Pumiceous rhyolitic peperite in ancient submarine volcanic successions

Pumiceous peperite is associated with a rhyolitic sill that intruded wet, unconsolidated, submarine stratified pumice breccia in the Cambrian Mount Read Volcanics, Australia. Other examples of pumiceous peperite described in this paper occur at the pumiceous base of a rhyolitic lava and the margins of a rhyolitic cryptodome complex in the Miocene Green Tuff Belt, Japan. Intervals of pumiceous peperite are thin (6 15 m), laterally discontinuous, massive, poorly sorted and the clast-to-matrix ratio varies significantly over short distances. They are composed of feldspar-phyric tube pumice clasts and domains of clastic sediment that include stratified pumice breccia, pumiceous siltstone and bioturbated mudstone. The sediment domains may be massive or may display relic depositional structures. Sediment adjacent to pumice clasts is silicified, possibly reflecting induration on contact with hot pumice. Pumiceous peperite has gradational contacts with in situ and clast-rotated pumiceous hyaloclastite and coherent pumiceous rhyolite. Gradational relationships between pumiceous peperite and coherent pumiceous rhyolite, pumice clasts with chilled margins and curviplanar surfaces, and abundant jigsaw-fit texture imply that quench fragmentation and autobrecciation were the dominant mechanisms of clast formation. The formation of pumiceous peperite is favoured by a combination of low confining pressure and delayed quenching allowing vesiculation. Pumiceous peperite may easily be misinterpreted or overlooked as it resembles other pumice-rich facies that are common in submarine volcanic successions. The correct identification of pumiceous peperite can provide evidence to constrain the timing of intrusive episodes in volcanic successions. 3 2002 Elsevier Science B.V. All rights reserved