bluuebunny/crossref_metadata_2025 huggingface dataset:
A flume study on the development and equilibrium morphology of current ripples in very fine sand
Authors: BAAS JACO H.
Published: June 2006

An empirical model is constructed for the development and equilibrium dimensions of small scale, unidirectional bedforms in sand with a median grain size of 0·095 mm, based on a series of steady flow experiments in a flume. Current ripples always attain a linguoid plan morphology with constant average height (13·1 mm) and wavelength (115·7 mm), provided that sufficient time is allowed for their formation. The development pattern of these ripples on a flat bed is independent of flow velocity, and involves four stages: (1) incipient ripples; (2) straight and sinuous ripples; (3) non‐equilibrium linguoid ripples, and (4) equilibrium linguoid ripples. Straight and sinuous ripples are non‐equilibrium bedforms at all flow velocities. The time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours. At flow velocities where washed ripples are stable, the equilibrium wavelength is similar to that of equilibrium linguoid ripples, but the equilibrium height rapidly decreases from 13·1 mm to zero towards upper stage plane bed conditions.

The results of the flume experiments correspond reasonably well with those of previous studies, provided that various complicating factors, such as different experimental methods, different sediment characteristics, shallow flow depths and non‐equilibrium runs, are accounted for.
Tuffaceous ephemeral lake deposits on an alluvial plain, middle Tertiary of central California
Authors: BARTOW J. ALAN
Published: June 2006

The Oligocene and Miocene Valley Springs Formation is one element in the middle to late Cenozoic alluvial fill of the Central Valley of California and represents a large fluvial depositional system that extended westward from sediment‐filled palaeovalleys in the high Sierra Nevada to a piedmont alluvial plain under the present Central Valley. The Valley Springs Formation consists largely of tuffaceous mudrock, tuffaceous sandstone, polymict conglomerate and rhyodacitic tuff. The most prominent lithofacies in the formation is yellowish grey or greyish yellow, tuffaceous mudstone and claystone characterized by crude, commonly wavy layering or bedding, an irregular fracture, and common clay‐lined partings, fissures, and small branching tubules.

Petrographic and X‐ray diffraction analyses of the composition, texture, fabric, mineralogy and organic content of an 8 m thick section of the tuffaceous mudrock lithofacies have provided evidence for an origin quite different from the earlier interpretations of this lithofacies as altered tuffs. Numerous rounded mudstone or claystone clasts in a detrital mud matrix, together with abundant freshwater microfossils (megaspores, chrysophyte cysts, diatoms and sponge spicules), indicates that the lithofacies is a mostly epiclastic sedimentary deposit. Root traces, fissuring, orientated clay skins lining pores and secondary surfaces, and mineralogical data are evidence of periodic exposure and desiccation.

The tuffaceous mudrock lithofacies of the Valley Springs Formation, interpreted in the context of the middle to lower regions of a piedmont alluvial system, most probably represents a complex of ephemeral lake and marsh environments on a low gradient alluvial plain. The inferred abundance of shallow lakes, ponds and marshes implies a climate that was wetter than the semi‐arid climate of the region today.
High energy coralgal‐stromatolite frameworks from Holocene reefs (Tahiti, French Polynesia)
Authors: CAMOIN GILBERT F. , MONTAGGIONI LUCIEN F.
Published: June 2006

Drill cores from Holocene reefs on Tahiti (French Polynesia) reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers. Laminated micritic crusts form coatings over coral branches or, more commonly, over related encrusting organisms throughout the cored reef sections; these crusts appear as a major structural and volumetric component of the reef framework. The microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry, the occurrence of microbial remains and stable isotope measurements. The reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 ± 170 yr bp to the present. The nature of the involved benthic communities, stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years. The causes of the predominance of microbial communities over actual encrusters (red algae, foraminifers) remain problematic and could be related to short term fluctuations in ecological parameters. Microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and, possibly, in strengthening the framework, due to rapid lithification. The record of similar microbial crusts in other Quaternary reef tracts suggests that microbial communities may have played a more prominent role in Quaternary reefs than presently recognized.
A note on the use of statistics in reporting detrital clastic compositions
Authors: HOWARD JEFFREY L.
Published: June 2006

Error polygons and error bars are commonly used graphical methods for showing the precision associated with means on triangular diagrams and orthogonal plots, respectively. If the error or precision associated with a mean, based either upon counts of individual components or upon count ratios, is to be defined using basic statistics, the confidence interval method should be used. Standard deviation alone, which has been frequently in the literature, is not an appropriate statistic to use for this purpose. The graphical use of confidence intervals permits a rapid visualization of statistically significant differences between means, and thus more quantitative description of detrital clastic composition. Previous investigators using the standard deviation method have underestimated the precision of their data when sample sizes are large, and overestimated precision with small samples.
Sedimentation in the lakes and marshes (Ahwar) of the Tigris‐Euphrates Delta, southern Mesopotamia
Authors: AQRAWI A. A. M. , EVANS G.
Published: June 2006

The fluvial plains of lower Mesopotamia crossed by the Tigris and Euphrates rivers are occupied by fresh‐brackish water lakes and extensive reed‐marshes dominated by Phragmites sp. and Typha sp. (together locally called Ahwar). Allochthonous sediment is mostly supplied by the Tigris and Euphrates seasonal floodwaters, and by winds which bring aeolian dust from the north‐west and sand during storms from the western deserts in summer. Other contributions to the sediment budget are made by in situ biological activity and by biochemical‐chemical processes within the sediments.

The sediments are dominantly silts, with silt comprising more than 60% of the sediment. The remainder of the sediment consists of varying quantities of clay and sand. Clay is more abundant in the lower parts of most of the analysed cores (30–80 cm in length), indicating a change in sediment supply and/or sedimentary environment. Well developed stratification is found only on the river levees and in some clay‐rich lake and marsh sediments. The remainder of the sediments are generally poorly stratified with irregular stratification and considerable bioturbation.

Mineralogically, the sediments consist mainly of calcite, quartz, dolomite and feldspar (particularly albite). Aragonite is present as a result of in situ production by macrofauna. The clay fraction (<2 μn) consists of smectite, illite‐smectite, illite, palygorskite, kaolinite and chlorite.

The total organic carbon (TOC) content is usually <5% except for some peaty surface layers of the freshwater lakes and marshes where TOC may rise to 10–20%. The sediments have a relatively high calcium carbonate content. 20–80%, the highest values only being found in mollusc‐rich horizons. The sediments contain high amounts of Ca, Mg and Sr and are generally low in Al, Ti. K. and V. Values for Fe, Co, Pb, Zn and Mn are close to the average for mudstones, whereas Cu and P are relatively low. The high Ni and Cr contents of the sediments are related to high contents in the source material of the major rivers and aeolian deposits. Multivariate statistical analysis shows that elemental components of surface and source materials correlate well with other sedimentary properties.

Aragonitic molluscs of various species dominate the macrofauna, but a microfauna of foraminifers, ostracods and diatoms is also present in the sediments.
Rare earth element geochemistry of dolomites in the Middle Devonian Presqu'ile barrier, Western Canada Sedimentary Basin: implications for fluid‐rock ratios during dolomitization
Authors: QING HAIRUO , MOUNTJOY ERIC W.
Published: June 2006

Rare earth elements (REE) were determined in fine, medium and coarse crystalline replacement dolomites, and for saddle dolomite cements from the Middle Devonian Presqu'ile barrier from Pine Point and the subsurface of the Northwest Territories and north‐eastern British Columbia. REE patterns of the fine crystalline dolomite are similar to those of Middle Devonian limestones from the Presqu'ile barrier. Fine crystalline dolomite occurs in the back‐barrier facies and may represent penecontemporaneous dolomitization at, or just below, the sea floor. Medium crystalline dolomite is widespread in the lower southern and lower central barrier. Medium crystalline dolomite is slightly depleted in heavy REE compared with Devonian marine limestones and fine crystalline dolomite, and has negative Ce and Eu anomalies. Medium crystalline dolomites replaced pre‐existing limestones or were recrystallized from earlier fine crystalline dolomites. During these processes, the REE patterns of their precursors were modified. Late stage, coarse crystalline replacement dolomite and saddle dolomite cements occur together in the upper barrier and have similar geochemical signatures. Coarse crystalline dolomites have negative Eu anomalies, and those from the Pine Point area also have positive La anomalies. Saddle dolomites are enriched in light REE and have positive La anomalies. The REE patterns of coarse crystalline dolomite and saddle dolomite differ from those of marine limestones and fine and medium crystalline dolomites, suggesting that different diagenetic fluids were responsible for these later dolomites.

Although massive dolomitization requires relatively large volumes of fluids in order to provide the necessary amounts of Mg2‐. dolomitization and subsequent recrystallization may not necessarily modify the REE signatures of the precursor limestones because of the low concentrations of REE in most natural fluids. Thus, relative fluid‐rock ratios during diagenesis may be estimated from REE patterns in the diagenetic and precursor minerals. Fine crystalline dolomites retain the REE patterns of their limestone precursors. In the medium and coarse crystalline dolomites the precursor REE patterns were apparently altered by large volumes of fluids involved during dolomitization. This study suggests that REE compositions of dolomites and their limestone precursors may provide important information about the relative amounts of fluids involved during diagenetic processes, such as dolomitization.
Subaerial exposure and deposition of shallowing upward sequences: evidence from stable isotopes of Purbeckian peritidal carbonates (basal Cretaceous), Swiss and French Jura Mountains
Authors: JOACHIMSKI MICHAEL M.
Published: June 2006

Purbeckian (lowermost Cretaceous) peritidal carbonates are characterized by open marine, lagoonal, intertidal and lacustrine facies arranged in Milankovitch‐type shallowing upward sequences. Shallowing upward sequences typically consist of 2–6 individual beds. The sequences may be (i) complete, (ii) incomplete or (ii) pedogenetically overprinted, reflecting the duration of subaerial exposure and/or the extent of erosion and pedogenetic modification at the cycle tops.

The stable isotopic composition of the peritidal micrites reveals homogenous δ18O values attributed to diagenetic stabilization in a meteoric, water‐buffered system. Carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences. Complete shallowing upward sequences consist of 4–6 individual carbonate beds. The carbon isotope values show a facies‐dependent pattern: open marine carbonate muds record enriched δ13C values of +0·28‰ while lagoonal (−0·82‰), intertidal (−2·46‰) and lacustrine micrites (−2·96‰) are increasingly depleted. This distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence. Incomplete sequences (2–5 carbonate beds) are characterized by depleted δ13C values below subaerial exposure surfaces that become progressively enriched in 13C with increasing depth. Pedogenetically overprinted sequences (1–3 carbonate beds) show strong 13C depletion throughout the sequence with little variation in the carbon isotopic composition. The depleted values (−4·5‰) of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas CO2.

The duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations, the completeness of the shallowing upward sequences and the carbon isotope pattern. The recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief. Otherwise, the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time‐stratigraphic interpretation.
Factors controlling aeolian sequence stratigraphy: clues from super bounding surface features in the Middle Jurassic Page Sandstone
Authors: HAVHOLM K. G. , KOCUREK G.
Published: June 2006

Super bounding surfaces (super surfaces) facilitate delineation of internal genetic architecture, or sequence stratigraphy, of aeolian sandstones and help in distinguishing controls on aeolian accumulation, hiatuses in accumulation and preservation of such sandstones. The Middle Jurassic Page Sandstone is an amalgamation of such sandstone units accumulated in a dry erg system, with water table below the surface, separated by super surfaces with features indicating an arid climate but a near surface water table. The Page Sandstone accumulated episodically with periods of aeolian accumulation followed by deflation to the water table; the water table fluctuated up and down through the sediment package. This rising water table did not directly control accumulation, but placed a limit on extent of deflation by stabilizing the substrate, thereby directly controlling preservation of the aeolian unit. A combination of relative water table behaviour, sediment supply and aerodynamic conditions upwind and within the erg controlled the thickness of genetic packages within the erg. A change in the pattern of deflation associated with super surface formation late in deposition of the Page Sandstone reflects a change in tectonic regime in the basin. The number of super surfaces and features on the super surfaces with the Page Sandstone suggest that the super surfaces represent more time than do the accumulations they enclose.
Threshold studies of gravel size particles under the co‐linear combined action of waves and currents
Authors: PANAGIOTOPOULOS I. , SYLAIOS G. , COLLINS M. B.
Published: June 2006

Gravel size sediment beds are tested under the combined influence of simulated wave action and co‐linear currents in a laboratory flume. Critical current speed, at threshold, increases with increasing size. Superimposed wave energy causes a small reduction in the unidirectional current energy. For low values of wave‐induced near bed current velocities, the resistance to erosion increases when the wave period decreases from 10 to 6 s. Finally, combined critical shear stresses are found to be lower than those predicted using the Shields curve, as modified for oscillatory flow. Grain protrusion is suggested as a mechanism to explain this divergence.
Determination of turbidity current erosional characteristics from reworked coccolith assemblages, Canary Basin, north‐east Atlantic
Authors: WEAVER P. P. E.
Published: June 2006

Turbidites contain mixtures of sediments of various ages. These sediments can include both material that was eroded to form the initial turbidity current plus additions derived from erosion of the sea floor during transport. It has been possible to interpret the age range of sediments incorporated into individual turbidites from the Madeira Abyssal Plain by examining the proportions of microfossil (coccolith) species that they contain. The pelagic record of coccoliths is well known for the Quaternary period and shows a succession of dominant species or acmes each lasting a few tens of thousand years. Hence, erosion of sediment representing more than a few tens of thousand years will produce coccolith mixtures not seen in the pelagic record, but dependent upon the age range of the sediments that were eroded. This age range can be estimated by comparison with synthetic ratios of coccolith species created by combining ratios of species from successively older layers in the pelagic record. These can then be compared with the ratios found in individual turbidites until a match is found. The results show age ranges of 54–500 kyr for the sediment mixture in seven turbidites from the Madeira Abyssal Plain. Since the volumes of these turbidites are also known, and accumulation rates in their source areas can be estimated, it is possible to determine both the thickness and the area of the eroded sediment mass that generated the turbidity current. Minimum depths of erosion on the north‐west African continental margin vary from 8 to 50 m and minimum areas eroded from 1600 to 5800 km2. None of the turbidites examined contains a significant excess of surface sediment, suggesting that, once formed, the turbidity currents that transported them were virtually non‐erosional, and that they travelled several hundred kilometres in this state.
Sediment particle size in the Hudson River Estuary
Authors: GIBBS R. J. , JHA P. K. , CHAKRAPANI G. J.
Published: June 2006

The particle sizes in bottom and suspended sediments from the ocean upstream for 80 km in the Hudson River Estuary were analysed. Several modes at 4, 8 and 30 μm were prevalent throughout the estuary. The average size of particles in bottom samples varied from 150 μm near the ocean to 15–20 μm in Haverstraw Bay. The bottom sediment size appears to be controlled by two components; (1) sand from the ocean and (2) the particles in the flocs.
High‐resolution sequence stratigraphy of a complex, incised valley succession, Cobequid Bay — Salmon River estuary, Bay of Fundy, Canada
Authors: DALRYMPLE ROBERT W. , ZAITLIN BRIAN A.
Published: June 2006

The post‐glacial succession in the Cobequid Bay — Salmon River incised valley contains two sequences, the upper one incomplete. The lower sequence contains only highstand system tracts (HST) deposits which accumulated under microtidal, glacio‐marine deltaic conditions. The upper sequence contains two, retrogradationally stacked parasequences. The lower one accumulated in a wave‐dominated estuarine environment under micro‐mesotidal conditions. It belongs to the lowstand system tract (LST) or early transgressive system tract (TST) depending on the timing and location of the lowstand shoreline, and contains a gravel barrier that has been overstepped and preserved with little modification. The upper parasequence accumulated in the modern, macrotidal estuary, and is assignable to the late TST. Recent, net progradation of the fringing marshes indicates that a new HST has begun.

The sequence boundary separating the two sequences was formed by fluvial incision, and perhaps also by subtidal erosion during the relative sea level fall. Additional local erosion by waves and tidal currents occurred during the transgression. The base of the macrotidal sands is a prominent tidal ravinement surface which forms the flooding surface between the backstepping estuarine parasequences. Because fluvial deposition continued throughout the transgression, the fluvial‐estuarine contact is diachronous and cannot be used as the transgressive surface. The maximum flooding surface will be difficult to locate in the macrotidal sands, but is more easily identified in the fringing muddy sediments.

These observations indicate that: (1) large incised valleys may contain a compound fill that consists of more than one sequence; (2) relative sea level changes determine the stratal stacking patterns, but local environmental factors control the nature of the facies and surfaces; (3) these surfaces may have complex origins, and commonly become amalgamated; (4) designation of the transgressive surface (and thus the LST) is particularly difficult as many of the prominent surfaces in the valley fill are diachronous facies boundaries; and (5) the transgression of complex topography may cause geologically instantaneous changes in tidal range, due to resonance under particular geographical configurations.
On the prediction of bed‐load sand transport rate in air
Authors: McEWAN I. K. , WILLETTS B. B.
Published: June 2006

This paper discusses the parameterization of aeolian mass transport rate beginning with a critical discussion of four different forms of the prediction equation. Following this, recent progress in understanding the physics of the aeolian system is used to suggest an alternative approach which provides some new insights into the prediction of mass flux. The primary object of the paper is not to add another transport rate equation to those already present in the literature but rather to discuss the transport system in relation to the physical processes showing, to some extent, why reliable transport rate prediction has so far been elusive.
Diagenetically altered sabkha‐type Pleistocene dolomite from the Arabian Gulf
Authors: CHAFETZ HENRY S. , RUSH PATRICK F.
Published: June 2006

Diagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha‐related dolomite deposits, these deposits comprise a thick (>56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13 and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precursor.
Microbiological effects on slope stability: an experimental analysis
Authors: MEADOWS AZRA , MEADOWS PETER S. , WOOD DAVID MUIR , MURRAY JOHN M. H.
Published: June 2006

A natural, pure quartz sand has been seeded with the bacterium Pseudomonas atlantica and the fungus Penicillium chrysogenum, and angles of avalanche and repose have been measured under water using a laboratory clinometer. The lowest angles of avalanche occur in freshly packed clean sediment (control), with the seeded sediments having higher values. Among the latter, the lowest angles of repose occur in the bacterial seeded sediments, and the highest in the fungal seeded sediments. The largest differences between the angle of avalanche and angle of repose occur in the bacterial seeded and media control sediments. The smallest differences occur in the fungal seeded sediment. In most cases the second angle of avalanche is lower than the first angle of avalanche, whilst the second angle of repose is higher than the first angle of repose.

The bacteria bind particles together with their extracellular polymeric material, while the fungus binds particles by holding them together with a network of hyphal filaments. In the bacterial seeded sediment growth is uniform over the sediment surface. In the fungal seeded sediment growth occurs as discrete colonies separated by bare sediment, and the fungal hyphae penetrate the sediment to a significant depth. On avalanching, the fungal colonies move down the slope with the hyphal filaments trailing behind them in the sediment. Overall, both the bacterium and the fungus increase slope stability. However, the fungal colonies maintain slope stability after avalanching more effectively than does the uniform bacterial growth. The results are discussed in relation to the wide range of biological effects that stabilize flat sediments and to laboratory and field studies on the stability of sediment slopes.
Lithofacies relations and palaeoecology of a Late Jurassic to Early Cretaceous fan delta to shelf depositional system in the Sierra Madre Oriental of north‐east Mexico
Authors: MICHALZIK D. , SCHUMANN D.
Published: June 2006

The La Casita Formation in north‐eastern Mexico represents a period of increased terrigenous clastic supply that is characteristic for the entire northern and western margins of the Gulf of Mexico Basin during latest Jurassic and earliest Cretaceous time. A prograding delta complex developed at the south‐eastern margin of the Coahuila Peninsula. Delta plain, delta front, prodelta and shelf areas show distinct lithofacies associations. Some of the most characteristic elements are fining upward and coarsening upward grain size trends, syndepositional deformational features, storm layers, phosphorites, fossiliferous carbonate concretions and an anaerobic biofacies.

A possible break in sedimentation is inferred for parts of the Valanginian. This may be interpreted as a sequence boundary between second order cycles. The Kimmeridgian ‐ Berriasian cycle appears to correlate with the Cotton Valley cycle of the Texas Gulf Coast.
Stratigraphical architecture at the muddy margin of the Cretaceous Western Interior Seaway, southern Utah
Authors: LEITHOLD ELANA L.
Published: June 2006

The Tropic Shale and correlative Tununk Shale Member of the Mancos Shale accumulated during Cenomanian‐Turonian time, within prodeltaic environments near the western margin of the Western Interior Seaway of North America. Stratigraphical and sedimentological analysis has revealed a detailed history of relative sea level change in the thick, fine grained succession. The Tropic and Tununk shales were deposited during the Greenhorn second‐order sea level cycle, over a time span of about 2–5 million years. In southern Utah, six depositional sequences are superposed upon the record of this long term sea level change. The sequences developed during third‐order relative sea level cycles of hundreds of thousands of years duration and are composed of at least 37 parasequences, arranged in retrogradational, aggradational and progradational parasequence sets.

The Tropic Shale and Tununk Shale Member accumulated just basinward of the axis of maximum subsidence of a foreland basin. Stratal geometries and facies distribution patterns in the succession indicate that in southern Utah the Greenhorn cycle was tectonically controlled. During the Greenhorn transgression and highstand, rapid rates of tectonic subsidence trapped terrigenous sediment to the west of the study area, in the more proximal foreland. At this time, hemipelagic facies accumulated at relatively slow rates in southern Utah and type 2 sequences developed during third‐order sea level cycles. In contrast, during the Greenhorn regression rates of thrust‐induced subsidence in the proximal foreland basin evidently slowed, and deltaic clinoforms prograded across the study area. At least one forced regression occurred in southern Utah at this time, and type 1 sequences developed.

The formation of type 1 sequence boundaries in the upper part of the Tropic Shale and Tununk Shale Member points to episodes of base level fall and indicates that the six third‐order sea level cycles recorded in the succession were not the result of changes in sediment supply alone. The third‐order cycles may have been a consequence of episodic tectonism. The timing of these cycles, however, suggests that development of sequences and parasequences in the Tropic Shale and Tununk Shale Member may have been related to orbital forcing in the Milankovitch band. Glacioeustasy or climatically related fluctuations in the amount of groundwater stored on continents may explain these high frequency sea level changes.
Early submarine cementation in fore‐reef carbonate sediments, Barbados, West Indies
Authors: STENTOFT NIELS
Published: June 2006

Bottom sediments from the sea floor west of Barbados between depths of 110 and 324 m are composed of nodular or crusted carbonate deposits. Individual biogenic sediment grains and the cemented aggregates, nodules and crusts are usually more or less altered by bioerosion and may support one or more generations of encrusting organisms.

On the basis of component analysis of the topmost part of the bottom sediments it is possible to recognize three facies: (1) a proximal slope facies down to a depth of about 140 m, rich in mollusc fragments, benthic foraminifera and bryozoans; (2) an upper distal slope facies between about 140 m and about 215 m, rich in benthic foraminifera, molluses and crustaceans; and (3) a lower distal slope facies from about 215 m to at least 300 m, dominated by molluscs, especially pelecypods, with subordinate scleractinians and tubes of the polychaeteLygdamus asteriformis.

The appearance and quantitative importance of the cemented aggregates is also related to these facles. In the proximal slope facies, only relatively few irregular and very porous nodules are found, whereas in the lower distal slope facies, aggregates are very common. Most aggregates are crust‐like with a smooth upper surface and a more or less irregular, knobby lower surface. The crusts are massive compared with the nodules of the proximal slope facies and, unlike the latter, the lower surfaces and walls of larger cavities are usually coated with Fe and/or Mn oxides. In the upper distal slope facies a gradual transition between the two types of aggregates is found.

Petrographical and morphological evidence, together with carbon and oxygen isotopic data, indicates that the nodules and crusts were formedin situby submarine lithification processes. Radiocarbon dating of two bulk samples suggests that the cementation took place during late Pleistocene and/or early Holocene.
Diagenesis of a mixed siliciclastic/evaporitic sequence of the Middle Muschelkalk (Middle Triassic), the Catalan Coastal Range, NE Spain
Authors: MORAD S. , AL‐AASM I. S. , LONGSTAFFE F. J. , MARFIL R. , ROS L. F. DE , JOHANSEN H. , MARZO M.
Published: June 2006

The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north‐east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north‐eastern parts of the basin are terminal alluvial fan deposits. South‐westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K‐feldspar and mixed‐layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed‐layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ18OSMOW values for microdolomite (+23.7 to +28.4%) and K‐feldspar overgrowths (+17.3 to +17.7%) suggest either low‐temperature, isotopic disequilibrium or precipitation from low‐18O porewaters. Low‐18O waters might have developed, at least in part, during low‐temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ18OSMOW=+21.5 to +25.6%o; δ13C= 7.7 to ‐ 5.6%o) and presence of haematite indicate infiltration of meteoric waters.
Biodegradation of clay minerals: laboratory experiments and results from Wadden Sea tidal flat sediments
Authors: ENGELHARDT H.‐J. , BROCKAMP O.
Published: June 2006

Laboratory experiments indicate that clay particles of the <2‐üm fraction can be considerably degraded by the bivalve Mytilus edulis: dickite, kaolinite, smectite, chlorite and illite particles are partially dissolved and rounded by the digestive processes. Structural changes and a decrease in crystallinity of kaolinites, dickites and illites are striking.

Investigations of clay minerals from tidal flat sediments highly populated by marine invertebrates, from marine suspensions and samples from profiles across Mytilis edulis beds on the North Sea coast of Germany show, however, that these materials have a uniform mineral composition. This uniformity of clay mineralogy can be explained by the hydrodynamic conditions in the nearshore area, where tidal currents and waves cause a periodic resuspension and transport of sediments introduced into the Wadden Sea from different sources (glacial, fluviatile). Processes of bioturbation also redistribute the sedimentary material. These mixing processes effectively erase any evidence of local enrichment of the biogenically degraded clays. However, the disordered clays should react more sensitively (i.e. have higher adsorption capacities for organic substances, hydroxides and metals) in sediments than untreated materials, so that biodegradation is of basic importance for clay diagenesis. During these processes, Pb, V, Ni and Fe are dissolved from the digested particles.
The decomposition of polymodal settling velocity distributions for a comprehensive sedimentological description of sand‐sized samples
Authors: MICHELS KLAUS H.
Published: June 2006

The composition of sediment samples and/or the influence of current activity on the sediment can be easily determined using the dissection of settling velocity distributions. A new dissection method for the analysis of settling velocity distributions has been developed. The initial settling velocity distribution is decomposed by means of settling velocity distributions of the main biogenic constituents, three planktic and three benthic foraminifera species. A divergence from the natural size distribution of a foraminifera species can be attributed to current influence. Thus, negative or positive skewness of current‐affected foraminifera populations indicate erosional or depositional sediment influence. The dissection of Holocene samples from the northern North Atlantic demonstrates the advantages of this method in the interpretation of sand‐size material.
Experimental verification of aeolian saltation and lee side deposition models
Authors: McDONALD RICHARD R. , ANDERSON ROBERT S.
Published: June 2006

We report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes. In steady sea‐breeze conditions on a 3‐m‐tall dune at Point Año Nuevo, California, we measured simultaneously the near‐brink wind speed and the deposition on both horizontal and lee face collector platforms. We then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process. Best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds. In addition, the total vertical number flux of grains is constrained to roughly 107−108 grains m−2 s−1 at shear velocities of 0.33–0.40 m s−1. The lee side deposition pattern, which shows the expected maximum in deposition rate at a distance of several decimetres from the brink, is also well fit by the model. In addition, simultaneous collection of horizontal and lee deposition patterns, along with the numerical simulation of these patterns, strongly implies that the windfield in the lee of this particular dune is best described as a non‐recirculating wake. Grainflows on the lee face are caused by failure of grainfall depositional bumps. Our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows. rather than to increase their size, implying that very large, thick grainflows require a different mechanism.
An experimental study of flow, bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models
Authors: BENNETT SEAN J. , BRIDGE JOHN S.
Published: June 2006

The nature of flow, sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size‐density sediment under equilibrium and non‐equilibrium (aggradational, degradational) conditions and compared with theoretical models. During each experiment, water depth, bed and water surface elevation, flow velocity, bed shear stress, bedload transport and bed state were continuously monitored. Equilibrium, uniform flow was established with a discharge of about 0.05 m3 s−1, a flow depth of about 0.01 m, a flow velocity of about 0.81–0.88 m s−1, a spatially averaged bed shear stress of about 1.7–2.2 Pa and a sediment transport rate of about 0.005–0.013 kg m−1 s−1 (i.e. close to the threshold of sediment transport). Such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment. Pebble clusters, bedload sheets and low‐lying bars were ubiquitous in the experiments. Heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits.

Aggradation was induced by (1) increasing the downstream flow depth (flume tilting) and (2) sediment overloading. Tilt‐induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross‐stratified deposit with downstream dipping pebbles (pseudo‐imbricated). and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate. These differences between pre‐ and post‐aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed. Sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles (imbricated), whereas the equilibrium flow and sediment transport conditions remained relatively unchanged. Degradation was induced by (1) decreasing the downstream flow depth (flume tilting) and (2) cutting off the sediment feed. Tilt‐induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions. The cessation of sediment feed produced degradation and armour development, a reduction in the mean water surface slope and flow velocity, an increase in flow depth, and an exponential decrease in bedload transport rate as erosion proceeded. A bedload transport model predicted total and fractional transport rates extremely well when the coarse‐grained (or bedform trough) areas of the bed are used to define the sediment available to be transported. A sediment routing model, MIDAS, also reproduced the equilibrium and non‐equilibrium flow conditions, total and fractional bedload transport rates and changes in bed topography and texture very well.
Diagenetic evolution of the Carnian Wetterstein platforms of the Eastern Alps
Authors: ZEEH S. , BECHSTÄDT T. , McKENZIE J. , RICHTER D. K.
Published: June 2006

The carbonate platforms of the Wetterstein Formation of the Eastern Alps (Drau Range and Northern Calcareous Alps) show a distinct facies zonation of reefs and lagoons. While some lagoonal areas were episodically emerged and formed lagoonal islands, others remained permanently flooded. The scale of near surface, meteoric or marine diagenesis was related to this lagoonal topography. At shallow burial depth, cementation was dominated by altered marine solutions, which additionally caused recrystallization of metastable constituents of the sediment and earlier marine cements (high magnesian calcite, aragonite) connected with a carbon and oxygen isotopic change to more negative values.

Deeper burial cementation shows a succession with two types of saddle dolomite and three types of blocky calcite. Carbon and oxygen isotopic values of these cements show a trend towards more negative values from the first to the last generation, in the following succession: clear saddle dolomite—zoned blocky calcite—cloudy saddle dolomite—post‐corrosion blocky calcite—replacive blocky calcite. Fluid inclusion studies of the carbonate cements are interpreted to indicate a deeper burial temperature development that first increases from 175 to 317°C, followed by a temperature decrease to 163–260°C, and subsequent increase up to 316°C, whereby the samples of the Drau Range always show the lowest values. Calculations of the isotopic composition of the water, from which the carbonate cements were precipitated, yielded positive δ18O values from 6.66 to 17.81%o (SMOW), which are characteristic for formation and/or metamorphic waters. Also, the isotopic compositions of the palaeofluids probably changed during deeper burial diagenesis, following the temperature development.
High‐resolution sequence stratigraphy of a Hettangian‐Sinemurian paralic succession, Bornholm, Denmark
Authors: SURLYK F. , ARNDORFF L. , HAMANN N.‐E. , HAMBERG L. , JOHANNESSEN P. N. , KOPPELHUS E. B. , NIELSEN L. H. , NOE‐NYGAARD N. , PEDERSEN G. K. , PETERSEN H. I.
Published: June 2006

Sequence stratigraphic interpretation of paralic successions is complicated by the complex interfingering of marine and continental strata. The successions may also include terrestrial extensions of marine parasequences and completely independent lacustrine parasequence analogues. Failure in recognizing the possible interbeddding of these two independent parasequence types may lead to construction of sequence stratigraphic schemes based on incompatible data sets. We have studied a Lower Jurassic paralic section from the Baltic island of Bornholm, situated in the Tornquist Zone, which demarcates the transition from the stable Precambrian Baltic Shield to the subsiding Danish Basin and Danish‐Polish Trough. The Hettangian‐Sinemurian Sose Bugt Member (Rønne Formation) of Bornholm includes lacustrine, fluvial and restricted marine, estuarine deposits reflecting the basin‐margin position. Biostatigraphic resolution is poor and a sequence stratigraphic interpretation of the paralic succession is far from straightforward. A multidisciplinary approach including facies analysis, recognition and lateral trading of key surfaces, palynostratigraphy, palynofacies, coal petrography, palaeopedology, clay mineralogy and source rock geochemistry is applied in order to obtain a high degree of precision in the interpretation of the paralic facies. In this way four sequences are recognized in the overall backstepping lacustrine to estuarine succession. Marine and marginal marine parasequences are distinguished from their purely lacustrine analogues, and an internally consistent sequence stratigraphic scheme is proposed. This is compared and tentatively correlated with fossiliferous marine sediments in the Danish Basin and with published eustatic cycle charts.
Observations of collisions of saltating grains with a granular bed from high‐speed cine‐film
Authors: RICE M. A. , WILLETTS B. B. , McEWAN I. K.
Published: June 2006

High‐speed photography was used to record saltating sand grains colliding with a horizontal, noncohesive bed of similarly sized grains. Impacting grain/bed interaction is discussed in general. The process, as observed from the films, is then described in terms of the apparent bed contact length (ABCL) and various parameters of the impacting grains and any ejected grains. Examples are given of typical behaviour of bed grains in response to impacting grains of different sizes. Saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments, so that previously buried grains become available for entrainment. This process is discussed in relation to the potential release of dust particles into the airflow.
Field measurements of the flux and speed of wind‐blown sand
Authors: GREELEY RONALD , BLUMBERG DAN G. , WILLIAMS STEVEN H.
Published: June 2006

A field experiment was conducted to measure the flux and speed of wind‐blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary‐layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0·5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an ‘egg‐box’ pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand‐trap systems used in previous experiments, 12 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high‐speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under‐collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0·5–7·0 m s−1, compared to a wind friction velocity of 0·32–0·43 m s−1 and a wind velocity of 2·7–3·9 m s−1 at the height of the particle measurements. Descending particles tended to exceed the speeds of ascending particles by ∼ 0·5 m s−1.