At about 20 sites in the Raton basin of Colorado and New Mexico and at least 10 sites in Wyoming, Montana, and western Canada, a pair of claystone units, an iridium (Ir) abundance anomaly, and a concentration of shock-metamorphosed minerals mark the palynological Cretaceous/Tertiary (K/T) boundary. The lower unit, the K/T boundary claystone bed, is generally 1 to 2 cm thick; the upper unit, the K/T boundary impact layer is, on average, 5 mm thick. This couplet is overlain by a coal bed 1 to 16 cm thick.
The boundary claystone in the Raton basin consists mainly of kaolinite and small amounts of illite/smectite (I/S) mixed-layer clay; but to the north at some sites in Wyoming, Montana, and Canada, it is more smectitic. Where the boundary claystone is kaolinite rich, it is similar to tonstein layers in coal. Typically, the claystone contains deformed vitrinite laminae, root-like structures, and plant impressions. The microscopic texture of the claystone is a polygonal framework of kaolinite filled with micrometer-size kaolinite spherules. The claystone is fragmental; at some sites, particularly in Wyoming, it contains centimeter-size kaolinite pellets, some of which contain carbonaceous plant material and millimeter-size goyazite spherules. The suite of trace elements in the claystone is, in general, similar to the suite of trace elements in tonsteins and to that in average North American shale, except that the boundary claystone contains anomalous amounts of platinum-group elements, including Ir (0.07 to 0.32 ppb).
The K/T boundary impact layer also consists of kaolinite and considerably more (I/S) mixed-layer clay than the boundary claystone. Commonly it is 3 to 8 mm thick, microlaminated, and contains planar laminae of vitrinite and ubiquitous kaolinite barley-shaped pellets similar to the “graupen” of tonsteins. The microscopic texture of the claystone as seen with a scanning electron microscope (SEM) generally consists of an open wavy framework similar to that of smectite and (I/S) mixed-layer clay. This texture is significantly different than the microspherulitic texture of the underlying K/T boundary claystone. The contact between the impact layer and boundary claystone is generally sharp, and locally it may be a diastem.
The impact layer and boundary claystone are similar chemically, but the former has slightly more Fe, K, Ba, Cr, Co, Li, V, and Zn than the latter. Iridium is most abundant in the impact layer (1.2 to 14.6 ppb); however, anomalous amounts of this element are found in carbonaceous shale and coal below or above the impact layer (0.1 to 2.0 ppb).
The fact that the boundary claystone and impact layer make up a regionally extensive unit suggests they are composed of primary air-fall impact or volcanic material. However, mineralogic and chemical evidence indicates that the K/T boundary claystone is not composed of altered impact ejecta because it essentially lacks shock-metamorphosed minerals and contains only a minor Ir anomaly. Moreover, it contains only a few parts per million of Ni; this is inconsistent with the idea that it is composed of altered impact material. Observational and chemical evidence disclosed that the shocked minerals and Pt-group elements, including Ir, are the only impact-related components in the impact layer. Seemingly, the boundary claystone and impact layers are not of volcanic origin because they essentially lack a coherent assemblage of volcanic crystals.
In the Raton basin, the boundary claystone, but not the impact layer, contains two types of spherules: kaolinite and goyazite (hydrous aluminum phosphate). Spherules composed of two Fe-rich minerals—jarosite or goethite—occur in the boundary clay-stone and impact layer. Kaolinite spherules are always solid and consist of a microspherulitic core encased in a thin shell of columnar kaolinite, whereas goyazite spherules are generally hollow and consist of a microlaminated colloform shell. Kaolinite spherules are large (0.1 to 0.5 mm) and relatively common (0.1 percent); in contrast, goyazite spherules are small (<0.1 mm) and rare (<0.01 percent). Kaolinite and goyazite spherules are dispersed widely in the claystone, but inexplicably they also occur in clusters. At two Wyoming sites, the boundary claystone contains abundant (as much as 30 percent) large (0.9 mm) goyazite spherules. Evidence indicates that the spherules formed by authigenesis.
Although the impact layer is mainly composed of clay, it contains a small amount, as much as 2 percent, of clastic mineral grains. About 50 percent of the grains are quartz, and about 50 percent of these contain multiple sets of shock lamellae. Quartzite and metaquartzite constitute 26 percent of the assemblage of clastic grains in the impact layer, and 30 percent of these contain multiple sets of shock lamellae. Clearly the source of the quartzite and metaquartzite grains was chiefly sedimentary, metasedimentary, and metamorphic rocks and not volcanic rocks as some have suggested. Unshocked grains of chert and chalcedony compose from 8 to 46 percent of the assemblage. Grains of shocked feldspar (oligoclase and microcline) and granite-like mixtures of quartz and feldspar are rare. The abundance of unshocked quartzite, metaquartzite, and chert in the impact layer as compared to their paucity in underlying Cretaceous rocks suggests that most of these grains were derived from continental supracrustal impact target rocks and are not locally derived material.
Shock-metamorphosed mineral grains are larger (mean = 0.15 to 0.20 mm) at North American K/T boundary sites relative to boundary sites elsewhere in the world (mean = 0.09 mm). Large (0.25 to 0.64 mm) grains are found at all Western Interior North American K/T boundary sites. Moreover, shocked minerals are several orders of magnitude more abundant at these sites as compared to elsewhere in the world. This information indicates unambiguously that the K/T impact occurred in North America.
The Manson, Iowa, impact structure may be the K/T impact site because of the mineralogic similarity of Manson area subsurface rocks and shocked K/T boundary minerals, the large size (36 km) of the structure, the compatible isotopic age (~66 Ma) of shocked granitic rock from the Manson structure and the K/T boundary, and the proximity of the Manson structure to North American K/T boundary sites that contain relatively abundant and large shock-metamorphosed minerals.
