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

Sites to See

Stop 2a: Starkville North K-T boundary site

A short distance on down the road is the Starkville North K-T boundary site. This first recognized Colorado location of the K-T boundary is now obscured by a landslide. The K-T sequence was exposed in a carbonaceous shale bed 20 in thick, 111 ft above pavement level at top of the high cut on the east side of I-25. The rocks exposed in the roadcut consists of interbedded channel sandstone and floodplain/backswamp siltstone, mudstone, carbonaceous shale, and coal deposits (Fig. 9).

Figure 9. Photograph of the Starkville North K-T boundary site showing the sedimentary units. The arrow at the top of the exposure shows the position of the K-T boundary sequence, but this upper part of the exposure is now covered by a landslide.

The contact between the Raton and Vermejo Formations is estimated to lie a few feet beneath the level of the service road. At the level of the palynological extinctions (Fig. 10), 6 ng/g (ppt) Ir was measured in the K-T boundary fireball layer, a dark, kaolinitic, carbonaceous layer 0.2 in thick that contains shock metamorphosed mineral grains (Pillmore and others, 1984; Pillmore and Flores, 1987). This layer, which consists mainly of kaolinite, contains sparse pollen yet marks the change in the fern spore-angiosperm ratio from 21 percent fern spores to more than 99 percent fern spores (Tschudy and others, 1984; Pillmore and Flores, 1987). The ratio recovers to 22 percent fern spores within 4 in above the shale bed. The fireball layer overlies the ejecta layer, a distinctive, light-colored layer of kaolinitic claystone 0.8 in thick that forms the lower part of the boundary claystone. The boundary claystone is overlain by a thin blocky coal bed about 2 in thick that appears to persist along the outcrop for a few miles. The boundary interval lies about 200 ft above the Trinidad Sandstone in a sequence of fine-grained deposits of the lower coal zone of the Raton Formation.

Figure 10. Diagram showing the lithology of the K-T boundary interval at the Starkville North K-T boundary site, about 3 mi south of Trinidad, Colorado. The large black dots show the variation in Ir concentration, the solid line and triangles show the fern-spore percentage, and the inset shows the detail of the boundary interval (From Tschudy and others, 1984).

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