Luminescence chronology of the great Khvalynian transgression from the Kosika section (Lower Volga)

At the present time the stages of the Caspian sea-level evolution during the largest Late QuaternaryKhvalynian transgression remains open. There have been attempts to establish an absolute chronologybased almost entirely on radiocarbon dating, often of individual shells. This approach is of course limitedto <۵۰ ka but in any case has given very conflicting results; as a result, no consensus view of thetransgression/regression history has evolved. In an attempt to resolve these problems, a major project(IGCP-۴۸۱) was established to re-date the more recent transgressions using radiocarbon. Unfortunately,most ages were again unexpectedly young and stratigraphically inconsistent (Yanina, ۲۰۱۴). Existingradiocarbon chronology does not allow resolving two transgressions (Early and Late Khvalynian). Basedon clear palaeontological and geomorphological evidence, these must be very different in age, but shellsassociated with both transgressions gave very scattered ages of between ۸ and ۵۰ ka (Arslanov et al, ۲۰۱۵).It is presumed that these difficulties arose because of reservoir effects and the very thin shell developed bymany fauna during cold periods. Another controversial issue is the stage between Early and LateKhvalynian – Enotaevka regression. Firstly identification at the Enotaevka section of Lower Volga wasmade in the middle of XX century and based on the description of thin subaerial strata between two marinehorizons containing Khvalynian marine fauna. Unfortunately later the particular described section was lostduring Volga erosion and construction on the coasts. Thus for the long time this important event describingdeep regression phase of the Caspian sea had no available section with During ۲۰۱۴-۲۰۱۸ fieldworks wemade an effort to find the section with clear continental sedimentation profile within two Khvalyniantransgressive series. And such deposits were identified at Kosika location, on the right valley side of theVolga River, ۴ km to the North of the Kosika village. The total thickness of s the outcrop is ۱۰ m. Theoutcrop is confined to the western base of the Baery knoll. The part of the outcrop with the clearest positionof the Enotaevka regression deposits is confined to the northern limit of the Baery knoll. Top of the sectionis represented by the upper Baery knoll strata (deflated sands), that passes through clear erosionalboundary to horizon of loess (~۹۵ cm) with clear evidence of soil-formation. Within these subaerialsediments a clear differentiation is observed in the grain-size: the upper layer (۰-۶۵ cm) is loamy (dustymedium loam), below - sandy (a silty light loam), alternating to the base of the separated stratum with asilty sandy loam. Within the subaerial part of the section, the profile of the paleosol is identified. The profileof the paleosol represented by a series of genetic horizons and reveals clear evidence of erosion.Apparently, the original profile was abraded during the Later Khvalynian transgression resulting inpreserved illuvial horizon. This strata passes into the marine clays with poor collection of shells (mostlyDreissena Sp). The applied luminescence dating allowed us to reconstruct the timing of the main stages ofthe environmental evolution during the second half of Khvalynian transgression. ۱۵ OSL-dates describesmain stages: re-worked Upper Khalyanian sands of the upper Baery knoll strata is dated ۸,۳±۰,۵ ka;paleosol formed in the Enotaevka regressive horizon dated ۱۲ – ۱۳ ka; Lower Khvalynian sediments ischaracterized with three dates ۱۹-۲۲ ka; the basis of the section (Hyrcanian horizon) is dated ۱۱۳-۱۳۳ ka.This research for the first time reviles the structure of Enotaerka regressive series of the Lower Volga anddescribes the numerical age of the main stages of the Late Quaternary environmental evolution of theregion. Research was supported by the Russian Science Foundation, project ۱۹-۷۷-۱۰۰۷۷.

Keywords:

Late Quaternary , OSL dating , Late Khvalynian transgression , Holocene , Caspian Sea