Mineralogy and element geochemistry of the Sohnari rocks of Early Eocene Laki Formation in the Southern Indus Basin, Pakistan: Implications for paleoclimate, paleoweathering and paleoredox conditions

Abstract

The Sohnari Member of the Early Eocene Laki Formation is massively deposited in the Southern Indus Basin of Pakistan and is considered a potential source rock to generate hydrocarbons. However, the detailed paleoclimatic, paleoweathering, and depositional conditions of the Sohnari Member have not been studied earlier. This research mainly discusses the detailed mineralogical (bulk and clay) and elemental geochemistry of the Laki Formation from two outcrop sections (Jhimpir and Lakhra) in the Southern Indus Basin, Pakistan. The bulk minerals, including quartz (low), hematite, calcite, halite, gypsum, and clay minerals such as kaolinite, chlorite, smectite and illite have been discussed here. These results demonstrate the paleo-environment of studied area was arid with enhanced saline and weak to strong oxidizing depositional conditions. The chemical index of alteration (CIA) values in Jhimpir and Lakhra sections are in the ranges of 41.30–97.93 and 22.30–96.19, respectively, indicating that the Sohnari sediments experienced weak to intense chemical weathering in the source area. The interpretation of the A–CN–K ternary diagram is consistent with the clay mineral contents in the studied sediments, which is characterized by the predominance of kaolinite, gibbsite and chlorite, demonstrating the weak to strong weathering state under warm and humid climatic conditions. The chemical indices such as Sr/Ba, δU, V/Cr, Ni/Co, and Cu/Zn, U/Th and Ba/Ga show that Sohnari rocks of Early Eocene Laki Formation underwent strong evaporation, oxic water column with warm to humid and minor contact of cold climatic conditions. Based on our present data, it can be concluded that the sediments of Sohnari Member of Laki Formation from Jhimpir and Lakhra areas of Southern Indus Basin in Pakistan are related to Indio-Eurasian collision and came from the Indian shield rocks that were deposited in a brackish water body with a minor contact of the freshwater oxidizing paleo-environment depositional conditions.