CONTROLS ON THE ORGANIC MATTER ENRICHMENT IN THE CENOMANIAN-TURONIAN BRIDGE CREEK LIMESTONE, WESTERN INTERIOR BASIN: PRELIMINARY INSIGHTS FROM ORGANIC PETROLOGY ANALYSIS

Although the limestone-marl bedding couplets in the Cenomanian-Turonian Bridge Creek Limestone (BCL) have been widely attributed to climate and environmental perturbations driven by Earth’s orbital cycles, the causes of short-term variations in organic matter (OM) enrichment in the BCL have not been extensively explored. Besides the high-resolution organic carbon content (%OC) data, previous studies have generally overlooked the organic petrographic composition—types and proportions of different macerals—of the BCL. To fill this gap, this study examined the BCL in the USGS 1 Portland core and at the Rock Canyon Anticline near Pueblo, Colorado. Organic petrology analysis was conducted on eight samples from the BCL within a detailed sedimentologic context established from the Portland core. The BCL focused in this study has overall low thermal maturity—0.55% to 0.60% vitrinite reflectance. Despite being deposited > 500 km offshore, terrigenous OM, including vitrinite and inertinite, constitutes 7-46% of total OM in all analyzed samples. All BCL samples contain dominant marine OM, including micrinized bituminite and less common alginite and liptodetrinite. The OM composition and characteristics, combined with %OC, suggest that the OM enrichment through the BCL is subject to different processes. The overall low OM enrichment of the lower BCL (up to Bed 97 sensu Cobban and Scott, 1972) is likely due to oxic degradation, dilution by biogenic carbonate particles and calcite cement, and volcanic ash input. The upper BCL is characterized by a small degree of terrigenous clastic dilution, reflected by the overall low proportion terrigenous OM. Marl beds show higher %OC and micrinized bituminite content than limestone beds. Micrinized bituminite is interpreted as sourced from seafloor erosion, which also supplies siliciclastic fines for the marl beds. The higher %OC of marl beds in the upper BCL is attributed to a higher sedimentation rate and burial efficiency of OM, supported by low bioturbation intensity. Direct examinations of OM in fine-grained sedimentary rocks can provide valuable insights into the causes of short-term variations in environmental conditions, which should be integrated to resolve the mechanism(s) of how orbital cycles modulate sedimentation in such fine-grained marine systems.