NEW RADIOMETRIC AGES FROM TUFFACEOUS STRATA IN THE IDAHO MEDICINE LODGE VALLEY INDICATE MULTIPLE EPISODES OF BASIN FORMATION

The Idaho Medicine Lodge valley is a Basin and Range extensional basin that lies west of the town of Dubois, in Clark County, Idaho. Medicine Lodge Creek runs from northwest to southeast roughly parallel to the trend of the Beaverhead normal fault. Idaho Medicine Lodge valley is on strike with the trend of the Muddy Creek, Montana Medicine Lodge, and Horse Prairie basins in Montana where ongoing investigations have demonstrated multiple episodes of Paleogene extension.

As mid-Eocene extension of the region occurred, short-lived basins developed and filled with fine-grained volcaniclastic sediment. Lack of through-going drainages in these short-lived basins resulted in more than 250 meters of tuffaceous sandstones, siltstones and paleosols. Strata in these basins then were gently tilted as extension continued, resulting in minor angular unconformity between Paleogene and Neogene rocks. In Miocene time, locally derived conglomerate, freshwater limestones and late Miocene ashflow tuffs and basalts armored the section, preserving friable tuffaceous sandstones and siltstones until Basin and Range extension and Snake River Plain subsidence allowed Medicine Lodge Creek to dissect the present valley.

New 40Ar/39Ar ages on sanidine mineral separates from dacitic to rhyolitic tuffaceous siltstones and sandstones in the Idaho Medicine Lodge valley yield late Eocene ages from measured sections at Spring Hollow (42.01 ± 0.31 Ma and 42.40 ± 0.15Ma), and Antelope Lakes (37.66 ± 0.10 Ma and 36.57 ± 0.17 Ma), and an Oligocene age from the base of a measured section at the head of the south fork of Deep Creek (30.23 ± 0.45 Ma). Lack of temporal, vertical and horizontal continuity between sections and lateral distance between sections indicate the development of multiple Eocene-Oligocene depocenters within the Idaho Medicine Lodge region. This reflects multiple episodes of extension.

New 40Ar/39Ar ages on groundmass concentrates from the canyon-filling Indian Creek basalt (5.59 ± 0.09 Ma) and the section-topping Indian Creek basalt (6.0 ± 0.10 Ma) indicate the presence of a Miocene depocenter, likely related to the development of the Heise eruptive episode of the Yellowstone hot spot.