Taphonomic analysis of shell concentrations in deltaic Miocene deposits of the South Caspian Basin, eastern Paratethys

Abstract

ABSTRACT The depositional environment and palaeoecological conditions of a Miocene delta in the Paratethys Basin have been reconstructed based on an integrated sedimentological, ichnological and taphonomic analysis of shell concentrations. Four facies associations, i.e. delta plain, distributary channel, delta front and prodelta, are recognised. The shell concentrations, which are a characteristic feature of the Miocene successions, display different taphonomic signatures in the various parts of the delta system. Delta plain shell concentrations are characterised by unabraded shells with abundant micro-borings and thick micrite envelops formed during colonisation windows and optimum ecological conditions such as good illumination, high nutrient availability and low sedimentation rate. Distributary channel shell concentrations consist mainly of highly abraded, highly fragmented, disarticulated shells that point to long-term high-energy hydrodynamic conditions during their formation. Sporadic shell concentrations in the delta front are composed of chaotic and highly fragmented skeletal elements and co-occur with predominantly suspension-feeding trace fossils such as Arenicolites, Diplocraterion and Skolithos. They reflect a high sedimentation rate and suspension organic particles in a high-energy depositional setting. Prodelta shell concentrations are characterised by moderately fragmented, parallel-oriented shells, preserved within thin-bedded sandstones, intercalated between thick mudstone packages. These concentrations were produced by storm-induced, offshore-directed high-density currents.