DEFORMED RIVER GRAVEL OF THE SOUTHERN BEAVERHEAD MOUNTAINS; NEOTECTONIC IMPLICATIONS

The Late Cretaceous Beaverhead Group crops out across much of the southwest Montana Rockies. The Group contains many facies, including syntectonic conglomerates. The Divide quartzite-cobble conglomerate unit has long been assigned to the Beaverhead Group but has never been directly dated. The Divide unit comprises poorly lithified to unlithified gravel that displays a consistent northerly transport direction. It is reportedly nearly 3 km thick. The 4.5 Ma Kilgore tuff crops out between in-situ beds of the gravel, and the gravel contains tuff cobbles, showing that deposition occurred into the Pliocene. Conflicts with age and source data suggest that the Divide unit is a much younger deposit than the Beaverhead Group. It may comprise a facies of the Miocene/Pliocene Sixmile Creek Formation.

Recent research suggests that a large Miocene river system followed grabens of the Basin and Range from Utah and Nevada to Montana. Cobbles of the Divide unit may define western and eastern branches of this river system. This interpretation more accurately explains age and provenance characteristics of the unit.

The unit contains well-rounded quartzite cobbles, typically poorly-cemented and clast-supported. Many cobbles exhibit pressure-solution pits, small faults, and slickenside lineations. The unit sits on the Continental Divide, and is directly bounded by the Snake River Plain to the south. Widespread, systematic orientations of the lineations shows a correlation with large-scale slump folds and regional horizontal velocity data. Growth faulting could explain the anomalous thickness of the deposit.

Preliminary fieldwork suggests that red and purple striped quartzites of the Brigham Group are a major source of the cobbles. Other likely sources include the the Valmy and Diamond Peak formations of Nevada and the Ordovician Kinnikinnick. Likely Nevada sources are absent higher in the section, suggesting termination of a western branch of the drainage following the passage of the Yellowstone hotspot. Late-stage sources are limited to the adjacent edges of the Snake River Plain, such as the Swan Valley. It is hypothesized that reworking of the Pinyon Formation of Wyoming provided much of the quartzite.