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Darwin's Great Patagonian Tertiary Formation at the Mouth of the Río Santa Cruz: a Reappraisal

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Darwin's Great Patagonian Tertiary Formation at the Mouth of the Río Santa Cruz: a Reappraisal 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.
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