IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
SLATE ISLANDS IMPACT STRUCTURE
Jens Ormö, Erik Sturkell, Carl Alwmark & Jay Melosh
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.
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:
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
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).
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.
Ground Exploration of the Slate Island Impact Structure
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.
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).
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.
Polymict Breccias. The dominant type of breccia on the Slate Islands is polymict breccia and this typically occurs as veins and dikes varying in width from ~5 cm to 5 m. The matrix is typically fine grained and grey in colour. It contains a wide variety of angular to sub-rounded clasts of different lithologies, ranging in size from <1 mm to 10s of centimeters. Some of the clasts can also be seen to contain shatter cones. (Kerrigan et al, 2014)
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.
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