The Hirnantian-Rhuddanian succession in East Central United States: New implications for latest Ordovician to early Silurian sea level

Abstract

A detailed understanding of the Late Ordovician to early Silurian glaciations and Hirnantian mass extinctions requires high resolution pattern and timing of sea level and climatic events. The record is particularly poorly known in the eastern USA, which was previously assumed to lack Hirnantian strata. Synthesis of new and previously reported biostratigraphic evidence and carbon isotope chemostratigraphy indicates that thin but widespread, fossiliferous upper Hirnantian strata are preserved in southern Ohio and southeastern Indiana. Elevated values of δ¹³C_carb, within and just beneath strata of the Ozarkodina hassi Zone, in siltstones and carbonates of the lower Whippoorwill Formation (new, herein) of the Cincinnati Arch in Ohio and Indiana, indicate the signature of the upper part of the Hirnantian isotopic carbon excursion (HICE). The presence of conodonts of the O. hassi Zone and brachiopods, corals, and a large dalmanitid trilobite belonging to the globally recognized Edgewood-Cathay fauna strongly supports a latest Hirnantian age for the Whippoorwill Formation. Sequence stratigraphy of the Whippoorwill Formation and the immediately overlying interval allowed for recognition of two thin (possibly 4th order) sequences in the Ohio and Indiana region (Hirnantian-2A (H2A), Centerville Member, and Hirnantian-2B (H2B), Belfast Member). The older Hirnantian (H1) sequence is absent in the greater Cincinnati region and probably truncated beneath the composite H2 or Cherokee Unconformity. Locally, the H2A and H2B sequences are truncated or completely removed beneath the higher Rhuddanian S1A erosion surface (base Brassfield Formation). In turn, the highly irregular lower Aeronian S2B erosion surface (base Oldham) can also locally cut deeply through lower Silurian, Hirnantian, and even upper Katian strata. Correlation of these sequences and their bounding unconformities across eastern North America reveals widespread sequences probably controlled by glacioeustasy during the latest Hirnantian and Rhuddanian.