The Decorah crater, also called the Decorah impact structure, is a possible
impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
located on the east side of the city of
Decorah in Iowa, United States. It is thought to have been caused by a
meteor
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mi ...
about wide which struck during the
Middle Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
Th ...
Period, circa 470 million years ago.
Description
The crater is estimated to be in diameter, covered by the
Winneshiek Shale.
There is no surface evidence of the impact, as the Winneshiek Shale is more than below the bottom of the
Upper Iowa River
The Upper Iowa River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed May 13, 2011 tributary of the Mississippi River in the upper Midwest of the United States.
Its headwaters rise ...
. The
impact event, equivalent to
1,000 megatons of TNT,
did not appear to penetrate the Earth's mantle, but it did push down the underlying Ordovician and Cambrian bedrock several hundred feet.
It may be one of several Middle Ordovician meteors that fell roughly simultaneously 469 million years ago, part of a proposed
Ordovician meteor event
The Ordovician meteor event was a dramatic increase in the rate at which L chondrite meteorites fell to Earth during the Middle Ordovician period, about 467.5±0.28 million years ago. This is indicated by abundant fossil L chondrite meteorites in ...
, including three confirmed impact craters:
Rock Elm crater in Wisconsin,
Slate Islands crater in Lake Superior, and
Ames crater in Oklahoma.
''Pentecopterus decorahensis''
The Middle Ordovician (
Darriwilian
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species ' ...
)
Winneshiek Lagerstätte sediments that filled in the crater contained a wide variety of unusual fauna. Among them was a newly discovered species of
eurypterid
Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is l ...
, ''
Pentecopterus decorahensis''. ''Pentecopterus'' was scorpion-like in appearance, and the largest predator known from that time, measuring nearly 6 feet in length. Its species name was derived from "Decorah."
See also
*
List of possible impact structures on Earth
This is a list of possible impact structures on Earth. More than 130 geophysical features on the surface of the Earth have been proposed as candidate sites for impact events by appearing several times in the literature and/or being endorsed by the ...
References
{{Impact cratering on Earth
Geology of Iowa
Impact craters of the United States
Landforms of Iowa
Landforms of Winneshiek County, Iowa
Ordovician impact craters
Possible impact craters on Earth