Abstract

Field evidence, combined with published and unpublished seismic data, supports a new, inferred, easternmost limit of Sevier thrusting during Late Cretaceous foreland deformation in east-central Utah. Imbricate thrust faults are exposed along the western margin of Cedar Mountain in the northern part of the San Rafael swell within the western bounds of the Colorado Plateau. The faults offset Jurassic Entrada, Curtis, Summerville, and Morrison formations and the Lower Cretaceous Cedar Mountain Formation. Strike separations ranging from a few metres to 564 m and stratigraphic separations of as much as 76 m are accommodated by bedding-parallel slippage along less competent shales and siltstones and by oblique ramping across more competent sandstones. Fault zones exhibit breccias, drag folds, chatter marks, and imbricate splay-faulted slivers. The kinematic mechanism appears to have been a layer-parallel shortening accomplished along nearly horizontal, nearly east-directed compression.

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