We have found that the Miocene-Pliocene Middle Siwalik succession of the Himalayan foreland basin is largely composed of coarse- to fine-grained sandstones with thin mudstone occurrences, while the Pliocene-Pleistocene Upper Siwalik succession contains medium to fine-grained dirty sandstone, and mudstone, along with conglomerates. The sand bodies show sheet geometry and the conglomerates show either sheet geometry or lenticular occurrences. The grain size variations in individual sand bodies and the gravel bodies are minimal along its length and width. Also, the conglomerates occurring as sheet bodies do not show variations in grain size in the Siwalik successions. This may be because the sedimentation was mainly controlled by the high precipitation in a monsoonal climate during Middle Siwalik (11 − 5 Ma). At this stage, the source area was low-lying and much away from the depositional sites resulting into low rate of sedimentation. Added uplift in the hinterland during Upper Siwalik sedimentation (5 Ma onwards) occurred due to reactivation of the Main Boundary Thrust. The uplift in the hinterland made the source area closer to the depositional basin resulting into high rate of sedimentation depositing conglomerates as braid-bars or fanglomerates of the megafans.
Singh et al. (Sun,) studied this question.