In Part 1 of this article, we saw how opal and the related process of fossilization (opalization), acting as a veritable window into the past, allowed us to reconstruct and better "visualize" the life forms that populated a small portion of Australia (Lightning Ridge and its surroundings) 100 million years ago. At the same time, it was possible to reconstruct the actual environmental conditions that existed in that area. A description was provided of opal's chemical and microstructural nature and how it enabled the preservation, through opalization, of what remained of the fauna and flora of that distant past. With this in mind, let's see what remains, thanks to opal, of what was once the so-called "biocenosis" of the opal fields and beyond, of another very famous mining town, that of Coober Pedy. We will discover, a short distance from our first destination (Lightning Ridge) and, at the exact same time, how the life forms present (biocenosis) and the reconstructed environmental conditions are substantially different from what we saw previously. One might think that Opal, perhaps also due to its non-rigid chemical-microstructural "setup," or rather, its non-membership of any of the seven crystalline systems (with all the constraints that such membership entails in terms of material "constraints"), was exceptionally versatile and adaptable to the most diverse environmental conditions in which it was called upon to "reconstruct" both fauna and flora (a small personal observation!!!).
In any case, the mining town of Coober Pedy has a brief (founded only in 1915 and currently entirely underground, "dugouts") but fascinating history, as are the fossil remains found in its opal fields. More specifically, the opal and its fossils are found in the most superficial and weathered portion of the so-called "Bulldog Shale," a clayey member (thinly layered shale) belonging to the Meer Formation and dating back to the Lower Cretaceous. These shales were deposited in marine facies conditions (Eromanga Sea). At the time of deposition of the organic remains (later fossilized by opal), the environmental conditions were typical of a temperate climate, characterized by significant temperature variations between winter and summer. The opalization of animal and plant remains occurred after they were buried in marine clay sediment (Bulldog Shale) and by the action of silica-rich groundwater, which flocculated the hydrated silica gel. The variety of opal mined in the Coober Pedy opal fields is white opal, or more commonly, white milky opal.
Opalization occurred mainly by "replacement" but also by "encrustation." The most common opalized fossil remains are those derived from the shells, valves, and "rostrums" of mollusks, and among vertebrates, those of the skeletons of marine reptiles such as plesiosaurs and ichthyosaurs. Among those belonging to plesiosaurs, the famous "Eric" stands out: the nearly complete and almost totally opalized skeleton (about 90%) of a plesiosaur found in the Coober Pedy opal fields in 1987 and currently preserved at the Australian Museum in Sydney.
Keywords : dugouts, opal fields, outback, bulldog shale, bentonitic, opalization, replacement, encrustation, silica, amorphous, fossil, crystalline, belemnites, plesiosaurs, ichthyosaurs, eric
Coober Pedy and its fascinating past
Coober Pedy is, as we were saying, a mining town founded in 1915 and located on the edge of the Stuart Mountain Range and almost in the centre of the Australian state of South Australia ( SA ). It is located approximately 850 km north of the capital Adelaide (see Fig.1 “Coober Pedy location from Google maps modified ”.), it has approximately 1,700 inhabitants who live (uniquely in the world) in the so-called “ dugouts ” or underground houses dug into the hills. This singular housing choice is a consequence of the prohibitive climate that occurs on the surface with temperatures of almost 50 C° during the day and which drop drastically during the night. The town of Coober Pedy is considered the “ Opal Capital of the World ” with almost 70 Opal fields that extend within its territory ( Fig.2; “Coober Pedy Opal fields”.) . It is thought that, from the beginning of the Opa le epic (1915) to today, around 250,000 wells have been dug in its territory for the search and extraction of the precious “ mineraloid ”. Only rarely, at the end of the extraction, have these wells been carefully “backfilled” and therefore, it is very dangerous to “walk” in the Outback (hinterland) of the town. The name of Coober Pedy itself is particular and suggestive as it seems to derive from the Aboriginal words “ Kupa ” (Coober) which apparently means “white man” and “ Piti ” (Pedy) which means “Waterhole” or simply “Hole” and therefore the whole thing translates as “White man's hole” referring to the miners who worked and lived underground. Initially, the name of the site was that of “ Stuart Range ” and was only changed in 1920 to Coober Pedy , during a district meeting.
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| Fig.1 “Coober Pedy location from Google maps modified ”. |
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| Fig.2; “Coober Pedy Opal Fields”. |
The first to explore the desert region of Coober Pedy was the Scotsman John McDouall Stuart back in 1858. He set out from near the town of Andamooka and headed north until he reached the mountain range that now bears his name, the “ Stuart Range, ” where, in February 1915, fourteen-year-old Willie Hutchinson first discovered opal. He was part of an expedition (led by his father Jim Hutchinson) consisting of a group of gold prospectors. Jim Hutchinson described, in an article in the Adelaide Chronicle , how his young son Willie returned to camp with a sack of opal, exclaiming: “Have a look here, Dad, I think you’ll find some good stuff there, I’ve found both opal and water, good water, there’s enough for us and the poor old hunchbacks to last at least a fortnight.”
In the early years of the Coober Pedy opal fields , miners and traders reached the area by railway, which stopped some distance from the opal fields at William Creek. Subsequently, the completion of the transcontinental railway provided direct access.
In the early days, mining was carried out in a "rudimentary" manner, with miners using shovels, pickaxes, and even pocket knives and pliers. The site was chosen based on the presence or absence of "floats," or small fragments of opal near the surface. Subsequently, deep digging began. The excavation was carried out by hand with the aid of shovels and pickaxes, to a depth varying from 3 to 10 meters. The shaft walls were lined with wood to stabilize them and prevent collapse. Miners used special winches to lower themselves to the bottom of the shaft or to clear it of waste soil or mullock (mullock is waste rock material containing metal scraps from opal mining equipment).
After that, always using shovels and pickaxes but also rudimentary explosives, they moved horizontally, following the vein of common Opal ( Potch ) with the hope that it would become precious Opal.
To facilitate the work, the government installed a large underground water tank in 1921. But in addition to the water shortage, the remoteness of the region itself and market fluctuations slowed the rise of Coober Pedy opal on the world market. This was until 1946, when the so-called " Eight Mile Field ," a huge deposit of precious opal in the Coober Pedy Outback , was discovered , reviving the industry.
Starting in the 1960s, opal mining in Coober Pedy became mechanized. Special drills (Calwed drills) began to be used for digging the pits, while horizontal excavation employed special excavation machines.
GEOLOGY
The geological setting of the territory in which Coober Pedy falls depends on the sedimentary-depositional "vicissitudes" of a large area located east of the "Great Artesian Basin" (GAB) , which extends for 300,000 km2, between the territories of central-southern Queensland , New South Wales and South Australia . It hosts a succession of sediments with a thickness of approximately 2,500 m. This thickness was formed in the period between the Lower Jurassic and the Lower Cretaceous (Albian).
Starting from the Middle-Upper Jurassic , the Coober Pedy area was affected by a phase of continentality which culminated in the deposition of huge thicknesses of silico-clastic material mixed with feldspar originating from the disintegration of the “ Stuart Mountain Range ” and forming part of the Cadna-Owie Formation Sandstones .
This sedimentary-depositional state continued until a good part of the Lower Cretaceous when, due to a significant phase of marine ingression " Eromanga Sea ", the deposition of "fine" sediments, i.e. clayey and with a purely horizontal disposition, began, resulting from the lack of active deformational tectonics, a typical lack of old intracratonic basins. This type of marine clayey sedimentation with the formation of thinly stratified argillites and/or shales and called " Bulldog Schist " reached its peak in the Aptian (also in the Lower Cretaceous ).
In the Coober Pedy area , at present, the clays and shales that make up the “ Bulldog Schist ” can be divided, depending on the depth from the ground level and the degree of alteration, into three main thicknesses.
A deeper layer, practically unchanged and made up of gray-brown, intact, well-compacted, low-porosity clays and shales known by miners as "mud." These clays are made up of smectite (a highly swellable clay mineral).
These are followed upwards by a layer of thinly stratified clays up to shales, medium weathered and grey or grey-mauve in colour.
Finally, at the top of the “ Bulldog Schist ” member we have a “partition” made up of highly altered, whitish and porous argillites and shales that miners improperly call “Sandstone” and made up of the clay mineral kaolinite . This layer, as we were saying, has been strongly altered by exogenous agents that have washed it out and bleached it or reduced it to various colours. This bleaching phenomenon in Coober Pedy (but also in the opal fields of another famous locality, that of Andamooka ) can extend down to a depth of 40 – 50 m (Robertson and Scott, 1990).
At the base of this last and most superficial layer of the Bulldog Shale , at the contact with the medium-altered layer, the Opal and its fossils are found . More precisely, the Opal is found in correspondence with thin layers of bentonite clays that are coeval and correlate with the “ Finch Clay Facies ” of Lightning Ridge . More precisely, the Opal is found more frequently along cracks, fault planes and cavities left by fossil remains . In the latter case, we have fossilization by opalization due to both “ replacement ” and “ encrustation ”. This second modality frequently allows the preservation of the internal structures of the fossilized organic remains . The formation of the Opal, we recall, seems to be linked to the cyclic fluctuations of the rather acidic groundwater, which have caused the dissolution of the silica from the arenaceous sediments, followed by its precipitation (in the form of hydrated silica gel ) within the above-mentioned structures.
The Opal mined in Coober Pedy is white Opal ( see Fig. 3 “White Opal from Coober Pedy”. ), although, in truth, much of it belongs to the “milky” variety known as “White Milky Opal” or more simply “Milky Opal”. The milky appearance of this Opal is due to the presence of liquid in its micropores.
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| Fig.3 “Coober Pedy White Opal”. |
LOWER CRETACEOUS BIOCENOSIS
The ecosystem around Coober Pedy reconstructed from opalized fossils found in the Bulldog Shale (Lower Aptian – Lower Albian) was decidedly different from the contemporary estuarine-lacustrine ecosystem of Lightning Ridge . In fact, in and around Coober Pedy we find ourselves in a marine environment ( Eromanga Sea ), although, as we will see, with some exceptions. Furthermore, opalized fossils of invertebrates and plants have not infrequently been found in the Bulldog Shale of the Coober Pedy area , in association with those of marine reptiles. The presence of large quantities of opalized wood suggests that in the Coober Pedy area we were in the vicinity of a heavily forested mainland. From there, the logs floated and were deposited in the purely marine sediments of the Bulldog Shale present around Coober Pedy .
We are here, essentially, in a marine environment of cold waters at high latitudes. In fact, as already mentioned, Australia (part of the Supercontinent Gondwana ) at that time was much closer to the South Pole than it is today (it is thought that it even fell at around 60 – 70° South latitude and that the coastal margins of the central-southern sector of the Eromanga Sea were subject to winter frosts with related ice accumulations. Again from paleoenvironmental reconstructions, it is estimated that the water temperature of the Eromanga Sea varied from maximum values in the summer half-year of 12°C to minimum values of around 2°C.
As regards invertebrates, the opalised fauna found is mainly made up of fish, molluscs, both gastropods of the Euspira genus ( see Fig. 4; “Opalised gastropods” ) and bivalves such as the Macc oyella and Cyrenopsis genera , but also cephalopods such as ammonites and belemnites (see Fig. 5; “Opalised rostrum of a belemnite”). The latter belong to the Peratobelus genus ( belemnites are squid-like molluscs whose fossils consist of their elongated, conical internal body called the “ rostrum ”).
Remains of echinoderms (both crinoids and ophiuroids ), relatives of the common sea urchin, are also found . These, especially the "articulated" ones, namely crinoids (with their typical "goblet shape"), easily disarticulate post-mortem, dispersing the small articulations (a sort of many small tesserae) in the sediment.
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| Fig.4; “Opalized gastropods”. |
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| Fig. 5; “Opalized belemnite rostrum”. |
Instead, as far as the opalized vertebrate fauna is concerned, it is represented by abundant remains of marine reptiles such as Plesiosaurs and Ichthyosaurs . But among the first finds of marine reptiles in the area of Coober Pedy , we have that of an Elasmosaurus (discovery occurred in 1928 near the Neales River) which was followed by that of a Pliosaur (just a tooth found in the 1980s by Pledge in the Bulldog Shale ). Elasmosaurus are an evolution of Plesiosaurs and are distinguished by having a disproportionately long neck. On the other hand, very common and differently distributed over time are, as mentioned above, the remains of Plesiosaurs and Ichthyosaurs . Plesiosaurs were marine reptiles with a long tail, a stocky belly and legs transformed into a sort of "paddles". Furthermore, they had a notoriously long and disproportionate neck (see Fig.6; “AI reconstruction of Plesiosaur”.) .
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| Fig.6; “AI reconstruction of Plesiosaur”. |
Instead, the Ichthyosaurs , or “fish lizards”, marine reptiles that evolved from pre-existing terrestrial forms, had an appearance, all things considered, similar to that of today's cetaceans.
In fact, although ichthyosaur fossils are regularly encountered in the Bulldog Shale , they are less common than plesiosaurs , which show both numerical and taxonomic dominance in most localities in South Australia . Virtually all Cretaceous ichthyosaur fossil material in Australia can be attributed to the genus Platypterygius . As many as five families of plesiosaurs have been recognised in the vicinity of Coobeer Pedy . A high percentage of these opalised plesiosaur remains are from juveniles found in the coastal areas of the Eromanga Sea . This may suggest that plesiosaurs favoured the shallow, cold, nutrient-rich waters near the coast at an early age. Furthermore, they used these shallow waters as “safe birthing grounds” and as refuges for young animals before facing life in the open sea.
It is still unclear why, alongside Plesiosaurs and Ichthyosaurs, the remains of other vertebrate species commonly associated with them, such as chimerids (i.e. cartilaginous fish distantly related to sharks), osteichthyan fish, sharks and marine turtles, are missing.
However, among the most famous Plesiosaurs for which it has been possible to reconstruct the skeleton with a certain completeness, we have: “ Eric ” , the Addyman Plesiosaur, the Opallionectes and 2 Cimoliosaurus.
Among the most famous Lower Cretaceous Plesiosaurs , we focus on “ Eric ” (discovered in the Opal fields of Coober Pedy in 1987) as it has an almost complete skeleton of which approximately 90% is made up of Opal. Although it was once “catalogued” as a Plesiosaur , it actually belongs to the “short-necked” Pliosaurs , or carnivorous marine reptiles (fish remains were found in association with the skeletal remains of this reptile) deriving from the Plesiosaurs but with a shorter neck (maximum 13 vertebrae) and a more hydrodynamic shape and the dimensions of a sea lion (length approximately 3.00 m). The “ Eric ” specimen is now preserved at the Australian Museum in Sydney (see Fig. 7; “Pliosaur Eric”.).
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| Fig.7; ” Pliosaurus Eric”. |
But in the Bulldog Shale , in the Coober Pedy area , remains of terrestrial reptiles such as those relating to the theropod dinosaur Kakuru kujani (Molnar and Pledge, 1980) have been found. Theropod dinosaurs are those dinosaurs, usually carnivorous, which present some characteristics typical of birds, which are supposed to have originated from theropods . These finds refer to the presence of a certain continental fauna in the coastal areas of the Eromanga Sea .
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