The sequence stratigraphic concept and the Precambrian rock record: an example from the 2.7–2.1 Ga Transvaal Supergroup, Kaapvaal craton

Abstract

Sequence stratigraphic concepts are applied to the 2.7–2.1 Ga Transvaal Supergroup, which constitutes the sedimentary floor to the Bushveld igneous complex. The Transvaal succeeds the foreland-fill of the Witwatersrand Supergroup and, in its lowermost portions, is partly synchronous with the Ventersdorp Supergroup lavas that followed upon the Witwatersrand sediments in the stratigraphic record. The unconformable contacts at the base and the top of the Transvaal Supergroup mark significant changes in the overall tectonic setting, which qualifies them as first-order sequence boundaries. Sedimentation within the Transvaal basin was controlled by cycles of extensional and/or thermal subsidence resulting in the accumulation of sequences, separated by stages of base-level fall generating sequence-bounding subaerial unconformities. The Transvaal first-order sequence is subdivided into five second-order unconformity-bounded depositional sequences, i.e. the Protobasinal, Black Reef, Chuniespoort, Rooihoogte–Timeball Hill and Boshoek–Houtenbek sequences. The span of time of these second-order sequences, averaging 130 Ma, is much longer than the duration of the second-order Phanerozoic cycles, which is explained by the less evolved and thus slower plate tectonic processes that dominated the Late Archaean–Early Proterozoic times. Each Transvaal second-order sequence preserves a variable number of systems tracts, mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities. In a complete succession, a sequence would include a basal lowstand systems tract (LST), followed by transgressive (TST), highstand (HST) and falling stage (FSST) systems tracts. The Protobasinal and Rooihoogte–Timeball Hill sequences only preserve the LST and TST, due to the strong post-depositional erosion. The Black Reef sequence is composed of LST, TST and HST. The Chuniespoort sequence preserves the TST, HST and FSST, but lacks the LST due to ravinement erosion. The Boshoek–Houtenbek sequence develops the complete succession of systems tracts. Common features may be emphasized between the same types of systems tract developed within different sequences. The LST includes high-energy coarse prograding facies, such as alluvial fans and fan–delta systems, that fill the irregularities of the pre-existing topography and lead to the peneplanation of the depositional areas. The TST may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments. The HST marks the normal regression of the previous marine environments, which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework. The FSST would normally include only basinal facies age-equivalent with the correlative subaerial unconformities. In our case study, the stages of base-level fall resulted invariably in fully nonmarine environments within the Transvaal area, generally leading to the development of subaerial unconformities. In particular circumstances, the nonmarine environment may still accumulate and preserve fluvial, alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base-levels. The interpretations of base-level rise and fall resulting from the application of the sequence stratigraphic concept to the 2.7–2.1 Ga Transvaal Supergroup enable estimation of sea level changes during deposition of this succession. With an understanding of local tectonic conditions on the Kaapvaal craton and of coeval sedimentation elsewhere in Africa, eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the Transvaal Supergroup.