Sedimentary environments in the western Kathmandu basin: Sedimentological analyses of past and present elements

The Kathmandu Basin is an amphitheatre-like basin having a centripetal drainage system originating from all its surroundings. The Paleo-Kathmandu Lake existed between the Pliocene and the Late Pleistocene as a consequence of upheaval of the southern mountains. Then the marginal sedimentation prevailed from all the directions of the lake. The fluvio-deltaic sedimentation forming typically the sandy foresets prevailed in the northern part, while alluvial fans with debris-flow sedimentation prevailed in the southern part of the basin. The western region of the Kathmandu Basin had a fan-delta system ranging from proximal to distal fan-delta sequences. The major transgression and then regression of the paleo-lake has been recorded. The scenario changed with the commencement of the Holocene, when the river drained out from the southern region of the basin and the lake disappeared. The present day depositional system constitutes fluvial environment in which river incises the valley slope and sculptures the fan-delta sequence into terraces of various levels. Such high terraces are vulnerable to streambank erosion and landsliding.