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1.
A.M. Harvey 《CATENA》1984,11(1):289-304
Similarities in Quaternary alluvial fan development are identified throughout southeast Spain. Pre-Wurm aggradation phases culminate in calcrete crusted upper fan surfaces, which are followed by Wurm dissection phases producing deep fanhead or through trenches. Deposits of the aggradation phases reflect source area size, relief and geology and in addition to proximal/distal facies variations show vertical decreases in debris flow and increases in gravel deposition. The dissection phases were complicated by cut and fill sequences forming within-trench terraces which show a southward decrease in complexity. The sequences are expressed in progressive changes in fan, terrace, and channel slopes and intersection point location. Fan morphological development during the Quaternary reflects long term progressive change, complicated by short term responses to climatic fluctuations and further comolicated by the locally variable effectiveness of trenching thresholds. 相似文献
2.
The cross-referencing of sedimentological, stratigraphic, geoarchaeological and radiocarbon data enables us to establish the geomorphologic evolution of the Late Holocene coastal flood plain of the Turia River. The Holocene marine transgression formed a coastal barrier lagoon environment in the coastal plain. During the Late Holocene, increased sediments supplied to the low reach of the river promoted: (a): the progradation of deltaic coastal sandy ridges on the coastline and (b): the aggradation of the floodplain on top of the sedimentary sequence. Four phases are clearly recognised in the geoarchaeological record of the flood plain. (1): High magnitude flooding events (2800 BP to 3rd century BC). (2): Slow riverbed aggradation and progressive levee formation (low energy flood regime) in Roman times (2nd century BC to 5th century AD). (3): Swampy environmental processes (6th to 9th centuries). (4): Second high-magnitude flooding phase (11th century). The navigable channel of the Roman and early Islamic periods became a wide and shallow braided channel in Medieval times. These phases are correlated with others detected by researchers in many areas of the Mediterranean region and with global Holocene climate events. 相似文献
3.
Late Quaternary alluvial fans and paleosols of the Kangra basin,NW Himalaya: Tectonic and paleoclimatic implications 总被引：1，自引：0，他引：1
The paper describes Late Quaternary pedosedimentary sequences of the alluvial fans from the Kangra basin of NW Himalayas for tectonic and paleoclimatic implications. In the proximal part of the Kangra basin three coalescing alluvial fans, namely Rait-Rihlu fans (~ 65 km2), Kangra fans (~ 200 km2), and Palampur fans (~ 170 km2) from west to east evolved due to reactivation of longitudinal and transverse faults and climatic changes during the Late Quaternary. The fans are characterised by subsidence of Rait-Rihlu fans, uplift of Kangra fans and tilting of Palampur fans. The thick (~ 90 m) pedosedimentary sequences exposed along the rivers characterise the dominant formative processes over the fans. The stream flow sediments dominate the Rait-Rihlu fans, whereas the debris flow sediments dominate the Kangra and Palampur fans. The fan sequences are also marked by the formation of strongly developed paleosols on loess (L1–L3 loess paleosols) and weakly developed paleosols on fluvial deposits in response to the tectonics and climate change in NW Himalayas. 相似文献
4.
Three representative soil profiles developed during the Late Quaternary in Northern Cilento (South Italy) were studied by optical microscopy and SEM-EDS observation of thin sections to examine soil features as evidence for past climatic fluctuations.Two of the profiles are polygenetic and the third monogenetic. All these have a clear interglacial imprint, whereas the polygenetic profiles also contain other features, which can be related to periglacial conditions during the stadial glacial periods although in midlatitude coastal areas. All the paleosols show a strong marine influence during development, which probably resulted from windblown processes or submergence during past interglacial phases. 相似文献
5.
The semi-arid Pulpi Basin is located within a 250 km long left-lateral shear zone between Alicante and Almería The Pulpi Basin itself is crossed by the N20E-striking Palomares Fault, active during the Neogene and Quaternary. In the Upper Pliocene a major epeirogenic uplift occurred which caused emergence of the basins. Simultaneously, a change from marine to terrestrial sedimentation started and the development of the present landscape began.The Quaternary relief development in the Pulpi Basin has been particularly influenced by tectonic disturbances and river captures, which have had contrasting effects on different parts of the basin. In the southern basin tectonically induced dissection of the oldest Quaternary glacis level was accompanied by the influence of the fluctuations in sea-level: both factors have caused continuing erosion. In the central Pulpi Basin Mid-Quaternary fluvial capture processes were important for the morphogenesis. Since the loss of the catchment areas in the neighbouring Huércal-Overa Basin the fluvial activity of the rivers in the western part has been limited. The transition from the distal end of the Mid-Quatemary glacis to the present valley floor takes place without a noticeable step; no younger fluvial terraces are missing. In the eastern part, the catchment area of the main river has hardly changed since its establishment during the Upper Pliocene. Besides the relics of the oldest glacis level no further glacis accumulation developed. During the Mid-Quaternary only a fluvial terrace was deposited. 相似文献
6.
Surface and buried Andosols and buried Luvisols of the Nevado de Toluca Late Quaternary tephra-paleosol sequence (Central Mexico) were studied to show whether these soils present an evolutionary sequence and to determine the pedogenic mechanisms and environmental factors involved in the evolutionary process. Micromorphological observations and mineralogical composition of fine sand and clay fractions were used to detect type and succession of soil-forming process. Some of the buried Andosols, defined as “intergrade” Andosols, have a predominantly blocky structure, humus-depleted areas, redoximorphic features and thin clay coatings in Ah horizons. Clay fractions of buried Andosols contain halloysite besides amorphous components, whereas in modern Andosols, allophane is dominant. Luvisols have micro-areas with granular structure and abundant phytoliths in the groundmass of Bt horizons assumed to be the relict Andosol features. Luvisol clay fractions are dominated by halloysite and kaolinite. Primary minerals show micromorphological weathering features in all studied soils being stronger in Luvisols; however, even in Luvisols, sand fractions consist mostly of unstable volcanic silicates. We hypothesise that the studied profiles form an evolutionary sequence: Andosols–“intergrade” Andosols–Luvisols; the soil transformation is supposed to be linked to progressive crystallisation of 1:1 clay minerals. Comparing the Nevado de Toluca paleosol properties with the existing data on volcanic soil climo- and chronesequences and assessing the regional paleopedological and lacustrine records of Quaternary paleoclimates, we concluded that wet/dry climatic oscillations took place during the formation of the studied paleosols. Rapid crystallisation of 1:1 minerals occurred during dry phases, which speeded up the Andosol to Luvisol transformation and made it independent from the primary mineral weathering status. The Andosol to Luvisol transformation accelerated by climatic fluctuations is thought to be a common soil evolutionary pathway in the subtropical and tropical regions of recent volcanism, which suffered contrasting precipitation oscillations in the Quaternary. 相似文献
7.
《CATENA》2001,42(1):39-57
The paper begins by examining the importance of southern African Quaternary environmental change. The Quaternary is characterised by repeated climatic changes of considerable amplitude; analysis of these fluctuations reveals the ubiquity of change and the fundamental dynamism of earth systems. Change is normal and, despite the fact that southern Africa was not subject to Quaternary glaciation per se, the influence of variations in, say, amount and seasonality of rainfall, has been very marked indeed. The Quaternary also represents the time period during which people have become a dominant environmental agent in the subcontinent; southern Africa has played a crucial role in the evolution of hominids and their development has been influenced strongly by accompanying environmental change. The relationship between geomorphology and climate in southern Africa is explored, revealing the degree and extent to which its landscapes are determined by changing environmental conditions, especially during the Tertiary and Quaternary; how apparent is the legacy of the past? This leads to a consideration of the types of geomorphological evidence, some more reliable than others, that can be utilised in order to reveal the details of Quaternary environmental change. Arid and semiarid landscapes appear to preserve more evidence of former environmental conditions, although high contemporary erosion rates and the paucity of long terrestrial-sedimentary sequences hinder their complete elucidation. Two case studies of Quaternary change in the subcontinent are presented, from the southwestern Cape and Namibia. These examples document the increasing intensity of human impact on landscapes to the extent that people now play the dominant geomorphological role, especially in semiarid and coastal areas. The conclusion offers pointers as to how geomorphological evidence of Quaternary change can be used to assist in the better management of contemporary and future environmental conditions. 相似文献
8.
Prof. Dr. Heinrich Rohdenburg 《CATENA》1983,10(4)
Because of the high infiltration capacity in natural rain forests or dry forests, there is no essential surface runoff with soil erosion and soil dislocation. At extremely poor rain forest sites one must consider that the soil chemistry is likely to induce a natural thinning of vegetation, which will lead to moderate surface runoff and a selective removal of clay, silt and fine sand. Even smaller man-made changes will cause — because of positive feedback — a considerable decrease of infiltration, as well as a corresponding surface runoff with related soil erosion and soil dislocation. Only then the recent soil erosion may be accepted as natural and climate-specific, when vegetation analyses have proven that all layers of the vegetation are undisturbed.Frequently, man-made or climatically induced regressive successions will damage preferably shallow-rooting plants. Then trees and bushes will dominate the pollen spectrum. Older erosional phases without an extensive destruction of the vegetation can thus be hidden in pollen profiles. They can be evidenced, however, by discordances, respectively by the accumulation of fine and partly also of coarse materials.Soil sediment sequences occur in all studied areas of the tropics and subtropics. They can be mainly ascribed to climatic changes. More detailed evaluations would necessitate facies- as well as frequency-analyses of these soil sediment sequences. Layers of coarse material, which are found close to, or on steep slopes, or on level surfaces, can be related to haphazardly distributed individual events of increased surface runoff. In very flat areas with predominant dense vegetation, also individual layers of coarse material, as e.g. residual debris, or shortly transported slope- respectively pediment debris, can indicate climatic periods of distinctly disturbed vegetation.Such periods can be explained by an increased frequency of randomly distributed individual events of higher rainfall intensity. The stability of the ecosystem, which depends on the “general climate” determines the threshold value, at which these individual events (miniature catastrophes) become geomorphologically active.In the presence of extreme soil moisture even the — with regard to surface runoff — very stable rain forest ecosystem can be attacked by landslides. Secondarily, then also the surface runoff can lead to increased erosion, as e.g. by slope incision. In most cases, probably a previous weakening of the ecosystem's stability will have taken place, such as a higher biopedo-climatic aridity of the “general climate”, which in turn could lead to primarily active surface runoff.It takes, however, numerous strong individual events, not to stabilize an ecosystem by progressive succession, but to destabilize it by regressive succession, so that surface runoff and the dislocation of fine and coarse material will set in, because of repeated events of less intensive rainfall. This state of an ecosystem means geomorphodynamic activity, whereas the vegetation-stabilized ecosystem stands for geomorphodynamic stability with distinct soil development. Soils and soil sediments are thus important indicators for the reconstruction of geomorphological effects of paleoclimates on slopes without concentrated surface runoff.In dry areas, which are characterized by thinner vegetation, repeated events of higher rainfall intensity can lead to geomorphodynamic activity and to regressive succession, even without a change of the “general climate”. An increase of the amount of precipitation coupled with a decreasing rainfall intensity would provoke the opposite effect: a strengthening of the ecosystem, stronger vegetation growth, and soil development.In periods of geomorphodynamic activity slopes can be entirely flooded and eroded by surface runoff, provided there is a sufficient runoff divergence due to transported coarse materials, or, in other words, when there is a high load ratio. When the load ratio is too small, then under all climates and on all geomorphologic elements, which have been shaped essentially by surface runoff, a concentration of the runoff and incision will take place. This could be so, either because the erosion capacity under a given soil protection turns out too weak, or because of too steep slope gradients related to a given catchment area.When the incision speed of such a concentrated runoff channel exceeds the lowering speed of its base level — which is frequently the case — then the following will result: its gradient, which is determined by the erosion, will become smaller, a concave longitudinal profile will develop, and finally the transport capacity will decrease, which means an increase of the load ratio. This will in turn increase the runoff divergence, and will thus lead — on all relief elements with runoff concentration — to a channel widening by lateral erosion. This process can become accelerated, when a climatic change towards higher rainfall intensities sets in, which will in turn increase the runoff divergence. In southern Brazil runoff channels of remarkable width are noticeable, not only on valley floors of smallest catchment areas, but also on steep slopes. In both cases, the high runoff divergence can be demonstrated — either in drillings, or in natural exposures — by the occurrence of coarse materials under subsequently deposited, fine sediments.In all climatic zones, this development can proceed until the interfluves are completely eliminated by lateral erosion. Then relief elements with unconcentrated runoff will develop, or rather be restored at a lower elevation, i.e. smooth slopes, respectively smooth pediments (Abtragungsfußfla¨chen). The further development is, on the one hand, characterized by a retrogressive flattening of these relief elements, and, on the other hand, by their growth, as neighbouring forms will unite. The final morphology will be one of a huge, smooth plain with a relatively small longitudinal gradient, a panplain (CRICKMAY 1933). It will develop over every substratum, and in all climatic zones. The prerequisites for such a development are, on the one hand, at least a temporary dominance of climatic conditions with geomorphodynamic activity, and, on the other hand, a relatively smaller lowering of the base level, as compared with the slope denudation. Thus, the development of panplains is favoured by areas of long-lasting tectonic stability or subsidence, or by periods without lowering of the sea level or transgression. It is, in addition, favoured by a previous deep weathering, or by a deep weathering, which continues alternatingly in periods of geomorphodynamic stability, i.e. by the development of easily removable saprolite on all types of rock.The previous stage of relief development with individual, associated plains — the pediplain, respectively the plain-facet-mosaic (Fla¨chenfacetten-Mosaik) — depends as associated form to a great extent on the former pattern of incision. This pattern depends, on the one hand, on a tectonic and petrographic weathering-conditioned differentiation, whereas on the other hand, there are distinct differences between zones of similar climate, respectively of a similar climatic sequence.In areas of the tropical savannas with wide-spread saprolite the stages of geomorpho-dynamic activity set in with a lowering of the river levels. The plains, however, are hardly dissected. When the load ratio grows because of climato-genetic reasons, then river bluffs will turn into independent slope pedimentation steps, which migrate uphill. But this will only be so, when the base level does not incise any stronger, because of the great speed of the slope denudation, which in turn results from the presence of easily removable substratum. The pediments of adjacent catchment areas will intersect near the interfluves, thus forming a pediplain. This process will be accelerated, as soon as the river incision reaches, at least in some places, the base of the saprolite. Then, any further linear erosion will be retarded.In areas of tropical savannas without saprolite, as well as in other arid areas with dominant coarse material, the slope denudation lags behind the linear incision. Incision of the rivers and dissection of older plains will be dominant. When, because of climato-genetic reasons, the load ratio grows, then relief elements with unconcentrated surface runoff will be restored, i.e. planation by lateral erosion will set in.In many areas of the nowadays perhumid tropics, the planation results from lateral erosion. At the beginning of a stage of geomorphodynamic activity, the ecosystem must probably have been stable to a great extent. Then, starting from disturbances, and also because of a low load ratio, the surface runoff must have initiated linear incision and a deep-reaching dissection of older plains.When the load ratio grows as a result of climato-genetic reasons, then new planation surfaces will develop by means of valley-floor-pedimentation processes, as in arid zones. These planation surfaces will, however, possess a much lower gradient, as there is — because of deep weathering — less coarse material involved, which consequently requires less transport work. Finally, the retrograding flattening of the longitudinal profile will also progress much faster. The system of plains consists of a mosaic of individual planation surfaces (plain-facets, Fla¨chenfacetten), which are generally sloping in the same direction. As it is an associated form of individual planation surfaces, it can be classified as a pediplain. Whereas pediplains still developed in the course of the younger Quaternary, or, respectively, developed again at lower elevations after a dissection, there is hardly any evidence of panplain development. In large parts of the world, especially in exhoric areas, a development towards increasing relief energy has occurred. The entire development can be characterized as a development sequence from panplain to pediplain, pediment, and terrace sequence. Depending on the tectonic conditions of any analysed area, on its petrographic and weathering-conditioned differentiation, and on its climate-zone-specific properties, this development has proceeded to different degrees. On the one hand, the entire development has been conditioned by the regression of the sea level since the Cretaceous period, which, although interrupted several times by transgressions, amounted up to several hundreds of meters. The development of panplains, respectively of very flat pediplains, during the older Tertiary was certainly favoured by either slow regression, or by transgression.On the other hand, a progressive planation development is possible, because of a clustering of periods of geomorphodynamic activity with especially high load ratios, i.e. with highly intensive planation processes, and with low tendencies towards dissection. The opposite is a sequence of periods of activity with small, respectively decreasing load ratios, which will favour the development of pediment sequences, and of terraces with decreasing extensions. The latter seems to be true for the younger Quaternary, which is characterized by a sequence of climatic cycles with a low potential for planation processes.With regards to the older development one must keep in mind that relief elements, which had developed during intermediate, less intensive periods of activity, were subsequently destroyed by periods with more intensive planation processes (principle of selection by intensity / Intensita¨tsausleseprinzip). Therefore, relief elements from older periods have only survived, when they either originated from very strong periods of activity, or from a sequence of equivalent periods of activity. This fact can lead to the deception that the climatic development has proceeded to ever weaker climatic cycles. The real relief development, however, can only be explained by the interference of tectonics, changes of the sea level, and the sequence of climates. The world-wide, parallel development was, to a great extent, controlled by the last two factors. A long-term shifting of the climatic zones has accelerated the relief development towards the formation of pediment-, respectively terrace sequences. Thus, for example, the Central European Quaternary valley incision can be traced to the regression of the sea level, as well as to the climatic change, which brought forth climatic cycles with less intensive processes of slope denudation, but with high intensities of linear erosion.The extremely deep-reaching weathering on panplains, respectively on pediplains, is not regarded to be responsible for planation processes, but, on the contrary, to be the result of too weak planation processes compared to the deep-reaching weathering. Very flat pediplains, but above all the panplains with their small gradients and extreme flatness are characterized — even in periods of stability with stable ecosystems — by far-reaching ground water currents and high ground water tables. Depending on the conditions of the catchment area, extreme concentrations of iron- and aluminium oxides and silica may result, whereas calcium carbonate may be deposited in arid areas. These deposits of oxides and carbonates are distinctly different from younger oxide- and carbonate concentrations, found at steeper, better-drained sites on younger surfaces. As in the case of the deep-reaching weathering, we do not regard this to be the expression of basically different process combinations, but to be the result of the different sites, which depend to a great deal on the existing relief and the relief development. 相似文献
9.
Morphologic features obtained from SRTM data, integrated with geologic information, are emphasized in this paper in order to provide the basis for understanding the development of the lowest Amazon drainage basin, focusing on the history of one of the largest Amazonian tributaries, the Tocantins River, and on the origin of the Marajó Island, throughout the Quaternary. This approach led to the recognition of a fan morphology related to the record of a tectonically controlled N/NW–S/SE orientated paleovalley cut down into Miocene and older rocks. The incised valley was fed by a paleo Tocantins River, which deposited its sediment load continuously to the north–northwest, reaching the Marajó Island and producing a deposit with a fan-morphology during the Plio–Pleistocene/Pleistocene. As characterized in the SRTM images, this channel system became abandoned due to capture by NE–SW orientated faults and establishment of the Pará River by W–E strike slip movements. This event, which probably took place in the Mid-Holocene, was responsible for the detachment of the Marajó Island from the mainland. 相似文献
10.
《CATENA》2005,59(2):187-204
The Clay Castles in the Hoarusib River valley are fine-grained deposits in a tributary immediately upstream of a narrow gorge. Sedimentological analysis reveals three lithofacies associations: (1) Channel Facies Association near the mouth of the tributary, (2) Back-Flow Facies Association in the middle section, and (3) Silt and Massive Sand Facies Association in the head of the tributary. Overall sedimentary architecture shows that aggradation of the main Hoarusib channel led to back flooding into the tributary and the deposition of massive fine-grained deposits. Decreased discharge competency, increased sediment load and the presence of narrow gorges within the main valley are suggested as reasons for the aggradation. Inter-fingering of desert-derived sediments channeled down the tributary and backflow sediments from the main river indicate an increase in both upland and desert rainfall during the course of sedimentation. Optically Stimulated Luminescence (OSL) dating of these deposits indicates formation between 44 and 20 ka, with the possibility that deposition occurred during two phases of rapid sedimentation at ca. 42 and 25 ka. 相似文献
11.
H. Faulkner 《Land Degradation \u0026amp; Development》1995,6(3):179-200
Since Moorish times, the lower slopes of the Sierra de Lujar in southern Spain have been intensively cultivated. Despite steep slopes and the highly erodible nature of the local micaschist/quartzite bedrock and derivative Quaternary sequences, accelerated erosion has largely been contained throughout this history by substantial, well-maintained rock terraces. However, since 1987, European Union (EU) agricultural policy has been to encourage the cultivation of certain crops by subsidising their expanded cultivation on a ‘per hectare’ basis. This has resulted in hasty clearance of native Mediterranean matorral on sites really unsuitable for the local staple crop (almonds), and erosion-prevention terraces are often absent or rudimentary. Gullying is now a very serious problem on some of these newly cleared sites. This paper identifies the lithological, topographic and agricultural policy controls on the consequent gully distribution, and then explores the pedological and hydrological controls on gully initiation on a single unterraced slope. Using these results alongside morphological data from gully surveys, a gully initiation and growth model is presented. This suggests that two types of gully are present in the area. ‘Type 1’ or upper-slope gullies are initiated by overland flows, which are relatively infrequent. By contrast, ‘type 2’ or slope-base gully initiation involves saturation and collapse in fossil landslip scars in Holocene colluvium. These two very different morphological systems may integrate through the terminal fan of the upslope gullies, in a manner described by Tiexeira de Oliveira 1990. The implications of this are that in the study area, sediment removal is associated with a very few highly productive sites. Contemporary management is briefly assessed against this model of geomorphic change in the area. 相似文献
12.
This article pertains to the fluvial dynamics of rivers in southern Québec, in particular to the aggradation and pedogenetic processes observed in floodplains affected by periodic floods. The frequency of flood events, notably along the Saint-François River and its main tributaries, leads to fine materials being frequently deposited on floodplains and affected development of alluvial soils. Particle size and physical and chemical analyses have led to a better understanding of the dynamics involved in the formation of floodplains and the development of soils related to this fluvial environment. Also, sedimentological analyses (layer texture and thickness, microstructure) combined with radiocarbon dating (14C) and isotopic methods (210Pb, 226Rd) enabled the determination of sedimentation rates along the floodplains. The radiocarbon dating results obtained from the organic layers buried in alluvial soils show relatively variable ages, i.e. between 2210 ± 60 and 30 ± 70 years BP. The data gathered reveal an active overbank deposition, which shows evidence of the high flood recurrence in many rivers of the basin. It is estimated that the annual floodplain aggradation ranges from 1.0 to 7.6 mm yr− 1 on average, which causes increases in floodplain elevations (vertical accretion). The sedimentation rates obtained using the constant rate supply dating model (210Pb) show average values that range from 2.1 to 10.7 mm yr− 1. Also, the presence of contaminated layers at the lower level (> 100 cm) of the floodplains suggested an active sedimentation rates along the rivers affected by floods. 相似文献
13.
清选装置是稻麦联合收获机上的重要工作部件，为了提高谷粒的清洁度和降低清选损失，风机出风口的气流流速应分布均匀。目前在稻麦联合收获机上大多采用的是低压中速离心式风机，其气流沿叶轮宽度方向分布的均匀性比较差。现研究的离心—轴流组合式清粮风机，试验得到的风速标准差为0.5727，变异系数为5%，稳定性系数为95%，比普通离心式风机变异系数低，风速的稳定性系数高。同时离心—轴流组合式风机的出风口风速分布情况是两端较低、中间较高，改变了普通离心式风机的出风口风速分布情况两端较高、中间较低的弊端。 相似文献
14.
Most spoils in the Lusatian lignite district have been afforested successfully. However, since soil chemical status remains very different compared to unmined locations even after decades it is still unclear if forest ecosystems develop to a self-sustainable status or if restoration will fail in the long term. To clarify this question water and element fluxes of different-aged red oak(Q. rubra L.) stands on both Quaternary pure sands and Tertiary, pyritic loamy sands were investigated and compared to a nearby, unmined location. Due to increasing interception and evapotranspiration, the deep percolation rates of red oak forests decline drastically within30 yr. Annual groundwater recharge is about 300 mm yr-1(41–46% of precipitation) initially. Deep percolation of the older stands decrease to less than 15% of precipitation, independent of substrate type. Tertiary substrates are characterized by intensive leaching of H, Fe, Al, Mg, Ca, NH4-N and SO4-S due to pyrite oxidation and therefore enhanced silicate weathering. Despite of drastically declining output rates with progressing ecosystem development, they remain remarkably higher as compared to Quaternary sands. However, despite differences in soil chemistry, the aged stands act as an almost complete sink for N, P and K. Compared to adjacent oak forests on undisturbed soils the stands on reclamation sites show very similar patterns with regard to N, P, K, Ca and Mg turnover. 相似文献
15.
C.J. Chartres 《Geoderma》1980,23(2):125-146
The field characteristics, texture, mineralogy and micromorphology of the soils of four chronologically separate river terraces of the River Kennet are described. The soils on the three uppermost terraces have illuvial (palaeoargillic) horizons, which have been intensely disturbed by periglacial activity. These horizons are overlain by similarly disrupted eluvial horizons mixed with aeolian silts derived from outside the Kennet catchment. The illuvial horizons most probably result from interglacial pedogenesis under climatic conditions warmer and wetter than at present, whilst the aeolian material was probably added to the soils during the last glacial period (Devensian). The lowest and youngest terrace is characterised by an undisturbed (normal argillic) soil developed in loams derived predominantly from the aeolian silts. Consequently, it is postulated that the soils of the lowest terrace owe their characteristics to a phase of postglacial pedogenesis.The results illustrate that several phases of pedogenesis can be recognised, which provide important information on the Quaternary development of the area. This is especially significant where floral, faunal, or archaeological evidence of environmental changes is unavailable. 相似文献
16.
Landform and stratigraphic information for the lower Mun River Basin, North-East Thailand and its adjacent areas assist in establishing the geomorphicevolution of the area. The present landforms and their associated stratigraphy are the result of both tectonic controls on river base levels and climatic changes within the Quaternary. Tectonically controlled changes in base level led to incision into the Cretaceous to Tertiary shales and sandstones (Mahasarakam Formation) to a depth going well below the present land surface and even below sea level, which was followed by the infilling of the valley system with alluvial sediments. Superimposed on these processes were late Pleistocene process changes during alternating drier and more humid phases which are reflected in both the shallow (up to −40 m) stratigraphy of the sediments and the landforms of the area. The most pronounced climatic change occurred after 20 000 yrs. BP when widespread desiccation was associated with aeolian activity and an increase in ground water salinity. These findings are consistent with evidence from other areas in the Australasian region which all show that in the Tropics the last glacial maximum coincided with considerably drier conditions than are found today. 相似文献
17.
Impacts of land use change on patterns of sediment flux in Weraamaia catchment,New Zealand 总被引：1，自引：0，他引：1
《CATENA》2005,64(1):27-60
Forest clearance between the 1890s and the early 1920s, subsequent scrub growth, and commencement of an afforestation program in 1979, modified the pattern and rate of sediment delivery to valley floors via shallow landslides and gully complexes in a steep headwater catchment (4.8 km2) in New Zealand. Analysis of the historical record, air photograph interpretation, and field survey indicates that both erosion types occurred across the catchment in the 1938 storm, aggrading channel beds and widening the active channel zone. In contrast, a 1 in 100 year event in 1988 (Cyclone Bola) induced numerous shallow landslides, but erosion of gully complexes was largely restricted to subcatchments that retained pasture, and the geomorphic impact of this event on channels was small. The changing volume and calibre of materials delivered to the valley floor, and the distribution of gully complexes, altered patterns and rates of channel adjustment after the events, and the resulting sediment flux. Development of gully complexes maintained coupling processes with channels for periods up to 102 years, forming wide channels in downstream reaches. Upstream–downstream connectivity along the trunk stream was altered by the formation of a large debris fan at the confluence with a tributary subjected to gully complex erosion. In contrast, slopes subjected to shallow landslides became decoupled from channels within 10 years, accelerating channel degradation and narrowing. Effective conveyance of a large volume of fine-grained materials promoted immediate aggradation of gentle-gradient channels downstream. As gully complex areas stabilized following an increase in forest and scrub cover, channel courses became significant sediment sources. Although shallow landslide activity will continue to induce intermittent aggradation in the future, it is inferred that average sediment yield will continue to diminish to levels approaching those experienced prior to clearcutting, and the pattern of sediment flux will recover by 2030. 相似文献
18.
S. M. Dabney Z. Liu M. Lane J. Douglas J. Zhu D. C. Flanagan 《Soil & Tillage Research》1999,51(3-4):219-231
Grass hedges are narrow (1–2 m wide) parallel strips of stiff, erect, grass planted near to or on the contour of fields but crossing swale areas at angles convenient for farming. They serve as guides for contour cultivation, retard and disperse surface runoff, cause deposition of eroded sediment, and reduce ephemeral gully development. After three years of tilled fallow between mixed-species hedges, the average grade of 18 m wide tilled strips between 1.5 m wide hedges was reduced from 0.068 to 0.052 as a result of surface lowering below hedges and on the shoulders of swale areas combined with increases in elevation above hedges. Annual surveys show progressive lowering of high spots and filling of low spots as contours lines more closely aligned with hedges. Survey data indicated annual erosion rates of nearly 250 t ha−1 year−1. Both RUSLE and WEPP over-predicted erosion rates, partly because backwater and slope modification affects were not considered. A tillage translocation model predicted enough soil movement to account for 30–60% of the observed changes. A combination of tillage translocation and water erosion/deposition provides the best explanation for the observed aggradation/degradation patterns. 相似文献
19.
玛纳斯河流域不同地貌单元弃耕地土壤盐分差异研究 总被引：7，自引：0，他引：7
本文在玛纳斯河流域中,选择冲洪积扇、冲积平原和干三角洲等不同地貌部位撂荒多年的弃耕地,分析土壤盐分动态变化及盐渍化类型的差异,探究土壤盐分的积聚特征,以期对弃耕地的生态重建提供参考依据。结果表明,自冲洪积扇缘带经冲积平原到干三角洲地区土壤盐分逐渐降低,土壤盐分表聚发生在冲洪积扇缘带和冲积平原中部,流域下游存在盐分向底层聚集的漏斗效应。7~10月份除冲洪积扇缘带外其他地貌类型区土壤盐分没有显著变化。流域表层盐渍土在冲洪积扇区以硫酸盐型或氯化物硫酸盐型为主,在冲积平原中部则为氯化物型,经冲积平原下部到干三角洲地区又逐渐演变为以氯化物硫酸盐型为主。 相似文献
20.
通过室内模型试验研究 ,对不同规模的水石流、稀性泥石流、黏性泥石流在堆积扇上输沙特征进行了研究。试验结果表明 ,不同类型泥石流在堆积扇上表现出不同输移、冲淤、含沙量演变特征 ,在堆积扇不同部位泥石流输沙特征也有差异。 相似文献