Offshore Shelf Research Articles

Depositional facies and the sequence stratigraphic control of a mixed‐process influenced clastic wedge in the Cretaceous Western Interior Seaway: The Gallup System, New Mexico, USA

AbstractDepositional facies have been hypothesized to be linked to sequence stratigraphic positions. Also, shoreline systems are built by mixed processes, including rivers, storms, fair‐weather waves and tides. Resolving the complexity of shoreline deposition requires detailed quantitative facies analysis with particular attention to heterolithic successions. In this study, 71 sections in a 130 km long outcrop belt of the Cretaceous Gallup Formation in the north‐west of the San Juan Basin were measured. Five major facies associations were identified using sedimentological and iconological interpretations, including offshore shelf, non‐deltaic shoreline sandstones, deltas, coastal bayline and fluvial. Each facies association also comprises subordinate facies. Depositional facies interpretations are placed in a high‐resolution sequence stratigraphic framework that allows for reconstructions of the palaeogeography of individual parasequence sets that demonstrate temporal and spatial evolution of facies associations and depositional processes. The results show that the Gallup is a mixed‐process‐controlled depositional system with fair‐weather and storm‐wave dominance, river influence and tide‐effect, contrasting with previous interpretations of a solely fair‐weather wave‐dominated environment. Depositional processes and the resultant facies change with sequence stratigraphic positions in response to relative sea‐level changes – particular facies are only deposited in certain systems tracts. Distinction and transition between non‐deltaic shorefaces and wave‐dominated deltas have also been documented in this study. Non‐deltaic shorefaces are characterized by homogeneous sandstones with a wide‐range bioturbation index and the absence of mudstones. Wave‐dominated deltas are subject to river influence and contain prodelta facies. This study shows the importance of detailed facies analysis with high‐resolution sequence stratigraphic control using outcrops for documenting sedimentary processes of shallow marine shoreline systems.

Controls on sediment distribution in the coastal zone of the central California transform continental margin, USA

We use >10,000 km of high-resolution seismic-reflection data together with multibeam bathymetry to document complex and highly variable post-Last Glacial Maximum (LGM) sediment distribution and thickness in the coastal zone (~10 m isobath to 5.6 km offshore) along a ~800 km section of central California's transform continental margin. Sediment thickness ranges from 0 (seafloor bedrock) to 64 m with a mean of 8.7 m. We delineate 25 coastal zone “sediment domains,” and group them based on common geomorphology and sediment occurrence. Thickest sediment occurs in “mountain front” and “large river” domains, which comprise 14.5% and 7.9% of the coastal zone and contain 30.1% and 18.2% of coastal zone sediment, respectively. In contrast, “small river” domains and “sediment-poor shelf” domains comprise 50.7% and 15.7% of the coastal zone and contain 18.4% and 12.7% of its sediment.The distribution and thickness of post-LGM sediment in the coastal zone is controlled by a combination of tectonics, sediment supply, and eustasy. Sediment is derived from a tectonically controlled coastal landscape of rapidly uplifting mountain fronts, more slowly uplifting marine terraces, and fault-bounded headlands and alluvial-estuarine troughs. Sediment supply is maximized along steep, landslide-prone, mountain fronts and at the mouths of large watersheds, and minimized along lower-relief, terraced coastal landscape drained by smaller rivers and creeks. In the offshore coastal zone, tectonics generates local uplifts and basins, and influences shelf width and gradient as well as the locations of some shelf-incised submarine canyons. Sea-level rise raises base level, drowns estuaries, creates accommodation space on the shelf (amount based on gradient), and isolates the heads of many submarine canyons at or near the shelfbreak. Comparison of shelf sediment volumes with estimates of “unaltered” watershed sediment supply reveals that a relatively small proportion of post-LGM sediment supply is preserved on the shelf offshore of some of the largest rivers. Sediments deposited in shoreline and shelf environments have limited preservation potential, and the most complete long-term geologic record of the post-LGM transgression and highstand is likely represented in slope and submarine fan deposits.

High-tech drilling of wells in offshore fields using import-substituting technologies

The article considers the relevance of increasing the efficiency of development of offshore shelf deposits in the North Caspian, using the example of the oil and gas condensate field named after Y. Korchagin. The development of offshore fields is associated with the use of the latest expensive technologies aimed at highly efficient development of facilities with hard-to-recover hydrocarbon reserves. The involvement of field operators in the development of expensive imported technologies leads to a significant increase in the cost of oil produced. In this regard, there is a need for testing and introducing similar domestic technologies into the offshore field development process to optimize the hydrocarbon production process.

Late Pleistocene to holocene palaeoclimates and palaeoenvironments inferred from palynofacies and dinoflagellates cysts in Santos Basin, offshore Brazil

We analyse Late Pleistocene to Holocene palynofacies and dinoflagellates in a sedimentary core (PC PRIMA 4), collected on the northern offshore shelf of the Santos Basin of Brazil, which covers the last 29,429 cal yrs BP. The aim of this study is to better understand changes in climate and oceanographic circulation that affected this region since the Last Glacial Maximum (LGM), and the 120 m rise in sea level that followed. The organic matter in the core shows characteristics consistent with an oxygenized proximal shelf with incursions of continental elements, which evolved into a more distal dysaerobic-anoxic environment over the studied time interval. An increase in phytoclast (opaque and non-opaque) abundance during the Last Glacial Maximum interval suggests enhanced river supply, driven by the increase in the hydrological cycle resulting from cold and humid climatic conditions. The increase of Amorphous Organic Matter from 16,437 cal yrs BP suggests the onset of anoxic bottomwater conditions that resulted of an increase in water depth. Together with the percentage increase of Operculodinium centrocarpum and the fall in Tuberculodinium vancampoae abundance, this implies the postglacial development of warmer currents. In summary, our results better document the climatic and oceanographic changes that occurred in this area during and following the LGM.

Scottish landform examples 45: Sgriob na Caillich: a landslide-sourced medial moraine on the Isle of Jura

ABSTRACTMedial moraines deposited by former glaciers and ice sheets are rare in Scotland. The most prominent example is the Sgriob na Caillich moraine, which consists of two to four parallel belts of angular quartzite boulders that extend northwestwards for over 3.5 km from the SW flank of Beinn an Oir on the Isle of Jura. The boulder belts extend to within 300 m of the present coastline, where they are truncated by a low bluff and raised marine terrace. The source of the moraine coincides with bedrock gullies and cliffs that represent the scars of former rock-slope failure(s), indicating that the moraine debris was sourced by one or more rockfalls or rockslides onto the ice surface after Beinn an Oir had emerged from the thinning ice cover as a nunatak. Exposure dating of boulders on the moraine indicate that it formed at 16.6 ± 0.8 ka, consistent with the timing of ice-sheet retreat in this sector. The alignment of the moraine indicates ice-margin retreat to the SE; as regional ice-sheet retreat across the adjacent offshore shelf was to the NE, this anomaly implies that a residual icefield became stranded on Jura during ice-sheet retreat.

Palynological analysis of Upper Ordovician to Lower Silurian sediments from the Diyarbakir Basin, southeastern Turkey

ABSTRACT This paper reports on a palynological analysis of 41 core and 21 cutting samples from a well drilled through an Upper Ordovician–Lower Silurian sequence, belonging to the Bedinan and Dadas formations, of the Diyarbakir Basin of southeastern Turkey. The samples yield abundant and well-preserved marine palynomorphs (acritarchs, chitinozoans and scolecodonts) although non-marine palynomorphs (spores/cryptospores) are extremely rare or absent. The Upper Ordovician sediments of the Bedinan Formation have low organic content but contain abundant palynomorphs whereas the Lower Silurian sediments of the Dadas Formation have high organic content, dominated by amorphous organic matter, with relatively rare palynomorphs. Three chitinozoan assemblages are identified and attributed a late Katian (merga Biozone), Hirnantian (moussegoudaensis Biozone) and Llandovery (?alargada Biozone) age. Two acritarch assemblages are identified and attributed a Katian–Hirnantian and Llandovery (Aeronian–Telychian) age. The chitinozoan and acritarch age determinations are compatible and suggest that the Bedinan Formation is of Katian–Hirnantian age and is separated by an unconformity from the Dadas Formation that is of Llandovery (Aeronian–Telychian) age. These findings confirm the presence of an unconformity at the Ordovician–Silurian transition in the southeastern Turkey. Palynofacies analysis suggests that the Bedinan Formation accumulated on an offshore shelf that was initially well oxygenated but became increasingly anoxic, whereas the Dadas Formation accumulated in an offshore basin that was anoxic. Palynomorph assemblages recorded in the Bedinan and Dadas formations indicate northern Gondwana affinity.

The Critical Importance of Rhodoliths in the Life Cycle Completion of Both Macro- and Microalgae, and as Holobionts for the Establishment and Maintenance of Marine Biodiversity

Rhodoliths are the main hard substrata for the attachment of benthic macroalgae in the NW Gulf of Mexico rubble habitats that are associated with salt domes, unique deep bank habitats at ~50-90m depth on the continental shelf offshore Louisiana and Texas. With the advent of additional sequencing technologies, methodologies for biodiversity assessments are now rapidly shifting to DNA metabarcoding, i.e. High Throughput Sequencing (HTS) of environmental DNA mixtures with standardized molecular markers, such as 16S V4, for rapid, cost-effective biodiversity measurement. We newly tested 16S V4 metabarcoding on endolithic portions of mesophtic rhodoliths exhibiting low phototroph colonization that revealed a hidden, cryptic algal diversity targeting spores, propagules, and unsuspected life history stages. We explored cryo-SEM as a potentially more informative method than regular SEM to minimize artefacts of sample preparation in the study of endolithic cell inclusions which brought to light a suite of microalgal stages. We were able to differentiate floridean starch from cellular inclusions. We associated the effect of anatomical growth pattern on presence or absence of cellular inclusions in biogenic rhodoliths. Analyses of combined 16S V4 metabarcodes and 16S Sanger sequences of two red algal orders, the Halymeniales and Bonnemaisoniales, increased the established record of diversity in the region. We view rhodoliths as marine biodiversity hotspots that may function as seedbanks, temporary reservoirs for life history stages of ecologically important eukaryotic microalgae, or as refugia for ecosystem resilience following environmental stress.

МИРОВАЯ ВЕТРОЭНЕРГЕТИКА МЕГАВАТТНОГО ДИАПАЗОНА МОЩНОСТЕЙ

Despite of the political and economic problems wind power continues its steady development, the main directions of which are presented in the paper. New production facilities and materials are being created, which contributes to increased efficiency by enlarging the unit capacity of wind power plants (wind turbines) and reducing the unit cost of energy production. A promising large-scale direction of modern wind power is offshore wind power plants operating on the offshore shelf, for which the production of 6–8 MW wind generators is developed. Many offshore wind turbines manufacturers prefer a direct-drive permanent magnets synchronous generator, while they managed to achieve 25 kg/kN·m for the torque to the mass ratio and are searching how to reduce wind plants cost by various ways. The most significant achievements in the industrial wind turbines production and wind plants construction, as well as promising projects providing non-standard solutions, including those based on superconductivity, are presented.

High frequency refraction/ reflection full-waveform inversion case study from North West Shelf offshore Australia

The robustness of diving wave Full-Waveform Inversion (FWI) has been proven in industry, but the effectiveness is limited by its penetration depth. To target deeper reservoirs, therefore, requires the application of reflection FWI. This paper presents a real data 25Hz VTI FWI case study from North-West Shelf (NWS) Australia utilizing full wave-field. Starting from a high-quality reflection tomography VTI model, a top-down approach has been adopted. Diving wave FWI updates the shallow, then reflection FWI is introduced to further update the deeper section. The updated FWI model demonstrates significant uplifts in resolution and consistency with the geology. Two promising aspects can be observed: (1) the fairly solid uplifts in mitigating the imaging challenges: FWI reduces wave-field distortions, leads to overall improved focusing, gather flatness, continuity, and better positioning in depth; and (2) uncovers geological features beyond imaging: high-resolution FWI delineates small shallow anomalies and velocity boundaries across faults, and reveals the strong acoustic impedance contrasts at reservoir level. It demonstrates FWI can aid both in reducing the velocity uncertainty as well as providing a geological interpretation.

Sediment transport by tropical cyclones recorded in a submarine canyon off Bangladesh

Frequent cyclones originating in the Bay of Bengal landfall on the delta coast of the Ganges and Brahmaputra rivers. The cyclones are well recorded in the sediments of a canyon that is deeply incised into the shelf offshore Bangladesh. The large mud supply by the two rivers forms temporary deposits on the innermost shelf, where they are mobilized by waves and currents during the passage of cyclones. The resulting, highly concentrated fine sand-silt-clay suspension is moved by wind-induced currents and eventually plunges into the shelf canyon. These gravity flows are deposited as graded beds on the broad canyon floor. In a 362-cm-long section of a dated sediment core covering the period from 2006 to 1985, nearly all 59 graded beds can be correlated with 42 cyclones observed in that period. The threefold decrease in the sedimentation rate of the last decade compared to the period from 1994 to 1954 is due to the decreased number and power of cyclones. Compared to the sediment transfer by cyclones, the input by local sediment slumps, tidal currents, and monsoonal floods is small. Thus, cyclones dominate the mobilization and distribution of sediment on the Bangladesh shelf. This sediment dispersal mechanism is probably also typical for other shelf areas crossed by tropical cyclones.

Platform drowning leading to cool-water carbonate deposition: evolution of a Late Ordovician (Turinian–Chatfieldian) mixed-sediment platform within the Taconic orogen (Long Point Group, Newfoundland Appalachians)

Late Ordovician (Turinian–Chatfieldian) drowning of a mixed carbonate–siliciclastic platform within the Taconic Orogen (Newfoundland Appalachians) is recorded by net deepening of an initial warm, shallow-water platform succession (Lourdes Formation) culminating in a metre-scale thick condensed interval that characterizes a drowning succession punctuated by storm deposits. Composition of transported material suggests that seaward drowning was coupled with back-stepping of a high-energy carbonate factory related to hinterland uplift and erosion that would eventually lead to drowning of the outer platform beneath marine-transported siliciclastic sediments (Winterhouse Formation). In the new offshore shelf setting, a sparse reciprocal stratigraphy of fine- to very coarse-grained phosphatic carbonate and mixed sediment is interpreted to document gravity-flow deposition downgradient from either a sustained or episodically developed high-energy cool-water carbonate source along the inner shelf. Transported carbonate was cemented rapidly at temperatures no warmer than 16 °C–23 °C, possibly within a seasonal oceanic thermocline. An upsection decrease in abundance of carbonate by the early Edenian is associated with a dramatic increase in siliciclastic supply. The Turinian–Edenian succession of platform drowning, oceanographic transition to cool-water carbonate production, and, later, its termination by increased siliciclastic supply reflects a first-order tectonic control proximal to uplift within the Taconic Orogen. Similar structural and oceanographic changes along the contemporary distal Laurentian margin provides the basis, with improved biostratigraphic control, for future analysis of the significance of proximal–distal stratigraphies in response to regional foreland tectonism.

Daily variation in the prokaryotic community during a spring bloom in shelf waters of the East China Sea.

ABSTRACTTo understand prokaryotic responses during a spring bloom in offshore shelf waters, prokaryotic parameters were measured daily at a station located in the middle of the East China Sea over a six-week period from March 25 to May 19. The site experienced a phytoplankton bloom in late April, triggering changes in prokaryotic abundance and production after a lag of approximately one week. Before the bloom, changes in prokaryotic composition were small. Both during the bloom and in the post-bloom period, successive changes among bacterial groups were apparent. A SAR11 group became more dominant during the bloom period, and diverse groups belonging to the Flavobacteriia occurred dominantly during both the bloom and post-bloom periods. However, bacterial community changes at the species level during the bloom and post-bloom periods occurred rapidly in a time scale of a few days. Especially, NS5, NS4 and Formosa bacteria belonging to Flavobacteriia and bacteria belonging to Halieaceae and Arenicellaceae families of Gammaproteobacteria showed a successive pattern with large short-term variation during the period. The changes in prokaryotic composition were found to be related to phytoplankton biomass and composition, as well as seawater temperature and variations in nutrients.

Past ice stream and ice sheet changes on the continental shelf off the Sabrina Coast, East Antarctica

Our understanding of the response of the Antarctic ice sheet to climate and ocean changes requires the improvement of current ice-atmosphere-ocean models and the accurate determination of boundary conditions such as ice thickness and extent at key time intervals, so that satellite gravity observations and isostatic models can be adjusted. However, large portions of the Antarctic margin remain understudied or lack suitable data. One key area where data are lacking, is the Sabrina Coast portion of the East Antarctic Ice Sheet (EAIS) margin where the Totten Glacier, which has the largest ice discharge in East Antarctica, is accelerating, thinning and loosing mass at high rates. In this work, we present the results of the first geological and geophysical marine survey to the continental shelf offshore of the Dalton Ice Tongue and Moscow University ice shelf, east of the Totten Glacier. The data presented include multibeam swath bathymetry and multichannel seismic, focusing on the sea floor morphology and sedimentary section above a regional angular unconformity separating pre- and post-Miocene glacial strata. Sea floor scouring and iceberg keel marks on the outer shelf, associated with gullies on the upper slope indicate that ice expanded in the past and grounded ~5 km from the shelf edge at ~450–500 mbsl, extending ~155 km north of the current Moscow University Ice Shelf. A nearly 1000 m deep area in the inner-middle shelf, oriented NW with paleo-ice flow direction indicated by mega scale glacial lineations (MSGL) and drumlins, is interpreted as a cross shelf glacial trough. A series of geomorphic associations on the north-eastern side of the glacial trough includes glacial lobes, grounding zone wedges (GZW), glacial lineations and transverse ridges, which indicates slower ice, grounding line stabilization and collapse. These geomorphic associations are organized in 4 four sets representing different past ice-flow configurations reflecting changes in ice flow direction, grounding line position, location of fast and slow ice areas, and retreat pattern. Some of the geomorphic features identified are compatible with the presence of an organized subglacial drainage, and others are with rapid grounding line collapse. A well-preserved series of GZWs occurring at different water depths implies they were formed during different glacial stages or cycles. The inferred diminishing ice thickness for consecutives GZWs indicates that the margin of the Antarctic ice sheet evolved to a less extensive coverage of the continental shelf through successive glacial stages or cycles.The identification of different ice flow configurations, evidence of subglacial water and past ice margin collapse indicates a dynamic ice sheet margin with varying glacial conditions and retreat modes. We observe that some of the described morphological associations are similar to those found in the Amundsen sea sector of the West Antarctic Ice Sheet (WAIS) where they are associated with ice sheet and ice stream collapse. Although further studies are needed to assess the precise timing and rates of the glacial processes involved, we conclude that there is enough evidence to support the hypothesis that the EAIS margin can behave as dynamically as the WAIS margin, especially during glacial retreat and icesheet margin collapse.

Submarine deglacial sediment and geomorphological record of southwestern Scotland after the Last Glacial Maximum

Understanding the style and pattern of retreat of the offshore sectors of the last British-Irish Ice Sheet (BIIS) is critical to any attempt to reconstruct its history following the Last Glacial Maximum (LGM). This paper presents a new seismo-stratigraphic analysis of Quaternary deposits on the inner continental shelf offshore of southwestern Scotland. It correlates these data with new high resolution seafloor bathymetry and sediment cores to reconstruct the post-LGM retreat dynamics of the Hebrides Ice Stream, a major outlet of the last BIIS which drained across the continental shelf offshore of northwest Britain. Two primary glacigenic units (Units III and IV) are observed in seismic sequences from the region. Unit III partly corresponds to the previously defined Barra Formation, but is re-interpreted here as a time-transgressive subglacial to ice-proximal deposit. On the mid-shelf, this unit comprises grounding-zone wedges (GZWs). Within inshore waters and sea lochs Unit III can be found at or near seabed, where it is associated with retreat moraines, as well as with proglacial outwash sediments near the Kintyre coast (RSL ~10 m OD). The younger Unit IV (equivalent to the Jura Formation) represents ice-proximal to hemipelagic conditions. Bathymetric data imaged streamlined subglacial landforms recording ice sheet flow onto the inner shelf and a variety of transverse landforms collectively interpreted as moraines recording episodic retreat. These new data indicate that during the last deglaciation of the shelf offshore of southwestern Scotland the retreat dynamics of the Hebrides Ice Stream followed three main stages: i) tidewater margin retreat punctuated by stillstands on the inner shelf, ii) topography-controlled fjordic retreat, with evolution from a coherent ice-sheet to separate tidewater glaciers, and iii) stabilisation at the transition from a tidewater to land-based ice margin.

Diagenesis and Compositional Partitioning of Quaternary Cool-water Carbonate Aeolianites: Southeastern Australia

Abstract Middle to late Pleistocene stranded carbonate sea beaches, adjacent aeolian dunes, and Pleistocene aeolianites together with Holocene marginal marine calcareous sediments across southeastern Australia contain the longest record of such deposition in the modern world, stretching back to at least ∼ 900 ky (MIS 23). The grains are/were derived from a suite of cool-water, heterozoan carbonate sediments produced on the adjacent offshore shelf. Most modern shelf carbonate sediments in the region today are aragonite-rich but the aragonite is presently slowly being dissolved on or just below the seafloor before the deposits enter the rock record. Beach-dune sediments are derived mainly from the shallow < 30 meters deep adjacent seafloor. They are mollusk-rich but with most bryozoan particles having been destroyed in the surf zone. The mollusks are dominantly aragonite. These nearshore sediments are swept onshore before any significant seafloor diagenesis can take place and so are compositionally different from the eventual aragonite-poor open-shelf sediments. Aragonite skeletal fragments in the dunes progressively dissolve with time under a semi-arid climate and produce diagenetic low-magnesium calcite meteoric cements, a well-known process common in tropical, aragonitic Pleistocene aeolianites. The loss of metastable minerals with time is matched by a progressive and well-understood change in stable isotopes. Such alteration in this environment, however, seems to take longer than in tropical carbonates, with lithification herein not beginning until ∼ 200 ka and complete cementation being achieved in ∼ 450 ky Thus, neither aeolianites nor offshore marine carbonates record the composition of the original sediment; aeolianites lack the important bryozoan constituents whereas neritic deposits lack the prolific mollusks. This partitioning, revealed for the first time, answers the conundrum as to why some cool-water neritic shelf and coastal sediments in the rock record are compositionally different.

Generation and Propagation of Nonlinear Internal Waves in Sheared Currents Over the Washington Continental Shelf

AbstractThe generation, propagation, and dissipation of nonlinear internal waves (NLIW) in sheared background currents is examined using 7 days of shipboard microstructure surveys and two moorings on the continental shelf offshore of Washington state. Surveys near the hypothesized generation region show semi‐diurnal (D2) energy flux is onshore and that the ratio of energy flux to group speed times energy ( ) increases sharply at the shelf break, suggesting that the incident D2 internal tide is partially reflected and partially transmitted. NLIW appear at an inshore mooring at the leading edge of the onshore phase of the baroclinic tide, consistent with nonlinear transformation of the shoaling internal tide as their generation mechanism. Of the D2 energy flux observed at the eastern extent of the generation region (133 ± 18 Wm−1), approximately 30% goes into the NLIW observed inshore (36 ± 11 Wm−1). Inshore of the moorings, 7 waves are tracked into shallow (30‐40 m) water, where a vertically sheared, southward current becomes strong. As train‐like waves propagate onshore, wave amplitudes of 25‐30 m and energies of 5 MJ decrease to 12 m and 10 kJ, respectively. The observed direction of propagation rotates from N of E to S of E in the strongly sheared region. Linear ray tracing using the Taylor‐Goldstein equation to incorporate parallel shear effects accounts for only a small portion of the observed rotation, suggesting that three‐dimensionality of the wave crests and the background currents is important here.

Study of Coating Corrosion Resistance for Shelf Structural Objects and Oil Industry Equipment

Results are given for a study of the corrosion resistance of a number of protective coatings for offshore structures and shelf zone constructions operating under conditions of total and periodic immersion in sea water.

Persistent Shoreline Shape Induced From Offshore Geologic Framework: Effects of Shoreface Connected Ridges

AbstractMechanisms relating offshore geologic framework to shoreline evolution are determined through geologic investigations, oceanographic deployments, and numerical modeling. Analysis of shoreline positions from the past 50 years along Fire Island, New York, a 50 km long barrier island, demonstrates a persistent undulating shape along the western half of the island. The shelf offshore of these persistent undulations is characterized with shoreface‐connected sand ridges (SFCR) of a similar alongshore length scale, leading to a hypothesis that the ridges control the shoreline shape through the modification of flow. To evaluate this, a hydrodynamic model was configured to start with the US East Coast and scale down to resolve the Fire Island nearshore. The model was validated using observations along western Fire Island and buoy data, and used to compute waves, currents and sediment fluxes. To isolate the influence of the SFCR on the generation of the persistent shoreline shape, simulations were performed with a linearized nearshore bathymetry to remove alongshore transport gradients associated with shoreline shape. The model accurately predicts the scale and variation of the alongshore transport that would generate the persistent shoreline undulations. In one location, however, the ridge crest connects to the nearshore and leads to an offshore‐directed transport that produces a difference in the shoreline shape. This qualitatively supports the hypothesized effect of cross‐shore fluxes on coastal evolution. Alongshore flows in the nearshore during a representative storm are driven by wave breaking, vortex force, advection and pressure gradient, all of which are affected by the SFCR.

Change in morphology and modern sediment thickness on the inner continental shelf offshore of Fire Island, New York between 2011 and 2014: Analysis of hurricane impact

Seafloor mapping investigations conducted on the lower shoreface and inner continental shelf offshore of Fire Island, New York in 2011 and 2014, the period encompassing the impacts of Hurricanes Irene and Sandy, provide an unprecedented perspective regarding regional inner continental shelf sediment dynamics during large storm events. Analyses of these studies demonstrate that storm-induced erosion and sediment transport occurred throughout the study area in water depths up to 30m. Acoustic backscatter patterns were observed to move from ~1m to 450m with a mean of 20m and movement tended to decrease with increasing water depth. These patterns indicate that both of the primary inner continental shelf sedimentary features in the study area, linear sorted bedforms offshore of eastern Fire Island and shoreface-attached sand ridges offshore of central and western Fire island, migrated alongshore to the southwest. The migration of the sorted bedforms represents the modification of an active ravinement surface and is thought to have liberated a significant volume of sediment. Comparison of isopach maps of sediment thickness show that the volume of modern sediment composing the lower shoreface and shoreface-attached sand ridges decreased by ~2.8×106m3 across the ~73km2 of common seafloor mapped in both surveys. However, a similar analysis for the relatively calmer 15-yr period prior to 2011 revealed significant accretion. This allows speculation that the shoreface-attached sand ridges are maintained over decadal timescales via sediment supplied through erosion of Pleistocene outwash and lower Holocene transgressive channel-fill deposits exposed on the inner continental shelf, but that the sand ridges also periodically erode and move to the southwest during large storm events. Analyses show that significant storm-induced erosion and sediment transport occurs far seaward of the 5 to 9m depth of closure assumed for Fire Island, where it is thought that an onshore-directed sediment flux from the inner continental shelf to the littoral system is required to balance the coastal sediment budget. It is also thought that the morphology of the shoreface-attached sand ridges controls the persistent shape of the adjacent shoreline through modification of incident waves. Thus, we suggest that the sediment dynamics of the inner continental shelf and both storm-induced and anthropogenic modification of the field of shoreface-attached sand ridges be considered in future coastal resiliency planning.

Sand ridge morphology and bedform migration patterns derived from bathymetry and backscatter on the inner-continental shelf offshore of Assateague Island, USA

The U.S. Geological Survey and the National Oceanographic and Atmospheric Administration conducted geophysical and hydrographic surveys, respectively, along the inner-continental shelf of Fenwick and Assateague Islands, Maryland and Virginia over the last 40 years. High resolution bathymetry and backscatter data derived from surveys over the last decade are used to describe the morphology and presence of sand ridges on the inner-continental shelf and measure the change in the position of smaller-scale (10–100s of meters) seafloor features. Bathymetric surveys from the last 30 years link decadal-scale sand ridge migration patterns to the high-resolution measurements of smaller-scale bedform features. Sand ridge morphology on the inner-shelf changes across-shore and alongshore. Areas of similar sand ridge morphology are separated alongshore by zones where ridges are less pronounced or completely transected by transverse dunes. Seafloor-change analyses derived from backscatter data over a 4–7 year period show that southerly dune migration increases in magnitude from north to south, and the east-west pattern of bedform migration changes ~ 10km north of the Maryland-Virginia state line. Sand ridge morphology and occurrence and bedform migration changes may be connected to observed changes in geologic framework including topographic highs, deflated zones, and sand availability. Additionally, changes in sand ridge occurrence and morphology may help explain changes in the long-term shoreline trends along Fenwick and Assateague Islands. Although the data presented here cannot quantitatively link sand ridges to sediment transport and shoreline change, it does present a compelling relationship between inner-shelf sand availability and movement, sand ridge occurrence and morphology, geologic framework, and shoreline behavior.