The Romagna Apennines,Italy: an eroded duplex

The study of clast composition carried out on the alluvial gravels of the Romagna Apennines of northern Italy has provided evidence for an extensive covering of allochthonous units (Ligurian nappe and Epiligurian succession) above the Miocene foredeep deposits (Marnoso‐Arenacea Formation), which has been subsequently eroded during the Late Miocene–Pleistocene uplift. This result is confirmed by the burial history outlined in the Marnoso‐Arenacea Formation through vitrinite reflectance and apatite fission‐track analyses. The Romagna Apennines represent, therefore, a regional tectonic window where the thrust system that displaced the Marnoso‐Arenacea Formation crops out. The geometric relations between this thrust system and the basal thrust of the Ligurian nappe, exposed at the boundaries of the Romagna Apennines (Sillaro Zone and Val Marecchia klippe), are consistent with a duplex structure. Thus, the Romagna Apennines thrust system is an eroded duplex. The duplex roof‐thrust corresponds to the surface of the synsedimentary overthrust of the Ligurian nappe on the Marnoso‐Arenacea Formation; the floor‐thrust is located in the pelagic pre‐foredeep deposits (Schlier Formation). Copyright © 2001 John Wiley & Sons, Ltd.     

New insight into the evolution of large-volume turbidity currents: comparison of turbidite shape and previous modelling results

The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log‐normal frequency distribution; a small number of flows dominated sediment supply to this non‐channelized basin plain. Turbidite sandstone within small‐volume (<0·7 km³) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger‐volume (0·7–7 km³) beds have a broad thickness maximum in their proximal part. Grain‐size trends within this broad thickness maximum indicate spatially near‐uniform flow for distances of ∼30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea‐floor gradient. Intra‐bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large‐volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea‐floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages.     

Turbidite bed thickness distributions: methods and pitfalls of analysis and modelling

Turbidite bed thickness distributions are often interpreted in terms of power laws, even when there are significant departures from a single straight line on a log–log exceedence probability plot. Alternatively, these distributions have been described by a lognormal mixture model. Statistical methods used to analyse and distinguish the two models (power law and lognormal mixture) are presented here. In addition, the shortcomings of some frequently applied techniques are discussed, using a new data set from the Tarcău Sandstone of the East Carpathians, Romania, and published data from the Marnoso‐Arenacea Formation of Italy. Log–log exceedence plots and least squares fitting by themselves are inappropriate tools for the analysis of bed thickness distributions; they must be accompanied by the assessment of other types of diagrams (cumulative probability, histogram of log‐transformed values, q–q plots) and the use of a measure of goodness‐of‐fit other than R², such as the chi‐square or the Kolmogorov–Smirnov statistics. When interpreting data that do not follow a single straight line on a log–log exceedence plot, it is important to take into account that ‘segmented’ power laws are not simple mixtures of power law populations with arbitrary parameters. Although a simple model of flow confinement does result in segmented plots at the centre of a basin, the segmented shape of the exceedence curve breaks down as the sampling location moves away from the basin centre. The lognormal mixture model is a sedimentologically intuitive alternative to the power law distribution. The expectation–maximization algorithm can be used to estimate the parameters and thus to model lognormal bed thickness mixtures. Taking into account these observations, the bed thickness data from the Tarcău Sandstone are best described by a lognormal mixture model with two components. Compared with the Marnoso‐Arenacea Formation, in which bed thicknesses of thin beds have a larger variability than thicknesses of the thicker beds, the thinner‐bedded population of the Tarcău Sandstone has a lower variability than the thicker‐bedded population. Such differences might reflect contrasting depositional settings, such as the difference between channel levées and basin plains.     

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.     

Spatio-temporal variability of bedload transport rate: analysis and 2D modelling approach

Field measurements to calibrate numerical bedload formulae are largely missing. Measurements using a Large Helley-Smith sampler were performed over a period of five years in the large Alpine Drau River, Austria. Our results reveal the high spatio-temporal variability of bedload transport rates. Commonly used bedload predictors poorly describe measured transport rates. Temporal and localised cross-sectional variation in bedload transport rates are observed in short time frames. To obtain significant mean values, the measurement period has to be extended to cover the existing bedload transport periodicity. The discrepancy between bedload transport measurements and simulation is partially explained by local hydraulic variations. The results can be improved, particularly for verticals where most of the bedload occurs, by relating measured transport rates to local hydraulic parameters. The incorporation of local cross-sectional parameters demonstrates the utility of 2D bedload models and their greater predictive power over similar 1D models.     

Guidelines for sampling bedload transport with minimum uncertainty

The stability of river channels and their suitability as habitat for aqueous organisms is strongly controlled by the rate of bedload transport. Quantification of bedload transport rates in rivers is difficult, not only because of the temporal variation in transport, but also because of the cross‐channel variation in transport. The objectives of this study were: (i) to determine the effect of cross‐channel variation in bedload transport on the uncertainty of width‐integrated transport rates; and to use this knowledge (ii) to improve guidelines for bedload sampling. This was done through a thorough statistical evaluation of stochastic and systematic uncertainties involved in bed‐load transport measurements. Based on this evaluation, new guidelines are presented for determination of the number of samples and sampling positions across the channel that are required for bedload measurements in several types of sand‐bed rivers and gravel‐bed rivers. The guidelines relate to bedload measurements made with pressure‐difference (Helley‐Smith type) samplers that require numerous bedload samples of short duration at several positions across the channel. The results show that generally more sampling positions across the channel are required in gravel‐bed rivers than in sand‐bed rivers. For gravel‐bed rivers with unknown cross‐channel distribution of transport, at least 10 sampling positions are recommended, whereas for most sand‐bed rivers five positions suffice. In addition, at least 12 short‐duration samples are required at each position to obtain bedload estimates with uncertainties below 20%. If the same level of uncertainty is desired in the case of high spatial and temporal variation in transport rates, the number of short‐duration samples needed per sampling position increases to 40.     

Distinctive erosional and depositional structures formed at a canyon mouth: A lower Pleistocene deep‐water succession in the Kazusa forearc basin on the Boso Peninsula,Japan

The canyon mouth is an important component of submarine‐fan systems and is thought to play a significant role in the transformation of turbidity currents. However, the depositional and erosional structures that characterize canyon mouths have received less attention than other components of submarine‐fan systems. This study investigates the facies organization and geometry of turbidites that are interpreted to have developed at a canyon mouth in the early Pleistocene Kazusa forearc basin on the Boso Peninsula, Japan. The canyon‐mouth deposits have the following distinctive features: (i) The turbidite succession is thinner than both the canyon‐fill and submarine‐fan successions and is represented by amalgamation of sandstones and pebbly sandstones as a result of bypassing of turbidity currents. (ii) Sandstone beds and bedsets show an overall lenticular geometry and are commonly overlain by mud drapes, which are massive and contain fewer bioturbation structures than do the hemipelagic muddy deposits. (iii) The mud drapes have a microstructure characterized by aggregates of clay particles, which show features similar to those of fluid‐mud deposits, and are interpreted to represent deposition from fluid mud developed from turbidity current clouds. (iv) Large‐scale erosional surfaces are infilled with thick‐bedded to very thick‐bedded turbidites, which show lithofacies quite similar to those of the surrounding deposits, and are considered to be equivalent to scours. (v) Concave‐up erosional surfaces, some of which face in the upslope direction, are overlain by backset bedding, which is associated with many mud clasts. (vi) Tractional structures, some of which are equivalent to coarse‐grained sediment waves, were also developed, and were overlain locally by mud drapes, in association with mud drape‐filled scours, cut and fill structures and backset bedding. The combination of these outcrop‐scale erosional and depositional structures, together with the microstructure of the mud drapes, can be used to identify canyon‐mouth deposits in ancient deep‐water successions.     

Distinguishing between fine-grained turbidites and contourites on the Nova Scotian deep water margin

Review of the criteria which have been proposed for distinguishing between the deposits of turbidity currents and bottom currents in deep water sedimentation shows no general agreement on their validity. It is important to compare finegrained turbidites and contourites, to recognize that different turbidity current and bottom current mechanisms exist, and that their deposits may be closely inter-bedded in a continental rise environment. Interbedded turbidites and contourites have been recognized in cores from the deep-water margin off Nova Scotia. The most useful criteria for distinguishing between the two deposits were found to be: (1) fining and sorting trends: perpendicular or parallel to the contours; (2) marked textural differences between interbedded turbidites and contourites indicating differences in source and transport distance; (3) mineralogy and textural composition: regional patterns indicating transport perpendicular or parallel to the contours; (4) grain fabric: indication of downslope or along-slope transport at the time of final deposition; (5) sedimentary structures: turbidites show a structural sequence and evidence of rapid burial; contourites are bioturbated and contain irregular lag concentrations of biogenic sand. Other criteria include grain-size parameters, and the regional setting, distribution and depositional rate of the various facies. With due care these criteria can be applied to other regions. Previously used characteristics of silt-laminae abundance, layer thickness, heavy mineral cross lamination, sorting, and the nature of bed contacts are not applicable to fine-grained turbidites and contourites. Compositional criteria depend on regional features.     

Comparisons between acoustic measurements and predictions of the bedload transport of marine gravels

Continuous, detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates (q_b) predicted by five bedload transport equations using measurements of the near-bed turbulent current flow. When mean flow data are used in these equations, total q_b estimates are similar to those measured. However, when instantaneous flow data are used, total q_b is over-estimated by approximately one order-of-magnitude. Based on the acoustic measurements, an empirical equation has been obtained that gives accurate estimates of total q_b over a tidal cycle and simulates well the intermittent characteristics of marine bedload transport.     

Geochemical mapping based on geological units: A case study from the Marnoso-arenacea formation (Northern Apennines,Italy)

Geochemical maps can provide us with much information on geology, earth surface processes and anthropogenic pressure and are valuable tools for ore prospecting and land management. Stream sediments represent an integral of the various possible sources of sediments upstream from the sampling point therefore there can be multiple signal sources but generally the prevailing signal source is the one related to bedrock geology. Stream sediments collected from active second-order channels including singular geological units, were selected in order to determine the geochemical characteristics of each unit. The aim of this study was to analyse their potential for using them to integrate geological interpretation and produce a geologically-oriented geochemical map. From the 770 samples collected for a regional geochemical mapping program, we selected 149 samples whose catchment basin included only one of the members recognized within the Marnoso–Arenacea formation. This Middle-Upper Miocene (Langhian–Tortonian) turbiditic unit forms the backbone of the Romagna Apennines and has been subdivided into 14 members according to age and lithostratigraphic criteria. The results indicate that there are marked differences in the composition of the members of the Marnoso Arenecea formation which indicate the provenance of the sediment and the palaeogeographic evolution of the units. By means of univariate and multivariate statistical analyses (Factor analyses) two main types of sediment compositions are identified: Tortonian members are characterized by sialic coarse grain- sediments while the Langhian–Serravallian members are richer in carbonate fraction, slightly enriched in a mafic contribution. This study elaborated the geochemical data from a geological point of view by integrating the information available in literature to spatially extend the interpretation based on limited site observation as for petrographic studies. In general, the geochemical map based on a geological unit could be a useful tool for carrying out the geological reconstruction of a complex area.     

Evaluation of bedload yield in gravel-bed rivers using scour chains and painted tracers: the case of the Esconavette Torrent (Southern French Prealps)

The event-based bedload yields of a small gravel-bed river (the Esconavette Torrent) have been concomitantly determined by surveying coarse sediment deposition in a trap and by monitoring the active layer of the bed and the displacement of painted tracers. The geometry of the active layer was obtained by means of scour chains and topographic resurveys. The cumulative bedload yield of 4 flow events measured in the trap and by the chain and tracer approach was respectively 174 and 153 m³. The consistency between those two field-based estimates confirms that the deployment of scour chains and tracers in gravel-bed rivers have the potential to provide a robust assessment of bedload transport. This potential theoretically depends on the spatial density of scour chains and the ability of the tracing technique to fit the grain size distribution of the active layer. The results demonstrate that a distance between scour chains that represents 10-15% of the active channel width is sufficient for a rather accurate determination of event bedload transport rate by reconstruction of scour and fill throughout a cross-section.     

In situ acoustic measurements of marine gravel threshold and transport

Measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles. The turbulence in the bottom boundary layer was measured using two electromagnetic current meters, and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material. Visual estimates of bedload were also obtained with an underwater TV camera. The acoustic technique, unlike a conventional bedload sampler, has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected. This has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions. The results of the study show that of the turbulent bursting events which contribute towards the Reynolds stress, only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material. The measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process.     

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.     

On the frequency distribution of turbidite thickness

The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log‐normal or negative‐exponential frequency distributions and power‐law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand‐interval thickness within each bed have a frequency distribution comprising the sum of a series of log‐normal frequency distributions. These strata were deposited predominantly in a basin‐plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log‐normal frequency distribution characterizes bed or sand‐interval thickness for a given basal grain‐size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log‐normal distributions is positively correlated with basal grain‐size. The power‐law exponent relating the basal grain‐size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power‐law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin‐bedded’ and ‘thick‐bedded’ turbidites noted by previous workers is explained as the result of a change in the power‐law exponent. This analysis supports the view that A and B divisions were deposited from high‐concentration flow components and that distinct grain‐size modes undergo different depositional processes. Summation of log‐normal frequency distributions for thin‐ and thick‐bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power‐law trend. Thus, the occurrence of both log‐normal and segmented power‐law frequency distributions can be explained in a holistic fashion. Power‐law frequency distributions of turbidite thickness have previously been linked to power‐law distributions of earthquake magnitude or volumes of submarine slope failure. The log‐normal distribution for a given grain‐size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain‐sizes present.     

Modeling Bedload Transport of Juvenile Bivalves: Predicted Changes in Distribution and Scale of Postlarval Dispersal

The potential consequences of bedload transport of postlarvae for patterns of distribution of marine invertebrates were explored by developing a bedload transport model for juvenile bivalves in a small estuary in New Jersey, USA. A simple numerical model of tidal current hydrodynamics was developed based on field measurements of shear stresses near the bottom. Burrowing behavior of bivalves was incorporated into the model of bedload transport by using estimates of entrainment rates of Gemma gemma and Mya arenaria in a laboratory flume, and jump lengths of the bivalves were estimated by methods previously developed for noncohesive particles. Based on the flood domination and strong gradient of shear stresses in the Navesink estuary, our model predicted that juvenile bivalves would accumulate in the center of the estuary, traveling up to several kilometers over 30 days. Field distributions of juvenile bivalves were consistent with the model predictions for other species of bivalves but not for G. gemma, for which field distributions of both <500- and >500-μm individuals were concentrated in the eastern end of the estuary. Differences between the bedload model and G. gemma distributions suggest that spatial variation in burrowing behavior or biological interactions are playing an important role in maintaining distribution patterns of this species in spite of high levels of bedload transport. This modeling approach is applicable to other juvenile benthic invertebrates that disperse as bedload and is a useful model against which to compare field observations of rates of transport and patterns of distribution and abundance.     

Stratigraphy and depositional setting of slurry and contained (reflected) beds in the Marnoso‐arenacea Formation (Langhian‐Serravallian) Northern Apennines,Italy

This work presents the stratigraphy and facies analysis of an interval of about 2500 m in the Langhian and Serravallian stratigraphic succession of the foredeep turbidites of the Marnoso‐arenacea Formation. A high‐resolution stratigraphic analysis was performed by measuring seven stratigraphic logs between the Sillaro and Marecchia lines (60 km apart) for a total thickness of about 6700 m. The data suggest that the stratigraphy and depositional setting of the studied interval was influenced by syndepositional structural deformations. The studied stratigraphic succession has been subdivided into five informal stratigraphic units on the basis of how structurally controlled topographic highs and depocentres, a consequence of thrust propagation, change over time. These physiographic changes of the foredeep basin have also been reconstructed through the progressive appearance and disappearance of thrust‐related mass‐transport complexes and of five bed types interpreted as being related to structurally controlled basin morphology. Apart from Bouma‐like Type‐4 beds, Type‐1 tripartite beds, characterized by an internal slurry unit, tend to increase especially in structurally controlled stratigraphic units where intrabasinal topographic highs and depocentres with slope changes favour both mud erosion and decelerations. Type‐2 beds, with an internal slump‐type chaotic unit, characterize the basal boundary of structurally controlled stratigraphic units and are interpreted as indicating tectonic uplift. Type‐3 beds are contained‐reflected beds that indicate different degrees of basin confinement, while Type‐5 are thin and fine‐grained beds deposited by dilute reflected turbulent flows able to rise up the topographic highs. The vertical and lateral distribution of these beds has been used to understand the synsedimentary structural control of the studied stratigraphic succession, represented in the Marnoso‐arenacea Formation by subtle topographic highs and depocentres created by thrust‐propagation folds and emplacements of large mass‐transport complexes.     

Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments

Co‐genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud‐poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud‐poor sandstone, siltstone and mudstone show features indicating progressive layer‐by‐layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co‐genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin‐plain settings. Co‐genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud‐rich sea‐floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (> 26 × 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run‐out on low‐gradient sea floor (< 0·1°) may simply be due to low yield strength (? 50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events.     

Variability of sedimentary sequence: numerical modeling of the deposition-erosion process

Reinterpretation of field studies of bedload transport allowed some power-law relations to be obtained between different quantities. They were taken as a basis for the construction of a numerical model simulating bedload transport. The deposition-erosion process is presented as a sequence of individual events with intensities distributed according to a power law, characteristic of the regime of self-organized criticality. The model output (sedimentary cross sections with the model age of each of its elements) is rich in features resulting from the non-linearity of the underlying process. Analysis of the results of a series of numerical experiments provided an estimation of scale invariance of model sedimentary structures in space and time. These data are tested against observed regularities of spaciotemporal variability of real sedimentary sequences. Good agreement of these data makes it possible to extrapolate the scaling relations obtained to larger scales.     

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.     

A method for the measurement of bedload sediment transport and passive faunal transport on intertidal sandflats

A simple and inexpensive sampler to measure bedload sediment transport in shallow subtidal or intertidal areas is described. The cylindrical sub-sediment trap with an aspect ratio of 20 (height: diameter) is an improvement over conventional bedload samplers which are difficult to use in shallow areas or fail to collect the biological material associated with bedload. Traps deployed on a low-energy intertidal sandflat for six months provided daily estimates of bedload transport (quartz grains: 0.001–40 kg m^?1 d^?1), passive infaunal transport (e.g., the bivalveMya arenaria, max: 800 ind m^?1 d^?1), and organic detrital flux (e.g., macrophyte fragments, max: 400 g dry wt m^?1 d^?1). Bedload rates estimated with traps were compared to predictions from a numerical bedload model to evaluate the trap’s collection and retention efficiency. A significant linear regression between observed (trap) and predicted (model) rates (r²=0.65, p<0.001, n=97) indicated that the traps were useful for the measurement of high- and low-frequency variability in bedload transport. Potential applications of the traps in benthic oceanography include recruitment and recolonization studies.