Trace-fossils of the glacial to postglacial transition in the El Imperial Formation (upper carboniferous), San Rafael Basin, Argentina

Abstract

This paper reports the first ichnologic investigation in the San Rafael Basin, carried out in the glacial to postglacial transition, recorded in the El Imperial Formation at the Rio Atuel canyon. Following previous published stratigraphic schemes, three units were recognized in the field, but the upper one is ichnologically sterile and not studied in this paper. The lower unit shows evidence of glacial influence but lacks marine indicators and was deposited in a shallow paleovalley. Almost exclusively, the ichnofossils include locomotion traces produced by arthropods (Diplichnites and Diplopodichnus) or molluscs (Archaeonassa). Palynomorphs are terrestrial and show evidence of intense transport and deterioration. The middle unit contains a more varied range of ethologic categories: pascichnia (e.g., Mermia, Gordia, Cochlichnus), repichnia (e.g., Paganzichnus, Diplopodichnus, Didymaulichnus, Diplichnites, Undichna) and fodinichnia (e.g., Treptichnus) documented in transgressive deposits that record the glacial retreat and early postglacial interval. Many previous detailed studies carried out in contemporary successions of the basins of western Argentina have been the object of intense debate about the marine or nonmarine nature of these successions. In the middle unit, ichnofossils occur together with palynomorphs that show degradation and pyritization and contain scarce acritarchs suggesting brackish rather than fresh water conditions in a fjord depositional system.