Contrasting Late Cretaceous–Palaeocene lithostratigraphic successions across the Bibai Thrust, western Sulaiman Fold–Thrust Belt, Pakistan: Their significance in deciphering the early-collisional history of the NW Indian Plate margin

Abstract

The Sulaiman Fold–Thrust Belt is a significant tectonic element in northwest Pakistan, located on the NW margin of the Indian Plate having a rift–drift-collision plate tectonic history. Late Cretaceous–Palaeocene successions in the western sector of this Belt conventionally comprise strata above the widespread pelagic Parh Limestone (Albian–Campanian) and below the fluvio-deltaic Ghazij Formation (Early Eocene). These successions display marked lithostratigraphic variations across the Bibai Thrust, long known to be an important structural feature in the western Sulaiman Belt. The successions on the foot wall, to the east and southeast of the Bibai Thrust (Quetta, Spera Ragha and Chinjun areas), exhibit the most variable lithostratigraphy, were deposited exclusively in shallow marine conditions and include volcanics associated with a local submarine edifice. They also display several internal disconformities, locally accompanied by oxidised units that attest to intermittent emergence. By contrast, coeval successions on the hanging wall, to the north and northwest of the Bibai Thrust (Urghargai–Mazu Ghar and Kach–Ziarat areas) are the products of deposition in deeper marine conditions and also include important volcanigenic contributions. These contrasting lithofacies accumulated respectively in widely separated proximal and distal sectors of the north-western margin of the Indian Plate and their current proximity is attributed to tectonic juxtaposition by relative southward translation of the distal facies along the Bibai and related thrusts. Involvement of Late Eocene limestones in these thrusts and their sealing by Mio-Pliocene fluvial sediments demonstrates that active translation on these thrusts continued at least into Late Eocene or Oligocene times.