ICHNOLOGY OF THE RÍO MAYER FORMATION, LOWER CRETACEOUS, SOUTHWESTERN GONDWANA, PATAGONIA, ARGENTINA

Abstract

The Río Mayer Formation was deposited during the Early Cretaceous in the Austral Basin (Santa Cruz Province,Argentina) and includes black shales with intercalated sandstones and marls. Abundant information is available on its palaeontologicalcontent, especially on ammonoids, but detailed ichnological studies are lacking. Sedimentological and ichnological data were collected inthe Río Guanaco area (49°57?11?S–72°04?56?W). Three sections were identified and described in detail. The lower Section was depositedduring the Berriasian–Valanginian and is composed of black shales. The middle section including black marls with intercalated black shaleswas deposited during the Valanginian–Hauterivian. The upper section is composed of black shales with frequent thin intercalations of finegrainedsandstones (Barremian–Albian). Six ichnogenera were identified: Zoophycos, Chondrites, Bergaueria, Ophiomorpha, Palaeophycus andTeredolites. The most abundant and constant is Zoophycos, arranged in three morphological groups. All these ichnogenera are grouped —inthe Río Mayer Formation— into two ichnoassociations. The first is found in the middle section, and is a typical Zoophycos ichnofacies withassociated Bergaueria. The second ichnoassociation appears at the top of the upper section, and contains a Zoophycos ichnofacies with associated Ophiomorpha and Teredolites. The unusual presence of Bergaueria in low oxygen offshore environments, exceeds the bathymetric range typical of this ichnogenus. The unusual presence of O. cf. rudis in outer shelf environments reveals the existence of doomed pioneers in the analyzed unit.