GLACIATION OF DEBRIS FLOW VALLEY, HOH SERH RANGE, MONGOLIAN ALTAI

Although the Mongolian Altai is the largest glaciated region in Mongolia, this area lacks well-studied paleoenvironmental proxy records of environmental change. Numerous glacial deposits are found in the region, including those in Debris Flow Valley (47°48'N, 90°59'E), located roughly 50 km northeast of the Mongolian-Chinese border. The most extensive glaciation in this valley occurred during the Last Glacial Maximum (LGM) based on moraine morphology and poor soil development on till. Cosmogenic ¹⁰Be dates from granitic erratics in nearby valleys will help to better determine age. Glacier movement in this area is unusual in that ice flowed south along a shallow trough parallel to the range front before splitting into southeastern and western lobes on opposite sides of the range crest. A large moraine was deposited at an elevation of 3110 m where the ice split. Terminal moraines are located at 2820 m and 2480 m in the southeastern and western valley, respectively. The wide variation in terminal moraine elevations is likely due to differences in aspect, slope, and valley morphology. Approximately 30 individual recessional moraines were deposited in Debris Flow Valley. Equilibrium Line Altitudes (ELAs) for the LGM, calculated using Accumulation-Area Ratio Method and Toe-to-Summit Altitude Method, range between 3140 m and 3370 m and are consistent with data from nearby valleys. Striations on bedrock indicate that the ice was at least partly warm based. A debris flow deposit roughly 3 km² in area extends 1.5 km west of the terminal moraine onto the floor of Deluun Valley. Above this, the terminal moraine and the oldest recessional moraine have been breached. Large, ripple-like landforms suggest catastrophic flooding when a lake behind the recessional moraine overtopped its dam and rushed westward depositing the huge debris flow. Reconstructing the glacial record in central Asia is important not only for understanding paleoenvironmental records, but also for assessing the sustainability of local communities dependent on glacial meltwater.