Steen River
Key facts
- Country
- Canada
- Diameter
- 25 km
- Age
- 91 ± 7 million years
- Location
- 59.50° N, 117.63° W
Nothing: the structure is buried about 200 m below the muskeg and forest of north-western Alberta.
Steen River is a buried impact crater about 25 km across in north-western Alberta, Canada. It formed in the Late Cretaceous, about 91 million years ago, and is now hidden beneath roughly 200 m of younger sediment.
How it was found. The structure first showed up in the 1960s as an unusual round pattern in magnetic surveys flown over the region. Oil and gas wells drilled into it then met jumbled and melted rock instead of the expected flat layers. Samples from the wells contain shocked quartz and feldspar, with microscopic features produced only by shock pressures, as well as impact breccia and melt rock, which together prove its origin.
What it hit. The target was a sequence of Devonian limestones, dolomites and evaporites, lying on Precambrian crystalline basement. The impact excavated through the sediments, and the rebound of the floor raised a central uplift of basement rock. After the impact, Cretaceous seas covered the site and buried it.
Oil and gas. Like several buried craters in North America, Steen River has interested the energy industry because broken rock around the rim can trap oil and gas. Small amounts of hydrocarbons have been found in the structure.
Why it matters. Steen River is one of a large number of North American craters discovered through oil and gas exploration, which has revealed impact structures that would otherwise never have been noticed. The impactor is unknown.
Did you know?
Steen River was first spotted as a round pattern in magnetic surveys flown over Alberta.
It lies about 200 m underground and cannot be seen at the surface.
Wells drilled into it brought up shocked and melted rock.