FLUVIAL AND LACUSTRINE PALEOENVIRONMENTS OF THE MIOCENE SIWALIK GROUP, KHAUR AREA, NORTHERN PAKISTAN

The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Path an Formations) in northern Pakistan records fluvial and lacustrine env ironments within the Himalayan foreland basin. Thick (5 m to tens of m etres) sandstones are composed of channel bar and fill deposits of low -sinuousity (1.08-1.19), single-channel meandering and braided rivers which formed large, low-gradient sediment fans (or 'megafans'). River flow was dominantly toward the south-east and likely perennial. Palaeo hydraulic reconstructions indicate that Chinji and Dhok Pathan rivers were small relative to Nagri rivers. Bankfull channel depths of Chinji and Dhok Pathan rivers were generally less than or equal to 15 m, and up to 33 m for Nagri rivers. Widths of channel segments (including si ngle channels of meandering rivers and individual channels around brai d bars) were 320-710 m for Chinji rivers, 320-1050 m for Nagri rivers, and 270-340 m for Dhok Pathan rivers. Mean channel bed slopes were on the order of 0.000056-0.00011. Bankfull discharges of channel segment s for Chinji and Dhok Pathan rivers were generally 700-800 m(3)s(-1), with full river discharges possibly up to 2400 m(3)s(-1) Bankfull disc harges of channel segments for Nagri rivers were generally 1800-3500 m (3)s(-1), with discharges of some larger channel segments possibly on the order of 9000-32 000 m(3)s(-1). Full river discharges of some of t he largest Nagribraided rivers may have been twice these values. Thin (decimetres to a few metres) sandstones represent deposits of levees, crevasse channels and splays, floodplain channels, and large sheet flo ods. Laminated mudstones represent floodplain and lacustrine deposits. Lakes were both perennial and short-lived, and likely less than 10 m deep with maximum fetches on the order of a few tens of kilometres. Tr ace fossils and body fossils within all facies indicate the former exi stence of terrestrial vertebrates, molluscs (bivalves and gastropods), arthropods (including insects), worms, aquatic fauna (e.g, fish, turt les, crocodiles), trees, bushes, grasses, and aquatic flora. Palaeoenv ironmental reconstructions are consistent with previous palaeoclimatic interpretations of monsoonal conditions.