Morphodynamique d'une dune sous-marine du détroit du pas de CalaisMorphodynamics of an undersea sandwave of the Dover Straits     

Déborah Idier  Axel Ehrhold  Thierry Garlan 《Comptes Rendus Geoscience》2002,334(15):1079-1085

This paper investigates the dynamics of sandwaves in relationship with tidal currents and weather conditions. The studied sandwaves are located in the Dover Straits and are covered by megaripples. These megaripples have migration speeds of $\text{1 }\text{m}\text{h}{}^{\mathrm{?1}}$ and slopes of 34°, suggesting the potential for avalanches to occur along the flanks of the sandwave. Tidal cycles without reversing currents were observed during stormy weather. Wind-induced currents lead to a unidirectional migration of megaripples and sandwaves. Well-defined areas without megaripples were observed and correlated with an increase in sandwave height. We propose interpretations for understanding sandwave saturation and migration. To cite this article: D. Idier et al., C. R. Geoscience 334 (2002) 1079–1085.  相似文献   

The morphological dynamics of intertidal megaripples in the Mawddach Estuary, North Wales, and the implications for palaeoflow reconstructions     

PIERS LARCOMBE  COLIN F. JAGO 《Sedimentology》1996,43(3):541-559

The morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow-bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi-diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects. Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s^?1 at spring tides, and 0.5 m s ^?1 at neaps, and bed sediment transport was also flood-dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases: 1 the neap mode-consisting of near-moribund two-dimensional (2-D) flood-orientated megaripples of wavelength c. 6 m and height c. 0.2 m; 2 a transitional mode-where, on rising tidal ranges, scour pits formed and developed into 3-D megaripples which underwent net migration with the flood tide; 3 the spring mode-consisting of 3-D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re-orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood-orientated megaripples during the flood tide. The data support previously reported separation of 2-D and 3-D megaripples at a depth to grain size ratio of 8000, and at a depth-mean velocity of the dominant tide (U_maxdom) of 0.75-0.8 m s^?1. A migration threshold exists at U_maxdom of c. 0.53-0.57 m s^?1. Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post-spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross-sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2-D to 3-D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.  相似文献   

The behaviour of intertidal sandwaves during neap-spring tide cycles and the relevance for palaeoflow reconstructions     

JOOST H. J. TERWINDT  MARGUERITE J. N. BROUWER 《Sedimentology》1986,33(1):1-31

Variations in migration distances and shape characteristics of sandwaves in relation to flow conditions were studied on the Ossenisse intertidal shoal in the Westerschelde estuary. The purpose was to analyse bedform behaviour, to establish the threshold and the time lags involved, to find differences in two- and three-dimensional sandwaves and to determine the implications for palaeoflow reconstructions. Sandwave migration is well correlated with the peak depth-averaged flow velocity of the dominant tide. Thus the latter parameter may be estimated from the thickness of the tidal bundles. Other flow parameters such as shear velocity, Chezy C or roughness length do not show a correlation with the migration and cannot be used in palaeoflow analysis. Flow depth does not correlate with sandwave height or with length. Consequently, neither sandwave height and length nor set height and length can be used for palaeoflow depth determination. Sandwaves start moving when the peak dominant flow velocity exceeds 0.5–0.6 m s^?1, and appreciable changes in shape occur at 0.75–0.8 m s ^?1. Complete reversal of sandwaves is accomplished if both the dominant and subordinate peak depth-averaged current velocities exceed 0.85 m s^?1. Two- and 3-D sandwaves appeared to have different stability fields in the velocity-depth diagram and in the diagram of the Froude number versus the depth-grain-size ratio. In addition the distinction between 2-D and 3-D sandwaves appeared to be related to a variability in current direction during periods of appreciable sand transport. There are also differences in sedimentary structures between the two types of sandwaves.  相似文献   

Megaripple dynamics and sediment transport in a mesotidal mangrove creek: implications for palaeoflow reconstructions     

P. LARCOMBE  P. V. RIDD† 《Sedimentology》1995,42(4):593-606

An experiment was conducted to study megaripple morpho dynamics on a sandy intertidal shoal in a mesotidal mangrove creek (Gordon Creek, Townsville, Australia). Tidal current velocity and depth were recorded with S4 current meters over a period of 35 tides. The tidal megaripples were 0.06–0.2 m in height and 1–2 m in wavelength, and their movement was monitored by (1) electromagnetic bed-elevation probes (which automatically recorded bed level every 2 min at three positions along the survey transect) and (2) daily surveying for 8 days around spring tidals. The tidal currents in Gordon Creek are ebb-dominated, with maximum depth-mean current velocities for the flood and ebb tides of 0.62 and 0.98 m s^?1 respectively. Significant bedload transport occurs only during spring tides, and only on the larger of the unequal semi-diurnal tides. Bedload transport is overwhelmingly in the ebb direction. Megaripple migration rates reach 5.6 m per tide in the ebb direction and up to 0.1 m min^?1 within individual tides. Within-tide ‘bedform transport rates’are up to 0.29 kg m^?1 s^?1. The results suggest that for reconstruction of palaeoflows from deposits of preserved fine- to medium-grained sandy tidal megaripples, it is valid to use a depth-averaged velocity of 0.5–0.6 m s^?1 as the migration threshold. Velocity thresholds associated with partial or complete reversal of megaripple asymmetry are invalid.  相似文献   

Migration and sedimentology of longshore sandwaves, Long Point, Lake Erie, Canada   总被引：1，自引：0，他引：1  

Robin G. D. Davidson-Arnott  Anthony G. Van Heyningen 《Sedimentology》2003,50(6):1123-1137

This paper describes the formation, migration and sedimentology of sandwaves along the distal end of Long Point, a 40 km long spit in Lake Erie. Some seven to nine sandwaves occur in a zone over the last 14 km of the spit. They are characteristically 50–100 m wide at the downdrift end, range in length from 350 m to > 1500 m and migrate alongshore at rates that are typically 100–300 m year⁻¹. Measurements over a 7-year period show two forms of alongshore sandwave migration: (1) a migratory jump; and (2) downdrift accretion. The migratory jump is commonly 200–500 m year⁻¹ and results from the onshore migration and welding of an inner nearshore bar to the downdrift end of the sandwave. This in turn leads to emergence of the bar over a distance of several hundred metres downdrift of the sandwave and isolation of the trough landward of the bar. Infilling of the trough abstracts large volumes of sediment from the local sediment transport system and may affect movement of the sandwave in the following year or movement of the next sandwave downdrift. Downdrift accretion commonly results in migration of 50–150 m year⁻¹ and results from the refraction of waves around the distal end of the sandwave and episodic accretion of small swash bars. This mechanism occurs less frequently and appears to reflect a local condition of lower sediment abundance, often triggered by a large migratory jump in the previous year. The process of bar emergence and infilling produces a distinct suite of sedimentary structures associated with the infilling of the landward trough and building of the sandwave berm. The initial shoreline perturbation that generates the sandwave results from onshore migration and welding of inner nearshore bars, and the development and growth of the sandwaves is promoted by refraction of highly oblique waves.  相似文献   

Internal structure of subtidal sandwaves revealed by high-resolution seismic reflection   总被引：1，自引：0，他引：1  

SERGE BERNÉ  JEAN-PAUL AUFFRET†  P. WALKER†‡ 《Sedimentology》1988,35(1):5-20

ABSTRACT
Bedforms of the Surtainville area, off the Cherbourg peninsula, include subtidal sandwaves, which are a good example of mega-structures associated with the strong tidal currents prevailing in the English Channel. A fine-scale study using a high-accuracy echo-sounder and side-scan sonar shows that some of these sandwaves have a crescentic shape and a strong asymmetry indicating a sand movement toward the north. The sandwaves range in height from about 3.5 m to 7.5 m, in width from 100 m to 500 m and in length from 70 m to 200 m; their internal structure, revealed by the simultaneous use of a high-resolution seismic source, is characterized by large 'foreset' beds dipping in the same direction as the lee sides of the sandwaves. Groups of foresets are limited by reactivation surfaces which we interpret as erosional surfaces created by subordinate tides. The presence of horizontal erosional reflectors inside the sandwaves and the truncation of the present-day profiles may reflect the effects of storms. The asymmetry of the tide in the area studied, shown by long-term current measurements, indicates that these sandwaves belong to classes III or IV of Allen's (1980a) classification; the observed structures correspond very well to the prediction of Allen's conceptual model, but we suggest that long term phenomena like equinox cyclicity, associated with storms, may be responsible for their origin rather than the neap-spring-neap tidal cycles responsible for the internal structure of intertidal bedforms.  相似文献   

Dynamics and facies model of a macrotidal sand-bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)     

ROBERT W. DALRYMPLE  R. JOHN KNIGHT  BRIAN A. ZAITLIN  GERARD V. MIDDLETON 《Sedimentology》1990,37(4):577-612

The 40-km-long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high-tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel-shaped geometry is developed, and by erosional or non-depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single-channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary. Longitudinal progradation will produce a 20-m-thick, upward-fining succession, the lower 1/2–2/3 of which will consist of cross-bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb-dominated portion of this unit will be finer grained than the flood-dominated part, and will contain trough crossbedding produced by 3-D megaripples; the flood-dominated areas, by contrast, will consist mainly of compound cross-bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb- and flood-dominated areas) will create packages of ebb cross-bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral-accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper-flow-regime parallel lamination. The succession will be capped by a 4-m-thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.  相似文献   

Evolution and mechanics of a Miocene tidal sandwave   总被引：3，自引：0，他引：3  

P. A. ALLEN  P. HOMEWOOD† 《Sedimentology》1984,31(1):63-81

A remarkable exposure of Miocene marine molasse in western Switzerland records the evolution of a tidal sandwave over a period of approximately 2 1/2 months. The sandwave is composed of tidal ‘bundles’ in which a sandwave reactivation stage and full vortex stage can be recognized for the dominant flow (ebb tide) and a rippled flood apron overlain by high water drape for the reversed flow. Bundle thicknesses vary systematically through neap–spring cycles, with a periodicity of 27 demonstrating the semi-diurnal lunar control of sedimentation. Waves were an additional component, especially when superimposed on flood tides, producing near-symmetrical combined-flow ripple marks in the flood apron. Tidal current velocities are estimated using critical shear velocities for entrainment, the ripple-dune transition and the dune-plane bed transition. Using appropriate estimates of roughness lengths and a logarithmic velocity law, maximum tidal speeds at 1 m above the bed were approximately 0·6 m sec^?1 for ebbs and up to 0·5 m sec^?1 for floods. The enhancement by waves of bed shear stress (τ_wc/τ of approximately 2 for 1 m high waves) under flood currents implies flood tidal velocities closer to 0·2–0·3 m sec^?1. Peak instantaneous bedload sediment transport rates using a modified Bagnold equation are nearly 5 times greater under ebb tides than floods. The average net sediment transport rate at springs (0·04 kg m^?1 sec^?1) is over 10 times greater than at neaps (0·002 kg m^?1 sec^?1). Comparison with transport rates in modern tidal environments suggests that the marine molasse of Switzerland was deposited under spatially confined and relatively swift tidal flows not dissimilar to those of the present Dutch tidal estuaries.  相似文献   

Downslope‐migrating sandwaves and platform‐margin clinoforms in a current‐dominated,distally steepened temperate‐carbonate ramp (Guadix Basin,Southern Spain)     

ÁNGEL PUGA‐BERNABÉU  JOSÉ M. MARTÍN  JUAN C. BRAGA  ISABEL M. SÁNCHEZ‐ALMAZO 《Sedimentology》2010,57(2):293-311

During the Late Tortonian, platform‐margin‐prograding clinoforms developed at the south‐western margin of the Guadix Basin. Large‐scale wedge‐shaped deposits here comprise 26 rhythms of mixed carbonate–siliciclastic bedset packages and marl beds. These sediments were deposited on a shallow‐water, temperate‐carbonate distally steepened ramp. A downslope‐migrating sandwave field developed in this ramp, with sandwaves moving progressively down the ramp to the ramp‐slope, where they destabilized, folded and occasionally collapsed. Downslope sandwave migration was induced by currents flowing basinwards. During the Late Tortonian, the Guadix Basin was open north to the Atlantic Ocean via the Dehesas de Guadix Strait and connected east to the Mediterranean Sea through the Almanzora Corridor. According to the proposed current circulation model for the Guadix Basin for this time, surface marine currents from the Atlantic entered the basin from the northern seaway. These currents moved counter‐clockwise and shifted the sediment on the ramp, forming sandwaves that migrated downslope. The development of platform‐margin prograding clinoforms by the basinward sediment‐transport mechanisms inferred here is known relatively poorly in the ancient sedimentary record. Moreover, these wedge‐shaped geometries are similar to those found in some shelves in the Western Mediterranean Sea and could represent an outcrop analogue to (sub)‐recent, platform‐margin clinoforms revealed by high‐resolution seismic studies.  相似文献   

Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation   总被引：6，自引：0，他引：6  

KEITH W. SHANLEY  PETER J. McCABE†  ROBERT D. HETTINGER† 《Sedimentology》1992,39(5):905-930

Detailed models already exist that outline physical and temporal relationships in marine and marginal marine strata. Such models are still in their infancy in alluvial deposits. Recognition of tidal and estuarine influence in fluvial strata is critical to the development of high resolution sequence stratigraphic correlations between marine and non-marine strata. Strata that have previously been interpreted as low energy meandering river deposits contain sedimentary and biogenic structures that suggest a tidal influence. These structures include sigmoidal bedding, paired mud/silt drapes, wavy and lenticular bedding, shrinkage cracks, multiple reactivation surfaces, inclined heterolithic strata, complex compound cross-beds, bidirectional cross-beds, and trace fossils including Teredolites, Arenicolites and Skolithos. Although none of these structures is unique to tidal processes, the preponderance of data suggests that fluvial systems have been affected by tidal processes well inland of coeval shoreline deposits. These deposits rarely form a significant proportion of a depositional sequence; however, their occurrence allows time significant surfaces to be extended for tens or even hundreds of kilometres inland from coeval shoreline deposits. In Turonian through Campanian strata exposed in the Kaiparowits Plateau of southern Utah, tidally influenced facies are recognized within at least two distinct stratigraphic levels that were deposited during periods of relatively rapid base level rise. These strata form part of an alluvial transgressive systems tract. Landward of each of the marine transgressive maxima, tidal facies are present in fluvial channels that are completely encased in non-marine strata at distances up to 65 km inland from a coeval palaeoshoreline. Our work suggests that such deposits may have gone unrecognized in the past, but they form a significant component of alluvial strata in many depositional sequences. Although these tidally influenced fluvial deposits may be difficult to recognize, they are temporally equivalent to marine maximum flooding surfaces and provide a chronostratigraphic correlation between alluvial and nearshore marine deposits.  相似文献   

A tidal-inlet complex in the Cretaceous epeiric sea of North America: Virgelle Member, Milk River Formation, southern Alberta, Canada     

R. J. CHEEL  D. A. LECKIE† 《Sedimentology》1990,37(1):67-81

The upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing-on-Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave-dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non-marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal-inlet complex. Most inlet sections consist of an erosional base overlain by a shale-pebble conglomerate, followed by cross-bedded sandstones which become finer-grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re-activation surfaces and herringbone cross-beds. The sequence through the tidal-inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb-dominated, flood-dominated, or mixed-tidal influence. Ebb-dominated sections commonly contain lateral accretion surfaces whereas flood-dominated sections contain tidal-ramp deposits. Back-barrier lagoon deposits are dominated by sandstones of an extensive flood-tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb-tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave-induced longshore currents acting on the regionally wave-dominated shoreline.  相似文献   

Wave-induced liquefaction: a modern example from the Bay of Fundy     

ROBERT W. DALRYMPLE 《Sedimentology》1979,26(6):835-844

Soft-sediment deformation features occur commonly on parts of intertidal sand bodies in Cobequid Bay, Bay of Fundy. These features are small- to intermediate-sized, slump-like bodies, 1-3 m² in area and located on the crest and upper stoss side of ebb megaripples. External modification of these slumps indicates that they formed before complete emergence. The deformed cross-bedding within these bodies extends to a depth of 0.15-0.35 m and shows that deformation occurred during slumping and flowage of liquefied sand down the megaripple stoss side. Field evidence and calculations strongly indicate that this liquefaction results from the impact of 0.1-0.3 m high waves breaking against the megaripple lee faces. Neither rapid drawdown of the water level nor earthquake shocks are reasonable alternative explanations. Indigenous wave activity provides an attractive substitute to tectonism as an explanation of soft-sediment deformation in ancient shallow-water sediments. Slow wave-induced compaction may also account for the relative scarcity of deformation structures in shallow marine sandstones.  相似文献   

Stokes surfaces and the effects of near-surface groundwater-table on Aeolian deposition   总被引：3，自引：0，他引：3  

STEVEN G. FRYBERGER  CHRISTOPHER J. SCHENK†  LEE F. KRYSTINIK‡ 《Sedimentology》1988,35(1):21-41

ABSTRACT Stokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near-surface water-tables (Stokes, 1968). A water-table forms a downward limit of scour through the cohesion of damp or wet sand near water-table, and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand-air interface. Study of modern analogues reveals that Stokes surfaces exist in a variety of depositional settings, including a coastal offshore prograding sand sea (Jafurah, Saudi Arabia); a coastal onshore prograding sand sea (Guerrero Negro, Mexico) and a continental sand sea (White Sands, New Mexico, USA). These modern analogues indicate that our concept of Stokes surfaces must be broadened to include the following: (i) modern analogues for Stokes surfaces described here cover areas on the order of 25 km². These may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas; (ii) Stokes surfaces occupy a continuum in scale from local to extensive, and erosional surfaces of different magnitude may be stacked closely in the sediments; (iii) Stokes surfaces, although erosional in nature, are commonly associated with deposits both above and below the Stokes bounding surface which plainly reveal the influence of a near-surface groundwater control on wind sedimentation. Moreover, the erosional relief of the bounding surface itself (as well as other features) reveals the influence of a groundwater-table; (iv) Stokes surfaces may be diachronous, representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface; (v) Stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments, including interdunes, tidal flats, lagoons, beaches, lakes and non-aeolian sabkhas. Finally, modern examples from different depositional settings suggest that while most Stokes surfaces have many features in common (such as erosional ridges due to early cementation), there are some features which may, with further study, be revealed to be distinctive of an individual depositional setting.  相似文献   

Time and space distribution of megaripples and associated bedforms, Middle Atlantic Bight, North American Atlantic Shelf     

D. J. P. SWIFT  G. L. FREELAND  R. A. YOUNG 《Sedimentology》1979,26(3):389-406

Three genetically distinct size classes of lower regime transverse bedforms have long been known from laboratory studies, and from studies of the intertidal zone; ripples, megaripples, and sand waves. These features are also present on the subtidal shelf surface of the Middle Atlantic Bight, and their distribution in time and space allows us to draw inferences concerning the time and space pattern of sediment transport. Transverse bedforms in the Middle Atlantic Bight occur in response to tidal flows at estuary and inlet mouths and on tide-dominated banks; on the shelf surface, however, they are primarily responses to wind-driven flows. Ripples are the most widespread of the three classes. They are current-formed during peak storm flows, but are probably remade as oscillatory wave ripples as the flow wanes. Megaripples are found primarily on the inner shelf, also as responses to peak storm flows. Sand waves of several metres amplitude occur on the inner shelf in the vicinity of topographic highs; low amplitude sand waves (< 2m), solitary or in trains, are widespread on the inner shelf. They survive through many seasons of storm flows. Megaripples are especially interesting as records of specific flow events. They are widespread on the inner shelf during the winter, occurring in fields up to several kilometres in diameter. On a portion of the Long Island inner shelf during December 1976, megaripple fields covered approximately 15% of the shelf surface. They tend to be erased during the succeeding summer months. Both megaripples (short-term response elements) and sand waves (long-term response elements) indicate that sand transport in the Middle Atlantic Bight is directed to the southeast, parallel with the regional trend of the isobaths.  相似文献   

A study of the dynamics of a marine sandwave   总被引：3，自引：0，他引：3  

D. N. LANGHORNE 《Sedimentology》1982,29(4):571-594

The movement of the crest of a sandwave was studied using cross-sectional profiles obtained from lines of sea-bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest. The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea-bed by storm waves, caused major reductions in crestal heights. Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.  相似文献   

The complex diagenetic history of discontinuities in shallow‐marine carbonate rocks: New insights from high‐resolution ion microprobe investigation of δ18O and δ13C of early cements     

《Sedimentology》2018,65(2):360-399

Sedimentary gaps are a major obstacle in the reconstruction of a carbonate platform's history. In order to improve the understanding of the early diagenesis and the succession of events occurring during the formation of discontinuity surfaces in limestones, secondary ion mass spectrometry was used for the first time to measure the δ ¹⁸O and δ ¹³C signatures of 11 early cement and fabric stages in several discontinuity surfaces from the Jurassic carbonate platform of the Paris Basin, France. Pendant cements show a high variability in δ ¹⁸O, which was impossible to detect by the less precise microdrilling method. The morphology of a given cement can be produced in various environments, and dogtooth cements especially can precipitate in marine phreatic and meteoric phreatic to vadose environments. Marine dogtooth cements and micritic microbially induced fabrics precipitated directly as low‐magnesium calcite in marine waters, as attested to by the preservation of their initial δ ¹⁸O and δ ¹³C signals. Five discontinuity types are recognized based on high‐resolution geochemical analyses, and their palaeoenvironmental history can be reconstructed. Two exposure surfaces with non‐ferroan pendant or meniscus cements formed in the oxidizing vadose zone. A hardground displays marine fibrous cements and non‐ferroan dogtooth cements that formed in a subtidal environment in oxidizing water. Two composite surfaces have undergone both marine and subaerial lithification. Composite surface 1 displays non‐luminescent ferroan dogtooth cements that precipitated in reduced conditions in seawater, followed by brown‐luminescent dogtooth cements characteristic of a meteoric phreatic environment. Composite surface 2 exhibits microbially induced fabrics that formed in marine water with abundant organic matter. The latter discontinuity, initially formed in a subtidal environment, was subsequently exposed to meteoric conditions, as evidenced by ferroan geopetal cements. A high‐resolution ion microprobe study is essential to precisely document the successive diagenetic environments that have affected carbonate rocks and discontinuities with a polygenic and intricate history.  相似文献   

La stratigraphie oligo-miocène et la surface d'érosion messinienne en mer Noire,stratigraphie sismique haute résolution     

Hervé Gillet  Gilles Lericolais  Jean-Pierre Rehault  Corneliu Dinu 《Comptes Rendus Geoscience》2003,335(12):907-916

The stratigraphy of the Black Sea western margin is revisited through seismic data acquired during two French-Romanian surveys. These data are calibrated by industrial and DSDP drillings; they display several major discontinuities regarded as Tertiary erosional surfaces. The major seismic discontinuity underlines the base of Miocene formations and corresponds to a composite surface including at least three erosional phases ranging from Oligocene to Pontian times. Moreover, a Messinian erosional surface is clearly identified. This is in agreement with the Hsü's proposition [Palaeogeogr. Palaeoclimatol. Palaeoecol. 29 (1979) 75–93] suggesting a Messinian sea-level drop of Black Sea related to the Messinian Salinity Crisis described in the Mediterranean Sea. To cite this article: H. Gillet et al., C. R. Geoscience 335 (2003).  相似文献   

On Bagnold's sediment transport equation in tidal marine environments and the practical definition of bedload   总被引：1，自引：0，他引：1  

CHANG-SHU YANG 《Sedimentology》1986,33(4):465-486

Bagnold's sediment transport equation has proved to be important in studying tidal marine environments. This paper discusses three problems concerning Bagnold's transport equation and its practical application:

• 1 Bagnold's suspended-load transport equation and the total-load transport equation with are incorrect from the viewpoint of energy conservation. In these equations the energy loss due to bedload transport has been counted twice. The correct form should be for suspended-load transport and for total-load transport with
• 2 The commonly used Bagnold's transport coefficient K varies as a non-linear function of the dimensionless excess shear stress, which can be represented best by the power law , where the coefficient A and exponent B depend on sediment grain size D. The empirical values of A and B for fine to medium grained sands are determined using Guy et al.'s (1966) flume-experiment data.
• 3 The sediment transport rates predicted from this equation are compared with bedform migration measurements in the flume and the field. This comparison shows that the sediment transport rates measured from bedform migrations are higher than the predicted bedload transport rates, but comparable to the calculated total-load (bedload plus intermittent suspended-load) transport rates. This indicates that bedform migration involves both bedload and intermittent suspended-load transport. As a logical conclusion, bedform migration data should be compared with Bagnold's total-load transport equation rather than with his bedload transport equation. In this respect the term ‘bed material’ might be more appropriate than the term ‘bedload’ for estimating sediment transport rate from bedform migration data.

The sediment transport rates predicted from this modified Bagnold transport equation are in good agreement with field measurements of bedform migration rates in four individual tidal marine environments, which cover a wide range of sediment grain size, flow velocity and bedform conditions (ranging from small ripples, megaripples to sandwaves).  相似文献   

A Pleistocene sandur in western Norway: facies relationships and sedimentological characteristics   总被引：1，自引：0，他引：1  

JON Y. LANDVIK  JAN MANGERUD 《Boreas: An International Journal of Quaternary Research》1985,14(2):161-174

At Godøya near Ålesund sequences of unconsolidated fine sand and silt below two till beds are interpreted as remains of a sandur. Two facies sequences dominate: One comprises erosional scours followed by horizontally and current-ripple laminated fine sand, massive silt and erosional scours. The other sequence differs by planar wedge-shaped cross-beds replacing the horizontal lamination. The planar cross-beds are assumed to represent migrating linguoid or transverse bars, with an orientation partly at a high angle to current ripples in the same beds. The frequent silt beds are interpreted as a result of rapid vertical accretion due to isostatic subsidence during deposition. A Middle Weichselian age is assumed from thermoluminescence, radiocarbon and amino acid dates.  相似文献   

Sandwaves on the Southeast Vietnam Shelf recorded by high resolution seismic profiles: formation and mechanism   总被引：1，自引：0，他引：1  

Viet Dung Bui  Alex Schimanski  Karl Stattegger  Phach Van Phung  The Tiep Nguyen  Tien Hai Nguyen  Trung Thanh Nguyen  Truong Thanh Phi 《Frontiers of Earth Science》2009,3(1):9-20

The application of high resolution seismic data using boomer sound source has revealed a wide distribution of large-scale bedforms (sandwaves) on the Southeast Vietnam continental shelf. Bedforms that are a few meters high in wave height and hundreds of meters long in wavelength are primarily developed in the inner shelf (20–40 m) and considered to be formed under the present-day marine hydrodynamic conditions. Those bedforms developed in the deeper water (120 m) of the northernmost part of the continent can be interpreted as the relict morphological features formed during the latest sea-level lowstand of the late Pleistocene period. Two sediment transport paths have been identified on the basis of the bedform’s leeward orientation: northeast-southwest (along-shore) and north-south (cross-shore). A quantitative bottom current map is constructed from sandwave dimensions, surface sediments and measurement data. The strongest current velocities that gradually decrease toward the southwest are indicated by large sandwaves in the north (field B). Water depth, surficial sediment composition and bottom current are three factors that control the development of bedforms.  相似文献