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

The Rio 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 palaeontological content, especially on ammonoids, but detailed ichnological studies are lacking. Sedimentological and ichnological data were collected in the Rio Guanaco area (49 degrees 57'11 '' S-72 degrees 04'56 '' W). Three sections were identified and described in detail. The lower Section was deposited during the Berriasian-Valanginian and is composed of black shales. The middle section including black marls with intercalated black shales was deposited during the Valanginian-Hauterivian. The upper section is composed of black shales with frequent thin intercalations of fine-grained sandstones (Barremian-Albian). Six ichnogenera were identified: Zoophycos, Chondrites, Bergaueria, Ophiomorpha, Palaeophycus and Teredolites. The most abundant and constant is Zoophycos, arranged in three morphological groups. All these ichnogenera are grouped-in the Rio Mayer Formation-into two ichnoassociations. The first is found in the middle section, and is a typical Zoophycos ichnofacies with associated 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.