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1.
The mechanisms of berm development along a microtidal-high energy beach is examined. Such a beach with medium-sized sand and monsoon wave-controlled profile at Valiathura, south-west coast of India, is selected for this study. The waves which very rarely fall below 1 m, often exceed 4 m during the monsoon period of May to October. The erosion-accretion pattern of the beach shows a cyclicity and the berm development is mainly due to the onshore migration and welding of longshore bars on to the beach following the monsoon rough season. The stages of berm development in the present microtidal beach are more or less similar to the model presented by Hine for a mesotidal case, except for the following intermediate additional stages. The longshore bar develops due to the erosion of beach when the wave steepness was above 0·04, gets flattened when it falls below 0·04, and then reforms nearer to the shoreline as a swash bar. This reformed bar gets divided and the inner bar gets welded on to the beach, followed by the outer bar developing the berm. During the onshore migration of the longshore bar and berm development the beach face becomes partially reflective with the surf scaling parameter, εb between 2·5 and 33. The inshore is dissipative with the inshore surf scaling parameter, εs?33. The offshore side of the longshore bar is partially reflective with its surf scaling parameter, εbar between 2·5 and 33. The breakers are spilling or plunging. Vertical growth of the berm is mainly due to the changes in swash-limit caused by the variations in wave steepness, breaker height and type. Vertical growth stops when the beach-face attains equilibrium with the grain size-wave energy relationship, and a wave steepness below 0·02 helps to sustain this state. 相似文献
2.
As an important reservoir type in the Bohai Bay Basin, China, lacustrine beach and bar sands which refer to the shallow water complex deposited mainly by nearshore, delta‐rim and buried hill‐related beaches as well as longshore bars were developed in a particular stage in the evolution of those faulted Cenozoic continental depressions. In the Chezhen Depression, for example, the Second Member of the Oligocene Shehejie Formation (abbr. as Es2 hereafter) formed during the rifting‐to‐thermal subsidence transitional stage. Although well developed in Es2, beach and bar sands are difficult to recognize owing to their relative thinness. The paper summarizes sedimentary characteristics of lacustrine beach and bar sands on cores and logs. Low‐angle cross‐stratification, swash stratification, as well as occasional small‐scale hummocky cross‐stratification resulted from storms can be observed in beach and bar sands. The paper distinguishes bars and beaches from each other in Es2 mainly based on the grain‐size, bed thickness, facies succession and log responses. In order to predict the distribution of beach and bars, a chrono‐stratigraphic correlation framework of Es2 in the study strata is established using a high‐resolution sequence stratigraphic approach. Es2 strata are sub‐divided into six medium‐scale cycles and the mapping of the high‐frequency cycles allows the geographic and stratigraphic distribution of both beach and bar sands to be predicted. The study shows that beach and bars are better developed in times of base‐level fall than in base‐level rise. Factors such as lake‐level fluctuation, sediment supply, palaeogeomorphology and palaeowind direction have exerted control on the formation and distribution of beach and bar sands. Finally, the genetic pattern of beach and bar sands in the Es2 unit has been constructed, which provides a foundation for the prediction of beach and bars reservoir in continental basins in general. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
3.
SEBASTIAN LINDHORST JÖRN FÜRSTENAU H. CHRISTIAN HASS CHRISTIAN BETZLER 《Sedimentology》2010,57(4):935-955
A sedimentary model for hooked spit depositional systems based on ground‐penetrating radar and sedimentological data is presented. The recurved main spit of Sylt Island (southern North Sea) is dominated by migrating sand dunes; the investigated hooked spit exhibits a system of foredune ridges, oriented perpendicular to the dunes of the recurved spit. The development of the hooked spit is related closely to the presence of an adjacent tidal inlet, where strong tidal currents and a steep bathymetry prevent a further northward progradation of the main spit and trigger a deflection from northerly‐directed to easterly‐directed net sediment transport. Ground‐penetrating radar data and shallow sediment cores reveal the sedimentary architecture of the hooked spit in high resolution and allow the proposition of a genetic stratigraphic model. It is shown that the growth of the hooked spit is controlled by the interplay of alongshore migrating foreshore beach drifts under fair‐weather conditions and strong erosional events, interpreted as the result of rare severe storms. These storms may excavate scarps in the backshore, which play an important role in the development of foredune ridges. Accelerator mass spectrometry 14C ages indicate an absolute age of at least 1300 years for the hooked spit, which possibly correlates with strengthened erosion of the main spit. In contrast to the main spit, where the sediment budget is negative nowadays, growth of the hooked spit beach accelerated significantly during the last decades. This effect can probably be attributed to enhanced beach‐nourishments updrift along the main spit and makes the investigated hooked spit a natural laboratory to study the influence of increasing sediment supply into a system developing under the conditions of sea‐level rise. The study shows that the same external forces lead to distinct progradational processes along one barrier‐spit system. 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.
陆相湖盆与海洋环境不同,具有水体浅、能量弱、湖进湖退频繁等特点。综合应用高精度层序地层学和地震沉积学等关键技术,通过中国东部及中西部典型陆相湖盆的勘探实践、野外露头解剖和现代沉积考察,总结了陆相湖盆三角洲—滩坝复合砂体的分布模式,并提出了复合砂体地质编图法。研究结果表明:(1)陆相湖盆的“湖—陆过渡沉积体系”不仅发育分流河道,还发育河口坝和沿岸砂坝,这些不同时期的分流河道、河口坝和沿岸砂坝交织叠加在一起,在古代地层和现代沉积中得以保存;(2)地形平坦处,水体浅、波浪作用不强、河流作用为主,形成河道砂体;湖岸较陡处,波浪作用强,分流河道砂体被波浪改造,形成沿湖岸分布的滩坝砂体;(3)在高频湖平面变化背景下,岸线的迁移导致沉积坡折的迁移,形成了多期垂直于岸线分布的树枝状三角洲分流河道砂体,和多期平行于岸线呈带状分布的沿岸滩坝砂体相互交织的陆相湖盆三角洲—滩坝复合砂体的特殊结构;(4)复合砂体地质编图法基于传统单因素法,以复合砂体分布模式为指导,主要针对发育在水体较浅、地形平坦开阔背景下的陆相湖盆三角洲—滩坝砂体。 相似文献
6.
Mikkel Fruergaard Bernadette Tessier Clément Poirier Dominique Mouazé Pierre Weill Suzanne Noël 《Sedimentology》2020,67(1):502-533
Physical stratigraphy, architecture and evolution of barrier systems in hypertidal environments (tidal range above 6 m) are understudied, and depositional controls are poorly understood compared with wave-dominated barrier systems and barrier spits in microtidal, mesotidal and macrotidal settings. Based on vibracores, ground-penetrating radar, radiocarbon and optically stimulated luminescence age data, the formation and evolution of the hypertidal barrier system of Pointe du Banc, north-western France has been reconstructed. The study shows that the barrier spit has a complex composite sedimentary architecture consisting of wave-dominated and tide-dominated sedimentary bodies. A morpho-sedimentary model is presented that demonstrates how barrier-spit progradation resulting from littoral drift was the main manner of spit elongation, whereas sediment convergence caused by landward migration of swash bars and seaward migration of tidal dunes caused the spit terminus to grow in height and width. These results suggest that long-term accretion rates varied considerably in response to changes in sediment supply. Variations in storminess together with the large-scale topography of the coast controlled the sediment supply and thus the evolution of the barrier system during latest Holocene. Despite architectural complexity, hypertidal barrier systems preserve records of past climate changes. 相似文献
7.
This paper describes the formation, migration and sedimentology of sandwaves along the distal end of Long Point, a 40 km long spit in Lake Erie. Some seven to nine sandwaves occur in a zone over the last 14 km of the spit. They are characteristically 50–100 m wide at the downdrift end, range in length from 350 m to > 1500 m and migrate alongshore at rates that are typically 100–300 m year−1. Measurements over a 7-year period show two forms of alongshore sandwave migration: (1) a migratory jump; and (2) downdrift accretion. The migratory jump is commonly 200–500 m year−1 and results from the onshore migration and welding of an inner nearshore bar to the downdrift end of the sandwave. This in turn leads to emergence of the bar over a distance of several hundred metres downdrift of the sandwave and isolation of the trough landward of the bar. Infilling of the trough abstracts large volumes of sediment from the local sediment transport system and may affect movement of the sandwave in the following year or movement of the next sandwave downdrift. Downdrift accretion commonly results in migration of 50–150 m year−1 and results from the refraction of waves around the distal end of the sandwave and episodic accretion of small swash bars. This mechanism occurs less frequently and appears to reflect a local condition of lower sediment abundance, often triggered by a large migratory jump in the previous year. The process of bar emergence and infilling produces a distinct suite of sedimentary structures associated with the infilling of the landward trough and building of the sandwave berm. The initial shoreline perturbation that generates the sandwave results from onshore migration and welding of inner nearshore bars, and the development and growth of the sandwaves is promoted by refraction of highly oblique waves. 相似文献
8.
9.
This study was conducted to relate the cross-shore distribution of longshore sediment transport and grain size characteristics to cross-shore and longshore current velocities on a sandy low-energy beach in a non-tidal embayment of the Baltic Sea. Simultaneous measurements of current velocities and amount of sand caught in streamer traps were made on 31 sampling runs on 6 d in April 1999 at three fixed sites including the swash zone on the upper foreshore, the lower foreshore, and the crest of the most landward of four bars. Spilling waves broke frequently on the bar but rarely on the lower foreshore, even during onshore wind speeds up to 11.0 m s−1. Waves always broke as plunging waves at the step at the base of the upper foreshore and were converted directly into swash. The greatest longshore current velocities in the swash occurred when wind speeds and water levels were greatest, but wind direction was nearly directly onshore. Longshore velocities were greater in the swash zone than at other sites except when relatively strong winds blew nearly parallel to the shoreline, causing a pronounced wind-induced current at the other two sites. Calculated longshore shear stress and rate of sediment trapped were highly correlated on the bar (r=0.90), less highly correlated in the swash zone, and least highly correlated (r=0.66) on the lower foreshore. Mean trapping rates in the swash were 14.6 times greater than on the lower foreshore and 7.2 times greater than on the bar. Greater trapping rates in the swash are attributed to the greater turbulence mobilizing sediments in the uprush and backwash. Little of the finer-grained sediment on the offshore sites was reworked under low energy conditions. The study reveals the dominance of swash transport on steep, reflective, low-energy beaches where wave energy dissipation takes place over small distances on the upper foreshore. 相似文献
10.
Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology. 相似文献
11.
The beach‐bar reservoir play has become an important exploration target within the Bohai Bay Basin, especially in the Boxing Sag within the Dongying Depression, where a large‐scale lacustrine beach‐bar oil pool has been discovered recently. The sedimentary characteristics, distribution patterns and formation mechanisms of beach‐bar sand bodies in the upper fourth member of the Eocene Shahejie Formation (Es4s) in the Boxing Sag were studied in detail based on seismic, well log data and core data. The Es4s in the Boxing Sag is composed of a third‐order sequence consisting of three systems tracts, i.e. a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Beach‐bar sand bodies were deposited widely in the basin during the lowstand systems tract period. The sandy beach‐bars are characterized by siltstones, fine‐grained silty sandstones interbedded with thin mudstone units. The presence of well‐developed sedimentary structures, such as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts, ripples, terrestrial plant debris and vertical burrows, suggests that beach‐bars were deposited in a relatively shallow water environment under the influence of strong hydrodynamics. Laterally, the sandy beach facies occurred as a more continuous sheet‐like body around the sandy bar in most parts of the sag. Stratigraphically, beach‐bars were distributed mainly in the lowstand systems tract and they were less well‐developed in the transgressive systems tract and highstand systems tract. Several factors were probably responsible for the occurrence of the large‐scale beach‐bars during the lowstand systems tract period, including: (i) a gentle palaeoslope and relatively weak structural activities; (ii) a shallow‐water condition with a strong hydrodynamic environment; (iii) high‐frequency oscillations of the lake level; and (iv) an abundant terrigenous clastic feeding system with multiple‐point and linear sediment sources. 相似文献
12.
滩坝是隐蔽油气藏最重要的沉积储集层类型之一,已成为陆相断陷盆地油气勘探的重要领域。现代沉积滩坝的研究是深入认识滩坝沉积相的有效手段。环湖考察研究发现青海湖近岸滩坝类型多样,主要发育了废弃三角洲前缘处滩坝、三角洲侧缘处滩坝、湖岸线拐弯处滩坝、开阔滨浅湖处滩坝和冲积扇改造处滩坝5种类型。通过对各种类型滩坝剖面的详细分层及沉积物室内矿物组成分析等,发现不同类型滩坝砂体沉积特征存在很大差异。整体而言,青海湖近岸滩坝沉积物粒度较粗,黏土含量低,结构成熟度较高,沉积构造多见平行层理、交错层理和冲洗交错层理,垂向上表现出上粗下细的反韵律特征。同时,选取了青海湖南岸规模较大的坝相,分别对坝主体、坝间和坝侧缘进行详细的探槽剖面分析,得出坝主体单旋回厚度较坝间和坝侧缘大,沉积物粒度也相对较粗,黏土含量最低,分选性也最好,是发育为隐蔽油藏最有利的区域。 相似文献
13.
通过对苏北盆地高邮凹陷古近系阜宁组一段38口取心井的岩心观察和薄片鉴定, 识别出岩性、结构、构造标志, 进一步分析了高邮凹陷阜一段的相标志和沉积展布特征。研究认为其具有浅水三角洲和滩坝相的沉积特征, 其中浅水三角洲可进一步分为浅水三角洲平原和浅水三角洲前缘2个沉积亚相, 前者包括分流河道、河口坝、天然堤3个沉积微相, 后者包含水下分流河道、水下天然堤、支流间湾、河口坝4个沉积微相。在湖平面频繁变化过程中, 浅水三角洲的水下分流河道向湖泊轴心方向推进, 形成多个叠置的朵叶体;而在浅水三角洲侧缘, 由于湖浪和沿岸流的改造, 形成多条沿湖岸线和浅水三角洲呈狭长带状展布的滩坝。在上述研究基础上, 建立了湖平面频繁变化时期浅水三角洲和滩坝混合沉积的沉积模式。 相似文献
14.
Several well-preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present-day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant-scale cross-bedded foreset unit, overlain by topset and beach units, while the second (Sequence II) consists of the foreset unit overlain by bar-trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell's (1966, 1972) model for spit-platform growth based on scaled wave tank experiments. The giant-scale cross-bedded unit corresponds to prograding of a coarse-grained subaqueous spit-platform while the topset, bar-trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and II reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse-grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant-scale cross-bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit-platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit-platform sequences allows precise determination of sea-level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant-scale cross-bedding to those already known from fluvial transverse, lateral and point bars, subtidal sand waves and Gilbert deltas. 相似文献
15.
古风力是一项重要的古气候指标,其定量恢复是一个难题。风作用于水体产生的波浪大小间接地反映了风力,能够为古风力的恢复提供思路。发育于破浪带和冲浪回流带的破浪沙坝、沿岸沙坝分别记录了破浪和冲浪过程,作者分别介绍利用古湖泊中发育的破浪沙坝和沿岸沙坝进行古波况和古风力恢复的原理和操作流程。(1)根据破浪沙坝的几何形态,可以将其厚度与破浪水深建立函数关系,而破浪水深又由破浪波高决定,因此破浪沙坝厚度可以恢复破浪波高,据此可以进一步根据波浪统计关系恢复有效波高、根据风浪关系恢复风力。此方法依托以下3个参数: 单期次的破浪沙坝厚度、破浪沙坝的基座坡角、古风程。(2)沿岸沙坝厚度近似记录了冲浪的极限高度,后者受控于有效波高,据此也可以恢复有效波高和风力。此方法依托以下5个参数: 单期次的沿岸沙坝厚度、古(平均)水深、古风程、古风向相对于岸线的入射角、组成沿岸沙坝的沉积物粒度。上述2种方法综合性较强,涉及古风向、古地形坡度、风程或盆地直径、古水深等参数的恢复,需要综合运用古地貌恢复、去压实校正、古岸线识别、古水深恢复等技术,并需要结合波浪理论。古湖泊滨岸带地层中保存有大量的滩坝沉积,利用其恢复古波况和古风况具有一定的应用前景,能够有助于更详细地重建沉积盆地的古地理背景。 相似文献
16.
Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan 总被引：1，自引：0，他引：1
The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts. 相似文献
17.
The dynamic sediment budget on the intertidal face of sandy surf beaches is influenced by interaction of swash/backwash flows with the beach watertable. Watertable height variations are coupled to tides and pass into the beach as a slow wave of diminishing amplitude and increasing lag. High-frequency pulses from the onshore wave train also propagate into the beach. The height at which the watertable outcrops on the beach face is affected by antecedent tide and wave history and influences balance of sand deposition and entrainment during swash and backwash. Experimental lowering of the watertable by pumping an array of wells induces sand deposition. Energy expended in pumping is less than 0.3% of the onshore flux in the wave train. 相似文献
18.
基于对惠民凹陷和东营凹陷沙四上亚段滩坝相的解剖研究成果,提出了适合这2个地区滩坝沉积体研究的分类方案。通过对勘探研究程度较低的惠民凹陷和勘探研究程度较高的东营凹陷滩坝沉积特征的对比分析,明确了惠民和东营凹陷沙四上亚段滩坝的沉积特征和展布模式受可容纳空间、湖平面升降与古岸线变迁、沉积水动力转换带、物源、古地貌及地层坡度等因素的控制。其中,起主要作用的是构造运动对凹陷形成所造成的沉积地形上的差异性、物质供给来源以及沉积水动力对滩坝砂体空间分布的控制。滩坝相的2种空间分布模式:东营凹陷单断单超水下低隆型滩坝分布模式和惠民凹陷中央隆起型滩坝分布模式。 相似文献
19.
Pleistocene Lake Bonneville of western Utah contains a variety of spits associated with shorelines and other features that formed between 21,000 and 12,000 14C yr BP. Field studies in the low-lying mountain ranges of the central portion of Lake Bonneville identified 17 spits of various types. The spits are connected to small mountain ranges and islands, vary in size from 0.02 to 0.5 km2, and are composed of coarse-grained, well-rounded, poorly-sorted sedimentary material. Sixteen of the 17 spits have a northeasterly to southwesterly orientation implying that winds were from the northwest to northeast, approximately 180° out of phase with modern winds in the eastern Great Basin. Lake Bonneville spit orientation is best explained as the result of persistent northerly winds caused by the high atmospheric pressure cell of the continental ice sheet and passage of low pressure extratropical storms south of the lake. Similar, strong persistent winds are a common feature of modern continental ice sheets and passing low pressure systems. If so, the North American jet stream tracked south of Lake Bonneville as recently as 12,000 14C yr BP, well past the height of the last glacial maximum. 相似文献
20.
青海湖现代滨岸沉积微相及其特征 总被引：6，自引：1，他引：5
本文将青海湖滨岸带沉积划分为沿岸砾沙坝、泥坪、沙滩、砾石滩、泻湖和水下风沙堆积六个微相。与国内外其它湖泊滨岸沉积相比,以发育沿岸砾沙坝和水下风沙堆积为其特征。其中规模较大的沿岸砾沙坝和沙滩分布于地形坡度不大（坡度１０°±）和波浪、沿岸流及风力作用强烈的滨岸地区,粒度概率曲线以推移和跳跃总体为主、悬移组份含量少和跳跃总具双跳跃式为特征,垂向层序由分选差（δ１＞１）的上部粗碎屑物与分选好（δ１＜１）的下部细碎屑物构成反韵律层;砾石滩主要分布于冲积扇或扇三角洲入湖地带和湖蚀岸边,以砾石均匀、磨圆较好、绝大多数砾石长轴平行湖岸线和最大扁平面倾向湖方向且倾角８°±为特征;泥坪主要分布于湖水动力弱、无大河流注入的湖湾、湖岸地形平缓以及坝后湖水滞流地带;水下风沙堆积以分选好、磨圆好、石英表面具风蚀凹坑、概率曲线呈上凸和双跳跃式总体以及粒度参数δ１＜１、Ｓｋ＝－０．１５～０．２１、Ｋｇ＝０．７５～１．１３和Ｍｄ＝０．２２～０．０２３ｍｍ为特征。本文对丰富湖泊沉积相内容具有一定意义。 相似文献