Geologic units in Lewis county, Idaho

Additional scientific data in this geographic area

Columbia River Basalt Group. (Miocene) at surface, covers 89 % 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).

Quartz diorite. (Cretaceous and Jurassic) at surface, covers 8 % of this area

Primarily quartz diorite, with subordinate diorite, gabbro, granite, and amphibolite; largely undeformed except near eastern part of Blue Mountains island-arc complex; wide age range (160-90 Ma). (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Tonalite and trondhjemite. (Cretaceous) at surface, covers 1 % of this area

Biotite- and hornblende-biotite tonalite and biotite-muscovite trondhjemite (120-110 Ma); rocks contain conspicuous magmatic epidote in plutons along eastern edge of arc complex. Includes tonalitic part of Hazard Creek complex northwest of McCall, Blacktail pluton southeast of Grangeville, and Sixmile Creek pluton southeast of Orofino. (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Intrusive rocks (Triassic and Permian). (Triassic and Permian) at surface, covers 0.8 % of this area

Diorite, tonalite, granodiorite, gabbro, norite, quartz diorite, and trondhjemite; basement of, and feeders to, volcanic rocks in Seven Devils Group (Wallowa terrane) and Olds Ferry terrane. (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Landslide deposits. (Quaternary) at surface, covers 0.5 % of this area

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

Sedimentary and volcanic rocks. (Jurassic and Triassic) at surface, covers 0.2 % of this area

Marine limestone and marble of the Martin Bridge Formation and calcareous mudstone and phyllite of the Hurwal Formation exposed west of Riggins and south of Lewiston; basaltic andesite, rhyolite tuff (~202 Ma), and conglomerate along Salmon River southwest of Grangeville; and rhyolite tuff at Pittsburg Landing (~198 Ma). (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Alluvial deposits. (Quaternary) at surface, covers 0.2 % 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).

Seven Devils Group. (Triassic and Permian) at surface, covers 0.1 % of this area

Basaltic to rhyolitic (largely mafic) arc-derived volcanic and volcaniclastic rocks of the Wallowa terrane; includes minor limestone. Composed of Windy Ridge, Hunsaker Creek, Wild Sheep Creek, and Doyle Creek formations. (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Missoula Flood deposits. (Pleistocene) at surface, covers 0.1 % of this area

Gravel, sand, and silt near Coeur d'Alene and Sandpoint that were carried by outburst floods from Glacial Lake Missoula (20-15 ka). Floods also flowed back up the Snake and Clearwater rivers to Kamiah. Missoula Flood slack-water deposits cover Lake Bonneville deposits in the Lewiston area. (Quaternary Sediments).

Coon Hollow and Weatherby formations. (Cretaceous and Jurassic) at surface, covers < 0.1 % of this area

Marine mudstone and subordinate conglomerate and sandstone of the Coon Hollow Formation south of Lewiston and turbiditic sandstone, mudstone, volcanic conglomerate, and andesite and rhyolite tuff of the Weatherby Formation north of Weiser. (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).