共查询到20条相似文献，搜索用时 640 毫秒
1.
The pattern of grainfall deposition in the lee of aeolian dunes 总被引：1，自引：0，他引：1
ROBERT S. ANDERSON 《Sedimentology》1988,35(2):175-188
ABSTRACT
A simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density.
The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits.
The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum. 相似文献
A simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density.
The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits.
The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum. 相似文献
2.
Basic types of stratification in small eolian dunes 总被引：5，自引：0，他引：5
RALPH E. HUNTER 《Sedimentology》1977,24(3):361-387
The thinnest recognizable strata in modern eolian dune sands can be grouped into six classes. They are herein named planebed laminae, rippleform laminae, ripple-foreset crosslaminae, climbing translatent strata, grainfall laminae, and sandflow cross-strata. Planebed laminae are formed by tractional deposition on smooth surfaces at high wind velocities. They are very rare in the deposits studied. Grainfall laminae are also formed on smooth surfaces, largely by grainfall deposition in zones of flow separation. They are much more common than planebed laminae, which they closely resemble. Eolian climbing-ripple structures are composed primarily of climbing trans-latent strata, each of which is the depositional product of a single climbing ripple. Climbing translatent strata that formed at relatively high or supercritical angles of ripple climb are typically accompanied by rippleform laminae, which are wavy layers parallel to the rippled depositional surfaces. Ripple-foreset crosslaminae, which are incomplete rippleform laminae produced when the angle of ripple climb is relatively low or subcritical, are rarely visible in eolian sands. Sandflow cross-strata are formed by the avalanching of noncohesive sand on dune slipfaces. Their form varies with slipface height and with other factors. 相似文献
3.
波—流相互作用是复杂水动力条件下流体相互作用的主要方式之一,由波—流相互作用形成的复合流沉积是目前沉积学在流体相互作用这一领域研究较多的一种沉积类型。以已有的文献为基础,对波—流相互作用下细砂级颗粒的运动机制进行了综述,预测了波—流相互作用的沉积特征,总结了复合流的沉积构造鉴别标志。取得的主要认识有: (1)波—流相互作用总体上属于衰弱流(waning flow)悬浮沉积,其微观的沉积机制可分为5种: 越过崩落点的喷射沉积(S1)、残余涡动沉积(S2)、未到崩落点的喷射沉积(S3)、背流面的崩落沉积(S4)、垂直降落沉积(S5);(2)波—流相互作用的沉积过程总体上受悬砂量和沉积时间的控制,5种微观沉积机制在不同的悬砂量和沉积时间条件下可形成不同的沉积机制组合,从而导致不同的底床形态;(3)复合流沉积构造鉴别标志主要有: 复合流波痕、复合流层理、爬升型复合流层理、不对称丘状交错层理、准平行层理和频繁交替的不能充分发育的浪成波纹层理与流水层理等6类。上述认识对于复杂水动力条件下的沉积学研究及对深水、浅水沉积环境的识别均具有重要的意义。 相似文献
4.
The origins and sedimentary features of grainfall-, avalanche-, and ripple-produced strata have been studied experimentally in a wind sedimentation tunnel. Rate of deposition, wind velocity and wind duration have been shown to control specific sedimentary features of these types of strata. Grainfall-produced strata were deposited on a horizontal surface, and surfaces sloping up to the angle of initial yield for dry sand (about 34°). Thickness of a grainfall-produced stratum depended upon rate of deposition and duration of a specific wind event. Grainfall-produced strata were both non-graded and graded. Graded strata were produced by changes in wind velocity which controlled size of sand in transport and flying distances of individual grains. Distinctive features of grainfall-produced strata are: (a) gradual thinning, or tapering downwind (e.g. down the simulated slipface and across the simulated interdune; (b) extreme variability of thickness from less than 1 mm (wind gusts of a few seconds) to 10 cm or more (sustained gusts). Aeolian avalanche-produced strata were formed when grainfall-produced strata steepened above the angle of initial yield and sheared downslope. A rapid transition in sedimentary features from top to bottom of the slipface characterized avalanche-produced strata of the slump degeneration type in dry sand derived from grainfall deposition. Fadeout laminae formed near the top of the simulated slipface and about 1 m farther down the slipface were flame structures and drag folds. Near the base of the slipface, the avalanche truncated and then overrode grainfall-produced deposits. Distinctive features of avalanche-produced strata for a 2.5 m long slipface are the deformation structures, a thickness of 1 or 2 cm, sandflow toes, and steep dip (34°). Each avalanche-produced stratum was roughly tabular in cross-section parallel to wind direction, with gradual pinchout upslope. Aeolian ripple-produced strata were deposited on horizontal surfaces, and surfaces sloping to as much as 28°. Thickness of a ripple-produced stratum depended upon rate of deposition, morphology of the ripple, and rate of ripple migration. A maximum thickness of several centimetres was observed for a single ripple-produced stratum. Shape and attitude of ripple foresets was controlled by ripple morphology. Distinctive features of aeolian ripple-produced strata are: (a) presence of ripple foresets; (b) abrupt changes in thickness of a stratum or pinchout over downwind distances of a few centimetres; (c) low average foreset-to-diastem angle (10–15°); (d) low ripple-climb angle (<10°). 相似文献
5.
Rippled flanks of longitudinal dunes and the slip faces of various dune types can be distinguished granulometrically by comparing the top to bottom trend of changes of sorting in the sand population of each individual slope. Flank sands, which are prone to sorting processes through migration of ripples alongslope, are always better sorted upslope whereas slipface sands, which are controlled by avalanche and grainfall of sands, become better sorted downslope. Considering the absolute values of the bottom samples, the sands of the slip faces are both better sorted and the size distribution more symmetrical than those of the rippled flanks. Applying this approach in investigating ancient sediments could improve palaeogeographical interpretations. 相似文献
6.
Particle size and geochemical data have been used to investigate the development of a large cliff-top dune at Rubjerg Knude, located on the western coast of Jutland, Denmark. Textural parameters and geochemical ratios provided useful indicators of the dune sediment provenance and mode of evolution of the dune. The dune sediments themselves showed no significant spatial particle size trends and reflect a number of processes, including grainfall, wind-ripple migration and avalanching (grainflow), which formed a high proportion of the deposits on both the stoss and lee sides of the present dune. Fine grainfall sediments, which have accumulated to form a sandplain in the lee of the dune, show fining and improved sorting with distance, and extend more than 2 km downwind of the dune crest. Comparison of the textural and geochemical data from Rubjerg Knude and other locations on the Jutland coast indicates that, although there is a contribution of sand to the dune from local marine sources, the main source of sand to the cliff-top dune and sand plain sediments has been provided by the wind erosion of the underlying cliffs, which are composed of Weichselian age sandy glaciofluvial and glaciolacustrine deposits. Optically stimulated luminescence dating indicated an apparent age for the sand at the base of 274 ± 14 years. If this date is reliable, it suggests that accumulation of the aeolian sand in this area began within approximately the last 300 years. Map and photographic evidence indicate that the modern high dune only began to form after 1885, apparently associated with an acceleration in the rate of coastal cliff retreat. 相似文献
7.
C. M. BELL 《Sedimentology》1991,38(2):289-300
Extensive occurrences of Upper Jurassic to Lower Cretaceous aeolian sandstones form part of a thick sequence of terrigenous red beds in northern Chile. These sediments accumulated in a N-S-elongated intermontane back-arc basin on the landward side of an active volcanic chain. This volcanic chain, which was produced by subduction beneath the continental margin, provided the source of almost all the clastic sediments. The aeolian sands formed isolated dune fields covering only part of the depositional basin at any one time. Cosets of cross-bedded sandstone up to 40 m thick represent the deposits of linear draas. Cross-bedded sets, which average between 0.5 and 1 m in thickness, were produced by the migration of small dunes down and along the gentle lee slopes of the draas. The majority of foreset laminations in the cross-bedded sets are thin, parallel and persistent. They are the product of the lateral migration of dunes which developed oblique to the prevailing winds. Foreset laminations dip at low angles (averaging 19°), well below the angle of repose of dry sand. Most of the laminations originated by tractional processes, rather than by grainflow (avalanching) or grainfall. This conclusion indicates that the dunes were gently undulating with no slip faces. The orientation of the direction of maximum dip of foreset laminations does not provide a reliable indicator of the palaeowind direction. Although individual sets or cosets commonly show a unidirectional pattern, significant variations have been recorded between sets of the same age at nearby locations and vertically between one coset and the next. Variations in vertical sections were produced by the superimposition of draas with different orientations. 相似文献
8.
Gilbert‐type deltas recording short‐term base‐level changes: Delta‐brink morphodynamics and related foreset facies 
下载免费PDF全文
下载免费PDF全文 Gilbert‐type deltas are sensitive recorders of short‐term base‐level changes, but the delta‐front record of a base‐level rise tends to be erased by fluvial erosion during a subsequent base‐level fall, which renders the bulk record of base‐level changes difficult to decipher from the delta‐front deposits. The present detailed study of three large Pleistocene Gilbert‐type deltas uplifted on the southern coast of the Gulf of Corinth, Greece, indicates a genetic link between the delta‐front morphodynamic responses to base‐level changes and the delta‐slope sedimentation processes. Sigmoidal delta‐brink architecture signifies a base‐level rise and is accompanied by a debrite‐dominated assemblage of delta foreset deposits, thought to form when the aggrading delta front stores sediment and undergoes discrete gravitational collapses. Oblique delta‐brink architecture tends to be accompanied by a turbidite‐dominated assemblage of foreset deposits, which are thought to form when the delta‐front accommodation decreases and the sediment carried by hyperpycnal effluent bypasses the front. This primary signal of the system response to base‐level changes combines further with the secondary ‘noise’ of delta autogenic variation and possible allogenic fluctuations in fluvial discharge due to regional climatic conditions. Nevertheless, the evidence suggests that the facies trends of delta foreset deposits may be used to decipher the delta ‘hidden’ record of base‐level changes obliterated by fluvial topset erosion. Early‐stage bayhead deltas may be an exception from the hypothetical model, because their narrow front tends to be swept by river floods irrespective of base‐level behaviour and their subaqueous slope deposits are thus mainly turbidites. 相似文献
9.
Grainfall deposition and associated grainflows in the lee of aeolian dunes are important in that they are preserved as cross‐beds in the geological record and provide a key to the interpretation of the aeolian rock record. Despite their recognized importance, there have been very few field, laboratory or numerical simulation studies of leeside depositional processes on aeolian dunes. As part of an ongoing study, the relationships among grainfall, wind (speed and direction), stoss sand transport rates and dune morphometry (height and aspect ratio) were investigated on four relatively small, straight‐crested transverse dunes at Silver Peak, Nevada. Between 55% and 95% of the total grainfall was found to be deposited within 1 m of the crest, and 84–99% within 2 m, depending primarily on dune size and shape. Grainfall decay rates on high dunes of large aspect ratio were observed to be very consistent, with a weak positive dependence on wind speed. For small dunes with low aspect ratios, grainfall deposition was more varied and decreased rapidly within 1 m of the dune crest, whereas at increased distance from the dune crest, it eventually approached the smaller decay rates observed on the large dunes. No dependence of grainfall on wind speed was observed for these small dunes. Comparison of field data with predictions from 1 ) saltation model of grainfall, based on the computation of saltation path lengths, indicates lack of agreement in the following areas: (1) deposition rate magnitude; (2) variation in decay rate with wind speed; and (3) the magnitude and location of the localized lee‐slope depositional maxima. The Silver Peak field results demonstrate the importance of dune aspect ratio and related wake effects in determining the rate and pattern of grainfall. This work confirms earlier speculation by 7 ) that temporary, turbulent suspension (or `modified saltation') of relatively large grains does occur within the dune wake, so that transport distances generally are larger than predicted by numerical simulations of `true' saltation. 相似文献
10.
We report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes. In steady sea-breeze conditions on a 3-m-tall dune at Point Año Nuevo, California, we measured simultaneously the near-brink wind speed and the deposition on both horizontal and lee face collector platforms. We then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process. Best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds. In addition, the total vertical number flux of grains is constrained to roughly 107?108 grains m?2 s?1 at shear velocities of 0.33–0.40 m s?1. The lee side deposition pattern, which shows the expected maximum in deposition rate at a distance of several decimetres from the brink, is also well fit by the model. In addition, simultaneous collection of horizontal and lee deposition patterns, along with the numerical simulation of these patterns, strongly implies that the windfield in the lee of this particular dune is best described as a non-recirculating wake. Grainflows on the lee face are caused by failure of grainfall depositional bumps. Our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows. rather than to increase their size, implying that very large, thick grainflows require a different mechanism. 相似文献
11.
Landscape and sedimentary response to catastrophic debris avalanches, western Taranaki, New Zealand 总被引：1，自引：1，他引：0
Catastrophic volcanic debris avalanches reshape volcanic edifices with up to half of pre-collapse cone volumes being removed. Deposition from this debris avalanche deposit often fills and inundates the surrounding landscape and may permanently change the distribution of drainage networks. On the weakly-incised Mt. Taranaki ring-plain, volcanic debris avalanche deposits typically form a large, wedge shape (in plan view), over all flat-lying fans. Following volcanic debris avalanches a period of intense re-sedimentation commonly begins on ring-plain areas, particularly in wet or temperate climates. This is exacerbated by large areas of denuded landscape, ongoing instability in the scarp/source region, damming of river/stream systems, and in some cases inherent instability of the volcanic debris avalanche deposits. In addition, on Mt. Taranaki, the collapse of a segment of the cone by volcanic debris avalanche often generates long periods of renewed volcanism, generating large volumes of juvenile tephra onto unstable and unvegetated slopes, or construction of new domes with associated rock falls and block-and-ash flows. The distal ring-plain impact from these post-debris avalanche conditions and processes is primarily accumulation of long run-out debris flow and hyperconcentrated flow deposits with a variety of lithologies and sedimentary character. Common to these post-debris avalanche units is evidence for high-water-content flows that are typically non-cohesive. Hence sedimentary variations in these units are high in lateral and longitudinal exposure in relation to local topography. The post-collapse deposits flank large-scale fans and hence similar lithological and chronological sequences can form on widely disparate sectors of the ring plain. These deposits on Mt. Taranaki provide a record of landscape response and ring-plain evolution in three stages that divide the currently identified Warea Formation: 1) the deposition of broad fans of material adjacent to the debris avalanche unit; 2) channel formation and erosion of Stage 1 deposits, primarily at the contact between debris avalanche deposits and the Stage 1 deposits and the refilling of these channels; and 3) the development of broad tabular sheet flows on top of the debris avalanche, leaving sediments between debris avalanche mounds. After a volcanic debris avalanche, these processes represent an ever changing and evolving hazard-scape with hazard maps needing to be regularly updated to take account of which stage the sedimentary system is in. 相似文献
12.
Open‐framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during a specific flow stage and sediment transport rate (either high, falling or low); (3) separation of sand from gravel in bedforms superimposed on the backs of larger bedforms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment size distribution (sandy gravel with a mean diameter of 1·5 mm) was kept constant, but flow depth, flow velocity and aggradation rate were varied. Bedforms produced under these flow conditions were bedload sheets, dunes and unit bars. The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation at the crest of bedforms, and further segregation of grain sizes during avalanching on the steep lee side. Sand in transport near the bed is deposited in the trough of the bedform, whereas bed‐load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the height of the bedform increases. Infiltration of sand into the gravel framework is of minor importance in these experiments, and occurs mainly in bedform troughs. The geometry and proportion of OFG in fluvial deposits are influenced by variation in height of bedforms as they migrate, superposition of small bedforms on the backs of larger bedforms, aggradation rate, and changes in sediment supply. If the height of a bedform increases as it migrates downstream, so does the amount of OFG. Changes in the character of OFG on the lee‐side of unit bars depend on grain‐size sorting in the superimposed bedforms (dunes and bedload sheets). Thick deposits of cross‐stratified OFG require high bedforms (dunes, unit bars) and large amounts of aggradation. These conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound‐bar tails and downstream of confluence scours. Increase in the amount of sand supplied relative to gravel reduces the development of OFG. Such increases in sand supply may be related to falling flow stage and/or upstream erosion of sandy deposits. 相似文献
13.
沙丘背风侧气流及其沉积类型与意义 总被引：6，自引：2，他引：6
在腾格里沙漠东南缘对现代沙丘表面气流、沉积过程的野外观测结果表明,由于区域气流、沙丘形态及其相互作用等的不同使沙丘背风坡气流发生变化,在此发现三种背风坡次生气流 :分离流、附体未偏向流和附体偏向流。前者以弱的反向流为特征多发生在横向气流条件下坡度较陡的背风坡;后二者具有相对高的风速,其中附体流多发生在坡度缓和的背风坡,其方向在横向气流条件下保持原来的方向,而在斜向气流作用下发生偏转且其强度为原始风入射角的余弦函数。根据背风坡气流方向及强度,作者阐述了不同区域气流环境中沙丘背风坡沉积过程、层理类型及特征,探讨了交错层产状与区域气流方向之间的关系. 相似文献
14.
J. R. L. ALLEN 《Sedimentology》1983,30(2):285-294
ABSTRACT The Lower Old Red Sandstone (Downton to ?Emsian) in southern Britain is a largely fluviatile sequence of increasing upward sand-dominance. The highest beds at two groups of localities include many sedimentation units composed of gravelly foresets in depositional continuity with overlying topsets of parallel-laminated sandstone. These units are thought to have been fashioned by humpback bars having a crest a considerable way upstream from the brink at the top of a gravelly slipface. The overall textural composition but internally segregated character of the bar units suggests that a bimodal sediment load of mixed sand and gravel was supplied at the upstream end of the bar, but that this load became texturally differentiated as it moved downstream. Differentiation is suggested to have occurred because the comparatively large and well-rounded gravel particles behaved on the sandy topset as though on a smooth surface, and were transported under similar flow conditions to the sand, much of which eventually lodged on the topset instead of being passed on, like the gravel, to the slipface beyond. A quantitative model is outlined which justifies the proposed gravel overpassing. In terms of the control of sedimentary structures exerted by grain size under laboratory conditions, the association of cross-bedding (gravel) with simultaneously formed parallel lamination (sand) seems to be a natural consequence of the efficient textural differentiation of the supplied load by the overpassing of the gravel component under a single flow condition. 相似文献
15.
Christopher M. Herbert Jan Alexander Kathryn J. Amos Christopher R. Fielding 《Sedimentology》2020,67(1):576-605
Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly-variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co-sets of cross-stratified and planar-stratified sets. The cross-stratified sets tended to down-climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down-climbing cross-stratified sets tended to pass into single planar cross-stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across-deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three-dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well-preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development. 相似文献
16.
We studied a large debris-avalanche deposit of Pleistocene age in the Tenteniguada Basin, Gran Canaria Island, Spain. This
deposit, which is well preserved because it is mostly covered by basanite lava flows, has distinctive matrix and block facies,
hummocky topography and internal structures typical of debris avalanches. However, neither syneruptive lavas nor some characteristic
features of volcanic debris-avalanche deposits, such as a stratovolcano edifice or a horseshoe-shaped crater, are present.
The occurrence of internal features characteristic of volcanic avalanche deposits could be attributed to the volcanic materials
involved in the movement rather than to the triggering of the avalanche during a volcanic eruption. The conditioning factors
are shown to be associated with specific structural and hydrological conditions, such as the presence of old volcanic domes,
strength reduction of the rocks, effective stress decrease, active gully erosion and water table rise during Pleistocene humid
episodes. We finally suggest that the possible triggering factor of the avalanche was a neighbouring volcanic or tectonic
earthquake. 相似文献
17.
Dariusz Krzyszkowski Anna Krzywicka Lucyna Wachecka‐Kotkowska Waldemar Sroka 《Boreas: An International Journal of Quaternary Research》2019,48(4):966-987
This article reports on an Early Saalian proglacial lake formed between the Scandinavian Ice Sheet and the front of the Sudeten Mountains, Poland. Sediments investigated at Mys?ów point to a transition from glacifluvial to glaciolacustrine environments. The bulk of the sediments was deposited in deep‐water Gilbert‐type deltas (A–E complexes). A delta plain (topset) gradually passes into a subaerial plateau and then a clastic shoreline and the subaquatic slope of a prograding delta (foreset). The glaciolacustrine lithofacies represent a number of lake‐basin environments, from marginal subaqueous slopes to distal parts of a subaqueous fan. Glaciolacustrine and glaciodeltaic deposits locally reach ?50–70 m in thickness. Analyses of A–E complexes indicate that the lake existed for more than 130 years and that its origin and evolution were closely connected with the ice front. This case study records lake sedimentation at an ice‐sheet margin with cohesionless gravity flows, turbidity currents, debris‐avalanching and, to a much lesser degree, parapelagic suspension fall‐out and ice‐raft dumping. In the initial stage, the lake extended more than 10 km to the south, and the deposition was relatively slow. In the second stage, recession of the ice sheet caused rapid growth of a delta. The third and ultimate stage coincided with the final glacial recession, with rapid deposition occurring only on the lake bottom. The model of the glaciolacustrine environment presented here may also be applicable to many other proglacial lakes in mountain areas. 相似文献
18.
P. A. CARLING 《Sedimentology》1996,43(4):647-664
Coarse-gravel bedforms which resulted from Pleistocene glacial outburst floods are identified as subaqueous dunes. Comparison of the morphology of these ‘fossil’ structures with modern dunes shows that the form of two-dimensional (2-D) transverse dunes and 3-D cuspate and lunate dunes developed in coarse gravels is comparable with sand-dune morphology within lesser-scale geophysical flows. The similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform. Internal structure indicates that flow over 2-D dunes was relatively uniform but over 3-D bedforms flow was locally variable. Flow separation and complex streaming of flow occurred over the steepest 3-D dunes. Cross-beds are thin and few approach the angle of repose; consequently most dunes did not migrate primarily by avalanching but by stoss-entrained gravel transported over the crests rolling-down and depositing on the lee slopes. Lee-side sediments are often finer than the stoss-slope sediments, which indicates the lee formed when flood power was waning. Some dunes were slightly planed-down during falling stage because lee-side cross-beds tend to be steeper than the angle of the preserved lee slope. However, silt-rich caps indicate that any height reduction was contemporary with the final deposition of foresets. Post-flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs. 相似文献
19.
LARS B. CLEMMENSEN MICHAEL HOUMARK-NIELSEN 《Boreas: An International Journal of Quaternary Research》1981,10(3):229-245
Inclined beds of sand, granules, pebbles and cobbles resembling a glacio-tectonically tilted sequence were shown by sedimentological studies to constitute the 10–12 m thick foreset beds of a glaciolacustrine Gilbert-type delta in Kyndby, North Sjælland, Denmark. The foreset beds are slightly curved, dip 10–28 SE, and display a bundlewise structure with alternating coarse-grained cobble-rich and fine-grained sandy units. The occurrence of ascending megaripple cross-bedding and climbing ripple cross-lamination in the sandy foresets can be ascribed to strong backflow currents formed by the lee-side vortice. The foreset beds are underlain by flat-lying fine-grained sand, silt and clay (bottomset beds), and are overlain transitionally or erosively by 2–3 m of flat-lying sand, pebbles and cobbles (topset beds). In the transition zone between foreset beds and topset beds, various delta distributary channel units occur. The delta probably formed in a partly ice-dammed lake in connection with the general retreat of a Weichselian advance from the north ('Norwegian ice') 相似文献
20.
Tsunamis versus storm deposits from Thailand 总被引：3，自引：0，他引：3
Along the Andaman (west) coast of Thailand, the 2004 tsunami depositional features associated with the 2004 tsunami were used to describe the characteristics of tsunamis in a place far away from the effect of both recent and ancient storms. The current challenge is that a lack of precise sedimentological characteristics have been described that will differentiate tsunami deposits from storm deposits. Here, in sedimentological senses, we reviewed the imprints of the sedimentological characteristics of the 2004 tsunami and older deposits and then compared them with storm deposits, as analyzed from the deposits found along the eastern (Gulf of Thailand; GOT) coast of Thailand. We discuss the hydraulic conditions of the 2004 tsunami and its predecessors, on the Andaman coast, and compare them to storm flows found on the coast of the GOT. Similar to an extensive tsunami inflow deposit, a storm flow overwash has very similar sedimentary structures. Well-preserved sedimentary structures recognized in sand sheets from both tsunami and storms include single and multiple normal gradings, reverse grading, parallel, incline and foreset lamina, rip-up clasts, and mud drapes. All these sedimentary structures verify the similarity of tsunami and storm inflow behavior as both types of high-energy flow start to scour the beach zone. Antidunes are likely to be the only unique internal sedimentary structures observed in the 2004 tsunami deposit. Rip-up clasts are rare within storm deposits compared to tsunami deposits. We found that the deposition during the outflow from both tsunami and storms was rarely preserved, suggesting that it does not persist for very long in the geological record. 相似文献