No abstract available

The force of gravity at any point on the earth's surface can vary by as much as 300 microgals (1 microgal = 1x10-8 m/s2) over a 6 hour period due to the gravitational attraction of the sun and the moon as they move in relation to the earth. This variation in gravity is called the earth tide. The earth tide is made up of the body tide, which is the direct gravitational effect of the sun and the moon; and the ocean load tide, which is the effect of the shifting mass of the earth's oceans also due to the sun and the moon. Comparison of five tidal prediction computer programs, two of which attempt to model the ocean load tide, with superconducting gravimeter observations at Mt Stromlo, ACT, has found that they all predicted the earth tide to within 6 microgals standard deviation of the observed tidal variation. The two programs that used an ocean tide model predicted the earth tide to better than 1.2 microgals standard deviation. Tidal gravity observations made at Sydney, Parkes and Cobar in NSW, and at Mt Stromlo in the ACT, were compared with predicted earth tides to determine the accuracy of these predictions, particularly for the ocean load tides, at both coastal and inland sites. All of the computer programs tested are capable of predicting the earth tide to sufficient accuracy for most relative gravity applications, such as for exploration purposes. Where accuracies of better than 10 microgals are required, such as with a portable absolute gravimeter or precise microgravity surveys, a program that uses a global ocean tide model is necessary. If better than 5 microgals accuracy is required, such as with precise absolute gravity measurements, a global ocean tide model is not sufficient and one that better suits the local region is required.

For the first time, the distribution of seabed geomorphic features has been systematically mapped over the Australian margin. Each of 21 feature types was identified using a new, 250 m spatial resolution bathymetry model and supporting literature. The total area mapped was >8.9 million km2 and included the seabed surrounding the Australian mainland and island territories of Christmas, Cocos (Keeling), Macquarie and Norfolk Islands. Of this total, the shelf is >1.9 million km2 (21.92%), the slope >4.0 million km2 (44.80%), and the abyssal plain/deep ocean floor >2.8 million km2 (32.20%). The rise covers 97,070 km2 or 1.08% of the margin. A total of 6,702 individual geomorphic features were mapped on the Australian margin. Plateaus have the largest surface area and cover 1.49 million km2 or 16.54%, followed by basins (714,000 km2; 7.98%), and terraces (577,700 km2; 6.44%), with the remaining 14 types each making up <5%. Reefs, which total 4,172 individual features (47,900 km2; 0.54%), are the most numerous type of geomorphic feature, principally due to the large number of individual coral reefs of the Great Barrier Reef. The geomorphology of the margin is most complex where marginal plateaus, terraces, trench/troughs and submarine canyons are present. Comparison with global seabed geomorphology indicates that the Australian margin is relatively under-represented in shelf, rise and abyssal plain/deep ocean floor area and over-represented in slope area, a pattern that reflects the mainland being bounded on three sides by passive continent-ocean rifted margins and associated numerous subsided marginal plateaus. Significantly, marginal plateaus on the Australian margin cover 20% of the total world area of marginal plateaus. The Australian margin can be divided into 10 geomorphic regions by quantifying regional differences in diagnostic features that can be used to infer broad-scale seabed habitats. The present study has application for the future management of Australia's ocean resources.

This paper was presented at the 2001: A Structural Odyssey. SGTSG Conference, Ulverstone TAS, February 2001

No abstract available

No abstract available

No abstract available

Compelling evidence is presented for the process of lipid sulfurisation in humic coal-forming environments. The production of reduced inorganic sulfides by sulfate-reducing bacteria during early diagenetic marine transgression enabled the selective sequestration of functionalised lipids in the polar and asphaltene fractions from the Eocene, marine-influenced Heartbreak Ridge lignite deposit, southeast Western Australia. Nickel boride desulfurisation experiments conducted on these fractions released small, but significant, quantities of sulfur-bound hydrocarbons. These comprised mostly higher plant triterpanes, C29 steranes and extended 17?(H),21?(H)-hopanes, linked by one sulfur atom at, or close to, sites of oxygenation in the original natural product precursors. These sulfurised lipids mostly derive from the same carbon sources as the free hydrocarbon lipids, the exception being the sulfurised extended hopanoids, which may be partially derived from a different bacterial source compared to the free hopanoids. These results indicate that the selectivity and nature of steroid and hopanoid vulcanisation in coal-forming mires is akin to that observed in other sedimentary environments. However, the diversity of sulfurised higher plant triterpanes is greater than that typically reported in immature coals. This selective preservation mechanism explains the formation of the structurally-related biomarkers in more mature sulfur-rich humic coals.

ASEG conference edition (Perth, March 2000)

The Lake Harris Komatiite in the central Gawler craton of South Australia is the first documented komatiite outside the West Australian craton and the easternmost occurrence of such primitive ultramafic rocks in Australia. A U-Pb zircon age of ca. 2520 Ga for the komatiitic sequence indicates a previously unknown period of mantle-plume activity in the Late Archean. An integrated program of airborne magnetic surveys, gravity surveys, and core drilling was successful in defining the distribution and volcanic architecture of the komatiitic flows and associated greenstones through an extensive thin cover of Cenozoic alluvial sediments. Surface exposure of the komatiitic rocks is restricted to one small outcrop near Lake Harris. The greenstones form a series of subparallel east-northeast-trending sinuous magnetic highs flanked by large ovoid to elongate magnetic highs and lows that correlate with Archean-Proterozoic granitic bodies associated with province-wide shear systems, similar to the Archean greenstone terranes in the Yilgarn craton of Western Australia. The steeply dipping greenstone sequence was metamorphosed to middle amphibolite facies during the ca. 2440 Ma Sleafordian orogeny and sheared during the ca. 1700 Ma Kimban and ca. 1540 Ma Kararan orogenies. The greenstones consist of komatiite cumulates (43-32% MgO, anhydrous), high to low Mg komatiite (32-18% MgO), komatiitic and tholeiitic basalt (<18% MgO), pyroxenite cumulates, felsic volcanic rocks, minor metasedimentary rocks, pyroclastic rocks, and rare banded iron formation. They extend over 300 km in three subparallel belts that appear to be isoclinally folded around east-northeast axes and tectonically dismembered to the south by the Yerda shear zone. Komatiitic rocks have been confirmed by drilling in all three belts, but the absence of outcrop and structural complexities prevent detailed stratigraphic correlations within and between the belts. The komatiitic rocks display a range of quenched and cumulus textures defined by the different habits of olivine and its alteration products. Trace sulfides (pyrrhotite, chalcopyrite, pentlandite, pyrite, marcasite, polydymite violarite, heazelwoodite, millerite) form very small (0.01-0.2 mm) single-phase disseminated grains and coarser disaggregated grains. Their distribution largely reflects metamorphic and serpentinization processes, with high Ni/S ratios and probable sulfur loss from the more magnesian parts of the flows. Rare composite pyrrhotite-pentlandite-chalcopyrite blebs (0.1-0.5 mm) characterize some low Mg flows. Locally, there is supergene pyrite-marcasite and native copper-bornite-chalcocite(?) assemblage infilling of late low-temperature serpentine-chlorite veinlets. Thick ponded lava lake and distal composite sheet flow facies have been identified from different parts of the komatiitic sequences. Systematic whole-rock and mineral chemical trends indicate that despite the effects of recrystallization and reequilibration during amphibolite-facies metamorphism, the original magmatic geochemical profiles are largely preserved. The whole-rock data for the Lake Harris Komatiite does not show any obvious Ni depletion during fractionation but indicate a strong olivine control in dominantly sulfur undersaturated environments. Low sulfur (100-600 ppm S) and high Pd + Pt (5-30 ppb) contents, and Ti/Pd ratios of 2 to 4 x 105 for the komatiitic rocks are similar to sulfur-undersaturated Archean komatiites hosting Ni-Cu-PGE deposits, i.e., there is little evidence for sulfur saturation in the sampled komatiites. Identification of a pre-2.5 Ga source of sulfur in the substrate would be a positive indicator of potential sulfur saturation of the lavas elsewhere in the greenstone belt and a possible target for mineralization.