Basalt lava flows; Early Miocene valley-filling basalt flows of subunit 1b; Columbia Plateau (IDTm1b;0)

State	Idaho
Name	Basalt lava flows; Early Miocene valley-filling basalt flows of subunit 1b; Columbia Plateau
Geologic age	Miocene
Original map label	Tm1b; Tm?b; JTRi
Comments	Imnaha Basalt (Tm1b) is the basal Formation of the Columbia River Basalt Group. It is tholeiitic basalt that filled canyons and valleys on the east and north margins of a topographically youthful to mature drainage basin for Miocene streams. It generally has plagioclase and olivine phenocrsysts, and massive columnar jointing.
Primary rock type	tholeiite
Secondary rock type	lava flow
Other rock types
Lithologic constituents	Major Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)porphyritic basalt with plagioclase and olivine phenocrysts
Map references	Bond, J.G., Kauffman, J.D., Miller, D.A., and Venkatakrishnan, Ramesh, 1978, Geologic map of Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, with contributions from U.S. Geological Survey, scale 1:500,000.
Unit references	Bond, J.G., Kauffman, J.D., Miller, D.A., and Venkatakrishnan, Ramesh, 1978, Geologic map of Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, with contributions from U.S. Geological Survey, scale 1:500,000. Mitchell, V.E. and Bennett, E.H., Geologic map of the Baker quadrangle, Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, Geologic Map Series, Baker 2o Quadrangle, scale 1:250,000. Rember, W.C. and Bennett, E.H., 1979, Geologic map of the Pullman quadrangle, Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, Geologic Map Series, Pullman 2o Quadrangle, scale 1:250,000. Gaston, M.P. and Bennett, E.H., 1979, Geologic map of the Grangeville quadrangle, Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, Geologic Map Series, Grangeville 2o Quadrangle, scale 1:250,000 Camp, V.E., Hooper, P.R., Swanson, D.A., and Wright, T.L., 1982, Columbia River basalt in Idaho: Physical and chemical characteristics, flow distribution, and tectonic implications: in Bill Bonichsen and R.M. Breckenridge, eds., Cenozoic Geology of Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology Bulletin 26, p. 55-75.
Geographic coverage	Adams - Clearwater - Idaho - Lewis - Nez Perce - Valley - Washington