Calvin
Key facts
- Country
- United States
- Diameter
- 8.5 km
- Age
- about 450 ± 10 million years
- Location
- 41.83° N, 85.95° W
Nothing: the structure lies about 1 km below the farmland of Cass County, south-western Michigan.
Calvin is a buried impact structure about 8.5 km across in Cass County, south-western Michigan, in the United States. It formed in the Late Ordovician, about 450 million years ago, on the floor of a shallow sea.
Discovery. The structure was found through oil exploration. Well logs and drill samples from oil wells in the area showed that the rock layers were disturbed in a round pattern, with a raised central area and a depressed ring around it. In the 1980s geologists studying these records concluded that it was a buried impact crater, and later study of samples found shocked quartz, which confirmed it.
What it hit. The target was a sequence of Cambrian and Ordovician sedimentary rocks, mainly sandstones and dolomites. The crater has a central uplift surrounded by a ring trough. Soon after the impact, sea floor mud and limestone filled the crater, and further layers buried it to a depth of roughly a kilometre.
Age. The age comes from the youngest rocks disturbed by the impact and the oldest that cover it, which are both Late Ordovician.
Oil. Several oil wells have produced from the structure, because broken rock in the rim and central uplift can hold oil and gas.
Why it matters. Calvin is one of a group of buried Ordovician impact craters in the American Midwest, from a period when the Earth seems to have been struck unusually often. The impactor is unknown.
Did you know?
Calvin was discovered from records of oil wells, not from anything at the surface.
It formed on the floor of a shallow Ordovician sea.
The structure lies about a kilometre underground.