PHREATOMAGMATIC TUFFS AT THE BASE OF THE COLUMBIA RIVER FLOOD BASALTS

Products of explosive basaltic volcanism have been recorded from several large igneous provinces [1]. The common occurrence of phreatomagmatic tuffs and other types of volcaniclastic deposit, especially in the basal portions of continental flood basalts, has been ascribed to tectonic settings controlling the availability of surface water, to eruption through volatile-rich subsurface rock formations, or to very high intrinsic magmatic volatile contents. In the Columbia River flood basalt province, early volcaniclastic rocks appear to be uncommon and have not previously been documented in detail. Here, we describe a sequence of phreatomagmatic basaltic tuffs near the base of the Imnaha Basalt, at Lookout Mountain in northeast Oregon. The tuffs contain fresh glassy lapilli; chemical analysis demonstrates affinities with the American Bar chemical lava type, consistent with the stratigraphic position of the tuffs. The Lookout Mountain sequence consists of two distinct tuffs with different characteristics. The lower unit is ash-dominated, with abundant juvenile and spatter clasts. The upper tuff consists of 26 m of massive basaltic tuff with abundant 1-4 mm glassy lapilli, basement lithic fragments up to 1 m in diameter, and vegetation casts, overlain by a 4 m thick sequence of pyroclastic density current and ashfall deposits, interbedded with fluvially reworked ash and lapilli. The overall character of the upper unit is 'wetter' than the lower tuff. In the upper unit, abundant rock fragments with lithologies identical to locally exposed basement indicate significant excavation of bedrock during a maar-type eruption. In neither case is the vent or a feeder dike exposed. Both units have the overall characteristics of phreatomagmatic deposits that are common products of smaller-scale volcanism, consistent with fortuitous interaction of rising magma with surface water or a shallow aquifer. This contrasts with most of the Imnaha Basalt, which was emplaced in a dominantly dry continental environment.

1. Ross, P.S., Ukstins Peate, I., McClinktock, M.K., Xu, Y.G., Skilling, I.P., White, J.D.L., and Houghton, B.F., 2005. Mafic volcaniclastic deposits in flood basalt provinces, A review: Journal of Volcanology and Geothermal Research, v. 145, p. 281-314.