Abstract

The Siwalik Group consists of a Neogene sedimentary succession 5-8 km thick, deposited as the Himalayan foreland basin developed following collision between the Indian and Eurasian continents. It consists of fluvial deposits composed of shale, siltstone, and sandstone in the lower part and thick sandstone and conglomerate in the upper part. This paper presents new petrologic data on the sandstone composition of two coeval petrofacies of Siwalik Group in two areas: the Potwar Plateau of northwestern Pakistan, and the Surai Khola and Bakiya Khola localities of western and southeastern Nepal. In both regions, detrital modes reflect a collisional-orogen provenance. Petrologic parameters indicate derivation from mid-crustal rocks and overlying sedimentary strata ascribed to different tectonostratigraphic units of the High Himalaya and Tibetan zone. The Siwalik sandstones of the Potwar Plateau (NW Pakistan) are composed of low- to medium-grade metamorphic and sedimentary detritus, and subordinately, of ophiolitic and volcanic detritus. Abundant metamorphic detritus consists of phyllite, fine-grained schist lithics, and coarse-grained gneiss derived from the High and/or Lesser Himalaya. Sedimentary detritus is represented by abundant limestone, radiolarian chert, siltstone, shale, very fine-grained quartzarenite, and other sandstone grains. The siliciclastic grains probably were eroded from older terrigenous sequences, belonging to the Tethys Himalayan and Lesser Himalayan zones, uplifted during the India-Eurasia collision. Ophiolitic, volcanic lithic grains and radiolarian chert reflect a provenance from the Indus suture and Trans-Himalayan belt. The sandstones of western and southeastern Nepal (Surai Khola and Bakiya Khola sections) have higher quartz and lower feldspar contents than those of northwestern Pakistan and contain sedimentaclastic (western Nepal) and metamorphiclastic sandstones (southeastern Nepal). Comparison of the data presented here with previous work on pre- and post-Siwalik sandstones of the Himalaya Range and of the remnant-ocean basins along both sides of the Indian Peninsula (also derived from the Himalayan suture belt) suggests abundant quartzolithic detritus from the Eocene to the present. Varying proportions of lithic populations in time are related to rapid uplift of the Himalayas and intense unroofing of sedimentary, metasedimentary and deeper terranes.

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