Ichnology from de Río Mayer Formation, Austral Basin, Patagonia Argentina

RICHIANO S., Y POIRÉ, D. G.

Mendoza, Argentina

Congreso; 18 International Sedimentological Congress; 2010

IAS

The Río Mayer Formation was developed during the lower Cretaceous in the Austral Basin (Santa Cruz Province, Argentina). It is deposited above either the El Quemado Complex or Springhill Formation and is composed of black shales with thin intercalated fine-grained sandstones, marls and limestones. Abundant information is available on its palaeontological content, especially ammonoids, but detailed ichnologycal studies are scarce. In the region of Río Guanaco area (S 49º 57’ 11’’; W 72º 04’ 56’’) sedimentologycal and ichnologycal data were collected. Tree main sections were identified and described in detail. The lower section is developed in the Berriasian - Valanginian and is composed by laminated black shales. The middle section is about 40 m thick of black marls with intercalated black shales developed between the Valanginian to Hauterivian. Finally the upper section is composed by black shales with frequent thin intercalations of fine sandstones (Barremian to Albian). Ichnologically, were identified 5 ichnogenus that contained 6 ichnospecies: Zoophycos isp., Chondrites targinoii, C. intricatus, Bergaueria perata, Ophiomorpha rudis and Teredolites isp. The most abundant and constant is Zoophycos isp., which following different authors (among others, Uchman, 1995, Miller, 2007),was classified in tree morphological groups. All of these ichnospecies are gruped in the Río Mayer Formation into two ichnoassociations. The first is found in the middle section, and is a typical Zoophycos ichnofacies with Zoophycos isp. and Chondrites isp. The second ichnoassociation is developed at the end of upper section, containing Zoophycos ichnofacies and Ophiomorpha rudis ichnosubfacies and Teredolites isp. These two last components are shown the link between the off shore to deep marine sedimentation and a littoral environment.