Beach ridge sets reflect the late Holocene evolution of the St Lucia estuarine lake system, South Africa

Greg A. Botha, Naomi Porat, Sylvi Haldorsen, Geoff A.T. Duller, Ricky Taylor, Helen M. Roberts

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

Sets of sandy beach ridges and intervening swales define shoreline sections of the shallow St Lucia wetland system within the iSimangaliso Wetland Park, World Heritage Site in northern KwaZulu-Natal province, South Africa. The sets comprise 3–10 beach ridges, the most prominent being 80–150 m wide and rising 0.5–2 m above the adjacent swales. The highest beach ridge crests elevated 3.2–4.6 m above mean sea level are furthest from the present shoreline and the lowest ridges, rising about 0.5 m above the swales, occur closest to the present mean lake level shoreline. This investigation assesses the genesis of the sandy beach ridges on five strand plain remnants within the estuarine lake. The ridges were topographically surveyed and their ages estimated using optically stimulated luminescence (OSL) dating. OSL dating reveals that the oldest beach ridge formed around 6240 years ago, with a sequence of beach ridges having accumulated during the period ~4000 to 1500 years ago reflecting coeval accretion around the lake. The present mean lake level shoreline was only reached within the past ~600 years. Sedimentation changed from marine-dominated to lacustrine deposition in the estuary during the period of beach ridge accretion. The dated beach ridges, supported by new radiocarbon dates of fixed biological indicators from Holocene intertidal zone settings on the coast, are used with published sea-level curves to set the context of periodic beach ridge accretion in the marine-linked estuarine lake. The dated ridges suggest that episodic regression of the estuarine lake shoreline occurred after a mid-Holocene sea-level highstand. The record of sea-level change is also reflected by the long-term natural shrinking and shallowing of the proto-St Lucia lagoon/estuarine lake in the context of reduced marine influences due to closure of former marine channels by progressive barrier dune accretion. The sequence of coeval beach ridges reflects a pulsed lowering of relative sea-level in the shallowing estuarine lake. Height differences between the coeval ridges at sites around the lake reflect local environmental controls including the range of wind fetch distances across shallow lake compartments, wave height and wind-induced seiche effects.

Original languageEnglish
Pages (from-to)112-127
Number of pages16
JournalGeomorphology
Volume318
DOIs
StatePublished - 1 Oct 2018
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2017 Elsevier B.V.

Funding

The research was funded by the Council for Geoscience and Norwegian NUFU projects ; 12/2002 and 16/2008 . Access to lake bed core logs and diatom data compiled by the late Bjorg Stabell is gratefully acknowledged. Caroline Fox, Lucas Myesa and the late Amos Myesa of KZN Wildlife assisted with boat access to Sengwane during the fieldwork. The GPR survey, ridge topographic survey and sampling phases were assisted by Charlie Bristow, Greg Mullins, Philo Schoeman, Brendan Clarke and Mxolisi Kota. The DGPS survey was undertaken with support from Carl Grossman of UKZN under a research permit issued by iSimangaliso Wetland Park Authority. Andrea Baker of Stellenbosch University made Mfabeni isotope data available.

FundersFunder number
Council for Geoscience and Norwegian NUFU16/2008, 12/2002

    Keywords

    • Beach ridge
    • Holocene
    • OSL
    • Sea-level

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