Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu-Natal shelf, south-east Africa

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.     

Submerged upper Holocene beachrock on San Salvador Island,Bahamas: implications for recent sea-level history

Sedimentological, petrographic and radiometric data from a submerged beachrock on San Salvador Island, Bahamas, provide new information about the Late Holocene sea-level history in this area.At French Bay, on the southern shore of the island, samples of beachrock collected at a depth of 1 m below low tide level yielded an average ¹⁴C age of 965 ± 60 years before present. These samples further display a well developed fenestral porosity and present an early generation of low Mg calcite meniscus cement. These features characterize intertidal and supratidal settings; they are not consistent with the present beachrock position and the reported Late Holocene sea-level history in the Bahamas. A 1.5–2m low stand of the sea about 1000 years ago would best explain the observed particularities of the French Bay beachrock.This example from San Salvador shows that the smooth trend of Late Holocene sea-level rise proposed by previous workers might be overprinted by high frequency, low amplitude fluctuations. Recognition of these fluctuations is fundamental when calculating rates of sea-level rise and evaluating the coastal response to a marine transgression. Correspondence to: P. Kindler     

广东达濠半岛近代高海滩岩的研究

广东达濠半岛现已发现距今三千多年的3个时期的海滩岩残留体,对它的认识有海滩岩与风成砂丘岩两种观点,笔者确定是海滩岩。本文从层面构造的流水线理、障碍物痕迹、波痕、鸟类足迹,岩层的韵律,胶结物的文石,微体古生物动物群,以及氧碳同位素的测定等多方面研究,认为该地海滩岩为热带、广盐度、与外海相通的滨海沉积物。 达濠海滩岩不是高海面的遗迹,而是海岸地壳抬升的证据。在构造格架上,位于北西西向强（震）隆起带上,上地幔隆起达2km。     

深圳西冲湾的海蚀地貌与海滩沉积研究

利用对深圳西冲湾两个岬角之间的海蚀地貌及其海滩沉积物的野外调查资料,鉴定出三级海蚀平台,对其分别进行了高程换算,并借助于海蚀拱门和海蚀刻槽的测定,以及对典型沙层沉积剖面序列的分析和14C测年,认为：虽然西冲湾海蚀平台的岩石类型是不易被侵蚀的花岗岩,但是此处的海蚀地貌（主要是海蚀平台）以及海滩沉积物（主要是海滩岩）可以作为全新世存在高海平面的证据。此外,针对我国海岸带人口密度大,人类活动对海岸影响程度大的现况,提出了未来研究南海北部海岸新构造运动存在的问题以及其解决方案。研究成果对于认识南海北部海岸全新世的构造运动、海平面变化以及与其他地区新构造运动的对比研究都具有重要的意义。     

Late Quaternary denudation of the Alps,valley and lake fillings and modern river loads

The coastal areas of the island of Delos, located at the centre of the Cyclades archipelago (Greece), are rich in submerged Hellenistic archaeological vestiges. This submersion can be explained by changes in relative sea-level: the recent 14C datings of submerged beachrock occurrences of Delos and the nearby islands of Mykonos and Rhenia suggest that the sea level was at about -2.5 m (± 0.5 m) around 400 BC [1, 2]. Such result has enabled to confirm and refine Negris'early-twentieth-century hypothesis that the submersion can be accounted for by the relative sea-level rise.

From this result, together with bathymetric maps, archaeological studies and stratigraphic data, the Hellenistic coastal landscapes on the western side of Delos have been reconstructed.

The Sacred Harbour (including the Agora of the Competaliasts) and the “Pointe des Pilastres” landscapes (located to the South) resembled those of the current Greek harbours: the paved walkways or esplanades bordering buildings or shops were separated from the sea by a beach onto which boats were drawn. The landscape of the “Maisons au flanc de la Colline” sector (located to the North) seems to have been different. These houses were located on a rocky platform, in a sector exposed to the north swell.     

Inland fields of dispersed cobbles and boulders as evidence for a tsunami on Anegada, British Virgin Islands

Marine overwash from the north a few centuries ago transported hundreds of angular cobbles and boulders tens to hundreds of meters southward from limestone outcrops in the interior of Anegada, 140?km east?Cnortheast of Puerto Rico. We examined two of several cobble and boulder fields as part of an effort to interpret whether the overwash resulted from a tsunami or a storm in a location where both events are known to occur. One of the cobble and boulder field extends 200?m southward from limestone outcrops that are 300?m inland from the island??s north shore. The other field extends 100?m southward from a limestone knoll located 800?m from the nearest shore. In the two fields, we measured the size, orientation, and spatial distribution of a total of 161 clasts and determined their stratigraphic positions with respect to an overwash sand and shell sheet deposit. In both fields, we found the spacing between clasts increased southward and that clast long-axis orientations are consistent with a transport trending north?Csouth. Almost half the clasts are partially buried in a landward thinning and fining overwash sand and none were found embedded in the shelly mud of a pre-overwash marine pond. The two cobble and boulder fields resemble modern tsunami deposits in which dispersed clasts extend inland as a single layer. The fields contrast with coarse clast storm deposits that often form wedge-shaped shore-parallel ridges. These comparisons suggest that the overwash resulted from a tsunami and not from a storm.     

Holocene cemented beach deposits in Belize

Two types of cemented beach deposits occur on reef islands off the coast of Belize. These are (1) intertidal beachrock that is dominantly cemented by marine aragonite and high-magnesium-calcite cements, and (2) supratidal cayrock that is cemented mainly by vadose low-magnesium-calcite cements. Besides differences in position relative to present sea level and resulting early diagenesic features, beachrock and cayrock can be distinguished on the basis of differences in composition, texture, geographical position, and age. Whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments, cayrock is enriched in benthic foraminifera. Intertidal beachrock is moderately to well sorted and well cemented, while supratidal cayrock is very well sorted, poorly cemented and friable. Beachrock occurs preferentially on windward beaches of sand-shingle Gays on the middle and southern barrier reefs and on the isolated platforms Glovers and Lighthouse Reefs. Cayrock only occurs on larger mangrove-sand Gays of the isolated platforms Turneffe Islands, Lighthouse Reef, and the northern barrier reef. ¹⁴C-dating of ten whole-rock and mollusk shell samples produced calibrated dates between AD 345 and AD 1435 for beachrock and between BC 1085 and AD 1190 for cayrock.

The large-scale distribution of beachrock in Belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution. Only on windward sides of cays that are close to the reef crest, where large amounts of seawater flush the beaches, considerable amounts of cements can be precipitated to produce beachrock. Cayrock forms due to cementation in the vadose zone and is only preserved on larger, stable mangrove-sand cays.     

闽南、粤东海岸带沉积物的沉积学、岩石学比较研究

本文通过对海岸带海滩砂、沙丘砂、砂丘岩、海滩岩和有关近岸海底砂的沉积学,岩石学的比较研究,分析其形成环境,发现沙丘岩沉积特征和历史沙丘相似;沙丘岩和海滩岩的识别有时不是主要从岩石学而可能从沉积学中去识别。兄弟屿比西海底硬地更可能是生物排出的钙质粘液胶结。     

Formation and preservation of an overstepped segmented lagoon complex on a high‐energy continental shelf

The duration of shoreline occupation at a given sea‐level, coastal response to sea‐level change and the controls on preservation of various shoreline elements can be recognized by detailed examination of submerged shorelines on the continental shelf. Using bathymetric and seismic observations, this article documents the evolution and preservation of an incised valley and lithified barrier complex between ?65 m and ?50 m mean sea‐level on a wave‐dominated continental shelf. The barrier complex is preserved as a series of aeolianite or beachrock ridges backed by laterally extensive back‐barrier sediments. The ridges include prograded cuspate lagoonal shoreline features similar to those found in contemporary lagoons. The incised valley trends shore‐parallel behind the barrier complex and records an early phase of valley filling, followed by a phase of extensive lagoonal sedimentation beyond the margins of the incised bedrock valley. Sea‐level stability at the outer barrier position (ca ?65 m) enabled accumulation of a substantial coastal barrier that remained intact during a phase of subsequent slow sea‐level rise to ?58 m when the lagoon formed. These lagoonal sediments are stripped seawards by bay ravinement processes which caused the formation of several prograded marginal cuspate features. An abrupt rise in sea‐level to ?40 m, correlated with melt‐water pulse 1B, enabled the preservation of thick lagoonal sediments at the top of the incised valley fill and preservation on the sea bed of the cemented core of the barriers. This situation is unique to subtropical coastlines where early diagenesis is possible. The overlying sandy sediment from the uncemented upper portion of the barriers is dispersed by ravinement, partly burying the ridges and protecting the underlying sediments. The high degree of barrier or shoreline preservation is attributed to rapid overstepping of the shoreline, early cementation in favourable climatic conditions and the protection of the barrier cores by sand sheet draping.     

Coastal circulation along the central west coast of India during cyclone Phyan: measurements and numerical simulations

Measurements and numerical simulations carried out off Mumbai coast for the period 22 October?C22 November 2009 showed significant variations in water level and currents during cyclone Phyan. Changes in the meridional component of the current velocity were found to be highly significant during Phyan. The strong southward flow due to the cyclone resulted in a complete nullification of a flood current and continued the net flow towards south for approximately 18?h. This enhanced the ebb flow by about 0.12?m/s off Worli and 0.16?m/s off Satpati. The shift in the cyclone generated currents towards north resulted in an enhancement of flood current by 0.3?m/s off Worli and 0.2?m/s off Satpati. During the first 7?h of Phyan, a strong current system associated with an anti-cyclonic eddy was formed off Mangalore, which propagated towards north up to Ratnagiri and disappeared while the cyclone entered the land. Water level variations of 0.38 and 0.44?m were observed due to Phyan off Worli and Satpati, respectively. Positive and negative surges, that is, a fall in water level up to ?20?cm and a rise up to 20?cm, were observed during Phyan and these were associated with shift in the wind direction.     

山东乳山海滩岩及其重要科学意义

乳山海滩岩,经过室内对微体化石、大化石和沉积岩石学研究,查明该岩体具有典型的海滩岩特征:含有两种文石胶结物、滨海相微体古生物和热带大化石。该岩体形成在全新世最佳气侯期,至少有3次成岩期,对研究古环境变化规律具有重要的科学意义。     

Géomorphologie de Tromelin,océan Indien

Tromelin is a small coral reef island (1 km²) located in the Indian Ocean, approximately 440 km east of Madagascar and 580 km north of La Réunion. Despite the presence of a permanent Météo France weather station on Tromelin since the 1950 s, a detailed geomorhological study of the island has never been undertaken. In this paper, we describe results from the 2008 field season, which enabled us to map and describe seven geomorphological zones on the atoll. (1) Tromelin's bathymetry is characterised by a pronounced submarine slope, which attains depths of 1000 m at only 2.5 km from the island's coastline. This geomorphology is typical of volcanic hotspots. (2) A coral platform borders the emerged crown of the island. This platform is exposed at low tide and has been fashioned in an ancient Pleistocene substratum. The substratum has been eroded by abrasive marine action to its present shape since the stabilisation of relative sea level around 6000 years ago. Erosion of the reef yields most of the biogenic sediment supply to the island. (3) The upper foreshore is characterised by outcrops of beachrock. These formations show a stratfication in slabs and dip gently towards the sea in the same manner as the contemporary clastic sediments. (4) Tromelin's beaches are divided into three sub-units, which reflect the different energy dynamics around the island, in addition to the transfer of sediment from the windward to the leeward side: (i) the southern storm beaches are characterised by coral blocks reworked during episodic high-energy events; they form boulder ramparts. Four perched rampart ridges have been recorded at ∼1 m, ∼3 m, ∼4.5 m and ∼7 m above mean sea level; (ii) transitional beaches are observed between the south and the north, constituting storm blocks and coarse sands and gravels; and (iii) sandy beaches are noted on the northern leeward side, comprising rounded gravels and coarse sands. The northern tip of the island is characterised by a sand spit (∼125 × ∼225 m), whose geometry varies based on the seasonal and meteo-marine contexts. (5) On Tromelin, a number of dune formations are observed. These are best developed in the north of the island due to the south-north translation of clastic sediments by aeolian wind action, and the trapping of sands by the well-developed vegetation. The height of the dunes varies between 10 cm (the southern micro-dunes) to 250 cm in the Northeast of Tromelin. (6) Exceptional waves have deposited storm tracts up to 250 m from the foreshore zone, comprising coarse gravels and coral blocks. The storm tract deposits are most prevalent on the windward side of the island, due not only to the high-energy dynamics, but also to the absence of vegetation cover in this area. (7) The southern depression covers an area of ∼40,000 m². At its lowest point it is only 1.2 m above sea level.     

海南岛全新世海滩岩的胶结作用与成岩环境的关系

我国海南岛及南海诸岛沿岸,广泛发育海滩岩。1980年我队赴海南岛考察现代沉积时,对海滩岩的分布、岩性特征及其与周围环境的关系进行了观察和采样。样品采自崖县鹿回头三亚湾水尾岭海蚀崖、西洲岛、小东海、东瑁岛、西瑁岛、天涯海角,乐东县莺歌海,文昌县渔业等地(图1)。有关的地质、地貌及岩性特征等,已有许多描述,对海滩岩的岩石学及成岩作用也有许多研究。本文侧重探讨海滩岩的胶结作用及与成岩环境的关系。     

Distribution of arsenic and mercury in subtropical coastal beachrock,Gulf of Mannar,India

An assessment of coastal pollution was made on the basis of trace element concentrations (arsenic — As, mercury — Hg) in the Gulf of Mannar. The beachrock samples were collected along the coastal tracts between Rameswaram and Kanyakumari. The samples were dried and digested to determine the As and Hg using atomic absorption spectrophotometer (AAS-air-acetylene and nitrous oxide method). The As and Hg accumulation status of the beachrock was assessed using geo-accumulation index values (I _geo). The accumulation of As and Hg in the beachrock ranges from 2.75 to 20.72 μg g⁻¹ and from 0.06 to 0.31 μg g⁻¹, respectively. The As and Hg concentrations in the beachrocks are compared with crustal average values and average of other region sediments. The possible source of the contamination is from atmospheric deposition and anthropogenic activities.     

Analysis of waves generated by Gongjiafang landslide in Wu Gorge, three Gorges reservoir, on November 23, 2008

At 4:40p.m. on November 23, 2008, the Gongjiafang slope collapsed on the north bank of Yangtze River in Wu Gorge of Three Gorges Reservoir. The 380,000-m³ sliding mass consisted mainly of cataclastic rock. A video record of the major sliding incident was analyzed using the general laws of physical motion. The analysis indicated that the maximum speed and maximum acceleration of the sliding mass were 11.65?m/s and 2.23?m/s², respectively, and that the maximum amplitude and the propagation velocity of the water wave near the landslide were 31.8?m and 18.36?m/s, respectively. Wave run-up investigation indicated that the maximum run-up on shore was 13.1?m, which declined to 1.1?m at Wushan dock 4?km away. The incident causes no casualties, but did result in economic losses of RMB five million. The numerical simulation model GEOWAVE was used to simulate and reproduced the impulse wave generated by the landslide; the results were in good agreement with the observed incident. The numerical simulation data were then applied to analyze the decay and amplification effects of the landslide wave in the river course. The field investigations and witness information provide valuable materials for the studies of landslide kinematics and impulse waves generated by landslides. In addition, the research results provide a useful reference for future similar waves generated by landslides in reservoirs.     

高寒富水地区-西藏那曲镇给水管道的安装及维修

西藏那曲镇是全世界海拔最高、含氧量最稀薄、最后一个安装自来水的地级城市。那曲镇海拔高度为4500m，冻土层深达2．8m，地下水丰富，基础开挖超过lm时就有地下水渗出，且土质多为流沙型，施工时容易引起塌方，施工环境恶劣。本文就那曲镇的具体情况介绍一下给水管道的安装及维修。     

Liquefaction potential evaluation of the quaternary alluvium, Western Ankara (Turkey)

Liquefaction of the soils is a major problem during and after earthquakes, and can lead to severe damage. A wide range of experience in dealing with natural hazards was gained in the capital city of Ankara after several earthquakes occured in Kaman (1938), Gerede (1944), and Duzce (1999), and in later events. The recent findings after the Orta (2000) and Bala (2005, 2007) earthquakes proved that Ankara should not be designated as a nonseismic hazard zone, as it has been previously. The Quaternary age alluvium is located in the nearby suburbs of Etimesgut, Macunkoy, Cuglu, Avdan, and Kazan. A high density of population and structures is spread over this alluvium, which contains a high percentage of cohesive soils associated with sand and gravel lenses. The depths of the groundwater level ranges between 6.0 and 8.0?m in west Etimesgut, 3.0?m in Macunkoy, 5.0 to 7.0?C8.0?m in Cuglu/Avdan, and 4.5?C7.0?m in southern Kazan. The average S wave velocities of the alluvium derived from the ReMi were 140?m/s in Etimesgut, 200?m/s in Macunkoy, 134?m/s in Cuglu/Avdan, and 200?m/s in Kazan, respectively. Liquefaction evaluation is conducted by standard penetration blow count, shear wave velocity, and threshold acceleration. Varying horizontal accelerations of the input motions in the soil stratigraphy were generated via the Quad4m program. Geotechnical investigation and proper soil improvement should be carried out in order to prevent structural damage.     

西藏措勤盆地上侏罗统萨波直不勒组的发现及其烃源岩

措勤盆地东部班戈县保吉乡各昌茶玉地区发现晚侏罗世珊瑚化石约10属15种及未定种,分别为Actinaraea?sp.,Allocoenia?sp.,A.matheyi(Koby),Clausastraea?sp.,Hydnophora?sp.,Latiastraea minima(Koby),Latiastraea?sp.,Meandraraea?sp.,Stylosmilia crassitheca Liao et al Li,S.corallina Koby,Thamnasteria?sp.,T.concinna(Goldfuss),Thecosmilia.baojiensis sp.nov.,Thecosmilia?sp.,Trocharaea cf.patelliformis Gregory等。化石指示各昌茶玉剖面第1—41层地层时代为晚侏罗世牛津期—基默里奇期。珊瑚化石产出的层位原来被认为是混杂在中侏罗世蛇绿岩中的中二叠统下拉组,本文根据剖面中化石及岩性组合特点将其修订为上侏罗统萨波直不勒组,并得出了该地区不存在原来所认为的中二叠统下拉组与中侏罗统蛇绿岩构造混杂现象的认识。本次萨波直不勒组与改则县物玛乡抢古村建组剖面相对比,其岩性更细、灰岩夹层更多,珊瑚化石更为丰富,代表了更远离陆源碎屑源区的沉积环境,进一步证明晚侏罗世冈底斯地区地势是从北向南逐渐加深的,措勤盆地在晚侏罗世处于活动型深海-半深海相和上斜坡相的沉积环境中。保吉地区萨波直不勒组顶部存在上斜坡相68 m厚黑色泥页岩,TOC(总有机碳)平均值为0.83%,属于中等-好烃源岩,剖面第20层12 m厚黑色泥岩TOC值为0.65%,属于中等烃源岩。萨波直不勒组顶部为上斜坡相沉积,分布范围应该比上覆的吐卡日组更广泛。经计算,措勤盆地内萨波直不勒组烃源岩的生烃量约为44.373亿吨。萨波直不勒组的泥页岩为措勤盆地提供了充足的油源保障,同时其中的碳酸盐岩夹层也可能是油气有利储层,建议对上侏罗统萨波直不勒组开展钻探取样工作,获取更为翔实丰富的有机地球化学参数。     

Delineation of shallow resistivity structure in the city of Burdur,SW Turkey by vertical electrical sounding measurements

The city of Burdur, which is built on an alluvium aquifer, is located in one of the most seismically active zones in southwestern Turkey. The soil properties in the study site are characterized by unconsolidated and water-saturated sediments including silty, clayey and sandy units, and shallow groundwater level is the other characteristic of the site. Thus, the city is under soil liquefaction risk during a large earthquake. A resistivity survey including 189 vertical electrical sounding (VES) measurements was carried out in 2000 as part of a multi-disciplinary project aiming to investigate settlement properties in Burdur city and its vicinity. In the present study, the VES data acquired by using a Schlumberger array were re-processed with 1D and 2D inversion techniques to determine liquefaction potential in the study site. The results of some 1D interpretations were compared to the data from several wells drilled during the project. Also, the groundwater level map that was previously obtained by hydrological studies was extended toward north by using the resistivity data. 2D least-squares inversions were performed along nine VES profiles. This provided very useful information on vertical and horizontal extends of geologic units and water content in the subsurface. The study area is characterized by low resistivity distribution (<150 Ωm) originating from high fluid content in the subsurface. Lower resistivity (3–30 Ωm) is associated with the Quaternary and the Tertiary lacustrine sediments while relatively high resistivity (40–150 Ωm) is related to the Quaternary alluvial cone deposits. This study has also shown that the resistivity measurements are useful in the estimation of liquefaction risk in a site by providing information on the groundwater level and the fluid content in the subsurface. Based on this, we obtained a liquefaction hazard map for the study area. The liquefaction potential was classified by considering the resistivity distributions from 2D inversion of the VES profiles, the types of the sediments and the extended groundwater level map. According to this map, the study area was characterized by high liquefaction hazard risk.     

Lateglacial and Holocene relative sea‐level changes and first evidence for the Storegga tsunami in Sutherland,Scotland

We reconstruct one of the longest relative sea‐level (RSL) records in north‐west Europe from the north coast of mainland Scotland, using data collected from three sites in Loch Eriboll (Sutherland) that we combine with other studies from the region. Following deglaciation, RSL fell from a Lateglacial highstand of +6?8 m OD (Ordnance Datum = ca. mean sea level) at ca. 15 k cal a BP to below present, then rose to an early Holocene highstand and remained at ca. +1 m OD between ca. 7 and 3 k cal a BP, before falling to present. We find no evidence for significant differential Holocene glacio‐isostatic adjustment between sites on the north‐west (Lochinver, Loch Laxford), north (Loch Eriboll) and north‐east (Wick) coast of mainland Scotland. This suggests that the region was rapidly deglaciated and there was little difference in ice loads across the region. From one site at the head of Loch Eriboll we report the most westerly sedimentary evidence for the early Holocene Storegga tsunami on the Scottish mainland. The presence of the Storegga tsunami in Loch Eriboll is predicted by a tsunami wave model, which suggests that the tsunami impacted the entire north coast of Scotland and probably also the Atlantic coastline of north‐west Scotland.