Reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the Sevier thrust belt suggests that initial thrust movement may be no older than Aptian age. This interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the Idaho-Wyoming and Utah thrust belts. A major episode of middle Cretaceous (Aptian–Cenomanian) subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent Sevier orogenic belt. An earlier subsidence event took place during Middle Jurassic time, more than 30 m.y. prior to the Cretaceous event, and may be the result of tectonic events to the west that are unrelated to thrust deformation in the Idaho-Wyoming and Utah thrust belts. We find no verifiable evidence to support previous interpretations that Sevier belt deformation and uplift began in Late Jurassic time.