Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New MexicoDiscussion of K/T BoundaryThe 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
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
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
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
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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