Abstract:
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.
