Forced regressive shoreface sandstone from Himalayan foreland: implications to early Himalayan tectonic evolution

Abstract

Sedimentology and sequence stratigraphic analysis of the ~ 31 Ma old marker White sandstone unit from the Subathu Sub-basin, NW Himalayan foreland, suggest it to be a forced regressive wedge (FRW) formed during the transition from the marine Subathu Formation to the continental Dagshai Formation. The FRW is bounded between the "Surf diastem" below and type 1 unconformity at the top and differs from RSME (regressive surface of marine erosion, occurring below) bounded FRWs described from other classical coastal/foreland settings. Correct identification of bounding surfaces of a FRW has an important implication to the estimation of rate of relative sea-level (RSL) fall. A faster rate of RSL fall, higher than the sedimentation rate, has been postulated for the erosion of the lower shoreface and RSME. Using the logged thickness of the Subathu/Dagshai transition zone including the White sandstone (bounded between the "Surf diastem" and unconformity), available chronology and eustatic sea-level fall (0.023 mm/year at 31 Ma), a higher RSL fall than the sedimentation rate (0.07 mm/year) has been inferred during the deposition of the White sandstone. Petrography of sandstones and their Sr and Nd isotopic compositions indicate a major provenance switch-over from dominant mafic/ultramafic to metamorphic source from White sandstone (~ 31 Ma) onwards attesting the link between hinterland tectonics, provenance and forced regression. The provenance switch-over at 31 Ma was earlier inferred to be driven by proto-Himalayan thrust propagation in the foreland. Using a simple isostatic model, on the contrary, a mechanism of accelerated surface uplift (at a rate of > 0.10-0.15 mm/year) is suggested for both provenance change and forced regression.