Sequence stratigraphy of the Precambrian Rooihoogte Timeball Hill rift succession, Transvaal Basin, South Africa

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1 Sedimentary Geology 147 (2002) Sequence stratigraphy of the Precambrian Rooihoogte Timeball Hill rift succession, Transvaal Basin, South Africa Octavian Catuneanu a, *, Patrick G. Eriksson b a Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3 b Department of Earth Sciences, University of Pretoria, Pretoria 0002, South Africa Received 31 December 2000 Abstract Third-order sequence stratigraphic analysis is performed on the Rooihoogte Timeball Hill second-order rift succession of the Paleoproterozoic Transvaal Basin, South Africa. This provides a case study for systems tract and sequence development during a time of glacio-eustatic fall, when accommodation was generated by subsidence related to syn-rift and post-rift tectonic processes. Two third-order depositional sequences have been identified, separated by a basin-wide subaerial unconformity. The lower third-order sequence includes the complete succession of lowstand, transgressive, and highstand systems tracts (LST, TST, and HST), whereas the upper third-order sequence only preserves lowstand and transgressive systems tracts. This indicates that the fall in base level associated with the upper second-order boundary of the Rooihoogte Timeball Hill sequence was of higher magnitude relative to the third-order subaerial unconformity, which is in agreement with the principles of boundary hierarchy based on the magnitude of base-level changes. The position of the lower boundary of the Rooihoogte Timeball Hill second-order sequence has been revised from the base of the chert breccias to the contact between the breccias and the overlying chert conglomerates. This is because a major tilting event occurred between the deposition of the two facies, which are genetically unrelated, and which are separated by a subaerial unconformity. The lithostratigraphic contact between the Rooihoogte and Timeball Hill formations is interpreted as a diachronous transgressive surface of erosion. In this interpretation, the Polo Ground Member of the Rooihoogte Formation may be coeval with the basal black shales of the Timeball Hill Formation, the two facies (fluvial and marine, respectively) forming together a transgressive systems tract. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Precambrian sequence stratigraphy; Second-order rift sequence; Third-order systems tracts; Transvaal Basin 1. Introduction * Corresponding author. Tel.: address: octavian@ualberta ca (O. Catuneanu). Sequence stratigraphy, which developed as a methodology for explaining the relationships of allostratigraphic units that fill a sedimentary basin, is currently one of the most actively evolving disciplines in sedimentary geology. Through the recognition of bounding surfaces, genetically related facies (systems tracts) can be identified. Lithofacies can then be correlated according to where each unit is positioned along an inferred curve that represents base-level fluctuations. The concepts of sequence stratigraphy have primarily been perfected from the study of Phanerozoic successions, which provide better preservation potential and time control for detailed stratigraphic analyses and correla /02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S (01)

2 72 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) tions (Vail, 1987; Posamentier et al., 1988, 1992; Van Wagoner et al., 1990; Hunt and Tucker, 1992; Miall, 1997; Plint and Nummedal, 2000). More recently, the principles of sequence stratigraphy have also been applied to genetic interpretations of Precambrian successions (e.g., Christie-Blick et al., 1988; Catuneanu and Eriksson, 1999; Catuneanu and Biddulph, in press) Concepts of sequence stratigraphy Comprehensive discussions of sequence stratigraphic concepts and their application to the Precambrian rock record are provided by Christie-Blick et al. (1988) and Catuneanu and Eriksson (1999). Briefly summarized below are key concepts relevant to this study. The various systems tracts and stratigraphic surfaces are defined relative to the base-level and transgressive regressive curves (Fig. 1). The two curves are offset by a time period equivalent to the duration of sediment-driven ( normal ) regressions, which depends on the ratio between the rates of baselevel rise and the sedimentation rates (see Catuneanu and Eriksson, 1999 for a more detailed discussion). Lowstand systems tracts form during early stages of base-level rise, when the rates of base-level rise are outpaced by sedimentation rates. As a result, a normal regression of the shoreline occurs. Typical products for lowstand systems tracts (LST) include amalgamated channel fills overlying subaerial unconformities, and lowstand deltaic deposits. Protected from subsequent erosion by the aggradation of overlying transgressive and highstand deposits, these LST deposits have a high preservation potential. Transgressive systems tracts form during accelerated base-level rise, when rates of base-level rise outpace sedimentation rates. As a result, a transgressive shift of the shoreline occurs, and retrogradation and vertical aggradation in both fluvial and shallow marine environments results. Fig. 1. Types of sequences, bounding surfaces and systems tracts defined in relation to the base-level and transgressive regressive curves (modified from Catuneanu et al., 1998). Abbreviations: TST transgressive systems tract; RST regressive systems tract; LST lowstand systems tract; HST highstand systems tract; FSST falling stage systems tract; SU subaerial unconformity; c.c. correlative conformity; MRS maximum regressive surface; MRS-c MRS-correlative (i.e., the nonmarine correlative of the marine MRS); MTS maximum transgressive surface; (A) positive accommodation; NR normal (sediment supply-driven) regression; FR forced (base-level fall-driven) regression.

3 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Highstand systems tracts form during late stages of base-level rise, when sedimentation rates outpace rates of base-level rise. Normal regression of the shoreline occurs, resulting in aggradation and progradation of both fluvial and marine deposits. Highstand deltaic deposits, bounded above by subaerial unconformities, are typical products. Highstand strata may have a low preservation potential due to erosion accompanying subsequent base-level falls. Subaerial unconformities develop in the nonmarine portion of the basin due to fluvial or wind degradation during stages of base-level fall. They may overlie fluvial or marine strata, but are overlain by nonmarine deposits. Transgressive surfaces of erosion, also known as ravinement surfaces, are scours cut by shoreface waves during the transgression of a shoreline. They are highly diachronous surfaces, separating fluvial strata below from shallow marine facies above. In areas of high preservation potential, ravinement surfaces may be entirely developed within transgressive systems tracts, which is why they are not represented in Fig. 1 (see Catuneanu and Eriksson, 1999, for discussion and illustration). Maximum regressive surfaces represent the boundary between a lowstand systems tract and an overlying transgressive systems tract. They are also known as conformable transgressive surfaces (Embry, 1995; Catuneanu et al., 1998). Maximum transgressive surfaces represent the boundary between a transgressive systems tract and an overlying highstand systems tract. A synonymous term is maximum flooding surfaces Aim of research This paper focuses on the Transvaal Basin of South Africa (Fig. 2), which preserves a 650-My record of Late Archaean to Early Proterozoic sedimentation. Previous work equated the sedimentary fill of the Transvaal Basin, that is, the Transvaal Supergroup, with a first-order depositional sequence bounded by subaerial unconformities generated in relation to major changes in the tectonic setting (Catuneanu and Eriksson, 1999; Fig. 3). The inferred curve of base-level changes for the Transvaal Basin allowed the further subdivision of the Transvaal first-order sequence into five second-order depositional sequences, that is, the Protobasinal, Black Reef, Chuniespoort, Rooihoogte Timeball Hill, and Boshoek Houtenbek sequences (Fig. 3). The purpose of this research is to increase the resolution of sequence stratigraphic analysis to the third-order level of cyclicity, for the case study of the Rooihoogte Timeball Hill second-order sequence. The motivation for doing this work is twofold: (1) no third-order sequence stratigraphic analyses have been performed so far on the Transvaal succession, and (2) the accumulation of the Rooihoogte Timeball Hill strata took place during a time of global glacio-eustatic fall (Young, 1995; Eriksson et al., 1998; Martin, 1999; Young et al., in press; Fig. 3), which provides a case study for systems tract and sequence development with accommodation generated by tectonic processes. This is particularly relevant in view of the core debate of sequence stratigraphy over the eustatic versus tectonic controls on accommodation and sequence development. 2. Geological background 2.1. Tectonic setting The Transvaal Supergroup overlies the c to 2.7-Ga Witwatersrand Supergroup in the stratigraphic record and constitutes the sedimentary floor to the Bushveld igneous complex (Fig. 3). It should be noted here, that the c. 2.7-Ga Ventersdorp Supergroup, which unconformably succeeds the Witwatersrand strata, is approximately coeval with the lowermost portion of the Transvaal Supergroup (e.g., Eriksson et al., in press). The 2714-Ma boundary between the Witwatersrand and Transvaal (Ventersdorp) Supergroups marks a significant change in the structural style of the receiving sedimentary basins. The Witwatersrand succession accumulated within a retroarc foreland basin developed in relation to the supracrustal loading associated with the initial phases of the Limpopo Orogeny (Winter, 1987; Stanistreet and McCarthy, 1991; Robb and Meyer, 1995), and probably also associated with collision of arc systems with the emerging Kaapvaal craton (Catuneanu, in press). In contrast, sedimentation within the Transvaal Basin was controlled by cycles of extensional and/or thermal subsidence separated by stages of uplift or glacioeustatic base-level fall (Catuneanu and Eriksson,

4 74 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 2. Outcrop distribution of the Transvaal lithostratigraphic units within the confines of the Transvaal Basin. Modified from Eriksson and Reczko (1995).

5 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) ). The upper boundary of the Transvaal Supergroup, that is, the 2050-Ma contact with the Bushveld complex, corresponds to another first-order tectonic event that terminated the evolutionary history of the Transvaal Basin. Bounded by these two and 2714-Ma contacts, the Transvaal Supergroup is interpreted as a first-order sequence related to the accumulation of sediment within the tectonic setting of the Transvaal and correlative basins. The five second-order depositional sequences of the Transvaal Supergroup correspond to distinct cycles of extensional and/or thermal subsidence, and are separated by second-order subaerial unconformities. Important to note is the cyclic repetition of tectonic settings within the succession of second-order sequences, indicating no major shifts in structural styles during the evolution of the Transvaal Basin. The accumulation of the Rooihoogte Timeball Hill second-order sequence took place during a full rifting cycle, with accommodation provided by syn-rift extensional subsidence (Rooihoogte time) followed by post-rift thermal subsidence (Timeball Hill time; Catuneanu and Eriksson, 1999; Fig. 3) Lithostratigraphy The Transvaal Supergroup comprises four main lithostratigraphic units, that is, the protobasinal (a nondescriptive term) rocks, Black Reef Formation, Chuniespoort Group, and Pretoria Group (Eriksson and Reczko, 1995; Fig. 3). Our stratigraphic objective is represented by the two lowermost formations of the Pretoria Group, that is, the Rooihoogte and Timeball Hill (Fig. 3). A generalized lithostratigraphic profile for the Rooihoogte and Timeball Hill formations is presented in Fig. 4. The basal contact of the Rooihoogte Formation, as well as the top contact of the Timeball Hill Formation, are both marked by major angular unconformities. These unconformities have been identified as second-order depositional sequence boundaries (Catuneanu and Eriksson, 1999), and their features are described in detail by Eriksson et al. (in press). The Rooihoogte Formation consists of three lithostratigraphic members, with a total thickness in excess of 400 m in the northwestern part of the Transvaal Basin. The lower Bevets Member includes coarse products of in situ weathering and alluvial sedimentation represented by chert breccias and conglomerates, respectively. The chert breccias have been traditionally considered as the basal part of the Pretoria Group, but they have been recently reassigned to the underlying Chuniespoort Group (Eriksson et al., in press). The revised contact between the Chuniespoort and Pretoria Groups is now taken at the unconformable limit between the Bevets breccias and conglomerates (Fig. 4). More details on the reasons for this suggested change are presented in Section 3 of this paper. Fig. 4 accommodates both old and new interpretations, preserving at the same time the integrity of the Bevets Member as defined in current literature. Overlying these basal coarse facies are the shale and Polo Ground sandstone members (Fig. 4), representing the products of lacustrine and fluvial sedimentation, respectively. The Timeball Hill Formation, with a thickness in excess of 1100 m in the northern part of the Transvaal Basin, also comprises three sedimentary members; these include the lower and upper shale members separated by a sandstone unit, the Klapperkop quartzite Member (Eriksson et al., 1994a; Fig. 4). Minor lenses of poorly sorted diamictites and wackes, ascribed to reworking of periglacial detritus have also been identified in the upper shale member (Visser, 1971). A variety of genetic facies associations are recognized in the formation: pelagic suspension deposits, distal and proximal turbidites, contourites, and lower and upper tidal flat deposits (Eriksson and Reczko, 1998). The close association between deeper marine and coastal facies is explained by a significant stratigraphic break that separates the lower mudstones from the overlying Klapperkop quartzite Member (Fig. 4). Thin stromatolitic carbonate interbeds in the Timeball Hill mudstones suggest that sedimentation took place within the photic zone (Eriksson and Reczko, 1998). In the southern part of the basin, the contact between the Rooihoogte and Timeball Hill formations is marked by a localized occurrence of highly altered lavas (i.e., the Bushy Bend lava Member, Eriksson et al., 1994b; Fig. 4). With an average thickness of about 30 m, these lavas are interpreted to reflect the eruption episode related to the transition from Rooihoogte rifting, to subsequent post-rift subsidence (Eriksson et al., in press).

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7 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 4 infers the relative chronologies of the lithostratigraphic members that build together the Rooihoogte and Timeball Hill Formations. Although this generalized vertical profile reflects the true relationships for individual data points, temporal overlaps between the timing of sedimentation of the various facies within the basin are most likely (Catuneanu and Eriksson, 1999) Palaeoclimatic background The available age constraints of the Rooihoogte and Timeball Hill formations indicate deposition within the span of the c to 2.2-Ga global glaciation (Young, 1995; Eriksson et al., 1998; Martin, 1999; Young et al., in press; Fig. 3). As evidence for extensive ice cover on the Kaapvaal craton is limited, sea levels were likely low and freeboard high (emergent, glaciated continents; Eriksson et al., in press). In addition, the cold temperatures would also have lowered the rates of weathering processes. In view of the low syn-glacial eustatic levels, subsidence to accommodate aggradation and epeiric drowning during the Rooihoogte Timeball Hill times must have been significant. This provides a case study where accommodation and sequence development were apparently primarily controlled by tectonic processes. 3. Sedimentary facies This section presents genetic interpretations for the sedimentary facies that comprise the Rooihoogte and Timeball Hill Formations. The outcrop distribution of the Rooihoogte Timeball Hill succession is illustrated in Fig Rooihoogte Formation Fig. 6 shows the location of the main data points and the associated vertical profiles for the Rooihoogte Formation. Sedimentary facies with regional extent include chert breccias, chert conglomerates, shales, and the Polo Ground sandstones Chert breccias The chert breccias form a discrete, wedge-shaped lithological unit that develops at the limit between the Chuniespoort and Pretoria Groups. This unit was originally assigned to the Rooihoogte Formation, and more recently re-interpreted as the time equivalent of the Duitschland Formation, at the top of the Chuniespoort Group (Eriksson et al., in press; Fig. 3). The sheet-like nature of the Chuniespoort carbonate and iron-rich units (Malmani Subgroup and Penge Formation, respectively, in Fig. 3) enables estimation of the stratigraphic loss related to the basal Pretoria unconformity, which is shown as a contour map of denudation in Fig. 7. Preserved thickness of the chert breccias, when superimposed on Chuniespoort denudation contours, exhibits a good correlation (Fig. 7), as expected for such residual products of in situ weathering. The in situ nature of the breccias, as borne out by their compositional similarity to varying underlying chemical sedimentary strata, was first observed by Button (1973). As shown by Eriksson et al. (in press), the Duitschland Formation largely comprises weathered Chuniespoort detritus that has been transported northwards from the uplifted southern area of the basin, and reworked to produce mainly fine marly facies. Possibly, formation of the Duitschland basin, presumably restricted to the northeast of the preserved Transvaal depository, was coeval with uplift of the southern Chuniespoort rocks. Chert breccias overlying the Chuniespoort chemical sediments in the south, and the Duitschland lithologies, may thus be time equivalents in addition to their inferred proximal distal relationship. If the chert breccias and Duitschland rocks may be correlated, then these strata represent the entire time gap (possibly up to 80 My; Eriksson and Reczko, 1995; Fig. 3) between the end of Chuniespoort chemical sedimentation and the onset of Pretoria Group deposition (beginning with the Rooihoogte conglomerates). Fig. 3. Lithostratigraphy, chronology, tectonic settings, paleoenvironments and inferred base-level changes for the Transvaal Supergroup. 1) from Armstrong et al. (1991); 2) from Eriksson and Reczko (1995); 3 5) from Walraven and Martini (1995); 6) from Eriksson and Reczko (1995); 7) from Walraven and Martini (1995); 8) from Harmer and von Gruenewaldt (1991). Wavy lines suggest unconformable contacts. Modified from Catuneanu and Eriksson (1999).

8 78 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 4. Lithostratigraphic profile of the Rooihoogte and Timeball Hill Formations (Pretoria Group). Modified from Eriksson and Reczko (1995). Wavy lines indicate unconformable contacts. Fig. 5. Outcrop distribution of the Rooihoogte Timeball Hill succession in the context of the Transvaal Basin. Modified from Eriksson and Reczko (1998).

9 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 6. Lithological field profiles of the Rooihoogte Formation Chert conglomerates The chert conglomerates represent alluvial fan and fan-delta deposits sourced from the north, which progressively downlap onto the underlying lithologies in a southward direction. The major occurrence (up to 250 m thick) of the chert conglomerate facies in the west of the basin is lobate in preserved geometry, and thickens northwards towards the source (Fig. 8). A much thinner lobe occurs in the northeast, where it overlies both Duitschland and older rocks. There is also a partial overlap between the areas of occurrence of chert breccias and chert conglomerates in the central part of the Transvaal Basin (Fig. 8). The conglomerates contain mostly chert pebbles, are matrix- and clastsupported (thus indicating, respectively, gravity flow and streamflow transport processes), and with pebbles that vary in size up to about cm, with sizes of 7 cm or less being most common (Fig. 6). The matrix of the conglomerates is generally sand-sized and siliceous, and the roundness of clasts varies between poorly rounded, subrounded, and well rounded. A very significant aspect is the change in topographic tilt at the boundary between the chert breccias and the overlying chert conglomerates (Fig. 8). The direction of tilt during the latest Chuniespoort times was from south to north, which explains the more pronounced weathering (thicker chert breccias) in the south. The progradation of the younger chert conglomerates took place on a topographic slope dipping to the south, which marks a change of approximately 180 in the direction of topographic tilt. This shows that the second-order sequence boundary that separates the Chuniespoort and Pretoria groups is related to a significant tectonic event that led to the reorganization of the Transvaal Basin. The chert breccias and the chert conglomerates preceded and succeeded this tectonic event, respectively, which indicates that they belong to sedimentary packages that are unrelated

10 80 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 7. Contour map showing the correlation between the occurrence of chert breccias and the thickness loss of the underlying Chuniespoort chemical deposits. Modified from Eriksson et al. (in press). Fig. 8. Isopach maps of the chert breccias (uppermost Chuniespoort Group) and chert conglomerates (lowermost Pretoria Group), showing the contrast in the direction of topographic tilt between the timing of deposition of the two facies. Modified from Eriksson et al. (in press).

11 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) genetically (i.e., different depositional sequences separated by a subaerial unconformity) Rooihoogte shales The shales of the Rooihoogte Formation exhibit horizontal stratification, graded laminae, ripple marks (siltstone interbeds), flaser lamination, and varve structures (Eriksson, 1988; Eriksson et al., 1991). This facies is interpreted to represent periglacial lacustrine sedimentation, with the depocenter in the western part of the Transvaal Basin (Eriksson and Reczko, 1995). The Rooihoogte shale Member displays a variable thickness, ranging from about 18 up to 250 m (Eriksson, 1988; Fig. 6). The boundary between the shales and the underlying chert conglomerates is a diachronous facies contact, as the two facies are partly age equivalent (Catuneanu and Eriksson, 1999). The conglomerates and shales together form a fining-upward genetic package that led to the peneplanation of the pre-existing karst topography. Paleogeographic reconstructions show the progradation of chert conglomerates via alluvial fans and fan-delta systems towards the south, into the standing body of water of the lacustrine environment (Eriksson and Reczko, 1995). The balance between fan progradation and lacustrine aggradation gradually shifted in the favor of the latter, in parallel with the denudation of the northern sediment source areas, which explains the overall fining-upward profile of the alluvial deltaic lacustrine systems tract (Catuneanu and Eriksson, 1999) Polo Ground quartzite Member This lithofacies is thin, generally varying from 6 to 10 m in thickness (Fig. 6). It comprises fine- to medium-grained ferruginous quartz wackes, with locally abundant lenses of very coarse pebbly lithic wackes, cm thick and 1 5 m wide. The pebbles consist of siltstone and silty, very fine sandstone, indicating erosion of the underlying lithofacies (Eriksson, 1988). These intraformational pebbles may be interpreted as rip-up clasts preserved at the base of braided channel fills, generated as the unconfined fluvial systems shifted laterally across their own overbank areas. The high-energy character of the interpreted braided systems is also confirmed by the observed sedimentary structures and textures. The sandstones exhibit common planar and trough crossbedding organized in macroforms up to 8 m wide and 70 cm thick, indicating the manifestation of downstream accretion processes, typical for multiple-channel, low-sinuosity systems. The sandstones are commonly granular in the far west of the basin, and contain both chert grains and feldspar. They are coarsest and most immature in the northwest of the basin, which was probably where they were most proximal (Eriksson, 1988). The contact between the Polo Ground sandstones and the underlying lacustrine shales appears to be conformable, locally represented by channel base scours. Taking the top contact of the underlying alluvial deltaic lacustrine systems tract as a time-line datum, the deposition of the Polo Ground fluvial sands in the west of the basin appears to be concomitant with the transgression recorded in the east by the basal black shales of the Timeball Hill Formation Timeball Hill Formation The Timeball Hill Formation is the product of dominantly marine and marginal marine sedimentation, being represented by fine-grained sedimentary strata (lower and upper shale members), and subordinate sandstones (medial Klapperkop Member). The regional distribution of these facies is illustrated in Fig. 9. The three members can be recognized around the preserved basin, and tend to have a sheet-like geometry (Eriksson and Reczko, 1998). The contact with the underlying Rooihoogte Formation is sharp, being represented by the ravinement surface at the base of the lowermost Timeball Hill transgressive black shales Lower shale Member The Lower Shale Member consists of a widespread basal black shale lithofacies, succeeded by rhythmically interbedded mudstones, siltstones, and fine-grained sandstones, often termed the rhythmite lithofacies, or lower mudstones, by previous researchers (Eriksson et al., 1994b). The marine black shales are inferred to have transgressed approximately from east to west (Eriksson and Reczko, 1998). This facies is typically laminated, with subordinate lenses of silty material with planar cross-laminations and current ripples. The black pigmentation is due predominantly to pervasive micro-

12 82 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 9. Fence diagram illustrating the distribution, thickness, and sheet-like geometry of the lithofacies identified by previous workers in the Timeball Hill Formation. Modified from Eriksson et al. (1994b) and Eriksson and Reczko (1998). The position of all localities in the context of the Transvaal Basin is indicated in Fig. 5. scopic iron minerals (mainly limonite after pyrite), with subordinate thin beds and laminae more intensely pigmented by flakes of carbonaceous material (Eriksson et al., 1994b). Above the transgressive black shale, the lower mudstones consist of a shallowing-upward succession of pelagic, distal delta-fed turbidites, and contourites, interpreted by Eriksson and Reczko (1998) as being deposited under highstand conditions. Dominant lithofacies include laminated and graded mudstones, and sheets of laminated and cross-laminated siltstones and fine-grained sandstones. These are compatible with the Te, Td, and Tc subdivisions of the low-density turbidity current systems (Eriksson and Reczko, 1998). Thin interbeds of stromatolitic carbonates support photic water depths up to about 100 m. Small lenses of coarse siltstone to very finegrained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits (Eriksson and Reczko, 1998) Klapperkop quartzite Member The arenaceous Klapperkop Member (Fig. 9) consists of an erosively based, generally upward-coarsening succession of tidally reworked braid-delta deposits, interpreted as lowstand facies by Eriksson and Reczko (1998). Eriksson and Reczko (1998) recognized two separate facies within the Klapperkop Member: (1) mature cross-bedded sandstone sheets, interpreted as lower tidal flat deposits; and (2) interbedded lenticular immature sandstones and mudstones, interpreted as medial to upper tidal flat deposits. The lower tidal flat deposits consist of sandstone beds with lateral extents of tens to hundreds of meters, and bed thicknesses of up to 5 m. These rocks are mostly fine- to medium-grained, and comprise quartz arenites with subordinate sublithic arenites, quartz wackes, and lithic wackes (Schreiber, 1990). Minor, thin mudstone interbeds are also found (Eriksson and Reczko, 1998). The sandstone sheets display planar and trough cross-bedding with varying proportions around the basin (Button, 1973; Key, 1983; Van der Neut, 1990; Schreiber, 1990). A few

13 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) herringbone cross-bed sets occur, as well as minor interference and bifurcating ripples, and rare mudcracked surfaces and preserved megaripples (Button, 1973; Schreiber, 1990). A relatively shallow water depositional setting is inferred for these cross-bedded sandstone sheets, supported by uncommon mudcracks and ripple marks. The presence of minor herringbone cross-beds, bifurcating, and interference ripples suggests tidal action. The textural and compositional maturity of these inferred tidal sandstone sheets points to a lower tidal flat setting where reworking would have been more prevalent (Eriksson and Reczko, 1998). The medial to upper tidal flat deposits are commonly interbedded with stacked lower tidal flat sandstone sheet successions up to 30 m thick (Eriksson and Reczko, 1998). The inferred upper tidal flat deposits comprise lenticular sandstone bodies, from less than 1 m to about 50 m in lateral extent, and up to about 50 cm thick, interbedded with finely laminated, micaceous mudstones. The sandstones are compositionally and texturally immature, mostly fine to medium grained. Locally, coarse-grained sandstones and even small pebble conglomeratic basal lags are observed (Eriksson and Reczko, 1998). The presence of interbedded sandstones and mudstones, ladderback and flat-top ripples, herringbone cross-strata, and mudcracks supports the varying energy levels and intermittent exposure typical of middle to upper tidal flats Upper shale Member The Upper Shale Member consists of a deepening-upward succession of suspension deposits and delta-fed turbidite fan systems interpreted as transgressive facies (Eriksson and Reczko, 1998). Subordinate occurrences of black shales and diamictites are recorded in the south of the basin, and arkosic sandstones in the north (Fig. 9). Typical for these upper mudstones are widespread soft sediment deformation structures (Eriksson et al., in press). The zone of disturbed mudstones extends over much of the eastern part of the basin, thinning to both north and south of an approximately central maximum preserved depth (below the upper contact of the formation) of deformation of c. 160 m (Button, 1973). The soft sediment deformation of the unconsolidated upper Timeball Hill facies is likely related to the tectonic instability that terminated the evolution of the Timeball Hill seaway. This tectonic event was interpreted to reflect conditions of pre-rift uplift that generated the second-order subaerial unconformity that separates the Rooihoogte Timeball Hill from the overlying Boshoek Houtenbek secondorder depositional sequence (Catuneanu and Eriksson, 1999). 4. Sequence stratigraphy Previous sequence stratigraphic analysis of the Transvaal Supergroup identified the Rooihoogte Timeball Hill succession as a second-order depositional sequence bounded by major subaerial unconformities (Catuneanu and Eriksson, 1999). This sequence accumulated during a stage of glacio-eustatic fall, with accommodation provided by syn-rift extensional and post-rift thermal subsidence. At a second-order level of stratigraphic cyclicity, the Rooihoogte Timeball Hill succession conforms with the definition of a depositional sequence, as it groups together a relatively conformable package of strata that are genetically related to one full tectonic cycle of rifting. As argued in the previous sections of this paper, the position of the lower second-order sequence boundary of the Rooihoogte Timeball Hill sequence should be revised from the base of the chert breccias to the contact of these breccias with the overlying chert conglomerates (Fig. 10). This is because the chert breccias are likely age equivalent with the Duitschland Formation of the Chuniespoort Group, and the major tectonic event leading to the basin inversion and the change in topographic tilt succeeded the timing of breccia formation and preceded the progradation of chert conglomerates. For this reason, the chert breccias and conglomerates are unrelated genetically and belong to different depositional sequences. The conformable character of the second-order Rooihoogte Timeball Hill succession is disrupted by the basin-wide erosional surface at the base of the Klapperkop quartzite Member. This basal Klapperkop unconformity has a markedly different character relative to the second-order sequence boundaries of the Rooihoogte Timeball Hill succession. The basal Rooihoogte and basal Boshoek unconformities (previously identified as second-order sequence boundaries: Catuneanu and Eriksson, 1999) are strongly

14 84 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Fig. 10. Sequence stratigraphic interpretation of the Rooihoogte Timeball Hill succession. Not to scale. The Rooihoogte and Timeball Hill Formations build together a second-order depositional sequence. This sequence is split by the basal Klapperkop basin-wide subaerial unconformity into two third-order depositional sequences, marked as (1) and (2) in this diagram. Abbreviations: LST lowstand systems tract; TST transgressive systems tract; HST highstand systems tract; (1) lacustrine facies (mudstone Member) of the Rooihoogte Formation; (2) Polo Ground sandstone Member of the Rooihoogte Formation; (3) Lower Shale Member of the Timeball Hill Formation, excluding the basal black shales; (4) Klapperkop Member of the Timeball Hill Formation; (5) Upper Shale Member of the Timeball Hill Formation. angular and erosional, being associated with major tectonic reorganizations of the basin. The amounts of downcutting are estimated to up to 800 and 250 m, respectively (Eriksson et al., in press; Button, 1973). The basal Klapperkop unconformity is not associated with any significant tectonic reorganization within the basin, and does not display angular relationships. For these reasons, although it is still uncertain at this stage how much downcutting took place due to the lack of angular relationships, we propose that this unconformity has a lower hierarchical order relative to the basal Rooihoogte and basal Boshoek surfaces. We therefore suggest that the basal Klapperkop unconformity is a third-order sequence boundary, which provides the basis for the sequence stratigraphic subdivision of the Rooihoogte Timeball Hill second-order sequence into two third-order depositional sequences. These two third-order sequences are marked as (1) and (2) in Fig Sequence (1) Sequence (1) includes the entire Rooihoogte Formation, plus the Lower Shale Member of the Timeball Hill Formation (Fig. 10). It corresponds to a period of time of continuous base-level rise, when a succession of lowstand, transgressive, and highstand systems tracts accumulated in the Transvaal Basin. The lowstand systems tract (LST) consists of a fining-upward succession of partly coeval lacustrine, fan-delta, and alluvial fan sediments, which includes

15 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) all the pre-polo Ground Member lithofacies of the Rooihoogte Formation. The progradation of alluvial fans and fan-deltas into the lacustrine environment took place from north to south, which indicates the direction of syn-depositional topographic tilt (Fig. 8). This initial stage of aggradation in the Pretoria Basin led to the peneplanation of the pre-existing karst topography that formed during extensive subaerial exposure at the top of the Chuniespoort Group. The lower contact of the LST coincides with the secondorder subaerial unconformity that bounds the Rooihoogte Timeball Hill succession at the base. The upper contact of the LST is represented by a maximum regressive surface (Fig. 10), which is associated with a tectonic re-organization of the basin and the debut of the subsequent transgression. The transgressive systems tract (TST) includes two lithostratigraphic units that are inferred to be partly age equivalent: the Polo Ground fluvial sandstones of the Rooihoogte Formation, and the transgressive black shales of the Timeball Hill Formation. There is a noticeable change in the direction of topographic tilt between the LST and the TST, as inferred from the chert conglomerate isopachs (Fig. 8), Polo Ground paleocurrents (Eriksson, 1988), and the direction of initial Timeball Hill transgression (Eriksson and Reczko, 1998). The transgression of the Timeball Hill seaway took place from east to west, which implies an easterly tilt in the basin. This in agreement with the paleodrainage patterns of the Polo Ground fluvial systems, with an overall flow along the strike of the basin in an easterly direction. The change in topographic tilt from the LST to the TST (southerly to easterly, respectively) is most likely related to differential subsidence in the basin. The bounding surfaces of the TST are represented by a maximum regressive surface, at the base, and a maximum transgressive surface, at the top (Fig. 10). The latter surface marks the contact with the overlying shallowing-upward succession of the lower Timeball Hill shales. The highstand systems tract (HST) is built by the shallowing-upward succession of the Timeball Hill Lower Shale Member (Fig. 10). This succession of fine-grained pelagic and low-density gravity flow facies is interpreted to represent aggradation in a marine environment during the normal regression of the shoreline. The HST is bounded at the base by the maximum transgressive surface (contact with the underlying transgressive black shales), and at the top by the third-order subaerial unconformity (contact with the overlying Klapperkop Member) Sequence (2) Sequence (2) includes the Klapperkop and Upper Shale members of the Timeball Hill Formation. It is bounded at the base and top by two basin-wide subaerial unconformities of third- and second-order, respectively (Fig. 10). This sequence preserves lowstand and transgressive systems tracts. The lowstand systems tract is represented by the tidally reworked braid-delta deposits of the Klapperkop Member (Fig. 10). The shallowing upward transition from lower tidal flat to medial and upper tidal flat settings suggests gradual normal regression in the braid-delta environment. This normal regression resulted in the aggradation of sandy lowstand deposits with a sheet-like geometry being developed across the entire Transvaal Basin. The lowstand aggradation lasted until the debut of the subsequent transgression, which is marked by an abrupt facies shift from lowstand sands to transgressive mudstones and shales. The contact between the lowstand and the overlying transgressive facies is represented by a wave-cut ravinement surface (Fig. 10). The transgressive systems tract is equated with the Upper Shale Member of the Timeball Hill Formation. This is a deepening-upward marine succession of pelagic and gravity flow deposits, topped by the upper boundary of the Rooihoogte Timeball Hill secondorder sequence (Fig. 10). The lack of a preserved maximum transgressive surface, as well as of an overlying highstand systems tract, indicates significant subaerial erosion and truncation associated with the upper boundary of the Rooihoogte Timeball Hill depositional sequence. The uppermost deep marine facies of the Timeball Hill Formation are sharply overlain by the high energy alluvial fan deposits of the Boshoek Formation (Fig. 3), indicating an abrupt change in the sedimentation regime across the sequence boundary. This secondorder subaerial unconformity may be related to a stage of pre-rift uplift that preceded the next second-order cycle of rifting in the Pretoria Basin (Catuneanu and Eriksson, 1999).

16 86 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) Discussion Precambrian successions are generally characterized by a scarce time control, due to the lack of a usable fossil record and the error margins associated with the radiometric dating of pre-phanerozoic rocks. This impairs sequence stratigraphic analyses at highfrequency temporal scales. This case study, as well as the previous lower resolution work in the Transvaal Basin (Catuneanu and Eriksson, 1999), shows, however, that sequence stratigraphy can be applied with a high degree of confidence at second- and third-order levels of stratigraphic cyclicity. The lack of a highresolution time control may be partly compensated by: (a) careful observations of the facies relationships of the basin fill, and (b) the study of changes in the direction of topographic tilt through time. Lateral and vertical facies changes allow for genetic interpretations of relative sea-level changes, as well as for the delineation of systems tracts. In addition, the shifts in the direction of topographic tilt help to constrain the age relationships between the various facies in the absence of other absolute or relative time indicators. For example, both the Polo Ground sandstones and the basal black shales of the Timeball Hill Formation accumulated during the same stage of easterly tilt, which differentiates them from the underlying systems tract dominated by a southerly topographic tilt. The value of applying the methods of sequence stratigraphy also consists in the better understanding of the nature and significance of the contacts between lithostratigraphic units. The most important lithostratigraphic contact in this case study is the boundary between the Rooihoogte and Timeball Hill formations. As inferred from our analysis, this contact is represented by a ravinement surface cut by waves in the upper shoreface during the transgression of the shoreline. This makes the Rooihoogte Timeball Hill contact a diachronous surface, with the rate of shoreline transgression, which develops within a transgressive systems tract. It is difficult to quantify the absolute and relative contributions of the different controls on accommodation, due to subsequent denudation and the intrusion of the Bushveld Complex (Eriksson et al., in press). We do know, however, that at the second-order level of the Rooihoogte Timeball Hill rifting episode, extensional and thermal subsidence rates outpaced the rates of glacio-eustatic fall to generate the necessary accommodation for sediment accumulation. It is still uncertain what caused the relative sea-level fall that resulted in the subaerial unconformity at the base of the Klapperkop Member. Temporary slowing down of subsidence, outpaced by the eustatic fall, or a temporary increase in the rate of eustatic fall, outpacing the subsidence rates, may both explain the generation of the third-order sequence boundary identified at the base of the Klapperkop Member. 6. Conclusions (1) The Rooihoogte Timeball Hill second-order sequence represents the depositional product of a rifting cycle in the Transvaal Basin, and accumulated during a stage of glacio-eustatic fall. This provides a case study for accommodation and sequence development controlled by tectonic processes. (2) Relative to previous research, the position of the lower boundary of the Rooihoogte Timeball Hill sequence is revised from the base of the chert breccias, to the contact between the breccias and the overlying chert conglomerates. This is because a major tilting event occurred between the deposition of the two facies, which are unrelated genetically and separated by a subaerial unconformity. The chert breccias formed through in situ weathering on a northerly dipping topographic slope, whereas the chert conglomerates represent the product of alluvial and delta fan progradation on a southerly dipping topographic profile. (3) The Rooihoogte Timeball Hill second-order sequence consists of two third-order depositional sequences separated by the subaerial unconformity at the base of the Klapperkop quartzite Member. The lower third-order sequence preserves lowstand, transgressive, and highstand systems tracts. The upper third-order sequence includes only a lowstand systems tract and a partially preserved transgressive systems tract. The missing highstand systems tract indicates strong erosional processes associated with the upper boundary of the Rooihoogte Timeball Hill secondorder sequence. (4) Secondary tectonic reorganizations within the basin, including changes in the direction of topographic tilt and changes in the subsidence rates,

17 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) occurred during the Rooihoogte Timeball Hill second-order cycle of rifting. Paleocurrents and stratal stacking patterns record a change in the direction of topographic tilt between the lowstand and the transgressive systems tracts of the lower third-order sequence, likely related to differential subsidence in the basin. The third-order subaerial unconformity at the base of the Klapperkop Member also suggests a shift in the balance between the rates of subsidence and eustatic fall, leading to a stage of relative sea-level fall. (5) The lithostratigraphic contact between the Rooihoogte and Timeball Hill formations is interpreted as a transgressive surface of erosion (ravinement surface) in sequence stratigraphic terms. This contact develops within a third-order transgressive systems tract, between the inferred coeval Polo Ground fluvial sandstones and the transgressive marine black shales, and is diachronous with the rate of shoreline transgression. (6) Sequence stratigraphy can be successfully applied to the analysis of Precambrian successions at least at the second- and third-order levels of cyclicity. The scarce time control may be partly compensated by careful observations of lateral and vertical facies relationships. Supplementary age constraints may be added by the changes through time in the direction of topographic tilt, which may be inferred from paleocurrents and stratal stacking patterns. Acknowledgements O.C. acknowledges financial support from the University of Alberta and NSERC Canada. P.G.E. is grateful for generous research support from the University of Pretoria and the National Research Foundation, South Africa. We thank Andrew Willis and John Hancox for their thoughtful reviews of the manuscript. In addition, we are grateful to the special issue subeditor, Pradip Bose, for his thorough handling of the manuscript, guidance, and constructive comments. References Armstrong, R.A., Compston, W., Retief, E.A., Williams, I.S., Welke, H.J., Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand Triad. Precambrian Res. 53, Button, A., A regional study of the stratigraphy and development of the Transvaal Basin in the eastern and northeastern Transvaal. PhD dissertation, University of Witwatersrand, Johannesburg. Catuneanu, O., in press. Flexural partitioning of the Late Archaean Witwatersrand foreland system, South Africa. Sediment. Geol. Catuneanu, O., Biddulph, M.N., in press. Sequence stratigraphy of the Vaal Reef facies associations in the Witwatersrand foredeep, South Africa. Sediment. Geol. Catuneanu, O., Eriksson, P.G., The sequence stratigraphic concept and the Precambrian rock record: an example from the Ga Pretoria Group, Kaapvaal craton. Precambrian Res. 97, Catuneanu, O., Willis, A.J., Miall, A.D., Temporal significance of sequence boundaries. Sediment. Geol. 121 (3 4), Christie-Blick, N., Grotzinger, J.P., von der Borch, J.P., Sequence stratigraphy in Proterozoic successions. Geology 16, Embry, A.F., Sequence boundaries and sequence hierarchies: problems and proposals. In: Steel, R.J., Felt, V.L., Johannessen, E.P., Mathieu, C. (Eds.), Sequence Stratigraphy on the Northwest European Margin. Norweg. Petrol. Soc., Spec. Publ. 5, pp Eriksson, P.G., Sedimentology of the Rooihoogte Formation, Transvaal Sequence. S. Afr. J. Geol. 91, Eriksson, P.G., Reczko, B.F.F., The sedimentary and tectonic setting of the Transvaal Supergroup floor rocks to the Bushveld Complex. J. Afr. Earth Sci. 21, Eriksson, P.G., Reczko, B.F.F., Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa. Sediment. Geol. 120, Eriksson, P.G., Schreiber, U.M., Van der Neut, M., A review of the sedimentology of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting. J. Afr. Earth Sci. 13, Eriksson, P.G., Engelbrecht, J.P., Res, M., Harmer, R.E., 1994a. The Bushy Bend lavas, a new volcanic member of the Pretoria Group, Transvaal Sequence. S. Afr. J. Geol. 97, 1 7. Eriksson, P.G., Reczko, B.F.F., Merkle, R.K.W., Schreiber, U.M., Engelbrecht, J.P., Res, M., Snyman, C.P., 1994b. Early Proterozoic black shales of the Timeball Hill Formation, South Africa: volcanogenic and palaeoenvironmental influences. J. Afr. Earth Sci. 18, Eriksson, P.G., Condie, K.C., Tirsgaard, H., Mueller, W.U., Altermann, W., Miall, A.D., Aspler, L.B., Catuneanu, O., Chiarenzelli, J.R., Precambrian clastic sedimentation systems. Sediment. Geol. 120, Eriksson, P.G., Altermann, W., Catuneanu, O., van der Merwe, R., Bumby, A.J., in press. Major influences on the evolution of the c Ga Transvaal basin, Kaapvaal craton. Sediment. Geol. Harmer, R.E., von Gruenewaldt, G., A review of magmatism associated with the Transvaal Basin implications for its tectonic setting. S. Afr. J. Geol. 94,