The northern KwaZulu-Natal coastal dunes rise to a height of more than 100m and a width of 2km between Lake Nhlabane and Cape St.-Lucia. This work documents detailed study of two cross-sections through the dunes and correlation of identified dune units over a 20km coastal stretch using confidential data from 6500 boreholes. The results provide useful constraints on the identification of dune units, their age relationships and spatial distribution.
Based on mineralogy, geochemistry and textural properties the sediment comprising the dunes can be grouped into 6 units (Units A to E and Unit K). Along the most inland part of the coastal dunes, the basal light grey 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 overlies Unit A and contains a mixture of sediments partly reworked from Unit A. Unit C is only found in the southern part of the study area. It has unique characteristics interpreted as an aeolian deposit reworked from local fluvial sediments. Luminescence dating indicates that the calcareous Unit D was derived from a nearby beach – foredune system between 15 000 and 11 000 BP, during the marine transgression following the last glaciation. The Holocene Unit E is also calcareous and is derived from the current beach – foredune system. All units, with the exception of Unit C, are interpreted to be derived from immature sediment from the Tugela River mixed with mature sediment from the continental shelf.
With the aid of sea level curves and the two luminescence dates, dune units were attributed an approximate age. The oldest Unit K, interpreted as the Kosi Bay Formation, and the older dune cordon (Dune Class 1) must be older than 200 Ka. 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). The “Holocene” dunes (Units D and E) are correlated to the Sibayi Formation.