Paper No. 19-6
Presentation Time: 9:15 AM
TIMING OF THE LOWER KELLWASSER EXTINCTION (LATE DEVONIAN) RELATIVE TO DARK SHALE DEPOSITION IN THE APPALACHIAN BASIN
The timing of a mass extinction should correspond with the timing of the environmental perturbation that drove it, but establishing the exact timing of extinction can be difficult, particularly in sections with substantial changes in facies. For example, many extinctions correspond with the development of black/dark shale, often taken as indicating low oxygen conditions. The disappearance of species at or near the base of the shale could indicate extinction driven by lowered oxygen (or associated conditions) or migration to more clement habitats, with extinction occurring later. We tested these possibilities for brachiopod extinctions in the Lower Kellwasser (LK) event (Frasnian, Late Devonian) in New York and Pennsylvania. This region was chosen because the sections are temporally expanded and represent a range of environments along an onshore-offshore gradient. Specifically, we have been correlating the extinction event into increasingly shallow-water settings, in part to test hypotheses on the timing of the extinction. In more distal sections in New York (Hornell, Cameron), the victims of the extinction all occur last below the Pipe Creek Shale (LK equivalent). Further onshore in northern Pennsylvania (Tioga), at least two of the victims of the extinction last occur in the shallower-water strata just above the Pipe Creek Shale. Well-preserved fossils have been located in fine grained strata, suggesting they are not reworked. These results reinforce the value of examining extinction events in a wide range of paleoenvironments. Because some victims postdate the dark shale, the LK extinction in this region is unlikely to have been driven entirely by anoxia, unless evidence of anoxia can be uncovered further upsection. Given preliminary δCorg records from these sections, we speculate that the timing of extinction of the species that post-date the development of dark shale is consistent with global cooling as a kill mechanism.