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Ancient Fluvial Systems
Alluvial and Destructive Beach Facies From the Archaean Moodies Group, Barberton Mountain Land, South Africa and Swaziland
The Moodies Group, approximately 3,300 m.y. in age, is the oldest relatively unmetamorphosed quartzitic assemblage of sediments presently known. This succession consists of a wide variety of sedimentary facies which accumulated inter alia in diverse alluvial and marginal marine depositional environments. A remarkable similarity is shown to exist between Holocene physical sedimentary processes and those operative during the Archaean.
A depositional model is proposed to relate the different alluvial sedimentary environments in space. Proximal alluvial plain deposits consist of matrix- and clast-supported conglomerates which were deposited, respectively, by mass flow and tractional processes. Mid-alluvial plain sandstones developed through vertical aggradation and mid-channel bar formation during falling stages of episodic floods. The most distal alluvial plain sediments comprise upward-fining channel-fill sequences enclosed within interlayered siltstones and shales of probable overbank origin. Vertical sequences of alluvial facies were determined by source area tectonics. During most active progradation alluvial plain sediments built directly onto shelf accumulations in the form of fan deltas. Marginal destructive swash bars developed on the fan deltas at the cessation of fluvial influx.
This investigation has shown that exposed granitic terrains existed greater than 3,000 m.y. ago, the erosion of which resulted in extensive subaerial alluvial sedimentation. The mineralogy of the fluvial sediments is indicative of an anoxygenic atmosphere at the time of their deposition. Braided fluvial processes predominated in the absence of levee stabilization by plant growth but a stable crust, on which prolonged reworking of sediments occurred, is indicated by the orthoquartzitic swash bar sandstones.
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