The aim of this dissertation was to understand lithogenic suspended sediment transport mechanisms and distribution in two river-influenced margins: The Cariaco Basin, Venezuela, and the Amazon Shelf, Brazil. Lithogenic sediment input in the Cariaco Basin is controlled by small mountainous rivers (SMR), while in the Amazon Shelf it is dominated by the Amazon River, the largest river in the world in terms of freshwater discharge (~20% of global riverine discharge). Optical transmissometer measurements were coupled with particulate organic matter (POM) observations to understand changes in the geochemical composition of suspended sediment and spatial/temporal distributions over the two regions of interest. In the Cariaco Basin sampling was conducted during the rainy seasons of September 2003, 2006 and 2008, and during the upwelling period (dry season) of 2009. Our results suggest that bottom nepheloid layers (BNL) originating at the mouth of the SMR discharging into the Cariaco Basin are a major delivery mechanism of terrigenous sediments to the basin's interior year-round. Intermediate nepheloid layers (INL) were also observed near the shelf break (~100m) and appear to effectively carry terrigenous material laterally from the shelf to deep waters, thereby providing a plausible supply mechanism of the terrestrial material observed in sediment traps, deployed >70 km offshore as part of the CARIACO Ocean Time-Series. These findings highlight the importance of small, local rivers in the Cariaco Basin as sources of terrestrial material. Indeed, the low isotopic composition of particulate organic carbon (δ13Corg, ~-30 - -24 ‰) carried by the BNL suggests that this material was continentally derived. BNL δ13Corg also changed with season, indicating that the geochemical composition of BNL reflects particle source. These nepheloid layers contained relatively low POM concentrations (average of 10%), agreeing well with published data, yet the fine sediment of the BNL may serve as mineral ballast, enhancing the sinking velocities of POC and thus increasing the efficiency of the biological pump in Cariaco. We suggest that during the transition between the upwelling and rainy season BNL deliver sediment to the deep Cariaco Basin in pulses. During upwelling, BNL are retained on the inner shelf by onshore Ekman transport associated with upwelling. The nepheloid layers are later released as the upwelling subsides; this, coupled with high river discharge rates, may explain the seasonal pulse of sediment observed at the end of the upwelling period (May) in the sediment trap array. The SMR in Cariaco also have the capacity to deliver large amounts of sediment to the Cariaco Basin during episodic events, such as earthquakes and floods. During September 2008 a sediment density flow was observed in the eastern Cariaco Basin, likely triggered by a magnitude 5.2 earthquake that occurred on August 11, 2008 off the city of Cumaná. Elevated suspended sediments near the bottom were observed at the mouth of the Manzanares Canyon (> 90 g m-2, over a depth of 165 m) and decreased to ~11 g m-2 (over a depth of 40 m) 42 Km away from the canyon's mouth at the CARIACO Ocean Time-Series site (10.5° N, 64.67° W). The sediment flux associated with this single event was ~ 10% of the total annual sediment flux that typically reaches the Cariaco Basin deep seafloor. Average carbon to nitrogen atomic ratios (C/N) as well as C and N isotopic composition confirm that most of the organic matter transferred by the sediment flow was of continental origin (C/N ratios of ~19.3, δ13C of -27.04 ‰, and δ15N of 6.83 ‰). The Manzanares River mouth is located at the head of the canyon, and likely supplies most of the fine grained sediments and fresh organic carbon that accumulate in the upper part of the canyon. This suggests that the canyon is an active depositional center, and its proximity to the Manzanares River and Cariaco Basin is critical for sediment supply offshore, which in turn can have a significant impact on the long-term sequestration of carbon into the deep basin. The nutrient and sediment biogeochemistry of the outer Amazon Shelf was studied in February-March 2010 to replicate observations made by the AmasSeds study in 1989-1991. These transects roughly corresponded to the AmasSeds Open Shelf (OS) and River Mouth (RM) transects. Onshore winds (~6 m s-1) contained the Amazon plume within ~120 Km of the coast; the plume was visible only in the mid-shelf stations located closest to the coast in the OS transect. Within the river plume, surface dissolved inorganic nutrient concentrations were near zero, except for silicates (4-6 μM). Coupled with oxygen supersaturation (AOU < 1), this suggested complete biological uptake of the major dissolved inorganic nutrients (N, P). Dissolved organic carbon (DOC) was also highest within the plume (average of 116 μM), decreasing to ~73 μM in oceanic waters. Total suspended solids (TSS) in surface waters within the plume were ~1-1.5 mg l-1, decreasing to ~0.2-0.3 mg l-1 in all other sampled stations both over the shelf and in deeper waters. TSS were highest within BNL (22-33 mg l-1) observed over the inner shelf; BNL were not observed outside the area of the Amazon plume. Suspended particulate organic carbon (POCsusp) showed a depleted δ13C isotopic signal (~-25 ‰ to -28 ‰) in surface and bottom waters, suggesting terrestrial provenance. Within the BNL, %POC was low (0.6-0.9%, as compared to 7-18% in surface waters), showing extensive and rapid decomposition of organic matter over the shelf. Atomic C/N ratios in particulate organic matter both in surface waters and within BNL were relatively close to Redfield's (8-14) and relatively stable over the area sampled. Particulate atomic organic carbon vs. particulate organic phosphorous (POC/POP) ratios were also low within the BNL (~110) and increased offshore (>500), suggesting a direct input of particulate P from the Amazon River or from reworked surface sediments. The fraction of POC in surface sediments was also low (0.73 ±; 0.56%; N = 5) and relatively uniform across the region sampled. We estimated instantaneous fluxes of 38.7 metric tons TSS s-1, 0.24 metric tons POC s-1 and 6.42 x 10-3 metric tons POP s-1 northwestward over an area extending between ~50 Km and 120 Km offshore. Our TSS estimates are 30% lower than those calculated by Nittrouer et al. (1986) during peak discharge of the Amazon. We also calculated that some 1.50 Tg yr-1 of DOC were being flushed northwestward along the outer shelf annually, which represent ~6% of the total DOC transported by the Amazon. By analyzing these two geographical settings it was possible to compare and contrast transport mechanisms of continentally-derived material and establish the relative importance of each mechanism in their different environment. There is still much to be understood regarding BNL in the Cariaco Basin, such as their role within the Manzanares Submarine Canyon with regards to sediment contribution and deposition. Additionally, during the last 30 years, anthropogenic influences on the small rivers around the Basin have significantly altered the drainage and sediment loads, yet reliable data to quantify the level of influence and change over time are not available. We need a better understanding of the natural variability of these small, tropical fluvial systems, trends and impact of episodic events, to better interpret the climate record stored at the bottom of the basin and predict future ecosystem changes in the region. In the Amazon Shelf, more accurate estimates of DOC, POC and POP fluxes northwestward are warranted. The magnitude of the Amazon River discharge dampens changes that have occurred in the last 20 years within the Amazon Basin, suggesting that historic Amazon Shelf sediment and carbon estimates are still valid. The data presented here adds to the growing body of literature that highlights the significance of river-influenced continental margins as sites of organic carbon deposition, remineralization export and sequestration.

This research investigates different scenarios of deltaic deposition, both in shelfbreak and ramp settings. I address four ancient cases with particular characteristics: 1) A shelf-margin case from the Eocene Battfjellet Formation, West Spitsbergen, Norway, in which deltas were able to migrate to the shelf-edge during rising and sea-level highstand conditions despite the low-supply character of the system (low progradation/aggradation rates compared to analogous margins), with consequent sand starvation on the slope and deeper areas of the basin. The delta system was overall wave-dominated, with restricted tide-influence at the mouth of the distributaries and more accentuated tide-influence during the transgressive transit of the deltas; 2) A shelf-margin case from the Pliocene paleo-Orinoco Delta System, Mayaro Formation, SE-Trinidad, in which high rates of sediment supply from the paleo-Orinoco River and exceptionally high subsidence rates due to growth-faulting, produced a spectacular stacking of sandstones on the outer shelf and shelf-edge areas, but with apparently limited sand delivery into deeper waters. The delta system was overall storm-wave dominated, with fluvial-influence in the lower segment of the system and some tide-influence in association with the fluvial-influence; 3) A case from a shallow-water ramp, Campanian Rock Springs Formation (Western Interior Seaway), in which deltas accumulated along relatively straight, north-south oriented shorelines highly impacted by wave-storm processes. Tide-influence was limited to the mouth of the distributaries, and fluvial deposits mostly developed within the coastal-plain areas; and 4) A case from the same ramp setting as (3) but in an outer-ramp site, Campanian Haystack Mountains Formation, in which a lowering in sea-level translated the delta system tens of kilometers eastwards into the basin. As a consequence of a shallower and narrower seaway, southerly-oriented tidal currents were enhanced and subsequently skewed or re-aligned the delta system to the south. The key contributions of this research concern (1) the feasibility of shelf-margin accretion during rising and highstand of sea level, (2) the critical importance of shelf width and sediment supply (and not only sea-level behavior) to bring deltas to the shelfedge, and (3) the possible tendency for tides enhancement in the distal reaches of shallow seaway ramps, caused by narrowing of the seaway and fault-topography enhancement during falling sea level. / text

Arc-continent collisions are believed to be an important mechanism for the growth of continents. Taiwan is one of the modern day examples of this process, and as such, it is an ideal natural laboratories to investigate the uncertain behavior of continental crust during collision. The obliquity of collision between the northern South China Sea (SCS) rifted margin and Luzon arc in the Manila trench subduction zone allows for glimpses into different temporal stages of collision at different spatial locations, from the mature mountain-belt in central-northern Taiwan to the 'pre-collision' rifted margin and subduction zone south of Taiwan. Recently acquired seismic reflection and wide-angle seismic refraction data document the crustal-scale structure of the mountain belt through these different stages. These data reveal a wide rifted margin near Taiwan with half-graben rift basins along the continental shelf and a broad distal margin consisting of highly-extended continental crust modified by post-rift magmatism. Magmatic features in the distal margin include sills in the post-rift sediments, intruded crust, and a high-velocity lower crustal layer that likely represents mafic magmatism. Post-rift magmatism may have been induced by thermal erosion of lithospheric mantle following breakup and the onset of seafloor spreading. Geophysical profiles across the early-stage collision offshore southern Taiwan show evidence the thin crust of the distal margin is subducting at the Manila trench and structurally underplating the growing orogenic wedge ahead of the encroaching continental shelf. Subduction of the distal margin may induce a pre-collision flexural response along the continental shelf as suggested by a recently active major rift fault and a geodynamic model of collision. The weak rift faults may be inverted during the subsequent collision with the continental shelf. These findings support a multi-phase collision model where the early growth of the mountain belt is driven in part by underplating of the accretionary prism by crustal blocks from the distal margin. The wedge is subsequently uplift and deformed during a collision with the continental shelf that involves both thin-skinned and thick-skinned structural styles. This model highlights the importance of rifting styles on mountain-building. / text

This research investigates the character and significance of shelf-edge deltas within the sedimentary source-to-sink system, and how variability at the shelf edge leads to different styles of deep-water deposition. Because the shelf-edge represents one of the key entry points for terrigenous sediment to be delivered into the deep water, understanding of the sedimentary processes in operation at these locations, and the character of sediment transported through these deltas is critical to understanding of deep-water sedimentary systems. The research was carried out using three datasets: an outcrop dataset of 6000 m of measured sections from the Permian-Triassic Karoo Basin, South Africa, a 3D seismic data volume from the Eocene Northern Santos Basin, offshore Brazil and a dataset of 29 previously published descriptions of shelf-edge deltas from a variety of locations and data types. The data presented highlight the importance of sediment instability in the progradation of basin margins, and deep-water transport of sediment. The strata of the Karoo Basin shelf margin represent river-dominated delta deposits that become more deformed as the shelf-edge position is approached. At the shelf edge, basinward dipping, offlapping packages of soft-sediment-deformed and undeformed strata record repetitive collapse and re-establishment of shelf-edge mouth bar packages. The offlapping strata of the Karoo outcrops record progradation of the shelf margin through accretion of the shelf-edge delta, for over 1 km before subsequent transgression. The Eocene Northern Santos Basin shelf margin, in contrast, exhibits instability features which remove kilometers-wide wedges of the outer shelf that are transported to the basin floor to be deposited as mass-transport packages. In this example, shelf-edge progradation is achieved through „stable. accretion of mixed turbidites and contourites. The data also emphasize the importance of the role of shelf-edge delta processes in the delivery of sediment to the basin floor. A global dataset of 29 examples of shelf-edge systems strongly indicates that river domination of the shelf-edge system (as read from cores, well logs or isopach maps) serves as a more reliable predictor of deep-water sediment delivery and deposition than relative sea level fall as traditionally read in shelf-edge trajectories or sequence boundaries. / text

Patterns of variability and the dynamics of the ocean circulation on the West Florida Shelf (WFS) are investigated using multi-year, shelf-wide oceanographic observations from moored Acoustic Doppler Current Profiler (ADCP) arrays,hydrographic cruises, High-Frequency (HF) radars, satellites, and coastal tide gauges.Novel neural network techniques, Self-Organizing Map (SOM) and Growing Hierarchical Self-Organizing Maps (GHSOM), are introduced as feature extraction methods in physical oceanography. The SOM is demystified and demonstrated to be a useful feature extraction method in a series of performance evaluations using artificial data sets comprising known patterns. It is then applied to velocity time series from moored ADCP arrays and to a joint HF-radar and ADCP data set, respectively, to extract patterns of ocean current variability, and it is shown to be a useful technique for extracting dynamically consistent ocean current patterns. The extracted characteristic patte rns of upwelling/downwelling variability are coherent with the local winds on the synoptic weather time scale, and coherent with both the local winds and thecomplementary Sea Surface Temperature (SST) patterns on the seasonal time scale. Thecurrents are predominantly southeastward during fall-winter and northwestward during summer. The GHSOM is used to describe the SST seasonal variation. As feature extraction methods, both the SOM and the GHSOM have advantages over the conventional Empirical Orthogonal Function method.The circulation dynamics are examined, first through depth-averaged momentum balances at selected locations and then via sea surface height (SSH) estimates across the inner shelf. Dominant dynamics of the shelf circulation are diagnosed and a method is discussed for estimating along-shelf currents from coastal sea level and wind data. Nontidal coastal sea level fluctuations are related to both the offshore SSH and the dynamical responses of the inner shelf to wind and bu oyancy forcing. The across-shelf distribution of the SSH is estimated from the velocity, hydrography, wind, and coastal sea level data.Subtracting the variability that may be accounted for by inner shelf dynamical responses yields a residual at the 50 m isobath that compares well with satellite altimetry data. This suggests the possibility of calibrating satellite SSH data on the continental shelf.

Fish sound production has been associated with courtship and spawning behavior. Acoustic recordings of fish sounds can be used to identify distribution and behavior. Passive acoustic monitoring (PAM) can record large amounts of acoustic data in a specific area for days to years. These data can be collected in remote locations under potentially unsafe seas throughout a 24-hour period providing datasets unattainable using observer-based methods. However, the instruments must withstand the caustic ocean environment and be retrieved to obtain the recorded data. This can prove difficult due to the risk of PAMs being lost, stolen or damaged, especially in highly active areas. In addition, point-source sound recordings are only one aspect of fish biogeography. Passive acoustic platforms that produce low self-generated noise, have high retrieval rates, and are equipped with a suite of environmental sensors are needed to relate patterns in fish sound production to concurrently collected oceanographic conditions on large, synoptic scales. The association of sound with reproduction further invokes the need for such non-invasive, near-real time datasets that can be used to enhance current management methods limited by survey bias, inaccurate fisher reports, and extensive delays between fisheries data collection and population assessment. Red grouper (Epinephelus morio) exhibit the distinctive behavior of digging holes and producing a unique sound during courtship. These behaviors can be used to identify red grouper distribution and potential spawning habitat over large spatial scales. The goal of this research was to provide a greater understanding of the temporal and spatial distribution of red grouper sound production and holes on the central West Florida Shelf (WFS) using active sonar and passive acoustic recorders. The technology demonstrated here establishes the necessary methods to map shelf-scale fish sound production. The results of this work could aid resource managers in determining critical spawning times and areas. Over 403,000 acoustic recordings were made across an approximately 39,000 km2 area on the WFS during periods throughout 2008 to 2011 using stationary passive acoustic recorders and hydrophone-integrated gliders. A custom MySQL database with a portal to MATLAB was developed to catalogue and process the large acoustic dataset stored on a server. Analyses of these data determined the daily, seasonal and spatial patterns of red grouper as well as toadfish and several unconfirmed fish species termed: 100 Hz Pulsing, 6 kHz Sound, 300 Hz FM Harmonic, and 365 Hz Harmonic. Red grouper sound production was correlated to sunrise and sunset, and was primarily recorded in water 15 to 93 m deep, with increased calling within known hard bottom areas and in Steamboat Lumps Marine Reserve. Analyses of high-resolution multibeam bathymetry collected in a portion of the reserve in 2006 and 2009 allowed detailed documentation and characterization of holes excavated by red grouper. Comparisons of the spatially overlapping datasets suggested holes are constructed and maintained over time, and provided evidence towards an increase in spawning habitat usage. High rates of sound production recorded from stationary recorders and a glider deployment were correlated to high hole density in Steamboat Lumps. This research demonstrates the utility of coupling passive acoustic data with high-resolution bathymetric data to verify the occupation of suspected male territory (holes) and to provide a more complete understanding of effective spawning habitat. Annual peaks in calling (July and August, and November and December) did not correspond to spawning peaks (March - May); however, passive acoustic monitoring was established as an effective tool to identify areas of potential spawning activity by recording the presence of red grouper. Sounds produced by other species of fish were recorded in the passive acoustic dataset. The distribution of toadfish calls suggests two species (Opsanus beta and O. pardus) were recorded; the latter had not been previously described. The call characteristics and spatial distribution of the four unknown fish-related sounds can be used to help confirm the sources. Long-term PAM studies that provide systematic monitoring can be a valuable assessment tool for all soniferous species. Glider technology, due to a high rate of successful retrieval and low self-generated noise, was proven to be a reliable and relatively inexpensive method to collect fisheries acoustic data in the field. The implementation of regular deployments of hydrophone-integrated gliders and fixed location passive acoustic monitoring stations is suggested to enhance fisheries management.

No description available.

The relationship between spawning stock biomass (SSB) and 3 spatial distribution metrics (SDMs), measuring range, concentration, and density, using fisheries independent survey data for 10 demersal Northwest Atlantic fish populations (9 species), show metrics of density offer the best correlate of SSB. The concave, positive relationship between high density area (HDA) and SSB indicates that a decline in HDAs beyond a spatial threshold is associated with disproportionately large SSB declines in 6 populations. HDAs might indicate highly productive areas and/or positive fitness consequences, enhancing the ability of individuals to successfully spawn, locate prey, and evade predators. HDAs can help to assess the status of a population’s spatial structure and serve as a spatial reference point. By comparing spatial reference point locations relative to existing biomass reference points (based on MSY), scenarios are described wherein spatial reference points contribute to biomass reference points and to a precautionary approach to fisheries management.

Turbulent dynamics at two sites (C and D) in a hypoxic zone on the Texas- Louisiana continental shelf were studied by investigating turbulence quantities i.e. turbulence kinetic energy (TKE), dissipation rate of TKE (E), Reynolds stress (τ ), dissipation rate of temperature variance (χ), eddy diffusivity of temperature (ν't), and eddy diffusivity of density (ν'p). Numerical models were also applied to test their capability of simulating these turbulence quantities. At site D, TKE, E, and τ were calculated from velocity measurements in the bot- tom boundary layer (BBL), using the Kolmogorov’s -5/3 law in the inertial subrange of energy spectra of vertical velocity fluctuations in each burst measurement. Four second-moment turbulence closure models were applied for turbulence simulations, and modeled turbulence quantities were found to be consistent with those observed. It was found from inter-model comparisons that models with the stability functions of Schumann and Gerz predicted higher values of turbulence quantities than those of Cheng in the mid layer, which might be due to that the former stability functions are not sensitive to buoyancy. At site C, χ, E, v’t, and ν’p were calculated from profile measurements throughout the water column, and showed high turbulence level in the surface boundary layer and BBL, as well as in the mid layer where shear stress was induced by advected non-local water above a hypoxic layer. The relatively high dissolved oxygen in the non-local water resulted in upward and downward turbulent oxygen fluxes, and the bottom hypoxia will deform due to turbulence in 7.11 days. Two of the four models in the study at site D were implemented, and results showed that turbulence energy resulting from the non-local water was not well reproduced. We attribute this to the lack of high-resolution velocity measurements for simulations. Model results agreed with observations only for χ and E simulated from the model with the stability function of Cheng in the BBL. Discrepancies between model and observational results lead to the following conclusions: 1) the stability functions of Schumann and Gerz are too simple to represent the turbulent dynamics in stratified mid layers; 2) detailed velocity profiles measurements are required for models to accurately predict turbulence quantities. Missing such observations would result in underestimation,

This geostrophic current-controlled Zululand/Natal shelf displays a unique assemblage of interesting physical, sedimentological and biological phenomena. The shelf in this area is extremely narrow compared to the global average of 75km, and is characterised by submarine canyons, coral reefs, and steep gradients on the continental slope. A shelf break occurs 2.1km to 4.1km offshore and the shelf can be divided into a northern region and a southern region based on the presence or absence of a defined shelf break. The southern shelf has a poorly-defined shelf break whilst the northern shelf has a well-defined break at -65m. The poor definition of the shelf break on the southern shelf can possibly be attributed to the presence of giant, climbing sand dunes offshore of Jesser Point at depths of -37m to -60m. The northern shelf has a series of coast-parallel oriented patch coral reefs which have colonised carbonate-cemented, coastal-facies sequences. The northern shelf can be divided into three distinct zones: inner-, mid-, and outer-shelf zones. The inner-shelf is defined as the area landward of the general coral reef trend, with depths varying from 0m to -I5m and having an average gradient of 1.1. The mid-shelf is defined by the general coral reef trend, varying from -9m over the shallow central axis of the reefs to -35m along the deep reef-front environments. The outer-shelf is seaward of the coral reefs and occurs at a depth range of -35m to - 65m. Gradients vary from 1° in the south to 2.5° in the northern part of the study area, and are steep compared to world average shelf gradient of 0.116°. Four submarine canyons occur in the study area and are classified as mature- or youthful-phase canyons depending on the degree to which they breach the shelf. The origin of these canyons is not related to the position of modern river mouths but can probably be linked to palaeo-outlets of the Pongola and Mkuze River systems. It is suggested that the canyons are mass-wasting features which were exploited by palaeo-drainage during regressions. The youthful-phase canyons appear to be mass-wasting features associated with an unstable, rapidly-deposited, progradational late Pliocene sequence and a steep upper continental slope. The mature-phase canyons were probably initiated by mass-wasting but have advanced shoreward, breaching the shelf, due to their link with the palaeo-outlets of the Pongola and Mkuze Rivers during late Pleistocene regressions. Evidence of modem canyon growth has been noted on numerous SCUBA diving surveys carried out on the canyon heads. These take the form of minor wall slumps and small-scale debris flows. The canyons are also supplied with large quantities of sand in the form of large-scale shelf subaqueous dunes generated and transported by the Agulhas Current. As these bedforms meet the canyons the sediment cascades down the canyon thalweg and causes erosion and downcutting of the canyon walls and floor thereby increasing the canyon dimensions. Late Pleistocene beachrock and aeolianite outcrops with or without an Indo-Pacific coral reef veneer are the dominant consolidated lithology on the shelf. These submerged, coast-parallel, carbonate cemented, coastal facies extend semi-continuously from -5m to -95m, and delineate late Pleistocene palaeocoastline events. The rock fabric of these high primary porosity lithologies shows grains floating in a carbonate cement with occasional point-contacts. Grains are mostly quartz (80-90%), minor K-feldspar and plagioclase (5-10%), and various lithic fragments. The rocks contain conspicuous organic grains including foraminifera, bivalve, echinoid, bryozoan, red algal, and occasional sponge spicule fragments; these commonly display replacement fabrics or iron-stained rims. The dominant sedimentary structures found in these sandstone outcrops include high-angle planar cross-bedding and primary depositional dip bedding. Palaeocurrent directions sngest a palaeoenvironment dominated by a combination of longitudinal and transverse dunes with wind directions similar to those observed forming the modem dune systems. Erosional features evident on the submerged beachrocks and aeolianites include gullies trending in two different directions and sea-level planation surfaces with or without the presence of potholes. The unconsolidated sediment on the shelf is either shelf sand, composed mainly of terrigenous quartz grains; or bioclastic sediment which is partially derived from biogenic sources. The quartzose sand from the inner-shelf is generally fine-grained, moderately- to well-sorted, and coarsely- to near symmetrically-skewed. Carbonate content is low, and varies between 4-13%. Quartzose sand from the outer-shelf is fine-grained, moderately- to well-sorted, and coarsely- to very coarsely-skewed. The inner-shelf quartzose sand is better sorted than the outer-shelf sand due to increased reworking of this sediment by the high-energy swell regime. Sediment from the shallower areas of the outer-shelf (< -50m) is better sorted than sediment from depths of greater than -50m. Generally wave-reworking of quartzose shelf sand from the Sodwana Bay shelf results in greater sediment maturity than that observed from geostrophic current effects or a combination of geostrophic and wave-reworking. This sediment was derived by reworking of aeolian and beach sediments, deposited on the shelf during the period leading up to the Last Glacial Maximum (15 000 - 18 000 years B.P.) when sea-level was -130m, during the Holocene (Flandrian) transgression. Bioclastic sediment on the Sodwana Bay shelf is defined as having a CaC03 content of greater than 20% and is a mixture of biogeoically-derived debris and quartzose sand. The distribution of bioclastic sediment in the study area is widespread, with reef-derived and outer-shelf-derived populations being evident. This sediment consists of skeletal detritus originating from the mechanical and biological destruction of carbonate-secreting organisms such as molluscs, foraminifera, alcyonaria, scleractinia, cirripedia, echinodermata, bryozoa, porifera. The reef-derived bioclastic population is confined to depths less than -40m in close proximity to reef areas, whereas the shelf-derived bioclastic population occurs at depths greater than -40m and is derived from carbonate-producing organisms on deep water reefs and soft-substrate environments on the shelf. Large-scale subaqueous dunes form in the unconsolidated sediment on the outer-shelf due to the Agulhas flow acting as a sediment conveyor. These dunes are a common feature on the Sodwana Bay shelf occurring as two distinct fields at depths of -35m to -70m, the major sediment transport direction being towards the south. The two dune fields, the inner- and outer subaqueous dune fields, are physically divided by Late Pleistocene beachrock and aeolianites ledges. A bedform hierarchy has been recognised. The larger, outer dune field appears to have originated as a system of climbing bedforms with three generations of bedforms being superimposed to form a giant bedform, while the inner dune field has a less complex construction. The largest bedforms are those of the outer dune field off Jesser Point, being up to 12 m high, 4 km long and 1.2 km wide. A major slip face, with a slope of 8° is present. Bedload parting zones exist where the bedform migration direction changes from south to north. Three bedload parting zones occur in the study area at depths of -60m, -47m and -45m; two in the inner dune field and one in the outer dune field. These zones are invariably located at the southern limits of large clockwise eddy systems. Such eddies appear to be the result of topographically induced vorticity changes in the geostrophic flow and/or the response to atmospheric forcing caused by coastal low-pressure system moving up the coastline. It has been demonstrated that the inner subaqueous dune sediment conveyor is not active all the time but only during periods . of increased current strength when the Agulhas Current meanders inshore. The smaller bedforms in the outer dune field undergo continuous transport due to the current velocity on the shelf edge outer dune field being higher than the velocity experienced on the inner dune field. The very large 2·D dune which forms the outer dune field is probably not active at present: this is inferred due to the shallow angle of the mega-crest lee slope (8°). The very large Sodwana Bay subaqueous dune fields may be compared with the very large, reconstructed, subaqueous dunes which occur in Lower Permian sediments of the Vryheid Formation, northern Natal. These Permian dunes are represented, in section, as a fine- to medium-grained distal facies sandstone with giant crossbeds. These large-scale bedforms are unidirectional, but rare directionally-reversed, climbing bedforms do occur, this directional reversal may be related to bedload parting zones. On the evidence presented in this thesis, it is proposed that these Permian subaqueous dunes may be ancient analogues of the modem subaqueous dune field on the Sodwana Bay shelf. Positive-relief hummocks and negative-relief swale structures are fairly common in the fine-grained, quartzose shelf sand at depths of -30m to -60m. These appear to be transitional bedforms related to the reworking by storms of medium 2-D subaqueous dunes. These hummocky structures may be the modem equivalent of hummocky cross-stratification noted in the geological record, and if so, they are probably the first to have ever been observed underwater. The occurrences of ladderback ripples on the Sodwana Bay shelf at depths of -4m to -17m, suggest that subtidal ladderback ripples may be more common than previously thought. Ladderback ripples are common features of tidal flats and beaches where they form by late-stage emergence run-off during the ebb tide. They are generally considered diagnostic of clastic intertidal environments. The mode of formation on the Sodwana Bay shelf is different from the classic late-stage emergence run-off model of intertidal occurrences, being a subtidal setting. Subaqueous observations indicate that ladderback ripples are not environment-specific, and that additional evidence of emergence is therefore necessary to support an intertidal setting in the rock record: ladderback ripples alone are insufficient to prove an intertidal environment. The coral patch reefs of the northern Natal coast are unique, being the most southerly reefs in Africa, and totally unspoilt. The Zululand reefs are formed by a thin veneer of Indo-Pacific type corals which have colonised submerged, late Pleistocene beachrocks and aeolianites. Two-Mile Reef at Sodwana Bay has been used to develop a physiograpbic and biological zoning model for Zululand coral reefs, which has been applied to other reefs in the region. Eight distinct zones can be recognised and differentiated on the basis of physiographic and biological characteristics. The reef fauna is dominated by an abundance of alcyonarian (soft) corals, which constitute 60-70% of the total coral fauna. The Two-Mile Reef zoning model has been successfully applied to larger reefs such as Red Sands Reef, and smaller patch reefs (Four-Mile and Seven-Mile Reefs) in the same general area. In this thesis extensive use has been made of Hutton's uniformitarian principles. Hutton's doctrine is particularly relevant to the study of depositional processes and relict shorelines. Coastal processes and weather patterns during the late Pleistocene were broadly similar to modem conditions enabling direct comparisons to be made. A computer-aided facies analysis model has been developed based on textural statistics and compositional features of carbonate-cemented coastal sandstones. Many attempts have been made to distinguish different ancient sedimentary depositional environments, most workers in this field having little success. The new method of facies reconstruction is based on: (1) underwater observations of sedimentary structures and general reef morphology; (2) a petrographic study of the reef-base enabling flve facies: aeolianite, backbeach, forebeach, swash, and welded bar facies to be recognised, which control the geomorphology of Two-Mile Reef; (3) cluster and discriminant analysis comparing graphic settling statistics of acid-leached reef-base samples with those of modem unconsolidated dune/beach environments. The results of this analysis demonstrated that the beachrocks and aeolianites on the shelf formed during a regression and that late Pleistocene coastal facies are similar to modem northern Zululand coastal environments, which have been differentiated into aeolian, backbeach, forebeach, swash, & welded bar. A late Pleistocene and Holocene history of the shelf shows that during the late Pleistocene, post Eemian regressions resulted in deposition and cementation of coast-parallel beachrocks and aeolianites, which define a series of four distinct palaeocoastline episodes with possible ages between 117 000 and 22 000 years B.P. The beachrock/aeolianites formed on the shelf during stillstands and slow regressions, and the gaps between these strandline episodes represent periods of accelerated sealevel regression or a minor transgressive phase which hindered deposition and cementation. The formation of these lithologies generated a considerable sediment sink in the nearshore zone. This reduced sediment supply and grain transport in the littoral zone during the Holocene, and probably enhanced landward movement of the shoreline during the Flandrian transgression. Prior to the Last Glacial Maximum, the beachrock/aeolianite sedimentary sequence was emergent and blanketed by shifting aeolian sands. The Pongola River, which flowed into Lake Sibaya, reworked the unconsolidated sediments on the shelf, and exploited the route of least resistance: along White Sands and Wright Canyon axes. The erosion resulting from fluvial denudation in Wright Canyon has caused this canyon to erode some of the beachrock/aeolianite outcrops which form palaeocoastline episode 2 and entrench the canyon to a deeper level; this eroded the shelf to a distance of 2km offshore. During the Flandrian transgression the unconsolidated sediment cover was eroded, exposing and submerging the beachrock/aeolianite sequence. Flandrian stillstands caused erosional features such as wave-planed terraces, potholes, and gullies to be incised into beachrock and aeolianite outcrops; these are seen at present depths of -47m, -32m, .26m, -22m, -17m to -15m, and -12m. High energy sediment transfers, in an onshore direction, resulted in the deposition of sand bars across the outlet of Lake Slbaya's estuary and the development of a 130m + coastal dune barrier on a pre-existlng, remnant Plelstocene dune stub. Sea-level stabilised at its present level 7 000-6 000 years B.P. and coral reef growth on the beachrock/aeolianite outcrops probably started at 5 000 years B.P. A minimum age for the formation of the northern Zululand coral reefs has been established at 3780 ± 60 years B.P. A mid Holocene transgression relating to the Climatic Optimum deposited a + 2m raised beach rock sequence. This transgression eroded the coastal dune barrier and caused a landward shoreline translation of approximately 40m. A minor transgression such as this can be used as a model for coastal erosion which will result from the predicted 1.5m rise in sea-level over the next century. This rise in sea-level could result in a 30m landward coastline translation of the present coastline, ignoring the influence that storms and cyclones will have on the coastline configuration. / Thesis (Ph.D.)-University of Natal, Durban, 1991.