The action of wind and water in a mid‐Cretaceous subtropical erg‐margin system close to the Variscan Iberian Massif,Spain

Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid‐Cretaceous outer back‐erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain‐size distributions, mud playa, ephemeral floodplain, pebble‐sand and cobble‐sand bedload stream, pebble–cobble‐sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub‐environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind‐driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back‐erg depositional system comprises an outer back‐erg close to the Variscan highlands, and an inner back‐erg close to the central‐erg area. The inner back‐erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back‐erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud‐cracked surfaces, resulting in largely structureless sandstones with boulder‐size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser‐grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back‐erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back‐erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non‐exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti‐clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well‐preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre‐Cretaceous times.     

Desert sand plain and sabkha deposits from the Bunter Sandstone Formation (L. Triassic) at the northern margin of the German Basin

Facies studies of well cores from the Bunter Sandstone Formation in the Tønder area, Denmark indicate, that the formation is composed of two desert sand plain sequences associated with sabkha and inland basin (lake?) mudstones. The lower desert sand plain sequence consists of subaerial sand flat deposits overlain by aeolian sand sheet and dune facies topped by interbedded aeolian and ephemeral river deposits. The upper desert sand plain sequence consists of ephemeral river deposits partly interbedded with and overlain by sabkha and inland basin mudstones. Two shoreline sandstones occur in the uppermost part. Both sequences are interpreted mainly in terms of tectonic subsidence of the basin and related upheavals of the source regions. The lower sequence represents a rather continuous progradation of the desert sand plain followed by a rapid transgression of the waters from the inland basin. The upper sequence represents brief periods of fluvial progradation followed by a gradual retreat of the river plain. The most distal part of the sand plain was finally reworked by weak wave-action.     

Field experiments of aeolian dust accumulation on rock fragment substrata

The effect of rock fragments and rock fragment cover on the accumulation of airborne dust was examined in a long-term field experiment in the Negev desert of Israel. Four parameters were studied: pebble eccentricity, pebble size, pebble flattening and cover density. The effect of these parameters on the accumulation of dust on the pebbles, on the accumulation of dust between and underneath the pebbles, and on total dust accumulation (pebbles+interpebble space) was measured separately. Accumulation on the pebbles increased as the pebbles became larger, less flattened and more elongated, and as cover density increased. Accumulation between and underneath the pebbles increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased (although the accumulation area available became smaller). Total dust accumulation increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased. Rock fragments act as a dust trap as dust accumulation on pebble-covered surfaces is several tens of times larger than dust accumulation on similar but pebble-free surfaces. However, less than 20% of the initially settled dust is protected from further erosion. In the Negev desert, dust erosion by wind is of the order of 10 times larger than dust erosion by water. The rate of long-term dust accumulation in the Negev is of the order of 15–30 g.m^?2.yr^?1. This is considerably lower than the rates proposed elsewhere in the literature, which are based upon over optimistic dust retainment percentages.     

Desert Pavement—the most ancient surface or a more recent development?

Desert pavement comprises a surficial layer of gravel clasts and cobbles underlain by dust and silt that mantles mature alluvial surfaces in arid regions around the world. Once thought to originate from either deflation or expansion and contraction of wet desert soils, desert pavement is now known to form in place from the aeolian accumulation of dust beneath a surface layer of gravel. Desert pavement affects the type and distribution of desert plants on arid land surfaces, as well as inhibits the activities of small burrowing animals. Considerable debate surrounds the age of pavement surfaces and how well they survive changes in climatic conditions. Some researchers maintain that most desert pavements developed during the Holocene, whereas others indicate that some of the oldest surface soils on the planet are desert pavements in the Middle East.     

Diagenetic features of Jurassic Fort Member sandstone,Jaisalmer formation,western Rajasthan

Jaisalmer Formation consists of 360m thick succession of medium to coarse grained sandstones with interbeds of shale, claystone and occasional lignite that rest over Lathi Formation, is the basal part of the Jaisalmer basin. The rocks are exposed amidst desert, low mounds and shallow stone quarries. Sandstones were deposited in shallow marine to deltaic environments. The studied sandstones consist of abundant quartz followed by feldspar, mica, chert, rock fragments and heavy minerals. The study mainly deals with identification of various diagenetic features such as compaction, cementation and porosity evolution. During mechanical compaction rearrangement of grains took place and point, long and suture contacts were formed. The sandstones are cemented by iron oxide, silica overgrowth, carbonate and clay. Porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution and alteration of feldspar, lithic fragments and pressure solution were the main source of quartz cements. The sandstones show good amount of existing optical porosity with an average of 7.19%. Porosity reduction is mainly due to early stage of mechanical compaction and subsequent pervasive calcite and iron oxide cementation. Further, porosity reduced due to deposition of clay cement.     

济南市历下区龙洞地区新近纪上新世一第四纪更新世白彦组地质特征

残存于济南市历下区龙洞一带下古生界出露区岩溶负地形中的砾岩体,总体呈浅褐灰色,由砾石及胶结物组成。灰岩、白云岩砾石,砾径大小不一,呈次棱角一次圆状,燧石砾、褐铁矿砾成熟度高、磨圆度好,表面光洁,呈蜡状光泽,钙质孔隙式及接触式胶结,已硬结成岩,砾岩体与周围呈角度不整合接触关系。通过对比研究,初步认定其为新近纪上新世一第四纪更新世白彦组沉积,这一新发现填补了该区岩石地层单位的一项空白,为研究地质构造发展史提供了重要的佐证。     

Factors controlling aeolian sequence stratigraphy: clues from super bounding surface features in the Middle Jurassic Page Sandstone

Super bounding surfaces (super surfaces) facilitate delineation of internal genetic architecture, or sequence stratigraphy, of aeolian sandstones and help in distinguishing controls on aeolian accumulation, hiatuses in accumulation and preservation of such sandstones. The Middle Jurassic Page Sandstone is an amalgamation of such sandstone units accumulated in a dry erg system, with water table below the surface, separated by super surfaces with features indicating an arid climate but a near surface water table. The Page Sandstone accumulated episodically with periods of aeolian accumulation followed by deflation to the water table; the water table fluctuated up and down through the sediment package. This rising water table did not directly control accumulation, but placed a limit on extent of deflation by stabilizing the substrate, thereby directly controlling preservation of the aeolian unit. A combination of relative water table behaviour, sediment supply and aerodynamic conditions upwind and within the erg controlled the thickness of genetic packages within the erg. A change in the pattern of deflation associated with super surface formation late in deposition of the Page Sandstone reflects a change in tectonic regime in the basin. The number of super surfaces and features on the super surfaces with the Page Sandstone suggest that the super surfaces represent more time than do the accumulations they enclose.     

Statistical analysis of the temporal stability of soil moisture in three desert regions of northwestern China

Soil moisture and its variations are key factors for understanding hydrological processes, which are characterized by a high temporal variability at different scales. The study was conducted at three field stations in the desert regions of northwestern China, where soil moisture measurements with gravimetric method were used to characterize the temporal stability of soil moisture using various statistical parameters and an index of temporal stability (ITS). The soils are a gray–brown desert soil at the Linze station, an aeolian sandy soil at the Fukang station, and a brown desert soil at the Cele station. Soil textures are accordingly sandy loam at Linze and Cele, and loamy sand at Fukang. The dynamic variation in soil moisture depends strongly on the rainfall pattern (amount and frequency) in these desert ecosystems. Soil moisture content is low and significantly different among the three desert ecosystems, with the maximum at the Linze station (6.61 ± 2.08 %), followed by the Cele (4.83 ± 0.81 %) and Fukang (3.46 ± 0.47 %) stations. The temporal pattern exhibits high variability because soil moisture is characterized by low temporal stability and a high coefficient of variation (CV). The standard deviation, CV, and ITS increase significantly with increasing soil moisture. Soil moisture displays a skewed frequency distribution that follows a logarithmic function at lower soil moisture but a log-normal distribution at higher values.     

Petrology of the Weissliegendes sandstones in the Harz and Werra-Fulda areas,Germany

The Weissliegendes and adjacent related Permian sediments from the Harz and Werra-Fulda areas are petrologically described. The Weissliegendes sandstones are classified as moderately sorted, multicycle subgraywackes and orthoquartzites. Petrographic data indicate that the Weissliegendes and Zechsteinkonglomerate were derived in large part from the underlying Rotliegendes. Textural and stratification data show that the Weissliegendes sandstones were deposited in a shallow marine depositional environment and that the aeolian dune origin for the sandstones should be rejected. Rotliegendes sands and gravels were reworked by the transgressive Zechstein Sea, producing the sands of the Weissliegendes and the lag gravels of the Zechsteinkonglomerate. These two closely related units are time equivalent facies which represent the initial stage of the Zechstein Series.     

光释光年代学对腾格里沙漠化机制及风沙物源的指示

腾格里沙漠地处东亚夏季风现代影响区的西北边缘,长期以来被认为是东亚沙尘暴多发中心之一。近年来的研究较明确地支持腾格里沙漠晚更新世以来的沉积环境发生了巨大变化,但该区沙漠化的机制仍需进一步研究。文章基于石英矿物的光释光测年技术,研究了腾格里沙漠南部腹地系列剖面的沉积年代,结合已有研究结果,重建了该区风沙堆积历史,获得如下认识：1）腾格里沙漠末次冰盛期以来的风沙堆积主要集中在16~10 ka及约1.5 ka以来两个时段;2）腾格里地区16~10 ka的风沙堆积发生在区域由干变湿的时段,而7.3 ka、5.1 ka和3.8~3.3 ka的风沙堆积发生于区域湿度较高背景下的干旱时段。由于这些时段的气候环境有利于风沙的堆积与固定而不利于风沙的产生,因而推断16.0~3.3 ka时段腾格里地区的风沙主要来自上风向地区的输入;3）腾格里大面积地区的沙丘/沙山堆积于最近0.37 ka以来降水较少的时段,略晚于区域高强度的人类工程活动,表明现今可见的沙丘/沙山地貌景观的形成是人类高强度工程活动触发、就地起沙的结果。

     

Calcretes in the Thar desert: Genesis,chronology and palaeoenvironment

The calcretes in the Thar desert occur in a variety of settings, including the piedmonts, sheetwash aggraded plains; and this study adds calcretes in regolith and colluvio-alluvial plains to the group of settings in which calcretes occur in the region. Field logs, morphological details and analytical data such as petrographic, cathodoluminescence and geochemical characteristics are described along with a discussion on their implications. Sand dunes and sandy plains dating to < 20 ka have weakly developed calcretes. The better-developed calcrete horizons occur in piedmonts, interdunes or in areas that have sufficient groundwater. Deep sections in the region show phases of calcrete development in aeolian sand aggradation at ∼ 150, ∼ 100, ∼ 60 and 27–14 ka. The extensive sheetwash plains have mature calcretes and date to mid-Pleistocene. Our studies indicate that these calcretes represent a hybrid process, where carbonate enrichment of the originally calcareous host occurred due to periodically raised groundwaters, and its differentiation into nodules occurred under subaerial environment i.e., after recession of groundwater. Deep sections also show a stack of discrete calcretes that developed in individual aggradation episodes with hiatuses as indicated by ESR dating results. Nodules display a multiplicity of carbonate precipi tation events and internal reorganization of calcitic groundmass. The process is accompanied by degradation and transformation of unstable minerals, particularly clays and with a neosynthesis of palygorskite. The ancient calcretes are dated from the beginning of the Quaternary to ∼ 600 ka and show more evolved morphologies marked by brecciation, dissolution, laminar growth on brecciated surfaces, pisolites and several generations of re-cementation. Mica/chlorite schists and such other rocks are particularly vulnerable to replacement by carbonate. In an extreme case, replacement of quartzose sandstone was observed also. The presence of stretches of alluvio-colluvial plains in an area presently devoid of drainage bespeaks of occasional high-energy fluvial regime, under a semi-arid climate. The mid-Pleistocene period saw a shift towards more arid climate and this facilitated sheetwash aggradation. Finally, during the late Pleistocene, aggradation of aeolian sands indicated a progressively drier climate. However, this does not find its reflection in stable isotope data. The amount of carbonate in the form of calcretes is substantial. The present studies indicate that aeolian dust or rainwater are minor contributors to the carbonate budget. A more important source was provided by the pre-existing calcretes in the sheetwash aggraded plains and detrital carbonate in the aeolian sediments. The original source of carbonate in the region, however, remains unresolved and will need further investigations. Electron spin resonance protocols for the dating of calcretes were developed as a part of this study and the results accorded well with geological reasoning     

Groundwater recharge in natural dune systems and agricultural ecosystems in the Thar Desert region, Rajasthan, India

Water and nutrient availability for crop production are critical issues in (semi)arid regions. Unsaturated-zone Cl tracer data and nutrient (NO₃ and PO₄) concentrations were used to quantify recharge rates using the Cl mass balance approach and nutrient availability in the Thar Desert, Rajasthan, India. Soil cores were collected in dune/interdune settings in the arid Thar Desert (near Jaisalmer) and in rain-fed (nonirrigated) and irrigated cropland in the semiarid desert margin (near Jaipur). Recharge rates were also simulated using unsaturated zone modeling. Recharge rates in sparsely vegetated dune/interdune settings in the Jaisalmer study area are 2.7–5.6 mm/year (2–3% of precipitation, 165 mm/year). In contrast, recharge rates in rain-fed agriculture in the Jaipur study area are 61–94 mm/year (10–16% of precipitation, 600 mm/year). Minimum recharge rates under current freshwater irrigated sites are 50–120 mm/year (8–20% of precipitation). Nitrate concentrations are low at most sites. Similarity in recharge rates based on SO₄ with those based on Cl is attributed to a meteoric origin of SO₄ and generally conservative chemical behavior in these sandy soils. Modeling results increased confidence in tracer-based recharge estimates. Recharge rates under rain-fed agriculture indicate that irrigation of 20–40% of cultivated land with 300 mm/year should be sustainable.     

Interaction between aeolian, fluvial and playa environments in the Permian Upper Rotliegend Group, UK southern North Sea

The Permian Upper Rotliegend Group in offshore UK Quadrants 42, 43, 47 and 48 comprises a sequence of mixed aeolian/fluvial/playa deposits. These deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments. The relative dominance of water vs. wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table/playa level driven by a combination of climatic change and syndepositional tectonics. The Rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water. The origin of these surfaces is mostly climatic, with periods of increased runoff resulting in fluvial incision, especially near active faults. During periods of reduced runoff, wind erosion of fluvial deposits occurred, with fluvially derived sand being reworked into expanding aeolian dune fields. Wind erosion also occurred as a rising water table isolated dunes from their sediment supply, resulting in deflation of dunes down to the water table. These surfaces formed in a basin that was subsiding. Thus, even in a background of overall increasing accommodation space, climatically driven falls in the water table allowed for periods of erosion. The occurrence of significant erosion, especially near syndepositional fault zones, resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations.     

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。     

Mineralogical characteristics and mineral economics of Sanu and Gotan SMS grade limestone deposits of Rajasthan

Two large, chemically similar, SMS grade limestone deposits occur in Sanu area of Jaisalmer and Gotan area of Nagaur, Rajasthan. The Sanu deposit belongs to Khuiala Formation of Tertiary sequence of lower Eocene age, while Gotan deposit belongs to Bilara Group of Marwar Supergroup of late Neoproterozoic to early Cambrian. These deposits are being developed and utilised as flux in the steel plants of India. The Sanu limestone of Jaisalmer is more suitable compared to Gotan and other limestone deposits of the country due to its superior and favourable physical properties. Mineralogically, it consists of fine-grained, sub-rounded to rounded grains of calcite mainly replacing tests of the larger foraminifera. While, the Gotan limestone is composed of fine to coarse-grained, angular to sub-rounded grains of calcite. In Sanu area, about 54% of the limestone produced is being generated as −30 mm grit during the process of sizing for SMS grade. This lower size of limestone is suitable for making cement but due to non-avaibility of rail transport facility from Sanu to Jaisalmer for a distance of about 60 km, it remains unutilised as mineral reject.     

青藏高原东缘盐源盆地古近纪风成沙丘及其古地理意义*

盐源盆地始新世丽江组沉积时期发育了一套具大型高角度交错层理的红色砂岩,其确切成因对于厘定青藏高原东缘古近纪古地理格局、恢复东亚新生代干旱带与古气候演化过程具有重要意义。文中通过对盐源盆地始新统丽江组红色砂岩的沉积特征进行系统分析,证明该套砂岩的岩性、粒度分布、石英颗粒表面特征与沉积构造等明显不同于水成沉积,而与典型风成沙丘相应特征完全一致,表明其为风成沙丘沉积。结合古近纪研究区的古气候条件、大气环流样式与大地构造背景,笔者进一步推断盐源盆地始新世风成沙丘可能沉积于中国古近纪干旱带内的沙漠环境。根据目前相关研究资料,确认青藏高原东缘存在一个厚度稳定的古近纪风成沙丘富集带。     

Braided rivers within an arid alluvial plain (example from the Lower Triassic,western German Basin): recognition criteria and expression of stratigraphic cycles

Based on a detailed sedimentological analysis of Lower Triassic continental deposits in the western Germanic sag Basin (i.e. the eastern part of the present‐day Paris Basin: the ‘Conglomérat basal’, ‘Grès vosgien’ and ‘Conglomérat principal’ Formations), three main depositional environments were identified: (i) braided rivers in an arid alluvial plain with some preserved aeolian dunes and very few floodplain deposits; (ii) marginal erg (i.e. braided rivers, aeolian dunes and aeolian sand‐sheets); and (iii) playa lake (an ephemeral lake environment with fluvial and aeolian sediments). Most of the time, aeolian deposits in arid environments that are dominated by fluvial systems are poorly preserved and particular attention should be paid to any sedimentological marker of aridity, such as wind‐worn pebbles (ventifacts), sand‐drift surfaces and aeolian sand‐sheets. In such arid continental environments, stratigraphic surfaces of allocyclic origin correspond to bounding surfaces of regional extension. Elementary stratigraphic cycles, i.e. the genetic units, have been identified for the three main continental environments: the fluvial type, fluvial–aeolian type and fluvial/playa lake type. At the time scale of tens to hundreds of thousands of years, these high‐frequency cycles of climatic origin are controlled either by the groundwater level in the basin or by the fluvial siliciclastic sediment input supplied from the highland. Lower Triassic deposits from the Germanic Basin are preserved mostly in endoreic basins. The central part of the basin is arid but the rivers are supplied with water by precipitation falling on the remnants of the Hercynian (Variscan)–Appalachian Mountains. Consequently, a detailed study of alluvial plain facies provides indications of local climatic conditions in the place of deposition, whereas fluvial systems only reflect climatic conditions of the upstream erosional catchments.     

Reconstruction of the late Quaternary environment of the lower Luni Plains,Thar Desert,India

Geomorphological processes in the Thar Desert of India are largely climate driven. In the lower reaches of the River Luni (the only major drainage system in the Thar Desert) a fluvio‐aeolian sequence was located at a site called Khudala. Sediments of this sequence represented a variety of depositional environments, namely aeolian, fluvially reworked aeolian, overbank deposits, gravels, and occasional evidence of pedogenesis. This provided a good opportunity to study aeolian–fluvial interaction in the region and for deducing climatic records. From the luminescence dating standpoint these sequences offered a good opportunity for a comparative study of thermoluminescence (TL), blue‐green light stimulated luminescence (BGSL) and infrared light stimulated luminescence (IRSL) on different mineral separates of identical provenance but deposited under different environments. Broadly, within experimental errors, the TL ages agreed with BGSL and IRSL ages on aeolian sands, but differed substantially in the case of fluvially reworked and proximally deposited sands and silts. The sequence provided a record spanning more than 100 ka, with an aeolian phase at > 100 ka, a channel activation phase between 70 and 30 ka and a phase of climate instability between 13 and 8 ka. This appears consistent with the records of monsoon performance during this period, which includes the Younger Dryas. It is also inferred that during the Last Glacial epoch, geomorphological processes in the Thar (both aeolian and fluvial) were dormant largely on account of their relationship with the southwest monsoon. Copyright © 2001 John Wiley & Sons, Ltd.     

科尔沁沙漠区风积沙动力压实特性研究

沙漠区修筑公路,优质路基填料匮乏。依据就近原则,选用沙漠区储量丰富的风积沙作为路基填料,分析其矿物成分、化学成分、酸碱度、含盐量与级配特征。进一步探究不同试验方法（击实试验、振动台法、表面振动压实仪法）与试验控制条件下,风积沙干密度受因素控制的变化规律与压实特征。风积沙击实曲线与振动压实曲线均呈现“振实-振松-再振实”造成的“多峰”特征, 振动法所得干密度一般优于对应含水率条件下击实试验所得干密度。振动台法在风积沙含水率接近饱和时振动的效果最好,风积沙的干密度最大,表面振动压实仪法3层填料的振动压实效果明显好于1层填料方式,进而可用于施工指导。     

Pleistocene gravels in the lower wye valley

The fluviatile Pleistocene of the lower Wye valley, below the Devensian glacial limit near Hereford, consists almost entirely of coarse gravels, situated both above and below the level of the modern flooplain. Those above the floodplain are largely composed of pebbles from the Ordovician and Silurian of east-central Wales. Around Hereford they are associated with well-preserved terraces; further downstream they are fragmentary and have also undergone some displacement. It is suggested that the high-level gravels comprise remants of four distinct aggradational units. Correlations are proposed with the Severn valley Pleistocene sequence, from which it is concluded that the lower Wye gravels represent a series of cold, perhaps glacial, periods ranging in age from pre-Hoxnian to Devension. This confirms the pre-Hoxnian age of fragmentary glacial deposits in northeast Herefordshire. Relations between the gravels and some geomorphological features are discussed, with special attention to abandoned incised meanders.