FIRST DOCUMENTATION OF EARLY WENLOCK IREVIKEN POSITIVE d13CCARB EXCURSION IN THE TYPE AREA OF THE NIAGARAN PROVINCIAL SERIES: IRONDEQUOIT LIMESTONE, ROCHESTER SHALE, DECEW DOLOMITE, GASPORT DOLOMITE, AND GOAT ISLAND DOLOMITE ARE ALL SHEINWOODIAN (EARLY WENLOCK) IN AGE
A high-resolution d13Ccarb stratigraphic study recently performed on strata of the Clinton and Lockport Groups in the Niagara River Power Project S-1 core (Lewiston, New York) resulted in the first-ever recognition in those strata of the most widely known and best-documented positive d13C excursion of the Silurian, the Early Wenlock Ireviken Excursion. A sharp increase in the d13Ccarb values (+2.96 to +5.55) near the Rockway Dolomite/Irondequoit Limestone contact marks the base of the Ireviken Excursion in the upper part of the Clinton Group, for it is known to have begun shortly following the extinction of Pterospathodus amorphognathoides amorphognathoides (just above the Llandovery/Wenlock boundary), which has its highest representation in western New York in the uppermost Rockway or basal Irondequoit. Apart from a slight decease in values in the lower Rochester Shale, d13Ccarb values are consistently around +5.00 in the Irondequoit Limestone, Rochester Shale, and DeCew and Gasport Dolomites. d13Ccarb values gradually return to baseline (approximately +2.50) in the lower part of the Goat Island Dolomite (Niagara Falls Member), marking the end of the Ireviken Excursion. The Ireviken Excursion is known to end within the middle Sheinwoodian Kockelella walliseri Zonal Group, a zone based on the range of the nominative species. That species has its highest representation in the lower part of the Ancaster Member of the Goat Island in western New York, just slightly above the level at which d13Ccarb values return to baseline. Since the Ireviken Excursion began in the earliest Sheinwoodian and ended well before the end of the Sheinwoodian, all of the strata containing a record of that excursion in western New York must also be Sheinwoodian (Early Wenlock) in age. A steady decline in d13Ccarb values of nearly 5.00 from baseline is recorded in the Ancaster and Vinemount Members of the Goat Island through the lower Eramosa Dolomite. A steady decline in d13Ccarb values continuing well after the Ireviken Excursion has never previously been documented, and remains enigmatic until further study.