The largest intact asteroid impact crater in the U.S. lies completely buried under the verdant cornfields of north-central Iowa, near a town called MANSON. The crater is about 23 miles in diameter, making it about 30 TIMES larger than Meteor Crater, in Arizona. The impact occurred during the Cretaceous Period, 74 Ma, and was caused by an asteroid or comet nucleus.
Crow Creek material contains "shocked quartz grains" which are produced when a high-energy shock wave generated by an impact passes through a quartz grain, creating thin regularly spaced zones of melting along preferred crystallographic planes. Extraterrestrial impacts are the only known natural force with sufficient energy to create these features.
Most scientists now believe that the very special layer of sediment called "Crow Creek," which can be found today exposed in isolated patches in South Dakota and Nebraska is impact ejecta from the Manson Impact. The Crow Creek sediment has been proven to contain the tell-tale shocked quartz and feldspar fragments blasted from the Manson asteroid impact. Because the crater itself is completely buried and there is none of the impact rock available at the surface, the Crow Creek material is the only available signature remnant of this large impact event.
Lithology of Crow Creek: Crow Creek consists of two distinct units of marlstone and a basal coarse layer. The material can contain light-gray, silty, quartz sand with gravel-sized particles of pink and white quartz and detrital carbonate. Rip-up clasts from the Sharon Springs Member can be found as rounded, black, noncalcareous mudstone clasts. Inoceramid shells mixed with detrital and skeletal carbonate, silt and fine quartz sand in a wackestone to packstone fabric can also be found. Smectite is the dominant clay mineral with minor kaolinite and illite. Black mudstone rip-up clasts can also be present. Quartz grains contain PDF (planar deformation features) which are only created by asteroid impacts.
Disc size: 45mm diameter X 15mm D, Crow Creek Manson Ejecta Material weight: 1.65 grams
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