Abstract

PARRAS, Ana  and  GRIFFIN, Miguel. Darwin's great patagoniantertiary formation at the mouth of the río Santa Cruz: a reappraisal. Rev. Asoc. Geol. Argent. [online]. 2009, vol.64, n.1, pp.70-82. ISSN 0004-4822.

Theclassical section described by Darwin at the mouth of the Río Santa Cruz was alandmark in the later stratigraphy of the marine Cenozoic of Patagonia.Sedimentological and paleontological studies of rocks from the Monte LeónFormation at the Darwin and Mount Entrance sections - mouth of the Río SantaCruz - suggest that they were deposited in an early Miocene inner shelf tosubtidal - or even intertidal at the top - environment. The base of thesections is covered and the top is unconformably overlain by Quaternarydeposits. The invertebrate fauna (mainly mollusks) mentioned herein - theearliest of which were collected by Darwin and described by Sowerby - werecollected from the two sections with a detailed stratigraphic control. Itincludes brachiopods, echinoderms, scaphopods, bivalves, and gastropods. Thesimilarity of this fauna with that one from the sections at Restinga Norte,Cabeza de León, Las Cuevas, Yegua Quemada and Monte Observación, all south ofthe study area, allow correlation with them. In addition to the equivalence ofthe faunas, sedimentological features at all these localities are also thesame. Therefore, all the shell beds involved should be referred to the PuntaEntrada Member of the Monte León Formation. The Monte Observación Member shouldbe restricted to its original use and included as a member at the base of theoverlying Santa Cruz Formation, outcrops at Cerro Monte León and CerroObservación.

Keywords : Darwin Section; MountEntrance Section; Mouth of Santa Cruz River; Paleoenvironments; Invertebrates; Patagonia.

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