VENT COMPLEXES AND SPATTER CONES OF THE ROZA FLOOD BASALT FLOW FIELD, COLUMBIA RIVER BASALT PROVINCE

The lava flow field of the 15 Ma Roza Member of the Columbia River Basalt Group is the one of the best documented examples of the products of a continental flood basalt (CFB) eruption. Its compositional variations, architecture, emplacement mechanisms, and environmental impact have been previously studied. It is also one of the few CFB flow fields to have a known vent system [Swanson et al., 1975]. We report results of new field studies on the proximal products of this 1300 km³ eruption that inundated an area in excess of > 40 000 km². Mapping of the northernmost exposed parts of the > 100-km-long NW-SE trending Roza fissure system has revealed new outcrops. Vent systems comprise extensive spatter cones and ramparts made up of agglutinated scoria and spatter. Cones and ramparts are up to 500 m in diameter, may have reached many tens of metres in height and outcrop over several km². Here we describe a representative vent. The lowest exposed parts of this vent comprise agglutinated spatter and scoria interbedded with discontinuous lenses of densely welded spatter. This is overlain by several units of columnar jointed welded spatter that are laterally persistent over 100’s of metres. The sequence is capped by an extensive sheet of densely welded spatter up to 20 m thick, which locally becomes lava-like with all outlines of original clasts lost. Dips of bedding within this upper unit vary rapidly and define cones and elongate mounds. The spatter cones and ramparts were partially buried by lava during the later stages of the eruption. The succession records proximal fallout from fire fountains that increased in intensity with time before commencing dominantly effusive activity. The proximal deposits are similar in scale and in lithofacies to those generated during smaller-volume basaltic fissure eruptions (e.g., 1783 Laki eruption, Iceland). Work is underway to trace these units away from the fissure system to establish eruption column heights and dispersal of fallout. The vent complex studied is thought to have formed near the end of the Roza eruption sequence, and the deposits are testimony that the eruptive vigor was still high even in later phases of activity.