Present day sedimentary processes on the northern Iberian shelf

The Galicia-Minho Shelf features two large mud patches, the Douro and the Galicia Mud patches. These are recent sediment bodies that have accumulated under a combination of conditions including: (1) abundant supplies of sediment; (2) morphological barriers that act as sediment traps; and (3) hydrographic conditions that favour the accumulation of fine sediment in these sinks. This paper describes the mechanisms controlling the deposition of the fine-grained sediment depositions and the processes that result in resuspension processes on the Galicia-Minho Shelf.Fine-grained sediments are provided from discharges from the river basins on the southern sector of the shelf, mainly the Douro and Minho rivers. Sediments are exported from river estuaries onto the shelf during episodic flood events. In contrast, most of the sediments originating from the Galician hinterland fail to contribute significantly to sedimentation on the shelf, because they are retained in the Galician Rías, which function as sediment traps.Sediments deposited on the shelf are frequently remobilized, particularly during southwesterly storms that coincide with downwelling conditions. Once in suspension the fine-grained sediments are transported northwards by the poleward flowing bottom currents and are eventually deposited on the Douro and Galicia Mud patches after a series of resuspension events. The locations of the two mud patches are strongly influenced by the shelf morphology.Fines already deposited on the mud patches are occasionally reintroduced into the system by large storm events. Some material from the Douro Mud patch and adjacent areas is re-deposited in the Galicia Mud patch. It is probable that sediments re-suspended from the Galicia Mud patch are carried off the shelf when storm events coincide with downwelling conditions.     

Reconstruction of paleo-wave conditions during the Late Pleistocene from marine terrace deposits, Monterey Bay, California

The Santa Cruz coastal terrace fringes much of the northern Monterey Bay region, California. It consists mainly of a regressive sequence of high-energy, barred nearshore marine sediments deposited during the last (Sangamonian) highstand of sea level. This sequence can be sub-divided into several depth-dependent facies on the basis of paleo-current data and vertical sequence of sedimentary structures. These include a lower shoreface facies deposited in 10–16 m water depth, an upper shoreface facies (including both a storm-dominated assemblage and a surf zone assemblage) deposited in 0–10 m water depth, and a foreshore facies deposited in the swash zone, up to 3.5 m above high tide.

The magnitudes of the storm events responsible for depositing these sediments were estimated by calculating paleo-wave heights using a variety of criteria (e.g., critical threshold equations, breaker depths, berm heights). In addition, the climate and paleogeography during the deposition of these sediments were essentially the same as today, allowing the use of present-day wave statistics to estimate the frequency of these storm events. The largest storms formed offshore-flowing currents (e.g., rip, wind-forced, and possibly storm-surge ebb currents) that resulted in the deposition of approximately 30% of the sediments seaward of the surf zone; however, the magnitude and frequency of these events are unknown. The remaining 70% of the sediment beyond the surf zone was deposited in response to smaller storm waves which were, on the average, at least 1.6 m high; such waves presently occur no more than 15% of the time. Sediments deposited during “fairweather” conditions (i.e., the remaining 85% of the time) have a low preservation potential, and are generally not preserved in this facies. In contrast, surf zone sediments were deposited by a variety of processes associated with waves whose maximum offshore heights were probably ≤ 2.2 m; such waves presently occur up to 92% of the time. Sediments within the swash zone were deposited by waves up to 3 m high, the largest of which presently occur approximately 2% of the time.

Most of the sediments were deposited by storms of intermediate magnitude and frequency; different facies, however, appear to preferentially record events of different recurrence intervals. In particular, surf zone sediments were deposited under relatively small storm and post-storm conditions, whereas sediments deposited farther offshore record increasingly larger, less frequent storm events. Relatively rare events (e.g., the 100 or 1000 yr events) do not appear to have significantly affected sedimentation in these nearshore environments.     

长江河口浮泥形成机理及变化过程

1976年以来，在长江河口盐水楔和最大浑浊带活动的河道进行了20余次现场观测，本文在现场观测资料基础上，确认长江河口浮泥由细颗粒泥沙组成，中值粒径在8－11.5um,小于2um的粘土占28.18%-36.39%,长江河口浮泥是悬沙在盐水混合环境中絮凝沉降于近义廾风暴潮再悬浮泥沙形成的高浓度浑水层，在成因类型上分为憩流浮泥，盐水楔浮泥和风暴潮浮泥，第1种在涨或落潮转流期低流时形成，规模大，厚度薄，第2种在盐水楔发育时形成，规模较小，厚度较大，第3种在大风后形成，规模大，厚度薄，第2种在盐水楔发育时形成，规模较小，厚度较大，第3种在大风后形成，规模大，范围广，若三者相遇，则浮泥厚度和范围最大，浮泥具有枯季，大小潮秽暴周期变化规律，长江河口河道多的浮泥层，浮泥层的变化与河口拦门沙的冲淤有良好的正相关。     

Combined effects of waves and plants on a mud deposition event at a mudflat-saltmarsh edge in the Bahía Blanca estuary

This study was carried out at the Bahía Blanca Estuary, Argentina, at the seaward edge of a saltmarsh. The saltmarsh-mudflat boundary in the study area shows sediment deposits at a higher elevation immediately seaward of the saltmarsh edge. We compared field determinations of water velocity, bed shear stress, wind wave conditions and variations of the bed elevation in the mudflat and within the Spartina alterniflora canopy at the seaward edge of a saltmarsh, and we evaluated the relative role of vegetation in the observed morphology. A mud deposition event that raised bed elevation in more than 5 cm occurred during the study period, with TSS concentrations > 500 mg l⁻¹, but simultaneous measurements performed on the bed levels confirmed that the sediments deposited did not originate from local resuspension within the edge of the canopy. In similar tidal cycles in terms of local wave activity and bed shear stresses at the sampling site, deposition occurred only with winds aligned with the azimuth of the Canal Principal, reaching a maximum fetch of more than 20 km in front of the sampling site.     

An overview of sedimentation on the amazon continental shelf

Sedimentological, geochemical, and physical-oceanographic studies of the Brazilian continental shelf near the Amazon River help provide a broad understanding of this major sediment dispersal system. Amazon River sediment accumulates as a subaqueous delta, with the most rapid accumulation (10 cm/yr) occurring near the seaward edge of the topset beds and in the foreset beds. Amazon River sediment is dispersed northwestward along the shelf and is transported beyond the Brazilian border. Radiographic studies of sediment cores delineate three sedimentary environments: interbedded mud and sand, faintly laminated mud, and bioturbated mud. The distribution of these environments is a function of proximity to the river mouth and of sediment accumulation rate.     

Sheet flow and suspension of sand in oscillatory boundary layers

after revisionTime-dependent measurements of flow velocities and sediment concentrations were conducted in a large oscillating water tunnel. The measurements were aimed at the flow and sediment dynamics in and above an oscillatory boundary layer in plane bed and sheet-flow conditions. Two asymmetric waves and one sinusoidal wave were imposed using quartz sand with D₅₀ = 0.21 mm. A new electro-resistance probe with a large resolving power was developed for the measurement of the large sediment concentrations in the sheet-flow layer. The measurements revealed a three layer transport system consisting of a pick-up/deposition layer, an upper sheet flow layer and a suspension layer.In the asymmetric wave cases the total net transport was directed “onshore” and was mainly concentrated in the thin sheet flow layer (< 0.5 cm) at the bed. A small net sediment flux was directed “offhore” in the upper suspension layer. The measured flow velocities, sediment concentrations and sedimenl fluxes showed a good qualitative agreement with the results of a (numerical) 1DV boundary-layer flow and transport model. Although the model did not describe all the observed processes in the sheet-flow and suspension layer, the computational results showed a reasonable agreement with measured net transport rates in a wide range of asymmetric wave conditions.     

A snapshot of suspended sediment and fluid mud occurrence in a mixed-energy embayment, Tijucas Bay, Brazil

Along the southern Brazilian coast, Tijucas Bay is known for its unique muddy tidal flats associated with chenier plains. Previous field observations pointed to very high suspended sediment concentrations (SSCs) in the inner parts of the bay, and in the estuary of the Tijucas River, suggesting the presence of fluid mud. In this study, the occurrences of suspended sediments and fluid mud were examined during a larger-scale, high-resolution 2-day field campaign on 1–2 May 2007, encompassing survey lines spanning nearly 80 km, 75 water sampling stations for near-bottom density estimates, and ten sediment sampling stations. Wave refraction modeling provided qualitative wave energy estimates as a function of different incidence directions. The results show that SSC increases toward the inner bay near the water surface, but seaward near the bottom. This suggests that suspended sediment is supplied by the local rivers, in particular the Tijucas. Near-surface SSCs were of the order of 50 mg l⁻¹ close to the shore, but exceeded 100 mg l⁻¹ near the bottom in the deeper parts of the bay. Fluid mud thickness and location given by densimetry and echo-sounding agreed in some places, although being mostly discordant. The best agreement was observed where wave energy was high during the campaign. The discrepancy between the two methods may be an indication for the existence of fluid mud, which is recorded by one method but not the other. Agreement is considered to be an indication of fluidization, whereas disagreement indicates more consolidation. Wave modeling suggests that waves from the ENE and SE are the most effective in supplying energy to the inner bay, which may induce the liquefaction of mud deposits to form fluid mud. Nearshore mud resuspension and weak horizontal currents result in sediment-laden offshore flow, which explains the higher SSCs measured in the deeper parts of the bay, besides providing a mechanism for fine-sediment export to the inner shelf.     

Facies, internal structures and sequences of modern Gironde-derived muds on the Aquitaine inner shelf, France

Seventy cores from the Aquitaine continental shelf were examined using radiographic and grain-size techniques in order to describe the sedimentary structures of the muddy deposits, and to evaluate their depositional processes. Four lithofacies are identified in this fine-grained deposit: (a) homogeneous silty sand, (b) interbedded homogeneous mud and sand, (c) silty-clayey mud, and (d) mottled mud. They show a logical pattern in relationship to the water depth and the distance from the coast.

Primary structures are present particularly in the landward and central portion of the mud fields, where the sediment is organized into sequences with a sharp-based erosional contact, overlain by a fining-up succession (centimetre to decimetre scale). The beginning of each of these is characteristic of a high-energy storm event, which is common on this shelf. The settling of suspended fine sediment corresponds to the flood estuarine discharge during quiet periods. Primary sedimentary structures decrease in the distal area where the muddy sediment is frequently reworked by infauna. Finally, primary structures and their preservation depend on the relative magnitudes of surface waves, storms, infaunal mixing and fluvial sediment deposition rates (i.e. floods).     

Sediment flux and equilibrium slopes in a barred nearshore

Estimates of time-integrated values of total (ITVF) and net (INVF) sediment volume flux and the associated changes in bed elevation and local slope were determined for a crescentic outer nearshore bar in Kouchibouguac Bay, New Brunswick, Canada, for eight discrete storm events. A 100 × 150 m grid of depth-of-activity rods spaced at 10 m intervals was used to monitor sediment behaviour on the seaward slope, bar crest and landward slope during the storms, at which time winds, incident waves and near-bed oscillatory currents were measured. Comparisons between storm events and between these events and a longer-term synthetic wave climatology were facilitated using hindcast wave parameters. Strong positive correlations between storm-wave conditions (significant height and total cumulative energy) and total volume flux contrasted strongly with the zero correlation between storm-wave conditions and net volume flux. ITVF values ranged up to 1646 m³ for the experimental grid and were found to have power function relations with significant wave height (exponent 2) and cumulative wave wave energy (exponent 0.4); values of INVF ranged from 0 up to 100 m³ for the same grid indicating a balance of sediment volume in the bar form through time. Sediment reactivation increased linearly with decreasing depth across the seaward slope and bar crest reaching maxima of 20 cm for the two largest storms; bed elevation, and thus slope, changes were restricted to the bar crest and upper landward slope with near zero morphological change on the seaward slope. The latter represents a steady-state equilibrium with null net transport of sediment under shoaling waves. Measurements of the asymmetry of orbital velocities close to the bed show that the energetics approach to predicting beach slope of Inman and Bagnold (1963) is sound. Gradients predicted vary from 0.01 to 0.03 for a range of angles of internal friction appropriate to the local sediment (tan ø = 0.3–0.6). These compare favorably with the measured seaward slope of 0.015 formed under average maximum orbital velocities of 1.12 m s⁻¹ (landward) and 1.09 m s⁻¹ (seaward) recorded during the period of the largest storm waves.     

渤海动力对黄河入海泥沙输移的影响

海洋动力过程对黄河入海泥沙的淤积和扩散具有决定性作用 ,利用三维斜压动力 -泥沙模型分析了黄河口外海域潮流动力变化与黄河入海泥沙淤积、扩散特性之间的响应关系。季节环流以及涨落潮的分布和变化很好地解释了黄河口南北浮泥的形成和消长变化以及黄河入海泥沙向海扩散的机理。结果表明 ,入海泥沙在洪季、枯季和表底层都具有不同的输移特性     

风暴过程中潮滩悬沙浓度和悬沙输运的变化及其动力机制——以长江三角洲南汇潮滩为例

悬沙浓度是淤泥质海岸重要的环境指标。为探讨潮滩悬沙浓度和悬沙输运对风暴事件的响应过程及其动力机制,于2014年9月"凤凰"台风过境前、中、后在长江三角洲南汇潮滩进行了现场观测,获得同步高分辨率的水深、波高、近底流速和浊度剖面时间序列(9个潮周期)。结果表明,风暴中平均和最大波高、波-流联合底床剪切应力、悬沙浓度和悬沙输运率可比平静天气高数倍;风暴期间高潮位低流速阶段悬沙沉降导致近底发育数十厘米厚的浮泥层(悬沙浓度大于10 g/L)。研究认为风暴事件中淤泥质海岸悬沙浓度和悬沙输运的剧烈变化其根本动力机制是风暴把巨大能量传递给近岸水体,进而显著增大波-流联合底床剪切应力,导致细颗粒泥沙再悬浮。     

Response of the suspended sediment transport system to continental shelf dynamics

Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to the slope and deep basins.     

绿色海堤的沉积地貌与生态系统动力学原理: 研究综述*

绿色海堤是传统结构工程与海岸生态系统共同组合而成的新型海堤, 用以应对未来海面上升、风暴加剧给低地海岸防护带来的挑战。需解决的问题主要有海岸生态系统消浪过程及生态系统在海堤体系中的配置方式。理论分析、现场观测、物模数模所获结果表明, 海岸生态系统确有显著的消浪功能: 1) 陆架泥区消浪, 其机制以再悬浮和浮泥运动为主, 底部摩擦为次; 2) 潮滩下部的粉砂细砂滩底部摩擦和推移质运动共同造成波能耗散, 而上部的泥滩则以再悬浮和悬沙输运为主; 3) 在盐沼、红树林、海草床等由植被构成的生态系统, 植物通过形态阻力、茎秆运动来阻滞水流、耗散波能, 其效能高于沉积物床面对波能的耗散; 4) 生物礁主要有珊瑚礁和牡蛎礁, 其消能作用主要通过床面摩擦和波浪破碎, 效能较高, 尤其是在风暴期间。生态系统如何成为海堤的有机组成部分, 尤其是侵蚀型海岸的生态位修复和绿色海堤整体设计, 还需进一步研究相关的科学问题: 与硬质工程结合的盐沼-牡蛎礁的适应性生物学; 未来环境变化条件下生态系统的稳定性; 绿色海堤生态系统空间配置及其与风暴事件的时间尺度匹配; 基于均衡剖面理论的海堤形态优化。     

Atmospheric forcing of fine-sand transport on a low-energy inner shelf: south-central Louisiana,USA

Hydrodynamic and sediment transport measurements from instrumentation deployed during a 54-day winter period at two sites on the Louisiana inner shelf are presented. Strong extratropical storms, with wind speeds of 7.8 to 15.1 m s^-1, were the dominant forcing mechanism during the study. These typically caused mean oscillatory flows and shear velocities about 33% higher than fair weather (averaging 12.3 and 3.2 cm s^-1 at the landward site, and 11.4 and 2.7 cm s^-1 at the seaward site, respectively). These responses were coupled with mean near-bottom currents more than twice as strong as during fair weather (10.3 and 7.5 cm s^-1 at the landward and seaward sites, respectively). These flowed in approximately the same direction as the veering wind, causing a net offshore transport of fine sand. Weak storms were responsible for little sediment transport whereas during fair weather, onshore sand transport of approximately 25-75% of the storm values appears to have occurred. This contradicts previous predictions of negligible fair-weather sediment movement on this inner shelf.     

Storm-generated,hummocky stratification on the outer-Scotian Shelf

Hummocky megaripples occur on Sable Island Bank, Scotian Shelf. Submersible observations show that the megaripples form during winter storms and are subsequently obliterated through bioturbation and fair-weather reworking. The megaripples of this study were underlain by a storm bed composed of: (A) a basal scoured and infilled gravel lag facies; (B) low-angle tangential crossbedding in gravel to coarse sand; (C) anisotropic hummocky stratification in medium sand; and (D) wave ripple cross-lamination in medium/fine sand. This sequence forms a tempestite bed created by a winter storm during our sampling program. Numerical simulation of bed conditions during the storm suggests that the hummocky megaripples and hummocky stratification formed together during late stages of storm decay from conditions of sheet flow. Near-bed wave motion during deposition exceeded steady currents by an order of magnitude.     

Hybrid sediment gravity flow deposits – Classification, origin and significance

The deposits of subaqueous sediment gravity flows can show evidence for abrupt and/or progressive changes in flow behaviour making them hard to ascribe to a single flow type (e.g. turbidity currents, debris flows). Those showing evidence for transformation from poorly cohesive and essentially turbulent flows to increasingly cohesive deposition with suppressed turbulence ‘at a point’ are particularly common. They are here grouped as hybrid sediment gravity flow deposits and are recognised as key components in the lateral and distal reaches of many deep-water fan and basin plain sheet systems. Hybrid event beds contain up to five internal divisions: argillaceous and commonly mud clast-bearing sandstones (linked debrite, H3) overlie either banded sandstones (transitional flow deposits, H2) and/or structureless sandstones (high-density turbidity currents, H1), recording longitudinal and/or lateral heterogeneity in flow structure and the development of turbulent, transitional and laminar flow behaviour in different parts of the same flow. Many hybrid event beds are capped by a relatively thin, well-structured and graded sand–mud couplet (trailing low-density turbulent cloud H4 and mud suspension fallout H5). Progressive bed aggradation results in the deposits of the different flow components stacked vertically in the final bed. Variable vertical bed character is related to the style of up-dip flow transformations, the distance over which the flows can evolve and partition into rheological distinct sections, the extent to which different flow components mutually interact, and the rate at which the flows decelerate, reflecting position (lateral versus distal) and gradient changes. Hybrid beds may inherit their structure from the original failure, with turbidity currents outpacing debris flows from which they formed via partial flow transformation. Alternatively, they may form where sand-bearing turbidity currents erode sufficient substrate to force transformation of a section of the current to form a linked debris flow. The incorporation of mud clasts, their segregation in near-bed layers and their disintegration to produce clays that can dampen turbulence are inferred to be key steps in the generation of many hybrid flow deposits. The occurrence of such beds may therefore identify the presence of non-equilibrium slopes up-dip that were steep enough to promote significant flow incision. Where hybrid event beds dominate the entire distal fan stratigraphy, this implies either the system was continually out of grade in order to freight the flows with mud clasts and clays, or the failure mechanism and transport path repeatedly allowed transmission of components of the initial slumps distally. Where hybrid beds are restricted to sections representing fan initiation, or occur more sporadically within the fan deposits, this could indicate shorter episodes of disequilibrium, due to an initial phase of slope re-adjustment, or intermittent tectonically or gravity-driven surface deformation or supply variations. Alternatively, changes between conventional and hybrid event beds may record changes in the flow generation mechanism through time. Thus the vertical distribution of hybrid event beds may be diagnostic of the wider evolution of the fan systems that host them.     

Waves, currents, sediment flux and morphological response in a barred nearshore system

A shore-normal array of seven, bi-directional electromagnetic flowmeters and nine surface piercing, continuous resistance wave staffs were deployed across a multiple barred nearshore at Wendake Beach, Georgian Bay, Canada, and monitored for a complete storm cycle. Time-integrated estimates of total (ITVF) and net (INVF) sediment volume flux together with bed elevation changes were determined using depth-of-activity rods.

The three bars, ranging in height from 0.10 to 0.40 m accreted during the storm (0.03 m), and the troughs were scoured (0.05 m). Sediment reactivation depths reached 0.14 m and 12% of the nearshore control volume was mobilized. However, the INVF value for the storm was less than 1% of the control volume revealing a near balance in sediment volume in the bar system. Landward migration of the inner, crescentic and second, sinuous bars occurred in association with an alongshore migration of the bar form itself; the outermost, straight, shore-parallel bar remained fixed in location.

The surf zone was highly dissipative throughout the storm (ε = 3.8 × 10²–192 × 10²) and the wave spectrum was dominated by energy at the incident frequency. Spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave, respectively. The former spectral peak was within the 95% confidence band for the spectrum while the latter contributed not more than 10% to the total energy in the surface elevation spectrum even near the shoreline.

During the storm wave height exceeded 2 m (H_s) and periods reached 5 s (T_{p k}): orbital velocities exceeded 0.5 m s⁻¹ (u_{rm s}) and were above the threshold of motion for the medium-to-fine sands throughout the storm. Shore-parallel flows in excess of 0.4 m s⁻¹ were recorded with maxima in the troughs and minima just landward of the bar crest.

The rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore-normal velocity field. These hydrodynamic parameters represent “steady” flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period. Increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential (erosion); decreases in these values spatially cause a decreasing net transport potential and thus deposition. These transport potentials are increased or decreased through the gravity potential induced by the local bed slope. Shore-parallel flow was important in explaining sediment flux and morphological change where orbital velocities, mean flows and flow asymmetries were at a minimum.     

辽东半岛东岸近海泥区悬沙浓度的时空分布及控制因素分析

鸭绿江作为典型的中小型河流,细颗粒沉积物从鸭绿江流域到辽东半岛东岸近海泥区的源汇过程有着独特的机制,而遥感解译是了解入海沉积物在河口-陆架输运过程和机制的有效手段之一。因此,本文通过在辽东半岛东岸海域进行了现场悬沙浓度测量,并与GOCI影像建立关系,反演了该地区2011年4月至2017年3月间的表层悬沙浓度,从而揭示了该海域表层悬沙浓度的时空特征,分析了潮流、大风天气和洪水对悬沙浓度变化的影响,在此基础上探讨了研究区的悬沙浓度输运格局。结果显示,研究区表层悬沙浓度空间差异大,河口附近是悬沙浓度高值区,可达500 g/m3以上,其他区域则悬沙浓度多小于20 g/m3。另外,研究区表层悬沙浓度枯季高,洪季低,有着显著的季节性变化。进一步分析表明,枯季大风天气引起的风浪作用增强,是导致表层悬沙浓度增高的主要原因;洪季虽然悬沙浓度较低,但流域洪水期间的悬沙浓度显著增加。此外,作为典型的中小型源-汇传输体系,沉积物从鸭绿江到辽东半岛东岸近海泥区总体上仍遵循"夏储冬输"的输运机制。     

Time and density-dependent properties of fluid mud suspensions,NE brazilian continental shelf

Fluid mud suspensions, defined as containing between 10 to 480 g/liter, occur in numerous estuarine and nearshore continental shelf environments. The quantities of sediment incorporated in fluid mud are enormous, and they must be considered to be of major importance in the transport and deposition of fine-grained sediment in these environments.This study was performed on the NE Brazilian Continental Shelf, a muddy coastline that reaches from the Amazon to the Orinoco, a distance of 1600 km. The fluid mud forms a thick (3 to 5 m) boundary layer that extends 100-km offshore and generally coincides with the 10-m depth contour.The fluid muds appear well-adjusted to the current regime on the NE Brazilian continental shelf. Near-bottom current velocities were seldom greater than 50 cm/sec during the sampling interval. Consequently, yield stress in fluid muds of 1.20 × 10³ kg/m³ and greater was seldom exceeded, and consolidation proceeded without interruption.     

The infralittoral prograding wedge: a new large-scale progradational sedimentary body in shallow marine environments

A progradational sedimentary body, the infralittoral prograding wedge (IPW), has been developing from the mean fair-weather wave-base level to the storm wave-base level between the onshore (beach) and the offshore (inner continental shelf) depositional zones along the Spanish coast during the Late Holocene. The main sedimentary body is composed of large inclined master beds which prograde seawards parallel to the shoreline, formed by sediments swept offshore by waves from shallow-water littoral environments. The inclined beds downlap onto finer-grained offshore sediments and, in turn, are overlain by shoreface deposits. The IPW is generated by downwelling storm currents and associated seaward transport of sediment. It represents a new depositional model for clastic wave-dominated coasts, and its identification requires a new subdivision of the nearshore environment. Received: 10 June 1999 / Revision accepted: 15 February 2000