Spatio-temporal distribution of igneous rocks and seismic facies analysis of buried volcanoes of the Prawn Platform, offshore Otway Basin

Volcanic rocks occur in different types of sedimentary basins, especially those evolving from lithospheric stretching. While volcanoes and other igneous rocks are widespread in the onshore Otway Basin, well-preserved volcanoes have not been documented in the offshore portion of the basin. Here, we analysed high-quality 2D and 3D seismic reflection datasets to investigate the spatio-temporal distribution of the igneous rocks in the Prawn Platform, offshore Otway Basin. Nineteen volcanoes, ranging from ~90-400 m in height and 1.8-6 km in diameter, occur at three stratigraphic levels: late Eocene, mid-Oligocene, and early Miocene. The igneous sills are relatively small (~0.2-11 km2), and located immediately beneath the volcanoes, implying a synchronous intrusion activity with the volcanoes. Seismo-geomorphological analysis indicates these are shield volcanoes fed by dykes. Distinct seismic facies characterise these buried volcanoes, including the chaotic central face that represents the main volcanic eruption centre, outward-dipping moderate amplitude reflections of tuff cone, and chaotic reflections at the distal flanks representing the pyroclastic mass-wasting deposits. Interestingly, seismic facies of interbedded extrusive and sedimentary rocks are mainly observed within volcanoes over 250 m high, and are associated with gullies along their flanks, indicating these volcanoes may have been subject to subaerial erosion. The discovery of these buried volcanoes extends our understanding of magmatism in the Otway Basin, especially regarding the offshore extension of the Older Volcanics.