Variability of the Glossifungites ichnofacies at the Lower Miocene boundary between the Sarmiento and Chenque Formations, Patagonia, Argentina

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

Trelew, Chubut, Argentina

Congreso; Ichnia 2004 - First International Congress on Ichnology; 2004

The ichnology of the boundary surface between the Sarmiento (Eocene- Oligocene?) and Chenque (Lower Miocene) Formations is analyzed in outcrops near Comodoro Rivadavia city, Chubut Province, Patagonia, Argentina. The Sarmiento Formation consists of tuffaceous continental deposits, while the overlying Chenque Formation is dominated by sandy and muddy strata that accumulated in shallow marine environments during the Miocene Atlantic transgressions. The analyzed contact represents a co-planar surface (amalgamated lowstand and transgressive surface of 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). At two localities (Bahía Solano and Infiernillo), the colonization surface was dominated by pseudo-borings assigned to bivalve activity and resembling Gastrochaenolites borings. The presence of Thalassinoides was also observed, although the abundance of these decapod excavations was subordinated to the bivalve structures. Analysis made at other localities revealed that Gastrochaenolites is not always present at the boundary, in contrast to Thalassinoides which is more regularly and widely distributed at all localities. This situation could be explained using information from modern analogues. Recent assemblages that colonize firmground surfaces seem to be controlled by three main parameters: sediment texture, substrate cohesiveness and bathymetry. In the Miocene example, sediment texture is approximately constant at all localities. Additionally, there is no apparent trend according to bathymetry in the studied sections. The observed variability of the Glossifungites ichnofacies is probably related to the firmness of the surface exhumed during the transgression. This is consistent with the ethology of the bivalve borers, which need hard to firm substrates for colonization.