Ridge and swale topography of the Middle Atlantic Bight,Nnorth America: Secular response to the Holocene hydraulic regime

The ridge and swale topography of the Middle Atlantic Bight was originally interpreted as a relict strand plain whose ridges reflect stillstands of the returning Holocene sea. However, close examination indicates that the ridges appear to be instead longitudinal bed forms, responses to a regime of intermittent, south-trending storm currents. Ridges may be initiated on the shore face and detached as the coast retreats to form fields of isolated ridges, or they may be molded into the shelf-transverse sand massifs that mark the retreat paths of littoral-drift depositional centers at estuary mouths and off cuspate forelands. The ridge and swale topography is thus a stable end configuration toward which a variety of near-shore constructional topographies have converged during the Holocene transgression.Morphologic evidence for readjustment of ridge topography to the deepening shelf flow field during the Holocene transgression is discernable. However, the extent to which the offshore topography continues to respond to hydraulic regime is unclear. The role of helical flow structure in the storm flow field remains to be documented. Resolution of these problems will require more detailed information of hydraulic process and substrate response on storm-dominated shelves.     

胶州湾口潮流沙脊特征

胶州湾内外的涨、落潮三角洲上发育有一定规模的潮流沙脊体系,通过多波束调查采集数据和浅剖、沉积物资料的分析,对沙脊特征进行了研究。湾内涨潮三角洲上分别分布有冒岛沙脊、中央沙脊和岛耳河沙脊;湾外落潮三角洲上分别发育了潮流沙脊大竹、南沙、北沙和位于主潮流通道末端的弧状沙脊。潮流沙脊体系的演变具有较为明显的继承性。沙脊以海侵时期发育的沙体为内核,沉积物组成以粒度较粗的砂质沉积物为主,物源基础为低海平面时期胶州湾盆地内堆积的河流三角洲相沉积物。据分析,沙脊现代物源较为匮乏,整体目前处于冲刷不淤的状态。在现代潮流水动力条件下,研究区潮流沙脊处于活动状态。     

Sand ridges on the inner Atlantic shelf of North America: Morphometric comparisons with Huthnance stability model

Sand ridge fields on the inner shelf of the Middle Atlantic Bight are generally thought to have formed in response to northeasterly storm flows as the shoreface underwent erosional retreat with postglacial sea-level rise. However, the hydrodynamic mechanism is poorly unerstood. Coastal boundary models see the ridges as responses of the seafloor to distortions in the flow induced by the coastal boundary. Stability models propose that an irregular initial topography will evolve toward an ordered array of bedforms in response to repeated flow events. The two classes of models are not mutually exclusive, nor are members within each class mutually exclusive. Results of measurements of ridge spacing on the inner Atlantic shelf of North America agree with the predictions of stability models.     

东海陆架冰后期潮流沙脊地貌与内部结构特征

东海陆架以宽平的地形、充分的陆源沉积物供应、快速沉降和强动力场为特征，中外陆架发育大规模潮流沙脊地貌。潮流沙脊走向大致为NW—SE向分布，与区域潮流主方向一致或成较小交角。东海陆架冰后期潮流沙脊以不对称横剖面为特征，陡坡倾向SW。沙脊内部发育典型的高角度前积斜层理，倾向与沙脊横剖面陡坡方向一致。这些斜层理可以划分为高达4组不同特征的组合，分别代表潮流沙脊发育的不同阶段，对应于冰后期海平面上升的不同时期。东海陆架潮流沙脊主体形成于冰后期海侵阶段，目前仍然受到陆架潮流场的影响，沙脊顶部为再沉积活动层。     

海底沙脊地貌的研究现状及进展

综述了国内外海底沙脊地貌的研究成果、技术方法以及东中国海沙脊研究中存在的问题。弓京港辐射沙脊群成因的研究依然是国内沙脊研究的焦点;东海中北部陆架沙脊地貌形成时期(海进与海退期)、沉积类型(现代、残留及潮流沉积)、沉积动力及沉积模式等研究尚存在较多争议;古潮流场的研究尚未得到足够重视。最新多波束探测数据显示,东海中南部外陆架60 m以深海域广泛分布线状沙脊群,其规模较前人预测的更大、范围更广,与长江口外喇叭状地形区中的沙脊地貌在空间分布特征和发展趋势上均存在差异。未来若能在利用多波束探测数据的基础上,对单道地震剖面、柱状样品以及钻孔等获取的多种资料进行综合分析,将更有利于对沙脊地貌的精细结构、时空展布规律以及成因机制和演化模式的研究。     

东海陆架西北部地貌分类及沉积环境演变研究

根据多次海洋调查的多波束、表层沉积物资料及中法合作海洋地质调查的浅地层剖面资料,对东海陆架西北部的地貌进行详细分类,探讨该区环境演化规律。分析结果表明,该区存在潮流沙脊,潮流冲刷槽、暗礁等多种地貌形态;该区发育多期潮流沙脊,反映了本区晚更新世以来经历了多次海进和海退过程。     

南黄海陆架沙脊的形成与演变

本文在对水深较大的滨外沙脊区钻孔、柱状样岩心详细观察的基础上,综合分析已有资料,并对该区浅地震地层剖面和水下沙脊的卫片进行了解译,提出南黄海陆架沙脊最早的形成年代为距今4000年,而现代的潮流沙脊形成于距今2000年以来的最近时期,潮流沙脊是在海面稳定、强潮流发育和大量物质供给的条件下形成和发展的,潮流沙脊的沉积物主要来自黄河和淮河;现代滨外沙脊具有向西北方向迁移、纵向延伸长度不断萎缩的动态演化特征。     

福建闽江口和九龙江口外线状沉积沙体特征

福建沿海强潮河口闽江口和九龙江口外均发育有一定规模的线状沉积沙体,通过多波束调查采集数据和浅剖、沉积物资料的分析,对此类沙体特征进行研究,初步认为其为潮流沙脊。结果表明:在平面分布上,闽江口外潮流沙脊走向近SW-NE向,沙脊局部连片;九龙江口外沙脊走向近ENEWSW向,沙脊末端有分叉现象。分析现代海洋环境作用并结合沙脊规模、水深和河口相对关系的研究显示,沙脊主体规模基本稳定,现代水动力仅对沙脊表面有改造作用。在潮流、波浪和近岸余流的共同作用下沙脊脊顶部略显平滑,两翼坡度较缓,其沉积物组成以粒度较粗的古河口砂质浅滩砂为主,沉积主体为全新世海平面上升时期,近岸河口环境的古水下三角洲遭受潮流侵蚀而成。     

Seismic geomorphology,architecture and genesis of Miocene shelf sand ridges in the Pearl River Mouth Basin,northern South China Sea

Multiple stages of large-scale shelf sand ridges, including the shoreface-attached and the offshore types, have developed in the Miocene successions on the mid-shelf region of the Pear River Mouth Basin, northern South China Sea. Utilizing a high-quality 3D seismic data set, accompanying 2D seismic profiles and well logs, the morphology, architecture and genesis of these shelf sand ridges have been systematically investigated in this study. The ridges are of very large scale, with the largest one having a maximum height of 64 m, a width of more than 20 km and a length of 37 km within the 3D survey area. Being mound-shaped, they also display obvious asymmetry character, with the ridge crest preferentially located on the SE side. Three main internal components, including the ridge front, central ridge and the ridge tail, have been recognized through careful anatomy analysis of the two most well-imaged ridges, each displaying distinct expressions on seismic amplitudes and geometries. In the plan view, most of the shelf sand ridges are generally NE–SW oriented and widening to the SW direction. Scouring features can also be clearly observed along the SW direction, including scour depressions and linear sandy remnants. On well logs, the shelf sand ridges are represented by an overall coarsening-upward pattern. Intervals with blocky sandstones are preferentially present on higher locations due to a differential winnowing process controlled by shelf topography.Plenty of evidence indicates that these ridges were primarily formed by the reworking of forced regressive or lowstand deltaic deposits under a persistent southwesterly flowing current during the subsequent transgression. This very current is a composite one, which is speculated to consist of winter oceanic current, SCSBK (South China Sea Branch of Kuroshio) intrusion onto the shelf and internal waves propagating from the Luzon Strait. Tidal currents might have contributed to the SE growth of the ridge. In response to the reglaciation of Antarctic ice-sheet and the closure of Pacific-Indian ocean seaway in the middle Miocene, the intensification of the North Pacific western boundary current was considered to have potential links to the initiation of the shelf sand ridges at ∼12 Ma. The development of shelf ridges was terminated and replaced by rapid deltaic progradation at ∼5.5 Ma.     

莺歌海油气资源开发区工程地质和灾害地质特征

根据单道地震、浅地层剖面、旁扫声纳和海底取样等实测资料,分析和评价了莺歌海油气资源开发区的工程地质和灾害地质环境。研究结果表明,研究区海底地形地貌较为复杂,存在潮流沙脊、侵蚀冲沟、海底沙波、麻坑、埋藏古河道和古湖泊、浅层气、埋藏珊瑚礁和滑塌断层等潜在的灾害地质因素,对海上石油平台和输油管线等工程设施构成直接或潜在的危害。根据地形、地貌和沉积物物理力学特征,将研究区划分为内陆架堆积平原区、陆架潮流沙脊区、内陆架侵蚀平原区、外陆架平原区和大陆坡区5个工程地质区。其中研究区东部的潮流沙脊区和东南部的陆坡区,海底地形复杂,活动性的潮流沙脊和断层发育,是海底工程建设的危险区,应引起高度重视。     

Anatomy of a shore face ridge system,False Cape,Virginia

The Middle Atlantic Shelf of North America is a broad sand plain, characterized by a subdued ridge and swale topography. Some ridges extend into or merge with the shore face.     

Calm-weather spring and neap tidal current characteristics on a shoreface-connected ridge complex in the German Bight (southern north Sea)

While shoreface-connected sand ridges may be molded by storm-generated waves and currents, calmweather counterparts may determine their longevity in the German Bight. Fair-weather flow measurements on shoreface ridges off Spiekeroog Island show that: (1) peak velocities (U₁₀₀ max) mostly range from 30 to 60 cm s^–1 and are flood asymmetric, except at neap tide when ebb flows are dominant in ridge troughs; (2) velocity contrast between accelerating and decelerating flow phases is higher for ebb than flood currents, suggesting intense interaction between inlet and shoreface ebb currents; and (3) tidal currents play a primary role in ridge maintenance.     

珠三坳陷西北部浅海陆架砂体时空演化及成因机制

为阐明珠三坳陷西北部珠江组一段上亚段浅海陆架砂体成因机制,综合地质与地球物理数据,首先搭建了五级层序地层格架,并以此为约束,开展浅海陆架砂体识别与定量描述,剖析其时空演化规律,进而讨论其成因机制。结果表明：（1）研究区珠江组一段上亚段可划分为4个五级层序,自下向上编号为FS4、FS3、FS2与FS1;(2)研究区发育潮流沙脊与滨外沙坝两种类型陆架砂体,两者整体呈NW-SE向展布,潮流沙脊主要分布于研究区西部,滨外沙坝则集中于东部;（3）FS4、FS3与FS2 3个五级层序中,潮流沙脊与滨外沙坝均呈现较大规模与较多数量,最上部FS1五级层序中,规模与数量达到最小;（4）沉积基准面（水动力）、同沉积地貌、沉积物碎屑供给等因素共同影响了潮流沙脊与滨外沙坝的发育规模、展布特征与时空演化规律等,综合构成了珠三坳陷西北部浅海陆架砂体的成因机制。     

Erosional shelf ridges in the mid-eastern Yellow Sea

In the mid-eastern Yellow Sea, closely spaced high-resolution seismic profiles and a 44-m-long sediment core (YSDP-104) were analyzed to reveal the internal structures and stratigraphy of the shelf ridges currently shaped by tidal currents. Three depositional sequences (sequences I, II and III in descending order) can be recognized. Sequence III, the substratum of the ridges, consists of coarse-grained sediments in the lower part (non-marine deposits) and tide-influenced muddy sediments in the upper part (probable transgressive to highstand systems tract). Sequence II represents internal ridge sediments, similar in character to sequence III, but is demarcated by an undulatory ridge topography. According to radiocarbon dating of marine muds, these sequences range in age from 47,000 to 28,000 years B.P., representing two cycles of short-term sea-level fluctuations during oxygen isotope stage 3. Sequence I consists mostly of late-Holocene transgressive sand veneer on the ridge surface. It also includes minor amounts of early-Holocene muddy sediments occasionally underlying the sand. Most of the ridges are presently undergoing erosion by tidal currents, forming widespread sand dunes on the entire surface.     

Large-scale current lineations on the central New Jersey shelf: Investigations by side-scan sonar

Two morphological orders of ridge and trough topography can be recognized on a terraced segment (at 37 m) of the central New Jersey shelf: (1) a first-order system with ridges to 14 m high, 2–6 km apart, in a Z-shaped pattern trending to the NNE, and (2) a second-order system with ridges 2–5 m high, 0.5-1.5 km apart and trends to the NE.Side-scan mapping together with submersible observations and bottom samples indicate a third-order system of large-scale current lineations which has been imprinted across the first- and second-order systems. The lineations are low relief forms (to 1.5 m high) which occur as elongate zones of textural contrast arranged in furrows, bands, patches and ribbons and display a uniform directional trend to the ENE.The morphology of the lineations appear to vary in response to the nature of the bottom. The lineations are narrow (10–25 m apart) and have no detectable relief in troughs and wider (to 75 m apart) and higher (to 1.5 m high) on ridges, especially second-order ridges of fine sand. Also revealed are wave ripple patterns and a pattern related to the outcropping of Pleistocene/Holocene units in trough bottoms and lower flanks.It is suggested that the first- and second-order systems developed during earlier stages of the Holocene transgression in response to a hydraulic regime of the inner shelf. The first-order system may have inherited some of its morphology from an older system, but did respond to a Holocene tidal regime adjacent to a major estuary. The second-order system developed in slightly deeper water, subsequent to the resumption of the transgression after the 37-m stillstand.The third-order lineations appear to be a response to the helical-flow structure within the flow field of a major shelf storm. Ridges of the central shelf may be maintained by alternate periods of oblique sweeping during storms, resulting in a net transport of fine sand out of the troughs and up on the ridges. Subsequent wave reworking returns the fine sand to the troughs.     

Erosional shelf ridges in the mid-eastern Yellow Sea

In the mid-eastern Yellow Sea, closely spaced high-resolution seismic profiles and a 44-m-long sediment core (YSDP-104) were analyzed to reveal the internal structures and stratigraphy of the shelf ridges currently shaped by tidal currents. Three depositional sequences (sequences I, II and III in descending order) can be recognized. Sequence III, the substratum of the ridges, consists of coarse-grained sediments in the lower part (non-marine deposits) and tide-influenced muddy sediments in the upper part (probable transgressive to highstand systems tract). Sequence II represents internal ridge sediments, similar in character to sequence III, but is demarcated by an undulatory ridge topography. According to radiocarbon dating of marine muds, these sequences range in age from 47,000 to 28,000 years B.P., representing two cycles of short-term sea-level fluctuations during oxygen isotope stage 3. Sequence I consists mostly of late-Holocene transgressive sand veneer on the ridge surface. It also includes minor amounts of early-Holocene muddy sediments occasionally underlying the sand. Most of the ridges are presently undergoing erosion by tidal currents, forming widespread sand dunes on the entire surface.     

东海陆架两期沙脊的时空对比东海陆架两期沙脊的时空对比

基于高分辨率的单道地震和多波束测深数据，识别并对比了东海陆架中部同一海区相距20余万年的层U14和层U2两期沙脊群，其中层U14期沙脊属于埋藏沙脊，位于东海海底以下90 m深处，推测属于距今320~200 ka的海侵体系域(TST)，沙脊顶界面是该期海侵的最大洪泛面(MFS)；层U2期沙脊位于东海陆架，属于衰退沙脊，系末次盛冰期(LGM)以来的TST，顶界面是LGM以来的MFS。尽管两期沙脊形成年代相距20余万年，地层层位相距近90 m，但是沙脊群总体走向一致，表明距今2×105 a以来东海陆架潮波基本格局稳定。从层U2期可识别出4个亚期沙脊，通过多波束海底地形图可识别出4组走向的沙脊，多亚期、多走向沙脊是LGM以来海平面阶梯状波动在海底地形演变过程中的响应证据。     

Sandbank occurrence on the Dutch continental shelf in the North Sea

Sandbanks, the largest of bed patterns in shallow sandy seas, pose a potential risk to shipping. They are also valuable elements of natural coastal protection, dissipating the energy of waves. In the Southern Bight of the North Sea, several sandbank areas have been reported in the literature. However, based on an objective crest–trough analysis of the bathymetry of the Dutch continental shelf, the present study shows that sandbanks are more widespread than commonly considered. These banks are relatively low, presumably explaining why they have not been documented before. This widespread occurrence of sandbanks in the North Sea is in agreement with theoretical predictions based on stability analysis techniques. The possible interference between large-scale human activity and low-amplitude open-shelf ridges implies that one should be careful not to overlook these patterns if none should appear in a preliminary (visual) assessment. The only part of the Southern Bight in which no ridges can be seen is a circular area with a diameter of about 50 km near the mouth of the river Rhine. Here, freshwater outflow affects the direction of tidal ellipses and residual flow, and suppresses the formation of open ridges.     

东海陆架两期沙脊的时空对比

基于高分辨率的单道地震和多波束测深数据,识别并对比了东海陆架中部同一海区相距20余万年的层U₁₄和层U₂两期沙脊群,其中层U₁₄期沙脊属于埋藏沙脊,位于东海海底以下90m深处,推测属于距今320~200ka的海侵体系域(TST),沙脊顶界面是该期海侵的最大洪泛面(MFS);层U₂期沙脊位于东海陆架,属于衰退沙脊,系末次盛冰期(LGM)以来的TST,顶界面是LGM以来的MFS。尽管两期沙脊形成年代相距20余万年,地层层位相距近90m,但是沙脊群总体走向一致,表明距今2×105/a以来东海陆架潮波基本格局稳定。从层U₂期可识别出4个亚期沙脊,通过多波束海底地形图可识别出4组走向的沙脊,多亚期、多走向沙脊是LGM以来海平面阶梯状波动在海底地形演变过程中的响应证据。     

Ridge development as revealed by sub-bottom profiles on the central New Jersey shelf

Closely-spaced 3.5 kHz seismic profiles were collected over the north-easterly trending ridge and swale system 50 km east-southeast of Atlantic City, New Jersey. They yield information on the Late Quaternary depositional history of the area, and on the origin of the ridge system. Four of the sub-bottom reflectors identified were sufficiently persistent to warrant investigation and interpretation. These reflectors, which have been cored, lithologically identified, and radiocarbon dated, are stratigraphically higher than the reflectors dealt with by the majority of previous studies. The upper three reflectors are definitely mid- and post-Wisconsin in age and present a record of the most recent glacial cycle. The upper three units associated with the observed reflectors appear to exert a pronounced influence on the bathymetry. The gently corrugated ridge system of Holocene sand is formed over the regionally flat-lying upper unit, an Early Holocene lagoonal silty clay. The characteristically flat, broad depressions of the area are floored by this lagoonal material. Locally, however, marine scour has cut through the silty clay into an underlying unit of unconsolidated fine Pleistocene sand. Several stages of trough development appear to be represented. After penetrating the lagoonal clay, troughs are initially narrow, but when incised through the sand into a lower, Pleistocene, silty-clay unit, the troughs become notably wider. As downcutting is inhibited by the lower clay, the upper clay is undercut as the trough widens in a fashion similar to a desert blowout.

The sub-bottom reflectors indicate that ridge development on the central shelf has involved aggradation as well as erosion. Some ridges seem to have grown by vertical and lateral accretion from small cores. The internal structure of other ridges suggests that they formed by the coalescence of several small ridges. Others appear to have undergone appreciable lateral migration.

The ridges appear to be in a state of continuing adjustment to the hydraulic regime of the deepening post-Pleistocene water column.