Doctoral Degrees (Geology)

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The seismic stratigraphy, geological evolution and CO2 storage potential of the offshore Durban Basin, South Africa.
(2017) Hicks, Nigel.; Green, Andrew Noel.
Abstract available in PDF file.
Biodiversity conservation and rural livelihoods : a comparative study of selected conservation approaches in Zimbabwe.
(2014) Chigonda, Tanyaradzwa.; Bob, Urmilla.
Historically, protected areas have operated as islands of biodiversity conservation in isolation from nearby communities. There is, however, a growing consensus that for protected areas to be more effective in conserving biodiversity, particularly in developing countries, they must incorporate the livelihood needs of poor local communities they often share boundaries with. This is because most of these communities historically pre-date the protected areas, have pre-existing rights to resources in them and have often been adversely affected by their designation. Successful protected area management thus depends on the collaboration, involvement and support of local communities. In this context, this study examines biodiversity conservation in Zimbabwe using two case studies, a private protected area (Malilangwe) and a community-conserved area (Mahenye) in terms of their livelihood impacts on local communities. The need to incorporate livelihoods goals into conservation areas in Zimbabwe has further been necessitated by the persistent failure of conventional post-independence rural development initiatives in the country. The study employed the mixed-methods approach in data collection and analysis involving both quantitative (questionnaire) and qualitative (interviews, group discussions and observation) techniques. Simple random sampling was used in selecting 150 households for questionnaire interviews from each of the two targeted communities adjacent to the conservation areas, while purposive and snowball sampling were employed in selecting key-informant interviewees. The Statistical Package for the Social Sciences (SPSS) was used in analysing quantitative data, while thematic analysis was used to analyse qualitative data. The study identifies various livelihood benefits and costs from the conservation areas to the local communities. There were some similarities and differences in the livelihood impacts of the protected areas. The main livelihood benefits from the conservation areas to the communities included the enhancement of income, health and education; in addition to improved environmental sustainability. Various hindrances to the flow of the livelihood benefits were also identified. Among the livelihood costs from the conservation areas to the local communities included, inter alia, loss of land and livelihoods, destruction of crops by wildlife, devouring of livestock by wildlife and human harassment by wildlife. Such costs were further exacerbated by lack of compensation from the conservation areas. The study recommends various measures for enhancing livelihood benefits from the conservation areas to the local communities which include, inter alia, compensation to communities for livelihood costs incurred from conservation, increased community involvement in conservation decision-making and a widening of the portfolio of livelihood-enhancing initiatives by the conservation areas. The main contribution of this study to the conservation-development discourse in Zimbabwe is that it has shown that, besides the much publicised communal areas management programme for indigenous resources (CAMPFIRE), other conservation approaches such as private protected areas can achieve similar, if not better, livelihood impacts on surrounding communities. The need for policy makers to promote other conservation approaches, besides CAMPFIRE, as alternative and equally effective vehicles for attaining rural development through conservation is thus apparent.
Rock fabric study of Karoo dolerite sills along the KwaZulu-Natal North Coast, South Africa : implications for the magma source.
(2015) Hoyer, Lauren.; Watkeys, Michael Keith.
Abstract available from the pdf file.
Tectonic history, microtopography and bottom water circulation of the Natal Valley and Mozambique Ridge, southwest Indian Ocean.
(2014) Wiles, Errol Avern.; Watkeys, Michael Keith.; Green, Andrew Noel.; Jokat, Wilfried.
This thesis focuses on aspects of the tectonic history, sediment delivery and subsequent sediment redistribution within the Natal Valley and Mozambique Basin of the southwest Indian Ocean. It aims to 1) better constrain the tectonic history of these basins based on anomalous seafloor features, 2) understand the timing, evolution and formative processes of sediment delivery systems within the Natal Valley and Mozambique Basin, 3) account for the redistribution of seafloor sediments within the southwest Indian Ocean. The southwest Indian Ocean opened during the Gondwana breakup event giving rise to two north/south orientated rectangular basins separated by the Mozambique ridge. Early research (1980’s) within these basins discussed basin features in terms of the available data at the time. By modern standards these data sets are relatively low resolution, and did not allow early researchers to fully account for the existence, development or evolution of many morphological features within the southwest Indian Ocean. This study uses recently acquired multibeam bathymetry and PARASOUND/3.5 kHz seismic data sets to describe and account for the geomorphology of the southwest Indian Ocean. Antecedent geology is discussed with respect to its development, in association with regional regimes, and role in provision of accommodation space and sediment redistribution within the study area. Sediment delivery pathways from the continental shelf to the deep marine basins are discussed, outlining the evolution of these systems under the control of antecedent geology and regional uplift. The redistribution of sediment is then discussed from the microtopography observed within the southwest Indian Ocean. Results show anomalous seafloor mounds in the northern Natal Valley, and extensional structures within the Mozambique Basin, are likely linked to the southward propagation of the East African Rift System. Dynamic current regimes and antecedent geology have played a significant role in the availability of sediment and subsequent delivery of sediment to the Natal Valley and Mozambique Basin via submarine canyons and channels. Once delivered to the basins, sediments are redistributed by deep and bottom water thermohaline Circulation. In the Natal Valley this is manifest as an atypical, current swept and winnowed, submarine fan (associated with the Tugela Canyon). While in the Mozambique Basin significant sediment wave fields reflect the influence of Thermohaline Circulation within this basin, and interaction with the seafloor. This relationship between Thermohaline Circulation and seafloor sediments has allowed existing deep and bottom water pathways to be better constrained and, in some instances, modified to better represent the actual circulation within specific regions of the study area.
The late cenozoic carnivora of the south-western Cape Province.
(1972) Hendey, Quinton Brett.; King, Lester Charles.
Abstract available in PDF file.
Flooding in KwaZulu-Natal : modelling, history and future aspects.
(2014) Botes, Zacheus Adriaan.; Green, Andrew Noel.; Smith, Alan.
The current state of flood modelling relies on statistical techniques revolving around either river or rainfall data that are used to produce an estimated flood return period (e.g. 1:100 year). These tend to ignore 1) the geological record as an archive of flood events; 2) the spatial distribution of flood producing weather systems; and 3) climatic cycles which may ultimately control episodes of flooding. This thesis developed a Flood Zone Model (FZM) model from existing Geographic Information Systems (GIS) datasets and GIS software models at a quaternary catchment (4th order basin) level. Model discharge estimates were derived from modified Regional Maximum Flood (QRMF) equations where it was found that flood elevations produced from QRMF estimated discharges could be directly related to the geological record of flood. This was achieved using Manning-derived calibration factors (CFs) based on reach slope. Comparison of the modelled flood elevation surfaces against the field data and available 1:100 year return period elevations showed R2 coefficients of 0.999 for all calibration factors. In one of the quaternary catchments investigated, geological evidence and discussion with local communities identified flood elevations attributed to flash flooding. On this basis, the Flood Zone Model was adapted to estimate peak discharges using the Rational Formula where it was found that the calibration factors were valid for flash flood modelling and that the flood elevations that resulted from flash flooding far exceeded the 1:100 year return period. To evaluate the spatial distribution of flood producing storms, daily rainfall data from KwaZulu-Natal (1890 - 2000) were gridded to produce regional storm event footprints. Storm events typically last between three to four days with the highest associated risk period from January to May. Flooding appears to be mostly influenced by migrating easterly waves. Compilation of all the storm event footprints defined five risk zones with a sixth zone at risk from tropical storms and cyclones. Comparisons between the annual regional storm event count and several climatic cycles show a significant correlation between the Pacific Decadal Oscillation (PDO) and regional storm event as a result of increased easterly wave activity. Assuming no change to the Pacific Decadal Oscillation cycle, increased periods of intense flooding will occur in the future.
Sedimentary models and geomorphological classification of river-mouths on a subtropical, wave-dominated coast, Natal, South Africa.
(1991) Cooper, James Andrew Graham.; Mason, Tom R.
Abstract available in PDF.
Tectonic evolution of western central Moçambique.
(2004) Bene, Bernardo Miguel.; Watkeys, Michael Keith.; Reinhardt, Juergen.
This thesis examines the geological history of western central Mocambique, with a particular emphasis on the enigmatic relationships between the Umkondo Group and the Frontier Formation. In the region the geological units can be broadly subdivided, from west to east, into the Archaean Basement Complex, the Umkondo Group, Gairezi Group and Proterozoic Barue Complex. The Basement Complex represents the eastern edge of the Archaean Zimbabwe craton that, near its margin, has been reworked by Kibaran and Pan-African events. The Umkondo Group comprises mainly sedimentary rocks that are weakly metamorphosed and mildly deformed. These rocks unconformably overlie the Basement Complex and are overthrust by the Gairezi Group in the east. The Gairezi Group is a highly metamorphosed and strongly deformed sequence subdivided into the quartziteschist Frontier Formation and the gneissschist Nhazonia Formation. This group tectonically overlies the reworked eastern margin of the Zimbabwe craton. The Barue Complex comprises all the geological units in the MQ9ambique Belt that was accreted to the Zimbabwe craton during the Kibaran event and reworked during the Pan-African event. Geothermobaromtery and structural signatures show an increase up to granulites facies and an increase in deformation from west to east across the Zimbabwe craton to the Barue Complex. No evidence could be found for a proposed sinistral strike-slip zone between the Zimbabwe craton and the Mocambique Belt. Structural and AMS data indicate the contact is an 8 Km wide duplex zone formed by the thrusting of the Mocambique Belt over the Zimbabwe craton. Evidence from single zircon SHRIMP ages indicates that the Gairezi Group sediments were derived from the Zimbabwe craton and Limpopo Belt. They were deposited either between 1800 Ma and 1430 Ma or between 1430 Ma and 1300 Ma, depending on the interpretation of the age of one discordant zircon. This event is interpreted as rifting of the Zimbabwe craton and formation of a passive continental margin. Geological and geochronological evidence show that this sequence was deformed and metamorphosed, and underwent uplift and erosion before deposition of the Umkondo Group. The sediments of the Umkondo Group were deposited in braided stream and shallow marine environments in a peripheral foreland basin. Before lithification they were intruded by dolerites sills dated at 1105 ± 5 Ma. Palaeocurrent data in previous studies indicate a source region to the west and north-west. The geochemistry presented here indicates a cratonic source and a recycled orogeny source while SHRIMP ages reveal that these sources were the Zimbawe craton and Limpopo Belt. The Umkondo Group -and the Ritschersflya Supergroup of western Dronning Maud Land, Antarctica, have been considered sequences that were originally deposited in the same sedimentary basin. However, geochemical evidence reveals that the Ritschersflya sediments were derived from an active arc that must have separated the two sequences. Consequently two separate basins must have formed during Kibaran times, one on the Zimbabwe craton and another on the Archaean Grunehogna Province in Antarctica. This needs to be taken into consideration in tectonic models for the Kibaran and in any reconstruction of Rodinia and Gondwana.
Aspects of Karroo vulcanicity in the Komatipoort area Lebombo.
(1979) Logan, C. T.
The petrology, mineralogy and geochemistry of the Karroo-age basaltic and rhyodacitic volcanics present in the Lebombo Belt near Komatipoort, Eastern Transvaal, Republic of South Africa are described. The volcanics comprise a succession of extrusives within the Komatipoort area, consisting of: (3) Rhyodacitic lavas and tuffs (2) Basalts (1) Olivine Basalts. These volcanics dip eastwards at angles between 10° and 40°, with dips increasing as the Eastern boundary of the area, the Mozambique border, is approached. Further Karroo-age volcanics, constituting the upper part of this succession, lie across the border in neighbouring Mozambique. Various intrusives, similar in composition to the extrusives, are also present in the area, commonly as north-south trending dykes forming part of a large dyke-swarm. These, as else\Olhere in the Lebombo, are considered to be feeders to the extrusives. Three relatively major intrusions occur in the area, the largest being the Komatipoort Intrusion, here interpreted as a 700m thick, sub-concordant, composite sheet-like body, ~onsisting of five major lithological units. From the base upwards these are:- . unit 1 - olivine gabbro, unit 2 - igneously laminated gabbro, unit 3- granophyric gabbro, unit 4 - granophyre, unit 5 - feldspathic gabbro. The olivine gabbro, the granophyre and the feldspathic gabbro appear to form separate intrusive bodies, but the igneously laminated gabbro and the granophyric gabbro, could represent the products of in situ differentiation. If so, the granophyric gabbro has suffered subsequent disturbance as shown by evidence including a homogeneous composition, reaction and corrosion textures and the presence of deformed pyroxene grains, which are described in detail. Another major intrusive of probable Karroo age is the Crocodile River Intrusion, which occurs to the north-east of the main mapped, and represents the southern- most end of a northsouth trending line of mafic intrusives, which parallel the Lebombo for approximately 200 km. The intrusion here has a dykelike form, and shows evidence of fractionation by crystal settling. A smaller, obviously composite intrusion occurs near the base of the basaltic sequence in the Komatipoort area, (the Basal Intrusion). Representative samples of a variety of extrusive and intrusive rock types have been analysed. These analyses include major and trace element determinations of a series of samples of the major units of the Komatipoort Intrusion. In 'addition, analyses of a number of minerals from several different rock types occurring in the Komatipoort Intrusion, are presented. The analytical data available for the Komatipoort volcanics confirms the previously known southward variation in the geochemistry of the basalts and the presence of bath a high and a low-Ng basalt series in the Komatipoort area. The majority of the basic rocks in the Komatipoort area belong to the low-Mg series. Compositional variation in this series may be explained largely by low-pressure fractionation of olivine and pyroxene only, despite the presence of abundant plagioclase phenocrysts. Some of the variation in the high-Mg series , basalts can be explained by the fractionation of olivine, and what appear to be relatively highly fractionated rocks formed in this manner occur in the area. The Lebombo volcanics ln general display a bimodal silica distribution and rocks with an intermediate silica content are rare. In the Komatipoort area intermediate rocks do occur in the, form of two classes of granophyre, (high and low silica varieties), present in the granophyre unit of the Komatipoort Intrusion. Liquid immiscibility is a possible mechanism for the formation of the two types of granophyre, and a widespread development of this process in intermediate magmatic liquids could provide an explanation for the ,scarcity of rocks of this composition in the Lebombo belt. Relatively few analyses of the rhyolitic volcanics are presented in this study, but it appears possible that those available could be representative of two processes, firstly, partial melting of the lower crust or upper mantle, and secondly, fractionation by crystallisation of the commonly observed phenocryst phases,(feldspar, pyroxene, quartz, magnetite), or addition of these phases to the magma.
The petrology and geochemistry of intrusions at selected nunataks in the Ahlmannryggen and Giaeverryggen, western Dronning Maud Land, Antarctica.
(1986) Krynauw, Johannes Reynhardt.; Hunter, Donald Raymond.; Wilson, Allan H.
The mid-Proterozoic Borgmassivet intrusions of western Dronning Maud Land intrude Archaean granites and a volcano-sedimentary sequence, the Ritscherflya Supergroup. They are SiO2-rich ultramafic to mafic tholeiitic rocks which occur as layered bodies of unknown dimensions and sills up to 400 m thick. This thesis records detailed field, petrographic and whole rock geochemical studies on Borgmassivet intrusions at three widely-spaced localities within an area of approximately 20 000 km in the Ahlmannryggen and Giaeverryggen. Annandagstoppane-Juletoppane-Forstefjell area: The Annandagstoppane intrusions form part of a layered body or bodies, of which only a small part is exposed. They consist of a medium-grained 'main suite ' of gabbronorites and minor anorthosites, and a Iyounger suite ' of quartz diorite pegmatites, basaltic dykes, fine- to medium-grained gabbroic sills and minor albitite veins. The main suite rocks are ortho-cumulates in which plagioclase, orthopyroxene and clinopyroxene were primocrysts which crystallized in situ. The dykes and sills show typical basaltic and doleritic textures. Geochemical evidence suggests that the main and younger suites may be consanguineous. Robertskollen-Krylen area: The layered complex at Robertskollen comprises a lower, rhythmically layered ultramafic unit, overlain by a mafic unit. Olivine, orthopyroxene and clinopyroxene are the dominant cumulus phases in the ultramafic rocks, whereas plagioclase, orthopyroxene and clinopyroxene fractionation occurred during crystallization of the mafic rocks. Metastable co-existence of quartz with olivine and anomalous incompatible trace element characteristics of the Robertskollen complex suggest contamination of the magma(s) by crustal material. The Krylen intrusions show petrographic and geochemical characteristics similar to those of the main suite at Juletoppane. A felsic dyke at Krylen may represent a rheomorphic product derived from Ritscherflya sediments. (iii) Grunehogna-Jekselen area: The Grunehogna sill is a medium-grained diorite of unknown thickness, characterized by varying amounts of plagioclase and amphibole and a high Fe-Ti oxide content. It is overlain by a 50 m-thick quartz diorite pegmatite. The overlying 400 m-thick Kullen sill varies in composition from gabbronorite to gabbro and diorite and shows evidence for contamination by crustal material. Vugs, evidence for fusion, destruction of sedimentary structures and deformation in sedimentary contact zones and xenoliths and the abundance of pegmatites suggest that the sills intruded wet, unconsolidated or partially lithified sediments. The Jekselen complex consists of quartz diorites intruding Ritscherflya sediments. Amygdales in the upper zones of the complex indicate the subvolcanic nature of the intrusion. Major, trace and rare earth element data of the Borgmassivet intrusions and the Straumsnutane basalts (Watters, 1969a, 1969b, 1972, pers. comm., 1985) show a strong regional coherence, indicating that the rocks may be consanguineous. Abundance ratio patterns ('spidergrams') of the intrusions and basalts are identical. They are consistent with crustal contamination, possibly leucotonalite, of the magmas during ascent. The present distribution of the Borgmassivet. intrusions and Straumsnutane basalts reflects emplacement at stratigraphically higher levels within the Ritscherflya Supergroup from west to east. Previous radiometric isotope studies suggested that the intrusions are approximately 1700 to 1800 Ma in age, but recent investigations show that the isotopic data are poorly understood and have to be re-evaluated. Some of the isotope characteristics may result from crustal contamination and alteration effects during intrusion into water-saturated sediments.
Geochemistry and structure of the archaean granitoid-supracrustal terrane, southeastern Transvaal and northern Natal.
(1987) Smith, Roric Gerard.; Wilson, Allan H.; Hunter, Donald Raymond.
Alkaline intrusives from the Tugela terrane, Natal metamorphic province.
(1991) Scogings, Andrew John.; Cooper, Mike.; Dunlevey, John N.
Three gneissose alkaline granitoid intrusives at Ngoye, Bulls Run and Wangu are described. They are located within the Nkomo Nappe of the Tugela Terrane, near the northern margin of the Natal Metamorphic Province. The Ngoye Complex comprises alkaline granites, with minor syenite and monzodiorite phases. According to modal am geochemical criteria the Ngoye granites range from peraluminous (muscovite-bearing), through metaluminous (biotite- and/or hornblende-bearing), to peralkaline (riebeckite-, aegirine- and magnetite-bearing). The granites are A-types according to their modal and geochemical characteristics. Rb-Sr isotopic data for the hornblende granites indicate an age of 1063 ± 17 Ma and the initial ratio (R๐ = 0.7025) provides evidence for derivation from a mantle source. Plotting of the Ngoye geochemistry on tectonic-discrimination diagrams suggests intrusion into rifted continental crust. It is concluded that the gneissose Ngoye granites constitute a deformed central complex, similar to anorogenic complexes in Nigeria and the Sudan. The Bulls Run Complex is situated 30 km west of the Ngoye Complex. A concentric outcrop pattern has been mapped, according to which an envelope of silica-saturated biotite-muscovite syenite surrounds a core of nepheline-bearing syenites. Minor intrusive phases include biotite-rich dykes, sovite carbonatite sheets, silica-oversaturated microsyenite dykes and feldspathic ijolite. The outer envelope of muscovite-rich syenite is interpreted as fenitised pelitic country rock. An alkali-lamprophyre origin is suggested for the biotite-rich dykes. Geochemically the syenites are predominantly miaskitic, apart from the microsyenite dykes which are mildly peralkaline. Rb-Sr isotopic data for the nepheline syenites indicate an age of 1138 ± 45 Ma (Ro = 0.70322). Carbonate separates from the carbonatites provide a similar low initial ratio (Ra = 0.70319) which supports a comagmatic mantle origin. A comparison is drawn between the Bulls Run Complex and miaskitic nepheline syenite gneisses in the mid-Proterozoic Grenville Province of canada. From this, it is suggested that the Bulls Run Complex is pretectonic and was intruded into the rifted passive margin of a continent. The Wangu Granite Gneiss is situated 3 km southwest of the Bulls Run Complex. The granites are fine grained and contain aegirine-augite and/or magnetite, and classify as alkali-feldspar granite. Peralkaline chemistry is characteristic of the Wangu granites, with trace-elenent contents indicating a distinct A-type signature. Biotite-rich mafic dykes intrude the southern part of the Wangu outcrop and, on the basis of major- and trace-element signatures, are suggested to be metamorphosed volatile-rich alkaline lamprophyres similar to those at Bulls Run. Geochemical similarities between the Wangu granites and certain comendites from the Kenya Rift are noted. It is suggested that the Wangu granites were emplaced as high-level dykes, within rifted continental crust. It is proposed that the Ngoye, Bulls Run and Wangu intrusives be united as the Nkwaleni Suite. Comparison of the Tugela Terrane with the Grenville Province reveals many similarities, particularly their mid- to late-Proterozoic age and the occurrence of pre-tectonic anorogenic continental magmatism. It is concluded that, unlike the current model which would have the Tugela Terrane as obducted ophiolite, these new data indicate that the Tugela Terrane is a metamorphosed continental rift system.
Structural evolution and tectonostratigraphy of the Kheis Orogen and its relationship to the south western margin of the Kaapvaal Craton.
(1999) Hilliard, Paul.; McCourt, Stephen.
The marine geology of the Aliwal Shoal, Scottburgh, South Africa.
(2012) Bosman, Charl.; Uken, Ronald.
This study represents the first detailed geological, geophysical and geochronological investigation of the continental shelf surrounding the Aliwal Shoal, ~5 km offshore of Scottburgh, in southern KwaZulu-Natal. Mapping of the seafloor geology using geophysics and direct observations from SCUBA diving transects were integrated with the seismic stratigraphy and constrained by new geochronological data. Four seismic stratigraphic units (A to D) were identified and interpreted with the subsequent sequence stratigraphic model consisting of four incompletely preserved stratigraphic sequences separated by three sequence boundaries (SB1 - SB3) comprising complex reworked subaerial unconformity surfaces. Sequence 1 is the deepest, subdivided by a basin-wide marine flooding surface (MFS1) into a lower Campanian (and possible Santonian) TST and an upper Maastrichtian combined regressive systems tract comprising HST/FRST deposits. SB1 follows Sequence 1 and spans most of the Tertiary representing multiple erosional events. Shelf sedimentation resumed during the Late Pliocene to early Pleistocene with deposition of Sequence 2, the shelf-edge wedge, which again was followed by erosion and non-deposition during the high frequency and amplitude Early to Middle Pleistocene sea-level fluctuations resulting in the formation of SB2. Sequence 3 consists of coast-parallel, carbonate cemented aeolianite palaeo-shoreline ridges of various ages overlying Sequence 1 and 2. Sequence 4 unconformably overlies all the earlier sequences and comprises a lower TST component displaying characteristic retrogradational stacking patterns and an upper local HST clinoform component showing progradation and downlapping. Inner and middle shelf TST units constrained between Sequence 3 ridges form thick sediment deposits showing a progression from lagoonal and lower fluvial-estuarine deposits, overlain by foreshore and shoreface sands, documenting the changing depositional environments in response to a sea-level transgression. Laterally, in the absence if Sequence 3 ridges, TST sediments comprise only a thin transgressive sand sheet. The upper HST component comprises a prograding shore-attached subaqueous-delta clinoform sediment deposit, the Mkomazi Subaqueous-Delta Clinoform (MSDC) which evolved in four stages. An initialization and progradation stage (Stage 1) (9.5 to 8.4 ka cal. B.P.) was interrupted by retrogradation (Stage 2) and backstepping of the system due to rapid sea-level rise between 8.4 to 8.2 ka cal B.P. Stage 2 backstepping of the clinoform controlled the subsequent overlying topset morphologies resulting in later stages inheriting a stepped appearance upon which shoreface-connected ridges (SCR’s) are developed. Stages 3 (8.2 to 7.5 ka cal. B.P.) and 4 (7.5 to 0 ka cal. B.P) show a change from ‘proximal’ topset aggradation to ‘distal’ foreset progradational downlap, linked to a change in the dominant sedimentary transport mechanism from aggradational alongshore to progradational cross-shore related to variations in accommodation space and the rate of sediment supply. Morphologically the MSDC is characteristic of sediment input onto a high energy storm-dominated continental shelf where oceanographic processes are responsible for its northward directed asymmetry in plan-view, for the lack of a well defined bottomset and for the re-organisation of its topset into very large SCR’s. The SCR’s are 1 - 6 m in height, spaced 500 to >1350 m apart and vary from 3 km to >8 km in length, attached on their shoreward portions to the shoreface between depths of -10 m to -15 m (average at -13 m) and traceable to depths exceeding -50 m, although the majority occur on the inner shelf between -20 m to -30 m. Several individual crests can be identified forming a giant shoreface-connected sand ridge field with a sigmoidal pattern in plan-view postulated to be a surficial expression of the subjacent retrogradational phase (MSDC Stage 2). SCR’s development occurred in two stages. Stage 1 involved deposition of sediment on the shoreface and ridge initiation during the MSDC Stage 2 retrogradational event. Sediment was reworked during sea-level rise generating clinoforms with proximal along-shore aggradation and distal across-shore progradation. This occurred during the last post-glacial sea-level rise from ca. 8.4 ka cal. B.P. SCR Stage 2 represents modern maintenance of the SCR system which is continually modified and maintained by shelf processes and consists of two physical states. State 1 considers SCR maintenance during fair-weather conditions when transverse ridge migration is dominant and driven by the north-easterly flowing counter current shelf circulation. State 2 considers SCR development during storm conditions when longitudinal ridge growth is suggested to occur as a result of storm return flows. Following the storm, the regional coast-parallel current system is restored and the fair-weather state then moulds the SCRs into a transverse bedform. Deposition on the MSDC is ongoing on a continental shelf that is still in a transgressive regime. The exposed seafloor geology comprises late Pleistocene to Holocene aeolianite and beachrock lithologies, deposited as coastal barrier and transgressive shoreface depositional systems. Extensive seafloor sampling was combined with a multi-method geochronological programme, involving the U-series, C14 and optically stimulated luminescence (OSL) to constrain the evolution of the aeolianite and beachrock complex. The Aliwal Shoal Sequence 3 ridge comprises three distinct aeolianite units (A1 to A3) which represent different types of dune morphologies deposited during the climatic and associated sea-level fluctuations of MIS 5. Units A1 and A2 deposited during the MIS 6/5e (~134 to ~127 ka cal. B.P.) transgression represent contemporaneous evolution of a coastal barrier system which consisted of two different dune forms associated with a back-barrier estuarine or lagoonal system. Unit A1 most likely originated as a longitudinal coastal dune whilst Unit A2 comprised a compound parabolic dune system that migrated into the back-barrier area across an estuary mouth/tidal inlet of the back-barrier system. The coastal barrier-dune configuration established by Unit A1 and A2 was most likely re-established during similar subsequent MIS 5 sea-level stands which during MIS 5c/b resulting in the formation of the back-barrier dune system of Unit A3. Palaeoclimatic inferences from Units A1 and A2 aeolianite wind vectors indicate a change from cooler post-glacial climates (lower Unit A1) to warmer interglacial-like conditions more similar to the present (upper Unit A1 and Unit A2). Unit A3 palaeowind vector data show variability interpreted to be related to global MIS 5c climatic instability and fluctuations. For Units A1, A2 and A3 pervasive early meteoric low-magnesium calcite (LMC) cementation followed shortly after deposition protecting the dune cores from erosion during subsequent sea-level fluctuations. Sea-spray induced vadose cementation in Units A1 and A2 may have been a key factor in stabilising dune sediment before later phreatic meteoric cementation. The final preserved Late Pleistocene depositional event in the study area was that of the storm deposit of beachrock Unit B5. Induration followed shortly after deposition by marine vadose high-magnesium calcite (HMC) cementation. Following deposition and lithification, Units A1, A2, A3 and B5 underwent a period of cement erosion associated with decementation and increased porosity due to either 1) groundwater table fluctuations related to the high frequency MIS 5 sea-level fluctuations and/or 2) carbonate solution due to complete subaerial exposure related to the overall MIS 4 - 2 sea-level depression towards the LGM lowstand. In addition to the decementation and porosity development Unit B5 also experienced inversion of the original unstable HMC cement to LMC. During MIS 4 to 2 the Aliwal shelf comprised an interfluve area which was characterised by subaerial exposure, fluvial incision of coast-parallel tributary river systems and general sediment starvation. Beachrock Units B1 to B4 were deposited in the intertidal to back-beach environments and subsequently rapidly cemented by marine phreatic carbonate cements comprising either aragonite or HMC. Unit B1 was most likely deposited at 10.8 ka cal. B.P., B2 at 10.2 ka cal. B.P, B3 at 9.8 ka cal. B.P and B4 <9.8 ka cal. B.P. thereby indicating sequential formation during the meltwater pulse 1b (MWP-1b) interval of the last deglacial sea-level rise. Unit B3 marks the change from a log-spiral bay coastal configuration established by Units B1 and B2 to a linear coastline orientation controlled by the trend of the pre-existing aeolianite units. This change in the morphology of the coastline is also documented by the shape of the underlying transgressive ravinement surface (reflector TRS, Sequence 4) which again was controlled by the subjacent sedimentary basin fill architecture and subsequent transgressive shoreline trajectory (Sequence 4). Sea-level rose at an average rate of 67 cm/100 years from B1 to B2 and 86 cm/100 years from B2 to B2 indicating an acceleration in the rate of sea-level rise supporting enhanced rates of sea-level rise during the MWP-1b interval which also seemed to have altered the coastal configuration and resulted in the closure of the southern outlet of the back-barrier estuarine system. Two cycles of initial aragonite followed by later HMC cement are tentatively linked to two marine flooding events related to different pulses of enhanced rates of sea-level rise during MWP-1b which are considered responsible for significant changes in the marine carbon reservoir ages. Comparisons of the U-series, C14 and optically stimulated luminescence (OSL) methods have shown OSL to be the most reliable method applied to dating submerged aeolianites and beachrocks. OSL not only provides the depositional age of the sediment but also does not suffer from open system behaviour, such as marine reservoir changes and contamination. Acoustic classification of the unconsolidated sediment samples resulted in the demarcation of 3 major acoustic facies, C to E, interpreted with sample analyses as quartzose shelf sand (C), reef-associated bioclastic-rich sand (D) and an unconsolidated lag and debris deposit (E). Grain size distribution patterns of the unconsolidated seafloor sediments indicate that the SCR system delivers fine and medium sand to the inner and middle shelf and imparts a general N-S trending pattern to the gravel and sand fractions. In addition grain size distributions support selective erosion of the seaward flank of the Sandridge with the remobilised sediment deposited in the Basin as low amplitude bedforms over the Facies E lag and debris pavement. The mud fraction is interpreted to be deposited by gravity settling from buoyant mud-rich plumes generated by river discharge. Integration of acoustic mapping, field observations and sample analyses indicate that the present distribution of the unconsolidated sediment is the result of a highly variable distribution of modern and palimpsest sediments which are continually redistributed and reworked by a complex pattern of bottom currents generated by the interaction of opposing oceanographic and swell driven circulation patterns.
Rock fabric study of the Northern Lebombo and Rooi Rand dyke swarms : regional and local implications.
(2012) Hastie, Warwick William.; Aubourg, Charles.; Watkeys, Michael Keith.
No abstract available.
The hydrogeology of Botswana.
(1974) Jennings, Christopher Mark Hubert.
Botswana covers an area of 570 000 km and has a population of about 600 000. It is estimated that three-quarters of the human and livestock populations are dependent on ground water, with an estimated 26 x 10 m of water from this source being used annually. Details of the physiography, climate, denudational and depositional surfaces and geology are given: Ground water occurs in both primary and secondary aquifers under both water-table and artesian conditions at varying depths from less than 1m to over 300m. The water-bearing properties of the various aquifers are described with the basalt/Cave sandstone contact providing the greatest number of successful boreholes and the sedimentary rocks of the Pretoria Group providing the highest yields (208,45 litres/minute). The shallowest water is found in the Kalahari Beds and the deepest in the Ecca Group. Detailed descriptions are given of the hydrogeology of the Central Kalahari, Lobatse, Orapa and Serowe. In the Lobatse area, estimates of the average monthly recharge rate have been made as well as estimates of the total storage capacity of the various ground-water basins. The percentage of annual rainfall contributing to ground water has also been calculated. An annual recharge rate has also been calculated for Serowe while the total amount of water in storage in the important Cave sandstone aquifer has been estimated. Hydrogeological details of the Kalahari Beds, basalt/Cave sandstone aquifer and Middle Ecca aquifers are given following core drilling programmes. ERTS photography was used to assist in obtaining a figure of 56 x 10 m of extractable water present in storage in the "sand rivers" of eastern Botswana. Detailed aquifer tests on a variety of aquifers are described and show that the secondary aquifers generally present often behave in a similar fashion to primary aquifer. Approximately 5 000 boreholes are presumed to be present in Botswana. It is estimated that 17% of the successful boreholes have yields in excess of 150 1/min. The siting of boreholes using geological/geophysical aids has resulted in an increase in the success rate of nearly 25%. New geophysical techniques for the location of ground water have been investigated and ground geophysical methods used include electrical resistivity, inductive and conductive electromagnetic, Afmag, self-potential and seismic reflection and refraction methods. The well-tried resistivity method remains the most successful technique but self-potential, Afmag and seismic methods have given encouraging results. Extremely detailed studies using environmental isotopes are described. These have enabled quantitative estimates of ground-water storage and turnover times to be made; have given round-water flow rates; have outlined areas of recharge; have enabled permeabilities to be calculated; have enabled a clearer picture of recharge mechanisms through the unsaturated zone to be built-up; and have provided important evidence of areas in which recent recharge has contributed to ground-water supplies. The studies have shown that measurable amounts of tritium are present over far wider areas than originally anticipated and thus more recharge is taking place than thought earlier from laboratory tests and hydrogeological considerations. In Lobatse a water balance model is proposed and calculations based on this model indicate that some leakage, hitherto unsuspected, between several of the ground-water basins, take place. The carbon-14 method has, in addition, helped outline areas of recharge (Central Basin, Lobatse) which tritium had failed to do and has shown by using combined 3H and l4C data that mixing of young and old waters takes place. In the Kalahari, radiocarbon has been used to calculate ground-water flow rates, permeability and transmissivity. The oldest ground water in Botswana has an age of 33 700 years. Isotopic studies in the unsaturated zone have shown that water moves given rates downward at a rate of between 31 and 41 cm per year. Studies of water levels in boreholes have shown that nearly all boreholes show responses which can be directly correlated with seasonal recharge and hence the nature and frequency of recharge can be estimated. In addition, storage capacity and safe yield have been estimated using long term water level changes and knowing the amount abstracted from the basin. The rapid responses shown in some boreholes indicates surprisingly rapid recharge. Two boreholes in Botswana showed effects of the Tulbagh earthquake on 29th September 1969. The disturbance of semi-diurnal fluctuations in boreholes could possibly be used as an early warning device to predict catastrophic earthquakes. A prelimlinary annual safe yield for ground-water supplies in Botswana is estimated to be 4 x 10 9 m3 per annum. Tree roots have been found in boreholes at depths greater than 68m. This emphasises the role vegetation can have in causing transpirational losses from ground-water supplies. Irrigation from boreholes is unlikely to be profitable unless exceptionally large supplies are obtained or water be present at very shallow depth. Underutilised boreholes, e.g. boreholes drilled specifically for cattle ranching, could also be profitably used for irrigation. Details of ground-water chemistry and examples of fresh water overlying saline, saline water overlying fresh, chemical stratification with depth and changes in quality with time are given. The distribution of fluoride rich waters in Botswana is also given. This thesis has therefore attempted to outline the current status of hydrogeological research in Botswana and it is hoped that this will lay the foundation for later, more detailed and quantitative, studies. These will become even more vital than at present, as it is estimated that all readily available surface water resources in eastern Botswana will be fully utilised by the late 1980's and the country will rely even more heavily on ground water than at present.
Sedimentary models for coal formation in the Klip River coalfield.
(1988) Christie, Angus David Mackay.; Tavener-Smith, Ronald.
The primary objective of this study was to establish sedimentary models for peat formation in the southern part of the Klip River coalfield during Ecca (Permian) times and to assess palaeoenvironmental controls on coal seam behaviour and distribution. In order to achieve this approximately 2 400 borehole logs and 25 field sections were collected. The coal-bearing Vryheid Formation records early to late Permian fluvio-deltaic sedimentation within the northeastern main Karoo basin. Three informal lithostratigraphic subdivisions, based on the investigations of Blignaut and Furter (1940, 1952), are proposed: the Lower zone, Coal zone and Upper zone. An examination of the structural framework and history of the northeastern Karoo basin reveals that the southern and western boundaries of the Klip River coalfield are defined by zones of rapid basement subsidence : the Tugela and Oannhauser Troughs respectively. There is some doubt as to the locality of the source area to the rivers emptying into the Ecca sea. Ryan (1967) postulated the "Eastern Highlands" situated off the present southeast African coast, but it is contended that the Swaziland area, situated no more than 200 to 300 km to the northeast of the Klip River coalfield, constituted a more plausible source area. The Lower zone represents sedimentation along a westerly to southeasterly prograding coastline dominated by high-constructive lobate or braid deltas, but also showing significant influence by wave processes. The Coal zone, which varies in thickness from 35 to 60 m, represents a major phase of coastal progradation and braided-river deposition on extensive alluvial plains. Significant coal seams formed only during periods of fluvial inactivity, the duration of which was dependent on source-area processes. Coal seam geometry and behaviour in the Klip River coalfield were not influenced by the depositional environments of associated clastic sediments. The following factors were found to have of profound influence in determining the extent, distribution and rate of peat accumulation: 1. Platform stability and temporal and spatial variations therein. 2. The absence or presence of penecontemporaneous clastic sedimentation. 3. Duration of periods of peat formation. 4. Lithology and topographic expression of clastic sediments underlying peat-forming swamps. The peat-forming phase of the Vryheid Formation was terminated by an extensive transgression brought about by an eustatic rise in basin water-level and/or an increased rate of platform subsidence.
Aspects of the engineering geology of Maputo City, Mozambique.
(2011) Vicente, Enoque Mendes.; Schreiner, Hilson Deneys.; Jermy, Colin A.; Richards, Nicholas.
The geological formations of Maputo City, which are mainly unconsolidated materials with soil like properties, are described in terms of their engineering geological and geotechnical characteristics with relevance to their distribution patterns and spatial trends. Problematic conditions such as collapse potential characteristics, loose aeolian sand dune deposits and loose sand plains characterize many of the materials. The geological characteristics combined with anthropogenic interference such as intensive urbanization with inappropriate land use, construction in sensitive areas like steep sandy slopes has led to many problems including slope stability. Foundation problems with building settlement and gully erosion also occur. The aim of this research was to study the engineering geological characteristics and the geotechnical properties of the geological formations of Maputo City and various related problems. Special relevance has been given to the understanding of three specific problems: building damage, gully erosion and slope instability. The geological formations are predominantly sandy (coarse to very fine sand) with very low clay content, are non-plastic and are classified as from the group SP-SM which are poorlygraded sand with silt. The majority of the materials are loose and normally consolidated with a high level of residual strength. Assessment of collapse settlement through double consolidation technique indicated soil compressibility and significant sensibility to collapse upon wetting. Truly collapsible soils that show full collapse of the soil structure were identified in 33% of the tested materials where the highest collapse behaviour reached values above 5%, predicted to cause moderate trouble in foundation design. Some of the bonded materials are bonded (evident in 67% of samples tested). Bonding was confirmed by comparing the compressibility of the undisturbed and remoulded samples. The remoulded samples showed a significantly higher compression than that of the bonded materials as part of the stress applied is carried by the bonds themselves, as the bonded material is stiffer than the same without bonds. The curves of the remoulded samples were used to establish the limit between the stable and meta-stable states of the material. A qualitative evaluation of the erosion susceptibility was investigated by physical tests such as the crumb test, shear strength and chemical indicators while a quantitative evaluation of the erodibility characteristics was obtained using a flume test. Some correlations were found between the results of various methods. Almost all samples that were found to be dispersive with ESP were also dispersive with TDS vs. %Na and SAR. Results of the flume erodibility test have very little correlation with the chemical properties related to dispersion revealing that the erosion susceptibility and gullying in Maputo City have more relation to the physical processes than to the dispersion related chemical properties of the soils. The positive identification of dispersive and erodible soils can only be carried out using a combination of various techniques. Therefore, a new rating system for erosion susceptibility of sandy soils combining the physical and chemical factors of dispersion is proposed including the flume test, crumb test, TDS/%Na, SAR and ESP. The proposed rating system was applied to the tested soils of Maputo City. Fifteen samples (83% of the rated samples) were classified with intermediate susceptibility to erosion while 3 samples (17%) were classified as having a low susceptibility to erosion. The highest rating scores were obtained by the same samples that showed dispersive behaviour with SAR, ESP and TDS/%Na. This group of samples was of intermediate erodibility in the flume test. The slope instability mechanisms observed in Maputo City are predominantly rotational failures with a mass of soil sliding along a curved surface of rupture followed by sand flow at the toe as failure occurs in the presence of excess water. Four groups of factors account for the slope instability problems in Maputo City: geomorphological causes, physical and meteorological causes, geological and geotechnical properties of soils, and anthropogenic causes. The mechanism of failure is mostly due to the loss of matric suction of soils in the presence of rainwater and possibly from destruction of bonding agents. Factors of safety values indicate that the slopes are generally unstable with the control being the slope angle. The slopes in the Polana-Caniço and Ferroviário Quarters show high factor of safety values but is the area most affected by slope instability. Slope failure in these areas is intrinsically caused by anthropogenic factors related to inappropriate land use planning. The gully sidewalls are unstable as the slope created is very steep. The slope at Friedrich Engels Avenue causes most concern due not only to the slope height and angle but also to the size and number of buildings constructed at the crest, mainly high rise buildings along the Julius Nyerere Avenue, the integrity of which could be threatened by a landslide event (this slope has recently been affected by active landslides).
Sedimentology, coral reef zonation, and late Pleistocene coastline models of the Sodwana Bay continental shelf, Northern Zululand
(1991) Ramsay, Peter John.; Mason, Tom R.
This geostrophic current-controlled Zululand/Natal shelf displays a unique assemblage of interesting physical, sedimentological and biological phenomena. The shelf in this area is extremely narrow compared to the global average of 75km, and is characterised by submarine canyons, coral reefs, and steep gradients on the continental slope. A shelf break occurs 2.1km to 4.1km offshore and the shelf can be divided into a northern region and a southern region based on the presence or absence of a defined shelf break. The southern shelf has a poorly-defined shelf break whilst the northern shelf has a well-defined break at -65m. The poor definition of the shelf break on the southern shelf can possibly be attributed to the presence of giant, climbing sand dunes offshore of Jesser Point at depths of -37m to -60m. The northern shelf has a series of coast-parallel oriented patch coral reefs which have colonised carbonate-cemented, coastal-facies sequences. The northern shelf can be divided into three distinct zones: inner-, mid-, and outer-shelf zones. The inner-shelf is defined as the area landward of the general coral reef trend, with depths varying from 0m to -I5m and having an average gradient of 1.1. The mid-shelf is defined by the general coral reef trend, varying from -9m over the shallow central axis of the reefs to -35m along the deep reef-front environments. The outer-shelf is seaward of the coral reefs and occurs at a depth range of -35m to - 65m. Gradients vary from 1° in the south to 2.5° in the northern part of the study area, and are steep compared to world average shelf gradient of 0.116°. Four submarine canyons occur in the study area and are classified as mature- or youthful-phase canyons depending on the degree to which they breach the shelf. The origin of these canyons is not related to the position of modern river mouths but can probably be linked to palaeo-outlets of the Pongola and Mkuze River systems. It is suggested that the canyons are mass-wasting features which were exploited by palaeo-drainage during regressions. The youthful-phase canyons appear to be mass-wasting features associated with an unstable, rapidly-deposited, progradational late Pliocene sequence and a steep upper continental slope. The mature-phase canyons were probably initiated by mass-wasting but have advanced shoreward, breaching the shelf, due to their link with the palaeo-outlets of the Pongola and Mkuze Rivers during late Pleistocene regressions. Evidence of modem canyon growth has been noted on numerous SCUBA diving surveys carried out on the canyon heads. These take the form of minor wall slumps and small-scale debris flows. The canyons are also supplied with large quantities of sand in the form of large-scale shelf subaqueous dunes generated and transported by the Agulhas Current. As these bedforms meet the canyons the sediment cascades down the canyon thalweg and causes erosion and downcutting of the canyon walls and floor thereby increasing the canyon dimensions. Late Pleistocene beachrock and aeolianite outcrops with or without an Indo-Pacific coral reef veneer are the dominant consolidated lithology on the shelf. These submerged, coast-parallel, carbonate cemented, coastal facies extend semi-continuously from -5m to -95m, and delineate late Pleistocene palaeocoastline events. The rock fabric of these high primary porosity lithologies shows grains floating in a carbonate cement with occasional point-contacts. Grains are mostly quartz (80-90%), minor K-feldspar and plagioclase (5-10%), and various lithic fragments. The rocks contain conspicuous organic grains including foraminifera, bivalve, echinoid, bryozoan, red algal, and occasional sponge spicule fragments; these commonly display replacement fabrics or iron-stained rims. The dominant sedimentary structures found in these sandstone outcrops include high-angle planar cross-bedding and primary depositional dip bedding. Palaeocurrent directions sngest a palaeoenvironment dominated by a combination of longitudinal and transverse dunes with wind directions similar to those observed forming the modem dune systems. Erosional features evident on the submerged beachrocks and aeolianites include gullies trending in two different directions and sea-level planation surfaces with or without the presence of potholes. The unconsolidated sediment on the shelf is either shelf sand, composed mainly of terrigenous quartz grains; or bioclastic sediment which is partially derived from biogenic sources. The quartzose sand from the inner-shelf is generally fine-grained, moderately- to well-sorted, and coarsely- to near symmetrically-skewed. Carbonate content is low, and varies between 4-13%. Quartzose sand from the outer-shelf is fine-grained, moderately- to well-sorted, and coarsely- to very coarsely-skewed. The inner-shelf quartzose sand is better sorted than the outer-shelf sand due to increased reworking of this sediment by the high-energy swell regime. Sediment from the shallower areas of the outer-shelf (< -50m) is better sorted than sediment from depths of greater than -50m. Generally wave-reworking of quartzose shelf sand from the Sodwana Bay shelf results in greater sediment maturity than that observed from geostrophic current effects or a combination of geostrophic and wave-reworking. This sediment was derived by reworking of aeolian and beach sediments, deposited on the shelf during the period leading up to the Last Glacial Maximum (15 000 - 18 000 years B.P.) when sea-level was -130m, during the Holocene (Flandrian) transgression. Bioclastic sediment on the Sodwana Bay shelf is defined as having a CaC03 content of greater than 20% and is a mixture of biogeoically-derived debris and quartzose sand. The distribution of bioclastic sediment in the study area is widespread, with reef-derived and outer-shelf-derived populations being evident. This sediment consists of skeletal detritus originating from the mechanical and biological destruction of carbonate-secreting organisms such as molluscs, foraminifera, alcyonaria, scleractinia, cirripedia, echinodermata, bryozoa, porifera. The reef-derived bioclastic population is confined to depths less than -40m in close proximity to reef areas, whereas the shelf-derived bioclastic population occurs at depths greater than -40m and is derived from carbonate-producing organisms on deep water reefs and soft-substrate environments on the shelf. Large-scale subaqueous dunes form in the unconsolidated sediment on the outer-shelf due to the Agulhas flow acting as a sediment conveyor. These dunes are a common feature on the Sodwana Bay shelf occurring as two distinct fields at depths of -35m to -70m, the major sediment transport direction being towards the south. The two dune fields, the inner- and outer subaqueous dune fields, are physically divided by Late Pleistocene beachrock and aeolianites ledges. A bedform hierarchy has been recognised. The larger, outer dune field appears to have originated as a system of climbing bedforms with three generations of bedforms being superimposed to form a giant bedform, while the inner dune field has a less complex construction. The largest bedforms are those of the outer dune field off Jesser Point, being up to 12 m high, 4 km long and 1.2 km wide. A major slip face, with a slope of 8° is present. Bedload parting zones exist where the bedform migration direction changes from south to north. Three bedload parting zones occur in the study area at depths of -60m, -47m and -45m; two in the inner dune field and one in the outer dune field. These zones are invariably located at the southern limits of large clockwise eddy systems. Such eddies appear to be the result of topographically induced vorticity changes in the geostrophic flow and/or the response to atmospheric forcing caused by coastal low-pressure system moving up the coastline. It has been demonstrated that the inner subaqueous dune sediment conveyor is not active all the time but only during periods . of increased current strength when the Agulhas Current meanders inshore. The smaller bedforms in the outer dune field undergo continuous transport due to the current velocity on the shelf edge outer dune field being higher than the velocity experienced on the inner dune field. The very large 2·D dune which forms the outer dune field is probably not active at present: this is inferred due to the shallow angle of the mega-crest lee slope (8°). The very large Sodwana Bay subaqueous dune fields may be compared with the very large, reconstructed, subaqueous dunes which occur in Lower Permian sediments of the Vryheid Formation, northern Natal. These Permian dunes are represented, in section, as a fine- to medium-grained distal facies sandstone with giant crossbeds. These large-scale bedforms are unidirectional, but rare directionally-reversed, climbing bedforms do occur, this directional reversal may be related to bedload parting zones. On the evidence presented in this thesis, it is proposed that these Permian subaqueous dunes may be ancient analogues of the modem subaqueous dune field on the Sodwana Bay shelf. Positive-relief hummocks and negative-relief swale structures are fairly common in the fine-grained, quartzose shelf sand at depths of -30m to -60m. These appear to be transitional bedforms related to the reworking by storms of medium 2-D subaqueous dunes. These hummocky structures may be the modem equivalent of hummocky cross-stratification noted in the geological record, and if so, they are probably the first to have ever been observed underwater. The occurrences of ladderback ripples on the Sodwana Bay shelf at depths of -4m to -17m, suggest that subtidal ladderback ripples may be more common than previously thought. Ladderback ripples are common features of tidal flats and beaches where they form by late-stage emergence run-off during the ebb tide. They are generally considered diagnostic of clastic intertidal environments. The mode of formation on the Sodwana Bay shelf is different from the classic late-stage emergence run-off model of intertidal occurrences, being a subtidal setting. Subaqueous observations indicate that ladderback ripples are not environment-specific, and that additional evidence of emergence is therefore necessary to support an intertidal setting in the rock record: ladderback ripples alone are insufficient to prove an intertidal environment. The coral patch reefs of the northern Natal coast are unique, being the most southerly reefs in Africa, and totally unspoilt. The Zululand reefs are formed by a thin veneer of Indo-Pacific type corals which have colonised submerged, late Pleistocene beachrocks and aeolianites. Two-Mile Reef at Sodwana Bay has been used to develop a physiograpbic and biological zoning model for Zululand coral reefs, which has been applied to other reefs in the region. Eight distinct zones can be recognised and differentiated on the basis of physiographic and biological characteristics. The reef fauna is dominated by an abundance of alcyonarian (soft) corals, which constitute 60-70% of the total coral fauna. The Two-Mile Reef zoning model has been successfully applied to larger reefs such as Red Sands Reef, and smaller patch reefs (Four-Mile and Seven-Mile Reefs) in the same general area. In this thesis extensive use has been made of Hutton's uniformitarian principles. Hutton's doctrine is particularly relevant to the study of depositional processes and relict shorelines. Coastal processes and weather patterns during the late Pleistocene were broadly similar to modem conditions enabling direct comparisons to be made. A computer-aided facies analysis model has been developed based on textural statistics and compositional features of carbonate-cemented coastal sandstones. Many attempts have been made to distinguish different ancient sedimentary depositional environments, most workers in this field having little success. The new method of facies reconstruction is based on: (1) underwater observations of sedimentary structures and general reef morphology; (2) a petrographic study of the reef-base enabling flve facies: aeolianite, backbeach, forebeach, swash, and welded bar facies to be recognised, which control the geomorphology of Two-Mile Reef; (3) cluster and discriminant analysis comparing graphic settling statistics of acid-leached reef-base samples with those of modem unconsolidated dune/beach environments. The results of this analysis demonstrated that the beachrocks and aeolianites on the shelf formed during a regression and that late Pleistocene coastal facies are similar to modem northern Zululand coastal environments, which have been differentiated into aeolian, backbeach, forebeach, swash, & welded bar. A late Pleistocene and Holocene history of the shelf shows that during the late Pleistocene, post Eemian regressions resulted in deposition and cementation of coast-parallel beachrocks and aeolianites, which define a series of four distinct palaeocoastline episodes with possible ages between 117 000 and 22 000 years B.P. The beachrock/aeolianites formed on the shelf during stillstands and slow regressions, and the gaps between these strandline episodes represent periods of accelerated sealevel regression or a minor transgressive phase which hindered deposition and cementation. The formation of these lithologies generated a considerable sediment sink in the nearshore zone. This reduced sediment supply and grain transport in the littoral zone during the Holocene, and probably enhanced landward movement of the shoreline during the Flandrian transgression. Prior to the Last Glacial Maximum, the beachrock/aeolianite sedimentary sequence was emergent and blanketed by shifting aeolian sands. The Pongola River, which flowed into Lake Sibaya, reworked the unconsolidated sediments on the shelf, and exploited the route of least resistance: along White Sands and Wright Canyon axes. The erosion resulting from fluvial denudation in Wright Canyon has caused this canyon to erode some of the beachrock/aeolianite outcrops which form palaeocoastline episode 2 and entrench the canyon to a deeper level; this eroded the shelf to a distance of 2km offshore. During the Flandrian transgression the unconsolidated sediment cover was eroded, exposing and submerging the beachrock/aeolianite sequence. Flandrian stillstands caused erosional features such as wave-planed terraces, potholes, and gullies to be incised into beachrock and aeolianite outcrops; these are seen at present depths of -47m, -32m, .26m, -22m, -17m to -15m, and -12m. High energy sediment transfers, in an onshore direction, resulted in the deposition of sand bars across the outlet of Lake Slbaya's estuary and the development of a 130m + coastal dune barrier on a pre-existlng, remnant Plelstocene dune stub. Sea-level stabilised at its present level 7 000-6 000 years B.P. and coral reef growth on the beachrock/aeolianite outcrops probably started at 5 000 years B.P. A minimum age for the formation of the northern Zululand coral reefs has been established at 3780 ± 60 years B.P. A mid Holocene transgression relating to the Climatic Optimum deposited a + 2m raised beach rock sequence. This transgression eroded the coastal dune barrier and caused a landward shoreline translation of approximately 40m. A minor transgression such as this can be used as a model for coastal erosion which will result from the predicted 1.5m rise in sea-level over the next century. This rise in sea-level could result in a 30m landward coastline translation of the present coastline, ignoring the influence that storms and cyclones will have on the coastline configuration.
The marine geology of the Northern KwaZulu-Natal continental shelf, South Africa.
(2009) Green, Andrew Noel.; Uken, Ronald.
This study proposes that the submarine canyons of the northern Kwazulu-Natal continental margin formed contemporaneously with hinterland uplift, rapid sediment supply and shelf margin progradation during the forced regression of upper Miocene times. These forced regressive systems tract deposits volumetrically dominate the shelf sediments, and comprise part of an incompletely preserved sequence, amongst which six other partially preserved sequences occur. The oldest unit of the shelf corresponds to forced regression systems tract deposits of Late Cretaceous age (seismic unit A), into which a prominent erosive surface, recognized as a sequence boundary, has incised. Fossil submarine canyons are formed within this surface, and underlie at least one large shelf-indenting canyon in the upper continental slope. Smaller shelf indenting canyons exhibit similar morphological arrangements. Late Pliocene deposits are separated from Late Cretaceous lowstand deposits (seismic unit B) by thin veneers of Late Palaeocene (seismic unit C) and mid to early Miocene (seismic unit D) transgressive systems tract deposits. These are often removed by erosive hiatuses of early Oligocene and early to mid Pliocene age. These typically form a combined hiatus surface, except in isolated pockets ofthe upper slope where late Miocene forced regressive systems tract units are preserved (anomalous progradational seismic unit). These sediments correspond to the regional outbuilding of the bordering Tukhela and Limpopo cones during relative sea level fall. Either dominant late Pliocene sediments (seismic unit E), or transgressive systems tract sediments which formed prior to the mid Pliocene hiatus, overlie these sediments. Widespread growth faulting, slump structures and prograding clinoforms towards canyon axes indicate that these sediments initiated upper slope failure which served to create proto-canyon rills from which these canyons could evolve. The association of buried fossil canyons with modern day canyons suggests that the rilling and canyon inception process were influenced by palaeotopographic inheritance, where partially infilled fossil canyons captured downslope eroding flow from an unstable upper slope. Where no underlying canyons occur, modern canyons evolved from a downslope to upslope eroding system as they widened and steepened relative to the surrounding slope. Statistical quantification of canyon forms shows a dominance of upslope erosion. Landslide geomorphology and morphometric analysis indicate that this occurred after downslope erosion, where the canyon axis was catastrophically cleared and incised, leading to headward retreat and lateral excavation of the canyon form. Trigger mechanisms for canyon growth and inception point to an overburdening ofthe upper slope causing failure, though processes such as freshwater sapping may emulate this pattern of erosion. It appears that in one instance, Leven Canyon, freshwater exchange with the neighbouring coastal waterbodies has caused canyon growth. The canyons evolved rapidly to their present day forms, and have been subject to increasingly sediment starved conditions, thus limiting their evolution to true shelf breaching canyon systems. Sedimentological and geomorphological studies show that the shelf has had minor fluvial influences, with only limited shelf-drainage interaction having occurred. This is shown by isolated incised valleys of both Late Cretaceous and Late PleistocenelHolocene age. These show classic transgressive valley fills of wave dominated estuaries, indicating that the wave climate was similar to that of today. The narrowness of the shelf and the inheritance of antecedent topography may have been a factor in increasing the preservation potential of these fills. Canyons thus appear to have been "headless" since their inception, apart from Leven Canyon, which had a connection to the Last Glacial Maximum (LGM) St Lucia estuary, and Wright Canyon, which had an ephemeral, shallow LGM channel linking it to the Lake Sibaya estuarine complex. Coastline morphology has been dominated by zeta bays since at least 84 000 BP, thus littoral drift has been limited in the study area since these times. The formation of beachrock and aeolianite sinks during regression from the last interstadial has further reduced sediment supply to the shelf. The prevalence of sea-level notching in canyon heads, associated with sea levels of the LGM indicates that canyon growth via slumping has been limited since that time. Where these are obscured by slumping in the canyon heads (Diepgat Canyon), these slumps have been caused by recent seismic activity. The quiescence of these canyons has resulted in the preservation of the steep upper continental slope as canyon erosion has been insufficient to plane the upper slope to a uniform linear gradient such as that of the heavily incised New Jersey continental margin. Progressive sediment starvation of the area during the Flandrian transgression has resulted in a small shore attached wedge of unconsolidated sediment (seismic unit H) being preserved. This is underlain by a mid-Holocene ravinement surface. This crops out on the outer shelf as a semi-indurated, bioclastic pavement. Thinly mantling this surface are Holocene sediments which have been reworked by the Agulhas Current into bedforms corresponding to the flow regime and sediment availability to the area. Bedforms are in a state of dis-equilibrium with the contemporary hydrodynamic conditions, and are presently being re-ordered. It appears that sediment is not being entrained into the canyons to the extent that active thalweg downcutting is occurring. Off-slope sediment loss occurs only in localized areas, supported by the dominance of finer grained Early Pleistocene sediments of the outer slope. A sand ridge from the mid shelf between Wright and White Sands Canyons appears to have been a palaeo-sediment source to White Sands Canyon, but is currently being reworked southwards towards Wright Canyon. The prevalence of bedform fields south of regularly spaced canyon heads is considered a function of hydrodynamic forcing of the Agulhas Current by canyon topography. These bedforms are orientated in a northerly direction into the canyon heads, a result ofnortherly return eddying at the heads of these canyons.

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