Paleothermometry of the Sydney Basin

Abstract

Evidence from overprinting of magnetizations of Late Permian and Mesozoic rocks and from the rank of Permian coals and Mesozoic phytoclasts (coal particles) suggests that surface rocks in the Sydney Basin, eastern Australia, have been raised to temperatures of the order of 200°C or higher. As vitrinite reflectance, an index of coal rank or coalification, is postulated to vary predictably with temperature and time, estimates of the paleotemperatures in the Sydney Basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin. These estimates give maximum paleotemperatures of present day surface rocks in the range 60–249°C, depending on factors such as location in the basin, the thickness of the sediment eroded, and the maximum paleogeothermal gradient. Higher coal rank and, consequently, larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the Tasman Sea on the basin's eastern margin. A theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperatures in the Sydney Basin. This estimate places the maximum paleotemperature in the range 250–300°C along the coastal region. Both coalification and thermal activation of magnetization models provide strong evidence of elevated paleo‐temperatures, which in places exceed 200°C, and the loss of sediment thicknesses in excess of 1 km due to erosion.

Analysis of vitrinite reflectance-burial depth relations in dynamic geological settings by the direct integration method.

“…(8) Substituting MIDDLETON and SCHMIDT'S (1982) constants into equation (7) we have logR=0.0289T+0.182logt-1.01.…”

Testing the use of viscous remanent magnetisation to date flood events

“…Some investigations of VRM (e.g., Kent, 1985;Smith and Verosub, 1994) have also assessed the VRM theory of Walton (1980) and Middleton and Schmidt (1982), which attempts to modify Equation (4) slightly to account for the effect of grainsize distributions. It is has been found that this model appears to work better than Equation (4) for long timescales, i.e, >>1 Myrs, but for shorter timescales, i.e., thousands of years, Equation (4) appears to be more applicable (Smith and Verosub, 1994).…”

“…We also considered the VRM theory of Walton (1980) and Middleton and Schmidt (1982), which requires the same input data as Equation (4), though it is slightly modified. When we applied this approach to our data, all the ages for the erratics were less than a year.…”

The effect of temperature-dependent thermal conductivity on the geothermal structure of the Sydney Basin

“…The Sydney Basin formation process consisted of a foreland-basin deposition component-a result of tectonic loading (the syn-orogenic phase), and a basin response to thrust-belt erosion (post-orogenic phase) (Heller et al 1988). The Sydney Basin is located between the New England and Lachlan Fold belts and consists of Permian-Triassic sedimentary sequence of at least 4 km thick (Middleton and Schmidt 1982). At its thickest point, the Sydney Basin is cross-cut by the northeasterly dipping frontal thrust fault of the Tamworth Arc of the New England Fold belt, and gradually thins towards the south (Conaghan et al 1982).…”