REEXAMINING THE ORDOVICIAN-SILURIAN BOUNDARY OF EASTERN NORTH AMERICA

The Hirnantian Stage of the Ordovician lasted for ~1.4 m.y. yet, despite its brief duration, encompassed at least two major and five minor episodes of glaciation in Gondwana and the extinction of roughly 85% of marine species. Until recently it was believed that Hirnantian age strata were absent throughout most of eastern North America. This was due to a lack of macroscopic index fossils and the presence of widespread unconformities in many latest Ordovician to Early Silurian sections. However, recent chemostratigraphic and biostratigraphic findings suggest that remnants of Hirnantian strata may remain in eastern North America.

Previous geochemical investigations of the Manitoulin Formation in Ontario by Bergström et al. revealed a positive δ¹³C_carb excursion, hypothesized to be the Hirnantian Isotopic Carbon Excursion or HICE. Our investigations continued to explore the Manitoulin and laterally equivalent strata that surround the current Ordovician and Silurian boundary in eastern North America. Detailed measurements and carbonate sampling were undertaken to identify δ¹³C_carb excursions, such as the HICE, and to correlate them across the region. Carbonate samples were collected from both drill core and outcrop that span across Indiana, Ohio, Kentucky, New York and Ontario. Sections in southern Ohio and Kentucky revealed a δ¹³C_carb excursion in the Centerville Member, which has been hypothesized to be the HICE. Extensive sampling through large sections of drill core also permitted the identification of a series of globally recognized Late Ordovician δ¹³C_carb excursions in the local stratigraphic record. Preliminary chemostratigraphic and sequence stratigraphic correlations of outcrop and drill core indicate the existence of a thin succession of Hirnantian strata sandwiched between two significant unconformities in eastern North America. The 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.