Stonewall anticline: An active fold on the Oregon continental shelf

Stonewall Bank is the site of a growing west-verging anticline striking north-northwest on the continental shelf at 44.5° N, southwest of Newport, Oregon. To the east are Pliocene-Pleistocene strata of the Newport syncline, onlapping eastward against gently west-dipping late Miocene and older rocks of the Oregon Coast Range. Folding of Stonewall anticline results in fine-grained middle Miocene strata being exposed at the sea floor along the anticlinal crest; rocks as old as Eocene are encountered in the Unocal P-0093-1 Grebe well, drilled at the anticlinal crest. Rates of folding are based on deformation of an unconformity between Pliocene and Miocene strata (PM unconformity) and of a stream channel that crossed Stonewall Bank during the last glacial maximum. The bed length of the unconformity is shortened across the Stonewall anticline and adjacent folds by about 400 m between structures west of Stonewall Bank and the Newport syncline. The PM unconformity has a vertical separation of about 1000 m between the anticline and the first syncline to the west. The horizontal shortening and vertical separation imply that Stonewall anticline is underlain by a blind reverse fault. Retrodeforming the PM unconformity shows that this fault dips 65°–70° E. A vertical separation of 1000 m on a fault with this dip yields a slip of 1070–1080 m along the fault. If folding of the PM unconformity is assumed to have begun 2–3 Ma, this would give a long-term slip rate of 0.4–0.6 mm/yr. If most folding began after deposition of the entire Pliocene-Pleistocene sequence, the slip rate would be 1.0–1.1 mm/yr. Stonewall anticline has arched the late Pleistocene lowstand wave abrasion platform since sea level underwent a rapid rise from 14.5 to 8 ka. This arch is crossed by an antecedent stream channel that is 275–550 m wide and is marked by side drainages and cut banks up to 12 m high. Warping of the platform on the eastern limb of the anticline has back-tilted the stream channel eastward toward its present onshore continuation, the Yaquina River. The platform slopes downward 10–13 m westward from the crest of Stonewall anticline. We estimate that the platform stopped abrading when sea level reached about −40 m at 11–12 ka. Assuming that the west slope of the platform is controlled by the same blind fault that produced the west dip of the PM unconformity, the Holocene slip rate on this fault would be 0.9–1.3 mm/yr, comparable to the long-term slip rate. Rupture of the entire 25 km length of the blind fault with 1 m of slip beneath Stonewall Bank could produce an earthquake with M_w=6.8 ± 0.25, which would result in peak ground accelerations close to 0.2g on the central Oregon coast.