Category Archives: Geological history

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

Fossil shells at Stansbury, South Australia, record a higher sealevel 125,000 years ago

Subsamples were taken of a collection of fossil shells recovered from a depth of around 3 m in trenches excavated in the Oyster Point Caravan Park by local contractors to improve drainage. Several of the fossils (Fig. 1) had been identified by SA Museum personnel and assigned to species including bivalves Katelysia scalarina and Sanguinolaria (Psammotellina) biradiata, and the large gastropod Turbo (Dinassovica) jourdani. All species are still living around the Australian coast, but these shells are clearly ancient and belong to a time when the coastal cliffs at Stansbury stood inland of the caravan park and the township and are now represented by the base of the hill that runs from the cemetery, northwards behind the town centre, and joins the current shore cliffs near the primary school oval. The seas, of which the fossil shells are a legacy, covered all of the lowland eastwards of these ancient cliffs. The cliffs themselves are in fact cut into much older marine deposits, as can be seen behind the jetty and elsewhere along the coast. These relate to the Tertiary period between 3 to 23 million years ago when much of Yorke Peninsula was inundated by sea.

Shell subsamples of two of the species (Katelysia scalarina and Sanguinolaria (Psammotellina) biradiata) were dated in the laboratories of the School of Earth & Environmental Sciences at the University of Wollongong by Professor Colin Murray-Wallace and his colleagues. They used a technique called Amino Acid Racemisation (AAR) and found that the shells are about 125,000 years old. Professor Murray-Wallace can be confident of this dating because he and his colleagues have much experience in determining the ages of ancient Quaternary coastlines of southern Australia and their fossils (see Further reading).

Sea levels 125,000 years ago (Fig. 2) were up to 2m above current sea level, as this time was part of an interglacial period (formally called the ‘Last Interglacial’) when ice in Antarctica and elsewhere had melted somewhat due to warmer global temperatures. This accounts for the encroachment of the seas into the embayment now occupied by much of Stansbury township, and the formation of the old cliff-line. The marine and coastal deposits generated at this time, and which occur widely around South Australian coasts, are referred to the Glanville Formation.

It might be of interest to note that several earth scientists, including Professor Murray-Wallace, have written a book on the coastal landscapes of South Australia. This is currently in press and should be available soon. It includes a chapter on the entire coast of Yorke Peninsula, including Stansbury. As well, a student from the School of Earth & Environmental Sciences at the University of Wollongong (Tsun-You Pan, visiting from Taiwan), and supervised by Professors Murray-Wallace and Bourman, has recently commenced a PhD research project on the Last Interglacial coasts and their deposits on southern Yorke Peninsula and may be able to report in future on his findings on these materials, including the Stansbury Caravan Park fossils.

Further reading

Bourman, R.P., Murray-Wallace, C.M. & Harvey, N. (2016, in press). Coastal Landscape of South Australia. University of Adelaide Press.

Ludbrook, N.H. (1984). Quaternary molluscs of South Australia. Handbook No. 9, 327pp. Department of Mines & Energy South Australia. (Government Printer: Adelaide).

Murray-Wallace, C.V., Bourman, R.P., Prescott, J.R., Williams, F, Price, D.M. & Belperio, A.P. (2010). Aminostratigraphy and thermoluminescence dating of coastal aeolianites and the later Quaternary history of a failed delta: The River Murray mouth region, South Australia. Quaternary Geochronology Vol. 5, pp28-49.

Zang, W-L, Cowley, W.M. & Fairclough, M. (2006). 1:250 000 Geological Series – Explanatory Notes. Maitland Special South Australia. Sheet S153-12 International Index. 62pp. Primary Industries and Resources SA (Government of South Australia).

Dr Tony Milnes, Earth Sciences, University of Adelaide

IMG_0272_cropped

Fig. 1 Assemblage of fossil shells found in excavation.

Fig 2

Fig. 2. Sea level curve for the past 130 000 years. Adapted from Lambeck and Chappell (2001). The thickness of the line of the curve is an expression of the degree of uncertainty of the calculated sea-levels. During the Last Glacial Maximum sea level was about 120 m lower than at present. The Last Interglacial warm period occurred about 130 000 to 120 000 years ago, when sea level was at least 2 m higher than at present. The present interglacial warm period (Stage 1) has existed for little more than the past 10 000 years. Source: Cann, J. (2014). Robe Geological trail. (Geological Society of Australia: South Australian Division).

 

Peeling back the layers under Adelaide

Siting Adelaide City

The decision on locating the city of Adelaide was made by Surveyor-General Colonel Light against the preference for a coastal site by the naval-trained Governor, Captain John Hindmarsh. In his Brief Journal Light wrote:

On examining the following day some distance up and down the river, I saw evident marks of the river overflowing its banks, and this made me resolve on the first site I had chosen, my instructions from Commissioners being peremptory as to the responsibility of this choice devolving upon myself – for although I was allowed to pay respect to the Governor’s opinion, yet my own judgement on this point was to be paramount and conclusive.

Light also noted his concern “with the best method of laying out the town according to the course of the river and the nature of the ground”. The chosen site “was on a beautiful and gently rising ground and formed altogether a better connection with the river than any other place” (MacDougall 1839).

Adelaide City is thus beautifully sited on the gently undulating plains between Gulf St Vincent and the distinctive crests of the Mt Lofty Ranges. The River Torrens, set in a valley, provides the scenic northern edge of the city and attracted early development due to the availability of fresh water and ready access to Port Adelaide.

Early Adelaide buildings were of imported timbers, or rammed clay and straw between timber slats (pug-and-pine), with a thatched or shingle roof. Although timber was in short supply, limestone was plentiful, more than half the town being upon a bed of limestone (Colwell & Naylor, 1974). A shallow limestone quarry was established on the riverbank where the Torrens Parade Ground now stands. Bricks were made from the alluvial red-brown clays excavated at Walkerville, Bowden, Brompton, and Croydon. Sand-washing was practised along the River to provide useful building material.

Adelaide is situated on the plains formed by the merged alluvial fans (red-brown clays, sands and gravels) deposited by numerous creeks emerging from the Mt Lofty Ranges (Fig 1). These creeks, as well as the River Torrens, have cut deep gorges, with rapids and waterfalls (Waterfall Gully) indicating a youthful topography resulting from the ongoing gradual uplift of the Ranges relative to the plains. Regular earthquake tremors, and rare stronger earthquakes support this view. Boreholes into hard rocks of the Ranges become distorted, showing that the crust is being squeezed towards the northwest, resulting in their slow uplift. The rate of uplift of the Ranges can be estimated as over 12m in the last 120,000 years.

Fig 1. Adelaide Plains and western slopes of Mount Lofty Ranges, including diagrammatic representation of alluvial fans associated with major streams (after Aitchison et al. 1954; Twidale 1976).

Fig 1. Adelaide Plains and western slopes of Mount Lofty Ranges, including diagrammatic representation of alluvial fans associated with major streams (after Aitchison et al. 1954; Twidale 1976).

While the main uplift has occurred along the curving Eden-Burnside Fault, forming the Hills-face zone, the Para Fault and other faults have also been active. These can be seen in the simplified block diagram of the Adelaide region, showing the general topography with the southward-tilted fault blocks (Fig 2). Hence, driving south along South Road the steep rise up Tapley Hill or the freeway is followed by the gentler slopes towards Noarlunga; and similarly after crossing the Onkaparinga River the steep rise is followed by the gentle slope into McLaren Vale and the Willunga Basin.

Fig 2. Simplified block diagram of Adelaide region showing general topography formed by tilted downfaulted blocks containing mainly marine Tertiary and younger sediments (45 million years to present) of St Vincent Basin (yellow).

Fig 2. Simplified block diagram of Adelaide region showing general topography formed by tilted downfaulted blocks containing mainly marine Tertiary and younger sediments (45 million years to present) of St Vincent Basin (yellow).

For most of the last million years, world sea levels were 40-60m below their present level, and Gulf St Vincent was largely the continuation of the Adelaide Plains. During this time the climate changed from seasonally wet to predominantly arid, creating the widespread calcareous soils and calcrete (used in early Adelaide buildings). Sea levels fluctuated regularly in response to the enormous continental ice caps in the northern hemisphere. During the warmer inter-glacial periods sea levels rose and the latest inundation of Gulf St Vincent took place from 9,000 to only 6,000 years ago, creating our present active coastline.

Underlying rock sequences

Numerous water bores and deeper geological drill holes have indicated that the sediments underlying the younger alluvial fans form part of a once extensive St Vincent Basin. During the Tertiary Period, from 45 to 5 million years ago, this Basin extended from the Mt Lofty Ranges west to Yorke Peninsula. Between 5 and 4 million years ago the eastern side of this Basin became separated into the Noarlunga, Willunga, and Meadows Embayments, along a set of curved faults (Fig 2).

The earlier Tertiary sediments of the St Vincent Basin, now best seen to crop out along the coast from Maslin south to Port Willunga, illustrate the environmental changes from the oldest river sands (North Maslin Sand), to coastal peat swamps (now lignite) and estuarine muddy sands. In turn these are overlain by fossiliferous shallow marine silts rich in sponge spicules (Blanche Point Formation), topped by mollusk-rich, bryozoal limestones (Port Willunga Formation).

The same sequence of Tertiary sediments is present directly under Adelaide, as seen in the detailed geological cross-section from North Adelaide south to Greenhill Road (Figs 3 & 4). Here these Tertiary strata are tilted southward, and increase in thickness to over 100m at Greenhill Road.

Overlying these tilted layers, is a sub-horizontal erosion surface (unconformity) overlain by 1-4m of fossiliferous shallow marine sandstones and sandy limestones (Hallett Cove Sandstone) usually well cemented, that was originally quarried at the current Torrens Parade ground.

Figure 3 shows a schematic E-W cross-section from the youngest coastal sediments at Le Fevre Peninsula up to the ancient rocks of the Mount Lofty Ranges. The Para Fault (Figs 1,2,3) lies just west of North Adelaide, west of West Terrace, Adelaide, and runs south towards Merino Rocks. It has elevated the eastern side some 25 meters relative to the western, allowing the River to create a well-defined valley at, and upstream of, the City. West of the Para Fault the Torrens regularly flooded the land and later spilled into the Torrens Reedbeds, now partly converted into Westlakes. Clearly the observations of Colonel Light ensured that the city of Adelaide was sited on elevated ground, above any risk of flooding.

Fig 3. Simplified cross-section from Grange eastwards to Mt Lofty Ranges showing effects of faulting on topography of Adelaide region. The hardest and oldest rocks forming Mt Lofty Ranges and “basement” to Adelaide city are Precambrian Adelaidean System (after Selby & Lindsay 1982).

Fig 3. Simplified cross-section from Grange eastwards to Mt Lofty Ranges showing effects of faulting on topography of Adelaide region. The hardest and oldest rocks forming Mt Lofty Ranges and “basement” to Adelaide city are Precambrian Adelaidean System (after Selby & Lindsay 1982).

The geological history involving this long Tertiary period can be best interpreted by the onset of extensive crustal stretching (extension) following the rapid separation of Australia from Antarctica – its Gondwana parent. Such stretching thinned the crust, 45 million years ago, allowing many subsiding basins to form, including Bass Strait, Murray Basin, Gulf St Vincent, Spencer Gulf and Eucla basins (part of the Great Australian Bight). As these basins subsided they were filled both with sediments eroded from the nearby hills and with major contributions from biogenic (plant and animal) material generated in the shallow seas (creating the fossiliferous limestones).

Fig 4.  N-S cross-section from North Adelaide directly south along King William Street to Greenhill Road showing tilted Tertiary strata (45-10 Ma = million years) under Adelaide, overlain by horizontal calcareous Hallett Cove Sandstone (4-2 Ma) and younger alluvial Hindmarsh Clay deposits. River Torrens has cut a shallow valley into underlying deposits.  (Alley & Lindsay, Ch19, in Drexel & Preiss, 1995, Fig 10.14).

Fig 4. N-S cross-section from North Adelaide directly south along King William Street to Greenhill Road showing tilted Tertiary strata (45-10 Ma = million years) under Adelaide, overlain by horizontal calcareous Hallett Cove Sandstone (4-2 Ma) and younger alluvial Hindmarsh Clay deposits. River Torrens has cut a shallow valley into underlying deposits. (Alley & Lindsay, Ch19, in Drexel & Preiss, 1995, Fig 10.14).

Around 5 million years ago the crustal stresses changed from extension (stretching) to compression. This affected the eastern side of St Vincent Basin by elevating and gently tilting the sedimentary layers southward, and forming the separated Noarlunga, Willunga, and Meadows Embayments. Erosion removed the upthrown edges of the tilted blocks, and deposited the thin Hallett Cove Sandstone on the eroded surface. Further compression in the last million years elevated the Mt Lofty Ranges allowing the westward flowing streams to create gorges, with extensive alluvial fans composed of clays, sands and coarse gravels (Hindmarsh Clay, Pooraka Formation etc.).

Most of the groundwaters available under Adelaide and its suburbs are derived from porous sand-rich strata belonging to the St Vincent Basin. Firm foundations for the high-rise office buildings in Adelaide are also sited in these sedimentary layers. Details of the engineering geology under Adelaide may be found in Selby and Lindsay (1982).

A much older story in the rocks

But there is an older story to tell: that of an intense glacial time some 290 to 270 million years ago (Carboniferous-Permian), when most of Australia was covered by thick ice caps, and it was part of a huge Gondwana supercontinent. Glacial remnants with exotic granite boulders and glacial striations indicate that the ice moved northwestward away from the once juxtaposed Antarctica. Patchy but numerous outcrops of these sediments remain exposed from Victor Harbor and the Inman Valley to Hallett Cove and beyond. They indicate that surprisingly limited landscape denudation has transpired since their deposition under glacial conditions over 270 million years ago.

An even more ancient story

If we were to strip away all of the blanket of Tertiary and Permian sediments described above, what do we see? We encounter the ancient hard rocks that today form the Adelaide Hills including the Hills-face Zone south to Marino Rocks and thence to Port Stanvac. In contrast to all younger sedimentary layers, these are now intensely folded, lithified and faulted, constituting the real “basement” to Adelaide’s geology.

Formed during a previous grand tectonic cycle, this ten-kilometer thick Adelaidean Sequence began depositing some 850 million years ago with the rifting and break-up of an ancient supercontinent of Rodinia. Apart from some basaltic volcanic flows, most of this thick sedimentary sequence was originally deposited as shallow marine sediments, like sandy deltas, limestone shoals with reefs, and deeper-water shales. At least two intervals of glacial marine sediments are included (eg. Sturt Tillite as at Sturt Gorge): these had formed as a vast new Pacific Ocean opened up where much of Eastern Australia now exists.

Preserved fossils indicate organic evolution from simple bacterial cells and green slime depositing limestones (stromatolites), to complex multicellular soft-bodied animals like sea-pens, flat worms, and jelly fish. These have made the Flinders Ranges world famous, resulting in the formal naming of the Ediacaran Period as a new geological time period. This very thick sequence ended with deposition of largely marine and richly fossiliferous Cambrian limestones and other sedimentary rocks (530 to 520 million years ago). Fossils included those of numerous molluscs, brachiopods, corals, trilobites, all in a great burst of evolution. These Cambrian sedimentary rocks outcrop today in Fleurieu Peninsula and on Kangaroo Island.

Around 500 million years ago the above sequence was folded, faulted an consolidated by east-west compression.  The burial and intensity of heat and pressure (metamorphism) increased eastward and southward where granites were intruded (eg. Reedy Creek, Victor Harbor).  Many Adelaide buildings are constructed from these very hard rocks (see also www.sa.gsa.org.au/Brochures/North_Terrace_final1.pdf).

This mountain building activity, called the Delamerian Orogeny, created a fold-mountain belt several kilometers high extending from Kangaroo Island, through Fleurieu Peninsula to Adelaide and north to the Flinders Ranges. The topography then was probably similar to that of the Pyrenees today.

A prolonged period of erosion followed when many kilometers of crust were removed. During this time most of eastern Australia lay under the ocean, slowly growing in continental crust.

Geological summary

The rocks under and around Adelaide can be grouped into four major sequences formed in distinct episodes, and taking different lengths of time. The oldest and hardest rocks form the deep basement to the city, and crop out in the Mt Lofty Ranges, the hills face zone, and coastal cliffs from Marino Rocks to Port Stanvac.

The second episode (Carboniferous-Permian) saw South Australia completely covered by a thick ice cap, with the ice spreading northwestward and grinding deep valleys into the landscape.

The third episode saw Australia separate from its Gondwanan supercontinent and as the thinned crust sagged, in the Tertiary, the St Vincent Basin subsided and began filling with river sands and coal swamps. Extensive warm shallow seas followed, depositing marine shelly and bryozoal limestones. These form both the aquifers of suburban Adelaide and foundations to the high city buildings.

The fourth episode, bringing Adelaide to its present situation, began about five million years ago with the onset of crustal compression that resulted in the present uplift of the Mt Lofty Ranges. The erosion of deep gorges and westward spread of the alluvial fans created the red-brown clays that form many soils of the Adelaide Plains.

References

  • Colwell M & Naylor A 1974. ADELAIDE An illustrated history. Lansdowne Press, Melbourne.
  • Drexel, JF & Preiss, WV (eds) 1995. The Geology of South Australia. Vol.2, The Phanerozoic. South Australia Geological Survey. Bulletin, 54.
  • MacDougall A 1839. A Brief Journal of the Proceedings of William Light, Late Surveyor-General of the Province of South Australia; With a Few Remarks on Some of the Objections that Have Been Made to them. The Royal Geographical Society of S.A. Inc. Library, Adelaide.
  • Selby J & Lindsay JM 1982. Engineering Geology of the Adelaide City Area. Dept. Mines and Energy, Geological Survey of South Australia, Bulletin, 51.
  • Twidale, CR 1976: Geomorphological evolution. In: Natural History of the Adelaide Region (eds. Twidale, CR, Tyler, MJ & Webb, BP). Royal Society of South Australia. Adelaide, pp. 43–59.

INFORMATION ON THE WEB: http://www.sa.gsa.org.au/Field_Guides.html

Article by Associate Professor V Gostin.  First published in bibliophile (ISSN 1033436X), 13 (4) December 2014.