INSIGHTS ON DEPOSITIONAL PROCESSES OF DEVONIAN-MISSISSIPPI­AN BLACK SHALES (CENTRAL APPALACHIAN BASIN, U.S.A.) FROM C-S-FE ANALYSIS WITHIN AN OPTIMIZED CYCLOSTRATIGRAPHIC FRAMEWORK

The Upper Devonian-lowermost Carboniferous of the Appalachian Basin in eastern North America is characterized by extensive organic-rich shales deposited in a stratified anoxic marine environment. C-S-Fe-system relationships can provide significant insights into depositional and early diagenetic processes in such environments, but recovering such information requires sampling and analysis in an optimal manner--specifically, at length scales shorter than the fundamental scale of paleoenvironmental variation in the depositional system of interest. In the present study of the Upper Devonian (Famennian) Cleveland Shale Member of the Ohio Shale and the Lower Mississippian (Tournaisian) Sunbury Shale, this fundamental scale is represented by decimeter-scale (~5-20 cm) compositional cycles that show strong variations in total organic carbon (TOC), sulfur (S), Fe_py/Fe_HR (a redox proxy), δ³⁴S_pyrite, and organic maceral proxies. High-resolution chemostratigraphic analysis of 21 such cycles revealed systematic variation at ~5 to 20 cm intervals. All proxies related to authigenic iron sulfides (e.g., S_pyrite, Fe_total and Fe_pyrite, Fe_py/Fe_HR, and δ³⁴S_pyrite) covary positively with total S and negatively with TOC within the dm-scale cycles. H₂S production via bacterial sulfate reduction was maximized within the high-TOC hemicycles owing to the greater availability of labile organic matter, and excess H₂S production was accommodated through an upward diffusive flux that resulted in higher S_pyrite concentrations and Fe_py/Fe_HR values and in strongly ³⁴S-enriched pyrite in the overlying TOC-poor intervals. The key to identifying these relationships in the present study was optimization of the sampling protocol. We strongly advocate the use of some form of compositional prescreening and targeted sampling approach in all chemostratigraphic studies in order to accurately characterize relationships among sedimentary components and to provide a superior understanding of high-frequency environmental dynamics within the depositional system.