Evolution of miocene fluvial systems in the himalayan foredeep through a two kilometer-thick succession in northern Pakistan

Abstract

Sedimentological variations through a kilometer-thick coarsening-upward succession in the Miocene Himalayan foredeep basin (fluvial Chinji and Nagri Formations of the Siwalik Group) are documented in the Chinji Village area on the Potwar Plateau of northern Pakistan. From the Chinji Formation upwards into the Nagri Formation: (1) the proportion of major sandstone bodies increase relative to mudstone-dominated beds; (2) average thickness and mean grainsize of channel deposits increase; (3) sediment aggradation rates increase; (4) there is a reported change in sediment providence; (5) paleocurrents are generally to the southeast and vary little upsection; and (6) paleosols are more distinct lower in the section where there is also more evidence of ponding of water on the floodplain. Smaller-scale (100 m thick) cyclic variations in the proportion of major channel sandstone bodies also occur in both formations. In the modern Himalayan Basin, sediment dispersal away from the mountain belt is associated with: (1) major rivers that flow along the basin axis; (2) large rivers that drain substantial areas of the mountain belt and deposit low relief sediment fans extending hundreds of kilometers into the basin; and (3) smaller rivers in fan and interfan areas that drain local areas and carry finer sediment loads. Sediment variations through the Chinji-Nagri Formation boundary can be related to a shifting of depositional environments within such fan and interfan areas. Major basin-axial rivers are not exposed in this area. It appears that kilometer-thick upsection variations in these deposits record the progradation of the sediment fan formed by a large river system (river discharge on the order of 10 4 m 3/s) over the floodplain of a smaller river system (river discharge on the order of 10 3 m 3/s). Large-scale vertical sediment variations over hundreds of meters to kilometers appear to reflect changes in the basin subsidence and the rate of sediment input to the basin over time scales on the order of 10 5–10 6 years. Upsection variations can not easily be related to climatic change or eustatic variations in sea level. The most likely explanation for upsection changes is tectonism. Deposition rates and shifting positions of river systems within the basin responded more quickly to changes in basin subsidence rate, whereas increases in the proportion of sandstone bodies and increases in floodplain slopes lagged behind. Deposit variations in the Siwalik Group clearly record variations between different-scale river systems delivering sediment to the basin as well as the overall basin evolution.