The Cretaceous-Tertiary boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals: Implications for the Cretaceous-Tertiary boundary impact-extinction theory

Abstract

In the Raton Basin and other Western North American Cretaceous-Tertiary (K-T) boundary sites, a pair of claystone beds, an overlying coal, an an iridium anomaly mark the boundary. The lower unit, the boundary claystone, is a 1- to 2-cm-thick kaolinitic tonstein that is generally free of quartz grains, has a micropherulitic texture, and contains a trace-element suite similar to average North American shale. Rare kaolinite and goyazite spherules in the claystone probably are not of impact origin. An equivalent of the boundary claystone does not occur at boundary sites outside of North America. The upper unit, the impact layer, is about 5 mm thick and consists chiefly of kaolinite; it is microlaminated and contains ubiquitous kaolinite pellets and vitrinite laminae. Its chemical composition is different than the boundary claystone, particularly its greater content of iridium. Shock-metamorphosed minerals are concentrated in the impact layer and generally are absent in subjacent rocks. The origin of the two units is seemingly closely related because they form a couplet at Western North American sites. However, evidence suggests that the claystone matrix of the units is not altered volcanic or impact-generated material. The impact layer contains up to 2 percent clastic mineral grains, 30 percent of which contain shock lamellae. Of these grains, quartzite, metaquartzite, and chert constitute about 60 percent and quartz the remainder. Grains of shocked feldspar and granite-like mixtures of quartz and feldspar are rare. The abundance of unshocked quartzite, metaquartzite, and chert in the impact layer and their paucity in underlying rocks suggests they are of impact origin. Evidence suggests that the shock-metamorphic minerals in the impact layer are not of volcanic origin. The Manson, Iowa, structure is proposed as the impact site because of the mineralogic similarity of Manson subsurface rocks and shocked K-T boundary minerals, the large size (35 km) of the impact structure, the compatible isotopic age of shocked granitic rock from the Manson impact structure and the K-T boundary (66 Ma), and the proximity of the Manson impact structure to North American boundary sites that contain relatively abundant and large shocked minerals.