The subsidence history and thermal state of the Eromanga and Cooper Basins

The Eromanga Basin, a major Jurassic-Cretaceous intracratonic basin in eastern Australia, and the underlying Permo-Triassic Cooper Basin are examined from two perspectives - the subsidence history and the present day thermal state.The backstripping method for subsidence analysis is assessed for sensitivity to the porosity/depth relationship and it is shown that the overall shape of subsidence curves are not affected, although the absolute magnitude of the values will be different. Assumptions regarding palaeo bathymetry, sea level variations and isostatic models are likely to introduce more significant errors, especially to individual values. Tectonic subsidence curves were obtained for 40 wells in the region. The Permian was a period of dominantly fault controlled subsidence and the Triassic-Jurassic subsidence phase is consistent with a thermally driven mechanism. It is not possible to tightly constrain the thermal time constant (50-200 m.y.) or the time of initiation of subsidence (260-180 Ma) of this latter subsidence phase. An unconformity in the Late Triassic is attributed to processes at the eastern plate margin superimposed on the subsidence history as rather than a change in the formation mechanism of the two basins. During the Cretaceous the simple subsidence pattern was interrupted and the subsidence rate increased rapidly. This is attributed to a relatively rapid influx of sediment from an active volcanic arc to the east of the region. A simple model of excess sediment influx is presented which predicts the the observed sediment thicknesses and explains the departure from the thermal subsidence trend. The model predicts a topography up to 200 m above the present day observations and an additional subsidence mechanism needs to be invoked, possibly related to continental margin rifting or phase transformations beneath the basin. This mechanism is~ however, poorly constrained from the available information. The proposed excess sedimentation model provides an explanation for the transgressiveregressive nature of the Cretaceous sequence without appealing to global sea level changes or continent wide uplift. As vitrinite reflectance observations are commonly the only available constraint on the thermal evolution of a sedimentary basin. an examination of the discriminatory potential of such observations was made. An inversion procedure was developed and results with synthetic data suggest that the early heat flow history of the Eromanga/Cooper Basins could not be adequately constrained. The present day geothermal gradient in the central Eromanga Basin region is up to 60"C/km, considerably higher than average continental values. Divided bar measurements on core samples, combined with downhole lithology information lead to a depth averaged thermal conductivity value of 2.0-2.2 wm-1K-1. However, unlike the subsidence analysis, estimates of the thermal conductivity are sensitive to the assumed lithologies. A reasonable range for the heat flow is about 75-120 m wm-2. In some wells higher heat flow is associated with granitic basement. Numerical modelling of the steady state thermal regime implies that much of the central southern Cooper Basin is underlain by granite. Individual granites have been previously dated at between 10 and 60 m. y. older than the oldest sediments and provide evidence for a thermal event prior to the commencement of subsidence. One 39 Ar I 40 Ar age spectrum for a basement granite presently at nearly 200"C supports conclusions previously drawn from vitrinite reflectance and, more recently, apatite fission track analysis that the present day elevated thermal state is a relatively recent(< 10 Ma) phenomenon. No detailed examination of this feature was undertaken but qualitatively it is considered that advective, as opposed to conductive, heat transfer is active as there is no topographic expression of a deep heat source