Organic and inorganic geochemical cyclicity of a Maastrichtian oceanic open-shelf carbonate source rock

Organic-rich source rocks are not only crucial for hydrocarbon exploration and production but also serve as valuable archives of past environmental conditions. This study investigates the Upper Cretaceous (Maastrichtian) source rocks present in the Al-Lajoun Basin of central Jordan, to identify geochemical compositional variability corresponding to the paleo-environmental conditions during deposition. To this end, a multifaceted approach using Rock-Eval, SGR, XRD, XRF, ICP-OES, SEM-EDX, and thin-section petrography is utilized to understand bulk organic and inorganic geochemical proxies. Based on the results, the Jordan source rock is characterized as organic-rich, Type IIS kerogen, and thermally immature source rock, representing three distinct cycles of organic matter distribution. Cycle 1 is characterized as organic-rich carbonate mudstones with an average total organic carbon (TOC) content of 17 wt.%. This cycle represents high organic matter productivity, anoxic bottom water conditions, and episodic detrital influx (clays and detrital quartz). Cycle 2 is characterized as silica-rich mudstones to wackestones with an average TOC of 15 wt%. This cycle reflects a shift from carbonate-dominated to silica-dominated biota, likely driven by increased nutrient supply and changing climatic conditions. These conditions resulted in high bioproductivity and highly reducing anoxic/euxinic bottom water conditions during deposition. Cycle 3 represents foraminiferal wackestones to packstones with an average TOC of 12 wt.%. This cycle is characterized by a relatively high detrital sediment input, with comparatively low organic matter productivity and anoxic bottom water conditions. The identified organic and inorganic geochemical variability between these cycles implies changing climatic conditions over the open shelf setting, which in turn implies changes in ocean currents impacting the upwelling system of the Tethys margin. Understanding this relationship between ocean currents, climate, and the geochemical composition is crucial for efficiently exploring and exploiting organic-rich source rocks. A regional correlation of these cycles and their geochemical signatures could provide a powerful tool to trace ocean currents and associated climate change along the Tethys margin during the early Maastrichtian.