WASHINGTON STATE U.S.A
Beneath the Flood Basalts of the Spokane Valley are the Precambrian sedimentary basement rocks of the Belt Supergroup, these rocks are visible above the basalt of the Columbia Plateau towards the far northeastern corner. The Belt Supergroup has been intruded by many widespread granite plutons, these were relatively young only being intruded in the Mesozoic (251 - 65 Million years ago) and Tertiary (66 - 2 Million years ago). A number of the plutons have combined into a batholith in the northwest section of the plateau, the Belt Supergroup has also been intruded by mafic dikes and sills of Precambrian, Mesozoic, and Tertiary age but they make up a minimal amount of the basement rocks.
All the basalt in the region is a part of a continental flood basalt province called the Columbia River Plateau and is the youngest flood basalt province on the planet, 99% of the eruptions are estimated to be between17 to 15 million years old in the Miocene epoch of the Quaternary period.
The Miocene Columbia River Basalt Group formed from fissures and several dike swarms located all throughout the Columbia Plateau which covers a 210,000 km3 region with dikes at an average of 8 meters in width and 100 to 1,000 meters in length. Basalt in the Spokane region erupted from the Chief Joseph Dike Swarm, located in the north end of the easternmost corner of all documented Dike Swarms.
The younger sediment in the Spokane Valley is comprised of a thin layer of Pleistocene glacial outwash from major flood events from paleo-lake Missoula. During the Pinedale Glaciation starting from 2.2 million years ago, the Cordilleran Ice sheets Purcell Trench Lobe and the Eastern Lobe dammed the Clark Fork River in a number of places which over time formed Lake Missoula in current day Montana. The lake had an estimated maximum volume of 2500 km3. The lake formed behind a major ice dam, that dam later catastrophically failed releasing water at 65 miles per hour, this would have drained the large lake in a matter of days. As the Cordilleran Ice Sheet advanced the ice dam returned, this dam built up and collapsed several times.
CHIEF JOSEPH DIKE SWARM
Dikes are a primary pathway for magma reservoirs at depth and the upper crust to reach the surface, dikes are the most common way that magma breaches the surface in all different volcanic regions, these are associated with large igneous provinces. The connection between dikes and lava flows still remains poorly understood and exposures are usually not in good condition in the Spokane region.
The Chief Joseph Dike Swarm is 50 miles (80 km) wide and trends to the northwest along with almost every dike in that swarm, it's also considered the largest in the plateau with an estimated 21,000 individual dikes. The dike swarm most likely extends into the southern Spokane region but exposures are limited as areas have been eroded out which makes finding exposures difficult. Only the uppermost section of the Columbia River Group which is the Yakima Basalt is visible in south-central Washington State.
The northernmost dike is visible in exposures along the canyons of the Snake River, from there it turns west into southeastern Washington. The area south is most likely the source of the Roza Member which is not visible anywhere near Spokane but it's still a member of the Yakima Basalt.
The Yakima Basalt is the upper formation of the Columbia River Group and was named in 1901. The composition of dikes in the Chief Joseph Swarm shows that most were feeders for the Lower and Middle Yakima Basalt Flows, these basalts have been exposed from major flood events.
The entire thickness of the basalt is unknown but its deepest recorded depth is currently is around 4,500 feet in a test well that was drilled within the Odessa area, towards the centre of the Columbia Plateau it most likely thickens and becomes even deeper.
The Yakima Basalt in the Spokane Valley is around 715 - 1000 feet thick interbedded with 25 - 45 feet of sedimentary material. This information was collected from several city water wells from inside the city and the outskirts of town.
The Yakima Basalt is comprised of 5 Members and 2 Beds:
.Vantage Sandstone Member
This is the oldest member of the Yakima Basalt, the member is comprised of a medium, friable quartz-feldspar mica sand and a weak cemented tuffaceous silt, clay and sand which also contains hornblende. The member has been oxidized and is a pale yellow where it has been exposed but is blue or green below. In the majority of locations, arkosic sand has been overlain by tuffaceous sedimentary rocks. Poorly carbonized twigs and other plant impressions are common in a black earthy zone at the top of the tuffaceous unit, the member is around 35 feet to 2 feet thick.
.Frenchman Springs Member
The Frenchman Springs Basalt is dark grey to black in colour with medium to fine-grained and sparsely porphyritic. The base layer of the member is comprised of a widespread zone of pillows, and palagonite, it also contains plagioclase, monoclinic pyroxene, opaque minerals, olivine, and a large amount of glass. The member is 250 feet thick and drops in thickness towards the north and northeast.
.Squaw Creek Diatomite bed
The bed is also present on top of the basalt of the Frenchman Springs Member, the bed was most likely deposited within a lake that was impounded by the Sentinel Gap Flow. It is 5 to 17 feet thick and grades westward into sandstone and pebble conglomerate which is composed of partly tuffaceous and partly granitic detritus.
The member consists of two unnamed flows, the lowest flow is relatively uniform in composition and thickness and covers an area of 20,000 square miles. The lower basalt is dark blue/grey and weathers to a deep red-brown colour, it is medium to coarse-grained, monoclinic pyroxene and porphyritic which contains plagioclase, it also contains iron-rich olivine, glass, and opaque minerals. The lowest flow mostly is columnar in thickness, with columns up to 5-10 feet in diameter which break into chips and slabs along horizontal upward grading platy joints normal to the visible columns, the upper zone is swirling plates.
.Priest Rapids Member
Above the Roza Member, four south-dipping flows on a homocline to the west of a reservoir near Priest Rapids. The member has a thickness of 220 ± 25 feet there, it thins to the north and eastward but thickens to the south towards Mabton. Basalt in the member is greyish-black, medium to coarse-grained, and nonporphyritic. Small phenocrysts are visible in some flows. The basalt weathers very easily compared to the other members, where it has recently broken a distinctive greenish cast is also visible which is caused by saponite and chalcopyrite. Advanced weathering is obvious as the basalt takes on a mottled brown with the outer surface is a very distinctive red/brown. The basalt contains monoclinic pyroxene, plagioclase, olivine glass, and opaque minerals.
.Quincy Diatomite Bed
The Quincy Diatomite Bed is a part of the Priest Rapids Member, it separates the Roza Member from the Priest Rapids Member above. To the southwest of the basin, the bed is 25 feet thick but up against the Frenchman Hills, it thickens to 35 feet thick. The bed varies laterally with diatomite, clay lenses, silt, layers of banded opal, and even opalized wood. The deposition environment was most likely a lake or a number of lakes, the deposition area must have been extremely extensive. The Diatomite bed is mostly visible in the Quincy Basin and was deposited in paleo-lake Quincy, the lake was impounded by one of the four Priest Rapids basalt flows and was most likely destroyed by another. Trees growing along the shoreline and floating in paleo-lake Quincy were then engulfed by the first flow of the Priest Rapids Member, these trees have been petrified with logs and stumps, some in an upright position.
.Saddle Mountains Member
Majority of the member ranges from black to light or dark grey in colour. Columnar Jointing is also visible, and the flow contains reddish subangular pebble and cobble-sized fragments. There have been four flows identified and the Saddle Mountains Member is intertongued with the below Ellensburg Formation.
WATERFALLS & RAPIDS
Within the Spokane Riverfront Park and along the Spokane River some rapids and waterfalls have formed from the erosion of the Yakima Basalt, and from major flood events. Columnar Basalt visible next to the waterfalls and rapids fractures in a way that creates steep drop-offs in some locations.
Water has exploited fractures in the basalt and has slowly started to erode backwards into Idaho, in the next few million years the falls will be gone from this location altogether.
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A. B. Griggs. (1976). The Columbia River Basalt Group in the Spokane Quadrangle, Washington, Idaho, and Montana. United States Geological Survey (USGS).
Laura DeGrey, & Paul K. (n.d.). Lake Missoula Floods. Digital Geology of Idaho.
Morriss, M. C., Karlstrom, L., Nasholds, M. W. M., & Wolff, J. A. (2020). The Chief Joseph dike swarm of the Columbia River flood basalts, and the legacy data set of William H. Taubeneck. Geosphere, 16(4), 1082–1106.
James W. Bingham, & Maurice J. Grolier. (1966). The Yakima Basalt and Ellensburg Formation of South-Central Washington. United States Geological Survey (USGS).