SEISMIC AND GRAVITY INVESTIGATION OF SEDIMENT DEPTH, BEDROCK TOPOGRAPHY, AND FAULTING IN THE TERTIARY DRUMMOND-HALL BASIN, WESTERN MONTANA

The Drummond-Hall basin is a NE-SW trending intermontane basin within Montana’s fold and thrust belt. It is located in the extensional system between the Bitterroot and Deer Lodge Valleys, both north-south trending Tertiary half-grabens. The bedrock in the Hall Basin consists of Cretaceous aged sedimentary rocks in the eastern portion with Precambrian Belt Supergroup thrust over Paleozoic rocks in the western side. We have been complementing geologic mapping of the area with a mix of seismic and gravity observations to better determine the geometry of the Tertiary faulting and sedimentation during the basin’s growth. Our complete Bouguer gravity data include 60 existing stations and 50 new observations spaced roughly 300 meters apart. The wide spacing maximizes coverage of the whole basin while sacrificing resolution of smaller fluctuations in bedrock depth. The resultant gravity model is consistent with the classic extensional structural style in the flanking Deer Lodge and Bitterroot valleys. Although the spatial dimensions of the Drummond-Hall Basin are smaller than those of the neighboring basins, the sediment depth appears to be greater. Seismic data include 8 refraction lines in three separate areas for about 0.5 km of new reversed seismic refraction data. These data reinforce the gravity model of depth to bedrock. The refraction data shows bedrock depth from 5-150 meters and normal faults with displacements of up to 10 meters. In the Coberly-Gulch section the seismic identification of a normal fault provides a new explanation for observed changes in surficial geology. Those data also constrain the age of basin-bounding faulting to the Miocene.