Structure and chronology of the Chocolate clays facies of the Lower Volga

سال انتشار: 1400
نوع سند: مقاله کنفرانسی
زبان: انگلیسی
مشاهده: 139

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شناسه ملی سند علمی:

IQA04_053

تاریخ نمایه سازی: 27 آبان 1400

چکیده مقاله:

During the Late Quaternary dramatic changes in relative sea-level are known to have occurred in theCaspian Sea. However, all previous attempts at resolving the uncertainty associated with the timing ofthese transgressive/regressive events using standard dating methods have produced inconclusive orcontroversial results. In the Lower Volga River region, Early Khvalynian marine deposits are situatedbetween the modern soil and Atelian subaerial strata (MIS ۳-۵), and are represented by readily identifiablemarine silty-clays with clear specific characteristics: fissile mudrock, in general with a well-developedstratified fabric, consisting of interbedded sub-mm laminae of silt. These laminated layers break into smallblocks, which, together with a red-brown colour, gives the material a characteristic ‘chocolate’ appearance– hence the widely-used name “Chocolate Clays”. In general, this type of facies in the Northern Caspian isfound in topographic depressions of various configurations and size, the largest of which is the LowerVolga River valley. As a result, the relative position of Chocolate Clays in the Lower Volga River sequencemay vary: in the northern part of the region, close to Volgograd, they are underlain by subaerial loess-soilsequences (Yanina et al. ۲۰۱۷) with a clear erosional contact. For the first time a reliable Late Pleistocenechronology has been derived using optically stimulated luminescence (OSL), and post-IR IRSL۲۹۰ analysisof quartz and K-feldspar grains extracted from ۲۰ sediment samples collected along the Lower Volga River.Analyses were conducted on multi-grain aliquots of the sand-sized fractions of loessic sediments, marineclays and the overlying modern soils from three exposed sections ~۵۰km downstream of Volgograd. Thesignals from all quartz samples were dominated by the fast component; there was no significant IRsensitivity, and no significant dependence of De on preheat temperature. The post-IR IRSL۲۹۰ signals fromthe ۵ samples examined also met all laboratory-based criteria for a reliable estimate of equivalent dose.Resetting of the luminescence signals was investigated based on the differential bleaching rates of quartzOSL and K-feldspar signals; we conclude that all signals were sufficiently reset prior to deposition(Kurbanov et al, ۲۰۲۱). Our results show unambiguously that the Early Khvalynian marine ChocolateClays present at all three sections were deposited post-LGM, between ~۱۳.۵ka and ~۱۷ ka ago. Theseages are further constrained by the overlying Kastanozem soils (۰.۷ to ۹.۱ ka) and underlying loess-soilseries (۱۹-۳۰ ka). Relative (Caspian) sea-level during the Early Khvalynian must have been well above thesampling altitude of ۹.۳۳ m (Srednyaya Akhtuba), ۸.۵۱ m (Raygorod) and ۶.۹۸ m (Leninsk) to explain theabsence of significant alluvial sand and to allow the deposition of the clay-sized particles of the ChocolateClays marker horizon. Using luminescence dating, we have obtained, for the first time, an unambiguousage for the Caspian Sea Early Khvalynian Chocolate Clay marker horizon. The Chocolate Clay horizon iswidespread throughout the Lower Volga River, and has previously been most recently dated using ۱۴C toboth before and after the LGM; our ages clearly place this important unit at the end of MIS ۲, after the LGM.It seems very likely that OSL has the potential to resolve many of the outstanding chronological questionsin the Ponto-Caspian basin. In particular, the young (۱۷-۱۳ ka) age for this unit, deposited by the largestLate Quaternary transgression, now provides a foundation for further research into the broader Eurasianpalaeogeographical mechanisms and implications of such an extreme sea-level rise (from about -۱۵۰ to >+۴۰ m) in just a few thousand years. Research was supported by the Russian Science Foundation, project۱۹-۷۷-۱۰۰۷۷.

نویسندگان

R Kurbanov

Institute of Geography, Russian Academy of Sciences, Moscow, Russia

A Murray

Nordic Laboratory for Luminescence Dating, Department of Geoscience, Aarhus University, DTU RisøCampus, DK-۴۰۰۰, Roskilde

T Yanina

Faculty of Geography, Lomonosov Moscow State University, Moscow