Date of Award
Master of Science
Robert Riding, Christopher Fedo
The interpretation of Archean marine depositional environments relies on precise evaluation of facies that reflect shifts in sediment distribution and accumulation, which in turn are driven by changes in accommodation, in situ carbonate precipitation and detrital deposition, and hydrodynamic energy conditions. In the Griqualand West sub-basin of the Transvaal Supergroup, the Ghaap Plateau Facies (Reivilo Formation) and Prieska Facies (Upper Nauga Formation) represent distinct facies belts (250 km apart) that both consist of cm-scale bedding alternations within primarily carbonate strata. In this study, facies descriptions and stratigraphic analyses are combined with limited petrographic and geochemical investigation to compare bedding style and environments of deposition. Centimeter-scale bedding couplets are composed of a lower interval consisting of coarsely recrystallized calcite (A-type laminae) and an upper interval consisting of finely crystalline dolomite that can contain detrital sediment (B-type laminae). Couplets are inferred to represent changes in depositional conditions that potentially reflect eustatic sea level changes that resulted in variation in both detrital input and energy conditions of carbonate environments.Petrographic and geochemical analysis of facies within the Reivilo and Upper Nauga Formations reveal pervasive diagenesis that limits interpretation of microscale fabrics. However, the degree of diagenesis is similar in A- and B-type components in both of the two localities. Stratigraphic analysis of cm-scale couplets suggests similarity in response to oscillating parameters but with differing magnitudes. Modified Fischer plots, which examine both the overall change in stacked couplet thickness through time, as well as changes in contributions from A- and B-components to couplet thickness, indicate that carbonate deposition was consistent within and between both facies belts, and that changes in detrital input played a primary role impacting couplet thickness. Strata of the Campbellrand Subgroup has previously been interpreted as a carbonate platform (Ghaap Plateau Facies) with a discrete, tectonically enhanced margin—reaching a penultimate relief of ~900 m—adjacent to deep slope and basinal (Prieska Facies) environments. This study, however, suggests that the Ghaap Plateau and Prieska facies accumulated carbonate in much more similar environments, which requires rethinking the current basin model.
Alexander, James Harvey, "Patterns within centimeter-scale bedding in carbonate rocks in the Archean Transvaal Supergroup, South Africa. " Master's Thesis, University of Tennessee, 2020.