RECONSTRUCTING HYDROGEN INDEX USING INORGANIC GEOCHEMISTRY IN THE UPPER CRETACEOUS-LOWER PALEOGENE MORENO FORMATION, CA, USA

Accurately reconstructing original Total Organic Carbon (TOC) in thermally mature rocks is essential for the correct application of geochemical proxies and understanding organic carbon burial through time. To reconstruct original TOC using empirical methods it is vital to have an accurate estimate of the original Hydrogen Index (HI). The two common empirical methods used are estimating HI using kerogen type or average HI values from immature rock elsewhere in the basin. This study tests the ability to use inorganic geochemical data to reconstruct original HI using the upper Cretaceous-lower Paleogene Moreno Formation from the San Joaquin Basin, California, USA as a case study. The study utilized sampled cores from the Moreno Formation (McGuire, 1988) that are thermally immature (Tmax avg. 419°C), thus preserving original HI values. First, inorganic geochemical data was produced (elemental abundances and iron speciation) for the same samples previously analyzed for organic geochemistry. These data suggest that bottom water conditions during deposition of the Moreno Formation were ferruginous (anoxic and non-sulfidic), with no development of sustained euxinia (anoxic and sulfidic). Next, a Random Forest analysis was implemented to analyze which inorganic geochemical variables best predict HI in the Moreno Formation. The most important proxies were found to be those for detrital input, marine productivity, and redox proxies for suboxic conditions. Finally, the model framework was used to predict HI values for three main study cores based on their inorganic geochemistry. These predictions were compared stratigraphically and statistically against the measured values and kerogen type and average HI methods for reconstructing HI and demonstrate that this new method has better predictive power than approaches based on single values.