Detrital composition of Triassic to Neogene sandstones in the Sulaiman Fold-Thrust Belt, Pakistan: implications for pre- and postcollisional evolution of the northwestern Indian margin

Authors: SHAZIA FAREED, AKHTAR KASSI, AIMAL KASI, NAMIK AYSAL

Abstract: Sandstones of the Triassic through Pliocene successions of the western Sulaiman Fold-Thrust Belt (SFTB), Pakistan, were studied, by polarizing microscope and using point-counting techniques, to determine systematic variations in detrital modes. The sandstones in the study area are categorized into pre- and postcollisional sandstone assemblages. The precollisional assemblage was deposited before the onset of collision of India with Eurasia (starting Late Cretaceous-Early Paleocene) and subcategorized as Wulgai- Mughal Kot, Bibai and Pab subassemblages. The postcollisional assemblage was deposited after the onset of collision and categorized as Ghazij and Urak subassemblages. The Wulgai-Mughal Kot subassemblage mostly comprises of highly quartzose sandstones of the Triassic Wulgai and Upper Cretaceous Mughal Kot formations. The Pab subassemblage consists of quartzose sandstone, with additional limestone and calcareous fossil fragments, of the Upper Cretaceous Pab Formation. Its detrital mode is craton interior, i.e. the crystalline basement of the Indian continent. The Bibai subassemblage is comprised of lithic arenites of the Middle-Upper Cretaceous Bibai Formation (Bibai Group) and is predominantly composed of mafic volcanic fragments, having a detrital mode of a magmatic arc. The Ghazij subassemblage, of the postcollisional assemblage, is comprised of lithic arenites of the Eocene Ghazij Formation, sourced mostly from a volcano-sedimentary terrain, derived from a recycled/transitional recycled orogen, indicating the initial stage of uplift (and sedimentation) of the successions including ophiolites of the suture belt. The Urak subassemblage is comprised of lithic arenites of the Miocene Uzda Pusha and Pliocene Shin Matai formations, and composed of a variety of sedimentary lithic fragments with ample quantities of quartz and feldspar. The detrital modes of this subassemblage indicate a recycle orogen derived from the collision orogen of the SFTB. The shift, in composition, provenance and detrital modes, from the pre- to postcollisional assemblages indicates beginning of the uplift and sedimentation of the older successions, including the hot spot-related volcanics of the Bibai Group. This study supports the notion that collision of the India with Eurasia initiated soon after the Late Cretaceous, resulting in the emergence of the older successions of the SFTB, reaching its culmination in Pliocene.

Keywords: Sulaiman Fold-Thrust Belt, sandstone, detrital modes, passive margin, Indian collision

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