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Crow Creek Marlstone outcrop

View on map:42.771668°N 98.067580°W

Comments

The Cretaceous Crow Creek Marlstone was deposited in a high energy water environment consisting of not only the usual carbonates and clay found in marlstone but sand-rich siltstone and relatively large quartz grains.  The marlstone is associated with the Pierre Shale which is exposed at the site.  The shale is indicative of deposition of clay particles at the bottom of a quiet seaway that once existed here.  So why the contrast in environments so closely related?  Scientists have discovered that the marlstone is the result of a meteorite impact in the region about 74 million years ago that hit the edge of the seaway causing the larger material to be deposited in the relatively deeper waters (where the shale was being deposited) that were located here.  A tsunami created by the meteorite impact was the origin of the deposit.  See the Manson impact site.

References

  • Page 116 - Dicka, A. B., 2012, 101 American Geo-sites you've gotta see: Mountain Press Publishing Co., 250p.
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