Soil type is the other important influence on farming practice in the region. The Mallee and Wimmera contain a variety of soil types, reflecting different sources of soil and soil-forming processes.
Method
Spatial data was downloaded from Geoscience Australia’s Geophysical Archive Data Delivery System (GADDS, http://www.agso.gov.au/gadds for gamma-radiometrics and magnetics) and the CGIAR Consortium for Spatial Information (http://srtm.csi.cgiar.org/ for Shuttle Radar Topography Mission 90m elevation data). Images were developed using the MapWindow GIS package (http://www.mapwindow.org) and converted to kmz files for display in Google Earth. The kmz files can be downloaded from http://www.malleefocus.com.au/PA/post/Spatial-data-for-Google-Earth.aspx.
The Gamma-radiometrics were measured by an airborne instrument, on transects 200-400m apart. Artifacts from the east-west flight pattern are prominent in the uranium and total count images, hence thorium and potassium images were chosen for interpretation.
Elevation
The focus paddocks are at the intersection of a number of geomorphological processes. This is perhaps best appreciated on elevation. To the southeast are the foothills of the Great Dividing Range (Figure 12), which have been the source of much of the material forming soils on the Wimmera and Southern Mallee plains. Across the area the concentric northwest-southeast strandlines mark the recession of the former Murrayvian Gulf as sea levels fell. This is the main source of much of the soil to the north.
Etched into this at low elevation is the former shoreline of the great Lake Bungunnia (at about 65m), formed when the Murray River (formerly flowing along the Douglas Depression to the west of Horsham) was dammed by uplift of the Padthaway High, about 3 million years ago. Soils in the Lake Bungunnia basin have been formed from the reworking of materials from the strandlines, and of a thick layer of clay sediment (originating from the Great Dividing Range).
After drainage of the lake (0.7 million years ago), many smaller lakes formed at low points. The beds, lunettes and drainage systems associated with these lakes add to the complexity of the landscape.
Uplift to the south associated with the Padthaway high and great dividing range means that the southern ends of strandlines have been elevated as much as 75-80m. The relatively ‘flat’ terrain of the Wimmera and Southern Mallee results from erosion of these, deposition of material eroded from the Great Dividing Range, and flooding/creek activity between.
The focus paddocks cover a range of this variation. The Culgoa, Donald and Minyip paddocks are all on soils of the Wimmera/Southern Mallee plain.
The Patchewollock paddocks are both among lunettes to the east of Lake Agnes (last flooded 1918) and the larger Lake Wirrengren (last flooded 1874). Both are on the west side of a lunette. The No Till paddock is on an older lunette, further east.
The Yaapeet No Till paddock is on the east face of the east-most lunette of Lake Albacutya, whereas the Yaapeet Till paddock is between a pair of low strandlines.
The Sea Lake paddocks are in the Tyrrell basin of Lake Bungunnia, but close to the Tyrrell Creek entrance to Lake Tyrrell. The No Till paddock is on a ‘billabong’ of the Tyrrell Creek, whereas the Till paddock is on higher ground between two small lunette dunes.
Figure 12. Elevation in the Wimmera-Mallee. Focus paddock locations are marked with black dots. M, B and H indicate Mildura, Birchip and Horsham. Data from the Shuttle Radar Topography Mission.
Gamma-radiometrics
Gamma-radiometrics measure the natural gamma-radiation from isotopes of potassium (40K), uranium (238U) and thorium (232Th) in soil near the surface (0-40cm). There are direct relationships between these isotopes and weathering processes, and spatial patterns can also be used to illustrate patterns of soil erosion and deposition (Cook et al. 1996). In particular, granites emit strongly for all isotopes, and highly weathered sands have little emission for all isotopes. Thorium has correlated strongly with topsoil clay (eg. Cattle et al. 2003). The half-lives of these isotopes are measured in the billions of years, so age is not a factor in interpretation.
The thorium image (Figure 13) shows the range of soil sources in the area. The Lachlan fold belt (Great Dividing Range) to the south east is the source of high thorium material, which can also be seen along the Murray River. High thorium material can also be seen along watercourses and associated floodplains, namely the Yarriambiack creek, through Warracknabeal and finishing northwest of Hopetoun, the Dunmunkle creek flowing through Rupanyup and west of the Minyip and Donald focus paddocks, the Richardson river finishing in Lake Buloke (and with traces of past water flow north through Birchip and probably to Lake Tyrrell), the Tyrrell and Lalbert creeks, and the Avoca river (running off the east of the image). Lunettes of many, but not all lakes are high in thorium (Lake Hindmarsh is a notable exception).
Low thorium soils are primarily the Lowan sands (Molineaux sands in South Australia), which enter from the west in two well-defined ‘fingers’. A third ‘finger’, further north (north of the Mallee highway), is less well defined and probably older, but extends right to the Murray. On the South Australian side of the border, apparent source regions for each ‘finger’ are successively further west and south (implying each was generated progressively as the Murrayvian gulf was retreating).
Analysis of the Lowan sands (Pell et al. 2001) suggests they are derived from erosion of the now Flinders Ranges, transported via the sea, whereas the surrounding Woorinen sands are derived from erosion of the Great Dividing Range. An older origin of the north-most Lowan sands (ie. when the coast was closer to the Great Dividing Range) would imply that the north-most Lowan sands would be more like the Woorinen sands than those further south. This may explain why the northern Lowan sands are less distinct on the thorium image, apart from having had more time to inter-mix the two groups.
The intermediate soils on the thorium image are the sands ofthe Woorinen formation (yellow on Figure13) and the heavier soils of the Shepparton formation (red). Soils more distant from water courses (in the Shepparton formation) may be lower in thorium because of eluviation (similar to leaching) of high-thorium clay from the topsoil.
The potassium image (Figure 14) is similar to the thorium image in its main features, but distinguishes less between the Lowan and Woorinen sands, and does not pick out water-related soil transport as well as the thorium image does. In this respect it may be more ‘true to texture’ for sands, whereas the thorium may be more ‘true to texture’ for clays.
Figure 13. Gamma-radiometrics Thorium in the Wimmera-Mallee. Focus paddock locations are marked with black dots. M, B and H indicate Mildura, Birchip and Horsham. Data from GeoScience Victoria.
Figure 14. Gamma-radiometrics Potassium in the Wimmera-Mallee. Focus paddock locations are marked with black dots. M, B and H indicate Mildura, Birchip and Horsham. Data from GeoScience Victoria.
Magnetics
Magnetics measure the depth to bedrock. Depth is indicated not by the measurement itself, but by the rate of change. Where magnetics change quickly, the object is closer to the surface; where they change slowly, the object is further from the surface.
According to this interpretation, the sharpest change occurs along the edge of the Padthaway high, in a line approximately from Horsham to Murrayville. There are also changes between Birchip and Sea Lake. Compared to other parts of the Australian content, bedrock in the Mallee is relatively shallow.
Figure 15. Magnetics in the Wimmera-Mallee. Focus paddock locations are marked with black dots. M, B and H indicate Mildura, Birchip and Horsham. Data from GeoScience Victoria.