A Geochemical and Mineralogical Investigation of Parasequences in the Camp Run Member of the Upper Devonian New Albany Shale
Spencer, Sarah; Schieber, Juergen
Shales of the Camp Run Member of the New Albany Shale reflect the interplay between sea level variations, terrigenous supply, and surface productivity of the Late Devonian inland sea. In order to understand the drivers for parasequence development we are developing an integrated depositional model that combines sedimentological observations, petrography, geochemistry, and quantitative mineralogy. Parasequences show sharp basal contacts with some erosion of previously deposited sediment, are ideally characterized by black-gray cycles of decimeter thickness, but may also consist entirely of black shale. Geochemical and XRD analysis shows that regardless of coloration, TOC decreases upwards and is accompanied by a decrease of pyrite and marcasite. Siderite, in contrast, is present throughout and does not differ significantly between bottom and top.
Black-gray cycles like in the Camp Run are often thought to reflect alternating anoxic and dysoxic bottom waters, but the distribution of redox sensitive minerals, marcasite in particular, suggests that throughout parasequence deposition the redox boundary was within the sediment and varied in depth intermittently. Overlying waters were not anoxic, but fluctuated in their degree of oxygen depletion. Distribution of pyrite and marcasite was controlled by higher abundance of readily metabolized marine OM (feeding sulfate reducers) in the bottom part of parasequences, whereas siderite distribution reflects the background terrestrial OM (feeding fermenters and methanogens) within the sediment that is linked to detrital flux. Thus, parasequence development reflects the upwards change in the mixing ratio between marine OM and terrigenous sediment supply (clay, silt, terrestrial OM).
Whereas in a "classical" parasequence the quartz/clay ratio would be expected to increase upwards, the highest quartz/clay ratios occur in the bottom-most portion of parasequences and then ratios drop to a lower level and stay rather uniform for the rest of a given parasequence. Probably due to extreme sediment starvation and winnowing at the very base of the parasequence, and the very distal nature of these deposits respectively. In addition, the laminated character of the basal black portions of parasequences suggests that bottom current deposition of flocculated organo-clay aggregates may have generated a sediment with better OM preservation potential due to intimate spatial association of labile marine OM and clays.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013