REEXAMINING THE δ13CCARB CHEMOSTRATIGRAPHY OF THE UPPERMOST ORDOVICIAN STRATA IN OHIO

This study examined the uppermost Ordovician Strata of Ohio, which includes the Waynesville, Liberty, Whitewater, Elkhorn Formations, in order to place relevant δ¹³C_carb excursions into stratigraphic context. Sampling through large sections of drill core permitted the identification of a series of globally recognized Late Ordovician δ¹³C_carb excursions in the local stratigraphic record. Previous studies identified three separate excursions of δ¹³C_carb (Waynesville, Whitewater and Elkhorn excursions) in the uppermost Ordovician Strata of Ohio, however this study recognized only two clear excursions, the Waynesville and Elkhorn excursions. Significant elevated values of δ¹³C_carb were not found in the Whitewater Formation of any drill core examined in this study. Other δ¹³C_carb anomalies were recognized in the drill cores. Low values δ¹³C_carb were found in the lower Liberty Formation throughout most of the drill cores and in outcrop. Elevated δ¹³C_carb found in the earliest Silurian aged Whippoorwill Formation were also identified. Based off a previous study by Bergström et al. of the Manitoulin Formation of Ontario, which is the Whippoorwill’s lateral equivalent, these elevated δ¹³C_carb values may suggest the presence of the Hirnantian Isotopic Carbon Excursion (HICE) directly overlying the Elkhorn Excursion. This hypothesis directly contradicts the century long presumption that Hirnantian aged strata was absent in the Ordovician record of the Appalachian Basin.

Our investigations continued to explore the Manitoulin, Whippoorwill and laterally equivalent strata that surround the current Ordovician/ Silurian boundary in eastern North America. Detailed measurements and carbonate sampling were undertaken to identify the hypothesized HICE and correlate it across the region. Preliminary chemostratigraphic and sequence stratigraphic correlations indicate the existence of a thin succession of Hirnantian strata sandwiched between two significant unconformities. The sandwiched strata with a putative HICE signature include the Manitoulin Formation (in part), Whirlpool Sandstone, and Centerville Member of the Whippoorwill Formation, all previously assigned a lower Silurian age. These form a thin but distinctive depositional sequence reflecting a Hirnantian glacioeustatic transgression.