The Roza Member, Columbia River Basalt Group; Chemical stratigraphy and flow distribution

The Roza Member of the Columbia River Basalt Group is one of the most distinctive units on the Columbia Plateau. Covering approximately 40,000 km 2in Oregon and Washington, the Roza Member comprises from 1 to 4 flow units consisting of 6 chemically distinct subtypes, delineated by systematic variations in Cr, Nb, Zr, P 2O 5, TiO 2, and CaO. The abundances of incompatible elements (Nb, Zr, P 2O 5, TiO 2) decrease while compatible elements (Cr, CaO) and plagioclase phenocryst abundances increase upward through the Roza stratigraphic succession. Stratigraphic relations between the 6 chemically defined subtypes are complex. In general, subtypes I-A and I-B form the base of the Roza in the eastern and north-central parts of the Columbia Plateau; II-A is the basal unit in the northwest and Columbia Gorge regions of the plateau; III forms the base of the Roza in the central plateau; and IV is the only unit present along the northeastern margin. The areal distribution of each subtype reflects the interaction of the constructional topography of older Columbia River basalt flows, the regional structure, geomorphology, and location and timing of activity along the 175-km-long linear vent system defined by earlier workers as the source of the Roza Member. The distribution of chemical subtypes within the dikes and vents correlates with the distribution of chemically defined subtypes, and indicates that only short segments of the vent system were active at any given time. Primary intraflow features and cooling-joint measurements suggest that the Roza cooled predominantly by a simple conductive process. Calculations based on the assumption of a conductive cooling regime indicate that decades elapsed between the eruptions of successive flows within the Roza Member. These temporal constraints are supported by a lack of saprolite horizons between Roza cooling units and an absence of geochemical variation due to magma reservoir processes, and are consistent with estimates of Columbia River basalt recharge rates.