The evolution of bornhardts in silicic volcanic rocks in the gawler ranges
Titel:
The evolution of bornhardts in silicic volcanic rocks in the gawler ranges
Auteur:
Campbell, E.M. Twidale, C.R.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 38 (1991) nr. 1 pagina's 79-93
Jaar:
1991-02
Inhoud:
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 that developed in two stages. The first involved subsurface weathering beneath a Jurassic planation surface. The weathering was deeper in fracture zones than on the fracture-bounded blocks. The stripping of regolith and the exposure of the rounded bedrock forms probably occurred mainly in the Early Cretaceous. Certainly it was essentially complete by the Early Tertiary since which time the hills have remained virtually unchanged. The bornhardts of the Gawler Ranges thus have their origins in the distant past, for some of the fractures to which they fundamentally owe their origin date from the Proterozoic, the weathering responsible for the exploitation of the regional fracture systems pre-dates the Cretaceous and the bedrock forms were substantially exposed during the Early Cretaceous.
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