Manchester State Park Case Study

Manchester State Park, Kitsap Peninsula, Washington: West Beach Outcrop Geology

Jason Ott

The west beach at Manchester State Park consists of roughly 200m of exposed outcrops with well bedded strata of consisting of the Blakeley formation, of Oligocene age, and poorly consolidated beds of Quaternary age. The northern boundary of the area of interest begins at the southern edge of Middle Point and area terminates at Clam Bay to the south.

The Blakeley formation consists of alternating sequence of coarse sandstones, mudstones, and sub-rounded/brecciated conglomerates. The formation strikes 269°, and dips 41°N. The sandstone and mudstone members are typically 30-50cm thick. The sandstones are immature poorly sorted greywacke, and the mudstones

The poorly sorted nature of the conglomerates, considered with the inclusion of wood fragments in the older conglomerate members and the graded sandstones and mudstones throughout the formation suggest deposition occurred through successions of debris flows. Presence of volcanics in the lithic fragments further indicate volcanic activity in the process of sedimentation as well—as debris flows associated with lahars are the likely source of the slope failures. Deposition environment was moderate to deep marine, as mudstone deposits require low energy depositional environment, but the style of sedimentation indicate deposition was not on a continental shelf. This is further supported by inclusion of the large overturned clast from an older member within the formation. Cross-bedding, graded bedding, and scouring surfaces provide 3 lines of evidence establishing the northern contact of the formation as the original upward oriented surface. Wood fragment inclusions, and mollusk fossils allow 2 methods of dating to confirm time of deposition. Presences of normal and thrust faulting in the outcrops indicate the formation underwent multiple episodes of deformation, including both compressional and tensional stresses. The Blakeley formation was deposited as a submarine fan in a tectonically