Depositional and Tectonic Framework of Lower to Middle Eocene Swauk Formation, Eastern Swauk Basin--A Model for Early Tertiary Basin Development Beneath Columbia Plateau

Stephen B. Taylor, Samuel Y. Johnson, Gregory T. Fraser, James W. Roberts

The Eocene Swauk basin formed between the Straight Creek and Entiat-Leavenworth fault systems and is one of several early Tertiary strike-slip basins in the Pacific Northwest. The basin is approximately 50 km wide, contains as much as 10,000 m of nonmarine deposits, and lies 30-70 km north of an area of recent hydrocarbon exploration in the Columbia Plateau region of central Washington. Sedimentologic and tectonic studies of exposed portions of the Swauk basin are useful for developing exploration plays to the south. The lower to middle Eocene Swauk Formation (> 4,800 m thick) comprises a large portion of basin fill and is the primary concern of our study.

Depositional characteristics of the Swauk Formation include: (1) rapid nonmarine deposition in alluvial-fan, braided-river, meandering-river(?), lacustrine-deltaic, and lacustrine environments, (2) abrupt lateral and vertical facies changes, (3) numerous fault-controlled reversals of paleo-drainage, and (4) arkosic detritus derived from plutonic and meta-plutonic source areas to the east, north, and west(?). The presence of interbedded pyroclastic deposits and intrusive mafic dikes implies high intra-basin heat flow. Postdepositional folding was controlled by wrench-style deformation along basin-margin faults.

These characteristics indicate strong and varied tectonic controls on the evolution of Swauk basin. Given the regional strike-slip setting and structural trends, a similar style of basin development seems likely for correlative early Tertiary strata to the south, beneath the Columbia River Plateau. Thus, our data provide a foundation for planning exploration strategies in this frontier province.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.