Rapid erosion of the coast of Sagar island, West Bengal - India

The coastal zone of the Sagar island has been studied. The island has been subjected to erosion by natural processes and to a little extent by anthropogenic activities over a long period. Major landforms identified in the coastal area of the Sagar island are the mud flats/salt marshes, sandy beaches/dunes and mangroves. The foreshore sediments are characterized by silty, slightly sandy mud, slightly silty sand and silty sand. Samples 500 m inland from high waterline are silty slightly sandy mud, and by clayey slightly sandy mud. The extent of coastline changes are made by comparing the topographic maps of 1967 and satellite imageries of 1996, 1998 and 1999. Between 1967 and 1999 about 29.8 km² of the island has been eroded and the accreted area is only 6.03 km². Between 1996 and 1998 the area underwent erosion of 13.64 km² while accretion was 0.48 km². From 1998 to 1999, 3.26 km² additional area was eroded with meager accretion. Erosion from 1997 to 1999 was estimated at 0.74 km² /year; however, from 1996 to 1999, the erosion rate was calculated as 5.47 km²/year. The areas severely affected by erosion are the northeastern, southwestern and southeastern faces of the island. As a consequence of coastal erosion, the mud flats/salt marshes, sandy beaches/dunes and mangroves have been eroded considerably. Deposition is experienced mainly on the western and southern part of the island. The island is built primarily by silt and clay, which can more easily be eroded by the waves, tides and cyclonic activities than a sandy coast. Historic sea level rises accompanied by land subsidence lead to differing rates of erosion at several pockets, thus periodically establishing new erosion planes.     

Beach erosion under rising sea‐level modulated by coastal geomorphology and sediment availability on carbonate reef‐fringed island coasts

This study addresses gaps in understanding the relative roles of sea‐level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for temporal relationships to late‐Holocene and modern sea‐level change. The study area shoreline on the north‐east coast of Oahu, Hawaii, is characterized by a series of kilometre‐long beaches with repeated headland‐embayed morphology fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand‐rich coastal strand plain, a common morphological setting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef platform during a period of late‐Holocene sea‐level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty‐eight per cent of headland beaches eroded over the past century at an average rate of ?0·12 ± 0·03 m yr^?1. In contrast, 56% of embayed beaches accreted at an average rate of 0·04 ± 0·03 m yr^?1. Given over a century of global (and local) sea‐level rise, the data indicate that embayed beaches are showing remarkable resiliency. The pattern of headland beach erosion and stable to accreting embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea‐level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea‐level rise.     

Depositional features in tourist beaches of Chennai Metropolis,SE coast of India: Inferences from grain size studies

The present study was undertaken to examine the depositional and transportation pattern in the tourist beaches of Chennai in SE coast of India. Twenty four samples were analyzed for textural parameters, Linear discriminate function (LDF), C-M plots and heavy/light minerals from beach, berm and dune regions. Results of textural parameters indicate that they are coarse grained in the three regions. Majority of the samples indicate that they are moderately well sorted, symmetrically skewed and leptokurtic. The LDF analysis allowed in identifying areas that are characterized by: (1) beach, (2) shallow agitated environment and (3) fluvial deposits. C-M plots indicate that the sediments in the beaches were deposited from the near shore environment. Heavy mineral analysis suggests that the heavy and light minerals were in the range of 0.5 to 16.9% and 82.08 to 98.14% and the light/dark layers are controlled by the prevailing tidal conditions in the coastal region. It is observed that sand is accumulating from the shallow agitated environment, while the transportation and deposition of sediments is mainly due to the interaction of tidal activities at the same time. C-M plots suggest that the sediments fall in the graded suspension and the saltation activities in the region also facilitate to understand the hydrodynamics of the region suggesting that they are deposited by high-energy activities.     

广东曲仁两套红层的重矿物组合特征与地层对比

本研究以广东曲仁煤田两套红色地层为对象。工作中各选择了中侏罗统和上白垩统一个完整的地层剖面,进行碎屑重矿物组合、标志特征,重矿物成因和演变,韵律特征和矿物或熟度等研究,初步查明了它们的蚀源母岩类型;揭示了重矿物组合和区域岩石类型变化的原因;指出以重矿物为标志的韵律性和标志层的存在;探讨了矿物成熟度在重矿物研究中的意义;进而提出了划分、对比两套红层的重矿物标志。     

Sea-level rise and shoreline retreat of the Nile Delta promontories,Egypt

Studies of the Nile Delta coast have indicated wide values of local subsidence, ranging from 0.4 to 5 mm/yr. Trend analysis of sea-level rise and shoreline retreat at two Nile Delta promontories have been studied. Records from tide gauges at Alexandria (1944–1989) and Port Said (1926–1987), north of the Nile delta coast, indicate a submergence of the land and/or a rise of the sea-level of 2 and 2.4 mm/yr, respectively. Dramatic erosion has occurred on some beaches of the Nile Delta. This is greatest at the tips of the Rosetta and Damietta promontories, with shoreline retreat up to 58 m/yr. Relationship between the shoreline retreat and sea level trends in terms of correlation analysis and application of the Bruun Rule indicates that the sea level rise has, by itself, a relatively minor effect on coastal erosion. The sea-level trend at the Nile delta coast is found to be only one of several effects on shoreline retreat. Major recent effects include a combination of cut-off of sediment supply to the coast by damming the River Nile and local hydrodynamic forces of waves and currents. Estimates of local future sea-level rise by the year 2100 at Alexandria and Port Said, respectively, is expected to be 37.9 and 44.2 cm. These expectations, combined with other factors, could accelerate coastal erosion, inundate wetlands and lowlands, and increase the salinity of lakes and aquifers.     

Spatial Variations in Particle Size and Magnetite Concentration on Cedar Beach: Implications for Grain-Sorting Processes, Western Lake Erie, Canada

<正>This study examined spatial variations in the concentration,grain size and heavy mineral assemblages on Cedar Beach(Lake Erie,Canada).Magnetic studies of heavy mineral-enriched,dark-reddish sands present on the beach showed that magnetite(～150μm) is the dominant magnetic mineral.Surficial magnetic susceptibility values defined three zones:a lakeward region close to the water line(Zone 1),the upper swash zone(Zone 2) and the region landwards of the upper swash zone (Zone 3).Zone 2 showed the highest bulk and mass susceptibility(κ,χ) and the highest mass percentage of smaller grain-size(250μm) fractions in the bulk sand sample.Susceptibility(i.e.κandχ) values decreased and grain size coarsened from Zone 2 lakewards(into Zone 1) and landwards (into Zone 3),and correlated with the distribution of the heavy mineral assemblage,most probably reflecting preferential separation of large,less dense particles by waves and currents both along and across the beach.The eroded western section of Cedar Beach showed much higher concentrations of heavy minerals including magnetite,and finer sand grain sizes than the accreting eastern section, suggesting that magnetic techniques could be used as a rapid,cost-effective way of examining erosion along sensitive coastline areas.     

福建闽江和九龙江现代沉积物重矿物特征及其物源意义*

通过对闽江和九龙江沿岸沉积物中重矿物的分析,发现重矿物组合与源区岩石具有极好的相关性。两条河流流域重矿物组合为不透明铁矿类—绿帘石—锆石—电气石—角闪石,特征矿物为绿帘石,含量高达原生透明重矿物比重的70%,其成因除与高级变质岩有关外,还与中酸性岩浆岩及其与围岩接触蚀变发生的绿帘石化有关。从重矿物组合、重矿物特征指数以及与锆石年龄谱系的比对分析发现,闽江流域重矿物源自闽西北武夷山前寒武纪的变质岩、闽东广泛出露的燕山期岩浆岩和接触变质岩,而九龙江流域重矿物源自闽西南的印支—燕山期花岗岩。闽江上游沉积物重矿物以源自高级变质岩的重矿物为特征,中下游由上游来源的重矿物和下游酸性岩浆岩及接触变质岩形成的重矿物共同构成;九龙江以印支—燕山期花岗岩中的副矿物组合为特征。研究结果显示,对于中小流域面积的河流,由于搬运距离有限,重矿物组合保存的源岩信息量大,可作为研究流域内构造演化和源汇对比的重要手段。     

Magnetic mineral transport and sorting in the swash‐zone: northern Lake Erie,Canada

A combined field and laboratory study in northern Lake Erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non‐magnetic sediment. Work was conducted on the cross‐shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement, in the field, of samples of magnetic (magnetite) and non‐magnetic (quartz and calcite) grains, respectively. Laboratory experiments examined how the burial of small, dense magnetic minerals is affected by the grain size of the non‐magnetic host material, and how grain burial affects magnetic susceptibility measurements at the surface. The field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser, non‐magnetic grains under low to moderate wave conditions, and subsequently were unable to move in the longshore or cross‐shore directions. The laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals, and that magnetic grain burial was most effective beneath coarser rather than finer non‐magnetic sand and, for the latter sediments, under less rather than more energetic conditions. The results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions, and subsequent settling, exposure and concentration in the upper swash zone during more energetic periods, when the non‐magnetic grains are eroded. It is probably during these erosional periods, when the magnetic minerals are exposed in fairly homogeneous deposits, that longshore and cross‐shore transport takes place.     

自然重砂成因矿物学特征及找矿指示作用

自然重砂是地质体经自然风化、剥蚀、搬运、沉积等地质作用而分离出的单矿物(或矿物组合)。自然重砂矿物晶体由于仍然保留有许多矿物成因信息,包括颜色、形态、化学成分、物理性质和矿物组合等特征,因此常用于追溯源区地质体或者找矿勘查。这种方法被认为是一种经济实用的找矿方法——自然重砂测量。本文基于全国自然重砂找矿的数据资料,系统梳理了自然重砂的矿物类型、出现频率及其分布等特点,分析了自然重砂的矿物组合和成因矿物学特征,研究了自然重砂矿物的源区烙印、搬运距离及标型指示矿物组合特征,探讨了自然重砂成因矿物学研究意义及其找矿应用前景。自然重砂矿物的颜色、形态及内部结构依然保留着明确的成因矿物学信息:其颜色和晶体形态具有继承性而与其源区母体联系起来,体现源区母体的特性;其矿物组成可分出残余原生重矿物(包括造岩矿物、副矿物、矿石矿物等)和新生重砂矿物两个部分,如赤铜矿、孔雀石等反映着源区岩性体的成分或者赋存状态特征,其矿物组合也在很大程度上继承原生共生矿物而体现诸如有无矿化等意义;重砂颗粒的磨圆度、边界光滑性等表面特征反映搬运距离,有利于明确响应源区母体或者物源,而具有良好的找矿指示意义。     

中国自然重砂分布特征及其意义

通过全国资源潜力评价,结合各省自然重砂资料和全国重砂资料汇总成果,对85个常见自然重砂矿物进行统计分析。研究表明,中国自然重砂矿物复杂多样,重砂矿物分布具有明显的区域性特点。造山带中自然重砂矿物产出数量相对较多且密度较大,而稳定地区重砂矿物相对较少。不同的自然重砂矿物及其组合具有明显的找矿信息,单矿物自然重砂可能来源于相对单一的矿石或岩石,其异常范围表明与其相关的矿产可能具有较好的成矿远景;矿物组合常显示出有关矿床的成因信息。自然重砂矿物与源区岩石或矿石具有密切的成因联系,对指导矿产勘查与研究具有十分重要的意义。     

Sea-level rise and shoreline retreat of the Nile Delta promontories, Egypt

Studies of the Nile Delta coast have indicated wide values of local subsidence, ranging from 0.4 to 5 mm/yr. Trend analysis of sea-level rise and shoreline retreat at two Nile Delta promontories have been studied. Records from tide gauges at Alexandria (1944–1989) and Port Said (1926–1987), north of the Nile delta coast, indicate a submergence of the land and/or a rise of the sea-level of 2 and 2.4 mm/yr, respectively.Dramatic erosion has occurred on some beaches of the Nile Delta. This is greatest at the tips of the Rosetta and Damietta promontories, with shoreline retreat up to 58 m/yr. Relationship between the shoreline retreat and sea level trends in terms of correlation analysis and application of the Bruun Rule indicates that the sea level rise has, by itself, a relatively minor effect on coastal erosion. The sea-level trend at the Nile delta coast is found to be only one of several effects on shoreline retreat. Major recent effects include a combination of cut-off of sediment supply to the coast by damming the River Nile and local hydrodynamic forces of waves and currents. Estimates of local future sea-level rise by the year 2100 at Alexandria and Port Said, respectively, is expected to be 37.9 and 44.2 cm. These expectations, combined with other factors, could accelerate coastal erosion, inundate wetlands and lowlands, and increase the salinity of lakes and aquifers.     

黄骅坳陷新近系馆陶组重矿物特征及物源区意义

黄骅坳陷馆陶组发育一套以辫状河沉积为主的沉积体系,储集物性较好,是研究区内的重要储油层。整个黄骅坳陷新近系物源研究还处于空白状态。本文重点利用统计学中的Q型聚类及R型聚类的方法对重矿物含量特征进行分析,划分出不同的重矿物组合区及相应的母岩类型。研究结果表明:根据重矿物含量特征可划分出A—D 4个不同的重矿物组合区,其中A、B、C区的母岩类型与周围燕山、沧县、埕宁3个古隆起的岩石类型能很好对应,重矿物稳定系数所揭示的物源方向也与3个古隆起位置相吻合;D区与前3个分异性很大,且重矿物稳定系数极高,应为远源搬运的沉积物,远于周围的古隆起,可能与古黄河沉积物有关。     

南海西部表层沉积物碎屑矿物分布特征及其物源

南海西部海域的物源研究程度相对较低,尤其对中段的物源有较大的争议。通常认为中部没有大河注入,其物源主要来自其南北的湄公河和红河。笔者通过对该区表层沉积物的系统取样及碎屑矿物分析鉴定认为,碎屑矿物主要分布于南部陆架区和中部陆坡区域。根据因子分析,南海西部碎屑矿物可以明显分为南部、中部、北部三区。南区陆架碎屑矿物主要由重矿物金红石、锐钛矿、白钛矿、锆石、透闪石、十字石及轻矿物石英、长石等组成,原岩应以岩浆岩类为主,包括部分变质岩;中区陆坡以十字石、透闪石、电气石、褐铁矿及黑云母、白云母为主,原岩可能主要为变质岩类;北区陆坡碎屑矿物少,组合特征不明显。各区不仅组合特征明显不同,且各区分界明显,表明其物质来源明显不同,南区物源主要来自红河和加里曼丹岛;中部海域的碎屑矿物组合与中南半岛大量出露的变质岩基本吻合,表明其物源主要来自中南半岛,而不是来自其南北的湄公河和红河。     

内蒙古五岔沟地区洮儿河流域重矿物特征分析

五岔沟洮儿河流域重矿物的分布在不同区域表现有明显差异性: 洮儿河主流和二道桥所在支流主要重矿物为橄榄石,查干郭勒河主要重矿物为褐铁矿,额木斯台沟主要以磁铁矿为主,胡斯台沟主要以绿帘石为主。不同矿物组合和分布特征反映出该流域重矿物成熟度低,受近源物质影响显著。根据河流矿物组合,主要划分为3 个矿物区,即洮儿河主流矿物区、洮儿河东部矿物区、洮儿河西部矿物区,其中洮儿河东部矿物区金属矿物含量异常,对于寻找有用矿产提供了一定的科学依据。     

鲁西隆起晚中生代盆地重矿物组合分析及其构造响应

本文以鲁西隆起区盆地及周边盆地砂岩中碎屑重矿物为研究对象,通过重矿物的稳定性分析,以及通过不同源区重矿物组合的差异,探讨了鲁西隆起及周边地区晚中生代以来的物源及构造演化过程。研究结果显示,鲁西盆地中生代早期早中侏罗纪时期主要源区为鲁西隆起,中晚侏罗纪时期主要物源为胶东地区和鲁西隆起,而早白垩世时期主要物源为胶东地区,其次才是鲁西隆起。晚白垩世沂沭断裂带盆地物源主要是鲁西隆起和胶东地区,鲁西隆起贡献量大于胶东地区。重矿物的稳定性也折射出源区构造演化历史。早-中侏罗世至中-晚侏罗世,鲁西盆地重矿物物源由鲁西隆起向胶东地区的显著转变,暗示着苏鲁造山带自三叠纪形成之后,经折返抬升,至晚侏罗世之前已经到达地表。重矿物的含量变化显示,早白垩世中期和晚白垩世中期鲁西盆地内沉积了大量的鲁西变质岩矿物组合,并且不稳定重矿物含量急剧增加,我们推测鲁西在早白垩世晚期、晚白垩世中期存在两次构造抬升。     

On the separation of fine particles by centrifugation

An investigation of recovering finely divided mineral particles by centrifugal forces has been carried out.A theoretical analysis of the separation efficiency for various heavy minerals with density ranging between 5 and 19.3 g/cm³ against silica particles has been made initially. The size range considered in this study was 1–5 microns. The separation efficiency of these heavy minerals from silica was found to show typically a recovery over 95% with 80% purity.A system containing cassiterite and silica particles with 1–5 microns has been chosen as a model system in experiments. The experimental results were in good agreement with the theoretical predictions. Better separations were observed at higher pH's than pH 4 due to better dispersion of these minerals at the higher pH range.     

沉积盆地物质来源综合研究——以南堡老第三纪亚断陷盆地为例

重矿物分析对于判断沉积盆地的物源方向和物源区的母岩性质是有效的,但具有局限性：①对无重矿物或者仅能提供极少量重矿物的母岩判别力较差;②它不能准确判断物源区沉积物的主要运移通道。因此,进行盆地物源分析还需要重视综合研究,即①通过重矿物的矿物学、重矿物组合研究,判断物源区的母岩类型,推测母岩岩性演化;②通过沉积岩矿物成分与结构研究,判别物源区是否存在重矿物无法辨认的母岩类型和沉积物的搬运距离;③通过沉积体系分析,尤其是砂分散体系的编图,能有效地判断物源区的方位和圈定各物源的具体影响范围;④利用古构造图判别物源区的主要沉积物运移通道及其方位;⑤编制沉积盆地－物源区古环境图,以此来弥补重矿物分析物源所带来的缺陷。     

江汉平原主要河流沉积物重矿物特征与物源区岩性的响应

通过对江汉平原主要河流沉积物的重矿物组合、特征矿物以及能够反映沉积物稳定状况、物源及成熟度的重矿物特征指数(ATi,GZi和ZTR)进行对比分析,发现在江汉平原范围内,长江和汉江及其长江主要支流的沉积物中重矿物特征具有显著的差异。长江的重矿物组合模型为：  锆石+绿帘石+辉石+绿泥石+金属矿物,特征矿物是锆石和辉石;   汉江的重矿物组合模型为：  绿帘石+角闪石+石榴石+绿泥石+金属矿物,特征矿物是角闪石、石榴石;   另外,清江、漳河、沮水和玛瑙河的重矿物组合及特征矿物也都完全不同。而且各水系的沉积物的重矿物特征与其源区的岩性分布显示出极好的相关性。研究表明在江汉平原利用重矿物特征及组合模型来进行物源示踪的方法开展水系演化研究是可行的。     

Heavy minerals of the Barakar Formation,Talchir Gondwana Basin,Orissa

Heavy mineral analysis has been carried out in the Barakar Formation of the Talchir Gondwana Bbasin, Orissa. The characteristic heavy minerals are garnet, zircon, tourmaline, rutile, biotite, chlorite, pyroxenes, hornblende, staurolite, sillimanite, apatite, epidote, sphene, spinel and siderite including opaques and leucoxene. These heavy minerals are divisible into four groups on the basis of principal component analysis and suggest derivation of Barakar sediments from pegmatite, acid and basic igneous as well as low- and high-rank metamorphic rocks lying to the south of the Talchir Gondwana Basin. Though the heavy mineral suites of all the sandstone samples are by and large similar, differences have been noticed in the frequencies of many heavy minerals in vertical succession. Cyclic nature and vertical fluctuation of heavy mineral frequencies can be ascribed to variation of the relief of the source area, sudden release of some of the minerals in the source region and/or existence of favourable geochemical condition to escape partial dissolution.