The Upper Cretaceous strata of the Western Interior of North America contain a rich molluscan fauna and abundant intercalated bentonites. This combination has permitted the development of a detailed time scale for the Late Cretaceous. Volcanic rocks are rare in the Lower Cretaceous of North America. Several isolated time-scale points have been reported for Lower Cretaceous rocks, but the Early Cretaceous time scale is not well defined.
Nearly all of the available isotopic ages are K-Ar mineral ages. Data have been recalculated using decay constants for K40 based on the refined activity data for K40 and the isotopic abundance of K40. The best current estimates for system and series boundaries are: Cretaceous-Tertiary boundary65 to 66 m.y.; Lower Cretaceous-Upper Cretaceous boundary96 m.y.; Jurassic-Cretaceous boundary138 ± 3 m.y.