bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evaluation of aeolian sand transport equations using intertidal zone measurements, Saunton Sands, England
Authors: SARRE R. D. 
Published: June 2006

ABSTRACTSand transport rates were measured using a vertical sand trap along the intertidal zone of a beach in North Devon, England, together with simultaneous monitoring of the wind speed on the beach and moisture levels in the surface layers of sand. The results of 88 sand trap samples in a wide range of wind speeds showed that moisture levels up to 14%, in the top 1–2 mm of the beach sand, have no discernible effect on the transport rates. Transport rates measured from areas of the beach where the moisture was below this level are compared with the rates predicted by seven expressions based on theoretical and wind tunnel research together with the empirical results of other published research. Measured transport rates range from 0.0001 to 0.22 kg m‐1 s‐1. The results indicate that expressions based on a power relation between the wind speed and the transport rate, and which include a threshold velocity term, provide the best estimates of the observed transport rates.
Applications of grain‐pivoting and sliding analyses to selective entrapment of gravel and to flow‐competence evaluations
Authors: KOMAR PAUL D. , LI ZHENLIN 
Published: June 2006

ABSTRACTModels of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water. Of special interest is whether such process‐based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods. A detailed derivation is undertaken, focusing first on the fluid flow and forces at the particle level. The resulting threshold equation for the particle‐level velocity is then modified to yield the mean entrainment stress for the flow as a whole. This approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit, the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape. The resulting threshold equations contain a number of coefficients (e.g. drag and lift) whose values are poorly known, but can be constrained by requiring agreement with the Shields curve for the threshold of grains in uniform deposits. If pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models, smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment. However, if reasonable modifications of those coefficients are made for expected field conditions, then the models yield good agreement with the data. Sliding models, where sorting is due entirely to projection distances of the grains above the bed, yield somewhat poorer agreement with the field data; however, the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends. The existing measurements lack documentation of the mechanisms of grain movement, so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding. In spite of such uncertainties, the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence.
Tidal inlet sequence, Sundance Formation (Upper Jurassic), north‐central Wyoming
Authors: UHLIR DAVID M. , AKERS ARTHUR , VONDRA CARL F. 
Published: June 2006

ABSTRACTThe 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.
Progradational gravel beach sequences in a moderate‐ to high‐energy, microtidal marine environment
Authors: MASSARI F. , PAREA G. C. 
Published: June 2006

ABSTRACTThe anatomy of progradational gravel beaches was analysed in two different tectonic settings: the strongly subsiding Messinian foreland basin of the eastern Southern Alps and the rising Pleistocene marine terraces of the outer margin of the Apenninic chain. Progradation took place near the mouths of fan‐delta or braid‐delta distributaries debouching in microtidal landlocked basins (palaeo‐Adriatic and palaeo‐Ionian seas). A wind‐wave climate, comparable to that of the present‐day, was characterized by periodic intense storm activity. Most of the beachface progradation is thought to have taken place during the post‐storm recovery and fair‐weather stages, whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly‐sorted and coarse‐grained gravels on the shoreface by storm‐intensified seaward‐trending flows and behind the highest berms by washover processes. Analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here. The characteristics of the Apenninic sequences suggest a strongly variable wind‐wave climate, with periodic changes from a relatively dissipative barred morphology during storms, accompanied by strong longshore currents and rip development, to a more reflective state, typical of recovery and fair‐weather stages. The Southern Alpine sequences, on the other hand, are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear‐ and gravity‐sorting in the lower‐beachface gravels. These features indicate a comparatively more reflective average state, due to location of beaches along the deeply embayed head of the palaeo‐Adriatic, and the very narrow directional width of the incoming wave spectrum, which was almost shore‐normal most of the time. In addition to well‐developed shape zonation of gravels, the common presence of wave‐generated gravel megaripples in the Southern Alpine sequences indicates the greater influence of long‐period waves, due to the greater available fetch distances. In both settings the cyclicity is thought to be genetically related to minor changes of sea‐level.
A Late Pleistocene coarse‐grained spit‐platform sequence in northern Jylland, Denmark
Authors: NIELSEN LARS HENRIK , JOHANNESSEN PETER N. , SURLYK FINN 
Published: June 2006

ABSTRACTSeveral well‐preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present‐day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant‐scale cross‐bedded foreset unit, overlain by topset and beach units, while the second (Sequence II) consists of the foreset unit overlain by bar‐trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell's (1966, 1972) model for spit‐platform growth based on scaled wave tank experiments. The giant‐scale cross‐bedded unit corresponds to prograding of a coarse‐grained subaqueous spit‐platform while the topset, bar‐trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and II reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse‐grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant‐scale cross‐bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit‐platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit‐platform sequences allows precise determination of sea‐level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant‐scale cross‐bedding to those already known from fluvial transverse, lateral and point bars, subtidal sand waves and Gilbert deltas.
Algodones dune field of southeastern California: case history of a migrating modern dune field
Authors: SWEET M. L. , NIELSON JAMIE , HAVHOLM K. , FARRELLEY JOHN 
Published: June 2006

ABSTRACTThe Algodones dune field of southeastern California is one of the largest active dune fields in North America. The dune field is migrating in an easterly direction, oblique to the resultant sand flow direction (S 24° E). The migration of the Algodones results from an interaction between regional winds and the dune field. This interaction generates a localized secondary flow that has caused the dune field to migrate in a direction oblique to the resultant sand flow direction.Four lines of evidence suggest that the Algodones has migrated in an easterly direction: (1) A ramp, interpreted as the trailing edge of the dune field, 35 m thick and 500 m wide composed of aeolian deposits that borders the western edge of the dune field. No similar deposits are found on the eastern (leading edge) margin of the dune field. (2) Leading‐edge sand‐sheet deposits are exposed in interdune areas within the dune field. These deposits are west of the modern leading‐edge sand sheet. (3) Across the breadth of the dune field sands are consistently coarser and more poorly sorted in the west and finer and better sorted in the east. This observation suggests that sand is transported from west to east. (4) Eastward migration of a large compound‐complex crescentic dune.If the dune field continues to migrate it will deposit a vertical sequence consisting of: a basal sand‐sheet deposit consisting of wind and water‐ripple laminae, small‐scale aeolian cross‐strata, and ephemeral stream (wadi) deposits; aeolian dune deposits consisting of medium‐scale aeolian compound cross‐strata; small‐scale simple sets of aeolian cross‐strata with highly variable dip directions; a sand sheet containing low‐angle wind‐ripple cross‐strata capped by a coarse sand lag super bounding surface.
Facies and origin of shales from the mid‐Proterozoic Newland Formation, Belt Basin, Montana, USA
Authors: SCHIEBER JÜRGEN 
Published: June 2006

ABSTRACTShales constitute more than 60% of the world's sediments, yet while facies models for sandstones and carbonates are at a high level of sophistication, the study of shales has clearly lagged behind. In the mid‐Proterozoic Newland Formation six major shale facies types, deposited in nearshore to basinal environments, are distinguished on the basis of bedding characteristics, textural features, and the proportions of silt, clay and carbonate. Textural features of these shale types are related to sedimentary environments as deduced from associated lithologies. The shales are undisturbed by bioturbation, and their textural and sedimentary characteristics reflect subaqueous growth of microbial mats, erosion and deposition by storms, deposition of flocculated vs. dispersed clays, continuous slow background sedimentation, winnowing by waves or currents, and subaerial exposure.
Turbidites and iron formations, Beardmore‐Geraldton, Ontario: application of a combined ramp/fan model to Archaean clastic and chemical sedimentation
Authors: BARRETT T. J. , FRALICK P. W. 
Published: June 2006

ABSTRACTSandy turbidites, grain flows, conglomeratic mass‐flows and oxide‐facies iron formation are present in the late Archaean Beardmore‐Geraldton terrain, a metasedimentary belt which extends for at least 80 km in an E‐W direction. The marine portion of this basin contains four lithofacies associations (LA): (1) Thinbedded, iron formation‐clastic sediment association. This association represents a continuum of deposit types containing iron formation; subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand/silt. (2) Thin‐bedded, turbidite‐dominated association. These sediments consist mostly of silt/sand beds which either show no vertical trends, or thin and fine upwards over a few metres. (3) Medium‐bedded, turbidite‐dominated association. Most of these sediments are medium to coarsegrained, vertically unstructured sand sequences with occasional structured intervals. (4) Thick‐bedded association. This is dominated by poorly graded sands up to 7–8 m in thickness. Sand beds are characterised by a thin basal zone of coarse sand and pebbles, a large central interval containing a mixture of medium and coarse sand, and a thin upper zone of fine sand/silt.The overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars. We infer a nearshore break in slope, locally with large channels (LA 4) extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans (structured portions of LA 3). However, many deltaic surfaces probably were not tapped by major channels, but merged downslope into a submarine ramp. Sediment was transported across the ramp by slump events and sheet‐like grain flows (unstructured portions of LA 3). Iron formation and LA 2 sediments probably accumulated both in upper‐mid ramp areas with low sediment delivery rates, and distal to fan‐ramp successions.As major streams on the braid plain changed position, associated submarine channels and slump‐fed ramp deposits also would have shifted laterally. This produced overlap of different facies associations in both the fan and ramp environments, which may explain why observed vertical trends in bedding are limited to several metres. We suggest that on narrow, active Archaean cratonic margins, additional non‐regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity, and seismic activity associated with the arc, also contributed to the lack of vertically structured sequences. Turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation.
Multiple paleosols of the late Albian Boulder Creek Formation, British Columbia, Canada
Authors: LECKIE DALE , FOX CATHERINE , TARNOCAI CHARLES 
Published: June 2006

ABSTRACTFifteen lithified paleosols, closely spaced in vertical sequence, occur in the top 90 m of the late Albian Boulder Creek Formation in the foothills of northeastern British Columbia, Canada. The paleosols have well‐developed profiles 0·5 to 1·5 m thick, including A, B and C horizons. The paleosols are characterized by their grey colour, cutans, vertical roots, peds, spherulitic siderite and absence of sedimentary structures. The paleosols formed during a period when one or more basin wide unconformities occurred as a result of either eustatic sea level fluctuations or local tectonic events. These unconformities represent the terrestrial record of a lowered base level which caused valley incision and decreased rates of sedimentation on the incised flood plain. The climate was humid to subhumid. Overall, the environment in which these soils developed was of low relief and subject to little erosion. The water table was high for part of the year but there is also evidence of periodic drying and oxidation of organic debris. Cumulatively, the paleosols in this interval may represent 150000 yr of non‐deposition.
Relationships between cyclicity and stromatolite form in the Late Proterozoic Bitter Springs Formation, Australia
Authors: SOUTHGATE PETER N. 
Published: June 2006

ABSTRACTStromatolite biostromes and bioherms in the lower two units of the Late Proterozoic Loves Creek Member of the Bitter Springs Formation represent shallowing upward and deepening upward sequences. In the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence. Ooid and/or peloid‐intraclast grainstones and small, irregular bulbous and columnar stromatolites characterize the basal, transgressive portion of cycles. Domal, columnar and stratiform stromatolites comprise the bulk of the cycle. These forms accreted in a gradually shallowing epeiric sea. Domal stromatolites predominate in the deeper parts of cycles. Here synoptic relief gradually increases upwards. Columnar and stratiform stromatolites predominate in the shallower parts of cycles, where synoptic relief rapidly diminishes upwards. In thin‐bedded dolo‐mudstones at the tops of cycles the co‐occurrence of desiccation cracks, tepee structures, scalloped dissolution surfaces, gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure. In contrast, stromatolites in a unit at the base of the Loves Creek Member accreted during a gradual rise in sealevel. Stratiform, columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence, but only the lower half of the shallowing upward cycle is represented. Synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit. Recognition of a deepening upward stromatolite sequence at the base of the Loves Creek Member, and a disconformity surface between this sequence and the underlying Gillen Member, permits palaeoenvironmental re‐interpretation of the Loves Creek Member as a single ‘large scale’ sea‐level cycle.
Three distinctive feeder‐lobe systems within one time slice of the Tortonian Tabernas fan, SE Spain
Authors: KLEVERLAAN KICK 
Published: June 2006

ABSTRACTExcellent exposures and the presence of lithological markers make it possible to reconstruct the fan facies pattern for a narrow (6000–60 000 yr), late Tortonian (7–8 Ma) time slice of a submarine fan complex that developed in the Tabernas basin. Unlike most classical fan models, which seem to refer to a single feeder‐lobe system, the reconstruction from the Tabernas basin reveals at least three distinctive juxtaposed feeder‐lobe systems: (I) a straight feeder valley terminating in a sandstone body consisting of stacked sand‐filled scours (sand‐rich system); (II) a straight feeder channel diverging in a broad front of nested scours in mudstones. These scours terminate in fine grained sheet‐like turbidite deposits (muddy system); (III) a sinuous channel complex extending far into the basin without ‘lobe’ deposits at its mouth (solitary system).Sedimentary features indicate that in the Tabernas basin, initial basin floor morphology and mass‐flow transport behaviour controlled and eventually stopped formation of the sand‐rich system (I), while depositional topography and slope instability controlled formation of the muddy system (II). The unusual narrowness (10–30 m) and length (8 km) of the solitary system (III) points to its confinement, possibly a result of an intrabasinal fault escarpment trending obliquely to the general direction of slope. Flow stripping (Piper & Normark, 1984) could then explain the absence of a lobe at the channel terminus.
Upper‐regime parallel lamination as the result of turbulent sediment transport and low‐amplitude bed forms
Authors: PAOLA CHRIS , WIELE STEPHEN M. , REINHART MARY A. 
Published: June 2006

ABSTRACTA 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.
Sedimentology of a saline playa in the northern Great Plains, Canada
Authors: LAST WILLIAM M. 
Published: June 2006

ABSTRACTCeylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present‐day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite.Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate‐clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na‐rich solution to a mixed Mg‐Na system. The most important post‐depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.