The Lower Siwalik foreland basin sedimentation in the Kumaun Himalaya (India): A transition from megafan setting to incised valley fills

Abstract

About 300–900 m thick the Lower Siwalik Subgroup exposed in the Kumaun Himalaya has been studied along three far spaced sections, which show variable stratigraphic changes in sedimentation pattern. Available magnetostratigraphic dates in the Kathgodam–Ranibagh section indicate Late Miocene age (12–10Ma) of the Lower Siwalik Subgroup. Detailed lithofacies analysis reveals four depositional settings namely, the Sandstone Lithofacies Association (Channel deposits), the Mudstone Lithofacies Association (Floodplain deposits), the Mottled Mudstone Lithofacies Association (Distal floodplain palaeosols), and the Mottled Siltstone Lithofacies Association (Interfluve deposits). Based on available data on channel pattern, lithofacies association, palaeocurrent pattern, and petrography, stratigraphic cyclicity is analyzed. The complete group is interpreted to be deposited in two Megacycles A and B. About 500 m thick, coarsening upward Megacycle A is internally made up of two Lithofacies sucessions I and II. The cycle is made up of meandering and anastomosing rivers towards the base (Lithofacies Succession I) that gradually evolves into braided rivers towards top (Lithofacies Succession II). This megacycle is inferred as prograding megafan deposit.In contrast, about 400 m thick Megacycle B, made up of two Lithofacies successions III and IV, is fining upward. This cycle is made up of thickly developed meandering – braided river systems at base interbedded with thick interfluve and distal flood plain deposits (Lithofacies Succession III) that gradually becomes thinner towards top, and river system changes to predominantly meandering type (Lithofacies Succession IV). The Megacycle B is inferred to be deposited in tectonically active basin, characterized by incision of rivers.