Thermal evolution and sediment provenance of the Cooper-Eromanga Basin: insights from detrital apatite.

Abstract

Despite the prolific hydrocarbon and geothermal potential within the Cooper-Eromanga Basin, the thermal history of the region has largely remained elusive. This study presents new fission track, U-Pb and rare earth geochemical data for apatite samples from five wells within the Cooper-Eromanga Basin. Based on these data, thermal history models were constructed and an apatite provenance study was carried out. The apatite samples taken from the upper Eromanga Basin sediments (Winton, Mackunda and Cadna Owie Formations) yielded a dominant population of early Cretaceous and minor population of late Permian – Triassic apatite ages that are (within error) equivalent to corresponding fission track age populations. Furthermore, the obtained Cretaceous apatite ages correlate well with the stratigraphic ages for each analysed formation, suggesting (1) little time lag between apatite exposure in the source region and sediment deposition; and (2) that no significant (>~100oC) reheating occurred after deposition. The apatites were likely distally sourced from an eastern Australian volcanic arc, (e.g. the Whitsunday Igneous Association), mixed with sediment sources from the New England and/or Mossman Orogens. Deeper samples (>2000m) from within the Cooper Basin (Toolachee Formation) yielded (partial) reset fission track ages, indicating heating to temperatures exceeding ~80-100oC after deposition. The associated thermal history models are broadly consistent with previous studies and suggest that maximum temperatures were reached at ~95-70 Ma as a result of progressive heating by sedimentary burial and/or radiogenic basement heat loss. The interpretation of subsequent late Cretaceous – Palaeogene cooling remains more enigmatic and may be related with enhanced thermal conductivity as a response to aquifer flow and/or cementation. Four of the five wells recorded a Neogene heating event: however, more data would be required to assess the significance of this more recent thermal perturbation.