Geology, Age and Extent of the Columbia River Basalts
The Columbia River Basalts were deposited between 17.5 and 6 million years ago and cover an area of approximately 164,000 km2, (see Figure 1). Five main episodes of volcanism occurred in western Idaho , central and southern Washington , and northern Oregon. The Imnaha Basalt was deposited first, followed by the Picture Gorge Basalt, the Grande Ronde Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt, (see Figure 2). It is believed that the fronts of the lava flows were several stories (approximately 30 meters) high as they flowed from the eruption center at speeds up to three miles per hour (Alt and Hyndman, 1985). These basalts erupted from long fissures in the ground, not from volcanic cones.
The first episode of volcanism produced the Imnaha Basalt. These lavas were erupted between 17.4 and 17.0 million years ago. At places in western Idaho the Imnaha Basalt is 900 meters thick, filling Miocene paleocanyons of the Clearwater River System.
The Imnaha Basalts account for approximately 5% of all the Columbia River Basalts, see (Figure 3). This type of basalt is characterized by visible grains of green olivine and greenish-white plagioclase. When it weathers it becomes rounded and smooth..
The Picture Gorge Basalt is not extensive in Idaho. The formation was deposited between 16 and 15 million years ago and is very limited in volume and is mostly covered by more recent flows. Some geologists believe that the Picture Gorge Basalt was emplaced during the same time as the Grande Ronde Basalt, leading them to believe that the Picture Gorge Basalt originated from a different source than the overlying Grande Ronde Basalt.
Accounting for approximately 90% of all the basalt in the Columbia River basalts, the Grande Ronde Basalt is the most extensive; covering 163,700 km2 (see Figure 4). The formation was deposited between 16.5 and 15.6 million years ago and buried many of the older flows. The weight of all the massive basalt caused the crust to subside, forming the Columbia embayment, or basalt-filled basin.
The Grande Ronde flows originated in southeast Washington and northeast Oregon in the Chief Joseph Dike Swarm, (see Figure 5). Some dikes are over 25 feet thick. Over 120 flows occurred during this episode producing tholeiitic basalt with no olivine and very few crystals visible. Glass, plagioclase and augite are the most common minerals in these basalts. When the Grande Ronde Basalt weathers, the basalts often produce sharp columnar jointing patterns that formed as the basalt cooled.
Figure 4. Map of Grande Ronde Basalt coverage area. Modified from Alt and Hyndman, 1985.
Figure 5. St. Joseph dike swarm. Simplified from Alt and Hyndman, 1985.
The Wanapum Basalt was deposited between 15.6 and 14.5 million years ago. Although it accounts for only approximately 5% of the total volume of Columbia River basalts, the formation onlapped much of the underlying Grande Ronde basalts, (see Figure 6). The Wanapum basalts are more silica rich than the older basalts, 58% vs. 56%. The appearance of these basalts varies. Some contain large prisms of pale green plagioclase as well as crystals of glassy olivine while others contain no olivine and only the green plagioclase.
Figure 6. Map of Wanapum Basalt coverage area. Modified from Alt and Hyndman, 1985.
The most recent event in the formation of the Columbia River Basalts was the deposition of the Saddle Mountains Basalt. These basalts flooded areas where subsidence was occurring due to the weight of the Grande Ronde Basalt. Although these basalts cover a large area, they account for less that 1% of the total volume of basalt and occur as very thin layers of basalt, (see Figure 7). The Saddle Mountains Basalt is even more silica rich than the Wanapum Basalt and also more deformed due compression and extensional events as the basalt cooled.
Figure 7. Map of Saddle Mountains Basalt coverage area. Modified from Alt and Hyndman, 1985.
According to paleomagnetic data, the Imnaha Basalt was deposited during a period of normal magnetic polarity. While the Picture Gorge Basalt was being deposited, a period of transition occurred. The paleomagnetic data shows that a reversal occurred before the Grande Ronde Basalt was formed, see Figure 2.
Methods of Lava Emplacement
The traditional view of lava emplacement holds that the lava flowed quickly from the point of eruption to where it cooled, taking only hours or days to cool. This view takes into consideration that there may be no present-day equivalent to the formation of the Columbia River Basalt Group. It is based on the texture and flow structures of the lava. According to Shaw and Swanson (1970), the amount of crystallization does not change from the fissure zone to the farthest reaches of each lobe of lava.
The newest idea of lava emplacement comes from Self and Thordarson (1996). They base their theory on the lava eruptions occurring presently in Hawaii from inflation of a ponded lava lake. The lava reaches the surface of the earth and then flows through lava tubes towards the outer edges of the lobe. As the lava flows, it inflates. Lava is added to the interior of the system. In order for the lava to flow, new lava must break through the exterior of the flow. This method allows the lava to flow long distances because it is insulated by the already cooling lava. Lava flows of this nature could take months or years to stop flowing and cool.
Both methods however, allow for cooling to occur, from both the bottom and the top of the flows. Two different jointing patterns occur in the Columbia River basalts to show the different cooling patterns. Cooling from the top down is shown by entabulature jointing. This jointing pattern occurs when very fine-grained basalt cools. The spacing between joints is approximately 10 cm. Cooling from the bottom up is evidenced by collonnade jointing which occurs when coarser basalt cools. Spacing in collonnade jointing is approximately 1 meter (Reidel and others, 2003).
Yakima Fold Belt
Folding and normal faulting was occurring at the same time as the Columbia River basalts were forming and continued after the end of the volcanic events. The Columbia Basin is broken into three major divisions; the Yakima fold belt, the Palouse subprovince and the Blue Mountain subprovince. Most of the tectonic activity took place in the Yakima fold belt (see Figure 8).
The Yakima fold belt covers 14,000 km2 of the western Columbia Basin. The fold belt formed after 10.5 million years ago from north-south compression. It is characterized by narrow ridges formed by anticlines and broad valleys formed by synclines, most of the anticlines in the southern half of the fold province have N50°E trend. Wavelengths of the folds range from 2-3 km to 20 km. Structural relief in the fold belt is typically less than 600 meters but varies laterally. The fold belt is cross-cut by faults in several places. After the belt formed it controlled the thickness of future basalt eruptions (Reidel and others, 2003).
Figure 8. Map showing Olympic Wallowa Lineament and the Yakima Fold Belt Subprovince. Simplified from Reidel and others, 2003.
Sedimentation in the Columbia River Basalt Region
Drainage patterns on the entire plateau changed because of the Columbia River Basalts. Smaller rivers could not cut through the basalt and many of these smaller rivers were dammed up by the basalt and formed lakes and ponds, such as Clarkia Lake in western Idaho, (see Figure 9), (Orr and Orr, 2002). These lakes and ponds became prime locations for the deposition of sediments as well as the deposition of plant and animal remains. Many plants, mammals and insects thrived in the areas surrounding these lakes. Soil often formed around the lakes between eruptions. Larger rivers also could not cut through the basalt. However, they re-routed themselves to travel alongside the edge of the basalt flows. The Snake River moved south several times during the formation of the Columbia River Basalts.
Sedimentary basins also formed during this time period. Three formed partially in Idaho; one near what is now Coeur d'Alene, one near Lewiston, and one near Weiser. Slow streams deposited many sizes of sediments into these basins. Volcanic ash from Miocene volcanoes also settled in the basins.
Lava also dammed rivers and created lakes; the most important of these lakes is Clarkia Lake. Clarkia Lake sediments are richly fossiliferous with abundant plant megafossils, insects, fish, mollusks, and freshwater sponges. (A 30 cm by 45 cm by 8 m section contained 10,749 identifiable plant megafossils.) The lake and its deposits formed mainly in the Neogene in central-western Idaho. Most fossils are found in bedding planes within the formation. The sediments are mostly finely laminated clays and silty clays which may indicate lacustrine varves formed seasonally in the lake. Layers of volcanic ash are also present. Most sediment layers fine and lighten in color upward and are capped by a layer of organic rich sediments (Smiley and Rember, 1979).
The fossils from Clarkia Lake are very well preserved, indicating little movement of bottom water as well as an anoxic environment on the bottom of the lake. Many plant fossils found in the Clarkia beds are preserved so well by the anoxic conditions that they retain original colors until exposed to oxygen. Fish fossils from the Clarkia area also exhibit indications of original color. The extraordinary preservation of these organisms indicates that as well as anoxic conditions at the bottom of the lake, there was little disturbance along the bottom to disrupt sediments or cause soft sediment deformation. The plants found in the fossil deposits indicated that this area had a humid climate, much like that of the present-day southern Appalachians. Plants include; bald cypress, poplar, alder, birch, chestnut, oak, tulip tree, magnolia, laurel, sycamore, and maple (Smiley and Rember, 1979).
Figure 9. Map showing approximate location of Clarkia Lake in western Idaho . Simplified from Smiley and Rember, 1979.
Alt, D. and Hyndman, D.W., 1995, Northwest exposures, a geologic story of the northwest, Chapter 34: Floods of Basalt: Mountain Press Publishing Company, Missoula, MT, p. 241-248.
Orr, W. N. and Orr, S. L., 2002, Geology of the Pacific Northwest, 2nd Edition, Chapter 10: Columbia River Plateau: McGraw-Hill Higher Education, New York, NY, p. 238-255.
Reidel, S.P., 1998, Emplacement of Columbia River flood basalt: Journal of Geophysical Research. Solid Earth, vol. 103, p. 27,393 – 27,410.
Reidel, S.P. and others, 2003, The Columbia River flood basalts and the Yakima fold belt, in Swanson, T.W. ed., Western Cordillera and adjacent areas: Geological Society of America Field Guide 4, p. 87 – 105.
Reidel, S.P. and others, 1989, The Grande Ronde Basalt, Columbia River Basalt Group; Stratigraphic descriptions and correlations in Washington, Oregon, and Idaho, in Reidel, S.P. and Hooper, P.R., eds., Volcanism and Tectonism in the Columbia River Flood-Basalt Province: Geological Society of America Special Paper 239, p. 21-53.
Smiley, C.J. and Rember, W.C., 1979, Guidebook and road log to the St. Maries River (Clarkia) fossil area of northern Idaho , Idaho Bureau of Mines and Geology Information Circular 33.
Smiley, C.J, and Rember, W.C, 1985, Physical setting of the Miocene Clarkia fossil beds, northern Idaho in Smiley, C.J, ed., 1985, Late Cenozoic History of the Pacific Northwest, interdisciplinary studies on the Clarkia fossil beds of northern Idaho; Allen Press, Lawrence, Kansas, 417 p.
Continue to Module 11 Snake River Plain Volcanics