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An integrated sedimentologic-chemostratigraphic study of the Late Devonian-Early Mississippian Chattanooga Shale in Kansas: high-resolution stratigraphy and organic matter accumulation

Abstract

The organic-rich Chattanooga/Woodford Formation was deposited in Kansas and Oklahoma during a third-order transgressive systems tract, corresponding both to a source rock and an unconventional hydrocarbon reservoir. Detailed sedimentologic and chemostratigraphic analyses in a core in Douglas County (KS) allowed subdividing the unit at different scales, thus setting the stage for determining the controls on organic matter accumulation. Lithofacies analysis was performed through description at millimetric-centimetric scale, aided by petrography. Chemostratigraphic data was collected using a hand-held X-Ray fluorescence device to determine the concentration of major (Al, Ca, Fe, Mg, K, P, S, Si, Ti) and minor/trace elements (Sr, Ba, Co, Ni, Cu, Zn, Zr, Mo). Elemental concentrations and indices were used as proxies for continental input, sedimentation rate, biogenic silica and degree of oxygenation during the time of deposition. Nine lithofacies were identified according to lithology, grain size, sedimentary structures, degree of bioturbation, types of organic matter and fissility. The vertical succession allowed the definition of three large-scale cycles composed of multiple high-resolution cycles. The large-scale sedimentologic cycles have a clear Ti, K, Fe, Mn, P, S, Zr and Si/Al signature, while the high-frequency cycles are reflected in Ba and Mo concentrations. Boundaries between large-scale cycles are clearly marked by P and S concentration peaks, which correspond to key stratigraphic surfaces. Large-scale cycles were interpreted as product of variations in the continental input and degree of oxygenation, while high-frequency cycles reflect changes in the degree of oxygenation, primary productivity and organic matter types within the succession. The coincidence between sedimentologic cycles and chemical signatures provides additional evidence for these interpretations. Al, Ti, and Zr can be used to indicate detrital input, while Si/Al ratio is a proxy of detrital vs. biogenic quartz, and Ba and Mo are proxies for primary productivity. The definition of chemostratigraphic tools that reflect variations in significant parameters, such as continental input, degree of oxygenation and primary productivity, coupled with the establishment of a high-resolution stratigraphic framework, may shed light on the mechanisms that control organic matter accumulation in mud-dominated settings, and provide a tool for predicting the distribution of organic-rich deposits.