Shelf Morphology Research Articles

Geomorphological features along the shelf of the southern Brazilian margin: Implications for shallow-water sediment transport induced by ocean currents

Chirp profiles collected on the southern Brazilian shelf were combined with sedimentological and hydrodynamic data to characterize shelf geomorphology, focusing on the development of outer shelf bedforms and their genetic links with major oceanic currents. Outer shelf sediment accumulations are interpreted as two scales of bedforms, such as dunes and sand ridges. In the São Paulo Bight, cuspate bedforms are bounded landward by a geomorphological scarp. The sector between Florianopolis and Mostardas also exhibits frequent asymmetric bedforms, whereas the inner- middle shelf is covered by semitransparent sheets. The southern Rio Grande Cone is characterized by stratified deposits covering vast outer shelf areas. Surficial sediment samples reveal a dominantly muddy composition and relatively high carbonate contents. Hydrodynamic data indicate a major influence of the Brazil Current over the São Paulo Bight. The prolific bedform outer-shelf environment is assumed to be mostly relict formed in a transgressive shallow-water setting. The sand ridges are believed to have evolved from a moribund stage characterized by morphological degradation to a relict stage, with preservation enhanced by fine-grained winnowing, coarse sediment armoring, and early cementation. Bedform migration seems to be occurring on the São Paulo Bight due to the presence of a nearly permanent meander of the Brazil Current. Over most of the inner shelf, the northward advection of sediments mainly led by the Brazilian Coastal Current has formed extensive fine-grained deposits. The occurrence of a relict shelf scarp may have conditioned the formation of the outer-shelf bedform environment. The vast spatial distribution of bedforms over most of the outer shelf seems comparable with other shelf areas, such as the south African shelf, dominated by the influence of unidirectional oceanic currents. Dune dimensions are lower on the Brazilian shelf, due to their relict character, the geomorphological configuration of the margin, and the scarcity of sand sources.

The geomorphology of Ireland’s continental shelf

ABSTRACT The Irish Shelf Seabed Geomorphological Map (ISSGM) (v2023) presented here, is the first high-resolution geomorphological map of the entire Irish continental shelf. This large-scale mapping exercise took advantage of the vast INFOMAR multibeam echosounder dataset, and used a protocol of semi-automated mapping techniques to accurately and rapidly extract seabed features. All previous mapping efforts and existing literature on the Irish shallow shelf geomorphology have also been collated and integrated in the map, critically evaluating the previous interpretations. An internationally standardised classification scheme was adopted, aligning the ISSGM (v2023) to other international geomorphological work. At a national level, this detailed geomorphological digital map is intended primarily as a resource to better inform multiple offshore activities and management of the marine environment. The map also acts as a baseline for future studies in marine geomorphology, as it identifies gaps in the knowledge and highlights areas of contentious interpretation that require further work. The map is available online on the Irish Marine Atlas (https://atlas.marine.ie – Geology Theme).

Multivariate examination of the sediment-deficient southeast Australian continental shelf

Continental shelves are the most morphologically variable element within the source-to-sink system owing to the numerous processes that influence their formation. A recent multivariate analysis of a global compilation of modern continental shelf data showed that much of the variability is related to tectonic setting, the degree to which the shelf has been glaciated, and carbonate production. While these factors play first-order roles in determining the morphology of shelves, other controlling mechanisms such as siliciclastic sediment supply, wave and tidal energy, bedrock lithology, and sea-level fluctuations are not as well understood. Here, we report findings from a detailed investigation of the southeast Australian shelf that explored how sediment distribution, wave energy, and bedrock lithology influence shelf morphology. The high-resolution analysis suggests that the southeast Australian shelf has 11 distinct shelf types. No strong relationships exist between the shelf attributes or shelf type with their onshore catchments. However, a substantial section boundary correlates with a bedrock contact between the Sydney Basin in the south and the New England Orogen to the north. South of this boundary, we propose that the shelf morphology reflects transgression with low sediment supply, whereas to the north, the morphology reflects transgression with higher sediment input. Although several factors contributed to this difference in shelf morphology, we suggest that sediment distribution and retention due to the active wave climate during the most recent transgression likely played a vital role.

Comparison of mandibular buccal shelf morphology between adolescents and adults with different vertical patterns using CBCT.

This retrospective study aimed to analyze the anatomical structure of the mandibular buccal shelf (MBS) in adolescents and adults with different vertical patterns to determine the optimal location for miniscrew insertion in orthodontic treatment. Cone-beam computed tomography (CBCT) scans of 230 patients were utilized for measurements. The morphology and thickness of alveolar bone at the MBS were measured. Two-way ANOVA and regression analysis were conducted to analyze the influencing factors on alveolar bone and cortical bone thickness. Age had a significant effect on alveolar bone thickness (level I: F = 62.449, level II: F = 18.86, p < 0.001), cortical bone thickness (level II: F = 18.86, p < 0.001), alveolar bone tilt (F = 6.267, p = 0.013), and second molar tilt (F = 6.693, p = 0.01). Different vertical patterns also influenced alveolar bone thickness (level I: F = 20.950, level II: F = 28.470, p < 0.001), cortical bone thickness (level I: F = 23.911, level II: F = 23.370, p < 0.001), and alveolar bone tilt (F = 27.046, p < 0.001). As age increased, the alveolar bone thickness at level I decreased by 0.096mm and at level II decreased by 0.073mm. Conversely, the thickness of alveolar bone at level I and level II increased by 0.06mm and 0.075mm, respectively. The cortical bone thickness at level I and level II increased by 0.024mm and 0.29mm, respectively. However, the alveolar bone thickness decreased by 0.931mm and 1.545mm at level I and level II, and the cortical bone thickness decreased by 0.542mm and 0.640mm at level I and level II, respectively. Age, different vertical patterns, alveolar bone inclination, and different shapes of MBS significantly affected the thickness of alveolar bone and cortical bone in the MBS area. Notably, only alveolar bone thickness and cortical bone thickness at level II were affected by age and different vertical patterns simultaneously. These findings can provide valuable insights for orthodontic practitioners in selecting the most suitable location for miniscrew insertion during treatment planning.

Depositional history of the subaqueous delta off the southeastern coast of Korea during the post-glacial period

The distal mud deposit (DMD) of the Korea Strait Shelf, located along the southeastern coast of Korea and detached from the main sediment source, the mouth of the Nakdong River, is considered a subaqueous delta system. The complex spatiotemporal changes in the stratigraphic architecture of the DMD were reconstructed, based on combined seismic and sediment core data, according to the three stages of post-glacial sea-level rise. During the early transgressive stage (from ca. 19 to 10.9 ka), transgression occurred at a relatively deeper northern outer shelf of the DMD, where Unit A began to be deposited in an aggradational stratal stacking pattern. During the late transgressive stage (ca. 10.9 to 8 ka), further sea-level rise provided sufficient accommodation for DMD, but an insufficient sediment supply from the Nakdong River might have prevented sedimentation. At the highstand stage (ca. 8 ka to the present), the increased supply of suspended sediments from the Nakdong River coupled with the entrance of the Tsushima Warm Current through the Korea Strait may have enabled the along-shore lobate progradation of Unit B on the flat and wide southern shelf. After appoximately 2 ka, as Unit B proceeded into the narrow, steep northern shelf, it started to prograde in a shore-normal direction, controlled by a rapid downslope decrease in near-bed shear stress. Over the last 700 years, sediments have been actively deposited at the depocenter of the DMD by frequent storm surges during the summer season. The reconstructed depositional history of the DMD demonstrates the importance of the pre-existing shelf morphology, along-shore currents, and storm surges in offshore sediment transport for a subaqueous delta in a microtidal shelf with a relatively small riverine sediment load.

Increasing Wave Energy Moves Arctic Continental Shelves Toward a New Future

AbstractArctic continental shelves, including the Alaskan Beaufort Shelf (ABS), are experiencing declines in sea ice coverage leading to increasingly energetic sea states and coastal erosion. In this study we investigated the morphologic response of the ABS to increasing wave energy, and how shelf profile adjustments modify wave energy propagating toward the coast. We developed a 2D cross‐shelf morphodynamic model using Delft3D and tested shelf response to a present‐day wave climate and a future Arctic wave climate projected under the RCP8.5 climate‐change scenario. Simulations lasting 1000 years were conducted for relatively steep (Flaxman Island, AK, slope 0.0008) and flat (Harrison Bay, AK, slope 0.0003) cross‐shelf profiles. We found that morphologic evolution and regulation of future waves depends primarily on existing shelf morphology. On the steeper profile, RCP 8.5 waves drove sediment erosion at 0–15 m water depth and redeposition at 15–30 m water depth. Over 1000 years, this redistribution of sediment from the inner to middle shelf resulted in a 7.6% reduction in wave heights at the 2 m isobath. This morphologic adjustment represented a regulatory feedback in which shallowing of the middle shelf led to attenuation of waves reaching the inner shelf. In contrast, effective wave attenuation across the flatter and wider Harrison Bay section limited cross‐shelf transport and morphologic change under both wave climates. Together our results suggest that coastal changes in response to the growing Arctic wave climate may be dependent on shelf morphology, and even mitigated in some regions by morphologic adjustment.

Character of advance and retreat of the southwest sector of the British-Irish Ice Sheet during the last glaciation

Relict landforms and sediments across former glaciated settings provide information about ice-sheet dynamics and can contribute to the understanding of the behaviour of contemporary ice masses, for which observations are limited in spatial and temporal extent. In this study, we focus on the shelf offshore southwest Ireland, in the Celtic Sea, which was once occupied by the Irish Sea Ice Stream (ISIS), the largest ice stream draining the southern portion of the marine-terminating British-Irish Ice Sheet (BIIS). Newly acquired high-resolution multibeam echosounder, sub-bottom and core data enabled the investigation of the shelf geomorphology and of the sedimentology and chronology of glacial and glacimarine sediments. A suite of drumlins records ice sheet flow from the coastline towards the central part of the shelf in southwest Ireland. Pre-existing highs in the seafloor topography promoted the formation of arcuate and transverse landforms interpreted as a grounding-zone wedge and moraines and they document episodic retreat of the ISIS across this portion of the shelf. Observed lithofacies show consolidated subglacial till and laminated fine muds. The sediments provide evidence of ice grounded ca. 30 km off the south-west Irish coastline with subsequent deglaciation occurring under glacimarine conditions. These new data refine the current reconstructions of the dynamics of the southern BIIS. They reveal for the first time the interplay of marine- and land-based ice and the presence of grounded ice offshore SW Ireland. This study highlights the importance of high-resolution data in revealing palaeo-landscapes as valuable analogues to test possible scenarios of modern ice sheet changes.

Sedimentological and morphological evidences of Meltwater Pulse 1B in the Southwestern Atlantic Margin

During the post-LGM deglaciation, global temperatures and rates of sea-level rise were influenced by centennial events driven by climate and ice volume variations, leading to cooling/warming climate stages, sea-level slowstands and meltwater pulses (MWP). Sea level fluctuations have shaped the shelf morphology by drowning coastal features and imposing shifts in coastal/shelf sedimentary environments worldwide. The influence of meltwater pulses in the South Atlantic is still not well established. Thus, here, we investigate morphological and sedimentological records of MWP-1B along two distinct areas of the Eastern Brazilian shelf (Espirito Santo and Abrolhos shelf), far-field sites in the western South Atlantic. Sedimentary core analysis, C14 dating, and multibeam data are combined to provide stratigraphic and morphological evidences of sea-level rise acceleration at the end of Younger Dryas, in the South Atlantic. Results of core analysis indicate a coastal environment around 12 kyrs BP and a major shift in sedimentary facies after 11.5 kyrs BP. In both cores, Unit 1 comprises terrigenous (low carbonate content) and mixed facies, and are interpreted as terrigenous/coastal/estuarine facies. Unit 2 is characterized by carbonate/mixed facies, interpreted as open marine deposits. Terrigenous and mixed estuarine/coastal facies (Unit 1) are replaced by open marine facies (unit 2) at 57.3 m in the Abrolhos Depression core (ABDR) and at 47.6 m in the Espirito Santo paleovalley shelf (PVS core). Morphological evidence shows a paleoshoreline at 60 m water depth associated with five drowned and partially filled incised shelf valleys. A depth x age analysis indicates that the sedimentary environment shift and the depth of paleoshorelines are associated with sea-level slowstand during Younger Dryas, followed by a sea-level rise acceleration, representing MWP-1B. These evidences in the western South Atlantic are combined with other morphological and stratigraphic results presented for South Hemisphere far field sites, indicating that the 60 m isobath is an average water depth marking sea-level stabilization during Younger Dryas, and that MWP-1B is potentially a global event.

Interactions between the North Brazilian Undercurrent (NBUC) and the Southwest Atlantic Margin. Implications for Brazilian shelf-edge systems

Western boundary currents influence the global ocean circulation and climate, whereas it locally reaches out differently to the continental margins. This study investigates the response of the North Brazilian Undercurrent (NBUC) over their interactions with the eastern and northern margins of the northeastern Brazil. The data set comprises regional bathymetry, in situ current measurements, CTD profiling, and simulations of oceanic currents using a regional model (CROCO). The physical analysis focuses on the months of February (austral summer) and August (austral winter). Horizontal and vertical structure of current velocity, temperature and salinity were correlated using maps and bathymetric profiles along the critical pathways of the NBUC. The eastern and northern margins are narrow and retracted with widths up to 40 km and steep upper continental slopes (14°). These margins are separated by the Touros High, which extends eastward up to 80 km offshore, with a shallow shelf (most parts are < 10 m) of approximately 55 km wide. The vertical sections of velocity show the NBUC shifting onto the Touros shelf in August and show that the strong trade winds during the austral winter handle the alongshore current circulation of this region. In the northern margin, the recirculation of the NBUC disturbs the base of the thermocline and part of these current fluxes toward the outer shelf region. The different patterns of the NBUC affect the sedimentary and ecological characteristics of eastern, Touros and northern outer shelves. Besides, the slope and outer shelf morphology might contribute to events of current-driven nutrient-enrichment and mixing processes at the base of the mixed layer. These results support the evidences that the western boundary current interactions by eddies and jets with the margin control the morpho-sedimentary and environmental conditions of the Brazilian outer shelf ecological system corridor.

Sequence stratigraphic evolution of the Kumano forearc basin during the last deglaciation: Influence of eustasy and tectonically-controlled shelf morphology on deep-marine sediment dynamics

We focus here on the different forcings that controlled the deep-marine sediment dynamics and associated stratigraphic pattern in the Nankai subduction zone, especially in the Kumano forearc basin. In this study, based on high-resolution subbottom profiler data, we investigate the Upper Pleistocene–Holocene deposits of the tectonically active Kumano forearc basin to better understand the role of glacio-eustatic forcing and tectonic-morphological controls on the sedimentation patterns in the basin. We recognize three distinct seismic units that characterize the different stages of the last to current interglaciation cycle. Chaotic debris- and mud-flow deposits, intercalated with turbidites accumulating mainly in the deep basin, are related to a lowstand stage when rivers were directly connected to canyon heads. The transgressive stage, covering the deglaciation period, is characterized by a high proportion of turbidites and a decrease in chaotic deposits. The flooding and development of the Ise Bay at ~10 ka, resulted in starvation of the deep basin where mostly hemipelagic deposits occur. However, some dilute flows still reached the proximal area of the basin depositing some turbidites. We identify an interplay between the global sea-level rise and the shelf morphology, both having a strong control on the deep-marine stratigraphic pattern of the Kumano Basin. The rise of sea level triggered a disconnection between the river mouth and canyon head, thus inducing a general retrogradation of sediment-gravity flow deposits throughout the deglaciation. The shelf morphology, which is mainly driven by tectonic uplift (narrow shelf) or subsidence (wide shelf), directly played on the connection between the river and deep-marine system and on the accommodation space in the shallow-marine environment. Although it does not appear as a dominant control, climatic control, especially the influence of the East Asian summer monsoon during deglaciation cannot be neglected. It most likely contributed to an increase in sandy sediment supply occurring at the beginning of the transgressive stage.The highlight of our study is related to the tectonically-controlled shelf morphology and its role on the deep-marine stratigraphic pattern. The northwestern area of the Kumano Basin, undergoing significant tectonic-related uplift, is defined by a narrow shelf (<10 km) and sustained sediment supply to the basin throughout the deglaciation and highstand period. However, under similar eustatic and climatic conditions, the eastern area characterized by the development of the subsiding Ise bay (wide shelf > 90 km), shows a decrease in sand supply to the basin toward the end of the deglaciation. A strong control of the shelf morphology on the deep-marine stratigraphic pattern is also observed in other forearc basins along the Nankai subduction zone undergoing similar eustatic and climatic conditions.

Wave-induced sediment and rhodolith mobility on a narrow insular shelf dominated by wave variability (Fernando de Noronha Archipelago, Brazil)

This study combines numerical modeling and empirical equations to define spatial fields of wave-induced sediment and rhodolith mobility in a narrow insular shelf that is subject to a seasonal wave regime: the Fernando de Noronha insular shelf. From analyses of the annual, summer and winter wave climates, we noticed an intense directional variability in swell waves coming predominantly from opposing directions between the studied seasons (northern swell in summer and southern swell in winter), reflecting higher wave energy on the north coast during summer and lower energy during winter. The longitudinal transport at the northern coast, which is predominantly oriented to the southwest, is also higher in the summer months and lower in the rest of the year. To describe the spatial variation in wave-induced mobility under summer and winter wave conditions, we consider empirical sediment movement equations, based on mean grain size and density values. Carbonate bioclastic sediments, which are predominant in the Fernando de Noronha shelf, are remobilized by waves throughout the entire shelf. However, in the nearshore zone, this movement occurs intensely for all sizes. The depth and shelf side affected by wave-induced mobility vary according to the wave climate variability that determines the direction and type of wave prevailing in the regime. When swell waves come from the north (summer scenarios), mobility is greater mainly on the northern shelf; however, when trade wind waves come from the southeast and swell waves come from the south (winter scenarios), greater movement is produced on the south face. Wave-induced sediment mobility occurs at depths of up to 60 m, near the shelf break, indicating the importance of the shelf morphology. The movement of rhodoliths, which presents a lower density and larger diameter than the sediments, eventually occurs under extreme wave conditions up to depths of approximately 30 m in areas that vary seasonally around the island according to the predominant wave direction. These characteristics are favorable to rhodolith development. With our results, we hope to better understand the wave action on the bottoms of high-energy island shelves by identifying the sediment movement patterns attributed to wave climate variability and morphological characteristics of the shelf.

Morpho-stratigraphic characterization of the southern shelf of Porto Santo Island (Madeira Archipelago): Insights for small-scale instability processes and post-LGM sedimentary architecture

Shelves surrounding volcanic islands represent only a small portion of the entire submarine edifice that can extend to a few thousand meters below sea level. Despite their small areas, shelves show a significant morphologic imprint made by the erosional, depositional, tectonic and volcanic processes when compared to those left on slopes or the submarine bases of islands. Therefore, shelf morphology can be used to improve our knowledge about the island evolution, especially the more recent events. In this study, we use multibeam bathymetry, side scan sonar and seismic reflection profiles of the southern shelf of Porto Santo Island in Madeira Archipelago to map its seabed morphology and seismic stratigraphy from the nearshore to the shelf edge. Based on morphological and seismo-stratigraphic differences, the study area has been divided in three sectors (Western, Central and Eastern). In general, the shelf consists of an erosive rocky surface mostly covered by sediments, and locally outcropping on the seafloor, especially on the Western and Eastern sectors. On these sectors two sets of submerged erosional terraces have been mapped at different depths (35–45 m, 50–75 m). The depth range of the terraces and their absence in the Central sector suggests that these were formed/modified at least by the Marine Isotope Stages (MIS) 5a-5d stillstands or even by older relative sea-level stillstands. Dykes, which are more resistant to erosion than the surrounding outcrops, have orientations that are similar to the volcanic and structural features present on the island (NNE-SSW and NW-SE). The sedimentary cover on the shelf is highly varied both in thickness and internal architecture, mainly due to different onshore sediment supply and available accommodation space. Two main seismic units have been recognized: the lower one (U1) is interpreted as a transgressive deposit, formed during the sea level rise after the Last Glacial Maximum (LGM); the upper one (U2) is interpreted as a modern highstand deposit. The Western sector is practically sediment-starved, except for a local thin sedimentary cover of U2 on the inner and outer shelf. The Central sector shows U2 as a sigmoidal-oblique progradational clinoform characterized by inner prograding geometry on top of U1. The Eastern sector shows U2 as sigmoidal clinoform extending over a large area and having a more aggradational component. The shelf edge shows an overall arcuate shape that strongly matches the coastline configuration, suggesting the occurrence of an old large-scale landslide event (LS1) prior to the shelf formation. Further incisions in the shelf edge of the Central sector are interpreted as resulting from mass-movements, classified as LS2 and LS3. LS2 is at least older than seismic unit U1 and LS3 formed during the late Holocene. The volume of the LS2 event (0.6 km3) suggests that it might have caused a hazardous tsunami if it failed as a single event. The recent LS3 events, due to their small magnitude were probably not hazardous. These observations provided a more comprehensive understanding of the recent evolution of Porto Santo insular shelf in terms of mass-movement processes and post-LGM sedimentary architecture.

Evolution of the inter-reef Halimeda carbonate factory in response to Holocene sea-level and environmental change in the Great Barrier Reef

The inter-reef Halimeda bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief and up to four times greater volume of calcium carbonate sediment than the nearby coral reefs during the Holocene. Covering >6000 km2, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian continental shelf geomorphology, neritic carbonate factory, and sedimentary archive of post-glacial environmental changes. However, the geochronological record of initiation and development of the Halimeda bioherm carbonate factory was poorly constrained and based on very few datapoints. A comprehensive age dataset is presented, comprising sixty-three new AMS radiocarbon measurements of Halimeda and foraminifera grains, mollusc shells and bulk soil from twelve inter-reef sediment cores, and ten previously published Halimeda ages. Facies transitions and environmental changes are recorded from lithological and palynological analyses spanning Last Interglacial Halimeda deposits, Last Glacial Maximum terrestrial palaeosols, transgressive mangrove vegetated shorelines, and the turn-on of the Halimeda bioherm carbonate factory. Mangrove pollen and coral records from the study area provide additional spatial, temporal and environmental context.Halimeda had established by 11.1 cal kyr BP, just 450 years after transgressive shelf inundation and approximately 1000 years earlier than previous inferred estimates. The outer-shelf carbonate factory was initially dominated by benthic foraminifera, then Halimeda was productive for at least 2100 years prior to the turn-on of Holocene coral reefs in the study area at 8.9 cal kyr BP from Boulder Reef (15.4°S, 145.4°E). Inter-reef Halimeda bioherms play a major role in carbon and nutrient cycling and potentially preserve a near-continuous geochemical record of northeast Australian Holocene oceanographic and climatic changes, filling spatial and temporal gaps not covered by coral and other marine sediment proxies.

Holocene morpho-stratigraphic evolution of a compound submarine deltaic system in front of the shelf-incising Almanzora and Garrucha Canyons (Palomares margin, southeastern Iberia)

The Holocene morpho-stratigraphic evolution of a compound submarine deltaic system linked with the Antas and Almanzora Rivers on the narrow (< 5 km) shelf along the Palomares margin (southeastern Iberia) has been reconstructed from the integrated analysis of geomorphology, seismo-stratigraphy and sedimentology. The shelf morphology is characterized by a 3–4 km wide deltaic body off the Almanzora River, where seafloor undulations and gullies are present. Larger and more incised gullies, crescent-shaped bedforms and pockmarks are recognizable at or close to the head of shelf-indenting canyons associated with the Almanzora-Garrucha canyon system.The seismo-stratigraphy displays a strong and irregular basal reflector overlain by a sedimentary wedge, showing variable thicknesses (1–25 m) and seismic characteristics along and across the shelf. The main depocenter of this wedge is not located off the Almanzora River mouth (i.e., the main sedimentary input in the study area) but along the southern shelf encompassing the Almanzora and Antas Rivers. Grain-size and sand compositional analyses show that silty sand with high terrigenous content (mostly quartz and mica) is mainly found on the submarine deltaic body. In contrast, sandy silt with variable ratios between terrigenous and biogenic contents is retrieved from the canyon head and associated gullies as well as from the surrounding mid-outer shelf.The evolution of the Antas-Almanzora compound delta has been divided into three evolutionary stages, mainly driven by the interplay between paleo-topography and riverine and marine processes and modulated by sea level changes during the Holocene. The origin of the present-day morphological features is also discussed, again highlighting a complex interaction between sedimentary gravity flows (mainly flash flood-generated hyperpycnal flows), storm-waves and shelf currents, fluid seepage and retrogressive slope failures.

Long‐Term Morphodynamics of a Coupled Shelf‐Shoreline System Forced by Waves and Tides, a Model Approach

AbstractSand ridges, with length scales of several km, are prominent features of the seafloor landscape of many sandy continental shelves. Knowledge about the extent to which these ridges influence the large‐scale (i.e., decadal and kilometer scales) morphodynamic evolution of the adjacent shoreline and vice versa (shelf‐shoreline morphodynamic coupling) is limited. The present work aims at quantifying this coupling by using a coupled nonlinear shelf‐shoreline model forced by tides and different wave conditions. Model results show that the presence of sand ridges on the shelf creates longshore non‐uniform wave patterns, which act as a forcing template for the morphodynamic development of the shoreline. The shelf‐shoreline coupling primarily works one way, meaning that the morphodynamic evolution of the shelf affects the evolution of the shoreline. When wave propagation is predominantly aligned with the long axis of the shelf ridges, the forced shoreline undulations are so prominent, that they affect the shelf morphology (significant two‐way coupling). Moreover, for those waves, the longshore spacing of the ridges is strongly imprinted on the shoreline morphology. Weaker shoreline undulations develop for waves that propagate more across the ridges and the weakest for time‐varying wave conditions with large variability in their angles of propagation. Model results compare fairly well with observations. Physical mechanisms underlying the different morphodynamic responses of the coupled shelf‐shoreline system to different wave conditions are also given.

Morphological evidences of eustatic events in the last 14,000 years in a far-field site, East-Southeast Brazilian continental shelf

The post-LGM transgression was marked by eustatic variability events, comprising periods of increased sea-level rise, triggered by meltwater pulses (MWP) on polar regions, preceded by periods of stillstand. Several studies highlight the influence of such events on the geomorphology of far-field continental margins worldwide by mapping drowned relict features. However, there is still a lack of information in the South Atlantic and a major discussion about the global distribution of MWP-1B. The present work aims to investigate morphological evidences in the east-southeast Brazilian continental shelf (Abrolhos and Espírito Santo shelf), by examining shelf morphology using regional bathymetric data and high-resolution seismic sections. The analytical strategy involved comparing depth signatures of interpreted relict features with a set of sea-level curves. The results suggested the predominance of depositional process in shoreline sectors under closer influence of fluvial discharges during the Younger Dryas (YD). During YD stillstand or low sea-level rise rates (~60 m deep), it was possible to recognize the occurrence of a lagoonal system with shallow water/shoreface features (sandbanks, sand ribbons, or other bedforms) associated with tidal channels in the Abrolhos Shelf Depression and erosive features such as coastal cliffs. In the Espírito Santo outer shelf, the YD coastal features were characterized by barrier island systems associated with incised valley mouths and possible fringing reefs, in a context of a marginal shallow marine environment. All these features tend to be preserved in the modern shelf morphology due to a fast increase in the accommodation space that defines the end of the YD due to the onset of MWP-1B. In general, morphological interpretation revealed that MWP-1B caused the overstepping or drowning of those coastal features. Erosive features, defining the YD period, were also identified far from the influence of fluvial input. In these areas, coastal erosive processes prevailed and are morphologically expressed as abrasion terraces developed since the YD onset ~70 m and features like a paleo-sea cliff. During MWP-1B, the presence of shallow water reefs in the Abrolhos shelf is hypothesized, experiencing backstepping until their complete submersion. Despite the lack of additional data on composition, age and 3D geometry of the features, the morphological approach proposed here strongly supports the concept that eustatic events, triggered mainly in the Northern Hemisphere, influenced the shelf geomorphology and sedimentary processes evolution in the South Atlantic, during the last deglaciation.

Rhodolith mobility potential from seasonal and extreme waves

Rhodolith beds can contribute large amounts of CaCO3 to coastal environments, playing an important role in Carbon sequestration. The spatial distribution of rhodoliths is a consequence of their sensitivity to light intensity, currents, wave action, sedimentation rates, and salinity, whereas mesophotic, low sedimentation quiescent conditions are optimal. Although there is evidence of a clear relationship between hydrodynamics and rhodolith morphology and mobility, there are few studies relating wave action and rhodolith mobility. We aim to assess the mobility potential by waves, of the rhodoliths of the Abrolhos Shelf (Brazil), that contains the largest rhodolith field in the world. Based on the local wave climate and its propagation through the shelf, the rhodolith mobility potential was evaluated. Rhodolith mobility potential distribution is directly related to shelf morphology and incoming waves characteristics. Despite being restricted to deeper areas of the shelf (>20 m), rhodoliths in the area are mobilized by wave action, especially from the more energetic waves associated with storms. Waves can mobilize rhodoliths in the area, with mobility potential distribution being controlled by shelf morphology and seasonal wave climate. Largest areas of mobilization are observed during energetic waves conditions associated to the passage of cold fronts.

Continental shelf morphology controlled by bottom currents, mud diapirism, and submarine slumping to the east of the Gaoping Canyon, off SW Taiwan

The Gaoping River-Gaoping Submarine Canyon (GPC) is one of the dispersal river systems in the world. Tens of tons of terrestrial sediments discharge per year at the mouth of the river, where gravity-driven flow can damage the telecommunication cable along the 260-km-long GPC; to the east of the GPC, the continental shelf has been eroded. To understand this geological phenomenon, high-resolution sparker seismic data around the continental shelf as well as parallel and across the GPC were collected. Deep-towed sub-bottom profiler (SBP) data and data from shipboard acoustic Doppler current profiler were also collected. Three major system tracts off southwest Taiwan were identified, based on which the coastline of the last glacial maximum was identified at approximately 138 m below the current sea level. Analysis of the deep-towed SBP profiles suggested sandy sediment overflows through breakages on the east bank of the GPC. A convergence of bottom currents with a speed of up to 1.5 m/s flowing southeastward has eroded the continental shelf intensively, forming the Xiaoliuchiu channel. Although a mud diapir is active to the east of the GPC and forms the Xiaoliuchiu islet, it has been eroded at the Xiaoliuchiu channel. The average erosion rate on the east bank of the GPC was estimated to be approximately 609 cm/ka in the past 8.2 ka; between the GPC and the Xiaoliuchiu islet, the continental shelf is wasted due to severe seabed slumping.

PROMINENT MARINE GEOLOGIST VOLODYMYR OLEKSANDROVYCH IEMELIANOV (TO THE 70TH ANNIVERSARY OF THE BIRTH)

PROMINENT MARINE GEOLOGIST VOLODYMYR OLEKSANDROVYCH IEMELIANOV (TO THE 70TH ANNIVERSARY OF THE BIRTH)

The role of shelf morphology on storm‐bed variability and stratigraphic architecture, Lower Cretaceous, Svalbard

AbstractThe dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.