Browsing by Author "Viljoen, J. H. A"

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    Piroklastiese afsettings van perm-ouderdom in die Hoof-Karookom met spesiale verwysing na die Collingham Formasie, Ecca Groep
    (Stellenbosch : Stellenbosch University, 1995) Viljoen, J. H. A; Verwoerd, W. J.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: This study comprises a geological and geochemical investigation of the K-bentonites (potassium-rich, illitic clay beds interpreted as altered volcanic ash) of Permian age in the Karoo Supergroup of the Main Karoo Basin. It also includes a sedimentological and stratigraphical study of the Collingham Formation (Ecca Group), which is the unit with the h.ighest concentration of K-bentonite layers. The 30 co 70 m thick Collingham Formation is conformably underlain by the white-weathering, carbonaceous Whitehill Formation and is overlain by the Ripon Formation in the southeast, the Vischkuil Formation in the southwest and the Tierberg Formation in the west and northwest. The formation consists essentially of alternating thin beds of hard, dark grey, siliceous mudrock and very thin beds of relatively softer yellow­ weathering K-bentonite beds. These sediments are sporadically interrupted by chert beds, and in the upper part of the formation by siltstone and very fine-grained sandstone beds. In the southern and southwestern part of its outcrop area the formation can be divided into three members, namely the Zoute KJoof, Buffels River and Wilgehout River. The basal two members are separated by the Matjiesfontein Chert Bed. Deposition of the Collingham Formation occurred as basin floor muds which settled out of suspension from under and upper flows and as pelagic and hemipelagic material. These deposits were sporadically interrupted by short-lived volcanic ash-falls. The upper part of the formation in the west consists of basin floor and outer fan deposits of a submarine fan. K-bentonite layers are distributed throughout the Karoo Basin, ranging stratigraphcally from the Dwyka Group through the Ecca Group up to and including the basal Beaufort Group. Three zones of relatively higher concentrations of K-bentonite occur, namely in the Prince Albert, Collingham and Abrahamskraal Formations. By using the K-bentonites as marker beds it was confirmed that the Dwyka/Prince Albert contact is younging southwards, that the Whitehill Formation is correlatable with the upper part of the Vryheid Formation and that the Wilgehout River Member is older that the Ripon Formation. Transport of the volcanic ash was from the south and southwest as aerially transported tephra originating from terrestrial Plinian eruptions. The K-bentonite consists mainly of illite, feldspar and quartz (except where it is silicified or enriched in iron oxides and/or carbonates). The maximum opserved grain size of the largest feldspar and quartz grains is 0.5 mm. Diageneticstructures indicate that a large percentage of the minerals that are recognised are diagenetic of origin. The diagenesis and very low grade metamorphism which the K-bentonite has undergone can be represented by the following reactions: Siliceous volcanic ash + H₂0 -> montmorillonite + silica (opal-A) + ions in solution -> montmorillonite + zeolite + silica (opal - CT) -> mixed-layer montmorillonite/illite + zeolite + silica (chert) - > illite + albite + quartz. Chemically the K-bentonite is depleted in Si0₂ and enriched in Al₂ (20-30%} and O (5-10%) in comparison with normal mudstones. Most of the movement of the components occurcd during the first phase i.e. when the volcanic glass was altered to montroorillonite. Only the following analysed components were apparently to a large extent immobile; A1₂0₃, Ti0₂, Zr, Nb, Y and Ga. The geochemistry indicates that the K-bentonites and silicified K-bentonites were originally acid (felsic) volcanic ash. The K-bentonite beds in the lower half of the Collingham Formation as well as the older ones show a within-plate granite geochemical signature, whereas the younger ones correspond better to a volcanic-arc granite association. All the K-bentonites seem to originate from a magma which was generated by the melting of crust that had already undergone a cycle of subduction-zone or continent-continent collision magmatism. The dacitic to rhyolitic volcanism was probably related to the broad Permian magmatic belt which at present crops out in southern South America and/or its eastward extension. This magmatism can probably be linked to tension and graben formation in Patagonia and was followed by southward-directed, and later northward­ directed subduction-related magmatism to the south of the African Plate. A counter-clockwise rotation of Patagonia can probably explain the tension in the west and the compression in the east to form the Cape Fold Belt. Although still tentative, the possiblity exists that the three K-bentonite-rich zones in the Karoo Supergroup can be correlated with the deformation events of the Cape Fold Belt. A preliminary single zircon age of 262 ± 4 Ma was obtained by R. Armstrong for the basal part of the Collingham Formation. Due to their mineralogical composition (illite instead of montmorillonite) the K-bentonites no longer possess the physical properties necessary for use as an industrial material (eg. drilling mud). There is, however, the possibility that the K-bentonites could be exploited as a source for potassium and aluminium, but at present this is not economically viable.