On the Caspian Lowland the Khvalynean sediments occur mostly close to the surface.
Younger (and higher in the sequence) are only the Holocene floodplain lacustrine and marine (New Caspian) sediments.
Authigenic minerals, such as iron hydroxide (up to 65%), gypsum (single crystals and aggregates), occasional glauconite, are less frequent (Chistyakova, 2001).
Clay minerals are represented by smectite, kaolinite, montmorillonite, chlorite, and hydromica.
They differ from under- and overlying layers in many characteristics (Badyukova, 2000; Chistyakova, 2001; Leonov et al., 2002).
They are separated from the Lower Khazarian series by continental Atelian layers synchronous to marine sediments of the Atelian regressive basin.Low content of carbonates in the clays or their complete absence suggest a cold climate, as solubility of carbonates increases at low temperatures, so that they may stay in the solution.On the other hand, abundance of chemogenic dispersed carbonates with no secondary changes recorded in the terrigenous pelitomorphic clays suggests under conditions of arid climate.Thickness of the chocolate clays and related Khvalynean sediments does not usually exceed a few meters (3—5 m), occasionally reaching 20—25 m and more.They are mostly confined to the Caspian Lowland, from the modern Caspian coast to the feet of bordering elevations (Ergeni, Obshchiy Syrt, Privolzhskaya, Stavropol); they are also found in the Volga and Ural estuaries.