Clark County, Idaho
l Mount nia ain enten Tpf C PPPMs s PPPMs TRs TRs Qa Ks PPPMs Ks Tpf Qb Ms DSOs Tpf Tpf Ks TRs o Contin Ks 44 30 enta Tpf l Div Tps ide Qs Tpf Zs Tpf Qa Tpf Tpf BeaverheadMtns QTs Tpf PPPMs QTs TKs Ms Tpf Ks Qa PPPMs Tps Ms Tpf 111o 40 Tpf Kilgore Zs QTs Qb Ms Tpf Ms 44o 20 OCs Tps Tpf QTs Spencer Zs Tpf Qs Ms Me Tmi Tpf dic Zs ine Lod Qs Zs ge C Qb r k 15 Plain Ms Tps Tpf Tpf Qs Tpf Zs PPPMs Tpf Tpf Small PPPMs Tpf Ms Dubois River B Tpf ir B Tpf c e Tpf h av C e 22 Tpf rh re e Ms Qs e a PPPMs k d Qa Snake V F Qs a a l Ms l u PPPMs e l Qw y t Tpf Lemhi Qs 28 Tpf Tpf 112o 10 Range
44o 00 Ms PPPMs Qb Qa Qa 0 5 10 miles 112o 50 0 8 16 kilometers Digital Atlas of Idaho, Nov. 2002 1:500,000 http://imnh.isu.edu/digitalatlas Compiled by Paul K. Link, Idaho State University, Geosciences Dept. http://www.isu.edu/departments/geology/ Clark County Clark County, one of the least populated of Idahos 44 counties, lies at the foot of the continental divide, and includes the cold, arid northeastern Snake River Plain. Quaternary basalt covers much of the area from Dubois north to Spencer, and supports sparse sagebrush grazing land. Alluvial areas are farmed with irrigation from the Snake River aquifer. Miocene and Pliocene felsic volcanic rocks, associated with the Heise and Yellowstone volcanic fields, occupy the area east of Spencer, and host Idaho opals. These formed when hot silica-rich water circulated through the young rhyolite lavas and tuffs. The Centennial Mountains, in the northeastern corner of the county, along the border with Montana, contain folded and thrust faulted Mesozoic and Paleozoic sediments, that lie above Paleoproterozoic basement rock. They belong to the Laramide province, where Mesoproterozoic and Neoproterozoic strata were never deposited beneath the basal Cambrian sandstone. West of Spencer are the Beaverhead Mountains, divided by the Medicine Lodge Creek from the Tendoy Range to the east along the Continental Divide with Montana. Limestones and tuffs of the Oligocene and Miocene Medicine Lodge Formation occupy the foothills on both sides of Medicine Lodge Creek. The Beaverhead Range extends north to Salmon and merges with the Bitterroot Range in southwest Montana, north of Lost Trail Pass. The range contains a thick package of folded and thrust faulted sedimentary rocks, including Mesoproterozoic Belt Supergroup (Apple Creek, Gunsight and Swauger formations), Neoproterozoic Wilbert Formation, and Paleozoic limestone and dolomite. The Tendoy Range is capped by the Cretaceous and Paleocene Beaverhead Conglomerate, which was shed from rising mountains to the north and west. Large rivers carried boulder conglomerate east to the Tendoy Range and much of southwest Montana. These quartzite clasts spread east to Jackson Hole, Wyoming and are now making their way back west again, carried by the Snake River. On the extreme southwest corner of the county is the dry Birch Creek valley and the southeast corner of the Lemhi Range. An active normal fault, the Beaverhead Range fault, bounds the east side of the Birch Creek Valley against the Beaverhead Range. Paleozoic carbonate rocks are exposed here. Written by P.K. Link, 9/02
Description of Map Units for Blackfoot County, Idaho
Qa Quaternary alluvial deposits 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. Qf Pleistocene silicic volcanic rocks; rhyolite lava and ash-flow tuff (includes Yellowstone Group). Qb Pleistocene basalt lava, 2 million to 12,000 years old, flows have some vegetation and surface weathering. QTb Pleistocene and Pliocene basalt lava and associated basaltic tuff (deposited close to basaltic vent). 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). Tgs Eocene granite, pink granite, syenite, rhyolite dikes, and rhyolitic shallow intrusive; last phase of the Challis magmatic event (46 to 44 Ma). Forms craggy scenic mountain landscape in central and northern Idaho. QTs Pleistocene and Pliocene stream and lake deposits; sand, gravel and mud; Lake Idaho sediments; Glenns Ferry Formation; Idaho Group. Tmi Miocene intrusive rock; gabbro and diabase sills (includes part of Salt Lake Formation). TKs Paleocene and Cretaceous sandstone and conglomerate (Beaverhead Formation). PPPMs Permian and Carboniferous sedimentary rocks (Snaky Canyon Formation). Ks Cretaceous sedimentary rocks. Js Jurassic sedimentary rocks.
TR s Triassic sedimentary rocks. PPPs Permian and Pennsylvanian sedimentary rocks. Ms Mississippian sedimentary rocks.
DSOs Devonian, Silurian, and Ordovician sedimentary rocks. Zs Neoproterozoic sedimentary rocks undivided.
Symbols
Geologic unit contacts with unit Overturned anticline: trace of axial Yp designation. 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 Interstate Route arrow indicates direction of plunge. 15 U.S. Route Syncline: trace of axial plane: large 95 arrow indicates direction of plunge. 1 State route