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

A distinctive monomictic breccia, composed of fluidal and blocky basaltic a ndesite clasts, occurs in a Cambro-Ordovician submarine volcanic succession in northern Queensland, Australia. Associated with this fluvial-clast bucc ia facies are coherent facies and coarse and fine breccia facies of the sam e composition. The fluidal-clast breccia facies is internally massive and > 250 m thick, v arying 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 eq uant or splintery in shape and identical in composition to the fluidal clas ts. The fluidal clasts strongly resemble subaerial volcanic bombs and are i nterpreted to be the products of submarine fire fountaining of relatively l ow-viscosity lava. The blocky clasts were mainly derived from disintegratio n of the fluidal clasts, by means of quench fragmentation. Coherent basalti c andesite intercalated with the fluidal-clast breccia represents co-geneti c lavas, dykes and irregular shallow intrusions. The coarse and fine brecci a facies is very thickly bedded, monomictic (basaltic andesite), poorly sor ted and class 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-fou ntain breccias by thick glassy margins on fluidal clasts, the lack of weldi ng and agglutination, and the distinctive association of highly vesicular, fluidal clasts with nonvesicular, angular, blocky clasts. Recognition of su bmarine fire-fountain breccias in volcanic successions constrains the erupt ion style, proximity (tens of metres) to source and environment of depositi on. (C) 2001 Elsevier Science B.V. All rights reserved.