Geology of the Sudbury District
The Sudbury District is underlain by rocks representing five different geologic environments, including: Archean aged (>2500 Ma) granitic, gneissic, metavolcanic and metasedimentary rocks; shelf sedimentary and volcanic rocks of the Paleoproterozoic aged (2500-1600 Ma) Huronian Supergroup; Mesoproterozoic aged (1600-1000 Ma) rocks of the Sudbury Structure; high-grade metamorphic rocks of the Mesoproterozoic aged Grenville and Grenville Front Tectonic Zone; and, Paleozoic aged (544-251 Ma) Ordovician and Silurian limestones and dolostone on Manitoulin Island.
Of the Archean provinces in the Canadian Shield, the Superior is the largest, the southern part comprising easterly trending belts, alternating for the most part between gneisses and greenstone. Archean rocks underlying the Sudbury District are of the southern Superior Province, and contain the Benny Greenstone Belt, along with smaller, fragmented greenstone terranes.
The Southern Province contains the Huronian Supergroup, a south-easterly thickening wedge of Paleoproterozoic, quartz-rich, shelf clastic sediments and subordinate mixed volcanic rocks 7-11 km thick which accumulated along the southern edge of the Superior Province. Lower strata of the Huronian include uranium-bearing conglomerate. The southeast part of the wedge was variably metamorphosed and deformed by folds and thrust faults pushed north during the Penokean Orogeny (2200-1700 Ma). In the Sudbury District, the Southern Province extends north-eastward from the Penokean fold belt north of Manitoulin Island along the southern margin of the Sudbury Basin and continues northeast around the eastern shore of Lake Wanapitei to the Cobalt embayment.
The Sudbury Basin and associated nickel-rich intrusion are thought to have resulted from a meteorite impact about 1850 Ma, probably the second largest of these events to have occurred on Earth. The impact scattered ejecta over an area of 1.6 million km2, extending more than 800 km from the event. The present day basin is about 60 km long and 30 km wide, a smaller portion of the 250 km round crater originally created. Subsequent geological processes such as the Grenville Orogeny have deformed the crater into the current smaller oval shape. The Sudbury Structure straddles the contact between the Superior and Southern provinces.
The main units characterizing the Sudbury Structure include the Sudbury Igneous Complex (SIC), the Whitewater Group, and footwall brecciated country rocks that include offset dikes and the Sublayer. The SIC is believed to be a stratified impact melt sheet composed from the base up of Sublayer norite, mafic norite, felsic norite, quartz gabbro, and granophyre. The Whitewater Group consists of a suevite and sedimentary package composed of the Onaping (fallback breccias), Onwatin, and Chelmsford formations in stratigraphic succession. Footwall rocks, associated with the impact event, consist of pseudotachylitic Sudbury breccia, footwall breccia, radial and concentric quartz dioritic breccia dikes (polymict impact melt breccias) and the discontinuous Sublayer.
Convergence of Grenville Province terranes (1300-1000 Ma) against the Southern Province completed assembly of the Canadian Shield. Along the northern boundary of the Grenville Province the orogenic Grenville Front is characterized by thickened crust and north-easterly trending structures pushed towards the northwest. Uplift and erosion have revealed rocks metamorphosed to upper amphibolite facies along the front.
The Paleozoic rocks of Manitoulin Island unconformably overlie Precambrian basement rocks of the Huronian Supergroup which outcrop in the eastern part of the island. The oldest Paleozoic rocks are of late Ordovician age (~465 Ma), overlain by Silurian aged (~420 Ma) strata deposited in warm, shallow seas just south of the equator. Dolostone of the Amabel Formation is quarried at the west end of the island by Lafarge Canada.
