The Columbia River flood basalt province: Stratigraphy, areal extent, volume, and physical volcanology

Abstract

The middle Miocene Columbia River Basalt Group is the youngest and smallest continental fl ood basalt province on Earth, covering over 210,000 km 2 of mainly Oregon, Washington, and Idaho, with an estimated basalt volume of ~210,000 km 3 . A well-established regional stratigraphic framework built upon six formations contains numerous fl ows and groups of fl that can be readily distinguished by their physical and compositional characteristics, thus producing mappable units, the areal extent and volume of which can be calculated and correlated with their respective feeder dikes. The distinct physical features that help to defi ne these units originated during their emplacement and solidifi cation, as the result of variations in cooling rates, degassing, thermal contraction, and interaction with their paleoenvironment. Columbia River Basalt Group fl ows can be subdivided into two basic fl ow geometries. Sheet fl ows dominate the basalt pile, but the earliest fl ows comprising the Steens Basalt and some of the Saddle Mountains Basalt fl ows are compound fl ows with elongated bodies composed of numerous, local, discontinuous, and relatively thin lobes of basalt lava. The internal physical characteristics of the voluminous sheet fl ows are recognizable throughout their extent, thus allowing mechanistic models to be developed for their emplacement. The emplacement and distribution of individual Columbia River Basalt Group fl ows resulted from the interplay among the regional structure, contemporaneous deformation, eruption rate, preexisting topography, and the development