Geology and Structural Evolution of the Foss River-Deception Creek Area, Cascade Mountains, Washington
AN ABSTRACT OF THE THESIS OF James William McDougall for the degree of Master of Science in Geology presented on Lune, icnct Title: GEOLOGY AND STRUCTURALEVOLUTION OF THE FOSS RIVER-DECEPTION CREEK AREA,CASCADE MOUNTAINS, WASHINGTOV, Redacted for Privacy Abstract approved: Robert S. Yekis Southwest of Stevens Pass, Washington,immediately west of the crest of the Cascade Range, pre-Tertiaryrocks include the Chiwaukum Schist, dominantly biotite-quartzschist characterized by a polyphase metamorphic history,that correlates with schistose basement east of the area of study.Pre-Tertiary Easton Schist, dominated by graphitic phyllite, is principallyexposed in a horst on Tonga Ridge, however, it also occurs eastof the horst.Altered peridotite correlated to Late Jurassic IngallsComplex crops out on the western margin of the Mount Stuart uplift nearDeception Pass. The Mount Stuart batholith of Late Cretaceous age,dominantly granodiorite to tonalite, and its satellite, the Beck lerPeak stock, intrude Chiwaukum Schist, Easton Schist, andIngalls Complex. Tertiary rocks include early Eocene Swauk Formation, a thick sequence of fluviatile polymictic conglomerateand arkosic sandstone that contains clasts resembling metamorphic and plutonic basement rocks in the northwestern part of the thesis area.The Swauk Formation lacks clasts of Chiwaukum Schist that would be ex- pected from source areas to the east and northeast.The Oligocene (?) Mount Daniel volcanics, dominated by altered pyroclastic rocks, in- trude and unconformably overlie the Swauk Formation.The
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