Lithofacies and age variation in the Fossil Hill Formation (Lower Silurian), southern Georgian Bay region, Ontario

The Fossil Hill Formation in the southern Georgian Bay region demonstrates considerable faunal and lithological variation. Well-defined distinctions exist between bedded chert-bearing, sparsely fossiliferous, argillaceous dolostones of the formation in the eastern Beaver Valley and relatively pure, fossiliferous, non-chert-bearing dolostones observed on the nearby southern Bruce Peninsula and Cape Rich Steps. Biostratigraphic studies and lithostratigraphic tracing through the intervening Bighead Valley suggest that the Fossil Hill Formation of the eastern Beaver Valley is correlative with the typical lower Pentamerus bank and the overlying coral-stromatoporoid biostrome of late Aeronian (Llandovery C1-C2) age observed on the southern Bruce Peninsula and Cape Rich Steps. Regionally, the Fossil Hill Formation exhibits significant age variation; biostratigraphically diagnostic brachiopods (Pentamerus oblongus, Pentameroides subrectus, and Plicostricklandia castellana) from the formation near Walters Falls indicate Telychian (Llandovery C4-C6) to early Wenlock age for part of the unit there. The highly localized preservation of Fossil Hill Formation strata of Telychian age in the Walters Falls area, along with contemporaneous facies changes noted in strata of Aeronian age and the apparent absence of Fossil Hill Formation strata at other localities, suggests that the paleotopography of the southern Georgian Bay region varied markedly during the mid to late Llandovery. A tectonic model that relates epeirogenic uplift associated with the Algonquin Arch in southern Ontario to the vertical rotation of fault-bounded lithospheric blocks, and additions to the model suggested herein, may explain the observed variations.