APPROXIMATING ABSOLUTE DEPTHS FOR DEVONIAIN-MISSISSIPPIAN BLACK SHALES IN THE NORTHERN APPALACHIAN BASIN, EASTERN UNITED STATES: IMPLICATIONS FOR EUSTATIC VS. TECTONIC CONTROL OF SEA LEVEL

Physical, paleontological, and geochemical bathymetric inidcators in the Devonian-Mississippian black shales of the northern Appalachian Basin suggest overall deepening with time, but offer little evidence of the actual depths involved. However, organization of the black-shale sequence into black shale-clastic wedge cycles and probable accumulation of the shales due to periods of rapid, foreland, tectonic subsidence allow absolute depth approximations by measuring the thickness of the clastic wedge from the top of the basinal black shale to a corresponding sea-level datum. Because some basins were underfilled and because the varying effects of compaction could not be considered, the estimates are minimal at best. Nonetheless, they provide order-of-magnitude estimates and reflect depths ranging from 80m to 310m during deposition of Lower Devonian through Lower Mississippian black shales in the northern Appalachian Basin. The depth estimates not only show a general deepening with time, but also reflect shallowing-upward, third-order cycles that coincide with unconformity-bound sequences containing one or more black shale-clastic wedge cycles. To determine the origin of these depth cycles, the positions of respective black-shale basins and paleopycnoclines were mapped in space and time. Results show that black-shale basins tracked the progress of the Acadian Orogeny as predicted by flexural models. However, analysis of actual versus predicted paleopycnocline positions for some basins shows that there were probably depth components related to eustacy and local structural activity that cannot be explained by flexural models. Although either wholly eustatic or wholly tectonic mechanisms are commonly employed to explain depth fluctuations inherent in the black-shale sequence, the above models suggest that both tectonic and eustatic mechanisms are involved. More important, however, is the fact that in some cases, it may be possible to determine the relative importance or eustatic versus teconic depth components.