SEDIMENTARY AND GEOCHEMICAL EVIDENCE FOR TERRESTRIAL ORGANIC MATTER INPUT DRIVING ANOXIA EVENTS FOUND IN LATE DEVONIAN BLACK-GRAY SHALE TRANSITIONS, APPALACHIAN BASIN, NORTH-CENTRAL OHIO

This study focuses on frequent stratigraphic transitions between gray shale (TOC< 1%) and black shale (TOC up to 9%) throughout a 140 m-thick sediment core extracted through the Late Devonian shale units in the northwestern part of the Appalachian Basin. In the study area, stratigraphic intervals up to several meters thick consist of thinly interbedded gray and black shales; these transition zones separate thicker intervals of predominantly gray shale or black shale used as the basis to define stratigraphic units. This study uses facies analysis, XRD analysis of clay minerals, TOC, extractable organic carbon, excitation-emission matrix fluorescence, XRF for major elements, and ICPES-MS for trace elements to compare and contrast the transition zones and thicker gray shale or black shale intervals. Turbidites and pelagic rhythmites are common in all of the shales, but the black shales are noted for abundant hyperpycnites, tempestites, and condensed successions where bioturbation extends into the black shales from overlying gray shales, and the black shales formed metallic concretions below that interface. The organic matter in the black shales is predominantly terrestrial in origin. Trace element V/Cr, Ni/Co, and Th/U ratios in the black shales are consistent with dysaerobic conditions in the overlying water column. One way of reconciling these observations is to attribute sediment TOC values to sediment dynamics (distributary lobe avulsion) of the Catskill Delta. Time intervals of increased terrestrial organic matter input may have resulted in both high TOC input and water column oxygen stress, similar to the “dead zones” of many modern deltas.