70 Revista de la Asociación Geológica Argentina 64 (1): 70 - 82 (2009)

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 uncon- formably 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 con- trol. 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 des- cripta 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 representan- tes 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 men- tion, 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 stratigra- phic sections showing identi- fied 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 laevis- cula 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, orienta- tion 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 stratifica- tion, 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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