The first stop will be to observe typical playa sediments on the surface of southern Lake Callabonna, including evaporative salts and clays. Small gullies nearby the lake comprise laminated gypsum crystals, which reflect the palaeohydrology of the area.
The stop also lies just north of the end of the lunette closest to the current shoreline of Lake Frome. From here we will be able to identify the scale and general morphology of one of the dominant landform types in the area. Four large, complex transverse dunes have been identified from satellite imagery within the area, and are oriented parallel to the current lake shoreline. The transverse ridges in the east contain quartz sands with minor clay aggregates, typical of lunette sediments. Lunettes are transverse dunes which form immediately downwind of playas, and contain sediments derived from the deflation of the dry lake floors. The morphological and sedimentological evidence suggests that the transverse dunes represent ancient shorelines of an ancestral Lake Frome which has retreated over time.
Longitudinal dunes extend downwind from these transverse ridges, and will be observed at the next stop. Just past Stop 1, we will pass through an area of irregular parabolic aeolian ridges and hummocks downwind of the transverse dune. These forms are significant in illustrating the genetic nature of the relationship between transverse and longitudinal dunes. These landforms quite often have a hybrid morphology and orientation.
Longitudinal dunes are elongated individual ridges of aeolian sediment, forming parallel to the resultant vector of the sand shifting winds (King, 1960). The longitudinal dunes are spaced approximately 300m apart and length varies from a few hundred metres to over twenty kilometres.
The longitudinal dunes northeast of Lake Frome are largely stable (Sprigg, 1982). Most dunes in the area have at least a small amount of mobile quartz rich sediment forming the crests. Desert dunes in the area (including transverse dunes) retain an internal structure consisting of multiple palaeosols overlying undisturbed, laminated aeolian sediments (Figure 3). The development and preservation of palaeosols is possible due to a clay component, which has important hydrologic implications requiring relatively high water tables to assist in clay deflation (Bowler, 1973). Furthermore, the multiple strata suggests that net aeolian accumulation was an episodic process, alternating with relatively more humid conditions conducive to soil development.
Figure 3. Cross section of a longitudinal dune
Cross section of a longitudinal dune.
Cross section of a typical longitudinal dune for the Lake Frome region, illustrating asymmetry of slope, including crestal slipface, and internal stratigraphy.
This is the southernmost point of interest on this excursion. The uncapped artesian bore which gives this site its name has, over decades of flow, produced significant amounts of erosion into the clay-bearing sediments. This enables us to observe a good example of active erosion, and almost the entire stratigraphic column, as we walk through the gully.
Intense levels of grazing around the artesian bore have assisted in the removal of vegetation on the dunes and subsequent reactivation of sediment.