New evidence on the structure and age of the Bestyakh Terrace of the Lena River (Ust-Buotama Outcrop)

The Lena River provides important records for understanding the Quaternary history of North-East Siberia. At present, the structure, origin and age of the elements of the Lena River valley remain unresolved. This article presents the results of lithofacies analysis and absolute dating of the Ust-Buotama section exposing the Fourth (Bestyakh) fill Terrace in the middle Lena River valley. Three stratigraphic units have been recognized in the section: lacustrine–alluvial deposits at depth of 120–85 m depth from the surface correlated with the Middle Pleistocene Mavra formation of Central Yakutia; eolian deposits of the Late Pleistocene D’olkuma formation (depth 85–23 m), and eolian deposits of the Holocene wind-blown dunes (from the depth of 23 m to the surface). First quartz and K-feldspar ages have been obtained for the section using luminescence dating. The age relations and standard tests have shown the reliability of the chronology obtained. This chronology suggests that sediments of the Mavra formation were deposited no later than 300 ka, and their stratigraphic position implies a preliminary correlation with the Tobolsk time of the Middle Pleistocene (MIS 11-9). Deposition of the D’olkuma formation took place from late MIS 3 (29–30 ka) to the late MIS 2 (14–15 ka), reflecting the period of eolian activity when sand dunes and sheets were formed. The periods of eolian accumulation alternated with deflation periods at the end of the Late Pleistocene. Short periods of stabilization of the eolian landscape are indicated in the section by poorly developed paleosoils. The uppermost part of the section consists of the Late Holocene dune sediments which accumulation started ~400 years ago during the Little Ice Age. These findings infer that the Bestyakh Terrace is not a fluvial terrace in the classic sense, but rather the remaining part of a complex deflational and depositional plain. Much of the terrace sediments appear to have been formed under subaerial conditions in cold, dry environments of the late Pleistocene.