SEQUENCE STRATIGRAPHY OF CENOZOIC CONTINENTAL ROCKS, SOUTHWESTERN MONTANA

Because the presently used Bozeman Group lithostratigraphy is difficult to apply to Cenozoic basin-fill rocks in the Jefferson, Beaverhead, Melrose, and Divide Valleys, southwestern Montana, we advocate the use of sequence stratigraphy. Surface sequence boundaries are in many cases marked by calcic paleosol zones; these zones can be projected into the subsurface and tied to seismic reflection data. Density/velocity contrasts between calcic paleosol zones and overlying nonpedogenic rocks generate bright reflectors that define sequence boundaries. Sequence boundary delineation is further enhanced by reflection termination patterns, similar to those found in marine strata. Sequence age calibration is based upon vertebrate fossil and radiometric age data. Five sequences, separated by basin-wide unconformities, are recognized. Most sequence-bounding unconformities are marked by paleosols, an occurrence not previously reported for Cenozoic strata of southwestern Montana. The sequences and their age ranges are (1) Bridgerian-Uintan (approximately 50 to 44 Ma) calc-alkaline volcanic flows and interstratified sedimentary rocks, (2) Duchesnian to Whitneyan (approximately 42 to 30 Ma) terrestrial sedimentary and volcanic rocks, (3) Arikareean (approximately 27 to 21 Ma) terrestrial sedimentary and volcanic rocks, (4) Barstovian to Blancan (approximately 16 to 4 Ma) terrestrial sedimentary and volcanic rocks, and (5) early Quaternary (approximately 1.8 Ma) to Holocene terrestrial sedimentary and volcanic rocks.