Master of Science
Brunton, Frank R.
Ontario Geological Survey
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.
Summary for Lay Audience
The Medina Group stratigraphy in southwestern Ontario includes, in ascending order: the Whirlpool, Manitoulin, Cabot Head, Power Glen, Devil’s Hole-Ball’s Falls, Grimsby, and Thorold formations. The depositional environment of these formations has undergone many changes over the Silurian age, resulting in a complex layering of these units. Some of the formations in the Medina Group do not have adequate descriptions and therefore confusion exists in relating the descriptions of these formations to other forms of data, such as geophysical logs and previous studies. 47 wells were chosen that compiled a set number of characteristics (e.g contained core, drill cuttings, and geophysical logs) that would aid in the description of the 7 formations. Once the core was described, the information was translated and correlated to geophysical log signatures. By combining detailed core logging and drill cutting descriptions with available geophysical borehole logs (gamma-ray and neutron) data, this combination of data develops criteria to describe formational contacts for future geologists to compare and study. The combination of this data can then help infer the environment that these formations were deposited in, aiding in the description of the geometry of the geology in the subsurface. This geometry can then aid in 3D modelling, or aid in the exploration of oil and natural gas in the subsurface of Ontario.
Paterson, Rhys H., "Subsurface Correlation of Lower Silurian Medina Group, Southwestern Ontario" (2022). Electronic Thesis and Dissertation Repository. 9024.