GSA SA Division Lunchtime Talk - The Geoscience Treasures of Flinders Uni

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Health Sciences Lecture Theatre Complex - Flinders University Bedford Campus

Flinders University

The Hub, Level 2

Bedford Park, SA 5042

Australia

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GSA SA Division Lunchtime Talk - The Geoscience Treasures of Flinders University Plus an Excursion to the Magpie Creek Geological Trail

About this Event

The GSA SA Division would like to invite GSA members and guests to a lunchtime meeting at Alere Function Centre, Flinders University. This lunchtime event will incorporate a short, informal seminar on the geology of the Flinders Uni Campus and the global importance of Sturt Gorge and Magpie Creek which was recognised in 1970 by the placement of a GSA plaque at a nearby rock face – the Sturt-Tillite-Tapley Hill Formation boundary. After the short talk, attendees will be invited to view the plaque and nearby key sites of interest which were deposited during vast climatic upheaval pre-emergence of life on Earth.

The schedule for the event is as follows:

12 noon – Register

12:15 – Lunch and network

12:30 – Welcome

12:35 – Wolfgang Preiss talk

12:50 - Colin Conor talk

1:05 – Q&A

1:15 – End of formal proceedings! Site visits (not compulsory, some walking involved)

1:45 - Farewell

THE GEOSCIENTIFIC TREASURES OF FLINDERS UNIVERSITY

Wolfgang Preiss

South Australia’s second university, Flinders, was built in the late 1960s in a commanding position on the steep escarpment of the Eden-Burnside Fault, overlooking Adelaide’s southern suburbs on the Adelaide Plains. Despite the geology of the site having been mapped in some detail by Reg Sprigg and his colleagues in the 1950s, the geoscientific features of the campus received little public attention until the early 1970s. At that time, the Geological Society of Australia, SA Division, recognised an important geological contact exposed in one of the recently excavated cuttings, and erected a small explanatory plaque upon it. However, the plaque was soon obscured by the vigorous Bougainvillea planted over the cutting and, with the university no longer teaching a geology degree, it faded from the university’s corporate memory.

Thanks to present geoscience research staff and students at Flinders University, interest in the geology of the campus has been rekindled. Why is this geology important not only for South Australia, but globally?

At the turn of the twentieth century, pioneer South Australian geologist Walter Howchin recognised certain rocks in the nearby Sturt Gorge as recording an ancient glaciation. These rocks are poorly bedded and consist of a mixture of pebbles, cobbles and boulders of many different rock types and origins set in a finer grained muddy, sandy matrix; they are known as tillite, a lithified equivalent of the Pleistocene tills Howchin was familiar with from his native Yorkshire. Many of the clasts are facetted and striated as a result of grinding against other rocks while encased in moving ice. There are also thinly bedded silty sediments, some of which contain pebbles that have been dropped from floating ice. Interbedded coarse-grained sandstones, commonly containing feldspar and other rock fragments, were deposited from rapidly flowing glacial meltwaters.

Howchin originally considered the sedimentary rocks of the Mount Lofty Ranges, including the Sturt Tillite, to be of Cambrian age. Later researchers such as Sir Douglas Mawson, Cecil Madigan and Reg Sprigg showed that these rocks underlie fossiliferous Early Cambrian sediments and are of late Precambrian age. At the same time, similar glacial sediments of late Precambrian age were recognised on most other continents, and today it is known that there are at least two, and in some places three, separate glaciations recorded during this time interval, now known as the Cryogenian Period.

Whether the glacial records are all of exactly the same age, reflecting discrete global glaciations, or represent separate local ice ages affecting different parts of the globe at different times, was hotly debated in the latter part of the twentieth century, and even after a great deal of research and age dating, remains controversial today. Palaeomagnetic studies of some glacial successions have shown that at least some glaciers existed in equatorial regions during the Cryogenian, but this observation has led to widely differing interpretations. At one extreme global glaciation has been proposed, and the popular term “snowball earth” has been applied to this theory. At the other end of the spectrum of interpretations, high obliquity of Earth’s orbit has been invoked to explain climatic extremes with glaciation at low latitude while the poles remain more temperate.

All known late Precambrian glacial successions are overlain by marine sediments that reflect a rapid post-glacial rise in sea level. Carbonate rocks deposited immediately after glaciation ended are referred to as ‘çap carbonates’ and play an important role in the Snowball Earth theory. At Flinders University, this critical contact is exposed in the cutting adjacent to the bus station, and is marked by the Geological Society’s plaque. Below the contact is the Sturt Tillite, comprising pebble-bearing siltstone and also a channel-filling arkosic grit deposited from a meltwater stream. Above the contact is the Tindelpina Shale Member, basal member of the Tapley Hill Formation. This very fine-grained sediment consists of extremely thinly laminated silty shale with thin interbedded flaggy fine-grained dolomite (‘cap carbonate’). These rocks when fresh are almost black in colour, due to contained organic matter, but at Flinders University they have been weathered and oxidised to pale grey, buff and purple colours. Importantly, the contact between the Sturt Tillite and Tindelpina Shale Member is an erosional disconformity surface. In other parts of South Australia, hundreds of metres of additional silty and sandy sediments (Wilyerpa Formation) intervene between the glacial tillite and the Tindelpina Shale Member, and these sediments record the gradual waning of the glaciation. The absence of Wilyerpa Formation in the southern Mount Lofty Ranges indicates either a hiatus in deposition, or deep erosion, or a combination of both, at this boundary. The significant delay in the post-glacial transgression and therefore in deposition of the cap carbonates after glaciation ended poses a serious difficulty for some aspects of the Snowball Earth theory.

Other important sites at Flinders University include the south wall of the car park, which is excavated into the hillside, exposing the pebbly and cobbly Sturt Tillite, weathered to pale yellow and brown colours, and the deeper cuttings near the eastern boundary of the Sturt Campus, where the tillite is a little less weathered and includes further channel-filling sandstones and grits.

When the plaque was erected the age of the sediments was estimated at 750 Ma, but this was a pure guess and is now know to be 100 million years too old. It is only in recent years that reliable dates have become available for the glaciation represented at Sturt Gorge and Flinders University. A thin layer of volcanic ash in the Wilyerpa Formation in the northern Flinders Ranges has yielded an accurate zircon U-Pb age of 663 Ma, providing a minimum age for the Sturt Tillite. It is uncertain how much older the base of the Sturt Tillite is, although preliminary data on detrital zircons suggest it is not very much older. By contrast, similar glacials in North America have been dated at 717 Ma. Much work remains to be done to establish the ages and durations of other Cryogenian glacial events around the globe, in order to evaluate competing hypotheses of palaeoclimatology.

AN EXCURSION DOWN INTO THE ANCIENT GLACIAL TILL OF THE STURT GORGE – THE MAGPIE CREEK GEOLOGICAL TRAIL

Colin Conor

The Sturt River valley (Warri Parri), and its tributary Magpie Creek, provides a traverse through a huge accumulation of glacial debris that was deposited in a marine basin, approximately about 650 million years ago. The rock, in its original unconsolidated state, is called till, but is renamed as ‘tillite’ once it has hardened. Tillite consists of a matrix of sand, silt and clay-sized particles, which, once rock, was turned into ‘flour’ by the grinding of glacial ice. Characteristically tillite also contains a wide range of different rock fragments measuring from few millimetres in size to tens or even in rare cases a hundred metres or more.

The oldest part of the Sturt Tillite overlies siltstones of the Burra Group some 2.4km southeast of where its youngest part is displayed at Flinders University. The Magpie Creek Geological Trail loops down into the Sturt River valley from Gorge Road and includes not only excellent examples of tillite, but also an interesting siltstone marker bed that separates earlier from later tillites. The Trail gives walkers the opportunity of viewing the Sturt Tillite in detail that encourages such questions as: why is the rock a rock and not friable like till, what are the different fragment-types in the matrix, where did they come from, when did all this happen, why was ‘Australia’ so cold, finally why does all this glaciomarine debris now forms hills in the Mount Lofty and Flinders Ranges? Answers to these questions can be provided by geological evidence easily visible along the Trail.

Note: This trip is being undertaken of your own free will. The Geological Society of Australia accept no liability for any injury or damage to property suffered during this excursion. Please do not hammer, damage, climb on or disturb the rock faces and local vegetation.

By registering I confirm that I will follow the instructions of the field leader and neither I or a member of my group will engage in at-risk behaviour.

Photos will be taken during this event and by registering for this event you agree to have your photo used for media and publications.

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Date and Time

Location

Health Sciences Lecture Theatre Complex - Flinders University Bedford Campus

Flinders University

The Hub, Level 2

Bedford Park, SA 5042

Australia

View Map

Refund Policy

No Refunds

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