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Twin Sisters Mountain, Whatcom Co., Washington, USA

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Latitude & Longitude (WGS84): 48° 42' 39'' North , 121° 59' 35'' West
Latitude & Longitude (decimal): 48.7108333333, -121.993055556

Dunites composing the majority of the Twin Sisters Mountains

*Cannon, B. (1975): Minerals of Washington, p.47,73,85

Mineral List

Mineral list contains entries from the region specified including sub-localities

8 valid minerals.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Paleocene - Cretaceous
56 - 145 Ma
Tertiary dunite intrusive rocks

Age: Phanerozoic (56 - 145 Ma)

Stratigraphic Name: Twin Sisters Dunite

Description: 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.

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.

Lithology: Major:{dunite}, Minor:{serpentinite}, Incidental:{pyroxenite}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Triassic - Jurassic
201.3 Ma
Twin Sisters Dunite of Ragan (1961)

Age: Early Jurassic (201.3 Ma)

Lithology: Ultrabasic (ultramafic) rocks

Reference: Washington State Department of Natural Resources, Division of Geology and Earth Resources. Washington Interactive Geologic Map: 1:100,000 scale. Washington Division of Geology and Earth Resources. [19]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

Localities in this Region

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