Factors controlling organic matter preservation and selected metals concentration in the Upper Devonian shales in southern North America (Chattanooga Shale, TN and Woodford Shale, OK).

摘要

Preservation of organic matter (OM) in the Chattanooga Shale was controlled by both high organic carbon flux, and anoxic bottom water. In the Dowelltown Member, OM preservation was controlled primarily by high organic carbon flux in relatively oxic bottom water. This organic carbon flux was very high in the lower Dowelltown, which facilitated preservation of higher percentages of OM while in the upper Dowelltown carbon flux was relatively low, resulting in less organic carbon preservation. During Gassaway time, the bottom water was generally more oxygen depleted than the Dowelltown Member. In the Gassaway Member, euxinic bottom water is believed to be the main control of the OM preservation and metal concentrations across much of the section. However, high frequency, short term events of euxina and vertical mixing were recognized in both Dowelltown and Gassaway, respectively. Preservation of metals and OM in the Woodford Shale was mostly controlled by the presence of euxinic bottom water. High OM productivity in the middle Woodford increased OM and metal preservation. On the other hand, the upper Woodford was mostly deposited during low OM productivity, in euxinic bottom water with a deficiency in iron. These conditions resulted in low metal concentrations in this section. Lower Woodford deposition was initially in euxinic bottom water that became more oxygenated bottom water upward. In general, intervals deposited in conditions of euxinic bottom water are enriched in Fe, S, Mo, V, and U, while sections that deposited in conditions of high OM productivity are enriched with Cu and Zn. However, sections deposited in conditions of Fe deficiency are characterized by less concentration of Fe and S. These elements can be released to the environment (water and sediment) during weathering of the host shale, a process that is accelerated by human disturbances.;The F/F boundary in Chattanooga Shale and Woodford Shale is characterized by euxinic conditions in the bottom water, which supports that contention that worldwide euxinia caused the mass extinction associated with the boundary. The D/C boundary in the Woodford Shale section is also characterized by euxinic bottom water during deposition.