Browsing by Author "De Villiers, John"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Open Access
    The Auborus Formation of the Bethanie district, South West Africa
    (1969) Miller, R McG; Martin, H; De Villiers, John
    A study of the southerly occurrence of Auborus sediments revealed a basal conglomerate varying in thickness from 100 ft. to 3300 ft. and lying unconformably on an undulating base of lavas and sediments of the Sinclair Formation. This is overlain by variable thicknesses of sandstone which contain a few interbedded conglomerate bands, most of which are wedge-shaped. Intercalated grit is present in both the sandstone and the conglomerate zones. Shale occurs in the north-west where the sediments reach a thickness of 8500 ft. Nama sediments overlie the formation unconformably. Deposition, after rapid erosion and decay, took place under warm humid conditions into a shallow, slowly subsiding, intracratonic basin. Inward dipping edges were produced by contemporaneous subsidence. Vertical post-depositional movements in the central section folded the beds of the whole succession in the west into vertical and overturned positions. Block faulting that followed had a meridional trend and elevated this central section. Roughly north-trending faulting has displaced the contact in the south-east, and two later sets of east-west faults occur in the north-east. Diagonal, release and tension joints were revealed by measurement. Fracture planes in conglomerate pebbles apparently show no relationship to nearby faults. Pebble, cobble and boulder conglomerates contain a great variety of rock types, which have mostly been identified as coming from nearby southerly sources. A matrix is always present. The sandstone is a uniform red colour, very fine-grained, well sorted, very well compacted and is felspathic and arkosic in character. Rock fragments are ubiquitous. The colouring matter is intergranular, crystalline hematite. Felspars and biotite show various stages of alteration. A little recrystallization of quartz has taken place. The main heavy minerals are ore (largely specularite), garnet, epidote, zoisite, clinozoisite, mica, monazite, zircon and tourmaline. Zircon has been classified on colour, roundness, inclusions and zoning; tourmaline on colour, roundness and inclusions. Bedding in the sands tone is from thin to laminated. The commonest sedimentary structures are cross-beds, mud cracks and clay-pellet impressions. Other features are ripple marks, parting lineations, flute casts and rain prints. The red colour was probably produced in the source area. Diagenetic processes have produced spots of decoloration; and some joints show decoloration. Chemical analyses have been made of the red beds and a decolored spot.
  • Thumbnail Image
    Item
    Open Access
    High-grade metamorphism and migmatizaof the Namaqua metamorphic complex around Aus in the Southern Namib Desert, South West Africa
    (1976) Jackson, MPA; De Villiers, John; Kroner, A
    Rocks of the Namaqua Metamorphic Complex are exposed in an area of 10 000 km² in the northwestern part of the Namaqua Mobile Belt east of Liideritz. The Garub sequence represents the oldest rocks in the Aus area and comprises a diverse group of layered rocks of mainly semi-pelitic, pelitic, mafic, calcareous and quartzose composition. These rocks have been metamorphosed to form marbles, calcitic gneisses, metaquartzites, biotite schists, sillimanitecordierite garnet gneisses, amphibolites and granolites with minor amounts of iron formation and magnesian rocks. The principal metasediments are concentrated in west-trending zones. A central zone of calcareous rocks broadens westwards and contains the largest known bodies of carbonate rocks in the Namaqua Metamorphic Complex. The calcareous zone is bounded in the north by a narrow beit of quartzose rocks and in the south, by a broad zone of aluminous rocks. Gradational rock types between these zones are compatible with original sedimentary lithofacies changes. Layered biotite gneiss of psammitic composition has been interfolded with units of the Garub sequence. Both these rock units are present as inclusions within a tonalitic augen gneiss.
  • Thumbnail Image
    Item
    Open Access
    Stratigraphy, igneous petrology and evolution of the Sinclair Group in southern South West Africa
    (1974) Watters, Brian Roydon; De Villiers, John
    The investigation of an area fringing the Namib Desert in southern South West Africa has led to the clarification of relationships between several geological units of the Late Precambrian Sinclair Group, and to the formulation of a petrogenetic scheme and geotectonic model of evolution. In the area studied, the oldest exposed unit of the Group is the hybrid and heterogeneous Haremub granite which is overlain by the predominantly arkosic Kunjas Formation. This is followed conformably by the Barby Formation, consisting predominantly of basic and felsic lava flows and various volcaniclastic beds including ash-flow tuff deposits. Basic lavas constitute the bulk of the succession and are typically highly porphyritic. A preliminary 'member' subdivision of the Barby sequence is suggested, based mainly on easily-recognisable field and petrographic characteristics. Gabbroic and noritic intrusives in the southwestern part of the area relate to the Barby basic magmatic phase. The following en bloc emplacement of the northwesterly elongated body of Spes Bona syenite was accompanied by intrusion of monzonitic and dioritic magma along fracture zones marginal to the syenite. The emplacement into high crustal levels of large volumes of the porphyritic/granophyric Nubib granite (1360±50 Ma) took place within a major northwest trending zone of crustal weakness which also marked the site of the developing Nam Shear Belt. This shearing imposed a strong but localised mylonitic imprint on all but the youngest intrusions of Nubib granite and on the Barby basic lavas invaded by the granite. Post-dating the Nubib granite and the main phase of shearing is the mixed sedimentary-volcanic Guperas Formation. Deposition took place mainly within a prominent north-south trending graben structure, and volcanic activity was apparently most intense in three fairly distinct centres. Ashflows and felsic lavas were extruded from vents, now preserved as plug-like bodies. The intrusion of dense composite swarms of .basic and felsic dykes took place toward the end of the Guperas phase. The Rooiberg granite (±1270 Ma) is intrusive into the Guperas. The Auborus Formation, comprising about 2,600 m of red felspathic sandstone and conglomerate represents the youngest unit in the evolution of the Sinclair Group. It is preserved in prominent north-south trending graben structures. It has been possible to postulate distinct parent magma types on the basis of major and trace element analyses carried out on sixty-seven samples from the Sinclair Group. The felsic rock-types can be differentiated into a relatively high-Ca group and a relatively low-Ca group, the latter being domi- .nant. The basic rock-types comprise both tholeiitic and calc-alkaline types and the latter are characteristically rich in potassium and associated elements, and have been classed as 'shoshonites'. There exists no direct genetic relationship between the basic and felsic rock-types.
  • Thumbnail Image
    Item
    Open Access
    Structural-metamorphic imprint on part of the Namaqua mobile belt in Southwest Africa
    (1977) Blignault, H J; De Villiers, John
    The cross-section of the part of the Namaqua Mobile Belt investigated, comprises tectonic domains differing in structural and metamorphic character. The upper-crustal Richtersveld Province is separated from the lower-crustal central zone by a marginal zone across which there is a sharp increase in P and T. A continuous prograde metamorphic zonation is established which is bounded on the low-grade side by the 'hornblende in' reaction and includes with increasing grade 'muscovite+ chlorite out', andalusite/sillimanite inversion, minimum melt line, •·epidote out', 'K-felspar + sillimanite in' and a brown hornblende zone. The PT conditions inferred for the K-felspar + sillimanite zone is in the order of 6 kb and 740°c. This metamorphic zonation is defined by the metamorphic peak at any one point and is associated with the early structures. Subsequent deformations indicate a continuous retrogression. The early kinematic event includes at least two phases of coaxial and coplanar folding giving rise to the main planar fabric which is interpreted as a shear surface. It is concluded that the first kinematic event constitutes a thrust regime. The latE kinematic event is represented in the central zone by two phases of macroscopic folding which yielded basin and dome structures. To accommodate the resultant lateral shortening in the central zone, the Kanabeam shear zone developed between the central zone and the more upper-crustal domains where the late phase folding is not developed. Two discrete magmatic events, yielding differentiated intrusive are closely related in time to the early kinematic event. These intrusives underlie at least 50 per cent of the area. The Vioolsdrif Suite (1900 Ma) is genetically related to the Orange River volcanics. The intersected volcanics (2000 Ma) and intrusives form the Vioolsdrif igneous complex which is correlated with the grey gneiss of the lower-crustal domains. In the high-grade central zone aluminous paragneisses structurally overlie the grey gneisses and are interpreted as a mudstone/wacke sequence. The early kinematic event, associated with thrusting, the main metamorphism and extensive intrusion, constitutes the main phase of the Namaqua tectogenesis which connnenced at least at about 1900 Ma. The late kinematic event is associated with lateral movement and shortening during the waning stages of the Namaqua tectogenesis at about 1000 Ma.
  • Thumbnail Image
    Item
    Open Access
    The tectonic development of the Namaqua mobile belt and its foreland in parts of the Northern Cape
    (1974) Vajner, Václav; De Villiers, John
    The Namaqua Mobile Belt extends from the south-western coast of Southern Africa through northern Namaqualand and adjacent parts of South West Africa to the south-western margin of the Kaapvaal Craton near Prieska. Its Precambrian tectonic development is characterised by several successive periods of deformation and metamorphism, the last of which - the Namaqua tectogenesis - occurred between c. 0,9 and 1,25 Ga B.P. The northern boundary of the mobile belt is the Namaqua front which, at different places along its length, appears as a metamorphic transition, as an oblique-slip fault, and as an interface between areas yielding radiometric ages of 2,5 - 2,9 Ga and 0,9- 1,25 Ga respectively. The foreland of the belt in the area under consideration is formed by the south-western marginal part of the Kaapvaal Craton and by the Kheis tectonic domain; the former comprises granitoids and metamorphites generally older than c.2,5 Ga (the Skalkseput Granite, the Draghoender Granite and the Swartkop sequence), as well as supracrustal cover-rocks ranging in age between c. 2,5 and 1,8 Ga (the Seekoebaard Formation, the Transvaal Supergroup and the Matsap Formation). The area of the latter is underlain by a sequence of metasediments and metavolcanics (the Kheis Group), which has been deformed and metamorphosed prior to the deposition of the Seekoebaard Formation (probably prior to the intrusion of the Draghoender Granite c. 2,9 Ga B.P.).