*The term structure is used to define an impact crater that is severely altered by erosion.
Type: Central peak
Age (ma): <300 a
Diameter: 4 km
Location: Quebec, Canada N 50° 41’ W 73° 53’
Shock Metamorphism: Shatter cones in dolomite and PDF in quartz grains (Ogilvie it al., 1984; Caty it al., 1976).
a Dating Method: No reliable age, the age estimate is based on stratigraphy. The structure’s survival, at such a comparatively small size implies an age of less than 300 million years (see text).
The absolute age of the Ile Rouleau Impact Structure is unknown within wide limits. As illustrated in this geologic schematic of the area, Ile Rouleau lies in Lake Mistassini, which is the center of a large basin of Proterozoic (2.5 b.y. to 543 m.a.) sedimentary rocks with considerable evidence of faulting and brecciation. The island is formed of argillaceous and micritic dolomite of member C of the lower member of the Albanel Formation of the Mistassini Group. The dolomite beds are considerably contorted and cut by small faults and breccia dikes which are unique in the Mistassini basin.
The bedrock is of the upper Aphebian (1.8 b.y.) Mistassini group which in turn is covered by (< 1.75 m.y.) Pleistocene Glacial deposits. The estimated 300 million year age of the structure was determined by this geological history of the area. Preservation of the structure for it’s the small size, suggests that it is fairly young (Caty 1976). At the estimated time of the impact the climate of the area was warm and moist. Severe continental collisions were causing the creation of mountains like the Urals. Ferns, fern-like trees and primitive conifers were among flora found in swamps and insects like cockroaches, flourished. Toward the end of this period, the first reptiles appeared.
Ile Rouleau has a 1 km diameter circular shape (most of the 4 km diameter structure is under the waters of Lake Mistassini) with a glacial tail pointing southwest. The structure does not appear to be closely related to the intersection of any major faults since no diatreme dikes, carbonatite bodies, or fenite zones are known in the Mistassini area. Ile Rouleau may be the exhumed central part of a somewhat larger structure, the rest of which lies under Lake Mistassini. The rocks of Ile Rouleau do not appear to be any more resistant to erosion than others of the local formation.
The micritic and argillaceous dolomite is favourable to their formation and all cone axis appear to point inward from around the island. The shatter-cones appear to be restricted to Ile Rouleau, as do the tectonic disturbances and the breccia dikes. Shatter-cones are one of the features produced by meteorite impacts most easily observed in the field. They are conical fractures generally produced in fine-grained rocks. Longitudinal striae with horsetail patterns, decorate the fracture surfaces. Complete cones in the structure are rare; segments of cones are common and frequently intersect each other. The angle of the apex of the cone is typically about 90 degrees, and they are 1 centimetre to 5 metres high. They can occur singly, or in clusters, and are produced during the impact when a shock-wave passes through the rock. The apex of the cone originally pointed towards the point of impact. Their orientation is usually changed by the post impact crater wall slump. Shatter-cones are positive evidence of a nearby meteorite impact, as they cannot be produced by other means, except nuclear explosions.
Marine magnetic and bathymetric data have been acquired in a 6.5 × 10 km area centred on the Ile Rouleau impact structure, Quebec, Canada. The deeply eroded crater is emplaced in weakly magnetic shaley dolomites of the Palaeoproterozoic Mistassini Group. Subaerial exposure of the structure consists of an island (Ile Mintunikus Misaupinanuch) in Lac Mistassini, which hosts shatter cones. Short-wavelength and linear NNE-trending magnetic anomalies that pervade the survey area are inferred to reflect the presence of magnetic glacial landforms on the lake floor.
The most prominent magnetic feature is a high-amplitude anomaly that wraps round the western side of the island. The anomaly correlates with an abrupt change in bathymetry but is not consistently circular about the island and its relationship to the impact is unclear.
To the east, a disjointed arcuate anomaly lies parallel to the island 100 m from shore. Its source may be interpreted to be either:
a locally thickened layer of magnetic glaciofluvial deposits, or
post-impact chemically induced magnetization along faults.
The abrupt variations in glaciofluvial thickness needed to account for the anomaly under scenario (1) are, however, hard to explain geologically and hypothesis (2) is therefore favoured. Regardless of which model is correct, both suggest that the arcuate anomaly coincides with the fault-bounded perimeter of a central peak. Two possible reconstructions for the original crater diameter, estimated from the curvature of the magnetic anomaly and using morphometric relationships, are 4.4 km and 6.8 km. The former is more realistic with regards to the topography and the lack of conspicuous impact-related deformation on adjacent Ile Manitounouc.
The island is crudely comma shaped. The round part of the comma (1 km in diameter) is ringed by cliffs 7 to 15 m high on the north and east shores, with a low rocky shoreline on the west side. The tip of the comma is a tail of glacial debris pointing to the southwest, and the center of the island is heavily bush covered.
The outcrops of this western arc have suffered no more deformation than is normal in the Mistassini Basin but the arcuate shape of Ile Manitounouc’s western shore interfacing Ile Rouleau is suggestive.
Numerous small faults are exposed along the shoreline, none of which appear to have a large displacement. At least two thrust faults are exposed in the northeast corner (image) of the island, where high cliffs expose structures of low dip. Several small normal faults are also present in these cliffs.
Shatter-cones are particularly well developed along the eastern and south-western portions of the island. This image illustrates the ruggedness of the east coast where a large number of these shatter-cones were discovered. Also note the clarity of the water in Lake Mistassini.
The results of the preliminary study of the Ile Rouleau Impact Structure suggest strongly that it is an astroblem (Caty 1976). This conclusion is based on:
The general circular form of the bedrock of the Island;
The general contorted and faulted aspect of the bedrock;
The presence and radial orientation of breccia dikes;
The presence of planar features in quartz;
The presence of shatter cones and their apparent orientation upward and inward towards the center of the island, and;
The absence of features that could be related to an endogenic process.
Ground Exploration of the Isle Rouleau Impact Structure
Following my impact crater exploration tradition, in the summer of 2007 I finally set foot on the Isle Rouleau Impact Structure. I was able to accomplish this with the help of my exploration partner, Eric Kujala (who has accompanied me on many other exploration trips).
Our canoe trip initiated from the village of Baie-du-Poste and it was a day and a half traveling time by canoe to the Isle Rouleau Impact Structure. We were very fortunate with the weather, as had there been any increase in the wind velocity the trip would easily have taken a day or so longer each way. The canoe was at maximum load for the trip.
As we approached the structure from the north-east, we documented a shoal which is approximately a kilometre (~0.6 mile) from Isle Rouleau. The shoal may be associated with the crater rim (my hypothesis only). Unfortunately, the shoal was deeply covered with glacial erratic rocks and I was unable to view any bedrock at the site. I was hoping to see signs of an impact here, but to no avail.
The target rocks are 1.8 Ga old Proterozoic aphebian sediments, with Isle Rouleau consisting of argillaceous and micritic dolomite of the Albanel Formation of the Mistassini Group (Grieve 2006). A breccia dyke, associated with impact shear in the target bedrock, is visible as the dark line in the lower right of the cliff image.
There are many loose shatter cones at the base of our landfall cliff. Shatter cones are shock-deformation features that form from impact pressures of typically 2-10 GPa up to ~30 GPa. 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.
After circumnavigating Isle Rouleau and upon our return to Baie-du-Poste, we had an interview with the government geologist posted there. We had an excellent conversation regarding the local geology (and we did brag a bit about our explorations!). She was gracious enough to give us a sample of breccia that she had recovered from the Isle Rouleau site. Thank you!
Analysis of known impact structures shows that three small North American craters of approximately end-Carboniferous date lie (within 1.3 km) on a great circle, suggesting a common provenance. These craters are Decaturville (6 km diam., 300 Ma), Des Plaines (8 km, 280 Ma) and Ile Rouleau (4 km, 300 Ma). A further 8 North American craters have similar ages. These could be secondary impact craters produced by fragments of a larger asteroid which broke up on impact following a low-angle oblique impact event. Statistically, this type of event should have happened at least once in Earth history (Hamill 2003).
Caty, J. L., Chown, E.H. and Roy,D.W., A new astrobleme: Ile Rouleau structure, Lake Mistassini, Quebec. Canadian Journal of Earth Sciences, v. 13, pp. 824-831. 1976.