Abstract: Continental Sequence Stratigraphy of Tertiary Strata in the Toston-Townsend Valley, Southwestern Montana

Debra L. Hanneman, Charles J. Wideman

Tertiary strata within the Toston-Townsend Valley of southwestern Montana range in age from Early Chadronian to Hemphillian. Our work in the Toston-Townsend Valley indicates that Tertiary strata contain a variety of lithologies that are strongly correlated to their location within a depositional basin. Thus, basin fill lithologies are repetitive throughout the Tertiary in response to several basin formation events. Because of the repetitive nature of Tertiary lithologies, we have employed a sequence stratigraphic approach to Tertiary stratigraphy for the Toston-Townsend Valley.

We have grouped the Tertiary strata into three sequences that are bounded by unconformities and age constrained vertebrate fossils. The unconformities that bound Tertiary sequences in the Toston-Townsend Valley are delineated by paleosols, erosional surfaces, and angular stratal relationships. The three Tertiary sequences correlate with Sequence 2 (approximately 37 to 32 Ma), Sequence 3 (approximately 29 to 21 Ma), Sequence 4 (approximately 16 to 5 Ma) as previously mapped in other southwestern Montana valleys.

Sequence 2 strata include mudstone, sandstone, and minor conglomerate that are interpreted as flood-plain and channel deposits. Sequence 3 lithologies range from clast-supported conglomerate of alluvial fan deposits to sandstone/mudstone units of flood-plain and channel-fill origin. Sequence 4 strata are similar to Sequence 2 strata in lithology and derivation.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana