QUATERNARY GEOLOGY OF THE TENGIS-SHISHID GOL REGION, KHOVSGOL, MONGOLIA

Last Glacial Maximum glaciers of the Tengis Gol, a north-south drainage of the Sayan Range in northern Mongolia, advanced far enough south to block the Shishid Gol. Terminal moraines from the Tengis glacier block two valleys on the south side of the Shishid Gol. The upper limit of ice along the lower Tengis Gol is established by maximum elevations of lateral moraines, erratics, and meltwater channels. Near the confluence of the two rivers, lateral moraines are perched at up to 250 m above the 1550-m valley floor. Granitic, greenstone, and limestone erratics are perched on late Cenozoic intracanyon basalt flow remnants on both sides of the Shishid Gol. The Shishid Gol drains the Darhad depression, a 3500-km2 basin and the site of the modern Tsagaan Nuur. Evidence for a Pleistocene lake in the Darhad depression includes shorelines 160 m above the valley floor, lakebeds, and possible ice rafted boulders. The Tengis glacier may have been responsible for damming this Pleistocene lake: shorelines are evident in the Shishid Gol east of the Tengis Gol, but were not observed west of the Tengis Gol. An ice dam of this size could have failed catastrophically, producing large-scale outburst floods. Cosmogenic-isotope ages on moraine boulders and AMS dates on bivalves in lake sediments just east of the Tengis Gol may help determine the age of the glaciation and of the lake.