Sequence stratigraphy and lithofacies of the middle Eocene upper McIntosh and Cowlitz Formations, geology of the Grays River Volcanics, Castle Rock - Germany Creek area, southwest Washington
A composite 732-rn thick section in Germany Creek of middle Eocene upper McIntosh and lower Cowlitz Formations, as well as the lower Grays River Volcanics, was studied using bio-, magneto- and lithostratigraphy to construct a sequence stratigraphic framework and assess hydrocarbon exploration potential. The upper McIntosh Formation forms a complete 3rd order depositional cycle that includes shallow marine, tide- and wave-dominated sandstones (upper McIntosh sandstone member) and overlying bathyal to outer-shelf siltstone (upper Mcintosh siltsone member). The upper McIntosh Formation coffelates to Chrons C19r (41.5 -42.5 Ma) and lower Chron 19n (41.5-41.2 Ma). A 31 order sequence boundary straddling Chron 19n (41.2-41.5 Ma) shows truncation of the highstand upper McIntosh siltstone by prograding lowstand shoreface parasequences of the lower Cowlitz Formation. The overlying (300 m thick) shoreface transgressive (TST) and highstand system tracts (HST) of the Cowlitz Formation range in age from 41.5-41.2 Ma to 39.35 ±0.36 Ma. The Cowlitz Formation HST of the study area chronostratigraphically correlates to the reservoir Clark arid
Wilson sandstone member of the Mist Gas Field Cowlitz Formation and to informal unit
1A and lower unit lB of the type-Cowlitz Formation, southwest Washington. The Grays River Volcanics of the study area are subdivided by an intraformational unconformity defining the upper (~150-m thick; 38.6-36.8 Ma) and lower (1250-m thick; 40.1-39 Ma) Grays River Volcanics subunits. Two subalkaline olivine- augite tholeiitic basalt flows of the lower Grays River Volcanics conformably overlying the Cowlitz Formation in Germany Creek were 40Ar/39Ar dated at 40.09 ±0.34 Ma and 39.35 ±0.36 Ma. The tholeiitic upper Grays River basalt unconformably overlies the lower Grays River Volcanics and the Cowlitz Formation. Numerous sub-parallel, northeast and minor northwest trending Grays River dikes (40Ar/39Ar dated at 39.98 ±0.29 and 39.56 ±0.41 Ma) attest to extension of the study area during the eruption of the
Grays River Volcanics. The Oligocene Lincoln Creek Formation (~70m) consists of bioturbated tuffaceous siltstone and interbedded silty sandstone with casts and molds of articulated mollusks. Geochemistry, normal polarity, and dikytaxitic texture of the 50-60 m thick Grande Ronde basalt are indicative of the Sentinel Bluffs unit (15.6 Ma) (Columbia River Basalt Group). The Arkansas Anticline and three fault sets: (1) north-northwest- and (2) northwest-trending dextral oblique-slip faults, and (3) subordinate northeast-trending conjugate sinistral oblique-slip faults are related to two periods of deformation. They are: (1) dextral transtension resulting in horst and graben style faulting, beginning 40-39.5 Ma and terminating prior to the eruption of the upper Grays River volcanics (38.6 Ma); (2) post-middle Miocene period of dextral transpression, including dextral reactivation of normal faults and associated broad regional northwest-trending folding (e.g. Arkansas Anticline). Small normal fault block similar to the Mist Gas Field provide structural traps throughout southwest Washington. Petrography of micaceous lithic arkoses of the upper McIntosh sandstone and Cowlitz Formation indicates reservoir-quality sandstones with
an estimated porosity of 16-19%. The most likely hydrocarbon exploration play involves
the subtidal to shoreface upper McIntosh Formation sandstone member.