C.P. Snyman

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.

Coal in South Africa

Abstract South Africa has more than 70 per cent of the coal resources of Africa, and coal forms the back-bone of the South African industry. In terms of the norms generally accepted for the Carboniferous coals of the Northern Hemisphere, South African coal has long been regarded as “abnormal”. However, these apparent abnormalities can be adequately explained in terms of the petrography which in turn reflects the conditions of peat formation and the subsequent metamorphism under a steep palaeogeothermal gradient. In common with other Gondwana coals South African coals are generally rich in material transitional between vitrinite and inertinite (sensu stricto). This transition material is partly reactive during technological processes like carbonisation, and is therefore regarded as semi-reactive. South African coals are generally low in sulphur, nitrogen and phosphorus, and in the case of the first two the contents are dependent on maceral composition and rank. On account of the low basisity of the coal ash the ash fusion temperatures are generally high and this is an advantage in most areas of coal utilisation. A review is given of adverse geological conditions affecting coal exploitation, and of methods that can possibly be used to recognise and predict of even eliminate these conditions for purposes of mine planning.

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.

The early proterozoic Mississippi Valley-type Pb-Zn-F deposits of the Campbellrand and Malmani Subgroups, South Africa

Pb-Zn-F deposits occur in the very late Archaean (2.55 Ga) shallow marine dolostone of the relatively undeformed Campbellrand and Malmani Sub-groups, which are overlain unconformably by the lower Proterozoic Postmasburg and Pretoria Group siliciclastics. They consist of stratiform deposits formed by replacement and porosity-filling, as well as pipes, ring-shaped and irregular bodies associated with collapse breccia. In the Transvaal basin the latter were generated during the karst denudation period between the deposition of the Chuniespoort Group (ending at ∼ 2.4 Ga) and of the Pretoria Group (starting at 2.35 Ga). A part of these mineralisations were overprinted by the metamorphism of the Bushveld Complex intrusion at 2.06 Ga. In the Transvaal basin, the age of the mineralisation is constrained between the start of the Pretoria Group deposition and the Bushveld intrusion. It is concluded that, although most of the mineralisations are characteristic of the Mississippi Valley-type, some of the northernmost occurrences, rich in siderite, are less typical. A classic genetic model is proposed. In an environment characterised by tensional tectonics and basin development, brines of basinal origin were heated by circulation into pre-Chuniespoort rocks, leached metals from the rocks they permeated, and rose as hydrothermal plumes. At relatively shallow depth they deposited minerals after mixing with water of surficial origin.

Mudrock geochemistry of the proterozoic pretoria group, transvaal sequence (South Africa): geological implications

Abstract Mudrocks of the 2300-2100 Ma Pretoria Group, Transvaal Sequence, differ from published data on Proterozoic mudrocks from other parts of the world. Most SiO2/Al2O3 and K2O/Na2O ratios lie outside the field of Proterozoic-Phanerozoic shales, with K2O/Na2O ratios becoming more widely spread with increasing stratigraphic height. Stratigraphic trends occur in the major element distribution within the Pretoria Group: Al2O3 and FeO (total) contents decrease upward, with a major change between the Silverton and Vermont Formations, whereas the opposite is true of MgO and CaO. The increase of the latter elements with stratigraphic height is possibly related to penecontemporaneous andesitic volcanism in the depository during the later stages of sedimentation. Higher Al2O3 and total iron contents towards the base of the group probably indicate more deeply weathered source materials, as is borne out by the Chemical Index of Alteration (Nesbitt and Young, 1982) of these older mudrocks. Mixed granitic to basaltic source rock composition is inferred from low Cr/Zr and intermediate TiO2/Al2O3 ratios. Variable Th/Sc ratios suggest that sedimentary recycling did not play a major part during deposition of the Pretoria Group. A decrease in the determined loss on ignition for stratigraphically younger mudrock samples can be related to a reduced carbonaceous mudrock component. Low boron contents point to a freshwater palaeoenvironment rather than a marine depository.

Sedimentary petrography of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

Abstract Sandstone petrography, geochemistry and petrotectonic assemblages of the predominantly clastic sedimentary rocks of the Early Proterozoic Pretoria Group, Transvaal Sequence, point to relatively stable cratonic conditions at the beginning of sedimentation, interrupted by minor rifting events. Basement uplift and a second period of rifting occurred towards the end of Pretoria Group deposition, which was followed by the intrusion of mafic sill swarms and the emplacement of the Bushveld Complex in the Kaapvaal Craton at about 2050 Ma, the latter indicating increased extensional tectonism, and incipient continental rifting. An overall intracratonic lacustrine tectonic setting for the Pretoria Group is supported by periods of subaerial volcanic activity and palaeosol formation, rapid sedimentary facies changes, significant arkosic sandstones, the presence of non-glacial varves and a highly variable mudrock geochemistry.

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.

Wind-blown Rocks and Trails on a Dry Lake Bed: An Alternative Hypothesis

Rocks with maximum dimensions between 5 and 50 cm, which have moved distances of 35 cm to 400 m over the dry, mudcracked upper surface of the seasonal lake at Magdalenasmeer in South Africa, have left well defined traces on the argillaceous material. This phenomenon is postulated to reflect a slick muddy lake surface that developed during the passing of a cold front in the middle of the winter season, and which developed as a result of dew precipitation related to high humidity, and two subsequent freezing nights. The resultant slick surface is thought to have represented a combination of damp mud polygons and ice-filled cracks in between, and movement of the rocks was as a result of gusts of wind on the second freezing night.