As part of a study to investigate relative sea level history from the Tertiary North West Shelf of Australia, we descibe a seismically imaged. stratigraphic architectural element not previously associated with prograding carbonate clinoforms. a strike-oriented trough on an early mid0Miocene sequence boundary. This trough located on a shallowly dipping carbonate ramp, is recognized on both 2-D and 3-D seismic data by truncation, reflector interruption and amplitude variations. It is composed of three individual segments, each up to 500 m wide and 60 m deep, wide variable cross-sections. Their combined length, orthogonal to the progradation direction, is ~ 8 km. The trough occurs where there is a slight basinward increase in dip, and a facies change on the ramp from sand-sized calcarenite to clay-sized calcilutite. It has both a cross-cutting and sub-parallel relationship with underlying Paleogene faults reactivated within a Mesozoic basin-forming trend. The trough, does not conform to known drainage patterns, or contour current incisions mapped in similar environments elsewhere. We propose that this seismically mapped trough is karst topography that developed as a result of preferential dissolution focused by heterogeneities within the exposed carbonate ramp. Stable isotope analyses conducted on bulk carbonate samples ~ 2 km updip, suggest a mixed, marine to slightly meteoric origin for associated porewaters. Furthermore, the sequence boundary on which the the trough is developed is contemporaneous with recognized subaerial exposure surfaces to the northeast and southwest. The presence of such a karst feature indicates a minimum fall in relative sea level at this location of 80-160 m during the early mid-Miocene.

Concern about the impact of ocean acidification on organisms secreting high magnesium calcite skeletons has led to renewed interest in the mineralogy of these organisms. The identification of minerals making up the skeletons of tropical coralline algae, and in particular the determination of the Mg-content of calcite, is most commonly performed with X-ray diffraction. This method, based on XRD peak position, attracted criticism in the past because it produced Mg-contents that were in some cases lower compared to those based on chemical analyses of the bulk sample (in solution). The recent discovery of dolomite and magnesite in living coralline algae skeletons in addition to Mg-calcite explained this issue, and it is our goal in the present study to reinstate XRD as a reliable, quick and affordable method for the study of the mineral make-up of coralline algae species. In this paper we review the history of mineralogical analyses on tropical coralline algae and identify physical preparation methods that can affect results. We build on existing XRD methods to develop simple sampling and analytical methods to identify the presence of dolomite and magnesite, and numerically assess peak asymmetry that is caused by the overlapping reflections of calcite, dolomite and magnesite. These methods do not require specialist crystallographic knowledge or expensive or time consuming processes. The additional information our methods produce can be used to study intra-cellular calcification, and helps to rapidly assess and compare the mineral make-up of large numbers of samples. We conclude that XRD should be an integral part of any mineralogical analysis of coralline algae skeletons, which may be composed of not only Mg-calcite (Ca1.0-0.6Mg0-0.4CO3), but also dolomite (Ca0.5Mg0.5CO3), magnesite (MgCO3) and aragonite (CaCO3).

This movie describes the main findings of two Geoscience Australia surveys to the Gulf of Carpentaria in 2003 (survey 238) and 2005 (survey 276). The story relates to the discovery of submerged coral reefs across the southern Gulf and how they were identified using new multibeam sonar technology. The age of the reefs was determined using drill-core samples collected from the reefs, and measured by the Uranium/Thorium method at the Australian National University. The submerged reefs in the Gulf were not known to exist prior to this study and their discovery adds a new coral reef province to be managed and protected as part of Australia's marine zone.

Dolomite is a magnesium rich carbonate mineral abundant in ancient coral reef formations [1-3] yet very little is found forming in modern sedimentary environments. For over 150 years this conundrum has led to various theories being put forward about dolomite formation, however none have solved the so called `Dolomite Problem'[1]. It has generally been considered a post-depositional diagenetic process [2, 3], despite little experimental success at replicating dolomite formation in normal sea water conditions [4]. Here we show dolomite is in fact forming with living crustose coralline algae Hydrolithon onkodes, a species growing prolifically in coral reefs globally. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls calcitised, but that the cell spaces are typically filled with magnesite, rimmed by dolomite, or both. Thus there are at least three mineral phases present (magnesium calcite, dolomite and magnesite) rather than one or two (magnesium calcite and brucite) as previously thought[5-7]. Both the magnesium calcite and dolomite phases comprise a continuum of magnesium to calcium compositions, whereas magnesite is near ideal composition. Using a mass balance approach we quantify potential dolomitisation of the coralline algae and can account for the total amount of dolomite found in a raised Pleistocene reef [2]. Our results are consistent with observed dolomites in coralline-algal rich environments in fossil reefs. This is the first time the presence of dolomite in living coralline algae has been confirmed.

The mapping of seabed environments is fundamental to successful fisheries management and environmental monitoring, however, there is an emerging need to better characterise habitats based upon appropriate physical parameters. In this study, relationships between seabed geomorphology and the distribution of benthic habitats were examined using multibeam sonar, underwater video, predicted wave energy, and sediment data for Esperance Bay, part of the Recherche Archipelago. This shallow (<50 m), high energy, biogenic sediment dominated environment is located in temperate southwestern Australia. Exposure to wave energy appears to determine the distribution of unconsolidated substrate, and is the most useful regional scale predictor of rhodolith and seagrass habitats. Although they are intermittently smothered by mobile sediments, limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. The distribution of rhodolith beds is related to poorly sorted sediments that contain high gravel, mud, and CaCO3 percentages. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised sediment transport and accumulation play a major role in increasing the diversity of inner shelf benthic habitats. This highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats.

<div>The bulk rock stable isotopes database table contains publicly available results from Geoscience Australia's organic geochemistry (ORGCHEM) schema and supporting oracle databases for the stable isotopic composition of sedimentary rocks with an emphasis on calcareous rocks and minerals sampled from boreholes and field sites. The stable isotopes of carbon, oxygen, strontium, hydrogen, nitrogen, and sulfur are measured by various laboratories in service and exploration companies, Australian government institutions, and universities, using a range of instruments. Data includes the borehole or field site location, sample depth, stratigraphy, analytical methods, other relevant metadata, and the stable isotopes ratios. The carbon (¹³C/¹²C) and oxygen (¹⁸O/¹⁶O) isotope ratios of calcareous rocks are expressed in delta notation (i.e., &delta;¹³C and &delta;¹⁸O) in parts per mil (‰) relative to the Vienna Peedee Belemnite (VPDB) standard, with the &delta;¹⁸O values also reported relative to the Vienna Standard Mean Ocean Water (VSMOW) standard. Likewise, the stable isotope ratio of hydrogen (²H/¹H) is presented in delta notation (&delta;²H) in parts per mil (‰) relative to the VSMOW standard, the stable isotope ratio of nitrogen (¹⁵N/¹⁴N) is presented in delta notation (&delta;¹⁵N) in parts per mil (‰) relative to the atmospheric air (AIR) standard, and the stable isotope ratio of sulfur (³⁴S/³²S) is presented in delta notation (&delta;³⁴S) relative to the Vienna Canyon Diablo Troilite (VCDT) standard. For carbonates, the strontium (⁸⁷Sr/⁸⁶Sr) isotope ratios are also provided.</div><div><br></div><div>These data are used to determine the isotopic compositions of sedimentary rock with emphasis on the carbonate within rocks, either as minerals, the mineral matrix or cements. The results for the carbonate rocks are used to determine paleotemperature, paleoenvironment and paleoclimate, and establish regional- and global-scale stratigraphic correlations. These data are collated from Geoscience Australia records, destructive analysis reports (DARs), well completion reports (WCRs), and literature. The stable isotope data for sedimentary rocks are delivered in the Stable Isotopes of Carbonates web services on the Geoscience Australia Data Discovery Portal at https://portal.ga.gov.au which will be periodically updated.</div>

New 2D seismic data acquired in the Mentelle Basin by Geoscience Australia in 2008-09 has been used for a seismic facies study of the post-rift succession. The Mentelle Basin is a large deep to ultra deep-water, frontier basin located on Australia's southwestern margin about 200 km southwest of Perth. The study focused on the post-rift sequences deposited following the breakup between Australia and Greater India. Stratigraphic wells DSDP 258 and DSDP 264 provide age and lithological constraints on the upper portion of the post-rift succession down to mid-Albian strata. The depositional environment and lithology of the older sequences are based on analysis of the seismic facies, stratal geometries and comparisons to the age equivalent units in the south Perth Basin. Fourteen seismic facies were identified based on reflection continuity, amplitude and frequency, internal reflection configuration and external geometries. They range from high continuity, high amplitude, parallel sheet facies to low continuity, low amplitude, parallel, subparallel and chaotic sheet, wedge and basin fill facies. Channel and channel fill features are common in several facies along with a mounded facies (probably contourite) and its associated ponded turbidite fill. A progradational sigmoidal to oblique wedge facies occurs at several stratigraphic levels in the section. A chaotic mound facies, probably comprising debrite deposits, has a localised distribution.

With the increasing need to extend mineral exploration undercover, new approaches are required to better constrain concealed geology, thereby reducing exploration risk and search space. Hydrogeochemistry is an under-utilised tool that can identify subsurface geology and buried mineral system components, while also providing valuable insights into environmental baselines, energy systems and groundwater resources. With this aim, 238 water bores spanning seven geological provinces in the Northern Territory and Queensland were sampled and analysed for major cations and anions, trace elements, stable and radiogenic isotopes, organic species, and dissolved gases. Here, we demonstrate the utility of this dataset for identifying carbonate-rich aquifers and mineral system components therein. First, we use trends in major element ratios (Ca+Mg)/Cl– and SiO2/HCO–3, then strontium isotope ratios (87Sr/86Sr), to define subpopulations that reflect both spatial and compositional differences. We then apply mafic-to-felsic trace element ratios (V/Cs and Cu/Rb) to reveal elevated base metal concentrations near Lake Woods caused by water–rock interaction with dolerite intrusions. Correlated Sr concentrations between groundwater and surface sediments suggest that the geochemical evolution of these mediums in carbonate-dominated terrains is coupled. Our work develops an approach to guide mineral exploration undercover via the characterisation and differentiation of groundwaters from different aquifers, resulting in improved identification of geochemical anomalies. <b>Citation:</b> Schroder, I., de Caritat, P. and Wallace, L., 2020. The Northern Australia Hydrogeochemical Survey: aquifer lithologies, local backgrounds and undercover processes. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

Lord Howe Island in the southwest Pacific Ocean is surrounded by a shallow (20 - 120 m) sub-tropical carbonate shelf 24 km wide and 36 km long. On the mid shelf a relict coral reef (165 km2) extends around the island in water depths of 30 - 40 m. The relict reef comprises sand sheet, macroalgae and hardground habitats. Inshore of the relict reef a sandy basin (mean depth 45 m) has thick sand deposits. Offshore of the relict reef is a relatively flat outer shelf (mean depth 60 m) with bedrock exposures and sandy habitat. Infauna species abundance and richness were similar for sediment samples collected on the outer shelf and relict reef, while samples from the basin had significantly lower infauna abundance and richness. The irregular shelf morphology appears to determine the distribution and character of sandy substrates and local oceanographic conditions, which in turn influence the distribution of different types of infauna communities.

High-CO2 gas fields serve as important analogues for understanding various processes related to CO2 injection and storage. The chemical signatures, both within the fluids and the solid phases, are especially useful for elucidating preferred gas migration pathways and also for assessing the relative importance of mineral dissolution and/or solution trapping efficiency. In this paper, we present a high resolution study focussed on the Gorgon gas field and associated Rankin trend gases on Australia's Northwest Shelf of Australia. The Gorgon field is characterized by a series of stacked reservoirs (Figure 1), and is therefore well placed to characterize CO2 migration, dissolution and reaction by looking at geochemical signatures in the different reservoirs. Hydrological data at the Gorgon field also suggests that many of the major faults possess very low transmissivities, which should prevent or limit mixing of reservoir fluids with different chemical imprints. The gas data we present here reveal correlatable trends for mole %-CO2 and --C CO2 both areally and vertically as observed by Edwards et al. (2007). We suggest that the observed relationships are imparted due to mineral carbonation reactions that occurred along the CO2 migration pathway. These results have important implications for carbon storage operations and suggest that under certain conditions mineral sequestration might occur over longer migration distances and on shorter timescales than previously thought.