Geophysics Of The Clarence Moreton Basin Biology Essay

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 about 48,000km2 and contains a maximal thickness of 3500m of tellurian Mesozoic deposits, which are bounded by Palaeozoic cellar blocks ( Pinder, 2001 ; Powell et al. , 1993 ; Wake-Dyster et al. , 1987 ; Wells and O’brien, 1995 ) . Following a brief compressional stage early in the Middle Triassic, the 2nd tensional period initiated development of a figure of little infra-basin=s, ( Esk Trough ; Ipswich and Tarong Basins ) ( Ties et al. , 1985 ) . The concluding tensional period commenced in the late Triassic clip and coincided with the deposition of the Clarence-Moreton Basin deposits overlying the older infra-basins ( Pinder, 2001 ; Ties et al. , 1985 ) .

The Clarence-Moreton Basin is divided into three north swerving sub-basins that are separated by cellars highs and distinguished by different grades of remission ( 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 sequence 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 approximately north swerving. The major basin constructions include the Kumbarilla ridge, Cecil Plains bomber 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 contemplation methods are used as the predominant tools to look into the Clarence-Moreton basins geometry, construction and sedimentations ( Ingram et al. , 1996 ) Seismic contemplation has detected geological fault, different sedimentary formation thicknesses, unconformities, deepness to cellar. Aeromagnetic and gravitation informations has provided information on the basins geometry. Heat flow surveies provide geothermic gradients in the basin whilst good log geophysical sciences has enabled stratigraphic correlativity across the basin. Ground perforating radio detection and ranging and electrical methods has non been employed in this paper.

SEISMIC REFLECTION

The quality of seismal informations varies from good to hapless depending on the age of informations and the energy beginning ( Ingram et al. , 1996 ) . A portion 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 mistake has normal supplanting connoting the Esk Trough is a half graben ( Figure 2 ) . This reading is non consistent with the synformal form of the strata up near the mistake. The greater thickness of strata in the flexible joint of the syncline is besides non consistent with a normal mistake that shallows with deepness. The thickness off from the mistake suggests a mistake that steepens with deepness ( Pinder, 2001 ) . Pinder 2001 has reinterpreted the seismal line BMR 85-16 with a prejudice towards explicating compaction events ( Figure 3 ) . Seismic line BOO-15 ( Figure 4 ) depicts two opposing push mistakes interpreted by the presence of an anticline characteristic instantly above corroborating compaction events ( Pinder, 2001 )

Gravity

Gravity studies have been conducted by AGSO with a station spacing of about 5km ( Ingram et al. , 1996 ; Wellman et al. , 1994 ) . Wellman et Al. ( 1994 ) suggest that the Bouguer gravitation anomalousness map ( Figure 5 ) shows some short-wavelength fluctuations in basin deepness at the Tweed, Focal Peak and Main Range Tertiary volcanic-intrusive centres and along the South Moreton Anticline ( Figure 1 ) whilst the long-wavelength characteristics of the basin show small or no correlativity with the gravitation anomalousnesss.

Due to the staying Moho consequence on the Bouger gravitation map, anomalousnesss of geologic involvement have been masked or distorted doing a qualitative gravitation reading hard if non unlikely ( Green et al. , 1998 ) .Gravitational anomalousnesss along the chief AGSO seismal line ( Figure 1 ) show that gravitation depressions coincide with narrow infra-basins that contain sedimentary sequences of 0.7 to 1.5 seconds two-way travel clip which are associated with soft droops ( Wellman et al. , 1994 ) . Ingram et Al. ( 1996 ) states that some gravitation anomalousnesss are besides believe to hold been generated by intrusive pyrogenic stones antecedently identified throughout the sedimentary subdivision.

Sedimentary troughs imaged in seismal contemplation profiles correlate with gravitation depressions whilst interpreted magnetic volcanic fill is consistent with magnetic highs ( Murray, 1990 ) . Figure 6 depicts a residuary gravitation anomalousness map was provided to the New South Wales Department of Primary Industries sketching the basin ( Sommacal et al. , 2008 ) .

Magnetism

Modern systematic aeromagnetic and gamma-ray radiometric studies were conducted by Geometrics and AGSO covering the entire span of the basin ( Ingram et al. , 1996 ; Wellman et al. , 1994 ) . The magnetic information was presented as a contour map by filtrating out wavelengths shorter than 0.2 grades ( Figure 7 ) . In countries of cellar outcrop, the short-wavelength magnetic anomalousnesss ( & A ; lt ; 10 km wavelength ) are by and large elongated due to lithological fluctuations seen across the work stoppage of steeply-dipping cellar. Small invasions found in cellar and sedimentary stones besides give short-wavelength anomalousnesss of changing forms and sizes ( Wellman et al. , 1994 ) .

Within the basin, most lava flows are inferred to be associated with four big shield vents ( Tweed, Focal Peak, Main Range and Bunya ) . High-amplitude anomalousnesss are found in these flows, foremost, from high evident magnetisation due to their thin extremely oxidised brecciate nature and secondly, because the lava flows had been profoundly dissected by vales bring oning magnetic anomalousnesss on the borders. Depths to magnetic cellar can be calculated from the medium-wavelength magnetic anomalousnesss, nevertheless, such computations are hard because of intervention from the high amplitude, short-wavelength anomalousnesss caused by Tertiary vents ( 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 electric resistance with depth runing from 1-10 ?m in the shallow constructions, 10-40 ?m in the Ipswich Basin stone types and 30-100 ?m in the implicit in pre-Carboniferous cellar constructions. The theoretical accounts for the two polarisations show good understanding in their basic superimposed construction, with the fluctuation in electric resistances between the two being linked to the dominant north-south swerving geological fault of the basin. Large mistake bars are seen between 300 m and 1000 m in the M-B profiles ( Figures 8, 9 ) which are consistent with the being of extremely folded and faulted constructions of the Marbug Formation and Helidon Sandstone. Divergence of the two polarizations at deepnesss of greater than 10 kilometer 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 built-in constituent of lithostratigraphic correlativity ( Wells and O’Brien, 1994 ) . A composite well subdivision exemplifying the characteristic gamma beam and sonic log response correlated with the typical petrology encountered in the Clarence-Moreton Basin can be seen in Figure 10 ( Ties et al. , 1985 ) .

HEAT FLOW

The heatflow theoretical account that best fits the apatite fission path informations and vitrinite coefficient of reflection informations can be seen in Figure 111. A minor addition in heatflow at about 200 Ma is straight related to the remission that allowed deposit to get down in the Clarence-Moreton Basin ( Haselwood, 2003 ) . Vitrinite coefficient of reflection profiles for crude oil geographic expedition wells suggest that between 1.5 and 2.5 kilometer of sedimentary strata have been removed from the basin ( Ties et al. , 1985 ) .

Summary

Different geophysical methods have been employed to detect the tectonic scenes the basin has experienced, its stratigraphy, its primary constructions and its overall geologic history. The Clarence-Moreton Basin has been lightly explored, many countries have ne’er been subjected to anything but shallow coal drillholes ( Ingram et al. , 1996 ) . Seismic contemplation surveies formed the primary footing for reading of the Clarence-Moreton tectonic history. Bouguer and Residual gravitation has given some penetration into the deepness of the Moho and cellar. The broad spacing of the gravitation Stationss across the basin do non supply sufficient control to observe major constructions therefore it is concluded that the anomalousnesss mapped are caused by denseness fluctuations within the cellar composite ( Ingram et al. , 1996 ) . Lithostratigraphic correlativity has been conducted via downhole geophysical methods to specify the sedimentary beds of the basin whilst heat flow through vitrinite analysis has determined the sum of deposit that has been eroded.