The Sudbury Basin (), also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geological structure in Ontario, Canada. It is among the oldest and largest known impact structures on Earth. The structure, the eroded remnant of an impact crater, was formed by the impact of an asteroid 1.849 billion years ago in the Paleoproterozoic era. The ores of the Sudbury Basin are known to contain nickel, copper, gold, silver, platinum, palladium, rhodium, iridium, and ruthenium. The Basin is located on the Canadian Shield in the city of Greater Sudbury, Ontario. The former municipalities of Rayside-Balfour, Valley East and Capreol lie within the Sudbury Basin, which is referred to locally as "The Valley". The urban core of the former city of Sudbury lies on the southern outskirts of the Basin. An Ontario Historical Plaque was erected by the province to commemorate the discovery of the Sudbury Basin.
Formation
The Sudbury Basin formed as a result of an impact of a large impact body approximately 10–15 km (6.2–9.3 mi) in diameter that occurred 1.849 billion years ago in the Paleoproterozoic era. Proposed released energies for the impact are 8.6×1023 Joules and 2.31×1024 J. Debris from the impact was scattered over an area of 1,600,000 km2 (620,000 sq mi) and thrown more than 800 km (500 mi); ejecta—rock fragments ejected by the impact—have been found as far away as Minnesota. Models suggest that for such a large impact, debris was most likely scattered globally, but has since been eroded. Its present size is believed to be a smaller portion of a 130-kilometre (81 mi) diameter crater that the meteor originally created. Subsequent geological processes have deformed the crater into the current smaller oval shape. Sudbury Basin is among the largest-known craters on Earth, after the 300-kilometre (190 mi) diameter Vredefort impact structure in South Africa, and the 180-kilometre (110 mi) diameter Chicxulub crater under Yucatán, Mexico. Geochemical evidence suggests that the impactor was likely a chondrite asteroid or a comet with a chondritic component.
Structure The full extent of the Sudbury Basin is 62 km (39 mi) long, 30 km (19 mi) wide, and 15 km (9.3 mi) deep, although the modern ground surface is much shallower. The main units characterizing the Sudbury Basin are as follows (in stratigraphic order): The footwall brecciated country rock including the offset dikes, The Sublayer, The Sudbury Igneous Complex (SIC), and the Whitewater Group. Footwall rocks, associated with the impact event, consist of Sudbury Breccia (pseudotachylite), footwall breccia, radial and concentric quartz dioritic breccia dikes (polymict impact melt breccias). The sub layer is the main zone of mineralization. The SIC is an elliptical shaped differentiated igneous body. Geographically it is common to differentiate the different areas of the SIC by the North Range, the East Range and the South Range which refer to the high topographic areas around the rim of the impact site. Stratigraphically the base of the SIC starts with quartz Norite capped by brown or green norite in the south range, and mafic norite capped by felsic norite in the north and east ranges. Overlying these norite layers is quartz gabbro followed by the Crows Foot granophyre and a transition layer of normal granophyre. The Whitewater Group consists of a suevite and sedimentary package composed of the Onaping (fallback breccias), Onwatin, and Chelmsford Formations in stratigraphic succession. Because considerable erosion has occurred since the Sudbury event, an estimated 6 km (3.7 mi) in the North Range, it is difficult to directly constrain the actual size of the diameter of the original transient cavity, or the final rim diameter. The deformation of the Sudbury structure occurred in five main deformation events (by age in millions of years):
the formation of the Sudbury Igneous Complex (1849 Ma), the Penokean orogeny (1890–1830 Ma), the Mazatzal orogeny (1700–1600 Ma), the Grenville orogeny (1400–1000 Ma), and the Lake Wanapitei impact (37 Ma).
Origin
Some 1.8 billion years of weathering and deformation made it difficult to prove that a meteorite was the cause of the Sudbury geological structures. A further difficulty in proving that the Sudbury complex was formed by meteorite impact rather than by ordinary igneous processes was that the region was volcanically active at around the same time as the impact, and some weathered volcanic structures can look like meteorite collision structures. Since its discovery, a layer of breccia has been found associated with the impact event, and stressed rock formations have been fully mapped. Reports published in the late 1960s described geological features that were said to be distinctive of meteorite impacts, including shatter cones and shock-deformed quartz crystals in the underlying rock. Geologists reached a consensus by about 1970 that the Sudbury Basin was formed by a meteorite impact. In 2014, analysis of the concentration and distribution of siderophile elements as well as the size of the area where the impact melted the rock indicated that a comet, rather than an asteroid, most likely caused the crater. The Sudbury Basin is located near a number of other geological structures, including the Temagami Magnetic Anomaly, the Lake Wanapitei impact crater, the western end of the Ottawa-Bonnechere Graben, the Grenville Front Tectonic Zone, and the eastern end of the Great Lakes Tectonic Zone, but the structures are not directly related to one another in the sense of resulting from the same geological processes.
Mining
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