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Flow‐field and palaeogeographic reconstruction of volcanic activity in the Permian Gerringong Volcanic Complex, southern Sydney Basin, Australia

2001; Taylor & Francis; Volume: 48; Issue: 3 Linguagem: Inglês

10.1046/j.1440-0952.2001.00865.x

1440-0952

L M Campbell, Patrick J. Conaghan, R. H. Flood,

Geology and Paleoclimatology Research

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, predominantly volcaniclastic, sediments emplaced under periglacial climatic conditions at high palaeolatitude. Four lower members of the Gerringong Volcanics (the Westley Park Sandstone, Blow Hole Latite, Kiama Sandstone and Bumbo Latite Members) were examined to elucidate the lava‐flow directions and coeval palaeophysiography. Field evidence indicates that the individual flows that comprise the compound‐flow‐units of the Blow Hole and Bumbo Latites advanced toward the north‐northwest and north, respectively, from an emergent island volcano or volcanic archipelago that developed offshore of the present‐day coastline, some tens of kilometres south‐southeast of Kiama. The field evidence incorporates descriptions and interpretations of features including vesicles and amygdales, lava tubes, pillow lavas and contact relationships between coherent lavas and sediments and brecciated lava. The similarity 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 and epiclastic deposits contributed to the formation of the volcanic edifice, as probably did hyaloclastic and pyroclastic and reworked pyroclastic deposits. Much fragmental volcanic material was dispersed into the local shallow‐marine environment by mass flows that were generated by a variety of mechanisms 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 basin floor. The Gerringong Volcanics constitute the oldest onshore‐preserved record of the regional onset of volcanolithic sediment influx into the Sydney Basin from the embryonic New England Orogen and herald the commencement of its foreland‐basin phase of development. Keywords: Gerringong Volcanicslavaslava tubespalaeogeographyPermianpillow lavasSydney Basinvesiclesvolcanic rocksvolcanism