An exceptional storm accumulation of nautilids in the Lower Cretaceous of the Neuquén Basin, Argentina

Abstract

An exceptional accumulation of nautilid shells of the species Cymatoceras perstriatum (Steuer) is reported and described in the Lower Cretaceous Agrio Formation of the Neuquén Basin (west-central Argentina). The bed represents a storm deposit in a shallow-water environment within the mid-ramp. The evidences of a storm-related origin of the bed come from the petrographic analysis and taphonomic features of the shells, specially the sedimentary infill pattern. The shells are dispersed in patches within the stratum, without any orientation relative to the bedding plane. It is proposed that the shells were floating after the death of the animals, although most of them have an almost complete living chamber. The presence of some heavily encrusted shells suggests that there is a mixing of specimens with different drift times. A variety of factors related to the origin of such high number of nautilid shells are discussed, including a transgressive stand system tract, the possible existence of a gregarious behaviour, changes in salinity and wind directions. Lower Cretaceous, Nautilids, Neuquén Basin, shell accumulation, storm deposit.