Abstract

Potholes are fascinating features in which one layer of the Bushveld Complex transgresses its footwall and forms a basin-shaped depression. A less severe disturbance is a roll. Generally, rolls are local changes of dip and strike, which seldom disrupt the mining operation significantly, but provide some insight into the understanding of potholes. A potholed layer may be developed within the pothole, although it is in a modified form when compared to its regional characteristics. We describe potholing of the UG2 Chromitite layer and secondary potholing of the Merensky Reef (pothole reef facies). In a UG2 pothole, the leader chromitite seams converge onto one another and then into the main seam before truncating the main seam and transgressing the underlying porphyritic feldspathic pyroxenite. Because the Merensky Reef and UG2 Chromitite layer are close together (18-36 m), and both are mined, it is possible to observe the effect of potholing on more than one layer. At Union Section a number of facies of the Merensky Reef in various pothole structures are developed, as well as a megapothole (900X1100 m) which affects the Merensky Reef (pothole reef facies), UG2 Chromitite layer, and UG1 Chromitite layer. The morphology of the reef within this megapothole provides valuable insight into the nature of potholes. The megapothole is also a milestone, in that portions have been successfully mined. The various types of Merensky Reef and UG2 pothole structures as well as their effect on mining are described.

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