Channelled erosion through a marine dump site of dredge spoils at the mouth of the Puyallup River, Washington State, USA

Channels are relatively common on river-mouth deltas, but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant (hyperpycnal) flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front. Channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in Commencement Bay, at the mouth of the Puyallup River. Shallow channels on the seaward upper surface of the dump site, away from any flows that could have been produced by delta front failures, suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform. The form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile. It suggests that the channel evolved by its steep front retreating, rather than by simple vertical entrenchment or diffusive-like evolution of the profile, a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom-travelling flows. Model calculations based on shear strengths back-calculated from the geometry of channel wall failures suggest that, if the main channel were eroded solely by hyperpycnal flows, their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure. Besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows, it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms.     

Numerical modeling of hyperpycnal flows in an idealized river mouth

Numerical experiments in an idealized river mouth are conducted using a three-dimensional hydrodynamics model (EFDC model) to examine the impacts of suspended sediment concentration (SSC), settling velocity of sediment and tidal mixing on the formation and maintenance of estuarine hyperpycnal flows. The standard experiment presents an illustrative view of hyperpycnal flows that carry high-concentrated sediment and low-salinity water in the bottom layer (>1.0 m in thickness) along the subaqueous slope. The structure and intra-tidal variation of the simulated hyperpycnal flows are quite similar to those previously observed off the Huanghe (Yellow River) mouth. Results from the three control experiments show that SSC of river effluents is the most important parameter to the formation of hyperpycnal flows. High SSC will increase the bulk density of river effluents and thus offset the density difference between freshwater and seawater. Low SSC of river effluents will produce a surface river plume, as commonly observed in most large estuaries. Both the settling velocity of sediment particles and the tidal mixing play an important role in maintaining the hyperpycnal flows. Increasing settling velocity enhances the deposition of sediment from the hyperpycnal layer and thus accelerates the attenuation of hyperpycnal flows, whereas increasing tidal mixing destroys the stratification of water column and therefore makes the hyperpycnal flows less evident. Our results from numerical experiments are of importance to understand the initiation and maintenance of hyperpycnal flows in estuaries and provide a reference to the rapidly decaying hyperpycnal flows off the Huanghe river mouth due to climatic and anthropogenic forcing over the past several decades.     

High-frequency turbidity currents in British Columbia fjords

The frequency of turbidity currents in Bute Inlet and Knight Inlet (British Columbia, Canada) was monitored. A prototype instrument (turbidity event detector) was deployed adjacent to prominent incised sea-floor channels. Approximately 25–30 turbidity currents occur annually. They appear closely correlated to periods of higher river discharge into the heads of the fjords. Two peaks in both discharge and turbidity current fequency occur, one in response to snow melt in late June–early July, the other to glacier melt in August. Virtually no turbidity currents were observed in winter. River mouth bars, channel deposits, and other deltaic sediments build up during lower discharge periods and are swept onto the steep delta front and into subaqueous channels, along with bedload, during floods.     

黄河清8出汊河口泥沙异重流特征及其时空分布

选用1996年9月5—6日黄河口新口门水文泥沙同步观测资料,采用距离倒数加权插值法(IDW)生成河口泥沙异重流的含沙海水密度、流速和含沙量随时间变化的等值线图。基于此,分析了黄河口泥沙异重流发育形成的时空变化特征,并结合遥感影像和水下地形图进一步探讨了泥沙异重流对河口沉积的影响。研究结果表明,黄河口(新口门)泥沙异重流发育频繁,且其形成时的含沙海水密度与老河口相似;泥沙异重流的空间分布与水下三角洲泥沙堆积体及河口沙嘴的形态存在明显相关,泥沙异重流的存在直接影响河口前缘泥沙的沉积,但对侧缘的影响不大;泥沙异重流形成初期,在低高潮涨潮阶段中的一次较小的涨落潮周期内其形成和消亡过程与潮流的涨落潮周期呈正向对应关系,且在这一时段的涨憩时泥沙异重流的泥沙含量达到最大;流速垂向分布为泥沙异重流形成初期流速小于上层海水流速,且层次分明,随其进一步向深海运动流速呈增大再减小的趋势,至三角洲前缘陡坡段其底层流速大干中上层海水流速。     

Geomorphology of the western Ionian Sea between Sicily and Calabria,Italy

In the westernmost Ionian Sea lies a steep, tectonically active marine basin influenced by turbidity currents generated by terrigenous river input from the adjacent mountains and strong tidal currents propagating through the Strait of Messina. Like many young marine rifts, the basin is lined by steep streams draining the uplifting coasts and supplying sediment across narrow shelves. However, unlike many rifts, this basin is semi-enclosed. The present study explores the seabed morphology and sediment structures in this complex environmental setting, based on multibeam sonar, chirp profiler and seismic reflection data collected in 2006. Offshore channels include many that can be directly linked to onshore streams, suggesting that hyperpycnal flows are important for their formation. Near the Strait of Messina in depths shallower than 400 m, the channels are subdued, plausibly explained as an effect of strong tidal currents. The Messina Channel is characterised by abundant mass-wasting features along its outer bends, particularly on the Calabrian side. Coincidence of the channel course with faults suggests that the channel is structurally controlled in places. The chirp profiles generally show only shallow penetration, the evidence for coarse texture being consistent with the steep gradient of the basin that inhibits deposition from turbidity currents. By contrast, some locally discontinuous mounds exhibiting layered sub-bottom reflectors in the chirp profiles are interpreted as modern levee deposits formed from channelised turbidity current overspill. Overall, this semi-enclosed basin shows little evidence of substantial accumulations associated with modern turbidity current activity, any contemporaneous sediment supply evidently bypassing the area to be deposited in the Ionian Trench; as a consequence, this trench should be an archive of local slope failure and flood events.     

Sedimentary geology of the Columbia River Estuary

A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m³s⁻¹). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging.Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5 phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5–2.0phi; 0.25–0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75–6.0phi; 0.016–0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephemeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions.The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6–8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations.A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these enviroments to the ecosystem of the estuary is discussed in subsequent papers in this volume.     

Geomorphology, sedimentary processes and development of the Zenisu deep-sea channel, northern Philippine Sea

The Zenisu deep-sea channel originates on the Izu-Ogasawara island arc, and disappears in the Shikoku Basin of the Philippine Sea. The geomorphology, sedimentary processes, and the development of the Zenisu deep-sea channel were investigated on the basis of swath bathymetry, side-scan sonar imagery, submersible observations, and seismic data. The deep-sea channel can be divided into three segments according to the downslope gradient and channel orientation. They are the Zenisu Canyon, the E–W fan channel, and the trough-axis channel. The sediment fill is characterized by turbidite and debrite deposition and blocky–hummocky avalanche deposits on the flanks of the Zenisu Ridge. In the Zenisu Canyon and the Zenisu deep-sea channel, sediment transport by turbidity currents generates sediment waves (dunes) observed during the Shinkai 6500 dive 371. The development of the Zenisu Canyon is controlled by a N–S shear fault, whereas the trough-axis channel is controlled by basin subsidence associated with the Zenisu Ridge. The E–W fan channel was probably affected by the E–W fault and the basement morphology.     

Internal tides and sediment movement on Horizon Guyot,Mid-Pacific Mountains

Internal tidal currents are the likely cause of erosional features such as current ripples, sand waves, and truncated bedding horizons on the sediment cap of Horizon Guyot. Current meter data obtained over a 9 month period in 1983–1984 at about 213 m above the guyot show that the tidal currents are anomalously strong for mid-oceanic depths, probably the result of topographically induced generation of internal tidal waves. An analysis of the initiation of motion of the foraminiferal sand by the internal tidal currents indicates that these currents, particularly during the months of March–May, are likely to transport the surficial sediment and generate the observed bedforms.     

Field expressions of the transformation of debris flows into turbidity currents, with examples from the Polish Carpathians and the French Maritime Alps

Various transformation mechanisms can generate turbidity currents from subaqueous debris flows. Different transformation mechanisms have been described and interpreted in the past from laboratory experiments and from deposits, but the two approaches have not generally been linked. This has made the genetic interpretation and comparison of deposits difficult. In this paper a generic classification scheme of debrite–turbidite couplets is proposed based on transformation mechanisms inferred from laboratory experiments. Five different flow types (called A–E herein) and their resulting deposits are detailed, but they are all part of a continuous spectrum, and a mixture of types is likely to be found in the field. Type A flows are strong, dense debris flows that undergo little transformation. Their deposit will be a debrite overlain by a thin turbidite, which is separated from it by a clear grain size break. Type B flows are weaker and can develop waves at the debris flow-turbidity current interface. The deposit will be a debrite with a wavy top overlain by a turbidite that is thicker than for type A flows. For type C flows, the interfacial waves will grow so much that the debris flow disintegrates into separate parts. The deposit will consist of debrite lenses encased in a turbidite. Type D flows will undergo even more mixing than type C flows so that the debrite parts will be mixed. Their deposit will be a turbidite with laterally varying areas of debrite characteristics near the bed. Type E flows will be so transformed that the debris flow character has disappeared and the flow is a turbidity current with high sediment concentration. The deposit will be largely turbiditic. The flow types and deposits will be illustrated with some examples from two field areas: the Polish Carpathians and the French Maritime Alps.     

Past and present sedimentary activity in the Capbreton Canyon, southern Bay of Biscay

Located in the south-eastern part of the Bay of Biscay, the Capbreton Canyon incises the continental shelf up to the 30 m isobath contour, and acts as a natural conduit for continental and shelf-derived sediments. EM1000 multibeam bathymetry shows two main features characterising the canyon — a deeply entrenched meandering channel, bordered by fluvial-like terraces constituting large sediment traps. A dataset of cores and seismic profiles together with a multibeam bathymetry map has enabled the characterisation of recent sedimentary activity in the axial channel and on the terraces. Data analysis evidenced the major role of the canyon head in recent sediment dynamics. This part of the canyon is a temporary reservoir for sediments, accumulated by coastal hydrodynamic processes. Exceptional climatic, tectonic or hydrodynamic events can mobilise the sediments and generate gravity-driven flows. Under the present-day sea-level highstand conditions, these flows are not powerful enough to bring their bedload to the deep sea, and are confined mainly to the upper part of the canyon. Turbidity currents model the axial channel pathway and are at the origin of terrace formation. Terraces in the Capbreton Canyon are not typical but rather are reduced to confined levees. Three factors control the vertical growth of a terrace: (1) the amount of overspilled sediments brought by turbidity currents, (2) hemipelagic sedimentation and (3) terrace height. The amount of sediment spilling over a terrace decreases with increased terrace elevation. Concurrently, the proportion of hemipelagic fallout depositing on a terrace increases. Terraces are considered to be fossil when the height of the terrace prevents further deposition by overspilling. The terraces studied in this paper are interpreted as having formed during the Holocene, implying that the sediment dynamics of the Capbreton Canyon is continuous through time. Highstand periods differ from lowstand periods because they show a decrease in the energy of erosive processes. Temporal variations in erosive and depositional processes in the canyon are controlled by the Adour River, which delivers large amounts of sediment to the system.     

Turbidite deposition and the development of canyons through time on an intermittently glaciated continental margin: The Bonanza Canyon system,offshore eastern Canada

Bonanza Canyon is a complex canyon system on the slope from the intermittently glaciated Grand Bank on the south side of Orphan Basin. A 3D seismic reflection volume, 2D high-resolution seismic reflection profiles and ten piston cores were acquired to study the evolution of this canyon system in relation to glacial processes on the continental shelf and the effects of different types of turbidity currents on the development of deep water channels. Mapped reflector surfaces from the 3D seismic volume show that the Bonanza Canyons developed in a depression created by a large submarine slide of middle Pleistocene age, coincident with the onset of glacigenic debris flows entering western Orphan Basin. Two 3–5 km wide, flat-floored channels were cut into the resulting mass-transport deposit and resemble catastrophic glacial meltwater channels elsewhere on the margin. Both channels subsequently aggraded. The eastern channel A became narrower but maintained a sandy channel floor. The western channel, B, heads at a spur on the continental slope and appears to have been rather passively draped by muds and minor sands that have built 1500-m wave length sediment waves.Muddy turbidites recorded by piston cores in the channel and on the inter-channel ridges are restricted to marine isotope stage (MIS) 2 and were deposited from thick, sheet-like, and sluggish turbidity current derived from western Orphan Basin that resulted in aggradation of the channels and inter-channel ridges. Sandy turbidites in channels and on inner levees were deposited throughout MIS 2–3 and were restricted to the channels, locally causing erosion. Some coincide with Heinrich events. Channels with well-developed distributaries on the upper slope more readily trap the sediments on Grand Bank to form sandy turbidity currents. Channel B dominated by muddy turbidity currents has wide and relatively smooth floor whereas channel A dominated by sandy turbidity currents has a sharp geometry.     

Inversely graded turbidite sequences in the deep Mediterranean: a record of deposits from flood-generated turbidity currents?

Turbidity currents generated during floods of small and medium rivers have been demonstrated to be an important process of sediment transport from continent to abyss. They produce fine-grained turbidite deposits. No deposit related to these flood-related turbidity currents has yet been described in the deep sea. In this paper, we present some unusual sandy to muddy turbidite beds cored in the Var turbidite system (NW Mediterranean). They show a coarsening-upward basal unit capped with a classical fining-upward unit which are related to the periods of increasing and decreasing discharge at the river mouth, respectively. The two units are separated by a contact which can be gradational to erosional. This intrabed contact is interpreted as resulting from erosion during peak flood conditions. This intrabed contact can be confused with classical basal contacts of turbidite beds. The frequency of hyperpycnal turbidite beds can be used to relate climatic changes inland to the deep-sea sedimentary record, as an increase corresponds to periods of enhanced flooding at the river mouth.     

Turbidity of waters over the Northwest Iberian continental margin

In OMEX-II-II, 9 cruises gathered optical data, principally by transmissometer. The distribution of optical turbidity caused by concentration of particulate matter (PMC) in the water column over the northern Iberian margin shows several features related to hydrography. It is concluded that a signal of PMC seen in Mediterranean Water (MW) found north of 42°N is not carried from its source at the Gibraltar Sill and Gulf of Cadiz because it is shown, using intermediate stations, that this turbid plume decays, mainly by fall out but also partly by mixing, to very low levels around southern Portugal. PMC maxima sometimes seen in MW on the northern Iberian margin are thus most likely to result from intermittent local resuspension by MW interacting with slope sediments. The highest turbidity is found over the upper slope and is the result of (i) shelf edge resuspension and off-shelf flow of turbid plumes, mainly between 100 and 300 m depth, and (ii) resuspension under the slope current aided by internal waves, in the depth range 500–800 m where the density gradient between ENACW and MW is maximal. Below the MW, flows are generally slow, and turbidity is low. The bottom nepheloid layer in deep water is also weak with PMC values <100 mg m-3. The focus of resuspension activity on the upper slope means that the region is an efficient exporter to the ocean of sediment that either escapes from the shelf or sinks to the bed from surface production. This accounts for upper slope sediments recorded in other studies as sandy or in places as rocky bottom.     

Sediment waves on the South China Sea Slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific Deep Water into the South China Sea

Using an integrated multi-beam bathymetry, high-resolution seismic profile, piston core, and AMS ¹⁴C dating data set, the current study identified two sediment wave fields, fields 1 and 2, on the South China Sea Slope off southwestern Taiwan. Field 1 is located in the lower slope, and sediment waves within it are overall oriented perpendicular to the direction of down-slope gravity flows and canyon axis. Geometries, morphology, and internal seismic reflection configurations suggest that the sediment waves in field 1 underwent significant up-slope migration. Field 2, in contrast, is located more basinward, on the continental rise. Instead of having asymmetrical morphology and discontinuous reflections as observed in field 1, the sediment waves in field 2 show more symmetrical morphology and continuous reflections that can be traced from one wave to another, suggesting that vertical aggradation is more active and predominant than up-slope migration.Three sediment wave evolution stages, stage 1 through stage 3, are identified in both field 1 and field 2. During stage 1, the sediment waves are built upon a regional unconformity that separates the underlying mass-transport complexes from the overlying sediment waves. In both of these two fields, there is progressive development of the sediment waves and increase in wave dimensions from the oldest stage 1 to the youngest stage 3, even though up-slope migration is dominant in field 1 whereas vertical aggradation is predominant in field 2 throughout these three stages.The integrated data and the depositional model show that the upper slope of the study area is strongly dissected and eroded by down-slope gravity flows. The net result of strong erosion is that significant amounts of sediment are transported further basinward into the lower slope by gravity flows and/or turbidity currents. The interactions of these currents with bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea and preexisting wavy topography in the lower slope result in the up-slope migrating sediment waves in field 1 and the contourites as observed from cores TS01 and TS02. Further basinward on the continental rise, turbidity currents are waned and diluted, whereas along-slope bottom (contour) currents are vigorous and most likely dominate over the diluted turbidity currents, resulting in the vertically aggraded sediment waves in field 2.The results from this study also provide the further evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in South China Sea at least since Quaternary.     

Reconstruction of turbidity currents in Amazon Channel

Quantifying the characteristics of the turbidity currents that are responsible for the erosion, lateral migration and filling of submarine channels maybe useful for predicting the distribution of lithofacies in channel fill and levee reservoirs. This paper uses data from a well-studied submarine channel in Amazon Fan in an attempt to reconstruct the velocity, thickness, concentration, duration, recurrence rates and vertical structure of turbidity currents in this long sinuous channel. Estimates of flow conditions are derived from the morphology of the channels and the characteristics of the deposits within them. In particular, the availability of information on the sediment distribution with respect to the channel topography at the time of deposition allows for insights into the vertical structure of the flow, a key property that has been so far poorly understood. Integration of flow constraints from well and seismic data or from detailed analysis of outcrop with numerical flow models is a critical step toward a complete understanding of the flow and associated deposits. Turbidity currents in sinuous submarine channels, exemplified by Amazon Channel, are found to last for tens of hours and occur on a regular, quasi-annual basis. Model results suggest that these flows had, on average, velocities ranging from 2 to 4 m/s in the canyon/upper fan which decreased to 0.5–1 m/s in the lower fan, travelling in excess of 800 km. The model turbidity currents were subcritical over most of the channel length, indicating a low degree of water entrainment and low rate of deceleration down the channel. The formation of such long, sinuous channels is intrinsically associated with frequent, long-duration, subcritical turbidity currents carrying a silt-dominated sediment load.     

Turbidites deposited on Southern Central Chilean seamounts: Evidence for energetic turbidity currents

Gravity cores obtained from isolated seamounts located within, and rising up to 300 m from the sediment-filled Peru–Chile Trench off Southern Central Chile (36°S–39°S) contain numerous turbidite layers which are much coarser than the hemipelagic background sedimentation. The mineralogical composition of some of the beds indicates a mixed origin from various source terrains while the faunal assemblage of benthic foraminifera in one of the turbidite layers shows a mixed origin from upper shelfal to middle-lower bathyal depths which could indicate a multi-source origin and therefore indicate an earthquake triggering of the causing turbidity currents. The bathymetric setting and the grain size distribution of the sampled layers, together with swath echosounder and sediment echosounder data which monitor the distribution of turbidites on the elevated Nazca Plate allow some estimates on the flow direction, flow velocity and height of the causing turbidity currents. We discuss two alternative models of deposition, both of which imply high (175–450 m) turbidity currents and we suggest a channelized transport process as the general mode of turbidite deposition. Whether these turbidites are suspension fallout products of thick turbiditic flows or bedload deposits from sheet-like turbidity currents overwhelming elevated structures cannot be decided upon using our sedimentological data, but the specific morphology of the seamounts rather argues for the first option. Oxygen isotope stratigraphy of one of the cores indicates that the turbiditic sequences were deposited during the last Glacial period and during the following transition period and turbiditic deposition stopped during the Holocene. This climatic coupling seems to be dominant, while the occurrence of megathrust earthquakes provides a trigger mechanism. This seismic triggering takes effect only during times of very high sediment supply to the shelf and slope.     

The Var turbidite system (Ligurian Sea, northwestern Mediterranean)—morphology, sediment supply, construction of turbidite levee and sediment waves: implications for hydrocarbon reservoirs

The Var turbidite system is a small sandy system located in the Ligurian Basin. It was deposited during the Pliocene-Quaternary in a flat-floored basin formed during the Messinian salinity crisis. The system was fed through time by the Var and Paillon canyons that connect directly to the Var and Paillon rivers. It is still active during the present sea-level highstand. Two main mechanisms are responsible for gravity-flow triggering in the Var turbidite system: (1) mass-wasting events affect mainly the upper part of the continental slope, in areas where volumes of fresh sediment delivered by rivers are highest, and result from the under-consolidation state of slope sediments and earthquakes, and (2) high-magnitude river floods resulting from melting of snow and convective rainfall during fall and spring seasons, and generating hyperpycnal turbidity currents at river mouths when the density of freshwater transporting suspended particles exceeds that of ambient seawater. Failure- and flood-induced gravity flows are involved through time in the construction of the Var Sedimentary Ridge, the prominent right-hand levee of the Var system, and sediment waves. Processes of construction of both the Var Ridge and sediment waves are closely connected. Sandy deposits are thick and abundant in the eastern (downchannel) part of the ridge. Their distribution is highly constrained by the strong difference of depositional processes across the sediment waves, potentially resulting through time in the individualization of large and interconnected sand bodies.     

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.     

Fan delta development and processes offshore a seasonal river in a seismically active region, NW Gulf of Corinth

High-resolution geophysical surveys (seismic, side-scan sonar) offshore of the Eratini River, a seasonally flowing river in the NW Gulf of Corinth, Greece, revealed a small fan delta with a variety of bottom features (blocky deposits, chutes and sediment instabilities). Considering the relatively small size of this river, however, these features could not be explained as being produced solely by river flow processes. Based on morphological features, the fan delta can be subdivided into a high- and a low-energy area. Sedimentation processes in the fan delta are associated with flood-derived sediment input, hyperpycnal flows which erode the fan surface, mud settling from suspension plumes, shelf sedimentation and sediment failures. The observed blocky deposits are considered to be the result of earthquake-induced mass flows in 1965 and 1995, whereas the chutes would be produced both by erosive mass flows and by hyperpycnal currents. The bulk block sediment volume has probably resulted from the 1965 earthquake. The 1965 evacuation zone and the related chutes were buried by the prograding fan delta. The main causative factor triggering the observed sediment instabilities is considered to be liquefaction, which is caused by (1) frequent earthquake-induced cyclic loading and (2) low sediment shear strengths created by rapid deposition during floods, in both cases associated with high pore-water pressures.