ABSTRACT

Nonmarine Mollusca are the most abundant animal macrofossils in uppermost Cretaceous and Paleocene continental strata in western North America. Their biochronologic potential, however, has yet to be realized due primarily to a lack of research focused on the determination of biostratigraphic ranges of molluscan taxa in thick sequences of strata temporally organized by mammalian local faunas into land-mammal ages. Cenozoic mammals are the present basis for the chronostratigraphic correlation of continental strata in North America (Archibald et al., 1987; Sloan, 1987). The eastern portion of the Crazy Mountains Basin of south-central Montana contains the thick sequence of nonmarine strata with multi-horizon occurrences of datable mammal local faunas, and numerous molluscan localities, necessary to permit construction of a detailed nonmarine molluscan biostratigraphy. A framework for a molluscan chronostratigraphy has been accomplished by placing all of the molluscan localities from the eastern portion of the basin into a composite reference section correlated with land-mammal ages. This column includes the revised stratigraphic placement of the nearly 100, almost entirely unpublished, historically collected molluscan localities, along with the over 200 localities discovered as part of the present research program. This locality record provides the basis for systematic, environmental, and paleobiogeographic studies of the molluscan fauna of the Crazy Mountains Basin.