| Manson Crater |
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No surface evidence exists due to coverage by Glacial Till and the site where the crater lies buried is now a flat landscape. But, hidden about 20 to 90 metres below the surface is a buried structure about 38 km in diameter. It lies under the southeast corner of Pocahontas County and extends under portions of three adjoining counties. That an anomalous structure underlaid the area was known from the early 1900s from unusual water well drill cuttings. A research investigation was started in 1955 and it was labeled a "cryptovolcanic structure" ( Volcanic - steam explosion). Further investigation was undertaken by Robert Dietz who proposed an impact origin in 1959 and by Nicholas Short in 1966 who produced evidence of Shocked Quartz grains which confirmed the Impact Origin of the structure. In 1991 and 1992 the U.S. Geological Survey along with others including the Iowa Geological Survey conducted detailed research in part to test the possible connection of the Manson Crater with the K-T Boundary Extinction Event . 40Ar/39Ar isotopic dating of core from the impact structure by Izett et al. (1993), however, gave an age of about 74 Ma, or about 10 Ma older than the K-T boundary. The impactor is considered to have been a stoney stony Meteorite about two kilometres in diameter. The impact disrupted Granite , Gneiss , and Shale s of the Precambrian basement as well as Sedimentary formations of Paleozoic age, Devonian through Cretaceous . REFERENCES
Izett, G.A., Cobban, W.A.. Obradovich,J.D., and Kunk, M.J., 1993 The Manson Impact Structure: 40Ar/39Ar Age and Its Distal Impact Ejecta in the Pierre Shale in South Eastern South Dakota: Science, v. 262, pp. 729-732.
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