IDAHO SOURCES FOR DETRITAL ZIRCONS IN LATE MIOCENE SIXMILE CREEK FORMATION, SW MONTANA

Detrital zircons separated from fluvial facies of the late Miocene Sixmile Creek Formation (SCF) from two localities in the SW Montana Ruby graben were ultimately derived from Idaho sources, in particular the ~12 Ma Twin Falls volcanic field. Sands sampled here were below the 6 Ma Timber Hill basalt and contain ca. 6 Ma zircons, constraining the depositional age.

The SCF fills the NE-trending Ruby graben with 200-400 m of fluvial and alluvial facies that are interbedded with numerous beds of tephra. The tephra beds were derived from silicic eruptions of Yellowstone hotspot track calderas, and establish a chronostratigraphy for the SCF (Perkins and Nash, 2002). Fluvial facies show systematic clast imbrication, channel morphologies, and grain size trends that indicate NE transport down a paleovalley along the axis of the graben for 200 km, from Lima to Townsend, MT (Sears and Thomas, 2007). Headwaters of the paleovalley may have reached 200 km farther SE into central Idaho, as previously interpreted from veined, black chert cobbles that have been correlated with distinctive Milligen chert source rocks in the Pioneer Mountains of central Idaho, and pumice cobbles that have been correlated with the Twin Falls or Picabo volcanic centers along the Snake River Plain (Sears and Thomas, 2007). Large detrital zircon populations at 9-12 Ma from ca. 6 Ma sands confirm the Twin Falls or Picabo volcanic centers as the ultimate sources of much tuffaceous sediment within the SCF. The proximate source for these sands is not well constrained. In any case, stream-rounded cobble-sized pumice clasts suggest 6 Ma fluvial transport from a proximal volcaniclastic source on the southwest (Pacific) side of the modern continental divide.

Small populations of Middle Mississippian (320 to 340 Ma) zircon grains are also present. These are enigmatic but suggestive of derivation from western assemblage rocks in the neighborhood of the Milligen chert.