Designation and Interpretation of Stratigraphic Sequences Within Upper Devonian Clastic Wedge, Northern West Virginia
Jonathan K. Filer
In recent years, a stratigraphic framework for the Upper Devonian distal black (organic) and gray (inorganic) shales of the west-central Appalachian basin has been developed by several workers. The contemporaneity of these units with certain units of the exposed equivalent rocks in New York and Ohio has been recognized. The abrupt and widespread transitions from gray to black shale facies in these exposed rocks have been recognized as resulting from rapid transgression.
In the subsurface of West Virginia, gamma-ray density logs can be used to trace certain black shale beds that extend east toward the center of the basin and mark rising sea levels. The black shale marker beds change facies to the east, where an equivalent gray shale persists within the coarser clastic wedge. Apparently, the base level rises that produced the black shale marker beds resulted in temporary nondeposition of coarser clastics in the central and eastern parts of the basin. Newly formed coastal environments, such as back-barrier lagoons and estuaries, would have served as sediment traps during these periods.
The resulting black and/or gray shale beds represent a synchronous deposit of relatively short duration, and may therefore be used to divide the clastic wedge into chronostratigraphic sequences. Some of the boundaries of these sequences may be placed within the standard time scale by reference to their New York equivalents; the age of others is uncertain at this time.
Facies and isopach maps of 10 chronostratigraphic intervals (ranging from the base of the Cashaqua Member to the top of the lower Huron Member in the west, and the Sycamore grit to the Speechley "sand" in the east) are useful for basin analysis.
During regressive phases, sediment was deposited in long prograding tongues apparently on a clastic "ramp." During transgressive phases, a significant shelf and slope system developed, resulting in sediments bypassing the shelf and/or upper slope and concentrating silt and sand along strike.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.