Temporal Variability, Migration Rates and Preservation Potential of Subaqueous Dune Fields Generated in the Agulhas Current on the Southeast African Continental Shelf

  1. Michael Z. Li2,
  2. Christopher R. Sherwood3 and
  3. Philip R. Hill4
  1. Burghard W. Flemming and
  2. Alexander Bartholomä

Published Online: 3 JAN 2013

DOI: 10.1002/9781118311172.ch11

Sediments, Morphology and Sedimentary Processes on Continental Shelves: Advances in Technologies, Research, and Applications

Sediments, Morphology and Sedimentary Processes on Continental Shelves: Advances in Technologies, Research, and Applications

How to Cite

Flemming, B. W. and Bartholomä, A. (2012) Temporal Variability, Migration Rates and Preservation Potential of Subaqueous Dune Fields Generated in the Agulhas Current on the Southeast African Continental Shelf, in Sediments, Morphology and Sedimentary Processes on Continental Shelves: Advances in Technologies, Research, and Applications (eds M. Z. Li, C. R. Sherwood and P. R. Hill), John Wiley & Sons, Ltd, Chichester, West Sussex, UK. doi: 10.1002/9781118311172.ch11

Editor Information

  1. 2

    Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2 Canada

  2. 3

    U. S. Geological Survey, Woods Hole, MA 02543-1598 USA

  3. 4

    Natural Resources Canada, Sidney, BC, Canada V8L 4B2

Author Information

  1. Senckenberg Institute, Suedstrand 40, 26382, Wilhelmshaven, Germany

Publication History

  1. Published Online: 3 JAN 2013
  2. Published Print: 26 JAN 2012

ISBN Information

Print ISBN: 9781444350821

Online ISBN: 9781118311172

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Keywords:

  • Southeast African continental shelf;
  • subaqueous dunes;
  • water depth;
  • grain size;
  • dune dimensions;
  • dune migration rates;
  • preservation potential

Summary

In the course of 53 months between 1978 and 1982, a 100 km long streamwise transect crossing two well-developed subaqueous dune fields located on the continental shelf southwest of East London (South Africa) at water depths between 80 and 100 m was surveyed four times by echo-sounder and side-scan sonar to obtain a time series on the basis of which bedform dynamics could be assessed (I: 12.02.1978; II: 10.08.1979; III: 15.05.1981; IV: 21.05.1982). During the second cruise, 75 sediment samples were recovered along the transect, mostly from dune crests. Fourteen years later, a fifth survey was carried out (V: 19.12.1995). On this occasion, 40 samples were recovered.

The two dune fields measured about 40 km and 35 km in length, respectively, and were separated by a ca. 20 km long interdune area devoid of bedforms. Typical dune dimensions ranged from <20 to >600m in wavelength and <1to >17m in height. Height/length relationships varied markedly (from 0.075 to 0.008), both spatially and temporally. Neither dune heights nor dune lengths correlated with water depth at any time. The largest dunes (up to 17.5 m high) were predominantly composed of well-sorted coarse-grained bioclastic material. Mean grain sizes ranged from >0.6mm to <0.2mm and maximum dune size generally increased with increasing grain size. Bioclastic carbonate contents of the sediment varied strongly, ranging from 40-98% in the upstream dune field and 12-75% in the downstream one.

Due to the high mobility of the dunes and limitations in navigational accuracy, it was not possible to track individual dunes from survey to survey. However, the well-defined boundaries of the large dune fields provided good markers from which the migration of the dune fields as a whole could be determined. On this basis, the following migration rates were calculated: 2.1 km or 3.85 m day-1 between survey I and II; 0.2 km or 0.23 m day-1 between survey II and III; and 0.5 km or 1.4 m day-1 between survey III and IV. Between survey I and IV, the mean migration rate thus amounted to 2.8 km or 1.8 m day-1. When including survey V, which was carried out 16 years and 4 months after survey II (the initial sampling survey of 1979), the long-term average migration rate amounted to 1.7 m day-1. Using a long-term mean migration rate of 1.5 m day-1, a dune field mobilized at the head of the ‘conveyor belt’ would require 822 years to pass through the 450 km long shelf section. Since the postglacial recovery of the Agulhas Current some 7 kyr BP, at least eight dune fields would thus have been able to pass through the system without leaving any trace on the shelf.