|
| << vorige | volgende >> |
Tijdschrift beschrijving
Alle jaargangen van het bijbehorende tijdschrift
Alle afleveringen van het bijbehorende jaargang
Alle artikelen van de bijbehorende aflevering
Details van artikel 15 van 16 gevonden artikelen
| Titel: |
The Sodium Group, South Africa: Reference section for Late Archaean-Early Proterozoic cratonic cover sequences |
| Auteur: |
Grobler, N. J. van der Westhuizen, W. A. Tordiffe, E. A. W. |
| Verschenen in: |
Australian journal of earth sciences |
| Paginering: | Jaargang 36 (1989) nr. 1 pagina's 41-64 |
| Jaar: |
1989-03 |
| Inhoud: |
The Ventersdorp Supergroup (∼2.7 Ga) consists of a thick pile of sedimentary and volcanic units which are widely distributed over a large part of the Kaapvaal craton. At the southwestern edge of the craton, in the T'Kuip Hills, a sequence of rocks termed the Sodium Group unconformably overlies foliated biotite granite (∼2.8 Ga), and occupies a half-graben along part of the craton margin. This succession is correlated with the Platberg Group of the Ventersdorp Supergroup. Initially piedmont-type arkosic sediments characterized by granite-boulder conglomerate were deposited as proximal facies. Higher in the sequence the grain size diminished and air-fall tuff became an important constituent mixed with the clastic component. Minor lenses of mafic lava and tuff also appeared in the arkose. Flows of felsic volcanics then followed, probably as high-temperature, volatile-poor ignimbrite. As elsewhere in the Ventersdorp Supergroup, mafic volcanism succeeded the felsic phase. This resulted in a varying unit of tholeiitic basaltic and andesitic-dacitic lavas with highly vesicular flow tops. This volcanism was characterized by repeated periods of quiescence, resulting in the deposition of arkose and also near the top, arkose, tuff, shale and carbonate in ephemeral lake environments. The uppermost part of the sequence is composed of vesicular basaltic lavas with pillow zones. Burial metamorphism caused pervasive alteration of the felsic and mafic volcanics of the Sodium Group, replacing some of the original minerals with a greenschist assemblage and changing the original chemical composition. Continued marginal faulting and uplift took place throughout the formation of the Platberg and Sodium Groups. The tectonic setting, conditions of formation and stratigraphy of the Ventersdorp Supergroup are remarkably similar to those of the Fortescue Group of the Pilbara craton in Western Australia. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
Details van artikel 15 van 16 gevonden artikelen
| << vorige | volgende >> |