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Rocks of Central Idaho

Central Idaho Black-Shale Mineral Belt
In the mountains around Sun Valley the stratigraphic units are different, and generally darker-colored, than in southeast Idaho thrust belt or in the area from the Lost River to the east. Here, in the Pioneer, Boulder, and Smoky Mountains, are thick Lower Paleozoic black shales, such as the Devonian Milligen formation, which occupies much of the low country east and west of the Wood River Valley. The Milligen contains mineralized black shales, from which rich silver-lead-zinc veins were mined following an exploration boom in the 1880s.

Large anticline (left) with small amplitude folds on right skyline, upper Paleozoic limestones, east side Lost River Range north of Borah Peak, view looks northwest, (July, 1990).
Keyser Creek anticline, above the Darby thrust, looking north in Little Grey's River, northwest Wyoming. Folded rocks are Jurassic Twin Creek Formation, (September, 1981).
Bare hills west of the Wood River Valley, looking northwest from north of Bellevue, (December, 1987). Colorado Gulch is immediately south of the kinked ridge. The rich silver and lead strikes of the Wood River Valley in the 1880s were mainly in black shales in these hills.

In the southern Pioneer Mountains are over 15,000 feet of Mississippian conglomerates and sandstones of the Copper Basin Formation, deposited in a deep-sea fan environment north and east of a highland that may have stood under the present Snake River Plain. Above the Milligen formation in the Wood River area are thick calcareous siltstones and sandstones of the Pennsylvanian and Permian Sun Valley Group (including the Wood River Formation), which were deposited in deep water west of a carbonate bank that existed in what is now the Lost River Range area.

Idaho Batholith
The Idaho batholith, generally of Late Cretaceous age (75-100 Ma) formed the roots of a continental volcanic arc, which is now eroded away. The magmatic activity was caused by the subduction of the Pacific Plate beneath North America during Cretaceous time. The batholith consists of two lobes, the southern Atlanta lobe and the northern Bitterroot lobe, separated by Proterozoic metamorphic rocks of the Salmon River arch.

Metamorphic Core Complexes
After intrusion of the Idaho batholith, there was a general relaxation of compression after Mesozoic crustal thickening. The general process of uplifting and thinning of over-thickened crust results in the juxtaposition of ductilely deformed high-grade metamorphic lower crust against brittlely deformed low-grade metamorphic upper crust. This process has produced the metamorphic core complexes which are present along the length of the North American cordillera. In southern and central Idaho the Pioneer Mountains and the Albion-Raft River Mountains are metamorphic core complexes.

Pioneer Mountains
At the same time as the Challis volcanic rocks were erupted, the Pioneer Mountains metamorphic core complex was rising. Low-angle extensional and strike-slip faults formed in the Boulder and Pioneer Mountains northeast of Ketchum. The general sequence of events was 1) Cretaceous intrusion ending by 70 million years ago; 2) formation of northwest-striking high angle faults as well as low-angle oblique-slip faults, ending about 45(?) million years ago; this faulting stripped sedimentary cover from the Pioneer Mountains; 3) volcanic activity of the Challis volcanic episode and faulting of the northeast-striking Trans Challis fault system; intrusion of the Summit Creek stock in the core of the Pioneer Mountains at about 48 million years ago; 4) intrusion of late-stage granite plutons (Sawtooth, Boulder, Pioneer and Smoky Mountains) and related rhyolite volcanism about 44 million years ago, 5) final uplift and unroofing of Pioneer Mountains core complex (37 to 34 million years ago); the Summit Creek stock was beheaded by this faulting and its upper portion moved northwestward by as much as 23 km.

The Albion Range and The Silent City of Rocks
In the Albion Mountains, the oldest rocks are gneisses of the Archean Green Creek Complex. These are overlain by several Proterozoic and Paleozoic sedimentary formations which have been much folded and faulted. The domal shape of the core complex is evident in the form of Mount Harrison south of Albion, on which the Pomerelle ski area is built (miller and others, 1983). The principle structural zone along which the core complex was uplifted south of Oakley is the Middle Mountain Shear zone, which occupies the valley of Birch Creek and much of Middle Mountain. This shear zone ceased moving about 35 million years ago.

The Silent City of Rocks, along the California Trail west of Almo, exposes the Almo Pluton, intruded about 32 million years ago, after uplift of the core complex had largely ceased (bandy, 1992). There are probably other plutons under the northern Albion Range, but these have not been exposed by erosion.