Geology of the Nez Perce Reservation
The Nez Perce Reservation lies within the confines of the geologic area known as the Clearwater Embayment or the Clearwater Plateau Sub-area. The Clearwater Embayment is one of the many geologic Sub-areas of the Columbia River Basalt Group. Making up the Clearwater Sub-area's 1.2 million acres are Idaho, Clearwater, Lewis, and Nez Perce Counties (Haggen IDWR 2000-2002).
Prior to the Miocene Epoch, the Clearwater Sub-area was constructed of Permian and Triassic sedimentary and volcanic rock from the Seven Devil's Complex, as well as Cretaceous granite from the Idaho Batholith. The Sub-area's basement rock is composed of the Precambrian Belt series metasedimentary complex. Towards the southwestern boundary this Belt series has been exposed in canyons by stream activity. Exposed deposits include metamorphosed rhyolite and pyroclastics, sand, silt, clay, gravel, and boulders. Some of these deposits, such as the gravel and clay, hold economic value.
Columbia River Basalt Group (Miocene)
Dominating the geology of the Clearwater Sub-area are the Columbia River Basalts. Although the greater Columbia River Basalt Group has been intensively studied, research specific to the Clearwater Sub-area is rather limited.
The Columbia River Basalt Group is composed of over 170,000 km3 of basalt erupted from 300 identified high volume basalt flows over a period of 11.5 million years (17.5-6.0 mya). Eighty-seven percent of these eruptions occurred over a period of 1.5 million years (17-15.5 mya). The Group is divided into five formations; the Imnaha Basalts, Grande Ronde Basalts, Picture Gorge Basalts, Wanapum Basalts, and Saddle Mountains Basalts. Each formation is separated by layered internal structures of porous, rubble-like flow-tops, variably jointed columnar basalt, and variably fractured flow bases. The Basalt's mineral composition mainly consists of plagioclase, pyroxene, and glass. Underlying the Sub-area are the Saddle Mountains, Wanapum, and Grande Ronde Basalts. Small portions of the older Picture Gorge Basalts are also found. Between layers of basalt are clay, sand, and gravel deposits, similar to the pre-Miocene deposits. These deposits are known as the Latah Beds, the result of eroded sediment deposited in bodies of water between basalt flows. As subsequent flows covered the bodies of water, the sediment was trapped, forming the current deposits of silt, clay, and gravel.
Faults and Folds
Following the deposition of Columbia River Basalts, the Clearwater Plateau experienced a period of structural deformation from the late Pliocene to early Pleistocene time period. Deformation from tilting and faulting created such features as the east trending Lewiston syncline and the northeast trending Craig Mountain anticline, which is dissected by the Limekiln Fault (also known as the Lime Point Fault and the Waha Escarpment). One other prominent fault is the Mount Idaho Fault, which is a northeast trending feature that forms the southern boundary of the Clearwater Plateau.
To further investigate the geology of north central Idaho or the Columbia River Plateau follow the links below.