Stability studies of surficial sediments in the Wilmington-Lindenkohl Canyons area,eastern U.S. margin

Stability analysis, based on infinite slope analysis and geotechnical data from a suite of 34 cores collected from the continental slope between Wilmington and Lindenkohl Canyons, indicates that the Quaternary surficial silty clay sediments on gentle slopes are stable; that sediment stability on steeper slopes (14°–19°) is marginal; and that on precipitous slopes (>50°) only a thin veneer of unconsolidated sediments can exist. Small earthquake-induced accelerations or the effects of internal waves can result in slope sediment instabilities.     

Sediment transport on subaqueous fan delta slopes, Britannia Beach, British Columbia

A high-resolution acoustic survey over a fjord side fan delta revealed distinctive bottom features resulting from slope instability processes. Delta-front chutes occurring on slopes of l3° are partially filled with radiating splays of coarse-grained sediment, apparently transported downslope by coarse-grained debris flows that originated on the subaerial slopes above the fan. Arcuate scarp patterns represent shallow successive, rotational slides, with numcrous small displacements of individual blocks and slabs of sediment. Blocky, ridged depositional areas occur at the base of the fan delta, but there is no evidence of long-distance mass movement farther downfjord.     

Sediment transport on subaqueous fan delta slopes,Britannia Beach,British Columbia

A high-resolution acoustic survey over a fjord side fan delta revealed distinctive bottom features resulting from slope instability processes. Delta-front chutes occurring on slopes of l3° are partially filled with radiating splays of coarse-grained sediment, apparently transported downslope by coarse-grained debris flows that originated on the subaerial slopes above the fan. Arcuate scarp patterns represent shallow successive, rotational slides, with numcrous small displacements of individual blocks and slabs of sediment. Blocky, ridged depositional areas occur at the base of the fan delta, but there is no evidence of long-distance mass movement farther downfjord.     

Depositional characteristics and spatial distribution of deep-water sedimentary systems on the northwestern middle-lower slope of the Northwest Sub-Basin, South China Sea

Based upon 2D seismic data, this study confirms the presence of a complex deep-water sedimentary system within the Pliocene-Quaternary strata on the northwestern lower slope of the Northwest Sub-Basin, South China Sea. It consists of submarine canyons, mass-wasting deposits, contourite channels and sheeted drifts. Alongslope aligned erosive features are observed on the eastern upper gentle slopes (<1.2° above 1,500 m), where a V-shaped downslope canyon presents an apparent ENE migration, indicating a related bottom current within the eastward South China Sea Intermediate Water Circulation. Contourite sheeted drifts are also generated on the eastern gentle slopes (~1.5° in average), below 2,100 m water depth though, referring to a wide unfocused bottom current, which might be related to the South China Sea Deep Water Circulation. Mass wasting deposits (predominantly slides and slumps) and submarine canyons developed on steeper slopes (>2°), where weaker alongslope currents are probably dominated by downslope depositional processes on these unstable slopes. The NNW–SSE oriented slope morphology changes from a three-stepped terraced outline (I–II–III) east of the investigated area, into a two-stepped terraced (I–II) outline in the middle, and into a unitary steep slope (II) in the west, which is consistent with the slope steepening towards the west. Such morphological changes may have possibly led to a westward simplification of composite deep-water sedimentary systems, from a depositional complex of contourite depositional systems, mass-wasting deposits and canyons, on the one hand, to only sliding and canyon deposits on the other hand.     

Sediment transport in Washington and Norfolk submarine canyons

A sediment study suggests that Washington and Norfolk canyons off the Mid-Atlantic States are not inactive, but have served periodically since the Late Pleistocene as conduits of sediment originating on the adjacent shelf and upper slope. Large quantities of sand occur in the canyon heads as thin beds and laminae, and on the continental slope as mixtures of sand (to $\text{>40\%}$), silt and clay that are extensively reworked by burrowing organisms. Sandy turbidites occur in the canyons on the rise. Basinward dispersal, from the outer shelf and uppermost slope, is recorded by heavy mineral suites and bioclastic components, primarily foraminifera of shallow marine origin, in the lower slope and upper continental rise canyon cores. The down-axis movement of material, presumably episodic, in the Holocene to recent results from offshelf spillover into canyon heads, failure on the steep walls bordering canyons on the slope, and resuspension by bottom currents.     

An investigation of diel synchronicity between water column marine snow concentration and the flux of organic matter in the Santa Barbara Channel,California

Diel variation in the concentration of marine snow (detrital aggregates >0.5 mm) in the surface ocean has been documented at several locations, but it is not clear whether this water column signal translates into a diel pulse in particle flux out of the upper mixed layer. In this field study we investigated the temporal relationship between the concentration of marine snow in the upper water column and carbon (C) flux as measured by a sediment trap at 100 m in the Santa Barbara Channel, CA. Camera profiles of marine snow displayed two opposing patterns: (1) higher nighttime total (i.e. cumulative) aggregate volume and (2) higher midday total aggregate volume. Increased nighttime total aggregate volume was only observed during a brief study in 1999 and was associated with increased daytime C flux. For the remaining deployments midday increases in total aggregate volume were observed but, depending on the deployment, were associated with (a) higher nighttime C flux, (b) higher daytime C flux, or (c) no diel pattern. Correspondence between water column aggregate concentration and sediment trap flux increased when average aggregate size exceeded a threshold volume of 0.5 mm³ (1.0 mm in diameter). Particles caught in sediment traps generally accounted for a small percentage of decreased marine snow particulate organic carbon (POC) in the upper 100 m. Other aggregate loss terms such as macrozooplankton grazing may dominate. When diel patterns in particle flux did occur, changes between day and night samples ranged from small (14%) to large (>200%). Diel variations in particle flux may impact mid-water and benthic ecology particularly animal grazing strategies, waste production, and reproductive cycles. Pulsed sedimentation may also create patchy vertical distributions of particle-associated biota and remineralization products and pulsed food delivery to the benthos.     

Sediment Stripping Correction to Marine Gravity Data

The knowledge of the bedrock topography (instead of the ocean-floor relief) is required in various geoscience studies investigating the evolution and structure of the oceanic lithosphere. The gross density structure and thickness of marine sediments were obtained from ocean drilling data or seismic surveys. Alternatively, marine gravity data corrected for the ocean and sediment density contrasts can be used for a detailed mapping of the bedrock topography. In this study, we compute and apply the sediment stripping correction to marine gravity data. The sediment density distribution is approximated by a 3-D density model derived based on the analysis of density samples from the Deep Sea Drilling Project. Methods for a spherical harmonic analysis and synthesis are utilized in computing the sediment stripping correction. Results show that this correction varies between 0 and 32 mGal. We also demonstrate that the approximation of heterogeneous sediment structures by a uniform density model yields large errors. The spectral analysis reveals a high correlation (>0.75) between the sediment-stripped marine gravity data and the bedrock topography. The application of the sediment stripping correction to marine gravity data enhanced the gravitational signature of the sediment-bedrock interface.     

Mass movements,geotechnical properties,and slope stability in the outer shelf—Upper slope,northwestern Aegean sea

Abstract

Subbottom profiling (3.5 kHz) in the uppermost slope of the outer shelf of the northwestern Aegean Sea has shown downslope slumping and intense sediment deformation as well as a succession of microslumping within the Late Pleistocene delta sequences. Postdeposition bottom erosion by currents and the overconsolidated sediments (Cu/Po > 1) indicate the termination of deformations a long time ago. Deformation processes are estimated to have been active 18,000–10,000 B.P. Large‐scale active downslope deformations with clockwise sediment block rotation have affected the area as well. These longer‐period deformations are associated with regional neotectonics and older unconformities (i. e., Plio‐Pleistocene) as potential slide planes.

Geotechnical properties reflect mainly textural variations and locally (within sapropelic layers) high organic matter content. Relatively high values of shear strength were measured (5–29 kPa) with intermediate sensitivities (2–5), whereas relatively low values of water content (33–81%) were found along the five selective sediment cores.

Infinity slope stability analysis revealed that the slopes in the study area are most stable, although theoretical evaluations (Cu estimated from linear regression analysis) indicate relative instability for the potential glide plane at 20 m depth. The outer shelf is covered by compact relict sands, and their gentle and low‐angle (<0.2°) slopes are the most stable region of the investigated area.     

南海珠江口盆地陆坡稳定性评价

In nature, a slope stability is determined by the ratio of a sliding resistance to a slide force. The slide force of a marine deep-water continental slope is mainly affected by sediment mechanics properties, a topography, and a marine seismic. However, the sliding resistance is mainly affected by sedimentary patterns and a sedimentary stress history. Both of these are different from case to case, and their impact can be addressed when the data are organized in a geographic information system（GIS）. The study area on the continental slope in Zhujiang River Mouth Basin in South China Sea provides an excellent opportunity to apply GIS spatial analysis technology for the evaluation of the slope stability. In this area, a continental slope topography and a three-dimension（3-D） topography mapping show a sea-floor morphology and the distribution of a slope steepness in good detail, and the sediment analysis of seabed samples and an indoor appraisal reveals the variability of a sediment density near the sea-floor surface. On the basis of the results of nine geotechnical studies of submarine study areas, it has worked out that an equivalent cyclic shear stress ratio is roughly between 0.158 and 0.933, which is mainly depending on the initial water content of sediment. A regional density, slope and level of anticipated seismic shaking information are combined in a GIS framework to yield a map that illustrates a continental slope stability zoning under the influencing factors in Zhujiang River Mouth Basin in the South China Sea. The continental slope stability evaluation can contribute to north resources development in the South China Sea, the marine functional zoning, the marine engineering construction and adjust measures to local conditions, at the same time also can provide references for other deep-water slope stability analysis.     

Geotechnical characterization of slump scars,eroded slopes,and correlated deposits on continental margins: Gulf of Lions,Western Mediterranean sea

Abstract

Vertical variations of geotechnical properties in the uppermost sediment layers characterize the main sedimentary processes acting on the construction and destruction of progressive‐type continental slopes. In the Gulf of Lions, the original thicknesses and distribution of the uppermost sedimentary layers of the continental slope and rise, which consist of Holocene muds overlying Pleistocene muds, have been greatly modified by erosion and several kinds of slope failure processes. Each process is typified through sets of geotechnical properties measured in the eroded or slumped sections and in the associated sediment accumulations.

In slump scars, the water‐rich Holocene muds lie on fine, overconsolidated, Pleistocene muds with high plasticity and low shear strength. In bottom current‐eroded slopes, where modern sedimentation is extremely reduced, the Pleistocene muds frequently outcrop and may sometimes be overlain by a very thin layer of Holocene muds. The Pleistocene muds of eroded slopes are overconsolidated and more silty and less plastic than the Pleistocene muds from slopes affected by slope failure, their shear strength being 10 times greater.

Deposits at the toe of slumps are very often formed by several superposed three‐layer units (triplets of interstratified Holocene, transitional, and Pleistocene layers) issued from retrogressive slumping occurring in the slump scars above their head area. The main body of each layer is then relatively undisturbed, showing the usual burial geotechnical gradients due to overburden pressure (i.e., decrease of water content and increase of unit weight and shear strength). At the toe of bottom current‐eroded slopes, a thick and homogeneous layer of Holocene muds overlies the Pleistocene muds; this Holocene layer has unappreciable burial depth gradient of its geotechnical parameters because of a high rate of modem and continuous deposition.     

Depth Correlated Benthic Faunal Quantity and Infaunal Burrow Structures on the Slopes of a Marine Depression

In the northern part of the Kattegat, western Sweden, a series of marine depressions remain since the last glaciation. One of these, the well-oxygenated Alkor Deep, is about 3 km long and 800 m wide and with a depth of 138 m. Random depth-stratified sampling was made along four transects on the slopes including benthic macrofauna (0·1 m²grab samples) and sediment profile imaging. A significant positive correlation was found between depth and the faunal variables abundance and biomass. Deposit feeders such as Maldane sarsi, Heteromastus filiformis andAbra alba were among the dominants and may have been supported by down-slope advected organic material. In many images, pockets and extensive burrows were seen in the sediment that appeared to be constructed by the crustaceans Calocaris macandreae and Maera loveni. The ecological significance of their irrigation of the sediment is discussed. Due to the faunal activity deep down in the sediments of the slopes, the mean apparent redox potential discontinuity (RPD) was found as deep as between 8·0 and 11·3 cm depth, and RPD was significantly positively correlated with water depth. On the slopes there appears to be a balance between the input of organic material and the capacity of the benthic organisms to assimilate that carbon.     

Natural abundances of 15N as a source indicator for near-shore marine sedimentary and dissolved nitrogen

Because organic matter originating in the euphotic zone of the ocean may have a distinctive nitrogen isotope composition (¹⁵N/¹⁴N), as compared to organic matter originating in terrestrial soils, it may be used to evaluate the relative nitrogen contribution to marine and estuarine sediment. The nitrogen isotope ratios of 42 sediment samples of total nitrogen and 38 dissolved pore-water ammonium samples from Santa Barbara Basin sediment cores were measured. The range of δ¹⁵N values for total nitrogen was $\text{+2.89 – +9.4‰}$ with a mean of $\text{+6.8‰}$ and for pore water ammonium, $\text{+8.2 – +12.4‰}$ with a mean of 10.2‰.The results suggest that the dissolved ammonium in the pore water is produced from bacterial degradation of marine organic matter. The range of δ¹⁵N values for total nitrogen in the sediment is interpreted as resulting from an admixture of nitrogen derived from marine ($\text{+10‰}$) and terrestrial (+2‰ marines. The marine component of this mixture, composed principally of calcium carbonate with smaller amounts of opal and organic matter, contains ～ 1.0% nitrogen. The terrestrial component, which comprises over 80% of the sediment, contains ～ 0.1% organically bound nitrogen and accounts for > 25% of the total nitrogen in Santa Barbara Basin sediment.     

Parameterization of nonlinear shallow water waves over sloping bottoms

Investigation of the bottom slope effects on the nonlinear transformation of irregular waves, which are generated based on JONSWAP spectra, is carried out in a physical wave flume with three slopes (β = 1/15, 1/30, 1/45). The slope effects on the estimation of representative wave height are examined first. To obtain a better estimation of wave height, the slope effect should be considered when slope is larger than 1/30. The nonlinear parameters (bicoherence, skewness and asymmetry) are estimated by using the wavelet-based bispectrum, and the empirical formulae regarding these nonlinear parameters as a function of the local Ursell number are derived based on the present data measured on each slope. The results indicate that the slopes have a negligible effect on the variations of the skewness. The fitted coefficients of the formulae for the other parameters on slope β = 1/15 are clearly different from the results on the slopes β = 1/30 and 1/45, indicating that slope influence on the parameterization cannot be ignored when β > 1/30. Hence, new formulae considering the slope effect are presented. Furthermore, the empirical formulae for the data in surf zone are recommended.     

陆架斜坡对内孤立波的动力响应特性试验研究

针对内孤立波在行进过程中遇到海底斜坡会对海底产生力的作用,不同坡度斜坡对内孤立波的动力响应应该存在差异.本文通过水槽中制造内波,对不同角度的斜坡对内孤立波的动力响应过程进行了研究.结果表明,内孤立波通过陆架斜坡上方,会造成斜坡沉积物超孔隙水压力的积累;在相同振幅条件下,缓坡沉积物动力响应的幅度比陡坡沉积物大;随着振幅的...     

Morphosedimentary features and recent depositional architectural model of the Cantabrian continental margin

Multibeam bathymetry, high (sleeve airguns) and very high resolution (parametric system-TOPAS-) seismic records were used to define the morphosedimentary features and investigate the depositional architecture of the Cantabrian continental margin. The outer shelf (down to 180–245 m water depth) displays an intensively eroded seafloor surface that truncates consolidated ancient folded and fractured deposits. Recent deposits are only locally present as lowstand shelf-margin deposits and a transparent drape with bedforms. The continental slope is affected by sedimentary processes that have combined to create the morphosedimentary features seen today. The upper (down to 2000 m water depth) and lower (down to 3700–4600 m water depth) slopes are mostly subject to different types of slope failures, such as slides, mass-transport deposits (a mix of slumping and mass-flows), and turbidity currents. The upper slope is also subject to the action of bottom currents (the Mediterranean Water — MW) that interact with the Le Danois Bank favouring the reworking of the sediment and the sculpting of a contourite system. The continental rise is a bypass region of debris flows and turbidity currents where a complex channel-lobe transition zone (CLTZ) of the Cap Ferret Fan develops.The recent architecture depositional model is complex and results from the remaining structural template and the great variability of interconnected sedimentary systems and processes. This margin can be considered as starved due to the great sediment evacuation over a relatively steep entire depositional profile. Sediment is eroded mostly from the Cantabrian and also the Pyrenees mountains (source) and transported by small stream/river mountains to the sea. It bypasses the continental shelf and when sediment arrives at the slope it is transported through a major submarine drainage system (large submarine valleys and mass-movement processes) down to the continental rise and adjacent Biscay Abyssal Plain (sink). Factors controlling this architecture are tectonism and sediment source/dispersal, which are closely interrelated, whereas sea-level changes and oceanography have played a minor role (on a long-term scale).     

Active slope failure,sediment collapse,and silt flows on the modern subaqueous Huanghe (Yellow River) delta

Post-depositional slope instability and bottom mass-movement processes strongly modify the progradational subaqueous slopes of the modern Huanghe (Yellow River) Delta. Wide, shallow gullies dissect the submarine slopes with gradients of 0.3 to 0.4°. Lower delta-front sediments experiencein situ subsidence, forming numerous collapse depressions. These processes are pronounced over much of the delta, incising and redistributing the most recently deposited silt-rich sediment. Principal causative factors include low sediment strengths created by rapid deposition in the delta during annual peak discharges from the river and severe bottom perturbations by surface storm-generated waves.     

Bulk density calibration for X-ray tomographic analyses of marine sediments

A calibration procedure is presented for generating bulk density profiles of intact marine sediment cores using X-ray computed tomography (CT). Developed using both artificial and natural marine sediment samples, the correlation is strong with an R ² value of 0.98. Similar experiments with carbonate sediments were also strongly linear, but offset from the terrigenous curve, illustrating the influence of sediment chemistry on scanner response. Comparison between newly computed densities for both sediment types with those from individually calibrated cores during our previous CT studies reveals good correspondence between the data sets. Received: 22 November 1998 / Revision received: 4 March 1999     

Morphological Features of Co-Rich Manganese Deposits and Their Relation to Seabed Slopes

Abstract

The morphological features associated with Co-rich manganese deposits, the size variations of nodules, and the occurrence of different substrates have been analyzed, to evaluate the influence of various seabed slope angles on the distribution of these features. The coverage and size of the crusts depend on their surface morphology and seabed topography, resulting in cobble-type, lineated, or step-like outcrops. Small nodules (1–4 cm in diameter) dominate all seabed slopes, with a few locations having nodules ranging from 1 to 8 or 1 to 10 cm. Sediments invariably occur as substrates for nodules and as cover for crusts, their coverage being inversely proportional to that of the nodules and crust outcrops.

Steeper seafloor areas have large crust outcrops exposed with no or few nodules and sediments associated with them. The intermediate slopes have a combination of nodules, sediments, and crusts in various proportions, depending on topography and gradient. Large-scale nodule occurrences, followed by sediment fields and crust outcrops on seabed slopes of < 3°, 3–7°, and > 15°, respectively, represent typical morphological distribution zones of the Co-rich manganese deposits on a seamount in the central Pacific Ocean. A transition zone between nodule-dominated fields and large crust outcrops occurs for slopes from 7° to 15°. This detailed study on distribution of Co-rich deposits gives a better understanding for purposes of their exploitation.     

Continental margin off Sergipe and Alagoas, northeastern Brazil: A reconnaissance geophysical study of morphology and structure

The stable continental margin of northeastern Brazil is unusually narrow, probably because of the small size and tropical character of the drainage basins of the hinterland, and correspondingly low rates of land erosion and marine sedimentation. The continental shelf, which is mainly a marine erosion surface, is also remarkably shallow, either because of upwarping or, more probably, because of the ineffectiveness of Pleistocene marine erosional processes on steeply sloping continental margins. Sediment accumulation is confined to the Sāo Francisco delta, seaward of which are fossil (?) lagoonal deposits, and to a poorly developed nearshore sand prism.The margin formed by seaward progradation of sediment on a subsiding basement, but the present morphology of the continental slope reflects chiefly Pleistocene canyon cutting and mass gravitational movements of sediment, which have exposed older strata in the upper slope. Beneath the continental slope is a magnetic anomaly (like the slope anomaly off the eastern U.S.A.), probably caused by a deeply buried dike of oceanic basalt, and apparently associated with a buried ridge which may have formed the seaward margin of the Sergipe—Alagoas Basin during the early history of the South Atlantic. Similar structures may be typical of the narrow easternmost part of the Brazilian margin.     

Return to ranger submarine slide,Baja California,Mexico

Ranger Slide is a modest (12 km³) slide deposit of Pliocene and younger sediment on the continental slope in northern Sebastian Vizcaino Bay, Mexico. A limited survey using a deeply-towed instrument shows that hummocky terrain immediately downslope from the slide scar consists of large blocks of semiconsolidated sediment, some exceeding a kilometer in length and 10⁷ m³ in volume. Most blocks have rotated, fallen apart, and/or deformed during movement. The form, structure, and processes related to emplacement of the blocks within the hummocky topographic zone of Ranger Slide may be common to many submarine slides on slopes involving semiconsolidated, terrigenous sediment.