The hydraulic interpretation of turbidites from their grain sizes and sedimentary structures

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.     

美国内布拉斯加州普拉特河河床沉积物渗透系数的现场测定

竖管测定法可现场测定河床的渗透系数.采用人工梯度法和自然梯度法两种方法,对普拉特河位于卡尼市(KearneyCity)东南河段河床的垂直、水平和任意方向的渗透系数进行了野外现场测定.10个测点的垂向渗透系数的平均值为30.51m/d,一个测点的水平方向渗透系数为97.2m/d,其各向异性比率约为3.结果表明:普拉特河床沉积物剖面上具有一定的各向异性,平面上为非均匀介质.     

Statistical and geostatistical features of streambed hydraulic conductivities in the Platte River, Nebraska

This paper presents streambed hydraulic conductivities of the Platte River from south-central to eastern Nebraska. The hydraulic conductivities were determined from river channels using permeameter tests. The vertical hydraulic conductivities (K _v ) from seven test sites along this river in south-central Nebraska belong to one statistical population. Its mean value is 40.2 m/d. However, the vertical hydraulic conductivities along four transects of the Ashland test site in eastern Nebraska have lower mean values, are statistically different from the K _v values in south-central Nebraska, and belong to two different populations with mean values of 20.7 and 9.1 m/d, respectively. Finer sediments carried from the Loup River and Elkhorn River watersheds to the eastern reach of the Platte River lowers the vertical hydraulic conductivity of the streambed. Correlation coefficients between water depth and K _v values along a test transect indicates a positive correlation – a larger K _v usually occurs in the part of channel with deeper water. Experimental variograms derived from the vertical hydraulic conductivities for several transects across the channels of the Platte River show periodicity of spatial correlation, which likely result from periodic variation of water depth across the channels. The sandy to gravelly streambed contains very local silt and clay layers; spatially continuous low-permeability streambed was not observed in the river channels. The horizontal hydraulic conductivities were larger than the vertical hydraulic conductivities for the same test locations.     

Assessing internal erosion with the modal decomposition method for grain size distribution curves

Acta Geotechnica - The modal decomposition method (MDM) is used to analyse the grain size distribution curve (GSDC) of a soil that was likely to produce internal erosion according to old criteria,...     

Interpretation of grain-size distributions from measured sediment data, Platte River, Nebraska

Breaks in the slope of log-probability plots of cumulative grain-size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south-central Nebraska. The break between suspension and intermittent suspension as determined from the bed-material curve coincides with the upper limit of the grain-size overlap between bedload particles and suspended-sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended-sediment particles. Although grain-size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended-sediment distributions increases with increasing discharge. The limits of grain-size overlap of bedload and suspended-sediment distribution curves associated with near-flood discharges most closely approximate the breaks in the bed material grain-size distribution.     

Sediments grain size and geo-chemical interpretation of three successive cutoff meanders of the Danube Delta,Romania

The paper presents the results of granulometric and geochemical analyses of the superficial bed sediments of the St. George branch, one of three Danube River distributaries. Three cutoff meander reaches (the Mahmudia, Dunavăţ de Sus (Upper Dunavăţ) and Dunavăţ de Jos (Lower Dunavăţ) meanders, noted here as M, DS and DJ meanders) was selected as an example to analyse the impact of the meanders rectification on the grain size and sediment quality. The cut-offs of the meanders by navigational canals since 1984–1988 caused dramatic changes in the distribution of velocities, discharge and sediment fluxes (Popa, 1997; Panin, 2003, Tiron and Provansal, 2010, Tiron Dutu et al., 2014). The correction of the studied meanders induced different hydro-sedimentary impacts: strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander in two cases (Mahmudia and Dunavăţ de Jos meanders) and slight modifications in Dunavăţ de Sus meander, where the natural channel still keeps almost its initial conveyance capacity. Sediment samples were acquired throughout several cross-sections of both natural and artificial canals of the three rectified meanders, in order to investigate the bed sediment characteristics. Grain size parameters (such as Median, Standard deviation, Skewness) show the predominance of fine fraction (silt and medium/ fine sand), relatively good sorted depending of the sample location (on the natural course of the branch, on the rectification canals or the former meanders. The statistical analysis of chemical data evidenced a high compositional variability, with concentration variation coefficients ranging between 27.4% – Sr and 178.3% – TOC (Total Organic Carbon). Generally, the highest concentrations of terrigenic compounds appear in the samples collected from the rectified natural canals. These results are validated by comparison with aDcp measurements made on each investigated profile.     

Types of geothermal curves and their interpretation

On the basis of an analysis of thermometric data from deep drill holes in Western Kazakhstan, characteristics of types of thermal curves (linear, convex, concave, etc.) are recognized from the nature of the change in the geothermal gradient. Angles of slope of the clusters of the thermal curves and their position on the graph are determined by the degree of the principal regions of supply and the heat flow from depth, the geologic construction, the lithologic-facies composition of the rocks, the hydrogeologic conditions, etc. Graphs of the clusters of thermal curves enable us to provide a comparative estimate of the effect of these factors OQ the geothermal regime of Individual sectors of the region, and to trace the change in depth of occurrence of the isothermal surfaces as well as the distribution of temperatures in various depth intervals. —Author.     

Geoarchaeology and late quaternary geomorphology of the middle south Platte River,Northeastern Colorado

Investigations were conducted along the middle South Platte River to better define the geomorphic contexts of Paleoindian sites and to reconstruct the alluvial and eolian geochronology. Paleoindian sites are associated with the Kersey terrace (the downstream equivalent of the Broadway terrace). The Kersey alluvium was deposited during Clovis occupation and the surface stabilized by 10,000 B.P. Post-Clovis sites post-date aggradation and stream downcutting may have started as early as 10,500 B.P. Subsequent floodplain development and downcutting formed the Kuner terrace (the possible downstream equivalent of the Piney Creek terrace) no later than 3000 B.P. and the Hardin terrace probably within the last 1000 years. Soils on the Kersey terrace are Ustochrepts (gravelly alluvium) or Haplustalfs (sandy and clayey alluvium). Soils on the Kuner terrace are cumulic Ustorthents and Ustochrepts. Soils on the Hardin terrace are Ustorthents with no obvious horizonation. Eolian sands began accumulating in the region by 10,000 B.P., but most are probably late Holocene deposits and are indicative of drier post-Pleistocene climate. Correlations with deposits in low order tributaries and other drainages can be difficult to make a) using soils because soil development varies as a function of parent material texture and b) because aggradation and degradation may be out-of-phase.     

从沉积速率和沉积物粒度看冰后期海侵以来珠江三角洲西江大鳌沙的形成

大鳌沙处于珠江三角洲西江河口的近口段,洪季西江河口的潮区界西线在其顶端附近。冰后期海侵以来,大鳌沙顶端的PRD05和中部的PRD04两个钻孔的沉积速率和沉积物粒度有着极大的差别。7 630 a BP以前,PRD04孔的沉积速率小于PRD05孔的沉积速率;但7 630 a BP以来,PRD04孔的沉积速率大于PRD05孔的沉积速率。从粒度分析看,埋深12.28 m以下,PRD04孔沉积物比PRD05孔的粗,分选性比PRD05孔的差;埋深12.28 m以上,PRD04孔的沉积物比PRD05孔的细,分选性相对比PRD05孔的好。分析表明,9 000～4 200a BP, 沉积物由南（中部）向北（顶部）输运,涨潮流冲蚀老沉积物,在喷出磨刀门通道后,流速迅速降低,粗颗粒泥沙先沉积,而细颗粒泥沙被涨潮流搬运至更北的区域沉积。近3500年以来,河流动力占优势,沉积物由北（顶部）向南（中部）输运。大鳌沙的形成与涨潮射流密切相关。涨潮射流口的位置在蛇地山和右岸纵向山地之间,宽约2200 m。     

Bed-thickness and grain size of turbidites

In measured sequences of limestone- and greywacke-turbidites the bed-thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed-thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain-size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed-thickness on maximum grain size is well defined and resembles a fall-velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee-forming processes.     

淮北平原中更新世以来古气候和沉积环境演变：来自涡河流域沉积物粒度和地球化学的证据

通过对毗邻中国南北地理分界线的第四系标准孔岩芯进行粒度和地球化学研究，探讨了淮北平原中更新世以来的古气候变化和沉积环境变迁。研究表明淮北平原中更新世气候在暖湿与干冷(温凉）频繁交替中总体经历了从暖湿趋于干冷，与中国东部季风区北部的温带季风气候变化趋势属同相位演变，而与受控于亚热带季风气候的长江中下游及淮河源区古气候演变模式总体呈反相位。晚更新世，气候总体经历了干冷(即温干—严寒—干冷）、温湿、转向干冷，却表现出与以长江中下游及淮河源区气候演化格局更相似，而与中国北方气候总体上由暖湿向干旱化转变步调不一致。进入全新世，淮北平原与中国南北方气候均回暖，降水相应增多。涡河流域GBK1标准孔区域沉积环境总体上经历了中更新世早中期从半深湖演化到浅湖、滨湖，到晚期湖泊快速收缩直至消失，演化为泛滥平原或河流；晚更新世以来主要为河流和泛滥平原。淮北平原区域气候变化总体上对全球变化有着积极的响应，或许因其地处中国南北地理分界线边缘，致使其气候又具区域独特性。     

Numerical models of hydraulic fracturing and the interpretation of syntectonic veins

The hydraulic fracture is modelled as an ellipse in an infinite elastic medium with an internal fluid pressure and loaded under biaxial stresses at infinity. The available stress function for this model has been evaluated numerically, and the magnitudes of the stresses generated around the crack calculated for a variety of loading conditions and crack orientations. Fracture initiation is predicted from the Griffith maximum tensile stress criterion. The location of the maximum tensile stress around the crack is recorded and it is found that for many conditions of applied stresses and crack fluid pressure, the hydraulic shear fracture has a symmetrically developed maximum tensile stress and fracture initiation will occur by growth along the direction of the crack. It is also predicted that fracture initiation will occur when the ratio of fluid pressure to applied least principal stress is considerably less than one. The elastic strain energy fields around elliptical hydraulic flaws have been calculated, and in particular, the change in strain energy upon introduction of a small flaw, and the change in strain energy upon growth of this flaw, have been investigated. The results allow an evaluation of the second part of the Griffith criterion-that fracture growth is accompanied by a decrease in strain energy-for hydraulic fractures. Changes in strain energy with small increases in fluid pressure provide a physical basis for dilatancy hardening and fracture instability. Quasi-static growth from a flaw is modelled by calculating changes in strain energy for unit increases in half length. The distinction between fractures which show an increasing and a decreasing rate of change in strain energy with increasing length, and between fractures which may only extend spontaneously for short distances and those which may show extensive spontaneous growth on the basis of the rate of change of strain energy with length, is made. A gradual drop in crack fluid pressure once the threshold for fracture initiation has been passed may promote the extent to which spontaneous crack growth occurs.The formation of syntectonic veins, particularly in rocks being deformed under low grade metamorphic conditions, is often the most abundant evidence of natural hydraulic fracturing in rocks. Commonly observed geometric features of syntectonic veins-length, simple tapering, symmetric and asymmetric forking, branching, irregular zig-zag traces, en échelon patterns—are discussed primarily with reference to the strain energy model for growth established, and the geometric variation is interpreted in terms of variation in applied stress and fluid pressure conditions and the rate of change of stored strain energy with crack growth. In particular, terminal branching arises when the minimum stress changes from a symmetric to an asymmetric location at the tip of a growing shear fracture, and terminal forking results when there is an increase in the energy release rate during crack growth, and may be symmetric or asymmetric depending on the location of the minimum stress at the crack tip at the time of forking.     

Description and interpretation of the composition of fluid and alteration mineralogy in the geothermal system,at Svartsengi,Iceland

The bulk composition and mineralogy of hydrothermally altered tholeiite, along with the composition and speciation of fluid, have been determined for a well-defined alteration zone at 240°C and 110 bars at Svartsengi, Iceland. Mass balances between the geothermal fluid and altered tholeiite, relative to a seawater/fresh water mixture and unaltered tholeiite, indicate the overall reaction per 1000 cm³ is: 1325 gm plagioclase + 1228 gm pyroxene + 215 gm oxide-minerals break down to form 685 gm chlorite + 636 gm albite + 441 gm quartz + 249 gm epidote + 266 gm calcite + 201 gm oxide-minerals + 15 gm pyrite, requiring an influx of 123 gm CO₂, 10 gm H₂S and 4 gm Na₂O and a release of 57 gm SiO₂, 35 gm FeO, 21 gm CaO, 8 gm MgO and 4 gm K₂O.Principal reactions, deduced from textural evidence, include Na-Ca exchange in plagioclase, precipitation of quartz, calcite and anhydrite, and formation of chlorite and epidote by reactions between groundmass minerals and fluid.Thermodynamic analyses of authigenic minerals and downhole fluid indicate that the fluid maintains a state close to equilibrium with the secondary mineral phases chlorite, epidote, albite, quartz, calcite, prehnite, anhydrite, pyrite and magnetite, whereas remnant primary labradorite and augite are out of equilibrium with the fluid.Water/rock ratios for the system are determined under a variety of assumptions. However, the open nature of the system makes comparisons with experimental and theoretical closed system studies ambiguous.     

祁连山北麓梨园河沉积物粒径的变化特征与分析

对河流沉积物粒径顺流变小这一现象的解释经历了从磨损到选择性沉积的转变,并发现混合粒径沉积物在搬运中会伴随有"隐藏效应",水力分选作用会随"隐藏效应"的增强而减弱;另外,河流河床经常会发生由砾质向砂质的过渡,这与砂含量增加及砾石与砂在临界起动力上产生的分化相关。这些理论都需要大量的野外观测进行论证。文章以祁连山北麓发育的梨园河为例,以河道水力学参数和河床沉积物粒径为主要研究对象,对梨园河现代河床沉积物粒径向下游变化的特征及其影响因素进行分析。分析结果显示：选择性沉积是梨园河沉积物粒径向下游逐渐变小的主要原因;砂含量向下游的增加导致河床对砾石的水力分选作用发生重要转变,并伴随砾质河床向砂质河床过渡;梨园河上半段的水力分选作用要比下半段强,这与"隐藏效应"向下游的增加有关;上半段的沉积速率比下半段的高,导致上半段沉积物粒径向下游变小的趋势更为显著。     

Coupled changes in sand grain size and sand transport driven by changes in the upstream supply of sand in the Colorado River: Relative importance of changes in bed-sand grain size and bed-sand area

Sand transport in the Colorado River in Marble and Grand canyons was naturally limited by the upstream supply of sand. Prior to the 1963 closure of Glen Canyon Dam, the river exhibited the following four effects of sand supply limitation: (1) hysteresis in sediment concentration, (2) hysteresis in sediment grain size coupled to the hysteresis in sediment concentration, (3) production of inversely graded flood deposits, and (4) development or modification of a lag between the time of a flood peak and the time of either maximum or minimum (depending on reach geometry) bed elevation. Construction and operation of the dam has enhanced the degree to which the first two of these four effects are evident, and has not affected the degree to which the last two effects of sand supply limitation are evident in the Colorado River in Marble and Grand canyons. The first three of the effects involve coupled changes in suspended-sand concentration and grain size that are controlled by changes in the upstream supply of sand. During tributary floods, sand on the bed of the Colorado River fines; this causes the suspended sand to fine and the suspended-sand concentration to increase, even when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand coarsens, and the suspended-sand concentration decreases independently of discharge. Also associated with these changes in sand supply are changes in the fraction of the bed that is covered by sand. Thus, suspended-sand concentration in the Colorado River is likely regulated by both changes in the bed-sand grain size and changes in the bed-sand area. A physically based flow and suspended-sediment transport model is developed, tested, and applied to data from the Colorado River to evaluate the relative importance of changes in the bed-sand grain size and changes in the bed-sand area in regulating suspended-sand concentration. Although the model was developed using approximations for steady, uniform flow, and other simplifications that are not met in the Colorado River, the results nevertheless support the idea that changes in bed-sand grain size are much more important than changes in bed-sand area in regulating the concentration of suspended sand.     

Uranium isotopes in the Platte River drainage basin of the North American High plains Region

The North and South Platte Rivers contribute sigficant concentrations of dissolved uranium to the Platte River system from the weathering of uraniferous rocks at their headwaters in Colorado and Wyoming. Uranium isotopes measured in three groups of samples collected from the Platte River and its major tributaries exhibted a wide range of concentrations (0.27–31.7 μg/l) and activity ratios (1.03–1.72) reflecting variations in the contributions of different sources under differing flow conditions. Locally elevated uranium concentrations and higher activity ratios were associated with groundwater sources during low flow periods. Uranium concentrations in surface runoff were lower than in baseflow and correlated significantly with dissolved solids. Uranium activity ratios in runoff were closer to equilibrium values than those in baseflow suggesting that dissolved uranium in runoff is less affected by disequilibrium processes. Mixing calculations using uranium concentrations and activity ratios indicate considerable variations in longitudinal and transverse mixing of tributary water with Plate River water, and temporal variations of groundwater contributions (baseflow and irrigation return flow) in the Plate River system.     

煤系高岭石的原始晶粒尺寸及结晶度对插层作用的影响

采用SEM、XRF、XRD和IR研究了大同、平朔和淮北煤系高岭石的结构、粒度及其特性,探讨了粒度大小对高岭石插层作用的影响及其机理。研究发现:粒度中等且结晶有序的平朔煤系高岭石插层率最高,其次为晶体粗大但结晶有序的大同高岭石,粒度最小且结晶无序的淮北高岭石插层率最低。研究认为高岭石原始晶粒粒度对高岭石插层作用有重要影响,中等粒度最有利于高岭石插层作用的进行,粒度过大或过小均不利于高岭石插层作用的进行,其原因是由于不同粒度的高岭石插层作用过程中导致的差异弹性变形引起的。结晶有序度对高岭石插层作用也有重要影响,结构无序不利于插层作用的进行。