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1.
Flume experiments were performed to study the flow properties and depositional characteristics of high‐density turbidity currents that were depletive and quasi‐steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine‐grained natural sand, very fine sand‐sized glass beads or coarse silt‐sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain‐size distribution within the deposits. The experimental turbidity currents formed distinct fan‐shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel‐levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two‐layer flow, with a low‐density turbulent layer moving on top of a dense layer with visibly suppressed large‐scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand‐laden, high‐density turbidity currents, contained massive sand in the central part grading to plane parallel‐laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse‐to‐normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward‐directed transport of relatively fine‐grained and low‐concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity‐current transport.  相似文献   

2.
The turbulent flow structure, suspended sediment dynamics and deposits of experimental sustained turbidity currents exiting a channel across a break in slope into a wide tank are documented. The data shed light on the flow evolution and deposit geometry of analogous natural channel‐fed submarine fans. Flows generated in a 0·3 m wide, sloping (0°, 6°, 9° or 20°) channel crossed an angular slope break and spread onto a horizontal tank floor. Flow development comprised: (i) channelized phase (unsteady channel flow developing into steady channel flow); (ii) initial lateral expansion phase (unsteady‐spreading wall jet phase); (iii) constant lateral expansion phase (steady‐spreading wall jet phase); and (iv) rapid waning phase. Phases (i) and (iv) are similar to laterally constrained turbidity currents, but phases (ii) and (iii) are considerably different from such two‐dimensional currents. Steeper channel slopes produced greater flow velocities and turbulence intensities, but these effects diminished markedly with distance from the channel mouth. Flow velocity vectors in the tank had similar patterns for all channel slopes, with a central core of faster velocity and narrow vector dispersion and slower flow with larger dispersion at the jet margins. Suspended sediment concentrations were higher within flow heads and dense basal layers in flow bodies. Time‐averaged acoustic backscatter data showed vertical concentration gradients, confirmed by siphon samples. The deposits comprised a thick central ridge, of similar order width to the channel mouth, with abrupt margins and a surrounding, very thin, fan‐like sheet. The ridge was coarser grained and better sorted than the original sediment, with grain‐size fining downstream, particularly over the fan‐like sheet. The formation of a central ridge suggests that, in the tank, vertical turbulent momentum exchange is more significant for sediment dynamics than spanwise momentum exchange due to lateral expansion. The streamwise elongate geometry of the ridge contrasts with conventional fan‐like geometry found with surge‐type turbidity flows, a result that has widespread stratigraphic and economic implications.  相似文献   

3.
Climbing dune‐scale cross‐statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium‐grained to coarse‐grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss‐depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing‐dune cross‐stratification related to the migration of lower‐flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion‐based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice‐proximal segment of the outwash plain. In erosion‐based sand sheets, climbing‐dune cross‐stratification results from unconfined mouth‐bar deposition related to expanding, sediment‐laden flows entering a water body. Within incised channels, climbing‐dune cross‐stratification formed over eddy‐related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing‐dune cross‐stratification is a combination between high‐magnitude flows and sediment supply limitations, which lead to the generation of sediment‐charged stream flows characterized by a significant, relatively coarse‐grained, sand‐sized suspension‐load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse‐grained sand fallout in sediment‐laden flows following flow expansion throughout mouth bars or in eddy‐related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss‐sides and generation of widespread climbing‐dune cross‐stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood‐dominated depositional systems of foreland basins.  相似文献   

4.
The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post‐depositional consolidation and soft‐sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain‐support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non‐cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle‐support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain‐to‐grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle‐support mechanism depends upon flow conditions, particle concentration, grain‐size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse‐grain tail (or dense‐grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large‐scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensu 4 ), in which fluid turbulence is the main particle‐support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge‐like flows and quasi‐steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge‐like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi‐steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening‐up units capped by fining‐up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near‐steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co‐exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.  相似文献   

5.
A process-based, forward computer model of turbidity current flow and sedimentation, termed the TCFS model, has been developed to trace the downslope evolution of individual turbidity flows. Details of the model itself have been presented in a preceding paper. We here outline a series of tests of the TGFS model. The sensitivity tests of the TCFS model to general geological controls reveal the quantitative relationship between these controls and the behaviour of turbidity flows and the geometry and textural features of the resulting turbidites. Experimental turbidity currents on relatively steep slopes accelerate more rapidly and reach higher velocities than those on gentle slopes. Flows with larger initial volumes have higher initial velocities, travel further downslope, and form beds of greater thickness and downslope extent than smaller flows. Experimental high-concentration flows with suspended-sediment concentrations of 25% accelerate more rapidly and reach higher downslope velocities than dilute flows with 5% suspended sediment. The higher velocities and enhanced hindered-settling effects of the high-concentration flows lead to much greater transport distances and reduced vertical and lateral sediment size grading in the resulting turbidites. Beds formed by experimental high-concentration flows are massive or show coarse-tail grading whereas beds formed by low-concentration flows show distribution-grading. Experimental flows fed by coarse sediment sources tend to deposit the bulk of their suspended sediment loads on the proximal slope, resulting in more rapid flow deceleration and sedimentation than flows fed by silt-rich, fine-grained sediment sources. Turbidites formed by coarse-sediment flows tend to have a wedge-shaped geometry, with low downslope extent and high surface relief, whereas turbidites formed by fine-sediment flows tend to have a tabular geometry, with greater downslope extent and lower surface relief. A specific geological test of the TCFS model is based on studies of modern turbidity currents in Bute Inlet, British Columbia, Canada. With the input initial and boundary conditions estimated from Bute Inlet, the model predicts the downslope velocity evolution of turbidity currents comparable to those of modern and ancient turbidity flows measured in Bute Inlet. Model-calculated vertical and downslope grain-size properties of turbidites are similar to those exhibited by surface and cored Bute Inlet turbidites. Model flows tend to decelerate more rapidly than some stronger turbidity currents in the Bute Inlet system, and model beds tend to decrease in grain-size downslope more rapidly than observed bottom sediments. This is probably because the TCFS model flows lacked clay, which is abundant in Bute Inlet; they do not fully simulate turbulent mixing of suspended sediments; and they better represent the unsteady, depositional stage of turbidity-currents than the preceding stage of more-or-less steady-flow conditions. These tests demonstrate that the TCFS model provides a semi-quantitative method to study the growth patterns of submarine turbidite systems. It can serve as a predictive tool for analysing the facies architecture of ancient turbidite systems through simulating multi-depositional events by improving its erosion function, and the compatibility between its numerical components.  相似文献   

6.
Deep‐water sandstone beds of the Oligocene Fusaru Sandstone and Lower Dysodilic Shale, exposed in the Buz?u Valley area of the East Carpathian flysch belt, Romania, can be described in terms of the standard turbidite divisions. In addition, mud‐rich sand layers are common, both as parts of otherwise ‘normal’ sequences of turbidite divisions and as individual event beds. Eleven units, interpreted as the deposits of individual flows, were densely sampled, and 87 thin sections were point counted for grain size and mud content. S3/Ta divisions, which form the bulk of most sedimentation units, have low internal textural variability but show subtle vertical trends in grain size. Most commonly, coarse‐tail normal grading is associated with fine‐tail inverse grading. The mean grain size can show inverse grading, normal grading or a lack of grading, but sorting tends to improve upward in most beds. Fine‐tail inverse grading is interpreted as resulting from a decreasing effectiveness of trapping of fines during rapid deposition from a turbidity current as the initially high suspended‐load fallout rate declines. If this effect is strong enough, the mean grain size can show subtle inverse grading as well. Thus, thick inversely graded intervals in deep‐water sands lacking traction structures do not necessarily imply waxing flow velocities. If the suspended‐load fallout rate drops to zero after the deposition of the coarse grain‐size populations, the remaining finer grained flow bypasses and may rework the top of the S3 division, forming well‐sorted, coarser grained, current‐structured Tt units. Alternatively, the suspended‐load fallout rate may remain high enough to prevent segregation of fines, leading to the deposition of significant amounts of mud along with the sand. Mud content of the sandstones is bimodal: either 3–13% or more than 20%. Two types of mud‐rich sandstones were observed. Coarser grained mud‐rich sandstones occur towards the upper parts of S3/Ta divisions. These units were deposited as a result of enhanced trapping of mud particles in the rapidly deposited sediment. Finer grained mud‐rich units are interbedded with ripple‐laminated very fine‐grained sandy Tc divisions. During deposition of these units, mud floccules were hydraulically equivalent to the very fine sand‐ and silt‐sized sediment. The mud‐rich sandstones were probably deposited by flows that became transitional between turbidity currents and debris flows during their late‐stage evolution.  相似文献   

7.
海底浊流在坡道转换处的流动及沉积的数值模拟   总被引：2，自引：1，他引：1  
郭彦英  黄河清 《沉积学报》2013,31(6):994-1000
根据一经多项试验数据验证的基于三维不可压缩流体Navier-Stokes方程和湍流 k-ε 模型的重力流数值计算的数学模型,模拟并分析了单粒径沉积物的海底浊流沿不同斜坡流至近似平坦坡的流动及沉积特征。模拟结果显示了有关海底浊流的一些重要特征:连续入流的浊流在斜坡上的流速随着斜坡的增大而增大,同时浊流厚度由于对环境水体的夹带而渐渐增厚,坡度越大,增厚越快;流至近水平坡时,流速均有明显的降低,但大斜坡入流依然保持相对较高的流速。在沉积方面,初步的模拟结果显示对给定的沉积物来说存在一相对应的临界坡度:当坡度小时,坡上沉积多,坡下少,这样整体的坡度有逐渐增大之势;当坡度大时,坡上沉积少或为侵蚀,而坡下沉积相对较多,坡度有整体减小之势。了解了不同坡度转换的浊流沉积的上述特点,对于我们根据实测的浊流沉积的剖面特征推测其形成的环境,进而推测相关油气储层的分布状况会有一定的参考作用。  相似文献   

8.
Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.  相似文献   

9.
Large roughness features, caused by erosion of the sea floor, are commonly observed on the modern sea floor and beneath turbidite sandstone beds in outcrop. This paper aims to investigate the effect of such roughness elements on the turbulent velocity field and its consequences for the sediment carrying capacity of the flows. Experimental turbidity currents were run through a rectangular channel, with a single roughness element fixed to the bottom in some runs. The effect of this roughness element on the turbulent velocity field was determined by measuring vertical profiles of the vertical velocity component in the region downstream of the basal obstruction with the Ultrasonic Doppler Velocity Profiling technique. The experiments were set up to answer two research questions. (i) How does a single roughness element alter the distribution of vertical turbulence intensity? (ii) How does the altered profile evolve in the downstream direction? The results for runs over a plane substrate are similar to data presented previously and show a lower turbulence maximum near the channel floor, a turbulence minimum associated with the velocity maximum, and a turbulence maximum associated with the upper flow interface. In the runs in which the flows were perturbed by the single roughness element, the intensity of the lower turbulence maximum was increased between 41% to 81%. This excess turbulence dissipated upwards in the flow while it travelled further downstream, but was still observable at the most distal measurement location (at a distance ca 39 times the roughness height downstream of the element). All results point towards a similarity between the near bed turbulence structure of turbidity currents and free surface shear flows that has been proposed by previous authors, and this proposition is supported further by the apparent success of a shear velocity estimation method that is based on this similarity. Theory of turbulent dispersal of suspended sediment is used to discuss how the observed turbulent effects of a single large roughness element may impact on the suspended sediment distribution in real world turbidity currents. It is concluded that this impact may consist of a non‐equilibrium net‐upwards transport of suspended sediment, counteracting density stratification. Thus, erosive substrate topography created by frontal parts of natural turbidity flows may super‐elevate sediment concentrations in upper regions above equilibrium values in following flow stages, delay depletion of the flow via sedimentation and increase their run‐out distance.  相似文献   

10.
Controlled laboratory experiments reveal that the lower part of turbidity currents has the ability to enter fluid mud substrates, if the bed shear stress is higher than the yield stress of the fluid mud and the density of the turbidity current is higher than the density of the substrate. Upon entering the substrate, the turbidity current either induces mixing between flow‐derived sediment and substrate sediment, or it forms a stable horizontal flow front inside the fluid mud. Such ‘intrabed’ flow is surrounded by plastically deformed mud; otherwise it resembles the front of a ‘bottom‐hugging’ turbidity current. The ‘suprabed’ portion of the turbidity current, i.e. the upper part of the flow that does not enter the substrate, is typically separated from the intrabed flow by a long horizontal layer of mud which originates from the mud that is swept over the top of the intrabed flow and then incorporated into the flow. The intrabed flow and the mixing mechanism are specific types of interaction between turbidity currents and muddy substrates that are part of a larger group of interactions, which also include bypass, deposition, erosion and soft sediment deformation. A classification scheme for these types of interactions is proposed, based on an excess bed shear stress parameter, which includes the difference in the bed shear stress imposed by the flow and the yield stress of the substrate and an excess density parameter, which relies on the density difference between the flow and the substrate. Based on this classification scheme, as well as on the sedimentological properties of the laboratory deposits, an existing facies model for intrabed turbidites is extended to the other types of interaction involving soft muddy substrates. The physical threshold of flow‐substrate mixing versus stable intrabed flow is defined using the gradient Richardson number, and this method is validated successfully with the laboratory data. The gradient Richardson number is also used to verify that stable intrabed flow is possible in natural turbidity currents, and to determine under which conditions intrabed flow is likely to be unstable. It appears that intrabed flow is likely only in natural turbidity currents with flow velocities well below ca 3·5 m s?1, although a wider range of flows is capable of entering fluid muds. Below this threshold velocity, intrabed flow is stable only at high‐density gradients and low‐velocity gradients across the upper boundary of the turbidity current. Finally, the gradient Richardson number is used as a scaling parameter to set the flow velocity limits of a natural turbidity current that formed an inferred intrabed turbidite in the deep‐marine Aberystwyth Grits Group, West Wales, United Kingdom.  相似文献   

11.
Uppermost Jurassic limestones of the South‐East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave‐induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave‐cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave‐induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present‐day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair‐weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100‐m‐thick wedge, which prograded over 3 km and was built up by the stacking of 5‐ to 20‐m‐thick cross‐stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high‐velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel‐like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave‐induced currents was quasi‐permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft‐sediment deformed on a hurricane‐dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.  相似文献   

12.
Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s?1 (equivalent to pure wind velocity of 6 to 8 m.s?1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s?1.  相似文献   

13.
Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south‐west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse‐grained and either ungraded or poorly graded, and were deposited by non‐cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub‐units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub‐units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi‐stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea‐level change.  相似文献   

14.
D. Uli&#;ný 《Sedimentology》2001,48(3):599-628
Deposits of coarse‐grained, Gilbert‐type deltas showing varying degrees of reworking of foresets by basinal currents were identified in Middle Turonian to Early Coniacian sandstones of the Bohemian Cretaceous Basin. The progradation of the deltaic packages, earlier interpreted as large‐scale subaqueous dunes, shelf ridges or subaqueous fault‐scarp ‘accumulation terraces’, was controlled by high‐ and low‐frequency, relative sea‐level changes in a relatively slowly subsiding, intracontinental strike‐slip basin. End‐member types of the Bohemian Cretaceous coarse‐grained deltas are deep‐water deltas, characterized by thick (50–80 m) foreset packages with steep (10–30°) foresets, and shallow‐water deltas, which deposited thin (<15 m) packages with foresets typically between 4° and 10°. The differences in thickness and foreset slope angle were controlled predominantly by the accommodation available during progradation. The depositional regime of the deltas was governed by (i) the fluvial input of abundant sand bedload, with a minor proportion of gravel; (ii) gravity flows, most probably caused by liquefaction of the upper part of the unstable foreset slope; and (iii) migration of sandy bedforms on the foreset slopes. The bedform migration was driven by unidirectional currents of possible tidal origin. Individual foreset packages represent systems tracts, or parts of systems tracts, of depositional sequences. A variety of stacking patterns of high‐frequency sequences exists in the basin, caused by low‐frequency relative sea‐level changes as well as by local changes in sediment input. Because of generally low subsidence rates, fluvial or beach topset strata were not preserved in the cases studied. The absence of preserved fluvial facies, which has been one of the main arguments against the fluvio‐deltaic origin of the sandstone bodies, is explained by erosion of the topsets during transgression and their reworking into coarse‐grained lags of regional extent covering ravinement surfaces.  相似文献   

15.
The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.  相似文献   

16.
Field measurements of the vertical structure of near-bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time-dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind-wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near-bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near-bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.  相似文献   

17.
The planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open‐channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density‐driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near‐bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open‐channels.  相似文献   

18.
《Sedimentology》2018,65(6):2149-2170
Hyperpycnal currents are river‐derived turbidity currents capable of transporting significant volumes of sediment from the shoreline onto the shelf and potentially further to deep ocean basins. However, their capacity to deposit sand bodies on the continental shelf is poorly understood. Shelf hyperpycnites remain an overlooked depositional element in source to sink systems, primarily due to their limited recognition in the rock record. Recent discoveries of modern shelf hyperpycnites, and previous work describing hyperpycnites deposited in slope or deep‐water settings, provide a valuable framework for understanding and recognizing shelf hyperpycnites in the rock record. This article describes well‐sorted lobate sand bodies on the continental shelf of the Neuquén Basin, Argentina, interpreted to have been deposited by hyperpycnal currents. These hyperpycnites of the Jurassic Lajas Formation are characterized by well‐sorted, medium‐grained, parallel‐laminated sandstones with hundreds of metre extensive, decimetre thick beds encased by organic‐rich, thinly laminated sandstone and siltstone. These deposits represent slightly obliquely‐migrating sand lobes fed by small rivers and deposited on the continental shelf. Hyperpycnites of the Lajas Formation highlight several unique characteristics of hyperpycnal deposits, including their distinctively thick horizontal laminae attributed to pulsing of the hyperpycnal currents, the extraction of coarse gravel due to low flow competence, and the extraction of mud due to lofting of light interstitial fluid. Recognition of shelf hyperpycnites in the Lajas Formation of the Neuquén Basin allows for a broader understanding of shelf processes and adds to the developing facies models of hyperpycnites. Recognizing and understanding the geometry and internal architecture of shelf hyperpycnites will improve current understanding of sediment transfer from rivers to deeper water, will improve palaeoenvironmental interpretations of sediment gravity‐flow deposits, and has implications for modelling potentially high‐quality hydrocarbon reservoirs.  相似文献   

19.
Heterozoan temperate‐water carbonates mixed with varying amounts of terrigenous grains and muddy matrix (Azagador limestone) accumulated on and at the toe of an inherited escarpment during the late Tortonian–early Messinian (late Miocene) at the western margin of the Almería–Níjar Basin in south‐east Spain. The escarpment was the eastern end of an uplifting antiform created by compressive folding of Triassic rocks of the Betic basement. Channelized coralline‐algal/bryozoan rudstone to coarse‐grained packstone, together with matrix‐supported conglomerate, are the dominant lithofacies in the higher outcrops, comprising the deposits on the slope. These sediments mainly fill small canyon‐shaped, half‐graben depressions formed by normal faults active before, during and after carbonate sedimentation. Roughly bedded and roughly laminated coralline‐algal/bryozoan rudstone to coarse‐grained packstone are the main lithofacies forming an apron of four small (kilometre‐scale) lobes at the toe of the south‐eastern side of the escarpment (Almería area). Channelized and roughly bedded coralline‐algal/bryozoan rudstone to coarse‐grained packstone, conglomerates, packstone and sandy silt accumulated in a small channel‐lobe system at the toe of the north‐eastern side of the escarpment (Las Balsas area). Carbonate particles and terrigenous grains were sourced from shallow‐water settings and displaced downslope by sediment density flows that preferentially followed the canyon‐shaped depressions. Roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope, whereas the rest of the carbonate lithofacies were deposited by high‐density turbidite currents. The steep escarpment and related break‐in‐slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse‐grained, low‐transport efficiency skeletal‐dominated sediment in the apron lobes. Accelerated uplift of the basement caused a relative sea‐level fall resulting in the formation of outer‐ramp carbonates on the apron lobes, which were in turn overlain by lower Messinian coral reefs. The Almería example is the first known ‘base of slope’ apron within temperate‐water carbonate systems.  相似文献   

20.
Modern and ancient tidal straits are the least well understood of all tide‐dominated depositional systems. To provide an increased understanding of these systems, a facies‐based depositional model is assessed by comparing multibeam surveys of three present‐day tidally dominated seaways with a number of superbly exposed Neogene‐to‐Quaternary strait‐fill successions of Calabria (south Italy). The model points out the existence of four depositional zones, laterally adjacent from the narrowest strait centre to its terminations, distributed along symmetrical or asymmetrical seaways. These zones, whose signature is recorded in four facies associations in the Calabrian tidal straits, are as follows: (i) the strait‐centre zone, associated with the tidal current maxima and where sediments are scarce or absent; (ii) the dune‐bedded zone, where sediments form dune complexes due to tidal flow expansion; (iii) the strait‐end zone, where currents decelerate accumulating thinly bedded, fine‐grained deposits; and (iv) the strait‐margin zone, where sediment massflows descend tectonically active, steep margins towards the strait axis. In ancient, tectonically confined, narrow seaways, these facies generate a distinctive deepening‐upward vertical succession, where tidal currents are the dominant process in the sediment distribution.  相似文献   

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