This paper describes the morphology, sequential development and general sedimentology of concave-bank benches on the Murrumbidgee River of southeastern Australia, and also notes their important role in floodplain formation on certain meandering rivers in western Canada. Benches form against the concave bank (cut-bank) of abruptly curving bends immediately upstream of the point of maximum curvature. As a result of flow deflection against the upstream limb of the convex bank, the channel widens here and produces a zone of expanded flow facilitating flow separation near the upstream limb of the concave bank. Sedimentation within this zone starts with a longitudinal-shaped bar of medium sand forming a platform isolated even at low flow by a narrow secondary channel against the concave bank. Aggradation of the longitudinal-shaped bar with fine sand, mud and organic matter permits the establishment of trees. Further sedimentation, particularly around the young trees, results in the formation of a fully developed bench isolated by the secondary channel from the remainder of the floodplain only during high flows. Observations on confined meandering rivers in western Canada provide evidence of substantial floodplain formation by concave-bank bench accretion, a process distinctly different in character to the more familiar mechanism of lateral point-bar accretion. Furthermore, the preservation of abundant organic debris means that extensive bench deposits may be a source of locally useful natural gas from within floodplain sediments. 相似文献
This paper investigates the nature and processes of sedimentation of allogenic cave deposits in the high relief, everwet karsts of montane New Guinea. Under the high intensity rainfall regime, episodic mass movements in small karst catchments provide a wide range of sediment textures from clayey gravels to fine clays. These allogenic sediments are deposited into pools of water within the caves, giving sedimentary structures analogous to turbidites. Diamictons within the cave relate to episodic mudflows in the catchment. These deposits move as fluidized masses in a manner similar to some esker deposits. Cross-stratified sediments are formed by dumping of pulses of sediment laden water into deep pools. Extremely fine-grained clays and muds accrete parallel to underlying surfaces following flood pulses. These deposits represent the last phase of catchment instability, when a small amount of slopewash occurs. Catchment processes are dominated by solution and episodic mass movements. When the thick root mat which masks the ground is disrupted, some slopewash occurs but it is not a major component in catchment processes. 相似文献
I. INTRODUCTIONAt present moot sediment transport models applied in engineering practice are based on equilibriumsediment transport approach, i. e. sediment--carrying Capacity is used to replace the actual sediment concentration (ref. 1 -- 9). However, the sediment--carrying capacity, in general, is not equal to sedimentconcentration, they may differ a lot especially for the case of reservoir sedimentation process and/orthe scouring process of river channel in the downstream of a reservoi… 相似文献
The concept of a ‘niveo-aeolian influence’ on the genesis of northwest European Pleistocene aeolian sand and loess deposits has been advocated in the past both to explain the presence and the absence of lamination. Field experiments were carried out on artificially deposited alternating layers of sand and snow, and of loess and snow, to investigate the role of a niveo-aeolian mode of deposition on resulting sedimentary structures. Field observations of sand plots revealed minor changes in surface morphology typical of denivation surfaces. The thin sections showed a characteristic porous structure. Deformations of the depositional structure and indications for erosion and redeposition were limited. However, the originally smooth aeolian sedimentary surface of the loess changed considerably. The thin sections showed also a porous, spongy structure. Indications were found for disturbances by frost action, liquefaction during melting of the snow and ice, and redeposition of loess by meltwater. The experimental results strongly suggest that sand and loess deposited as niveo-aeolian material are characterized both by a porous structure, and that the original sedimentary structure of the sand is not affected by snowmelt, whereas loess becomes non-laminated. Thus, the intercalation of snow during sedimentation of sand and loess does not in itself induce a laminated structure. 相似文献
This study assesses historic overbank alluvial sedimentation along a low-gradient reach of West Fork Black's Fork in the northern Uinta Mountains, Utah. In this previously glaciated setting, an alluvial floodplain that is approximately 400 m wide by 1500 m long has been modified by the combined effects of valley morphometry and the recent history of clear-cut logging during the late 19th and early 20th Centuries. To quantify the effects on sedimentation and flow conveyance, three natural streambank exposures were sampled and analyzed for nuclear bomb fallout 137Cs. The distribution of 137Cs within the three profiles suggests that a remnant outwash terrace exerts a first-order control over the deposition of overbank alluvium. Upstream from a constriction in the floodplain caused by the terrace remnant, as much as 40 cm of overbank alluvium has been deposited since the beginning of clear-cut logging. Immediately downstream of that constriction, no evidence exists for any overbank sedimentation during that same period. Vibracore samples and Oakfield soil probe sampling throughout the study reach quantified the geographic extent and thicknesses of the historic alluvial package. Flood conveyance through the study area was modeled using the U.S. Army Corps of Engineers HEC-RAS modeling program. Model simulations were run for modern conditions (using surveyed topography) and for prehistoric conditions (using the modern topography less the historic alluvial package determined by 137Cs analyses). Model results indicate that the floodplain constriction caused a significant impediment to flood conveyance at even modest discharges during prehistoric conditions. This promoted ponding of floodwaters upstream of the constriction and deposition of alluvium. This has increased bank heights upstream of the constriction, to the point that under modern conditions 1- to 5-year recurrence interval floods are largely confined within the channel. These results confirm the validity of this new approach of combining 137Cs dating of alluvial sediments with HEC-RAS flow modeling to compare flood conveyance along a single stream reach prior to and since an abrupt change in alluvial sedimentation patterns. 相似文献
The Gulf of Corinth is a graben, which has undergone extension during the Late Quaternary. The subsidence rate is rapid in the currently marine part whereas uplift now affects a large part of the initially subsiding area in the North Peloponnese. In this paper, we document the rates of subsidence/uplift and extension based on new subsurface data, including seismic data and long piston coring in the deepest part of the Gulf. Continuous seismic profiling data (air gun) have shown that four (at least) major oblique prograding sequences can be traced below the northern margin of the central Gulf of Corinth. These sequences have been developed successively during low sea level stands, suggesting continuous and gradual subsidence of the northern margin by 300 m during the Late Quaternary (last 250 ka). Subsidence rates of 0.7–1.0 m kyr− 1 were calculated from the relative depth of successive topset to foreset transitions. The differential total vertical displacement between the northern and the southern margins of the Corinth graben is estimated at about 2.0–2.3 m kyr− 1.
Sequence stratigraphic interpretation of seismic profiles from the basin suggests that the upper sediments (0.6 s twtt thick) in the depocenter were accumulated during the last 250 ka at a mean rate of 2.2–2.4 m kyr− 1. Long piston coring in the central Gulf of Corinth basin enabled the recovery of lacustrine sediments, buried beneath 12–13.5 m of Holocene marine sediments. The lacustrine sequence consists of varve-like muddy layers interbedded with silty and fine sand turbidites. AMS dating determined the age of the marine–lacustrine interface (reflector Z) at about 13 ka BP. Maximum sedimentation rates of 2.4–2.9 m kyr− 1 were calculated for the Holocene marine and the last glacial, lacustrine sequences, thus verifying the respective rates obtained by the sequence stratigraphic interpretation. Recent accumulation rates obtained by the 210Pb-radiometric method on short sediment box cores coincide with the above sedimentation rates. Vertical fault slip rates were measured by using fault offsets of correlated reflector Z. The maximum subsidence rate of the depocenter (3.6 m kyr− 1) exceeds the maximum sedimentation rate by 1.8 m kyr− 1, which, consequently, corresponds to the rate of deepening of the basin's floor. The above rates indicate that the 2.2 km maximum sediment thickness as well as the 870 m maximum depth of the basin may have formed during the last 1 Ma, assuming uniform mean sedimentation rate throughout the evolution of the basin. 相似文献
The combined micropaleontological (spores and pollen, diatoms, benthic foraminifers), lithologic, and isotopic-geochemical analysis of sediments from the northern shelf of the Sea of Okhotsk recovered by hydrostatic corer from the depth of 140 mbsl elucidated environmental changes in this part of the basin and adjacent land areas during the last 12.7 thousands cal. years. Geochronological scale of the core is established using the acceleration mass-spectrometry method for radiocarbon dating of benthic Foraminifera tests. The first insignificant warming in the northern part of the sea after glaciation occurred in the mid-Boreal time (9.6 ka ago) but not at the onset of the Holocene. The strongest warming in the region took place in the mid-Atlantic epoch to reach climatic optimum in the second half of the Subboreal (6 to 2.5 ka ago). A cooling in the northern shelf and adjacent land areas is established at the beginning of the Subatlantic (2.5 ka). A comparison of results obtained for Core 89211 with dated hydrological and climatic changes in central and southern parts of the Sea of Okhotsk (Gorbarenko et al., 2003, 2004) is used for a high-resolution analysis of climatic fluctuations in the study region and other areas of the basin during deglaciation and the Holocene. 相似文献