Showing papers in "South African Journal of Geology in 2016"

Th-U-Pb zircon geochronology of the Palaeoproterozoic Hartley Formation porphyry by six methods, with age uncertainty approaching 1 Ma.

Abstract: The Palaeoproterozoic Hartley Formation in the Olifantshoek Group was deposited in one of the rift-related Waterberg (sensu lato) red bed basins which formed on the Kaapvaal Craton after the 2.05 Ga Bushveld intrusions and coeval thermal event. The age of these basins is not well constrained due to the shortage of directly dateable rock types. The Hartley Formation contains rare quartz-porphyry lavas interbedded with the dominant basalts and these provide the means to date the formation by analyses of zircon. In this work zircon from one sample has been dated by six Th-U-Pb methods, namely Laser Ablation ICP Quadrupole Mass Spectrometry, Laser Ablation ICP High-resolution Mass Spectrometry, Laser Ablation ICP Multicollector Mass Spectrometry U-Pb (also Lu-Hf), Nordsim Ion probe U-Pb and Th-Pb; and Krogh method ID-TIMS. Our precise ages give a combined age of 1915.2 ± 1.1 Ma. Including one published ion probe date from the only other known occurrence of quartz porphyry, the results only agree if the quoted analytical errors are increased by 20%, which gives a combined result of 1915.6 ± 1.4 Ma. This is considered a reliable, precise and accurate age for the Hartley Formation and supersedes the published Kober method 207Pb/206Pb age of 1928 ± 4 Ma. The new Lu-Hf zircon data, supported by published whole rock Sm-Nd and Rb-Sr data, suggests that both the dominant basalts and the rare quartz porphyries of the Hartley Formation were derived from mafic source rocks which had been in the crustal domain from Archaean times. By contrast with the intracratonic rifts of the other Waterberg Basins, the Olifantshoek Supergroup reflects the development of a western passive margin as the Archaean Kaapvaal Craton rifted and drifted. This was followed by accretion of the Rehoboth Province along the Kalahari Line, accompanied by the development of the east-vergent Kheis Province thrust complex. This created a larger cratonic block against which the 1.2 Ga collisions of Namaqua-Natal terranes impacted. The Kheis Province now yields ~1.17 Ma cooling ages, reflecting the Namaqua collisions, but the true age of the Kheis event is still enigmatic.

Geological constraints on Archean (3.22 Ga) coastal-zone processes from the Dycedale Syncline, Barberton Greenstone Belt

Abstract: Geological constraints bearing on Paleoarchean surface environments are rare but critical, establishing the physiological consequences of a very different physical environment for early life, including a different tectonic style, closer Earth-Moon interaction, different chemical composition of atmosphere and hydrosphere, higher weathering intensity, and to validate geochemical proxy data. Strata relevant to these topics can be studied in uniquely accessible, subvertically dipping beds of the Moodies Group (ca. 3.22 Ga) within the tightly folded, steeply plunging western Dycedale Syncline of the Barberton Greenstone Belt. They display a large variety of environment-specific sedimentary structures in a small (ca. 1 km 2 ) but excellently exposed area, including microbial mats, abundant shoreline and tidal structures and early diagenetic pedogenic nodules. These are set among conglomerates, sandstones, minor lava flows and banded-iron formation within an initially deepening-, then shallowing-upward sequence, including (from base to top) braided-fluvial floodplain, sandy shoreline, and estuarine and protected tidal environments, some of which were temporally choked by volcanic ash. Shorelines were (semi-)arid, had fluctuating groundwater levels and moderate to high tides. Nevertheless, diverse microbial communities gained footholds in this challenging setting, including sulfate reducers in the shallow subsurface, photosynthesizers along high-energy shorelines, and photoferrotrophs or other Fe-metabolizing chemolithautotrophs in lagoonal or offshore settings. Facies analysis and regional structural geology suggest that the Dycedale area formed a minor, structurally controlled basin within a southward-facing fold-and-thrust belt which became intensely deformed during subsequent steepening. Overall, the datasets derived from this area allow the detailed description of interactions between early Earth’s volcanism, atmo- and hydrosphere and place the emergent biosphere into physical settings that are observable to such detail in only few other places worldwide.

Geology and U/PB geochronology of the Gamtoos Complex and lower Paleozoic Table Mountain Group, Cape Fold Belt, Eastern Cape, South Africa

Abstract: The Cape Fold Belt (CFB) along the southern coast of South Africa contains several tectonic windows that expose low-grade “basement” rocks as inliers that are inferred to be late Neoproterozoic in age subjected to tectono-metamorphism during the Pan African Saldanian Orogeny (ca. 650 to 550 Ma). Whilst a Saldanian tectonic history along the northwest to southeast trending western branch of the CFB has been well documented, such Neoproterozoic deformation along the east-west trending southern branch of the CFB is not established. This work presents new field observations and SHRIMP U/Pb geochronologic data from the Kleinrivier Sequence within the Gamtoos Inlier exposed along the eastern-most known tectonic window, near Port Elizabeth, and which is here renamed the Gamtoos Complex. The study area includes 3 tectonic sequences from the Gamtoos Complex (“pre-Cape”) and 2 sequences from the overlying Table Mountain Group of the Cape Supergroup. The lower sequences of the Gamtoos Complex comprise north-east verging thrust packages within a regional antiform, all of which have a common oriented planar tectonic fabric (S 2 ) coincident with those in the lower Paleozoic rocks. Phyllites from the Kleinrivier and upper Sardinia Bay sequences (spanning the pre-Cape and Cape boundary) are similarly affected by a slaty cleavage (S 1 ), crenulation cleavage (S 2 ) and multiple generations of quartz veins. U/Pb data are from zircons of metasediments and igneous rocks from the Gamtoos Complex and from the overlying rocks of the Table Mountain Group. Meta-greywackes of the Kleinrivier Sequence (youngest zircon: 523 ± 6 Ma) are probably Cambrian in age. However, some units from the Kleinrivier Sequence are intruded by felsic sills with a Concordia age: 530.2 ± 4.4 Ma and therefore require further U/Pb detrital zircon analyses. Mafic sills and dykes also intrude the Kleinrivier Sequence and were subsequently deformed and contain S 2 . Boulder conglomerates in the Sardinia Bay Sequence (youngest matrix zircon: 521 ± 6 Ma) are separated from the Kleinrivier Sequence by an angular unconformity. It also contains granite-boulders (Concordia age: 530.2 ± 4.7 Ma), and is overlain by the feldspathic psammites and quartzites of the Sardinia Bay Sequence with zircons as young as 510 ± 7 Ma (e.g. middle Cambrian). In total, 75% of the detrital zircons from all sequences date between early-Mesoproterozoic and early-Neoproterozoic (ca. 1455 to 852 Ma) and are likely sourced from the gneisses of the Namaqua-Natal province that are known to underlie the CFB, whilst the Neoproterozoic to mid-Cambrian zircons (ca. 828 to 431 Ma) have been sourced from Pan African mobile belts, possibly the Mozambican and/or Saldanian Belts to the east and west, respectively. Such “Pan African” detrital zircons are more prevalent in the Sardinia Bay Sequence and form the dominant component in the Lower Table Mountain Group, suggesting a lesser source influence from the Namaqua Natal Mobile Belt. The conglomerates at Sardinia Bay possibly represent mid-Cambrian rift sequences, similar to the Klipheuwel Group that overlie the Malmesbury Sequence and Cape granites along the western branch of the CFB. The Sardinia Bay Sequence has zircons equivalent to those of the Peninsula Formation (youngest zircon: 516 ± 5 Ma), and therefore should be linked to represent the lower Table Mountain Group.

Primary magmatic amphibole in Archaean meta-pyroxenite from the central zone of the Limpopo Complex, South Africa

Abstract: Mapping of a section along the Sand River in the high-grade metamorphic Central Zone of the Limpopo Complex, South Africa, delineated meta-peridotite, meta-pyroxenite and meta-gabbro occurring as conformable layers in the host supracrustal rocks. The ultramafic to mafic rocks form part of the Palaeoarchaean Messina layered intrusion. Modal mineralogy of the pyroxenite is typical of an amphibole websterite. Although metamorphic re-equilibration of the pyroxenite is obvious, no new metamorphic mineral had formed. Detailed evaluation of grain margins between the prominent minerals (amphibole, clinopyroxene, orthopyroxene and olivine) indicated two types of grain margins, with their mutual relation used to support the poly-metamorphic nature of overprint on the pyroxenite. Inclusion-host relations between the prominent minerals helped to infer the original igneous order of crystallization. Petrographic and mineral chemical analyses permitted distinction of two varieties of amphiboles, the earlier occurring as polygonal grains as well as inclusions in olivine and pyroxenes, and the later occurring along grain margins, often replacing clinopyroxene. Two varieties of magnetite were also recognized, an earlier Cr-rich variety occurring as inclusions and interstitial grains, and a later Cr-poor variety occurring together with serpentine. Application of geothermobarometers indicate metamorphic re-equilibration conditions (~830 to 879°C; 5.3 to 6.1 kbars). Comparison with compositional characteristics of primary and secondary/metamorphic amphiboles reported from Archaean ultramafic rocks occurring elsewhere, Alaskan-type layered intrusions, and metasomatised rocks from the vicinity of the study area, helped to place constraints on the nature of amphiboles reported here. The identification of primary magmatic amphiboles in the meta-pyroxenite implies likely formation from a hydrous parental magma in the Archaean mantle.

The effects of clay diagenesis on petrophysical properties of the lower Cretaceous sandstone reservoirs, Orange Basin, South Africa

Abstract: Authigenesis of clay minerals occurs during rock-fluid interactions in the subsurface, and inevitably occludes the pore spaces of clastic reservoirs. The presence of authigenic clay minerals defines the pore geometry of clastic reservoirs, which could impact on drilling and production of hydrocarbons by affecting subsurface pressure, and reducing hydrocarbon recovery, respectively. The latter has made clay diagenesis study a major component of reservoir quality assessment studies, and consequently an important area of research. Here we explore how clay diagenesis affects the basic petrophysical properties (porosity, permeability and saturation) of lower Cretaceous sandstone reservoirs of two wells (AU-1 and KH-1) along the west coast of South Africa. We use petrographical (SEM and thin-section analysis), geochemical (Pore water chemistry, XRD and CEC), and geophysical (wireline logs interpretation) tools in our study. Clay minerals present include kaolinite, illite, and montmorillonite. Illite occurs as the grain coating mineral, and kaolinite as pore filling mineral. Average porosity values of 8.5% (9.6%), effective porosity of 3.3% (9.5%), permeability range of 0.001 to 0.03 mD (0.005 to 0.2mD), and average water saturation of 51% (27%) from wells AU1 and KH1, respectively, were obtained. The low resistivity, poor porosity and permeability values recorded are attributed to the presence of the dominant clay minerals (illite, kaolinite and montmorillonite) within the reservoir intervals. Generally, the dominant cement in the sediment is quartz with calcite and pyrite in traces. The presence of calcite and glauconite in the reservoir rocks is an indication that the sediments were deposited in shallow marine conditions. It is concluded that all these minerals likely contributed to the low values encountered for porosity and permeability, as well as the moderate water saturation levels in the reservoir intervals.

Platinum-group element concentrations in base-metal sulphides from the Platreef, Mogalakwena Platinum Mine, Bushveld Complex, South Africa

Abstract: Base-metal sulphides (BMS) are important host minerals for platinum-group elements (PGE) in orthomagmatic Ni-Cu-PGE deposits. The PGE distribution and concentration in the BMS provide important genetic, exploration and mineral processing information. Therefore, detailed mineralogical work and an in situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) analysis of trace elements were conducted on the BMS of mineralised samples from the Overysel and Sandsloot localities of the Mogalakwena Platinum Mine. These localities access the Platreef of the northern Bushveld Complex. Platinum-group element enrichment trends in the BMS are similar in the Overysel and Sandsloot samples, although the overall PGE concentrations in the Overysel BMS are lower than those in the Sandsloot BMS. A general decoupling of the PGE from the BMS due to fluid activity, as advocated by other authors for the Sandsloot reef lithologies, was not observed in the samples studied. A major portion of the PGE mineralisation is associated with BMS as is indicated by the ‘moderate’ (Overysel r = +0.62) to ‘very good’ (Sandsloot r = +0.79) PPGE-S bulk-rock correlation, whereas an excellent PPGE-Ni correlation (especially at Overysel) indicates that pentlandite is a major PGE-carrier. These results are corroborated by the trace element contents of the BMS and by mass balance calculations. For instance, the LA-ICP-MS BMS study revealed average concentrations of 65 to 165 ppm Pd and 0.17 to 13 ppm Rh at Overysel, and 156 to 412 ppm Pd and 18 to 57 ppm Rh at Sandsloot in pentlandite of both the mineralized pyroxenites. In contrast, chalcopyrite and pyrrhotite are almost devoid of PGE. Platinum and the IPGE (Ir, Os, Ru) are very low in the BMS and, therefore, are thought to be hosted in discrete PGM grains. This conclusion is supported by the mass balance calculations for Pt and by the actual presence of PGM grains in both sample suites. The presence of discrete PGM grains explains the calculated gap between the whole-rock PPGE concentrations and the PPGE concentration in the mineral phases as revealed by the mass balance calculations. The PPGE concentrations in the BMS and the complementary mass balance calculations are in general agreement with those of the Merensky Reef and the UG-2, in both of which large proportions of the whole-rock Pd and Rh are hosted by pentlandite, whereas Pt and the IPGE mainly occur in discrete PGM rather than being hosted by BMS. However, comparison of our results with those of the Merensky Reef and the UG-2 indicates that the average maximum Pd and Rh concentrations in pentlandite are distinctly lower in the Platreef.

Lithostratigraphy of the Abrahamskraal Formation (Karoo Supergroup), South Africa

Abstract: The Middle Permian Abrahamskraal Formation is the lowermost formation of the lower Beaufort Group (Adelaide Subgroup, Karoo Supergroup) in the main Karoo Basin. It has been amalgamated with the lithologically similar and age-equivalent Koonap Formation and now occupies the entire southern part of the main Karoo Basin. It consists of greenish-grey and less common reddish-brown mudrock and subordinate light grey fine-grained sandstone, arranged in fining-upward, 1st to 3rd order cycles that range in thickness from a few metres to a few tens of metres. It reaches a maximum thickness in the southwest part of the basin (2200 to 2565 m) and thins northeastward with younging of the lower contact with the underlying Ecca Group and overlap of the overlying Middleton Formation onto the Ecca Group in the vicinity of Petrusville. The Abrahamskraal Formation contains several arenaceous zones in the southwest part of the basin, which increase in thickness towards a source area located along the southwest margin of the basin. The sedimentary facies represent deposition on a vast alluvial plain with lateral and downstream accretionary sand bodies in fluvial channels and floodbasin and subordinate lacustrine muds and silts in the extensive interchannel areas. Biostratigraphically, the Abrahamskraal Formation falls within the Tapinocephalus Assemblage Zone. However, along the southwestern margin of the basin, this zone is underlain by the Eodicynodon Assemblage Zone.

A geochemical contribution to the discussion about the genesis of impact-related pseudotachylitic breccias: Studies of PTB in the Otavi and Kudu Quarries of the Vredefort Dome support the “In Situ Formation” hypothesis

Abstract: This work is aimed at contributing to the understanding of the formation of massive pseudotachylitic breccias (PTB) in impact structures. In the past this has been debated as being due to either melting of locally available country rocks or a combination of injection of impact melt from a higher level of the impact structure plus assimilation of other, including locally derived, material. Two occurrences of massive PTB in the Otavi and Kudu quarries of the outer crystalline core of the Vredefort Dome (South Africa) have been investigated petrographically and chemically. As shown in many previous studies of PTB, lithic and mineral clast populations only support derivation of PTB from local precursor material (granitic gneiss and amphibolite in the case of Kudu Quarry and various granitoids and a dolerite/amphibolite component in Otavi Quarry). The new major and trace element chemical systematics of melt rock and possible local precursors are fully consistent with this petrographic finding: in both cases PTB chemistry is readily explained by derivation from directly adjacent country rock, or mixtures of locally occurring granitoids and amphibolite. Harmonic least-squares mixing (HMX) calculations also do not indicate that additional components, such as a Vredefort Granophyre-like impact melt intrusive phase, contributed to the formation of these PTB. Absence of evidence (such as significant displacements along PTB developments) for the origin of PTB by friction melting along significant faults/shear zones is also recorded. As melting immediately after shock wave propagation (end of early compression stage of cratering), which is widely considered as the genetic process for shock vein formation in impact-affected rocks, including meteorites, does not apply to formation of such voluminous PTB, only local melting due to rapid decompression upon central uplift formation, followed by melt pooling in dilation sites, can be called upon to satisfactorily address PTB formation.

Petrographic, geochemical and petro-physical analysis of the Collingham Formation near Jansenville, Eastern Cape, South Africa – potential cap rocks to shale gas in the Karoo

Abstract: The Lower Ecca Group of the Karoo Basin potentially contains significant unconventional gas, hosted in gasshales, of which the most significant sequence is the organic rich shales of the Whitehill Formation. The Whitehill Formation is directly overlain by clastic and volcaniclastic rocks of the Collingham Formation. Due to the potential for the upward migration of hydrocarbons from the gas-shales it is important to establish the integrity of the overlying formation. Here we report on the mineralogical, geochemical and physical properties of the Collingham Formation to test its ability to act as a cap rock to the underlying carbon-rich, potentially gas-bearing shales of the Whitehill Formation. Fresh core samples from a borehole drilled through the Lower Ecca Group near Jansenville, in the Eastern Cape, South Africa, were analysed for their mineralogical and geochemical content, using thin-section microscopy, XRD, XRF, TOC and SEM. The main rock types of the Collingham Formation contain three pore types (intrapore, interpore and microfractures) that are predominantly nanopores (<1 micron). The small size of pores, low permeability values, and their mineral content suggest that the Collingham Formation formed in a marine environment, subsequently modified through burial and regional metamorphism. Physical properties, determined through mercury porosimetry include porosity, permeability and density. Results show that the large proportion of clay minerals, a low TOC, the fine-grained nature of lithologies, a low porosity, a lack of permeability, a moderate fracturability, average density values, and the laminate nature of the formation, favour it as a suitable cap rock. We conclude that, although the characteristics of the formation favour it as a caprock, and therefore a sealing sequence to the Whitehill Formation, our results apply only to the Collingham Formation affected by the Cape Fold Belt. Further work is needed farther into the basin, to reveal its potential as a regional cap rock.

A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa – Part I: stratigraphic and structural features

Abstract: Who would have thought that the southern part of ‘stable, old South Africa’ was in fact ‘not-so-stable’, and that there has been spectacular tectonic activity in the Cape Fold Belt as recent as the early Holocene, about 10,327 ± 755 years ago? Large-scale surface rupture, with at least 2 m of vertical geomorphic offset, has been recorded along the Kango Fault over a lateral distance of at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the western end of the Baviaanskloof in the Eastern Cape. Although the Kango Fault scarp was reported in the mid-1970’s and late-1980’s, detailed palaeoseismic investigation was not undertaken until recently, when the chief energy supplier for South Africa, Eskom, rekindled investigations to establish a number of nuclear power plants along the South African coastline. This paper reports the results of the first palaeoseismic trench excavated across the Kango Fault during the initial phase of the nations’ current New Build programme. The location and date of the most recent event (MRE) is reported, as well as the style of faulting and local stress direction. These palaeoseismic data are derived from carefully-mapped logs of an 82 m long, 5 m deep trench, supported by 12 optically-stimulated luminescence (OSL) dates of stratigraphic horizons exposed in the trench. Various features of the stratigraphy, pedology and structure in the trench sidewall are described in detail, particularly those leading to the interpretation of the MRE. Development of a shear fabric in the cobble- to small boulder-sized alluvium at the main fault zone indicates that interpretation of local neotectonic fault style and stress direction should be made at least 4 to 5 m below the free face of the fault scarp. Retro-deformation of the trench log clearly defines the MRE and distinguishes apparent vertical displacement from the actual displacement of 2.0 ± 0.06 m. It also reveals that 1.9 m of surface extension occurred during the rupture, as well as the depositional processes that subsequently buried the 32 m wide graben formed at the site.

Termination of BIF deposition in the Paleoproterozoic: the Tongwane Formation, South Africa

Abstract: The Tongwane Formation (~2.4 Ga) conformably overlies banded iron formations (BIF; Penge Iron Formation) on the Kaapvaal Craton, South Africa. As such, it provides a unique window into depositional processes and environmental conditions in the aftermath of major Archean-Paleoproterozoic BIF deposition, and on the eve of irreversible environmental oxygenation in the Great Oxidation Event (GOE, ~2.35 Ga). This study presents the first sedimentological and bulk-rock geochemical characterization of the Tongwane Formation to provide a sedimentological and stratigraphic framework for further studies of early Paleoproterozoic environments. The Tongwane Formation is 220m thick and consists from the base up of shales, siliceous mudstones with local BIF facies, interbedded mudstones and dolomites, and a massive dolomite unit at the top. Strata record the progressive shallowing of depositional environments from deep shelf (BIF) to a wave-swept carbonate ramp. Intervening slope environments record increased detrital sedimentary input in the form of shales and distal turbidites. The carbonate ramp had a distally steepened margin as documented by an important margin collapse breccia. Extension due to seismic forces and/or slope steepening caused progressive deformation of slope deposits, from slumping and fracturing through sedimentary boudinage, to brecciation, and mass wasting. Termination of BIF deposition could have been related to (a) shutdown of Fe-precipitating processes, (b) shutdown of the hydrothermal Fe source, (c) shallowing of environments to restrict BIF deposition to deeper parts of the basin, (d) masking of Fe deposition by increased detritus, or a combination of these. Although a partial or complete shutdown of the Fe source or of Fe precipitating processes cannot be excluded, the weight of evidence from the Tongwane Formation favors external factors such as relative sea level fall and Fe dilution by increased detrital input as the main drivers for the BIF-carbonate transition. All samples fall on a mixing curve between hydrothermal and detrital end members, and despite metamorphic overprint, a weak hydrothermal signature is observed up to below platform deposits. These results stress the importance of understanding sedimentary factors in studies of Archean-Paleoproterozoic environments.

The Saldanha Bay Volcanic Complex: Clarifying the Cambrian geology of the Postberg - Saldanha Area, West Coast, South Africa

Abstract: Re-examination of the field relations and thin-section petrography of the rocks presently known as the Saldanha quartz porphyry show that the entire sequence, on the northern side of Saldanha Bay on the West Coast of South Africa, is pyroclastic in character. Thus, the area around Saldanha Bay is a large, volcanic complex consisting of the Postberg and Saldanha eruption centres. We propose that this complex, of Cambrian age, should be named the Saldanha Bay Volcanic Complex. The low-Ti Saldanha Ignimbrite and the high-Ti Jacobs Bay Ignimbrite are confined mainly to the Saldanha centre (probably a volcanic cauldron) and represent two chemically distinct series. The Jacobs Bay Ignimbrite magmas were emplaced throughout the eruptive history of the Saldanha centre but overly the uppermost sections of the more felsic Saldanha Ignimbrite. Small amounts of the Jacobs Bay and Saldanha Ignimbrites are also present in the Postberg centre but the geochemical data suggest that this structure is mainly filled with the more felsic and chemically distinctive Plankiesbaai and Tsaarsbank Ignimbrites.

Early Permian diamond-bearing proximal eskers in the Lichtenburg/Ventersdorp area of the North West Province, South Africa

Abstract: Diamond-bearing gravels of the Lichtenburg-Ventersdorp area of the North West Province are associated with north-south orientated sinuous ‘runs’ that occur almost entirely on a flat erosional surface of the Malmani dolomites (Transvaal Supergroup) at some 1,500 m elevation. East to west, this dolomite plain measures 150 km, and north-south it is on average 40 km wide. This unconformity, which first developed before the Pretoria Group sedimentation over a period of at least 80 Myr, is marked by siliceous breccias (palaeo-karst infill) and conglomerates (reworked breccias). It was exhumed in pre-Karoo and post-Gondwana times. Glacial pavements and remnants of thin Lower Karoo sediments are also found on this polyphase surface. The gravels that make up these ‘runs’ and sinkholes directly or indirectly linked to these runs, are coarse-grained, very poorly-sorted, and are best described as diamictites. The ‘runs’ are narrow, elongated, generally positive ridges that meander across the dolomite surface and are up to 30 km long and between 80 to 300 m wide. They have always been regarded as post-Cretaceous drainage features linked to southward-flowing river systems. Diamonds were discovered in these ‘runs’ and they have produced some 12 million carats. However, no Cainozoic fossils or artefacts have ever been found in almost 90 years of mining. From new field evidence, geomorphological studies, age dating from inclusions in diamond and zircon and clay analyses, it is proposed that these coarse-grained runs represent proximal palaeoeskers of the last deglaciation of the Dwyka continental ice sheet, that are preserved on this ancient ‘palimpsest’ surface. The age of the deposit is constrained by two populations of agate within the diamictites that are linked to two separate volcanic units of the Pretoria Group. In addition, the youngest crustal zircon ages from the gravels are 1 Ba, but mantle zircons from Lichtenburg suggest that these have been derived from Cambrian age kimberlites. Analysis of inclusions in diamond support a Neoproterozoic to Cambrian source for the diamonds, so the absence of diamonds from Mesozoic kimberlites and Cainozoic fossils within the gravels support the conclusion that the runs are of Karoo age.

Palaeoenvironment and provenance in the early Cape Basin of southwest Gondwana: sedimentology of the Lower Ordovician Piekenierskloof Formation, Cape Supergroup, South Africa

Abstract: The lowermost unit of the Table Mountain Group (Cape Supergroup), the Ordovician Piekenierskloof Formation is a siliciclastic succession that formed during the initial stages of the Cape Basin development in southwest Gondwana. This contemporary sedimentological re-evaluation of the depositional history and provenance of the Piekenierskloof Formation provides new insights into the palaeoenvironment and tectonic setting of the early Cape Basin. Outcrop-based sedimentary facies analysis revealed distinctive depositional features, dominated by large trough cross-bedding, which suggest a shallow, perennial, sand-bed braided fluvial style with bedload transportation in roughly 1 to 2.5 m deep and relatively wide channels. This fluvial style was possibly influenced by the global absence of substrate stabilizing terrestrial flora in pre-Devonian rivers. Based on an integrated approach of clast count, framework petrography and palaeocurrent studies, the provenance analysis show that the source rocks of the Piekenierskloof Formation were likely situated in source areas characterized by two main rock types: (1) a dominant low metamorphic rank metasedimentary rock suite with parent rocks of variable grain-size, and (2) minor plutonic rocks. The data also suggest that the parent rocks were locally intruded by quartz veins and/or pegmatites. Consistent palaeoflow to the southeast and east further indicates that these source areas were located to the northwest and west of the clastic sediment depocentres in the Cape Basin of southwest Gondwana in the Early Ordovician. Accessible outcrops of the Piekenierskloof Formation are mostly laterally limited and sparsely distributed, which hinder stratigraphic correlations, and therefore inferences of tectonic activity during the sedimentation in the Early Ordovician cannot be convincingly made. For a better understanding of the Early Palaeozoic evolution of the Cape Basin in southwest Gondwana, systematic outcrop studies acquiring high-resolution digital imagery using unmanned aerial vehicles (UAVs) and terrestrial laser scanning, coupled with subsurface facies analysis should be undertaken. The Piekenierskloof Formation and the rest of the Table Mountain Group, with their exceptionally extensive, but rather inaccessible cliff faces, are excellent candidates for such high-resolution digital-based studies.

Multi- and hyperspectral spaceborne remote sensing of the Aggeneys base metal sulphide mineral deposit sites in the Lower Orange River region, South Africa

Abstract: New tools and algorithms for geological remote sensing are developed and verified at test sites throughout the world in preparation of the German hyperspectral satellite Mission (EnMAP), which is an Environmental Mapping and Analysis Program. The Aggeneys Cu-Pb-Zn deposit, situated in the arid north western part of South Africa, represents a unique field laboratory for testing these new tools. Here spaceborne hyperspectral data covering the Swartberg, Big Syncline and Gamsberg area were collected by the Hyperion sensor. New synergies between multispectral and hyperspectral spaceborne data can be demonstrated, such as the Iron Feature Depth index (IFD), which has recently been proposed for mine waste mapping in the North West Province of South Africa and for gossan detection at Haib River in South Namibia. The work presented here explores the potential of the IFD for gossan mapping and characterization at Gamsberg and Big Syncline, from EO-1 ALI and Landsat-8 OLI data together with mineral maps from expert systems such as the United States Geological Survey (USGS) Material Identification and Characterization Algorithm (MICA), and first results from EnMAPs EnGeoMAP algorithm. Field spectroscopic measurements and field sampling were carried out to validate and calibrate the results from the expert systems and the IFD. This ground truthing is a necessary complementary step to link the results from the expert systems and the IFD to in-situ field spectroscopy. Future mineral exploration initiatives may benefit from the techniques described here, because they can significantly narrow the expensive, exploration activities such as hyperspectral airborne data, field activities and drilling, by identifying the most promising mineral anomalies in an area from the spaceborne data.

A deep electrical conductivity structure of the southern Barberton Greenstone Belt, South Africa, derived from magnetotelluric measurements

Abstract: The Barberton Greenstone Belt (BGB) in South Africa is one of the few well-preserved, albeit deformed and complex volcano-sedimentary remnants from the Paleoarchean, and thus an excellent locality to study the formation and evolution of the early Earth’s crust. Due to the significant amounts of resources, especially gold in shear zones, the BGB has been extensively studied by geologists for almost 100 years. While the surface geology is well known, only a few geophysical studies have been conducted to investigate the deeper architecture of the BGB and its granitoid surroundings. Here we describe the results of a Magnetotelluric (MT) survey that was conducted over two field seasons to image the subsurface electrical conductivity distribution of geological units of the southern BGB, and to locate dykes, faults and shear zones that are imprints of subsequent tectonic processes. Specifically, mineralization along the shear zones is predicted to reveal high electrical conductivities, in contrast with highly resistive adjacent mafic to ultramafic rocks. The MT station layout of our survey was planned to allow for 2D and 3D interpretation, although it was expected that the 2D inversion models might not be adequate to reveal the expected complex subsurface geology of the BGB and its surrounding region. However, both 2D and 3D inversion results show electrically conductive structures that appear to correlate well with surface traces of known fault zones such as the Inyoka-Saddleback fault system. High resolution 2D conductivity images along selected profiles suggest that some faults might continue further south into the granitoids of the Mpuluzi batholith, implying that the batholith was emplaced along faults (Inyoka-Saddleback fault system and/or Komati Fault), or that a younger fault cuts across the pre-existing batholith. This is contrasted by 3D models that reveal deep-reaching (>10 km) resistive structures beneath the intrusive bodies within the BGB and surrounding batholiths. These results suggest that the granitoids are not disrupted by shear zones, and may imply that episodes of predominant magmatic emplacement have affected the BGB in large parts. A network of conductive faults, especially in the central part of the BGB, suggests that tectonic processes along shear planes have also shaped the BGB, and may have provided pathways for fluids creating zones of gold mineralization.

Statistical evaluation of seismic event location accuracy by the South African National Seismograph Network over four decades

Abstract: The South African National Seismograph Network operated by the Geophysics Competency of the Council for Geoscience.

Evaluating crop yields, crop quality and soil fertility from organic and conventional farming systems in South Africa’s southern Cape

Abstract: Climate change, water scarcity and degraded soils jeopardise the ability of agriculture to ensure food security. The dependence on agrochemicals and monoculture practices in agriculture is not only environmentally destructive and expensive; these practices also produce foods of poor nutritional value. Here we report on the initial results of a long-term comparative controlled experiment at the NMMU Saasveld campus in George to test the differences in yield and nutritional quality of crops grown under different agricultural practices. A baseline study showed considerable variation in the soil fertility of the experimental site. As anticipated from the soil analysis results, the baseline crop had varying growth from one replication to the other. Initial organic treatment yields were lower and initial nutritional levels were inconclusive. The experimental site accommodates a randomised complete block design with four replications for three agricultural treatments: conventional, organic and controlled. The crops included cabbage ( Brassica olaracea, var. capitata L. ), sweet potato ( Ipomea batatas ) and cowpeas (Vigna unguiculata). Our results show that the cabbage and cowpea yields grown under organic conditions were 20% and 24% less than conventional yields, respectively.

Development of a conceptual geohydrological model for a fractured rock aquifer in the Karoo, near Sutherland, South Africa

Abstract: An aquifer in the semi-arid environment of Sutherland, South Africa is examined. The characterization of the saturated geological unit was completed by mapping associated dolerite dykes and fractured outcrops of Beaufort Group sedimentary rocks. This was complimented by pumping tests in order to better understand the fracture connectivity that was found to decrease with distance from a prominent dyke. Groundwater samples and precipitation samples in the study area were analysed for stable isotopes. Drill chips of selected boreholes were examined in order to also better understand the nature of the subsurface at the contact of the dolerite and a suspected subsurface feature identified through satellite imagery. The results are combined into a conceptual geohydrological model to gain insight into the groundwater flow dynamics of the area. Findings suggest preferential recharge occurs in the study area with higher hydraulic infiltration related to the river beds and fracture sets in the study area.

A proposed mathematical model of thermal variations on the HartRAO Lunar Laser Ranging telescope for enhanced test of Earth-Moon system dynamics

Abstract: A new geodetic observatory at Matjiesfontein will house a 1-metre aperture Lunar Laser Ranging (LLR) optical telescope to acquire millimetre Earth-Moon distance measurements. Large optical telescopes utilised for laser ranging observations of Earth-Moon distance, with sub-centimetre level accuracy can suffer from structural deformations induced by thermal variations, wind loadings and/or gravitational forces. These variables need to be measured to enable correction of the resulting pointing errors. This paper presents the proposed locations of temperature sensors on the composite structure of the Hartebeesthoek Radio Astronomy Observatory (HartRAO) LLR 1-metre optical telescope. Furthermore, we describe a mathematical model for (i) obtaining thermal measurements from a network of temperature sensors, (ii) interpolating the temperatures across the telescope composite structure, and (iii) predicting thermally-induced structural deformations. Currently, the available thermal simulation results of the LLR telescope tube assembly indicate isotherms with gradients of about 1°C followed by thermal deformations that vary between 2.9 μm and 40.7 μm along the optical tube axes. The physical implementation of the above mathematical model is part of the next phase of this research work coupled with experiments to derive thermally-induced pointing errors. The errors will be incorporated into the steering control software to compensate for thermally-induced misalignment of the telescope axes.

Fractal geometry of the fault network across the Soutpansberg Mountains, Limpopo, South Africa

Abstract: Earth systems and landforms are defined by complicated patterns and structures that hold key information for many important naturally-controlled systems (e.g. minerals, energy and natural hazards). A deeper understanding of these systems remains elusive, in part due to the complexity in defining their geometry. Fractal analysis (FA) provides a method of characterising patterns that are seemingly complicated beyond conventional geometry principles. FA has been actively applied to faulted regions in an attempt to resolve multi fracture-controlled processes. In this study we apply FA to the Soutpansberg Mountains (SM) to better characterise its extensive fracture network (most notably regional faults) covering many orders of scale, and to test whether the results mimic known geological complexities. In addition, we test whether there is any link between the fractal geometry and the occurrence of Cu-mineralisation and groundwater hot springs. We apply the box-counting technique to determine the fractal dimension (D), lacunarity (LC) to quantify the concentration (density) of fracturing, and test for multifractal (MF) affinity. The results of the FA correspond well with the known complex geology across the SM; areas with higher D-values and an affinity toward MF behaviour correspond to regions of the SM displaying at least two fault orientations with possibly two directions of shear displacement (i.e. normal and transcurrent). In addition, lower LC values correspond to regions of the SM that exhibit a greater degree of faulting. Also, regions with higher fractal complexity relate to areas with the highest concentration of Cu-mineralisation and highest-temperature hot springs. This suggests that principles of fractal geometry play a role in the interconnectivity within the brittle upper crust. We thus confirm that FA can be applied together with regional strain controls to better understand dynamics of natural and induced fracture propagation and recommend that this be further applied to exploration within the Earth’s Critical Zone.

Magnetic evaluation of the palaeothermal variation across the Karoo Basin, South Africa

Abstract: The Karoo Basin of South Africa is of economic importance for its large coal reserves but has in recent years also been in the spotlight due to the possibility of extensive shale gas reserves. Reconstruction of the thermal history of the Karoo Basin is essential for evaluating the potential hydrocarbon generation within this Late Carboniferous – Middle Jurassic sedimentary basin. Magnetic techniques provide an alternative approach in comparison to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper variations in the thermal history across the Karoo Basin as a result of heating by the Karoo LIP are evaluated using different magnetic “geothermometers”. These include palaeomagnetism (baked contact test), thermomagnetic analysis (alteration index method) and anisotropy of magnetic susceptibility (AMS). Although these techniques were successful in identifying a variation in metamorphic effects adjacent to contact aureoles, only the alternating index (A 40 ) provides a means of estimating peak temperatures. Our results indicate a regional elevation of palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. This study shows that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining in strata between these intrusions. An increase in the paleotemperatures from 200°C in the southwest to 400°C in the northeast of the basin is observed. We hypothesize that this trend is mainly due to differences in thermal conductivity of the different sedimentary rock types across the basin as the Karoo Basin transgresses from tight low porosity marine shales in the south and southwest towards more lacustrine mudstone and porous sandstone towards the northeast.

Petrology, geochemistry and fluid inclusion analysis of altered komatiites of the Mendon Formation in the BARB4 drill core, Barberton greenstone belt, South Africa

Abstract: The 3.33 to 3.26 Ga Mendon Formation in the Palaeoarchaean Barberton greenstone belt, South Africa, forms the uppermost unit of the Onverwacht Group. It is dominated by ultramafic volcanic rocks interbedded with thin layers of cherty sediments that show pervasive alteration, including widespread serpentinisation, silicification and chert and quartz veining. The BARB4 drill core of the ICDP Barberton drilling project exposes a unique section through the Mendon Formation in the Manzimnyama Syncline. The komatiites are pervasively altered to an assemblage comprising quartz, chlorite, carbonate, talc, biotite, and, locally, plagioclase, K-feldspar, muscovite, amphibole, stilpnomelane and ankerite. The overlying sediments are made up of banded iron formations and rare beds of siliciclastic rocks. Though the altered komatiites are pervasively silicified, SiO 2 contents do not exceed 58 wt.%, and their major and trace element geochemistry is similar to other komatiitic rocks of the Mendon Formation, particularly those of the M2v-member. Quartz veins and, less commonly, quartz-carbonate and quartz-carbonate-plagioclase veins are found throughout the core. Overall, composition and texture of the veins differ from primary and early diagenetic veins found in silicified komatiites elsewhere in the Barberton greenstone belt. In the BARB4 drill core, the veins are generally coarse-grained, and the immediate wall rocks are locally foliated along the vein margins. In addition, the δ 18 O values of vein quartz range from 14.1 to 15.3‰, significantly lower than the values typically found in veins on the modern seafloor that formed during low temperature hydrothermal seafloor alteration (~22 to 32‰). Fluid inclusions in vein quartz are homogeneous two-phase (L+V) aqueous inclusions that occur in trans- and intragranular trails and clusters. Intragranular and isolated fluid inclusions have a similar homogenisation temperature (T h ) of 130 to 200°C, with most data ranging between 145 and 175°C. Salinities cluster in three different groups of high (20 to 27wt.% NaCl equiv.), medium (10 to 15wt.% NaCl equiv.) and low salinity (0.3 to 1.5wt.% NaCl equiv.). The composition and microthermometric characteristics of the fluid inclusions analysed within the drill core show similarities to those found in quartz veins in silicified komatiites of the Mendon Formation, which are interpreted to have been entrapped during metamorphism. P-T calculations based on fluid inclusion microthermometry reveal conditions of 230 to 400 MPa and 250 to 400°C. Similar conditions of 240 to 270°C have been obtained using oxygen isotope thermometry, assuming a metamorphic fluid with a δ 18 O value of 6‰. Collectively, the δ 18 O values, together with the texture and composition of the veins, are interpreted to indicate a metamorphic origin of the veins. The presence of high salinity inclusions indicates the occurrence of a highly saline fluid that locally mixed with the dominant lower salinity fluids. The high salinity might have been derived from fluid circulation through evaporites.

A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa– part II: earthquake parameters

Abstract: The location and estimated magnitude of a recent surface rupturing earthquake along the eastern part of the Kango Fault traversing the southern Cape Fold Belt, South Africa, is reported here, as well as the minimum recurrence interval and maximum slip rate. These palaeoseismic data are derived from analysis of the mapped logs of an 82 m long, 5 m deep trench excavated across the fault, supported by 12 optically stimulated luminescence (OSL) dates. The tectonic event that formed the observed 2.0 ± 0.06 m free-standing fault scarp at the trench site had a moment magnitude estimated between 6.97 ± 0.01 M and 7.18 ± 0.01 M. According to the recently established Environmental Scale of Intensity, the rupture was devastating (ESI Io = XI). It would have been accompanied by strong ground shaking felt across most of South Africa by the ancestors of the indigenous people groups who experienced the well-known 6.3 M Ceres-Tulbagh event of 29 September 1969 (currently South Africa’s largest and most damaging earthquake). This latter strike-slip event occurred along the western end of the same Ceres-Kango-Baviaanskloof-Coega (CKBC) fault system, but was not accompanied by significant surface deformation. In contrast, the Kango ‘Toorwater’ earthquake reported here displaced the land surface vertically, by over 2 m in places. While the fault scarp extends laterally for at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the start of the Baviaanskloof in the Eastern Cape, it is unlikely the entire surface rupture length originated during only the most recent event. The palaeoseismic data reported here have contributed to a seismic source characterization model used to update the existing seismotectonic model for South Africa, first established in the mid-1990’s, and revised at various intervals since. The presence of a potentially active, capable, seismogenic structure within a low-seismicity intraplate stable cratonic region suggests that the current 5.3 to 6.3 M m max for the south and south eastern Cape must be reviewed, should these data be included in a revised Probabilistic Sesimic Hazard Assessment (PSHA) for critical structures in the region.

Lithostratigraphy of the Floriskraal Formation (Witteberg Group), South Africa

Abstract: The Lower Carboniferous, probably Tournaisian, Floriskraal Formation is part of the Witteberg Group (Cape Supergroup) of South Africa. Together with the underlying Kweekvlei and overlying Waaipoort Formations, it forms part of the Lake Mentz Subgroup. The Floriskraal Formation essentially comprises mud-, silt- and sandstones arranged as two-to-four laterally continuous upward coarsening successions suggested to have been deposited in a storm-wave dominated shallow-marine setting. It contains a low diversity trace fossil assemblage, reworked vascular plant debris, and possibly acanthodian fish fossils. There are no know stratigraphic equivalents in South Africa.

Iphakade is Earth Stewardship Science

Abstract: ### Inkaba yeAfrica and !Khure Africa – the past Two collaborative bilateral Earth System Science Programmes - Inkaba yeAfrica and !Khure Africa - have been active for 12 and 6 years with Germany and France, respectively, and have been generously supported by DST and NRF throughout this time (see www.inkaba.org ). More recently the two programmes merged into one as the bilateral spirits of these programmes started to fade. It is therefore timely to move on and establish a South African Programme that can stand on its own, and attract international collaborations on the basis of its indigenous research excellence in one of Earth’s most attractive natural and enigmatic social laboratories. From a small beginning in 2004 with 10 students, the programmes 10 years later catered for 181 students in some 31 projects; and by 2015, 224 students had graduated. The largest student participation continues to be at MSc level (with a ratio of Honours:MSc:PhD at 1:6:5). In terms of gender, the total number of female students in 2014 was 43%; and 64% of the students were black. On average, the cost of running this program over a decade has been about R24k per student, including 1 or 2 student conferences/workshops per annum. The success of the project to-date has largely been due to dedicated mentoring by senior researchers and university support (in 2014, 12 South African universities and the Council for Geoscience participated); and to the near 24/7 ukuphatha (liaison) and management for the student interests by Ms Elronah Schaap over the last decade. Building on the two bilateral programmes we have now combined the research goals and capacity building aims of the two Earth Systems Science programmes into one unified endeavour that we have called –’Iphakade’ – an isiKhosa word that best encapsulates ‘Earth Stewardship Science’. It is perhaps a reflection of the dedications and hard work …

Using Sm-Nd systematics to constrain sediment sources of the Ikorongo Group, north-eastern Tanzania

Abstract: Shales and siltstone samples from the Ikorongo Group of north-eastern Tanzania yield T DM ages of between 2049 Ma and 2597 Ma and ɛ Nd (0) ranging from ca Ȓ283 to Ȓ173 These ages are interpreted as representing the mean mantle extraction ages of the protolith of the Ikorongo rocks The data are consistent with most of the detritus being derived from lithologies of the Archaean Musoma-Mara Greenstone Belt The T DM data also suggest participation of the Mozambique Belt rocks, although less in proportion when compared to the cratonic protolith Mixing calculations indicate that the samples’ protolith can be modelled as mixtures of detritus from cratonic granitoids (422%) and andesites (404%) of the MMGB with those from Proterozoic granulites (174%) of the Mozambique Belt The fact that the basin received material from the Pan-African Mozambique Belt, which is known to have cooled below ~300°C at about 540 Ma, points to a Cambrian or younger age for deposition of the Ikorongo strata

Mineralogical and geochemical characteristics of emeralds from the Leydsdorp area, South Africa

Abstract: The world’s oldest emerald deposit occurs in the Gravelotte-Leydsdorp area of the Limpopo Province of South Africa. Emeralds have been mined from the main emerald working, the Cobra pit, since 1927, although emeralds were discovered in the area much earlier. The emerald mineralisation is closely linked with albitite pegmatites, with the emeralds confined almost entirely to metasomatic zones at the margins of strongly deformed pegmatoid bodies in biotite, talc and actinolite schists. While the surrounding Murchison greenstone belt has been extensively studied, very little geochemical information is available for the emeralds in the deposit. This paper documents the geochemical characteristics of 57 low-quality emeralds from the Leydsdorp area, Murchison Greenstone Belt. The emeralds range in size from 3 to 12 mm and were studied using both visual examination and electron microprobe analysis with 960 microprobe analyses on 54 specimens. The emeralds in the present study are subhedral to euhedral with a well-formed prismatic habit, and range in colour from light green to a dark green or bluish green. A number of samples are characterised by colour zoning, typically comprising of a green to dark green transition from core to rim, while small colourless areas within crystals are also seen. Many of the emeralds examined are practically opaque owing to the presence of very abundant mineral (and fluid) inclusions. Numerous small cracks and veins are also present. A wide variety of inclusion species are encountered, comprising dominant biotite, apatite and quartz, as well as a host of other minerals including hematite, pyrite, pyrrhotite, zircon, molybdenite and galena. SiO 2 content varies between 64 and 67 wt%, Al 2 O 3 range from 13 to 17 wt%, MgO concentrations vary between 0.08 and 3.46 wt% and Na 2 O contents range from 0.76 to 2.33 wt%. The Cr content of Leydsdorp emeralds range up to 1.07 wt% Cr 2 O 3 , with an average of 0.23 wt%. Samples are often heterogeneous with a maximum variation of 0.17 to 0.9 wt% Cr 2 O 3 observed in one specimen. Darker green crystals and dark green patches in individual crystals have significantly higher Cr contents compared to light green crystals and crystal sections. An inverse relationship between Al and the amount of Cr, Mg, Na and Cs is seen, while substitution of Al by Fe is also evident. The major and minor element chemistry of the emeralds are broadly similar to other emerald deposits worldwide. The major chromophore appears to be Cr as opposed to V. Na contents are somewhat higher than the world-wide average and this, along with the inclusion suite, could serve to distinguish these emeralds from other emerald deposits.

Pore pressure prediction of some selected wells from the Southern Pletmos Basin, offshore South Africa

Abstract: An accurate pore pressure prediction is vital in any risk management plan during the exploration, exploitation and development of wells in the petroleum industry. It is therefore important to understand the factors controlling these pore pressures namely, sediment compaction, overburden, lithology characteristics, hydrocarbon generation, tectonic stress, thermodynamic effects, osmosis and clay mineral transformation, which are all influenced by physical, geological, geochemical and mechanical processes. In this study we present the results of data selected from three wells: Ga-N1, Ga-W1 and Ga-AA1, which were all drilled within the Southern Pletmos sub-basin, offshore South Africa. The pore pressure in the wells was predicted using the Ben Eaton method by creating a depth dependent Normal Compaction Trend (NCT), using resistivity and sonic wireline logs. 2D seismic data were used to develop depth imaging of the pre-drilling pore pressure predictions; this was done using seismic interval velocity data and pressure data to obtain a reflection tomography extraction grid map. Detailed depth plots of overburden gradient (OBG), the Effective Stress (ES), Fracture Gradient (FG), Fracture Pressure (FP), Pore Pressure Gradient (PPG), and the Predicted Pore Pressure (PPP) were thus derived for the three wells. The overburden density varies from 2.09 gm/cc to 2.24 gm/cc between the wells, while the PPP changes from 3,405 psi to 5,062 psi within the selected reservoir intervals. The results show that most of the reservoir intervals of the three wells experience normal pore pressures (between 3000 and 4000 psi), except the reservoir intervals between depths 1868.73 m to 1875.40 m and 1880.3 m to 1887.3 m, respectively, in well Ga-W1, where significant overpressure in the pores exceeding 4000 psi was demonstrated (normal pore pressures are between 3000 and 4000 psi). The tomography extraction grid map that was generated from 2D seismic data of the wells was used to delineate the depth imaging of the pore pressure, in both overpressure and normal pressure formations before drilling, yielded good results.

Breccia formation during intrusion of a dolerite sill: an example from Sheffield Beach, KwaZulu-Natal North Coast, South Africa

Abstract: The breccia documented here is the first recorded occurrence of a sub-volcanic dolerite breccia developing due to the intrusion of overlapping dolerite sill segments in the Karoo Igneous Province (KIP) along the KwaZulu-Natal North Coast, South Africa. Recognising these breccias can assist in indicating the crustal level at which the dolerite sills intruded, as fragmentation of the country rocks will not occur beyond a certain depth. The breccia developed when two overlapping sill segments intruded into water-saturated Permian country rocks creating a laterally-extending dolerite-hosted breccia. The characteristics of the breccia, such as clast angularity and size, have been used to identify the fragmentation process of the rocks and infer the environment where it formed. Fragmentation of the lithified country rock is considered to be a consequence of heating between the sills causing vapourisation of the pore fluids, resulting in a dramatic increase of pressure beyond the yield strength of the rock and lateral propagation of the fragmentation. This brecciation process indicates that the dolerites intruded at a high crustal level and were thus intruded into a thin or thinning crust, possibly related to the rifting phase of the breakup of the supercontinent Gondwana.