The Glaciolacustrine Terrace in the Izborsk-Malskaya Valley (the North-West of Russia): Composition, Age, and Origin

Studying the Izborsk-Malskaya Valley, the authors focus on the origin of glaciolacustrine terraces in overdeepened valleys widespread in the Late Valdai (Ostashkov, Weichselian) glaciation areas. In the northern part of the Izborsk-Malskaya Valley were found fragments of the terrace (56–57 m a.s.l.). They are adjacent to the lower parts of its slopes and composed of thin-laminated sands. This terrace could be referred to as the alluvial or kame type. Detailed field and laboratory investigations have made it possible to study the thin-laminated sand formation. It was accumulated in the proglacial lake during the Izborsk–Malskaya valley deglaciation. Later, sand strata were divided by water flow causing the fragments to set apart. The incised stream channel should be associated with the larger water flow rather than the modern Obdekh River. It is likely that dissection of the primary glaciolacustrine surface and fluvial erosion in the valley bottom occurred at the bottom during the proglacial lake discharge approximately 13.3 ka (OSL). The ice-dammed lake was drained relatively rapidly. For this reason, the composition and facies in the upper part of the glaciolacustrine deposits are not variable. Furthermore, additional terraces are absent in the incised stream channel. Thus, the terrace has a glaciolacustrine origin, while its slopes – fluvial erosional one. The new data on the terrace formation process in the northwestern part of the East European Plain differ from the current concepts. Along with the previously described mechanisms of glaciolacustrine terrace formation (abrasion activity of former proglacial lakes, gradual downcutting, or ice-contact slope formation of kame terraces), we should bear in mind the possibility of incision powerful water flows during catastrophic discharges of ice-dammed lakes. Our study of the terrace in the Izborsk-Malskaya Valley gives a clear indication that other similar valleys should be thoroughly examined to revise their age and mechanism of terrace formation.