Boris F.F. Reczko

The sedimentary and tectonic setting of the Transvaal Supergroup floor rocks to the Bushveld complex

Abstract The Palaeoproterozoic Transvaal Supergroup floor to the Bushveld complex comprises protobasinal successions overlain by the Black Reef Formation, Chuniespoort Group and the uppermost Pretoria Group. The protobasinal successions comprise predominantly mafic lavas and pyroclastic rocks, immature alluvial-fluvial braidplain deposits and finer-grained basinal rocks. These thick, laterally restricted protobasinal sequences reflect either strike-slip or small extensional basins formed during the impactogenal rifting and southeasterly-directed tectonic escape, which accompanied collision of the Zimbabwe and Kaapvaal cratons during Ventersdorp times. The erosively-based sheet sandstones of the succeeding Black Reef Formation reflect northwand-directed compression in the south of the basin. Thermal subsidence along the Ventersdorp Supergroup and Transvaal protobasinal fault systems led to shallow epeiric marine deposition of the sheet-like Chuniespoort Group carbonate-BIF platform succession. After an estimated 80 Ma hiatus, characterized by uplift and karstic weathering of the Chuniespoort dolomites, slower thermal subsidence is thought to have formed the Pretoria Group basin. Widespread, closed basin alluvial fan, fluvial braidplain and lacustrine sedimentation, as well as laterally extensive, subaerial andesitic volcanism (Rooihoogte to Strubenkop Formations), gave way to a marine transgression, which laid down the tuffaceous mudrocks, relatively mature sandstones and subordinate subaqueous volcanic rocks of the succeeding Daspoort, Silverton and Magaliesberg Formations. Poorly preserved post-Magaliesberg formations in the Upper Pretoria Group point to possible compressive deformation and concomitant rapid deposition of largely feldspathic detritus within smaller closed basins.

Architectural elements from Lower Proterozoic braid-delta and high-energy tidal flat deposits in the Magaliesberg Formation, Transvaal Supergroup, South Africa

Abstract Three architectural elements are identified in the Lower Proterozoic Magaliesberg Formation (Pretoria Group, Transvaal Supergroup) of the Kaapvaal craton, South Africa: (1) medium- to coarse-grained sandstone sheets; (2) fine- to medium-grained sandstone sheets; and (3) mudrock elements. Both sandstone sheet elements are characterised by horizontal lamination and planar cross-bedding, with lesser trough cross-bedding, channel-fills and wave ripples, as well as minor desiccated mudrock partings, double-crested and flat-topped ripples. Due to the local unimodal palaeocurrent patterns in the medium- to coarse-grained sandstone sheets, they are interpreted as ephemeral braid-delta deposits, which were subjected to minor marine reworking. The predominantly bimodal to polymodal palaeocurrent trends in the fine- to medium-grained sandstone sheets are inferred to reflect high-energy macrotidal processes and more complete reworking of braid-delta sands. The suspension deposits of mudrocks point to either braid-delta channel abandonment, or uppermost tidal flat sedimentation. The depositional model comprises ephemeral braid-delta systems which debouched into a high-energy peritidal environment, around the margins of a shallow epeiric sea on the Kaapvaal craton. Braid-delta and tidal channel dynamics are inferred to have been similar. Fine material in the Magaliesberg Formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early Proterozoic.

Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa

Abstract Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone–very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings.

Early proterozoic black shales of the Timeball Hill Formation, South Africa: volcanogenic and palaeoenvironmental influences

Abstract Black shales in the Early Proterozoic Timeball Hill Formation exhibit a widespread dark grey colour due to disseminated iron minerals, predominantly limonite after pyrite, with subordinate thin beds and laminae more intensely pigmented by finely disseminated flakes of carbonaceous material. Andesitic-basaltic volcanism is thought to have provided the source of iron and sulphur for the ferruginous colouration in a basal and uppermost black shale facies. Sulphate-reducing bacteria around volcanic vents possibly produced the organic matter for the darker beds. Turbiditic rhythmites and succeeding fluviodeltaic sandstones overlie the basal black shale facies and associated Bushy Bend lavas. In the south west of the basin both rhythmites and sandstones have black shales as either thin interbeds or matrix material suggesting the possibility of continued fumarolic emissions in this region. A repetition of the rhythmite facies again shows an association with subordinate thin black shale interbeds in the southwest of the basin and this facies is succeeded by a second occurrence of the black shale facies, underlying the Hekpoort Andesite Formation. The observed association of black shales with turbiditic rhythmites, lavas lacking pillow structures and fluviodeltaic sandstones suggests water depths which varied from shallow to a few hundreds of metres. Some potential for volcanogenic massive sulphide and sedimentary exhalative ore deposits exists in the black shale as there is volcanic rock associated at the base of the formation.

Petrography and Geochemistry of Sandstones Interbedded with the Rooiberg Felsite Group (Transvaal Sequence, South Africa): Implications for Provenance and Tectonic Setting

ABSTRACT The 2150 Ma Rooiberg Felsite Group, Transvaal Sequence, contains thin sandstone interbeds within its 3500-5000 m thick volcanic succession. Whereas uppermost feldspathic arenites and wackes are thought to represent reworked felsitic material, quartz arenites and subordinate lithic sandstones, present throughout most of the Rooiberg sequence, comprise mainly reworked sedimentary detritus, probably belonging to the underlying Pretoria Group. Sandy braided-river systems probably transported clastic material into the basin from relatively stable source areas, subject to intense chemical weathering. Geochemistry, and sandstone petrography indicate that mixing of Pretoria Group detritus with sediment derived from erosion of felsitic material occurred within the basin during late Rooiberg ti es. The inferred hiatuses in volcanism represented by the predominant siliceous sedimentary interbeds appear to have been of relatively short duration and occur throughout much of the Rooiberg stratigraphy. Upper arkosic sandstones indicate longer breaks in volcanism as Rooiberg eruptions came to an end. The sandstones provide evidence compatible with an impact origin for the Rooiberg Felsite Group, and for its successor, the Bushveld Complex.

A re-evaluation of the volcanism of the Palaeoproterozoic Pretoria Group (Kaapvaal craton) and a hypothesis on basin development

Abstract The Palaeoproterozoic (approximately 2.3-2.1 Ga) Pretoria Group, Transvaal Supergroup, contains three main volcanic units, i.e the basal Bushy Bend Lava Member, the medial Hekpoort Formation and the Machadodorp Volcanic Member. Field relationships and geochemistry of the latter two volcanic successions have similarities with continental flood basalts (CFB), such as comparable trace element patterns and a distinct niobium anomaly. The genesis of the Pretoria Group volcanic rocks is inferred to be related to processes in a replenished, fractionated, tapped, assimilated (RFTA) magma chamber, involving an asthenospheric source, replenishing and tapping of the magma chamber and contamination of the primary partial melts by crustal material accompanied by fractional crystallization. The proposed basin development of the Pretoria Group, thought to have been related to asymmetric stretching of crust and lithosphere, may have been complicated further by decoupling of the extension in the sedimentary cover from the basement below, due to the presence of over-pressured basal shales. The tectonic setting of the Pretoria Group is inferred to lie in the rift-to-intracratonic-sag-type continuum.

The Kanye axis, Kaapvaal craton, southern Africa: a postulated Archaean crustal architectural element inferred from three-dimensional basin modelling of the lower Transvaal Supergroup

Abstract The Kanye axis of Late Archaean/Palaeoproterozoic rifting, volcanism and sedimentation, is postulated mainly from three-dimensional basin modelling of the lower Transvaal Supergroup, carried out using DATAMINE software. Acid lavas and intrusives of the pre-Transvaal Kanye suite, which occur in eastern Botswana adjacent parts of South Africa and further eastwards below the Dennilton fragment in the Bushveld complex, he on the inferred axis. Both lowermost Transvaal volcano-sedimentary successions and the overlying sheet sandstones of the Black Reef Formation have major axes of subsidence and deposition along this proposed Kanye trend in the DATAMINE basin-fill models. Geophysical data also support the suggested Kanye trend, as does the location of younger intrusive bodies such as the Pilanesberg and Goudini alkaline complexes in South Africa. Three-dimensional basin modelling using DATAMINE software provides a powerful tool in evaluating the tectono-sedimentary history of ancient Precambrian basins, such as that of the Late Archaean/ Palaeoproterozoic Transvaal Supergroup of southern Africa.

An interpretation of boron contents within a Palaeoproterozoic volcano-sedimentary succession: Pretoria Group, Transvaal Supergroup, South Africa

Abstract Boron contents of sedimentary rocks were measured in three profiles through the Palaeoproterozoic Pretoria Group. Enhanced boron values occur at a number of stratigraphic levels across much of the preserved basin, and only these anomalies are considered significant. Evaluation of the various possible factors controlling these high boron concentrations suggests that inherited boron, sedimentation rate, detrital tourmaline, silica and carbonate dilution, metamorphism and evaporitic borates were relatively unimportant. A major role is indicated for the preferential uptake of boron by clay minerals, and for both andesitic volcanic and related hydrothermal activity as a source of boron. The dilution of boron concentrations due to organic coating of clay minerals may also be important in the case of the Pretoria Group. Stratigraphically expressed boron anomalies which cannot be explained by these various factors, are inferred to represent enhanced palaeosalinity of the waters of deposition. The suggested marine transgression is supported by sedimentological data.

Some evidence for the base-metal potential of the Pretoria Group: stratigraphic targets, tectonic setting and REE patterns

The Timeball Hill and Silverton Formations of the 2.1–2.3 Ga Pretoria Group have regional lithological associations which are thought to have been favourable for the genesis of stratiform sulphide deposits. The observed association of carboniferous and pyritic black shales, tuffaceous material, stromatolitic carbonates and inferred turbidity current deposits is common in stratiform sulphide deposits of the sedimentary exhalative group. Massive sulphides in the Silverton Formation are compatible with a syngenetic brine discharge, probably related to deep fracture systems. The basal shales of the Timeball Hill Formation are significantly enriched in base-metals and Ba. Interlayered tuff beds at this stratigraphic level have PGE-contents of up to 1 g/t. The REE-geochemistry of Pretoria Group sedimentary rocks supports hydrothermal activity as an important factor in both stratigraphic units.