Pumiceous peperite in a submarine volcanic succession at Mount Chalmers, Queensland, Australia

Abstract

Pumiceous peperite comprising irregularly shaped apophyses of feldspar-phyric rhyolitic tube pumice and siltstone occurs within well-bedded volcaniclastic sandstone and siltstone facies of the Early Permian Berserker beds at Mount Chalmers, Australia. The tube pumice structure is preserved where sericite or silica have replaced the glass of vesicle walls and vesicles have been infilled by silica. In some instances, the peperite occurs gradationally above or below intervals of coherent feldspar-phyric rhyolite that are also predominantly pumiceous. The siltstone in the pumiceous peperite is texturally homogeneous, locally vesicular and contains shards and crystal derived fromdisintegration of the pumiceous rhyolite. Pumiceous rhylite and peperite occur at various positions in the stratigraphy and may represent interconnected intrusive digits or lobes. Intrusion of the lobes was accommodated by expansion of the pore water and possible fluidisation of the host sediment, resulting in local destruction of bedding. The lobes developed chilled margins at contacts with wet sediment and inflated in response to vesiculation and the supply of new magma. Cooling of the lobes was possibly accompanied by development of microfractures in the glassy vesicle walls. Rupture of the chilled margin and propagation of fractures into the interior could have temporarily and locally depressurised the lobes, resulting in failure, disintegration and mixing with the adjacent wet and/or steam-rich sediment. Hot pumiceous rhyolite in lobe interiors may have interacted directly with the wet sediment and been dismembered by quench fragmentation and/or steam explosions. Bubbles of magmatic gas and/or steam were trapped in the sediment that mixed with the pumiceous rhylite. The development of pumiceous texture in the sills was favoured by emplacement beneath a thin cover of wet sediment in a relatively shallow, submarine shelf setting in which the confining pressure was sufficiently low to permit vesiculation. This setting was also important in limiting the extent of degassing of the pumiceous rhyolite during cooling.