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

Sites to See

Stop 1: Long Canyon K-T boundary site

Walk about 300 to 400 yards up the trail to the south end of the Long Canyon K-T boundary site, an extended exposure of the K-T boundary sequence similar to the Madrid Road K-T boundary site. On the left of the trail are the floodplain/backswamp boney coal and carbonaceous shale beds and interbedded siltstone and mudstone beds that contain the K-T boundary. This sequence is overlain by the prominent splay-sandstone ledge at the top of the exposure. Continue along the trail for a hundred yards or so and observe one of the best exposures of the K-T boundary sequence in the world. The section shown in Figure 7 was measured behind the park bench about midway along the exposure.

Figure 7. Click image to see a larger version.

The rocks in the slope are typical of the mudstone and siltstone and thin coal beds found in the lower coal zone of the Raton Formation (fig. 7). Near the top of the slope, immediately beneath the thick sandstone ledge is the K-T boundary claystone bed, overlain by a 1- to- 2-in-thick coal bed and underlain by 7- to 8-in of brownish-orange-weathering carbonaceous shale and boney coal typical of the Madrid area. This coal sequence suggests that the ejecta cloud from the impact deposited a thin bed of glassy debris in a large coal-forming swamp mire where it eventually altered to kaolinitic clay. The thin coal bed indicates that plant material that accumulated following the impact was inundated by a splay deposit of sandstone as determined by its nearly flat base, which is overlain by a splay-channel sandstone.

Figure 8. Photograph of the Long Canyon K-T boundary site showing mudstone and siltstone beds in the lower part overlain by the carbonaceous coaly sequence of the K-T boundary interval and the white K-T boundary claystone (shown by arrow) directly beneath a 7-to-8 foot thick ledge-forming sandstone.

Return to the parking lot and retrace the route to the entrance of the Trinidad Lake dam. Turn right and proceed across the dam through Starkville across I-25 and turn right along the service road to Stops 2a and b, the Starkville K-T boundary sites.

Proceed south on the service road past waste piles of several abandoned coal mines in the Vermejo Formation about 2 miles to the Starkville K-T boundary sites. The prominent channel sandstone in the roadcut at about 1 mi rests directly on the upper Starkville coal bed, which is approximately 65 ft above the Trinidad Sandstone.

Previous: Trinidad to the Long Canyon site
Next: Stop 2a - Starkville North K-T boundary site

Back to Sites to See