THE FRASNIAN/FAMENNIAN BOUNDARY IN SHALLOW MARINE FACIES OF NEW YORK, USA, WITH PRELIMINARY ANALYSES OF FAUNAL DISTRIBUTIONS

The Frasnian/Famennian (F/F) transition is characterized by mass extinction and major geochemical perturbations. In New York State, the Upper Devonian is represented by the Catskill clastic wedge, and the shallow marine facies of this thick sedimentary package preserve an abundant benthic fauna. However, faunal changes across the F/F boundary have never been studied in detail (bed-by-bed) in these facies, in part because the boundary has never been precisely located. Here, we narrowly constrain the placement of the F/F boundary at several stratigraphic sections in Steuben County, largely based on the stratigraphic distribution of brachiopods. In previous studies of offshore sections (in which conodonts were abundant, but macrofauna were scarce), Over (1997) placed the boundary within a thin black shale bed within the Java Formation, slightly below (~ 1-3 meters) the base of the regionally extensive Dunkirk Shale. In the Steuben County sections, a potentially correlative bed is also located within several meters of the base of the Dunkirk Shale and represents the last transgressive pulse before the major transgression of the Dunkirk. It is variably developed as a dark, silty shale or a dark, silty mudstone. Brachiopod taxa previously described as victims of the F/F extinction (e.g., Spinatrypa) do not occur above this bed, although they persist up to it. Other taxa, which are presumably survivor taxa (e.g., Stainbrookia), continue to occur in immediately overlying strata. A simple analysis of confidence intervals on brachiopod stratigraphic ranges is consistent with the placement of the extinction horizon slightly below the Dunkirk Shale. Paleoenvironmental changes through the sections (e.g., sediment type, water energy, etc.) exerted strong controls on the occurrences of at least some brachiopod taxa, as described by a non-metric multi-dimensional scaling analysis of taxon abundances. The New York record provides a rich data set for future analyses of the Frasnian/Famennian extinction.