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Three basement trends, defined by the 1.0-0.5 Ga foldbelts of weak crust that wrap around the 1 Ga Namaqua-Natal Belt and >2.5 Ga Kaapvaal Province, provide a tub-shaped template that was impressed on succeeding structures up to the Cretaceous breakup of Pangea along the present divergent margins. The pattern is reprinted during the Ordovician-Devonian deposition of the Cape Supergroup in grabens on the northwest and northeast linked by an east-west depositional axis and during the Permian and Triassic development of the Cape Fold Belt along the eastwest trend linked with intermittent uplifts to the northwest (Atlantic upland) at a syntaxis around Cape Town and to the northeast (Eastern upland) at a syntaxis in the (restored) Falkland Islands.The inception of the Karoo (Gondwanan) Sequence in the latest Carboniferous (290 Ma) reflected the Gondwanaland-wide relaxation of the Pangean platform in sags (Karoo terrain) and rifts (Zambezian terrain). The first appearance of tuffs from a convergent arc in the Sakmarian (ca. 277 Ma) marked the onset of a foreland basin. Material derived from the south included a small component of mainly rhyodacitic tuff which persisted to the end of Beaufort deposition, when the presumed southern magmatic arc became extinct. Karoo deposition expanded northward over the interior beyond that of the confined pre-Gondwanan Cape Sequence. The axis of maximum thickness of the Permian-Triassic foredeep remained near the South Crop of the Karoo Basin; the parallel drainage axis migrated northward from an initial distance of 80 km during Dwyka deposition through 400 km during Ecca deposition and 550 km during Beaufort to a final 1,000 km during Stormberg deposition. The increasing separation of foredeep and drainage axis reflects the widening during the growth of the Cape Fold Belt of the southern depositional flank of the Karoo Basin at the expense of the starved northern cratonic side. Only during Stormberg deposition did the northern craton match the Cape Fold Belt as a source of voluminous sediment. Downloaded from 224 J. J. Veevers and Others Renewed and more extensive deposition in the Late Triassic corresponds to a singularity in Pangean history: terminal compression of foldbelts (Cape Fold Belt, Bowen-Sydney Basin, Canning Basin) and widespread subsidence, mainly in rifts that prefigured the divergent margins of the Atlantic and Indian Ocean regions. The subsequent Karoo volcanism reflects the increased activity of Pangean hotspots. This synopsis of the geologic history of southern Africa, south of 26°S, focuses on the Permian-Triassic Karoo Sequence of the Karoo Basin in its Gondwanan setting. The Karoo Sequence (Figs. 1-3) is a component of the Pangean Supersequence of the Gondwanaland cratonic province of Pangea (Veevers, 1990). In the latest Carboniferous (290 Ma), the Karoo Sequence started to accumulate in the oval-shaped Karoo Basin and narrow, fault-affected, Zambezian basins. From 277 Ma, the Karoo Basin became a foreland basin that subsided along a zone of inherited (pre-Gondw...

The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects.

ARTICLE INCLUDES: Supplementary material × Data set FUNDING:None South Africa intends to mitigate its carbon emissions by developing renewable energy from solar, wind and hydro, and investigating alternative energy sources such as natural gas and nuclear. Low-enthalpy geothermal energy is becoming increasingly popular around the world, largely as a result of technological advances that have enabled energy to be harnessed from relatively low temperature sources. However, geothermal energy does not form part of South Africa's future renewable energy scenario. This omission may be related to insufficient regional analysis of potentially viable geothermal zones across the country. We considered existing subsurface temperature and heat flow measurements and performed solutebased hydrochemical geothermometry to determine potentially anomalous geothermal gradients that could signify underlying low-enthalpy geothermal energy resources. We correlated these findings against hydro/geological and tectonic controls to find prospective target regions for investigating geothermal energy development. Our results show a significant link between tectonic features, including those oncraton, and the development of geothermal potential regions. In addition, potential regions in South Africa share similarities with other locations that have successfully harnessed low-enthalpy geothermal energy. South Africa may therefore have a realistic chance of developing geothermal energy, but will still need additional research and development, including new temperature measurements, and structural, hydrogeological and economic investigations. Significance:• The regional low-enthalpy geothermal energy potential of South Africa should be further researched for consideration of low-enthalpy geothermal energy as a renewable energy option. IntroductionSouth Africa is the leading carbon emitter in Africa and has one of the highest rates of emissions of nations in the world. 1 This status can be linked to South Africa's vast coal resources, which are an important contributor to the local mining sector and also account for more than 80% of South Africa's energy generation.1 South Africa intends to reduce its carbon emissions by producing about 40% of the country's total energy through renewable sources by 2030.1 This goal will be achieved mostly through solar-, wind-and hydro-generated forms of energy and largely accelerated by a Renewable Energy Independent Power Producer Procurement Programme, which has attracted considerable private-sector investment. 1 Renewable energy alone will not meet South Africa's growing energy demands and therefore the country will also consider additional large-scale coal-fired energy, nuclear energy and energy produced from shale gas. 1 Low-enthalpy geothermal energy is becoming increasingly popular around the world.2 This popularity is largely because it requires geothermal gradients as low as ca 40 °C/km, which may be found in many global settings. South Africa does not have any active or recent volcanism and is situat...

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