Abstract

Isopachous and lithofacies maps of the Mississippian, Pennsylvanian, and Permian Systems of Idaho show that upper Paleozoic rock sequences and structures for several time-stratigraphic intervals north of the Snake River Plain differ from those south of the plain. Lower Mississippian rocks north of the plain largely consist of western derived (Antler highland), coarse orogenic clastics deposited in a foreland basin environment juxtaposed on the east by a contemporaneous inner cratonic platform carbonate bank. South of the plain, Lower Mississippian rocks are dominated by sediments of a starved basin environment deposited west of an inner cratonic platform carbonate bank. Upper Mississippian rocks north of the plain consist of western-derived detritus deposited in a narrow shoaling foreland basin trough adjacent to a carbonate bank that extended eastward to the inner cratonic platform margin on which shallow marine and restricted marine mudstones, sandstones and limestones were formed. South of the plain, Upper Mississippian carbonate bank limestones and marine dark shale of the outer cratonic platform were formed west of the shallow water and restricted marine mudstones, sandstones and carbonate rocks of the inner cratonic platform.

In middle Pennsylvanian time, the Copper Basin highland arose north of the Snake River Plain and separated the Wood River basin on the site of the former Antler highland on the west from a carbonate bank on the east that extended to the margin of the inner cratonic platform. Large amounts of fine-grained sand were deposited simultaneously on the inner cratonic platform. South of the plain, in middle Pennsylvanian time the Bannock highland became submerged, and sandstones and sandy carbonates were formed across it, in the western Sublett basin, and on the eastern inner cratonic platform. The Copper Basin highland did not extend into southeastern Idaho.

In Early Permian time north of the plain, the Copper Basin highland continued to occupy a position between a deepening Wood River basin on the west and an extensive carbonate bank on the east, on the outer cratonic platform. No record of sediments of this age is present on the eastern inner cratonic platform. To the south, the Sublett basin continued to subside and receive large amounts of sand, and then shoaled in middle Early Permian time. East of the Sublett basin, thinner shelf sand accumulations formed on both the inner and outer cratonic platforms.

In late Early and early Late Permian time north of the plain, a persistent Copper Basin highland probably separated the Phosphoria sea from deeper marine waters of the Wood River basin on the west. Phosphoria-equivalent rocks of continental rise affinities are present in central Idaho west of the Wood River Formation. South of the plain the Phosphoria sea extended across the entire area, including both inner and outer cratonic platforms. Contemporaneous oceanic and island-arc volcanogenic assemblages present in and adjacent to western Idaho probably were accreted to the continent in Mesozoic time.

No large Upper Paleozoic strike-slip movements seem necessary along the axis of the Snake River Plain to account for the present distribution of facies and thicknesses of Mississippian, Pennsylvanian, and Permian rocks on the two sides of the plain according to this interpretation of sedimentary trends. The differences that are observed suggest that the rock sequences and structures were at least as distant during Paleozoic time as they are now.