J.B. Hartung and R.R. Anderson

The Geological Society of America
Special Paper 302, 1996, p. 31-44
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.


"Where the experience of the individual student fails him in his narrow field, the labors of others come to his relief, and thus even the problems of a single county, a single township, may, and do become intelligible as forming at least a part of some wider, vaster whole." (Macbride, 1905, p. 230)

Early geologists did not recognize the Manson Impact Structure as the product of a hypervelocity impact, but geologic anomalies produced by the impact were observed. The subsurface escarpment that defines the Manson structural rim in one area of the crater was identified from water well data before 1870. By 1905 the bedrock underlying the town of Manson (near the center of the structure) was known to be anomalous. During the drilling of a new town well an uncharacteristically thick shale sequence lying directly on granitic rock was recognized in place of the Paleozoic strata that normally constitute bedrock in north-central Iowa. Two drill cores were obtained in 1953 near the center of the structure; the first was a shale breccia, the second, crystalline rocks, presumably basement rocks lying directly under Pleistocene glacial deposits. The anomalous geology in the Manson area was first interpreted as a deeply-incised river channel deposit, later as a cryptovolcanic structure, and most recently as an impact structure. Magnetic, gravity, seismic, and aerial photographic surveys of the Manson area were undertaken to ascertain the subsurface structure. Several investigations were conducted to determine the age of the Manson impact. The best upper limit obtained prior to the initiation of the recent research coring project was based on the cooling of microcline and found to be very near the accepted age of the K-T boundary. With the information and interpretations described in this paper, the Iowa Department of Natural Resources - Geological Survey Bureau and U.S. Geological Survey initiated in 1991 a joint research core drilling program to investigate the Manson Structure. The results of investigations of the core materials recovered, and related research, (many reported in the papers that follow in this volume) have already advanced our understanding of the Manson Impact Structure beyond the historical perspective presented here.


Macbride, T.H., 1905, The geology of Emmet, Palo Alto and Pocahontas Counties: Iowa Geological Survey Annual Report, v. 15, p. 227-276.