The Fort Hill quadrangle lies along the b<>undary between the Idaho-Wyoming thrust belt and the Green River Basin. A sequence of Paleozoic and Mesozoic sedimentary rocks about 21:l,OOO feet thick has been intensely deformed along the front of the thrust-belt mountain system. Only Cambrian to Ordovician and Triassic to Cretaceous rock units are exposed now within the quadrangle. Middle and upper Paleozoic rocks are covered by partly deformed, but well exposed, Tertiary strata that help date some of the tectonic events.Drilling within the quadrangle has provided data which help outline the third dimension of structures map~. Westerly dipping lower Paleozoic to Mesozoic units overrode gently folded qretaceous rocks eastward along the Hogsback fault in ~aleocene time; all movement along this fault in the quadrangle ehded before latest Paleocene time, for the fault trace is overlain by the Chappo Member of the Wasatch Formation. In outcrops the dip of the Hogsback fault is about 50° ; but in the subsurface the dip declines westward to 10°, as shown in boreholes. The Hogsback fault is a bedding-plane glide surface that rode on claystones near the middle of the Cambrian Gros Ventre Formation; lateral transport eastward may have been about 20 miles.Above the Hogsback fault are three other thrust faults, here named, in ascending order, the Fort Hill, the Meridian, and the Pine Ridge faults. All three are interpreted as slices of the Hogsback fault that cut across the section moderately abruptly. Movements al()Jlg these faults are not well dated but may have been in latest Paleocene and earliest Eocene time, when detritus in the Chappo Member of the 'Vasatch Formation was deposited. Movement may have been later on the higher slices than on the lower.Below the Hogsback fault is the La Barge thrust fault, along which the Cretaceous Hilliard and Frontier Formations overrode the Tertiary Hoback(?) and the Cretaceous Adaville(?) and Hilliard Formations. Movement along this fault was in early, possibly middle early, Eocene time.Thrusting was followed in the area by block faulting in late early Eocene time--which may explain, in part, intertonguing of the Wasatch with the Green River• Formation-and later.Billions of cubic feet of natural gas and millions of barrels of oil have been produced from stratigraphic traps, in part controlled by structure in the northeastern part of the quadrangle and adjoining areas. Lenses and tongues of Cretaceous and lower Tertiary sandstone bounded by claystone and mudstone have been especially productive. Many more such stratigraphic traps are likely to be found in Cretaceous, Tertiary, and possibly also in Paleozoic rock units within the region.Other natural resources of the area include helium, oil shale, coal, and phosphate.

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2 2 2 3 3 3 4 Geophysical surveys___________-___-_-_______l._-___-E4 Gravity survey and map_______________________ Aeromagnetic survey and map_________________ Interpretation of local geophysical anomalies__________. Gem Valley.___________________________________ Blackfoot lava field____________-______________-. Upper Blackfoot River drainage_________________. Local anomalies in the ranges-___________________ Regional gravity anomalies.

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