The Cretaceous-Tertiary boundary in the Raton Basin, New Mexico and Colorado
Charles L. Pillmore and R. Farley Fleming
The Cretaceous-Tertiary (K-T) boundary is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (late Cretaceous and Paleocene) at more than 20 sites in the southern and east-central parts of the Raton basin. The boundary occurs at the top of a kaolinitic claystone layer, commonly referred to as the "K-T boundary claystone," in an interval of coal and carbonaceous shale. The claystone has also been found and correlated with remarkably similar claystone layers that occur at sites in the Powder River Basin in Wyoming, and north through Montana into Saskatchewan and the Red Deer Valley of Alberta, Canada—a distance of more than 1600 km. Throughout the region, the K-T boundary claystone displays a distinctive characteristic: it is always overlain in sharp contact by a 0.1- to 0.2-in.-thick laminated bed of claystone, termed the K-T boundary impact layer, that contains shocked quartz and high concentrations of iridium—both signatures of asteroid impact. In contrast, shocked quartz grains are extremely rare and Ir markedly less abundant in the underlying K-T boundary claystone.
Field relations suggest that both the boundary claystone and the impact layer resulted from an asteroid impact(s). The wide distribution of this thin couplet of beds suggests a fallout origin, and the ubiquitous, sharp, uncontaminated contact between the two units indicates a genetic relationship and nearly simultaneous deposition. However, a simple graded-bed fallout model is precluded by the presence of the coarser shocked grains at the top of the boundary claystone bed.
- Pillmore, Charles L.; Fleming, R. Farley, 1990, The Cretaceous-Tertiary boundary in the Raton Basin, New Mexico and Colorado, in: Tectonic development of the southern Sangre de Cristo Mountains, New Mexico, Bauer, Paul W.; Lucas, Spencer G.; Mawer, Christopher K.; McIntosh, William C., New Mexico Geological Society, Guidebook, 41st Field Conference, pp. 327-331.