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1.
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   

2.
The Magdalena River is noted for its high discharge of river sediment and its importance as the sediment source for a large delta complex and downdrift coastal sand bodies. The emplacement of jetties, completed in 1935 to stabilize the river mouth, contributed to major changes in the downstream coastal sand bodies. The western delta front retreated an average 65 m/yr. Puerto Colombia spit detached and migrated toward Puerto Colombia at rates of 230–430 m/yr, ultimately running into the town's quay and port facility. Galerazamba spit alternately elongated and shortened over the short term, leading to the destruction or damage of coastal town sites. Isla Cascajo acted as a significant sand trap with nearly 12 km2 of accretion over a 47-year period. Sand is now bypassing the tombolo, and the accretion zone continues migrating southwest. The small Punta Canaos spit also has shown significant accretion since 1974. The changes imply high rates of sediment transport; furthermore their growth is probably dependent on jetty-caused alterations of wave patterns, causing remobilization of shelf sands as well as delta-derived sand.Understanding sand body evolution and behavior is important to future development of the northern Colombia coast. Placement of port facilities, recreational beaches, tourist villages, and related support facilities on these sand bodies, as well as utilizing the sand bodies for aggregate, beach nourishment sands for other areas, or heavy mineral resources will require significant planning.  相似文献   

3.
Morphology and internal structure of sandwaves in the Bay of Fundy   总被引：1，自引：0，他引：1  
Intertidal sandwaves in the Minas Basin and Cobequid Bay, Bay of Fundy, occur under a wide range of conditions (mean grain size 0.274-1.275 mm; velocity strength index (V1)0.46-3.34; and velocity symmetry index (V2) 0.011-0.294), and they vary from symmetrical to strongly asymmetrical in cross-section. Heights and wavelengths average 0.81 and 37.9 m respectively. They are straight to weakly sinuous and laterally continuous in plan, occasionally show crestal branching reminiscent of wave ripples, and are commonly skewed relative to the strongest currents because of differential migration rates along their length. The average migration rate is 0.11 m/tidal cycle. Megaripples occur on each sandwave crest, at least during spring tides, but the areal extent, sinuosity and size of the megaripples increases as the dominant current speed increases. The megaripples have heights averaging 24% of the sandwave height, are oriented perpendicular to the fastest dominant currents, and have life spans of several tidal cycles. They are believed to be in quasi-equilibrium with the sandwaves and play a key role in sandwave dynamics and internal structure formation: periods of lee face steepening and rapid forward migration (megaripple crest at sandwave brink) alternate with times of non-deposition or erosion and slowed or reversed migration (trough at brink). Dominant-current cross-bedding predominates in the two intergradational varieties of translation structure observed: Inclined Cross-Bedding—decimetre-scale cross-beds separated by gently inclined (9° average) erosional surfaces; and Large-Scale Foresets—cross-beds with thicknesses greater than half the sandwave height, interrupted by weakly erosional to conformable discontinuity surfaces. These are overlain by a vertical growth or repair structure, Complex Cross-Bedded Cosets, that consists of nearly equal volumes of dominant- and subordinate-current cross-beds stacked without a preferred set-boundary dip. The translation structures correspond well to forms predicted by Allen (1980a, fig. 8) but the inclined set boundaries and discontinuity surfaces (master bedding planes) are produced by megaripple troughs rather than by current reversals. Consequently, Allen's regime diagram is unable to predict structure occurrences. The repair structures suggest that ‘curvature-related mass-transport’ (Allen, 1980a, b) is important in tidal sandwave maintenance, although it is not necessarily responsible for sandwave initiation.  相似文献   

4.
Physical stratigraphy, architecture and evolution of barrier systems in hypertidal environments (tidal range above 6 m) are understudied, and depositional controls are poorly understood compared with wave-dominated barrier systems and barrier spits in microtidal, mesotidal and macrotidal settings. Based on vibracores, ground-penetrating radar, radiocarbon and optically stimulated luminescence age data, the formation and evolution of the hypertidal barrier system of Pointe du Banc, north-western France has been reconstructed. The study shows that the barrier spit has a complex composite sedimentary architecture consisting of wave-dominated and tide-dominated sedimentary bodies. A morpho-sedimentary model is presented that demonstrates how barrier-spit progradation resulting from littoral drift was the main manner of spit elongation, whereas sediment convergence caused by landward migration of swash bars and seaward migration of tidal dunes caused the spit terminus to grow in height and width. These results suggest that long-term accretion rates varied considerably in response to changes in sediment supply. Variations in storminess together with the large-scale topography of the coast controlled the sediment supply and thus the evolution of the barrier system during latest Holocene. Despite architectural complexity, hypertidal barrier systems preserve records of past climate changes.  相似文献   

5.
This study evaluates the impacts resulting from the construction of two large-scale detached breakwater systems on the Nile delta coast of Egypt at Baltim and Ras El Bar beaches (~18.3 km shoreline length). The two protective systems were installed in a water depth of between 3 and 4 m and consist of 17 units in total (each ~250 m long). A comprehensive monitoring program spanning the years 1990 to 2002 was implemented and included beach-nearshore profiles, grain size distribution of seabed sand and information related to the background coastal processes. Evaluation of these systems concentrates on the physical impacts on coastal morphodynamics, mitigation and their design implications. The beach and nearshore sedimentation (erosion/accretion patterns) and grain texture of seabed sediment in the study areas have been substantially disrupted due to the interruption of longshore transport by the shore-parallel detached breakwaters. Rate of shoreline and seabed changes as well as alongshore sediment volume have been substantially affected, resulting in accretion in the breakwater landward sides (tombolo or salient) followed by downdrift erosion. The preconstruction beach erosion at Baltim (–5 m/year) and at Ras El Bar (–6 m/year) has been replaced, respectively, by the formation of sand tombolo (35 m/year) and salient (9 m/year). On the other hand, beach erosion has been substantially increased in the downdrift sides of these protective systems, being –20 m/year at Baltim and –9 m/year at Ras El Bar. Further seaward, the two protective systems at Baltim and Ras El Bar have accumulated seabed sand at maximum rates of 30 and 20 cm/year and associated with downdrift erosion of –45 and –20 cm/year, respectively. Strong gyres and eddies formed in the breakwater gabs have drastically affected swimmers and subsequently caused a significant number of drownings each summer, averaging 35 and 67 victims/year at Baltim and Ras El Bar beaches, respectively. This study provides baseline information needed to help implement mitigation measures for these breakwater systems.  相似文献   

6.
Landsat enhanced thematic mapper imagery (ETM) of 2002 and aerial photography of 1955, combined with published charts and field observations were used to interpret coastal changes in the zone between Kitchener drain and Damietta spit in the northeastern Nile delta, previously recognized as a vulnerable zone to the effects of any sea level rise resulting from global warming. The interpretation resulted in recognition of several changes in nine identified geomorphological land types: beach and coastal flat, coastal dunes, agricultural deltaic land, sabkhas, fish farms, Manzala lagoon, saltpans, marshes and urban centers. Reclamation of vast areas of the coastal dunes and of Manzala lagoon added about 420 km2 to the agricultural deltaic land. About 48 km2 of backshore flats, marshes, salt pans and Manzala lagoon have been converted to productive fish farms. The main urban centers have expanded; nearly 12.1 km2 have been added to their areas, and new urban centers (Damietta harbor and the New Damietta city) with total area reach of ~35.3 km2 have been constructed at the expense of vast areas of Manzala lagoon, coastal dunes, and backshore flats. As a consequence of human activities, the size of Manzala lagoon has been reduced to more than 65%. Shoreline changes have been determined from beach profile survey (1990–2000), and comparison of 1955 aerial photographs and ETM satellite image of 2002 reveal alongshore patterns of erosion versus accretion. The short-term rate of shoreline retreat (1990–2000) has increased in the downdrift side of Damietta harbor (≃14 m/year), whereas areas of accretion exist within the embayment of Gamasa and in the shadow of Ras El Bar detached breakwaters system, with a maximum shoreline advance of ~15 m/year. A sandy spit, 12 km long, has developed southeast of Damietta promontory. These erosion/accretion patterns denote the natural processes of wave-induced longshore currents and sediment transport, in addition, the impact of man-made coastal protection structures.  相似文献   

7.
The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers (210Pb and 137Cs), we observe no significant change in overall accretion rates (0.27–0.69 cm year?1) compared to historical averages (0.24–0.60 cm year?1), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27–0.45 cm year?1) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41?±?0.07 cm year?1). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the long-term survivability of these marshes.  相似文献   

8.
Spit systems are seldom recognized in the pre‐Quaternary sedimentary record compared to their common occurrence along present‐day coasts and in Quaternary successions. This lack of recognition may partly be due to the lack of widely accepted depositional models describing the facies characteristics of spit systems and their subaqueous platforms in particular. The Skagen spit system is a large active system that began to form 7150 yr bp and from 5500 bp to Recent times it has prograded 4 m year?1 and accumulated 3·5 × 109 m3 of sand. The spit system provides a unique opportunity for establishing a well‐constrained depositional model because uplift and erosion have made large windows into the preserved facies, while active spit‐forming processes can be examined at the young prograding end of the same system. The depositional model presented here thus builds on excellent outcrops, surface morphology, a well‐defined palaeogeography and detailed C14 age control supplemented with observations from continuous well cores and profiles obtained by ground‐penetrating radar and transient electromagnetic surveys. The factors that have governed the development of the spit system, such as relative sea‐level change, wave and current climate, tidal range, sediment transport and depositional rates are also well‐understood. The sedimentary facies of the spit system are grouped into four principal units consisting from below of thick storm sand beds, dune and bar‐trough deposits, beach deposits and peat beds. These four units form a coarsening and shallowing upward sand‐dominated succession, up to 32 m thick, which overlies offshore silt with a transition zone and is topped by a diastem overlain by young aeolian dune sand. The sedimentary structures and depositional processes are described in detail and integrated into a depositional model, which is compared to other spit systems and linear shoreface models.  相似文献   

9.
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 1 mh?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   

10.
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   

11.
ABSTRACT Lateral migration of the Keurbooms Inlet along its barrier causes stratigraphic stacking of the different sedimentary units in the inlet area thus accumulating an inlet sequence under the newly formed barrier. Deposition in the inlet is controlled by wave and tidal action. Migration is caused by sediment accretion on the downdrift side of the inlet and erosion on its updrift side. The migration direction is therefore in the opposite direction to the longshore current. This is comparatively rare as most inlets with a tendency to migrate do so in the same direction as longshore drift. Tidal Current directions in the inlet are highly variable and not bipolar.
The inlet-associated environment is divided into a seaward wave-dominated zone and a landward tide-dominated zone. These zones probably have laterally interfingering facies. Only the lower part of the sequence is likely to be preserved. Recognition of ancient analogues in the rock record may be obscured by the unidirectional palaeocurrent pattern of the lower part of the inlet sequence.  相似文献   

12.
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   

13.
A coincidence of the Beeswax galleon shipwreck (ca. A.D. 1650–1700) and the last Cascadia earthquake tsunami and coastal subsidence at ∼A.D. 1700 redistributed and buried wreck artifacts on the Nehalem Bay spit, Oregon, USA. Ground‐penetrating radar profiles (∼7 km total distance), sand auger probes, trenches, cutbank exposures (29 in number), and surface cobble counts (49 sites) were collected from the Nehalem spit (∼5 km2 area). The field data demonstrate (1) the latest prehistoric integrity of the spit, (2) tsunami spit overtopping, and (3) coseismic beach retreat since the A.D. 1700 great earthquake in the Cascadia subduction zone. Wreck debris was (1) initially scattered along the spit ocean beaches, (2) washed over the spit by nearfield tsunami (6–8 m elevation), and (3) remobilized in beach strandlines by catastrophic beach retreat. Historic recovery of the spit (150 m beach progradation) and modern foredune accretion (>5 m depth) have buried both the retreat scarp strandlines and associated wreck artifacts. The recent onshore sand transport might re‐expose heavy ship remains in the offshore area if the wreck grounded in shallow water (<20 m water depth of closure). © 2011 Wiley Periodicals, Inc.  相似文献   

14.
A sedimentary model for hooked spit depositional systems based on ground‐penetrating radar and sedimentological data is presented. The recurved main spit of Sylt Island (southern North Sea) is dominated by migrating sand dunes; the investigated hooked spit exhibits a system of foredune ridges, oriented perpendicular to the dunes of the recurved spit. The development of the hooked spit is related closely to the presence of an adjacent tidal inlet, where strong tidal currents and a steep bathymetry prevent a further northward progradation of the main spit and trigger a deflection from northerly‐directed to easterly‐directed net sediment transport. Ground‐penetrating radar data and shallow sediment cores reveal the sedimentary architecture of the hooked spit in high resolution and allow the proposition of a genetic stratigraphic model. It is shown that the growth of the hooked spit is controlled by the interplay of alongshore migrating foreshore beach drifts under fair‐weather conditions and strong erosional events, interpreted as the result of rare severe storms. These storms may excavate scarps in the backshore, which play an important role in the development of foredune ridges. Accelerator mass spectrometry 14C ages indicate an absolute age of at least 1300 years for the hooked spit, which possibly correlates with strengthened erosion of the main spit. In contrast to the main spit, where the sediment budget is negative nowadays, growth of the hooked spit beach accelerated significantly during the last decades. This effect can probably be attributed to enhanced beach‐nourishments updrift along the main spit and makes the investigated hooked spit a natural laboratory to study the influence of increasing sediment supply into a system developing under the conditions of sea‐level rise. The study shows that the same external forces lead to distinct progradational processes along one barrier‐spit system.  相似文献   

15.
《Sedimentary Geology》1999,123(3-4):199-218
Gravelly shoreline deposits of the latest Pleistocene highstand of Lake Lahontan occur in pristine depositional morphology, and are exposed in gravel pits along Churchill Butte in west-central Nevada. Four environments differentiated at this site are alluvial fan/colluvium, lakeshore barrier spit, lake lower-shoreface spit platform, and lake bottom. Lakeshore deposits abut, along erosional wave headcuts, either unsorted muddy to bouldery colluvium fringing Churchill Butte bedrock, or matrix-supported, cobbly and pebbly debris-flow deposits of the Silver Springs fan. The lakeshore barrier spit is dominated by granule pebble gravel concentrated by wave erosion of the colluvial and alluvial-fan facies. The lakeward side of the barrier consists of beachface deposits of well-sorted granules or pebbles in broad, planar beds 1–10 cm thick and sloping 10–15°. They interfinger downslope with thicker (10–25 cm) and less steep (5–10°) lakeward-dipping beds of fine to medium pebble gravel of the lake upper shoreface. Interstratified with the latter are 10–40-cm-thick sets of high-angle cross-beds that dip southward, alongshore. Higher-angle (15–20°), landward-dipping foresets of similar texture but poorer sorting comprise the proximal backshore on the landward side of the barrier. They were deposited during storm surges that overtopped the barrier berm. Gastropod-rich sand and mud, also deposited by storm-induced washover, are found landward of the gravel foresets in a 15-m-wide backshore pond. Algal stromatolites, ostracodes, and diatoms accumulated in this pond between storm events. The lake lower shoreface, extending from water depths of 2 to 8 m, consists of a southward-prograding spit platform built by longshore drift. The key component of this platform is large-scale sandy pebble gravel in 16° southward-dipping `Gilbert' foresets that grade at a water depth of about 6–7 m to 4°-dipping sandy toesets. A shift from bioturbated lower-shoreface sand and silt, to flat and laminated lake-bottom silt and mud, occurs between water depths of 10–40 m and over a shore-normal distance of ≥250 m. This lake-bottom mud facies, unlike the others, is areally expansive.  相似文献   

16.
Sediment accretion was measured at four sites in varying stages of forest-to-marsh succession along a fresh-to-oligohaline gradient on the Waccamaw River and its tributary Turkey Creek (Coastal Plain watersheds, South Carolina) and the Savannah River (Piedmont watershed, South Carolina and Georgia). Sites included tidal freshwater forests, moderately salt-impacted forests at the freshwater–oligohaline transition, highly salt-impacted forests, and oligohaline marshes. Sediment accretion was measured by use of feldspar marker pads for 2.5 year; accessory information on wetland inundation, canopy litterfall, herbaceous production, and soil characteristics were also collected. Sediment accretion ranged from 4.5 mm year?1 at moderately salt-impacted forest on the Savannah River to 19.1 mm year?1 at its relict, highly salt-impacted forest downstream. Oligohaline marsh sediment accretion was 1.5–2.5 times greater than in tidal freshwater forests. Overall, there was no significant difference in accretion rate between rivers with contrasting sediment loads. Accretion was significantly higher in hollows than on hummocks in tidal freshwater forests. Organic sediment accretion was similar to autochthonous litter production at all sites, but inorganic sediment constituted the majority of accretion at both marshes and the Savannah River highly salt-impacted forest. A strong correlation between inorganic sediment accumulation and autochthonous litter production indicated a positive feedback between herbaceous plant production and allochthonous sediment deposition. The similarity in rates of sediment accretion and sea level rise in tidal freshwater forests indicates that these habitats may become permanently inundated if the rate of sea level rise increases.  相似文献   

17.
Spatial and quantitative analysis of infilling processes of the tide‐dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post‐Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea‐level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet‐like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea‐level rise of 4 to 7 mm yr?1. Before migration of the Tone River at 5 ka, the tide‐dominated bay in the Nakagawa Valley was filled by upward‐fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide‐dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward‐coarsening deltaic sediments.  相似文献   

18.
The morphology and dynamics of modern gravel shorefaces are poorly documented. This hinders the interpretation of possible ancient counterparts. A comparative study of a modern (Chesil Beach, England) and an ancient (Baytree Member of the Cardium Formation, Alberta) gravel shoreface shows that the two systems are very similar close to and above sea-level, with a high (about 1 m) gravel plunge step lying below plane-bedded sands and gravels of the beachface. The shoreface at Chesil Beach is dominated by asymmetrical gravel wave ripples. These are oriented offshore near the toe of the shoreface, and onshore in shallower depths. This may reflect offshore movement during storms and landward reworking during fair weather. The Baytree Member is over 12 m thick and comprises over 80% conglomerate. Conglomerate is decimetre-bedded, massive or cross-bedded, with sets over 60 cm thick produced by gravel bedforms migrating alongshore. It is interbedded with discontinuous cm- to dm-bedded sandstones which may be cross-bedded. Pebble fabric and cross-bed orientation both indicate strong alongshore sediment transport. Near the base of the section, pebble orientations suggest that gravel wave-ripples developed below the zone of strong longshore flows. Differences between these two examples may be attributed to different directions of wave approach.  相似文献   

19.
The morphology and sediment dynamics of the 1500 km-long coast of South America between the mouths of the Amazon and the Orinoco Rivers are largely dependent on the massive suspended-sediment discharge of the Amazon, part of which is transported alongshore as mud banks. These mud banks have an overwhelming impact on the geology, the geomorphology, the ecology and the economy of this coast. Although numerous field investigations and remote sensing studies have considerably enhanced our understanding of the dynamics of this coast over the last three decades, much still remains to be understood of the unique functional mechanisms and processes driving its evolution. Among the themes that we deem as requiring further attention three come out as fundamental.The first concerns the mechanisms of formation of individual mud banks from mud streaming on the shelf off the mouth of the Amazon. An unknown quantity of the fluid mud generated by offshore estuarine front activity is transported shoreward and progressively forms mud banks on the Amapá coast, Brazil. The volume of each mud bank can contain from the equivalent of the annual mud supply of the Amazon to several times this annual sediment discharge. The mechanisms by which individual banks are generated from the Amazon turbidity maximum are still to be elucidated. Areas of research include regional mesoscale oceanographic conditions and mud supply from the Amazon.The second theme is that of variations in rates of migration of mud banks, which influence patterns of coastal accretion. Research emphasis needs to be placed on the analysis of both regional meteorological-hydrodynamic forcing and distant Atlantic forcing, as well as on the hydrology of the large rivers draining the Guyana Shield. The rivers appear to generate significant offshore deflection of mud banks in transit alongshore, through a hydraulic-groyne effect. This may favour both muddy accretion on the updrift coast and downdrift mud liquefaction with probably lessened muddy deposition.The third theme concerns sand supply by the Guiana Shield rivers. The rare sand deposits are important in providing sites for human settlements and routes and for nesting by marine turtles. The limited presence of sand bodies on this coast may reflect ‘mud blanketing’, a hypothesis that requires verification through high-resolution seismic analyses of shelf deposits and coring operations. The large Guiana Shield rivers, especially in Surinam and Guyana, have supplied sand for the construction of significant bands of cheniers, probably enhanced by the afore-mentioned downdrift hydraulic-groyne effect on hindered mud deposition. In all the three themes of this future research agenda, two central elements are the sediment input of the rivers of the Amazon basin, starting with the massive mud supply from the Amazon catchment itself, followed by sand inputs by the Guiana Shield rivers and their river-mouth effects on mud banks.  相似文献   

20.
Thick bay‐fill sequences that often culminate in strandplain development serve as important sedimentary archives of land–ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground‐penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes – notably a reduction in wave energy in response to coastal embayment infilling – in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5·8 ka, the 75 km2 Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi‐enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressive–regressive sequence, and backed by a highstand barrier‐island. The strandplain is immediately underlain by 5 to 16 m of seaward‐thickening, fluvially derived, Holocene‐age, basin‐fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand‐dominated to mud‐dominated; (ii) beachface slopes decrease from >11° to ca 7°; and (iii) progradation rates increase from 0·4 to 1·8 m yr?1. Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea‐level fall and sediment infilling and a decrease in onshore wave‐energy transport from 18 to 4 kW m?1. The combination of allogenic (sediment supply, falling relative sea‐level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records. These findings demonstrate the complexities in architecture styles of highstand and regressive systems tracts. Furthermore, this article highlights the diverse internal and external processes and feedbacks responsible for the development of these intricate marginal marine sedimentary systems.  相似文献   

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