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 amo- nites. 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 inte- rest 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.