Dictyodora-Nereites dominated ichnofauna in the Silurian−Devonian turbidite-like deposits of central-western Gondwana (San Rafael Block, Argentina): New stratigraphic and bathymetric constraints

Abstract

The Río Seco de los Castaños Formation (San Rafael Block) holds one of the few middle Palaeozoic (Silurian−Devonian) records worldwide of a Nereites-Dictyodora dominated ichnofauna with sedimentological features such as oscillatory and combined-flow deposits. This record is associated with microbial mats in fine-grained intervals between turbidite-like deposits documented above the storm wave base. In this work, a detailed sedimentological study permitted to divide the unit into two informal sections separated by an erosional unconformity attributed to a sea level fall. Prograding turbidite-like sand-grained deposits document gravitational turbulent flows. The tectonostratigraphic framework allowed to disregard a submarine fan, whereas the unconformity does not support an autocyclic deltaic evolution. Palaeocurrents and palaeoslope indicators to the SW suggest a high source area to the NE, that contrasts with the well-established E-W trend of most of the units of similar age in the Cuyania terrane. Deposition was probably deflected by subaqueous highs with the development of mini-basins most likely structurally controlled. The ichnological analysis allowed to recognise ichnotaxa and intergradation cases between ichnospecies previously unknown for the unit, adding to the most diverse record of Dictyodora and Nereites in central-western Gondwana. Chondrites intricatus, Chondrites targionii, Chondrites?, Dictyodora zimmermanni, and specimens resembling Dictyodora liebeana are recorded for the first time. Nereites missouriensis is documented in intergradation with Nereites jacksoni, whereas Dictyodora scotica is recognised in intergradation with Dictyodora atuelica and Dictyodora tenuis. A decrease in ichnodiversity throughout the section could be related to variation in environmental parameters between the lower and upper sections. The ichnofacies assignment is discussed in terms of ethological categories and ichnodiversity. Some of the defined ichnoassemblages are assignable to a non-standard record of the Nereites Ichnofacies, and one in particular to the Nereites Ichnosubfacies, while another shows some characteristics coincident with the Zoophycos Ichnofacies. A transitional case between ichnofacies for the ichnological content of the unit as a whole, as previously reported in other units, cannot be disregarded.