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Physical characteristics of the Karoo sediments and mode of emplacement of the dolerites

Department of Geology, University of Stellenbosch, South Africa, e-mail: janvanzijl{at}yahoo.com

The mode of emplacement of the dolerites and their relation with the overlying basalts are revisited in the light of new information provided by a recent resistivity study of the structure of the Karoo basin and augmented by an analysis of age and palaeomagnetic data. The different styles of intrusion from the bottom to the top are explained in terms of anisotropy, lithology and increasing upward bending stresses accompanied by a decrease in overburden thickness. The lowermost zone 3 consisting of flat lying dolerite sills of large extent occurs in well-laminated, homogeneous shales with a high degree of anisotropy. The thick dolerite-rich middle zone 2 extending from the Upper Ecca Subgroup through the Beaufort Group and partially into higher Formations is characterised by the occurrence of large basin structures of dolerite. A model, which assumes a central dyke as source and based on the principle that the propagating magma front always follows the path of least resistance is developed to explain the formation of a basin structure including the steepening and flattening of the rim. The model also explains the variations in shape and size such as are encountered in the field by taking into consideration the structure of the nexus of lenticular sandstones intercalated with shales and mudstones occurring throughout this zone. The dominant style of intrusion of, at least, the upper part of zone 1 that extends from the surface to a depth of about 700 m is that of dykes and steeply dipping sheets. Another striking feature of this zone is its dolerite-poor nature. An analysis of the magnetic polarities of the dolerites and basalts leads to the conclusion that the reversely polarized dolerites, which only occur in zone 1 were the feeders to the outpouring of the basalts. The younger normally polarized dykes that cut through the entire basalt succession are associated with the period of cooling and contraction following the main phase of dolerite intrusion. The implications, aided by well resolved age determinations are that the duration of intrusion of the Karoo magmatic event was short. Finally, the significant increase in both the amount of dolerite and the thickness of the dolerite bearing layer towards the east supports the view of a magma source in that direction.