ИСТИНА

Chocolate clays are one of the most common facies of the Lower Khvalynian deposits in the Lower Volga River valley. They were deposited during the Early Khvalynian transgression of the Caspian Sea. These clays consist mainly of clay with lenses sand and silt that contains mollusc shells. Here we present the results of 69 radiocarbon ages of chocolate clays derived from mollusc shells (Didacna, Monodacna and Dreisenna species). Radiocarbon ages were calibrated to calendar years using the CALIB 7.0.4. (IntCal13) program (Stuiver and Reimer, 1993). The materials were collected from the key sections (Svetly Yar, Raigorod, Sr. Akhtuba, Cherny Yar, Tsagan-Aman, Kopanovka, Enotaevka, etc.) in the Lower Volga region. Radiocarbon dates of chocolate clays reveal ages between 20-11 cal ka BP. 28 of these ages lie between 14,1-13,5 cal ka BP. The ages at the base of the chocolate clays are 16-14,5 cal ka BP and at the top are 14-12,5 cal ka BP. Few dates correspond to 11 and 20 cal ka BP. The radiocarbon ages of the bottom part of the chocolate clays in the southern key sections are 16,5-14,8 cal ka BP, meanwhile in the Northern sections they are 14,1-13,8 cal ka BP. The results suggest that the sand layers with mollusc shells in the bottom of the chocolate clays were deposited between 16,2-14,7 ka BP, which correspond to the Oldest Dryas cold event, while during 14,5-14,1 ka BP chocolate clays with massive structure were accumulated. This time corresponds to the Bølling warm period. The ages of upper and middle sand horizons that contain molluscs Didacna ebersini, Dreissena polymorpha are estimated between 14,1- 13,8 ka BP, which correspond to the Older Dryas cold event. Deposits of sandy clay sediments that contain molluscs of Dreissena polymorpha reveal the ages of 13,8-12,8 ka BP and correspond to Allerød warm period. The results show that the variability of the litho-facial composition of the Early Khvalynian region sediments in the Lower Volga could potentially depend on the Late Pleistocene climatic regimes. The abundance of molluscs prevailed during the cold events when sandy layers were deposited. In the Bølling warm period when massive chocolate clays were deposited the molluscs disappeared.