PRESQU’ILE IMPACT STRUCTURE*

by: Charles O’Dale

* The term “structure” is used to define an impact crater that is severely altered by erosion.

• Type: Central peak
• Age (ma): <500^a CAMBRIAN
• Diameter: 24 km
• Location: Quebec, Canada. N 49° 43′ W 74° 48′
• Shock Metamorphism: Shatter cones best developed in massive, fine-grained rhyodacites, up to 60 cm long (Higgins and Tait, 1990). Neither PDF in quartz or other minerals nor evidence of microbrecciation has been found at this impact site.

^a Dating Method: No reliable age, the age estimate is based on the high degree of erosion.

An estimated 3000 m of vertical erosion has taken place in this area over the eons and has completely erased the original crater structure. The target rocks are Precambrian, crystalline Archean aged tonalite and metabasalts of the Abitibi greenstone belt. The documented gravity and aeromagnetic fields in this region are dominated by signatures related to the Presqu’ile tonalite intrusion, and show no evidence of a cryptic circular structure (MERQ, 1982; MRN, 1976, 1978).

The absence of high-pressure shock metamorphism from the floor of the structure that would be produced by a crater of this size, indicates a prolonged post-impact and preglacial erosion event. The age of the impact is difficult to establish, though it is clearly post-metamorphic (approximately 2.7 Ga) and pre-glacial. Together this data suggests a minimum age for the impact event of several hundred million years (Higgins and Tait, 1990).

Aerial Exploration

The low relief of the impact remnant lake Presqu’ile is displayed in the above image taken from the north west. The absence of high-pressure shock metamorphism from the floor of the structure that would be produced by a crater of this size, indicates a prolonged post-impact and preglacial erosion event.

The six km diameter circle of shatter cones was used to determine the original size of the crater. If the lake is taken as the center of the impact and considering that shatter cones die out at 0.5 X diameter (Robertson, 1975) then the crater probably had an original diameter of 24 km (Higgins et al, 1990).

Shatter-cones are evidence of a nearby meteorite impact. The only other way to produce these geological features is by large explosions such as a nuclear explosion. The shatter cone in this image (left) is one example found in the Presque’ile structure (Grieve 2006).

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 – the most recent at Isle Rouleau).

The surrounding topography of Presqu’ile is one of rolling hills without any indication of a crater rim. The suggestive circular shape of Lac de la Presqu’ile is the only indication of a cosmic collision event in this area. The discovery of 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.

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.

On our second day of exploring the Presqu’ile structure, we explored the eastern area of the structure. From the eastern lake we paddled further to the east via a small creek toward the bedrock where the original shatter cones were discovered. Via canoe, the trip to the eastern shatter cones was about a 10 kilometres (~6.2 miles) from our campsite.

We had to “climb” over some rapids to get to our shatter-cone-in-bedrock-destination. 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.SIDE NOTESyenite/Pluton Circular Feature – SKOOTAMATTA SYENITE**

The Skootamata Syenite is a large, roundish body centred in Anglesea Township. The body covers an area of approximately 30 km² , and underlies a large part of Skootamatta and Sheldrake Lakes. Access is provided by county roads east from Highway 41 and the town of Cloyne.Geology

The Skootamatta Syenite is centrally located in the Elzevir Terrane. The body is slightly elongate to the south-southwest with a length of 7.5 km and a width of 4 to 6 km. The Skootamatta Syenite has not been studied in detail. Mapping of Anglesea Township was undertaken by Meen (1944) and was later followed up by the work of Moore and Morton (1986) who cover only the far eastern part of the pluton. These two sources provide all the known information on the Skootamatta body.

**The syenite is a light grey to pale pink, massive equigranular rock. There is a weak foliation in places. The syenite is coarse- to medium-grained for the most part with a minor decrease in grain size towards the contacts. Mineralogy consists of microcline, albite and biotite with accessory sphene, apatite and magnetite. The very low colour index is suggestive of a mafic content of 5% or less. Quartz is present in very minor amounts, varying from trace to 5%, and can only be detected in thin section (Moore and Morton 1986).

SKOOTAMATTA SYENITE**

The Skootamata Syenite is a large, roundish body centred in Anglesea Township. The body covers an area of approximately 30 km2 , and underlies a large part of Skootamatta and Sheldrake Lakes. Access is provided by county roads east from Highway 41 and the town of Cloyne.