Unlocking paleolatitudinal secrets of the early Cretaceous by rare earth element imprints: Implications for seawater chemistry, depositional environments, and paleoclimate in the Talhar Shale, Lower Indus Basin, Pakistan

Abstract

The early Cretaceous in Pakistan facilitated the deposition of extensive shale deposits, notably the thick (∼70 m) Talhar Shale, which is widespread across the Lower Indus Basin (LIB) and is considered a significant hydrocarbon source with rich hydrocarbon potential. Despite its petroleum significance, the original seawater signatures of the Gondwana domain in the LIB remain underexplored. This research aims to fill this gap by using total organic carbon content and elemental geochemistry to present novel geochemical signatures that elucidate the paleodepositional history, sedimentary origin, paleoclimate, and tectonic framework of the Talhar Shale. Our results show moderately higher organic richness (average 1.81 wt. %), with total rare earth elements (ΣREE) averaging 284 ppm—exceeding various global shale deposits and the upper continental crust. Moreover, the weak correlations between key elemental proxies suggested a limited or no diagenetic impact on the REE of the Talhar Shale, making them reliable for paleoenvironmental interpretation. Elevated ΣREE values and lower Y/Ho ratios suggest warmer, more humid paleoclimatic conditions with freshwater influence during Talhar Shale deposition. From a global perspective, this suggests lower salinities at mid-latitudes and brackish conditions at higher latitudes and near the equator. This interpretation is further supported by narrow and stable δCe anomaly and Ce_Index indicate suboxic water conditions throughout the deposition, whereas more oxic conditions prevailed at the poles. Elevated LREE/HREE ratios and various trace elemental proxies imply a likely felsic origin for Talhar Shale, while paleotectonic discrimination highlights the complexity of the tectonic setting and demands a more comprehensive geochemical analysis. These findings justify the need for further exploration of the Talhar Shale to refine its palaeoenvironmental significance and broader implications on a global scale.