Sedimentology, stratigraphy and geological history of part of the northern KwaZulu-Natal coastal dune cordon, South Africa.
Abstract
The northern KwaZulu-Natal coast is backed by a continuous aeolian dune cordon that rises
in places, to a height of more than 100 metres and a width of 2 kilometres. This MSc thesis
documents the geomorphology of the area, as well as the mineralogical, geochemical and
textural variation of nine boreholes within a small part of the coastal dune cordon between Lake
Nhlabane and Cape St.-Lucia. The results provide useful constraints on the identification of
individual beach and aeolian dune systems, their age relationships and spatial distribution.
Aeolian dunes within the coastal dune cordon were studied using aerial photographs and
grouped into five dune classes that reflect their relative age. These comprise 1) a system of
highly weathered dunes inland of the present coastal dune cordon, that are thought to represent
older dune cordons; 2) a system of weathered and reworked dunes located on the most inland
portion of the coastal dune cordon; 3) a less altered, large field of linear parallel dunes located in
the northern part of the study area; 4) a system of large scale parabolic dunes; and 5) a system of
coastal, relatively unweathered small parabolic dunes.
Mineralogy, geochemistry, texture and SEM analysis of borehole samples revealed a
complex internal structure within the present coastal dune cordon. In the most inland part of the
dune cordon, a basal light grey unit (Unit K) presents similar characteristics to the Kosi Bay
Formation. This is overlain by Unit A, comprising beach and dune systems, characterised by a
very high heavy mineral content. Unit A also forms the basal unit of the central and coastal
portions of the dune cordon. Unit B contains a mixture of reworked sediments from Unit A and
younger sediments. Aeolian Units D and E form the upper part of the dune cordon. Units D and
E were derived from beach - foredune systems and contain a high carbonate bioclast content.
All units are interpreted to be derived from immature sediment from the Tugela River and
mature sediment from the continental shelf. In the southern part of the study area, an additional
unit (Unit C) with unique characteristics has been interpreted as an aeolian deposit reworked
from local fluvial sediments. The units identified from their sedimentological characteristics can
be directly correlated to the regional dune classes identified from the geomorphology.
Luminescence dating of two calcareous dunes was undertaken, revealing that only the
sediment of the small coastal parabolic dunes (Dune Class 5, Unit E2) is of Holocene age. The
deposition of the large field of linear dunes (Dune Class 3, Unit D2) took place between 15 000
and 11 000 BP, during the marine transgression following the last glaciation. Luminescence
dating also indicated that both dunes were subject to at least one major reworking event.
A study on the weathering characteristics of the dunes can be used to attribute a relative age
to the nine sedimentological units. With the help of sea level curves and the two luminescence
dates, the nine units were attributed an approximate absolute age and regrouped into four
sediment packages thought to broadly represent four interglacial periods. The three younger
packages are attributed to the penultimate interglacial (lower part of Unit A), last interglacial
(upper part of Unit A, Units B and C) and "Holocene" interglacial (Units D and E). Hence the
northern KwaZulu-Natal coastal dune cordon under study represents a complex stacking of
three generations of coastal dune cordons, and appears to be constituted of sediments with age
ranging from at least two hundred thousand years ago to present.
The oldest sediment package (Unit K), interpreted as the Kosi Bay Formation, and the older
dune cordons (Dune Class I) must be older than 200 000 years, which is older than considered
by previous studies. The "Holocene" dune cordon (Units D and E) is interpreted as the Sibayi
Formation.