Australia: The Land Where Time Began
Cretaceous to Recent
Most Australian landscapes have been exposed to weathering and erosion for many millions of years under tectonic conditions that were very stable, the result being seen in the heavily leached, infertile soils. Not all landscapes in Australia are ancient, though an area that was previously believed to be young, the Southeastern Highlands (Andrews, 1911; Brown, 1969); Hill, 1975), has since been found to be comparatively ancient (Wellman, 1987; Bishop, 1988; Taylor et al., 1990a).
The Australian continent has had its present outline since some time between 150 and 50 Ma. (Wilford & Brown, in Hill, 1994, Chap.2), though many of Australia's landscapes are much older. The Tasman Line of Veevers (1984) divides the tectonic provinces of Australia into 2 groups. To the west of the line, blocks of Precambrian age dominate, with fold belts that have been overlain by thin basins of Phanerozoic age. To the east, it is Phanerozoic fold belts that dominate, being overlain by thin basins that are younger. These fundamental geological divisions correspond roughly to the landscape regions, the Precambrian blocks to plateaux, with elevations of up to about 500 m and the fold belts to uplands that reach up to 2000 m elevation, the basins to lowland plains, mostly less than 200-300 m. The nature of the drainage systems of the present varies in accordance with the major landform regions.
The drainage systems in the western part of the continent tend to be uncoordinated, coordinated drainage being mostly restricted to the margins of the continent, especially the area of the eastern Phanerozoic fold belt. The Eastern Highlands presently receive more rain that the western 2/3 of the continent. According to Veevers (1984), this has been the case since the beginning of the Cenozoic. The distribution of lignitic sediments in southern, western and central regions indicate that in the early Tertiary the western parts may have been as wet as those in the east. Veevers suggested that drainage systems of the Tertiary were not much different from those of the present.
Not much is known about Australian soils before the Cenozoic, but the history of soils from the start of the Cenozoic has been reconstructed, based on some soils from this time that have been found. Soils form as a result of the interaction between parent material, climate, erosion and deposition, the deposition being generally tectonically controlled.
The western region, the ancient 2/3 of the continent, is made up of a number of cratonic blocks of Precambrian age, composed of granite, metamorphic and sedimentary rocks. These blocks are the Yilgarn-Pilbara, Kimberley, Arnhem-Pine Creek-McArthur-Mt. Isa and the Musgrave-Amadeus-Arunta Blocks. These blocks correspond to the major landscape provinces. Throughout most of the Phanerozoic, prominent landscape elements have been centred on these regions, most of the sediment entering the surrounding basins coming from them.
The landscapes of the blocks in the western region has been greatly modified since they were first exposed to the elements. The surface of the Yilgarn-Pilbara Plateau has been lowered by at least 600-700 m throughout the Phanerozoic (Veevers, 1984). It is believed a large portion of this occurred during the Late Carboniferous and Permian, when the plateau was scoured by glaciers moving from the south to the north. Glacial pavements of this age have been found in the Pilbara region (Veevers & Well, 1962), where there are valleys that have been carved by glaciers. Glacial deposits of Permian age have been found in valleys on the northern Pilbara (Butt, 1989). Between the Permian and Cretaceous, large amounts of sediment were deposited into the Perth Basin. It has been suggested that much of the sediment deposited in the Mesozoic may have originated in uplifted parts of the basin. Denudation rates of 4.5-5 m per million years have been calculated by van de Graaf (1981), suggesting a considerable amount of uplift during the Phanerozoic. It doesn't appear likely that much uplift occurred after the glaciation of the Permian, tills from that age being found on the surface. Present-day chains of lakes that formed between the late Mesozoic and the Eocene, involving about 100 m of erosion, represent relict drainage of the Yilgarn, according to Jackson & van de Graaf (1981). The drainage has been dated as of Permian age by R.P. Langford, G.E. Wilford & E.M. Truswell, which accords with other data. It has been suggested that erosion has been no more than a few cm/million years during the late Cenozoic (van de Graaf, 1981). Based on this, it seems the Yilgarn Plateau has been uplifted and eroded significantly during the Palaeozoic and the early Cenozoic, since which it has been extremely stable.
The Kimberly Block and the Arnhem Block, the Pine Creek Inlier, McArthur Basin and the Mt Isa Block, collectively largely composed of metamorphic and granitic rocks and geosynclinal sediments, all of Precambrian age, underlie the Kimberley Plateau, the Northern Australian Plateau and the Carpentaria Fall, all formed landscapes throughout much of the Phanerozoic (Veevers, 1984). These Precambrian high landscapes have been uplifted though much of the Phanerozoic, the erosion material from them being deposited in sediments in the marginal basins (Hill, 1994, Fig.5-2). The coastline of the area that existed through much of the Phanerozoic is not much changed at the present. The only exception was during the marine transgression of the Early Cretaceous, when the sea covered the Daly Basin, McArthur Basin and the Pine Creek Inlier for a brief period. When the sea retreated it left behind quartzose sheet sand that has been mostly removed since then (Skwarko, 1966). 'This indicates the long-term lateral stability of these cratonic areas over the last 500 Ma or so' (G. Taylor in Hill, 1994). Material derived from the Kimberley Plateau in the north has been found in glacial debris of Permian age in the Fitzroy Trough. This indicates that, like the Yilgarn plateau, the Kimberley Plateau underwent some glacial erosion during the Late Carboniferous and Permian. It has been shown by Young (1986) that solution weathering could have substantial effects in the sandstones of Devonian age in the Ord Basin, as well as the physical removal of detritus, suggesting that similar chemical effects were probably as important in the denudation of the ancient cratons.
The highest elevation found in cratonic areas of the western region from the Precambrian are found in the Central Australian Ranges, some peaks reaching up to 1500 m. They are comprised of 3 geological entities, the Musgrave Block and the Arunta Block from the Precambrian, and the Amadeus Basin from the late Proterozoic-Palaeozoic. The Tanami Block, the Birrindudu Basin and Hall's Creek Province, all of which are composed of rocks of sedimentary, metamorphic and granitic type, connect these regions to the Kimberley Block in the northwest. Around the margins of the Central Australian Ranges younger sedimentary basins of Phanerozoic age overlie them. They are also partially divided by Palaeozoic sediments of the Amadeus Basin. Around the Aranda Block are marginal basins containing sediments of Cambrian and Devonian age (Veevers, 1984). This indicates that it had reached high elevations during the Cambrian, its uplift being complete by the Devonian. The surrounding basins received sediment from both blocks. Evidence that glaciers eroded the Central Australian Ranges was found in the form of glacial pavements in the highlands and glacial sediments in the surrounding basins. A southeasterly trending drainage network was carved out by erosion during the Mesozoic and early Cenozoic, then later partially filled with sediment (Twidale & Harris, 1977). Features such as Uluru and the Olgas (Kata-tjuta) were formed as a result of this drainage, which proves the antiquity of these landscape elements. The continuing role of tectonism in these landscapes is illustrated by the occurrence of minor fault-bounded basins of Cenozoic age.
The core of the Eyre Peninsula and the Gulf Ranges region is made up of the Gawler Block, the Willyama Block, both of Cambrian age, with the Adelaide Fold Belt and the Stuart Shelf. The rocks making up these regions are of a number of types, volcanics, metasediments, flat-lying and folded sediments, and granites, all surrounded by basins containing sediments of Phanerozoic age, lapping on the older cratonic platforms. Since the Late Carboniferous, this region has formed high ground on and off (Veevers, 1984), though there were highlands in parts of the area long before this time. Since their formation, the marginal basins of the Phanerozoic have been receiving sediments from this high ground. Significant erosion occurred in the highlands in the Permian when they were scoured by glaciers. In the region of the Gulf Ranges, renewed faulting occurred in the late Cainozoic that resulted in the present form (Callan & Telford, 1976).
The Australian Eastern Highlands is a complex, relatively high region, about 400-500 km wide, reaching heights above 500 m, many parts reaching above 1000 m, that stretch from North Queensland to Tasmania. The drainage of the highlands is asymmetric, with short river networks, of steep gradient, flowing to the coast on the east. To the west, the river networks are long, low gradient streams with long histories of sedimentary deposits in the marginal basins on the western flanks of the highlands. They are dominated by geosynclinal deep and shallow marine sediments, of Early to Middle Palaeozoic age and granitoids with abundant basalts of Cenozoic age.
The age of the Eastern Highlands is uncertain. It was believed that most uplift occurred since the Late Miocene, mainly based on data from the southern highlands (Andrews, 1911; Browne, 1969; Hill, 1975). Since then, detailed studies of the highlands, and the basins that flank them, has found that there can be large differences between the northern ranges, in Queensland and northern New South Wales, and the southern ranges, in southern New South Wales, Victoria and Tasmania. The erosional and uplift history of the full length of the highlands was reviewed by Wellman (1987). The southern section of the highlands was reviewed by Bishop (1989). The southern section of the highlands had a height of at least 600 m in the Eocene, as shown by Ollier (1977). The highlands were demonstrated by Taylor et al. (1985, 1990a) to have a height of at least 600 m, and probably 800 m, in the Palaeocene. Further studies (Macumber, 1978; Wooley, 1978; Veevers, 1984; Brown, 1989) found that sediments deposited in the Gippsland Basin and the Otway Basin, of late Mesozoic age, originated in the southern highlands, and in the Palaeocene they were being deposited in the Murray Basin. The coastal strip of southern New South Wales had been established by the middle Tertiary, though probably earlier (Nott et al., 1991; Young & McDougall, 1982). Sediments from the Late Cretaceous and Palaeocene were found in basins along the northern coast of Tasmania (Williams, 14989). These studies make it clear that by the Late Cretaceous or earliest Tertiary the southern sector of the highlands had been uplifted enough to make the highlands a major source of sediments deposited in the basins that flank them.
According to G. Taylor (in Hill, 1994), the southern sector of the highlands was uplifted by the Mesozoic, but there is insufficient evidence to determine how much earlier the uplift occurred. A major uplift is indicated about 95 Ma by the lack of sediments in the flanking basins older than the late Mesozoic (Jones & Veevers, 1982), and supported by Wellman, (1987). On the basis of a passive isostatic rebound model, Lambeck & Stephenson (1986) argued that the highlands were uplifted in the late Palaeozoic, rising more or less continuously since that time, responding to unloading by erosion. Periodic uplift through the Cenozoic was proposed by Jones & Veevers (1982), in association with volcanism and sedimentation, as well as sea level fluctuations in the flanking basins. It is believed the Tasmanian sector of the highlands probably began developing its present form in the Triassic, being well established by the Late Cretaceous (Williams, 1989). By the Cenozoic, the southern sector of the highlands was well established, whichever model is accepted.
Mesozoic sediments on the highland summit of the northern sector of the Eastern Highlands suggests uplift occurred after the Early Cetaceous in Queensland and following the Triassic near Sydney (Wellman, 1987). In the Eromanga Basin, which lies along the western flank of the highlands, andesitic debris is found abundantly in sediments as young as the Cenomanian (90 Ma) that derived from volcanoes to the east of the present coastline (Veevers, 1984), indicating that the highlands were not present until after this time. The shedding of sediments into the flanking basins by the Eocene is indicated by the presence of quartzose sediments above the surface, of Late Cretaceous-Palaeocene age, that is deeply weathered (Doutch, 1976). 3 periods of basaltic volcanism and uplift, during the Palaeocene-Eocene, Late Oligocene- Mid Miocene and Pliocene-Quaternary, were identified by Grimes (1980), that were similar to those identified by Jones & Veevers (1982) in the southern sector. According to Doutch (1976), basaltic volcanism, with associated uplift, occurred in North Queensland as late as 13,000 years ago. Retreat of the Great Escarpment in the Ebor area of the New England region, New South Wales, is believe to post date the formation of the Ebor Volcano 18.5 Ma (Gleadow & Ollier, 1987). Ollier (1982) has argued that uplift is later than 18 Ma in this region. It has been suggested that the Sydney basin was uplifted and eroded between 180 and 100 Ma (Schmidt & Embleton, 1976).
It is believed the southern sector of the Eastern Highlands is probably older than the northern sector, but with landscapes of considerable antiquity occurring in both sectors. These ancient landscapes are overlain in many places by younger landscapes. Widespread basaltic landscapes are a common feature along most of the Eastern Highlands. They are highly dissected, but have frequently preserved the topography and soils that the lavas flooded across (Bishop, 1988). The ages of these basalts vary along the ranges, being of Holocene age in Victoria and Late Cretaceous in central Queensland and northern New South Wales. Overall, the eruption rate was more or less constant throughout the Cenozoic. Until about 35 Ma lava field volcanism was dominant, after which central volcanic eruptions became the dominant form of volcanism (Wellman & McDougall, 1974).
Many of the sedimentary basins flanking the highland regions formed in the Palaeozoic, but most were covered by the sea during marine transgressions in the Early to Mid Cretaceous. Sedimentation alternated between marine and terrestrial throughout the period of marine transgressions, as sea levels fluctuated (Morgan, 1980) or uplift occurred, as the early process of separation from Antarctica got under way (Veevers, 1984; Wilford & Brown, Hill, 1994, Chapter 2). Shallow seaways separated the highland areas during the marine phases. It has been suggested by Wasson (1982) that separation would have increased spatial and habitat diversity over the continent. The marginal habitats of the shallow epicontinental seas would have had an increase in habitat diversity as the global sea levels fluctuated during this period. Limited tectonism following the Cretaceous resulted in the basins having low-relief surfaces after the seas retreated during the late Aptian, a condition that remained little changed through most of the Cenozoic. Many of the major basins, such as the Eromanga Basin, the Canning Basin and the Officer Basin have areas where remnants of the surfaces from the mid-Tertiary have been preserved. Cenozoic sediment now covers much of the Cretaceous surface, especially in many of the eastern and southern basins.
During the Palaeocene, post Mid-Cretaceous deposition began in the Murray Basin, continuing to the present. It has been suggested that sedimentation began with the uplift of the Eastern Highlands, and the subsidence of the basin (Jones & Veevers, 1982). 3 depositional cycles have been recognised, during the Palaeocene-Early Oligocene, Oligocene-Early Middle Miocene, and Late Miocene-Pliocene. The 3 depositional cycles are separated from each other by phases in which the basin was eroded. During each cycle there were marine incursions, the most extensive of which was during the Late Miocene. There was a wide range of habitats during each cycle, deposition taking the form of marine carbonate platforms through to deposits of lagoonal, tidal and deltaic origin, and riverine plains with ;lakes. Brown (1983, 1985, 1989) has suggested that the cycles were caused by sea level changes and associated with basinal isostatic adjustment, though the actual cause is uncertain. According to Taylor (in Hill, 1994), the effects of these variations in sea level were restricted to the basin, Though the valleys of streams draining the margins of the Eastern Highlands cut deeper at times of low sea level, when sealevels were high the valleys were backfilled because of the reduced potential for erosion (Macumber, 1978). During the Late Miocene-Early Pliocene, a prograding series of beach ridges formed, with intervening deposits of quartzite and fluvial sands (Parilla sand). In the western part of the basin the ridges are prominent features. The basin has been dominated by alluvial, lacustrine and aeolian activity associated with the alternating cold and warm conditions of the last 2.5 million years. Though water availability has varied during the period, the variation has not been in a systematic way with regard to temperature (Bowler, 1978), except that at this time conditions were generally arid, unlike the humid conditions that had prevailed since the Late Cretaceous.
During the Eocene and Miocene, the Eucla Basin was dominated by the deposition of limestone. Following the withdrawal of the Cretaceous seas, rivers draining into the Eucla Basin from the Yilgarn and Musgrave Blocks were established. In the Eocene, at times of high sea level, these channels were inundated by the sea, the channels were alluviated, deposits of sand, lignite, spongolite and limestones being laid down across the southern Yilgarn Block (Jones, 1990). The river channels of the Officer Basin and the southwestern parts of the Eromanga Basin filled with quartzose alluvium (Lampe Beds) and alluvium and marginal marine sediments (Pidinga Formation: Benbow et al., 1982) at times of high sea level. Chains of playas now lie along the palaeochannels on the Yilgarn, and across the Officer Basin and the southwestern parts of the Eromanga Basin. Uplift of the Eucla Basin occurred after deposition of a thick limestone deposit had formed, resulting in the flat surface of the Nullarbor Plain following the withdrawal of the sea.
The only Tertiary record in the Canning Basin is seen in shallow drainage lines, between low hills of deeply weathered Cretaceous rocks, where there are minor alluvial and lake deposits. The palaeodrainage systems were buried in the Quaternary by longitudinal dunes. Following the retreat of the Eromanga Sea during the Albian (95 Ma), the Winton Formation was deposited in the Eromanga Basin, the largest of the flanking basins. Volcanogenic sandstone, shale and coal, deposited between 95 and 90 Ma forms the terrestrial sheet of the Winton Formation. Over the following 20-30 million years the low relief surface was deeply weathered (the Mornay Profile of Idnurm & Senior, 1978). The deposition of the Eyre Formation, and its equivalents, during the Palaeocene and Eocene, that consisted of quartz-rich fluvial sediments, halted the weathering phase.
Another weathering phase occurred when a break in the deposition, leading to the formation of silcrete (the Cordillo Silcrete of Wopfner, 1974; the Cordillo Surface of Doutch, 1976). Warping and uplift occurred after the deposition of the Eyre Formation, during the Oligocene, in which the major landscape elements that are now found in the basin were formed.
The subsidence of the depressions of Lake Eyre and Lake Frome was another result of this warping, with sedimentation occurring in substantial lakes in the Miocene. The Etadunna Formation (and its equivalents) was another deposit that formed in depressions in other parts of the basin. The Etadunna Formation is composed of muds, carbonates and evaporites that were deposited in streams and lakes. Following the formation of the Etadunna Formation, the warping continued, resulting in another round of deep weathering (the Canaway Profile and its equivalents, the Curalie Profile and the Strathgordon Profile (Doutch, 1976). Fluvial and lacustrine sediments continued to accumulate in the warped areas. The Cooper Syncline beneath the Channel Country of Cooper Creek and Lake Yama Yama, are examples of this (G. Taylor in Hill, 1994). In southern Queensland there are many other drainage lines that are also examples (Senior et a., 1978). In these deposits, the sediments reach as much as 160 m in depth, being composed of mudrocks, thin layers of gypsum, quartz sandstones, conglomerates, above the Etadunna Formation. fluvial and lacustrine sedimentation still occurs in these drainage lines. Deeply weathered and silcreted low ridges and mesas of the Cretaceous age Eyre Formation, separate the drainage lines. Especially in the Simpson Desert and the Strzelecki Desert in the southwest, fluvial and lacustrine sedimentation continued through the Quaternary, as dunefields spread across the interfluves.
In the lakes, evaporites and clastics accumulated in the drier phases, and muds and sands accumulated in the rivers in the wetter phases.
Post-Cretaceous records of marine and terrestrial sedimentation, punctuated by breaks of tectonic or eustatic origin, are found in similar flanking basins such as the Gippsland Basin, the Otway Basin and the Perth Basin.
The regolith, all the fragmented material above bedrock, the the soil being uppermost part (Ollier, 1988), the nature of which is determined by the surface processes acting on the rocks, such as the climate, erosion rate, length of time it has been forming, and the rock type or sediment on which it has developed.
In the Late Carboniferous and Early Permian, the last glaciation to affect Australia produced glacial pavements and valleys in the Eastern Highlands of Victoria (Craig & Brown, 1984) and the ancient plateaux in western Australia. Following the retreat of the glaciers the continent was in high latitudes, but the climatic conditions were good enough for plants to flourish up until the Jurassic. About 160 Ma the climate began drying, but humidity rose again about 140 Ma, allowing the forests to be widespread (Frakes et al., 1987). The temperatures during the Mesozoic are uncertain, Frakes (1987) suggesting the climate was generally temperate, with 'cooler than expected temperatures' in the Early Cretaceous. According to Francis (1990), the temperatures in the southwestern Eromanga Basin agreed with those proposed by Frakes. Between 175 & 40 Ma Australia was inside the Antarctic Circle (Wilford & Brown, in Hill, 1994). Evidence was provided by Taylor et al., (1990b) that during the Late Palaeocene the climates in the southern sector of the Eastern Highlands were cool.
Three warm-cool cycles have been identified throughout the Tertiary, Late Cretaceous-Middle Eocene, Middle Eocene-Early Oligocene and Early Oligocene-Late Miocene (Frakes et al., 1987). Between the Middle Pliocene and the beginning of the Quaternary these cycles were replaced by a drift towards more aridity, a cycle of about 100,000 years from cold to warm and back again. Over these cycles the humidity also varied, not always in line with the temperature swings (Frakes et al., 1987). The development of regolith is closely related to the history of groundwater, that is mostly related to climate and geology.
The nature of the landscape and regolith are strongly affected by the rock type in various parts of the continent. Deep regoliths tend to be found in areas where there are rocks containing labile minerals, such as igneous felspathic sedimentary rocks. Thin regoliths and soils are usually found above limestones or quartzose sedimentary rocks.
Since the Late Cretaceous, parts of the continent have experienced minor tectonic activity that, together with changes of sea level during the period (Frtakes et al., 1987), have resulted in much of the regolith being stripped from the Eastern Highlands and the cratonic blocks, or increasing deposition in the flanking basins and the cratonic bock margins.
There are few places where regoliths that formed before the Late Cretaceous are known in Australia. An example of such known regoliths are the glacial material and pavements that were formed by glaciers in the Permian. Most known regoliths date from the Cretaceous when the epicontinental seas regressed. Since then, there are 2 known episodes of major regolith formation, the 1st from the Late Cretaceous to the Late Miocene and the 2nd beginning about 10-6.5 Ma. The first episode produced deep leteritic weathering under humid conditions, and most believe with warm temperatures (e.g. Frakes et al., 1987; Butt, 1988; Ollier, 1988), though with cooler phases (Frakes et al., 1987). The evidence, based on micropalaeontological records, onshore and offshore, and oxygen isotope values, was summarised by Frakes et al.(1987).
The 2nd phase began at a time of rapid lowering of sea level that occurred when the Antarctic ice sheets expanded. The climate had entered a phase of cyclical glacial and interglacial periods by 2.5 Ma, after the retreat of the Tertiary seas. In these cycles the warm periods were associated with wetter conditions and the cold periods with dry, windy times. In central Australia, the acidic deep weathering was replaced by alkaline weathering conditions that produced widespread evaporites and calcareous regoliths over the last 2.5 million years.
Deeply weathered in situ land surfaces or sedimentary deposits cover large areas of Australia. In the basins of Eastern Australia there were a number of episodes of ferruginisation (Doutch,, 1976; Frakes et al., 1987; Grimes, 1980).
Various weathering events have been found in depositional terrains that are, at least partially, separated from each other by alternating sedimentary events. Between 90 and 6.5 Ma the continent was tectonically stable, but it has been found that these stable conditions didn't result in a continuous period of weathering, there were periods when deep weathering events occurred in distinct episodes, that apparently coincided with the warmer parts of the 3 climatic cycle that occurred in the Tertiary (Frakes et al., 1987). These warm phases also coincided with times of uplift in the Eastern Highlands (Jones & Veevers, 1982). It has been proposed by Frakes, el al., (1987) that the climate of the Late Cretaceous was cool, and together with evidence presented by Francis (1990), Taylor et al., (1990a,b) and Bird & Chivas (1988), indicates that deep weathering may not require warm conditions, as was commonly believed. There is also evidence from Iceland (Gislason et al., 1990), and is suggested by Reynolds (1971), that very rapid chemical denudation can occur in cool to cold climates. When the evidence of deep weathering is looked at over the whole continent it becomes apparent that, on the whole, there is a correlation between warm conditions and deep weathering, even after allowing for dating inaccuracies.
In the deeply weathered rock there is a gradual change between the upper surface and the parent rock, the classic 'laterite' profile for Western Australia (Walther, 1915). There is an uppermost layer is a ferruginous crust - ferricrete, laterite or bauxite. below this is a layer of white to pale-coloured matrix with ferruginous red mottling. Between this layer and the bedrock is rock with a decreasingly kaolinite-rich saprolite (weathered in situ and retaining the fabric of the original rock) (G. Taylor in Hill, 1994), pallid at the top of the zone grading to least pallid at the bedrock.
There is a great amount of variability among deeply weathered profiles. Among these deep weathering profiles are those in which the transported upper horizons are unrelated to the underlying saprolite (e.g. Churchward & Bettenay, 1973; Milnes et al., 1985; Taylor & Ruxton, 1987). Some have the upper zones removed, said to be stripped (e.g. Senior, 1979; Butt, 1989; Ollier et al., 1988). Some have upper zones that have been silicified (e.g. Ollier, 1988) while other have been partially stripped, having a silcrete in the pallid zone (e.g. Senior, 1979; Ollier, 1988). Some have stripped profiles overlain unconformably by silcreted sediments e.g. Senior, 1979; Taylor & Ruxton, 1987; Ollier et al., 1988).
Across the entire Australian continent there are widespread areas with deeply weathered profiles, though they are not as common in Tasmania. In places such as Monaro in New South Wales, ranging from 2 m on the basaltic lavas to 10s of metres on surfaces that have been exhumed, previously being beneath basalts from the early Tertiary. In places, such as southwest Queensland, they can be more than 100 m thick (senior, 1979). Most of the deeply weathered landscapes are in the form of mesas or low plateaux, having been protected to some extent by ferricrete or silcrete caps, during long periods of erosion. The wide valleys separating them have cut into weathered rocks. The erosion has resulted from several processes such as tectonic, eustatic and climatic, or even a combination of all 3. The erosion products, mostly kaolinite and quartz, have been deposited in the lower parts of the landscape, such as valleys, but also in alluvial sheet sediments that are widely distributed in places such as the Eyre Formation and the Glendower Formation. Sandplains cover some areas of cratonic blocks, such as the Yilgarn Block. It is believed these result from kaolinite solution from deep weathering profiles, the solution-resistant quartz sand being left (Butt, 1985).
Highly leached acid soils, mostly kaolinite, quartz and sesquioxides, form on deeply weathered materials, and sediments derived from them. Soil types usually associated with these parent materials are listed in Table 5.1 (G. Taylor in Hill, 1994). The parent material of these soils are mostly old (fig 5.8, G. Taylor in Hill, 1994). It is believed Australian vegetation has gown on a substrate of these soils since the early Tertiary. These soils were more widespread in the early Tertiary than at present because they were heavily eroded in the later Tertiary and Quaternary, as well as partially buried during the Quaternary, which would have resulted in them playing a major role in the evolution of the Australian flora.
Thick regoliths have not developed beneath quartzose and limey sediments in landscapes that have been deeply weathered The predominant weathering form in quartz sandstone terrains, such as the eastern Kimberley and Arnhem Land Plateau, has been solution weathering. The result has been the development of landscapes of a karst type in which the form of weathering is mostly solution channels forming along structural weaknesses in the rock. In the Bungle Bugle Ranges (Young, 1986) and the Ruined City in Arnhem Land (Jennings, 1983), both extreme examples, the result of weathering is the formation of tower karst landscapes. With the exception of some valleys containing quartz sand soils, there are minimal amounts soils in this type of landscape. The marine limestones beneath the Nullarbor Plain is a karst landform with discontinuous drainage that is typical of limestone terrains, in spite of the comparatively short period it has been exposed to terrestrial weathering. Thin calcareous loams, with extensive calcrete development, are the principal soil type on the Plain (Northcote & Wright, 1982).
An intensification of the atmospheric circulation pattern that was similar to that of the present, that led to the development of mid-latitude dry deserts, resulted from the expansion of Antarctic ice sheets and the associated abrupt drop in sea surface temperatures, that occurred at the close of the Miocene. There was a transition over the next 3 million years of landscapes and vegetation from the humid climate of the Tertiary to more arid conditions in the Quaternary (Frakes et al., 1987). At about 2.4 Ma, evidence has been found from widespread parts of the continent, of alkaline weathering associated with post-Tertiary aridification. Sine then, warm and cool climate phases have been associated with widespread formation of alluvial and lacustrine deposits and aeolian activity, with alluviation and erosion in highland areas, and Hillslope instability.
Before the last glacial-interglacial cycle, that began about 120,000 years ago, the record is not good for the Australian Quaternary, but the record for the last cycle is much better. Bowler (1982), Wasson & Clark (1987) and Frakes et al. (1987) have described landscape and regolith development over this period. At the start of the last cycle, between 120,000 and 60,000 years ago, climates and landscapes were similar to those of the present. Stabilised longitudinal dunes dominated inland Australia (Frakes et al., 1987). Fine sediments were carried from the Eastern Highlands across the Murray Basin along westerly flowing major streams, and dry lake beds occurred scattered across the landscape. When the ice began to accumulate in the Northern hemisphere, between about 60 and 36 Ma, there was an increase in runoff from the Eastern Highlands, coarser sandy sediments being carried across the Murray Basin, and the inland lakes filled (Bowler, 1978). The longitudinal dunes were reactivated at this time during a brief drying phase (Bowler & Wasson, 1983), though wetter conditions soon returned, continuing until about 25,000 years ago, with the return of cooler, drier conditions.
Much of central and western Tasmania was covered by glaciers from about 30,000 - 15,000 years ago, during the last glacial maximum, as was a limited area around Mt. Kosciusko. Glacial landscapes and moraines remained when the glaciers retreated. At this time the rainfall on the inland parts of Australia decreased (Bowler & Wasson, 1983). The longitudinal dunefields, that had been stabilised, were reactivated by the increased wind velocities that occurred during the dry period, the dunes reaching their present dimensions. These dunefields formed downwind of major rivers, which were the sources for the components of the dunes. This process even occurred in places in the high country, such as Canberra. Clay-rich lunettes formed on the leeward edge of inland lakes, as well as in Tasmania (Frakes et al., 1987). In Tasmania, dunefields formed in many coastal areas at this time. In the arid inland areas of the continent, as the lakes dried, the clays that deflated from them formed lunettes containing increasing amounts of salt and gypsum. Blankets of dust and salt were deposited in many parts of Australia as a result of the strong winds of the period. It was at this time that many of the desert loams and parna (loess) formed. In the high country they were added to the soil profiles (Walker et al., 1988).
Hillslope erosion occurred over wide areas of the Eastern Highlands during these cool, arid periods, and alluvial fans formed, as well as the alluviation of valleys with gravels and sands. After about 15,000 years ago the hillslopes were relatively stable, and deposits of fine alluvium formed in the valleys (Walker & Butler, 1983). The pedzolic and earth soils on these sand and gravel deposits, and prairie soils that formed on fine alluvium, were mostly reds and yellows.
Alkaline weathering conditions were produced in the arid areas during the arid phase, as occurred in the other cycles of the Quaternary. Expanding clays with high cation exchange capacity were eroded from weathering profiles, that had formed in the Eastern Highlands under different conditions, and deposited on the adjacent lowlands in increased amounts. The result was the soils typical of much of Australia at the present, alkaline, saline and cracking clays. Uniform red siliceous sands were produced from the dunes. It could be expected that the vegetation would have been impacted by the change from leached acid soils to soils that were alkaline, carbonate-rich and salt-rich that occurred in the Quaternary. The impact would have been compounded by the climatic instability and changes in the availability of water.
Sea level oscillations of 100-200 m were associated with the climatic oscillations of the Quaternary. The complex set of coastal environments and landscapes of the present are the result of the changing sea levels of the Quaternary. It was during this period that many of the large coastal plains were formed, especially since the glacial maximum. It at this time that the complex soil landscape patterns of the complex coastal dune systems were developed.
The climatic conditions of the present developed after the last glacial maximum, becoming established by about 10,000 years ago. The sealevel fluctuated throughout the Quaternary, but by about 6,000 years ago had stabilised at about the present level.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|