Depositional origin of tuffaceous units in the Pliocene Upper Siwalik Subgroup, Jammu (India), NW Himalaya

Abstract

During late Pliocene times, extrabasinally derived acidic volcanic ashes were deposited as distal pyroclastic fallout in upland interfluvial–lacustrine settings in the northwestern part of the Siwalik basin. These ash beds occur as a bentonitized tuff band and tuffaceous mudstones in the Jammu region of India. We located and described 12 outcrops of this conspicuous, geographically widespread bentonitized tuff band and tuffaceous mudstone association at the same stratigraphic level, coincident with the Gauss–Matuyama boundary at 2.48 Ma. This bentonitized tuff band and tuffaceous mudstone association represents a stratigraphic marker horizon in the diachronous and sporadic vertebrate fauna-yielding Siwalik strata. The claystone, siltstone, and sandstone units and embedded bentonitized tuff band and tuffaceous mudstone beds represent a coarsening-upward sequence reflecting deposition in lacustrine proximal and distal bottomsets, pro-delta foresets, and mouth-bar facies capped by fluvial topsets. This study reports a chain of four contemporaneous, palaeo-lake basins at this stratigraphic level, which ranged in length from 2 to 7 km.