Sedimentology of the Middle Siwalik Subgroup of Mohand Area, Dehra Dun Valley, India

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Abstract
The Middle Siwalik Subgroup of Mohand area (1800 m thick) is divided into three units: (1) sandstone-mudstone interval (340 m), (2) thickly bedded multi storied sandstone (1060 m), and (3) conglomerate-sandstone-mudstone interval (400 m). Three major facies are recognised, namely: conglomerate, sandstone, and mudstone facies. Sandstone is the dominant litho-facies and displays frequent erosional surfaces along which mud-clasts are present. The common sedimentary structure is trough and planar cross-stratification. Palaeo-flow measurements of trough cross-stratification show prominent modes in SE and NW directions. However, planar cross-stratification shows a high order of deviation in mean vector azimuth from trough cross-stratification. The fining-up in grain-size from erosional surface (Se facies) to mudstone (Fm facies) through trough and planar cross-stratified sandstone (St-Sp facies) and ripple drift laminated sandstone (Sr facies) has been recognised. The mudstone intervals are almost negligible, while mud-clasts and suspension load in the sandstone (channel facies) are high (to to 25%). The absence of mudstone unit in multi storied sandstone complex may be attributed to the extensive and frequent avulsion on the large braided alluvial fan. This study reveals that sedimentation pattern of multi storied complex is similar to that of modern radial fan of Kosi river.

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