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1.
Deriving transport pathways in a sediment trend analysis (STA)   总被引：1，自引：0，他引：1  
Sediment trend analysis (STA) is a technique that enables patterns of net sediment transport to be determined by relative changes in grain-size distributions of all naturally occurring sediments. In addition, STA can determine the dynamic behaviour of bottom sediments with respect to erosion, accretion or dynamic equilibrium. The data requirements for STA are the full grain-size distributions taken from sediment grab samples collected at a regular spacing over a particular area of interest. Two types of methods are presently used in the derivation of the transport pathways: a line-by-line approach, in which transport pathways are determined by searching for sample sequences in which the distributions change, in a statistically acceptable manner; and various vector approaches, in which each sample is compared to neighbouring sites from which a vector sum is calculated.The basic assumption for STA is that the processes that cause sediment transport will affect the statistics of sediment distributions in a predictable way. In reality, this type of analysis is complicated through the inclusion of a number of uncertainties, or noise. The goal of STA is to extract the information (the transport pathways) from the noisy signal (the grain-size distributions), an approach that is made difficult because neither the nature of the information nor the noise is known. Because of this, obtaining results by simply applying its theory in a “black-box” approach may provide poor solutions.The line-by-line approach to STA draws from communications theory to achieve a solution. In some communications systems, the information from many sources is combined into one signal. The resulting signal is, from a statistical viewpoint, nothing but noise. The extraction of specific information assumes that information is indeed present, and determines if that assumption is consistent with the received signal. For STA, the procedure is to assume a transport direction over an area comprising many sample sites. From this assumption, the predicted sediment trends are compared with the pathways determined by the actual samples. The assumed transport direction is repeatedly modified until the best fit is achieved.Vector techniques may be very helpful to provide insights and guidance to the line-by-line approach. However, the approach is ultimately necessary to finalize a credible solution as well as providing important further information on the dynamic behaviour of the sediments, and to delineate specific sediment transport environments.  相似文献   

2.
The subtidal sediments of four coastal environments (beach, berm, surf zone and nearshore sea floor) of Kuala Terengganu have been studied in terms of their textural characteristics and depositional process. Sampling and statistical analyses of the grain size distributions of beach, surf zone and shallow sea-floor sediments at various locations along the coastline show that they varied in their mean grain size as well as values of sorting and skewness. The statistical parameters of grain size distributions thus allowed recognition of distinct sub-environments. The size distribution of such sediments suggests that the various processes that are responsible for their transportation and deposition are wave, current and wind action. The different energy conditions and the morphology of each coastal environment seem to be reflected in the characteristics of the sediments in each environment.  相似文献   

3.
Grain size is a fundamental property of sediments and is commonly used to describe sedimentary facies and classify sedimentary environments. Among the various conventional techniques utilized to determine grain‐size frequency distributions, sieving is the most widely applied procedure. The accuracy of such analyses is, among other factors, strongly dependent on the sieving time. However, despite a substantial amount of research in this field, optimal sieving times for different types of sediments have, to date, not been established. In this article, the influence of sieving time on grain‐size analyses of medium‐grained microtidal and mesotidal beach and dune sands has been determined. To assess the precision of important textural parameters, such as median grain size, sorting, skewness and kurtosis, an error analysis was carried out for different sieving times (2, 5, 10, 15 and 20 minutes). After calibrating the analytical and sampling methodologies, significant deviations were registered when sieving time was less than 10 minutes. However, such deviations were very small and grain‐size distributions remained almost identical for sieving times of 10 minutes and longer, relative errors being as low as 0% in some cases.  相似文献   

4.
Most techniques for estimating settling velocities of natural particles have been developed for siliciclastic sediments. Therefore, to understand how these techniques apply to bioclastic environments, measured settling velocities of bioclastic sedimentary deposits sampled from a nearshore fringing reef in Western Australia were compared with settling velocities calculated using results from several common grain‐size analysis techniques (sieve, laser diffraction and image analysis) and established models. The effects of sediment density and shape were also examined using a range of density values and three different models of settling velocity. Sediment density was found to have a significant effect on calculated settling velocity, causing a range in normalized root‐mean‐square error of up to 28%, depending upon settling velocity model and grain‐size method. Accounting for particle shape reduced errors in predicted settling velocity by 3% to 6% and removed any velocity‐dependent bias, which is particularly important for the fastest settling fractions. When shape was accounted for and measured density was used, normalized root‐mean‐square errors were 4%, 10% and 18% for laser diffraction, sieve and image analysis, respectively. The results of this study show that established models of settling velocity that account for particle shape can be used to estimate settling velocity of irregularly shaped, sand‐sized bioclastic sediments from sieve, laser diffraction, or image analysis‐derived measures of grain size with a limited amount of error. Collectively, these findings will allow for grain‐size data measured with different methods to be accurately converted to settling velocity for comparison. This will facilitate greater understanding of the hydraulic properties of bioclastic sediment which can help to increase our general knowledge of sediment dynamics in these environments.  相似文献   

5.
ABSTRACT It is generally believed that grain-size distributions can be broken down into subpopulations which may be characteristic of a specific mode of transport, some source signature, or the nature of sediment packing in the depositional process. This paper evaluates the variation at a sample site of subpopulation parameters of settling velocity distributions derived using a non-linear least squares algorithm for curve dissection. Results incorporate artificial samples constructed using spherical glass beads in the 125-840 μm range and natural samples collected as close as possible from a single sedimentation unit in marine, fluvial and aeolian environments. If subpopulation distributions from the same sample site are compared then 90% of artificial bead samples and 65% of natural sands have cumulative curves diverging from each other by less than 20%. Similarity in means and standard deviations of subpopulations collected at the same site was not easily obtained. However, if it is important to be able to identify a specific process or source signature correctly, then 86% of all subpopulations from the same natural sample site grouped together. If subpopulations of grain-size distributions are to be used for process identification, it is advisable to analyse more than one sample from a site.  相似文献   

6.
 Bottom sediments were collected from seven Texas reservoirs to assess the spatial distribution of sediment types in reservoirs. The sediment samples were analyzed for water content, organic content, and grain-size distributions. The reservoirs are characterized by distinct water content patterns (referred to in this paper as Types I, II, and III) that reflect the lithologic units comprising their drainage area. The water content, organic content, and percent mud decreases from Type I (shale, marl) through Type II (limestone-marl-sandstone) to Type III (granite-gneiss-schist-sandstone) reservoirs. Reported elemental concentrations in the reservoir sediments are consistent with concentrations in the dominant rocks within their drainage area. While water depth accounts for part of the spatial distribution, sediment source explains the overall distribution of sediment types. Coarse-grained source rocks, multiple source rocks, and multiple tributaries which input sediment at different points contribute to an inconsistent sediment type distribution. Depending on the sediment source, sand and gravel-size sediments are present in the deeper regions of some reservoirs. This disrupts the classical morphometry (water depth, slope) controlled sediment distribution pattern (coarser sediments in shallower regions and finer sediments in deeper regions) of natural lakes. Thus, the location of tributaries and the type of sediments they carry are the dominant factors that control the sediment type distribution in reservoirs. Received: 16 June 1998 · Accepted: 24 August 1998  相似文献   

7.
Genetically meaningful decomposition of grain-size distributions   总被引：2，自引：0，他引：2  
All spatio-temporal grain-size patterns in sediments can be characterized by a mathematical representation of (un)mixing. This implies that an inverse model of (un)mixing would be ideally suited to obtain genetically meaningful interpretations of observed grain-size distributions (GSDs). GSDs are therefore often decomposed into theoretical end members by parametric curve-fitting procedures. Many researchers have been tempted to use goodness-of-fit measures as a means of justifying such decompositions in the absence of generic process-based models of end-member GSDs. A critical examination of parametric curve fitting through a series of numerical experiments shows that the goodness-of-fit of an approximation may be a poor guide to its genetic significance. The genetic interpretation of GSDs is a poorly constrained problem that cannot be solved without taking into account the geological context of GSDs, which may be captured by the covariance structure of grain-size classes across a series of GSDs sampled in a contiguous area. Curve-fitting methods cannot exploit this geological context, which explains why the geological relevance of curve-fitting results obtained in black-box mode is questionable. The desired genetic interpretation of GSDs can be obtained by applying the end-member-modelling algorithm EMMA to a series of GSDs simultaneously. Many end-member GSDs estimated by EMMA do not conform to one of the popular theoretical GSD models. Consequently, parametric curve fitting with theoretical distributions is more likely to obscure than to reveal the existence of genetically significant grain-size populations in sediments, especially if such populations are present in small proportions.  相似文献   

8.
通过对长江口、杭州湾、舟山海域及东海陆架4个海区表层沉积物样品的粒度敏感组分分析,发现东海陆架与长江口具有相似的三级组分物质组成,杭州湾与舟山海域具有相似的两级组分物质组成.综合4个区域粒级组分分布特征,采用湿分法将4个海域表层沉积物样品分成6个粒级:＜0.004 mm,0.004～0.025 mm,0.025～0.063 mm,0.063～0.125 mm,0.125～0.250 mm和＞0.250 mm,分别提取各级组分和全样进行有机碳及同位素测试.定量分析各级组分有机碳含量、来源及物质组分,除杭州湾海域粗粒级外,基本上＜0.004 mm的粘土组分有机碳含量最高;富集在杭州湾海域粗颗粒中的有机质主要来源于陆源植物碎屑.沉积物颗粒大小、物质组成类型是不同粒级有机质富集的主要控制因素.  相似文献   

9.
ABSTRACT A measure of grain shape is needed for incorporation in calculations of the behaviour of grain populations (for example during transport by fluids). Many shape measures have been proposed, most of them for application to single grains rather than to populations. In this paper three such shape parameters are evaluated for samples taken by size fraction from each of three parent sands. The chosen parameters are the maximum projection sphericity of Sneed & Folk (based on triaxial measurements made on the grains), the dynamic shape factor of Briggs (based on settling velocity in water), and rollability, after Winkelmolen (based on rolling behaviour in a specially mounted rotating cylinder).
It is shown that the Sneed & Folk parameter and rollability both discriminate clearly between the shape characteristics of the three sands over the size range 150-500 μm. Moreover the discrimination of the two parameters is mutually consistent. However, dynamic shape factor gives results which for sizes smaller than 300 μm are inconsistent with those of the other two methods and which do not discriminate reliably between the populations. This is inevitable because the differences between drag on spheres and on other shapes become very small at Reynolds Numbers corresponding to those which obtain in settling tests on grains smaller than 300 μm.  相似文献   

10.
 Here we investigate geochemical characteristics of sediment in different compartments of a karst aquifer and demonstrate that mobile sediments in a karst aquifer can exhibit a wide range of properties affecting their contaminant transport potential. Sediment samples were collected from surface streams, sinkholes, caves, wells, and springs of a karst aquifer (the Barton Springs portion of the Edwards (Balcones Fault Zone) Aquifer, Central Texas) and their mineralogy, grain-size distribution, organic carbon content, and specific surface area analyzed. Statistical analysis of the sediments separated the sampling sites into three distinct groups: (1) streambeds, sinkholes, and small springs; (2) wells; and (3) caves. Sediments from the primary discharge spring were a mix of these three groups. High organic carbon content and high specific surface area gives some sediments an increased potential to transport contaminants; the volume of these sediments is likely to increase with continued urbanization of the watershed. Received: 13 April 1998 · Accepted: 6 October 1998  相似文献   

11.
Some of the methods used in the resolution of mixed normal distributions are discussed under three headings: analytical, graphical, and numerical methods. Attention is given to their applicability in the analysis of grain-size data as derived from sieving. Comparisons are made by applying several methods to published data. It is concluded that the numerical methods offer most scope, especially the method of nonlinear least squares. Some analyses of beach sediments, using this method, are presented. The adoption of a convention for the number of individuals in the sample increases ease of interpretation.  相似文献   

12.
Laser diffraction is now widely used for particle size distribution analysis of sediments and soils. The technique can be very precise, and offers advantages of speed and cost over many other methods when used to analyse mixtures of sand, silt and clay. This study presents the results of an experimental investigation that examined the sensitivity of the Beckman-Coulter LS230 instrument to mixtures of different grain populations and differences in particle shape. The instrument was found to have high sensitivity to coarse particles in a finer matrix (detection threshold 1–2%), but much lower sensitivity to finer particles in a coarser mixture (detection threshold 12–17%). Experiments using near-spherical ballotini showed that laser analysis provides very similar values to dry sieving for the mean, median and mode, but for a range of natural sand samples values for the mean, median and mode were offset by 8–21%, with an average of ca 15%, compared with sieving. Analysis using a Beckman-Coulter RapidVUE instrument, which provides both size and shape information, provided support for the hypothesis that the differences between laser analysis and sieving are partly attributable to the effects of particle shape. However, an additional factor is the way in which the laser software interprets the optical diffraction data. The software predicts a high degree of log-normality in the size distribution, such that highly skewed, truncated or bimodal samples are poorly represented. Experiments using sieved fractions of ballotini indicated that, even with near-perfectly spherical particles, the particle size distribution predicted by the laser software includes a relatively large percentage of particles outside the sieve class limits.  相似文献   

13.
Settling rates of natural sand-size particles are influenced, to some extent, by their shapes and this may be an important factor in using settling rates to estimate grain size. In order to gauge the sensitivity of this influence, two natural sand populations from the Mesozoic Nubian Sandstones of Southern Israel, with a high probability of being similar in their bulk shape characteristics, were examined in ¼φ sieved fractions for their shape characteristics and settling rates. Fine surface features (roundness and surface roughness) were evaluated using Fourier shape analytical methods. Significant differences in bulk shape were detected and their influence on settling rates was measured empirically in a settling tube. The most marked differences were in the coarse grain sizes and, to a lesser extent, in the intermediate sizes. Sampling of raw settling data at closely-spaced time intervals yielded high-resolution grain size frequency plots which were usually polymodal in nature. Subtle shape contrasts, which are an important influence on settling rates, are thus an important consideration when working at this level of sensitivity. Natural sand populations which have followed a more varied provenance or process pathway could be expected to have even greater contrasts in settling rates than the samples analysed here. Thus it is recommended that the bulk shape factor should be taken into account in order to minimize errors in the conversion of settling times to grain size. An easy method, outlined in this paper, is through the establishment of an empirically derived calibration curve for each individual suite of sand undergoing analysis. Sieved ¼φ samples, derived from a split of the total composite sample undergoing analysis, forms the basis of the calibration and hence a correction factor converting sieve diameters to true diameters must be applied. In this research, nominal section diameters were obtained optically through an image analyser.  相似文献   

14.
Previous attempts to deduce the flow parameters of turbidity currents from their deposited sediments have focused on applications of criteria for the suspension or autosuspension of the grains and on hydrodynamic interpretations of the characteristic Bouma sequence of sedimentary structures. There has been a considerable diversity of opinion, however, as to how the transport criteria relate to the observed deposited grain-size distribution, and no attempt has been made to determine whether the separate analyses of deposited sediments and sedimentary structures agree as to the magnitudes of the evaluated flow parameters. Such analyses are performed on a turbidite from the Capistrano Formation (Miocene-Pliocene) of California. This turbidite is normally graded from medium sand at its base to very fine sand and silt at the top, and has the complete sequence of Bouma structures. Only a small degree of cementation has occurred so that samples from the layer could be disaggregated and grain sizes determined both by sieving and sedimentation balance analyses. It was decided to employ the grain-suspension criterion for the calculation of the flow conditions at the time of deposition, published experiments on the pipe-flow of suspensions having demonstrated that this criterion is one of deposition versus non-deposition of grains according to their settling velocities. The published work relating types of sedimentary structures to the sediment grain size and either the flow power or Shield's dimensionless stress is used to evaluate the flow parameters from the observed Bouma sequence. All methods employed yield estimates of the mean flow velocity and bed stress (force per unit bottom area). The evaluated flow parameters for the transition from a flat bed to ripples (Bouma B to C divisions) are nearly an order of magnitude greater than obtained from the grain-suspension criterion where the calculations are based on the median grain size of the deposited sediments. Agreement results only if the calculations utilizing the suspension criterion are based on nearly the coarsest grains deposited at any instant, a procedure that is difficult to justify with the expected sediment deposition from a waning current. Inclusion of other factors, such as possible lags in sediment deposition or ripple formation beneath the decelerating flow, provide no firm explanation as to the cause of the discrepancy between the results based on the two approaches, and in nearly all cases their inclusion would increase the difference. Something is amiss with our procedures for the hydraulic interpretation of sediments which can be resolved only by further study.  相似文献   

15.
High-resolution records of the natural radionuclide230Th were measured in sediments from the eastern Atlantic sector of the Antarctic circumpolar current to obtain a detailed reconstruction of the sedimentation history of this key area for global climate change during the late Quaternary. High-resolution dating rests on the assumption that the230Thex flux to the sediments is constant. Short periods of drastically increased sediment accumulation rates (up to a factor of 8) were determined in the sediments of the Antarctic zone during the climate optima at the beginning of the Holocene and the isotope stage 5e. By comparing expected and measured accumulation rate of230Thex, lateral sediment redistribution was quantified and vertical particle rain rates originating from the surface water above were calculated. We show that lateral contributions locally were up to 6.5 times higher than the vertical particle rain rates. At other locations only 15% of the expected vertical particle rain rate were deposited.  相似文献   

16.
《Sedimentology》2018,65(5):1761-1776
Grain shape plays an important role in textural analysis of sedimentary grains. Textural analysis helps to determine the formation, transportation and deposition processes of sedimentary rocks. However, there is a lack of standardized methodology for quantitative characterization of grain shapes. The utility of fully automated image analysis for grain shape measurement is assessed in this paper. This research aimed to identify the most useful shape parameters for textural characterization of populations of grains and determine the relative importance of the parameters. A key aspect of this study is to determine whether, in a particular sedimentary environment, textural maturity of the samples can be ranked based on their grain shape data. Furthermore, discrimination of sedimentary depositional environments is explored on the basis of grain shape. In this study, 20 loose sediment samples from four known depositional environments (beach, aeolian, glacial and fluvial) were analysed using newly implemented automatic image analysis methods. For each sample, a set of 11 shape parameters were calculated for 200 grains. The data demonstrate a progression in textural maturity in terms of roundness, angularity, irregularity, fractal dimension, convexity, solidity and rectangularity. Furthermore, statistical analysis provides strong support for significant differences between samples grouped by environment and generates a ranking consistent with trends in maturity. Based on novel application of machine learning algorithms, angularity and fractal dimension are found to be the two most important parameters in texturally classifying a grain. The results of this study indicate that textural maturity is readily categorized using automated grain shape parameter analysis. However, it is not possible to absolutely discriminate between different depositional environments on the basis of shape parameters alone. This work opens up the possibility of detailed studies of the relationship between textural maturity and sedimentary environment, which may be more complicated than previously considered.  相似文献   

17.
近些年,定量化火成岩结构研究表明,利用常规的岩矿鉴定设备,获取不同尺度的火成岩二维岩相学照片,通过肉眼识别矿物颗粒,并借助图像处理和结构分析软件,可以准确地量化火成岩的结构特征。本文结合近些年国内外同行的研究成果,对火成岩二维定量化结构分析方法中常用的多种观测方式优缺点进行了总结。粒度在毫米级以下的火成岩的定量化结构参数,可以用偏光显微镜下的透射光、反射光、阴极发光和电子探针背散射成像中的两种或两种以上观测方式进行分析,并具有较高的精度和准确度。粒度小于0.03 mm的各种镁铁质矿物可用反射光和背散射图进行分析,灰度近似的镁铁质矿物可以利用图像处理软件赋予不同的彩色,提高颗粒间的辨识度。常规偏光显微镜下不易区分的长英质矿物和多数副矿物可用偏光显微镜阴极发光进行分析。粒度在毫米级以上的造岩矿物可以用光片或野外测量的方式进行定量分析。为了方便相关领域学者使用火成岩二维定量化结构分析方法,本文详细列出了具体的分析步骤,并结合一个玄武岩样品中的橄榄石斑晶数据结果,重点分析以下4个方面的问题:(1)如何准确识别矿物颗粒边界;(2)矿物含量和形态的确定;(3)分析区域面积和颗粒数的确定;(4)不同晶体群的区分。分析结果表明,颗粒数100~500颗时,晶体粒度分布(CSD)的截距和斜率、矿物含量、定向程度和粒状矿物的三轴比在误差范围内没有显著区别,但颗粒最大长度和聚集程度会被低估。当颗粒数小于300颗时,晶体空间聚集程度的R值会被高估0.05~0.2,这一点在以往的研究中没有得到充分重视。当颗粒数大于500颗时,所有结构参数都趋于稳定,且精度和准确度都会显著提高。目前多数研究者提供的结构参数往往与观测和统计方式有关,缺乏对应的原始数据,不方便同行间的对比研究,建议学者今后发表相关成果时,提供详细的分析步骤和最原始的数据。分析步骤重点说明包括:(1)聚集矿物边界的识别和处理方式;(2)晶体三维形态的确定方法,样品间CSD参数的变化是否是由形态参数变化引起;(3)能够准确识别的矿物颗粒最小粒度;(4)利用颗粒数较多的样品选取较小的不同区域重复分析3到5个不同区域,评估样品的均一性,并据此估计样品的分析精度。原始数据方面包括:(1)提供同一个样品至少一个不同区域的分析结果,如果是多个作者的研究成果,建议提供至少两人独立分析的结果用来评估数据的精度和准确度;(2)文章正文或附件中应该提供每个样品不同粒度间隔的颗粒数,样品原始的高分辨率矿物轮廓描绘图或图片分析的相关原始参数。火成岩出现复杂晶体群时,定量化的结构参数往往体现的是多种晶体群的混合特征,并且与不同晶体群的比例有关。未来的研究需要结合多种观测方式和微区成分分析重点识别不同晶体群的结构参数,对粒度和成分近似的多种晶体群的识别,还需要开发更多有效的方法,这对准确认识火成岩结构多样性的成因和岩浆作用过程都有重要意义。  相似文献   

18.
The grain-size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments. The existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain-size fractions in the transport layer under given hydraulic conditions. The efficiency of these models was tested with the observed data. Subsequently, the patterns of observed grain-size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions (pdf), such as, log-normal, log-hyperbolic and log-skew-Laplace. Tests indicated that a log-skew-Laplace distribution fitted best for 49%, a log-hyperbolic for 31%, and a log-normal for 20% out of forty-five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness. The results of this study would be useful to specify the grain-size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments.  相似文献   

19.
The autocorrelation technique for estimating grain-size from digital images of sand beds has been extended and validated for use on coarse sand (0·7 mm) and gravel (up to ∼20 mm). A number of aspects of the technique have been explored and some potential improvements suggested. Autocorrelation is just one suitable statistical method sensitive to the grain-size of sediment in digital images; four additional techniques are presented and their relative merits discussed. A collective suite of techniques applicable to the general problem of grain-size estimation from digital images of sediment might broaden the applicability to more sedimentary environments, as well as improve its accuracy. These techniques are compared using a large data set from a gravel barrier beach in southern England. Based on over 180 samples, mean grain-size of sieved and imaged sediments correspond to within between 8% and 16%. Some theoretical aspects of the spatial arrangement of image intensity in digital images of natural sediments are addressed, including the fractal nature of sediments in images, which has potential implications for derivation of grain-size distributions from images of sand-sized material through segmentation and thresholding. The methods outlined in this contribution may also find application in further uncovering the geometric structure of sediment beds, as well as in the simulation of sedimentation processes.  相似文献   

20.
Grain-size parameters derived from the mechanical analysis of acid-insoluble residues from mixed terrigenous-shell beach sediments from Auckland, New Zealand, are more effective for interpreting the depositional processes, and for characterizing the beach environment on textural scatter plots, than are parameters based on the analysis of the total beach samples. This results mainly from the wide variations in size of hydraulically equivalent skeletal carbonate grains in the sediment, especially in the coarser size grades, but may also be accentuated by the local origin and susceptibility to alteration of the shell material. Compared with the total sediment grain-size distribution, the insoluble residues from the beach deposits are characteristically finer-grained, better sorted, and more consistently coarse-skewed and leptokurtic. The grain-size distribution characteristics of insoluble residues from several stratigraphic units in a sequence of Oligocene mixed terrigenous-skeletal shelf sediments in the South Auckland region distinguish depositional mechanisms and environmental energy levels for each unit that are consistent with interpretations made on the basis of carbonate petrography and palaeontology. With regard to the bulk content of terrigenous mud in the insoluble residues, the grain-size distribution of the fraction coarser than 4φ is alone diagnostic of the energy-time trends in these sediments. The grain-size parameters of the acid-insoluble residues in modern and ancient mixed terrigenous-skeletal carbonate sediments may provide more reliable criteria for distinguishing and characterizing the depositional environment of these deposits than do the parameters obtained from the size distribution of the total grain population.  相似文献   

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