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

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Stop 4: The Sugarite K T boundary site

At the Sugarite site the coal bed is 72 in thick and contains two thin partings: a thin carbonaceous shale 15 in above the base of the coal, and the kaolinite rich K-T boundary claystone, 12 in thick, 6 in below the top (Fig. 15). As shown in the diagram (Fig. 16), the boundary claystone contains Ir concentrations of 2.7 ng/g and also marks the disappearance of the Proteacidites assemblage and change in fern spore/angiosperm pollen ratios (Pillmore and others, 1984; Gilmore and others, 1984; and Pillmore and Flores, 1987).

Figure 15. Photograph of the upper part of the Sugarite coal bed showing the position of the K-T boundary claystone bed below the knife.

Palynology of the Sugarite K-T boundary site

The palynology of the K-T boundary at the Sugarite site was first studied by R. H. Tschudy of USGS on the basis of samples collected by C. L. Pillmore. Tschudy located the boundary within the coal using the palynological extinction datum (Pillmore and others, 1984). This was an unexpected result because, in prior investigations in the Raton basin, the boundary was found below a thin coal bed, never within coal. These results had important implications because they showed clearly that the palynological K-T boundary datum is independent of lithofacies.

The original collection consisted of 15 samples from within and above the Sugarite coal bed. The original samples, which were supplemented by 13 others collected later, became the basis for a more detailed study of the palynological and paleobotanical changes across the boundary at this unique site (Nichols and others, 1985). The total interval eventually studied encompassed 1.6 m of section, including the entire thickness of the Sugarite coal bed and carbonaceous shale intervals above and below it (Fig. 16).

Figure 16. Diagram showing the position of the K-T boundary claystone bed in the Sugarite coal bed. Inset shows the variation in Ir values; the double peak at the coal-boundary claystone contacts suggests mobilization of the Ir from the boundary claystone into the coal.

Both the original study and the detailed study of the Sugarite site utilized palynology to locate the boundary -- specifically, the disappearance of the Proteacidites assemblage. At Sugarite this assemblage was found to include not only the previously reported species Proteacidites retusus, "Tilia" wodehousei, Trisectoris sp., and Trichopeltinites sp., but also Aquilapollenites reticulatus, Libopollis jarzenii, Liliacidites complexus, and Tricolpites microreticulatus. These species are well known from the uppermost Maastrichtian at K-T boundary sites elsewhere in the Western Interior of the United States and Canada (Nichols and Fleming, 1990). In all, 48 palynomorph taxa were identified, and about 20 percent disappeared at the K-T boundary at the Sugarite site.

The occurrence of a species of Aquilapollenites pollen at Sugarite is especially interesting. Several other species of the genus Aquilapollenites disappear at the K-T boundary at localities north of those in the Raton basin (Nichols, 1996), but in the Raton basin only Aquilapollenites reticulatus is present, and it is quite rare. In contrast, Proteacidites retusus and perhaps other species of the genus Proteacidites are quite common in the Raton basin. At Sugarite, Proteacidites pollen is present in almost every sample below the boundary but in none above it. Two important patterns are revealed by these observations: (1) as already noted, Proteacidites pollen represents species that became extinct abruptly at the K-T boundary; and (2) pollen evidence shows that the vegetation varied in composition from south to north in latest Cretaceous time, yet all plant communities were similarly and simultaneously affected by the impact event.

In addition to the extinction of the pollen species named, the K-T boundary at Sugarite is marked by the fern-spore "spike," the anomalous abundance (here up to 70 percent) of fern spores (mostly Cyathidites sp.) in samples just above the extinction horizon. As interpeted by Tschudy and others (1984), the fern-spore spike is evidence of the temporary replacement of a "normal" plant community by one dominated by an opportunistic species -- one that can quickly reoccupy a devastated landscape (see also Fleming and Nichols, 1990).

The detailed study at Sugarite (Nichols and others, 1985) involved analysis of dispersed fragments of leaf cuticles as well as pollen and spores. Cuticle fragments, which represent leaf fossils not otherwise preserved in coal, are recovered by laboratory techniques similar to those used in preparing palynologic samples. The palynologic and dispersed-cuticle data suggest that in latest Cretaceous time an ecological transition - a hydrosere - was taking place in the mire that eventually formed the Sugarite coal. A mire community dominated by dicot angiosperms was changing to one dominated by monocots, probably palms. Sphagnum moss was also becoming more common in the mire. This gradual change in plant communities was abruptly terminated by the K-T extinction event. Ferns dominated the mire immediately after the K-T event. Samples from above the coal show that, although palms survived extinction, dicot angiosperms returned to dominance as dicots recolonized the area and the fern-dominated vegetation was replaced by the profoundly and permanently altered plant communities of earliest Tertiary time.

The Sugarite K-T boundary site remains unique in many ways. Further studies of the palynology and micro-paleobotany are continuing in conjunction with isotopic studies.

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