Intra- and extrabasinal controls on fluvial deposition in the Miocene Indo-Gangetic foreland basin, northern Pakistan |
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| Authors: | MICHAEL J ZALEHA |
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| Institution: | Department of Geological Sciences, Binghamton University, Binghamton, New York 13902–6000, USA |
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| Abstract: | The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records evolving fluvial systems within the Himalayan foreland basin. Sedimentological variations are evaluated with respect to local, regional, and global controls on fluvial deposition and basin filling. Thick (5 m to tens of metres) sandstones are composed of channel bar and fill deposits of low-sinuosity, meandering and braided rivers which formed large, low-gradient sediment fans (or ‘megafans'). River flow was dominantly toward the south-east. The proportion of thick sandstones varies in all Siwalik sections on three scales, and reflects similar variations in palaeochannel size and grain size: (1) small-scale variations are generally tens of metres thick, and reflect the alternation of thick sandstones (channel-belt deposits) and mudstone-dominated strata (overbank deposits) through the section; (2) medium-scale variations are roughly one-hundred to a few hundreds of metres thick, and primarily correspond to changes in channel-deposit thickness, but also to the degree of superposition of channel deposits and/or to changes in the number of channel-belt deposits per unit of section; and (3) large-scale variations (formation-scale) are greater than one km thick, and primarily correspond to changes in channel-deposit thickness. Time-scales of small-, medium-, and large-scale variations appear to be on the order of 104, 105 and 106 years, respectively. The Chinji-Nagri transition is characterized by increases in channel-deposit proportion, sandstone thickness, palaeochannel size and discharge, mean grain size of sandstones, and sediment accumulation rates; and a decrease in avulsion period. The Nagri-Dhok Pathan transition is characterized by decreases in channel-deposit proportion, sandstone thickness, palaeochannel size and discharge, mean grain size of sandstones, and avulsion period; and a further increase in sediment accumulation rates. Formation boundaries across the Potwar Plateau decrease in age toward the west. The Chinji-Nagri transition ranges in age from ～ 10·9–12·7 Ma, and the Nagri-Dhok Pathan transition ranges in age from ～9·3–10·1 Ma. Small-scale variations are attributable to repeated river avulsions triggered by autocyclic processes and/or mountain-front tectonism (e.g. faulting, earthquakes). Medium-scale variations are attributable to local changes in the position of large sediment fans, also triggered by autocyclic processes and/or mountain-front tectonism. The Chinji-Nagri transition records the diversion or establishment (possibly due to river piracy) of a larger river system in the area. River diversion or piracy probably took place within the mountain belt and is attributable to increasing and spatially variable mountain-belt uplift rates, and possibly the development of associated mountain-front deformational structures. The Nagri-Dhok Pathan transition records the diversion of the larger river system out of the area and the establishment of a smaller river system. This diversion is attributable to progressively increasing rates of mountain-belt uplift and basin subsidence. The regional palaeoclimate throughout the time interval studied was apparently constant, and eustatic sea level changes apparently had no effect on deposition in the area. |
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