A stratigraphic model to account for complexity in aeolian dune and interdune successions

Wet aeolian systems, in which the water table or its capillary fringe are in contact with the accumulation surface, such that moisture influences sedimentation, are well‐known from modern aeolian systems and several ancient preserved successions are recognized from outcrop. One common mechanism by which accumulation of wet aeolian system deposits occurs is via a progressive rise in the relative water‐table level that is coincident with ongoing dune and interdune migration, the angle of dune climb being determined by the ratio between the rate of relative water‐table rise and the rate of downwind migration of the bedforms. Accumulations of wet aeolian system deposits tend to be characterized by units of climbing dune strata separated by units of damp or wet interdune strata. For simple geometric configurations, where the size of the dune and interdune units, the rate of bedform migration and the rate of aggradation all remain constant over space and time, the resulting accumulation has a simple architecture characterized by sets of uniform thickness inclined at a constant angle. However, the dynamic nature of most aeolian dune systems means that such simple configurations are unlikely in nature. The complexity inherent in these systems is accounted for here by a numerical model in which key controlling parameters, including dune and interdune wavelength and spacing, migration rate and aggradation rate, are allowed to vary systematically both spatially (from a dune‐field centre to its margin) and temporally (in response to changes in sediment availability or water‐table level). The range of synthetic stratigraphic architectures generated by the model accounts for all the best‐known examples of aeolian dune and interdune stratigraphic configurations documented from the stratigraphic record. Modelling results have enabled the erection of a scheme for the classification of dune system type whereby the many elaborate stratal architectures known to exist in nature can effectively be accounted for by only four parameters that are allowed to vary over space and time: dune and interdune wavelength and spacing, rate of bedform migration and rate of accumulation. Results have applied implications, including the modelling of reservoir heterogeneity and the prediction of fluid flow pathways of hydrocarbons, water, CO₂ and contaminants in subsurface reservoirs and aquifers, in which low permeability interdune units might act as baffles or barriers.     

The origin of bounding surfaces in ancient aeolian sandstones

Three orders of aeolian bounding surface are arranged in a hierarchy based on their extent and regularity. First order surfaces are the most extensive. They are flat-lying bedding planes cutting across all other aeolian structures and are attributed to the passage of the largest aeolian bedforms—draas—across an area. First order surfaces cut across second order surfaces, which are gentle to moderately dipping surfaces bounding sets of cross-strata. Second order surfaces are attributed to the passage of dunes across draas, or to longitudinal dunes migrating across the lower ice slopes of draas. Third order surfaces bound bundles of laminae within coscts of cross laminae and are due either to local fluctuations in wind direction and velocity or to changes in airflow patterns caused by configurational changes in dune patterns. All these bounding surfaces could be explained by wind variations and dune migration, but the rates of dune migration relative to probable sediment deposition rates are incompatible with this general explanation of the form and spacing of the bounding surfaces. The concept of climbing bedforms of different hierarchical order together with subsidence provides a better explanation. Analogous bounding surfaces in aqueous bedforms have already been attributed to climbing bedforms of differing hierarchical order.     

Major bounding surfaces in aeolian sandstones—a climatic model

Large areas of fixed, vegetated dunes occur in the Sahel and southern margin of the Sahara. These dunes were active during Late Pleistocene times and have been stabilized as a result of a change to more humid, less windy conditions in the Holocene. The stabilized surfaces, which are in part of erosional origin, are of regional extent. Within aeolian sand sequences, such surfaces form the highest order bounding surfaces. Since regional bounding surfaces of this type occur in all the world's major deserts, it is suggested that they should also be present in some ancient aeolian sandstone sequences.     

Evidence for Pleistocene wet aeolian dune and interdune accumulation,S. Pedro da Maceda,north‐west Portugal

Outcrops and cored/counter‐flushed boreholes in the coastal area between Espinho and Aveiro (north‐west Portugal) were investigated to reconstruct the changing patterns of sedimentation during the Late Pleistocene–Holocene. To obtain a common comparison basis, the grain‐size data from outcrop and borehole samples were analysed. The outcrops and the cored parts of the boreholes were dated by radiocarbon and optically stimulated luminescence. The results show that, on top of pebble‐rich beds of fluvial origin, a wet aeolian dune and interdune environment was active during the later part of the Pleistocene, turning to dry aeolian at the transition to the Holocene. The data indicate also that aeolian accumulation was controlled by vegetation changes (climate) and groundwater table fluctuations. During the Holocene, a podzol formed on the Pleistocene dunes and extensive vegetation precluded major aeolian accumulations. Remobilization of sand started again because of human deforestation and – last but not least – the Little Ice Age.     

Collision behaviors of barchans in aeolian dune fields

In this paper, the probability of the collision behaviors of barchans in aeolian dune fields and the processes of three collision types, i.e., coalescence, breeding and solitary-wave like behavior, are analyzed by the scale-coupled model of dune fields. Moreover, the influence of wind speed, sand diameter, sand supply, the height ratio between two barchans, and the angle between the dune trend and wind direction, as well as the distance between the centerlines of two barchans on the collision results of barchans is demonstrated.     

The Permian Weissliegend of NW Europe: The partial deformation of aeolian dune sands caused by the Zechstein transgression

The Weissliegend is a European sandstone unit of largely late Early Permian age. It is underlain by the Early Permian Rotliegend red desert sandstones and is overlain by the conventionally accepted basal bed of the Zechstein-the bituminous marine shales of the Kupferschiefer. The Weissliegend sandstones are characteristically white or grey in colour and have been recognised beneath the North Sea, in Germany and in Poland. Equivalents, which are red or yellow in colour, occur in NE England and at the southern edge of the Moray Firth Basin in Scotland.From an examination of cliff and quarry exposures in Britain, and of drill cores from southern North Sea gas wells, it is now believed that the bulk of the Weissliegend sandstones (and their equivalents) were originally deposited as aeolian dunes. These dune sands, however, were later modified by a widespread event, the Zechstein transgression, which caused their partial homogenisation, the creation of large-scale soft-sediment deformation structures, and the local and minor reworking of some of the dune flanks.The preferred mechanism of deformation is interpreted as: (1) entrapment of large pockets of air within the bodies of the dunes by flanking and overlying wetted dune sands; (2) venting of the air pockets when the rising internal air pressures overcame the weight of the hydrostatic head of water and the capillary (cohesive) strength of the overlying wetted sands; (3) the rapid replacement of air by water, which caused liquidisation of the original dune laminae; and (4) the associated collapse and final consolidation of the sands into a tigher packing configuration.Deformations seem to be more developed in former transverse dunes than in seif dunes. The reason may be that the relatively tightly packed low-angle accretion bedding common on the flanks of seif dunes is more resistant to deformation than the looser avalanche sands that form a major part of transverse dunes. Limited reworking of former dune sands was probably best developed on the steep lee slopes of transverse dunes and the steeper upper slopes of seif dunes.The lack of reddening of the Weissliegend sandstones-proper is attributed to a combination of their accumulation above the Rotliegend water table, to the rapidity of the Zechstein transgression, and to the anoxic state of the early Zechstein sea floor. The Weissliegend sands, unlike the underlying Rotliegend into which they grade, were thus never in a diagenetic environment that was conducive to reddening.Finally, it is recommended that the term Weissliegend be dropped in any formational sense. It should only be retained for the Weissliegend proper, and their equivalents, to denote a complex facies association dominated by (1) the uppermost Early Permian Rotliegend dune sands (now partly deformed) that lay above the water table just prior to the Zechstein transgression, together with (2) the minor erosional marine products caused by that transgression. The latter, sensu stricto, are Zechstein sandstones of earliest Late Permian age.     

Experimental investigation of cephapirin adsorption to quartz filter sands and dune sands

Batch experiments were performed to investigate cephapirin (a widely used veterinary antibiotic) adsorption on various size sands of low total organic carbon content (0.08–0.36 wt%). In the aqueous concentration range investigated (11–112 μmol/L cephapirin), adsorption to nearly pure quartz filter sands (0.50–3.35 mm diameter) is low. Isotherms are S-shaped and most display a region of minimum adsorption, where decreased adsorption occurs with increasing solution concentration, followed by increased adsorption at higher concentrations. Cephapirin adsorption to quartz-rich, feldspar-bearing dune sands (0.06–0.35 mm diameter), and the smallest quartz filter sand investigated (0.43–0.50 mm), can be described by linear sorption isotherms over the range of concentrations investigated. Distribution coefficients (K _d) range from 0.94 to 3.45 L/kg. No systematic relationship exists between grain size and amount of adsorption for any of the sands investigated. Cephapirin adsorption is positively correlated to the feldspar ratio (K-feldspar/(albite + Ca-plagioclase). Feldspar-ratio normalization of distribution coefficients was more effective than organic carbon normalization at reducing variability of K _d values in the dune sands investigated.     

Stoss-side mud-drapes: deposits of interdune pond margins

Arcuate mud-drapes have been observed in an interdune area in southern Tunisia. They are believed to have been formed by deposition from ponded water around aeolian dunes during flood events. They are preferentially preserved on the stoss-side of the dunes as a result of subsequent dune migration, and exhibit extensive evidence of animal tracks. They are useful in illustrating dune-interdune dynamics and recording the type of animals present.     

An empirical model of aeolian dune lee-face airflow

Airflow data, gathered over dunes ranging from 60-m tall complex-crescentic dunes to 2-m tall simplecrescentic dunes, were used to develop an empirical model of dune lee-face airflow for straight-crested dunes. The nature of lee-face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee-face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low-speed (0–3O% of crestal speed) back-eddy flow, typically occur leeward of steep-sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment. As each of the three types of lee-face flow is defined by a range of wind speeds and directions, the nature of lee-face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross-strata.     

浑善达克沙地的光释光测年研究

光释光测年技术以光可以使在半导体结晶固体中被俘获到光敏陷阱中的电子释放出来这一原理为依据,以样品在被覆盖前,它们的光释光信号都已经晒退回零为假设前提进行测年的。而风成沉积物是最符合这一假设的,所以光释光测年对于风成沉积物非常适合。本文对采自浑善达克沙地样品中的石英粗颗粒进行光释光测年,试图重建沙地的古环境演化过程。本文研究的样品分别采自浑善达克沙地的中部SG点(42°40'N,115°57'E)和东部YS点(43°14'N,116°49'E)。采样时,先挖大约30cm深,然后用锤子将金属小筒打入,拔出后,小筒用黑色塑料和胶带封死。到实验室后,小筒顶端大约1cm厚的样品用来测量     

Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK

Abstract New and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics, rate of water table rise and/or periodicity of interdune flooding. This is illustrated with an example from the Mid‐Triassic (Anisian) Helsby Sandstone Formation, Cheshire, UK. Lenses of damp and wet interdune strata exhibit an intertonguing, transitional relationship with the toe‐sets of overlying aeolian dune units. This signifies dune migration that was contemporaneous with water table‐controlled accumulation in adjacent interdunes. Downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding, during which accumulation of dune strata continued relatively uninterrupted. This contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface. Architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction, enabling a quantitative three‐dimensional reconstruction of genetically related aeolian dune and interdune elements. Sets of aeolian dune strata are composed of grainflow and translatent wind‐ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless, sinuous‐crested parent bedforms, together with lee‐slope reactivation under non‐equilibrium flow conditions. Silty‐mudstone and sandstone interdune units are characterized by wind ripple‐, wavy‐ and subaqueous wave ripple‐laminae, desiccation cracks, mud flakes, raindrop imprints, load casts, flutes, intraformational rip‐up clasts and vertebrate and invertebrate footprint impressions and trackways. These units result from accumulation on a substrate that varied from dry‐ through damp‐ to wet‐surface conditions. Interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding. Red silty‐mudstone beds of subaqueous origin pass laterally into horizontally laminated wind‐ripple beds indicating a progressive transition from wet‐ through damp‐ to dry‐surface conditions within a single interdune.     

Size—frequency distribution of longitudinal dune rippled flank sands compared to that of slipface sands of various dune types

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.     

The study of relative density in some dune and beach sands

A new method was developed for the measurement of relative density in natural sand deposits. The method is based on hardening sand in the field, so that undisturbed samples can be obtained, and the later removal of the organic hardener in the laboratory. In the study of two coastal dunes use was made of “peels” which reveal in detail the internal structure of the sand. A definite relationship was established between relative density of the dune sand, its internal structure and mode of deposition. Very high relative densities, approaching and even exceeding 100%, were found in sand deposited by accretion on the top of the dunes. Low to very low relative densities were measured in slip deposits formed by avalanching of sand on the leeward slope of the dunes. High densities in accretion deposits are attributed to sand movement by saltation taking place during the accumulation of the sand and the very high kinetic energies expended during this transportation process. Low densities in the slip deposits are attributed to the process of avalanching which involves minimum energy expenditure. A more or less constant relative density of around 80% was found in the stretch of beach sand studied.     

循环荷载下砂土的剪切硬化边界面本构模型

基于临界状态土力学框架,建立了一个适用于往返循环荷载作用的砂土边界面本构模型。采用无纯弹性域假设,认为受到反向荷载的瞬时土体就产生塑性变形,砂土的弹性区域退化为一个点。屈服面为倒子弹头型,由于砂土孔隙比与压力之间不存在惟一对应的关系,使得屈服面大小无法与体积应变直接耦合,故采用塑性偏应变而不是剑桥模型那种塑性体应变作为硬化参数。流动法则采用加入状态参数的修正的Rowe应力剪胀关系,体现了依赖状态的剪胀思想。屈服面大小的比值 反映了塑性模量的演化,并推导了 的表达式。只用1套参数,该模型就能合理地模拟砂土在不同密度和固结压力下循环荷载的应力-应变关系曲线。     

Physical properties of aeolian sediments within major dune corridor in Kuwait

A total of 212 sand samples were collected from aeolian surface sediments in a major dune corridor, northwestern Kuwait. Five main physical properties were taken into consideration in analyzing aeolian samples, namely color, shape, roundness, particle morphometry, and surface area. The analysis of quartz particles by the scanning electron microscope shows the influence of transportation on the microfeatures of individual particles. The surface deposits are dominantly unimodal and occasionally bimodal. It is obvious that coarse and medium sand are the dominant size fractions within aeolian deposits. The results according to the Munsell color system between downwind and upwind values show no variations. Also, the average percentages of roundness subclasses for upwind and downwind samples are similar (6.7%). The average values (area, equivalent diameter, perimeter, and elongation) for aeolian samples in downwind are slightly higher than in upwind and Al-Dibdibba Formation samples, but the values overlap at 1 standard deviation. The interrelationship diagrams show that the shapes of the particles within samples in the three groups are running in the same trend. In general, the northwestern (upwind) particles show more mechanical and chemical features compared with southeastern particles (downwind). The slight variation between aeolian (upwind and downwind) samples is attributed to the transportation effect of these particles. This led to a conclusion, based on the overall results of physical properties, that the coarse and medium particles (about 80% of the whole aeolian sample) are dominantly derived from local sources.     

Geochemical evidence of aeolian deposits in European soils

Hafnium (Hf) and zirconium (Zr) concentrations measured in over 4100 agricultural soil samples from Europe were assessed with the focus on their relationship to the distribution of aeolian deposits, such as loess and coversands. Comparison of extractable (aqua regia; ICP‐MS) and total (XRFS) concentrations shows that only 1.0 to 1.7% of the total Hf and Zr is chemically extractable because of the resistant nature of their host minerals. Resistate minerals, such as zircon, are commonly found in the predominantly silty fraction of loess deposits. In this study a statistical analysis of total Hf and Zr soil data from areas with and without loess was carried out to derive threshold values of 10 mg kg^?1 (Hf) and 318 mg kg^?1 (Zr). These values were subsequently applied across the project area in an attempt to indicate the presence of aeolian deposits. The spatial distribution of above‐threshold concentrations suggests a more extensive and coherent loess belt across central and eastern Europe, providing additional evidence of loess across Brittany, Aquitaine and near the Vosges mountains in France as well as in the basins of central and northern Spain. Above‐threshold concentrations were also detected in loess regardless of its thickness, emphasising the importance of the abundance of zircon in the upper part of the soil profile rather than the actual thickness of the deposit. Soil data however, failed to indicate various loess facies within most of the Pannonian Basin, suggesting that this approach only works where deposits contain a sufficient amount of zircon. This may also explain why the extensive coversands across northern Germany and Poland were largely undetected. This study demonstrates that continental‐scale soil geochemical data can help identify and map the distribution of zircon‐rich loess and coversand, and subsequently enhance and improve current knowledge of the extent of these deposits.     

辽西地区风积砂土振动液化特性的试验研究

本文对辽西地区风积砂土地震液化特性进行了实验研究,对围压、固结比、密度和细粒含量进行了分析,得出了它们对抗液化强度的影响规律,同时分析了不同细粒含量和干密度对砂土振动孔压发展模式的影响规律.在偏压固结情况下,振动孔压的发展趋势可以用双曲线对其进行模拟;在均压固结条件下,振动孔压的发展趋势和H.B.Seed、张建民等人研究的振动孔压发展趋势的曲线形态相一致.通过实验分析表明细粒含量、密度等都会对孔压的增长规律有一定影响.