E. M. Campbell

The evolution of bornhardts in silicic volcanic rocks in the gawler ranges

Most bornhardts (or domical inselbergs) are developed in granitic rocks. Some have evolved in massive sediments (sandstone, conglomerate, limestone) but few, if any, have been reported from volcanic terrains. Yet bornhardts dominate the landscape of the Gawler Ranges, located in the interior of South Australia, where domes of dacite and rhyolite are developed on fracture‐defined blocks. Their profiles are associated with the development of sheet structure. Columns of rock are also prominent components of hillslope profiles. The volcanics were extruded in Middle Proterozoic times (1592 ± 2 Ma) and the columnar cooling joints and the orthogonal sets, the latter at various scales, developed soon afterwards. The sheet fractures developed after the columnar joints and probably later than the regional orthogonal joints; it has not been possible to determine their age but they pre‐date the Late Jurassic planation and deep weathering of the massif. The field evidence suggests that the bornhardts are etch forms th...

Pre-Quaternary landforms in the low latitude context: the example of Australia

Abstract Direct effects of Quaternary glaciation and periglacial activity affected only comparatively small areas of southeastern Australia. Certainly, volcanicity continued in a few districts, extensive new planation surfaces were formed, dunefields were widely developed, and there were important developments at the coastline and offshore, during this period, but many pre-Quaternary terrains persist in the contemporary landscape. Tertiary volcanic plains and plateaux are widespread in the Eastern Uplands and duricrusted (ferruginised, silicified) remnants, some of them folded, are characteristic of many parts of the central and western sectors of the continent. Drainage systems of similar age are also increasingly recognised. Older, Gondwanan elements also feature prominently but especially in the Eastern Uplands and on the Craton. Some of these ancient elements are exhumed but others are epigene and etched, though the so-called epigene surfaces are mostly of etch type. The survival of the ancient epigene and etch forms is attributed to resistance of bedrock, through drainage, unequal activity and reinforcement mechanisms.

The early Cretaceous marine transgression and its significance for landscape interpretation

SUMMARY Significant elements of the Australian landscape date from Mesozoic or earlier times. Australia did not separate completely from other Gondwanan components until Early Tertiary times and these Mesozoic and older elements can therefore be regarded as Gondwanan. During the separation and northern drift of the continent and particularly in Late Jurassic and earlier Cretaceous times the sea invaded and spread across much of the erstwhile landmass. The associated sediments not only covered and preserved much of the pre‐existing land surface, but they also augmented the effects of thalassostatic loading of the basins, causing further subsidence. Hinge lines developed near the coastal zones of the times, so that subsidence of the basins caused adjacent land masses to rise. Many old land surfaces have been re‐exposed at the former oceanic margins, but epigene forms are preserved high in the relief on the uplifted blocks. They survive partly because, as Crickmay (1976) suggested, rivers effectively erode a...

The gawler ranges, South Australia: an unusual volcanic massif

SUMMARY The volcanic residuals of the Gawler Ranges together form an extensive massif that in its gross morphology differs markedly from most exposures of silicic volcanic rocks. The upland developed in two stages, the first involving differential fracture‐controlled subsurface weathering, the second the stripping of the regolith. As a result, an irregular weathering front was exposed, with domical projections prominent. These bornhardts are etch forms, and they are of considerable antiquity. The differential weathering of the rock mass reflects the exploitation of various fracture systems by shallow groundwaters. Orthogonal fracture systems at various scales, sheet fractures and columnar joints control the morphology of the bornhardts in gross and in detail. The exploitation of the structural base, which was established in the Middle Protero‐zoic, probably took place throughout the Late Proterozoic and the Palaeozoic, though only minor remnants of the Proterozoic land surface remain. The major landscape ...