A GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF CRESCENT FORMATION BASALTS IN THE SOUTH MOUNTAIN AREA OF THE OLYMPIC PENINSULA, WA

Exposed at South Mountain, WA, are some of the uppermost units of the early to middle Eocene Crescent Formation, a voluminous basaltic complex in the Olympic Peninsula. The sequences of basalts, breccia, and sediments in this area suggest that flows erupted subaerially, transitioned laterally to pillow basalts, and finally broke up as breccia that filled a graben to the east. Geochemical analyses show that the Crescent basalts resemble enriched mid-ocean ridge basalts, but do little to discriminate the geotectonic setting for the eruption of these lavas. The presence of a graben is consistent with the marginal rifting hypothesis for the origin of the Crescent Formation. We suggest that the Crescent Formation represents the extrusive product of marginal rifting off the North American continent in a transtentional stress regime sourced to the obliquely subducting Kula-Farallon plate complex.