Discovery of Sediments Indicating Rapid Lake-level Fall in the Late Pleistocene Gokarna Formation, Kathmandu Valley, Nepal : Implication for Lake Terrace Formation

摘要

Sediments indicating a rapid fall in lake level have been discovered in the late Pleistocene Gokarna Formation, Kathmandu Valley, Nepal. The indicators are observed along a widely traceable erosional surface in this formation, and is characterized by (1) gently inclined (ca. 10°) tabular cross-stratified sand beds of delta front deposits consisting of coarser material and showing gradual decrease in elevation of its top to the progradation direction, (2) an antidune cross-laminated sand bed that interfingers with the delta front deposit, and (3) an approximately 5 m-deep erosional depression filled with convolute laminated sand beds and recognized at a location distal to that where deposits (1) and (2) were found. The early phase of rapid lake level fall caused minor erosion of the delta plain deposits by fluvial processes, introducing a higher rate of progradation of the delta front and resulting in the accumulation of deposit (1). The delta emerged as dry land due to further lowering of the lake level. The antidune cross-laminated sand bed shows evidence of having accumulated from a high-velocity stream that may have formed as the lake water drained from the delta front during the lowering of lake level. When the lake level fell below the level of the topographic high created by delta accumulation prior to the fall, incised valleys may have formed and part of them may have been filled with sediment at that time. The rapid fall in lake level is interpreted to have been the result of lake-wall failure, which would have occurred at the gorge outlet as the only discharge path for the basin. The initial rise of lake level causing accumulation of terrace sediments may have been due to the formation of a plug at this outlet, attributable to mass movement along the gorge.