Paleoredox and lithofacies assessments in Deepwater intervals of the Monterey Formation, Santa Maria Basin, California: Insights from organic sulfur geochemistry

Abstract

Although the Monterey Formation has been studied extensively for its petroleum system and Miocene climate insights, debates persist regarding its paleo-redox conditions. Furthermore, its characteristic lithofacies offer a natural laboratory for developing tools to differentiate siliceous and calcareous rocks in deepwater environments. This study evaluated the potential of the Sulfur Index (SI = mg S_org/g TOC), measured by Rock-Eval 7S, as a proxy for assessing paleoredox conditions by comparing the SI with iron speciation data. Additionally, high-resolution molecular analyses were performed to investigate possible relationships between sulfur compounds and rocks with different carbonate content. The findings indicate higher SI values and higher concentrations of benzothiophenes over dibenzothiophenes in the siliceous lithofacies compared to the calcareous lithofacies. This suggests that the mineral matrix plays a crucial role in influencing the process by which sulfur-rich kerogen undergoes cracking when placed under thermal stress. The SI may support a paleoredox interpretation for the Monterey Formation, differing from previous interpretations based only on iron speciation. This alternative interpretation involves a more extensive water-column euxinia, distinct from the seasonal euxinia suggested in previous work. This study represents a practical and pioneering workflow based on sulfur data derived from Rock-Eval 7S for paleoredox and lithofacies assessments, opening avenues for further research.