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Fluvial and lacustrine palaeoenvironments of the Miocene Siwalik Group, Khaur area, northern Pakistan
Author(s) -
ZALEHA MICHAEL J.
Publication year - 1997
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1997.tb01529.x
Subject(s) - geology , fluvial , crevasse , channel (broadcasting) , foreland basin , geomorphology , alluvium , group (periodic table) , levee , paleontology , overbank , structural basin , hydrology (agriculture) , geotechnical engineering , engineering , electrical engineering , chemistry , organic chemistry
ABSTRACT The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records fluvial and lacustrine environments within the Himalayan foreland basin. Thick (5 m to tens of metres) 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. Palaeohydraulic 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 ≤ 15 m, and up to 33 m for Nagri rivers. Widths of channel segments (including single channels of meandering rivers and individual channels around braid 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 segments 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 discharges 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 the largest Nagri braided 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 floods. 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. Trace fossils and body fossils within all facies indicate the former existence of terrestrial vertebrates, molluscs (bivalves and gastropods), arthropods (including insects), worms, aquatic fauna (e.g. fish, turtles, crocodiles), trees, bushes, grasses, and aquatic flora. Palaeoenvironmental reconstructions are consistent with previous palaeoclimatic interpretations of monsoonal conditions.