Diagenesis of the B chalk, B marl, and Fort Hays member of the Niobrara Formation, Denver Basin, Colorado

Abstract

A diagenetic assessment of the B chalk, B marl and Fort Hays Member of the Niobrara Formation, in eight cores and one outcrop sample, revealed that progressive diagenetic alteration occurred with increasing burial depth and increasing temperatures. Cathodoluminescence petrography, scanning electron microscopy, geothermal data, and stable oxygen isotopes provided insight into the varying diagenetic changes that have occurred. An increase in temperature lead to a decrease in porosity and permeability from greater accumulations of calcite cement through recrystallization and reprecipitation (seen through SEM), yielding more depleted isotope (δ18O) values as the system became more closed. Early diagenesis of the Fort Hays Member was supported by little evidence of compaction or pressure solution, as well as early lithification of the carbonate mud. As a result, the Fort Hays presented more enriched δ18O values than were seen in the chalks and marls. Evidence for early diagenesis was also present in the cathodoluminescence portion of the study, where the Fort Hays was brightly luminescent in comparison to the chalks and marls, and elemental data showed high concentrations of Mn2+ (700+ ppm). The marls, compared to the chalks, yielded δ18O values that were attributed to two possible scenarios 1) lower permeabilities resulting from more clays and greater cementation, that prevented ion exchange between fluids and sediments; and/or 2) more opportunities for pressure solution exist in the marls, and, therefore, dissolved carbonate is transported from sites of dissolution in marly intervals to sites of precipitation in neighboring chalks under higher temperature conditions. Utilizing calcite-water fractionation curves and corrected bottom hole temperatures for the DJ basin, it was found that the isotopic composition of Late Cretaceous ocean water in the CWIS relative to standard mean ocean water (SMOW) may have been closer to 3‰, which is heavier than the values previous workers have published.