Sandstone is the most significant rock type in the Natal Group, and extends widely within the Marianhill Formation. Due to its prevalence within the Natal Basin it is widely used as aggregate material. It is therefore important to study the physical properties and their relationship to the mechanical properties. The literature on this topic is rather limited. This study constitutes an investigation of the published data, and the application of self-organizing maps (SOMs) and data-mining techniques to extract new knowledge and yield further insight into the relationships between the petrographic, physical, and mechanical characteristics of sandstone. SOM-based analysis distinguished three clusters of sandstones, based on petrographic, physical, and geotechnical characteristics, which led to the identification of different lithofacies. Significant parameters dictating cluster identification are the type of grain contact, void space, packing density, and amount of silica cement.

INTRODUCTION

The sandstones studied fall under the Natal Group, which is a succession of reddish-grey sandstones with subordinate siltstones, shales, and conglomerates, regarded as a molasse deposit that accumulated 490 Ma ago (Thomas et al., 1992). The great majority of the sediments of the Natal Group were deposited in a fluvial environment and represent a terrestrial succession, interrupted by occasional shallow marine transgressions during which quartz arenites were deposited, Marshall (1994). The thickness of the Group ranges from 45 to 600 m, of which the Newspaper Member comprises some 80%. Marshal (1994), recognized two formations within the Natal Group, the lower Durban Formation and the upper Marianhill Formation, which is subdivided into three members; from oldest to youngest the Tullini, Newspaper, and Westville members. The Newspaper Member comprises 90% of the Marianhill Formation and occurs over the whole of the Natal Group basin. The most important sedimentary feature is the planar cross-bedding.

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