STRATIGRAPHY, GEOCHRONOLOGY, AND FAULTING IN THE SOUTHERN CRESCENT FORMATION, OLYMPIC PENINSULA, WASHINGTON STATE

The basement of the Coast Ranges from southern Oregon to southern Vancouver Island is composed of the accreted oceanic terrane known as Siletzia. The northern portion of this large igneous province, the Crescent Formation, is best exposed in the Olympic Peninsula of Washington state. New geologic mapping, geochemical data, a composite stratigraphic column, and recently published high-precision U-Pb zircon dates from silicic tuffs and detrital zircons (Eddy et al., 2017) show that the Crescent Formation is composed of 3 members. The Lower Crescent member (LC) consists of ~3.6 km of MORB-like submarine basalts ranging in age from ≥53.2 Ma to ≤51.1 Ma; the rocks were deformed post 51.1 Ma, perhaps during docking, before being buried by up to 185 m of basaltic turbidites. The Upper Crescent member (UC) consists of ~2.7 km of more enriched basalt flows. The top 0.6 km of the UC has shelly beach deposits and alternating subaerial and submarine flows. A date of 48.4 Ma on a silicic tuff marks the transition to subaerial volcanism. Both the LC and UC are heavily intruded by feeder dikes. The third member of the Crescent Formation, herein called the Crescent Assemblage (CA), consists of ~3.6 km of submarine basalt flows interbedded with and floored by the Blue Mountain Unit (BMU), a continentally-derived submarine fan with a local provenance (Einarsen, 1987). Detrital zircons in a BMU sandstone near the base of the CA yield a maximum depositional age of 47.8 Ma. This youngest member of the formation was thrust beneath the other Crescent rocks by ongoing subduction sometime after 47.8 Ma. The lack of subaerial strata and feeder dikes in the CA suggests that the lavas erupted off-axis and west of the Kula-Farallon ridge before being thrust beneath the rest of the formation, essentially making it one of the earliest-arrived packets of Olympic Core Rocks. The location of the thrust fault, indicated by an abrupt chemical transition and the absence of feeder dikes, is best constrained near Dusk Point and it is called the Dusk Point fault here. A similar chemical transition in the Dosewallips area, and similar young age dates in the lowest Crescent rocks 80 km to the north near Discovery Bay, strongly suggest that the Dusk Point fault is a regional structure comparable to the Hurricane Ridge fault in scale and in origin.