摘要:

ABSTRACTBagnold's sediment transport equation has proved to be important in studying tidal marine environments. This paper discusses three problems concerning Bagnold's transport equation and its practical application:1Bagnold's suspended‐load transport equation and the total‐load transport equation with are incorrect from the viewpoint of energy conservation. In these equations the energy loss due to bedload transport has been counted twice. The correct form should be for suspended‐load transport and for total‐load transport with2The commonly used Bagnold's transport coefficientKvaries as a non‐linear function of the dimensionless excess shear stress, which can be represented best by the power law , where the coefficientAand exponentBdepend on sediment grain sizeD. The empirical values ofAandBfor fine to medium grained sands are determined using Guyet al.'s (1966) flume‐experiment data.3The sediment transport rates predicted from this equation are compared with bedform migration measurements in the flume and the field. This comparison shows that the sediment transport rates measured from bedform migrations are higher than the predicted bedload transport rates, but comparable to the calculated total‐load (bedload plus intermittent suspended‐load) transport rates. This indicates that bedform migration involves both bedload and intermittent suspended‐load transport. As a logical conclusion, bedform migration data should be compared with Bagnold's total‐load transport equation rather than with his bedload transport equation. In this respect the term ‘bed material’ might be more appropriate than the term ‘bedload’ for estimating sediment transport rate from bedform migration data.The sediment transport rates predicted from this modified Bagnold transport equation are in good agreement with field measurements of bedform migration rates in four individual tidal marine environments, which cover a wide range of sediment grain size, flow velocity and bedform conditions (ranging from small ripples

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00755.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTA numerical model describing two‐dimensional, shallow water, long wave propagation has provided estimates of the co‐oscillating palaeotides in the upper Devonian Catskill Sea. This sea was open to the ocean along the present Gulf Coast and, moving clockwise, was circumscribed by the Transcontinental Arch, the Old Red Sandstone Continent, and the rising Appalachian Orogen. Using the present best estimates of basin bathymetry and open‐ocean tidal range, the model indicates high mesotidal to low macrotidal ranges for the Catskill Shelf along the ancestral Appalachians. The Mid‐Continent Shelf, centred on Iowa, is considered to have been microtidal. These conclusions are not sensitive to reasonable variations in shelf geometry and support the hypothesis that at least some epicontinental seas could have been tide‐

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00756.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTBathonian algal limestones from the Duntulm Formation of the Great Estuarine Group, western Scotland, are subdivided into three microfacies: cryptalgal laminites, thrombolites and porostromate‐micrites. The distribution of early diagenetic, carbonate spherulites and cement fringes is fabric selective. Spherulites are common in the thrombolitic microfacies, whereas cement fringes were best developed upon algal nodules with porostromate microstructure. Syndiagenetic micrites, spherulites and cement fringes have stable carbon isotopic ratios indicative of abundant, organic‐derived HCO3incorporation (δ13C values of −0·05 to −15‰). Enhanced magnesium concentrations in these fabrics, relative to neomorphic and burial‐diagenetic spar cements, suggest an original Mg‐calcite mineralogy, probably with precipitation under the influence of microbial systems. Brecciated, early‐diagenetic fabrics and localized pockets of calcite pseudomorphs after gypsum were probably formed during subaerial exposure. Sulphate supply was probably from storm‐derived, saline lagoon‐water washed on to supralittoral stromatolitic flats. These inundations also allowed the temporary establishment of green algae and encrusting foraminifera. The combined geological and geochemical information suggests a palaeoenvironment somewhat similar to Recent, sub‐tropical humid zone, c

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00757.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTA considerable portion of the upper member of the Mid‐Proterozoic Newland Formation, Meagher County, Montana, consists of carbonaceous silty shales (striped shale facies). This type of shale facies is common in Proterozoic basins and is host to several major base metal deposits. The striped shales were deposited in a subtidal setting, basinward of carbonates characterized by cryptalgal laminites, mudcracks and flat pebble conglomerates. The carbonaceous silty shales are considered remnants of benthic microbial mats. Irregular internal laminae, patterns of particle trapping, mechanical deformation during penecontemporaneous soft‐sediment deformation and filamentous microbiota provide evidence for this interpretation. The dolomitic clayey shale contains graded silt‐clay couplets, and these are interpreted as storm layers. Modern subtidal microbial mats can only survive under special conditions, but in the Proterozoic, it is suggested that benthic microbial mats colonized the shallow seafloor during periods of low sediment input, leading to the formation of carbonaceous s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00758.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe mechanisms of berm development along a microtidal‐high energy beach is examined. Such a beach with medium‐sized sand and monsoon wave‐controlled profile at Valiathura, south‐west coast of India, is selected for this study. The waves which very rarely fall below 1 m, often exceed 4 m during the monsoon period of May to October. The erosion‐accretion pattern of the beach shows a cyclicity and the berm development is mainly due to the onshore migration and welding of longshore bars on to the beach following the monsoon rough season. The stages of berm development in the present microtidal beach are more or less similar to the model presented by Hine for a mesotidal case, except for the following intermediate additional stages. The longshore bar develops due to the erosion of beach when the wave steepness was above 0·04, gets flattened when it falls below 0·04, and then reforms nearer to the shoreline as a swash bar. This reformed bar gets divided and the inner bar gets welded on to the beach, followed by the outer bar developing the berm. During the onshore migration of the longshore bar and berm development the beach face becomes partially reflective with the surf scaling parameter, εbbetween 2·5 and 33. The inshore is dissipative with the inshore surf scaling parameter, εs≫33. The offshore side of the longshore bar is partially reflective with its surf scaling parameter, εbarbetween 2·5 and 33. The breakers are spilling or plunging. Vertical growth of the berm is mainly due to the changes in swash‐limit caused by the variations in wave steepness, breaker height and type. Vertical growth stops when the beach‐face attains equilibrium with the grain size‐wave energy relationship, and a wave steepness below 0·02 he

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00759.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTA new method for analysing observed aeolian sand transport rate profiles of the kind obtained by Williams is presented. The method involves a mathematical model of aeolian saltation. Detailed information about the saltation process can be calculated from the transport rate profile by means of this model. The method is used to perform a re‐analysis of Williams' trap data. Among the main findings of this analysis is that the grain borne shear stress appears to be a smaller fraction of the total shear stress than assumed by Bagnold&Owen in their theories of aeolian saltation. Other findings are that the probability distribution of the jump height of the grains does not depend much on the wind speed once the saltation is established, and that the vertical component of the mean launch velocity decreases with the grain size. It is approximately inversely proportional to the grain diameter. Our estimates of the landing angles indicate that estimates of the impact angles obtained from photographically recorded trajectories are too small due to biased sampling. The influence of grain shape on the transport characteristics is mainly due to changes in the grains' ability to jump when hitting the bed. It is found that angular grains have a lower mean jump height than spherical grain

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00760.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTSeveral infilled glacial meltwater channels of Anglian age are described from SE Suffolk, U.K. These channels are up to 4km long, 150m wide and 8m deep. They possess a consistent pattern of infill comprising glacifluvial sands and gravels overlain by subglacial water‐lain fine sands and silts; these pass conformably upwards through thinly interstratified tills and sands into basal tills. The channels were formed at the ice‐margin. Their sedimentary infill indicates overriding by the ice‐sheet and subsequent subglacial depos

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00761.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTReplacement of originally aragonite mollusc shells by pyrite commonly occurs in the Lower Oxford Clay. Petrographic studies show the shells to have constituted complex microenvironments in the sediment. A range of replacement textures is found showing a variable amount of solution of the original aragonite. Three distinct textures were found in crushed pyrite‐replaced ammonite shells from heavily pyritized concretions.(1) A texture reflecting the original shell structure due to the replacement of the organic shell‐matrix by pyrite.(2) An ovoid texture seen at several stages of replacement reflecting processes occurring at discrete centres of sulphate reduction.(3) Euhedral crystals lining cracks and fractures in the shell.Three types of replacement are found in small gastropods and bivalves from shell bed, some of which may relate to those seen in the ammonites.(1) Replacement of organic shell‐matrix by pyrite preserving good shell‐microstructure.(2) Replacement showing outwardly good preservation of morphological features but inwardly only the gross structure, such as growth lines, is preserved.(3) Replacement of the shell in a matrix of euhedral pyrite leaving only lines of carbonate inclusions marking the margins of the shell.The replacement textures and types appear to be dependent on the initial structure of the shell and the access of iron and sulphate into the shell.Early stages of replacement appear to proceed by pyrite formation within the organic matrix of the shell, with little or no solution of the carbonate, this produces textures which faithfully mimic the original shell microstructure. It is thought that the lack of carbonate solution is due to a limited availability of iron, brought about by the less intensively reducing nature of the sediment. Later stages of replacement are promoted by the cracking and fracturing of the shell and are, generally, not as faithful to the original shell structure. This is due to the greater availability of iron as the sediment becomes more reducing with

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00762.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTCement‐porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high free‐surface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small ‘v’‐shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore‐filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00763.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00764.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY