INTEGRATING SEDIMENTOLOGY, MAGNETOSTRATIGRAPHY AND 40AR/39AR AGE DATING TO RESOLVE THE REGIONAL SEQUENCE STRATIGRAPHY OF THE MIDDLE EOCENE UPPER MCINTOSH AND COWLITZ FORMATIONS, SOUTHWEST WASHINGTON

Despite the approximately 70 BCF production of the middle Eocene Cowlitz Formation arkosic shoreface sandstones of northwest Oregon (Mist Gas Field), there has been limited exploration in the genetically related units of nearby southwest Washington. The complex stratigraphy of the middle Eocene Oregon-Washington Coast Range resulting from deltaic sedimentation, basaltic volcanism, and ongoing forearc deformation, has hindered development of this exploration frontier. This field-based study was undertaken to determine the stratigraphy of the middle Eocene sedimentary units of the Germany Creek area, southwest Washington, enabling refined regional surface-subsurface high-resolution stratigraphic correlations between the type-section Cowlitz and McIntosh Formations of Washington and the Cowlitz and Hamlet Formations of northwest Oregon. Based on detailed lithofacies descriptions of three measured sections (1160 m total), sampling of macro- and microfossils, magnetostratigraphy and two 40Ar/ 39Ar dates, the local stratigraphy was correlated to related regional units and used to interpret the middle Eocene 3rd order unconformity-bound sequences and depositional systems. The subtidal to bathyal upper McIntosh and Hamlet Formations correlate to Chron C19r (41.5 – 42.5 Ma) and lower Chron C19n (41.5 – 41.2 Ma) consisting of a complete 3rd order depositional cycle. A sequence boundary between the bathyal highstand upper McIntosh and prograding shoreface lowstand Cowlitz Formations correlates to Chron 19n (41.2 – 41.5 Ma). The overlying highstand shoreface parasequences of the study area Cowlitz Formation range in age from 41.5 – 41.2 Ma to 39.35 ±0.36 Ma (conformably overlying 40Ar/ 39Ar dated basalt flow); to as young as Chron C17n (36.5 – 38.2 Ma) at the type-locality.