The geology and geochemistry of the Palaeoproterozoic Makganyene diamictite Stéphane Polteau, John M. Moore and Harilaos Tsikos

Abstract The Palaeoproterozoic Earth experienced a global glacial event at 2400 Ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans. The Transvaal Supergroup in the Griqualand West Basin, South Africa, hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes. The focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic Makganyene Formation, in order to constrain its palaeoenvironmental settings.

The Makganyene Formation forms the base of the Postmasburg Group and has been regarded as resting on an erosive regional unconformity throughout the Northern Cape Province. Systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal. The Makganyene Formation is, in fact, conformable with underlying formations of the Koegas Subgroup in the deep southern Prieska basin and rests on an unconformity only on the shallow Ghaap platform to the north-east. The Makganyene Formation displays lateral facies changes that reflect the palaeogeography of the study area, and the advance and retreat of ice sheets/shelves.

Geochemical investigations of glacial strata of the Makganyene Formation demonstrate that underlying banded iron formations of the Transvaal Supergroup acted as the main clastic source for the diamictite detritus. Geographic variations in bulk composition of the diamictites correlate well with field observations, and show that sorting processes were controlled largely by the morphology of the palaeobasin. Carbon isotope results emphasize the transitional nature of the Makganyene Formation in terms of the environmental conditions that resulted in widespread global glaciation in the Palaeoproterozoic.

On the basis of the above geological evidence, it is proposed that the Transvaal Supergroup in the Northern Cape Province represents a continuous depositional event that lasted approximately 250 Ma and hence provides a unique opportunity for assessing the transitional changes experienced by the Palaeoproterozoic Earth.

1. Introduction Palaeomagnetic data obtained from Western Australia, India, North America, Scandinavia and South Africa (Evans et al., 1997) have indicated that during the interval 2500–2200 Ma, the Earth experienced extreme climatic variations related to the development of ice sheets at sea level in equatorial palaeolatitudes (Kirschvink et al., 2000 and Hoffman and Schrag, 2002). This cold-climate event is represented in South Africa by the Makganyene Formation of the Transvaal Supergroup, a unit that, for all its significance in Palaeoproterozoic palaeoclimatic interpretations, has received little attention up till now (Visser, 1971, Visser, 1981, Visser, 1999 and De Villiers and Visser, 1977).

The aim of this study was to assess the stratigraphic settings and geochemical signature of the Makganyene Formation in order to constrain environmental changes in the Palaeoproterozoic marked by this glacial event. Geological aspects of the Makganyene Formation, including lateral facies changes within the glacial diamictites and stratigraphic relationships with the underlying and overlying formations, are presented here together with bulk and stable-isotope geochemical data from the diamictites and associated banded iron-formations (BIFs). Based on this study, a depositional model is presented for the Makganyene Formation and the Transvaal Supergroup in the Griqualand West Basin, along with implications for iron and manganese deposition.

2. Regional stratigraphic settings The Transvaal Supergroup in the Griqualand West Basin in the Northern Cape Province has conventionally been subdivided into the basal Ghaap Group and the overlying Postmasburg Group. The two groups have been correlated with the Chuniespoort and Pretoria Groups respectively in the Transvaal Basin and are reported to be separated by a major unconformity (Beukes, 1983 and Beukes and Smit, 1987). More recently, an alternative model was proposed that questions the lateral correlations between the Postmasburg and Pretoria Groups and their unconformable relationship with underlying strata (Moore et al., 2001).

The Ghaap Group is subdivided into the Schmidtsdrif, Campbellrand, Asbestos Hills and Koegas Subgroups, whereas the Postmasburg Group into the Makganyene and Ongeluk Formations and overlying Voëlwater Subgroup (Table 1). Brief descriptions of all these units follow below.

2.1. Ghaap Group

2.1.1. Schmidtsdrif Subgroup

The base of the Schmidtsdrif Subgroup unconformably overlies either crystalline Archaean basement or lavas of the Ventersdorp Supergroup (Beukes, 1983). This Subgroup consists of shales, quartzites, siltstones and lava. The latter gave a single-zircon Pb-evaporation age of 2642 ± 3 Ma (Walvaren and Martini, 1995). The predominantly clastic nature of the Schmidtsdrif Subgroup contrasts with the rest of the Ghaap Group, which consists of a thick succession of stromatolitic carbonates (dolomites and limestones of the Campbellrand Subgroup) overlain by transgressive BIFs of the Asbestos Hills Subgroup (Grobbelaar et al., 1995).

2.1.2. Campbellrand Subgroup

The onset of widespread carbonate accumulation was reportedly diachronous across the Kaapvaal Craton. Based on radiometric ages (2555 ± 19 Ma, SHRIMP U–Pb zircon) obtained from volcanic tuff layers, Altermann (1997) and Altermann and Nelson (1998) deduced that drowning started in the south-western Prieska sub-basin of the Griqualand West Basin that was separated from the eastern Ghaap platform by a 950 m high escarpment, resulting from active faulting in the Griquatown area (Fig. 1). As relative sea level rose, deep-water shales of the Naute Formation were deposited in the Prieska sub-basin while shallow-water carbonate precipitation commenced on the Ghaap platform and in the Transvaal Basin (Altermann, 1997).

Fig. 1. Geological sketch map showing the distribution of the Ghaap and Postmasburg Groups over the Griqualand West Basin in the Northern Cape Province. Localities investigated are the circled letters from A to G. Map after Horstmann and Hälbich (1995).

2.1.3. Asbestos Hills Subgroup

The conformably overlying Asbestos Hills Subgroup consists predominantly of BIFs and has been subdivided by Beukes (1983) into a lower orthochemical rhythmically banded Kuruman Formation and an upper allochemical clastic-textured and shallow-water Griquatown Formation. A tuffaceous layer from the Kuruman Formation has a SHRIMP U–Pb zircon age of 2465 ± 5 Ma (Pickard, 2003) and the Griquatown Formation has a calculated SHRIMP U–Pb zircon age of 2432 ± 31 Ma (Trendall et al., 1990).

2.1.4. Koegas Subgroup

The Koegas Subgroup is only developed in the south-western Prieska sub-basin and consists of a mixed sequence of terrigeneous clastic sediments with subordinate iron-formations and dolomitic bioherms (Beukes, 1983). The formations and corresponding rock types are described in Table 1 and one Pb/Pb age of 2415 ± 6 Ma was determined for the Rooinekke Formation (Kirschvink et al., 2000).

During this study, a diamictite lens was located near the base of the Doradale Formation of the Koegas Subgroup on the farm Klooffontein 332 (locality A; Fig. 1). It appears identical to typical Makganyene diamictites in clast size, shape, type and distribution and has a fine red matrix, identical to certain diamictites in the Makganyene Formation, implying a similar glacial origin.

The uppermost portion of the Koegas Subgroup comprises the Naragas and Rooinekke Formations. The 70–100 m-thick Naragas Formation consists of 1–2 m-thick massive and cross-bedded chloritic siltstone beds alternating with 10–20 m-thick quartz-chlorite mudstone units. The Rooinekke Formation, at the top of the Koegas Subgroup, consists mainly of BIFs and stromatolitic bioherms, and hosts a modest manganese deposit of supergene origin at the Rooinekke mine (Dirr and Beukes, 1990). In the south-western part of the basin, the Rooinekke Formation has a thickness of 70 m and progressively thins out north-eastward towards Griquatown where it disappears completely together with the underlying Naragas and Doradale Formations (Fig. 2).

Fig. 2. (A) Simplified geological map of the Griquatown area. (B) Profile corresponding to the section line AB.

The transition from the Rooinekke Formation to the overlying Makganyene Formation is transitional and 1 m-thick diamictite beds are commonly interbedded with the upper Rooinekke BIF (Fig. 3). This feature, taken together with the presence of a glacial diamictite lens at the base of the Koegas Subgroup, indicates that there is a close environmental relationship between the Koegas Subgroup and the overlying Makganyene Formation.

Fig. 3. Typical section of the Makganyene Formation in the Sishen area. The nature of the basal contact of the Makganyene Formation is transitional with interbedded diamictites and BIFs. The upper contact is also transitional with interbedded diamictites and volcanic tuffs.

2.2. Postmasburg Group

2.2.1. Makganyene Formation

The base of the Postmasburg Group is developed throughout the Griqualand West Basin and is represented by the glaciogenic Makganyene Formation. Profiles of the Makganyene Formation and enclosing stratigraphic units were examined at selected localities in the basin and are presented in Fig. 6. A major regional unconformity has been reported to exist at the base of the Makganyene Formation (Beukes, 1983), separating the Postmasburg Group from the underlying Ghaap Group. According to Beukes (1983), the Koegas Subgroup was entirely removed from the Ghaap platform by ice-scouring during the Makganyene glacial event and is only preserved in the Prieska sub-basin. However, no striated pavements have been observed (Visser, 1971, Visser, 1981, Visser, 1999 and De Villiers and Visser, 1977) in support of this statement. The observed transitional relationships with the Rooinekke Formation in the Prieska sub-basin described above (Fig. 3) appear to contradict regional-unconformity models.

The Makganyene Formation displays extreme thickness variations, from 3 m near the Orange River, to 70 m near Kuruman and to 500 m in a borehole near Postmasburg (Visser, 1971; this study). The Makganyene Formation contains a variety of rock types including massive to coarsely bedded diamictites, sandstones, shales, BIFs and stromatolite bioherms. The diamictite clasts consist essentially of chert, BIF, sandstone and occasional carbonates set in a fine ferrugineous matrix. Clast shapes range from angular to sub-rounded, are heterogeneously distributed within the matrix and are generally 0.5–30 cm in diameter, though larger clasts are occasionally reported (Polteau, 2000 and Polteau, 2005). Large chert clasts display conspicuous glacial striations (Visser, 1999 and Eyles and Januszczak, 2004).

Rock-type associations depend on the exact location within the depositional basin. The Prieska sub-basin contains an intermixed association of clastic and chemical sediments (localities A–D, Fig. 1). The clastic facies is represented by the glacial diamictites, while (bio-)chemical facies corresponds to stromatolitic manganese-bearing carbonate bodies (bioherms), commonly about 5 m long and 3 m thick (Fig. 4A–C), that consist of alternating thinly bedded and contorted manganese-rich carbonate laminae. The chemical sedimentary portion of the Makganyene Formation thins out and incorporates chert clasts towards the Ghaap platform (Fig. 4A and B). At the transition between deep and shallow facies (locality E, Fig. 1), the carbonate bodies are replaced by coarse sandstone lenses that can be massive, laminated and/or cross-bedded. Associated with this lateral facies transition are numerous striated pebbles (Fig. 4D). On the shallow Ghaap platform (locality G, H for Sishen and I for Rissik farm near Hotazel on Fig. 1), only clastic sediments are present and the sandstone lenses coarsen northwards.

Fig. 4. (A and B) Clast bearing stromatilitic bioherms within the Makganyene diamictites. (C) Clast-free stromatolitic bioherm within the Makganyene diamictites. (D) Large striated pebble from the Makganyene Formation at the Griquatown hinge zone.

Fig. 2 is a simplified geological map of the Griquatown area and shows that active faults, as suggested by Beukes (1983), were not responsible for the lateral facies changes and pinch-outs observed in the Koegas Subgroup and Makganyene Formation. The Griquatown Fault may have been an active fault zone during deposition of the Campbellrand Subgroup (Altermann, 1997 and Altermann and Nelson, 1998), but during Koegas and Makganyene times it acted more as a hinge zone. Field mapping indicated that the Koegas Subgroup is confined to the deeper Prieska sub-basin and wedges out onto the Ghaap platform (e.g. at locality E, Fig. 1). The fine-grained nature of the Koegas sediments as they pinch out and the lack of coarse debris-flow or breccia-type sediments adjacent to the hinge zone do not support an active fault model.

The basal contact of the Makganyene Formation changes sharply from a conformity with the underlying Koegas Subgroup in the south-western Prieska sub-basin (localities A–D and H, Fig. 1) to an unconformity with the underlying Asbestos Hills Subgroup on the shallow Ghaap platform facies (localities G and I, Fig. 1), separated by a hinge zone (locality E, Fig. 1). The unconformity on the Ghaap platform is not significantly transgressive as demonstrated by the observation that the Makganyene Formation rests on a single, 50 m-thick member of the Asbestos Hills Subgroup over a 200 km north–south strike distance between Hotazel and Griquatown (Beukes, 1980). However, the dominance of BIF clasts from the Asbestos Hills Subgroup in the Makganyene diamictites demonstrates the erosive nature of the contact.

The presence of dropstones in both underlying and interbedded Koegas BIFs emphasizes the transitional nature of the basal contact in the deep basin (Fig. 3). The diamictite lens occurring at the base of the Koegas Subgroup indicates the onset of glacial climatic conditions even prior to the development of the main Makganyene diamictite episode. The Koegas Subgroup may well represent a deep-water facies of the Makganyene glacial event—an observation supported by the progressive thinning of the massive diamictites to the west.

2.2.2. Ongeluk Formation

A 1–3 m-thick tuffaceous turbidite unit consisting of 5–40 cm-thick graded beds characteristically separates the massive diamictites of the Makganyene Formation from an overlying 900 m thick succession of continental flood-type basaltic andesites of the Ongeluk Formation that includes widespread pillow lavas indicating a marine setting (Grobler and Botha, 1976 and Cornell et al., 1998). According to Moore et al. (2001) and Pickard (2003), the 2222 Ma Rb–Sr age of the Ongeluk Formation (Cornell et al., 1998) is not robust and no SHRIMP U–Pb age could be determined by Dorland (2004). The upper contact of the Ongeluk Formation with the overlying Voëlwater Subgroup is transitional over approximately 20 m and comprises hyaloclastites, black volcanic tuffs and red haematitic jaspilites interbedded with diamictite and BIF (Fig. 5).

Fig. 5. (A and B) Location of boreholes from the Kalahari Manganese Field in the farm Rissik intersecting the upper Ongeluk Formation and the base of the Hotazel Formation. The profiles (C and D) display rapid facies changes over short distances (50 m).

2.2.3. Voëlwater Subgroup

The Voëlwater Subgroup is preserved only in the northern part of the Griqualand West Basin (locality I, Fig. 1) and is divided into two distinct formations, the basal Hotazel and upper Mooidraai Formations. BIF and diamictite lenses from the base of the Hotazel Formation conformably overlie the Ongeluk Formation (Fig. 5), apparently resting on an undulating surface of Ongeluk lava (Kirschvink et al., 2000). The BIF facies varies from Fe-oxide- to Fe-silicate-rich and contains a substantial calcareous component. The diamictites are relatively fine grained and contain small rounded carbonate clasts with a maximum observed diameter of 2 cm, set in a stilpnomelane-rich matrix. The presence of potential dropstones indicates that the diamictites are probably glacial in origin (Kirschvink et al., 2000; this study).

The hyaloclastite/diamictite-bearing basal part of the Hotazel Formation grades upward into the more typical Hotazel Fe–Mn succession as described by Tsikos and Moore (1997) and Tsikos et al. (2003). The Hotazel iron formation is characterised by alternations of chert-rich and magnetite-rich bands of thicknesses generally between 1 mm to 1 cm, and can be broadly subdivided into a lower, carbonate-poor member (modal carbonate typically less than 5%) and an upper, carbonate-rich one (modal carbonate generally >15%). Commonly small modal amounts of Fe-silicate minerals (greenalite, minnesotaite, stilpnomelane) are present essentially throughout the iron-formation succession. The carbonate-poor iron-formations enclose the lowermost, thickest and thus most economic of a total of three discrete manganese-rich units that consist of braunite, Mn-carbonate (Mn-rich calcite and kutnahorite) and lesser fine-grained haematite.

A conformable, and usually gradational contact separates the Hotazel from the Mooidraai Formation (Tsikos, 1999 and Tsikos et al., 2001). The latter comprises Fe-bearing limestone, dolomite and subordinate chert, and represents the uppermost preserved part of the Transvaal Supergroup in the Northern Cape Province. A carbonate Pb–Pb age of 2394 ± 26 Ma has been obtained from a dolomitized portion of the Mooidraai Formation (Bau et al., 1999). The Voëlwater Subgroup is overlain unconformably by the Gamagara and Mapedi Formations of the late Palaeoproterozoic Olifantshoek Supergroup (Beukes and Smit, 1987). Radiometric dating of the Hartley Basalt Formation that stratigraphically succeeds the Mapedi/Gamagara Formations has constrained the latter part of the Olifantshoek Supergroup to an age not lower than 1900 Ma (Cornell et al., 1998).

3. Geochemistry

3.1. Sample selection and methods

A total of 75 diamond-drillcore samples were selected for bulk-rock geochemical analyses from a variety of lithologies, ranging from BIFs to clastic diamictites and volcanic tuffs. The samples come from two different stratigraphic intervals—the Makganyene Formation and the base of the Hotazel Formation. The boreholes were located in the Sishen (locality A, Fig. 1: borehole GA107) and Hotazel areas (locality I, Fig. 1: boreholes BG5, BH171/91 and REX41). These two localities represent the central and northern parts of the Griqualand West Basin respectively. Additional data from Polteau (2000) for the Matsap (southern part of the basin) and Sishen areas have been incorporated, in order to assess geochemical variations related to geographic locations.

Each sample had its large clasts (>3 cm) removed (where present), then were crushed in two stages (swing mill and automatic agate pestle and mortar) and analyzed for 10 major elements and fifteen trace elements using standard X-Ray Fluorescence (XRF) procedures (Norrish and Hutton, 1969). Analyses were carried out on a Philips PW 1480 X-ray spectrometer in the Geology Department at Rhodes University. All analytical runs were calibrated using a variety of international and in-house standards, and were corrected for dead time, background, spectral line interference, mass absorption and instrumental drift.

In addition to bulk geochemical analyses, stable (C, O) isotope determinations were performed on the carbonate fractions of 8 samples from the Makganyene diamictite and interbedded BIF (drill-cores GA107 and GA171), as well as on 12 selected samples of BIF, diamictite (ca. 1 m thick) and hyaloclastite that collectively characterize the basal part of the Hotazel Formation (drill-core REX41). These samples complement previous isotopic studies on the underlying Asbestos Hills Subgroup BIFs (Klein and Beukes, 1989 and Beukes and Klein, 1990) and overlying Voëlwater Subgroup BIFs (Tsikos, 1999 and Tsikos et al., 2003). Carbonate minerals present in all samples were calcite and dolomite/ankerite in variable modal amounts, and occasional minor siderite was also present.

Isotopic compositions were obtained using a Finnigan MAT 252 mass spectrometer in the Department of Geological Sciences, University of Cape Town. Carbon dioxide was evolved by reacting between 30 and 110 mg of bulk-rock powder from each sample (depending on the total amount of carbonate minerals as determined by XRD methods) in 100% H3PO4 at 25 °C for 4 h (for calcite) or 50 °C for 8 h (for ankerite/dolomite). Results are presented in Table 3, while Fig. 8 presents a compilation of isotopic data from this study and from the literature (Klein and Beukes, 1989, Beukes and Klein, 1990, Tsikos, 1999 and Tsikos et al., 2003).

4. Results

4.1. Bulk compositional variations

The bulk composition of Makganyene diamictites is similar to that of the underlying BIFs (Table 2, Fig. 7), albeit with higher Al2O3 and TiO2 values. MnO, MgO and CaO concentrations increase steadily from the base to the top of the Makganyene diamictites, whereas Fe2O3 concentrations decrease sympathetically (Fig. 8). Increases in MgO and CaO might reflect an increase in the modal carbonate component of the diamictites, as indicated by the presence of stromatolitic bioherms in the upper part of the Makganyene Formation in the Prieska sub-basin. They may also be interpreted as indicating (at least partly) an increased carbonate clast contribution from Campbellrand Subgroup dolomitic source to the diamictites (Fig. 7). Zr is the only trace element having a constant concentration throughout the diamictite stratigraphy. Transitional metals such as Ni, Cu and Cr show a moderate upward increase, which indicates a possible corresponding increase in mafic source material that is confirmed by the presence of interbedded tuffaceous layers at the Makganyene-Ongeluk formations contact (average compositions shown in Table 2).

Fig. 6. Combined field profiles with letters corresponding to localities investigated (shown in Fig. 1). The Makganyene Formation basal contact grades from an unconformity on the Ghaap platform to a correlative conformity in the Prieska basin.

Fig. 7. Comparison between the Makganyene and Hotazel diamictites with the BIFs from the Griquatown/Rooinekke and Hotazel Formations. Values have been normalized to the volcanic tuffs from this study to further compare the diamictites with a possible

volcanic origin.

Fig. 8. Section of the Transvaal Supergroup mega-cycle displaying symmetrical lithologies and δ13C curve centered on the Makganyene/Ongeluk Formations. The stratigraphic variations of Fe, Mn and Ca are displayed for the diamictites from the Makganyene and base of the Hotazel Formations. The δ13C curves compiled from Klein and Beukes (1989), Beukes and Klein (1990), Tsikos (1999) and this study where the number of samples n is indicated in the stratigraphic column and the horizontal bars represent the range of values. The δ13C displays a climatic first order variation and two short transgressive second-order variations.

The diamictite at the base of the Hotazel Formation (borehole REX41) exhibits SiO2, Fe2O3 and MnO concentrations in similar relative proportions to BIFs from the Asbestos Hills Subgroup. The diamictites from the base of the Hotazel Formation have a similar major-element composition compared to the average composition of the Makganyene diamictites (Fig. 7, Table 2).

4.2. Provenance considerations

The three most probable source rocks present on the Kaapvaal Craton have been placed at the apices of the ternary diagram in Fig. 9, and correspond to BIFs from the Asbestos Hills Subgroup (Fe2O3 + MnO); dolomitic carbonates from the Campbellrand Subgroup (CaO + MgO); and igneous/volcanic rocks and/or clastic sediments from the Schmidtsdrif Subgroup, Ventersdorp Supergroup and/or Archaean Kraaipan granite-greenstone terrane (Al2O3 + Na2O + K2O). Average compositions of the Makganyene diamictites from the Matsap, Sishen and Hotazel areas and the younger Hotazel Formation diamictites are plotted on this diagram (Fig. 9). As expected, all samples of the Makganyene diamictite plot close to the BIF apex and display geochemical variations according to their exact geographic location. The variations correspond to increasing, though minor, carbonate concentrations in the Sishen-Hotazel area whereas the diamictite from the Matsap area plots more on a trajectory towards the igneous/volcanic/clastic sedimentary apex. The Hotazel Formation diamictite also plots near the BIF apex, along the carbonate-BIF tie-line.

Fig. 9. Ternary diagram representing the three most likely sources for the the Makganyene and base of the Hotazel diamictites.

The igneous/volcanic apex does not differentiate between a mafic or felsic source. Certain trace elements of a commonly mafic derivation (such as Sc, Co, Ni and Cr) when plotted against Zr which is indicative of a felsic origin, may help identify the exact nature of the source. Typical mafic and felsic trends were constructed using data from Taylor and McLennan (1985) and represent averages for Archaean mafic and felsic igneous end-member compositions. All the inter-element diagrams (Fig. 10) demonstrate that the Makganyene diamictite has an intermediate (dacitic-andesitic or granodioritic-dioritic) or mixed mafic-felsic source trend. This could be indicative of a source from the Ventersdorp Supergroup or the Kraaipan granite-greenstone terrane.

Fig. 10. Whole-rock igneous signature of selected samples of the Makganyene and base of the Hotazel diamictites displayed in bivariate plots. Averages M and F are from Taylor and McLennan (1985).

By comparison, the Hotazel diamictite shows a greater shift towards a mafic source signature in its composition (Fig. 10). This is reasonable since this diamictite is immediately underlain by basaltic andesites of the Ongeluk Formation and is interbedded with hyaloclastites. The glacial sedimentary processes depositing the Makganyene diamictites were probably punctuated by the extrusion of the Ongeluk lavas and, when they resumed, they presumably incorporated material from the underlying lavas in the Hotazel diamictite matrix, giving the diamictite a more mafic composition.

4.3. Stable isotope trends

Stratigraphic records of the stable isotope composition of carbon in Precambrian carbonate-rich sedimentary successions have been widely used as proxies for palaeoenvironmental reconstructions (Knoll et al., 1986, Schidlowski, 1988, Kaufman et al., 1990, Beukes et al., 1990, Des Marais et al., 1992, Derry et al., 1992, Klein and Beukes, 1993, Des Marais, 1994, Karhu and Holland, 1996, Condie et al., 2001 and Tsikos et al., 2003). A critical prerequisite for such applications is the preservation of primary depositional isotopic signals, a condition

commonly not met in Precambrian strata due to strong diagenetic overprinting and/or other post-depositional effects. In fact, it has been variously proposed (e.g. Baur et al., 1985, Kaufman et al., 1990 and Tsikos et al., 2003) that carbonate carbon isotope depletion in Palaeoproterozoic iron formations is most likely to have resulted from coupled oxidation of organic carbon and reduction of ferric precursors during sub-oxic diagenesis.

Stable isotope results from both diamictite- and BIF-hosted carbonates of this study range from −3.4‰ to −15.8‰ (data shown in Table 3) similarly indicate that diagenetic recycling of organic carbon must have had an influence on the observed δ13C data. Notwithstanding this limitation, the composite carbon isotope curve of Fig. 8 effectively reflects the symmetrical sedimentary succession of rock types in the Transvaal Supergroup of the Griqualand West Basin:

carbonate→BIF→diamictite→mafic lavas→diamictite→BIF→carbonate

By placing the glacial diamictites at the centre of this broad symmetry. This general first-order negative trend (solid line in Fig. 8) also records a major climatic cycle from relatively warm conditions (carbonate deposition) to progressively colder conditions (BIF) which culminated in glacial sedimentation (Makganyene Formation), before a return to a progressively warmer environment of BIF/Mn and finally carbonate deposition (Voëlwater Subgroup). The warming cycle from the end of the Makganyene diamictites to the Mooidraai carbonates lasted at the most 10–15 Ma, and is constrained by the ages of the Rooinekke Formation (2415 ± 6 Ma) and Mooidraai carbonates (2394 ± 26 Ma).

This broad cycle contains two second-order negative isotopic incursions developed within the BIFs of the Kuruman Formation and the base of the Hotazel Formation (dashed lines in Fig. 8). These incursions probably reflect the temporary establishment of relatively deeper water conditions associated with corresponding transgressive events, which resulted in the relative paucity of marine carbonate precipitation and the deposition of typical orthochemical BIF.

5. Depositional model In the light of the results presented in the preceding sections and available age data from the literature, the full stratigraphic sequence of the Transvaal Supergroup in the western (Prieska) part of the Griqualand West Basin represents a continuous depositional succession deposited over approximately 250 Ma (2650–2400 Ma, see Table 1). A proposed palaeo-depositional model is illustrated in Fig. 11 and comprises five stages that successively correspond to:

(A) Stable platform phase with warm climatic conditions that consists of the Schmidtsdrif and Campbellrand Subgroups.

(B) Transgressive, sag phase accompanied by climatic cooling that includes the Kuruman BIFs and terminates around the same stratigraphic point where the first δ13C negative incursion also occurs (Fig. 8).

(C) Regressive, uplift, basin-fill phase with climatic cooling that includes the Griquatown, Koegas and ultimately the glacial Makganyene Formation.

(D) Transgressive rift/sag phase developed during a progressively warmer regime; this would have commenced with the Ongeluk lava eruption and coincides with the δ13C negative incursion recorded in the lower portion of the Hotazel BIF.

(E) Gradual, regressive uplift phase in a progressively warmer climate, represented by the transition from the Hotazel Fe–Mn formation to the overlying Mooidraai carbonates.

Fig. 11. Depositional model for the Transvaal Surpergroup in the Northern Cape Province.

Post-magmatic, post-rifting thermal subsidence allowed the Schmidtsdrif Subgroup to accumulate from continental incised valley fill to shallow marine environments. The subsequent deposition of the thick Campbellrand carbonate succession is related to tectonic stability, continuous creation of accommodation space and progressive drowning of clastic sources that characterize typical ‘highstand’ conditions (Eriksson et al., 2001). The stromatolitic carbonates of the Campbellrand Subgroup are thus representative of a highstand system tract (Catuneanu and Eriksson, 1999 and Eriksson et al., 2001) (Fig. 11) during which the Griquatown fault zone (Beukes, 1983) separated the north-eastern Ghaap platform from the deeper Prieska sub-basin to the south-west (Altermann, 1997 and Altermann and Nelson, 1998). The Campbellrand and Malmani Subgroups of the Transvaal Supergroup constitute the earliest significant platform carbonate successions (2650–2500 Ma) and would have formed in a shallow (and probably relatively warm) depositional setting, with isotopic signatures reflecting closely those of the overlying water-mass. This geographically and temporally extensive episode of carbonate deposition would have acted as a major sink for atmospheric CO2 (Moore et al., 2001) that dominated the Archaean atmosphere and probably resulted in progressive lowering in average surface temperatures.

The overlying orthochemical BIF of the Kuruman Formation of the Asbestos Hills Subgroup (Beukes and Klein, 1990 and Altermann and Nelson, 1998) represents a sag phase with progressive deepening of the basin, as no drowning unconformity separates them from the underlying Campbellrand carbonates (Catuneanu and Eriksson, 1999). The onset of widespread BIF deposition was triggered by the establishment of a temperature-controlled stratification of the water column with cold, reduced, S-free bottom waters, conducive to the precipitation of typical BIFs (Huston and Logan, 2004). At that time, the Griquatown fault zone would have been inactive, and is now considered to represent a hinge zone.

The succeeding allochemical BIFs of the Griquatown Formation indicate the initiation of deep-water-fan sedimentation whereas the overlying clastic-sediment-dominated Koegas Subgroup points to the progressive advancement of submarine-fan sedimentation into the Prieska sub-basin. This basin-filling episode was associated with regression and climate degradation, culminating with the deposition of the shallow-water Rooinekke stromatolites and the Makganyene bioherms and glacial diamictites.

The Koegas Subgroup is confined to the deeper Prieska portion of the basin to the south-west of the Griquatown hinge zone as conformable submarine fan accumulations, while the Ghaap platform was exposed to subaerial erosion (Beukes, 1983) possibly due to uplift along the Vryburg arch. The presence of a diamictite lens near the base of the Koegas Subgroup implies the onset of glacial climatic conditions and development of continental glaciers. The Koegas Subgroup corresponds to a falling-stage system tract (Fig. 11) which was triggered by eustatic regression that caused the formation of a subaerial unconformity on the Ghaap plateau and its correlative conformity in the basin.

At the onset of deposition of the Makganyene Formation, progression of glacial conditions allowed for ice-shelf formation with an intermixed association of glacial diamictites and calcareous stromatolitic bioherms in the Prieska sub-basin. The stromatolitic bioherms thin out and incorporate chert clasts towards the shallow Ghaap platform. Approaching the Griquatown hinge zone, the carbonate bioherms are replaced by massive, laminated and/or cross-bedded coarse sandstone lenses, and numerous striated pebbles are present in the diamictites. On the Ghaap platform, only clastic sediments are present and the sandstone lenses coarsen northwards. The Makganyene Formation corresponds to a typical lowstand system tract (Fig. 11) bound by the subaerial unconformity and its correlative conformity at the base, and a conformable transgressive surface obscured by the Ongeluk Formation at the top.

The glacial event that resulted in the deposition of the Makganyene Formation was interrupted by the eruption of the conformably overlying Ongeluk andesitic lavas. This period of flood-basalt activity is thought to represent a relatively short-lived event coincidental to the sedimentary conditions prevailing in the Griqualand West Basin. It also marks the waning of the Makganyene glacial episode, probably due to the increased expulsion of volcanic greenhouse gases (e.g. CO2) into the Palaeoproterozoic atmosphere (Tsikos and Moore, 1997, Tsikos, 1999, Polteau, 2000, Polteau, 2005 and Moore et al., 2001). Both volcanic and glacial conditions continued intermittently after cessation of the major flood-basalt activity, as witnessed by the thin hyaloclastite and diamictite units at the base of the Voëlwater Subgroup.

The eruption of the Ongeluk flood basalts was initiated by a rifting event that caused rapid sagging of the basin and the onset, once again, of a deep-water orthochemical BIF sedimentary cycle at the base of the Hotazel Formation. The gradual transition from carbonate-poor to carbonate-rich Hotazel BIF and ultimately to the overlying carbonates of the Mooidraai Formation indicates a general shallowing-upward trend under progressively warmer climatic conditions. Episodic deposition of manganese carbonate-rich sediments in three discrete cycles would have occurred during respective second-order transgression-regression events.

6. Conclusions The Makganyene Formation represents a singular cold climate event within the Transvaal Supergroup in the Griqualand West Basin of South Africa. Stratigraphically, the Makganyene strata rest on an apparent unconformity in the shallow Ghaap platform but were deposited conformably on the Koegas Subgroup in the confined deep Prieska sub-basin. The transition between the two environments is located at the Griquatown hinge zone which separates a mixed diamictite and stromatolitic bioherm association to the south-west in the Prieska sub-basin from a diamictite containing striated pebbles and sandstone lenses on the Ghaap platform to the north-east.

The lack of a time gap between the deposition of the Ghaap and Postmasburg Groups in the Griqualand West Basin argues against correlations of the Makganyene Formation with the Rooihoogte Formation at the base of the Pretoria Group in the Transvaal Basin. On similar grounds, stratigraphic correlation between the Postmasburg Group with the Pretoria Group appears also to be invalid.

Bulk-rock geochemical results reveal that BIF was the main source rock for the Makganyene Formation, together with minor carbonate and intermediate igneous components. Data also confirmed that the facies changes observed in the field were controlled by the morphology of the palaeobasin. Isotope results reflect palaeoenvironmental processes across the entire Transvaal Supergroup on two different scales. A first-order symmetrical climatic cycle is evident with higher δ13C values recorded in Campbellrand and Mooidraai carbonates (warm) that enclose progressively lower values in the Kuruman/Griquatown and Voëlwater BIFs (cold) and in the Makganyene diamictite (glacial). Two second-order negative incursions coincide with the Kuruman and Hotazel BIF stratigraphic intervals. These prominent incursions in the broad symmetrical climatic cycle correspond to marine transgression events that brought about the deposition of typical orthochemical BIF.

Acknowledgements Authors thank the Society of Economic Geologist (Grant McKinsky), Kumba Resources, SAMANCOR and ASMANG who kindly provided borehole material, all the farmers who allowed access to the outcrops, Prof. Goonie Marsh for XRF analyses at Rhodes University and John Hepple for technical support. The authors also wish to express their gratitude to Sharad Master, two anonymous reviewers, and Karen Webb and Bradley G. Philips whose comments greatly improved the manuscript.

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