Maceral composition and origin of organic matter input in Neoproterozoic–Lower Cambrian organic-rich shales of Salt Range Formation, upper Indus Basin, Pakistan

The appropriate thermal maturity (oil window or lower) of the Neoproterozoic rocks is a prerequisite for preserving robust and primary biological assemblages. The Neoproterozoic–Lower Cambrian organic-rich shales present in the upper part of Salt Range Formation, Pakistan, with abundant organic matter (OM) (total organic carbon (TOC) up to 46%) at relatively low thermal maturity (protobitumen reflectance, BRo = 0.2–0.4%), represents by far the lowest-thermal maturity Neoproterozoic–Lower Cambrian rocks analyzed to date. The detailed petrographic description of Neoproterozoic–Lower Cambrian organic matter in the current study was lacking from previous source-rock studies of coeval strata from Oman, Eastern Siberia, Australia, and elsewhere. Eight organic-rich shales of the Salt Range Formation from three different outcrops have been analyzed for maceral composition and source rock characterization using organic petrography, scanning electron microscopy and standard source rock assessment techniques. The organic petrography reveals the organic-rich shales are predominantly composed of alginites (lamalginite and telalginite), bituminite, protobitumen, and mineral-bituminous groundmass. Overall, the maceral assemblages suggest the deposition of these shales took place under a salinity-stratified water-column in a shallow restricted basin, where both oxygenic and anoxygenic modes of primary production were operative. The top oxygenated part of the water-column was populated with the phototropic mat-forming cyanobacterium, whereas the lower, low-dissolved oxygen zone was dominated by algal mats and sulphate reduction bacteria. The rare and scattered thucholites present within the Salt Range Formation shales were derived from the radiation-induced polymerization of adjacent OM by the radioactive grains. The heat from the radioactive grains, however, found to have a negligible effect on the overall thermal maturity of these shales. •OM in Neoproterozoic shales comprised mainly of alginites, bituminite, protobitumen, and mineral-bituminous groundmass.•Microbial degradation by sulphate reduction bacteria in low-dissolved oxygen settings lead to the formation of protobitumen.•Both, oxygenic and anoxygenic modes of primary production of OM were operative during the deposition of Salt Range shales.•Radioactive detrital grains have a negligible effect on the overall thermal maturity of Neoproterozoic shales.