User:MargaretRDonald/sandbox/Bassandean Sands

The dune systems of the Swan Coastal Plain in south-western Australia have a complex origin. The younger succession (Quindalup and Spearwood) have a primary carbonate-rich origin (McArthur & Bettenay 1974; McArthur 2004; Hearty & O’Leary 2008). In these, following dune deposition, carbonate is progressively leached from the upper horizons, eventually leading to a thick (i.e. many metres) siliciclastic sand overlying a karst weathering profile (McArthur & Bettenay 1974; Bastian 1996). The older succession (Bassendean) is often characterized by deep siliciclastic sands that are not associated with underlying carbonate lithologies, and appear to have been primarily deposited as quartz-rich sand lacking a significant carbonate component (Kendrick,Wyrwoll & Szabo 1991). Still, reference soils from all dune systems have a similar particle size distribution, with 73% (range 52–94%) coarse sand (200–2000 lm), 23%(range 3–43%) fine sand (20–200 lm), 2% (range 1–3%) silt (2–20 lm) and 2% (range 1–3%) clay (<2 lm) (McArthur 2004).

The Bassendean dunes are the dune facies of the underlying Ascot Formation; the latter represents a series of depositional events associated with a progading shoreline (Kendrick,Wyrwoll & Szabo 1991). The cover of white quartz-dominant sand can attain thicknesses of 15–20 min some places (Kendrick,Wyrwoll&Szabo 1991), although our preliminary field observations suggest thinner accumulations of sand around Jurien Bay. Due to their considerable age (early Pleistocene to late Pliocene, i.e.>2 Ma; Kendrick,Wyrwoll & Szabo 1991), the Bassendean dunes have lost their distinct dune topography over most of their width, and around Perth only the westernmost (youngest) dunes have amorphological expression (Bastian 1996). Some authors have suggested that the Bassendean dunes may have originated as calcarenites (similarly to the Quindalup and Spearwood dunes), but that these have been leached for so long that carbonate has been entirely dissolved, leaving behind thick accumulations of quartz-dominant sand (McArthur & Bettenay 1974; Playford, Cockbain & Low 1976; Bettenay 1984; Bastian 1996). However, Kendrick, Wyrwoll & Szabo (1991) point to a clear distinction between the early Pleistocene (Ascot Formation), where sediments show higher quartz content, and the strong carbonate environment of the middle Pleistocene (Tamala Limestone), presumably reflecting higher carbonate productivity in the shelf environment from where the sand was sourced. Therefore, the Bassendean sands probably had much lower initial carbonate content than the Quindalup and Spearwood dunes. As outlined above, the Bassendean dunes are considerably older than the Spearwood dunes (Kendrick, Wyrwoll & Szabo 1991; Bastian 1996; Tapsell, Newsome & Bastian 2003), although the details of the age structure of the three marine ⁄ dune successions remain to be firmly established. Over most of the Swan Coastal Plain, the Bassendean dunes have a width of c. 6–8 km (McArthur & Bettenay 1974), but near Jurien Bay they abruptly narrow to about 2 km at their widest (Griffin&Burbidge 1990).

Coastal deposits along the western coastal margin of Australia, a region of relative tectonic stability, record Plio-Pleistocene events and processes affecting the inner shelf and adjacent hinterland. Tectonic deformation of these deposits is more apparent in the Carnarvon Basin, and rather less so in the Perth Basin. The most complete record comes from the Perth Basin, where units of Pliocene and Pleistocene ages are well represented. In the Perth Basin, the predominantly siliciclastic Yoganup Formation, Ascot Formation and Bassendean Sand represent a complex of shoreline, inner shelf and regressive-dune facies equivalents, the deposition of which began at an undetermined stage of the Pliocene, through to the Early Pleistocene. The deposition of this sequence closed with a major regression and significant faunal extinction. Bioclastic carbonates characterize the Middle and Late Pleistocene of the Perth and Carnarvon basins. Fossil assemblages include a distinct subtropical element, unknown from the Ascot Formation and suggesting a strengthening of the Leeuwin Current. The estuarine arcoid bivalve Anadara trapezia characterizes assemblages of Oxygen Isotope Stages 5 and 7 in the Perth and Carnarvon basins, where it is now extinct. Deposits of Substage 5e (Perth Basin) also record a southerly expansion of warm-water corals and other fauna consistent with shelf temperatures warmer than present. New uranium-series ages on corals from marine sequences of the Tantabiddi Member, of the Bundera Calcarenite of the western Cape Range are consistent with the ‘double peak’ hypothesis for levels of Substage 5e but the evidence remains less than conclusive. Initial uranium-series dates from the Bibra and Dampier formations of Shark Bay indicate that both derive from the Late Pleistocene. These numerical ages contradict previous interpretations of relative ages obtained from field studies. The age relationship of the units requires further investigation.

The Swan Coastal Plain is bounded on the east by the Darling Fault, on the north by a subsidiary fault running north-west from Bullsbrook, and on the south by the Collie-Naturaliste Scarp. The Swan Coastal Plain is formed almost entirely of depositional material from wind (aeolian) or river (fluviatile) activity.

Views on the origin of sandplains in Western Australia remain controversial with debate focusing around three different models of formation. These are in situ, aeolian and in situ formation with local remobilisation by wind or colluvial transport. The only recent work on the subject to date espouses a dominantly aeolian origin. New work from a detailed study on the Victoria Plateau is described and demonstrates the applicability of utilising a range of evidence in understanding the origin of sandplains in Western Australia. Field investigations show a strong association of sandplain with sandstone and an absence of sand on non?arenaceous geology in similar and adjacent topographic settings. Grainsize, mineral magnetic analysis and heavy?mineral spectra show the Victoria Plateau to be a heterogeneous body of sand. These findings coupled with a lack of internal sedimentary structures are not consistent with an aeolian origin for the sandplain. Furthermore, scanning electron microscopy, grainsize and heavy minerals also demonstrate a clear link between bedrock and overlying sandplain. These data support the hypothesis that Western Australian sandplains are mostly the product of in situ weathering. Such findings question whether the origin of sandplains can be satisfactorily deduced without such a range of data.

The Swan Coastal Plain consists of a series of geomorphic entities running parallel to the coastline. It comprises of the Pinjarra Plain and three dune systems of different ages of deposits, whose soils are at different stages of leaching and soil formation. The Pinjarra Plain is a piedmont deposit formed from the rivers that entered the Coastal Plain on its eastern side (Mattiske, 1995). It is characterised by medium-textured, fairly well drained soils and gentle slopes. Immediately to the west is the Bassendean dune system, which consists of slightly leached grey sands devoid of virtually all nutrients. This dune system is generally gently undulating, although the dunes do reach an altitude of 60m in the north (Mattiske, 1988). Between the dunes is a series of extensive seasonal swamps with peaty soils and moist flats underlaid by organic hardpans. These minor variations in topography are also reflected by differences in the depth of the watertable, which in tum, influences the vegetation (Mattiske, 2003). The Spearwood dune system occurs to the west of the Bassendean dune system, and is generally higher and steeper, and consists of a core of calcium-rich aeolinite with a limestone capping. The two soils associated with this area are the Karrakatta and Cottesloe soil profiles, and are separated by a series of lakes, including Lake Joondalup and Lake Nowergup on the western side of the Gnangara Mound (Mattiske, 2000). West of the Spearwood dune system is the Quindalup dune system, consisting of a narrow belt of unconsolidated calcareous sand (Gentilli, 1947).

A detailed study of the soils of the southern part of Gnangara Mound in the Swan Coastal Plain was carried out to determine their soil-water characteristics and their leaching capacity to nutrients and pesticides. Physical, chemical and hydraulic characteristics of the topsoil (0–15 cm) and the subsoil (40–50 cm) were measured at 21 sites representing the major soils under the different landuses in the area. The results show that Bassendean Sands have higher coarse sand particles and consequently higher hydraulic conductivity than Spearwood Sands. The Bassendean Dunes generally have low relief; minor variations in topography translate into variable depths to watertable, which are the basis for division into soil mapping units. For example the Gavin soil has higher organic carbon content than all other soils sampled. The Spearwood Dunes are divided mainly on the depth of soil over the limestone substrate and the incidence of karst features. The Spearwood and Jandakot soils have lower coarse sand and lower carbon content, while the Karrakatta soils have the least amount of organic carbon. Detailed soil maps were compared with GIS-produced hydrogeomorphic maps. The results show that the distribution of HGUs in the catchment is controlled by the geological formations on which they were developed. The results also show that the hydrogeomorphic maps can be used in the absence of detailed soil maps to classify the catchment into areas that have similar soil characteristics. Filtering capacity of the soils is dependant on organic material, clays and other minerals. Based on these criteria, Spearwood Sands have the highest filtering capacity, followed by Bassendean and Quindalup.

THE GNANGARA MOUND is part of the Swan Coastal Plain which was formed mainly of depositional material either from fluviatile or aeolian origin. The plain consists of a series of geomorphic entities, which are sub-parallel to the present coastline. The most easterly feature of the Mound is a series of laterite covered spurs forming the foothills of the Darling Scarp. Stretching from the foot of the hills is the relatively flat Pinjarra Plain built up of alluvium of varying ages. It is up to 13 km wide and is terminated sharply at its western edge by a series of coastal sand dunes: the Bassendean Dune System in the east, followed by the Spearwood Dune System, with the Quindalup Dune System fringing the present coastline (McArthur and Bettenay 1960).

The generalised surface geology of the study area is mainly based on the soils consisting of the Guildford clay in the east, the Bassendean and Spearwood sands in the middle, and the Tamala limestone and Safety Bay sand (Quindalup Dune System) on the beach.

The alluvial terrain includes the drainage system which discharges from the Gnangara Mound. The north and northeast drainage lines, which discharge into Gingin Brook, are mapped as Gingin Brook Complex (GG). The eastern drainage system towards Ellen Brook and the Swan River have been mapped as the Yanga unit (Ya). The Bassendean Dune System is generally of low relief with minor variations in topography; this means that the watertable is at variable depths, and this variation is the basis for division into soil mapping units. The landscape comprises 20 m high ridges and permanent open-water lakes. The flat or gently undulating terrain supporting Gavin soils (G) has less than 5 m relief. The hills and ridges (Jandakot soils (Ja) generally have more than 5 m relief, with one area of exceptionally high ridges mapped as Jandakot-Steep. The dunes probably accumulated as shoreline deposits and coastal dunes during interglacial periods of high sea-level and consist of lime sand, quartz sand and minor fine-grained, black, heavy mineral concentrations. The carbonate material has been completely leached, leaving dunes consisting entirely of quartz sand.

The Spearwood Dune System is classified mainly on the basis of depth of soil over the limestone substrate and the incidence of karst features. Limestone is exposed or covered with shallow soil (Limestone (Kls)) on the hills and ridges. In lower positions, the sand may be several metres thick and is mapped as the Karrakatta unit: the sand may be yellow almost to the surface (Karrakatta Yellow (Ky)), or grey in the surface layers (Karrakatta Grey (Kg)). In the karst depressions the slopes are mapped as Spearwood (Sp). The depressions often have permanent lakes (W) with poorly-drained areas, mapped as Beonaddy (B) around the edge.

The Quindalup Dune System occurs mainly west of the Wanneroo Road; the mapping units include four phases (Q1–4), unstable sand (Qu), deep calcareous (Qp) and shallow calcareous (Qs). Filtering capacity of the soils is dependant on organic material, clays and other minerals. Based on these criteria, Spearwood Sands have the highest filtering capacity, followed by Bassendean and Quindalup.