bluuebunny/crossref_metadata_2025_split huggingface dataset:
The influence of peripheral sabkhas on the geochemistry and sedimentology of a Tunisian lagoon: Bahiret el Biban
Authors: MEDHIOUB KHALED , PERTHUISOT JEAN‐PIERRE 
Published: June 2006

ABSTRACTBahiret el Biban is a restricted lagoon in an arid climate. The general geological setting is briefly analysed, especially with respect to the relations between the water body and its evaporitic bordering areas, i.e. supratidal sabkhas.The geochemical behaviour of the lagoon waters has been studied over a 2‐year period following floodings. In spite of some exchange with the open sea, there is a general tendency towards increasing salinity, calcium sulphate impoverishment and a consequent increase of the Mg/Ca ratio. Thus, unless large floodings occur, the basin waters undergo an evaporitic evolution.Sedimentation and early diagenesis are controlled largely by this geochemical pattern, but also by exchange with the open sea and by the water depth. Organic matter derived from the sea is deposited on entering this highly saline basin and locally produces sediments which could eventually become source rocks.Bahiret el Biban is a possible model which may be of considerable interest in the reconstruction of the early stages of the evolution of evaporitic basins.
Debris flow (olistostromes) and slumping on a distal passive continental margin: the Palombini limestone–shale sequence of the northern Apennines
Authors: NAYLOR MICHAEL A. 
Published: June 2006

ABSTRACTAn olistostrome accumulation up to 530 m thick occurs in the Casanova area of the northern Apennines. It lies within or above the calciturbiditic Palombini limestone‐shale sequence, and is part of the allochthonous Vara Complex—sediments originally deposited on oceanic crust. The olistostromes are poorly sorted, monomict, matrix‐supported, submarine debris flow deposits with rigid plugs. They have a compactional foliation and a compaction‐modified, planar clast fabric created during flow. Although diachronous in the main part of the area, the olistostromes have a vertical gradational contact with the overlying slumped Palombini, in which recumbent asymmetric fold hinges and trains and slump boudins are present. Many criteria indicate soft‐sediment deformation. Fold asymmetry indicates a uniformly SW‐dipping palaeoslope. The textural gradation from slumps to olistostrome beds, plus slump folds and boudins as olistostrome clasts show that the olistostromes are dismembered slumps. In vertical sections, variations in limestone petrography, volume percentage and size of clasts confirm that the olistostromes were derived from the Palombini as a series of bed‐by‐bed slumps keeping pace with sedimentation of the Palombini. From olistostrome clast sizes and bed thicknesses, a depositional slope of ∼ 4° is estimated. The olistostromes are not precursor sediments shed from advancing nappes, as in the Bracco ridge model of some authors; rather, they were formed at the foot of a distal, block‐faulted passive continental margin, long before nappe emplacement.
The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia
Authors: WARREN JOHN K. 
Published: June 2006

ABSTRACTWell exposed, often laminated gypsum sequences occur in many Quarternary salt lakes in southern South Australia. The gypsum in the salt lakes is classified by increasing grain size into gypsite, gypsarenite and selenite. The salt lakes are classified by age and hydrological setting into coastal salinas which are Holocene sea‐water fed groundwater lakes, and continental playas which are late Pleistocene endorheic basins. A study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes. As the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited. Application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non‐salina gypsum deposits.
Lacustrine deltas in a Mesozoic alluvial sequence from Camp Hill, Antarctica
Authors: FARQUHARSON G. W. 
Published: June 2006

ABSTRACTSedimentary rocks of late Mesozoic age exposed at Camp Hill, northern Antarctic Peninsula, are associated with calc‐alkaline volcanic rocks. They represent deposition on a fault‐controlled floodplain, with marginal alluvial fans, on a volcanic arc. Finely laminated mudstone and occasional graded laminae were deposited from suspension and by density underflow currents, respectively, in small shallow lakes. Thickening‐ and coarsening‐upward sandstone bodies overlying the lake deposits are interpreted as lacustrine deltas of which two types are preserved: (1) Gilbert‐type with steep foresets and (2) mouth‐bar type which lack steep foresets. Sections through the latter type reveal the presence of sub‐environments characteristic of fluvial‐dominated marine deltas, i.e. prodelta, distributary mouth‐bar and distributary channel. Abandoned mouth‐bars resulting from avulsion are recognized. It is suggested that the processes which operated during formation of the mouth‐bar deltas resulted from hyperpycnal flow. By contrast, the Gilbert‐type delta is thought to be the consequence of a reduced inflow of suspended sediment causing homopycnal flow, and thorough mixing of the river and lake waters.
Rare earth elements in the sedimentary cycle: a pilot study of the first leg
Authors: BASU ABHIJIT , BLANCHARD DOUGLAS P. , BRANNON JOYCE C. 
Published: June 2006

ABSTRACTA pilot study with Holocene fluvial sands was undertaken in order to evaluate the effects of source rock composition and climate on natural abundances of rare elements (REE) in the first leg of the sedimentary cycle. We have analysed the medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in Montana (semi‐arid), Georgia (humid), and in South Carolina (humid). Despite compositional differences between parent plutons the REE distribution patterns (but not the total absolute abundances) of the daughter sands are very similar. Averages of the three areas have a La/Lu ratio of ∼ 103 showing a depletion of heavy REE with respect to an ‘average granite’ (La/Lu = 79) or the composite of North American Shales (NAS; La/Lu = 55). Also, the Eu/Sm ratio in sands from these areas is ∼ 0·22 which is very close to the NAS ratio of 0·21. However, the overall REE distribution of these sands is not similar to that of NAS in any way. We conclude that the major rock type, but neither its minor subdivisions nor the climate (in the source area), controls the REE distribution patterns in first cycle daughter sands. But, the total and the parent rock‐normalized abundances of REE in sands from the humid areas are much lower than those in sands from the arid areas.
Bedform and stratification characteristics of some modern small‐scale washover sand bodies
Authors: SCHWARTZ ROBERT K. 
Published: June 2006

ABSTRACTWashover sand bodies commonly develop along microtidal coastlines in beach/barrier island or spit settings. Wave runup, usually in conjunction with an abnormally high water level, may overtop the most landward berm of the beach and the foredune crest, if one exists, to produce overwash and subsequent runoff across the more landward subaerial surface. Two main elements of the resulting deposit are the washover fan and runoff channel. Newly formed, small‐scale washover deposits were examined along the Outer Banks, North Carolina, near Pt Mugu, California, and at Presque Isle (Lake Erie), Pennsylvania. The fans were formed in response to unidirectional landward transport, and the runoff channels in response to unidirectional flow usually in a landward direction, but sometimes in shore‐parallel then seaward direction. Where overwash carried across the fan surface and entered a pond or lagoon, a small‐scale delta (microdelta) developed. In this case, the washover fan consisted of two subfacies, the wetted, but ‘subaerial’ part of the fan and the subaqueous washover delta. Flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification.High velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low‐angle (landward dipping) planar stratification which comprised the major part of the fan. Similarly, rhomboid forms were produced by high velocity sheet flow across the fan surface. Where flow carried into a standing body of water, delta‐type foreset strata developed. For this case, the lateral structural sequence was subhorizontal, planar stratification merging landward into landward dipping, delta (tabular) foreset strata. In the runoff setting, where flow became channelized and continuous, both upper‐flow and lower‐flow regime currents were typical. Upper‐flow regime bedforms included antidunes, standing waves, and plane beds. The most commonly observed lower‐flow regime bedforms included microdelta‐like bars, low‐amplitude bars, linguoid ripples, and sinuous‐crested current ripple trains.The sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment.
Depositional environments of the Senonian chert, phosphorite and oil shale sequence in Israel as deduced from their organic matter composition
Authors: BEIN AMOS , AMIT ORA 
Published: June 2006

ABSTRACTIn the Upper Cretaceous sequence of the Negev (southern Israel) the organic matter in phosphorites and cherts differs from that associated with oil shales in its higher content of humic substances and lower kerogen content, and in its more intensive microbial alteration. The n‐alkane distribution pattern of the oil shales, phosphorites and cherts indicates that marine biota, probably algae, are the main organic precursors of their organic matter. In some of the oil shales, however, some contribution of terrestrial organic matter is also evident. Similar high phytane/pristane ratios indicate that the organic matter in the oil shales as well as in the phosphorites and cherts accumulated under reducing conditions. The main differences in the organic matter composition are attributed to early diagenetic processes rather than to different biotic precursors or to late modifications due to temperature‐induced maturation. The depositional model suggested for the sequence involves upwelling conditions at the boundary between the deep Tethys and the shallow shelf, which induced high organic productivity deep into the inner shelf. Bottom water circulation enabled intensive microbial alteration of the organic matter, followed by a winnowing process leading to phosphorite formation. Since humification is considered an oxygen‐consuming reaction, these processes favoured the formation of oxygen‐enriched humic substances and the oxidation of humic substances already present. These humic substances are relatively resistant to further alteration and their conversion into kerogen is thus retarded. Subsequently, syndepositional tectonic activity resulted in the introduction of less saline water, restriction of bottom‐water circulation and the establishment of a density stratification in the water body. Consequently, aeration of the bottom layer and the sediments was inhibited, microbial alteration was reduced and later winnowing processes were prevented. Such conditions favoured the formation of kerogen directly, rather than through humic substances, and also favoured the preservation of most of the organic matter in the form of oil shale deposits instead of phosphorites.
Saltation threshold on Earth, Mars and Venus
Authors: IVERSEN J. D. , WHITE B. R. 
Published: June 2006

ABSTRACTNew formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles. A low‐air‐density wind tunnel was used to extend the range of previous investigations and to separate the effects of Reynolds number and interparticle forces of cohesion. The new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the Earth, Mars, and Venus.
The oxygen and carbon isotope composition of Carboniferous fossil components: sea‐water effects
Authors: BRAND UWE 
Published: June 2006

ABSTRACTThe aragonitic molluscs and lime‐mud of the Pennsylvanian Buckhorn asphalt (Deese Group) of southern Oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea‐water. In addition, δ18O values indicate that the pelecypods precipitated their shells during the warmer months of the year. The coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year. For the orthocone nautiloids, the δ18O values suggest that they precipitated their shells in deeper/cooler water.The low‐Mg calcite brachiopods of the Mississippian Lake Valley Formation of New Mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea‐water.The δ18O and δ13C values of the Buckhorn and Lake Valley faunas, in conjunction with other published results, suggest that Carboniferous sea‐water was, on a average, depleted in δ18O by 1·5 ± 2‰, PDB, relative to Recent sea‐water. However, the δ13C value of +2.6 ± 2‰, PDB, for average Carboniferous sea‐water is similar to that of Recent ocean water.Early diagenetic alteration of metastable carbonates probably occurs in a meteoric‐sea‐water mixing zone. In this zone the oxygen and carbon isotopic compositions of these components are increased by about 2‐4‰, PDB over their marine composition.
Geometry and origin of large‐scale deformation structures in some ancient wind‐blown sand deposits
Authors: HOROWITZ DANIEL H. 
Published: June 2006

ABSTRACTLarge‐scale deformational features that disrupt the cross‐bedded strata of some ancient wind‐blown dune sand deposits (Jurassic Aztec and Navajo Sandstones, U.S.A.) can betraced laterally for at least tens of metres. Information from four exposures leads to an idealized deformation style characterized by (1) a ‘head’ portion marked by collapse features, (2) a middle portion marked by a thrust ramp or a large recumbent fold, and (3) a ‘toe’ portion marked by a planar shear zone with small recumbent folds, drag folds, and possibly small horizontal shear faults. An hypothesis involving earthquake‐induced liquefaction and collapse of the dunes is proposed. Sands liquefied during earthquakes cannot support an unequal surface load, resulting in the collapse of surface dunes above the liquefied substrate. Compensatory lateral squeezing of liquefied sand and buried strata produces the shear zone in the ‘toe’ portion. Engineering studies suggest that buried strata underlying interdunal lows, where over‐burden stress is least, will be most susceptible to liquefaction. Other considerations suggest that it will be the steeper lee sides of advancing surface dunes that collapse, squeezing liquefied sand forward or downwind, i.e. away from the lee slope of surface dunes. If the liquefied condition persists, load structures can be formed when unliquefied layers founder into the liquefied substrate.
Depositional model for a chenier plain, Gulf of Carpentaria, Australia
Authors: RHODES E. G. 
Published: June 2006

ABSTRACTThe southern coast of the Gulf of Carpentaria, Northern Australia, consists of a broad chenier plain which is comparable in size and morphology to those reported on the coasts of Surinam and south‐western Louisiana. The Carpentaria chenier plain has, at some locations, prograded over 30 km since the Middle Holocene by deposition of low‐tide muds over subtidal muds during periods of increased sediment input by the rivers. Chenier ridges are formed during periods of reduced sediment flux by wave induced sorting and redistribution processes. An alternation of increased and decreased sediment flux from fluvial sources has occurred several times during the upper Holocene. Such variation in sediment supply is believed to be related to temporal fluctuations in rainfall over the drainage basin which surrounds the southern and eastern Gulf.
Temperate carbonate sediments of Northern Spencer Gulf, South Australia: a high salinity ‘foramol’ province
Authors: BURNE R. V. , COLWELL J. B. 
Published: June 2006

ABSTRACTCool‐water skeletal carbonate sediments are forming in Spencer Gulf, South Australia, an area of high salinity and moderate tidal range. Four environments can be distinguished: deeper marine areas (10–20 m); shallow subtidal platforms and banks (2–10 m); intertidal and supratidal zones; and coastal springs and lakes fed by saline continental groundwaters. The sediments are predominately bioclastic carbonate sands; muddy sediments occur in protected intertidal environments. The most common grain types are gastropods, bivalves, foraminifera, coralline algae and quartz. Indurated non‐skeletal carbonate grains have not been seen. Composition of the sediment varies little between environments, but considerable textural variation results from variation in the stability of the substrate, hydrodynamic conditions, depth of water, period of tidal inundation, supply of terrigenous grains, temperature, and salinity. The Spencer Gulf data suggests that temperature, and particularly minimum temperature, controls the distribution of skeletal and non‐skeletal grain associations in high‐salinity environments. The textures of the sedimentary facies of Spencer Gulf closely parallel those of equivalent environments in warm‐water carbonate provinces.
Critical remarks on grain parameters, with special emphasis on shape
Authors: WINKELMOLEN A. M. 
Published: June 2006

ABSTRACTTo answer the question under what dynamic conditions a sediment has been deposited, one needs to describe the properties of the particles in parameters that are relevant to the processes. This requirement is not always fulfilled by the parameters commonly used for facies description like ‘size’. The three functional, relevant parameters are mass, density (or volume) and shape.Most depositional processes are a balance between resistive forces like weight, which act on the mass of the grains and tractive forces, which act on the grain's surfaces. To understand the transport processes, both sides of this balance should be quantified independently.It is argued that to quantify the resistance side of the transport balance, the particles must be described in terms of mass‐frequency distributions and densities. For the evaluation of the tractive part, the grain's surface area should be known. Since shape is the unique parameter which determines the amount of surface area of a volume, the particle's shape and the shape distribution should be quantified.Shape is a very complex property. Contrary to the widespread opinion that ‘shape’ is a dimensionless parameter, it is argued that shape is a volume/surface ratio and has moreover vectorial properties. It is shown that ‘shape’ essentially is a length tensor.A method is given to quantify the shape parameter. Since shape has the physical dimension of length, it is involved in scaling and should be accounted for when comparing different grain sizes.Methods to measure functional shape are briefly discussed.
Sheet‐flow deposits of graded beds and mudstones on an alluvial sandflat‐playa system: Upper Triassic Blomidon redbeds, St Mary's Bay, Nova Scotia
Authors: HUBERT J. F. , HYDE M. G. 
Published: June 2006

ABSTRACTThe 400 m of Blomidon redbeds accumulated in a semi‐arid rift valley in the subtropics. At St Mary's Bay, these redbeds are 64% sandy mudstone (playa mudflats), 25% graded beds (sandflats at the toes of alluvial fans), 10% fissile claystone (playa lakes), and 1% channel sandstone (stream channels). Flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north‐east as sheet flows on sandflats and playa mudflats. Deceleration of the sheet flows deposited graded beds 2–83 cm in thickness on the sandflats and thin layers of mud on the playas. Nine sequences, consisting of arrangements of six lithologies, compose 90% of the graded beds. In order of decreasing abundance, these are: ripple cross‐laminated siltstone → horizontally laminated mudstone; fining‐upward, ripple cross‐laminated siltstone; ripple cross‐laminated sandstone → horizontally laminated mudstone; cross‐bedded sandstone horizontally laminated mudstone; ripple cross‐laminated sandstone → ripple cross‐laminated siltstone; rippledrift cross‐laminated siltstone horizontally laminated mudstone; fining‐upward, ripple‐drift cross‐laminated siltstone; cross‐bedded sandstone → ripple cross‐laminated siltstone; and cross‐bedded sandstone → ripple cross‐laminated siltstone → horizontally laminated mudstone. The sheet flows, perhaps up to 1 m in depth, had a high concentration of suspended load. Deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration. There are 32 thinning and fining‐up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone. The cycles range in thickness from 1·3 to 13·3 m, averaging 4·6 m. Each cycle is initiated by avulsion to a new active channel network on a fan. Gradual abandonment of the channel network produces the thinning and fining‐up cycle. The cycles are grouped in three 60–70 m fining‐up megacycles. Upwards within each megacycle, the packages of sandy mudstone compose a progressively larger proportion of the cycles. Each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor. Subsequent erosion gradually lowered the relief to yield a fining‐up megacycle.
The dynamics of a river bend: a study in flow and sedimentary processes
Authors: BRIDGE J. S. , JARVIS J. 
Published: June 2006

ABSTRACTComprehensive field measurements of flow and sedimentary processes have been made with the aid of stable scaffolding bridges spaced along the length of a bend of the River South Esk, Scotland.At river stages between about two‐thirds full and bankfull, channel width, mean depth and mean flow velocity at a cross‐section vary little in the streamwise direction. Flow resistance reaches a maximum at these stages, and the bed topography is stable and in equilibrium with flow and bedload transport. Stable flow geometry is thus related in some way to energy conservation, and to maximization of flow resistance.Detailed observations over a large range of river stages of mean velocity distributions, secondary circulation, water surface configuration, bed shear stress and resistance to flow, bed configurations and bed load transport rates agree with much (but not all) of the comparable published experimental studies and selected theoretical work.Generalized physical models of flow and sediment transport in natural curved channels (Engelund, 1974; Bridge, 1977) are demonstrated to be sound in basis and can simulate the bend studied very well. Although there is a pressing need for further development of these models, the results lend confidence to their use in simulating ancient river sedimentation.Sediment deposited on point bars is the result mainly of bedload transport over a range of near‐bankfull stages. The areal distribution of grain‐size characteristics and bed configurations at these stages give rise, with lateral deposition, to vertical facies sequences that vary substantially in the streamwise direction.