Abstract

This paper presents the first stratigraphic subdivision of the Natal Group rocks that is applicable to the entire preserved basin. The Natal Group consists of two formations, each representing a tectonic cycle -- a lower Durban Formation and an upper Mariannhill Formation. The former is subdivided into the Ulundi, Eshowe, Kranskloof, Situndu, Melmoth, and Dassenhoek Members, and the latter into the Tulini, Newspaper, and Westville Members. The sedimentary rocks comprising these units are greyish red, and consist of conglomerates, sandstones, siltstones, and shales. The generally accepted correlation between the Natal Group in KwaZulu-Natal and the Msikaba Formation in Pondoland (Transkei) is considered unlikely. The Msikaba Formation contains Devonian plant fossils, whereas the Natal Group rocks have an 40 Ar/ 39 Ar age of 490 Ma (i.e. Ordovician). At the Dweshula High, just north of Port Shepstone, Natal Group rocks are absent and the Karoo-age Dwyka Group rests directly on basement. It is suggested that the Dweshula High now marks the southern limit of the Natal Group depositional basin. Palaeocurrent data indicate a major provenance for the Natal Group was to the northeast in what is now Mozambique. Pan-African tectonics provided molasse which was deposited, as the Natal Group, in a subsiding elongate (northeast-southwest-trending) foreland graben, under fluvial conditions. Evidence for contemporaneous volcanism and tectonic activity is presented. Fragmentation of Gondwana removed the eastern part of the depositional basin. The occurrence of pressure solution phenomena in the Ulundi Member conglomerates is described. The tensile strength of fractured quartzite clasts in the same unit is used to estimate a minimum thickness for the Natal Group of 1300 to 2600 m at the time of deposition. This is considerably greater than the preserved thickness.

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