CENOZOIC STRATIGRAPHY IN WESTERN MONTANA: CURRENT STATUS AND A WAY FORWARD

Lithostratigraphy is an approach currently used by some for Cenozoic stratigraphy in western Montana. Cenozoic lithostratigraphic units are part of the Bozeman Group, with lower Tertiary strata placed in the Renova Formation and upper Tertiary deposits comprising the Sixmile Formation. The characterizing lithologic features of this two-fold formal lithostratigraphy are that strata within the Renova Formation are predominantly fine-grained and the Sixmile Creek Formation contains mainly coarse-grained rocks. The difficulty in applying this stratigraphy is that basin strata lithology commonly exhibit abrupt lateral lithologic changes, with predominantly fine- and coarse-grained units being often repeated throughout the stratigraphic column. Consequently, Bozeman Group lithostratigraphy does not provide a solid mappable basis for working with Cenozoic strata and has resulted in muddled geologic interpretations for these basins.

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.