Browsing by Department "Department of Geological Sciences"

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    A 3D fault seal analysis study conducted in the Ibhubesi Gas Field Offshore the West Coast of South Africa
    (2018) Ntombela, Nondumiso; Kahle, Beth
    A three dimensional fault seal analysis study of the AK fault situated offshore of the West Coast of South Africa is presented. This study is aimed at informing the development plan of the Ibhubesi gas field with regards to the compartmentalization of the reservoirs, by understanding whether a key fault, the AK fault facilitates hydrocarbon migration to the Ibhubesi field reservoirs or whether it provides a seal. In order to address this research aim, a seismic interpretation of the area was carried out and combined with an interpretation of well data to construct a 3D structural model from which the fault seal analysis was carried out. Juxtaposition analysis was used to determine the lithologies that had been juxtaposed across the fault. Fault clay was determined using the Shale Gouge Ratio (SGR) algorithm and permeability and threshold pressure were also used to quantify the fault seal capacity. The results from the SGR were correlated to the across fault pressure difference and plotted on a reference diagram which is a comparison of sealing faults vs. leaking faults. This diagram indicates that faults with an SGR greater than 25% and threshold pressures greater than 8bars, have a high potential to seal. The AK fault SGR results range between 25 and 55 % with a threshold pressure of up to 20bars. These results indicate that the AK fault is likely to be a sealing fault.
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    4D seismic analysis: discriminating between saturation and pressure changes form a data acquired Offshore Equatorial Guinea
    (2017) Msolo, Andile Lucky; Kahle, Beth; Gidlow, Maurice; Kahle, Richard
    4D seismic analysis is used to understand the performance of a producing oil field in order to increase production and mitigate risk. This is done by interpreting changes in water saturation and changes in effective pressure in the reservoir, which allows production to be maximized safely. In this project, I determined saturation and pressure changes in an oil field offshore Equatorial Guinea. I used Landrø's technique as a basis for the analysis, however Landrø's technique requires the presence of reservoir samples. For this project, rock samples of the reservoir were not available; this led me to modify Landrø's methodology using approximations based on well-accepted rock physics relationships. I tested this new methodology on a synthetic model, which gave encouraging results, after which it was applied to real seismic data. Pressure and saturation changes estimated in the reservoir indicate that the northern part of the reservoir experienced a decrease in pore pressure. The saturation changes over the northern part of the reservoir are not prominent. The central and southern parts of the reservoir show an increase in water saturation accompanied by an increase in pore pressure. The results are consistent with the production and injection history of this area. A comparison between saturation before production, saturation changes after production and pressure changes after production indicates an area (on the northern part of the reservoir) which shows fluid anomalies before production and does not show any production (or 4D) effects. New production wells could potentially be drilled in this area to increase production.
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    A contribution to the study of older granites in the Nigerian Precambrian with special reference to the Tegina granite
    (1960) Truswell, J F; Truswell, John Francis
    Rocks of the Older Granite suite occur throughout the Nigerian Precambrian. In part 1 of this thesis an example, the Tegina granite, is examined in detail. In part II the suite as a whole is described and discussed. The granite is an elongate mass covering an area of 49 square miles. Structural analysis indicates its vertical emplacement under internal pressure. The pressure was maintained during the formation of joints and secondary structures. Three petrological units are recognised in the granite. (1) The Main granite, a porphyritic biotite granite. Granodiorite and quartz-diorite in it represent local or marginal contamination. (2) Similar rocks are developed in the zone of banded foliation. In addition, there are remnant bands of granodiorite providing evidence that the 'granite' was formed as granodiorite subsequently 'granitised' with the formation of late phenocrysts of microcline microperthite. (3) The later biotite-muscovite microgranite at Karaya.
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    A geochemical study of metasomatised peridotite and marid nodules from the Kimberley pipes, South Africa
    (1987) Waters, Frances Gillian; Erlank, Tony
    A comprehensive study has been made of a large collection of metasomatised peridotite, and MARIO (Mica-Amphibole-ButileIlmenite-Qiopside) xenoliths from the Kimberley pipes, with the aim of obtaining an improved understanding of enrichment processes operating in the sub-continental lithospheric mantle. The metasomatised peridotite suite is divided into garnet peridotites (GP), which contain no texturally equilibrated or primary phlogopite, garnet phlogopite peridotites (GPP), which constitute the most abundant peridotites, phlogopite peridotites (PP) and phlogopite K-richterite peridotites (PKP). Diopside can be present in all four groups. PKP rocks may also contain exotic incompatible element-rich Crtitanates such as lindsleyite and mathiasite, Nb-Cr rutile, ilmenite and armalcolite. Petrographic and chemical evidence presented here suggest that metasomatism increases progressively in the sequence GPGPP-PP-PKP, with the PKP group being richest in Fe, Ti and incompatible elements such as K, Na, Rb, Ba, Sr, Zr, Nb and the LR.EE. Mineral compositions change progressively from the garnet-bearing rocks to the PP and PKP rocks, showing decreases in Al 2o 3 and cr2o 3 contents, and increases in FeO and Tio2 contents. Data from this study show that most PKP and some PP rocks were derived from Al-deficient harzburgites. Other PP rocks probably had garnet-bearing precursors, as they display chemical similarities to GPP rocks, and typically contain aggregates of phlogopitediopside-spinel which are interpreted and modelled as being garnet replacement textures. Pressures and temperatures of equilibration of the peridotites in this study, combined with published experimental mineral stability data, suggest that metasomatism increases in intensity upwards in the subcontinental lithospheric mantle (SCLM) over a considerable depth interval from _170 km to Sr isotope data suggest that the metasomatism is young (1 Ga) have been recorded by Nd isotopes. New and published peridotite mineral and whole-rock Nd and Sr isotopic compositions range from moderately depleted to highly enriched and are interpreted in terms of mixing between variably enriched "ancient" SCLM and young metasomatic fluids with isotopic compositions close to Bulk Earth values. A rnineralogic expression of the ancient enrichments which might suggest that they resulted from older metasornatic events, cannot be clearly identified, but is best represented by enriched Nd-isotopic compositions of nonmetasornatic garnet. Rare phlogopites with low 1 43 Nd/144Nd ratios are interpreted as the product of complex mixing between the young metasomatic fluids and old enriched SCLM, rather than as older phlogopite. A group of unusually diopside-rich peridotites (±Phlogopite, ±ilmenite), are described and discussed. They are probably not directly related to the main group of metasomatised peridotites. Their chemical differences and greater pressures and temperatures of equilibration are consistent with formation by interaction between a diapir of asthenospheric melt and surrounding peridotitic mantle at the base of the SCLM. Comparison with chemical and isotopic data from the literature for Cr-rich "granny-smith" diopside rnegacrysts and glimmerites (phlogopite-diopside-ilmenite aggregates), suggests that they may have close genetic links with the diopside-rich suite. MARIO nodules are modally dominated by phlogopite, Krichterite and diopside, with lesser ilmenite and rutile, and accessory minerals such as calcite, barite and apatite. Olivine is absent, as is (with one exception) orthopyroxene, which serves to distinguish MARIO rocks from highly metasornatised peridotites. Relative modal proportions, textures and grain-size of MARIO rocks vary considerably, both within and between nodules. Textures suggest that they are igneous cumulates. The maximum stability depth of Krichterite restricts their depths of crystallisation to <120km. Data obtained in this study show that MARIO mineral compositions are Fe and Ti rich compared to most peridotites, and are much poorer in Cr, Al, Mg and Ni. Bulk compositions are alkali- and magnesian-rich (K20=4-9 weight%, Mg0=20-25 weight%), with moderate to high concentrations of i ncompatible trace elements. New and published MARIO mineral Nd and Sr isotopic measurements ranges from slightly depleted to highly enriched compositions. They are interpreted in terms of mixing of recent (phlogopite metasomatism is presented. The model proposes that there was recent input of metasomatic fluids at two distinct levels in the sub-continental mantle lithosphere. (1) Deep level fluids were generated at the base of the subcontinental lithosphere between 170-200 km depth - l l I ) possibly evolved from failed asthenospheric melts. The intensity of metasomatism progressively increased as these fluids percolated upwards to _100 km depth, and the GPP and some of the PP rocks were formed. (2) Shallow level metasomatic fluids were evolved during the crystallisation of MA.RID magmas ponded at 60-100 km depth. They were released into the surrounding SCLM, and formed the PKP and some PP rocks. These MA.RID-derived fluids appear to have overprinted the deep-derived metasomatism in places; some PP rocks have metasomatic signatures with characteristics of both events, which is interpreted as evidence for the passage of both types of fluid through them. Various lines of evidence including variations in mineral and whole-rock chemistry, suggest that both types of fluids contained Si, Al, Fetotal, Fe 3+, Ti, ±Ca, incompatible elements such as K, Na, Rb, Ba, Sr, Zr, Nb, V, LREE, s, and possibly F and Cl. The MARIO-derived fluids may have been generally richer in these elements, as the PKP rocks are more intensely metasomatised than even the most metasomatised GPP. u and related PP rocks. The behaviqr of Ba appears to have been different in the two sources/fluids - the deeper-derived fluids produced Ba-rich phlogopites, whereas the MARID magmas and MARID-derived fluids resulted in the formation of Ba-poor phlogopites in MA.RID and PKP rocks. Modelling of fluid compositions using published crystal/melt and crystal/fluid partition coefficients and PKP mineral trace element abundances is inconclusive but suggests that the MARIDderived fluids (and possibly the deep-derived fluids as well), were strongly LREE-enriched with REE patterns comparable to lamproites or kimberlites, and with moderately high Rb and Ba contents. However, inferred concentrations of Sr in the metasomatic fluids are of the order of 1% or more. The PKP rocks have attracted more attention in the literature than the GPP and PP groups, due to their spectacular J metasomatic assemblages. However, it is worth noting that they are relatively rare (~10% of peridotites from the Kimberley pipes). The ¥.LA.RID-derived metasomatism was probably intense, but strongly localised. In contrast the GPP and PP rocks are extremely abundant at Kimberley (_50% and _30% respectively) and were equilibrated over a large depth interval (170-100 km). The deep-derived metasomatism therefore appears to have been much more pervasive, and although less intense (most GPP rocks have 1-2% phlogopite) , it is considered to be a more significant phenomenon in the context of mantle metasomatic processes, especially as it appears to predate host kimberlite formation and emplacement.
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    A metasomatic origin for late Archean eclogitic diamonds: Implications from internal morphology of diamonds and Re-Os and S isotope characteristics of their sulfide inclusions from the late Jurassic Klipspringer kimberlites
    (2004) Westerlund, K J; Gurney, J J; Carlson, R W; Shirey, S; Hauri, E H; Richardson, S H
    The majority of the Klipspringer eclogitic diamonds display complex crystal zonation with zones of N- and H-rich cubic diamond alternating with zones of N-poor octahedral diamond. These growth features were likely related to different levels of carbon supersaturation in the diamond forming fluid(s). The diamonds are younger than their host rock and formed through differential flux of fluids into the eclogite. Re-Os isotope systematics of sulfide inclusions yield an isochron age of 2554 ± 150 Ma. The age overlaps the break-up of the Vaalbara supercontinent, the formation of the Ventersdorp lavas and a major re-activation of the Thabazimbi-Murchinson lineament into which the Klipspringer kimberlites intruded 148 ± 4 Ma ago. While a separate, relatively older generation of Klipspringer diamonds (Westerlund and Gurney, 2004) may be related to a proposed ~2.9 Ga craton-wide eclogitic diamond formation event (Shirey et al., 2001; 2002), a temporal connection between the current diamonds and the Ventersdorp lavas is inferred. Sulfur isotope compositions (δ34S = −1.8 to +2.4 ‰) of the sulfide inclusions overlap mantle values and large S isotope excursions indicative of altered MORB and associated sediment protoliths are absent. The data are compatible with slightly altered oceanic basalt or sub-lithospheric mantle sources for the S in these inclusions. The radiogenic initial Os isotope composition of the sulfide inclusions (187Os/188Os of 0.187 ± 0.046; γOs = +69) is not compatible with an origin of the PGE in the sulfides from the Ventersdorp lavas. The diamond growth characteristics along with the S and Os isotopic composition of their sulfide inclusions could be accounted for by the remobilization of local fluids in or proximal to the pre-existing eclogite host rock. We propose that the event that generated the Ventersdorp lavas may have raised the ambient lithospheric temperature and triggered fluid remobilization and the formation of the diamonds.
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    A multi-isotopic geochemical investigation of the Lower Zone, Bushveld Complex, South Africa: implications for a crustal component for parental magmas
    (2020) Edwards, Hunter R; Howarth, Geoffrey H
    The current study focuses on the Lower Zone of the Bushveld Complex, South Africa using multiple geochemical and isotopic systems to determine the origin of crustal signatures, i.e. crustal assimilation or recycled crust in the mantle source, present throughout the Rustenburg Layered Suite (RLS) such as elevated  18O values. These geochemical and isotopic systems include major elements, trace elements, highly siderophile elements, oxygen isotopes, 87Sr/86Sr, 143Nd/144Nd, and Os-Os isotopes. Samples come from the Nooitgedacht Borehole 2 (NG2) at Union Section of the western limb of the Bushveld Complex, which sampled the Lower Zone. The 87Sr/86Sri (0.7043 – 0.7086) and Ndi values (-7.40 - -4.97) calculated in this study are in agreement with published data for the Bushveld. The majority of NG2 samples contain  18O greater than mantle peridotite (5.50‰) and MORB (5.70‰), in which NG2  18O ranges from 5.60 up to 8.00‰ for olivine, orthopyroxene, and clinopyroxene separates. These high  18O values suggests the Bushveld magmas sourced a crustal reservoir, either through crust assimilation or recycled crustal materials in the mantle source. This is the first study utilizing the Re-Os isotope system for the Lower Zone. The Osi values for the NG2 suite range from -4.37 to +35.9, which overlap with published data for the Critical Zone and the Platreef, the only previous Re-Os studies on the Bushveld. However, there are no previously reported negative Osi values for the Bushveld. The range in Osi values for the NG2 samples suggest mixing of at least two geochemical reservoirs. In addition to Lower Zone NG2 samples, sample NG2-773.65 is a chilled margin sample at the base of the NG2 borehole that contains high  18O ( 18O = 9.42 – 9.78‰) and radiogenic Osi (Osi = +62.5), in which the crust and/or recycled crust in the mantle source caused these high values. Sample NG2-734.64 contains the second lowest  18O ( 18O = 5.67‰) and most unradiogenic Osi (Osi = -4.37) values for the NG2 suite, evidence for a harzburgitic SCLM (H-SCLM) mantle source component. A lack of correlations for Osi values with  18O and 87Sr/86Sri values are indices for crustal assimilation processes. This lack in correlation may suggest a crustal component in the mantle is more likely than assimilation of the crust during ascent of the magmas toward the surface. The geochemical data presented in this study suggest the Lower Zone parent magma had a H-SCLM mantle source component in addition to the assimilation of the crust and/or the eclogitic SCLM (E-SCLM).
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    A subduction origin for komatites and cratonic lithospheric mantle
    (2004) Parman, Stephen W; Grove, Timothy L; Dann, Jessc C; De Wit, Maarten J
    We present a model in which the generation of komatiites in Archaean subduction zones produced depleted mantle residues that eventually formed the highly depleted portions of the Kaapvaal lithospheric mantle. The envisioned melting process is similar to that which has formed boninites in Phanerozoic subduction zones such as the Izu-Bonin-Mariana arc. The primary differences between the Archaean and Phanerozoic melting regimes are higher mean melting temperatures (1450 versus 1350 °C) and higher mean melting pressures (2.5 versus 1.5 GPa) for the komatiites. The komatiites from the Komati Formation in the Barberton greenstone belt are mafic enough to have produced the depletion seen in most Kaapvaal granular peridotite xenoliths. However, the most highly depleted Kaapvaal xenoliths require an even more Mg-rich magma than the Komati komatiites (Kk). Samples of boninite mantle residues from the fore-arc of the Marianas subduction zone are nearly as depleted as the Kaapvaal cratonic mantle, indicating that buoyant, craton-like mantle is being produced today. We speculate that production rates of cratonic mantle were greater in the Archaean due to the greater depth of melting for komatiites (relative to boninites) and greater worldwide arc length. The high production rates and high buoyancy of the komatiite mantle residues gave rise to the rapid growth and stabilization of the Kaapvaal craton in the Archaean.
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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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    African river basins : their present geometry and recent past as a framework for their evolution
    (2004) Stankiewicz, Jacek; De Wit, Maarten
    Fractals and scaling laws abound in nature, and it is said that geometry of river networks and basins is an epitome of this. This study investigates how on the tectonically unique African continent, scaling parameters, and in particular deviations from 'perfect fractal patterns' relate to parameters like underlying geology, climate, and vegetation through which the river flows. Stream and basin patterns are also used to reconstruct the past network geometry of rivers, and to shed some light on the drainage evolution of major African rivers.
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    Age, paragenesis and composition of diamonds and evolution of the Precambrian mantle lithosphere of southern Africa
    (2004) Shirey, Steven B; Richardson, Stephen H; Harris, Jeffrey W
    Two decades of diamond research in southern Africa allow the age and average composition (C isotope and N abundance) of diamonds and the dominant paragenesis (peridotitic versus eclogitic) of their syngenetic silicate and sulfide inclusions to be reviewed on a cratonwide scale. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa and Jwaneng have Re-Os isotopic ages that range from ~2.9 Ga to the Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (±1%) that the mantle lithosphere shows at depths within the diamond stability field (150 to 225km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm-Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds. The converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds support a model whereby mantle that became part of the keel of the oldest continental nuclei was created by middle Archean (~3.3 to ~3.2 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of eclogitic sulfide inclusions in the ~2.9 Ga age population links late Archean subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely-distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.
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    Alkaline igneous rocks of the coastal belt, south of Luderitz, South West Africa : a petrological study
    (1973) Marsh, Julian Saville
    The Luderitz Alkaline Province, as it is at present known, comprises the subvolcanic central complexes of Drachenberg, Pomona, and Granitberg. An attendant dyke swarm strikes NE-SW and crops out between the latitudes of 27°00' and 27°30' S. Stratigraphic indications (now confirmed by a K/Ar age from Granitberg) are that the Luderitz Province is early-Cretaceous in age and therefore older than the Klinghardt phonolites (Eocene) as well as the smaller melilitite and nephelinite intrusions. Granitberg is a circular foyaite complex, in the centre of which is preserved a large fragment of sedimentary rocks that originally formed the roof of the intrusion. The foyaites have been emplaced into the feldspathic sandstones and dolomites of the Bogenfels Formation, and three major intrusive phases can be recognised. The first phase produced chilled nepheline syenites beneath the roof of the intrusion. These chilled rocks grade downwards into coarse-grained foyaites. The second phase was the emplacement of the Inner Foyaite which crystallized as a cylindrically zoned plug, capped by a zone of layered, laminated, and xenolithrich foyaites. The third phase was the emplacement of the Outer Foyaite, into which the Roof Zone and the Inner Foyaite foundered. The Outer Foyaite is zoned with a miaskitic core, and an agpaitic outer zone.
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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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    Anatomy of a pressure-induced, ferromagnetic-to-paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds
    (2011) Gilder, Stuart A; Egli, Ramon; Hochleitner, Rupert; Roud, Sophie C; Volk, Michael W R; Le Goff, Maxime; de Wit, Maarten
    Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50% over that of initial conditions by 2 GPa, and then decreases until only 33% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ∼1.5 GPa, multidomain pyrrhotite obeys Néel theory with a positive correlation between coercivity and remanence; above ∼1.5 GPa, it behaves single domain-like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth’s surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used.
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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.