Odale-Articles-SlateIslands

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

SLATE ISLANDS IMPACT STRUCTURE

The Slate Islands impact structure in northern Lake Superior.
The Slate Islands impact structure (circled) is located in northern Lake Superior ~10 km south of Terrace Bay and ~150 km east of Thunder Bay, Ontario. This Landsat image illustrates the Slate Island archipelago’s relative position in northern Lake Superior with the archipelago shown magnified on the upper right of the image (Courtesy NASA).
  • Age (ma): 436 Ma ± 3
  • Diameter: 32 km
  • Location: Ontario, Canada. N 48° 40' W 87° 00'
  • Shock Metamorphism: Shatter cones (up to 10 metres high), a variety of microscopic shock metamorphic features, impact glasses and brecciation, quartz fragments exhibiting planar deformation features (PDF) in pseudotachylites (Dressler et al, 1997).
  • Dating Method: 40Ar-39Ar release spectrum of a pseudotachylite. The age of the youngest target rocks indicate that the Slate Islands structure has a maximum age of 800 or 1100 Ma. Based on similarities of the Slate Islands Crater’s erosion level with that of the Charlevoix structure in Quebec a proposed age of ~350 Ma was estimated. A lamprophyre dike that had been subjected to impact deformation was dated by the K-Ar method for antigorite at 310 ± 18 Ma and for phlogopite at 282 ± 11 Ma. Two 40Ar-39Ar release spectra on two pseudotachylite samples suggest an age of ~436 Ma for the Slate Islands impact (Dressler et al, 1999). This investigation concentrated on one rock type, namely, vein quartz, to eliminate effects rock types may have had on the formation of shock features.

Possible link to the Ordovician Extinction
The red dot represents the approximate area of the possible multiple impact in the late Ordovician Period.
The Slate Islands impact structure is the eroded remnant of a 30-32 km-diameter complex impact structure located in northern Lake Superior, Ontario, Canada. Target rocks are Archean supracrustal and igneous rocks and Proterozoic metavolcanics, metasediments, and diabase. A wide variety of breccias occurs on the islands, many of which contain fragments exhibiting shock metamorphic features. Aphanitic, narrow and inclusion-poor pseudotachylite veins, commonly with more or less parallel boundaries and apophyses branching off them, represent the earliest breccias formed during the compression stage of the impact process. Coarse-grained, polymictic elastic matrix breccias form small to very large, inclusion-rich dikes and irregularly shaped bodies that may contain altered glass fragments. These breccias have sharp contacts with their host rocks and include a wide range of fragment types some of which were transported over minimum distances of similar to 2 km away from the center of the structure. They cut across pseudotachylite veins and contain inclusions of them. Field and petrographic evidence indicate that these polymictic breccias formed predominantly during the excavation and central uplift stages of the impact process. Monomictic breccias, characterized by angular fragments and transitional contacts with their host rocks, occur in parautochthonous target rocks, mainly on the outlying islands of the Slate Islands archipelago. A few contain fragmented and disrupted, coarse-grained, polymictic elastic matrix breccia dikes. This is an indication that at least some of these monomictic breccias formed late in the impact process and that they are probably related to a late crater modification stage. A small number of relatively large occurrences of glass-poor, suevitic breccias occur at the flanks of the central uplift and along the inner flank of the outer ring of the Slate Islands complex crater. A coarse, glass-free, allogenic breccia, containing shatter-coned fragments derived from Proterozoic target rocks (upper target strata), observed at two locations may be analogous to the 'Bunte Breccia' of the Ries crater in Germany. At one of these locations, this breccia lies close to a crater suevite deposit and at the other, it overlies parautochthonous, monomictic breccia.
The Slate Islands impact structure is indicated in this eastward looking Shuttle image by a circle representing the approximate crater rim. Lake Huron (containing the Can-Am suspected impact structure) and Lake Erie are also visible in the background. (Courtesy NASA)
The ~7-km-wide Slate Islands group represents the heavily eroded central peak of a ~32 km diameter (from bathymetric data) complex meteorite crater. It is not known if the present height of the central peak island is the result of stratigraphic uplift only or of uplift followed by partial collapse of the central peak and erosion. Target rocks consist of three main groups of Archean and Proterozoic supracrustal and intrusive rocks, about 2.7 Ga and 1.8 Ga and 1.1 Ga old respectively. Heterogeneous melt bodies are located within heavily brecciated units of the Slate Islands central uplift peak (Dressler, 1997; Halls, 1976; Sharpton, 1996).

Specific impact breccia types in the target rocks are related to the various phases of the impact process as follows:

  • Type A pseudotachylitesa, compression type, central uplift and excavation (clastic matrix breccias), and crater modification (monomict, parautochthonous breccias).
  • Type B pseudotachylitesa may have formed during the excavation and late crater modification stages.
  • Bunte Breccia and suevite deposits occur on the flanks of the central uplift or on the inner flanks of the annular trough.

The Slate Islands impact breccias are superbly exposed, much better than breccias in most other terrestrial impact structures. Observations, including those indicative of multiple and sequential processes, provide insight on how impact breccias form and how they relate to the various phases of the impact process. Eventually they will lead to an improved understanding of planetary impact processes (Dressler et al, 1997).

a Pseudotachylite - a breccia having the aspect and the black color of a volcanic rock (a tachylite). It is formed when a high pressure from an impact is applied to country rocks and then abruptly released. This causes the rock along and within fracture lines or faults to partly melt. The fractures or faults containing the pseudotachylite are welded shut as soon as the motion created by the impact stops. Microscopic shock metamorphic features, shatter cones, impact glasses and pseudotachylites were formed during the contact and compression phase of the impact process. Polymict, clastic matrix breccia dikes, suevite, and bunte breccia contain fragments that were formed during the excavation and central uplift stage of the impact process when target rocks were in a cohesionless state allowing long-range fragment mixing. Subsequent stress is supported by the pseudotachylite as though it had never been active. The entire period of activity of a fracture or 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.


Slate Islands Topography

The water depth around the Slate Islands impact structure depicted on this aeronautical chart.
Topographical Map of the Slate Islands Impact Structure.
On this aeronautical chart of the Slate Islands meteorite crater area (left), the inserted circle around the central peak represents the approximate location of the crater rim. A characteristic of a complex crater is a ring graben (depressed ring) surrounding its central peak. I have sketched, from information transposed from a nautical chart of the area, a >100 metre deep depression that forms a semi-circle ring graben around the central peak of the meteorite crater. Indicated within the depression is an area that is >125m deep. The small “X” I penciled in on the chart is the position of a relatively shallow ~25 metre depth in the area that corresponds to the rim of the crater. The water outside of the ring graben averages from 30 to 70 metres in depth. The non-concentric annular moat may have been the result of a low angle, oblique impact. The impactor is estimated to have been 1-1.5 km diameter contacting at a velocity of ~20-25 km/sec. The image inserted on the chart, taken from over the islands, indicates the distance from the central peak to the crater rim immediately to the north at the shore of Lake Superior.

Aerial Exploration

The Slate Islands impact structure - north.

As I approached the archipelago from the north-east at the vantage point of >100 metres over the water (image right), I received an appreciation for the power of the geological forces that created the area of the islands forming the crater’s central peak. The Slate Islands were designated as a Provincial park in 1985 and are home to the largest known herd of unpredated woodland caribou. Biological cycles determine their numbers, now approximately 400 (approx. 13 caribou per sq km).

The Slate Islands impact structure - north from 2000'.
The Slate Islands impact structure - east.
This image of the Slate Islands was taken while I was over the approximate place of the structure's northern crater rim (the shore of Lake Superior) looking south. The Slate Islands archipelago lies close to the northern rim of the North American mid-continental rift system of Proterozoic (~1.1Ga) age which is characterized by mafic (dark colour, heavy element) rocks of the Keweenawan Supergroup. They overlie 1.8 – 1.9 Ga Animikie Group ironstones and siltstones, which in turn horizontally overlie deformed and metamorphosed Archean supracrustal and igneous rocks that are ~2.7 Ga. The thickness of the Proterozoic rocks in the target area at the time of the impact is unknown.

I did a low and over close to the north-west shore of the crater’s central peak to get these images of the cliffs in that area. I wanted to get close up images of the giant shattercones that are along this shore. These images only make me want to get a closer look.

This Google Earth image of the Slate Islands impact structure depicts the crater that is underwater, note the circular trench around the central peak islands.

Ground Exploration of the Slate Island Impact Structure

2006

The Slate Islands impact structure from ground level on the north shore of Lake Superior.
Our explorations of the Slate Islands impact structure are depicted on this nautical chart, BLACK = via canoe, RED = on foot.
My ground exploration of impact structures project continued in July of 2006 when Eric Kujala and I explored the north area of the Slate Islands Impact Structure by canoe. From the north coast of Lake Superior looking south, the Slate Islands impact structure can be identified (from the ground and the air) 15 kilometres away. The main purpose of the expedition was to find and document impact shock features on the islands.


We chartered a ride across the open water between Terrace Bay and the Slate Islands Structure as the waves would easily overwhelm our loaded 18 foot canoe. In three days of exploring we paddled over 35 kilometres and in addition I explored over 10 kilometres on foot. On this Slate Islands structure map, the route we followed by canoe is indicated by the black lines and the ground explorations are indicated by the red lines. The numbers indicate locations of interest (see text).


Shock waves of >4 ± 2 GPa from the bolide impact and target rock compression caused the formation of shatter cones in the Slate Islands’ crystalline rocks. Shatter cones occurring on all of the islands indicate that a shock pressure of about 3 GPa was the minimum shock pressure that all Slate Islands target rocks were subjected to. Some very spectacular >10-m-long shatter cones are exposed on the islands.

Yours truly posing on the largest discovered shatter cone on this planet, at the Slate Islands impact structure.
Eric pointing at an in situ shatter cone at the Slate Islands impact structure.
In the image at left, at location #1, is one of the largest known exposed shattercones on earth. It has a circumference of about 20 metres at its base. To give you an idea of the scale of the structure, that is me standing and hanging on to shatter cone cliff. This particular cone is located 2 to 4 km from the point of impactor contact on Slate Islands. It had been rotated after impact in response to uplift at the crater center. The image on right illustrates the striations that are typical of shatter cones and that run throughout this cliff. These particular shatter cones are formed in felsic metavolcanic rocks.

The Slate Islands in northern Lake Superior represent the eroded remains of a complex impact crater, originally 32 km in diameter. New field studies there reveal allogenic crater fill deposits along the eastern and northern portions of the islands indicating that this 500–800 Ma impact structure is not as heavily eroded as previously thought. Near the crater center, on the western side of Patterson Island, massive blocks of target rocks, enclosed within a matrix of fine-grained polymict breccia, record the extensive deformation associated with the central uplift. Shatter cones are a common structural feature on the islands and range from <3 cm to over 10 m in length. Although shatter cones are powerful tools for recognizing and analyzing eroded impact craters, their origin remains poorly constrained (Sharpton, 1996).

In situ shatter cones on Mortemer Island west within the Slate Islands impact structure.
In situ shatter cones on Mortemer Island west within the Slate Islands impact structure.
The shattercones that are illustrated here are exposed at location #2 on the western tip of Mortimer Island. They developed in mafic Keweenawan metabasalts and Proterozoic metasediments (Dressler et al, 1995). Shatter cones form from impact pressures of typically 2-10 GPa and up to ~30 GPa, and is the only distinctive and unique impact shock-deformation feature that develops on a megascopic scale (e.g., hand sample to outcrop scale).
In situ impact breccia on Patterson Island east within the Slate Islands impact structure.
The breccia deposits illustrated here are from location #3 and are typical for almost all the other breccia deposits I found on the islands. Impact breccias are made up of fragments of the target rocks, containing various ratios of impact melt and shocked mineral inclusions.

We had a very pleasant three day stay on the islands that we shared with Canada’s most southern caribou herd. The wild animals were fairly tame, one caribou sauntered by within 3 metres of our campsite without a sideways glance. The islands are also home to an arctic plant species usually found 1,600 km north or in a sub alpine environment. The challenging conditions found especially on the outer islands shorelines have supported these plants following the retreat of the last continental glaciers approximately 10,000 years ago.

Apparently there is a fortune in gold and copper deposits in the eight main islands.

There are other areas of the islands yet to explore, but unfortunately, they are exposed to the wind and waves from Lake Superior. It will require a larger vessel than an 18 foot canoe to safely continue the explorations. I will be returning someday with a larger boat.


2013

An expedition to the Slate Island structure was organized for the RASC 2013 GA. Katrina Ince-Lum from RASC Toronto was on that expedition and took the following images (presented here with permission). From left to right: 1. Breccia, 2&3 Mafic Shattercones.



References

Dressler, B. O., Sharpton, V.L., Schnieders, B. and Scott,J., New Observations at the Slate Islands Impact Structure, Lake Superior. Ontario Geological Survey, v. Miscellaneous Paper 164, pp. 53-61. 1995.

B.O. Dressler & V.L. Sharpton: Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada. TECTONOPHYSICS, 1997 Vol.275, No.4, pp. 285-311.

Dressler, B. O., Sharpton, V. L., Copeland, P., Slate Islands, Canada: A mid-size, complex impact structure. Geological Society of America Special Paper 339, p. 109-124, 1999.

B.O. Dressler & V.L. Sharpton: Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada. TECTONOPHYSICS, Vol.275, No.4, pp. 285-311, 1997.

Halls, H. C., Grieve, R. A. F., The Slate Islands: A probable complex meteorite impact structure in Lake Superior. Canadian Journal of Earth Sciences, v. 13, pp. 1301-1309. 1976.

Sharpton, V. L., Dressler, B.O., Herrick, R.R., Schnieders, B. and Scott,J., New constraints on the Slate Islands impact structure, Ontario, Canada. Geology, v. 24, pp. 851-854. 1996.

Earth Impact Database


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