Last glacial “coastal” dunes in Eastern Australia and implications for landscape stability during the Last Glacial Maximum

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

Last glacial "coastal" dunes in Eastern Australia and implications for landscape stability during the Last Glacial Maximum Bruce Thorn a, Patrick Hesp ~,1, Edward Bryant b "Department of Geography, University of Sydney, Australia bDepartment of Geography, Universityof Wollongong, Australia Received 26 January 1993; revised and accepted 24 February 1994

Abstract

The Last Glacial Maximum or LGM (25,000-15,000 yr B.P.) has been recognized in Australia as a period of increased dryness, coolness, continentality and windiness compared to earlier and later periods. Enhanced aeolian activity during the LGM occurred in arid and semi-arid regions of western, central and eastern Australia. The east coast has been considered to have been better watered and vegetated. However, at a number of sites from Tasmania to North Queensland, there is evidence for extensive aeolian instability of coastal sand deposits. Dating by radiocarbon and thermoluminiscence techniques has supported morphologic and stratigraphic evidence of dune formation during the LGM under the influence of westerly (offshore) winds in the southern sector of the east coast (i.e. south of 31°S latitude) and southeast winds to the north. It is now apparent that vegetation cover on sandy surfaces was quite patchy during the LGM. Sand surface instability under conditions of strong west or southeast winds promoted linear and/or parabolic dune development. This suggests greater concentration of forests in more discrete, protected sites along the eastern escarpment than was previously considered by palaeoecologists. More widespread drier and cooler climatic conditions operated even in coastal regions on expanded continental shelves at this time. Stabilization of areas of active dunes became more likely as sea levels rose, reduced windiness occurred, and precipitation increased as sea surface temperatures began to rise in the Holocene.

I. Introduction

Dunefields now fully stabilized by vegetation occur on m a n y humid coasts. Cooper (1958) has described the morphology of stabilized coastal dune types and hypothesized on their origin (see also Hesp and Thom, 1990). However, relatively little work has been undertaken on the age of those coastal dunes whose orientation and morphology may not be consistent with present-day wind patterns and dune-forming conditions. Recently, Carver and Brook (1989) reviewed the 1 Present address: Department University of Singapore.

of Geography,

National

0031-0182/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved S S D I 0031-0182(94)00018-4

evidence for late Pleistocene palaeowind directions based on an analysis of dunefields along the Atlantic Coastal Plain, USA. Although they concluded that these new stabilized dunes "do not necessarily indicate arid conditions", they pointed to environmental conditions which permitted the localized free movement of sand at m a n y sites. A similar problem exists along the humid coast of eastern Australia. The Last Glacial Maximum, or L G M (25,000-15,000 yr B.P.), has been the focus of considerable geomorphological, palynological and archaeological attention in Australia over the past 20 years, embracing a diversity of fieldwork in a variety of environments (Thorn and Wasson, 1991;

230

B. Thom et al./ Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

Dodson, 1992). Several studies in the 1970's and 1980's were directed to this period when aridity, coolness and continentality were at their greatest (see Bowler, 1976, 1983; J. Hope, 1983; G. Hope, 1989; Chappell, 1991; Colhoun, 1991). The extent of arid and semi-arid environments in Australia around 18,000-22,000 yr B.P. was very much greater than earlier in the Last Glacial, or during the Holocene (Bowler, 1976; Wyrwoll, 1979). Sea level was lowered to a level of - 1 2 0 m to - 1 5 0 m (Chappell and Shackleton, 1986; Shackleton, 1987) and the degree of "continentality" was markedly increased such that the area of Australia and New Guinea was increased by up to 25% (Williams, 1984; Colhoun, 1991). Indications are that sub-tropical high pressure systems formed during full glacial conditions were more intense with increased westerly wind velocities affecting a much greater area than at present (Sprigg, 1959, 1979; Galloway, 1965; Bowler, 1976; Wasson, 1986). Temperatures were lower than at present by between 4-7°C (Bowler et al., 1976) and precipitation for most regions was lower by up to 50% of present values (Colhoun, 1991). The last major lunette building episode has been dated to between 20,000 and 15,000 yr B.P. across a wide area of the continent, and there is evidence that desert dune expansion took place over a similar period (Williams, 1973; Bowler, 1976; WyrwoU, 1979; Jennings, 1975; Wasson, 1983; Sprigg, 1978, 1979; Nanson et al., in press). Lake levels were at their lowest or even dried up during this period (Bowler, 1983, 1986; Singh, 1983; Singh and Geissler, 1985; Kershaw, 1983). Increased aerosol transport took place across the continent and into adjacent oceans on both west and east coasts (Thiede, 1979; De Angelis et al., 1987). The impact of the Last Glacial Maximum has not been investigated in the coastal zone of the eastern sector of the Australian continent. Of course much of the land of this zone is now beneath the waters of the continental shelf. This area was drowned and partially reworked by transgressing seas from depths of over 120 m during the Postglacial Marine Transgression which terminated approximately 6000-6500 yr ago (Thom and Roy, 1985; Chappell, 1991). In recent years coring of shelf "lakes" in Bass Strait and the Gulf of

Carpentaria has revealed new information about environmental conditions during times of lower sea level (e.g. Torgenson et al., 1988). However, exposed coastal deposits of the Last Interglacial and previous interglacials were subjected to full glacial climatic conditions and reworked by terrestrial processes; in many areas these deposits have not been greatly modified by postglacial marine or terrestrial conditions except to become vegetated. Dune and barrier systems of the southeast and eastern coast, from Tasmania to far north Queensland, contain fragmentary records of environmental change which must now be examined in relation to our knowledge of changes in glacial, periglacial, riverine, lake, playa/lunette and desert dune environments of the mountains and the interior of the continent.

2. Port Stephens-Myall Lakes area during the LGM

2.1. Background The area that has received most attention along the eastern coast in terms of reconstruction of a palaeo-environmental history is the stretch of coast between Newcastle and Seal Rocks, the so-called Port Stephens-Myall Lakes area (see Thom et al., 1981; Thom et al., 1992). Fig. 1 depicts the location and general morphology of this embayed section of coast with drowned river valleys infilled to varying degrees with Quaternary sediments (see Roy and Thom, 1981, 1991, for a broad view of the evolution of the shelf and coast of New South Wales during the Tertiary and Quaternary). Barriers of pre-Holocene age are well-developed in this region (Inner Barriers) and have been viewed as Last Interglacial in age (equivalent to Stage 5e) based on the interpretation of U-Th and amino acid racemization dates of corals and shells (see Thorn et al., 1992, for details on the geochronology of the area). The Pleistocene barriers contain deeply leached soils with thick B horizons of humate-impregnated sand extending below sea level. Radiocarbon dating of the humate has yielded a history of pedogenesis consistent with the age of barriers being Last Interglacial (Thom

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B. Thom et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

et al., 1992). However, new TL ages from the Forster-Tuncurry area to the north suggest that parts of some Inner Barriers may be both younger than Last Interglacial Stage 5e and of Penultimate Interglacial age (Roy et al., 1992). The surface morphology of the Inner Barriers in places has been reworked by aeolian processes. The primary ridge and swale topography of relict foredunes (formerly termed sand beach ridges) which reflects the progradation of the barrier during a period of sea level at or above present sea level, has been partially reconstructed into a complex of linear and parabolic dunes oriented c. west to east (see Fig. 2). At present the dunefields on Inner Barriers are well-stabilized by either heath or low woodland dominated by Eucalyptus, Angophora and Banksia tree species. There is no evidence of aeolian instability. Swales are commonly water-logged with thin peat lenses accumulating under a wet heath or swamp vegetation (Myerscough and Carolin, 1986). In contrast, the mostly vegetated Outer Barriers of Holocene age possess some active dunes whose morphology is determined by variable coastal and offshore winds (Thorn et al., 1992). Although westerly (offshore) winds can be important in moving dune sand on the Outer Barriers, the predominant onshore winds associated with coastal storms from the east and southeast are mainly responsible for transporting beach and foredune sand landwards to form the massive mobile dune sheets of Newcastle Bight and Fens (Fig. 1) (Hesp and Thorn, 1990). The question then arises as to how and when the surficial dunes of Inner Barriers, so well vegetated today, were formed. Indirect radiocarbon evidence suggested a late Last Glacial age, but until thermoluminescence (TL) dating became available, the ages were essentially estimated (Thorn et al., 1981, 1992). The orientation and morphology of the dunes strongly suggests westerly flow at the time of formation. This orientation agrees with dune orientation inland at a similar latitude (32°S) (Bowler, 1986; Chappell, 1991). It is also consistent with dunes of similar character observed in other coastal areas of southern Australia including Gippsland by Bowler (1986, fig. 5.1) and Chappell and Thorn (pers. comm.); in the Coorong-Naracoorte-Ouyen region of South

Australia (Sprigg, 1976, 1979); and by Sprigg (1976) and Bowden (1983) in northeast Tasmania. Although Bowler (1986) recognised the existence of dunefields along the coast in the late Pleistocene he was not sure of their age or able to speculate on their palaeo-environmental significance. A more widespread view as recently stated by Dodson et al. (1992, p. 118) is that "At the glacial maximum forest was limited to the eastern seaboard and in the south to areas now mostly offshore", implying extensive coastal vegetative coves during the LGM. This paper will argue that significant landscape instability, especially aeolian activity, occurred throughout eastern Australia during much of the LGM. Evidence for this comes mainly from the Newcastle Bight-Myall Lakes area; however, a range of dune morphologies supported by some dates from southeastern Tasmania to northern Queensland appears to substantiate the conclusions.

2.2. Morphology of the dunefields Three sites have been identified within the Newcastle-Myall Lakes region as containing dunes considered to be Last Glacial in age. These are areas on the Inner Barrier in Newcastle Bight, the dunes in the Tomaree Hills area, and the Inner Barrier in Upper Myall-Broadwater and Eurunderee embayments (Fig. 1).

Newcastle Bight Inner Barrier The Inner Barrier of the Newcastle Bight embayment attains a maximum length of 32 km measured from its southwestern tip where it is tied to a Permian bedrock "headland" to an outcrop of Carboniferous volcanics in the northeast. The barrier decreases in width from 5-6 km northwards to 3-4 km. The general elevation of much of the Inner Barrier is c. + 10 m MSL. However, there is considerable variation in relief with dune heights ranging from a maximum of 60 m to a minimum of around 5 m (see Thom et al., 1992, for more detail ). At its eastern end, the Inner Barrier is characterized by a relict foredune plain. The location and trend of ridges is shown in Fig. 1. The maximum

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Fig. 2. Pattern of dune ridges on the western portion of the Pleistocene (Last Interglacial) barrier in Newcastle Bight. Linear and parabolic dunes reflect west to east sand transport on a partially vegetated sand surface during the Last Glacial Maximum.

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B. Thomet al./Palaeogeography,Palaeoclimatology,Palaeoecology111 (1994) 229-248

length of the ridge pattern is 15 km. In width, the plain nowhere exceeds 3.5 km. The trend of the ridges generally parallels the present coast. The ridge pattern to the west is obscured by east-west trending dunes, which post-date relict foredune formation. At the western or Tomago end, the Inner Barrier of the Newcastle Bight embayment is characterized by hummocky ridges of sand (Fig. 2). These ridges are covered by a dune woodland community which is typical of well-drained, highly podzolized soils. Soils of the dunes have been classified as Type 3 soils by Thom et al. (1981). These soils are typified by A1 horizons up to 0.5 m thick, moderate to deep, light grey A2 horizons (1-3 m), and dark brown to black humic-rich, deep Bh horizons (+ 5 m). Sand size ranges from fine to medium, and is predominantly quartzose in composition. The barrier surface at the western end is an aeolian reworked sand plain initially formed by beach progradation and the formation of foredunes. Although foredune-ridge morphology is not visible on the ground or on aerial photographs, the alignment and structure of heavy mineral seams beneath the surface confirms the beach progradation origin of the western part of this Inner Barrier. Fig. 2 indicates that the sand ridges on the reworked sand plain form a series of elongated dunes trending east-west for the most part, although there are some at the extreme westem end which trend more northwest-southeast. They occur mainly on the periphery of the western part of the Inner Barrier and cut across the northeastsouthwest trend of foredune ridges where the latter can still be seen at the eastern end (Fig. 1). A detailed study of the morphology of 26 of these vegetated dunes has shown that most exceed 3 m in height and range from 500 m to 3000 m in length (Thom et al., 1992). Smaller undulations also trend east-west. Steepest slopes are on the southern side of the barrier bordering the interbarrier depression, with maximum slopes of 25-28 °. The dune ridges taper distinctly from a broad western end to a sharp eastern point. In plan they have the appearance of a string of beads. Crest lines rise and fall by as much as 10 m in descending to low, relatively broad cols separating rounded hillocks. Many ridges are quite linear, but others

possess a more arcuate plan shape open to the west. This is particularly so of two large dunes on the north side of the barrier. These features could be regarded as parabolic dunes. The Inner Barrier extends eastwards to a prominent 60 m high bedrock hill at Lemon Tree Passage overlooking Port Stephens (shown as L T P on Fig. 1). In this vicinity the bedrock is mantled by a complex of sand dunes which climb the bedrock and also extend around the base of the hill. Type 3 soil profiles occur in these dunes (Thorn et al., 1981). Tomaree Hills

The Tomaree Hills area is characterised by a cluster of prominent bedrock hills (Carboniferous volcanics) at the southeast end of Port Stephens (Fig. 1). The area varies in relief from lowland swamps and sand plains in the western part, to rocky peaks partially enveloped by aeolian sand in the east. In the western part of the segment, the sand-swamp surface is generally within a few metres of MSL. However, to the east, the terrain mostly lies above 30 m with numerous bedrock hills exceeding 100 m in elevation. Sand dunes of Tomaree are all well vegetated (Eucalyptus pilularis, Banksia serrata dominant) and often enclose Melaleuca-rich swamps. In the flatter, western portion of the Tomaree segment a distinctive west to east alignment of ridges is observed, forming a complicated maze of curved ridges. The characteristic arcuate forms have the appearance of a degraded parabolic dune complex. Individual ridges over-ride and merge with one another across a broad sand plain. At the time of dune formation, the enclosed depressions were deflation surfaces; the depressions are now flooded and were active when sea level was lower, becoming "drowned" during the Postglacial Marine Transgression thus forming "water-table window lakes". The dune sands are well-podzolized (Type 3 soil, Thom et al., 1981). An origin as nested parabolics involving the reworking of older Pleistocene sands is suggested for this dune field with effective winds being from the west. In the eastern portion of the Tomaree Hills, prominent sand ridges rising to -t-50 m MSL, wrap themselves around the eastern and southern

B. Thom et al./Palaeogeography, Palaeoclimatology,Palaeoecology111 (1994) 229-248

sides of bedrock peaks. Many of these ridges enclose depressions against the bedrock hills, and in the "valleys" between the hills. Elsewhere the bedrock is mantled by a veneer of dune sand except along the rocky shorelines and summits of peaks. Locally, the sand surface between dune ridges is swampy. An interdune surface slopes to the west where it merges with the western Tomaree dune-swamp complex. The Upper Myall-Broadwater embayment The Upper Myall valley occupies the trough of the MyaU Syncline of Carboniferous age, and accordingly follows the general northeast-southeast strike of the region. The bedrock valley is quite narrow averaging 4 km in width. Fig. 1 shows how this valley opens out into an interbarrier lagoon enclosed by the Outer Barrier. The Upper Myall valley and the eastern and western margins of the lake are bordered by Inner Barrier relict foredune plains. The Inner Barrier that formed along the northeast shore of Broadwater Lake is interrupted at its southern end by a parabolic dune complex. This complex extends into the adjacent Eurunderee embayment. The dunes are nearly 2 km in length and are aligned west-east. They consist of two narrow ridges up to 10 m in height, forming a nested set, one inside the other. Both enclose water-filled depressions. These have formed on the deflation surface of a pre-Holocene parabolic dune as water-table window lakes. Sedges and Melaleuca trees occur within these depressions. Sediments consist of well-sorted quartzose sands which are deeply leached. Drilling on the parabolic ridge 10 m above the foredune ridge plain revealed a well-sorted, leached quartz sand, medium to fine in texture overlying a humate-impregnated sand 11 m below the surface. Dateable material at the contact was not encountered. The source of these aeolian sands appears to originate from the deflated Inner Barrier surface.

235

Soils Last Interglacial (ca. 120,000 yr B.P.) and Holocene landforms in the region can be inferred by the degree of soil development (Thorn, 1965; Thorn et al., 1981, 1992). Last Interglacial soils developed on low, swampy relict foredune plains (Type 2 of Thom et al., 1981) tend to display deeply leached light grey A2 horizons and deep, semi-indurated B horizons (Bh to 10 m) containing thick humate rich columns or concretions. In contrast, early Holocene ( ~ 6000-9000 yr B.P.) moderate to high relief dune soils (Type 4 of Thom et al., 1981) tend to display somewhat shallower A horizons, moderate B horizons (3-4 m) with incipient columns and pipes varying in colour from light yellow to dark brown. Soils of the dunes described above at Newcastle Bight, Tomaree and Broadwater are defined as Type 3, and display deep (to 3 m), leached, light grey A2 horizon and deep B horizons (+ 5 m) with dark brown to black, humic-rich distinct columns containing leached white sand pipes up to 10 cm in diameter (Thom et al., 1981). The degree of soil development indicates that the dunes therefore straddle a temporal zone between the Last Interglacial and the early Holocene.

2.3. Age of the dunefields

Stratigraphic position The linear and parabolic dunes and aeolian sheets on the Inner Barrier at Newcastle Bight overlie a Last Interglacial relict foredune plain surface (Thom et al., 1992). The boundary separating the Last Interglacial relict foredunes/strand lines and the aeolian dune sheet is relatively sharp. In addition, drilling within the dune sheet has revealed the continuation of swash zone placer concentration of heavy minerals and the thick groundwater podzol within the Last Interglacial relict foredunes extends beneath the linear dune field (Thom et al., 1981; Thom et al., 1992). Similarly, the stratigraphic and spatial position of the parabolic dune complex adjacent to Broadwater Lake indicates that it, in part, overlies, and has reworked the Last Interglacial relict foredune plain.

A variety of evidence and dating methods have been utilised to determine the evolution and age structure of the dune terrains described above.

Thermoluminescence and radiocarbon ages The chronology of the dunes in the Newcastle Bight-Tomaree area were determined using five

B. Thorn et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

236

thermoluminescence (TL) dates supported by two radiocarbon determinations. TL dating is based upon the measurement of TL energy acquired by crystalline minerals while they are buried within a sedimentary unit. The method therefore requires that during transport prior to burial most, if not all, of the previously accumulated TL energy is removed by exposure to the solar ultra-violet component. Following deposition, TL energy once again builds up at a rate dependent upon the radiation flux delivered by the long-lived isotopes of uranium, thorium and potassium found in most sediment, with the fluxes from rubidium and cosmic radiation playing a lesser role. The technique used here is essentially the combined regenerative additive method of Readhead (1984) for the 90-125 pm quartz-grain fraction. A more complete description of the technique as used in the New South Wales coastal environment is presented in Bryant et al. (1990). The field collection of samples and laboratory analysis are identical to those reported recently in this journal (Nanson et al., 1993). Briefly, samples were collected at least 30 cm from stratigraphic or soil horizon boundaries and immediately wrapped in black plastic. Sample radiation dose levels were measured using calibrated thick-source alpha counting and atomicemission spectroscopy with secular equilibrium in the decay chain assumed. A cosmic ray contribution of 150 uGy yr-1 was assumed. As the radiation flux is partly absorbed by groundwater, estimating the long term average water content of sediment can be problematic. Water contents were

assessed as being equivalent to the moisture content of each sample at the time of sampling. Extended age plateaux over the 325-475°C temperature range indicate that partial bleaching was not a problem; however, surface corrections could not be applied to the ages because no modern recently transported sediments were available at the sample sites. Check dating for similarly aged aeolian sediments along this coast suggests that this correction is no more than 2 Ka (Bryant et al., 1992). The TL ages reported should thus be considered maxima. At a site near Williamtown the TL ages range from 17.7+3.7 Ka for the A2 horizon, 20.3+5.6 Ka for the B horizon in a soil profile at the centre of the site, to 30.0+5.7 Ka for a B horizon in the northern section of the site (Table 1). The northern B horizon age could be a stratigraphically slightly older deposit. Two radiocarbon dates on freshwater peats nearby, buried by advancing linear/ parabolic dunes, provided ages of 11,300+250 yr B.P. (Gak-1700) and 13,700___600 yr B.P. (Gak-1701 ). These indicate that some dune activity was taking place towards the end of the Pleistocene. The dunes in the Lemon Tree Passage area have been TL dated in a range from 16.1 + 3.6 Ka to 20.5 ___3.4 Ka (Table 1). Radiocarbon dates from the Tomaree Hills area on organic materials which have been buried by dune sand have dated as "background". A thermoluminescence age on dune sand sampled at Nelson Bay, however, provided an age of 14.2__ 1.6 Ka. At nearby Shoal Bay an apparently stratigraph-

Table 1 Thermoluminiscence ages and sites Newcastle Bight-Tomaree areas Sample no.

W1007 W1008 W1009 W1010 Wl011 WI012 W1013 W1014 W1015

Location

Lemon Tree Passage Lemon Tree Passage Lemon Tree Passage Lemon Tree Passage Nelson Bay Shoal Bay Williamtown Williamtown Williamtown

Dune type

dune mantle dune mantle dune mantle dune mantle dune mantle dune hillslope linear ridge linear ridge linear ridge

Paleodose (Grays)

8.0 ___1.3 16.3 + 1.5 10.4+0.8 9.8 + 1.2 23.3-+2.3 200+43 8.8+ 1.3 11.0___2.6 13.5 -+ 1.1

K (%)

0.05 0.05 0.09 0.10 0.35 0.14 0.05 0.08 0.07

Specific activity Th and U chains (Bq/kg)

Annual Dose

Standard deviation

(u Grays)

TL age (x 1000 yr)

14.0_+4 11.7_+4 12.9_+4 15.3+4 56.3+4 47.2+4 14.3+4 15.5_+4 10.3+4

495+76 452+77 507+75 562+75 1640+75 1202 + 72 501___77 543 ___75 443+76

16.1 36.0 20.5 17.4 14.2 168 17.7 20.3 30.5

+3.6 +6.9 +3.4 +3.2 +1.6 + 37 +3.7 + 5.6 +5.7

B. Thorn et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229~48

ically older dune deposit provided an age of 168_+37 Ka. As noted earlier, and discussed in detail by Thorn et al. (1992), the Lemon Tree and Tomaree areas are very complex, the topography and stratigraphy indicating the existence of a suite of dunes in the region which ranges from early Holocene to the penultimate Glacial in age.

3. Other dunefields in eastern Australia

Fig. 3 shows sites which contain dunes of possible Last Glacial Maximum age in eastern Australia (see also Table 2). Two different types of sites exist: first, there are those in the coastal zone where the dunes have formed from sand deposited during previous higher and lower sea levels; and second, there are those where dunefields represent reworked source-bordering lake or river sands, or mobilisation of weathered sands from sandstone or granitic sources. Of all the sites listed in Table 2, only seven have ages which clearly indicate that the sands were mobile during the LGM. This includes an area in southeast Tasmania, the areas discussed above near Newcastle and Myall Lakes, an area offshore of Evans Head, the northern Queensland dunes, the Lake George area on the Southern Tablelands in New South Wales and the sites recently described by Nott and Price (1991) those in the Southern Tablelands of New South Wales. For all the other sites, the dunes possess morphological and pedological characteristics, and in some cases indirect chronological evidence, which links them to an event(s) during the Last Glacial Maximum. There are also sites bordering coastal rivers which possess dunes which could be of LGM age or even older. It is not the purpose of this paper to describe each site in detail, but to indicate what features exist in support of the hypothesis that aeolian instability occurred in certain locations during the Last Glacial Maximum along a vast stretch of the eastern part of the continent. A variety of sand sheets and low relief dunes occur on the elevated river terraces and lower hillslopes throughout the lower Derwent Valley near Hobart in southeast Tasmania (Sigleo and Colhoun, 1982). Radiocarbon dating, together

237

with stratigraphic and archaeological work indicates that these aeolian deposits were formed by northwesterly winds in a drier climatic phase during the Last Glacial Maximum. In coastal northeastern Tasmania, there are extensive linear, sub-parabolic and parabolic dunefields and lunettes, the sediments of which could have been derived from last Interglacial marine sands and Last Glacial alluvial deposits (Sprigg, 1979; Bowden, 1983; Colhoun, 1983). The dunes lie in a west-east orientation, and although they have not been dated, morphologic and stratigraphic work indicates that they are Last Glacial in age (Sprigg, 1979; Bowden, 1983; Bowden and Colhoun, 1984). Similar dunefields occur on Flinders Island (Sigleo and Colhoun, 1982). Deflation basins and dunes also occur on Bruny Island in Tasmania and may be of Last Glacial age (Colhoun, pers. comm.). On King Island in Bass Strait, new and old parabolic dune systems dominate the morphology of the west coast and are orientated west-east (Jennings, 1959). Several of these dunefields occur as cliff-top dunes (Jennings, 1967). Whilst Jennings (1959) considered most of the old dunes were formed during high and falling sea levels prior to the Holocene, some of the cliff-top dunes and dunefields were possibly formed during the low sea level stand in the Last Glacial Maximum (Lampert, 1983; J. Hope, 1983). On Kangaroo Island in South Australia, Pleistocene consolidated and calcrete covered aeolianites extend west-east at orientations up to approximately 45 ° different from that of the Holocene coastal dunes (Sprigg, 1979). Adjacent lunettes also display orientations which indicate the influence of westerly and northwesterly winds during formation. Sprigg (1979) states that both the Pleistocene coastal dunes and lunettes are glacial phase deposits. Between the Mt Lofty Range in South Australia, the Murray River and the Great Dividing Range in Victoria lies an extensive system of coast-parallel barrier sequences which extend in age from Pliocene to Holocene (Sprigg, 1959, 1976, 1979). These barriers have been extensively modified and buried in places by aeolian sheets, which, according to Sprigg (1976, 1979) were formed during glacial

B. Thorn et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

238

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B. Thom et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

239

Table 2 Dunefields of possible last glacial age in Eastern Australia Site

Dune-forming wind

Age (yr B.P.)

Reference

Coastal Derwent R. SE Tasmania NE Tasmania

W W

20,000-14,000 ND

Flinders Island King Island Kangaroo Island L. Wellington, Gippsland Bairnsdale-Dutson Downs

W W W W W

ND ND ND ND ND

Newcastle-MyaU Lakes Pt Macquarie-Crowdy Head Evans Head Evans Head (offshore) Cooloola Fraser Island Shoalwater Bay Cape Bedford/Cape Flattery Shellburne Bay Lizard Island

W SW SE ? SE SE SE SE SE SE

31,000-12,000" ND ND 18,000 ND ND ND 24,000-18,000" 29,000-17,000" ND

Sigleo and Colhoun, 1982 Sprigg, 1979 Bowden, 1983 Sigleo and Colhoun,1982 Jennings, 1959 Sprigg, 1979 Chappell, pers. comm. Bowler, 1986 Jenkins, 1968 this paper (Table 1) this paper this paper Colwell and Roy, 1983 Thompson, 1981 Ward, 1977 Thompson, 1981 Lees et al., 1990 Lees et al., 1990 this paper

Inland Shoalhaven catchment Lake George region

N W

19,000"* 23,000-16,000

Nott and Price, 1991 Coventry and Walker, 1977

Hunter River

W

ND

Galloway and van de Graft, 1962

14C dates unless otherwise stated *Th and 14C dates **All TL dates ND: not dated

low sea level events under a strong westerly wind regime. Extensive marine and fluvial sand deposits of late Tertiary and Pleistocene age occur throughout east Gippsland where they have been variously mapped by Jenkin (1968) and others as marine, aeolian and fluvial features. Jenkin (1968) made several references to the modification of preexisting landforms by aeolian action such as at Sperm Whale Head on what could be interpreted as a Last Interglacial barrier (p. 86), and in the Gelliondale area (p. 95 and his plate V). Crescentshaped ridges in the eastern or downwind side of new well-vegetated deflation basins were interpreted as "lunettes" and compared with similar fea-

tures from more arid parts of Victoria (Hills, 1939; Sprigg, 1979). Unpublished field studies by Bowler, Chappell and Thom support the interpretations of Jenkin (1968) that pre-Holocene sandy terrain in east Gippsland was extensively modified by strong westerly winds during a more arid climatic phase than exists at present. Aeolian dunes are also found on the terraces of the Hawkesbury River (Simonett, 1950), and the lower Hunter River which enters the sea at Newcastle (Fig. 3); these dunes may have formed during the Last Glacial Maximum (Galloway et al., 1962) or be even older. Whilst modem sourcebordering dunes are forming in, or adjacent to some of the east coast river systems at present

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B. Thorn et al./ Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

(Page, 1971), the presence of vegetated dunes suggests that Last Glacial Maximum aeolian sand transport was possible in coastal river valleys in eastern Australia. The mid north coast of New South Wales in the vicinity of Crowdy Head and Point Plomer (Lat 31020' ) contains extensive Inner Barrier deposits of assumed Last Interglacial age. Aerial photographs of both sites show the existence of linear and sub-parabolic dune ridges which cut across the trend of degraded foredune ridges. The dune ridges trend WSW to SW in these areas. No ridges with similar trend occur north of Point Plomer or Crescent Head. However, large well-vegetated parabolic dunes occur on Pleistocene barriers near Hat Head (Lat 31003') with a more southerly trend, and farther north at Evans Head (Fig. 3) there is a more S to SE trend to the parabolic dunes. All these dunes are well-podzolized and contain a vegetation cover with elements in common with the late Pleistocene dunes of the Newcastle area. Drilling on the continental shelf off Evans Head at a depth of 53 m below MSL has encountered plant root material dated 18,070 ___280 (HV-10783, Colwell and Roy, 1983; see also Kudrass, 1982). They considered the material found in muds and muddy sands to have been "probably deposited in a perched interdune swamp which was subsequently at least partly infilled by dune sands blown in from the surrounding countryside" when the surface was well above contemporary sea level (Colwell and Roy, 1983, p. 5). Large sand accumulations occur on the coast and as dune islands in southern Queensland as at Cooloola and North Stradbroke, Moreton and Fraser Islands, and at Shoalwater Bay (Thompson, 1981; Ward, 1977). These dune systems consist of nested sets of SE to NW oriented parabolic dunes. Studies indicate that at least eight periods of dune building have taken place over a period that may extend from before Last Interglacial through to the late Holocene (Thompson, 1981; Tesan-Kella et al., 1990). Further north, at Cape Bedford, Cape Flattery and Shellburne Bay (Fig. 3), parabolic dunes, oriented SE-NW, form nested complexes which range

in age from late Holocene to possibly pre-Last Interglacial (Lees et al., 1990). Three dunebuilding events have been positively identified at 2600-1800 yr B.P., 8500-7000 yr B.P. and 24,000-18,000 yr B.P. The middle event is possibly related to coastal erosion and initiation of dune mobilisation by the postglacial marine transgressing seas as proposed by Cooper (1958), Thom (1978) and Pye and Bowman (1984). The 24,000-18,000 age dunes appear to represent a period of dune formation during the Last Glacial Maximum when, with lower sea levels, sands present on the exposed continental shelf were available for dune building (Lees et al., 1990) in a more arid and/or windier climate regime. Vegetated dune sands on Lizard Island in far north Queensland could also have been deposited during the Last Glacial Maximum. This island is now surrounded by Holocene fringing reefs. It is unlikely that the sand was driven over the bedrock during the Postglacial Marine Transgression as this was a period of active reef growth on the Great Barrier Reef (Hopley, 1982; Marshall and Davies, 1984). However, confirmation of the age of these sands and those at many other localities await further dating.

4. Environmental conditions during the last glacial maximum in southeast Australia

4.1. Orientation of dunefields Fig. 4 depicts the likely occurrence of west to east and SE to NW aeolian features in the late Quaternary aeolian landscape of Australia. The existence of continental linear and parabolic dunes with these orientations have been known for some time (Sprigg, 1979; Bowler, 1976, 1986; Wasson, 1986). They form part of the complex anti-cyclonic network of dunes which covers nearly two-thirds of the continent. The relationship of W to E and SE to NW oriented dunes along the eastern coast of Australia (including Tasmania) to other aeolian features of the continent has taken more time to be recognized. Field identification in the more heavily

241

B. Thom et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

J 130"E

120"E

l 140"E

/ 150"E

~10'S

I .... I

ShellburnaBay Cape Flattery

\

20"S

L-20"S

"

Kangaroo I . ~

i ~

Lunetteorientations Orientation of sand sheets and longitudinaldunes Major dust paths East coast linear, longitudinal & parabolic dune orientations

110"E

j

120"E

v

o ooo. SCALE

~

Gippsland

~

130"E

140"E

I

I

Tasmania 0

150"E

/

I '~'s

ti

160"E

/

Fig. 4, Wind trends as reflectedin the orientation of various aeolian features of presumed Last Glacial Maximum in Australia. wooded section of Australia may have inhibited recognition. Nevertheless, there is enough data now available to show west to east oriented dune features of possible L G M age from Tasmania and nearby islands, southeast South Australia (including Kangaroo Island), across Victoria including Gippsland, in the Southern Highlands o f New South Wales, and along parts of the New South Wales coast to as far north as Crowdy Head (latitude 31°25'S). In northern New South Wales extending along the Queensland coast to Cape York are SE to N W dunes which appear to be part of the same aeolian system (Fig. 4). If they

are then they would be linked to an atmospheric circulation pattern covering most of the continent at the time of formation. Although not all the dunes have been accurately dated, they possess strong indications of being formed or reactivated during the LGM. The direction of sand movement suggested by these features is always from the westerly quadrant (270_+45 ° ) in the southern half of the continent (i.e. south of c. 31°S). The features include lunettes, linear and longitudinal dunes, parabolic dunes, deflation sheets, source-bordering dunes, or simply sand sheets to the east of some source area.

242

B. Thorn et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

Fig. 4 suggests that their orientations are consistent in the same latitudinal band, 31°S to 42°S, with desert longitudinal dunes, lunettes and dust paths (Sprigg, 1979; Wasson et al., 1988; McTainsh, 1989). In regard to the desert dunes, Sprigg (1979) noted that a feature of the more southerly Australian deserts was a present-day tendency for minor destruction and or directional modification under the prevailing wind regime. He stated that this was particularly so between 27-38°S latitude where in the subcoastal southeast South Australia, Kangaroo Island, the central Murray Malice, the Lake Eyre Basin and the Simpson Desert, presentday sand drifts cut across the pre-existing fossil dunes (by up to 70 ° in the MaUee). In northern Australia and on the northeast coast of Tasmania, the trends of the sub-fossil and recent dunes coincide. Sprigg (1979) argued from these dune trends (and other data) that the zonal westerlies extended as far north as 27°S during the glacial period and were therefore 10° or more further north than at present. The "roaring forties" were thus the roaring thirties (McTainsh, 1989). Sprigg's (1979) argument for a northerly latitudinal shift in the Sub-Tropical Anticyclone Belt has been supported by Wasson (1986), who considers that a 5° northerly shift would explain anomalies in the divergence between longitudinal dune orientation and modern wind resultants in the southern Mallee and southern Strzelecki dunefields. In southeast Australia, from both inland and coastal locations, it is inferred that aeolian activity was mainly driven by the zonal westerly wind circulation during the LGM. Dating on the coast near Newcastle and on the Southern Tablelands (Nott and Price, 1991; see later discussion) suggests localised instability of sandy surfaces at this time. Zonal westerlies appear to have been very effective at least as far north as latitude 31°S, and as far as can be determined at this stage, were considerably stronger as well as being more frequent than at present (Wasson, 1986, 1989). The question then arises as to the nature of atmospheric and soil conditions at this time which made it possible for surface mobilisation and west to east sand transport to take place in southern latitudes and southeast to northwest in more northern latitudes.

4.2. Conditionsfor duneformation The existence of dune features at present-day coastal sites which have a "terrestrial" as distinct from a "coastal" origin is not unusual in Australia. On the humid southeast coast of Australia, linear and parabolic dunes of "terrestrial" origin largely overlie Last Interglacial barrier sands. They represent the localised reworking of those sands. There has been no large-scale spread of aeolian sand from vast inland sand sheets to the coast as there was in northwestern Western Australia (Jennings, 1968). This is to be expected for three reasons, one, the lack of a desert dunefield in immediate proximity to the east coast; second, the localized occurrence of sand sources given the embayed rocky terrain of the Eastern Highlands and adjacent coastal plain; and third, the assumed higher moisture content of soils and greater vegetation cover of the surrounding landscape. Although it now appears that much of the Eastern Highlands was covered by semi-arid grassland and steppe vegetation at the LGM (Hope, 1989; Dodson et al., 1992), palynologists working on vegetation histories for this period have long assumed that the eastern seaboard possessed a forest cover (see Hope, 1989, his fig. 1, and fig. 6.1 of Dodson et al., 1992, p. 121). By implication climatic conditions along the east coast would have generated sufficient moisture to allow tree cover to persist. On the well-drained dune sands of Pleistocene age today a mixture of Eucalyptus, Angophora and Banksia species form a low woodland to forest cover depending on the degree of podzolisation and drainage of soils. Yet for the linear and parabolic "terrestrial" dunes to have formed on the Inner Barriers, these vegetation types must have become disturbed at certain sites. Some plant cover clearly remained as indicated by preserved foredune topography, a relic of Last Interglacial age. However, there is no local palynological evidence to support the hypothesis that the present-day woodland and forest types on the dunes were replaced by patchy grassland and/or heath. Yet for threshold wind strengths at the sand surface to be reached which permitted large-scale sand movement, it is most likely that a heath or

B. Thom et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

woodland or grassland cover would have become quite patchy during the LGM exposing bare sand. Further evidence supporting the existence of a patchy plant cover is derived from the dating of humic colloids (or humate) in the ground-water podzol beneath the Inner Barrier near Williamtown. The dates suggest a hiatus in the accumulation of humate during the LGM (Thom et al., 1992). Water tables are currently perched on the humate (or sandrock as it is locally known) creating swampy conditions in the swales between dune ridges. As humate was less well developed near the surface at 20-25,000 B.P., then water tables were at greater depths in the sand. This would mean less soil moisture to help retain a plant cover during the onset of drier conditions associated with strong westerly winds, and with less plant cover then less organic material was available to supply the deep B horizon of the ground-water podzol (Type 2 soil of Thom et al., 1981). Evidence from a variety of sites from inland and southern Australia clearly suggests that the climate was drier and windier during the LGM, and that by extension eastwards such conditions may have prevailed across the Highlands onto the coast. At Ulungra Springs in the Mendooran area, New South Wales, (~31°50'S, 149°E), on the western margin of the Great Dividing Range, there is some indication that in the period 25,000-10,500 yr B.P. a semi-arid chenopod shrubland dominated the flora (Dodson and Wright, 1989). The pre-existing and presently existing vegetation was, and is, an open forest or woodland dominated by ironbark eucalypts with a heath understorey. A decrease in rainfall of 300 mm/yr and a significant shift of the arid and semi-arid zones eastward is inferred (Dodson and Wright, 1989). McTainsh's (1989) map (following Williams, 1984) of arid zone limits at 18,000 yr B.P. indicates that the arid zone boundary shifted some 250 km eastwards at ,-~32°S and 120 km eastwards at ,~30°S. Dodson and Wrights' (1989) research indicates that the arid boundary would in fact have been 100 km further east of McTainsh and Williams arid boundary line. Presumably the semi-arid zone would have extended eastwards into the Eastern Highlands (see Hope, 1989).

243

At Barrington Tops the vegetation was probably shrubland with patches of woodland prior to 12,000 yr B.P. (Dodson, 1987), and at Lake George grasslands dominated formerly temperate forest regions, and the treeline fell to 670 m asl (at present it is 2000 m) during the period 20,000-8000 yr B.P. (Singh et al., 1981; Singh, 1983; Singh and Geissler, 1985; Coventry and Walker, 1977). Aeolian deposits form extensive sheets on the eastern slopes of the Lake George Basin. They range in age from 23,000 to 1900 yr B.P. with an extensive phase in the period 23,000-16,000 yr B.P. during falling lake levels (Coventry and Walker, 1977). Strong westerly winds are indicated. Aeolian dunes also occur in the upper and middle catchment of the Shoalhaven River on the southeastern portion of the Great Dividing Range (Galloway, 1969; Nott and Price, 1991). The dunes occur across an area of over 800 km 2 and form gently undulating sand sheets, linear dunes, climbing and falling dunes on west facing slopes. TL ages of the dunes suggest that two phases of dune activity occurred over the period 19,000-6000 yr B.P. with a period of stability between 14 and 18 ka. Since the dune sediments are derived from both source-bordering river point bars and granite slope deposits, much drier conditions coupled with a significantly reduced vegetation cover are indicated (Nott and Price, 1991). Some (if not all) of the lunettes and clay sheets found in the CoomaNimmitabel region (Pillans, 1987) may also have been initiated during these times. Hope (1983) has suggested that in southern Australia the treeline was not simply depressed, but that much of the region was unfavourable for closed woodland or forest growth. Precipitation may have been about 40% of present levels and temperatures less than 6°C below present. At Wyrie Swamp near-coastal swamps were dry during the period 25,000 and 10,000 yr B.P. (Dodson, 1977), and at nearby Lake Bullenmerri, a semi-arid vegetation formation similar to that found today in Western New South Wales and northwest Victoria existed by 15,000 yr B.P. (Dodson, 1979; Dodson and Hope, 1983). This indicates a region where precipitation may have been generally less than 400ram/yr. Further evidence for aridity in

244

B. Thorn et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229-248

Tasmania during the LGM is found in the work of Markgraf et al. (1986). In northeast Australia, at Lynch's Crater in the period 25,000-19,000 yr B.P., dryland vegetation was dominated by a sparse cover of sclerophyll vegetation or grassland, and all rainforest had been eliminated from the area (Kershaw, 1978, 1983). Overall, the palaeoecological and geomorphological data, when taken together for the LGM, show eastern Australia, including Tasmania, to be drier, windier and less well vegetated than what preceded it during the late Pleistocene, as well as in the postglacial period (Dodson et al., 1992). 4.3. East coast climate during the L G M

The basic foundation for developing a conceptual model of climate for southeastern Australia, including the coastal zone, during the LGM is the palaeo-environmental evidence for greater aridity and a windier, colder climate as discussed above. The pollen records and the existence of dunefields, in particular, point to localized landscape instability and cold steppe subalpine vegetation. Shrubland-open woodland would have been confined to levels below 600 m at latitudes north of c. 35°S (Dodson et al., 1992). The dunefields suggest a predominance of westerly winds lacking in moisture (summer winds?) with wind speeds far in excess of the present-day westerlies (Wasson, 1989). Southeast wind speeds in northern New South Wales into Queensland may also have been considerably greater than at present as indicated by the development of parabolic dunes up to 4 times the length of contemporary dunes in some areas. The persistence of an intense anti-cyclonic system over central-eastern Australia with steeper baroclinic gradients to south and to north than occured in postglacial times has been suggested by Derbyshire (1971) which could help explain the palaeoenvironmental conditions of the LGM. There are five characteristics of the LGM climate which favour aeolian landscape instability along the east coast: (1) Airflow for most of the time was zonal with stronger west to east winds than occurs at present; it is possible that westerly airflow was intensified

during summer months thus accompanied by high evaporation rates and low rainfall producing conditions inimical to plant growth (cf. Bowler, 1983). (2) The dominant high pressure zone extended eastwards towards New Zealand restricting the frequency of east coast lows which bring rainfall to the coastal zone (Holland et al., 1987). (3) Sea surface temperatures were much lower and gradients of sea surface temperatures and weaker than they are during the Holocene, again inhibiting east coast low development. (4) Global circulation patterns in the lower as well as at upper tropospheric levels could have been inherently more stable during the LGM due to the forcing effect of the more northerly position of the Polar Front, and enhanced baroclinicity between the Front and the Intertropical Convergence Zone. (5) Windier conditions with the probability of increased aridity would have led to increased risk of fires which could have kept forest vegetation and undergrowth thinned and/or restricted to isolated patches. Under such conditions, colder winters and more arid summers with increased moisture stress and stronger westerly winds would have occurred. In conjunction with increased continentality (the coastline at - 1 2 0 m, 18,000 years ago was 20-100 km offshore of the present coastline), this climate would foster a retreat of rainforest and woodlands from sandy surfaces to more moist sites with higher soil moisture content and less exposure to westerly winds. Open woodland sites would have had a lower percent surface cover, and in combination with the increased risk of fires (thinning the undergrowth), would have had a higher potential for exposure of bare sand surfaces and sediment movement. A general conclusion can now be developed from this reconstruction of LGM conditions. Dodson et al. (1992, fig. 6.1) and Hope (1989, fig. 1) show maps indicating forest conditions along the entire east coast. We suggest that forest refugia were much less extensive than depicted on these maps, restricted to small areas within the Great Dividing Range below a considerably depressed treeline, and isolated moist areas along the coast adjacent to rivers near local water sup-

B. Thorn et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 229 248

plies (see Dodson and Thorn, 1992) or in sheltered locations (eg. cliff slopes on the Illawarra Scarp). We conclude that widespread semi-arid environments occurred along the eastern fringe during the LGM where both localised aeolian dune formation took place in a drier, moisture deficient, windier climate. The patchy nature of the evidence along such a vast coastal region, large parts of which are now submerged on the continental shelf, makes it difficult to obtain a complete reconstruction of palaeo-environmental conditions during the LGM. Clearly conditions were sufficiently different than in the Holocene to trigger the initiation and development of large linear and parabolic dunes which can be related to palaeo-wind directions. Many of these dunes became stable from about 12,000 to 15,000 years ago and have not experienced any further instability. It is possible that the process of plant stabilization took place under conditions of reduced windiness, especially from the west, accompanying the deterioration of an intense anticyclonic system as the Polar Front moved south and overall baroclinicity declined in the postglacial period. A similar situation involving a reduction in wind strength may have taken place along the Atlantic Coastal Plain (Carver and Brooks, 1989) where dunes associated with the LGM also became vegetated as watertables rose, flooding the deflation basins in dunefields and further encouraging the spread of more moisture tolerant plants. These processes along coastal plains on eastern sides of continents would have been further facilitated by rising sea levels and a return to more moisturebearing onshore winds as sea-surface temperatures began to rise in the Holocene.

Acknowledgements The authors wish to sincerely thank their colleagues Peter Roy and Mike Shepherd for many stimulating discussions and joint research on the Newcastle-Myall Lakes area, David Price for the thermoluminescence dating, the Departments of Geography, University of Sydney and the University of Wollongong for continued support,

245

Sandra Donnelly for typing and Peter Johnson for cartography.

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