Geochemical evidence of depositional and diagenetic processes associated with elevated organic-matter content and gamma-ray intensity in the Paleozoic New Albany Shale, USA

The New Albany Shale was deposited in a marine setting within the interior cratonic Illinois Basin during Middle Devonian to Early Mississippian time. The New Albany Shale is a major regional source of hydrocarbons and contains unusually high contents of organic carbon (Corg) and pyrite. In this study, data are presented from sequentially extracted fractions of sulfur for the Ellsworth, Clegg Creek, and Camp Run members of the New Albany Shale using core samples from a well drilled in Pike County in 2006. Sulfur geochemistry of whole-rock samples, metal concentrations, and visual microscopy of organic particles (macerals) were used (i) to understand the depositional environment of the New Albany Shale, (ii) to investigate linkages between shale intervals with high gamma ray and shifts in sulfur isotopic compositions, and (iii) to explore using proportions of the metals as indicators of the depositional environment.;The studied members of the New Albany are characterized by low values for the isotopic compositions of sulfur in pyrite inferred to reflect sulfide production by bacterial sulfate reduction under open-system conditions. The Camp Run Member and the lower portion of the Clegg Creek Member are characterized by similar sulfur isotopic compositions of pyrite. On the other hand, isotopic data gradually change upward from the middle portion of the Clegg Creek Member into the Ellsworth Member in the study core and indicate increasing variation in depositional conditions. Extreme depletion in 34S for sulfides in some samples could reflect alternating oxic/suboxic or anoxic conditions with large shifts in redox conditions and associated opportunities for microbial disproportionation of intermediate sulfur species in accumulating sediment. Systematic variation in sulfur isotopic values for pyrite from repeated chrome-reduction extraction could result from different pyritization processes through time during early diagenesis. Corg, acid-insoluble sulfur (Sai), degree of pyrtization (DOP), maceral compositions, and metal-ratio paleoredox indices are the basis to conclude that the Camp Run Member and the lower portion of the Clegg Creek Member were deposited under poorly oxygenated and moderately reducing conditions in accumulating sediment compared to the upper portion of the Clegg Creek Member and the Ellsworth Member.;The presence of extractable sulfate in Camp Run and Clegg Creek members is inferred to result from gradual oxidation of sulfides in the subsurface over hundred millions of years. In contrast, the overlying Ellsworth Member is dominated by sulfate that likely originated from sea water trapped in carbonate-associated sulfate (CAS). Correlation between sulfur isotopic compositions, gamma ray intensity, and uranium concentration are consistent with abiotic oxidation of pyrite by radiolysis in the Clegg Creek Member.