NEW EVIDENCE ON THE AGE AND FORMATION CONDITIONS OF PLEISTOCENE DEPOSITS IN THE EASTERN PART OF THE TUNKA DEPRESSION
F.E. MAKSIMOV1, L.A. SAVELYEVA1, S.A. LAUKHIN2, A.Yu. PETROV1, S.S. POPOVA3, S.I. LARIN4, D.V. KOBYLKIN5, V.Yu. KUZNETSOV1,6, V.A. GRIGORYEV1, S.B. LEVCHENKO1, K.S. YAKIMOVA7
1St. Petersburg State University, 199034, St. Petersburg, Universitetskaya Naberezhnaya, 7-9, Russia
maksimov-fedor@yandex.ru
2Russian State Geological Prospecting University, 117997, Moscow, ul. Miklukho-Maklaya, 23, Russia
valvolgina@mail.ru
3Komarov Botanical Institute, 197376, St. Petersburg, ul. Professora Popova, 2, Russia
celenkova@gmail.com
4Earth Cryosphere Institute, Tyumen Scientific Center, Siberian Branch, Russian Academy of Sciences, 625026, Tyumen, ul. Malygina, 86, Russia
silarin@yandex.ru
5V.B. Sochava Institute of Geography, Siberian Branch, Russian Academy of Sciences, 664033, Irkutsk, ul. Ulan-Batorskaya, 1, Russia
agrembrandt@inbox.ru
6Herzen State Pedagogical University of Russia, 191186, St. Petersburg, Naberezhnaya reki Moiki, 48, Russia
v_kuzya@mail.ru
7Shirshov Institute of Oceanology, Russian Academy of Sciences, 117997, Moskow, Nahimovskiy pr., 36, Russia
yakikarina@gmail.com
Keywords: четвертичные отложения, впадина байкальского типа, геохронология, уран-ториевый метод, радиоуглеродный метод, палеоботанический метод, Quaternary deposits, depression of the Baikal type, geochronology, uranium-thorium method, radiocarbon method, paleobotanical method
Abstract
This article presents the results of a comprehensive geochronological study of organic-rich sediments lying at the base of the sand suite in the well-known reference Belyi Yar II section on the left bank of the Irkut river (Tunka depression, Southwestern Cisbaikalia). It was found that in the 1960s, a pack of lake-marsh sediments from the lower part of the section was assigned to the upper Eo-Pleistocene, and the overlying sands to the Samarovo period of the Middle Pleistocene. Later, based on the first 14 C datings, the stratigraphy of these sediments was revised. The lower part of the sediments was attributed to the Kargian time, and the overlying sands to the Sartanian glaciation. However, the questions concerning the genesis, age and conditions of forma tion of these sediments remain controversial. In particular, the individual dates of 14 C differ greatly different from one another; therefore, they require further verification. A renewed study of the lower part of the Belyi Yar II section discovered peat and gyttjas deposits of alluvial origin. It was shown that their numerical age is beyond the 14 C dating method limits. Paleobotanical data showed that the gittya was formed in conditions of a relatively warm and humid climate, and the overlying layers accumu lated in the cooling and more xerophytic condition. Evaluation of the 230 Th/U isochronous age of these organogenic deposits, combined with palynological data, suggested that the bottom of the sand layer in the Belyi Yar II section was not formed during the Kargian time (MIS 3), as was thought previously, but during MIS 5. The gittya could have formed during MIS-5c, where as the overlying peat layers formed, possibly, at the transition to MIS-5b.
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