Trace fossils of the glacial to postglacial transition in the El Imperial Formation (Late Carboniferous), San Rafael Basin, Argentina

PAZOS, PABLO, DI PASQUO, MARÍA DE LAS MERCEDES, AMENÁBAR, CECILIA R.

SEPM Special Publication

Society for Sedimentary Geology

Lugar: Tulsa; Año: 2007 vol. 88 p. 137 - 147

This paper reports the first ichnological 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 stratigraphical schemes three sections were recognised in the field, but the upper one is ichnologically sterile and not studied in this paper. The lower section shows evidence of glacial influence but lacks marine indicators and was deposited in a shallow palaeovalley. Ichnofossils include almost exclusively locomotion traces produced by arthropods (Diplichnites-Diplopodichnus) or molluscs (Archaeonassa). Palynomorphs are terrestrial and show evidence of intense transport and deterioration. The middle section contains a more varied range of ethological categories:   pascichnia (e.g., Mermia, Gordia, Cochclichnus), 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 section ichnofossils may appear with palynomorphs that show degradation and pyritization and contain scarce acritarchs suggesting brackish rather than fresh water conditions in a fjord depositional system.