Christmas Island is an Australian territory lying south of Java inthe Indian Ocean, at about 10°30'S and 105°40'E. It lies on oceanic crust of Late Cretaceous age, is moving north at 7cm/year, andis being raised as it climbs the bulge on the southern flank of the Java Trench. The island itself consists of Cainozoic volcanics and limestone, and has been extensively mined for Pliocene phosphate. It covers an area of 140 km2 , and rises 360 m above sea level.
Australia has declared a 200 mile Fisheries Zone around the island, and the aim of this BMR investigation is to assess the seabed morphology, sediment thickness, and offshore mineral resources in a future Exclusive Economic Zone. This information will be of particular value to the Department of Foreign Affairs and Trade, when Australia negotiates a Christmas Island seabed boundary with Indonesia to the north.
Present knowledge indicates that oceanic crust is generally at 5000-6000 m around Christmas Island, and that it is overlain by 100-300m of pelagic sediment which thickens northward toward the Java Trench. A number of volcanic ridges trend generally northeast or north-northeast, and are as shallow as 1200 m below sea level. Christmas Island itself sits on such a ridge. Shallow-water limestones and manganese oxide crusts have been dredged from the ridges. Deepseacoring programs show that pelagic foraminiferal ooze and marl give way to siliceous (diatom-radiolarian) ooze and red clay below 5000 in water depth. Volcanic ash from Indonesia is an additional component of the sediment.
Reconnaissance sampling has shown that manganese nodules are quite common in the deep sea, and that they carry moderate grades of the valuable metals, copper (Cu), nickel (Ni) and cobalt (Co). In a fairly similar geological setting to the west, in the central Indian Ocean, India has pioneer investor status for a nodule mine site of 150,000 square kilometres. At this site the grade of Cu+Ni+Co is about 2.55%,and nodule abundance is 5-7.5% of wet nodules per square metre, figures which suggest that the site has long-term economic potential.
The present project will commence with a 28-day geoscience cruise of R.V. "Rig Seismic" from 7 January to 4 February, 1992. The plan isto acquire about 2500 km of high-resolution reflection seismic and bathymetric data, to define seabed morphology and to allow regional mapping of sediment thickness and facies. The seismic data will beused as the basis of a sampling campaign to investigate sediment type, manganese nodule abundance and metal grade on the deepsea floor, and manganese crust thickness and metal grade on the volcanic ridges. The end result will be a comprehensive review of the geology and mineral resources of the Christmas Island offshore zone.
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