bluuebunny/crossref_metadata_2025 huggingface dataset:
Bedform and stratification characteristics of some modern small‐scale washover sand bodies
Authors: SCHWARTZ ROBERT K.
Published: June 2006

Washover 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

In 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

New 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

The 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

Large‐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

The 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

Cool‐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

To 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

The 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

Comprehensive 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.
Peat‐marl deposition in a Holocene paludal‐lacustrine basin—Sucker Lake, Michigan
Authors: TREESE KATHY L. , WILKINSON BRUCE H.
Published: June 2006

Exceptionally thick (over 15 m) deposits of peat which fill the northern end of Sucker Lake basin in northcentral Michigan document the volumetric importance of allochthonous organic material in a modern coal‐forming environment. Organic debris that originates in, and is derived from, a highly vegetated floodplain immediately upstream is deposited as a lakeward‐prograding lobe that exhibits features typical of most lacustrine Gilbert deltas, but is composed almost entirely of organic material. This system overlies an additional 7 m of nearly pure, brecciated lacustrine carbonate, deposited as shallow lake‐margin benches and emplaced into the deep basin centre by gravity sliding prior to deltaic progradation.

In the southern end of the basin, bottomset beds of fine silt‐size organic phytoclasts onlap distal facies of progradational bench carbonates, which originate (in shallow water) through calcite encrustation about stems of the dominant macrophyte Chara. With continued sedimentation, a stratigraphic succession in which allochthonous organics will overlie pure, allochthonous lacustrine carbonates and in part be overlain by autochthonous carbonates will characterize the northern end of this Holocene system. In the southern end around the basin margins, however, autochthonous carbonates will entirely underlie allochthonous organics.

Numerous continuous cores (up to 22 m) through these units document: (1) the importance of sources of allochthonous organic debris in modern coal‐forming complexes, (2) the genetic relationship between nearly pure calcareous and nearly pure carbonaceous facies within these paludal‐lacustrine settings, and (3) the complexity of interrelationships between the several component facies within such continental sedimentary systems.
Shape effects in aeolian grain transport
Authors: WILLETTS B. B. , RICE M. A. , SWAINE S. E.
Published: June 2006

Wind tunnel experiments are reported, the results of which show that grain transport rate is responsive to characteristic grain shape. It is relatively less sensitive to changes of sorting unless these involve the fraction smaller than 50μm. A cubic relationship between transport rate and velocity gradient appears to be valid only for well worked natural grains having high sphericity.
A stable isotope study of the Mercia Mudstones (Keuper Marl) and associated sulphate horizons in the English Midlands
Authors: TAYLOR S. R.
Published: June 2006

ABSTRACT Widespread evaporitic sulphate horizons occur in the frequently dolomitic Mercia Mudstones (Upper Triassic) of the English Midlands. The mudstones were deposited on an extensive peneplain which had areas of upstanding minor relief (Charnwood Massif, Derbyshire—Pennines, London Platform). Horst and graben structures formed during the early and middle Triassic, controlled late Triassic sedimentation rates and peneplain slope directions. A stable isotope study (S,C,O) of the sulphates and dolomites of the Trent Formation indicates that during the deposition of the lower Fauld Member grabens such as the Need wood Basin in Staffordshire and areas marginal to a graben such as Newark in Nottinghamshire had a marine brine regime with minor continental input. Using constraints applied by sulphate concentrations of modern waters in arid environments, the ratio of volume of marine input to volume of continental input lay between 3:1 and 4:1. The horst area (East Leake, Nottinghamshire) was strongly influenced by continental brines which derived sulphate from the exposed Carboniferous Hathern Anhydrite Series. A predominantly continental brine regime existed across the whole area during deposition of the overlying Hawton Member. Periodic marine influxes gave rise to thin sulphate horizons whilst continental run‐off was occasionally sufficient to form temporary lakes. The Blue Anchor Formation (Rhaetian) was deposited in a lacustrine environment with waters of mixed marine and continental origin. Salinities were lower than those of the preceding Trent Formation brines.
Braidplain deposition of the Upper Triassic Molteno Formation in the main Karoo (Gondwana) Basin, South Africa
Authors: TURNER BRIAN R.
Published: June 2006

ABSTRACT The Upper Triassic (Carnian?) Molteno Formation in the main Karoo (Gondwana) Basin, South Africa forms a northerly thinning, intracratonic clastic wedge comprising sandstones, shales and coals occurring within thick (up to 140 m) laterally persistent fining‐upward sequences. These sequences were deposited by braided streams draining an alluvial plain which may have been built on to the distal slopes of alluvial fan complexes of glacial outwash type. Geometric relations between sequences indicate three phases of tectonic activity. The lowermost fining‐upward sequence in the south accumulated against a rising mountain front; cessation of movement and an eastward shift in the main locus of tectonism and sedimentation was followed by renewed uplift and basinwide progradation of the second fining‐upward sequence adjacent to a fault‐block granite terrain located close to the present south‐east coastline of South Africa. This is believed to be the granite at the eastern end of the Falkland Island Plateau, an interpretation consistent with its position on most continental reconstructions and the fracture zone marking its northern scarp face. Faulting is attributed to the first phase of extension prior to continental breakup. The sourceward recession and lack of gross fining‐upward trends shown by the uppermost fining‐upward sequences is accounted for by limited back‐faulting of the still active basin margin. Cessation of activity and further basin margin recession occurred with deposition of the overlying floodplain deposits (Elliot Formation) which were distal equivalents of the braided alluvial plain.
Water escape structures in the context of a depositional model of a mass flow dominated conglomeratic fan‐delta (Abrioja Formation, Pliocene, Almeria Basin, SE Spain)
Authors: POSTMA GEORGE
Published: June 2006

ABSTRACT Pebbly sediments of the shallow marine Abrioja fan‐delta show pockets (bowl‐shaped structures, partly filled with pebbles) and pillars (elongate structures, filled with sand and pebbles). These structures are most abundant in pebbly sediments deposited on a steep slope (ca. 25°‐10°) and are absent in conglomerates deposited on a slope of ca. 6° and less, although they are present in the pelitic top of these beds.

The pocket and pillar structures are interpreted as fluid escape structures originating from local liquefaction and fluidization, processes which are favoured by rapid deposition, rapid sediment accumulation, the presence of less permeable layers and an immature sediment texture.

These conditions are met in conglomeratic fan‐deltas, which have steep slopes with immature sediments. It is concluded that the presence of fluid escape structures in conglomeratic sediments may indicate a steep depositional slope.
Environmental discrimination based on quartz grain surface textures: a statistical investigation
Authors: CULVER S. J. , BULL P. A. , CAMPBELL S. , SHAKESBY R. A. , WHALLEY W. B.
Published: June 2006

ABSTRACT The relative abundance of thirty‐two surface features identified on the surfaces of quartz sand grains from eight coded samples was noted independently by five scanning electron microscopists. These data (reduced to binary form) were the subject of canonical variate analyses which discriminated clearly between all samples. Thirteen variables were important in distinguishing between the eight samples. This indicates that there are no single key surface features which, on their own, would allow rapid environmental discrimination. The contention that a multiplicity of surface features should be recorded for the purpose of environmental discrimination is, therefore, upheld.

Operator variance, although considerable in the recognition of individual surface features, is negligible in discrimination of samples based on binary data. The SEM technique of analysis of quartz grain surface textures is a reliable and statistically valid means of discriminating between samples from different environments.
Transitional submarine fan deposits from the late Precambrian Kongsfjord Formats submarine fan, NE Finnmark, N. Norway
Authors: PICKERING KEVIN T.
Published: June 2006

ABSTRACT Three transitional submarine fan environments are recognized in the late Precambrian, 3‐2 km thick Kongsfjord Formation in NE Finnmark, North Norway, namely: (1) middle to outer fan; (2) fan lateral margin, and (3) fan to upper basin‐slope deposits. Middle to outer fan deposits have a high proportion of sandstones, typically showing Bouma Tbede with Ta in the thicker beds. Deposition was mainly from sheet flows with rare shallow channels. Middle to outer fan deposits are an association of sandstone packets less than 10 m thick but commonly only a few metres thick, interpreted as channels or lobes. Interchannel and fan fringe deposits occur as discrete packets of beds between the thicker bedded and coarser grained channel or lobe deposits. Fan lateral margin deposits are recognized on the basis of their stratigraphic position adjacent to inner/middle fan deposits. They are characterized by: (a) a relatively high proportion of fine‐grained sandstone/siltstone turbidites compared to other major fan environments; (b) relatively small channels oriented at various angles to the regional basin slope; (c) lobes associated with channels, and (d) abundant clastic dykes and other soft‐sediment deformation. Fan lateral margin deposits are distinguished from the outer fan/basin plain successions on account of the very high proportion of siltstone turbidites comparable with middle fan inter‐channel deposits. Fan to upper basin‐slope deposits occur at the top of the formation as an alternation of sandstone turbidites, most of which are laterally discontinuous, and very thin‐bedded upper basin‐slope siltstones with slide deposits.
Eocene fan delta‐submarine fan deposition in the Wagwater Trough, east‐central Jamaica
Authors: WESCOTT WILLIAM A. , ETHRIDGE FRANK G.
Published: June 2006

ABSTRACT The Wagwater Trough is a fault‐bounded basin which cuts across east‐central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I‐unfossiliferous massive conglomerates; Facies II—channelized, non‐marine conglomerates, sandstones, and shales; Facies III‐interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin‐bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta‐submarine fan deposits. Facies associations suggest that humid‐region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided‐fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta‐front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope‐basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid‐fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor.
Diagenetic density inversions and the deformation of shallow marine chert beds in Israel
Authors: FINK JONATHAN H. , RECHES ZE'EV
Published: June 2006

ABSTRACT Chert folds and ‘dyke’‐like boudinage in the Senonian Mishash Formation of Israel have orientations consistent with contemporaneous tectonic patterns whereas their amplitudes are anomalously high relative to strains calculated for adjacent layers. We suggest several means by which density inversions associated with chert diagenesis might amplify gentle structures whose wavelengths and orientations were determined by regional stresses.
Gravel overpassing on humpback bars supplied with mixed sediment: examples from the Lower Old Red Sandstone, southern Britain
Authors: ALLEN J. R. L.
Published: June 2006

ABSTRACT The Lower Old Red Sandstone (Downton to ?Emsian) in southern Britain is a largely fluviatile sequence of increasing upward sand‐dominance. The highest beds at two groups of localities include many sedimentation units composed of gravelly foresets in depositional continuity with overlying topsets of parallel‐laminated sandstone. These units are thought to have been fashioned by humpback bars having a crest a considerable way upstream from the brink at the top of a gravelly slipface. The overall textural composition but internally segregated character of the bar units suggests that a bimodal sediment load of mixed sand and gravel was supplied at the upstream end of the bar, but that this load became texturally differentiated as it moved downstream.

Differentiation is suggested to have occurred because the comparatively large and well‐rounded gravel particles behaved on the sandy topset as though on a smooth surface, and were transported under similar flow conditions to the sand, much of which eventually lodged on the topset instead of being passed on, like the gravel, to the slipface beyond. A quantitative model is outlined which justifies the proposed gravel overpassing. In terms of the control of sedimentary structures exerted by grain size under laboratory conditions, the association of cross‐bedding (gravel) with simultaneously formed parallel lamination (sand) seems to be a natural consequence of the efficient textural differentiation of the supplied load by the overpassing of the gravel component under a single flow condition.
A technique for preparing high water content clayey sediments for thin and ultrathin section study
Authors: CHIOU W. A. , SHEPHARD L. E. , BRYANT W. R. , III M. P. LOONEY
Published: June 2006

ABSTRACT The study of sediment microfabric is fundamental to the understanding of various sediment properties including shear strength, compressibility, and permeability, evaluating depositional environments and investigating clay diagenesis. A technique is presented which preserves the particle integrity of fine‐grained sediments while permitting examination of the same sample using light and electron microscopy techniques. The technique involves drying the sediment sample using a critical point drying apparatus and then impregnating it using low viscosity SPURR epoxy. Standard thin sectioning techniques are used for light microscopy, whereas ultrathin sections are made using a microtome for transmission electron microscopy work. Scanning electron microscopy investigations are performed on samples prior to impregnating.
The fluvial and pyroclastic deposits of the Cagayan Basin, Northern Luzon, Philippines—an example of non‐marine volcaniclastic sedimentation in an interarc basin
Authors: MATHISEN M. E. , VONDRA C. F.
Published: June 2006

ABSTRACT The Cagayan basin of Northern Luzon, an interarc basin 250 km long and 80 km wide, contains a 900 m thick sequence of Plio‐Pleistocene fluvial and pyroclastic deposits. These deposits are divided into two formations, the Ilagan and Awidon Mesa, and three lithofacies associations. The facies, which are interpreted as meandering stream, braided stream, lahar, and pyroclastic flow and fall deposits, occur in a coarsening upward sequence. Meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits; lahars and ignimbrites. The coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent Cordillera Central volcanic arc. Uplift of the arc resulted in the progradation of coarser clastics further into the basin, the development of an alluvial fan, and migration of the basin depocentre away from the arc. The coarsening of the pyroclastic deposits reflects the development of a more proximal calc‐alkaline volcanic belt in the maturing volcanic arc. The Cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins. This is because the tectonic and volcanic processes which controlled sedimentation in the Cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins.
The genesis of lateral accretion deposits in recent intertidal mudflat channels, Solway Firth, Scotland
Authors: DE MOWBRAY TESSA
Published: June 2006

ABSTRACT The pattern of lateral accretion on an intertidal point bar is a response to the suspended sediment concentrations to which it is exposed. In summer, high sediment concentrations occur only during shallow (ebb) channel flows; deposition therefore takes place mainly on the point bar toe. In winter, high concentrations obtain at all depths and deposition therefore occurs on the upper point bar slopes. Accretion of the point bar toe is limited by scouring resulting from the high rainfall runoff of winter. High suspended sediment concentrations have little effect, however, if flow velocities are too high to allow deposition. Deposition is dependent on the non‐uniformity of curved channel flow and is often confined to the downstream (ebb‐sense) portion of the point bar. The lateral accretion deposits form a series of wedge‐shaped units. Each unit represents one year's deposition, bounded by erosion scarps produced during successive winters. The base of the point bar deposit shows a gradual aggradation, keeping pace with the build‐up of the adjacent interchannel flats.
The relationship of sediment transport rates and paths to sandbanks in a tidally dominated area off the coast of East Anglia, U.K.
Authors: LEES BARBARA J.
Published: June 2006

ABSTRACT Sand transport measurements of bedload and suspended load in the Sizewell‐Dunwich Banks area, East Anglia have shown that the suspended mode is dominant. The depth‐integrated spring tidal residual is 5.66 g cm−1 sec−1, although the neap rate is only one‐fifth of this. The calculated bedload transport rates also vary, from 0.012 to 0.040 g cm−1 sec−1, correlating with changing meteorological conditions.

In order to predict the bedload sediment circulation pattern from midwater current meter measurements, five sediment transport equations were calibrated, using fluorescent dyed sand. Yalin's relationship gave the best estimates. The bed shear stress was determined by extrapolating the velocity profile as a power law relationship, with an exponent equal to 0.1, from midwater down to 2 m and as a lognormal profile from 2 m to the sea‐bed. Roughness length values appropriate to the substrate were used.

Although bedload transport residuals are mainly to the south, the banks trend northwards from the coast and have also elongated in this direction. This is thought to be in response to the dominance of the suspended sediment transport. It is suggested that a tidal residual eddy mechanism is responsible for the banks’maintenance, similar to the process operating in Start Bay, Devon. The well‐documented westward movement of the banks is likely to be related to wave processes.
Submarine debris flow deposits from the Upper Carboniferous Dwyka Tillite Formation in the Kalahari Basin, South Africa
Authors: VISSER J. N. J.
Published: June 2006

ABSTRACT Four types of sediment gravity flow deposits occur interbedded with rhythmite shale, siltsone, mudstone and minor diamictite in a 230‐m thick Carboniferous glacial sequence. Shear and plug zones are present in the cohesive debris flow deposits which have a diamictic texture. The high‐density turbidity flow deposits which consist of coarse to medium‐grained clastics, are characterized by both normal and reverse grading. The medium to fine‐grained low‐density turbidity flow deposits show normal grading and consist of Bouma units A, B, D and E. Deposition occurred by gravity flow, suspension settling and minor basal melt‐out during ice retreat along the southern margin of the Kalahari Basin. Immediately basinwards of the ice grounding line a proximal diamictite facies consisting predominantly of cohesive and high‐density turbidity flow deposits, and minor melt‐out tills formed. A distal sedimentary facies of graded sandstone and siltstone units deposited by low‐density turbidity flows and suspension settling of muds with ice‐rafted debris is located basinwards.
Dynamic processes acting on a longitudinal (seif) sand dune
Authors: TSOAR HAIM
Published: June 2006

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 is higher than on the crest line or the windward flank and longitudinal sand transport occurs. When the angle is less acute (> 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.
Chemical controls of cathodoluminescence of natural dolomites and calcites: new data and review
Authors: FAIRCHILD IAN J.
Published: June 2006

ABSTRACT Calcites and dolomites from the late Precambrian Bonahaven Formation of Scotland exhibit luminescence when MnCO3>0.03 wt % (dolomite) or >0.06 wt % (calcite) and FeCO3<2‐2‐5 wt % (dolomite). Comparison with other published data shows close similarity in the position of Fe‐quenching in dolomite, but more variability in the position of Mn‐activation in calcites and dolomites and the onset of Fe‐quenching in calcites. These anomalies are thought to relate to chemical controls subsidiary to the dominant factors of Fe and Mn concentration. It is not practical to make a useful comparison between operating conditions of different luminescence instruments because of differences in design and an inability to measure routinely the current actually on the specimen surface.
Facies, environments and sedimentary cycles in the Middle Eocene, Bracklesham Formation of the Hampshire Basin: evidence for global sea‐level changes?
Authors: PLINT A. G.
Published: June 2006

The Bracklesham Formation is of Middle Eocene age and occurs throughout the Hampshire Basin of southern England. The basin is elongated east‐west and filled with Lower Tertiary sediments. Its southern margin is marked by either large, northward‐facing monoclines, or faults, both of which underwent differential movement, with uplift of the southern side throughout the Middle Eocene. The Bracklesham Formation, which is up to 240 m thick, shows pronounced lateral facies changes with dominantly marine sediments in the east passing to alluvial sediments in the west. Four principal sedimentary environments: marine, lagoonal, estuarine and alluvial are distinguished. Marine sediments comprise six facies including offshore silty clays and glauconitic silty sands, beach and aeolian dune sands, and flint conglomerates formed on pebble beaches. Offshore sediments predominate in the eastern part of the basin, as far west as Alum Bay, where they are replaced by nearshore sediments. Lagoonal sediments comprise four facies and formed in back‐barrier lagoons, coastal marshes and, on occasions, were deposited over much of the basin during periods of low salinity and restricted tidal motion. Five estuarine facies represent tidal channels, channel mouth‐bars and abandoned channels. These sediments suggest that much of the Bracklesham Formation was deposited under micro‐ to meso‐tidal conditions. Alluvial sediments dominate the formation to the west of Alum Bay. They comprise coarse to fine sands deposited on the point‐bars of meandering rivers, interbedded with thick sequences of laminated interchannel mudstones, deposited in marshes, swamps and lakes. Extensive layers of ball clay were periodically deposited in a lake occupying much of the alluvial basin. In alluvial areas, fault movement exposed Mesozoic rocks along the southern margin of the basin, the erosion of which generated fault‐scarp alluvial fan gravels. Locally, pisolitic limestone formed in pools fed by springs emerging at the faulted Chalk‐Tertiary contact. In marine areas, flint pebbles were eroded from coastal exposures of chalk and accumulated on pebble beaches and in estuaries. From other evidence it is suggested that older Tertiary sediments were also reworked. The Bracklesham Formation is strongly cyclic and was deposited during five marine transgressions, the effects of which can be recognized throughout the basin in both marine and alluvial areas. Each of the five transgressive cycles is a few tens of metres thick and contains little evidence of intervening major regression. The cycles are thought to represent small‐scale eustatic sea‐level rises (‘paracycles’) superimposed upon a major transgressive ‘cycle’ that began at the base of the Bracklesham Formation, following a major regression, and was terminated, at the top of the Barton Formation by another major regression. This major cycle can be recognized world‐wide and may reflect a period of rapid northward extension of the mid‐Atlantic ridge.
Bar development in a fluvial sandstone (Westphalian ‘A’), Scotland
Authors: KIRK MARTIN
Published: June 2006

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.
Measurements of beach pebble attrition in Palliser Bay, southern North Island, New Zealand
Authors: MATTHEWS ERIC R.
Published: June 2006

The beaches of Palliser Bay, exposed to oceanic waves from the south, are composed of mixed sand and gravel derived from mountain ranges of greywacke and argillite that rise above both sides of the bay. Beach gravel tracer experiments, each using up to 75 tonnes of limestone pebbles, were conducted at three sites. Increase in the roundness of these tracer pebbles at each site during the year of observation was compared with the increase in roundness and rate of weight loss of limestone and greywacke pebbles in a laboratory tumbler. The comparison enabled estimation of the rate of attrition of the natural beach pebbles. A weight loss of 41% per year was found for pebbles at the most exposed site and 15 and 7% at the other two sites.