Abstract

A composite stratigraphic sequence, constituting the upper portion of the Lower Palaeozoic Natal Group, is well exposed on the beach at Park Rynie. The total sequence is 72.29 m thick and consists mainly of medium- to very coarse-grained feldspathic sandstone with lesser amounts of mudstone and siltstone. This sequence can be subdivided into thirteen cyclothems; of which eleven are fining upward, one coarsening upward, and one holocyclothem. The eleven fining-upward cyclothems represent cyclic episodes of braided channel deposition and abandonment and suggest that the channels were frequently shifting and, therefore, creating a wide channel belt in the lower reaches of the braided river system. The holocyclothem records building up and abandonment of transverse bars which, probably, were affected also by a short-lived marine incursion. The coarsening-upward cyclothem 13, showing conspicuous large-scale tabular cross-bedding and textural rhythmic couplets, shows the development process of a river-mouth bar from its initial stage, through an active stage, into a mature stage. Based on the lithology, sedimentary structures, sand body geometry, and stacked facies architecture, ten sedimentary facies and seven facies associations are recognized. These record the existence of seven major sedimentary environments, namely, braided channel, flooding sheet-flow, migrated channel, crevasse splay, overbank or abandoned channel, transverse bar, and river-mouth bar. The nature of sedimentary facies and facies architecture strongly suggests that the stratigraphic sequence of the Natal Group at Park Rynie Beach is a transitional sequence recording a change in depositional processes from a braided river to a river-mouth bar. This new information suggests that the shoreline for the upper portion of the Natal Group was located at the Park Rynie area, rather than at Hibberdene or Port Shepstone as proposed by earlier studies. Based on the Park Rynie section, integrated with other observations, the author proposes that the reddish colour, and the triplex and duplex facies architectures could be used as identification characteristics for fluviatile sediments.

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