Biochemostratigraphy of the Eramosa Formation in southwestern Ontario, Canada

Abstract

The lithostratigraphic term “Eramosa” was introduced in Ontario more than a century ago to include a distinctive package of thin- to medium-bedded, black to medium-brown dolostones that make up key cuesta faces and railway roadcuts along the Eramosa River in the City of Guelph, southwestern Ontario, Canada. This stratigraphic unit makes up part of a stacked carbonate succession that constitutes one of the most economically significant Paleozoic sedimentary rocks in Ontario. The strata assigned to the Eramosa have a complex history of lithostratigraphic study, and the relative age, regional lithostratigraphic relationships, and varied depositional environments of the Eramosa were poorly understood. This research, which combines conodont biostratigraphy and carbonate carbon (δ13Ccarb) isotope stratigraphy to generate a detailed chronostratigraphic framework for the Eramosa Formation in southwestern Ontario, is part of a regional-scale surface and subsurface mapping initiative of the Silurian strata by the Ontario Geological Survey. Dolostone samples from Wiarton and the City of Guelph, Ontario, yielded three biostratigraphically important conodonts: Ozarkodina sagitta rhenana, Kockelella walliseri, and Kockelella ortus ortus. The carbonate carbon (δ13Ccarb) isotope data from the studied intervals record the descending limb of the Sheinwoodian (Ireviken) positive δ13Ccarb isotope excursion, including a distinctive positive shift in δ13Ccarb isotope values typical of records of the descending limb of the excursion in Laurentia and Baltica. Aldridgeodus minimus was also recovered from these conodont faunas and co-occurs with Kockelella walliseri, below the last occurrence of Ozarkodina sagitta rhenana (Lower Kockelella walliseri Zone), suggesting that the range of Aldridgeodus minimus should be extended lower into the Sheinwoodian Stage.