Browsing by Author "Fuller, A O"
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- ItemOpen AccessContributions to the geology of the Table Mountain Group(1988) Thamm, A G; Fuller, A OModern analysis of Table Mountain Group sediments began with I. C. Rust's D.Sc. thesis "On the sedimentation of the Table Mountain Group in the western Cape Province" in 1967. Rust defined the stratigraphy of the Table Mountain Group, produced computer generated isopach and palaeocurrent maps for each formation and attempted palaeoenvironmental analyses based on what data he had available. For work dated prior to 1967 the reader is directed to Rust's excellent review in Chapter 2 of his thesis. The thesis served as a basis for Rust's later published work on the Cape Supergroup. Current published palaeoenvironmental models of the lower Table Mountain Group (the Piekenierskloof, Graafwater and Peninsula Formations) are based on a transgressive fluvial - littoral - shallow shelf model (Tankard et al., 1982) following earlier facies and palaeoenvironmental analyses (Tankard and Hobday, 1977: Rust, 1977; Hobday and Tankard, 1978: Vos and Tankard, 1981). The validity of this model has recently been questioned (Turner, 1986; 1987) although no comprehensive alternative has been proposed to date. The sedimentology of the upper Table Mountain Group i.e. the Pakhuis, Cedarberg, Rietvlei, Skurweberg and Goudini Formations (the latter three the newly named Nardouw Subgroup) has not been studied systematically. Good progress has recently been made on the fossil content of the Cedarberg Formation (Gray et al., 1986; Cocks and Fortey, 1986) and palaeoenvironmental analyses initiated in the Nardouw Formation. This thesis documents contributions to the geology of the Table Mountain Group. It is not the intention of the author to present an extensive overview and treatise on the lower Table Mountain Group, but rather to concentrate on three topics that can provide some insight into Table Mountain Group geology. The following three topics were selected 1) Petrology and Diagenesis of lower Palaeozoic sandstones in the s.w. Cape Sandveldt (Clanwilliam and Piketberg Discricts). 2) Palaeoenvironmental indicators in the Faroo Member, (Graafwater Formation) at Carstensberg Pass, R364. 3) Facies analysis of conglomerates and sandstones in the Piekenierskloof Formation: Processes and implications for pre-Devonian braid-plain sedimentology. These topics form the basis of the thesis.
- ItemOpen AccessThe geometry of some Beaufort group sandstones and its relationship to uranium mineralisation(1981) Stuart-Williams, Vivian Le Quesne; Fuller, A OFrom field evidence gained whilst working on the Ptistetognathus/ Diictodon Assemblage Zone west of Beaufort West it is found that three discrete sandstone types called: A) straight channel sandstones, B) low sinuosity channel sandstones and C) transitional sandstones can be identified, in what has previously been considered as high sinuosity channel facies association sediments. Palaeocurrent analysis has demonstrated that the transitional sandstones were high sinuosity and were the larger fluvial systems; the straight channel and low sinuosity channel sandstones were generally much smaller and had as their names imply a much lower sinuosity. The palaeo-variability of current vectors in these systems, in the study area, is such that a standard deviation of greater than +40° is considered diagnostic of the transitional sandstones. Moreover in the study area only this type of sandstone is of importance in uranium exploration. From the palaeocurrent data and the reduction-oxidation states of the sandstone types it is proposed that the transitional sandstones represent semi-perennial fluvial systems flowing across an arid intracratonic basin, whilst the straight and low sinuosity channel sands are intrabasinal tributaries of the transitional sandstones. From this study of the sandstone geometry has evolved a new model of uranium mineralisation. It is proposed that the mineralisation is syngenetic and generated by reduction of uranyl carbonates on carbonaceous material. The carbonaceous material must however be lying closely below or within a weak REDOX front, since it is vital to transport complexes in an oxidising environment, and yet such an environment will not allow reduced uranium to be preserved for any length of time, as the carbonaceous material on which it reduced will eventually oxidize. Such a REDOX front, it is proposed, is created by the coalescence of two discrete sands. The upper sand is an oxidising active channel. The lower sand has been buried for some time and is weakly reducing due to anaerobic breakdown of its carbonaceous material by bacteria. Fluids in the two sands mix at the point of coalescence and uranium in transport in the upper sand is transported to and fixed at the REDOX front by carbonaceous material in the lower sand. Borehole data suggests that the correlation between coalescence of two discrete sandstones (the lower being a transitional sandstone) and mineralisation is very good. It is believed that the three sandstone types, whilst representing major fluvial systems and their tributaries, were undergoing water loss along their length such that distal portions of even the major (transitional) sandstones were likely ephemeral. Evidence for this is found in the arid climate, lack of in-channel vegetation, dominance of flat-bedded sandstones and clear evidence for bedload transport.
- ItemOpen AccessAn outline of the inshore submarine geology of Southern South West Africa and Namaqualand(1971) O'Shea, Desmond O'Connor; Fuller, A OAn outline of the inshore submarine geology of the south western coast of southern Africa is presented. The study is derived from diamond prospecting operations carried out between 1964 and 1970 in the shallow waters between Walvis Bay in South West Africa and the Olifants River mouth in Namaqualand, Republic of South Africa - a distance of approximately 1 000 km (600 miles). The area can be conveniently subdivided into three regions from north to south: (i) Tidal Diamond's Concession (T.D.C.) from Sandwich Harbour to Hottentot Bay. (ii) Marine Diamond Corporation's Concession (M.D.C.) from Luderitz to the mouth of the Orange River. (iii) Southern Diamond's Concession (S.D.C.) from the Orange River mouth to the mouth of the Olifants River.
- ItemOpen AccessThe petrography and major element geochemistry of the phosphorite nodule deposits on the Agulhas Bank, South Africa(1971) Parker, Robin James; Fuller, A O; Willis, J PDredging operations carried out on the Agulhas Bank have proved the existence of a widespread phosphorite nodule deposit, considered to be essentially in situ. The pebble to boulder sized nodules recovered have been classified into two conglomeratic and three non-conglomeratic classes. The latter classes comprise (i) phosphatized microfossiliferous limestones (N I class); (ii) phosphatized highly ferruginous microfossiliferous limestones (N II class); and (iii) nodules composed of a poorly sorted mixture of quartz, glauconite and microfossil grains set in a micrite/collophane cement (N III class). Surface to centre phosphatization effects have been observed in some N I nodules. The first conglomeratic variety (C I class) is noted for abundant, often highly irregularly shaped, enclosed N I class phosphorite pebbles set in a matrix that is similar to the N III phosphorite type. The second conglomeratic variety (C II class) is similar to the first, but it is characterised by the inclusion of pebble sized microfossiliferous internal cast of macrofossils, as well as the presence of macrofossil shell debris. X-Ray diffraction studies have shown that the prime phosphate mineral present is francolite, a carbonate fluorapatite, while optically this mineral has been identified as cellophane. An X-Ray diffraction peak-pair technique has indicated an average 5.5% CO₂ concentration in the apatite phase of the phosphorites. Studies on the major element geochemistry of the various phosphorite classes has shown that the bulk geochemistry of the nodules corresponds to the dominant mineralogy and that variations in the bulk geochemistry of the nodules from within a given group reflects variations in the concentration of diluent allogenic minerals. A sympathetic relationship exists between the Na and S concentrations in the phosphorites, and this has been related to substitution effects in the phosphate mineral phase. The average P₂O₅ concentration determined for the Agulhas Bank phosphorites is 16.18%. The N I and N II phosphorite classes are considered to have originated as a result of limestone phosphatization involving a calcite-to-francolite replacement process. Many of the sedimentological features exhibited by the texturally heterogenous N III, C I and C II class nodules are incompatible under normal hydrodynamic conditions, suggesting an unusual depositional environment. In order to explain these features it has been proposed that the nodules were originally lime mud rich sediments and that the conglomeratic varieties were deposited by submarine transporting agencies such as mud-flows, turbidity currents and/or tidal/storm wave surges. Bioturbation may have been responsible for the mixing of lime mud and coarser material to produce the poorly sorted non-conglomeratic N III phosphorite class. Replacement of the calcitic lime mud by francolite .s considered to be the prime mechanism responsible for the phosphate mineralization and lithification of these N III, C I and C II class phosphorites.
- ItemOpen AccessPetrology and petrography of samples from two Algoa Basin cores(1973) Stewart, Allan Graham; Fuller, A ONinety-four samples from two cores drilled in the Uitenhage Group of the Algoa Basin were analysed. The group of sedimentary rocks comprises the Enon, Kirkwood and Sundays River Formations and consists of continental (Enon and Kirkwood Fms.) and marine (Sundays River Fm.) deposits typical of a transgression sequence into an intermontane valley. Only the Kirkwood and Sundays River Formations were intersected in the boreholes but in one - AD 1/68 'basement' of Bokkeveld Group is reached, while the other borehole, CO 1/67, did not reach 'basement'. The Kirkwood Formation sediments are typically red shales interbedded with drab, fine sandstones (wackes) while the Sundays River Formation is typically drab fine to very fine silty sandstones and gray shales of a marine delta environment. Grain size analyses utilizing wet sieving and pippeting methods showed that two populations of grains are dominant, namely fine to very fine silty sand and clay size material. From the granulometric analyses statistical parameters (e.g. mean, standard deviation, skewness, kurtosis) were calculated and plotted in various combinations. Distinctive trends are thus revealed indicating a mixing of the two sediment populations in varying proportions. Compaction has resulted in an increase with depth in specific gravity (which varies from about 2,5 to +2,6 gm cc⁻³). Studies of the heavy mineralogy reveals a dominance of garnet (two varieties, colourless and pink) with zircon, sphene, rutile and some others in considerably lesser amounts. Opaque grains are also present, sometimes in dominant amounts. X-ray diffraction analyses of the <2 μm clay fraction showed that illite is dominant in both the marine and terrestrial deposits and that chlorite is abundant to infrequent, while montmorillonite is more prevalent in the continentally deposited rocks. Selected clay samples were photographed with both the Transmission and the Scanning Electron Microscopes. Thin sections of the arenaceous samples reveal that those which are carbonate cemented are relatively free from matrix, while those which are uncemented are matrix-rich. This latter situation can be ascribed to the breakdown, after burial, of the commonly occurring rock fragments which frequently constitute about one-third of the sandstones. The problem of red beds is considered, and the red pigmentation found in the Kirkwood Formation is believed to be due to oxidation of iron after deposition of the sediment. Intense weathering in the upland source area is not a suitable explanation for the formation of the Kirkwood Formation red beds. The basin as a whole is considered with attention being focussed on the provenance areas (believed to be the Cape and Lower Karroo Supergroups), the dispersal, depositional environments and the lithification and diagenesis. Finally, the economic potential is briefly considered and some suggestions for future research are put forward.
- ItemOpen AccessThe sedimentology, geochemistry and diagenesis of West Rand Group sediments in the Heidelberg area, Transvaal(1980) Camden-Smith, P M; Fuller, A OThis study deals with the West Rand Group (formally the Lower Division of the Witwatersrand System) sediments within an area of approximately 500 km east of Heidelberg. The aim of the study was to interpret from the stratigraphy, lithology, petrology and sedimentary structures, the type of processes which were involved in the deposition of the arenaceous units of the West Rand Group, the current dispersal pattern and the probable, equivalent modern day depositional environment. The extent of diagenesis and metamorphism was investigated by using two white mica techniques. The chemistry of the shales was related to its mineralogy and a detailed study of the geochemical profile below the West Rand/Central Rand unconformity was undertaken. The West Rand Group east of Heidelberg is made up of a thick (3400 m) succession of alternating arenaceous and argillaceous units. It has traditionally been subdivided into three Subgroups - the Hospital Hill, Government and Jeppestown. Each Subgroup is divided into three formations on the basis of laterally persistent markers. Facies analysis has indicated that deposition occurred in the following environments in the different Formations: (1) the first shale horizon of the Orange Grove Quartzite Formation marks a change from "high" energy-wave dominated to lower energy - tide dominated conditions. The basal conglomerates and overlying trough crossbedded facies are interpreted as either platform beach or inlet deposits while ebb tidal deltas and local storm deposits characterise the rest of the arenaceous succession (2) the shales and thin sublitharenites of the Park- Town Shale Formation were formed by suspension deposition of mud alternating with periodic sand influxes while the banded iron formations and magnetite rich zones probably represent distal shelf muds. (3) the 'sago'-textured units of the Brixton Formation were deposited by storm ebb surge currents in conjunction with tidal currents (4) at the base of the Promise Quartzite Formation offshore (sub- tidal shelf) marine conditions prevailed. Nearshore sequences and finally a braided stream setting of the Platte type is interpreted for the rest of the succession (5) the poor outcrop of 'tillite' in the Coronation Shale Formation made it impossible to delineate the facies associated with the till (6) the immature subgreywackes of the Witpoortjie Formation below the Government Reef represent subtidal shelf and inshore tidal flat deposits. The Government Reef marker represents an ancient beach deposit with onshore migrating sandbars and ripples moving in response to shoaling waves. The Blue Grit marker is interpreted as either a fossil submarine rockfall or a cannon and fan valley deposit (7) the shales of the Jeppestown Subgroup represent proximal shelf deposits while the sandstones formed as a response to tidal, shelf and possibly fluvial processes. The Weber crystallinity index indicates that the mineral assemblage (white mica, chlorite and quartz) are low grade. The West Rand Group's Hb(rel) value of 150 corresponds to an approximate temperatures of 290°C. The study areas baric constraints are similar to the conditions for the Hercynian metamorphism in the eastern Alps. The relative amounts of clay mineral present in a sample was calculated from its bulk chemistry. Chlorite, illite and muscovite (in that order) are the major clay phases present. The trace element abundance indicates that the source rocks for the West Rand Group in the study area are similar in petrology to the source rocks of the Fig Tree sediments. The trace elements - Sr, Ni, Rb, ca, Zn, Cr - can be used to discriminate the shales of the West Rand, Central Rand and Fig Tree Groups. The Jeppestown shale immediately below the Main Conglarerate has a geochemical profile that has traits of a palaeosol which has subsequently been modified by diagenesis and the percolation of ground waters. Two models are proposed for the deposition of the Hospital Hill Subgroup and the Government Subgroup by integrating the writer's detailed facies analysis approach with the work done by previous workers in other outcrop areas.
- ItemOpen AccessThe genesis of the blue amphibole asbestos of the Union of South Africa.(1961) Genis, Jacob Hendrik; Genis, Jacob Hendrik; Fuller, A OThe blue amphibole asbestos, crocidolite, which occurs as interbedded seams in banded ironstones of the Lower Griquatown stage of the Transvaal System in the Northern Cape Province, is the finely fibrous form of the soda-amphibole riebeckite. Despite the widespread occurrence of the Precambrian type of banded ironstones, crocidolite is a mineral of rare occurrence and is only developed to a relatively minor extent in South Africa, Western Australia and Central China. The composition, structure and properties of riebeckite in general and of crocidolite in particular are discussed and four new chemical analyses are given. Particular attention is paid to the mode of occurrence of fibrous riebeckite and it is suggested that the name "crocidolite" be reserved for the asbestiform riebeckite which occurs interbedded with banded ironstones. The general geology, lithology and mineralogy of Precambrian banded ironstones are described and their distribution in space and time is discussed. It is found that no banded ironstones are known to be younger than 1000 million years. This fact is related to special conditions of atmosphere, surface temperature and biological development which existed during the so-called Primitive Period of the Precambrian, a period which lasted from approximately 3000 million years to 1000 million years ago. The banded ironstones of the Transvaal System are described in some detail and seven new chemical analyses as well as trace element data are given. The genesis of banded ironstones is discussed at some length and particular attention is paid to the authigenesis of riebeckite. It is concluded that banded ironstones were deposited in seasonally fluctuating, typically "non-aggressive", fresh to brackish water lakes which were fed by sluggish, mature rivers. The solutions of iron and silica, alkaline earths and clay colloids carried by these rivers were derived from basic igneous rocks by chemical weathering in a tropical, monsoon-type climate. Recognisable detrital material is virtually absent from the banded ironstones of the Transvaal System, but it is thought that the stilpnomelane layers are aeolian deposits and that their composition gives some indication of the material which remained behind as "lateritic" soils in the source area. It is suggested that both crocidolite and riebeckite were formed by the low temperature dehydration, in situ, of an ordered precursor which could have been a clay mineral similar in structure to attapulgite, but containing ferrous and ferric ions in the octahedral layer. This clay mineral acquired sodium by cation exchange during dry periods when the depositional lakes were enriched in sodium. It is found that the actual formation of crocidolite is completely unrelated to dynamic stress. It formed only where layers of proto-riebeckite were in close contact or traversed by magnetite layers. It grew by the diffusion of proto-riebeckite through the magnetite layers, its transformation to riebeckite during its passage, and final addition to fibre growth points in contact with the magnetite layer. This process was initiated by slight temperature gradients in the magnetite layers and was propagated by a type of thermal autocatalysis. The temperature gradients in the magnetite layers resulted from their superior heat conductivity combined with differences in depth of burial of the strata. Economic deposits of crocidolite formed where a sufficient number of layers of protoriebeckite were in contact with magnetite layers which maintained or repeatedly attained the requisite temperature levels. The superimposed economic deposits of the Kuruman area, which appear to be related to folding, are due to an early period of folding, possibly even slumping during deposition, which took place prior to the formation of crocidolite and caused the thickening of the protoriebeckite layers in the crests and troughs of folds by plastic flow from the flanks. Three appendices, giving details of chemical and spectrographic analyses and of chemical experiments carried out, are attached.