School of Geography, Earth and Atmospheric Sciences - Theses

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    The thermal history of the Bowen Basin (Qld.) : an Apatite Fission Track Study
    Marshallsea, Susan Jane. (University of Melbourne, 1988)
    The thermal history of the Bowen Basin has been studied using apatite fission . track analysis (AFTA). Previous vitrinite reflectance studies had shown that the magnitude of the maximum paleotemperatures experienced by outcropping Permo-Triassic sediments had varied across the basin. From the AFTA data the timing and magnitude of maximum paleotemperatures has been constrained and a detailed thermal history for the basin has been elucidated and placed in a geological context. The extensive vitrinite reflectance data for the basin has also afforded an opportunity to compare and contrast AFTA with this more conventional paleotemperature indicator. Outcropping Late Permian Rangal Formation and correlatives were analysed primarily because of their widespread distribution, volcanogenic nature and relatively synchronous time of deposition. Fission track ages of apatites from these units vary from ~250 Ma to around 100-120 Ma. The apatite fission-track ages of ~250 Ma are indistinguishable from the Late Permian stratigraphic age of the units and indicate that fission tracks in these samples have not been appreciably annealed. The track length distribution of these samples indicate that the samples have resided at temperatures of less than �50�C since deposition and the apatite fission track ages are regarded as reflecting the provenance of these sediments from contemporaneous Late Permian volcanism. A good trend between apatite fission track age and mean track length for the Rangal Formation and equivalents is apparent. This trend, together with progressive changes in the track length distribution and the single grain ages, shows that the decrease in apatite fission track age from ~250 Ma to ~100-120 Ma reflects an increasing degree of annealing at progressively higher paleotemperatures, prior to cooling in the Early Cretaceous. This interpretation is consistent with the vitrinite reflectance results and suggests that presently outcropping rocks have been subjected to paleotemperatures of between 50-120�C and above. An AFTA study of Permian and Triassic samples from deep petroleum wells in the Denison Trough also reveals a period of elevated paleotemperatures prior to early Cretaceous cooling, and allows constraints on paleogeothermal gradients at the time of maximum paleotemperatures. Estimated paleogeothermal gradients are around ~30�C/km implying that elevated paleotemperatures were due to increased depths of burial and that cooling in this area was due to uplift and erosion of 1-2 km of post-Triassic section. A number of igneous intrusions outcrop in the northern region of the basin. These intrusions give Early Cretaceous apatite, sphene and zircon fission track ages and were emplaced at the same time as maximum paleotemperatures were experienced across the basin, although the distribution of intrusions does not directly correlate with the region of highest paleotemperatures. Both the intrusions and the sediments have experienced a similar cooling history. The cooling is interpreted to reflect a period of regional uplift and erosion beginning in the Early Cretaceous. A compressional event has recently been recognized in the central regions of the basin and it is thought that the uplift observed from the AFTA data and the intrusive activity may be associated with this event. The AFTA results suggest that between 1-3 kilometres of sediment has been eroded from the basin since the Early Cretaceous, with the central region of the basin experiencing the greatest degree of uplift. The eroded sequence is thought to be equivalent to the Early Triassic to Jurassic sediments preserved to the south and west in the Galilee and Surat Basins. Former extension across the Bowen Basin of these units is consistent with their present-day distribution. The conclusions from the AFTA data are also consistent with the geology of the coastal region, to the east of the basin, which indicates significant early-mid Cretaceous tectonism, and with widespread emplacement of Early Cretaceous intrusions. A good correlation between vitrinite reflectance and apatite fission track age is apparent in the outcrop samples with a trend from old apatite fission track ages and low vitrinite reflectance values (~0.3 Romax) to young apatite fission track ages (~100-120 Ma) and high vitrinite reflectance values (~2.0 Romax). An investigation of the relative merits of the various published models for predicting vitrinite reflectance values in the Bowen Basin shows that only one of the models used was able to predict the observed vitrinite reflectance data. However, it is concluded that in general the published vitrinite reflectance models are unable to predict the observed vitrinite reflectance values from the thermal history derived independently from AFTA. This is thought to reflect limitations in the formulations of these models.