CATASTROPHE AND CONTROVERSY:
The Sudbury Meteorite Impact Structure, Canada

Bevan M. French
Research Collaborator
Department of Mineral Sciences, MRC 119
Smithsonian Institution
Washington, DC 20560

The Sudbury Basin in southern Ontario, 60 km long by 27 km across, is the topographic expression of a larger geological structure, about 1.8 Ga old, that has been a focus of continuous controversy since its discovery over a century ago. A wide variety of mechanisms, both internal and external, have been proposed to explain its unusual characteristics: (1) extensive shattering of older basement rocks around the Basin; (2) a thick unit of "volcanic" breccias within the Basin; (3) a large associated igneous body (the Sudbury Nickel Irruptive); (4) immense copper-nickel-iron ore deposits at the base of the Irruptive.

In the 1960s, the discovery of unique shock-metamorphic effects (shatter cones, planar deformation features (PDFs) in quartz, etc.) provided solid evidence that the Sudbury structure was produced by a meteorite impact event. The distribution of shock effects at Sudbury is similar to the patterns observed at other large impact structures. Deformed older rocks outside the Basin contain features consistent with relatively low shock pressures (5-10 Gpa or 50-100 kb): shatter cones, kink bands in biotite, and basal PDFs in quartz. Within the Basin, the Onaping Formation, now identified as a fallback breccia from the impact event, contains clasts of basement rock with features indicative of even higher shock pressures (10-50 GPa): multiple PDFs in quartz and feldspar, melting of refractory minerals such as sphene, and formation of heterogeneous airborne glassy bodies identical to the Fladen from the younger and well-preserved Ries Crater, Germany.

Even as an impact origin for Sudbury has become increasingly accepted, our picture of the structure has changed, and controversy has continued. Better understanding of the impact process indicates that the present Sudbury Basin is only a small part of a much larger structure perhaps 200 km in diameter, produced by the impact of a bolide about 10 km across (about the size of the impactor involved in the Cretaceous-Tertiary extinction event) and highly deformed and metamorphosed by subsequent tectonic events. The Basin is surrounded by a large area of deformed rocks cut by unusual pseudotachylite breccias ("Sudbury Breccia") which formed the floor of the original structure. A further combination of geochemical, geophysical, and theoretical studies now support the idea that the entire volume of the Sudbury Irruptive was produced as impact melt during the impact event and has not been derived from internal melting processes, a theory which has left the origin of the Irruptive's rich ore deposits as a source of even greater argument.

After more than a century of controversy, the impact origin of Sudbury has been well -established, but major questions remain: the detailed mechanics of the impact event, the original shape and extent of the structure, the formation and emplacement of the Irruptive, the nature and extent of subsequent deformation, and the origin of the ores. Other exciting issues extend far beyond Sudbury and include: understanding the full role of meteorite impact in Earth history, the involvement of impact in other large terrestrial igneous structures, and the use of Sudbury as a model for understanding the impact structures and the rocks produced on the Moon and other planets. The recent (1994) discovery of fullerenes ("buckyballs") in carbonaceous material in the Onaping Formation has added even newer issues to Sudbury: the formation of carbon compounds in meteorite impact events and the possible role of meteorite impacts in the origin of life on Earth.

Suggested Further Reading:

French, B.M. (1968), Sudbury Structure, Ontario: some petrographic evidence for an origin by meteorite impact, in Shock Metamorphism of Natural Materials. Ed. by B.M. French and N.M. Short, Baltimore, MD, Mono Book Corp., pp. 383-412.

French, B.M. (1972), Shock-metamorphic features in the Sudbury Structure, Ontario: a review. Geol. Assoc. Canada Spec. Paper 10, pp. 19-28.

Pye, E.G., A.J. Naldrett, and P.E. Gilbin (1984) (eds.), The Geology and ore deposits of the Sudbury Structure, Ontario Geol. Survey Spec. Vol. 1, 603 p.

Grieve, R.A.F. (1994), An impact model of the Sudbury Structure, Ch. 11 in Lightfoot, P.C., and A.J. Naldrett (eds.), Proceedings of the Sudbury-Noril'sk Symposium, Ontario Ministry of Northern Devel. and Mines, Spec. Vol. 5, pp. 119-132.

Becker, L., J.L. Bada, R.E. Winans, J.E. Hunt, T.E. Bunch, and B.M. French (1994), Fullerenes in the 1.85-billion-year-old Sudbury impact structure, Science, v. 265, pp. 642-645.