SCANNING ELECTRON MICROSCOPY OF DEVONIAN MILLBORO BLACK SHALE AND IMPLICATIONS FOR GROUNDWATER POLLUTION AND PALEOENVIRONMENTAL RECONSTRUCTION

Located within the Appalachian Basin, is the Devonian-aged Millboro black shale. Such shales have been extensively studied for their source-rock potential. In addition, such pyritiferous shales can pose risks via acid rock drainage (ARD). Research has shown that patterns of pyrite morphologies and concentrations can be used as proxies for bottom-water conditions during accumulation of such sediment. Here we present research of pyritization and paleoenvironment reconstruction of the Middle Devonian Millboro Shale exposed near Radford in Virginia.

Five stratigraphic sections were extracted from outcrop using a portable, concrete-cutting saw. A total of 113 samples were collected and analyzed using a SEM. Specific goals of the SEM analysis included – evidence for tectonic overprinting, types of weathering and secondary mineralization and a survey of pyrite crystal morphologies, their abundance and ichnofabric indices.

Results indicate that Millboro Shale is dominated by tectonically-induced fracturing with joint-sets facilitating seepage of water which in turn facilitated chemical weathering and dissolution of pyrite preferentially along crystal edges and corners. Slickensides and related stress features attest to post-depositional deformation of such shales concomitant with secondary mineralization of gypsum, calcite and quartz. Oxidation of iron is common as to be expected in an ARD scenario.

SEM results show that most pyrite is octahedral and euhedral to subhedral. Such pyrite is concentrated along stratigraphically-continuous intervals on a mm-scale and laterally traceable over tens of meters. Pyrite is typically associated with infilled worm tubes and burrows, nodules, plant fragments and depauperate/stressed faunal elements. Size criteria suggest a benthic environment that was prone to fluctuations between lower dysoxic (weakly oxygenated bottom water) and anoxic (no oxygen in bottom water for extended periods of time) paleoenvironmental conditions.