Australia: The Land Where Time Began

A biography of the Australian continent 

Cretaceous Australia - 144-65.4 million years go

The Cretaceous has been called a time of change, because the changes that took place during this period on a global scale were enormous. Australia was still attached to Antarctica and ancestral New Zealand was still attached to Marie Byrd Land in Antarctica. The southern continents were beginning to break from Gondwana, and the effects on the world were enormous. As the continents moved from Gondwana the new arrangement of the land masses led to new circulation patterns in the oceans and the atmosphere. The result was a huge effect on the climate of the world.

In the Early Cetaceous there was a global sealevel rise as the continents separated. Around the world vast areas of land were flooded. At 110 Ma the sealevel reached its maximum height, after which it rapidly dropped, and by the Late Cretaceous most continental areas were again dry land. Resulting from the plate movements, mountain building surged in many places. It was at this time that the Rockies of North America, the Andes of South America and the Alps of Europe were uplifted.

There was a sudden cooling phase at the start of the Cretaceous, the severity or length of which is unknown. For the rest of the period the temperatures continued to rise so that most of the Period was characterised by a warm to hot climate, the mean annual temperatures being about 10-15^o C warmer than at present. The temperature gradient between the tropics and the poles was about half the present gradient. It is believed tropical and subtropical conditions extended much further south and north than now, possibly up to 70^o at its greatest extent, and the poles had a warm temperate climate. It is even estimated that the abyssal water was about 15^oC compared with 2^o C at the present. Ocean circulation was sluggish and had almost no vertical zonation.  During the Period there appear to have been times of great aridity around the world, as evidenced by widespread evaporite deposits that have been found in a band either side of the Equator that are believed to have extended to possibly 45^o on either side of the Equator. The Late Cretaceous was the warmest time in the Phanerozoic Era. At the end of the Period there was a sudden cooling that ushered in a regime of fluctuation of climatic patterns that continued up to the most recent ice age.

Tectonic movements were raising and lowering blocks of crust at the time of the rising sea level, leading to flooding of areas locally where raising sealevels flooded lowering crust. As a result, new environments were becoming available for colonisation even before the epicontinental seas began to retreat. Following the retreat of the seas the areas exposed contained salt marshes, swamps, dunes, and areas of sand that had been the sea bed. This appearance of large areas of new land was on a very large scale around the world as the epicontinental seas regressed. There was then a burst of evolution as the flora and fauna invaded the new niches. The flowering plants had been adapting to the conditions and when this opportunity arose diversified rapidly and became the dominant type of vegetation. The Angiosperms originated prior to the disintegration of Pangaea, then Gondwana allowed them to become established on all the continents and evolve into the plants of the present.

There were great changes in the flora and the fauna of the world. During this Period the flowering plants came to dominate the vegetation and among the fauna, Dinosaurs reached their peak and were declining by the time of the mass extinction event that occurred at the close of the Cretaceous, and the mammals started on the road to dominance of the fauna.

Australia

According to the authors³ at the start of the Cretaceous, the Berriasian, Australia maintained a continuous landsurface with India along what is now western coast, its eastern coast being the Queensland Plateau and the Lord Howe Rise, though it was geographically isolated. In the Canning Basin, Carpentaria Basin, Eromanga Basin and the Surat Basin river systems drained to the northwestern and northeastern coasts. Up until it separated from India rivers flowed through western Australia’s southern Perth Basin.

A complex series of rivers and lakes was supported by the southern rift valley that had formed between Antarctica and Australia during the Jurassic. The Otway Basin, Gippsland Basin and the Bass Basin of the present received vast amounts of sediment from these rivers.

About 140 Ma, in the Valanginian, India finally split from Australia and much of the western margin of the continent was inundated by the ocean. About 130 Ma, in the Barremian, large areas of Eromanga Basin and the Carpentaria Basin of northeastern Australia was flooded by the sea following a major global sealevel rise, at which time the sea covered terrestrial environments that had survived the inundation of the western coast of 140 Ma. The result of this was the geographic isolation of terrestrial organisms that led to the production of large numbers of endemic and relictual flora and fauna. Terrestrial habitats in central and Western Australia, the Queensland Plateau and the southern rift valley, that partially flooded to the west, but largely remained landlocked, were increasingly restricted by continued sealevel rises during the Aptian.

Extensive crustal folding and volcanic activity in the Great Dividing Range of the present now provided new coastal environments. As the inland sea regressed, being replaced by vast areas of brackish lagoons and estuaries in the Surat Basin, about 100 Ma in the late Albian these areas expanded. Further east, coal-forming swamps that formed such deposits as the Styx River Coal Measures of the present, and rivers drained to the east entering the sea off the outboard coastal margin of the Queensland Plateau. Along the western and southwestern coasts of the present-day Victoria there were areas of inundation that persisted, receding by the Turonian about 93 Ma, the only remnants of the great inland sea at this time were shallow embayments along the coast of Western Australia. The northern branch of the inland sea remained in the area of Darwin and nearby offshore islands, Bathurst Island and Melville Island, where is deposited sediments of the Bathurst Island Group, then receding during the Turonian

These islands and the smaller landmasses to the north, Timor, Moluccas and Celebes, became dry land. Active crustal movements, such as the beginning of a phase of mountain building events continuing into the Cainozoic, are widely believed to be responsible for the widespread seaway regression that occurred.

Western Australia and central Queensland are the areas from which fossils from the Upper cretaceous are best known, these deposits preserving evidence of both marine and terrestrial assemblages. The remains in Western Australia preserve shallow marine faunas from along the western margin of the forming Indian Ocean. These fossils are mostly derived from limestone, marl and chalk that are exposed along the coast. The chalk is a friable rock formed from the shells of unicellular foraminifers that are part of the plankton. The most notable deposits include the Molecap Greensand to the north of Perth in the Gingin region of the Perth Basin, Gingin Chalk in the northwestern Murchison River region, and the Miria Formation in the Cardabia-Giralia Ranges in the eastern and central parts of the Carnarvon Basin in the northwest of Western Australia. The rocks in many places the rocks have been described by the authors³ as extremely fossiliferous, recording the marine life in the Australian region close to the end of the Cretaceous about 65 Ma.

The situation is different for the terrestrial faunas of Australia in the Late Cretaceous, the Winton Formation of Queensland being the main source of information from the Late Cretaceous.

Separation

Now it was Australia's turn to break from Antarctica, bringing an end to Gondwana. The rifting took a long time to complete, not being completely separated until the Early Tertiary. Sea levels rose and central Australia was submerged by a last inland sea. At this time southern and central Queensland were a series of islands in this inland sea. The dinosaur fossils that are known are mostly in the sediments deposited by this inland sea over much of Queensland, New South Wales and South Australia. It is believed that the fossils found were dead dinosaurs washed into the shallow seas during floods.

According to the authors³ at the beginning of the Cretaceous, about 145 Ma (Berriasian) Australia was the eastern high-latitude extremity of Gondwana, 50^o-85^o S. In the west of the continent the future Australia and future India formed a continuous landsurface. In the east of the continent the Queensland Plateau and the Lord Howe Rise comprised the eastern margin. Most of the continent was landlocked as it had been in the Jurassic.

Narrow marine straits had formed in the rifts that had developed between Australia and India in the west and between Australia and Antarctica in the south by 140 Ma in the Valanginian. 130 Ma during the Barremian rising global sealevels, combined with downwarping that affected the Carpentaria Basin, Eromanga Basin and the Surat Basin led to a major marine transgression that covered large areas of central-eastern parts of the continent. At this time there were also marine transgressions in the north-western section of the Carnarvon Basin and the coastal margin of the Perth Basin resulting in the opening of new habitats in the shallow epicontinental marine environment. The Australian continent was mostly inundated by about 120 Ma as the sealevel rise continued during the Aptian. In the west of the continent the northwestern Kimberley Region remained above sealevel, as did the Queensland Plateau in the east of the continent that was volcanically active. In the south of the continent the southeastern margin was still continuous with Antarctica. Open ocean in the south was connected to the open ocean ocean in the west by the epicontinental sea that extended from the western Perth Basin and Carnarvon Basin and by a broad strait across the Nullarbor Plain in the south.

In the east of the continent the vast epicontinental sea connected the Gulf of Carpentaria to the Great Artesian Basin and across the Northern Territory, and in the Surat Basin a narrow southeastern channel around the Queensland-New South Wales border.

The sea surface temperatures dropped to near freezing when the climate began to cool during the Aptian. Isotopic and sedimentary analyses has indicated that average palaeotemperatures at sea level were about 12^o C. According to the authors³ there appears to have been extensive ice build-up along the coasts and along the southern margin of the inland seaway during long winters at high latitude. The palaeotemperatures rose by about 115 Ma in the early-middle Albian as the extreme conditions became more temperate. Along the coastlines of Western Australia and the Northern Territory isolated inundations remained as the epicontinental sea regressed. In the central east of the continent the sea remained throughout the early and middle Albian until about 105 Ma, finally retreating to the north towards the Gulf of Carpentaria by the earliest part of the Late Cretaceous about 99 Ma in the Cenomanian.

During the Cretaceous, Australia was still attached to Gondwana, but as the process of separation got under way rift valleys formed a complex system along the lines of separation of the 2 continents. It had been thought that at the time Australia was between 40^o and 75^o S, later evidence suggested that it was actually between 50^o and 85^o S. For the first 50 million years of the Cretaceous the sealevel had been rising and this caused the flooding of the Tasman Depression as it continued its subsidence. Low-lying central and southern basins adjacent to the Depression were also flooded at this time, as were West Australian basins where the Amadeus Transverse Zone had previously been. The continent had been separated into 4 blocks of land by the vast expanse of the Eromanga Sea.

The marine deposits that were laid down in this epicontinental sea now outcrop, or are present beneath the surface, over about 1/3 of the continent. Deposits of terrestrial origin that formed at the start and towards the end of the period are not common.

The Maryburian Orogeny, a major phase of mountain building that occurred in coastal Queensland, was the final act of the Tasman Orogen.

Along the southern margin basins were formed by the rifting that was under way accumulating large amounts of sediment throughout the Cretaceous. The accumulations of Bass Strait contained coal deposits. In the Gippsland, Otway and Bass Basins the rifts were slowly evolving, and continued to accumulate sediments and volcanic detritus. The sea inundated the rift zone progressively from west to east, and by late in the Period Tasmania was the last connection to Antarctica.

Throughout the Cretaceous the break-up of Gondwana picked up speed. The western margin of Australia was becoming progressively delineated as sea floor spreading began near Carnarvon and Perth on the edge of the Westralian Depression. India began breaking away from the western arm of the Depression, its northward movement beginning at about 125 Ma. At the same time Africa broke from Antarctica and began rotating away from India.

By the time the Tasman Sea began opening about 80 Ma the southeast margin of the continent was established. The whole of the outer edge of Gondwana, comprising the Lord Howe Rise and the Ancestral New Zealand landmass, extending from the Campbell Plateau to New Caledonia, began moving away from the east coast of Australia. The movement ceased about 60 Ma and the distance separating the 2 has remained constant ever since. The tectonic events involved in the opening of the Tasman Sea led to the tilting of the eastern margin of Australia and the uplift of the Great Divide. Antarctica was at the South Pole during the Cretaceous and has remained there as the other continents moved north, Australia being the last block to remain attached after the others had separated, though the beginnings of separation had started.

During the Cretaceous, Australia was close to the South Pole, but its climate was warm-temperate and humid, but south-eastern Australia is believed to have been temperate to cool at the time. By the Early Cretaceous the first angiosperms (flowering plants) had begun to appear among the dominant conifers, pines and ferns. At the Koonwarra site in eastern Victoria, some of the oldest flower fossils in the world have been found. The fish deposits at the Koonwarra site show evidence of what is believed to have been a winter fish kill in the Early Cretaceous and the ice-rafted boulders found in the bed of the Eromanga Sea from this time indicate that sea ice must have been present, at least in winter. Winters would have been cold at this time, with glaciers probably on the high ground. The cool to cold seas off the southeastern coast is also indicated by study of the plankton from that time. The vegetation, having evolved during warm humid times of the Jurassic, would have been under stress during this cold phase. The stress increased as the epicontinental sea expanded as sea levels rose.

Most Australian rocks from the Mesozoic are of Cretaceous age. The coasts of Queensland and northern Western Australia were inundated by major marine transgressions, resulting in a shallow inland sea that covered central Queensland and large areas of the the Northern Territory. By the Middle Cretaceous marine incursions had flooded large areas of the continent form the north, west and south until the landmass had been divided into a series of 4 main islands in what was then the Eromanga Seas. The very large amounts of sediment deposited in the Eromanga Sea cover vast areas of central Queensland, northern New South Wales, central South Australia and the northwest of Western Australia. A rift valley had formed as the separation of Australia from Antarctica got under way, though the margin of the southeast of the continent was still land-locked. This resulted in the deposition of sediment by inland rivers, as well as isolated subsurface deposits in northern New South Wales, that together form 1 of the few instances of rocks containing non-marine fossils from the Lower Cretaceous³.

The Eromanga Sea had retreated by the beginning of the Late Cretaceous, all that remained of the inundation was a series of low-lying lakes and swamps. Along the coast of Western Australia and far northern Australia there were still areas of marine inundation that continued until the latest part of the Cretaceous, and when they finally retreated they left calcareous rock deposits that are rich in fossils in the far northwest of Western Australia³.

The Nanutarra Formation and the Birdrong Sandstone are among the earliest fossil-bearing sequences in the Lower Cretaceous of Australia. Vertebrate and invertebrate fossils have been found in these units that are exposed as weathered cliffs in the Murchison River region in the Carnarvon Basin in the northwestern part of Western Australia. In the Eromanga Basin and the Carpentaria Basin incorpoating the Bulldog Shale and Wallumbilla Formation in South Australia and northern New South Wales/southern Queensland respectively, have produced much richer assemblages dating to the Lower Cretaceous. Especially in areas around Coober Pedy and Andamooka in South Australia and White Cliffs in New South Wales, areas that were the bed of the inland sea, some of the most spectacular fossils found in Australia occur, many of which have been completely replaced by opal. In mine workings, open cut and underground, these fossils are typically found alongside such indicators of palaeoclimate low-temperature such as ice-rafted boulders.

The fossil assemblages display a marked change from the sediments of the Aptian to the younger Albian, such as the invertebrates appearing to have decreased significantly in diversity, and among marine vertebrates there appears to have been a rearrangement of faunal composition, such as plesiosaurs being replaced by ichthyosaurs and marine turtles. The authors³ suggest that the combination of widespread marine regression with climatic warming might have been an effective catalyst for these changes, though they say the causes are unclear. In central-eastern Australia marine deposits from the areas covered by the epicontinental sea have produced the best-known faunas from the Albian. The Toolebuc Formation and Allaru Mudstone have both produced many very well-preserved fossils mainly from weathered blacksoil plains around the central-northern Queensland towns of Boulia, Hughenden and Richmond.

In the late Albian, about 110 Ma, this region was covered by shallow marine environments and is one of the richest known fossil sequences from the Mesozoic of Australia.

Significant Australian deposits and localities from the Cretaceous - marine

In the northwestern part of the continent the onset of shallow marine conditions are recorded by the Nanutarra Formation and the Birdrong Sandstone in the middle-late Neocomian (Hauterivian-Barremian).

• Nanutarra Formation 
• Bulldog Shales
• Birdrong Sandstone
• The Rolling Downs Group
• Marine Invertebrates
• Sharks and bony fish
• Sea turtles
• Ichthyosaurs
• Plesiosaurs
• Early Cretaceous – non-marine animals

Significant Lower Cretaceous deposits – non-marine

A series of sandstones and mudstones that are finely laminated, outcropping in the Gippsland Basin and Otway Basin, coastal basins of southeastern Australia, including the Wonthaggi Formation/Koonwarra Fossil beds (Strzelecki Group), as well as the Eumeralla Formation (Otway group) that is stratigraphically younger. In the mid-Cretaceous these units were deposited in a floodplain that formed as a result of rifting between Antarctica and Australia. The authors³ suggest the sediments were probably derived from a volcanically active region associated with the Tasman Sea formation, as they are typically of volcanic origin.

Wonthaggi Formation

A progressive change in the setting of deposition from one that was mostly meandering to a braided river system is recorded in the Wonthaggi Formation and the Eumeralla Formation. Channel deposits with thick accumulations of rapidly deposited cross-bedded sediments formed by flash floods or flows from seasonal snowmelt are characteristic of the Wonthaggi Formation that is the older of the two. Lenses of fine sand derived from successive river channels characterise the younger Eumeralla Formation. Unlike the environment of the other 2 sites, the Koonwarra Fossil Beds were composed of very fine-grained clays that were deposited in a low-energy lake.

Fission-track and microfossil methods have been used to determine the age of the Wonthaggi Formation/Koonwarra Fossil Beds and the Eumeralla Formation. An age of middle Valanginian-Aptian is suggested by the most recent work on the Wonthaggi Formation/Koonwarra Fossil Beds, though the latter are considered to be of Barremian-Aptian age. The Eumeralla Formation is believed to be from the upper Aptian-lower Albian.

Griman Creek Formation

In the Surat Basin of southeastern Queensland and northern New South Wales there are outcrops of the Griman Creek Formation (Rolling Downs Group). The interbedded sandstones, siltstones and mudstones of this unit are rich in fossils of non-marine molluscs, such as hyriid bivalves, thiiarid and viviparid gastropods, as well as impressions of plant roots. A freshwater to brackish-estuarine origin is suggested for these deposits by the cross-bedding they preserve. Weathering has bleached white some horizons of the Griman Creek Formation that is best seen in areas around Lightning Ridge where opal is present in the clay-rich sediment. At Lightning Ridge the opal is found in concretionary structures called ‘nobbies’, as well as a replacement in shell, wood or bone. It is not well known how the opal at Lightning Ridge formed, though one suggestion has linked it to the precipitation during the Cretaceous of silica from acidic groundwater by bacterial action.

The age of the Griman Creek Formation has been determined by the rich microfossil assemblages to be of the lower-middle Albian. It has been difficult to establish a reliable date for the opal-bearing strata at Lightning Ridge because of leaching of the opal-bearing strata.

Throughout much of the Jurassic-Cretaceous southeastern Australia was located near the Antarctic circle, based on palaeolatitude estimates. At the time the Wonthaggi Formation/Koonwarra Fossil Beds and Eumeralla Formation were deposited they were located about 60-80^o S. For the Otway Group there are specific estimates of 66.8^o S, and for the sediments of the Strzelecki Group, 77.8^o S. Based on the preserved sedimentary structures, stable isotopes and palaeofloras it is believed they were deposited under a very cool to cold climate, possibly with freezing during winter. It is believed the Griman Creek Formation was deposited at a high latitude zone about 60^o S, during a warmer, probably temperate phase near the middle-late Albian. The average palaeotermperatures of the southeastern Australian region were about 12^o C, though in southeastern Australia continental surface air temperatures may have dropped to a low as 0^o C. It has been suggested that there is possible evidence of permafrost in Victorian sediments from the Cretaceous and in South Australia, glacial Tillites.

• Land Plants
• Invertebrates – non-marine
• Sharks & Bony fish - non-marine
• Lower Cretaceous Amphibians
• Lower Cretaceous Turtles - non-marine
• Lizards
• Crocodiles
• Dinosaurs
• Pterosaurs
• Birds
• Relict therapsids
• Cretaceous Mammals

Late Cretaceous

Bathurst Island Group

Molecap Greensand & overlying Gingin Chalk

Miria Formation

Winton Formation

The Winton Formation is the unit that has produced almost all the terrestrial fossil record in Australia of the Upper Cretaceous. According to the authors³ the Winton Formation reaches a maximum depth of 1.2 km. Identification of outcrops of the Formation is often made difficult by low topographic relief, deep weathering and a cover of oxidsed ironstone. The Winton Formation that is believed to be of uppermost Albian to lower Cenomanian age based on assemblages of invertebrates and microfossils, forms the uppermost section of the Rolling Downs Group. In the uppermost beds the pollen record indicates that deposition continued for some time after the lower Cenomanian and into the early Turonian, at least in central Queensland, the central section of the basin.

The Winton Formation is believed to have been deposited by river systems that drained inland towards lakes and swamps that were permanent. Included among the macrofossils found in this formation are freshwater unionoid bivalves, insects, aquatic vertebrates such as fish and reptiles, as well as dinosaurs that are known from body fossils and trackways.

• Land Plants
• Invertebrates - marine and non-marine
• Sharks and fish  
• Turtles
• Ichthyosaurs
• Plesiosaurs
• Marine lizards - mosasaurs and dolichosaurs
• Crocodiles
• Dinosaurs
• Pterosaurs

Cretaceous plants

The earliest "generalist" Angiosperm pollen known in the fossil record of Australia is from about 110 million years ago, the Albian, about 14 million years after the age of the earliest known pollen of this type which was found in west Gondwana and Laurasia. Pollen of Ilex (Holly), Proteaceous-type and Nothofagus brasii (a southern beech) appear in the fossil record near the end of the Late Cretaceous. In the areas where pollen is known, the vegetation was still dominated by conifers. There were already several modern conifer types present abundantly. Among the pollens present were Podocarp pollen, Huon Pine, Rimu (the common New Zealand species, also found in Tasmania). The celery-top pine (Phyllocladus) is present. There were abundant Araucarian pollens like those of Kauri (Agathis), Hoop Pines, Bunya Pines and Norfolk Pikes. There was also pollen like that of the modern Callitris.

Animals from the Cretaceous are widespread throughout inland Queensland, as well as in the opal fields of South Australia and New South Wales. Much of the sediment load from the large river systems that developed as Australia split from Antarctica was deposited in the lower parts of the growing rift between the 2 continents, off the western and eastern coasts of Victoria. It is in the Early Cretaceous sediments of the Strzelecki Group and Otway Group that the Victorian dinosaurs have been found. More recently, the youngest known labyrinthodont fossils have been found in these deposits.

Cretaceous reptile fossil deposits are very rare in Western Australia, the few that have been found were in the form of outcrops in the Carnarvon Basin. Some dinosaur bones being found in the Late Cretaceous Miria Formation, and in the Birdrong Sandstone, there were some partially articulated ichthyosaurs and plesiosaurs from the Early Cretaceous.

Dinosaur Evolution⁴

Sources & Further reading

1. John A Long, Dinosaurs of Australia and New Zealand, University of New South Wales Press
2. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
3. Kear, B.P. & Hamilton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life from the southern continent, CSIRO Publishing.
4. Paul, Gregory S., 2010, The Princeton Field guide to Dinosaurs, Princeton University Press.

Links

1. Ptychodontid Sharks: Late Cretaceous Shell Crushers
2. Ancient dolphin-like ichthyosaurs may have been wiped out by climate change