THE IDAHO BATHOLITH AND YELLOWJACKET FORMATION

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THE IDAHO BATHOLITH AND YELLOWJACKET FORMATION










INTRODUCTION

The Idaho Batholith, located in central Idaho, has some of the most rugged and remote terrain in North America. The Idaho Batholith extends north from the Frank Church Wilderness Area, through the Salmon-Challis national Forest, and terminates in north-central Idaho (Figure 1). It extends east from the Washington-Idaho border to the Idaho-Montana border.

The Idaho Batholith has had historical importance for Idahoans since Statehood in July , 180 (Arrington, 14). The Batholith served as a near-impassable obstacle for the early northern Statesmen, especially in the winter months. In order to attend State House and Senate meetings in Boise, Statesmen would generally travel to Seattle where they would then take a ship down the coast, up the Columbia River, and along the Snake River (Arrington, 14). This was a long and arduous trip, forcing the Statesmen to be away from home for months, and in many cases, all winter long. This caused a little discontent between the northerners and the rest of the state, to say the least.

The rugged terrain of the Idaho Batholith is the result of millions of years of violent geologic events and is the roots of an eroded away continental volcanic arc of late Cretaceous age (Alt and Hyndman, 15). The geologic history has been of great interest to geologists and students of geology for decades, and the rugged terrain and wild waters of the Salmon River have been an enjoyment for outdoorsmen for decades. This paper will discuss the origin of the Idaho Batholith and one of the most significant geologic formations in the Batholith, the Yellowjacket Formation. The Yellowjacket Formation is one of the most studied, yet most controversial, units of the Batholith.

Figure 1. Distribution of known and inferred Yellowjacket Formation (modified from Evans, 1).

PRECAMBRIAN THROUGH NEOGENE HISTORY

In order to understand the origin of the Idaho Batholith, the early geologic history of the West Coast, particularly Idaho, must be considered. In the late Precambrian Era, the West Coast, including Washington, Oregon, and California, were on the ocean basin floor and Idaho was on the miogeoclinal sediment shelf. Montana, Wyoming and the rest of North America were a shallow marine environment. As the Pacific oceanic plate subducted under the North American plate, a volcanic island arc, with a back-arc basin between the ancestral Washington-Idaho area, was built. This event occurred during the Ordovician and Early Devonian periods and created what is now called the Columbian Arc. As time progressed, the Farallon Plate began to subduct under the ocean basin floor. This created the North Cascade Island Arc, and Washington began to take shape. Further subduction of the Farallon Plate forced Washington closer to Idaho, as accretion and associated geologic forces started to build mountains, and the mountainous terrain of Idaho began to take shape.

THE IDAHO BATHOLITH

The Idaho Batholith is actually two lobes, the southern Atlanta lobe and the northern Bitterroot Lobe, separated by the Proterozoic metamorphic rocks of the Salmon River Arc (Lewis et al., 1). These two huge granitic plutons were forced to the surface during mountain building events in the late Cretaceous (75 Ma to 100 Ma). They have since been eroded and carved into a rugged and wild landscape by the forces of nature including wind, water, and ice. After the Batholith intrusion, the compressional forces began to relax. As noted by Link and Phoenix, 14), the processes of uplift and thinning of the thickened crust resulted in the high-grade metamorphic lower crust deforming against the low-grade metamorphic upper crust. This produced the metamorphic core complexes found along the length of the North American Cordillera. For example, the Pioneer Mountains are one of these metamorphic core complexes.

THE YELLOWJACKET FORMATION

The Middle Proterozoic Yellowjacket Formation is located in east-central Idaho within the Idaho Batholith (Figure 1). This formation is divided into three units, including the lower, middle, and upper units (Figure ). As noted by Evans (1), the lower unit is primarily made up of gray and greenish-gray siltite and argillaceous siltite with minor carbonate strata. The siltite is finely laminated and cross-laminated with bedding typically ranging from 5 to 0 centimeters. Directional and oscillatory ripple marks are well preserved. This unit was most likely deposited in a subaerial or shallow-water environment. The Hoodoo Quartzite, originally thought to be part of the lower unit but now recognized as a separate and distinct unit, forms a 1,100 m thick tongue in the lower unit and. Bedding is generally massive and ranges from 8 to 100 cm.

The middle unit is composed of light gray pairs of siltite or fine quartzite layers with alternating black argillite layers ranging from 0.5 to 10 cm (Evans, 1). On the east side of the White Ledge Fault, the middle unit has been preserved, but on the west side of the fault it has been removed by erosion. Silt and sand dikelets penetrate down into cracks that have formed in the argillite layers. This unit was probably deposited in a shallow marine environment.

Figure . Simplified geologic map of the Salmon River Mountains between Yellowjacket and Salmon (Evans, 1).

The upper unit is made up of light to dark gray, very fine to medium-grained feldspar and biotite quartzite with planar cross beds and a hummocky cross-stratification (Evans, 1). The bedding ranges from 10 to 100 cm thick. Ripple marks range from a few centimeters to tens of centimeters, with the high-energy features indicating a shallow marine to intertidal depositional environment.

The Yellowjacket Formation is best known for its cobalt, copper, and gold deposits mined by the Blackpine and Salmon Canyon Copper Company mines (Evans, 1). Geochemical studies indicate two stratigraphically distinct zones of mineral strata located in the upper middle unit and the lower upper unit. However, the age of the Yellowjacket Formation is still in contention. Pb07/Pb06 ratios indicate ages of 1,670 Ma and 1,700 Ma for two different samples of zircon from biotite, but U-Pb methods suggest an age of 1,70 Ma (Evans, 1). Still other dating methods indicate dates ranging from 71 Ma to 75 Ma (Evans, 1). These dating inconsistencies serve to confirm that more studies are required before reliable dates can be assigned to the Yellowjacket Formation.

CONCLUSION

The historical, geographical, and geological significance of the Idaho Batholith has been well-established by historians and geologists alike. The rugged terrain of the Batholith and the wild waters of the Salmon River are enjoyed by outdoor enthusiasts all over the world. The origin of the Batholith continues to be of great interest to professional geologists and students of geology. Understanding the stratigraphical relationships between the geological units such as the Yellowjacket Formation within the Batholith presents a challenge for geologists, and much more work is needed to accurately establish dates for these units.

REFERENCES

Evans, K. V., 1, Yellowjacket Formation of East-Central Idaho, Belt Symposium III, Montana Bureau of Mines and Geology, Special Publication 11, p. 17-0.

Lewis, R. S., Burmester, R. F., and Bennett, E. H., 1, Metasedimentary Rocks

Between the Bitterroot and Atlanta Lobes of the Idaho Batholith and Their

Relationship to the Belt Supergroup, Belt Symposium, III, Montana Bureau of

Mines and Geology, Special Publication 11, p.10-144.

Link, P. K., and Phoenix, E. C., 14, Rocks, Rails, and Trails Pocatello, Idaho State University, 18 p.

Arrington, L. J., 14, History of Idaho Moscow, University of Idaho Press, 5 p.

Alt, D., and Hyndman, D. W., 15, Northwest Exposures A Geologic Story of the Northwest Missoula, Mountain Press Publishing Company, 44 p.

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