Clast shape and textural associations in peperite as a guide to hydromagmatic interactions: Upper Permian basaltic and basaltic andesite examples from Kiama,Australia

Upper Permian shallow marine siltstone and sandstone units of the Broughton Formation are intercalated with basaltic and basaltic andesite sheets at Kiama, New South Wales. Parts of the two sheets examined in this study display peperite texture. The lower example (Blow Hole Latite Member) can be divided into two units with peperitic contacts suggesting their intrusion into wet unconsolidated sediments of the overlying Kiama Sandstone Member. The Bumbo Latite Member overlies the Kiama Sandstone Member and has been interpreted by previous workers as a lava. Well‐developed columnar joints cut the interior of the sheets. Along contacts with sedimentary facies and peperitic dykes which penetrate the sheets, columnar joints merge into a several metre‐wide zone of blocky jointing, pseudo‐pillows and hyaloclastite. In peperitic facies, sandstone or siltstone fills joints and fractures that define pseudo‐pillows, polyhedral joint blocks and columns (closely packed fabric) or sediment matrix‐rich breccia contains fragments and apophyses of basalt and basaltic andesite (dispersed fabric). Along some contacts, peperite with dispersed fabric passes through a zone of closely packed peperite into coherent facies. Alternatively, closely packed peperite passes directly into coherent facies. Examples of peperite with more than one clast type (globular, blocky, platy), and involving sedimentary matrix of constant grain‐size, are common. In some examples, globular surfaces formed during an early, low‐viscosity phase of magma emplacement into wet sediment. Planar and curviplanar fractures cut some globular surfaces suggesting that these formed slightly later as the magma became more viscous (cooler) and/or vapour films at the magma‐sediment interface broke down. However, the complexities of peperite, in respect to clast types, abundances and distribution, as well as grainsize and structures in the sedimentary component, suggest that a spectrum of fragmentation and mixing processes were involved in fragmenting the sheets. Many peperitic domains include poorly and strongly vesicular parts, resulting in apparent polymictic breccias. Vesiculation of the sheets is interpreted to have occurred in two phases: an early degassing of primary magmatic volatiles and a later, scoria‐forming event, both of which progressed as the magma mixed with unconsolidated sediment. During the later phase, magma incorporated limited amounts of steam from the wet sediment and a vesicular front propagated out into the magmatic component. Confining pressures were insufficient to prevent vesiculation of the magma or to suppress fluidisation of the host sediment along magma‐sediment contacts, but large enough to inhibit large‐scale steam explosivity. Displacement of sediment along contacts may have reduced confining pressures sufficiently to promote vaporisation of pore water, and induce local vesiculation of the magma.