Preliminary Report on Basement Complex Rocks of Iowa

Yoho, W. H.

doi:10.17077/0578-6045.1002

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“…Samples from cores and cuttings from water wells that penetrated into crystalline rocks in the Manson area were examined by Herbert Yoho (1967) as a part of his research on the Precambrian rocks of Iowa. He noted the abundance of cataclastic rocks and suggested their possible relationships to the Manson structure.…”

Section: Investigation Of Early Coresmentioning

confidence: 99%

A brief history on investigations of the Manson impact structure

Hartung¹,

Anderson²

1996

Special Paper 302 the Manson Impact Structure Iowa Anatomy of an Impact Crater

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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 near Gilmore City 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 is composed of 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 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.

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Section: Investigation Of Early Coresmentioning

confidence: 99%