BRAIDED CHANNEL SYSTEM IN THE PALEOGENE BEAVERHEAD INTERMONTANE BASIN: A LONGITUDINAL SEGMENT IN THE PALEOMISSOURI HEADWATER SYSTEM OF SOUTHWEST MONTANA

The Beaverhead Basin is a wide northeastward intermontane basin located between the Pioneer and Ruby Mountains, paralleling the fabric of Laramide and frontal Sevier structures. The projected stratigraphic position of a large boulder-cobble braided fluvial system below adjacent Eocene strata and the detrital zircon ages of sand within the conglomerate indicate a depositional age equivalent to that of the Paleogene Renova Fm. Renova strata make up most of the earliest post-contractile record of sedimentation within intermontane basins of the dissected and extensionally reactivated Sevier-Laramide orogen.

Internal architecture of the dominantly framework-supported conglomerate consists of subhorizontal planar strata and low-angle bar-top beds and trough cross-stratified channel bodies typical of braided systems. Maximum thickness ranges up to 20 m and maximum measured width of laterally truncated deposits is about 400 m that, together with diagnostic clast composition and ½ m boulder dimensions, indicate a widespread, long-distance, high-energy system. Clast imbrication documents northward paleoflow into the paleo-Jefferson Basin. Clast composition of dominantly feldspathic Proterozoic quartzite and rare shear-zone metamorphic clasts and Swauger Fm quartzites indicate headwater source areas in southwesternmost Montana and north-central Idaho. Downstream (northward), at the juncture of the Ruby and Beaverhead Basins, clast composition shows an addition of abundant Archean clasts from the Ruby and southern Tobacco Root Mountains. Further downstream, in the Jefferson and Three Forks Basins, similar braided-fluvial conglomerates show a dilutional decrease in Proterozoic quartzite and an increase in quartzofeldspathic sand and granitic clasts, Cretaceous volcanics, and Paleozoic-Mesozoic sedimentary clasts reflecting increasing abundance of corresponding source rocks in adjacent paleouplifts.

Overall, the Beaverhead braided system served as an axial through-going system in the paleo-upper Missouri network that drained headwaters in southwesternmost Montana and north-central Idaho and interconnected with axial trunk-fluvial systems in the Divide, Jefferson, and Three Forks intermontane basins prior to juncture with the northward-flowing paleo-Missouri River.