Upper Ordovician (Sandbian-Katian) sub-surface stratigraphy of the Cincinnati Region (Ohio, USA): transition into the Sebree Trough

摘要

Facies and carbon isotope analysis of a series of four drill cores from Cincinnati, Ohio provide an opportunity to assess Late Ordovician paleoenvironmental variability along the margin of the Sebree Trough, an intracratonic basin associated with Taconian far-field tectonics. The cores provide stratigraphic data for an approximately 50km long NE-SW transect that roughly parallels the margin of the trough. The cores span the upper High Bridge Group through much of the equivalents of the Lexington Formation, and in one case into the overlying Cincinnatian strata (Kope). The cores were logged at centimeter-scale for Iitho-, bio-, and tapho-facies. A number of marker beds that are present in the cores, including K-bentonites, fossil epiboles (e.g., Prasopora bryozoans), and deformed beds, have been previously documented in the central Kentucky outcrop belt. We also sampled one core for carbon isotopes as a means of providing additional constraints on correlation into the central Kentucky outcrop belt. The facies succession in the lower half of the cores matches that of the lower members of the Lexington Formation in Kentucky (Curdsville, Logana, and lower Grier members). The upper half of the cores contain a more shale-rich facies succession, recording offshore environments that were substantially deeper than those of the Lexington Platform of the central Kentucky outcrop belt. These results corroborate previous studies, which indicated that the Sebree Trough expanded during the earliest Katian. Two positive carbon isotope excursions identified from outcrops of the Logana Member and the Macedonia bed on the Lexington Platform, are clearly recognizable in the sampled Cincinnati region drill core. We believe that one or both of these excursions are equivalent to the Guttenberg carbon isotope excursion (GICE) of the upper Mississippi Valley.