Chocolate clays of the Northern Caspian Sea Region: Distribution, structure, and origin

The chocolate clays commonly comprise a part of the Lower Khvalynian sequence in the northern Caspian Lowland and in the Volga region. The mode of occurrence varies from continuous to patchy mosaic. All the clay deposits are confined to Pre-Khvalynian depressions of various origins. There are several sub-facies distinguishable within the chocolate clay facies: mono-clayey (typologic), stratified sandy-clayey and silty-clayey. Judging from specific features of lithology, geomorphic position, mode of occurrence, mollusc fauna composition, and radiocarbon dates, the chocolate clays represent a specific facies of the Lower Khvalynian sediments and cannot be considered as an individual stratigraphic unit.