THE LATE PLEISTOCENE PALEOESTUARY LITHOFACIES OF THE EARLY KHVALYNIAN BASIN (THE MIDDLE VOLGA RIVER VALLEY, RUSSIA)

The Early Khvalynian basin was one of the largest transgression events in the history of the Caspian Sea during the Pleistocene when the sea level of the basin reached +45-50 m asl and occupied an area of 872 000 km² (Svitoch, 2014). Transgression waters formed up to 700 km long and 30-40 km wide paleoestuary within the Lower and Middle Volga River valley. During this event unique lithofacies presented by dark brown chocolate clay were deposited. According to radiocarbon and OSL dating the age of deposition of chocolate clay is between 25-13 ka. During the fieldwork lower khvalynian deposits in the outcrops and boreholes of the Middle Volga River valley were studied. Four main lithofacies were identified basis on sedimentary structures, visual characteristics, colours, grain-size. Lithofacies 1 - Laminated chocolate clay is presented in most studied sections in the Middle Volga River valley. Lithofacies are presented by an alternation of chocolate-reddish clay 5-10 cm thick with 1 to 5 cm thick of light yellow-brown silt and grey-yellow fine-grained sand. The thickness of deposits varies from 20 cm to 1.5 m. The upper part of chocolate clay was affected by cryogenic processes and can be identified by 10 to 50 cm thick paleoschlieren structure. The upper part of lithofacies is often affected by reddish oxidation. Lithofacies 2 - Massive chocolate clay composed of 30 cm to 4 m thick homogeneous dark brown faintly laminated clay usually with vertical fissures. This event is marked as one of the largest stages of clay deposition in the Caspian Sea during the Late Pleistocene. Lithofacies 3 - Cross-laminated sand consists of 10 to 30 cm thick brown-yellow and grey-yellow fine to coarse-grained sand with thin 0.2-1 cm sand layers that are affected by iron oxidation. The sand layer in the lower part consists of 5-10 cm thick combined flow ripples. Lithofacies 4 - Well-laminated grey and brown clay composed of 0.5 to 3 m thick alternation of olive-grey and light-brown clay and silt. Olive-grey clay layer is usually 20-50 cm thick and contains thin 0.5-1 mm thick silt laminae that are similar to lacustrine deposits with varves. Light-brown clay is characterized by alternation with 1-2 cm thick silt layers. The main contribution of water supply and fine material to the paleoestuary of the Early Khvalynian basin depended on the regime of the Volga River drainage basin during the Late Pleistocene climatic fluctuations. This research was supported by a grant from the Russian Foundation for Basic Research (№ 20-05-00608).