The Columbia River Basalt Group—From the gorge to the sea

Abstract

Miocene fl ood basalts of the Columbia River Basalt Group inundated eastern Washington, Oregon, and adjacent Idaho between 17 and 6 Ma. Some of the more voluminous fl ows followed the ancestral Columbia River across the Cascade arc, Puget-Willamette trough, and the Coast Range to the Pacifi c Ocean. We have used fi eld mapping, chemistry, and paleomagnetic directions to trace individual fl ows and fl ow packages from the Columbia River Gorge westward into the Astoria Basin, where they form pillow palagonite complexes and mega-invasive bodies into older marine sedimentary rocks. Flows of the Grande Ronde, Wanapum, and Saddle Mountains Basalts all made it to the ocean; at least 33 fl ows are recognized in the western Columbia River Gorge, 50 in the Willamette Valley, 16 in the lower Columbia River Valley, and at least 12 on the Oregon side of the Astoria Basin. In the Astoria Basin, the basalt fl ows loaded and invaded the wet marine sediments, producing peperite breccias, soft sediment deformation, and complex invasive relations. Mega-invasive sills up to 500 m thick were emplaced into strata as old as Eocene, and invasive dikes up to 90 m thick can be traced continuously for 25 km near the basin margin. Mega-pillow complexes up to a kilometer thick are interpreted as the remains of lava deltas that prograded onto the shelf and a fi lled submarine canyon southeast of Astoria, possibly providing the hydraulic head for injection of invasive sills and dikes at depth.

Alteration, mass analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province

“…This is consistent with the petrographic observations of Benson and Teague (1982), who noted that clinopyroxene and magnetite appeared to be most affected by alteration, with plagioclase less so. This is also in agreement with the observations of Wells et al (2009), who characterized plagioclase in rocks showing evidence of chemical alteration as having a fresh appearance.…”

“…Overall various field and laboratory studies of water-rock interaction support the interpretation that Fe 2+ is mobile under anoxic and reducing conditions. The depletion of FeO with increasing mass loss is in agreement with the observations of Wells et al (2009), who attributed low FeO n (<11 wt%) in GRB lavas to alteration. The strong iron depletion is also in agreement with petrographic observations of borehole samples by Benson and Teague (1982), who noted that Fe-rich minerals, clinopyroxene and magnetite, are altered to a greater degree than feldspar.…”

“…Evarts (2008, personal commun. ) and Wells et al (2009) recognized chemical modifications to GRB lavas, notably iron depletion, and applied an empirically derived, minimum FeO threshold to screen samples deemed weathered. It is important that they noted that many flows with a fresh appearance (e.g., dark gray color, unaltered plagioclase) were nonetheless chemically altered.…”

Igneous Rock Associations 15. The Columbia River Basalt Group: A Flood Basalt Province in the Pacific Northwest, USA

“…Over the last 40+ years the CRBG has been extensively analyzed for major and minor oxides, trace elements and isotopes and, with these data along with other field criteria, have been used to establish a regional-scale, mappable stratigraphy (e.g. Wright et al 1973Wright et al , 1989Swanson et al 1979;Reidel 1982Reidel , 1998Reidel , 2005Beeson et al 1985;Mangan et al 1986;Reidel et al 1989a, b;Wells et al 1989Wells et al , 2009Hooper 2000;Hooper et al 2007). The geochemical and isotopic data have also been invaluable for understanding the origin of CRBG magma (e.g.…”

Cretaceous basalts of the High Arctic large igneous province at Axel Heiberg Island (Canada): Volcanic stratigraphy, geodynamic setting, and origin

“…Particularly notable at the East Fiord section is the occurrence of two invasive flow units at the base of the section (“i1” and “i2” in Figures c and ), first recognized by Muecke, Williamson, and MacRae (). Invasive flows represent initially surface‐erupted flows that plough into and intrude soft sediments due to their higher relative density (Wells, Niem, Evarts, & Hagstrum, ). At East Fiord, these flows are the first in the succession and include large (tens of metres) sediment inclusions/rafts and metre‐scale lobe‐like and pillow‐like structures at protrusions (apophyses) of the basaltic flow into the overlying dark mudstones with localized blocky basaltic clasts at the contacts (Figures c,d).…”