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Structural features of the Bokkeveld Group (Cape Supergroup)
in the Eastern Cape, South Africa
Gideon Brunsdon 1 , P.W.K. Booth 2 , J.A. Mondon 3
1. NMMU, South Africa, gideon.brunsdon@nmmu.ac.za
2. NMMU, South Africa, peter.booth@nmmu.ac.za
3. NMMU, South Africa, jean-luc.mondon@nmmu.ac.za
ABSTRACT
Previous studies of the Bokkeveld Group focussed mostly on the sedimentation and sedimentary features of this
group with a view to interpreting the depositional environment. This study aims to contribute new information on
the structural features of the Bokkeveld Group north-west of Uitenhage in the Eastern Cape, where there is
evidence of folding and thrust faulting, similar to that occurring in the under- and overlying groups of the Cape
Supergroup.
Field work to date shows that predominantly argillaceous beds of siltstones and mudstones, with lesser arenites and
thin conglomerates make up the Bokkeveld Group. This group conformably overlies the Table Mountain Group of
predominantly arenaceous beds. Both groups have been folded into asymmetric anticlines and synclines, with a
prominent, south-dipping pervasive cleavage developed in the Bokkeveld Group. The fissile nature of argillaceous
beds on outcrop have required that very careful observations of the all important bedding-cleavage relationship in
the field are made, so that correct interpretations of facing direction of strata can be applied. Thrust faults showing
fore-and backthrusts in the Bokkeveld Group are interpreted to have formed during a main deformation phase of
the Cape Orogeny, during the Permian.
Samples of mainly argillaceous rocks have been collected and mineral compositions identified with the aid of a
petrographic microscope and a scanning electron microscope (SEM). Bedding in argillaceous shales of the Gydo
Formation shows a graded texture with layers of coarse grained silicate minerals alternating with fine grained
chlorite. Chlorite, muscovite and quartz make up the major minerals in the shales, and accessories include zircon,
Rare Earths minerals and opaques (iron ore).
In places bedding planes dip predominantly to the northeast, but this is probably a function of poor outcrop because
bedding does dip in the opposite direction elsewhere in the study area, indicating the presence of folding in the
Bokkeveld Group. A pervasive cleavage dips consistently to the southeast, and appears to be thrust fault related. It
is however possible that some of the cleavage development might also be closely related to folding in the
Bokkeveld Group. This relationship needs to be carefully verified through further microscopic studies. The results
of this study will inevitably point to the fact that strata have been disrupted by structures such as folds, thrust faults
and cleavages to the extent that the stratigraphic order of the Bokkeveld Group in this area will have to be reassessed.
KEYWORDS: Bokkeveld Group, cleavage, folded, faulted strata
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