Australia: The Land Where Time Began
Australian Palaeogeography Cretaceous to Recent
During the Phanerozoic Australia had 3 regimes, the Uluru, 570-320 Ma, the Innamincka, 320-90 Ma, and the Potoroo, 97-0 Ma. Each of these was a complex of uniform plate tectonic and palaeoclimatic events at a similar or slowly changing latitude that produced deposits with a sequence of distinct facies, bounded by disconformities at the margins and stratigraphic gaps in the interior. The Uluru of the continental interior is predominately low-latitude, shallow-water deposits, that include marine carbonates. The Innamincka regime is composed of high-latitude, non-marine, including glacial, deposits. The Potoroo sequence is increasingly lower latitude deposits, almost all of which are confined to the margins. A change from a Chilean- to Mariana-type subduction is marked by the boundary between the Innamincka and the Potoroo regimes. The Innamincka regime resembles the Gondwana sequence of peninsula India. In general, the Australian regimes resemble those of other component landmasses of Gondwana. The Gondwanan regimes reflect those from the rest of the world, being part of 2 Phanerozoic super-cycles, each about 400 million years long. The first covered the period from the Palaeozoic to the Early Mesozoic and the 2nd from the Late Mesozoic to the Cenozoic, each of which reflected a cycle of mantle convection, expressed in variation over time of plutonism, and so the CO2 concentration of the atmosphere, and the swings from greenhouse to icehouse conditions, as well as continental congregation and dispersion, Pangaea, and eustatic sea level changes, and from marine platform sediment at times of high sea level to non-marine during periods of low sea level.
Regimes were a concept originally devised for dealing with the historical evolution of North America (Sloss, 1963), and employed by Haq et al.(1987). Veevers introduced the concept of regimes to discuss the geological history of Australia. Two of Veevers' regimes are, the Innamincka, that ended in the Cenomanian, and the Potoroo, up to the Recent. There are 2 features that identify the boundary between the regimes, during the Cenomanian there was a major retreat of the sea from central Australia, and a widespread depositional change from sediments of terrestrial origin to biogenic carbonate sediment.
Innamincka Regime- the Cretaceous 320-99 Ma
Terrestrial deposits were being deposited at the start of the Cretaceous, as they had been during the Jurassic, predominantly in active graben structures around the margins of Australia, and the large depression in the interior of the continent that formed the Great Artesian Basin. This was the last time such thick deposits of detrital sediments accumulated in Australia. It marked the end of the tensional development of depocentres resulting from the separation from Gondwana. It was also the last long interval of detrital sedimentation in Australia, as the conditions changed in the mid-cretaceous to a predominantly biogenic carbonate regime in the marine environment. After this time most deposition occurred on subsiding continental margins lacking a basin style of geometry.
At this time, there was major volcanic activity around southern and southwestern Australia, as seen in the sediments, for which volcanic rocks were a major source. An example is seen in the southwestern Australia, the Bunbury Basalt, that extruded onto an erosional surface at this time, though elsewhere there is not much evidence such as volcanic plugs, flows and volcaniclastic sediments.
There were apparently large, active river systems across the entire continent during the Cretaceous, indicating a wet climate with high levels of precipitation. Large quantities of sediment from large active river systems were being deposited in northwestern Australia, southern part of the west coast and the Great Australian Basin, as well as basins in the southeast, the Otway Basin and Gippsland Basin (Strzelecki Basin), material of terrestrial origin, often with additional input of andesitic volcanic origin connected with the tension of the separation from Gondwanan, which in some cases was the predominant component of the sediment. The 2 main depositional sites were in central Australia and in a series of small basins along the southern coast. Drainage from the Antarctic and Australian sides of the growing rift deposited sediment in basins along the central south coast. Sediment was also added by drainage from salt lake system in Western Australia and South Australia. This drainage from the salt lake system is believed to be a relict from this time, though it may possibly be from an earlier time. In the Perth Basin, and offshore, there was also active tectonism at this time, during which the Darling was markedly uplifted, which resulted in the addition of increased amounts of coarse detrital sediments, with fresh feldspars, into the Perth Basin. This has been interpreted as resulting from the presence of active short path streams that were rapidly eroding the steepened slopes. The relatively small depocentres of the offshore Perth Basin contain very thick sedimentary deposits. The eastern slopes of the ranges were characterised by streams that were much longer and slower moving, that carried finer sediments to the Eucla Basin. The main depocentre during the final stages of the Innamincka Regime was the Central Australian depression in which the bulk of the sediment from this time accumulated.
As the Indian Ocean began to open, deposition of sediments was restricted to the west coast. As the sea entered the progressively eastward opening rift between Antarctica and the present-day southern margin of Australia, and from a marine incursion (Vananginian-Hauterivian), initially north-south, into the Great Australian Bight from the north (Fig 3.3, Hill, 1994), marine sediments accumulated in the new areas covered by the spreading waterways. Prior to the Aptian, marine microfossils occur sporadically in sediments along the margins. The sediments of Aptian age in the Eucla Basin contains the first major evidence of advancing marine conditions, as the earliest marine transgression between Antarctica and Australia. The continent of Australia was separated into 3 large islands, as marine conditions spread across the continent during the Aptian-Albian (Figs. 3.5, 3.7, Hill, 1994) as the continent was gradually inundated. The extent of this incursion had not occurred previously in the Phanerozoic or since it regressed.
Radial drainage developed on these islands, generally to a depocentre south of the central south coast. Deposition was into very widespread into shallow water in intracratonic basins. It has been suggested the transgression of the Aptian-Albian may actually have been 2 transgressions, separated by a small regression, as indicated on the Australian sea level curve on Fig. 3.2 (Hill, 1994). Reducing conditions existed over wide areas, with radiolaria an important component in the sediments, though planktonic foraminifera were rare, or small, and of low diversity if present. (Scheibnerova, 1976). The abundant molluscs are present over wide areas, indicating the widespread nature of this marine phase (Ludbrook, 1966). By the end of the Albian, marine conditions were mostly restricted to small areas of the Great Australian Basin and northwestern Australia. Marine conditions retreated from central Australia for the last time at the end of the Early Cretaceous. Frakes et al. (1987a) has documented the position of the coastline throughout the Cretaceous. The position of this part of the Cretaceous has been summarised by Hill (1994, Figs. 3.6 & 3.7).
The microfossil evidence suggests that the marine conditions over much of the Aptian-Albian may have been cool. This would be consistent with the location of the continent at this time, the southeastern quarter being inside the Antarctic Circle. According to Douglas (1969), the climate was cool, with a rainfall of 760 - 1140 mm/year, and a seasonal dry season in southeastern Victoria for most of the Early Cretaceous. It was suggested by Waldman (1971) there were cold winters and ice cover in restricted parts of the environment during part of this period. Similar conditions were also proposed by Frakes & Francis (1988), with sea ice more generally present in winter, and that this ice deposited sediment. Glendonites were detailed in the Bulldog Shale by Sheard (1990), who proposed very cold to frigid conditions for the time. Jell & Duncan (1986) agreed with Waldman's hypothesis, but disputed the need for winter ice. Across Australia, conditions were generally cool, with possibly a gradient to cooler temperatures towards the southeast, but there is no evidence for widespread glaciation nearby. The results of studies of fossil from the Early Cretaceous of Victoria were reviewed in detail by Rich et al. (1989), including data from other sources. They made a summary, indication that the polar atmosphere of the Early Tertiary may have had a temperature range of -5 - +8 C. They found evidence that there was a seasonal climate that was cool and humid, which agreed with earlier hypotheses. Other suggested climates for the period are at odds with this conclusion of a cool climate scenario (Barron et al, 1981; Hallam, 1984,1985). They propose that the Cretaceous climate was ice free.
Potoroo Regime - the Cretaceous 99 Ma - Recent
At the change of regimes of 99 Ma, when the Innamincka Regime gave way to the Potoroo Regime, there are a number of events that characterise the changeover. On the western margin, detrital sediment was replaced by biogenic carbonate deposits, and India changed from slow northwestward motion to rapid northward divergence. On the southern margin, volcanogenic sediment of Early Cretaceous age is overlain by quartzose sediment of Late Cretaceous age. Long-lived subduction was replaced by transtension (extensive and transverse shear) of the east coast, the interior of the continent rising faster than the eustatic sea level rise. Large amounts of sediment were transported to the southwest to the Ceduna depocentre from passive margin highlands (Veevers, 1984; Gurnis et al., 1998). The 99 Ma event was interpreted by Veevers (1991) as a change in subduction along the eastern margin from a Chilean- to a Mariana-type with back-arc spreading.
Lithgow-Bertelloni & Richards (1998) suggested that the "Hawaiian-Emperor bend was changed by a sudden change of the Pacific-Australian margin from transform to subduction along pre-existing transform faults."
Climatic conditions from central Australia are not well known for this period, the evidence available comes mostly from the marine sections of the west coast, with some evidence coming from sections along the south coast that are not as well known. The result is that over the period of the Cretaceous-Cenozoic, the interpretation of the climate of central Australia are made with less confidence than those of the west coastal regions.
A 5 million year interregnum was proposed by Veevers (1984) between the Innamincka Regime and Potoroo Regime. Hill (1994) has included this period in the later Cretaceous history. Deposition changed from marine to non-marine in the central part of the Great Australian Basin after the sea retreated at the end of the Aptian. This change didn't occur in the most northerly parts around the edge of the Gulf of Carpentaria. The pattern of climate and palaeogeography brought about by this change in deposition origin has remained in place up to the present. There is evidence suggesting that the level of river activity, and probably precipitation, declined at this time, as the sea was entering the rift between Antarctica and Australia. The change from terrigenous to biogenic carbonate deposition along the west coast indicates less sediment being delivered by rivers to marine deposits, indicating a change to drier conditions on the land. There was a marine regression in Australia during the Cenomanian, along the west coast, the position of the coastline during each sedimentation cycle in the Late Cretaceous along the western margin was very uniform (Hill, 1994, Fig 3.8). The Gulf of Carpentaria and the Eucla Basin were the only areas where the position varied significantly, the coastlines during the Cenomanian in these areas being relicts of the Albian transgression. The variation of the position of the coastline along the south coast during the Cenomanian was produced by the history of the separation of Australian from Antarctica. It was at this time that the Tasman Sea began to open, separating New Zealand from Australia, and spreading began between Australia and Antarctica.
At this time there were several areas in eastern Australia where volcanism occurred, but they have not been fully dated or completely understood. The Cenozoic volcanism originated in the Late Cretaceous. Volcanic rocks in Queensland and New South Wales have been dated to about 70 Ma, though they are not widespread. Central Victoria has rocks of Group 1 of the same age (Johnson, 1989). At Cape Portland in Tasmania, there are lavas that are believed to possibly have been extruded at either the earliest part of the interval or a bit earlier (Sutherland & Corbett, 1974).
Apart of the Winton Formation, of Cenomanian age, and possibly the Mt Howie Sandstone (Hill, 1994, Fig 3.9), most of continental Australia was bypassed by sediment of this age. The Eastern Highlands in Queensland and southeast Australia were sources for the sediments. It is believed a large amount of volcanic debris from the Eastern Highlands was deposited, some in the Winton Formation, and possibly the depocentre on the central south coast. It had previously received sediment from Antarctica as well as Australia, but by this time was no longer receiving the Antarctic component. In other places the terrigenous component (derived from terrestrial rocks), that was mostly volcanogenic, changed to a more quartzose continental source.
It is believed the ancestral Murray River and Darling River probably originated at this time, combining with other rivers to override the now dramatically less important central Australian depocentre, now carrying the sediment from about 2/3 of the continent to the central south coast. The drainage divide that had been present earlier, now moved north, almost to the gulf of Carpentaria, changing the drainage of the continent. Much of the region appears to have been subject to intense weathering, that could indicate humid conditions and warmer temperatures than had prevailed earlier. According to global reconstructions for the time by Hallam (1984), this is consistent with changes that occurred globally.
According to Douglas et al., (1976), the cool conditions that were characteristic of the earlier Cretaceous didn't change until the Santonian, at which point the climate became more humid and warmer through to the close of the Cretaceous. The coal swamps weren't as widespread as they had been in earlier parts of the Mesozoic, but their presence indicates that the climate was very humid. According to Dettmann (1981), this time was humid, but with cool temperatures, but Truswell suggested that the 'the temperate elements like Nothofagus' evolved in the Late Cretaceous, based on evidence from the Otway Basin and the Gippsland Basin.
The pre-existing basin geometry along the western margin changed to the deposition of a sediment wedge that thickened in the seaward direction with a prograding (the growth seaward of a river delta) geometry. The Carnarvon Basin and the offshore Northwest Shelf (Apthorpe, 1979) are the best sections known (Hill, 1994), as well as from the Perth Basin, where sporadic records exist (Quilty, 1978; McNamara et. al., 1988). The foraminifera that dominate the biogenic carbonate deposits are the only component of the sediment that have been studied in detail and published.
Glassner (1969) recorded large foraminifera from New Guinea, so presumably the area was close to areas with tropical conditions, but fossils of tropical species, such as rudistid bivalves, coral reefs or large foraminifera, are not found in Australian waters from this time, as are species indicating cold water. Most sediments from the Late Cretaceous contained many diverse planktonic foraminifera, with features such as double keels, that indicate warm to tropical conditions. Faunas are from Sliter's (1976) Transitional or Tethyan Faunal Provinces. The temperature of oceans was much higher than in the earlier Cretaceous. The faunas appear to have been Transitional in the Santonian-Campanian, and Tethyan at other times, indicating subtropical-tropical conditions. Biogenic carbonate sediments predominate, but a fine terrigenous component is found in all of them. There is evidence of active drainage systems in the form of sandstones in places in the Perth Basin and more widely around the Kimberley Block, especially in the northern part.
Cenozoic (Cainozoic - closer to the original Greek root kainos = recent)
This is the period from 66.4 Ma to the present. Rocks of this age are widespread throughout Australia, and include sedimentary and volcanic rocks, as well as soil and laterite, which includes rock types such as silcrete. The Cenozoic climate was not much changed from that of the regime that had become established in the Late Cretaceous, but as time passed there was an increasing trend towards biogenic carbonate deposition in the marine environment, and there was reduced, though still important, internal drainage.
Sedimentary sections have not been found in Australia that continued from the Cretaceous to the Cenozoic, therefore no evidence has been found that could indicate if the environmental conditions changed in a way that could lead to the major extinctions that occurred in Australia, as they occurred around the world at the end of the Cretaceous. The marine biota around Australia suffered the same fate as the marine biota globally. It was at this time that Australia began moving away from Antarctica, picking up speed (Cande & Mutter, 1982) as the last connection with Antarctica was finally completely severed.
It was also at this time that the biogenic carbonate regime progressively expanded along the margin of southwest Australia, and continued spreading all the way to the east coast along the southern edge of the continent, and eventually all the way up the east coast. When the south coast was uplifted, the deposition of sediment in the central south coast was stopped. The Murray Basin and the Birdsville Basin also formed at this time.
The record of the Cenozoic in Australia has been divided into 4 distinct intervals. This division is based on the concept of 'unconformity-bounded stratigraphic units', or sedimentation cycles (Ki Hong Chang, 1975). This system of division has been used by a number of authors such as Quilty (1977, 1980, 1982), McGowran (1979), Veevers (1984), Frakes et al. (1987b), and Apthorpe (1989), Hill (1994).
It has been proposed by Savin et al. (1975; Savin, 1977) that on a global scale this was a time of warm marine conditions in which the temperatures at the ocean floor were probably about 14-15 C. The water temperatures were higher at the Palaeocene-Eocene boundary than they have reached since that time. Following a short period near the boundary of the Cretaceous and the Tertiary, the temperatures had been gradually rising throughout the Palaeocene. Less difference between polar and tropical climatic zones than is the case at the present is indicated by marine faunal evidence. The palynological evidence was comprehensively reviewed by Kemp (1981) for reconstruction of Australian climates during the Tertiary. The results of this study indicate that westerly winds would have controlled precipitation levels in southern Australia. It also indicated that the circulation pattern would have occurred less regularly and been more chaotic further north in the inland regions. This evidence of warm marine conditions fits with the belief that it was a time of high humidity.
In southeastern Queensland and northern New South Wales, there was a a considerable amount of volcanism that produced a diverse range of materials. Group 1 and Group 2 volcanics were emplaced in central and southwestern Victoria. In southeastern Tasmania this was also a time of minor source of volcanics, the geometry indicating that the present Tasmanian topography was generated before this time (Johnson, 1989).
It had been suggested that this was a time of low significant drainage (Quilty, 1982), but much new evidence now indicates a change in this view is required. This cycle has been divided into 2 parts by Apthorpe (1989) (her subcycles 1A and B). Apthorpe also produced evidence for a gradient in northern northwestern Australia from a 'relatively wet climate in the south' to a drier climate in the northeast. The earliest part of the later sub cycle coincides with the most widespread part of the marine transgression that occurred in this cycle near the earliest Palaeocene. According to Apthorpe, the conditions in subcycle B were initially warmer than in subcycle A1, the northeast having hot arid conditions, with precipitation that may have been seasonal. The waters of the Carnarvon Basin area may have been warm temperate to subtropical, possibly with a coastal countercurrent similar to the present Leeuwin Current. Apthorpe found evidence for significant cooling of oceanic temperatures, as well as increasing runoff from the hinterland of the Carnarvon Basin, in the Early Eocene, at the end of subcycle 1B. This cooling occurred immediately after the warmest marine conditions globally (Savin et al., 1975; Savin, 1977; Haq, 1981; Kennett & Barker, 1990). The conditions remained arid, tropical in the Bonaparte Basin in the north. According to Apthorpe, this cooling appears to have occurred some time prior to the global cooling described by Savin (1977). However, the timing of the global cooling is not based on very precise dating, and it is possible that the northwestern Australian evidence may be the best available (Hill, 1994), apart from the oxygen-isotope studies such as those carried out by Kennett & Barker (1990). It was suggested by Davies et al. (1991) that this scenario also applied to northeastern Australia. The Early Eocene was a time when northeastern Australia experienced the warmest conditions in the Palaeogene (though cooler than the present) when colonial corals could grow. It is believed temperatures were also warm at the time sedimentation was occurring in the Perth Basin (Quilty, 1974a,b), associated with significant rainfall and runoff (Kemp, 1978).
Evidence has been found of considerable precipitation and runoff in southeastern Australia (Deighton et al., 1976). Kemp (1978) suggested the conditions were temperate with high rainfall. Cool, moist conditions in the Otway Basin during the Palaeocene are mentioned (Harris, 1965). It is believed that Murray River, as well as many of its tributaries, including the Darling River, possibly existed since the Late Palaeocene (Stephenson & Brown, 1989), but its mouth has continually moved throughout the period of its existence, as the coastline changed position, which was in tern determined by variations in sealevel and structural movements, controlling the extent of marine transgressions that occurred in the Murray Basin.
In the Birdsville Basin, central Australia, is the thin, widespread Eyre Formation. This is believed to indicate a 'warm temperate with seasonally high precipitation' climate (Wopfner et al., 1974). It was the depocentre of a significant drainage system. The material from this region has been analysed in much more detail by Sluiter (1991) which has allowed a differentiation of a Late Palaeocene-earliest Early Eocene climate in which the mean annual temperature (MAT) ranged between 18-19 C., with rainfall of about 1400 mm/yr., and an Early Eocene climate that was warmer, about 20 C and higher precipitation. The temperatures had declined to about 17-18 C and rainfall to about 1500 mm/yr by the Middle Eocene. Though the temperatures were consistently higher than in the Gippsland Basin, they varied over time.
Middle-Late Eocene (Fig. 3.11, Hill, 1994)
The Middle-Late Eocene was apparently a time of decreasing oceanic water temperatures, both surface and bottom water dropping by about 5o C. Oxygen isotope studies indicate that the timing of the temperature changes was slightly different or had different gradients. All the major reviews agree on this point (Savin et al., 1975; Savin, 1977; Grant-Mackie, 1979; Mcgowran, 1979, 1989). A major stepwise change to heavier ocean water occurred near the close of the previous cycle, that was followed by the general decrease (Kennett & Barker, 1990). It has been said that this change that occurred in the Early/Middle Eocene transition is the 'most striking feature in the oxygen isotope record' in the Indian Ocean (Oberhansli, 1986).
Apart from Monaro, northern Queensland and throughout New South Wales, don't appear to have experienced more than minor volcanism at this time. It was mostly during this time that the Group 2 volcanics of southern Victoria were produced, that are considered to have been voluminous in the totality of Australian volcanoes. Volcanic rocks from this time have been found in Tasmania that occurred earlier than the main episode of volcanism that occurred in Tasmania in the Cenozoic (Johnson, 1989).
The sedimentary record from Australia dated to this time is better than those from most times in the Late Cretaceous-Cenozoic, and there is a wide distribution of the sediments. During this period, Australia had moved far enough from Antarctica to allow the establishment of the Southern Ocean, though the link with the Pacific ocean was still not completely open south of Tasmania. As Australia moved further north (Hill, 1994, Fig. 3.3) it moved into a different regime of atmospheric circulation (Kemp, 1981), in which the influence of the westerlies decreased, even along the southern parts of the continent.
Marine carbonate regimes had become established along the western coast, now they extended into the Eucla Basin and the South Australian basins. Between the sediments deposited during the Palaeocene-Early Eocene and the Middle-Late Eocene there is a major hiatus along the western and southern margins of the continent. Apthorpe (1989) suggested there could possibly be continuous sequences in parts of the Northwest shelf. Quilty (1977) introduced a simple 2 subcycle concept of cycle 2 that has since been revised for southern Australia, especially by McGowran (1989), relating 4 transgressions within the cycle to intervals of high sea level (with a variety of causes) and of warmer water. According to Apthorpe (1989), these cannot be differentiated in northwestern Australia. Temperatures are believed to have decreased globally during this interval, though around Australia there was a short reversal of this trend in the later Eocene. Large areas of the western margin experienced widespread tropical conditions in times when sedimentation was occurring, penetrating as far as the Bremer Basin, based on the occurrence there of larger foraminifera (Cockburn, 1967). Though generally warm, along the south coast evidence has been found of periodic intrusions of cool water (McGowran & Beecroft, 1985). Along the northeastern margin, the growth of larger foraminiferids indicates that the conditions were warm (Davies et al. 1991) in the early part of the Middle Eocene, after which the conditions deteriorated to the point where no tropical carbonates accumulated before the Miocene.
Vegetation data are very sparse for the northern half of the continent, but in the south (Blackburn, 1981) and the western coast, extending to the northwest and into central Australia (Lange, 1982; Truswell & Harris, 1982) the vegetation was subtropical to tropical rainforest. Distribution maps of the various vegetation groups she had differentiated for this time, that included coastal mangroves, were presented by Truswell, (1990). The hypothesis of deep weathering before the laterite and silcrete surface material, that is very widespread throughout the southern and western parts of Australia, formed, is consistent with the pattern she presented. According to Apthorpe (1989), some of the foraminiferal faunas are more consistent with warm temperate conditions, urging caution in the application of the tropical concept to the northwest, suggesting that rainfall decreased throughout the interval in that region. She has differentiated more terrigenous subcycles in the Middle Eocene and more carbonate-rich subcycles in the Late Eocene. This is consistent with the views of Quilty (1982). Internal drainage to the Birdsville Basin continued to contribute sediment that accumulated in central Australia to form sandstone. This is all consistent with the warm-hot humid climate hypothesis.
It is believed there may have been less tropical conditions in southeastern Australia, changes in marine water temperature being reflected in vegetation changes. There appears to have been a lack of tropical forms in the southeast by the Late Eocene (Kemp, 1981). According to Martin (1989), there was rainforest in this area, with high humidity throughout the year, Possibly with rainfall reaching about 1500-2000 mm/yr. According to MacPhail & Truswell (1989) the flora was 'complex evergreen forest dominated by Nothofagus' with high year round precipitation.
A transition occurred during this period, from what had been a more or less continuous regime from the Cretaceous to the Eocene, to a completely different regime in the circulation patterns in both ocean and atmosphere affecting both Australia and Antarctica, lading directly to the climate of the present. It was in the Middle Oligocene that the final narrow gaps between Australia and Antarctica, as well as the opening of the Drake Passage between the southern tip of South America and the northernmost point on the Antarctic Peninsula. This finally allowed the complete, unhindered flow of ocean currents in the Southern Ocean. Antarctica was now completely isolated, allowing the frigid climate of that continent to develop more fully. Australia was now isolated from both South America and Antarctica.
In this period volcanism occurred on the central Queensland coast, New South Wales highlands and central and eastern Victoria. In Tasmania, the main volcanics of the Cenozoic were formed, long after the uplift of the eastern highlands. The volcanic rock, as well as the forms volcanism, varied greatly at this time.
Significant cooling occurred at the start of the period, then at the end, very warm conditions prevailed for a short time. Sea levels varied widely, with transgressions occurring on multiple occasions. It is believed to have been a time when the marine conditions varied widely around Australia, possibly the most varied conditions up to that time. It has been suggested that it would have been a time when there would have been a large variation in precipitation and runoff because of the variations in water temperature of the ocean and the varying sea level. Shackleton & Kennett (1975) have suggested, based on oxygen isotope studies, that in the marine environment south of Australia the conditions remained relatively stable following a large change at the boundary between the Eocene and the Oligocene.
Sedimentation around Australia underwent a major hiatus at the start of the Oligocene (Quilty, 1977, 1982) as described by Apthorpe (1989) and Quilty (1975). This appears to have coincided with the time when extensive areas of laterite surface formed over areas of Western Australia (Johnstone et al., 1973), that is thought to probably indicate seasonal humidity. The sea level appears to have fallen at the end of the Eocene, rising again in the Middle Oligocene, based on the history of sedimentation around Australia in the Oligocene. According to Apthorpe (1989), this differs greatly from the findings of Vail et al., (1977) and Haq et al., (1987), who suggest the exact opposite to the Australian pattern. There was a major cooling in the ocean around Antarctica, and in the Indian Ocean generally, based on data from the DSDP (Oberhansli, 1986), and ODP data from around Antarctica (Kennett & Barker, 191990), at the same time that evidence from Australia suggests a drop in sea level. As pointed out by Hill (1994).
Evidence indicates that the glacial phase of Antarctica began in the Oligocene, with the development of sea ice, but the development of an icecap at this time is less certain. Temperature gradients between the tropics and the poles were increased as a result of this cooling, leading to higher wind speeds. The lack of warm water morphological features on planktonic foraminifera indicate that globally, the Oligocene appears to have been a cool interval.
At this time Australian sedimentation was mostly restricted to the coastal regions, only extending further inland in the Murray Basin (Ludbrook, 1961). At the end of the Early Miocene, 2 subcycles are recognised, that appear to be separated by a brief regression. (N6-N7, according to Apthorpe (1989) and Quilty (1982)). The later of the 2 subcycles produced deposits that are more widespread.
Large warm water foraminifera are again found around northwestern Australia as a major new transgression occurred in the Late Oligocene (Chaproniere, 1984). The faunas indicate tropical conditions across northern Australia from this time, when Australia began to collide with Southeast Asia, and structural development, possibly of the Kimberley Block and some offshore structures, as a result. Other parts of the Australian margin show evidence of the transgression (Quilty, 1972, 1974b; Brown, 1986; Shafik, 1991). There is not a lot of evidence from this interval over much of the continent. Sub-tropical to tropical waters surrounded Australia at the close of the Early Miocene, Lepidocyclina faunas becoming established, for a short period, in nearly all marine sedimentary basins from this age.
Conditions were temperate to cool off northeastern Australia early in the interval, with no tropical carbonates forming (Davis et al., 1991). Warmer conditions returned in the Early Miocene, and coral reefs grew once again.
Humid, cool temperate conditions are indicated by palynological evidence from southern and southeastern Australia during the Oligocene (Harris, 1971; Stover & Partridge, 1973; Kemp, 1978,1981. Truswell, 1990). In central Tasmania, at an elevation of about 700 m, humid, conditions are indicated in the Late Oligocene to Early Miocene by micro- and macro-fossil analysis (Macphail, 1991). A temperature decrease from those of the Eocene (Truswell, 1990) is suggested by vegetation records, that may have been associated with some degree of lowered humidity. Nothofagus forests expanded, replacing other forest types, as well as some alpine forests at this time. In the Murray Basin region there appears to have been a decrees in rainfall as the close of the Oligocene approached (Martin, 1989), with rainforests continuing to be the dominant type of vegetation. The area was found by Macphail & Truswell (1989) to have had the greatest degree of habitat diversity in Australia at this time.
Significant vegetation and drainage in Central Australia occurs for the last time in the Middle Miocene. Evidence has been found in vertebrate deposits of the arid Centre of rainforests lining watercourses, as well as large lakes and more open grassland (Kemp, 1978; Callen, 1977; Stirton et a;, 1967). All this suggests a humid climate originating in the northern reaches of the westerlies, and it is believed that the conditions since that time have never been as equable, the climate changing to more seasonal, with a marked dry period (Martin, 1989). Macphail & Truswell, (1989) have supported Martin's conclusion. The result was decreasing importance of Nothofagus-dominated forests, and the spread of 'drier and more open forest types' (Macphail & Truswell, 1989), with fire becoming increasingly important in the environment (Martin, 1989).
Late Miocene-Recent Fig. 3.13 in Hill, 1994
According to Galloway & Kemp (1981), there is a dearth of evidence from Australia that is required for interpreting the climate of the continent through the Pliocene and later. Global patterns, carbonate sections from the deep sea and from Antarctica are being used to infer about the Australian climate in this period.
Australia continued drifting north at about 6.5-7 cm/yr from Antarctica (McDougall & Duncan, 1988; Quilty, 0993), (about 6 cm/yr in relation to hotspots). As Australia continued to collide with Southeast Asia it became possible for a limited amount of faunal and floral influence from Asia to reach Australia. It was at this time that the oceanic and atmospheric circulation patterns that continue to the present were established, as the gap between Australia and Antarctica widened., simultaneously narrowing the gap between Australian and Asia. These patterns varied widely in the Quaternary, open vegetation becoming the dominant pattern in Australia as the grasslands expanded. There was rapid alternation between rainforest and sclerophyllous vegetation in some parts of the continent (Truswell, 1990). The area covered by rainforests gradually decreased as the aridification of the continent continued, though throughout the Quaternary it appears the total area covered by rainforest has remained approximately constant.
In this interval it was northern Queensland and eastern Victoria, where the volcanism was concentrated, with some in the New South Wales highlands. Tasmania had little volcanic activity at this time, and what the was proved to be the last episode of volcanism on the island. The Western Plains of Victoria, and structures such as Mt Schank and Mt Gambier, were formed by a considerable amount of volcanism in western Victoria and southeastern South Australia. As late as 25,000 years ago there was a considerable amount of volcanism, and the latest eruptions finished about 4,500 years ago.
As Antarctica entered its present glacial phase about 2.6-2.4 Ma the temperature of the ocean to the south of Australia plummeted (Shackleton & Kennett, 1975), even in parts of the ocean well north of the Antarctic Convergence. Around Australia, as the volume of the runoff reaching the ocean decreased, associated with the increasing aridification of the continent, and possibly the soil-binding effect of the spreading grasslands, marine sediments became more carbonate rich. Tropical conditions continued around northern Australia, and during the cooler Late Miocene, reef growth was migrating north (Davies, 1991). The reef growth moved back south in the Pliocene. It was also the time when the pattern of high rainfall being restricted to the southeast developed, continuing to the present.
Some clues have been found in marine data from northwest Australia about changes that occurred in the marine environment (Quilty, 1974b), planktonic percentage changes and cooling ratio patterns in the planktonic foraminifera indicate the temperatures were higher in the Early Pliocene than in the Late Miocene and the Late Pliocene and later. The data also indicate that there was a marked lowering of water depth of the continental shelf area, that could have resulted from a fall in global sea level or from a migration of the continental shelf edge over the area. Apthorpe (1989) has shown that the influence of tectonics strongly controls the diachronous nature of the section base, documenting in detail what is known of rocks from this time to the north of the sections recorded by Quilty (1974b). She has also shown that throughout the interval, sea level fluctuations were an important influence on sedimentation, as well as the strong tidal patterns that are well known in the region at present. It is possible that some of these changes may link to global changes, but at the time of writing time differentiation is inadequate at some key times (Hill, 1994).
For a short period in the Early Pliocene Nothofagus-dominated rainforests returned to the Murray Basin (Martin, 1989), with the return of high, non-seasonal rainfall, called the 'warm, wet phase' by Truswell (1990) in the Lake George region. During the Middle and Late Pliocene the area began to dry out to its present condition, the Nothofagusi rainforests being replaced by wet sclerophyll forest. It is believed the change to drier conditions in the Murray Basin early in the Late Pliocene may have coincided with the changes recorded in the Lake George region by McEwen Mason (1991), when the modern regime became established. Between 2.5 and 0.7 Ma Lake Bungunnia formed (Brown, 1989b) when structural movements dammed the southern reaches of the Murray River (Stephenson & Brown, 1989). The Murray River reached its present form when the dam was breached at the end of this interval.
Angiosperm fossils dated to the Pliocene have been found in Antarctica by Hill & Truswell, suggesting that Antarctica had a warm interval at about the same time.
The present polar glaciation pattern became established probably abut 2.6-2.4 Ma in the Late Pliocene. It has been suggested that this was caused by features of the Earth's orbit, the Milankovitch hypothesis (Imbrie, 1984). According to this pattern, a cycle in which ice cap volume increased for about 100,000 years, leading to an associated drop in sea level. In the last 2 glacial maxima this drop was about 130 m lower than the present sea level. The glaciations were followed by a collapse of the ice cap, mostly in the Northern Hemisphere, over about 10,000 years. It is believed the cycle switches back to increasing ice volume after about 5,000-10,000 years. 17 of these cycles have been identified by Fink & Kukla (1977) in the last 1,8 million years.
The history, as it applies to Australia, has been summarised by Williams (in Veevers, 1984), referring to a change in near-surface temperature of about 5-10o C between glacial and interglacial phases in each cycle, usually over about 10,000 years. The rate of change is more rapid than any other change that occurred, as well as when compared with any changes proposed for the the present 'greenhouse' change. The pattern of change from glacial to interglacial is seen in the environment as arid phases, with high wind speed and blowing dust, reaching a maximum of spread of arid areas and a maximum intensity at the glacial maximum, similar to the conditions that existed about 18,000 years ago, and sea surface temperatures were lower the south, west and northwest, the numbers of tropical cyclones was greatly reduced, increased amounts of dust blown form the land was deposited in marine sediments. In Tasmania and the Kosciusko area the high country had higher snowfall with some significant glaciation. Precipitation was lower in the lee of the Tasmanian snowfield, leading to a higher level of aridity. At these times, the sea was low enough for a land bridge to form between the mainland and Tasmania in the south and New Guinea in the north. The interglacials were intervals in which the environmental conditions were similar to those of preset-day Australia.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|