PALEOENVIRONMENTAL RECONSTRUCTION OF A SHALLOWING-UPWARD, MARINE SHALE SEQUENCE IN SOUTHWESTERN VIRGINIA: CASE OF THE DEVONIAN MILLBORO SHALE

Devonian black shales in the Appalachian region of North America are traditionally thought of as being deposited in relatively calm anoxic to dysoxic bottom-water conditions. However, recent research has questioned such a degree of uniformity in paleoenvironmental conditions specifically for the Millboro Shale in Virginia. We used a mineralogical and sedimentological approach for investigating variations in the level of benthic oxygenation that affected sedimentation in a gradually shallowing-upwards, near-shore, marine environment.

Two stratigraphic sections were extracted – a Lower and an Upper that were correlated across 30 meters. The Lower section spanned 363 centimeters whereas the Upper section consisted of 229 centimeters of strata excavated with a portable saw. A total of 32 polished hand-samples were processed using a lap polisher and analyzed for sedimentary macro- and micro-structures on a centimeter scale. Parameters included stratigraphic heterogeneities in shale type, ichnofabric indices (levels of bioturbation), degree and morphologies of pyrite distribution and macro- and micro-sedimentary structures indicative of benthic energy.

Results indicate that distribution of pyrite was controlled by two different environmental settings in both the Lower and Upper sections. The first involved reign of a generally low-energy environment wherein disseminated diagenetic pyrite was intermixed within the relatively undisturbed clay-rich sediment. High pyrite levels were observed to be associated with greater lamination and low ichnofabrics indices.

The second setting attests to presence of higher energy, wave action leading to churning of sediment and rapid settling of pyrite on sharp lenses of reworked matrix as in a graded bed. Such a setting is manifested by high pyrite levels associated with discrete, sharp erosional surfaces. A model for paleoenvironmental reconstruction will be presented.