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Cretaceous sedimentary rocks in the Mitchell inlier of central Oregon accumulated in a forearc basin within a poorly understood part of the North American Cordillera. Sedimentary units are broadly to tightly folded and include the Albian Hudspeth Formation (marine mudstone and turbidites) and the Cenomanian Gable Creek Formation (submarine-fan conglomerate and sandstone). Previous and new mapping in the Toney Butte area reveals interference of the NE-trending Mitchell anticline and the NW-trending, basement-cored Toney Butte anticline. The entire ∼1.2–1.4 km thick Main Mudstone member of the Hudspeth Formation pinches out across the Toney Butte anticline, and the overlying Gable Creek Formation is laterally continuous and rests depositionally on basement rock in the core of the fold. Two leading hypotheses are considered to explain these relationships: (1) Pinch-out of the Main Mudstone member is due to preexisting erosional paleotopography, and all folds are Tertiary in age; or (2) the Toney Butte anticline initially grew during deposition of the Main Mudstone member, was overlapped by the Gable Creek Formation, and was further folded and rotated after deposition of Cretaceous strata. Based on critical map and stratigraphic relationships, we favor the second hypothesis. Accounting for ∼37° of postbasinal clockwise rotation, the data are interpreted to record growth of a basement-cored anticline due to north-south crustal shortening during Albian sedimentation in a large forearc basin. We infer that this folding episode resulted from regional transpression associated with northward translation of accreted terranes in the Blue Mountains along the western U.S. Cordilleran margin.

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