ETD Collection

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Subject: search.filters.subject.Karoo Supergroup

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    Stratigraphy and sedimentary environments of the Late Permian Dicynodon Assemblage Zone (Karoo Supergroup, South Africa) and implications for basin development
    (2016) Viglietti, Pia Alexa
    The Dicynodon Assemblage Zone (DiAZ) spans the last three million years of the Late Permian (Lopingian) Beaufort Group (Karoo Supergroup). Fluvio-lacustrine conditions covered the entire Karoo Basin during this period, preserved as the rocks of the Balfour, Teekloof, and Normandien formations. However widely separated exposures and few dateable horizons make correlating between lithostratigraphic subdivisions difficult. Here a revised litho- and biostratigraphic framework is provided for the Upper Permian DiAZ. The Balfour Formation’s Barberskrans Member (BM) is renamed due to identifying the Oudeberg Member and not the BM at the current type locality (Barberskrans Cliffs). It is renamed Ripplemead member (RM) after Ripplemead farm 20 km north of Nieu Bethesda where it outcrops. The Teekloof Formation’s Javanerskop member and Musgrave Grit unit in the central Free State Province are regarded mappable units whereas the Boomplaas sandstone (BS) may represent a unit that is a lateral equivalent to the Oudeberg Member. Palaeontological and detrital zircon data suggest none of these locally persistent sandstone horizons correlate temporally. Three index fossils that currently define the DiAZ (Dicynodon lacerticeps, Theriognathus microps, and Procynosuchus delaharpeae) appear below its lower boundary and disappear below the Permo-Triassic Boundary (PTB), coincidentally with the appearance of Lystrosaurus maccaigi. The base of the DiAZ is redefined, with the revived Daptocephalus leoniceps and T. microps re-established as the index fossil for the newly proposed Daptocephalus Assemblage Zone (DaAZ), and is subdivided into two subzones. Da. leoniceps and T. microps’ appearance define the lower and L. maccaigi defines the base of the upper subzone. The same patterns of disappearance are observed at the same stratigraphic interval throughout the basin, despite the thinning of strata northward. Additionally wetter floodplain conditions prevailed in the Lower DaAZ than in the Upper DaAZ which likely reflects climatic changes associated with the Permo-Triassic mass extinction (PTME). Palaeocurrent and detrital zircon data demonstrate a southerly source area, and recycled orogen petrography indicates the Cape Supergroup is the source of Upper Permian strata. Dominant late Permian zircon population supports the foreland nature of the Karoo Basin. Orogenic loading/unloading events are identified by two fining-upward cycles, separated by a diachronous third-order subaerial unconformity at the base of the RM and Javanerskop members. Sediment progradation northwards was out-of-phase with the south and wedge-shaped. Distributive fluvial systems depositing sediment within a retroarc foreland basin best explains these observations. Lithostratigraphic beds and members are recommended for use as local marker horizons only in conjunction with other proxies, such as index fossils or radiometric dates in future studies.
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    The evolution of marginal-marine systems of the Amibberg formation, Karasburg Basin, Southern Namibia: implications for Early-Middle Permian palaeogeography in South Western Gondwana
    (2015-05-07) Berti, Michael
    The Karasburg Basin is situated in southern Namibia and preserves a heterogeneous succession of Karoo Supergroup strata up to 1000m thick. The uppermost preserved succession in this basin is the Amibberg Formation which is 250m thick and consists of intervals of sandstone, siltstone and mudstone. This study uses facies analysis, sequence stratigraphy and petrography to determine the palaeogeography and provenance for the Amibberg Formation. This is then used to establish environmental variability across the Karasburg – Aranos – Main Karoo basins and to define an equivalent of the Amibberg Formation in the Main Karoo Basin. Detailed stratigraphic logging of five outcrop localities has led to the identification of seven distinct lithofacies and two dominant ichnofacies (Cruziana and Skolithos). These lithofacies include: 1) Massive, laminated and bioturbated mudstones interpreted as offshore deposits (OS); 2) Bioturbated siltstones and sandstones which are representative of offshore-transitional environments (OST); 3) Interbedded sandstones and siltstones also interpreted as offshore-transitional deposits (OST) and generated by river-fed hyperpycnal plumes; 4) Sharp based, massive sandstones interpreted as being deposited on the distal lower shoreface (dLSF); 5) Non-amalgamated hummocky cross-stratified (HCS) and wave rippled sandstones interpreted as distal lower shoreface deposits (dLSF); 6) Amalgamated HCS and wave rippled sandstones interpreted as proximal lower shoreface deposits (pLSF); and 7) Soft-sediment deformed (SSD) sandstones and siltstones occurring in close juxtaposition with dLSF and pLSF deposits. The vertical arrangement of these lithofacies shows a general coarsening and shallowing upward trend. Overall the rocks of the Amibberg Formation consist of wave-dominated shoreface deposits with significant influence by tidal processes. Petrographically, the sandstone samples fall into the class of quartz and feldspathic wackes and are sourced from craton interior provenances. Geochemical analysis of mudstones and nodules indicate high levels of microbial activity under predominantly oxic conditions during the deposition of the Amibberg Formation. Five poorly defined 4th order T-R cycles are observable within the strata of the Amibberg Formation. Large regressive intervals are capped by thin transgressive tracts and these cycles are interpreted to have formed due to eustatic processes. Overall, the Amibberg Formation represents a regressive shoreline. iii Based on the mean palaeocurrent vectors a NNE-SSW palaeoshoreline orientation is deduced and the shoreface must have occupied a palaeohigh on the northern side of the western Cargonian Highlands. This emergent highland acted as an extensive headland and assisted in the connectivity of the Karasburg and Aranos basins, with partial connectivity with the Main Karoo Basin during the Early Permian. Based on this study, the Amibberg Formation is considered an equivalent of the Waterford Formation in the Main Karoo Basin based on similar: stratigraphic position; thickness; sedimentary structures; trace fossil assemblages and stacking patterns.
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    Stratigraphy and basin modelling of the Gemsbok Sub-Basin (Karoo Supergroup) of Botswana and Namibia
    (2011-06-22) Nxumalo, Valerie
    The Gemsbok Sub-basin is situated in the south-western corner of the Kalahari Karoo Basin and extends south from the Kgalagadi District of Botswana into the Northern Cape (South Africa); and west into the Aranos Basin (southeast Namibia). The Sub-basin preserves a heterogeneous succession of Upper Palaeozoic to Lower Mesozoic sedimentary and volcanic rocks of the Karoo Supergroup. Because the succession is largely covered by the Cenozoic Kalahari Group, the stratigraphy of the succession is not as well understood as the Main Karoo Basin in South Africa. Most research in the Gemsbok Sub-basin is based on borehole data. This study focuses on the intrabasinal correlation, depositional environments and provenance of the Karoo Supergroup in the Gemsbok Sub-basin in Botswana and Namibia. Based on detailed sedimentological analyses of 11 borehole cores of the Karoo Supergroup in the Gemsbok Sub-basin of Botswana and Namibia, 8 facies associations (FAs) comprising 14 lithofacies and 2 trace fossil assemblages (Cruziana and Skolithos ichnofacies) were identified. The facies associations (FA1 to FA8) correspond to the lithostratigraphic subdivisions (the Dwyka Group, Ecca Group, Beaufort equivalent Group, Lebung Group [Mosolotsane and Ntane formations] and Neu Loore Formation) of the Karoo Supergroup. Sedimentological characteristics of the identified facies associations indicate the following depositional environments: glaciomarine or glaciolacustrine (FA1, Dwyka Group), deep-water (lake or sea) (FA2, Ecca Group), prodelta (FA3, Ecca Group), delta front (FA4, Ecca Group), delta plain (FA5, Ecca Group), floodplain (probably shallow lakes) (FA6, Beaufort Group equivalent), fluvial (FA7, Mosolotsane and Neu Loore formations) and aeolian (FA8, Ntane Sandstone Formation). The Dwyka Group (FA1) forms the base of the Karoo Supergroup in the Gemsbok Subbasin and overlain by the Ecca Group deposits. Three types of deltas exist within the Ecca Group: fluvial-dominated; fluvial-wave interaction and wave-dominated deltas. The Gemsbok Sub-basin was characterised by rapid uplift and subsidence and high sediment influx during the deposition of the Ecca Group. Petrographic and geochemical analyses of the Ecca Group sandstones revealed immature arkose and subarkose type sandstones dominated by angular to subangular detrital grains, sourced from transitional continental and basement uplifted source areas. The sandstones of Ntane Sandstone Formation are classified as subarkoses and sourced from the craton interior provenances.