The Oregon Basin and Range is the northern region of the Basin and Range Province, which extends from Oregon all the way into Mexico. Covering eight states, the Basin and Range is continually undergoing tectonic displacement. The process began about 20 m.y.a. with the final subduction of the Farallon Plate and the East Pacific Rise. This event halted the subduction process and created stress along the west coast as the Pacific Plate began to collide with the continent and pull it in a northwesterly direction.
The force from the moving seafloor stretched and thinned the continental crust creating a series of faults in the inland areas. Elevated temperatures from magma under the inland margin reduced the density of the crust making it more buoyant. As a result of the loss in density, uplifting occurred along the fault lines causing one side to rise and tilt and the other to drop. This combination of stretching and uplift occurred over millions of years to create an expanse of fault scarp mountains and long flat valleys covering a third of the western states.
The Sights of Oregon's Outback
Oregon's Basin and Range, known as the Oregon Outback, extends from the Cascade Mountains to the Owyhee Uplands in the east. It is bordered by the High Lava Plains to the north and runs south into California and Nevada. Most of the rocks in this region are thick layers of basalt which flooded the area between 20 and 16 m.y.a. Numerous eruptions from the Steens volcano produced flows of lava that covered thousands of square miles and in some areas is over a hundred feet thick. Tectonic displacement of the region began taking place about 10 m.y.a. creating the Brothers Fault zone and such prominent features as Abert Rim, Hart Mountain and Steens Mountain.
Abert Rim & Lake Abert
Located about 20 miles north of Lakeview, Oregon, USA, Abert Rim is one of the highest fault scarps in the U.S. Rising to an elevation of 7,548 feet, the rim is considered one of the longest exposed scarps in North America. At the base of the rim is Lake Abert, a shallow akali lake that is fed by Chewaucan River. Having no outlet, the lake contains high concentrations of sodium carbonates and alkali, which forms as white crystals along the shoreline. During the Pleistocene Epoch, a large pluvial lake stretched northwest from the rim's base covering over 461 square miles.
Agates at Hart Mountain
Hart Mountain is a fault-block mountain that sits about 20 miles southeast of Abert Rim. Considered as a single mountain, its highest elevation is 8,017 feet at Warner Peak. The mountain and its eastern basin were designated as a National Antelope Refuge in 1936 and is home to pronghorn antelope and bighorn sheep. The Warner Valley spreads out towards Abert Rim on the west side. At the western base of the mountain sits the Warner Wetlands Reserve, which is managed by the Bureau of Land Management (BLM).
Quality specimens of agate and jasper can be found at Flook Lake on the east side of the mountain, while fire opal and porcelain jasper may be found in some of the ravines on the west side. Digging is not allowed in the refuge. However, up to 7 lbs of surface material can be collected per person on a daily basis.
Sunstones at Rabbit Hills
Rabbit Hills lies in between Abert Rim and Hart Mountain and is the primary location for Oregon Sunstones. Sunstones are phenocrysts (large crystals) of plagioclase feldspar that are embedded within the basaltic rock. Designated as the state Gem of Oregon, sunstones range in color from yellow to green to red. Microscopic crystals of copper within the stones create a "schiller" effect, which produces a luster or glow emanating from within the stone. There are three mines that operate in the area that allow the public to dig for a fee. However, the BLM has four square miles set aside for free collection. Digging by hand is allowed, but mechanical digging equipment is prohibited.
Dry Creek Jasper at Barnes Rim
Barnes Rim is located about 23 miles west of Lakeview, Oregon, USA in the Fremont National Forest. The rock here is rhyolite from Quartz Mountain. Good specimens of agate, jasper and some opal can be found on the east side of the rim in the Dry Creek area. Nice quality Fire Opal can be mined for a fee at Juniper Ridge on the western side. The mine operates by reservation only and takes only small groups of people, who must be escorted to the site.
The Rugged Steens Mountain
Steens Mountain sits about 30 miles north of the Oregon/Nevada border. At 9,700 ft, it is the highest fault-block mountain in southeastern Oregon. Deep cut ravines and trenches show strong evidence of glacier activity during the last ice age. The majority of the rock is basalt. However, large areas of andesite cover the eastern face and pockets of granite can be found in the south. Thundereggs, agate and jasper can be found on the east side, north of the Alvord Hot Springs. In the ravines to the northwest, the basalt contains large flat crystals of feldspar that appear in fascinating designs.
An Actively Hot Spot
Though hidden from view, the ground underneath Oregon's Basin and Range is volcanically active as is evidenced by the large numbers of hot springs that are found throughout the entire region. Whether the activity is fueled by the Juan De Fuca Plate or the Yellowstone Hot Spot is still open to debate. The fact remains that the land is presently in a state of quiet unrest. Each year uncovers more of its hidden wonders in the form of beautiful gems, fascinating minerals and unusual rocks.
References:
Alt, David D, and Hyndman, Donald W. Roadside Geology of Oregon, Missoula: Mountain Press, 1978. Print.
Romane, Garret, Gem Trails of Oregon, Gem Guides Book Co., Baldwin Park, 2008
Licciardi, Joseph M., Chronology of latest Pleistocene lake-level fluctuations in the pluvial Lake Chewaucan Basin, Oregon, USA, Journal of Quaternary Science, John Wiley & Sons LTD, 2001
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