Calabrian (Eopleistocene) micromammal assemblages from the lacustrine and fluvial deposits of the Southern Trans-Urals and chronological position of some regional stratigraphic units

摘要

In the Southern Trans-Urals, the Quaternary deposits dated back to the Early Pleistocene (2.58-0.77 Ma) are scarce, incomplete, and often severely eroded. Data scarcity is a major challenge for accurate geological correlation and for integration of regional data into continent-wide stratigraphic frameworks. We provide a review of stratotypes and type localities for regional stratigraphic subdivisions of the Eopleistocene (the units correlated with the Calabrian stage of the Western European stratigraphical timescale, 1.8-0.77 Ma) with particular focus on small mammals and their biochronological significance. Subdivisions of the Eopleistocene which have been officially accepted in Russia include three regional chronostratigraphic units (Horizons) starting from Uvel'ka to Chumlyak, and Sarykul. We show that the Calabrian deposits are characterized in the Southern Trans-Urals by sequential changes in the evolutionary level of micromammal assemblages that could be directly correlated with the mammal-based biochronological zones MQR of Russia (Vangengeim and Tesakov, 2008; Tesakov and Titov, 2013) within the interval from MQR 10 to MQR 8. Analysis of micromammal assemblages reveals incongruent positioning of the officially accepted Uvel'ka, Chumlyak, and Sarykul' Horizons in the regional stratigraphic chart. The revealed incongruences appear to be a result of the difficulty to delineate chronostratigraphic units using the highly eroded and reworked sediments of fluvial origin. The complete cycles of alternating fluvial-lacustrine sedimentation and subaerial exposure provide the most reliable sections to establish the chronostratigraphic units of the Calabrian stage of the Early Pleistocene in the Southern Trans-Urals. The position of Chumlyak and Sarykul' Horizons in the regional stratigraphic chart of the Southern Trans-Urals is proposed to be changed. The validity of the Uvel'ka Formation as a chronostratigraphic unit should be further verified, with a particular focus on faunal proxies (mollusks, ostracods, and small mammals).