From OttawaRasc

IMPACT CRATER EXPLORATIONS

Sudbury Impact Structure

Age (ma): 1850 +/- 50
Diameter: 250 km (estimated)
Location: Ontario, Canada. N 46° 36' W 81° 11'
Shock Metamorphism:
- shatter cones (up to 3 m in length);
- PDF in quartz, feldspar and zircon grains;
- overturned collar rocks of South Range structure, and;
- brecciation of country rocks occurring up to 80 km from the Sudbury Igneous Complex.
Dating Method: U-Pb dating on zircons in the Sudbury Igneous Complex, radioactive dating of the melts and the hydrothermal deposits.

Introduction

While documenting the Sudbury Meteorite Crater from my airplane and from the ground, I realized that for a non-professional geologist the geographic features that state that “this area is a meteorite crater” are not obvious. For this reason the initial content of my article will be a geologic description of the remnants of the present day Sudbury Meteorite Crater (highlighted by the circle in this topographic image). I will also document the evidence stating that the anatomy of the Sudbury complex was formed as the result of a cosmic collision. In this way I hope to better explain my aerial and ground images of the crater.

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).

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.

Formation of the 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. [Note 2]

Illustrated in this sketch (courtesy of James E. Mungall) is a view of the sequence of events that may have produced the current structural relations between the SIC and the Huronian outliers (Huronian sediments were deposited between 2450 and 2219 Ma on the subsiding margin of the Superior craton). The transgressive nature of the passive margin produced a sequence which onlapped and thinned progressively toward the northwest. The Blezardian orogeny caused the formation of basement-cored tight folds in the metasediments, which were peneplained and submerged by 1850 Ma. At 1850 Ma a large impactor created a transient crater at least 100 km in diameter and 30 km deep somewhere in the vicinity of the current SIC. Within about ten minutes of the impact, the crater had rebounded and collapsed into its final form. Inward collapse of the transient crater walls was accomplished along detachment surfaces, now preserved as anastomosing networks of pseudotachylite-filled faults (Sudbury Breccia) tens of km in length. Lateral collapse and structural uplift in the center worked together to form a crater approximately 200 km in diameter. The South Range Shear Zone (SRSZ) line on the sketch is the transition from pristine North Range to deformed South Range of the SIC and occurs over a distance of less than 20 km. [Note 1]

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.

This geologic schematic of the Sudbury structure (courtesy of F. Brunton) illustrates the present day remnant of the Sudbury Meteorite Crater comprising of:

the surrounding brecciated footwall rocks of both the Superior and southern Structural Geologic Provinces extending up to 100 km away from the present-day position of the Sudbury Igneous Complex (SIC);
the Sudbury Igneous Complex (that formed as a result of impact-triggered magmatism, or deep crustal melting); and
the Sudbury Basin within the SIC, comprising rocks of the Whitewater Group (found only in the interior of the SIC). The Whitewater Group consists of the Onaping, Onwatin and Chelmsford Formations [Note 2].

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.

In this aerial radar image of the Sudbury Structure, the 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). [Note 3]

When the crater is viewed from the altitude of the International Space Station, only the SIC within the Sudbury Structure (highlighted by the oval in the landsat image) is identifiable as being related to an impact event. The structure is located in central Ontario, north of Georgian Bay and north-west of Lake Nipissing [Note 4]. The city of Sudbury is located to the south-east of the SIC.

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. [Note 2]

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).

AERIAL TOUR

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.

This image was taken from the north of the structure while flying over the Superior province country rock. In the immediate foreground is the area containing the Matachewan dykes and the Sudbury Breccia followed by the north rim of the SIC comprising of the different minerals of the lower and upper zones. Here at the time of impact, a 1 km cross section of country rock surrounding the crater was instantaneously melted, forming about 31,000 cubic kilometers of impact melt! This represents approximately six times the volume of lakes Huron and Ontario combined, and nearly 70 percent more than the melt at Chicxulub (Pope, Geo Eco Arc Research). In the background is the internal bowl shaped portion of the SIC containing the Whitewater group.

I took this image above the SIC north rim. The internal edge of the SIC is illustrated here by the Vermillion River which is immediately adjacent to the internal north rim of the SIC and meanders through the relatively flat area of the Whitewater Group. All geology enclosed by the SIC is a result of fall-back, fall-out and wash-back.

This image of the floor of the Sudbury Structure was taken from about 3000’ over the center of the structure looking east over the Proterozoic rocks of the Whitewater group. The deepest mine shaft in the Sudbury complex, the Creighton Deep Project, is more than twice as deep as the altitude where this image was taken from! The northern rim of the SIC is visible in the left of the image with Lake Wanapitei in the left background. The town of Val Therese is in the foreground with the town of Hanmer just behind and to the east. I took the ground image (below) of the SIC north rim from just west of Hanmer. Garson Lake visible to the extreme right of the image is situated in the center of the southern rim of the SIC. The long axis of Garson Lake points at the Sudbury airport which is at the south east SIC origin of the South Range Shear Zone (SRSZ). It is barely visible in the haze in the background. The meandering river visible to the north (left) of Garson Lake is approximately over and running parallel to the SRSZ.

This lower altitude image is looking north east from directly over the north-east corner of the Whitewater Group. The relatively flat geology of the Whitewater Group is terminated by the sharp north east rim of the SIC. In the background beyond the SIC is Lake Wanapitei. Under the Sudbury Basin are thousands of kilometres of drifts (lateral tunnels) and shafts (vertical to inclined tunnels) cut into the SIC to extract nickel. If these tunnels were placed end-to-end across Canada, it would almost be possible for someone to drive, bike or walk from coast to coast underground!

The south rim of the SIC illustrated in this image is not as well defined as the relatively intact northern rim. The infamous Sudbury “stack”, visible in the foreground, rests on the Huronian supergroup south of the SIC. The SIC south rim is visible as the “mound” behind the stack and the “bowl” of the internal Sudbury Structure is visible in the background. To give an excellent perspective of the size of the structure, the north rim of the SIC is barely discernable just below the horizon in the far background!

This area has the single largest magmatic nickel source in the world. The Creighton Deep Project is currently mining and actively exploring well below the 7500-ft. level, maintaining its status as the deepest working mine in the western hemisphere. The size of the underground workings at Creighton dwarfs all man-made structures on the surface of the Earth. The No. 9 vertical shaft is between 4-5 times higher than the CN Tower!

The Sudbury Neutrino Observatory is housed in a cavern as large as a 10-story building, in the deepest section of the Creighton Mine.

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. [Note 2]

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)

GROUND TOUR

The next series of images will document my ground tour of the Sudbury Meteorite Crater that was guided by geologist Frank Brunton. We started north-west of Windy Lake outside of the SIC in the country rock and worked our way south-east along highway 144 to the center of the crater. This route gives a most fascinating display of the changing geology throughout the SIC feature caused by the impactor.

Surrounding Brecciated Footwall Rocks

Midcrustal 2.4 to 3 billion year old migmatites (a rock that incorporates both metamorphic and igneous materials) of the Levack Gneiss Complex are found immediately outside of the north rim of the Sudbury Igneous Complex (SIC).

Shatter cones have been reported up to 15 kms away from the periphery of the SIC. The cones commonly point toward the centre of the Sudbury basin, indicating that the Sudbury crater structure has undergone considerable erosion since the impact occurred 1.85 billion years ago. [Note 2] This 18cm shattercone was found outside the SIC basin in the Sudbury Structure country rock and was given to me by Frank Brunton. Shatter cones are shock-deformation features that form from impact pressures of typically 2-10 GPa up to ~30 GPa (the GPa, or gigapascal, is a unit of pressure that corresponds to 9900 times atmospheric pressure). They represent the only distinctive and unique shock-deformation feature that develops on a megascopic scale (e.g., hand sample to outcrop scale). They appear in outcrops as distinctively curved striated fractures that typically form partial or complete conical structures (image). They are commonly found beneath impact crater floors, usually in the central uplifts of complex impact structures, but they may also be observed in isolated rock fragments within brecciated units.

This image of the shattered (brecciated) bedrock is taken just north-west of Windy Lake on highway 144. When driving into the SIC from the north this is the first indicator of an impact event. The pulverization of these footwall rocks illustrates the deformation of the local bedrock that immediately followed impact.

Black pseudotachylite Matachewan Dykes are found throughout this area in the rock cuts along the highway. These dykes predate the formation Sudbury Meteorite Crater and possibly offered a weakness in the Levack Gneiss. Pseudotachylite Sudbury Breccia (SB), a breccia having the aspect and the black color of a volcanic rock (a tachylite), was formed within these dykes when the high pressure from the meteorite impact was applied to these rocks and then abruptly released. This caused the rock along and within these dykes to partly melt. The dykes containing the pseudotachylite were welded shut as soon as the motion created by the impact stopped. Subsequent stress was supported by the fault as though it had never been active. The entire period of activity of a fault filled with pseudotachylite may be measured in minutes. (e.g., Pseudotachylite is a rock type formed by friction-induced melting, during very rapid deformation) (Philpotts 1964; Maddock 1983).

The pulverized and melted country rock injected into the pink gneisses has similar chemistry to the derived gneisses. 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)

In this image the SB is illustrated as the black pulverized (by the impact) country rock injected into the pink gneiss. SB pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of SB are found up to 100 km north of the SIC (the toe of my boot is for scale). [Note 1]

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.

The middle of the Matachewan Dykes are mostly finer grained while toward the outside we find pyroxenes (any of a group of igneous-rock-forming silicate minerals that contain calcium, sodium, magnesium, iron, or aluminum, usually occur in short prismatic crystals or massive form, are often laminated, and vary in color from white to dark green or black). A strange unexplained characteristic of the dykes are the green flecks superimposed on the rocks, shown here in this image.

Another terrestrial example of this type of breccia is found in the Vredefort meteorite crater in South Africa.

Sudbury Igneous Complex (SIC)

Approximately 1 km south from Windy Lake is a hill containing the North Range lower zone of the SIC. This area experienced an instantaneous melt at impact 1.8 billion years ago. These salt and pepper coloured felsic norite rocks consist of medium course crystalline, igneous textured plagioclase containing white feldspar and dark pyroxenes & mica. The lower zone of the SIC is 500 metres thick.

Just over the crest of the hill and a bit further into the structure are the pinkish tinged rocks of the SIC North Range upper zone. Like the lower zone of the SIC, these rocks experienced an instantaneous melt at impact. These rocks are three parts granophyric intergrowth (interlocked wedge shaped quartz and feldspar crystals) to one part plagioclase feldspar plus biotite, amphibole, chlorite and opaque minerals. The upper zone is 900 metres thick. The colour and texture differences between the upper and lower zone of the SIC is caused by the different rates of cooling after the impact.

Whitewater Group

Onaping Formation

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.

Immediately interfacing the upper SIC is the grey Whitewater breccia that contains many large angular rock fragments floating in a glass like amorphous rock. These fragments are the fallback particles from the surrounding Huronion supergroup country rock that were deposited immediately after the impact. Basically these rock fragments went up hundreds of km and then hours later “plopped” into this still molten rock. Note the large fragment in the lower right of the image that is hydrothermally altered and surrounded by a “chilled margin” (a mineralized area around the fragment caused by a hydrothermal vent).

Further into the structure is the darker Whitewater breccia containing smaller rock fall-back fragments originating from the igneous quarts granite north range footwall. Here the breccia indicates the introduction of carbon. A biogenic origin of the carbonaceous material (soot) found in the black Whitewater Group is theoretically caused by the evaporation/condensation from the hot impact fireball and/or from a later global cloud. The colour of the rock is not uniform indicating that the carbon is not uniformly distributed.

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 [Note 2].

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.

Chelmsford Formation

The Chelmsford Formation comprises about 850 metres of mostly wacke and siltstone, essentially sedimentary rocks deposited over the Whitewater Group after the impact. In the image note the ripples encased in the rock caused by water flowing back and forth. Paleocurrent studies indicate that the predominant flow direction was to the southwest, parallel to the long axis of the Sudbury Basin.

This image of the top surface of the Chelmsford Formation illustrates striations on the sedimentary rocks caused by the glaciers. Note the circular carbonate concretions that are caused by ground fluids passing through the carbon rich mud rocks that would have been full of organics. The anaerobic waters caused chemical precipitation exchanges forming these things around a nucleus of organic material. A combination of the ground water and the chemical nature of the organic material in the particular layers, determines the size. (Frank Brunton – private discussion)

The rim of the SIC north wall is visible in the background while facing north a couple of km west of Hanmer and situated in the center of the Sudbury Structure.

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.

Notes

J.E. Mungall and J.J. Hanley: ORIGINS OF OUTLIERS OF THE HURONIAN SUPERGROUP WITHIN THE SUDBURY STRUCTURE. Department of Geology, University of Toronto.
Frank Brunton: THE FACTS-SIGNIFICANCE OF SUDBURY GEOLOGY MINING HISTORY. 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.
The close proximity of the Sudbury and 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.
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.
In the Landsat image observe Lake Nipissing to the east of the Sudbury Structure. There are two circular features on Lake Nipissing whose geological features are unexplained. They are documented in the following two images taken from the north coast of the Lake Nipissing looking south. At the extreme east of Lake Nipissing, by the town of Callander, is a circular feature that resembles a caldera. In the eastern area of Lake Nipissing are the Manitou Islands, a series of small islands placed in the form of a circle. A third feature, a semicircular indentation into the north coast of Lake Nipissing at Meadowside, was probably caused by wave action erosion of the soft coast material.