Browsing by Author "Wiles, Errol Avern."

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Marine geology of the East London continental shelf.
(2017) Dlamini, Nontuthuzo Patricia.; Green, Andrew Noel.; Wiles, Errol Avern.; Dladla, Nonkululeko Nosipho.
This dissertation examines the marine geology of the continental shelf offshore East London, on the east coast of South Africa. High-resolution seismic, multibeam bathymetric and backscatter tools are employed to reveal the stratigraphic, geomorphic and oceanographic controls on the shelf development. Eight seismic units (A-H) are revealed and comprise Campanian-age limestones of the Igoda Formation at their base, with an overlying transgressive stratigraphic package associated with the last deglaciation. A subaerial unconformity transects the shelf and is infilled by Late-Pleistocene to Holocene-age material of Unit C. Overlying the subaerial unconformity in other places are isolated shoreface deposits of Unit B. Unit D comprises a series of aeolianites and beachrocks which form palaeo-shorelines at -100 and -60 m. They are mantled to landward by the back-barrier deposits of Unit E, and to seaward by the disaggregated barrier deposits of Unit F. Unit G comprises shoreface deposits and is interfingered with Unit H, a series of rhodoliths that mantle the modern day seafloor. Multibeam data reveal extraordinarily preserved palaeo-shorelines which are the outcrop expression of Unit B. The most seaward of these form barrier islands and associated back-barrier segmented coastal waterbodies that evolved to planform equilibria before being overstepped. These are bordered by large, well-preserved parabolic dunefields that signify planform equilibrium with high-rates of sediment supply. These shorelines formed during the Bǿlling-Allerod stillstand and were overstepped by Melt Water Pulse (MWP) 1-A. A -60 m shoreline is preserved as an isolated drumstick barrier, and a series of cuspate spits that are welded onto palaeo-embayments in Gondwana-aged bedrock. These formed during the Younger Dryas slowstand and were overstepped by MWP-1B. Underfilled incised valleys are still exposed at the seafloor along these palaeo-embayments and formed due to rapid transgression and limited marine sediment supply during the conditions associated with MWP-1B. They are currently being filled by the submerged prodeltas of the contemporary drainage systems. Backscatter data reveal eight acoustic facies (A-H). These units all show marked current sweeping of the shelf, with dredge samples revealing gravels that fill in erosional furrows, or form streamers and ribbons. The AMS C14 dating of the rhodolith fields of Unit H indicates that the vigorous Agulhas Current has continuously swept the shelf since ~7400 years BP, post MWP 1-B. This has caused the sediment starvation of most of the shelf, and has transported much of the available sediment to the deep sea via the shelf-indenting canyon systems of the area.
Submarine canyon evolution of the Southwest Cape continental margin.
(2017) Palan, Kreesan Jonah.; Green, Andrew Noel.; Wiles, Errol Avern.; Sink, Kerry.
Submarine canyons are diverse geomorphological systems that are characterised by a wide variety of geomorphic and sedimentary processes. The complexities that arise during their evolution reveal changes in the tectonic and eustatic setting that actively sculpt continental margins. Newly acquired high-resolution bathymetry reveals 15 submarine canyon systems, most of which were previously undocumented, and a large fluid seep/pockmark field off the Cape continental margin of the west coast of South Africa. These are hosted in the Orange Basin, South Africa’s largest gas producing basin. High resolution 2D seismic reflection and borehole data were used to establish a general seismic stratigraphy in which eight units are defined (Seismic Facies 1 – 8). Five key unconformity bounding surfaces are delineated (surfaces A – E) and related to major fluctuations in sea-level. Surface A marks the Albian sedimentation in the basin, B defines the Turonian – Conacian boundary and is imprinted by the first palaeo-canyons of the area, C characterises the Maastrichtian – Danian boundary and correlating with another episode of canyon formation, D marks the Palaeocene – Eocene surface, above which an assumed Oligocene canyon system was formed, and E defines the Mid-Miocene unconformity correlating to a pulse of uplift of the hinterland and further canyon incision. The modern-day canyons observed from multibeam bathymetry are suspected to have initiated in the Pliocene. The contemporary canyon morphologies vary, with many canyon features yet to be described in the literature. These morphologies are broadly classed into linear, sinuous, hooked and shelf-indenting types. Pockmarks are situated in close proximity to the sinuous, hooked and shelf-indenting canyon types and were quantified using hydrological extraction techniques to a total of 2219. These pockmarks represent the terminus of stratigraphic fluid migration from an Aptian gas reservoir, evidenced in the form of blowout pipes and brightened reflectors. Various pockmark morphologies are exhibited including circular, elongate, crescentic, composite and stringed-types. This pockmark morphological diversity is explained through localised bottom current controls which modify a point-sourced circular pockmark to establish the more complicated morphologies. The morphometric analyses of the canyons suggest contemporaneous down- and upslope eroding paradigms, that later were dominated by the influences of vertical fluid flows and gas seepage. It is proposed that fluid flow plays a key role in establishing the morphological variability of canyons along the Cape continental margin. Vertical fluid migration within the study area has the potential to mobilise sediments, evidenced by the occurrences of blowout pipes, pockmarks and neighbouring mass wasting deposits. The youngest (or most immature) canyons are considered to be the linear-types, produced by the amalgamation of intra-slope rills and with a notable absence of fluid flow features. Succeeding these are the sinuous canyons, their sinuous form dictated by the spatially irregular control of fluid flow on the sea-floor stability. The hooked canyons are defined by their arcuate heads and dense pockmark associations, suggesting further fluid flow interaction around the canyon head, producing erosion patterns associated with neither up, nor downslope mass wasting. A single shelf indenting/breaching canyon is observed. This is considered the most mature canyon system. The meandering mid-components of this canyon formed by fluid-interactions, however slumping may have been of sufficient magnitude to have extended beyond the pockmark fields, the canyon head thus gaining access to sediment flows from the upper slope. The head then subsequently retrogressed beyond the shelf-break to its present position. This thesis provides the first opportunity for a glimpse into seafloor fluid venting and escape features from the South African margin and how they affect canyon morphologies.
Tectonic history, microtopography and bottom water circulation of the Natal Valley and Mozambique Ridge, southwest Indian Ocean.
(2014) Wiles, Errol Avern.; Watkeys, Michael Keith.; Green, Andrew Noel.; Jokat, Wilfried.
This thesis focuses on aspects of the tectonic history, sediment delivery and subsequent sediment redistribution within the Natal Valley and Mozambique Basin of the southwest Indian Ocean. It aims to 1) better constrain the tectonic history of these basins based on anomalous seafloor features, 2) understand the timing, evolution and formative processes of sediment delivery systems within the Natal Valley and Mozambique Basin, 3) account for the redistribution of seafloor sediments within the southwest Indian Ocean. The southwest Indian Ocean opened during the Gondwana breakup event giving rise to two north/south orientated rectangular basins separated by the Mozambique ridge. Early research (1980’s) within these basins discussed basin features in terms of the available data at the time. By modern standards these data sets are relatively low resolution, and did not allow early researchers to fully account for the existence, development or evolution of many morphological features within the southwest Indian Ocean. This study uses recently acquired multibeam bathymetry and PARASOUND/3.5 kHz seismic data sets to describe and account for the geomorphology of the southwest Indian Ocean. Antecedent geology is discussed with respect to its development, in association with regional regimes, and role in provision of accommodation space and sediment redistribution within the study area. Sediment delivery pathways from the continental shelf to the deep marine basins are discussed, outlining the evolution of these systems under the control of antecedent geology and regional uplift. The redistribution of sediment is then discussed from the microtopography observed within the southwest Indian Ocean. Results show anomalous seafloor mounds in the northern Natal Valley, and extensional structures within the Mozambique Basin, are likely linked to the southward propagation of the East African Rift System. Dynamic current regimes and antecedent geology have played a significant role in the availability of sediment and subsequent delivery of sediment to the Natal Valley and Mozambique Basin via submarine canyons and channels. Once delivered to the basins, sediments are redistributed by deep and bottom water thermohaline Circulation. In the Natal Valley this is manifest as an atypical, current swept and winnowed, submarine fan (associated with the Tugela Canyon). While in the Mozambique Basin significant sediment wave fields reflect the influence of Thermohaline Circulation within this basin, and interaction with the seafloor. This relationship between Thermohaline Circulation and seafloor sediments has allowed existing deep and bottom water pathways to be better constrained and, in some instances, modified to better represent the actual circulation within specific regions of the study area.