REVISED STRATIGRAPHY OF THE LOWER SIMCOE GROUP OF SOUTHERN ONTARIO AND CORRELATION WITH THE CINCINNATI ARCH

Upper Ordovician sedimentary rocks in the Simcoe District of southern Ontario, Canada, dominated by shallow-water subtropical carbonates, are poorly exposed at the surface but provide key insights into paleoenvironments and sea level fluctuations from Chazyan to Richmondian age. These rocks are well exposed in several quarries in the district but contacts between certain stratigraphic units are rarely exposed and few formations are continuously exposed in any given locality. The Simcoe Group (Black River – Trenton) of southern Ontario was studied and documented from the mid-19th century to the mid-20th century. However, some of these units have been overly lumped and subsequent advances in sequence stratigraphy and chemostratigraphy reveal a need for revision. Here we present new data on high-resolution sequence stratigraphy and carbon isotope chemostratigraphy. On this basis, we propose revised correlations and nomenclature for rocks of the Simcoe Group and redefine stratigraphic contacts between members of the Bobcaygeon Formation of Liberty (1969) based on extensive field study and comparison with equivalent strata of the Cincinnati Arch.

The Bobcaygeon Formation (Upper Ordovician, Black Riveran-Trentonian) of southern Ontario represents a composite unit on the basis of sequence stratigraphy, lithostratigraphy, chemostratigraphy, and paleontological evidence; it contains one or more major erosional unconformities, disparate facies, and markedly distinct faunas. Most of the present Coboconk, previously considered to be the lower member of the Bobcaygeon, should be elevated to the status of formation as it represents the uppermost portion of the Sandbian M4 sequence and is bounded at the top by a regional erosional disconformity. The middle and upper members of the Bobcaygeon should be reverted to their original definition as the Kirkfield Formation. The upper Bobcaygeon can be further divided into two members (middle and upper Kirkfield), placing a 1–2 m thick sharply-based grainstone at the base of each member, which thus correspond to small scale depositional sequences. Based upon carbon isotope chemostratigraphy as well as biostratigraphy and allostratigraphy, these units are correlated with equivalent strata and sequences (Katian M5A [upper cycle] and M5B) of the Cincinnati Arch region.