The Columbia Basin, also known as the Columbia Plateau, is a vast area in eastern Washington, southwestern Idaho, and northern Oregon. The physiographic province is characterized by incised rivers, extensive plateaus, and anticlinal ridges rising to 4,000 feet above sea level. The region is underlain by Miocene Columbia River Basalt Group rocks and interbedded Neogene terrestrial sediments.
Data about what lies under the Columbia River basalts are sparse. Along the Idaho border south of Spokane, steptoes that once were mountain tops consist of Precambrian Belt Supergroup sedimentary rocks and metamorphosed Cretaceous granites. These mountains were enveloped by Miocene basalts so that only the summits remain above the lava flows. Deeply weathered granites support a clay mining industry, and a cassiterite deposit is known just south of Spokane.
Even less is known about the pre-Miocene basement in the central and western parts of the Columbia Basin. The only information available is from seven petroleum exploration wells that have penetrated the basalt and from projections of geology from the margins of the basin. Along the margins, Paleogene fault-bounded basins are filled with thick sequences of arkose, volcaniclastic rocks, and coal. Drilling has demonstrated that in a general way these sedimentary basins extend southward under the Columbia River basalts. The subsurface geology changes as one reaches the Snake River. A 1987 exploratory well drilled 20 miles northeast of Pasco penetrated a thin Paleogene crystal tuff section before encountering Triassic or Jurassic chloritic metamorphic rocks at an approximate depth of 8,000 feet.
The Columbia basin province is best defined by the areal extent of the Miocene Columbia River Basalt Group rocks. These basalts, which are present in the Blue Mountain uplift as well as in the Columbia Basin, cover 36 percent of the entire state. The group consists of four flood basalt formations, starting with the Imnaha Basalt at 17.5 Ma, followed by the Grande Ronde Basalt (16.5 to 15.6 Ma), the Wanapum Basalt (15.6 to 14.5 Ma), and lastly the Saddle Mountains Basalt (14.5 to 6 Ma). On the basis of geophysical evidence, the basalts are known to reach a maximum thickness of 16,000 feet in the Pasco Basin. An exploratory gas well (BN 1-9) drilled southeast of Vantage penetrated 11,500 feet of these basalts before entering Paleogene sediments. The Columbia Basin province is modest in size in comparison with other continental flood basalts such as the Deccan basalts of India; however, the basin has been studied in greater detail than any other such basalt accumulation in the world. This is true because the basalts were once seriously considered for deep geologic disposal of the nation's high-level nuclear waste.
The four formations of the Columbia River Basalt Group consist of 38 units or members that in the three-state area (Washington, Idaho, and Oregon), cover 63,208 square miles (163,700 km2), and have a volume of 41,820 cubic miles (174,300 km3). The greatest volume of basalts was erupted before 15.5 Ma. These flows have similar appearances; techniques have been developed, however, to fingerprint individual basalt units using whole-rock geochemistry and magnetic polarity. Within the Grande Ronde Basalt, individual flows exceed 480 cubic miles (2,000 km3) in volume. The flows were extruded from vents and northwest-trending fissures east of Pasco and in the southeast corner of the state. The flows were extremely fluid, and as a result a number of them reached the Pacific Ocean via the ancestral Columbia River drainage.
There were periods of quiescence between major extrusive events. Erosion would take place, and tuffs, sandstones, and conglomerates would be deposited on top of basalt flows. In some areas, lake beds formed. Forests developed during these periods. Another basalt flow would follow, infilling canyons, engulfing trees, and covering lake sediments. These sedimentary horizons have such names as the Latah Formation (near Spokane), the Vantage Interbed with its opalized and silicified wood, and the Ellensburg Formation consisting of volcaniclastics shed from the growing Cascades. The state is a major producer of diatomite from lake beds that were covered by flows. Ginkgo Petrified Forest State Park is an excellent place to view petrified trees. In 1975, the state legislature designated petrified wood as the state gem.
In the western portion of the Columbia Basin, the flows of the Columbia River Basalt Group have been folded into a series of giant anticlines that strike east-west to southeast-northwest. This region is called the Yakima Fold Belt subprovince. The anticlines have steep dips north of the fold axis and overturned beds in numerous locations. The south flanks of the anticlines have gentle dips. Folding was initiated during middle to late Miocene time and has continued to this day. Crossing the Columbia Basin is a northwest-trending tectonic feature called the Olympic Wallowa Lineament. Right-lateral displacement on this transform structure influenced the formation of the Pasco Basin.
The Rattlesnake Hills anticline produced 1.3 billion cubic feet of natural gas from the Columbia River Basalt Group during the years 1929-1941. The state drilling depth record was reached by well BN 1-9 on the Saddle Mountains anticline with a total depth of 17,518 feet. It took from 1982 to 1984 to drill and test the well. This well encountered considerable natural gas but not enough to make production profitable. Besides natural gas, the Columbia Basin is also known for deep ground-water circulation bringing heated waters close to the surface. Such waters are being utilized for geothermal heating purposes at Ephrata, Othello, Yakima, and Walla Walla.
During the Pliocene and the Pleistocene, gravel, sand, silt, and clay were deposited in lakes or by aggrading streams and rivers in depressions such as the Pasco Basin, where 1,000 feet of sediment lies on top of the basalt. Glacial outwash during the Pleistocene produced huge volumes of wind-blown silt called loess. It blankets much of the Columbia Basin and in places is up to 200 feet thick. The loess-rich soils of the Palouse subprovince provide ideal conditions for growing wheat, making southeast Washington one of the major grain-producing regions of the world.
The Columbia Basin was the scene of the greatest catastrophic floods ever documented in the geologic record. The Pleistocene Cordilleran ice sheet advanced south into Idaho, damming the Clark Fork River at the Montana border. A huge impoundment, called Lake Missoula, formed. The lake had the volume of present-day Lake Michigan and was 2,000 feet deep at the dam. The ice dam repeatedly gave way between 12,700 and 15,300 years ago, releasing waters that caused unprecedented flooding. Water raced down the Spokane Valley and spread out over the Columbia Basin. The maximum flow rate is estimated at 15 cubic miles (62.5 km3) per hour--a rate 15 times the combined flow of all the rivers of the world. During the floods the surface of the land was greatly modified. Anastomosing channels were cut through the loess blanket and into basalt, leaving a jumbled topography of coulees, buttes, mesas, dry water falls, hanging valleys, and giant ripples. These geomorphic features are known collectively as the Channeled Scablands. The events are called the Great Spokane Floods.
The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.