Timing data on a frontal thrust system indicate that the Sevier thrust wedge of central Utah advanced rapidly to its ultimate structural front in ∼30 m.y., between early Albian and Campanian time. It then persisted by out-of-sequence thrusting for an additional 25 m.y., until the early Eocene. The key to the history of the frontal thrust is a piggyback basin that formed on the hanging wall. Unconformity-bounded sedimentary units within the basin thin both westward and eastward onto structures generated by thrust faulting. Up to 860 m of upper Campanian to lower Eocene strata accumulated between these structures, which consist of a ramp anticline in the west and a thrust duplex in the east. The newly recognized interaction of coeval deposition and thrust deformation permits revised limits on the inception and longevity of the frontal thrust system.

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