The Sudbury Basin, also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geologic structure in Ontario, Canada. It is the second largest known impact crater or astrobleme on Earth, as well as one of the oldest.
The basin is located on the Canadian Shield in the city of Greater Sudbury, Ontario. The former municipalities of Rayside-Balfour and Valley East 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.
The Sudbury Basin is located near a number of other geological structures, including the Temagami Magnetic Anomaly, the Lake Wanapitei impact crater and the western end of the Ottawa-Bonnechere Graben, although none of the structures are directly related to each other in the sense of resulting from the same geophysical processes.
The full extent of the Sudbury Basin is 62 km long, 30 km wide and 15 km deep, although the modern ground surface is much shallower. It was created as the result of a 10 km meteorite impact that occurred 1.85 billion years ago in the Paleoproterozoic era. Debris was scattered over an area of 1.6 million square kilometers and travelled over 800 kilometers away — rock fragments ejected by the impact have been found as far as Minnesota. Its present size is believed to be a smaller portion of a 250 km round crater that the bolide originally created. Subsequent geological processes have deformed the crater into the current smaller oval shape. Sudbury Basin would then be the second largest crater on Earth, after the 300 km Vredefort crater in South Africa, and larger than the 170 km Chicxulub crater in Yucatán, Mexico.
The main units characterizing the Sudbury Structure can be subdivided into three groups: 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 Sudbury Breccia (pseudotachylite), footwall breccia, radial and concentric quartz dioritic breccia dikes (polymict impact melt breccias), and the discontinuous sublayer. Because considerable erosion has occurred since the Sudbury event, an estimated 6 km in the North Range, it is difficult to directly constrain the actual size of the Sudbury crater, whether it being the diameter of the original transient cavity, or the final rim diameter.
The deformation of the Sudbury structure occurred in four main deformation events (by age):
In 1856, while surveying a baseline westward from Lake Nipissing, provincial land surveyor Albert Salter located magnetic abnormalities in the area that were strongly suggestive of mineral deposits. The area was then examined by Alexander Murray of the Canadian Geological Survey, who confirmed "the presence of an immense mass of magnetic trap".
Due to the then-remoteness of the Sudbury area, however, Salter's discovery did not have much immediate impact. The later construction of the Canadian Pacific Railway through the area, however, made mineral exploration more feasible and finally resulted in the development of a mining settlement in 1883 after blasting connected to the railway construction revealed a large concentration of nickel and copper ore at the Murray Mine site.
As a result of these metal deposits, the Sudbury area is one of the world's major mining communities. The region is one of the world's largest suppliers of nickel and copper ores. Most of these mineral deposits are found on the outer rim of the basin.
Due to the high mineral content of its soil, the floor of the basin is among the best agricultural land in Northern Ontario, with numerous vegetable, berry, and dairy farms located in the Valley. However, due to its northern latitude, it is not as fertile as agricultural lands in the southern portion of the province. Accordingly, the region primarily supplies products for consumption within Northern Ontario, and is not a major food exporter.
An Ontario Historical Plaque was erected by the province to commemorate the discovery of the Sudbury Basin.