Category Archives: Geological history

What’s the significance of the large granite erratics scattered through the Inman Valley in South Australia?

Overview

The earliest studies of glacial sediments and landforms of the Inman Valley, starting with Selwyn (1859), made much of the smoothed and striated bedrock pavements (which we now know to have been generated by the westward movement of ice sheets from continental regions that abutted southern Australia around 300 million years ago), and the large granitic and other erratics scattered throughout the valley.   Much of the available information at the time, and hypotheses attached to it, were summarized by Professor Walter Howchin in 1926.  Howchin’s map of the Inman Valley showed the locations of large boulder erratics (principally coarse grained, porphyritic granite similar to that exposed along the coast at Encounter Bay) and striated bedrock pavements, on which the directions of movement of the glacial ice could be measured.  As might be expected, in-situexposures of glacigene sediments were observed directly overlying the pavements in some of these localities.  Howchin remarked on the fact that changes in the courses of the Inman River and subsidiary streams over time continued to variously expose and also obscure rock pavements and overlying glacial deposits, and this situation has continued to the present day.

As well as at sites at which conspicuous large erratics occurred and striated bedrock pavements were located, Howchin assigned the sedimentary fill throughout the Inman Valley and adjacent areas to ‘Permo-Carboniferous’ glacigene deposits. This was partly due to borehole data that indicated diamictitic fill above bedrock in parts of the valley, but also because there were exposures of diamictite associated with the scattered striated pavements and erratics shown on his map (Fig. 1).  This view was promulgated by successive geologists and dominates even the most recent geological maps of the area.  However, as pointed out by Bourman & Milnes (2016), the fill in the valley is complex.  It includes the remnants of glacigene deposits extensively eroded and reworked during Mesozoic and Cainozoic times as well as younger fluvial and alluvial deposits such as the peaty sediments of Pleistocene marshlands and sandy sediments resulting from post-European settlement erosion and aggradation.  The soils map of the area (Fig. 2) is a good indication of this complexity as it presents at the modern landsurface.

171009_Howchin erratics map_enhanced_cropped
Enhanced map of ‘greater’ Inman River valley by Howchin (1926) showing his locations of erratics (red dots) and striated pavements (with directions of ice movement – purple arrows). Region coloured in yellow was assigned to ‘Permo-Carboniferous glacial’ deposits; other coloured areas are bedrock of various types and ages.
171005_erratics vs soils_cropped
Map of soils in the Inman Valley & surrounds showing locations of erratics (red dots – Howchin 1926; purple dots – recent field observations). Yellow indicates the dominant soil type – ‘G3: Thick sand over clay’ which corresponds closely to Howchin’s ‘Permo-Carboniferous glacial’ deposits and is promulgated on recent geological maps on which the soil mapping was based. Red = areas of ‘L1: Shallow soil on rock’ where bedrock is exposed or close to the surface on the steep slopes. Green = areas of ‘K: shallow to moderately deep acidic soils on rock’. Brown colours = areas of ‘F2: Sandy loam over poorly structured brown or dark clay’ soils, ‘E3: Brown or grey cracking clay’ soils, and F1: Loam over brown or dark clay in the modern stream valleys.

Erratic strewnfields

Recent field observations demonstrate that the strewnfields of large, mostly granite erratics (Howchin’s 1926 map, to which there are more recently added occurrences shown in Fig. 2), essentially pinpoint outcrop or subcrop of in-situglacigene diamictite from which they have been recently exhumed, or within which they still remain partly encased.  The diamictite is generally plastered over smoothed and striated Cambrian Kanmantoo Group bedrock and may be ‘lodgement till’.  Good examples of this are on the beach at Rosetta Bay, in dam excavations east of Mt Alma road, in the Inman River channel at Inman Valley township and for some kilometres eastwards, and at the site known as ‘Glacier Rock’.  Elsewhere, Permian glacigene sediments have long been eroded and substantially reworked, and the granite erratics that have been exhumed from them have largely disintegrated and the weathering products dispersed.  The stages in this process can be observed in some granite erratics now being exhumed from glacigene diamictite. Weathering and disintegration of granite erratics in association with Permian glacial diamictites does not occur to anywhere near the same extent in coastal environments, such as on beaches at Rosetta Bay, just north of Port Vincent and at Port Moorowie on Yorke Peninsula, and at Hallett Cove south of Adelaide.

Diamictites

Howchin (1926) referred to the diamictite as ‘.. glacial sandstone and boulder clay..’ and it is quite distinctive, as shown in the accompanying photographs.  In locations near the coastline, for example in Rosetta Bay, the diamictite tends to be bluish in colour, with some bleaching and iron-staining, suggesting that weathering is not pervasive.  In the main part of the Inman Valley, however, and particularly in the river channel, exposures are generally yellowish-white and bleached of colour.  In the earliest reports (Tate et al., 1898), the glacigene sediments were described as being to be so dark in colour that they were thought to be potentially coal-bearing, and this led to exploration drilling of three bores in Back Valley by the Victor Harbour Coal Company.  Carbonaceous glacigene sediments are known elsewhere, for example at Port Moorowie on southern Yorke Peninsula, but have not been observed recently in the Inman Valley. The beds of glacial origin that Howchin (1926)2referred to as being typically ‘.. tenacious blue clays..’  have also not been observed recently although the diamictite that is periodically exposed at low tide in Rosetta Bay and which underlies the conspicuous lags of large granite erratics, is bluish in colour.

The sandy-clay matrix of bluish-coloured diamictite is dominantly quartz, with feldspars and muscovite or biotite.  Unexpectedly, in the samples examined so far, the clay is dominated by poorly crystalline 14Åmontmorillonitic material: no kaolinite was observed.

 

 

1.  Strewnfield of large granite erratics west of Mt Alma.  2.  Lag of granite & metamorphic rock erratics on diamictite, Rosetta Harbor.  3.  Lag of granite & other erratics over diamictite, bed of Inman River, east of village.  4.  Disintegrating granite erratic eroding from diamictite, Strangways Hill.  5.  Large granite erratic in diamictite on glaciated pavement, Glacier Rock.  6.  Large granite & other erratics embedded in diamictite on glaciated pavement, bed of Inman River, east of village.  7.  Bluish sandy-clay diamictite (‘lodgement till’) beneath lag of granite erratics, Rosetta Harbor.  8.  Bluish sandy-clay diamictite with embedded granite & other clasts, Rosetta Harbor.

Summary

The strewnfields of large granite and other erratics in the Inman Valley are considered to represent vestiges of extensive Permian glacial diamictite.  Remnants of these sediments in localities along the north-central and eastern parts of the valley have been exposed close to the ice-moulded bedrock walls and floor that have been progressively exposed by erosion.  As downwasting continues, it is expected that the large granitic erratics now exposed in the boulder lags will gradually weather and disintegrate, as is common in terrestrial environments.  New occurrences could emerge if riverine erosion exposes more of the original bedrock valley. On the other hand, rising sea-levels may trigger aggradation and the burial of the now exposed strewnfields of erratics, the associated diamictites, and the underlying glaciated bedrock pavements.

Although there have been many investigations of facets of the Permian glaciation, including landforms and sedimentary deposits, starting as early as Selwyn (1859)1, evidence of post-Permian geological processes in the Inman Valley up until the Quaternary has not been recognised.  Some soil mapping linked with Howchin’s (1926) observations and more recent data reported by Bourman & Milnes (2016)2is the most recent information.  Opportunities to discover more of the history of this complex landscape clearly exist.

References

Selwyn R.C., 1859, Geological notes of a journey in South Australia from Cape Jervis to Mount Serle, No. 20, p. 4.

Howchin W., 1926, Geology of the Victor Harbour, Inman Valley and Yankalilla districts, with reference to the great Inman Valley glacier of Permo-Carboniferous age. Transactions of the Royal Society of South Australia, 50, p. 89-116.

Bourman, R.P. & Milnes, A.R., 2016, The geology and landforms of the Inman River Catchment.  Report to Inman River Catchment Landcare Group, Government of South Australia Department of Environment, Water and Natural Resources, December 2016.  237pp.

Tate R., Howchin W., David T.W.E., 1898, On the evidence of glacial action in the Port Victor and Inman Valley districts, South Australia.  Report of Research Committee No. 5, Australasian Association Advancement Science, 7thmeeting, Sydney 1898, p.  114-127.

Dr Tony Milnes

The beach cliffs north of Stansbury

North of Stansbury on Yorke Peninsula (South Australia), towards Port Vincent, there is an interesting hike along the beach at low tide.  Prominent cliffs of yellowish fossiliferous limestone overlain by reddish sand and mottled clay are in places capped by white carbonate-rich silts and hard limestone called ‘calcrete’ (1).  The cliffs are up to 20m high except where broken in a few places by gullies that mark once active, short streams.  At high tide the sea laps at and erodes the base of the cliffs across a shore platform cut into the limestone. Version 2

The limestone beds (Port Vincent Limestone) were deposited in Oligocene to Miocene times, roughly 20-30 million years ago.  The overlying reddish sands and clays (Hindmarsh Clay) are probably around 700,000 years old (Pleistocene age) and the white carbonate (lime) capping is even younger.

At the Stansbury jetty, the cliffs are mostly of Port Vincent Limestone.  The original bedding in the limestone is outlined in places by thin rubbly layers and elsewhere by shell-rich beds.  The limestone was deposited in shallow seas that once occupied the St Vincent Basin. Conspicuous amongst the fossils are whole-shell sea-urchins (echinoderms), bivalves (clams and oysters) and bryozoa in a sandy matrix that is largely made of shell fragments (4).  The cliffs show spectacular evidence of former caves, sinkholes, pipes and other solution features: these are obvious because they are filled with mottled green, yellow and red sandy clays (2, 3).   Undercutting by the sea has progressively collapsed the limestone to expose these structures and in places wash out the clays.  The caves and pipes characteristically have smooth surfaces dating from a time when lime-rich solutions seeping over their walls precipitated calcite on evaporation. Version 2

Version 2

Version 2

In the section from Beach Point (Long Beach) northwards, the Port Vincent Limestone is overlain by Hindmarsh Clay, a thick band of mottled green-yellow clays and deep red sandy clays.  In contrast to the marine origin of the limestones, these were deposited during the Pleistocene in fluvial environments.  The boundary between the two formations is important as it represents a large time break of several million years, during which the seas retreated from St Vincent Gulf to well south of Kangaroo Island.  As this was happening, in the Late Tertiary period, the changing conditions exposed the limestones at the landsurface and subjected them to erosion, weathering and dissolution to generate a significant karst (caves, sinkholes, pipes, and the like) landscape.  Major riverine and lake environments later became established in the former gulf.  The river systems drained highlands to the north and generated widespread alluvial and lacustrine deposits of sand and clays over the karst limestone, filling the caves and sinkholes to produce the remnants we see in the cliffs today.

It’s interesting to speculate as to whether the remains of any fauna (including megafauna) occur within this cave and sinkhole landscape.  It did, after all, form at about the same time as the extensive cave systems in the limestones of the South East and the Nullarbor which contain abundant mammalian and other fossils.

A feature of the Hindmarsh Clay is the occurrence in places of white bands of the mineral alunite (potassium aluminium sulfate).  Towards the northern end of Long Beach it was sufficiently abundant for local entrepreneurs to begin to mine it to produce potassium sulfate fertilizer (according to a report by RL Jack, Assistant Government Geologist, in 1918),  which was in short supply during the First World War, but the venture failed.   Remnants of mining operations can still be seen at the base of the cliffs behind sand dunes (5).  The alunite was formed from a reaction between acid groundwater and potassic clays in the sediments.  The process in detail is unclear, but the occurrence of alunite in superposed near-horizonal seams suggests that appropriate conditions may have related to flow of local acid groundwater.Version 2

Above the Hindmarsh Clay in most of the cliff-line is a ‘blanket’ of younger Pleistocene lime deposits.  These consist of unconsolidated silts with interlayers of hardened calcrete.  The youngest calcrete is exposed as sheets over large areas of the landsurface on this part of Yorke Peninsula, and was exploited in earlier days for the local production of quicklime.  It remains an impediment to cropping but is progressively being crushed and disaggregated by farmers using heavyweight rollers hauled behind large tractors.  The unconsolidated carbonate silts are considered to be a loess-like aeolian deposits whereas the calcrete bands are likely to represent the remnants of ancient soils that formed at successively younger times.

What else can we note about the geology of the Stansbury cliffs?

Projecting the top of the cliffs seawards provides an indication of the former extent of the Pleistocene landscape that has now been disrupted and incised as the St Vincent ‘valley’ was drowned by rising seas.

Fallen blocks accumulated at the base of the cliffs is the result of mass collapse of limestone and calcrete in the cliffs caused by back-wasting under the attack of waves.  Some rock fragments are very large; others are small and part of a lag concentrated by tides at the back of beach.  Many are angular, which indicates that they have relatively recently fallen from the cliff; some are rounded and smoothed which indicates that they have been washed around in the surf zone for a long period of time.   It’s interesting to speculate about the time that it takes the cliff to retreat under the attack of the rising sea.  Only observations over the long-term (mapping of fallen blocks or reference to old photographs to compare with the present) might answer this question.  However, most cliff erosion is likely to take place at comparatively rare times of severe storms and powerful wave and wind attack, whereas little change occurs under usual weather conditions.

The cliffs in some places are coated with a ‘wash’ of sediment from higher in the sequence, and this sometimes obscures the geology and makes it difficult to pinpoint breaks in the rock sequence. This coating varies from a thin surface veneer to substantial talus deposits, and usually occurs in places where the cliff is less exposed to wind and waves.  As well, in places, the red sand and clay fill in the old caves and pipes has spilled out from the cliffs where erosion has exposed them.

Collapsing masses of rock, spilling of sands and clays from less coherent deposits, breaking apart and exposure of karst features, and wash over cliff surfaces, are all components of cliff retreat in this region.

It’s interesting that the cliffline can be traced from actively eroding beach cliffs north of the jetty at Stansbury to a subdued hillslope at the back of the town to beach cliffs again south of the cemetery. The townsite is actually an embayment that was filled by shallow water shelly limestones (deposited around 10,000 years ago in seas that were at a slightly higher level than today) and covered by more recent sand dunes.  It seems to have been a quirk of nature that preserved the embayment, in much the same way that winds, tides and seawater circulation on this eastern side of the gulf produced distinctive, recurved sand and mud spits that protect other bays like Stansbury.

Further reading.

RP Bourman, CV Murray-Wallace, N Harvey (2016)  Coastal Landscapes of South Australia.  (University of Adelaide Press: Adelaide).  423pp.

AR Crawford (1965)  The Geology of Yorke Peninsula.  Bulletin No. 39.  Department of Mines, Geological Survey of South Australia. 139pp.

RL Jack (1918)  Alunite deposits, Section A. Hundred of RamsayInSouth Australia Department of Mines, Mining Review No. 28 for the half-year ended June 30th, 1918.  pp51-53.

AR Milnes, JT Hutton (1983)  Calcretes in Australia – a Review.  In ‘Soils: an Australian Viewpoint’, Chapter 10, 119-162. (CSIRO, Melbourne/Academic Press, London).

WJ Stuart (1970)  The Cainozoic stratigraphy of the eastern coastal area of Yorke Peninsula, South Australia.  Transactions of the Royal Society of South Australia 94, 151-178.

Dr Tony Milnes – Earth Sciences, University of Adelaide

Gulf St Vincent & Adelaide beaches

Over the last million years in southern Australia, sea levels have fluctuated between about 120m below present sea level, during the Ice Ages, to about 2m above during the interglacials.  As humans first arrived in Australia more than 50,000 years ago, the sea level fluctuated between 60 and 40m below present, and vast coastal areas were exposed for settlement.  Gulf St Vincent has a maximum depth of 40m so, during those times, it had the form of a flat landscape of alluvial plains veneered with calcrete and covered by thin sandy soils and sporadic desert dunes gently sloping towards a central oblong saline swamp (Fig. 1).

Around 30,000 years ago the Earth plunged into a severe Ice Age that lasted until about 15,000 years ago, with sea levels dropping to 120m below present.  A warming climate then caused rising sea levels, so that by around 9,700 years ago the sea encroached into the land to form the Gulf (Fig. 2).  Continued climate warming and further sea level rise progressively filled the Gulf.  The warm shallow seas enabled prolific growth of algae and marine grasses that were nurseries to a rich ecology of fish, shellfish, starfish, sponges, calcareous algae and some corals.  Sand-sized foraminifera were abundant, especially in the nearshore and intertidal zones (corresponding to the mangrove-rich areas north of Adelaide).

By around 6,500 years ago the sea had reached its present level and the Gulf achieved its current shape.  Abundant shells and shell grit were deposited to form a meter-thick veneer of calcareous sediment covering the now drowned landscape.  Predominant south-southwesterly winds created waves that eroded these sediments, for example around Port Norlunga and Maslins Beach, and quartz sand winnowed from them mixed together with more recent shell grit to create the modern beaches (Figs 3, 4).  Some quartz sands came from more ancient deposits that are now exposed at Hallett Cove.

Frequent southwesterly storms drove coastal sands northward from Marino Rocks and Glenelg to form a coastal dune series that enclosed the Patawalunga and Westlake depression.  This wave-dominant dune system continued to grow northward creating the present Le Fevre Peninsula where around 10m of sand has accumulated (Fig. 5).

European settlement since 1836 has altered the coastal zone by building on or removing most sand dunes, thus allowing storms to attack roads and residences. Placement of rocky rip-rap with the aim of protecting the shoreline has in fact increased wave turbulence and lowered the beach profile by removing additional sand.  Replenishment of beach sands to maintain amenity is a continuing Government-funded activity.  Tennyson Dunes is one of a few tiny remnants that provide a glimpse of the original Adelaide metropolitan coastline.

References

Bourman RP, Murray-Wallace CV, Harvey N. 2016. Coastal Landscapes of South Australia.Available as a free ebook from www.adelaide.edu.au/press.

Fuller MK, Gostin VA. 2008. Recent coarse biogenic sediments of Gulf St Vincent.InSA Shepherd et al. (eds) Natural History of Gulf St Vincent.  Ch.3 , 29-37. (Royal Society of South Australia).

Harvey N, Belperio AP, Bourman RP. 2001.  Late Quaternary sea-levels, climate change and South Australian coastal geology.  InV Gostin (ed.) Gondwana to Greenhouse: Australian Environmental Geoscience. Geol. Soc. Aust. Spec. Publ. 21, 201-213.  (Geological Society of Australia: Sydney).

James NP, Bone Y. 2011. Neritic Carbonate Sediments in a Temperate Realm, (Springer Science+Business Media).

www.tennyson.org.au

Author: Dr VA Gostin

Paradigms in astronomy & Earth history are not absolute

Not a week passes that a new astronomical discovery is announced in the popular as well as the scientific press.  Our vision and appreciation of our living and cosmic environment is continually changing.  Old, long-held views – for example, that our solar system is the only example of planetary systems – continue to be replaced by new ideas and concepts.  During the 1960s and 1970s, we witnessed a similar paradigm shift from the notion of a very solid, relatively stable Earth, on which oceans and continents would periodically subside or emerge from beneath sea-level, to a vigorous Plate-Tectonic Earth, on which oceans are continually created and removed.  We now know that light and buoyant continents are essentially carried as ‘passengers’ on the ever-circulating, dense, underlying mantle.  It took decades before most (but not all) earth scientists were convinced by the evidence for this concept.

New astronomical techniques

Recent astronomical discoveries that challenge former theories are particularly fascinating.  Looking out into the far reaches of our Universe, researchers have now detected a radio signal from the first stars formed.  Because such distant red-shift signals fall in the FM radio electromagnetic spectrum, Judd Bowman, of the Arizona State University’s discovery team, had to use the Murchison Radio-astronomy Observatory in the ultra-quiet region of Western Australia, to detect the weak signal.  This discovery, as indeed most astronomical information, comes to us via exploration of the whole electromagnetic spectrum.

In contrast, a whole new way of exploring the universe by observing gravitational waves (first reported in April 2016) has recently been proved.  Andrew Grant has called this the beginning of multi-messenger astronomy (Physics Today, October 2017).  Seconds after the gravity wave recorders (LIGO and VIRGO) detected gravitational waves, the Fermi Gamma-Ray Space Telescope spotted a gamma-ray burst.  Many observatoriesconfirmed this event. The observations suggest that gamma-ray bursts result from colliding ultra-dense neutron stars, the enormous energy release from which is sufficient to create heavy elements like gold and uranium via the fusion of lighter elements.

Colliding-galaxies

This stunning image, taken by the Hubble Space Telescope, shows the individual galaxies UGC 1810 (right) and UGC 1813 (left) in the process of colliding. Together, this pair of interacting galaxies is known as Arp273. Image courtesy of NASA/ESA/HHT

In the past year, there have also been breakthroughs in the study of cosmic rays that continually bombard the earth.  While most are light protons, some are ultra-high energy cosmic rays made of iron nucleii that may originate from supermassive black holes in the centre of distant galaxies.  In addition to cosmic rays, our planet is immersed in charged particles emanating from the Sun in what is termed the solar wind.  This reacts with the Earth’s magnetic field and creates numerous transitory phenomena including the aurorae, sprites and lightning.  The solar wind controls spaceweather and climate.  On the other hand, retention of the Earth’s atmosphere is generally attributed to theplanet’s strong magnetic field which prevents widespread stripping of the volatile gasses by the solar wind.  A review of the Earth’s electromagnetic environment was published by Catherine Constable in 2016 (Surveys in Geophysics37, p27-45).

When some massive stars are at the end of their life cycle they become supernovae that violently explode, expelling gas at high speed into space.  These explosions are strong enough to hurl matter across vast distances into neighbouring galaxies.  Our own Milky Way galaxy has apparently grown significantly by capturing material from its satellite galaxies, namely the Small and Large Magellanic Clouds (A. Woodward, New Scientist, August 2017). We clearly live in a complex interactive universe, but the time scales are vast in comparison to the brevity of human experience.

Collapse of a single planetary paradigm

The old paradigm of planetary origins based on our Solar System has collapsed with the discovery of hundreds of other planets with very different orbits.  An article by Ann Finkbeiner (Planets in Chaos: Nature, July 2014) describes this astronomical puzzle.  Planets in their early years collided, interacted, migrated and grew in size, before ultimately settling into some relatively stable arrangement. This turbulent and chaotic impact history during the early assemblage of planets resulted in the expulsion of some planets into the cold emptiness of space, away from their parent stars.  Some stars, like our own Sun, have large families of planets.  As of December 2017, researchers have identified 3,567 exoplanets.  Two telescopes planned to be launched in 2018 will search for signs of other planets by observing their crossings in front of parent stars (J.N. Winn, Scientific American, March 2018, p26-33).

Meteors and impacts on Earth

Recent observations confirm that meteorite impact on Earth is a dynamic process that continues and directly influences our environment.  The planet is continually showered by meteors of various sizes from microscopic space dust and visible sand-sized meteorite grains, up to fragments, some kilometers in diameter, of rarer asteroids or comets.

Some asteroids with unusual fractured shapes and compositions have recently whizzed past our planet on hyperbolic (i.e. non-returning) orbits, and continued onto other stellar rendezvous (K. Cooper, New Scientist, February 2018).  The estimated number of such interstellar objects may be in the thousands.  More relevant of course, have been those visitors that have made direct hits on Earth. Much of Earth’s history since Proterozoic times has in fact been shaped by catastrophic meteorite impacts that have triggered mass extinctions of living creatures and other biological effects.  The immense impact some 63 million years ago that spelt the extinction of dinosaurs, as well as ammonites and other marine creatures, is well known, as perhaps is the atmospheric explosion at Tunguska, in central Siberia only a century ago (1908).  Recognition of the importance of major meteorite impacts on Earth history has been yet another paradigm shift in our understanding.

A cometary catastrophe

Less well known is the discovery that fragments of a disintegrating ~100km-diameter comet collided with the Earth some 12,800 years ago in what is known as the Younger Dryas period (named after a signature Arctic flower).  The collision triggered a rapid return to glacial conditions which lasted about 1,400 years, interrupting the gradual warming of the planet after the Last Glacial Maximum around 20,000 years ago.  In a two-part publication in 2018, Wendy Wolbach (and 31 co-authors) presented a detailed analysis of evidence of this most unusual climatic episode gathered over the last decade (W.S. Wolbach et al.,Journal of Geology126, p165-184 & p185-205).  Data was gathered from ice-cores in Greenland, Russia and Antarctica as well as from lake, marine and terrestrial sediments.  Contemporaneous layers of charcoal and dust in these geographically dispersed cores confirm this cosmic impact event.  These specific layers are enriched in platinum and other impact-related elements.  They also contain glassy spherules and nano-diamonds, and are anomalously high in ammonia, nitrate, and other compounds that represent a major period of extensive biomass burning.  Sea levels rose a few metres due to major melting of the North American Ice Cap and this surge of fresh water disturbed the oceanic circulation that began a period of cooling.

YDB field

YDB = Younger Dryas boundary field. graphic from C.R. Kinzie et al. The Journal of Geology, 2014, v122, p475–506.

Evidence points to numerous fragments of a disintegrating comet detonating above and/or colliding with ice-sheets, oceans, and land on at least four continents centred on North America.  The radiant and thermal energy from multiple explosions triggered extensive wildfires that are estimated to have burned about 10% of the planet’s biomass, considerably more than that accompanying the meteorite impact that caused the demise of the dinosaurs.  The burning created long-lived atmospheric soot, blocking most sunlight and creating an ‘impact winter’ and acid rain.  The reduced vegetation caused a major crisis in the ecosystem and may have contributed to many megafaunal extinctions including mammoths, mastodons, ground sloths and American horses, along with many birds and smaller mammals.  Human population declined for about a thousand years and the demise of the Clovis hunters ensued.  This synchronicity of multiple events makes the Younger Dryas interval one of the most unusual climatic/ecological episodes during the last two million years.  It also raises the importance of supporting the Near Earth Asteroid Survey in defence of future serious impacts on our planet.

Our changing paradigms

The rapid acquisition of new and exciting knowledge about astronomy and Earth history requires paradigm shifts in our thinking and interpretation.  We should always be prepared for new scientific observations and revelations and continue to adapt our ideas and concepts to better explain our cosmic and earthly environments.

Dr Vic Gostin

Tate Museum attracts young scientists

The Tate Museum, at the University of Adelaide, is considered to be the largest and best Australian university geology museum.  Although established in the 1880s by Professor Ralph Tate, the foundation Elder Professor of Natural Sciences at the University (1875 – 1901), the Museum was named in his honour in 1902, after his death.  It was moved into its current location in the Department of Earth Sciences’ Mawson Laboratories in the early 1950s.  The history of the Museum, which is an important part of University Collections, has been well documented by Dr Barbara Kidman.

The extensive assemblage of minerals, meteorites, Antarctic rocks and memorabilia relating to Sir Douglas Mawson’s expeditions, as well as rocks and fossils that record important aspects of Australian and South Australian geology, are presently being catalogued and recorded using digital tools not available in past decades.  New displays are also being designed.  Much of the work is being undertaken by volunteers in lieu of a Museum Curator, a position that has not existed in the Mawson Laboratories for many years.

DSCN2140

Tate Museum with specimens housed in cabinets  matching those made for the first-generation of the museum in the 1880s.

Visitors to the Museum, which occupies a ‘foyer’ to the Mawson Lecture Theatre, include University students attending lectures, attendees at Learned Society meetings, parties of school students and off-the-street visitors. All express delight that a historical Museum such as this exists.  Research students and staff from within the University, and also overseas, are surprised at the breadth of the mineral and meteorite collections, and occasionally request subsamples for specific research purposes.  In this manner, the Museum has an ongoing role in assisting new research and, at the same time, receiving new data and information about its specimens.

Visits by parties of school students are especially noteworthy.  On Tuesday 8th May, twenty enthusiastic young (7-12years) Science Club students from the Woodcroft State School visited the Museum.  Dr Vic Gostin, who often hosts visitors to the Museum, talked to them about the work of Professor Ralph Tate, who studied and maintained collections of Type Specimens of molluscs (including fossil forms) found in South Australia.  As well, his analysis of the ancient glacial features on Fleurieu Peninsula eventually led to a total paradigm shift to ‘Plate Tectonic’ theory.  The students were encouraged to visit the Hallett Cove Conservation Park to view the scientific evidence.  Dr Gostin also pointed out the pioneering exploratory work of Sir Douglas Mawson in Antarctica, as illustrated by displays of his Antarctic specimens and memorabilia.

A special display of hand-sized meteorites and Australites enabled the students to ask questions and become aware of the asteroid/space connection.  Finally, they were allowed to handle and be photographed with the Nakhla Martian meteorite and so, in great excitement, ‘got their hands on another planet’!

The 90-minute visit of this keen group of Science Club students was peppered with exclamations and insightful questions, including many about the variety of spectacular minerals on display.

The Tate Museum and its collections are recognised by many visitors as an important University asset and scientists like Dr Gostin, who know of its history, say that it generates great interest and provides encouragement for current and potential future earth scientists.

Additional reading:

Kidman, B P (2015).  Ralph Tate, his Natural History Museum at the University of Adelaide and the ‘Tate Museum’.  Historical Records of Australian Science, 2015, 101-121. (CSIRO Publishing).

Dr Tony Milnes, Dr Vic Gostin

The Giles Complex intrusions, central Australia

Long-term Research Program initiated by Professor Bob Nesbitt between 1963 – 1970 in the Department of Geology & Mineralogy, The University of Adelaide

R.W. Nesbitt, Emeritus Professor, University of Southampton, UK (Nov 2017)

Brief overview

The Giles Complex is an iconic geological province straddling the junction of South Australia, Western Australia and Northern Territory.  It was explored by Reg Sprigg and his colleagues in the 1950s as part of a mining company (Southwestern Mining) evaluation of its mineral potential and the SA sector was later mapped by the South Australian Geological Survey in the late 1950s.  These early geological studies were essentially exploratory, setting out the distribution of the major rock types, but they provided little detail of the geological evolution and origin of these ancient rocks.  In 1963, the area, being a remote and scientifically challenging geological province, provided an exciting challenge to a small University of Adelaide group.  An important consideration at the time was the fact that as a University-based group we were not inhibited by State boundaries which allowed us to examine the whole igneous province on both sides of the WA-SA border.  Several years of field studies, petrological, mineralogical and geochemical research were undertaken by me and my colleagues and post-graduate students in the Department of Geology & Mineralogy.  The results were summarized in post-graduate research theses, reported at National and International conferences, and published widely in scientific journals (list attached).  The theses and rock samples collected over the many field seasons, together with the respective thin and polished sections for petrographic study, are archived in the Mawson Collection in the Mawson Building.

This comprehensive suite of studies was largely completed in the early 1970s with later follow-up isotopic studies by Chris Gray when based at the ANU and later at La Trobe.  The area was re-surveyed by the Australian Geological Survey Organisation (AGSO, Commonwealth Government) in 1987 and 1990 (AGSO Bulletin 239, 1996) which built on the work of Adelaide University.

Later work by the Geological Survey of Western Australia (http://www.dmp.wa.gov.au/Geological-Survey/West-Musgrave-Province-21418.aspx) was restricted to the Western Australian sector of the Complex.  In South Australia, further studies have been significantly restricted because access is controlled by the local indigenous population.

image001

Simplified geological map showing the location and distribution of the Giles Complex Intrusions (after Nesbitt et al, 1970)

 

Scientific significance & outcomes

The major outcomes of the work carried out by the Adelaide University group can be summarized as follows:

  • The intrusive rocks of the Giles Complex were emplaced as a series of individual mafic-dominated sheets of varying dimensions, some as large as 25km in length and 4km thickness.  The present outcrop area occurs over an area of about 2,500 sq.km.
  • The intrusions were emplaced at varying depths in the crust with those in the east of the Complex being at deep crustal depths progressing to shallow depths in the west.  The mapping and subsequent laboratory studies demonstrated that the Giles Complex rocks present an east to west vertical section of continental crust with the volcanics (at Tollu in Western Australia) representing the final extrusive sequence.  Petrographic studies by Goode and Moore demonstrated that the layered intrusions in South Australia were emplaced at pressures equivalent to 30 to 40 km depth.  Such pressures indicate that the intrusions were emplaced near the base of the continental crust with subsequent geological events bringing them to their present surface position.
  • The Adelaide group, working with isotope geochemists at the Australian National University (Compston & Nesbitt 1967) were the first to determine the age of the Giles Complex rocks as 1060 Ma.  This age has been subsequently verified by The Geological Survey of Western Australia using the latest zircon dating techniques (1040 to 1090 Ma) and AGSO in Canberra (1080 Ma)
  • The intrusions were emplaced into already deformed high-grade gneisses and granulites representing at least one previous major tectonic event and, after emplacement, were subsequently deformed into varying orientations with some (e.g. Mt Davies) being overturned.
  • Studies by Moore (1973) and Goode (1978) confirm that shortly after consolidation, magma chambers in the east suffered high temperature-high pressure strain in localised areas.  These zones (sometimes more than 100 metres across) point to major deformation events deep in the crust which were responsible for the disruption of the original intrusions.  Such zones are marked by spectacular gneissic deformation structures where most of the original minerals have been totally recrystallized leaving residual highly deformed crystals or augen within a fine-grained groundmass.
  • Field studies demonstrate that during cooling, the magma-crystal mix behaved like aqueous sediments producing characteristic structures such as cross-bedding, slumping, load structures and ripple marks.  This phenomenon was modelled by Goode in a series of important papers (1967a, b, c).  Using this model allowed us to determine the original orientation of the magma bodies prior to the deformation event.  Laboratory studies on fractionation trends in mineral groups also confirmed this interpretation (e.g. Kleeman & Nesbitt 1967).
  • In several areas, the contacts of the intrusions, particularly in the east, are well exposed.  Given that the intrusions crystallised from high temperature magmas (> 1100°C) one would expect a strong cooling reaction where the magma reacted to the host country rock.  The fact that this reaction is surprisingly muted indicates that the temperature difference was small and this in turn indicates the host rocks were at high pressure at emplacement.   Field and petrographic studies at the margins of Mt Davies has revealed the presence of incipient melting producing granophyre veins and inclusions.  On-going research using laser ICPMS isotopic techniques is aimed at understanding the degree of involvement of the host granulite rocks.

Ongoing research

The next stage of research is to understand how these intrusions fit into the evolution of continental Australia.  The presence of such large quantities of magma in the continental crust is indicative of a major mantle melting event and may provide a model for the Large Igneous Provinces (LIPS) which mark major tectonic events in several continents (e.g. the Deccan and Siberian Traps).

Publications & theses from the Giles Complex team 1964 – 2007

Publications

Collerson, K.D., Oliver, R.L. & Rutland R.W.R. (1972).  An example of structural and metamorphic relationships in the Musgrave Orogenic Belt, central Australia.  J. geol. Soc. Aust. 18, 379-394.
Compston, W. & Nesbitt, R.W. (1967).  Isotopic age of the Tollu Volcanics, W.A.   J. geol. Soc. Aust. 14, 235-238.
Facer, R.A. (1967).  A preliminary study of the magnetic properties of rocks from the Giles Complex, central Australia.  Australian J. Science 30, 237-238.
Facer, R.A. (1970).  Magnetic properties of the Giles Complex, central Australia. Search 1, 76-77.
Facer R.A. (1971).  Magnetic properties of rocks from the Giles Complex, central Australia.  Royal Society of NSW Journal and Proceedings 104, 45-61.
Facer, R.A. (1971).  Intrusion and magnetization of the Giles Complex, central Australia. Geophysical Journal of the Royal Astronomical Society 22(5), 517-520.
Goode, A.D.T. & Krieg G.W. (1967).  The geology of the Ewarara Intrusion, Giles Complex, central Australia. J. geol. Soc. Aust. 14, 185-194.
Goode, A.D.T. & Nesbitt, R.W. (1969).  Granulites and basic intrusions of part of the Eastern Tomkinson Ranges, central Australia.  Spec. Pub. Geol. Soc. Aust. 2, 279-281.
Goode, A.D.T & Moore A.C. (1975).  High pressure crystallisation of the Ewarara, Kalka and Gosse          Pile intrusions, Giles Complex, central Australia.  Contr. Mineral. Petrol. 51, 77-97.
Goode A.D.T. (1975).  A transgressive picrite suite from the western Musgrave Block, central Australia.  J. geol. Soc. Aust. 22, 187-194.
Goode, A.D.T. (1976a).  Small scale primary igneous cumulus igneous layering in the Kalka layered intrusion, Giles complex, central Australia.  J. Petrol. 17, 379-397.
Goode, A.D.T. (1976b).  Sedimentary structures and magma current velocities in the Kalka layered intrusion, central Australia.  J. Petrol. 17, 546-558.
Goode A.D.T. (1976c).  Vertical igneous layering in the Ewarara layered intrusion, central Australia.  Geol. Mag, 114, 365-374.
Goode, A.D.T. (1977).  Flotation and remelting of plagioclase in the Kalka intrusion, central Australia: petrological implications for anorthosite genesis.  Earth & Planetary Science Letters 34 (3), 375-380.
Goode, A.D.T. (1978).  High temperature, high strain rate deformation in the lower crustal Kalka intrusion, Central Australia.  Contr.Mineral. Petrol. 66, 137-148.
Gray, C.M (1977).  The geochemistry of central Australian granulites in relation to the chemical and isotopic effects of granulite facies metamorphism.  Contr. Mineral. Petrol. 65, 79-89.
Gray, C M. (1978).  Geochronology of granulite-facies gneisses in the Western Musgrave Block, Central Australia.  J. Geol. Soc. Aust. 25, 403-414.
Gray, C.M. (1987).  Strontium isotopic constraints on the origin of Proterozoic anorthosites.  Precambrian Research 37, 173-189.
Gray C.M. & Compston W. (1978). A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites.  Geochim. Cosmochim. Acta 42, 1735-1747.
Gray, C. M. & Goode, A.D.T. (1981).  Strontium isotopic resolution of magma dynamics in a layered intrusion.  Nature 294, 155-158.
Gray, C.M. & Goode, A.D.T. (1989).  The Kalka layered intrusion, Central Australia: a strontium isotopic history of contamination and magma dynamics.  Contr. Mineral. Petrol. 103, 35-43.
Gray, C.M., Cliff, R.A. & Goode, A.D.T. (1981).  Neodymium-strontium isotopic evidence for extreme contamination in a layered basic intrusion.  Earth Planet. Sci. Letts 56, 189-198
Kleeman, J.D. & Nesbitt, R.W. (1967).  X-ray measurements on some plagioclases from the Mt. Davies Intrusion, South Australia.  J. geol. Soc. Aust. 14, 39-42.
Moore, A.C. & Goode, A.D.T (2007).  Petrography and origin of granulite‐facies rocks in the Western Musgrave Block, Central Australia.  J. geol. Soc. Aust. 25, 341-358.
Moore, A.C. (1968).  Rutile exsolution in orthopyroxene.  Contr. Mineral. Petrol. 17, 233-236.
Moore, A.C. (1969).  Corona textures in granulites from the Tomkinson Ranges, central Australia.  Spec. Publ. Geol. Soc. Aust. 2, 361-366.
Moore, A.C. (1970).  Descriptive terminology for the textures of rocks in granulite facies terrains.  Lithos 3, 123-127.
Moore, A.C. (1971a).  Corundum-ilmenite and corundum-spinel associations in granulite facies rocks from central Australia.  J. geol. Soc. Aust. 17, 227-230.
Moore, A.C. (1971b).  Some aspects of the geology of the Gosse Pile Ultramafic intrusion.  J. geol. Soc. Aust. 18, 69-80.
Moore, A.C. (1971c).  Mineralogy of the Gosse Pile ultramafic intrusion, central Australia.  Plagioclase.  J. geol. Soc. Aust. 18, 115-126.
Moore, A.C. (1971d).  Mineralogy of the Gosse Pile ultramafic intrusion, central Australia.  Pyroxenes. J. geol. Soc. Aust. 18, 243-258.
Moore, A.C. (1973).  Studies of igneous and tectonic textures and layering in the rocks of the Gosse Pile intrusion, central Australia.  J. Petrol. 14, 49-80.
Nesbitt, R.W. & Kleeman, A.W. (1964).  Layered intrusions of the Giles Complex.  Nature 203, 391-393.
Nesbitt, R.W. & Talbot, J.L. (1966).  The layered ultrabasic and basic rocks of the Giles Complex, central Australia.  Contr. Mineral. Petrol. 13, 1-11.
Nesbitt, R.W. (1966).  The Giles Complex, an example of a deeply eroded volcanic zone.  Bull. Volcanogique 29, 271-282.
Nesbitt, R.W., Goode, A.D.T., Moore, A.C. & Hopwood, T.P. (1970).  The Giles Complex, central Australia; a stratified sequence of mafic and ultramafic intrusions.  Geol. Soc. S. Africa Spec. Publ. 1, 547-564.
Oliver, R.L., Collerson, K.D. & Nesbitt, R.W. (1969).  Precambrian geology of the Musgrave Block.  Excursion Guide No 13, ANZAS 1969, 37-40.

PhD theses

Bell, T.H. (1973).  Mylonite development in the Woodroffe Thrust, central Australia.  Unpubl. PhD thesis University of Adelaide.
Collerson K.D. (1972).  High grade metamorphic and structural relationships near Amata, Musgrave Ranges, central Australia.  Unpubl. PhD thesis University of Adelaide.
Facer, R.K. (1969).  Magnetic properties of the Giles Complex, central Australia. Unpubl. PhD thesis University of Sydney.
Goode A.D.T. (1970).  The petrology and structure of the Kalka and Ewarara layered basic intrusions, Giles Complex, central Australia.  Unpubl. PhD thesis University of Adelaide.
Gray, C.M. (1971).  Strontium isotopic studies in granulites.  Unpubl. PhD thesis Australian National University.
Moore A.C. (1970).  The geology of the Gosse Pile ultramafic intrusions and the surrounding granulites, Tomkinson Ranges, Central Australia.  Unpubl. PhD thesis University of Adelaide.

Honours theses

Barnes, L. (1968).  The petrography and geochemistry of some high grade metamorphic rocks from the Mt Davies-Giles region, central Australia.  Unpubl. Honours thesis University of Adelaide.
Blight D.F. (1969).  The geology, petrology and geochemistry of an area south of Tollu, W.A.  Unpubl. Honours thesis University of Adelaide.
Bowden, P.R. (1969).  Geology of the Tollu area Western Australia.  Unpubl. Honours thesis University of Adelaide.
Coin, C.D.A. (1970).  A study of the granulite facies terrain near Amata.  Unpubl. Honours thesis University of Adelaide.
Goode, A.D.T. & Kreig, G.W. (1965).  The geology of the Ewarara intrusion, Giles Complex, central Australia.  Unpubl. Honours thesis, University of Adelaide.
Gray, C.M. (1967).  The geology, petrology and geochemistry of the Teizi meta-anorthosite.  Unpubl. Honours thesis University of Adelaide.
Kleeman J.D. (1964).  Studies on the X-ray diffraction, analysis and geochemistry of plagioclase from the Mt Davies igneous intrusion.  Unpubl. Honours thesis University of Adelaide.
Miller, C. (1966).  A geochemical study of clinopyroxenes from the igneous intrusion South Davies, N.W. South Australia.  Unpubl. Honours thesis University of Adelaide.
Smith, P.C. (1970). The geology of the Hinckley Ranges, W.A.  Unpubl. Honours thesis University of Adelaide.
Steele, R.J. (1966).  Gravimetric investigation of the Mt Davies and Gosse Pile intrusions of the Giles Complex.  Unpubl. Honours thesis University of Adelaide.
Yong, S.K. (1964).  The distribution of trace elements Ni, Cu, Sr, Cr, and Mn in the Mt Davies basic intrusion of South Australia.  Unpubl. Honours thesis University of Adelaide.

New report

Professor Bob Bourman and I have just submitted a report to the Inman River Catchment Landcare Group (southern Fleurieu Peninsula, South Australia) entitled ‘The geology and landforms of the Inman River Catchment‘.  Some funds in support of the project came from the Regional Landcare Facilitator Programme, an initiative of the Australian Government’s National Landcare Programme.  Our time in researching the subject and writing the report was a voluntary effort.  front-page

The general aim of the project was to prepare an overview of the geology and geomorphology of the Inman Catchment.  This was to provide a basis for improving local knowledge and awareness of how landscape and landforms have changed (and continue to change) according to landuse and land management practices.  We enlisted the help of landowners and gained new insights via their responses to a wide-ranging questionnaire.

The report can be downloaded via the following link:  http://www.victor.sa.gov.au/page.aspx?u=856

Dr Tony Milnes, Earth Sciences, University of Adelaide