Cretaceous Volcanogenic and Miocene Calcareous Strata Dredged from the Deepwater Gippsland Basin on R/V Franklin Research Cruise FR 11/98

In 1998, Franklin Cruise FR11/98 recovered 18 dredge hauls in deep water in the Gippsland Basin. The dredge hauls were sited on the basis of seismic reflection profiles and morphological features. The study provided information on the lithologies, ages and paleo-environments of the little-known deepwater Gippsland Basin. The rocks and sediments fall broadly into four categories: volcanics of probable Late Cretaceous age, volcaniclastics and labile sediments of Late Cretaceous age, Neogene marly calcareous sediments, and calcareous oozes of the Quaternary to Holocene. Minor ferromanganese nodules and crusts from several deepwater stations are of no economic potential, being high in SiO2 and remarkably low in copper and cobalt. Volcanics were confined to the three easternmost dredges (present water depths 3300-3800 m) from a rifted block elongated west-northwest and just inboard of the continent-ocean boundary. They consist of basalt, hyaloclastite, breccia, scoria and volcaniclastic sandstone. Because, these volcanic rocks occur on an isolated ridge they cannot have derived pebbles and clasts from younger sequences. The rocks are not dated but may have been laid down during the Tasman Sea rifting phase in the Turonian to Coniacian. We hypothesise that lava flows and domes formed on a coastal plain and in shallow water. Normal vesicular flows formed on dry land, and some weathered to scorias. In water they broke up to form volcaniclastic mass flow deposits such as hyaloclastites. Some of the volcaniclastics apparently became intermingled with soft clays and lime muds, because the interstices are now filled with zeolites, clay minerals and calcite. No Early Cretaceous rocks (Strzelecki Group) age were recovered, suggesting that they were not deposited east of the Gippsland Rise (~149?30?E). Immature labile rocks of the Late Cretaceous (Emperor and Golden Beach Subgroups of the Latrobe Group) were recovered in eleven dredges on the outer continental margin (present water depths 800-2040m). Palynological ages are Turonian to Campanian (~90 Ma to ~74 Ma). Thin to medium bedded labile sandstone, siltstone and mudstone (and their weathered variants) are carbonaceous in part. Some beds are burrowed and mottled or contain cross-lamination, ferruginous nodules, trace fossils, load casts, ripple marks and plants. Marine macrofossils are generally absent. These rocks were apparently deposited rapidly in coastal and marine environments, in the rift involving eastern Australia, Lord Howe Rise, and the Gippsland Basin. Palynology documents the onset on marine conditions, and rapid subsidence between ~90 Ma and ~86 Ma, as the Tasman Sea entered. Silts and clays were deposited in a deep freshwater lake in the Early to mid Turonian, deep marine carbonates in the Santonian, and deep marine muds in the Campanian. Marine calcareous rocks of the post-Eocene Seaspray Group were recovered in eight dredges (present water depths 680-2800 m): medium to very fine grained calcarenites, calcisiltites and calcareous mudstones, composed largely of molluscan debris, foraminifers and clay. They are often poorly bedded, with some thin to medium bedding. Quartz, feldspar, clay clasts and muscovite are common. Mottling shows that bioturbation was widespread, and organic debris includes wood and leaves, sponge spicules and echinoderm spines. Foraminifera date the older rocks as early to middle Miocene. Microplankton indicate deep-water deposition.