Four new coreholes in the western annular trough of the buried, late Eocene Chesapeake Bay impact structure provide samples of shocked minerals, cataclastic rocks, possible impact melt, mixed sediments, and damaged microfossils. Parautochthonous Cretaceous sediments show an upward increase in collapse, sand fluidization, and mixed sediment injections. These impact-modified sediments are scoured and covered by the upper Eocene Exmore beds, which consist of highly mixed Cretaceous to Eocene sediment clasts and minor crystalline-rock clasts in a muddy quartz-glauconite sand matrix. The Exmore beds are interpreted as seawater-resurge debris flows. Shocked quartz is found as sparse grains and in rock fragments at all four sites in the Exmore, where these fallback remnants are mixed into the resurge deposit. Crystalline-rock clasts that exhibit shocked quartz or cataclastic fabrics include felsites, granitoids, and other plutonic rocks. Felsite from a monomict cataclasite boulder has a sensitive high-resolution ion microprobe U-Pb zircon age of 613 ± 4 Ma. Leucogranite from a polymict cataclasite boulder has a similar Neoproterozoic age based on muscovite 40 Ar/ 39 Ar data. Potassium-feldspar 40 Ar/ 39 Ar ages from this leucogranite show cooling through closure (∼150 °C) at ca. 261 Ma without discernible impact heating. Spherulitic felsite is under investigation as a possible impact melt. Types of crystalline clasts, and exotic sediment clasts and grains, in the Exmore vary according to location, which suggests different provenances across the structure. Fractured calcareous nannofossils and fused, bubbled, and curled dinoflagellate cysts coexist with shocked quartz in the Exmore, and this damage may record conditions of heat, pressure, and abrasion due to impact in a shallow-marine environment.

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.

Miocene sedimentary rocks in northern Colorado record evidence of late Cenozoic deformation, including folding, uplift, and normal faulting. Faults with late Cenozoic movements are localized along zones of Laramide faulting, and many have movements in an opposite direction from their Laramide movements. The Miocene formations in northern Colorado and their stratigraphic equivalents in Wyoming and Nebraska include the Browns Park, North Park, and Troublesome of northwest Colorado and the Arikaree and Ogallala of northeast Colorado. These formations, which formerly were much more extensive, are mainly nonorogenic eolian and fluvial siltstone and sandstone as much as 900 m thick. In the White River Plateau, Grand Mesa, State Bridge, and Middle Park areas, the sediments are interlayered with, or intruded by, basalts that are remnants of a much more extensive volcanic field than is preserved today. Deformation accompanied and followed deposition of Miocene sediments and basalts, as shown by (1) deposition of Miocene rocks in a paleovalley cut prior to 25 m.y. along the axis of the Uinta arch and normal faulting later than 9 m.y. ago in the eastern Uinta Mountains, (2) major uplift later than 10 m.y. ago of Miocene rocks of the White River Plateau and folding of Miocene basalt in the State Bridge area, (3) faulting of Miocene rocks on the west flank of the Park Range, (4) faulting of Miocene rocks indicating renewed deformation along the trace of the Williams Range thrust (Laramide ancestry) in Middle Park, (5) faulting of Miocene rocks along the Blue River, suggesting uplift of the Gore Range, and (6) sharp folding of Miocene rocks in the North Park syncline and faulting in Saratoga valley.