Ichnology of Marginal Marine Facies of the Agrio Formation (Lower Cretaceous, Neuquén Basin, Argentina) at its Type Locality

This study includes a thorough systematic analysis of the ichnological record in the uppermost section of the Agua de la Mula Member of the Agrio Formation (Mendoza Group) in the Neuquén Basin (Argentina). The Bajada del Agrio section represents an environment evolving from marine to marginal-marine. Five main facies were defined: (1) an oolitic-skeletal bar; (2) transgressive pelites and coquinas (bioclastic conglomerates); (3) progradational coarsening-upward sandstones; (4) heterolithic deposits; and (5) heterolithic and carbonatic deposits. Fluctuating salinity and/or hypersalinity is inferred in the open tidal-flat deposits (facies 4). Sixteen ichnotaxa were identified and three types of trace fossils were described using open nomenclature because of the poor preservation of the material. Three ichnoassemblages are described. The section yielded trace fossils of callianasid decapods, palaeotaxodont bivalves, annelids and small tetrapods. Paleoenvironmental information drawn from trace fossils is coincident with sedimentologic evidence. It also highlights the current lack of accurate ichnological models for marginal marine hypersaline or fluctuating-saline environments, and suggests that sharp salinity changes in the environment not necessarily implie a decrease in ichnodiversity.