Multivariate analysis of sedimentary environments using grain-size frequency distributions

Multivariate statistics were used to characterize and test the effectiveness of grain-size frequencies as environmental discriminators. Sediment from the following two depositional systems along eastern Lake Michigan were studied: (1) a closed system with respect to available grain sizes (Little Sable Point), and (2) an open system (Sleeping Bear Point-Manitou Passage). Principal components analysis shows that grain-size distributions are composed of two or more subgroups that reflect surface creep bedload, mixed suspension bedload, and uniform suspension. Discriminant function and principal latent vector analyses of the Little Sable Point environments show that, when available sediment is limited with respect to grain size (0.5 φto 3.0 φ), similar size distributions can occur in environments supposedly characterized by different energy conditions. Sediment in the Sleeping Bear Point-Manitou Passage system is not restricted to available grain sizes and the environments discriminated very well (α < 0.001). The grain-size distributions are such that they reflect differences in energy conditions within the environments. It is apparent that the grain sizes available to a depositional system control to a great extent the effectiveness of environmental discrimination.     

The link between grain‐size components and depositional processes in a modern clastic lake

It is widely recognized that lake sediment grain‐size distributions tend to be polymodal and consist of two or more grain‐size components. However, for specific cases, the genesis of each component usually is poorly understood. In this study, the grain‐size components of the surface sediments of Hulun Lake, Inner Mongolia, were partitioned using the log‐normal distribution function method and the relationship between the identity of each grain‐size component and the hydraulic condition of the lake was investigated in order to relate the constituent components to specific depositional processes in the lake. The data indicate that the modern clastic sediments of Hulun Lake contain six distinct unimodal grain‐size distributions representing six grain‐size components. Each of the components retains its identity including modal size, manner of transportation and environment of deposition, although the relative percentage varies with the hydraulic conditions throughout the lake. These components are specified from fine to coarse modes as long‐term suspension clay, offshore‐suspension fine silt and medium to coarse silt, and nearshore‐suspension fine sand, saltation medium sand and traction coarse sand. The percentage contribution of several grain‐size components interpreted as being indicative of nearshore environments is shown to be correlated negatively with water depth across the modern lake bed; this suggests that the proportion of these components in core data might be useful as a proxy for water depth. This possibility was tested using a sediment core from Hulun Lake where high percentages of the nearshore grain‐size components were found to be correlated with low regional precipitation reconstructed from the pollen profile of the same core. The coincidence of two independent proxies does not only demonstrate the validity of log‐normal distribution function in partitioning polymodal sediments but reveals the potential of lake sediment grain‐size components for the research of lake‐level fluctuations during the geological past.     

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.     

Current‐aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso‐arenacea Formation

Turbidite sandstones of the Miocene Marnoso‐arenacea Formation (northern Apennines, Italy) display centimetre to decimetre long, straight to gently curved, 0·5 to 2·0 cm regularly spaced lineations on depositional (stratification) planes. Sometimes these lineations are the planform expression of sheet structures seen as millimetre to centimetre long vertical ‘pillars’ in profile. Both occur in the middle and upper parts of medium‐grained and fine‐grained sandstone beds composed of crude to well‐defined stratified facies (including corrugated, hummocky‐like, convolute, dish‐structured and dune stratification) and are aligned sub‐parallel to palaeoflow direction as determined from sole marks often in the same beds. Outcrops lack a tectonic‐related fabric and therefore these structures may be confidently interpreted to be sedimentary in origin. Lineations resemble primary current lineations formed by the action of turbulence during bedload transport under upper stage plane bed conditions. However, they typically display a larger spacing and micro‐topography compared to classic primary current lineations and are not associated with planar‐parallel, finely laminated sandstones. This type of ‘enhanced lineation’ is interpreted to develop by the same process as primary current lineations, but under relatively high near‐bed sediment concentrations and suspended load fallout rates, as supported by laboratory experiments and host facies characteristics. Sheets are interpreted to be dewatering structures and their alignment to palaeoflow (only noted in several other outcrops previously) inferred to be a function of vertical water‐escape following the primary depositional grain fabric. For the Marnoso‐arenacea beds, sheet orientation may be linked genetically to the enhanced primary current lineation structures. Current‐aligned lineation and sheet structures can be used as palaeoflow indicators, although the directional significance of sheets needs to be independently confirmed. These indicators also aid the interpretation of dewatered sandstones, suggesting sedimentation under a traction‐dominated depositional flow – with a discrete interface between the aggrading deposit and the flow – as opposed to under higher concentration grain or hindered‐settling dominated regimes.     

A comparison of grain‐size analysis methods for sand‐dominated fluvial sediments

Grain‐size distribution is a fundamental tool for interpreting sedimentary units within depositional systems. The techniques assessed in this study are commonly used to determine grain‐size distributions for sand‐dominated sediments. However, the degree of consistency and differences in interpretation when using a combination of grain‐size methods have not yet been assessed systematically for sand‐dominated fluvial sediments. Results obtained from laser diffraction, X‐ray attenuation and scanning electron microscopy grain‐size analysis techniques were compared with those obtained from the traditional sieve/hydrometer method. Scanning electron microscopy was shown to provide an inaccurate quantitative analysis of grain‐size distributions because of difficulties in obtaining representative samples for examination. The X‐ray attenuation method is unsuitable for sand‐dominated sediments because of its upper size range of only 300 μm. The consistently strong correlation between the laser diffraction results and the sieve/hydrometer results shows that these methods are comparable for sand‐dominated fluvial sediments. Provided that sample preparation is consistent, the latter two methods can be used together within a study of such sediments while maintaining a high degree of accuracy. These results indicate that data for sand‐dominated fluvial sediments gained from the long‐established sieve/hydrometer method can be compared with confidence to those obtained by modern studies using laser diffraction techniques.     

Climbing‐ripple successions in turbidite systems: depositional environments,sedimentation rates and accumulation times

Climbing‐ripple cross‐lamination is most commonly deposited by turbidity currents when suspended load fallout and bedload transport occur contemporaneously. The angle of ripple climb reflects the ratio of suspended load fallout and bedload sedimentation rates, allowing for the calculation of the flow properties and durations of turbidity currents. Three areas exhibiting thick (>50 m) sections of deep‐water climbing‐ripple cross‐lamination deposits are the focus of this study: (i) the Miocene upper Mount Messenger Formation in the Taranaki Basin, New Zealand; (ii) the Permian Skoorsteenberg Formation in the Tanqua depocentre of the Karoo Basin, South Africa; and (iii) the lower Pleistocene Magnolia Field in the Titan Basin, Gulf of Mexico. Facies distributions and local contextual information indicate that climbing‐ripple cross‐lamination in each area was deposited in an ‘off‐axis’ setting where flows were expanding due to loss of confinement or a decrease in slope gradient. The resultant reduction in flow thickness, Reynolds number, shear stress and capacity promoted suspension fallout and thus climbing‐ripple cross‐lamination formation. Climbing‐ripple cross‐lamination in the New Zealand study area was deposited both outside of and within channels at an inferred break in slope, where flows were decelerating and expanding. In the South Africa study area, climbing‐ripple cross‐lamination was deposited due to a loss of flow confinement. In the Magnolia study area, an abrupt decrease in gradient near a basin sill caused flow deceleration and climbing‐ripple cross‐lamination deposition in off‐axis settings. Sedimentation rate and accumulation time were calculated for 44 climbing‐ripple cross‐lamination sedimentation units from the three areas using TDURE, a mathematical model developed by Baas et al. (2000) . For T_c divisions and T_bc beds averaging 26 cm and 37 cm thick, respectively, average climbing‐ripple cross‐lamination and whole bed sedimentation rates were 0·15 mm sec^?1 and 0·26 mm sec^?1 and average accumulation times were 27 min and 35 min, respectively. In some instances, distinct stratigraphic trends of sedimentation rate give insight into the evolution of the depositional environment. Climbing‐ripple cross‐lamination in the three study areas is developed in very fine‐grained to fine‐grained sand, suggesting a grain size dependence on turbidite climbing‐ripple cross‐lamination formation. Indeed, the calculated sedimentation rates correlate well with the rate of sedimentation due to hindered settling of very fine‐grained and fine‐grained sand–water suspensions at concentrations of up to 20% and 2·5%, respectively. For coarser grains, hindered settling rates at all concentrations are much too high to form climbing‐ripple cross‐lamination, resulting in the formation of massive/structureless S₃ or T_a divisions.     

The action of wind and water in a mid‐Cretaceous subtropical erg‐margin system close to the Variscan Iberian Massif,Spain

Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid‐Cretaceous outer back‐erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain‐size distributions, mud playa, ephemeral floodplain, pebble‐sand and cobble‐sand bedload stream, pebble–cobble‐sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub‐environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind‐driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back‐erg depositional system comprises an outer back‐erg close to the Variscan highlands, and an inner back‐erg close to the central‐erg area. The inner back‐erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back‐erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud‐cracked surfaces, resulting in largely structureless sandstones with boulder‐size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser‐grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back‐erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back‐erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non‐exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti‐clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well‐preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre‐Cretaceous times.     

Validation and implications of an energy‐based bedload transport equation

A recently developed bedload equation (Abrahams & Gao, 2006) has the form i_b = ωG^3˙4, where i_b is the immersed bedload transport rate, ω is the stream power per unit area, G = 1?θ_c/θ, θ is the dimensionless shear stress and θ_c is the associated threshold value for the incipient motion of bed grains. This equation has a parsimonious form and provides good predictions of transport rate in both the saltation and sheetflow regimes (i.e. flows with low and high θ values, respectively). In this study, the equation was validated using data independent of those used for developing it. The data represent bedload of identical sizes transported in various steady, uniform, fully rough and turbulent flows over plane, mobile beds. The equation predicted i_b quite well over five orders of magnitude. This equation was further compared with six classic bedload equations and showed the best performance. Its theoretical significance was subsequently examined in two ways. First, based on collision theory, the parameter G was related to the ratio of grain‐to‐grain collisions to the total collisions including both grain‐to‐grain and grain‐to‐bed collisions, P_g by P_g = G², suggesting that G characterizes the dynamic processes of bedload transport from the perspective of granular flow, which partly accounts for the good performance of the equation. Moreover, examining the ability of two common equations to predict bedload in gravel‐bed rivers revealed that G can also be used to simplify equations for predicting transport capacities in such rivers. Second, a simple dimensionless form of the equation was created by introducing B = i_b/ω. The theoretical nature of the term B was subsequently revealed by comparing this equation with both the Bagnold model and two commonly used parameters representing dimensionless bedload transport rates.     

Variability in bedform morphology and kinematics with transport stage

Bedform geometry is widely recognized to be a function of transport stage. Bedform aspect ratio (height/length) increases with transport stage, reaches a maximum, then decreases as bedforms washout to a plane bed. Bedform migration rates are also linked to bedform geometry, in so far as smaller bedforms in coarser sediment tend to migrate faster than larger bedforms in finer sediment. However, how bedform morphology (height, length and shape) and kinematics (translation and deformation) change with transport stage and suspension have not been examined. A series of experiments is presented where initial flow depth and grain size were held constant and the transport stage was varied to produce bedload dominated, mixed‐load dominated and suspended‐load dominated conditions. The results show that the commonly observed pattern in bedform aspect ratio occurs because bedform height increases then decreases with transport stage, against a continuously increasing bedform length. Bedform size variability increased with transport stage, leading to less uniform bedform fields at higher transport stage. Total translation‐related and deformation‐related sediment fluxes all increased with transport stage. However, the relative contribution to the total flux changed. At the bedload dominated stage, translation‐related and deformation‐related flux contributed equally to the total flux. As the transport stage increased, the fraction of the total load contributed by translation increased and the fraction contributed by deformation declined because the bedforms got bigger and moved faster. At the suspended‐load dominated transport stage, the deformation flux increased and the translation flux decreased as a fraction of the total load, approaching one and zero, respectively, as bedforms washed out to a plane bed.     

Quantifying particle aggregation in sediments

Sediments often occur as non‐normal size distributions composed of discrete, partially aggregated particle populations. These populations reflect provenance, dispersal pathways and their depositional environments. Recent experimental laboratory studies describing mud flocculation in turbulent marine systems prompted this investigation of the potential of aggregates to record size‐sensitive transport dynamics in a terrestrial fluvial system. Here, sediment‐size distributions in their natural condition of particle–aggregate mixtures are analysed by parametric statistics. A practical and freely available decompositional approach is outlined and field tested, which allows sediment to be viewed in both its conventional particulate form and as its naturally occurring mixture of transport‐stable aggregates and elementary particles. From a sequence of upward‐fining slack water couplets in the Flinders Ranges, South Australia, it is demonstrated that the characteristics, provenance and depositional history of fine‐grained sediments consisting of particle‐aggregate mixtures can best be understood fully by quantifying aggregation.     

The partitioning of the total sediment load of a river into suspended load and bedload: a review of empirical data

The partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology, sedimentation engineering and sedimentology. Bedload transport rates are notoriously hard to measure and, at many sites, only suspended load data are available. Often the bedload fraction is estimated with ‘rule of thumb’ methods such as Maddock’s Table, which are inadequately field‐tested. Here, the partitioning of sediment load for the Pitzbach is discussed, an Austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available. The available data show large scatter on all scales. The fraction of the total load transported in suspension may vary between zero and one at the Pitzbach, while its average decreases with rising discharge (i.e. bedload transport is more important during floods). Existing data on short‐term and long‐term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested. The partitioning averaged over a flood can vary strongly from event to event. Similar variations may occur in the year‐to‐year averages. Using published simultaneous short‐term field measurements of bedload and suspended load transport rates, Maddock’s Table is reviewed and updated. Long‐term average partitioning could be a function of the catchment geology, the fraction of the catchment covered by glaciers and the extent of forest, but the available data are insufficient to draw final conclusions. At a given drainage area, scatter is large, but the data show a minimal fraction of sediment transported in suspended load, which increases with increasing drainage area and with decreasing rock strength for gravel‐bed rivers, whereby in large catchments the bedload fraction is insignificant at ca 1%. For sand‐bed rivers, the bedload fraction may be substantial (30% to 50%) even for large catchments. However, available data are scarce and of varying quality. Long‐term partitioning varies widely among catchments and the available data are currently not sufficient to discriminate control parameters effectively.     

新疆焉耆盆地开都河不同河型段砂砾质沉积特征与差异分析*

新疆焉耆盆地开都河自察汗乌苏水电站流入博斯腾湖的河口之间,发育单一物源供给下的山间河段、辫状河段、曲流河段、顺直河段以及三角洲平原顺直型分流河道段等多种类型河道沉积。通过探坑挖掘与观察、砂砾质沉积物结构的测量、碎屑组分与重矿物分析以及数据统计分析等方法,分析开都河不同类型河段的砾石质沉积特征与搬运距离关系、砾质与砂质组合特征、砂质碎屑组分与沉积构造特征及变化等,认为沉积地形与坡度、沉积物组成以及气候条件等因素控制了不同河段类型的变化与沉积特征的差异。同时,建立了不同河型段砾石径变化与沉积搬运距离的定量关系。在此基础上,统计了在干旱气候与充沛物源供给条件下,开都河不同河型沉积中的有利储集体分布范围及其比例关系数据,可为陆相湖盆河流沉积相图的编制提供重要的参考依据。     

Linking submarine channel–levee facies and architecture to flow structure of turbidity currents: insights from flume tank experiments

Submarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.     

Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia

Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m³ s^?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m³ s^?1). About 3·7 × 10⁶ tonnes of suspended sediment and 3 × 10⁵ tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.     

Flow, sediment transport and bedform dynamics over the transition from dunes to upper-stage plane beds: implications for the formation of planar laminae

Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms^-1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near-bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low-relief bedwaves on upper-stage plane beds are ubiquitous and give rise to laterally extensive, mm-thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves.     

Fine‐grained meandering systems of the Lower Permian Clear Fork Formation of north‐central Texas,USA: Lateral and oblique accretion on an arid plain

Facies models that adequately represent the diverse range of fine‐grained fluvial systems are currently lacking from the literature. In this paper, the spectrum of these systems on the arid plains of western equatorial Pangea is explored, as well as the source and nature of the fine‐grained sediments. Eight fluvial elements in the Early Permian Clear Fork Formation of north‐central Texas represent channel systems up to 7 m deep with coarse basal deposits, three types of lateral‐accretion deposits and sandstone sheets, with laminated, disrupted and massive mudstones laid down in abandoned channels and on floodplains. The three fine‐grained fluvial styles represent a continuum between two end‐members: sustained lateral accretion of bedload composed of quartzose sediments and mud aggregates on point bars, and oblique accretion of suspended sediment on steep accretionary benches and banks with limited lateral migration. This spectrum is controlled, in part, by grain size and the proportion of suspended to bedload sediments. The presence of rarely documented swept ripples on exhumed accretion surfaces is attributed to rapid decline in water levels and downstream re‐entry of overbank floodwaters into the channel. Rill casts, roots and disrupted mudstones low down in channel bodies indicate periods of near‐dryness. Laterally extensive sheet sandstones were formed by episodic flows in broad, sandbed channels. The fluvial sediments were primarily intrabasinally sourced with extrabasinal sediments brought in during major floods from upland source areas or reworked from local storage in the basin, representing a supply limited system. The upward change in cement composition from mainly calcite and ankerite to dolomite and gypsum with minor celestine implies increasingly saline groundwater and progressive aridification, supporting Late Palaeozoic palaeoclimatic models. By integrating petrographic data with sedimentology, a plethora of information about ancient landscapes and climate is provided, allowing a fuller comparison between the Clear Fork Formation and modern dryland alluvial plains.     

The relationship of sediment transport rates and paths to sandbanks in a tidally dominated area off the coast of East Anglia, U.K.

ABSTRACT Sand transport measurements of bedload and suspended load in the Sizewell-Dunwich Banks area, East Anglia have shown that the suspended mode is dominant. The depth-integrated spring tidal residual is 5.66 g cm⁻¹ sec⁻¹, although the neap rate is only one-fifth of this. The calculated bedload transport rates also vary, from 0.012 to 0.040 g cm⁻¹ sec⁻¹, correlating with changing meteorological conditions.
In order to predict the bedload sediment circulation pattern from midwater current meter measurements, five sediment transport equations were calibrated, using fluorescent dyed sand. Yalin's relationship gave the best estimates. The bed shear stress was determined by extrapolating the velocity profile as a power law relationship, with an exponent equal to 0.1, from midwater down to 2 m and as a lognormal profile from 2 m to the sea-bed. Roughness length values appropriate to the substrate were used.
Although bedload transport residuals are mainly to the south, the banks trend northwards from the coast and have also elongated in this direction. This is thought to be in response to the dominance of the suspended sediment transport. It is suggested that a tidal residual eddy mechanism is responsible for the banks'maintenance, similar to the process operating in Start Bay, Devon. The well-documented westward movement of the banks is likely to be related to wave processes.     

Direct numerical simulation of bedload transport using a local, dynamic boundary condition

ABSTRACT Temporally and spatially averaged models of bedload transport are inadequate to describe the highly variable nature of particle motion at low transport stages. The primary sources of this variability are the resisting forces to downstream motion resulting from the geometrical relation (pocket friction angle) of a bed grain to the grains that it rests upon, variability of the near‐bed turbulent velocity field and the local modification of this velocity field by upstream, protruding grains. A model of bedload transport is presented that captures these sources of variability by directly integrating the equations of motion of each particle of a simulated mixed grain‐size sediment bed. Experimental data from the velocity field downstream and below the tops of upstream, protruding grains are presented. From these data, an empirical relation for the velocity modification resulting from upstream grains is provided to the bedload model. The temporal variability of near‐bed turbulence is provided by a measured near‐bed time series of velocity over a gravel bed. The distribution of pocket friction angles results as a consequence of directly calculating the initiation and cessation of motion of each particle as a result of the combination of fluid forcing and interaction with other particles. Calculations of bedload flux in a uniform boundary and simulated pocket friction angles agree favourably with previous studies.     

Incipient vertical traction carpets within collapsed sinkhole fills

Small vertically oriented traction carpets are reported from the collapsed sandy fills of 100 m deep Devonian limestone sinkholes underlying the Lower Cretaceous Athabasca oil sands deposit in north‐eastern Alberta, Western Canada. Dissolution of 100 m of underlying halite salt beds caused cataclysmic collapse of the sinkhole floors and water saturated sinkhole sand fills to descend very rapidly. Turbulent currents flushed upper sinkhole fills of friable sandstone blocks and disaggregated sand and quartz pebble for tens of metres. Laminar deposits with inverse grading accumulated as many as six to eight curvilinear entrained pebble streaks, 10 to 30 cm long, vertically impinged against the sides of descending collapse blocks. These deposits were initiated as vertically oriented early stage traction carpets that interlocked fine sand grains and inversely graded overlying pebbles entrained below the dilute overlying turbulent flows. Vortexes that flushed these sinkhole fills and induced these depositional processes may have lasted only seconds before the very rapid descents abruptly halted. Some of the fabrics were suspended vertically in‐place and preserved from unlocking and obliteration. These small fabrics provide insight into the instability and ephemeral character of the transition from strong gravity‐driven grain falls to very early stages of traction carpet formation. These short‐lived deposits of very thin sand layers resulted from sufficient incipient frictional freezing that grain interlocking overcame, however briefly, the strong gravity drives of the vertical falls that would have otherwise dispersed grains and obliterated any organized fabric patterns. Tenuous frictionally locked grains were also suspended at the centres of hyperbolic grain fall flows that briefly developed between turbulent flow eddies, some of which were fortuitously preserved. Some of these suspended grain locking zones passed downward onto the relatively more stable surfaces of the rapidly descending block surfaces. The morphogenesis of these early stage traction carpets differ from more fully developed deposits elsewhere because of their short‐lived transport, dynamic instability and vertical orientation.     

Kaolinite deposition from moving suspensions: The roles of flocculation,salinity, suspended sediment concentration and flow velocity/bed shear

Understanding how mud moves and deposits is essential for conceptualizing the dynamic nature of surface environments and their ancient counterparts. Experimental study has largely been pursued by civil engineers, using kaolinite as an active ingredient. Yet, applying their data to the physical comprehension of mudstone sedimentology is hampered by multiple flume configurations between labs, and data sets tailored to specific engineering needs. The need for a better grasp of underlying processes is acute, given recent flume studies that show that moving suspensions form large bedload floccules, migrating floccule ripples and bed accretion under currents capable of moving sand grains. To advance mudstone sedimentology, integrated study of suspended sediment concentration, salinity and bed shear stress on the deposition of floccules is crucial. Described here is a set of tightly controlled experiments that explored suspended sediment concentrations from 70 to 900 mg/l, freshwater, brackish and marine salinities, flow velocities in the 5 to 50 cm/s range (equivalent to 0.01 to 0.58 Pa bed shear), measured the size of in-flow and bedload floccules, and the critical velocity of sedimentation that marks the onset of sustained bedload accumulation. The critical velocity of sedimentation of kaolinite clays is in the 26 to 28 cm/s flow velocity range (0.22 to 0.25 Pa), appears insensitive to a wide range of suspended sediment concentrations and salinities, and coincides with the formation of sand-size bedload floccules. Further decrease of flow velocity/bed shear stress is accompanied by a steady increase in the size of bedload floccules. Large bedload floccules appear to form in the high-shear basal part of the flow, a phenomenon requiring further investigation. Better understanding of the mechanisms that facilitate mud deposition from moving suspensions is critical for more realistic assessments of the depositional conditions of mud and mudstones, as well as for refining predictive models for the flux of fine-grained sediments across the Earth's surface.