CHALLENGING THE PARADIGM: PALEOREDOX AND LITHOFACIES REVELATIONS FROM ORGANIC SULFUR IN DEEPWATER MONTEREY FORMATION, SANTA MARIA BASIN, CALIFORNIA

The Miocene of the East Coast of California was marked by a unique combination of tectonic activity, eustatic sea-level changes, oceanographic current shifts, and climatic events. In this context, the deepwater sediments of the Miocene Monterey Formation represent both challenges and opportunities for groundbreaking research. Some of the challenges arise from the difficulty of sampling very thin individual layers, low sedimentation rates, presence of condense sections, and the intricate basins' bathymetries. The interplay of physical, chemical, and biological processes resulted in the development of distinctive lithofacies in a complex geological system, evidenced by an asynchronous transition between lithofacies across various basins. This characteristic makes the Monterey Formation an ideal natural laboratory and represents an excellent opportunity for studying deepwater paleoenvironments. This research sheds light on the development of new understandings of deepwater facies in Santa Maria Basin by examining the potential applications of organic sulfur measured using programmed pyrolysis (Rock-Eval 7S) in conjunction with other geochemical data. This study proposes a novel paleoredox interpretation for the Monterey Formation, indicating more extensive water-column euxinia than previously thought. Moreover, the findings reveal a higher concentration of benzothiophenes in siliceous lithofacies compared to calcareous lithofacies, suggesting a significant influence of the mineral matrix on sulfur-rich kerogen's thermal cracking process. Consequently, the ΣDBT/ΣBT ratio (dibenzothiophenes to benzothiophenes ratio) emerges as a promising tool for differentiating siliceous and calcareous rocks in deepwater environments. This innovative workflow based on sulfur data from Rock-Eval 7S and molecular analysis not only challenges prior interpretations and paradigms but also paves the way for new research directions in paleoredox and lithofacies assessments.

*Abstract adapted from paper submitted to the International Journal of Coal Geology