Stratigraphy of the mesoproterozoic aggeneys terrane,western Namaqua mobile belt, South Africa

摘要

The stratigraphy of the Aggeneys Terrane is discussed, one of at least eleven tectono-stratigraphic terranes of the Namaqua mobile belt. The severe deformation of the region caused isoclinal folding, structural duplications and excisions everywhere, presenting a formidable obstacle to regional correlation of the stratigraphie sequences. Geological mapping of lithological types proves unable to characterize the supracrustal rocks. For compilation of the various metasedimentary sequences, detailed sequence mapping is required. The lateral continuity of characteristic sequence packages on dekametre scale and the systematic recording of defined polarity, enables unequivocal correlation and classification of the metasediments. The deformation of the Aggeneys Terrane resulted in different east-west trending strain domains: a northern zone of lower strain (zone I) with nearly uninterrupted stratigraphy, and a southern zone of higher strain (zone II) with fragmented stratigraphy wherein the supracrustal successions are preserved in mega sheath folds and fold nappes only. Because of the economic interest in Cu-PbZn deposits of one formation (the Gams Formation), most detailed studies have in the past been restricted to the disrupted zone. The complete stratigraphie column can however be compiled in the zone of lower strain, where it is apparent that the metasediments may be divided into six units. The units of the Aggeneys Subgroup (mentioned in descending order) are the Koeris, Gams, Hotson, T'hammaberg, Skelmpoort and Wortel Formations. In the zone of disrupted stratigraphy (zone H), the portions of the various formations preserved in specific fold nappe structures can be recognized easily in spite of faciès variations, excisions and structural juxtapositions. Facies variations include differences in thickness and non-deposition. To illustrate all distributional aspects, maps and interpretations are presented for the regions around Swartberg (Black Mountain) and the Soutkloof fold nappe in the Aggeneys Hills (Aggeneysberge). It is concluded that the interpretational system employed for the Aggeneys Terrane, has powerful predictive properties. The system provides convincing modeling of the mechanisms controlling the stratigraphie development of the entire western Namaqua-Natal mobile belt.