Scale Bedforms Research Articles

Experiments on Oscillatory-Flow and Combined-Flow Bed Forms: Implications for Interpreting Parts of the Shallow-Marine Sedimentary Record

Abstract Analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm-like oscillatory and combined flow. Experiments were carried out in a large wave tunnel and used a range of sand sizes (fine and very fine), wave periods (10.5 and 8 s), oscillatory velocities (0 to 125 cm/s), and unidirectional velocities (0 to 25 cm/s). At low unidirectional velocities (≤10 cm/s), addition of an increasing collinear oscillatory flow caused the bed to evolve from small-scale (wavelength 100 cm), symmetric, orbital ripples, to plane bed. At higher unidirectional velocities (>10 cm/s), a similar trend was noted, but ripples were more asymmetric. Three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions. These diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent. Distinctive features of small-scale asymmetric combined-flow ripples generated are: a 3D planform, round crest, and convex-up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a boxy appearance. Similarly, large-scale asymmetric combined-flow ripples had broad and round crests, convex-up stoss sides, and compressed due to scouring at the toe of the stoss side. Hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities. Hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed-form scale and symmetry. Hummocks were more prevalent at longer oscillatory periods and in finer-grained sediment. Stratification produced by synthetically aggrading hummocky bed profiles closely resembles hummocky cross-stratification. With the introduction of only a small unidirectional-flow component, hummocks evolve into downstream-migrating large-scale asymmetric ripples, and the resultant cross-stratification becomes similar to that produced by unidirectional-flow dunes. Accordingly, these experiments suggest that much of the hummocky cross-stratification observed in the stratigraphic record is produced by storm-generated long-period oscillatory-dominant combined flows. Inasmuch as long-period, high-energy waves require deep, wide basins to form, hummocky cross-stratification may therefore serve as a useful indicator of deposition in unrestricted, open-water conditions.

Modelling fine-scale heterogeneity for optimal history matching from aeolian gas reservoirs, Neptune Field, UKCS

Improved matching of reservoir simulation model results to production data in gas fields allows improved reservoir management and optimization of hydrocarbon production. In high net:gross aeolian sandstones this requires the full range of scales of permeability heterogeneity to be represented in a model. This necessitates the modelling and appropriate upscaling of rock properties, including the millimetre-scale lamina effects within the aeolian dune slipface facies, the contrast between dune apron and slipface facies and the impact of first- and second-order bounding surfaces within the dune complexes and the super-bounding surfaces that separate them. The Neptune Gas Field lies on the northwest margin of the Southern North Sea gas basin and in Block 47/4b and extends into Blocks 47/5a and 42/29. Gas is reservoired in the Lower Leman Formation which in the Neptune Field, comprises a succession of stacked dry aeolian sandstones and thin, wet interdune deposits. Permeability varies significantly at the lamina and bedform scale and these variations impact the performance of production wells in the field. A deterministic reservoir simulation model was constructed prior to development drilling, with permeability determined from porosity–permeability transforms based on core analysis data. The collection of continuous bottom-hole pressure (BHP) data from three of the development wells showed that this model failed to match accurately the rate of pressure decline whilst flowing, and over-predicted the build-up pressure when the wells were shut in. Review of the sedimentological information revealed that heterogeneity in the aeolian sands was being lost in the modelling of permeability and the coarse upscaling. To test whether the poor dynamic history match was due to the deterministic model not incorporating the permeability heterogeneity, a fine-scale geocellular model was built to characterize the expected geological heterogeneity and then upscaled for flow simulation. To accommodate the range in scales of heterogeneity present, the modelling was undertaken in two principal steps. First, the lamina-scale effects were modelled to determine average flow properties perpendicular and parallel to dune migration directions. The results of the lamina-scale heterogeneity modelling were then incorporated into petrophysical models of dune bodies that were characterized by low permeability dune apron facies and high permeability base of dune slipface facies that grade upwards into moderate permeability slipface facies. Upscaling of the stochastic permeability grid captured the impact of the fine-scale heterogenetity and gave BHP responses at the development wells in better agreement with the production data than any deterministic permeability distribution modelled. The stochastic rock property distributions have, thus, been adopted as the base case in all future history match and reservoir performance prediction studies.

Late Pleistocene alluvial plain sedimentation in Lower Narmada Valley, Western India: Palaeoenvironmental implications

Late Pleistocene fluvial sediments that were deposited in a slowly sinking basin are now exposed as 30–50 m high incised vertical cliffs all along the Lower Narmada Valley in western India. The exposed fluvial deposits have been classified into two sediment packages, alluvial fan sediments overlain by alluvial plain sediments. The alluvial plain sequence has not been studied previously. It consists mainly of sands and silts and is dominated by overbank deposits. Occurrence of large scale bedforms in the alluvial plain sequence points to the existence of a large sand bed river in an alluvial plain setting. The major sedimentary facies in stratigraphic order include large channel fills, giant epsilon cross bedded strata, overbank fines occurring in horizontal, massive and undulatory stratified forms associated with crevasse splay and backswamp deposits, and a reddish brown palaeosol overlain by thinly stratified sands and silts at the top of the exposed sediment succession. Large sized channel fills occur at two stratigraphic levels, which are morphologically similar and are indicative of high rates of deposition and avulsion. The large channel fill structures and the giant epsilon cross bedded strata indicate a large single channel river that was consistently 10–15 m deep and about 70–80 m wide even during the dry seasons. These dimensions are larger than those of the present day Narmada River at low discharge levels. The overbank sediments indicate rapid deposition through frequent overbank floods and floodplain aggradation by a laterally shifting river. Available chronologic data suggests that the reddish brown palaeosol correlates with a regional phase of pedogenesis in the alluvial plain of Gujarat prior to the Last Glacial Maximum (LGM). The thinly stratified sands and silts overlying the palaeosol were deposited by a considerably depleted but perennial river during the arid phase of the Last Glacial Maximum. Overall, the alluvial plain sediments of the Lower Narmada valley, particularly those below the palaeosol, have been attributed to a hyper-avulsive large river with low sinuosity whose high discharge levels were determined primarily by a large catchment area further to the east and not by the semiarid climate prevailing in the Gujarat alluvial plain during the upper part of the Late Pleistocene. The study concludes that the Narmada River has maintained a large catchment at least since the last 100 ka, however, the river was characterised by a much bigger channel during much of the Late Pleistocene with discharge levels higher than the present day.

Scale and the detection of land‐use effects on morphology, vegetation and macroinvertebrate communities of grassland streams

Summary1. Land‐use studies are challenging because of the difficulty of finding catchments that can be used as replicates and because land‐use effects may be obscured by sources of variance acting over spatial scales smaller than the catchment. To determine the extent to which land‐use effects on stream ecosystems are scale dependent, we designed a whole‐catchment study of six matched pairs (pasture versus native tussock) of second‐order stream catchments, taking replicate samples from replicate bedforms (pools and riffles) in each stream.2. Pasture streams had a smaller representation of endemic riparian plant species, particularly tussock grasses, higher bank erosion, a somewhat deeper layer of fine sediment, lower water velocities in riffles, less moss cover and higher macroinvertebrate biodiversity. At the bedform scale, suspendable inorganic sediment (SIS) was higher in pools than riffles and in pasture streams there was a negative relationship between SIS and the percentage of the bed free of overhanging vegetation. Differences between stream reaches (including any interactions between land use and stream pair) were significant for SIS, substrate depth and characteristics of riparian vegetation. There were also significant differences between replicate bedforms in the same stream reaches in percentage exotic species in overhanging vegetation, percentage moss cover, QMCI (Quantitative Macroinvertebrate Community Index – a macroinvertebrate‐based stream health index) and macroinvertebrate density.3. Significant differences among stream reaches and among replicate bedform units within the same reach, as well as interactions between these spatial units and land‐use effects, are neither trivial nor ‘noise’ but represent real differences among spatial units that typically are unaccounted for in stream studies. Our multi‐scale study design, accompanied by an investigation of the explanatory power of different factors operating at different scales, provides an improved understanding of variability in nature.

Sedimentology of cold-climate aeolian sandsheet deposits in the Askja region of northeast Iceland

Active aeolian sandsheets cover an area of 270 km 2 in the Askja region of NE Iceland. The sandsheet is divided into (i) a deflationary upwind part that is characterised by deflation lags, ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles, (ii) an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick, where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars, and (iii) a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields. Trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet. These sinuous-crested, oblique bedforms attain heights of 0.35 m and downwind spacings of 6 m. They are composed principally of low-density, pebble-grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby Askja volcanic centre. The distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial, ice-margin fluvial, aeolian and volcaniclastic processes under the influence of a distinctive climatic regime. Sediment supply at the upwind sandsheet margin, adjacent to the Dyngjujökull glacier, increases during summer periods when rates of glacial ablation are at a maximum. Longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat, and by periodic eruptions of volcaniclastic debris from the Askja volcanic centre. The availability of sediment for aeolian transport is controlled by the level of the groundwater table, the depth and continuity of the permafrost layer in the proglacial area, the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months. The transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods. Both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969. The suite of aeolian landforms developed within the Askja region, as defined by sediment texture and composition, and bedform scale, morphology and spatial distribution, reflects a special set of controlling circumstances, which mean that the Askja Sandsheet deposits differ significantly from those described from elsewhere.

Numerical modeling of flow processes over gravelly surfaces using structured grids and a numerical porosity treatment

This article describes the development and validation of a method for representing the complex surface topography of gravel bed rivers in high‐resolution three‐dimensional computational fluid dynamic models. This is based on a regular structured grid and the application of a porosity modification to the mass conservation equation in which fully blocked cells are assigned a porosity of zero, fully unblocked cells are assigned a porosity of one, and partly blocked cells are assigned a porosity of between 0 and 1, according to the percentage of the cell volume that is blocked. The model retains an equilibrium wall function and an RNG‐type two‐equation turbulence model. The model is combined with a 0.002 m resolution digital elevation model of a flume‐based, water‐worked, gravel bed surface, acquired using two‐media digital photogrammetry and with surface elevations that are precise to ±0.001 m. The model is validated by comparison with velocity data measured using a three‐component acoustic Doppler velocimeter (ADV). Model validation demonstrates a significantly improved level of agreement than in previous studies, notably in relation to shear at the bed, although the resolution of model predictions was significantly higher than the ADV measurements, making model assessment in the presence of strong shear especially difficult. A series of simulations to assess model sensitivity to bed topographic and roughness representation were undertaken. These demonstrated inherent limitations in the prediction of 3‐D flow fields in gravel bed rivers without high‐resolution topographic representation. They also showed that model predictions of downstream flux were more sensitive to topographic smoothing that to changes in the roughness parameterization, reflecting the importance of both mass conservation (i.e., blockage) and momentum conservation effects at the grain and bed form scale. Model predictions allowed visualization of the structure of form‐flow interactions at high resolution. In particular, the most protruding bed particles exerted a critical control on the turbulent kinetic energy maxima typically observed at about 20% of the flow depth above the bed.

Habitat scale and biodiversity: influence of catchment, stream reach and bedform scales on local invertebrate diversity

Although many studies have investigated the influence of environmental patterns on local stream invertebrate diversity, there has been little consistency in reported relationships between diversity and particular environmental variables. Here we test the hypothesis that local stream invertebrate diversity is determined by a combination of factors occurring at multiple spatial scales. We developed predictive models relating invertebrate diversity (species richness and equitability) to environmental variables collected at various spatial scales (bedform, reach and catchment, respectively) using data from 97 sampling sites dispersed throughout the Taieri River drainage in New Zealand. Models based on an individual scale of perception (bedform, reach or catchment) were not able to match predictions to observations (r 0.01, between observed and predicted equitability and species richness). In contrast, models incorporating all three scales simultaneously were highly significant (P < 0.01; r = 0.55 and 0.64, between observed and predicted equitability and species richness, respectively). The most influential variables for both richness and equitability were median particle size at the bedform scale, adjacent land use at the reach scale, and relief ratio at the catchment scale. Our findings suggest that patterns observed in local assemblages are not determined solely by local mechanisms acting within assemblages, but also result from processes operating at larger spatial scales. The integration of different spatial scales may be the key to increasing model predictability and our understanding of the factors that determine local biodiversity.

Sand Waves in the Messina Strait, Italy

In the present contribution the presence of sand waves in the Messina Strait, Italy, is analysed, on the basis of unpublished data gathered during two different surveys carried out in 1991 and 1992. The study is carried out both from a sedimentological and morphological viewpoint, and in relation to the hydrodynamics in the Strait. Sand waves in two areas are examined: the first one lies just outside the northern part of the Strait, where the flow is rather complex, while the second one lies farther south in the Strait, where the tide characteristics force the flow essentially to oscillate along one direction. As a matter of fact, the dunes are made up with rather coarse sand; therefore, even where current direction varies significantly, only the highest currents are able to mobilise sediment. However, the differences in flow conditions play a significant role on the generation of smaller scale bed-forms (megaripples) in the first area, as obtained from a FFT analysis, and a likely sand bank in the second one.A morphometric analysis showed that there is not a remarkable variability of these geometric characteristics at constant depth in the two areas. Steepness does not change much as well, and it appears always very low, thus suggesting a likely negligible effect of flow separation on these sand wave morphodynamics.Time for the examined bed-forms to re-orient has been computed, on the basis of a defect dynamics theory, with the aim of partially investigating the risk for buried cables and pipelines to become exposed. Such time has been estimated as more than one century for both areas, thus implying that it does not play a significant role on problems concerning duct and cable cross design in the Messina Strait.

Proximal–distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain)

Abstract Facies analysis of the upper Kimmeridgian rocks in the outcrops located near Ricla (Zaragoza province, northeast Spain) and the integration of the resultant data in a broader context (the northern part of the Iberian Basin), has produced two general models showing the facies distribution and the processes that controlled the sedimentation in the Kimmeridgian carbonate ramp. Using these two models the transition from shallow to relatively deep environments of the carbonate ramp is examined in detail. Model 1 corresponds to the development of a mixed carbonate-siliciclastic ramp during a slow rise and stillstand of sea level (Sequence 1-HST), whereas Model 2 represents the growth of a pure carbonate ramp during a rapid rise of sea level (Sequence 2-TST). Carbonate production was higher in the shallow ramp domains (coral reefs and oolitic shoals in Model 1 and reefs in Model 2) than in deeper domains, where there is no indication of significant pelagic or benthic production. The activity of unidirectional return flows induced by winter storms and hurricanes, played an important role in the redistribution of the sediment across the ramp, generating different coarse-grained deposits. In the inner and mid-ramp settings dunes, lower scale bedforms and tempestites occur in Model 1, and storm lobes, bars and tempestites in Model 2. Moreover, a significant bulk of the carbonate mud produced in shallow areas would eventually be resedimented in the outer ramp as suspended load in the density currents. Stillstand of sea level in Model 1 involved a rapid progradation of the inner and proximal mid-ramp carbonate and siliciclastic facies. The rapid relative sea level rise of Model 2 is determined by the dominance of the carbonate facies and by the presence of aggradational geometries in the transitional area between shallow and deep-ramp domains. The presence of relatively thick sections in the outer-ramp settings (instead of condensed sections, as observed in Model 1) during times of sea level rise (Model 2) can mainly be explained by the increase of the shallow production in the reef dominated areas.

Recent drumlins, flutes and lineations at Vestari-Hagafellsjökull, Iceland

AbstractIn the foreland of the Vestari-Hagafellsjökull glacier, Langajökull, central Iceland, three scales of subglacial bed forms are found: drumlins, flutes and lineations. The smaller bed forms are superimposed upon the larger. These three bed forms probably formed as a result of the presence of a rigid suhglacial core producing a lee-side low-pressure area into which sediment flowed and collected. The cores of the drumlins are tumuli (lava blisters), whilst the cores of the flutes and lineations were large clasts.Differences between the three scales of bed forms are that: (a) the flutes and lineations are far more mobile features as their cores could move within the deforming layer, whilst the drumlin bedrock cores were fixed; (b) the lineations formed in association with a clast pavement.The change in scale of the bed forms may he related to changes in thickness of the deforming layer and size of the core. At this site, smaller bed forms were superimposed upon larger bed forms and these may have formed as the deforming layer progressively thinned in association with glacial retreat.

Recent drumlins, flutes and lineations at Vestari-Hagafellsjökull, Iceland

AbstractIn the foreland of the Vestari-Hagafellsjökull glacier, Langajökull, central Iceland, three scales of subglacial bed forms are found: drumlins, flutes and lineations. The smaller bed forms are superimposed upon the larger. These three bed forms probably formed as a result of the presence of a rigid suhglacial core producing a lee-side low-pressure area into which sediment flowed and collected. The cores of the drumlins are tumuli (lava blisters), whilst the cores of the flutes and lineations were large clasts.Differences between the three scales of bed forms are that: (a) the flutes and lineations are far more mobile features as their cores could move within the deforming layer, whilst the drumlin bedrock cores were fixed; (b) the lineations formed in association with a clast pavement.The change in scale of the bed forms may he related to changes in thickness of the deforming layer and size of the core. At this site, smaller bed forms were superimposed upon larger bed forms and these may have formed as the deforming layer progressively thinned in association with glacial retreat.

The development of small scale bedforms in tidal environments: an empirical model for unsteady flow and its applications

ABSTRACTDimensions and plan morphology of current ripples are generally considered to vary with flow velocity and grain size. Recently, however, it has been shown that for sand of D50=0.095 and 0.238 mm the equilibrium dimensions are identical at all velocities within the stability field of ripples and that the plan form of equilibrium ripples is linguoid. On this basis, an empirical unsteady flow model has been developed and tested with flume experiments in order to predict ripple development in natural depositional environments. The model includes the development of washed‐out ripples and upper stage plane bed. The unsteady flow model explains the development and preservation of small scale bedforms in various tidal environments more accurately than previous models. Such bedforms can serve, therefore, as indicators of prevailing hydrodynamic conditions.

Dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves

Bedform dimensions, bed position changes, near-bed velocities and suspended sand concentrations are analyzed from measurements at a single location on a macro tidal beach in the south west of England. The study was conducted in 0.5-2.25 m water depth under both swell and wind-generated storm waves with both weak and strong currents present. Bed positions and suspended sand concentrations were measured using the 3 transducers of a multi-frequency acoustic backscatter sensor with 5 mm vertical resolution. Two distinct bed types, based on wavelength (λ), were observed, each with two or more subtypes possible: (I) small-scale bedforms ( λ < 20 cm) which include two-dimensional pre-vortex (1), vortex (2), post-vortex (3) and three-dimensional vortex (4) forms; (II) large-scale bedforms ( λ > 20 cm) which include two-dimensional (5) forms and three-dimensional vortex (6) forms. Small-scale forms were dominant under non-breaking conditions while the large-scale forms occurred under both non-breaking and breaking waves; the large-scale forms dominate under breaking conditions. Both types, but particularly low steepness forms, were highly mobile with maximum horizontal migration rates of 5 cm min −1. Large (upto 15 cm) and rapid (upto 3.0 cm min −1) changes in vertical bed elevation were also observed in association with the development and migration of large-scale forms. Large-scale bedforms were also highly variable spatially, often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes. Observations suggest these forms are present even under high energy surf zone conditions (wave Shields ⩾ 1). Models for predicting ripple dimensions did not perform well in this environment. This lack of agreement reflects the complex hydrodynamic regimes associated with random (grouped) shoaling and breaking waves together with the presence of currents, often at large angles to the waves, as well as the rapid rates of change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment. Suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration. Estimates of transport rates are subject to potentially large errors (upto 30%) without compensation for bed elevation changes relative to sensor position.

Bedform dimensions and migration rates under shoaling and breaking waves

Ripple wave lengths, heights and migration rates have been measured during mainly swell wave conditions on a macrotidal beach in the U.K. Three high frequency acoustic backscatter sensors were used to follow the changes of the bed position at three locations while a fourth was used in a side-scan mode to examine the wave lengths and movements of the bedforms over a 1 m shore-normal section. Two scales of bedforms are clearly visible; low amplitude (∼0.5–2cm) ripples with wavelengths of 7–20 cm and larger bedforms, usually correlating with lunate megaripples (LMR) observed by divers and discussed by O sborne and V incent, 1992, Proceedings of the 23rd Conference on Coastal Engineering ASCE, pp. 2321–2331, which are 3–8 cm in height and 0.3–0.8 m wavelength. Both forms coexist outside the breakpoint under shoaling waves but in the surf zone the smaller ripples can be destroyed by plunging breakers and high orbital current speeds. The larger bedforms occurred in the surf zone during all the tidal cycles for which measurements were made and appear to be significant surf zone features. No significant correlation was observed between the wave lengths of the small ripples and the orbital excursion A 0 nor with the wave Reynolds number, Shields number or Mobility number, but the migration rate was significantly correlated (at 99%) with all these parameters. Conversely, for the larger bedforms, significant correlation was found for the wave lengths but not the migration rates. We conclude that deployment of high frequency ABS in a side-scan mode is a simple and relative effective method for obtaining bedform measurements in the surf zone.

Formation and Changes of Multiple Sand Bars and Influence of Fine Sand Bed

Decreasing process of bar modes, which was found in basic experiments on plain bed and is regarded as a fundamental characteristic in formative process of multiple bars, is discussed with initial bar amplification of various modes evaluated by an existing linear stability theory. Specific features of multiple bar formation with micro scale bedforms are summarized from experiments carried out on a fine sand bed, where the mode decreasing process was also observed. Validity of the existing stability theory is examined to apply to prediction of changes in multiple bar modes on ripple-covered fine sand beds.

Floodplain sedimentology of the Squamish River, British Columbia: relevance of element analysis

ABSTRACTElement analysis of modern‐day floodplains provides a framework for characterizing associations amongst depositional forms, the processes responsible for them and their local depositional environment. From interpretation of the spatial association of elements, mechanisms of floodplain evolution can be analysed. The Squamish River, in southwestern British Columbia, is a high‐energy, gravel‐based river, which exhibits a distinct downstream gradation in channel planform type. The floodplain sedimentology of this river is evaluated using an element approach. Five elements, defined on the basis of their morphological outline, position within sediment sequences and sedimentological character, describe the floodplain sedimentology: (i) top‐stratum, (ii) chute channel; (iii) ridge; (iv) bar platform; (v) basal channel gravels. The sedimentological composition of each element is described. Each of these units relates directly to morphostratigraphic units which make up contemporary bars of the Squamish River. Associations among facies defined at the bedform scale, morphostratigraphic units on bar surfaces and elemental floodplain features are described and explained.The vertical stacking arrangement of elements is analysed in trenches (dug perpendicular to the main channel) and in bank exposures. Two elemental sedimentology models are proposed. In the first model, bar platform sands are discontinuous above basal channel gravels. Chute channel, ridge and proximal topstratum elements form thick sequences above. The second model is characterized by sequences in which distal top‐stratum deposits are observed. In these instances, bar platform sands are better preserved beneath the distal top‐stratum element, with proximal top‐stratum elements above. The applicability of these models is determined primarily by position on the floodplain. Chute channel reworking of floodplain sediments and replacement by top‐stratum elements is the dominant process marginal to contemporary bars. Sites in which channel avulsion has resulted in preservation of distal top‐stratum deposits in the midsequence of the present‐day channel banks determine the occurrence of the second model. Although channel planform style changes down‐valley in the study reach from braided to meandering, these two models apply in each reach. It is concluded that processes operative at the element scale, rather than the channel planform scale, determine floodplain sedimentology.

Bar Sedimentology of the Squamish River, British Columbia: Definition and Application of Morphostratigraphic Units

ABSTRACT In a 20-km river reach of the Squamish River, British Columbia, in which channel planform changes progressively downstream from braided to meandering, there is a corresponding down-valley change in bar type from mid-channel compound bars to bank-attached compound bars to point bars. On each of 10 bars studied, within-bar facies and particle size trends relate directly to the spatial distribution of local-scale depositional environments on bar surfaces. Four fluvial morphostratigraphic units, defined in terms of their surface morphology and sedimentary characteristics, are differentiated: bar platform, chute channel, ridge, and remnant floodplain. The morphology, scale, and sedimentologic character of each morphostratigraphic unit are described. Compound bars are composed of extensive bar platform units dissected by chute channels, with remnants of other morphostratigraphic units. Within-bar facies and particle size trends are highly irregular, whether viewed down-bar, across-bar or vertically. Sediment sequences on point bars exhibit both around-the-bend and lateral trends. Bar platform units are transitional laterally to alternating ridge and chute features away from the main channel, with floodplain sequences beyond. Analysis of bar sedimentology using fluvial morphostratigraphic unit analysis provides a more reliable basis for palaeoenvironmental reconstruction than do existing procedures based on facies associations at the channel bedform scale.

River Channels: Environment and Process

1. Rivers: environment, process and form: Keith S.Richards (Department of Geography, University of Cambridge) 2. Spatial adjustments to temporal variations in flood regime in some Australian rivers: Robin F.Warner (Department of Geography, University of Sydney) 3. The effect of active tectonics on alluvial river morphology: Daniel I.Gregory (Water Engineering and Technology Inc., Colorado) and Stanley A.Schumm (Department of Earth Sciences, Colorado State University) 4. Modelling fluvial systems: rock, gravel and sand bed channels: Alan D.Howard (Department of Environmental Science, University of Virginia) 5. River Channel adjustment - the downstream dimension: D.Knighton (Department of Geography, University of Sheffield) 6. Hydraulic and sedimentary controls of channel pattern: Rob Ferguson (Department of Environmental Science, University of Stirling) 7. Bed forms and clast size changes in gravel bed rivers: B.J.Bluck: (Department of Geology, University of Glasgow) 8. Mechanics of flow and sediment transport in river bends: William E.Dietrich (Department of Geology and Geophysics, University of California) 9. Channel boundary shape - evolution and equilibrium: T.R.H. Davies (Agricultural Engineering Department, Lincoln College, Canterbury) 10. Small and medium scale bedforms in gravel bed rivers: Pamela S.Naden (School of Geography, Leeds University) and Andrew C.Brayshaw (BP PLC, Brittanic House) 11. Measuring and modelling bedload transport in channels with coarse bed materials: James C.Bathurst (NERC Water Resource Systems Research Unit, Department of Civil Engineering, University of Newcastle-upon-Tyne) 12. The classification and characterization of rivers: M.P.Mosley (Ministry of Works and Development, New Zealand) 13. Bed stability in gravel streams, with reference to stream regulation and ecology: P.A.Carling (Freshwater Biological Association, Cumbria) 14. Applied fluvial geomorphology: river engineering project appraisal in its geomorphological context: K.S.Richards, (Department of Geography, University of Cambridge) with D.Brunsden, (Department of Geography, King's College London) D.K.C.Jones, (Department of Geography, LSE) and M.McCaig (formerly Geomorphological Services Ltd., Bucks.).

Transitions in a Fluvial System--Abo Formation, Southeastern Nacimiento Mountains, New Mexico: ABSTRACT

The Abo Formation (Wolfcampian) in the southeastern Nacimiento Mountains, New Mexico, is an alluvial plain red-bed sequence which records the evolution of a fluvial system within the distal reaches of a large clastic wedge. Sediment was derived from the southern Uncompahgre uplift, with a paleoflow to the southwest. The Abo Formation consists of mudstone, feldspathic sandstone, and intraformational conglomerate. Three major lithofacies associations, representing distinct fluvial depositional settings, may be distinguished. The lowermost division (Unit A) (100+ m, or 330+ ft, thick) is mudstone dominant with isolated sandstone bodies showing cutbanks and lateral accretion surfaces. Ridge-and-swale features were noted on an exhumed point bar surface. The sandstones commonly show a progressive upward decrease in the scale of internal bedforms with a lower scour surface containing intraformational conglomerate. Trough cross-bedding is the dominant bedform in the sandstones with common planar-tabular and flat-bedded interbeds. The uppermost sandstones, characteristic of levee and crevasse-splay sands, are ripple cross-bedded with abundant bioturbation and some plant roots. The overbank mudstones contain numerous pedogenic horizons characterized by calcrete nodules (representative of all stages of evolution), vein networks, pseudo-anticlinal structures, slickensides, color mottling, and bioturbation. Vertebrate remains are found in the mudstones. The middle division (Unit B) (approximately 70 m, or 230 ft, thick) is characterized by thick multilateral channel sandstones and thinner mudstones. The sandstones are coarse grained and contain common intraformational conglomerate. Large-scale trough cross-stratification is the dominant bedform. Pedogenic horizons are present but less common than in unit A. The uppermost division (Unit C) (approximately 30 m, or 100 ft, thick) is entirely composed of reddish-orange, very fine-grained sandstone. Flat bedding is the dominant bedform. High aggradation is indicated by abundant dish structures in the sandstones and aggradational ripple cross-bedding. Mudstone is rare and preserved as thin laminae. End_Page 466------------------------------ The fluvial sequence from Unit A to Unit B is thought to represent the transition from a fine-grained to a coarse-grained meander-belt system. The transition to Unit C reflects a change to flash-flood, ephemeral-stream controlled sedimentation. Allocyclic phenomena are suggested as probably cause of these fluvial transitions. End_of_Article - Last_Page 467------------

Seabed Morphology of George V Continental Shelf, Antarctica: ABSTRACT

Seabed processes at high latitudes are known to involve ice and ice-related processes. Antarctic sedimentologic studies have outlined the glacial domination of major modern sedimentary processes and topographic features of the Antarctic shelf. The troughs, basins, and shoals of the shelf exhibit glacial and slump deposits, turbidites, and preglacial outcrops. The bed forms associated with the glacial and ice-related processes, as well as with the slumping and current-related processes, have not yet been defined. This study investigates the smaller scale bed forms and their relation to seabed processes. The outcome of a cruise during January 1984 to the George V continental shelf is expected to show the major influence of icebergs in determining the modern morphology of the shoals and bank tops. The bed forms related to grounding of such iceblocks will help define the nature of ice/sea-floor interactions. We postulate that significant oceanic energy is expended on the sea floor through grounding of ice on the banks during the winter when wave and current activity under the ice is minimized. The depth of these shoals may be controlled in part by the maximum dominant keel depths of the ice shelf and of glacially calved iceblocks. On the basin slopes and floors, glacial, mass-failure, and current-related morphologies are expected to dominate. End_of_Article - Last_Page 452------------