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Dr. Benjamin Dattilo
Department of Geosciences
Indiana University – Purdue University Fort Wayne
patchiness of both litho- (grain size and mineral composition) and biofacies (fossil content of a rock). Shallow water environments also favor long-ranging generalist species that are not useful for correlation. Correlation may be further complicated by reciprocal sedimentation (sediment occurs in different places during different phases of sea level change), with proximal (close to sediment source) sedimentation and distal (far from sediment source) sediment starvation during sea-level rise alternating with proximal erosion and distal sedimentation during sea-level fall.
The Ordovician Maysvillian transect from Cincinnati, OH to Central Kentucky displays a transition from subtidal (up to a few meters deep) to peritidal facies (area above water at low tide and below at high tide). The sub-Bellevue unconformity (stratigraphic gap) is a major feature of the section in Cincinnati where erosion truncates several beds within the Miamitown Shale. Generally the Fairview–Bellevue section in Cincinnati is correlated to the Calloway Creek Formation (subtidal), and Tate (peritidal) and Gilbert (subtidal) members of the Ashlock Formation in Kentucky. While there are theoretical sequence stratigraphic considerations in making such a correlation, the precise position of the sub-Bellevue unconformity has proven difficult to pinpoint within this succession. There are particularly few clues to correlation in the relatively monotonous, fossil-poor peritidal lithologies of the Tate.
Bergström et al. (2010) established some potentially useful carbon isotope excursions in this interval based on a composite Cincinnatian section, with the Maysvillian section sampled near Maysville, Kentucky. The recovery following the Fairview excursion particularly appears to mark the location of the sub-Bellevue unconformity in the Bergström section. We refined the isotope curve for the upper Kope–Mt. Auburn Member in the Maysville area.
To test the ability to correlate from deep subtidal into more shallow water facies, we sampled four other sections along a subtidal to peritidal transect. The Fairview excursion can be tracked into the deep subtidal near Cincinnati, OH, but to the south an extended positive to negative trend through the Tate may indicate that it correlates to a similar but more abrupt recovery associated with the Bellevue. This finding may be evidence of an expanded peritidal section deposited during a time when sediment starvation was occurring in the more distal part of the basin.
Thus, while carbon isotopes have been found useful for matching relatively thin strata worldwide, more intensive sampling of this region shows that the carbon isotope curve is more complex than previously thought. Global correlations depend on fossils, which are found in deeper water-deposits. We hypothesize that the consistent sampling of sub-tidal sediments, combined with the differential distribution of sediments during different phases of sea-level cyclicity introduces a systematic bias in which time periods are sampled.
Earth Sciences | Physical Sciences and Mathematics
Freeman, Rebecca L.; Fischer, Sarah; Schramm, Thomas J.; Brett, Carlton E.; Mosser, Sasha L.; Blair, Michael; and Chakraborty, Suvankar, "Can carbon isotopes constrain high-resolution stratigraphy of Ordovician shallow water facies in the Cincinnati, Ohio Region?" (2015). 2015 IPFW Student Research and Creative Endeavor Symposium. 20.