Dataset: The Geology of the Kenn Plateau Off Northeast Australia: Results of Southern Surveyor Cruise SS5/2004 (Geoscience Australia Cruise 270)


Description

In the Late Cretaceous, the Kenn Plateau was part of the Maryborough Basin to the west and the Capricorn Basin to the north. It separated from Australia from the earliest Paleocene to the Middle Eocene by moving northeastward overall along the Cato Fault Zone and rotating 45 degrees anticlockwise. The West Bellona Fracture Zone splays off the Cato Fracture Zone and separates the western part of the plateau, with west to northwest trending structures, from the eastern part of the plateau, with northeast trending structures. The northeast trending structures correspond in trend to the fracture zones in the oceanic crust of the northern Tasman Basin, indicating that they have a common origin.

The Kenn Plateau consists of about 140,000 km sq of thinned continental crust and it is surrounded by ocean basins, largely floored by oceanic crust, to the north, west and south. To the east are the limestone banks of the Early Miocene Lord Howe hotspot chain. A geoscience survey by R.V. Southern Surveyor in 2004 showed that the plateau is highly complex, consisting of basement ridges, intervening troughs, and the Oligocene hotspot volcanoes of the Tasmantid chain in the west. On this survey, multibeam-sonar swath-bathymetry was recorded continuously, and 3090 km of multichannel seismic data and magnetic data were also recorded. In addition, twelve dredge hauls recovered sedimentary rocks, many of which have been dated micropaleontologically as Eocene and younger.
Rifting of the pre-existing continental basement rocks, and the overlying Mesozoic siliciclastic sedimentary rocks, thinned the crust, and formed Late Cretaceous and Cainozoic rift basins that trend ENE overall. The thinning led to subsidence, but this was complicated by Early Oligocene and Early Miocene volcanic heating and build-ups, and Eocene compression. Overall, subsidence has averaged ~50 m/m.yr. since the Early Oligocene. Three major sequences occur. The lowermost is the most poorly understood, but appears to consist of up to 2500 m of Upper Cretaceous and Lower Paleocene siliciclastic sediments deposited in the troughs. These detrital sediments grade into marine limestones, both upward and into basinal areas.

Unconformably overlying the earlier sequence is a few hundred metres of Middle and Upper Eocene chalk, some of it highly siliceous, with the silica apparently derived from radiolarians. Compression during the Eocene formed anticlines in the troughs in places. Faults generally show major displacement only in the Cretaceous to Eocene sequences. Above another unconformity is a draped Oligocene to Recent sequence of chalk, containing few siliceous microfossils. It is relatively thin and often sits directly on basement on the highs, but is up to 700 m thick in the troughs. Virtually the entire area is blanketed in calcareous pelagic ooze.

The Oligocene volcanic edifices of the Tasmantid chain formed at 30-34 Ma, earlier than the calculated onset of carbonate build-up of about 15-30 Ma. Erosion had probably removed most of the volcanic edifice above sea level before build-ups started to form. Cool water carbonates apparently formed here first (30 Ma) and somewhat later on the Marion Plateau (25 Ma), perhaps because the Kenn Plateau seamounts subsided beneath the sea earlier than did the Marion Plateau with its thicker continental crust. Limestone ages suggest that average reefal growth rates were 40-110 m/m.yr. Reefal growth kept up on some seamounts until the present day, but not on others.

The Kenn Plateau, as expected, appears to have little petroleum resource potential. However, its varied environments reefs, slopes of basement rocks, volcanics, limestone and soft sediments, and bathyal ridges and deepwater troughs may well be significant environmentally.

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