Pompeii to Stabiae: downcurrent versus substrate-induced variations of the AD 79 Vesuvius pyroclastic current deposits and their impact on human settlements

Santangelo Ileana1, Scarpati Claudio1, Perrotta Annamaria1, Sparice Domenico2, Fedele Lorenzo1, Chiominto Giulia1, Muscolino Francesco2, Rescigno Carlo3, Silani Michele3, Massimo Osanna2

1Department of Earth, Environmental and Resources Sciences, University of Napoli Federico II, Napoli, Italy; 2 Parco Archeologico di Pompei, Pompei, Italy; 3Dipartimento di Lettere e Beni Culturali, Università della Campania Luigi Vanvitelli, Santa Maria Capua Vetere, Italy

The AD 79 Vesuvius eruption buried the Roman towns around the volcano under several metres of pyroclastic materials. The destruction of these Roman towns allows volcanologists to build models that can provide valuable information on the extent and type of damage that a future Plinian eruption could cause in urbanized areas. In order to fully understand these phenomena, volcanologists need to observe the sequence of volcanic layers (stratigraphic reconstruction) that buried the city during the AD 79 eruption and which of these layers are associated with damage and victims. This study reports the results of a collaboration between the Archaeological Park of Pompeii and the University of Naples Federico II to document the stratigraphic sequence and the distribution of damage and victims unearthed by new excavations in the archaeological sites of Pompeii and Stabiae. A systematic survey of all exposed pyroclastic sequences allowed us to study in detail the distribution and lateral facies variations of the different stratigraphic units. The deposit of stratified ash forming the upper part of the pyroclastic succession, was studied in detail to define the downcurrent variations of its sedimentological features and how these were influenced by urban structures. Pronounced lateral variations are observed in the upper part of the sequence at Pompeii, mainly consisting of a pyroclastic density current (PDC), stratified ash deposit, that ranges in thickness from few tens of centimetres to two metres. In this case, thin, massive ash layers can be traced laterally into thick, poorly sorted, ash and lapilli layers, with well-developed sedimentary structures. All PDC layers, except the lowermost, are dispersed across the entire Pompeii area, although some are missing locally as a result of the erosive action of the following PDC. The layer associated with the most destructive impact on the Roman buildings shows a strong lateral variation in thickness (0 to 330 cm) and sedimentary structures. Where it is less than 30 cm thick, the deposit is fine-grained and thinly stratified. Where it thickens, the lower part is rich in coarse pumice lapilli and locally shows well-developed stratifications, while the upper part shows an internal arrangement of alternating layers of fine and coarse ash forming progressive dunes. Upwards, ash deposits show rare pumice lapilli clasts and diffuse accretionary lapilli. This ash sequence is interstratified with four well-sorted, thin lithic-rich layers that exhibit mantling structures of fall deposits. At Stabiae, the ash PDC deposit ranges in thickness from 70 to 160 centimetres. Its internal structure shows the same types of stratification observed at Pompeii. Ash layers thicken and show lateral lithofacies variations where the pumice deposit thins and close to standing walls. It is proposed that the urban structures affect the structure of the deposit much more than the variations induced by the increase in the distance from the eruptive vent.


Biography

“I obtained my bachelor degree and master degree in Geology at University of Federico II, Naples, Italy. With an experimental master thesis in Volcanology I investigated the pyroclastic sequence of the tuff cone of Miliscola, a pre-caldera, monogenetic volcano that partially crops out in the Campi Flegrei volcanic field. As a student, I took part in some summer schools with volcanology theme like the “XI School of Volcanology AIV Bruno Capaccioni” and the “Etna International Training School of Geochemistry 2019 – Science meets practice”.

A new approach to integrate passive seismic HVSR depth models in magnetotelluric (MT) 1D inversion to characterize the cover-basement interface.

Suriyaarachchi, Nuwan1,2 , Giraud, Jeremie1,2, Seille, Hoel3 , Jessell, Mark1,2, Lindsay, Mark1,2, Hennessy, Lachlan4, Ogarko, Vitaliy5

 1Centre of Exploration Targeting, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Perth  6009 WA, Australia; 2Mineral Exploration Cooperative Research Centre (MinEx CRC), School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Perth 6009 WA, Australia; 3CSIRO Deep Earth Imaging FSP, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington 6151 WA, Australia; 4Anglo American, Group Discovery and Geosciences, 201 Charlotte Street, Brisbane 4000 QLD, Australia; 5International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Perth 6009 WA, Australia

Electromagnetic methods are useful tools to understand cover-basement interface in many aspects. They could provide valuable information to exploration targeting, mineral prospectivity mapping and in some cases. They can also provide volume constraints for groundwater and energy resource estimates with a fraction of the cost of drilling. In this study, we use passive seismic Horizontal to Vertical Spectral Ratios (HSVR) to reduce the uncertainties of detecting cover-basement contact in Magnetotelluric (MT) depth-resistivity models. 

We invert MT apparent resistivity and phase using Occam’s inversion algorithm to obtain resistivity-depth model. Generally, Occam’s inversion produces the smoothest model that fits the observations (data) with a certain target misfit. But the smoothest model may not fully describe a realistic resistivity structure, such as thin resistivity layers and sharp resistivity contrasts (eg. impermeable basement), which is critical to identify the cover-basement transition and thus interface. For this study, we expect to control the MT depth-resistivity model smoothness (or roughness) without producing unrealistic models. For that, we brought forward the importance of detection of cover-basement interface prior to 1D MT inversion. Our approach is to use a prior interface-depth model to support interface prediction by adjusting the model roughness values in Occam’s 1D inversion. This prior interface-depth model is generated from co-located or reasonably placed passive seismic-HVSR models, which give robust results to detect possible interfaces up to 1500m depths.

To test our approach, we created a synthetic homogenous two-layer cover-basement case with 20 Ω m cover resistivity, 1000 Ω m basement resistivity and a cover-basement interface at 1000m depth. The 1D forward response (from 104 to 10-3 Hz) was calculated and 5% random noise was added to the synthetic data. Firstly, a cover-basement interface depth interval is estimated using synthetic HVSR model data. Then we performed 1D MT inversions. The inversions are regularized using a series of roughness penalty values ranging from 0 (no penalty) to 1.00 (maximum penalty) within this hypothetical cover-basement transition depth interval.  Thinner MT depth mesh was used at the HVSR derived cover-basement interface region to identify accurate resistivity variations.

Preliminary results on the synthetic test show that the depth roughness penalty values between 0.05-0.25 reveal sharp resistivity contrasts consistent with the true depth to cover basement interface. We tested a spectrum of cover-basement depths and cover basement resistivity scenarios. We expect to test the procedure further for multi-layer synthetic cases and analysed the results statistically to validate our approach. Additional priori information, such as borehole data and stratigraphy data will be used to improve the HVSR-based constrain for the 1D MT (real data) inversion.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia. The work has been supported by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/xxx.


Biography

Nuwan is a PhD candidate working with Mark Jessell, Jeremie Giraud, Mark Lindsay, Hoel Seille, Lachlan Hennessy and Vitaliy Ogarko. He is working to Integrate magnetotelluric data and Passive seismic data to characterize the cover-basement interface. This project is a collaboration between MInexCRC and Loop consortium

The urban geochemical baseline of Canberra: Does it provide dirt on criminals?

Aberle, Michael1, de Caritat, Patrice2,1, McQueen, Ken1, Hoogewerff, Jurian1

1National Centre for Forensic Studies, Faculty of Science and Technology, University of Canberra, Canberra, Australia; 2Geoscience Australia, GPO Box 378, Canberra, Australia

Topsoil is a common material that may be transferred to people and objects prior to, during, or after perpetrating a criminal activity. Traditionally the use of soil material in law enforcement operations involves one-to-one comparison of recovered soil evidence with reference samples collected from known areas of interest. In casework and intelligence applications where this contextual information is not available, properties of the recovered evidence may be used to triage geographical regions as areas of low and high interest. If sufficient relevant spatial information is available, this approach may provide forensic intelligence to better focus operational resources on areas of interest. Here, spatial variability is both a strength and a weakness, with research required to determine 1) which parameters are sufficiently discriminatory at the chosen spatial scale, and 2) the effect of contributing anthropogenic and geogenic sources on forensic soil provenancing applications.

 To investigate these issues, a high-density (1 site/km2) geochemical survey has been conducted to map the compositional variation of ~700 urban topsoil (0-5 cm depth) samples across Canberra, Australian Capital Territory. As a study location, Canberra represents a large city (population > 450k) with a system of urban open green spaces and bushland reserves, as well as minimal heavy industrial activity. Thus, in addition to law enforcement applications, the effect of urbanisation on the environment can be studied without significant masking by industrial pollutant sources. Using standard protocols for urban geochemical surveys, including extensive quality control measures, the samples have been prepared in a fusion flux matrix and characterised for bulk elemental geochemistry using X-Ray Fluorescence and Inductively Coupled Plasma – Mass Spectrometry analysis of total acid digests. Bulk mineralogy has been determined on a subset of samples using powder X-Ray Diffraction.

The results demonstrate that the geochemistry of the topsoils is strongly influenced by the dominant lithological units of the underlying bedrock. While the concentrations of known anthropogenic elements are mainly below thresholds for health-based investigation (with 1 or 2 sites marginally at threshold), there is evidence of diffuse anthropogenic contributions, particularly in older suburbs and light industrial areas.

For provenancing applications, a number of different approaches have been suggested to ‘match’ a recovered sample to a map. Typically, these involve comparing the properties of the recovered sample to those in each ‘target’ survey grid cell and attributing statistical significance to some measure of ‘overlap’ at an arbitrary inclusion/exclusion threshold (e.g. 95%). By instead comparing each grid cell to a series of questioned samples from within and outside the survey boundary, and weighing each probability by the probability of observing the grid cell value in the total dataset in a likelihood ratio approach, we have demonstrated that regions of interest may be reduced in a more conservative manner better suited to forensic provenancing than previous approaches.

Further merits and challenges of provenancing topsoil from urban environments will be presented, notably the significance and impact of displacement and introduction of topsoil material from other areas on developing forensic soil surveys, as well as determining the source of questioned samples.


Biography

Michael Aberle is currently a PhD candidate at the National Centre for Forensic Studies, University of Canberra. Together with industry partners, his research primarily focuses on evaluating the forensic utility of a fit-for-purpose, high-density soil geochemical survey of the Australian Capital Territory for forensic provenancing of bulk and trace soils.

Where Will Australia’s Next Volcano Erupt? : Wider Perspectives

Sutherland Frederick L.1, Graham Ian T.2

1Geosciences, Australian Museum, 1 William Street, Sydney, NSW 2010, Australia; 2School of Biological Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia

East Gondwanan Cretaceous late Mesozoic thermal rifting initiated near-continuous Australian intraplate basaltic volcanism that inevitably will continue. Researchers mostly expect that the next eruption will likely occur within the young SW Victoria-SE South Australia or NE Queensland basalt fields. These areas preserve many eruptive features, although the southern fields seem past peak activity, while the northern fields seem closer to peak activity. This presentation looks beyond these areas and examines isolated eruptive events within the last 5 Ma elsewhere throughout Eastern Australia. These sites forecast that a new volcano may well erupt in an unexpected area.

Queensland: Young isolated basalt sites include 3.7 Ma nephelinites at Silver Plains, over 200 km NW of the main NE Queensland basalt provinces. These fields have been linked to a cryptic mantle plume below the adjacent Coral Sea floor. The isolated South Barnard offshore islands are ~ I Ma old pyroclastic cones and intrusive dykes, SE of the main Atherton basaltic field. A further eruption there would create shallow marine explosive activity. Mount St Martin, northern Bowen Basin, is a unique   isolated 2.4 – 3.1 Ma lava-capped pyroclastic vent of hybrid basanite-trachyte, much younger than    adjacent basalts and rhyolites of the Nebo Province. In western Qld, in the Winton-Longreach region, 3He/4He and 87Sr/86Sr studies on ground water discharges and an underlying slow seismic anomaly suggested sub-surface cooling basaltic intrusions. In SE Qld, an age-decreasing trend of isolated young basaltic fields extends 200 km SW from Bundaberg to Brigooda. It predicts potential further eruption to the SW, but this may depend on the lithospheric depth.

New South Wales: Young 2 – 5 Ma reset zircon megacrysts eroded from basaltic eruptives fringe outskirts of older basaltic areas in several regions. In rare cases they occur in diatremes, as at Gloucester River, SW of Barrington Tops shield volcano. Otherwise, they concentrate in alluvial deposits on the eastern flank of that volcano. In Tobins Camp lead SE of Yarrowitch, 2.7 Ma zircons in the isolated deposit are 40 Ma younger than adjacent Yarrowitch basalts.  At Oban and Uralla, 2 – 3 Ma zircons are ~20 Ma younger than adjacent basalts. In SW NSW, bentonite ash beds in 1.6 – 2.4 Ma Murray Basin strata at Arumpo seem linked to proximal eruptive sites.

Victoria:  In eastern Victoria, the Uplands basalts incude 2 and 4 Ma flow events, while to the NW near Toombullup alluvial deposits contain 2 Ma reset zircons.

Tasmania-West Tasman Sea:  Mantle CO2 discharges in NW Tasmania and seismic activity east of Flinders Island and in the Central Tasman Sea floor mark predicted dormant plume positions for the East Australian plume array. Recent research voyages have now located young sea mounts at the predicted plume nodes for the Tasmantid and Lord Howe seamount chains.

Summary: These widespread young activities offer a complex dynamic scenario for future eruptions.


Biography

Frederick Lin Sutherland, BSc (Hons), MSc (Univ.Tasmania), PhD (James Cook Univ.), worked at Queen Victoria Museum, Tasmanian Museum in Tasmania, James Cook Univ., Townsville, Qld, The Australian Museum, Univ. Western Sydney, now Senior Fellow, Geosciences, Australian Museum. Researches involve igneous rocks, Australian Geology, gemstones, zeolitic suites, mass extinctions.

What is Under the Antarctic Ice: An Integrated Study of U-Pb, O and Lu-Hf Isotopes

Miss Bei Chen1, Mr Ian Campbell1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

Antarctica is the central piece in the Gondwana jigsaw, connecting Australia, India and Africa. Little bedrock is exposed in Antarctica with over 98 % of the continent covered by ice. Its geology can provide new insights into the relationship between Antarctica and its neighbours and elucidate its role in the amalgamation and breakup of the Gondwana supercontinent.

Detrital zircons separated from IODP holes drilled around Antarctica have been analysed for U-Pb, O and Lu-Hf isotopes. U-Pb results show major detrital zircon crystallization peaks at ca. 250, 550, 950 and 1200 Ma, with a minor peak at 1600 Ma. They broadly correlate with younger peaks in the Australian detrital zircon population, but the older Australian peaks are missing. By far the largest peak, at ca. 550 Ma, is interpreted to represent zircons derived from the Transgondwana Supermountain formed by the collision between East and West Gondwana. Unlike previous studies, based on 40Ar/39Ar dating from hornblende and biotite, our data show a significant ca. 250 Ma peak, indicating that Antarctica was affected by an event of Pangaea age.

Oxygen isotope in zircons display a step increase at the end of the Archean, consistent with the temporal evolution of zircon δ18O as recognized by Valley et al. (2005). δ18O values of ca. 500 Ma group from Antarctica cover a large range (4.9-11‰), similar to the range of δ18O in ca. 500 Ma detrital zircons from Australia. Interestingly, zircons of ca. 100 Ma from West Antarctica are unusual in having δ18O less than the mantle value, implying crystallization from a felsic magma produced by melting wet basalt. Lu-Hf isotopes of these detrital zircons show that they were derived from juvenile crust, which formed around 100 Ma. εHf values of the ca. 550 Ma zircons show the largest variation (-18 to 10), and in this respect are similar to zircons of the same age from Australia. Arc Mantle Hf model ages reveal three major periods of growth of Antarctic continental crust: 300-500, 1200-1600 and 2000-2300 Ma, and indicate that the growth of the Antarctic continental started to form after the Archean. An unexpected outcome of this study is that it showed that Antarctica is younger than the other continents we have investigated both in terms of its model ages, and the orogenic events that have affected it.

Reference: Valley, J. W., Lackey, J. S., Cavosie, A. J., Clechenko, C. C., Spicuzza, M. J., Basei, M. A. S., … & Peck, W. H. (2005). 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contributions to Mineralogy and Petrology, 150(6), 561-580.


Biography

2016 – Present: Ph.D. student Research School of Earth Science, Australian National University

2013 – 2016: M.S. in Geochemistry State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

2009 – 2013: B.S. in Geology College of Earth Sciences, Jilin University, China

Using Trace Element Chemistry of Magnetite as an Indicator Mineral at the Starra Iron-Oxide Copper Gold Deposits, Northwest Queensland

Mr Max Hohl1, Assoc. Prof. Shaun Barker1, Dr. Jonathan  Cloutier1, Dr. Jeffrey Steadman1

1CODES – Centre of Ore Deposits and Earth Sciences, University of Tasmania, Hobart, Australia

Iron Oxide – Coper – Gold (IOCG) deposits are hydrothermal ore deposits that occur worldwide and globally account for significant amounts of copper, gold, and uranium. They are defined by large scale potassic, sodic and iron alteration that often extend several kilometres from the mineralised centre, making it difficult to differentiate between fertile and barren alteration systems. In recent years, it has been suggested that trace element chemistry of alteration minerals may be used to discriminate fertile from barren hydrothermal systems. In this aims to test the discrimination potential of magnetite at the Starra Au-Cu system which is hosted in the Eastern Fold Belt of the Mount Isa Inlier in Northwest Queensland (Australia). Five deposits occur along a circa 6 kilometres long interval. The mineralisation is spatially associated with magnetite-hematite dominated ironstones along the Starra shear, which are focused at the contact between the Answer Slate to the west and the Staveley Formation to the east. Magnetite is the dominant iron oxide mineral in the ironstones and define a strong magnetic anomaly that can be detected for more than 20 kilometres. Incorporation of trace elements in magnetite is influenced by physicochemical parameters at time of precipitation such as temperature and redox, making it ideal to record process-based information. In addition, the extensive nature of the ironstones at Starra makes it an ideal location to study the trace and major elements variations in magnetite with increasing distance to the ore bodies.

At Starra, mineralisation zones are associated with variable magnetite overprinted by hematite. New laser ablation ICP-MS of magnetites from distal, proximal, and mineralised setting reveals that magnetite spatially associated with the mineralisation contain lower V content compared to distal magnetite, suggesting higher fO2 conditions in mineralised areas. Hematite within in ironstones replaced the pre-existing sedimentary lithology retains the original texture as shown by different crystal orientation and distinct trace element concentrations in hematite. Scheelite inclusions in magnetite support reduction of hematite to magnetite due to relative incompatibility of W in magnetite compared to hematite. Together, these indicate a complex evolution of prevailing redox conditions during formation of the IOCG system.

Our findings support previous formation models that suggest the main controlling factor on the Cu-Au precipitation in IOCG systems is the oxygen fugacity fO2 of the hydrothermal fluid. At Starra, previous genetic model suggested that oxidized fluids interacted with magnetite and were reduced, leading to reduction of sulphates to bisulphides and precipitating Cu.


Biography

Max Hohl completed his master’s degree in Germany, working on porphyr copper deposits in Northern Greece. In September 2019 he started his PhD degree at CODES working on the Starra Cu-Au deposits in the Mt. Isa Inlier in Northwest Queensland.

Using macrofossils to interpret peatland facies of Miocene brown coals, Gippsland Basin, southeastern Australia

Dr Anne-Marie Tosolini1, Dr Vera Korasidis2, Associate Professor Malcolm Wallace1, Dr  Barbara Wagstaff1, Professor Robert Hill3

1School of Earth Sciences, The University Of Melbourne, Australia, 2Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, U.S.A., 3Environment Institute, University of Adelaide, Adelaide, Australia

Coals rarely preserve fossil leaves due to catabolic, fungal, bacterial, and/or root disturbance mechanisms that degrade leaf material deposited on the surface of swamps.  Well-preserved leaf macro- and meso-fossils (cuticles) have, however, been recovered from the Miocene Morwell 1 brown coal seams in the Gippsland Basin, southeast Australia. Three separate mechanisms of preservation facilitated the accumulation of these floral Lagerstättan, with the unifying condition for all mechanisms that plant material is delivered directly to the anaerobic catotelm and avoids the degrading aerobic acrotelm. Well-preserved leaf material found in these coals is, thus, derived from leaf litter that falls into low-energy acidic and anoxic water-filled depressions that lie below the water table.

Cyclic successions within individual coal seams reflect repeated lithotype cycles in the peat swamps that represent peatland aggredation and record relative drying or terrestrialization events. Six facies occurring within the cycles are defined by colour, texture, gelification and weathering: laminated dark, dark, medium dark, medium light, light and pale. A full lithotype cycle was located within both the lower M1B and upper M1A seams in the Loy Yang Open Cut Mine, Latrobe Valley, where each facies was sampled, analysed for macrofossils where present, which yielded leaves, wood and seeds, then macerated for mesofossils, which yielded leaf cuticles. 

Laminated dark facies contain abundant rushes (Typhaceae/Restionaceae), common coral-ferns (Gleicheniaceae) and a high abundance of charcoal (but lack recognisable cuticles), suggesting deposition in open, well-lit, inundated, emergent to meadow marsh environments. Dark facies contain abundant Kauri leaves (Araucariaceae: Agathis yallournensis) and Blue Quandong endocarps (Elaeocarpaceae: Elaeocarpus); cuticle assemblages are dominated by gymnosperms (abundant Agathis yallournensis; common Podocarpaceae: Dacrycarpus, Dacrydium). These arborescent taxa suggest deposition in periodically inundated environments of a forested bog. Medium dark and medium light facies are dominated by angiosperms: abundant Elaeocarpus endocarps and Oleinites; common Agathis yallournensis and Myrtaceae leaves; and abundant cuticles of Proteaceae (Banksia laevis, possible Orites) and lilly pilly (Myrtaceae: Syzygium); and notably contain tawheowheo (Parachryphiaceae: Quintinia). Deposition of these facies was in an angiosperm-dominated forested bog environment. Light and pale facies contain abundant Casuarinaceae leaves and cuticles (and rare Quintinia) that represent shallowing upwards and drying of the swamps into ombrogenous forest bog, with some open canopy areas, and finally to the formation of oxidised soils.

The macro- and mesofossil elements of these six different facies within repetitive cycles in the M1B and M1A brown coals, thus, represent changing floral composition as a direct result of changes in substrate wetness during peatland aggradation and the evolution of wetland/peatland systems through the Early to Middle Miocene. Facies progression reflects development of the peatland from fire-prone marsh environments to an angiosperm-dominated, ombrogenous forested bog and supports previous palynological, sedimentological and charcoal analyses. These multidisciplinary analyses have overturned previous theories that charcoal represents the driest facies, whereas in fact, charcoal forms from the burning of fire-tolerant and flammable species in the marsh environments. Modern peat swamp analogues of these cyclic facies successions are found on the South Island of New Zealand and support the “dry-light”, shallowing upwards, facies model.


Biography

Dr Anne-Marie Tosolini, palaeobotanist, Lecturer, School of Earth Sciences, University of Melbourne. Dr Vera Korasidis, palynologist and Postdoctoral Fellow, Smithsonian Institution. Associate Professor Malcolm Wallace, Chair in Sedimentology, University of Melbourne. Dr Barbara Wagstaff, Honourary palynologist, University of Melbourne. Professor Robert Hill, Director, Environment Institute, University of Adelaide.

Understanding the nature transition of the Late Jurassic formations of the Surat Basin through borehole image logs using cumulative dip plots.

Claudio Luiz de A V Filho1, Kasia Sobczak2, Heinz-Gerd Holl2, Suzanne Hurter2 and Paulo Vasconcelos1

1School of Earth and Environmental Science, the University of Queensland, Brisbane, Qld, Australia; 2UQ Centre for Natural Gas, the University of Queensland, Brisbane, Qld, Australia

The Surat Basin located on the border of Queensland and New South Wales is one of the most prominent coal-bearing basins in Australia. Over the past decade, the basin has become one of
Australia’s largest and most productive coal seam gas (CSG) provinces, with major reservoirs discovered in the Middle – Late Jurassic Walloon Coal Measures.

The Walloon Coal Measures are directly overlain by the Late Jurassic Springbok Sandstone, which has been characterised as a regional aquifer. This raises major environmental concerns regarding the connectivity between the two formations and potential impact of coal seam gas exploration and production from the Walloon Coal Measures on local groundwater reservoirs in the Springbok Sandstone. Therefore, there is a pressing need for a good understanding of the relationship between the two formations, and accurate modelling of reservoir interconnectivity. Despite the growing interest in the Springbok Sandstone, however, the nature of the boundary between the two formations remains unclear. Thus, this study aims to better characterise the Walloon-Springbok contact using highresolution image log analysis in five wells in the north-eastern Surat Basin. Measured bedding-plane orientations are presented on cumulative dip plots that allow precise location of shifts in dip directions and angles related to unconformities, sequence boundaries and structural features.

The analysis revealed the presence of a distinct inflection point on the cumulative dip plot between the Walloon Coal Measures and the Springbok Sandstone in only one of the analysed wells. This inflection point reflects a shift in bed orientation that may indicate an unconformity. Conversely, the remaining four wells did not show a significant change in dip magnitude and azimuth at the base of the Springbok Sandstone, suggesting that there is no major or regional unconformity, contrary to previous studies. Additionally, several significant inflection points were observed in the Walloon Coal Measures in all five wells, indicating possible major time gaps in sediment accumulation within the formation, rather than at the formation boundary. These findings largely support recently published depositional ages from U-Pb zircon dating of tuffs that suggest the presence of an unconformity within the Walloon Coal Measures.

The image log analysis used in this study provides new constraints on depositional continuity and unconformity distribution in the CSG-producing formations of the Surat Basin. A more extensive
application of this approach across the basin in the future will allow assessing the magnitude and regional extent of the identified unconformities.


Biography

Claudio Luiz de A Vieira Filho is a PhD Candidate at the University of Queensland in Australia. His research focuses on characterising the transition between Middle to Upper Jurassic gas bearing coals and the overlying geological formation in the Surat Basin. His interests are basin analysis, sedimentology and chronostratigraphy.

Tracking plumbing system architecture in age-progressive intraplate volcanoes in Eastern Australia.

Tapu, Al-Tamini1, Ubide, Teresa1, Vasconcelos, Paulo1

1School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia

Cenozoic age-progressive volcanism in eastern Australia crops out as the longest age-progressive continental track of shield volcanoes that extends for ~2000 km. The so-called ‘central volcanoes’ show a southward-younging trend that has been related to the motion of the Australian plate over one or several stationary mantle plumes. Central volcanoes developed in regions of contrasting lithospheric thickness and have distinct eruptive volumes, however, the relationships between regional context and extent of volcanic activity remain poorly constrained.

Here we apply a high-resolution geochemical-geochronological approach to investigate differences in the time spans of volcanism and the architectures of the magma feeder systems at depth. We investigate central volcanoes Ebor, Nandewar, and Canobolas, located in regions of different lithospheric thickness and with varied volumes of magma from 50 to 300 km3.

The rocks are porphyritic to aphyric basalts to trachytes and rhyolites, including 5-40 vol.% phenocrysts of plagioclase, clinopyroxene, and minor olivine. Clinopyroxene-melt thermobarometry indicates crystallization at 10-3 kbar (25-10km depth) and 800-1150°C. Rhythmic-oscillatory zonations in plagioclase (An55) and clinopyroxene (Mg#70) suggest steady-state growth in deep reservoirs undergoing continued magma supply and differentiation of compositionally similar magmas. K-feldspar and green-clinopyroxene (Mg#25-45) are interpreted as antecrysts recycled from pockets of fractionated melts. Successive mafic magma influx before eruption generated growth of mafic mineral zones (plagioclase An65; clinopyroxene Mg#75) over partially resorbed, sieved, and/or patchy cores (An35-50 and Mg#55-65, respectively). 40Ar/39Ar geochronology indicates that the volcanoes were episodically replenished over timescales of ~0.1Ma, and the eruptive activity lasted for ~3 – 1.5 Ma.

The combined geochemical and geochronological studies suggest three spatially separated but genetically linked volcanoes were fed through comparable plumbing system architectures. Rhythmic mafic recharge and fractionation controlled the lifespans and tempos of eruptive activity.


Biography

The primary research area of Mr. Al-Tamini focuses on igneous petrology, volcanology, geochemistry. His current research work aims to understand the long term tempos of eruptive activity and evolution of magma plumbing system architecture in plume related intraplate volcanoes in Eastern Australia by applying high-resolution petrology, geochemistry, and geochronology techniques.

Trace element geochemistry of sphalerite from polymetallic sulfide mineralization in Betul belt, Central Indian Tectonic Zone, India.

Mr Bishnu Mishra1, Pitambar Pati, Muduru Dora

1Indian Institute Of Technology Roorkee, Roorkee, India

Sulfide mineralization in Betul belt (BB) in the central India tectonic zone (CITZ) is one of the critical zinc-enriched polymetallic sulfide mineralization in India. Stratiform ore bodies hosted within the volcano-sedimentary units were moderately conceptualized to be a volcanic-hosted massive sulfide (VHMS) type deposit by earlier workers. As it is not yet wholly realized whether the deposit is genetically less fertile or underexplored, uncertainty remains in its future course of exploration. In this study an effort has been made to provide insights into various genetic aspects of mineralization, including major elements content, the enrichment of trace elements, and consequently, their broad exploration significance. Therefore, sphalerite trace element geochemistry has been studied using electron probe microanalyzer (EPMA) to obtain total major elements and laser ablation inductively coupled mass spectroscopy (LA-ICP-MS) to measure trace elements concentration. The trace elements such as Pb, Mn, Co, Cu, Ga, Ge, Ag, Cd, In, Sn, Sb, Bi, along with Fe in bulk sphalerite specimens have been analyzed using LA-ICP-MS technique. Thereafter, the dataset has been investigated using a multivariate statistical procedure called principal component analysis (PCA). This study shows that sphalerite in the BB are relatively abundant in Fe content ranging from 4.58 wt% to 11.10 wt% with mean 7.54 wt%. Trace elements like Mn and In show comparatively high concentration with a mean value of 3533.31 ppm and 33.73 ppm respectively. On the other hand, Ga, Ge and Ag content are depleted in the sphalerite with a mean value of 0.58 ppm, 0.36 ppm and 0.90 ppm respectively. Subsequently, the ore-forming temperature is conservatively and separately estimated using the geothermometers devised by Kullerud, 1953 and Frenzel et al., 2016, which range from 3190C to 5560C and 374.100C to 402.250C respectively. This study suggests the sulfide mineralization in the BB is high temperature, magmatic-hydrothermal origin. The enrichment of elements is predominantly controlled by the ore-forming temperature and host lithology. We observed the effect of metamorphism and recrystallization of the BB sphalerite. We encouraged to target basement exposed areas for further exploration activities. Pre to syn mineralization deformation structures are considered to be useful to locate the mineralization. This study also significantly added the degree of confidence to the growing consensus and typified BB sulfide mineralization as a VHMS type of deposit.

Keywords: Sphalerite, LA-ICP-MS, trace element geochemistry, Sulfide Mineralization, VHMS, Betul Belt.


Biography

Bishnu Prasad Mishra completed his B.Sc. from Utkal University, Odisha in Geology, and M.Sc. from Indian Institute of Technology Roorkee, Roorkee. At present, he is pursuing a Ph.D. with MHRD Fellowship with the supervision of Dr. Pitambar Pati in the Department of Earth Sciences, IIT Roorkee.

1238

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.