Browsing by Author "Green, Andrew Noel."

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Flooding in KwaZulu-Natal : modelling, history and future aspects.
(2014) Botes, Zacheus Adriaan.; Green, Andrew Noel.; Smith, Alan.
The current state of flood modelling relies on statistical techniques revolving around either river or rainfall data that are used to produce an estimated flood return period (e.g. 1:100 year). These tend to ignore 1) the geological record as an archive of flood events; 2) the spatial distribution of flood producing weather systems; and 3) climatic cycles which may ultimately control episodes of flooding. This thesis developed a Flood Zone Model (FZM) model from existing Geographic Information Systems (GIS) datasets and GIS software models at a quaternary catchment (4th order basin) level. Model discharge estimates were derived from modified Regional Maximum Flood (QRMF) equations where it was found that flood elevations produced from QRMF estimated discharges could be directly related to the geological record of flood. This was achieved using Manning-derived calibration factors (CFs) based on reach slope. Comparison of the modelled flood elevation surfaces against the field data and available 1:100 year return period elevations showed R2 coefficients of 0.999 for all calibration factors. In one of the quaternary catchments investigated, geological evidence and discussion with local communities identified flood elevations attributed to flash flooding. On this basis, the Flood Zone Model was adapted to estimate peak discharges using the Rational Formula where it was found that the calibration factors were valid for flash flood modelling and that the flood elevations that resulted from flash flooding far exceeded the 1:100 year return period. To evaluate the spatial distribution of flood producing storms, daily rainfall data from KwaZulu-Natal (1890 - 2000) were gridded to produce regional storm event footprints. Storm events typically last between three to four days with the highest associated risk period from January to May. Flooding appears to be mostly influenced by migrating easterly waves. Compilation of all the storm event footprints defined five risk zones with a sixth zone at risk from tropical storms and cyclones. Comparisons between the annual regional storm event count and several climatic cycles show a significant correlation between the Pacific Decadal Oscillation (PDO) and regional storm event as a result of increased easterly wave activity. Assuming no change to the Pacific Decadal Oscillation cycle, increased periods of intense flooding will occur in the future.
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.
The marine geology of the Northern KwaZulu-Natal continental shelf, South Africa.
(2009) Green, Andrew Noel.; Uken, Ronald.
This study proposes that the submarine canyons of the northern Kwazulu-Natal continental margin formed contemporaneously with hinterland uplift, rapid sediment supply and shelf margin progradation during the forced regression of upper Miocene times. These forced regressive systems tract deposits volumetrically dominate the shelf sediments, and comprise part of an incompletely preserved sequence, amongst which six other partially preserved sequences occur. The oldest unit of the shelf corresponds to forced regression systems tract deposits of Late Cretaceous age (seismic unit A), into which a prominent erosive surface, recognized as a sequence boundary, has incised. Fossil submarine canyons are formed within this surface, and underlie at least one large shelf-indenting canyon in the upper continental slope. Smaller shelf indenting canyons exhibit similar morphological arrangements. Late Pliocene deposits are separated from Late Cretaceous lowstand deposits (seismic unit B) by thin veneers of Late Palaeocene (seismic unit C) and mid to early Miocene (seismic unit D) transgressive systems tract deposits. These are often removed by erosive hiatuses of early Oligocene and early to mid Pliocene age. These typically form a combined hiatus surface, except in isolated pockets ofthe upper slope where late Miocene forced regressive systems tract units are preserved (anomalous progradational seismic unit). These sediments correspond to the regional outbuilding of the bordering Tukhela and Limpopo cones during relative sea level fall. Either dominant late Pliocene sediments (seismic unit E), or transgressive systems tract sediments which formed prior to the mid Pliocene hiatus, overlie these sediments. Widespread growth faulting, slump structures and prograding clinoforms towards canyon axes indicate that these sediments initiated upper slope failure which served to create proto-canyon rills from which these canyons could evolve. The association of buried fossil canyons with modern day canyons suggests that the rilling and canyon inception process were influenced by palaeotopographic inheritance, where partially infilled fossil canyons captured downslope eroding flow from an unstable upper slope. Where no underlying canyons occur, modern canyons evolved from a downslope to upslope eroding system as they widened and steepened relative to the surrounding slope. Statistical quantification of canyon forms shows a dominance of upslope erosion. Landslide geomorphology and morphometric analysis indicate that this occurred after downslope erosion, where the canyon axis was catastrophically cleared and incised, leading to headward retreat and lateral excavation of the canyon form. Trigger mechanisms for canyon growth and inception point to an overburdening ofthe upper slope causing failure, though processes such as freshwater sapping may emulate this pattern of erosion. It appears that in one instance, Leven Canyon, freshwater exchange with the neighbouring coastal waterbodies has caused canyon growth. The canyons evolved rapidly to their present day forms, and have been subject to increasingly sediment starved conditions, thus limiting their evolution to true shelf breaching canyon systems. Sedimentological and geomorphological studies show that the shelf has had minor fluvial influences, with only limited shelf-drainage interaction having occurred. This is shown by isolated incised valleys of both Late Cretaceous and Late PleistocenelHolocene age. These show classic transgressive valley fills of wave dominated estuaries, indicating that the wave climate was similar to that of today. The narrowness of the shelf and the inheritance of antecedent topography may have been a factor in increasing the preservation potential of these fills. Canyons thus appear to have been "headless" since their inception, apart from Leven Canyon, which had a connection to the Last Glacial Maximum (LGM) St Lucia estuary, and Wright Canyon, which had an ephemeral, shallow LGM channel linking it to the Lake Sibaya estuarine complex. Coastline morphology has been dominated by zeta bays since at least 84 000 BP, thus littoral drift has been limited in the study area since these times. The formation of beachrock and aeolianite sinks during regression from the last interstadial has further reduced sediment supply to the shelf. The prevalence of sea-level notching in canyon heads, associated with sea levels of the LGM indicates that canyon growth via slumping has been limited since that time. Where these are obscured by slumping in the canyon heads (Diepgat Canyon), these slumps have been caused by recent seismic activity. The quiescence of these canyons has resulted in the preservation of the steep upper continental slope as canyon erosion has been insufficient to plane the upper slope to a uniform linear gradient such as that of the heavily incised New Jersey continental margin. Progressive sediment starvation of the area during the Flandrian transgression has resulted in a small shore attached wedge of unconsolidated sediment (seismic unit H) being preserved. This is underlain by a mid-Holocene ravinement surface. This crops out on the outer shelf as a semi-indurated, bioclastic pavement. Thinly mantling this surface are Holocene sediments which have been reworked by the Agulhas Current into bedforms corresponding to the flow regime and sediment availability to the area. Bedforms are in a state of dis-equilibrium with the contemporary hydrodynamic conditions, and are presently being re-ordered. It appears that sediment is not being entrained into the canyons to the extent that active thalweg downcutting is occurring. Off-slope sediment loss occurs only in localized areas, supported by the dominance of finer grained Early Pleistocene sediments of the outer slope. A sand ridge from the mid shelf between Wright and White Sands Canyons appears to have been a palaeo-sediment source to White Sands Canyon, but is currently being reworked southwards towards Wright Canyon. The prevalence of bedform fields south of regularly spaced canyon heads is considered a function of hydrodynamic forcing of the Agulhas Current by canyon topography. These bedforms are orientated in a northerly direction into the canyon heads, a result ofnortherly return eddying at the heads of these canyons.
Nearshore morphological changes and their relation to wave-induced forcing at Isipingo embayment, KwaZulu-Natal.
(2017) Shanganlall, Arissa.; Green, Andrew Noel.; Loureiro, Carlos Manuel.
The nearshore zone is one of the most active sedimentary environments on the continental shelf, frequently impacted by energetic wave conditions and storm–generated waves and flows. Despite its proximity to the beach and shoreline, the nearshore is a difficult domain to study and little is understood with regards to its response to storm waves and the agents responsible for the morphological evolution of the region. Previous research has provided insight into the nearshore, however high-resolution approaches to gain a three dimensional understanding of these environments are still lacking. This is especially true in geologically constrained environments as coastal embayments. Advances in the acquisition and analysis of high- resolution multibeam bathymetry of the nearshore, coupled with wave modelling techniques, can provide an improved understanding of coastal response to high energetic wave conditions and the subsequent morphological changes that result from the impact of storm events. Detailed nearshore multibeam bathymetric surveying conducted at the Isipingo embayment before and after the 2017 winter season provided a framework to analyse the seasonal morphological changes that were driven by winter storms along the KwaZulu-Natal coastline. High-resolution bathymetric grids were implemented in the hydrodynamic model SWAN (Simulating Waves Nearshore) to simulate the nearshore wave field of Isipingo embayment for a variety of wave conditions. Morphological changes were evaluated in conjunction with wave-induced forcing to determine the potential for sediment mobilisation. Spectral wave modelling results of the wave field and bed shear stresses agree with the observed morphological changes. Significant erosion and deposition occurred in the shallower regions (5 m to 14 m) of the study area and along the northeast and southwest sections of the embayment. Modelling results presented in this study indicate that the spatial variation and distribution of orbital bottom velocities and bed shear stresses are strongly dependent upon the bathymetric configuration of large-scale bedforms and the magnitude of the nearshore wave field. Consequently, the greater the energy of the wave conditions (i.e. as a result of major storm events) the greater the wave-induced forcing, which causes the change of the bedforms and the morphological evolution of the nearshore. Large-scale features such as shoreface-connected ridges, rippled scour depressions and large subaqueous dunes tend to increase the significant wave height and the bed shear stresses acting on the seabed. Thus, waves are focussed towards the NE headland resulting in an extensive zone of erosion in the embayment. The main geological constraints at the embayed region of Isipingo are imposed by the NE and SW headlands and the shoreface-connected ridges. Such constraints drive the development of topographically controlled rips against the NE headland, set up by the prevalent south-north longshore current, and the shadowing effect of the SW headland also contributes to the observed erosional (NE region) and accretion (SW region) patterns on the nearshore. The hydrodynamic forcing of rippled scour depressions and shoreface connected ridges control the persistence of the non-stationary rip currents of the NE section of the embayment. As a consequence, the absence of rip activity at the centre of the embayment creates a relatively stable region. Additionally, the study reveals that the patterns of morphological change in the nearshore mimic those of the apparent rotation of the beach, explained by the breathing mode described from other embayments. Model simulations of extreme storm events depict that the morphological change is driven by an energetic wave field, and the wave induced orbital motions and bed shear stresses have a strong bathymetric imprint. This study demonstrates the usefulness of high-resolution bathymetry in understanding nearshore morphological change. In providing a complete three dimensional image of changes at incredibly high-resolutions, and with high (2-3 month) temporal resolution, an increased level of understanding of the nearshore changes seaward of bars can be garnered. This includes rip-dynamics, erosion patterns, geological control on wave-forcing and seasonal accretion/erosion monitoring. These are areas which have not yet received attention in this level of detail and in which it is demonstrated that good results can be obtained.
Sediment dynamics of the Amatikulu Estuary, Central KwaZulu-Natal Coast, South Africa.
(2010) Le Vieux, Alain.; Green, Andrew Noel.; Uken, Ronald.
The seismic stratigraphy, geological evolution and CO2 storage potential of the offshore Durban Basin, South Africa.
(2017) Hicks, Nigel.; Green, Andrew Noel.
Abstract available in PDF file.
Seismic, geochemical and sedimentological characteristics of storm deposits from the Durban continental shelf, South Africa.
(2015) Dixon, Shannon Louise.; Green, Andrew Noel.
The Durban shelf is a wave-dominated, high energy setting, characterised by submerged shorelines at depths of ~60 m, backed to landward by low-relief backbarrier depressions. The exposure at the seafloor of a back-barrier/lagoon complex, coupled with the general high wave energy of the shelf makes for a unique opportunity to examine records of marine storminess, preserved as tempestites in the shelf stratigraphy. This thesis examined a variety of ultra-high resolution seismic data, coupled with multibeam bathymetry, core and sedimentological data. High resolution XRF, XRD and other provenance proxies were also examined in order to reveal cycles of storminess that have impacted the lower shoreface offshore Durban. These are integrated with a rigorous geochronological framework. The seismic sections revealed two distinct packages which comprise the unconsolidated shoreface (Unit A and B). These are further subdivided into an upper five packages (Sub-unit B1-B5 and sub-unit A-Ai). Cores that penetrated this sediment enabled a further correlation with that of the bounding surfaces, sediment compositions and the nature of individual sub units. Unit A and Ai are considered incised valley fills corresponding to organic-rich fine sand of the central basin and flood tide delta deposits with a distinctly higher terrigenous sediment signature in comparison to the overlying sediment packages. The tidal ravinement surface (tRS) is restricted to the incised valley where it separates unit A and Ai. The wave ravinement surface (WRS) truncates the incised valley fills and is overlain by unconsolidated material of unit B. Sub-units B3 and B4 are storm associated deposits which are of particular interest to this study. Sub-unit B3 comprises a number of high energy deposits, namely mudballs; these deposits consist of organic rich material indicative of storm winnowing of an exposed muddy backbarrier (such as presently occurring along sectors of the adjoining coastal plain). This is corroborated by the geochemical analyses of the mudballs which displays significantly higher concentrations of terrigenous elements (Si, Al, K, Ti and Rb), in comparison to the surrounding sediment, indicative of a terrigenous sediment origin. The centre of the mudball was dated at 9 850 ± 50 cal yrs BP. The outer veneer dates to 3 835 ± 35 cal yrs BP and represents the final phase of deposition in the lower shoreface. The mudballs are encased in coarse sediment, dominated by Ca and Sr elemental concentrations, suggestive of a marine origin. Sub-unit B4 consists of alternating horizons of storm generated gravel horizons displaying increased marine elemental signatures, interbedded with finer sediment with increased terrigenous concentrations indicative of fair-weather conditions. It was found that horizons of coarser material had higher elemental signatures of Ca and Sr indicating a predominantly marine input into the system. These horizons are intercalated with finer material with distinctly higher concentrations of elements associated with terrigenous source material that represents fair-weather suspension settling of terrestrial materials. Based on modelling of the largest experienced contemporary marine storm (Hs = 8.5 m), it is clear that storm waves do not significantly rework gravelly sediment on the lower shoreface, especially in the areas of smooth seafloor where the cores are situated. Bathymetry of the area shows no contemporary evidence for storm scour or gravel deposition. As the palaeo-tempestites date to a time when sea level occupied a similar position to that of today, it is logical to assume these represent much larger storm events than are commonly experienced. This study shows for the first time a period of increased storminess in the Indian Ocean between 6 480 ± 40 cal yr BP to 4 595 ± 35 cal yr BP; a time linked to a strongly positive Indian Ocean Dipole (IOD) anomaly and increased sea surface temperatures (SST). With further global warming, it appears that Durban may be more vulnerable to large marine storms as the associated changes in atmospheric circulation patterns and oceanic currents promote the formation of positive IOD phenomena.
Seismic-stratigraphic models for late Pleistocene/Holocene incised valley systems on the Durban continental shelf.
(2013) Dladla, Nonkululeko Nosipho.; Green, Andrew Noel.
This dissertation examines the Durban continental shelf of the east coast of South Africa from a seismic and sequence stratigraphic perspective. High resolution seismic data reveal eleven seismic Units (A-K) offshore the Durban continental shelf comprising several partially preserved sequences. Unit A is the lower most unit, comprising Permian age shale of the Pietermaritzburg Formation. An early Santonian age is assigned to Unit B. The ages of Units C and D are indeterminate. Unit E is considered late Maastrichtian in age. Units F to I are assigned a late Pliocene age and represent an aggradational progradational shelf-edge wedge. Unit J comprises calcite cemented late Pleistocene/Holocene shoreline deposits which display morphologies similar to planform equilibrium shorelines on modern coasts. Unit K caps the stratigraphy and comprises a seaward thinning, shore-attached wedge of Holocene age. The lower portions of Unit K comprise the fills of an extensive LGM age incised valley network. A widespread network of incised valley systems on the continental shelf offshore Durban was recognised and examined; the evolution of which were compared over time. These incised valleys represent the shelf extension of the main river systems in the area, namely, the Mgeni, Mhlanga and Mdloti rivers as well as those that drain into the Durban Harbour complex. In the study area late Pleistocene/Holocene aged valleys occur together with a subsidiary series of late Pliocene isolated valleys. Valleys of the last glacial maximum (LGM) of ~ 18 Ka BP exhibit simple fills and have intersected and reworked or completely exhumed the late Pliocene incised valleys. Only isolated examples of these late underlying Pliocene valleys are apparent. Twenty five prominent incised valleys are recognised within the study area and occur predominantly in the mid-outer shelf. These valleys mainly incise into Cretaceous age rock, except for a few incisions occurring within Permian age shale of the Pietermaritzburg Formation. Six seismic units (Units 1-6) comprise the infill material within the late Pleistocene/Holocene incised valleys, and on the basis of their architecture are interpreted to correspond with a succession from high energy basal fluvial deposits, low-energy central basin fines, mixed-energy estuarine mouth plug deposits, clay-rich flood deposits through to capping sandy shoreface deposits. The LGM aged fills in particular have volumetrically thick fluvial deposits, the result of increased gradient and stream competence during the LGM. The youngest valleys show a situation of differential evolution along the valley length due to varying rates of sea level rise in the Holocene. Initially, rapid sea level rise caused drowning and overstepping of the outer segment of the incised valley. During the late Holocene, slower rates of sea level rise caused shoreface ravinement of the inner-mid segments of the valley and created an imbalance between accommodation space and sediment supply, producing different facies architectures in the valleys. This differential exposure to accommodation has resulted in a sedimentological partitioning between tide-dominated facies in the outer valley segment and river dominated facies in the inner segment. Due to significantly wider exposed coastal plain during lowstand intervals, the rivers in the study area avulsed and coalesced on this lowstand surface and thus possess no defined drainage patterns. A crenulate shaped subsurface knickpoint occurs at a depth of ~ 50 m, and is considered to have formed by initial slow ravinement processes that graded the antecedent shelf, followed by overstepping and preservation of the knickpoint during meltwater pulse 1B.
Structure and evolution of the Zinkwazi and Umdloti barrier spit and inlet systems along the KwaZulu-Natal coastline, South Africa.
(2017) Pillay, Talicia.; Green, Andrew Noel.
Abstract available in PDF file.
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.
Submerged shoreline sequences on the KwaZulu-Natal shelf : a comparison between two morphological settings.
(2013) Salzmann, Leslee.; Green, Andrew Noel.
Holocene shoreline sequences and associated shelf stratigraphy are described from a high gradient, high wave energy shelf offshore the central KwaZulu-Natal and northern KwaZulu-Natal coastlines. These are examined using high resolution single-channel seismic and multibeam bathymetric means in order to describe the shallow stratigraphy and seafloor geomorphology of each area. The development and preservation of two distinct planform shorelines at -100 m (northern KwaZulu-Natal) and -60 m (northern KwaZulu-Natal and central KwaZulu-Natal) is described. The shallow seismic stratigraphy of northern KwaZulu-Natal comprises three seismic units (Units 1-3) corresponding to calcarenite barriers (Unit 1), back barrier lagoonal sediments (Unit 2) and the contemporary highstand sediment wedge (Unit 3). At intervening depths between each shoreline the shelf is characterised by erosional surfaces that reflect ravinement processes during periods of slowly rising sea level. Where shorelines are not preserved, areas of scarping in the ravinement surface at depths coincident to adjoining shorelines are apparent. These areas represent rocky headlands that separated the sandy coastal compartments where the shorelines formed and are a function of the high gradient. In central KwaZulu-Natal where the shelf is notably wider and gentler, shoreline building was more intense. Five major seismic units are identified (Units 1-5) with several subsidiary facies. The formation of the -60 m barrier complex (Unit 2) in central KwaZulu-Natal was accompanied by the simultaneous formation of a back-barrier system comprising lake-lagoon depressions (Unit 3) and parabolic dune fields aligned to the local aeolian transport direction, formed on a widened coastal plain. On the seaward margins of the barrier, gully and shore platform features developed coevally with the barrier system. Several relict weathering features (Unit 4) are associated with the barrier and reflect similar processes observed in contemporary aeolianite/beachrock outcrops on the adjacent coastline. The two submerged shoreline sequences observed are attributed to century to millennial scale periods of stasis during which shoreline equilibrium forms developed and early diagenesis of beachrock and aeolianite occurred. These extensive phases of shoreline development are thought to have occurred during periods of stillstand or slowstand associated with the Bølling-Allerod Interstadial (~14.5 ka BP) and the Younger Dryas Cold Period (~12.7-11.6 Ka BP). Shoreline preservation in such an environment is considered unlikely as a result of intense ravinement during shoreline translation, coupled with the high energy setting of the KwaZulu-Natal shelf. Preservation of both the 100 m and 60 m shorelines occurred via overstepping where preservation was promoted by particularly rapid bouts of relative sea-level rise associated with meltwater pulses 1A and 1B (MWP-1A and -1B). This was aided by early cementation of the shoreline forms during stillstand. Differences in shelf setting have led to variations in the style of barrier preservation and associated transgressive stratigraphies between the central KwaZulu-Natal and northern KwaZulu-Natal shelves. The main differences include a much thicker post-transgressive sediment drape, higher degrees of transgressive ravinement and an overall simplified transgressive system’s tract (TST) architecture on the steeper and narrower continental shelf of northern KwaZulu-Natal. In comparison, the central KwaZulu-Natal shelf’s 60 m shoreline complex reflects more complicated equilibrium shoreline facets, large compound dune fields formed in the hinterland of the shoreline complex, higher degrees of preservation and a more complicated transgressive stratigraphy.
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.