Authors / CoAuthors
Exon, N. | Bernardel, G. | Cortese, A.
Abstract
Weather conditions during the survey were generally very good with only a few days of seas of about two metres. Technically and scientifically, the balance of the program changed when the seismic (and magnetic) program was reduced to one third of the expected kilometres because of the failure of the compressor. Fortunately, two of four critical lines running WNW-ESE were acquired (Table 1). Data acquisition rates (average of 200 km/day) were tolerable and seismic data quality was good. A whale watch was kept in accord with the requirements of the Department of the Environment and Heritage, but no whales were seen. The dredging program was increased to take advantage of the reduced seismic program, and most Mellish Rise sites were located either on the two new seismic lines or on pre-existing BMR continental margin survey seismic lines. A number of sites on the Kenn Plateau made use of seismic data from last year's Southern Surveyor Cruise SS5/04. The need to use BMR seismic lines moved the dredging balance to the western half of the area. Of the 44 dredges attempted, 37 (85 %) produced valuable results (Table 3). The swath-mapper was invaluable in designing dredge plans. The coring program of 5 cores (Table 2) produced three moderately successful cores, but was disappointing overall. The two seismic lines extend right across the Mellish Rise and reveal how the area has been affected by tension but not compression, with high blocks 50-100 km across separated by heavily sedimented graben of similar width. Satellite bathymetry and gravity maps, and the total seismic data set show that structural trends bounding the blocks are NW-SE, N-S and NE-SW. Numerous smaller horsts rise above the broad highs. Dredges from the Coriolis Ridge and the Selfridge Rise, both on the northern Kenn Plateau, are dominated by silicic volcanics of continental origin, siliciclastic sediments, and shallow marine carbonates (some reefal). Basaltic volcanics are rare. The continental volcanics may be rift-related (Upper Cretaceous to early Eocene). The calcarenites may be Eocene and Oligocene in age. Dredges from the generally deeper water (thinner crust) Mellish Rise are different, being dominated by basaltic volcanics and hyaloclastites, although silicic volcanics, siliciclastic sediments, and shallow marine carbonates (some reefal) occur. Two phases of volcanism, rift related (Upper Cretaceous to early Eocene) and hotspot related (late Eocene-Oligocene) may well be present. Three dredges from a southern protrusion of the Louisiade Plateau, which is not necessarily genetically related to that plateau, contain basaltic volcanics and hyaloclastites, silicic volcanics, siliciclastic rocks, and shallow marine carbonates in an assemblage like that of the Mellish Rise. Until exhaustive laboratory studies of the rocks are carried out, the above generalisations remain speculative. In the end, the volcanics could be related to any of four known periods of volcanism: ? The Late Jurassic (145-135 Ma) subduction-related volcanism of the Graham?s Creek Formation in the Maryborough Basin: tuffs, agglomerates and volcanic breccias, overlain by trachyte and rhyolite flows, overlain by basaltic andesite and dacite. ? The Early Cretaceous (125-115 Ma) explosive rift-related volcanism of the Whitsunday and Cumberland Islands: dacite, rhyolite, and andesitic ignimbrite. ? The assumed Late Cretaceous to Paleocene rift-related volcanism of the Marion Plateau (drilled in ODP Leg 194): altered basalt flows and volcaniclastic breccias and conglomerate. ? The Late Eocene to Early Oligocene hotspot volcanism of the Tasmantid chain: basalts and hyaloclastites.
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nonGeographicDataset
eCat Id
61845
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
ACT
2601
Australia
Keywords
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- External PublicationArticle
- ( Theme )
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- marine
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
Publication Date
2005-02-28T00:00:00
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geoscientificInformation
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Marine facility voyage summary on CSIRO marine division WEB site