Subsurface Correlation of Lower Silurian Medina Group, Southwestern Ontario
Abstract
The Medina Group succession in southwestern Ontario includes, in ascending order: the Whirlpool, Manitoulin, Cabot Head, Power Glen, Devils Hole-Balls Falls, Grimsby, and Thorold formations. Subsurface mapping in central and eastern Lake Erie has resulted in the recognition of three stratigraphic packages within the Medina Group based upon the integration of sedimentologic, stratigraphic and geophysical borehole logs. The interplay between changing paleoenvironments, sediment provenance, and regional tectonics and associated sea level fluctuations has resulted in the accumulation of a complex mosaic of mixed siliciclastics and carbonates. Manitoulin Formation carbonates and Whirlpool Formation siliciclastics display consistent thicknesses across the study area. Sitting disconformably above these strata are the Cabot Head/Power Glen grey/green shales with thin quartz sandstone stringers. This shale-dominated succession is overlain by the Devils Hole Formation pink/grey, slightly dolomitic medium to fine grade quartz sandstones and the Ball’s Falls dolostones and mixed dolomitic shales. A subregional phosphatic nodule horizon, recognized in the Niagara Region as the Artpark phosphate bed, has been correlated across the study area. The overlying Grimsby Formation is characterized by interbedded red and green sandstones, siltstones, and shales, with multiple bryozoan biostromes in the western part of the study area. The greenish-grey quartz arenites of the Thorold Formation cap the Medina Group succession and with the underlying Grimsby Formation represent a historically significant gas play in southwestern Ontario.