Sedimentation Rates, Basin Analysis and Regional Correlations of Three Neoarchaean and Palaeoproterozoic Sub-Basins of the Kaapv

ELSEVIER Sedimentary Geology 120 (1998) 225–256

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

a, b Wladyslaw Altermann Ł, David R. Nelson a Institut fu¨r Allgemeine und Angewandte Geologie, Ludwig-Maximilians-Universita¨t, Luisenstraße 37, D-80333 Mu¨nchen, Germany b Geological Survey of Western Australia, Department of Mines, 100 Plain Street, Perth, W.A., Australia Received 29 April 1997; accepted 26 June 1997

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 sešdimentation rate fšor 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 š2000 m of carbonate and subordinate shale sedimentatišon 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

Ł Corresponding author. E-mail: [email protected]

0037-0738/98/$ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 3 4 - 7 226 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 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.  1998 Elsevier Science B.V. All rights reserved.

Keywords: basin analysis; sedimentation rates; Archaean; Proterozoic; Kaapvaal craton; SHRIMP

1. Introduction volcanic indolence. These two conditions are basic prerequisites for chemical or bio-chemical precipi- In the absence of biostratigraphic markers, high- tation. In the presence of clastic detritus, microbial precision isotopic data on the age and duration of organisms that facilitate carbonate precipitation can sedimentation are essential aspects of the study of be buried or swept away from the sediment surface Archaean and Proterozoic sedimentary basins. Pre- and from the water column, and inorganic precipita- cambrian siliciclastic basins containing thousands tion is hindered by the attachment of metal ions like of metres of sedimentary ﬁll are often bracketed Ca and Fe to mineral grains. The scarcity of clastic by rare and imprecise stratigraphic data, and lat- detritus thus also allows purely chemical precipi- eral lithostratigraphic correlations lack arguments tates like Banded Iron Formations (BIF) to develop. other than similar facies development. As a conse- It is certainly not coincidental, that large Precam- quence, poorly constrained basin models and equiv- brian BIF provinces are often underlain by carbonate ocal tectonic interpretations are commonly presented platforms. Hence, the conspicuous carbonate (shale) for Precambrian sediments. Precambrian carbonate and BIF association must be explained not only in basin-ﬁlls are equally vulnerable. More particularly, terms of palaeoenvironmental atmospheric and hy- the carbonate sedimentary processes and the mech- drospheric evolution (Eriksson et al., 1998), but also anism of carbonate precipitation are generally not as a function of basin development (Simonson and well understood for the Archaean (see discussions Hassler, 1997). Comparisons of sedimentation and by Grotzinger, 1989, 1990; Sumner and Grotzinger, subsidence rates of clastic and chemical sedimen- 1996). Although stromatolites and microbial remains tary basins of the Precambrian and Phanerozoic, as are known from older deposits, the earliest large car- attempted here, may reveal important aspects of tec- bonate platforms apparently developed in intracratonic history, rates of erosion and sediment transport, tonic basins, following cratonic stabilization. This genesis of mineral deposits and the evolution of was until recently ascribed to the Palaeoprotero- carbonate precipitating microbiota. zoic, around 2.5–2.0 Ma ago (Grotzinger, 1989). The Kaapvaal craton of southern Africa hosts With the development of new dating techniques, three major Archaean to Palaeoproterozoic sub- it has now become apparent that the earliest large basins, in which clastic and chemical sediments carbonate platforms developed during the Neoar- and igneous rocks accumulated. The Transvaal basin chaean, between 2700 Ma and 2500 Ma (Jahn et in the Transvaal geographic region, the Griqualand al., 1990; Arndt et al., 1991; Hassler, 1993; Barton West basin in the Northern Cape Province of South et al., 1994). Consequently, the time span between Africa and the Kanye basin of Botswana share cratonization and subsequent carbonate basin devel- lithostratigraphically similar deposits which uncon- opment is now believed to be shorter, with less than formably cover the 2.7 Ga old volcanic Ventersdorp 1.0 billion years separating the formation of granite– Supergroup (Armstrong et al., 1991). In this con- greenstone terranes at around 3.5 Ga to 3.0 Ga from tribution the Kanye basin is not discussed and the the formation of huge stromatolitic platforms in the Griqualand West basin is subdivided into the Prieska Neoarchaean (Beukes, 1986; Altermann and Her- sub-basin and Ghaap Plateau sub-basin, because of big, 1991; Jahn and Simonson, 1995; Altermann and their different development. Carbonates are volumet- Siegfried, 1997). The rise of these platforms was rically dominant rocks in the Prieska, Ghaap Plateau made possible by the widespread absence of clas- and Transvaal sub-basins and, together with thin, tic input during periods of tectonic quiescence and lowermost siliciclastic rocks, form the base of the W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 227

Transvaal Supergroup, being overlain by BIF de- and Wotherspoon (1995) and in Altermann (1997). posits. The iron-rich chemical precipitates are in turn General stratigraphy is shown in Figs. 1–3 and 7. overlain by a thick sequence of predominantly clastic sediments. Similar volcano-sedimentary basin devel- 2. Regional geology and stratigraphy of opment can be deduced in other Archaean cratonic Griqualand West terranes, but especially well on the Pilbara craton of Western Australia, where the lithostratigraphic suc- The Vryburg Formation of the Schmidtsdrif Sub- cession is strikingly similar to that of the Kaapvaal group (Beukes, 1979) of the Ghaap Group (Fig. 1) craton (Cheney, 1996). is the lowest stratigraphic unit above the unconfor- At ﬁrst glance, the three sub-basins discussed mity cutting into the 2709 Ma (Armstrong et al., here host mainly chemical sediments, and thus might 1991) Ventersdorp Supergroup lavas in Griqualand appear unsuitable for a special volume on Precam- West. This formation consists of shales, quartzites, brian clastic depositional systems. Nevertheless, we siltstones and lava. According to the South African feel that the sediments discussed herein impressively Committee for Stratigraphy (SACS, 1980), it cor- demonstrate the interplay of clastic and chemical relates with the Black Reef Quartzite Formation in sedimentation and its appearance in the geologic Transvaal (Fig. 7). A lava in the Vryburg Formation record of the Precambrian. Moreover, the over- was dated by Walraven et al. (in press) at 2642 3 whelming presence of the chemical sediments in Ma. Stromatolitic carbonates of the upper Schmišdts- the discussed sections is misleading. As our calcula- drif and succeeding Campbellrand Subgroups con- tions and age data demonstrate, clastic sedimentation formably cover the Vryburg Formation. A tuff band played a major role at different times in differ- in the upper part of the Gamohaan Formation, at ent sub-basins. In some areas pelitic sedimentation the top of the Campbellrand Subgroup (Figs. 1, 3 dominated the environment for periods longer than and 7), was dated by Sumner and Bowring (1996) 50 m.y., with only short intervals occupied by car- at 2521 3 Ma, giving a good approximation of bonate sediments. Because of different compaction the minimšum age of the Ghaap Plateau carbonates. behaviour, however, carbonates apparently dominate The carbonates are overlain by shales and subse- the sedimentary record. Upon decompaction, silici- quently by the Kuruman and Griquatown BIF of the clastic sediments would make up between one third Asbestos Hills Subgroup (Fig. 1). The Griquatown and half of the sedimentary section below the BIF. BIF has an age of 2432 31 Ma (Trendall et al., The discussion of the development of the intracra- 1990). The Koegas Subgršoup of mainly siliciclastic tonic Griqualand West–Transvaal basin is based on deposits is conformably superimposed on the BIF new age data presented herein, and on novel facies sediments, and is covered by the Makganyene glacial interpretation of the sediments in question (Alter- deposits of the Postmasburg Group with a regional mann, 1997; Altermann and Siegfried, 1997). Subse- unconformity (Figs. 1 and 7). Again unconformably, quently, we argue the possible processes responsible the 2222 13 Ma old (Cornell et al., 1996) Ongeluk for the basin development and the widespread accu- basaltic ašndesite formation covers the glacial tillite mulation of siliciclastic, biochemical and chemical (Altermann and Ha¨lbich, 1991). sediments. We also compare our data and inter- The only continuous section through the pretation to other Precambrian examples from the Schmidtsdrif and Campbellrand strata is preserved in literature in an attempt to elaborate the principal the Kathu drillcore. Altermann and Siegfried (1997) aspects of sediment accumulation for chemical and give a detailed description and facies interpretation clastic deposits during the Precambrian. Throughout of the sediments in the drillcore (Fig. 3). The entire this contribution we use the detailed stratigraphic Archaean sediment pile, in the core, with a total subdivision of Beukes (1980a), but with some mod- thickness of almost 3000 m, exceeds by far the 1900 iﬁcations for the Prieska sub-basin of Griqualand m thickness deduced from outcrops (Beukes, 1980a). West. A detailed discussion of various depositional This thickness increase is attributed to lateral facies and stratigraphic models for the Griqualand West variation and to differing sedimentary conditions, but and Transvaal carbonates is presented in Altermann also to a minor extent, to faulting and folding and 228 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

Fig. 1. Simpliﬁed geological map of the Griqualand West sub-basin and its relative geographic position with respect to the Transvaal sub-basin. Sample location for the four analyzed samples and for the sample dated by Barton et al. (1994) are shown. The sample dated by Sumner and Bowring (1996) was taken south of Kuruman. Note Prieska region southwest of Griquatown fault zone; Ghaap Plateau region northeast of it. W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 229

Fig. 2. Composite stratigraphic section through the Schmidtsdrif and Campbellrand (Nauga Formation) Subgroups between Prieska and Westerberg (Fig. 1). Lithology and facies interpretation for each member and formation are briefly summarized, and the position of the dated samples and their ages are given. Other ages are from the literature or calculated using compacted sedimentation rates. The sedimentation rates given in Bubnoff units are for decompacted sediments. Note that the section is disrupted in the middle to save space in the figure. 230 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 231 to the difficulty of thickness measurements in poorly flux, evident from the increase in shale content. This outcropping formations. increase culminated in the deposition of the Naute In a facies distribution model developed by Shales, followed by precipitation of BIF of the As- Beukes (1980a), for the Campbellrand Subgroup bestos Hills Subgroup, which date between around carbonates, two different facies realms in the south- 2500 and 2432 Ma (Trendall et al., 1990, 1995; western and northeastern part of the Griqualand Barton et al., 1994). West basin are separated by a synsedimentary hinge, The detailed sedimentology, geochemistry and the Griquatown growth fault. North of this fault, petrography of tuffs from the Campbellrand Sub- the Reivilo to the Kogelbeen Formations form the group are described by Altermann (1996a). Separa- ‘Ghaap Plateau Facies’ sequence of stromatolitic tion of fine and coarse grains in tholeiitic tuffs of the carbonate platform sediments (Beukes, 1980a). The Nauga Formation carbonates suggests deposition in Monteville and Gamohaan Formations, respectively, shallow water, perhaps a few metres to 40 m depth. at the base and at the top of the Campbellrand Sub- A tuff layer close to the top of the Nauga Formation group, north of the fault zone (Figs. 1, 3 and 7), were carbonates was dated by the SHRIMP U–Pb method interpreted as basinal, shelf, or endoclastic basinal on zircons, at 2552 11 Ma (Barton et al., 1994). facies framing the platform. South of the Griquatown Proximal tuffs were šfound within the Nauga Forma- fault zone, these formations pass into the basinal tion, near Prieska. The tuffs thin out and become Nauga Formation (compare Figs. 2, 3 and 7), which finer-grained towards the north and away from the includes the entire carbonate section of the Camp- peritidal flats described by Altermann and Herbig bellrand Subgroup accumulated south of the fault. (1991). Altermann (1996a) suggested that the vol- A thick sequence of shales (Naute Shale Member) canic centres were located along the southwestern with some chert beds of great lateral continuity cov- margin of the Transvaal sea, to the south and south- ers the Nauga Formation carbonates. The difference west of the present margin of the Kaapvaal craton. in thickness between the basinal carbonates south Volcanoes might have formed islands and the craton of the Griquatown fault (600 m) and the platform and the epeiric basin probably extended farther to north of the fault (1600 m on the Ghaap Plateau) is the southwest, into areas now occupied by younger striking. Together with Beukes’ (1980a) depositional Proterozoic mobile belts (Altermann and Ha¨lbich, model, this difference tempted Grotzinger (1989) to 1991). Zircons collected from these tuffs are the hypothesize a possible relief of 950 m between the source for the new age data presented herein. base and the top of the Campbellrand platform, at New investigations of the Nauga Formation show the time of its terminal drowning. rapid lateral and vertical facies changes within the Altermann and Herbig (1991) proposed an alter- lower part of this formation. Vertically, the Nauga native model in which the intracratonic Griqualand Formation can be subdivided into five informal mem- West basin experienced its highest subsidence rates bers, as illustrated in Fig. 2 (Kiefer et al., 1995; in its central parts, north of the Griquatown fault. The Altermann, unpubl. data). subsidence was matched by stromatolitic growth and (1) A mixed siliciclastic and carbonate clastic carbonate accumulation (building the Ghaap Plateau) member at the base of the formation. and thus, shallow marine conditions prevailed. South (2) A peritidal member, consisting of widespread of the Griquatown fault, peritidal flats often exposed tufted Stratifera-like mats with abundant palisade to erosion prevented the accumulation of a thick structures intercalated with loferite beds and tidal pile of carbonate strata. The decline in carbonate channel carbonate sand bodies. sedimentation was accompanied by siliciclastic in- (3) A chert member follows, in which the tidal flat

Fig. 3. Brief lithological description and stratigraphic subdivision (Altermann and Siegfried, 1997) of the borehole drilled at Kathu, Sishen (Fig. 1). The ages of the formations were dated on samples from outcrops remote from Kathu, and are thus tentatively correlated here on lithostratigraphic grounds. The sedimentation rates given in Bubnoff units are for decompacted sediments. Note that the section is disrupted in the upper part (thick dyke intrusion) to save space. 232 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 features give way to deep lagoonal platy dolmicrites, by defining stratigraphic units on the basis of stroma- dolarenites and microbial laminites. Three laterally tolite morphology and on carbonate facies and pet- persistent chert marker horizons are intercalated. rography. Such a lithostratigraphic approach is only (4) An overlying proto-BIF member consists applicable if the cyclicity and hydrodynamic condi- mainly of carbonates with some coiled thin micro- tions were uniform across the entire basin. In this cor- bial mats (Kiefer et al., 1995) resembling those in the relation, the Gamohaan Formation, at the top of the chert member. It comprises three laterally persistent Campbellrand Subgroup, passes northeastward into BIF-like horizons (proto-BIF of Button, 1976). the Frisco Formation, at the top of the Malmani Sub- (5) The approximately 150 m thick Naute Shale group carbonates in Transvaal. The Monteville and member covers the carbonates. These finely lami- Reivilo Formations of Griqualand West interfinger nated shales, intercalated with rare thin tuffites and with the Oaktree and Monte Christo Formations at the prominent chert beds, represent deposition on the base of the Malmani Subgroup (Beukes, 1986, fig. 7). shelf, probably below the storm wave-base. 3. Sample localities and description 2.1. Correlation to Malmani Subgroup in the Transvaal Four samples were processed for zircon dating. The sampling sites are shown in Fig. 1. No continuous outcrops exist between the sedi- (1) Sample WA92=4 was collected from the up- ments of the preserved Transvaal basin and of the permost tuff bed of the Gamohaan Formation, at Griqualand West basin, although the two sub-basins the Kuruman Kop peak, north of the town Kuru- share the same basement of Ventersdorp Supergroup man (Fig. 1). This stratigraphic level was correlated volcanics (Fig. 1). The Black Reef Quartzite Forma- by Beukes (1980a) with the stratigraphic position of tion (Fig. 7) is generally accepted as the Transvaal the sample dated by Barton et al. (1994) and of the basin equivalent of the 2642 Ma old Vryburg Forma- sample WA93=12 described below. The stratigraphic tion (lower Schmidtsdrif Subgroup) in Griqualand section through the Kuruman Kop was recorded and West, for both formations unconformably cover the depicted in detail by Ha¨lbich et al. (1992, fig. 10). Ventersdorp Supergroup (compare Figs. 2, 3 and 7; The sample is from the upper Gamohaan Formation, SACS, 1980). The upper Schmidtsdrif Subgroup is from lithofacies ‘e’ (microbial laminites, grainstones commonly correlated with the Oaktree Formation and shales) of Ha¨lbich et al. (1992), and lies ap- at the base of the Malmani Subgroup in Transvaal proximately 40 m below the nearly 30 m thick Tsi- (Altermann and Wotherspoon, 1995). Tuffs in the neng member (Beukes, 1980b), which represents a upper Oaktree Formation were dated at 2550 3 Ma transition from carbonate to BIF sedimentation. The (U–Pb on zircons) by Walraven and Martini š(1995). stratigraphic thickness to the massive Kuruman IF Like the Ghaap Plateau facies, the Malmani Sub- proper is around 75 m. It is probably the same tuff group carbonates also consist of several formations. bed as that dated by Sumner and Bowring (1996) at These formations were grouped into genetic units 2521 3 Ma. The horizon is 45 cm thick and consists and attributed by Clendenin (1989) to transgression– of thršee graded, fine lapilli to ash tuff intervals with regression cycles. The first two transgressive cy- thin dolarenitic interlayers, and with some tuffaceous cles are documented in the lower Monte Christo admixture. The pure tuff beds are interpreted as fall- Formations of the lower Malmani sediments. The out tuffs, as they are normally graded, lack Bouma in- upper three formations of the Malmani carbonates tervals and there is a general absence of layers resem- (Lyttelton, Eccles and Frisco Formations) reflect to- bling turbidites within this facies (Altermann, 1996a). gether the third major transgression, followed by the Over 50 zircons were recovered from about 7 deposition of the Penge Iron Formation (fourth trans- kg of rock. The zircons are morphologically ho- gressive cycle) which correlates with the Kuruman mogeneous, short- to long-prismatic (100–150 µm), BIF in Griqualand West. idiomorphic, pink and clear. Rare inclusions are Beukes (1986) correlated the Campbellrand Sub- present in some of the zircons. group with the Malmani Subgroup in the Transvaal, (2) Sample WA93=41 was collected from an out- W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 233 crop along the road from Douglas to Niekerkshoop, imately 60 m below the Naute Shale member, was on the farm Suiversfontein and is from the upper Mon- taken 6 m below the three prominent chert marker teville Formation, Ghaap Plateau, adjacent to the Gri- horizons of the chert member (Figs. 1 and 2). These quatown fault. The carbonate facies at the sampling three chert horizons are very uniformly distributed area is of cross-bedded and finely laminated dolaren- between Nauga and Kliphuis. The tuff band dated by ites. The volcaniclastic band in these dolarenites is 20 Barton et al. (1994) pinches out and is not present cm thick. A few thin shale beds are intercalated in the in the Kliphuis area. On the farm Klein Naute, mid- lower part of the outcrop, together with three promi- way between Nauga and Kliphuis (Fig. 1), 28 m of nent Fe-rich chert beds, that are brecciated in places sedimentary section separate these two tuff horizons. along a minor fault (approximately 10 m below the tuff About 30 zircons were recovered from 7.35 kg band). The breccia exhibits weak Pb (galena) miner- of rock. The zircons are morphologically similar, alization. Above the tuff band, cross-bedded dolaren- equant to long-prismatic, idiomorphic, 100–200 µm ites pass upward into stromatolitic mats. The micro- long, pink and dim. Rare inclusions are present in bial lamination builds lateral linkage between small some of the zircons. conical to sub-conical columns. Abundant, cm-large (4) Sample WA93=15 was collected on the farm fenestral cavities filled by calcite and rarely by quartz, Engelwildgeboomfontain, at Prieska, close to the are irregularly distributed in the columns and between Kliphuis farm boundary. It is from the same sec- the laminae. The bioherms resemble thyssagetacean tion as WA93=12 and stratigraphically about 230 stromatolites, as described by Hofmann and Masson m below it, within the peritidal member of the (1994). The overall facies is interpreted as shallowing- Nauga Formation. The section (shown in Fig. 2) upward, entirely subtidal, but with upward decreasing does not outcrop continuously and has been as- hydrodynamic energy. The volcaniclastic bed itself sembled from several shorter sections, measured by shows no internal sedimentary structures apart from ‘Jacob’s staff’, and only a few tens to hundreds of a faint lamination. Because different zircon popula- metres apart. This was necessitated by folds and tions were found in this sample, it may represent a faults displacing the measured sections of strata rel- reworked sediment, such as a tuffite. This interpreta- ative to each other. Approximately 10 m of strata, tion is consistent with the nature of the cross-bedded judged from detailed mapping, are missing between dolarenites directly above and below the tuffite. the measured sections from the Engelwildgeboom- About 25 zircons were recovered from 8.5 kg of fontain and Kliphuis farms, and are probably of shale sample material. The sample was rich in pyrite. Two that makes no outcrops. The tuff bed sampled here is of the zircons were well rounded and abraded and only 5 cm thick and roughly correlative of Beukes’ of orange-brown colour. These were not analyzed. (1980a) ‘tuff 4’ from the ‘Central Dolomite Zone’ Other zircons were broken, long- or short-prismatic, (Beukes, 1980a, fig. 22). This zone is characterized xenomorphic, between 50 and 100 µm long, and by microbial laminites with Stacked Hemispheroids- some of them were abraded (subangular to sub- Inverted (SH-I) structures and interpreted as peritidal rounded). They exhibit common inclusions and all to supratidal Stratifera-like biostromes (compare Al- were pink and turbid. termann and Herbig, 1991). (3) Sample WA93=12 was collected on the farm Over 100 zircons were recovered from 5.0 kg Kliphuis, at Prieska, from the uppermost tuff bed of rock. The zircons are equant to long-prismatic, in the carbonates, below the Naute Shales. It comes idiomorphic, 100–150 µm long, pink and clear, with from the top of the chert member, 10 m above the some inclusions. three prominent chert marker horizons of the Nauga Formation. In the measured section, it is located 48 4. Analytical procedures m below the Naute Shale member and almost at the same stratigraphic level as the sample dated by Bar- Samples were crushed in a jaw crusher and bro- ton et al. (1994). However, the sample dated by Bar- ken to <2 mm particle size in a cylindrical rolling ton et al. (1994) from the farm Nauga, approximately mill, and then passed through a 180 µm sieve. The 30 km northwest of the farm Kliphuis and approx- sieved fraction was processed using a Wilfley ta- 234 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 ble. The heavy mineral fraction was further purified such as that arising from the inclusion of analyses of using a Frantz magnetic separator and methylene io- slightly older xenocryst zircons or zircons that may dine. Zircons were hand picked from the resulting have lost small amounts of radiogenic Pb. Chi-square mineral fraction, mounted in epoxy and sectioned values for grouped analyses of less than or equal to approximately in half, and the mount surface was unity indicate that scatter about the weighted mean then polished to expose the grain interiors. value determined for the grouped analyses can be ac- U–Th–Pb measurements were made using the counted for by analytical sources of error alone. A ion microprobe SHRIMP-II at Curtin University of chi-square value significantly greater than unity indi- Technology, employing operating and data-process- cates that analyses are not normally distributed about ing procedures similar to those described by Comp- the weighted mean value and that other (geologi- ston et al. (1984) and Williams et al. (1984). Pb=U cal) sources of error are present within the grouped ratios were determined relative to that of the stan- population. In these cases, the 95% confidence error dard Sri Lanka zircon CZ3, which has been assigned is based on the observed scatter about the weighted a 206Pb=238U value of 0.0914 corresponding to an mean 207Pb=206Pb ratio of pooled analyses. age of 564 Ma. Reproducibility of the Pb=U ratio of the standard was better than 1:6%; this uncertainty 5. Analytical results is included in the quoted ašnalytical errors. Errors given on individual analyses are based on counting 5.1. WA92=4 statistics and are at the 1¦ level; those given on pooled analyses are at 2¦; or 95% confidence. Ages Analytical data are summarized in Table 1 and cited are based on weighted mean 207Pb=206Pb ratios. shown on a conventional concordia plot in Fig. 4. Features such as zircon morphology (size, shape, All analyses plot within the error of the concordia, zonation, etc.) and chemistry (U and Th contents, or are only slightly discordant. Sixteen analyses of Th=U ratios), degree of discordance of each analy- 16 zircons gave a 207Pb=206Pb age of 2516 4 Ma š sis and evidence of radiogenic Pb loss were taken into (95% confidence). This is regarded as the crystalliza- account in the assessment of the validity of pooled tion age of the zircons and the age of the tuff layer. analyses. Dates were determined using the mean One analysis (4.1) had a slightly lower 207Pb=206Pb 207Pb=206Pb ratios determined from pooled analyses. ratio corresponding to an age of 2476 9 (1¦) š Individual analyses were weighted according to the Ma. This analysis is probably of a zone which has inverse square of the individual analytical error (based experienced some post-crystallization loss of radio- on counting statistics) of the analysis, for the de- genic Pb. Cathodoluminescence imaging of the zir- termination of the weighted mean 207Pb=206Pb ratio con growth zones reveals no abnormalities at the of pooled analyses. Analyses more than 2¦ from analyzed site. If this analysis is included in the sta- the weighted mean value were treated as ošutliers and tistical calculations, the weighted mean 207Pb=206Pb deleted from the pool, and the weighted mean value age is 2513 4 Ma, and thus insignificantly differ- š then recalculated. This process was repeated until all ent from the calculated age of 2516 4 Ma. One š pooled analyses were within 2¦ of the weighted analysis (3.1), indicated an early Palaeozoic age and mean value and the remaining špooled data were nor- is believed to be a contaminant introduced during mally distributed about the mean. Where there was sample preparation, and is not discussed further. no obvious justification (based on zircon morpho- logical or chemical differences) for deletion of out- 5.2. WA93=41 liers and their deletion did not significantly affect the age and error obtained, the outliers were retained A total of sixteen analyses were obtained on four- within the pooled population used to determine the teen zircons. The analyses fall into four statistically weighted mean date and error. A chi-square test was distinguishable age groups (Fig. 4). applied to grouped analyses in order to assess the rel- Group 1, consisting of seven spots on five zir- ative effects of analytical sources of error, such as cons (0.1, 3.1, 3.2, 4.1, 6.1, 6.2, 8.1), has a pooled counting statistics, and geological sources of error, weighted mean 207Pb=206Pb age of 2637 30 Ma. š W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 235 e 4 l p š m 6 a 1 s 5 n 2 i f s o n o n c o r i i t z a l e u c h l t a f c o e e g c a n a e d h t r o n c i n o d c e - d r u e l v c o n i t . t n t o e x r n e a t , p 1 e p . h a 4 t e n n i h o c T d r e i . s s z s n s u o i c c s r 4 i i = d z 2 d e 9 n h A t a f W n o e n v i n i o g x i t o e i r s b a o d p s e r m d o r a o r c h e s c i e ¦ p h 2 o T t h t o . i t s i x w e g t s n e i e g h a w t o e n h l i i s h , d s e w e s , l s p ¦ u 1 c m s a i e s r d a d e s e r t z a o y l l e p a u n e q i a h t n r h n u c i o e f t n e g w h n o t i h z r s y o l s f a e s n x t a o o l b d p r n o a a r i r s d e r m l o l e c l a n b t o o a r c h p t l a e e l n t b o o i i t s N s n . e o ) t v p x n , e o t 1 C 4 e = . e 3 s 4 ( 9 . a g A i M F W 236 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 Pb ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5 6 9 6 6 3 7 0 1 2 4 8 8 3 9 0 3 8 7 ) ) ) ) ) ) ) ) 0 1 ) ) ) ) 3 ) 206 1 1 2 1 1 2 1 2 6 3 4 3 3 3 3 4 4 1 1 7 7 6 6 6 6 7 6 1 1 9 6 6 8 1 5 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( / ( = ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ¦ 6 5 1 2 4 7 6 7 6 7 1 2 6 9 4 4 2 8 8 3 1 4 8 4 2 4 7 2 7 6 6 4 8 9 6 Pb 1 3 7 3 3 6 0 9 1 2 0 3 4 6 5 4 7 4 0 6 1 2 2 1 1 1 0 0 3 2 7 1 2 0 0 1 5 5 7 7 6 6 6 7 4 6 7 6 4 6 6 5 5 5 5 5 5 5 5 5 5 5 5 5 5 4 5 5 5 5 5 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 207 . 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 U ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 9 9 6 9 9 3 0 1 9 7 4 0 8 7 1 0 0 8 6 3 3 3 3 3 3 3 3 1 2 4 3 3 4 5 3 235 1 1 2 1 1 2 2 2 3 2 3 3 2 2 3 3 3 1 1 1 1 1 1 1 1 1 1 2 2 1 1 1 1 1 1 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ) ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( = ¦ 5 8 0 3 7 0 7 5 7 8 8 6 2 6 4 0 0 1 0 3 5 0 8 4 3 7 7 7 7 5 7 7 5 0 3 Pb 1 5 5 9 7 9 6 0 1 6 2 3 1 5 3 0 5 8 1 1 1 9 0 8 9 7 7 8 7 6 3 0 9 6 5 1 5 6 7 7 6 6 7 7 4 6 7 7 5 7 7 6 3 5 3 5 4 5 4 4 4 4 4 4 4 4 5 4 4 4 5 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 207 ( 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 U ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5 8 4 9 8 9 8 9 8 9 5 5 9 3 4 0 9 0 3 3 6 7 7 7 6 8 7 7 7 7 7 7 7 2 7 238 3 3 4 3 3 3 3 3 3 3 4 4 3 4 4 4 2 3 2 4 2 2 2 2 2 2 2 2 2 2 2 2 2 4 2 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ) ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( = ¦ 8 9 1 9 0 3 0 2 8 4 7 6 1 0 4 1 5 5 0 6 6 6 4 4 9 4 4 1 0 0 5 4 4 0 9 Pb 1 7 6 7 2 4 1 2 1 1 5 4 1 6 4 8 5 9 1 3 9 8 9 6 1 7 1 6 7 5 7 2 4 6 1 0 5 7 8 8 7 7 7 7 5 6 7 8 6 8 7 7 1 5 0 3 3 4 4 4 3 5 4 4 4 4 4 4 4 4 5 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 206 ( 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 h ) ) ) ) ) ) ) ) T ) ) 5 ) ) ) ) ) 4 ) 1 0 9 2 1 5 ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) a 9 7 6 4 8 1 2 2 3 3 0 9 3 5 7 0 8 9 6 2 0 8 7 9 6 9 8 8 8 7 5 7 7 0 6 232 5 6 1 7 5 9 8 9 1 9 2 1 1 1 1 1 6 7 3 6 4 3 3 3 2 3 3 3 3 3 3 3 3 6 3 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( M ) ( ( ( = ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ¦ 6 2 6 7 5 5 0 0 9 1 6 2 4 5 0 6 6 3 5 8 1 0 3 3 2 0 2 0 4 7 0 5 3 7 9 e Pb 1 2 3 3 9 0 9 3 9 9 3 1 3 1 5 3 9 4 1 6 7 3 2 4 3 9 8 9 5 2 0 7 8 4 1 3 g 4 6 9 6 5 3 4 5 2 4 3 3 2 7 6 4 1 5 6 3 2 4 4 3 0 4 3 4 4 4 3 2 4 4 4 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 A 208 ( 2 2 1 2 2 2 2 2 1 2 2 2 2 2 2 2 2 2 2 ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5 6 2 8 7 4 9 3 5 4 0 0 5 5 2 0 4 7 7 0 8 6 6 8 3 7 7 6 6 6 6 7 6 6 5 1 1 3 1 1 2 1 2 5 3 5 4 3 3 4 4 4 1 1 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... Pb ...... 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 206 ) 9 8 7 9 2 1 8 1 2 1 7 8 0 7 1 7 = 5 0 1 9 0 8 6 0 1 5 3 6 0 7 5 6 9 4 9 7 1 8 8 1 5 4 7 7 5 8 8 1 0 9 1 8 5 1 6 2 5 6 5 5 5 6 6 6 5 5 4 4 7 5 ¦ 6 7 8 8 8 7 8 8 5 7 8 7 6 8 7 7 6 6 7 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 Pb 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ...... š 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 207 ( 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 8 4 7 5 7 4 4 0 2 1 7 5 4 1 7 3 4 0 8 4 4 3 7 7 1 4 0 0 2 6 5 2 2 8 8 3 7 9 1 9 2 2 3 9 3 8 8 1 4 8 0 0 5 3 8 7 3 0 1 5 3 2 0 1 6 0 9 5 4 0 2 9 5 9 6 0 8 0 2 4 8 2 4 9 2 9 0 0 5 3 4 5 5 5 5 5 5 5 5 4 5 4 6 3 5 2 3 2 2 2 3 2 3 4 3 4 4 3 3 4 3 3 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 ...... 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 U 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 6 8 5 3 4 0 2 8 9 2 9 3 0 6 3 9 7 5 7 4 4 3 7 5 2 0 0 2 4 1 4 7 8 9 8 235 ) 2 1 7 5 7 7 5 0 5 1 0 9 5 4 3 0 = 3 9 8 9 2 6 3 7 2 6 1 6 4 0 4 3 2 6 9 d 8 0 0 4 3 3 7 9 5 0 2 0 4 9 3 0 e 2 5 2 5 0 0 9 3 8 7 3 8 6 2 6 8 6 6 7 ¦ t 3 6 7 3 2 7 3 4 3 3 8 5 3 7 3 6 4 8 7 3 0 8 6 3 8 6 7 5 6 4 4 5 1 4 8 Pb 1 c ...... e 1 4 2 4 3 2 3 3 0 2 3 3 1 3 3 2 9 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 š r 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ( 207 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r o ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) c 1 7 1 3 9 3 0 1 7 1 7 8 2 2 5 4 9 2 6 9 9 2 1 0 3 1 2 1 2 0 1 1 9 0 2 8 0 9 9 8 9 9 9 8 9 0 0 9 0 0 9 b 6 6 9 4 5 6 6 6 6 6 6 6 6 6 6 6 5 1 6 0 1 0 0 0 0 0 0 0 0 1 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 P 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... U ...... 204 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( , 238 s d ) 8 1 4 8 7 5 9 0 8 4 2 9 0 7 0 2 = 3 7 1 1 9 8 5 3 9 6 1 5 9 7 0 4 4 4 8 o e 1 1 6 0 9 7 4 3 7 9 1 7 1 3 0 2 7 5 0 0 7 2 5 4 6 5 7 2 6 6 1 5 6 3 5 i ¦ s t 9 6 3 5 2 2 2 2 7 0 3 4 1 5 4 3 7 0 5 7 4 7 6 5 7 6 6 6 6 5 6 6 4 5 7 s Pb 1 a 4 5 5 5 5 5 5 5 4 5 5 5 5 5 5 5 4 4 4 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 u r ...... š c 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ( 206 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 c s i i ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) d m 3 4 8 2 3 0 6 2 4 2 7 5 7 6 7 9 4 7 4 9 2 1 1 1 2 1 1 1 1 0 0 1 4 4 0 s o 3 9 3 4 3 5 4 5 7 5 1 0 7 8 9 5 4 3 3 1 2 2 2 2 2 2 2 2 2 2 2 2 1 3 2 t e 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 l 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a h 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 p 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... T d 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 m e ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( t ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( a 232 a s l ) 5 4 1 8 9 8 8 7 5 8 4 3 8 0 0 5 4 0 8 9 7 2 5 3 6 6 9 4 5 4 7 5 5 0 = 2 r u 7 6 9 2 1 5 7 6 0 7 1 2 5 6 9 1 2 2 4 5 6 7 8 2 0 5 8 7 6 4 9 8 5 7 8 ¦ u c 2 5 3 4 3 2 2 3 2 2 2 2 1 4 3 3 3 1 2 8 1 2 2 2 3 2 2 2 2 2 1 2 5 2 2 Pb 1 l o 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 a f ...... š 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 208 ( C 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 e h t b P m o 204 r = f 2 8 0 4 8 4 6 5 5 8 1 5 6 0 7 4 4 8 5 2 7 7 0 1 3 5 5 9 5 9 7 2 1 5 6 6 8 6 5 7 9 9 6 2 1 0 2 6 1 7 7 s 7 3 4 8 1 3 0 0 0 0 3 4 1 6 6 9 2 8 3 Pb 7 2 3 4 4 2 6 3 2 5 5 7 5 8 8 2 n 8 4 4 5 8 9 0 4 8 5 5 8 6 3 3 6 7 7 2 1 1 3 2 1 1 1 1 o 2 9 2 7 8 3 6 4 7 6 9 6 4 7 0 1 c 206 8 1 1 r i . z f b o corr 5 4 7 4 9 0 2 6 5 7 1 2 8 6 5 4 3 5 5 5 2 2 7 5 4 1 0 0 1 8 1 1 0 6 4 P ) s 9 1 6 8 1 3 5 7 0 7 6 9 5 2 3 4 2 1 0 5 4 3 2 5 5 6 8 9 5 3 1 7 1 3 6 . e ...... m Pb s d 7 0 9 4 9 6 5 1 3 9 1 7 1 1 0 5 5 3 6 8 9 8 2 7 7 6 9 3 2 2 8 4 1 1 2 p m a y 8 3 3 6 5 2 7 8 4 2 4 3 3 7 6 5 3 2 0 8 0 4 5 1 5 3 4 5 3 3 7 6 4 5 1 l p F 204 R ( 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 a n a a g U . 5 6 4 6 2 5 3 4 3 2 7 6 4 2 2 7 5 2 6 6 1 9 8 7 5 9 0 0 9 6 3 0 3 9 2 u = . b 6 6 4 8 9 5 5 5 5 9 5 8 5 6 6 4 4 3 4 4 5 4 4 5 9 4 5 5 4 5 5 5 1 4 5 a h ...... P m Fm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 N T 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 – F , h . 8 2 1 5 8 5 3 5 8 7 3 6 4 1 n ) T b 0 9 9 4 4 2 1 6 7 1 5 5 6 5 4 1 1 2 9 3 9 8 9 8 4 2 8 5 5 1 4 7 9 1 9 ille a – ...... m m v a 8 0 0 0 2 7 3 6 2 3 3 3 4 3 0 7 7 1 7 4 1 7 0 0 9 4 6 7 3 7 4 4 3 5 8 U p t h h 6 2 6 4 3 2 6 8 5 8 2 7 4 2 3 5 9 0 1 4 1 3 4 3 8 4 2 2 6 7 4 4 5 2 4 p r o e 1 1 T ( 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 e b m h ) o a 5 6 6 2 6 2 6 1 6 2 0 9 9 0 7 2 5 2 2 3 2 9 5 8 8 0 6 0 3 1 2 1 5 2 1 r Monte C ...... m p G . 5 4 7 0 3 0 1 0 3 2 7 3 7 7 1 9 6 3 1 1 9 0 2 7 9 4 5 3 5 8 3 0 4 6 6 p o . 2 5 0 4 7 7 6 5 2 9 9 9 4 1 2 7 4 6 4 1 1 1 8 9 2 9 9 5 5 3 1 7 9 2 5 8 r p 41. 4 1 1 1 1 1 1 U ( 3 2 2 2 2 2 1 2 2 2 3 3 2 2 2 2 2 c 1 i = = = 3 2 3 n e m i l 9 9 9 1 1 1 1 1 1 2 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 a ...... b n A A A . r a 0 1 2 3 1 2 4 5 6 7 9 5 6 6 7 8 9 1 2 0 1 2 3 3 4 2 3 4 5 6 7 0 1 0 1 r o 1 1 1 1 1 1 2 2 W I G W 1 1 T 1 1 1 1 W n W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 237 Pb ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 2 ) 7 4 4 0 3 2 2 2 9 8 7 2 0 7 4 1 ) ) 3 8 7 8 4 8 2 0 2 0 6 3 6 8 8 206 7 9 1 9 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 9 9 1 1 1 1 1 3 5 4 5 3 5 3 1 4 2 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( / = ¦ 6 2 6 1 0 3 4 2 9 2 0 4 0 8 9 8 7 1 3 6 6 1 9 7 4 6 0 1 9 7 1 9 5 6 3 9 9 Pb 1 9 9 9 6 8 7 7 0 7 8 7 8 0 8 8 7 5 4 4 4 4 4 0 7 9 4 7 3 2 8 3 2 3 2 4 6 0 5 5 5 5 5 5 5 6 5 5 5 5 6 5 5 5 5 5 5 5 5 5 6 5 5 5 5 5 5 4 5 5 4 5 5 5 5 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 207 . U ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 6 7 8 4 9 8 9 7 8 8 7 8 1 0 0 7 5 2 5 5 4 4 8 0 0 8 6 8 6 9 5 3 7 4 6 0 1 235 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 1 1 2 1 1 1 1 1 2 2 1 1 2 3 2 3 2 3 2 1 3 2 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ) = ¦ 1 3 4 6 1 6 2 4 0 1 8 9 2 5 1 7 9 7 5 9 8 0 3 3 3 9 4 4 0 8 0 8 1 6 5 3 1 Pb 1 7 7 8 1 0 7 8 8 9 8 8 6 8 7 2 6 2 4 7 2 4 2 8 9 7 3 2 6 8 1 3 2 6 2 3 4 1 5 5 5 5 6 5 5 5 5 5 5 5 5 5 6 5 5 5 4 5 5 5 5 5 5 5 5 5 5 5 5 5 4 5 3 1 4 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 207 ( U ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 3 4 7 7 5 6 4 5 5 5 4 7 6 8 4 8 2 7 8 8 8 5 7 5 0 8 6 1 4 5 1 6 1 4 3 8 5 238 3 3 2 3 3 3 3 3 3 3 3 3 3 3 3 2 3 2 2 2 2 3 3 3 3 2 3 4 3 3 3 3 3 2 2 2 3 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ) = ( ¦ 8 8 2 9 9 2 1 3 8 1 0 0 9 2 2 4 4 3 9 0 9 9 4 0 6 9 7 5 6 6 9 5 3 5 4 7 7 Pb 1 3 4 6 2 7 9 6 0 7 1 5 6 5 6 5 9 5 9 0 5 3 4 1 7 4 2 6 0 4 5 2 2 9 2 0 2 9 5 5 4 6 5 5 5 6 5 6 5 5 5 6 5 4 5 3 5 5 4 5 6 5 5 5 4 6 6 5 5 5 4 5 1 7 2 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 2 206 ( h ) ) T ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5 ) ) 1 ) ) ) ) ) a 4 6 6 1 6 7 0 6 3 4 2 5 0 1 0 3 1 1 9 2 8 8 5 3 0 6 9 2 1 8 2 2 7 0 1 0 1 232 4 4 3 7 5 5 5 5 5 5 5 7 7 8 5 4 5 3 4 4 3 5 7 5 7 6 3 8 1 5 5 1 5 7 3 7 5 M ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ) = ( ¦ 5 6 5 8 8 9 4 8 8 1 9 7 5 2 4 5 0 6 7 5 1 5 7 6 8 2 6 6 6 5 7 5 8 7 1 8 8 e Pb 1 2 3 5 4 9 7 4 1 4 7 3 2 9 6 4 5 2 5 3 1 2 3 6 2 7 8 2 2 6 5 5 7 2 3 4 2 7 g 5 5 3 5 4 4 5 6 5 5 5 6 4 6 3 4 5 2 4 5 4 5 5 5 4 4 4 5 6 2 5 3 4 4 9 8 4 š 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 2 A 208 ( ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 8 0 9 7 4 4 9 3 3 2 2 9 8 8 2 0 6 4 1 9 9 3 8 3 8 8 5 8 2 8 1 9 2 2 6 9 8 0 1 0 1 1 1 0 1 1 1 1 1 1 1 1 1 2 1 1 0 0 1 1 1 1 1 1 3 5 3 5 2 5 3 1 4 2 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... n Pb i 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 g ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( r 206 a ) 0 6 3 2 6 6 3 2 5 3 7 3 1 2 1 9 3 5 8 8 3 3 0 9 8 8 2 3 1 0 4 2 1 9 5 2 2 = 4 3 0 2 1 1 6 2 2 1 2 4 3 3 2 9 8 8 8 8 8 5 2 3 3 8 1 7 7 3 7 7 8 6 8 1 5 ¦ m 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 6 6 6 6 6 6 7 7 7 7 6 7 6 6 6 6 6 5 6 6 7 6 Pb 1 r 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ...... o š r 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 207 ( r e ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 0 2 3 2 0 5 2 3 7 8 8 4 5 8 9 4 4 9 1 6 4 6 7 1 8 9 7 5 0 7 3 0 1 4 0 4 0 ¦ 7 3 1 1 4 8 4 8 3 1 1 4 0 6 3 1 2 3 7 4 4 6 3 2 1 0 1 7 2 9 0 9 4 2 7 7 7 1 9 0 6 7 6 1 4 5 1 7 6 7 7 4 2 3 1 2 1 1 5 8 2 5 2 4 1 0 4 4 3 6 8 0 5 4 2 1 2 1 1 1 2 2 2 2 1 2 1 1 2 2 2 2 2 2 2 2 1 2 2 2 2 2 4 3 4 3 2 2 4 1 2 2 n ...... i 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 U h ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( t i 2 3 4 0 4 3 5 1 2 9 0 1 9 8 7 0 3 2 8 2 6 0 2 1 9 7 2 1 6 3 8 8 1 9 5 3 9 235 w ) 3 1 9 1 1 8 7 8 7 6 0 0 3 6 1 2 3 1 1 2 6 8 5 8 5 3 3 5 7 3 9 3 9 1 6 1 8 = d e 7 0 1 3 6 9 2 0 2 6 8 7 9 5 4 1 4 1 9 5 1 0 2 5 4 0 8 8 8 9 3 5 2 9 6 5 4 e ¦ t r 5 6 9 4 9 6 6 2 5 0 2 0 2 9 6 7 7 8 7 5 7 0 7 8 7 6 1 0 4 6 9 0 0 2 9 2 7 Pb 1 c a ...... e 1 1 0 0 0 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 0 8 7 9 š s r 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ( 207 r e o g ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) c a 7 7 2 0 3 0 3 8 9 4 4 4 5 5 1 3 9 1 1 9 5 4 0 6 0 2 8 1 2 4 6 9 1 7 1 2 1 7 7 6 6 6 8 8 8 7 6 7 6 6 8 8 8 7 8 8 7 8 6 8 8 8 8 6 8 5 8 9 7 7 4 6 7 8 b d 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 n P 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a ...... U s 204 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( o , i 238 s t ) 4 7 0 4 3 8 2 2 7 3 1 7 2 5 0 9 9 4 4 1 4 2 1 0 8 3 4 4 0 0 4 7 6 2 1 4 1 = o a 2 4 5 9 2 1 7 1 5 2 6 5 5 3 2 4 7 7 9 5 7 6 5 0 9 4 0 0 6 8 7 6 9 7 8 9 2 i r ¦ t 8 8 6 4 7 9 8 1 8 7 8 7 8 0 9 9 8 9 9 8 8 6 8 0 8 8 8 8 8 9 0 8 7 0 2 7 7 Pb 1 a c 4 4 4 4 4 4 4 5 4 4 4 4 4 5 4 4 4 4 4 4 4 4 4 5 4 4 4 4 3 4 5 4 4 3 4 4 4 r i ...... š 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 206 ( c m i o ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) t m 4 6 1 7 1 0 9 6 8 4 9 7 4 0 1 2 8 2 1 9 2 2 2 2 0 9 7 9 7 6 5 2 7 8 8 9 2 a o 2 2 2 1 2 3 3 4 2 2 2 2 2 4 3 3 2 3 3 2 4 2 3 4 3 3 3 2 1 4 6 3 6 3 2 3 3 t d 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a h e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 t ...... T d a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 e l ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( t ( ( ( u 232 a c l ) 1 7 6 1 2 4 4 8 0 2 8 1 5 4 6 5 2 3 6 8 8 5 6 2 1 5 7 9 8 1 0 1 7 6 4 1 6 = l u 3 3 3 8 7 4 1 0 3 9 2 8 2 4 1 0 4 8 5 3 8 7 3 5 3 0 0 4 0 3 1 8 4 4 0 8 7 a ¦ c 3 3 2 1 2 3 3 4 2 2 3 2 3 3 3 3 3 3 3 3 3 2 3 3 3 3 3 3 0 3 4 1 2 9 3 2 2 c Pb 1 l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 a t ...... š a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ( 208 C h t b e P t o 204 N = . 0 2 8 9 5 7 1 2 4 8 2 5 5 5 5 5 7 4 9 0 4 2 4 8 4 0 9 8 6 1 8 0 5 1 2 2 0 n 5 3 8 0 0 3 1 7 3 7 7 2 1 7 9 9 2 4 3 3 2 2 2 5 1 5 9 1 1 9 4 2 1 7 1 7 5 Pb o 9 7 1 7 2 4 4 2 5 3 2 5 1 8 7 9 4 4 8 7 4 2 0 5 8 4 7 6 8 9 9 8 9 4 8 9 4 i 5 7 2 3 4 3 3 6 3 1 2 4 2 2 3 7 5 6 4 7 2 9 3 3 4 1 t 2 206 p i r . c s . e b corr d m 4 3 1 6 4 6 9 1 6 1 5 1 7 8 8 5 0 8 6 9 3 8 3 9 2 0 2 0 7 1 4 2 8 8 5 8 4 P ) t 0 2 1 1 2 3 9 2 7 1 1 6 5 4 8 0 1 3 2 9 9 9 2 6 0 3 5 4 9 0 2 3 3 8 3 5 4 F ...... m x Pb d 8 3 9 2 1 0 4 8 9 8 5 8 6 0 6 2 0 3 1 5 7 6 2 7 8 4 8 3 4 3 9 8 8 5 3 0 4 a e p a t 1 0 6 3 3 9 7 7 3 3 6 7 1 1 1 3 7 8 2 1 4 7 5 7 6 7 6 3 3 6 1 5 3 6 4 2 3 g p 1 1 1 1 204 R ( u e e a s U N 5 7 1 9 3 6 7 6 0 1 4 4 2 2 1 0 8 4 6 8 3 8 2 0 1 2 2 6 7 2 8 6 9 0 3 8 9 , = , e 7 7 8 6 8 7 9 7 7 7 6 7 6 6 5 7 9 5 6 4 0 5 7 7 2 1 0 8 4 6 5 7 5 8 6 4 0 . h ...... r b 0 0 0 2 0 1 7 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 1 2 0 1 0 0 0 0 3 0 1 1 T u m d ) e l ) 6 8 1 6 1 6 0 1 1 0 4 9 6 6 4 7 7 0 5 1 0 2 3 8 3 1 2 9 3 1 0 0 0 8 3 8 9 ...... c a d m 8 5 2 2 7 6 3 3 0 8 0 7 9 2 8 6 8 1 8 9 4 9 6 6 7 9 6 4 o 0 7 4 5 3 3 6 2 6 e d p r i h 6 9 8 8 8 3 4 3 8 8 7 9 1 4 4 9 9 3 1 4 6 3 8 4 8 3 8 3 0 5 4 2 3 4 7 0 4 u t p p i 1 1 1 6 1 2 1 2 1 T ( n r i l t e ) a n 3 2 1 1 5 2 9 8 0 9 0 5 7 6 4 0 5 9 6 5 4 1 1 9 2 3 1 8 1 1 0 1 1 0 7 7 9 P c ...... i m o . t 9 5 6 4 8 9 8 0 5 6 6 5 4 2 0 8 9 9 0 8 3 3 2 5 6 4 2 3 7 3 8 3 2 6 8 9 1 c p 5 y ( 8 3 1 3 2 1 5 7 0 7 6 1 4 3 3 4 5 3 4 3 8 3 4 2 1 2 2 0 6 1 7 2 6 3 5 9 3 p l 1 1 1 1 1 1 2 1 1 2 1 1 1 1 1 1 1 1 2 U ( a 1 = n 3 n e a i l 9 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 3 4 1 1 1 1 1 1 1 2 3 1 1 1 1 1 1 1 1 1 2 a r ...... b A . r o a 6 7 0 1 4 5 6 7 8 9 0 1 3 5 6 6 6 6 7 8 4 5 0 1 2 2 2 2 4 7 9 3 4 5 6 9 9 r F G T n 1 1 2 2 2 2 2 2 2 2 3 3 1 1 1 1 1 1 1 1 W 1 1 1 1 1 1 1 238 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

Group 2, consisting of five spots on five zircons 5.4. WA93=15 (1.1, 2.1, 7.1, 9.1, 12.1), has a pooled weighted mean 207Pb=206Pb age of 2718 26 Ma. The analytical data obtained for this sample are Group 3, two spots ošn two zircons (10.1, 11.1), summarized in Table 1 and shown on a concordia has a pooled weighted mean 207Pb=206Pb age of plot in Fig. 4. Eighteen analyses on eighteen grains 2555 19 Ma. gave an age of 2588 6 Ma (95% confidence). This Grošup 4, consisting of analyses 5.1 and 13.1, has a is interpreted as the dšeposition age of the tuff. pooled weighted mean 207Pb=206Pb age of 2455 32 š Ma. 6. Regional interpretation of age-dated samples All analyses plot within error of the concordia or are slightly reverse discordant (Fig. 4). Groups 1 and Sample WA92=4, from the uppermost tuff layer 2 are interpreted to provide the ages of older for- at the Kuruman Kop peak, was dated at 2516 4 mations eroded and redeposited within this reworked Ma. This tuff band is therefore at least 22 Ma, šand volcanic layer. The age of 2555 19 Ma (95% con- up to 44 Ma younger than the uppermost tuff layer fidence) is regarded as the besšt approximation of in the carbonates at Prieska, some 250 km south of the age of deposition of the tuffite layer. This age Kuruman (sample WA93=12; 2549 7 Ma). This age has been reported as 2555 11 Ma by Altermann difference indicates that the Gamohšaan Formation is, (1996b, 1997) and Altermanšn and Nelson (1996); re- at least in its uppermost part, significantly younger calculation of the pooled weighted mean 207Pb=206Pb and therefore not correlative of the Nauga Formation age, however, results in 2555 19 Ma. Analyses carbonates, but of the Naute Shale member, that belonging to Group 4 may reflecšt some post-crystal- was deposited between 2549 7 Ma and the 2500 lization loss of radiogenic Pb in these two zircons. Ma Kuruman BIF. From thešdiscussion below, it Alternatively, these zircons may be contaminants. becomes clear that the stromatolitic formations of the Ghaap Plateau facies below the Gamohaan and above 5.3. WA93=12 the Monteville Formation (WA93=41, 2555 19 Ma) must also largely fall into the time of the dšeposition Twenty-two analyses were obtained on sixteen of Naute Shale member. grains from this sample. The results are summarized The age of 2516 4 Ma on WA92=4 is within an- in Table 1 and shown on a concordia plot in Fig. 4. alytical error of theš2521 3 Ma date acquired for a Some analyses were discordant, indicating recent tuff band sampled south ošf Kuruman, probably from loss of radiogenic Pb. Twenty-one analyses of fifteen the same stratigraphic position within the Gamohaan grains gave an age of 2549 7 Ma (95% confidence). Formation (Sumner and Bowring, 1996). The strati- This is the best estimate ofšthe crystallization age of graphic thickness between the WA92=4 sample and the zircons in the tuff and is equivalent to the deposi- the Kuruman BIF is around 75 m (Ha¨lbich et al., tional age of the tuff. One analysis (14.1) indicated a 1992, fig. 10) and it can be speculated that, with Palaeozoic age and this zircon is interpreted to be a a bulk sedimentation rate of 2 m=Ma to 4 m=Ma contaminant. (Barton et al., 1994) for a carbonate, shale and BIF The age of 2549 7 Ma determined for WA93=12 succession, the Kuruman BIF sedimentation in this from the uppermosšt tuff of the Nauga Formation area started about 2500 Ma to 2480 Ma ago. This at Prieska, is within error of the age of 2552 11 is consistent with the calculations by Barton et al. Ma determined by Barton et al. (1994), usingšthe (1994), for the onset of BIF sedimentation in the SHRIMP I in Canberra, for a tuff from a similar Prieska area, and with the zircon age data of Trendall stratigraphic level from Nauga Farm (see discus- et al. (1995). sion below). The two samples are taken about 30 Sample WA93=41 yielded different morphologi- km apart. They are vertically separated by approxi- cal and age populations of partly abraded and broken mately 30 m of chert and carbonate sediments, with zircons, consistent with its interpretation as a possi- WA93=12 being the stratigraphically higher sample bly reworked tuffaceous sediment (i.e. tuffite). The and from the chert member of the Nauga Formation. complex age structure is difficult to interpret. The W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 239 sample is from the upper Monteville Formation, error margins of the Oaktree Formation age (2550 3 the lowest formation of the Campbellrand Subgroup Ma; Walraven and Martini, 1995), at the base ofšthe carbonates on the Ghaap Plateau, and according to Malmani Subgroup, in an identical litho-stratigraphic Altermann and Siegfried (1997) in their study of the position, in the Transvaal basin (Beukes, 1986). This Kathu borehole, this unit is about 2000 m below age is also within the error margins of the 2549 7 the top of the Gamohaan Formation. The Campbell- Ma age of sample WA93=12 from the top of the chšert rand Subgroup sediments total 2460 m thickness in member of the Nauga Formation (compare Figs. 2 the borehole and the Monteville Formation is 540 and 7). On the basis of this result, the Campbellrand m thick, while the upper Gamohaan Formation has Subgroup carbonates on the Ghaap Plateau above been removed by erosion (compare Fig. 3). the Monteville Formation accumulated within a time The oldest age group in this sample, 2718 26 span of about 50 Ma, as did the Naute Shales in the Ma, coincides with the age of the Ventersdorp Sušper- Prieska area. group (2714 8 and 2709 4 Ma; Armstrong et al., Sample WA93=12 from the chert member of the 1991). Zirconšs of this age šgroup are therefore inter- upper Nauga Formation was dated at 2549 7 Ma. preted as sedimentary detritus from the Ventersdorp Sample WA93=15 from the middle Nauga Fošrmation volcanics. The Ventersdorp lavas underlie uncon- was dated at 2588 6 Ma. Prior to this time, almost formably the Schmidtsdrif Subgroup and were also half of the peritidšal Nauga Formation carbonates locally exposed to erosion during the time of deposi- had been accumulated (Fig. 2). As the upper Nauga tion of the Monteville Formation. Alternatively, these Formation is thus only slightly younger than the up- zircons may have been deposited in the Schmidts- per Monteville Formation (Fig. 3), the older Nauga drif Subgroup and subsequently redeposited in the carbonates cannot be correlated with the Campbell- Monteville Formation. rand Subgroup on the Ghaap Plateau. As the upper The age of 2637 30 Ma is very close to the age Oaktree Formation in the Transvaal basin was dated of 2642 3 Ma deštermined for the Vryburg lavas at 2550 3 Ma (Walraven and Martini, 1995), the (Walravešn et al., in press) and is, therefore, too old carbonatše formations there between the Oaktree and to represent the Monteville Formation. Thus, most the BIF units must also be younger than the Nauga likely, this age group also reflects the age of some Formation carbonates, and are thus rather correlative source area of siliciclastic debris. If Ventersdorp of the Naute Shales, assuming an age for the BIF in Supergroup rocks were exposed during Monteville all basins of 2500 Ma (Trendall et al., 1995). times, then the Vryburg Formation may also have been exposed in the vicinity. This age may thus 7. Implications for depositional rates of Archaean indicate the existence of a locally developed uncon- sediments formity between the Monteville Formation and the Schmidtsdrif Subgroup. A possible source area for Various types of calculation of depositional rates this detritus can be inferred in the Vryburg rise, have been made by different authors for variable northeast of the sampling site. Precambrian formations. Barton et al. (1994) defined The age group of 2555 19 Ma in the sample the sediment accumulation rate as the amount of sed- WA93=41 is interpreted as pršoviding the depositional iment vertically accumulated over a given period of age for this upper Monteville Formation tuffite. It is, time, irrespective of possible unconformities. Gener- however, based on two zircons only. It is younger ally, however, sediment accumulation rate (SAR) is than the age of 2642 3 Ma of the Schmidtsdrif defined as: š lavas below the carbonates, as dated by Walraven et Ws SAR al. (in press), and older than sample WA92=4 from D .A t/ Ð the Gamohaan Formation. This interpretation is also where Ws is the weight of sediment deposited during supported by a similar age obtained by Jahn et al. time t, over an area A. (1990), for stromatolitic carbonates approximately at Sedimentation rate (SR) is defined as: the same stratigraphic level (2557 49 Ma, Pb–Pb h on carbonate). The age of 2555 19šMa is within the SR š D t 240 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 where h is the uncompacted thickness of the sed- carbonates is probably negligible, as evidenced by imentary section and t is the duration of its de- the excellent form preservation of stromatolites and position. Hence, sedimentation rate is calculated in microfossils, carbonate muds and arenites undergo 1 1 Bubnoff units (B mm ka$ or m Ma$ ) and ac- considerable mechanical compaction, mainly in the D 2 1 2 cumulation rate is expressed in g m$ a$ or t m$ first 200 to 300 m of burial. Thickness reduction 1 Ma$ (Einsele, 1992). in carbonate muds can exceed up to 50%, and in Workers such as Arndt et al. (1991), Barton et carbonate sands, up to 30%, within this overburden al. (1994), Walraven and Martini (1995) and Barley range (Goldhammer, 1997). Here, for the reason of et al. (1997) did not correct the sedimentation rates lack of quantitative data, we conservatively estimate for compaction, but discussed the possible alteration a thickness reduction of only 20% for carbonate of the sedimentary record by intraformational ero- muds and sands. Our conservative estimate is sup- sional gaps or by times of non-deposition, and the ported by manifold signs of early lithification, found possible effects of compaction. Archaean and Pro- by many authors (Klein et al., 1987; Altermann and terozoic sedimentation rates may thus be calculated Herbig, 1991; Altermann and Wotherspoon, 1995; for compacted sediment, defined here as compacted Sumner and Grotzinger, 1996). In this estimate we sedimentation rate (cSR): have also summarized carbonate muds and sands into one category to facilitate the calculations. This h cSR .x/ seems reasonable because the mud-to-sand ratio is D t fairly high (probably >5 : 1; Altermann and Herbig, where h.x/ equals the thickness of the sediment 1991; Altermann and Siegfried, 1997) and because column, irrespective of post- and syn-depositional carbonate sands tend to lithify more readily due to alteration, and t is the time period during which the their greater initial porosity. column formed. Siliciclastic pelites may compact from >80% orig- Thickness correction for compaction and other inal porosity to about 10%, arenites (and coarse diagenetic influences is complex and can only be tuffs) compact from about 45% porosity to 20%, but estimated for maximum values in the present case. these values can increase significantly with increasing Chemical crystallization in pore space and recrystal- amounts of pelitic matrix. Considering the high over- lization of sediment particles, but especially of car- burden of thousands of metres of sediment, of in part bonates, during diagenesis, can increase the sediment very high density (average density of BIF approxi- 3 thickness. This occurs, for instance, when aragonite mates 3100 kg m$ ), we assume 70% compaction for is transformed to calcite. On the other hand, pres- shales and 25% compaction for sandstones. sure dissolution may result in a thickness decrease. Silicified carbonates (cherts) can be treated as The amount of such changes, however, is very dif- carbonates sensu lato. Early silicification leads to ficult to quantify, and can be judged only from thin excellent preservation of the stromatolite morphol- sections, which cannot be examined for every part ogy and of microfossils, and therefore compaction of the sediment column. Such detailed information is probably negligible. Late diagenetic or post-dia- does not exist yet for the rocks under discussion. genetic silicification usually does not alter the mor- Stylolitization is, however, visible virtually in every phology of bioherms significantly. Therefore, silici- outcrop and thin section examined. Pressure dissolu- fied carbonates are treated here as uncompacted. The tion can result in up to 20–35% thickness reduction amount of compaction in other silicified sediments in carbonates and therefore must be assumed also cannot be ascertained because the processes and tim- for the rocks under consideration. In the following ing of silicification were not investigated in detail. calculations we compensate, however, only for a Several periods of silicification are known, however, conservative estimate of 5% of thickness reduction for the sediments in question, most of them probably by pressure dissolution, as applicable to the Ghaap of very late, post-diagenetic stage (Altermann and Group carbonates in Griqualand West because of a Wotherspoon, 1995). lack of any quantitative investigations. Compaction in BIF and primary cherts is most Although mechanical compaction in stromatolitic difficult to substantiate. The literature does not offer W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 241 any standard figures for BIF compaction, although carbonates, the rapid freshwater cementation, how- compaction is clearly evident in structures such as ever, drastically increases the resistance of these sed- pods, pillows, billows and macules (Trendall and iments to erosion and thus, shallow water carbonates Blockley, 1970; Trendall and Morris, 1983; Findlay, behave in this respect very differently to siliciclas- 1994). Siliceous oozes contain up to 80% water, but tic rocks (Dravis, 1997). Cherts and BIF deposits lithify faster than muds, and therefore probably com- are resistant to erosion and their chance of expo- pact less. The Red Sea siliceous, Fe-rich oozes, and, sure is less due to the generally deeper depositional in some respects, BIF-like silica-rich ore sludge can environment, although their lithification is orders of have pore water contents in excess of 70–90% (We- magnitudes slower than that of peritidal carbonates. ber-Diefenbach, 1977). Therefore, for the Kuruman In most depositional environments, the estimated BIF, we assume up to 90% compaction. This is in sedimentation rate, when calculated over a long pe- accordance with Trendall and Blockley (1970), who riod of time (>100 ka), will only approximate the assumed compaction of up to 95% for generation actual sedimentation (SR) or sediment accumula- of genetic models based on deposition of seasonal tion (SAR) rate. In the Precambrian, because of the varves for the Hamersley BIF. This implies that 300 ‘poor’ time resolution of 5 Ma and greater, this m of BIF represents a thickness of about 3000 m of difference is of major impo¾rtance, especially in tidal original sediment, but reflects a much lower subsi- flat or other marginal marine to fluvial deposits, dence, assuming BIF deposition at roughly 100 m to which are typically sites of discontinuous sedimen- 200 m water depth (Klein and Beukes, 1989). tation. In fossil tidal flats, for example, generally In all calculations of basin subsidence, the amount only less than 50% of the actual sedimentation is of compaction must be taken into consideration. recorded. About 60% to 90% of the time covered Hence, decompacted sedimentation rates do not di- by a sediment column is characterized by erosion rectly reflect the rate of subsidence, but are rather a and=or non-deposition (Drummond and Wilkinson, function of compaction and subsidence. Compaction 1993; Osleger, 1994). Therefore, by implication, the as assumed above reflects the final stage of lithifica- preserved sediments reflect only a fraction of the tion, disregarding gradual thickness decrease related observed time span, and corrections are necessary to a growing overburden, and concomitant dewater- for times of erosion and non deposition. In pre- ing or dissolution. For a proper basin analysis, back- vegetational depositional systems, exposed horizons stripping of the sedimentary pile, where the gradual and times of non-deposition are especially difficult changes of sediment and water column over the layer to recognize because of the lack of typical environ- are restored step by step, is necessary. However, be- mental markers. The sedimentation rate, as defined cause of the lack of data on periods of exposure above, is rather a direct function of the average basin and of age data within the sedimentary column, and subsidence rate and sediment supply within the given data on the burial and thermal history for these Ar- time limits. Wider time limits covering broader fa- chaean to Proterozoic sub-basins (Altermann, 1997), cies variation result in an average rate that is remote our attempts to backstrip the sedimentary columns from the true rates of deposition for the particular were unreliable. The sedimentation and subsidence sedimentary units. rates given here are thus probably in the lower range of the real figures, and should be regarded as mini- 7.1. The Nauga Formation and Schmidtsdrif mum calculations based on conservative estimates of Subgroup in the Prieska sub-basin compaction. The preservation potential of sediment varies with 7.1.1. Sedimentation rates for the Naute Shale, the its composition and with the depositional environ- proto-BIF and chert members of the Nauga ment. Evidently, shallow water, peritidal environ- Formation ments are predisposed to frequent erosion and non- Barton et al. (1994) calculated sedimentation rates deposition, while deep water sediments are usually of 2–4 m=Ma (cSR) for this carbonate, shale and exposed only during pronounced sea-level low stands BIF succession. About 180 m of sedimentary rock, or periods of tectonic uplift. In modern shallow water consisting of 50 m of carbonate (mainly muds) and 242 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

Fig. 5. Comparison of decompacted sedimentation rates for Griqualand West and for modern sedimentary facies associations (from Einsele, 1992). Note that the Griqualand West rates are calculated over extremely long periods of time and times of non-deposition and erosion for the Nauga Formation, the Ghaap Plateau and the Schmidtsdrif Subgroup are neglected here (see discussion in text). However, the deep subtidal to shelf sediments (BIF, Naute Shale) have comparable sedimentation rates to modern carbonate and siliciclastic shelf sediments and black shales. chert sediments of the proto-BIF member and 130 rates of around 10 B for the proto-BIF member and m of shales and cherts of the Naute Shale member the Naute Shale member of the Nauga Formation can of the Nauga Formation, separate the 2549 7 Ma be derived. This is comparable to modern black shale sample WA93=12 from the base of the Kšuruman accumulation rates (Fig. 5). The 2552 11 Ma sam- BIF (Fig. 2). The facies vary from below-wave-base ple of Barton et al. (1994) is 30 m stršatigraphically photic zone carbonates to below-storm-wave-base below the site of WA93=12 (¾2549 7 Ma) (Fig. 2). shales. Somewhat farther to the southeast, the shales These ages agree within their assigšned analytical er- reach their maximum thickness of 170 m. The cherts rors, and indicate that the samples were deposited in the Naute Shale vary between 15 m and 40 m in within 21 Ma of each other, at maximum. During thickness and exhibit at least two regional, and up this time, at least 30 m of sediment accumulated. to seven local horizons of intraformational breccias Consequently, sedimentation rates must have been at as well as disconformities (Altermann, 1990; Kiefer least 1.5 B to 30 B (in the case of identical age), for et al., 1995), that mark erosional or non-depositional these below-wave-base, photic zone carbonates and time intervals. Correction for compaction of about cherts, which also include intraformational breccias 130 m of shale and 35 m of carbonate mud, on and disconformities. average, and for carbonate dissolution, results in a sediment column of over 500 m. 7.1.2. Sedimentation rates for the upper Nauga The deposition of the Kuruman Banded Iron For- Formation carbonates (peritidal member) mation started at around 2500 Ma (Barton et al., Sample WA93=15 was taken approximately 230 1994; Trendall et al., 1995). Thus, sedimentation m below WA93=12, within the same measured strati- W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 243 graphic section. The age of 2588 6 Ma is on average 7.1.3. Sedimentation rates for the lower Nauga 39 Ma older than the 2549 7 šMa date determined Formation (basal to lower peritidal member) for WA93=12 (Fig. 2). Thešresulting sedimentation There are no continuous outcrops from the mea- rate (cSR) for the carbonates separating the two sam- sured section containing the samples WA93=15 and ples is between approximately 4 B and 9 B, with WA93=12 down to the base of the Nauga Forma- an average of 6 B. When corrected for compaction tion. The section measured through the Schmidtsdrif and dissolution and for the intercalated shales, which Subgroup and the overlying lower Nauga Formation together constitute less than 10% of the stratigraphic (Fig. 2) was assembled from several shorter sections section, the total decompacted sediment thickness northwest of Prieska and correlated with the help increases to around 300 m and the SR to about of tuff horizons. It represents the average lithology 8 Bubnoff, on average. This is extremely low for and sediment thickness, which may differ substan- carbonates. The lower facies are peritidal, passing tially locally. The lavas encountered at the base of upward, within the uppermost 20 m, into subtidal, this section (Vryburg Formation) are presumably, on below-wave-base carbonate deposits with condensed lithostratigraphic grounds, time equivalent to the lava sedimentation (Fig. 2). It can be assumed that, in dated by Walraven et al. (in press), at 2642 3 Ma. the peritidal member, up to 90% of the time repre- The base of the Nauga Formation carbonatesšis some sented by this sediment section is not recorded in the 280 m below the 2588 6 Ma tuff bed. Using the beds, but in the contacts between the sedimentary above cSR of 6 B on avešrage, calculated for peritidal layers. Erosional surfaces and desiccation features carbonates, this base must be around 2635 Ma old. were described by Altermann and Herbig (1991) in Because the facies and the lithologies are largely these deposits. However, even if corrected for 90% similar below and above the dated tuff bed, such an of missing record (thus, assuming the extreme case approach seems reasonable. The decompacted sedi- that the 300 m of sediment represent only 10% of mentation, including 10% of shale in the section, is the time of 39 Ma, and multiplying the sediment calculated to be 350 m of sediment and an SR of column by 10), a sedimentation rate of only 60 B to about 8 B is indicated, or 80 B assuming 90% of the around 115 B is achieved. This is at least ten times time as representing non-deposition and erosion. lower than the growth rate of modern carbonate reefs (Fig. 5) and about four times lower than the 400 7.1.4. Sedimentation rates for the Schmidtsdrif B reported for Holocene stromatolites at Shark Bay Subgroup at Prieska (Chivas et al., 1990). This discrepancy is probably The sediments of the Schmidtsdrif Subgroup caused by the long time interval covered by the sec- above the Vryburg lava consist of around 40 m tion and by the presence of condensed sediments in of carbonate, 80 m of shale and 40 m of coarser sili- its upper part. The assumption that 90% of the time ciclastics (Fig. 2). When corrected for compaction, is represented by layer boundaries is necessary for this accounts for around 375 m to 400 m of sedi- comparison to modern growth rates of stromatolitic ment (depending on the locally varying proportions carbonates, which are observed and calculated for of sediment type) deposited in roughly 7 Ma, on much shorter time intervals than dealt with in the average .2642 3–2635), and gives a SR of 50–60 present case. The above example, when compared to B. Again, correšction for times of non-deposition and Phanerozoic deposits, for instance, represents the du- erosion, which are common in this facies, should be ration of the entire Triassic system. The compacted allowed, resulting in possible ﬁgures of up to 600 sedimentation rates of below 10 B are comparable to Bubnoff, in good agreement with modern tidal to the classic Jurassic carbonate sedimentation in Ger- deltaic sediments (Fig. 5). The total thickness of the many, when calculated for the total thickness of the sedimentary pile between the 2588 6 Ma sample entire system (compare with Bosscher and Schlager, and the Vryburg lavas approximatesš450 m (Fig. 2) 1993). and covers a time span of 65 Ma to 51 Ma. An average cSR of 8 m=Ma can be calculated for this section of peritidal carbonates and marginal marine to ﬂuvial siliciclastic rocks. 244 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

7.2. The Campbellrand and Schmidtsdrif Subgroups carbonate platforms (Chivas et al., 1990) and reefs in the Ghaap Plateau sub-basin (Fig. 5).

7.2.1. Sedimentation rates for the platformal 7.2.2. Sedimentation rates for the Schmidtsdrif carbonates of the Campbellrand Subgroup Subgroup in the Kathu borehole Less than 1600 m of predominantly stromatolitic The Vryburg Formation in the Kathu core is at platform carbonates separate the Monteville Forma- least 277 m thick (the base was not reached by tion (2555 19 Ma) from the 2516 4 Ma uppermost the drill) and consists of shales and quartzites, with GamohaanšFormation in field outcšrops on the Ghaap subordinate dolarenites and shaly dolomites (Fig. 3). Plateau. The minimum time for deposition is thus 16 These are interpreted as shallow shelf to deep la- Ma and the maximum time available, 62 Ma. This goonal deposits. In outcrop, the Vryburg Formation implies sedimentation rates between approximately is at most 100 m thick and consists of wavy-lami- 26 B and 100 B (cSR). However, in the Kathu bore- nated, intertidal stromatolitic dolomites and calc- and hole, the Campbellrand Subgroup is 2460 m thick, dolarenites, which interfinger with, and pass upward with the upper part of the Gamohaan Formation into siliciclastic facies. The overlying Boomplaas removed during Palaeozoic erosion, and it is thus Formation is 185 m thick in the Kathu borehole. significantly thicker than estimated from outcrops It consists of black shales, transported oolite beds (Fig. 2). The general facies association, nonethe- and crypt-microbial laminites, and is thus interpreted less, does not differ significantly from that observed as upper shelf facies, deeper than the platformal in outcrops (compare Beukes, 1980a; SACS, 1980; carbonates and in situ oolites observed in surface Altermann and Siegfried, 1997). The Reivilo, Fair- outcrops, where this formation is no more than 100 field, Klipfontein Heuwel, Papkuil, Klippan, Kogel- m thick. The overlying Lokammona Formation is been and Gamohaan Formations of the Campbell- 55 m thick in the borehole (Fig. 3) and comprises rand Subgroup in the borehole consist of several black shales with minor tuff and dolomite interca- generally shallowing-upward cycles, of various stro- lations. The thickness and lithology of the Lokam- matolitic carbonate facies and some shale, chert and mona Formation in outcrop are very similar. In both rare tuff intercalations (Fig. 3). These formations core and outcrop, the Lokammona is interpreted as (the age of 2555 19 Ma is for the uppermost part of a transgressive phase over the Boomplaas platform the Monteville Fšormation; Fig. 3) total around 2000 (Beukes, 1979; Altermann and Siegfried, 1997). The m in thickness, only about 5% of this being shale overlying Campbellrand Subgroup starts with the and an equally small portion of the carbonates being Monteville Formation, which in the Kathu borehole non-stromatolitic calcareous mudstones and aren- is 540 m thick and contains domal stromatolites, ites (Fig. 3). The decompacted thickness estimate is thick pyritic shale intercalations, a lava flow a few about 2500 m, and implies sedimentation rates of metres thick and, in the upper part, small columnar 40 B to 156 B. Sedimentation rates in this range stromatolites, dolarenites and oolites. A shallowing- are known from Phanerozoic tidal flats and, although upward platformal carbonate association was inter- within the lower limits thereof (Fig. 5), are in good preted for this borehole section by Altermann and agreement with sites of low subsidence rate (Scholle Siegfried (1997). The Monteville Formation is sig- et al., 1983; Einsele, 1992). The section is continu- nificantly thinner and of overall shallower platformal ous and no evidence for exposure or disconformities character in surface outcrops (Beukes, 1980a). was recognized in the drillcore; however, they should Using the age of 2555 19 Ma for the upper be expected, at least, in the peritidal facies of this Monteville Formation and tšhe age of 2642 3 Ma section, and have been described in outcrop (Eriks- for the Vryburg lavas (Walraven et al., inšpress), son and Truswell, 1974; Beukes, 1986). Correction 109 Ma to 65 Ma separated the deposition of the for up to 90% of the time in the peritidal facies lower Schmidtsdrif and the lower Campbellrand being of non-deposition or erosion, results in sedi- Subgroups. Only 250 m of sedimentary rocks on mentation rates from 400 B to more than 1500 B, in the Ghaap Plateau (SACS, 1980) separate the top of agreement with modern growth rates of stromatolitic the Vryburg Formation from the top of the Mon- W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 245 teville Formation. The same stratigraphic interval is non-deposition between the Black Reef and the dated represented by a nearly 800 m thick sediment pile at Oaktree tuff. The suggestion, that a significant period Kathu. For the borehole section, a sedimentation rate of non-deposition is partly responsible for the low (cSR) of 7 B to 12 B results and for the outcrops, Bubnoff numbers, may also be true for the Nauga 2 B to 4 B are calculated. When decompacted, the Formation of Griqualand West, where brecciated shelf sediments in the Kathu borehole (340 m of cherts are present within the Naute Shales, between shales, only a few metres of quartzite, and 460 m of the carbonate and BIF sediments (Altermann, 1990). carbonate with a high proportion of mudstones and For the five peritidal carbonate formations of the dolarenites; Altermann and Siegfried, 1997) reflect 1500 m to 1800 m thick sequence of Malmani Sub- approximately 1700 m of sediment and concomi- group carbonates (Button, 1972), a cSR of 26 B to 32 tant sedimentation rates of 16 to 26 B; these are B results, when the base of the Malmani Subgroup is comparable to modern black shale and carbonate assumed to be 2556 Ma and the base of the succeed- shelf deposits (Fig. 5). This section in the borehole ing BIF is taken as 2500 Ma (Trendall et al., 1995). (Boomplaas, Lokammona and Monteville Forma- When decompacted, sedimentation rates comparable tions) is interpreted as entirely subtidal, reflecting to those calculated for the Ghaap Plateau, in the mainly below-wave-base shelf facies and, thus, times order of <100 B result. of non-deposition were probably negligible. For the 277 m thick Vryburg Formation in the borehole, 7.4. Banded iron formations in the Transvaal and decompacted thickness accounts for around 650 m, Griqualand West sub-basins: sedimentation rates including 5% quartzites, 50% shales and 45% carbonate muds. However, because the hole did not Sedimentation rates for the Kuruman and overly- reach the base of the formation, and as the time of ing Griquatown BIF of the Asbestos Hills Subgroup initiation of the sedimentation is not known, sedi- are difficult to determine, because suitable age deter- mentation rates for the complete section cannot be minations are not available. Additionally, folding and calculated. thrusting complicates correlation of BIF units across the Griqualand West basin (Altermann and Ha¨lbich, 7.3. The Malmani Subgroup and Black Reef 1991). The base of the Kuruman BIF in Griqualand Formation in the Transvaal sub-basin: West and of its Transvaal correlative, the Penge BIF, sedimentation rates is around 2500 Ma (Trendall et al., 1995), whereas the base of the Griquatown BIF in Griqualand West Walraven and Martini (1995) dated the upper is 2432 31 Ma (Trendall et al., 1990). The thickness Oaktree Formation, at the base of the Malmani Sub- of BIF šbetween the two dated tuffaceous horizons is group in the Transvaal sub-basin, at 2550 3 Ma, estimated to be 210 m (Beukes, 1980b; Barton et al., and calculated a cSR of 8 m=Ma for the Našuga For- 1994). This implies sedimentation rates of 2 B to 6 mation carbonates and of 17 m=Ma for the Ghaap B, as were calculated by Arndt et al. (1991) and Bar- Plateau carbonates. They estimated the base of the ton et al. (1994) for a mixed lithological succession Malmani Subgroup in the central Transvaal sub- of carbonates, shales, BIF and chert. Upon decom- basin to be 2556 Ma, and thus 86 Ma younger than paction, the BIF sediments reflect a thickness of the correlated Vryburg and Black Reef Formations 2100 m and sedimentation rates of 20 B to 60 B, the (Fig. 7). The top of the Chuniespoort Group (carbon- latter thus being comparable to uncompacted pelagic ates and BIFs) was estimated to be between 2472 sediments (Fig. 5) (Mu¨ller and Mangini, 1980). and 2400 Ma, depending on the varying thickness More recently, Barley et al. (1997) published new of the preserved sediments. Two possible explana- age data for volcanic rocks within the Hamersley tions were given by Walraven and Martini (1995) for Range of the Pilbara craton in Western Australia, the very low sedimentation rates of the quartzites of and derived sedimentation rates (SR) of 30 B and the Black Reef Formation: (a) the formation is not more, for the BIF and shales included in this suc- a correlative of the Vryburg Formation, but signif- cession. This is an order of magnitude higher than icantly younger; (b) there is a significant period of previous calculations of sedimentation rates (3–4 B) 246 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 for a BIF, volcanic, shale and carbonate rock succes- basin (Fig. 3), and of the upper Nauga Formation sion from the same region (Arndt et al., 1991), or carbonates at Prieska with the Monteville Forma- from South Africa (Barton et al., 1994), and in good tion on the Ghaap Plateau and Oaktree Formation agreement with the results presented here for the in the Transvaal (Fig. 7), requires a new sedimen- decompacted Kaapvaal craton BIF. However, if as tation scheme and a new model for basin develop- suggested by Barley et al. (1997), this sedimentation ment. The Boomplaas Formation above the Vryburg was related to enormous volcanic and hydrother- Formation established the first carbonate platform mal activity in the Hamersley basin, differences in between 2642 and 2588 Ma ago. This was subse- intensity of volcanism may explain divergent BIF quently transgressed by the Lokammona Formation accumulation rates in other basins. shales (Beukes, 1979) and followed, on the Ghaap Earlier calculations of the sedimentation rate of Plateau, by the Monteville Formation when platfor- BIF resulted in higher figures than presented above. mal conditions returned at around 2555 Ma. Before Trendall and Blockley (1970, p. 298) arrived at 2588 6 Ma, almost half of the tidal flat carbonates basin subsidence rates of 2000 to 6000 years per in thešPrieska area had already accumulated. In the one foot (50 m to 150 m in one million years) for Transvaal basin and in the Ghaap Plateau sub-basin, the Fortescue and Hamersley Group basins of the the Oaktree and Monteville carbonate sedimenta- Pilbara craton. Their calculations for the compacted tion commenced only prior to 2550 Ma and 2555 sedimentation rate for BIF were based on microband Ma, respectively. There is as yet no evidence for counting (varve model) and the inferred quantities continuous and stable carbonate basin development of annual Fe deposition in the basin. For the Dales in the Transvaal sub-basin prior to about 2556 Ma Gorge member BIF, a cSR of 20 B to 70 B can be (Walraven and Martini, 1995). This suggests a trans- deduced from Trendall and Blockley’s (1970, p. 262) gression, progressing from the west or southwest calculation of 2000 to 3000 years of deposition time towards the east or northeast. for one Knox or Calamina cyclothem (a common The major transgressive step at around 2550 Ma cyclic sequence of banding types in the BIF, and drowned the tidal flats in the southwest and shifted on average 7 cm and 14 cm thick, respectively, the main site of carbonate sedimentation to the north in the above calculation). This cSR is an order of and east. At the time of the subtidal and below- magnitude higher than the calculations based on wave-base carbonate sedimentation at Prieska, car- isotopic age data, as presented here. bonate sediments of the Monteville and Oaktree For the Kuruman BIF, a weighted average mi- Formations had accumulated under platform con- croband thickness of 0.58 mm was calculated by ditions. Continuous subsidence in the basin centre Klein and Beukes (1989, p. 1772), and considered was matched by stromatolitic growth (Altermann to represent an annual varve. Under this assumption and Herbig, 1991) under predominantly subtidal a compacted sedimentation rate (cSR) of 570 B was conditions, locally passing into supratidal settings derived. However, with such a high cSR, the 210 m (Eriksson and Truswell, 1974). An area of crustal of Kuruman BIF separating the two dated horizons updoming like the Maremane or Ganyesa Domes of 2500 Ma and 2432 31 Ma would have been could have served as the source for clastic sediments deposited within less thašn 0.5 m.y. At Prieska, where intercalated with the Ghaap Plateau carbonates, and the Kuruman BIF approximates 750 m thickness, the also may have been a barrier between the intracra- time of deposition would be less than 1.5 m.y., when tonic basins. Smith et al. (1990) investigated Sm–Nd disregarding possible tectonic duplication suggested isotopes in shales at the base of the Griqualand West by Altermann and Ha¨lbich (1991). and Transvaal basin sequences, and found profound differences in the geochemistry and isotopic char- 8. Basin history and tectonic interpretation acteristics of the sediments in the two sub-basins. These differences probably reflect different source The correlation of the Naute Shale member of the areas for the Vryburg and Black Reef shales (Smith Prieska sub-basin (Figs. 1 and 2) with the Reivilo et al., 1990; Barton and Hallbauer, 1996). to Gamohaan Formations of the Ghaap Plateau sub- Carbonate deposition on the Ghaap Plateau and W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 247 in the Transvaal lasted for at least 30 m.y. longer deltaic intercalations (Eriksson and Reczko, 1998). than in the Prieska area, where the Naute Shales The cSR for these sediments thus ranges between 3 were deposited between roughly 2550 Ma and 2500 B and 7 B, but decompacted sedimentation rate is in Ma. The source area for the mud influx must have range of 20 B, assuming a shale to sandstone ratio been located farther to the south or west. Otherwise, of 5 : 1 (Eriksson et al., 1995). Despite including a the large amount of pelitic detritus would have over- regional unconformity, these sedimentation rates are whelmed the stromatolitic platform carbonates in the comparable with those of modern deltas and delta- north and east, if not derived from this SW direction. front shales and turbidites (Fig. 5). As demonstrated by Altermann (1996a), the tuffs in The decompacted sediment thickness of the var- the Nauga Formation originated in the southwest. ious age-bracketed sections of the Schmidtsdrif and However, it is now clear that the tuffs in the upper Campbellrand Subgroups are plotted against their Campbellrand Subgroup, on the Ghaap Plateau, are upper age boundaries, in order to obtain a sedimen- younger and not correlative of tuffs in the periti- tation curve (Fig. 6). The diagrams in Fig. 6A and dal member of the Nauga Formation. Volcanism in Fig. 6B are based on time-level plots, as proposed the southwestern Griqualand West sub-basin could by Friend et al. (1989), but modified to suit our have accompanied rifting and thermal uplift. Erosion purposes. These curves are not subsidence curves, in these parts of the basin could have exposed the because back-stripping has not been carried out, but source area required for derivation of the more than when compared to estimated depth of deposition, 100 m of Naute Shale member, and for the occa- subsidence can be deduced easily from these dia- sional shale intercalations accumulated on the Ghaap grams. The Prieska area (Fig. 6A) and the Kathu Plateau. borehole (Fig. 6B) are treated separately. The es- From about 2500 Ma to 2432 Ma, BIFs of the timated depth of deposition for the Naute Shales Kuruman and Penge Formations were deposited at and BIF is below storm wave-base, in water depth >100 m depth, following a substantial deepening greater than 100 m. The maximum depth is difficult of the basin, to below storm wave-base (Klein and to determine but could be greater than the 200 m Beukes, 1989). During deposition of the Griquatown assumed. The sedimentation rates (SR) for both ar- BIF, a shallowing-upward cycle commenced (Beukes eas are plotted against their upper age boundaries in and Klein, 1990). In southwestern Griqualand West, Fig. 6C. Although in both areas the density of age the BIFs are conformably followed by the Koegas data is different and our decompaction data are cer- Subgroup, a succession mainly comprising shales, tainly imprecise, the different shapes of the curves cherts and arkoses, with minor BIF and carbonate in Fig. 6A and Fig. 6B can only be interpreted as deposits. These shallow marine to deltaic sedimen- reflecting differing depositional histories for the two tary rocks are locally over 600 m thick and are cov- areas. This is even more apparent in Fig. 6C, where ered, above a low angular unconformity (Altermann the discrepancy in sedimentation rates between 2550 and Ha¨lbich, 1991), by glaciogenic tillites in turn Ma and 2500 Ma is striking. This difference is due to disconformably followed by the 2222 Ma Ongeluk varying lithology of highly compactible, slowly ac- lavas (Cornell et al., 1996). In the Transvaal, the cumulating shales versus poorly compactible, rapidly Penge BIF is unconformably covered by the Pretoria growing carbonates, and due to different absolute Group siliciclastic rocks, with a hiatus of perhaps amounts of subsidence. more than 100 Ma. Thus, sedimentation rates for When evaluating the subsidence in both areas, a deposits younger than the BIF cannot be calculated. similarity in timing becomes apparent. For the time It seems certain, however, that between the 2350 Ma period between 2550 and 2500 Ma, subsidence ap- old Bushy Bend lavas (F. Walraven, pers. commun., pears to be greatest in both Fig. 6A and Fig. 6B. in Eriksson et al., 1995) at the base of the Preto- Two concave-upward parts of the curve connect- ria Group of the Transvaal and the 2222 Ma old ing the bars of estimated depth of deposition are Hekpoort–Ongeluk lavas, at least 400 to 800 m of recognizable in Fig. 6A. This curve shape can be mudrock and subordinate sandstones were laid down interpreted as typical of a rift basin or passive con- in a deep periglacial turbiditic basin, with some distal tinental margin. For the Kathu borehole (Fig. 6B), 248 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

Fig. 6. Curves for decompacted thickness of sediment and estimated depth of its deposition for the Prieska area (A) and the Ghaap Plateau (B). Differences in subsidence and compaction (due to varying sediment types) can be deduced from comparison of both curves. Greatest subsidence is implied by the thickness of the less compactible carbonates of the Ghaap Plateau, which remain at shallow depositional depth between 2555 Ma and 2516 Ma (B). Approximately at the same time, mainly highly compactible shales were accumulated in the Prieska area and the depth of deposition for the shales increased to below storm wave-base (A). Since the overlying BIFs were deposited at 100 m to 200 m depth (compare discussion in text), but are highly compactible (around 90%), subsidence must have been less effective than compaction during BIF deposition. This is emphasized in (C), where the poorly compactible shallow-platform carbonates of the Ghaap Plateau are shown to have the highest sedimentation rate. Note that the shaded and solid bars in (C) mark different areas of deposition, while in (A) and (B) they indicate different scales of decompacted sediment thickness and depth of deposition. W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 249 a single rift-related curve shape extends for an ad- of metres per million years, as calculated for the de- ditional 100 m.y. backwards in time. The curve, posits of the Kaapvaal craton, are known from young however, is based on less age-data, which may be tidal flats and carbonate shelves of low subsidence the reason for this divergent behaviour. Rift basin rate (Scholle et al., 1983; Einsele, 1992). or passive continental margin-related subsidence is Because of the calculated sedimentation rates and typically a slow and long-lasting activity (McCabe, the conspicuous lack of coarse-clastic sedimentary 1991). Interpretation of a rift basin–passive margin rocks in the entire Griqualand West and Transvaal analogue for the Griqualand West basin is suggested sub-basin successions, the subsidence rates reflected by the marked scarcity of clastic debris, other than in the major transgression episodes and follow- suspension deposits in the Campbellrand and As- ing sedimentation cycles must have been moder- bestos Hills Subgroups, and also supported by the ate. It is evident that stromatolitic growth-rates un- presence of basaltic volcanism within the carbon- der favourable conditions should have been able ates (Altermann, 1996a). Passive margins are the to match the subsidence rate. Archaean stroma- preferred site of formation of carbonate platforms. tolites and carbonates, however, differ from their The acceleration of subsidence during the period of Neoproterozoic and Phanerozoic counterparts in the 2550 Ma to 2500 Ma might be due to overlapping scarcity of coarse pelletal carbonate sands, due pre- mechanical and thermal subsidence. Usually, the ini- sumably to the lack of organisms producing such tial mechanical subsidence is accompanied by higher pellets or carbonate skeletons, that could be worn heat flow due to stretching, which is then followed to produce carbonate arenites (Grotzinger, 1989). by thermal subsidence due to cooling of the stretched Therefore, Archaean stromatolites are mainly finely asthenosphere (Einsele, 1992). Where both processes laminated and trap rare carbonate detritus between overlap, faster subsidence would be expected. the column branches, whereas younger stromatolites tend to trap and bind carbonate sands also along 9. Implications for sediment deposition in the the microbial lamination. This may lead to the de- Neoarchaean velopment of finer lamination and slower growth in Archaean stromatolites. Klein et al. (1987) and From the calculations above, it is difficult to un- Altermann and Schopf (1995) demonstrated the con- derstand why the growth of stromatolitic carbonate spicuous lack of detrital grains in microfossiliferous platforms was incapable of continuing after ma- stromatolites of the Campbellrand Subgroup. Addi- jor transgressions. A paradox of drowned carbonate tionally, in the photomicrographs presented by Klein platforms like that postulated for many modern and et al. (1987), minute aragonite needles can be ob- Phanerozoic carbonates (Schlager, 1981) can be de- served in the Siphonophycus transvaalensis mats and tected also in the Neoarchaean. Modern carbonate filaments, evidence that such microbiota, after decay, accumulation can match subsidence rates or relative could contribute only to the production of micrite. sea-level increases of up to 500 m=Ma (Aigner et al., Grotzinger (1989) proposed that carbonate precipi- 1989; Bosellini, 1989; Chivas et al., 1990; Bosscher tation could also have been triggered indirectly by and Schlager, 1993). Over short duration, the accu- photosynthetic decrease of the CO2 content of the mulation rates of carbonates may be even higher, seawater, in the presence of cyanobacterial bioherms. in excess of 1000 m=Ma. The drowning of carbon- Such ‘chemical’ precipitation might be the reason ate platforms in the Phanerozoic requires subsidence for the slow sedimentation rates of Archaean car- rates or sea-level rise in excess of 4000 m=Ma, at bonates compared to modern reefs (Fig. 5). Sumner least for a short period of time, until the basin floor and Grotzinger (1996) proposed a purely chemical sinks below the zone of euphotic activity (Schlager, subtidal precipitation of carbonate in a saturated ma- 1989). Sedimentation rates in the order of a few rine environment for parts of the Griqualand West metres per million years are typical of Phanerozoic and other Precambrian deposits, which would prob- oceanic pelagic deposits that lack terrigenous sedi- ably account for even lower sedimentation rates. ment influx and which are fed only by planktonic On the other hand, there is ample evidence, for rain. Sediment deposition in the order of a few tens the cyanobacterial and bacterial communities, that 250 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 their metabolism was similar to that observed today must have been bound by clay or carbonate minerals (Schopf and Klein, 1992; Schopf, 1993). In support and no BIFs were precipitated. Ha¨lbich et al. (1992), of this, Lanier (1986) calculated organic production have demonstrated that the shales of the carbonate– rates for microfossils from the Malmani Subgroup BIF transition have comparable Fe contents to the stromatolites in the range of modern microbial mats, BIF units. and thus the growth rates of Precambrian and modern stromatolites should be comparable, provided that a 10. Conclusions similar sediment binding or precipitation mechanism operated. The calculated decompacted sedimentation Long periods of non-deposition and erosion have rates of over 150 B for the stromatolitic carbonates of been assumed for peritidal and associated silici- the Ghaap Plateau also support comparable growth clastic and carbonate facies deposits of the Kaap- rates for modern and Precambrian stromatolites. vaal craton. Decompacted sedimentation rates were The apparent slow drowning may be due to an nonetheless calculated using conservative estimates inability of the microbial organisms secreting or fa- of compaction and dissolution, and no back-strip- cilitating the precipitation of carbonate to cope with ping was performed. The results obtained probably possible climatic changes or changes in the chemical underestimate the true sedimentation rates. They are environment. The latter could have been influenced generally comparable to Phanerozoic deposits, when by increased hydrothermal activity, as suggested by observed over similarly long periods of time, and, BIF geochemistry (Klein and Beukes, 1989). For with consideration of times of non-deposition, to the climate, it can be speculated that the fixation of modern sedimentation rates. CO2 in the first giant carbonate platforms of Africa, Four long-term (millions of years) transgression– Australia, northern America and India reduced the regression cycles can be recognized in the Griqua- greenhouse effect and led to lower temperatures, as land West sub-basin and can be partly correlated to discussed for the Upper Precambrian by many in- Transvaal sub-basin deposits. (1) The Vryburg For- vestigators (e.g. Eriksson et al., 1998). As no higher mation and the Boomplaas carbonate platform sedi- organisms directly secreting carbonate were present, ments represent, respectively, the first transgression growth rates comparable to those of coral reefs, for and a succeeding long-duration shallowing-upward example, seem unlikely in the Early and Middle Pre- cycle in Griqualand West. (2) The second transgres- cambrian. The drowning of the carbonate platforms sive step is marked by the Lokammona shales and the was thus probably facilitated by the development overlying shallowing-upward (regressive) cycle rep- of unfavourable conditions for stromatolitic growth. resented by the lower Nauga Formation. (3) The up- The drowning of the carbonates of the Nauga For- per Nauga Formation (chert member) together with mation (2549 Ma) and of the Gamohaan carbonate the Monteville and Oaktree Formations mark the platform (2516 Ma) coincide with two ‘events’, re- third transgressive phase, followed by a long-term spectively: the introduction of pelitic sediment into shallowing-upward regressive cycle with the devel- the basin from the southwest, and the increase of Si opment of a stromatolitic carbonate platform that and Fe content in the sediments and by inference in formed the Campbellrand and Malmani Subgroups. the seawater, from volcanic and hydrothermal activ- (4) The fourth transgressive step is the development ity (Klein and Beukes, 1989; Barley et al., 1997). of the Kuruman and Penge BIF-pelagic basin in all As no thick shales separate the Gamohaan Forma- provinces. The Griquatown BIF and the overlying tion from the Kuruman BIF, shale sedimentation Koegas siliciclastics mark the fourth regressive cy- alone cannot be responsible for the drowning of the cle, not preserved in the Transvaal sub-basin, due to carbonate platforms. Other factors, such as climatic a possibly 100 m.y. long period of erosion before the changes may thus have hastened the end of carbon- Pretoria Group sediments were laid down (Eriksson ate deposition. The increasing Fe and Si content led et al., 1995). finally to precipitation of BIF when pelitic sediment An attempt to correlate these four cycles in the deposition was insignificant. During pelitic (and car- Prieska, Ghaap Plateau and Transvaal areas is sum- bonate) sedimentation, the Fe dissolved in the water marized in Fig. 7. In this figure, the transgression– W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 251

Fig. 7. Comparison of recognizable second-order transgression–regression cycles for the Prieska, Ghaap Plateau and Transvaal areas. The most complete curve can be sketched for the Prieska area and the least complete for the Transvaal area, due to varying density of age data and preservation of the sedimentary column. Nevertheless, similar development for the upper part of the curve can be seen (2550 Ma to 2432 Ma approximately). Note the uncertainties in the shape of the curves and the differences in facies and lithology at Prieska compared to the Ghaap Plateau and Transvaal basin areas, as discussed in the text. The cited ages are: 2222 13 Ma for Ongeluk š Formation (Cornell et al., 1996); 2350 Ma for Bushy Bend lavas in the Timeball Hill Formation (F. Walraven, pers. commun., in Eriksson et al., 1995); 2432 31 Ma for the lower Griquatown BIF (Trendall et al., 1990, and discussion by Barton et al., 1994); 2500 Ma for š the base of the Kuruman and Penge BIF (Trendall et al., 1995); 2516 4 Ma for the upper Gamohaan Formation; 2521 3 Ma for the š š upper Gamohaan Formation (Sumner and Bowring, 1996) and 2555 19 Ma for the upper Monteville Formation on the Ghaap Plateau š (this work); 2549 7 Ma for the chert member and 2588 6 Ma for the peritidal member of the Nauga Formation at Prieska (this work); š š 2550 3 Ma for the upper Oaktree Formation in the Transvaal sub-basin (Walraven and Martini, 1995); 2641 3 Ma for the Vryburg š š Formation on the Ghaap Plateau (Walraven et al., in press); 2709 4 Ma for the upper Ventersdorp Supergroup (Makwassie quartz š porphyry, Armstrong et al., 1991). The other formations are not dated and their correlation across the different sub-basins is uncertain (see text). The sedimentary section between the Vryburg (Black Reef) Formation and the BIF (including Koegas Subgroup at Prieska) represents a first-order cycle as recognized by Cheney (1996). Cycles of third and fourth order, as described by Clendenin (1989), are not shown in this figure. They may be included in the two transgressions of the Schmidtsdrif Subgroup or between the Monteville and the Reivilo Formations. Because of lack of age data, however, their duration can only be calculated using compacted sediment thickness. 252 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 regression cycles 1 and 2, as described above, are not cycles in Archaean and Palaeoproterozoic basins. recognized in the Transvaal sub-basin due to the lack For example, the conspicuous stromatolite cyclic- of age data, and due to the uncertain age and cor- ity observed in many formations of the Transvaal relation of the Black Reef Formation. For the same Supergroup (Eriksson and Truswell, 1974) may be reason, the third cycle (the Oaktree Formation trans- attributed to fourth- and fifth-order cycles, of 0.1– gression) cannot be separated from the Black Reef. 0.5 m.y. and 0.02–0.1 m.y. duration, respectively The role of the Black Reef Formation in this scenario (Mitchum and van Wagoner, 1991). and its possible correlation to the Vryburg Forma- Comparison of the Transvaal Supergroup depos- tion are equivocal and require further confirmation itories with other Archaean and Palaeoproterozoic (see discussion by Walraven and Martini, 1995). The basins is hindered by the lack of sufficient data. The upper Black Reef encompasses a transgressive shale only basin equally well investigated as the Kaap- cover, which grades into the overlying carbonates of vaal craton sub-basins, is the Hamersley basin of the Oaktree Formation (Clendenin, 1989). Shallow the Pilbara craton, Western Australia. Both basins platform carbonates are developed above the Oak- are of comparable age, contain comparable sedi- tree and Monteville Formations. Only at Prieska, mentary fills, and have even been considered to be did deposition of the Naute Shale member remain parts of the same original cratonic depositional sys- predominantly below the wave-base, perhaps with tem (Trendall, 1968; Button, 1976; Cheney, 1996). the exception of the silicified and brecciated inter- First-order cycles (>50 Ma), on the Kaapvaal cra- vals, which may correlate with some regressions in ton, can be identified in the four sequences recog- the Monte Christo Formation and below the Lyttel- nized by Cheney (1996). In his interpretation, the ton Formation in Transvaal (Clendenin, 1989). The lower Transvaal Supergroup (>2432 Ma) is the sec- fourth cycle leading to the deposition of banded iron ond first-order cycle, the first being represented by formations, is recognizable in all three sub-basins. sedimentary and volcanic rocks of the Ventersdorp The shape of the transgression–regression curves in Supergroup. Based on our interpretation presented Fig. 7 is inferred because the duration of these events here, we would rather extend this second first-order and their relative intensity (i.e. the amount of relative tectonically driven cycle to include the Koegas Sub- sea-level rise or fall) are not known. Slow transgres- group of Griqualand West, and thus to have an upper sions followed by rapid regressions are, however, age limit of <2432 Ma (Griquatown BIF) for this typical of Phanerozoic sea-level fluctuations (Cloet- second cycle. However, as Cheney (1996) correctly ingh et al., 1985). stated, with more data the sequences will have to be These four large-scale transgressive–regressive redefined in the future, and the boundaries will shift (shallowing-upward) cycles are in the order of tens with the identification of other sequences. The last of millions of years duration, and must therefore two first-order cycles recognized in the Transvaal be attributed to second-order sequences of crustal Supergroup, the Pretoria and the Rooiberg Groups, evolution (Vail et al., 1991). They represent ma- are not discussed here, but it should be emphasized jor regional transgressions and regressions and build that Cheney (1996) was also able to identify three sequence cycles that can be subdivided into se- of the four unconformity-bounded sequences on the quences comprising third-order system tracts, and Pilbara craton of Western Australia. Thus, our sec- into fourth-order parasequences. Cyclicity above the ond-order cycles identified herein may be found also second-order (duration of less than 3 m.y.) can be de- within the Hamersley Group, if Cheney (1996) is duced from lithological columns, such as the Kathu correct. From published data (Trendall et al., 1990, borehole, and differ regionally in their facies, as 1995; Arndt et al., 1991; Barley et al., 1997) it is, comparisons to adjacent realms demonstrate (e.g. on however, evident that sedimentation rates and lithos- the Ghaap Plateau: Altermann, 1997, Altermann and tratigraphic sequences of both cratonic basins are Siegfried, 1997; or in the Transvaal basin: Clen- similar across the Archaean–Proterozoic boundary. denin, 1989). More detailed facies work and precise Analyses of subsidence and sedimentation rates age data are needed for identification and correlation for Precambrian sedimentary basins are still an ex- of system tracts, parasequences and Milankovitch ception, compared to the much more regularly pub- W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 253 lished pure facies descriptions and lithostratigraphic and Palaeoproterozoic basins is also possible. The correlations. To our knowledge, the most recently evolution of Precambrian biochemical, chemical and published attempt to draw decompacted sedimenta- siliciclastic sedimentary basins imply long-term first- tion rate and subsidence curves for Precambrian sed- and second-order cyclicity, as in the Phanerozoic, imentary basins was by Maynard and Klein (1995). and thus similar crustal processes and analogous These authors investigated the subsidence history of rates of denudation. Similarly, the biological and the clastic Witwatersrand basin, underlying the Ven- chemical processes of carbonate sedimentation in tersdorp Supergroup forming the basement to the varying facies realms appear comparable to their rocks discussed herein. The Witwatersrand deposits Phanerozoic and modern carbonate facies equiva- are >2800 Ma and consist mainly of sandstones lents, and thus a similar metabolism and evolution- and shales with subordinate conglomerates, and have ary stage of carbonate-fixing stromatolitic microbial been interpreted as having been laid down within a organisms are inferred. strike-slip modified retroarc foreland-basin. Because the Witwatersrand sediments contain the largest gold Acknowledgements deposits known, it has attracted the most attention from sedimentologists (e.g. Bickle and Eriksson, Zircon analyses were carried out on the Sensitive 1982; Burke et al., 1986; Winter, 1987; Stanistreet High-Resolution Ion Microprobe mass spectrome- and McCarthy, 1990) and a large amount of data and ter located at Curtin University of Technology. The manifold interpretations have been presented. The SHRIMP II laboratory is supported by the Aus- calculations by Maynard and Klein (1995), using a tralian Research Council. We thank especially J.R. computer program to correct for compaction and the de Laeter, Allan Kennedy, Bob Pidgeon and Alec load of the sediment fill, resulted in subsidence rates Trendall for their kind support and the Geological of 40 m=Ma to 50 m=Ma, on average, for the Do- Survey of Western Australia for excellent sample minion and West Rand Groups of the Witwatersrand mounting. WA was supported by the German Re- Supergroup. search Foundation (DFG) grant DFG Al 295=3-3 A similar exercise was performed for the 1100 and by a stipendium to the Curtin University and Ma to 1050 Ma old White Pine Cu deposit of¾north- greatly enjoyed the organization and collaborative ern Michigan, USA (Maynard and Klein, 1995). spirit at this institution, but especially the com- Rates of subsidence for the sedimentary phase of pany of Frank So¨llner (IAAG-LMU). Frank helped the basin were calculated to be greater than 80 at various stages of the investigations, with endless m=Ma for shales, sandstones and conglomerates. discussions and contributed substantially to our suc- These rates were, however, calculated on a much cess. Pat Eriksson patiently waited for this paper. smaller observational scale, of less than ten million Alec Trendall also corrected an early version of the years, separating the available age-data points. The manuscript and helped with many critical remarks basin was interpreted as a rift-related basin with sev- and suggestions. Pat Eriksson and an anonymous eral episodes of mechanical subsidence. Although a referee critically reviewed and improved contents direct comparison of our calculations with those per- and style of this contribution. formed by Maynard and Klein (1995) is not possible, because of the lack of back-stripping in our calcu- References lations, the results in all three Precambrian basins published so far (and obviously from the Pilbara Aigner, T., Doyle, M., Lawrence, D., Epting, M., von Vliet, sedimentary successions, as well as from the lower A., 1989. Quantitative modeling of carbonate platforms: some Pretoria Group) are similar, and are also comparable examples. Soc. Econ. Paleontol. Mineral. Spec. Publ. 44, 28– to Phanerozoic sedimentation and subsidence rates. 37. Precambrian siliciclastic and chemical sediments Altermann, W., 1990. Facies development at the Archean– Proterozoic boundary, along the southwestern rim of the Kaap- accumulated at rates comparable to their younger vaal craton, South Africa. In: Glover, J.E., Ho, S.E. (Eds.), 3rd equivalents. From the calculated sedimentation rates, Int. Archean Symp., Perth. Geoconferences (W.A.) Inc., Perth a conclusion as to the subsidence rates of Archaean ed. Vol. Extended Abstracts, pp. 299–301. 254 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

Altermann, W., 1996a. Sedimentology, geochemistry and palaeo- West Sequence, South Africa: Implications for Early Protero- geographic implications of volcanic rocks in the upper zoic rock accumulation rates. Geology 22 (4), 343–346. Archean Campbell Group, western Kaapvaal Craton, South Beukes, N.J., 1979. Litostratigrafiese onderverdeling van die Africa. Precambrian Res. 79, 73–100. Schmidtsdrif-Subgroep van die Ghaap-Groep in Noord-Kaap- Altermann, W., 1996b. Discussion on ‘Zircon Pb–evaporation land. Trans. Geol. Soc. S. Afr. 82, 313–327. age determinations of the Oak Tree Formation, Chuniespoort Beukes, N.J., 1980a. Stratigrafie en litofasies van die Camp- Group, Transvaal Sequence: implications for the Transvaal– bellrand-Subgroep van die Proterofitiese Ghaap-Groep, No- Griqualand West basin correlations’. S. Afr. J. Geol. 99 (3), ord-Kaapland. Trans. Geol. Soc. S. Afr. 83, 141–170. 337–338. Beukes, N.J., 1980b. Lithofacies and stratigraphy of the Kuruman Altermann, W., 1997. Sedimentological evaluation of Pb–Zn and Griquatown Iron-Formations, Northern Cape Province, exploration potential of the Precambrian Griquatown Fault South Africa. Trans. Geol. Soc. S. Afr. 83, 69–86. Zone in the Northern Cape Province, South Africa. Mineral. Beukes, N.J., 1986. The Transvaal Sequence in Griqualand West. Deposita 32, 382–391. In: Anhaeusser, C.R., Maske, S. (Eds.), Mineral Deposits of Altermann, W., Ha¨lbich, I.W., 1991. Structural history of the Southern Africa, Vol. I. Geological Society of South Africa, south western corner of the Kaapvaal Craton and the adjacent Johannesburg, pp. 819–828. Namaqua Realm: New observations and reappraisal. Precam- Beukes, N.J., Klein, C., 1990. Geochemistry and sedimentology brian Res. 52, 133–166. of a facies transition — from microbanded to granular iron- Altermann, W., Herbig, H.G., 1991. Tidal flat deposits of the formation — in the early Proterozoic Transvaal Supergroup, Lower Proterozoic Campbell Group along the southwestern South Africa. Precambrian Res. 47, 99–139. margin of the Kaapvaal Craton, Northern Cape Province, Bickle, M.J., Eriksson, K.A., 1982. Evolution and subsidence of South Africa. J. Afr. Earth Sci. 13 (3/4), 415–435. early Precambrian sedimentary basins. R. Soc. Philos. Trans. Altermann, W., Nelson, D.R., 1996. Neoarchean carbonate plat- A 305, 225–247. form development: An example from the Kaapvaal Craton, Bosellini, A., 1989. Dynamics of Tethyan carbonate platforms. Soc. Econ. Paleontol. Mineral. Spec. Publ. 44, 3–13. South Africa. 17th Int. Assoc. Sedimentol. Reg. African– Bosscher, H., Schlager, W., 1993. Accumulation rates of carbon- European Meeting, March, 1996, Sfax, Tunisia, Abstr. Vol., ate platforms. J. Geol. 101, 345–355. pp. 8–9. Burke, K., Kidd, W.F.S., Kusky, T.M., 1986. Archean foreland Altermann, W., Schopf, J.W., 1995. Microfossils from the basin tectonics in the Witwatersrand, South Africa. Tectonics Neoarchean Campbell Group, Griqualand West Sequence of 5, 439–456. the Transvaal Supergroup, and their paleoenvironmental and Button, A., 1972. Algal stromatolites of the early Proterozoic evolutionary implications. Precambrian Res. 75, 65–90. Wolkberg Group, Transvaal sequence. Economic Geology Re- Altermann, W., Siegfried, H.P., 1997. Sedimentology, facies de- search Unit, Univ. Witwatersrand, Johannesburg, Info. Circ. velopment and a type-section of an Archean shelf-carbonate 69, 9 pp. platform transition, Kaapvaal craton, as deduced from a deep Button, A., 1976. Transvaal and Hamersley basins — Review of borehole at Kathu, South Africa. J. Afr. Earth Sci. 24 (3), basin development and mineral deposits. Miner. Sci. Eng. 8 391–410. (4), 262–293. Altermann, W., Wotherspoon, J.McD., 1995. The carbonates of Cheney, E.S., 1996. Sequence stratigraphy and plate tectonic the Transvaal and Griqualand West Sequences of the Kaapvaal significance of the Transvaal succession of southern Africa craton, with special reference to the Lime Acres limestone and its equivalent in Western Australia. Precambrian Res. 79, deposit. Mineral. Deposita 30 (2), 124–134. 3–24. Armstrong, R.A., Compston, W., Retief, E.A., Williams, I.S., Chivas, A.R., Torgersen, T., Pollach, H.A., 1990. Growth rates Welke, H.J., 1991. Zircon ion microprobe studies bearing on and Holocene development of stromatolites from Shark Bay, the age and evolution of the Witwatersrand Triad. Precambrian Western Australia. Aust. J. Earth Sci. 37, 113–121. Res. 53, 243–266. Clendenin, C.W., 1989. Tectonic Influence on the Evolution of Arndt, N.T., Nelson, D.R., Compston, W., Trendall, A.F., Thorne, the Early Proterozoic Transvaal Sea, Southern Africa. Ph.D. A.M., 1991. The age of the Fortescue Group, Hamersley Thesis, Univ. Witwatersrand, Johannesburg, 376 pp. Basin, Western Australia, from ion microprobe zircon U–Pb Cloetingh, S., McQueen, H., Lambeck, K., 1985. On a tectonic results. Aust. J. Earth Sci. 38, 261–281. mechanism for regional sealevel variations. Earth Planet. Sci. Barley, M.E., Pickard, A.L., Sylvester, P.J., 1997. Emplacement Lett. 75, 157–166. of large igneous province as a possible cause of banded iron Compston, W., Williams, I.S., Meyer, C., 1984. U–Pb formation 2.45 billion years ago. Nature 385, 55–58. geochronology of zircons from lunar breccia 73217 using a Barton, E.S., Hallbauer, D.K., 1996. Trace-element and U–Pb sensitive high mass-resolution ion microprobe. J. Geophysical isotope composition of pyrite types in the Proterozoic Black Res. 89, B252–B534. Reef, Transvaal Sequence, South Africa: Implications on gen- Cornell, D.H., Schu¨tte, S.S., Eglington, B.L., 1996. The On- esis and age. Chem. Geol. 133, 173–199. geluk Basaltic Andesite Formation in Griqualand West, South Barton, E.S., Altermann, W., Williams, I.S., Smith, C.B., 1994. Africa: submarine alteration in a 2222 Ma Proterozoic sea. U–Pb zircon age for a tuff in the Campbell Group, Griqualand Precambrian Res. 79 (1/2), 101–124. W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256 255

Dravis, J.J., 1997. Rapidity of freshwater calcite cementation — tions on the age of the Ventersdorp Supergroup. Geology 18, implications for carbonate diagenesis and sequence stratigra- 1211–1214. phy. Sediment. Geol. 107 (1/2), 1–10. Kiefer, R., Franck, C., Klemm, D.D., Altermann, W., 1995. Drummond, C.N., Wilkinson, B.H., 1993. Aperiodic accumula- Stratigraphic subdivision and facies distribution of the Camp- tion of cyclic peritidal carbonate. Geology 21, 1023–1026. bellrand and Asbestos Hills Subgroups near Prieska, Griqua- Einsele, G., 1992. Sedimentary Basins Evolution, Facies and land West. Centennial Geocongress, Geol. Soc. S. Afr., April, Sediment Budget. Springer, Berlin, 628 pp. Johannesburg, Abstr. Vol. II, pp. 1154–1157. Eriksson, K.A., Truswell, J.F., 1974. Tidal flat associations from Klein, C., Beukes, N.J., 1989. Geochemistry and sedimentology the lower Proterozoic carbonate sequence in South Africa. of a facies transition from limestone to iron-formation depo- Sedimentology 21, 293–309. sition in the early Proterozoic Transvaal Supergroup, South Eriksson, P.G., Reczko, B.F.F., 1998. Contourites associated with Africa. Econ. Geol. 84, 1733–1774. pelagic mudrocks and delta fed turbidites in the Early Protero- Klein, C., Beukes, N.J., Schopf, J.W., 1987. Filamentous micro- zoic Timeball Hill Formation, Transvaal Supergroup, South fossils in the Early Proterozoic Transvaal Supergroup: Their Africa. Sediment. Geol. 120, 319–335, this volume. morphology, significance, and paleoenvironmental setting. Pre- Eriksson, P.G., Hattingh, P.J., Altermann, W., 1995. An overview cambrian Res. 36, 81–94. of the geology of the Transvaal Sequence and Bushveld Com- Lanier, W.P., 1986. Approximate growth rates of Early Protero- plex, South Africa. Mineral. Deposita 30 (2), 98–111. zoic microstromatolites as deduced by biomass productivity. Eriksson, P.G., Condie, K.C., Tirsgaard, H., Mueller, W.U., Alter- Palaios 1, 525–542. mann, W., Miall, A.D., Aspler, L.B., Catuneanu, O., Chiaren- Maynard, J.B., Klein, G.D., 1995. Tectonic subsidence analysis zelli, J.R., 1998. Precambrian clastic sedimentation systems. in the characterization of sedimentary ore deposits: Examples Sediment. Geol. 120, 5–53, this volume. from the Witwatersrand (Au), White Pine (Cu), and Molango Findlay, D., 1994. Diagenetic boudinage, an analogue model for (Mn). Econ. Geol. 90, 37–50. the control on hematite enrichment in iron ores of the Hamers- McCabe, P.J., 1991. Tectonic controls on coal accumulation. Soc. ley Iron Province of Western Australia, and a comparison with Geol. Fr. Bull. 162, 277–282. Krivoy Rog of Ukraine, and Nimba Range, Liberia. Ore Geol. Mitchum, R.M., van Wagoner, J.C., 1991. High-frequency se- Rev. 9, 311–324. quences and their stacking patterns: sequence-stratigraphic ev- Friend, P.F., Johnson, N.M., McRae, L.E., 1989. Time-level plots idence of high-frequency eustatic cycles. Sediment. Geol. 70, and accumulation patterns of sediment sequences. Geol. Mag. 131–160. 126, 491–498. Mu¨ller, P.J., Mangini, A., 1980. Organic carbon decomposition Goldhammer, R.K., 1997. Compaction and decompaction algo- rates in sediments of the Pacific manganese nodule belt by rithms for sedimentary carbonates. J. Sediment. Petrol. 67, 230Th and 231Pa. Earth Planet. Sci. Lett. 51, 94–114. 26–35. Osleger, D., 1994. Aperiodic accumulation of cyclic peritidal Grotzinger, J.P., 1989. Facies and evolution of Precambrian car- carbonate: Comment and reply. Geology 22, 479–480. bonate depositional systems: Emergence of the modern plat- South African Committee for Stratigraphy (SACS), 1980. form archetype. Soc. Econ. Paleontol. Mineral. Spec. Publ. 44, Stratigraphy of South Africa, Part 1 (Comp. L.E. Kent). 79–106. Lithostratigraphy of the Republic of South Africa, South Grotzinger, J.P., 1990. Geochemical model for Proterozoic stro- West Africa=Namibia, and the Republics of Bophuthatswana, matolite decline. Am. J. Sci. 290-A, 80–103. Transkei and Venda. Handbook Geol. Surv. S. Afr. 8, 690 pp. Ha¨lbich, I.W., Lamprecht, D.F., Altermann, W., Horstmann, U.E., Schlager, W., 1981. The paradox of drowned reefs and carbonate 1992. A carbonate-banded iron formation transition in the platforms. Geol. Soc. Am. Bull. 92, 197–211. early Proterozoikum of South Africa. J. Afr. Earth Sci. 15 (2), Schlager, W., 1989. Drowning unconformities on carbonate plat- 217–236. forms. Soc. Econ. Paleontol. Mineral. Spec. Publ. 44, 15– Hassler, S.W., 1993. Depositional history of the Main Tuff Inter- 25. val of the Wittenoom Formation, late Archean–early Protero- Scholle, P.A., Bebout, D.G., Moore, C.H., 1983. Carbonate depo- zoic Hamersley Group, Western Australia. Precambrian Res. sitional environments. Am. Assoc. Pet. Geol. Mem. 33, 345– 60, 337–359. 440. Hofmann, H.J., Masson, M., 1994. Archean stromatolites from Schopf, J.W., 1993. Microfossils of the Early Archean Apex Abitibi greenstone belt, Quebec, Canada. Geol. Soc. Am. Bull. Chert: New evidence of the antiquity of life. Science 260, 106, 424–429. 640–646. Jahn, B-m., Simonson, B.M., 1995. Carbonate Pb–Pb ages of Schopf, J.W., Klein, C., 1992. The Proterozoic Biosphere: A the Wittenoom Formation and Carawine Dolomite, Hamersley Multidisciplinary Study. Cambridge University Press, New Basin, Western Australia (with implications for their correla- York, 1348 pp. tion with the Transvaal Dolomite of South Africa. Precambrian Simonson, B.M., Hassler, S.W., 1997. Was the deposition of Res., 72: 247–261. large Precambrian Iron Formations linked to major marine Jahn, B.-m., Bertrand-Sarfati, J., Morin, N., Mace, J., 1990. Di- transgressions? J. Geol. 104, 665–676. rect dating of stromatolitic carbonate from the Schmidtsdrif Smith, C.B., Duane, M.J., Clendenin, C.W., 1990. Sm–Nd iso- Formation (Transvaal Dolomite), South Africa, with implica- topic systematics of shales from the Chuniespoort and Ghaap 256 W. Altermann, D.R. Nelson / Sedimentary Geology 120 (1998) 225–256

Groups, Transvaal basin. Geocongress ’90, Cape Town, pp. of Gondwana BIFs: Progress report from recent zircon U–Pb 518–520. results. 3rd Aust. Conf. Geochronology. Curtin University ed. Stanistreet, I.G., McCarthy, T.S., 1990. Middle Ventersdorp Vol. Abstracts, Perth, pp. 1–3. graben development north of Carletonville goldﬁeld and its Vail, P.R., Audemard, F., Bowman, S.A., Eisner, P.N., Perez- relevance to the evolution of the Witwatersrand Basin. S. Afr. Cruz, C., 1991. The stratigraphic signatures of tectonics, J. Geol. 93, 272–288. eustasy and sedimentology. In: Einsele, G., Ricken, W., Sumner, D.Y., Bowring, S.A., 1996. U–Pb geochronologic con- Seilacher, A. (Eds.), Cycles and Events in Stratigraphy. Am. strains on deposition of the Campbellrand Subgroup, Transvaal Assoc. Pet. Geol. Mem. 26, 617–659. Supergroup, South Africa. Precambrian Res. 79 (1/2), 25–36. Walraven, F., Martini, J., 1995. Zircon Pb-evaporation age de- Sumner, D.Y., Grotzinger, J.P., 1996. Herringbone calcite: Pet- terminations of the Oak Tree Formation, Chuniespoort Group, rography and environmental signiﬁcance. J. Sediment. Res. A Transvaal Sequence: implications for Transvaal–Griqualand 66 (3), 419–429. West basin correlations. S. Afr. J. Geol. 98 (1), 58–67. Trendall, A.F., 1968. Three great basins of Precambrian banded Walraven, F., Beukes, N.J., Retief, E.A., in press. 2.64 Ga zircons iron formation deposition: a systematic comparison. Geol. Soc. from the Vryburg Formation, Griqualand West: implications Am. Bull. 79, 1527–1544. for the age of the Transvaal Supergroup and accumulation Trendall, A.F., Blockley, J.G., 1970. The iron formations of the rates in carbonate=iron formation successions. Precambrian Precambrian Hamersley Group, Western Australia. Geol. Surv. Res., in press. Bull. 119, 1–366. Weber-Diefenbach, K., 1977. Geochemistry and diagenesis of Trendall, A.F., Morris, R.C., 1983. Iron-formation: Facts and Recent heavy metal ore deposits at the Atlantis-II-deep (Red problems. Developments in Precambrian Geology 6. Elsevier, Sea). In: Klemm, D.D., Schneider, H.-J. (Eds.), Time- and Amsterdam, 558 pp. Strata-Bound Ore Deposits (1st ed.), Springer, Berlin, pp. Trendall, A.F., Compston, W., Williams, I.S., Armstrong, R.A., 419–436. Arndt, N.T., McNaughton, N.J., Nelson, D.R., Barley, M.E., Williams, I.S., Compston, W., Black, L.P., Ireland, T.R., Foster, Beukes, N.J., de Laeter, J.R., Retief, E.A., Thorne, A.M., J.J., 1984. Unsupported radiogenic Pb in zircon: a case of 1990. Precise zircon U–Pb chronological comparison of the anomalously high Pb–Pb, U–Pb and Th–Pb ages. Contrib. volcano-sedimentary sequences of the Kaapvaal and Pilbara Mineral. Petrol. 88, 322–327. Cratons between about 3.1 and 2.4 Ga. In: Glover, J.E., Ho, Winter, H. de la R., 1987. A cratonic foreland model for Wit- S.E. (Eds.), 3rd Int. Archean Symp., Perth. Geoconferences watersrand Basin development in a continental back-arc plate- (W.A.) Inc., Perth ed. Vol. Extended Abstracts, pp. 81–83. tectonic setting. S. Afr. J. Geol. 90, 409–427. Trendall, A.F., de Laeter, J.R., Nelson, D.R., 1995. Chronology