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 faunayielding 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.

Occurrence of a Likely Tuff Bed between the Middle and Upper Siwaliks, Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan

“…As the Himalayas have no reported syn-and lateorogenic volcanism, there should be an outside source of volcanism to contribute volcanic ash to Siwaliks (Johnson et al, 1982). It seems that volcanic eruptions of Dacht-e-Nawar complex in Afghanistan may have contributed ashfall deposits to Upper Siwaliks in Potwar plateau and Kashmir basin (Johnson et al, 1982;Bhat et al, 2008). This is also indicated by similar fission track ages of 2.86 m.y.…”

“…Thus, this tuffaceous deposit seems to correlate with Taunsa ash bed both mineralogically and compositionally. Based on analogous occurrences of volcanic ash both in Pakistani and Indian Siwaliks (Johnson et al 1982;Bhat et al, 2008), it can be inferred that an equivalent volcanic ash may also occur in Eocene rocks in Pakistani Himalayas, and possibly also in Sulaiman Range. Such as an older volcanic ash occurring within the pre-Siwalik units of the Sulaiman Range might have led to the origin of the Taunsa tuff upon reworking.…”

Sedimentology and paleoenvironments of a new fossiliferous late Miocene-Pliocene sedimentary succession in the Rukwa Rift Basin, Tanzania

“…The presence of erosional upper surfaces of conglomeratic units and associated discontinuous carbonate lenses suggest channel abandonment and subaerial exposure. Deposition in the upper (sandstone-mudstone) interval of facies 2B most likely resulted from remobilized pyroclastic flow/fallout deposits from diluted hyperconcentrated fluvial flows (Bhat et al, 2008). The presence of wavy and ripple cross-laminated intervals (Fig.…”

“…Facies 2B is interpreted to represent fluvially reworked pyroclastic deposits. This interpretation is based on the high degree of sediment rounding and the interbedding of coarse-grained tuffaceous sandstone with devitrified ash (bentonite) and bentonitic mudstone deposits (e.g., Bhat et al, 2008). The ash-rich matrix and poorly-sorted nature of the conglomerate is indicative of debris flow-driven sedimentation.…”

“…Textural features such as a high degree of rounding of individual silt-to fine-sand grains suggest sediment derivation from fluvial-reworked pyroclastic deposits in the hinterlands (e.g., Cas and Wright, 1987;Nichols and Fisher, 2007;Bhat et al, 2008). Horizontally laminated beds (facies Fbh) were deposited by sediment settling from suspension.…”