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Geologic Units

Quaternary alluvial deposits
Gravels and terraces on western Snake River Plain
Quaternary surficial cover, fluveolian cover on Snake River Plain, alluvial fans (Snake River Group)
Quaternary windblown deposits, dunes, loess
Bonneville Flood gravels
Pleistocene basalt lava
Pleistocene and Pliocene stream and lake deposits
Pliocene and Upper Miocene stream and lake deposits (Salt Lake Formation, Starlight Formation, Idaho Group)
Pliocene and Upper Miocene felsic volcanic rocks, rhyolite flows, tuffs, ignimbrites (includes Moonstone rhyolite)
Pliocene and Upper Miocene basalt (Starlight Formation, Salt Lake Formation)
Eocene Challis Volcanic Group, volcanics and volcaniclastics
Miocene basalt (basalt of Weiser and basalt of Cuddy Mtn.) (split with Tpb is at 5 Ma) (includes rocks shown as Tpb (Bond, 1978) in Owyhee County and Mt. Bennett Hills
Miocene basalt (Columbia River Basalt Group)
Oligocene volcanics (Potlatch volcanics and Salmon Falls Creek volcanics)
Miocene felsic volcanic rocks (Idavada volcanics), includes rocks designated as Tmf (Bond, 1968) in Owyhee County and Mt. Bennett Hills
Cretaceous granite and granodiorite of the 2-mica suite (Idaho batholith)
Upper Paleozoic sedimentary rocks 

  Map Key
Geologic units with unit designation.
Normal Fault: certain; dashed where approximately located; dotted where concealed.
Thrust Fault: certain; dashed where approximately located; dotted where concealed.
Detachment Fault: certain; dashed where approximately located; dotted where concealed.
Interstate Route.
U.S. Route.
State Route.
Location of Rockwalk rock from the county.
Feature location.

Owyhee County

Owyhee County covers a huge area in southwest Idaho, south of the Snake River. It contains the wilderness of the Owyhee Plateau and the narrow canyons of the Bruneau and Jarbidge rivers. In the northwest it contains Cretaceous granodiorite near Silver City, an outlier of the Idaho batholith, that intrudes Paleozoic sedimentary rocks. Silver City was site of a mining boom in the middle 19th century.

The bulk of the county is underlain by voluminous rhyolite lava flows and ignimbrites erupted from the Bruneau-Jarbidge eruptive center around 16 million years ago. These high temperature, high volume rhyolite eruptions are unique in the world. They were produced by the Snake River Plain hotspot, as it sat under southwest Idaho in middle Miocene time. Pliocene basalt covers the rhyolite in much of the eastern and southern parts of the plateau.

The margins of the Owyhee Plateau expose Miocene lake and fluvial sediments, and basalt flows, deposited in Lake Idaho and its margins. Along the Snake River are Quaternary fluvial sediments and basalt lava flows.

The entire county is laced with northwest striking faults, mainly normal faults, dipping north, toward the western Snake River Plain graben.

P.K. Link, 10/02

Additional Reading

Rocks Rails and Trails: page
Challis Volcanic Group & Intrusive Rocks
References on Idaho Geology

Click here to see a correlation of geologic units, and the associated time scale.

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