CHEMOSTRATIGRAPHY OF THE DEVONIAN WOODFORD AND MISSISSIPPIAN BARNETT FORMATIONS AND ASSOCIATED PALEOCEANOGRAPHIC CONDITIONS, SOUTHERN LAURENTIA

Paleoceanographic change on the southern margin of Laurentia during late Devonian and Mississippian time has been studied extensively, but a comprehensive understanding of timing, duration, and intensity of change and its linkages to contemporaneous records of epicratonic deposition (e.g., Bakken, Chattanooga, Ohio shales) is elusive. A thick (~300-feet) chemostratigraphic record of late Devonian Woodford deposition and the overlying 280-feet-thick succession of Mississippian Barnett accumulation are presented. Both successions are organic-rich (TOC > 3%), and large-scale changes in silica (as proxied by a quartz-to-clay ratio: Si/K) are interpreted to reflect highly variable biosiliceous exports from the water column to the deep basin, which may reflect eustasy-induced changes in export efficiency, or indicate the variability of radiolarian bloom conditions brought about by changing nutrient availability. Overall, accumulation of both successions occurred under relatively low oxygen conditions, and throughout much of the combined interval it can be demonstrated that depositon occurred under euxinic conditions. Molybdenum concentrations (a proxy for euxinia) are highest in the lower portion of the middle Woodford (>400 ppm), and lowest throughout much of the overlying Barnett succession (seldom rising above 20 ppm), suggesting a distinct difference in oceanic circulation and/or chemistry. While the enhanced Mo content during Woodford deposition may reflect increased drawdown due to deep water-mass euxinia associated with high fluxes of organic matter to the seafloor, it may also reflect a greater connectivity to the global ocean that provides a steadier source of redox-sensitive trace elements. The Devonian-Mississippian record, collected at a 2-inch sample spacing, is used to define highly-resolved changes in sedimentary chemofacies whose vertical stacking patterns can help 1) document shifts in depositional modes (e.g., less/more sulfidic conditions, high/low biosiliceous fluxes, more/less detrital inputs) within a dark-colored, fine-grained succession, and 2) build a more comprehensive understanding of the evolution in sedimentary conditions along the southern edge of Laurentia.