Flow-field and palaeogeographic reconstruction of volcanic activity in thePermian Gerringong Volcanic Complex, southern Sydney Basin, Australia

The Broughton Formation, the basal part of the lower Upper Permian (ca 264- 263 Ma) Gerringong Volcanics of the Kiama area of the southern Sydney Basin , comprises intercalated shoshonitic lava flows and shallow-marine, predomi nantly volcaniclastic, sediments emplaced under periglacial climatic condit ions at high palaeolatitude. Four lower members of the Gerringong Volcanics (the Westley Park Sandstone, Blow Hole Latite. Kiama Sandstone and Bumbo L atite Members) were examined to elucidate the lava-flow directions and coev al palaeophysiography. Field evidence indicates that the individual flows t hat comprise the compound-flow-units of the Blow Hole and Bumbo Latites adv anced toward the norih-northwest and north, respectively, from an emergent island volcano or volcanic archipelago that developed offshore of the prese nt-day coastline, some tens of kilometres south-southeast of Kiama. The fie ld evidence incorporates descriptions and interpretations of features inclu ding vesicles and amygdales, lava tubes, pillow lavas and contact relations hips between coherent lavas and sediments and brecciated lava. The similari ty of the coherent lavas and volcaniclastic components in the intercalated sediments and in the underlying uppermost Berry Siltstone indicate that the lavas and the volcaniclastic material shared a common source. Lava flows a nd epiclastic deposits contributed to the formation of the volcanic edifice , as probably did hyaloclastic and pyroclastic and reworked pyroclastic dep osits. Much fragmental volcanic material was dispersed into the local shall ow-marine environment by mass flows that were generated by a variety of mec hanisms on the volcano's subaerial and subaqueous slopes. Redistribution of the volcanogenic sediment and erosion of the lava flows was influenced by longshore marine currents that flowed predominantly northwards across the b asin floor. The Gerringong Volcanics constitute the oldest onshore-preserve d record of the regional onset of volcanolithic sediment influx into the Sy dney Basin from the embryonic New England Orogen and herald the commencemen t of its foreland-basin phase of development.