Sudbury Basin

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, the western end of the Ottawa-Bonnechere Graben, and the eastern end of the Great Lakes Tectonic Zone although none of the structures are directly related to each other in the sense of resulting from the same geophysical processes.

Formation and structure

NASA World Wind satellite image of the Sudbury astrobleme

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 basin formed as an impact from a bolide approximately 10–15 km (6.2–9.3 mi) in diameter that occurred 1.849 billion years ago in the Paleoproterozoic era.

Image taken from Space Shuttle Challenger in 1984/1985

Debris from the impact was scattered over an area of 1,600,000 km² (620,000 sq mi) and traveled over 800 km (500 mi) away — rock fragments ejected by the impact have been found as far as Minnesota. Models suggest that for such a large impact, debris was most likely scattered globally, but has since been eroded away. Its present size is believed to be a smaller portion of a 250 km (160 mi) 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 (190 mi) Vredefort crater in South Africa, and larger than the 170 km (110 mi) 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 (3.7 mi) 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 five main deformation events (by age):

1. formation of the Sudbury Igneous Complex (1849 Ma)
2. the Penokean orogeny (1890-1830 Ma)
3. the Mazatzal orogeny (1700-1600 Ma)
4. the Grenville orogeny (1400 - 1000 Ma)
5. the Lake Wanapitei impact (37 Ma)

Disputes over origin

Even as late as the mid-1970s, geologists were not fully certain of the Sudbury basin having meteor origins. The educational television programme Planet of Man—created for Canadian universities to use in introductory geology classes—mooted the possible volcanic origins of the Sudbury geological structures.

Considering how old the evidence is for a meteor impact, some 1.8 billions of years of weathering and other geological processes made it difficult to prove with 1970s era technology that a meteor was the cause of the Sudbury geological structures. Since then an ash and conglomerate layer has been found associated with the impact event and stressed rock formations have been fully mapped.

The cause of the ongoing difficulty with proof of the meteor event was that the region was volcanically active around the same time as the impact—and some weathered volcanic structures can look like meteor collision structures given the right conditions.

Modern uses

The large impact crater filled with magma containing nickel, copper, platinum, palladium, gold, and other metals. 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 Geological Survey of Canada, who confirmed "the presence of an immense mass of magnetic trap".

Due to the then-remoteness of the Sudbury area, 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. The development of a mining settlement occurred 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. 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.

See also: Economic geology

External links

• Earth Impact Database
• Aerial Exploration of the Sudbury Crater
• Fallbrook Gem and Mineral Society - Sudbury Structure page
• ¹⁹⁰Pt– ¹⁸⁶Os and ¹⁸⁷Re– ¹⁸⁷Os systematics of the Sudbury Igneous Complex, Ontario. Study by John W. Morgan et al. Geochimica et Cosmochimica Acta, Vol. 66, No. 2, pp. 273–290, 2002.