Nicholson
Key facts
- Country
- Canada
- Diameter
- 12.5 km
- Age
- 389 ± 7 million years
- Location
- 62.67° N, 102.68° W
Nicholson Lake, in the tundra of the Northwest Territories, with a large central island that is the eroded central peak of the crater.
Nicholson is an impact structure about 12.5 km across in the remote tundra of the south-eastern Northwest Territories, in Canada, mostly filled by the waters of Nicholson Lake.
Discovery. Evidence of impact was found in 1965, during studies of the Canadian Shield. Geologists found shatter cones, cone-shaped fractures that form only under shock, in gneiss on the western shore, and impact breccias, rocks made of broken and partly melted fragments, on a promontory and on islands in the lake. Microscopic study found planar deformation features in quartz, confirming an impact.
What it hit. The impactor struck ancient crystalline rocks of the Canadian Shield, mainly gneiss and granite more than 2.5 billion years old, which at the time were covered by a layer of Ordovician limestone and other sedimentary rocks. That cover has since been almost entirely removed by erosion, but fragments of it survive inside the impact breccias, preserved by the crater.
Structure. Erosion and ice sheets have worn the structure down so far that no rim survives; the surrounding land lies within about 10 m of the lake level. The large central island, rising about 50 m above the water, is the eroded central uplift, where the crater floor rebounded after the impact.
Age. Argon dating of the impact rocks gives an age of about 389 million years, in the Middle Devonian, with an uncertainty of about 7 million years.
Why it matters. Nicholson shows how a crater can preserve fragments of rock layers that have vanished from the surrounding landscape. The impactor is unknown.
Visiting. The lake is extremely remote and can be reached only by aircraft.
Did you know?
The large central island in Nicholson Lake is the crater's eroded central peak.
Fragments of limestone that has vanished from the region survive inside the crater's breccias.
No trace of the crater rim remains.