CONTINENTAL SCALE INTERMITTENT PHOTIC ZONE EUXINIA DURING DEPOSITION OF DEVONIAN-MISSISSIPIAN BLACK SHALES OF NORTH AMERICA

This study uses biomarkers to determine the stratigraphic and geographic extent of photic-zone euxinia during deposition of Middle Devonian-Early Carboniferous black shales on the North American craton. Organic geochemical data are compared to other geochemical and sedimentological data (e.g. stable carbon isotopes and ichnofacies) to constrain water column structure and dynamism during black shale deposition in equatorial regions. Black shale samples from the New Albany Shale Fm. collected from four different localities in the Illinois Basin have been analyzed for biomarkers (molecular fossils). Preliminary data indicate that all black shale samples from the Blocher, Selmier, Sweetland Creek, Grassy Creek, and Camp Run members of the New Albany Shale contain biomarkers derived from green sulfur bacteria, including isorenieratane. The presence of isorenieratane is unambiguous evidence for photic zone euxinia (anoxic, sulfidic waters reaching into the photic zone). Green sulfur bacteria biomarkers were also identified in Devonian sediments and oils of the Western Canadian and Williston basins in previous studies. This suggests that photic zone anoxia was recurrent and widespread in epicontinental seas of the North American craton. To further test this hypothesis, samples from the Appalachian and Michigan basins are being analyzed. However, macroscopic bioturbated horizons indicate that water column stratification was not a permanent feature of Devonian-Mississipian basins of North America. Two green shales of the Sweetland Creek and Hannibal/Saverton members from the Illinois basin did not contain isorenieratene derivatives, a result in agreement with their burrowed texture.