Sedimentary facies and soft-sediment deformation structures in the late miocene-pliocene Middle Siwalik subgroup, eastern Himalaya, Darjiling District, India

摘要

The Himalayan fold-and-thrust belt has propagated from its Tibetan hinterland to the southern foreland since ～55 Ma. The Siwalik sediments (～20 - 2 Ma) were deposited in the frontal Himalayan foreland basin and subsequently became part of the thrust belt since ～ 12 Ma. Restoration of the deformed section of the Middle Siwalik sequence reveals that the sequence is ～325 m thick. Sedimentary facies analysis of the Middle Siwalik rocks points to the deposition of the Middle Siwalik sediments in an alluvial fan setup that was affected by uplift and foreland-ward propagation of Greater and Lesser Himalayan thrusts. Soft-sediment deformation structures preserved in the Middle Siwalik sequence in the Darjiling Himalaya are interpreted to have formed by sediment liquefaction resulting from increased pore-water pressure probably due to strong seismic shaking. Soft-sediment structures such as convolute lamination, flame structures, and various kinds of deformed cross-stratification are thus recognized as palaeoseismic in origin. This is the first report of seismites from the Siwalik succession of Darjiling Himalaya which indicates just like other sectors of Siwalik foreland basin and the present-day Gangetic foreland basin that the Siwalik sediments of this sector responded to seismicity.