Decorah
Key facts
- Country
- United States
- Diameter
- 5.6 km
- Age
- about 470 million years (Middle Ordovician)
- Location
- 43.31° N, 91.77° W
Nothing: the crater lies beneath the town of Decorah, Iowa, and the bed of the Upper Iowa River, filled with shale and covered by younger rock.
Decorah is a buried impact crater about 5.6 km across beneath the town of Decorah in north-eastern Iowa, in the United States. It formed about 470 million years ago, in the Middle Ordovician, when a shallow sea covered the area. It is famous less for the crater itself than for the extraordinary fossils that collected inside it.
Discovery. In 2008 and 2009 geologists of the Iowa Geological Survey, studying rock samples from water wells, found a layer of unusual shale and broken rock that did not belong to the normal sequence. Later work showed that the shale filled a round basin. Airborne surveys that measure how well the ground conducts electricity revealed its shape, and in 2013 microscopic study found shocked quartz in breccia from the basin floor, confirming that it was an impact crater.
What it hit. The impactor struck Ordovician and Cambrian sedimentary rocks, sandstones and carbonates, on the floor of a shallow sea.
A fossil treasure house. After the impact, the crater became a sheltered, poorly oxygenated hollow on the sea floor. The mud that settled in it, now the Winneshiek Shale, preserved the remains of animals in exceptional detail. Among them is *Pentecopterus*, a sea scorpion (a eurypterid) up to about 1.7 m long, one of the largest predators of its time, named after an ancient Greek warship. The shale also contains early fish and many other creatures.
Why it matters. Decorah shows how a crater can create a special environment that preserves a snapshot of ancient life. The impactor is unknown.
Did you know?
The giant sea scorpion Pentecopterus was found in shale filling the Decorah crater.
The crater was discovered in water-well samples, not from anything at the surface.
It lies under a town and a river and cannot be seen.