SUDBURY IMPACT STRUCTURE – GROUND EXPLORATION

SUDBURY IMPACT STRUCTURE -GROUND EXPLORATION (SIC)

A.Y. Jackson Lookout – Sudbury 2018-12

A view across the eroded and deformed Sudbury crater along Highway 144 from the south rim to the north rim.(Natural Resources Canada and Ontario Geological Survey)
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.

SHATTERCONES

This 18 cm shattercone was found just outside of the Sudbury Igneous Complex (SIC). The discovery of shatter cones confirmed that a large meteorite impact caused the formation of the Sudbury Igneous Complex (Gibson, Spray 1998).
After 1.85 Billion years the striations on the shatter cone illustrated above are still recognizable.
Sudbury shattercone (courtesy Martin Schmieder)
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 (Brunton). 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.

SHATTERED ROCK

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.

PSEUDOTACHYLITE

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 . 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.

The Sudbury impact structure – black pseudotachylite.
The Sudbury impact structure – black pseudotachylite.

Black pseudotachylite Matachewan Dykes are found throughout the rock cuts along the highway outside the SIC. 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).

Sudbury 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 pseudotachylite here is injected into the pink gneiss country rock (the toe of my boot is for scale).

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)

Pseudotachylyte petrogenesis: Constraints from the Sudbury impact structure

Abstract

Pseudotachylytes and their host rocks from the North Range of the 1.85 Ga Sudbury impact structure have been investigated using analytical scanning electron microscopy, electron microprobe analysis and XRF spectrometry. The results show that the pseudotachylytes were produced in high-speed slip zones by the frictional comminution and selective melting of wall rock lithologies. The preferential assimilation of hydrous ferromagnesian phases during frictional melting produced relatively basic melts, leaving the more mechanically resistant quartz and, to a lesser extent, plagioclase as included mineral clasts. Three distinct assemblages are identified within the pseudotachylytes: (a) pre-impact (>1.85 Ga) rock and mineral clasts derived from host lithologies; (b) a syn- to immediately post-impact (1.85 Ga), rapidly cooled, quartz + sanidine + labradorite + phlogopitic biotite matrix assemblage, formed due to crystallization from a melt at 800–900°C and (c) a post-impact (<1.85 Ga) retrograde assemblage which overprints both clasts and matrices. Field evidence indicates that most pseudotachylyte formed in large-displacement fault systems during gravitational collapse of the impact-generated transient cavity. The Sudbury pseudotachylytes, like endogenic pseudotachylytes, were generated by frictional melting on fault surfaces. The difference is primarily one of scale. Large (km) displacements occurring on impact-induced ring faults can generate immense volumes of friction melt resulting in spectacular pseudotachylyte bodies up to 0.5 km thick and more than 10 km long. (Spray, Thompson 1996)

SUDBURY BRECCIA

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 Sudbury impact structure – Matachewan Breccia Dykes.
The Sudbury impact structure – felsic norite breccia rocks.  “Norite” – composed of intergrown crystals of light-coloured minerals (feldspar) and dark-coloured magnesium-iron-silicate minerals (pyroxene), giving the rock a salt-and-pepper
texture, formed by crystal growth from melted rock (igneous rock – gabbro),

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.

The pinkish tinged rocks of the SIC North Range upper zone of the SIC North Range upper zone.
These “breccia” rocks experienced an instantaneous melt at impact. They 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).

The Sudbury impact structure – darker Whitewater breccia. 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 (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.

Chelmsford Formation

The Sudbury impact structure – 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.
The Sudbury impact structure – top surface of the Chelmsford Formation. 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 Sudbury impact structure – SIC north wall. The rim of the SIC north wall is visible in the background. I  took this image 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.

 

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