Sedimentary characteristics of baer knolls deposits in the volga river delta

Abstract

Baer knolls (BK) are elongated ridges often close to the sub-latitudinal orientation sometimes spatially isometric that are widespread in the entire Northern Caspian Region up to 0 m a.s.l. (the upper limit of the Late Khvalynian sea transgression). The goal of this study was to establish the genesis of BK based on interpretation of textural and lithologic characteristics of sediments and dating the material composing these landforms. Research has led to the following conclusions that BK have been formed during the transition of Late Khvalynian and Early Holocene time. Sediments of BK consists of three lithofacies (LF1, LF2, LF3). Chocolate clay (CC) and Volga alluvium were significant sources of material for knolls formation. Nonetheless, for lithofacies 1, it was also sandy material lying below the CC. The BK material cannot be attributed to the aeolian genesis because of its lithological, faunal and geochemical characteristics. The knolls formed in brackish subaquatic conditions of the lagoon floor, where a low-energy currents occurred due to the descent of Late Khvalynian basin waters through the Manych Strait. Thus, BK are analogues of river bedforms appearing as the result of turbulent flow, like ripples and river dunes, where, in concordance with the accumulation of sandy material and detritus of redeposited shells, clay particles were deposited under the mixing of the brackish water of the lagoon and the fresh water of the rivers flowing into it.