Abstract

CARMONA, Noelia B.; PONCE, Juan José; MANGANO, María Gabriela  and  BUATOIS, Luis A.. Variability of the Glossifungites ichnofacies at the boundary between the Sarmiento Formation (middle Eocene- early Miocene) and Chenque Formation (early Miocene) in San Jorge Gulf, Chubut, Argentina. Ameghiniana [online]. 2006, vol.43, n.2, pp.413-425. ISSN 1851-8044.

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.

Keywords : Trace fossils; Glossifungites ichnofacies; Chenque Formation; Miocene; Patagonia; Transgression; Firmground.