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The role of shelf morphology and antecedent setting in the preservation of palaeo-shoreline (beachrock and aeolianite) sequences: the SE African shelf

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Abstract

On the SE African shelf, a submerged shoreline at a depth of 60 m is examined and its attributes compared between two shelf sectors with different morphologies, yet similar energy regimes. The aim is to assess the controls of antecedent conditioning on shoreline development and later preservation from transgressive ravinement. Using a combination of multibeam bathymetry and single-channel seismic profiles, the stratigraphy and morphology of the shoreline is investigated. Low-gradient bedrock examples reveal several distinctive seismic facies, including onlapping chaotic reflector packages which are interpreted as calcarenite rubble fields. These palaeo-shorelines possess planform equilibrium morphologies, including parabolic dunes and blowout forms along with relict shore platforms. They are strongly associated with incised valleys of last glacial maximum age which underlie the shoreline locations; these provide wide, back-barrier accommodation space during transgression. In contrast, palaeo-shorelines on the steeper-gradient shelf have a simpler stratigraphic arrangement. They are not as well preserved, are generally covered by thick drapes of sediment, and lack the elaborate planform morphologies of their lower-shelf gradient equivalents. Isolated incised valleys and the steep bedrock gradient limit accommodation space. The comparison indicates that antecedent bedrock slope and available accommodation are amongst the dominant controls on overstepping, and thus potential preservation, of palaeo-shorelines on the shelf. Lower-gradient shelves not only promote rapid shoreline translation but, together with wide, sandy back-barrier accommodation, also foster larger barrier volumes. In suitable climates such as in the Mediterranean and other sub-tropical areas, the ensuing shoreline stability promotes rapid and effective cementation of the barrier. In comparison, steep bedrock profiles with limited back-barrier accommodation have much lower preservation potential. Transgressive ravinement is more focussed on steep slopes, effectively removing more material during the ravinement process. The more dynamic environment may also reduce the effectiveness of diagenesis. The potential of beachrock and aeolianite palaeo-shorelines as submerged sea-level indicators may be optimal in low-gradient settings in Mediterranean to subtropical environments.

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Acknowledgements

We gratefully acknowledge funding from the University of KwaZulu-Natal and the National Research Foundation (NRF) via the African Coelacanth Ecosystem Programme (ACEP; grant number 97968). We further acknowledge the National Ports Authority who gave their kind permission to publish the datasets collected from the south of Durban. In this regard, we thank SUBTECH’s surveying department. Errol Wiles, Enviromap and KZN Boat Hire assisted us in the collection of the WASSP and seismic datasets from Durban, and are acknowledged accordingly. Ryan Palmer of ACEP is thanked for the ROV imagery from the Umdloti shoreline. Prof. B.W. Flemming, Dr. M.T. Delafontaine and two anonymous reviewers are thanked for their constructive comments.

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Authors and Affiliations

  1. Geological Sciences, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban, South Africa

    Andrew N. Green, J. Andrew G. Cooper & Leslee Salzmann

  2. School of Environmental Sciences, Centre for Coastal and Marine Research, University of Ulster, Cromore Road, Coleraine, BT52 1SA, UK

    J. Andrew G. Cooper

  3. Marine Geoscience Unit, Council for Geoscience, Bellville, Cape Town, South Africa

    Leslee Salzmann

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  1. Andrew N. Green
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Correspondence to Andrew N. Green.

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Green, A.N., Cooper, J.A.G. & Salzmann, L. The role of shelf morphology and antecedent setting in the preservation of palaeo-shoreline (beachrock and aeolianite) sequences: the SE African shelf. Geo-Mar Lett 38, 5–18 (2018). https://doi.org/10.1007/s00367-017-0512-8

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