IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
PRESQU'ILE IMPACT STRUCTURE*
* The term "structure" is used to define an impact crater that is severely altered by erosion.
Poorly exposed segments of shatter cones are exposed at three points on Lac de la Presqu’ile in Obatogamau basalts or basalt xenoliths in tonalite. The “X” indicators mark the areas where shatter cones were recovered from within the lake area. This image of Lac de la Presqu’ile was taken from the east approximately 1000’ above the ground.
complex meteorite crater due to the 24 km estimated diameter. For terrestrial craters the transition between simple and complex classification is 4 km in crystalline rocks.
This structure was one of the most fascinating geological sites that I had explored from the air. Reflecting on the extent of the surface erosion that had occurred in this area, the original land surface was probably hundreds of feet ABOVE the altitude where my airplane was flying when I took these images!! The island-like peninsula in the lake is probably the shattered basement rock remnant which originally was several tens of meters under the central peak of the crater.
Ground Exploration of the Presqu’ile Impact Structure
Following my impact crater exploration tradition, in the summer of 2007 I was finally able to explore the Presqu’ile impact structure. I accomplished this with the help of my exploration partner, Eric Kujala (who has accompanied me on many other exploration trips).shatter cones in the local bedrock confirmed that this structure was created by an impact event. 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.
Isle Rouleau structure and we completed a short little "recce" in the west Presqu'ile lake before nightfall. We were exploring in the south end of the west lake when a fairly substantial line squall almost caught us in the open. We high-tailed it in our canoe to the lee of one of the islands and waited the storm out. We sat there for awhile discussing life, the universe and everything but the rain didn't stop. So I thought, what the heck, I'm getting out of the canoe and have a look at a couple of these rocks. The first rock I picked up was a shatter cone! I don't think this area has ever been explored, the shatter cones were not disturbed.
I also found a rock containing quartz crystals. At home in my lab with my personal microscope, I did a substantial search within these crystals for any planar deformation features (pdf). My initial search did not detect pdf. Substantial processing of the sample would be required to complete a detailed study of these crystals to possibly identify pdf within this quartz sample.
After climbing a series of rapids we arrived at the site of the original “bedrock shatter cone” discovery. These shatter cones and shatter-coned surfaces occur within massive meta-basalt and rhyodacite 5 km east of Lac de la Presqu’ile. The cones have apical angles of ~90° and their apparent position is vertical (Higgins and Tait, 1990). It was very satisfying to find the bedrock site where the original discovery of the shattercones was made. It was these shattercones that made the firm assessment that the Presqu’ile structure was the result of a cosmic collision.
Richard A.F. Grieve, Impact Structures in Canada. Geological Association of Canada, 2006.
Michael Higgins and Larry Tait, A possible new impact structure near Lac de la Presqu’ile, Quebec, Canada” Meteoritics 25, 235-236,1990.
Ministere d'Energie et Resources du Quebec, Leve aeroporte INPUT dans le region du lac Doda, 1982.
Ministere des Resources Naturelles, Quebec, Resultats d'un leve INPUT, dans la region de la Dauversiere, 1976.
Ministere des Resources Naturelles, Quebec, Resultats d'un leve INPUT, dans la region d'Opermisca, 1978.1975.
Robertson, P.B., Zones of shock metamorphism at the Charlevoix impact structure, Quebec: Geological Society of America Bulletin. V. 86 p 1630-1638,