NEW INSIGHTS INTO OLD SHALES: UNDERSTANDING DEPOSITIONAL PROCESSES AFFECTING PRESERVATION OF THE MILLBORO BLACK SHALE IN A DEVONIAN MARINE BASIN

The Millboro Shale unit of the Middle Devonian, Appalachian Basin, has long been understood as a classic organic-carbon rich, clay deposit which accumulated in a epicontinental, anoxic benthic environment. Microbial sulfate reduction by sulfur-reducing bacteria consumed marine organic carbon and led to formation of pyritiferous horizons in this shale. Relatively little, however, is known about just how constant, or consistent, such a paleoenvironment was at the time these beds accumulated.

This study investigates a series of three correlative stratigraphic sections in outcrops of the Millboro Shale near Radford in SW Virginia to reconstruct the Devonian sea-bottom and the dominant biogeochemical processes that affected sediment accumulation in this part of the Appalachian Basin. Field observations included use of a 10 square centimeter grid placed over the shale beds to determine the frequency and size distribution of pyrite thus creating a Density of Pyritization (DOP) curve. Saw-cut slabs were also obtained for intervals that were highly laminated and precluded easy recognition of pyritiferous intervals. Such slab samples were then cut and polished to facilitate observation of the DOP. Additionally all samples, on a centimeter scale, were analyzed for their ichnofabrics and level of bioturbation. Lastly, thin sections were used to determine sedimentary structures on a millimeter scale and for understanding the nature of sedimentary deposition versus erosion events.

Results reveal a surprising, counter-intuitive, difference in the DOP values between the darker, finely laminated shale layers versus the lighter-colored, massive, intensely bioturbated, mudstones. The laminated shale intervals characterized by little to no bioturbation contain mostly finely-disseminated, microscopic, framboidal pyrite (as observed under a Scanning Electron Microscope) with a generally low volume but a high total surface area. The bioturbated mudstones, in contrast, reveal larger pyrite mineralizations (mostly as nodules) with an overall low total surface area but a high volume. A new paleoenvironmental model is proposed which highlights the role of terrestrially-derived organic carbon and elevated benthic levels of oxygenation in formation of the shale and mudstone lithologies at this location.