NEW INSIGHTS INTO THE TRILOBITE AND CONODONT BIOSTRATIGRAPHY OF THE MIDDLE-UPPER DEVONIAN GENESEE GROUP IN EASTERN NEW YORK STATE

During the Middle Devonian (Givetian) Global Taghanic Biocrisis, numerous trilobite families underwent extinction including the Homalonotidae. In northeastern North America, the last occurrence of homalonotid trilobites (Dipleura dekayi) has previously been reported in strata deposited during the final stages of the Taghanic Biocrisis (semialternans Zone –latest Middle Givetian) including the Tully Formation of the northern Appalachian Basin and the Petoskey and Thunder Bay formations of the Michigan Basin. Globally observed low-oxygen conditions associated with the Taghanic Biocrisis, represented by the Geneseo and Antrim black shales in the Appalachian and Michigan basins, respectively, have been interpreted to be at least in part responsible for the local extinction of the Homalonotidae and other trilobite families. Recent geologic mapping in eastern New York State (northern Appalachian Basin) has documented the occurrence of Dipleura dekayi in siliciclastic-dominated, nearshore post-Taghanic strata that were deposited below fair-weather wave base. Although these strata have been mapped as Genesee Group, they have yet to be assigned a formation-level designation. An integrated stratigraphic approach, including litho-, sequence-, and conodont biostratigraphy has provided new insight into the stratigraphic succession in the eastern Genesee Group, representing marine shelf through non-marine settings. Lithostratigraphic correlation places the occurrence of Dipleura dekayi above the level of the Fir Tree Limestone of western, offshore sections. Application of a sequence stratigraphic model has enabled the identification of time-rich intervals such as flooding surfaces and sequence boundaries that have yielded conodonts. Preliminary conodont biostratigraphic data suggests that Dipleura dekayi persisted into at least the latest Middle Devonian Lower subterminus Zone (=Lower disparilis Zone). Preliminary mapping further suggests that Dipleura dekayi may have even survived into the Late Devonian. Similar to other northern Appalachian Basin taxa that persisted through the Taghanic Biocrisis, Dipleura dekayi apparently found intra-basinal refuge in oxygenated, nearshore shelfal settings.