Saint Martin
Key facts
- Country
- Canada
- Diameter
- 40 km
- Age
- 227.8 ± 1.1 million years
- Location
- 51.78° N, 98.53° W
Very little: low, forested and boggy ground between Lake Manitoba and Lake Winnipeg. Gypsum quarries near Gypsumville work deposits laid down in the old crater.
Saint Martin is a partly buried impact structure about 40 km across in the Interlake region of Manitoba, in central Canada. It formed in the Late Triassic, about 228 million years ago, when dinosaurs were just beginning to spread.
What it hit. The target was a sequence of flat-lying Palaeozoic limestones and dolomites resting on Precambrian granite and gneiss. The impact punched through the sediments into the crystalline basement, and the rebound of the floor raised a central uplift of basement rock. Later, the eastern half of the structure was eroded away, while the rest was buried by red beds and evaporites. Gypsum, a soft mineral formed by evaporating sea water, collected in the crater basin and is quarried near the village of Gypsumville.
Proof and discovery. The structure was recognised in the late 1960s from its circular pattern on geological maps and from drill holes that met disturbed rock. Cores contain breccias and impact melt rock, rock that was completely melted by the impact and then cooled, together with quartz showing shock features. Argon dating of the melt gives the age.
A chain of craters? In 1998 scientists proposed that Saint Martin, Manicouagan and Rochechouart, together with smaller structures in North Dakota and Ukraine, formed within hours of each other from the fragments of a single broken-up object, like the comet Shoemaker–Levy 9 that hit Jupiter in 1994. Since then, more precise dating has given ages several million years apart for some of these craters, so most scientists now think they are not a single chain.
Visiting. The crater is not obvious on the ground. As everywhere, ask permission before entering private land or quarries.
Did you know?
Gypsum quarried near Gypsumville was deposited in the basin left by the impact.
Saint Martin was once proposed as one of a chain of craters formed on the same day, an idea that newer dating has weakened.
Half of the structure has been eroded away and the rest is buried.