Browsing by Author "Bordy, Maria Emese"

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    Diagenetic, thermal and provenance histories of the Permian lower Ecca Group based on two newly drilled boreholes in the western and eastern main Karoo Basin, South Africa
    (2020) Geel, Claire; Bordy, Maria Emese
    Fine-grained sedimentary rocks of the Lower Permian lower Ecca Group in the Main Karoo Basin (MKB) in southern Africa have been identified to form a possible unconventional hydrocarbon reservoir, the gas potential of which is still relatively unknown. The lower Ecca Group is comprised of flysch-like successions of the Prince Albert, Whitehill and Collingham formations, which were impacted by diagenesis as well as contact and regional and metamorphism. Studies of fine-grained sedimentary rocks are notoriously complex as mechanical breakdown and diagenesis result in mineralogical changes and loss of source rock information. Therefore, a variety of analytical techniques are required for their thorough investigation. In this study, we analyse Permian rocks from two boreholes (KZF-1 and KWV-1) that were drilled ~ 830 km apart in the western and eastern MKB, respectively, and compare their composition, thermal maturity and petrophysical characteristics to better assess the hydrocarbon potential of the lower Ecca Group. In addition, whole rock geochemistry is used to reconstruct the palaeo-environment and provenance settings. Lastly, we investigate the influence of shale composition, porosity, well pressure and temperatures on their geomechanical properties such as compressive strength and elastic moduli. Sedimentary rocks in both boreholes were affected by burial and regional metamorphism (linked to orogenic events), but the sedimentary rocks in BH KWV-1 were also altered by contact metamorphism due to dolerite intrusions in the Early Jurassic. Major mineralogical differences between the boreholes include: 1) Fesilicate (greenalite); carbonates (rhodochrosite and dolomite) and Mn nodules (birnessite) being found only in BH KZF-1; and 2) metamorphic minerals such as garnet, cordierite, staurolite being found only in BH KWV-1. The results show that these rocks are over mature as evidenced by the low quantity of free hydrocarbons (S1 peak; 0.02–0.06 mg/g) and potential to release hydrocarbons (S2 peak; 0.06– 0.14 mg/g) and low hydrogen index (HI) values (2.40–167 mg HC/g TOC) from Rock Eval pyrolysis accompanied with high vitrinite/bitumen reflectance (BH KZF-1 is VRo= 4 and BH KWV-1 is BRo= 5). The total organic carbon (TOC) content is the highest in the Whitehill Formation in both boreholes (BH KZF-1: 5.17 wt%; BH KWV-1: 4.87 wt%). Burial diagenesis significantly reduced interparticle porosity and most of the measurable porosity comes from intraparticle nano-pores confined to organic matter v and dolomite. Meso- and macro-porosity are limited to phyllosilicates, pyrite framboids and microfractures. The Chemical Index of Alteration (CIA) is highest for the Prince Albert Formation (72– 78) as the palaeo-environment changed from an ice-house to a green-house, postglacial period in southern Gondwana. Enrichment of trace elements in BH KZF-1 and BH KWV-1 are in order: Mn > Ba > Zn > Co > Cu > V > Rb > Cr > Ni; and for BH KWV-1: Mo > Ba > Cu > Zn > Mn > V > Co > Cr > Rb > Ni. Geochemical proxies used in determining levels of anoxia were: V/Cr, V(V+Ni), Ni/Co and Fe-S-TOC plots. Salinity was deduced using Rb/K ratios and with identification of potential benthic foraminifera and minerals such as apatite, birnessite and phosphate nodules, which are assumed to be syn-sedimentary in origin. Overall, the geochemical results indicate that the depositional conditions fluctuated with respect to anoxia, salinity levels, and that while the sedimentation rates were low and the bio-productivity level was high, at least episodically. Provenance was investigated using Discriminant Function Diagrams, Principle Component Analysis Results (PCA), Zr/Ti ratios and the Index of Compositional Variability (ICV). Results indicate that the Prince Albert and Whitehill formations in the western MKB comprises sediment that was mostly sourced from the Cambrian to Late Carboniferous Cape Granite Suite, Cape Supergroup, the Dwyka Group and possibly the Precambrian Kango Group (Saldania Belt) and/or the Namaqua-Natal Belt. The Prince Albert and Whitehill formations in the eastern MKB have a mafic signature, indicating that these sediments may have been derived from the PermoCarboniferous Dwyka Group, Precambrian Kaapvaal Craton, Natal Belt (Tugela Terrane), Natal Group and the Pan-African Mozambique and Maud Belts. The mudstone and wackes in the Collingham Formation were mostly sourced from the quartz-arenites and granites that were most likely associated with the Cape Fold Belt, the orogen immediately adjacent to and largely coeval with the MKB. Geomechanical results from BH KZF-1 show that the lower Ecca Group has a high proportion (~ 50–70 vol%) of mechanically strong minerals (e.g., quartz, feldspar, pyrite), ~ 30–50 vol% weak minerals (e.g., clay, organic matter) and up to ~ 0–50 vol%, highly variable, intermediate components (e.g., carbonates). Constant strain rate deformation experiments (T ≤ 100 °C; p ≤ 50 MPa) and compressional tests performed perpendicular and parallel to bedding show that the Prince Albert Formation is the strongest and most brittle followed by the Collingham Formation and then the Whitehill Formation. Triaxial compressive strength as well as static Young's vi moduli increased with increasing hard minerals and decreased with increasing mechanically weak minerals and porosity. On comparison with European and American shales, the shales in the lower Ecca Group are geomechanically stronger and more brittle, and thus may allow for fracture propagation without rock breakdown under pressure. The lower Ecca Group varies from the western to the eastern MKB in mineralogy, thermal maturity, palaeo-depositional environment and provenance. These observations directly challenge the previous assumptions that these black shales were deposited under predominantly anoxic conditions. Indeed, during deposition, the levels of anoxia appear to have been inconsistent across the MKB and were likely dependent on the changing primary bio-productivity levels, sedimentation rates and basin morphology. Neither borehole contained significant gas, likely due to degasification by dolerite intrusions in the Early Jurassic and structural deformation during syn- and post-Karoo times. In summary, this study quantitatively confirms that the lower Ecca Group has the potential to be an easily frackable, unconventional shale gas reservoir. However, this study also attests to the compositional and geomechanical complexity of these Karoo rocks, and this inevitably translates to a gas potential that is expected to vary significantly across the basin.
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    Evaluation of tridactyl theropod tracks in southern Africa: quantitative morphometric analysis across the Triassic–Jurassic boundary
    (2020) Abrahams, Miengah; Bordy, Maria Emese; Knoll, Fabien
    In the Mesozoic, dinosaur abundance and diversity steadily increased from the Carnian to the Triassic booming soon after the end Triassic Mass-Extinction event (ETE), marking a key period in archosaur history. In southern Africa, the Triassic–Jurassic Boundary (TJB) is contained in the richly fossiliferous, fluvio-lacustrine-aeolian Upper Triassic to Lower Jurassic strata of the Stormberg Group. More specifically, the middle Norian – Pliensbachian Elliot and Clarens formations (upper Stormberg Group) of main Karoo Basin, host a diverse tetrapod osteological and ichnite record. Due to an absence of high resolution radioisotopic age determinations, the exact stratigraphic placement of the TJB remains unknown. Although diverse ichnofossils attributed to Saurischians and Ornithischians are preserved in the Stormberg Group, the record is dominated by isolated tridactyl tracks that can be assigned to common ichnogenera like Grallator, Eubrontes and Kayentapus. Ideally, these track morphologies would reflect the trackmaker's autopod morphology, but complex interactions between the trackmaker and tracking substrate may affect the final footprint shape. Tracks with a high morphological preservation grade may be used to infer information (e.g., body length, hip height, weight) about the trackmaker, which is especially useful when skeletal remains are scarce, as is the case with theropod body fossils in the Elliot and Clarens formations. Herein, we present the findings of an extensive southern African field-based study to quantify the morphological variation of Grallator, Eubrontes and Kayentapus tracks across the TJB in the upper Stormberg Group. Furthermore, this study produced the first detrital zircon Uranium–Lead (U-Pb) LA-ICPMS ages of the major ichnosites from this region, and confirmed that the TJB is within the Elliot Formation, near the boundary of the lower and upper Elliot Formation (i.e., near the contact of the informal lEF and uEF). Across this contact, the considered tridactyl tracks become more abundant, larger and have a less pronounced medial digit projection. These morphological changes are gradational, with tracks from the Clarens Formation being distinct to tracks from the lEF, while the uEF tracks being intermediate between the lEF and Clarens Formation. A decrease in the mesaxony (Dp/TS ratio) and a decrease in medial digit projection relative to track length can be detected in both small and large tridactyl tracks. These apparent trends in the upper Stormberg Group are consistent with global tridactyl trends, which suggest an overall increase in theropod abundance and body size across the Jurassic. Moreover, the reason for the less prominent medial pedal digit is linked to a better weight distribution across the autopod in the increasingly larger theropods. Last but not least, Grallator, Eubrontes and Kayentapus ichnogenera which may be attributed to at least three different groups of theropod-like trackmakers, suggest a higher palaeo-diversity and abundance of tridactyl dinosaurs in southern Africa than is known from the osteological record.
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    Fluvial facies architecture and provenance history of the Abrahamskraal-Teekloof formation transition (lower Beaufort group) in the main Karoo Basin
    (2016) Paiva, Francisco; Bordy, Maria Emese
    The Middle Permian lower Beaufort Group in the southern main Karoo Basin comprises of the fluvio-lacustrine Abrahamskraal and the overlying Teekloof and correlative Middleton Formations. These units are mainly made up of mudrocks with subordinate fine-grained sandstone-rich intervals. The objective of this mainly qualitative facies analysis study is the evaluation of the vertical and lateral abundances of the facies associations in order to decouple the preserved tectonic and climatic signals in the formations. The results on the stratigraphic trends should aid paleo-environmental, paleontological, and basin analysis studies, and improve our understanding of reservoir dynamics (e.g., permeability, connectivity) when used as outcrop analogues for subsurface fluvial reservoirs. High resolution facies mapping showed that flat-topped, laterally continuous (for 10s of Kms) channel-belt complexes are the most common external architecture of the channel sandstones in all formations.
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    Palaeoenvironmental change from the Hettangian to Toarcian in Moyeni (Quthing District), southwestern Lesotho
    (2018) Haupt, T'Nielle; Bordy, Maria Emese
    The upper Karoo Supergroup of southern Africa encompasses the Late Triassic-Early Jurassic geological evolution of southern Gondwana. The environment is known to have shifted from wet swamps to a semi-arid fluvio-lacustrine and then to an aeolian system, preceding the outpouring of the Karoo continental flood basalts. The study area at Moyeni in SW Lesotho, exposes the richly fossiliferous Lower Jurassic upper Elliot and Clarens Formations (Stormberg Group) as well as the Barkly East Formation (lowermost Drakensberg Group). With the aid of integrated geological methods (e.g., field mapping, sedimentology, ichnofacies analysis), this research aims to document Early Jurassic palaeoenvironmental and tectonic changes in this region. The upper Elliot Formation comprises very fine- to fine-grained sandstone units with either massive or with ripple cross-lamination, low-angle cross-bedding as well as sandy siltstone with in-situ carbonate nodules, desiccation cracks, bone fossils and a diverse ichnofauna (e.g., algal mats, vertebrate burrows, tracks of theropods, ornithischians, amphibians, crocodylomorphs). Sedimentological evidence indicates that in the Hettangian the area was prone to flash floods and drying in a low energy depositional system with small rivers and shallow lakes. The bulk of the Clarens Formation comprises very thick, tabular, fine- to medium-grained, massive to large-scale cross-bedded arenites. However, the lower part of the Clarens also preserves thin-bedded mudstones and theropod tracks. These suggest a change in the Sinemurian from a wet to a dry desert with large, down-wind and eastward migrating sand dunes. It also appears that in spite of climatic changes these Hettangian Sinemurian ecosystems sustained a diverse biota. Within the lowermost Drakensberg Group, interbedded with lava flows and pillow lavas, massive to cross-stratified sandstone units are common. These beds typically thin and fine upward and, at least locally, terminate in surfaces that are covered by symmetrical ripple marks. These suggest that by the Toarcian the land surface was again covered by temporary lakes and streams into which the lavas outpoured during the early stages of the Karoo volcanism. The study area is structurally deformed by ~ESE-WNW trending normal faults, some of which appear to have been active already during Clarens Formation times. This syn -sedimentary faulting is interpreted as part of the growing body of evidence for the changeover from a compressional to extensional tectonic regime during the Early Jurassic and may be the first signs of Gondwana break-up.
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    Recalibrating the breakup history of SW Gondwana: the first U-Pb chronostratigraphy for the Uitenhage Group, South Africa
    (2019) Muir, Robert Anthony; Bordy, Maria Emese
    Syn-rift deposits often provide the only means to determine the processes for initiation and evolution of rift basins and passive margins. The structurally preserved erosional remnants of several rift basins that formed during the Mesozoic breakup of Gondwana are located within the southern Cape region of South Africa. These onshore basins contain the Suurberg and Uitenhage Groups, which are predominantly continental, taphrogenic, fossiliferous strata interbedded with volcaniclastics. Their significance in the Gondwanan breakup events is poorly understood due to a lack of precise and accurate radioisotopic ages. The development of SW Gondwana into the modern passive margins of southern Africa, South America and Antarctica, as well as the evolution of life recorded in the regional strata are difficult to evaluate without a high resolution chronostratigraphic framework. By integrating field observations with U-Pb geochronology of over 4000 detrital and primary volcanic zircons from pyroclastic, mixed-origin volcaniclastic and sedimentary rocks by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS), this thesis presents the first radioisotopic ages for the Uitenhage Group and provides a new chronostratigraphic framework for the onshore Jurassic – Cretaceous in the southern Cape. To further improve precision and accuracy, a selection of crystals from four pyroclastic deposits in key stratigraphic positions were selected for single-zircon Chemical Abrasion – Thermal Ionisation Mass Spectrometry (CA-TIMS) analysis to minimize the effects of Pb-loss and constrain depositional age uncertainties to < 1%. These new age constraints show that the Suurberg Group was deposited rapidly during the emplacement of the Karoo and Ferrar Large Igneous Province in the Early Jurassic (Pliensbachian) and likely predates the main phase of rifting in the southern Cape, whereas the Uitenhage Group was deposited over a prolonged (> 40 Ma) period from the Early Jurassic into the Early Cretaceous and records two phases of rifting: an initial Jurassic episode that roughly coincides with the separation of East and West Gondwna and is contemporaneous with widespread volcanism in SW Gondwana; and a subsequent period of renewed rifting during the Early Cretaceous opening of the South Atlantic. Trace element geochemical and zircon morphological assessments indicate that the volcanic source that supplied ash into the growing rift basins in the southern Cape during Gondwana breakup was situated in modern-day Patagonia and the Antarctic Peninsula, which were proximal to the southern Cape in the Jurassic and Early Cretaceous. The valuable geological framework presented in this dissertation illustrates the complexity of long-lived, rift-basin sedimentation and highlights the importance of high-resolution chronostratigraphy when investigating the tectonic, palaeontological and palaeogeographical records from the final moments of a unified Gondwana.
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    Reinterpreting vintage geophysical data from the Algoa and Gamtoos Basins, South Africa: an integrated sequence stratigraphic framework since the middle Mesozoic
    (2021) Makhubele, Marvel Hope; Bordy, Maria Emese
    Sequence stratigraphy is a branch of stratigraphy that is concerned with how genetically related geological successions are deposited in time and space. This requires the integration of diverse types of datasets (drill core, outcrop, wireline, reflection seismic surveys, etc.) to build robust depositional models, the cornerstones of stratigraphic frameworks. Although the application of sequence stratigraphy has been a successful tool to predict the lithology of geobodies in the petroleum industry, terminology is inconsistently used by the different schools of thought to define stratigraphic surfaces. This has resulted in multiple sequence stratigraphic models that interpret the same data differently. The limited exploration, to-date, and poor dataset quality have impeded the understanding of the geological evolution of the offshore Algoa and Gamtoos Basins in the southern Cape region of South Africa. To reconstruct the main geological events in the area since the late Early Jurassic, we integrated vintage borehole and seismic data as well as key outcrop observations, generated contemporary gross depositional environment models for the basin fill, and tested the applicability of different sequence stratigraphic models. The studied stratigraphic interval formed since the inception of Gondwana break-up, in syn- and post-rift systems that were increasingly dominated by marine processes, especially in the distal hanging walls. Marine incursions are detected in the Upper and Middle Jurassic in the Algoa and Gamtoos Basins, respectively. However, the severely eroded Algoa Basin syn-rift succession, exacerbated by poor data quality, makes it challenging to understand the timing of the marine incursion in this compartmentalized half-graben. Sedimentation within these half-grabens primarily occurred above the hanging walls, whilst the footwalls (i.e., basement highs) formed the dominant sediment source areas. The geological characteristics of the studied synrift succession prevents the application of the depositional sequence stratigraphic or the tectonic system tracts models. Because subaerial unconformities (SUs) in the distal syn-rift sequence are not detectable, a diachronous, northward advancement of the shoreline until the late Valanginian can be postulated. The observations in the syn-rift sequence, which is bound by a basal SU, followed by third and fourth-order transgressive and regressive cycles and a second-order maximum flooding surface at the top, can be explained with a modified genetic sequence stratigraphic model. In the transitional to drift phase interval, from Hauterivian to Holocene, the successions are bound by third-order SUs and their correlative conformities. In the successions without evidence for subaerial exposure in the drift successions, flooding surfaces are used as sequence-bounding stratigraphic contacts, validating the applicability of the genetic and transgressive-regressive sequence stratigraphic models for this upper part of the studied stratigraphic interval. This study reaffirms the notion that while the sequence stratigraphic concept is model independent, sequence models are sensitive to depositional scale and data resolution. Moreover, it also reiterates that sequence boundaries should not be limited to subaerial unconformities, but rather to correlative surfaces that bound genetically related sedimentary successions.
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    Sedimentology of the Cape Town boulder beds
    (University of Cape Town, 2024) Akkaş, Tuğçe; Bordy, Maria Emese
    Gravelly sediment layers in fossil beaches around the Cape Peninsula and False Bay in South Africa are assumed to be Pliocene in age and are essential for reconstructing the ancient sedimentary dynamics along the shoreline in the greater Cape Town region. The cobble- to boulder-size clasts in these fossil beaches point to a genesis that can be linked to the erosion of local rocky shores during hurricanes and “super storms”. This mode of formation seems similar to the Pliocene fossil beaches located at different elevations around the world (aka 'the Pliocene sea-level paradox'). Although mapped c. 100 years ago, to date, no modern sedimentological study has been conducted on Cape Town's fossil beaches. Clast characteristics (e.g., clast size, sorting, roundness, composition) of the gravelly layers had been quantified in the field and by the processing of field images using ImageJ software. Our results show that the fossil beaches are dominated by cobble-sized orthoquartzite clasts and display a variety of percussion marks. Originating locally from the Ordovician Peninsula Formation, these clast-supported, rounded clasts decrease in size from east to west, with the maximum clast diameter of >3.2m being recorded at Kogel Bay in False Bay. While this sedimentological study of Cape Town's fossil beaches elucidates the ancient marine dynamics during their genesis, linking them with other Pliocene fossil beaches requires further investigation through their age assessment. Irrespective of their age, the sedimentological properties and stratigraphic position of the fossil beaches above the modern sea level show that during sedimentation not only was the relative sea level higher by up to 30 m, but also that these deposits formed in powerful marine events that are often associated with rising global temperatures.
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    The Karoo sand sea in changing climates: Early Jurassic interdune lakes and erg dynamics in southern Africa
    (2022) Head, Howard Vincent; Bordy, Maria Emese
    Aeolian systems have a particular sensitivity to climate change, and where these are preserved in the geological record, they can provide insights into palaeo-climatic drivers of erg dynamics through time. This thesis investigates the sedimentary geology of the southern extension of a vast ancient desert system that was active over Pangaea and formed part of one of the largest known sand seas in Earth's history. The Lower Jurassic Clarens Formation in the main Karoo Basin of southern Africa has been interpreted as an aeolian succession that is dominated by massive sandstones in the lower and upper parts of the unit, whereas large-scale cross-bedded sandstones are mostly associated with its middle part. Lenticular mudstones and sandstones with ripple marks, ripple crosslamination, horizontal lamination and desiccation cracks have also been reported from the lower and upper parts of the succession. Based on the stratigraphic distribution of the sedimentary facies, the formation is informally subdivided into 3 zones that seem to reflect a wet-dry-wet climate megacycle. To date, the age of the formation is based on a very small detrital zircon dataset, the bioand chronostratigraphy of the conformably underlying Elliot Formation and the radioisotopic dates of Drakensberg Group basalts that are conformably overlying the Clarens Formation. Thus far, few investigations have targeted the evolution of this Early Jurassic fluvio-lacustrine-aeolian system, and the origin of massive sandstone facies that dominates parts of the formation. Therefore, this study re-evaluates the origin and basinal distribution of the sedimentary facies, and the spatiotemporal controls on erg development during the deposition of the Clarens Formation. Moreover, this study provides the first robust chronostratigraphic framework and provenance history for this iconic Early Jurassic aeolian system of southwestern Gondwana. The wet-dry-wet climate megacycle is not only corroborated herein, but using maximum depositional ages from detrital zircons, these climatic changes are now better dated. The results show a Late Sinemurian wet phase, an larly Pliensbachian dry phase and a late Pliensbachian wet phase during Clarens times. Newly documented field evidence shows that ponds and large lakes coexisted with aeolian dunes, with sand sheets, loess plains and fluvial systems having been established downwind. The size of the late Sinemurian ponds and lakes were controlled by their proximity to the erg margin, such that small ponds developed close to the erg centre, and large lakes were established along the erg margin. By the Early Pliensbachian, an intensification of the arid conditions led to the increased availability of sediments for aeolian entrainment, resulting in expansion of the erg eastward. Wet conditions ensued once again in the Late Pliensbachian, and lead to the contraction of the erg and expansion of loess plains with seasonally wet interdunes. Sediment, mostly supplied from the west, was overwhelmingly recycled from pre-existing, Karoo-age deposits and sedimentary rocks associated with the Pan-African orogeny, although evidence for some non-sedimentary and young, syn-Clarens sources are also present. The youngest zircon population show subtle temporal and spatial trends that can be used as proxies for the erg development. The late Sinemurian signal is identified in the lower zone in the south of the basin and is incorporated in subsequent zones in the central and northern parts of the basin, suggesting that, although the main wind direction was from the west northwest, the basin was filled from south to north. This newly established temporal framework of the wet-dry-wet climate cycle in the Clarens Formation corresponds well with the palaeoclimatic trends interpreted for the Tethyan margin of Gondwana, signifying that this may have been a global trend in the Early Jurassic.