Faculty Advisor

Clark, Ken and Mike Valentine

Area of Study

Science and Mathematics

Publication Date

Summer 2019


The mafic rocks of the Olympic Peninsula, Washington, are part of an accreted terrane known as Siletzia which experienced transpressional stresses as far as 50 Ma ago in the early Eocene. The Peninsula has an accretion-thrust marine sedimentary interior and a mafic volcanic periphery juxtaposed along the Hurricane Ridge fault; a terrane-scale thrust fault. The mafic Crescent Formation (CF) can be subdivided into two units: The Lower Crescent member (LC) and the Upper Crescent member (UC) as defined by Tabor and Cady (1978). The LC consists of submarine basalt flows that have composition similar to mid-oceanic ridges with zircon fission-track age dates ranging from ≥53.2 Ma to ≤51.1 Ma while the lavas of the UC are mostly ocean island basalts; the youngest flows around 48.4 Ma (Clark, 2019) in age. New geochemical data and age dates collected by Ken Clark (2019) in the CF show that a section of mafic material is younger and chemically distinct from the material sitting above it. The third and newer section, called the Crescent thrust sheet (CT) is thrust beneath the LC and contains basaltic material composition closer to ocean island basalts. The Blue Mountain Unit (BMU) within the CT has been dated to around 47.8 Ma (Clark, 2019). This study seeks to provide magnetic data across the geochemical boundary to the west of Dusk Point in order to locate magnetic anomalies potentially relating to the Crescent thrust and to determine the extent of the boundary between the CT and LC by chemical analysis.

Evidence for the Crescent thrust includes magnetic intensity anomalies and changes in rock composition near Dusk Point in the Olympic National Forest in Mason County. High intensity readings accompanied mafic intrusions in outcrops which were sampled along with other massive and sheared pillow flows. These samples were analyzed via ICP procedure as well as scanned for trace elements. Enriched Y/Nb ratios are found in the CT and UC while less enriched Y/Nb are found in the LC member. Magnetic transects taken across the chemical break show potential subsurface structures related to thrust faulting due to the similarities in magnetic profiles and anomalies in one transect reappearing along places where the geochemical break was proposed to extend to. Along Transects F101, F102, F103, and F202 are repeated depressions of 150-200nT (nanoTeslas) before and after a rise of 200-400nT. F202, having the most detailed data so far, shows a sharper drop in intensity than in other transects parallel to it and this is where I believe the fault lies due to a change in where the drop appears in other transects.

We suggest that the differences in composition and magnetic anomalies in the Dusk Point area may be the result of a separate package of submarine basalt flows (the Crescent thrust sheet) being thrust beneath older flows, making a third member of the Crescent Formation. In addition, a previously unrecognized thrust fault, tentatively named the Dusk Point Fault, bounds the chemically and magnetically different units in the area. More chemical sampling is underway to better define the extent of this fault and the CA unit.



University of Puget Sound