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Master of Science (MS)
Babcock, R. Scott (Randall Scott)
Engebretson, David C.
Ellis, Ross C.
A stratigraphic section developed for the Bremerton rocks in the Kitsap Peninsula suggests formation by rifting in a marine environment. Basal gabbro, dated by 40Ar/39Ar at 49.8 Ma plus or minus 0.8 Ma, and associated mafic to felsic plutonics, appear to be the source of a mafic dike complex that composes 100% of the stratigraphic level above the plutonics. These dikes are the apparent feeders to overlying submarine and subaerial volcanics. The previously unrecognized submarine sequence consists of interbedded basaltic breccia, tuffs, basalt flows, and basaltic sandstone, siltstone, and conglomerate. Approximately 1 km of columnar basalt flows cap the sequence.
Structures in the Bremerton rocks suggest that as many as four deformations may have affected these rocks. In the middle Eocene, extension to the north or northwest caused rifting and emplacement of the mafic dikes and lavas; several faults show that this extension was joined or followed by northeast-directed compression. Small faults and shears may have formed later by compression to the northwest. Reactivation of one of these faults and emplacement of northwest-striking Cascade arc(?) dikes suggest the latest deformation was extension to the northeast.
The gabbro and basalts have chemistry transitional between N-type MORB and enriched ocean island basalt. Large-ion-lithophile to high-field-strength trace element ratios are similar to those of back-arc basin basalt. Felsic plutonics are enriched in elements (especially Zr) indicative of an arc influence. Several dikes intruding the gabbro are chemically indistinguishable from P-type (plume) ocean island basalt.
Stratigraphic sections were developed in the basalts of the Crescent Formation and for basalts near Port Townsend in the Olympic Peninsula. Comparison to the subaerial basalts near Bremerton shows that the latter are quite similar to the upper subaerial Crescent Formation basalts and Port Townsend basalts. New chemical data for 16 flows of the Olympic Peninsula basalts are also similar to that of the Bremerton rocks.
Stratigraphic and chemical similarities imply that rocks of all three areas are coeval. Deposition of the Crescent Formation basalts on a continentally-derived fan, stratigraphic and structural characteristics of the Bremerton rocks, and geochemical data on rocks of ail three areas suggest that these rocks formed by rifting of the western margin of the North America continent. Following rifting and basalt generation, the rocks may have moved north due to oblique convergence of the oceanic and continental plates, where they were thrust under Vancouver Island along the Leech River Fault. Thrusting moved outboard of the basalts and underlying fan; continued convergence thrust marine sediments that make up the Olympic core rocks beneath the basalts and underlying fan.
Bremerton rocks, Crescent formation basalts
Western Washington University
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Clark, Kenneth P., "The Stratigraphy and Geochemistry of the Crescent Formation Basalts and the Bedrock Geology of Associated Igneous Rocks Near Bremerton, Washington" (1989). WWU Graduate School Collection. 652.