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1.
The St George Group consists of peritidal carbonate rocks deposited on the continental shelf of North America bordering the ancient Iapetus Ocean. These Lower Ordovician rocks are similar to other lower Palaeozoic limestones and dolostones that accumulated in epeiric seas and veneer cratonic areas worldwide. A wide variety of facies in the St George is grouped into seven lithotopes, interpreted to represent supratidal, intertidal and shallow, high- and low-energy subtidal environments. Rapid lateral facies changes can be observed in some field exposures, and demonstrated by correlation of closely spaced sections. The stratigraphic array of these lithotopes, although too irregular to be simplified into shallowing-upward cycles, suggests that they were deposited as small tidal flat islands and banks. Shallow subtidal areas around islands generated sediment and permitted tidal exchange. Tidal flat islands were somewhat variable in character at any one time, and evolved with changing regional hydrographic conditions. The St George rocks suggest an alternative theory of carbonate sedimentation in large, shallow epeiric seas, namely as small islands and banks built by processes that operated in a tidal regime. Furthermore, this island model provides a framework for a mechanism of cyclic carbonate sedimentation, by which small-scale, peritidal cycles represent tidal flat islands that accreted vertically and migrated laterally as local sediment supply from neighbouring subtidal areas waxed and waned during relatively constant subsidence.  相似文献   

2.
Many methods have been used to study wind erosion, a common phenomenon in arid and semi-arid regions. This paper documents a 2006 field study in which sediment samplers and steel pins were used to measure soil erosion, and sand drift potential was used to estimate wind erosion and transport in the eastern part of Ningxia Province, China. Wind energy in the study area was found to be moderate and sediments were transported from northwest to southeast. Sediment transport differed depending on the surface type: sediment transport was greatest in areas of shifting dunes, with cultivated land, semi-fixed dunes, and fixed dunes showing progressively less transport. Sediment transport was lowest in shrubland areas. The depth of erosion was 66 in cultivated areas, 33 in shifting dunes, 28 in semi-fixed dunes, 20 in fixed dunes, and 3 mm in shrublands. The extensive erosion of sediment in cultivated lands and the resultant decrease in surface height results in serious degradation and loss of soil fertility.  相似文献   

3.
Hydrodynamics and sediment dynamics of The Wash embayment, eastern England   总被引：6，自引：0，他引：6  
Water and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 106 tonnes yr?1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ～ 1·4 × 104 tonnes yr?1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals.  相似文献   

4.
The number of bacteria in sediments of a North Carolina salt marsh was determined by direct counts with epifluorescent illumination and acridine orange stain. Cell number decreased from 8.36–10.90×109 cells/cm3 of sediment at the surface to 2.19–2.58×109 cells/cm3 of sediment at a depth of 20 cm. No significant difference was found among four stations located on a transect which crossed the marsh and spanned subtidal to intertidal sediments.  相似文献   

5.
The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1·5 m over subtidal platforms and a mean tidal range of about 1·0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1·0 to 5·0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point‐bar beds do not show sigmoidal geometries, but consist of horizontally‐bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in‐channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point‐bar deposits point out that the curvature‐induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the ‘classical’ fining‐upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally‐bedded platform sediments is interpreted here as due to bar top wave‐winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar ‘apron‐like’ accumulations, where sand accumulated through avalanching processes and mud settled down from suspension.  相似文献   

6.
Two data sets, one from surface sediment samples obtained from a subtidal sand wave and another consisting of three wave-height records representing different oceanographic conditions, are employed to test the application of maximum entropy (ME) and optimal number of class interval (K) concepts. Each data set was modified further to obtain a frequency distribution determined by eight unequal-size class intervals. Both original and modified sets were treated by a reliable statistical method. Comparison of relative entropies and results from statistical treatments show the advantage of the transformation of any original data set by means of ME and K methods before analyzing it further.Contribution 127, Instituto Argentino de Oceanografia.  相似文献   

7.
The history of Quaternary sedimentation in the subtidal Wash is described using high-resolution seismic profiles. The Pleistocene sequence is divided into three depositional units, comprising Anglian till overlain by possible Late Devensian subglacial scour fill and lacustrine sediments. These latter sediments may provide further evidence for a lake in the Wash impounded by ice along the Lincolnshire–Norfolk coast. The Holocene sequence is divided into six depositional units, each truncated by the one above. Estuarine sediment resting on a marine flooding surface forms the earliest unit. This sediment was partially eroded by migration of the shoreface as the marine flooding progressed landward. The following four units comprise sand and gravel banks deposited on the erosion surface. Bank deposition was followed by an episode of tidal scour caused either by increased tidal current velocities following reclamation of the Fenland or by breakdown of postulated former offshore barriers. The youngest and most extensive Holocene unit rests on the scoured surface and comprises several types of deposit. These are: large sand banks around the periphery of the subtidal area with sediment extending seawards into two NE–SW aligned troughs; low sand banks on a central ridge dividing the troughs and partially covering the sediments in the troughs; thick gravels towards the mouth of the Wash; muddy sediments forming drapes over the sand in the centre of the Wash. The data provide information on the variety of processes related to the advance and retreat of Pleistocene ice sheets in eastern England and the subsequent Holocene marine flooding of the Wash–Fenland embayment. The Holocene sequence reveals periods of widespread sedimentation separated by periods of both local and regional erosion, with possible implications for climatic and hydrodynamic change. © 1997 John Wiley & Sons, Ltd.  相似文献   

8.
In order to understand sequence development and sea-level fluctuations during the late Middle Cambrian to early Furongian on the North China epeiric platform, the present study focuses on a unique, subtle erosion surface of an extensive (approx. 100 km), strongly deformed limestone bed in the uppermost part of the Gushan Formation, Shandong Province, China. The Gushan Formation and the overlying Chaomidian Formation consist mainly of shales and a variety of carbonates that were deposited in subtidal environments (e.g., deep subtidal, shallow subtidal, shoreface/shoal, subtidal microbial flat, and restricted platform interior). Three third-order depositional sequences (S1–3) are identified, each of which comprises a thin transgressive systems tract (TST) and a relatively thick highstand systems tract (HST). Each sequence is bounded by a drowning unconformity (SB1), a subaerial unconformity (SB2), or a surface of submarine erosion (SB3). The upper sequence boundary (SB2) of sequence 1 (S1) is represented by a subtle erosion surface of an extensive, deformed limestone bed with a wide variety of soft-sediment deformation structures (e.g., lime mudstone breccias, chaotic wacke-packstone laminae and fragments, homogenized oolites, and clastic dykes), and is overlain by small sporadic microbial buildups and an extensive bioclastic grainstone bed. The deformed limestone was formed during early diagenesis by differential deformation processes (brecciation, liquefaction/fluidization, and injection) which were most likely induced by pore-water overpressure during the period of rapid sea-level fall. Despite the lack of subaerial exposure features (e.g., paleokarst, paleosol, etc.), the characteristics of the erosion surface (cutting well-lithified sediment below), the missing of a significant geological record (the Prochuangia biozone), and the worldwide correlatable positive carbon isotope excursion collectively indicate that the erosion surface developed under conditions of subaerial exposure after contemporaneous marine cementation of the deformed sediment. The missing of the Prochuangia biozone is most likely due to non-deposition at a subaerial hiatal surface. The erosion surface was submerged as a result of subsequent rise in sea level, where sporadic microbial buildups formed under suitable conditions. Freshly deposited, winnowed, shell-dominated transgressive lag deposits (containing Chuangia trilobite fragments, brachiopod shells, and abundant glauconite grains) formed with continued rise in sea level, which became, in turn, overlain by shale-dominated facies. The unique combination of the subtle erosion surface (sensu stricto a subaerial unconformity) and the underlying deformed limestone bed provides an important criterion for recognizing the subtle changes in relative sea level on shallow epeiric platforms.  相似文献   

9.
A numerical model of fine-grained sediment dispersion in the New York Bight of the North American continental shelf is presented. Large amounts of waste material have been dumped in this region and the dispersal patterns of this material are of great interest to environmental managers. The model assumes that fine sediment resuspension is determined by surface wave activity and that transport is determined by tidal currents. Considering surface wave activity to be a random process reduces sediment motion to a random walk which is governed by a diffusion equation. The diffusion equation is solved numerically by an implicit time difference, finite element algorithm for a number of initial conditions. Initial conditions corresponding to ocean dumping sites show patterns of dispersal controlled by the geometry of the study region and the anisotropy of the tides. Material dumped at currently used dump sites reaches sensitive coastal areas before it leaves the continental shelf. Examination of the diffusion coefficients suggests an alternative dump site for fine-grained material. This dump site is relatively near sources of dumped material but produces minimal impact on coastal areas. This site should be considered as a possible alternative to expensive direct disposal at deep ocean sites.  相似文献   

10.
Systematic morphological changes of the coastline of the outer Yangtze River mouth in response to storms versus calm weather were documented by daily surveys of tidal marshes and flats between April 1999 and May 2001 and by boat surveys offshore during this and earlier periods. The largest single event during 1999 to 2001 was Typhoon Paibaian, which eroded the unvegetated tidal flat and lower marsh and led to accretion on the middle-to-upper marsh and in the subtidal channel. The greatest erosion of 21 cm occurred at the border between the marsh and the unvegetated flat due to the landward retreat of the marsh edge during the storm. Strong waves on the flats increased suspended sediment concentration by 10–20 times. On the upper marsh, where the frequency of submergence by astronomical tides is only 3%, Typhoon Paibian led to 4 cm of accretion, accounting for 57% of the net accretion observed over the 2-yr study. Typhoon Paibian led to 4 cm of accretion, accounting for 57% of the net accretion observed over the 2-yr study. Typhoon Paibian and other large storms in the 1990s caused over 50 cm of accretion along the deep axis of the river mouth outlet channel. During calm weather, when hydrodynamic energy was dominated by tides, deposition was centered on the unvegetated flats and lower, marsh with little deposition on the high marsh and erosion in the subtidal channel. Depositional recovery of the tidal flat from typhoon-induced erosion took only several days, whereas recovery of the subtidal channel by erosion took several weeks. A conceptual model for the morphological responses of tidal marshes, flats, and subtidal channels to storms and calm weather is proposed such that sediment continually moves from regions of highest near-bed energy towards areas of lower energy.  相似文献   

11.
A new sediment profile imaging (SPI) instrument, CHEM-SPI, was developed for in situ two-dimensional measurements of biogeochemical solutes using fluorosensor foils in sediments and overlying waters. The CHEM-SPI system was used to simultaneously measure vertical sections of pH, O2, and pCO2 distributions in subtidal, surface deposits of Long Island Sound, NY. Images are readily obtained in 5?C15 min with inexpensive LED excitation and commercial grade digital cameras having typical pixel resolution of ~50?C100 ??m over areas >150 cm2 sediment. Seasonal examples of in situ deployments of the instrument revealed extensive horizontal and vertical heterogeneity of pH distributions. pH dynamics were associated with complex biogenic structures in the upper few centimeters of marine sediment and the pulsed input of organic matter during the spring bloom period. The pH beneath the sediment?Cwater interface was dramatically depressed by the bloom input of organic matter but macrofaunal activity otherwise dominated pH variations in the bioturbated zone. The CHEM-SPI system allows direct quantitative confirmation of biogeochemical patterns previously inferred qualitatively from color patterns in visible SPI images. The instrument is sufficiently adaptable in design to accommodate new optical sensor foils for other chemical variables.  相似文献   

12.
淤长型泥质潮滩双凸形剖面形成机制   总被引：1，自引：0，他引：1       下载免费PDF全文
以江苏中部淤长型泥质潮滩为例,建立了基于过程的潮滩动力地貌演变数学模型,研究了在沿岸潮流作用、供沙充分情况下双凸形潮滩横剖面形成机制。在向岸方向,流速自潮下带至低潮位线附近急速减小,潮间带区域流速则缓慢减小,泥沙在流速急变区域迅速堆积形成上凸点。潮下带中部在小潮期的淤积量大于大潮期冲刷量,总体淤积率较高;加之潮间带中部较弱的落潮流不足以将其上风处底沙掀动并向海输运,导致落潮后期潮下带上部含沙量小、沉积率相对较低,最终在潮下带中部形成下凸点。随着滩面淤长抬升,上、下凸点位置逐步向高、低潮位线附近移动。与前人关于双凸形剖面形成机制的定性分析成果相比,尚有不一致之处,需通过现场观测等进一步探讨。  相似文献   

13.
The relations among geochemical parameters and sediment microbial communities were examined at three shoreline sites in the Prince William Sound, Alaska, which display varying degrees of impact by acid-rock drainage (ARD) associated with historic mining of volcanogenic massive sulfide deposits. Microbial communities were examined using total fatty acid methyl esters (FAMEs), a class of compounds derived from lipids produced by eukaryotes and prokaryotes (bacteria and Archaea); standard extraction techniques detect FAMEs from both living (viable) and dead (non-viable) biomass, but do not detect Archaeal FAMEs. Biomass and diversity (as estimated by FAMEs) varied strongly as a function of position in the tidal zone, not by study site; subtidal muds, Fe oxyhydroxide undergoing biogenic reductive dissolution, and peat-rich intertidal sediment had the highest values. These estimates were lowest in acid-generating, intertidal zone sediment; if valid, the estimates suggest that only one or two bacterial species predominate in these communities, and/or that Archeal species are important members of the microbial community in this sediment.  相似文献   

14.
Deposits from a Middle Weichselian transgression, the Mezen Transgression, are found in coastal sections in the Mezen and Chyorskaya Bays, northwestern Russia. The marine event is bracketed between two ice advances from the Barents and Kara Sea shelves and dated by Optically Stimulated Luminescence (OSL) to around 60 kyr BP. The deposits represent a shallowing upward succession from offshore marine to intertidal coastal environments. Relative sea-level maximum was at least 40 m above the present owing to significant isostatic subsidence. The sedimentary record is dominated by shallow-marine, subtidal deposits bounded below by an erosion surface representing a downward shift in facies and above by subaerial exposure. The succession reflects deposition during forced regression due to isostatic uplift. A rapidly aggrading succession of subtidal deposits at one site suggests a relative sea-level rise or stillstand superimposed on the isostatically controlled sea-level fall. The rhythmic tidal deposits allow identification of semi-monthly to yearly cycles, providing an estimate of the sedimentation rate of 39 cm/year. This implies a high sediment yield and a rapid relative sea-level rise. We correlate this signal with the rapid eustatic sea-level rise at the end of OIS 4 known from deep-sea records.  相似文献   

15.
Tropical coastal seascapes are biodiverse and highly productive systems composed of an interacting mix of habitats. They provide crucial ecosystem services that support people’s livelihoods, yet key components of these seascapes remain unstudied. We know little about the deep (>2 m) subtidal areas of tropical estuaries, because, due to gear restrictions, there have been no detailed studies of the habitats they contain and the fish that use them. Consequently, potentially important functions and linkages with surrounding habitats remain unknown. Using unbaited videos, an approach capable of sampling the full breadth of benthic habitats and whole fish assemblages, we investigated patterns of fish occupancy of the deep subtidal habitats (2–20 m) in one of Australia’s largest tropical estuaries. We identified 19 taxa not previously recorded from estuaries of tropical eastern Australia, along with 36 previously identified estuary taxa. Three recognisable fish assemblages were associated with distinct benthic habitat types: open bottom fine sediment, seagrass and structurally complex rocky areas. In deep water, habitats often overlooked in shallow water become important, and there are sharp differences in habitat function. Deep subtidal habitats are potentially an important zone for direct interaction between estuary and marine fauna, with a range of consequences for intertidal habitat use and nursery ground functioning. The interface between marine areas and the shallow-water estuary may be richer and more complex than previously recognised.  相似文献   

16.
The effect of predation by blue crabs and fishes on all species of infaunal macrobenthos of a subtidal sandy bottom in the York River, Virginia, was investigated by manipulative field experiments. Wire mesh cages were used both to exclude all large predators from and to confine different predators to small areas of the natural bottom. After 2 months, many species showed significant differences between the various treatments. There were similar patterns of species responses in three separate years. Species tended to fit one of two categories. Those species which had tough tubes, which lived deep in the sediment, or which could quickly retract deep into the sediment were shown experimentally not to change much in abundance regardless of whether predators were excluded or included. These species were generally the numerical dominants in the natural community—e.g.Peloscolex gabriellae, Heteromastus filiformis, Spiochaetopterus oculatus, andPhoronis psammophila. Other species which lived near the surface or exposed on the surface responded to experimentally altered predation intensity with large changes in density. These species were either uncommon or only sporadically abundant in the natural community—e.g.Polydora ligni, Streblospio benedicti, Mulinia lateralis, andLyonsia hyalina. This evidence indicates that the abundant species in the natural community are abundant because they avoid predators.  相似文献   

17.
Scientists investigating wetland soil vertical development have required a high precision, high accuracy technique to measure wetland surface elevation change over time. The Surface Elevation Table (SET), capable of millimeter-scale precision, has proven effective in supporting hypothesis-based inquiry into mechanisms of wetland vertical development. More recently, SET sampling stations have been established in many areas to monitor wetland elevation change in an effort to gauge wetland resilience to accelerated sea level rise. Unfortunately, each SET sampling station covers an area of about 1 m2, and the technique was not designed to monitor elevations across broad, hectare-sized spatial scales. The purpose of this study is to investigate an alternative method for obtaining repeated, high-precision measurements of wetland elevation and elevation change that could easily be extended to the hectare scale. This study compares the precision and accuracy of a novel digital barcode leveling technique to the now well-accepted SET for the purposes of measuring wetland surface elevation change. Results suggest that leveling can provide wetland elevations with similar vertical precision as the SET over much larger spatial scales and with an instrument that costs about the same as the SET.  相似文献   

18.
分析了潮滩冲淤监测的几种常见方法,其中标志桩法、SET和RSET法、沉降板法、人工示踪砂法、PEEP和PEEP-3T法、ALTUS和ADV法等主要测量某点或某断面上的冲淤量,3s技术结合地形图或海图实现了大范围潮滩面上的冲淤变化测量。借鉴海洋、海岸实时监测系统的原理和实践经验,提出了潮滩冲淤实时监测系统的设计方案,该系统数据采集子系统包括潮滩高程、景观图像、地物光谱、气象和水动力等因子的数据综合采集,数据通过网络连接传输到实验室,以实现地貌变化和驱动因素的同步获得和综合分析,并通过地物光谱信息获取,为遥感数据解译提供基础资料。  相似文献   

19.
Depositional model for a chenier plain, Gulf of Carpentaria, Australia   总被引：1，自引：0，他引：1  
E. G. RHODES 《Sedimentology》1982,29(2):201-221
The southern coast of the Gulf of Carpentaria, Northern Australia, consists of a broad chenier plain which is comparable in size and morphology to those reported on the coasts of Surinam and south-western Louisiana. The Carpentaria chenier plain has, at some locations, prograded over 30 km since the Middle Holocene by deposition of low-tide muds over subtidal muds during periods of increased sediment input by the rivers. Chenier ridges are formed during periods of reduced sediment flux by wave induced sorting and redistribution processes. An alternation of increased and decreased sediment flux from fluvial sources has occurred several times during the upper Holocene. Such variation in sediment supply is believed to be related to temporal fluctuations in rainfall over the drainage basin which surrounds the southern and eastern Gulf.  相似文献   

20.
The purpose of this study is to investigate the effects of salt marsh geomorphology on the distribution of surface metal content. Surface sediment samples (46) were taken along two transects across an intertidal salt marsh island. Abundance of Ca, Al, Fe, Ni, Cr, Cu, Zn, Rb, Sr, As, and Zr vary up to 300?% with statistically significant differences along transects and between transects. Along transect, metal abundances are strongly influenced by elevation and distance from the subtidal channel. Between transects, salt marsh morphodynamics, relative marsh age, and water sources seem to affect metal distribution. These observations lead us to hypothesize that at least three scales of variability can be expected for assessing salt marsh sediment metal distributions. Larger-scale variations associated with salt marsh sediment sources and age, and another related to local salt marsh geomorphic structure. Therefore, studies that characterize salt marsh metal loading should explicitly consider the variability imposed over a range of spatial scales.  相似文献   

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