Mark Lackie

Upper crustal structure of the Tamworth Belt, New South Wales: Constraints from new gravity data

New gravity data along five profiles across the western side of the southern New England Fold Belt and the adjoining Gunnedah Basin show the Namoi Gravity High over the Tamworth Belt and the Meandarra Gravity Ridge over the Gunnedah Basin. Forward modelling of gravity anomalies, combined with previous geological mapping and a seismic-reflection transect acquired by Geoscience Australia, has led to iterative testing of models of the crustal structure of the southern New England Fold Belt, which indicates that the gravity anomalies can generally be explained using the densities of the presently exposed rock units. The Namoi Gravity High over the Tamworth Belt results from the high density of the rocks of this belt that reflects the mafic volcanic source of the older sedimentary rocks in the Tamworth Belt, the burial metamorphism of the pre-Permian units and the presence of some mafic volcanic units. Modelling shows that the Woolomin Association, present immediately east of the Peel Fault and constituting the most western part of the Tablelands Complex, also has a relatively high density of 2.72 – 2.75 t/m3, and this unit also contributes to the Namoi Gravity High. The Tamworth Belt can be modelled with a configuration where the Tablelands Complex has been thrust over the Tamworth Belt along the Peel Fault that dips steeply to the east. The Tamworth Belt is thrust westward over the Sydney – Gunnedah Basin for 15 – 30 km on the Mooki Fault, which has a shallow dip (∼25°) to the east. The Meandarra Gravity Ridge in the Gunnedah Basin was modelled as a high-density volcanic rock unit with a density contrast of 0.25 t/m3 relative to the underlying rocks of the Lachlan Fold Belt. The modelled volcanic rock unit has a steep western margin, a gently tapering eastern margin and a thickness range of 4.5 – 6 km. These volcanic rocks are assumed to be Lower Permian and to be the western extension of the Permian Werrie Basalts that outcrop on the western edge of the Tamworth Belt and which have been argued to have formed in an extensional basin. Blind granitic plutons are inferred to occur near the Peel Fault along the central and the southern profiles.

Gunnedah Basin 3D architecture and upper crustal temperatures

The Gunnedah Basin in New South Wales has long been an important coal and gas resource, but limited information exists on the temperature structure or crustal architecture at depth to enable development of its geothermal potential. Here we combine gravity modelling, seismic-reflection surveys and borehole drilling results to develop a 3D depth to basement structural map and geological model of the basin. The 3D structure of the Gunnedah Basin is characteristic of a typical intracontinental rift basin. Gravity modelling of the Lachlan Fold Belt basement, using borehole and seismic-reflection controls, shows a 2–3.5 km-deep approximately north–south-oriented channel between the basement highs of the Rocky Glen Ridge in the west and Boggabri Ridge in the east. Extensional basal volcanics during the Late Carboniferous–Early Permian fill this channel. Borehole data and gravity modelling show up to 1 km of Permian to Jurassic sedimentary rocks overlying the rift volcanics. Preliminary thermal modelling, incorporating the geological model and limited deep borehole temperatures, indicates temperatures at the top of basement are in the range 105–165°C.

Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history

Garnet-bearing schists and migmatites sampled from the high-T, low-P Wongwibinda Complex in the New England Fold Belt, northern New South Wales, contain S1 and S2 assemblages that are inferred to have formed within error of each other at T = 700 and 650°C, respectively, and P = 400 and 380 MPa, respectively. Garnet grains commonly display a zoning profile that includes a flat unzoned interior with narrow (<350 μm) rims of variable composition. We interpret the unzoned cores as resulting from elemental homogenisation at peak D1 metamorphic conditions and the narrow rims (with increased Mn) as resorbed grain edges that formed during retrograde conditions (D2 and thereafter). The retrograde overprint is nearly pervasive across the complex and is most notable nearer to shear zones and intrusive rocks that cut S1, including the Hillgrove Plutonic Suite. A gravity traverse across the complex determined the Wongwibinda Fault is best modelled with a dip of 65° towards the west but did not identify any substantial concealed mafic plutons, suggesting that the heat source for the shallow crustal thermal perturbation is not imaged beneath the complex today.

Deep 3D structure of the Sydney Basin using gravity modelling

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.

Practical considerations: making measurements of susceptibility, remanence and Q in the field

Here we consider how measurements of magnetic susceptibility, magnetic remanence and Königsberger ratios (Q) can be made in the field. A basic refresher is given on how induced magnetisation differs from remanent magnetisation and what distinguishes multidomain from single domain behaviour of magnetite particles. The approximation of an infinite half-space, which is the usual assumption for using most handheld susceptibility meters, is experimentally investigated and it is found that a block 100 × 100 × 60 mm is the minimum requirement for the meters tested here. The susceptibilities of chips of a dacite, an andesite and a spilite (altered basalt) are also experimentally investigated for a range of chip sizes from a few mm down to 200 μm. The relationship is quite flat until very small grain sizes are reached where the susceptibility either decreases or increases, which is interpreted as an indication of the grain-size fraction where the magnetite resides. Making susceptibility measurements on bags of rock chips is investigated and guidelines given. The temperature of susceptibility meters is also found to be a factor and five meters have been tested for temperatures from 0°C to 50°C, the stated operating range of most meters. Finally Breiner’s method to separate induced magnetisation from remanent magnetisation using a field magnetometer is discussed. A new fluxgate based pendulum instrument to allow a more controlled implementation of Breiner’s method is also described. The measurement of magnetic susceptibility, magnetic remanence and Königsberger ratios can be made in the field, either using a total field magnetometer or a new portable fluxgate device that is described. Various problems of using a magnetic susceptibility meter on non-ideal rock, core and chip samples can be avoided.

Building 3D geological knowledge through regional scale gravity modelling for the Bowen Basin

Regional scale gravity modelling is an effective and fast way to gain geological understanding of large scale structures like the Bowen Basin. Detailed deep 3D geological knowledge has become an important component of many types of exploration and resource modelling. Current interest in the Bowen Basin for geothermal exploration highlights the need for a complete basin scale model which is compatible with thermal modelling software. The structure of the Bowen Basin is characteristic of a typical asymmetrical extensional rift basin, with up to 5 km of sediment overlying the basement. By combining gravity modelling, calibrated by boreholes and seismic reflection profiles, we produce geologically reasonable 3D surfaces and structures to create a model of the Bowen Basin. This model is the final part in the completion of the 3D Sydney–Gunnedah–Bowen Basin system geological model and provides both an important framework from which detailed thermal models can be derived and a platform from which to expand with new information.

Locating an ice-covered Antarctic landfill using ground magnetometry

Abstract At former Antarctic research stations, legacy waste often remains in situ and concealed by ice. Consequently, the location, characteristics and potential environmental impact associated with legacy waste remains poorly documented. This study applies ground magnetometry to map the spatial extent of the landfill at the abandoned Wilkes Station. Magnetic anomalies indicate that the landfill extends north-west to south-east and is close to, and perhaps prograding into, the ocean. The landfill is characterized by large magnetic variations of > 1500 nT with asymmetrical magnetic anomalies which suggest variable orientations of material and random dumping. Magnetic susceptibilities > 0.02SI units beyond the landfill area reveal elevated magnetic properties of the basement geology. However, a contrast in anomaly shape reliably distinguishes large anomalies generated by landfill material. Surface and subsurface melt streams (observed at the shoreline) flowing from the survey area suggest elevated potential for metal contamination of the nearshore and marine environment. The survey demonstrates a cost-effective and non-invasive method for gathering information to guide the clean up of landfills beneath ice.

A Monte Carlo approach to constraining uncertainties in modelled downhole gravity gradiometry applications

Gravity gradiometry has a long legacy, with airborne/marine applications as well as surface applications receiving renewed recent interest. Recent instrumental advances has led to the emergence of downhole gravity gradiometry applications that have the potential for greater resolving power than borehole gravity alone. This has promise in both the petroleum and geosequestration industries; however, the effect of inherent uncertainties in the ability of downhole gravity gradiometry to resolve a subsurface signal is unknown. Here, we utilise the open source modelling package, Fatiando a Terra, to model both the gravity and gravity gradiometry responses of a subsurface body. We use a Monte Carlo approach to vary the geological structure and reference densities of the model within preset distributions. We then perform 100 000 simulations to constrain the mean response of the buried body as well as uncertainties in these results. We varied our modelled borehole to be either centred on the anomaly, adjacent to the anomaly (in the x-direction), and 2500 m distant to the anomaly (also in the x-direction). We demonstrate that gravity gradiometry is able to resolve a reservoir-scale modelled subsurface density variation up to 2500 m away, and that certain gravity gradient components (Gzz, Gxz, and Gxx) are particularly sensitive to this variation in gravity/gradiometry above the level of uncertainty in the model. The responses provided by downhole gravity gradiometry modelling clearly demonstrate a technique that can be utilised in determining a buried density contrast, which will be of particular use in the emerging industry of CO2 geosequestration. The results also provide a strong benchmark for the development of newly emerging prototype downhole gravity gradiometers. In this paper, we describe the utilisation of numerical models to simulate the downhole application of gravity gradiometry in order to benchmark the impact of a density contrast at depth, with specific application to monitor a density contrast consistent with the sequestration of CO2 displacing brine water in an abandoned oil reservoir.

An assessment of the gravity signature of the Windmill Islands, East Antarctica

Abstract A gravity survey was conducted on the Windmill Islands, East Antarctica, during the 2004–05 summer season. The aim of the study was to investigate the subsurface geology of the Windmill Islands area. Ninety-seven gravity stations were established. Additionally, 49 observations from a survey in 1993–94 were re-reduced and merged with the 2004–05 data. A three-dimensional subsurface model was constructed from the merged gravity dataset to determine the subsurface geology of the Windmill Islands. The main country rock in the Windmill Islands is a Garnet-bearing Granite Gneiss. A relatively dense intrusive charnockite unit, the Ardery Charnockite, generates the dominant gravity high of the study area and has been modelled to extend to depths of 7–13 km. It has moderate to steep contacts against the surrounding Garnet-bearing Granite Gneiss. The Ardery Charnockite surrounds a less dense granite pluton, the Ford Granite, which is modelled to a depth of 6–12 km and creates a localized gravity low. This granitic pluton extends at depth towards the east. The modelling process has also shown that Mitchell Peninsula is linked to the adjacent Law Dome ice cap by an ‘ice ramp’ of approximately 100 m thickness.

Geophysical characterisation of a blind I-type pluton emplaced within the Bundarra Suite S-type granites of the New England Batholith

Geophysical data are presented that characterise a blind pluton, the Mountain Home Pluton (MHP), which intrudes the southern portion of the Bundarra Suite (BS), 30 km northeast of Bendemeer, New South Wales. A positive magnetic anomaly within the non-magnetic granites of the BS (Banalasta and Pringles Monzogranites) was previously identified as a sub-surface intrusion. Interpretation of new gravity data and analysis of aeromagnetic data are used to infer the depth, size, density, magnetic susceptibility and likely petrology of the pluton. The best-fit model indicates that the MHP is very similar to the Looanga Monzogranite, a felsic member of the Moonbi Suite of the New England Batholith (NEB) that intrudes the BS 5–7 km southeast of the MHP. The top of the MHP is inferred to lie about 1 km beneath the surface and the pluton extends to a depth of at least 6 km. Our model furthermore suggests that the southwestern margin of the MHP is subvertical, whereas a shallower dip (<45°) towards the north is proposed for the northeastern surface of the pluton. A north-trending dyke swarm, identified on the basis of linear positive magnetic anomalies, may be related to the MHP. This swarm of more than 20 relatively magnetic dykes extends out to about 10 km north from the pluton. Magnetic modelling of the dykes indicates that susceptibility values of the dykes are probably very similar to the range of the MHP, and also suggests the width of individual dykes (also not known to be exposed at the surface) to be at most a few tens of metres. A petrographic examination of the intruded BS granites at the surface suggests that metamorphic zoning as seen in mineralogical characteristics may be related to the underlying pluton.