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Relationship Between Sedimentological Processes and Climate Cycles During Transgressive Ravinement in the Codell Sandstone and Fort Hays Limestone of Colorado and Western Kansas


Understanding the relationships between paleogeography, climate, and sedimentary processes is crucial to understanding the geology of unconventional reservoirs. This study explores these relationships by incorporating measured outcrop sections, core descriptions, and X-Ray Fluorescence (XRF). Data trends reflect complex variations in the depositional environments through time, which are especially apparent in the transition from the upper Codell Sandstone to the Fort Hays Limestone. The latest Turonian to Coniacian contact between the upper Codell Sandstone and the Fort Hays Limestone marks the onset of the Niobrara Transgressive-Regressive cycle. This basal transgressive package is marked by alternating siliciclastic and carbonate cycles that reflect variations in sedimentary processes during transgression. Sand in the upper Codell was derived by reworking of the underlying shelfal substrate during eastward transgression of the shoreline. Basinward of the shoreline, the Fort Hays chalk was derived by biogenic processes during dryer climate cycles while wet cycles produced influxes of terriginous mud, depositing thin (1–5 cm thick) marls which punctuate the Fort Hays. This contact has traditionally been described as unconformable. However, cores and outcrops in this study show that this contact is only regionally erosive, displaying gradation over a 0.2–0.5 m transition. This indicates a gradual mixing of the two sediment sources with conformable changes in proximity to marine ravinement (clastic sediment supply) and carbonate productivity during sea-level transgression. In addition, XRF data displays a gradual vertical decrease of quartz with corresponding increases in calcite over the contact, further supporting the interpretation of a conformable contact. XRF data also suggests an increase in the relative amount of terriginous material deposited during mud-rich marl cycles within the Fort Hays. These are interpreted to represent smaller scale climate cycles, with wet periods resulting in increased runoff and terriginous sedimentation, capable of periodically overwhelming local carbonate production. Resulting marls occur every 0.5–1.0 m throughout the Fort Hays. These overprinted cycles can be observed in the regular cyclicity of chemical variations throughout the overlying section. Understanding these variations increases the understanding of the Niobrara depositional system, and its response to sea-level change and climate variations across the basin.