Biostratigraphic zone data compiled from outcrop and cored sections in Colorado and New Mexico are used for correlation of rock units below and above the Cretaceous-Tertiary boundary. Data localities are in the Sand Wash basin, Williams Fork Mountains, Grand Hogback, and Piceance Creek basin, northwestern Colorado; Denver basin, eastern Colorado; Raton basin, southeastern Colorado and northeastern New Mexico; San Juan Mountains and northern San Juan basin, southwestern Colorado; and San Juan basin, northwestern New Mexico.

Reference sections of intertonguing marine and nonmarine rocks of Campanian and Maastrichtian age are present in northwestern and eastern Colorado. Coexisting ammonite and palynomorph zones allow definition of upper Campanian, lower Maastrichtian, and upper Maastrichtian palynomorph intervals on the basis of the ammonite-based stages. Likewise, coexisting vertebrate (dinosaur and mammal) and palynomorph zones allow definition of upper Maastrichtian and lower Paleocene palynomorph intervals on the basis of the vertebrate-based stages.

The most precise Cretaceous-Tertiary boundary definitions are based on vertebrates and palynomorphs in the Denver basin and on palynomorphs associated with an iridium anomaly in the Raton basin. The resulting stage intervals and Cretaceous-Tertiary boundary can be correlated (but only approximately and incompletely) to the southern San Juan basin, New Mexico, where disagreements exist concerning the ages of formations, continuity of deposition, and presence or absence of a boundary unconformity.

This paper confirms old, and offers new, evidence supporting the interpretation of a substantial boundary unconformity, or possibly two unconformities, being present in the San Juan basin, with variable amounts of Maastrichtian (to early Paleocene?) time not represented. Furthermore, some rocks assigned a late Maastrichtian age by paleomagnetic means are no younger than late Campanian and early Maastrichtian, according to biostratigraphic correlations.