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1.
The 1800-km2 Okavango alluvial fan of northern Botswana represents an unusual depositional setting in which peat-forming perennial swamps (6000 km2) occur in a region of aeolian and semi-arid sedimentation within an incipient graben of the East African Rift. A channel system distributes water and sediment on the fan surface but cannot contain seasonal flood water, which spreads laterally from the channels through permeable channel margins, sustaining the flanking swamps. All sediment introduced is deposited on the fan. A detailed study of sediment movement and associated hydrological conditions in the channels was undertaken to examine sediment dispersal. Bedload greatly exceeds suspended load (at least by a factor of four). Vegetation and peat form permeable levees which confine the channels. In the upper reaches, two-way exchange of water occurs between channel and swamp depending on the season, but on the fan itself, channels lose water to the swamp. Bedload measurements reveal that the channel system is in a state of grade disequilibrium, with interspersed depositional and erosional reaches. Deposition of most of the incoming bedload occurs on the upper portion of the fan in a meandering and anastomosed channel system, but on the midfan, deposition of bedload occurs by channel-bed aggradation, at a rate of up to 5 cm yr–1. Further down slope, the channel enters a large lake where all remaining bedload is deposited. The presently observed sedimentation patterns may be due to a recent disturbance of the fluvial system, either by avulsion or neotectonics.  相似文献   

2.
Studies, spanning a 3 year period, of Westphalian C strata exposed in an active quarry have enabled three dimensional reconstruction of a lacustrine delta complex. The sequence exhibits a complex history of lake infilling by sediment introduced by intermittent high energy, low sinuosity distributary channel flows. Deposition in the small 0.2 km2 lake prior to delta formation was dominated by organic matter and typified rheotrophic swamp conditions. Large lycopods colonized the swamp floor. The lake was filled by a prograding delta which comprised six horizontally and vertically stacked delta lobes. The presence of lycopods aided sediment accumulation. Fluctuations in river discharge and consequent lake level rise and fall exerted a fundamental control on delta progradation and aggradation. Flooding during low lake levels first produced erosion on the existing lobe followed by a lake level rise which created accommodation for aggradation of a new delta lobe. Further lobe erosion and low stand lobe formation occurred during lake level fall. The area's proximity to alluvial fans resulted in hyperconcentrated flood flow within the distributary channels. The occurrence upon in-channel surfaces of plant colonization, including lycopods, testifies to the ephemeral nature of the flow. The lacustrine delta complex formed at the front of a terminal alluvial fan. Northward progradation of the alluvial fan was achieved by the capture and infilling of lakes by northerly flowing distributary channels.  相似文献   

3.
Geoarchaeological studies of alluvial fans in the northern Sny Bottom (Upper Mississippi River valley) focused on distal fan lobe processes, the effects of small-scale landscape change on prehistoric locational decisions, and the resulting structure of the archaeological record. Aerial imagery together with coring and trenching of paleochannels on distal lobes found that frequent channel avulsion (1) produced abrupt landform/habitat changes and (2) shifted the primary loci of sediment deposition on fans through time. Analysis of wood charcoal from prehistoric occupations indicates that vegetation succession during post-avulsion overbank aggradation along a new channel was characterized by a shift from Fraxinus-dominated to mesophytic forest. Results also indicate that use of fans by Early Woodland (ca. 2550–2100 B.P.) and other foragers centered on portions along active channels. Shifting depositional loci on fans led to surfaces and depositional units of varying age, and hence a complex stratigraphic record of Holocene occupations.  相似文献   

4.
内蒙古平庄盆地黑水地区上白垩统孙家湾组上段沉积相为冲积扇相,宏观上分为近端砾质辫状平原、远端砂质辫状平原及扇前洪泛平原3个亚相,进一步细分为泥石流、砾质辫状河道、砾质坝、砂质辫状河道、砂质坝、洪泛细粒、泥炭沼泽等微相。孙家湾组上段可划分为3个亚段,第一亚段为干旱_潮湿过渡气候下形成的退积型序列的旱地扇沉积;第二亚段为潮湿气候下形成的退积型序列的湿地扇沉积;第三亚段为潮湿气候下形成的进积型序列的湿地扇沉积。黑水地区的孙家湾组上段经历了早期退积型旱地扇—中期退积型湿地扇—晚期进积型湿地扇的过程。沉积相对该区铀成矿具有控制作用,铀矿化主要以不连续、局部富集的形式赋存于远端砂质辫状平原前缘的砂质辫状河砂体中,泥炭沼泽和泥质含量少的泥石流沉积中有少量存在。泥炭沼泽沉积所形成的泥岩及泥质含量高的泥石流沉积充当了局部隔水层,但由于泥炭沼泽沉积较薄,以及泥石流沉积的不均一性,致使铀矿（化）不连续、不稳定。  相似文献   

5.
A three‐dimensional numerical model of sediment transport, erosion and deposition within a network of channel belts and associated floodplain is described. Sediment and water supply are defined at the upstream entry point, and base level is defined at the downstream edge of the model. Sediment and water are transported through a network of channels according to the diffusion equation, and each channel has a channel belt with a width that increases in time. The network of channels evolves as a result of channel bifurcation and abandonment (avulsion). The timing and location of channel bifurcation is controlled stochastically as a function of the cross‐valley slope of the floodplain adjacent to the channel belt relative to the down‐valley slope, and of annual flood discharge. A bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value. The floodplain aggradation rate decreases with distance from the nearest active channel belt. Channel‐belt degradation results in floodplain incision. Extrinsic (extrabasinal, allogenic) and intrinsic (intrabasinal, autogenic) controls on floodplain dynamics and alluvial architecture were modelled, and sequence stratigraphy models were assessed. Input parameters were chosen based on data from the Rhine–Meuse delta. To examine how the model responds to extrinsic controls, the model was run under conditions of changing base level and increasing sediment supply. Rises and falls in base level and increases in sediment supply occurred over 10 000 years. Rising base level caused a wave of aggradation to move up‐valley, until aggradation occurred over the entire valley. Frequency of bifurcations and avulsions increased with rate of base‐level rise and aggradation rate. Channel‐belt width varied with water discharge and the lifespan of the channel belt. Wide, connected channel belts (and high channel‐deposit proportion) occurred around the upstream inflow point because of their high discharge and longevity. Less connected, smaller channel belts occurred further down‐valley. Such alluvial behaviour and architecture is also found in the Rhine–Meuse delta. During base‐level fall, valley erosion occurred, and the incised valley contained a single wide channel belt. During subsequent base‐level rise, a wave of aggradation moved up‐valley, filling the incised valley. Bifurcation and avulsion sites progressively moved upstream. Relatively thin, narrow channel belts bordered and cut into the valley fill. These results differ substantially from existing sequence stratigraphy models. The increase in sediment supply from upstream resulted in an alluvial fan. Most bifurcations and avulsions occurred at the fan apex (nodal avulsion), and channel belts were the widest and the thickest here (giving high channel‐deposit proportion) due to their high discharge and longevity. The width and thickness of channel belts decreased down‐valley due to decreased discharge, longevity and aggradation rate. This behaviour occurs in modern alluvial fans. Intrinsic controls also affect floodplain dynamics and alluvial architecture. Variation of aggradation rate, bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years, compared with 10 000 years for extrinsic controls. This variation is partly related to local aggradation and degradation of channel belts around bifurcation points. Channel belts were preferentially clustered near floodplain margins, because of low floodplain aggradation rate and topography there.  相似文献   

6.
深水扇储层的沉积特征是油气勘探开发中的一个热点问题。通过分析珠江口盆地白云凹陷荔湾3-1地区珠江组深水扇的沉积特征,明确了研究区具备深水扇形成的有利条件,受白云凹陷总体构造沉积背景影响,形成了富砂型的深水扇。本区深水扇是多种流体动力学机制共同作用的结果,碎屑流、浊流及底流相互作用与复合,形成了平面上具水道复合朵体的分布特征。在建立深水扇沉积模式的基础上,提出了水道加朵体型深水扇沉积微相划分方案,研究区储层以重力流水道、天然堤及滑塌朵体为主;而水道逐渐由相对近源的宽浅下切型变为远离物源的窄深型,其深度与天然堤宽度呈正比。  相似文献   

7.
Morphological and sedimentary records at the exit of Brahmaputra River at Pasighat in the NE Himalaya inform about the climate–tectonic interplay during the past ca. 15 ka. The geomorphology of the area comprises (1) fan terrace T3, (2) a high‐angle fan (3) terrace T2, (4) terrace T1 and (5) a low‐angle fan. Geomorphic consideration suggests that the fan terrace T3 and high‐angle fans are the oldest units and were coeval. The low‐angle fan is the youngest geomorphic unit. Sedimentological studies and optically stimulated luminescence chronology suggest that (i) fan terrace T3 formed between 13 and 10.5 ka and comprised multiple events of debris flows separated by the aggradation as channel bars in a braided river environment; (ii) the high‐angle fan formed during 15–10 ka and comprises channel bar aggradation in braided river conditions; (iii) terrace T2 formed during 10–8 ka due to aggradation in a braided channel environment with lesser events of debris flows; (iv) terrace T1 formed during <7 and 3 ka took place as bars of the braided river. Sudden coarsening of the sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka; and (v) the low‐angle fans dated to <3 ka formed due to aggradation in a small tributary joining the Brahmaputra River. This implies a phase when the main channel of the Brahmaputra did not flood regularly and the tributaries were actively aggrading. The sedimentation style and incision of these geomorphic units responded to contemporary climatic changes and uplift in the Siwalik range along the Himalayan Frontal Fault. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

8.
The Okavango Delta of semi-arid northern Botswana is a large alluvial fan (22,000 km2) covered by permanent and seasonal swamps from which 96% of the annual discharge is lost by evapotranspiration. Many small islands (1ha) within the permanent swamps are the sites of accumulation of sodium carbonate salts and many contain saline pans. The associated alkaline soils are toxic to vegetation. An understanding of the processes involved in alkalinization could be of potential benefit to long-term conservation planning in this unique ecosystem. The relation between soil chemistry and mineralogy, and swamp and groundwater chemistry were investigated on an island in the swamps. The study revealed that the water table beneath the island is depressed and swamp water enters the groundwater regime of the island from the margins and below, and flows toward the centre. The water becomes progressively more saline, initially owing to transpiration by trees and ultimately by evaporation in the central parts of the island. As a result of increasing salinity, amorphous SiO2 and magnesium calcite precipitate in the soils beneath the marginal zone of the island, raising the land surface, while the more soluble alkali carbonates are concentrated in the centre of the island as surface crusts and brine ponds. Leaching of these salts into the soil during the rainy season and gravity-driven flow of saline brines in the dry season causes the downward movement of Al and Fe in the central zone of the island. K-feldspar and possibly amorphous allophane develop in the deeper soils under the central zone of the island.  相似文献   

9.
Deposits of the ancestral Rio Grande (aRG) belonging to the Camp Rice Formation are preserved and exposed in the uplifted southern portion of the Robledo Mountains horst of the southern Rio Grande rift. The sediments are dated palaeomagnetically to the Gauss chron (upper Pliocene). The lower part of the succession lies in a newly discovered palaeocanyon cut into underlying Eocene rocks whose margins are progressively onlapped by the upper part. Detailed sedimentological studies reveal the presence of numerous river channel and floodplain lithofacies, indicative of varied deposition in channel bar complexes of low‐sinuosity, pebbly sandbed channels that traversed generally dryland floodplains and shifted in and out of the study area five times over the 1 Myr or so recorded by the succession. Notable discoveries in the deposits are: (1) complexes of initial avulsion breakout channels at the base of major sandstone storeys; (2) common low‐angle bedsets ascribed to deposition over low‐angle dunes in active channels; (3) palaeocanyon floodplain environments with evidence of fluctuating near‐surface water tables. Sand‐body architecture is generally multistorey, with palaeocurrents indicative of funnelling of initial avulsive and main fluvial discharge from the neighbouring Mesilla basin through a narrow topographic gap into the palaeocanyon and out over the study area. An avulsion node was evidently located at the stationary southern tip to the East Robledo fault during Gauss times, with aRG channels to the north flowing close to the fault and preventing fan progradation. Subsequent Matuyama growth of the fault caused (1) deposition to cease as the whole succession was uplifted in its footwall, (2) development of a thick petrocalcic horizon, and (3) fan progradation into the Mesilla basin. Parameters for the whole aRG fluvial system are estimated as: active single channels 2 m deep and 25 m wide; valley slope 0·24–0·065°; maximum mean aggradation rate 0·05 mm year–1; major channel belt avulsion interval 200 ky; individual channel recurrence interval 100 ky; minimum bankfull mean flow velocity 1·54 m s–1, minimum single‐channel discharge 77 m3 s–1, bed shear stress 22·3 N m–2; and stream power 34·3 W m–2.  相似文献   

10.
Stream channels in the Paria River basin were eroded and partially refilled between 1883 and 1980. Basin-wide erosion began in 1883; channels were fully entrenched and widened by 1890. This erosion occurred during the well-documented period of arroyo cutting in the Southwest. Photographs of the Paria River channel taken between 1918 and 1940 show that the channel did not have a floodplain and remained wide and deep until the early 1940s. A thin bar (<50 cm), now reworked and locally preserved, was deposited at that time. Basin-wide aggradation, which began in the early 1940s, developed floodplains by vertical accretion. The floodplain alluvium, 1.3–3 m thick. consists of two units recognizable throughout the studied area. An older unit was deposited during a time of low flow and sediment yield whereas the younger unit was deposited during times of high flow, sediment yield, and precipitation. Tree-ring dating suggests that the older unit was deposited between the early 1940s and 1956, and the younger between 1956 and 1980. The units are not time transgressive, suggesting that deposition by knickpoint recession was not an important process. High peak-flood discharges were associated with crosion and low flood discharges with aggradation. The erosional or aggradational mode of the streams was determined principally by peak-flood discharge, which in turn was controlled by precipitation.  相似文献   

11.
The Villanueva Complex is a lacustrine alluvial fan - fan delta system which accumulated on the northern margin of the Guadix Basin during the Pliocene. Five transitional zones can be distinguished from proximal to distal areas: proximal alluvial fan, mid-fan, fan fringe (transition zone), proximal fan delta and distal fan delta (lacustrine). This paper focuses on sedimentation in the fan fringe and the proximal fan delta areas, where the effects of fluctuations in the base level (lacustrine level) are more easily observed. The stratigraphical succession is here characterized by an alternation of fine lacustrine sediments (mudstones and siltstones) and gravels. The gravels appear as isolated channels, stacked channels, lenses and sheets. The isolated channels, which have a V-shaped profile and better developed wings towards the top, appear in the fan fringe zone and in the fan delta. The stacked channels originate in the fan fringe zone and evolve laterally and distally to isolated channels. The lenticular gravel bodies (lenses) are well represented throughout the fan delta and present small channels at the base, indicating a radial flow pattern. Finally, the sheets are characteristic of the fan fringe zone. The first episodes of channel incision occurred at lowstand lake level. The channels and lenses developed in rising lake level conditions, and sheet deposit took place at highstand lake level. Although the final form of the lithosome was heavily controlled by the fluctuations in lake level, it depended on the relative proportions of gravel and silt sedimentation. The gravel-siltstone interface therefore represents an equilibrium surface between the proportions of sedimentation of these lithologies. The bodies mentioned above are organized in coarsening and thickening upwards sequences tens of metres thick, in which a distal to proximal evolution can be observed from isolated channels to lenses or from stacked channels to sheets. The building and thickness of these sequences were controlled by fluctuations in the lake level. An initial fall in base level caused lengthening of the channels and entrenchment from distal to proximal areas, and a continuous increase in supply due to erosion in the drainage basin installed on the subaerial fan. When the lake level rose, lobes were formed at the channel endings and overbank processes were made possible, thus generating radial channels, whose levees were formed by amalgamation with the levees of the main lens channels. At the same time, sheets were formed in proximal areas, where the available amount of sediment was greater.  相似文献   

12.
The morphodynamics of a river flood on a fan delta and its resultant stratigraphic and sedimentary signatures have been studied by means of a flume experiment under controlled boundary conditions. The experiment revealed that deposition was dominant in flood periods when the channels were highly loaded with sediments. In contrast, erosion was dominant in periods of low flow. Mouth bars were formed when a subaqueous channel began to backfill. The development of a mouth bar began with progradation in the down‐dip direction and proceeded by aggradation, then retrogradation and finally transverse growth. A channel bifurcated in multiple stages by sequentially forming mouth bars or by simultaneously forming arrays of mouth bars. During the bifurcation, the diffluent point moved upstream, which resulted in channel migration and the development of a delta lobe. Flood events triggered fan‐delta front slide‐slump deposits.  相似文献   

13.
通过对新疆现代白杨河冲积扇的地貌和现代沉积进行调查,发现冲积扇表面发育两种不同类型的河道,一种为季节性河道,另一种为暂时性河道。暂时性河道内水流占有率小于50%直至接近于0,主要由暴雨形成突发性洪水造成,季节性河流河道内流水占据率为50%左右,输出水流特征介于暂时性河道和常年性河流河道之间。白杨河主河道属于季节性河道,河道占冲积扇表面面积2.1%,沉积物以砾石质为主,颗粒粗、磨圆度高、分选较好,泥质含量低、叠瓦状排列特征明显,沉积物具有向下游变细的趋势,河道形态沿程变化明显。暂时性河道占冲积扇表面面积97.9%,沉积物粒度相对较细,磨圆度低、分选差、泥质含量高,河道规模向下游减小,分叉增多。季节性河道以河道径流为主,暂时性河道主要以片流、泥石流等方式搬运沉积物。季节性河道沉积物主要来自上游较远的源区,暂时性河道的沉积物来源于冲积扇扇根附近基岩风化形成的倒石锥,塌积扇和山地泥石流沉积,一部分来自于对冲积扇原有沉积物的改造、搬运和再沉积。季节性河道是形成冲积扇扇体的主要动力,暂时性河道主要对冲积扇起改造作用。研究深化了对干旱地区冲积扇沉积过程和沉积特征的认识,丰富了冲积扇的沉积模式。  相似文献   

14.
Several Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain-size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep-tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain. On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s?1 and flow velocities of about 0.75 m s?1. Using this flow velocity and channel dimensions, sediment concentration (～2×10?3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2. On the mid-fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross-flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand-rich late Pleistocene and early Holocene turbidity currents on the fan.  相似文献   

15.
以珠江口盆地白云凹陷钻井、测井、地震和古生物资料为基础,结合前人研究成果,系统的分析了白云凹陷晚渐新统到早中新统沉积相发育特征及凹陷沉积充填演化过程。研究结果表明,珠海组下部发育大型陆架边缘三角洲沉积,地震反射特征表明该三角洲发育三期;钻遇水下分支河道、支流间湾、河口坝及远砂坝微相,沉积物以富砂为特征,发育冲刷-充填构造、递变层理及较粗的水平潜穴等多种构造;珠海组上部为浅海相,沉积物以海相砂泥岩互层为主,此时期陆架坡折带位于白云凹陷南坡;珠江组沉积时期,海平面升降旋回频繁,陆架坡折带迁至凹陷北坡。随着古珠江流域扩大,沉积物输入量增加,在珠江组下部发育了深水扇,沉积物以细-粗砂岩为主,夹少量粉砂岩及深海泥岩,发育颗粒流、液化流、浊流及碎屑流等四种主要的重力流,钻遇内扇水道,中扇废弃水道及水道间漫溢沉积,外扇深海泥沉积;珠江组上部为半深海相,沉积物以深海泥岩为主。   相似文献   

16.
Exceptional exposures of Permian basin floor fans (fans 3, 4) and a slope fan (fan 5) in the Tanqua Karoo foreland basin of South Africa have enabled an investigation of the relation between the pinch-out geometries and fan architecture. The pinch-out geometry of fan 3 is characterized by the down dip transition from thin to medium bedded sheet deposits to pinch-out fingers, which are overlain by younger prograding sheet deposits. This geometry reflects the progradational stacking pattern of the fan. In contrast, the fan 4 pinch-out fingers consist of stacked channel fills in the same conduit. This pinch-out configuration relates to the dominant aggradational style observed on the mid and distal parts of fan 4. Fan 5 represents a slope fan comprising an axial channel conduit, which branches down slope into three distributary channels. The distal fan is characterized by larger channel fills, which may represent bypass channels to other basin floor fans. The very thick-bedded nature of the youngest channel fill unit suggests early bypass followed by retrogradation as indicated by the presence of thinner bedded heterolithic channel fill deposits along the axial conduit. Although some of the massive pinch-out channels exhibit basal scour, their depositional morphology suggests that they mainly originated due to the infill of subtle topographic depressions by low concentration turbidity currents. Instead of describing these features as channel fills, the use of the term pinch-out fingers is preferred.  相似文献   

17.
Many cities around the world are developed at alluvial fans. With economic and industrial development and increase in population, quality and quantity of groundwater are often damaged by over-exploitation in these areas. In order to realistically assess these groundwater resources and their sustainability, it is vital to understand the recharge sources and hydrogeochemical evolution of groundwater in alluvial fans. In March 2006, groundwater and surface water were sampled for major element analysis and stable isotope (oxygen-18 and deuterium) compositions in Xinxiang, which is located at a complex alluvial fan system composed of a mountainous area, Taihang Mt. alluvial fan and Yellow River alluvial fan. In the Taihang mountainous area, the groundwater was recharged by precipitation and was characterized by Ca–HCO3 type water with depleted δ18O and δD (mean value of −8.8‰ δ18O). Along the flow path from the mountainous area to Taihang Mt. alluvial fan, the groundwater became geochemically complex (Ca–Na–Mg–HCO3–Cl–SO4 type), and heavier δ18O and δD were observed (around −8‰ δ18O). Before the surface water with mean δ18O of −8.7‰ recharged to groundwater, it underwent isotopic enrichment in Taihang Mt. alluvial fan. Chemical mixture and ion exchange are expected to be responsible for the chemical evolution of groundwater in Yellow River alluvial fan. Transferred water from the Yellow River is the main source of the groundwater in the Yellow River alluvial fan in the south of the study area, and stable isotopic compositions of the groundwater (mean value of −8.8‰ δ18O) were similar to those of transferred water (−8.9‰), increasing from the southern boundary of the study area to the distal end of the fan. The groundwater underwent chemical evolution from Ca–HCO3, Na–HCO3, to Na–SO4. A conceptual model, integrating stiff diagrams, is used to describe the spatial variation of recharge sources, chemical evolution, and groundwater flow paths in the complex alluvial fan aquifer system.  相似文献   

18.
Channel modifications from 1968 to 1969 on the South Fork Forked Deer River in western Tennessee have caused upstream degradation, downstream aggradation, and bank failures along the altered channels, adjacent reaches, and tributaries. The result of these adjustments is a general decrease in gradient as the channel attempts to absorb the imposed increase in energy conditions created by channelization. Headward degradation at a rate of approximately 2.57 km/yr on the South Fork Forked Deer River caused from 1.52 m to about 3.14 m of incision over a 13.5 km reach from 1969 to 1981. As a consequence of substantially increased sediment supply, approximately 2.13 m of aggradation was induced downstream of this reach during the same period. This accumulation represents a 60% recovery of bed level at the downstream site since the completion of channel work in 1969. Gradient adjustment with time is described by exponential decay functions. The length of time required for adjustment to some new quasi-equilibrium condition is computed by these decay functions and is about 20 years from the completion of channel work. Adjusted slopes are less than predisturbed values, probably because straightened channels dissipate less energy by friction, allowing more energy for sediment transport. An equivalent sediment load, therefore, can be transported at a considerably gentler slope. The predisturbed slope exceeds the adjusted slope by an order of magnitude on the downstream reach of the South Fork Forked Deer River.  相似文献   

19.
《Geodinamica Acta》2013,26(2):145-152
The understanding of deep-water turbidite systems implies a preliminary detailed analysis of the architectural elements which compose them. Using 3D seismic data, three architectural elements are recognized including a new one: the “meandering erosional nested channels”. The spatial organisation and the relative stratigraphic position of these “elementary bricks” allow to define four stages which form the sedimentary history of the distal part of a upper Miocene turbidite system of the Lower Congo basin: 1, depositional stage with frontal splay development; 2, erosional channel and prograding system; 3, depositional stage with vertical aggradation of the channel and 4, abandonment phase with channel avulsion.  相似文献   

20.
准噶尔盆地西北缘三叠系干旱型冲积扇储层内部构型特征   总被引：7，自引：0，他引：7  
以克拉玛依油田一中区为例,综合现代沉积、岩芯、露头、密井网等资料,应用层次分析法深入解剖干旱型冲积扇内部构型。以冲积扇相带划分为基础,构建7~9级构型要素分级体系,明确构型要素的成因、规模及样式,建立精细的干旱型冲积扇构型模式。研究表明:①根据岩性组合、沉积构造等特征将冲积扇依次划分为槽流、片流、辫流及径流4个相带;②槽流带主体为宽带状槽流沉积体,由多期槽流砾石体垂向叠置而成,单期槽流砾石体顶部在间洪期受到小股牵引流侵蚀改造,发育网状的流沟系统;③片流带主体为扇状片流砂砾体,由多期片流朵体侧向—垂向复合而成,朵体顶部披覆的泥质夹层是稳定的渗流屏障;④辫流带主体为宽带状辫流水道,由洪水水道和间洪水道两种单一水道侧向复合而成,洪水水道规模较大、侧向迁移频繁、切割叠置关系复杂、物性较好,在油田注水开发过程中极易形成水窜通道,而间洪水道规模较小,侧向切叠程度较低、物性也较差;⑤径流带主体为漫流细粒沉积,在漫流细粒沉积的背景上发育径流水道,其水道规模小、物性差且易尖灭。  相似文献   

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