Geophysics Of The Clarence Moreton Basin Biology Essay

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The Clarence-Moreton Basin is an intracratonic basin located along the eastern coastline of southern Queensland and Northern New South Wales (Figure 1) (Finlayson et al., 1988; McElroy, 1969; Powell et al., 1993). It covers approximately 48,000km2 and contains a maximum thickness of 3500m of terrestrial Mesozoic sediments, which are bounded by Palaeozoic basement blocks (Pinder, 2001; Powell et al., 1993; Wake-Dyster et al., 1987; Wells and O'brien, 1995). Following a brief compressional phase early in the Middle Triassic, the second tensional period initiated development of a number of small infra-basin=s, (Esk Trough; Ipswich and Tarong Basins) (Ties et al., 1985). The final tensional period commenced in the late Triassic time and coincided with the deposition of the Clarence-Moreton Basin sediments overlying the older infra-basins (Pinder, 2001; Ties et al., 1985).

The Clarence-Moreton Basin is divided into three north trending sub-basins that are separated by basements highs and distinguished by different degrees of subsidence (Wells and O'brien, 1995). The Cicil Plains Sub-basin is separated from the Laidley Sub-basin by the Gatton arch (Figure 1) whilst the Logan Sub-basin separated from the two by the South Moreton anticline (Figure 1) (Pinder, 2001). The sedimentary succession in the Clarence-Moreton Basin is divided into two, the Bundamba Group and the post-Bumdamba group (O'Brien et al., 1994). The Bundamba Group is broken into the Woogaroo Subgroup, which is further divided into Laytons Range and aberdare Conglomerates, overlain by the Raceview Formation, and Ripley Road Sandstone; whilst the Marburg subgroup includes the Gatton Sandstone overlain by the Koukandowie Formation. The post-Bundama Group units include the Walloon Coal Measures, Kangaroo Creek Sandstone, Woodenbong beds and the Grafton formation.

Structures present in the Clarence-Moreton Basin are roughly north trending. The major basin structures include the Kumbarilla ridge, Cecil Plains sub basin, Gatton arch, Laidley Sub-Basin and the South Moreton anticline and the Logan Sub-basin. The basin has a shallow dip towards the sub-basin depocentres with steeper dips being overprinted (Korsch et al., 1989; O'Brien et al., 1994)

Seismic reflection methods are used as the predominant tools to investigate the Clarence-Moreton basins geometry, structure and deposits (Ingram et al., 1996) Seismic reflection has detected faulting, different sedimentary formation thicknesses, unconformities, depth to basement. Aeromagnetic and gravity data has provided information on the basins geometry. Heat flow studies provide geothermal gradients in the basin whilst well log geophysics has enabled stratigraphic correlation across the basin. Ground penetrating radar and electrical methods has not been employed in this paper.

SEISMIC REFLECTION

The quality of seismic data varies from good to poor depending on the age of data and the energy source (Ingram et al., 1996). A part of the Bureau of Mineral Resources (BMR) conducted a deep seismic profile across the Surat and Clarence-Moreton basin. Korsch 1999 has interpreted that the Ipswich fault has normal displacement implying the Esk Trough is a half graben (Figure 2). This interpretation is not consistent with the synformal shape of the strata up near the fault. The greater thickness of strata in the hinge of the syncline is also not consistent with a normal fault that shallows with depth. The thickness away from the fault suggests a fault that steepens with depth (Pinder, 2001). Pinder 2001 has reinterpreted the seismic line BMR 85-16 with a bias towards explaining compression events (Figure 3). Seismic line BOO-15 (Figure 4) depicts two opposing thrust faults interpreted by the presence of an anticline feature immediately above confirming compression events (Pinder, 2001)

GRAVITY

Gravity surveys have been conducted by AGSO with a station spacing of approximately 5km (Ingram et al., 1996; Wellman et al., 1994). Wellman et al. (1994) suggest that the Bouguer gravity anomaly map (Figure 5) shows some short-wavelength variations in basin depth at the Tweed, Focal Peak and Main Range Tertiary volcanic-intrusive centers and along the South Moreton Anticline (Figure 1) whilst the long-wavelength features of the basin show little or no correlation with the gravity anomalies.

Due to the remaining Moho effect on the Bouger gravity map, anomalies of geologic interest have been masked or distorted making a qualitative gravity interpretation difficult if not improbable (Green et al., 1998).Gravitational anomalies along the main AGSO seismic line (Figure 1) show that gravity lows coincide with narrow infra-basins that contain sedimentary sequences of 0.7 to 1.5 seconds two-way travel time which are associated with gentle sags (Wellman et al., 1994). Ingram et al. (1996) states that some gravity anomalies are also believe to have been generated by intrusive igneous rocks previously identified throughout the sedimentary section.

Sedimentary troughs imaged in seismic reflection profiles correlate with gravity lows whilst interpreted magnetic volcanic fill is consistent with magnetic highs (Murray, 1990). Figure 6 depicts a residual gravity anomaly map was provided to the New South Wales Department of Primary Industries outlining the basin (Sommacal et al., 2008).

MAGNETICS

Modern systematic aeromagnetic and gamma-ray radiometric surveys were conducted by Geometrics and AGSO covering the total span of the basin (Ingram et al., 1996; Wellman et al., 1994). The magnetic data was presented as a contour map by filtering out wavelengths shorter than 0.2 degrees (Figure 7). In areas of basement outcrop, the short-wavelength magnetic anomalies (<10 km wavelength) are generally elongated due to lithological variations seen across the strike of steeply-dipping basement. Small intrusions found in basement and sedimentary rocks also give short-wavelength anomalies of varying shapes and sizes (Wellman et al., 1994).

Within the basin, most lava flows are inferred to be associated with four large shield volcanoes (Tweed, Focal Peak, Main Range and Bunya). High-amplitude anomalies are found in these flows, firstly, from high apparent magnetization due to their thin highly oxidized brecciate nature and secondly, because the lava flows had been deeply dissected by valleys inducing magnetic anomalies on the edges. Depths to magnetic basement can be calculated from the medium-wavelength magnetic anomalies, however, such calculations are difficult because of interference from the high amplitude, short-wavelength anomalies caused by Tertiary volcanoes (Wellman et al., 1994).

A magnetotelluric deep-sounding was carried out 10km south of Ipswich. The profiles seen in Figures 8 and 9 show an increased resistivity with depth ranging from 1-10 Ωm in the shallow structures, 10-40 Ωm in the Ipswich Basin rock types and 30-100 Ωm in the underlying pre-Carboniferous basement structures. The models for the two polarizations show good agreement in their basic layered structure, with the variation in resistivities between the two being linked to the dominant north-south trending faulting of the basin. Large error bars are seen between 300 m and 1000 m in the M-B profiles (Figures 8, 9) which are consistent with the existence of highly folded and faulted structures of the Marbug Formation and Helidon Sandstone. Divergence of the two polarisations at depths of greater than 10 km may be associated with the West Ipswich Fault and Moreton Anticline (Chant and Hastie, 1988; Chant and Hastie, 1990)

WELL LOG DATA

Geophysical logs are an integral component of lithostratigraphic correlation (Wells and O'Brien, 1994). A composite well section illustrating the characteristic gamma ray and sonic log response correlated with the typical lithology encountered in the Clarence-Moreton Basin can be seen in Figure 10 (Ties et al., 1985).

HEAT FLOW

The heatflow model that best fits the apatite fission track data and vitrinite reflectance data can be seen in Figure 111. A minor increase in heatflow at approximately 200 Ma is directly related to the subsidence that allowed sedimentation to commence in the Clarence-Moreton Basin (Haselwood, 2003). Vitrinite reflectance profiles for petroleum exploration wells suggest that between 1.5 and 2.5 km of sedimentary strata have been removed from the basin (Ties et al., 1985).

SUMMARY

Different geophysical methods have been employed to discover the tectonic settings the basin has experienced, its stratigraphy, its primary structures and its overall geologic history. The Clarence-Moreton Basin has been lightly explored, many areas have never been subjected to anything but shallow coal drillholes (Ingram et al., 1996). Seismic reflection studies formed the primary basis for interpretation of the Clarence-Moreton tectonic history. Bouguer and Residual gravity has given some insight into the depth of the Moho and basement. The wide spacing of the gravity stations across the basin do not provide sufficient control to detect major structures therefore it is concluded that the anomalies mapped are caused by density variations within the basement complex (Ingram et al., 1996). Lithostratigraphic correlation has been conducted via downhole geophysical methods to define the sedimentary layers of the basin whilst heat flow through vitrinite analysis has determined the amount of sediment that has been eroded.

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