THE UPSLOPE LIMITS OF TRANSGRESSIVE, LATE DEVONIAN-BASAL MISSISSIPPIAN BLACK SHALES ON THE CATSKILL DELTA PRODELTA SLOPE: PHYSICAL CHARACTERISTICS AND CONTROLS

Recent fieldwork shows that several prominent black shale units (Late Famennian Cleveland Member, Early Mississippian Sunbury Shale), long known in Ohio sections, have been found in northwestern Pennsylvania. Likewise, thinner black shale units (earliest Famennian Point Gratiot Bed; Hanover Formation; medial Famennian Lyons Road Bed; Ellicott Formation) are also found to be widespread.

The newly named Point Gratiot Bed, containing the F-F extinction horizon, is a regionally widespread, compact, hard, black shale layer. In the easternmost sections where it has been identified confidently, it remains thin relative to associated gray, prodeltaic, slope deposits. By contrast, several thicker, highstand-late highstand black shale units, exemplified by the overlying Dunkirk Formation, expand greatly deltaward, passing upslope into microrhythmic, black-gray, “zebra facies” or more uniform dark gray shale.

The Lyons Road Bed, Cleveland Shale, and Sunbury Shale all grade upslope into condensed, microbioturbated, dark gray (dysoxic) siltstone beds which are notably coarser and thinner than associated gray shale units. These transitional, dysoxic, siltstone layers grade further into macrobioturbated and current-rippled fine sandstone beds in neritic shelf facies, but remain thin relative to surrounding units. The occurrence of multiple microbioturbated siltstone beds within the Cleveland black shale in northeast Ohio, suggests that these thin layers are time-rich and represent minor transgressive, sediment-starvation events within a larger highstand context. These condensed, coarser layers appear to merge upslope with residual laminated black shale relegated to thin partings between dark siltstone beds. Bioturbation, combined with current-related sediment-bypass in a sediment-starved slope setting, is believed to account for these thin, coarse beds.