THE METASEDIMENTS OF THE AGGENEYS TERRANE IN THE NAMAQUA MOBILE BELT - SEDIMENTARY RESPONSE TO EXTENSIONAL-COMPRESSIONAL VARIATIONS IN A CONTINENTAL ENVIRONMENT

The Aggeneys Subgroup, Bushmanland Group, consists of six formations w hich exhibit a relatively wide variety of sedimentological responses t o changing tectonic conditions. A high percentage of quartz arenites i ndicate a granitic provenance, which is in agreement with the persiste nt tholeiitic affinity of associated volcanites. The sedimentological evidence is interpreted to be the result of two periods of crustal ext ension separated by a period of compression. The first extensional per iod resulted in deposition of offshore to shoreline deposits (Wortel a nd Wilputs Formations). The crustal relaxation caused by the transitio n to compressional conditions, is characterized by the deposition of l ow-energy deposits, namely black shales (distinctive in the Skelmpoort and T'hammaberg Formations). Compressional stress field response is r epresented by feldspathic delta-front sediments (Hotson Formation) dep osited in a contractional basin. The second period of relaxation (reve rsal of the stress field) once again resulted in low-energy deposits, such as banded iron formation associated with calc-silicate rock (Gams Member of the Hotson Formation). The stratabound sulphide deposits ar e interpreted to be the result of fumarolic activity preceding widespr ead volcanism. Extensional conditions gave rise to the deposition of f luvial deposits interbedded with tholeiitic lavas (Koeris Formation). When all the sedimentological properties are taken into account, the A ggeneys Subgroup could possibly have been deposited in a retro-arc for eland basin.