Onset of aridity and dune-building in central Australia: sedimentological and magnetostratigraphic evidence from Lake Amadeus

Palaeogeography, Palaeoclimatology, Palaeoecology, 84 (1991): 55-73

55

Elsevier Science Publishers B.V., Amsterdam

Onset of aridity and dune-building in central Australia: sedimentological and magnetostratigraphic evidence from Lake Amadeus X . Y . C h e n a' 1 a n d C . E . B a r t o n b

aDepartment of Biogeography and Geomorphology, The Australian National University, Canberra, ACT 2601, Australia bBureau of Mineral Resources, P.O.Box 378, Canberra, ACT2601, Australia (Received January 27, 1989; revised and accepted May 15, 1990)

ABSTRACT Chen, X.Y. and Barton, C.E., 1991. Onset of aridity and dune-building in central Australia: sedimentological and magnetostratigraphic evidence from Lake Amadeus. Palaeogeogr., Palaeoclimatol., Palaeoecol., 84: 55-73. Sediments from Lake Amadeus, a groundwater discharge playa in central Australia, comprise a surface playa sequence (the Winmatti Beds), varying between 0.6 and 3 m in thickness, overlying a long sequence of fairly uniform fluvial-lacustrine clays (the Uluru Clay). The latter extends down to at least 15 m (the maximum depth cored during this study), and possibly down to 65 m below the present playa surface. In the cores studied the Brunhes-Matuyama boundary (0.73 Ma) was identifiable in all the upper sediments, typically at a depth of between 1 and 2 m, but below this there are two plausible chronological interpretations of the palaeomagnetic data. Deposition rates are very low, typically no more than 1.5 cm/ka in the Uluru Clay Beds, and down to 0.2 cm/ka in the Winmatti Beds. These rates suggest that the fluvial-lacustrine phase lasted for at least 5 Ma. The boundary between the Uluru Clay and the Winmatti Beds bears a close correspondence to the boundary between the Blanchetown Clay (a lacustrine sequence) and the Tyrrell Beds (a playa sequence) in palaeo-lake Bungunnia, in what is now the Murray Basin in southeastern Australia. However the transition occurs at about 0.9 Ma (or possibly 1.6 Ma) at Lake Amadeus, whereas it has been dated palaeomagnetically at less than 0.73 Ma (probably 0.5 Ma) in Lake Bungunnia. Records from other locations in southeastern Australia likewise support a major change from wetter to drier conditions at, or shortly after, the Matuyama-Brunhes polarity transition. Thus the current evidence from Lake Amadeus indicates that in central Australia the onset of aeolian and saline gypseous deposits, characteristic of arid-zone facies, may have pre-dated the corresponding change in the southeastern part of the continent by about 0.4 Ma, or possibly by about 1.1 Ma. In common with most playas in central Australia, Lake Amadeus is surrounded by a ring of gypseous dunes derived from material deflated from the surface of the playa. Magnetostratigraphic evidence from Auger Island, in the middle of Lake Amadeus, suggests that the oldest gypseous dune formation on the island must have pre-dated the start of the Jaramillo subchron at 0.98 Ma (or possibly the start of the Brunhes epoch, 0.73 Ma), thus making these the oldest dated dunes in Australia. The age of the Auger Island dunes is probably characteristic of other gypseous dunes around the margins of playas in central Australia.

Introduction

ing p r e s e n t - d a y e n v i r o n m e n t s a n d c l i m a t i c e v o l u tion. In A u s t r a l i a , Q u a t e r n a r y h i s t o r i e s h a v e b e e n

The Quaternary history of environmental c h a n g e s p r o v i d e s the b a c k g r o u n d f o r u n d e r s t a n d -

well e s t a b l i s h e d for southeastern Australia ( B o w l e r , 1982; B o w l e r a n d W a s s o n , 1984; S i n g h a n d Geissler, 1985; B o w l e r , 1986), c o a s t a l N o r t h Q u e e n s l a n d ( K e r s h a w , 1985), a n d T a s m a n i a ( C o l h o u n , 1985; B a r b e t t i a n d C o l h o u n , 1988). In s o u t h -

~Present address: Geography Department, The University of Wollongong, P.O.Box 1144, Wollongong, NSW 2500, Australia 0031-0182/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

56

eastern Australia, Miocene humid environments with high lake levels culminated in drying at about 5 Ma, close to the Miocene-Pliocene boundary. A drier climate with seasonally dry lakes prevailed during most of the Pliocene. Reactivation of lakes occured at about 2.5 Ma at the Gauss-Matuyama boundary (Bowler,1986), heralding the beginning of Quaternary oscillatory fluctuations. The full development of arid environments, indicated by the first production of saline, evaporitic lake facies and dune successions, occurred within the last 0.7 Ma (Bowler, 1982). Relatively little is known about major palaeoenvironmental changes in central Australia (CLIMANZ, 1983) and the time-relationships with corresponding changes in the rest of Australia. This is partly because of the poor access to this vast region, and partly because of difficulties in the interpretation of the limited geological record available. Climatic events in the heart of the continent are important for understanding the Quaternary history of Australia as a whole. Playa lakes (dry salt lakes, that rarely contain surface water) are abundant in central Australia. Sedimentary records from playa lakes provide one of the few means at our disposal for understanding the hydrological and climatic history of arid regions. Establishment of a chronological framework for such records is therefore of great importance, but has so far proved to be difficult. The paucity of dateable materials limits the use of radiocarbon dating, and the time range covered by the radiocarbon method is little more than 40,000 years. Thermoluminescence dating has been successfully applied to aeolian sediments (e.g. Gardner et al., 1987; Chen et al., 1990), but is seldom practicable for salt lake sediments that usually have a complicated hydrological history. Magnetostratigraphy has considerable potential in such environments. The method can provide a means of stratigraphic correlation as well as absolute dating of the time of acquisition of magnetic remanence via the geomagnetic reversal timescale (we have used the timescale of Berggren et al., 1985). Magnetostratigraphy also offers a suitable time resolution - - in the order of 105 to 106 years during the Quaternary and Neogene. Two problems must usually be faced. Firstly, that of resolv-

X.Y. CHEN AND C.E. BARTON

ing the primary (detrital) component of magnetic remanence in sediments that have been subjected to weathering and alteration processes. Exposure to the atmosphere, cyclic wetting and drying, migration of fluids through the sediment column, and diagenesis will all tend to obliterate the primary magnetization. The second problem arises from the binary nature (normal and reverse) of the magnetic polarity time scale. It is often difficult to correlate uniquely a magnetostratigraphic profile with the magnetic polarity timescale, particularly when sedimentation rates are irregular with possible hiatuses and when no additional constraining chronostratigraphic evidence is available. Several dry and ephemeral lake sediment sequences in Australia have been the subject of magnetostratigraphic studies (Singh et al., 1981; An Zhisheng et al., 1986; Chivas et al., 1986) . In general it is nearly always possible to identify the Brunhes-Matuyama (B/M) boundary (0.73 Ma), in some cases it is possible to identify the Jaramillo (0.98-0.91 Ma) and Olduvai (1.88-1.66 Ma) subchrons and the Matuyama-Gauss boundary (2.47 Ma), and it is seldom possible to extend the stratigraphy much beyond 3 Ma. There are some exceptions where longer records are obtainable, notably from Lake George (McEwen-Mason, 1991. Sedimentary sequences in Australian playas consist mainly of clastic materials, ranging from clays to sands, with remarkably few salts. The most significant salt deposit is a 1 m halite layer in Lake Eyre (Johns, 1963). Gypsum commonly occurs scattered within the sediment or as thin layers. The generally low sedimentation rate in playas, and frequent exposure of playa sediments to subaerial conditions produce magnetic properties that differ somewhat from those of normal lacustrine and marine sediments. Post-depositional physical and chemical alteration are more common and it is usually much more difficult to identify the primary component of magnetization. It is not surprising that clay-rich facies, deposited under relatively deep-water conditions (several metres or more), invariably preserve a much better palaeomagnetic record than sand-rich facies and gypseous sediments. Because of the natural water content of typical sediments and their friable nature, ther-

57

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

represented by Karinga Creek (Fig. 1). These playas are the remnants of a Tertiary system that was linked with the Lake Eyre drainage system to the east. Today Karinga Creek forms a tributary junction with the Finke River that drains south towards Lake Eyre. The disruption of the Tertiary drainage system was probably due to a major climatic change. Slight tectonic movements also had an influence. An uplifted area is recognized close to the east end of the playa that blocks not only the modern surface drainage, but also the groundwater flow (Lloyd and Jacobson, 1987). The George Gill Range, about 70 km north of the playa, along the north edge of the Amadeus lowland, consists mainly of Palaeozoic sedimentary rocks comprising sandstone, conglomerate and siltstone. Ayers Rock and Mt. Olga, about 50 km to the south, are the closest major bedrock outcrops (of arkose and conglomerate respectively). Further south, the northern fringe of the Petermann and Musgrave Ranges, flanking the southern edge of the lowland, are mainly gneiss, granite and sedimentary rocks. The Amadeus lowland itself is covered by Quaternary sand dunes and some calcrete layers, with a few small bedrock mounds. The playa is underlain by the Bitter

mal demagnetization is difficult to perform and is much more time-consuming than alternating field (AF) demagnetization. However, in zones where chemical alteration and iron staining are readily apparent, thermal cleaning is necessary in order to isolate stable characteristic directions of magnetic remanence (Singh et al., 1981; Barton, 1988). This paper presents the results of a sedimentological and paleomagnetic study carried out on cores recovered from the unconsolidated sediments of Lake Amadeus, a playa in central Australia. The objective was to further our understanding of major Quaternary climatic and environmental changes in Central Australia, and their time-relationships with corresponding changes in less arid, and better studied, regions of Australia. Lake Amadeus

Lake Amadeus is a groundwater-discharge playa in central Australia, about 120 km long and 4-10 km wide. The playa lies within the Amadeus lowland region that trends east-west through the Central Australian Ranges (Jennings and Mabbutt, 1977). Lake Amadeus forms the largest of a chain of playa lakes along a palaeodrainage line now

131o'00 '

Mncdonnell Ranges ALICE

L.Mac~a I ~

SPRINGS

LAKEAMADEU~ Mt O~a~ - ~

km

150

Karinga Ck

A~USgrQvaganga8 ~

N

L. Eyre Fig.l. Location map showing relationship of Amadeus playa to Karinga Ck., a palaeo-channel of the Finke drainage system. The 500-m height contour is shown.

58

X.Y. CHEN AND C.E. BARTON

Springs Formation - - a Proterozoic evaporitic sequence of limestone, dolomite, shale and gypseous siltstone (Wells et al., 1970). An 88-m drill hole (Fig.2) penetrated 65 m of unconsolidated clays and reached the dolomitic limestone of the Bitter Springs Formation (unpublished core log, Cundill, Meyers and Associates Pty.Ltd.). Rainfall in the area is about 250mm/yr, (averaged over 6 nearby meteorological stations), and the pan evaporation rate in Alice Springs is about 2880 mm/yr (averaged over 17 years). Rainfall in the catchment either infiltrates to the groundwater system or is lost by evaporation, and rarely leads to overland flow. Water reaches the playa by direct precipitation onto its surface and

a.

~

~

~

0

/

~

~

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~

km

20

88mc o r e ~

÷ Longcore

[ ~ ] Gypseous dune

~

•

~

[ ~ Gypsumground

Short core

km

5

Sandplain

I - 7 Salt flat

[~

Sand dune

Sulphidelowland

Fig.2. Core sites and surface geomorphological features at Lake Amadeus.

occasionally, after heavy rains, by minor discharge from small creeks. The playa floor is generally dry. After heavy rains, surface water may appear locally, usually covering less than 10% of the total area of the playa, and may stay for a few days. The groundwater beneath the playa floor has a salinity of around 250 g/1.

Geomorphicfeatures Based on salt-crust features, subsurface sediments and depth to the water table, the playa surface can be classified into three main types. Gypsum-ground, characterized by a thick crust (up to 5 cm thick) overlying banded gypsum and sandy clay. The water table depth is typically around 40 cm. Sulphide-lowland, characterized by lower relief and a black sulphide stained layer (up to 3 cm thick) across the surface, with gypsum and mud beneath. The water table is shallower than 30 cm. Salt-flat, characterized by a thin (about 1 cm thick), white crust with sand, clay and minor gypsum beneath. The water table is usually deeper than 40 cm. The playa shorelines are highly irregular and there are many islands in the playa (Fig.2). Surrounding the playa shorelines and on the islands in the playa, there are two, and in some places, three rings of gypseous dunes. These dunes have a surficial white, microcrystalline indurated gypcrete layer, about 0.5-1 m thick and many are partially or totally covered by more recent aeolian deposits of siliceous sand. The sediments beneath the surficial gypcrete layer consist principally of coarse sand-sized gypsum crystals (typically more than 30%) and fine quartz sand, with minor (typically less than 5%) clay which commonly coats the quartz sand and gypsum crystals. The gypsum crystals are lenticular and discoidal in habit, similar to those in the gypsum-ground of the playa, and slightly corroded and fractured (indicative of a transport origin rather than in situ growth). In some exposures the gypseous, sandy sediments show well-developed cross bedding. These features suggest that the marginal gypseous dunes were derived from material deflated from the surface of the main playa (Chen, 1989). Beyond the playa

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

margins the dunes are composed essentially of siliceous sands.

Coring and sediment features In 1984, during the Australian National University's SLEADS (Salt Lakes, Evaporites and Aeolian DepositS) drilling program, three 15-m cores (AM1, AM2 and AM3), were taken from the playa (Fig.2). AM2 and AM3 were collected about 1.5 km apart on the salt-fiat and gypsum-ground respectively. AM 1 was recovered about 7 km away on the salt-fiat at the entrance to the narrower eastern section of the playa. A light-weight piston corer and hollow-flight auger drilling-rig (Jacro 350) were used and cores were recovered in sections of 50 cm length. Complete recovery was achieved in the clay layers but in sandy and gypseous sediments core recovery was incomplete in some sections. For example, more than 1 m of hard gypcrete was augered from the middle of AM1. Up and down directions were recorded on the cores, but no relative azimuthal orientation was obtained. Several short cores (Fig.2), penetrating to 72-180cm below the playa surface, were taken using an 80 mm-diameter plastic tube and piston. Three were recovered from the salt-fiat (SC105, SC107 and SC26) and seven were recovered from the gypsum-ground (SC101, SC102, SC104, SC106, SC203, SC204 and SC205). Two additional short cores were collected at Auger Island - - one from a claypan on the island (SC201, length 125 cm) and one from the gypsum-ground of the main playa adjacent to the island (SC202, length 107 cm). Compression of the sediment during coring appeared to have been negligible for both long and short cores. However, deformation may have occurred within a few sandy and gypseous layers, as indicated by oblique laminations. Lithological logs of the cores are shown next to the plots of palaeomagnetic data in Figs. 6-10. The sediment sequence of the playa, as shown by the 15-m cores, consists of two major units: an upper sandy unit and a lower clay sequence. The upper unit, about 0.6 m in core AM1, and nearly 3 m in both AM2 and AM3, is variable in lithology,

59

consisting of sub-layers of aeolian sand, abundant gypsum crystals (commonly >1 mm in size) and sandy clays. The lower clay sequence consists of uniform clays and thin, intercalated layers of finegrained (< 0.2 mm) gypsum. The upper sandy unit and the lower clay sequence are termed here the Winmatti Beds and Uluru Clay, respectively. The following four sediment types occur in the cores. (a) Clay (typically > 70%) with minor silt and a very small fraction of sand (commonly < 1.5%). The clay consists of kaolin and illite; and the silt portion consists mainly of quartz with minor magnetite. The prominent feature of the clay is its colour variation. The dominant colour is brown (7.5 YR, 4/6), with minor olive (5Y, 5/4) and reddish (5YR, 3/4) layers, and slight mottling in several thin horizons. (b) Sand, commonly a reddish colour, consisting mainly of quartz with minor magnetite, zircon and ilmenite. The size distribution is similar to that of the nearby dune sand, with a peak in the fine sand class (3.0-3.5 q~). (c) Sandy-clay and clayey-sand. These occur in the top layer of the Uluru Clay and as minor layers in the Winmatti Beds. The grain size distribution lies mainly within the fine sand and clay size classes, and colour varies from grey to brown to red. (d) Gypseous sediments. Gypsum abundance, gypsum crystal-size and morphology are variable, and intermixing with sand and clay is common.

Magnetic measurements One long core (AM3) and 6 short cores (SC26, SC105, 107, 201, 202, and SC204) were sampled at intervals of approximately 20 cm or less. Other cores were sampled at greater intervals for comparative purposes. Specimens were taken using 8 cm 3 cubic plastic pots which were pressed into the moist, soft sediments by hand. Progressive demagnetization was used to identify the components of magnetic remanence of the sediments. AF demagnetization was used whenever possible because it is less time-consuming. However, for reddish and highly weathered sediments, which commonly contain hematite, thermal de-

60

X.Y. C H E N A N D C.E. BARTON

magnetization proved to be necessary. Specimens for thermal demagnetization were first d i e d at room temperature, then removed from their pots. The friable sediments were stuck together by painting with a flame-proof coating that contains 10.7% of non-volatile components of titanium dioxide, silicates and silicone resin (trade mark VHT, from Sperex Corp., Calif., USA). After measurement of the natural remanent magnetization (NRM) of all specimens in a cryogenic magnetometer, progressive demagnetization was carried out on selected pilot specimens. Eight specimens were thermally demagnetized at steps of 50, 100, 150, 200, 250, 300, 400, 500 and 700°C, and about 40 specimens were AF demagnetized (in a tumbler) at steps of 50, 100, 150, 200, 250, 300, 400, 500 and 700 oersted (Oe). Of the remaining specimens, about 130 were AF demagnetized at 200, 300, 400, and 500 Oe, and 23 were thermally demagnetized at 200, 300, 400 and 500°C. Initial

(low field) magnetic susceptibility was measured for all specimens.

Palaeomagnetic reliability ratings The results of progressive demagnetization for each specimen were displayed as a plot of normalized intensity of magnetization against peak demagnetization field (or temperature), a stereographic projection of the magnetic directions after each step, and an orthogonal (Zijderveld) plot showing projections of the magnetic vector onto horizontal and vertical planes (Fig.3). The reliability of the palaeomagnetic results for each specimen are classed according to how clearly a primary magnetic polarity signal can be identified. R e l i a b i l i t y 5 - - reliable. After viscous remanent magnetization (VRM) is removed, a single stable component is left (Fig.3a).

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: : : S

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horizontal

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vertical

Fig.3. Plots of typical progressive demagnetization results. Plots (a) through (d) are for samples with reliability classifications 5 through 2, respectively,

61

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

A

R e l i a b i l i t y 4 - - relatively reliable. The demagnetization curve may be complicated and directions variable, but stable components can still be recognized, and inclination stays either positive or negative (Fig.3b). R e l i a b i l i t y 3 - - usable. The demagnetization curve is complicated and directions are too variable to show clearly any stable components. However, the inclination of the hard magnetization keeps the same sign, with only occasionally a single exception (Fig.3c). R e l i a b i l i t y 2 - - unreliable. Directions are highly scattered, but a dominant inclination polarity still exists (Fig.3d). R e l i a b i l i t y 1 - - unusable. Directions are scattered and show no dominant polarity. Of the 201 specimens treated, 84 (42%) were assigned reliability 5, 50 (25%) were assigned reliability 4, 29 (14.5%) were assigned reliability 3, 15 (7.5%) were assigned reliability 2, and 23 (11%) were assigned reliability l.

Influence of lithology on magnetic properties Magnetization of the sediments resides mainly in the ferromagnetic minerals that exist in the sediments as detrital grains, or as a product of diagenetic chemical processes. The mineralogy, size, shape and amount of ferromagnetic minerals determine the magnetic properties of the sediments. Because of the correlation between mineral assembly and lithology, the magnetic properties also correlate with the lithology. Magnetite, as the major heavy mineral in Lake Amadeus sediments (based on XRD analysis), is the dominant ferromagnetic mineral in most specimens. However, weathering is an important diagenetic process within the playa, where the sedimentation rate is very low and the playa floor is often exposed to high-temperature air. Hematite is a common ferromagnetic mineral formed under such conditions and is commonly indicated by reddish colouring. Figure 4 shows mean magnetic susceptibility and N R M intensity values for various types of sediment. N R M intensity correlates both with sediment grain-size and with colour: the intensity is progressively greater for sediments with larger

Colour I

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Fig.4. Summaryof (A) NRM intensity(in/aG) and (B) magnetic susceptibility(in pG/Oe) of sediment types. grain size and more intense reddening. Gypsum is non-magnetic so that gypseous sediments tend to have low intensities of magnetization. Susceptibility is generally greater where there is more reddening (Fig.4B), which may indicate the existence of hematite, but could also show the presence of more primary magnetic oxides (titano-magnerites), and hence reflect more source material for producing weathering products. Mean reliability classifications for different types of sediment are summarized in Fig.5. This shows, Colour

Grey & Olive

Red

2.4 (AF)

Sand

3.8 (TH)

a,

*-~ Clag& "~ Sand

Brown & Mottled

3.7 (AF)

3.2 (AF) 4.3 (TH)

U

E

,=,.

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3.3 (AF)

3.3 (AF) 2.5 (TH)

2.9 (AF)

2.5 (AF)

Fig.5. Average reliability classifications of various sediment types for AF and thermal demagnetization.

62

firstly, that AF demagnetization is more effective in obtaining reliable results from brown clays and clay-rich sediments, but less effective for sand, red clay and gypseous sediments. Secondly, thermal demagnetization is more effective in obtaining reliable results from sand and red sandy-clay, but is less effective for. brown clay. This may be due to the hematite in the red sediments and sand.

X.Y. CHEN AND C.E. BARTON Dec. 0

Depth

(m)

~

The short cores from both the salt-flat and the gypsum-ground regions penetrate only the Winmatti Beds which is a fairly typical playa sequence of interbedded sand, gypsum crystals and grey or red sandy-clays. The three short cores from the salt-flats show normal polarity throughout with high reliability ratings (Fig.6). One specimen from SC105 shows a low positive inclination, but has low reliability

360 -90 l

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Palaeomagnetic results and interpretation

Palaeomagnetic results for each core are plotted in Figs. 6-10. The characteristic directions of magnetization plotted in the figures were usually obtained after AF or thermal demagnetization to approximately half the initial N R M intensity. The peak AF and thermal steps corresponding to this are typically around 200 Oe and 100°C respectively. Specimens with reliability 1 have been discarded and those with ratings of 2 must be treated with scepticism. Ratings of 5 and 4 are considered to represent reasonably good data, although the cleaning procedure would not have been capable of removing a very strong-hematitic weathering overprint. Reliability ratings are shown on the inclination plots. The declination data for long cores are of little use because the cores were recovered in short (50 cm) sections without relative azimuthal orientation. This does not apply to the short cores which were recovered as single lengths. The geocentric axial dipole inclination for the latitude of Lake Amadeus is ___43°. This is the expected inclination of the time-averaged geomagnetic field in either polarity state, and is shown by the dashed lines on the figures (the present-day inclination at Lake Amadeus is -57°).

Incl.

SC26

Inten.

Sus. pG/Oe

pG

o

0

360 -90

(m)

10 100 A J

i

I

lOO i

,\

1.0

13

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I lad 'Dec.

SC107 Depth

o

Incl.

360 -90

Inten. pG

90

, , , , , , ,

Sus. pG/Oe

1 10 100 10 =

i

i

i

i

100 i

7._~

1.o

~:-~

lad Clay

[ ]

Sand

~

Sandy clay

[ ]

Gypsum

Fig.6. Lithology and palaeomagnetic results for short cores, SC26 (upper), SC105 (middle) and SCI07 (lower). Reliability classifications are marked next to the data points.

(2) and is discounted. We conclude that all three sequences were deposited during the Brunhes normal chron. Cores SC105 and SC26 span the poorly recovered top parts of nearby long cores AM2 and AM 1, respectively. The seven short cores from the gypsum-ground became partly disturbed during coring and transportation to the laboratory due to the abundance of loosely compacted gypseous sediments. Although the cores can be correlated lithologically, the palaeomagnetic data were mostly inconclusive, and had low reliability ratings. There is some

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

evidence that the Brunhes-Matuyama boundary occurs at a depth of about 1 m within a gypseous clay zone. Long cores

Cores AM2 and AM3 are similar in stratigraphy. A major lithologic change occurs at around 3 m depth. The lower unit (the Uluru Clay) is represented by a long sequence of homogeneous clays intercalated by thin gypsum layers. The quartzsand content is less than 3% throughout most of the unit, but towards the top of the Uluru Clay there is a transitional sandy-clay layer, several decimetres thick, containing about 20% quartz sand. The Uluru Clay was found to be devoid of fossil remains, suggesting that it was deposited under relatively shallow-water fluvial-lacustrine conditions. Considering the long homogeneous nature of the unit this interpretation was somewhat unexpected. The upper unit (the Winmatti Beds) consists of two major cycles of sand overlain by gypsum crystals with clay (Figs. 7 and 8). In core AM 1 from the eastern neck of the lake, the Uluru Clay contains many more gypseous layers, and the Winmatti Beds are only about 60 cm thick. Core A M 3

This 15-m core was taken from the gypsumground in the centre of the playa (Fig.2). Recovery was incomplete in the upper 2.5 m. The results (Fig.7) show that both intensity and susceptibility in the upper 2.5 m (Winmatti Beds) vary greatly, being low in the gypseous layer and very high in the reddish sand layer. In the Uluru Clay sequence below 3 m depth, the intensity of magnetization fluctuates, being higher in the reddish layers and lower in the olive and brown layers. However, the magnetic susceptibility stays remarkably constant. This suggests that the bulk magnetic composition of the sediments is relatively homogeneous, and that the small fraction of grains carrying the magnetic remanence is preferentially influenced by irregularities in weathering effects. There are two plausible polarity interpretations (labelled a and b on Fig.7). In each, the B/M boundary is at a depth of about 1 m. Its position cannot be established more accurately as the sedi-

63

ments at this depth have high gypsum contents and give relatively low palaeomagnetic reliability ratings. Location of the B/M boundary at about 1 m depth is consistent with results from the short cores from the gypsum-ground. The upper 3 m of the core appears to contain several hiatuses, which probably represent long periods of playa deflation accompanied by formation of gypsum dunes along the playa edges (Chen, 1989). Hence the lengths of the magnetostratigraphic zones in the upper sequence are not in proportion to the magnetic polarity time-intervals. The first normal polarity event below the B/M boundary must be either the Jaramillo (interpretation a) or the Olduvai subchron (interpretation b). The event coincides with the change of lithology from the Uluru Clay to the overlying Winmatti Beds. The well-defined normal section at the bottom of the core must then represent either the Olduvai subchron (interpretation a) or the Gauss chron (interpretation b). In the absence of independent age control, either from other dating methods or from sedimentological arguments, it is not possible to choose between the two interpretations.

Core A M 2

This 15-m core was taken from the salt-flat to the southwest of AM3 (Fig.2). Palaeomagnetic sampling was restricted to intervals of about 50 cm. The results (Fig.8) show similar features to AM3 in the lower clay sequence, with the same polarity transitions recorded at almost the same depths. The upper parts are slightly different. The top specimen in AM2 has negative inclination, whereas core AM3 at the corresponding depth has positive inclination. However, the negative inclination in AM2 is shown only by a single specimen and the exact stratigraphic correlation of the Winmatti Beds between AM2 and AM3 is uncertain. Adjacent short core SC105, and short core SC107 also taken from the salt-flat, show normal (Brunhes) magnetization to depths of 106cm and 138cm respectively. We conclude that the B/M boundary in core AM2 must lie between 1 and 2 m depth. Below this boundary, polarity interpretations a and b for core AM3 are applicable.

64

X.Y. CHEN AND C.E. BARTON

Interpretat ion

Inclination

Intensity

AM3 b

a

-90.0

90.0 0.1 i I

2

3

4

0.

~ ' ~ - ;~

M

M

.fA~& ~-;,'. ~ t ~.

Old.

Jar.

6

~3 S)i I

2

A'.' iJ'~~

M

M

-~--

M

M

--~3! ~

;'Z//, //7/, ~

"(,';_;

i

I L

5 bl

/IIZ 7

21

I I I I i

I

8¢o 8

B/M

10

I

15 aIM

1

Susceptibility (uG/Oe) 'I 00.0 0.1 1 10 100.0

I

M

M I

8

>g:'

,,...,,:

M

M

9

11

I

M

~,7",,~

' "" [7'S; 7/21 //1"/; g;,L,z,.

12

i

14 15

I

I

M

<--Ls

M

4

M

I I

M

: ". :..

13

':/#f

M/G

.... 74//,

G

:. ,,-:, G

15j

O=d.

J;',

l

4'X4

M

i lad

and

s

5k'5 1

. . . . .

~S

4

13

,x,,.

Clay

lad ~

Gypsum

~

Sandy clay or clayey sand

~ Z ~ Earthy gypsum

Fig.7. Lithology and palaeomagnetic data for long core AM3, with two possible polarity interpretations (a and b). On the magnetic polarity column black denotes normal polarity, white denotes reverse polarity and hatched denotes uncertain polarity.

Core AM1 Because of abundant gypsum and gypcrete layers in core AM 1, the sampling interval used was coarser than in AM2. The results (Fig.9) show a more complicated pattern of inclination behaviour, particularly between 3 and 9 m depth. The susceptibility plot shows again that the bulk magnetic composition is very uniform. A major boundary exists at about 9 m, above which positive inclina-

tion is dominant and below which negative inclination is dominant (down to 13.5 m). We take this to be the Matuyama-Gauss boundary for the following reasons. Firstly, the nearby short core SC26 shows normal polarity within the top 68 cm, representing the Brunhes chron. The upper part of AM 1 underlies this and is therefore probably of Matuyama age. Secondly, the sedimentation rate at the site of

65

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

b

a

90.0 0.1

-90.0

B 1

~._ :::::": !

,,

,,

I I I

I I I

B/M

/4

4

Jar.

£

3

,-\.

6

////,

M

i

M

~5;

I

I I I

:,-27

I I

M

7

I

f

I I ~

2t~

5':r, 5

1 i i i

5~

,-x,q i,-

I

I

(

"--*.4

',5/

~T~.(. ~L.2

Old.

I

I j i I

M 3

Susceptability Intensity (,uG/Oe) 0,G) 1 10 100.0 1 10 100.00.1

Inclination

Interpretation AM2

M

5)!

s%',

I

1"~5 8

I I

IIII,

i

5~

I

=

"_%<

9

t

M 10

;)5

I

"-,-

5¢=

I

M

i

I////

;55

I

.-,<

iI

/

3I

\

I

t

5~!

I

M

M

I I

13

////,

I I

M/G

/11/,

Old, G

I I

J /¢

sY" k3

M

JI ~

I ~ i

15 • ~-', ~Sand

2

I i

Clay

I

~ [~

Gypsum

I

~

I

Sandy clay or clayey sand

Earthy gypsum

Fig.8. Lithology and palaeomagneticdata for long core AM2, with two possible polarity interpretations (a and b).

AM l, in the narrower eastern part of the playa basin which is closer to the relatively uplifted zone, is expected to be lower than at the sites of AM2 and AM3 which are closer to the centre o f sedimentation. Several pits and auger holes proved that in the eastern part of the playa near AM 1, the Uluru Clay sequence is first encountered at depths of 50-100 cm, compared to 300 cm in cores AM2 and AM3. Moreover, the abundant gypcrete layers indicate a marginal position, as illustrated

by some nearby small pans (Chen, 1989), where the sedimentation rate is expected to be lower. We therefore conclude that the major normal section in the sequence represents the Gauss chron, with the Gauss-Gilbert boundary (3.4 Ma) at about 13.5 m. The single specimen with positive inclination at 11.7 m depth probably falls within either the Mammoth subchron (3.18-3.08 Ma) or the Kaena subchron (2.99-2.92 Ma). The interpretation of the two low negative inclination results

66

X.Y. CHEN AND C.E. BARTON

Interpretation AM1

-90.0

0

2

;o:~

3

".*..~

/

Inclination

Intensity

Susceptibility (pG/Oe) 10 100.0 0.1 1 10 100.0

O,G) 90.0 0.1

1 I

I

I

I

I

I

4

'//-(6 4

-~?/

6

,' ~.'~

M/G

\-t

11

;,~:

-,'h ,~r,,

I

1 2 , ,~..-,

(3

i

:;r):

~

I

15

Sand

1 ~

[">4

I

/

I

I

Clay

i

', / ~

I

Gypsum

[~

Gypcrete (or earthy gypsum)

Fig.9. Lithology,palaeomagneticdata and polarity interpretation for long core AM1. within the Matuyama is less clear. The lower one, at 6.3 m, has a low reliability rating and may be spurious. The upper one at 4.4 m could lie within the Olduvai, but further evidence from additional cores is needed. Auger island

Auger Island, near the centre of the main playa (Fig.2), is nearly 2 km in diameter, and is constructed of gypseous dunes most of which are covered by siliceous sand crests. There are three small pans (playas) in the depressions among the

dunes, with surfaces at the same level as that of the main playa. The largest pan (coring site 201), 800 m long and 300 m wide, is located in the western part of the island and is separated from the main playa by its two rings of gypseous dunes. The short core from the pan on Auger Island (SC201) and another from the main playa adjacent to the island (SC202) were expected to provide clues to the mode and age of formation of the gypseous dunes forming the island. The core SC202 from the main playa near Auger Island stops in the Winmatti Beds and does not reach as far as the distinctive Uluru Clay. Core SC201 from the

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

reliability results were obtained (Fig. 10). All specimens from SC202 show negative characteristic inclinations and are therefore of Brunhes age, in agreement with results from the Winmatti Beds recovered in the other short cores from the main playa. Inclinations in core SC201 are negative and close to the geocentric axial dipole field value at depths down to 72cm, and also at about 105-112cm. Below 125 cm the core is reversely magnetized.

pan on Auger Island is somewhat different. The clay below 120 cm corresponds to the Uluru Clay. Above this is'a sequence of clayey-sand and sands that are distinct in both lithology and thickness from the Winmatti Beds. The Uluru Clay sequence below 120 cm in SC201 seems to be the basement for the early gypseous dune formation which isolated the pan. Nearly all the specimens from the cores were subjected to thermal demagnetization, and high

Incl.

Dec.

I nterpretation SC201 Depth (m)

b

a

B

B

B

B

360

,0 . . . . . .

-90 ,

0 ,

B/M

1.0

B

,

,

t

M M

,

,

i

M

i

i

L

i

i

t j

Jar.

B/M

base~

90

,

Inten. Sus. pG p G/Oe 1 10 100 10 100

14

Excursion? M

Dune

67

4

I

lad

lad

Dec. SC202

Incl.

0

Depth

i

(m)

,

,

i

,

,

360

-90

,

.t

90 i

,,

,

,

,

Inten. Sus. pG pG/Oe 1 10 100 10 100 i

i

i

i

i

i

I I I I I I I

~,,:2C"

=

I I

1.0

4

,°,t .~.A ¢ I Clay

I

I

lad

~

Sand

~

Sandy clay

~

Gypsum

Fig.10. R e s u l t s f o r s h o r t c o r e s , SC201 f r o m the small p a n o n A u g e r I s l a n d ( u p p e r ) a n d S C 2 0 2 f r o m the salt flat j u s t to the w e s t o f the island (lower).

68 Between the two intervals of normal polarity there are five specimens with low inclinations and high reliability ratings. Three possible explanations for this are (1) the sediments acquired their magnetization during a geomagnetic excursion, (2) the sediments have a reverse primary magnetization from which a normal (probably Brunhes age) overprint has not been completely removed, and (3) the directions result from physical disturbance of the core. The possibility that the shallow inclinations arise from the classical "inclination error" phenomena is discounted on the grounds that there is an approximately 180° shift in relative declination coincident with the shallow inclinations. Note that the shallow-inclination interval lies completely within a zone of homogeneous clayeysand and is not associated with any obvious lithological changes. If we assume that the deposition rate in the clayey-sand unit is typical of this environment (i.e. very slow), then the first explanation would require a geomagnetic excursion of extremely long duration (at least ten times longer than any that have hitherto been proposed). The approximately 180° shift in declinations in the shallow-inclination interval lends some support to explanation (2). However, there is no obvious evidence of either chemical or physical alteration to support either (2) or (3). If we assume that the sediments in the shallowinclination interval were reversely magnetized initially and retained a partial normal overprint, then the B/M boundary is probably at about 73 cm depth and the short normal interval at about l l0cm corresponds to the Jaramillo subchron (interpretation a, Fig.10). If, on the other hand, we attribute the shallow-inclination interval to a geomagnetic excursion (with or without core disturbance), then the B/M boundary must be at about l l 0 c m depth (interpretation b, Fig.10). Based on the currently available evidence our view is that interpretation a is the most likely. However, there is clearly a need for information from additional cores to resolve the ambiguity. The sequence of events that probably led to the formation of the island are illustrated in Fig.11. The starting point is a flat dry lake surface at, or shortly after, the onset of the arid conditions that

X.Y. CHENAND C.E. BARTON

A - , ~..,.,~-/J~.. /~_,,'/~,;. ~ -,,~.Lx - " -,'~-,,=.--.

-~-x

'L' ~ <'~~'.~.~'~'~;"~

~Short

B

core

C

~Sand

~ ~

Clay

Gypsum crystals

Fig. 11. Illustration of island formation.(A) Playa surface before formation of the gypseous dunes. (B) Deflation in the playa forms an island of gypseous dunes, with an isolated small pan. (C) Effect of different sedimentation rates in the main playa and in the isolated small pan.

terminated the Uluru Clay unit (Fig. 11A). A group of dunes started to nucleate about an obstruction, and, by a series of deflation episodes, led to the growth of the dunes and formation of the interdunal pan (Fig.liB). It is our contention that during successive dune-building episodes the pan would have been scoured out and there would have been little opportunity for sediment accumulation. Subsequently less energetic environmental conditions prevailed: the dunes stabilized and developed a gypcrete surface layer, while sediment started to accumulate in the main playa (the Winmatti Beds) and in the pan (Fig. 11C). Because of the different sedimentary environments there is no direct correspondence between sedimentary features seen in the two sequences above the Uluru

O N S E T O F A R I D I T Y A N D D U N E B U I L D I N G IN C E N T R A L A U S T R A L I A : L A K E A M A D E U S

Clay. It is possible that the surface material from the dry lake was lost by deflation before Auger Island started to form. Thus there may be a hiatus between the Uluru Clay and the pan sediments in core SC201. Based on the above model of formation of Auger Island, the palaeomagnetic data for SC201 show that the start of the pan sequence coincides either with the lower Jaramillo subchron (interpretation a) or with the B/M boundary (interpretation b). Hence the last major phase of dune building must either predate 0.98 Ma, or predate 0.73 Ma, respectively. The sharp contact at 125 cm in SC201 probably represents a hiatus of unknown duration. This is below, and quite distinct from, the reverse-to-normal transition and therefore does not affect the above conclusion. Further discussion and conclusions

Sediments in Lake Amadeus have provided information about environmental conditions in central Australia during the last 3 million years. There are two sedimentary units present in the upper 15 m that was cored: a long homogeneous sequence of clays (the Uluru Clay formation) overlain by a 1-3 m thick playa sequence (the Winmatti Beds). Sedimentation rates are very low, typically no more than 1.5 cm/ka for the Uluru Clay and down to 0.2 cm/ka in the Winmatti Beds (see below). The Uluru Clay unit probably extends well below 15 m. It is lightly mottled and weathered in places, it contains occasional gypsum layers and it is devoid of fresh-(deep)-water fossil remains. For these reasons we believe that is was deposited under predominantly shallow-water fluvial-lacustrine conditions. The reported existence of 65 m of similar uniform unconsolidated clays in the 88 m drill core suggests that the Uluru Clay unit was deposited over a very long period of time - roughly 5 million years if we assume that the palaeomagnetically determined sedimentation rate for the upper 15 m is characteristic of the entire sequence. The development of a shoreline ring of gypseous dunes, such as those found around Lake Amadeus, appears to be a common feature of playas in

69

central Australia. The smaller playas in the Karinga Creek palaeodrainage system are also ringed by gypseous dunes, as is Lake Lewis (Napperby), 250 km to the northeast. Likewise, aerial photographs of Lake Mackay, a large playa 350 km northwest of Lake Amadeus, also suggest the presence of gypseous dunes surrounding the shoreline. This feature is in marked contrast to the crescent-shaped dunes (lunettes), with high clay contents, which develop around the leeward sides of playas in the Mallee Plains of southeastern Australia. The reasons for this difference in clay content, and why central Australia playas are ringed by marginal dunes, whereas those from SE Australia develop only lunettes on the down-wind sides are only poorly understood. Presumably there is a connection with location within the anticyclonic wind pattern that dominated early Quaternary arid landforms over much of Australia: central Australia, being nearer the focus of the rotating wind pattern, would have been subject to greater variability in wind direction. J. M. Bowler (pers. comm., 1988) has pointed out that marginal rings of gypseous dunes are characteristic of deflation from groundwater-discharge playas, whereas surface-drainage playas develop clay-rich lunettes. Oscillations in the level of the watertable must play a fundamental role in producing gypseous source material for dune formation found predominantly (though not exclusively) around the margins of playas in central Australia. Surfacedrainage playas, on the other hand, are relatively rich in clay and depleted in gypsum as source materials for deflation, and develop only clay lunettes. Despite the influence of weathering and postdepositional alteration, more than 80% of the specimens studied appear to provide primary palaeomagnetic polarity data. As a general rule AF demagnetization is more effective in obtaining primary magnetization directions in brown and grey clays and in clay-rich sediments, whereas thermal demagnetization is more effective in obtaining reliable results from red sandy-clays and sandy sediments. A magnetostratigraphic summary is shown in Fig.12. The surficial sediments of the playa were deposited during the Brunhes normal chron. This

70

X.Y. C H E N A N D C.E. B A R T O N

A u g e r Island SC201

m t@

~

o

E SC202 !.:.'~

AM2

SClO SClO;

0

b

AM3 sc2o,4

~

AM1

"r"

,

sc2,6

II

I

®

~ (u

4

....

6

\ \ b\

Q.

8 @

"\

M \ \

.~ 'to ,~, 12 ® 14

B Brunhes

16

M Matuyama J Jaramillo O Olduvai

-3.40 Gi

Ga Gauss Gi Gilbert

Fig. 12. Summary plot showing correlation and polarity interpretation of the cores. is consistent with thermoluminescence ages detained for marginal dunes and the correlation between the dune and playa stratigraphies (Chen, 1989; Chen et al., 1990). For each of the three cores AM2, AM3, SC201 there are two plausible magnetic polarity interpretations below the B/M boundary. In the absence of any independent chronological control it is difficult to choose between them. Our preference is labelled (a) in each case in Figs. 7, 8 and 10, although the alternative interpretations (b) are almost equally plausible. The Brunhes-Matuyama boundary occurs at a depth of less than 1 m at the AM 1 site, and within a depth-range from 1 to 2 m at the AM2 and AM3 sites. The beginning of the Winmatti Beds coincides either with the end of the Jaramillo subchron (0.91 Ma) or with the end of the Olduvai subchron (1.6 Ma). The Uluru Clay sequences in AM2 and AM3 are very similar, not only in lithology but also magnetically. They have a very uniform bulk magnetic composition, shown by the

nearly constant magnetic susceptibility. These 15 m sequences go back to at least the Olduvai subchron, and possibly into the Gauss chron. The sedimentation rate in the narrower eastern part of the playa (east of site AM1) is appoximately half that at sites AM2 and AM3. In core AM I the Matuyama-Gauss boundary occurs at about 9 m depth (2.47 Ma) and the base of the core penetrates the Upper Gilbert chron (3.4 Ma). This is the oldest material recovered. The gypseous dunes forming Auger Island must have resulted from a period of deflation of material from the playa surface. If the model of formation of the island illustrated in Fig. 11 is correct, the palaeomagnetic data from core SC201 from the pan on the island indicate that the last major phase of dune building on the island must either predate 0.98 Ma (interpretation a) or predate 0.73 Ma (interpretation b). Our personal preference is for interpretation a, although further evidence and a more detailed

ONSET OF A R I D I T Y A N D D U N E B U I L D I N G IN C E N T R A L A U S T R A L I A : LAKE A M A D E U S

from corresponding records from other Australian sites. One of the best Quaternary records from SE Australia (An et al., 1986) comes from ancient Lake Bungunnia, which extended over an area of approximately 68,000 km 2 in the Murray Basin and was responsible for producing the extensive lacustrine Blanchetown Clay formation and the overlying Tyrrell Beds (a saline gypseous playa sequence). The Gauss-Matuyama boundary occurs within the Blanchetown Clay sequence and corresponds to a discomformable horizon, indicated by evidence of oxidation along a bedding plane, that implies a major change from relatively dry to wetter conditions. Similarly, at Lake George, the Gauss-Matuyama boundary corresponds to a major depositional change from a long period of slope-debris deposition to a deep-water facies indicating the reactivation of the lake under wetter conditions (Singh et al., 1981). However, in

palaeomagnetic study of additional cores is needed to clarify the chronology. The current palaeomagnetic evidence indicates that Auger Isand is composed of the oldest dated sand dunes in Australia regardless of which interpretation is chosen. It is reasonable to assume that the gypseous dunes around the margins of the main playa are similar in age. Average sedimentation rates for core AM3 are shown in Fig.lY Sedimentation rates for core AM2 are similar. No allowance is made for deflation or hiatuses which are indicated by aeolian sand layers, soil structures and sharp boundaries in the Winmatti Beds (Chen, 1989), and which lower the apparent mean sedimentation rates (Fig. 13). This magnetostratigraphy and its major implications for the Quaternary history of the region show some interesting similarities and some differences 0.73Ma I Jaramillo

Brunhes

4 5-J

2

•

71

1.66Ma IOIduvai

2.47Ma Gauss

°

~

. . . . . . . . . . . . . 0.444~ , ~ ~. :. .0.~ . . . . 6-. . . .~ . ~._~._ . .

oo

.:7'.

Q.

~,, ?L,

..9°

8

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

");'~ \ \ \ \ \ V \

interpretation a

interpretation b~\ \ \ \

12

\.

14

'~,l'F

Fig.lY Plot of magnetic reversal stratigraphy of core AM3 against the palaeomagnetic time scale (Berggren et al., 1985), showing interpreted sedimentation rates (in cm/103 yr).

72

Lake Amadeus the transition from the Tertiary to the Quaternary (conveniently taken as the GaussMatuyama boundary at 2.5 Ma, Kukla, 1987) occurs within the homogeneous Uluru Clay and does not correspond to any significant change in either the sedimentary environment, or, by implication, the climatic regime in central Australia. The Uluru Clay and the overlying Winmatti Beds of Lake Amadeus bear a strong correspondence with the Blanchetown Clay and the overlying Tyrrell Beds of Lake Bungunnia. The Blanchetown Clay consists of micaceous sandy clay, commonly laminated and containing elements of freshwater flora and fauna, being a lacustrine sediment of a well-developed permanent lake. However, the Uluru Clay does not contain any fossils, does not show any laminations, and has intercalated thin gypseous layers indicative of intermittant saline conditions. The Uluru Clay is therefore interpreted as a fluvial-lacustrine sediment of a seasonally dry lake. Despite this difference, in both sites the transition from the clay to the overlying playa sequence indicates a major shift from relatively wet conditions to higher aridity and more aeolian activity. This transition occured in Lake Amadeus at, or before about 0.91 Ma (interpretation a) or possibly at, or before about 1.6 Ma (interpretation b). This is significantly earlier than in Lake Bungunnia where the termination of the Blanchetown Clay post dates the B/M boundary (0.73 Ma), and is estimated to be at about 0.5 Ma (An et al., 1986). The Quaternary aeolian landforms in the southeastern part of Australia probably developed within the last 0.7 Ma (Bowler, 1982; Wasson, 1986). Bowler (1976) speculated that aeolian desert dunes might exist in central Australia significantly earlier. This study of Lake Amadeus provides the first evidence that Bowler's speculation may be correct, and that the onset of aeolian and saline gypseous deposits in central Australia may have occured 0.4 Ma earlier, and possibly 1.1 Ma earlier than in the southeastern part of the continent. Salt lakes abound over much of Australia. It is now important to establish whether records from other salt lakes in central Australia, e.g. Lake Lewis (Napperby), confirm the sequence and timing of environmental changes indicated by the first

X.Y. CHEN AND C.E. BARTON

results from Lake Amadeus. It may well be possible to extend the record out to the margins of the continent.

Acknowledgements We thank Mr D. Edwards for help in laboratory work and thank Mr D. Kelleher for field assistance. Dr J.M.A. Chappell read the manuscript and provided discussions. Dr J.M. Bowler initiated this project and subsequently provided guidance and discussions. Barton acknowledges permission to publish this paper from the director, Bureau of Mineral Resources. The 15-n~ cores used in this research were provided by the SLEADS project of the Australian National University.

References An Zhisheng, Bowler, J. M., Opdyke, N. D., Macumber, P. G. and Firman, J.B., 1986. Palaeomagnetic stratigraphy of Lake Bungunnia: Plio-Pleistocene precursor of aridity in the Murray Basin, southeastern Australia. Palaeogeogr., Palaeoclimatol., Palaeoecol., 54: 219-239. Barbetti, M. and Colhoun, E. A., 1988. Reversed magnetization of glaciolacustrine sediments from western Tasmania. Search, 19: 151-153. Barton, C.E., 1988. Magnetostratigraphy on a timesale of 0-106 years: secular variation, geomagnetic excursions and reversals. Geol. Soc. Aust. Abstr., 21: 53-54. Berggren, W. A., Kent, D. V., Flynn, J. J. and Van Couvering, J.A., 1985. Cenozoic geochronology. Geol. Soc. Am. Bull., 96: 1407-1418. Bowler, J.M., 1976. Aridity in Australia: age, origins and expression in aeolian landforms and sediments. Earth-Sci. Rev., 12: 279-310. Bowler, J. M., 1982. Aridity in the late Tertiary and Quaternary of Australia, In: W.R. Barker and P . J . M . Greenslade (Editors), Evolution of the Flora and Fauna of Arid Australia. Peacock, Adelaide, pp. 35-46. Bowler, J. M., 1986. Quaternary landform evolution. In: D. N. Jeans (Editor), Australia, a Geography. Sydney Univ. Press, 2nd ed., 1, pp. 117-147. Bowler, J.M. and Wasson, R.J., 1984. Glacial age environments of inland Australia. In: J.C. Vogel (Editor), Late Cainozoic Palaeoclimates of the Southern Hemisphere. Balkema, Rotterdam, pp. 183-208. Chen, X.Y., 1989. Modern processes and evolution of Lake Amadeus, central Australia. Thesis. Aust. Natl. Univ., 387 pp.(unpublished). Chen, X. Y., Prescott, J. R. and Hutton, J. T., 1990. Thermoluminescence dating on gypseous dunes of Lake Amadeus, central Australia. Aust. J. Earth Sci., 37: 93-101. Chivas, A.R., De Deckker, P., Nind, M., Thiriet, D. and Watson, G., 1986. The Pleistocene palaeoenvironmental r e -

ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: LAKE AMADEUS

cord of Lake Buchanan: an atypical Australian playa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 54: 131-152. CLIMANZ, 1983. Proceedings of the first CLIMANZ conference. Dep. Biogeogr. Geomorphol. RSPacS, ANU, Canberra. Colhoun, E.A., 1985. Glaciations of the West Coast Range, Tasmania. Quat. Res., 24: 39-59. Gardner, G. J., Mortlock, A. J., Price, D. M., Readhead, M. L. and Wasson, R. J., 1987. Thermoluminescence and radiocarbon dating of Australian desert dunes. Aust. J. Earth Sci., 34: 343-357. Jennings, J. N. and Mabbutt, J.A., 1977. Physiogral~hic outlines and regions. In: D.N. Jeans (Editor), Australia, a Geography. Sydney Univ. Press, pp. 38-53. Johns, R.K., 1963. Investigation of Lake Eyre. Geol. Surv. South Aust. Rep. Invest., 24, 41 pp. Kershaw, A. P., 1985. An extended late Quaternary vegetation record from north-eastern Queensland and its implications for the seasonal tropics of Australia. Proc. Ecol. Soc. Aust., 13: 179-189. Kukla, G., 1987. Loess stratigraphy in central China and correlation with an extended oxygen isotope stage scale. Quat. Sci. Rev., 6:191 220.

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Lloyd, J. W. and Jacobson, G., 1987. The hydrogeology of the Amadeus Basin, Central Australia. J. Hydrol., 93: 1-24. McEwan-Mason, J., 1991. Late Cenozoic magnetostratigraphy and preliminary palynology of Lake George, N. S. W. In: M. A. J. Williams, P. De Deckker and A. P. Kershaw (Editors), The Cenozoic in Australia: A Re-appraisal of the Evidence. Geol. Soc. Aust. Publ., 18, in press. Singh, G. and Geissler, E. A., 1985. Late Cainozoic history of vegetation, fire, lake levels and climate, at Lake George, New South Wales, Australia. Philos. Trans. R. Soc. Lond., B311: 379-447. Singh, G., Opdyke, N.D. and Bowler, J.M., 1981. Late Cainozoic stratigraphy, palaeomagnetic chronology and vegetational history from Lake George, N. S. W. J. Geol. Soc. Aust., 28: 435-452. Wasson, R.J., 1986. Geomorphology and Quaternary history of the Australian continental dunefields. Geogr. Rev. Jap., 59 (Ser.B): 55-67. Wells, A. T., Forman, D.J., Ranford, L.C. and Cook, P.J., 1970. Geology of the Amadeus Basin, Central Australia. Bur. Min. Resour. Aust. Bull., 100, 222 pp.