Geologic units in Cassia county, Idaho

Additional scientific data in this geographic area

Alluvial-fan deposits. (Quaternary) at surface, covers 36 % of this area

Gravel and subordinate sand and silt deposited at mouths of canyons; largest fans are in Basin and Range Province in east-central and southeastern Idaho. (Quaternary Sediments).

Rhyolite (Miocene). (Miocene) at surface, covers 17 % of this area

Rhyolite tuffs and flows (14-8 Ma); includes Juniper Mountain volcanic center, tuff of Little Jacks Creek, Cougar Point Tuff, tuffs in the Mt. Bennett Hills, and Arbon Valley Tuff Member of Starlight Formation. Volcanic sources include Owyhee-Humboldt (13.8-12.0 Ma), Bruneau-Jarbidge (12.5-11.3 Ma), Twin Falls (10.0-8.6 Ma), and Picabo (10.2 Ma) volcanic centers. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Sedimentary rocks (Permian and Pennsylvanian). (Permian and Pennsylvanian) at surface, covers 11 % of this area

Marine phosphorite, shale, and chert of Phosphoria Formation, fine-grained sandstone and mudrock of Wells, Quadrant, Amsden, and Shedhorn formations, and fine-grained sandstone, carbonaceous mudstone, and limestone of the Snaky Canyon Formation and Sun Valley and Oquirrh groups. Located in south-central and eastern Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Basalt (Pleistocene and Pliocene). (Pleistocene and Pliocene) at surface, covers 11 % of this area

Flows and cinder cones of olivine tholeiite basalt in and near Snake River Plain. Largely Pleistocene (<2.6 Ma) but includes flows as old as 3 Ma. Covered with 1-3 m (3-10 ft) of loess. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Windermere Supergroup. (Cambrian and Neoproterozoic) at surface, covers 6 % of this area

Upper part consists of shallow marine and fluvial quartzose sandstone, and minor siltite, shale, and limestone of the Brigham Group of southeast Idaho, Wilbert Formation of east-central Idaho, and quartzites of unknown affinity west of Borah Peak. More highly metamorphosed equivalents are quartzite, metaconglomerate, metasiltite, calc-silicate marble, and schist of Gospel Peaks successions C and D in central Idaho; mature quartzite, biotite schist, and minor calc-silicate rocks of Syringa metamorphic sequence east of Moscow; and schist and quartzite in the Albion Range (Elba Quartzite, schist of Upper Narrows, quartzite of Yost, schist of Stevens Spring, quartzite of Clarks Basin, schist of Mahogany Peaks, and Harrison Summit Quartzite). Lower part consists of diamictite, immature sandstone, and bimodal volcanic rocks related to continental rifting. Includes Pocatello Formation (700-665 Ma), formation of Leaton Gulch near Challis, Shedroof Conglomerate in extreme northwest Idaho, and metamorphic equivalents (schist, marble, calc-silicate rocks, metaconglomerate, and ~686 Ma metavolcanic rocks) of Gospel Peaks successions A and B in central Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Sedimentary rocks associated with Basin and Range extension. (Quaternary, Pliocene, and Miocene) at surface, covers 4 % of this area

Fluvial, fan, and lacustrine deposits and intercalated volcanic rocks of the Basin and Range Province (~16-2 Ma); consolidated to weakly consolidated sandstone, siltstone, arkose, conglomerate, mudstone, tuffaceous sediment, basalt, basaltic tephra, and rhyolite tuff. Includes deposits of Lake Idaho (Idaho Group) in western Snake River Plain and Salt Lake Formation deposited in Basin and Range Province of east-central Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Alluvial deposits. (Quaternary) at surface, covers 4 % of this area

Deposits in valleys consisting of gravel, sand, and silt. Includes younger terrace deposits. May contain some glacial deposits and colluvium in uplands. (Quaternary Sediments).

Loess deposits. (Pleistocene) at surface, covers 3 % of this area

Wind-deposited quartzose and calcareous silt in southeastern Idaho. Only shown where thick enough to mask older units; commonly 7-15 m (23-50 ft) thick. (Quaternary Sediments).

Older rhyolite, latite, and andesite. (Miocene) at surface, covers 2 % of this area

Older rhyolite, latite, and andesite (Miocene)—Rhyolitic domes and tuffs, and subordinate latite flows (17-14 Ma); includes rhyolites of Silver City and tuff of Flint Creek in the Owyhee Mountains and rhyolite of Timber Butte northeast of Emmett. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Metamorphic rocks. (Paleoproterozoic and Archean) at surface, covers 2 % of this area

Pend Oreille granodiorite gneiss near Priest River (~2650 Ma); schist, gneiss, and subordinate quartzite along North Fork Clearwater River; Kelly Creek granite gneiss northeast of Pierce (~2670 Ma); orthogneiss in Pioneer Mountains (~2600 Ma and 695 Ma); granite gneiss, granite, schist, and amphibolite of Green Creek complex in Albion Mountains (~2600 Ma); granite gneiss in Beaverhead Mountains east of Leadore (~2450 Ma); and marble, quartzite, schist, and amphibolite of uncertain age near Henrys Lake. (Mesoproterozoic to Archean Basement Rocks).

Granite. (Oligocene) at surface, covers 1 % of this area

Biotite, muscovite-biotite, and muscovite granite of the Almo pluton (29 Ma) at City of Rocks National Reserve and Castle Rocks State Park southeast of Oakley. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Lake Bonneville deposits. (Pleistocene) at surface, covers 1 % of this area

Silt, clay, sand, and gravel deposited in and at margins of former Lake Bonneville (33-11 calibrated ka) and sand and gravel deposited in giant flood bars by outburst floods from the lake (17.4 calibrated ka). The 1,575-m (5,170 ft) Lake Bonneville shoreline was used to determine the maximum extent of the lake deposits. Flood deposits follow Bonneville flood path from near Downey and the Portneuf River westward along the Snake River to Lewiston. They include sand and silt deposited in slack-water areas to 740 m (2,430 ft) elevation in the Boise, Weiser, Payette, and Snake river drainages. (Quaternary Sediments).

Sedimentary rocks associated with flood basalts. (Miocene) at surface, covers 0.7 % of this area

Fluvial and lacustrine deposits associated with Columbia River Basalt Group and equivalent basalts (17-8 Ma); consolidated to weakly consolidated sandstone, siltstone, arkose, conglomerate, claystone, and tuffaceous sediment; subordinate intercalated basalt and rhyolitic tuff. Includes Payette and Sucker Creek formations in southwestern Idaho, sediments associated with basalt of Weiser in western Idaho, and Latah Formation in northern Idaho. Includes sedimentary rocks of uncertain origin in southwest corner of Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Glacial deposits. (Pleistocene) at surface, covers 0.3 % of this area

Till and outwash consisting of gravel, sand, silt, and clay. Formed by valley glaciers at higher elevations and by the Cordilleran ice sheet in northern Idaho. Includes deposits of several glacial episodes. Includes rock glacier deposits and some modern alluvium derived from reworked till and outwash. (Quaternary Sediments).

Sedimentary rocks (Devonian to Ordovician). (Devonian to Ordovician) at surface, covers 0.2 % of this area

Marine dolostone and limestone and sandstone in east-central and southeastern Idaho (Summerhouse, Kinnikinic, and Swan Peak sandstones, Fish Haven, Laketown, Jefferson, Three Forks, and Darby formations) and deep-water carbonaceous mudrocks of Phi Kappa, Trail Creek, and Milligen formations east of Ketchum. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Landslide deposits. (Quaternary) at surface, covers < 0.1 % of this area

Unsorted gravel, sand, and clay of landslide origin; includes rotational and translational blocks and earth flows. (Quaternary Sediments).

Salt Lake Formation and other Basin & Range valley-filling alluvial, lacustrine, and volcanic materials (Middle Miocene to Late Pliocene) at surface, covers < 0.1 % of this area

Salt Lake Formation and other Basin & Range valley-filling alluvial, lacustrine, and volcanic materials. Valley fill is more than 8,000 feet thick in places and includes salt masses under the Sevier Desert.

Younger tuffaceous sedimentary rocks (Pliocene and Miocene) at surface, covers < 0.1 % of this area

Tuffaceous and other young Tertiary sedimentary rocks. Most of these rocks are sedimentary with a strong volcanic component - a few are tuffaceous with a strong sedimentary component. This unit includes rocks originally mapped as the High Rock sequence in Washoe County; the Horse Camp Formation in northern Nye County; the Esmeralda Formation in Mineral and Esmeralda Counties; older lake beds in Lincoln County; the Belted Range Tuff; the Indian Trail Formation (now abandoned); Timber Mountain, Paintbrush, and Crater Flat Tuffs; Wahmonie and Salyer Formations in southern Nye County; the Siebert Tuff in Esmeralda County; the Muddy Creek Formation in Clark County; and the Thousand Creek and Virgin Valley “beds” in Humboldt County; and other unnamed units. It corresponds to units Ts3 and Tts from the 1978 State map. It is present in all counties.

Salt Lake Formation? (Middle Miocene to Late Pliocene) at surface, covers < 0.1 % of this area

Tuffaceous sediments, mudstone, sandstone, conglomerate.

Cratonal Sequence - Marine siltstone, limestone, and conglomerate (Middle (?) and Lower Triassic) at surface, covers < 0.1 % of this area

Unit consists of marine deposits of siltstone, sandstone, claystone, mudstone, limestone, and conglomerate (Stewart and Carlson, 1978). It includes rocks assigned to the Moenkopi and Thaynes Formations and related unnamed rocks in northern Nevada (Stewart, 1980). It crops out in the eastern part of the State in Elko, White Pine, Lincoln, and Clark Counties.

Alluvium and colluvium (Quaternary) at surface, covers < 0.1 % of this area

Alluvium and colluvium

Oquirrh Group (Pennsylvanian to Permian) at surface, covers < 0.1 % of this area

Little metamorphosed allochthonous remnants.

Older alluvial deposits (Quaternary) at surface, covers < 0.1 % of this area

Older alluvial deposits

Younger silicic ash flow tuffs (Miocene) at surface, covers < 0.1 % of this area

Includes units mapped as the High Rock sequence on the Washoe North map; the Timber Mountain, Paintbrush, Crater Flat, and Belted Range Tuffs, and Indian Trail Formation (now abandoned) on the Nye South map; the Thirsty Canyon Tuff on the Nye South and Esmeralda maps; and other unnamed units. Locally it includes tuffaceous sedimentary rocks interstratified with tuffs. It is present in the northernmost part and southernmost parts of the State, and is not exposed in the central region. It corresponds to unit Tt3 on the 1978 State map, although a few rocks also mapped as Trt on the 1978 State map also are included. It is present in Clark, Churchill, Washoe, Nye, Lincoln, Lyon, Douglas, Carson, Esmeralda, Elko, Humboldt, Pershing, and Mineral Counties.