SEISMIC AND GRAVITY INVESTIGATION OF SEDIMENT DEPTH, BEDROCK TOPOGRAPHY, AND FAULTING IN THE TERTIARY FLINT CREEK BASIN, WESTERN MONTANA

The Flint Creek basin is a NE-SW trending intermontane basin within Montana’s fold and thrust belt. It is located in an 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 up to 1 kilometer deep. Seismic data include 8 refraction lines in three separate areas for about 1 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 30 meters. These two methods indicate a large basin bounding normal fault with simultaneous lateral fault movement on the Lewis and Clark structural line creating a small deep basin.