Geological and soil determinants

Knowledge of the locations and properties of geology and soil has been used to map and manage salinisation on King Island and in the wider Australian landscape (Sections 2.1 and 3.2).

The spatial location of salinity classes over 15,420 hectares of the island was constructed at a scale of 1:238,000 (Map 2). At this scale it is possible to determine the association between salinity in the top 350 cm of soil and the island’s geology mapped at 1:250,000 (Figure 14, Section 3.2).

Consistently low levels of salinity were associated with the locations of Pleistocene old dunes and Holocene new dunes and Proterozoic siltstone. Topography is an important determinant for the location of salinisation on Proterozoic shale, with highly- and

extremely-saline levels occurring in topographic depressions. Areas classed as highly- and extremely-saline are associated with Quaternary sediments and Proterozoic granite. Salinity on Quaternary sediments relate to their location in the northern half of the island, i.e. being in areas of lower rainfall and lower topography (Figure 10 and Figure 13, respectively). The likely major source of salts onto these areas is from sea spray,

discussed previously.

On the granite, high levels of salinity were measured toward the top of hills, as well as at valley depressions. As such, landscape morphology is a poor predictor for the location of salinisation, as was evidenced at Lake Flannigan (Map 3), in the Bungaree Creek and Pass River catchments (Map 5) and around Porky Creek inland of Currie (Map 6). This

The high salinity levels on the Proterozoic granite are likely related to their primary mineralogy, because this dictated the secondary mineral weathering assemblage.

Weathered granite provides good physical and chemical properties to enable salinisation. Gresham (1972) describes the west coast granite as a granodiorite, i.e. generally of low silica compared to the general granite compositions seen in south east Australia. Near surface weathering of the sodium and potassium feldspathic (and biotite) component of granite would result in the formation of clays (Twidale, 1982). The layered properties of clay lattice allows for cation exchange of surface potassium with sodium from percolating water. Concentrations of sodium at clay exchange site would occur in a high sodium chloride environment from salts sourced from sea spray, which are transported by rainfall into the soil. Essentially, the fine-textured, high surface area nature of the weathered granite-derived clay results in the soil to be weakly leached, and therefore, exhibits high storage capacity through cation exchange at the surface of resultant clay colloids. It would appear that this does not occur to the same extent in the weathered zones on the island’s other geological units. This was supported by Gunn (1985) who found that the materials associated with soils of high salinity at several sites on the Australian mainland comprised biotite, hornblende and potassium, sodium and calcium feldspars derived from volcanic and granitic rocks.

The assertion in the previous section that salt accession from weathering alone is unlikely to comprise a significant component to the island’s total salt budget is sustainable on the King Island Proterozoic granite. However, blurring this concept is that weathering rock can take up more salt under more saline conditions, and then re-release it under more fresh conditions.

The depth trend in soil salinity at the sample holes drilled at the top, mid- and at the bottom of slopes suffering from salt scalding on Proterozoic granite at Lake Flannigan (Map 3) and on Proterozoic shale at South Road (Map 7) reveal that the difference in salinity in the whole soil average is only minor (Figure 46). A greater proportion of salt is stored in mid-slope granite topsoil and has been flushed on the shale soil. It is this difference between the patterns of salinisation on the two geological units that has been revealed by the depth attenuation properties of the EM31 meter (Figure 27, Section 4.2). Since the EM31 was used at the height of 0.9 m above the soil surface, it is most

On Proterozoic shale this characteristic has allowed the use of terrain analysis from DEMs to predict salinisation within areas not directly assessed by the EM31. This is not the case for soils formed on granite, where there has been less flushing of salt from mid- slope topsoil. As such, topography alone is a poor predictor of salinisation to 350 cm depth, and highly saline soil is likely to occur in all areas of the landscape. This supports findings by Engel et al. (1987) and Barrett-Lennard and Nulsen (1989) who also

concluded that whilst salinised land often developed in the topographically lower parts of the landscape, topography alone is insufficient to predict the location of all salinised areas. Prediction of soil salinisation from geology and DEMs appear in section 6.5.

top slope mid

bottom of slope 5 10 15 20 25 topsoil (ave. 0 - 50 cm depth) S o il sa lin ity ( E C e - d S

/m) whole soil average

(0 - 350 cm depth)

Soil on granite (Lake Flannigan)

5 10 15 20 25 30 35 30 35

Soil on shale (South Road) top slope mid

bottom of slope

whole soil average (0 - 350 cm depth)

topsoil

(ave. 0 - 50 cm depth)

Figure 46: Salinity storage and slope position on Proterozoic granite at Lake Flannigan and Proterozoic shale at South Road. In this case, topsoil is the top 50 cm of soil.

6.4. Salinisation models on the various geomorphic landforms