Source bordering sand dunes are found at the eastern and northern margins of palaeochannel systems in the Murray Basin of eastern Australia. Their sediments are dominated by fine to medium sand but often contain a minor clay component that gives rise to red and dark brown soil colours and may be manifested in the profile as multiple clay-rich (argic) bands. In the Murrumbidgee sector of the Riverine Plain Page et al. (1996) noted that source bordering sand dunes were associated with three of the major palaeochannel systems dated by thermoluminescence (TL): the Coleambally (80 to 100 ka), Kerarbury (35 to 60 ka) and Yanco (12 to 20 ka).

Upstream of the Riverine Plain, in the mid-catchment reach of the Murrumbidgee near Wagga Wagga, low bedrock hills confine the alluvial valley fill to a width of 2 to 4 kilometres. Here, fossil source bordering dunes were first described by Sturt (1813). Subsequent studies by Beattie (1972) and Chen and McKane (1997) described the sediments and soils of the dunes but did not establish a chronology of formation beyond noting that they appeared to correlate with Beattie's (1972) Yarabee depositional phase.

The present study investigated the stratigraphy and TL chronology of the Wagga Wagga dunes at quarry exposures and auger sites. Near Wagga Wagga the dunes occur both as mounds on the floodplain surface and as drapes over adjoining bedrock ridges. In places, the dune sands are on-lapped by more recent alluvium. Fieldwork and sampling were carried out at Millwood, Clarendon, Glenfield and Yarragundry Dunes (Figure 1).

At all sites a thin artefact-bearing post-European surficial layer overlay an aeolian depositional sequence which rested, in turn, upon either floodplain alluvium or clay-rich hillslope soils. At Yarragundry, three aeolian units separated by soil groundsurfaces were identified. These are named, in increasing order of age and on the basis of the type sites, the Clarendon, Glenfield and Yarragundry Units. The separation of three aeolian units is strongly supported by the TL dates (Figure 2).

The Clarendon Unit occurs at all field sites as a loamy sand or sandy loam with a brownish black to reddish brown soil profile overlying yellow brown medium sands with well-developed large-scale cross beds. Argic layers are occasionally found in the Clarendon Unit but they rarely exceed 5 mm in thickness. TL dating of the Clarendon Unit suggests that it was deposited between about 15 and 25 ka at about the time of the Last Glacial Maximum (LGM) in Oxygen Isotope Stage 2. At Clarendon Quarry the Clarendon Unit rest directly on fluvial silts and pebbly sands from which it appears to have been derived given that two TL dates on the fluvial deposits are not statistically distinguishable from those in the overlying aeolian material. At the other three sites the Clarendon Unit rests unconformably on the Glenfield Unit which is characterised by a truncated bright red soil with strongly developed argic layers that may be either accordant or discordant with the primary dune bedding. Where the argic layers occur deeper in the profile they are typically conformable with the primary dune bedding. When traced up the cross-bed planes they tend to thicken and darken in colour and then merge with a complex zone of criss-crossing sub-horizontal wavy bands that are discordant with the primary bedding. The argic bands, which are typically from 10 to 30 mm thick but occasionally exceed 50 mm, are separated by sand bands averaging 100 mm thick. The absence of appreciable silt in the argic layers is typical of aeolian dust deposits in Australia (Dare-Edwards, 1984) and consistent with the transportation of fine sand-sized clay aggregates from a distant source. At Glenfield and Yarragundry Quarries the Glenfield Unit contains carbonate, either as fine earth, soft glaebules or vertical rhizomorphs. The clay and carbonate content of this unit is consistent with the pedogenic reworking of the clay aggregates and carbonate in the profile.

The seven TL dates from the Glenfield Unit show a strong clustering between approximately 35 and 60 ka with only one outlier at 78.5 ka. At Millwood Dune the TL ages clustered between 37.2 and 44.2 ka over a vertical sequence of more than six metres sugggesting a rapid sand accumulation. Here, the Glenfield Unit rested unconformably on a mottled and pedogenically well-structured heavy clay thought to represent Beattie's (1972) Brucedale groundsurface.

At Yarragundry Quarry, the Glenfield Unit yielded dates of 53.6 and 57.6 ka and unconformably overlay a unit with bright red colours and appreciable clay. TL dates in this lower Yarragundry Unit of 86.6 and 111 ka indicate an earlier phase of aeolian activity. The Yarragundry Unit unconformably overlies a mottled medium clay which, on the basis of soil characters, is thought to represent the Brucedale groundsurface.

TL dating of the source bordering dunes near Wagga Wagga confirms that they were deposited since the last Interglacial at 125 ka (Martinson et al., 1987) and were derived from sand from local bedload dominated rivers very different from the intrenched suspended load systems of today. However, the calcareous clay component of the dunes appears to have been derived from a source region to the west, probably during active phases of swale deflation in the Mallee dunefield (Bowler and McGee, 1978). Given the irregular morphology of the argic bands it seems likely that they are produced by mobilisation and illuviation of the clay rather than direct deposition. It is unlikely that phases of sand and dust accretion were synchronous. The last major phase of dust accretion probably occurred in Oxygen Isotope Stage 2.

The stratigraphic separation of the Clarendon, Glenfield and Yarragundry phases of dune formation is strongly supported by the TL dates which form three distinct clusters with little error overlap (Figure 2). A comparison of the Wagga Wagga dune ages with the TL chronology for the Murrumbidgee sector of the Riverine Plain shows a strong correlation respectively between the Yarragundry and Coleambally Units, the Glenfield and Kerarbury Units and the Clarendon and Yanco Units. Although this correlation between dune building phases in the mid and lower Murrumbidgee Valley is not surprising, it does provide a useful corroboration of the reliability of the TL dating method for aeolian and fluvial sand deposits in the Murray Basin.

It is now clear that Beattie's (1972) Yarabee phase of dune formation extended over approximately 100 000 years and includes at least three soil surfaces, two of which are normally buried. In terms of the Oxygen Isotope record, Yarabee sand dune formation occurred mainly in Stages 5, 3 and 2 in association with enhanced fluvial activity by rivers carrying significantly more bedload than those of the present.