Geochemical investigation of low latitude black shale intervals of the Lower to Middle Jurassic succession, Indus Basin, Pakistan

The Lower to Middle Jurassic sedimentary succession is dominated by siliciclastics with a significant amount of black shales in the Indus Basin, Pakistan. Several outcrop samples have been studied using an integrated approach to interpret the conceptual depositional setting from carbon and oxygen isotopes (delta C-13& delta O-18), organic geochemistry, and palynofacies with major and trace element analysis. For interpretation of trace element data, various single and elemental ratios have been used in this research to unlock the geological history of the studied strata. Ti/Al is 1.96 for high-potential source rock and 7.82 for non-potential source rock, and Cr (less than 1) indicates low clastic input with low oxygen for stratified and stagnant water. The ratios of V/(V + Cr), V/(V + Ni), V/Mo, V/Ni, (Cu + Mo)/Zn, Mo/Al, isotopic values of delta C-13 and delta O-18 and besides the V/Cr elemental ratio, all proxies indicate that there are oxygen-depleted anoxic conditions at high potentials, while in non-potential source rock, these ratios show oxic to sub-oxic settings. In addition to the trace element correlation with total organic carbon, the influx of organic matter is determined by the palynoafacies analysis, which indicates mixed terrestrial and marine organic influx in high-potential source rock and vice versa. Furthermore, the studies of palynofaceis DFPF A-D and SFPF A-B suggest that the depositional setting of black shale occurred in the anoxic proximal to distal shelf. The results suggest that the regional and local occurrence of black shale during the Lower to Middle Jurassic and its geological condition were addressed, and these play an important role in its depositional and paleooceanographic setting in the Eastern Tethys.

Automated summary

This study provides insights into the depositional setting and geological history of black shales in the Indus Basin. Analysis of trace elements and isotopic ratios indicates the presence of oxygen-depleted anoxic conditions in high-potential source rock and oxic to sub-oxic settings in non-potential source rock. Palynofacies analysis confirms a mixed terrestrial and marine organic influx. The depositional setting of black shale is found to occur in the anoxic proximal to distal shelf.