ИСТИНА

Intoduction The Ergeni Upland lies in north-west Caspian Lowland. This area divide the Volga River and the Don River basins. During Khvalynian period Ergeni bench was modified by transgression and regression. The structure and development history of Khvalynian period of the Ergeni bench have been sporadically known in the north part and most studies have been investigated in south part. This region is the classical area of the maximum Early Khvalynian transression that deposited marine sediments containing species facies chocolate clays and fauna. Methods The investigated area includes Gryaznaya, Yalmata and Arshan’-Zelmen river valleys. Generally, the thickness of Khvalynian sediments is about 6–8 m, but a thickness of about 10–20 m may be found in topographic pre-Khvalynian depressions different in origin, outlines, and size (Britsyna, 1954, Moskvitin 1962). Field study included comprehensive investigations of the Khvalynian sections. The range of applied methods included: geomorphological, lithologic-facies and malacological analyses. The altitudes were measured by GPS. Khvalynian sedimets were sampled from outcrop wall in Gryaznaya (~ 28 m a.s.l., 47°56’45.70” N, 44°37’04.71” E), in Yalmata (~ 34 m a.s.l., 47°52’34.66” N, 44°36’48.97”E), in Arshan’-Zelmen (~32 m a.s.l., 47°35’47.01” N, 44°36’00.60” E) and quarry near Tundutovo (~32 m a.s.l., 47°56’59.54” N, 44°37’25.36” E). For local correlation, Early Khvalynian sediments of Tundutovo section along with their content of chocolate clays and mollusc fauna have been used as a stratigraphic marker. First Results Four sections and their sedimentary environment have been described. Lower Khvalynian layers are found at hypsometric levels of 18 to 30 m. Sea level not to exceed to 36 m because of absence higher marine terraces. During the Early Khvalynian, this region characterized quiet depositional environment. The Khvalynian deposits in the Tundutovo, Yalmata valleys are dominated by the chocolate clays that occur usually at the center of the sequence. Chocolate clays include sand and silt interbeds with shells of index species Didacna protracta.