| Summary: | Includes bibliographical references. 2017 Spring. The Late Cretaceous (89.5-82 Ma) Niobrara Formation is the primary target for hydrocarbon development in many fields throughout Colorado. The formation is composed of two members: the basal Fort Hays Limestone and the overlying Smoky Hill Member. Historically, production has centered on the Smoky Hill Member but recent success in the Fort Hays Limestone has sparked interest in this underlying unit. Compared to the Smoky Hill Member, the Fort Hays Limestone is markedly different in lithology, sedimentology, and elemental composition. Through detailed core descriptions, measured outcrop sections, XRF analysis, thin section petrography, and FESEM analysis, this study develops a better understanding of the unique depositional history that characterizes the Fort Hays. In order to assess regional changes, five cores and five outcrops were utilized to represent the Fort Hays on a depositional basin scale. Sedimentologically, the Fort Hays Limestone is characterized by rhythmic bedding of chalk and marl which represent climatic fluctuations that result from perturbations in the Earth’s orbit around the sun (i.e. Milankovitch cycles). Chalks were deposited during times of arid climate when siliciclastic input from the Sevier highlands was minimal and carbonate production was enhanced. Chalks are characterized by extensive bioturbation suggesting well-oxygenated benthic conditions. Conversely, marls were deposited under humid conditions where increased terrestrial runoff from the Sevier highlands diluted ongoing carbonate production. Marls are characterized by a laminite sedimentary fabric suggesting that benthic conditions were less-oxygenated and unable to support benthic fauna. Ichnological observations reveal that the Fort Hays Limestone is characterized by high frequency cycles of rapid oxygenation and deoxygenation. In the field, outcrops were measured for total Fort Hays and individual bed thicknesses and described for lithologic changes. Western outcrops exhibit changes in lithology from base to top that suggest an overall rise in sea level. Scour surfaces in the westernmost outcrops suggest a higher energy environment during marl deposition. The absence of these surfaces to the east in Hays, Kansas suggests a quieter, lower energy environment. Elemental data from X-ray fluorescence (XRF) were used to generate spatial elemental concentration maps for the Western Interior Seaway during Fort Hays time. This data suggests that detrital elements (Al, Si, Zr, and K) are primarily sourced from the west and northwest with an additional southwest source during humid climates. Carbonate elements were brought up from the south via Gulfian currents. Finally, a manganese anomaly found exclusively in the Fort Hays appears to originate from the north. The element is thought to be enriched in sea water at this time as a result of increased sea floor spreading and carried down to the basin via Arctic currents. The Fort Hays Limestone is characterized by a unique depositional history that resulted in the regional lithological and geochemical trends observed throughout this study. Changes in climate, benthic oxygenation, sea level, and energy are recorded in the Fort Hays. Each of these factors played a critical role in the supply, delivery and movement of elements throughout the basin.
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