A Paleomagnetic Assessment of the Age of Manson Crator

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Age dating of rocks from manson crater in central Iowa indicates that this crater was formed in the same time frame as the K/T boundary, and the impacted strata are of the correct compositions to have generated the shocked minerals found in the boundary clay layer worldwide, but most abundantly and of the largest grain sizes in the North American boundary clays. Although Ar40/Ar39 dating places the crater age at 65.7+1 m.y., a 4-sample paleomagnetic study reported the crater rocks to have only normal polarity magnetization. Because the K/T boundary is found worldwide in a reversed polarity interval, the normal polarity results have led to the inference by some that the Manson impact did not happen at the K/T boundary. However, the study was exceedingly small, and moreover, the samples displayed some ambiguous magnetization behavior. This study involves a two-fold paleomagnetic study of the rocks in Manson Crater to determine the polarity and age of magnetization. First is a detailed study of the magnetization of the impact breccia. Both the matrix magneti- zation and a "conglomerate test" of the clast magnetizations potentially can reveal the polarity of the geomagnetic field at the time of the Manson impact, hence whether or not the crater was formed in the same polarity as the K/T boundary. Further the magnetostratigraphy of the post-impact, as well as the pre-impact sedimentary sequences will be studied to further narrow the age range of the impact event. The magnetostratigraphy study also has the potential to narrow the uncertainty of the radiometrically determined age of Manson crater from 2 m.y. to less than 0.5 m.y. It is important to determine whether the Manson impact is an indication that the K/T boundary phenomena are the result of multiple impacts.