Australia: The Land Where Time Began
Jurassic Australia - 199-145.5 million years ago
The Jurassic opened with the landmasses still forming Pangaea. With the exception of part of western North America, that remained covered by shallow seas, continental crust of Pangaea was above sea level. On the edges of this submerged area the Sierra Nevada Mountains were being uplifted during this period. As the Jurassic progressed the first rumblings of the breakup of Pangaea began to gather strength as the tectonic plates were reactivated and began the process of separation. Rift lines had formed along the margins of the rifts where they were to separate, large quantities of basalts and dolerites poured out along these lines. The rocks that formed at this time are found in all the former members of Gondwana. In Tasmania they can be seen at the mouth of the Derwent River.
The beginnings of the separation of Gondwana from Pangaea was soon followed by rifts between the southern continents that comprised Gondwana. It was at this time that the Australian continent was delineated. By the end of the Jurassic the rocks of the ancestral New Zealand landmass had been raised above sea level on the eastern margin of Gondwana.
During the Jurassic Gondwana began to break up with major continents beginning to drift away. Africa and South America began the breakup, then India rifted away from Antarctica. By the close of the Jurassic Australia was the only continent still joined to Antarctica. The first to arrive, the last to leave. The changing arrangement of the landmasses produced major changes in climate and ocean circulation, greatly affecting the global climate.
Globally, climates were warm to hot, the high latitudes being warm and humid. In a band about 40o either side of the Equator it is believed the climate was seasonal, with long dry times alternating with rainy periods.
The Jurassic was the Age of Dinosaurs and the Age of Conifers and Ferns. The vegetation was made up of the gymnosperms - conifers, cycads, Ginkgophytes and seed-ferns, and ground ferns, tree-ferns and fern allies, the herbaceous horsetails and clubmosses. The vegetation of the Early Cretaceous was of a similar type to this. Some evidence from Laurasia suggests plant diversity may have been declining towards the close of the Jurassic. Some think the apparent decline may indicate the vegetation had reached a stage of stagnation. Some believe the genetic variability of the flora was gradually being lost under such a long period of unchanged conditions, making them more vulnerable to being replaced, when conditions changed, by new, more efficient plant types. It has been suggested that this apparent stagnation may have been conducive to the rapid rise of the Angiosperms in the Late Cretaceous.
The marine microfossil record shows that during the Jurassic Dinoflagellates and Foraminifera were abundant in the plankton of the time. Modern Ammonite families were flourishing and some more primitive Ammonite families became extinct and Belemnites were abundant. The insects flourished and diversified on the land, flies and bees appearing in the fossil record for the first time.
The Australian continent was a stable landmass between 35o S and 65o S palaeolatitude that formed a wide peninsula on the southern margin of Gondwana. The Meso-Tethys Ocean bordered the northwestern margin of Australia and along the southwest coast India was separated from Australia by a complex rift valley system. In the east were the Queensland Plateau and the Lord Howe Rise that were tectonically active. Extending to the south this volcanically active region incorporated in its southern part New Zealand's Western Province and New Caledonia's west coast terrain as a shallow marine margin that was near-shore. The exposed land surface extended to New Guinea that formed the offshore shelf of the Australian region3.
During the Jurassic the marine incursions that occurred were mostly limited, though in the Perth, Carnarvon and Canning basins, a progressive sea level rise that peaked about 170 Ma in the Bajocain of the Middle Jurassic resulted in large shallow marine embayments in these Western Australia basins.
In the rift valley separating India from Australia there were also coal swamps. The authors³ suggest that the presence of these swamps indicates conditions that were wetter and possibly tending towards a climate that was monsoonal, and that a climate with distinct dry season was beginning to develop.
155-145 Ma in the Kimmeridgian-Tithonian, Late Jurassic, rifting commenced between Australia and Antarctica, and marine conditions existed in the northwestern part of the Canning Basin. This led to the formation of a system of deep valleys in which there were lakes, rivers and forests that persisted into the Early Cretaceous. From the Lower (Pliensbachian) to the Callovian (Middle Jurassic) deposits formed that are the best documented fossiliferous strata of Jurassic age in Australia. Marine and continental deposits are known from the Upper Jurassic, but they are more sporadic and contain fewer fossils, as well as generally being restricted to the Jurassic-Cretaceous boundary.
Intermittently sedimentation occurred during the Jurassic that was mostly of terrestrial origin and major river systems were mainly located in the lowland regions of the Clarence-Moreton, Surat and Eromanga basins of the southern part of Queensland and the northern part of New South Wales. These had expanded into a vast network of coal swamp and lacustrine areas by 185 Ma, the Pliensbachian of the Early Jurassic, that were subject to alternating periods of high-energy and low-energy stream and terrestrial conditions. In the central, northwestern and western parts of the continent there were uplands, making them the principal source of the sediments that were added to fluvial deposits throughout the Jurassic. Tasmania and northeastern Queensland also had mountainous areas.
Australia was still at high latitudes during the Jurassic, the main driving force for adaptation being winter nights that were months long. Dinoflagellates were becoming abundant in the ocean basins that were formed by rifting at the margins of the continent. Dolerites and basalts being erupted along the rift lines were the first indications of the changes to come as Australia began to separate from Antarctica. These erosion-resistant rocks are seen as some dramatic features, especially in Tasmania.
The first step in the process of separation was fragments of crust began moving away from the coast of Western Australia. It is though possible that prior to this separation the western part of Thailand may have been situated beside the northwestern sector of the Westralian Depression where the rifting started, though proof of this is lacking.
Late in the Period there was a marine transgression on the northwestern margin of the continent. Rifting and faulting had been occurring for millions of years before the seafloor began spreading. These rifts involved a tearing and shearing component that resulted in parallel ridges and troughs at an angle to the continental margins, not in a single straight line. These basins filled with sediment and became traps for hydrocarbons. In the Perth Basin activity along the major Darling Fault led to the accumulation of sediments in a deep trough. In the Middle Jurassic there was a brief marine incursion into this trough.
From the Middle Jurassic there was faulting and rifting along the southern margin of Australia in preparation for the separation of Australia from Antarctica. This was the last link severed when Gondwana finally broke up. Late in the Jurassic there were active volcanic vents (diatremes) near the eastern margin of the Sydney Basin that is thought to have been connected with the impending opening of the Tasman Sea that separated the outer margin of the supercontinent from the main landmass. During the Jurassic the Tasman Orogen, having been stabilised, was being eroded by major river systems that drained mainly east to the sea north of Brisbane. There was a broad series of interconnected depressions, each of which had its own river, lake and swamp systems. Combined they covered a vast expanse of country. Between the early part of the Middle Jurassic and the earliest Cretaceous, over 5 tectonic cycles that were associated with the break-up of eastern Gondwana, all these systems were converted to the Tasman Depression, a major structural unit. High ground was raised to the northwest and the south in pulses of activity as the depression subsided.
Four of these cycles occurred in the Jurassic, as catchments were raised, rejuvenated rivers eroded the high ground, carrying large quantities of coarse sediments to the depression. The gradients of the rivers were reduced as erosion lowered the catchments, silting up the lakes and swamps, after which fine sediments accumulated. A rapid phase of erosion began following another uplift. At the end of the Jurassic, lagoons and deltas formed along the northeast margin of the depression during the 4th cycle. Global sea level rise affected the drainage patterns leading to the development of these features. In the following earliest Cretaceous, the 5th cycle was truncated when the whole depression was flooded by a global sea level rise.
To the east of Gondwana a volcanic arc was present on an active zone of interaction between the proto-Pacific plate and the Gondwanan plate. During the tectonic events associated with the uplift of the ancestral New Zealand landmass above sealevel from the Middle Jurassic the outer margin of Gondwana was bent into an S-shape (orocline) in the Tasman-Antarctica region. Volcanic ash from these volcanoes contributed to the sediments being deposited in the Tasman Depression.
The sediments of the Tasman Depression contain the main aquifers of the Great Artesian Basin, and the reservoir rocks where the accumulation of oil and gas in the Roma, Moonie and Jackson fields.
In northeast New South Wales and southeast Queensland coal swamps developed along the lower reaches of the east-draining rivers in the Middle to Late Jurassic. North of Perth coal swamps developed.
The Amazon is the closest modern analogue to the environment of the Tasman Depression in the Jurassic and the Early Cretaceous. It covered a vast area of low relief with large, slow-flowing rivers, lakes and swamps. It was covered with luxuriant vegetation as a result of the warm and humid climate. Unlike the present-day Amazon, there were no Angiosperms, the forests being composed of conifers, cycadophytes, ginkgophytes and ferns. The Tasman Depression in the Jurassic was unique, having luxuriant vegetation like the Amazon area but with summers during which the sun never set and winters with months of continuous darkness.
During the Jurassic, freshwater sediments, with shallow marine incursions, accumulated near Geraldton and the Kimberley district, along the western margin of Australia. In central Queensland and northern New South Wales a large inland river system developed. The sediments deposited in these freshwater rivers and lakes of this drainage basin contained fish fossils at several localities, but amphibian and reptile bones are rare.
Based on oxygen isotope studies of the sediments from this time it is believed the climate of Western Australia was warm. Queensland seems to have been cooler and wetter, coal swamps formed in southern Queensland. The vegetation was very diverse, but cold-weather species dominated the flora. There were widespread conifer forests with an understorey of ferns, seed ferns and cycads. Beetles and other insect fossils from the Middle Jurassic have been found near Geraldton, Western Australia.
In Australia Jurassic rocks are mostly of terrestrial origin, the sediment deriving from mountain ranges that had formed during the Palaeozoic in eastern Australia, and were mostly deposited in lake and swamp systems. The beginnings of the Gulf of Carpentaria of the present are indicated by marine transgressions in northwest Queensland that extended close to the coast of the present. By the Middle Jurassic most of the western margin of the continent had been covered by persisting substantial marine inundations. The environments from this time are indicated by scattered
These basic Jurassic floral elements have been shown to exist across the continent throughout the remainder of the period, based on correlations of spores and pollen, as well as macrofossils in strata dating to the Jurassic-Cretaceous boundary in the Gippsland Basin of southern Victoria and the Perth Basin and Canning Basin in Western Australia.
Many Jurassic fossil invertebrates have been found in Australia, but land animals are rare. The Talbragar River site in western New South Wales contains faunas typical of global sites from this time, including early teleostean bony fish like Leptolepis, and many primitive forms such as the palaeoniscoids.
In Australia plant macrofossils have been found in deposits from the Lower to the Upper Jurassic. The floras of Australia were generally similar to those in other parts of Gondwana. The canopy was dominated by araucarian and podocarp conifers, and the understory was composed of cycadophytes (cycads and related plants), ginkgoaleans (ginkgos), pteridophytes (ferns), horsetails (equicetaleans), mosses and liverworts (bryophytes), clubmosses (lycophytes). That some palaeolatitudinal variability between the dominant vegetative types was present is suggested by macrofloras of Western Australia and Queensland during the Lower Jurassic. Plant material from theIn Australia plant macrofossils have been found in deposits from the Lower to the Upper Jurassic. The floras of Australia were generally similar to those in other parts of Gondwana. The canopy was dominated by araucarian and podocarp conifers, and the understory was composed of cycadophytes (cycads and related plants), ginkgoaleans (ginkgos), pteridophytes (ferns), horsetails (equicetaleans), mosses and liverworts (bryophytes), clubmosses (lycophytes). That some palaeolatitudinal variability between the dominant vegetative types was present is suggested by macrofloras of Western Australia and Queensland during the Lower Jurassic. Plant material from the Cockleshell Gully Formation in the Perth Basin, palaeolatitude 25-35o S, characterises the floras from Western Australia, displaying the influence of warm currents from equatorial regions of the Meso-Tethys Ocean. In the east of the continent the assemblages from the Evergreen Formation and the Precipice Sandstone in the Surat Basin are more similar to more seasonal vegetation of 40-75o S palaeolatitude.
Kear & Hamilton-Bruce suggest that rising sealevels and the onset of climatic conditions that were cooler and more equable in later times could possibly have been responsible for the latitudinal zonation patterns of the Early Jurassic being less pronounced later in the period.
Several Australian deposits have produced temnospondyl amphibians. A mandible fragment from the Marburg Formation on the banks of the Brisbane River, southeast Queensland, was named Austropelor wadleyi in 1941. A feature of this specimen is the arrangement of tooth sockets which decrease in size from the back to the front of the jaw. Such a feature is characteristic of brachyopoids, a group of temnospondyls from the Triassic to the Cretaceous of central Asia, India, South America, South Africa, and Australia. The Westgrove Ironstone Member, Evergreen Formation in southeastern Queensland, produced another amphibian, Siderops Kehli, of which a skeleton has been found that is almost complete, only missing parts of the limbs and tail. At 2.5 m long this was a large crocodile-like aquatic temnospondyl, a derived member of the Chigutisauridae (a brachyopoid family). Its head was almost 650 mm wide with upward facing orbits and along the side of the jaw were large lance-shaped teeth. Kear & Hamilton-Bruce suggest it probably lived in freshwater rivers and lakes, the same habitat occupied by freshwater plesiosaurs.
Dinosaurs are known from the Jurassic of Australia from body fossils that are sporadic and footprints, of which the most famous is probably, according to Kear & Hamilton-Bruce, a skeleton of Rhoetosaurus brownei, a primitive herbivorous sauropod similar to Brontosaurus, that is incomplete, only most of the hind leg, partial pelvis and vertebral series having been found. A number of diagnostic characters are shared between Rhoetosaurus and cetiosaurids, an archaic group of sauropods that are known from deposits of Lower-Middle Jurassic age from England, North America, South America, and especially China. Among these characters are rudimentary pleurocoels, structures that are believed to function in reducing the weight of the bone without compromising its structural strength. Of the cetiosaurids, Rhoetosaurus has been specifically allied with the Shunosaurinae that had a bony club at the end od a short stiffened tail. Kear & Hamilton-Bruce suggest that this Australian sauropod may have had a defensive tail club as did the Chinese shunosaurines3.
In the Colalura Sandstone, of marine origin, in the Perth Basin contained scattered dinosaur bones of Bajocian, Middle Jurassic, age. The bones in this deposit are those of animals that were washed out to sea from the nearby coastal land. Included among these bones are a tail vertebra of the indeterminate sauropod from the Bringo Shale that had similar proportions to the cetiosaurid tail vertebra. Ozraptor subotai is a species that has been named from a partial tibia found in the Bringo railway cutting. According to Kear & Hamilton-Bruce this is an intriguing dinosaur as it cannot yet be assigned to any known family, as it differs from all other orders3.
Dinosaur footprints are known from the Lower Jurassic of Australia in the Precipice Sandstone and the Razorback Beds of southern Queensland, trackways of sauropods, both large and small (Anomoepus) and indeterminate theropods. Near Balgowan in southeastern Queensland the Walloon Group Coal Measures contain extensive dinosaur trackways of Middle Jurassic (Bajocian-Bathonian) age. Among the dinosaurs indicated by these trackways are theropods and a number of ornithopods (Changeipus bartholomaii) a quadrupedal thyreophoran. Included among the latter are small and large Eubrontes forms up to 710 mm long, suggesting according to Kear & Hamilton-Bruce large carnosaurs that could be up to a body length of about 12 m3.
Queensland fossil sites from the Jurassic are the main depository of the rare terrestrial vertebrates, some are also found in Western Australia. Included in the fauna are Rhoetosaurus, a large sauropod dinosaur, some bones of a theropod dinosaur, some aquatic reptiles like plesiosaurs, and Siderops, one of latest known labyrinthodont. Fossil trackways in Queensland demonstrate that there were many other dinosaurs that have yet to be discovered, including large carnivorous dinosaurs.
At Lune River in Tasmania there is a flow that overlies a zone of abundant petrifactions, agatised tree-fern trunks, trunks of conifers and rare specimens of fruiting bodies of Bennettitalean Cycadophytes in which the plant material has been replaced by minerals down to the molecular level, resulting in complete preservation of cellular structure. Because of the beauty of the agate fossils at this site little remains in situ, most of the material having been taken by collectors.
Dinosaurs lived in Australia during the Jurassic, even though they would not have been expected because of the climate at such high latitudes were long periods of darkness. The plants at the base of the food chain were evergreen, merely becoming dormant during the dark winters when sunlight was not available for photosynthesis, so there would have been food for herbivores throughout the year.
Rheitrosaurus, one of the oldest known sauropods, has been found in freshwater deposits in Queensland. Theropods also are present in the Queensland deposits, as are plesiosaurs.
Austrosaurus mckillopi see Winton Formation
Siderops kehli, Uppermost Early Triassic, Westgrove Ironstone member of Evergreen Formation, south central Queensland.
An almost complete skeleton of Siderops was found that was 2.5 m long, one of the largest and most complete amphibian skeleton known from Australia.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|