New Data on the Age of the Early Khvalynian Transgression of the Caspian Sea

Abstract

This study presents results of the first detailed absolute chronology of the Early Khvalynian transgression of the Caspian Sea based on optically stimulated luminescence (OSL) dating. Study was carried out for special facies of the Khvalyninan deposits–Chocolate Clays, widely distributed in the Northern Caspian Lowland. In the studied reference sections of the Lower Volga region (Srednyaya Akhtuba, Raygorod, Leninsk), Chocolate Clays are of considerable thickness, three subfacies are distinguished in their structure: a unit of interbedded sands and loams; unit of unified dense clays; and a unit of characteristic clays with thin interlayers of silt, forming a characteristic fine-platy structure. The first unit reflects the conditions of the initial penetration of the Khvalynian waters into the studied area, the second unit reflects conditions of a deep-sea bay, while the third unit reflects the time of regression, with a significant influence of alluvial processes on sedimentation. OSL chronology was obtained for both quartz and feldspar grains using modern measuring protocols. Our dating results clearly indicate that formation of the Lower Khvalynian marine deposits in the northern part of the Lower Volga region took place between 27 and 14 ka. New data on the age of the Khvalynian deposits are further confirmed by dating of the overlying Kastanozem soils (9.6–0.7 ka) and the underlying loess-paleosol series (37–19 ka). High-resolution luminescence dating for the first time distinguished four stages in the development of the Khvalynian transgression in the Lower Volga region: the initial penetration of waters to a level of ~5 m (27 ka); stage of stabilization and gradual level rise from ~5 to ~7 m (27–20 ka); the stage of active sea level rise and movement of the coastline northward (16–15 ka); and the final regressive phase with a drop in sea level (15–14 ka).