NMH incisor (ex RCS2228); M16568 fragmen- Videnskavernes Selskabs Skrifter. 5 Raekke. M16556 and ex RCS2209); detached pro- tary teeth (ex RCS 2229). Naturvidensakbelig og Mathematisk Afde- ximal and distal ends of right tibia and ling 12(4): 351-381. fibula (ex NMH M16559 and ex RCS Recibido: 25 de agosto de 2008 Smith-Woodward, A. 1900. On some remains of 2215); right femur (ex NMH M16561 Aceptado: 7 de noviembre de 2008 160 Revista de la Asociación Geológica Argentina 64 (1): 160- 169 (2009)

YOUNG DARWIN AND THE ECOLOGY AND EXTINCTION OF PLEISTOCENE SOUTH AMERICAN FOSSIL MAMMALS

Sergio F. VIZCAÍNO1, Richard A. FARIÑA2 and Juan Carlos FERNICOLA3

1 División Paleontología Vertebrados, Museo de La Plata, La Plata, Argentina. CONICET. Email: [email protected] 2 Departamento de Paleontología, Facultad de Ciencias, Universidad de la República, Montevideo. Uruguay. Email: [email protected] 3 Sección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", Buenos Aires. Argentina. Email: [email protected]

ABSTRACT During his two years in South America Charles Darwin became fascinated not only with the lush vegetation of Brazil, but also with the gigantic Pleistocene mammals that he found in the drier areas of Uruguay, and in the pampas and Patagonian coast of Argentina. These findings included various ground sloths and glyptodonts among xenarthrans, and hoofed herbivores like Toxodon and Macrauchenia, in addition to horses and small rodents. He concluded that the general assumption that large animals require luxuriant vegetation was false and that vitiated the reasoning of geologists on some aspects of Earth's history. He also reflected on the evident changes that occurred in the continent, the extinct fauna of which suggested to him an analogy to southern parts of Africa. He wondered about our ignorance of biological traits in extinct creatures and the reasons for their extinction. Thus, not only did Darwin inspire phylogenetic studies on fossil mammal lineages, he also opened a gate to the research on their behaviour, physiology and extinction; i.e., their palaeobiology. Whereas the first approach was largely developed in South America beginning about the second half of the 19th century due to the intellectual influence of Florentino Ameghino, palaeobiology became a much more recent line of work, in apparent relation to innovations in methodology and technology. This contribution provides an overview of recent contributions on the palaeobiology of Pleistocene fossil mammals of South America as attempts to provide answers for Darwin's questions.

Keywords: Darwin, Ecology, Extinction, South America, Mammals.

RESUMEN: El joven Darwin y la ecología y extinción de los mamíferos fósiles sudamericanos. Durante los dos años que Charles Darwin estuvo en América del Sur no sólo se deslumbró con la profusa vegetación de Brasil, si no también con los gigantescos mamíferos pleistocenos que colectó en áreas más secas de Uruguay y en la pampa y la costa patagónica de Argentina. Sus hallazgos incluyeron distintos perezosos y gliptodontes, ungulados herbívoros como Toxodon y Macrauchenia, además de caballos y pequeños roedores. Darwin, desechó la presunción general de que los grandes animales requieren una exuberante vegetación, reconociendo que la misma condicionó las interpretaciones que hicieran los geólogos sobre algunos aspectos de la historia de la Tierra. También reflexionó sobre los cambios acaecidos en el continente, cuya fauna le sugirió una clara analogía con lo observado en el sur de África. Darwin se preguntó acerca de nuestro desconocimiento de las características biológicas y de las causas que llevaron a su extinción. Así, no sólo inspiró el estudio filogenético de distintos linajes de mamíferos fósiles, si no que abrió la puerta a las investigaciones sobre su comportamiento, fisiología y extinción; i.e. su paleobiología. En América del Sur, la influencia intelectual de Florentino Ameghino permitió apuntalar el estudio filo- genético de los mamíferos durante la segunda mitad del Siglo XIX, mientras que el fortalecimiento de la paleobiología se dio en tiempos recientes en relación con innovaciones metodológicas y tecnológicas. Esta contribución provee una visión global de las contribuciones realizadas en paleobiología de mamíferos pleistocenos de América del Sur como un intento de respon- der los cuestionamientos que se hiciera Darwin.

Palabras clave: Darwin, Ecología, Extinción, América del Sur, Mamíferos.

INTRODUCTION started to develop an interest in natural tical plants, the beauty of the flowers, the glossy history some time before, while studying green of the foliage, but above all the general When HMS Beagle reached the coast of medicine at Edinburgh University. The luxuriance of the vegetation, filled me with ad- Brazil in February, 1832, Charles Darwin overwhelming power of South American miration." (Voyage of the Beagle, Chapter was a 22-year old theology student at biodiversity that greeted the young Char- I, Feb. 1832) Cambridge University and his ambition les Darwin led him to declare that "The However, it was not only the luxuriance was to become a rural pastor. He had elegance of the grasses, the novelty of the parasi- that influenced his mind but also the les- Young Darwin and the ecology and extinction of Pleistocene south american ... 161

ser plains of the south and their fossils. cal sequence, as Lamarck had proposed, very great physical changes can have taken place Actually, he became fascinated by the dis- but a tree with asymmetric branches (see in the nature of the Country. What then has covery of gigantic Pleistocene mammals Huxley and Kettlewell 1965). In his own exterminated so many living creatures?…We during his journeys in Uruguay, and the words: "to my view, in S. America parent of are so profoundly ignorant concerning the physio- Pampean region and Patagonian coast of all armadilloes might be brother to Megatherium logical relations, on which the life, and even Argentina, before leaving the continent - uncle now dead" (Darwin, 1837-1838 in health (as shown by epidemics) of any existing in May 1834 (see letters sent by Charles Barret 1960). Woodward (1987) stated species depends, that we argue with still less Darwin to Caroline Darwin and John that by that time Darwin was also fami- safety about either the life or death of any Stevens Henslow in Burkhardt and Smith liar with adaptation (The red notebook extinct kind" (Voyage of the Beagle, 1985, p. 276 and 280 respectively). As of Charles Darwin in Herbert 1980, p. Chapter IX, Jan. 1834). developed more fully in another contri- 67), another important biological issue In this way, Darwin not only triggered bution (see Fernicola et al., 2009) the ex- related to evolution, but an idea then the studies on the genealogical interpre- traordinary zoological collection of fossil associated with Natural Theology (Paley tation of the fossil mammal lineages, but and extant specimens made by Charles 1802), which argued that every organism also opened a gate to the research on Darwin during his voyage to South Ame- was intentionally perfectly designed to its their behaviour, physiology and extinc- rica was studied by an important group particular life conditions by God. tion; in others words, on their palaeobio- of naturalists who published their con- But it was not only the idea, means and logy. Phylogenetic studies, combined clusions between February 1838 and Oc- processes of evolution that impressed with morphological and taxonomic ana- tober 1843 in the "The Zoology of the Voy- Darwin. During his journey between lyses, flourished in South America, parti- age of H.M.S. Beagle". All fossils mammals Buenos Aires and Santa Fe he wrote "We cularly beginning during the latter half of included in this work were studied by may therefore conclude that the whole area of the the 19th century due to the intellectual Richard Owen, who recognized the Pampas is one wide sepulchre for these extinct influence of Florentino Ameghino, but South American ungulates Toxodon platen- quadrupeds" (Voyage of the Beagle, Chap- palaeobiology became a much more sis and Macrauchenia patachonica, a fossil ter VII, Oct. 1833). By that time his recent line of work, in apparent relation horse identified as Equus sp., the masto- sharp mind had already noted "That large to innovations in methodology and tech- dont Mastodon angustidens, and the ground animals require luxuriant vegetation has been a nology. Indeed, Pleistocene South Ame- sloths Glossotherium sp., Mylodon darwinii, general assumption, which has passed from one rican fossil mammals show a greater Scelidotherium leptocephalum, Mega-lonyx jef- work to another. I do not hesitate, however, to morphological diversity than their living fersonii, Megatherium cuvierii, and the glyp- say that is completely false; and that it has vitia- counterparts, as they include representa- todonts Glyptodon clavipes, Hoplophorus eu- ted the reasoning of geologists, on some points of tives of great body size and very peculiar phractus among xenarthrans, as well as great interest in the ancient history of the world" features. Their peculiarity and general some smaller forms like rodents. The ta- (Chapter V, of the Beagle, Aug. 1833, lack of modern analogues have encoura- xonomic history of several of these taxa The red notebook of Charles Darwin in ged creative palaeobiological approaches is complex, including issues as varied as Herbert 1980, p. 54). that will be outlined below. nomenclatural problems (e.g. Megatherium In January 1834, Darwin collected re- The aim of this contribution is to over- cuvierii nomen illegit.) to the mixing of mains of Macrauchenia in Patagonia near view the recent contributions on the specimens of different species or genera San Julián, in what today is Santa Cruz palaeobiology and palaeoecology of the assigned to the same species (e.g. Mega- province. He made inferences about the Pleistocene fossil mammals of South therium americanum), an issue that is trea- environment in which this beast lived America, in order to investigate if we can ted by Fernicola et al. (2009). and reflected on its extinction: "Mr. provide answers for the questions that It is widely accepted that by the time Owen... considers that they form part of an ani- the young Darwin made himself when Darwin boarded the Beagle he had been mal allied to the guanaco or llama, but fully as he first collected them. influenced by Lamarck's ideas on evolu- large as the true camel. As all the existing mem- tion (see Woodward 1987). Also, that the bers of the family of Camelidae are inhabitants RECONSTRUCTING giant fossil quadrupeds he found were of the most sterile countries, so we may suppose PALAEOBIOLOGY OF significant toward the development of was this extinct kind… It is impossible to reflect SOUTH AMERICAN his evolutionary theory, as very early they without the deepest astonishment, on the changed PLEISTOCENE MAMMALS suggested to him that the similarities bet- state of this continent. Formerly it must have ween extinct and living forms should be swarmed with great monsters, like the southern Palaeobiologists are interested in recons- explained by the existence of common parts of Africa, but now we find only the tapir, tructing the form of the fossils as living ancestors, and that the transformation of guanaco, armadillo, capybara; mere pigmies com- animals, their habitat, ecological role, be- species to a large degree was not a verti- pared to antecedents races... Since their loss, no haviour, and basic biology. Vizcaíno et al. 162 S. F. VIZCAÍNO, R. A. FARIÑA AND J. C. FERNICOLA

(2004) and Vizcaíno et al. (2008) describe a basic protocol for palaeobiological studies that identifies three biological attributes that are essential for each taxon: size, diet and usage of substratum or type of locomotion. Such principles have been used for the last three decades (e.g., Andrews et al. 1979, Van Couvering 1980, Reed 1998, etc.) though not fully applied to South Aerican mammals. Morphological study of the masticatory Figure 1: Life recons- and locomotor apparatuses allows pre- truction of Lestodon sp. dictions on the movements for which the The different size of the apparatuses are optimized. In addition, tusks suggest sexual analyses of mastication are useful for dimorphism. Drawing by Néstor Toledo. formulating hypotheses about the diet of Scale=100 cm. the organism, while analyses of the locomotor apparatus allow inferences about 2008, and references therein) title of the largest of them: the giant the type of locomotion or preferences in It was not until the second part of the ground sloths Megatherium americanum and the usage of substratum: runner, hopper, 1990s that authors began to apply biome- Eremotherium laurillardi whose body mas- digger, burrower, etc. Obviously, these chanic, morpho-geometrical, and eco- ses must have reached between three and two aspects, added to body size, yield morphological methods to the study of six tonnes, depending on the approach relevant data for the interpretation of an morphology as part of a major project used (Casinos 1996, Fariña et al. 1998), organism in a palaeobiological context. aimed at understanding the great palaeo- and the mastodont Stegomastodon superbus, Palaeomammalogists have largely applied biological diversity of the South Ame- estimated at four or five tonnes. The my- actualism, according to which past events rican extinct forms. These results allowed lodontid ground sloth Lestodon armatus are surmised by analogy with currently the development of novel interpretations (Fig. 1) follows closely, with an estimated observable processes assuming that fossil of their modes of life that, coupled with mass of three to four tonnes, while the species had similar habits to their current palaeoenvironmental data (geology, pa- other my-lodontids are smaller: between relatives. However, when phylogenetic laeoclimatology and reconstruction of one and two tonnes for Glossotherium affinity is not very close or fossil lineages palaeovegetation), provide insightful in- robustum and Mylodon darwini, and with possess morphologies not represented in formation on the paleoecological context Scelidotherium leptocephalum, at 900 kg, extant species (Vizcaíno et al. 2004), this in which these animals existed. falling just short of the megamammal ca- methodology does not provide reliable tegory (Bargo et al. 2000). Among glypto- results, a circumstance that is particularly Body size donts the largest was Glyptodon clavipes, applicable to the mammalian faunas that individuals which may have reached evolved in relative isolation in South Body size has a remarkable influence on nearly two tonnes. Others, though sma- America during a good part of the Ter- an animal's life because it can be correla- ller, were also very large mammals: Doe- tiary. Vizcaíno et al. (2004) and Vizcaíno ted, among other features, with metabo- dicurus clavicaudatus at 1400 kg, Panochthus et al. (2008) provide accounts on the re- lism, limb bone dimensions and biome- tuberculatus at (1100 kg), and Glyptodon reti- construction of palaeobiology through chanics of locomotion, or particular so- culatus at 850 kg. the application of the "form-function corre- lutions for food intake. Body mass in There were also non-xenarthran giants, lation approach" (Radinsky 1987), accor- Pleistocene xenarthrans was estimated such as the camel-like Macrauchenia and ding to which function can be inferred using scale and computer generated (geo- the rhinoceros-like Toxodon (Fig. 2), who- from form, to make good use of the metric) models, and allometric equations se body masses must have surpassed the main sources of information like fossili- (see Vizcaíno et al. 2008 and references one tonne limit (Fariña and Álvarez zed bones and teeth (though indirect evi- therein). Using these approaches, Fariña 1994, Fariña et al. 2005). The Carnivora, dence can also be used). Form-function et al. (1998), Bargo et al. (2000) and Chris- in turn, reached impressive sizes, al- relationships can be studied through dif- tiansen and Fariña (2003) estimated the though well below one tonne, with the ferent approaches, like functional mor- masses of most Lujanian megamammals. sabre-tooth Smilodon populator and the phology, biomechanics and ecomorpho- Among the late Quaternary mammals of short faced bear Arctotherium spp. attai- logy (see definitions in Vizcaíno et al. South America, three species rival for the ning sizes of three or four hundred kilo- Young Darwin and the ecology and extinction of Pleistocene south american ... 163

grams (Fariña et al. 1998). well as a complete geometric and biome- Feeding apparatus chanical analysis of the footprints assig- Limbs, locomotion and habits ned to this species found in Pehuén-Có, Functional morphology and biomecha- Buenos Aires Province, Argentina. Bipe- nics have been applied to the study of Biomechanical studies performed in the dalism also implies that the forelimb feeding in a wide range of xenarthrans last decade on large glyptodonts and could have been free to perform activi- (Vizcaíno et al. 2008, and references the- ground sloths provided insight into the ties other than locomotion. Fariña and rein). Within the Pleistocene armoured capacity of the limb bones to withstand Blanco (1996) tested the possibility that forms, plant-eating was determined in bending forces, forearm extension and the forearms of Megatherium americanum eutatines (Vizcaíno and Bargo 1998), velocity, bipedalism or digging abilities. were designed for optimizing speed pampatheres (Vizcaíno et al. 1998, De Within cingulates, Fariña's (1995) analy- rather than strength of extension, and Iuliis et al. 2000) and glyptodonts (Fariña ses of limb bones and locomotory habits concluded that such a trait may have and Vizcaíno 2001), although different in some glyptodonts indicated that femur been associated with a potentially aggres- kinds of herbivory may have occurred in strength indicators of large Pleistocene sive use of the animal's large claws. Bar- each group. The studies also revealed forms were equivalent to those of large go et al. (2000) analysed limb proportions that some cingulates evolved mechanical living mammals capable of galloping (i.e. and resistance to bending forces in mylo- solutions not present in any related taxa, buffalos and rhinos), but values of the dontids to infer their locomotor adapta- and do not have current analogues that humerus were similar to those of ele- tions. The analysis indicates that some of can be used as models to investigate and phants, which cannot gallop. The muscu- them were well adapted for strenuous ac- interpret adaptations of lineages without lar insertions suggest that glyptodonts tivities in which force was enhanced over living representatives. For instance, the were able to adopt bipedal postures to velocity, such as digging. Based on this masticatory apparatus in glyptodonts un- perform strenuous activities, such as the work, Vizcaíno et al. (2001) considered derwent a telescoping process that placed intraspecific fighting proposed by Fariña these taxa as possible builders of large it well below the cranium (Fariña 1985, (1995). Pleistocene burrows reported in the 1988), creating problems in the way that For the ground sloths, different speciali- Pampean region (Zárate et al. 1998). stresses produced by mastication were zations may have been derived from a As for the strange hoofed mammals co- absorbed by the mandible and implying primitive quadrupedal way of locomo- llected by Darwin, their athleticism se- unusual jaw mechanics (Fariña and Viz- tion in both main lineages of Pleistocene ems to have been impressive in the case caíno 2001). forms, megatheriids and mylodontids. of Toxodon, which must have been capa- Bargo (2001), Bargo et al. (2006a, b), and The giant sloth Megatherium americanum ble of fast locomotion, and even more Bargo and Vizcaíno (2008) studied the has been formally proposed as bipedal impressive in Macrauchenia, albeit for dif- masticatory apparatus of the large South based on ichnologic and biomechanical ferent reasons. Indeed, limb bone American Pleistocene ground sloths. Jaw evidence (Aramayo and Manera de Bian- strength of the latter was larger if mea- mechanics, morphogeometric analyses, co 1996, Blanco and Czerwonogora sured transversely rather than anteropos- and the correlation between cranio-den- 2003). The latter includes analyses of bo- teriorly, which has been interpreted as tal variables (hypsodonty, dental occlusal dy size, speed, Froude number, indicator the capability of suddenly turning while surface area and relative width of the of athletic ability, bending and resistance being pursued by a predator (Fariña et al. muzzle) and diet, all suggested probable moments of the vertebral column, as 2005). niche differentiation among ground sloths based on dietary categories. While the masticatory pattern of mylodontids is rather generalized with a clear antero- medial powerstroke, as previously proposed by Naples (1989), Megatherium americanum was well adapted for strong, mainly vertical biting. This information, in addition to tooth shape, suggests that teeth were mainly used for cutting, rather than grinding, and that fibrous food was not the main dietary component. Figure 2: Life reconstruction of Toxodon sp. Hypsodonty is the relative increase in Drawing by Néstor crown height of a tooth. It has been tra- Toledo. Scale=100 cm. ditionally viewed as a response to dietary 164 S. F. VIZCAÍNO, R. A. FARIÑA AND J. C. FERNICOLA

shifts toward abrasive vegetation, al- sotherium robustum and Lestodon armatus) Pleistocene-early Holocene) megamam- though recent work indicates that evolu- had a square, non-prehensile upper lip mals, from the perspective of the ecolo- tion of hypsodonty is also due to the hig- that, coupled with the tongue, were used gical implications of their body sizes. her prevalence of grit and dust in more to pull out grass and herbaceous plants Based on an estimation of population open environments (Bargo et al. 2006a (mostly bulk-feeders). Sloths with na- density derived from body sizes Fariña and references therein). Bargo et al.'s rrow muzzles (Mylodon darwini, Scelidothe- (1996), as Darwin realized 160 years be- (2006a) comparative analyses of eleven rium leptocephalum and Megatherium america- fore, emphasized that the fauna contai- species of Pleistocene sloths suggest that num) had a cone-shaped and prehensile ned a significant diversity of large herbi- differences in hypsodonty may be explai- upper lip (Fig. 3) that was used to select vores. Conversely, according to Fariña, it ned by diet, habitat and behaviour. particular plants or plant parts (mixed or did not contain a proportionally diverse Among mylodontids, hypsodonty was selective feeders). suite of large carnivores. Assuming a ba- unlikely due solely to dietary preferences, A morphofunctional approach has been sal metabolism in agreement with the such as grazing. As mentioned above, less often applied to other Lujanian mam- body size of the beasts, the food energy some mylodontids were capable diggers mals because they are more readily com- required to sustain the fauna was calcula- that likely dug for food, and ingestion of parable to living analogues. The upper ted. The results suggest that based on the abrasive soil particles probably played a incisors of Toxodon were strongly arched, requirements of the species under consi- considerable role in shaping their dental whereas the lower ones were horizontally deration (i.e., mammals over 10 kg), they characteristics. Geographical distribu- arranged. Their great lateral expansion alone must have needed about 1.8 mega- tions of the megatheriids Eremotherium gave the lower jaw a giant, spade-like joules per square metre per year (hereaf- and Megatherium indicate differing habi- appearance. In Macrauchenia the retracted ter, MJ m-2 year-1) of the vegetation which tats as possible factors in hypsodonty dif- position of the large, elliptical nostrils sustained them. Since a primary produc- ferences. Vizcaíno et al. (2006) investiga- suggests the presence of a trunk. Consi- tivity of 7.3 MJ m-2 year-1 is considered ted the relationship between dental dering morphological and isotopic evi- excellent for modern open field ecos- occlusal surface area (OSA) and diet, and dence, they have been mainly considered ystems, it is difficult to explain how the other biological factors in fossil xenarth- as grazers and mixed-feeders respectively smaller species of mammals could have rans. They found that for most fossil (MacFadden and Shockey 1997). Among survived, let alone reptiles, birds, insects xenarthrans OSA is smaller than expec- horses, Hippidion shows a narrower naso- and other consumers. If the Lujanian ted compared to extant herbivorous maxillar region, with well developed pre- plains had been as productive as the mammals of equivalent body size. With- orbital fossae and a retracted nasal notch, African savannah is today, about 38 MJ in xenarthrans, cingulates show the hig- a combination of features that has been m-2 year-1, the consumption efficiency hest OSA values, suggesting more exten- interpreted as an adaptation to more clo- would fall to little more than 3%, which sive oral food processing than in tardi- sed habitats, such as a savanah. Also, Hip- is a typical value for modern grassland grades. Among ground sloths, mylodon- pidion shows teeth with relatively lower systems. tids have extremely low OSA values, sug- degree of folding in the enamel and less However, available evidence points to a gesting low efficiency in oral food pro- hypsodont than Equus (Amerhippus), different scenario. The fossiliferous Gue- cessing that was probably compensated which suggest a diet less rich in silica. rrero Member of the Luján Formation for by high fermentation in the digestive Congruently, biogeochemical data sug- was deposited between about 20 to 10 tract, and/or lower metabolic require- gest that the species of Equus (Amerhip- thousand years before present, when the ments. On the other hand, Megatherium pus) had a more grazing diet than Hip- Last Glacial Maximum (LGM) was esta- americanum has an OSA as high as, or pidion (MacFadden et al. 1996, Mac blished and, due to the extensive glacia- even higher than, that expected for a Fadden and Shockey 1997, MacFadden et tion in the Andes, the climate was much mammal of its size, which indicates hig- al. 1999). dryer and decidedly cooler than present her oral food processing, lower fermen- conditions in that region (Clapperton tation capacity, and/or higher metabolic PALAEOECOLOGY 1983). Different sources of evidence are requirements. congruent with this paleoclimatic inter- Other features besides teeth are also im- As for palaeoecological interpretations, pretation (Cantú and Becker 1988, Tonni portant. For instance, Bargo et al. (2006b) there have been several approaches to 1990). As noted above, this was precisely used muzzle shape and facial musculatu- the study of Cenozoic South American Darwin's (1839) intuition about the re reconstructions to develop models of faunas, including feeding habits, locomo- landscape being less luxuriant than today. food intake in five species of South Ame- tion, and trophic relationships. Fariña However, it seems that it was even more rican Pleistocene giant ground sloths. (1996) analysed the trophic relationships arid than surmised by the great naturalist. Ground sloths with wide muzzles (Glos- of the South American Lujanian (late Current biogeographic reconstructions Young Darwin and the ecology and extinction of Pleistocene south american ... 165

nating large ungulates for varied morphological reasons, Fariña (1996) proposed that ground sloths were opportunis- tic carrion eaters. This challenging view, in turn, renewed interest in other ecological topics, such as niche partitioning in the Pleistocene and reinterpretation of the systematics of some South American Carnivora, among others. For instance, Vizcaíno's (2000) brief analysis of plant resource exploita- tion among sympatric Lujanian herbivorous armoured xenarthrans suggested that the main dietary difference among these cingulates was in the coarseness of the vegetation they were capable of processing. Bargo (2001) and Bargo et al. (2006b) proposed a niche differentiation Figure 3: Life reconstruction of the head of Megatherium sp. Drawing by Néstor Toledo. Scale=50 cm. among Lujanian ground sloths based on the different degrees of ability for plant for the period of the LGM show that the der but winters up to 4ºC colder). More selection due to muzzle morphology. Pampean plains underwent intense aeo- importantly, the aridity must have been Moreover, Vizcaíno et al. (2006) propo- lian activity that redeposited large masses higher, with rainfall considerably lower, sed that, like living sloths (see Gilmore et of silt and fine sand of periglacial origin. about 350 mm per year as compared with al. 2008 and references therein), mylo- Southwest from the rivers where the the nearly 900 mm current for that area. dontids had very low metabolism, which sand was trapped in, a sand-sea in the These figures are considered as approxi- suggests they were probably neither par- southwestern half of the Pampas was mations, given the likely influence of ticularly abundant nor did they require as formed, as well as a broad loessic belt other factors, such as the well-known much food as originally calculated. over the remainder of the area. Also, the high edaphic quality of the Pampean re- A recent revision of the bears from remains of the still-extant mammals, i.e., gion, and perhaps due to the impact of South America proposed that during the those whose habitat preference can be the local water bodies, a topic discussed Late Pleistocene there were three species safely assigned, belong to species confi- below. Thus, primary productivity in the of bears (Soibelzon 2004), instead of ned to Central and Patagonian faunistic mid-latitude Lujanian might have been one as considered by Fariña (1996). Bears provinces (Tonni 1985, Prado et al. 1987, higher on average than that in today's may have acted as large scavengers, Alberdi et al. 1989). The same can be said Choele-Choel area but it does not seem which was probably true for other carni- about the birds of this age (Tonni and likely that it could have been higher than vores such as felids and canids as well, Laza 1980), and analyses of pollen and the most productive present-day cattle forcing reexamination of Fariña's (1996) ostracods have yielded congruent results field of Uruguay, and, hence, it must estimates of trophic diversity, at least to a (Quattrocchio et al. 1988, Markgraf 1989, have been dramatically lower than the certain extent. Prevosti and Vizcaíno Prieto 1996). African savannah. As a consequence of (2006) reviewed carnivore richness in the According to Iriondo and García (1993), this reasoning, Fariña (1996) suggested Lujanian of the Pampean Region, descri- the shift was about 750 km south west that the coexistence of so many large bing the palaeoecology of these species relative to present conditions. Hence, the herbivores in a poor environment led to (including their probable prey choices) place where the current city of Luján lies strong competition for resources. It is and assessing the available information would have had climatic conditions simi- worth to mention here that by the on taphonomy, carnivore ecology, and lar to the climate that exists currently in Pleistocene large carnivorous birds, like macroecology to test the hypothesis of the northern Patagonian locality of the phorusrhacids and theratorns had "imbalance" of the Río Luján fauna. Choele-Choel (39ºS, Río Negro provin- long before become extinct in the They found that the carnivore richness ce), whose climogram indicates a lower Pampean region. He concluded that so- of the Río Luján fauna comprises five mean annual temperature (2.5-3ºC less me of the mammals previously conside- species: Smilodon populator, Panthera onca, than at the present), with more marked red strict herbivores might have been Puma concolor, Arctotherium tarijense, and seasonality (summers only about 1ºC col- flesh-eaters to some degree. After elimi- Dusicyon avus, plus two other species that 166 S. F. VIZCAÍNO, R. A. FARIÑA AND J. C. FERNICOLA

may be added when the Lujanian of although not all were equally successful young Darwin's questions and comments Buenos Aires province is included: or ventured equally far in their new land. on such matters. As conclusions of this Arctotherium bonariense and Canis nehringi. According to a proposal long held for the article, his assertions will be considered With the exception of D. avus and A. tari- first half of the 20th century (Matthew one by one and our current point of view jense, these are hypercarnivores that could 1930, Webb 1976, Simpson 1950), the will be added. prey on large mammals (100-500 kg) and evolutionarily advantaged northern "We may therefore conclude that the whole area juveniles of megamammals (>1000 kg). mammalian lineages, on their triumphal of the Pampas is one wide sepulchre for these Smilodon populator could also hunt larger path from their alleged evolutionary cra- extinct quadrupeds", said Darwin, and time prey with body mass between 1000 and dle in central Asia to all the corners of has corroborated this statement in its 2000 kg. The review of the "imbalance" the globe, outcompeted their South fullest sense. Beginning with the collec- hypothesis reveals contrary evidence and American counterparts, driving them to tions made by Francisco Muñiz and allows the proposal of alternative hypo- extinction or forcing them to take refuge Da´maso Larrañaga, as well as the subse- theses. If high herbivore biomass occu- in marginal habitats. In the last part of quent work of the Ameghino brothers, rred during the Lujanian, a higher density that century, some authors critically revi- the Museo de La Plata, the Museo Ar- of carnivores could be supported. sed the success of the North American gentino de Ciencias Naturales "Bernar- immigrants in South America (Marshall et dino Rivadavia", the Museo Nacional de EXTINCTION al. 1982, Marshall 1988, Webb 1985), and Historia Natural de Montevideo and a Webb (1991) provided an ecogeographic multitude of smaller museums exhibit The study of extinctions has become a model that explains the assymmetrical exquisite specimens of those amazing particularly relevant issue in palaeonto- results of the land-mammal interchange fossil beasts, as well as of many other re- logy. Over the last few decades, analyses between both land masses. An analysis of mains that are housed in collection of biodiversity during the history of life the pattern of Pleistocene extinction rooms. and its ups and downs, both gradual and (Lessa and Fariña 1996) revealed that "That large animals require luxuriant vegeta- sudden (Sepkoski 1978, 1979, Raup and large body size, rather than continent of tion has been a general assumption, which has Sepkoski 1984) have become the cor- origin, was the leading factor in determi- passed from one work to another. I do not hesi- nerstone of the way we look into the ning which mammals went extinct. tate, however, to say that is completely false; and deep past. Moreover, the spectacularity Another interesting issue is that of that it has vitiated the reasoning of geologists, on of mass extinction and of the non-actua- human impact, as it relates both to the some points of great interest in the ancient his- listic proposals of extraterrestrial causes timing of the peopling of the Americas tory of the world", claimed the young trave- (Álvarez et al. 1980) attracts the attention and to ethical, environmentally oriented lling naturalist. It seems he was absolu- of academics as well as of the general issues. Up to present, evidence of this tely right: as far as modern evidence is public. human impact was scarce, although some concerned, climates in middle latitudes The extinction of the giant mammals has remains do show interesting connections. seem to have shifted some 750 km long been attributed to the purported Particularly, a clavicle of a mylodontid towards the South-West since pleniglacial competition that followed the interchan- ground sloth found in Arroyo del times, and hence the places where this ge of mammalian contingents across the Vizcaíno, Uruguay, shows marks assig- impressive fauna must have lived then in Panama isthmus when it emerged some ned to human activities. This specimen, a rather arid environment, as suggested three million years ago. Known as the as well as others, have been dated at by studies on small mammals, invertebra- Great American Biotic Interchange, this about 29,000 years before present, a tes, pollen and sediments, although more asymmetrically reciprocal invasion brought much older age than the 12,000 or 13,000 precise biogeographic and ecological carnivores (cats, dogs, mustelids, and ybp currently accepted for human pre- hypotheses are yet in great need. bears), insectivores, rodents (in addition sence in the Americas (Arribas et al. 2001, "Mr. Owen... considers that [the remains of to the already present caviomorphs), rab- Fariña and Castilla 2007). what today is called Macrauchenia] form part of bits, mastodons, tapirs, horses, camels, an animal allied to the guanaco or llama..." deer and peccaries to South America, ANSWERS TO DARWIN'S This statement would have been right if where many of those groups still thrive. QUESTIONS referred to the fossil camelid Palaeolama. On the other hand, some South Ame- However, Macrauchenia is not a camelid rican lineages trekked north across the As noted above, we intend here to over- but a Litoptern, a group not related to bridge: opossums, caviomorph rodents, view the palaeobiological and palaeoeco- any living order of mammals and its toxodonts, glyptodonts, armadillos (in- logical contributions on the Pleistocene phylogenetic relationships are yet to be cluding pampatheres), anteaters and mammals of South America and assess established. Nevertheless it is worth to ground sloths, and platyrhine primates, their measure of progress in answering consider his reflection on the palaeoenvi- Young Darwin and the ecology and extinction of Pleistocene south american ... 167

ronments: "As all the existing members of the great numbers the struggle for life. As Croft reviewed the manuscript. Néstor family Camelidae are inhabitants of the most for the giant mammals that inspired his Toledo made the life reconstructions. sterile countries, so may we suppose was this thoughts as a young traveller, he was mo- kind…It is impossible to reflect without the dee- re than eager to accept they no longer WORKS CITED IN THE TEXT pest astonishment, on the changed state of this lived, although the question why it may continent. Formerly it must have swarmed with have happened did not surface frequently Alberdi, M.T., Menegaz, J.L., Prado, J.L. and great monsters, like the southern parts of enough, probably accepting Cuvier's Tonni, E.P. 1989. La fauna local de Quequén Africa, but now we find only the tapir, guanaco, point of view on the subject. Salado - Indio Rico (Pleistoceno tardío) de la armadillo, capybara; mere pigmies compared to Our knowledge has increased conside- provincia de Buenos Aires, Argentina. As- antecedents races... Since their loss, no very great rably since then. Today, we pay enor- pectos paleoambientales y bioestratigráficos. physical changes can have taken place in the mous attention to this subject, due to our Ameghiniana 25: 225-236. nature of the Country". Darwin's astonis- growing concern about present, dwin- Alvarez, L.W., Alvarez, W., Asaro, F. and Michel, hment is to be shared. As is evident from dling biodiversity, our fascination with H.V. 1980. Extraterrestrial cause for the Cre- the preceding sections, not so long ago, the spectacular extraterrestrial causes taceous-Tertiary extinction: Experimental in geological terms, the mid latitude that have been proposed, and the on- results and theoretical interpretation. Science plains of south eastern South America going human impact on megamammals. 208: 1095-1108. were home to perhaps the most specta- However, we end this article with one Andrews, P., Lord, J.M. and Evans, E.M.N. 1979. cular mammalian fauna of all times. How last reflection. Patterns of ecological diversity in fossil and an environment of apparently such a low "We are so profoundly ignorant concerning the modern mammalian faunas. Biological Jour- productivity could support so many her- physiological relations, on which the life, and nal of the Linnean Society 11: 177-205. bivores remains a mystery. On the other even health (as shown by epidemics) of any exis- Aramayo, S.A. and Manera de Bianco, T. 1996. hand, advancements in studies in astro- ting species depends, that we argue with still less Edad y nuevos hallazgos de icnitas de mamí- nomical forcing and palaeoclimatology safety about either the life or death of any feros y aves en el yacimiento paleo iconológi- partially refutes Darwin's impression, as extinct kind" (Voyage of the Beagle, co de Pehuen-Co (Pleistoceno Tardío), Pro- the nature of the country has changed Chapter IX, Jan. 1834). Science has made vincia de Buenos Aires, Argentina. Asocia- since its coldest period during the LGM considerable progress since those words ción Paleontológica Argentina, Publicación (approximately 18 kybp) and sea level has were written by the person who changed Especial 4: 47-57. risen more than 100 m, causing vast areas the way humankind views itself and the Arribas, A., Palmqvist, P., Pérez-Claros, J.A., Cas- to become submerged. Ironically, the rest of nature. However, we should be tilla, R., Vizcaíno, S.F. and Fariña, R.A. 2001. large beasts went extinct when the clima- persuaded that his claim, which emphasi- New evidence on the interaction between te became less arid and the vegetation zed ignorance, be taken as a call to conti- humans and megafauna in South American. more luxuriant. That leads us to the nue his approach. In this context, mo- Publicaciones del Seminario de Paleontología following question: What then has exter- dern research on the marvellous beasts de Zaragoza 5: 228-238. minated so many living creatures? that roamed South America during the Bargo, M.S. 2001. The ground sloth Megatherium Extinction, a major topic in current evo- Pleistocene has indeed continued for- americanum: skull shape, bite forces, and diet. lutionary thinking was surprisingly not a ward on the path set out by Darwin, con- In Vizcaíno, S.F., Fariña, R.A. and Janis, C. major issue for Darwin. References to tributing to the understanding of evolu- (eds.) Biomechanics and Paleobiology of Ver- the subject are scarce, although impor- tion intermingled with ecology and phy- tebrates. Acta Paleontologica Polonica (Spe- tant, in his writings and we are left to logeny. Thus, in the process of expan- cial Issue) 46: 41-60. wonder whether he accepted the ideas ding our knowledge, as often happens in Bargo, M.S. and Vizcaíno, S.F. 2008. Paleobiology common in his time of catastrophes science, further ignorance has been reve- of Pleistocene ground sloths (Xenarthra, (Cuvier's global revolutions) having wi- aled. However, we hope that this will Tardigrada): biomechanics, morphogeometry ped species from the Earth. On the other only whet our appetite for the acquisition and ecomorphology applied to the mastica- hand, his proposal, many years later as a of more knowledge. tory apparatus. Ameghiniana 45:175-196. settled gentleman living in the quiet com- Bargo, M.S., Vizcaíno, S.F., Archuby, F.M. and fort of his Kent home, that species were ACKNOWLEDGEMENTS Blanco, R.E. 2000. Limb bone proportions, originated by means of natural selection strength and digging in some Lujanian (Late has been interpreted as the foundation of We want to acknowledge the editors for Pleistocene-Early Holocene) mylodontid modern views about extinction. In other inviting us to participate in this volume. ground sloths (Mammalia, Xenarthra). Jour- words, species were easily viewed as Gerry De Iuliis read critically an early nal of Vertebrate Paleontology 20: 601-610. prone to extinction if their individuals version of the manuscript. Susana Bargo, Bargo, M.S., Vizcaíno, S.F. and Kay, R.F. 2004. were not fit, if their members lost in Teresa Manera, Christine Janis and Darin Evidence for Predominance of Orthal Mas- 168 S. F. VIZCAÍNO, R. A. FARIÑA AND J. C. FERNICOLA

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DARWIN AT PUENTE DEL INCA: OBSERVATIONS ON THE FORMATION OF THE INCA'S BRIDGE AND MOUNTAIN BUILDING

Victor A. RAMOS

Laboratorio de Tectónica Andina, FCEN, Universidad de Buenos Aires - CONICET. Email: [email protected]

ABSTRACT The analyses of the observations of Charles Darwin at Puente del Inca, during his second journey across the High Andes drew attention on two different aspects of the geological characteristics of this classic area. Most of his descriptions on the characteristics and the origin of the natural bridge were not published, mainly due to his poor impression of Puente del Inca. However, the application of the uniformitarian principles shows that it was formed as an ice bridge associated with snow and debris avalanches later on cemented by the minerals precipitated by the adjacent hot-water springs. Darwin's observations on the complex structural section at Puente del Inca, together with his findings of shallow water marine fossil mollusks in the thick stratigraphic column of the area interfingered with volcanic rocks, led him to speculate on several geological processes. Based on his geological observations, Darwin argued on the mountain uplift, the subsidence of the marine bottom, the episodic lateral growth of the cordillera, and their association with earthquakes and volcanic activity, which was an important advance in the uniformitarian hypothesis of mountain uplift proposed by Charles Lyell. Darwin was able to recognize the episodic nature of mountain uplift, and based on these premises he concluded that the Andes were still undergoing uplift. Taken as a whole, his ideas anticipated in many years some of the premises of the geosynclinal theory, and current hypothesis on foreland migration of the fold and thrust belts.

Keywords: Andes, Subsidence, Volcanism, Mountain uplift, Lateral growth, Ice-bridge.

RESUMEN: Darwin en Puente del Inca: observaciones sobre la formación de Puente del Inca y el levantamiento de las montañas. El análisis de las observaciones geológicas realizadas por Charles Darwin en Puente del Inca, durante su segundo cruce de la alta cordillera de los Andes, atrajo la atención a dos diferentes aspectos de las características geológicas de esta clásica área. La mayor parte de sus descripciones sobre las características y el origen de este puente natural no fueron publicadas, principalmente debido a su pobre impresión sobre el Puente del Inca. Sin embargo, la aplicación de principios uniformitaristas muestra que fue for- mado como un puente de hielo asociado con avalanchas de nieve y detritos y posteriormente cementado por los minerales precipitados por las termas adyacentes de agua caliente. Las observaciones de la compleja sección estructural en las vecinda- des de Puente del Inca, junto con sus hallazgos de moluscos marinos fósiles de aguas someras en la potente columna estrati- gráfica del área intercalada con rocas volcánicas, lo condujeron a especular sobre diversos procesos geológicos. Basado en sus observaciones geológicas, Darwin argumentó sobre el levantamiento de las montañas, la subsidencia de los fondos marinos, el crecimiento lateral y episódico de la cordillera, y su asociación con terremotos y actividad volcánica, que fue un importante avance en las hipótesis uniformitaristas de levantamiento de montañas propuestas por Charles Lyell. Darwin fue capaz de reconocer la naturaleza episódica del levantamiento de montañas, y basado en estas premisas, concluyó que los Andes esta- ban todavía creciendo.Tomadas en conjunto, sus ideas se anticiparon en varios años a algunas de las premisas de la teoría geo- sinclinal y las hipótesis actuales de la migración de las fajas plegadas y corridas.

Palabras clave: Andes, Subsidencia, Volcanismo, Levantamiento de montañas, Crecimiento lateral, Puente de hielo.

INTRODUCTION Darwin's mind during his research and see Figs. 1 and 2). It is well documented observations in South America and later that he considered himself a geologist, It is well established in the biological consolidated after his return to Britain. receiving in his early days a strong in- science community that Darwin's theory However, his important ideas on the for- fluence of the uniformitarian ideas of on the evolution of the species is a miles- mation of the mountains and his hypo- Charles Lyell (Judd 1909). It is also tone in the comprehension of Life, and thesis on the origin of the Puente del important to state that after his Beagle that this theory started to develop in Inca are not well known (Inca's Bridge, journeys around the world, he contribu- Darwin at Puente del Inca: observations on the formation ... 171

ted to improve and exerted a strong influence on Lyell's ideas. When we read that in 1838 he chose to characterize himself this way: "I a geologist" (Herbert 2005, p. 2), it should not come as a surprise that his monumental contribution - as stated in his Geological Observations in South America (Darwin 1846) - to the evolution of the Patagonian Coast and the High Andes is still a milestone in the comprehension of the geological history of these regions. There are several contributions highligh- ting his geological thoughts, his ideas on mountain building, and his geological descriptions of the many different geological environments visited (see for example Judd 1909, Rhodes 1991, Herbert 2005, and references therein). In the present contribution I examine only his observations and his thoughts - first in the Figure 1: Location map of Puente del Inca in the High Andes of Mendoza. origin of the bridge, and later on his ca's Bridge). The earliest description was published in a later edition of Darwin's ideas on the uplift and deformation of by Schmidtmeyer in 1820 to 1821, al- Voyage (Darwin 1890, Fig. 4). the Andes - gained during his excursion though the illustration was drafted by A. New samples of the thermal springs we- to the Puente del Inca region. Aglio, a lithographer that had never seen re obtained by another British traveler, the bridge. In spite of his unrealistic re- Charles Brandt in 1827, and the chemical ORIGIN OF PUENTE presentation, the two major springs are analyses were done by the famous physi- DEL INCA depicted in the southern part of the cist and chemist of that time Michael valley (Fig. 3a, after Schmidtmeyer 1824). Faraday (Brandt 1827), whose results The pioneer observations The second description was by John were analyzed by Darwin (1846). Miers in 1826, and he again related the However the first geologic description The first mention of the natural bridge bridge to the two thermal springs and and hypothesis on the origin of the brid- in the headwaters of Río Mendoza is at- presented the first analyses of the com- ge were proposed by Darwin, in spite of tributed to Alonso de Ovalle in 1646. position and temperature of the hot wa- the poor impression that the bridge pro- This natural bridge later drew the atten- ter (Fig. 3b). Darwin had a copy of Miers' duced on him: "When one hears of a Na- tion of several 19th Century travelers, (1826) book in the library of the Beagle, tural bridge, one pictures to oneself some deep & who in their reports described what be- and followed his trail on his way back to narrow ravine across which a bold mass of rocks gan to be called the Puente del Inca (In- Chile. Another picture of the bridge was has fallen, or a great archway excavated. Instead

Figure 2: Present view to the southeast of Puente del Inca, High Andes of Mendoza. Note the thermal springs developed on the southern side where the ruins of the old hotel bath are preserved (photo by Marcelo G. Armentano). 172 V. A RAMOS

Figure 3: a) First picture of Puente del Inca. Note the two thermal springs on the southern side of the valley as depicted by the lithographer A. Aglio (after Schmidtmeyer 1824); b) Puente del Inca and hot mineral springs described by Miers (1826). Note the ravines developed behind the thermal springs where nowadays are the ruins of the hotel baths (compare with Fig. 2).

of all this the Incas bridge is a miserable object" the stratified (A) rock & a confused mass is form of spherical balls. They burst forth, as do Darwin's Diary of the Beagle (1831- very distinct & this latter is different from the those of Cauquenes, and probably those of Vi- 1835) reproduced by Keynes (2001). general character of the plain (B).- This Inca's lla Vicencio, on a line of elevation" (Darwin In his original field notebook he depicted bridge is truly a sight not worth seeing" 1846, p. 190). a pencil sketch (Figs. 5a and b) and des- (Darwin's The Diary of the Beagle 1831- As a common factor in all these descrip- cribed "Incas bridge irregular hilly plain of 1836, pages 553-554, see Fig. 6). tions it is clear that Darwin recognized valley filled up with pebbles & detritus a fan of Later on in Darwin's Journal of Resear- that the river first excavated its channel, ferruginous cellular Tufa covering a part: the ri- ches he said "April 4th.- ...when one hears of and later the thermal springs cemented ver having cut as far as (x). continued to scoop a natural Bridge, one pictures to oneself some the gravels. He also recognized that the out to the Southward; rubbish (B) fell down deep and narrow ravine, across which a bold bridge gravels where some kind of strati- from plain I supported circled [page 202a] (m) mass of rock has fallen; or a great arch hollowed fied crust in contrast with the northern whilst river, continued forming arch. - the oblique out like the vault of a cavern. Instead of this, side, where colluvial debris was falling junction is very evident (horizontal & confined) the Incas Bridge consists of a crust of stratified from a disorganized deposit. He identi- plain generally horizontal gravel this not so; shingle,[page 335] cemented together by the fied the obliquity between the river valley hence rubbish: - My hypothesis of Tufa is that deposits of the neighbouring hot springs. It appe- and the bridge, interpreting a younger it was deposited after valleys excavated & just ars, as if the stream had scooped out a channel age for the bridge. He accepted that con- before sea retired; matter before that generally on one side, leaving an overhanging ledge, which cretions were formed were water drips, deposited - hence Tufa from these Springs ex- was met by earth and stones falling down from explaining the long stalactites descending tends above ... in the slope, above their level: the opposite cliff. Certainly an oblique junction, from the roof of the bridge. He conside- Springs hot - violent emission of gaz: Concre- as would happen in such a case, was very distinct red the origin of the bridge so simple tions, where water drips" (Darwin field note- on one side. The Bridge of the Incas is by no that "this Inca's bridge is truly a sight not worth book, pages 201-202a). means worthy of the great monarchs whose name seeing". His sketches were not published The most complete description is preser- it bears" (Darwin 1845, p. 334-335). until recently (Keynes 2001), and there- ved in the Diary of the Beagle (1831- In his geological observations he quoted fore his detailed descriptions were igno- 1836) where he described "the bottom of "at this place (Puente del Inca), there are red in subsequent publications. the valley is nearly even & composed of a mass some hot and cold springs, the warmest having a of Alluvium; on one side are several hot mine- temperature, according to Lieut. Brand (Travels, Modern proposals on the origin ral springs, & these have deposited over tha peb- p. 240), of 91°; they emit much gas. According bles [page 554] a considerable thickness of to Mr. Brande, of the Royal Institution, ten cu- Most of the subsequent visitors recogni- hard stratified Tufa; The river running in a bical inches contain forty-five grains of solid zed - as Darwin did - the natural origin of narrow channel, scooped out an archway beneath matter, consisting chiefly of salt, gypsum, carbo- Puente del Inca, with the only exception the hard Tufa; soil & stones falling down from nate of lime, and oxide of iron. The water is of Christiano Junior (1902). This author the opposite side at last met the overhanging part charged with carbonic acid and sulphuretted claimed that perhaps it was an old Inca & formed a bridge. The oblique (D) junction of hydrogen. These springs deposit much tufa in the bridge built with wood and rattan (bejuco, Darwin at Puente del Inca: observations on the formation ... 173

a climbing wild plant used by the Incas to make ropes), that after several centuries was cemented by the thermal springs and thus obtaining its present shape. Schiller (1907) described the deposits involved in the formation of the bridge, although his hypothesis on its origin is known from other people's references (see Sekelj 1947, Monteverde 1947). His hypothesis was that a crust growth from the thermal springs reached the opposite margin of the valley by means of slow lateral precipitation, a criteria accepted by Reichert (1924, 1929, p. 51). The bridge was again studied by Kittl (1941), who improved Darwin's original hypothesis. He postulated that the river was deflected southwards, increasing the incision that favored the slumps from the Figure 4: Puente del Inca southern margin, later on cemented by as illustrated in a new edi- the thermal springs. This proposal was tion of Darwin's Voyage refined by Monteverde (1967), who also (Darwin 1890). supported the excavation of the river, but did not accept the lateral growth. He region in the summer of 1983. The ava- postulated that the gravels of the bridge lanches reached the opposite side of the were prior to its formation and were pre- valley producing a run-up (see Fig. 7). served from glacial erosion. He claimed Thawing concentrated the gravels of the that the excavation was enhanced by late- avalanche in the upper surface, and these ral erosion coming from the northern si- gravels are supported in a mud matrix. de, which deflected the river valley south- This process hardens the gravel conglo- wards. See the analyses of Ramos (1993) merate and allows crossing these tempo- for further details in these hypotheses. rary bridges even with loaded mules. If that happens near a thermal spring, ce- Present knowledge mentation of the gravels with precipitation of sulphates and carbonates would To analyze the origin of Puente del Inca be possible, as observed in Puente del it is necessary to refer to the original Inca. Figure 5: Puente del Inca after Darwin: a) Re- ideas of Darwin and to apply Lyell's Puente del Inca is the only natural bridge production of Darwin's pencil sketch from his field notebook, April 5th, 1835 (with the permis- (1835) classical concept: "the present is the of its type presently standing, but collap- sion of English Heritage); b) Trace of the au- key of the past". From time to time, abun- sed bridges have been observed in the thor from the original sketch (see explanation dant and frequent snowfall during the headwaters of Río Plomo. In the sources on the text). winter is followed by the ENSO (El of Río Morado de Las Toscas, a river that Niño Southern Oscillation) over the high joins the southern margin of the Río subtropical Andes (Compagnucci and Plomo one kilometer upstream of the Vargas 1998). This abundant and fre- Refugio Las Toscas, Padva (2000) descri- quent winter snowfall produces nume- bed travertines and other deposits asso- rous ice bridges that last until the follo- ciated with thermal springs. There is evi- Figure 6: Puente del Inca after Darwin: Small wing summer, and sometimes even for dence in these deposits that the traverti- sketch in the margin with letter A, B, C, and D two or three years. I had the opportunity nes reached the opposite margin, but drawing in his Diary of the Beagle (1831-1836). to observe several ice bridges along the they are now collapsed. Horcones, Las Cuevas and Blanco rivers It is interpreted that during the last gla- doza valley were covered by snow avalan- in the vicinities of the Puente del Inca ciation the headwaters of the Río Men- ches developing a series of ice bridges as 174 V. A RAMOS

the ones depicted in figure 7. After thawing, only those with an extra hardening were preserved, as the one cemented by thermal spring waters. Figure 8 illustrates the different stages in the development of Puente del Inca. As established by Rubio et al. (1993), the precipitation of carbonates and sulphates is controlled by the presence of cya- nobacteria. These blue-green algae produce thin layers of carbonates coating the surface. The hot water flux is linked to the amount of seasonal rainfall in the region, which controls the temperature and the concentration of the spring waters. During the dry seasons, fluvial and aeolian erosion dominate over carbonate and sulphate precipitation, thus drying and cracking the bridge. These periods alternate with more humid ones that produce an intense coating of the bridge structure. A delicate equilibrium between erosion and precipitation, sometimes modified by anthropic activity, preserved the bridge until nowadays. In spite of Darwin's negative impression of Puente del Inca, the comprehension of its genesis is another good example that the "the present is a key of the past". No doubt that if Darwin would have seen an ice bridge he would have not hesitated in applying the uniformitarian ideas he de- fended - alongside Lyell - to understand the processes that formed the bridge.

ANDES MOUNTAIN BUILDING Figure 7: a) Ice bridge in the Río Blanco, a few kilometers south of Puente del Inca as seen in the summer of 1983. An avalanche during a hard winter remains for one or two years. Note the concen- There is no doubt on the importance tration of the gravel covering the ice and the run-up formed by the avalanche on the right side of the bridge; b) Partially collapsed ice bridge located upstream of Puente del Inca in the Río Cuevas, that the journey across the Andes had for during the same field season. Darwin's geological thought. He stated in a letter to her sister: "I returned a week ago vation of 12,000 feet bears so different an as- back to Valaparaíso, were important in from my excursion across the Andes to Men- pect from that in the lower country...To a geolo- several aspects of the development of doza. Since leaving England I have never made gist, also, there are such manifest proofs of ex- his ideas on mountain building. In this so successful a journey...how deeply I have enjo- cessive violence; the strata of the highest pinna- area the complexity of the structure yed it; it was something more than enjoyment; I cles are tossed about like the crust of a broken competes in impressiveness with the cannot express the delight which I felt at such a pie" (Burkhardt and Smith 1985). extraordinary exposures of the marine famous winding-up of all my geology in South Darwin's observations on the geology sequences interfingered with volcanic America. I literally could hardly sleep at nights around the Puente del Inca region, when rocks. The stratigraphic sequence is repe- for thinking over my day's work. The scenery he examined the Mesozoic sections at ated by a series of thrusts from Puente was so new, and so majestic; everything at an ele- both sides of the valley (Fig. 9) coming del Inca up to the drainage divide along Darwin at Puente del Inca: observations on the formation ... 175

the present border with Chile. Figure 10 shows Darwin's interpretation and the present understanding of that structure. Note how Darwin was aware of the complex structure of Río Horcones and the fine details in the classical Puente del Inca Section. He described in detail the anticline and the fault of Los Horcones as follows: "A little further on, the north and south valley of Horcones enters at right angles our line of section; its western side is bounded by a hill of gypseous strata [F], dipping westward at about 45°, and its eastern side by a mountain of similar strata [G] inclined westward at 70°, and superimposed by an oblique fault on another mass of the same strata [H], also inclined westward, but at an angle of only about 30°: the complicated relation of these three masses [F, G, H] is explained by the structure of a great mountain-range lying some way to the north, in which a regular anticlinal axis (represented in the section by dotted lines) is seen, with the strata on its eastern side again bending up and forming a distinct uniclinal axis, of which the beds marked [H] form the lower part"(Darwin 1846, p. 189). This is the overturned faulted anticline nowadays recognized in Los Horcones valley as shown in figure 10b. The observation of these sections in- Figure 8: Schematic evolu- fluenced his geological thoughts in seve- tion of Puente del Inca based on Ramos (1993) and ral central facts. I would like to emphasi- Aguirre-Urreta and Ramos ze the followings aspects. (1996). Note that when the avalanche is compacted and Evidence of uplift melting starts, the pebbles are concentrated in the upper part, where they are As Charles Lyell, Darwin was impressed slowly cemented by the pro- by the localized uplift denoted by the evi- ducts of the thermal spring. Compare stage 2 and 3 with dence of variation of sea level through the ice bridges of figure 7. time. The example of the Temple of Se- rapis near Naples that was the frontispie- During his examination of the sedimen- interpreted as evidence of submarine ce illustration of the "Principles of Geology" tary and volcanic sequences he found re- volcanism, matched early 19th Century was clearly showing uplifts in the order mains of fossil shells in his level No. 3 a ideas on mountain building. Darwin's pa- of tens of meters, only affecting a small few hundred meters to the southeast of per "On the connexion of certain volcanic phae- portion of the coast. But if we compare the bridge (see location (I-3) in Fig. 9), nomena, and on the formation of mountain- these small variations with what he was almost 3,000 meters above sea level (Dar- chains and volcanoes, as the effects of continen- seeing during his journeys across the win 1846). Some specimens fallen from tal elevations" (Darwin 1838) read in the Andes - where marine "shells that once were the outcrop were identified as Gryphea,a Geological Society on the 7th of March, crawling in the bottom of the sea" are now typical benthic mollusk of Jurassic - produced a deep impact in the members standing over 10,000 feet above sea level Cretaceous age. He associated this uplift of the society. As already established by - we can understand his exciting com- to some sort of injection of the volcanic Rhodes (1991), he favored the slow for- ments. Puente del Inca was a key area for rocks. The observation of fossiliferous mation of the mountain chains, based on this observation by him. strata interfingered with the volcanic sills, the relation between the small elevations 176 V. A RAMOS

Figure 9: Geologic map of Puente del Inca (after Ramos 1988 and Cegarra and Ramos 1996) with indication of some fossil localities (I-3, I-10) described in the text. Legend: Cat: Carboniferous slates; Trch: Triassic Choiyoi volcanics; Jurassic: Jlm: La Manga Formation carbonates where level No. 3 was described; Y: Auquilco Gypsum; Jt: Tordillo Formation redbeds; JKvm: Vaca Muerta Formation black shales; Cretaceous: Km: Mulichinco Formation redbeds; Ka: Agrio Formation limestones and sandstones; Kd and Kv: red sandstones and volcanic rocks; Most of these rocks are interfingered with volcanic flows and sills; Tsm: synorogenic Miocene deposits; Q: Quaternary Alluvium. in the order of a few meters produced by accepted in the current theories of that we may safely conclude that the bottom of the sea his observed earthquake displacements epoch (Whewell 1839, Rhodes 1991). on which the shells lived, subsided, so as to recei- of Concepción, and the supposed subse- ve the superincumbent submarine strata: and quent volcanic activity in the Osorno Evidence of subsidence this subsidence must have taken place during the volcano. A few months later in the Pre- existence of these shells; … The conclusion of a sidential Address of 1839 Whewell - then Another important contribution of Dar- great subsidence during the existence of these cre- the President of the Society - expressed win derived from his observations in the taceo-oolitic fossils, may, I believe, be extended to the antagonism between the Uniformita- Puente del Inca region was his analysis of other districts" (Darwin 1846). rians and the dominant Catastrophists the sea bottom subsidence. He claimed His reasoning on an active subsidence views of society at that moment. Dar- that "the fossils … from the limestone-layers in anticipated James Hall's similar ideas of win's ideas quoting that "the formation of the whitish siliceous sandstone, are now covered 1857, who proposed - based on the sub- mountain chains and volcanoes, which he (Dar- … by strata, from 5,000 to 6,000 feet in thick- sidence inferred for the Appalachians' win) conceives to be the effect of gradual, small, ness. Professor E. Forbes thinks that these shells during Paleozoic times - that the great and occasional elevation of continental masses" probably lived at a depth of … 180 to 240 feet; sediment load caused crustal failure and contrast with the paroxysmal turbulence … in this case, as in that of the Puente del Inca, downwarp, opening the way to geosyncli- Darwin at Puente del Inca: observations on the formation ... 177

ne theory (Hall 1857). quakes originated in fractures, gradually mum glacial; the ice was covered by ava- Darwin used as reference the stratigra- formed in the earth's crust, and were lanches debris from the adjacent slope; phic thickness between the first fossil accompanied by movements of the land then during thawing the bridge was sup- bed at (I-3), and the youngest one located on either side of the fracture (Darwin ported by the mud matrix of the conglo- at (I-10) (see location in Fig. 9). The pre- 1838, 1846). In conclusion he boldly merates, which subsequently were ce- sence of several packages with similar advanced the view "that continental eleva- mented by the minerals of the hot-ther- fossil shells from the top to the bottom tions, and the action of volcanoes, are phenome- mal springs. of the marine sequence was for Darwin na now in progress, caused by some great but clear evidence of a great subsidence. slow change in the interior of the earth; and, the- b) Mountain building When combined with the observation of refore, that it might be anticipated, that the for- The observations that led Darwin to in- the thick conglomerates of the Tunuyán mation of mountain chains is likewise in pro- fer gradual, small, and episodic mountain Formation these packages indicated to gress: and at a rate which may be judged of by uplift combined with periods of impor- him a second, and more recent period of either actions, but most clearly by the growth of tant subsidence, together with the per- subsidence. This perception of renewed volcanoes." (Darwin 1838, pages 654-660). ception of lateral growth of a mountain subsidence and subsequent uplift was the chain to the foreland, are the base of embryonic stage of his global mountain CONCLUDING REMARKS modern concepts in orogeny. The evi- uplift hypotheses later discussed. dence connecting episodic earthquakes The region of Puente del Inca was very and volcanic activity, partially seen by Evidence of lateral growth important in Darwin's geological obser- Darwin, but also mentioned to him by vations and considerably contributed to some other witnesses, were complemen- As quoted by Giambiagi et al. (2009) clast his post-fieldtrip interpretations of the tary to understand the processes associa- provenance analyses of the synorogenic collected data. His work there can be ted with the uplift of the Andes. How- conglomerates of Miocene age provided summarized under two different aspects, ever, the notion that the uplift was pro- the key for Darwin's claim for an episo- i.e., the origin of the Puente del Inca and duced by a succession of earthquakes, an dic and lateral migration of the mountain his contributions to mountain building. active process still ongoing, was the clue uplift. The finding of the succession of for the comprehension that the Andes marine fossils derived from the Cordi- a) Puente del Inca are still undergoing uplifting, and one of llera Principal at the base, and clasts from Although Darwin was not impressed at the best examples of application of uni- the high grade metamorphic basement of all with the natural bridge of Puente del formitarian ideas in tectonics. It is easy to Cordillera Frontal above in the sequence, Inca, he spent some time analyzing its understand the satisfaction of Charles clearly indicated to him the lateral and origin. For him this had been a simple Lyell with the new evidence drawn from episodic growth of the Andes. This con- process associated with lateral growth Darwin's observations, and the close cept was analyzed in the same region and cementation aided by hot-spring wa- friendship developed by these two scien- many years later by Polanski (1964, ter. Most of his observations were left tists in the Geological Society, that at that 1972), who also arrived at the same con- unpublished in his field notebook and in time was dominated by Catastrophists. clusions about the foreland migration of his Diary of the Beagle (1831-1836). His There is no doubt now that Darwin's and the uplift during Andean deformation. basic ideas were right, but because they Lyell's ideas slowly pervaded the geologi- This notion is part of the ideas about remained unpublished they were not cal community, and that in those early fold and thrust belt deformation as infe- considered for many years by later years Darwin was a geologist trained rred in modern plate tectonics. authors that hypothesized on the genesis mainly by several years of fieldwork and of the bridge. The origin of the bridge is observations obtained during his rese- Association with volcanoes simple to understand when Darwin's arch in Argentina, where the Andes pla- ideas on uniformitarian processes are yed a central role in his hypotheses. In 1838 Darwin read at a meeting of the combined with actual observations of Geological Society perhaps the most im- hard winters with exceptional snowfall, ACKNOWLEDGMENTS portant of all his geological papers, re- such as in those seasons preceding the El lating deformation, earthquakes, and Niño Southern Oscillation. The sequen- The author wants to acknowledge the mountain uplifts. After describing the ce of processes involved in the forma- access to the field notebooks to English great earthquakes which he had experien- tion of the bridge is straightforward, if Heritage and to Marcelo G. Armentano ced in South America, and the evidence classical uniformitarian concepts are for his consent to use the panorama of of their connection with volcanic out- applied. First it was formed as an ice Puente del Inca. Roberto Page, Carlos bursts, he proceeded to show that earth- bridge, probably during the last maxi- Costa, Suzanne M. Kay and Miguel 178 V. A RAMOS a) Classical structural cross-section of Darwin (1846); by drawn to the Uspallata valley the Cumbre Pass b) Present interpretation of the structure based on Cegarra and Ramos (1996). Figure 10: Cerro Almacenes correlates with k in Darwin's cross section. Darwin at Puente del Inca: observations on the formation ... 179

Griffin contributed with their reviews to the years 1832 to 1836. Smith Elder and Co., Polanski, J. 1964 . Descripción geológica de la improve the preliminary version of the 280 p., London. Hoja 25a Volcán San José, provincia de Men- manuscript. Thanks are also due to the Darwin, C.R. 1890. Journal of researches into the doza. Dirección Nacional de Geología y Mi- staff of the Laboratorio de Tectónica natural history and geology of the countries nería, Boletín 98: 1-94, Buenos Aires. Andina for many years of cooperation visited during the voyage round the world of Polanski, J. 1972. Descripción geológica de la and work in the region. H.M.S. Beagle under the Command of Cap- Hoja 24a-b Cerro Tupungato, provincia de tain Fitz Roy. R.N. John Murray (A new edi- Mendoza. Dirección Nacional de Geología y WORKS CITED IN THE TEXT tion with illustrations by R.T. Pritchett of pla- Minería, Boletín 128: 1-110, Buenos Aires. ces visited and objects described), 598 p., Ramos, V.A. 1988. The Tectonics of the Central Aguirre-Urreta, M.B. and Ramos, V.A. 1996. Áre- London. Andes: 30° 33°S latitude. In Clark, S., Burch- as de interés. In Ramos, V.A. et al., Geología Giambiagi, L., Tunik, M., Ramos, V.A. and Go- fiel, D. and Suppe, J. (eds.), Processes in de la Región del Aconcagua, Provincias de doy, E. 2009. The High Andean Cordillera of Continental Lithospheric Deformation, Geo- San Juan y Mendoza. Dirección Nacional del central Argentina and Chile along the Piu- logical Society America, Special Paper 218: Servicio Geológico, Anales 24: 471-480, Bue- quenes Pass-Cordón del Portillo transect: 31-54. nos Aires. Darwin's pioneering observations compared Ramos, V.A. 1993. Geología y estructura de Brandt, C. 1827. Journal of a voyage to Peru; a with modern geology. Revista de la Asocia- Puente del Inca y el control tectónico de sus passage across the Cordillera of theAndes, in ción Geológica Argentina 64(1): 44-54. aguas termales. 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Tra- London. South America. Being the third part of the bajo Final de Licenciatura, Universidad de geology of the voyage of the Beagle, under Buenos Aires, (unpublished), 87 p., Buenos Recibido: 23 de septiembre de 2008 the command of Capt. Fitzroy, R.N. during Aires. Aceptado: 6 de noviembre de 2008