117 o0133 43o4100 Owyhee County, Qg Homedale Idaho Tps Qg 78 Qg 0 5 10 miles Marsing
S 0 8 16 kilometers Tmb n a
k Tps e Tmf R iv e Tps r
TmfTps 95 Tmf Tms Tmb
Tfm Kgd Tpf Tmf Qa Tmb Kgd Tpb Murphy Qb Qs Kgd Tps S n a k Qs Qw e Kgd Tps Ri Tms Tpb v er Tmf Qb Tmf QTs QTs Qb Qs Tmb Kgd
78 Silver Tpb C.J. Stri City Tpf Grand View ke Re Tmb Tpb serv War Eagle o i Mountain Tpb r 115 o2700 QTs Qb Tmb 42o5600 Tpb Tpf Qg QTs Qa Tmf Tpb Tmb Qg Qs Qg Tmf QTs Qw Qg QTs Qg Digital Atlas of Idaho, Nov. 2002 Qa Bruneau Qa QTs http://imnh.isu.edu/digitalatlas Qs Compiled by Paul K. Link, Qs Idaho State University, Geosciences Dept. Qs http://www.isu.edu/departments/geology/ Kgd Tps Tps QTs Qa Ki Qw Qw o Tps 115 0200 Tmb Qs 42o4600 B Pzu r Tpb Tpb u Qs Tcv Qw n Kgd e QTs a u Qg Pzu Tpb R
Tpb i Tpf v Tpb e
Tmf Tpb r Tpf Tps Tpb Qa Qs Tmf Tpf Tpf Tpb Tpb Tpb Tpb Tpb Qg 51 Tpb Tpf Tpf Tpb Tpb Tpf Tpb Tpb Tpf Tpf Tpb Tpb Qs Grasmere Tps Tpf Tpb Qs Tpf Tpf Tpb
Tpb J a Tpb r Juniper Butte Owyhee River b Tpf i d Tpb ge Tpf R iv Tpb e r Tpf Riddle Qs Tpf Tpb Tpb Tpb Tpb Tpb B Tpb r u n Tpb e Tpb a Murphy Tpb Tps Qa u Tpf R Hot Srings
Qa i
Tpb v Tpb Tpf e
Duck Valley Indian Reservation r 41o5945 41o5956 Tpb Tpb Tpb 115 o0200 117 o0144 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
Description of Geologic Units for Owyhee County, Idaho
Qa Quaternary alluvial deposits Qg Quaternary gravels; forming terraces above modern stream levels, mainly mapped on western Snake River Plain. Unit generally represents detrital glacio-fluvial systems. Qs Quaternary surficial cover, including colluvium, fluvial, alluvial fan, lake, and windblown deposits. Included fluveolian cover on Snake River Plain, (Snake River Group). Qw Quaternary windblown deposits; sand dunes and loess. Qbf Bonneville Flood gravels, including boulder and gravel bars north and west of Red Rock Pass through Marsh Valley and along Snake River west of Pocatello to Hells Canyon. Town of Lewiston is underlain by Bonneville gravels. Qb Pleistocene basalt lava, 2 million to 12,000 years old, flows have some vegetation and surface weathering. QTs Pleistocene and Pliocene stream and lake deposits; sand, gravel and mud; Lake Idaho sediments; Glenns Ferry Formation; Idaho Group. Tps Pliocene and Upper Miocene stream and lake deposits (Salt Lake Formation, Starlight Formation, Idaho Group). Tpf Pliocene and Upper Miocene felsic volcanic rocks, rhyolite flows, tuffs, ignimbrites. (in Owyhee County and Mt. Bennett Hills, this should be Tmf). Tpb Pliocene and Upper Miocene basalt (includes parts of Starlight Formation and Salt Lake Formation) (in Owyhee County and Mt. Bennett Hills, this should be Tmb). Tcv Eocene Challis Volcanic Group, volcanics and volcaniclastics; Older andesitic lavas, intermediate age dacite lava and tuff and younger rhyolite flows and tuffs; 51 to 44 Ma. (Includes Potato Hill and Kamiah volcanics of northern Idaho). Tmb 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. Tcr Miocene basalt (Columbia River Basalt Group); flood basalt, extensively exposed in western Idaho; fed by fissures, many of which are near the Idaho-Oregon border. Flowed eastward up valleys cut into the Idaho mountains. Tov Oligocene volcanics; Potlatch volcanics, basalt and trachytic pyroclastic rocks [alkali-rich basalts]) and Salmon Falls Creek volcanics [andesites]. Tmf Miocene felsic volcanic rocks, rhyolite lava, ignimbrite, fallout tuff (Idavada volcanics), includes rocks designated as Tmf (Bond, 1968) in Owyhee County and Mt. Bennett Hills.
Kgd Cretaceous granitic rocks of the 2 mica suite. Idaho batholith and related plutons; granite and granodiorite that contains both muscovite and biotite. Sodium (Na) rich. Intruded between 80 and 65 Ma. Pzu Upper Paleozoic sedimentary rocks.
Symbols Yp Geologic unit contacts with unit designation. Overturned anticline: trace of axial plane. Normal fault: certain; dashed where Overturnedsyncline: trace of axial approximately located; dotted where plane. concealed. Location of ISU Rockwalk rock Thrust fault: certain; dashed where from each county. approximately located; dotted where concealed. Cities Detachment fault: certain; dashed Feature location where approximately located; dotted where concealed. Roads Anticline: trace of axial plane: large 15 Interstate Route arrow indicates direction of plunge. U.S. Route Syncline: trace of axial plane: large 95 arrow indicates direction of plunge. 1 State route