Tidal Sand Research Articles

Unincised fluvial and tide-dominated estuarine systems from the Mesoproterozoic Lower Tombador Formation, Chapada Diamantina basin, Brazil

The Mesoproterozoic Lower Tombador Formation is formed of shallow braided fluvial, unconfined to poorly-channelized ephemeral sheetfloods, sand-rich floodplain, tide-dominated estuarine, and shallow marine sediments. Lowstand braided fluvial deposits are characterized by a high degree of channel amalgamation interbedded with ephemeral, intermediate sheetflood sandstones. Sand-rich floodplain sediments consist of intervals formed by distal sheetflood deposits interbedded with thin layers of eolian sandstones. Tide-dominated estuarine successions are formed of tide-influenced sand-bed braided fluvial, tidal channel, tidal sand flat and tidal bars. Shallow marine intervals are composed of heterolithic strata and tidal sand bars. Seismic scale cliffs photomosaics calibrated with vertical sections indicate high lateral continuity of sheet-like depositional geometry for fluvial–estuarine successions. These geometric characteristics associated with no evidence of incised-valley features nor significant fluvial scouring suggest that the Lower Tombador Formation registers deposition of unincised fluvial and tide-dominated systems. Such a scenario is a natural response of the interplay between sedimentation and fluctuations of relative sea level on the gentle margins of a sag basin. This case study indicates that fluvial–estuarine successions exhibit the same facies distributions, irrespective of being related to unincised or incised-valley systems. Moreover, this case study can serve as a starting point to better understand the patterns of sedimentation for Precambrian basins formed in similar tectonic settings.

Single-cell genomics reveals potential for magnetite and greigite biomineralization in an uncultivated multicellular magnetotactic prokaryote.

For magnetic orientation, magnetotactic bacteria biosynthesize magnetosomes, which consist of membrane-enveloped magnetic nanocrystals of either magnetite (Fe3 O4 ) or greigite (Fe3 S4 ). While magnetite formation is increasingly well understood, much less is known about the genetic control of greigite biomineralization. Recently, two related yet distinct sets of magnetosome genes were discovered in a cultivated magnetotactic deltaproteobacterium capable of synthesizing either magnetite or greigite, or both minerals. This led to the conclusion that greigite and magnetite magnetosomes are synthesized by separate biomineralization pathways. Although magnetosomes of both mineral types co-occurred in uncultured multicellular magnetotactic prokaryotes (MMPs), so far only one type of magnetosome genes could be identified in the available genome data. The MMP Candidatus Magnetomorum strain HK-1 from coastal tidal sand flats of the North Sea (Germany) was analysed by a targeted single-cell approach. The draft genome assembly resulted in a size of 14.3 Mb and an estimated completeness of 95%. In addition to genomic features consistent with a sulfate-reducing lifestyle, we identified numerous genes putatively involved in magnetosome biosynthesis. Remarkably, most mam orthologues were present in two paralogous copies with highest similarity to either magnetite or greigite type magnetosome genes, supporting the ability to synthesize magnetite and greigite magnetosomes.

Influence of the back-barrier basin length on the geometry of ebb-tidal deltas

The characteristics of ebb-tidal deltas are determined by the local hydrodynamics. The latter depend, among others, on the geometry of the adjacent back-barrier basin. Therefore, interventions in the back-barrier basin can affect the geometry of ebb-tidal deltas. In this study, the effect of the length of the back-barrier basin on the sand volume and spatial symmetry of ebb-tidal deltas is quantified with the use of a numerical model. It is found that the length of the back-barrier basin affects the tidal prism, the amplitude and phase of the primary tide and its overtides, and the residual currents that, together, determine the sand volume of the ebb-tidal delta. In particular, it is found that no unique relationship exists between tidal prism and sand volume of an ebb-tidal delta. The spatial symmetry of ebb-tidal deltas is also found to be affected by the length of the back-barrier basin. This is because the basin length determines the phase difference between alongshore and cross-shore tidal currents. The numerical model results give a possible explanation for the changes that are observed in the geometry of the ebb-tidal deltas that are located seaward of the Texel Inlet and Vlie Inlet after the closure of the Zuiderzee.

Late Quaternary stratigraphic evolution on the outer shelf of the East China Sea

The East China Sea is a typical marginal sea and is characterized by strong land–sea interaction and paleoenvironmental changes during the late Quaternary with sea-level fluctuations. In this study, late Quaternary sedimentary stratigraphy and facies on the outer shelf of the East China Sea were reconstructed by using high-resolution seismic reflections and combining evidences from lithology, microfossil assemblages, element geochemistry and geochronology of borehole SFK-1. Sedimentary sequence consists of nearshore facies of forced regression formed during the late marine isotopic stage 3, tide-influenced fluvial and estuarine facies during the last glacial maximum, incised-valley fill and estuarine-tidal flat facies during the deglacial transgression, buried and quasi-active tidal sand ridge facies during the postglacial and Holocene periods.The sea-level fluctuation and paleo-river and sea interaction primarily controlled the stratigraphic framework and sedimentary facies on the outer shelf during the late Quaternary. This study confirms the occurrence of paleo-river channels and relating deposits on the outer shelf. Paleo-fluvial deposits accumulated during the last glacial maximum albeit with strong tidal reworking, and tide-dominated estuarine facies are recognized during the early deglacial transgression. The previously-recognized delta facies might not have formed on the outer shelf because of the low sediment supply and/or strong tidal reworking.

The role of suspended load transport in the occurrence of tidal sand waves

AbstractTidal sand waves are dynamic bed patterns which are formed by the complex interaction between hydrodynamics, sediment transport, and geomorphology. Field data from the southern North Sea reveal that sand waves are absent where suspended load transport is the dominant transport mode. In order to understand the mechanisms responsible for the absence of sand waves, we study the influence of suspended load transport on the formation of tidal sand waves with a numerical process‐based geomorphological model (Delft3D). Model simulations are presented in which the vertical eddy viscosity and sediment diffusivity are both spatially and temporally variable (k‐ε turbulence model). First, it is shown that the preferred wavelength of sand waves for a relatively large grain size increases by the inclusion of suspended sediment, while for a relatively small grain size the flat bed is stable and no sand waves evolve. Second, it is shown that suspended load transport causes the suppression of long sand waves, resulting in a finite range of wavelengths that experience growth. Finally, by varying flow velocity amplitude and grain size, critical conditions for sand wave formation are found, i.e., conditions for which sand waves are marginally generated.

基于GIS技术的南黄海辐射沙脊群海域水质分布特征与评价

使用Arcgis软件对南黄海辐射沙脊群附近海域2007年春秋两季表层水样的水质进行了分析，包括砷(As)、汞(Hg)、铜(Cu)、锌(Zn)、镉(Cd)、铅(Pb)、铬(Cr)七种重金属的含量分布变化。用Hakanson潜在生态危害指数法(HPERI)对该海域7种重金属进行生态风险评价，旨在为江苏沿海辐射沙脊区水质健康和海域生态环境保护提供科学依据，并为江苏沿海的开发利用提出合理化建议。研究结果发现，春秋两季As、Cd、Cr、Cu 4种重金属含量均低于国家海水水质I类标准；Pb和Zn在部分区域符合II类标准，其余各处符合I类标准；Hg在沙脊沉积区含量较高，春季为IV类海水，秋季为超IV类海水，在潮流通道含量较低，春季为I类海水，秋季为II、III类海水。秋季重金属较春季普遍含量更高，且整体分布呈现南移趋势。Hakanson潜在生态危害指数法评价结果表明，春秋两季该海域均以Zn的潜在生态危害最大，除秋季Zn存在强生态危害以外，其余重金属在春秋两季均为轻微生态危害；综合潜在生态风险以沿岸河口、港口附近和沙脊沉积区处为最高，表明人类活动是影响该海域重金属分布的主要因素；春季潜在生态风险属轻微，秋季高值区属中等风险。We used Kriging modelling in Arcgis 10.1 to find out the distribution characteristics of water qual-ity in Radial Tidal Ridge in South Yellow Sea during spring and autumn in 2007, focusing on 7 heavy metals as As, Hg, Cu, Zn, Cd, Pb, Cr. Hakanson Potential Ecological Risk Index (HPERI) was used to assess extents of heavy metals in this area to propose better suggestions for Jiangsu coastal development. According to our study, contents of heavy metals were higher and more enriched in south in autumn than in spring. Monsoon had a significant impact on distribution of those heavy metals in surface water in study area. Zn was found to contribute the most to the pollution status while other heavy metals were found to cause only slight ecological risk based on HPERI method. RI was high in costal Estuary and port area, where human impacts were significant, or sedimentary area of sand ridges, where tidal dynamics were weaker, which suggested that there was a tendency that pollution caused by human activities had turned into the most important impacting factor of the water quality in the Radial Tidal Sand Ridge in South Yellow Sea. And RI showed that the potential ecological risk was low in spring while high in autumn.

Evolution of sedimentary environments of the middle Jiangsu coast, South Yellow Sea since late MIS 3

An evolutionary model of sedimentary environments since late Marine Isotope Stage 3 (late MIS 3, i.e., ca. 39 cal ka BP) along the middle Jiangsu coast is presented based upon a reinterpretation of core 07SR01, new correlations between adjacent published cores, and shallow seismic profiles recovered in the Xiyang tidal channel and adjacent northern sea areas. Geomorphology, sedimentology, radiocarbon dating and seismic and sequence stratigraphy are combined to confirm that environmental changes since late MIS 3 in the study area were controlled primarily by sea-level fluctuations, sediment discharge of paleo-rivers into the South Yellow Sea (SYS), and minor tectonic subsidence, all of which impacted the progression of regional geomorphic and sedimentary environments (i.e., coastal barrier island, freshwater lacustrine swamp, river floodplain, coastal marsh, tidal sand ridge, and tidal channel). This resulted in the formation of a fifth-order sequence stratigraphy, comprised of the parasequence of the late stage of the last interstadial (Para-Sq2), including the highstand and forced regressive wedge system tracts (HST and FRWST), and the parasequence of the postglacial period (Para-Sq1), including the transgressive and highstand system tracts (TST and HST). The tidal sand ridges likely began to develop during the postglacial transgression as sea-level rise covered the middle Jiangsu coast at ca. 9.0 cal ka BP. These initially submerged tidal sand ridges were constantly migrating until the southward migration of the Yellow River mouth to the northern Jiangsu coast during AD 1128 to 1855. The paleo-Xiyang tidal channel that was determined by the paleo-tidal current field and significantly different from the modern one, was in existence during the Holocene transgressive maxima and lasted until AD 1128. Following the capture of the Huaihe River in AD 1128 by the Yellow River, the paleo-Xiyang tidal channel was infilled with a large amount of river-derived sediments from AD 1128 to 1855, causing the emergence of some of the previously submerged tidal sand ridges. From AD 1855 to the present, the infilled paleo-Xiyang tidal channel has undergone scouring, resulting in its modern form. The modern Xiyang tidal channel continues to widen and deepen, due both to strong tidal current scouring and anthropogenic activities.

Sediment sorting along tidal sand waves: A comparison between field observations and theoretical predictions

A site-by-site comparison between field observations and theoretical predictions of sediment sorting patterns along tidal sand waves is performed for ten locations in the North Sea. At each site, the observed grain size distribution along the bottom topography and the geometry of the bed forms is described in detail and the procedure used to obtain the model parameters is summarized. The model appears to accurately describe the wavelength of the observed sand waves for the majority of the locations; still providing a reliable estimate for the other sites. In addition, it is found that for seven out of the ten locations, the qualitative sorting process provided by the model agrees with the observed grain size distribution. A discussion of the site-by-site comparison is provided which, taking into account uncertainties in the field data, indicates that the model grasps the major part of the key processes controlling the phenomenon.

Modeling tidal sand wave formation in a numerical shallow water model: The role of turbulence formulation

Tidal sand waves are prominent dynamic bottom features in shallow sandy seas. Up to now, the processes controlling the formation of these bedforms have only been studied in stability sand wave models, in which geometry, boundary conditions and turbulence models are schematized. In this paper we present simulations of sand wave formation and migration with a numerical shallow water model (Delft3D), in which we restrict us to bedload transport and analyse the initial formation stage only. First, it is shown that the reproduction of the basic sand wave formation mechanisms in a numerical shallow water model requires careful treatment of model geometry, initial profile, vertical resolution and lateral boundary conditions. Second, an intercomparison between the Delft3D model and a nonlinear stability sand wave model is performed. Next, we compare the results for two of the built-in turbulence models: constant vertical eddy viscosity model (commonly used in stability sand wave models) and a more advanced spatially and temporally variable vertical eddy viscosity model (k–ε turbulence model). Finally, the model results are compared with field data on sand wave length. The k–ε turbulence model shows good agreement with the field data, whereas the constant vertical eddy viscosity model overestimates the wave length of the sand waves considerably.

Comparison of Tidal Sand Ridges between the Subei Shoal and the West Korea Bay

In the Yellow Sea, two large terrains of tidal sand ridges developed, respectively, in the Subei Shoal and the West Korea Bay. Sediment supply, hydrodynamics and bedforms are critical factors to form them. In this paper, sediment composition, morphology of sand ridges, etc. are analyzed in terms of relevant factors and subsequently their formation mechanisms and evolution processes are careful compared.

Geologically-based permeability anisotropy estimates for tidally-influenced reservoirs using quantitative shale data

The effect of the vertical to horizontal permeability ratio ( k v / k h ) on many displacement properties is significant, making it an important parameter to estimate for reservoir models. Simple ‘streamline’ models have been developed which relate k v / k h at the reservoir scale to shale geometry, fraction and vertical frequency. A limitation of these models, especially for tidally-influenced reservoirs, is the lack of quantitisative geological inputs. To address this lack of data, detailed shale characteristics were measured, using Lidar point clouds, from four different tidally-influenced reservoir analogues: estuarine point bar (McMurray Formation, Alberta, Canada), tidal sand ridge (Tocito Sandstone, New Mexico), as well as both unconfined and confined tidal bars (Sego Sandstone, Utah). Estuarine point bars have long ( x̄ = 67.8 m) shales that are thick and frequent relative to the other units. Tidal sand ridges have short shales ( x̄ = 8.6 m dip orientation) that are thin and frequent. Confined tidal bars contain shales that are thin, infrequent and anisotropic (x– = 16.3 m dip orientation). Unconfined tidal bars contain nearly equidimensional shales of intermediate length ( x̄ = 18.6 m dip orientation) with moderate thicknesses and vertical frequency. The unique shale character of each unit results in a different distribution of estimated k v / k h values. Estuarine point bars have lower average k v / k h values ( x̄ = 8.2 ×10 −4 ) than any other setting because of the long shales they contain. Tidal sand ridges have short, but frequent shales, which results in moderate k v / k h estimates ( x̄ = .011). Estimates of k v / k h are typically highest in confined tidal bars ( x̄ = .038), which contain anisotropic and infrequent shales. Unconfined tidal bars have moderate lengths and frequency resulting in k v / k h estimates averaging 0.004. The results of this study highlight the link between heterogeneity, reservoir architecture and flow parameters.

A REVIEW OF THE LATE QUATERNARY SEDIMENTOLOGICAL STUDIES ON THE OUTER SHELF OF THE EAST CHINA SEA

The current study of Late Quaternary stratigraphy,paleo-river channels,channel-fillings and tidal sand ridges on the outer shelf of the East China Sea are summarized in this paper.Key questions,such as the boundaries of MIS 2 and MIS 4,the different schemes of sequence stratigraphic architectures,the existence and evolution of the paleo-Yangtze River during LGM,and the formation of the tidal sand ridges on the continental shelf of the East China Sea are highlighted and discussed.Comments and suggestions are provided in the paper for the study of the Late Quaternary sedimentation on the East China Sea in the future.

Relations between geomorphic form and sedimentologic-stratigraphic variability: Holocene ooid sand shoal, Lily Bank, Bahamas

Although the linkages among surface sediments, geomorphic forms, and hydrodynamics in Holocene ooid tidal sand shoals have been evaluated recently, how these factors are reflected in the geomorphic evolution and stratigraphic record of shoals is less constrained. Yet, such understanding is essential to developing meaningful predictive conceptual models of three-dimensional architecture of ancient reservoir analogs. Integrating remote-sensing imagery, high-frequency seismic data, and core characterization from Lily Bank, a modern tidally dominated Bahamian ooid shoal in which sedimentologic processes are well documented, reveals the stratigraphic record of geomorphic change. An irregular, gently dipping rocky surface (interpreted as the top Pleistocene) with no pronounced topographic high underlies the Holocene oolitic succession. A 6-m (20-ft)–thick poorly sorted, gravelly muddy sand with few ooids overlies this basal surface. This lower interval is overlain by sand with an upward increase in proportion of ooids, sorting, and grain size. The uppermost unit, present only under active bar forms, is well-sorted oolitic medium sand with accretionary foresets. Sediments vary stratigraphically and geomorphically; the lower unit is finer and less well sorted than the upper units, and in the oolitic upper unit, sediment size and sorting on bar crests are distinct from bar flanks. Collectively, these results suggest that a marked antecedent bump is not necessary for occurrence of ooid shoals and that the stratigraphic record of analogous ooid shoal systems may preserve clues of geomorphic position, as well as geobody size and orientation.

Petrographic Study of the Sandstones of Eze-Aku Formation (Turonian) in Abaomege, Southern Benue Trough, Nigeria.

The Turonian Eze-Aku Formation in the study area uncomformably overlies the Albian Asu-River Group and unconformably underlies the Nkporo Group (Campano-Maastrichtian) in Afikpo Synclinorium. The petrographic study of the sandstones in Abaomege was carried out to determine the provenance, paleoclimate and source area tectonics for the sandstone facies. The lithofacies encountered in the area consist of fine to medium grained calcareous sandstones which correlate the sediments of Turonian Amasiri Sandstone and alternating sequence of light grey to dark grey shales, and siltstone. Petrographic analysis of the sandstone suggests that they are mineralogically mature and are essentially quartz arenites. Analysis and interpretation of sedimentary structures, mostly the cross beds, and also the structures of quartz grains indicates that the sandstones were derived primarily from granitic Basement Complex rocks from the Oban Massif, under humid climate. The paleocurrent plots for the sandstones show bimodal distribution pattern, suggesting a shore environment of deposition. The sandstones in Abaomege area are part of the sediments of the Late Cenomanian-Turonian Eze-Aku Formation of the Middle Cretaceous which outcrops at eastern flank of the Abakaliki Anticlinorium, Southern Benue Trough Nigeria. There has not been enough work done to establish the provenance regions, paleoclimatic setting and source area tectonics for the sandstones of Eze-Aku Formation in Abaomege area, southern Benue Nigeria. This study seeks to carry the petrography study and paleocurrent analysis of the sandstone and its implications on the provenance, sediment direction flow and paleoclimatic setting. (1) established an unconformity between the Turonian Eze-Aku Formation and the Campanian - Maastrichtian Nkporo Formation in the Afikpo Synclinorium. (2) reviewed the stratigraphy of the Eastern Nigeria, although some of the original names he adopted have been changed by ((3); (4)) in their later work. According to their report on tectonic activities, which occurred during the Cretaceous period, resulted in a series of folds in the sequence of the Asu River Group, the Eze - Aku Formation and created the Abakaliki Anticlinorium. Regional study of Pre - Santonian sandstones including the sandstones of the study carried out by ((5); (6)); they suggested that the sandstones are petrologically arkosic quartz arenites derived from the granitic/metamorphic basement rocks. An interest in paleocurrent analysis can be noted in the work of (7) which suggested that since cross bedding of any scale has paleocurrent value, all that is required is that its orientation be systematically measured and mapped. Paleocurrent indicators are orientated sedimentary structures interpreted to have been deposited by ancient flows. Cross beds slip face, pebble imbrications, parting lineation, tool marks and groove casts, and ripple crest orientation are all examples of possible paleocurrent indicators (8). Some paleocurrent indicators are unidirectional - that is, their shape provides unique information about the direction of the ancient paleoflow. (9) studied the Amasiri Sandstones in the Abaomege, Akpoha and Afikpo areas and noted that the sediment were derived from east of the study area and were deposited as tidal sand ridges in shallow marine environments. The Eze-Aku Formation outcrops out on both sides of the Asu River Group (Fig. 1). It is composed of calcareous shale, siltstone and thin sandy or shelly limestone as well as calcareous fine to medium grained sandstones (3). The Eze-Aku Formation has been dated to be generally of Lower Turonian age on the basis of diagnostic ammonites and microfauna (3). Cenomanian forams have also been recovered from the formation. It has an estimated thicknesss of 2100m (10). Table 1 shows the stratigraphic position of the Eze-Aku Formation in the southeastern Nigeria. Based on the study, the sandstones were suggested to be quartz arenites and the sediments were derived from the Basement Complex rocks of Oban Massif in a humid climate.

Topographic Mapping of Offshore Sandbank Tidal Flats Using the Waterline Detection Method: A Case Study on the Dongsha Sandbank of Jiangsu Radial Tidal Sand Ridges, China

The waterline detection method (WDM) based on satellite images is one of the most effective methods for constructing digital elevation models (DEMs) for tidal flats. The general practicability of the WDM has been demonstrated in previous studies for small areas. This article attempts to generate a DEM over a large offshore sandbank with a tidal flat area of more than 620 km2 by integrating the conventional WDM with a hydraulic model and multitemporal satellite images. For illustration purposes, a case study was conducted at the Dongsha sandbank, one of the largest sandbanks among the radial tidal sand ridges of Jiangsu Province, China. In this study, 34 multitemporal satellite images acquired at different tidal phases in 2006 were analyzed to detect the waterlines. The water level at each satellite overpass moment was simulated by a south Yellow Sea hydraulic model, and discrete waterline points were combined with the corresponding water levels to produce a topographic map. Compared with the DEM produced during the same period by the airborne light detection and ranging (LiDAR) data, the DEM derived from WDM effectively contains the average height error within 47 cm, which is satisfactory. All of these findings may be useful for researchers and local authorities, as the findings could be used as a reference for sandbank evolution research or to support environmental management, coast protection, and storm forecasting.

Delta dynamics: Effects of a major earthquake, tides, and river flows on Ciénega de Santa Clara and the Colorado River Delta, Mexico

The intertidal portion of Mexico's Colorado River Delta is a dynamic environment subject to complex interactions of tectonic, fluvial, and tidal forces at the head of the Gulf of California. We review the historical interactions of these forces, use sequential satellite images, overflights, ground observations, and interferometric synthetic aperture radar (InSAR) data to study the effects of the 2010 Mw 7.2 El Mayor-Cucapah Earthquake on changing patterns of tidal inundation within the Delta, and assess effects of these changes to the fluvial/hydrological regime of the Colorado River estuary and nearby Ciénega de Santa Clara wetland. The objectives of this study are to highlight for environmental scientists, land managers, and ecological engineers the contribution of tectonic forces in shaping the intertidal Delta environment and to provide information on the effects of the 2010 earthquake which will be of practical value in planning and designing management measures and restoration projects for the estuary and Ciénega.The Colorado River estuary is at present blocked by a tidal sand bar which restricts access by marine species to the upper estuary and obstructs the flow of fresh water into the lower estuary. Located 13km east of the estuary, the Ciénega is a 6000ha wetland supported by agricultural drain water from Arizona and Mexico. South of the Ciénega is the Santa Clara Slough, an unvegetated 26,000ha basin subject to periodic inundation from the northern Gulf's high amplitude tides, which have historically reached the margins of the Ciénega several times each year.The El Mayor-Cucapah earthquake ruptured the previously unknown Indiviso Fault which extends into the intertidal zone just west of the Ciénega. The Ciénega experienced only minor surface deformation having no direct effects to the wetland. Most of the significant ground movement and surface deformation occurred west of the Indiviso Fault adjacent to the estuary, where portions of the intertidal flats underwent extensive liquefaction, northward coseismic displacement and post-seismic subsidence. These surface deformations changed the pattern of tidal inundation, triggering development of a new system of natural tidal channels and creating conditions favorable for installation of projects to restore connectivity between the upper and lower estuary. The changed pattern of tidal inundation may also have contributed to an observed reduction in the occurrence of tidal flooding along the southwestern margin of the Ciénega following the earthquake.

Toward a Method of Constructing Tidal Flat Digital Elevation Models with MODIS and Medium-Resolution Satellite Images

ABSTRACT Liu, Y.; Li, M.; Mao, L.; Cheng, L., and Li, F., 2013. Toward a method of constructing tidal flat digital elevation models with MODIS and medium-resolution satellite images. The terrain of tidal flats often varies remarkably over space and time, but few existing methods are capable of producing fine–spatiotemporal resolution digital elevation models (DEMs) for tidal flats. This research aims to fill the knowledge gap and propose a 6-step method to construct tidal flat DEMs by incorporating both moderate-resolution imaging spectroradiometer and medium-resolution satellite images. The six steps include: waterline extraction, water level simulation, height assignment, median filtering, waterline point fusion, and interpolation to DEMs. A case study was conducted at the Dongsha sandbank—one of the largest sandbanks among the radial tidal sand ridges of Jiangsu Province, China. The resultant DEMs were compared with the light detection and ranging–based DEMs for vertical accuracy assessment. The result...

Holocene peat deposits in the German Bight, southern North Sea – extending the reconstruction of the postglacial transgression seaward

A set of 125 vibrocores collected on the basis of a dense grid of high and very high resolution seismic coverage over a tidal sand bank (the Middelkerke Bank) provides a unique opportunity to reconstruct the stratigraphy and sedimentary facies succession of such a large tidal sand body. Five sedimentary facies are distinguished. They range from clay to pebble and display various types of bedding and lithologies corresponding to different depositional environments. A succession of four Late Quaternary units are defined on the basis of the stratigraphic framework given by the seismic grid. The first unit represents the infill of a palaeovalley system by tidally influenced sedimentary facies. It is followed by extensive and thick lagoonal or sub-tidal deposits. The third unit reveals the development of a landward prograding storm-dominated sand bar. This unit is dominated by clean fine sand. The last unit corresponds to the present-day tidal sand-bank. It shows the typical offshore tidal sand bank facies (cross-bedded sand) prograding in an offshore direction. Like other banks located in a more offshore position, the Middelkerke Bank is a composite bank incorporating facies which do not occur in `classic' tidal sand banks.

Palaeogeographic evolution of the Nazaré-Salgado coastal zone (Portugal) during the Holocene

A detailed analysis of the Chirp (2–7 kHz) profiles, core sediments and 14C ages from an isolated mound and adjacent area in the southern Yellow Sea off the Jiangsu coastal area reveals a highly complex development of transgressive and highstand deposits during the last postglacial sea-level rise. In water depths of 50–60 m, the terrace consists of thin transgressive sediments directly above transgressive surface. The isolated mound (40.7 km long and 29.6 km wide) occurs above 40–50 m in water depth, and is detached from the Jiangsu and East China Sea tidal sand ridges. It comprises the SSE-ward prograding sandy unit (PU) and the overlying muddy flank unit (FU) at the outer margin. The terrace with thin transgressive sediments was most likely formed when sea level rose rapidly from −58 to −45 m around about 11.6 ka. As rate of sea-level rise was attenuated from −45 to −36 m between ca. 11.6 and 9.6 ka, the nearly flat lower boundary of the PU was produced at water depths of 37–42 m by a ravinement process. Subsequent rapid sea-level rise from −36 to −16 m between ca. 9.6 and 9.1 ka created accommodation for deposition of the PU. Sea-level rise stagnated from −16 to −10 m between ca. 9.1 and 7.5 ka during which the Huanghe River apparently shifted south of the Shandong Peninsula and discharged directly into the western Yellow Sea. The combination of slow sea-level rise and sediment supply from the early Holocene Huanghe delta by the southward-flowing coastal currents most likely favoured the formation of the SSE-ward prograding transgressive sandy PU. The highstand muddy FU overlying the PU was accumulated after ca. 7.5 when sea level approached its present position and the Huanghe River shifted back to the north. Development of transgressive and highstand deposits in the study area during the last postglacial sea-level rise was highly variable in time and space in response to changes in rate of sea-level rise together with variations in sediment supply caused by the shifting of the Huanghe River.

Late Quaternary evolution of tidal sand ridges in the southern coast of Korea

Late Quaternary evolution of tidal sand ridges in the southern coast of Korea