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Ancient Fluvial Systems
Fluvial Deposits of the Ecca and Beaufort Groups in the Eastern Karoo Basin, Southern Africa
The Permian-Triassic Ecca and Beaufort Groups of the eastern Karoo Basin display several distinct fluvial facies, including channels of the delta plain, mixed-load high-sinuosity channel units and braided bedload stream deposits. Ecca sediment influx was mainly from the north and east, and shoal water deltas prograded across a broad, shallow platform which deepened southward into a flysch basin. The lower delta plain was traversed by multiple branching channels up to 4 m deep. These channels filled both by the downstream migration of solitary, large-scale bedforms, and by deposition from progressively smaller bedforms representing waning flow. Channels of the upper delta plain are up to 15 m deep and display point bar lateral accretion structures, with intrasets decreasing in scale upward. Associated topstratum and interfluvial deposits are silty and carbonaceous with irregular crevasse-splay sandstones, and are commonly overlain by coal of probable allochthonous origin. Along the updip basin margin similar coal-bearing cycles of the alluvial plain pinch out laterally against basement ridges.
The succeeding Beaufort basin was characterized by the development of a subsiding foreland related to the tectonically-active Cape Fold Belt to the south. A molasse-type wedge attained a maximum thickness of 1,000 m along the southern depo-axis, thinning northward where it prograded across a lacustrine shelf. The more distal portions of this clastic wedge resembled smaller lobes which developed along the northeastern basin margins, and comprise mixed-load, high sinuosity channel deposits. The coarser grained proximal channel facies consist of (I) braided stream deposits which are dominated by solitary sets of planar cross-beds, with associated plane-beds and trough cross-bed cosets, and (2) deposits made up entirely of upper flow regime plane-beds, probably recording flash floods in ephemeral streams.
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