ALBERT

Description: On the last page of his 1937 book “Our Wandering Continents” Alex Du Toit advised the geological community to develop the field of “comparative geology”, which he defined as “the study of continental fragments”. This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean. In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the 〉100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc. A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called “ARCs”) and their Deformed equivalents (called “DARCs”), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana). A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian – Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift. Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called “Mauritia”, and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge “jumps”. This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.

Description: Recent investigations of the shale gas potential in the main Karoo Basin have concentrated on the Whitehill Formation within the Ecca Group. This study focuses on the shale gas potential of the underlying Prince Albert Formation using the parameters of volume porosity, permeability, total organic carbon (TOC), vitrinite reflectance and Rock-Eval data. Shale samples were retrieved from three surface localities in the southern part of the main Karoo Basin and from core of three boreholes drilled through the Prince Albert Formation near Ceres, Mervewille and Willowvale. The sampling localities occur near the borders of the prospective shale gas areas (“sweet spots”) identified for the Whitehill Formation. Kerogen was found to be Type IV with hydrogen indices less than 65 mg/g. Shale porosities are between 0.08 and 5.6% and permeabilities between 0 and 2.79 micro-Darcy, as determined by mercury porosimetry. TOC varies between 0.2 and 4.9 weight % and vitrinite reflectance values range from 3.8 to 4.9%. Although the porosity and TOC values of the Prince Albert Formation shales are comparable with, but at the lower limits of, those of the gas-producing Marcellus shale in the United States (porosities between 1 and 6% and TOC between 1 and 10 weight %), the high vitrinite reflectance values indicate that the shales are overmature with questionable potential for generating dry gas. This overmaturity is probably a result of an excess depth of burial, tectonic effects of the Cape Orogeny and dolerite intrusions. However, viable conditions for shale gas might exist within the “sweet spot” areas, which were defined for the Whitehill Formation.

Description: The type area of the Copperton Formation is on the farms Vogelstruisbult 104, Somuspan 105 and Dooniespan 108 in Prieska District. Outcrop is poor and the type material is preserved in exploration borehole cores from the Prieska Copper Mines and the Annex Cu-Zn deposits. It is highly deformed and variably metamorphosed. Thus it is a lithodemic unit, but interpreted as a supracrustal sequence and described as a formation including lithologically distinct members. The Copperton Formation comprises a wide range of rock types including metabasic and intermediate gneisses with minor amounts of metapelitic and calc-silicate rocks. Metamorphic parageneses generally reflect amphibolite facies metamorphism, but granulite and retrograde greenschist facies zones also occur. The protoliths are interpreted as an arc-related volcano-sedimentary package and the Smouspan Member metadacite is dated at 1284 ± 9 Ma. Members are distinguished as follows: The Magazine Member is dominated by calc-silicate rocks which are dominant in outcrop but rarely found in borehole cores. The Smouspan Gneiss is a fairly homogeneous hornblende-biotite intermediate gneiss which is up to 400 meters thick in an isoclinal fold structure. It comprises the footwall to the Prieska Copper Mines Member, in which the massive sulphide orebody occurs, enclosed in an alteration assemblage comprising dark gedrite fels and strongly foliated, leucocratic quartz-perthite-sillimanite gneiss. The ore is interpreted as a volcanogenic massive sulphide deposit formed in a Mesoproterozoic island arc system. The Vogelstruisbult Member is the hanging wall unit, comprising mainly laminated amphibolites and metapelites, but also containing a variety of rock types including hornblende gneiss, biotite gneiss, chlorite schist, and calc-silicate gneiss. Away from the Prieska Mines orebody, a similar variety of rock types is found, and not subdivided but classified as Copperton Formation, a mappable unit. The same assemblage of rock types, including massive sulphide mineralization, was intersected in drill holes on the farms Kielder (portion of Doonies Pan 108), Eierdop Pan and Kantienpan to the north. The Copperton Formation is the southernmost unit of the Areachap Group which is exposed between Prieska Copper Mines and Areachap Mine north of Upington, where the Jannelsepan and Bethesda formations occur. The Copperton Formation is partly obscured in many places by Dwyka Group tillite cover which thickens southwards. A sequence of structural and metamorphic events affected the Copperton Formation and Areachap Group during the 1.2 to 1.0 Ga Namaqua-Natal orogeny. These involved collision of the Areachap Terrane with the Kaapvaal-Rehoboth Craton at about 1220 Ma, a thermal and deformational event coeval with the continental-scale Umkondo mantle event at about 1100 Ma, followed by uplift, erosion and the development of right-lateral shear zones of the Doornberg Lineament, with cooling below 300°C by 920 Ma. A Cambrian peneplain developed in the region which was first covered by Nama Group sandstones, then glaciated and covered by Permian Dwyka Group tillites which are presently being eroded to expose the Copperton Formation.

Description: The Prince Albert Formation is a mudstone-dominated unit, which is located in the central and southwestern part of the Main Karoo Basin and the southernmost part of the Kalahari Basin in South Africa. It is Early Permian (Artinskian to early Kungurian) in age and is stratigraphically located between the underlying Dwyka Group and the overlying Whitehill Formation. In the Main Karoo Basin, its regional extent is limited to the cut-off boundary of the Whitehill Formation along a line from Hertzogville in the Free State to Coffee Bay in the Eastern Cape. Northeast of this boundary, it correlates with the lower part of the undifferentiated Ecca Group known as “Ecca Shales” and in the northern part of the basin, north of a line from Bloemfontein (Free State) to Harding (KwaZulu-Natal), with the Vryheid and Pietermaritzburg formations. The Prince Albert Formation is generally between 50 and 200 m thick, including the type area around Prince Albert, where a thickness of about 145 m was measured. It is thicker (230 to 497 m) in the region between Brandvlei and Jansenville and thins northeastwards to between about 30 and 60 m in the area between Kimberley and East London. In the Kalahari Basin, the formation is only 25 to 50 m thick, due to post-Karoo erosion of its upper part. The Prince Albert Formation contains fossils of marine invertebrates, palaeoniscoid fish, sharks, sponge spicules, foraminiferans, radiolarians, acritarchs, fragments of wood and leaves, together with ichnofossils in the form of fish trails, arthropod trackways and invertebrate burrows. High Rb/K ratios in the mudstones also indicate a marine shelf environment with suspension settling of mud being the predominant depositional process. Sedimentation was initiated during a major transgressive event following final melting of Dwyka Group associated ice sheets in southern Gondwana in the Late Palaeozoic. Water depths in the basin were probably about 400 m, but shallowed northeastward. In the proximal part of the basin northeast of Kimberley, sandstones in coarsening-upward successions represent prograding deltaic deposits derived from an adjacent source area to the north.

Description: Faults and dolerite dykes within Basement- and Karoo-aquifers in northern Mozambique may increase groundwater occurrence but may also be barriers to groundwater flow. Should observation boreholes drilled into regional and local faults as well as dykes show a response to aquifer testing, it would be deduced that these hydrogeological discontinuities are not barriers to groundwater flow. The approach adopted for this study included a sequential process involving data acquisition through a hydrogeological fieldwork programme consisting of geophysical surveys, borehole drilling, aquifer testing, and groundwater level monitoring. The Zambezi Border and geological contact faults were characterised by high variability in hydraulic properties. Aquifer testing resulted in drawdown in observation boreholes as well as a reduction in piezometric surface in the installed vibrating wire piezometers located in different aquifer units, indicating the Zambezi Border- and geological contact-faults were not barriers to groundwater flow. Not all the northwest-southeast trending dykes acted as barriers to groundwater flow, as there were discreet intervals with relatively high permeability present.

Description: Population and economic growth within the Durban Metropolitan region in eastern South Africa have increased the demand for water supply. Though the region’s water supply comes mainly from surface water sources, the ever-increasing demand means that all available water supply sources including groundwater will be looked at, particularly in the peri-urban areas. However, the state of the groundwater resource in the region is poorly understood. This study aims to contribute towards improved understanding of the state of groundwater resources in the Metropolitan District through an integrated hydrogeological, hydrochemical and environmental isotope investigations. Results of the hydrogeological and hydrogeochemical characterization identified at least five hydrostratigraphic units of varying hydraulic and hydrochemical characteristics: the weathered and fractured basement aquifers of the Mapumulo Group, Oribi Gorge, Mzimlilo and Mkhomazi Suites characterized by average borehole yield and transmissivity (T) of 1.2 l/s, and 3.9 m2/day, respectively, and hydrochemical facies of Ca-Mg-HCO3;the fractured Natal Group sandstone characterised by average borehole yield and hydraulic conductivity (K) of 5.6 l/s and 2.8 m/day, respectively and with Na-Mg-HCO3-Cl dominant water type;the fractured aquifers of the Dwyka Group diamictite and tillite characterized by average borehole yield of 0.4 l/s, transmissivity of 1.3 m2/day and Na-Cl-HCO3 dominant water type;the Vryheid Formation of the Ecca Group characterized by average borehole yield of 2.5 l/s, T of 4.9 m2/day, K values 0.17 m/day, and Na-Cl-HCO3 water type. The Pietermaritzburg Formation of the Ecca Group is characterized by a shale lithology with very low borehole yields and average transmissivity of 0.28 m2/d with Na-Ca-Cl dominant water type. It is considered as an aquiclude than an aquifer;the intergranular aquifer of the Maputaland Group which comprises the Bluff, Berea type sands and harbour beds (recent alluvium and estuarine deposits). These units collectively have average borehole yield of 14.8 l/s, transmissivity of up to 406 m2/day and a mainly Na-Cl-HCO3 hydrochemical signature. The region receives mean annual precipitation (MAP) of 935 mm/yr of which an estimated 6.6% recharges the various aquifers. Environmental isotope data (2H, 18O and 3H) indicated that groundwater is recharged from modern precipitation. High concentrations of tritium, as high as 92 T.U., measured around landfill sites, indicates groundwater contamination from leachate leakage posing a risk to human and environmental health.

Description: The Late Archaean to Early Proterozoic Malmani Subgroup comprises of dolomites and limestones forming part of the Chuniespoort Group within the Transvaal Supergroup, outcropping as an arc structure east of the Pretoria Group along the Limpopo and Mpumalanga escarpment. These rocks form a fractured karst aquifer in the area and have a high degree of heterogeneity and anisotropy. The aquifers are unconfined to semi-confined, with compartmentalisation by dolerite dykes being a possible effect (if the dykes are large and extensive enough) due to the dykes acting as aquitards or barriers to groundwater flow. The contact zones between the dolomite formations and dolerite dykes are usually fractured however, and along with any other faults and fractures result in preferential dolomite dissolution and the development of groundwater flow paths in the area. Borehole yields ranges between 2 to 5 l/s and potentially 〉10 l/s per borehole in the vicinity of large regional fractures or dolerite intrusions. Groundwater from the Malmani Subgroup generally meets the drinking water quality standards for major constituents and it is of Mg-Ca-HCO3 nature. Groundwater development within this particular hydrostratigraphy is linked to potential well field target zones that take cognisance of various surface water-groundwater interaction affecting surface water discharge rates as well as groundwater over-abstraction concerns. Preliminary results have indicated that given a groundwater potential of 44 hm3/a, the aquifer will be able to support abstractions of up to 29 hm3/a if systematically developed adaptively and could be used and managed conjunctively with surface water to alleviate the pressure on the already stressed Olifants Water Management Area.

Description: The properties of residual dolomite, sometimes termed wad, are variable and uncertain. It represents the insoluble residue after dissolution of dolomite and is commonly found in the Neoarchaean Malmani Subgroup of the Chuniespoort Group (South Africa). This study comprised triaxial tests, crumb tests, X-Ray diffraction and fluorescence spectroscopy, foundation indicators, stereo-microscope imagery and permeability testing of the represented formations outcropping in the northeastern portions of South Africa. Results concur that residual dolomite is not typically dispersive, has low density that can be below that of water, mostly grades in the silt fraction, has high plasticity indices with low to high liquid limits, and has hydraulic conductivities in the order of 1x10-6 m/s. This new knowledge database contributes to our understanding of the flow through these systems and to how ingress scenario subsidences and sinkholes can possibly occur.

Description: The Malmesbury Group has often been overlooked as a useable source of groundwater. This is due to its previously generalised low yields and relatively higher salinities of groundwater found in these weathered to fresh sedimentary and metasedimentary rocks. However, during a period of severe drought in the Western Cape of South Africa from 2016 to 2018, the Malmesbury Group was investigated for groundwater more intensively, mainly out of necessity by those who had little other choice geologically. This study presents two case studies from distinctly different geological and hydrogeological zones within the Malmesbury Group. A selection of 57 boreholes that have been drilled into the Malmesbury Group were used for this study. Of these, 30 boreholes are in close proximity to Paarl while the remaining 27 boreholes are located in the Cape Flats. Although these areas are noticeably different, both sites revealed that the Malmesbury Group could serve as a host to a high yielding aquifer, under certain geological conditions. Borehole yields of up to 9 l/s and 21 l/s were recorded at the Cape Flats and Paarl sites respectively. However in all cases the high yields were due to high yielding fractures in the bedrock, rather than the inherent bedrock properties. At the Paarl site, a correlation is seen between aquifer parameters and their proximity to an inferred extension of the Wellington – Piketberg Fault and Shear Zone from published geological maps. Boreholes in both the Cape Flats and Paarl sites that did not intersect these fractures were found to have similar bedrock yields to previous Malmesbury Group generalisations (〈0.5 l/s).

Description: The current study aimed to determine the factors affecting groundwater recharge and its spatial distribution in the Rietvlei wetland located near Cape Town. To achieve this, the subsurface material was logged during the construction of eight shallow wells, complimented with field observations, and surveying the dug wells. The water stemming from these wells was sampled and analysed for Oxygen 18 and Deuterium to determine the source of groundwater in Rietvlei wetland. Downhole salinity logs of the wells were also undertaken, and rainfall samples were analysed for the stable isotopes. Isotopic results indicate that groundwater stems from rainfall, with the exception of Well 8 which has been influenced by the river due to its proximity to the surface water body. Additionally, results obtained indicate a distinct relationship between elevation and soil structure. Shallow profiles were mainly dominated by medium to fine sands, silty sand, and clay. Using the water table fluctuation method, this was found to have a direct impact on the spatial distribution of groundwater recharge on an event basis. It was therefore concluded that both lithology and topography have a direct effect on recharge rates and water quality.