Montagnais

Key facts

Country

Canada (Atlantic shelf)

Diameter

45 km

Age

50.5 ± 0.8 million years

Location

42.88° N, 64.22° W

Montagnais is a buried crater about 45 km across on the continental shelf off Nova Scotia, in eastern Canada. It formed about 50.5 million years ago, in the early Eocene, when an asteroid or comet fell into the shallow sea that covered the edge of the continent.

Found by the oil industry. The structure was discovered by chance. In the 1970s an exploration well, drilled to look for oil and gas on the shelf, passed through strange broken rock, and seismic surveys showed a circular structure with a central peak. In 1987 geologists published the evidence that it was an impact crater, making Montagnais the first impact structure identified on a continental shelf.

What it hit. The impact struck sedimentary layers of the shelf, sandstones and mudstones of Cretaceous age, lying on older Palaeozoic metamorphic rocks and granites. Cuttings and cores from the well contain shocked quartz with planar deformation features, the microscopic signature of a shock wave, together with breccia and rock that was melted by the impact. Dating that melt rock gave the age.

Structure. Seismic profiles show the classic form of a complex crater: a central uplift about 11 km wide, surrounded by a ring of down-dropped blocks and an outer rim. After the impact the sea washed sediment back in, and later layers buried the whole structure.

Why it matters. Most of Earth's surface is ocean, so marine impacts should be common, yet few are known because sea-floor craters are hard to find and sea floor is constantly recycled. Montagnais is one of the best examples of an impact into a continental shelf. The impactor is unknown.

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