Abstract

Five regional Cenozoic unconformity-bounded sequences in southwestern Montana correlate with five regional Cenozoic unconformity-bounded sequences in central Washington. The strata in both areas are continental. The bounding unconformities are variously marked by extensive paleosol formation, irregular topography developed on erosional surfaces, and/or angular discordance of strata. Sequence identification in southwestern Montana is based largely on paleosols, biostratigraphy, and seismic reflection data. In central Washington, changes in lithology and provenance have been important for sequence identification.

The correlation of the five sequences highlights the similarity in the Cenozoic stratigraphic record throughout the northwestern United States. Specifically, interregional unconformities occur at about 55 Ma, 37 Ma, 30 Ma, 20 Ma, and 4 Ma. Minor differences in the ages of the bounding unconformities in southwestern Montana and in central Washington may be due to differences in the timing of geologic events or to insufficient data. The major lithologic difference is that in the approximately 17 to 4 Ma sequence, the Columbia River Basalt dominates the Washington sequence whereas clastic strata, numerous tuffs, and only minor basalt, occur in southwestern Montana.

The application of sequence stratigraphy allows the correlation of geographically separate strata, some of which even may have had different source areas. The Cenozoic sequences of southwestern Montana and central Washington extend into the rest of the Pacific Northwest, the Great Plains, and much of the Cordillera. The wide extent of these sequences will elucidate interregional Cenozoic history.