Soft-sediment deformation structures in the sub-Himalayan Middle Siwalik Subgroup, Lish River section, India

Abstract

The Mio-Pliocene Middle Siwalik Subgroup, in the Lish River Section in the Darjiling District of the Eastern Himalaya, consists of sandstone, mudrocks, heterolithic units and conglomerates preserving six distinct associations of soft-sediment deformation structures at different stratigraphic levels. The foreland basin sediments of the Himalayan orogen were deposited in alluvial fan settings. Deformation structures present are: folds of variable geometry, pseudonodules, water escape features, flame structures and chaotic laminae. Liquefaction and/or fluidization of sediments in the soft state are inferred to have been responsible for the origin of these structures. Both the depositional palaeoenvironment and seismic tremors generated due to thrusting activities in the hinterland situated to the north of the then Middle Siwalik basin contributed to triggering the liquefaction/fluidization and creation of the softsediment deformation structures within the Middle Siwalik sediments.