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1.
Measurements were made of tidal currents, bed sediment particle sizes, and bedform dimensions at the South Branch and the South Channel of the Changjiang estuary, China, during the dry season in 1997 and the flood season in 1998. The near bottom current speed and direction were measured by a mechanical current meter for 10 h in 1997. The near surface current speed and direction were measured by a Current Meter of Endeco/YSI Inc. 174 SSM for 14 h in 1997 and 1998. Nine bed sediment samples were taken and their particle sizes were analyzed with sieves and siphons. The bedforms were nautically detected by an echo sounder and a side scan sonar. Results show that the ebb tides had larger near-bottom and near-surface current speeds and longer durations than the flood tides, in which the former occurred during the flood season in 1998 and not in the dry season in 1997. The bed sediments were composed of coarse silts and very fine sands during the dry season but of fine sands during the flood season. Bedforms were dominated by ebb tidal currents, the height∶length ratios of dunes and lee face angles were low, and heights and lengths were larger during the flood season in 1998. The ebb and flood tidal currents, bed sediment sizes, and dune morphology were largely controlled by the seasonal runoff variations. A new tentative boundary might be proposed for natural dunes in very fine sand with the availability of additional field data in the future.  相似文献   

2.
Clay-drape couplets on subaqueous dunes have been regarded as a diagnostic feature of the subtidal environment since Visser's seminal paper (1980). The new observation of clay-drape couplets in the intertidal zone on a present day tidal bar of the Gironde estuary shows that they are not restricted to the subtidal zone.
In the intertidal zone, low-tide slack-water clay drapes are deposited in the bottomsets of the dominant current dunes when the muddy water retained in the troughs is absorbed into the sand during the emergence of the intertidal bar. They drape emergence run-off ripples generated by the drainage currents in the bottomsets. High-tide slack-water clay drapes are deposited over the entire dune surface and are preserved on the lee side of the dunes and in the bottomsets. They drape the subordinate current ripples. Low-tide and high-tide slack-water clay drapes enclose one thin rippled sand layer (the subordinate current bundle) and are isolated from other adjacent clay-drape couplets by the dominant current bundle.
The clay-drape couplets deposited in the intertidal zone can be distinguished from their subtidal counterparts on the basis of two morphological differences:
1. In the intertidal zone, the low-tide clay drape is only present in the bottomsets of the dunes, whereas in the subtidal zone equivalent clay drapes are also present on the lower part of the lee side of the dunes.
2. In the intertidal zone, low-tide clay drapes are deposited in the bottomsets of the dunes over emergence run-off ripples oriented in the direction of the drainage currents (i.e. in a direction normal to the tidal currents). Conversely, in the subtidal zone, the equivalent clay drapes are typically deposited over ripples oriented in the tidal-current direction (ebb or flood). There is a difference of polarity of 90° between the intertidal and subtidal small-scale bedforms draped by the low-tide slack-water drapes.  相似文献   

3.
《Sedimentary Geology》2006,183(3-4):159-179
In the macrotidal Severn estuary, UK, the dynamics of intertidal fine-gravel dunes were investigated. These dunes are migrating across a bedrock platform. Systematic observations were made of hydraulic climate, geometry, migration rates and internal sedimentary structures of the dunes. During spring tides, the ebb flow is dominant, dunes grow in height and have ebb orientated geometry with bedrock floors in the troughs. During neap tides, a weak flood flow may dominate. Dunes then are flood orientated or symmetrical. Neap dune heights decrease and the eroded sediment is stored in the dune troughs where the bedrock becomes blanketed by muddy gravel. During spring tides, instantaneous bed shear stresses reach 8 N m− 2, sufficient to disrupt a 9 mm-gravel armour layer. However, a sustained bed shear stress of 4 N m− 2 is required to initiate dune migration at which time the critical depth-mean velocity is 1 m s− 1. Ebb and flood inequalities in the bed shear stress explain the changes in dune asymmetry and internal structures. During flood tides, the crests of the dunes reverse such that very mobile sedimentary ‘caps’ overlie a more stable dune ‘core’. Because ebb tides dominate, internal structures of the caps often are characterised by ebb orientated steep open-work foresets developed by strong tidal currents and some lower angle crossbeds deposited as weaker currents degrade foresets. The foresets forming the caps may be grouped into cosets (tidal bundles) and are separated from mud-infused cores of crossbeds that lie below, by reactivation and erosion surfaces blanketed by discontinuous mud drapes. The cores often exhibit distinctive muddy toe sets that define the spacing of tidal cosets.  相似文献   

4.
Carling  Williams  Gölz  & Kelsey 《Sedimentology》2000,47(1):253-278
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5.
Current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments. There are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides. As a consequence, the ability to predict bedform response to variable flow is rudimentary. Using high‐resolution multibeam bathymetric data, this study explores the dynamics of a dune field developed by tidally modulated, fluvially dominated flow in the Fraser River Estuary, British Columbia, Canada. The dunes were dominantly low lee angle features characteristic of large, deep river channels. Data were collected over a field ca 1·0 km long and 0·5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet, yielding the most comprehensive characterization of low‐angle dunes ever reported. The data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs, then increasing as the tide rises and the flow velocities decrease. Bedform lengths do not appear to respond to the changes in velocity caused by the tides. Changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity, indicating that changes in the bedform geometry lag changes in the flow. The data reveal that lee angle slope responds directly to suspended sediment concentration, supporting previous speculation that low‐angle dune morphology is maintained by erosion of the dune stoss and crest at high flow, and deposition of that material in the dune trough.  相似文献   

6.
The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D50 of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D50 of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D50 < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport.  相似文献   

7.
A study of the dynamics of a marine sandwave   总被引：3，自引：0，他引：3  
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   

8.
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   

9.
The oolitic ironstones of the Minette were deposited during Toarcian/Aalenian times in a nearshore environment of the Paris Basin. The sedimentary sequence comprises up to 13 coarsening upward depositional cycles. The development of the cycles and different facies types are described. The iron ooids accumulated in a subtidal environment under the effects of tidal currents. Sand waves, which form an important part of the Minette sediments, were deposited under both time-velocity symmetrical and time-velocity asymmetrical tides. From the study of their internal structures the time-velocity patterns of the currents, current speeds (up to 0±9m s?1) and net transport rates have been estimated (29 g m?1 s?1 in the case of large-scale sand waves controlled by time-velocity symmetrical tides). Sand wave heights and calculated near-bed current speeds suggest a mesotidal regime. Sedimentary facies include large-scale and waves, subtidal shoals with channels and an offshore muddy shelf. Conclusions as to bathymetry are drawn from ichnofaunal associations and from the study of wave ripple marks. Finally a depositional model of the Minette iron formation is proposed which may be applicable to other oolitic ironstones.  相似文献   

10.
Coarse-gravel bedforms which resulted from Pleistocene glacial outburst floods are identified as subaqueous dunes. Comparison of the morphology of these ‘fossil’ structures with modern dunes shows that the form of two-dimensional (2-D) transverse dunes and 3-D cuspate and lunate dunes developed in coarse gravels is comparable with sand-dune morphology within lesser-scale geophysical flows. The similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform. Internal structure indicates that flow over 2-D dunes was relatively uniform but over 3-D bedforms flow was locally variable. Flow separation and complex streaming of flow occurred over the steepest 3-D dunes. Cross-beds are thin and few approach the angle of repose; consequently most dunes did not migrate primarily by avalanching but by stoss-entrained gravel transported over the crests rolling-down and depositing on the lee slopes. Lee-side sediments are often finer than the stoss-slope sediments, which indicates the lee formed when flood power was waning. Some dunes were slightly planed-down during falling stage because lee-side cross-beds tend to be steeper than the angle of the preserved lee slope. However, silt-rich caps indicate that any height reduction was contemporary with the final deposition of foresets. Post-flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs.  相似文献   

11.
Large symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near-bed velocity and bed load transport near dune crests result in crest rounding. Long, low-angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near-bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume-based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows.  相似文献   

12.
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   

13.
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   

14.
The continental shelf of the State of Rio Grande do Norte, Brazil, is an open shelf area located 5°S and 35°W. It is influenced by strong oceanic and wind-driven currents, fair weather, 1·5-m-high waves and a mesotidal regime. This work focuses on the character and the controls on the development of suites of carbonate and siliciclastic bedforms, based on Landsat TM image analysis and extensive ground-truth (diving) investigations. Large-scale bedforms consist of: (i) bioclastic (mainly coralline algae and Halimeda) sand ribbons (5–10 km long, 50–600 m wide) parallel to the shoreline; and (ii) very large transverse siliciclastic dunes (3·4 km long on average, 840 m spacing and 3–8 m high), with troughs that grade rapidly into carbonate sands and gravels. Wave ripples are superposed on all large-scale bedforms, and indicate an onshore shelf sediment transport normal to the main sediment transport direction. The occurrence of these large-scale bedforms is primarily determined by the north-westerly flowing residual oceanic and tidal currents, resulting mainly in coast-parallel transport. Models of shelf bedform formation predict sand ribbons to occur in higher energy settings rather than in large dunes. However, in the study area, sand ribbons occur in an area of coarse, low-density and easily transportable bioclastic sands and gravels compared with the very large transverse dunes in an offshore area that is composed of denser medium-grained siliciclastic sands. It suggests that the availability of different sediment types is likely to exert an influence on the nature of the bedforms generated. The offshore sand supply is time limited and originates from sea floor erosion of sandstones of former sea-level lowstands. The trough areas of both sand ribbons and very large transverse dunes comprise coarse calcareous algal gravels that support benthic communities of variable maturity. Diverse mature communities result in sediment stabilization through branching algal growth and binding that is thought to modify the morphology of dunes and sand ribbons. The occurrence and the nature of the bedforms is controlled by their hydrodynamic setting, by grain composition that reflects the geological history of the area and by the carbonate-producing benthic marine communities that inhabit the trough areas.  相似文献   

15.
Bedform climbing in theory and nature   总被引：7，自引：0，他引：7  
Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.  相似文献   

16.
Aeolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Palaeozoic strata. These rocks have been interpreted as a low latitude signal of glacio-eustatic sea level fluctuations and have not been previously reported from the Mesozoic or from other episodes of earth history generally believed to have been non-glacial. Numerous lenticular bodies of cross-stratified oolite lie near the contact between the lower and upper members of the mudstone-dominated lower Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north-central Wyoming, USA. The lenses, up to 12 m thick, contain sedimentary structures diagnostic of aeolian deposition. Inversely graded laminae within thick sets of cross-strata were deposited by climbing wind ripples. Adhesion structures and evenly dispersed lag granules are present in flat-bedded strata at the bases of several of the oolite bodies. Thin sections reveal abundant intergranular micrite of vadose origin. The lenses appear to represent virtually intact, isolated aeolian bedforms that migrated across a nearly sand-free deflation surface. When the Sundance Sea transgressed the dunes, a thin (<1 m thick), wave-rippled, oolite veneer formed on the upper surface of the aeolianite. Previous workers, primarily on the basis of sedimentary structures in the veneer, interpreted the oolite lenses as tidal sand bodies. The dunes provide clear evidence of widespread subaerial exposure on the crest and north flank of the Sheridan Arch. This structural high was delineated by previous workers who demonstrated thinning of pre-upper-Sundance Formation strata and localized development of ooid shoals. Ooids that formed in shoals on the windward (southern) side of the palaeohigh were exposed and deflated during lowstand. Thin, scour-filling ooid grainstone lenses that crop out in the southern part of the study area represent remnants of the marine beds that sourced the aeolianites. Farther north (down-wind), oolitic dunes prograded over thinly laminated lagoonal silts. When relative sea level began to rise, the uncemented dunes were buried under fine-grained marine sediment as the lee side of a low-relief island was inundated.  相似文献   

17.
Gravel antidunes in the tropical Burdekin River, Queensland, Australia   总被引：4，自引：0，他引：4  
The geological record is punctuated by the deposits of extreme event phenomena, the identification and interpretation of which are hindered by a lack of data on contemporary examples. It is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle-transporting, high-velocity currents, and therefore, their characteristics are poorly documented. The deposits of such flows are exposed however, in the dry bed of the Burdekin River, Queensland, Australia following tropical cyclone-induced floods. Long wave-length (up to 19 m) gravel antidunes develop during short (days) high-discharge flows in the upper Burdekin River (maximum recorded discharge near the study reach over 25 600 m3 s?1 in February 1927). Flood water levels fall quickly (metres in a day) and flow is diverted away from raised areas of the river bed into subchannels, exposing many of the high-stage bedforms with little reworking by falling-stage currents. Gravel bedforms were observed on the dry river bed after the moderate flows of February 1994 (max. 7700 m3 s?1) and January 1996 (max. 3200 m3 s?1). The bedforms had wave-lengths in the range 8–19 m, amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak-discharge flow direction. The gravel fabric and size sorting change systematically up the stoss and down the lee faces. The antidune deposits form erosive based lenses of sandy gravel with low-angle downstream dipping lamination and generally steep upstream dipping a-b planes. The internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses. The erosive based lenses of low-angle cross-bedded gravel with steep upstream dipping a-b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes. The antidune gravel lenses are associated with thick (to 1 m) high-angle cross bed sets. Ancient antidune gravel lenses may be diagnostic of episodic high-discharge conditions and particularly when they are associated with high-angle cross-bedded gravelly sand they may be useful for palaeoenvironmental interpretation.  相似文献   

18.
The Neoproterozoic Kansapathar Sandstone of the Chattisgarh basin, a shallow marine shelf bar sequence, consists of mineralogically and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. The sediments were transported, reworked and deposited in subtidal environments by strong tidal currents of macrotidal regime as well as storms, and accumulated as discrete shoaling-upward features, separated from each other by muddy to low-energy sandy deposits. The sandbodies developed into shoaling up linear bars, often more than a kilometre in length, through accretion of thick cross-stratified units in transverse directions under the influence of ebb and flood tidal currents, as well as in longitudinal direction affected by southeasterly flowing along-shore currents. The aggrading upper surfaces of the bars experienced protracted reworking by strong oscillatory wave currents leading to extensive development of subaqueous 2D or 3D dunes mantled with lag pebble deposits at different points. With continued shoaling and progradation, the bars amalgamated into large sandstone sheets with the development of high energy beach deposits and coastal sand flats in the uppermost part of the sequence. The presence of rill marks, flat-topped ripples, wrinkle marks, desiccation cracks and adhesion warts point to intertidal conditions with intermittent exposure. The high energy sandstone bars overlie a thick mudstone-dominated shelf sequence across a sharp interface indicating rapid change in the sea-level, provenance, rate of sediment generation and sediment input, and circulation condition in the shelf. A quiet muddy shelf was replaced by a major sand-depositing environment with strong, open marine circulation. An interplay of tidal currents, oscillatory wave currents and storm currents generated a complex flow pattern that varied in time and space from bimodal-bipolar to strongly unimodal flows. Close parallelism of wave ripple crests, trend of linear bars and unidirectional flows suggest that the elongate bars were parallel to sub-parallel to the coastline, and were strongly influenced by along shore drift. The inferred coastline was broadly N-S. The large-scale structures in the bar sandstones, emplacement of vast amount of sand and migration of large bedforms under strong macrotidal currents collectively indicate that the Kansapathar shelf was intimately connected with an open ocean basin towards north-northwest.  相似文献   

19.
Evolution and mechanics of a Miocene tidal sandwave   总被引：3，自引：0，他引：3  
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 (τwc/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   

20.
Largescale ripples in the meandering lower Wabash River of Illinois and Indiana, U.S.A., include scroll bars and three dunelike bed forms (dunes, sand waves, and transverse bars). Scroll bars are lobate crested, asymmetrical in stream-wise vertical profile, usually solitary, and oriented approximately normal to local channel strike. They form by passive flow expansion downchannel from locally emergent topographic highs, face and lie near inner banks of meander bends, enjoy a high preservation potential as leveelike ridges of ridge-and-swale topography, and migrate only during relatively low stream discharges, when water depth over bar crests is less than 0·5 m. Dunes correspond to dunes of the flow-regime classification and rarely are solitary or superimposed. Sand waves may be symmetrical or asymmetrical, are always superimposed by dunes, occur in depths greater than 4 m and in bed material coarser than 1 mm mean size, and develop at bankfull and flood flows. Transverse bars migrate in depths less than 5 m in straight reaches and near inner banks of bends, display crestal dunes, and correspond to the bars of Costello (1974) and to the sand waves of Boothroyd (1969). Hydrodynamic regimes of scroll bars and transverse bars differ from that of dunes. The omnipresence of dunes upon stoss-sides of sand waves confirms the existence of an equilibrium superimposition of dunelike largescale ripples. Depth-velocity-size diagrams appear to be a valid representation of empirical stability fields of dunelike largescale ripples in deep unsteady nonuniform aqueous flows. Stability fields of dunes and sand waves overlap greatly. Velocity profiles demonstrate an absence of leeside flow separation over dunes and an appearance (rare) over transverse bars only when the ratio of trough depth to crest depth exceeds two. Dune stratification displays (1) largescale trough cross-strata, (2) thinning of sets as bed-material size increases, and (3) an orientation within 20° of local channel strike. Transverse bars show avalanche sets up to 2 m thick, with reactivation surfaces. Scroll bars display thick avalanche sets separated by reactivation structures consisting of erratically oriented smallscale trough cross-strata. Avalanche sets of scroll bars and of transverse bars are oriented 50–150° from and within 50° of, respectively, local channel strike.  相似文献   

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