J. A. Bourne

Episodic exposure of inselbergs

Flared slopes, breaks of slope, tafoni, and gently inclined platforms occur together in the piedmont zone of many inselbergs. Similar assemblages of forms at various levels above the present hill-plain junction are interpreted as former piedmont zones that have been abandoned as the inselbergs have been episodically exposed. Correlation of these old piedmont assemblages with paleosurface remnants of adjacent plains and uplands suggests that parts of the inselbergs are tentatively dated as Mesozoic, and many of the lower whalebacks are considered to be of Tertiary age. Most of the discussion is concerned with Eyre Peninsula in South Australia, a region that diastrophically is and has been relatively stable and that has suffered progressive erosion.

The subsurface initiation of some minor granite landforms

Abstract Flared slopes originate as concave weathering fronts beneath the regolith, and several other minor granite landforms found in association with them (tafoni, Rillen) may be initiated subsurface also. Other features, particularly incipient gnammas and gutters, have been observed on surfaces that have only recently been cleared of granite debris in situ.

Origin and age of bornhardts, Southwest Western Australia

Most of the granitic residuals of the Wheat Belt of southwestern Western Australia are bornhardts, with some nubbins developed at the western margin and occasional poorly developed castellated forms. Their origin and age can be deduced from their structure and their relationship to a weathered (lateritic) land surface and various palaeochannels. The bornhardts are massive and most stand lower than local palaeosurface remnants. They are best interpreted as having formed by differential fracture density controlled weathering beneath the weathered land surface in pre‐Eocene times. They were exposed by the stripping of the regolith beginning in the Eocene. Many are clearly stepped, indicating that their exposure took place not all at once, but episodically. A few bornhardts stand higher than the weathered land surface. They pre‐date the Eocene and the stepped morphology preserved on some suggests that their crests are much older.

The use of duricrusts and topographic relationships in geomorphological correlation: conclusions based in Australian experience

Though there are many possible pitfalls, duricrusted surfaces provide morphostratigraphic markers which are useful in preliminary extrapolation and correlation at the local and regional scales, especially if other factors, such as landscape evolution and topographic relationships, are also considered. Particular reference is made to ferruginous and siliceous cappings. Close stratigraphic dating of the duricrusts allows regional differentiation, but correlations are preferably intra- rather than inter-regional, and are at best general, and involve age-ranges rather than narrow time-frames. Problems broached include type of landscape development, the relation of surface and duricrusts, and the temporal equivalence of duricrust facies, i.e., duricrusts of similar chemical tendencies but contrasted morphology. The roles of duricrusts in relief inversion, surface preservation and climatic interpretation are also mentioned.

Active dislocations in granitic terrains of the Gawler and Yilgarn cratons, Australia, and some implications

Examination and monitoring of bornhardts and of quarry exposures on the Gawler Craton exposed on northwestern Eyre Peninsula, South Australia, and in the Yilgarn Craton of the southwest of Western Australia have shown, first, that the bornhardts of those regions are complex features, with each topographic form including several structural domes; second, that the rock masses are in compression along one axis so that one set of steeply-inclined fractures has not been penetrated by water and hence is not weathered; third, and in contrast, sheet fractures, which are tectonic features, continue to be arched and thus are readily infiltrated by groundwaters so that the adjacent rock is consistently weathered; fourth, the rock masses continue to be stressed so that neotectonic forms are still developing. These observations confirm that bornhardts are developed on rock compartments which are in compression and that they are two-stage or etch forms, and that sheet fractures and A-tents, or popups, are tectonic in origin.

Stepped landscapes and their significance for general theories of landscape development

The reasons for attributing stepped inselbergs to episodic exposure are critically reviewed and problems discussed. The evolution ofsome residuals can be dated by correlation with palaeosurface remnants preserved in adjacent terrains. The age and evolution of landscapes implied by stepped landform assemblages are incompatible with most conventional models of landscape evolution. Most examples are taken from Australian sites but reference is made to comparable forms in southern African with the implication that the principles derived from Australian landscapes are also applicable elsewhere.

Dolines of the Pleistocene dune calcarenite terrain of western Eyre Peninsula, South Australia: a reflection of underprinting?

Abstract A field of Middle and Late Pleistocene coastal foredunes occupies much of western Eyre Peninsula, South Australia. The rolling surface reflects the morphology of the stacks of calcarenite dunes that underlie the area. Hardpan calcrete is well developed in relation to the present, as well as to earlier dune surfaces. The region is a typical karst in that surface drainage is lacking. There are a few shallow and short caves but solution pipes and dolines are abundantly developed. Some dolines, including several of the larger forms occur high in the local topography and are also aligned in groups. They are attributed to underprinting, to the diversion of groundwaters into fractures in the pre-Pleistocene basement and the concentration of solution in the limestone above such zones. Low permeability calcrete horizons within the dune sequence have probably disturbed groundwater circulation and also form a stable framework preventing major collapse, and preserving both dolines and caves.

Flared slopes revisited

Flared slopes are smooth concavities caused by subsurface moisture-generated weathering in the scarp-foot zone of hillslopes or boulders. They are well represented in granitic terrains but also developed in other massive materials such as limestone, sandstone, dacite, rhyolite, and basalt, as well as other plutonic rocks. Notches, cliff-foot caves, and swamp slots are congeners of flared slopes. Though a few bedrock flares are conceivably caused by nivation or by a combination of coastal processes, most are two-stage or etch forms. Appreciation of the origin of these forms has permitted their use in the identification and measurement of recent soil erosion and an explanation of natural bridges. Their mode of development is also germane to the origin of the host inselberg or bornhardt and, indeed, to general theories of landscape evolution. But certain discrepancies have been noted concerning the distribution and detailed morphology of flared slopes. Such anomalies are a result of structural factors (sensu...

On the origin of A-tents (pop-ups), sheet structures, and associated forms

A-tents are also known as pop-ups and by several other local names. They consist of raised slabs or plates of various thicknesses and origins. Laminae are caused by weathering, and spall plates or slabs may be caused in part by freeze-thaw acting on water held in pre-existing partings which are, however, like the sheet fractures that define the thicker sheet structures, probably of tectonic origin. A-tents are the result of the buckling of such laminae, plates and slabs. Again, tectonism in the form of compressive horizontal stress, appears the most likely and common causation, though decreases in lithostatic pressure consequent on unloading and, in particular cases, surficial expansion of plates caused by the intense heat of fires, may also contribute to rock failure and rupture. All A-tents are of Holocene age and are, therefore, neotectonic forms. Some have developed in living memory, and some are known to be developing from blisters. There is some evidence of continued dislocation along sheet fractures and of the transformation of blisters or arches into A-tents under the influence of continued lateral compression. This review demonstrates, therefore, that A-tents, sheet structures, and associated forms share a common heritage.

Pediments and alluvial fans: Genesis and relationships in the western piedmont of the Flinders Ranges, South Australia

Pediments and alluvial fans coexist in the western piedmont of the Flinders Ranges. The major representatives of the two landforms are interpreted as members of an evolutionary sequence. Pediments are formed first and are due to planation by short streams debouching from the quartzite and limestone ranges of the western part of the upland. The streams simultaneously deposited spreads of coarse allochthonous detritus which protects and preserves these covered pediments. Most pediments are dissected, though distributary bar plains, some of which are construed as incipient covered pediments, are not. On the other hand, those streams which extend eastwards, beyond the zone of quartzitic and limestone ridges and into the mainly argillaceous terrains of the core of the upland, carry large volumes of mixed clay, sand, cobbles and boulders, and have deposited alluvial fans on leaving the uplands, either in the scarp foot or on leaving the confines of rock outcrops and debouching on to alluvial plains. Thus, wheth...