- GeoRef, Copyright 2004, American Geological Institute.
The origin of the quartz-arenite units of the Natal Group has long been controversial. At present there are two prevailing hypotheses, either that they are lacustrine sediments which owe their maturity to aeolian processes prior to deposition or that they are of near-shore shallow-marine origin. However, published evidence for these hypotheses is limited to general discussion; detailed documentation is lacking. Here the problem is reconsidered, based on an integrated study involving stratigraphy, lithology, sedimentary structures, depositional facies, and grain-size analysis. The occurrence of seventeen types of sedimentary structures in the quartz arenites, including herringbone cross-bedding, rhythmic couplets, tidal bundles, lenticular and flaser bedding, symmetrical and asymmetrical ripples, and desiccation cracks support a shallow-marine origin and allows for recognition of tidal flat, tidal channel, and subtidal facies. This interpretation is supported by facies stacking, grain-size analysis, and high mineralogical and textural maturity of the sediments. The tidalites occur at two stratigraphic levels and represent two marine incursions, which are believed to have been from the east. The relatively uniform thickness and wide geographic distribution of the tidalites indicate a lack of topographic relief to the lower reaches of the braided alluvial plain on which the arkosic sediments of the Natal Group were deposited.