U.M. Schreiber

A review of the sedimentology of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

Abstract The sedimentary rocks of the Early Proterozoic Pretoria Group form the floor rocks to teh 2050 M.a. Bushveld Complex. An overall alluvial fan-fan-delta - lacustrine palaeoenvironmental model is postulated for the Pretoria Group. This model is compatible with a continental half-graben tectonic setting, with steep footwall scarps on the southern margin and a lower gradient hanging wall developed to the north. The latter provided much of the basin-fill detritus. It is envisaged that the southern boundary fault system migrated southwards by footwall collapse as sedimentation continued. Synsedimentary mechanical rifting, associated with alluvial and deltaic sedimentation (Rooihoogte-Strubenkop Formations) was followed by thermal subsidence, with concomitant transgressive lacustrine deposition (Daspoort-Magaliesberg Formations). The proposed half-graben basin was probably related to the long-lived Thabazimbi-Murchison and Sugarbush-Barberton lineaments, which bound the preserved outcrops of the Pretoria Group.

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.

Alternative marine and fluvial models for the non-fossiliferous quartzitic sandstones of the Early Proterozoic Daspoort Formation, Transvaal Sequence of southern Africa

Abstract This paper discusses some of the problems related to the palaeoenvironmental interpretation of non-fossiliferous, early Precambrian, recrystallised quartzitic sandstones, using the Early Proterozoic Daspoort Formation, Transvaal Sequence of southern Africa as a case study. These cross-bedded and planar stratified rocks have been interpreted previously as shallow marine deposits, based on limited studies of areas with well-exposed, relatively undeformed outcrops. This postulate rests largely on the apparently mature nature of the recrystallised sandstones and their thin bedding. Examination of outcrops throughout the preserved basin, including those which have been deformed and metamorphosed, reveals the presence of subordinate immature sandstones. Lateral facies relationships permit an alternative distal fan-fluvial braidplain model to be proposed. This is compatible with collected palaeocurrent data, thicknes trends and results of thin section petrography.

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.

An early proterozoic braid-delta system in the Pretoria group, Transvaal sequence, south Africa

Abstract The Dwaal Heuwel Formation of the Proterozoic Pretoria Group in the eastern Transvaal, South Africa, was deposited in a large-scale braid-delta system. Predominant immature to mature, cross-bedded sandstone probably represents fluvial braidplain deposits, whereas minor conglomeratic sandstone and mudrock are interpreted as proximal alluvial fan deposits and shallow water deposits, respectively. Sandy stream-flow deposits dominated the braidplain succession, whereas mass flow sedimentation occurred in the northernmost fans systems, the latter acting as source regions for the fluvial detritus. A distal lacustrine basin is represented by a laminated mudrock facies which occurs in the southern portion of the study area.

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.