GLACIALLY DAMMED LAKE DEPOSITS IN THE TANGTSE RIVER VALLEY, LADAKH, INDIA

Holocene incision rates in Transhimalayan Ladakh, near the west end of the Tibetan plateau, appear to exceed longer-term tectonic exhumation rates. One factor that may complicate matters is the storage and release of sediment from glacially dammed lakes. Field study and geologic mapping of surficial geology via remotely sensed data were used to assess lacustrine deposits in the Tangtse river valley, previously thought to have accumulated in a landslide-dammed lake. These deposits are composed of unconsolidated silt and clay interbedded with sandy turbidites, capped by coarser prograding delta sediments. Convulted soft-sediment folding in several layers is consistent with concurrent activity of the nearby Karakoram Fault. The deposits are about 25 m thick near the town of Tangtse but thicken to over 100 m downstream. Except for a few older possible shorelines up to ~4340 m, the highest shorelines and lacustrine deposits reach an elevation of ~4260 m, as much as 440 m above the present-day floodplain. This is about the same height as the spillway into the Pangong basin, suggesting that water may have been routed "upstream" onto the Tibetan Plateau, opposite previously documented drainage events out of the Pangong basin. Above 4260 m, older, now dissected alluvial and colluvial deposits appear to have aggraded to the lake highstand level. Our mapping suggests this lake was in fact glacially dammed, based on the presence of a large, upstream-dipping kame terrace, at an elevation of 4200-4320 m, near the mouth of the valley, where the Tangtse and Shyok rivers merge. Consistent with this interpretation, moraines astride the Shyok river near this confluence (at ~3570 m) can be found as high as 4500-4700 m. Geochronology was not part of the present study, but, based on published cosmogenic exposure ages, this lake was most likely active at roughly 20-25 ka. The older, higher shorelines may correlate with a 4320 m highstand documented in the Pangong basin at 35-45 ka, perhaps during an earlier glacial damming episode. It is not immediately clear, however, why established basement incision rates increase following this massive impoundment of lacustrine sediments.