SEQUENTIAL DOMING AND FAULTING ALONG THE MARGIN OF THE YELLOWSTONE HOT SPOT TRACK IN SW MONTANA: PARAMETERS TO TEST THE PLUME HYPOTHESIS FROM NEOGENE SIXMILE CREEK FORMATION

The Neogene Sixmile Creek Formation (SCF) constrains initiation of NW-trending normal faulting and doming on the flank of the Yellowstone hot spot track in the Blacktail Deer Creek area of SW Montana. In this area, the SCF is a 2-300 m thick, structurally concordant sequence that includes: a) several tephra beds derived from calderas along the Snake River Plain, b) stream-cobble conglomerate with distinctive clasts of central Idaho provenance, c) welded tuff of Walcott (?), d) 6-Ma Timber Hill basalt, and e) a conglomerate of 4.3-Ma tuff of Kilgore (?) clasts. It is capped with angular unconformity by pediment gravel and 2.0-Ma Huckleberry Ridge tuff. The SCF accumulated in a broad river valley along the Middle Miocene Ruby graben (RG), which trended NE for >200 km across SE ID and SW MT prior to being dissected by NW-trending late Neogene faults on the hot spot shoulder. As the hot spot migrated from the Picabo to the Yellowstone volcanic fields, drainage was successively diverted into the RG along new NW-trending graben valleys that split the NW flank of the dome. Tephra from the Picabo field may have been diverted into the RG down the Lemhi Valley at ca. 10 Ma, the 6 Ma Timber Hill basalt may have flowed into the RG from the Medicine Lodge Valley of Idaho on the flank of the Heise volcanic dome, and the 5.6, 4.9, and 4.0 Ma basalts of Little Table Mountain, Lima, and Lone Butte, respectively, are remnants of basalt flows from the Heise field that flowed down the Red Rock graben. Tuff of Kilgore (?) cobbles may have been transported down the Red Rock graben, then down the RG. As the NW-trending faults grew in length and displacement, drainage was diverted out of the RG. The distinctive central-Idaho cobbles abruptly disappear from the study site by 4.3 Ma, indicating that NW-trending faults on the Heise volcanic dome had truncated the RG at the continental divide. After deposition of the Kilgore (?) cobble conglomerate, the RG was tilted and block-faulted at the study site, truncated by the NW-trending Blacktail graben, unconformably overlapped by pediment gravel derived from the rising Gravelly and Snowcrest Ranges to the east, and capped by the Huckleberry Ridge tuff. The tilting dates initiation of doming of the Yellowstone volcanic field between 4 and 2 Ma, providing constraints for plume models.