APPROXIMATING WATER DEPTHS DURING DEPOSITION OF DEVONIAN–MISSISSIPPIAN BLACK SHALES IN THE NORTHERN APPALACHIAN BASIN

Water depths during deposition of Devonian-Mississippian black shales in eastern North America have been estimated at anywhere from “knee-deep” to thousands of meters, but both end-member estimates are probably incorrect. Similarly, physical, paleontological, and geochemical bathymetric indicators suggest overall deepening with time, but offer little evidence of the depths involved. However, occurrence of the black-shale sequence in the northern Appalachian Basin as alternating units of black shale and intervening coarser clastic wedges not only reflects the cyclic nature of Acadian tectonism and subsidence, but also provides an internal means of approximating water depths during black-shale deposition. These black shales clearly accumulated in a foreland-basin setting in which each black-shale unit represents an episode of rapid, foreland, flexural subsidence below the pycnocline (anaerobic conditions) and subsequent infilling of the basin with shales and coarser clastics into higher, dysaerobic and aerobic parts of the water column. Due to the largely, one-time nature of each flexural subsidence event, measuring the thickness of the infilling clastic wedge from the top of the basinal black shale to a sea-level datum in the overlying coarser-clastic counterpart provides an estimate of absolute basin depth. Because some basins were underfilled and because the varying effects of compaction cannot be easily resolved, the estimates are probably minimal at best. Nonetheless, the exercise provides order-of-magnitude estimates of depths through time and shows depths ranging from 80 to 310 m during deposition of Early Devonian-through-Early Mississippian black, organic-rich muds in the northern Appalachian Basin. The estimates not only show a general deepening with time, but also reflect shallowing-upward, third-order cycles that coincide with the timing of unconformity-bound sequences containing one or more, black shale-clastic wedge cycles.

Increasing depths over time in large part likely reflect the cumulative effects of tectonic loading, but the Devonian was also a time of rising sea level, and this, together with a unique paleogeographic setting, probably ensured large areas of enhanced organic productivity and the deep, stratified waters necessary to preserve it as basinal black shales.