The "Cretaceous World" represents a critical period in Earth's geological history, marked by dynamic environmental and tectonic changes that profoundly influenced sedimentary systems and global biogeochemical cycles (Skelton, 2003). Key global events during this period include widespread oceanic anoxic events, the formation of oceanic red beds, intense volcanism associated with large igneous provinces (LIPs), global greenhouse phases, and rapid cooling events (Bodin et al., 2023; Keller, 2008). Moreover, this crucial period globally witnessed the widespread deposition of organic-rich marine sediments attributed to carbon cycle perturbations and high primary productivity (Hassan et al., 2023; Song et al., 2024). The early Cretaceous Talhar Shale (a key member of the Lower Goru Formation) is extensively deposited in the Lower Indus Basin (LIB) of Pakistan, with a thickness of 60–110 m. Despite its geological significance, the Talhar Shale has not been thoroughly investigated, particularly through integrated geochemical analysis to elucidate its composition, depositional environment, and hydrocarbon potential. This research seeks to bridge this knowledge gap by employing a comprehensive geochemical approach combining both organic and inorganic analysis, including Total Organic Carbon (TOC), Rock-Eval pyrolysis, X-ray fluorescence (XRF), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). This study aims to unravel the source rock potential, sediment provenance, paleoproductivity, redox conditions, and paleoclimate of the Talhar Shale; with the objective of identifying the key mechanisms driving OM accumulation and constructing a novel depositional model.
The TOC content in the Talhar Shale samples ranges from 1.0-6.3 wt.% (mean 1.81 wt.%). Rock-Eval pyrolysis results show S₁ values (free hydrocarbons) ranging from 0.14 to 2.14 mg HC/g rock, and S₂ values (hydrocarbon generative potential) from 0.97 to 9.28 mg HC/g rock. The Tmax values range between 442°C and 472°C, with calculated vitrinite reflectance (Ro) values from 0.8% to 1.34%. Kerogen typing using the Van Krevelen diagram indicates the dominance of Type II kerogen, known for its ability to yield both liquid and gaseous hydrocarbons. These parameters suggest that the Talhar Shale possesses good to excellent source rock quality and can be considered a prospective petroleum source rock. Major element geochemistry indicates that SiO₂ is the dominant oxide (~55%), surpassing other major oxides in abundance. The trace element enrichment factor (EF), calculated relative to the Upper Continental Crust (UCC), categorizes elements into two groups: depleted (EF < 1.0) and enriched (EF > 1.0), with most elements exhibiting enrichment relative to the UCC. Additionally, the total rare earth element (ΣREE) concentrations range from 217 to 370 ppm (mean = 284 ppm), which is significantly higher than those of the UCC and various global shale deposits.
Provenance analysis based on trace and REE geochemistry indicates that the sediments were predominantly derived from a felsic, silica-rich source. Tectonic discrimination analysis further suggests that the Talhar Shale sediments were sourced from an active continental margin or island arc setting. Such tectonic environments likely facilitated rapid sedimentation and quick burial, promoting enhanced preservation of organic matter. The weathering indices (CIA and CIW) exceed UCC averages, pointing to moderate chemical weathering under warm, humid climatic conditions. The paleoclimate interpretation is further supported by higher ΣREE values, low Y/Ho ratios, and C-values (0.38–0.77), indicating a semi-humid to humid environment during deposition. Paleosalinity indicators, such as Sr/Ba ratios (~0.42) and Rb/K×1000 values (2.2–3.9), suggest low salinity levels indicative of freshwater conditions, which likely influenced bioproductivity and the preservation of OM. On a broader paleogeographic scale, however, brackish conditions are generally associated with equatorial and higher latitudes, whereas lower salinities tend to prevail in mid-latitude regions. A higher average P content (0.09%) compared to the earth's crust, coupled with a positive correlation between TOC and phosphorus, supports enhanced paleo-bioproductivity. The Ni/Al (4.16), Cu/Al (2.5), and Zn/Al (18.7) ratios also point toward moderate to high productivity, corroborated by elevated Babio values (~764 ppm). Evidence of hydrothermal influence during deposition is supported by Ba/Sr ratios (mean 4.87) and elemental plots (e.g., Ni-Co-Zn ternary, Mn-10(Cu+Co+Ni)-Fe, and Co/Zn-Ni+Co+Cu). Hydrothermal fluids could have stimulated nutrient availability and enhanced productivity, further promoting OM accumulation.
Redox-sensitive elements such as V, Ni, U, and Co were employed to reconstruct the paleoredox conditions of the Talhar Shale. Elemental ratios, including V/Cr (~1.1), Ni/Co (~3.08), V/Sc (~8.14), and U/Th (0.14–0.20), collectively suggest deposition under predominantly oxic to suboxic conditions. Additional proxies, such as δU (0.60–0.74) and δCe (0.89–0.92), further support suboxic redox conditions. Regionally, the Early Cretaceous depositional environment appears to exhibit a latitudinal gradient, with more oxic conditions prevailing at higher latitudes and a transition toward suboxic to anoxic settings closer to the equator (Fig. 1a-c). Based on the integration of organic and inorganic geochemical data, we propose a depositional model where OM accumulation in the Talhar Shale was driven by the interplay of multiple factors: high primary productivity, freshwater input, warm-humid paleoclimate, low salinity, tectonically enhanced sedimentation, and suboxic conditions (Fig. 1d). These insights have broader implications for understanding early Cretaceous depositional systems and can guide hydrocarbon exploration strategies in analogous settings across the globe.
 

Cretaceous, Talhar Shale, Lower Indus Basin, Paleoenvironment, Paleolatitudes, Geochemistry