Stable isotope geochemistry of authigenic clay minerals from late Permian coal measures, Queensland, Australia: Implications for the evolution of the Bowen Basin

Oxygen and hydrogen isotope analyses were carried out on authigenic clay minerals from Late Permian coal measures of the Bowen Basin (Australia). In the northern Bowen Basin, the oxygen isotope compositions of the mixed-layer illite/smectite show significant irregular variations with respect to depth, which parallel the changes in the extent of the illitisation reaction and are interpreted as reflecting changes in water/rock ratio in turn related to permeability. The δO and δD values of illite-smectite and kaolinite in the northern Bowen Basin and the calculated fluid isotopic composition (δO = -3‰ to +1‰; δD = -70‰ to -90‰) in equilibrium with these clays are considerably lower than those typically reported for deeply buried sedimentary basins. These stable isotope data, together with relatively high inferred palaeotemperatures (up to 235°C) and abnormally high geothermal gradients are consistent with a hydrothermal origin for clay mineral formation in the northern Bowen Basin. The hydrothermal system is interpreted to be a result of the Late Triassic extensional tectonic regime, which developed in large parts of eastern Australia and affected the northern part of the Bowen Basin. In the southern Bowen Basin, by contrast, clays are more enriched in O and deuterium, which is explained by lower temperatures (in the shallow coal measures) and a significant enrichment in the fluid isotopic composition (δO = -3.6‰ to +5.6‰, δD = -66‰ to -35‰) under low water/rock ratio conditions, especially in deeper pelitic rocks. (C) 2000 Elsevier Science B.V. All rights reserved

The thermal history of the Bowen Basin, Queensland, Australia: Vitrinite reflectance and clay mineralogy of Late Permian coal measures

The thermal history of the Bowen Basin (Queensland, Australia) has been investigated using vitrinite reflectance data and clay mineralogy. Vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the Bowen Basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest Middle Triassic-earliest Late Triassic as previously believed. The results of the present study indicate that the development of a zone of high heat flow in the latest Late Triassic had a major control on the thermal history of the Bowen Basin. High palaeogeothermal gradients estimated in the northern Bowen Basin are interpreted to result from convective heat transfer during a hydrothermal event. Variable heat distributions due to localised fracture-enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures. The estimated palaeogeothermal gradients in the southern Bowen Basin also indicate high heat flow in the lower sections of the stratigraphy. Sections in the southern Bowen Basin, however, are believed to reflect a rock dominated semi-closed system with low water/rock ratio, where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction. The correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite/smectite (I/S) mixed-layer clays in the northern Bowen Basin. This phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks. The relationship between I/S expandability and vitrinite reflectance for the Bowen Basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the Bowen Basin. The effect of thin igneous intrusions on clay mineral reactions is very limited. Intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies. This further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration, in contrast to organic maturity indicators. These differences between mineral and organic parameters aid in the identification of local contact metamorphic effects. (C) 2000 Elsevier Science B.V. All rights reserved

K-Ar evidence from illitic clays of a Late Devonian age for the 120 km diameter Woodleigh impact structure, Southern Carnarvon Basin, Western Australia

Woodleigh is a recently discovered impact structure with a diameter of 120 km, and thereby represents the third largest proven Phanerozoic impact structure known after Morocweng and Chicxulub. K-Ar isotopic studies of fine-grained authigenic illitic clay minerals (< 2 μm), considered to be impact-induced hydrothermal alteration products, indicate a Late Devonian (359 ± 4 Ma) age for the impact. Other evidence reported for Late Devonian extraterrestrial impacts include the strong iridium anomaly in the Canning Basin, Western Australia, and microtektites and elemental anomalies (including iridium) in South China. Given the large diameter of the Woodleigh impact structure and its relative proximity to iridium anomalies also of Late Devonian age in eastern Gondwana basins, environmental effects of the Woodleigh impact event are a likely contributor to a biotic crisis in the Late Devonian

Linking CO2 degassing in active fault zones to long-term changes in water balance and surface water circulation, an example from SW Turkey

Calcite veins are commonly found at shallow depth (a few metres below the surface) in damage zones of active normal fault systems in southwest Turkey. Although earlier studies demonstrated the link between the vein formation and seismicity, the association of near-surface carbonate precipitation with climate-driven hydrological conditions (water table, amount of precipitation and evaporation, and water discharge) is poorly understood. In this study, using the U/Th dating method we investigate the timing of vein formation and the interrelationship between tectonic and climatic-related hydrological processes. Carbonate precipitation is interpreted to occur as a result of sudden pressure drops and CO release after earthquake-induced fracturing. Vein formation mostly occurred during glacial periods, which coincide with slow growth rates and higher oxygen isotope values of speleothems from the Eastern Mediterranean region. We relate these episodes to reduced winter rainfall due to the decrease in westerly flow in SW Turkey. These somewhat drier conditions influenced the chemical composition of circulating water, creating conditions conducive to carbonate precipitation and sealing of damage zones. These conditions also facilitated CO accumulation and overpressure build-up in the rupture zones. Failure of the faults resulted in the release of large volumes of CO -rich fluids and the generation of shallow carbonate veins. It is proposed that during phases of increased winter rainfall CO is advected to the surface and discharged as passive degassing when meteoric water circulation is enhanced. While regional tectonics is the ultimate driver of fault activity and fracture formation, climate-driven near-surface hydrological changes may have played an important role in modulating CO -rich fluid circulation and surface discharge