Comparative Cretaceous Molluscan Biochronology and Biogeography, Pacific Coast and Western Interior Regions of North America

Ammonite biochronology provides the principal means of correlation of Cretaceous stratigraphic sequences of the Pacific coast and Western Interior (WI) regions of North America. Traditional interpretations of Cordilleran geology suggest the evolving orogenic belt formed a substantial barrier blocking east-west faunal mixing, resulting in establishment of two distinct faunal provinces. Cretaceous faunas of the WI are thus closely related to those of Europe and North Africa while those of the Pacific coast share affinity with faunas of the circum- and Indo-Pacific regions. In addition, the Pacific coast region differed from the WI during Cretaceous time in the development of geographically-restricted basins dominated by clastic sedimentation and subject to an extended history of Cretaceous and younger structural fragmentation. The biochronologic zonal sequence for the WI is significantly more refined than that for the Pacific coast, traditionally explained by differential evolutionary of endemic faunas; however, other factors, such as paleoecologic variation between the two regions, the possibility of unrecognized disconformities in the Pacific coast successions, and different taxonomic perspectives, may also be responsible. Paleobiogeographic analysis generally supports the interpretation of distinct molluscan faunas on eastern and western sides of the Cordilleran region for most of Cretaceous time, although periodic, short-term trans-Cordilleran marine linkages are suggested, typically at times of sea-level high-stand. Faunal assemblages in the northern WI reflect mixing with Arctic faunas while those in the south show Tethyan influences. In contrast, Pacific coast faunas from California northward are dominated by high-latitude faunal elements, presumably reflecting migration of cool-water forms southward on prevailing ocean current systems. New work on latest Cretaceous Pacific faunas, however, largely from Baja California, shows significant, previously unrecognized mixing of North Pacific, WI, and Tethyan index ammonites in the Campanian and Maastrichtian, indicating that southern faunal interchange increased late in the Cretaceous..