Major Rock Types of the Spokane Area

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Here is a generalized stratigraphic diagram illustrating the major geologic units of the Spokane region:

(diagram)

The Basement: The oldest rocks in the area make up what geologists collectively term the “basement”. These very old metamorphic rocks of Precambrian age (1.4 BY+), include the Hauser Lake gneiss (Weis, 1968), and are what all younger rocks in the Spokane area either sit upon or intrude. These high-grade metamorphic rocks often contain the mineral sillimanite and commonly exhibit intense deformation either in the form of isoclinal folding or mylonitic fabric development. Griggs (1973) correlated the protoliths of these metamorphic rocks to the Pritchard Formation of the lower Belt Supergroup; the actual age of metamorphism is still somewhat problematic.

Plutonic Rocks: Abundant plutonic rocks ranging from quartz monzonite to granite in composition can be seen from the west side of Mt. Spokane through the Mead and Dartford areas (Weissenborn & Weis, 1976, Derkey, et al., 1998). The plutonic rocks contain large crystals of quartz, feldspar and mica, with some of the associated pegmatite dikes containing abundant red garnets. These rocks range from Cretaceous to Eocene in age (Derkey, et al., 1998).

Latah Formation: Sitting unconformably upon the crystalline basement rocks are the weakly lithified sedimentary rocks of the Miocene-aged (14-16?Ma) Latah Formation. The Latah is easily identifiable by the presence of locally kaolinite clay-rich layers and abundant plant fossils (Pardee & Kirk, 1926). Some of the better fossil collecting localities are found within Deep Creek of Riverside State Park near the 7-Mile area north of Spokane. Patient attempts can yield nearly complete sequoia, maple and cypress fossils suggesting that Miocene climates of the Spokane region were indeed different than the present. Studies of the Latah sediments and flora indicate a shallow lake-type depositional environment.

Columbia River Basalts: Lava flows of the Columbia River Basalt Group occur throughout the Spokane region. These fine-grained basalt lavas were the result of voluminous outpourings of lava from fissures south of Spokane and Cheney from approximately 17 Ma to 6 Ma (Hooper & Swanson, 1987). The “CRB’s “, as they are often termed, typically sit on top of the Latah Formation but in some localities occur as invasive flows into the Latah (Robinson, 1991). An excellent locality to view an invasive flow is along the abandoned rail line just south of Thorpe Road in the SW1/4 of S25, T25N, 42E.

Missoula Flood Deposits: Unconsolidated Pleistocene-aged (approx. 10-20,000 years) gravel and sand deposits underlie much of the Rathdrum Prairie (east of the Spokane Valley), Spokane Valley and the Little Spokane River basin to the north. The Spokane aquifer, the sole source of drinking water for Spokane, is composed of these deposits. The flood deposits are also the major source for construction aggregate materials in the region. The great Missoula Floods were the result of numerous catastrophic outbursts of water from ancient Lake Missoula to the east. Volume of the peak floods has been estimated at approximately 10 times all of the world’s rivers combined flow! These floods inundated the Spokane Valley with huge torrents of water that ultimately flowed west across eastern Washington eroding the famous Scablands channels. For an excellent discussion on the Missoula Floods and the formation of the Spokane aquifer, see Molenaar, 1988.

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