冲击压实技术在处治湿陷性黄土地基中的应用

基于冲击压实技术在某高速公路工程中处理湿陷性黄土地基的应用实践,通过土工试验和现场沉降观测试验,对冲击压实前后地基土的湿陷性、干密度、孔隙比、压缩模量及路基的沉降变形随土层深度、碾压遍数的变化规律进行了系统的研究。研究表明,冲击压实能使土的物理力学性质得到很大改善,路基的整体强度得到均匀提高,使得地基产生很大的沉降,是传统压实机械所达不到的。因此,冲击压实技术在处治湿陷性黄土地基中是有效的和实用的,而且生产效率高,有很好的发展和应用前景。     

Tracing and dating recent chemical transfers in weathering profiles by trace-element geochemistry and U---U---Th disequilibria: the example of the Kaya lateritic toposequence (Burkina-Faso)

The paper presents a synthesis of the approach recently developed to constrain the nature and the age of recent element mobility in weathering profiles. The approach relies on a comparison of variations of trace elements and U-series disequilibria in weathering profiles. It is exemplified in the case of old lateritic profiles from the Kaya toposequence, Burkina Faso. Trace elements and ²³⁸U---²³⁴U---²³⁰Th disequilibria have been analysed in whole rock samples from two pits located in very contrasted topographical positions. Trace element data show that the whole toposequence is marked by an intense chemical remobilization, including uranium, from the cap to the lower part of the profiles. ²³⁸U---²³⁴U---²³⁰Th disequilibrium data outline that all the levels of the profiles are affected by recent U---Th fractionations, and that each level of the toposequence is marked by U gains and losses. The chronological approach developed in this paper leads to an age of about 400 kyr for the dismantling of the iron cap, and ages ranging from 0 to 400 kyr for U accumulation in the pink clay horizon of the profiles. The depth repartition of U accumulation in the profile implies that the remobilisation processes in this toposequence varied through time, on time constants similar to those of climatic variations. To cite this article: F. Chabaux et al., C. R. Geoscience 335 (2003).     

Changes in soil physico-chemical and microbiological properties during natural succession on abandoned farmland in the Loess Plateau

The re-establishment of natural species-rich heath lands on abandoned farmland is one of the main measures in soil erosion control in the Loess Plateau of China. So, it is important to understand how the vegetation and soil properties develop after land abandonment. The objective of this study was to determine how physico-chemical properties, microbial biomass, and enzyme activities changed for abandoned farmland with an age sequence of 0, 1, 5, 7, 10, 15, 20, 25, 30, 40 and 50 years in Zhifanggou watershed (8.27 km²), Shaanxi Province, NW China. The results of this study indicate that species succession after land abandonment in the Zhifanggou watershed on the Loess Plateau resulted in a significant improvement in soil chemical and microbiological properties. Soil organic C, total N, available N and K, soil microbial biomass C, N and P, as well as alkaline phosphatase, catalase, saccharase, and cellulase activity increased with time since plantation establishment increased. In contrast, soil bulk density, pH, and polyphenol oxidase activity decreased after farmland abandonment. Urease and α-amylase decreased until 15 years at the early phase of species succession, and then increased. However, there was no significant change in total P and available P during the restoration. Results only implied the tendency that the herbage was developing toward shrub. Although secondary succession plays an important role which improved soil properties after farmland abandonment, the values of these parameters were still much lower than native forest in 50 years. Thus, vegetation recovery after farmland abandonment in a semi-arid environment would be slow and the improvement of soil properties in the Loess Plateau is likely to require a considerably long period of time.     

Carbon losses in soils previously exposed to elevated atmospheric CO2 in a chaparral ecosystem: Potential implications for a sustained biospheric C sink

Between 1996 and 2001 an experimental set up in a chaparral community near San Diego, CA, examined various plant and ecosystem responses to CO₂ concentrations ranging from 250 to 750 μl l^− 1. These experiments indicated a significant increase in soil C sequestration as CO₂ rose above the ambient levels. In 2003, two years after the cessation of the CO₂ treatments, we returned to this site to examine soil C dynamics with a particular emphasis on stability of specific pools of C. We found that in as little as two years, C content in the surface soils (0–15 cm) of previously CO₂ enriched plots had dropped to levels below those of the ambient and pretreatment soils. In contrast, C retained in response to CO₂ enrichment was more durable in the deeper soil layers (> 25 cm deep) where both organic and inorganic C were on average 26% and 55% greater, respectively, than C content of ambient plots. Using stable isotope tracers, we found that treatment C represented 25% of total soil C and contributed to 55% of soil CO₂ efflux, suggesting that most of treatment C is readily accessible to decomposers. We also found that, C present before CO₂ fumigation was decomposed at a faster rate in the plots that were exposed to elevated CO₂ than in those exposed to ambient CO₂ levels. To our knowledge, this is the first report that allows for a detail accounting of soil C after ceasing CO₂ treatments. Our study provides a unique insight to how stable the accrued soil C is as CO₂ increases in the atmosphere.     

贵州喀斯特山区植被演替对土壤有效性氮磷含量及酶活性的影响

对贵州南部茂兰喀斯特山区不同植被类型的土壤进行调查与采样分析,探讨植被演替过程中土壤生物有效性氮磷含量及酶活性的变化。结果表明:喀斯特植被从阔叶林向灌木林及灌草丛的演变过程中,土壤全氮、活性有机磷含量出现下降;而速效氮、硝态氮、速效磷及水溶性磷的含量出现显著的降低,灌草丛土壤中速效氮、硝态氮、速效磷及水溶性磷的平均含量分别比原生阔叶林土壤减少28.09%、65.44%、69.42%和81.71%;同时,土壤脲酶、蛋白质酶、磷酸酶活性分别比原生阔叶林土壤减少74.72%、63.08%和50.32%。在喀斯特山区土壤生物有效性氮磷含量的变化明显地受到植被群落演变的影响,森林退化主要导致土壤硝态氮及水溶性磷数量的明显减少。      

Late Glacial-Holocene sequence of Lake Saint-Point (Jura Mountains, France): Detrital inputs as records of climate change and anthropic impact

A sediment sequence (SP05, 12.5 m long) was taken from the deep zone of Lake Saint-Point (850 m a.s.l.). Sedimentological analyses highlight two main contrasted periods of sedimentation: the Last Glacial Maximum (LGM)/Late Glacial characterized by high silicates and quartz contents the Holocene dominated by the carbonated fraction. At the beginning of the Holocene (11 400 years cal. BP), silicates fraction flux abruptly decreased. The shift between the Late Glacial and the Holocene periods may be explained by forest development in the catchment. From 10 200 to 6800 years cal. BP, silicates and detrital carbonate fractions remained stable before they progressively increased steady till 5000 years cal. BP. Both increases cannot be totally attributed to an anthropic impact since pollen data indicate continuous anthropic activities only dated back from 3000 years cal. BP. They thus resulted from a dominant climatic control. From 5000 years cal. BP, silicates content still increased while detrital carbonates input became steady due to a change in pedogenetic processes affecting the catchment. During the last millennium, silicates and detrital carbonate decreased, probably due to pastureland development.     

Quantitative evaluation of soil processes using in situ-produced cosmogenic nuclides

In situ-produced cosmogenic nuclides provide a means for quantitative evaluation of a wide range of weathering and sediment transport processes. Although these nuclides have received attention for their power as geochronometers of surface exposure, it may be argued that they are more broadly suited for study of surface processes. In many environments, they may be used to evaluate collapse, erosion, burial, bioturbation, and creep, as well as providing a qualitative basis for distinguishing allochthonous from autochthonous materials. In addition, these nuclides can provide quantitative information on rates of erosion on scales of landforms and drainage basins. Here, we review the systematics of cosmogenic nuclide production within the Earth's surface, and present field examples demonstrating the utilization of in situ-produced cosmogenic nuclides distributions for evaluation of a range of soil evolution processes. To cite this article: E.T. Brown et al., C. R. Geoscience 335 (2003).     

南亚热带森林植被恢复演替中土壤有机碳组分及其稳定性

以鼎湖山森林植被恢复演替过程中的松林(初期)、混交林(中期)和季风林(后期)为研究对象,通过测定其土壤总有机碳(TOC)、易氧化有机碳(ROC)和不易氧化有机碳(NROC)及凋落物13C-NMR分析,以期阐明森林植被恢复演替过程中土壤有机碳组分变化规律及其原因。结果表明:(1)3个森林土壤ROC含量差异不显著;由松林向季风林演替过程中,ROC占TOC的比例下降。松林新鲜凋落物层的烷氧基碳含量(57.03%)高于季风林(49.10%)和混交林(54.50%)。(2)3个森林土壤NROC含量差异显著,大小顺序为:季风林混交林松林。混交林和季风林凋落物半分解层和已分解层的惰性指数明显高于松林。TOC趋势与NROC一致。南亚热带森林由松林向季风林恢复演替过程中,土壤有机碳的稳定性增加,惰性有机碳的持续积累可能是季风林和混交林土壤TOC积累的一个重要过程。     

Rivers, chemical weathering and Earth's climate

We detail the results of recent studies describing and quantifying the large-scale chemical weathering of the main types of continental silicate rocks: granites and basalts. These studies aim at establishing chemical weathering laws for these two lithologies, describing the dependence of chemical weathering on environmental parameters, such as climate and mechanical erosion. As shown within this contribution, such mathematical laws are of primary importance for numerical models calculating the evolution of the partial pressure of atmospheric CO₂ and the Earth climate at geological timescales. The major results can be summarized as follow: (1) weathering of continental basaltic lithologies accounts for about 30% of the total consumption of atmospheric CO₂ through weathering of continental silicate rocks. This is related to their high weatherability (about eight times greater than the granite weatherability); (2) a simple weathering law has been established for basaltic lithologies, giving the consumption of atmospheric CO₂ as a function of regional continental runoff, and mean annual regional temperature; (3) no such simple weathering law can be proposed for granitic lithologies, since the effect of temperature can only be identified for regions displaying high continental runoff; (4) a general law relating mechanical erosion and chemical weathering has been validated on small and large catchments. The consequences of these major advances on the climatic evolution of the Earth are discussed. Particularly, the impacts of the onset of the Deccan trapps and the Himalayan orogeny on the global carbon cycle are reinvestigated. To cite this article: B. Dupré et al., C. R. Geoscience 335 (2003).     

Calpionellid zonation of the Jurassic–Cretaceous transition in North-Atlasic Tunisia. Updated Upper Jurassic stratigraphy of the ‘Tunisian trough’ and regional correlations

The analysis of calpionellid associations from jebels Amar and Jédidi sections in North-Atlasic Tunisia provides, for the first time, a precise biozonation of the Jurassic–Cretaceous transition succession. In the light of the new data obtained and considering recently published results, the age of Upper Jurassic formations is clarified, allowing correlations with the Tunisian ‘Dorsale’ and the North–South Axis successions. Within the Maghrebides' range, sections from the external zones correlated to the Tunisian successions are quite distinctive from their equivalent in the internal zones. Both have evolved in different palaeogeographic domains related to the early structuration of the northwestern and southwestern Tethys margins. To cite this article: M. Boughdiri et al., C. R. Geoscience 338 (2006).     

Boreal peatland margins as repository sites of long-term natural disturbances of balsam fir/spruce forests

A multidisciplinary, high-resolution paleoecological study (Lepidoptera and plant remains, macroscopic charcoal, pollen) was conducted on a 4000-yr peat monolith extracted from the margin of an ombrotrophic peatland on Anticosti Island (Gulf of St. Lawrence, eastern Canada) to reconstruct the long-term natural disturbances (insect outbreaks, forest fires) of a balsam fir/spruce forest. We hypothesized that an activity of insect defoliators (spruce budworm, hemlock looper) was the main disturbance factor of conifer forests during the Late Holocene. The earliest remains of spruce budworm and hemlock looper were found ca. 3220 and 2350 cal yr BP, respectively. Peaks of insect head capsules occurred from ca. 1640 to ca. 625 cal yr BP. Low balsam fir pollen concentrations during this period suggest a lengthy episode ( 1000 yr) of high insect activity, resulting in extensive fir dieback and mortality. The long-term dynamics of the pristine balsam fir/spruce forests were mainly governed by the activity of insect defoliators. The limited extent and possibly the low occurrence of forest fires in the maritime environment of Anticosti Island allowed the development of mature coniferous stands propitious for insect infestations. Insect head capsules appeared to be a useful and effective tool for establishing insect presence and activity during the Holocene.     

Les argiles bleues du Cambrien inférieur de Saint-Pétersbourg et leur fissuration. Implications pour des stockages souterrains

The Lower Cambrian Saint Petersburg blue clays are composed of predominant illite and chlorite, sometimes accompanied by kaolinite. The <0.1 μm fraction has a high content of illite–smectite mixed layers. Particle-size distribution is more than 50% of clay particles and about 30% of silts. These blue clays correspond to plastic (and soft) clays; they may be compared to the Callovian clays of Bure (France), where storage of natural waste is envisaged. To cite this article: M. Arnould et al., C. R. Geoscience 334 (2002) 1135–1140.

Résumé

Les « argiles bleues » du Cambrien Inférieur de Saint-Pétersbourg sont constituées d'illite dominante et de chlorite, avec parfois présence de kaolinite. Des interstratifiés illite/montmorillonite sont très abondants dans la phase <0,1 μm. Du point de vue granulométrique, outre plus de 50% de particules argileuses, il existe une phase silteuse de l'ordre de 30%. Du point de vue pétrophysique, ce sont des argiles plastiques, de consistance molle. Elles sont subhorizontales et ont jusqu'à 116 m d'épaisseur. Outre l'absence de métamorphisme, malgré leur âge, leur caractère le plus remarquable est leur réseau de fracturation, bien observable en carrière. Les joints sont nets, sans remplissage ni cimentation. Les plans verticaux sont particulièrement développés. Le volume unitaire des blocs de matrice ne dépasse guère 1 m³. Des traces d'oxydation témoignent de circulations d'eau. Ces argiles silteuses anciennes peuvent aider à mieux connaı̂tre et comprendre les argilites silteuses épigénétiques calloviennes du site de Bure (Haute-Marne, France), où un stockage souterrain de déchets nucléaires est envisagé. Pour citer cet article : M. Arnould et al., C. R. Geoscience 334 (2002) 1135–1140.     

Application of structural equation modeling for assessing relationships between organic carbon and soil properties in semiarid Mediterranean region

Restoration of atmospheric carbon in soils has principal many good effects. Arid lands cover more than 40 % of the global earth area, but only stock 16 % from the global carbon stock. It seems to be a suitable solution for this environmental issue, but still all variables controlling organic carbon in such kinds of soil, was ignored. This study aims to develop two models of organic carbon under clayey and sandy soils in semi-arid Mediterranean zones basing on physical and chemical soil properties. For establishing both models, structural equation modeling was used. For modeling organic carbon, two Tunisian soil databases composed from clayey and sandy soils made respectively, of 450 and 602 soil horizons were used. Using the two databases for all properties, the principal component analysis shows two components for clayey soil; (i) chemical properties and bulk density and (ii) physical properties. For the sandy soil it reveals two components; (i) chemical properties and (ii) physical properties. According to the derived components for each soil category, two models have been built. Structural equation modeling results show that clayey model has proved that organic carbon was controlled by chemical properties and bulk density more than physical properties and sandy model has proved that organic carbon was controlled by chemical properties more than physical properties. The root mean square errors of approximation were 0.079 and 0.050 for the clayey and sandy models, respectively. Then these two models were validated with two other databases from Tunisian dryland soils.     

Late Holocene vegetation and fire dynamics from a savanna-forest ecotone in Roraima state,northern Brazilian Amazon

Two sediment cores from Mauritia flexuosa palm swamps have been studied by pollen and charcoal analysis. The cores Fazenda Cigana (FC) and Terra Indígena Aningal (TIA) were taken from a savanna-forest ecotone area in the Roraima State, northern Brazilian Amazon. Based on 5 radiocarbon dates, these records allow the reconstruction of the vegetation fire and climate dynamics during the past 1550 years. At the FC site was recorded a higher proportion of forest cover, suggesting local wetter climatic conditions favorable for forest expansion, especially by gallery forests, between 1550 and 1400 cal yr BP. Stands of M. flexuosa started to establish on the site indicating sufficient soil moisture. From 1400 to 1050 cal yr BP, forest cover retreated while savanna, and the Mauritia palm swamp expanded considerably. The FC site was marked by savanna and Mauritia cover with a slight increase of forest between ca. 1050 and 900 cal yr BP. From 900 to 300 cal yr BP the savanna and palm swamp taxa became dominant and the forest area decreased. At the TIA site the savanna cover was dominant between 1200 and 1000 cal yr BP. From 1000 to 700 forest expanded while savanna and Mauritia palm swamp reduced. Between 700 and 300 cal yr BP savanna and Mauritia palm swamp increased and forest area decreased. The high amount of charred particles found in the sediments, indicate fires with a marked increase between 1400 to 1000 cal yr BP (FC site) and 700 to 300 cal yr BP (TIA site), and probably caused the retreat of forest cover during these two time intervals. The relatively lower fire activity after 300 cal yr BP until present-day favored the increase of forested area at both TIA and FC sites. The arrival of the European settler and the subsequent introduction of cattle, is suggested as the main reason for the decrease of fire in the study region. The results point the fire caused by indigenous people as the principal controlling factor for forest and savanna dynamics during the past 1550 years.     

Effect of Biochar Produced from Mesquite on the Compaction Characteristics and Shear Strength of a Clayey Sand

Biochar is a carbon-rich and low microbial degrading material obtained after pyrolysis of biomass in the absence or limited content of oxygen. The impact of biochar on hydraulic properties of soil is extensively studied in agricultural and geotechnical or geoenvironmental engineering for potential application in bioengineered structures. While a little study is conducted to assess its effect on soil mechanical properties, especially shear strength. However, the effect of biochar on the combined shear strength and compaction characteristics of soil is not studied. The shear strength of biochar amended soil is thought to be related to the compaction characteristics. In addition, the effect of biochar on the shear strength of the soil is soil and biochar specific. In this study, an attempt was made to investigate and better understand the effect of biochar on the shear strength and compaction characteristics of a clayey sand for potential application in bioengineered structures. Standard proctor, direct shear and unconfined compression tests were conducted on bare soil and soil amended with 5, 10 and 15% (w/w) biochar. The experimental results revealed that the amendment of biochar from 5 to 15% (w/w) decreased the dry density and increased the shear strength parameters such as cohesion (c) and angle of internal friction (ϕ) of the soil. While it decreased the undrained shear strength (c_u) at higher compaction density (> 0.95MDD) and increased at lower compaction density (< 0.9MDD). Thus, in undrained condition, the initial compaction density has a strong influence on the shear strength of biochar amended soil. In addition, the increased c and ϕ and decreased dry density in biochar amended soil is observed to increase the stability of slopes (hypothetical). The increase of c and ϕ is believed to be due to the roughness and active chemicals (functional groups) present on the surface of the biochar. The decreased c_u at higher density is believed to be due to the lubricating effect by the higher water content.

     

强夯碎石土地基土体结构的试验研究

通过现场容重试验和注水试验研究了碎石土地基的密度和渗透性在强夯处理前后的变化,探讨了强夯对土体结构的影响.试验结果表明：强夯能有效改善碎石土的结构,且浅部明显,随深度增加表现出逐渐不明显的趋势;强夯作用下,土体结构的改变在3m以上区域为颗粒填充所致,3m以下则为土体孔隙压缩.     

A Late Hauterivian short-lived anoxic event in the Mediterranean Tethys: the ‘Faraoni Event’

A Late Hauterivian interval (127.5 Ma), called the ‘Faraoni Event’, which is characterised by the deposition of deep-marine black shales in the Mediterranean Tethys, is demonstrably of sufficient geological brevity to be qualified as an anoxic event. This event lies within the Pseudothurmannia catulloi ammonite subzone, coincides with the extinction of the calcareous nannofossil species Lithraphidites bollii, and records an increase in a globular planktonic foraminifer. High quantities of marine organic matter were preserved in pelagic successions from northern and central Italy, Switzerland, southeastern France, southern Spain and probably elsewhere in the Mediterranean Tethys and Atlantic Ocean. Carbon-isotope stratigraphy from Tethyan and Atlantic sections shows a minor positive excursion in the uppermost part of the Hauterivian and Lowermost Barremian, suggesting accelerated extraction of organic carbon from the ocean reservoir just after the ‘Faraoni Event’. The duration of this short event is less than 100 ka according to cyclostratigraphy and coincides with a third-order sea-level rise. It is likely that similar forcing mechanisms responsible for global OAEs operated during this short time interval. To cite this article: F. Baudin, C. R. Geoscience 337 (2005).     

Soil quality indicator of oxisols grown with sugarcane and native forest in northeastern São Paulo state,Brazil

The evaluation of soil quality is an important tool for degradation monitoring and sustainable management implementation. The objective of this study was to measure physical and chemical soil properties to set soil quality and validate a model of soil quality indicator in latosols (oxisols) under sugarcane cropping and native forest. The study was carried out in the cities of Araras, Santa Ernestina, and Guariba in São Paulo State, Brazil. We collected 24 samples of disturbed and undisturbed soil at 0.0–0.10 m layer from three areas grown with sugarcane and neighboring locations under native woodland. We assessed the following soil properties: (a) chemical—pH in CaCl₂, organic matter (OM), phosphorus (P), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), potential acidity (H?+?Al), aluminum (Al³⁺), and sulfur (S); (b) physical—macro- and microporosity, soil bulk density (Ds), aggregate stability, mean weight diameter (MWD), rill (K_r) and interrill (K_i) global erodibility, shear stress (τc), and magnetic susceptibility (MS). Data underwent multivariate statistics to identify the properties that denote soil quality and to set their weights within the functions of soil quality indicator (SQI). This study showed that the multivariate analysis was efficient in determining which physical and chemical properties were most sensitive, of which we can mention total sand, MS, clay, microporosity, Mg, Ca, pH, and OM. We can therefore conclude that the quality indicators of soils grown with sugarcane were lower than those under forest were, showing the need for adoption of conservation management practices.     

半干旱岩溶区油松林显著促进碳酸盐岩溶蚀

植被覆盖通过促进碳酸盐岩风化吸收大气CO2, 在实现碳中和中具有重要作用, 但不同植被类型的影响强度仍不清楚。为揭示半干旱区植被类型对碳酸盐岩风化的影响规律, 以一个典型岩溶小流域为研究区, 通过系统的植被调查和野外溶蚀试片试验, 详细对比了不同植物群系的碳酸盐岩溶蚀率及其影响因素, 并探讨了不同分类层次植被的溶蚀率的差异。结果表明, 半干旱岩溶区碳酸盐岩溶蚀率在植被型组层次为森林>草地>灌丛, 在演替的早期减弱、后期促进溶蚀; 群系层次的对比发现油松(Pinus tabuliformis)林内碳酸盐岩溶蚀率最高, 是其它群系的5倍至近30倍; 不同群系对溶蚀率的影响强度的大小得以明确; 溶蚀率与土壤CO2浓度(pCO2)、土壤温度和土壤含水量任何单一因素无相关性, 而与三者匹配性有较好的对应关系, 与湿润区明显不同; 植被演替通过增强对溶蚀环境因子匹配性的调控能力, 促进碳酸盐岩的溶蚀。半干旱区油松林显著促进碳酸盐岩溶蚀, 进行群系层次的广泛对比研究可以更好揭示植被类型与碳酸盐岩溶蚀之间的关系, 为提高岩溶生态系统恢复的碳汇量提供有效指导。     

Geomorphological-sedimentological studies of landform types and modern placer deposits in the savanna (Southern Malawi)

Semi-arid climatic conditions were responsible for the evolution of the vast savanna that stretches across central Africa and for the evolution of placers in this area that carry gemstones, rare earth element minerals and zircon- and titanium-bearing minerals. In combination with a polystage peneplanation, chemical weathering contributed to the emplacement of four different types of placer deposits during the Cenozoic in Malawi. Eight landform types have been defined by their geomorphological form, geometry, grain size parameters, stratification, fabric and rock contacts. Each type describes the landscape of the savanna and can be related to the emplacement of placer types within this morphoclimatic zone. Residual to eluvial placers, represented geomorphologically by landform type V, came into existence under strong chemical weathering during a period of alternating peneplanation and valley incision into soil and saprolite. Gentle valley-side slopes or elongated interfluves have a low gradient favorable for the formation and preservation of eluvial to colluvial placers (landform type Ic). Fluvial placers become economic only on outwash plains of tributaries near the confluence with their trunk rivers, beyond the point of inflection (landform types III, II). In the lower reaches of fluvial drainage systems around shallow lakes, fluvio-deltaic placers evolved during periods of rapid uplift and regression of the lake (landform type IV). Other landform types established throughout this study are cast in the role of ‘ore guides’ or, alternatively, have a negative impact on the likelihood of placer formation. The key associations of landforms discussed in this paper may be used as a geomorphological-sedimentological tool in search of modern continental placer deposits in areas where semi-arid climatic conditions occur(red) during the Cenozoic.