bluuebunny/crossref_metadata_2025 huggingface dataset:
On measuring time‐averaged velocities in open channel flow
Authors: BARTHOLDY JESPER
Published: June 2006

The problem of finding the necessary measuring time to determine the mean velocity in open channel flow is treated. The result shows an ‘on the safe side’ relation between measuring time and standard deviation of measured velocities.
Impressions of algal mats from the Middle Proterozoic Belt Supergroup, northwestern Montana, U.S.A.
Authors: HORODYSKI ROBERT J.
Published: June 2006

Irregular, low relief bedding‐plane markings which probably represent impressions of thin, wrinkled algal mats have recently been detected in mudstones from the 1100 m.y. old Snowslip Formation, Belt Supergroup, Glacier National Park, Montana. Algal‐mat impressions are rarely reported from ancient strata, yet algal mats are common in certain modern environments. Originating primarily in moist settings and in relatively quiescent shallow‐water settings, such structures may be of some value in interpreting ancient depositional environments.
A study of the dynamics of a marine sandwave
Authors: LANGHORNE D. N.
Published: June 2006

The movement of the crest of a sandwave was studied using cross‐sectional profiles obtained from lines of sea‐bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest.

The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea‐bed by storm waves, caused major reductions in crestal heights.

Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.
Quantitative mineralogical analysis of carbonate sediments by X‐ray diffraction: a new, automatic method for sediments with low carbonate content
Authors: ROSELLE PHILIPPE
Published: June 2006

A new X‐ray diffraction method has been developed whereby the weight percentages of aragonite and low and high‐magnesium calcite are determined from the integrated peak areas of samples. Peak areas are measured by a step scanning method. The weight percentages of MgCO3 in calcite are determined from the angular position of the calcite peak. This technique uses a direct calculation method which simplifies the preparation of the samples and the calibration processes and increases the quality of the results. The fully automatic method uses a desk‐top computer to guide the diffractometer and to carry out the necessary calculations. Tests on precision and accuracy of the method indicate that results with less than ± 4% error (mineral %) and ± 0.6% error (MgCO3%) are obtainable for all samples even those with a low (10%) carbonate content.
A carbonate submarine fan in a fault‐controlled basin of the Upper Jurassic, Betic Cordillera, southern Spain
Authors: RUIZ‐ORTIZ P. A.
Published: June 2006

ABSTRACT The middle member of the Loma del Toril formation (Kimmeridgian‐Lower Tithonian, Intermediate units, Betic Cordillera) consists of up to 250 m of resedimented carbonate material. Three units have been distinguished. The lower, Unit A, is composed of conglomerates that are interpreted as deposited in a major valley on the lower slope of a basin margin. Unit B, calcarenites with some conglomerate intercalation, is interpreted as distributary channel deposits and Unit C, calcarenites, as the result of poorly developed depositional lobes of a submarine fan. The three units form a recessional sequence. They cannot be related to a transgression because the Kimmeridgian‐Lower Tithonian in the Prebetic zone, where epicontinental sediments exist, is clearly regressive. The upper member of the Loma del Toril formation, made up of pelagic limestones with sporadic calcarenites or even thin conglomerate intercalations, is best interpreted as a basin plain facies. Lateral facies relationships suggest that down‐faulting of the basin floor controlled the development of the fan. The scarce occurrence of turbidite beds in the basin plain facies, the prevailing channelized facies and the obvious lack of overbank deposits, suggest a transport system of low efficiency, with fan deposition at the base of slope. The underlying Jurassic strata cropping out along fault scarps, coeval carbonate shelf material, and upper slope deposits were the main sources of turbiditic resediments. With respect to basin morphology sedimentary processes and fan geometry, this Jurassic turbidite basin can be compared with the modern California continental borderland. Ancient analogues have been described by Reinhart (1977) and Price (1977).
A rendzina from the Lower Carboniferous of South Wales
Authors: WRIGHT V. PAUL
Published: June 2006

ABSTRACT A thin calcrete‐crust horizon from the Lower Carboniferous Llanelly Formation of South Wales consists of two parts; an upper laminated unit and a lower peloidal unit. The former is interpreted as a subaerial stromatolite and the latter as an A horizon of a palaeosol. Comparisons are made with the A horizons of rendzinas and it is concluded that the calcrete‐crust represents a complete rendzina profile. This fossil rendzina contains abundant evidence of a soil fauna in the form of fecal pellets and small burrows.
Time‐ and method‐dependent size distributions of fine‐grained sediments
Authors: NELSEN TERRY A.
Published: June 2006

ABSTRACT With increased interest in fine‐grained sediments it is imperative that a firm basis exist for comparative studies of these cohesive sediments. In this study the size distributions of continental slope and rise muds are shown to be dependent on both method and duration of sample pretreatment. Statistical analysis of 171 size distributions indicates that of the four most frequently used sample preparation methods (soak, stir, shake and ultrasonify) the time required to reach a ‘terminal’ distribution beyond which no ‘fine‐shift’ was detected varied from 15 minutes to > 90 hours for a given sample solely as a function of preparation method. Data from this study also indicate that sample preparation by simple soaking alone will probably yield fine‐grained sediments which, when analysed by pipette or microsedimentation accumulation balance, undergo a continuous change in size distribution during analysis and may never reach a ‘terminal’ distribution during the analysis time. On the other hand stirring and ultrasonification were shown to be the most rapid and consistent methods for obtaining a sample's‘terminal’distribution.
Asymmetry of Zoophycus burrows as a way‐up criterion
Authors: PUDSEY CAROL J.
Published: June 2006

ABSTRACT Zoophycus burrows from Neogene hemipelagic sediments in the equatorial Atlantic contain an asymmetric spreite fill. The long limb of each lamella is down‐section. This is useful as way‐up evidence in folded sequences.
Aeolian stratification and facies association in desert sediments, Arran basin (Permian), Scotland
Authors: CLEMMENSEN LARS B. , ABRAHAMSEN KJELL
Published: June 2006

ABSTRACT Permian aeolian sediments on the island of Arran are divisible into dune (including draa) and interdune deposits. Both types display a distinctive and unusually wide variation in grain size. The dominant features of the dune deposits are grainfall lamination, sandflow lamination, and inverse graded lamination associated with ripple‐form lamination and normal graded lamination. The flat‐lying aeolian interdune deposits are characterised by granule and sand ripples, horizontal lamination in coarse sand and granules, plane bed lamination and inverse graded lamination. Associated structures include ripple‐form lamination and deflation lags. Three types of trace fossil associated with completely bioturbated horizons occur in some low‐angle dune and interdune deposits.

The aeolian facies interfinger with alluvial fan deposits giving rise to three recognizable facies belts. Marginal aeolian deposits are associated with fluvial conglomerates and are dominated by interdune deposits and occasionally very thin barchan deposits (set height 3‐37 cm). Intermediate aeolian deposits are characterized by interbedded crescentic dune, small draa (dune set height 5 cm‐4.5 m) and interdune deposits, and rare fluvial and lake sediments. Basinal aeolian deposits are dominated by draa deposits (dune set height 0.2‐28 m) associated with rare interdune sediments. Transverse dunes and draas were moved by north‐eastern palaeowinds towards the foot of the alluvial fans. The aeolian sediments were deposited in a fault‐bounded desert basin.
Microbial processes in the Negev phosphorites (southern Israel)
Authors: SOUDRY DAVID , CHAMPETIER YVES
Published: June 2006

ABSTRACT Microscopic and SEM observations indicate that cyanobacterial activity was important during the formation of the Campanian Negev phosphorites. One aspect of such activity is the development of phosphorites through the binding of phosphate particles. These sediments often demonstrate a conspicuous smooth cryptalgal lamination. The algal binder was fossilized as intertwined phosphatic sheaths that coat and seal off the phosphate grains. Different types of sheaths representing the remnants of an algal community were found. Thick apatite overgrowth commonly deforms the microbial fossils converting them into barely recognizable forms. Fabrication of phosphate particles is another aspect of the microbial activity. Phosphate algal coated grains are a common constituent of the Negev phosphorites. The algal coating consists of stacked phosphatic tubes and colonies of coccoid unicells with an apatite infilling. Fragmented phosphatic sheath bundles form another type of phosphate particle produced by algae. Obliteration of algal structures is common, resulting in undifferentiated groundmasses of cryptocrystalline apatite. The extremely close connection between microbial structures and phosphatization processes suggests cyanobacterial participation in phosphorus enrichment.
Quantitative interpretation of an evolving ancient river system
Authors: BRIDGE JOHN S. , DIEMER JOHN A.
Published: June 2006

Multistorey sandstone bodies described from the Upper Devonian‐Lower Carboniferous of Kerry Head (Ireland) are interpreted as deposits of aggrading, perennial, river channels migrating laterally across alluvial plains. Point bars displayed surface features such as scroll bars, chute channels and chute bars. Relatively uncommon channel fills are both coarse‐ and fine‐grained.

Quantitative interpretation of the sandstone bodies was accomplished by comparison with a physical model that predicts the sedimentology of single point bar deposits developed in channels of prescribed geometry and hydraulics. This analysis reveals that the separate storeys (point bars) in each sandstone body were deposited in a single channel belt in which channel geometry and hydraulics varied little with time (order of 103 yr) and space (order of 103 m). Two southerly flowing rivers of markedly different size were responsible for all sandstone bodies: bankfull widths, depths and mean velocities of both rivers varied little with time (order of 105 yr), implying a stable climatic setting. Channel sinuosities were usually 1.15–1.2 throughout the succession. Both rivers decreased in mean channel slope as time progressed, in association with a rising base‐level and a shoreline encroaching from the south.

Using Bridge & Leeder's (1979) alluvial stratigraphy model, the nature and distribution of channel sandstone bodies relative to overbank deposits in the succession can be explained by an average (compacted) floodplain deposition rate of about 0.005 m yr−1, if avulsion occurred with a frequency of about once every 103 yr. Local variation in the relative amount of channel sandstone in the succession is probably due to local tectonic control of deposition.
Transport by wind of granular materials of different grain shapes and densities
Authors: WILLETTS BRIAN
Published: June 2006

Aeolian transport rates were measured for three sands: a quartz sand (relative density 2.68, sphericity 0.73), a shelly sand (relative density 2.64, sphericity 0.59, carbonate content 67%), and granular aluminium oxide (relative density 3.95, sphericity 0.67). Low sphericity depresses the transport rate, particularly at high wind speeds; high density also does so but the effect is more marked at low transporting wind speeds.

The sand of low sphericity undergoes longer saltations than the other materials, but is dislodged less frequently than they are and is transported less freely in a given wind. Of the more spherical materials, the denser was the least effective at extracting energy from the wind for grain transport in the range of our experiments. Both of the more spherical materials showed evidence of a fairly sudden transition of transport behaviour at modest wind speed. It is inferred that this marks the transfer of the function of grain dislodgement from direct wind action to inter‐granular collision. The transition did not occur in experiments on the material of lower sphericity.

For a given transport rate, wind speed near the bed is highest for the grains of low sphericity (by a considerable margin) and lowest for the more compact quartz grains. Propensity for transport by wind is greatest for the quartz sand, less so for the heavy material and least for the material of low sphericity.
Skeletal halites from the Jurassic of Massachusetts, and their significance
Authors: PARNELL J.
Published: June 2006

Pseudomorphs after skeletal halite occur in a bed of Lower Jurassic lacustrine shale at Holyoke, Massachusetts. The pseudomorphs include pagoda types similar to those described by Southgate and show dissolution furrows. However the shales were deposited in a permanently subaqueous rather than an emergent environment, and the pseudomorphs accordingly grew in brine‐saturated sediment. Evaporation to dryness is not therefore essential to the genesis of skeletal halite.
Deposition of the Bear Gulch Limestone: a Carboniferous Plattenkalk from central Montana
Authors: WILLIAMS LORETTA ANN
Published: June 2006

The Carboniferous Bear Gulch Limestone of the central Montana Big Snowy Trough is a lithographic limestone analogous to the plattenkalk carbonates of the Mesozoic European/Middle Eastern Tethyan belt. It contains an excellently preserved chitinous and phosphatic marine fauna.

Petrographic and field studies synthesized with palaeontological data provide insights into depositional history. Tectonic activity initiated development of small en echelon basins. Rims of these basins were sites of high primary productivity. Photosynthesis by subtidal cyanobacterial mats and algae caused in situ precipitation of micrite. Cyanobacteria and algae also provided a food source for benthic and nektonic organisms, which provided food for predators. Respiration in turn provided carbon dioxide for continued photosynthesis. Thus a balanced ecosystem evolved.

Productivity in the water column used up available oxygen, resulting in dysaerobic to anaerobic conditions in the bottom waters and sediments. This plus high sedimentation rates of carbonate ooze transported downslope, aided in preservation of whole‐body fossils.

There are no likely modern analogues known for the Bear Gulch Limestone. However, comparisons with Mesozoic plattenkalke demonstrate that it is most closely analogous to those deposited in shallow water back‐reef areas of the Tethyan carbonate platform, rather than to those deposited in the deeper water shelf‐edge environment. This is in agreement with the water depth of roughly 40 m estimated for the Bear Gulch Limestone.
A comparison between Shields' threshold criterion and the movement of loosely packed gravel in a tidal channel
Authors: HAMMOND F. D. C. , HEATHERSHAW A. D. , LANGHORNE D. N.
Published: June 2006

Observations of the threshold of movement of loosely packed gravel in a tidal current are described. For gravel with equivalent ‘spherical’ diameters D in the range 0.2 ≲D≲ 5.0cm the critical friction velocity u*c, corresponding to the initiation of sediment transport, is given by u*c=7.0 D0.2. At large values of D within the quoted range, the value u*c is significantly lower than would be obtained by a Shields experiment (u*c∞D0.5). By comparing our values of u*c with those obtained under well‐controlled laboratory conditions, the discrepancy with Shields is shown to be due to the open spacing between, and exposure of, individual pebbles on the seabed. By comparing our results with those from upland gravel streams and flume experiments, it is suggested that Shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid.
Note on boundary effects on stream meandering
Authors: NAKAGAWA TAKEO , HOTSUTA MASASHI
Published: June 2006

This paper provides some additional evidence supporting the necessary (but insufficient) condition for the formation of stream meandering proposed by Nakagawa:
image

where Me is a non‐dimensional parameter, τs, and τb are the average bank and bed shear stress respectively, ps and pb are the average bank and bed wetted perimeter of a half‐channel respectively, and K≃0.2 is the critical value of the parameter estimated from experimental data.

Provided the criterion is satisfied, the main thread of the stream meanders in the non‐erodible channel, and the maximum amplitude amax, the angle a between the channel central axis and oblique crest line of the surface wave, and the mean wavelength L of the main thread decrease as the non‐dimensional parameter Me increases.
The fluviolagoonal palaeoenvironment in the Rhine/Meuse deltaic plain
Authors: VAN DER WOUDE J. D.
Published: June 2006

Detailed lithostratigraphical, palynological and 14C analyses have served to reconstruct the palaeoenvironment during two main, mid‐Holocene, regional phases of increased fluvial clay and sand deposition in the Rhine/Meuse deltaic plain. The investigated location lies in a broad area transitional to the upstream fluviatile and the downstream marine (lagoon/tidal flat) deposition zones. The palaeoenvironment appears to have been extensive, permanent open water intersected by the wooded natural levees of many small river channels. The term fluviolagoon is proposed for this typical environment, as illustrated here by three‐dimensional oblique landscape drawings. A present‐day example is found in the Magdalena river deltaic plain, Colombia.
Holocene stratigraphy and depositional history of the Narragansett Bay System, Rhode Island, U.S.A.
Authors: McMASTER ROBERT L.
Published: June 2006

ABSTRACT

An extensive seismic reflection survey has been used to gain further knowledge of Holocene stratigraphy and depositional history in the Narragansett Bay System (NBS).

The early Holocene stream‐dissected surface beneath the NBS is interpreted as having been flooded by the Holocene sea in a manner suggested by Oldale & O'Hara's (1980) sea‐level‐rise curve. The sea initially is believed to have penetrated the pre‐NBS East Passage trunk valley about 9000 yr BP and subsequently spread landward via the trunk valley and its branches.

The Holocene sediments display stratigraphic relationships that differ spatially. At passage mouths, the basal unconformity is inferred to be covered over with some 3 m of paralic and 5 m of marine sands and silts separated, by a transgressive unconformity. In contrast the interior sequences reveal (a) a valley section up to more than 15 m thick in which the regressional unconformity is overlain by probable lower fluvial and/or estuarine sand‐silt facies that commonly grades upward to an estuarine silt‐clay facies and (b) an interfluve section in which a basal transgressive unconformity is blanketed by an estuarine nearshore sand‐silt facies that locally may change upward to a silt‐clay facies.

Primarily Holocene silt‐clay accretion, produced by sedimentation processes associated with net non‐tidal estuarine circulation, infilled the evolving NBS. Depositional bodies, lenticular in shape and comprised of 12 m or more of sediment, developed in lowlands near Gould Island (˜9000 yr BP), in Upper Narragansett Bay (˜7500 yr BP), around Hope Island (˜7500 yr BP) and in Mt Hope Bay (˜6250 yr BP) with an average minimal sedimentation rate of 1.6–2.2 mm yr‐1. Silt‐clay deposition, commonly gas‐bearing, has buried the basal relief in most of the NBS upper and middle portions except for middle East Passage.

A comparison of NBS sedimentation with that of Chesapeake, Delaware and Hudson Estuaries shows that the estuaries to the south have accumulated more sediment over a slightly longer period yet, with the exception of the higher rate in the Hudson Estuary, the sedimentation rates appear to be similar.
Variations in sublittoral sediments and their associated macro‐infauna in response to inner shelf processes; Swansea Bay, U.K.*
Authors: SHACKLEY SUSAN E. , COLLINS MICHAEL
Published: June 2006

ABSTRACT

The purpose of this investigation was to examine the relationships between sublittoral sediments and their associated macro‐infaunas in a coastal embayment. Samples of a fine‐grained surficial deposit were collected at monthly intervals from Swansea Bay, located in the northern Bristol Channel. The area is one of high tidal and wave energy. Mean spring and mean neap tidal ranges are 8.6 and 4.1 m respectively, and the area is subjected to wave action from locally‐generated winds and swell from the Atlantic Ocean. Maximum near‐bed tidally‐ or wave‐induced currents are of the order of 1 ms‐1.

The surveys have provided the factual basis for a model to explain the presence in a sublittoral deposit of allochthonous bivalves of an inshore, muddy‐sand Abra community and an offshore Spisula subcommunity of the Venus communtiy. Further, a mechanism is advanced to explain the rapid burial of the allochthonous bivalves by coarse‐grained sediment influx during storms and the subsequent localized deposition of mud.
Early Pleistocene glacial outwash deposits in the eastern San Joaquin Valley, California: a model for humid‐region alluvial fans
Authors: CHERVEN VICTOR B.
Published: June 2006

ABSTRACT

The early Pleistocene Laguna and Turlock Lake Formations and China Hat and Arroyo Seco Gravels along the east side of the San Joaquin Valley, California, were deposited in alluvial fans and marginal lakes. Upward‐coarsening sequences of silt‐sand‐gravel record westward progradation of glacial outwash fans from the Sierra Nevada into proglacial lakes in the San Joaquin Valley. Distinctive sedimentary features delineate lacustrine, prodelta, and delta‐front facies within fan‐margin deposits and lower, middle, and upper‐fan facies within alluvial‐fan deposits.

The lacustrine facies consists of a few metres of thinly and evenly bedded, rhythmically laminated claystone and clayey siltstone in varved couplets. Draped lamination, sinusoidal lamination, and load and pillar structures occur in some beds. Siltstone and claystone grade upward to slightly thicker wavy beds of siltstone and very fine‐grained unconsolidated sand deposited in a prodelta setting. Convolute laminae within deformed steeply dipping foreset beds suggest slumping on the prodelta slope. The prodelta facies grades up to the delta‐front facies, which consists of burrowed and bioturbated cross‐bedded fine sand. Deltaic deposits are 5–6 m thick.

The lower‐fan facies forms the base of the fan sequence and consists of several metres of irregularly bedded, laminated, oxidized siltstone and fine sand. The middle‐fan facies consists of cross‐bedded, medium‐grained to gravelly sand‐filled channels cut into the lower‐fan facies. Interbedded lens‐shaped siltstone beds 2 m thick and several metres across were deposited in abandoned channels. The upper‐fan facies consists of moderately to strongly weathered clayey gravel and sand containing pebble imbrication and crude stratification. Argillization during post‐depositional soil formation has blurred the distinction between mud‐supported debris‐flow deposits and clast‐supported channel deposits, but both are present in this facies.

The deposits described here demonstrate the need for additional fan models in order to incorporate the variety of deposits developed in alluvial fan sequences deposited in humid climates. In previous models based on arctic fans, debris flows, abandoned channels, or widespread siltstone beds are not present in fan sequences, nor are marginal lacustrine and deltaic deposits well represented.
Calcitic, aragonitic and mixed calcitic‐aragonitic ooids from the mid‐Proterozoic Belt Supergroup, Montana
Authors: TUCKER MAURICE E.
Published: June 2006

A great variety of ooid types occurs within the Siyeh and Snowslip Formations of the mid‐Proterozoic Belt Supergroup, Montana. Cortical layers are inferred to have been composed either of calcite in a radial‐concentric or radial‐with‐dark‐rays fabric or, aragonite in a radial or concentric fabric. The calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite. Some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure. Others consist of calcite spar without inclusions, or columnar calcite which grew radially from the nucleus, commonly a calcitic ooid. Some ooids were wholly composed of calcite, others were of aragonite, but two phase ooids were common, mostly consisting of an inner calcitic part and an outer aragonitic part. Probable microdolomite inclusions suggest a high Mg content of the calcitic cortical layers. The depositional environment of these oolites was probably analogous to Baffin Bay, Texas, where a similar range of ooid types is forming today.
An application of scanning electron microscopy of quartz sand surface textures to the environmental diagnosis of Neogene carbonate sediments, Finestrat Basin, south‐east Spain
Authors: CATER J. M. L.
Published: June 2006

The Finestrat Basin, 25 km NE of Alicante, contains a poorly understood sequence of Neogene sediments that lack an indigenous fauna. Study of the surface textures of euhedral quartz grains, probably derived by first‐cycle weathering from an underlying Triassic mudstone diapir, provides new data on the depositional environment of the basin. The possible influence of alluvial fans is recorded by mechanical breakage features in these non‐glaciated sediments. The distribution and abundance of chemical etch pits suggest their diagenetic origin following non‐marine deposition, while the density and abundance of mechanically induced ‘V’‐shaped pits seems typical of fluvial and low‐energy littoral environments. The presence of silica precipitated during transport, such as silica globules, suggests a silica‐rich transport medium and a depositional environment that was not highly alkaline. In combination with independent sedimentological evidence, these data suggest that the sediments collected in a lake after derivation by rivers and alluvial fans from the diapir.

It is concluded that the abundance and superposition of features must be noted for each grain in order to yield useful results, and that the mechanisms acting on each grain be deduced before attempting to identify specific environments.
Interpretation of grain‐size distributions from measured sediment data, Platte River, Nebraska
Authors: ESCHNER THOMAS R. , KIRCHER JAMES E.
Published: June 2006

Breaks in the slope of log‐probability plots of cumulative grain‐size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south‐central Nebraska. The break between suspension and intermittent suspension as determined from the bed‐material curve coincides with the upper limit of the grain‐size overlap between bedload particles and suspended‐sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended‐sediment particles. Although grain‐size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended‐sediment distributions increases with increasing discharge. The limits of grain‐size overlap of bedload and suspended‐sediment distribution curves associated with near‐flood discharges most closely approximate the breaks in the bed material grain‐size distribution.
The importance of maintaining horizontal sieve screens when using a Ro‐Tap
Authors: METZ ROBERT
Published: June 2006

Sieve size analysis, using a Ro‐Tap shaking machine, for horizontal and non‐horizontal sieve screen nests are compared. The non‐horizontal sieve condition results in more sediment being retained on the topmost sieve, with less sediment reaching certain sieves within the nest. It is thus essential to maintain the nest of sieves perfectly horizontal.
Multiple flow and support mechanisms and the development of inverse grading in a subaquatic glacigenic debris flow
Authors: BROSTER BRUCE E. , HICOCK STEPHEN R.
Published: June 2006

Subaquatic glacigenic debris flows of Late Wisconsinan age occur as lobes within an ice‐marginal glaciomarine lithofacies at Victoria, British Columbia. Flow was initiated by release of supraglacial debris during an interval of glacial ablation prior to advance and deposition of lodgement till. Many of the lobes developed a common morphology during deposition, consisting of an outer layer surrounding an inner core of rafted material.

In the largest lobe reworking of the sediment during flow produced improved sorting, a coarsening of mean grain size, and a shift in skewness toward the negative. This occurred simultaneously within the core and outer layer as a result of different transformations in mechanisms of flow and support. Normal grading, produced in the outer layer, was conveyed around the nose and buried in an inverse position along the base of the flow. These data enable us to present a model that may be applicable to similar flows elsewhere.

The flow studied demonstrates that transformations between flow types and sediment support mechanisms can occur simultaneously and serially in subaquatic debris flows.
Evolution of a provenance as revealed by petrographic analyses of Cretaceous formations in the Queen Charlotte Islands, British Columbia, Canada
Authors: YAGISHITA KOJI
Published: June 2006

Two contrasting marine sedimentary facies, the Haida Formation of sandy and argillaceous sediments and the conformably overlying Honna Formation of gravelly sediments were formed within different types of tectonic basins during mid‐ to Late Cretaceous time. The sediments of both formations were derived from the east. Sandstones from the two formations show characteristics of mature magmatic arc provenance and are classified as lithic and feldspathic arenites. However, the Honna sandstones are more feldspathic and less quartzose than the Haida sandstones. The Honna sandstones have many volcanic rock fragments (VRF) but the Haida sandstones do not. Feldspars of the VRF‐rich Honna sandstones, however, do not seem to have been derived from the breakdown of volcanic rocks. The observed petrographic differences between the two formations can be ascribed to a process in which the volcanic cover and the basement rock denudation took place simultaneously in the source area during deposition of the Honna Formation.
Discrimination between depositional environments using grain‐size analyses
Authors: EL‐ELLA RAMADAN ABU , COLEMAN JAMES M.
Published: June 2006

Grain‐size analyses were performed on 84 samples from six environments in the Burdekin River Delta, NE Queensland, Australia. The data were subjected to a stepwise discriminant function analysis and canonical analysis which showed that the grain‐size characteristics of the sediments are sufficiently distinctive to distinguish the six different environments of deposition. The resulting ‘reference set’ was then tested by plotting the grain‐size characteristics of 66 samples from different environments of the Mississippi River Delta, Louisiana and Galveston Island, Texas. Seventy‐four per cent of the North American samples were correctly classified.
The prodelta environment of a fjord: suspended particle dynamics
Authors: SYVITSKI JAMES P. M. , ASPREY K. W. , CLATTENBURG D. A. , HODGE GORDON D.
Published: June 2006

The dynamics of suspended particles within a fjord's estuarine circulation are investigated and the results compared with larger non‐enclosed prodelta environments. In the upper prodelta, the seaward‐flowing river plume flows over the ambient marine water depositing much of the initial riverine suspended load. Sedimentation is dominated by coarse silt and fine‐grained sand particles with coarseness determined by the tidal and fluvial stage. Particles less than 10 μn have similar settling velocities regardless of size because they settle in flocs: the settling velocity at a water depth of 5 m is 30 m day‐1 and increases with depth so that at 30 m the particles settle at 100 m day‐1. For larger particles, the downward settling velocity enhancement due to flocculation decreases with increasing grain size. Hydraulic sorting allows the preferential settling of feldspar and quartz over mica.

Particle dynamics in the lower prodelta are dependent on the character of the freshwater wedge that thins seaward of the upper prodelta. The vertical flux of particles is controlled by biogeochemical interactions such as pelletization of fine particles and flocculation (which occurs within rather than below the surface layer in contrast to the upper prodelta). The pellets are produced by indiscriminate filter feeding zooplankton. Across the lower prodelta the suspensate character, recognized in the composition of both flocs and pellets, changes from a dominance of mineral grains to that of autochthonous organic matter. The interaction of bacteria with the suspended particles increases with depth and seaward distance. At depth, the mucoid filaments form stable interconnecting webs.

Particle concentration in the surface layer decreases at a rate proportional to the negative one‐half and three‐halves power of the distance seaward over the upper and lower prodelta, respectively. This relationship is hypothesized as being universal for large marine deltas dominated by buoyancy flow dynamics, regardless of the levels of initial riverine particle concentration or their composition.
Experimental study of sand streaks formed in turbulent boundary layers
Authors: WEEDMAN SUZANNE D. , SLINGERLAND RUDY
Published: June 2006

Fine‐ to medium‐grained sand transported as bedload moves in lanes parallel to the flow that are thought to be preserved as parting lineation. A series of six flume experiments was designed to discover the morphology and spacing of these lanes, here called sand streaks, as functions of local shear velocity, U* (9 × 10‐3 to 4.8 × 10‐2 m s‐1), depth (5 × 10‐2 and 9.5 × 10‐2 m), mean grain diameter (150, 200, 290, 1380 μm), and sediment bedload concentration (0.0–0.39). Low U* flows produce predominantly straight, non‐intersecting sand streaks, moderate U* flows produce sub‐parallel and en échelon sand streaks, and moderate to high U* flows produce wavy sand streaks and secondary streaks with a spacing an order of magnitude larger. The wavy sand streaks are thought to be composed of sand grains in suspension close to the bed. An upper grain‐size limit for the sand streak structure occurs at a grain size between 290 and 1380μm. The spacings of the fine‐and medium‐grained sand streaks, at low to moderate U* (0.9 × 10‐2 to 3 × 10‐2m s‐1), are similar to those predicted for low‐speed fluid streaks, although the fine‐grained sand forms more closely‐spaced streaks than the medium‐grained sand. The spacings of sand streaks formed at moderate to high U* and at bedload concentrations greater than 0.15, are wider than those predicted for the low‐speed fluid streaks. The wider spacing is thought to reflect a new type of flow immediately above the moving bed layer in which the formation of low‐speed streaks is inhibited. This results from an increase in either grain concentration or grain size. The spacing of parting lineation, also wider than that predicted for low‐speed streaks, may reflect this.
Sedimentology and budget of a Recent carbonate mound, Florida Keys
Authors: BOSENCE DANIEL W. J. , ROWLANDS ROYSTON J. , QUINE MARK L.
Published: June 2006

The sedimentology of a Recent carbonate mound is investigated to further our understanding of mound building communities, surface and subsurface mound sediments, and the overall sediment budget of mounds. Nine sedimentary facies of the surface of Tavernier mound, Florida Keys are described. These sediments are composed of Neogoniolithon, Halimeda, Porites, mollusc and foraminiferal grains, and lime mud. Muds rich in aragonite and high magnesian calcite show little mineralogical variation over the mound surface. Geochemical evidence suggests that the mud is mainly formed from breakdown of codiacean algae and Thalassia blade epibionts. Production rates of the facies are established from in situ growth rate experiments and standing‐crop surveys. Annual calcium carbonate production is c. 500gm‐2, intermediate between reef and other bay and lagoonal environment production rates in the Caribbean. The internal structure of the mound, studied from piston cores and sediment probes, indicates that seven facies can be identified. Five of these can be related to the present‐day facies, and occur in the upper part of the mound (gravel‐mound stage). The remaining two facies, characterized by molluscs and aragonite‐rich muds, occur in the lower part of the mound (mud‐mound stage), and are most similar to facies from typical Florida Bay mud mounds. Mangrove peats within the mound indicate former intertidal areas and C14 dates from these peats provide a time framework for mound sedimentation. The mound appears to have formed because of an initial valley in the Pleistocene surface which accumulated mud in a shallow embayment during the Holocene transgression.

A sediment budget for the mound is presented which compares production rates from present‐day facies with subsurface sediment masses. During the mud mound stage production rates were similar to accumulation rates and the mound was similar to the present‐day mounds of Florida Bay. During the gravel mound stage (3400 yr BP‐present day), conditions were more normal marine and the establishment of Porites and Neogoniolithon on the mound increased production rates 10% over accumulation rates. This excess sediment is thought to be transported off the mound to the surrounding seabed. Models are proposed which divide carbonate mounds on the basis of internal versus external sediment supply. Comparisons are made with other Recent and ancient mounds. Similarities exist between the roles of the biotic components of late Palaeozoic mounds but major differences are found when structures and early diagenesis are compared.
Botryoidal aragonite and its diagenesis
Authors: AISSAOUI DJAFAR M.
Published: June 2006

Botryoidal aragonite is a spectacular growth‐form occurring as mamelons, up to 100 mm in diameter. Three examples of this particular carbonate cement have been discovered in two distinct areas: in New Caledonia, small‐scale mamelons have been recognized within Pleistocene reefal terraces at Ouvea, an Island of the Loyalty archipelago, and in the Red Sea, large‐scale mamelons of botryoidal aragonite exist within Pleistocene reefal terraces along the Um Gheig region of the Egyptian Coast. In addition, a similar botryoidal cement, partly dolomitized but exhibiting aragonite relics, occurs within a Miocene reef in the same region.

Mamelons of botryoidal aragonite are isolated and/or coalescent but grow only on fixed substrates. They occur within cavities of varied origin. Their fabrics are characterized by fans of elongate euhedral crystals of aragonite fibres. Botryoidal aragonite can be preceded or followed by other types of cements or internal sediment.

Despite similar mineralogy, petrography and ultrastructure, there are differences between the fabrics of the Pleistocene botryoids from the Red Sea and Ouvea and the Miocene botryoids from the Red Sea. The former are of submarine origin as confirmed by the strontium content (8500–10,500 ppm) and isotopic composition (δ18O between ‐0.10 and +0.19% PDB). The latter, related to a Miocene karst, are rich in strontium (average 13,600 ppm), and have an isotopic composition (average δ18O ‐10.50% PDB) indicative of non‐marine precipitation.

Diagenesis of these botryoidal aragonites consists of slight dissolution for Pleistocene botryoids and mineralogical transformation for the Miocene botryoids. The latter exhibit the diagenetic sequence aragonite → calcite → dolomite. The aragonite to calcite transformation is a dissolution‐reprecipitation process, the void distribution and size influencing the distribution and the size of the replacement calcite crystals.
Mixed siliciclastic and carbonate sediments: a proposed first‐order textural and compositional classification
Authors: MOUNT JEFFREY
Published: June 2006

Sediments that are composed of mixtures of siliciclastic and carbonate material occur in both the modern and the ancient. To date, there is no commonly‐used classification of this type of sediment. A descriptive, first‐order classification system is proposed here that is applicable to most mixed sediments and their lithified equivalents. The sediments are composed of four components: (1) siliciclastic sand (sand‐sized quartz, feldspar, etc.), (2) mud (mixtures of silt and clay), (3) allochems (carbonate grains such as peloids, ooids, bioclasts and intraclasts > 20 μm in size), and carbonate mud or micrite (< 20 μm in size). These components define a tetrahedra whose subdivision produces eight general classes of mixed sediment. The name of a class reflects both the dominant grain type and the most abundant antithetic component. For example, a rock that contains greater than 50% siliciclastic material, of which most is sand‐sized, and subordinate allochems is termed an allochemic sandstone. Other rock names include micritic sandstone, allochemic and micritic mudstone, sandy or muddy allochem limestone and sandy or muddy micrite. Prefixes and adjectives can be added to these terms to include dolomitic sediments, conglomeratic textures, and biogenic constituents.
Isolated stationary carbonate platforms: the Middle Triassic (Ladinian) of the Marmolada area, Dolomites, Italy
Authors: BLENDINGER WOLFGANG
Published: June 2006

As a result of a phase of extensional tectonics in the western Tethyan region, a horst and graben topography formed during the Middle Triassic (Ladinian) in northern Italy. Horsts were sites of shallow water carbonate sedimentation, while pelagic and volcaniclastic sediments were deposited in the grabens. Two carbonate platforms approximately 500 m thick can be distinguished in the Marmolada area of the Dolomites: the Marmolada platform proper, which covered an area of 6 km2, and the Costabella platform, which extended for about 12 km in a NW‐SE direction and was about 3 km across.

The facies of these isolated platforms reflect the influence of storms from the SW. Windward platform margins were characterized by a marine sand belt of skeletal and aggregate grainstones with a dominant platform directed cross‐stratification. The central portions of the platforms were occupied by supratidal sand cays which are made up of storm washovers. Leeward parts of the platforms are composed of shallow subtidal sand flat deposits. Laterally discontinuous reefs chiefly composed of various calcareous algae are developed at the outer margins of the platforms. Along windward margins, reefs may form a belt several hundred metres wide; along leeward margins their width is commonly reduced to some tens of metres. Foreslope talus breccias surround the platforms. Clinoform bedding showing basinward dips of 30°‐40° is typical of this facies belt, which is approximately 2 km wide. Basinal sediments, only some tens of metres thick, are radiolarian micrites. Abundant sediment‐gravity‐flow deposits expand the basinal sequence at the toes of windward margins and were probably triggered by storm return flows.

Synsedimentary faults striking both NNE‐SSW and NW‐SE separate the bedded platform limestones from flank deposits and reefs. They account for the stationary nature of the platforms. Neptunian dykes show preferred NNE‐SSW and E‐W trends. Sinistral displacements are associated with NW‐SE trending faults. Depressions in the basins, filled with red, turbiditic pelagic sediments, show N‐S trends and are probably compressional in origin. The structural pattern may have resulted from oblique, NW‐SE oriented extension of the E‐W trending Middle Triassic graben zone of the Dolomites.

In the Ladinian of the Dolomites, the stationary platform type can be distinguished from a retrograding type, whereas continuously prograding platforms apparently did not develop.
Sedimentation during carbonate ramp‐to‐slope evolution in a tectonically active area: Bowland Basin (Dinantian), northern England
Authors: GAWTHORPE ROB L.
Published: June 2006

The Bowland Basin (northern England) contains a series of carbonates and terrigenous mudstones deposited during the Ivorian to early Brigantian. Two regional depositional environments are indicated by facies and facies associations. Wackestone/packstone and calcarenite facies indicate deposition in a carbonate ramp environment, while lime mudstone/wackestone, calcarenite and limestone breccia/conglomerate facies, often extensively slumped, represent a carbonate slope environment. Stratigraphic relations suggest that the depositional environment evolved from a ramp into a slope through the Dinantian.

Two main sediment sources are indicated by the sequence; an extra‐basinal terrigenous mud source and a supply of carbonate from the margins of the basin. Deposition from suspension and from sediment gravity flows, in situ production and remobilization of sediment during sedimentary sliding were important processes operating within the basin.

Periods of enhanced tectonic activity in the late Chadian to early Arundian and late Asbian to early Brigantian are indicated by basin‐wide horizons of sedimentary slide and mass flow deposits. Both intervals were marked by a decline in carbonate production resulting from inundation and uplift/emergence. The first of these intervals separates deposition on a seafloor with gentle topography (carbonate ramp) from a situation where major lateral thickness and facies variations were present and deposition took place in a carbonate slope environment. The second interval marks the end of major carbonate deposition within the Bowland Basin and the onset of regional terrigenous sedimentation.