PALEOENVIRONMENTAL SIGNALS RELATING TO RECOGNITION OF THE LATE DEVONIAN FRASNIAN-FAMENNIAN STAGE BOUNDARY IN THE FOREKNOBS FORMATION OF VIRGINIA AND WEST VIRGINIA

Analysis of twenty roadcut exposures of the Foreknobs Formation and equivalent rocks in the central and northern Appalachian basin have been used to map the Frasnian-Famennian (F/F) stage boundary as defined by the F/F extinction event. Biostratigraphic analyses by McGhee (1977) and McGhee and Dennison (1980) used macrofossils to define F/F boundary and extinction. These works were based on several sections along a northeast trend from Hightown, VA, to LaVale, MD, and then extended into Pennsylvania by Warne and McGhee (1991). The Pound Member, a thin conglomeratic sandstone, of the Foreknobs Formation, was used as the maker horizon for the F/F boundary. The Pound lithostratigraphic-biostratigraphic horizon is fairly consistent as these locations are distributed along approximate paleodepositional- paleoenvironmental strike within the Catskill clastic wedge.

Later investigations at sections that trend northward from Elliott Knob, VA, to Elkins, WV, suggest that the Pound Member is time transgressive and that rapidly changing water depth associated with the progradation of the Catskill clastic wedge have influenced the recognition of the F/F extinction event and boundary. Locations in the east, nearer the Acadian highlands, shallowed so rapidly that the marine fauna was forced westward into the basin well before the end of the Frasnian. In the western portion of the basin, some locations may have been so deep as to act as a refugium for Frasnian stage fossils after the extinction event while others show a distinct termination horizon of Frasnian fossils without an introduction of Famennian stage fossils followed by very rapid shallowing to terrestrial facies. Therefore the rapidly changing nature of the Catskill clastic wedge presents a challenge in recognizing the F/F boundary based on macrofossils that more consistent environments, such as those associated with black shales, may not.