THE INFLUENCE OF SEDIMENTARY PROCESSES ON ELEMENT DISTRIBUTION IN THE DEVONIAN NEW ALBANY SHALE OF THE ILLINOIS BASIN

The question of the role of anoxia in the formation of black shales has generated considerable scientific debate. Various geochemical proxies have been proposed to address this issue, but so far the search for the ideal anoxia proxy has proven elusive. Geochemical data and derived proxies were obtained on 41 samples collected from a complete core of the Late Devonian New Albany Shale (1-3 Kavanaugh, Daviess County, Indiana). Stratigraphic trends of proximality (e.g., Ti/Al), and paleo-redox (e.g., degree-of-pyritization (DOP), V/Cr, Mn/Al, Fe/Al) proxies were examined and correlated with variations in lithology, sedimentary structures, degree of bioturbation, and thin section petrography. This revealed how sedimentary processes (e.g., erosion and lag formation, resuspension, bottom currents, and bioturbation) may influence elemental distribution in black shale successions.

For example, elevated values of DOP (>0.75), and V/Cr (>2) associated with depleted values of Mn/Al have been commonly considered to indicate anoxic to anoxic-sulfidic deposition. Anoxia indicated by these geochemical parameters contrasts, however, with observations of bioturbation in analyzed samples. Moreover, examination of corresponding thin sections revealed various sedimentary features indicative of erosion and winnowing (e.g., conodont enrichment, reworked Tasmanites cysts, burrows filled with material that differs in composition from any overlying strata). Reworking and winnowing in these samples also resulted in an enrichment of reworked pyrite (as framboids, fecal pellets, and fills of Tasmanites cysts), a fact that readily explains the high DOP and Fe/Al values without a need for postulating anoxia. These same samples also show elevated Ti/Al values. Although typically interpreted as an indicator of greater proximity to a coastline/clastic input (and by implication of shallower water), relative Ti enrichment is equally well explained by heavy mineral enrichment due to reworking. Resuspension of organic matter during reworking also facilitated the oxidation of labile organic matter and could have led to the observed TOC reduction in these supposedly anoxic sediments.