Stratigraphic architecture resulting from dune interactions: White Sands Dune Field,New Mexico          下载免费PDF全文

Sarah C. Brothers  Gary Kocurek  Thomas C. Brothers  Ilya V. Buynevich 《Sedimentology》2017,64(3):686-713

Aeolian dune interactions provide the dynamics for field‐scale pattern emergence and evolution within a set of boundary conditions. Although morphologies for a spectrum of dune interactions are recognized, associated stratigraphic architectures are unknown and have probably been misidentified in the rock record. A unique data set for the White Sands Dune Field in New Mexico (USA) allowed for a detailed analysis in which the morphological evolution of defect and bedform repulsion interactions is chronicled over a decadal time‐series of images and coupled with the resulting stratigraphic architecture, documented from cross‐strata exposed in interdune areas and ground‐penetrating radar imaging of dune interiors. Defect and bedform repulsions represent a class of interactions in which the faster‐migrating dune termination or defect (defect repulsion), or pair of defects (bedform repulsion), collides with the target dune downwind. Results document that during the collision, the defect(s) of the impactor dune recombine(s) with a segment of the target dune, and the redundant target dune segment is ejected as a parabolic‐shaped ejecta dune. The ejecta dune assumes a more barchanoid shape as it migrates downwind. The interaction architecture consists of lateral truncation of the target set by an interaction bounding surface. Defect cross‐strata tangentially approach the surface in plan‐view, and downlap onto the surface in cross‐section. The orientation of the defect cross‐strata is at an acute angle to the trend of the interaction surface. Orientations of the defect cross‐strata, which represent the defect approach angle, and the target dune cross‐strata, which represent the general dune migration direction, diverge at a high angle. Defect cross‐strata typically consist of wind‐ripple laminae, in contrast to the target set that may house grainflow cross‐strata. In the transport direction, the erosional interaction surface curves to become subparallel to subjacent and superjacent cross‐strata where the defect and target unify into a single lee face.  相似文献   

Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)     

Flavia Girard  Rmy Deschamps  Jean‐Franois Ghienne  Stphane Rouss  Jean‐Loup Rubino 《Sedimentology》2019,66(3):1042-1066

To elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre‐Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late‐Ordovician to Silurian outcrops at the Algeria–Libya border, in order to define the glacio–isostatic rebound and relative sea‐level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo‐relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea‐level changes. Deepest marine domains record a forced regression of 40 m of sea‐level fall resulting from an uplift caused by a glacio‐isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi‐type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo‐relief. Sedimentology and high‐resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea‐level changes curve for the studied succession. Estimates suggest a relatively short (<10 kyr) duration for the glacio‐isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).  相似文献   

Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition     

Jeremy G. Venditti  Jeffrey A. Nittrouer  Mead A. Allison  Robert P. Humphries  Michael Church 《Sedimentology》2019,66(6):2538-2556

A distinct suite of sand bedforms has been observed to occur in laboratory flows with limited sand supply. As sand supply to the bed progressively increases one observes sand ribbons, discrete barchans and, eventually, channel spanning dunes; but there are relatively few observations of this sequence from natural river channels. Furthermore, there are few observations of transitions from limited sand supply to abundant supply in the field. Bedforms developed under limited, but increasing, sand supply downstream of the abrupt gravel–sand transition in the Fraser River, British Columbia, are examined using multi‐beam swath‐bathymetry obtained at high flow. This is an ideal location to study supply‐limited bedforms because, due to a break in river slope, sand transitions from washload upstream of the gravel–sand transition to bed material load downstream. Immediately downstream, barchanoid and isolated dunes are observed. Most of the bedform field has gaps in the troughs, consistent with sand moving over a flat immobile or weakly mobile gravel bed. Linear, alongstream bedform fields (trains of transverse dunes formed on locally thick, linear deposits of sand) exhibit characteristics of sand ribbons with superimposed bedforms. Further downstream, channel spanning dunes develop where the bed is composed entirely of sand. Depth scaling of the dunes does not emerge in this data set. Only where the channel has accumulated abundant sand on the bed do the dunes exhibit scaling congruent with previous data compilations. The observations suggest that sediment supply plays an important, but often overlooked, role in bedform scaling in rivers.  相似文献   

Evidence for a change in wind regime during the Last Glacial Maximum from the Sydney region     

B. G. Thom 《Australian Journal of Earth Sciences》2019,66(2):279-288

This paper presents the results of a small number of dates from sand samples collected at building sites in the eastern suburbs of Sydney. OSL ages of well-sorted, quartz sand ranges from 31 to 24?ka from within well-developed podzol soil profiles within a metre of the surface. The source of this sand appears to be from the east off the inner continental shelf during a period just prior to the maximum of the Last Glacial Maximum (LGM, Marine Isotope Stage 2) when sea-level was approximately 70–90 m below present level. During a later phase of the LGM, there is evidence from other coastal sites in central NSW and Gippsland, Victoria, that the dominant wind was from the west consistent with evidence from arid and semiarid Australia. Dune orientations in northern NSW into Queensland and other evidence at this time appear to reflect winds from the southeast quadrant. This study highlights the existence of a potential pivot area in dominant wind direction during the LGM between sustained westerly flow in southern area including Victoria and northeast Tasmania and southeast flow of northern NSW into Queensland. In central NSW, the circulation switched from easterly in Stage 2 to westerly as the glacial stage intensified in the LGM sometime after ca 25?ka.  相似文献   

沙漠地区人工固沙植被对土壤温度与土壤导温率的影响   总被引：13，自引：2，他引：13  

王新平  李新荣  张景光  周海燕  Berndtsson R 《中国沙漠》2002,22(4):344-349

通过分析人工固沙植被区与流动沙丘区不同部位 (沙丘迎风坡、丘间低地、沙丘背风坡 )各层土壤温度梯度分布 ,及其月平均值变化与日变化特点 ,计算不同地表覆盖 (人工植被与裸沙区 )条件下的沙土导温率。结果表明 ,沙坡头地区 7～ 10月平均地温变化的振幅 ,在地面约为 16 2℃ ,振幅变化随深度的递增而迅速减小。在 0 0 5m深度处 ,地温年振幅为地面振幅的一半。 7～ 10月中地温最高最低出现的月份 ,随深度增加都呈滞后现象 ,但滞后的情况两者并不相同。另外 ,在 2 0cm以下深度内 ,7～ 10月月平均温度都高于 0℃。当观测深度按算术级数增加时 ,地温日振幅则以几何级数减小。尽管地温振幅日变化因天气情况不同大有差别 ,在相同的气象条件 (连续四日晴天 )下的研究发现 ,下垫面的影响作用不容忽视 ,尤其是由于植被覆盖度的差异而引起的地温日振幅与地温剖面分布变化。根据 1992年 8月观测记录 ,流沙区丘间地温日振幅最高达 2 6 2℃ (表层 ) ,最小值只有 1 8℃ (1m深 )。固沙植被区丘间低地最高日振幅为 2 5 9℃ (表层 ) ,最低值 4 3℃ (1m深 )。植被区地表层温度变幅小于流沙区 ,而深层温度波动显著。在 5cm深度处 ,植被区地温降至最低值的时间滞后于流沙区 2h以上 ,2 0cm处植被区地温达到最大值的时间比  相似文献   

Topographic changes associated with coastal dune blowouts at island beach state park,New Jersey     

Paul A. Gares 《地球表面变化过程与地形》1992,17(6):589-604

Topographic changes in two blowouts located in Island Beach State Park, New Jersey, USA were monitored over the winter of 1981-1982. Elevation changes were measured with erosion pins, and sediment traps placed at comparable locations in each blowout monitored the amount of sand moved by the wind. Discrete wind events were identified from regional data, and morphological data for the intervals with the highest onshore and offshore wind speeds are examined in detail. Vegetation is the primary influence on the development of the two blowouts. Blowout A is characterized by eroding sidewalls, a stable base, and an accreting blowout rim. High rates of sediment transport occur through the blowout throat which results in accretion on the vegetated rim. This blowout is an active sediment transfer system. Vegetation causes a large amount of deposition in the throat of blowout B. As vegetation was buried over the winter, the area of deposition migrated inland. Sidewall erosion also occurred in blowout B. Little change was recorded on the blowout rim. Blowout B is a recovering system where sediment is delivered to the blowout floor from the beach by onshore winds and from the blowout rim by offshore winds where it is stabilized by vegetation. The development of foredune blowouts is governed largely by vegetation cover on the dune crest and by sidewall erosion during offshore and onshore winds. Blowout recovery depends on vegetation growth and sediment deposition in the throat, and on the role of the sidewalls as sources of sediment which is deposited elsewhere within the system. Foredune blowouts are dynamic systems in which positive feedbacks in sediment availability and vegetation growth lead to a cycle of development and closure.  相似文献   

Palaeoclimate,stratigraphy and prehistory in north and west Rajasthan     

D. P. Agrawal  P. S. Datta  Zahid Hussain  R. V. Krishnamurthy  V. N. Misra  S. N. Rajaguru  P. K. Thomas 《Journal of Earth System Science》1980,89(1):51-66

Quaternary formations in western Rajasthan are of fluvial, locustrinal and aeolian origins. Fossil gravel ridge of Jayal in Nagaur District is one of the earliest quaternary formations so far recorded in the region. A rich lower palaeolithic habitation-cum-workshop site has been discovered in association with the gravel ridge. Multiple evidence for climatic change in the area during the quaternary period is discussed.  相似文献   

Study of Sand Dunes and Their Effect on Desertification of Cultivated Lands in Shaanxi Province, China Using Remote Sensing Techniques     

Mushtak Talib Jabbar  HU Guangdao  Chen Jianguo 《中国地质大学学报(英文版)》2002,13(3)

About half of the arid and semi-arid lands in the world are deserts that comprise various types of aeolian sand dunes deposits. In Shaanxi Province, aeolian sand dunes cover considerable areas of the Yulin desert and northern Jinbian. Sand dunes are moving in the main wind direction and converting some agricultural area to wasteland. Remote sensing of sand dunes helps in the understanding of aeolian process and desertification. Remote sensing data combined with field studies are valuable in studying sand dunes, regional aeolian depositional history. In particular, active and inactive sand dunes of the north Shaanxi Province were studied using remote sensing and geographic information system. In this study, we describe the Landsat thematic mapper (TM) images, covering north Shaanxi Province, which were used to study the distribution, shape, size, trends, density and movement of sand dunes and their effect on desertification of cultivated lands. Estimation was made depending on soil erodibility factor (Ⅰ) and local climatic factor (C) during the period (June to September). The result indicates that soil erosion caused sand drift of 8.957 5, 7.03 ton for Yulin and Jinbian, respectively. The mean sand dunes movement rate were 4.37, 3.11 m, whereas, monthly sand dune advance rate were 1.092 5, 0.777 5 m, for the two locations, respectively. The study reveals that cultivated lands extended obliquely to the direction of sand dune movement are extremely affected, while other segments that extend parallel to the direction of the movement are not affected. Accordingly the north Shaanxi Province was divided into areas of different classes of potential risk. Moreover, blown sands and sand movement from neighboring highlands also affect the area of western desert.  相似文献   

Preservation of cross-strata due to the migration of subaqueous dunes: an experimental investigation   总被引：1，自引：0，他引：1  

Suzanne F. Leclair 《Sedimentology》2002,49(6):1157-1180

This experimental investigation examined the controls on the geometry of cross‐sets formed by subaqueous dunes. A range of steady, unidirectional flow conditions spanning the field of dune existence was investigated, and aggradation rate ranged from 0 mm s^?1 to 0·014 mm s^?1. Data from an ultrasonic depth profiler consist of high‐resolution temporal and spatial series of bed profiles from which dune height and length, migration rate and the depth of trough scour were measured. Cross‐set thickness and length were measured from sediment peels. The size and shape of dunes from an equilibrium assemblage change continuously. Individual dunes commonly increase in height by trough scouring and, occasionally, by being caught‐up by the upstream dune. Both types of behaviour occur suddenly and irregularly in time and, hence, do not appear to depend on dunes further upstream. However, dune climbing or flattening is a typical response of dunes that disappear under the influence of the upstream dune. All types of behaviour occur at any flow velocity or aggradation rate. Successive dune‐trough trajectories, defined by dunes showing various behaviours, affect the geometry of the preserved cross‐sets. Mean cross‐set thickness/mean dune height averages 0·33 (±0·7), and mean cross‐set length/mean dune length averages 0·49 (±0·08), and both show no systematic variation with aggradation rate or flow velocity. Mean cross‐set thickness/mean cross‐set length tends to decrease with increasing flow velocity and Froude number, therefore allowing a qualitative estimation of flow conditions. Quantitative analysis of the temporal changes in the geometry and migration rate of individual dunes allows the development of a two‐dimensional stochastic model of dune migration and formation of cross‐sets. Computer realizations produced stacks of cross‐sets of comparable shape and thickness to laboratory flume observations, indicating a good empirical understanding of the variability of dune‐trough trajectories. However, interactions among dunes and aggradation rates of the order of 10^?2 mm s^?1 should be considered in future improved models.  相似文献   

Formation of the complex linear dunes in the central Taklimakan Sand Sea,China     

Xunming Wang  Zhibao Dong  Jiawu Zhang  Jianjun Qu 《地球表面变化过程与地形》2004,29(6):677-686

The formation of the complex linear dunes in the central Taklimakan Sand Sea is discussed based on analyses of wind regimes, sand grain size distributions on the topography of the dunes, and a combination of geomorphic and geophysical investigations into the morphology of the dunes. Complex linear dune formation is shown to have ?ve stages. Analysis clearly shows that under the control of wind regime, sand supply and other factors, the simple linear dunes move sideways while they evolve. This is the main cause for the formation of complex linear dunes in the central Taklimakan Sand Sea. We have not collected enough evidence to show whether the complexity of the complex linear dunes is left over from previous wind regimes or whether the previous wind regimes had different dominant wind directions compared to those of modern winds. The evolutionary processes of complex linear dunes in the region partly support the theory of ‘barchan evolution’ but do not support the ‘roll‐vortex’ and ‘bimodal wind regime’ hypotheses. After the complex linear dunes were developed, the local wind regime and the other controls such as sand supply suggest it is possible for them to maintain their linear shape. The evolutionary process discussed is limited to the region indicated in this paper, and may not be applicable to the whole Taklimakan Sand Sea. There are different evolutionary processes in different dune?elds because of variations in the factors that control complex linear dune formation. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献