David J. Cullen

Sedimentation dynamics and redox iron-cycling : controlling factors for the apatite-glauconite association on the East Australian continental margin.

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Autochthonous rocks from the Chatham Rise, east of New Zealand

Abstract Certain rocks, obtained by dredging in the central region of the Chatham Rise, are considered to occur in situ or to be derived from sources nearby. It is possible to distinguish them from ice-rafted, erratic rock fragments previously described from the same area. Details are given of the autochthonous rocks, and their significance with respect to the structure of the Chatham Rise is bridly discussed. Some of the rocks occur on, or close to, a newly discovered shallow submarine ridge, here named the Matheson Bank.

Submarine evidence from New Zealand of a rapid rise in sea level about 11,000 years B.P.

Abstract The proximity of a 35-fathom (64 m) submerged shoreline, approximately 11,000 year old, and an 8-fathom (14 m) submarine peat horizon, dated at 9,300 ± 80 years B.P., in the vicinity of Foveaux Strait indicates an abrupt increase in the rate of rise of sea level between these two dates. Published radiocarbon analyses of peat and shell from terrestrial sites elsewhere in New Zealand support this hypothesis, and facilitate construction of a generalized curve depicting sea level fluctuations in the New Zealand region during the last 11,000 years.

Phosphorite associations on seamounts in the tropical southwest Pacific Ocean

Abstract Recent surveys have revealed two distinct types of submarine phosphorite on seamounts between latitudes 6°–22°S in the southwest Pacific Ocean. The more widely distributed type, comprising carbonate—fluorapatite and calcite of typical marine facies, is found on a number of isolated seamount peaks at depths between 1000–1650 m. A markedly different assemblage, in which dolomite is associated with carbonate—fluorapatite, has been discovered at depths of 550–1100 m on two flat-topped guyots on the northern margin of the Fiji Plateau. The latter assemblage is strongly reminiscent of the “insular” type of phosphorite that occurs subaerially on several tropical Pacific islands and atolls. It is possible that similar phosphorite deposits may lie beneath biogenic carbonate sediments on a series of shallow banks in line with the guyots. The mineral associations, geochemistry, textures and geological settings indicate that the guyot phosphorites may well represent submerged insular deposits, while the phosphorites on the isolated seamounts are entirely of marine origin.

A tectonic analysis of the south-west Pacific

Abstract The regional structure of the south-west Pacific is attributed to the spreading and segmentation by sub-crustal convection currents of a Paleozoic—Mesozoic geosynclinal belt marginal to the Australian craton, with migration of the separated sialic fragments northward and eastward into the Pacific Basin. The source of the convection currents presumably lay beneath the mid-ocean rises (Indian-Antarctic and Pacific-Antarctic Ridges) south of Australia and New Zealand. During Cenozoic times Australia has moved northward relative to the New Zealand Plateau, the migration of the latter having a greater eastward component that allowed opening of the Tasman Basin. As a result of these differential movements the geosynclinal belt extending from New Zealand across the north-east margin of the Australian craton has been subjected not only to crustal spreading and stretching in a north-easterly direction with anticlockwise rotation, but also to north-south extension and tensional rifting. The New Guinea sect...

The antipodes fracture zone, a major structure of the south‐west pacific

Summary The steep Antipodes Scarp, along the eastern boundary of the Campbell Plateau south‐east of New Zealand, is attributed to dextral tear‐faulting within a NE‐SW belt, the Antipodes Fracture Zone, which also truncates the eastern end of the Chatham Rise. A complementary zone of sinistral movement, the Waipounamu Fracture, separates the Campbell Plateau and Chatham Rise from mainland New Zealand. The origin of these fracture zones is linked with that of the parallel Alpine Fault of South Island, and is related to a phase of NE‐SW crustal compression that dominated the New Zealand region during the Mesozoic era. It is suggested that this compression resulted from the north‐eastward “drift” of the Australian craton and the simultaneous elevation of the Darwin Rise in the central Pacific.

Autochthonous rocks of the bounty islands region, south-west Pacific ocean

Abstract The Bounty Islands lie in the south-west Pacific, about 850 km east of southernmost New Zealand, and close to the eastern margin of the submerged quasicontinental crustal segment known as the Campbell Plateau. The islands themselves consist of coarse granodiorite, but autochthonous rocks dredged from the adjacent sea floor include quartz-feldspar porphyries, dioritic intrusives, and sedimentary rocks of greywacke facies. The granodiorites have been radiometrically dated as early Jurassic; the acidic and intermediate intrusives are presumably either syngenetic or younger. The greywackes are petrographically comparable with rocks of late Precambrian (?)-early Paleozoic age on the New Zealand mainland. The occurrence of a very similar assemblage of igneous and sedimentary rocks in Marie Byrd Land favours the supposed original proximity of Campbell Plateau and westernmost West Antarctica.

The significance of a glacial erratic from the Chatham Rise, east of New Zealand

Abstract The recognition of an undoubted glacial erratic within a heterogeneous collection of rock fragments dredged from the Chatham Rise raises the possibility of extensive and long-continued deposition of rock debris by drifting icebergs in this area. The specimen is a red feldspathic sandstone of a type not known in New Zealand but showing some resemblance to rocks of the Beacon Sandstone of Antarctica.

The age of glauconite from the Chatham rise, east of New Zealand

Summary Radiometric analysis indicates that a granular variety of glauconite, widespread on the Chatham Rise, is derived from late Tertiary sediments. Other forms of glauconite, associated with pumice, phosphorite nodules, and glacially‐transported erratic blocks, are of Quaternary age. These are quantitatively unimportant, and may have originated as a result of redistribution of components from the older glauconite.

Uranium-series isotopic studies of marine phosphorites and associated sediments from the East Australian continental margin

The uranium-series disequilibria of phosphorites, sediments, and coexisting solitary corals from the outer continental shelf and upper slope off eastern Australia have been determined in an attempt to relate the genesis of phosphorites in this area to an absolute time scale. The234U/238U,230Th/234U, and231Pa/235U activity ratios in the phosphorites are internally consistent, and indicate that the apatite component of the phosphorites has remained a closed system with respect to U, Th, and Pa, except for very minor losses of234U. These losses do not affect the230Th/234U-derived ages, and comprise only 1–2% of the total234U activity, significantly less than the losses observed in phosphorite nodules from off Peru-Chile. A morphological subdivision of the phosphorites into either non-ferruginous or ferruginous is supported by the isotopic data. Earthy, friable non-ferruginous phosphatic nodules range in age from 2.5 to > 250 kyr; Holocene ages were obtained from five different locations in water depths ranging from 365 to 450 m. In contrast, the uranium-series isotopes in well-indurated ferruginous nodules occurring in the same general area, but in shallower ( < 350 m) water, are in radioactive equilibrium, indicating that their ages exceed 800 kyr. Solitary corals (Caryophyllia planilamellata Dennant 1906) are associated with non-ferruginous nodules on the upper slope, and commonly contain internal phosphatic molds. Several corals and their internal molds from one location were dated by uranium-series methods; the coral ages ranged from 17 to 20 kyr, and were typically 10–15 kyr older than their respective phosphatic internal molds. These results are consistent with the sedimentological observations, and provide independent evidence that geologically meaningful ages can be obtained from marine phosphorites. Phosphate deposition off eastern Australia appears to have been largely continuous throughout the late Quaternary, rather than being restricted to times of high sea-level, as reported for phosphorites off Peru-Chile.