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100300 m

VermeĆ³ Formation

0115 m

Trinidad Sandstone

090 m

Pierre Shale

550580 m

Figure 7.3 Stratigraphic nomenclature for Upper Cretaceous and Paleocene strata in the Raton Basin.

later in the early Paleocene to a somewhat more diverse angiosperm-dominated forest that was still lower in diversity than that of the latest Maastrichtian. The Paleocene climate remained warm, but became much wetter. Within a few million years after the K-T boundary, forests with rainforest physiognomy

Boundary Raton Basin
Figure 7.4 The K-T boundary claystone (at arrow) visible even at a distance. Clear Creek North locality, Colorado.

were present. Mires in which coal-forming peat accumulated persisted from Maastrichtian to Paleocene time in the Raton Basin region.

Research on the microstratigraphic palynology of the K-T boundary began when Robert Tschudy of the US Geological Survey identified the palynological K-T boundary in a drill core as part of a search for the iridium anomaly in nonmarine rocks (Orth et al. 1981). Orth and his team obtained the Los Alamos-York Canyon Core (locality 42) at the York Canyon mine in New Mexico (Figure 7.2). Palynology bracketed the K-T boundary within an interval of about one meter, and gamma-ray spectrum analysis pinpointed an iridium anomaly of 5.6 ppb. Following that discovery, further palynological and nuclear geochemical (neutron activation) analyses were conducted on samples collected at 2.5-cm intervals. Results of those analyses showed that characteristic Cretaceous pollen species present low in the core abruptly disappeared precisely at the level of the peak concentration of iridium. Paleomagnetic analysis later confirmed the reversed polarity of the interval that included the K-T boundary (Shoemaker et al. 1987).

A short time after the discovery of a K-T boundary in the York Canyon Core, the boundary was located in outcrop exposures in the basin (Orth et al. 1982). The basin continued to be a prolific source of K-T boundary localities, and by 2003, about 25 had been discovered - almost all of them through the field work of Charles ("Chuck") Pillmore of the US Geological Survey. Of these, 13 have been fully documented by palynological and iridium analyses (Table 2.1).

Figure 7.5 The K-T boundary claystone (just below jackknife). A thin coal bed that lies above the claystone at this locality has been scraped away to expose the claystone layer. The jackknife is about 10 cm in length. Starkville North locality, Colorado.

The Raton sections have been the site of numerous ancillary and illustrative analyses. These include the dating of zircons from the boundary layer to show both the age of the target rock and the age of the impact event, effectively fingerprinting the Raton K-T horizon to the Chicxulub source (Kamo and Krogh 1995). Other putative K-T boundaries were identified solely by the distinctive impactite layer (Figure 7.5), and all but one of them have been verified by palynology (Table 2.1). The 13 fully documented localities are York Canyon Core (the discovery core), City of Raton (also known as Old Raton Pass), Sugarite, North Ponil, Dawson North, Crow Creek, Starkville North, Starkville South, Clear Creek North, Clear Creek South, Madrid, Berwind Canyon, and Long Canyon. The first six listed are in northeastern New Mexico; the last seven are in southeastern Colorado. For decriptions of Raton Basin outcrop localities, see Pillmore et al. (1984), Pillmore et al. (1988), Pillmore and Fleming (1990), Pillmore et al. (1999), and Nichols and Pillmore (2000). Selected Raton Basin localities are discussed below because they provide insights into the history of plants at the K-T boundary. The records from three localities: Starkville South, Sugarite, and City of Raton, epitomize palynological data from the K-T boundary in the Raton Basin (Figure 7.2).

Starkville South (locality 49) is the locality at which Tschudy et al. (1984) first found the fern-spore spike in a K-T boundary outcrop locality, shortly after it had been observed in the Los Alamos-York Canyon Core. At Starkville South, the K-T boundary is in a claystone layer just beneath a thin (5 cm) coal bed (Figure 7.6). A layer of flaky shale only millimeters in thickness at the top of

Radar Allemand Amiens

Figure 7.6 Typical K-T boundary interval in the Raton Basin showing boundary claystone layer (at tip of hammer) overlying shaly mudstone containing Maastrichtian pollen and spores, and overlain by thin coal bed and shaly mudstone containing Paleocene pollen and spores. Starkville South locality, Colorado.

Figure 7.6 Typical K-T boundary interval in the Raton Basin showing boundary claystone layer (at tip of hammer) overlying shaly mudstone containing Maastrichtian pollen and spores, and overlain by thin coal bed and shaly mudstone containing Paleocene pollen and spores. Starkville South locality, Colorado.

the claystone layer yielded a 56 ppb iridium anomaly, the strongest ever measured in continental rocks in North America (Pillmore et al. 1984). The palyno-logical extinction level is marked by the abrupt disappearance of characteristic Maastrichtian palynomorphs of what Pillmore, Tschudy, and others designated as "the Proteacidites assemblage." (North American species previously assigned to Proteacidites have been reassigned to a new genus named in honor of Robert Tschudy, Tschudypollis.) Members of the Tschudypollis (Proteacidites) assemblage are listed in Table 7.1. Species of the genus Tschudypollis are by far the most common palynomorphs in the samples below the K-T boundary. About 19% of the total Maastrichtian palynoflora disappears at the boundary. The percentage of the palynoflora that becomes extinct is low compared with that in the Williston Basin, and, in fact, the list of Raton Basin K taxa is short compared with that for North Dakota, largely because of the paucity of Aquilapollenites species (Table 6.1). At Starkville South, coal and mudstone just above the boundary claystone contain the fern-spore spike (Figure 7.7). Spores of a single species of the genus Cyathidites overwhelmingly dominate assemblages within a 10-cm interval above the K-T boundary with a peak abundance greater than 99%. This contrasts strongly with the spore content of assemblages from mudstone below the boundary, which are composed of 22-36% fern spores of several species. The boundary claystone layer itself is barren of palynomorphs.

Table 7.1 Palynomorph taxa whose extinctions mark the K-T boundary in the Raton Basin, Colorado and New Mexico

Aquilapollenites mtchedlishvilii Srivastava 1968 [= A. reticulatus (Mtchedlishvili 1961)

Tschudy and Leopold 1971] Ephedripites multipartitus (Chlonova 1961) Yu, Guo, and Mao 1981 Libopollis jarzenii Farabee et al. 1984 Liliacidites complexus (Stanley 1965) Leffingwell 1971 ''Tilia'' wodehousei Anderson 1960 Trichopeltinites sp.

Tricolpites microreticulatus Belsky, Boltenhagen, and Potonie 1965 [= ''Gunnera'']

Trisectoris costatus Tschudy 1970

Tschudypollis retusus (Anderson 1960) Nichols 2002

Tschudypollis thalmannii (Anderson 1960) Nichols 2002

Tschudypollis spp. [= "Proteacidites"]

Cyathidites spores constitute 80% of the assemblage in the lower part of a 5-cm-thick coal bed overlying the boundary claystone, and 77% in the upper part. The peak abundance of fern spores (all species) is 99.5%, just above the coal. The percentage of fern spores drops close to the pre-boundary level as angiosperm pollen reappears in mudstone above the coal. This mudstone contains leaves of Paranymphaea crassifolia, a taxon that appears immediately above the K-T boundary from the Raton Basin all the way north to Saskatchewan.

The geologic setting of the K-T boundary at Sugarite (locality 44) is quite different from that of the other Raton Basin localities (Figure 7.2). At Sugarite, the boundary is 18 cm below the top of a 183-cm-thick coal bed (Figure 7.8). An iridium anomaly of 2.7 ppb (Pillmore et al. 1984) forms a double spike, indicating migration of iridium from the boundary claystone layer into the coal above and below it (Pillmore et al. 1999). About 17% of the characteristic Maastrichtian palynomorph taxa present in the coal below the K-T boundary disappear at the boundary. A fern-spore spike assemblage is present above the boundary and is composed of up to 78% fern spores, most of them the single species of Cyathidites.

The significance of the Sugarite locality for understanding the nature of the palynological record of the K-T boundary is that both the pollen extinction level and the fern-spore spike assemblage are present entirely within a coal bed. The occurrence of these phenomena at Starkville South and most other Raton Basin localities where a coal bed lies just above the boundary might suggest that certain pollen taxa disappear because of the transition from a clastic lithology to coal. The presence of coal marks a change in depositional environments inhabited by differing plant communities; an abundance of fern spores might

Figure 7.7 Diagram showing K-T boundary interval at the Starkville South locality with iridium concentrations (black dots) and percentages of fern spores (triangles connected by line) (modified from Pillmore et al. 1999). The shaded area is the ''fernspore spike,'' which is composed predominantly of a single species, Cyathidites dia-phana (illustrated). Reprinted by permission.

Figure 7.7 Diagram showing K-T boundary interval at the Starkville South locality with iridium concentrations (black dots) and percentages of fern spores (triangles connected by line) (modified from Pillmore et al. 1999). The shaded area is the ''fernspore spike,'' which is composed predominantly of a single species, Cyathidites dia-phana (illustrated). Reprinted by permission.

be indicative only of a mire paleoenvironment. Sugarite disproves that interpretation and demonstrates that the pollen species that disappear at the K-T boundary were produced by plants that became extinct, and that following the extinction, the first plant communities to emerge in earliest Paleocene time were composed primarily of a low-diversity group of surviving species.

Another study of note from Sugarite is the measurement of the stable carbon isotope 513C by Beerling et al. (2001) that showed an appreciable 2 per mil negative excursion just above the boundary. The negative excursion was interpreted as indicative of collapse of the global carbon cycle following the K-T boundary event. A similar excursion is known from marine K-T boundary sections where it is relatively well understood. The mechanisms driving the carbon isotope excursion in this terrestrial section are not fully known, but the

Ir in ppb ,logar ithmic scale (dotted line)

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