LAC COUTURE IMPACT CRATER
by: Charles O’Dale
- Type: Central peak
- Age (ma): 425 ± 25 a – SILURIAN
- Diameter: 8 km
- Location: Quebec, Canada. N 60° 08′ W 75° 20′
- Shock Metamorphism: Shatter cones and PDF in quartz and feldspar grains.
a Dating Method: Four samples of impact melt from the western side of the lake give Ar40-Ar39 ages ranging from 400 to 450 Ma (Bottomley et al., 1990).
Lac Couture General Area: Subdued topography in the Canadian Shield. The area is tundra, with little vegetation, and has been glaciated. The target rocks are crystalline.
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.
The Lac Couture Meteorite Crater is located 145 km south west of the Pingualuit Crater in northern Quebec. It is illustrated in the above Landsat image as the large circular lake visible just over half way down the left margin. The submerged impact structure occupies the central island free portion of the lake. All vestiges of a rim have been removed by glaciation and only remnants of the melt rock and breccia remain (Robertson, 1965). The target rock is Precambrian crystalline in the Superior structural province consisting of granitic to granodiorite Archean gneisses. A significant amount of sediment accumulated on the crater floor before glaciation had scraped most of it out. The original ground plane is approximately 200 m above the present surface (Beals et al., 1967).
The 120 m deep Lac Couture meteorite crater containing a 25 m central uplift is central to the 8 km diameter central island-free area of Lac Couture. This implies the structure is a complex meteorite crater. Rock breccia found on some islands on the west shore is believed to have been dredged from the lake by glacial action. Analysis of this breccia has shown the micro-fracturing of individual crystals typical of impact craters (Grieve 2006).
We approached the Lac Couture Meteorite Crater from the north east after spending the previous night at Salluit, one of the most northern villages in Quebec (Side Note). The geology in this area of Quebec is like a moonscape and almost completely lifeless. We noted many caribou tracks but unfortunately did not see the vast herds that apparently are up there somewhere.
We departed toward Kuujjuaq for fuel (the nearest place where it was available) to continue our explorations (next stop, Clearwater craters).
In this article I mentioned the village of Salluit, about 200 km north of Lac Couture, where we refueled and spent a cold August night. We arrived there after exploring the Pingualuit Crater(aerial exploration in GOZooM) and were so impressed with the beauty of the area that I wanted to share it with you.
The village of Salluit (right) is neatly tucked into this valley. The airport is on top of the 1000’ hill to the right (south) of the village. You can barely make out the road we walked down to visit the village later that evening. We had a good workout walking back up that hill!
Beals, C. S., Dence, M.R. and Cohen,A.J.,Ottawa Dominion Observatory Publications, v. 31, pp. 409-426. 1967.
Bottomley, R. J., York, D. and Grieve,R.A.F., 40Argon-39Argon dating of impact craters. Proceedings Lunar and Planetary Science Conference 20th, pp. 421-431. 1990.
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Grieve, R.A.F., Impact Structures in Canada. Geological Association of Canada, 2006.
Robertson, P. B., Petrography of the bedrock and breccia erratics in the region of Lac Couture, Quebec. Master’s Thesis, Pennsylvania State University, University Park. 1965.