Correlation of the Waco Member of the Alger Shale Formation (Silurian; Llandovery; Telychian) in east-central Kentucky and south-central Ohio

摘要

New and published stratigraphic data are integrated herein to resolve the age and correlation of the Waco Member (Alger Shale Formation), a problematic Telychian lithostratigraphic unit exposed in east-central Kentucky (United States). This interval is correlated with strata presently assigned to the Dayton Member of the Drowning Creek Formation in south-central Ohio and north-central Kentucky, where recent conodont studies refer these beds to the Pterospathodus eopennatus Superzone, a conclusion that is reinforced by an interval of elevated stable carbon isotope (δ~(13)C) values, consistent with the lower Telychian "Valgu" Excursion. This correlation is strengthened by observations of facies and new carbonate carbon isotope (δ~(13)C_(carb)) data generated from localities in south-central Ohio and, for the first time in publication, from the type area of the Waco in east-central Kentucky. It is now clear that both lithostratigraphic units are substantially older than the recently redefined Dayton Formation of western Ohio, and a formal revision of nomenclature is needed. A series of revised correlations are proposed: the basal carbonate bed of the Waco in its type area is correlated to the lower "white" division of the "Dayton Member" in southern Ohio; an interval of fossiliferous shales, found in typical exposures of the Waco, is correlated to the upper "orange" division of the "Dayton" in outcrops further north. Continued use of the term "Dayton Member of the Drowning Creek Formation" at sections south and east of Greene County, Ohio, should be abandoned in favour of "Waco Member of the Alger Shale".