The nature and timing of the Middle Devonian Kacak bioevents in the Marcellus Subgroup of the Appalachian Basin

Intervals of faunal turnover in the fossil record are often preserved at unconformities, suggesting that their apparent abruptness is an artifact of geologic discontinuities. A detailed investigation of the Middle Devonian Kacak Bioevents of the Appalachian Basin focused on stratigraphically complete sections of eastern New York State to further elucidate the nature and timing of faunal turnover where not cloaked by an unconformity. The Lower Kacak Bioevent is fairly well constrained to the lower portion of the East Berne Member of the Mount Marion Formation. The Upper Kacak Bioevent is less well constrained to within the middle to upper East Berne Member, however. A thorough dissection of the East Berne Member has yielded the more precise timing of the initial Hamilton Fauna incursion, occurring at the level of the thin, fossiliferous, Dave Elliot Bed (DEB). The occurrence of the goniatite Tornoceras aff. mesopleuron below the DEB constrains the Hamilton incursion to the earliest Givetian age. The DEB fauna is composed of a single taxonomic holdover from the older Stony Hollow Fauna, while the entire rest of the DEB fauna carries over into the remaining Hamilton E-E (ecological-evolutionary) subunit. From the data at hand, this faunal incursion does not represent a gradual or step-wise incursion. When studied in the thickest, most conformable, stratigraphically complete sections preserved, this incursion occurs as an abrupt event. The pattern of faunal change preserved is a two-step extinction and turnover, with extinction occurring at the onset of dysoxic flooding (Lower Kacak Bioevent), followed by the rapid incursion of new fauna associated with sea-level fall (Upper Kacak Bioevent) in the area under consideration.