The stratigraphy of the Natal Group.

Abstract

Research for this project involved the first systematic field and laboratory investigation over the entire known portion of the Natal Group depositional basin, from just north of Hlabisa in the north, to Hibberdene in the south, and, on a reconnaissance basis, as far south as the Mtamvuna River near Port Edward. The development of a stratigraphy for the Natal Group is traced through the work of previous researchers, who worked in specific areas. The SACS compilation was inadequate, and this unsatisfactory situation was addressed in presenting the first workable stratigraphic subdivision of the Natal Group for the whole of the basin. There are two proposed formations, each representing a cycle of sedimentation - a lower Durban Formation and an upper Mariannhill Formation. These are subdivided into the Ulundi, Eshowe, Kranskloof, Situndu and Dassenhoek Members, and the Tulini, Newspaper and Westville Members, respectively. They are generally greyish red in colour, and consist of conglomerates, sandstones, siltstones and shales. The generally accepted correlation between the Natal Group in KwaZulu-Natal and what was hitherto considered as its time-equivalent in Pondoland has been disproved. Consequently, the supposed interdigitation/transition between these two assemblages, along with the hypothesis that the Kranskloof and Dassenhoek quartz-arenite Members were littoral deposits formed during a marine transgression/regression cycle, is no longer accepted. A provenance to the northeast is proposed, based on palaeocurrent data. Pan-African mountain-building in what is now Mozambique provided molasse sediments which were laid down in an elongate (NE - SW) foreland graben basin. The age of this was determined as 490 Ma, from 4°Ar;J9Ar step-heating on micas extracted from argillaceous samples. Contemporaneous volcanism, as reflected in the presence of volcanic glass (sericitised) shards, is reported. The Natal Group is a molasse deposit, derived some 490 Ma ago from a Pan African orogenic event in southern Mozambique, and deposited in a foreland graben, the Natal Trough, during continued subsidence. Activity of this trough is seen to have continued from Pan African to Permian times. This assemblage rests on the basement, and is overlain by the Dwyka Group. Only part of the basin survives on the African continent, the unknown portion being removed during the fragmentation of Gondwana. The southern limit of the Natal Group is at the Dweshula High, near Port Shepstone, which, together with basin tilting, is seen to have been instrumental in causing the deposition of this assemblage. It is suggested that fluvial activity and debris flow processes led to the deposition of the conglomerates of the Ulundi, Tulini and Westville Members, whereas braided rivers of the Platte and Bijou Creek types deposited the arenaceous and argillaceous sediments which now constitute the other members. The climate was probably semi arid, with ephemeral streams. Shape parameters of the conglomerate clasts point to a fluvial environment. The dividing-line between monomict (quartz) Facies A to the south, and polymict Facies B to the north, of the Tulini Member, was found to coincide with the edge of the craton in this area. The common occurrence of pressure solution phenomena is described. The tensile strength of fractured quartzite clasts in the Ulundi Member is used to estimate a minimum thickness for the Natal Group of 1300 to 2600 m - considerably greater than the present thickness. This estimate supports the hypothesis that much of the Natal Group was removed by erosion during the 200 million year period between the cessation of Natal Group deposition and the onset of Dwyka glaciation, and indeed, by the glaciation itself. The Westville. Member is thus seen as the basal unit of a third cycle of sedimentation, all of which, except the remnants of the Westville Member, have been eroded away.