THE CRESCENT FORMATION ALONG LAKE CUSHMAN'S NORTHERN SHORE, OLYMPIC PENINSULA, WASHINGTON: A STRATIGRAPHIC, CHEMICAL AND STRUCTURAL STUDY

The Crescent formation (Cf) is an unusually thick sequence of subaqueous and subaerial basalts interbedded with sedimentary layers which formed 53-45 Ma. The Cf along with the Olympic Core terrane composes the bulk of the Olympic Mountains on the Olympic Peninsula, Washington. Previous workers have recognized three main members of the formation. The lower Cf is characterized by subaqueous, pillow basalts which are interbedded with, and underlain by, sedimentary rocks of the Blue Mountain unit while the upper Cf tends to be massive with jointed basalts. The Blue Mountain unit is a continentally-derived fan primarily composed of shales and slates. Foraminifera can be found in many of the interbedded sedimentary layers. In this study, I examined and mapped the rocks of the lower Cf along Lake Cushman's northern shore to add to the chemical and structural database on the lower Cf.

I collected twenty three samples for ICP-ES, SEM, petrographic, stratigraphic and structural analysis. ICP-ES analysis indicates that the lower Cf is enriched in calcium and strontium but contains lower silica values than neighboring basalts. Petrographic analysis shows amygdaloidal weathering with secondary minerals beginning to replace primary minerals such as plagioclase and pyroxene. There is also a considerable amount of weathering on many of the basalt samples causing indistinct grain boundaries. A stratigraphic column indicates unusually thick lava flows consistent with the rest of the Cf, as well as evidence for the presence of the Blue Mountain unit. Structural data was plotted on stereonets and displays a distinct boundary between the Skokomish River shore and Lake Cushman shoreline sections of the study area. The Skokomish River shore was more highly faulted and strikes were shallower than those found on the Lake Cushman shoreline.

Preliminary analysis indicates that rocks of the Lake Cushman area are similar to those studied in other parts of the Cf, as their chemical data points to formation in a rift setting. This area has undergone a substantial amount of weathering, faulting, and low-grade metamorphism. The orientation of mapped faults indicates that there has been strike slip motion which may have caused a bend in Lake Cushman itself.