Geology of the Southwestern Ontario District

Bedrock Geology

With the exception of a small area of the Precambrian Canadian Shield in the extreme northeastern corner of the District, Southwestern Ontario is underlain by sedimentary rocks of Paleozoic Age. (350 – 500my)

All rock units are flat-lying and undeformed with a gentle regional dip to the southwest. The sedimentary rocks that comprise the Paleozoic succession were largely deposited in shallow marine environments, at times when the area was covered by a series of inland seas.

Rocks on the southeast edge of the District were influenced by sedimentation in the foreland Appalachian Basin. Rocks in the northwest portion of the District form a major portion of the east-side of the Michigan Basin. The Algonquin Arch, a broad basement ridge, dissects the District in a southwest orientation separating the two basins. The basins and arch controlled the distribution and accumulation of sediments. The thickness of Paleozoic strata over the Precambrian basement increases in a southwesterly direction from zero to a maximum of 1350 m near Sarnia. Paleozoic rocks are, from west to east, Devonian, Silurian and Ordovician in age.

Ordovician

Precambrian gniess and felsic igneous rocks of the Grenville Province are overlain by Ordovician age rocks. The Shadow Lake Formation consists of red and green, dolomitic and sandy shale, generally 2 to 3 m in thickness, unconformably resting on Precambrian basement. The overlying succession of limestones, carbonate mudstones, calcareous shales and non-calcareous shales of the Gull River, Bobcaygeon, Verulum and Lindsay Formations represent relatively continuous deposition. These units comprise the Simcoe Group.

The Simcoe Group is overlain by sediments of the Blue Mountain, Georgian Bay and Queenston Formations. The Blue Mountain Formation is a blue-grey-brown, non-calcareous shale. The Georgian Bay Formation, which underlies much of Toronto, is a blue-grey shale with minor siltstone and limestone. The Queenston Formation is an easily weathered, maroon-coloured shale with interbeds of grey-green shale, limestone and siltstone. It forms the base the Niagara Escarpment. The Niagara Escarpment is a prominent erosional cuesta, with an east-facing scarp, which bisects the District in a southeast orientation.

Silurian

Silurian age units are exposed along the face of the Niagara Escarpment and in an outcrop belt extending up to 70 km southwest of the Escarpment. The Cataract Group, at the base of the Silurian succession, consists of the Whirlpool, Manitoulin, Cabot Head and Grimsby Formations. Fossiliferous dolostones of the Manitoulin Formation overlie the Whirlpool sandstone north of Hamilton. The overlying unit, the Cabot Head Formation, consists of shale with calcareous sandstone, limestone and dolostone interbeds. The maroon sandstones and shales of the Grimsby Formation comformably overlie the Cabot Head Formation on the Niagara Peninsula. Overlying the Cabot Head shales on the Bruce Peninsula are a succession of carbonates and shales termed, in ascending order, the Dyer Bay, Wingfield and St. Edmund Formations.

In the Niagara Peninsula, in ascending order, the Thorold, Neahga, Reynales, Irondequoit, Rochester and Decew Formations are assigned to the Clinton Group. Most of these units pinch out or are truncated by unconformities to the west and north. Clinton Group units are a succession of sandstones, shales, argillaceous dolostones, dolomitic limestones, limestones and siltstones. The Lockport Formation unconformably overlies the Clinton Group. The dolostone and dolomitic limestone rocks of the Lockport Formation form the brow and main cliffs of the Niagara Escarpment.

In southern Ontario outside of the Niagara Peninsula and Lake Erie areas, carbonate deposition predominated during the equivalent time period. Fossil Hill Formation is the lateral transition of the Reynales Formation. The Fossil Hill Formation is a fossiliferous dolostone. Overlying are the thick, extensive dolostones of the Amabel Formation. The white weathering, thick-bedded Amabel dolostones form the main cliffs of the Niagara Escarpment north of Hamilton. Amabel Formation is approximately equivalent to the Lockport Formation on the Niagara Peninsula.

Across the District, the Guelph Formation overlies the Lockport and Amabel Formations. The Guelph Formation consists of reef and inter-reef deposits, characterized by tan, sugary, fossiliferous dolostone. After a probable unconformity, evaporites, evaporitic carbonates and shales of the Salina Formation were deposited. The Salina Formation is conformably overlain by dolostones of the Bertie and Bass Island Formations, the youngest Silurian strata in the District. The Bertie Formation is an Appalachian Basin unit. The lateral equivalent Bass Island Formation is a Michigan Basin unit.

Devonian

The early Devonian in southern Ontario was characterized by an extended period of erosion. Silurian strata is separated from the Oriskany Formation by a disconformable contact. In most areas, the Bois Blanc Formation is the oldest Devonian stratum. The Bois Blanc Formation is a cherty limestone in the Appalachian Basin and dolostone in the Michigan Basin. The Bois Blanc Formation is overlain by Onondaga Formation limestone in the Niagara Peninsula and the Detroit River Group elsewhere in southwestern Ontario. In ascending order, the Sylvania, Amherstburg and Lucas Formations are included in the Detroit River Group. The Sylvania sandstone is restricted to the subsurface in the Windsor area. The Amherstburg Formation is a relatively thick bituminous limestone. It is conformably overlain by the Lucas Formation of interbedded dolostone and limestone strata. In the Algonquin Arch area, the Lucas Formation consists of a high-purity limestone. The overlying Dundee Formation is the subcrop strata for broad area between lakes Erie and Huron. The Dundee Formation is a fossiliferous, micitric limestone. In a small area north of lake Erie, the Dundee Formation is overlain by the black, organic-rich shales of the Marcellus Formation.

A sharp change from carbonate to shale-dominated strata begins above the Dundee and Marcellus Formations. The overlying Hamilton Group consists of mudstones and shales with thin carbonate horizons. The formations constituting the Hamilton Group are, in ascending order, Bell, Rockport Quarry, Arkona, Hungry Hollow, Widder and Ipperwash. An unconformity separates the Hamilton Group from the overlying Kettle Point Formation. The Kettle Point Formation consists of a siliciclastic organic-rich shale. It is disconformably overlain by the Port Lambton Group. This group of clastic rocks consists mainly of grey and black shales and sandstones. In southern Ontario, Port Lambton Group strata are restricted to the subsurface in an area south of Sarnia.

Economic Geology

In 2008, the Southwestern Ontario District had over 40 non-aggregate mineral quarries and mines in operation. The following minerals and rocks were extracted - salt (rock and brine), gypsum (wallboard), shale (brick), limestone (cement, lime, aggregate), dolostone (dolime, aggregate, building stone), sandstone (building stone), gneiss (building stone) and oil and gas. Unconsolidated aggregate from various glacial deposits is extracted from numerous pits across the district.