Flocculation and sedimentation in the ACE Basin, South Carolina

Extremely rapid deposition of fine-grained sediment has been observed near an artificially created channel between the Edisto and Ashepoo Rivers in South Carolina. Accumulation rates on the order of 0.05 m mo⁻¹ formed a region of fluid mud known as the Mud Reach that was present during a study carried out in May 1998. Subsequent surveys in March and May 1999 found no evidence of fluid mud in this area. Image analysis of photographs of the suspended sediment from 1998 showed that the water column was populated by flocs with a median diameter on the order of 0.3 mm which tended to remain in suspension over the tidal cycle, and flocs on the order of 0.8 mm which remained in the near bottom region except at maximum current speeds. In 1999, both the size and concentration of flocs at the surface remained the same, but the size and abundance of flocs near the bottom was reduced. Disaggregated inorganic grain size analysis of the suspension and samples from a core collected in the Mud Reach indicated that in 1998 the accumulation of material on the bottom was the result of floc settling and that in spite of highu _s values, little sorting of this material was occurring. Grain size analysis of samples deeper in the core showed that the rapidly floc-deposited mud was underlain by coarser, well-sorted sediment, confirming that the Mud Reach was a transient feature. The observed flocculation processes may play a significant role in the trapping of fine-grained sediment throughout this region.     

Late quaternary sediments in Lake Zürich,Switzerland

Lake Zürich occupies a glacially overdeepened perialpine trough in the northern Middlelands of Switzerland. A total of 154.4 m of Quaternary sediments and 47.3 m of Tertiary Molasse bedrock has been cored from the deepest part of the lake, some 10 km south of the city of Zürich. Some 16.8 m of gravels and sands directly overlying the bedrock include basal till and probably earliest subglacial fluvial and lacustrine deposits. These are overlain by 98.6 m of fine-grained, glacial-aged sediments comprising completely deformed proglacial and/or subglacial lacustrine muds, separated by four basal mud tills. The lack of interglacial sediments, fossils, and other datable material, and the presence of severe sediment deformation and unknown amounts of erosion prevent the establishment of an exact chronostratigraphy for sediments older than the upper mud till. Above it some 8.6 m of lacustrine muds were deposited, folded, faulted, and tilted during the final opening of the lake at about 17,500–17,000 years ago. Superimposed are 30.4 m of final Würm and post-glacial sediments comprising (from oldest): cyclic proglacial mud, thick-bedded and laminated mud, a complex transition zone, laminated carbonate, laminated marl, and diatom-calcite varves. These sediments reflect changing catchment and lacustrine conditions including: glacial proximity, catchment stability, lake inflow characteristics, thermal structure, chemistry, and bed stability. Average sedimentation rates ranged from 11 cm yr⁻¹ immediately after glacier withdrawal, to as low as 0.4 mm yr⁻¹ as the environment stabilized. The lack of coarse outwash deposits separating the fine-grained glaciolacustrine sediments from a corresponding underlying basal till suggests that deglaciation of the deep northern basin of Lake Zürich was by stagnation-zone retreat rather than by retreat of an active ice-front.     

Selenium Behavior in San Francisco Bay Sediments

Sediment and porewater samples (1997–1999) were collected in the Northern Reach of the San Francisco Bay and Sacramento–San Joaquin Delta for determinations of sedimentary selenium and its chemical speciation. Total sedimentary selenium increased with depth, with approximately 50% of the sedimentary selenium as elemental selenium and 35% as organic selenide. Porewater total dissolved selenium increased with depth in the estuary and Delta, and fluxes out of the sediments were calculated at 0.01 and 0.06 nmol cm⁻² year⁻¹ for the estuary and Delta, respectively. Present-day sediment–water exchange of dissolved selenium and internal transformations cannot explain the observed increase in total sedimentary selenium with depth. However, mass balance calculations demonstrate that the increase in total selenium with depth may be linked to higher dissolved selenium concentrations in the water column in the 1980s, suggesting that the sediments could be used as historical recorders of selenium in the estuary.     

Remotely sensed observations of the turbidity maximum in the highly turbid Humber Estuary, UK

The Humber Estuary, UK, divides into the Ouse and Trent estuaries at the so-called Apex within its upper reaches. Remotely sensed Compact Airborne Spectrographic Imager (CASI) images and boat measurements were used to observe a strong turbidity maximum in the upper Humber and Ouse during a spring tide in November 1995. Surface suspended particulate matter (SPM) concentrations during the late ebb, as estimated from the CASI data, increased from approximately 6 to 13 g I⁻¹ moving up-estuary into the Ouse. Greater SPM concentrations (∼10 g I⁻¹) were evident in the deeper channels of the Ouse, compared with shallower areas, possibly due to faster ebb currents there and differential down-estuary advection of the turbidity maximum. Ribbons, or streaks, of lower SPM and slightly cooler waters were observed. It appears that slightly cooler and lower turbidity waters from the confluent Trent estuary remained fairly distinct for distances of approximately 2 km down-stream of its confluence with the upper Humber and Ouse. These waters eventually broke into ribbon-like or streak-like structures within the higher SPM-laden and slightly warmer waters of the Humber. They were discernible for more than 5 km down-estuary of the confluence of the Humber, Ouse, and Trent. Boat measurements showed that the turbidity maximum occurred over a fairly restricted region of the upper Humber, between about 20 to 50 km from the tidal limit at high water. The turbidity maximum’s sediment load was largely suspended in the water column during stronger currents. SPM rapidly settled close to the bed during high water and low water slack periods. At these times, SPM concentrations in a thin, near-bed layer were >60 g I⁻¹ in the turbidity maximum region of the Ouse and >30 g 1⁻¹ in the upper Humber (where channel volumes were much greater). SPM within the turbidity maximum comprised very fine-grained material and its low organic content demonstrated that the SPM was essentially mineral, clastic sediment derived originally from erosion and decay of crustal rocks.     

Diurnal variation in rates of calcification and carbonate sediment dissolution in Florida Bay

Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution. On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production (or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO₃ m⁻²d⁻¹. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210 to −1.900 g CaCO₃ m⁻² night⁻¹. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration. Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr⁻¹ and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida Bay.     

赣江南昌段江心洲沉积特征

通过赣江南昌段一江心洲的实地考察,分析了江心洲不同部位沉积物特征、落淤层发育特征、江心洲发育过程。江心洲头部水动力最强,沉积物较粗,连续性最好;侧翼也以粗粒沉积物为主,但含有大量泥砾;顶部和尾部发育横向连续的落淤层。落淤层易板结,边缘龟裂,被河水打碎搬运。保留下来的落淤层分布局限,头部和侧翼均不易保存,顶部和尾部保存较好。另外,保存下来的落淤层近于水平,对非均质性的影响主要为垂向的,侧向连通性好。     

全新世渤海泥质沉积物地球化学特征

对渤海泥质区柱状沉积物的粒度与化学成分的分析及AMS 14C测年数据表明,全新世以来本区泥质沉积物的物质来源与沉积环境较为稳定.R型因子分析得出三种主要组合类型,以Ca、Ti和Mn为代表,分别对应黄河物质影响、陆源细粒物质输入和海洋自生作用.其中西南部与中部泥形成于约6000 a B.P.以来,黄河物质影响较强,主要在潮余流的作用下搬运后沉积下来.北部泥自早全新世之前便已开始形成,且主要沉积于高海面之前,受滦河物质作用相对较强.南部和北部泥质沉积年代存在明显差异,受水深、离岸距离、海平面高度等多种因素影响.尽管距黄河口较近,但区内沉积速率远低于中国近海其他泥质体,这与渤海环流状况及泥沙运移方向关系密切.     

Benthic Oxygen Consumption and Organic Matter Turnover in Organic-poor, Permeable Shelf Sands

The high permeability of sediments and strong near-bottom currents cause seawater to infiltrate the surface layers of Middle Atlantic Bight shelf deposits. In this study, sandy sediment cores from 11 to 12 m water depth were percolated with filtered seawater on shipboard. Sedimentary oxygen consumption (SOC) increased non-linearly with pore water flow, approaching maximum rates of 120 mmol m⁻² d⁻¹ (May 2001) or 75 mmol m⁻² d⁻¹(July 2001). The addition of acetate to the inflowing water promptly enhanced the release of dissolved inorganic carbon (DIC) from the cores. DIC production rates were a linear function of acetate concentration, ranging from 100 to 300 mmol m⁻² d⁻¹ without substrate addition to 572 mmol m⁻² d⁻¹ with 100 mM acetate. The sediments also hydrolyzed a glucose pseudopolymer, and the liberated glucose prompted an increase of SOC. Our results suggest that decomposition rates of organic matter in permeable sands can exceed those of fine-grained, organic-rich deposits, when water currents cause advective interstitial flow, supplying the subsurface microbial community with degradable material and electron acceptors. We conclude that the highly permeable sand beds of the Middle Atlantic Bight are responsive within minutes to hours and efficiently operate as biocatalytical filters.     

Speciation and phytoavailability of heavy metals in sediments in Nanjing section of Changjiang River

Heavy metal pollution and their fractionations in the sediments of Changjiang River in Nanjing Reach was monitored for cadmium (Cd), lead (Pb), zinc (Zn), chromium (Cr), and copper (Cu). Moreover, the biological enrichment of metals by riverine plants was studied. The results demonstrated there were highly significant variations among different sampling stations for the concentrations of tested metals. The highest range was for Cu (38.8–120.4 mg kg⁻¹), followed by Cr (74.4–120.0 mg kg⁻¹), Zn (80.9–121.1 mg kg⁻¹), Ni (26.0–55.5 mg kg⁻¹), Pb (15.8–46.7 mg kg⁻¹) and Cd (0.28–0.48 mg kg⁻¹). Cd was the element with highest biological enrichment factor (BEF). The highest BEF of Cd in Erigeron bonariensis reached 3.0, indicating a significant Cd enrichment in this aquatic plant. In addition, 60% of Cd was found in reducible fraction and exchangeable and acid-soluble fraction, which was consistent with its high mobility. The consistency of Cd fraction in sediment and suspended particle indicated they came from the same source. Accumulated Cd concentration calculated according to the release curve showed significant relativity with the total Cd concentration in the sediment.     

Sedimentation of particulate matter in the Dona Paula Bay, West Coast of India during November to May 1995–1997

Data on hydrography, nutrients, suspended particles, and sedimented particles were collected at weekly intervals from November to May during 1995 to 1997 at a station in the coastal waters of Dona Paula Bay, India. Suspended and sedimented particles were analyzed for total suspended matter (SPM), total sedimented particulate matter (TPM), particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophylla (chla), and diatom abundance. Variations in hydrography and nutrients influenced the quantity and composition of sedimented particles. The TPM, POC, PON, and chla fluxes showed small-scale seasonal variations and were higher in the summer (February to May) than in the winter (November to January). Resuspension of carbon accounted for approximately 25% of the gross POC and was highest in April 1997 (45%). The mean net POC flux was 197±90 mg C m⁻² d⁻¹ and accounts for 4.6% of the TPM. The average C∶N (w∶w) ratio of the sedimented material was 13.2±6.6. The POC:chla ratio was relatively higher in the sedimented material as compared to the suspended material. The particulate carbon reaching the bottom sediment was 39% of the primary production. The low organic carbon concentration (approximately 0.1% of dry sediment) in the sediments implies that about 98% of the sedimented carbon was either consumed at the sedimentwater interface or resuspended/advected before it was finally buried into the sediments.     

Distinction between sortable silts and aggregated particles in muddy intertidal sediments of the East Frisian Wadden Sea, southern North Sea

In muddy sediments, the distinction between sortable silt and aggregated silty clay is important for the understanding of fine particle dynamics because both have different hydraulic properties. The Wadden Sea of the southern North Sea is severely depleted in fine-grained sediments mainly due to high energy levels along the diked coastline. As a result, muddy sediments are restricted to a narrow belt along the diked mainland shore. In the present study, the mechanism by which this mud is deposited and how floc deposition and break-up are reflected in the size distribution, has been investigated. For this purpose, surficial sediments from four intertidal nearshore transects were monitored and repeatedly sampled in the course of two years. High-resolution grain-size analyses were performed by an automated settling tube and a Sedigraph particle analyser for the sand and mud fractions, respectively.The grain size frequency distributions of the fine fractions demonstrate that the Wadden Sea muds are composed of two subpopulations, a well-sorted coarse silt and an unsorted silty clay population. A depletion of grain size around 8 μm (7 phi) demarcates the grain-size boundary between the two populations, suggesting that the finer mud population (< 8 μm) is deposited in the form of flocs and aggregates which are hydraulically equivalent to the local sands and coarser silts. Floc break-up and reconstitution in response to seasonally changing energy regimes lead to apparent seasonal sedimentation patterns in the back-barrier tidal basins. Furthermore, in the course of sample preparation, the flocs and aggregates are broken down into their constituent particles. This mechanical artefact in the size distributions produces an artificial seasonal fining/coarsening pattern. It was found that the comparison of clay/silt and < 8 μm/63 − 8 μm particle ratios are good indicators of floc behaviour. Higher ratios are found in mixed flats which are relatively protected from wave action, thereby promoting deposition of flocs. In addition, progressive size sorting and mixing processes along the transects are recognized in the frequency distributions of sands. The skewness pattern shows a landward decrease in positive values, which is exactly opposite to previously reported patterns, suggesting progressive winnowing of fine particles caused by increased wave action over the last decade. This effect of climate change further promotes depletion of fine-grained sediments in the basin.     

Water quality analysis of a freshwater diversion at Caernarvon, Louisiana

Since 1991, Mississippi River water has been diverted at Caernarvon, Louisiana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km² of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, total Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton Sound from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and decreased salinity concentrations in the estuary due to the diversion. Spring and summer water quality transects indicated rapid reduction of nitrite + nitrate and total suspended sediment concentration as diverted Mississippi River water entered the estuary, suggesting near complete assimilation of these constituents by the ecosystem. Loading rates of nitrite + nitrate (5.6–13.4 g m⁻² yr⁻¹), total nitrogen (8.9–23.4 g m⁻² yr⁻¹), and total phosphorus (0.9–2.0 g m⁻² yr⁻¹) were calculated along with removal efficiencies for these constituents (nitrite + nitrate 88–97%; total nitrogen 32–57%; total phosphorus 0–46%). The low impact of the diversion on water quality in the Breton Sound estuary, along with assimilation of TSS over a very short distance, suggests that more water may be introduced into the estuary without detrimental affects. This would be necessary if freshwater diversions are to be used to distribute nitrients and sediments into the lower reaches of the estuary, in an effort to compensate for relative sea-level rise, and reverse the current trend of rapid loss of wetlands in coastal Louisiana.     

Mercury enrichments in core sediments in Hugli–Matla–Bidyadhari estuarine complex,north-eastern part of the Bay of Bengal and their ecotoxicological significance

Mercury concentrations (Hg_T) in fine-grained fraction (<63 μm) of core sediments of the Hugli–Matla–Bidyadhari estuarine complex, India were analyzed. Results revealed a wide range of spatial variations (<4–93 ng g⁻¹ dry weight) with a definite enhancement level at the lower stretch of the estuarine complex infested with mangrove plants, which might act as a sink to Hg_T. An elevated concentration of Hg was encountered in surface/subsurface layer of the core in majority of the cases resulting from physical, biogenic and postdepositional diagenetic processes that remobilized and resuspended the metal from deeper sediments. A strong positive correlation was observed between the Hg and clay fraction content of the sediments, while the correlations of Hg with Al, Fe and Mn were poor. Based on the index of geoaccumulation (I _geo) and effects range-low (ER-L) value, it is considered that the sediments are less polluted and thus there is less chance of ecotoxicological risk to organisms living in sediments.     

A computer-interfaced sedigraph for modal size analysis of fine-grained sediment

ABSTRACT
A system has been developed allowing the direct storage on disc of size data from the X-ray scanning settling tube SediGraph 5000ET. Processing of the cumulative percentage data output to produce smooth frequency curves is described. This allows rapid (15 min) and precise (±M 0.1ø) discrimination of size modes, peak heights, median and silt and clay percentages of fine-grained sediments. An example of a mud turbidite is given showing systematic changes in bulk and modal properties, not visible by eye. Lack of size segregation in the > 1 m thickness of this turbidite shows that it may have been deposited from a highly concentrated flow, rather than from a dilute, waning turbidity current. Such flows may explain ungraded/poorly graded, muddy sediments also known as homogenites, unifites or megaturbidites by other authors.     

Whiting-related sediment export along the Middle Miocene carbonate ramp of Great Bahama Bank

Modern aragonite needles are present all along the modern leeward margin of Great Bahama Bank (ODP Leg 166), while Middle Miocene sediments contain needles only in more distal areas (Sites 1006 and 1007). In contrast to the rimmed, flat-topped platform topography during the Plio-Pleistocene, the Miocene Great Bahama Bank morphology is a carbonate ramp profile. This might imply a different location and precipitation type for Miocene aragonite needles. In this study, aragonite needles in Miocene sediments were isolated using a granulometric separation method. Furthermore, the isolation of the various carbonate components enables the identification of primary versus diagenetic components. The Miocene aragonite needles are concentrated in the finest granulometric sediment fractions (<12 μm). The fraction-specific geochemical analyses (δ¹³C, δ¹⁸O and Sr elemental abundance) represent useful tools to assess the possible sources of the aragonite mud. The geochemical variation of the fractions, rich in pristine aragonite needles, and the characteristics of the needle morphology point to whiting phenomena as the main sediment source and algal fragmentation as a minor component. Both components indicate shallow-water environments as the main sediment source area. Ramp-top-related fine-grained particles now present at distal sites were likely exported as suspended material similar to present-day transport mechanisms. The scarcity of needles at proximal sites is probably linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded. The granulometric separation approach applied here enables a better characterization of the finest carbonate particles representing an important step towards the discrimination between primary and diagenetic fine-grained components.     

Nile floods recorded in deep Mediterranean sediments

Clastic mud beds rich in continental organic matter are observed recurrently in the Nile deep-sea turbidite system. They formed during flooding periods of the river similar to those that induce sapropel formation and occurred during periods of increased density stratification of the eastern Mediterranean. The very fine-grained flood deposits are intercalated within pelagic sediments, sapropels and Bouma-type turbidites. These flood deposits form by the successive reconcentrations of surface (hypopycnal) plumes by convective sedimentation, which in turn generate a fine-grained low-energy hyperpycnal flow. Sea-level high stands seem also to favor hypopycnal plume formation and increase clastic mud bed formation. Consequently, these muddy clastic beds provide a direct link between deep-marine sedimentary records and continental climatic change through flood frequency and magnitude.     

Dissolved fluoride in the Lower Ganges-Brahmaputra-Meghna River system in the Bengal Basin, Bangladesh

The dissolved fluoride (F⁻) in the Lower Ganges-Brahmaputra-Meghna (GBM) river system, Bengal basin, Bangladesh, was studied during 1991–1993 to determine its distribution and source in the basin, and its annual flux to the Bay of Bengal. The concentration of dissolved F⁻ varied between 2 and 11 μmol l⁻¹ with statistically significant variations both spatially and temporally in the basin. Such variations are attributable to the geology of the individual subbasins (Ganges, Brahmaputra and Meghna), dilution by rainwater during monsoon and groundwater contribution to the river systems during dry season. Correlation coefficients among F⁻ and major cations and anions suggest diverse inorganic processes responsible for regulating the concentration of F⁻ in these river systems. However, fluorite seems to be one of the major sources of dissolved F⁻. The concentration of F⁻ in the Lower GBM river system is low compared to the rivers draining Deccan Plateau and arid regions of the subcontinent, for example, Yamuna and its tributaries. However, it is within the range of most of the other Peninsular and Himalayan rivers. The GBM system contributes about 115×10³ tonnes year⁻¹ of dissolved F⁻ into the Bay of Bengal, and thus accounts for about 3% of the global F⁻ flux to the oceans annually. Received: 19 May 1999 · Accepted: 11 October 1999     

东海陆架泥质区沉积地球化学比较研究

利用东海陆架泥质沉积区和周围砂质沉积区的表层沉积物元素含量和海水悬浮沉积物总量分析资料,对泥质区的沉积地球化学进行了比较研究。结果表明东海不同泥质区的沉积地球化学过程很不相同。浙江近岸泥质区沉积物元素以Fe、Mn、Zn的含量高,Na、Sr、Al、Ca的含量低为特征,其地球化学特征与长江沉积物非常一致。近岸泥质区的沉积物基本来源于长江输入海的沉积物,在台湾暖流的作用下沉积在浙江沿岸。远岸济州岛西南泥质区沉积物以Na、Sr的高含量,较高的Al、Ca含量和低的Fe、Mn、Zn含量为特征,其地球化学特征与黄河源沉积物有好的相关性,但有一定的混合源性质,该泥质区的沉积物主要来源于黄河源的细颗粒沉积物,由黄海沿岸流搬运到该区并沉积下来。沉积物在搬运沉积过程中,元素粒度分异作用明显。东海陆架泥质区沉积地球化学过程的控制性影响因子是东海环流系统和物源,其中台湾暖流对东海陆架远岸和近岸泥质区的沉积地球化学分区具有关键作用。     

鄂尔多斯盆地南部三叠系延长组湖相重力流沉积细粒岩及其油气地质意义

随着页岩油气勘探开发和相关领域研究的不断深入,细粒沉积物的搬运和沉积已成为当前沉积学研究的热点问题之一,但中国中生代湖泊环境中的泥质重力流沉积尚未引起应有的关注。通过岩心观察、薄片鉴定等手段及综合研究,分析了鄂尔多斯盆地晚三叠世湖相泥质重力流沉积特征,探讨了其形成机制与成因分类。鄂尔多斯盆地三叠系延长组湖相泥页岩结构类型多样,发育泥质块体流沉积、泥质碎屑流沉积、泥质浊流沉积和泥质异重流沉积等多种重力流沉积类型。按照泥质含量将重力流划分为砂质重力流、泥质重力流和混合重力流3种亚类,并根据成因将重力流划分为滑塌体、碎屑流、浊流及异重流等4种亚类;结合成因和泥质含量,将重力流沉积共划分为12种类型。滑塌岩、碎屑岩分布于三角洲前缘斜坡脚附近;浊积岩、异重岩广泛分布于三角洲斜坡至沉积中心。认为泥质沉积物可以在强水动力条件下搬运-沉积;重力流沉积细粒物质在湖相沉积中占据很大的比例;泥质重力流对泥页岩中的碎屑物质、黏土矿物及有机质的搬运和沉积起到重要作用,因而对于页岩油气的生烃、储集性能和压裂工艺研究具有重要意义。     

Slope failures and stability analysis of shallow water prodeltas in the active margins of Western Greece,northeastern Mediterranean Sea

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10–15 m below seafloor. Shear planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the steeper (2^°–6^°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone. The foresets of the HST prodelta deposits display high water content (30–80%), low bulk density (1.4–1.9 g cm⁻³) and relatively low values of undrained shear strength (3–20 kPa). The water content of the HST distal muddy bottomsets is relatively higher (50–110%) and bulk density relatively lower (1.3–1.7 g cm⁻³) with low values of shear strength (2–10 kPa). The shear strength of the gas releasing sediment layer displays lower values (2–9 kPa) relative to the overlying, post failure, muddy sediments of the late 100–300 years. Slope stability was calculated using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments. This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6–29.6% g for the HST foresets and 12.4–14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs) for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments of the lower part of the HST foresets to the upper part of early HST bottomsets.