Quantifying lateral and distal variability within hybrid beds, case studies from Central and Northern Italy

Brooks, Dr Hannah1, Steel,Dr Elisabeth2

1 School of Earth Sciences, 253-283 Elgin St, University of Melbourne, Carlton Victoria, Australia, 2 Department of geological sciences and geological engineering, Bruce Wing/Miller Hall, 36 Union Street, Queen’s University, Kingston, Ontario, Canada.

Hybrid beds or linked debrites are deposits that form under bi- or tri-partite flow conditions, involving both turbulent and laminar flow conditions. Often, hybrid beds occur with distal or lateral flow transformation following significant entrainment of a muddy substrate and/or declining turbulent energy. Hybrid beds have been noted to make up significant proportions of deposits within basin floor setting worldwide, most commonly within the distal fringes of lobe systems. The study examines dip and lateral variations in facies and architecture in hybrid beds using detailed facies analysis of selected sections within the Castagnola, Marnoso-Arenacea, and Gottero Formations, deposited within three different basins within central and northern Italy. Sections within these systems were selected where hybrid beds could be traced out laterally or down-dip for several metres to several kilometres. In total 407 samples were taken for laser-diffraction grain-size analysis. Samples were selected through beds at 20 cm intervals and across/ down-dip at 10’s of metre to several kilometre intervals. Layers within beds were classified into facies divisions which were ran through ‘EMMA’ End Member Modelling Analysis, this splits beds into an optimum number of end members that when combined create the trends found within the dataset. This method is utilized to establish patterns of changes within beds laterally and down-dip which are otherwise difficult or too complex to be quantified from field data alone.

This detailed study of facies, architectural and grain-size changes within the targeted deposits will help to establish how flow processes varied as flows spread laterally and moved downstream. Through quantifying the amount of mud within the matrix and clasts at any one time within the flow it is possible to interpret how and when turbidites and hybrid beds erode and incorporate sediment from underlying substrate. Initial results from EMMA indicate that using three end members for each formation is an optimum number for observing basinal trends within beds. These end members constitute a sand-rich unimodal end member, a silt-rich unimodal end member, and a bimodal poorly sorted end member, interpreted to represent a high-density turbidite, a low-density turbidite and a debrite respectively. Recognised trends within the data include a decrease laterally in the sand-rich unimodal end member within the Marnoso-Arenacea section, interpreted as an increased distance from the flow input or the presence of a lateral basin slope. Through application of this methodology in basins with well-established bed correlation it is possible to provide novel understanding that will significantly augment traditional field techniques. It is evident that laser diffraction grain-size analysis and EMMA are vital tools in furthering our understand of process sedimentology and should be applied more widely.


PhD in Sedimentology, University of Leeds, UK, studying Permian deepwater sediments in the Karoo Basin, South Africa. Undertook postdoc in Chiba University, Japan studying Pliocene deepwater outcrops, and Queens University studying hybrid beds in central and northern Italy. Now, postdoc at University of Melbourne studying Neoproterozoic rocks in Flinders Ranges.

Correlation of stratigraphic sequences to evaluate downstream transitions within the Wonoka canyon at Umberatana syncline, South Australia

Giles, Sarah M.1, Christie-Blick, Nicholas 1

1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA

Mid-Ediacaran (~580 Ma) paleocanyons as much as 1 km deep in the Wonoka Formation of South Australia are associated with the largest carbon-isotope excursion in Earth history, the Shuram anomaly. Widely interpreted as submarine, the canyons are thought by many to be comparable to those found at modern continental margins. New data from the northern Flinders Ranges reinforce an alternative hypothesis: that the Wonoka canyons were subaerially incised as a result of evaporative drawdown in a temporarily isolated marine embayment at the onset of the Gaskiers glaciation. Critical supporting evidence has emerged in the canyon-fill at Umberatana syncline, where four oblique cross-sections interpreted as a single sinuous canyon are being independently analyzed. The three incisions studied so far are characterized at the base by ~ 30-m-thick conglomerate-based cycles that are bounded by laterally persistent erosion surfaces and fine upwards into sandstone, siltstone, and minor carbonate (canyon-marginal tongues). Nine such cycles are confidently correlated between the first two incisions (Fortress Hill and Mt. Curtis), and at least plausibly related to seven cycles identified in the third incision (Muccabaloona south) on the basis of 168 measured sections and high-resolution physical stratigraphic mapping. Our correlation of cycles is based on similarities in facies stacking and stratigraphic position. Greater variability in cycle thickness at Muccabaloona south is attributed to facies changes within some cycles in the direction of sediment transport from channelized boulder conglomerate to pervasively rippled sheet-like sandstone and siltstone event layers, as well as a northward increase in structural complexity. The local erosional relief at cycle boundaries is comparable across the three outcrops: 3-6 m at Fortress Hill, 5-10 m at Mt. Curtis, and 3-7 m at Muccabaloona south. The observed stratigraphic organization and details of the facies lead us to interpret the diffusely stratified, channelized conglomerates as fluvial, and the prevalently rippled and laminated tabular sandstones/siltstones as deltaic. The lack of dish structures and nested channels in the sandstone facies, as well as the apparent absence of classical turbidites interstratified with hemipelagic mudstone further supports our fluvial-deltaic interpretation rather than a deep marine interpretation. An upward transition to primarily siltstone event layers interstratified with cm-scale carbonate couplets and monolithologic breccias is interpreted to represent canyon drowning. New field work is planned at the fourth incision (Muccabaloona north) to further test our model. We expect fewer conglomerate-based cycles, and additional fining overall. The correlation of stratigraphic cycles across all four incisions will allow us to evaluate the length scale of downstream facies transitions, which we expect to be much more abrupt in the subaerial incision model than the deep marine canyon interpretation.


Sarah Giles is a Ph.D. student at Columbia University in New York City, USA. Sarah’s Ph.D. research integrates geologic mapping, sedimentology, stratigraphy, isotope geochemistry, and geochronology to evaluate the origins, timing, and stratigraphic context of the mid-Ediacaran Shuram carbon isotope excursion in South Australia and eastern California.

Mineralisation around Mount Adrah, New South Wales: new observations by the Geological Survey of NSW

Wang, Yamei; Forster, David; Cronin, Dan; Montgomery, Karen; Blevin, Phil

1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia

This study summarises previous exploration in the Mount Adrah area and presents the results of new petrographic, sulfur isotopic and HyLogger hyperspectral analyses of intrusion-related gold mineralisation centred on the Hobbs Pipe intrusion at Mount Adrah, New South Wales. The work was conducted as part of the five-year East Riverina Mapping Project, which improved geological understanding to aid mineral exploration and enable informed land use decisions in the region.

The Hobbs Pipe deposit contains about 770 000 oz of gold and is interpreted to be an intrusion-related gold deposit. Two main styles of mineralisation are present: disseminated pyrite–gold, developed within a cylindrical, zoned intrusion (Hobbs Pipe); and narrow, high-grade gold reefs located nearby. Drillhole GHD009 (total depth 1,312 m) provides a representative section through the Hobbs Pipe deposit and associated rocks.

Reconnaissance petrology was undertaken on twenty thin sections collected from drillhole GHD009. Three main phases have been identified: a felsic monzogranite core, an intermediate quartz monzodiorite to diorite outer core, and an intermediate–mafic diorite to gabbro rim. Disseminated pyrite-gold mineralisation in the Hobbs Pipe deposit is mainly hosted in the felsic monzogranite core.

Twenty-eight sulfur isotope results were obtained; twenty-seven from drillhole GHD009 and one from White Deer Reef, intersected in drillhole GHD011. δ34S results mainly fall within a relatively narrow range and suggest a single, magmatic-dominated source of sulfur.

HyLogger hyperspectral analysis of drillhole GHD009 was conducted at the WB Clarke Geoscience Centre, Londonderry, New South Wales. Chlorite, white mica and dark mica type all show mineralogical changes near lithological contacts and/or alteration which relates to gold mineralisation.

The geology and alteration assemblages in conjunction with new petrology, hyperspectral data and sulfur isotopes suggest that both the disseminated mineralisation within the core of the intrusive stock and the distal high-grade gold-bearing reefs are related to the same mineralising event, and probably the same fluid source.


Dr Yamei Wang has broad experience in minerals exploration. She also has a strong academic background, with a PhD in structural geology. She joined Geological Survey of NSW in February 2019 and her current role is Senior Geoscientist in Mining & Exploration Assessment team in the Survey.

QEMSCAN and PGE geochemistry to track sulfide saturation, magmatic evolution and fertility of porphyry suites (on the example of Mount Hagen, Papua-New Guinea)

Misztela, Monika1, Campbell, Prof. Ian1

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

Sulfide saturation is believed to play an important role in porphyry systems fertility. It can also determine the type of ore in an economic deposit. During the early stages of magma evolution Cu, Au and Pd behave incompatibly, and they are concentrated in the melt by fractional crystallisation. If sulfide saturation occurs early, the chalcophile elements are trapped in sulfide phases and locked in an underlying magma chamber, where they are able to enter the fluid phase, which results in a barren system. However, if sulfide saturation occurs later, after or shortly before volatile saturation, the metals are able to enter the fluid phase and form an economic deposit.

Platinum Group Elements are sensitive indicators of sulfide saturation due to their high partitioning into immiscible sulfide melts. Cu and Au also partition into sulfide melts but with lower partition coefficient, so that PGE are more sensitive to any changes in the system. Furthermore, their solubilities in hydrothermal fluids are lower; they are less mobile than Cu or Au, so their concentration in rocks reflect magmatic rather than hydrothermal processes.

QEMSCAN (Quantitative Evaluation of Materials by Scanning Electron Microscopy) can provide valuable information when studying magmatic suites. It provides high-resolution maps and images of mineral and elemental distributions, porosity structure maps, the density of samples and most importantly, quantitative mineral and elemental analyses.

Mount Hagen is an arc system that could potentially be related to a porphyry deposit. Its favourable location and proximity to other deposits was the initial motivation for undertaking this project. Eighteen samples, covering a compositional range from 2 to 11.5 wt.% MgO, were analysed for the PGE. Thirteen rocks were selected for detailed petrological description and mineral quantification by QEMSCAN. Quantitative reports were used to plot the cumulative abundance of major mineral phases as a function of whole-rock MgO. The results show that significant changes occur at ca. 6 wt.% MgO. Hornblende appears as a primary phase, which is attributed to major input of wet magma into the system at that time. Analysis of olivine cores revealed a reversal in their compositional trend at 6 wt.% MgO, which confirmed this hypothesis. Pd, Pt and Au analysis showed that concentrations of these elements initially increase, up to about 8.5 wt.% MgO, followed by a decrease. The change at 8.5 wt.% Mg is interpreted to mark the start of sulfide saturation, and it correlates well with the similar change in the Cu trend. At about 6 wt.% MgO there is a reversal of the trend, followed by a second decline, which can also be explained by a new magma influx to the system.

Mount Hagen was an open system, with at least one major magma recharge into the chamber. The magma experienced early sulfide saturation, which makes it unlikely that this system produced economic mineralisation.


Monika Misztela is a PhD student at the Research School of Earth Sciences, Australian National University interested in petrological and geochemical studies. Monika comes from Poland, where she did her Bachelor`s and Master`s degree in Economic Geology at the AGH University of Science and Technology in Krakow.

Corundum Conundrum


1Unsw, Kensington, Australia

Volcaniclastics and basalts are widely spread across the Anakie Gemfields in Central Queensland, Australia. A principal elevator of sapphires to the surface is widely believed to have been eruptions from Hoy Province volcanoes, followed by disintegration and release of sapphire from lava flows. However, it is noteworthy that volcaniclastic rocks are usually associated with areas where intensive sapphire mining has taken place. Although corundum has been reported from some Hoy plugs, it is extremely rare but is more commonly found by miners in volcaniclastics (called clinker by local miners). Crucially, although numerous basalt clasts of various textural types occur within the mined gravels, despite a hundred years of mining miners have not found any corundum within these clasts. The volcaniclastics from this field vary widely in mineralogy, texture, and chemistry. A surprising result is that some samples comprise over 50% dolomite, with some containing much higher amounts of around 70%. The volcanic origin of these is supported by anomalously high levels of zirconium and titanium. In addition, these alkaline rocks sometimes contain abundant small equant-shaped flakes of biotite, now altered to clays. This raises an important question: were the dolomite-rich volcaniclastics erupted from one or more carbonatite-type volcanos? And were these also the source for the corundums? Also, what were the sources for the diverse suites of zircons and spinels found in the gem gravels? The timing of events is complicated by the range in ages of Hoy plugs from 67 Ma to 14 Ma. The most likely scenario is that lavas flows initially protected earlier volcaniclastics from erosion. After removal of the flows, the volcaniclastics were free to release corundum to the waterways. Trace element geochemistry for seven plugs and four volcaniclastic samples suggests that they are genetically related. Matters are complicated by an undocumented basalt field high in the Drummond Range, west of the sapphire fields where clinker was also observed, along with possible volcaniclastics immediately below the basalt flows.


Forty six years experience within the resources sector including working as a  petrologist, exploration geologist, equity analyst and portfolio manager, commodity analyst, gold fund manager and minerals economist.

U-Pb ages and aluminium concentrations of colourful sapphire-related zircon megacrysts from Far North Queensland:  Merlot, Rosé and Champagne

Charlotte M Allen1, Erica J Porter1 and Mitchell DeBruyn2

1Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, 2School of Mechanical, Medical and Process Engineering, Faculty of Science, Queensland University of Technology, Brisbane

Detrital sapphires are commonly found with megacrystic zircon but it is only co-location that links these free grains.  If related, zircon can constrain the circumstances by which nature creates corundum. Trail and co-workers suggested that zircons from high-Al-activity magmas (peraluminous ones) have Al contents > 4ppm, a step-jump up from Al contents from zircons from metaluminous magmas.  Aluminium is generally not considered to crystallize in zircon because of radius and charge but was included in the 24 isotopes analytical list. Twenty-seven 25 micron laser ablation spots were targeted on 3 colour groups from detrital zircon megacrysts associated with sapphires of Scot’s Camp near Undara National Park, a Lava Plains locality.  Three spots on each of three grains in each colour group were analysed by Agilent 7900 ICPMS.  Zircons were cathodoluminescence and backscatter imaged. Reported dates are concordant 206Pb/238U ages using Temora as calibrant with no Th-disequilibrium correction. Trace element concentrations (TE) are based on NIST 610 with Si as internal standard and assumed 15.22 wt% concentration.

Champagne grain #1, wholly, proved to be older than the rest at 2716 Ma (1stdev, n=3) but its cathodoluminescence, morphology and TE are not distinctive. Two other Champagne spots gave 5.5 and 4.1 Ma, ages older than Merlot grains which are consistent at 2.60.2 Ma (n=7). Rosé grains give the same age (2.90.5 Ma, n=5).

TE among the 9 grains have somewhat grain-specific compositions which is true of aluminium.  Merlot, Rosé and Champagne have average Al contents of 98, 2619 and 3323 ppm (1stdev), respectively.  The 271 Ma grain (Champagne#1) contains variable Al (4 to 78 ppm) whereas #2 is consistent at 812 ppm (n=3).  Compare these averages to our reference zircons and sources: Temora, (quartz-monzodiorite), Plešovice, (alkalic granulite), and 91500, a detrital megacryst provisionally associated with syenite (?). Respectively, they give 1.81.0, 8935 and 7.72.4 ppm Al.  The Lava Plains megacrysts, indeed, were generated from Al-rich sources.

Sapphire-associated zircons tend to be TE-poor as in a 13.6 Ma example from Mt Weldborough, NE Tasmania. Uranium is <100 ppm in Champagne and Rosé but Merlot has U from 200 to 1250 ppm save one and thus gives the more precise age. Except for Al and P, Merlot has greater TE contents.  Zircon classification based on Yb, U, and Hf indicate these are continental and/or kimberlitic zircon types. Features of note are small to zero Eu anomalies and positive Ce anomalies of 100-300 for Merlot and Rosé but 10s for Champagne (method after Blundy and Wood). These Lava Plains zircons are decidedly not-mantle-like, particularly in their Zr/Hf (>>37, the chondritic value).

The dominant age of 2.6 Ma is in the range of the oldest Lava Plains eruption ages, however, to find a morphologically non-distinctive Permian megacryst among them means that nature has repeated herself in providing the odd conditions to generate both sapphire and zircon megacrysts, crystals that have been brought to the surface by alkalic basalts of similar or much younger age. Permian is a common age among Queensland sapphire-related zircons.


Charlotte is the Director of the Central Analytical Research Facility, IFE.  She is interested in zircons, their ages and trace elements.  As collateral material from a process engineering problem, Mitch identified the zircons as interesting, and Erica named, imaged and analyzed them. Potential reference materials?

Trace element sector zoning in clinopyroxene as a function of undercooling: An experimental evaluation in trachybasaltic magmas

MacDonald, Alice1, Ubide, Teresa1, Masotta, Matteo2, Mollo, Silvio3, Pontesilli, Alessio4

1University Of Queensland, Brisbane, Australia, 2University of Pisa, Pisa, Italy, 3Sapienza University of Rome, Rome, Italy, 4University of Otago, Dunedin, New Zealand

Clinopyroxene chemistry is increasingly being utilised to investigate magmatic processes, due to its ability to record an extensive history of physiochemical changes in the host magma. However, clinopyroxene chemistry is not only influenced by pressure, temperature, and magma composition, but also by kinetic effects that may generate compositional zoning, such as sector zoning. Previous experimental work has highlighted the role of undercooling (ΔT = T­liquidus – Tsystem) on crystal morphology and major element chemistry of sector-zoned clinopyroxene. In this regard, the spatial distribution of trace elements in clinopyroxene remains relatively underexplored.
Here we present trace element data collected by laser ablation ICP-MS mapping of experimental sector-zoned clinopyroxenes crystallised from a trachybasalt melt representative of one of the most primitive magmas ever erupted at Mt Etna volcano (Italy). Experiments were conducted an isobaric pressure of 400 MPa, and a range of temperatures (1050 – 1200°C) and H2O contents (0-4 wt.%). Undercooling was attained by cooling the experiments from a starting temperature (1300°C) above the clinopyroxene liquidus at a rate of 80°C/min to a resting temperature (Tsystem)  resulting in a range of ΔT (23-173°C).
Our results indicate that clinopyroxene crystals show different styles of zoning across the entire range of ΔT, where zones enriched in Al are also enriched in HFSEs and REEs. At ΔT < 40 °C, clinopyroxene is sector-zoned with distinct Al-poor hourglass and Al-rich prism sectors. At ΔT = 75 – 123°C, skeletal morphologies dominate as crystal growth transitions from interface controlled to diffusion limited. These crystals are comprised of Al-rich skeletons and Al-poor overgrowths. At ΔT = 123-173°C, clinopyroxene is primarily dendritic, with subtle Al zoning.
The overall correlation between Al and trace element composition is attributed to charge-balancing mechanisms. Highly charged cations are favourably incorporated into the M1 and M2 sites with increasing Al, to compensate for the substitution of Si for Al in the tetrahedral site. 
Thermodynamic modelling of lattice strain parameters for 3+ cations in the M2 site (REEs + Y) illustrates that the strain-free partition coefficient (D0) is strongly correlated with ΔT. Conversely, the optimum radius (r0) and Young’s modulus (E), remain constant across our dataset.
The application of trace element calibrations to natural samples from Mt Etna supports the growing conception that sector zoning in clinopyroxene is related to low degrees of ΔT, whereas microlites crystallised at moderate degrees of ΔT. Our new experimental data could bring crucial new insights into magmatic processes which occur under polythermal and polybaric crystallisation regimes, in the lead up to volcanic eruptions. 


I am a current PhD candidate at the University of Queensland, supervised by Dr. Teresa Ubide. My research is focused on investigating sector-zoned clinopyroxenes from volcanic settings, through the analysis of natural and experimental samples using high-resolution techniques.

The Distribution and Origin of Hydrogen Sulphide Gas in the Triassic Montney; unconventional Play, British Columbia and Alberta, Canada

Chalmers, Gareth1, Bustin, Amanda2 and Bustin, Marc2

1 University of the Sunshine Coast, Sippy Downs, Australia, 2 University of British Columbia, Vancouver, Canada

The distribution and origin of hydrogen sulphide (H2S) within the Triassic Montney Formation of the western Canadian sedimentary basin (WCSB) were investigated in British Columbia and Alberta, Canada. Hydrogen sulphide is a toxic gas that can be co-produced with hydrocarbons and impacts well economics and the environment. Even small amounts of H2S can impact hydrocarbon operations by depositing ‘elemental sulphur’ within pipelines and compressors as observed in Australia and overseas.

This study has mapped the H2S concentration in the Upper, Middle and Lower sections of the Montney Formation as operators are drilling multi-directional well pads within three zones of a 200 m reservoir. The Montney Formation has tested or produced H2S gas at concentrations between 0.001% and 22%. The stratigraphic and lateral variation in the H2S concentration can be inexplicable.

Sulphur available to generate H2S includes sulphide oxidation, decomposition of well-completions surfactants, bacterial sulphide reduction, kerogen cracking or fluid migration of sulphate ions from sulphur-rich evaporites. The isotopic ratios of sulphur and oxygen will depend on the source and the formation pathway of the H2S gas and these ratios can be used to help model H2S gas generation. Samples were collected from the Triassic Charlie Lake, Doig, Montney formations and the Devonian Nisku, Elk Point and Muskeg formations within areas of sour wells. Organic matter, sulphate and sulphide minerals were isolated using chemical and physical mineral separation techniques. These samples were analysed for sulphur and oxygen isotopes at the Ján Veizer Stable Isotope Laboratory, University of Ottawa (Ontario, Canada). Sulphur and oxygen isotopic ratios from sulphate minerals within the Montney Formation and the Charlie Lake Formation have a range between 9.0 to 18.0 ‰ V-CDT and -5.0 to 19.0 ‰ V-SMOW, respectively. These isotopic ratios differ from the sulphur and oxygen isotopic ratios from sulphate minerals sampled from Devonian rock sources which vary between 18.0 to 30.0 ‰ V-CDT and 12.0 to 30 ‰ V-SMOW, respectively. The sulphur isotopic ratio measured from H2S gas of producing Montney Formation wells varies between 9.3 and 20.9 ‰ V-CDT.

Preliminary results from isotopic analyses suggest that the sulphur that generated H2S in the Montney Formation is from Triassic sulphates or a mixture of Triassic and Devonian sources and not solely from Devonian rocks as first expected. It is postulated that the sulphate ions have migrated through localised fractures into the Montney Formation and then the sulphate is used to generate H2S. Another possibility is the H2S gas formed in the Charlie Lake Formation and/or Devonian rocks and then migrated into the Montney Formation. Textural relationships between the reservoir rock and the sulphate minerals is currently being examined which will provide key data for creating a H2S generation model for the Montney Formation.


Gareth Chalmers has used a multidisciplinary approach to investigate coal seam gas and shale reservoirs since 2002 at University of British Columbia, Canada. He also worked at Shell to develop the Duvernay shale play. Gareth now lectures at the University of the Sunshine Coast and is researching Australian gas reservoirs.

Structural analysis of wavecut platform structures, Shellharbour, New South Wales

Lennox, Paul1, Goff, James1,2, Edwards, David1 & Coates, Ashlie1,3

1The University of New South Wales, Sydney, Sydney, Australia, 2School of Ocean and Earth Sciences, University of Southampton, Southampton, UK, 3Department of Industry, Innovation and Science, Canberra, Australia

Bedrock-sculptured features on the wavecut platform south of Shellharbour, NSW have been structurally assessed as some consider they have been formed by a southeast to northwest directed (mega)tsunami. These features have formed within the up to 120 m thick, Late Permian Upper Bumbo Latite unit of the Gerringong Volcanics.

The research focused on two subareas; the southern side of Bass Point (Bushranger Bay) where apparent s-forms due to tsunami-sculptured rock platforms expose so called “muschelbruche” having a relief of decimeters to several metres and Atcheson Rock one kilometre to the southwest. At Atcheson Rock the orientation of the channel and the so called vortex nearby; a semi-circular depression with raised internal feature were assessed with respect to the orientation of the local jointing, layering and flow foliation.

The muschelbruche vary in size from a few metres across to tens of metres across. The orientation of bedrock-sculptured features south of Bushranger Bay display various relationships to the flow foliation and jointing. In some cases the sides or raised back of the “muschelbruche” are aligned subparallel to either the flow foliation, joint strike or some combination. The long axis of the muschelbruche are not predominantly aligned northwest-southeast as expected if they were generated by a single (mega)tsunami from the southeast.

The cross-cutting ? Cenozoic dyke at Atcheson Rock weathers to a channel in the basalt and is along strike of a similar eroded dyke and channel on the coastline south of Bushranger Bay. The strike of this dyke is subparallel to the strike of flow foliation and the dominant jointing in the basalt.   Some assert that the canyon containing this dyke is not structurally controlled. Normal coastal erosion of this dyke has caused Atcheson Rock headland to almost be cut off from the mainland. The Cenozoic dyke has intruded subparallel to the dominant northeast to southwest striking joints and flow foliation in this region. Some consider that the vortex pit on the south side of Atcheson Rock was eroded by a tsunami from the southeast causing a whirlpool over 10m wide and characterised by a central plug of bedrock 5m high. The presence of a  rounded but north-south elongated depression in the basalt (“vortex”) can be explained by weathering related to the pervasive localised NNE-SSW and ~ NW-SE striking jointing, gently southeast-dipping layering in this locality but not the NE-SW or NW-SE striking flow foliation.

The orientation, scale and development of various coastal erosional features including channels, scour-like structures (muschelbruche) and depressions near Shellharbour, NSW can be explained through normal coastal processes without the necessity to invoke (mega)tsunami-related scouring from the southeast.


Paul Lennox has over thirty years experience teaching undergraduates and undertaking field-based map-based research in Eastern Australia. He has published papers on deformation in granites and their enclosing country rocks in the Lachlan Orogen   and  the  tectonic development of the Southern New England Orogen.

Displacement of mega boulders across coastal rock platforms south of Sydney, Australia, by big storm waves generated by East Coast Lows

Bann, Dr Glen1, Lau, Dr Annie2

1University of Wollongong, Wollongong, Australia, 2University of Queensland, St Lucia, Australia

This research reports on a number of large boulders, or blocks, displaced across coastal rock platforms between Kiama and Snapper Point, south of Sydney, Australia. All boulders are derived from underlying fine to coarse-grained sandstones. A boulder described previously (Bann 2012), weighing  ~10 tonnes, had been transported 42m across the rock platform and rotated 110o from its original in-situ position. Before and after photos show evidence of more recent movements, ~1m sideways and rotating it back 50o in 2016, coinciding with an ‘East Coast Low’ that impacted the whole NSW shoreline in June 2016. It was moved again in 2017 a further 30o and ~30cm sideways with one end lifted ~15cm to rest it on a ledge, then in July 2020 following another East Coast Low, another 10o rotation and ~20cm movement occurred off and away from the ledge. Three smaller boulders nearby of ~1 to 2 tonne each also get tossed around with these events. Two other much larger boulders weighing ~100 and ~180 tonnes also reported movement previously from 2016 (Bann et al. 2018), to the north of Jervis Bay moved ~10m upslope and rotated ~15 degrees for the smaller one and a few metres up slope for the larger one. The smaller boulder has also been flipped over at some stage given by reversed cross-bedding and inverted sedimentary grading. Slight movement of these two has also been detected since 2016. Examples from other localities include the placement of blocks weighing a few tonnes onto an elevated rock platform, then removed at a later date, a block weighing about 1 tonne lifted and moved onto a higher ledge on a cliff about 20m from the platform sea edge and 5m elevation, and a boulder weighing about 10 tonnes on a rock platform lifted with tree logs emplaced and wedged firmly beneath it before settling on them. Satellite imagery was used to identify approximate movement times and then correlated with storm wave data obtained from wave buoys located offshore at Port Kembla and Batemans Bay. This provided timing in addition to wave heights, allowing approximate wave height constraints for wave height formulas. None of the boulders appear to have been pushed or slid across the actual platform, rather, all boulders show evidence of being lifted during displacement, hence by saltation or suspension. Calculations suggest that waves of considerable height and wave length are required to displace these boulders across the platform, either in the one event, or in incremental steps, larger than the actual heights from the wave buoy data show. The findings indicate that large storm waves are capable of shifting very large boulders, such as those generated by East Coast Lows which frequent the coast during the winter months. Although a few previous authors use boulder movements as evidence that the east coast experiences occasional large tsunamis, tsunamis are not required to transport these large boulders on coastal platforms. Clearly, coastal lowland environments south of Sydney are prone to occasional very large storm waves and needs to be prioritized in coastal hazard and sustainable management plans, particularly applicable to climate change and future sea level rise.


This is ongoing research which originated in 2012 when looking for intrusions and tuffs along the coast –  noticing big boulders in places they shouldn’t be, then, noticing these boulders are moving around regularly, without leaving scrape marks, meaning, the boulders are effectively floating…


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.