oa South African Journal of Geology - The Achab Gneiss : a "floor" in Bushmanland or a flaw in Namaqualand?

Volume 89 Number 2
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590



An enigmatic topic in the Namaqualand Metamorphic Complex of southern Africa is the elusive "floor" on to which the mid-Proterozoic Aggeneys Subgroup was deposited. In Bushmanland, grey gneisses were considered suitable candidates for basement around the Namiesberg and have been termed the Achab Gneiss. Further work has revealed this lithology to be a suite of quartzo-feldspathic gneisses which is more widespread than previously suspected and, in part, may be correlated with the augen gneiss around the Aggeneysberge. Geochronological data demonstrate that the Achab Gneiss is older than the Aggeneys Subgroup, but structural evidence indicates that a thrust zone separates these two units. Such a horizontal tectonic regime has been recognized as responsible for juxtaposing rocks of different ages elsewhere in Namaqualand. The preferred interpretation in this region is that the Achab Gneiss intruded a "pre-Bushmanland" sequence and represents a "floor" to the Aggeneys Subgroup. Subsequently, an early complex fold-and-thrust event resulted in the development of imbricate structures by utilizing contacts between units of contrasting viscosity as zones of movement.

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