IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
SUDBURY IMPACT STRUCTURE
Large Meteorite Impacts and Planetary Evolution II, a major portion of the data quoted in this documentation of the Sudbury impact structure, was gleaned from this particular volume that contains many articles on the Sudbury Igneous Complex.
The Sudbury structure shows that large volumes of subsurface magma can be generated by impact. The Sudbury structure is a large (D ~200 km, 1850 Ma), deformed and eroded impact crater, whose central region was occupied by melt. An eight-year multidisciplinary study by Stoffler et al. (1994) concluded that the impact excavated deep into the crust, almost to the mantle (~30 km), before collapse and rebound. The present eccentric shape is due to subsequent tectonism. The melt (> ~12,000 km3), possibly superheated, formed by impact melting of crust within just a few minutes. The magma differentiated by gravity settling of crystals and immiscible sulfides to produce hundreds of metres of noritic cumulates (norite is a type of gabbro). Early formed pyroxene and sulfides were swept into basal depressions to form mineralised norite, overlain by slowly cooled igneous-textured rocks with differentiated compositions (Stoffler et al, 1994).
There is no record of volcanism at Sudbury but it may have been spectacular. The high temperatures implied by coexisting immiscible melts and mafic magmas are comparable to those of many large igneous intrusions, representing the mid- to upper-crustal reservoirs feeding surface volcanism. At Sudbury, the presence of pseudotachylites (veins of shock-induced glassy rock), contact zone breccias and an array of peripheral shock features is well established. Any mantle melt component is thought to have been small, but could have been delivered almost instantly via crust-spanning dykes with rapid post-crater closure (Price 2001).
Rapid closure of fractures may explain the absence of feeders in impact-induced melt bodies such as Sudbury. The Sudbury nickel deposits are crudely concentrated around the margins of the impact cavity and form the largest nickel mining district ever mined. The source of the nickel could be the impactor in terms of mass balance, although isotope data suggest a crustal source for the accompanying sulphur. In other impact craters, there is evidence for associated downwards and outwards injection of magma, forming dykes, breccias, and pseudotachylites, and for the establishment of vigorous hydrothermal systems. The Sudbury rootless impact melt, the likelihood of superheat, and the formation of immiscible sulphides are valuable lessons for mainstream igneous petrology and global ore prospecting. (Adrian P. Jones 2005)
The Sudbury Structure comprises a 200-250 km multi ring impact basin formed at 1.85 Ga. The core of the structure is elliptical, 60 x 30 km, containing a layered 2.5 km thick impact melt sheet, referred to as the Sudbury Igneous Complex (SIC). The SIC was formed by differentiation of the impact melt pool at the probable main contact point of the impactor.
I want to thank Frank Brunton and James E. Mungall for their assistance and allowing me to quote from their published papers (listed at the end of this article). |
This is Frank Brunton beside my chariot, C-GOZM immediately after our aerial exploration of the crater. Note the F18 in the right background. The Sudbury airport is on the east rim of the SIC and directly over the South Range Shear Zone.
Formation of the original crater
The Sudbury Structure is situated within a unique Geotectonic setting in northeastern Ontario, being sandwiched between:
- the Archean-age (>2.5 billion-year-old) Superior Geologic or Structural Province, situated to west and north of the structure, and;
- the Proterozoic-age (>1.9 billion-year-old) Southern Geologic or Structural Province Huronian Supergroup, deformed by the (1.9 billion –year-old) Penokean orogeny , and situated to west, south and east of the Sudbury Structure.
The boundary of the Proterozoic-age (~1 billion-year-old) Grenville Geologic Province presently lies approximately 10 km to southeast of the SIC. The Grenville orogeny occurred 800 million years after the Sudbury Crater was formed. The SW-NE trend of the Grenville Front Structural Zone, which delineates the northernmost margin of the Grenville Structural Province, is roughly parallel to the long-axis of the SIC.
Segments of the Crater
The Sudbury Structure is interpreted to represent the tectonized and deeply eroded remnant of a multi-ring or peak-ring impact basin (Stoffler et al). Approximately 4 km of erosion over the eons has obliterated the crater rim. Tectonism has possibly deformed the original crater into an ellipse. The subsequent metamorphism in the structure is tied to tectonic activity such as collision of continents and folding and thrusting up of crustal rocks. A zone of deformation (shatter cones and rock metamorphism) has been documented to 74 km from the SIC.
The Sudbury Igneous Complex (SIC)
Thick sheets of melted rocks line the bottom of many large meteor craters. Some of these impact melts are derived from the release of kinetic energy at impactor contact that is converted to heat. Also, rocks lying kilometers deep within Earth are often on the verge of melting but are prevented from doing so by the immense pressure from the weight of the material lying above them. A large impactor would blast away this weight, releasing the pressure on the buried rocks and causing the underlying minerals to melt.
The impact melts may not fully cool for hundreds of thousands of years. In the meantime, water from the environment and the heat from the newly exposed rocks can combine to form hydrothermal systems in the heavily fractured rocks in and around the crater. Scientists believe such warm mineral-rich venues could have played a role in the early development of life on Earth. (Science News: 3/9/02, p. 147) Evidence of the hydrothermal systems is documented in my ground tour.
The SIC is this type of large melt sheet produced from crustal melting resulting from a cosmic impact. The target rocks, which remained within the crater after the impact, ponded to form a sub horizontal sheet of magma and differentiated as it cooled. It is currently exposed as an elliptical 60 x 30 km, 2.5 km thick remnant of the original impact melt sheet and consists, from bottom to top, of inclusion-rich, in places ore-bearing, quartz diorite sub layer, norite, quartz gabbro, and granophyre layers, and, within the target rocks surrounding the SIC, the quartz dioritic offset dikes.
Lake Wanapitei Meteorite Crater is visible adjacent to the distorted east edge of the SIC oval. Quantitative evidence stating that the deformation of the SIC in this area was caused by the Wanapitei impact has not been documented to date (2004). The close proximity of these two impact structures is strictly coincidence. The Wanapitei crater occurred over 1.8 billion years after the Sudbury impact. (Image courtesy of Earth Impact Database, 2003). (see: Side Note below)
The Whitewater Group
The SIC is overlain by the 1.8 km thick Onaping Formation. It consists of impact melt breccia, suevite and reworked suevite from:
- Fall-back (collapse of the original crater) and Fall-out (impact debris) forming a 2 km post impact sediment over the SIC melt rock; and,
- Wash-in – post impact sediment (the impact happened in a shallow sea).
The rock fragments in the breccias of the Onaping Formation are from the impact target Archean and Proterozoic rocks of the Superior and Huronian Provinces of the Canadian Shield (Brunton).
The Onaping Formation is covered by 600 metres of argillites and minor exhalative carbonates and cherts of the Onwatin formation. This formation occurred during a period of quiescence after the impact basin formation.
The end of this quiet period was signaled by the abrupt appearance of the 850 metre-thick siliciclastic turbidites (sedimentary deposits settled out of muddy water carrying particles of widely varying grade size caused by turbidity currents) of the Chelmsford Formation (Rousell, 1972, 1984), which have been interpreted as a flysch apron deposited in the foredeep ahead of an advancing late Penokean mountain front (Young et al. 2001).
The next series of images will document my aerial exploration the crater, first from the north outside of the SIC, and gradually working across the structure to the south. Most of my images were from approximately 2000 feet above the ground.
The 200-m-thick impact melts found within the Sudbury Crater are a treasure trove of minerals. More than $1 billion of metal ores including those bearing nickel, platinum, and copper are mined from the melts each year. Isotopic analyses show that the metals come from Earth's crust, not from the meteorite that fell from space. Before the impact melt solidified, the deep, thick blend of light silicates and dense metal ores—which didn't mix well with each other—separated into two layers, according to density, just like oil and vinegar do. This ancient segregation makes mining today much easier (Brunton).
The hydrothermal system created by the Sudbury impact also dissolved minerals containing copper and other metals from a broad area and then concentrated them in rich veins. (Richard Grieve, Natural Resources Canada in Ottawa)
Surrounding Brecciated Footwall Rocks
The pulverized and melted country rock injected into the pink gneisses has similar chemistry to the derived gneisses (RIGHT). The greenish rock is secondary alteration of clays and micas from existing minerals within the rock. The brecciated zones fall along structures inferred as super faults or fault-controlled structures. The crustal rocks underlying the crater would have experienced substantial fracturing from the impact and the shear concentration of the billions of tonnes of the metal-rich rock that subsequently formed could have easily have created more fracturing while sinking down deep into the crust. (F. Brunton – private correspondence with the author)
Most of the SB dikes dip vertically or steeply and apparently have no obvious preferred orientation with respect to the present shape of the Sudbury Structure.
Characteristics of the Sudbury Breccia:
- concentrated within 5 km of the SIC;
- formed by dynamic means during very rapid deformation. (e.g., post impact friction-induced melting during the extensive and very rapid deformation and brecciation of the footwall rocks);
- a two component rock consisting of a fine-grained to aphanitic matrix surrounding inclusions of host-rocks and minerals;
- comprised of mineral and rock fragments derived predominantly from wall rocks, set within a typically dark, microcrystalline to fine grained matrix, generated by grinding and frictional melting; and
- mostly associated with the 2.5 billion year old Matachewan dykes. In the immediate area of the shattered bedrock are samples of the Matachewan dykes.
Ten pseudotachylyte samples from the North Range of the 1850 Ma Sudbury impact structure have been analyzed by the 40Ar/39Ar laser spot fusion method. Field and petrological evidence indicate that the pseudotachylytes were formed at 1850 Ma by comminution and frictional melting due to impact-induced faulting. The cryptocrystalline to microcrystalline grain size (<30 μm) of the pseudotachylyte matrices and the predominance of orthoclase as the main K-bearing phase, have rendered the rocks particularly susceptible to Ar loss. The age determinations range from ∼1850 to ∼1000 Ma, with some samples yielding multiple ages that cannot be correlated with known geological events in the area. However, if the finite-difference algorithm of Wheeler (1996) is used to calculate combined Ar loss and the accumulation of radiogenic Ar for the K-bearing phases, it is possible to reproduce the range of observed ages. The model infers that the long-term volume diffusion of Ar has occurred and that, as a result, the Ar system cannot be treated with a conventional closure temperature approach. The algorithm requires burial of the impact structure to 5–6 km depth and 160–180 °C at 1850 Ma, followed by exhumation at ∼1000 Ma. These ages may be equated with two events: Penokean thin-skinned overthrusting in the North Range, immediately following impact, and exhumation ∼850 Ma later, coincident with the Grenville orogeny to the southeast. The results suggest that, contrary to previously accepted paradigms, the North Range has been affected by a protracted period of postimpact, low-grade thermal metamorphism. If these events also involved tectonic shortening within the North Range (as has been documented for the South Range), then the original size of the Sudbury impact structure has been underestimated.(Spray et al, Feb 2010)
Another terrestrial example of this type of breccia is found in the Vredefort meteorite crater in South Africa.
Sudbury Igneous Complex (SIC)
Further into the structure is the Whitewater Group, a 1400 m thick section consisting of fall-back of the original country rocks that has been hydrothermally altered. Ground water had seeped into faults caused by the impact, the water boiled creating hot springs through the Whitewater Group.
The impact probably occurred in a shallow sea as there is evidence of water flow-back in the top layers of the Whitewater Group. The quantity of “breccia fall-back” specifies that the fall-back segment of the impact lasted a substantial amount of time (perhaps hours) before the appearance of the returning tsunami.
The size of the Sudbury structure implies that the hydrothermal venting continued for thousands of years after the impact. The rocks of the Whitewater Group comprise (oldest-to-youngest): initially glass-rich breccias of the Onaping Formation, carbonates and argillites of the Vermilion and Onwatin formations, and arkosic sandstones and wackes of the Chelmsford Formation (Brunton).
Bucky balls (soccer-ball-shaped molecules of 60 carbon atoms) possibly of extraterrestrial origin and with traces of helium and argon gas trapped inside were found in this breccia.
The controversy over the origin of the Sudbury Structure and the Sudbury Igneous Complex was ongoing before the beginning of mining in the area. My father was involved in the mining industry before the onset of WWII and he always thought that the structure was somehow involved with volcanic activity. I remember being in high school when my science teacher mentioned that he had recently read a paper hypothesizing that the Sudbury Structure may be the result of a meteorite impact. Also that it had happened long before any life had evolved to survive on land (evolution was not allowed to be taught in Ontario schools at that time!). I found this idea fascinating and I think that this was probably where my interest in meteorite craters originated.
The magnitude of scientific information describing the Sudbury Structure over the years has amplified my desire to fully explore this crater and others. This project was one of my lifelong dreams realized and I am still amazed at the magnitude of the “event” that created this structure.
|Wanapitei impact structures is strictly coincidence. The Sudbury impact happened over 1.8 billion years before the one at Wanapitei. In another coincidence, relatively nearby in northern Quebec’s Canadian Shield, is another double impact site, the Clearwater East and Clearwater West impact structures. The Clearwater impacts, shown in this image, are related and simultaneously occurred 290 million years ago. Image courtesy of Earth Impact Database, 2003.|
Jones, A.P., 2005. Meteorite Impacts as Triggers to Large Igneous Provinces, Elements, Vol 1, PP. 277-281.
Maddock, R.H., 1983. Melt origin offault-generated pseudotachylytes demonstrated by textures, Geology, Vol 11, no 2.
J.E. Mungall and J.J. Hanley: ORIGINS OF OUTLIERS OF THE HURONIAN SUPERGROUP WITHIN THE SUDBURY STRUCTURE. Department of Geology, University of Toronto.
Ostermann, M., Scharer, U., Deutsch, A., 1996. Impact melt dikes in the sudbury multi-ring basin: Implications from uranium-lead geochronology on the Foy Offset Dike.
Philpotts, A.R. Origin of Pseudotachylites, American Journal of Science, Vol 262 1964.
Price, G.D., Price, N.J., Decarli, P.S., and Clegg, R.A., Fracturing, thermal evolution and geophysical signature of the crater floor of a large impact structure: The case of the Sudbury Structure, Canada. Geological Society of America Special Papers, 2010, 465, p. 115-131, 2001.
Sharpton, V.L., Dressler, B.O. (editors), Large Meteorite Impacts and Planetary Evolution II,
Spray, J.G., Thompson, L.M., Kelley, S.P., 4 FEB 2010 - Laser probe argon-40/argon-39 dating of pseudotachylyte from the Sudbury Structure: Evidence for postimpact thermal overprinting in the North Range - Article first published online: 4 FEB 2010
Stöffler, D., Deutsch, A., Avermann, M., Bischoff, L., Brockmeyer, P., Buhl, D., Lakomy, R. and Müller-Mohr,B., The formation of the Sudbury Structure, Canada: Towards a unified impact model. Geological Society of America Special Paper 293, pp. 303-318. 1994.
Young, G.M., Long, D.G.F., Fedo, C.M., Nesbitt, H.W.,Paleoproterozoic Hunonian basin: product of a Wilson cycle punctuated by glaciations and a meteorite impact. Department of Earth Sciences, University of Western Ontario, 2001.