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The Geology And Petrology Of The Fifes Peak Formation In The Cliffdell Area, Central Cascades, Washington, Brad A. (Bradley Allen) Carkin

WWU Graduate School Collection

The Fifes Peak Formation in the Cliffdell area is comprised of two structurally, lithologically and geochemically distinct members separated by a pronounced unconformity. The older member (Edgar Rock member) is comprised of highly brecciated lava flows, coarse laharic breccia and lesser volcaniclastic sediments and tuffs. A thickness of at least 1,800 m is exposed near Edgar Rock. K-Ar ages indicate a latest Oligocene age of about 24 to 27 Ma. Lava compositions range from basalt to dacite, but are mostly basaltic andesite. These lavas are typically highly porphyritic and contain an anhydrous phenocryst assemblage of plagioclase, olivine, minor clinopyroxene and …