LAC COUTURE IMPACT CRATER

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.

^aDating 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).

The Lake Couture impact structure is visible as the open water left centre of this image. The Pingualuit Impact Crater (AKA – New Quebec Crater, AKA – Chubb Crater) is the small dot in the upper right centre. (NASA)

The Lake Couture impact structure is visible left in this Landsat image of the area.

Lac Couture impact crater in northern Quebec is image centre right . The coast of Hudson Bay is left in the Google image.

Lac Couture impact crater north rim from GOZooM, we are above the tree line.

LAC COUTURE QUEBEC, CANADA

Subdued topography in the Canadian Shield. The area is tundra, with little vegetation, and has been glaciated. The target rocks are crystalline.

Specific Features: Crater occupies a circular lake -13 km in diameter which contrasts sharply with the linear and irregular lakes in the area. The central, island-free portion of the lake is -8 km wide and is taken as the diameter of the crater. There is a small central peak which is submerged. Erosion has removed all traces of the rim. There is some indication of increased fracturing in the local bedrock surrounding the structure.

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.

Paleogeography of Baltica and neighboring cratons at the time of the increased cosmic bombardment following the ~470 Ma asteroid breakup event illustrating the resulting known craters (red dots). Clearwater East is dated to this event (~460–470 Ma). Light blue color represents areas of shallow epicontinental seas, and dark blue areas of deep ocean. This distribution may, however, have varied somewhat due to periodical transgressions and regressions of the sea. The timeline documents the related meteorite falls (black dot and line).

First known Terrestrial Impact of a Binary Asteroid from a Main Belt Breakup EventJens 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.

The red dot represents the approximate area of the Lac Couture impact 425 million years ago in the Silurian Period. Life was beginning to emerge from the oceans and colonize the land.

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).

This high altitude aerial view of the 16 km diameter Lac Couture shows a fringe of islands surrounding a central island-free area of the lake.

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).

Aerial Exploration

From Salluit (top of chart) we headed south to the Lac Couture impact crater, bottom left in the aeronautical chart. We refueled in Salluit after visiting the Pingualuit crater, circled right on the chart.

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.

The Lac Couture impact structure has experienced substantial glacial erosion since impact and all that remains is a circular water filled hole in the bedrock that contains the meteorite crater. GOZooM from the west,

The Lac Couture impact structure – south west. We circled the crater in a counter-clockwise direction from the north-east around the western rim of the crater and exiting to the south-east. The crater images are from the north, north-west and south respectively and give you an appreciation for the immensity of the impact that happened here.

The micro-fractured breccia that confirmed this feature as an impact site was recovered from the islands pictured in the image from the north-west.

The Lac Couture impact structure – south east. We looked back to take this image while exiting the area to the east. This south-east view illustrates the terrain type in the foreground that was penetrated by the meteorite leaving this deep circular lake.

We departed toward Kuujjuaq for fuel (the nearest place where it was available) to continue our explorations (next stop, Clearwater craters).

Side Note

Sugluk Inlet taken from over Hudson Strait looking south. The Salluit village is half way down this fiord on its east coast.

The village of Saluit in northern Quebec.

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!

The “survival” accommodations that we erected at the Salluit airport, and yes, that is August snow beside the tent! When the sun went down, so did the temperature, to almost 0º C.

Yours truly posing beside the “bilingual” STOP sign in Salluit, Quebec

References

[see – METEORITE]

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.

Earth Impact Database