Paleochannel and paleohydrology of Middle Siwalik (Pliocene) fluvial system in Northern India – A paper in Journal of Earth System Science

This paper in Journal of Earth System Science deals with paleochannel and paleohydrology of the Middle Siwalik (Pliocene) fluvial system in Northern India. In recent years, fluvial sedimentologists have carried out numerous studies to estimate quantitative hydrodynamics of ancient fluvial systems, particularly, their morphology and hydrology. The methods for reconstructing the morphology and hydrology employ either several groups of empirical relationships derived from modern fluvial environments or dynamic models of fluvial flow and sediment transports in alluvial reaches.

Late Cenozoic fresh water molasses sediments (+6000 m thick) deposited all along the length of the Himalayan foreland basin, in northwestern Himalayas form the Siwalik Supergroup. The paper reports the results of the paleodrainage and paleohydrology of the Middle Siwalik subgroup of rocks, deposited in non-marine basins adjacent to a rising mountain chain during Pliocene. Well-exposed sections of these rocks have provided adequate paleodrainage data for the reconstruction of paleochannel morphology and paleohydrological attributes of the Pliocene fluvial system.

Cross-bedding data has been used as inputs to estimate bankfull channel depth and channel sinuosity of Pliocene rivers. Various empirical relationships of modern rivers were used to estimate other paleohydrological attributes such as channel width, sediment load parameter, annual discharge, and channel slope and flow velocity. Computed channel depth, channel slope and flow velocity are supported independently by recorded data of scour depth, cross-bedding variability and Chezy’s equation.

The estimates indicate that the –

• Middle Siwalik sequence corresponds to a system of rivers, whose individual channels were about 400 m wide and 5.2–7.3 m deep; the river on an average had a low sinuous channel and flowed over a depositional surface sloping at the rate of 53 cm/km.
• The 700-km long Middle Siwalik (Pliocene) river drained an area of 42925 sqkm to the north–northeast, with a flow velocity of 164–284 cm/s, as it flowed generally south–southwest of the Himalayan Orogen.
• Bed-load was about 15 per cent of the total load of this river, whose annual discharge was about 346–1170 cum/s normally and rose to approximately 1854 cum/s during periodic floods.
• The Froude number of 0.22 suggests that the water flows in the Pliocene river channels were tranquil, which in turn account for the profuse development of cross-bedded units in the sandstone.
• The estimated paleochannel parameters, bedding characteristics and the abundance of coarse clastics in the lithic fill are rather similar to the modern braided rivers of Canada and India such as South Saskatchewan and Gomti, respectively.

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