Pseudo-tidal sedimentation in a non-tidal shelf environment (Southeast African continental margin)
Flemming, B.W. (1988). Pseudo-tidal sedimentation in a non-tidal shelf environment (Southeast African continental margin), in: de Boer, P.L. et al. (Ed.) Tide-influenced sedimentary environments and facies. Extended versions of papers presented at the Symposium on Classic Tidal Deposits, held August 1985 in Utrecht, Netherlands. pp. 167-180
In: de Boer, P.L. et al. (Ed.) (1988). Tide-influenced sedimentary environments and facies. Extended versions of papers presented at the Symposium on Classic Tidal Deposits, held August 1985 in Utrecht, Netherlands. D. Reidel Publishing: Dordrecht. ISBN 90-277-2622-1. ix, 530 pp., meer
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| Abstract |
Up to about 1970 the fossil occurrence of large-scale crossbedding produced by subaqueous bedforms were mostly attributed to either fluvial or tidal origin. Alternative environments were rarely considered due to the lack of appropriate modern analogues. The subsequent discovery of large non-tidal bedforms on open shelves exposed to ocean currents and wind driven circulation contributed significantly towards a better distinction between tidal and non-tidal environments. In this paper evidence is presented which may again complicate the issue. On the southeast African continental shelf the interaction of strong geostrophic flow (Agulhas Current) with the local shelf morphology and coupled with wind and swell driven counter currents, has produced a variety of depositional features which would normally be associated with tide-dominated environments. Amongst these are bedload partings, large symmetrical transverse bedforms (dunes, megaripples), inflected crests on asymmetrical transverse bedforms (dunes, megaripples), bimodal bedform orientations, the inferred presence of reactivation surfaces and large submerged spit-bar complexes, resembling prograding tidal sand shoals. The features do not occur in isolation but in close association with each other at specific localities on the shelf. Sediment distribution patterns in these are not different from those found on shelves exposed to strong tidal currents. If observed in the fossil record, such deposits and their internal structures could therefore suggest a tidal origin. The only nonambiguous argument against a tidal setting is, by implication, the absence of lunar cycles in the cross-bedded sets. |
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