Coalescence megafan: multistorey sandstone complex of the late-orogenic (Mio-Pliocene) sub-Himalayan belt, Dehra Dun, India

Abstract

In the Himalayan foreland basin, a prominent multistorey sandstone complex, in the Middle Siwalik Subgroup, is exposed along the southern flank of the Dehra Dun valley in the Mohand anticline. Extending laterally for ∼ 40 km, the Middle Siwalik Subgroup consists of sandstone-mudstone couplets (300–450 m thick) in the lower part and the multistorey sandstone complex (900–1200 m thick) in the upper which forms a gross coarsening upward sequence. The major lithofacies are trough cross-stratified sandstone, planar cross-stratified sandstone, ripple drift laminated sandstone, horizontal laminated sandstone and massive sandstone with subordinate amounts (<20%) of mudstone and conglomerate. The multistorey sandstone complex is punctuated by several erosional surfaces over which abundant mud clasts are present. The trough cross-stratification exhibits a dominantly southward palaeoflow with much variation in the southeast and southwest quadrants. However, at outcrop level the palaeoflow data reveal internal consistency. This spatial and temporal variability in palaeoflow direction indicates the existence of two major drainage systems of coalescing alluvial megafan type. The estimated palaeochannel sinuosity is of the order of 1.04 to 1.23 with a return period of 4 to 8 × 103 years.Palaeoflow variability, facies distribution, frequent avulsion and the coarsening upward sequence collectively suggest the progradation of coalescing alluvial fans towards the south and southwest. This was principally controlled by thrust movements and differential basin subsidence at the margin of the Himalayan orogen.