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

Sites to See

Stop 3: The Raton Pass K-T boundary site

The Raton site is at the top of a saddle about 1 mi west of Raton on Southwell Mountain Road (the old Raton Pass Road). The roadcut exposes the first outcrop discovery of the Ir anomaly (about 1 ng/g) in the Raton basin. The anomaly is coincident with the disappearance of the Proteacidites assemblage. It is at the top of a 1-in-thick bed of rusty-weathering kaolinitic claystone about 7 to 8 in below the base of a thin coal bed. The boundary here is about 150 ft above the Trinidad Sandstone.

Figure 13. The K-T boundary exposed at the Raton Pass site. The boundary claystone layer is visible about midway between the top of the sign and the shrubs on the bank behind the sign.

At the Raton Pass site three coal beds lie in an 11-ft-thick sequence of mudstone, siltstone, and carbonaceous shale. These three coal beds can be quite definitely correlated with the Sugarite coal bed on the basis of the presence of the K-T boundary in both sections. The boundary claystone occurs in a detrital claystone sequence and does not directly underlie a coal bed, as is the usual case elsewhere in the basin -- in fact, its position beneath the coal bed ranges from 1.5 to 8 in. The boundary is below the base of the uppermost coal bed, which is 6 in thick; a 16-in-thick zone of coal and carbonaceous shale lies about 6 ft beneath the boundary, and a 28-in-thick coal bed is at the base of the sequence. This coal zone appears to be correlative with the coal bed mined at Sugarite, about 3 mi across the high ridge to the northeast.

Figure 14. Closer view of the Raton Pass site. The knife blade is at the boundary claystone layer. Note position of claystone layer 7-8 in below the thin coal bed at the top of the photo.

After examining the K-T boundary at the Raton site, return to Raton for the night.

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