SILURIAN CONODONT BIOSTRATIGRAPHY AND δ13C STRATIGRAPHY OF THE ERAMOSA FORMATION, SOUTHWESTERN ONTARIO, CANADA

The Eramosa lithofacies were named in Ontario almost a century ago, but the relative ages and regional lithologic and stratigraphic relationships of these strata are still poorly understood. This dolostone succession is among the most economically significant Paleozoic sedimentary rocks in Ontario. It has been given formational rank, been allocated as an upper member of the underlying Amabel Formation, and been designated as the basal member of the overlying Guelph Formation. Recently completed regional mapping and core logging along the Niagara Escarpment reveals the following: a disconformable relationship exists between the Eramosa and the underlying Amabel Formation; at least two significant diastems are present in the Eramosa strata; and the contact between the Eramosa and the overlying Guelph lithofacies varies from transitional to disconformable regionally.

This study is the first attempt to carry out a combined biostratigraphic and chemostratigraphic study of the Eramosa lithofacies in southwestern Ontario. This research will provide the detailed stratigraphic framework required to determine a more accurate age for this lithofacies, help clarify the stratigraphic relationship of the Eramosa lithofacies in this region with strata currently recognized as the Eramosa in the remainder of Ontario and western New York, and facilitate correlation into the Michigan Basin. Preliminary results indicate that the Eramosa lithofacies in the Wiarton area of the Bruce Peninsula (Ontario Highway 6 outcrop and Ledgerock Quarry composite stratigraphic section) is early Wenlock (early to middle Sheinwoodian) in age. Samples from the Wiarton area record the latter part of the positive δ¹³C excursion recognized as the Ireviken Excursion and have yielded Ozarkodina sagitta rhenana. The Eramosa lithofacies from two drill cores in the Guelph area, located more than 100 km to the south, appear to record the positive δ¹³C excursion recognized as the Mulde Excursion, thereby indicating a late Wenlock (Homerian) age. A recent report of Ctenognathodus cf. murchisoni in the Eramosa Lagerstätte at Hepworth, located 15 km south of Wiarton, suggests that the biota at this locality is within the Ctenognathodus murchisoni Zone, and therefore the Eramosa lithofacies in this area would also be late Wenlock (Homerian) in age.