Siwaliks, an introduction

Siwaliks is indicated by the yellow line in the following map (map from Chauhan, P.R.):

• The Siwaliks are made up of mudstones, siltstones, sandstones, and conglomerates. They were deposited in a variety of fluvial environments including piedmonts, outwash plains, channels, floodplains, and oxbow lakes.

• They comprise a fining-upwards sequence on the scale of individual cycles but a coarsening-upwards succession as a whole.
• The deposits are frequently divided into the Lower, Middle, and Upper Siwaliks ranging in age from Middle Miocene to Early Pleistocene. Such lithological subdivisions are diachronous and the equivalent units become successively younger from the northwest towards the southeast.
• The thickness of a given lithostratigraphic unit of the Siwaliks may change significantly in its lateral direction (i.e., within a few tens of kilometres). It is noticed mainly in the Middle and Upper Siwaliks (e.g., in the Arung Khola area, Kamala River basin, and the Sahajpur–Godavari area).
• The Upper Siwaliks exhibit disconformable contacts in a number of places. In a few locations, they are transgressive over the Middle and Lower Siwaliks indicating past prodigious tectonic movements.
• The Siwaliks are very incompetent (soft) and have buckled into noncylindrical plunging folds with curved hinges in map view (i.e., concave or convex towards the foreland) implying some amount of shortening in the direction tangential to the fold belt.
• There are areas with overturned Siwalik strata. Such beds are sometimes found just at the boundary with the Terai plain.
• The Siwalik beds are frequently truncated by north-dipping as well as south-dipping thrust faults. Like the folds, the thrusts too are convex or concave towards the foreland. There are also some tear faults disrupting these thrusts in the direction essentially perpendicular to their strike.
• Some intermontane valleys are located within the Siwalik belt.
• There are a few Pre-Siwalik outliers in central and east Nepal.
• Contrary to a general belief, there is no single and continuous Himalayan Frontal Fault or Main Frontal Thrust  outcropping throughout the Nepal Himalaya and beyond; in a number of locations the Siwalik folds abruptly disappear in the Terai plain