Sedimentology and Tectonic Setting of Polymictic Conglomerates in a Rapidly Subsiding Miocene-Pliocene Foreland Basin: Kohat Plateau, Northern Pakistan
Abstract
More than 6km of Neogene molasse sediments of the Rawalpindi and Siwalik Groups accumulated in the western Himalayan (Kohat) foreland basin, northern Pakistan, in response to active basin subsidence and orogenic uplift. In the Kohat Plateau, the Siwalik Group consists of a coarsening upward sequence of silt in the lower pan (Chinji Formation), sand in the middle part (Shakardarra Formation) and conglomerate (Janak Conglomerate Formation) in the upper part, This formation was previously named as the Indus Conglomerate Formation by Abbasi and Friend (1989), but I now propose that it should be called the Janak Conglomerate Formation. This new name is proposed because of the danger that the previous name will be confused with stratigraphic terms used in the Indus Suture Zone of Ladakh, India, where the terms Indus Group, Indus Formation, Indus Molasse and Indus Flysch have all been used in recent years (e.g. Sñvastava, et al., 1979; Searle et al., 1990; Searle, 1996). The name of Janak belongs to a village located on the northwestern edge of the outcrop area of the formation, at 71o38ʹ30ʺE, and 33o14 ʹ 30ʺN (Fig. 2).
The Janak Conglomerate Formation is exposed over an area of 400km2, and is about 1500m thick. The conglomerate/sandstone is in the lower part and increases to in the middle and upper parts. The formation is laterally variable but consists of three main lithofacies i.e., a) crudely stratified to horizontally bedded conglomerate (Gh), b) massive, clast supported conglomerate (Gm), and c) planar cross-stratified conglomerate (Gp). In lithofacies Gp, some of the cross-sets are up to 7m thick suggesting channels deep as 14m or more. However, the average thickness of the cross-sets are 3-5m, suggesting channels 6-10 m deep.
The progradation of the conglomerates over the finer grained formations is probably mainly a response at this depositional site on the Indian plate, to the progressive movement of the plate towards the uplifted Himalaya. But in the Kohat area, the late Miocene progradation of the conglomerates is distinctly earlier than elsewhere in the western Himalayan foreland basin. The conglomerates pass laterally into the finer grained facies of the Nagri and Dhok Pathan formations to the east and south of the Kohat basin, and also pass into finer grained sediments to the west. This suggests that it was in this area that the largest river system entered the western foreland, about 10 ma, approximately along the course of the present-day Indus river.
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