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1.
ABSTRACT Mixed‐sand‐and‐gravel beaches are a distinctive type of coarse‐clastic beach. Ground‐penetrating radar (GPR) and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed‐beach deposits at Aldeburgh in Suffolk, south‐east England. The principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site. The application of radar stratigraphy allows the delineation of both bounding surfaces (radar surfaces) and the intervening beds or bed sets (radar facies). The deposits of the main backshore berm ridge consist of seaward‐dipping bounding surfaces that are gently onlapped by seaward‐dipping bed sets. Good correspondence is observed between a sequence of beach profiles, which record development of the berm ridge on the backshore, and the berm ridge's internal structure. The beach‐profile data also indicate that backshore berm ridges at Aldeburgh owe their origin to discrete depositional episodes related to storm‐wave activity. Beach‐ridge plain deposits at the study site consist of a complex, progradational sequence of foreshore, berm‐ridge, overtop and overwash deposits. Relict berm‐ridge deposits, separated by seaward‐dipping bounding surfaces, form the main depositional element beneath the beach‐ridge plain. However, the beach ridges themselves are formed predominantly of vertically stacked overtop/overwash units, which lie above the berm‐ridge deposits. Consequently, beach‐ridge development in this progradational, mixed‐beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface. Interannual to decadal variations in wave climate, antecedent beach morphology, shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions.  相似文献   

2.
Guichen Bay on the south‐east coast of South Australia faces west towards the prevailing westerly winds of the Southern Ocean. The bay is backed by a 4 km wide Holocene beach‐ridge plain with more than 100 beach ridges. The morphology of the Guichen Bay strandplain complex shows changes in the width, length, height and orientation of beach ridges. A combination of geomorphological interpretation, shallow geophysics and existing geochronology is used to interpret the Holocene fill of Guichen Bay. Six sets of beach ridges are identified from the interpretation of orthorectified aerial photographs. The ridge sets are distinguished on the basis of beach‐ridge orientation and continuity. A 2·25 km ground‐penetrating radar (GPR) profile across the beach ridges reveals the sedimentary structures and stratigraphic units. The beach ridges visible in the surface topography are a succession of stabilized foredunes that overlie progradational foreshore and upper shoreface sediments. The beach progrades show multiple truncation surfaces interpreted as storm events. The GPR profile shows that there are many more erosion surfaces in the subsurface than beach ridges on the surface. The width and dip of preserved beach progrades imaged by GPR shows that the shoreface has steepened from around 2·9° to around 7·5°. The changes in beach slope are attributed to increasing wave energy associated with beach progradation into deeper water as Guichen Bay was infilled. At the same time, the thickness of the preserved beach progrades increases slightly as the beach prograded into deeper water. Using the surface area of the ridge sets measured from the orthophotography, and the average thickness of upper shoreface, foreshore and coastal dune sands interpreted from the GPR profile, the volume of Holocene sediments within three of the six sets of beach‐ridge accretion has been calculated. Combining optically stimulated luminescence (OSL) ages and volume calculations, rates of sediment accumulation for Ridge Sets 3, 4 and 5 have been estimated. Linear rates of beach‐ridge progradation appear to decrease in the mid‐Holocene. However, the rates of sediment accumulation calculated from beach volumes have remained remarkably consistent through the mid‐ to late Holocene. This suggests that sediment supply to the beach has been constant and that the decrease in the rate of progradation is due to increasing accommodation space as the beach progrades into deeper water. Changes in beach‐ridge morphology and orientation reflect environmental factors such as changes in wave climate and wind regime.  相似文献   

3.
探地雷达(GPR)在海南岛东北部海岸带调查中的应用 *   总被引：2，自引：0，他引：2  
对海南岛东北部海岸带调查采用了探地雷达(GPR)研究海岸带沙体的结构、展布和沉积序列,取得了很好的效果。文章通过和有限的露头资料对比以及对反射波形态、结构的研究,确定了海滩脊、海岸沙丘的反射特征以及潜水面的位置。雷达图像显示五龙港古海滩脊由亚水平、不连续、高振幅和透镜状反射波组成;木兰头海岸沙丘已受到人类活动的扰动,短的、陡倾斜反射可能代表未受扰动的海岸风成沙的前积层。研究表明探地雷达是一种可靠、快速和经济的地球物理方法,在砂砾质海岸可产生高质量、高分辨率和连续的反射剖面,值得加以推广。  相似文献   

4.
《Sedimentology》2018,65(3):721-744
Storm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.  相似文献   

5.
四川盆地高石梯—磨溪地区埃迪卡拉系灯影组广泛发育一套微生物碳酸盐岩。以磨溪8井区灯影组四段(灯四段)为例,通过精细解析高频向上变浅序列的岩石学特征,发现灯四段微生物云岩及序列建造具有如下特征: (1)向上变浅序列主要由凝块石云岩、叠层石云岩和(含微生物)泥晶云岩3类构成,伴以少量微生物粘结颗粒云岩;(2)完整的典型向上变浅序列自下而上由(含微生物)泥晶云岩(A段)、平直状叠层石云岩(B段)、小型丘状叠层石云岩(C段)、分散状凝块石云岩(D段)、格架状凝块石云岩(E段)、微生物粘结颗粒云岩(F段)等6个岩性单元构成,但大量的序列由A-C-E、A-C-D、B-D、B-C-D-E等不完整的岩性单元构成,且序列顶底皆以凝块石云岩/(含微生物)泥晶云岩、叠层石云岩/(含微生物)泥晶云岩等不平整的岩性突变面或暴露面区分;(3)埃迪卡拉系微生物碳酸盐岩发育于浪基面之上至平均海平面附近的相对高能区域,且凝块石云岩较叠层石云岩形成的环境能量高。该研究结果不仅揭示出较高能的浅水环境控制了规模性微生物岩分布这一规律,而且对区域储集层预测具有指导意义,同时也因建立了有别于现代微生物的环境分布模式而具有重要的沉积学意义。  相似文献   

6.
滩坝作为发育于滨岸带非常重要的沉积储集层类型之一,已成为隐蔽油气藏勘探的目标。为进一步明确峡山湖东岸滨浅湖滩坝沉积模式,以现代沉积考察为基础,结合探地雷达技术和粒度分析资料,对东岸研究区滩坝砂体的沉积特征以及控制因素进行详细分析。结果表明: 峡山湖东岸主要发育风成沙丘及滩坝2种沉积体系。早期风成沉积物以发育连续性好的大型板状交错层理细砂为主,分选较好,粒度概率曲线以两段式为特征;滩坝沉积物以发育连续性差的板状交错层理、平行层理中砂为主,主要为反粒序或者复合粒序,具冲刷构造,岩性较混杂并以砂泥薄互层为特征,发育大量植物根系,粒度概率曲线以三段式为主,跳跃组分分为2段,分选较好。探地雷达资料表明研究区下部主要以厚层层状反射结构砂体为主,与风成沙丘沉积反射特征较为一致,环境敏感粒度组分对滩坝沉积较强的水动力作用有较好的指示。总体上,风力作用控制了风成砂体的沉积发育并且改变了古地貌条件,风浪作用冲刷改造了早期砂体,为滩坝的发育提供了物源,控制了滩坝发育的规模和地理位置,可概括为“风运—湖改”滩坝沉积模式,可为滩坝体系的识别和有利储集层预测提供指导。  相似文献   

7.
Alluvial fans serve as useful archives that record the history of depositional and erosional processes in mountainous regions and thus can reveal the environmental controls that influenced their development. Economically, they play an important role as groundwater reservoirs as well as host rocks for hydrocarbons in deeply buried systems. The interpretation of these archives and the evaluation of their reservoir architecture, however, are problematic because marked heterogeneity in the distribution of sedimentary facies makes correlation difficult. This problem is compounded because the accumulated sedimentary deposits of modern unconsolidated fan systems tend to be poorly exposed and few such systems have been the focus of investigation using high‐resolution subsurface analytical techniques. To overcome this limitation of standard outcrop–analogue studies, a geophysical survey of an alpine alluvial fan was performed using ground‐penetrating radar to devise a scaled three‐dimensional subsurface model. Radar facies were classified and calibrated to lithofacies within a fan system that provided outcropping walls and these were used to derive a three‐dimensional model of the sedimentary architecture and identify evolutionary fan stages. The Illgraben fan in the Swiss Alps was selected as a case study and a network of ca 60 km sections of ground‐penetrating radar was surveyed. Seven radar facies types could be distinguished, which were grouped into debris‐flow deposits and stream‐flow deposits. Assemblages of these radar facies types show three depositional units, which are separated by continuous, fan‐wide reflectors; they were interpreted as palaeo‐surfaces corresponding to episodes of sediment starvation that affected the entire fan. An overall upward decline in the proportion of debris‐flow deposits from ca 50% to 15% and a corresponding increase in stream‐flow deposits were identified. The uppermost depositional unit is bounded at its base by a significant incision surface up to 700 m wide, which was subsequently filled up mostly by stream‐flow deposits. The pronounced palaeo‐surfaces and depositional trends suggest that allocyclic controls governed the evolution of the Illgraben fan, making this fan a valuable archive from which to reconstruct past sediment fluxes and environmental change in the Alps. The results of the integrated outcrop–geophysical approach encourage similar future studies on fans to retrieve their depositional history as well as their potential reservoir properties.  相似文献   

8.
Clastic, depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution: climate, sea‐level, and the frequency of major meteorological and oceanographic events. This study seeks to use one such record from a southern Brazilian strandplain to highlight the potentially‐complex nature of coastal sedimentological response to small changes in these drivers. Following a 2 to 4 m highstand at ca 5·8 ka in southern Brazil, falling sea‐level reworked shelf sediment onshore, forcing coastal progradation, smoothing the irregular coastline and forming the 5 km wide Pinheira Strandplain, composed of ca 500 successive beach and dune ridges. Sediment cores, grab samples and >11 km of ground‐penetrating radar profiles reveal that the strandplain sequence is composed of well‐sorted, fine to very‐fine quartz sand. Since the mid‐Holocene highstand, the shoreline prograded at a rate of ca 1 to 2 m yr?1 through the deposition of a 4 to 6 m thick shoreface unit; a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies; and an uppermost beach and foredune unit. However, the discovery of a linear, 100 m wide barrier ridge with associated washover units, a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3·3 to 2·8 ka during the otherwise regressive developmental history of the plain. The protected nature of Pinheira largely buffered it from changes in precipitation patterns, wave energy and fluvial sediment supply during the time of its formation. However, multiple lines of evidence indicate that a change in the rate of relative sea‐level fall, probably due to either steric or ice‐volume effects, may have affected this coastline. Thus, whereas these other potential drivers cannot be fully discounted, this study provides insights into the complexity of decadal‐scale to millennial‐scale coastal response to likely variability in sea‐level change rates.  相似文献   

9.
Over 1 km thick Mesozoic sedimentary sequence is exposed over a wide area in the Upper Indus basin of north Pakistan along the western margin of the Indian Plate. The Mesozoic sequence is comprised of clastic facies in the lower part, while carbonate facies are dominant in the upper part. About 200 m thick mixed sequence of interbedded sandstone, siltstone, clay, and carbonaceous shale represents the lower Jurassic Datta Formation in the Salt and Trans Indus Ranges in North Pakistan. The Datta Formation constitutes important reservoir horizons in a number of oil fields in the western Himalayan foreland basins where it is encountered at a depth of about 4 km in various wells. The Datta Formation is described from different parts of the range front to understand the internal architecture of various sedimentary facies and their depositional system. The thickness and lithofacies assemblages of the Datta Formation change in different parts of the range front as well as in subsurface of the Upper Indus basin. The Datta Formation represents a coarsening upward deltaic sequence in most parts of the basin. On the basis of lithological variations and sedimentary structures, a number of depositional facies have been recognized which include channel belt facies, floodplain/abandoned channel facies, swamp facies, and lagoonal facies. Further north, in the Kalachitta and Hazara regions, the siliciclastic facies change to more complex assemblages of interbedded bauxite, silcrete, marl, and some limestone. These sediments represent deposition in a delta-plain setting of a fluvial-dominated delta with northwestward flowing channels.  相似文献   

10.
通过现场实地踏勘、拍照、开挖探槽、利用卫星图解译等方法,对乌伦古湖环布伦托海区域和吉力湖北部乌伦古河现代三角洲地区的湖泊滨岸沉积环境和沉积体系进行了现代沉积调查。研究表明乌伦古湖滨岸沉积环境可以划分为基岩型湖岸、砾质湖岸、砂质湖岸、泥质湖岸等4种类型,发育山前基岩型湖岸、侵蚀基岩型湖岸、砾质冲积扇-扇三角洲、砾质辫状河三角洲、砾质滩坝、砂质滩坝、砂质三角洲、风成沙丘和泥质沼泽等9种滨岸沉积体系。山前基岩湖岸分布在布伦托海的北部,主要发育小型塌积扇、倒石锥和狭窄的湖滩。侵蚀型基岩湖岸位于布伦托海西岸和东北角地区,发育湖滩宽20~40 m。砾质冲积扇-扇三角洲沉积体系分布在布伦托海西北部25.8 km狭长区域,表现为一系列冲积扇-扇三角洲体系在山前形成裙边状展布的辫状平原,顺流向长5~15 km。砾质辫状河三角洲体系发育在布伦托海西部,砾质滩坝发育在砾质三角洲前缘,沉积物一般为中砾和粗砾,泥质含量低。现代乌伦古河三角洲位于吉力湖北部,沙丘广泛分布在布伦托海东部的三角洲平原。砂质滩坝发育在布伦托海东岸南部地区,滩坝带宽30~100 m,发育大量障碍痕、冰划痕。泥质沼泽占据湖岸总长度29.22 km,沼泽地带植物繁茂,水动力微弱,泥质和有机质含量高。根据卫星照片推测乌伦古湖水位可能发生过3次较大的下降,现代乌伦古河三角洲可能经过了4个发育阶段,但目前缺乏地质年代学证据。构造格局控制了湖泊边界的地形地貌特征,平行构造线走向容易形成规模较大的沉积体系,垂直构造走向形成的沉积体系规模较小。寒旱地区湖泊周缘入湖河流较少,具有季节性和暂时性特点,洪水泥石流、塌积扇等重力沉积体系比较发育。湖泊封冻是寒旱区湖泊区别于温暖地区湖泊的重要特征。在相同气候背景下,源汇地区的高差和河流的流程、流量决定了沉积物的供给总量和沉积体系的特征。湖盆边界形态影响沿岸流的发育,也影响湖泊风动力方向和强度。乌伦古湖滨岸沉积体系的多样性对研究古代湖泊滨岸沉积体系具有重要的启发,开展湖泊滨岸沉积环境和沉积体系调查对完善陆相湖盆沉积体系模式,对发现新的储层类型,对重建湖泊古地理环境具有重要的意义。  相似文献   

11.
The internal architecture of raised beach ridge and associated swale deposits on Anholt records an ancient sea level. The Holocene beach ridges form part of a progradational beach ridge plain, which has been interpreted to have formed during an isostatic uplift and a relative fall in the sea level over the past 7700 years. The ridges are covered by pebbles and cobbles and commonly show evidence of deflation. Material presumably removed from the beach ridges and adjacent swales form the present dune forms on Anholt. Ground-penetrating radar (GPR) reflection lines have been collected with 250 MHz shielded antennae across the fossil ridge and swale structures. The signals penetrate the subsurface to a maximum depth of ～ 10 m below the fossil features. The GPR data resolve the internal architecture of the beach ridges and swales with a vertical resolution of about 0.1 m. GPR mapping indicates that the Holocene beach ridges are composed of seaward-dipping beachface deposits as well as minor amounts of inland dipping deposits of wash-over origin. The beachface deposits downlap on underlying shoreface deposits, and we use these surfaces as markers of a relative palaeo-sea level. The new data indicate that the highest relative sea level at about 8.5 m was reached 6500 years ago; 700 years later the relative sea level had dropped 0.7 m indicating a change in the relative sea level around 1 mm/year. This fall in the relative sea level most likely records the influence of an isostatic rebound causing younger beach ridge deposits to indicate lower sea levels.  相似文献   

12.
南盘江盆地的早—中三叠世地层中 ,沉积相形成复杂而有序的空间变化和时间变化。二叠纪与三叠纪之交的大规模台地淹没事件之后 ,三叠纪碳酸盐台地从缓坡型演化为镶嵌陆架型 ;空间上 ,从连陆台地到浊积盆地相分异特别明显 ,而且在盆地中发育若干的孤立台地 ,特别是南宁和靖西一带 ,孤立台地上早三叠世的鲕粒滩以及连陆台地边缘的礁滩相灰岩更是引人注目。尽管不同相带的三级沉积层序相序组构千差万变 ,并且它们的形成时限也不尽相同 ,但是由其所表征的相对海平面变化则具有大致的同步性 ,因此在早—中三叠世地层中可以识别出 6个三级沉积层序。以地层记录中的两种相变面和两种穿时性为基本要素 ,可以建立南盘江盆地早—中三叠世的层序地层格架。  相似文献   

13.
The internal structure of coastal foredunes from three sites along the north Norfolk coast has been investigated using ground‐penetrating radar (GPR), which provides a unique insight into the internal structure of these dunes that cannot be achieved by any other non‐destructive or geophysical technique. Combining geomorphological and geophysical investigations into the structure and morphology of these coastal foredunes has enabled a more accurate determination of their development and evolution. The radar profiles show the internal structures, which include foreslope accretion, trough cut and fill, roll‐over and beach deposits. Foredune ridges contain large sets of low‐angle cross‐stratification from dune foreslope accretion with trough‐shaped structures from cut and fill on the crest and rearslope. Foreslope accretion indicates sand supply from the beach to the foreslope, while troughs on the dune crest and rearslope are attributed to reworking by offshore winds. Bounding surfaces between dunes are clearly resolved and reveal the relative chronology of dune emplacement. Radar sequence boundaries within dunes have been traced below the water‐table passing into beach erosion surfaces. These are believed to result from storm activity, which erodes the upper beach and dunes. In one example, at Brancaster, a dune scarp and erosion surface may be correlated with erosion in the 1950s, possibly the 1953 storm. Results suggest that dune ridge development is intimately linked to changes in the shoreline, with dune development associated with coastal progradation while dunes are eroded during storms and, where beaches are eroding, a stable coast provides more time for dune development, resulting in higher foredune ridges. A model for coastal dune evolution is presented, which illustrates stages of dune development in response to beach evolution and sand supply. In contrast to many other coastal dune fields where the prevailing wind is onshore, on the north Norfolk coast, the prevailing wind is directed along the coast and offshore, which reduces the landward migration of sand dunes.  相似文献   

14.
福建海坛岛北部海岸发育一座覆盖淡灰黄色风成砂层、相对高度达30 m的大沙山,外貌很像风成大沙丘,但同时又具有冲积扇形态。其成因和沉积环境有待考证,而探明大沙山内部沉积构造是判别其成因的主要途径之一。在野外考察的基础上,运用探地雷达（GPR）对大沙山内部沉积构造进行探测,获得约20 m深度范围内的沉积构造图像数据。结果表明:大沙山中上部发育由棱角状砾石或粗砂细砂组成的沉积旋回和沟槽沉积构造,显示出冲积扇上部的沉积构造组合特点;中下部层面中交替呈现弧形沉积构造,大沙山南侧发育有向南倾斜28°～30°的前积层理。结合大沙山西侧冲沟剖面沉积构造、大沙山表面的棱角状砾石特征以及现代风沙地貌分布规律,初步认为大沙山是山前多期沟谷洪流携带的碎屑物沉积形成的冲积扇残余堆积体,沉积间隙受海岸带风沙活动影响。  相似文献   

15.
The Campanian Cliff House Formation represents a series of individually progradational shoreface tongues preserved in an overall landward-stepping system. In the Mancos Canyon area, the formation consists of four, 50- to 55-m-thick and 10- to 20-km-wide sandstone tongues, which pinch out landwards into lower coastal plain and lagoonal deposits of the Upper Menefee Formation and seawards into offshore shales of the Lewis Shale Formation. Photogrammetric mapping of lithofacies along the steep and well-exposed canyon walls was combined with sedimentary facies analysis and mapping of the detailed facies architecture. Two major facies associations have been identified, one comprising the mostly muddy and organic-rich facies of lagoonal and lower coastal plain origin and one comprising the sandstone-dominated facies of shoreface origin. Key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of depositional processes. The stratigraphic surfaces (master ravinement surface, shoreface/coastal plain contact, transgressive surface, maximum flooding surface and the sequence boundary) allow each major sandstone tongue to be divided into a simple sequence, consisting of a basal transgressive system tract (TST) overlain by a highstand system tract (HST). Within each sandstone tongue, a higher frequency cyclicity is evident. The high-frequency cycles show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion. The complexities of the Cliff House sandstone tongues are believed to reflect changes in the rate of sea-level rise combined with the responses of the depositional system to these changes. Synsedimentary compaction, causing a thickness increase in the sandstone tongues above intervals of previously uncompacted lagoonal/coastal plain sediments, also played a role. This study of the facies architecture, geometry and sequence stratigraphy of the Cliff House Formation highlights the fact that there may be some problems in applying conventional sequence stratigraphical methods to landward-stepping systems in general. These difficulties stem from the fact that no single stratigraphic surface can easily be identified and followed from the non-marine to the fully marine realm (i.e. from the landward to the basinward pinch-out of the sandstone tongues). In addition, the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface.  相似文献   

16.
This study highlights lithofacies and biofacies characteristics of the open coast tidal flat near Daman on the eastern flank of Gulf of Khambhat. Sedimentological and biological observation record six facies within the tidal flat area including older beach, beach face, sand flat, mud flat/mixed flat, sand bar and beach rock. Distinct sedimentary structures, foraminiferal assemblage and bioturbation intensity characterize each facies. A wide variety of wave and current generated sedimentary features characterize the sand flat facies. Semiconsolidated sands of older beach running parallel the coastline at a level higher than the present beach face possibly records the latest sea level highstand. The beach rock reflects early cementation of sands in tropical environments. Foraminifera are widely distributed in sand flats, mixed flats and mud flats and grouped into two biofacies — Ammonia-Elphidium-Quinqueloculina biofacies (sand flat and mixed flat) and Trochammina-Miliammina biofacies (mud flats). The beach face and sand bar facies contain forminifera reworked from sand flat and mud/mixed flat. Seasonal variation in depositional style is marked by deposition of fresh mud deposited over large areas of the intertidal flat during monsoon time, most of which is washed away by waves and current actions well before the onset of the next monsoon.  相似文献   

17.
Beach ridges in macrotidal environments experience strong multi‐annual to multi‐decennial fluctuations of tidal inundation. The duration of tide flooding directly controls the duration of sediment reworking by waves, and thus the ridge dynamics. Flume modelling was used to investigate the impact of low‐frequency tidal cycles on beach ridge evolution and internal architecture. The experiment was performed using natural bioclastic sediment, constant wave parameters and low‐frequency variations of the mean water level. The morphological response of the beach ridge to water level fluctuations and the preservation of sedimentary structures were monitored by using side‐view and plan‐view photographs. Results were compared with the internal architecture of modern bioclastic beach ridges in a macrotidal chenier plain (Mont St. Michel Bay, France) surveyed with ground‐penetrating radar. The experimentally obtained morphologies and internal structures matched those observed in the field, and the three ridge development stages identified in ground‐penetrating radar profiles (early transgressive, late transgressive and progradational) were modelled successfully. Flume experiments indicate that flat bioclastic shapes play a key role in sediment sorting in the breaker zone, and in sediment layering in the beach and washover fans. Water level controls washover geometry, beach ridge evolution and internal structure. Low water levels allow beach ridge stabilization and sediment accumulation lower on tidal flats. During subsequent water level rise, accumulated sediment becomes available for deposition of new washover units and for bayward extension of the beach ridges. In the field, low‐frequency water level fluctuations are related to the 4·4 year and 18·6 year tidal cycles. Experimental results suggest that these cycles may represent the underlying factor in the evolution of the macrotidal chenier coast at the multi‐decadal to centennial time scale.  相似文献   

18.
Ground‐penetrating radar surveys across the southern end of the Boco Plain, western Tasmania, revealed a complex sequence of Quaternary glacial and non‐glacial sediments. The subsurface imaging supported previous suggestions of a complex Boco Plain palaeotopography that incorporates a range of depositional environments and multiple constructional events. The ground‐penetrating radar technique enabled imaging of the sediments to 20 m depth, and permitted identification of different sedimentary facies and constructional events due to the significant contrast in dielectric constant within and between the sediments and bedrock. The bedrock and sediment stratigraphy are in broad agreement with drillcore records from the southern end of the Boco Plain and indicate the utility of the method in the initial stages in the investigation of Pleistocene sedimentary sequences of this type.  相似文献   

19.
Thick till sheets deposited during the Quaternary form significant aquitards in many areas of North America. However, the detailed sedimentary heterogeneity and architecture and depositional history of till units are not well understood. This study utilizes architectural element analysis to delineate the internal sedimentary architecture of the Tiskilwa Formation exposed at two outcrop sections in north‐central Illinois, USA. Architectural element analysis facilitates systematic delineation of sedimentary architecture based on the nature of facies contacts and change in facies associations, delineation of unit geometries and understanding of depositional processes at different scales of resolution; making architectural element analysis suitable for the sedimentological analysis and palaeoenvironmental reconstruction of subglacial deposits. Eleven facies types are identified in this study, including sand, gravel and diamict facies that record a suite of subglacial depositional processes. Detailed analysis of facies contacts (bounding surface hierarchy) and change in facies associations allows the delineation of five architectural elements, including coarse‐grained lens, coarse‐grained sheet, mixed zone, diamict lens and diamict sheet elements. The spatial arrangement and genetic interpretation of elements, and their spatial relationship with fifth‐order bounding surfaces, allows the delineation of five larger scale architectural units (‘element associations’), which can be mapped in the local study area and record at least three stacked successions of meltwater accumulation and till deposition. The results of this study can be utilized for architectural analysis of till sheets and provide insight to groundwater flow pathways through till in the study area and elsewhere.  相似文献   

20.
Shoreface sandstone deposits within the Early Carnian part of the Snadd Formation of the Norwegian Barents Sea can be traced for hundreds of kilometres in the depositional strike direction and for tens of kilometres in the depositional‐dip direction. This study uses three‐dimensional seismic attribute mapping and two‐dimensional regional seismic profiles to visualize the seismic facies of these shoreface deposits and to map their internal stratigraphic architecture at a regional scale. The shoreface deposits are generally elongate but show variable width from north‐east to south‐west, which corresponds to a sediment source in the northern part of the basin and a southward decrease in longshore sediment transport. The Snadd Formation presents an example of how large‐scale progradational shoreface deposits develop. The linear nature of its shoreface deposits contrasts with more irregular, cuspate wave‐dominated deltaic shorelines that contain river outlets, and instead implies longshore drift as the main sediment source. In map view, discrete sets of linear features bounded by truncation surfaces scale directly to beach ridge sets in modern counterparts. The shoreface deposits studied here are characteristic in terms of scale and basin‐wide continuity, and offer insight into the contrast between shallow marine deposition under stable Triassic Greenhouse and fluctuating Holocene Icehouse climates. Findings presented herein are also important for hydrocarbon exploration in the Barents Sea, because they describe a hitherto poorly understood reservoir play in the Triassic interval, wherein the most prominent reservoir plays have so far been considered to be found in channelized deposits in net‐progradational delta‐plain strata that form the topsets to shelf‐edge clinoforms. The documented presence of widespread wave‐dominated shoreface deposits also has implications for how the relative importance of different sedimentary processes is considered within the basin during this period.  相似文献   

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