bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, north‐west Canada
Authors: SAMI TERRY T. , JAMES NOEL. P. 
Published: June 2006

ABSTRACTThe Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform‐to‐basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south‐east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow‐water rimmed shelf, (2) shallow‐water open shelf, (3) shallow‐water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud.These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea‐level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level.The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.
Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size
Authors: ORTON G. J. , READING H. G. 
Published: June 2006

ABSTRACTShort term variability in delta form and process can be partly explained by the relative strength of hydraulic parameters such as river discharge, discharge variability, wave energy flux and tidal range. However, the calibre or grain size is also important. The amount, mode of transport and grain size of the sediment load delivered to a delta front have a considerable effect on the facies, formative physical processes, related depositional environments and morphology of the deltaic depositional system. The available grain size influences (1) the gradient and channel pattern of the fluvial system on the delta plain; (2) the mixing behaviour of sediment as it discharges into the ambient basin waters at the river mouth; (3) the type of shoreline, whether reflective or dissipative, and its response to both wave energy and tidal regime; and (4) the deformation and resedimentation processes on the subaqueous delta front. Long term aspects of deltaic sedimentation, including a few generalized relationships between sediment supply and physiographic setting, are briefly introduced. The need for further detailed research on modern and ancient deltaic dispersal systems is emphasized, and specific suggestions are given for future research.
The geometry and organization of fluviodeltaic channel sandstones in the Jurassic Saltwick Formation, Yorkshire, England
Authors: MJØS RUNE , PRESTHOLM EDWARD 
Published: June 2006

ABSTRACTWithin the Middle Jurassic Saltwick Formation of Yorkshire, England, 65% of the fluvial channel sandstones (n= 22) have width to thickness ratios in the range 17–30. More than 90% lie in the range 17–67 and are classified as narrow sheet channel sandstones. A broad sheet channel sandstone (c. 15 m thick, 3 km wide, width to thickness ratio of 200) occurs at the base of the formation in the proximal part of the studied depositional system. The thicknesses of the channel sandstones are in the range 4 m to more than 22 m, with 90% in the range 4–12 m. The widths of the channel sandstone sections are in the range 100–800 m, with 80% between 100 and 400 m. In plan view the channel sandstones display relatively constant thicknesses across the main part (80%) of the channel body.Channel sandstones are usually multistorey. The storeys are separated by major scour surfaces, with depths of scour ranging from 2 m to 8–10 m. The individual storeys are 4–13 m thick, and transverse and oblique storey sections have lateral extents of 60–480 m. Vertical stacking of storeys is accompanied by a decrease in the width to thickness ratio of the channel sandstones, whereas lateral stacking of storeys is accompanied by an increase. Mudstones and siltstones within channel sandstone bodies occur at the top of storeys and were in most cases deposited during channel abandonment.The Saltwick Formation was deposited during increasing accommodation forming a transgressive systems tract. The channel sandstone to interchannel sediment ratio is highest in the lower parts of the transgressive systems tract. The quantitative geometry and architecture data from the Saltwick Formation may be useful for describing and modelling humid delta plain hydrocarbon reservoirs of transgressive systems tracts.
The statistics of counting clasts in rudites: a review, with examples from the upper Palaeogene of southern California, USA
Authors: HOWARD JEFFREY L. 
Published: June 2006

ABSTRACTThe relative proportions of gravel sized particles of different lithology in rudaceous sedimentary rocks are generally determined in the field by clast counting. Clast counts are usually carried out qualitatively in order to assess sedimentary provenance. However, a review of the statistical aspects of clast counting suggests that this technique also can be applied quantitatively, and to investigate a variety of other objectives during basin analysis. Geographical and stratigraphical changes in the relative proportions of clasts can be quantified statistically and used to characterize sediment dispersal patterns in space and time, respectively. Statistical comparisons between clast assemblages can be used as a tool to match up rock units. This approach may help to constrain tectonic or suspect‐terrane models, or to document sediment recycling. Both counting and sampling errors contribute to the total probable error of a clast count. Sampling error results from the uneven distribution of clasts in outcrop, perhaps caused by selective sorting. Counting error arises from a count of some number less than the total number of clasts in the population. Sampling and counting errors can be minimized by counting in closely spaced subsets, and by counting a total of at least 400 clasts, respectively. Thus, a useful procedure is to count four closely spaced subsets of 100 each, and combine the results for a total of 400. Point counting should not be used because differences in particle size produce biased results. A better method is to count all clasts above some minimum size within a specified area of outcrop. Analysis of upper Palaeogene non‐marine conglomerates composing part of the Sespe Formation in California, using confidence intervals, hypothesis testing, analysis of variance, ratio analysis and varietal studies, demonstrates that useful statistics can be derived by counting clasts.
Grain scale simulations of loose sedimentary beds: the example of grain‐bed impacts in aeolian saltation
Authors: HAFF PETER K. , ANDERSON ROBERT S. 
Published: June 2006

ABSTRACTSediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role. In order to illustrate the modelling of collections of frictional, inelastic sedimentary grains with the particle dynamics method (PDM), we use the grain impact process in aeolian saltation as a specific example. In PDM, all the forces on each particle are evaluated at a sequence of small time‐steps, and the Newtonian equations of motion are integrated forward in time. Interparticle forces at grain contacts are treated as springs with prescribed stiffness (normal force) and by a Coulomb friction law (tangential force); particle inelasticity is represented by spring damping. The granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties, and the integration time‐step. We find that for the range of impact speeds and impactor masses relevant to aeolian settings, grain splashes are relatively insensitive to grain stiffness, grain inelasticity and grain friction, and that the pattern of ejection from the bed is largely controlled by bed microtopography. A large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process. The splash function representing these statistics is then available for use in calculations over longer time‐scales, such as the evolution of the saltation curtain. The details given here will enable the interested reader to adapt PDM modelling to other types of clastic sedimentary systems.
Geometry and kinematics of dunes during steady and unsteady flows in the Calamus River, Nebraska, USA
Authors: GABEL SHARON L. 
Published: June 2006

ABSTRACTThe geometry and kinematics of river dunes were studied in a reach of the Calamus River, Nebraska. During day‐long surveys, dune height, length, steepness, migration rate, creation and destruction were measured concurrently with bedload transport rate, flow depth, flow velocity and bed shear stress. Within a survey, individual dune heights, lengths and migration rates were highly variable, associated with their three‐dimensional geometry and changes in their shape through time. Notwithstanding this variability, there were discernible changes in mean dune height, length and migration rate in response to changing discharge over several days. Changes in mean dune height and length lagged only slightly behind changes in discharge. Therefore, during periods of both steady and unsteady flow, mean dune lengths were quite close to equilibrium values predicted by theoretical models. Mean dune steepnesses were also similar to predicted equilibrium values, except during high, falling flows when the steepness was above that predicted.Variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period. However, the calculated rates of change of height of individual dunes vary considerably from those measured. Rates of dune creation and destruction were unrelated to discharge variations, contrary to previous results. Instead, creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate.Observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses, but not at higher bed shear stresses when suspended sediment transport is significant.
Experimental studies of the fluidization of layered sediments and the formation of fluid escape structures
Authors: NICHOLS R. J. , SPARKS R. S. J. , WILSON C. J. N. 
Published: June 2006

ABSTRACTExperiments demonstrate that fluid escape structures can be produced as a result of unstable fluidization behaviour where a lower base layer of granular material is inhibited from fluidizing by the presence of an overlying non‐fluidizing top layer. Before the base layer can fluidize the weight of the overlying material must be balanced, and this is accomplished by base layer material pressing against the bottom surface of the confining top layer forming a static layer. This static layer allows the top layer to lift away from the base layer which is then free to fluidize. A water‐filled crack forms below the static layer and, as this grows, instability causes the static layer and top layer to bend and conical voids to form below the antiformal sections. Rupture occurs at the apex of the water void, allowing the underlying water and fluidizing material to burst out through the top layer. The fluidized base layer material then flows through the rupture until all of this material, except that in the static layer, is deposited above the previously overlying layer and a stable fluidization system results. The top layer material is bent upwards around the rupture, and the resulting pillar‐type escape structure is preserved if flow then ceases. The vigour of the burst‐out is greatest when the base layer material has a grain size 15% of the top layer material. If the base layer grain size is less than 8% of the top layer then base layer material will pass through the top layer pore spaces, without forming an escape structure. If cohesive material is present, escape structures form when a layer of fine grained cohesive material overlies a layer of cohesionless material. At low flow rates small pipes with scattered angular bends pierce the top layer, and base layer material passes through them. The base layer material is ejected on to the top layer and builds up around the mouth of each pipe to form constructional structures, sand volcanoes. This is in contrast to the cohesionless experiments, where the weight of material being deposited on the top layer caused an ejecta‐filled depression to form around the rupture. If flow then ceases both the pipes and the sand volcanoes are preserved. At high flow rates, where the base layer fluidizes, the top cohesive layer becomes fragmented. Small fragments circulate within the fluidizing base layer and are preserved as floating clasts. Large fragments sink to the bottom of the fluidizing base layer. Erosion of the bottom surface of these larger fragments causes this surface to become convex downward. The experimentally derived structures are similar to pillar‐type structures observed in the field and the processes described can be used to investigate the development of these structures. Fluidization experiments also demonstrate the genesis of dish structures, and the cohesive behaviour can be applied to the deformation of these structures after initial formation.
Determining fluid source and possible pathways during burial dolomitization of Maryville Limestone (Cambrian), Southern Appalachians, USA
Authors: SRINIVASAN KRISHNAN , WALKER KENNETH R. , GOLDBERG STEVEN A. 
Published: June 2006

ABSTRACTDetailed petrographic analyses along a depositional transect from a carbonate platform to shale basin reveals that dolomite is the principal burial diagenctic mineral in the Maryville Limestone. This study examines the role of burial dolomitization of subtidal carbonates. Dolomite occurs as a replacement of precursor carbonate and as inter‐ and intraparticle cements. Four different types of dolomite are identified based on detailed petrographic and gcochemical analyses.Type I dolomite occurs as small, irregular disseminations typically within mud‐rich facies.Type II dolomite typically occurs as inclusions of planar euhedral rhombs (ferroan), 5–300 μm in size, in blocky clear ferroan calcite (meteoric) spar. Type II dolomite is non‐luminescent. Type I and II dolomite formed during shallow to intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals 10–150 μm in size occurring as thin seams along stylolites and as thick bands a few millimetres in width. This dolomite consists of dominantly non‐luminescent rhombs and, less commonly, orange luminescent and zoned rhombs. Type IV dolomite consists of baroque or saddle‐shaped, 100–1500 μm crystals, and is non‐luminescent. Type IV dolomite formed during the period of maximum burial.Types III and IV dolomite increase in abundance downslope. Type III dolomite contains 1.2–2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends, decreasing from 3.5–4.5 wl% Fe and 0.1–0.3 wt% Mn in the west (slope/basin) to 1.5–2.5 wt% Fe and less than 600 ppm Mn in the east (shelf margin), a distance of approximately 60 km. Spatial trends in Fe and Mn distributions in Type IV saddle dolomite, suggest a west‐east fluid flow during late burial diagenesis. Types III and IV dolomite have a mean δ18O value of ‐ 7.8%00 and a mean δ13C value of + 1.1%00 (relative to the PDB standard). Based on a range of assumed basinal water composition of 2.8%00 SMOW, temperatures calculated from δ18O values of Types III and IV dolomite range between 75 and 160°C. 87Sr/86Sr data for Types III and IV dolomite range from 0.7111 to 0.7139. These values are radiogenic when compared to Cambrian marine values and are consistent with the presence of a diagenetic fluid that interacted with siliciclastic sediments.The distribution of Palaeozoic facies in the southern Appalachians indicates a Cambrian shale source for the fluids, whilst burial curves suggest a Middle Ordovician age for burial fluid movement.
Aerodynamic entrainment threshold: effects of boundary layer flow conditions
Authors: WILLIAMS JON J. , BUTTERFIELD GRAEME R. , CLARK DAVID G. 
Published: June 2006

ABSTRACTExperimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (1) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment.In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity (U*t), were calculated using the momentum integral method.Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi‐organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process.For each grain fraction, values of U*t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF for U*t required by the model and for future stochastic treatment of the threshold condition.
The effect of surface slope on saltation threshold
Authors: IVERSEN JAMES D. , RASMUSSEN KELD RØMER 
Published: June 2006

ABSTRACTA new wind tunnel has been constructed to study the mass transport properties of wind‐blown sand. This report on the first set of experiments in the new wind tunnel concerns the effect of slope on threshold friction speed. The results of this series of static threshold experiments (and one dynamic threshold experiment) for a range of particle diameters and bed slope angles show that, provided the effects of Reynolds number variation and interparticle cohesive force are accounted for, the static friction angle α is independent of slope and close in value to the measured static angle of response.
Synchronous detachment folds and coeval sedimentation in the Prepyrenean External Sierras (Spain): a case study for a tectonic origin of sequences and systems tracts
Authors: MILLAN H. , AURELL M. , MELENDEZ A. 
Published: June 2006

ABSTRACTThis paper reports on the structural and sedimentary evolution of the middle to late Eocene of the Prepyrenean External Sierras (southern Pyrenees, Spain). The initiation, duration and kinematics of a set of growth structures that developed in a shallow marine depositional setting is documented. The detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. The sedimentary units coevally deposited with these growth structures are arranged in four depositional sequences. Their boundaries correspond to flooding surfaces which grade basinwards into correlative conformities. They are also indicated by the presence of both angular unconformities and onlap geometries. Each depositional sequence generally consists of two systems tracts. The lower one, or transgressive systems tract, is formed by up to 400 m of azoic marls deposited in outer ramp areas. The upper one, or highstand systems tract, mainly consists of shallow siliciclastic and carbonate facies, up to 200 m thick, deposited in middle to inner ramp areas. These depositional sequences are interpreted to be controlled by regional tectonic pulses. An increase of tectonic activity resulted in the flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue marls (i.e. transgressive systems tract). The overlying highstand systems tract developed following periods of diminished tectonism, with the consequent growth and progradation of shallow carbonate platform facies.
Late Quaternary sediment, sediment mass flow processes and slope stability on the Scotian Slope, Canada
Authors: MOSHER DAVID C. , MORAN KATHRYN , HISCOTT RICHARD N. 
Published: June 2006

ABSTRACTThe study area, just to the west of the Verrill Canyon on the Scotian Slope, eastern Canada, exhibits both large and small scale sediment mass movement features. Study of high resolution seismic reflection and sidescan sonar data shows that a large portion (approximately 70%) of the near surface sediment (<20 m) in the area has undergone erosion, rotational slumping and internal deformation. Remoulded sediment observed in physical properties profiles of piston cores and sediment deformation structures are further evidence of slumping. Small scale mass flow events are recorded by abundant turbidites and debris flow deposits noted in piston cores. Sediment physical properties are highly dependent on sediment type (lithofacies). Frequent facies changes, both temporally and spatially, make correlation between cores difficult.Although the small scale mass movement events correlate with glacial recession on the continental shelf and lower relative sea levels, the triggering mechanisms for the large scale events are less obvious. Slope stability analyses indicate that, at present, the seabed is stable. The most plausible explanation for large scale slope failures in this region are ground accelerations related to earthquake shock. Our analyses demonstrate that it is unlikely that large magnitude, distant earthquakes, such as those previously proposed in the Laurentian Slope Seismic Zone (LSP) model, could initiate failure of sediment in the study region. Our data support the interpretation that more frequent, lower magnitude earthquakes, closer to the study region, as previously proposed in the Eastern Slope Experimental Source Zone (ESX) model, are the likely causes of large scale slope failures. Furthermore, excess pore pressures resulting from shallow gas and/or high sedimentation rates during deglaciation contribute to slope failure.