Tertiary dunite intrusive rocks

An intimate mixture of saxonite and dunite, partly serpentinized; Twin Sisters mountain area of Whatcom and Skagit Counties. Predominantly dunite and associated serpentinite in Jumbo Mountain area of central Snohomish County.
State Washington
Name Tertiary dunite intrusive rocks
Geologic age Late Cretaceous
Lithologic constituents
Igneous > Plutonic > Ultramafic > Peridotite > Dunite
Metamorphic > Metaigneous > Serpentinite
Igneous > Plutonic > Ultramafic > Pyroxenite
Comments May be referred to no more specifically that as peridotite; may also be reported as harzburgite. Occurs as pods, lenses, and "tectonically emplaced bodies" (Dragovich and others, 2002); Twin Sisters Dunite is a "plate-like body" (Tabor and others, 1994). Pyroxenite occurs very rarely.
Stratigraphic units Twin Sisters dunite

Huntting, M.T., Bennett, W.A.G., Livingston, V.E.Jr., and Moen, W.S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.

Dragovich, J.D., Logan, R.L., Schasse, H.W., Walsh, T.J., Lingley, W.S., Jr., Norman, D.K., Gerstel, W.J., Lapen, T.J., Schuster, J.E., Meyers, K.D., 2002, Geologic Map of Washington-Northwest Quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-50, scale 1:250,000.

Tabor, R.W., Haugerud, R.A., Booth, D.B., Brown, E.H., 1994, Preliminary Geologic Map of the Mount Baker 30- by 60-Minute Quadrangle, Washington: U. S. Geological Survey Open File Report 94-403, 55p., 2 pls.

Tabor, R.W., Booth, D.B., Vance, J.A., Ford, A.B., Ort, M.H., 1988, Preliminary geologic map of the Sauk River 30 by 60 minute quadrangle, Washington: U. S. Geological Survey Open File Report 88-692, 51 p., 1 pl.

NGMDB product
Counties Skagit - Snohomish - Whatcom