I. Tonguç Uysal

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

Abstract 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.

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

Abstract 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 (

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks, Bowen Basin, Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay. minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite-smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic (R greater than or equal to 3) illite-smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic (R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less. illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu)(sn) ratios show negative correlations with the illite content in illite-smectite and positive correlations with the delta(18)O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu)(sn) ratios and illite content in illite-smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/ rock ratio conditions. Increasing HREE enrichment with delta(18)O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin. Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins

K-Ar and Rb-Sr dating of authigenic illite-smectite in Late Permian coal measures, Queensland, Australia: Implication for thermal history

Abstract K–Ar and Rb–Sr isotopic studies were carried out on authigenic illitic clay minerals in Late Permian coal measures from the Bowen Basin (Australia), in order to determine the timing of maximum paleotemperatures, which were responsible for coal maturation and coal seam gas generation. The results indicate two major thermal events affected the Bowen Basin region, at 205–215 and 140–155 Ma. The narrow range of age data from different size fractions, lithologies and stratigraphic depths indicates episodic, short-lived thermal events, rather than gradual temperature increase due to progressive burial as previously believed. The earlier, thermal event in the latest Triassic postdates maximum burial of the Bowen Basin strata, which occurred during the Middle to Late Triassic. The ages of 205–215 Ma correspond with the published evidence for regional Late Triassic extension in eastern Australia. The younger (140–155 Ma), thermal phase during the Late Jurassic–Early Cretaceous is related to the initial rifting and associated widespread igneous activities prior to break-up of Gondwana. The K–Ar dates in conjunction with vitrinite reflectance data as paleotemperature indicators indicate that the younger thermal event occurred at lower temperatures than the earlier one, except in the shallow part of the southern Bowen Basin. This event is recorded exclusively in less illitic, R=0 illite–smectite (I–S) mixed-layer samples mainly in the shallow part of the southern Bowen Basin (Baralaba Coal Measures).

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

An 80 kyr-long continuous speleothem record from Dim Cave, SW Turkey with paleoclimatic implications for the Eastern Mediterranean

Speleothem-based stable isotope records are valuable in sub-humid and semi-arid settings where many other terrestrial climate proxies are fragmentary. The Eastern Mediterranean is one such region. Here we present an 80-kyr-long precisely-dated (by U-series) and high-resolution oxygen (δ18O) and carbon (δ13C) records from Dim Cave (~36°N) in SW Turkey. The glacial-interglacial δ18O variations in the Dim Cave speleothem are best explained in terms of changes in the trajectories of winter westerly air masses. These are along a northerly (European) track (isotopically less depleted) during the early last glaciation but are gradually depressed southward closer to the modern westerly track along the North African coast (more depleted) after c.50 kyr and remain in the southern track through the Last Glacial Maximum. The southward displacement of the westerly track reflects growth of the Fennoscandian ice sheet and its impact on westerly wind fields. Changes in δ13C are interpreted as reflecting soil organic matter composition and/or thickness. δ13C values are significantly more negative in interglacials reflecting active carbonic acid production in the soil and less negative in glacial times reflecting carbonate rock values. Several Heinrich events are recorded in the Dim record indicating intensification of westerly flow across this part of the EM.

The evolution of intraplate fault systems in central Turkey: Structural evidence and Ar‐Ar and Rb‐Sr age constraints for the Savcili Fault Zone

The Savcili Fault Zone represents one of the most prominent regional-scale intraplate fault systems in central Turkey, recording the collisional events following the closure of Neo-Tethys in the eastern Mediterranean region. It consists of anastomosing reverse/thrust faults with WNW-ESE direction that placed rocks of the Central Anatolian Crystalline Complex on Paleogene sedimentary units. Structural measurements and kinematic indicators show that faults within the Savcili Fault Zone (SFZ) have top to the NE and NW sense of brittle deformation. Stable isotope (δ18O and δ D) and trace element data indicate that fault gouge illites precipitated from deep basinal brines. These fluids were mobilized during phases of compressional deformation and migrated upward along thrust faults toward shallow brittle deformation zones. Rb-Sr and Ar-Ar geochronology of fault gouges in two cataclastic zones demonstrates age variability for two different dating techniques (Rb-Sr: 40.9 ± 1.5 Ma and 22.9 ± 1.3 Ma; Ar-Ar: 46.45 ± 0.25 Ma and 29.8 ± 0.13 Ma). We argue that Rb-Sr dating provides ages more closely reflecting the timing of fault movements because of potential contamination of illite by excess 40Ar. Accordingly, the SFZ was active during at least two phases; the middle Eocene and late Oligocene to early Miocene, which is consistent with the relative age constraints suggested by field relationships. Geochronology combined with structural field evidence indicates a rapid change in stress regime from extension to contraction at ∼40 Ma that continued until at least ∼23 Ma. Direct dating of brittle faulting provides a prolific approach for determining the absolute timing of tectonic events in areas that have largely relied on indirect information.

Hydrogen and 40Ar/39Ar isotope evidence for multiple and protracted paleofluid flow events within the long‐lived North Anatolian Keirogen (Turkey)

We present a new approach to identifying the source and age of paleofluids associated with low-temperature deformation in the brittle crust, using hydrogen isotopic compositions (δD) and 40Ar/39Ar geochronology of authigenic illite in clay gouge-bearing fault zones. The procedure involves grain-size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to δD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of todays North Anatolian Fault (NAF). δD values of the authigenic illite population, obtained by extrapolation, are −89 ± 3‰, −90 ± 2‰, and −97 ± 2‰ (VSMOW) for samples KSL, RES4-1, and G1G2, respectively. These correspond to δD fluid values of −62‰ to −85‰ for the temperature range of 125°C ± 25°, indistinguishable from present-day precipitation values. δD values of the detrital illite population are −45 ± 13‰, −60 ± 6‰, and −64 ± 6‰ for samples KSL, G1G2, and RES4-1, respectively. Corresponding δD fluid values at 300°C are −26‰ to −45‰ and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 ± 3.4 Ma (authigenic) and 95.8 ± 7.7 Ma (detrital) for sample G2 and 24.6 ± 1.6 Ma (authigenic) and 96.5 ± 3.8 Ma (detrital) for sample RES4-1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that todays NAF incorporated preexisting, weak clay-rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time.