SLATE ISLANDS IMPACT STRUCTURE
by: Charles O’Dale
- Type: Central peak
- Age (ma): 436 Ma ± 3a – SILURIAN
- 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).
a 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 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 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 ~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.
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
As I approached the archipelago from the north-east in GOZooM at the vantage point of >100 metres over the water, 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 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.
Ground Exploration of the Slate Island Impact Structure
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. 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.
At location #1, in McGreevy Harbour, 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.
We identified an outcrop of Archean felsic metavolcanic rock in McGreevy Harbour (Fig. above) exhibiting one confirmed shatter cone, located closest to the shoreline, that is at least 10 m long. Several other large, conical features are obvious on the nearvertical walls of the outcrop, but steep slopes and thick scree prohibited our reaching these features to confirm their origin. Nonetheless, these features appear identical to the confirmed shatter cone in terms of scale, morphology, and orientation, and a similar genesis seems probable. We are aware of no reports of similarly large shatter cones from other impact craters; however, this does not mean such megacones are unique to the Slate Islands. (Sharpton et al 1996)
These particular shatter cones are formed in felsic metavolcanic rocks.
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).
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.
RASC Expedition – 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). [top-bottom] Breccia, Mafic Shattercones.
ABSTRACT: Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Målingen crater, formed by the impact of a binary, presumably ‘rubble pile’ asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
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