Columbia River Basalt Stratigraphy in the Pacific Northwest
In Cooperation with the Oregon Water Resources Department
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Flow interiors are the dense massive basalt found between interflow zones and are characterized by their cooling joints. The joints are typically very narrow and 77 to >99% filled with secondary minerals (Lindberg, 1989). Voids are usually not interconnected. The two most common styles are columnar-blocky and entablature-colonnade jointing.
Columnar-blocky jointing (below) is usually associated with thinner (more fluid) flows and typically displays a mostly vertically oriented, poorly to well-formed polygonal columns that can range from 1 ft to more than 10 ft in diameter. The vertical columns are often cut by horizontal to subhorizontal cooling joints.
Columnar-blocky jointing, Frenchman Coulee, west-central Columbia Plateau, Washington (photograph by Terry Tolan)
Entablature-colonnade jointing (below) is usually observed in more viscous (thicker) flows. The entablature portion displays a pattern of numerous, irregularly jointed, small columns to randomly oriented cooling joints that abruptly overlie a thinner zone displaying well-developed columnar jointing. The transition zone between the entablature and the basal colonnade may be very narrow, generally less than 0.5 ft in width. Typically, the entablature is thicker than the basal colonnade, often comprising at least two-thirds of the total flow thickness. While the entablature-colonnade jointing style is commonly observed in CRBG flows, it is a very uncommon jointing pattern for lava flows elsewhere in the world.
Entablature-colonnade jointing near Crab Creek Valley, Lincoln County, Washington (photo by Terry Tolan)
Lindberg, J.W., 1989, A numerical study of cooling joint width and secondary mineral infilling in four Grande Ronde Basalt flows of the central Columbia Plateau, Washington, 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. 169-185.
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