Soft sediments deformation structures: Implication for draining of Paleo-Kathmandu Lake

Abstract

Kathmandu basin is one of the Quaternary intermontane basins in the central Nepal Himalaya. It is bounded by several faults on both southern and northern margins. The basin is filled with Plio-Pliestocene terrestrial sediments and their characteristics indicate four types of unconsolidated successions within the basin. These are before lake succession, during lake succession, draining stage lake succession and fluvial succession. Late Pleistocene aged Sunakothi Formation crops out along the southern part of the basin. It is a typical fluviolacustrine delta succession that extended from 1390 m in the southern margin to nearly 1300 m toward center of the basin. It is composed by poorly consolidated sand, gravelly sand, silt and mud beds. Various soft-sediment deformation structures occur in the formation, especially in fine- to medium –grained sands, silts and mud: load structures, flame structures, clastic dikes (sand dike), disturbed layers, convolute beds, slumps and synsedimentary faulting. The deformation mechanism and driving force for the soft-sediment deformation are related, essentially, to gravitational instability, dewatering, liquefaction and brittle deformation. Field data and the wide lateral extent of the structures as well as regional geological data show that most of the deformation is related to seismicity and the structures are interpreted as seismites. In addition, there have also been experimental studies undertaken by various authors within the different sedimentary basin.

Soft-sediments deformation structure in Kathmandu basin are mainly considered to be part of the initial diagenetic changes of the sediments and include: Slump structure which occurred on the slope like delta-front area, dewatering structures which occurred by the processes of upward escape of water commonly due to loading, load structures which occurred due to density contrasts between sand and underlying wet mud. The existence of seismites in the Sunakothi Formation is evidence of continuing tectonic activity in the study area during the late Pleistocene and is a main factor for draining of the Paleo- Kathmandu lake water.