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Sonic Velocity, Submarine Canyons, and Burial Diagenesis in Oligocene-Holocene Cool-Water Carbonates, Gippsland Basin, Southeast Australia

¹ School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3010; m.wallace{at}earthsci.unimelb.edu.au
² School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3010; grh{at}unimelb.edu.au
³ School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3010; j.daniels{at}nam.nl
⁴ School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3010; sjgall{at}unimelb.edu.au
⁵ School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3010; a.smith14{at}pgrad.unimelb.edu.au

Malcolm Wallace obtained a Ph.D. in 1988 from the University of Tasmania on the sedimentology and diagenesis of Devonian reefs in the Canning Basin, Western Australia. He worked as a research fellow at the University of Adelaide on the sedimentology and geochemistry of the Late Proterozoic Acraman impact ejecta horizon (1988-1991). He is presently a senior lecturer in sedimentology at the School of Earth Sciences, University of Melbourne. His current research interests include the stratigraphy, sedimentology, and diagenesis of Tertiary successions in the southeast Australian basins, together with diagenesis and mineralization in sedimentary carbonate successions worldwide.Guy Holdgate has a Ph.D. in Gippsland Basin coal geology (Monash University, Australia, 1997) and an M.Sc. degree in geology (Victoria University of Wellington, New Zealand, 1972). Since 1990, he has consulted for several companies on a variety of Australian and New Zealand exploration ventures in oil, gas, coal, and minerals. He is the principal of Guy Holdgate and Associates Pty. Ltd. and a director of Tyers Petroleum Pty. Ltd. Since 1998, he has been a research associate at the School of Earth Sciences, Melbourne University on carbonates in the offshore Gippsland Basin. He has worked as a consulting geologist (1970-1971) and with Geological Survey of Victoria basin studies (1973-1977), and was coal geology head at the Exploration and Geological Division, State Electricity Commission, Victoria (1977-1990).

Jim Daniels graduated from the School of Earth Sciences, University of Melbourne in 1991 and completed an M.Sc. degree in Antarctic paleontology at the University of Otago, New Zealand, in 1996. In 1997, with an Australian Postgraduate Award scholarship, he began work on a Ph.D. at the School of Earth Sciences, University of Melbourne, studying the seismic stratigraphy and sedimentology of the submarine canyons in the Seaspray Group, offshore Gippsland Basin. He has spent two summer field seasons in Antarctica and participated as a shipboard scientist on the 1998 Ocean Drilling Program Leg 178 off the Antarctic Peninsula. He is a member of AAPG and SEPM.

Stephen Gallagher received his Ph.D. in 1992 (University College, Dublin) on Lower Carboniferous stratigraphy, biostratigraphy, and foraminifera. He is the paleontology lecturer at the School of Earth Sciences, Melbourne University. His research interests include the biostratigraphy of macro- and microfossils in carbonate sediments. He has previously worked as a part-time consultant for Conodate Ltd., Ireland (1990-1994) and as a micropaleontologist for The Geochem Group Ltd., United Kingdom (1991-1993).

Andrew Smith graduated in 1997 from the School of Earth Sciences, Melbourne University, with a first class honors degree in geology. He is currently undertaking a Ph.D. in geology on marine foraminifera and biostratigraphy of the Seaspray Group, Gippsland Basin. He was a participant on the joint Melbourne/Sydney universities and Australian Geological Survey Organisation 1998 marine geology cruise in Bass Strait.

Burial diagenetic processes largely control sonic velocity and porosity destruction in the Oligocene-Holocene Seaspray Group carbonates of the Gippsland Basin. Extensive cementation of the carbonates begins at around 300-500 m burial depth, and most macroporosity is filled by calcite cement at a burial depth of around 1.5 km. Sonic velocity data from well logs are strongly correlated with the burial diagenetic processes operating in the carbonates. Above 300-500 m burial depth, sonic velocity increases rapidly with depth, with porosity loss being dominated by mechanical compaction. Below 500 m burial depth, the sonic velocity increases at a lesser rate, with porosity loss being dominated by pressure solution and calcite cementation. At these greater depths, sonic velocity (and therefore porosity) is a function of burial depth, carbonate content, and, to a lesser extent, sediment age.

Lateral sonic velocity variations within the Seaspray Group are associated with submarine canyon facies. The submarine canyons are filled by sediments that have a higher carbonate content than the surrounding succession. During burial, these carbonate-rich canyon-fill sediments are more rapidly cemented by calcite, producing higher sonic velocities than the surrounding strata. The resultant velocity variations cause problems in time-depth conversion.