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.