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1.
This paper evaluates the influence of natural sand particle characteristics on the amount and particle-size distributions of dust produced by aeolian abrasion. It contrasts with previous studies of aeolian abrasion by conducting experiments using: (i) whole sand samples, as opposed to selected size fractions; (ii) natural, mature dune sands, rather than artificial or freshly crushed material; and (iii) weathered sands that have acquired a superficial clay coating, instead of grains with clean surfaces. Whilst previous research has found clear, positive relationships between particle size, sorting, roundness and the amount of dust produced by aeolian abrasion, the relationships determined in this study show some variation according to the geomorphological context from which the original samples were obtained. The most important factor affecting the amount and particle-size characteristics of the dust produced was the presence of a clay coating on the grain surface that is removed by the abrasion process. The dust produced by this mechanism had a modal size of 2–5  μ m and material <10  μ m comprised up to 90% of the particles produced.  相似文献   

2.
《Sedimentology》2018,65(4):1322-1330
This study examines the role of quartz sand in the production of dust using mixtures of quartz sand from the Sahara and diatomite aggregates from the Bodélé Depression in Chad. An aeolian abrasion chamber is used to reproduce the physical processes of aeolian abrasion and test the hypothesis that the breakdown of saltating diatomite flakes as they collide in saltation, and with the surface, is the most prolific mechanism of dust production (auto‐abrasion). This hypothesis is tested against the competing hypothesis that a hard, higher‐density quartz sand impactor is required to abrade fine‐grained sediments to generate dust. The results show that dust can be produced by a mixture of saltating diatomite and quartz sand particles. However, quartz sand is not required for saltating aggregates to produce dust. Indeed, these results, which used a mixture of very coarse‐grained aggregate (1 to 2 mm diameter) with fine quartz sand, indicate that the addition of quartz sand can decrease dust production. For a very coarse aggregate (1 to 2 mm), a pure diatomite aggregate produced the most dust, although using a coarse‐grained aggregate (0·5 to 1·0 mm) with a mixture of 20% quartz and 80% aggregate was found to produce the most dust overall. The results of this study confirm the auto‐abrasion hypothesis for the breakdown of diatomite particles in the Bodélé Depression, which is the single biggest source of atmospheric mineral dust on Earth.  相似文献   

3.
Outcrops of the Early Jurassic Navajo Sandstone in southern Utah and northern Arizona, south-western USA are being actively eroded by sand-laden, south-westerly winds. Small-scale stepped topography with risers facing into the wind develops even on steep canyon walls when wind-swept grains strike the rock at a low angle. Photosynthetic, endolithic microbes directly underlie most outcrop surfaces; the crusts formed by these organisms are essential to formation of the small-scale steps. Wind erosion of highlands also forms troughs and pits that are tens of metres across. The pits have deeply scalloped, overhanging walls, and contain central domes surrounded by 'moats' filled with dune sand. Wind erosion of aeolian sandstone is favoured by a positive feedback mechanism in which grains that are liberated from outcrops by impacting particles become a fresh supply of pre-sorted abrasive particles for further attack.  相似文献   

4.
Dissected sand sheets and flow-aligned sand shadows occur near the summit of Muckish Mountain, Co. Donegal. The sand is of medium to fine size and moderately sorted to moderately well sorted. Sand transport by northerly winds is indicated by the location and morphology of the deposits. The source of the sand is a series of friable quartzite beds immediately below the northern edge of the summit plateau. Minor additions of gravel and very coarse sand, derived from the disintegration of plateau clasts, were probably also incorporated within the deposits by aeolian action, although surface wash associated with heavy rain or snowmelt may have mobilized these particles. The absence of diagnostic aeolian transport textures on quartz grain surfaces reflects the short distance/duration of transport. 14C dates indicate two phases of sand sheet accumulation: one between c. 5,300 ande. 2,650 B.P. and again after c. 1,910–1,760B.P. The sand shadows have formed within the last hundred years in response to the construction of small cairns across the plateau. Sand sheet dissection suggests erosion is currently occurring, but this began prior to the late nineteenth century. Present-day sand accumulation is also apparent from the widespread scatter of grains trapped by surface Vegetation. The sand represents the first recognition of aeolian deposition in the uplands of Ireland.  相似文献   

5.
Aeolian (wind) erosion is most common in arid regions. The resulted emission of PM10 (particulate matter that is smaller than 10 μm in diameter) from the soil has many environmental and socioeconomic consequences such as soil degradation and air pollution. Topsoil resistance to aeolian transport highly depends on the surface composition. The study aim was to examine variations in PM10 fluxes in a desert-dust source due to surface composition and topsoil disturbance. Aeolian field experiments using a boundary layer wind tunnel alongside soil composition analysis were integrated in this study. The results show variations in PM10 fluxes (ranging from 9.5 to 524.6 mg m?2 min?1) in the studied area. Higher wind velocity increased significantly the PM10 fluxes in all surface compositions. A short-term natural disturbance caused changes in the aggregate soil distribution (ASD) and increased significantly PM10 emissions. Considering that PM10 contains clays, organic matter, and absorbed elements, the recorded PM10 fluxes are indicative of the potential soil loss and degradation by wind erosion in such resource-limited ecosystems. The findings have implications in modeling dust emission from a source area with complex surfaces.  相似文献   

6.
Dry lakes, degraded sandy grasslands, abandoned farmland and mobile dunes which are widely distributed throughout the arid areas of northern China have been investigated in this work. Gain-size distribution of the surface sediments of Manas lake in Junggar basin, Juyan lake in the Alxa plateau, Zhuye lake in Minqin basin and most deserts (such as Mu Us desert, Otindag desert, Horqin desert and Hulun Buir desert) in China have been analyzed. The results show clay with particle sized <10 μm on the surface sediments of dry lakebed and sandy grassland developed from dry lakebed, respectively, account for >60% and ∼50% of the total mass. Since the tiny particles on the surface of abandoned farmland are blown away easily and rapidly, the content of clay particles in Minqin basin is <14%. The grain-size distribution of mobile dunes in northern China mainly consists of particles >63 μm and few particles <10 μm. Consequently, although sand/dust storms originate primarily in the western deserts, the gobi areas of the Alxa plateau, the north and east of Hexi Corridor and in central Mongolia, the widely distributed dry lakebeds, sandy grasslands and abandoned farmland adjacent to the deserts also contribute to aeolian dusts. Hence, the material sources for sand dust storm in East Asia include inland deserts, but also dry lakes, sandy grasslands and abandoned farmland, which are widely distributed throughout the arid inlands of northern China.  相似文献   

7.
A series of experiments was conducted to determine the potential for aeolian abrasion of natural dune sands to produce fine particles (< 125 µm) by (1) the release of resident fines; (2) spalling, chipping and breakage of particles; and (3) the removal of grain surface coatings. Parent samples were obtained from the surfaces of four active continental dunes and abraded using a glass ‘test tube’ chamber for up to 120 h. The fine particles produced by this abrasion process were trapped at varying time intervals and subject to detailed particle‐size analyses using a Coulter Multisizer. The abrasion of untreated parent samples produced fine particles in one of two main size classes, < 10 µm and > 50 µm, but when the parent sample was sieved to exclude particles < 250 µm, relatively more material in the range 10–50 µm was produced. For unsieved parent samples, the size range associated with the dominant mode varied according to the length of the abrasion time. The coarsest mode (> 63 µm) was dominant during the first 16 h of abrasion, then became less significant and is thought to be associated with the release of resident fines into suspension. The finest mode (< 10 µm) was absent or very weak during the first 16 h of abrasion, then became more significant and, in some instances, dominated the distribution as abrasion continued. Removal of grain surface coatings is the main source of fine material < 10 µm, and this may be a significant source of fine material in areas where sands are dominated by subrounded and rounded particles. By comparison with previous studies of aeolian particle abrasion, these natural dune sands produced very low quantities of fine material (by weight), but their spatial extent makes them potentially a significant source of dust‐sized particles at the global scale.  相似文献   

8.
Arid and semi‐arid environments are important sources for the atmospheric loading of PM10 (particulate matter <10 μm), although the emission of this material is often limited by surface crusts. This study investigates the emission and vertical flux of PM10 from a clay‐crusted playa, with and without saltating grains to abrade the surface. Using a portable field wind tunnel, it was found that, despite disturbance to the surface, the emission of PM10 decays rapidly without abrasion. Only in the presence of saltating grains was PM10 continuously liberated from the surface, such that the emission rate (the total amount of PM10 emitted from the surface expressed as a horizontal flux) varied linearly with the saltation transport rate. Although the emission of PM10 was found to depend on saltation abrasion, past studies have tended to focus on the relationship between the vertical flux of PM10 (the amount of PM10 being transported vertically through the boundary layer) and the shear velocity. In this study, the vertical flux of PM10 was found to vary with the shear velocity to the power of 2·14. Although the vertical PM10 flux is a proportion of the emission rate (the horizontal flux), no statistically significant relationship was observed between the emission rate and the shear velocity. The disparity of these results is explained by the lack of a consistent relationship between the shear velocity and the saltation transport rate in this supply‐limited environment. This suggests that the observed relationship between the vertical PM10 flux and the shear velocity is a spurious correlation, resulting from the use of shear velocity to calculate the vertical dust flux. It is thus concluded that shear velocity is not an appropriate variable for emission modelling in supply‐limited environments and that improvements in dust emission modelling will only be realized if the abrasion process is the focus of a concerted research effort.  相似文献   

9.
Quaternary desert loess and sandstone-loessite relationships in the geological record raise questions regarding causes and mechanisms of silt formation and accretion. In the northern Sinai-Negev desert carbonate terrain, only sand abrasion in active erg could have produced the large quantities of quartzo-feldspathic silts constituting the late Quaternary northwestern Negev loess. In the continuum of source (medium to fine sand of dunes) to sink (silts in loess) the very fine sand is unaccounted for in the record. This weakens the sand abrasion model of silt formation as a global process. Here, we demonstrate that, as predicted by experiments, abrasion by advancing dunes generated large quantities of very fine sand (60-110 μm) deposited within the dune field and in close proximity downwind. This very fine sand was generated 13-11 ka, possibly synchronous with the Younger Dryas under gusty sand/dust storms in the southeastern Mediterranean and specifically in the northern Sinai-Negev erg. These very fine sands were washed down slope and filled small basins blocked by the advancing dunes; outside these sampling basins it is difficult to identify these sands as a distinct product. We conclude that ergs are mega-grinders of sand into very fine sand and silt under windy Quaternary and ancient aeolian desert environments.  相似文献   

10.
The northwestern part of India, which includes the Thar desert on its western side, is a hot and arid region with intense aeolian activity and transport of aerosols by the prevailing SW-W summer winds. Different size fractions of aerosols (free fall = FF, suspended particulate matter = SPM, PM10 = <10 μm, and >10 μm) from air were sampled simultaneously at four locations along the dominant wind trajectory for ∼600 km. These aerosols and deposited surface sediments were characterized for their texture, mineralogy and geochemistry including REE and Sr isotopes. Within each size fraction, a bimodal distribution of grain size was observed. Quartz is the dominant mineral followed by K-feldspar, mica, calcite, chlorite and plagioclase. Garnet, amphibole, titanite and zircon are some of the identified heavy minerals. All samples, particularly those collected during summer, are compositionally homogeneous, including in their REE geochemistry, and are similar to average upper continental crust (UCC). However, in the winter aerosol samples, large deviations from the UCC composition are observed. This is attributed to meteorological parameters such as low wind velocity and temperature inversions in the winter season. During winter, secondary non-silicate and anthropogenic materials become important sources to Ca, Na, Mg, K, Ba and Ni budget; also reduction in the uptake and transport of heavy minerals lowered the concentration of Ti, Zr, Y, Cr, and REE in the aerosols. Geochemical coherence among aerosols, deposited surface sediments and the Thar sands, and the limited Sr isotopic data indicate that the Thar sediments and certain lithotectonic units of the Himalayan orogen are the proximal and distal crustal sources, respectively, for the aerosols in this region. Prevailing aridity and strong summer winds, and the presence of river alluvium in the Thar act together to transport silt rich dust, the removal of which could be a possible mechanism of ongoing desertification.  相似文献   

11.
Wind tunnel experiments were carried out with respect to the vertical distributions of wind-blown sand flux and the processes of aeolian erosion and deposition under different wind velocities and sand supplies above beds with different gravel coverage. Preliminary results revealed that the vertical distribution of wind-blown sand flux was a way to determine whether the gobi sand stream was the saturated one or not. It had different significances to indicate characteristics of transport and deposition above gobi beds. Whether bed processes are of aeolian erosion or deposition was determined by the sand stream near the surface, especially within 0–6 cm height, while the sand transport was mainly influenced by the sand stream in the saltating layer above the height of 6 cm. The degree of the abundance of sand supply was one of the important factors to determine the saturation level of sand stream, which influenced the characteristic of aeolian erosion and deposition on gravel beds. Given the similar wind condition, the sand transport rates controlled by the saturated flow were between 2 and 8 times of the unsaturated one. Those bed processes controlled by the saturated flow were mainly of deposition, and the amount of sand accumulation increased largely as the wind speed increased. In contrast, the bed processes controlled by the unsaturated flow were mainly of aeolian erosion. Meanwhile, there was an obvious blocking sand ability within the height of 0–2 cm, and the maximal value of sand transport occurred within the surface of 2–5 cm height.  相似文献   

12.
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.  相似文献   

13.
Heavy aeolian deposition is one of the most threatening natural hazards to oases in arid areas. How an oasis affects aeolian deposition is tightly related to the local ecological environments. To examine the effects of oasis on aeolian deposition under different weather conditions, monthly aeolian deposition from April 2008 to March 2009 and additional samples during dust storms in April and May 2008 were collected at four sites along the Qira oasis. The monthly ADRs (aeolian deposition rates) varied greatly with seasons and sites, ranging from 19.4 to 421.2 g/m2/month and averaging 198.8 g/m2/month. Aeolian deposition in the oasis was composed dominantly of sand and silt. Based on the variations of ADRs from the four sites, it can be found that the oasis exhibits two different effects on aeolian deposition under different weather conditions. During dust storms, the oasis demonstrates a significantly shielding effect due to the obstruction of the oasis-protection systems, resulting in most aeolian particles being deposited at the windward side of the oasis. While during non-dust storm periods with weak winds, the oasis exhibits an “attracting” effect on aeolian deposition, leading to a higher ADR inside the oasis. Owing to the annual ADR is dominated by the non-dust storm ADR in Qira, the oasis seems to become an important aeolian deposition area caused by the “attracting” effect of the oasis.  相似文献   

14.
Temporal variation of PM10 using 2-year data (January, 2007–December, 2008) of Delhi is presented. PM10 varied from 42 to 200 μg m−3 over January to December, with an average 114.1 ± 81.1 μg m−3. They are comparable with the data collected by Central Pollution Control Board (National Agency which monitors data over the entire country in India) and are lower than National Ambient Air Quality (NAAQ) standard during monsoon, close to NAAQ during summer but higher in winter. Among CO, NO2, SO2, rainfall, temperature, and wind speed, PM10 shows good correlation with CO. Also, PM10, PM2.5, and PM1 levels on Deepawali days when fireworks were displayed are presented. In these festive days, PM10, PM2.5, and PM1 levels were 723, 588, and 536 μg m−3 in 2007 and 501, 389, and 346 μg m−3 in 2008. PM10, PM2.5, and PM1 levels in 2008 were 1.5 times lower than those in 2007 probably due to higher mixing height (446 m), temperature (23.8°C), and winds (0.36 ms−1).  相似文献   

15.
The Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3-D outcrops allow a detailed study of structures and sequenses in the erg body. The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well-developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low-angle dipping toesets, and upper steeply dipping foresets. The foresets reveal significant across-crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross-bedding that documents strong along-crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along-crest transport of sand. The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along-crest transport of sand. The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.  相似文献   

16.
克里雅河流域风成物质的粒度分析与讨论   总被引：6，自引：0，他引：6       下载免费PDF全文
杨小平 《第四纪研究》1999,19(4):373-379
本文分析研究了昆仑山北坡克里雅河流域风成物质的粒度特征。该流域四大地貌景观带的地表沉积物表层样品的机械组成差别明显,黄土的粒度组成以粉砂为主,比我国黄河中游地区的黄土粗得多,亚砂土的主要成分为微砂,沙漠地区以细砂为主。除戈壁带以"非地"形式出现以外,由沙漠中心向南至昆仑山,在西北风和东北风的作用下,风成物质的粒度由粗变细。流域出露的黄土主要是在晚更新世和全新世沉积的。  相似文献   

17.
Sediment data from the Bahamian Santaren carbonate drift reveal the variability of trans‐Atlantic Saharan dust transport back to about 100 ka bp (Marine Isotope Stage 5·3) and demonstrate that carbonate drifts are a valuable pelagic archive of aeolian dust flux. Carbonate drift bodies are common around tropical carbonate platforms; they represent large‐scale accumulations of ocean‐current transported material, which originates from the adjacent shallow‐water carbonate factory as well as from pelagic production, i.e. periplatform ooze. Subordinately, there is a clay‐size to silt‐size non‐carbonate fraction, which typically amounts to less than 10% of the sediment volume and originates from aeolian and fluvial input. Sedimentation rates in the 5·42 m long core GeoHH‐M95‐524 recovered 25 km west of Great Bahama Bank in the Santaren Channel ranges from 1·5 to 24·5 cm ka?1 with lowest values during the last glacial lowstand and highest values following platform re‐flooding around 8 ka bp . These sedimentation rates imply that carbonate drifts have not only the potential to resolve long‐term environmental changes on orbital timescales, but also millennial to centennial fluctuations during interglacials. The sediment core has been investigated with the aim of characterizing the lithogenic dust fraction. Laboratory analyses included X‐ray fluorescence core scanning, determination of carbonate content and grain‐size analyses (of bulk and terrigenous fraction), as well as visual inspections of the lithogenic residue; the age model is based on oxygen isotopes and radiocarbon ages. Data show that the input of aeolian dust in the periplatform ooze as indicated by Ti/Al and Fe/Al element ratios abruptly increases at 57 ka bp , stays elevated during glacial times, and reaches a Holocene minimum around 6·5 ka bp , contemporary to the African Humid Period. Subsequently, there is a gradual increase in dust flux which almost reaches glacial levels during the last centuries. Grain‐size data show that the majority of dust particles fall into the fine silt range (below 10 μm); however, there is a pronounced coarse dust fraction in the size range up to 63 μm and individual ‘giant’ dust particles are up to 515 μm in size. Total dust flux and the relative amounts of fine and coarse dust are decoupled. The time‐variable composition of the grain‐size spectrum is interpreted to reflect different dust transport mechanisms: fine dust particles are delivered by the trade winds and the geostrophic winds of the Saharan Air Layer, whereas coarse dust particles travel with convective storm systems. This mode of transport ensures continuous re‐suspension of large particles and results in a prolonged transport. In this context, grain‐size data from the terrigenous fraction of carbonate drifts provide a measure for past coarse dust transport, and consequently for the frequency of convective storm systems over the dust source areas and the tropical Atlantic.  相似文献   

18.
We analyzed Rb-Sr-Nd isotope ratios of mineral dust in total aerosol load collected with rainwater continuously from 1998 to 2006 at the summit of Mt. Sefuri, northern Kyushu, southwestern Japan. During this period, the total mass of the dust generally increased in late winter, peaked in early spring, and then decreased.87Sr/86Sr in atmospheric mineral dust varied from 0.7096 to 0.7180, and εNd(0)CHUR from −19.9 to −3.5. During heavy deposition periods, the dust had high 87Sr/86Sr isotope ratios and low to middle εNd(0)CHUR values, respectively. These compositions are comparable to those of sand and loess in arid areas of Northeast China, Takla Makan and Western Beijing. Such particles were transported by westerlies from those areas to northern Kyushu in winter and spring. In summer and autumn, the isotopic compositions of the dust varied greatly; however, during light deposition periods, the Sr isotope composition was low. In these seasons, the contributions to the dust from Japanese soils and volcanic ash, transported by southern winds, were relatively larger than in winter and spring because of decreased mineral dust particle transport from the continent. Nevertheless, fine sandy desert particles and loess in general accounted for most mineral dust deposition in northern Kyushu year-round, even in summer. Local soils derived from weathered granite and volcanic ash were minor components only.The net mass of water-insoluble inorganic matter in the collected mineral dust fluctuated from year-to-year; deposition on Mt. Sefuri was relatively large before 2001, decreased from 2002 to 2005, and increased greatly in spring 2006. These year-to-year differences probably reflected changes in the strength of the westerlies and in climate conditions in the arid source areas.  相似文献   

19.
Agriculture tillage can result in the high concentration of particulate matter with an aerodynamic diameter of 10 μm or less (PM10) that can cause serious health problems. To understand how different agriculture tillage methods and wind conditions affect the transmission and distribution of PM10, four model runs were performed using the high resolution Weather Research and Forecasting model coupled with a chemistry component (WRF-Chem). In these runs, the observed emission rates under the conventional and combined tillage methods and different wind speeds were inputted into WRF-Chem. The simulated results show that the WRF-Chem model can reasonably capture the meteorological conditions at 500 m horizontal resolution over an agricultural field in California. The atmospheric concentration of particulate matter increases significantly with an increase in the emission area. Substantial reduction, 50%, of aerosolized PM10 dust emissions can be achieved by using combined tillage, when considered under the same meteorological conditions when compared to that caused by the conventional tillage method. Using the same conventional tillage emission rates, the lower velocity wind produces larger airborne concentrations of pollutants than does a stronger wind. Conversely, a stronger wind distributes the particulate matter over a larger area though with a diminished concentration when compared to a weaker wind. The atmospheric concentration of particulate matter was found to have a direct relationship to its emission intensity and area and wind conditions.  相似文献   

20.
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.  相似文献   

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