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Abstract:

The Oligocene to Recent sediments of the Seaspray Group in offshore Gippsland Basin consist of about 20,000 km3 of marls and limestones deposited in temperate latitudes between 58° and 38°S. The Seaspray Group can be subdivided into four lithological units, using cores and sidewall cores from wells in the Central Deep region.

Unit I, of Oligocene to Early Miocene age, overlies the Latrobe Group and is characterized by hemipelagic mudstones in the Central Deep. These sediments accumulated under deep marine conditions at accumulation rates ranging from <10 m/my at the base of the unit to 80 m/my at the top.

Unit II marks a change in depositional processes near the Early/Middle Miocene boundary. Through most of the Central Deep, the sediments are interpreted as fine-grained, carbonate-rich turbidites, comprising bioclastic wackestones and packstones deposited on the continental slope. Al- lochems were partly derived from the adjacent shelf, and accumulation rates significantly increased to a maximum of 220 m/my. At the western end of the Central Deep, outer shelf sediments accumulated.

Unit III represents most of Middle Miocene time. The base of this unit coincides with major sea-level falls which initiated submarine canyon- cutting. Slope sediments infill most of these channels and are characteristically wackestones and packstones with very abundant bioclastic debris. Closer to shore the sediments become coarse-grained, porous, quartzose bioclastic packstones and grainstones, deposited probably on a high-energy shelf. Sedimentation rates were very rapid, peaking at 250 m/my.

The Late Miocene to Recent Unit IV represents open shelf sedimentation and consists of coarse-grained highly fossiliferous wackestones and packstones. Sedimentation rates fall to under 100 m/my over most of the basin, although higher accumulation rates occur at the shelf edge.

Initiation of carbonate deposition in the basin appears to be related to the establishment of deep ocean currents south of Australia. Maximum Middle Miocene accumulation rates may reflect the presence of the Subtropical Convergence at the same latitude as the Gippsland Basin, aided by a general increase in surface productivity in the Pacific Ocean at the time.

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