Two-polarity magnetization in the Manson impact breccia

A preliminary paleomagnetic study of the impact breccia matrix and clasts has produced surprising results--nearly antipodal normal and reversed polarity magnetic vectors are observed in different portions of the core. Near-antipodal magnetizations within a segment of matrix and within individual samples rule out core inversion as the explanation of the dual polarity. In both the dense and the sandy matrix breccias, the magnetizations of clasts and matrix within the same core segment are identical; this negative 'conglomerate test' indicates that magnetization originated after impact. Paleomagnetic study of the Manson Impact Structure is an attempt to refine the Ar-40/Ar-39 age (65.7 +/- 1 m.y.) that suggests Manson to be a Cretaceous-Tertiary boundary impact. Refinement is possible because the boundary occurs within a reversed polarity interval (29R) of only 0.5 m.y. duration. The two breccia types in the Manson structure were both examined: one of a very dense matrix and apparently partially melted, and the breccia stratigraphically below it of granular or 'sandy' chloritic matrix. Samples were taken from the matrixes and a wide variety of clast compositions, including granite, diabase, gneiss, amphibolite, and melted granite. Currently, measurements have been made on 22 samples, using 30-35 steps of either alternating field (AF) or thermal demagnetization.