Document Type: Original Article

Authors

1 Institute of Geology Punjab University, Lahore-54590, Pakistan

2 School of Earth and Environment, the University of Leeds, LS2 9JT, UK

Abstract

Deposits of the Warchha Sandstone in the Salt Range, Pakistan are characterised by a range of fluvial facies and architectural elements that together preserve a record of both the proximal and distal parts of a meandering river system that drained the northern margin of Gondwanaland. Several fining-upward cycles are recognised and completely preserved cycles can be divided in to three parts; a lower part composed of an erosive base with gravel- and coarse sand-grade trough cross-bedded facies, a middle part composed of planar cross-bedded, ripple cross-laminated and horizontally laminated sandstone facies, and an upper part composed predominantly of horizontally laminated and massive mudstone facies. Nine architectural elements are recognised within these cycles and these record the presence of channels, downstream and laterally accreting barforms, laminated sand sheets, crevasse splays, levees, over-bank floodplain units and shallow lakes. A broad range of sedimentary structures is recognised, including different forms of bedding, cross bedding, ripple marks and stratification, channels, flute casts, load casts, desiccation cracks, rain prints, conein- cone structures, a variety of concretions and bioturbation. The occurrence and abundance of these structures varies in a systematic manner throughout the vertical thickness of the succession. Cross bedding is the most prominent and consistent sedimentary structure, including various trough and planar varieties. The clasts are mainly of plutonic and low-grade metamorphic origin, with an additional minor sedimentary component. Textural properties of the sandstone are fine- to coarse-grained, poorly to moderately sorted, sub-angular to sub-rounded and with generally loose packing. Based on modal analyses, the sandstone is dominantly a sub-arkose to arkose. Detrital constituents of this formation are mainly composed of monocrystalline quartz, feldspars (more K-feldspar than plagioclase) and various types of lithic clasts. XRD and SEM studies indicate that kaolinite is the dominant clay mineral. Detailed palaeocurrent analysis reveals a broad unimodal palaeocurrent pattern within each cycle but significant changes in local migration direction between each vertically stacked cycle, supporting the notion of a high-sinuosity system with an overall dominant flow direction to the north-northwest. Petrographic analysis indicates the provenance of the Warchha Sandstone to have been the Aravalli Range to the southeast and the Malani Range to the south of the Salt Range, suggesting northward transport across a broad alluvial plain towards the margin of the Tethys Ocean in the north.

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