Browsing by Subject "Geological Sciences"

Now showing 1 - 20 of 135
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    Aeromagnetics of selected continental areas flanking the Indian Ocean : with implications for geological correlation and reassembly of Central Gondwana
    (2000) Sahu, Bijay Kumar
    Reassembling continental fragments of Gondwana has been a subject of interest to many since almost the beginning of the last century. As a result, the broad relative position of the major continental fragments and their dispersal history is well understood using marine magnetic anomalies, coastline geometry, surface geology and limited geophysics. Uncertainty still prevails in reassembly of central Gondwana fragments flanking the Indian Ocean. This thesis aims at utilising geophysical constraints to corroborate an fine-tune the reconstruction of these fragments supported by geological evidence.
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    Alteration reactions which affect the geochemistry of kimberlite and its xenoliths, and the search for unaltered mantle materials
    (1994) Berg, Gerhard Wolfgang
    [Volume 1 - needs to have volume 2 incorporated.] This thesis records a series of attempts to elicit from bulk kimberlites and their xenoliths, geochemical and mineralogical information meaningful with respect to the primary composition of their source in the mantle. Some of these attempts were successful and some controversial, whilst others were modified significantly to cater for secondary alteration processes (metasomatic events) which took place either in the mantle, or during, or after the emplacement of the rocks in the crust. These secondary events document those stages of the history of the rocks which overwrite and disguise their primary state. It is important to understand the details of these secondary alteration processes so that their record is not erroneously attributed to the primary state of the rocks in the mantle.
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    The amendment of acid soil with an ettringitic waste and its effects on plant growth
    (1995) Tomlinson, Ian Rory; Fey, Martin V; Willis, James
    Associated with ESKOM's ash water beneficiation programme is the precipitation of an ettringitic waste from highly alkaline, saline water. The waste is dominated by ettringite (CauAJ4(OH)24(S04) 6.52H20) with calcite (CaC03) as a minor phase (17.7% for the sample used in this study). Apart from the presence of calcite, the ettringite itself is alkaline due to the presence of OH ions. Following a submission that the waste had potential as an ameliorant of acid soil, research into this possibility was initiated. Following the determination of a calcium carbonate equivalent (HCl-CCE) value of 78% using the HCl back titration method of Horwitz (1980), an incubation experiment was initiated using three acid soils of contrasting characteristics: a so-called Silvermine sand, Kranskop A and Kranskop B soils. The effects on soil acidity of ettringitic waste were compared with analytical grade calcite. Soils (50g samples) were incubated with the two alkaline amendments for two weeks, following which pH(KCl), pH(H20) and KCl-extractable acidity were determined. Ettringitic waste led to apparently lower levels of acidity neutralization for corresponding treatments set on an HCl-CCE basis. This difference was minimized with the highly buffered, sesquioxide and organic-rich Kranskop A soil which could be attributed to the greater reactivity of the ettringitic waste with organically-complexed acidity together with the "self-liming" effect of so4 in sesquioxide-rich soils (sensu Reeve & Sumner, 1972). The waste showed progressively less neutralization with Kranskop B and Silvermine soils apparently in response to a decline in buffering capacity of these soils.
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    An integrated geochemical and microbiological investigation of sulphate reduction in hypersaline pans
    (2007) Porter, Donovan; Roychoudhury, Alakendra; Cowan, Don
    Geochemical and microbiological methods were used to derive a holistic picture of sulphate reduction in five hypersaline pans in South Africa. Sulphate reduction rates were determined using a radioactive tracer (35 So/-) technique. This was applied to determination of in situ sulphate reduction rates, the effect of increased salinity on the activity of sulphate-reducers, the determination of kinetic parameters for sulphate uptake, the effect of temperature on sulphate reduction and the determination of favoured organic substrates. Such measurements were supported by the collection of pertinent geochemical data from pan sediments. The sulphate-reducing microbial community was quantified by competitive polymerase chain reaction. The structure of the microbial community was studied by denaturing gradient gel electrophoresis, from which bands were excised for DNA sequencing. Denaturing gradient gel electrophoresis patterns were analysed statistically by cluster analysis and principal components analysis. High in situ rates of sulphate reduction (up to 3684 nmol.cm-3 .daf1 ) were measured, showing strong correlations to salinity and sulphate concentrations. Rather than inhibiting sulphate reduction, slurry experiments showed increased sulphate reduction rates with increased salinity. Optimum salinities were 272-311 at hypersaline pans and 134-244 at highly saline pans. The use of compatible solutes, not K+, for osmoprotection, was inferred. Half-saturation constants measured here ( 64-780mM) are the first reported for hypersaline sites and were much higher than previously measured in other environments, implying greater regulation of sulphte transport. Values for apparent activation energy were within a narrow range (28-62 kJ.mor1 ) and similar to those measured in other environments. Sulphate reduction rates rarely increased significantly after addition of organic substrate solutions and there were no narrow substrate preferences. It was inferred that sulphate-reducers generally had sufficient organic carbon in situ. However, the use of acetate and n-butyrate suggests that complete oxidation of organic matter was more widespread than predicted on bio-energetic grounds. This was supported by DNA sequence data suggesting the presence of members of the completely-oxidising Desulfobacteraceae. Changes in bacterial numbers paralleled changes in sulphate reduction rates. Significant correlations between geochemical and microbiological data inferred from statistical analysis of denaturing gradient gel electrophoresis data revealed the importance of salinity, sulphate concentration and organic matter reactivity as determinants of SRB community structure.
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    An investigation into the geomorphology of the Hebron Fault, Namibia, using a satellite-derived, high-resolution digital elevation model (DEM)
    (2019) Salomon, Guy; Sloan, Alastair
    The Hebron fault scarp in southern Namibia is 45 km in length with an average height of 5.5 m and a maximum height of 8.9 m. Namibia is a Stable Continental Region (SCR) — a slowly deforming area within a continental plate. The country also has little recorded seismicity with the largest earthquake on the International Seismological Center (ISC) catalogue being MW 5.4. If the Hebron fault scarp was formed in a single event, this would represent a MW 7.3 earthquake. SCRs do occasionally experience large earthquakes, however, the recurrence intervals between these events is much larger than in rapidly deforming areas. Consequently, studying palaeo-earthquakes allows the record of seismicity to be extended and the characteristics of SCR events to be better understood. These studies may help refine the Mmax estimates required for seismic hazard assessment. Previous work on Hebron has been limited to field descriptions and theodolite survey scarp heights. Furthermore, there have been several interpretations of the fault mechanism and number of rupture events. This study produces a high-resolution Digital Elevation Model (DEM) via stereophotogrammetry using pan-sharpened Worldview-3 satellite imagery (0.31 m resolution). The DEM was used for several geomorphological analyses. These included measuring the scarp height at 160 locations along its length, measuring river channel displacements and identifying knickpoints along river profiles. Results indicate that the scarp formed from a normal, dip-slip fault that ruptured in a single event. This scenario would imply a high slip-to-length ratio. A comparison of other SCR fault scarps in the literature was made which shows that Hebrons’ slip-to-length ratio falls within the values found on other SCR faults. This study also discusses the implications of results for seismic hazard assessment in the region. Due a poor seismic record, probabilistic seismic hazard analysis (PSHA) will calculate a low seismic risk for Namibia. As large earthquakes can occur in SCRs, deterministic seismic hazard analysis (DSHA) can be used to inform policy makers of the worst case scenarios.
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    Analysis of effect of using estimated shear wave data as compared to measured compressional and shear wave sonic log
    (2010) Oghenekohwo, Felix Onovughe; Smith, George
    This study is aimed at developing a workflow, and ultimately a model, for quantitative interpretation of sonic and seismic data. Measured data collected at the point of logging can be fraught with errors that can lead to wrong interpretation. One of such data is the shear wave velocity which in most cases is collected with the compressional wave velocity. The measured shear wave velocity log may contain errors that are due to drilling conditions, mud invasion etc. It may also contain cycle skips and might contain a lot of missing data and information. It is because of the poor quality of this type of log that has often made well log analysis companies and log interpreters neglect the measured shear wave log and subsequently generate or create an estimated shear wave log which they use for interpretation and modelling to check how the amplitudes vary with increasing offsets, among other uses.The workflow presented in this study considers the effect of working with the measured data, a reprocessed shear wave log and a locally estimated shear wave log. Specific correction procedures for invasion of the logs was done and synthetic seismograms were created for each type after correction for comparison to a 3D seismic data. The results of this study suggest that oil based mud invasion can cause significant problems to sonic logs especially the shear wave log. It also suggests that, if a shear wave log is of low or bad quality, a reprocessed shear wave log would be better for interpretation and modeling rather than a locally calibrated shear wave log or an estimated shear wave log using global predictions. The conclusion is evident from the synthetics generated using the measured shear wave data and the estimated shear wave data.
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    Apophyllite weathering and the aqueous geochemistry of a Karoo breccia pipe
    (2002) Cavé, Lisa Caryn; Fey, Martin; Jackson, Graham; Nordström, Kirk
    Apophyllite is a phyllosilicate mineral commonly associated with hydrothermal environments. It is a major secondary phase occupying fractures in a dolerite breccia pipe on the farm Kopoasfontein, outside Calvinia in the western Karoo, South Africa. Groundwater abstracted from this breccia is uncharacteristically alkaline (pH 10) and high in fluoride (11 mg/L) for a natural water and the link between the groundwater chemistry and breccia pipe minerals has been investigated.
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    Applicability of rock physics models in conjunction with seismic inverted data to characterize a low poro-perm gas-bearing sandstone reservoir for well location optimization, Bredasdorp Basin, SA
    (2015) Adrian, Jorge Isaac; Smith, George
    The primary focus of this dissertation is to develop a predictive rock physics theory that establishes relations between rock properties and the observed seismic and to present the results of different seismic characterization techniques to interpret a tight gas sand reservoir off the south coast of South Africa using as input rock physics analysis and inverted seismic outcomes. To perform the aims and goals of this study a workflow that involves the execution of three main processes was implemented: (1) rock physics modelling, (2) a simultaneous seismic inversion, and (3) seismic reservoir characterization techniques. First, a rock physics model was generated as a bridge between the seismic observables (density, Vp and Vs) and reservoir parameters such as fluid content, porosity and mineralogy. In situ and perturbational log - derived forward modelling was performed. Both in situ and perturbational forward modelling were used to generate synthetic seismic gathers, which were used to study the AVA attribute responses. Overall, the effect of fluid fill on this tight gas sand seismically is modest compared with the effect of porosity changes. Second, there follows a detailed description of a workflow implemented to simultaneously invert P and S pre - stack seismic data. The derived elastic properties (acoustic impedance, Vp/Vs and density) were then used in combination with the rock physics analysis to characterize seismically the reservoir. The predicted acoustic impedance and Vp/Vs volumes show a good tie with the log data. However, the density outcome was of limited quality compared with the two mentioned above. Finally, using outcomes from rock physic s analysis and/or inverted data, four seismic techniques to characterize the reservoir were conducted. The techniques involved are: (1) AVO cross - plotting to generate a good facies property based on AVO attributes (intercept - gradient) and rock physics in the area of study , (2) rock physics templates (RPTs) to compute discrete rock property volumes (litho - Sw, litho - porosity) using a collection of curves that cover all possible "what if" lithology - fluid content - porosity scenarios for the reservoir and the inverted data, (3) a lithological classification to calculate litho - facies probability volumes based on a litho - facies classification using petrophysical cut - off s , multivariate probability functions (PDFs) and inverted data, and (4) an extended elastic impedance (EEI) inversion to derive rock property volumes (Vclay, porosity) based on AVO attributes (intercept, gradient). Despite differences in the input and theory behind each technique, all outcomes share parallels in the distribution of good and poor facies or reservoir and non - reservoir zones.
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    Aspects of the chemistry of some stony meteorites
    (1971) McCarthy, T S
    [pg 20,67 missing] Ten eucrites, four howardites, three enstatite achondrites, eight HL chondrites, the one known chassignite and a chondrite inclusion from the Cumberland Falls achondrite have been analysed for major, minor and some trace elements (viz. Fe, Mn, Ca, Cr, P, Si, Al, Mg, Na, K, Sr, Zr, Y, Ba and Ni) using X-ray fluoresence analysis. A fusion technique was used to determine the more abundant elements, while trace constituents were determined on powdered samples. The data obtained have been critically examined from the point of view of both precision and accuracy, and found to be of a high standard for most elements. Inter-element relationships amongst the diogenites, howardites and eucrites suggest that two distinct processes were involved in the genesis of these meteorites. Theories for the origin of these meteorites have been examined in the light of these inter-element relationships. It is concluded that mixing of diogenitic and eucritic material was responsible for the formation of howardites, while diogenites and eucrites appear to have been formed by processes of igneous differentiation from a single parent magma. The unique achondrite Chassigny may also be a product of this process. Some speculations concerning the composition of the parent; material from which these meteorite types crystallized are presented. The enstatite achondrites are apparently unrelated to the previously mentioned meteorite types. They do, however, show close affinities with the enstatito chondrites. It would appear that the two types are not related by igneous differentiation, but were probably formed by the same processes. The chondrite fragment removed from the Cumberland Falls achondrite has a composition which suggests that it is a member of the genetic sequence represented by enstatite chondrites and achondrites. The Vigarano and Ornans sub-types of the HL chondrites were observed to exhibit slight but distinct differences in bulk composition in the form of higher Ca and Al abundances in the Vigarano sub-type. Superimposed on these differences are more dramatic chemical differences which appear to be associated with the recrystallization which some of these meteorites have undergone. These differences include loss of alkali metals, Ca and Al.
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    Aspects of the geology, geochemistry and metamorphism of the lower orebody, Broken Hill deposit, Aggeneys
    (1993) Hoffmann, Dennis; Frimmel, Hartwig E
    The Broken Hill deposit, Aggeneys, is a metamorphosed stratiform Cu-Pb-Zn-Ag sulphide deposit situated in the mid-Proterozoic supracrustal sequence of the Bushmanland Subprovince in the Namaqualand Metamorphic Complex. The deposit comprises two superposed orebodies, each consisting mainly of massive sulphide lenses and iron formation which are hosted within a metapelitic schist close to major quartzite horizons. This study is concerned with the tectonically lower orebody (LOB). The iron formation is magnetite-rich and contains varying proportions of (Fe,Mn)-rich silicates (garnet, amphibole, olivine, orthopyroxene, pyroxenoid), quartz and Cu-Pb-Zn-sulphides. These minerals occur in mm- to 5 cm-thick bands and are often traceable over tens of metres. The well preserved banding is considered to represent bedding. Five different varieties of silicate-rich mesobands in the iron formation can be distinguished based on the predominant mineral assemblage: (a) amphibole-olivine-quartz +/- garnet, (b) amphibole-quartz, (c) garnet-apatite-quartz +/- amphibole, (d) garnet-apatite-quartzorthopyroxene, (e) pyroxferroite-quartz +/- amphibole and (f) quartz. These rocks all contain magnetite, and Ba-rich biotite is common but is not always present.
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    Chemical and mineralogical changes associated with leachate production at Kriel Power Station ash dam
    (1996) Bezuidenhout, Nico
    ESKOM is the biggest producer of electricity in Southern Africa and generated 22mt of ash from its coal burning power stations in 1994. Of the 22 mt of ash only 5% was utilized and the rest disposed of in either landfills or ash dams. Various environmental impacts are associated with the disposal of ash including loss of usable land, dust pollution, water pollution and effects on vegetation. Current research into the environmental impact of fly ash disposal has focused on the dissolution of fly ash and the physical and chemical characteristics of unreacted fly ash particles. Few studies have investigated the chemical and mineralogical transformations that occur during the weathering of ash and little is known concerning secondary mineral formation associated with weathered ash from large ash deposits. The objective of this study was to investigate the dissolution characteristics of fly ash particles, the physical and chemical alteration of fly ash in a real weathering environment and the formation of secondary weathering products associated with ash dams. The research was based on a core section that was drilled through the ash dam of Kriel power station, precipitate samples associated with the water that is used to transport the ash to the ash dam and samples of the ash water and leachate.
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    Chemical, physical and mineralogical properties associated with the hardening of some South African fly ashes
    (1999) Campbell, Alex E; Fey, Martin V; Willis, James
    Coal combustion for power generation is relied on heavily in South Africa. The main residue from the combustion process is fly ash, with about 22 million tons being produced in 1994, of which only about 1 million tons is utilised, the remainder requiring disposal as waste. Various environmental impacts are associated with the disposal of fly ash in surface impoundments, including loss of usable land, contamination of groundwater, and production of wind-blown particulates, effects on vegetation and aesthetic impacts. Ultimately the disposal of fly ash in the mined-out underground workings is highly sought after, as it may limit many of the impacts which fly ash induces on the environment. The main purpose of this study was to assess the degree of hardening of four South African fly ashes from power stations at Kriel, Lethabo, Matla and Sasol in terms of their chemical, physical and mineralogical properties. Samples were taken of fresh and unweathered ashes from each power station. Unweathered ash samples analysed by XRFS were found to have high contents of Si0 2 (49.9-56.3%), Al 2 0 3 (27.2-31.5%), CaO (4.2-8.6%Y and Fez03 (3.1-3.9%) with varying contributions from loss on ignition (L.O.I.) (0.4-4.4%) and MgO (1.1- 2.2%). The major mineral phases detected in unweathered ash by XRD include quartz, mullite and glass, with smaller contributions from hematite and lime. Laser particle size analysis and liquid limit determination (Atterberg tests) were also conducted. The exceptionally high L.O.I of Sasol ash (4.4%) was attributed to the presence of partially burned coal, based on DTA and TGA investigation. Modulus of rupture experiments were conducted on briquettes (dimensions 7.0 x 3.5 x 1.0 em) of unweathered ash from Kriel, Matla, Lethabo and Sasol, which were wetted to their liquid limit and cured for 24 h under ambient conditions. The tests indicated that there is a strong linear relationship between total Ca content and the modulus of rupture for 3 of the 4 unweathered ashes. The fly ash from Kriel deviates from this relationship, providing evidence that other chemical (e.g. form of Ca) and physical factors (e.g. particle size) may also play a role in hardening.
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    Chemostratigraphic correlation and Pb-Pb dating of carbonate sequences in the external Gariep belt and Kango inlier of the Saldania belt in Namibia and South Africa
    (2000) Fölling, Peter Gerhard; Frimmel, Hartwig E
    Worldwide most Neoproterozoic sequences are characterised by an intimate association of glaciogenic diamictite with marine carbonate deposits, indicating possibly rapid climatic changes from icehouse to greenhouse conditions. Such sequences also occur in the Pan-African Gariep Belt and in the Kango inlier of the Saldania Belt in Namibia and South Africa. Correlation of these sequences with others has been problematic because of poor age control, structural complications and poor exposure in the Saldania Belt.
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    Classification and petrogenesis of the Tongo dike-01 from the Tongo-Tongoma cluster, Sierra Leone: constraints from bulk rock geochemistry
    (2021) Mathafeng, Katleho; Howarth, Geoffrey
    The Man Craton in West Africa, like the Kaapvaal Craton in southern Africa, hosts diamondiferous kimberlites. However, West African kimberlites are commonly micaceous and unusual relative to archetypal South African kimberlites. Petrographically, they appear more similar to orangeites (aka Group II kimberlites), which represent a type of olivine-lamproite. A suite of 14 representative samples from the Cretaceous Tongo dike-01, Sierra Leone have been analysed for their bulk-rock major and trace elements as well as Sr-Nd-Pb isotope geochemistry. The primary objectives of this study are: 1) provide detailed petrographic observations of the dike, 2) classify the dike relative to kimberlites worldwide, 3) constrain the geochemical effects of secondary processes on bulk-rock analyses, 4) provide an estimate of the close-to-primary parent magma composition, and 5) constrain the petrogenesis of these diamondiferous rocks. The major element chemistry of the Tongo dike-01 reflects concentrations that are similar to both archetypal kimberlites and orangeites. Major elements such as SiO2 (~28.20 ± 3.90 wt. %) and CaO (~ 12.50 ± 1.80 wt. %) display archetypal kimberlite concentrations whereas Cr2O3 (~0.20 ± 0.01 wt. %) and P2O5 (~1.65 ± 0.60 wt. %) resemble those that define orangeites. The high abundance of phlogopite in this dike is illustrated by the high bulk-rock concentrations of K2O (~3.03 ± 0.50 wt. %) and Al2O3 (~4.08 ± 1.00 wt. %), similar to those of orangeites. Like the major element geochemistry, the trace element geochemistry of the Tongo dike-01 also displays mixed archetypal kimberlite and orangeite traits. Trace elements such as Nb (~365.0 ± 50.4 ppm) and Y (~18.77 ± 6.60 ppm) possess concentrations that are similar to kimberlites whereas Rb (~160.0 ± 14.8 ppm) and Th (~36.22 ± 5.30 ppm) resemble orangeites. Trace element ratios are no different, ratios such as Ce/Pb (16-82), Ba/Nb (1-8), La/Nb (0.6-1.2) and La/Sm (11-13) resemble those of kimberlites while La/Yb (280-520) are more similar to orangeites. However, unlike major and trace element geochemistry, the Sr-Nd-Pb isotope geochemistry of the Tongo dike-01 solely resembles those of archetypal kimberlites (87Sr/86Sr)i ~0,7039, (206Pb/204Pb)i ~18.88, (208Pb/204Pb)i ~40.02 and (143Nd/144Nd)i ~0.51253 ± 0.00001. Prior to interpretation of primary processes and parent magma composition estimates, the effects of secondary processes were first evaluated. These secondary processes include crustal contamination, ilmenite contamination and olivine entrainment/fractionation. Samples that had experienced these secondary processes were excluded and a suite of unaltered/least contaminated samples was compiled in order to constrain the close-to-primary magma composition of the Tongo dike-01 and interpret primary petrogenetic processes effecting the kimberlites. To determine a representative parent magma composition, a total of six out of the fourteen samples were excluded from consideration. The estimated close-to-primary magma composition for the Tongo dike-01 is suggested to be SiO2 ~28.20 ± 3.90 wt. %, Fe2O3 ~10.20 ± 1.70 wt. %, TiO2 ~1.70 ± 0.30 wt. %, Al2O3 ~4.08 ± 1.00 wt. %, K2O ~3.03 ± 0.50 wt. %, La ~363.12 ± 25.43 ppm, Gd ~15.87 ± 4.20 ppm, Yb ~1.020 ± 0.06 ppm, ( 87Sr/86Sr)i ~0.7039, ( 206Pb/204Pb)i ~18.88, ( 208Pb/204Pb)i ~40.02 and ( 143Nd/144Nd)i ~ 0.51253 ± 0.00001. Although the petrography and major element concentrations are similar to orangeites found in South Africa, the trace element and Sr-Nd-Pb isotope geochemistry of the Tongo dike-01 reflects a kimberlite composition. Thus, the Tongo dike-01 is more consistent with being classified as a relatively rare type of ‘mica-rich' kimberlite rather than orangeite. Kimberlites from around the world derive from the same asthenospheric mantle reservoir and their major element chemistry is controlled by the compositions/mineralogy of the lithospheric mantle assimilated during kimberlite evolution. The similarity of the trace element ratios and Sr-Nd-Pb isotopes of the Tongo dike-01 in this study relative to archetypal kimberlites worldwide strongly implies that the Tongo dike-01 derives from the same asthenospheric reservoir as these kimberlites, although mineralogically the Tongo dike-01 is different and has a different parent melt major element composition. This is interpreted to reflect the contribution of lithospheric mantle material that is mineralogically different to that assimilated by archetypal kimberlites during the ascent of the Tongo dike-01 parent magma through the sub-continental lithospheric mantle (SCLM). In the case of the Tongo dike-01, its primary melt is K2O-rich and must have assimilated more K2O-rich material in the SCLM. Such material is typically present as metasomatic products, e.g., Phlogopite-Ilmenite-Clinopyroxene (PIC) xenoliths observed in South Africa kimberlites. These xenoliths tend to possess abundant phlogopite. Thus, the main difference between the Tongo kimberlite and archetypal SA kimberlites is the fact that Tongo kimberlite assimilated more K2O-rich metasomatised material in the SCLM during its evolution.
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    A comparison of the Nickel and the conventional geothermometers with respect to the Jagersfontein and the Matsoku kimberlite peridotite xenolits
    (1998) Mofokeng, Sipho Wiseman; Le Roux, A P; Tredoux, M
    The accuracy of the experimental (Canil, 1994; T-Canil) and the empirical (Ryan et al., 1996; T-Ryan) calibrations of the Ni geothermometer has been evaluated on two suites of geochemically and geothermobarometrically well characterised mantle xenoliths from Matsoku and Jagersfontein by comparison to the more commonly used conventional geothermometers. The two published calibrations of the Ni geothermometer are in agreement to within ±500C in the temperature range of ~900°C to 1200°C. Outside this temperature range, the two calibrations differ by between 75 and 150°C. The importance of the Ni geothermometer in diamond exploration and the studies of the mantle makes the resolution of this discrepancy very important. In addition to issues of calibration, errors in the determination of trace levels of Ni abundances in garnets may affect the accuracy of the Ni geothermometer. A 'reliable' Ni in garnet dataset was, therefore, required to minimise errors associated with Ni compositions used in temperature determination by Ni geothermometry. Thus, Ni compositions of garnets determined by PIXE, LA-ICP-MS and SIMS were compared to select the most 'reliable' dataset. Four matrix-matched secondary garnet standards were developed for the cross-checking and testing of the accuracy of data. The standards were developed by multi-method analyses, which included PIXE (using both the GeoPIXE and GUPIX software programmes for data reduction), LA-ICP-MS and solution ICP-MS. PIXE and LA-ICP-MS data were found to agree to within their 2σ errors of, respectively, 2 to 10% and 4 to 12% for a concentration range of ~15 to 112 ppm Ni. However, PIXE analyses were found to be superior in terms of smaller beam width, allowing several repeat analyses, and analysis of small and altered garnets thereby producing a relatively larger dataset.
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    Contributions to the geology of the Table Mountain Group
    (1988) Thamm, A G; Fuller, A O
    Modern 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.
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    Contributions to the Study of Lithospheric Deformation and Seismicity in Stable Continental Regions
    (2022) New, Thomas; Sloan, Alastair
    Recently, the field of geophysics has seen increasing recognition of the unique character of deformation and seismicity in stable continental regions (SCRs). However several important questions remain understudied. What controls the locations of earthquakes in SCRs? How well do observations, in SCRs, of elastic strain accumulation and release correlate with each other? How well do they correlate with stresses and geological proxies for rheological variation? The ultimate goal of this study was to better understand stable continental regions like southern Africa, where large earthquakes occur despite not being near plate boundaries, for example the 2017 Mw 6.5 earthquake in Moiyabana, Botswana. One way of studying the stress and strain in stable continental regions is by understanding the surface deformation of the region. This deformation is easily studied using global navigation satellite system (GNSS) velocity data. One of the biggest difficulties when it comes to GNSS data is that it isn't collected on a regular grid, but rather as irregular data points that need to be interpolated. This research investigated multiple interpolation methods and recommended two methods that best replicate the original velocity field (using a well populated dataset from Southeast Asia). These interpolated GNSS data can then be used to determine deviatoric strain in a region, which can in turn be fed into numerical stress models. However, limited GNSS data exist across southern Africa, and therefore topographic data was used to calculate the gravitational potential energy, and in turn the body stress and deviatoric stress for the region. This study also investigated how this deviatoric stress (or deviatoric strain) can be more accurately calculated on a spherical rather than a flat surface, which is particularly important over large study areas. Across southern Africa, data show that deviatoric stress lined up with stress data within mobile belts. This suggests that in these weaker mobile belt crust (such as the Namaqua-Natal and Damara-Chobe belts), gravitational collapse is the dominant driver of deformation, which is in line with conclusions that have been made in previous literature. In other regions, deviatoric stress vectors and stress data do not coincide and therefore there are other forces at play. These observations are obviously restricted by limited data coverage; it remains an open question if areas that have increased deviatoric stress due to gravitational collapse, which are also aligned with the orientation of weak zones, will have elevated strain in the long term.
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    Controls on the diversity of the fault slip styles at the brittle-ductile transition: examples from the Cape Fold Belt, Nuy Valley, South Africa
    (2019) de Carvalho, Antónia; Diener, Johann; Fagereng, Åke
    Crustal deformation models have a first-order rheological division, with pressure-dependent brittle deformation predominating at shallow depths, and temperature-dependent viscous deformation occurring in the deeper levels of the crust. The brittle-ductile transition zone separates these two regimes, it occurs at approximately 350°C for quartz and it is characterised by mixed-mode brittle and viscous deformation. Complex fault zones exhumed to the surface may preserve evidence that can explain the mechanics and the complex slip behaviour of faults. Fault rocks response to applied shear stress is affected by environmental conditions during deformation (such as temperature and pressure), composition of fault zone, fluid presence and strain rate. Thus, the interplay of these factors determines the slip style of a specific fault and may lead to multiple slip styles that overprint each other. The Nuy Valley area in Worcester, Western Cape, South Africa, exposes a section through the deeper parts of the Cape Fold Belt, where the Malmesbury Group schists experienced thrust faulting in response to crustal shortening. Individual thrust faults are manifested in different ways, with quartz-cemented breccias, limestone mylonites, abundant quartz veining and cataclasites attesting to faulting occurring by a diversity of slip style, which allows investigating how the interplay of the controlling factors lead to the observed diversity of fault rock. Through mineral equilibria modelling, the pressure-temperature conditions under which faulting occurred was determined to lie between 5 - 8 KPa and 250 - 420C, with fluid content lines indicating low amounts of dehydration during peak metamorphism. The exhumed fault being analysed in this study was active at 10 - 15 km deep at 25C.km-1 geothermal gradient. The temperature over this transition is relatively constant and short ranged throughout geological evolution of Worcester and the cyclic superposition of ductile and brittle deformation and change in slip styles along fault zones as found in Nuy Valley cannot be justified by ambient temperature and pressure oscillations. Lithotype and competency of wallrocks play an essential role in deformation partitioning by being crucial determinants of rheological properties, and accounts for the coexistence of brittle and ductile fabrics but not for cyclic overprint of slip styles. Fluid presence is evidenced by an intense network of quartz veins and hydraulic breccias and contributes to the weakening and strengthening of wallrock during deformation. Slip style diversity in the study area is considered to the result of the interplay of compositional variabilities, fluid flow and strain rate variations associated with the seismic cycle.
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    Cratonic eclogite xenoliths - formation and evolution of the subcontinental lithospheric mantle
    (2018) Radu, Ioana-Bogdana; Harris, Chris; Cottin, J-Y; Moine, B N
    The most extensive evidence for the evolution of the primitive crust is preserved at the base of the continental roots (140 to 200 km depth). However, this is completely inaccessible and hard to evaluate, except through the study of mantle xenoliths brought to the surface by kimberlite magmas. Most mantle xenoliths undergo kimberlitic/carbonaceous metasomatism prior to their entrapment, altering their primary composition. Despite a very complex history and a very low abundance (180 nodules), sampled in four main localities from the Siberian (Obnazhennaya, Udachnaya) and South African (Jagersfontein, Roberts Victor) cratons. Major and trace element compositions have been analysed in representative bimineralic (garnet (gt)-omphacite (cpx)), coesite-, kyaniteand corundum-bearing eclogites. Metasomatized eclogites (Type I) have a higher alkali- (Na2O in garnet and K2O in omphacite) and LREE, LILE-content. Non-metasomatized (Type II) eclogites include two main compositional groups, derived from different protoliths. The first group (Type IIA) is typically more magnesian, enriched in LREE and has δ18O values from 3.73 to 7.50 ‰, with positive Sr anomalies and corresponding to a low pressure-temperature layer. The whole-rock composition is consistent with a basaltic protolith. The second group (Type IIB) is more sodic and aluminous, depleted in LREE and has δ18O values of 2.35 to 3.59 ‰, corresponding to equilibrium at high pressure and temperature. The whole-rock trace element composition is consistent with a pyroxenitic protolith. Eclogites that contain coesite, kyanite and corundum (coe-ky-cor) are typically characterized by jadeite-rich clinopyroxenes with positive Eu and Sr anomalies and grossular-rich garnets with corresponding positive Eu and negative Sr anomalies. Additionally, corundum-bearing samples are overall LREE-depleted. Pressure-temperature estimates indicate coe-ky-cor-bearing eclogites equilibrated in the lowermost part of the cratonic keel, and the reconstructed whole-rock trace element composition corresponds to a very depleted gabbroic protolith. Corundum-bearing eclogites often show topotactic exsolution textures hosted in omphacite, consisting of a more calcic garnet ± kyanite/zoisite. Based on Si, Al and Mg diffusion profiles it can be inferred that exsolution was controlled by chemical exchange in an isochemical system. Similarly, an interpretation can be made for the variable HREE fractionation of intermediate composition between the exsolutions and the host omphacite, as evidence for intermineral partitioning. The formation of zoisite (1.7 % H2O) from a precursor clinopyroxene requires a significant amount of hydrogen (as H+ , OH- or H2O) incorporated in omphacite at mantle conditions. Calculated water content of omphacite, based on Fourier transform infrared spectrometry (FTIR) analyses, varies from ~930—1410 to ~1100—1680 ppm by weight H2O, according to different absorption coefficients. Primary garnets are typically anhydrous (<7 ppm H2O), whereas coarse exsolutions contain between 165—1950 ppm H2O. Reconstructed estimates for whole-rock water content (~310—890 ppm H2O) for the Obnazhennaya eclogites are significantly higher than those of the surrounding peridotites. The variability of δ18O in garnet among 41 xenoliths, shows a bi-modal distribution with median values at 3.57 ‰ and 5.68 ‰ and strong correlation (r = 0.96) between garnet and omphacite. The δ18O values and the reconstructed whole-rock trace element compositions indicate an oceanic crustal protolith for mantle eclogite xenoliths. This is consistent with the subduction of a hydrothermally altered, basaltic to websteritic sequence of an incompatible-element-depleted oceanic crust. Furthermore, although eclogites represent a small fraction of the upper mantle, they may be an important water reservoir at the base of the cratonic root. High water content in the lithospheric mantle would have major consequences for the longevity of the cratonic keel, forphysical and chemical properties in minerals, partial melting, mantle rheology and electrical conductivity and the global water cycle.