The Transvaal Supergroup (South Africa) records evidence of trace oxygen production in late Neoarchaean strata, approximately 200 million years before the 'Great Oxidation Event' (GOE) which is recorded within the Palaeoproterozoic Duitschland Formation (Transvaal Supergroup) between ~2.42 and 2.32 Ga. It is hypothesized that there was a secular rise in oxygen concentrations between the late Neoarchaean and the GOE which may be recorded within the 'mid-Transvaal' Supergroup (Tongwane Formation, Duitschland Formation, Koegas Subgroup). This project has integrated field sedimentology, petrography and geochemistry to build new or revised depositional and diagenetic frameworks for each of these successions and has assessed palaeoredox conditions using carbon isotopes and rare earth element and yttrium (REY) patterns and anomalies. Despite a complex paragenetic history, including medium-grade contact metamorphism, the Tongwane Formation preserves primary (or near-primary), carbon isotope (delta13Ccarb = ~0 ± 2 ‰VPDB) and REY patterns that are consistent with Palaeoproterozoic seawater. No anomalously positive delta13Ccarb values or cerium (CeSN) anomalies are preserved, suggesting limited build-up of free O2. The lower Duitschland Formation preserves previously undocumented lithofacies variations and an angular mid-Duitschland unconformity (which is contemporaneous with the GOE). A new depositional model is proposed; facies assemblages and geometries are consistent with deposition of a wave-influenced Gilbert fan delta deposited in an isolated depocentre created by localised extensional fault subsidence. Lower Duitschland Formation limestones and dolomites show depleted delta13Ccarb and delta18Ocarb values and marine REY patterns which lack CeSN anomalies. Negative delta13Ccarb values suggest incorporation of 12C from organic matter during early diagenesis. There is no evidence of significant free oxygen production. The Koegas Subgroup is unconformably overlain by glacial strata of the Postmasburg Subgroup; the two successions are not intercalated and therefore not synchronous. Marine REY signals with positive Ce anomalies are recorded in delta13Ccarb depleted, stromatolitic dolomite exposed on the farms Taaibosfontein and Sandridge. Small magnitude positive anomalies are likely calculation artefacts, though anomalies >30 % may reflect redox stratification. Neoarchaean cuspate stromatolites of the Gamohaan Formation record trace element distributions - imaged using synchrotron-based XRF techniques - that map to primary microbial structures are not attributable to syndepositional or diagenetic mineralisation processes. Thus they may prove to be indicators of specific microorganisms and metabolic processes, e.g. photosynthetically relevant metals (e.g. Mn, Cu, Ni) mapped in biogenic structures may serve as a 'fingerprint' of cyanobacterial oxygenic photosynthesis. Overall, no evidence is seen for a secular rise in oxygen in the mid-Transvaal. However, depositional frameworks and diagenetic processes have been determined and the retention of marine signals established within the Tongwane, Duitschland and Koegas successions. Therefore the findings of this project constitute a robust framework for future palaeoredox studies of the mid-Transvaal Supergroup.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:728237 |
| Date | January 2017 |
| Creators | Warke, Matthew |
| Contributors | Schroeder, Stefan |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/stratigraphic-and-geochemical-framework-of-the-palaeoproterozoic-rise-in-atmospheric-oxygen-transvaal-supergroup-south-africa(b0aa0021-946c-4f01-bf4e-297611aa2ec1).html |
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