Variabilidad de la icnofacies de Glossifungites en el contacto entre las Formaciones Sarmiento (Eoceno-Oligoceno) y Chenque (Mioceno temprano) en el Golfo San Jorge, Chubut, Argentina

CARMONA, N.B.; PONCE, J.J.; MÁNGANO, M.G.; BUATOIS, L.A.

AMEGHINIANA

Año: 2006 vol. 43 p. 413 - 425

0002-7014

This study focuses on the ichnologic and stratigraphic characteristics of the boundary surface between the Sarmiento (middle Eocene- early Miocene) and Chenque Formations (early Miocene). The analyzed outcrops are located near Comodoro Rivadavia city, Chubut Province, Patagonia, Argentina. The studied boundary represents a co-planar surface of lowstand erosion and transgressive erosion that produced exhumation of deposits belonging to the Sarmiento Formation, allowing the development of the Glossifungites ichnofacies. This ichnofacies is characteristic of stiff and dewatered sediments (firmgrounds) and comprises a particular suite of substrate-controlled trace fossils. In the studied outcrops (Astra, bahía Solano and Infiernillo), this boundary surface was colonized by decapod crustaceans that constructed galleries (Thalassinoides isp.) and bivalves that constructed dwelling structures (Gastrochaenolites ornatus). Although in all these localities the ichnotaxonomic composition does not show significant variations, important paleoecological differences between the trace fossil suites preserved in each outcrop can be inferred. Recent assemblages that colonize firmground surfaces seem to be controlled by three main parameters: sediment texture, substrate firmness and bathymetry.In the Miocene examples, the observed variability of the Glossifungites ichnofacies was probably controlled by the firmness of the surface exhumed during the Leonense transgression. Analysis of the ichnologic content, truncation of trace fossils, and the relationship between biogenic structures suggests a complex history for this erosive surface, involving successive events of ravinement erosion and benthic colonization.