Fluvial and tidal sedimentation in the 3000 Ma Mozaan basin,South Africa |
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| Authors: | MB Watchorn |
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| Institution: | Department of Geology, University of Natal, Pietermaritzburg 3201 South Africa |
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| Abstract: | The 3000 Ma Mozaan Group comprises three interacting sedimentary environments: (1) braided alluvial plain, (2) offshore shelf, and (3) tidalites. The predominance of planar cross-bedded sandstones in the fluvial sequence indicates that sedimentation was dominated by southerly accreting transverse bars in a distal braided river system. However, the development of lenticular conglomerates suggests the occasional formation of longitudinal gravel bars, probably during periods of high discharge. Argillaceous sediments, some of which are ferrugineous represent an interplay between clastic sedimentation and chemical precipitation on a prograding shelf. The abnormal thickness of these shelf deposits is attributed to facies stacking. The absence of barrier beach deposits suggests that the marginal environment was controlled by macrotidal conditions. Two varieties of tidal sequences are recognized. The first, which forms the classic upward-fining succession, contains ubiquitous ebb-dominated paleocurrents and is interpreted as having been influenced by tidal asymmetry. The other, which lacks the mixed flat unit, was apparently generated by tidal currents flowing parallel to the coastline. The nature of the Mozaan sediments suggests at least incipient cratonization in eastern South Africa 3000 Ma ago. |
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