model for the combined flow origin of hummocky cross stratification many shallow marine sandstones are characterized by gently dipping trough cross stratification there appear to be significant similarities between this low angle cross stratification and typical hummocky cross stratification and it is proposed that much of what is currently recognized as hummocky cross stratification is related to low relief megaripple bed forms that are equilibrium structures for fine grained sand under combined wave surge and unidirectional currents the geometry that obtains when these bed forms migrate is low angle trough cross stratification high rates of sedimentation from incipient suspension and limited lateral migration of individual bed forms promote preservation of convex upward depositional surfaces hummocks between troughs highly unsteady flow leads to abundant internal scour and drape surfaces neither random scour and drape nor oscillatory currents seem sufficient to generate hummocky bed forms

horizontal lamination and the sequence of bed phases and stratification under upper flow regime conditions two forms of horizontal laminae have been produced under upper flow regime conditions in a flume by deposition on a plane bed and on low in phase waves upper plane bed laminae consist of approximately equal numbers of fining upward fu and coarsening upward cu textural laminae heavy minerals when present are typically within or form cu laminae in horizontal laminae formed under low in phase waves fu laminae may predominate and are significantly thicker than cu laminae heavy minerals are associated with the tops of some fu laminae and are absent from cu laminae deposits aggraded on plane beds and low in phase waves both display low angle 13 14 upstream grain imbrication upper plane bed horizontal laminae are attributed to the effects of the burst sweep cycle on local bedload transport bursts form fu laminae and sweeps form cu laminae some fu laminae in in phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest heavy minerals are concentrated in the tops of these fu laminae due to their enrichment on the crests of the migrating bed waves other fu and cu laminae in in phase wave deposits probably formed by the action of bursts and sweeps as on upper plane beds a sequence of bed phases and stratification formed under upper flow regime conditions is described based on the observations here combined with descriptions from the literature with increasing flow strength the gradual sequence of bed phases is 1 plane bed 2 low relief downstream migrating inphase waves increasing in amplitude and wavelength with increasing flow strength 3 stationary in phase waves 4 upstream migrating including breaking wave antidunes the sequence of stratification formed under these bed phases is 1 upper plane bed horizontal lamination 2 in phase wave horizontal lamination to in phase wave foreset cross laminae 3 in phase wave drape laminae 4 antidune backset cross laminae

geometry of ripples and dunes a kinematic theory for lower regime bedform shape is presented and examined in the light of extensive bedform data observed in large prototype channels depending upon the extent of bedform maturity the theory predicts uniquely defined nondimensional shapes for upstream faces these shapes are borne reasonably well by the observed data in spite of large variation in bedform size the length of upstream face relative to the wavelength is randomly distributed with most probable value of 0 614 which is close to the value 0 667 predicted by the theory for mature ripples and dunes

the initiation of sedimentary furrows by standing internal waves longitudinal helical vortices are generally considered to be the cause of longitudinal sedimentary bedforms in tidal oscillatory flows however it is not clear how a regular system of vortices will become fixed for long enough to establish the bedforms it is proposed that the presence of standing internal waves with axes parallel to the tidal current have provided the basic flow pattern on which longitudinal helical vortices develop in southampton water the observed furrows then developed in a pattern determined by the standing internal waves

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

the bouma division a and the possible duration of turbidity currents abstract experimental data arnott and hand 1989 combined with theoretical arguments suggest that the transition in a turbidite from division a to division b occurs at a deposition rate of unsupported grains of about 6 7 10 4 m s 1 in the case of fine medium quartz sand a second rate may be calculated from properties measured at the transition from division b to division c allen 1971b these rates together permit an estimate to be made of the order of magnitude of the duration of deposition from a turbidity current the calculated order of a few tens of minutes for a mio pliocene turbidite described and interpreted hydraulically by komar 1985 is in broad agreement with his independent estimate based on the sequenc of sedimentary structures

earthquake magnitude frequency epicentral distance and soft sediment deformation in sedimentary basins abstract empirical equations available for earthquake magnitude frequency and the maximum epicentral distance of liquefied sites versus magnitude are combined with the characteristic instantaneous sediment deposition rate at a station in a sedimentary basin to form a model for the local stratigraphic frequency of earthquake induced soft sediment deformation structures both the style of sedimentation and the characteristics of the earthquake source combine to determine the pattern of incidence of the structures in a basin the model is probably most directly applicable to the geodynamic interpretation of fluvial basins in which substantial volumes of readily liquefied sands accumulate at high local short term rates

skin friction behind isolated hemispheres and the formation of obstacle marks we have measured the mean magnitude and direction and the rms fluctuation intensity of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long downstream of reattachment the mean skin friction magnitude is about 20 greater than its free stream value along the centre line and comparably reduced to either side of it the horizontal divergence of the skin friction vector field is positive divergent along the centre line and negative convergent to either side of it neither of these conditions favours development of a ridge longer than the separation length along the centre line the development of ridges or tails many obstacle heights long commonly observed in nature requires considerable modification of the simple sediment free wakes we have studied

the formation of shadow dunes abstract an investigation of the modification of airflow by a discrete semi circular roughness element in this case vegetation the resulting flow structure and the formation of pyramidal shaped shadow dunes is presented two erect pioneer sand dune grasses festuca littoralis and ammophila arenaria were used in the experiments which were conducted in both the field and a wind tunnel for a range of wind velocities 4 20 m sec 1 the three dimensional flow structure about the vegetation is dominated by horizontal separation characterized by symmetrically opposed reversing vortices which form within a triangular wake a vertical separation envelope is also formed over the vegetation pyramidal shadow dunes are formed within this flow structure when aeolian sand transport occurs shadow dune height is dependent on element basal width and repose angle of the sand shadow dune length is dependent on element basal width and wind velocity the results are extrapolated to include all semi porous to non porous semicircular discrete roughness elements the role of shadow dune formation in the development of foredunes and related environments is examined

early permian rotliegendes palaeowinds of the north sea abstract the classical desert sediments of the early permian upper rotliegendes were deposited in a post variscan basin that extends from eastern england to the russo polish border which has been referred to as the southern permian basin these sediments comprise four main depositional facies fluvial wadi aeolian sabkha and lacustrine the lacustrine facies includes important bedded halites the southern permian basin is separated from the smaller northern permian basin by the fragmented mid north sea ringkobing fyn system of highs and beyond the grampian spur lies the even smaller moray firth basin these latter basins contain rock sequences similar to those of the southern basin with the exception that bedded halite has not been recognised in the lacustrine facies the rotliegendes of the southern basin apparently conforms to deposition in a northern hemisphere tradewind desert similar to the modern southern sahara apart from regional palaeomagnetic considerations and the occurrence of sediments of a non arid nature south of the variscan highlands the strongest evidence supporting this interpretation is the pattern of palaeowind directions deduced from the orientations of dune bedding this is seen in both outcrop and core and derived from the continuous dip meter logs of many north sea wells by plotting the poles to the bedding attitudes on polar nets palaeowind directions have also been deduced from the dip meter logs of wells drilled in the northern and moray firth basins in general they indicate winds that blew in a direction opposed to those of the southern basin thus an area of high barometric pressure seems to have been located over the mid north sea high when the bedding attitudes in a well are roughly unidirectional an origin on a transverse dune is normally indicated with some wells the indicated palaeowind direction is regionally anomalous in many cases this can be rectified if the well is presumed to have drilled down through one flank of a seif dune the indicated palaeowind is thus changed by up to 90 and then commonly fits the regional pattern these early permian desert basins were smaller and closer to the equator than the modern sahara their location in the centre of a continent between the caledonian and variscan mountain ranges suggests the analogy of a more tropical equivalent of the central asian takla makan desert

velocity measurements close to rippled beds in oscillatory flow the results are reported of velocity measurements in oscillatory flow over rippled beds velocities were measured with a laser doppler anemometer in both an oscillating tray rig and an oscillatory flow water channel both self formed and artificial ripples were examined in addition some measurements were made with an apparently plane bed with intense sediment motion the experimental results were compared with the predictions of kalkanis 1964 kajiura 1968 bakker 1973 1974 and sleath 1974 the closest agreement between theory and experiment was obtained with the method of sleath measurements of the eulerian drift velocities showed drift towards the adjacent ripple crest in the immediate vicinity of the bed and away from the crest further out

the pattern of grainfall deposition in the lee of aeolian dunes a simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance the deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity the velocity being weighted by its probability density the resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink beyond which deposition rates fall off roughly exponentially an important length scale emerges that characterizes this decay with distance from the brink the length increasing with wind velocity and decreasing with grain diameter it is shown that this length scale is on the order of one metre for typical grain size and wind conditions that this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes the position of a pivot point on the lee slope may be predicted separating source regions from accumulation regions for grainflow avalanche deposits the calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits but they provide the initial geometry for grainflow avalanches the initial failures should coincide with the steepest gradient in grainfall deposition slightly downslope from the grainfall maximum

bar development in a fluvial sandstone westphalian a scotland the fluvial sandstone beneath the mill coal in the westphalian a of scotland erosively overlies a lake mudstone slightly erosive surfaces within the sandstone traceable for over 200 m are used to divide it into two types of major sedimentary units termed type a and type b type a sand units are approximately 200 m wide up to 7 m thick convex upward and lenticular in all directions the constituent cosets overlap to the ene and dip mainly at 1 2 downcurrent nnw but locally at 10 15 where thickest type a sand units display a vertical facies sequence commencing with trough cross bedded and massive sandstone overlain by a thick zone of ripple cross lamination a thin zone of trough cross beds and a variably eroded silt drape up to 0 4 m thick attenuated lateral margins are dominated by flat bedded sandstone with primary current lineation type a sand units are interpreted as deposits which were accreted on to a large fluvial bar during successive flood events the bar is thought to have had a similar topographic significance to sand waves described from the brahmaputra and slip face bounded bars observed in the south saskatchewan river palaeocurrents measured from trough cross bed sets 0 3 1 0 m thick within type b sand units indicate flow to the wsw perpendicular to the palaeoflow direction measured from type a units in sections perpendicular to the wsw flow direction type b units are lenticular and in ene wsw trending sections they can be traced for over 80 m at a constant thickness type b sand units are interpreted as the product of low stage channels which flowed across bar fronts and tops the sandstone described herein is interpreted as a braided type river deposit but is atypical because it is fine grained and has an internal structure dominated by ripple cross lamination and upper phase plane beds the palaeoriver is thought to have been of low sinuosity 7 10 m deep with a high suspended load and large rapidly fluctuating discharge at low stage a braided type flow pattern developed around submerged bars the regime of the palaeoriver was probably controlled by the fine sediment grain size and humid tropical climate

flow separation and the internal structure of shadow dunes abstract in the sand undersaturated aeolian terrains of southern kuwait the only dune bedforms are nebkhas and shadow dunes both of which are confined to the surfaces of evaporitic pans shadow dunes are aerodynamic bedforms with a quasi pyramid profile and caused by horizontal flow separation to the lee of impermeable barriers the nebkhas their internal structure consists of a bipolar azimuths and bimodal dip distributions of tabular planar grainfall cross strata with a chevron like pattern in the dune crest and is formed by variable secondary winds and b alternating reactivation surfaces probably representing third order bounding planes on each slip face this structure is developed in an essentially prevailing unidirectional wind field with minor fluctuations about a vector mean with a rising water table causing sand sheets to migrate over and cover the evaporitic pans these cross strata could be preserved in ancient sequences the internal geometry of shadow dunes could be of potential value in estimating regional palaeowind directions and in palaeogeographic reconstructions of sand starved desert terrains

sedimentary furrows and organized flow structure a study in lake superior investigations of the geology and physical limnology of a field of sedimentary furrows in lake superior have provided information on the characteristics of these bedforms and their relationship to the wind driven currents and associated turbulence structures in the bottom boundary layer the results generally support the existing conceptual model for furrow formation furrows in the study area 2 km2 at 100 m depth are spaced 20 l 00 m apart and exceed 800 m long the troughs being 0 5 m deep and 3 5 m wide sediment cores collected by submersible are poorly sorted cohesive terrigenous silts with a mean grain size of 20 pm relative to sediments in the interfurrow area those in the trough contain higher fractions of sand and coarse debris and accumulate at a rate 15 slower strong bottom currents speed 6 cm s l usually flow within 20 of the furrow direction and are most frequent between late october and early february when wind forced currents reach the bottom annual peak speeds approach 30 cm s l the relationship of the current and temperature records to the wind field is consistent with predictions of coastal jet theory profiling current meters deployed on either side of a furrow showed near bottom cross stream flow converging over the furrow with divergence higher in the boundary layer when the current speed exceeded 6 cm s l a preliminary numerical model exhibits a cross stream flow pattern consistent with the profiler observations

aeolian granule ripple deposits namibia granule ripples are a common feature of most dunefields yet they have seldom been recognized in ancient deposits although granule ripples are common in erosional settings such as windward slopes of dunes or scour surfaces in interdunes they nevertheless migrate laterally and leave distinctive deposits that can be recognized in ancient rocks these deposits have characteristics of type b sand sheet deposits including poured in texture curving ripple trough tangential coarse grained foresets irregular silty layers well sorted coarse and fine layers either horizontal or within foresets and fine layers in ripple troughs wind tunnel experiments suggest that under low velocity wind conditions granule ripples grow to a significant degree as parasites dependent on saltation of fine sand grains whose impact moves the larger grains of the granule ripple although the depositional surface of granule ripples is commonly coated with a layer of coarse grains this is in most places only a few grains thick underlying deposits commonly have a poorly sorted or poured in texture this texture results from an admixture of fine grains that fall among the spaces between the larger grains during deposition

the morphology of permian palaeodunes a reinterpretation of the bridgnorth sandstone around bridgnorth england in the light of modern dune studies abstract an examination of the permian bridgnorth sandstone formation around bridgnorth shropshire has indicated that it probably accumulated in a combination of transverse and barchanoid draas with superimposed oblique crescentic and linear dunes under the influence of a fluctuating wind regime three major recurrent facies associations were identifiable isolated transverse draa with superimposed oblique crescentic and linear dunes barchanoid draa with superimposed oblique crescentic and linear dunes and bimodal sand sheets with isolated dome dunes the overall long term wind direction was probably from the east and appears to have controlled draa orientation the orientations of the superimposed dunes however were probably related to medium term wind fluctuations from a more northerly quarter possibly modified by the draa bedforms short term wind reversals may have reworked slipfaces as shown by the presence of internal scour surfaces within dune and draa foresets

giant desiccation fissures filled with calcareous eolian sand hermosa formation pennsylvanian southeastern utah abstract at two stratigraphic intervals within the upper member of the upper pennsylvanian hermosa formation calcareous eolian sand fills downward tapering fissures that are as much as 18 cm wide and 5 7 m deep fissure fillings define orthogonal polygons 10 m or more in diameter one of the host beds is primarily composed of subtidally deposited limestone the other is a thinly laminated nonmarine red siltstone both systems of fissure fillings are directly overlain and underlain by large scale cross stratified calcareous eolianites the limestone host bed contains chert pseudomorphs after gypsum compaction of host rocks contorted fissure fillings and caused doming of eolian strata over each fissure platy mineral grains in fissure fillings are aligned subparallel to bedding in the host rocks supporting the view that the fissures were passively filled rather than forcefully injected these ancient fissure systems are similar in scale and pattern to those that define giant desiccation polygons in numerous great basin playas the pennsylvanian fissures like their holocene counterparts probably formed when groundwater tables dropped from shallow levels within fine grained impermeable deposits into underlying aquifers greatly decreasing the extent of the capillary fringe our study of the fissures and host rocks supports the hypothesis that carbonate grains within the eolianites were deflated from uncemented marine sediments that were broadly exposed during regressive intervals

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

classification of large scale subaqueous bedforms a new look at an old problem sepm bedforms and bedding structures abstract a symposium entitled classification of large scale flow transverse bedforms was convened at the 1987 mid year meeting of sepm in austin texas with the purpose of examining the problems involved in classifying large subaqueous flow transverse bedforms developed in fluvial intertidal and marine environments and recommending changes in nomenclature the consensus of the participants is that despite the wide spectrum of morphologies of large scale flow transverse bedforms excluding antidunes they all occupy a similar position in the lower flow regime sequence between ripples and upper plane bed the wide variety of forms is a reflection of secondary effects such as channelization fluctuating water levels and unsteady and reversing flows the bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1 000 m the symposium panel proposes therefore that they should have only one name dune dune is preferred as it has historical precedence over other terms in use such as megaripple and sand wave the term dune should be modified by rimary descriptors of shape i e 2 d or 3 d and size based on spacing small 0 6 5 m medium 5 10 m large 10 100 m or very large 100 m and the adjective subaqueous when it is important to distinguish them from eolian dunes the panel recommends a morphologically based classification that is descriptive with an underlying genetic rationale second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature

derecho and mcs development evolution and multiscale interactions during 3 5 july 2003 abstract from 3 to 5 july 2003 during the bow echo and mesoscale convective vortex experiment bamex multiple mesoscale convective systems mcss 1 and 2 and derechos derechos an as a bw and be progressed across a preferred upper midwest corridor the derechos evolved in a favorable synoptic scale environment however the environmental details associated with each derecho such as the characteristics of the initial surface boundary the formation position relative to the upper level jet stream the presence of an upper level mesoscale disturbance and the cape shear environment varied from derecho to derecho the mcss and derechos composed three distinct convective episodes multiple mesoscale interactions between the mcss and derechos and the environment altered the character and longevity of these episodes the first convective episode consisted of derecho a which formed from merging derechos an and as northern and southern systems respectively the 200 hpa deep cold pool associated with d

swaley cross stratification produced by unidirectional flows bencliff grit upper jurassic dorset uk unusual bedforms occur in the bencliff grit at osmington mills dorset the geometry of cross strata in stacked predominantly fine grained sandstone units is dominated by low angle troughs with locally perfectly preserved antiforms which wedge out onto climbing low angle erosion surfaces many of the antiforms hummocks and troughs swales appear to have grown at similar rates of sediment fall out others show draped antiforms and thickening into swales in both cases laminae are concordant with the lower bounding surfaces of the cross sets demonstrating that vertical accretion was the dominant process this geometry together with the conspicuous absence of angle of repose cross strata give the bedforms a resemblance to amalgamated hummocky cross stratification or swaley cross stratification many workers consider amalgamated hummocky cross stratification and swaley cross stratification to be identified with storm sedimentation on the shelf to shoreface thereby bracketting the structures both in terms of process and environment however the structures in the bencliff grit suggest that such a precise interpretation of amalgamated hummocky cross stratification or swaley cross stratification is inadvisable the geometry of the cross stratification suggests deposition under a highly sediment charged but essentially unidirectional flow this study therefore supports recent views that hummocky and swaley styles of cross stratification are in reality a form of low angle trough cross stratification produced primarily by unidirectional flows clearly since undulatory forms of stratification are substrate and process dependent and not environmentally specific great care must be taken in the interpretation of palaeoenvironmental settings especially where exposures are limited as for instance in borehole cores it is unlikely that core data can be used to identify bedforms analogous to hummocky cross stratification or swaley cross stratification unless boreholes are highly deviated

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

morphology and genesis of asymmetric adhesion warts a new adhesion surface structure abstract adhesion surface structures have been studied during their formation on a fluvial bar in east greenland two main types occurred adhesion ripples and asymmetric adhesion warts adhesion ripples formed on moist surfaces their crests lay transverse to the wind direction and they migrated by trapping dry wind blown sand on their steep fronts asymmetric adhesion warts new structure formed because of falling moisture content by preferred upwind migration of small protuberances on the adhesion ripples the protuberances were apparently inherited from an initial rain sculpturing of the bar surface the asymmetric adhesion warts here described for the first time were elongate parallel to the wind associated with steep upwind facing fronts and commonly displayed sand shadow tails tapering in a downwind direction a study of devonian flood basin deposits hornelen basin norway revealed the existence of adhesion surface structures very similar to their modern analogues the devonian examples were associated with rain sculptured surfaces which are believed to have controlled the morphology of the adhesion surface structures as in the modern example the orientation of the ancient adhesion surface structures is here used for determination of the palaeowind which blew from the ene

origin of polygonal fractures in sand uppermost navajo and page sandstones page arizona abstract polygonal fractures in eolian sandstones occur along the upper surface of the navajo sandstone and along four additional surfaces within the page sandstone near page arizona the polygons are largely rectangular and range from 1 to 6 m across most of the fractures are less than 1 5 m deep and fracture widths range from hairlines to tens of centimeters or more fracture fill most commonly shows vertical layering and in some cases a series of layers is evident symmetrical about the midline of the fracture other fractures are filled with contorted or structureless deposits subhorizontal laminae and displaced or brecciated host strata in some cases surfaces marked by polygonal fractures can be shown to have been topographically low at the time of deposition and the fracture sandstone below these low areas contains calcite or siliceous nodules and siliceous cement interpreted as contractional in origin the polygonal fractures formed on an exposed evaporite encrusted surface in a manner similar to ice wedges irregularities in these fractures are attributed to erosion by wind or water evaporites evidenced by anhydrite inclusions pore shapes megaquartz replacement and pseudomorphs of evaporitic minerals provided the cohesion and contraction that caused the tensional stresses necessary for polygonal fracturing stresses were the result of thermal contraction such surfaces are thought to represent hiatuses in eolian sand sea development and probably represent deflation to near the water table during net deflationary conditions

on the curl of desiccation polygons abstract two qualitative mechanisms differential water loss and differential contraction because of a textural gradient normal or inverse have been advanced in the literature to explain the curvature of desiccation crack polygons a geometrical model is described which shows how these mechanisms operate to produce either concave up or convex up polygons depending on the direction of the gradient of water loss or shrinkage capacity the model shows that the angle subtended by the edges of a dried polygon increases with the ratio of the width to the thickness of the polygon thus explaining the thin tightly enrolled mud curls widely reported from fluvial environments it also reveals that the radius of curvature of a curled polygon increases with the initial thickness of the dried layer but declines with the differential shortening across it calculations based on the model are compatible with field and laboratory observations

unsteady flow in straight alluvial streams part 2 transition from dunes to plane bed the effect of suspended sediment on the modification of individual dunes in unsteady channel flow is analyzed the model is an extension of that developed by fredsoe 1979 in which the change of the dune dimensions was analyzed assuming that the transport of sediment mainly occurs as bed load by including suspended sediment it is possible to describe dune behaviour at high sediment transport rates for which the dunes are washed out so the bed at sufficiently high bed shear stress becomes plane the model is compared with laboratory experiments and the agreement is reasonable the effect of temperature variations on dune dimensions is incorporated in the analysis

experiments on bed configurations in fine sands under bidirectional purely oscillatory flow and the origin of hummocky cross stratification abstract we made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods t and maximum orbital velocities um in two closed flow ducts 0 1 m and 0 4 m wide the motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits the ripples produced were classified geometrically as two dimensional 2d or three dimensional 3d in plan view geometry and dynamically as oscillatory current ripples dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself and reversing current ripples dependent on he existence of the current in one direction or the other during the oscillation runs in the small duct with effectively 0 19 mm and 0 30 mm sand showed the familiar small 2d oscillatory current ripples at short t and low um passing into much larger ripples with spacing up to 2 m at periods of 6 20 s and velocities up to 1 m s with no apparent limit on size with increasing t and um even in such a narrow duct there was some evidence of 3d geometry of the large ripples runs in the large duct with effectively 0 11 mm sand at a single period of about 9 5 s showed a progression from small 2d oscillatory current ripples through small 3d reversing current ripples to large round crested 3d oscillatory current ripples with prominent superimposed 3d to 2d reversing current ripples the large 3d ripples which increased in spacing to 1 3 m at the h ghest attainable um of 1 m s shifted in position and changed in size with time seemingly at random causing substantial local and temporary erosion and deposition at a given point on the bed there was no net aggradation during the runs but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3d ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium and then plotting the profiles one above the previous with a constant upward increment the resulting stratification is strikingly similar to much hummocky cross stratification in ancient sandstones this suggests that some hummocky cross stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3d oscillatory current bed forms develop from a planar bed during strong but waning flow

hummocky cross stratification from the lower jurassic hasle formation of bornholm denmark abstract the exposed part of the lower pliensbachian hasle formation is composed of coarse siltstone and fine grained sandstone and is hummocky cross stratified throughout the stratification can be described as amalgamated h and or hf divisions the dott bourgeois ideal sequence for hummocky cross stratification is slightly modified so that the basal pebbly lag is extended to include the graded and laminated pebbly portion at the base of the h division the suggestion to include a parallel laminated p division below the h division is not followed the hummocks of the hasle formation are mainly of erosional nature but hummocks representing growing bed forms have been observed the formation was deposited in an open marine shelf 1 2 km from the fault controlled coast lines the hummocky cross stratification was probably formed in depths of 10 40 m by combined flow storm currents representing along coast geostrophic flows storms were frequent and the coastline faced a wide epeiric sea with a fetch towards the west of possibly 1000 km the hettangian of bornholm represents an overall transgressive or deepening sequence passing from lacustrine over fluvial to tidal flat deposits the recognition of hummocky cross stratification in the hasle formation thus indicates that the deepening continued in the pliensbachian this conforms with published curves on eustatic sea level changes

distribution morphology and origin of sedimentary furrows in cohesive sediments southampton water patches of sedimentary furrows are developed at several locations in the cohesive estuarine sediments of southampton water water depth 1 12 m these furrows apparently result from short periods of erosion followed by long periods of deposition although all the furrows are similar regularly spaced parallel troughs 0 5 15 m wide aligned with the dominant current furrows in different patches have different characteristics in some areas furrow width is 1 5 1 15 of furrow spacing termed narrow whereas in other areas furrow width is about 1 2 of the spacing termed wide narrow furrows have developed where sediment accumulation rates are greater than 3 6 cm yr 1 wide furrows where accumulation rates are lower cockle shells and other coarse sediments concentrated on the furrow floors and on floors of smaller 2 10 cm wide minifurrows play an important role in furrow formation and evolution as they act to widen the furrows when mobilized during current episodes uniform sedimentation across the profile during slack periods tends to narrow the furrow some of the larger furrows have remained in the same position for 12 years while mini furrows have duration scales of a few months or less well developed furrows are also found in a recently dredged channel bedforms similar to those described here may be preserved in the sedimentary record while no analogues to the larger furrows are presently known minifurrows may be morphologically similar to the gutter casts described from ancient rocks

reconstruction of ancient sea conditions with an example from the swiss molasse ancient sea conditions can be estimated from the grain size spacing and steepness of preserved ripple marks the element of greatest uncertainty in such reconstructions is the relationship between near bed orbital diameter of water particles and the ripple spacing this relationship is simple for vortex ripples of high steepness but is problematical for the low steepness forms known as post vortex rolling grain or anorbital ripples the existence field for wave ripples is between the threshold velocity for sediment movement and the onset of sheet flow most low steepness forms occurring close to the bed planation threshold a range of maximum period of formative waves can be obtained using combinations of orbital diameter and orbital velocity assuming linear wave theory to be a reasonable approximation probable wave heights wave lengths and water depths can be investigated using the transformation of wave parameters in shallowing waters and the constraints on wave dimensions provided by the wave breaking condition given reasonable estimates of wave height crude estimates of wave power allow a comparison of ancient wave influenced sequences with modern counterparts wave ripple marks preserved in the upper marine molasse of western switzerland have been investigated results which are in agreement with regional geology suggest deposition in a seaway of approximately 100 km width where moderate period waves t 3 6 s were generated the depositional facies belts were adjusted to the prevailing waves tides and fluvial outflows

upper regime parallel lamination as the result of turbulent sediment transport and low amplitude bed forms a series of laboratory experiments has been carried out in which parallel laminated deposits were produced from an upper regime plane bed the laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area 0 3 m by 1 5m fluctuations in bed elevation were measured both during deposition and at equilibrium much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence as most theories for lamina formation would require we suggest instead that extremely low amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination all the lamination produced in the laboratory experiments was normally graded using high speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain diameters thick followed by the slow sieving out of a well packed surface layer of finer sand the initial deposition is the result of small scale turbulent fluctuations in boundary shear stress the sieving out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition we term this process glazing the alignment of small scale turbulent scour and fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination

dish structures in some paleocene deep sea sandstones norwegian sector north sea origin of the dish forming clays and their effect on reservoir quality abstract the clay mineralogy of dish structured sandstones supports the intrastratal rather than the interstratal redistribution of clays during the formation of dish structures authigenic fe chlorite is the predominant constituent clay mineral in the dish structures interbedded mudstones comprise illite smectite with minor illite and chlorite dish structured horizons cause a marked reduction in average vertical permeability

sand ripples under sea waves part 2 finite amplitude development in the present paper the authors formulate a theory to predict the time development of sand ripples characterised by small but finite amplitude under the action of surface gravity waves the theory is based on a weakly nonlinear stability analysis of a flat sandy bottom subject to viscous oscillatory flow the parameters of the problem namely the reynolds number of the flow and the reynolds and froude numbers of sediments are assumed to fall within a neighbourhood of the critical conditions determined in blondeaux 1990 the analysis can predict the actual ripple height wavelength and profile when flow separation is absent i e for the case of rolling grain ripples assuming sleath s 1984 criterion for separation the values of the relevant parameters at which transition from rolling grain ripples to vortex ripples occurs are predicted a comparison between theoretical findings and experimental data supports the validity of the present theory

why deposits of longitudinal dunes are rarely recognized in the geologic record dunes that are morphologically of linear type many of which are probably of longitudinal type in a morphodynamic sense are common in modern deserts but their deposits are rarely identified in aeolian sandstones one reason for non recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long term sand transport direction thereby depositing cross strata that have unimodal cross bed dip directions and consequently resemble deposits of transverse dunes dune parallel components of sand transport can be recognized in ancient aeolian sands by examining compound crossbedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along crest direction over the large dunes

runoff microdeltas a potential emergence indicator in cross bedded sandstones abstract microdeltas produced by slope controlled runoff are useful emergence indicators in cross bedded tidal and fluvial sandstones because they occur abundantly have high preservation potential and are identifiable in cross section each microdelta normally consists of a lenticular shaped linguoid cross bed that sits within a scour at the base of a larger cross bed delta thickness approximates the relief on the base of the overlying set while their lateral extent is normally less than 1 m the deltas are closely associated with reactivation surfaces in the overlying set and are generally oriented perpendicular to the over and underlying cross beds the lowest occurrence of microdeltas in a sequence provides an estimate of the low water level the identification of which should great y aid in the quantification of paleotidal or stage fluctuation ranges

late pliocene and pleistocene climatic oscillations and monsoon upwelling recorded in sediments from the owen ridge northwestern arabian sea

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

hummocky cross stratification is not produced purely under progressive gravity waves hummocky cross stratification is a type of bedding in sedimentary rocks caused by the draping of sand around low steepness three dimensional bedforms this distinctive form of stratification has been thought to be diagnostic of the former action of storm waves and hence its occurrence has been used to reconstruct ancient palaeogeographies and marine processes here i examine theoretical and empirical aspects of hummocky cross stratification pertaining to its origin and argue that the notion that it was produced purely under progressive storm waves is incompatible with observed hummock spacings and possible values of the ratio a under waves a is the amplitude of oscillation near the bed and is the bedform spacing my view is supported by recent observations on naturally occurring hummocky megaripples from the sea floor of the inner atlantic shelf of north america

wave formed coarse grained ripples and their relationship to hummocky cross stratification abstract wave formed coarse grained ripples cgr form in water depths ranging from 3 to 160 m on transgressive surfaces in upper and lower shoreface sediments and in finer grained offshore transition deposits cgr have crest spacings of 0 25 3 0 m with amplitudes of 5 35 cm internal stratification is rarely preserved but where present crossbeds dip en echelon away from the ripple crest and individual laminae feather out away from the crest waves that form cgr commonly have wavelengths on the order of 2 5 m with periods of 8 to 14 sec these are not large storm events especially on a geological scale cgr commonly occur in close association laterally and vertically with hummocky cross stratification it is postulated that if sediment is medium to very coarse grained sand or pebbly sand then two dimensional cgr will be formed from oscillatory or oscillatory dominant flow if however sediment is very fine to fine grained sand then three dimensional hummocky cross stratification will be formed the wave conditions under which cgr form are similar to those which would form hummocky cross stratification it is only the grain size that is different reported and calculated wave conditions for hummocky cross stratification are almost identical to those for cgr modern cgr crests trend parallel to the local shoreline and bathymetry as such ancient ripple crests are a useful tool for basin reconstruction

experimental study of sand streaks formed in turbulent boundary layers fine to medium grained sand transported as bedload moves in lanes parallel to the flow that are thought to be preserved as parting lineation a series of six flume experiments was designed to discover the morphology and spacing of these lanes here called sand streaks as functions of local shear velocity u 9 10 3 to 4 8 10 2 m s 1 depth 5 10 2 and 9 5 10 2 m mean grain diameter 150 200 290 1380 m and sediment bedload concentration 0 0 0 39 low u flows produce predominantly straight non intersecting sand streaks moderate u flows produce sub parallel and en echelon sand streaks and moderate to high u flows produce wavy sand streaks and secondary streaks with a spacing an order of magnitude larger the wavy sand streaks are thought to be composed of sand grains in suspension close to the bed an upper grain size limit for the sand streak structure occurs at a grain size between 290 and 1380 m the spacings of the fine and medium grained sand streaks at low to moderate u 0 9 10 2 to 3 10 2m s 1 are similar to those predicted for low speed fluid streaks although the fine grained sand forms more closely spaced streaks than the medium grained sand the spacings of sand streaks formed at moderate to high u and at bedload concentrations greater than 0 15 are wider than those predicted for the low speed fluid streaks the wider spacing is thought to reflect a new type of flow immediately above the moving bed layer in which the formation of low speed streaks is inhibited this results from an increase in either grain concentration or grain size the spacing of parting lineation also wider than that predicted for low speed streaks may reflect this

oscillatory ripple geometry ripple geometry provides a bed roughness which governs the bed shear stress and the profile of eddy viscosity or diffusion coefficient required for the description of the sediment concentration profile or rate of sediment transport an analysis is made of ripple geometries using known tabulated laboratory and field data as well as water tunnel data obtained by the writer three different types of ripples can be identified sand light sediment and oscillating bed ripples they reflect different effects of the fluid shear stress and fluid acceleration on bed sediments it is also shown that when the bed shear stress or the amplitude of the water particle motion relative to the sediment size is small the resulting vortices are more regular with low turbulence intensity and the generated ripples are two dimensional they are called growing or vortex ripples which are strongly influenced by the amplitude of the water particle motion at the other extreme when the bed shear stress or the amplitude of the

climbing zibars of the algodones abstract zibars are coarse grained low relief slipfaceless eolian bedforms that occur on sand sheets and within interdune corridors of many eolian sand seas zibar surface features consist exclusively of wind ripples and local shadow and shrub coppice dunes associated with vegetation a study of zibars located on the southwestern margin of the algodones dune field california shows that zibars and their associated interzibar areas are migrating features oriented transverse to oblique to the resultant sand transport direction zibars leave deposits formed by deposition on the zibar lee face the zibar shape modifies the near surface airflow to produce a subtle grain size change across the zibar interzibar surface flow expansion not separation leeward of the zibar crest causes the lee face deposits those preserved upon bedform migration to have the finest grain size on the zibar interzibar surface sediment coarser than that deposited on the zibar lee face is transported more slowly and less frequently and thus becomes concentrated on the zibar stoss slope and ultimately becomes interzibar deposits migration of zibars and interzibar areas can produce two characteristic depositional sequences depending upon the angle of climb for low angles of climb a sequence of stacked amalgamated interzibar deposits is preserved zibar deposits are absent amalgamated interzibar deposits consist of low angle usually less than 5 wind ripple laminae of medium to coarse sand given higher angles of climb deposits of both the entire interzibar and zibar lower lee face are preserved low angle interzibar deposits of medium to coarse sand are overlain tangentially by finer grained fine to medium sand wind ripple laminae dipping less than 15 of the zibar lee face lee face wind ripple laminae are sharply truncated and overlain by the succeeding set of interzibar wind ripple deposits this zibar interzibar sequence is distinctive and allows for the detailed interpretation of some ancient sand sheet deposits as left by migrating zibars a possible example of ancient zibar deposits is present in the entrada sandstone jurassic of southwestern colorado

ripple formation in combined transdirectional steady and oscillatory flow measurements are described of the geometry of ripples formed on beds of sand exposed to a steady current at right angles to an oscillatory flow four different sands were studied the oscillation was produced by an oscillating tray set into the bed of a steady flow flume it was observed that straight crested ripples formed by oscillatory flow would usually develop a serpentine form when the superimposed steady current exceeded a certain limit for amplitudes of the tray velocity u less than about 0 38 m s 1 this limit corresponded to u c 31 where c is the shear velocity measured just upstream of the oscillating tray it is suggested that the serpentine form is caused by the interaction of vortices carried back and forth between adjacent ripples on this assumption the wavelength of the serpentine form would be proportional to the product of period of oscillation and near bed steady current velocity the present measurements appear to support this hypothesis although there is also evidence that the wavelength is influenced by preferred spacing patterns between vortices the measurements also show the ratio of the amplitude of the serpentine form to its wavelength to be approximately constant empirical relationships are derived relating ripple geometry to flow and sediment properties it is observed that the influence of reynolds number and sediment properties on the geometry is very weak it is suggested that this is typical of ripples formed with relatively low sediment transport rates it is also found that under the present experimental conditions the ripple spacing in the direction of oscillation is almost independent of the magnitude of the steady current and in close agreement with the wavelengths previously measured in an oscillating water tunnel this suggests that the additional inertia effects associated with oscillating tray rigs were not sufficient to affect bed geometry under the present test conditions

diffusion of sediment on the lee of dune like bedforms theoretical and numerical analysis theoretical and empirical analyses of flow structure sediment transport and sediment size characteristics at the crest of dune like bedforms indicate that it is possible to describe at least semi quantitatively the diffusion and deposition of sediment on the leeside of such structures a numerical program based on this analysis simulates the grain size distribution and deposition rate on the leeside of dunes for specified flow conditions and bed material evaluation of flow and sediment variables through the numerical simulation program shows that flow velocity flow depth and sediment size have a strong influence on the deposition rate and texture of leeside sediment before avalanching sorting of the bed material in particular appears to exert a strong control on both the grain size and the deposition rate gradients

evidence of synsedimentary tectonic movements in the triassic halite of cheshire during the triassic north west europe was subjected to tensional stresses which resulted in the formation of a complex system of rapidly subsiding grabens and wrench faulted basins this pattern of regional crustal extension which is part of the larger scale mesozoic break up of the pangean megacontinent is related to the triassic opening of the tethys ocean in southern europe and rifting in the arctic north atlantic and is a prelude to the jurassic opening of the southern north atlantic1 great thicknesses of generally continental sediments often with salt deposits accumulated in the triassic basins which were mainly avolcanic in western britain a complex series of fault bounded basins extended from the western approaches and channel area northwards to the irish sea and includes the worcester graben and the cheshire basin the present study of the halite formations in the cheshire basin shows evidence for penecon temporaneous fault movements these were a feature of triassic times but it is rare that such movements can be detected so precisely as is the case here

grain fabric in hummocky cross stratified storm beds genetic implications abstract a detailed analysis of grain fabric in samples from hummocky cross stratified hcs storm beds suggests that these shallow marine sandstones were deposited from waning combined oscillatory unidirectional flows particle a axis orientations in the deposits in plan view parallel to visible lamination vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators sole marks and subparallel to regional shoreline this alignment of a axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline imbrication of particles varies in both the horizontally laminated and hcs intervals of the hcs stor beds due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow in the basal horizontally laminated interval of an hcs storm bed grain imbrication angles vary about a mean of approximately 13 from the horizontal dipping into the paleoflow inferred from sole marks this preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow however quasi cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally laminated interval in hcs intervals imbrication angles vary about a mean that is parallel to visible laminae suggesting th t oscillatory flow had become predominant the sequence of structures and changing patterns of fabric through these hcs storm beds is consistent with deposition under waning combined flows erosional scour forming the sharp base and unidirectional sole marks occurred in response to combined flows with an effective unidirectional component the unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally laminated sands but waned and became negligible relative to a strong oscillatory component during formation of hcs

the dynamics of star dunes an example from the gran desierto mexico observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the gran desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type such interactions lead to deposition of sand in the central parts of the dune giving rise to its pyramidal shape as well as to some extension of the linear arms the major arms of the dune studied are oriented ne sw or transverse to summer sse and winter nnw winds an avalanche face up to 10 m high develops during the course of each season flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune where it is deposited as wind ripples migrate into zones of locally reduced flow velocity reattachment of the separated flow occurs on the lower part of the n or s arms parallel to the flow spring westerly winds move sand obliquely up the s and n arms of the dune and outwards on the e arm large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms formation of star dunes in the gran desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge convergent leeside secondary flows are developed which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune examples of star dunes at different stages of their development can be documented

pot and gutter casts from the chapel island formation southeast newfoundland abstract member 2 of the chapel island formation of southeast newfoundland contains a diverse suite of erosional structures known as pot and gutter casts these are particularly abundant in a lithofacies deposited in a nearshore zone characterized by erosion and sediment bypass during storm emplacement of sands the steep to overhanging walls of both pot and gutter casts imply rapid filling during the same storm events that created these depressions erosion is thought to have taken place under strong offshore directed unidirectional flow storm surge or other relaxation flow as with many sandstone tempestites the initial stages of the storm events were dominated by strong and in this case highly erosive unidirectional flow and the latter stages by oscillatory flow as evidenced by wave diagnostic lamination in the sandy infills pot casts have geometries and soles indicating vortex flow similar to that which forms potholes in bedrock in glaciated regions gutter casts that lead into potholes do so at their edge a requirement for generating rotary currents within the depression pot casts are commonly tilted relative to paleohorizontal and their direction of plunge is either upcurrent or downcurrent variation in geometry of the erosional structures from the chapel island formation and from erosional structures in general can be accounted for by the following substrate type diagenetic history of substrate and flow parameters such as pattern of water motion velocity and intermittency of flow

liquefaction features on the fraser delta evidence for prehistoric earthquakes sand dykes and sills and vented sand boils on the fraser river delta and adjacent serpentine river floodplain in southwestern british columbia record one or more major liquefaction events the source of the dykes is a shallow subsurface saturated sand unit that was deposited in foreslope and distributary channel environments during holocene progradation of the fraser delta the dykes cut steeply through a crust of delta topset muds and locally flatten out as sills in overlying peaty sediments at two sites evidence was found for venting of sand onto a subaerial or intertidal surface liquefaction and upward movement of sand and water locally deformed the intruded sediments causing some subsidence and uplift of the delta surface all observed liquefaction features are younger than ca 3500 bp and at least some are younger than 2400 bp it is not clear however whether they formed during one or several separate events a review of possible causes suggests that the liquefaction features probably are seis

tidal inlet sequence sundance formation upper jurassic north central wyoming the sandstones and coquinas of the upper 20 m of the sundance formation are interpreted as a tidal inlet back barrier shoal and sandy tidal flat sequence deposited at the close of marine jurassic sedimentation in north central wyoming the barrier strandline maintained a generally e w trend as it prograded to the north the lateral migration of inter barrier tidal inlets along the regressive shoreline of the late sundance sea caused the coquinas and sandstones of the uppermost sundance formation to be deposited as tabular laterally extensive units tidal bundles sigmoidal reactivation surfaces herringbone cross lamination and abundant mud drapes within the sandstones are evidence of considerable tidal influence during the deposition of the uppermost sundance formation earlier models which attach an offshore environment of deposition to the sequence do not explain the tabular geometries of the sandstone and coquina units and their conformable stratigraphic relationship with the overlying non marine sediments of the morrison formation

pennsylvanian to jurassic eolian transportation systems in the western united states peterson f 1988 pennsylvanian to jurassic eolian transportation systems in the western united states in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 207 260 the direction of sediment transport in eolian sandstones of permsylvanian to jurassic age was interpreted from crossbedding resultants vector means obtained from studies of eolian rocks in the western u s supplemented by data from the few eolian units of eastern north america these were compiled from the published or unpublished theses hterature from unpublished fidd data contributed by colleagues or from measurements made for this study in addition new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies regionally the crossbedding indicates northeasterly northerly or northwesterly winds present coordinates from pennsylvanian through most of middle jurassic time a rather abrupt change in wind directions occurred in late middle jurassic time late part of the cauovian age when westerly wind patterns developed by the late jurassic the winds shifted to southwesterly calculations of the consistency factor vector mean strength made from region wide analyses of the resultants indicate fairly unidirectional winds from the permsylvanian through the early jurassic middle jurassic circulation was more varied judging from crossbedding studies in the lower part of the entrada sandstone crossbedding in upper jurassic eolian rocks of wyoming and south dakota yielded a random pattern but upper jurassic rocks farther south on the colorado plateau and adjoining areas show a return to a fairly unidirectional pattern comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns however the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns although they do not cause noticeable horizontal deflections in wind patterns small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field

daily cycles in coastal dunes abstract daily cycles of summer sea breezes produce distinctive cyclic foreset deposits in dune sands of the texas and oregon coasts in both areas the winds are strong enough to transport sand only during part of the day reach a peak during the afternoon and vary little in direction during the period of sand transport cyclicity in the foreset deposits is made evident by variations in the type of sedimentary structure the texture and the heavy mineral content of the sand some of the cyclic deposits are made up entirely of one basic type of structure in which the character of the structure varies cyclically for example the angle of climb in a climbing wind ripple structure may vary cyclically other cyclic deposits are characterized by alternations of two or more structural types variations in the concentration of fine grained heavy minerals which account for the most striking cyclicity arise mainly because of segregation on wind rippled depositional surfaces where the ripples climb at low angles the coarsegrained light minerals which accumulate preferentially on ripple crests tend to be excluded from the local deposit daily cyclic deposits are thickest and best developed on small dunes and are least recognizable near the bases of large dunes

storm surge ebb origin of hummocky cross stratified units of the andrews mountain member campito formation lower cambrian white inyo mountains eastern california the andrews mountain member of the campito formation consists of complexly interbedded and regionally metamorphosed feldspathic quartzites siltstones and mudstones of latest precambrian and early cambrian age extensive outcrops of the member occur in the white inyo mountains of eastern california the andrews mountain member accumulated in a shallow water offshore depositional system within the broadly subsiding cordilleran miogeocline the sedimentology of the member suggests that both storm and fairweather processes were involved in its formation the following idealized sequence of sedimentary structures is recognized within individual beds 1 an erosional base with localized flute and load casts 2 fine grained sands containing parallel and low angle cross lamination hummocky cross stratification rare penecontemporaneous deformation and internal channels and discontinuities 3 these free sands grade into silts and very fine sands containing climbing ripples wave current ripples flaser bedding and locally abundant bioturbation and 4 the uppermost portions of the bed grade into bioturbated muds with cruziana ichnofossil assemblages and local occurrences of linguoid symmetrical and interference ripples the sedimentary structures in these sequences reflect deposition by high energy storm events the flows generated by these events are interpreted to be the product of the ebb of large storm surges an inferred scenario of storm deposition can be reconstructed for the andrews mountain member shelf 1 initial erosion and suspension of fairweather sediments in response to long period storm waves 2 as a low pressure system moves onshore and dissipates a sediment charged return bottom flow is generated that flows across the shelf and further erodes and entrains material 3 fine sands are deposited as parallel laminated and hummocky cross stratified units as the competence and capacity of the flow decreases 4 attenuation of the surge leads to low flow regime deposition of climbing ripples and where deposition is within storm wave base wave current ripples and flaser bedding 5 with the resumption of fairweather conditions softbodied benthic communities recolonize muddy substrates which are locally reworked by tidal and oceanic currents rather than depositing sand sheets over extensive areas multiple storm surges are inferred to have moved sand incrementally across the andrews mountain member shelf thinning fining upward and thickening coarsening upward sequences may reflect changes in both the intensity of the flows and the proximity of a local sand source

neap spring cycles in a subrecent tidal channel fill 3665 bp at schoorldam nw netherlands abstract neap spring tidal sequences are described from an intertidal channel fill the channel fill consists of a larger scale thinning up and fining up sequence of parallel laminations on a smaller scale more or less regular thickening up to thinning up sequences are formed separated from each other by more or thicker clay laminae peat detritus or echinocardium spicules deeper lying sequences contain about 28 laminae higher up the sequences become more irregular and contain gradually a lesser number of laminae these thickening up to thinning up sequences are interpreted as neap spring tidal sequences formed by semi diurnal flood tidal currents hence ca 28 laminae per 14 days three types have been found a cm thick sand clay alternations b ripple like sand mud alternations and c mm thick sand clay alternations these types show transitions to each other type a was deposited at higher current velocities and type c at lowest current velocities type b features are not linsen and flaser bedding but ripple like features which during flood only moved one cross lamina followed by one slackwater lamina clay peat or spicules they are called rippled tidal couplets the pattern of neap spring sequences is compared with recent hw water levels in the north sea and related to mean high water levels mhw and mean sea level msl

internal structure and surface geometry of longitudinal seif dunes abstract a mutual relationship exists between the morphology of longitudinal seif dunes and their dynamics and internal structure the basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank the magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line after heavy rain exposed the internal structure of the dune two kinds of depositional unit were found the main unit forms in the meandering area on both sides of the dune the other secondary unit is a narrow belt on both sides and parallel to the crest line the two units have different strikes because the laminae deposited on the dune are of the three primary stratification types climbing translatent strata sand flow cross strata and grainfall laminae the range of dips is wide 10 to 30 these dips accord with topographic profiles across the dune a model of the internal structure of a longitudinal dune has been developed from the field data the model shows the erosional and depositional areas along this linear dune

storm influenced inner shelf sand lobes in the caradoc ordovician of shropshire england abstract a sequence through the middle caradoc ordovician of shropshire shows the horderley sandstone with thick beds of hummocky cross stratified sandstone succeeded by the cheney longville flags consisting of interbedded mudstones siltstones and sandstones some of which also exhibit hummocky bedding in the upper formation the mudstones are commonly bioturbated but also have many wave rippled silty horizons the interbedded thick siltstones and sandstones all lack current ripples and appear to have been rapidly deposited as a result of storm episodes the siltstone and sandstone beds can be separated into two types type 1 beds are massive or faintly planar laminated and have good lateral continuity type 2 beds are lenticular coarser well laminated and have hummocky bedding the f rmer are believed to have been deposited from suspension and the latter from powerful bottom traction currents flowing offshore during storm surges two varieties of hummocky bedding have been recognized within the sequence the more usual type has laminae mantling a hummocky erosional topography a second variety confined to the cheney longville flags has laminae that accreted upwards from a planar base to produce undulating bedforms wavelength up to 3 m wave height up to 15 cm that have symmetrical internal laminae that are form concordant the bedforms resemble wave ripples in cross section but are about an order of magnitude larger than usual wave ripples in the cheney longville flags the argillaceous sediments and their fauna show virtually no change through 80 m of the succession the type 1 beds however increase in proportion upwards whereas the type 2 beds show two strong maxima because the overall environment apparently remained nearly constant and there is no evidence for progradation we interpret the two developments of type 2 beds as reflecting two localized lobes of sand carried offshore into an inner shelf environment by strong laterally restricted offshore currents analogous to but on a larger scale and more powerful than rip currents the independent upward increase in type 1 beds might reflect regional changes in paleogeography which caused a progressive exposure of the shoreface to an oceanic wave climate

wrinkle marks an intertidal sedimentary structure due to aseismic soft sediment loading abstract the structures have a wide distribution intertidally in the macrotidal severn estuary and inner bristol channel occurring on the active sedimentary surface chiefly in association with a millimetre scale sand over mud doublet the sand surface is marked by depressions and ridges of a wide variety of shapes and orientations relative to slope that are matched antithetically by ridges and depressions on the mud sand interface below the sand layer consequently takes the form of a series of touching to detached pillow shaped masses whose centres lie 2 4 sand layer thicknesses apart wrinkle marks originate soon after the emergence of the sedimentary surface into the atmosphere and are considered to be soft sediment deformation structures on account of their 1 occasional displacement and or severance of early emergence surface organic trails and feeding marks 2 similarity in shape and internal structure to pseudo nodules recorded from the field and produced experimentally and 3 association with gravitationally unstable sediment layers in settings that favour the vigorous seepage of pore water through water logged sediments owing to their origin at the sedimentary surface wrinkle marks appear to be preserved mainly as surface markings on eroded and discordantly reburied mud sand interfaces

sedimentary structures sorby and the last decade henry clifton sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures together with their use as palaeocurrent indicators research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant particularly as regards the physical explanation of structures and to present significant challenges and opportunities perhaps the most pressing needs are for a better understanding of 1 bedforms in gravels silts and carbonate sediments 2 tidal and especially sand wave bedding 3 hummocky and swaley cross stratification and 4 soft sediment and dewatering structures in turbidites many sedimentary structures present a little exploited opportunity to quantify process rates and define short time periods from the rock record

a sequence of soft sediment deformation dewatering structures in late quaternary subaqueous outwash near ottawa canada abstract a sequence of soft sediment deformation structures occurs in late quaternary sediments near ottawa canada the sediments are attributed to deposition on subaqueous outwash fans in the champlain sea at the submerged front of the retreating wisconsin glacier in upward sequence the deformed units comprise convolute stratification ball and pillow structures and dish structures deformation is attributed to slumping on the subaqueous fans and or to the melt out of ice blocks which had been buried by fan sediments the sequence of structures is interpreted in terms of a series of events as follows 1 1 local fluidization of low permeability sediments formed convolute stratification disrupted anticlines acted as vertical diapirs through which fluidized sediment ascended 2 2 the diapirs penetrated overlying more cohesive sediments forming ball and pillow structures 3 3 dish structures formed in silty fine sand which had passed upward through the diapirs and between the ball and pillow structures in some cases the silty sand moved downslope as a liquefied sediment flow and dish structures were deformed to give oval plan forms with long axes normal to the local paleoslope

analytical method for estimating palaeowave climate and water depth from wave ripple marks analytical expressions are developed for evaluating palaeodepth and palaeowave conditions from wave ripple marks palaeodepth and palaeowave climate estimates can be extremely powerful predictors in reconstructions of vertical movements related to basin subsidence local and regional ancient wave climate trends and the basin shape the analytical expressions developed herein are applied to wave ripple marks preserved in the lower marine molasse of switzerland western austria and southern germany the results suggest that in contrast to traditional concepts of an eustatic sea level change the lower marine molasse regression was a result of mainly depositional processes the subsidence rate greatly exceeded the rate of sea level fall over a wide geographical area furthermore the widening of the lower marine molasse basin to the ene is well documented by the increase of the ancient wave period and wave power calculated from wave ripple marks found in storm deposits

short paper hummocky cross stratification in the downton of the welsh borderland hummocky cross stratification is recorded from the classic upper silurian whitcliffe group ludlow series downton group pridoli series boundary section at ludlow shropshire this necessitates a revision of previous interpretations of palaeoenvironments which infer either a tidal mudflat depositional setting evidence for tidal influence is lacking instead the importance of storm activity probably on a shoreface is emphasized the absence of articulate brachiopods and other open marine faunal elements in the downton strata is attributed to changes in salinity in the restricted downton sea shortly before its elimination by acadian uplift the boundary between the upper whitcliffe formation ludlow and the overlying downton castle sandstone formation pridoli formerly proposed as the stratotype section for the base of the fourth series of the silurian system is designated at ludford corner ludlow shropshire so 51237413 fig 1 bassett et al 1982 and references therein it has received considerable attention with particular emphasis on faunal changes through the section in addition to its stratigraphic significance the section occurs at a crucial position in the transition from marine to non marine environments associated with the acadian late caledonian inversion of the welsh basin woodcock et al 1988 the downton castle sandstone formation in the ludlow area overlies shelly calcareous siltstones of the top ludlow upper whitcliffe formation and comprises a coarsening and cleaning upward sequence about 15 to 20 m thick bassett et al 1982 coastal mudflat deposits of the temeside shale formation lie directly above but are not seen at ludford corner the basal part of the

constraints on the formation of pliocene hummocky cross stratification in calabria southern italy from consideration of hydraulic and dispersive equivalence grain flow theory and suspended load fallout rate abstract hummocky cross stratification hcs occurs in two pliocene progradational nearshore marine to fluvial sequences in the town of guardavalle calabria southern italy the hcs is notable because of its shallow depth of deposition 2 5 m and its coarse grain size up to coarse sand individual units of hcs consist of in ascending order three subfacies which are characterized by distinctive internal structure a massive subfacies a centimeter scale laminated meso laminated subfacies and a millimeterscale laminated micro laminated subfacies the massive subfacies consists of normally graded coarse to fine sand 1 98 1 85 in beds a few cm to 25 cm thick these beds have flat to broadly undulatory bases and hummocky swaley tops the mesolaminated subfacies consists of sets of 0 8 to 1 cm thick laminae of normally graded medium to fine sand 2 3 that drape the underlying massive subfacies thicken gradually toward swales and then flatten upward the micro laminated subfacies comprises sets of several 2 to 8 ram thick planar to slightly undulatory laminae that are characterized by mica rich and mica poor zones individual micro laminae are inversely graded from silt and very fine sand at their hases to fine to medium sand at their tops 2 84 2 95 the upper few tenths of individual lamin e are highly enriched up to 56 in biotite grains most of which are aligned parallel to the lamination the most likely mode of deposition of the massive and meso laminated subfacies was rapid settling from suspension from increasingly unsteady flows the bulk of the sediment was probably imported to the depositional site by unidirectional flows during major storms hummocky swaley laminae did not develop during deposition of the massive subfacies because of the high rate of suspended load fallout which probably was controlled by the relatively coarse grain size the grains composing the microlaminae are in dispersive not hydraulic equivalence the thickness grain size relationships within micro laminae satisfy predictions of grain flow theory and indicate that the laminae were produced by collapse of traction carpets under the influence of currents on the order of 80 150 cm s deposition of the guardavalle hcs took place in four stages 1 initial plane bed 2 growth of hummocky bedforms while was very high probably under the influence of combined orbital and unidirectional currents 3 draping of the hummocky bedforms by meso laminae during a period of moderately high high orbital velocity but lower unidirectional current velocity and 4 reworking of the seafloor by powerful orbital currents under conditions of relatively low and minor unidirectional currents preservation of the guardavalle hcs in a very shallow marine environment was promoted by the microtidal storm wave dominated chara ter of the pliocene ionian coast of calabria

bedding types in holocene tidal channel sequences knik arm upper cook inlet alaska abstract uplifted convoluted and horizontal to subhorizontal beds of varying thickness in intertidal silt as old as 3 280 90 yr b p are exposed in the banks of tidal channels of unknown depth in the intertidal zone in knik arm of upper cook inlet internal discordances may occur both within convoluted beds and between convoluted and horizontal to subhorizontal beds at the base of many convoluted beds there is a rapid gradation upward into laminae which are severely deformed that is in some places the contortions appear to have originated along a single bedding plane where the convoluted sequences are truncated by nearly horizontal sequences the distortion must have resulted from syndepositional or postdepositional events prior to their burial by the overlying beds various forms of gravitational and tidal processes caused the deformation of the knik arm deposits events triggering the movement of the sediment include any or all of the following 1 seismic activity 2 sediment or ice loading 3 wave induced liquefaction 4 pore pressure changes brought on by changing levels of tidewater 5 undercutting of sediments by channel migration and 6 freezing and thawing

horizontal laminae formed under upper flow regime plane bed conditions horizontal laminae have been examined from the fluvial whirlpool sandstone silurian modern intertidal sand bars and a flume textural variations which define laminae were determined by averaging all grain long axes lengths in contiguous rectangular areas 2 mm wide by 0 2 mm high up through thin sections cut normal to bedding textural laminae were identified as fining upward fu and coarsening upward cu from trends seen in plots of average grain long axes versus height above an arbitrary datum fu and cu laminae occur in random sequence and have an average thickness of less than 1 mm with cu laminae being slightly thinner than fu laminae heavy mineral sheets which appeared as continuous layers in outcrop and in trenches consistently occurred in cu laminae lateral correlation diagrams constructed across thin sections showed that some textural laminae were continuous for the 15 mm over which measurements were made while others pinched out laterally across the measured distance heavy m