Stonewall anticline: An active fold on the Oregon continental shelf

Abstract

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