bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbonate banks and slump beds in the Upper Cretaceous (Upper Turonian‐Santonian) of Haute Normandie, France
Authors: KENNEDY W. J. , JUIGNET P.
Published: June 2006

Large scale sedimentary structures present in the Upper Turonian to Santonian chalks of Haute Normandie (northern France) represent the remains of a carbonate bank complex which formerly extended over an area of at least 1500 km2. Cliff exposures along the Channel coast from St Valéry‐en‐Caux to Cauville and along the Seine from Sandouville to Lillebonne show sections of banks up to 50 m high and 1500 m across, their internal structures picked out by hardgrounds, nodular chalks and horizons of burrow flint.

Associated with banks are slump sheets up to 20 m thick, slump scars, sedimentary breccias, injection phenomena and faults contemporaneous with sedimentation. Later diagenetic features include extensive dolomitization and silicification.

These structures compare closely with the Waulsortian banks of the Palaeozoic, and bryozoan bioherms known from the Upper Cretaceous and Palaeocene of Denmark. Frame‐building, sediment trapping and stabilizing organisms are absent, and bank development and stabilization was probably due to a plant covering, either algal or of marine angiosperms. Banks generated much of their own sediment, whilst a pelagic constituent (calcareous nannofossils and Foraminiferida) is also present.

The distribution of the bank complex is related to a basement controlled swell area, whilst the life of the complex was limited to a relatively shallow water, regressive episode in the predominantly transgressive Upper Cretaceous history of the region.

Les falaises littorales du Pays de Caux comprises entre Antifer et St Valèry‐enCaux, et les affleurements de la basse vallée de la Seine permettent d'observer des formations du Turonien supérieur‐Sénonien inférieur qui présentent des stratifications irrégulières soulignées par de nombreux hardgrounds, des horizons de craie noduleuse et des cordons de silex. Ces structures sont identifiées à des accumulations de calcilutite et calcarénite sous forme de bancs sous‐marins dont la hauteur peut atteindre 50 m et qui couvrent une surface supérieure à 1500 km2; ils apparaissent au‐dessus de hardgrounds subhorizontaux qui indiquent un haut‐fond régional stable. Des glissements sous‐marins sont associés à ces bancs et engendrent des niveaux avec des déformations souples atteignant 20 m d'épaisseur. Des brèches apparaissent localement et contiennent des blocs basculés de hardgrounds fragmentés lors du glissement; on y observe aussi de petites failles intrasédimentaires et des phénomènes d'injection. Aucun organisme constructeur ou capable de piéger et retenir le sédiment n'a été observé. La stabilisation de ces bancs serait due à une couverture végétale (algues ou angiospermes marines) dont on sait qu'elle peut disparâitre sans laisser de trace lors de la fossilisation. La croissance de ces bancs serait réalisée par un apport de sédiment comprenant une part de nourrissage autochtone comme cela existe pour les bancs récents en eau peu profonde, associée au dépôt d'une fraction pélagique.
Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers
Authors: EYNON GEORGE , WALKER ROGER G.
Published: June 2006

Facies relationships in Pleistocene braided outwash deposits in southern Ontario demonstrate the presence of a large braid bar with adjacent side channel. The core of the bar is up to 6 m high, and consists of crudely horizontally stratified gravels. Downstream from the core is the bar front facies, consisting of large gravelly foresets up to 4 m high, rounded off in many places by reactivation surfaces. Upstream from the core is the bar stoss side facies consisting of several sets (individually up to 35 cm thick) of tabular cross‐bedding, arranged in coarsening‐upward sequences. The stoss side—core—bar front relationships are continuously exposed in one 400 m long quarry face which is cut almost parallel to the palaeoflow direction. A transverse quarry face shows the side channel facies, which consists of trough cross‐bedded sands. Gravel layers can be seen to finger from the main gravelly bar into the sandy side channel, but they do not reach the base of the channel. This surprising relationship indicates that gravel moved only in the topographically higher parts of the system. After deposition in the side channel, and growth upstream and downstream from the bar core, the entire system aggraded. Crudely horizontally stratified, and imbricated gravel sheets were laid down as a bar top facies.

Grain size analyses indicate strongly bimodal distributions, implying that much of the sand in the spaces between pebbles and boulders filtered in after the gravel had been deposited. This interpretation is strengthened by velocity calculations—mean velocities in excess of 300 cm/s would be needed to roll the gravel as bed load, but at such a velocity, a large amount of sand would be transported entirely in suspension.

In a final section of the paper, our results are combined with other work on braided systems in an attempt to formulate a more general facies model.
Packing and resistance to compaction of shells
Authors: ALLEN J. R. L.
Published: June 2006

The separated valves of bivalve molluscs, certain gastropods, and many brachiopods closely resemble the regular geometrical form known as the shell. The packing concentration of natural accumulations of such organic shells may be placed within bounds with the aid of packing models making use of ordered arrangements of equal geometrical shells, either conical, cylindrical or spherical. The low packing concentrations, comparable with 0·1–0·2, indicated by these models are confirmed by experiments using the shells of four common British species of bivalve or gastropod mollusc. Packings of these shells have a substantial intrinsic strength, and experimentally appear able to support without failing sedimentary overburdens equivalent to loads comparable with 1000 kg mass/m2. The observed and predicted low packing concentrations suggests that natural shell beds can hold relatively very large amounts of pore fluids or mineral cements. The resistance to compaction of the packings means that the high original porosities of natural shell beds have a good chance of being permanently preserved.
A scanning electron microscope study of surface textures of quartz grains from glacial environments
Authors: WHALLEY W. BRIAN , KRINSLEY DAVID H.
Published: June 2006

Surface textures of quartz sand grains from several glacial environments at the Feegletscher, Switzerland examined by means of scanning electron microscopy are described. The difference between supraglacial and subglacial material is very slight. At the moment the interpretation of these textures must be done with care until the full statistical relation to environments can be ascertained. This does not necessarily invalidate earlier investigations although it does mean that glacial and extraglacial environments in the vicinity of glaciers are apparently not distinguishable. Examination of surface precipitation features suggests a sequence of events which can be used to help discriminate between different ages of deposits in an area. A similarity between some of the surface debris from moraine samples and those seen in loess and quickclay deposits is also suggested.
Inlet sequence: a vertical succession of sedimentary structures and textures created by the lateral migration of tidal inlets
Authors: KUMAR NARESH , SANDERS JOHN E.
Published: June 2006

Tidal inlets, narrow, comparatively deep gaps between barrier islands, are channels through which tidal currents flow vigorously as the water enters and leaves the backbarrier lagoons, bays, and intertidal flats and marshes. Many geologists have compared tidal inlets to river channels and have speculated that if an inlet shifts laterally, it should deposit a distinctive sequence of sediments, analogous to the point‐bar sequence left by a meandering stream channel.

Fire Island Inlet, located 56 km east of New York City, has migrated WSW at a mean rate of 64 m/year during the period 1825–1940. Waves approaching the coast, predominantly from the southeast, have shifted sediment along the shore toward the WSW. Deposition of sediment on the east side of the inlet has forced the tidal currents to erode the west side of the inlet, thus causing lateral migration.

Because hydraulic conditions vary from the channel floor to the subaerial part of the spit which is present on the ENE side of the channel, sedimentary structures and textures vary systematically with depth. We have determined the various sedimentary environments associated with the modern Fire Island Inlet, sampled and described the sediments from these environments, and have collected samples from corings made on that part of Fire Island through which the inlet has migrated.

On the basis of our studies, we propose an inlet sequence which is formed by the lateral migration of a tidal inlet. The sequence includes five major units, as follows on p. 492.

The sediments belonging to various units in this sequence have been identified in four borings made on those parts of Fire Island through which the Fire Island Inlet has migrated since 1825.

This sequence should be applicable to other inlets also. We think that the boundary between deep channel and shallow channel units remains relatively fixed at −4.5 m, whereas the thickness of the deep channel unit is determined by the depth range between −4.5 m and the total depth of the inlet. Hence, the main source of variation in the inlet sequence will be the thickness of the deep‐channel unit.

Most of the sediments of the inlet sequence are incised below mean low water; hence they will almost certainly be preserved in the geologic record, even if all other associated sediments from barrier environments located above mean low water are not preserved. Because of the great variability possible in rates of lateral migration of inlets along the shore compared with the rates of barrier displacement perpendicular to the shore, inlet sediments may be preserved as elongate lenses, or as widespread blankets. The shape of inlet deposits reveals much about the behaviour of barriers during a submergence. Hence, inlet sediments should shed new light on sediments of the continental shelves and on basal transgressive sands in the geologic record.
Structure and directional properties of some valley sandur deposits in southern Iceland
Authors: BLUCK B. J.
Published: June 2006

Sediments in the valley sandurs of Markarfljot and Landmannalaugar accrete in complex lateral (transverse) bars. Directional structures within the bars are more variable in orientation than the channels which contain them. This greater variability is mainly due to the diversification of the flow, at the low stage, as it moves through the accretion topography of the sandur. Different directional structures preferentially record different stages of the flow: imbrication and lineation form mainly in response to the high flow stage, and have a low degree of dispersion; cross‐stratification records the low stage of flow, and has a wide dispersion. Cross‐stratification is produced in four main ways: by small deltas, by megaripples, by ripple topped bars, and by migrating accretionary bar banks. Small deltas form at low flow stage; accretionary banks at high.
Statistical methods for analysis and mapping of flysch‐type sediments
Authors: SCHLEIGER N. W.
Published: June 2006

To present a model for flysch deposition involves the simultaneous representation of many parameters. The most significant of these are arenite‐lutite bed thickness ratio, mean and variance of arenite‐bed thickness, mean palaeocurrent direction and the primary directional structures yielding them.

Field and flume data suggest that arenite‐bed thickness decreases exponentially away from source, with a complementary increase in siltstone bed thickness in a turbidite fan.

If HA, HL. are arenite and lutite bed thicknesses in a couplet (the unit of bedding at a flysch outcrop), then

where m and R are constants of the outcrop in the turbidite wedge. Field evidence suggests m= 1, so that R‐values allow for a classification of couplets at an outcrop or within a zone. Maximum HA, HL. values are indicative of proximal deposition from turbidites, and usually provide transverse directional structures, as does (HA) max.

Arenite bed thickness can be considered as thick beds or thin beds according to whether they exceed a chosen thickness, say t cm. If 15 cm be chosen, the percentage thick arenite‐bed thickness HA1 and thin arenite bed thickness percentage, HA2 can be represented with HL. (siltstone bed thickness percentage) over twenty‐five couplets, as one point on a triangular diagram. The mean palaeocurrent direction Nφ° E is represented as an arrow through the point, a colour (point or arrow) being used to distinguish the directional structure (flute, ripple, groove, foreset, etc.) producing it.

Flysch examples from Ordovician, Silurian and Devonian in Central Victoria are thus interpreted in terms of proximal and distal emplacement. In cases where bedding detail is clear, A + B, C and D + E divisions substitute for HA1HA2 and HL. respectively, for a unit of 1 m thickness of outcrop on the triangular diagram.

If thick arenite bed thickness percentage be plotted against total arenite bed thickness percentage, the linear plot of points for outcrop samples results on log‐log paper. Limiting lines of the form log HA1= m log HA+ log C (m, C constants) demarcate the field. Parallel and equally spaced lines with values C1, C2, C3,…Cn subdivide this into sub‐fields. If the determinations for HA1 in any one sub‐field be plotted on a locality map considering only MAXIMUM HA1 values, and the HA1H values conveniently contoured, trends emerge which closely agree with the modal palaeocurrent directions in the case of the Llandovery Deep Creek Flysch 25 km NNW of Melbourne, Australia.
Geochemical and mineralogical influence of Ordovician Island Arcs on epicontinental clastic sedimentation. A study of Lower Palaeozoic sedimentation in the Oslo Region, Norway
Authors: BJÖRLYKKE KNUT
Published: June 2006

The mineralogy and geochemistry of clastic sediments in the 1000 m thick Lower Palaeozoic marine sequence of the Oslo Region is discussed on the basis of 200 chemical (XRF) and mineralogical (X‐ray diffractometer) analyses. The composition of these sediments is considered to largely be a function of the relative supply from land of the continental shield (Baltic Shield) and from island are systems to the NW in the Trondheim Region. The relative abundance of chlorite and illite is discussed in light of recent knowledge about their distribution in modern oceans.

Middle Cambrian to Lowermost Ordovician black shales contain only illite as the dominant clay mineral and have a high potassium content. These sediments are thought to be derived from land on the continental shield, which during this period probably had a low relief and a warm climate where chlorite is not stable. Chlorite is introduced first in the Arenigan (L. Ordovician) and this is interpreted as evidence for transport of clastic chlorite from a developing island arc system in the Trondheim Region to the west. The highest chlorite/illite ratio in the Middle and Upper Ordovician sediments of the Oslo Region corresponds to a Taconic orogenic phase. The content of clastic chlorite in the Lower Palaeozoic of the Oslo Region is thus shown to be a sensitive indicator of palaeoenvironment and tectonic evolution of the Caledonian geosyncline.

This conclusion is also supported by a systematic increase in Mg, Fe, Ni, and Cr from Lower to Middle and Upper Ordovician beds. High Cr values exceeding 0–1 % Cr in the Middle/Upper Ordovician shales are due to small grains of clastic chromite resembling those in the serpentinites of the Trondheim Region. This also suggests transport from exposed island arcs onto the epicontinental shelf.
Tidal flat associations from a Lower Proterozoic carbonate sequence in South Africa
Authors: ERIKSSON K. A. , TRUSWELL J. F.
Published: June 2006

An association of structures is documented from a Lower Proterozoic carbonate sequence in South Africa: this is dominated by finely laminated material which may display flat (smooth), small domical (blister), crinkled (tufted) and pustular (mamillate) surface. The lamination may display overfolded structures and contain flat encapsulated features. Lens‐like flat‐pebble breccias are common, while edgewise breccias were seen less frequently. Small‐scale features suggestive of vertical algal moulds were also recorded. This facies is considered to be analogous to contemporary tidal flat sedimentation developed in specific settings at Shark Bay in Western Australia, and at Abu Dhabi where it is the dominant growth form. This analogy is most notably clear in relation to the protected embayments at Shark Bay, and as a result, an inner intertidal to marginal supratidal environment is suggested for the specific horizons under consideration from the Transvaal Dolomite.

Other structures are associated with this facies. These include larger domes, coarser bedding, oolites, ripple‐marks and, in one case, columnar stromatolites. Using the basic facies as an environmental datum, a model is developed in which these latter structures extend into the outer intertidal zone. They are also considered to be associated with varying states of turbulence in the intertidal regime.
New interpretations of Great Salt Lake ooids and of ancient non‐skeletal carbonate mineralogy
Authors: SANDBERG PHILIP A.
Published: June 2006

Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non‐skeletal grains) or to paramorphic replacement (proposed for non‐skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non‐skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non‐skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non‐skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic‐Cretaceous time and by abundant younger, Mg‐poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non‐skeletal carbonates was probably during early to middle Cenozoic time.
The size and shape of small‐scale current ripples: an experimental study using medium sand
Authors: BANKS N. L. , COLLINSON J. D.
Published: June 2006

Flume experiments with medium sand confirm the increasing complexity in the shape of small‐scale current ripples with increasing flow velocity for constant depth. Experiments suggest that a measure of ripple shape (the ratio of wavelengths of transverse to streamwise features, λ‐x/λ‐z) has a more complex relationship with the flow property (Fr, H̄/d̄) (where Fr is Froude Number of the flow and H̄/d̄ is relative roughness), than was previously realized. It is suggested that hydraulic properties of the flow at the sediment water interface have a more general relationship with ripple form than do properties of the whole flow such as Froude and Reynolds Numbers. Properties of the whole flow tend to separate data into depthrelated curves at shallow flows where the free water surface influences the structure of the turbulence.
A reconnaissance of the sedimentology of lower Silurian mudstones, English Lake District
Authors: PIPER DAVID J. W.
Published: June 2006

Polished slabs and thin sections of lower Silurian graptolitic mudstones and interbedded barren mudstones show three main lithologies are present (a) unfossili‐ferous green mudstones, sometimes with indistinct silt laminae, (b) a similar black pyritic lithology, with rare graptolites, (c) striped mudstones, with prominent carbonaceous and silt laminae, and common graptolites. The abundance of silt laminae suggests that the striped mudstone facies represents the highest energy depositional conditions. Comparison of sedimentary structures with modern deep sea muds suggests this facies is distal turbidite. Graptolite preservation was favoured by rapid burial.
A carbonate island barrier from the Great Oolite (Middle Jurassic) of central England
Authors: PALMER TIMOTHY J. , JENKYNS HUGH C.
Published: June 2006

Over part of south‐central England the widespread shallow‐water carbonate sedimentation represented by the White Limestone Formation (Great Oolite Group: Middle Jurassic), culminated locally with the formation of a sun‐cracked, stromatolitic micrite indicating a supratidal environment. The emergent area can be mapped as a broad strip running at an angle to, and offshore from, the coast‐line of the ancient Anglo‐Belgian landmass. As a result of the emergence of this carbonate area a polarization of environments occurred. To the north‐east a series of clay‐rich sediments containing lignite and a brackish and fresh‐water fauna were laid down: a lagoonal environment is indicated. To the south‐west fully marine shelf carbonates continued to be deposited. We suggest that isolation of the northerly area of reduced salinity from the open sea was due directly to emergence of this carbonate area, which can thus be specifically interpreted as an island barrier or spit.
Experimental authigenesis of phyllosilicates from feldspathic sands
Authors: DIVIS ALLAN F. , McKENZIE JUDITH ANNE
Published: June 2006

Hydrothermal experiments with primary detrital components of feldspathic sands (orthoclase, albite, quartz, and calcite) were conducted to simulate possible diagenetic changes in geosynclinal sedimentary accumulations and the geothermal reservoir of the Imperial Valley area, California. Phyllosilicate and zeolite mineralization was produced at 200°C and 1 Kb Ph2o and at 300°C and 1 and 3 Kb Ph2o. Scanning electron microscope examination of the detrital grains shows the development of authigenic minerals and solution features. Phyllosilicate development occurred as dense surface coatings on orthoclase crystals in concentrated brines and as scattered grain clusters in dilute brines. Cation concentration is considered to be a controlling factor in phyllosilicate formation and growth.

During formation the phyllosilicate crystals appear to utilize the surficial feldspar lattice structure as a preferred growth site. Electron diffraction studies indicate the crystals are a 1 Md mica similar to illite. Initial phyllosilicate formation occurs principally on orthoclase in systems containing this mineral, but is disseminated on other mineral surfaces in systems without orthoclase.

This experimental development of authigenetic illite via the destruction of potassium feldspar may offer a potential mechanism to help explain the resulting mineralogy of diagenetic processes occurring in natural sediments such as in feldspathic sands and argillaceous sediments.
Cyclic characteristics of coal‐bearing sediments in the Bochumer Formation (Westphal A 2) Ruhrgebiet, Germany
Authors: CASSHYAP SATYENDRA M.
Published: June 2006

Cyclic characteristics of the Bochumer coal measures have been studied statistically using first‐order embedded Markov chain analysis on data from thirty‐two boreholes. Separate tally matrices which were made for the Lower, Middle and Upper units by summing the tally matrices for all the boreholes, were used to produce various transition probability matrices, which were then subjected to the chi‐square test. Results strongly suggest that the Bochumer succession was deposited by a Markovian mechanism, and that the sequence represents cyclic sedimentation.

The preferential upward transition path for lithologic changes that can be derived in the total area and in individual logs is: coal\rightarrow shale\rightarrowsiltstone\rightarrowsandstone\rightarrowsiltstone\rightarrowrooty bed\rightarrowcoal. Among the noteworthy features are a two‐way transition between siltstone and sandstone, which implies frequent interbedding of the two lithologies, and the transition, up out of siltstone to rooty beds. There are, however, minor variations, locally, either by way of subsidiary transitions or in the extent to which the above transitions can be followed. When tested for stationarity through time and space, the Markov chain analysis yields results which show the upward sequence of lithologic states (facies) to be stationary at individual localities, but non‐stationary when the entire data are tested for the whole area. The relationship of the number of coal‐bearing cycles to total thickness of strata and average thickness of coal‐cycles is examined using linear regression. Significant positive relationships are present between the number of cycles and total thickness, and significant negative correlations occur between the number of cycles and their average thickness.

The results of the study have been used to infer sedimentological evolution of the succession and sediment cyclicity. Environmentally, the Bochumer cycles fit suitably into the concept of deltaic cycles.
Size‐sorting during suspension transportation—lognormality and other characteristics
Authors: SENGUPTA SUPRIYA
Published: June 2006

Grain size distributions of the suspended loads above a bed of bimodal size distribution (size range 2‐00‐0.04 mm) were studied in a laboratory flume at water velocities varying from 42 to 160 cm/s. With increase of velocity the phi (logarithmic) size distribution of the suspended particles (at 5‐20 cm above the bed) changed from a strongly skewed to a nearly symmetrical, unimodal form (nearly lognormal) through an intermediate bimodal stage. At low velocity the skewness of the distribution changed from positive to negative with increase of height. The experiments indicate that lognormality of‘weight frequency’ distribution of grain sizes is a transitional feature, attained through size sorting within a critical range of velocity and height above a sand bed of a given composition.

The observed changes in the size distribution patterns were effected by a differential rate of increase in weight in the different size classes in suspension with increase of flow velocity. The phenomenon could be explained by the equation of relative suspension concentration which relates the relative concentration of a suspended particle of a particular diameter to the flow velocity of the turbulent fluid and the height of suspension above the bed.
Turbiditic and non‐turbiditic mudstone of Cretaceous flysch sections of the East Alps and other basins
Authors: HESSE REINHARD
Published: June 2006

Recognition of the occurrence and extent of hemipelagic and pelagic deposits in turbidite sequences is of considerable importance for environmental analysis (palaeodepth, circulation, distance from land, hemipelagic or pelagic versus turbidite sedimentation rates) of ancient basins. Differentiation between the finegrained parts (E‐division) of turbidites and the (hemi‐) pelagic layers (F‐division of turbidite‐pelagite alternations) is facilitated in basins where carbonate turbidites were deposited below the carbonate compensation depth (CCD) such as the Flysch Zone of the East Alps but may be difficult in other basins where less compositional contrast is developed between the fine‐grained turbidites and hemipelagites. This difficulty pertains particularly in Palaeozoic and older basins. For Late Mesozoic‐Cenozoic oceans with a relatively deep calcite compensation level three other types of turbidite basins may be distinguished for which differentiation becomes increasingly more difficult in the sequence from (1) to (3): (1) terrigenous turbidite basins above the CCD; (2) carbonate turbidite basins above the CCD; (3) terrigenous turbidite basins below the CCD.

Criteria and methods useful for the differentiation between turbiditic and hemipelagic mudstone in the Upper Cretaceous of the Flysch Zone of the East Alps include calcium carbonate content, colour, sequential analysis, distribution of bioturbation, and microfaunal content. In modern turbidite basins clay mineral content, organic matter content, plant fragments, and grain‐size (graded bedding, maximum grain diameter) have reportedly also been used as criteria (see Table 3).

Deposition of muddy sediment by turbidity currents on weakly sloping sea bottoms such as the distal parts of deep‐sea fans or abyssal plains is not only feasible but may lead to the accumulation of thick layers. Contrary to earlier speculation it can be explained by the hydrodynamic theory of turbidity currents, if temperature differences between the turbidity current and the ambient deep water as well as relatively high current velocities for the deposition of turbiditic muds (an order of magnitude higher on mud surfaces than commonly assumed) are taken into consideration. The former add to the capacity of turbidity currents to carry muddy sediment without creating a driving force on a low slope.
Wave and tidal current sorting of shelf sediments southwest of England
Authors: CHANNON R. D. , HAMILTON D.
Published: June 2006

The relative roles of waves and tidal currents in transporting bottom sediment on the continental shelf off Lands End, southwest England, are evaluated by study of (a) sediment grain size in relation to boundary layer measurements in tidal currents, (b) regional variation in sediment parameters in relation to peak tidal and wave‐induced currents, and (c) visual observation of bedforms.

(a) The sediments are mainly zoogenic sands. The average hydraulic equivalent median diameter is Mdφ=1.40φ (medium grade sand), and two‐thirds of the median grain sizes fall between 0.97φ and 1.83φ. The linear bottom current which will just move this range of sizes is exceeded only slightly by the highest tidal drag velocities Ū* measured in the area. Thus, sediment movement by tidal currents alone is restricted to areas of high bed roughness and strong peak tidal flows. In contrast, wave‐induced oscillatory currents at 100 m depth (typical of the area) attain sufficient speed to disturb the same particle sizes over 3% of the time. This includes storm periods when much greater velocities occur.

(b) The average Mdφ of the sediment decreases southwest and northeast from south of the Lizard. This correlates well with the pattern of maximum tidal current speeds, suggesting that tidal currents control the areal distribution of sediment median grain size.

Most sediments are well sorted (mean σi=0.48φ). Sorting improves at shallower depths but does not improve in areas of faster tidal currents, suggesting that wave‐induced currents exert the major control on sorting.

Silt and clay proportions increase west of the Scilly Isles and are influenced by both wave and tidal currents.

(c) Photographs and television pictures show that symmetrical bedforms due to wave action are dominant north and west of the western Channel. Asymmetric bedforms are more common in the western Channel itself, where tidal currents and bed roughness are both high.

Results are used to construct a sediment transport model for the study area. Since medium grade well sorted sands occur in depths of over 100 m, many ancient, extensive, well sorted sand sheets may have been deposited at depths greater than previously suspected.
Interrelationships between bed morphology and bed‐material transport for a small, gravel‐bed channel*
Authors: LARONNE J. B. , CARSON M. A.
Published: June 2006

Bed conditions (micro‐relief, textural associations and packing structural arrangements) in the gravel‐bed channel of Seale's Brook are shown to be closely interrelated; various categories are identified and related to mode of bed material transport and deposition.

Entrainment of bed material, commonly treated as a simple function of particle weight and channel hydraulics, is also shown to be strongly affected by varying and variable bed conditions. In particular, the classic concept of competence appears to be of restricted utility in such channels; resistance of bed material to fluid drag and to particle impact is augmented, over large parts of the channel bed, by its interlocking structure, made possible by the wide range in particle calibre, and by the characteristic disc and blade shapes of the slate debris.

Particle mobility, as indicated by distance of travel of labelled bed material, is only partly a function of particle weight; indeed, although particle mobility decreases from small pebbles to large cobbles, it also decreases for the finest bed material (very small pebbles). This appears to be explicable, partly in terms of the ease of entrainment (and duration of travel), and, partly in relation to the ease of transport of material over an uneven channel bed surface. Particle mobility is greatest for material in open and infilled structures and smallest for sediment in tight structural arrangements. Local bed slope also exerts an influence on the probability of particle entrainment and on particle mobility.

The findings emphasize the need for combining sedimentological and engineering approaches to bed material transport in coarse‐bedded channels, and, at the same time, illustrate some of the reasons for the existence of indeterminacy in the modelling of bed‐material transporting processes.
Phytoclasts as provenance indicators in the Belgian septaria clay of Boom (Rupelian age)
Authors: VANDENBERGHE NOËL
Published: June 2006

Phytoclasts and coal particles, present in small quantities in a clay sediment, can be used as source rock indicators. The measurement of their reflectivities revealed characteristic low values typical for British Carboniferous coals, outcropping in Northern England, which may be a source area of the sediment. This technique may be a valuable complementary method to traditional mineralogical provenance studies.
Continental sedimentation, soil genesis and marine transgression in the basal beds of the Cretaceous in the east of the Paris Basin
Authors: MEYER ROBERT
Published: June 2006

In the eastern Paris Basin, on the boundary of two French ‘départements’, Marne and Meuse, the Cretaceous deposits begin with Wealden facies assigned to the Valanginian. This is overlain by marine Hauterivian and Lower Barremian.

Several techniques have been used for studying these deposits: light‐microscope, scanning electron microscope, grain‐size analysis, chemical and mineralogical analysis.

Environmental interpretations of the various facies are based on sedimentological, geochemical and pedological criteria (fossil soils are present). Clay minerals are dominantly illite and kaolinite. In the soil horizons kaolinite is dominant. A mixed‐layer illite‐smectite appears at the base of the transgression. Under marine influence a more stable illite dominant suite is formed.

The Wealden sediments appear to have been deposited in a very mature flood‐plain. Part of the sequence was subjected to weathering in situ by soil processes under a warm climate with repeated wetting and drying. Paraconformably above, the marine Hauterivian is the product of a shallow sea that tended to form lagoons. A true marine environment was established in the Lower Barremian, when mixing homogenized the sediments.
Recent sand volcanoes in the sand of a dike under construction
Authors: NEUMANN‐MAHLKAU PETER
Published: June 2006

Sand volcanoes are small volcano‐like features. They range in diameter from a few centimetres to several metres. Five volcanoes with a maximum diameter of 2 m were observed during the construction of a dike on the coast of Northwestern Germany. The cones reached heights of about 15 cm with craters in the middle.

The rise of the volcanoes is explained by de‐watering of sediment.
Tidal‐shelf sedimentation: an example from the Scottish Dalradian
Authors: ANDERTON ROGER
Published: June 2006

The Jura Quartzite, a formation of probably late Precambrian metasediments over 5 km thick from the Caledonian belt in Southwest Scotland, has been divided into a coarse and three fine facies. The former comprises cross‐bedded sands with some laminated sands and silt horizons, interpreted as the deposits of shallow marine tidal dunes and other bedforms together with some beach units. Deposition from suspension of silt and sand formed climbing dunes while largescale erosion produced flat or channelled surfaces. The fine facies comprise laterally persistent, parallel and cross‐laminated sand units from millimetres to decimetres thick, interbedded with muds. The coarse and fine facies can be finely interbedded, the former sometimes filling decimetre deep, straight channels, cut in the latter. The fine facies exhibit structures indicative of deposition from decelerating currents and are interpreted as shallow marine storm deposits.

The facies are compared with a model developed from published observations on modern shelf areas. Zones of erosion, large and small dunes, flat bedded sand and mud are considered to be the end product of a wide spectrum of tidal and storm conditions. During severe storms the fair weather tidal dunes may be modified or washed out, new dunes may be initiated downcurrent of the normal dune field while storm‐sand layers are deposited in the distal zones. Hence, the nature of the preserved sediment blanket reflects the rare severe storm event rather than normal tidal conditions.

The Jura Quartzite was deposited in a tidal gulf intimately connected with an ocean basin. The north‐northeast directed palaeocurrent modes are probably roughly parallel to the coastline.
Palaeocurrent and palaeohydrologic analysis of some vertical profiles through a Cretaceous braided stream deposit, Banks Island, Arctic Canada
Authors: MIALL ANDREW D.
Published: June 2006

A study of directional variability has been carried out on five vertical profiles containing a total of approximately 195 superimposed planar crossbed sets. The latter are interpreted as the deposits of braid bars. Detailed measurements on one set revealed within‐set variance of 93. Within river tract, between‐bar variance was obtained by calculating a ten‐point moving average for each profile, and subtracting the actual azimuth at each point from the moving average value. Variance at this level is 980, which compares closely with data obtained from modern braided streams. Variability in the moving average azimuth is interpreted as the result of meander migration in the entire channel system. Sinuosity can be estimated from this variability, using a geometrical approximation. It ranges from 1.03 to 1.13.

Changes in stream competency with time are indicated by vertical changes in directional variance and set thickness. When accompanied by changes in mean azimuth, as in one of the present five profiles, the data suggest successive deposition by two distinct stream systems.

A Markov chain analysis of the data sequence indicates a weak memory effect. Short sequences of bar deposits were formed by streams which varied little in orientation, and these are separated from one another by sequences showing random directional fluctuation.

The hydrology of the Isachsen rivers was investigated using Schumm's (1968a, b, 1969, 1972) empirical relationships for modern rivers. Owing to inherent uncertainties in these relationships and a wide margin of error in the input data (principally associated with estimates of depth and width of the Isachsen streams), the results have a validity only at the order‐of‐magnitude level. Individual Isachsen rivers are estimated to have had drainage areas of between 5000 and 30,000 km2; this information is of use in reconstructions of palaeogeography and tectonic history.
Depositional environment and development of Danian bryozoan biomicrite mounds (Karlby Klint, Denmark)
Authors: THOMSEN ERIK
Published: June 2006

The lower Danian bryozoan mounds exposed in the cliff at Karlby are described. Analyses of some sixty samples taken from a single mound reveal that the limestone is composed of about 30% of bryozoan fragments in a fine grained matrix. The texture and the grain‐size distribution show that two distinct sedimentary facies can be recognized: (1) the northwestern flank and the basins with fine bryozoan fragments, a relatively small amount of matrix, and grain‐supported texture; (2) the southeastern flank and the summit with coarse bryozoans, a larger amount of matrix and partly mud‐supported texture.

It is shown that the differences in the size of the bryozoan fragments are due to adaptations to changes in the water‐movement rather than a result of significant transport. Thus, currents were roughly from the southeast producing more agitated water on the southeast flank and the summit and more gentle water on the northwest flank and in the basins. The unusual relationship between the velocity of the water‐movement and the sedimentary facies is caused by an influence of the bryozoans on the depositional environment. On the southeast flank the higher current velocity favoured the growth of bryozoans, resulting in a relatively dense cover that was able to trap and bind the matrix. On the northwest flank the currents were slower and less favourable for growth, and the ability of the bryozoans to bind the sediment was correspondingly smaller.
The Cretaceous Talme Yafe Formation: a contour current shaped sedimentary prism of calcareous detritus at the continental margin of the Arabian Craton
Authors: BEIN AMOS , WEILER YEHEZKIEL
Published: June 2006

The Cretaceous (Albian‐Turonian) Talme Yafe Formation is a huge prismshaped accumulation (more than 3000 m thick, about 20 km wide, and at least 150 km long) of calcareous detritus at the northwest continental margin of the Arabian Craton (Israel). Its sources of clastic material are biochemical carbonates and skeletal fragments derived from rudistid reefs deposited on the wide epicontinental platform located east of the accumulation site. Transport of the material from the shelf platform over the edge onto the slope was probably done by storms and by tidal and seasonal currents. Downslope movement was by nepheloid layers and by gravity‐induced currents (turbidity currents?) whereas contour currents are considered the main dispersing and shaping agent active on the slope proper. No clear‐cut evidence indicating turbidites was perceived; however, they would be expected to be found west of the study area, on the abyssal plain where they are usually ponded.
The mineralogy and geochemistry of the sediments of northwestern Lake Victoria
Authors: MOTHERSILL JOHN S.
Published: June 2006

In the nearshore area of northwestern Lake Victoria a thin strip of quartzarenite sand occurs which grades lakeward into silty clay consisting of quartz with subordinate amounts of K‐feldspar, plagioclase, kaolinite, illite, vermiculite and organic matter.

Varimax matrix determinations of the element concentrations in the lake‐bottom sediments extractable by aqua regia indicate that: (1) there is a strong association of Cr, Cu, Zn and Ni; (2) there is a strong association of Fe, Mn and Co; (3) Cu and Ni show moderate to slight associations with organic matter; (4) Ca is relatively independent of the other elements.

The general decrease in the pH values of the surface and bottom waters outward from the lake shore, with consistently higher values for the surface water relative to the bottom waters, results from decreasing levels of photosynthetic activity.
A reconnaissance of the Recent sediments of the Ranns of Kutch, India
Authors: GLENNIE K. W. , EVANS G.
Published: June 2006

The Ranns of Kutch are flat salt‐covered desert areas (sabkhas) which are just above the normal tidal range and may be regarded as supratidal flats. They are flooded annually by the storm tides of the Southwest Monsoon. As the waters recede and evaporate they leave behind a crust of halite, and gypsum crystals grow within the clays and sands. The increase in salinity of the interstitial waters as they are traced inland is reflected in the higher boron content of the clays. The Mg2+/Ca2+ratio of these waters increases from 3 on the coast, to 240 in the evaporite environment of the shoreline of Pachham Island.

Much of the sediment of the Ranns was probably once derived from the Indus and Nara rivers which used to flow into the western end of the Great Rann. Clay is now probably carried in by the monsoon storm tides after being transported along shore from the Indus, and also from the rivers of Kutch and Kathiawar. Coarser siliciclastic sediment is carried into the eastern inland portion of the area by the Luni and other intermittently flowing rivers and streams. Some sand and silt is blown into the Ranns from the surrounding hilly areas. Foraminifera are found concentrated in the sandier fractions of the sediments, having been transported there primarily by tidal currents and also by the wind. One species ofAmmoniais almost the only fauna found in the sediments of the very saline environment around Pachham Island.
Rollers and ripples in sand, streams and sky: rhythmic alteration of transverse and longitudinal vortices in three orders
Authors: FOLK ROBERT L.
Published: June 2006

Sediment ripples are caused by systematically‐spaced transverse roller vortex systems in a moving fluid undergoing shear. With greater shear, these transverse rollers change over into longitudinal (helicoidal) vortices. This is the basic cause for the change from so‐called ‘lower flow regime’ conditions to ‘upper flow regime’ conditions. All characteristics of these two regimes (sediment transport rate, bed form, sedimentary structures) are logically explained by attributing them to change in type of vortex system. For currents depositing sediments, there are three orders of magnitude of vortices, each order beginning with transverse rollers, passing through festoon to longitudinal rollers. A chaos zone (antidunes) ensues, followed by resumption of transverse rollers that are five to ten times as large as those in the previous order. Features of river sediments, marine sands, turbidites, desert sand dunes, sky, and stars are satisfactorily explained by this model.