D.Phil. / The Mesoarchean Pongola Supergroup, cropping out in the southeastern region of the Kaapvaal craton, is one of the oldest known supracrustal successions in the world. It represents an erosional remnant of a once extensive cratonic cover sequence. The succession is subdivided into the lower Nsuze Group, a volcano sedimentary succession, and the Mozaan Group composed mainly of siliciclastics and minor volcanics. The Mozaan Group is host to the world’s oldest known glacial deposit. The Pongola Supergroup is well preserved both geochemically and structurally. Outcrops extend from the northern part of KwaZulu-Natal and Mpumalanga Province in South Africa into Swaziland. This study presents results of an integrated stratigraphic, sedimentological, geochemical, geochronological, and paleomagnetic investigation of the Pongola Supergroup in Swaziland and drill cores from the Nongoma Graben in KwaZulu-Natal. The Nsuze Group displays marked cyclicity between volcanic and siliciclastic rocks that were probably deposited in an intracratonic ‘sag and dome’ basin. The volcanicsedimentary cycles are thought to represent periodic heat loss from a hot regional mantle beneath the Kaapvaal craton. In the sag basins the rate of subsidence was gradual and sedimentation marked by near shore deposition. Volcanism in the Agatha Formation was episodic and displays a cyclicity of 2-14Myr in duration. Magmatic eruption was marked by the development of a low crustal level magma chamber. Crustal contamination trough assimilation and fractional crystallization at these low crustal level magma storages is recorded by compositional bimodal volcanism of basaltic andesite-rhyodacite and andesite-rhyolite association. After cessation of the volcanism of the Nsuze Group, subsequent development of the Pongola basin was marked by thermal subsidence and marine transgression in an epicratonic basin, at the time of deposition of the Mozaan Group. The Mozaan Group overlies the Nsuze Group with an erosive unconformity developed over an in situ weathering profile, i.e. a paleosol. The Mozaan Group consists of alternating quartzite, shale, conglomerate, iron-formation. Three units of contemporaneous flood basalt, namely the Tobolsk, Gabela and Ntanyana formations are interbedded with the siliciclastic deposits in the upper part of the succession. Unimodal paleocurrent directions based on fluvial quartzites indicate initial provenances to the south and north. This indicates that the basin morphology was in a form of a trough. Eventually paleocurrent distribution patterns turn southeast, and marine flooding extended further towards the west, to form the Greater Witwatersrand Basin, which was then modified by development of a foreland basin towards the west and northwest. This suggests that the present outcrops of the Mozaan Group represent mere remnants of an extensive basin. Siliciclastic deposits of the Mozaan succession harbours a wealth of information on the crust-forming events that affected the Kaapvaal craton. Detrital zircons from quartzites and diamictite samples yield ages that record magmatic events that extend from the early (ca. 3.6Ga) to middle (2.89Ga) Archean. Late magmatic events were coeval with the development of the Mozaan basin. The Mozaan succession also is host to the oldest known glacial deposits, namely the Klipwal and the Mpatheni Member diamictites. The absence of deformation, erosional contacts and the presence of incorporated underlying material suggest that grounded glaciers were not the mode of deposition of these diamictites, but that the glacial deposits can be explained as gravity flows from collapse of oversteepened slopes of low relief glaciomarine setting that degenerated in more distal part of the basin into clast poor turbidity flows. Analysis of shale and matrix composition in diamictites show that mechanical erosion processes dominated the source area. CIA values range between 70 and 81, suggesting negligible chemical weathering in the sediment source area. CIA values increase only in the upper part of the stratigraphy, i.e. Ntanyana Formation. Incompatible to compatible trace element ratios are low and suggest that mafic and ultramafic rock dominated the source area. This, perhaps, demonstrates that the greenstone belts were a major source of detritus to the Mozaan basin. Well-constrained paleomagnetic data sets acquired from the Klipwal Member diamictite and Tobolsk lava give a very good estimate of the paleogeographic setting of the Kaapvaal craton during the deposition of the Mozaan Group. The Klipwal diamictite was apparently deposited at high-paleolatitude setting of 48°. The craton then moved slightly to the north to latitude ~43° at the time of eruption of the Tobolsk lavas. From the results it appears that available geochemical classification schemes based on the composition of Phanerozoic volcanic rocks are not suitable to classify the lavas of the Agatha Formation unequivocally. To arrive at any tectonic model for these igneous rocks it is necessary to consider stratigraphic relationships, physical volcanology and geochemical characteristics of the rock succession. On the other hand, the wellconstrained paleomagnetic data indicate that the global climate system in the Mesoarchean was similar to modern day earth where glacial deposits are constrained largely to Polar Regions.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:14843 |
| Date | 28 January 2009 |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0018 seconds