A team of doughty FOBIF individuals are currently developing a field guide to the geology of our local region, prompted by frequent requests for such a guide, and the popularity of FOBIF’s existing publications. Supported by professional geologists, the aim is to produce a guide that sheds light on the various major local rock types and the geological processes that formed them, via a series of site visits. “Go here, look at this, note these features, here’s the story that explains what you see”.
Since geological processes are by nature slow, the team has decided to test-drive draft selections from the guide in a series of articles on the website. First up are the oldest rocks in our region known as the Ordovician rocks. Readers are invited to send feedback to info@fobif.org.au . Please note: diagrams are rough sketches only.
The Ordovician rocks
The Ordovician Period (485–444 million years ago) is named after the Welsh ‘Ordovices’ tribe, Wales being the first place where rocks of this period were studied. The oldest rocks found In the Mount Alexander region are of Ordovician age.
Site visit 1: The Anticlinal Fold
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| The Anticlinal Fold in Lyttleton Street east, adorned (some might say desecrated) with a plaque on its apex |
Our geological journey begins on the north side of Lyttleton Street in Castlemaine, some thirty metres to the east of Urquhart Street, where we find an exposure of Ordovician rocks in the steep hillside. There are several geological observations to be made here. The rocks are in distinct layers. Each layer is clearly defined and sits on top of the one below it. Geologists call these layers ‘beds’. The rock beds do not lie flat but form an elegant arch. Sitting on top of the arch is a plaque which assumes that the average reader has a working knowledge of mining geology.
The inscription, headed “ANTICLINAL FOLD”, tells us that “This fine exhibit was disclosed when Lyttleton St East was constructed in 1874. Saddle reefs occur in similar folds of the sandstones and slates on lower geological horizons”
The heading itself holds two key geological terms. To geologists, a fold is a wave or zig-zag shape in rock beds. In our anticlinal fold we note that the beds in the fold sides lie at a steep angle. Anticline is the name geologists give to an upwards fold.
The remainder of the inscription appears to be strangely technical for a notice intended for the casual reader. It refers to ‘reefs’. Do not expect to find coral. In mining geology a reef is a formation where gold is likely to be found. More on this later. Noting the date (1874) we remind ourselves that geology played a very important role in the wealth of early Castlemaine.
How the Ordovician Rocks formed
The story of the Ordovician rocks in south-eastern Australia starts some 520 million years ago. At that time, Australia was not an isolated landmass but part of the supercontinent of Gondwana, The eastern coastline of Gondwana lay roughly north-south along a line passing through present-day Mildura and Ararat.
A coastal mountain range lay along Gondwana’s eastern margin. Rivers carried sand, silt and mud from the mountains to the shallow sea along Gondwana’s coastline. The debris settled along the continental shelf as thick deposits of soft sediment. Earth tremors periodically upset the unstable mass, triggering sudden collapses, causing huge volumes of loose sediment to violently slide down along a series of submarine channels into deeper waters.
[source: Wikipedia]
The collapsing and fast-moving sediment is called a turbidity current. Modern turbidity currents travel at more than 60 km/hour and can break submarine cables.
Thousands of these violent events led to thousands of layers accumulating on the ancient seafloor – a single layer resulting from one event is called a turbidite. Each turbidite layer was buried by a newer turbidite until the ocean floor grew thicker by about 3000 metres. Typically, each turbidite is less than one metre thick and is made up of a section of sandstone topped by a thin layer of fine mudstone.
Looking at the Lyttleton Street anticlinal fold. We see three sandstone beds stacked on top of each other. Each bed formed as a turbidite. If we examine each bed closely we can see the rock grading from coarser grains at the base of the bed to finer grains and near the top, in some places, we might even see some fine wavy structures. These represent the finer material settling out as the violent current lost its energy at the end of the flow.
Turbidites can take on different forms depending on the energy and sediment supply of the system; while some turbidites are mostly composed of sandstone, others are mostly mudstone.
Sometimes, in quiet periods when there were no earth tremors, coarser sandy material built up close to shore and fine mud was carried out into deep water, gently settling to form mudstone. The gradual accumulation of organic material added carbon to the sediments, giving the rocks the dark colours. These quiet conditions were ideal for preservation of fossils known as graptolites in the fine, dark mud. More on graptolites at the end of the chapter
The Gondwanan sedimentation process gradually spread eastwards over many millions of years to where Castlemaine now stands. The oldest known turbidites in this region are about 485 million years old and over some 40 million years an immense depth of sand and mud layers accumulated on the ocean floor. The horizontal seafloor layers were gradually compressed into rock beds under the weight of overlying sediments.
All this information and interpretation of what we observe has been pieced together by geologists over many years. The Ordovician rocks of the Mount Alexander region have been studied in great detail because they are the primary source of all the gold that has been and is still being found in the region. But how the rocks formed from turbidity currents, carrying avalanches of sandy and muddy sediment down submarine channels to deep water over some 40 million years, is only part of the story. Taking another look at the Anticlinal Fold, the obvious question posed is: ‘Why are the rocks bent into a steep arch, an anticline?’
Upheaval of the Ordovician Rocks
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Geologically, the process of laying down layers of sand and mud on a sea floor leads to a horizontal stack of sandstone and mudstone rock layers, the oldest layer at the bottom and the youngest at the top. Clearly huge-scale forces have been at work to mess up the original horizontal layering and shape them into structures like the anticlinal fold. Where did those forces come from? The answer is nothing less than the earth-scale forces of plates of the earth’s crust moving against each other.
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About 445 million years ago in the slow shuffling dance of the earth’s crust, a tectonic plate to the east of Gondwana began to move towards it, exerting sideways pressure on the Gondwanan ocean floor. Over millions of years the sediments were squashed by the eastern plate, crumpling them into tight, parallel folds and raising them to form new land on the eastern edge of Gondwana. An ocean floor about 1500 kilometres wide was crushed into a series of tightly folded beds less than half the width and 15 to 20 kilometres thick. Under east-west pressure, the folds formed with their axes running north-south and this is reflected in the north-south ridges characteristic of the local Ordovician rock landscape today.
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| Typical Ordovician landscape. View from Main Road Chewton |
Returning to our study of the Anticlinal Fold, we note that Lyttleton Street lies in an east-west direction, hence our fold has been sliced through neatly at ninety degrees to the fold axis, revealing its beautiful symmetry. One can almost feel the tectonic plate advancing inexorably from the east, against the Gondwanan plate edge, crumpling the layers of rock that lay on the Gondwanan ocean floor century by century, millennium by millennium, mega-year by mega-year.
Where there’s an up-fold, an anticline, there’s usually a down-fold on either side. A downfold is called a syncline. Stepping back from the anticline, (watching out for traffic) then looking to the right, or east along the rock exposure, we note that the rock layers leaning against the anticline gradually become steeper. There’s a bit of a break then we see a continuation of the rock layers, but this time they’re leaning away from the anticline. What has happened here? Looking down to the ground we spot a vital clue; a rock face showing a V shape of rock beds. This marks a syncline.
The anticlinal fold and its attendant syncline are just one of thousands of folds across the region.
Gradually the folded rocks were raised above sea level to form new land, We can romanticise and imagine an immensely high mountain chain forming, but geologists believe that due to the slow speed of the process (as opposed to the current day Indian plate crashing into Asia, resulting in the Himalayas), the new land never rose to great heights. It was steadily eroded by wind, water and ice, with several kilometres of material removed between then and now. The ridges of Ordovician rocks seen in the landscape today are the worn down remains of those ridges that began to emerge from the ocean some 445 million years ago.
Gold in the rocks
Before we leave the anticlinal fold, we’ll pay attention to the inscription one more time, in particular its reference to ‘reefs’. What are they and how did they form?
While the rocks were being squashed and folded, under the intense pressure, cracks, known as faults formed in the hardened rocks. Towards the end of the folding process hot, watery fluids carrying dissolved quartz and gold were forced up through the rock along the fault lines. In time they solidified to form gold-bearing quartz veins. These veins, known by the miners as reefs, are the primary source of all the gold in the region.
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| Large quartz vein in Campbell Street embankment |
A saddle reef is quartz that gathers between rock beds along the top part of an anticline, hence the reference to a saddle. Sometimes these were rich sources of gold and were keenly searched for by miners, although they were less likely to be fruitful in the Castlemaine goldfield. On the other hand, Bendigo’s saddle reefs have been reliably rich sources of gold, historically making the Bendigo goldfield one of the richest in the world.
Fossils
Fossils are the preserved remains or imprints of animals or plants found in rocks. In the Ordovician rocks, fossils called graptolites are found in the black mudstone beds, which formed in quiet, turbidity current-free conditions.
Graptolites were twig-like structures harbouring colonies of minute animals that free-floated in the ocean, feeding on plankton. Graptolites are found worldwide in rocks of Ordovician age. Due to their rapid evolution into a wide variety of different forms, they have proved to be very valuable tools for determining the relative ages of rocks, and for identifying rocks of similar age in different locations
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| Examples of the many graptolite forms |
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Click on image for info/order page
Click on image for info/order page
