Wladyslaw Altermann

Sedimentation rates, basin analysis and regional correlations of three Neoarchaean and Palaeoproterozoic sub-basins of the Kaapvaal craton as inferred from precise U–Pb zircon ages from volcaniclastic sediments

Abstract Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of 2588±6 Ma and 2549±7 Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of 2516±4 Ma for the upper Gamohaan Formation and 2555±19 for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

Abstract The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

Tidal flat deposits of the Lower Proterozoic Campbell Group along the southwestern margin of the Kaapvaal Craton, Northern Cape Province, South Africa

Lower Proterozoic stromatolites and associated clastic carbonate deposits of the Campbell Group, from the southern margin (Prieska area) of the Kaapvaal Craton, northern Cape Province, are described. Contrary to previous interpretations (Beukes, 1978; 1980a) shallow subtidal to supratidal facies are recognised and discussed in regional context. An alternative model for the facies development of the Campbell Group is proposed.

Sedimentology and facies development of an Archaean shelf: carbonate platform transition in the Kaapvaal Craton, as deduced from a deep borehole at Kathu, South Africa

Abstract A lithological description and a facies interpretation of Neoarchaean rocks, as penetrated by the 3672 m deep borehole at Kathu, Northern Cape Province, South Africa are presented here. The core penetrated all the formations of the Schmidtsdrif Subgroup and the overlying Campbellrand Subgroup of the Ghaap Group, Transvaal Supergroup. The core contains the longest continuous section (> 3000 m) through the worlds oldest giant carbonate platform and records invaluable information on Archaean development of sedimentary cover sequences. It is proposed that the core may serve as the type section for the Lower Transvaal Supergroup rocks in Griqualand West. In the core, the Schmidtsdrif Subgroup, comprising the Vryburg, Boomplaas and Lokammona Formations, is significantly thicker than deduced from surface outcrops. The lithology of these three formations is of mixed siliciclastic and carbonate composition and is thus similar to the outcrop lithology. The depositional conditions are interpreted as having been of shelf to subtidal character and appear to have been generally deeper than those inferred from the surface outcrops. The Campbellrand Subgroup is also of greater thickness in the core than known from outcrop. The Monteville Formation, at the base of the Campbellrand Subgroup, marks shallowing-upward conditions, followed by stromatolitic platform carbonates of the Reivilo, Fairfield, Klipfontein Heuwels, Papkuil, Klippan, Kogelbeen and Gamohaan Formations. Within these platform carbonates, several transgression-regression episodes are recognised. The overall inferred depth of deposition was similar to conditions interpreted from surface outcrops on the Ghaap Plateau, but perhaps with less tidal influence. Only for the Kogelbeen and Gamohaan Formations are somewhat shallower water conditions deduced compared to those previously interpreted from outcrops on the Ghaap Plateau, which is attributed to the Maremane upwarp. A model of a generally westward deepening basin, but with a locally developed palaeohigh at the Maremane Dome, is proposed to explain the inferred depositional patterns observed in the Kathu core and their differences to the surface outcrop facies.

Atomic force microscopy of Precambrian microscopic fossils

Atomic force microscopy (AFM) is a technique used routinely in material science to image substances at a submicron (including nm) scale. We apply this technique to analysis of the fine structure of organic-walled Precambrian fossils, microscopic sphaeromorph acritarchs (cysts of planktonic unicellular protists) permineralized in ≈650-million-year-old cherts of the Chichkan Formation of southern Kazakhstan. AFM images, backed by laser-Raman spectroscopic analysis of individual specimens, demonstrate that the walls of these petrified fossils are composed of stacked arrays of ≈200-nm-sized angular platelets of polycyclic aromatic kerogen. Together, AFM and laser-Raman spectroscopy provide means by which to elucidate the submicron-scale structure of individual microscopic fossils, investigate the geochemical maturation of ancient organic matter, and, potentially, distinguish true fossils from pseudofossils and probe the mechanisms of fossil preservation by silica permineralization.

The oldest fossils of Africa – a brief reappraisal of reports from the Archean

Abstract From the Archean sedimentary rocks of Africa numerous reports of cellularly preserved microfossils were published since 1960. Until the first report of authentic microfossils from the Barberton Greenstone Belt [M.M. Walsh, D.R. Lowe, Nature 314 (1985) 530], however, none of these reports stood up to the criteria for confirming authenticity and originality of Archean microfossils. According to these criteria, only less than 10 microfossil reports from the Archean of Africa can be recognised as indeed cellularly preserved microbes. All of them are from Southern Africa, namely from the territory of the Republic of South Africa. Nevertheless, these few occurrences provide evidence for a very advanced evolutionary stage of Archean microbiota and are thus key contributions to our knowledge of the early life on Earth. They demonstrate the presence of coccoid and filamentous bacteria and cyanobacteria in the Archean and also confirm the development of aerobic and anaerobic photosynthesis and of biogenic calcium precipitation in the Archean. Therefore, the contribution of the African continent to the knowledge of Archean biology and the early evolution of life is of immense value to paleobiological science.

A Chronostratigraphic Division of the Precambrian: Possibilities and Challenges

Abstract: This chapter provides a review of events through Precambrian Earth history, with the aim of providing an up-to-date foundation on which to construct a chronostratigraphic revision of the Precambrian time scale. The guiding principles used to develop a revised Precambrian time scale follow Cloud’s vision to “…seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe…”, and apply Gould’s historical principles of directionality and contingency.

Sedimentology, geochemistry and palaeogeographic implications of volcanic rocks in the Upper Archaean Campbell Group, western Kaapvaal craton, South Africa

Abstract Tuffs and lava interbedded in Campbell Group carbonates and shales are investigated for their sedimentological and geochemical properties. The most proximal tuffs occur at the southwestern margin of the Kaapvaal craton, in Griqualand West. They were deposited in a shallow-marine to tidal carbonate environment from hydroclastic eruptions. The tuffs along this margin are subdivided into two main facies types: (1) graded fallout-lapilli to ash tuffs, containing accretionary lapilli; and (2) current-reworked, laminated and rippled lapilli to ash tuffs. Calculation of water depth from settling velocities of tuff particles indicates deposition in depth of a maximum of some 30 m. The tuff beds generally thin out towards the north and northwest, but distal tuff layers are widespread in the Campbell Group and allow for some lateral correlation within regionally narrow limits. Single zircon age dates suggest that correlation over the entire basin is unreliable. Two geochemical groups of tuffs are tentatively distinguished. One group includes andesites and basalts and tends to concentrate in higher stratigraphic levels than the other, which consists exclusively of basalts. Volcanism was of a tholeiitic, within-plate character, situated in a rifted cratonic setting that failed to develop an oceanic basin. Geochemical similarity to some Karoo volcanic rocks is striking. A cratonic rift basin model for the Griqualand West depository in which carbonate sedimentation in shallow environments prevailed is discussed. The sedimentation of carbonates was periodically interrupted by hydroclastic eruptions along a rifting graben or half graben system.

A Chronostratigraphic Division of the Precambrian

Abstract: This chapter provides a review of events through Precambrian Earth history, with the aim of providing an up-to-date foundation on which to construct a chronostratigraphic revision of the Precambrian time scale. The guiding principles used to develop a revised Precambrian time scale follow Cloud’s vision to “…seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe…”, and apply Gould’s historical principles of directionality and contingency.

Process development for elemental recovery from PGM tailings by thermochemical treatment: Preliminary major element extraction studies using ammonium sulphate as extracting agent

Mine tailings can represent untapped secondary resources of non-ferrous, ferrous, precious, rare and trace metals. Continuous research is conducted to identify opportunities for the utilisation of these materials. This preliminary study investigated the possibility of extracting major elements from South African tailings associated with the mining of Platinum Group Metals (PGM) at the Two Rivers mine operations. These PGM tailings typically contain four major elements (11% Al2O3; 12% MgO; 22% Fe2O3; 34% Cr2O3), with lesser amounts of SiO2 (18%) and CaO (2%). Extraction was achieved via thermochemical treatment followed by aqueous dissolution, as an alternative to conventional hydrometallurgical processes. The thermochemical treatment step used ammonium sulphate, a widely available, low-cost, recyclable chemical agent. Quantification of the efficiency of the thermochemical process required the development and optimisation of the dissolution technique. Dissolution in water promoted the formation of secondary iron precipitates, which could be prevented by leaching thermochemically-treated tailings in 0.6M HNO3 solution. The best extraction efficiencies were achieved for aluminium (ca. 60%) and calcium (ca. 80%). 35% iron and 32% silicon were also extracted, alongside chromium (27%) and magnesium (25%). Thermochemical treatment using ammonium sulphate may therefore represent a promising technology for extracting valuable elements from PGM tailings, which could be subsequently converted to value-added products. However, it is not element-selective, and major elements were found to compete with the reagent to form water-soluble sulphate-metal species. Further development of this integrated process, which aims at achieving the full potential of utilisation of PGM tailings, is currently underway.