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A problem with the impact hypothesis for the Cretaceous/Tertiary (K/T) mass extinction is the apparent absence of an identifiable impact site. The Manson impact structure is a candidate site because of its size (the largest such structure recognized in the United States); in addition, the largest and most abundant shocked quartz grains at the K/T boundary are found relatively close by, and its age is indistinguishable from that of the K/T boundary.

The region of northwest central Iowa that contains the Manson impact structure is covered by Quaternary glacial deposits, which are underlain by Phanerozoic sedimentary rocks (mostly flat-lying carbonates) and Proterozoic red clastic, metamorphic, volcanic, and plutonic rocks. In a circular area about 22 mi (35 km) in diameter around Manson, Iowa, this normal sequence is absent or “disturbed.” Within the structure, three roughly concentric zones of rock associations have been identified: an outer zone of displaced strata, a zone of completely disrupted strata, and a central area in which basement igneous and metamorphic rocks have been uplifted at least 1,220 ft (4,000 m). Gravity, magnetic, and seismic refraction surveys readily identify the central uplift within the structure. Manson is established as an impact structure based on its circular shape, its central uplift, and the presence of shocked quartz within the granitic central uplift.

Paleontological evidence, a fission track age, and preliminary 40Ar/39Ar dating all allowed a K/T boundary age for the Manson structure. Improved 40Ar/39Ar age spectra may be interpreted in terms of samples that were incompletely degassed during heating due to the Manson impact. An age of 65.7 ± 1.0 Ma was obtained, a value indistinguishable from that of the K/T boundary.

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