Changes in the depositional system of the Paleo-Kathmandu Lake caused by uplift of the Nepal Lesser Himalayas

Sedimentological and palynological studies on a series of slimes taken from a drill-well in the central part of the Kathmandu Basin and the Lukundol Formation at the southern margin of the basin indicate that the depositional environments of the Paleo-Kathmandu Lake changed at around 1 Ma. In the central part of the basin, the abrupt appearance of a fossiliferous 4 m thick sand bed, containing abundant fish teeth and gastropod opercula, and shell fragments, in an otherwise open-lacustrine mud sequence, suggests that a lowering of the water level occurred at about 1 Ma. The common occurrence of the green alga Pediastrum in the overlying mud beds implies that the lake remained shallow after the deposition of the sand bed. Changes in the depositional system of the Paleo-Kathmandu Lake at about 1 Ma are also recorded in the Lukundol Formation. Granitic gravel and detrital muscovite flakes, which are common in the Lower and Middle Members, disappear from the Upper Member. Paleocurrent directions in the Lower and Middle Members show flow from the north and east, whilst in the Upper Member they change to flow from the south. Sedimentary facies change from marginal lacustrine in the Middle Member, to a braided river facies in the Upper Member. These changes occurred at around 1 Ma, at the base of the Upper Member. They seem to have been caused by the initiation of rapid uplift of the Mahabharat Lekh, which was due to faulting and underthrusting along the Main Boundary Thrust System.