Sequence Stratigraphy of the Vaal Reef Facies Associations in the Witwatersrand Foredeep, South Africa
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Sedimentary Geology 141±142 02001) 113±130 www.elsevier.nl/locate/sedgeo Sequence stratigraphy of the Vaal Reef facies associations in the Witwatersrand foredeep, South Africa Octavian Catuneanua,*, Mark N. Biddulphb aDepartment of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alta., Canada T6G 2E3 bGeology Department, Anglogold, Private Bag 5010, Vaal Reefs 2621, South Africa Accepted 19 January 2001 Abstract Facies analysis and sequence stratigraphic methods resolve the correlation and relative chronology of Vaal Reef facies associations at the Great Noligwa and Kopanang mines in the Klerksdorp Gold®eld, Witwatersrand Basin. Four depositional sequences, separated by major erosional surfaces, build the Vaal Reef package. Lowstand, transgressive and highstand systems tracts 0HST) are identi®ed based on stratal stacking patterns and sedimentological features. The core of the Vaal Reef deposit is represented by a Transgressive Systems Tract 0TST), which is built by shoreface 0proximal) facies at Great Noligwa and inner shelf 0distal) facies at Kopanang. The proximal TST includes the G.V. Bosch reef and the overlying MB4 quartzites. These facies are correlative to the Stilfontein conglomerates and quartzites, respectively, at Kopanang. The overlying Upper Vaal Reef is part of a Lowstand Systems Tract 0LST). It is only preserved at Kopanang, with a thickness that equals the difference in thickness between the proximal and distal transgressive facies. This suggests that the Upper Vaal is an erosional remnant that accumulated on a gently dipping topographic pro®le. Traditionally, the MB4 quartzite has been considered the hanging wall relative to the Vaal Reef. We infer that these quartzites are merely associated with the G.V. Bosch conglomerate, and interpret that they are coeval with the Stilfontein quartzites, and older relative to the Upper Vaal facies. From a genetic viewpoint, the true hanging wall to the Vaal Reef begins with the Zandpan gravel lag that extends across the entire study area. Paleocurrent analysis of shoreface and ¯uvial systems indicate a curved shoreline and a northern elevated source area. The coastline underwent north±south transgressive and regressive shifts during Vaal Reef time. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Vaal Reef; Sequence stratigraphy; Witwatersrand foredeep; Shoreline shifts 1. Introduction 0systems tracts) can be identi®ed. Lithofacies can then be correlated according to where each unit is positioned Sequence stratigraphy, currently one of the most along an inferred curve that represents base-level ¯uc- actively evolving disciplines in sedimentary geology, tuations. Application of sequence stratigraphic prin- is a powerful tool for explaining the relationships of ciples can be important for evaluating the origin of allostratigraphic units that analyzes the sedimentary sedimentary-hosted mineral deposits. In the present response to base-level changes. Through the recog- study, we examine deposits in the Klerksdorp Gold- nition of bounding surfaces, genetically related facies ®eld along the northwestern margin of the Witwaters- rand Basin 0Fig. 1). Data were obtained from three * Corresponding author. Fax: 11-780-492-6569. contiguous mines 0Great Noligwa, Kopanang, Moab E-mail address: [email protected] 0O. Catuneanu). Khotsong; Fig. 1) operated by Anglogold. Most data 0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0037-0738001)00071-9 114 O.Catuneanu, M.N.Biddulph / Sedimentary Geology 141±142 32001) 113±130 South Africa Johannesburg Carletonville Klerksdorp Welkom r e iv l R Vaa 0 255075100km 0 5km Fig. 1. Area of study in the context of the Witwatersrand Basin. O.Catuneanu, M.N.Biddulph / Sedimentary Geology 141±142 32001) 113±130 115 WITWATERSRAND SUPERGROUP STRATIGRAPHICCOLUMN Formation Subgroup Ventersdorp Contact Reef Pretoria Group L V Mondeor E V Elsburg UENC Turffontein ANSVAA Chuniespoort Gold Estates SEQ TR Group Robinson CENTRAL RAND GROUP V V Booysens V Pniel Group V Krugersdorp GROUP V Johannesburg V Luipaardsvlei V Randfontein UPER Platberg V Main Group V RP S Blyvooruitzicht V V Maraisburg V RSDO Roodepoort Klipriviersberg V V v v Group v Crown v v VENTE V Jeppestown V Babrasco Rietkuil UP O Central Rand Koedoeslaagte Group Afrikander UPERGR Elandslaagte WEST RAND Palmietfontein GROUP Government SRAND S West Rand Tusshenin Group Coronation ATER Promise WITW Bonanza Dominion Group Brixton Hospital Hill Parktown Basement Orange Grove Granites v v Quartzite v Lava Conglomerate Shale Diamictite Fig. 2. Generalized stratigraphy of the Witwatersrand Supergroup in the context of the overall Precambrian geology of central South Africa. 116 O.Catuneanu, M.N.Biddulph / Sedimentary Geology 141±142 32001) 113±130 are from Great Noligwa and Kopanang mines as the outpace rates of base-level rise. `Normal' regression Moab Khotsong Mine deposit is currently being of the shoreline occurs, resulting in aggradation and developed and is only scheduled to begin production progradation of both ¯uvial and marine deposits. in the year 2003. Highstand deltaic deposits, bounded above by subaerial Gold bearing conglomerates of the Witwatersrand unconformities, are typical products. Highstand strata Basin have generally been regarded as alluvial fan and may have a low preservation potential due to erosion ¯uvial in origin 0e.g. Pretorius, 1976, 1981; Minter, accompanying subsequent base-level falls. 1978), a view that has persisted in part because the overall genetic stratigraphic setting has not been fully 2.2. Bounding surfaces considered. Herein, we focus on the Vaal Reef Subaerial unconformities develop in the nonmarine deposit, preserved as a relatively thin 0few meters) portion of the basin due to ¯uvial or wind degradation laterally continuous, mature placer, split into a during stages of base-level fall. They may overlie number of facies. Based on existing and new data, ¯uvial or marine strata, but are overlain by nonmarine we use sequence stratigraphic methods to correlate deposits. between the Great Noligwa and Kopanang mining Regressive surfaces of marine erosion are the areas, and to suggest a new model for the chronology marine equivalent of the subaerial unconformities. of events leading to the formation of the preserved They are also formed during the `forced' regression stratigraphic architecture. of a shoreline accompanying a fall in base-level, and are represented by scoured surfaces cut by waves in an 2. Sequence stratigraphic concepts attempt to re-establish an equilibrium shoreface pro®le. They usually separate underlying deeper Comprehensive discussions of sequence strati- marine strata from overlying shallower marine strata. graphic concepts and their application to the Precam- Transgressive surfaces of erosion, also known as brian rock record are provided by Christie-Blick et al. `ravinement surfaces' 0RS), are scours cut by shore- 01988) and Catuneanu and Eriksson 01999). Brie¯y face waves during the transgression of a shoreline. summarized below are key concepts relevant to this They are highly diachronous surfaces, separating study. ¯uvial strata below from shallow marine facies above. Maximum ¯ooding surfaces represent the boundary 2.1. Systems tracts between a TST and an overlying HST. Lowstand Systems Tract 0LST) form during early stages of base-level rise, when rates of base-level rise 3. Geological overview are outpaced by sedimentation rates. As a result, a `normal' regression of the shoreline occurs. Typical The Witwatersrand Basin is underlain by a 3.1 Ga products for LSTs include amalgamated channel ®lls and older granite-greenstone basement that comprise overlying subaerial unconformities, and lowstand the Kaapvaal Craton 0Fig. 1; Barton et al., 1986; deltaic deposits. Protected from subsequent erosion Myers et al., 1990; Hartzer et al., 1998). The age of by the aggradation of overlying transgressive and the Witwatersrand Supergroup 0Figs. 2 and 3) is highstand deposits, these have a high preservation constrained by U±Pb zircon ages obtained from potential. volcanic rocks in the underlying Dominion Group Transgressive Systems Tract 0TST) form during 03074 ^ 6 Ma; Armstrong et al., 1991) and the accelerated base-level rise, when rates of base-level overlying Ventersdorp Supergroup 02714 ^ 8Ma; rise outpace sedimentation rates. As a result, a trans- Armstrong et al., 1991). Only one age 02914 ^ 8Ma) gressive shift of the shoreline occurs, and retrograda- has been determined for the Witwatersrand Basin tion and vertical aggradation in both ¯uvial and itself 0upper West Rand Group 0Armstrong et al., shallow marine environments results. 1991; Hartzer et al., 1998). Despite low-grade regional Highstand Systems Tracts 0HST) form during late metamorphism 0Stevens et al., 1996), primary sedi- stages of base-level rise, when sedimentation rates mentary features are well preserved. O.Catuneanu, M.N.Biddulph / Sedimentary Geology 141±142 32001) 113±130 117 Formations Members Lithology Venterspost Ventersdorp Contact Reef Elsburg Reefs Mondeor Bastard Reef GROUP GE4 - GE7B Elsburg IN SUB Dennys Reef FFONTE Gold Estates Gold Estates Reef GE8 UP TUR Buffelsfontein GE9/10 Robinson Crystalkop GRO “C” Reef Doornkop MBA D Booysens MB1 AN MB2 R Krugersdorp MB3 Vaal Reef & MB4 this RAL Mapaiskraal MB5 & Mizpah study GROUP } Basal grit NT G SUB Luipaardsvlei E Hartebeesfontein MB6 C SBUR MB7 - MB9 Livingstone Reef OHANNE Randfontein MB10 J Main Shale marker MB11