Geologic units in Jefferson county, Idaho

Additional scientific data in this geographic area

Basalt (Pleistocene and Pliocene). (Pleistocene and Pliocene) at surface, covers 57 % 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).

Alluvial deposits. (Quaternary) at surface, covers 17 % 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 12 % 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).

Windblown sand deposits. (Quaternary) at surface, covers 12 % of this area

Fine- and medium-grained sand dunes in southern Idaho; includes Bruneau and St. Anthony dune fields. (Quaternary Sediments).

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

Rhyolite tuffs and flows of Heise volcanic field (6.6-4.5 Ma; includes Blacktail Creek, Walcott, Conant Creek, and Kilgore tuffs), and rhyolite domes and flows of Magic Reservoir area (6.6-3 Ma). (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Alluvial-fan deposits. (Quaternary) at surface, covers 0.2 % 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 (Pleistocene). (Pleistocene) at surface, covers < 0.1 % of this area

Rhyolite tuffs, flows, and domes; includes Yellowstone Group (2.0-0.6 Ma) and isolated domes on Snake River Plain and north of Soda Springs (less than 2.0 Ma). (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).