Discovery of large-scale buried volcanoes within the Cenozoic succession of the Prawn Platform, offshore Otway Basin, southeastern Australia

• Published in: Marine and petroleum geology 2021-01, Vol.123, p.104747, Article 104747
• Main Authors: Niyazi, Yakufu, Eruteya, Ovie Emmanuel, Warne, Mark, Ierodiaconou, Daniel
• Format: Article
• Language: English
• Subjects: Buried volcanoes; Igneous sills; Offshore Otway Basin; Seismic reflection data; Volcanic seismic facies
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2020.104747

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 2-D and 3-D seismic reflection datasets to investigate the origin and distribution of the enigmatic, kilometre-scale buried mound-shaped structures in the Prawn Platform, offshore Otway Basin. Detailed seismic characterisation enabled the identification of 19 mounds, ranging from ~90–400 m in height and 1.8–6 km in diameter. Relatively small (~0.2–11 km2) igneous sills are associated with these mounds. Based on their external geometries and internal seismic architectures, we interpret these mounds as dyke-fed shield volcanoes. Distinct seismic facies characterise the buried volcanoes, including the main volcanic eruption centre, tuff cone, and pyroclastic mass-wasting deposits. 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 erosion. The volcanoes occur at three stratigraphic levels: late Eocene (~37 Ma), mid-Oligocene (~27–29 Ma), and early Miocene (~20 Ma), within the age of the Older Volcanics of the southern Australian margin. We propose that this newly discovered volcanism in the offshore Otway Basin was caused by edge-driven convection (similar mechanism to adjacent onshore volcanism), associated with the fast spreading rate of the Southern Ocean since the late Eocene (~40 Ma). The discovery of these buried volcanoes extends our understanding of magmatism in the Otway Basin, especially regarding the offshore extension of the Older Volcanics. •First evidence of km-scale buried volcanoes in the offshore Otway Basin.•Volcanoes occur at three levels: late Eocene, mid-Oligocene, and early Miocene.•Gullies along the flanks of some volcanoes suggest post-eruption erosion.•Seismo-geomorphological analysis indicates these are shield volcanoes fed by dykes.•These volcanoes show a new geographical extension of the Older Volcanic Province.