TECTONIC HISTORY OF THE COOS BAY BASIN AND ITS RELATIONSHIP TO PACIFIC NORTHWEST ‘SILETZIA’ TECTONICS

The Cenozoic Coos Bay Basin in SW Oregon is a north-plunging synclinal trough that overlies the southwestern margin of ‘Siletzia’, the accreted basaltic basement of the Cascadia forearc. We have examined the Eocene to Pleistocene history of the Coos Bay Basin and compared it to the post-accretion history of rotation and northward transport of ‘Siletzia’ as described for the rest of the forearc (Wells and others, 2014).

Structural and sedimentologic analyses within the Coos Bay Basin document four phases of tectonic activity with the sequential formation of accommodation space and deposition of sediment followed by deformation along the convergent plate boundary. Fault and fracture patterns clearly define primary stress orientations within each tectonic phase, with progressive overprinting by each successive deformation. These outcrop scale patterns are reflected in sub-regional lidar, gravity and geologic maps.

The deformational phases are:

1. Mid Eocene to Oligocene extension with associated deposition of the Beds of Sacchi Beach-Coaledo-Bastendorff-Tunnel Point units, followed by -30⁰ to -60^o SW-NE σHmax folding that is consistent with regional margin-parallel extension recorded by Tillimook-Yachats basaltic volcanism in the forearc.

2. Late Oligocene to Mid-Miocene folding forming the South Slough Syncline with deposition of the Tarheel Formation, followed by contraction with thrusting and -5⁰ NW-SE σHmax folding that is consistent with northward transport of the forearc and its south-to-north shortening against the Canadian Coast Range Buttress.

3. Late Miocene to Early Pliocene extension with associated deposition of the Empire Formation, followed by <-5^o SW-NE σHmax folding that appears to be a local tectonic pulse with regional Late Pliocene contraction SW-NE σHmax resulting in -25⁰ folding, coeval with extensional activity in the Great Basin and northward translation of the Sierra Nevada.

4. Modern uplift and faulting of marine terraces N-S σHmax that is consistent with ongoing coastal tectonic activity along the Cascadia forearc.

The Coos Bay Basin structural patterns are similar with published observations in other basins from the Newport Basin of Oregon northward to the Puget Lowlands of Washington and suggest a consistent pattern of deformation along much of the Cascadia forearc.