Cyclic sediment waves on western slope of the Caspian Sea as possible indicators of main transgressive/regressive events

摘要

New high-resolution seismic reflection profiles collected on the western slope of the Caspian Sea during cruises of RV "Rift" in 2004, 2006 and 2007 allowed the identification of regional sedimentary processes formed its present morphologic and structural image. The upper, gentle part of the slope, from the shelf edge to 400 m water depth, was formed in the Pleistocene-Holocene. Its architecture is dominated by very prominent wavy features, sedimentary wave bedforms, which were formed mostly by density gravitational (turbidity) streams downslope during lowstands of the Caspian Sea, i.e. during the regressive intervals, and affected by bottom currents partly. This specific sedimentary process was slowed during transgressive phases. Downsection, several distinct generations of the sediment waves are distinguished within the upper sedimentary sequence. Sampling by gravity corer characterized the uppermost undulation only. Therefore, several deeper generations of the sediment waves distinguished within the upper sedimentary sequence were compared with the available drilling results from boreholes in the shallow-water northern Caspian Sea shelf, by means of extrapolation and correlation of horizons along the available seismic profiles. Three strong wavy reflectors on the western slope were identified on the shelf as unconformities, marking the Atelian, Enotaevian, and Mangyshlakan regressions. Lacking submarine boreholes, a rough estimation for general regional seismic-stratigraphy of the upper sedimentary section on the western slope of the Caspian Sea is proposed from the stratigraphic identification of the observed sediment waves.