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The Cretaceous-Tertiary (K-T) boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals; implication concerning the K-T boundary impact-extinction theory

1987; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr87606

2332-4899

G. A. Izett,

Geology and Paleoclimatology Research

At 20 localities in the Raton Basin of Colorado and New Mexico and a few other Western North American Cretaceous-Tertiary (K-T) boundary sites, a pair of thin claystone layers, an iridium abundance anomaly, and a concentration of shock-metamorphosed minerals mark the paleontologic boundary.The lower unit is informally called the "K-T boundary claystone," and the upper unit is informally called the "K-T boundary impact layer."This stratigraphic couplet is generally overlain by a coal bed, informally called the "K-T boundary coal bed" that ranges from 4 to 16 cm in thickness.In the Raton Basin, the boundary claystone is a 1-to 2-cm-thick tonstein composed of kaolinite and a small amount of illite/smectite (I/S) mixed-layer clay.Scanning electron microscopic (SEM) study shows that the claystone mainly consists of a polygonal boxwork filled with micrometer-sized kaolinite microspherules.The lower part of the claystone contains angular fragments of cryptocrystalline kaolinite nearly as large as 1.0 mm.Typically, small amounts of carbonaceous material including horizontal vitrinite laminae, subvertical carbonaceous structures, plant impressions, swirls of vitrinite, and millimeter-sized rounded fragments of cellular fusinite occur in the claystone.In Saskatchewan, Canada, the claystone contains inclusions of yellow amber, as much as 2.5 mm in diameter.The claystone has a traceelement content similar to average North American shale, except that it contains about 0.07 to 0.32 ppb iridium.An equivalent of the K-T boundary claystone seemingly does not occur at marine K-T boundary sites outside of North America.Solid kaolinite and hollow and solid goyazite spherules, 0.05 to 1.2 mm in diameter, occur in the K-T boundary claystone but not in the overlying impact layer.Goyazite spherules are rare and widely scattered in the claystone in the Raton Basin, but are common in a 1-cm-thick goyazite-enriched layer at the top of the claystone at Wyoming K-T boundary sections.In Wyoming, they compose as much as 30 percent of the goyazite-enriched layer.SEM and thin section study of the goyazite and kaolinite spherules indicates that they formed authigenically and are not of impact origin.The upper unit, the K-T boundary impact layer, is typically 5 mm thick in the Raton Basin and elsewhere in Western North America, whereas it is only about 1 mm thick at Caravaca, Spain, and Stevns Klint, Denmark.The claystone consists chiefly of kaolinite and various amounts of I/S mixed-layer clay.Typically the claystone is microlaminated and contains planar laminae of vitrinite and ubiquitous kaolinite pellets.The SEM texture of the claystone is, in general, similar to smectite clay and is different from the underlying microspherulitic boundary claystone.The contact between the impact layer and the underlying boundary claystone is generally sharp and records a significant change in depositional regime.Of importance to the impact-extinction theory is the fact that the impact layer contains as much as 2 percent clastic mineral grains, 30 percent of which contain multiple intersecting sets of planar lamellae clearly of shockmetamorphic origin.Only one such concentration of shock-metamorphosed minerals has been found near the K-T boundary.Of the grains that contain shock lamellae, 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 target materials of impact origin.The shocked minerals and an unknown phase (perhaps originally an aerosol) that carried iridium are the only impact-related components in the claystone.Two sources of information, one from the compositional types of K-T boundary shock-metamorphosed minerals and the second from the study of pumice fragments in silicic pyroclastic rocks eliminates the possibility that the shock-metamorphosed minerals are of volcanic origin.The global size distribution of shock-metamorphosed mineral grains in the impact layer suggests that the K-T impact occurred in North America.At Western North American K-T boundary sites, the mean size of shocked quartz grains is as follows: Raton Basin, Colorado and New Mexico (0.20±0.06 mm to 0.16±0.06mm), Teapot Dome, Wyoming (0.14±.04 mm), Brownie Butte, Montana (0.15±0.05 mm), Alberta, Canada (0.26±0.06 mm).Rare grains, as long as 0.50 to 0.64 mm, are found at all Western North American sites.The mean size of 100 shocked quartz grains in the impact layer at Caravaca, Spain, is 0.09±0.03mm, and the range is 0.04-0.19mm, considerably less than the mean size of shocked quartz grains at North American K-T boundary sites (0.14-0.26 mm).Forty shocked quartz grains from upper Eocene glassy impact sediment in DSDP site 612 off the New Jersey Coast range in size from 0.1 to 0.5 mm and average 0.26±0.1 mm.The chemical composition of the impact layer and boundary claystones are similar, except that the former contains slightly more iron, potassium, barium, chromium, copper, lithium, vanadium, and zinc than the latter.Like the boundary claystone, the impact layer claystone has a low content of nickel and cobalt, only a few ppm.Radionuclide activation analyses of rocks that span the K-T boundary show that iridium is generally greatest in the K-T boundary impact layer; nevertheless, anomalously large values also can occur in carbonaceous-rich layers, particularly coal, either below or above the impact layer.Amounts of iridium vary from 1.2 to 14.6 ppb; these amounts are considerably more (5 to 66 times) than those in the underlying boundary claystone.The surface concentration of iridium varies considerably at localities only a few kilometers apart, from 8 to 120 ng/cm .Because shockmetamorphosed minerals are concentrated in the impact layer, the assumption was made that the iridium also was originally concentrated in this layer.During diagenesis of the sediments, some iridium was probably mobilized and transported away from the impact layer and concentrated in adjacent carbonaceous-rich sediments.The origins of the K-T boundary claystone and K-T boundary impact layer are seemingly closely related because they form a stratigraphic couplet at numerous Western North American sites.However, considerable observational and chemical evidence suggests that the claystone matrix of the units is not altered volcanic or impact-generated material.The most important observational fact that bears on the origin of the K-T boundary claystone is that shock-metamorphosed minerals are restricted essentially to the K-T boundary impact layer and are not found in the underlying K-T boundary claystone.It is difficult to understand how the boundary claystone could originate from impact material that would be entirely free of clastic mineral grains, in particular, shock-metamorphosed minerals.The Manson, Iowa, structure is proposed as the K-T 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.Objections can be raised that the Manson, Iowa, impact structure is not the K-T boundary impact site because it is too small.Inherent in this reasoning are the assumptions that the composition, mass, strike-angle of the asteroid are known.None of these assumptions may be warranted, and, thus, the Manson impact structure is a viable candidate site for the K-T boundary impact.The killing mechanism for the K-T extinction event may not have been the lofting of large volumes of dust into the atmosphere and the ensuing global darkness as was proposed by the Alvarez team.Instead the killing mechanism may have been the generation of large volumes of nitrogen oxides and carbon dioxide produced in the atmosphere from the K-T impact.Acid rain generated during the impact would have a disastrous effect on plant life, particularly in the Western Interior of North America, and the biota of the photozone of the oceans.An increase in the atmosphere of impact-generated carbon dioxide may have caused a green house effect and resulting global warming.