Marine strata of Permian, Late Triassic, Jurassic, and Late Cretaceous age have an aggregate thickness of 35,000–40,000 feet in the Suplee-Izee district of central Oregon. This deformed but unmetamorphosed eugeosynclinal sequence is composed largely of material derived from within the eugeosynclinal belt and consists dominantly of andesitic debris and of detritus eroded from internal tectonic lands. Unconformities and coarse conglomerates show that epeirogeny or orogeny and subaerial erosion occurred at least locally during nearly every age of the standard geologic column from the middle of the Permian until late in the Cretaceous.
If the history of the Suplee-Izee district is considered a spot sample typical of the western Cordilleran eugeosynclinal region, two general interpretations are suggested: (1) The term “eugeo-syncline” is applied generally to a mass of stratified rocks accumulated during a complex history of diastrophism, volcanism, and sedimentation in separate, short-lived downwarps or basins. The term is not applied in practice to any particular topographic feature or to a single, simple tectonic feature. (2) Sinking of the eugeosynclinal pile resulted largely from transfer of volcanic magmas from the depths to the surface. This volcanism was coupled with crustal foundering into the sites of magma generation.