Petrographic and geochemical evidence for determining the provenance of the Nari Formation, Pakistan

Abstract

The geochemical and petrographic characteristics of the Nari Formation (NF) have been thoroughly examined at the Haji Haroon section in the Lower Indus Basin (LIB), located in the Northern Laki Range, approximately 20 kilometres south of Therhi village, Sehwan Sharif. The NF is predominantly composed of sandstone, multicoloured clays, shale and limestone in the Haji Haroon section. The geochemical analysis of major elements using the automated Scanning Electron Microscopy Energy Dispersive Spectrometer (automated SEM-EDS) reveals that quartz is the dominant mineral in all the studied sandstones, typically comprising over 75% and reaching up to 90% and the second dominant phase mineral is calcite which varies from sample to sample. In the basal part of the NF sample, labelled HHS-02, consists of calcitic limestone, with calcite making up more than 85% of the weight (%). The previous studies in Lower Indus Basin shows that the basal part and upper part of the Nari Formation are formed in fluvial environments. However, our study contradicted the existing paradigms and revealed novel insights into the depositional environment of the NF. The current petrography, SEM-EDS, and field observations suggest that the NF at the Haji Haroon section in the Northern Laki range does not solely originate from fluvial processes. Instead, the basal part of the formation is marine in origin, while the upper part is fluvial. The basal part is characterized by transitional environments, which may be deltaic or beach deposits due to the presence of significant amounts of both quartz and calcite. The samples bear an elevated amount of quartz, classifying them as quartz arenite. Therefore, it is determined from the shape of clastic quartz grains that the sediments presumably originated in the western highlands rather than the northern Himalayas or the Indian shield, and based on the existing rock fragments.