Seismostratigraphy of Buried Pleistocene Deltas on the Mangyshlak Threshold, Caspian Sea

The change in sedimentation conditions from shelf to deep-water, including deltas and underwater cones of outflow, can be traced on the regional profiles of the Mangyshlak Threshold. It is possible to confidently identify several continuous seismic horizons. Upper part of sedimentary cover consists of two types of sediments with essentially different acoustical image, i.e. seismic sequences. The first type of sediments appears to be accumulated under enough quiet hydrodynamic conditions of high seas far from coastline, i.e. they are marine sediments. On the contrary, the second acoustical pattern evidences active lithodynamics inhering in shallow-water and subaerial environment, which are affected essentially by fluvial processes. The characteristic lenslike architecture of the sediments allows us to interpret them as deltaic and avandeltaic sequences, which were accumulated during lowstand stage of the Caspian Sea. According to geographical location of the buried valleys and deltaic sequences, one can assume that the discussed accumulative units relate rather to the paleo-Ural River.