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.

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)

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

• Type A pseudotachylites^a, compression type, central uplift and excavation (clastic matrix breccias), and crater modification (monomict, parautochthonous breccias).
• Type B pseudotachylites^a 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).

The water depth around the Slate Islands impact structure depicted on this aeronautical chart.

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 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.

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).

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.

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 impact breccia on Patterson Island east within the Slate Islands impact structure.

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.