| Faunal records in the Carboniferous Upper Freeport roof shales of the Appalachian Basin indicate low-salinity, nonmarine deposition. High ratios of organic carbon (Corg) to pyrite sulfur (Spy) in these shales help confirm suspected deposition in a lacustrine setting. However, exceptions do exist, and high sulfur contents were observed in some samples. Because of these high Spy values, the current model for this system favors diffusional overprinting by marine sulfate following a transition from freshwater to marine deposition. Such overprints have diagnostic sulfur isotope character, which can be distinguished readily from pyrite formation under primary marine conditions or secondary overprints associated with transitions from oxic marine to euxinic marine conditions.; By contrast, the euxinic setting of the Cariaco Basin shows diffusion of HS- into sediments that were once oxic marine. The corresponding isotopic signature in these muds mimics the light delta 34S signature of sulfide in the modern water column. The Midcontinent, which changed from oxic marine (gray shales) to euxinic marine (black shales) in the Pennsylvanian, shows Corg to Spy ratios that are indicative of oxic and euxinic marine deposition. (Abstract shortened by UMI.)... |
