THE MEDICINE LODGE BEDS: NEW RADIOMETRIC AGES FROM IDAHO MEDICINE LODGE VALLEY RESTRICT DEPOSITION TO THE PALEOGENE BASIN-FORMING EVENT

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 occupies the footwall of the northwest-striking Beaverhead normal fault. Idaho Medicine Lodge valley is on strike with the Muddy Creek, Montana Medicine Lodge, and Horse Prairie extensional basins in Montana that developed in response to the Paleogene rift forming event of Janecke et al. (1994).

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).

During mid-Eocene extension, basins developed and were filled with tuffaceous sandstones, siltstones and paleosols. These rocks are known as the Medicine Lodge beds, terminology that should be restricted to Eocene and Oligocene volcaniclastic, fluvial and lacustrine strata. Ongoing extension tilted these Eocene and Oligocene strata creating a small but recognizable angular unconformity between Paleogene and Neogene rocks. Basin and Range extension and Snake River Plain subsidence exposed the Medicine Lodge beds in the Idaho Medicine Lodge valley and allowed Medicine Lodge Creek to incise the valley to its present depth.

Tectonism and volcanism related to the development of the Snake River Plain to the south of the Idaho Medicine Lodge generated locally derived conglomerate, freshwater limestones, ashflow tuffs (rhyolite of Edie School, 6.6 Ma) and basalts during Miocene time (ca 15 to 6 Ma). These Miocene rocks, which we have informally named "the formation of Lidy Hot Springs," should not be considered part of the Medicine Lodge beds, as they are genetically unrelated to the Eocene-Oligocene Medicine Lodge beds.

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 ponded north of the Heise eruptive center.