Author Archives: tony milnes

About tony milnes

Currently Visiting Research Fellow in Geology & Geophysics, The University of Adelaide. Formerly a General Manager (Environmental Strategy) at Rio Tinto (ERA Limited) and a Chief Research Scientist at CSIRO Land & Water. See also: https://www.researchgate.net/profile/Anthony_Milnes https://www.linkedin.com/in/tony-milnes-7a696923/

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).

 

A conversation with the community about Mining and environmental management

A context for Yorke Peninsula in South Australia

This article was the basis of a talk to a community group (‘Friends of Gulf St Vincent’) based in Adelaide, South Australia , which is committed to improving the ecology and amenity of the Gulf St Vincent biozone.  It was one of several talks given at a gathering of members of the Group and the local community in the Community Hall at Pine Point, a small village on the eastern coast of Yorke Peninsula.  The focus of the meeting was the possible impacts on Gulf St Vincent of an impending open-cut iron ore-copper-gold-uranium mine nearby (Hillside Mine), but broader issues of environmental impacts and controls were discussed.  Having earlier prepared and submitted a response to the Government regulator on the proposing Company’s submission of a mine and environmental management plan, my presentation at this meeting was broadly to overview mining and mine environmental management on Yorke Peninsula (in 20 minutes or so).

When faced with making a presentation on mining and mine environmental management, which is one of the issues that local communities are finding it increasingly difficult to deal with, I find it interesting and somewhat enlightening to take a step back and look at some history.  Community perspectives on industry, and especially coexisting with large-scale industry, has changed in Australia over the decades.  Perhaps with a historical perspective in mind, it might be possible to map a pathway forward to a productive and acceptable coexistence?

The following notes for my presentation are basically dot-points from a series of ‘slides’ which summarise the approach I’d taken in my presentation.  I’m not sure how it went.

Mining & agriculture started early

  • Both industries significantly changed landscapes.  Mining – local but intensive disturbance.  Agriculture – regionally extensive forest clearing but largely surface disturbance
  • Communities benefited from & adapted to both enterprises – but regulation, attitudes & circumstances changed with time
  • ‘Small’ farmer has gone; properties now amalgamated into large enterprises with fewer workers; rural towns & services have dwindled
  • ‘Mines’ neither larger nor necessarily closer to population centres now than in the past are being viewed as environmentally damaging & socially disruptive

Some history of mining on Yorke Peninsula

  • Copper mined from Wallaroo & Moonta from ~1860. Intermittent mining in 1930s & 1940s. Renewed exploration & from 1989 – 1994
  • Parara Mine (west of Ardrossan) operated for Cu-Au in the 1870s; the Hillside & Harts Mines were opened further south at about the same time in a similar geological setting
  • Salt mined & exported from Yorketown, Port Vincent & Edithburg from ~1874. Later developments at Price & Stenhouse Bay
  • Gypsum mined around Yorketown (1870s), Stenhouse Bay & Marion Bay (1890s) for export for plasterboard production
  • Calcrete mined almost everywhere for local building stone & lime mortar.  Lime kilns were very common & lime was exported to Adelaide.
  • Marine limestones mined for export as flux in Port Pirie Pb-Zn smelters from ~1896, mostly from quarries adjacent to ports
  • Cement produced & exported to Adelaide from Tertiary limestone at Stansbury from ~1913 & quarrying continues to this day at Klein Point
  • Dolomite produced from Cambrian limestone at Curramulka since ~1930s & from Ardrossan since 1948.  Exported as refractory for steel furnaces in Newcastle & Port Kembla
  • Construction sand now mined from a Tertiary paleochannel near Price for Adelaide building industry

Some history of agriculture

  • First agriculture ~1846 at Stansbury on an ‘Occupation license’
  • First pastoral lease in 1851 at Wallaroo
  • Agriculture on Yorke Peninsula as a whole expanded from ~1869 with land clearing & crop production.  Establishment & growth of port towns followed for export of goods; inland settlements & towns were established later
  • Poor yields during early days of agriculture addressed by superphosphate additions to soils starting ~1892. High demand for superphosphate.  Imported phosphate from Nauru (high Cd) & Christmas Island (high U) – widespread additions to South Australian soils
  • New barley crop varieties introduced ~1901
  • Extensive clearing of forest to produce additional agricultural land at ‘bottom end’ of the peninsula from 1950s – significant Cu & Mn deficiencies corrected by additives in superphosphate
  • Widespread modern use of ‘direct-drilling’ in croplands with increases in use of herbicides, pesticides, fertilisers

Getting back to mining – what about minerals exploration?

  • Basically, minerals exploration is controlled by geology. Main search areas are in the ancient basement rock complexes.  Gawler Craton – Olympic Dam orebody in central South Australia, Wallaroo-Moonta mines in northwestern Yorke Peninsula; Curnamona Province in northwestern South Australia – Broken Hill mines.

Old crustal elements form the foundations of South Australia and some of the State’s largest orebodies are found in them.  (Map from Preiss W V et al. 2002 MESA Journal 27, 39-53; http://bit.ly/1Tt4KZI)  Cratons

  • In Australia all mineral deposits are owned by the Crown & the Government approves applications to explore (& possibly later to mine) – on conditions

Exploration

  • Much exploration & analysis utilises remote sensing data – well before field work starts
  • Exploration leases are granted on application to Government
  • Drilling of target areas follows much deliberation, assessment, sampling & analysis (high costs)
  • Success in finding an economic orebody is very lowAirborne magnetics give clear indications of the geological makeup and structure (‘bones’) of the land at various depths beneath the surface (red = most magnetic rock; blue = least magnetic rocks). (Map from SARIG)

Airborne magnetics give clear indications of the geological makeup and structure (‘bones’) of the land at various depths beneath the surface (red = most magnetic rock; blue = least magnetic rocks). (Map from SARIG)

 

 

Radiometric K

Airborne radiometrics provide unique information about the distribution of radioactive elements (K,Th,U) in surface rocks and soils from gamma ray signals (red = most potassium; blue = least potassium). (Map from SARIG)

Regulations

  • If, after exploration, an orebody is discovered and assessed to be ‘economic’, Government approves (or not) an application to mine on conditions based on submission of a ‘comprehensive’ formal Proposal or Plan
  • Legislation drives the process & the outcomes: Mineral Resources Division (Department of State Development) in South Australia is the regulator
  • Consultation with landowners & communities (particularly at the exploration stage) can be inadequate.  Community angst leading to ‘outrage’ is a common consequence
  • Environmental management guidelines (& community expectations) can be ‘downplayed’ in favour of ‘public good’
  • Company (& shareholders), Government (through royalties & taxes) & Community (employment, local Company spend, services) can all benefit from a mining operation. What about landowners?  How is best to benefit neighbouring landholders?
  • BUT legislation (& regulation) is generally inadequate in terms of environmental management & rehabilitation (post-mining) – can lead to significant environmental legacy
  • Information about what can & can’t be ‘done’ should be readily available & clearly explained to communities.  How? By whom?
  • Community lobby can change legislation: best practice environmental management ‘guidelines’ should become ‘requirements’?
  • Cost of mining projects must include the full cost of rehabilitation of project areas to something like that existing pre-mining according to best practice guidelines – not currently the case .  Many mining projects would not proceed if this was the case!
  • Mining companies & legislators should include community representatives on site-specific environmental management committees that operate for the term of the project and have ‘teeth’?
  • Community groups must remain active & vigilant?

Summary

  • Historically, mining has predated agriculture & other enterprises to ‘kick-start’ local economies, usually in ‘outback’ areas
  • In time, agriculture & pastoral pursuits generally ‘subsume’ land after mining ceases, even though there are on-going environmental legacies
  • Different forms of community development attach to mining & agriculture – community attitudes/perspectives change (and will continue to do so)
  • Legislation drives environmental regulation – community attitudes & perspectives can help to change legislation
  • Knowledge is key – monitoring & acceptance (or not) of impacts of any enterprise on local & regional landscapes ultimately falls to the community
  • Change is inevitable – alertness, communication, regulation, adaptability, science all help

Some mining operations currently on southern Yorke Peninsula

Ardrossan dolomite quarry & port facility  Ardrossan

 

 

 

  

Klein Point limestone quarry & port operations  Klein Point

 

 

 

 

Stenhouse Bay gypsum operations  Stenhouse Bay

 

 

 

 

Price salt pans  Price saltpans

 

 

 

 

Dr A R Milnes

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.

Draft Letter to the Editor, ‘Yorke Peninsula Country Times’: proposed Hillside Mine near Pine Point, Yorke Peninsula, South Australia

I’ve been following with interest the proposed development of the Hillside Mine by Rex Minerals, as well as both the concerns and support from within the community. Nick Perry’s articles and the accompany Letters to the Editor in the Tuesday August 12 issue of the ‘Yorke Peninsula Country Times’ cover the range of views. There is no doubt that the State is desperate for investment which generates jobs, and this clearly ‘flavours’ the decision by the Government and the mining regulator to approve the proposed mine on the basis of ‘overall public benefit’.

However, there is another dimension to the proposed mine and this is likely to have a lasting negative impact: the proposed mine pits will not be rehabilitated.

Open-cut mining operations are generally destructive of land by their very nature. Open-cut, base-metal mining operations are additionally problematic in having the potential to generate and ‘leak’ acid and toxic metals, like copper.

‘Enlightened’ mining companies wanting a lasting positive interaction with the communities in which they operate work their mine plan so that they can strategically place metallic and acid-producing wastes at depth in the pit (effectively a geological containment) and backfill with benign waste rock and soil. Thus, the landscape has a chance to ‘recover’ somewhat and gradually return to something like a pre-mining condition in the long term.

Much of the land on and around the proposed mine, as in other parts of Yorke Peninsula, has been farmed by some families for up to 140 years. Hillside might operate for a decade and a half. So the community gets stuck, for the long term, with an open pit that will eventually part-fill with water and is likely to be contaminated by base metals. The land will affectively have little or no value. It is likely to require ongoing monitoring to ensure that contamination does not leak offsite and also regular intervention from both the safety and environmental perspectives.

Mining companies effectively ‘borrow’ lands from communities to advance their business, pay their taxes, make returns to shareholders, and hopefully inject money and jobs into local regions. I think that they also have an ethical obligation to return the lands that they’ve effectively ‘borrowed’ (albeit some companies may purchase property for the life of the operation) in good condition so that there is some long-term value to the community. At the outset, mining companies clearly register community concerns about rehabilitation and contamination through statutory community consultation programs. Consequently, they have the opportunity to research and develop mining strategies and plans that give them the best opportunity to rehabilitate the landscape at the end of mine life. ‘We can’t afford to backfill pits because it would make the proposed mine uneconomic’ is not an appropriate response.   Why should communities be left with this legacy?

Mining regulators must make it very clear well ahead of project initiation that they may permit mining operations that potentially leave the community (and the State) with the costs for maintaining the long-term legacies of open-cut, base-metal mining operations. We are now only just seeing the advance of short-term mine operations into productive agricultural areas and towards urban settlements in this State. We’re also seeing the signs of community outrage that must be addressed if all are to benefit from State development projects such as mining.

A Lesson Learned

Field observations

As part of ongoing studies of the nature and distribution of Permian glacigene sediments on Fleurieu Peninsula, we were shown to a location just below the plateau surface near Spring Mount, west of ‘Minnawarra’ Homestead (Fig. 1).  Here, on a north-facing spur high in the landscape, adjacent to scattered outcrops of extensively weathered and ferruginised bedrock, there was a surface scatter of rounded cobbles and pebbles cascading down towards a small dam in the valley. We would normally be looking for just this type of geological occurrence as evidence for Permian glacigene sediments from which the cobbles and pebbles (erratics, outwash gravels) would have been eroded. This site was somewhat removed from the usual locations of Permian sediments that are common nearby at lower elevations in the Inman Valley to the south, and in the Hindmarsh Tiers and Myponga valleys to the north. Nevertheless, the scatter of rounded clasts was distinctive.

Figure 1:  Location map.  Site studied marked by red star.

Figure 1: Location map. Site studied marked by red star.

On closer examination, the clasts were commonly of quartzite and fine-grained gneissic rocks but we could not identify any granite cobbles: boulders and cobbles of Encounter Bay Granites are very common in the Permian deposits along the northern margins of the Inman valley, for example. In addition, many of the cobbles and pebbles here were somewhat oblate in shape.

We traced the scatter of clasts downhill towards a small dam. On the eastern side of the dam a shallow cutting had been made in the hillslope to provide vehicular access to the dam.

Figure 2:  Pebbles and cobbles eroding from a bleached and weathered sand-silt material exposed at the base of a Xanthorrhea.

Figure 2: Pebbles and cobbles eroding from a bleached and weathered sand-silt material exposed at the base of a Xanthorrhea.

Figure 3:  Scatter of pebbles and cobbles eroding from a bleached, weathered and somewhat ferruginised sand-silt material.  Note the oblate character of many of the cobbles.

Figure 3: Scatter of pebbles and cobbles eroding from a bleached, weathered and somewhat ferruginised sand-silt material. Note the oblate character of many of the cobbles.

Various exposures en route to the cutting, for example on the downslope side of a Xanthorrhea (grass-tree), revealed cobbles and pebbles weathering from a bleached, weathered and generally poorly consolidated sand-silt matrix (Fig. 2), which is very like the situation we have observed in Permian glacigene deposits.  First observations on approaching the cutting by the dam confirmed these relationships (Fig. 3).  At the cutting, however, the actual field relationships of the clasts and the matrix are more clearly seen and the clasts tend to have a well defined ‘imbricate’ orientation in the host matrix (Fig. 4).  Immediately to the east in the cutting the relationships become clear:  the clasts are actually contained within steeply dipping weathered bedrock.  Their oblate shape and orientation is a function  of deformation of the bedrock and they are aligned parallel to the steeply dipping layering represented by schistosity roughly parallel to bedding (Fig. 5).  The weathering of the bedrock, which is related to its occurrence in close proximity to the deeply weathered pre-Tertiary summit surface of Fleurieu Peninsula, and which is particularly characteristic of the area around Spring Mount, has extensively altered the rock matrix but apparently little affected most of the contained clasts.

Some outcrops in the small tributary immediately to the west of the dam are of weathered and ferruginised schist and gneiss, but there are no rock clasts evident.

Figure 4:  Pebbles and cobbles with a clearly defined ‘imbricate’ habit within the bleached, weathered and somewhat ferruginised sand-silt matrix.  Again note the oblate character of many of the cobbles.

Figure 4: Pebbles and cobbles with a clearly defined ‘imbricate’ habit within the bleached, weathered and somewhat ferruginised sand-silt matrix.

Figure 5:  Pebbles and cobbles, now seen as significantly deformed parallel to the cleavage in steeply dipping bedrock which has been strongly altered by weathering.

Figure 5: Pebbles and cobbles, now seen as significantly deformed parallel to the cleavage in steeply dipping bedrock which has been strongly altered by weathering.

Clearly, the geology is not reflective of Permian glacigene deposits but of weathered and altered bedrock most likely to be the basal Proterozoic conglomerate that unconformably overlies the Barossa Complex basement inlier in this region. Reference to the geological map (Fig. 6) confirms this possibility. The essentially unweathered conglomerate, which is also spectacularly deformed, is well-exposed in the Inman Valley at Grey Spur, just south of the Spring Mount locality. The same formation (assigned to the Aldgate Sandstone; SARIG mapping) is exposed on the coastline at Lady Bay, south of Normanville, but the deformation here has been very intense and the contained cobbles and pebbles are significantly deformed.

Conclusion

The ‘lesson learned’ is that all is not as it initially may seem in field geology, and jumping to conclusions is not recommended. A close examination of field relationships in any locality, together with questioning of initial conclusions and gathering of all available evidence, might actually uncover an interesting story that would otherwise be missed.

Figure 6:  Geological map showing site location (red star) and distribution of basal Proterozoic conglomerate (Nol Aldgate Sandstone). Geology as follows:  Orange-brown (Lb) = basement Barossa Complex; dark brown (NoI, Nds, Nl etc) = Proterozoic; pale brown (Eec, Eeb etc) = Cambrian Kanmantoo Group; blue (CP-j) = Permian glacigene sediments; orange (T) = undifferentiated Tertiary weathered  zone materials; yellow (Q) = undifferentiated Quaternary alluvials.

Figure 6: Geological map showing site location (red star) and distribution of basal Proterozoic conglomerate (Nol Aldgate Sandstone). Geology as follows: Orange-brown (Lb) = basement Barossa Complex; dark brown (NoI, Nds, Nl etc) = Proterozoic; pale brown (Eec, Eeb etc) = Cambrian Kanmantoo Group; blue (CP-j) = Permian glacigene sediments; orange (T) = undifferentiated Tertiary weathered zone materials; yellow (Q) = undifferentiated Quaternary alluvials.

A R Milnes

R P Bourman

Where do these rocks come from?

In the streets and gardens of Poznan in Poland there are often red and pink coloured rocks.

Stone wall with pink & red coloured rocks

Stone wall with pink & red coloured rocks

I asked my grandfather where all the nice rocks came from.  My grandfather, who is a geologist and who knows all about rocks, told me that these rocks came from Norway.  “Norway? … this is very far!”  I asked him “How did they get here?”

He told me that once there was a time when it was very cold and Norway was covered by a giant ice sheet, more than 2 km thick!!!

The ice slid slowly to the south, tore rocks away from the country and carried the rocks with it.  Later the weather became warm again, the ice melted and the rocks were liberated.  In that way the Norwegian rocks settled down in Poland … after a journey of 1000 km over 50000 years!!!

Ice flow from Norway

Ice flow from Norway

 

 

 

Guest author:  Alexia Thiry, 9 years.  Address:  4Hands, nr 4(17) / 2013-11-24 The International School of Poznan Monthly (Poland).  Guest grandfather:  Dr Medard Thiry, Centre des Geosciences, Mines ParisTech, 35 rue St Honore, 77305 Fontainebleau, France.