Abstract: Cenozoic Geology of the Sage Creek-Blacktail Area, Southwestern Monatana

Debra L. Hanneman, Charles J. Wideman

The Sage Creek-Blacktail (SCB) area contains five Cenozoic sequences ranging in age from approximately 54 Ma to the present. Volcanic flows, pyroclastic units, and volcaniclastic deposits ranging in age from approximately 54 Ma to 42 Ma comprise Sequence 1. Sequence 2 and 3 (approximately 37 Ma to 32 Ma and approximately 30 Ma to 22 Ma, respectively) include mostly rocks of fluvial/floodplain origin. Sequence 3 units are limited in geographic distribution and most likely represent paleovalley fill. Sequence 4 (approximately 17 Ma to 5 Ma) is also comprised of predominantly fluvial deposits. Sequence 5 (approximately 2 Ma to present) is made up of pyroclastic units, limestone hot spring deposits, and alluvium.

Numerous Cenozoic extensional faults occur throughout the SCB area. Listric normal faults are the most common structures. Steeply dipping northeast trending normal faults are present adjacent to the Snowcrest Range, and bound a northeast trending graben herein termed the Little Sage Creek Graben. Northwest trending, steeply dipping normal faults occur in the northeastern part of the SCB area. Cenozoic tectonism occurred primarily from 54 Ma to 42 Ma, and again from 17 Ma to 5 Ma.

The Cenozoic structural pattern delineated for the SCB area can be used to predict pre-Cenozoic structure. We suggest that Cenozoic listric normal faults indicate the subsurface presence of Sevier style thrust faults. If this is a correct interpretation, then the thrust belt does not end in the Tendoy Mountains, but extends across the SCB area, and includes the Blacktail Mountains.

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