Geologic units in Ada county, Idaho

Additional scientific data in this geographic area

Basalt (Pleistocene and Pliocene). (Pleistocene and Pliocene) at surface, covers 55 % 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).

Sediments and sedimentary rocks (Pleistocene and Pliocene). (Pleistocene and Pliocene) at surface, covers 16 % of this area

Older gravel, sand, and silt deposited in fans, streams, and lakes. Includes older terrace gravels and Tuana Gravel northwest of Twin Falls. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Sedimentary rocks associated with flood basalts. (Miocene) at surface, covers 9 % 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).

Granodiorite and two-mica granite. (Cretaceous) at surface, covers 7 % of this area

Granodiorite and granite containing biotite, commonly with muscovite; includes bulk of Atlanta lobe (85-67 Ma) and isolated plutons in northern Idaho (107-67 Ma). (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive 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).

Fluvial and lake sediment. (Quaternary) at surface, covers 4 % of this area

Largely fine-grained sediment, in part playa deposits of evaporative lakes; includes Snake River Group and Bruneau Formation in central Snake River Plain and glacial lake deposits in Bonners Ferry area of northern Idaho. Also includes travertine and tufa northeast of Bancroft and Lake Thatcher sediments in the Gem Valley south of Grace. (Quaternary Sediments).

Alluvial-fan deposits. (Quaternary) at surface, covers 3 % 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).

Sedimentary rocks associated with Basin and Range extension. (Quaternary, Pliocene, and Miocene) at surface, covers 2 % 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).

Basalt (Pliocene and Miocene). (Pliocene and Miocene) at surface, covers 0.7 % of this area

Flows and cinder cones of olivine tholeiite basalt, and shallow basalt intrusives (~15-3 Ma); includes basalt in Owyhee County and southwest of Twin Falls, basalt of Weiser (basalt to andesite), basalt of Cuddy Mountain (alkali basalt and picro-basalt) north of Cambridge, basalt in Mount Bennett Hills north of Gooding, andesite at Square Mountain near Magic Reservoir, and Cub River diabase sill east of Preston. Includes gabbro at depth in cross section D-D’-D”. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Challis intrusive rocks. (Eocene) at surface, covers 0.3 % of this area

Shallow roots of Challis volcanic field. Older suite of granodiorite and quartz monzodiorite and subordinate diorite, granite, and subvolcanic dacite; includes Jackson Peak, Beaver Creek, Marsh Creek, and Summit Creek stocks (49-45 Ma). Younger suite of granite and minor syenite and subvolcanic rhyolite; includes Sawtooth, Casto, Bungalow, and Lolo Hot Springs plutons (47-43 Ma). (Eocene Challis Magmatic Complex and Related Sedimentary Rocks).

Lake Bonneville deposits. (Pleistocene) at surface, covers 0.2 % 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).

Landslide deposits. (Quaternary) at surface, covers 0.2 % of this area

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

Rhyolite (Miocene). (Miocene) at surface, covers 0.1 % 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).

Columbia River Basalt Group. (Miocene) at surface, covers < 0.1 % of this area

Large-volume lava flows of tholeiitic basalt, basaltic andesite, and subordinate andesite in western Idaho; consists of Imnaha Basalt (17.5-16.5 Ma), Grande Ronde Basalt (16.5-15.6 Ma), Wanapum Basalt (15.6-14.5 Ma), and Saddle Mountains Basalt (14.5-6 Ma). Includes porphyritic basalt and basaltic andesite in western Owyhee County. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).