风成砂沉积和古气候研究

作为一类常见的陆相沉积岩类型,风成砂岩产出于太古代到新生代的岩石记录中。风成砂沉积的形成与古气候、古环境密切相关,因而地质历史中的风成砂沉积是研究古气候和古地理环境的重要窗口。本文回顾了国内外风成砂沉积的研究进展,着重讨论了气候因素控制下风成砂沉积的成因及其形成过程。当前,国际研究注重风成沙丘形成过程的推理和模拟以及风成砂的沉积成岩过程,在风成沙丘形成过程、计算模拟、沉积保存的四维时空模型、风成相等方面取得了许多新认识。鉴于国际研究动态,国内需要在地层记录中鉴别和剖析风成砂的宏观和微观形态特征基础上,加强风成砂沉积动力学过程研究,增强对风成砂沉积(微)环境的理解和认识。     

Aeolian dune‐field development in a water table‐controlled system: Skeiđarársandur,Southern Iceland

Aeolian dune fields characterized by partly vegetated bedforms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on Skei?arársandur, Southern Iceland. The largest aeolian dune complex on the sandur covers an area of 80 km² and is characterized by four distinct landform types: (i) spatially isolated aeolian dunes; (ii) extensive areas of damp and wet (flooded) interdune flat with small fluvial channels; (iii) small aeolian dune fields composed of assemblages of bedforms with simple morphologies and small, predominantly damp, interdune corridors; and (iv) larger aeolian dune fields composed of assemblages of complex bedforms floored by older aeolian dune deposits that are themselves raised above the level of the surrounding wet sandur plain. The morphology of each of these landform areas reflects a range of styles of interaction between aeolian dune, interdune and fluvial processes that operate coevally on the sandur surface. The geometry, scale, orientation and facies composition of sets of strata in the cores of the aeolian dunes, and their relationship to adjoining interdune strata, have been analysed to explain the temporal behaviour of the dunes in terms of their mode of initiation, construction, pattern of migration, style of accumulation and nature of preservation. Seasonal and longer‐term flooding‐induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds. Most of the dunes are currently undergoing active construction and migration and, although sediment availability is limited because of the high water table, substantial aeolian transport must occur, especially during winter months when the surface of the wet interdune ponds is frozen and sand can be blown across the sandur without being trapped by surface moisture. Bedforms within the larger dune fields have grown to a size whereby formerly damp interdune flats have been reduced to dry enclosed depressions and dry aeolian system accumulation via bedform climb is ongoing. Despite regional uplift of the proximal sandur surface in response to glacial retreat and unloading over the past century, sediment compaction‐induced subsidence of the distal sandur is progressively placing aeolian deposits below the water table and is enabling the accumulation of wet aeolian systems and increasing the likelihood of their long‐term preservation. Wet, dry and stabilizing aeolian system types all co‐exist on Skei?arársandur and the dunes are variously undergoing coeval construction, accumulation, bypass, stabilization and destruction as a result of interactions between localized factors.     

内蒙巴丹吉林沙漠成因的粒度分析和热发光测年新证据

巴丹吉林沙漠位于中国内蒙古自治区的西部,是中国第三大沙漠。过去通常认为该沙漠中沙丘的可能沙源来自附近的各种沉积物和岩石,包括弱水河扇的沉积物,弱固结的二叠纪页岩、白垩纪砂岩和砾岩以及戈壁阿尔泰的古老岩石。根据对风成砂的粒度观测,有证据表明在沙漠内部的风成砂沉积朝东南方向具有较弱的但又确实的变细趋势。这与沙丘横脊线的SW-NE向排列以及坡面的南东指向是一致的。因此,可以认为西北部的弱水河冲积扇是巴丹吉林沙漠风成砂的最可能的沙源。在西北部风成床沙覆盖了较老弱水河扇起源的冲积物。这种沉积物的热发光测年范围在190～100 ka BP之间。由于测量中对沉积物漂白可能不完全以及对其中平均古水含量估算均存在不确定性,同时由于对沙漠该地区风成作用开始之前堆积的沉积物进行了测年,这一数据范围代表了最大的年龄。另外三个风成砂测年结果分别接近133.66和22 ka,代表有关巴丹吉林沙漠西北部风成作用开始的最小年龄。沙漠的沙丘高度平均为200～300m,但在东南部偶尔可达450m。一些学者曾提出风成床沙覆盖了一个陡峭岛山突起的假说,来解释这些异常的空间。本次研究发现,在研究区之下是一个产状水平的白垩纪扇砾岩和砂岩的台地,可以断定在沙漠东南部呈现台地地貌,但进一步向北该台地明显延伸到这些沙丘之下。因此现在可以认为巴丹吉林沙漠中高的沙丘是不同区域气候和地貌因素相互作用的结果,而不是覆盖一个陡峭的岛山突起。对弱水河冲积扇作为巴丹吉林沙漠的主要源区的证实强调区域环境的重要性。在全新世,沿河流的绿洲植被在某种程度上阻碍了冲积扇提供沙源。现在,河西走廊的农业用水量极大地危及沿河森林,因此沿着作为天然拦沙阱的弱水河,维持足够的河水流量来保护区域性密集的沙丘植被,一定会避免具有重大威胁的沙的活动性增加。     

Aeolian architecture, bedform climbing and preservation space in the Cretaceous Etjo Formation, NW Namibia

Sets of aeolian cross‐strata within the Cretaceous Etjo Formation of NW Namibia are bounded by a hierarchy of surfaces, the origin of which are ascribed to one of four processes related to aeolian bedform and erg behaviour. The base of the main aeolian succession is characterized by a basin‐wide erosional supersurface that formed in response to a period of aeolian deflation before the onset of the main phase of erg building. Interdune migration surfaces formed by draa migration are planar in sections parallel to the palaeowind and are inclined at up to 5° in an upwind direction (SW). Perpendicular to the palaeowind, interdune surfaces form 500‐m‐wide troughs, signifying crestline sinuosity within the original bedforms. Superimposition surfaces are inclined at 5–10° in a downwind direction and indicate the migration of crescentic oblique dunes over larger, slipfaceless transverse draa. Reactivation surfaces associated with minor changes in dune slipface orientation are distinct from other bounding surface types because overlying cross‐strata lie parallel to them, rather than downlap onto them. Analysis of the geometry of these bounding surfaces, together with the orientation of the cross‐strata within the sets that they bound, has enabled the detailed morphology of the original bedforms to be reconstructed. The maximum preserved thickness of individual aeolian sets varies systematically across the basin, from 52 m in the basin depocentre to only 8 m at the basin margin. The set architecture indicates that this spatial variation is primarily the result of decreased angles of bedform climb at the basin margin, rather than the presence of smaller bedforms. Similarly, a temporal reduction in the angle‐of‐climb, rather than a reduction in bedform size, is considered to be responsible for an upward decrease in preserved set thickness. Reductions in bedform climb angle reflect progressive loss of accommodation space as the accumulating erg filled the basin.     

Aeolian Geomorphology in the Era of Deep Space Exploration

The exploration of "Deep Sea, Deep Earth and Deep Space" provides opportunities and challenges for the development of geoscience, and geographical science begins to meet the climax of deep space exploration represented by Mars. In China, Martian exploration will be launched in 2020, which will carry out global and comprehensive surrounding exploration of Mars, patrol detection in some local areas, researchers need to be well prepared for the study of planetary geosciences including aeolian geomorphology based on these coming data. Aeolian geomorphology is divided into three stages based on the development history and trend: classical research focusing on single dune observation, modern research with earth system ideology and future research mainly on extra-terrestrial planets. The characteristics of each developing stage were summarized, and we believe that the planetary aeolian research will come naturally. Then, the development and achievement of planetary aeolian research are summarized, the existing problems and future developing trend were also discussed here. Study shows that there are many types of aeolian landforms on Mars, Venus and Titan, and the aeolian process is the most active modern surface process. Aeolian geomorphology in different planets has good similarity, but the difference is also obvious, which means that they have similar formation mechanism, but different formation and evolution conditions, therefore, the theory of aeolian geomorphology will be improved and enriched gradually through the comparative study on different planets. There are obvious advantages in revealing the formation laws and mechanism of aeolian geomorphology in extraterrestrial planets because of the simple formation conditions, and the research of aeolian geomorphology in the era of deep space exploration is in the ascendant.     

A Surface Model for Aeolian Dune Topography

A surface model for aeolian bedform topography is adapted from a surface model of subaqueous bedform topography. The aeolian bedform surface model is developed using a uniform grid with a cell-centered finite volume approximation of the sediment continuity equation. The resulting modeling framework approximates the dynamic motions of aeolian bedform topography driven by bedform field boundary conditions. The numerical model is applied to simulate bedforms growing from unimodal and bimodal transport regimes from both a fixed elevation (sediment source area) and within a domain with fully periodic boundary conditions. The rates at which modeled aeolian bedforms grow and morphologically mature are sensitive to the chosen boundary conditions. Video files of model simulations and source code for the presented aeolian bedform surface modeling framework are available in supplemental materials. The aeolian bedform surface model code is malleable and readily modified for exploratory study of dynamic bedform topography that inherits morphological traits from aeolian bedform field boundary conditions.     

海岸沙丘表面现代风成沙地球化学元素分异的典型研究--以河北昌黎黄金海岸横向沙脊为例

在我国海岸沙丘主要分布地区河北昌黎黄金海岸选取代表性的海岸横向沙脊为研究对象,对其典型断面上沙丘表面不同部位的21种地球化学元素的含量进行了比较分析。结果表明,Co、Zr和Mg三种元素在海岸沙脊表面不同部位的含量差异明显且与海滩沙相比含量差异较大,Cr、Pb、Ti、Nb、As、Mn、Zn和Ba等8种元素的含量差异程度中等,但Y、Ca、Fe、Na、Al、Sr、P、K、Rb和Si等10种元素的差异并不明显,而11种差异程度中等以上元素在海岸横向沙脊迎风坡脚、迎风坡、脊顶、背风坡、背风坡脚等部位的含量变化特征并不一致,呈现出不同的分异模式,其主要原因是受沉积环境、化学元素性质、粒度特征及营力的影响等。     

黄河源风沙沉积及意义

考察发现黄河源发育有广泛的现代、古代风沙沉积和沙漠化草地。地貌、沉积和年代学的分析表明,这些沙丘都是就地形成的半成熟沙丘,粒度粗,分选中等,形成于四个主要时期,即倒数第二次冰期末、末次冰期最盛期,全新世寒冷时期和现代,为青藏高原及邻区黄土提供了丰富的粉尘。     

Shaler: in situ analysis of a fluvial sedimentary deposit on Mars

This paper characterizes the detailed sedimentology of a fluvial sandbody on Mars for the first time and interprets its depositional processes and palaeoenvironmental setting. Despite numerous orbital observations of fluvial landforms on the surface of Mars, ground‐based characterization of the sedimentology of such fluvial deposits has not previously been possible. Results from the NASA Mars Science Laboratory Curiosity rover provide an opportunity to reconstruct at fine scale the sedimentary architecture and palaeomorphology of a fluvial environment on Mars. This work describes the grain size, texture and sedimentary facies of the Shaler outcrop, reconstructs the bedding architecture, and analyses cross‐stratification to determine palaeocurrents. On the basis of bedset geometry and inclination, grain‐size distribution and bedform migration direction, this study concludes that the Shaler outcrop probably records the accretion of a fluvial barform. The majority of the outcrop consists of large‐scale trough cross‐bedding of coarse sand and granules. Palaeocurrent analyses and bedform reconstruction indicate that the beds were deposited by bedforms that migrated towards the north‐east, across the surface of a bar that migrated south‐east. Stacked cosets of dune cross‐bedding suggest aggradation of multiple bedforms, which provides evidence for short periods of sustained flow during Shaler deposition. However, local evidence for aeolian reworking and the presence of potential desiccation cracks within the outcrop suggest that fluvial deposition may have been intermittent. The uppermost strata at Shaler are distinct in terms of texture and chemistry and are inferred to record deposition from a different sediment dispersal system with a contrasting provenance. The outcrop as a whole is a testament to the availability of liquid water on the surface of Mars in its early history.     

Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes

The Hornby Bay Group is a Middle Proterozoic 2.5 km-thick succession of terrestrial siliciclastics overlain by marine siliciclastics and carbonates. A sequence of conglomeratic and arenaceous rocks at the base of the group contains more than 500 m of mature hematitic quartz arenite interpreted to have been deposited by migrating aeolian bedforms. Bedforms and facies patterns of modern aeolian deposits provided a basis for recognizing two sequences of aeolian arenite. Both sequences interfinger with alluvial—wadi fan conglomerates and arenites deposited by braided streams. Depositional processes, facies patterns and paleotopographic position of the arenites are consistent with modern sand sea dynamics.Distal aeolian facies in both sequences are composed of trough crossbed megasets deposited by climbing, sinuous-crested, transverse dunes. Megasets comprise a gradational assemblage of tabular to wedge-planar cosets formed by deflation/reactivation of dune lee slopes and migration of smaller superposed aeolian bedforms (small dunes and wind ripples). Megasets in the proximal facies are thinner, display composite internal stratification and have a tabular-planar geometry which suggests that they were formed by smaller, straight-crested transverse dunes. Most stratification within the crossbeds is inferred to have formed by the downwind climbing of aeolian ripples across the lee slopes of dunes.Remarkably few Precambrian aeolian deposits have been reported previously. This seems anomalous, because most Precambrian fluvial sediments appear to have been deposited by low sinuosity (braided) streams, the emergent parts of which are prime areas for aeolian deflation. Frequent floods and rapid lateral migration of Precambrian humid climate fluvial systems probably restricted aeolianite deposition to arid paleoclimates. Thus the apparent anomaly may reflect non-recognition and/or non-preservation of aeolianites and/or variations in some aspect of sand sea formation and migration unique to the Precambrian. Reconstruction of the Hornby Bay Group aeolianites using recently developed criteria for their recognition suggests that the latter reason did not exert a strong influence.     

Characteristics and Formation Mechanism of Wind Streaks on Mars

Wind streaks with two-dimensional plane shapes are a collective term for a variety of aeolian features that display distinctive albedo surface patterns and they do not have three-dimensional shape. Wind streaks are widely distributed on Mars， and are good proxy indicators of the surface wind regime， and even of global circulation patterns on Mars. However， the study on wind streaks has been largely ignored for a long time. Based on published studies， this paper summarized the types， morphology and formation mechanism of wind streaks. According to the relationship between albedo and obstacles， wind streaks can be divided into six basic types： bright wind streaks， dark wind streaks， mixed-tone wind streaks， splotches and related wind streaks， dune shadow wind streaks and frost wind streaks， of which the bright and dark streaks are the most common and representative， for they are the most abundant types of variable features on Mars. Wind streaks are primarily distributed in the latitudinal zone between 60°S and 60°N with little difference among different types， and they have many shapes such as tapered， fan， oval and parallel shapes due to the diverse obstacles. Considering the relationship between sediment characteristics and aeolian erosion and deposition， bright wind streaks are generally depositional with a consensus and dark streaks are erosional with a controversy. In the absence of Martian meteorological observation data， the retrieval of surface wind regime based on the orientation of wind streaks has good reliability， which helps to understand the modifications of Martian surfaces by wind in the geological context.     

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.     

风沙地貌形态动力学研究进展

风沙地貌是广泛分布于干旱、半干旱,甚至部分湿润地区,由风力作用形成的一种地貌类型。风沙地貌学是研究在风力作用下物质运动形成的地表形态特征、空间组合规律及其形成演变的科学,是地貌学中以风为外营力形成的地貌为对象的分支学科。风是风沙地貌学研究的基础,其贯穿整个风沙地貌学研究。风况决定了风沙地貌的形态特征、空间组合特征和演化过程,同时,沙丘表面气流和风沙流控制沙丘的形态演化过程和移动过程。风沙地貌经过100多年的发展,在沙丘形态特征、动力学过程等方面取得了长足发展。从风沙地貌观测方法、分析方法和形态动力学角度出发,总结了近年来风沙地貌形态、形成风况以及动力学方面的研究进展。随着新技术的发展,全站仪、三维地形扫描仪等新的形态观测设备开始应用于风沙地貌形态测量,使得大范围风沙地貌形态精准测量成为可能,为风沙地貌形态动力学研究提供精确的地形特征资料。同时,三维超声风速仪等高频风速观测仪器也广泛应用于风沙地貌动力学观测,从而探讨风沙地貌形态—近地层气流的互馈机制。但是,针对具体的分析方法,如风况与沙丘形态的对应关系,近地层气流的分析方法以及形态—气流互馈关系等方面,目前还没有好的解决办法。     

Ancient and modern cold-climate aeolian sand deposition: A review

Although partly active aeolian sand sheets and dunes cover large areas in the zones of (dis)continuous permafrost, little precise information is available about the influence of cold-climate conditions on modern aeolian processes. This means that palaeoenvironmental reconstructions in the stabilised, mainly Late Pleistocene dune fields and cover sand regions in the ‘sand belts’ of the European Lowlands and the Northern Great Plains of the USA and Canada, are necessarily still based on ancient evidence. Cold-climate wind deposits are typically derived from areas of abundant sediment supply like unvegetated flood plains, glacial outwash plains, till plains and lake shores. The common parabolic and transverse dune forms resemble those observed in temperate regions. Although a variety of periglacial features has been identified in Late Pleistocene dune and cover sands none of them indicate that permafrost is crucial to aeolian activity. Specific structures in aeolian strata permit tentative interpretation of the moisture content of depositional sand surfaces, the nature of annual sedimentation cycles and the processes by which strata were deposited and/or contorted. But surprisingly little is known about the role of vegetation in the process of sand accumulation. Dunes are most informative with respect to reconstructions of past wind regimes, which offer important data for verification of palaeoclimatic simulations.     

Aeolian Deposits in the Yarlung Zangbo River Basin,Southern Tibetan Plateau: A Brief Review

The Yarlung Zangbo River is located in the southern Tibetan Plateau. Loess since the geological history and modern aeolian sand dunes are widely developed in the basin, bearing rich information on the aeolian processes and environmental evolution. In this work, we reviewed the main research progress of aeolian deposits in the Yarlung Zangbo River basin in recent years, and discussed the distribution characteristics, accumulation age, sediment characteristics and provenance and the possible formation mechanism of aeolian deposits and the history of aeolian activity. The results show that loess is mainly developed on the river’s terraces and some of the hilltops, and mostly formed since the last glacial maximum and mainly developed since Marine Isotope Stage 1 (about 14 ka BP), while aeolian sand deposit is mainly developed in the river’s wide valley, with relatively short accumulation time. The sediment particles mainly underwent mechanical transport and physical weathering, but chemical weathering was weak. Although the physical and chemical properties of sediments show some spatial changes, there still exist obvious local characteristics, which are closely related to the nearby surface sediments. Under the background of aeolian source, the local-source genesis of loess is generally recognized. The surface loose material at the bottom of valley is main material source of aeolian deposits. The evolution of aeolian activity is relatively complex. In addition to the regional climate change, the local environment is also an important factor affecting the aeolian activity and its regional difference. Based on the previous studies, we suggest that the systematic study on the physical and chemical properties of aeolian sediments, climate change since the Holocene and the impact of dust emission on human living environment should be strengthened in the future research.     

Aeolian dune interactions and dune‐field pattern formation: White Sands Dune Field,New Mexico

Pattern formation is a fundamental aspect of self‐organization in fields of bedforms. Time‐series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian dunes at White Sands, New Mexico, interact and the dune pattern organizes in systematically similar ways as wind ripples and subaqueous dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off‐centre collision; (vi) defect creation; and (vii) dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off‐centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of dunes, crest length, defect density, crest spacing and dune height), dune migration rates, and the type and frequency of dune interactions within a 3500 m box transect from the upwind margin to the core of the dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo‐lake topography. Pattern evolution is most strongly coupled to the pattern parameters of dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the dunes become further apart and have fewer defects.     

沙丘背风侧气流及其沉积类型与意义

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.     

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.     

Research Methodology of Martian Aeolian Geomorphology

Martian exploration is the focus and hot topic of deep space exploration, and China implemented the first Martian exploration Program in 2020. Aeolian process is the most extensive and active landform process on the surface of Mars, and has been an important part of Martian research. Sustainable development of Martian aeolian geomorphology research requires the support of theoretical system and research methodology, and research methodology is a key issue when field observations are impossible. We analyzed the research methods of Martian aeolian geomorphology from three aspects: methodology, approach, and application of modern technology. Methodology must focus on the dialectical unity of induction and deduction, reductionism and holism. Research approach includes exploration and numerical simulation, and Mars-like aeolian geomorphology study on Earth is also a common approach. Taking full advantage of remote sensing observations and detection technologies is an important basis for the development of Martian aeolian research. Simulation experiments have been an important part of aeolian geomorphology research. Since the 1980s, the United States, Europe, and Japan have successively built Martian wind tunnels to study various aircrafts in Martian atmosphere. In the absence of field observation, wind tunnel experiment and numerical simulation play an important role in studying the evolution and formation process of aeolian landform and the Martian environment.     

Archean to Recent aeolian sand systems and their sedimentary record: Current understanding and future prospects

The sedimentary record of aeolian sand systems extends from the Archean to the Quaternary, yet current understanding of aeolian sedimentary processes and product remains limited. Most preserved aeolian successions represent inland sand‐sea or dunefield (erg) deposits, whereas coastal systems are primarily known from the Cenozoic. The complexity of aeolian sedimentary processes and facies variability are under‐represented and excessively simplified in current facies models, which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour, and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes. Archean and Neoproterozoic aeolian successions remain poorly constrained. Palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts. During the Triassic, widespread desert conditions prevailed across much of Europe. During the Jurassic, extensive ergs developed in North America and gave rise to anomalously thick aeolian successions. Cretaceous aeolian successions are widespread in South America, Africa, Asia, and locally in Europe (Spain) and the USA. Several Eocene to Pliocene successions represent the direct precursors to the present‐day systems. Quaternary systems include major sand seas (ergs) in low‐lattitude and mid‐latitude arid regions, Pleistocene carbonate and Holocene–Modern siliciclastic coastal systems. The sedimentary record of most modern aeolian systems remains largely unknown. The majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes, whereas successions representing linear and star dunes remain under‐recognized. Research questions that remain to be answered include: (i) what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology; (ii) what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation; (iii) can sophisticated four‐dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation; and (iv) is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of Earth history when large‐scale aeolian systems were active, or has the evidence to support the existence of other major desert basins been lost for many periods throughout Earth history?