Effects of topography on lofting gravity flows: Implications for the deposition of deep-water massive sands

Hyperpycnal flows are generated in the marine environment by sediment-laden fresh water discharge into the ocean. They frequently form at river mouths and are also generated in proximal ice-melting settings and are thought to be responsible for transporting a large proportion of suspended river sediment onto and off the continental shelf. Hyperpycnal flows are an example of gravity currents that display reversing buoyancy. This phenomenon is generated by the fresh water interstitial fluid being less dense than that of the ambient seawater. Thus after sufficient particles are sedimented the flow can become positively buoyant and loft, forming a rising plume. Here we present results from physical scale-modelling experiments of lofting gravity currents upon interaction with topography. Topography, in the form of a vertical obstacle, triggered a localised lofting zone on its upstream side. This lofting zone was maintained in a fixed position until the bulk density of the flow had reduced enough to allow lofting along its entire length. The obstructed lofting zone is associated with a sharp increase in deposit thickness. By inference these experimentally established lofting dynamics are applied to improve understanding of the potential for hyperpycnal flows to deposit deep-water massive sands. This study provides a depositional mechanism by which large volumes of sand can be deposited in the absence of traction and the fines removed, leaving thick deposits of structureless sand with a low percentage of mud. This conceptual model for the first time provides a framework by which the geometries of certain deep-water massive sands may be predicted within specific depositional and basinal settings. This is crucial to our understanding of massive sand deposits in modern and ancient turbiditic systems and in the commercial evaluation of hydrocarbon potential of such sedimentary successions.     

Facies analysis of Pleistocene sediments from the Bannock Basin area: A key to the reconstruction of its tectonic evolution

Facies analysis was carried out on 21 selected cores from the Bannock area in order to investigate the relationships between sedimentation and tectonism. Bannock Basin is a large, > 3500 m (uncorrected) deep subcircular depression near the deformation front of the Mediterranean Ridge facing the Sirte Abyssal Plain. The basin is divided into several sub-basins aligned along a rim-syncline surrounding a central elevated area (salt dome?). High-density brines and anoxic sediments occupy the deepest part of the depressions.

The facies distribution is controlled by the bottom configuration in that pelagic facies typically occur on plateaus and domes, debris-flow deposits in base-of-slope settings, and turbidites in basinal settings. The facies distribution may therefore be used to reconstruct the evolution of the various parts of the rim-syncline.

Erosional gaps may be related to tectonism or may be features created by the passage of major turbiditic events.

The results of our study of the facies associations, and of the rates of sediment accumulation, indicate that the collapse of the eastern part of the rim-syncline pre-dates the collapse of the western part. The western basins are deeper and larger than the eastern ones and (unlike the latter) are aligned at the foot of a strike-slip fault with a vertical offset of at least 700 m.     

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.     

High-resolution sequence stratigraphy of clastic shelves VI: Mixed siliciclastic-carbonate systems

High-frequency sequences composed of mixed siliciclastic-carbonate deposits may exhibit either vertical or horizontal changes between siliciclastics and carbonates. Vertical facies shifts occur between systems tracts and define a ‘reciprocal sedimentation’ pattern, typically consisting of transgressive/highstand carbonates and forced regressive/lowstand siliciclastics, although variations from this rule are common. Mixed systems with lateral facies change, usually typifying transgressive and/or highstand systems tracts, may exhibit proximal siliciclastics and distal carbonates or vice-versa, although variations may also occur along depositional strike. The marked variability of mixed siliciclastic-carbonate sequences makes the definition of a universal sequence stratigraphic model impossible, as the composition and geometries of systems tracts may change considerably, and sequence stratigraphic surfaces and facies contacts may vary in terms of occurrence and physical expression. However, some resemblance exists between siliciclastic sequences and mixed sequences showing lateral facies changes between siliciclastics and carbonates. In particular, these mixed sequences display 1) a stratal architecture of the clastic part of the systems tracts that is comparable to that of siliciclastic deposits, 2) a dominant role of the inherited physiography and of erosional processes, rather than carbonate production, in shaping the shelf profile, and 3) a local lateral juxtaposition of siliciclastic sandstones and carbonate bioconstructions due to hydrodynamic processes. These observations are helpful in predicting the location of porous and potential sealing bodies and baffles to fluid flow at the intra-high-frequency sequence scale, and ultimately they are useful for both petroleum exploration and production.     

Normal faulting vs regional subsidence and sedimentation rate

Normal faults occur in a variety of geodynamic environments, both in areas of subsidence and uplift. Normal faults may have slip rates faster or slower than regional subsidence or uplift rates. The total subsidence may be defined as the sum of the hangingwall subsidence generated by the normal fault and the regional subsidence or uplift rate. Positive total subsidence obviously increases the accommodation space (e.g., passive margins and back-arc basins), in contrast with negative total subsidence (e.g., orogens). Where the hangingwall subsidence rate is faster than the sedimentation rate in cases of both positive and negative total subsidence, the facies and thickness of the syntectonic stratigraphic package may vary from the hangingwall to the footwall. A hangingwall subsidence rate slower than sedimentation rate only results in a larger thickness of the strata growing in the hangingwall, with no facies changes and no morphological step at the surface. The isostatic footwall uplift is also proportional to the amount and density of the sediments filling the half-graben and therefore it should be more significant when the hangingwall subsidence rate is higher than sedimentation rate.     

Lacustrine massive mudrock in the Eocene Jiyang Depression,Bohai Bay Basin,China: Nature,origin and significance

Massive mudrock refers to mudrock with internally homogeneous characteristics and an absence of laminae. Previous studies were primarily conducted in the marine environment, while notably few studies have investigated lacustrine massive mudrock. Based on core observation in the lacustrine environment of the Jiyang Depression, Bohai Bay Basin, China, massive mudrock is a common deep water fine-grained sedimentary rock. There are two types of massive mudrock. Both types are sharply delineated at the bottom and top contacts, abundant in angular terrigenous debris, and associated with oxygen-rich (higher than 2 ml O₂/L H₂O) but lower water salinities in comparison to adjacent black shales. In addition, type 1 is laterally isolated and contains abundant sand injections and contorted layers formed in the depositional process, but type 2 exactly distributes in the distal part of deep water gravity-driven sandstone units, and shows scoured bases, high-angle mineral crytsals, and fining-upward trend. It is suggested that type 1 is a muddy mass transport deposit (MMTD) formed by slide, slump, and/or debris flow, and type 2 is a turbiditic mudrock deposited by settling from dilute turbidity currents. A warm and humid climate and high subsidence rate are two main triggering events. Because of its mass movement nature, MMTD preserves the mineralogic composition and organic matter characteristics of the source sediment. By contrast, dilute turbidity currents are able to greatly entrain biochemically-formed micrite and planktonic organisms from the water column, and deposit them in the turbiditic mudrock. Because of their different ability to deposit organic matter, MMTD have poor or fair source rock potential, but the turbiditic mudrock is able to be a potentially effective source rock. The minerals in the massive mudrock are disorganized and chaotic, which cause fractures to develop in various directions, thereby, enhancing the vertical migration of oil and gas molecules to horizontal wellbore in shale reservoir exploitation.     

Spatio-temporal evolution of velocity structure,concentration and grain-size stratification within experimental particulate gravity currents

We describe a flume study of the spatio-temporal evolution of particulate gravity currents. Time series of the vertical structure of flow in terms of the forward component of velocity, flow concentration and grain-size of suspended sediment were co-measured at a different position along the flume for each of a series of nominally identical flows. The data are combined to show the spatial evolution of a single idealised flow. Such a flow, fed into the flume from an overhead reservoir, first propagates as a quasi-steady jet. The subsequent evolution of coherent spatial and temporal trends in velocity, grain-size and concentration are developed by the internal action of the flow itself, rather than being inherited from the flow generation mechanism. Quasi-steady input currents evolved down flume into surge-type flows that wax very rapidly upon arrival then progressively wane. Thus the velocity history of currents at source may differ from that experienced downstream, indicating that flow steadiness measured (or interpreted) in downstream positions may not necessarily be indicative of the flow generation mechanism. The fore-most parts of the flow travelled more rapidly than the hind-most parts and thus gradually drew further away; as a consequence, the duration of the experimental currents systematically increased along the length of the flume (the flows ‘stretched out’). Natural scale flows may therefore wane more rapidly in proximal regions than distal ones. This may impact upon sedimentation style, with higher sedimentation rates potentially more prevalent in proximal than in distal regions. Grain-size maxima are recorded in the head of the current. Convincing evidence of coarse tail lag behind the head is only patchily developed. Within the flow body a consistent pattern of upward fining then coarsening is observed. This can be related to an upward flux of coarse particles from the head, which subsequently settle downwards into the body. At the natural scale such a phenomenon may have implications for the development of coarse tail grading.     

A model for Meddy formation

A particular feature of “Meddies” is that the water within them has stable gradients of salinity and temperature. In this paper a model for a dense turbulent plume falling in a stationary, linearly-stratified environment is described. The density of the plume is assumed to be a linear function of two components, while the ambient density gradient is due to a gradient in only one of these components (labelled component one). The ratio of the contributions to the source buoyancy flux of the two components within the plume is identified as the important parameter for this type of flow. After deriving the steady equations for the plume, a simplified model for the filling process which creates the Meddy is described, which shows how a layer of fluid may be formed with stable gradients in both components in the lower part of the intrusion. The intrusion is shown to be less stratified than the surrounding fluid, with relatively weak gradients in component one (identified with temperature in the Meddy) that vary from unstable at the top to stable at the bottom of the intrusion and a fairly constant (stable) gradient in the second component (salinity for a Meddy). If the stable gradients in a Meddy are due solely to the filling process then, during formation, the top of the Meddy must become cooler than the surrounding water at the same level. The observed stable temperature gradient in Meddies is likely to be due to mixing processes after their formation, whereas the stable salanity gradient is mainly due to the filling process during formation.     

Downstream evolution of turbiditic channel complexes in the Pab Range outcrops (Maastrichtian,Pakistan)

The Pab Formation consists of deltaic and turbiditic sediments which were deposited during the Late Maastrichtian on the Indo-Pakistani passive margin. The margin geometry has been restored in the Pab Range from a regional transect 120 km long. Two superposed turbiditic systems onlap the slope carbonates and completely pinch-out southward. The lowest turbiditic system (Lower Pab) is a sand-rich basin floor fan, which consists of sand-rich channel complexes distally passing to lobes northward. This basin floor fan is overlain by a mud-rich slope fan formed during the subsequent sea-level rise, which drowned the shelf. The upper turbiditic system (Upper Pab) is a sand-rich slope fan, formed during the progradation of a deltaic system in the shelf setting. It consists of prograding tabular lobes passing upward to conglomeratic channels, and thins out northwards. The Lower Pab turbiditic system consists of three channel complexes (LP1, 2, 3) organised in a backstepping succession. Each channel complex has a multi-storey internal architecture, resulting from the amalgamation of several individual turbiditic channels. Five major facies associations have been determined in the LP3 channel complex. FA-1 corresponds to polygenic and monogenic debris-flows, FA-2 to high-density gravelly or sandy turbidites, FA-3 to by-pass deposits, FA-4 to thin-bedded turbidites (spill-over lobes and levees) and FA-5 to hemipelagites. The downstream evolution of the LP3 channel complex can be studied from canyon to mid-fan settings. Where it is confined in the canyon, the channel complex is 50 m thick and 1 km large, and shows a high sand/shale ratio. The development of overflow deposits is limited and occurs only at the top of the channel complex. At the canyon mouth, the channel complex is still deeply incised but overflow deposits start to expand laterally as a result of the decreased confinement. By-pass facies here are well-developed, and are related to hydraulic jump processes. In the mid-fan setting, the channel complex widens and the sand/shale ratio decreases. Erosion at the channel base is less developed, whereas internal and external levees are well-developed. Spill-over lobes form the last stage of the channel complex infill. The internal geometry of the channel complexes is a result of a complex interaction between lateral confinement, by-pass and lateral migration processes.     

Facies,provenance and paleoclimate interpretation using spectral gamma logs: Application to the Lower Cretaceous of the Scotian Basin

Spectral gamma logging has been useful in some settings for correlation of wells, for identification of sediment facies, and for interpretation of hinterland paleoclimate. We have taken advantage of the hundreds of geochemical analyses of sandstones and shales from conventional core that are available from the Scotian Basin to test the relationship of the elements K, U and Th measured by spectral gamma to sedimentary facies. These elements are also known to be useful in characterising provenance of sediment to different parts of the basin. The geochemical and spectral gamma data both show that there are subtle differences in K, U and Th in different lithofacies, such as higher Th/U in low sedimentation rate facies, but they are not sufficiently distinctive to identify the lithofacies of any unknown interval. The main control on abundance of these elements is the effect of grain size, that is the proportion of sand to mud in sediment. Volcanic sources have a high K/Th ratio, and early Albian sediments with relatively high volcanic component have unusually high K/Th. In distal shale-prone successions, the grain size effect is sufficiently small that climate controlled variations in Th/K ratio are recognised in the Early Cretaceous, with the ratio being higher at times of humid weathering on land. The variations in Th/K ratio through time are broadly similar to those known from Western European sections, notably with topmost Hauterivian and topmost Aptian humid events. Overall, this study provides an assessment of the use and pitfalls of spectral gamma logs in a deltaic succession.     

Facies system of the Eastern Barents Sea since the last glaciation to present

Twenty-two sediment cores raised from the central and eastern parts of the Barents Sea have been studied to reconstruct the evolution of the facies system since the Late Weichselian glaciation. Multiproxy records reveal four lithostratigraphic units, which reflect major development stages of paleoenvironments. Age control is provided by 23 AMS ¹⁴C dates for Holocene sections of four cores. Continental moraine deposits of the last glaciation are overlain by proximal glaciomarine facies of the initial deglaciation phase. During this phase, the Barents Sea ice sheet detached from the ground resulting in seawater penetration into troughs, iceberg calving, deposition of IRD and fine-grained glacier meltwater load in newly formed marine basins. The main deglaciation phase is characterized by pulsed sedimentation from various gravity flows resulting in accumulation of distal glaciomarine facies comprising laminated clay and sand sequences with minor IRD. Redistribution of fine-grained suspended matter by bottom currents and brine-induced nepheloid flows combined with biogenic processes and minor ice rafting caused facies diversity of the Holocene marine sediments. The Holocene facies of shelf depressions reflect rather high, but variable productivity responding to climate changes and variations of Atlantic water inflow into the Barents Sea.     

Turbidites and their association with past earthquakes in the deep Çınarcık Basin of the Marmara Sea

Two gravity cores (CAG-3 and C-15) from the tectonically active, 1,276-m deep Çınarcık Basin of the Marmara Sea each include three sandy turbiditic mud units (1 mm–2 cm thick) with sharp basal contacts. The high benthic foraminifer content of these units suggests that the sediments were transported by turbidity currents from the upper slope region. These units represent the thin edges of turbidites thickening towards the subsiding north-eastern part of the basin, and contain quartz, detrital calcite, intact shells and shell fragments, smectite, pyrite framboids, muscovite, biotite, epidote and garnet. Their clay fractions are more enriched in smectite than those of adjacent layers. AMS 14C ages (957±43 a.d. and 578±31 a.d.) of two upper and middle turbiditic units in core C15 overlap with the historical İstanbul-Thrace (intensity=10) and İstanbul-Kocaeli (intensity=9) earthquakes of 26 October 986 and 15 August 553, respectively. This overlap, together with sedimentological characteristics, strongly suggests that the turbiditic units are related to the tectono-seismic activity of the North Anatolian Fault. The age of the lowest turbiditic unit in core C-3 was found to be 6,573±87 a b.p. (calendar) by AMS 14 C. In terms of chronostratigraphic relationships and lithological composition, the turbiditic units in core CAG-3 cannot be correlated with those in C15. This can be explained by gravity-controlled sedimentation causing wedging out of turbidites towards the edge of the basin.     

Controls on regional variability in marine pore-water diagenesis below the seafloor in Upper Jurassic–Lower Cretaceous prodeltaic sandstone and shales,Scotian Basin,Eastern Canada

Diagenesis in the uppermost Jurassic to Lower Cretaceous deltaic sandstones and shales of the Scotian Basin is an important control on reservoir quality. Ferruginous zone (sub-oxic) marine pore-water diagenesis controls the initial formation of Fe²⁺-silicates that are the precursors of grain-rimming chlorite that preserves porosity. This study assesses the regional controls on the type of marine pore-water diagenesis by studying the sedimentology, mineralogy, and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin. Coated grains preserve a record of whether marine pore-water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone. Four types of coated grain were distinguished, each with a different mineral paragenesis. Mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite, and the correlation of P with Fe or Ti. Pyrite and Fe-calcite are found where the sulphidic zone is more significant than the ferruginous zone. Ferruginous zone diagenesis was common in low-sedimentation rate retrogradational sediments with low organic carbon, and in delta-front turbidites and river-mouth sandstones. Estuarine, tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower Fe:Ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products. The relative contribution of colloidal iron (hydr)-oxides appears greater in distal low-sedimentation rate environments. Where large changes in sedimentation rate occurred at ravinement surfaces, the underlying progradational rocks have evidence of ferruginous zone diagenesis, whatever their facies. Rapid upward migration of the pore-water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore-water zones. This study has shown that the availability of Fe and organic carbon varying in a complex manner in marine deltaic sediments, but that the resulting diagenesis by marine pore-water can be predicted from facies and paleogeographic setting.     

The Golo submarine turbidite system (east Corsica margin): morphology and processes of terrace formation from high-resolution seismic reflection profiles

New high-resolution seismic reflection data collected along the eastern margin of Corsica have been analysed to describe the morphology of the turbidite systems located seaward of the Golo River mouth. The boomer data reveal that there is not only one turbidite system directly associated with the river, but four additional, non-coalescing systems which grew simultaneously. In the south, the system has the typical morphology of a turbidite deposit rich in mud and sand with a well-developed meandering canyon and channel morphology. In the north, they have the morphology of sand-rich turbidite systems with shorter straight channels. The southernmost deposits are interpreted to represent a more advanced stage of turbidite system development. Terraces, recognised by their particular seismic facies on boomer profiles which clearly differs from the surrounding levee facies, are observed in the channel meanders. They are interpreted as confined levees built by vertical accretion due to deposition from low-energy flows. Despite limited penetration, boomer seismics are demonstrated to be a useful complement to lower-resolution sparker data. The boomer data are superior (1) for the characterisation of fine-grained turbidite deposits by extending seawards the limits of the turbidite systems commonly defined by the acoustic response of sands, (2) in demonstrating the persistence of turbiditic processes farther towards the basin, and (3) for proposing conceptual models for the formation of terraces in fan valleys.     

Contrasting facies patterns in subtropical and temperate continental slope sediments: inferences from east Australian late Quaternary records

Studies of latest Quaternary continental slope sediments at two localities on the east Australian margin have revealed markedly different responses to late Quaternary sea level fluctuations. Offshore of Noosa, in the sub-tropics, the sediment is predominantly a mixture of fine metastable carbonate, siliciclastic material, and pelagic carbonate. Important features of the stratigraphy include a siliciclastic-dominated facies deposited relatively slowly during the last glacial lowstand (sedimentation rate ≤8 cm/ka), and a calcareous facies, rich in metastable carbonate, deposited more rapidly during the late post-glacial transgression (sedimentation rates 15–24 cm/ka). Highstand and transgressive sedimentation rates are greater than lowstand rates by a factor of 2.5–6 due to increased shelf carbonate productivity after flooding of the mid-shelf. Off Sydney, in temperate latitudes, continental slope sediment is largely a mixture of fine siliciclastic material and pelagic carbonate. Mean sedimentation rates range from 2 to 5 cm/ka over the last four oxygen isotope stages, with mean glacial/interstadial rates higher than Holocene rates by a factor of ∼1.36. This largely reflects the transfer of siliciclastic mud from the shelf to the slope during sea level regression. In both localities, facies changes on the slope are not related to specific sea level states (e.g. lowstand facies, transgressive facies, etc.), but reflect instead the interaction of changing sea level with shelf morphology.     

Role of effective permeability distribution in estimating overpressure using basin modelling

Overpressure generation is a function of the rates of sedimentation, compaction, fluid generation from kerogen and dehydration of minerals, and most importantly the lateral distribution of permeability within a basin as this controls lateral drainage. Sedimentary basins, however, are typically highly heterogeneous with respect to primary sedimentary facies, diagenesis and tectonic development. While fluid flow models based on idealised homogeneous basins may further our understanding of the processes that influence overpressure development, the results are very sensitive to the distribution of rock properties, particularly permeability. The absolute permeability of sedimentary rocks varies from more than 1 Darcy to less than 0.01 nanodarcy (nD) (10⁻¹¹ Darcy).     

Transport of terrestrial organic matter in the Ogooué deep sea turbidite system (Gabon)

In order to define the nature and distribution of the organic matter (OM) preserved in the modern Ogooué deep sea turbidite system (Gabon), bulk geochemical techniques (Rock-Eval pyrolysis, elemental and isotopic analyses) and palynofacies were applied to three piston cores collected in the Cape Lopez Canyon and lobe and on the continental slope, north of the canyon.The hemipelagic sedimentation in the study area is characterized by high accumulations of well-preserved OM (∼2-3 wt. TOC %). Bulk geochemical and palynofacies analysis indicate both a marine and terrestrial origin of the OM. Contribution of the marine source is higher on the slope than in the canyon and lobe.OM accumulation in turbidites is strongly controlled by the combined influence of the Cape Lopez Canyon and littoral drift. In the canyon and lobe, turbidites show generally low TOC content (0.5 wt. %) and OM is oxidized. The origin of the OM is interpreted as both marine and terrestrial, with a higher contribution of continental source versus marine source. The low TOC contents are due to the large siliciclastic fraction transported by the littoral drift and diverted in the Cape Lopez Canyon during high energy processes (e.g. storms) which tend to dilute the OM in the turbidites. Transport by long-shore currents and/or turbiditic flows leads to oxidation of the OM.On the continental slope located north of the Cape Lopez Canyon, large amounts of OM are deposited in turbidites (up to 14 wt. %). The OM is predominantly derived from terrestrial land plants and has not been subjected to intense oxidation. These deposits are characterized by high hydrocarbon potential (up to 27 kg HC/t rock), indicating a good potential as gas-prone source rock. Because Cape Lopez Canyon captures a significant part of the sediment transported by the littoral drift, the siliciclastic sedimentary flux is reduced north of the canyon; OM is thus concentrated in the turbidites. Variation in TOC content within turbidite laminae can be explained by the burst and sweep deposition process affecting the boundary layer of the turbulent flow.This study confirms that gravity flows play a preponderant role in the accumulation and preservation of OM in deep water and that deep sea turbidite systems could be regarded as an environment where organic sedimentation occurs.     

On the role of bottom roughness in overflows

Overflows play an important role in the downwelling limb of the oceanic thermohaline circulation. In light of the recognition that some overflows are not homogenous but exhibit a vertical density structure, and details of topography influence the pathways of some overflows, the impact of topographic roughness on the product property distribution is explored using the 3D non-hydrostatic spectral element model Nek5000. Numerical experiments are carried out by varying bottom roughness amplitude and ambient stratification parameters, in a regime where equilibrated product water masses are formed in a non-rotating environment.Our main finding is that bottom roughness can influence the overflow product distribution such that the highest salinity classes are removed and neutral buoyancy level is attained higher up in the stratified ambient water column. It is also shown that the form drag coefficients in overflows over rough bottom can be much larger than the skin drag coefficient over smooth bottom. To our knowledge, form drag has never been measured in oceanic overflows. As such, these numerical experiments imply that such measurements would be useful for a better understanding of overflow dynamics. It is also found that the ratio of source and product overflow mass transports is robust to changes in bottom roughness. This appears to happen because the distribution of entrainment is totally different in the case of rough bottom. Entrainment tends to initiate earlier (due to vertical motion induced by topography) and terminate earlier (due to development of form drag) than that over smooth topography.     

Flow processes and sedimentation in submarine channel bends

Turbidity currents in sinuous submarine channels are an important mechanism for transporting terrestrial sediments to deep water, and their deposits are of increasing importance as hydrocarbon exploration targets. Despite this, the architecture and dynamics of submarine channel systems are not well understood. Analogies are often drawn with fluvial systems due to similarities between their planform shapes even though differences in channel evolution and hydrodynamics have been noted. A key question is the nature of deposition within submarine channel bends; in particular at inner bends where point bars form in alluvial meandering rivers. Recent experimental and numerical work has demonstrated that the fluid dynamics of submarine channel bend flow are markedly different from rivers. Notably, a reversal in the orientation of secondary (helical) flow at bend apices occurs in submarine channels. The potential influence of these differences in fluid dynamics on deposition within submarine channel bends is investigated herein. We report the results of a series of physical experiments in which solute-driven gravity currents were run through pre-formed sinuous channels containing mobile beds. These experiments reveal sedimentation patterns characterised by accumulation zones downstream of bend apices and erosion zones at outer bends. These patterns are broadly analogous to the point bars and outer-bank pools observed in meandering rivers, demonstrating that the longitudinal flow component dominates over the cross-stream component, as also occurs in rivers. However, the data suggest that the reversal in direction of the cross-stream flow component compared with subaerial flows is important in determining the position and morphology of ‘point bars’ relative to bend apices. From analogy with fluvial compound channels, and fluvial theory, this reversal in secondary flow cell orientation is also expected to influence the spatial variations of grain size in submarine channel ‘point-bar’ deposits.     

胶州湾沉积环境演变的分析

从剖析表征沉积环境演变的生物学、地质动力学以及地球化学指示因子入手,系统总结了近年来胶州湾沉积环境演变研究的进展,揭示了诸如有孔虫、沉积物粒度、生源要素、污染物及有机质等众多指示因子以及它们的相关组合与环境演变的关系。综合分析表明,胶州湾沉积速率较低,属于缓慢淤积的海湾,是一个典型的潮汐汊道系统。胶州湾的沉积环境演变分为4个阶段:陆相、三角洲沼泽相、海陆过渡相和海湾相。在过去的50 a中,特别是近20 a来,由于受人类活动与工业排污的影响,胶州湾的沉积环境发生了显著的变化,海域面积大大减少,陆源输入已经成为胶州湾沉积物的主要来源,沉积速率不断增加,局部海域特别是河口附近污染加重。这些变化都是通过各种生物、地质动力和地球化学因子来综合体现的。