Intra‐ and extrabasinal controls on fluvial deposition in the Miocene Indo‐Gangetic foreland basin, northern Pakistan

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 lo4, lo5 and lo6 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 z 10.9-12.7 Ma, and the Nagri-Dhok Pathan transition ranges in age from z 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 0 1997 International Association of Sedimentologists 369 370 M. J. Zaleha 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...

The structure and rate of late Miocene expansion of C4 plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan

“…In the Potwar area, the chronostratigraphy is based on paleomagnetism, supplemented by isotopic dating of rare volcanic ash beds (Johnson et al, 1982a(Johnson et al, , 1982b(Johnson et al, , 1985. The formations are characterized by different proportions of sand versus mudstone, refl ecting shifts through time in the geographic position of largeto small-scale rivers and their alluvial deposits as well as changes in foreland basin subsidence and sediment input (Burbank and Beck, 1991;Willis, 1993aWillis, , 1993bKhan et al, 1986Khan et al, , 1997Zaleha, 1997). Studies of the fl uvial sedimentology of the Miocene through Pleistocene deposits document the changing infl uence of mountain (Himalaya)-sourced ("blue-gray") versus lowland-sourced ("buff") rivers (Raynolds, 1981;Behrensmeyer and Tauxe 1982;Burbank and Beck, 1991;Barry et al, 2002).…”

Ecological changes in Miocene mammalian record show impact of prolonged climatic forcing

“…The paleoenvironmental record of this study, inferred from sedimentary facies, paleosol features, and stable isotopes of carbon and oxygen in pedogenic carbonates and mammal teeth, documents substrates, vegetation, and climate over 5 myr. The Miocene flood basin was traversed by large rivers-on the scale of major tributaries to the modern Indus and Ganges-and smaller flood plain channels (18)(19)(20). Paleosol sequences of variable maturity indicate the presence of forest, woodland, and grassland vegetation that was stable for decades to thousands of years (12,14).…”

Middle Miocene pedological record of monsoonal climate from NW Himalaya (Jammu & Kashmir State), India

“…The wet and humid climate is attributed to initiation of a monsoonal system in Asia caused by uplift of the Tibetan Plateau/Himalaya. Sedimentary and petrographic evidence indicates an increase in precipitation in the Himalayan region at 10.5 Ma (Kumar et al, 2003;Metivier et al, 1999;Zaleha, 1997;Ghosh and Kumar, 2000) due to major uplift. Carbon isotope studies of palaeosol from elsewhere (Kangra valley, India) suggest that between 10.5 and 6 Ma, the floodplain was dominated by C 3 plants (Sanyal et al, 2004) reflecting the intensification of monsoons.…”