PYRITE IN ORGANIC-CARBON-RICH BLACK SHALES: FRIEND OR FOE!

Devonian Black shales have been extensively studied for deciphering paleoclimates and paleoenvironments of sediment deposition, accumulation and diagenesis. Such shales typically contain a variety of organo-metallic, polysulfide compounds including the ubiquitous pyrite whose Degree of Pyritization has often been linked with levels of benthic and subsurface oxygenation. This paper highlights the utility of a integrated sedimentological, mineralogic, paleoceanographic and engineering or environmental approach for investigating such black shales vis-a-vis their role in causing environmental problems.

The Middle Devonian Millboro and the Needmore Shales in West-central and South-west Virginia were studied with regard to determining the specific role of micro-morphologies and geochemisry of pyritiferouis phases in aiding landslides and slope stability problems. Analyses involved included a determination of Total Organic Carbon, Degree of Bioturbation via polished slabs, macro- and micro-sedimentary structures via thin-section petrology and variations in morphology and crystal habits using Backscatter (BSEM) and Scanning Electron Microscopy (SEM) and Energy Dispersive Analytical X-ray (EDAX). Results, to date, indicate that a) Original fluxes of labile organic carbon to this paleoenvironment and levels of benthic oxygenation were dominant controls on the Degree of Bioturbation preserved variably in the Millboro and the Needmore Shales, b) The pyritiferous phases exhibited a variety of morphological structures identifiable under the SEM/BSEM/EDAX which shed new light on ways via which varying levels of pyritization can control planes/zones of structural weakness in such sedimentary units, and c) Engineering studies of the structural strength of such rock units must take into account both the total amount of pyritiferous phases and the nature of crystal clusters in terms of their size, shape and density in order to effectively evaluate the impact of such shales in slope stability problems. Examples of analyses from the study will be shared along with recommendations for modifying future studies of similar shale units in other geological terrains.