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Desert pavement development on the lake shorelines of Lake Eyre (South), South Australia
Geomorphology 100 (2008) 154–163
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Desert pavement development on the lake shorelines of Lake Eyre (South), South Australia Asma Al-Farraj ⁎ P.O. Box 17771, Geography Department, U.A.E. University, Al Ain, United Arab Emirates
A R T I C L E
I N F O
Article history: Received 30 October 2005 Received in revised form 1 September 2006 Accepted 6 October 2007 Available online 15 May 2008 Keywords: Desert pavement Salt weathering Lake shore Lake Eyre (South) South Australia
A B S T R A C T To the southwest of Lake Eyre (South), South Australia, silcrete boulders exposed by the erosion of the surrounding fine sediments undergo mechanical weathering to form desert pavement. Successive palaeoshorelines of Lake Eyre have exposed an age-related sequence of different stages in the weathering of the boulders. This study investigates desert pavement development in this saline environment. In addition, it attempts to develop a model for the development of desert pavement following exposure of the silcrete boulders, based on palaeo-lake shorelines dated from previous studies. Seven stages can be recognised corresponding to stages of soil and pavement development. Prior to stage one is the actual exposure of the boulder as the result of erosion by wave action at the lake shoreline or by erosion as the lake level falls during desiccation. At stage-1 the upper surface of the boulder breaks up through mechanical weathering (salt weathering), while the rest of the boulder is still buried. At stage-2 the surface fragments fall to the edge of the stone and expose more of the stone, which continues to break-up. There is no soil development in stages 1 and 2. By stage-3 most of the stone is exposed and broken up, making a minihill. At this stage soil development begins with the accumulation of sandy soil between the rock fragments. At stage-4 the stones form small cones and the soil is more developed. It is sandy with a typical of colour 10 YR 6/6. At stage-5 the stones forming the small cone are completely fragmented. Stone fragments at the centre are very angular but smoother at the edges of the mini-hill as the result of weathering (etching by chemical processes?). Soil texture is silty/sand and soil colour is 7.5 YR 6/6. At stage-6 the surface is nearly flat. The soil is sandy/silt and soil colour is between 7.5 YR 5/6 and 7.5 YR 5/8. Stage-7a is the gibber plain phase, composed of small well rounded stones, as a result of continued etching of the edges of the fragmented stones. The soil is silty, and the soil colour is between 5 YR 5/6 and 5 YR 5/8. Stage-7b is also gibber plain, with small well rounded stone fragments but where the soil has been replaced by crystalline gypsum. This sequence differs from sequences described in other areas, especially on alluvial fan or terrace surfaces. This may be partly due to the different origin of the clasts, as “pre-weathered” silcrete boulders, and partly due to the importance of chemical weathering by “etching” in this salt-rich environment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Desert pavement development involves the in situ mechanical weathering of clasts. Most desert pavements described in the literature are formed of clasts derived either from the weathering of bedrock or from alluvial fan or terrace gravels. The pavement surfaces are modified by weathering processes involving fracturing and reorganising the clasts. These processes are influenced by temperature, moisture and salt (Yaalon, 1970; Al-Farraj, 1996; Al-Farraj and Harvey, 2000). Pavement development is intimately related to soil
⁎ Tel.: +971 506622909; fax: +971 3 7679199. E-mail address: [email protected]. 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.10.029
formation and development and is therefore time-dependent. McFadden et al. (1998) have studied in detail the vesicular layer and carbonate collars on desert soils and pavement. They made several observations; 1) dust accumulation and concomitant soil development are genetically linked to stone pavement formation; 2) wind activity and Quaternary climatic change influence the evolution on the vesicular fine-grained A horizon (Av); 3) changes in Av horizon increasingly influence soil infiltration and carbonate translocation and accumulation; 4) soil bulk composition is strongly influenced by Av horizon formation; and 5) late Pleistocene soils have more texturally mature Av horizons and a more limited rate and depth of leaching. These processes differ from the simple surface winnowing of fine material that produces a deflation pavement (Cooke and Warren, 1973; Goudie and Wilkinson, 1977; Williams and Greely, 1984; Breed
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et al., 1997), as occurs on surfaces where aeolian sand and fine gravels occur together (Al-Farraj, 2004). Desert pavements of both types have different local names; in Australia they are called gibber plains or stony mantles. Desert pavement characteristics have been used to indicate the relative age of geomorphic surfaces in arid regions. Such studies include those of McFadden et al. (1998, 1989) and Harvey and Wells (2003) in the USA, Amit and Gerson (1986) and Amit et al. (1993) in Israel, and Al-Farraj (1996, 2004) and Al-Farraj and Harvey (2000) in The United Arab Emirates. These studies all deal with pavements developed on alluvial depositional surfaces, and identify the older surfaces as being characterised by more mature pavements, comprising well sorted, angular clasts of small clast size, and with a packed interlocking surface texture (Al-Farraj and Harvey, 2000). The Lake Eyre basin, South Australia, is an area of active salt weathering. Desert pavements occur around the margin of the modern lake on a suite of surfaces created by erosion during former high stands of the lake. This offers a rather different context for the development of desert pavements; a source material provided by the erosional exposure of silcrete boulders and an extremely saline weathering environment rich in gypsum and halite. In this context both mechanical and chemical weathering processes may be effective. Mechanical salt weathering is considered as one of the quickest and most effective weathering processes in hot arid environments (Doornkamp and Ibrahim, 1990). Over the last century, many field observations and laboratory investigations have suggested that it is an important mechanism of rock breakdown (Cooke and Smalley, 1968; Amit et al., 1993; Cooke et al., 1993; Goudie, 1993; RodriguezNavarro and Doehne, 1999; Goudie et al., 2002). Goudie and Cooke
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(1984) demonstrated that the rocks are weathered rapidly in arid areas in response to saline material. Cooke (1981) outlined three main mechanical processes in salt weathering which include: 1) growth of salt crystals as a result of evaporation and/or cooling of saline solutions; 2) hydration as an agent changing the volume of salt crystals; and 3) thermal expansion of salt crystals. In addition, chemical absorption of the salt by the rock and soil materials may also result in volume changes that can cause disintegration of rock (Cooke, 1981) as well as bringing about chemical change within the materials. However, these processes depend on local environmental conditions (Beaumont, 1968; Chapman, 1980; Cooke, 1981; Sperling and Cooke, 1985; Rodriguez-Navarro and Doehne, 1999), particularly on the mineralogy of the salt concerned. This study investigates desert pavement development in this saline environment. In addition, it attempts to develop a model for the development of desert pavement since exposure of the silcrete boulders, based on the age of palaeo-lake shorelines derived from previous studies. The primary aim of this paper is to assess whether a similar time-dependent sequence of pavement development can be recognised here, as has been recognised on alluvial fan and wadi terrace fluvial depositional surfaces in the UAE (Al-Farraj, 1996; AlFarraj and Harvey, 2000). The secondary aim of this paper is to assess the extent to which the method developed earlier can characterize age relationships on these different pavements. In addition to the measures used previously, as it was obvious in the field that pavement development involved changes in micro-topography, the micro-topography of the surfaces was also recorded. This is shows a range from an exposed boulder through low hills of rock fragments to a uniform pavement surface.
Fig. 1. General geology map of the study area.
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2. Regional physical setting The Lake Eyre basin drains into two lakes; Lake Eyre (North) which is larger and located to north of the lake depression, and Lake Eyre (South) which is smaller and located to the south of the depression (Fig. 1). The study area is located to the southwest of Lake Eyre (South), and extends between Curdimurka to the east and opposite Swan Island to the west (Fig. 2). Lake Eyre, including both Lake Eyre (North) and Lake Eyre (South), is one of the largest salt pans in the world. It is subjected to major flooding after monsoon rains. The total area of Lake Eyre is 9330 km2, Lake Eyre (North) has an area of 8038 km2 and Lake Eyre (South) of 1300 km2 only. They are linked together by the Goyder channel, which has a length of 13 km and maximum width of 1.2 km. The water may flow either way, depending on the runoff sources (Bonython, 1955a; Dulhunty, 1978; Allan et al., 1986; Dulhunty, 1990). Lake Eyre is located in the most arid part of Australia (Fig. 1). Average annual rainfall ranges between 125 and 150 mm and annual evaporation rate ranges between 2500 and 3000 mm per year. Mean maximum summer temperature is 36 °C in January, and it may reach as high as 50 °C. Mean maximum winter temperature is 17 °C in July and it may reach as low as 5 °C. However, the drainage system headwaters are located in more humid monsoonal regions (Bonython and Mason, 1953). Most of the flood waters come from the northern and northeastern part of the catchment, which has annual rainfalls of 370–500 mm (Dulhunty, 1990). The Lake Eyre (South) salt crust is up to 20 m thick. It has been precipitated dominantly from water that flooded though the Goyder channel from Lake Eyre (North). Lake Eyre (North) has a 50 m thick salt crust (Dulhunty, 1974, 1978). However, the salt may also flow in the opposite direction by solution in northward-flowing groundwater, or occasionally when Lake Eyre (South) overflows into Lake Eyre (North) (Allan et al., 1986). The salt is ultimately derived largely from the weathering of exposed rocks within the catchment area (Bonython, 1955a,b). The salt had been transported into the lake as a solution in river water and shallow groundwater (Dulhunty, 1977; 1990), and has accumulated since the lake dried out during the late Pleistocene through to the Holocene. In addition, other environmental factors suggested by Johnson (1980) include: low topographic relief
and a predominantly endorheic drainage system have resulted in the retention of connate salts; conditions of arid climate have produced wind deflation hollows with the surficial cover of alluvium removed to expose the bedrock of the catchment; and subsequent seepage of saline groundwater containing the reworked salts into the depressions produces the characteristics salt lakes. The salt is similar to sea salt, dominantly of NaCl but with a lower potassium content (Wopfner and Twidale, 1967). The salt crust consists of sodium chloride (90– 95%), magnesium sulphate (5–7%), magnesium chloride (b4%) and calcium sulphate up to (b2.5%) (Bonython, 1955b; Bonython and King, 1956). 3. Regional geological setting The southwest of Lake Eyre (South) is a plateau of Quaternary fluvial and lacustrine sediment dissected by short streams (2 km in length), which expose the underlying deeply weathered materials. Most of the geological studies of the area deal with the Proterozoic and Palaeozoic strata, which are not exposed, but underlie the basin. However, these bedrocks are exposed in the headwaters of the streams that drain into the lake (Fig. 1) (Krieg et al., 1990, 1991; Magee et al., 1995). These and overlying younger rocks in the study area (Fig. 2) were subjected to deep chemical weathering producing silcrete crusts, particularly on Miocene palaeo-lake sediments and shoreline deposits (Croke, 1997). Following tectonic subsidence of the Lake Eyre region in the Late Cainozoic, continental sedimentation continued through to the Pleistocene (Wopfner and Twidale, 1967; Tedford and Wells, 1990). The continental sedimentary phases have been described by Krieg et al. (1990) and divided into three phases; two in the Tertiary and one in the Quaternary. The late Palaeocene–Eocene was a period of river deposition of fine sand, gravel and deposition of fine-grained laminated clay in swamps. From the late Oligocene to the middle Miocene very large, relatively shallow lakes covered much of the basin. These lakes were fed by meandering rivers with extensive floodplains (Krieg et al., 1990, 1991). The middle Miocene and Pliocene were times of silicification (Krieg et al., 1990, 1991), producing silcrete crusts (Goudie, 1973) on the lake and shoreline sediments. During this period the Lake Eyre basin was affected by widespread epeirogenic
Fig. 2. Locations of the sites examined in the study area.
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Fig. 3. Cross-section of the investigated sites in the study area.
subsidence with uplift of the margins of the basin. Since then the area has subject to only minor local movement. Presently, low gradient ephemeral channels drain from low elevation hills at the margin of the Lake Eyre basin. The major catchments to the northeast are fed with water by summer monsoon rains. Smaller catchments drain into Lake Eyre (South) from the Willouran Ranges. These ephemeral streams have very variable discharge both seasonally and yearly (Magee et al., 1995). Three major streams cut through the Willouran Mesozoic bedrock and drain into Lake Eyre (South); i.e. Gregory, Poole and Welcome/Frances, and the Warriner, Margaret and Stuart streams as well as small streams cut through the Tertiary plateau to the west (see Fig. 1). During global climatic oscillations of the late Quaternary associated with glacial and interglacial cycles, Australia underwent climatic oscillations between wetter and drier periods than the present day climate (Thomas, 1974; Ollier, 1975; Nanson et al., 1992). During wetter climates Lake Eyre became significantly larger than at present and more than 25 m deep. During arid periods the lake dried completely and sediment deflation took place, with the level of deflation controlled by groundwater (Magee et al., 1995). During the wetter periods flooding occurred (Bonython and Fraser, 1989), with relatively small amounts of sediment input (Gillespie et al., 1991). During dry periods small amounts of sediment have been removed by deflation, because the water table is relatively stable (Magee et al., 1995). Fragments of the Miocene silcrete crusts have been preserved as large boulders (N1 m long axes) within the lacustrine sediments
(Figs. 3 and 4). These boulders are believed to have been weathered by deep chemical weathering (Thomas, 1974; Ollier, 1975). During lake highstands, and by stream erosion at times of falling lake levels, these boulders have been exposed at the surface throughout the Late Pleistocene. During lake highstands wave action has eroded the palaeo-lake sediments, in some places creating low cliffs up to 10 m high (Dulhunty, 1975), and exposed the boulders. During falling lake levels stream erosion has acted to further expose the boulders. The exposed boulders have since been fragmented by mechanical weathering (salt-induced) to form the desert pavements, during which salt weathering may have been an important process. Around the modern lake margin are a series of low lake terraces preserving concentric former lake shorelines. Ages of the shorelines increase with elevation above the modern lake (Dulhunty, 1975). These shorelines provide a time framework for the exposure of the boulders and the initiation of desert pavement development on the lake terraces. 4. Methodology A survey of the rock pavement proprieties was carried out in the study area at 5 locations (Fig. 2). Locations were selected across the palaeo-shoreline sequences to be representative of the range of the rock pavement development in the area, and on the basis of accessibility of the exposed surfaces. At these locations 17 exposed surfaces were examined. At all sites scaled photographs of 1 m2 quadrats of the exposed surfaces were taken in the field.
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Fig. 4. Examples of desert pavement on the exposed surfaces, a–h youngest to oldest in the study area. Please note: mini-hills in c, and cone-hills in e.
Soil texture and colour were observed in the field to differentiate between otherwise similar desert pavement surfaces (Table 1). Soils have been characterised as follows. 1) Absent: at locations where the boulder is just exposed on active lake shorelines, and in locations where the boulder is still intact. 2) Immature: at locations where no horizon development is present, the soil is composed of weathered
materials from the boulders together with recently accumulated dust, but no evidence is present of CaCO3 accumulation. This material has depths of up to 10 cm, a sandy texture, and shows little reddening (Munsell colour: 10 YR 7/2, light grey). 3) Moderate: at locations where the soil profile shows discernable horizon development, with early stages (1 or 2) of CaCO3 accumulation. This material has depths
A. Al-Farraj / Geomorphology 100 (2008) 154–163 Table 1 Soil maturity and elevation of sample sites above the lake bed Stage
Site
Elevation (m)
Soil depth (cm)
Soil colour
Soil texture
1 2
5.7 3.3 2.3 5.6 5.5 4.2 3.2 2.2 5.4
0.5 4⁎ 5⁎ 4 4.5 5 6 5 8
0 0 0 10 40 40 40 40 50
No soil No soil No soil Sand Sand Sand Sand Sand Silty sand
1.2 5.3 3.1 5.2 4.1 2.1 1.1 5.1
10 11 11 15 15 15 15 22
50 60 70 90 90 80 100 ?
No soil No soil No soil 10 YR 7/2 (light grey) 10 YR 6/4 (light yellowish brown) 10 YR 5/6 (yellowish brown) 10 YR 5/6 (yellowish brown) 10 YR 5/6 (yellowish brown) Between 7.5 YR 5/6 (strong brown) and 7.5 YR 6/6 (reddish yellow) 7.5 YR 5/6 (strong brown) Between 7.5 YR 5/6 and 7.5 YR 5/8 7.5 YR 5/6 light brown 5 YR 5/6 (yellowish red) 5 YR 5/8 (yellowish red) 5 YR 5/8 (yellowish red)) 5 YR 5/6 (yellowish red) No soil: crystalline gypsum
3 4
5
6 7a
7b
Silty sand Sandy silt Sandy silt Silt Silt Silt Silt
⁎ These two locations are higher than more developed pavements, however they are on surfaces recently exposed by stream erosion.
of up to 40 cm, a sandy texture, with soil colour ranging between 10 YR 6/4 (light yellowish brown) and 10 YR 5/6 (yellowish brown). 4) Mature: showing distinctive horizons with later stages of CaCO3 accumulation (3–4). The soil profile has a depth of 60–100 cm, and the soil texture ranges between sandy silt to silt. The soil colour ranges between 7.5 YR 5/6 (light brown) and 5 YR 5/6 (yellowish red). Quantitative measurement of clast size, angularity, and sorting on the pavement surfaces was carried out from the photographs. From each surface photograph length and width axes of the largest 25 stones were measured, using the same methods as used by Al-Farraj (1996) and Al-Farraj and Harvey (2000). The angularity of the same 25 stones on each surface was recorded using the Powers Angularity Chart (Tucker, 1996). Percentage cover of selected stone sizes was
159
calculated by systematically sampling 100 points from a grid drawn onto the photograph and recording clast size at each sample point (AlFarraj, 1996; Al-Farraj and Harvey, 2000). This uses the same methodology that was used to characterize the pavement development on fluvial depositional surfaces in UAE, although the origin of the clasts that form the pavements is different. 5. Results and interpretation The recently exposed silcrete boulders range in size up to about 1.5 m in length, 1 m in width and 0.70 m in thickness. During the earlier breakup stages these boulders form topographic domes (mini-hills) about 0.5 m above the surface. The fragments around the domes occupy a diameter of about 7 m. As weathering proceeds these mini-hills become cones, with heights initially between 0.9 and 1.1 m, larger than those of the mini-hills because of exposure of the full depth of the boulders, with the fragments scattered around a diameter of up to 17 m. For site 1.1 the cone height is unusual as it is exposed on the edge of a channel. This is as a result of the boulder base being eroded entirely and the fractured fragments rolling down the slope by gravity making the cone and its fractured fragments appear to be higher than the original boulder thickness. As these cones degrade, the surface relief becomes only 10 cm in height but the diameter ranges between 17 and 18 m (see Table 2 for more details). On the gibber plain both the boulder boundaries disappear and the surface relief is reduced to between 1 and 2 cm, as with normal desert pavement. The particle length on the top and around these boulders, and on the gibber plain, ranges between 10 cm and 1 cm, from youngest to oldest. The particle sizes reflect the time of exposure rather than any other factor (e.g. parent material). The 17 exposed surfaces have been classified into 7 types/stages according to the soil maturity (colour and texture) and elevation above the lake bed (Table 1; Figs. 3–5). This shows 7 different pavement types/stages: stage 1 is the youngest and stage 7 is the oldest. Stage 1 occurs at a height of 0.5 m above the present lake bed. No soil is developed at this stage. It is characterised by a microtopography inherited from the boulders. This type appears at location 5.7 (Fig. 2 and 3). Stage 2 occurs at a height of 1 m on a surface recently exposed by stream erosion, but 4 m above the present lake bed
Table 2 Desert pavement particle size characteristics, clast angularity, and pavement relief, derived from field observations and measurements made from photographs of 1 m2 sample quadrats on 17 surfaces Stage
1 2 3 4
5 6 7a
7b a b c d e
Site
5.7 3.3 2.3 5.6 5.5 4.2 3.2 2.2 5.4 1.2 5.3 3.1 5.2 4.1 2.1 1.1 5.1
Largest 1–25 clasts a-axes (cm)a
Percentage cover by clasts with b-axis (cm)b
Clast angularity: largest 25 clastsc
Surface topography
1
2
5
10
25
N 10
N7
N5
N4
VA 0
A1
SA 2
SR 3
R4
WR 5
Mnd
10.1 10.7 9.9 16 10.5 7.2 7.1 7.7 16.3 12 11 11.1 8.8 8.2 7.7 9.5 8.8
9.3 6.7 9.4 12.6 10.1 6.6 6.7 6.6 13.5 12 8.7 10.4 7.2 8 6.6 7.9 5.7
8.3 6.1 7.7 10.3 7.7 5.5 6.2 6 9.2 9.7 6.6 9.1 5.7 6.1 6 6.1 5.3
6.4 5.5 6.5 9.5 6.9 4.1 5.1 4.3 7.9 8.7 6.1 7.8 4.9 5.6 4.3 5.1 5.1
3.3 3.8 2.9 4.4 3.1 2.7 2.5 1.9 4.9 4.5 4 3.2 3 3.8 1.9 2.9 3.1
8 12 2 05 4 3 3 2 6 12 2 3 2 0 0 0 1
15 19 5 8 7 6 5 3 12 20 5 5 5 2 1 2 6
17 21 15 13 12 9 10 6 15 24 8 13 11 9 5 7 10
19 25 21 16 15 16 18 12 17 27 17 19 21 20 14 15 18
25 13 13 12 12 13 11 11 12 10 8 0 5 4 0 1 0
– 12 12 10 12 8 9 13 8 9 9 0 5 6 0 2 0
– – – 3 1 4 5 1 5 6 8 12 7 8 11 12 12
– – – – – – – –
– – – – – – – –
– – – – – – – –
– – 6 8 7 7 5 5
– – 7 – – 7 5 6
– – – – – – – 2
0 0.48 0.48 0.64 0.56 0.64 0.76 0.6 072 0.84 1.0 2.8 1.72 1.72 2.84 2.44 2.6
B B B M M M M M C C F F G G G G G
e
Surface relief (m)
Surface diameter (m)
0.3 0.4 0.35 0.5 0.6 0.7 0.6 0.8 0.9 1.1 0.1 0.2 0.02 0.02 0.02 0.015 0.02
3.5 4 4 7 8 9.5 8.5 10 15 17 17 18 – – – – –
a-axis (cm) for the 1st 2nd, 5th, 10th and 25th largest stone within the 1-m2 sample quadrats. Percentage cover of the quadrats occupied by clasts N 4, 5, 7, and 10 cm b-axis. Clast angularity of the 25 largest clast, classified on powers roundness scale; VA = very angular, A = angular, SA = sub-angular, SR = sub-rounded, R = rounded, WR = well rounded. Average values derived from allocating a scale of VA = 0 to WR = 5. B = Upper part of boulder exposed and breaking up; M = “mini-hill”; C = cone-hill; F = almost flat surface; G = flat surface, “Gibber plain”.
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Fig. 5. Model of the salt weathering and desert pavement processes.
(Table 1).This surface has no soil (location 3.3; Fig. 2 and 3). Stage 3 occurs at a height of 4 m above the lake bed and comprises a soil of grey sandy materials (location 5.6; Fig. 2). Stage 4 occurs at an average height of 5 m above the present lake bed and has a sandy soil with a colour of 10 YR 6/4 (locations 5.5, 1.2, 2.2, 3.2, 3.2; Fig. 2). Stage 5 occurs at an average height of 9 m above lake bed and has a more mature soil with a soil colour of 7.5 YR 6/6 (location 1.2, 5.4; Fig. 2). Stage 6 occurs at a height of 11 m above present lake bed, with a mature soil with identifiable horizon development and a soil colour of 7.5 YR 5/6 (locations 5.3 and 3.1; Fig. 2). Stage 7a occurs at a height of 15 m above the present lake bed with soil colour 5 YR 5/6 (locations 5.2, 4.1, 3.1, 2.1 and 1.1; Fig. 2), This stage represents the mature “gibber plain” desert pavement. At one site (location 5.1; Fig. 2), a Stage 7 pavement occurs, 22 m above present lake bed, that has little soil but instead has inter-granular spaces filled with crystalline gypsum. This is characterized as Stage 7b. The soil characteristics (colour and texture) have been used to differentiate between the surfaces (see above). The soil characteristics beneath the pavement surfaces and the elevations have been summarized in Table 1. Except for sites 4 and 5, which have higher elevations than more developed pavements, the stages and the soil development are closely related to height, and time as McFadden et al. (1998) have suggested. The reason for the two exceptions of sites 4 and 5 is that these two sites have been more recently exposed as a result of channel activity (Fig. 3).
Table 2 summarizes the desert pavement clast size characteristics based in part on the methodology used by Al-Farraj (1996) and Al-Farraj and Harvey (2000) but also including observations on microtopography. Length and width axis measurements of the largest 25 clast within each quadrat demonstrate little difference between the surfaces, expressed either by a-axis measurements or by b-axis distributions. This may be for two reasons; 1) the high rate of weathering, where even young surfaces have small stone fragments (to be discussed later in relation to both clast origin and weathering regime); or 2) the quadrat position — for consistency most of the samples were taken of the exposed surface at the centre of a fragmented boulder. In this zone the shattered fragments are highly similar; any smaller clasts may have moved to the edges of the boulder circumference (Fig. 4). The roundness characteristics (Table 2) derived from the 25 largest clasts show a decrease in angularity with age from stage 1 to stage 7. This is in contrast to many previous findings, and may relate to chemical weathering of the clast edges (to be discussed below). 6. Discussion These results are in marked contrast with those for age-related pavement characteristics in UAE (Al-Farraj, 1996; Al-Farraj and Harvey, 2000) or those reported in other pavement studies (e.g. McFadden et al., 1989). There are two possible reasons for this. The first reason
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could be that this is not an age-related sequence at all, and all the pavement samples have been developed over the same period of time. This is unlikely because there are clear age-related trends in soil development, micro-topography and an unexpected trend of decreasing angularity with apparent age. Furthermore, these sites themselves are located across the age range of former lake shorelines (see below). The second reason is related to the possibility of a different pavementforming mechanism between here and the sites of the UAE and elsewhere. Over last five decades, several filling and drying periods have been recorded at Lake Eyre (Bonython and Mason, 1953; Dulhunty, 1975; Tetziaff and Bye, 1978; Dulhunty, 1984, 1986; Allan et al., 1986). Three major fillings occurred in 1949, 1974 and 1984 together with a number of minor fillings (Dulhunty, 1990). Madigan, when he reached the Lake Eyre for the first time in 1929, found the Lake dry and that made him assume it was never wet. His observation was the perceived wisdom until the 1949–50 flooding which was the first known and well documented flood. However, there are records of earlier instances when the lake held water, that have been highlighted by Bonython and Mason (1953) as follows. In 1840 a flood was reported by E.J. Eyre, (Lake Eyre was named after him). He believed the lake might be a great inland sea. More water reached the lake in the 1890s, and in 1922 the Royal Australian Air Force reported that one third of the lake was flooded. Recorded rain data suggested that water might have reached the lake in 1890, 1906, 1916, 1917 and 1920 (Bonython and Mason, 1953). Dulhunty (1975) demonstrated three prehistoric fillings in Lake Eyre to levels of 280, 160, and 70 cm above the 1974 filling. He suggested dates of 3000, 1500, 500 years before present. Krieg et al. (1990, 1991) suggested three Pleistocene phases of fluvial deposition in Lake Eyre and southern Lake Eyre (South). Nanson et al. (1992) have dated these surfaces using thermoluminescence (TL) and U/Th dating. The dates given are between 130,000 and 100,000 years for the oldest phase, between 25,000 and 40,000 for the middle phase, and 18,000 for the youngest phase is (Hubbard, 1995a,b). During the late Pleistocene/Holocene small streams have cut into the sediments. Initiation of these small streams together with filling and drying of the lake exposed the underlying boulders. Different times of boulder exposure have resulted in different stages of weathering and pavement development. Boulders which are exposed for longer periods of time are more weathered than those which have been exposed more recently. Combining these chronological data (Table 3) with desert pavement development data on the lake shorelines (Table 2; Fig. 4) a model for desert pavement development rate and processes in this area can be put forward (Fig. 5). Pre-stage 1 is the actual exposure of the boulder as a result of erosion by lake flooding then drying; such unweathered boulders as these are exposed at the present time on the modern lake shore (Fig. 4). At stage 1 the upper surface of the boulder is broken up while the rest of boulder is still buried by the lacustrine sediments (e.g., location 5.7; Figs. 2, 4 and 5). At this stage, two processes appear to
Table 3 Summary of surface chronology provided from the literature Stage Suggested date Reference 1
3 4 5 6
Less than 200 years Less than 500 years 500 years 1500 3 ka 18 ka
7 7a
25–40 ka 100–130 ka
2
According to A.J. Eyre flood record in 1840 According to floods record and weathering rate compared with Dulhunty (1975) estimation Estimated by Dulhunty (1975)
Dated by Nanson et al. (1992) using thermoluminescence (TL) and U/Th.
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operate: 1) horizontal cracks occur, caused by insolation heating and cooling; 2) vertical cracks extend into the interior of the boulder, which may be caused by salt weathering exploiting previous weaknesses within the rock. That could be the pattern produced by salt weathering. At this stage there is no soil development and the stone fragments are very sharp. A suggested age for this stage is less than 200 years according to the flood record of A.J. Eyre. At stage 2 the upper surface of the boulder is broken up horizontally, and the inner part of the rock breaks up vertically (See Figs. 4 and 5) (e.g. location 5.6; Fig. 2). Again there is no soil development, no soil horizon development and no soil colour change. Soil absence may be because it is an unstable surface because of frequent flooding. At this stage and stone fragments are very sharp. A suggested age is less than 500 years, on the basis of estimated dates from Dulhunty (1975). By stage 3 most of the stone is exposed, creating what is described here as a ‘mini-hill’ topography (Figs. 4 and 5; Table 2). At this stage there is the beginning of soil development, with a profile of grey sandy materials, colour 10 YR 6/2 (pale brown) (e.g. location 5.5; Fig. 2). A suggested age for this pavement stage could be 500 years or more, again according to Dulhunty's (1975) estimation of shoreline age. At stage 4 the stones make a mini-hill and the soil is more developed but still sandy with a colour of 10 YR 5/6 (yellowish brown), (e.g. location 5.4, Fig. 2). A suggested age for this pavement stage would be 1500 years or more according to Dulhunty's (1975) estimation of shoreline ages. By stage 5 the stones making the degraded cone-hill are all fragmented. Stone fragments at the centre are very sharp, while those at the edges are more rounded (perhaps as a result of etching). Soil texture is silty sand and soil colour is 7.5 YR 6/6. This stage can be recognized at locations 1.2 and 5.3 (Fig. 2). A suggested age for this pavement stage could be 1500 years or more according to Dulhunty's (1975) shoreline age estimates. By stage 6 the surface is nearly flat, the boulders have disintegrated into rounded clasts. The soil is sandy silt and soil colour is 7.5 YR 5/6. This surface has been dated to 18 ka by Nanson et al. (1992) using thermoluminescence (TL) and U/Th dating. Stage 7a is the “gibber plain” phase with small well rounded stones and a silty soil. Soil colour is 5 YR 5/6. Again this surface has been dated by Nanson et al. (1992) using (TL) and U/Th, with a given date of 25–40 ka. Stage 7b also is characterised by gibber plain well rounded clasts, but it has no soil (see Amit et al., 1993; McFadden et al., 1998). Instead the pore spaces are infilled by crystalline gypsum. The soil has become engulfed by pedogenic gypsum. This surface has been dated by Nanson et al. (1992) to between 100 and 130 ka. In general, these dates are in accord with other dates for the rates of soil development on other desert regions (see soil profile description above) (e.g. Amit and Gerson, 1986; Amit et al., 1993). The age gap between stages 1–4 and 5–7 may be related to the likelihood that the more recent surfaces (stages 1–4) have retained more details of changes than the older surfaces (stages 5–7). The two main differences between the sequence observed here and those reported in the USA and the UAE are the very limited changes in clast size and the decreasing, as opposed to increasing, clast angularity with increasing surface age on these Australian pavements. Two main reasons can be proposed in explanation of these differences. First, the origin of the clasts differs. The other studies referred to above have dealt primarily with pavement development of alluvial fan or fluvial gravel deposits within which the original clasts inherit particle size and angularity characteristics from those depositional environments and are progressively shattered by mechanical weathering processes to show reduced particle size and increased angularity with increasing pavement age. In this study the source materials are silcrete boulders derived from older deep weathering profiles. In this sense they may have been subjected to pre-weathering processes involving brecciation and re-cementation during silcrete formation (Goudie,
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1973; Goudie and Pye, 1983), creating weaknesses within the boulders whose spacing was related to silcrete formation. Thus, on weathering following exposure at the surface, fractures may develop following previous fracture lines rather than in relation to modern stress patterns induced by surface exposure. The result may be that mechanical breakdown produces relatively small, uniformly-sized, highly angular clasts, that are perhaps less prone to further size reduction by shattering. Secondly, the weathering environments appear to differ. The Lake Eyre environment is a highly saline environment increasing the effectiveness of salt-induced mechanical weathering, and within which pH may be sufficiently high to cause “etching” of the clast edges by chemical processes. The high rate of saltinduced mechanical weathering may contribute to the rapidity of the initial breakdown of the boulders. Etching of the clast edges on the older pavement surfaces accounts for the decrease in angularity with surface age. It may be significant that in the Atacama Desert, Chile, also a highly saline environment (Berger, 1997), clasts on desert pavement surfaces also show evidence of chemical weathering (Anne Mather, pers. comm.). 7. Conclusion A sequence of desert pavement development can be recognised around the margins of Lake Eyre (South). Exhumed silcrete boulders weather to provide the clasts for pavement formation. The timescale for pavement development can be related to the palaeo-lake shoreline sequence. Successive stages in the pavement sequence can be related to stages in soil development and to progressive changes in microtopography. Seven stages of pavement development can be recognised, from the initial exposure of the boulders to total break-up and organisation of the clasts into a mature “gibber plain” desert pavement. The pavement development sequence recognised here differs from those recognised in other arid regions, particularly the USA and the UAE, in that there is relatively little, if any, progressive reduction in clast size, after the initial salt-induced mechanical weathering of the silcrete boulders, and a progressive decrease rather than an increase in angularity. The reasons for these differences appear in part to relate to the origin of the clasts as the products of disintegration of silcrete boulders, and in part to the highly saline weathering environment. In this environment not only does the abundance of salt lead to rapid saltinduced mechanical weathering on exposure of the boulders, but appears to lead also to chemical weathering in the form of etching of the clast edges during the later stages of pavement development. It is clear that the morphology and development of desert pavements as a whole reflects both the parent material and interactions between different weathering processes. Acknowledgements I'm grateful to Martin William and Peter Gill and for their assistance in making the field work for this investigation successful. I also would like to acknowledge Adelaide University for providing the vehicle for the field trip. I am also grateful to Adrian Harvey for feedback on this work. I also would like to thank Ahmad Massasati to his help with the figures. Finally I would like to thank the referees L.D. McFadden and A. Yair for the valuable comments which have enriched the discussion. References Al-Farraj, A., 1996. Late Pleistocene Geomorphology in Wadi Al-Bih northern UAE and Oman: with special emphasis on Wadi terrace and alluvial fans. PhD. Thesis, University of Liverpool, 363pp. Al-Farraj, A., 2004. Review of desert pavement types in the UAE: development and implications for rates and processes of formation. Joint International Geomorphology Conference, 18–20 August 2004, Glasgow, Abstracts Volume, p. 37. Al-Farraj, A., Harvey, A.M., 2000. Desert pavement characteristics on Wadi terrace and alluvial fan surface: Wadi Al-Bih UAE and Oman. Geomorphology 35, 279–297. Allan, R.J., Bye, J.A.T., Hutton, P., 1986. The 1984 filling of Lake Eyre South. Transactions of the Royal Society of Australia 110, 81–87.
Amit, R., Gerson, R., 1986. The Evolution of Holocene Reg (gravelly) soils in deserts — an example from the Dead Sea region. Catena 13, 59–79. Amit, R., Gerson, R., Yaalon, D.H., 1993. Stages and rate of gravel shattering process by salts in desert Reg soil. Geoderma 57, 295–324. Beaumont, P., 1968. Salt weathering on margin of the Great Kavir, Iran. Geological Society of America Bulletin 79, 1683–1684. Berger, I.A., 1997. South America, In: Thomas, D.S.G. (Ed.), Arid Zone Geomorphology: Process, Form and Change in Drylands, 2nd ed. Wiley, Chichester, pp. 543–562. Bonython, C.W., 1955a. Lake Eyre, South Australia. The great flooding of 1949–50. Report of the Lake Eyre committee. Royal Geographical Society of Australia, South Australia Branch. Griffin Press, Adelaide, pp. 27–36. Bonython, C.W., 1955b. The salt of Lake Eyre—its occurrence in Madigan Gulf and its possible origin. Royal Society of South Australia. Transactions 79, 66–92. Bonython, C.W., Mason, B., 1953. The filling and drying of Lake Eyre. Geographical Journal 119, 321–330. Bonython, C.W., Fraser, A.S., 1989. The great filling of Lake Eyre in 1974. Geographical society. Australia (South Australia Branch), Adelaide. 120 pp. Bonython, C.W., King, O., 1956. The occurrence of native sulphur at Lake Eyre, Royal Society of South Australia. Transactions 79, 121–129. Breed, C.S., McCauley, J.F., Whitney, M.I., 1997. Wind erosion forms. In: Thomas, D. (Ed.), Arid zone Geomorphology. Belhaven Press, London, pp. 437–464. Chapman, R.W., 1980. Salt weathering by sodium chloride in the Saudi Arabian desert. American Journal of Science 280, 116–129. Cooke, R.U., 1981. Salt weathering in deserts. Proceeding of the Geologist Association, London 92, 1–16. Cooke, R.U., Smalley, I.J., 1968. Salt weathering in deserts. Nature 220, 1226–1227. Cooke, R.U., Warren, A., 1973. Geomorphology in Deserts. Batsford, London. 322 pp. Cooke, R.U., Warren, A., Goudie, A.,1993. Desert Geomorphology. UCL press, London, p. 526. Croke, J., 1997. Australia, In: Thomas, D.S.G. (Ed.), Arid Zone Geomorphology: Process, Form and Change in Drylands, 2nd ed. Wiley, Chichester, pp. 563–573. Doornkamp, J.C., Ibrahim, H.A.M., 1990. Salt weathering. Progress in Physical Geography 14, 335–348. Dulhunty, J.A., 1974. Salt crust distribution and lakebed conditions in southern areas of Lake Eyre North. Royal Society of South Australia. Transactions 98, 125–134. Dulhunty, J.A., 1975. Shoreline shingle terraces and prehistoric fillings of Lake Eyre. Royal Society of South Australia. Transactions 99, 183–188. Dulhunty, J.A., 1977. Salt crust solution during fillings of Lake Eyre Royal Society of South Australia. Transactions 101, 147–151. Dulhunty, J.A., 1978. Salt transfers between North and South Lake Eyre. Royal Society of South Australia. Transactions 102, 107–112. Dulhunty, R., 1984. When the Dead Heart beats lake Eyre lives, 2nd ed. J.A. Dulhunty, Seaforth, N.S.W. 92p. Dulhunty, J.A., 1986. 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Chemical Sediments and Geomorphology. Academic Press, London. Goudie, A.S., Wilkinson, J., 1977. The Warm Desert Environment. Cambridge University Press, Cambridge. 88 p. Goudie, A.S., Wright, E., Viles, H.A., 2002. The roles of salt (Sodium nitrate) and fog in weathering: a laboratory simulation of conditions in the northern Atacama Desert, Chile. Catena 48, 255–266. Harvey, A.M., Wells, S.G., 2003. Late Quaternary variations in alluvial fan sedimentologic and geomorphic processes, Soda Lake basin, eastern Mojave Desert, California. In: Enzel, Y., Wells, S.G., Lancaster, N. (Eds.), Palaeoenvironments and Palaeohydrology of the Mojave and Southern Great Basin Deserts. Sp. Paper, vol. 368. Geological Society of America, pp. 207–230. Hubbard, N.N., 1995a. In search of regional palaeoclimates: Australia, 18,000 yr BP. Palaeogeography, Palaeoclimatology, Palaeoecology 116, 167–188. Hubbard, N.N., 1995b. An integrated method for reconstructing regional palaeoclimates: Australia, 18,000 YR BP. Palaeogeography, Palaeoclimatology, Palaeocology 116, 141–166. Johnson, M., 1980. The origin of Australia's salt lakes. New South Wales Geological Survey-Records 19, 221–266. Krieg, G.W., Callen, R.A., Gravestock, D.I., Gatehouse, C.G., 1990. Geology. In: Tyler, M.J., Twidale, C.R., Davies, M., Wells, C.B. (Eds.), Natural History of the Northeast Desert. Occasional Publications, vol. 6. Royal society of South Australia, Adelaide, pp. 1–26. Krieg, G.W., Roger, P.A., Callen, R.A., Freeman, P.J., Alley, N.F., Forbes, B.G., 1991. Explanatory notes for the Curdimurka 1:250 000 geological map. Magee, J.W., Bowler, J.M., Miller, G.H., Williams, D.L.G., 1995. Stratigraphy, sedimentology, chronology and Palaeohydrology of Quaternary lacustrine deposits at Madigan Gulf, Lake Eyre, South Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 113, 3–42. McFadden, L.D., Ritter, J.B., Wells, S.G., 1989. Use of multiparameter relative-age methods for estimation and correlation of alluvial fan surfaces on a desert piedmont, eastern Mojave Desert, California. Quaternary Research 32, 276–290.
A. Al-Farraj / Geomorphology 100 (2008) 154–163 McFadden, L.D., McDonald, E.V., Wells, S.G., Anderson, K., Quade, J., Forman, S.L., 1998. The vesicular layer and carbonate collars in desert soils and pavement: formation, age and relation to climate change. Geomorphology 24, 101–145. Nanson, G.C., Price, D.M., Short, S.A., 1992. Wetting and drying of Australia over the past 300 ka. Geology 20, 791–794. Ollier, C., 1975. Weathering. Longman, London. 297 p. Rodriguez-Navarro, C., Doehne, E., 1999. Salt weathering: influence of evaporation rate, supersaturation and crystallization pattern. Earth Surface Processes and Landforms 24, 191–209. Sperling, C.B., Cooke, R.V., 1985. Laboratory simulation of rock weathering by salt crystallization and hydration processes in hot, arid environments. Earth Surface Processes and Landforms 10, 541–555. Tedford, R.H., Wells, R.T., 1990. Pleistocene deposits and fossil vertebrate from the ‘Dead Heart of Australia’ Mem. Queensland Museum 28, 263–284.
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Tetziaff, G., Bye, J.A.T., 1978. Water balance of Lake Eyre for the flooded period January 1974–June 1976, vol. 102. Royal Society of South Australia, Adelaide, pp. 91–96. Thomas, M.F., 1974. Tropical Geomorphology, A study of weathering landform development in warm climates. MacMillan, London. 302 p. Tucker, M.E., 1996. Sedimentary Rocks in the Field. Wiley, Chichester. 153 p. Williams, S., Greeley, R., 1984. Desert pavement study at Amboy, California. Planetary Geological Program 169–170. Wopfner, H., Twidale, C.R., 1967. Geomorphological history of the Lake Eyre basin. In: Jennings, J.N., Mabbut, J.A. (Eds.), Landform Studies from Australia and New Guinea. Australian National University Press, Canberra, A.C.T., pp. 119–143. Yaalon, D.H., 1970. Parallel stone cracking a weathering process on desert surfaces. Bucharest, Geological Institute of Technical Economics Bulletin 18, 107–110.
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Desert pavement development on the lake shorelines of Lake Eyre (South), South Australia Asma Al-Farraj ⁎ P.O. Box 17771, Geography Department, U.A.E. University, Al Ain, United Arab Emirates
A R T I C L E
I N F O
Article history: Received 30 October 2005 Received in revised form 1 September 2006 Accepted 6 October 2007 Available online 15 May 2008 Keywords: Desert pavement Salt weathering Lake shore Lake Eyre (South) South Australia
A B S T R A C T To the southwest of Lake Eyre (South), South Australia, silcrete boulders exposed by the erosion of the surrounding fine sediments undergo mechanical weathering to form desert pavement. Successive palaeoshorelines of Lake Eyre have exposed an age-related sequence of different stages in the weathering of the boulders. This study investigates desert pavement development in this saline environment. In addition, it attempts to develop a model for the development of desert pavement following exposure of the silcrete boulders, based on palaeo-lake shorelines dated from previous studies. Seven stages can be recognised corresponding to stages of soil and pavement development. Prior to stage one is the actual exposure of the boulder as the result of erosion by wave action at the lake shoreline or by erosion as the lake level falls during desiccation. At stage-1 the upper surface of the boulder breaks up through mechanical weathering (salt weathering), while the rest of the boulder is still buried. At stage-2 the surface fragments fall to the edge of the stone and expose more of the stone, which continues to break-up. There is no soil development in stages 1 and 2. By stage-3 most of the stone is exposed and broken up, making a minihill. At this stage soil development begins with the accumulation of sandy soil between the rock fragments. At stage-4 the stones form small cones and the soil is more developed. It is sandy with a typical of colour 10 YR 6/6. At stage-5 the stones forming the small cone are completely fragmented. Stone fragments at the centre are very angular but smoother at the edges of the mini-hill as the result of weathering (etching by chemical processes?). Soil texture is silty/sand and soil colour is 7.5 YR 6/6. At stage-6 the surface is nearly flat. The soil is sandy/silt and soil colour is between 7.5 YR 5/6 and 7.5 YR 5/8. Stage-7a is the gibber plain phase, composed of small well rounded stones, as a result of continued etching of the edges of the fragmented stones. The soil is silty, and the soil colour is between 5 YR 5/6 and 5 YR 5/8. Stage-7b is also gibber plain, with small well rounded stone fragments but where the soil has been replaced by crystalline gypsum. This sequence differs from sequences described in other areas, especially on alluvial fan or terrace surfaces. This may be partly due to the different origin of the clasts, as “pre-weathered” silcrete boulders, and partly due to the importance of chemical weathering by “etching” in this salt-rich environment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Desert pavement development involves the in situ mechanical weathering of clasts. Most desert pavements described in the literature are formed of clasts derived either from the weathering of bedrock or from alluvial fan or terrace gravels. The pavement surfaces are modified by weathering processes involving fracturing and reorganising the clasts. These processes are influenced by temperature, moisture and salt (Yaalon, 1970; Al-Farraj, 1996; Al-Farraj and Harvey, 2000). Pavement development is intimately related to soil
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formation and development and is therefore time-dependent. McFadden et al. (1998) have studied in detail the vesicular layer and carbonate collars on desert soils and pavement. They made several observations; 1) dust accumulation and concomitant soil development are genetically linked to stone pavement formation; 2) wind activity and Quaternary climatic change influence the evolution on the vesicular fine-grained A horizon (Av); 3) changes in Av horizon increasingly influence soil infiltration and carbonate translocation and accumulation; 4) soil bulk composition is strongly influenced by Av horizon formation; and 5) late Pleistocene soils have more texturally mature Av horizons and a more limited rate and depth of leaching. These processes differ from the simple surface winnowing of fine material that produces a deflation pavement (Cooke and Warren, 1973; Goudie and Wilkinson, 1977; Williams and Greely, 1984; Breed
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et al., 1997), as occurs on surfaces where aeolian sand and fine gravels occur together (Al-Farraj, 2004). Desert pavements of both types have different local names; in Australia they are called gibber plains or stony mantles. Desert pavement characteristics have been used to indicate the relative age of geomorphic surfaces in arid regions. Such studies include those of McFadden et al. (1998, 1989) and Harvey and Wells (2003) in the USA, Amit and Gerson (1986) and Amit et al. (1993) in Israel, and Al-Farraj (1996, 2004) and Al-Farraj and Harvey (2000) in The United Arab Emirates. These studies all deal with pavements developed on alluvial depositional surfaces, and identify the older surfaces as being characterised by more mature pavements, comprising well sorted, angular clasts of small clast size, and with a packed interlocking surface texture (Al-Farraj and Harvey, 2000). The Lake Eyre basin, South Australia, is an area of active salt weathering. Desert pavements occur around the margin of the modern lake on a suite of surfaces created by erosion during former high stands of the lake. This offers a rather different context for the development of desert pavements; a source material provided by the erosional exposure of silcrete boulders and an extremely saline weathering environment rich in gypsum and halite. In this context both mechanical and chemical weathering processes may be effective. Mechanical salt weathering is considered as one of the quickest and most effective weathering processes in hot arid environments (Doornkamp and Ibrahim, 1990). Over the last century, many field observations and laboratory investigations have suggested that it is an important mechanism of rock breakdown (Cooke and Smalley, 1968; Amit et al., 1993; Cooke et al., 1993; Goudie, 1993; RodriguezNavarro and Doehne, 1999; Goudie et al., 2002). Goudie and Cooke
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(1984) demonstrated that the rocks are weathered rapidly in arid areas in response to saline material. Cooke (1981) outlined three main mechanical processes in salt weathering which include: 1) growth of salt crystals as a result of evaporation and/or cooling of saline solutions; 2) hydration as an agent changing the volume of salt crystals; and 3) thermal expansion of salt crystals. In addition, chemical absorption of the salt by the rock and soil materials may also result in volume changes that can cause disintegration of rock (Cooke, 1981) as well as bringing about chemical change within the materials. However, these processes depend on local environmental conditions (Beaumont, 1968; Chapman, 1980; Cooke, 1981; Sperling and Cooke, 1985; Rodriguez-Navarro and Doehne, 1999), particularly on the mineralogy of the salt concerned. This study investigates desert pavement development in this saline environment. In addition, it attempts to develop a model for the development of desert pavement since exposure of the silcrete boulders, based on the age of palaeo-lake shorelines derived from previous studies. The primary aim of this paper is to assess whether a similar time-dependent sequence of pavement development can be recognised here, as has been recognised on alluvial fan and wadi terrace fluvial depositional surfaces in the UAE (Al-Farraj, 1996; AlFarraj and Harvey, 2000). The secondary aim of this paper is to assess the extent to which the method developed earlier can characterize age relationships on these different pavements. In addition to the measures used previously, as it was obvious in the field that pavement development involved changes in micro-topography, the micro-topography of the surfaces was also recorded. This is shows a range from an exposed boulder through low hills of rock fragments to a uniform pavement surface.
Fig. 1. General geology map of the study area.
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2. Regional physical setting The Lake Eyre basin drains into two lakes; Lake Eyre (North) which is larger and located to north of the lake depression, and Lake Eyre (South) which is smaller and located to the south of the depression (Fig. 1). The study area is located to the southwest of Lake Eyre (South), and extends between Curdimurka to the east and opposite Swan Island to the west (Fig. 2). Lake Eyre, including both Lake Eyre (North) and Lake Eyre (South), is one of the largest salt pans in the world. It is subjected to major flooding after monsoon rains. The total area of Lake Eyre is 9330 km2, Lake Eyre (North) has an area of 8038 km2 and Lake Eyre (South) of 1300 km2 only. They are linked together by the Goyder channel, which has a length of 13 km and maximum width of 1.2 km. The water may flow either way, depending on the runoff sources (Bonython, 1955a; Dulhunty, 1978; Allan et al., 1986; Dulhunty, 1990). Lake Eyre is located in the most arid part of Australia (Fig. 1). Average annual rainfall ranges between 125 and 150 mm and annual evaporation rate ranges between 2500 and 3000 mm per year. Mean maximum summer temperature is 36 °C in January, and it may reach as high as 50 °C. Mean maximum winter temperature is 17 °C in July and it may reach as low as 5 °C. However, the drainage system headwaters are located in more humid monsoonal regions (Bonython and Mason, 1953). Most of the flood waters come from the northern and northeastern part of the catchment, which has annual rainfalls of 370–500 mm (Dulhunty, 1990). The Lake Eyre (South) salt crust is up to 20 m thick. It has been precipitated dominantly from water that flooded though the Goyder channel from Lake Eyre (North). Lake Eyre (North) has a 50 m thick salt crust (Dulhunty, 1974, 1978). However, the salt may also flow in the opposite direction by solution in northward-flowing groundwater, or occasionally when Lake Eyre (South) overflows into Lake Eyre (North) (Allan et al., 1986). The salt is ultimately derived largely from the weathering of exposed rocks within the catchment area (Bonython, 1955a,b). The salt had been transported into the lake as a solution in river water and shallow groundwater (Dulhunty, 1977; 1990), and has accumulated since the lake dried out during the late Pleistocene through to the Holocene. In addition, other environmental factors suggested by Johnson (1980) include: low topographic relief
and a predominantly endorheic drainage system have resulted in the retention of connate salts; conditions of arid climate have produced wind deflation hollows with the surficial cover of alluvium removed to expose the bedrock of the catchment; and subsequent seepage of saline groundwater containing the reworked salts into the depressions produces the characteristics salt lakes. The salt is similar to sea salt, dominantly of NaCl but with a lower potassium content (Wopfner and Twidale, 1967). The salt crust consists of sodium chloride (90– 95%), magnesium sulphate (5–7%), magnesium chloride (b4%) and calcium sulphate up to (b2.5%) (Bonython, 1955b; Bonython and King, 1956). 3. Regional geological setting The southwest of Lake Eyre (South) is a plateau of Quaternary fluvial and lacustrine sediment dissected by short streams (2 km in length), which expose the underlying deeply weathered materials. Most of the geological studies of the area deal with the Proterozoic and Palaeozoic strata, which are not exposed, but underlie the basin. However, these bedrocks are exposed in the headwaters of the streams that drain into the lake (Fig. 1) (Krieg et al., 1990, 1991; Magee et al., 1995). These and overlying younger rocks in the study area (Fig. 2) were subjected to deep chemical weathering producing silcrete crusts, particularly on Miocene palaeo-lake sediments and shoreline deposits (Croke, 1997). Following tectonic subsidence of the Lake Eyre region in the Late Cainozoic, continental sedimentation continued through to the Pleistocene (Wopfner and Twidale, 1967; Tedford and Wells, 1990). The continental sedimentary phases have been described by Krieg et al. (1990) and divided into three phases; two in the Tertiary and one in the Quaternary. The late Palaeocene–Eocene was a period of river deposition of fine sand, gravel and deposition of fine-grained laminated clay in swamps. From the late Oligocene to the middle Miocene very large, relatively shallow lakes covered much of the basin. These lakes were fed by meandering rivers with extensive floodplains (Krieg et al., 1990, 1991). The middle Miocene and Pliocene were times of silicification (Krieg et al., 1990, 1991), producing silcrete crusts (Goudie, 1973) on the lake and shoreline sediments. During this period the Lake Eyre basin was affected by widespread epeirogenic
Fig. 2. Locations of the sites examined in the study area.
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Fig. 3. Cross-section of the investigated sites in the study area.
subsidence with uplift of the margins of the basin. Since then the area has subject to only minor local movement. Presently, low gradient ephemeral channels drain from low elevation hills at the margin of the Lake Eyre basin. The major catchments to the northeast are fed with water by summer monsoon rains. Smaller catchments drain into Lake Eyre (South) from the Willouran Ranges. These ephemeral streams have very variable discharge both seasonally and yearly (Magee et al., 1995). Three major streams cut through the Willouran Mesozoic bedrock and drain into Lake Eyre (South); i.e. Gregory, Poole and Welcome/Frances, and the Warriner, Margaret and Stuart streams as well as small streams cut through the Tertiary plateau to the west (see Fig. 1). During global climatic oscillations of the late Quaternary associated with glacial and interglacial cycles, Australia underwent climatic oscillations between wetter and drier periods than the present day climate (Thomas, 1974; Ollier, 1975; Nanson et al., 1992). During wetter climates Lake Eyre became significantly larger than at present and more than 25 m deep. During arid periods the lake dried completely and sediment deflation took place, with the level of deflation controlled by groundwater (Magee et al., 1995). During the wetter periods flooding occurred (Bonython and Fraser, 1989), with relatively small amounts of sediment input (Gillespie et al., 1991). During dry periods small amounts of sediment have been removed by deflation, because the water table is relatively stable (Magee et al., 1995). Fragments of the Miocene silcrete crusts have been preserved as large boulders (N1 m long axes) within the lacustrine sediments
(Figs. 3 and 4). These boulders are believed to have been weathered by deep chemical weathering (Thomas, 1974; Ollier, 1975). During lake highstands, and by stream erosion at times of falling lake levels, these boulders have been exposed at the surface throughout the Late Pleistocene. During lake highstands wave action has eroded the palaeo-lake sediments, in some places creating low cliffs up to 10 m high (Dulhunty, 1975), and exposed the boulders. During falling lake levels stream erosion has acted to further expose the boulders. The exposed boulders have since been fragmented by mechanical weathering (salt-induced) to form the desert pavements, during which salt weathering may have been an important process. Around the modern lake margin are a series of low lake terraces preserving concentric former lake shorelines. Ages of the shorelines increase with elevation above the modern lake (Dulhunty, 1975). These shorelines provide a time framework for the exposure of the boulders and the initiation of desert pavement development on the lake terraces. 4. Methodology A survey of the rock pavement proprieties was carried out in the study area at 5 locations (Fig. 2). Locations were selected across the palaeo-shoreline sequences to be representative of the range of the rock pavement development in the area, and on the basis of accessibility of the exposed surfaces. At these locations 17 exposed surfaces were examined. At all sites scaled photographs of 1 m2 quadrats of the exposed surfaces were taken in the field.
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Fig. 4. Examples of desert pavement on the exposed surfaces, a–h youngest to oldest in the study area. Please note: mini-hills in c, and cone-hills in e.
Soil texture and colour were observed in the field to differentiate between otherwise similar desert pavement surfaces (Table 1). Soils have been characterised as follows. 1) Absent: at locations where the boulder is just exposed on active lake shorelines, and in locations where the boulder is still intact. 2) Immature: at locations where no horizon development is present, the soil is composed of weathered
materials from the boulders together with recently accumulated dust, but no evidence is present of CaCO3 accumulation. This material has depths of up to 10 cm, a sandy texture, and shows little reddening (Munsell colour: 10 YR 7/2, light grey). 3) Moderate: at locations where the soil profile shows discernable horizon development, with early stages (1 or 2) of CaCO3 accumulation. This material has depths
A. Al-Farraj / Geomorphology 100 (2008) 154–163 Table 1 Soil maturity and elevation of sample sites above the lake bed Stage
Site
Elevation (m)
Soil depth (cm)
Soil colour
Soil texture
1 2
5.7 3.3 2.3 5.6 5.5 4.2 3.2 2.2 5.4
0.5 4⁎ 5⁎ 4 4.5 5 6 5 8
0 0 0 10 40 40 40 40 50
No soil No soil No soil Sand Sand Sand Sand Sand Silty sand
1.2 5.3 3.1 5.2 4.1 2.1 1.1 5.1
10 11 11 15 15 15 15 22
50 60 70 90 90 80 100 ?
No soil No soil No soil 10 YR 7/2 (light grey) 10 YR 6/4 (light yellowish brown) 10 YR 5/6 (yellowish brown) 10 YR 5/6 (yellowish brown) 10 YR 5/6 (yellowish brown) Between 7.5 YR 5/6 (strong brown) and 7.5 YR 6/6 (reddish yellow) 7.5 YR 5/6 (strong brown) Between 7.5 YR 5/6 and 7.5 YR 5/8 7.5 YR 5/6 light brown 5 YR 5/6 (yellowish red) 5 YR 5/8 (yellowish red) 5 YR 5/8 (yellowish red)) 5 YR 5/6 (yellowish red) No soil: crystalline gypsum
3 4
5
6 7a
7b
Silty sand Sandy silt Sandy silt Silt Silt Silt Silt
⁎ These two locations are higher than more developed pavements, however they are on surfaces recently exposed by stream erosion.
of up to 40 cm, a sandy texture, with soil colour ranging between 10 YR 6/4 (light yellowish brown) and 10 YR 5/6 (yellowish brown). 4) Mature: showing distinctive horizons with later stages of CaCO3 accumulation (3–4). The soil profile has a depth of 60–100 cm, and the soil texture ranges between sandy silt to silt. The soil colour ranges between 7.5 YR 5/6 (light brown) and 5 YR 5/6 (yellowish red). Quantitative measurement of clast size, angularity, and sorting on the pavement surfaces was carried out from the photographs. From each surface photograph length and width axes of the largest 25 stones were measured, using the same methods as used by Al-Farraj (1996) and Al-Farraj and Harvey (2000). The angularity of the same 25 stones on each surface was recorded using the Powers Angularity Chart (Tucker, 1996). Percentage cover of selected stone sizes was
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calculated by systematically sampling 100 points from a grid drawn onto the photograph and recording clast size at each sample point (AlFarraj, 1996; Al-Farraj and Harvey, 2000). This uses the same methodology that was used to characterize the pavement development on fluvial depositional surfaces in UAE, although the origin of the clasts that form the pavements is different. 5. Results and interpretation The recently exposed silcrete boulders range in size up to about 1.5 m in length, 1 m in width and 0.70 m in thickness. During the earlier breakup stages these boulders form topographic domes (mini-hills) about 0.5 m above the surface. The fragments around the domes occupy a diameter of about 7 m. As weathering proceeds these mini-hills become cones, with heights initially between 0.9 and 1.1 m, larger than those of the mini-hills because of exposure of the full depth of the boulders, with the fragments scattered around a diameter of up to 17 m. For site 1.1 the cone height is unusual as it is exposed on the edge of a channel. This is as a result of the boulder base being eroded entirely and the fractured fragments rolling down the slope by gravity making the cone and its fractured fragments appear to be higher than the original boulder thickness. As these cones degrade, the surface relief becomes only 10 cm in height but the diameter ranges between 17 and 18 m (see Table 2 for more details). On the gibber plain both the boulder boundaries disappear and the surface relief is reduced to between 1 and 2 cm, as with normal desert pavement. The particle length on the top and around these boulders, and on the gibber plain, ranges between 10 cm and 1 cm, from youngest to oldest. The particle sizes reflect the time of exposure rather than any other factor (e.g. parent material). The 17 exposed surfaces have been classified into 7 types/stages according to the soil maturity (colour and texture) and elevation above the lake bed (Table 1; Figs. 3–5). This shows 7 different pavement types/stages: stage 1 is the youngest and stage 7 is the oldest. Stage 1 occurs at a height of 0.5 m above the present lake bed. No soil is developed at this stage. It is characterised by a microtopography inherited from the boulders. This type appears at location 5.7 (Fig. 2 and 3). Stage 2 occurs at a height of 1 m on a surface recently exposed by stream erosion, but 4 m above the present lake bed
Table 2 Desert pavement particle size characteristics, clast angularity, and pavement relief, derived from field observations and measurements made from photographs of 1 m2 sample quadrats on 17 surfaces Stage
1 2 3 4
5 6 7a
7b a b c d e
Site
5.7 3.3 2.3 5.6 5.5 4.2 3.2 2.2 5.4 1.2 5.3 3.1 5.2 4.1 2.1 1.1 5.1
Largest 1–25 clasts a-axes (cm)a
Percentage cover by clasts with b-axis (cm)b
Clast angularity: largest 25 clastsc
Surface topography
1
2
5
10
25
N 10
N7
N5
N4
VA 0
A1
SA 2
SR 3
R4
WR 5
Mnd
10.1 10.7 9.9 16 10.5 7.2 7.1 7.7 16.3 12 11 11.1 8.8 8.2 7.7 9.5 8.8
9.3 6.7 9.4 12.6 10.1 6.6 6.7 6.6 13.5 12 8.7 10.4 7.2 8 6.6 7.9 5.7
8.3 6.1 7.7 10.3 7.7 5.5 6.2 6 9.2 9.7 6.6 9.1 5.7 6.1 6 6.1 5.3
6.4 5.5 6.5 9.5 6.9 4.1 5.1 4.3 7.9 8.7 6.1 7.8 4.9 5.6 4.3 5.1 5.1
3.3 3.8 2.9 4.4 3.1 2.7 2.5 1.9 4.9 4.5 4 3.2 3 3.8 1.9 2.9 3.1
8 12 2 05 4 3 3 2 6 12 2 3 2 0 0 0 1
15 19 5 8 7 6 5 3 12 20 5 5 5 2 1 2 6
17 21 15 13 12 9 10 6 15 24 8 13 11 9 5 7 10
19 25 21 16 15 16 18 12 17 27 17 19 21 20 14 15 18
25 13 13 12 12 13 11 11 12 10 8 0 5 4 0 1 0
– 12 12 10 12 8 9 13 8 9 9 0 5 6 0 2 0
– – – 3 1 4 5 1 5 6 8 12 7 8 11 12 12
– – – – – – – –
– – – – – – – –
– – – – – – – –
– – 6 8 7 7 5 5
– – 7 – – 7 5 6
– – – – – – – 2
0 0.48 0.48 0.64 0.56 0.64 0.76 0.6 072 0.84 1.0 2.8 1.72 1.72 2.84 2.44 2.6
B B B M M M M M C C F F G G G G G
e
Surface relief (m)
Surface diameter (m)
0.3 0.4 0.35 0.5 0.6 0.7 0.6 0.8 0.9 1.1 0.1 0.2 0.02 0.02 0.02 0.015 0.02
3.5 4 4 7 8 9.5 8.5 10 15 17 17 18 – – – – –
a-axis (cm) for the 1st 2nd, 5th, 10th and 25th largest stone within the 1-m2 sample quadrats. Percentage cover of the quadrats occupied by clasts N 4, 5, 7, and 10 cm b-axis. Clast angularity of the 25 largest clast, classified on powers roundness scale; VA = very angular, A = angular, SA = sub-angular, SR = sub-rounded, R = rounded, WR = well rounded. Average values derived from allocating a scale of VA = 0 to WR = 5. B = Upper part of boulder exposed and breaking up; M = “mini-hill”; C = cone-hill; F = almost flat surface; G = flat surface, “Gibber plain”.
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Fig. 5. Model of the salt weathering and desert pavement processes.
(Table 1).This surface has no soil (location 3.3; Fig. 2 and 3). Stage 3 occurs at a height of 4 m above the lake bed and comprises a soil of grey sandy materials (location 5.6; Fig. 2). Stage 4 occurs at an average height of 5 m above the present lake bed and has a sandy soil with a colour of 10 YR 6/4 (locations 5.5, 1.2, 2.2, 3.2, 3.2; Fig. 2). Stage 5 occurs at an average height of 9 m above lake bed and has a more mature soil with a soil colour of 7.5 YR 6/6 (location 1.2, 5.4; Fig. 2). Stage 6 occurs at a height of 11 m above present lake bed, with a mature soil with identifiable horizon development and a soil colour of 7.5 YR 5/6 (locations 5.3 and 3.1; Fig. 2). Stage 7a occurs at a height of 15 m above the present lake bed with soil colour 5 YR 5/6 (locations 5.2, 4.1, 3.1, 2.1 and 1.1; Fig. 2), This stage represents the mature “gibber plain” desert pavement. At one site (location 5.1; Fig. 2), a Stage 7 pavement occurs, 22 m above present lake bed, that has little soil but instead has inter-granular spaces filled with crystalline gypsum. This is characterized as Stage 7b. The soil characteristics (colour and texture) have been used to differentiate between the surfaces (see above). The soil characteristics beneath the pavement surfaces and the elevations have been summarized in Table 1. Except for sites 4 and 5, which have higher elevations than more developed pavements, the stages and the soil development are closely related to height, and time as McFadden et al. (1998) have suggested. The reason for the two exceptions of sites 4 and 5 is that these two sites have been more recently exposed as a result of channel activity (Fig. 3).
Table 2 summarizes the desert pavement clast size characteristics based in part on the methodology used by Al-Farraj (1996) and Al-Farraj and Harvey (2000) but also including observations on microtopography. Length and width axis measurements of the largest 25 clast within each quadrat demonstrate little difference between the surfaces, expressed either by a-axis measurements or by b-axis distributions. This may be for two reasons; 1) the high rate of weathering, where even young surfaces have small stone fragments (to be discussed later in relation to both clast origin and weathering regime); or 2) the quadrat position — for consistency most of the samples were taken of the exposed surface at the centre of a fragmented boulder. In this zone the shattered fragments are highly similar; any smaller clasts may have moved to the edges of the boulder circumference (Fig. 4). The roundness characteristics (Table 2) derived from the 25 largest clasts show a decrease in angularity with age from stage 1 to stage 7. This is in contrast to many previous findings, and may relate to chemical weathering of the clast edges (to be discussed below). 6. Discussion These results are in marked contrast with those for age-related pavement characteristics in UAE (Al-Farraj, 1996; Al-Farraj and Harvey, 2000) or those reported in other pavement studies (e.g. McFadden et al., 1989). There are two possible reasons for this. The first reason
A. Al-Farraj / Geomorphology 100 (2008) 154–163
could be that this is not an age-related sequence at all, and all the pavement samples have been developed over the same period of time. This is unlikely because there are clear age-related trends in soil development, micro-topography and an unexpected trend of decreasing angularity with apparent age. Furthermore, these sites themselves are located across the age range of former lake shorelines (see below). The second reason is related to the possibility of a different pavementforming mechanism between here and the sites of the UAE and elsewhere. Over last five decades, several filling and drying periods have been recorded at Lake Eyre (Bonython and Mason, 1953; Dulhunty, 1975; Tetziaff and Bye, 1978; Dulhunty, 1984, 1986; Allan et al., 1986). Three major fillings occurred in 1949, 1974 and 1984 together with a number of minor fillings (Dulhunty, 1990). Madigan, when he reached the Lake Eyre for the first time in 1929, found the Lake dry and that made him assume it was never wet. His observation was the perceived wisdom until the 1949–50 flooding which was the first known and well documented flood. However, there are records of earlier instances when the lake held water, that have been highlighted by Bonython and Mason (1953) as follows. In 1840 a flood was reported by E.J. Eyre, (Lake Eyre was named after him). He believed the lake might be a great inland sea. More water reached the lake in the 1890s, and in 1922 the Royal Australian Air Force reported that one third of the lake was flooded. Recorded rain data suggested that water might have reached the lake in 1890, 1906, 1916, 1917 and 1920 (Bonython and Mason, 1953). Dulhunty (1975) demonstrated three prehistoric fillings in Lake Eyre to levels of 280, 160, and 70 cm above the 1974 filling. He suggested dates of 3000, 1500, 500 years before present. Krieg et al. (1990, 1991) suggested three Pleistocene phases of fluvial deposition in Lake Eyre and southern Lake Eyre (South). Nanson et al. (1992) have dated these surfaces using thermoluminescence (TL) and U/Th dating. The dates given are between 130,000 and 100,000 years for the oldest phase, between 25,000 and 40,000 for the middle phase, and 18,000 for the youngest phase is (Hubbard, 1995a,b). During the late Pleistocene/Holocene small streams have cut into the sediments. Initiation of these small streams together with filling and drying of the lake exposed the underlying boulders. Different times of boulder exposure have resulted in different stages of weathering and pavement development. Boulders which are exposed for longer periods of time are more weathered than those which have been exposed more recently. Combining these chronological data (Table 3) with desert pavement development data on the lake shorelines (Table 2; Fig. 4) a model for desert pavement development rate and processes in this area can be put forward (Fig. 5). Pre-stage 1 is the actual exposure of the boulder as a result of erosion by lake flooding then drying; such unweathered boulders as these are exposed at the present time on the modern lake shore (Fig. 4). At stage 1 the upper surface of the boulder is broken up while the rest of boulder is still buried by the lacustrine sediments (e.g., location 5.7; Figs. 2, 4 and 5). At this stage, two processes appear to
Table 3 Summary of surface chronology provided from the literature Stage Suggested date Reference 1
3 4 5 6
Less than 200 years Less than 500 years 500 years 1500 3 ka 18 ka
7 7a
25–40 ka 100–130 ka
2
According to A.J. Eyre flood record in 1840 According to floods record and weathering rate compared with Dulhunty (1975) estimation Estimated by Dulhunty (1975)
Dated by Nanson et al. (1992) using thermoluminescence (TL) and U/Th.
161
operate: 1) horizontal cracks occur, caused by insolation heating and cooling; 2) vertical cracks extend into the interior of the boulder, which may be caused by salt weathering exploiting previous weaknesses within the rock. That could be the pattern produced by salt weathering. At this stage there is no soil development and the stone fragments are very sharp. A suggested age for this stage is less than 200 years according to the flood record of A.J. Eyre. At stage 2 the upper surface of the boulder is broken up horizontally, and the inner part of the rock breaks up vertically (See Figs. 4 and 5) (e.g. location 5.6; Fig. 2). Again there is no soil development, no soil horizon development and no soil colour change. Soil absence may be because it is an unstable surface because of frequent flooding. At this stage and stone fragments are very sharp. A suggested age is less than 500 years, on the basis of estimated dates from Dulhunty (1975). By stage 3 most of the stone is exposed, creating what is described here as a ‘mini-hill’ topography (Figs. 4 and 5; Table 2). At this stage there is the beginning of soil development, with a profile of grey sandy materials, colour 10 YR 6/2 (pale brown) (e.g. location 5.5; Fig. 2). A suggested age for this pavement stage could be 500 years or more, again according to Dulhunty's (1975) estimation of shoreline age. At stage 4 the stones make a mini-hill and the soil is more developed but still sandy with a colour of 10 YR 5/6 (yellowish brown), (e.g. location 5.4, Fig. 2). A suggested age for this pavement stage would be 1500 years or more according to Dulhunty's (1975) estimation of shoreline ages. By stage 5 the stones making the degraded cone-hill are all fragmented. Stone fragments at the centre are very sharp, while those at the edges are more rounded (perhaps as a result of etching). Soil texture is silty sand and soil colour is 7.5 YR 6/6. This stage can be recognized at locations 1.2 and 5.3 (Fig. 2). A suggested age for this pavement stage could be 1500 years or more according to Dulhunty's (1975) shoreline age estimates. By stage 6 the surface is nearly flat, the boulders have disintegrated into rounded clasts. The soil is sandy silt and soil colour is 7.5 YR 5/6. This surface has been dated to 18 ka by Nanson et al. (1992) using thermoluminescence (TL) and U/Th dating. Stage 7a is the “gibber plain” phase with small well rounded stones and a silty soil. Soil colour is 5 YR 5/6. Again this surface has been dated by Nanson et al. (1992) using (TL) and U/Th, with a given date of 25–40 ka. Stage 7b also is characterised by gibber plain well rounded clasts, but it has no soil (see Amit et al., 1993; McFadden et al., 1998). Instead the pore spaces are infilled by crystalline gypsum. The soil has become engulfed by pedogenic gypsum. This surface has been dated by Nanson et al. (1992) to between 100 and 130 ka. In general, these dates are in accord with other dates for the rates of soil development on other desert regions (see soil profile description above) (e.g. Amit and Gerson, 1986; Amit et al., 1993). The age gap between stages 1–4 and 5–7 may be related to the likelihood that the more recent surfaces (stages 1–4) have retained more details of changes than the older surfaces (stages 5–7). The two main differences between the sequence observed here and those reported in the USA and the UAE are the very limited changes in clast size and the decreasing, as opposed to increasing, clast angularity with increasing surface age on these Australian pavements. Two main reasons can be proposed in explanation of these differences. First, the origin of the clasts differs. The other studies referred to above have dealt primarily with pavement development of alluvial fan or fluvial gravel deposits within which the original clasts inherit particle size and angularity characteristics from those depositional environments and are progressively shattered by mechanical weathering processes to show reduced particle size and increased angularity with increasing pavement age. In this study the source materials are silcrete boulders derived from older deep weathering profiles. In this sense they may have been subjected to pre-weathering processes involving brecciation and re-cementation during silcrete formation (Goudie,
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1973; Goudie and Pye, 1983), creating weaknesses within the boulders whose spacing was related to silcrete formation. Thus, on weathering following exposure at the surface, fractures may develop following previous fracture lines rather than in relation to modern stress patterns induced by surface exposure. The result may be that mechanical breakdown produces relatively small, uniformly-sized, highly angular clasts, that are perhaps less prone to further size reduction by shattering. Secondly, the weathering environments appear to differ. The Lake Eyre environment is a highly saline environment increasing the effectiveness of salt-induced mechanical weathering, and within which pH may be sufficiently high to cause “etching” of the clast edges by chemical processes. The high rate of saltinduced mechanical weathering may contribute to the rapidity of the initial breakdown of the boulders. Etching of the clast edges on the older pavement surfaces accounts for the decrease in angularity with surface age. It may be significant that in the Atacama Desert, Chile, also a highly saline environment (Berger, 1997), clasts on desert pavement surfaces also show evidence of chemical weathering (Anne Mather, pers. comm.). 7. Conclusion A sequence of desert pavement development can be recognised around the margins of Lake Eyre (South). Exhumed silcrete boulders weather to provide the clasts for pavement formation. The timescale for pavement development can be related to the palaeo-lake shoreline sequence. Successive stages in the pavement sequence can be related to stages in soil development and to progressive changes in microtopography. Seven stages of pavement development can be recognised, from the initial exposure of the boulders to total break-up and organisation of the clasts into a mature “gibber plain” desert pavement. The pavement development sequence recognised here differs from those recognised in other arid regions, particularly the USA and the UAE, in that there is relatively little, if any, progressive reduction in clast size, after the initial salt-induced mechanical weathering of the silcrete boulders, and a progressive decrease rather than an increase in angularity. The reasons for these differences appear in part to relate to the origin of the clasts as the products of disintegration of silcrete boulders, and in part to the highly saline weathering environment. In this environment not only does the abundance of salt lead to rapid saltinduced mechanical weathering on exposure of the boulders, but appears to lead also to chemical weathering in the form of etching of the clast edges during the later stages of pavement development. It is clear that the morphology and development of desert pavements as a whole reflects both the parent material and interactions between different weathering processes. Acknowledgements I'm grateful to Martin William and Peter Gill and for their assistance in making the field work for this investigation successful. I also would like to acknowledge Adelaide University for providing the vehicle for the field trip. I am also grateful to Adrian Harvey for feedback on this work. I also would like to thank Ahmad Massasati to his help with the figures. Finally I would like to thank the referees L.D. McFadden and A. Yair for the valuable comments which have enriched the discussion. References Al-Farraj, A., 1996. Late Pleistocene Geomorphology in Wadi Al-Bih northern UAE and Oman: with special emphasis on Wadi terrace and alluvial fans. PhD. Thesis, University of Liverpool, 363pp. Al-Farraj, A., 2004. Review of desert pavement types in the UAE: development and implications for rates and processes of formation. Joint International Geomorphology Conference, 18–20 August 2004, Glasgow, Abstracts Volume, p. 37. Al-Farraj, A., Harvey, A.M., 2000. Desert pavement characteristics on Wadi terrace and alluvial fan surface: Wadi Al-Bih UAE and Oman. Geomorphology 35, 279–297. Allan, R.J., Bye, J.A.T., Hutton, P., 1986. The 1984 filling of Lake Eyre South. Transactions of the Royal Society of Australia 110, 81–87.
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