A Paleomagnetic Assessment of the Age of Manson Crator

爱荷华州中部曼森陨石坑岩石年龄测定 表明这个陨石坑的形成时间 K/T边界，冲击层位正确 这些成分产生了在地球上发现的震动矿物质， 世界各地的边界粘土层，但最丰富的和 北美边界粘土中最大的粒度。 虽然 Ar 40/Ar 39测年表明陨石坑的年龄为65.7+1百万年，四样本 古地磁研究报告说，陨石坑岩石只有正常的 极性磁化 由于K/T边界已找到， 在世界范围内的一个反向极性间隔，正常极性 结果导致了一些人的推论，曼森的影响 并没有发生在K/T边界。 然而，这项研究是 非常小，而且，样品显示出一些 模糊的磁化行为。 这项研究涉及两个方面 对曼森陨石坑的岩石进行古地磁研究， 极性和磁化年龄。 首先是详细研究 冲击角砾岩的磁化强度。 矩阵磁力- 碎屑磁化的“砾岩试验” 有可能揭示地球磁场的极性， 曼森撞击的时间，因此无论陨石坑是 形成与K/T边界相同的极性。 进一步 撞击前后的磁性地层学 将研究沉积序列，以进一步缩小年龄 撞击事件的范围。 磁性地层学研究也有 有可能缩小辐射测量的不确定性 确定曼森陨石坑的年龄为200万年小于50万年。 重要的是要确定曼森的影响是否是一个 表明K/T边界现象是 多重影响

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.