Tectonic history, microtopography and bottom water circulation of the Natal Valley and Mozambique Ridge, southwest Indian Ocean.

Doctor of Philosophy in Geology. University of KwaZulu-Natal, Durban 2014.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

Shoreline variability and coastal vulnerability: Mossel Bay, South Africa

Coastal erosion may cause significant damage to property and infrastructure with far reaching socio-economic consequences. Assessing the site-specific shoreline dynamics is fundamental to understand the morphodynamic behaviour of a particular coastal area, as well as the associated coastal hazards. However, changes in shoreline position, even when significant, are not necessarily associated with increased coastal hazards. In this contribution we investigate the impact of short-term changes in shoreline position within a crenulated embayment of Mossel Bay. The 30 km-long embayment, located in the Western Cape region of South Africa, lies in a high-energy wave-dominated, micro-tidal setting. Mossel Bay is heavily populated and experiences an influx of tourists year-round. Much of the coastal community and infrastructure lies within 25–40 m of the foredune toe. Georeferenced Landsat 7/8 and Sentinel 2A scenes are used to manually digitise shoreline position in ArcMap, using the “wet/dry” line as a shoreline position proxy. The Digital Shoreline Analysis System was then used to generate shoreline change statistical metrics. Wave conditions were modelled using SWAN wave model, implemented using a nested grid approach with a high-resolution (10 m) inshore grid, and a lower resolution (50 m) offshore regional grid. The nearshore wave field during mean and storm conditions was obtained along the 15 m isobaths along the entire embayment. The embayment’s orientation in relation to the prevailing swell direction results in significant alongshore variability in nearshore wave conditions; wave heights increase towards the east along the embayment. This variability in wave forcing is reflected by the changes in shoreline position in both long and short-term, computed using the end-point rate method. However, the areas of higher shoreline change are not those experiencing the worst detrimental effects. Over the long-term, the present-day Mossel Bay embayment is relatively stable, with no significant signs of extensive accretion or erosion. However, rapid migration the shoreline is documented on a seasonal scale (short-term) with significant change proximal to river mouths, areas influenced by megacusps, and regions where the highly dynamic shoreline behaviour is constrained by rocky platforms and unable to freely adjust to variations in forcing. Thus, Mossel Bay is divided into three sub-cells in terms of coastal processes and coastal vulnerability with hazards associated with the location of such infrastructure rather than the specific patterns of shoreline change

Reversing transverse dunes: Modelling of airflow switching using 3D computational fluid dynamics

Airflow dynamics across dune surfaces are the primary agent of sediment transport and resulting dune migration movements. Using 3D computational fluid dynamic modelling, this study examined the behaviour of near surface airflow travelling over transverse (reversing) dunes on a beach system. Wind direction was modelled in two opposing directions (both perpendicular to dune crestline) to investigate surface alteration of flow on the dune topography. Surface shear stress, velocity streamlines and potential sediment flux were extracted from the modelling. The work shows that under SW winds the surface (under the configuration measured) underwent almost 10% more aeolian flux than with opposing NE winds of the same magnitude. Differences were also noted in the airflow behaviour with SW winds staying attached to the surface with less turbulence while NE winds had detached flow at dune crests with more localised turbulence. The work provides detailed insights into how 3D airflow behaviour is modified according to incident flow direction of reversing dune ridges and the resulting implications for their topographic modification. These dune types also provide interesting analogues for similarly scaled Transverse Aeolian Ridges found on Mars and the findings here provide important understanding of flow behaviour of such landforms and their potential movement

The value of multibeam bathymetry in marine spatial planning in South Africa: A review

Given a growing global population and shift to embrace the blue economy, a need for marine spatial planning (MSP) has emerged in South Africa to sustainably resolve the rising conflicts over the use of marine and seabed resources and services. A well-developed marine spatial plan yields numerous ecological, social and economic benefits. These are achieved through mediating between spatially conflicting economic drivers’ interests (e.g. commercial fishing, tourism, mining), preventing their activities from compromising thresholds of an environment’s sustainability. Within the MSP framework, high-resolution geospatial datasets are required to document and describe the seabed in the highest possible detail. At any scale, integrated analysis of seabed geomorphology and habitats is anticipated to greatly improve the understanding of ecosystem functioning from a multidisciplinary perspective, whilst improving MSP procedures and management of marine space. South Africa is the first of few African countries to have an approved and implemented MSP framework, but is still somewhat behind globally in implementing large-scale regional hydroacoustic surveys to cover the country’s vast offshore territory. The deficiency of hydroacoustic surveys is perhaps due to a relative lack of funds and poor communication about the value of multibeam echo-sounder (MBES) derived data, whilst marine geoscience remains a scarce skill in the country. This review paper presents a geological perspective of MSP and explores (1) the value that seabed mapping offers MSP specifically and (2) the need to increase seabed mapping with MBES, using a recently initiated project from the South African east coast as a case study. Significance: The collected MBES data (our case study) provides unprecedented seabed detail of the complex reef habitat and adjacent areas within specific management zones of the uThukela Banks Marine Protected Area. We reveal seabed features and their spatial distribution at a scale not possible using earlier (singlebeam) seabed mapping techniques. These high-resolution data will enable a better understanding of east coast marine habitats whilst contributing to improved spatial management of areas within and around the uThukela Banks Marine Protected Area

Zambezi continental margin: allocyclic and antecedent controls on sediment transport in the Mozambique Channel.

Sediment delivery to the abyssal regions of the oceans is an integral process in the source to sink cycle of material derived from the hinterland. How sediments are transported down-slope, and where they are deposited has implications for the mass balance of the upper lithosphere, hydrocarbon reserves, climate archives and sequence stratigraphic models. The Zambezi River, the largest in southern Africa, delivers vast amounts of material to the continental shelf, submarine Sofala/Zambesia Bank. The Sofala/Zambesia Bank acts as a staging area for this riverine input prior to its redistribution toward the abyssal plains of the Mozambique Channel. Much of this material is said to be directed into the submarine Zambezi Valley and Channel. Until this study, however, the sediment transfer routes between the Sofala/Zambesia Bank and abyssal plains of the Mozambique Channel have been quite poorly understood and remain unconstrained. The aim of this contribution is to better constrain sediment transport pathways to the abyssal plains using the latest, regional, high resolution multibeam bathymetry data available, taking into account the effects of bottom water circulation, antecedent basin morphology and sea level change. Results show that sediment transport and delivery to the abyssal plains is discreetly partitioned into southern, central and northern domains. This sediment partitioning is primarily controlled by changes in continental shelf and shelf break morphology under the influence of a dynamic anticyclonic inshore circulation system. However, changes in base level have an overarching control on sediment delivery to particular domains at various sea levels. A direct consequence of these controlling factors is limited sediment delivery to the submarine Zambezi Valley and Channel under present-day conditions, with increased activity envisaged during regression. Furthermore, the “on-off” switching of discrete domains along strike is a sequence stratigraphic concept generally not previously considered in the shelf-slope-abyssal continuum. The proposed sediment transport routes, under varied sea level scenarios, provide a framework which relates shallow to mid depth studies with those focused on the deep regions of the Mozambique Channel providing the first inclusive account of shelf to abyssal sediment transport in the region

Surface expression of microplate boundary kinematics: an isolated abyssal hill in the Mozambique Channel

In this contribution, high resolution multibeam swath bathymetry and PARASOUND sediment echosounder data are used to describe a region within the distal part of the central Mozambique Channel. The study area marks a transition from abyssal plain to abyssal hill type morphology within the sediment-rich Mozambique Fan and associated with a zone of extension in response to East African Rift System kinematics. Hosted within the abyssal hill lies an east-west orientated, elongate (80 km × 11 km) depression (relief of ca.175 m). Multibeam bathymetry and PARASOUND data show that the region surrounding the depression is variable in geomorphology including rugged irregular seafloor and sediment waves. Low gradient, smooth sea floor dominates the abyssal plain which returns several, distinct, sub-parallel sub bottom echoes. The flanks of the abyssal hill are marked by seafloor undulations likely evidence of bottom-current controlled geomorphology, and mass wasting deposits. The floor of the depression is characterised by hyperbolic echoes commonly associated with very rugged seafloor and basement outcrop with little sediment cover. The present-day geomorphology of the study area is the product of deep-seated ocean circulation and soft sediment deformation superimposed upon the antecedent geological framework, influenced by present-day kinematics of the East African Rift System. Faulting associated with these kinematics is manifest at the seafloor as the elongate steep-flanked depression; the result of an extensional regime expressed across the Mozambique channel from south-southwest to north-northeast. This contribution highlights the local, marine, ramification of a continental-scale largely terrestrial tectonic regime

Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel

Sediment delivery to the abyssal regions of the oceans is an integral process in the source to sink cycle of material derived from adjacent continents and islands. The Zambezi River, the largest in southern Africa, delivers vast amounts of material to the inner continental shelf of central Mozambique. The aim of this contribution is to better constrain sediment transport pathways to the abyssal plains using the latest, regional, high-resolution multibeam bathymetry data available, taking into account the effects of bottom water circulation, antecedent basin morphology and sea-level change. Results show that sediment transport and delivery to the abyssal plains is partitioned into three distinct domains; southern, central and northern. Sediment partitioning is primarily controlled by changes in continental shelf and shelf-break morphology under the influence of a clockwise rotating shelf circulation system. However, changes in sealevel have an overarching control on sediment delivery to particular domains. During highstand conditions, such as today, limited sediment delivery to the submarine Zambezi Valley and Channel is proposed, with increased sediment delivery to the deepwater basin being envisaged during regression and lowstand conditions. However, there is a pronounced along-strike variation in sediment transport during the sea-level cycle due to changes in the width, depth and orientation of the shelf. This combination of features outlines a sequence stratigraphic concept not generally considered in the strike-aligned shelf-slope-abyssal continuum

The Zambezi Channel: a new perspective on submarine channel evolution at low latitudes

The submarine Zambezi Channel is the deep, stable, north-south orientated, lower portion of a channel system draining the continental slope of central Mozambique; transporting material southwards into the Mozambique Channel and Basin, southwest Indian Ocean. Using recently collected Multi Beam Echo Sounder and PARASOUND data we discuss the geomorphology of the Zambezi Channel. This system is enigmatic in that the main channel is stable, with low sinuosity despite being at a low latitude where rivers seasonally deliver fine grained sediment. A further enigma is that system does not now continue upslope to the Zambezi River, the largest river in southern Africa. Instead this river flows into the northern Mozambique basin to the south-west of the small channels. The Zambezi Channel is compared to small-scale physical models in an attempt to better understand the geomorphology of the channel. The geomorphological features of the main channel show a quite remarkable resemblance to an analogue model produced within a purely erosive environment. To explain these enigmas, it is proposed that geomorphology of the main Zambezi Channel was produced by periodic, high-volume pulses of flood water, and associated sediment, from the Zambezi River, the second largest river in Africa. These events are considered to be due to minor tectonic movements along the Chobe Fault in the Kalahari that permitted the draining of several palaeo-lake systems between the Early Pleistocene through to the early Mid-Pleistocene. Such repetitive draining of palaeo-lakes would have produced flooding comparable to glacial dam bursts. Such events would deliver significantly more sediment laden flood water to the region than “normal” flow conditions. We hypothesise that these significant flood events have influenced the geomorphology of the Zambezi River to the extent that it is no longer comparable to other low-latitude systems, and exhibits characteristics akin to high-latitude systems with highly variable sediment input