Fluidal-clast breccia generated by submarine fire fountaining, Trooper Creek Formation, Queensland, Australia

Abstract

A distinctive monomictic breccia, composed of fluidal and blocky basaltic andesite clasts, occurs in a Cambro-Ordovician submarine volcanic succession in northern Queensland, Australia. Associated with this ftuvial-clast buccia facies are coherent facies and coarse and fine breccia facies of the same composition. The ftuidal-clast breccia facies is internally massive and >250 m thick, varying only in the ratio of fluidal clasts to blocky clasts. Fluidal clasts range in size from 2 cm to 170 cm, and have moderately to highly vesicular cores and thick (up to 1 cm), non-vesicular, formerly glassy rims. Blocky clasts are highly vesicular to non-vesicular, <2 cm, angular, dominantly equant or splintery in shape and identical in composition to the ftuidal clasts. The ftuidal clasts strongly resemble subaerial volcanic bombs and are interpreted to be the products of submarine fire fountaining of relatively low-viscosity lava. The blocky clasts were mainly derived from disintegration of the fluidal clasts, by means of quench fragmentation. Coherent basaltic andesite intercalated with the ftuidal-clast breccia represents co-genetic lavas, dykes and irregular shallow intrusions. The coarse and fine breccia facies is very thickly bedded, monomictic (basaltic andesite), poorly sorted and clast supported. This facies is interpreted to have been generated by periodic gravitational collapse of unstable accumulations of the fluidal-clast breccia facies. Subaqueous fire-fountain breccias are distinguished from subaerial fire-fountain breccias by thick glassy margins on ftuidal clasts, the lack of welding and agglutination, and the distinctive association of highly vesicular, f1uidal clasts with nonvesicular, angular, blocky clasts. Recognition of submarine fire-fountain breccias in volcanic successions constrains the eruption style, proximity (tens of metres) to source and environment of deposition.