Petrographic composition, sedimentary structures and palaeocurrent analysis in Northern Gondwana: The Lower Permian Warchha Sandstone of the Salt Range, Pakistan

摘要

The Warchha Sandstone is a Lower Permian fluvial succession present in both outcrop and subsurface throughout the Salt Range and the Potwar Basin of Pakistan that originally accumulated in a palaeogeo-graphic setting adjacent to the northern margin of Gondwana. Sandstone beds are feldspatho-quartzose, including dominantly monocrystalline quartz, more K-feldspar than plagioclase, and mainly plutonic and low-grade metamorphic rock fragments. Twenty-eight fining-upward cycles, composed of conglomerate, sandstone, siltstone and claystone are identified. A varied range of sedimentary structures is recognised, including different forms of cross-bedding, ripple marks, flute casts, load casts, desiccation cracks, rain prints, cone-in-cone structures, and a variety of types of concretions and bioturbation. The occurrence and abundance of these structures varies in a systematic manner throughout the vertical thickness of the succession. Sedimentary structures, palaeocurrent data and lithofacies arrangement indicate deposition in a high-sinuosity meandering river system. Detailed palaeocurrent analysis reveals a broad uni-modal palaeoflow within each cycle with dominant flow direction having been towards the north-northwest, but with significant changes in local bedform migration direction between each cycle. The northward flowing river transported sediments from the Aravalli and Malani Ranges that lay to the south to the Salt Range, northwards to the Tethyan proto-ocean in the north.