Geological Controls on Magnetic PropertiesInterpretation of magnetic surveys in terms of geology is hampered by poor correspondence between broad lithological categories and magnetic properties, and by lack of knowledge of the geological factors that influence the magnetisation of rocks. Magnetic petrology is the integrated application of rock magnetic and conventional petrologic techniques to identify and characterise the magnetic minerals in rocks. This information elucidates the factors that produce, alter and destroy magnetic minerals and thereby influence the bulk magnetic properties of the rocks and their associated magnetic anomalies. Improved understanding of magnetic petrology is therefore essential for maximising the geological information that can be obtained from magnetic anomaly patterns.

SUMMARY This study provides reliable, precisely defined and well‐dated Early Permian (286 ± 6 Ma) palaeomagnetic poles for Australia from the Mount Leyshon Intrusive Complex (MLIC) and the Tuckers Igneous Complex (TIC). Both complexes are associated with prominent negative magnetic anomalies, indicating the presence of rocks carrying stable remanence of reverse polarity, with a Koenigsberger ratio greater than unity. The characteristic remanence carried by the intrusive phases and by locally remagnetized, contact‐metamorphosed host rocks is always of reverse polarity, consistent with acquisition during the Permo‐Carboniferous (Kiaman) Reverse Superchron. The corresponding palaeopoles confirm that Australia occupied high latitudes in the Early Permian. The pole positions are: MLIC: lat. = 43.2 °S, long. = 137.3 °E; dp = 6.0°, dm = 6.4°; Q= 6; TIC: lat. = 47.5 °S, long. = 143.0 °E, dp = 6.0°, dm = 6.6°; Q= 6. Permian palaeomagnetic overprinting is detectable at considerable distances from the MLIC (2–3 km), well beyond the zone of visible alteration. The primary nature of the Early Permian palaeomagnetic signature is established by full baked contact/aureole tests at both localities. Other new data from Australia are consistent with the poles reported here. Comparison of the Australian, African and South American Apparent Polar Wander Paths (APWP) suggests that mean Permian and Triassic poles from West Gondwana, particularly from South America, are biased by remagnetization in the Jurassic–Cretaceous and that the Late Palaeozoic–Mesozoic APWP for Gondwana is best defined by Australian data. The Australian APWP exhibits substantial movement through the Mesozoic. Provided only that the time‐averaged palaeofield was zonal, the Early Triassic palaeomagnetic data from Australia provide an important palaeogeographic constraint that the south geographic pole was within, or very close to, SE Australia around 240 Ma. The new Early Permian poles are apparently more consistent with Pangaea B‐type reconstructions of Gondwana and Laurussia than with the Pangaea A2 configuration. This may be partly an artefact of reconstruction problems within Gondwana, as systematic differences between approximately coeval, apparently reliable, Permo‐Carboniferous poles from Africa, South America and Australia are evident in standard Gondwana reconstructions. These discordances require a tighter fit of the southern continents, suggesting that some attenuation of continental margins, not accounted for in the reconstructions, has occurred during breakup of Gondwana, or that the fit between East and West Gondwana needs to be substantially modified. If stretching of continental margins during breakup of supercontinents is a general phenomenon, it may help to ameliorate, but not solve, the long‐standing controversy regarding Pangaea reconstructions. Although alternative Pangaea reconstructions, such as Pangaea B, may reconcile poles from Laurussia with Australian poles in the Late Carboniferous–Early Permian, no plausible reconstruction can bring th...

Paleomagnetic analysis of sediment samples from Ocean Drilling Program (ODP) Leg 133, Site 820, 10 km from the outer edge of the Great Barrier Reef, is undertaken to investigate the mineral magnetic response to environmental (sea level) changes. Viscous remanent magnetization (VRM) of both multidomain and near-superparamagnetic origin is prevalent and largely obscures the primary remanence, except in isolated high-magnetization zones. The Brunhes/Matuyama boundary cannot be identified, but is expected to be below 120 mbsf. The only evidence that exists for a geomagnetic excursion occurs at about 33 mbsf (-135 k.y.). Only one-half the cores were oriented, and many suffered from internal rotation about the core axis, caused by coring and/or slicing. The decay of magnetic remanence below the surface layer (0-2 mbsf) is attributed to sulfate reduction processes. The magnetic susceptibility (K) record is central for describing and understanding the magnetic properties of the sediments, and their relationship to glacio-eustatic fluctuations in sea level. Three prominent magnetic susceptibility peaks, at about 7, 32, and 64 mbsf, are superimposed on a background of smaller susceptibility oscillations. Fluctuations in susceptibility and remanence in the "background" zone are controlled predominantly by variations in the concentration, rather than the composition of ferrimagnetics, with carbonate dilution playing an important role (type-A properties). The sharp susceptibility maxima occur at the start of the marine transgressions following low stands in sea level (high δ 18 θ, glacial maxima), and are characterized by a stable single-domain remanence, with a significant contribution from ultra-fine, superparamagnetic grains (type-C properties). During the later marine transgression, the susceptibility gradually returns to low values and the remanence is carried by stable single-domain magnetite (type-B properties). The A, B, and C types of sediment have distinctive ARM/K ratios. Throughout most of the sequence a strong inverse correlation exists between magnetic susceptibility and both CaCO 3 and δ 18 θ variations. However, in the sharp susceptibility peaks (early transgression), more complex phase relationships are apparent among these parameters. In particular, the K-δ ls O correlation switches to positive, then reverts to negative during the course of the late transgression, indicating that two distinct mechanisms are responsible for the Åf-δ 18 O correlation. Lower in the sequence, where sea-levelcontrolled cycles of upward-coarsening sediments, we find that the initial, mud phase of each cycle has been enriched in high-coercivity magnetic material, which is indicative of more oxic conditions. The main magnetic characteristics of the sediments are thought to reflect sea-level-controlled variations in the sediment source regions and related runoff conditions. Some preliminary evidence is seen that biogenic magnetite may play a significant role in the magnetization of these sediments.

A detailed deep 3D geological model is an important basis for many types of exploration and resource modelling. Renewed interest in the structure of the Sydney Basin, driven primarily by sequestration studies, geothermal studies and coal seam gas exploration, has highlighted the need for a model of deep basin geology, structure and thermal state. Here, we combine gravity modelling, seismic reflection surveys, borehole drilling results and other relevant information to develop a deep 3D geological model of the Sydney Basin. The structure of the Sydney Basin is characteristic of a typical intracontinental rift basin, with a deep north-south orientated channel in the Lachlan Fold Belt basement, filled with up to 4 km of rift volcanics, and overlain with Permo-Triassic sediments up to 4 km thick. The deep regional architecture presented in this study will form the framework for more detailed geological, hydrological and geothermal models.

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