Browsing by Author "Bordy, Emese M"

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    Palaeoenvironments and position of the Precambrian–Cambrian boundary within the lower Vanrhynsdorp Group of South Africa: sedimentary facies analysis, U-Pb series zircon geochronology and micropalaeontology
    (2018) Harrison, Bianca; Bordy, Emese M; Taylor, Wendy L
    The Vanrhynsdorp Group is a mainly fluvio-marine siliclastic succession that outcrops in the northwestern part of South Africa. The critical Precambrian-Cambrian boundary falls within the group, however the depositional environments across the boundary, its exact stratigraphic position and nature are unresolved. The group was deposited in the Vanrhynsdorp Basin, which has been shown to be the southernmost extension of the Nama Foreland Basin. Consequently, the Vanrhynsdorp Group has been correlated with the world-famous Nama Group, which features diverse Ediacaran-Cambrian fossils. To date, no body fossils have been discovered in the Vanrhynsdorp Group. Through U-Pb dating of detrital zircons using LA-ICP-MS, radiometric ages for the middle part of the Vanrhynsdorp Group (Besonderheid Formation) were obtained in a preliminary study of this project. The radiometric data, yielding a maximum depositional age of 524 to 528 Ma from the youngest zircon grain population, indicated that the Precambrian-Cambrian boundary is stratigraphically lower in the group than it was thought before. To further constrain the age of the lower Vanrhynsdorp Group, and by extension the position of the Precambrian-Cambrian boundary, several detrital zircon samples were processed for age determination from the succession in this study. In addition, using sedimentary facies analysis, the lateral and vertical facies variation in this lower part of the group were (re)documented in order to refine the palaeoenvironmental setting. The current results suggest a dominantly shallow marine, partly storm-dominated depositional environment for the lowermost units as opposed to the previous interpretations of dominantly alluvial settings. Because of the global importance of the Ediacaran-Cambrian transition for diversification of marine biota in the Cambrian, addressing these palaeoenvironmental inconsistencies is the vital outcome of this study. By integrating our sedimentological and geochronological results, the project presents an improved understanding of the depositional history of the Vanrhynsdorp Group during the critical Ediacaran-Cambrian transition.
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    Position of the Triassic-Jurassic boundary in South Africa and Lesotho : a multidisciplinary approach aimed at improving the chronostratigraphy and biostratigraphy of the Elliot Formation, Stormberg Group
    (2016) Sciscio, Lara; Bordy, Emese M; De Kock, M O
    The continental red bed succession of the main Karoo Basin in South Africa and Lesotho, the Elliot Formation (Stormberg Group, Karoo Supergroup), is a significant stratigraphic unit for the regional and global understanding of the Late Triassic - Early Jurassic evolution of terrestrial vertebrate faunas, however, the temporal resolution of its biostratigraphy is inadequate for detailed regional and global correlations. The main aim of this dissertation is to build a more comprehensive chronostratigraphic framework of the Late Triassic - Early Jurassic Elliot Formation by combining and constraining its biostratigraphy with new results obtained using magnetostratigraphic techniques. This dissertation presents magnetostratigraphic data from ten measured stratigraphic sections in the Elliot Formation across the main Karoo Basin in South Africa and Lesotho.
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    The sedimentary petrology of carbonate nodules in the Elliot Formation, Karoo Supergroup, main Karoo Basin (South Africa)
    (2016) Moodley, Adam; Bordy, Emese M
    In South Africa, fossils found in the upper part of the Elliot Formation (Stormberg Group, Karoo Supergroup) are often associated with genetically poorly-constrained carbonate nodules. The origin of carbonate nodules i.e., pedogenic versus diagenetic, is important as pedogenic carbonate nodules can be used as palaeoclimate indicators, while diagenetic nodules carry limited palaeoclimatic information on the depositional setting. This research aims to characterize the carbonate nodules of the Elliot Formation macroscopically, petrographically and geochemically and to establish a diagnostic set of criteria to enable the differentiation between pedogenic and diagenetic nodules and/or diagenetic overprint. The research techniques employed in this study range from a) macroscopic field observations of the stratigraphic relationships of the nodules to the sedimentary features of the host rocks; b) sedimentary petrography of the textural features in the nodules; and c) X-ray diffraction for the assessment of the clay composition trapped within the nodules as compared to host rocks. Macroscopic field observations have shown that carbonate nodules found in the UEF are strongly associated with host rocks that contain pedogenic features such as root traces, burrows, colour mottling, and desiccation cracks, and thus are suggestive of ancient soils. However, the microscopic analysis of the nodules reveal no evidence for biological activities but rather a range of abiotic features such as septarian cracks, circumgranular cracks, and micronodules which are more likely have resulted from physicochemical processes that may have occurred during diagenesis. Clay minerals identified by X-ray diffraction include illite, muscovite, and montmorillonite confirm the generation of the sediments under arid to semi-arid climatic conditions.
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    Sedimentology and depositional environment of a marine target, Southern Namibia: 3D stratigraphic architecture and diamond mineralisation potential
    (2018) Runds, Megan Jean; Bordy, Emese M
    The world's largest diamond placer lies on the southwestern coast of Namibia, within the restricted area known as the Sperrgebiet, and comprises, amongst others, a series of Plio-Pleistocene to Holocene littoral deposits preserved onshore for >120 km northwards from the Orange River mouth. Through comprehensive seismic and sedimentological analysis, this study provides the first attempt at linking the well-documented onshore diamondiferous deposits of the Sperrgebiet with the submerged landscape of a diamondiferous marine target, called the Purple Target Area (PTA), situated beneath up to 70 m of seawater, some 3.5 km offshore of the onshore deposits. Four seismic units (A - D) have been identified on 2D seismic reflection profiles, and calibrated to lithological data from several boreholes. These have then been integrated into a detailed landscape evolution model for the PTA. The principal controls on the stratigraphic development of the PTA are rate of sediment input and relative sea-level (RSL) fluctuations. The latter is defined as the interplay of vertical tectonic changes in accommodation space and eustatic sea-level (ESL) fluctuations. The most noteworthy phase of deposition within the PTA basin is the normal regressive coarse gravel beaches trending shore parallel, with the primary gravel barrier and its preserved coeval back-barrier deposits exceeding 7 km in length. The approximate age of the PTA gravel beaches, due to a lack of absolute age constrains, are estimated with reference to their correlated water depths (palaeo-bathymetry) in relation to the ESL curve. Based on this approximation, the PTA gravel beaches have survived the transgression and erosive wave-ravinement processes associated with the Late Pleistocene-Holocene, which occurred between 19 and 7 ka before present. The preservation of the primary gravel barrier beach complex is linked here to a rapid RSL rise during the last transgression, namely Meltwater Pulse 1B. The smaller gravel beaches that are landward of the main barrier were partially preserved through overstepping, but endured more intense ravinement associated with the rapid RSL rise. The primary gravel barrier complex preserved the same cross-beach variability in clast shape sorting (disc/blades and spherical clasts consistent with the middle section and seaward section of the gravel beach, respectively) and back-barrier lagoonal facies identified laterally across the locally known ED barrier beach complex, preserved onshore 65 km south of the PTA. The sedimentary facies correspondence between the onshore and offshore deposits presents a rare opportunity to study the emplacement and preservation processes of gravel beach deposits on a high energy shelf. In addition, the diamond mineralisation potential of the offshore gravel beaches can be assessed with respect to the well-documented onshore diamondiferous beaches.
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    Sedimentology and ichnology of the Mafube dinosaur track site (Lower Jurassic, eastern Free State, South Africa): a report on footprint preservation and palaeoenvironment
    (2016) Sciscio, Lara; Bordy, Emese M; Reid, Mhairi; Abrahams, Miengah
    Footprint morphology (e.g., outline shape, depth of impression) is one of the key diagnostic features used in the interpretation of ancient vertebrate tracks. Over 80 tridactyl tracks, confined to the same bedding surface in the Lower Jurassic Elliot Formation at Mafube (eastern Free State, South Africa), show large shape variability over the length of the study site. These morphological differences are considered here to be mainly due to variations in the substrate rheology as opposed to differences in the trackmaker’s foot anatomy, foot kinematics or recent weathering of the bedding surface. The sedimentary structures (e.g., desiccation cracks, ripple marks) preserved in association with and within some of the Mafube tracks suggest that the imprints were produced essentially contemporaneous and are true dinosaur tracks rather than undertracks or erosional remnants. They are therefore valuable not only for the interpretation of the ancient environment (i.e., seasonally dry river channels) but also for taxonomic assessments as some of them closely resemble the original anatomy of the trackmaker’s foot. The tracks are grouped, based on size, into two morphotypes that can be identified as Eubrontes -like and Grallator -like ichnogenera. The Mafube morphotypes are tentatively attributable to large and small tridactyl theropod trackmakers, possibly to Dracovenator and Coelophysis based on the following criteria: (a) lack of manus impressions indicative of obligate bipeds; (b) long, slender-digits that are asymmetrical and taper; (c) often end in a claw impression or point; and (d) the tracks that are longer than broad. To enable high-resolution preservation, curation and subsequent remote studying of the morphological variations of and the secondary features in the tracks, low viscosity silicone rubber was used to generate casts of the Mafube tracks.
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    Spatial and temporal variations in the geometry and composition of the Permian Whitehill Formation South Africa
    (2017) Chukwuma, Kenneth; Bordy, Emese M
    The Lower Permian Whitehill Formation (WHF) is an important hydrocarbon resource unit in the main Karoo Basin where it occurs in outcrops and up to 4 400 m in the subsurface and thus it is distributed over an area in excess of 260 000 km² in the southern half of South Africa. Although the formation is composed predominantly of black laminated carbonaceous shales, earlier studies detected significant spatiotemporal variability in its stratigraphy and composition, particularly the distribution of its organic carbon content across the basin. Because these stratigraphic variations and compositional heterogeneities remained largely uninvestigated, there are conflicting interpretations of not only the hydrocarbon potential of this resource unit but also the paleoenvironmental conditions that prevailed in the Karoo Basin during its accumulation. Following the recent global proliferation of unconventional hydrocarbon resources originating in organic-rich shale successions, the WHF is seriously being considered for gas shale exploitation in South Africa. Consequently, re-characterizing and explaining the spatiotemporal variations in its geometry and composition would be invaluable to the energy industry as well as the larger scientific community. With this aim, this study applied a combination of field descriptions, vintage borehole data, micro- to nano-scale petrographic observations, and multiple geochemical data so that a more critical understanding of the sedimentological controls responsible for the variability can be established. Using this integrated approach, five primary sedimentary facies (F1-F5, i.e., stratigraphic subunits in the WHF) were identified, which show specific and systematic variations in nature and content of organic carbon, stable isotopic composition (of δ¹³Corg and δ¹⁵N), C/N ratio, major and trace elemental enrichment, nature and content of iron sulfides, quartz texture, and CIA across the basin. The lower dark grey to black thinly laminated pyritic, carbonaceous fine shales (facies F1 and F2) contain up to 16.5 % TOC, δ¹³Corg of -15.57‰, δ¹⁵N of 12.49‰, C/N ratio of 1.50, average CIA of 68.11, Rb/K (x10⁻³) and Sr/Ba ratios of 6.56 and 0.67, respectively. Relative to average shale, this unit is up to 6.27 and 3.11 times richer in Mo and Fe, respectively. The organic materials in this facies comprise Tasmanites cysts, colonial algae cells, and amorphous macerals and occur in well-defined laminae (lamalginites) as well as in discrete organic domains loosely associated with mineral grains (organo-minerallic aggregates). At least 25% of the silicate in this facies is of early diagenetic origin, possibly derived from alteration of air-fall volcanic ash. Iron sulfides occur dominantly in form of framboidal aggregates of pyrite. Marcasite in form of lags cement and nodules is also reported. A binary mixture of organic matter and phosphorites with botryoidal textures is also abundant. The upper medium to light grey calcareous-siliceous silty lenticular shale (F3-F5) contain up to 2.04% TOC, δ¹³Corg of -24.71‰, δ¹⁵N of 4.93‰, C/N ratio of 17.62, average CIA of 74.33, Rb/K (x10⁻³) and Sr/Ba ratios of 3.83 and 0.36, respectively. Relative to average shale, this unit was up to 2.65 and 0.43 times richer in Mo and Fe, respectively. Their organic macerals comprise disarticulated plant remains in disseminations with few amorphous macerals. At least 85% of the quartz content is of detrital origin likely sourced from the basin margins and transported to the basin by the action of bottom-hugging currents. Few iron sulfides occur dominantly in form of octa- and euhedral pyrite grains. The data presented in this thesis suggest that the lower WHF (subunits F1 and F2) may have accumulated in a marine setting with high bioproductivity of organic carbon delivered in form of flocculated organo-minerallic aggregates (pelagic snow) onto an anoxic seabed overlain by dysoxic to oxic waters. Reduced terrigenous input, presence of phosphorites, increasing CIA and increasing δ¹³Corg values with higher TOC point to a depositional setting that resulted from an interplay of sea-level highstand and climatic warm-ups. In contrast, the upper WHF (subunits F3-F5) was deposited largely under non-marine conditions with OM in the company of great detrital debris sourced from terrestrial settings, transported into the basin mainly in form of fluid mud flows and deposited above storm wave base. The presence various body and trace fossils, as well as the variations in bioturbation styles and intensities in subunits F3-F5, indicate that colonisation of the basin by invertebrate and vertebrate organisms is related to the different stages in the oxygenation of the sediment-water interface in an increasingly oxygenated setting.
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    Taphonomy, palaeoecology and taxonomy of an ophiuroid-stylophoran obrution deposit from the Lower Devonian Bokkeveld Group, South Africa
    (2017) Reid, Mhairi; Taylot, Wendy; Bordy, Emese M
    The Lower Devonian Voorstehoek Formation is a fossil-rich, siliciclastic unit (Ceres Subgroup, Bokkeveld Group, Cape Supergroup) in South Africa. This Emsian unit contains a highly endemic benthic fossil biota characteristic of the cool to cold water Malvinokaffric Realm of southwestern Gondwana. The palaeontological and sedimentological investigations of the Voorstehoek Formation suggest that deposition took place in a shallow marine environment within the storm influenced, proximal part of an offshore transition zone. A relatively diverse, ophiuroid–stylophoran assemblage, well-preserved in the Karbonaatjies obrution bed, was excavated at the study site in the Hex River Pass, Western Cape. In this study the taphonomy, taxonomy and the palaeoautecology of Palaeozoic ophiuroids and stylophorans was investigated using micro CT scans. Over 60 samples were scanned, manually segmented and stitched together to create a virtual 3D model of a portion of the Karbonaatjies obrution bed. This method allowed for the determination of the degree of fossil articulation, fossil orientation and faunal counts, without damaging the delicate echinoderm fossils. Furthermore, the ability to digitally analyse the fossil-rich bed has revealed an echinoderm assemblage composed of over 700 articulated ophiuroids dominated by a proposed new genus and species Gamiroaster tempestatis, over 145 articulated mitrate stylophorans Paranacystis cf. petrii Caster, 1954 and eight Placocystella africana (Reed, 1925). Taphonomic analysis of this ophiuroid–stylophoran assemblage indicates this obrution deposit formed due to rapid burial that smothered a potentially gregarious community during a single storm event. Additionally, the admixture of skeletal debris and intact echinoderms present in the Karbonaatjies obrution bed reflects a complex history with significant time-averaging. This unique assemblage provides a taphonomic window into the marine ecosystems of the Early Devonian, including the structure of an unusual, echinoderm-dominated benthic community that forms part of a much wider fossil biota from the Falkland Islands and Precordillera of Argentina, which formed part of SW Gondwana.