FOUR DISTINCT MID-EOCENE AND OLIGOCENE FLUVIAL SYSTEMS, GRASSHOPPER AND BEAVERHEAD BASINS, SW MONTANA

Four samples from middle Eocene to Oligocene sandstones SW Montana were analyzed for detrital zircons using 60 random-grain samples analyzed by SHRIMP. The data suggest that streams in the Grasshopper half-graben draining the footwall, hanging wall, and along the basin axis carried very different populations of detrital zircons, and that all of these were different than the pre-existing Eocene strata east of the Grasshopper basin.

Eocene strata east of the Grasshopper basin sample reflect a distinct fluvial system in the footwall of the Oligocene rift. Uintan or Duchesnian, mid-Eocene, pre 38.7 Ma sand from the Mantle Ranch fossil locality in the Bone Basin Member of the Renova Formation contains a strong population of 85 to 100 Ma detrital zircons. These zircons overlap in ages from the Atlanta Lobe of the Idaho batholith, exposed over 100 km to the southwest.

Within the Grasshopper basin, west of the Muddy-Grasshopper detachment fault, three distinct provenance systems are recognized. The two-mica arkose of the SSE flowing axial stream (Arikareean, > 27.7 Ma ⁴⁰Ar/³⁹Ar age from overlying basalt) in the Grasshopper basin contains a sharp peak of 70 to 83 Ma detrital zircons, and no Grenville grains. This suggesting transport from the northwest, from latest Cretaceous plutons, of similar age to the Bitterroot lobe of the Idaho batholith. The Chief Joseph pluton in the Anaconda Range is one candidate.

The eastern facies of the Grasshopper basin contains a mixed population of detrital zircons, with Grenville and Archean grains, and a strong peak at 72 to 81 Ma, similar to the south-flowing arkosic stream. This system likely drained the Cretaceous Cold Springs volcanic field and Archean and Proterozoic rocks exposed west of Dillon in the Bannack-Argenta Hills.

The western quartzite pebble-cobble facies of the Grasshopper Basin contains nearly the same detrital zircon populations as the Mesoproterozoic upper Belt Supergroup (Missoula Group) and ties the deposits to a source west of the basin, in the Grasshopper thrust sheet that lacks Cretaceous granites.

These data suggest that in mid-Eocene time, prior to opening of the Grasshopper basin, regional drainages may have flowed to the northeast, but during the Oligocene main phase of Grasshopper Basin deposition, a trunk stream drained south-southeast along the axis of the rift zone.