AQUEOUS WEATHERING OF COLUMBIA RIVER BASALTS: CONTRASTING STYLES CONTROLLED BY CLIMATE AND FLOW ARCHITECTURE

We are studying basalt alteration under varying climatic and hydrologic conditions to determine how these conditions control secondary mineralogy. The Columbia River Basalt Group includes 18-6 Ma lavas covering much of eastern Washington, northern Oregon, and north-central Idaho. The lavas are up to several km thick, and include numerous interbedded sediments and paleosols. The older basalts were emplaced during the warm, wet conditions of the Miocene Climatic Optimum and many flows were emplaced into water or wet sediments, producing glassy pillow complexes and hyaloclastites that rapidly weather to form palagonite and/or nontronite. Where basalt intruded very wet sediments, peperites formed, producing mixtures of Al-rich sedimentary clays and glassy basalt fragments altered to palagonite / nontronite. Flows overrunning less wet sediments show less evidence for explosive activity but are frequently altered at the basalt-sediment contact to produce nontronite clay. Weathering of older flows produced lateritic soil layers up to several meters thick containing montmorillonite, kaolinite, and Fe oxides; they are underlain by saprolites containing nontronite and opaline silica. Alteration within saprolites is strongly controlled by basalt porosity and permeability including vesicularity and the presence of cracks or joints. In the late Miocene, the regional climate transitioned to cooler, drier conditions that have persisted to the modern day. Unlike older basalts, capping basalt flows have undergone little chemical weathering. Formation of clay minerals is limited to cracks, joints, and vesicles that acted as conduits or traps for water. Nontronite clay and opaline silica fill cavities; Al-rich clays are limited to halloysite which fills some larger cracks. These observations show how both climate and flow structure and emplacement conditions control the mineralogy of weathering basalts.