bluuebunny/crossref_metadata_2025 huggingface dataset:
Burial dolomitization of the Woo Dale Limestones Formation (Lower Carboniferous), Derbyshire, England
Authors: SCHOFIELD KEVIN , ADAMS ANTHONY E.
Published: June 2006

Dolomite occurs in open‐shelf packstones and grainstones of the Vincent House Member of the Woo Dale Limestones Formation (Lower Carboniferous) in the Wye Valley area, east of Buxton, Derbyshire, Central England. Two stages of dolomitization can be recognized. Stage 1 consists of limpid, non‐luminescent dolomite lacking detectable iron and manganese. Stage 2 comprises cloudy, inclusion‐rich, orange‐red luminescing dolomite with significant iron and manganese. The contact between the two stages is irregular and suggests a hiatus between dolomite growth stages. Dolostones lacking any unreplaced limestone may contain up to 35% calcite cement rilling intercrystal porosity. This cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals. In some cases, precementation compaction of the dolomite mosaic caused fracturing of the crystals. Sherds of dolomite on the floors of pore‐spaces then provided the nucleus for growth of calcite cement.

The petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth, during burial of the Woo Dale Limestones in the Late Carboniferous. Dolomitizing solutions, expelled from basinal shales, moved up‐dip along permeable limestones or along the sub‐Carboniferous unconformity. Fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the Carboniferous Limestone in the Wye Valley area.
Facies sequences on a carbonate ramp: the Carboniferous Limestone of South Wales
Authors: WRIGHT V. PAUL
Published: June 2006

During early Carboniferous times a major sea‐level rise led to the development of an extensive carbonate ramp over what is now South Wales. Differential subsidence and sea‐level changes resulted in distinctive facies sequences in the ramp succession and a model is offered which recognizes three distinct geomorpho‐tectonic settings; inner, mid‐ and outer ramp. The inner ramp zone occurs in the more landward part of the province and was an area undergoing little or no subsidence. The sequence is dominated by oolitic grainstones and peritidal limestones representing shoal and back shoal environments. The peritidal units are transgressive deposits consisting of stacked asymmetrical shallowing‐up cycles. The sequence contains many subaerial breaks and tectonic uplift resulted in base‐level changes and fluvial incision. The mid‐ramp zone sequence is intermediate in thickness between the inner and outer ramp successions and consists mainly of bioclastic limestones deposited below fairweather wave base. Sedimentation periodically exceeded sea‐level rise and subsidence, and regressive (progradational) oolitic sand bodies developed, the thickest of which are stacked units with up to four individual sand bodies. Storm processes were of major importance in this setting. The outer ramp zone is represented by a thick sequence of muddy bioclastic limestones deposited below storm wave base and major Waulsortian reef‐mounds also developed. None of the shallowing phases seen in the other ramp zones can be detected in this sequence. Subsidence and eustatic sea‐level rise seem to have been the major controls on deposition but the recognition of eustatic sea‐level falls is difficult. The detailed facies model for ramp carbonates presented here may be applicable elsewhere in the geological record.
A storm and tidally‐influenced prograding shoreline—Upper Cretaceous Milk River Formation of Southern Alberta, Canada
Authors: McCRORY VERNON L. C. , WALKER ROGER G.
Published: June 2006

The Santonian‐Campanian Milk River Formation of Southern Alberta represents the transition from an open shelf, through a storm‐dominated shoreface into a non‐marine sequence of shales and sandstones, with coal. The open shelf deposits consist of interbedded bioturbated mudstones with sharp‐based hummocky cross‐stratified sandstones. There are no indications of fairweather reworking of the sandstones, which are therefore interpreted as having been deposited below fairweather wavebase. The shoreface sequence consists of a 28 m thick sandstone. It has a very sharp, loaded base, and is dominated by swaley cross‐stratification, a close relative of hummocky cross‐stratification. Angle of repose cross‐bedding is preserved in scattered patches only in the top 5 m of the sand body. Channels up to 180 m wide and 7 m deep are cut into this sand body, with channel margins characterized by lateral accretion surfaces. Regional dispersal trends, as well as local palaeocurrent readings suggest flow toward the NW. Within the channels there is some herringbone cross‐bedding and at least two examples of neap‐spring bundle cycles, suggesting that the channels are tidally‐influenced. Above the channels there is a sequence of carbonaceous shales with in situ root casts and lignitic coal seams. No marine, brackish or lagoonal fauna was identified, and the sequence appears to represent a distal floodplain.

The sequence from interbedded hummocky cross‐stratified sandstones and bioturbated mudstones into a 10–20 m thick, sharp‐based shoreface sandstone characterized by swaley cross‐stratification is uncommon. The scarcity or absence of angle of repose cross‐bedding in the shoreface, and the dominance of swaley cross‐stratification suggests that the shoreface was so storm‐dominated that almost no fairweather record was preserved. Other examples of swaley cross‐stratified shorefaces are reviewed in the paper.
Facies sequences of a semi‐arid closed basin: the Lower Jurassic East Berlin Formation of the Hartford Basin, New England, U.S.A.
Authors: DEMICCO ROBERT V. , KORDESCH ELIZABETH GIERLOWSKI
Published: June 2006

The Lower Jurassic East Berlin Formation exposed in the centre of the Hartford Basin can be divided into six facies: (1) laminated black mudstone is composed of very finely‐laminated, organic‐bearing clay‐stone with common millimetre‐scale lenses of dolomitic siltstone; (2) planar laminated mudstone is commonly mudcracked and composed of thickly‐laminated, red, green or grey mudstone with common centimetre‐scale lenses of sandstones; (3) disrupted mudstone has a complex, desiccation‐cracked fabric; (4) planar‐ and large‐scale trough cross‐stratified sandstones are composed of moderately well‐sorted medium‐ to coarse‐grained arkoses; (5) small‐scale, cross‐stratified silty sandstones with common climbing‐ripple structure; and (6) interbedded sandstones and mudstones which commonly carry desiccation cracks.

Mudstone facies are organized into repetitive, metre‐scale facies sequences which change gradationally upwards from laminated black mudstones to planar‐laminated mudstones to disrupted mudstones. Facies sequences have sharp tops and bottoms and record increasing desiccation upwards. There are 15 such cycles in the upper 100 m of the formation in central Connecticut. They record long periods of dry playa mudflat aggradation punctuated by the rapid expansion and contraction of perennial lakes.

The sandy facies occur as single, decimetre‐scale sedimentation units or as two or more stacked sedimentation units up to 1 m thick. These record sheet floods across ephemeral floodplains.
A sedimentation pattern in Anglian marginal meltwater channels from Suffolk, England
Authors: MATHERS S. J. , ZALASIEWICZ J. A.
Published: June 2006

Several infilled glacial meltwater channels of Anglian age are described from SE Suffolk, U.K. These channels are up to 4km long, 150m wide and 8m deep. They possess a consistent pattern of infill comprising glacifluvial sands and gravels overlain by subglacial water‐lain fine sands and silts; these pass conformably upwards through thinly interstratified tills and sands into basal tills. The channels were formed at the ice‐margin. Their sedimentary infill indicates overriding by the ice‐sheet and subsequent subglacial deposition.
Ooids in Purbeck limestones (lowermost Cretaceous) of the Swiss and French Jura
Authors: STRASSER ANDRÉ
Published: June 2006

ABSTRACT Ooids occurring in the shallow‐water Purbeckian carbonate sediments of the Jura mountains can be grouped into six types. Gradations from one type to another and coexistence of the various types are common.

Type 1 ooids are small and well rounded. They display fine concentric micritic laminae. In many cases their cortices are dissolved and replaced by void‐filling spar. Microsparitic neomorphic replacement occurs locally. Type 2 ooids are large and have irregular shapes. They show fine micritic laminae and occasional layers of fine‐radial crystals. They commonly evolve into oncoids. Ooids of type 3 display many fine‐radial cortical laminae and are patchily micritized. They are medium in size and mostly well rounded. This type of ooid may pass into large, irregularly shaped coated grains. Type 4 ooids have 1 to 4 cortical laminae with a fine‐radial structure and patchy micritization. They are medium in size and well rounded. Type 5 ooids have only one lamina with a coarse‐radial structure. They are small and well rounded. Associated are spherical grains containing bundles of elongate crystals. Ooids of type 6 show superpositions of two or more different, radial and or fine micritic laminae. The cortical structure may also change laterally in the same lamina.

The preferential dissolution of type 1 ooid cortices to form oomoulds indicates a primary composition of unstable carbonate. Sedimentological features and comparison with modern ooid occurrences point to formation on high‐energy sandbars in normal‐marine waters. Type 2 ooids grew in marine‐lagoonal environments with quiet water and abundant cyanobacteria. The radially structured ooid cortices of types 3, 4 and 5 show no dissolution features. This implies that they were originally composed of stable carbonates, or that an unstable carbonate phase was transformed into a stable one at an early stage of diagenesis. Type 3 ooids occur together with marine faunas and indicate high water energy. Ooids of type 4 and type 5 originated probably from relatively quiet water of variable salinity.

Coexistence of different ooid types and mixed forms of type 6 implies gradual or rapid changes in hydrodynamic, geochemical and microbiological conditions which were a feature of the Purbeckian depositional environments.
Sample site variation in settling velocity distribution subpopulations using curve dissection analysis
Authors: BRYANT EDWARD
Published: June 2006

ABSTRACT It is generally believed that grain‐size distributions can be broken down into subpopulations which may be characteristic of a specific mode of transport, some source signature, or the nature of sediment packing in the depositional process. This paper evaluates the variation at a sample site of subpopulation parameters of settling velocity distributions derived using a non‐linear least squares algorithm for curve dissection. Results incorporate artificial samples constructed using spherical glass beads in the 125‐840 μm range and natural samples collected as close as possible from a single sedimentation unit in marine, fluvial and aeolian environments. If subpopulation distributions from the same sample site are compared then 90% of artificial bead samples and 65% of natural sands have cumulative curves diverging from each other by less than 20%. Similarity in means and standard deviations of subpopulations collected at the same site was not easily obtained. However, if it is important to be able to identify a specific process or source signature correctly, then 86% of all subpopulations from the same natural sample site grouped together. If subpopulations of grain‐size distributions are to be used for process identification, it is advisable to analyse more than one sample from a site.
A comparison of some rapid techniques for the measurement of density in soft sediments
Authors: HILTON J. , LISHMAN J. P. , MILLINGTON A.
Published: June 2006

ABSTRACT Four different methods of measuring density in soft sediments have been compared with the density bottle method. Measurement of the wet sediment volume was found to be the step which limited accuracy. Use of open calibrated rings during the slicing of a core overestimated the volume. This problem could be avoided if a lid was placed on the ring during slicing, but volume measurement could still be biased by the presence of gas bubbles. Measurement of the volume of sediment after dispersion in a liquid medium was found to overestimate the volume unless the bias, introduced by difficulties in reading the final meniscus level, was subtracted from each measured volume. A cut off syringe was shown to be capable of estimating the volume of wet sediment with a high precision if reasonable precautions were taken.
Patterns of sediment composition of Jamaican fringing reef facies
Authors: BOSS STEPHEN K. , LIDDELL W. DAVID
Published: June 2006

Recent carbonate sediments from Jamaican north coast fringing reefs were collected along three parallel traverses in the vicinity of Discovery Bay. Each traverse extended from near shore across the back reef, reef crest, and fore reef to a depth of 75 m. Relative abundances of the biotic constituents vary between sites, reflecting general patterns of reef community composition. The sediment is dominated by highly comminuted coral fragments (27·1% to 63·1%), plates of the calcareous green alga Halimeda (0·4% to 38·7%), coralline algae (4·7% to 16·2%) and the encrusting foraminiferan Homotrema rubrum (0·7% to 9·5%), with lesser amounts of other taxonomic groups (non‐encrusting foraminifera 1·3–5·5%; molluscs 1·4–7·0%; echinoderms 0·9–5·0%). Coral fragments, coralline algae and particles of Homotrema rubrum dominate the sediments of the shallow portions of the fore reef (5–15 m), whereas plates of Halimeda are most abundant in sediments from the back reef and deeper portions of the fore reef ( 24 m). Q‐mode cluster analysis, using sediment constituent data, resulted in the delineation of four reef biofacies over the depth range of this study (1–75 m).
Variations in the surface texture of suspended quartz grains in the Loire River: an SEM study
Authors: MANICKAM S. , BARBAROUX L.
Published: June 2006

Statistical size distribution and scanning electron microscopic studies of suspended sand grains in the Loire River at Montjean, France were carried out over a period of a year (hydrological cycle) to discern seasonal variations. The sand fraction in suspension is better sorted during winter (average mean = 0.69 mm, median = 0.65–0.95 mm, sorting value, σ= 1.1–1.35) and is dominated by quartzo‐feldspathic minerals. During summer, on the contrary, this fraction is rich in mica minerals and is poorly sorted (average mean = 1.21 mm, median = 0.33–0.95 mm, sorting value, σ= 1.4–2.0). The grain size of the coarser fraction shows a tendency to increase with river discharge up to an optimum discharge of about 1000 m3 s‐1 and thereafter decreases.

Scanning electron photomicroscopy of quartz grains from the suspended sand population indicates that both mechanical and chemical features occur. The former (conchoidal fractures, mechanical fractures and breakage) dominate in the samples collected near the water surface during winter floods and the latter (solution pits, vermicular features, silica flowers, neogene silica and diatoms) in the samples from the bottom during low summer flows. During summer, chemical action takes place on sediments prior to or upon their deposition. Sediments that are resuspended during winter floods undergo mainly physical processes (attrition/abrasion). Mixed surface features are, therefore, observed during average and low river discharge. Inheritance of these surface features from the source area is, however, not completely excluded.

Thus, the history of quartz grains in suspension can be reconstructed from SEM exoscopic studies: therefore, it can be proposed that the fluvial quartz grain surface textures result from a combination of alternating chemical and physical processes.
Depositional environments of marine‐dominated bedded halite, Permian San Andres Formation, Texas
Authors: HOVORKA SUSAN
Published: June 2006

The interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology. Facies studies, supplemented by geochemical data, define an evaporite shelf depositional environment for the halite of the San Andres Formation of the Palo Duro Basin, Texas Panhandle, that is significantly different from modern and previously described ancient halite‐precipitating environments such as playas, salinas, sabkhas, and barred basins.

A coastal palaeoenvironmental setting for the San Andres halite is defined by its facies position between normal marine‐shelf carbonates and aeolian deposits. The San Andres brine pool extended over more than 10 000 km2 but was very shallow. Perennial brine‐pool conditions (thick sequences of halite) alternated with ephemeral brine‐pool conditions (halite, haloturbated mudstone‐halite, mudstone interbeds, and microkarst). The depositional sequence in the perennial brine pool includes (1) flooding and minor dissolution of previously deposited halite, (2) precipitation of a thin lamina of gypsum, and (3) evaporation and precipitation of halite. Bottom‐growth forms of halite, including chevrons and vertically elongated crystals, are well preserved. Cumulates of foundered, floating crystals and rafts were probably abundant but have been almost completely recrystallized.

Mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine‐pool deposition and the establishment of subaerial conditions. Vertically elongated karst pits originate at the palaeosurfaces, cross‐cut primary brine‐pool fabrics, and are filled with geopetal sediment and coarse halite cement. Mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity. The geochemistry of halite from both ephemeral and perennial brine‐pool environments reflects the frequent influx of marine brine, indicating that communication with the marine environments to the south was ‘good’ considering that restriction was sufficient to precipitate halite.
Upper Cretaceous Muti Formation: transition of a Mesozoic nate platform to a foreland basin in the Oman Mountains
Authors: ROBERTSON A. H. F.
Published: June 2006

The Upper Cretaceous (Turonian‐Campanian) Muti Formation (Sayja Member) documents the transition from a passive continental margin to a foreland basin, related to overthrusting of continental margin and ophiolitic nappes derived from the Tethys ocean. Upper Cretaceous northeastward subduction culminated in collision of a trench with the Arabian margin. As the trench docked with the margin the lithosphere was flexed, forming a peripheral bulge that migrated cratonward with time. The platform edge was initially uplifted (Turonian) and deeply eroded, creating the ‘Wasia‐Aruma break’. After passage of the peripheral bulge subsidence began, with accumulation first of ferruginous crusts on hardgrounds. Lime‐muds were then deposited on a deepening unstable sea‐floor, along with phosphatic nodules and crusts (Turonian‐Coniacian). Passage of the overthrust load over the Arabian continental‐margin edge downflexed the lithosphere (Santonian‐Campanian), resulting in drastic foundering of the old shelf edge to form a foredeep. Upper platform horizons collapsed as slump‐sheets and debris‐flows. Limestone blocks and lithoclastic debris‐flows were shed by mass‐wasting of the already deeply eroded old platform edge. Mud and silt were derived from the uplifted Arabian continent and deposited by mainly gravitational processes in a foredeep below the C.C.D. Subsidence of the Arabian platform edge allowed the Semail ophiolite nappe finally to override the Muti basin (late Campanian) with little internal deformation. Submarine emplacement is suggested by the absence of ophiolitic detritus in the Muti Formation. The stratigraphic evolution of the Muti Formation is in good general agreement with a model of the transition of an old, thermally mature, passive continental margin to a foreland basin, where the emplaced load is submerged.
Reverse flow in turbidity currents: the role of internal solitons
Authors: PANTIN H. M. , LEEDER M. R.
Published: June 2006

Pickering & Hiscott, (1985) have demonstrated amply the presence of reverse‐flow units within the thick‐bedded calcareous wacke (TCW) beds of the turbiditic Cloridorme Formation (Middle Ordovician, Gaspé Peninsula, Quebec, Canada). These reverse‐flow units are underlain and overlain by units which reveal flow in the primary (obverse) direction.

In this paper, a model is proposed for this reverse flow, based on the probable nature of the primary turbidity flow. It appears that the initial flow was highly elongated (thickness h≪ length L), with h∼ 500 m, velocity U∼ 2 m s‐1 and sediment concentration C∼ 1·25%o. The rate of momentum loss of the flow is estimated by means of a useful parameter which we call the ‘drag distance’, symbol dD, defined by

image

where h and L are the thickness and length of the flow, respectively; cCd is a combined drag coefficient representing friction on the bottom and at the upper interface; and fCd is a form‐drag coefficient related to the shape and size of the head. dD is the distance travelled by a current of constant h and L, flowing over a horizontal bottom and obeying a quadratic friction law, for an e‐fold reduction in velocity.

Simple considerations, confirmed by our own experiments (described in this paper), show that such an elongated turbidity current cannot be reflected as a whole from an adverse slope: when the nose of the current reaches the slope, it forms a hump, which surges backwards and sooner or later breaks up into a series of internal solitons. The latter, probably numbering 4–7, will cause reverse flow at a given point as they pass by, provided that the residual velocity in the tail is not too great. Flow in the original (obverse) direction will be re‐established after the passage of the solitons. Quiescent periods in front of, between and behind the solitons, when soliton‐associated currents cancelled out the residual obverse flow, would allow the deposition of thin mud‐drapes.

Additional flow reversals observed in a few of the TCW beds cannot be explained readily by the re‐passage of solitons, since wave breaking at the ends of the basin would cause massive energy loss; internal seiches are the preferred explanation for these later reversals.
Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub‐basin and the North Sea Central Graben
Authors: JØRGENSEN NIELS OLUF
Published: June 2006

The oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub‐basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%o to +3.00%oδ13C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near‐surface drilled sections fall in the range from —0.50%o to — 2.00%oδ18O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub‐basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea.

The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature‐induced isotopic re‐equilibration in the water‐rock system. In these sequences the oxygen isotope values range from − 1.50%o to — 7.50%oδ18O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure‐dissolution, recrystallization, reprecipitation and ion‐exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re‐equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore‐water controlled isotopic re‐equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re‐equilibration process increasingly becomes influenced by the rock‐introduced isotopic change in the composition of the formation water.
The use of burrows to detect compaction and sliding in fine‐grained sediments: an example from the Cretaceous of S.E. France
Authors: GAILLARD CHRISTIAN , JAUTEE ERIC
Published: June 2006

In the French Southern Alps, alternating pelagic limestones and marls of Hauterivian age contain a dense network of burrows. Differences between the original and preserved forms of the burrows provide evidence of sediment deformation. (a) Total compaction, from early dewatering of sediment to deep burial compression, caused flattening of burrows parallel to bedding. Its relation to carbonate is measured, and a tentative mathematical relation is proposed, (b) Synsedimentary slumping is shown by total disordering of the bioturbation network in the displaced layers. (c) A more characterized deformation is occasionally visible only on the upper part of some limestone beds. The sliding is indicated by the dip of burrows in the same direction as the flow. (d) Similar embryonic flows occur in the middle of rare limestone beds. It is thus suggested that this can account for some of the double beds that occur in pelagic alternations of calcareous and marly beds.
Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control
Authors: EMEIS K.‐C. , RICHNOW H.‐H. , KEMPE S.
Published: June 2006

Hydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO2 partial‐pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO2 partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO3 as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits.
Aeolian saltation threshold: the effect of density ratio
Authors: IVERSEN JAMES D. , GREELEY RONALD , MARSHALL JOHN R. , POLLACK JAMES B.
Published: June 2006

Saltation threshold data from three wind tunnels and from hydraulic flumes are presented to show that the dimensionless threshold friction speed for small particles is a continuous function of particle‐to‐fluid‐density ratio. In addition, the dimensionless threshold speed is a function of the grain‐friction Reynolds number and an interparticle force term. The variation with density ratio seems to be due to the relative energy with which particles impact other particles to initiate saltation.
The anatomy of a deep water mud‐mound complex to the southwest of the Dinantian platform in Derbyshire, UK
Authors: BRIDGES PAUL H. , CHAPMAN AMANDA J.
Published: June 2006

ABSTRACT

An elongate Waulsortian mud‐mound complex developed at Dovedale on a ramp to the southwest of a developing carbonate platform in Derbyshire during Chadian (early Viséan) times. The complex occupied an area of approximately 6 km2 and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin.

Five mound associated facies have been identified: mound core, mound flank (fine), mound flank (coarse), intermound (fine) and intermound (coarse). The mound core facies is a massive skeletal wackestone with comminuted sponge debris, foraminifera, ostracodes and crinoid debris set in a matrix of clotted micrite. The mound flank sediments display moderately inclined bedding surfaces. While the mound flank (fine) contains sponge debris, the mound flank (coarse) is dominated by articulated crinoid columnals, and includes algal‐encrusted micritized intraclasts and coarse peloids. The well‐bedded intermound (fine) facies is bituminous and micritic while the intermound (coarse) facies is composed of skeletal‐peloidal‐intraclast grainstones which locally contain calcified algae.

Although the fauna is diverse, the density of colonization by metazoans was low and the supply of macrofossil debris modest. The clotted micrite texture is interpreted as the product of micro‐organisms which precipitated and trapped fine‐grained sediment. The mud‐mound complex is dominated by the bathymetric assemblages B and C proposed by Lees, Hallet & Hibo which on their model of the Belgian Waulsortian, indicate depths of between 220 and 280 m. Intercalation of assemblages B/C and C/D on the northern margin of the complex is interpreted as the result of local storm disturbance. A deep water drift is postulated to explain the NW‐SE alignment of the complex which probably fitted the ‘export model’ of Bosence, Rowlands & Quine. Beneath the sediment surface, phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy. Mound instability triggered the opening of fissures which filled with crinoid debris, peloids, indurated lithoclasts and micrite.
Sand waves, Echinocardium traces and their bathyal depositional setting (Monte Torre Palaeostrait, Plio‐Pleistocene, southern Italy).
Authors: COLELLA A. , D'ALESSANDRO A.
Published: June 2006

ABSTRACT

Stacked cross‐sets, up to 2.5 m thick, produced by sand wave migration and meniscate trace fossils produced by Echinocardium cordatum, both considered in the literature as typical of shallow‐water marine depositional settings, commonly occur in the bathyal Plio‐Pleistocene deposits of Monte Torre (Calabria, southern Italy).

The Plio‐Pleistocene sediments form two coarsening‐upward depositional sequences, separated by an unconformity and by a palaeobathymetric gap of at least 300 m. The lower sequence passes upwards from hemipelagic marls and thin‐bedded turbidites to thick‐bedded sandy turbidites, then to sand wave deposits alternated with sandy turbidites, and finally to base‐of‐slope megabreccias. Facies characteristics and relationships, and the occurrence of deep‐sea faunal associations, indicate deposition in the bathyal zone. The facies of the upper sequence reflect a fan‐delta environment, no deeper than a few tens of metres.

The depositional setting of the lower sequence, where the sand wave deposits and meniscate trace fossils occur, appears to have been a tectonically controlled seaway, connecting the Tyrrhenian and Ionian Seas. This seaway became progressively narrower with time, evolving into a strait. The overall coarsening‐upward trend reflects the upward transition from a low to a high‐energy environment, possibly caused by the tectonic narrowing of the seaway. Deposition and erosion from high‐concentration turbidity currents and from tidal bottom currents were important processes. Periods of tectonic activity, producing first the uplift of the seaway margins and culminating with the uplift of the strait sequence itself, are marked by‐scattered rockfall deposits.

The strait setting, causing the development of powerful, oxygenated bottom currents, produced optimal conditions in the bathyal zone for the colonization of sandy bottoms by a single infaunal r‐selected species, Echinocardium sp.
Eocene spheroidal dolomite from the western Sirte Basin, Libya
Authors: AMIRI‐GARROUSSI K.
Published: June 2006

Scanning electron micrographs of crypto‐crystalline (< 4 μm) dolomites found in cores taken from the lower Eocene Facha Member reservoir of the Zella and Aswad fields in Libya show that they consist of tiny globular bodies mixed with anhedral crystals, producing a chalky texture. The sedimentary sequence indicates deposition in upper intertidal/supratidal arid environments and in saline lagoons. The crypto‐crystalline dolomite is facies‐controlled and probably formed as a primary precipitate, similar to those reported from present‐day subaqueous environments in the Coorong of South Australia and Holocene supratidal crusts off Florida.

Such chalky‐textured dolomites could form local fluid‐migration barriers within hydrocarbon reservoirs.
Cycle of earthquake‐induced aggradation and related tidal channel shifting, upper Turnagain Arm, Alaska, USA
Authors: BARTSCH‐WINKLER SUSAN
Published: June 2006

Aggradation of intertidal sediment in upper Turnagain Arm, a macrotidal estuary located in southern Alaska near Anchorage, began because of subsidence caused by sediment consolidation and tectonic lowering of the land during the earthquake of March 27, 1964. At the head of the Arm, decrease in percent sand of surface sediments with time and return of rodents, pulmonate gastropods, trees, bushes, and grasses to the intertidal region, indicate that the rate of sediment deposition has slowed and the Upper Turnagain Arm area has returned to a state of preearthquake equilibrium only 17 years after the event. Comparison of channel locations before and after the earthquake indicates the possibility of regional postseismic tilting of the estuary.
A preliminary study of the dynamics of a modern draa, Algodones, southeastern California, USA
Authors: HAVHOLM KAREN G. , KOCUREK GARY
Published: June 2006

A transverse crescentic draa in the Algodones dune field, California, was monitored for a year using surface process mapping, aerial photography and supplemental wind measurement. The draa is oriented by the long‐term resultant wind, whereas its superimposed features are in equilibrium with the bedform‐modified secondary airflow. Surface airflow and the movement of superimposed bedforms is typically oblique or parallel to the draa brinkline, particularly on the lee slope. Comparison of measurements of draa movement and sand deposition on the lee slope, with expected rates calculated from wind data and draa size, confirm that there is a significant component of sand flow parallel to the draa brinkline. The internal structure being generated at the base of the draa lee slope is inferred from the surface processes active there. Within the space of a kilometer two types of compound cross‐strata, separated by an area of simple cross‐strata, are being produced. This has significant implications for interpretations of ancient aeolian strata. Variations in internal structure types found in lateral sequence may be generated by one complex bedform, and these cross‐strata may be simple or compound. Second‐order bounding surface orientations indicate resultant primary palaeowind directions; compound cross‐strata dip directions indicate secondary flow conditions. The existence of cross‐strata dip directions oblique or perpendicular to the second‐order surface indicates longitudinal secondary flow on the lee face, but not necessarily a longitudinal or oblique draa. Without further detailed knowledge about various draa configurations and behaviour, stratification attributed to draas can be used only to interpret activity on the lower draa lee face.
Flow, sediment transport and bedform dynamics over the transition from dunes to upper‐stage plane beds: implications for the formation of planar laminae
Authors: BRIDGE JOHN S. , BEST JAMES L.
Published: June 2006

Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms‐1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near‐bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low‐relief bedwaves on upper‐stage plane beds are ubiquitous and give rise to laterally extensive, mm‐thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves.
Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf
Authors: DOMINGUEZ LAURA L. , MULLINS HENRY T. , HINE ALBERT C.
Published: June 2006

Analyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (<4 m), coarse‐grained, relict sediment cover, along with limited reef development, and a relatively deep (20–30 m) margin indicating that it has been unable to ‘keep‐up’ with Holocene sea‐level rise.

Early flooding at relatively high rates of sea‐level rise (4 m kyr‐1, 5–8 × 103 yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch‐up’ with continued sea‐level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank‐margin reentrants.

In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea‐level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open‐marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs.
Mixed‐water and hydrothermal dolomitization of the Pliocene Shirahama Limestone, Izu Peninsula, central Japan
Authors: MATSUMOTO RYO , IIJIMA AZUMA , KATAYAMA TETSUYA
Published: June 2006

The early Pliocene Shirahama Limestone is a grainstone‐packstone principally composed of fragments of algae, bryozoa, and echinoderm and subordinate volcanic rocks. The limestone was variously dolomitized and the regional distribution of dolomite is patchy. Dolomite occurs as isolated crystals filling pores, moulds, and solution vugs, and mosaic aggregates replacing bioclasts. Calcite occurs as rim and pore‐filling sparry cements, and as calcareous skeletons. Isotopically, the dolomites are classified into a heavy oxygen group (−2 to − 3.5%0 PDB) and a light oxygen group (−5.5 to − 7.5%0 PDB). Calcite associated with heavy oxygen dolomite has δ18O of − 6.5 to −8.5%0 PDB, whereas those associated with light oxygen dolomite have a wide range from −7.5 to −14%0 PDB. Calcite in dolomite‐free limestone has an oxygen isotopic composition of −2 to −8.5%0 PDB. Textures, chemistry, and isotopic evidence indicate that heavy oxygen calcite formed in freshwater, and heavy oxygen dolomite in a meteoric‐marine mixture of 10–30% seawater. Light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30–65°C.

Petrographic features, and both isotopic and chemical evidence suggest that the Shirahama Limestone was exposed to freshwater soon after deposition. Subsequently blocky calcite precipitated (Stage I). The limestone was locally submerged in the meteoric‐marine mixture due to gradual subsidence or eustatic movement. This led to the precipitation of heavy oxygen, zoned dolomite and dolospar (Stage II). Hydrothermal alterations occurred in the area a few Myr ago, and related hydrothermal fluids and mixed meteoric‐hydrothermal waters caused dedolomitization of some zoned dolomite, partial dissolution of vuggy dolomite, precipitation of limpid dolomite and recrystallization of some earlier dolomites (Stage III). Zeolites were also precipitated from these fluids. Finally, the Shirahama Limestone was exposed again to freshwater and sparry calcite precipitated to plug some of the remaining pores (Stage IV).
Erg margin of the Permian White Rim Sandstone, SE Utah
Authors: CHAN MARJORIE A.
Published: June 2006

The Permian White Rim Sandstone of the Canyonlands National Park, Utah, contains a wide variety of sedimentary structures and features that largely result from stages in erg migration and marine influence on an erg margin. Three spatially distinct lithological and depositional facies are recognized and can be distinguished as informal units within the formation. The aeolian dune facies is composed predominantly of fine‐grained cross‐stratified sandstone of the White Rim erg. This facies is the most widespread and comprises the bulk of the formation. Within the aeolian dune facies are small subfacies that represent interdune deposits. A sheet sand facies, composed of parallel‐bedded sandstone, makes up a significant part of the lowest part of the White Rim Formation. This facies appears to have been the precursor or leading (progradational) edge to the main erg system. The final facies is a reworked or veneer facies of rippled to disturbed sandstone that is localized in its extent. It is restricted to the upper few metres of the formation and is transitional in some places to the Triassic Moenkopi Formation. This veneer facies contains many structures which indicate marine reworking as well as periods of desiccation or subaerial exposure.

Some previous interpretations of the White Rim Sandstone have tended to classify the whole formation as one depositional setting. It is clear that at the margin of a sand sea, as shown in the White Rim Sandstone, there are transitional facies due to the interactions with other environments. Additionally, variation in the stratigraphic relationships of facies can be related to stages of erg migration. Erg margin deposits preceded central erg development. Erg initiation occurred during a probable relative sea level low. Sea level influence is recorded at the top of the formation because erg termination accompanied a relative sea level high with cut‐off of sand supply. Transgression of the Permian Kaibab Sea over the White Rim erg was probably the main process in preservation of original dune topographic relief. Sea level fluctuations also may have affected distribution of facies and the complexities of structures at the erg margin. Subsequent fluvial reworking of the veneer facies may have obliterated Late Permian features during lowest Triassic Moenkopi deposition.
Textural characteristics and genesis of the aeolian sediments in the Kuwaiti desert
Authors: KHALAF FIKRY
Published: June 2006

Several types of aeolian deposits have been recognized in Kuwait: (a) smooth sand sheets that resemble desert floor sand, (b) immobile sands that include rugged vegetated sand sheets and wadi fill deposits, and (c) mobile sands that form active sand sheets and sand dunes. Simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types. The four size parameters, namely, mean size, sorting, skewness and kurtosis, were calculated. Scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands. Active sand sheet deposits can also be recognized because they are usually located between the two end members–smooth sand sheets and dune sands. Size parameters change with location regardless of their types. Coarsening and positive skewness usually increase downwind. Mineralogical and textural characteristics of the aeolian deposits in Kuwait revealed that they are mostly derived from the lower Mesopotamian muddy flood plain deposits, the sand fraction of the Al‐Dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts. Distribution of depositional and deflational areas indicates that the northern desert of Kuwait is characterized by a positive sand budget, whereas the southern desert has a negative sand budget.