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Sequence Stratigraphy of the Precambrian Rooihoogte–Timeball Hill Rift Succession, Transvaal Basin, South Africa

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Sequence Stratigraphy of the Precambrian Rooihoogte–Timeball Hill Rift Succession, Transvaal Basin, South Africa Sedimentary Geology 147 (2002) 71–88 www.elsevier.com/locate/sedgeo Sequence stratigraphy of the Precambrian Rooihoogte–Timeball Hill rift succession, Transvaal Basin, South Africa Octavian Catuneanu a,*, Patrick G. Eriksson b aDepartment of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3 bDepartment 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 mentary geology. Through the recognition of bounding surfaces, genetically related facies (systems tracts) can Sequence stratigraphy, which developed as a meth- be identified. Lithofacies can then be correlated accord- odology for explaining the relationships of allostrati- ing to where each unit is positioned along an inferred graphic units that fill a sedimentary basin, is currently curve that represents base-level fluctuations. The con- one of the most actively evolving disciplines in sedi- cepts of sequence stratigraphy have primarily been perfected from the study of Phanerozoic successions, * Corresponding author. Tel.: +1-780-492-6569. which provide better preservation potential and time E-mail address: octavian@ualberta ca (O. Catuneanu). control for detailed stratigraphic analyses and correla- 0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0037-0738(01)00188-9 72 O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) 71–88 tions (Vail, 1987; Posamentier et al., 1988, 1992; Van duration of sediment-driven (‘‘normal’’) regressions, Wagoner et al., 1990; Hunt and Tucker, 1992; Miall, which depends on the ratio between the rates of base- 1997; Plint and Nummedal, 2000). More recently, the level rise and the sedimentation rates (see Catuneanu principles of sequence stratigraphy have also been and Eriksson, 1999 for a more detailed discussion). applied to genetic interpretations of Precambrian suc- Lowstand systems tracts form during early stages cessions (e.g., Christie-Blick et al., 1988; Catuneanu of base-level rise, when the rates of base-level rise are and Eriksson, 1999; Catuneanu and Biddulph, in outpaced by sedimentation rates. As a result, a ‘‘nor- press). mal’’ regression of the shoreline occurs. Typical products for lowstand systems tracts (LST) include 1.1. Concepts of sequence stratigraphy amalgamated channel fills overlying subaerial uncon- formities, and lowstand deltaic deposits. Protected Comprehensive discussions of sequence strati- from subsequent erosion by the aggradation of over- graphic concepts and their application to the Precam- lying transgressive and highstand deposits, these LST brian rock record are provided by Christie-Blick et al. deposits have a high preservation potential. (1988) and Catuneanu and Eriksson (1999). Briefly Transgressive systems tracts form during acceler- summarized below are key concepts relevant to this ated base-level rise, when rates of base-level rise out- study. The various systems tracts and stratigraphic pace sedimentation rates. As a result, a transgressive surfaces are defined relative to the base-level and shift of the shoreline occurs, and retrogradation and transgressive–regressive curves (Fig. 1). The two vertical aggradation in both fluvial and shallow marine curves are offset by a time period equivalent to the 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. O. Catuneanu, P.G. Eriksson / Sedimentary Geology 147 (2002) 71–88 73 Highstand systems tracts form during late stages of ces, that is, the Protobasinal, Black Reef, Chunies- base-level rise, when sedimentation rates outpace poort, Rooihoogte–Timeball Hill, and Boshoek– rates of base-level rise. ‘‘Normal’’ regression of the Houtenbek sequences (Fig. 3). The purpose of this shoreline occurs, resulting in aggradation and progra- research is to increase the resolution of sequence dation of both fluvial and marine deposits. Highstand stratigraphic analysis to the third-order level of cyclic- deltaic deposits, bounded above by subaerial uncon- ity, for the case study of the Rooihoogte–Timeball formities, are typical products. Highstand strata may Hill second-order sequence. The motivation for doing have a low preservation potential due to erosion this work is twofold: (1) no third-order sequence accompanying subsequent base-level falls. stratigraphic analyses have been performed so far on Subaerial unconformities develop in the nonmarine the Transvaal succession, and (2) the accumulation of portion of the basin due to fluvial or wind degradation the Rooihoogte–Timeball Hill strata took place dur- during stages of base-level fall. They may overlie ing a time of global glacio-eustatic fall (Young, 1995; fluvial or marine strata, but are overlain by nonmarine Eriksson et al., 1998; Martin, 1999; Young et al., in deposits. press; Fig. 3), which provides a case study for systems Transgressive surfaces of erosion, also known as tract and sequence development with accommodation ‘‘ravinement surfaces’’, are scours cut by shoreface generated by tectonic processes. This is particularly waves during the transgression of a shoreline. They relevantinviewofthecoredebateofsequence are highly diachronous surfaces, separating fluvial stratigraphy over the eustatic versus tectonic controls strata below from shallow marine facies above. In on accommodation and sequence development. areas of high preservation potential, ravinement sur- faces may be entirely developed within transgressive systems tracts, which is why they are not represented 2. Geological background in Fig. 1 (see Catuneanu and Eriksson, 1999, for discussion and illustration). 2.1. Tectonic setting Maximum regressive surfaces represent the boun- dary between a lowstand systems tract and an over- The Transvaal Supergroup overlies the c. 3.0- to lying transgressive systems tract. They are also known 2.7-Ga Witwatersrand Supergroup in the stratigraphic as ‘‘conformable transgressive surfaces’’ (Embry, record and constitutes the sedimentary floor to the 1995; Catuneanu et al., 1998). Bushveld igneous complex (Fig. 3). It should be noted Maximum transgressive surfaces represent the here, that the c. 2.7-Ga Ventersdorp Supergroup, boundary between a transgressive systems tract and which unconformably succeeds the Witwatersrand an overlying highstand systems tract. A synonymous strata, is approximately coeval with the lowermost term is ‘‘maximum flooding surfaces’’. portion of the Transvaal Supergroup (e.g., Eriksson et al., in press). The 2714-Ma boundary between the 1.2. Aim of research Witwatersrand and Transvaal (Ventersdorp) Super- groups marks a significant change in the structural This paper focuses on the Transvaal Basin of South style of the receiving sedimentary basins. The Witwa- Africa (Fig. 2), which preserves a 650-My record tersrand succession accumulated within a retroarc of Late Archaean to Early Proterozoic sedimentation.
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