Carswell
Key facts
- Country
- Canada
- Diameter
- 39 km
- Age
- about 115 ± 10 million years
- Location
- 58.45° N, 109.50° W
From the air, a ring of low dolomite cliffs about 30 km across encircling a core of ancient crystalline rock, set in the forests and lakes of northern Saskatchewan.
Carswell is an eroded impact structure about 39 km across in the far north of Saskatchewan, Canada, on the western side of the Athabasca Basin. It is one of the few craters that has been mined for uranium.
What it hit. The target was the thick Athabasca sandstone, more than a billion years old, with a layer of dolomite (a carbonate rock) on top, all resting on even older Precambrian granite and gneiss. The rebound of the crater floor lifted the basement rocks by more than a kilometre to form a central core about 20 km wide. Around it, the tilted dolomite now forms a ring of hills and cliffs, the most obvious feature of the structure.
Age. The age is poorly known. Argon dating of the impact melt points to roughly 115 million years, in the Early Cretaceous, but with an uncertainty of at least 10 million years, and some estimates are older.
Proof of impact. Carswell has shatter cones, cone-shaped fractures in rock that form only under the shock of an impact, together with quartz grains showing microscopic planar deformation features, and bodies of impact breccia and melt rock. Geologists recognised the structure as a probable impact in the 1960s, and the evidence soon became conclusive.
Uranium. The Athabasca Basin contains some of the world's richest uranium ores, and the impact lifted ore-bearing rocks near the old surface of the basement into reach. The Cluff Lake mine, inside the structure, produced uranium from 1980 until 2002, and the site has since been restored.
Why it matters. Carswell shows how an impact can rearrange a region's geology and bring deep mineral deposits within reach of miners. The impactor is unknown.
Did you know?
The Cluff Lake uranium mine operated inside the Carswell structure for more than twenty years.
Rocks in the centre were lifted more than a kilometre by the rebound of the crater floor.
A ring of dolomite cliffs marks the structure, clearly visible from the air.