CENOZOIC STRATIGRAPHY IN WESTERN MONTANA: CURRENT STATUS AND A WAY FORWARD
A way forward for western Montana Cenozoic stratigraphy is the use of sequence stratigraphy. Cenozoic continental strata in western Montana basins can be separated into five sequences that can be delineated in the field based upon unconformable relationships with other strata and by their capping of mature paleosols. In ascending order, approximate age ranges for these informally designated sequences are: (1) 50 to 44 Ma, (2) 38 to 32 Ma, (3) 27 to 21 Ma, (4) 16 to 4 Ma, and (5) 2 Ma to present time. A regional unconformity separates an early Eocene paleolandscape from the overlying five Cenozoic sequences. The early Eocene unconformity and Sequence 1 are delineated by ultisols; all other younger sequence boundaries have bounding calcic paleosols. The sequences are presently age constrained by existing isotopic ages and by North American Land Mammal ages derived from contained fossil vertebrate assemblages. A new effort to more tightly age constrain the sequences includes six 40Ar/39Ar isotopic ages ranging from about 28 Ma to 2 Ma for tuffs in the Blacktail-Ruby valleys, southwest Montana, and ten tuff samples from other southwest Montana localities that are currently undergoing isotopic age analysis. Ultimately, Cenozoic sequence stratigraphy provides a better framework for both inter- and intra-valley correlation and yields a more accurate basin formation time frame.