Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New Mexico

Discussion of K/T Boundary

The Cretaceous-Tertiary (K-T) boundary has been placed at different stratigraphic horizons by different workers. Modern workers believe the boundary is now more accurately placed than ever, based on three lines of independent and corroborating evidence: palynology; trace element chemistry; and mineralogy. Lee (1917) originally placed the K-T boundary at the unconformity at the base of the Raton Formation. Later, Brown (1962) identified Cretaceous plant fossils in the lower part of the Raton, at a site about 3.5 mi north of Trinidad, Colo., and indicated that the boundary should be placed at least 50 ft above the base of the formation. In 1967, the late R.H. Tschudy of the USGS bracketed the K-T boundary on the basis of palynology between two coal beds about 270 ft above the base of the Raton Formation in a core hole drilled at York Canyon, New Mexico (R.H. Tschudy, written comm., 1967, and Pillmore, 1969). The position of the boundary was precisely established by Tschudy four years later as reported in Orth and others (1981). In the Raton basin, this extinction horizon coincides with the top of a 0.5-to 1.0-in-thick claystone bed termed the boundary claystone (Orth and others, 1981; Pillmore and others, 1984; Tschudy and others, 1984). This horizon also coincides with an anomalous concentration of Ir and shock metamorphosed minerals (Orth and others, 1981; Izett and Pillmore, 1985a, 1985b) and a sudden change in the relative proportion of fern spores to angiosperm pollen (Tschudy and others, 1984). This unique claystone bed has been found at more than 25 sites throughout an area of about 1000 mi² in the east central and southern parts of the Raton basin

Palynology
In the northern part of the Rocky Mountain region, the palynological K-T boundary was originally defined on the basis of the disappearance of fossil pollen of Proteacidites spp. and most species of Aquilapollenites (Leffingwell, 1970; Tschudy, 1970). This horizon occurs within a few feet above the disappearance of dinosaurs. In the Raton basin, the K-T boundary is defined solely on the basis of palynology due to the absence of dinosaur fossils. Aquilapollenites pollen occurs only rarely in the southern part of the Rocky Mountain region and Tschudy (Tschudy, 1973; Orth and others, 1981; Tschudy and Tschudy, 1986) used the extinction of the species "Tilia" wodehousei, Trisectoris, and Trichopeltinites sp. in addition to the extinction of Proteacidites spp. to locate the K-T boundary in the Raton basin. For discussions of the Raton basin, these taxa are herein called the Proteacidites assemblage.

Figure 4. Fossil pollen of latest Cretaceous age from the Raton basin. Top row, left to right: Tschudypollis ("Proteacidites") retusus, Trisectoris sp., "Tilia" wodehousei; bottom row, left to right: Liliacidites complexus, Aquilapollenites reticulatus, Libopolllis jarzenii.

Tschudy and others (1984) concluded that the K-T boundary event, the hypothesized asteroid impact, caused massive destruction of vegetation, disrupted the terrestrial ecosystem, and resulted in the extinction of the plants that produced the Proteacidites assemblage. Plants that survived exhibit three basic patterns of survival. The first pattern is shown by pollen of Kurtzipites spp., which is common in the latest Cretaceous, survives the K-T boundary event, and persists into the Paleocene until it disappears about the middle Paleocene (Tschudy and Tschudy, 1986). Psilastephanocolpites sp. exemplifies the second pattern. This species is rare in the Cretaceous but becomes more abundant in the Paleocene, perhaps because the plants that produced this particular fossil pollen were better adapted to the new ecological conditions. The last pattern is characterized by species little affected by the K-T boundary event and includes such fossil pollen as Ulmipollenites sp. and Pandaniidites radicus. The patterns of extinction and survival within the Raton basin indicate that different plant species responded in different ways to environmental stress caused by the K-T boundary event.

Detailed palynological sampling across the K-T boundary revealed the presence of anomalously abundant fern spores just above the extinction level, which appears to be unique to the K-T boundary event. This abundance of fern spores (termed the "fern spike") occurs in K-T boundary sections from the Raton basin to south-central Saskatchewan (Tschudy and others, 1984; Nichols and others, 1986; Tschudy and Tschudy, 1986). In Cretaceous assemblages of the Raton basin, fern spores usually constitute 15-30 percent of the palynomorph assemblages. Just above the K-T boundary claystone, the fern spore percentage increases dramatically to as much as 99 percent. Usually the percentage returns to the 15-30 percent level within 3-5 in above the boundary.

The fern spike is an unusual palynological assemblage in comparison with typical Upper Cretaceous and Paleocene palynological assemblages found in nonmarine rocks (Fleming and Nichols, 1990). Comparison of these assemblages from just above the K-T boundary at many localities with typical assemblages reveals that the fern-spore spike is characterized by: (1) relatively abundant spores, ranging from 70 percent to 100 percent of the assemblages (in contrast with 10 percent to 40 percent for typical Upper Cretaceous and Paleocene assemblages in the same sections); (2) dominance of only one of a few species at each locality; (3) restriction of the anomaly to a layer 0-6 in above the K-T boundary (usually only an inch or two above the boundary); (4) independence of lithology (the anomaly occurs in coal, carbonaceous shale, and mudstone); and (5) isochroneity (based on palynological and geochemical evidence) and (6) wide distribution (from northern New Mexico to south-central Saskatchewan, a distance of approximately 810 mi). Within the Raton basin, comparison of three K-T boundary localities reveals the pattern of relative abundance of fern spores and the independence of lithology characteristic of this unique assemblage. (See Fig. 5).

Figure 5. Fern-spore relative abundances from three K-T boundary localities in the Raton basin. Left: Starkville North section, Colorado. Middle: Sugarite section, New Mexico. Right: Raton Pass section, New Mexico. (Black = coal; white = mudstones and shales; xxxx = K-T boundary claystone; T = Tertiary; K = Cretaceous).

Tschudy and others (1984) pointed out the importance of this phenomenon with respect to the destruction of terrestrial vegetation. They attributed the "fern spike" to early colonization of the devastated landscape by ferns. The temporary dominance of ferns at the K-T boundary is due to the "early arrival of wind-dispersed spores, the removal of competitors, and the known tolerance of ferns to soils deficient in mineral nutrients" (Tschudy and others, 1984, p. 1031). In general, palynological observations of patterns of extinction and survival suggest that the terrestrial ecosystem was stressed by a significant, though geologically brief, event (Tschudy and others, 1984; Tschudy and Tschudy, 1986).

Paleobotany
Early in the K-T boundary controversy, Hickey (1981) asserted that the megafloral record was inconsistent with the hypothesis that a catastrophe caused terrestrial extinctions. However, in their joint paper in 1990, Johnson and Hickey reversed this position by presenting new evidence that the megafloral change is about 80 percent and that it coincides with a peak in palynofloral extinctions and the occurrence of Ir and shock-metamorphosed mineral grains. They state that "the results of their analysis of the terrestrial plant record are compatible with the hypothesis of a biotic crisis caused by extraterrestrial impact at the end of the Cretaceous" (Johnson and Hickey, 1990, p.433).

Wolfe and Upchurch (1986) analyzed fossil leaves and dispersed fragments of leaf cuticles from K-T boundary sequences in the Raton Formation. Their results suggest a brief low-temperature excursion (mean temperature near 0°C) that caused a mass kill and ecological disruption of terrestrial vegetation at the K-T boundary. Leaf size and shape indicate that a major increase in precipitation occurred across the boundary. Their conclusions are consistent with the bolide impact hypothesis.

The K-T Boundary Claystone Bed
The boundary claystone bed resembles a tonstein (kaolinitic claystone partings thought to result from alteration of volcanic ash beds in acidic coal swamps), but, unlike typical tonsteins, it usually weathers to a lighter, pinkish, color and exhibits a fine-grained to amorphous texture and a distinctive hackly to conchoidal fracture. The claystone is mostly gray and grayish pink to grayish yellow and commonly contains tiny specks and thin contorted lenses or layers of organic matter, especially near the margins. Small spheroidal structures can be seen on fracture surfaces of some specimens and in thin section. X-ray diffractograms show that, like many tonsteins, the boundary claystone is nearly pure, well-crystallized kaolinite with lesser amounts of randomly stratified illite-smectite clay and some quartz and feldspar (Pollastro and others, 1983; Pollastro and Pillmore, 1987). However, the boundary claystone is texturally and chemically different from typical Raton basin tonsteins. As seen in ultrathin section, it is fine grained to amorphous but may exhibit an imbricate fabric and relics of small bubbles in a fine crystalline matrix of kaolinite. Microspherules (40-120 microns in diameter) consisting of calcium, aluminum, strontium, cerium, rare earth elements, and phosphorus (similar to goyazite, a hydrous strontium alumino-phosphate in composition; microprobe analysis by Ralph Christian, USGS, 1984) have been observed in samples of the boundary claystone from the Raton site. These phosphatic spherules are rarely seen at other sites but they form discrete layers in the boundary claystone at the Dogie Creek and Teapot Dome localities in the Powder River Basin in Wyoming (Izett, 1990). Smit (1984) has referred to similarly shaped grains in the boundary claystone from the Raton basin and other areas as microtektite-like structures, implying an impact origin. The microtektite grains from Raton basin sites are mostly spheroidal to subspheroidal and resemble dull to shiny resinous little balls under the microscope. Some are hollow. Under the scanning electron microscope, they have an uneven surface texture and commonly are pitted by irregularly shaped cavities. On the basis of textures and shapes observed in goyazite spherules in the Powder River Basin, that are identical to those seen in microtektites, the spherules are thought to result from the alteration of glassy ejecta material (microtektites) blown out of the crater during the K-T impact event.

Figure 6. Photomicrograph by G.A. Izett of a 0.21-mm diameter shock metamorphosed quartz grain from the Starkville South site. The grain is mounted in index oil on the needle (dark part of photograph) of a spindle stage. The two sets of planar lamellae that are prominent in the photograph are strong evidence of impact origin as no comparable lamellae have been observed in rocks of volcanic origin, only those related to impact and underground atomic explosions (Izett, 1990). Shocked quartz grains have been observed in K-T boundary layers worldwide.

High concentrations of Ir and shock metamorphosed mineral grains, both compelling evidence of impact origin (Bohor and others, 1984; Izett, 1990) occur in a discrete layer at the top of the boundary claystone. This layer was called the flaky shale layer by Pillmore (Pillmore and others, 1984), the K-T boundary impact bed by Izett and Bohor (1986), and, later, the fireball layer by Hildebrand and Boynton (1988). The shocked grains consist mainly of quartz, with rare microcline and plagioclase. The shock metamorphosed quartz grains contain as many as nine intersecting sets of closely spaced planar features per grain (Izett, 1990). Figure 6 is a photograph by Izett of one of the shocked grains of quartz from the Starkville South site, showing two sets of planar lamellae.

Geochemistry
Both the ejecta layer of the boundary claystone and tonsteins found in coal beds are high-alumina clays that characteristically contain about 32 percent Al₂O₃ (Gilmore and others, 1984). Ir abundance anomalies as high as 56 ng/g (56 x 10^-9 g/g, about 8000 x background) have been measured in samples from the fireball layer at the top of the boundary claystone collected from the study area (Pillmore and others, 1984). In comparison, background Ir concentrations of only 0.004-0.040 ng/g are observed in tonsteins and other beds of coal and shale not associated with the boundary (Gilmore and others, 1984). In addition, titanium, scandium, vanadium, chromium, and antimony in the boundary claystone are enhanced by factors of about four or more over their concentrations in all other Raton basin tonsteins that were analyzed by Gilmore and others (1984). Table 1 shows a comparison of these and other elements in the boundary claystone with those in tonstein beds in the Raton basin (Gilmore and others, 1984). The boundary claystone has been found at scattered localities from Cimarron, New Mexico, to Red Deer Valley, Alberta, remarkably consistent in chemical composition and always in direct contact with the high Ir, shocked-quartz bearing fireball or K-T boundary impact layer (Izett, 1990). The Ir abundance anomalies in the boundary claystone bed, its wide geographic extent, and its unusual textural and chemical character indicate that the boundary claystone bed was derived from a different source than were the tonstein beds. It is widely accepted now that the boundary claystone consists primarily of altered glass ejecta material from the Chicxulub impact site on the northern tip of the Yucatan Peninsula in Mexico and the fireball layer contains material from the vaporized bolide together with some vaporized target material.

Table 1. Elemental abundances in the thin kaolinitic K-T boundary claystone bed compared to elemental abundances in kaolinitic tonstein beds found in coal beds above and below the K-T boundary in the Raton basin. From Gilmore and others, 1984.

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