A study of the geology of the Vaalbank and Lady Frere areas in the Karoo Basin of South Africa using gravity, magnetic and gamma-ray spectrometry data: implications of groundwater potential

The Vaalbank and Lady Frere areas belong to the Elliot, Molteno and Burgersdorp Formations, that are part of the sedimentary succession of the Beaufort Group in the Karoo Basin of South Africa. There is Quaternary alluvium covering the mudstone, sandstone and shale rocks of the Beaufort Group. Dolerite and gabbro sills of the Karoo age have intruded the whole area. A number of investigations were carried out for the study namely field observations, petrographic, X-ray diffraction, airborne gamma-ray spectrometry, aeromagnetic and gravity analyses. Twenty four rock samples were collected in the field from fresh outcrops of the Karoo dolerites, Burgersdorp and Molteno formations. The measured densities and porosities show that the Karoo dolerite suite has the highest average dry density of 2.771 g/cm3 and the lowest average porosity of 3%. The Burgersdorp Formation has an intermediate average density of 2.458 g/cm3 and an intermediate average porosity of 5.4%. The Molteno Formation has the lowest average density of 2.190 g/cm3 and the highest average porosity of 14.5%. Petrographic and geochemical investigations were carried on 19 rocks samples collected in the field to determine the mineralogy character using microscopic and XRD analyses. Both petrographic and X-ray diffraction analyses revealed that the sandstones of the study area are composed of quartz as their primary mineral and feldspars as the secondary mineral. The dolerite consists of euhedral structured minerals such as magnetite, pyroxene, plagioclase and amphibole. The gamma ray maps show high spectrometric values are associated with the Burgersdorp Formation, alluvium cover and dolerite dykes. The low spectrometric values are associated with dolerite sills and ring complexes. The boundaries of the sill/ring complexes are clearly delineated by the K/U ratio and ternary maps. The magnetic maps show ring like patterns of magnetic high anomalies that are due to the sills/ring complexes whilst the numerous, narrow, linear magnetic highs are due to faults and dolerite dyke swarms. The majority of the dykes trend in a NW-SE direction and a few dykes are in a NE-SE direction. The magnetic power spectrum indicates four depths of the magnetic sources at average depths of 0.13 km, 1.60 km, 2.83 km and 12.40 km. The magnetic depth iii slices maps show that both the circular/ring like and linear anomalies extend from shallow to deeper depth of 12.4 km. The gravity map is dominated by a long wavelength gravity anomaly that increases from about -120 mGals in the northwestern corner of the study area to -110 mGals in the southeastern corner of the study area. Results from 2½D gravity modelling show that this long wavelength anomaly is due to a horizontal Moho at a depth of 47 km in the study area, which shallows outside the study area near the coast. Superimposed on it are short wavelength gravity anomalies due to low density sediments and high density dolerite intrusions. The simple 2½D models generated from five profiles traversing the study area reveal that the sediments of the Karoo Supergroup extend from the surface down to a depth 5 km below sea level attaining a maximum thickness of about 6½ km. These sediments are dykes, sills and ring complexes. The dykes and sills/ring complexes identified from gamma ray spectrometry, magnetic and gravity data results are likely to be associated with fractures, joints, cracks and fissures in the host rocks, which form preferential pathways for groundwater transmission. Hence, they could be suitable targets for groundwater

Modal composition and tectonic provenance of the sandstones of Ecca Group, Karoo Supergroup in the Eastern Cape Province, South Africa

Petrography of the sandstones of Ecca Group, Karoo Supergroup in the Eastern Cape Province of South Africa have been investigated on composition, provenance and influence of weathering conditions. Petrographic studies based on quantitative analysis of the detrital minerals revealed that the sandstones are composed mostly of quartz, feldspar and lithic fragments of metamorphic and sedimentary rocks. The sandstones have an average framework composition of 24.3% quartz, 19.3% feldspar, 26.1% rock fragments, and 81.33% of the quartz grains are monocrystalline. These sandstones are generally very fine to fine grained, moderate to well sorted, and subangular to subrounded in shape. In addition, they are compositionally immature and can be classified as feldspathic wacke and lithic wacke. The provenance characteristics suggest the influence of plutonic and metamorphic terrains (meta-magmatic arc) as the main source rock with minor debris derived from recycled sedimentary rocks. The latter revealed that the compositional immaturity of the sandstones is a result of weathering or recycling and short transport distance. The weathering diagrams and semi-quantitative weathering index indicate that the Ecca sandstones are mostly from a plutonic source area, with climatic conditions ranging from arid to humid. The detrital modal compositions of these sandstones are related to back arc to island and continental margin arc. These results, therefore, support previous studies that infer foreland basin setting for the Karoo Basin

Grain size statistics and depositional pattern of the Ecca Group sandstones, Karoo Supergroup in the Eastern Cape Province, South Africa

Grain size analysis is a vital sedimentological tool used to unravel the hydrodynamic conditions, mode of transportation and deposition of detrital sediments. In this study, detailed grain-size analysis was carried out on thirty-five sandstone samples from the Ecca Group in the Eastern Cape Province of South Africa. Grain-size statistical parameters, bivariate analysis, linear discriminate functions, Passega diagrams and log-probability curves were used to reveal the depositional processes, sedimentation mechanisms, hydrodynamic energy conditions and to discriminate different depositional environments. The grain-size parameters show that most of the sandstones are very fine to fine grained, moderately well sorted, mostly near-symmetrical and mesokurtic in nature. The abundance of very fine to fine grained sandstones indicate the dominance of low energy environment. The bivariate plots show that the samples are mostly grouped, except for the Prince Albert samples that show scattered trend, which is due to the either mixture of two modes in equal proportion in bimodal sediments or good sorting in unimodal sediments. The linear discriminant function analysis is dominantly indicative of turbidity current deposits under shallow marine environments for samples from the Prince Albert, Collingham and Ripon Formations, while those samples from the Fort Brown Formation are lacustrine or deltaic deposits. The C-M plots indicated that the sediments were deposited mainly by suspension and saltation, and graded suspension. Visher diagrams show that saltation is the major process of transportation, followed by suspension

Geochemistry of sandstones and shales from the Ecca Group, Karoo Supergroup, in the Eastern Cape Province of South Africa: Implications for provenance, weathering and tectonic setting

Geochemical compositions of twenty-four sandstone and shale samples from the Ecca Group were analysed to decipher their provenance, paleoweathering conditions and tectonic setting. The shales have high Fe2O3, K2O, TiO2, Ce, Cu, Ga, La, Nb, Nd, Rb, Sc, Sr, Th and Y content more than the sandstones, whereas, sandstones are higher in SiO2, Hf and Zr than the shales. The positive correlations of Al2O3 with other elements as well as the abundance of Ba, Ce, Th, Rb, Zn and Zr suggest that these elements are primarily controlled by the dominant clay minerals. Tectonic discrimination diagrams revealed that the sandstones and shales are mostly of quartzose sedimentary provenance, suggesting that they were derived from a cratonic interior or recycled orogen. The binary plots of TiO2 versus Ni, TiO2 against Zr and La/Th versus Hf as well as the ternary diagrams of V-Ni-Th*10 indicate that the shales and sandstones were derived from felsic igneous rocks. A-CN-K (Al2O3-CaO-K2O) ternary diagram and indices of weathering (CIA, CIW and PIS) suggest that the granitic source rocks underwent moderate to high degree of chemical weathering. The CIA values range between 24.41% and 83.76%, indicating low to high weathering conditions. The CIW values for the studied sandstones and shales range from 25.90 to 96.25%, suggesting moderate to high intensive chemical weathering. ICV values for the sandstones and shales vary from 0.71 to 3.6 (averaging 1.20) and 0.41 to 1.05 (averaging 0.82), respectively. The k2O/Na2O ratios for the studied samples vary from 0.71 to 8.29, which reveal moderate to high maturity. The plot of CIA against ICV shows that most of the shales are geochemically mature and were derived from both weak and intensively weathered source rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance

Diagenesis and Reservoir Properties of the Permian Ecca Group Sandstones and Mudrocks in the Eastern Cape Province, South Africa

Diagenesis is one of the most important factors that affects reservoir rock property. Despite the fact that published data gives a vast amount of information on the geology, sedimentology, and lithostratigraphy of the Ecca Group in the Karoo Basin of South Africa, little is known about the diagenesis of the potentially feasible or economically viable sandstones and mudrocks of the Ecca Group. This study aims to provide an account of the diagenesis of sandstones and mudstones from the Ecca Group. Twenty-five diagenetic textures and structures were identified and grouped into three stages that include early diagenesis, burial diagenesis and uplift-related diagenesis. Clay minerals are the most common cementing materials in the sandstones. Smectite, kaolinite, and illite are the major clay minerals that act as pore lining rims and pore-filling materials. A part of the clay minerals and detrital grains was strongly replaced by calcite. Calcite precipitates locally in the pore spaces and partially or completely replaced clay matrix, feldspar, and quartz grains at or around their margins. Precipitation of cements and formation of pyrite and authigenic minerals occurred during the early diagenetic stage. This process was followed by lithification and compaction which brought about an increase in tightness of grain packing, loss of pore spaces, and thinning of bedding thickness due to overloading of sediments and selective dissolution of the framework grains. Mineral overgrowths, mineral replacement, clay-mineral transformation, dissolution, deformation, and pressure solution occurred during burial diagenetic stage. After rocks were uplifted, weathered and unroofed by erosion, this resulted in decementation and oxidation of iron-rich minerals. The rocks of the Ecca Group were subjected to moderate-intense mechanical and chemical compaction during their progressive burial. Intergranular pores, secondary dissolution, and fractured pores are well developed in the sediments of the Ecca Group. The presence of fractured and dissolution pores tend to enhance reservoir quality. However, the isolated nature of the pore linkage makes them unfavorable producers of hydrocarbons, which at best would require stimulation. The understanding of the space and time distribution of diagenetic processes in these rocks will allow the development of predictive models of their reservoir quality, which may contribute to the reduction of risks involved in hydrocarbon (oil and gas) exploration

Hydrological Evaluation of the Groundwater Potential in the Fractured Karoo Aquifer Using Magnetic and Electrical Resistivity Methods: Case Study of the Balfour Formation, Alice, South Africa

The study is aimed at evaluating the groundwater accumulations present in Alice using magnetic and electrical resistivity measurements to examine the trends of structural elements and characterize the groundwater resource for borehole drilling. The magnetic maps show a low magnetic linear structure moving northwest to southeast direction, which may be caused by fractures. The linear high intensities were probably caused by dolerite dykes, while dolerite sills caused broader high-intensity areas. The depth slices show that the near-surface magnetic structures are visible to a depth of about 19 m, and the deep-seated structures are found at a depth of about 31 m, possibly deeper. Twenty-five vertical electrical soundings (VES) of the Schlumberger array were measured with AB/2 varying between 1.5 m and 250 m across the study area. The VES interpretation showed four geoelectric layers composed of HK and HA curve types. The geoelectric layer’s thicknesses are (1) topsoil from 0.4 to 1.8 m, (2) weathered layer from 0.8 to 17.5 m, and (3) weathered/fractured layer from 9.9 to 143.9 m; the third layer could be the productive water-bearing zones, and (4) bedrock layer has an infinite thickness. The layers have resistivity values of 20-5752 Ωm, 3-51 Ωm, 136-352 Ωm, and 44-60428 Ωm, respectively. A correlation of the VES with the borehole log indicated a well-matched result. The magnetic and electrical resistivity surveys provided a detailed subsurface structure and helped identify possible fractures that could act as a passage for groundwater

Petrography, modal composition and tectonic provenance of some selected sandstones from the Molteno, Elliot and Clarens Formations, Karoo Supergroup, in the Eastern Cape Province, South Africa

The Late Triassic - Early Jurassic non marine clastic sediments of the Molteno, Elliot and Clarens Formations were studied to deduce their mineralogy and tectonic provenance. The study is based on road-cut exposures of the formations in the Eastern Cape Province of South Africa. Petrographic studies based on quantitative analysis of the detrital minerals shows that the clastic sediments (mostly sandstones) are predominantly made up of quartz, feldspars, and metamorphic and igneous rock fragments. Among the main detrital framework grains, quartz constitutes about 62-91%, feldspar 6-24% and 3-19% of lithic fragments. The sandstones can be classified as both sublitharenite and subarkose. Although, most of the sandstones (> 70 %) plotted in the sub-litharenite field. Petrographic and XRD analyses revealed that the sandstones originated from granitic and metamorphic rock sources. The QFL (Quartz-feldspar-lithic fragments) ternary diagrams indicate that the sandstones were derived from recycled or quartzose source rocks reflecting a craton interior or transitional continental setting which probably came from the Cape Fold Belt. This possibly revealed that most of the sandstones might have been derived as a result of weathering and erosion of igneous and metamorphic rocks in the Cape Supergroup. The study has revealed the depositional environments, and provide a basis for the description and interpretation of the sedimentology of the Molteno, Elliot and Clarens Formations

Diagenesis and rock properties of sandstones from the Stormberg Group, Karoo Supergroup in the Eastern Cape Province of South Africa

Diagenesis is one of the factors that affect reservoir quality in sandstones. Knowledge of diagenetic transformation and how it impacts the development of porosity in reservoirs rocks is thus key to successful mineral exploration. To date, little is known about the diagenesis of the uranium-hosted sandstones of the Stormberg Group, Karoo Basin, South Africa. Petrographic study, scanning electron microscopy (SEM) aided with energy dispersive X-ray (EDX), and X-ray diffraction (XRD) analyses were employed for the diagenetic study of the sandstones. This study aims to provide an account of the diagenesis of these rock units and how it impacts their reservoir quality. The diagenetic processes that have affected the sandstones are cementation, compaction, recrystallization, replacement, mineral overgrowth and dissolution. These processes have passed through early, late and uplift-related diagenesis. Formation of authigenic minerals and precipitation of the mineral cements occurred during different diagenetic phase but mostly during the early diagenetic stage. This stage was followed by lithification, which resulted in increased in grain packing, loss of pore spaces and thinning of bedding. Quartz and feldspar overgrowths, chloritization, muscovitization, concave-convex contacts, recrystallization, albitization and suture contacts are present in the late diagenetic stage. The uplift-related diagenetic stage was affected by calcitization, grain deformation and fracturing, dissolution and saussuritization. All these diagenetic processes largely affected the porosity and permeability of the sandstones. Primary and secondary porosities were both observed in the sandstones. In general, there is no single diagenetic process that is controlling the pattern of porosity evolution in the sandstones. On the other hand, the presence of fractured and dissolution pores tend to increase the porosity, thus enhancing the reservoir quality. This study shows that diagenesis has significantly altered the original petrologic characteristics of the sandstones which have induced considerable deterioration and heterogeneity in the reservoir quality of the Stormberg Group sandstones in the Main Karoo Basin of South Africa

Textural characteristics, mode of transportation and depositional environment of the Cretaceous sandstone in the Bredasdorp Basin, off the south coast of South Africa: Evidence from grain size analysis

A total of 92 representative sandstone samples of the Bredasdorp Basin in boreholes E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 have been investigated for their grain size characteristics. Grain size textural parameters and their cross plots, linear discriminate functions (LDFs), C–M (C = first percentile and M = median) diagram and log–probability plots were calculated and interpreted to understand the mode of transportation and hydrodynamic conditions and also to unravel the depositional environments of the sediments. The grain size textural parameters revealed that the Bredasdorp sandstones are unimodal, predominantly fine-grained, moderately well-sorted, mesokurtic and near symmetrical. The bivariate plots of grain size textural parameters indicate that the depositional environments had been influenced mainly by river/beach/coastal dune conditions. The LDF plots show that the sediments are turbidity current deposits in a shallow marine environment. The C–M diagram revealed that the studied sandstones were mainly deposited by traction currents and beach process. In addition, the grain size log–probability curves and C–M diagram show the predominance of suspension and saltation modes of sediment transportation. Based on the inter-relationship of the various statistical parameters, it is deduced that the Bredasdorp Basin are mainly shallow marine deposits with signature of beach and coastal river processes