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1.
The ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware-Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross-sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression. A second aspect is the change in bottom sediment characteristics that accompanies these large-scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high-frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea-floor. A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large-scale bedforms. Finally, flow was measured and transport calculated on the same ridge during a one-month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43-year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration. Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat.  相似文献   

2.
Sediment transport and trapping in the Hudson River estuary   总被引：3，自引：0，他引：3  
The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i.e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neapetides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.  相似文献   

3.
夏鹏  孟宪伟  丰爱平  李珍  杨刚 《沉积学报》2015,33(3):551-560
气候变化造成的海平面上升是迫使红树林向陆迁移的主要驱动力, 而其自身通过捕沙促淤不同程度的减缓了海平面上升速率的影响。基于广西典型红树林区8根短柱的210Pb测年和含水率分析, 以考虑/未考虑沉积物压实作用为研究情景, 通过对比研究红树林区潮滩地表高程抬升速率和相对海平面上升速率的大小关系, 揭示当前海平面上升对广西红树林向陆/向海迁移的驱动机制。研究发现:未考虑压实作用下的沉积速率约是考虑压实作用下沉积速率的1.00~1.34倍(平均1.12倍), 压实作用明显;压实沉积速率介于0.16~0.78 cm/a, 其底层压实沉积速率与潮滩地表高程抬升速率相等。压实作用下, 英罗湾和丹兜海红树林区的地表高程抬升速率小于相对海平面上升速率;与未考虑压实作用得到的结论相悖。由于广西红树林海岸大都建有防波堤, 限制了红树林向陆的迁移;表明英罗湾和丹兜海的红树林正面临海平面上升的威胁。压实作用校正与否对地表高程抬升速率与相对海平面上升速率相当的区域尤为重要。  相似文献   

4.
Estuarine tidal mudflats form unique habitats and maintain valuable ecosystems. Historic measurements of a mudflat in San Fancsico Bay over the past 150 years suggest the development of a rather stable mudflat profile. This raises questions on its origin and governing processes as well as on the mudflats’ fate under scenarios of sea level rise and decreasing sediment supply. We developed a 1D morphodynamic profile model (Delft3D) that is able to reproduce the 2011 measured mudflat profile. The main, schematised, forcings of the model are a constant tidal cycle and constant wave action. The model shows that wave action suspends sediment that is transported landward during flood. A depositional front moves landward until landward bed levels are high enough to carry an equal amount of sediment back during ebb. This implies that, similar to observations, the critical shear stress for erosion is regularly exceeded during the tidal cycle and that modelled equilibrium conditions include high suspended sediment concentrations at the mudflat. Shear stresses are highest during low water, while shear stresses are lower than critical (and highest at the landward end) along the mudflat during high water. Scenarios of sea level rise and decreasing sediment supply drown the mudflat. In addition, the mudflat becomes more prone to channel incision because landward accumulation is hampered. This research suggests that sea level rise is a serious threat to the presence of many estuarine intertidal mudflats, adjacent salt marshes and their associated ecological values.  相似文献   

5.
The Pacific margin of the Antarctic Peninsula to the southwest of the Hero Fracture Zone (HFZ) is a former subducting margin which became inactive following the arrival of ridge crest segments of the Antarctic–Phoenix ridge at the margin during the Tertiary. In contrast, the part of the margin to the northeast of the HFZ remains active. Tertiary convergence was approximately perpendicular to the margin and ongoing motion is thought to have the same orientation. A new seismic reflection profile running along Boyd Strait, just northeast of the landward projection of the HFZ, shows major structural components similar to those typically observed along the margin to the southwest of the HFZ. In order of increasing proximity to the margin, these components are: the inner shelf, the shelf basin, the mid-shelf basement high (MSBH), and the outer shelf. The continuation of the post-subduction margin structures to the active margin suggests that the boundary between crust with passive and active margins characteristics is not sharply defined. Our postulated scenario for tectonic evolution along Boyd Strait is that: (1) before the arrival of the last ridge crest segment to the southwest of the HFZ, the inner shelf and the shelf basin were part of a Cretaceous–Tertiary arc and forearc area, (2) after the arrival, thermal effects resulting from interaction of the ridge crest with the margin just southwest of the HFZ lead to the formation of the MSBH to the northeast, but MSBH uplift in Boyd Strait did not prevent concurrent cross-shelf sediment transport contributing to development of an extensive outer shelf on the seaward flank of the MSBH, (3) Recent extension in Bransfield Strait, a marginal basin to the northeast of the landward projection of the HFZ, has caused about 10 km of seaward deflection in the strike of the part of the MSBH to the northeast of the projection of the HFZ.  相似文献   

6.
A coupled hydrostatic and morph-dynamic model COMCOT-SED was used to investigate the morphological change in Lhok Nga bay during the 2004 Indian Ocean tsunami, and the coupled model predicted the thickness of tsunami deposits in agreement with the measured ones. The relationship between the characteristics of tsunami deposit and flow hydrodynamics was discussed in details. Phenomena such as landward thinning in deposit thickness, landward fining in grain size, and fining upwards in grain size are commonly used to identify tsunami deposits and were examined in this case study. We also discussed the effects of sediment supplies and the constraints that can be put on the earthquake parameters using the information derived from tsunami deposits. This study shows that the model presented in this paper is capable of simulating extreme tsunami events (tsunami wave height ~30?m) in a large domain and that forward models of tsunami sediment transport can be a promising tool to help tsunami geologists understand tsunami deposits.  相似文献   

7.
A process-based numerical model is applied to investigate sediment transport dynamics and sediment budget in tide-dominated estuaries under different salt marsh erosion scenarios. Using a typical funnel-shaped estuary (Ribble Estuary, UK) as a study site, it is found that the remobilization of sediments within the estuary is increased as a result of the tidal inundation of the eroded salt marsh. The landward export of the finest sediment is also intensified. The relationship between salt marsh erosion and net landward export of sediments has been found to be non-linear—with the first 30% salt marsh erosion causing most of the predicted export. The presence of vegetation also influences the sediment budget. Results suggest that vegetation reduces the amount of sediment being transported upstream. Again, the trapping effect of salt marsh in terms of plant density is non-linear. Whilst a vegetated surface with a stem density of 64 plants/m2 decreased the net landward export of very fine sand by around 50%, a further increase in stem density from 64 to 512 plants/m2 had a relatively small effect.  相似文献   

8.
The Taitao ophiolite of southern Chile lies 10 km from the buried extension of the Peru-Chile trench, and less than 50 km from the present position of the Nazca/South America/Antarctica triple junction. Plio-Pleistocene radiometric and paleontologic ages indicate its formation during ridge subduction, and an ultramafic rock, gabbro, sheeted dike, volcanic and sedimentary rock psuedostratigraphy suggests formation by typical accretionary processes for oceanic lithosphere. Yet major and trace element data show that mafic dikes and volcanic units are transitional from MORB to IAT, and there are abundant silicic volcanic units of calc-alkaline character that have high LIL element and light REE concentrations relative to oceanic plagiogranites. Sr and Nd isotopic data are consistent with that of modern oceanic suites, even though having a greater internal variability. Silicic volcanic units show the more enriched Sr and depleted Nd isotopic ratios relative to dike and gabbro samples. In addition to chemical distinctions, paleobathymetric data support a shallow water origin for some of the upper volcanic units and, assuming local compensation, suggest crustal thicknesses of continental proportion. In the vicinity of the Taitao ophiolite, and extending some 40 km landward of the plate margin, are a series of silicic stocks, sills, and plutons that were intruded into the forearc at the time of ridge collision and ophiolite generation. These calc-alkaline I-type granitoids are light REE enriched and have Sr and Nd isotopic compositions similar to those of the main volcanic chain 200 km landward. Chemically, some of the silicic intrusions are indistinguishable from volcanic units of the ophiolite. In general, major, trace, REE, and isotopic variations of both the ophiolite and the distributed intrusions are atypical of simple fractionation trends for basaltic liquids. Intermediate to silicic units lie along mixing hyperbolae between Taitao gabbro and either forearc sediment or metamorphic basement on a Nd---Sr correlation diagram, and these two crustal components support, respectively, either a 10–25% or 5–10% assimilation. Shutdown of magmatism, and therefore probably partial melting as well, appears to occur within 40 km of the trench, roughly spanning the depth interval for the disappearance of the plagioclase-lherzolite stability field as the zone of mantle upwelling is overridden by an increasing thickness of continental lithosphere. A deeper and more landward absence of partial melt related to the subducted ridge is supported by the correlation of the shutoff and re-initiation of arc volcanism over the northern and southern trailing edges of the postulated subcontinental asthenospheric window. Here, as well as elsewhere in the circum-Pacific, the general restriction of magmatism related to ridge subduction to near-trench settings supports a shallow (0 to 15 km) shutoff mechanism for adiabatic decompressive melting and a rather abrupt return to single-phase (solid) convective rise of mantle into an evolving asthenospheric window.  相似文献   

9.
Environmental settings on the Ionian coastal plain and inner shelf seaward of Locri‐Epizephiri in Calabria, Italy, differed markedly before, during, and following settlement by the Greeks. Sediment core analyses and geophysical surveys in this study support recent archaeological findings and the hypothesis that the margin may once have served as a harbor and/or shipyard. The subsurface Holocene stratigraphy records that (1) the shoreline advanced to a maximum landward position before Greek settlement, then regressed offshore to what is now the inner shelf before once again migrating landward. These marked coastal shifts were triggered primarily by land uplift and probable subsidence offshore along this structurally active Calabrian Arc segment. Associated with this are: (2) a sediment fining‐upward sequence in Greek–Roman time that indicates only partial protection of the coastal area, and (3) possible presence of subsurface structures seaward of the city wall in a sector now positioned ∼200 m offshore. © 2010 Wiley Periodicals, Inc.  相似文献   

10.
本研究利用SMT法和七步连续提取法对桂林市会仙岩溶湿地中典型土地利用方式（水稻田、果园、荒地）的土壤以及河流底泥中磷的赋存形态进行研究。结果表明:岩溶土壤总磷含量大小为底泥＞果园＞水稻田＞荒地;底泥中Ex-P和Fe-P所占比例较大;荒地土壤中Or-P和Res-P为主要磷形态;水稻田和果园土壤中Fe-P所占比例较高。人为干扰明显的果园表层土壤有较大的磷淋溶风险;荒地土壤中磷的生物可利用性低,体现了其缓冲固持湿地水体中磷的生态功能。大量可溶性磷经淋溶迁移富集于底泥中,是河流富营养化的风险源。七步法表现出在提取多种复杂结合态磷上的优势,对岩溶土壤中Ca-P的提取更充分有效,更适用于研究岩溶土壤不同形态的磷分布特征。   相似文献   

11.
位于浙江省宁波市大榭岛的大榭遗址,是迄今所发现的中国最早的史前海盐业遗址,与一般遗址沉积地层不同,其主体区系人工堆筑土台。本研究在遗址发掘过程中,采集了泥质和砂质2种类型堆土、废弃陶器堆中的白色硬化物和盐灶坑表层白色硬化物,同时采集本岛自然堆积物,进行了粒度、无机元素地球化学和重矿物分析,以探讨堆土的原料构成、古人的制盐工艺及制盐活动对堆土地层产生的影响。研究结果显示: 来自盐灶遗迹附近的7个泥质堆土样品,其中5个具有陆相元素富集特征,但是后期受海水影响,另外2个显示滩涂盐泥特征;废弃陶器堆中的白色硬化物元素组成与盐泥相似,盐灶坑表层白色硬化物具高Ca、Sr、Ba含量,推测是烧煮卤水产生的沉淀物。砂质堆土的粒度和重矿物组成显示其属于河流冲积物,且具强烈的化学风化特征,推测其原始地层不仅限于全新统。该研究显示了钱山漾文化时期古人利用岛屿上的潮上带、山麓黄土、河流冲积物等原材料在滩涂营建土台,并在土台上利用“淋卤煎炼”工艺从事制盐活动。  相似文献   

12.
洋中脊热液硫化物勘探技术的滞后,严重制约了对海底热液硫化物资源的勘探开发。以我国西南印度洋硫化物勘探合同区域采集的25个表层沉积物样品为研究对象,基于主量、微量和稀土元素检测数据,采用元素含量特征、相关性分析、元素对比值、特征元素三角图解,以及稀土元素分馏特征值等手段,开展沉积物热液信息研究。结果表明:样品所代表的大部分研究区域内主要为钙质生物沉积,部分样品元素地球化学特征受沉积物中玄武岩风化碎屑的影响,龙旂热液区的部分样品中表现出一定的热液迹象,稀土元素分馏特征和配分模式、（Al+K）?Mg?（Fe+Mn）三角图解可较好地指示热液活动。研究不仅为我国在西南印度洋的硫化物勘探提供基础数据参考,同时也是对海底热液硫化物勘探的沉积物地球化学找矿理论和方法的初步探索。  相似文献   

13.
During landward migration, ridge and runnel systems are subjected to asymmetric oscillatory and/or unidirectional flow regimes, depending on the stage of development reached by these systems. In the early stages of evolution, when the ridge is situated in the upper shoreface, the whole system is subjected to asymmetric oscillatory flow. The runnel is under lower flow regime conditions and the ridge may be under upper or lower flow regime according to water depth and wave energy. Later, when the ridge has migrated to a position on the foreshore, the runnel is largely under a unidirectional lower flow regime while the ridge itself is under oscillatory upper flow regime. When the ridge welds to berm, it is largely emergent and exposed to high-tide swash action under upper flow regime conditions. The runnel is eventually filled with sand and transformed into a low-lying area. All these types grade laterally into each other. One or more ridge and runnel systems can occur at the same time. Wave energy, tide level and position of the ridge control the variations in the characteristics of the ridge and also the position of the zones of bedforms found at the upper shoreface.  相似文献   

14.
苏北海岸带潮成辐射砂脊群的形成及其古地理意义   总被引：3，自引：3，他引：3       下载免费PDF全文
南黄海潮成辐射砂脊群的面积约为20000km2，以160° 的角度从弓京港向海展开。它与以弓京港为顶点的辐聚辐散潮流场相伴而生。60余个钻孔揭示，毗邻海区辐射砂脊体系的江苏沿岸平原上存在一个面积约3000 km2潮成砂区，其顶点位于东台，同样呈扇形以130°的角度向东展开。在潮成砂区内潮成砂质沉积单元位于冰后期海侵型砂坝-湖沉积层之上，二者之间具明显的冲刷面。砂坝-湖沉积层位于晚更新世基底硬粘土层之上，二者之间有较长的沉积间断。潮成砂沉积层上覆潮坪沉积层，二者呈渐变关系。以潮成砂层底部的侵蚀面为界，其下为海侵序列，其上为海退序列。古潮流的研究揭示，潮成砂区内同样存在辐聚辐散的古潮流场，其顶点位于东台附近。由此推断，沿海平原的潮成砂区内也是辐射状潮成砂脊体系，它形成于全新世海退时期。由于长江和黄河三角洲的前展，以东台为顶点的潮成砂脊体系逐渐暴露成陆。陆上和海域潮成辐射砂脊群形成于相同的潮汐动力环境，但处在不同的发育阶段，前者形成于全新世中期，后者发育于全新世晚期。矿物分析揭示，陆上和海区的潮成辐射砂脊体系主要由长江和黄河沉积物组成，其中长江沉积物由南向北运移，且时间较早；黄河沉积物由北向南运移，时间较迟，这种泥沙的运移趋势一直延续至今。随着海平面上升趋于减缓，长江三角洲增长，江苏海岸线向外推进，苏北潮成砂区逐渐出露成陆。1128年黄河由苏北入海，大量的黄河沉积物的加入，加快了本区海岸线的推进速度。潮成辐射砂脊体系与辐聚辐散的潮流场相伴而生，全新世最大海侵以来，辐聚辐散的潮流场的位置曾经历三次变化，第一次以长江古河口湾为顶点，第二次位于现今陆上潮成砂区，第三次位于以弓京港为顶点的现代海域，代表了潮成辐射砂脊体系发育的三个阶段。只是长江古河口湾的潮成辐射砂脊体系由于河流的巨大改造作用，可能未很好保存，至今未发现典型的辐射砂脊体系。  相似文献   

15.
Unlike for subaerial settings, the impact of subaqueous relay ramps on sediment dispersal is still poorly understood. A combination of analogue laboratory experiments in a sandbox with numerical flow calculations is used to simulate relay ramp topographies on rifting continental margins and to analyse the resulting turbidity current pathways and their deposits. Various scenarios are investigated, including inflow perpendicular and oblique to the relay ramp axis as well as flow constrained by an incised channel on the ramp and by a landward‐directed tilt of the ramp. Without channelling, most sedimentation takes place on the basin floor because the bulk of the flow follows the steepest gradient down the fault and into the rift basin. With a channel along the relay ramp, significant flow occurs initially down the ramp axis, but channel spillover and basinward ramp tilting combine to redirect much of the sediment down the fault slope into the basin. When the relay ramp has a landward‐oriented tilt, most of the current flows down the ramp and deposits its sediment load there and at the foot of the ramp. However, also here a considerable amount of the flow is shed over the hanging wall fault and into the basin, forming a secondary depocentre, while ponding redistributes thin deposits over a wider area of the basin. The quantitative dependence of these results on the specific ramp geometries remains to be investigated further but may bear great importance for refined sedimentary models in subaqueous rifted settings as well as for hydrocarbon exploration therein.  相似文献   

16.
Offshore sediment characteristics of the 2004 tsunami were identified from a shallow core collected from the Chennai Coast, India. The depositional sequence clearly distinguishes four different processes: mixed facies (post-tsunami): 0–8 cm; tsunami return flow facies (TRFF): 8–20 cm; tsunami landward flow facies: 20–44 cm; and pre-tsunami facies: 44–64 cm, which all took place during and after the tsunami event. The coarse-grained nature and higher carbonate in the TRFF indicate that considerable sediment load was transported from the beach/land area to the offshore region during the return flow of tsunami waves. The relatively greater abundance of benthic foraminiferal species in the core sample suggests that the taxa were transported from deeper regions of the inner shelf regions of Bay of Bengal region. The depositional characteristics in this region can be utilized for future comparative studies from this region as well as in other offshore regions affected by tsunamis with sequence-based studies on local topography.  相似文献   

17.
Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ??350°C hydrothermal fluids. The high-temperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources >600 times the total known VMS reserves on land and a copper resource which would last >6,000?years at current production rates.  相似文献   

18.
Backwater tidal sloughs are commonly found at the landward boundary of estuaries. The Cache Slough complex is a backwater tidal region within the Upper Sacramento–San Joaquin Delta that includes two features that are relevant for resource managers: (1) relatively high abundance of the endangered fish, delta smelt (Hypomesus transpacificus), which prefers turbid water and (2) a recently flooded shallow island, Liberty Island, that is a prototype for habitat restoration. We characterized the turbidity around Liberty Island by measuring suspended-sediment flux at four locations from July 2008 through December 2010. An estuarine turbidity maximum in the backwater Cache Slough complex is created by tidal asymmetry, a limited tidal excursion, and wind-wave resuspension. During the study, there was a net export of sediment, though sediment accumulates within the region from landward tidal transport during the dry season. Sediment is continually resuspended by both wind waves and flood tide currents. The suspended-sediment mass oscillates within the region until winter freshwater flow pulses flush it seaward. The hydrodynamic characteristics within the backwater region such as low freshwater flow during the dry season, flood tide dominance, and a limited tidal excursion favor sediment retention.  相似文献   

19.
In the northeastern Mediterranean Sea, Pliocene to Quaternary depocentres have formed in extensional basins bounded by splays of the East Anatolian Transform Fault. This tectonic regime is superimposed on a Miocene and older back-arc environment, that experienced late Miocene compression along the Misis-Kyrenia thrust, which now lies in the middle of the extensional zone. The thrust zone is now represented by a narrow horst that appears to be bounded by strike-slip faults. Pliocene-Quaternary extension took place on listric fault fans that are orthogonal to the bounding transform splays and sole at a Messinian evaporite horizon, and on some deeper-soling listric faults parallel to and near the bounding faults. The rapid extension has resulted in progressive landward migration of paleoshorelines and low depositional gradients. Glacio-eustatic fluctuations in shoreline positions strongly influenced sediment distribution. Most sediment dispersion was from deltaic plumes, with turbidites of minor significance. Depocentres landward of the maximum seaward extent of paleoshorelines were formed almost entirely by tectonic subsidence. Minor deep-water depocentres, controlled by halokinesis, accumulated mud turbidites during extreme low-stands of sea-level.  相似文献   

20.
Subaqueous dunes are formed on the KwaZulu-Natal outer-shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of ? 35 to ? 70 m. The net sediment transport direction is south, but short-period reversals form northward-migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large-scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of ? 60, ? 47 and ? 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record.  相似文献   

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