Petroleum exploration in younger over older thrust plates in southeastern Idaho

West of the Paris and Putnam faults (Bear River and Portneuf Ranges, Idaho), thrust plates of younger strata overlie older with tectonic omissions as great as 7 km, in contrast to eastern foreland thrusts of older over younger strata with repetitions of about 6 km. Folds in the western plates are broad, open, and upright in contrast to tight asymmetric folds in eastern plates. Three major thrust plates are recognized from extensive but incomplete mapping. (1) An uppermost and most extensively exposed plate of Oquirrh strata (Pennsylvanian and Permian) with Manning Canyon Shale (Mississippian and Pennsylvanian) at its base lies discordantly on middle Paleozoic units and extends from the Albion Range 100 km east to the Bannock Range. (2) A middle plate of mainly Ordovician to Middle Cambrian strata lies on upper Precambrian quartzite and argillite of the Brigham Group in the Bannock Range between Pocatello and Utah. Ordovician strata in the Raft River and Albion Ranges lie on schist and quartzite of unknown age. (3) A lower plate of mainly upper Precambrian (Mutual and Camelback Mountain) quartzite in the Bannock Range overlies Precambrian rocks of the Scout Mountain Member of the Pocatello Formation, 3 km lower in the stratigraphic sequence. The middle plate locally overlaps the lower, placing Ordovician limestone directly on Scout Mountain strata. Scout Mountain rocks are more intensely sheared, mylonitized, and metamorphosed (greenschist) than any east of the Raft River Range, which poses the problem of whether they are autochthonous. Prolific Mesozoic hydrocarbon reservoirs of the foreland belt are thus unlikely to be encountered by drilling farther west. Moreover, these thrust plates are cut by younger basin-and-range faults that have enormous apparent stratigraphic throws, further complicating local structure and hydrodynamic history. Reported moderate to high paleotemperatures reflect a complex thermal history with undefined implications for maturation. Nevertheless, regional stratigraphic relations suggest the presence of Paleozoic hydrocarbon source beds and reservoirs, although exploration will be difficult.