PALEOENVIRONMENTAL RECONSTRUCTION OF THE MID-MIOCENE BEAVERHEAD GRABEN IN SOUTHWEST MONTANA

In recent years, numerous studies have been conducted on the mid-Miocene Ruby Graben in southwest Montana (Fritz and Sears, 1993, Thomas et. al., 1995, Landon and Thomas, 1999), but few studies have focussed on the mid-Miocene Beaverhead Graben (Dawkins, 1996). During the fall semester of 2004, the Depositional Environments class at the University of Montana-Western measured and described seven sections in the mid-Miocene Six Mile Creek Formation between Dillon and Sheridan, Montana to determine the paleoenvironments, paleogeography and geohistory of the Beaverhead Graben. They conducted an extensive provenance study that consisted of paleoflow, grain size and grain composition analyses in order to determine the flow patterns in the graben and test models that suggest drainage reversals associated with the Yellowstone hotspot. The sedimentological data show that from approximately 15 to 4.0 Ma, fluvial systems deposited sediment into the axial drainage of a northeast-trending graben to form the Six Mile Creek Formation. The graben was flanked on the east by highlands (i.e., the ancestral Ruby Range) that shed fluvial and debris flow deposits into the graben down a northwest-trending alluvial fan. The provenance data show that the axial drainage of the graben was from the southwest to the northeast, and that no reversals in this drainage pattern occurred during the time that deposition was active in the graben. Starting around 4 Ma, northwest-trending extension cut the northeast-trending Beaverhead Graben to form the existing topography and drainage patterns in southwest Montana.