Time-dependent changes for Midcontinent Pennsylvania black shales, U.S.A.

Abstract

Three types of Midcontinent North American black shale types have been recognized based on metal content, organic carbon contents, sulfur isotopes, and various other sedimentological and paleontological characteristics that may reflect fluctuations in bottom-water oxygenation during deposition. Near-shore, Middle Pennsylvanian black shales, exemplified by the Mecca Quarry Shale Member of Indiana, typically yield degree of pyritization (DOP) values near 0.65, the highest measured in the study, consistent with relatively, but not fully anoxic conditions of sedimentation. DOP values for Heebner-type shales, most prevalent in the Upper Pennsylvanian, average 0.27–0.35 barely in the dysaerobic range, which is difficult to reconcile with an origin for black shale dependent upon upwelling bottom water. Positive correlations exist between DOP, metal values and total organic carbon (TOC) in black shale units through the Pennsylvania. These changes can be related to organic carbon production and published sea-level fluctuations, suggesting a change in black shale character in the Midcontinent from the Middle to Late Pennsylvanian. Consistently decreasing values for DOP and TOC through time suggest the likelihood of a progressive ventilation or improved bottom-water oxygenation accompanied by a reduced rate of input from terrestrial organic matter during this time interval.