To the Issue of the Age of the Terrace Complexes of the Chuya and Katun Rivers, Russian Altai: Should the Sartan Cryochron Be Excluded from the Epochs of Ice-Dammed Lakes and Cataclysmic Outburst Floods?

The upcoming edition of new stratigraphic scheme of Quaternary deposits of the Altai–Sayan mountain province is based on the concept of cataclysmic draining of the Pleistocene ice-dammed lakes that occurred prior to 90 ka (MIS-5 and earlier). Our geochronological (OSL, 14 C) and paleontological data argue against excluding the Sartansky cryochron (MIS-2) from the epochs of the Altai glaciogenic fluvial catastrophes. The last cataclysmically drained ice-dammed lake in the Chuya Depression existed after 37 ka. The Kurai Depression during MIS-2 was filled with water already by 25 ka and was drained at ~19–16 ka. Draining of the last ice-dammed lake in the Kurai Basin correlates with the onset of the formation of the section of catafluvial-lacustrine cyclites in the Inya River valley (a right tributary of the Katun River) at 17–16 ka (MIS-2). Previously, this section was suggested as a parastratotype of the Saldzhar catafluvial deposits accumulated during MIS-5 in the new edition of the Altai–Sayan stratigraphic scheme. OSL dates (16–12 ka) were obtained for deposits at the margin of the 70-m “Saldzhar” terrace of the Katun River upstream of the mouth of the Chuya River. The paleontological findings of lacustrine fauna in situ allow us to reconsider the concept of the exclusively catafluvial genesis of the Saldzhar deposits of the 60-m terrace of the Katun River, near the mouth of its left tributary, the Sema River. The OSL date (32 ka) of deposits at the base of this terrace indicates the onset of its formation not earlier than the end of MIS-3. The section proposed as the stratotype of the post-Saldzhar Maly Yaloman alluvium with the age 90 ka does not meet the requirements of the Stratigraphic Code, because the contact with the Saldzhar deposits has not been revealed here and genesis of deposits is controversial. At present, 14 С and OSL dates of the Saldzhar sequence and post-Saldzhar alluvium form two non-overlapping clusters, older than 90 and 35–12 ka, which can be connected both with methodological features of age dating and with insufficient geological knowledge. Only one of the eight IRSL and OSL dates obtained for Inya sequence deposits, which compose high (up to 300 m) bars in the Chuya and Katun river valleys, falls within the MIS-6 time, which is insufficient to establish its age as MIS-6 and older. To make correct stratigraphic reconstructions, it is necessary not only to expand a dataset obtained by different geochronological methods but also t