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Surface geology and gravity data suggest the presence of a major subsurface ramp zone in the Paris (Putnam) thrust beneath the mountain ranges south of Pocatello, Idaho. Immediately to the west a major thrust plate of mainly upper Paleozoic rocks (Hansel allochthon) overlies the Paris plate and conceals any evidence of the proposed ramp. Minor petroleum exploration work has been conducted in the general area of the ramp.
At localities in the Bannock Range, Blue Spring Hills, and Deep Creek Mountains, surface geology may indicate the presence of a steep ramp on the underlying Paris thrust. Recent mapping at the north end of the Deep Creek Mountains suggests the presence of an anticline west of Bannock Peak that has been disrupted by Basin and Range faults. This anticline lies at the structural contact between the area of upper Paleozoic rocks to the west (Hansel plate) and the area underlain by lower Paleozoic and Precambrian strata to the east (Paris plate). A similar structural-stratigraphic relationship is present locally in the Bannock Range and Blue Spring Hills. Restored cross sections, which have Basin and Range fault offsets removed, show that inference of a ramp on the underlying decollement a each locality helps to explain the observed structural-stratigraphic associations.
Preliminary analysis of gravity data supports the existence of a major thrust ramp beneath the mountain ranges south of Pocatello. A progressive eastward decrease in Bouguer anomaly values associated with pre-Tertiary rocks across the thrust belt begins in the Bannock Range, and may indicate that tectonic duplication of the sedimentary section has occurred as far west as the Bannock Range. The progressive eastward decrease in Bouguer values may reflect tectonic thickening associated with a significant ramp on the underling decollement.
Limited petroleum exploration work has been conducted along the exposed western margin of the Paris thrust plate and in the adjacent Hansel plate. Although near-surface strata in the Paris plate in the Bannock Range area lack hydrocarbon potential, it is possible that favorable stratigraphic units exist beneath the thrust at depth. The Hansel plate deserves additional exploration attention in that potential reservoir facies of the thick upper Paleozoic sequence are present in folded structures in the Blue Spring Hills and Sublett Range. Furthermore, the Manning Canyon Shale, which is a possible hydrocarbon source, is widespread in the Hansel plate. Structural complexity and high heat flow in the area have been the main deterrents to hydrocarbon exploration.
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