Abstract The Late Devonian-Early Carboniferous Drummond Basin of central Queensland, Australia is one of the oldest units of the New England Fold Belt. Samples of authigenic clay minerals have been collected from across the Drummond Basin and analysed using mineralogic, isotopic and geochronologic techniques. X-Ray diffraction analysis on 36 samples selected from six exploration wells was carried out on volcaniclastic and argillaceous sandstones and mudstones of the Drummond Basin sequence in order to ascertain authigenic illite content. Sixteen samples were found to be suitable for K-Ar dating purposes, containing abundant illite and illite/smectite mixed layers (I/S). K-Ar age dating was carried out on these selected samples yielding three age groups at 300, 250 and 200 Ma. Temperature of formation was estimated for the authigenic illite and illite/smectite minerals, based on illite crystallinity and illite content of illite/smectite mixed layers. Two groups of temperatures have been delineated, with a high temperature assemblage of between 275¨¬C-320¨¬C and a low temperature assemblage of approximately 100¨¬C. When compared with the age groups, it is seen that the older event was typically hotter. Stable isotope compositions for the authigenic clay minerals were analysed with ¥äD and ¥ä18O values of between -68¢¶ to -117¢¶ and +7.5¢¶ to +14.4¢¶ (SMOW) respectively. Isotopic compositions for the fluid in equilibrium with the clay minerals were calculated using palaeotemperatures estimated from the clay mineralogy and range between -90¢¶ and -41¢¶ and -0.6¢¶ and +8.7¢¶ (SMOW) for hydrogen and oxygen respectively. These compositions are more depleted in deuterium and more enriched in 18O than those previously reported for basins in the region. The data also produced three groups of fluid compositions, which correlate with the age distributions delineated by the K-Ar geochronology. The first fluid composition, which correlated with the Permo-Carboniferous age (~300 Ma) is interpreted to result from a mixing of Permo-Carboniferous meteoric water with highly evolved formation water trapped within the Drummond Basin since deposition of the basin. The regional extensional event at the Permo-Carboniferous boundary, which initiated the opening of the Bowen Basin to the east of the Drummond Basin allowed meteoric water to percolate deep into the basin, enabling the growth of illite rich clay minerals at this time. The second fluid group is considered to represent the evolution of Late Triassic meteoric water migrating through the Drummond Basin during another extensional period in the region at approximately 200 Ma. This event precipitated a new period of clay mineral growth in the basin (reflected in the K-Ar dates), raised the geothermal gradient in the region and extended the crust, forming new sedimentary basins within the New England Fold Belt. The third assemblage of fluid compositions were found to be very similar to the Late Triassic group, with ¥äD and ¥ä18O values plotting very close to the Late Triassic meteoric water evolution trend. This is interpreted as representing the same fluid as that which produced the Late Triassic compositions and therefore the 250 Ma ages may represent partial resetting of older ages by the Late Triassic thermal episode. However, if it is assumed that these K-Ar ages are not old ages partially reset by a thermal event, the stable isotope data could represent a mixing of Mid-Late Permian meteoric water with formation water trapped in the basin. This event may represent the regional compressive Hunter-Bowen event and may be responsible for dewatering the basin before the Late Triassic extensional episode.
| Identifer | oai:union.ndltd.org:ADTP/253723 |
| Creators | Morrison, Christopher Stedman |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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