--> Progressive Burial Diagenesis of the Upper Cretaceous Niobrara Formation, Denver Basin, Colorado

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Progressive Burial Diagenesis of the Upper Cretaceous Niobrara Formation, Denver Basin, Colorado

Abstract

The Upper Cretaceous Niobrara Formation was deposited in areas of Colorado, Wyoming, Nebraska, and Kansas in the Cretaceous Western Interior Seaway. A diagenetic assessment of the B chalk, B marl (Smoky Hill Member), and Fort Hays Member of the Niobrara Formation, in eight cores and one outcrop locality, revealed that progressive diagenetic alteration occurred with increasing burial depths and concomitant increasing temperatures. Standard and cathodoluminescence (CL) petrography, scanning electron microscopy, geothermal data, elemental data, and stable oxygen isotope data were used to characterize diagenetic changes from outcrop to the deepest part of the basin in Wattenberg field. Overall, increasing temperatures resulting from greater burial led to a decrease in porosity and permeability through calcite recrystallization, pressure solution, and calcite cementation. Isotope values (δ18O) became more depleted with depth as the system became more closed and dominated by temperature fractionation. The Fort Hays Member was subjected to early diagenesis, with little evidence of compaction or pressure solution, as well as early lithification of carbonate mud. As a result, the Fort Hays presented more enriched δ18O values than those of the chalks and marls of the Smoky Hill. Early diagenesis in the Fort Hays is also supported by CL observations, with significantly brighter yellow-orange luminescence in comparison to Smoky Hill chalks and marls, and Fort Hays elemental data show 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 Upper Cretaceous ocean water in the Western Interior Seaway may have been closer to 3‰ VSMOW, which is heavier than the values previous workers have published.