Bio and Chemostratigraphic Record through the Upper Kellwasser Anoxic Event (Appalachian Basin)
University of California, Riverside, Department of Earth Sciences Riverside, California, USA
Most researchers agree that oxygen is limited in association with Late Devonian biotic crises because of the pervasiveness of black shale preservation in Devonian Age stratigraphic units, but the duration and extent of marine anoxia, and its association to depressed origination rates and targeted extinction, are not well-constrained. Increasingly, it is recognized that black shales do not represent homogenous conditions, but may in fact capture a range of bottom water redox conditions. Potential evidence for rapid fluctuations justifies a high resolution study of the bio- and chemostratigraphy of Devonian black shale units, specifically tracking the ichnofabric index and lipid biomarker assemblage at a mm-cm scale through putative persistently anoxic extinction intervals.
In the Upper Kellwasser black shale unit of the Hanover Formation in western New York State (the Appalachian Basin), the ichnofabric index (ii) varies from ii1-5. Laminated intervals are typically thin (<7cm) and are not persistent. Rapid variation in the amount of bioturbation is observed, which is consistent with rapidly fluctuating bottom water oxygen levels. Initial lipid biomarker analyses suggest that the extracted bitumens are syndepositional and not significantly affected by contamination; additionally, no appreciable amounts of aromatic carotenoid biomarkers diagnostic for green sulfur bacteria have been detected yet. This indicates that photic zone euxinia may not be a ubiquitous environmental backdrop during deposition of Late Devonian shales in epeiric seas as the existing organic geochemical literature currently suggest.
AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects