Mark A. Kleffner

Conodont biostratigraphy of the upper part of the Clinton Group and the Lockport Group (Silurian) in the Niagara Gorge region, New York and Ontario

Twenty-nine conodont species are represented in formations comprising the upper part of the Clinton Group and the Lockport Group at four localities in Niagara County, New York, and adjacent Ontario, Canada. Four previously unrecognized species of Ozarkodina represented in two samples from the Oak Orchard Formation are described. Ozarkodina sagitta rhenana (Walliser) and O. sagitta sagitta (Walliser) appear to form an evolutionary lineage in which the latter evolved from the former during the late early Wenlockian by slight modification of the Pa element. Elements in the other corresponding positions in their respective apparatuses are virtually identical. The Williamson Shale, Irondequoit Limestone, Rochester Shale, and DeCew Formation of the Clinton Group belong in the upper part of the Pterospathodus amorphognathoides to the middle part of the Ozarkodina sagitta sagitta Chronozone, and are early to early middle Wenlockian in age. The Gasport, Goat Island, Eramosa, and Oak Orchard Formations of the Lockport Group belong in the middle part of the O. sagitta sagitta to the lower part of the Ancoradella ploeckensis Chronozone, and are early middle to late Wenlockian in age.

Late Ordovician-Early Silurian delta C-13 chemostratigraphy in the Upper Mississippi Valley: implications for chronostratigraphy and depositional interpretations

A pioneer study of the previously unknown delta C-13 chemostratigraphy in the Ordovician/Silurian boundary interval in eastern Iowa and northeastern Illinois resulted in the discovery of the Hirnantian Isotope Carbon Excursion (HICE). The presence of this major isotope excursion in the Mosalem Formation in Iowa and the Wilhelmi Formation in Illinois, which indicates that the excursion interval in these units is of Hirnantian (latest Ordovician) rather than Early Silurian age, necessitates a revised chronostratigraphic classification of these units. Although the precise level of the Ordovician/Silurian boundary remains somewhat uncertain in the absence of the diagnostic graptolites, it is herein placed in the upper part, but well below the top, of the Mosalem Formation and at the top of the Wilhelmi Formation. During a major regression following the deposition of the Maquoketa Shale, the upper part of the latter elastic unit was in some places deeply eroded, resulting in a topographically dissected landscape with upland areas separated by wide incised valleys. During a subsequent late Hirnantian transgression, these palaeovalleys were gradually filled with marine sediments, but the upland areas were not transgressed until earliest Silurian times. The new chemostratigraphical evidence is in good agreement with the available biostratigraphical data, especially from corals, conodonts, and brachiopods. A preliminary chemostratigraphical study of the presumably coeval Edgewood Group successions in Pike County, northeastern Missouri failed to document any heavy delta C-13 values characteristic of the HICE and some, or all, of the Hirnantian values obtained there may be diagenetically overprinted.

Sequence boundaries and chronostratigraphic gaps in the Llandovery of Ohio and Kentucky: The record of early Silurian paleoceanographic events in east-central North America

New and published data are integrated herein to resolve the age and stratigraphic relationships for problematic strata of the Aeronian and Telychian (Llandovery; Silurian) in Ohio and Kentucky (USA). At least two major depositional sequences were traced along the eastern flank of the Cincinnati Arch; these are separated by a regionally angular unconformity with complex topography. Underlying units are progressively truncated to the northwest while overlying strata change facies, condense, and onlap in the same direction. The basal unit of the upper sequence is the Waco Member of the Alger Shale Formation in Kentucky and southern Ohio and the Dayton Formation in western Ohio. A persistent, positive carbonate carbon isotope (δ13Ccarb) excursion associated with the mid-Telychian Valgu Event is recognized in the upper subunit of the Waco Member; the absence of a comparable signal in the Dayton Formation corroborates interpretations that it is significantly younger. The correlations proposed here can be used to understand the nuanced depositional history and chronostratigraphic completeness of the lower Silurian in eastern North America. This framework can be used to characterize sea-level history and local conditions that prevailed during global paleoenvironmental events.