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The Geochemical and Mineralogical Expression of Sea Level Cycles, Upper Devonian Woodford Shale, Permian Basin


The Upper Devonian Woodford Shale, Permian Basin, west Texas, is the black shale formation in North America that probably represents the longest time span, from uppermost Givetian (Middle Devonian) to lowermost Mississippian. That time span of approximately 22 Myr comprises a complex sea level history, with multiple 3rd order sea level cycles superimposed on a 2nd order sea level fall. It includes the Frasnian-Famennian boundary, which is generally interpreted to represent a major 3rd fall and transgression that triggered anoxic conditions and mass extinction. The second order cycle is recorded by the appearance of phosphate nodules in mudstone, overlain by bioturbated organic-lean mudstone. Third order sea level cycles are manifested by bundling of non-mudstone lithofacies, including carbonate beds deposited as turbidites, chert beds and radiolarian laminae. In this context, major element chemistry and mineralogy are examined for systematic variation relative to sea level cycles in two wells, one that is near a major carbonate platform and distal to a source of siliciclastic sediment and a second that is farther from the carbonate platform and somewhat more proximal to the source of siliclastics. Most striking is a systematic increase in the quartz to clay ratio (decreasing Al2O3/SiO2) in the well farthest from the source of siliciclastics; this is interpreted as a biogenic silica signal, responding to increased silica concentration in a smaller water mass as the basin became restricted. In the well closer to the source of siliciclastics, the Al2O3/SiO2 ratio increases in the upper part of the formation, where the biogenic silica signal is swamped by influx of clays in the 2nd order low stand. Third order sea level cycles are indicated by quasi-periodic peaks in Al2O3/SiO2 ratios, where high values are interpreted as low stands. Interspersed between these peaks are distinct spikes in Ba, which are interpreted as manifestations of maximum flooding surfaces, when low sedimentation rates forced redox fronts deeper below the sediment surface. While shale geochemistry and mineralogy can be interpreted in the framework of sequence stratigraphy, it is unlikely that geochemical composition provides the basis for easy interpretation of stratigraphic sequences. Instead, robust sequence stratigraphic interpretations require the careful integration of sedimentological interpretations and detailed geochemical sampling.