A STRATIGRAPHIC STUDY OF BENTHIC PALEOECOLOGY FROM THE FRASNIAN/FAMENNIAN TRANSITION (LATE DEVONIAN) IN NEW YORK

On a global basis, the Frasnian/Famennian (F/F) transition is marked by extinction, geochemical perturbations, and dramatic paleoecologic changes. Here, we examine this time interval in New York based on bed-level collections of macrofossils from F/F boundary sections that represent a variety of siliciclastic paleoenvironments. The ultimate goal of this project is to address the dynamics of the F/F extinction in this region. In previous work on stratigraphic sections from offshore paleoenvironments, Over (1997) placed the F/F boundary in a thin bed of black shale in the uppermost Java Formation, just below the base of the Dunkirk Shale. Our field area is further onshore and conodont biostratigraphy has not been possible, but a correlative bed marking the F/F boundary appears to be present in the uppermost Java. The stratigraphic ranges of brachiopods are consistent with this interpretation: purported victims of the F/F extinction (e.g., Spinatrypa sp.) persist into this bed at several localities, but do not occur above it. Ongoing geochemical analyses should help verify the position of the F/F boundary, but for now, several lines of evidence constrain it to within several meters. At some locations, there is a pronounced change in fossil assemblage composition several meters before the inferred position of the boundary, from Ambocolia- and productid-dominated to orthid-dominated (Stainbrookia and Schizophoria sp.). The orthid-dominated assemblage continues through the inferred position of the F/F boundary to the base of the Dunkirk Shale. At other locations, there is no pronounced change in assemblage composition during the uppermost Java Formation, other than the disappearance of Spinatrypa and perhaps a couple other taxa. Fossil assemblages from the lower Famennian of New York are discussed in comparison with the Frasnian assemblages. We have also collected fossils from purported boundary sections in Pennsylvania. Located further to the east, these locations represent paleoenvironments that were closer to the paleoshoreline. However, these collections require further study, and the position of the F/F boundary in these sections is less certain.