bluuebunny/crossref_metadata_2025_split huggingface dataset:
Isotope geochemistry, petrology and depositional environments of the diatomite‐dominated Tripoli Formation (Lower Messinian), Sicily
Authors: BELLANCA A. , CALDERONE S. , NERI R. 
Published: June 2006

ABSTRACT The Tripoli Formation (Lower Messinian) in Sicily includes diatomites irregularly alternating with marl and carbonate beds and lies, stratigraphically, between the Tortonian pelagic marls and the evaporitic Calcare di base. The relationships between mineralogy, textural features and oxygen‐carbon isotopic compositions of carbonate components point to a wide variability of depositional conditions and suggest that Tripoli sedimentation occurred in small basins characterized by periodic and marked restriction from the open sea.The isotopic values of calcite and dolomite in the diatomites suggest an evolution from normal marine towards more restricted environments. Evaporating conditions are also indicated by the occurrence of anhydrite, length‐slow chalcedonic quartz and moulds of gypsum. In a more advanced stage, the precipitation of heavy δ180 dolomite in the interstitial pores of fossil‐poor diatomites denotes an environment with highly evaporated water. Mixing of meteoric and marine waters, on the other hand, might have favoured the precipitation of a dolomite characterized by relatively low δ180 and δ13C values.The deposition of marl and carbonate beds alternating with or overlying the diatomites took place in an environment with highly evaporated marine waters on the basis of δl18O values of dolomite (up to + 9.10‰) and aragonite (up to + 5.83‰), occurrence of evaporitic minerals and lack of fossils. The presence at these levels of calcite with extremely negative δ13C values (down to ‐ 38.40‰), filling gypsum moulds, suggests activity of sulphate‐reducing bacteria. Some aragonitic marls, however, bear evidence of deposition in relatively normal marine conditions.
Sedimentological effects and palaeoecological implications of the tube‐building polychaete Lanice conchilega Pallas
Authors: CAREY DREW A. 
Published: June 2006

ABSTRACTThe tube‐building polychaete Lanice conchilega forms intertidal mounds in association with macroalgae. This assemblage produces structures that can record tidal and seasonal cycles in the stratigraphic record. They mark low‐tide levels because the assemblage occurs below neap low tide and many of the structures form when water drains off the assemblage.Mounds are created by disturbance of the sediment surface around aggregations of the assemblage and accumulation of sediments within the aggregations. The initial aggregations may be relict patches of adult worms, new clumps of juveniles or both. Juvenile worms and drift algae easily settle and survive in high‐density patches, whereas predators and waves more readily disturb low‐density patches. Algae and tubes extend through accumulated sediment and create a new, higher surface. Through this interactive process the high‐density patches increase height rapidly while the low‐density patches erode. Regardless of density, when the initial distribution is regular the surface remains flat, rising evenly or eroding evenly. Accumulation and erosion increases during spring tides and decreases during neap tides.Mound development also follows a seasonal pattern. The recruitment of juvenile worms in spring facilitates algal settlement and initiates mound buildup. The rate of tube‐building and algal growth increases in early summer as the erosive forces of storms decline. This leads to the greatest development of mounds in late summer just before the macroalgae begin to die. The loss of algae coincides with autumnal storms, and causes catastrophic erosion of the mounds into relict patches of dormant tube‐worms. Patches of tubes that survive the winter enhance spring recruitment and renewal of mound development.The shape of the mounds is often obscured during burial and preservation. Because of this, it may be difficult to distinguish the original form of ancient mounds without contemporaneous cementing organisms. But cycles of deposition and erosion are recorded in Lanice tubes and other biogenic structures. These structures can be used in conjunction with physical structures to define tidal height, seasonality or current regime. In general however, the presence of tube aggregations per se is not diagnostic of a specific hydraulic environment.
Interference phenomena between particle flattening and particle rounding in free vertical sedimentation processes
Authors: GOOSSENS DIRK 
Published: June 2006

ABSTRACTIn this paper interference phenomena between particle flattening and particle rounding during the free vertical sedimentation of natural particles are investigated. Particle flattening is measured by the Corey Shape Factor (CSF), particle rounding by the Powers Index (P). Using Dietrich's equation, the effect of the flattening and the effect of the rounding on the terminal fall velocity of the particle can be studied separately. This is done by examining the influence of particle flattening and particle rounding on the Particle Sedimentation Coefficient (PSC), which is the ratio of the nominal diameter Dn of the particle to the equivalent sedimentation diameter Db. It appears that particle flattening and particle rounding are interdependent so that interference phenomena occur. In the case of positive interference, the combined effect of flattening and rounding on the PSC is larger than the sum of the effects of both parameters separately. If, on the other hand, the combined effect is smaller than the sum of the single effects, negative interference occurs.For fine‐grained particles, the more the shape of a particle differs from a sphere in one variable (flattening or rounding), the higher will be the influence of the other variable on the free vertical fall. For coarse‐grained particles, the inverse occurs. For very fine‐grained particles (Dn < 200 μm (air) or < 370 μm (water)) the influence of particle rounding on free vertical settling is negligibly small. For coarser grained particles, flattening is the most important parameter if the grains are strongly flattened. If not, rounding will be the governing parameter.
Quantitative X‐Ray diffraction of carbonate sediments: mineralogical analysis through fitting of Lorentzian profiles to diffraction peaks
Authors: MARTINEZ B. , PLANA F. 
Published: June 2006

ABSTRACTA new automatic method for quantitative mineralogical analysis of carbonate sediments by X‐Ray diffraction is described. Percentage analysis has been carried out by means of the reference intensities method from integrated areas of the diffraction peaks. Diffraction peaks were fitted by means of Lorentzian profiles. The addition of an internal standard allows the determination of data on the stoichiometric disequilibrium of the carbonate phases and the existence of non‐carbonate phases and/or non‐crystalline phases simultaneously. Relative errors of the quantitative estimates of carbonate phases are always less than 15%, even though real percentage differences between actual and calculated values are less than 3·8% in all cases.
Ultra‐fine structures and genesis of the Campanian Negev high‐grade phosphorites (southern Israel)
Authors: SOUDRY DAVID 
Published: June 2006

ABSTRACTUltra‐fine structures of the Negev high‐grade phosphorites provide valuable clues to unravelling the genesis of these rocks, the question of their differential areal distribution, and the biosedimentary mechanisms involved in their considerable enrichment. Peculiar to these phosphorites is an intergranular phosphate matrix, for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro‐organizations. The phosphate particles fixed by this filamentous meshwork also consists of internally organized and non‐organized packed microbial remains of different types.Analogies between the fabrics of the matrix and the corpuscles lead to conception of a two‐stage depositional scheme for these phosphorites, based on rhythmical repetition of two sedimentary mini‐events, in a slightly oscillating, very shallow marine system—(1) a low‐energy event of microbial colonization of the Mishash bottoms, followed by early phosphatization of the organic structures mainly in marginal situations; and (2) a higher‐energy event which broke up the phosphatic mats into debris, redepositing them as clastic layers in nearby basinward sites, while becoming bound by a new meshwork of filamentous microphytes. The differential phosphatization of the intergranular microbial binder, again occurring mostly in marginal localities, produced highly enriched phosphorites. Minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks. Examining the structural and textural features, the validity of the sedimentary mechanisms of Recent phosphorite formation for the Campanian Negev rocks is discussed.
A numerical technique for analysing pebble‐form variations, applied in Loch Earn, Scotland
Authors: MCMANUS JOHN , ALRASOUL ALI H. A. 
Published: June 2006

ABSTRACTThe lack of a suitable method of comparing the form assemblages of pebbles in naturally occurring populations was identified during studies of the shorelines of Loch Earn. In the absence of a precise intercomparison system between the verbal descriptors, a numerical method is introduced based on the Sneed & Folk (1958) grid. This permits mean, single and double pebble‐form field scores to be calculated. Average form values for ‘compactness’, ‘platiness’ and ‘elongateness’ are computed from rotations of the grid, and simple numerical values are derived to describe mean pebble populations. The technique enables some quantification of pebble‐form diversity. Each form of analysis is demonstrated using real data from lake shorelines.
Diagenetically enhanced bedding in argillaceous platform limestones: stratified cementation and selective compaction
Authors: BATHURST ROBIN G. C. 
Published: June 2006

ABSTRACTA field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the U.K. Lithologies include grainstones, packstones, wackestones and lime mudstones. The rocks are of Silurian, Carboniferous and Jurassic ages. Additional information was obtained from other limestones in the U.K., the U.S.A., Canada and continental Europe. The beds are parts of sequences composed of couplets of strata,fissile limestonesalternating withhard limestones.In the fissile limestones the effects of mechanical compaction and pressure‐dissolution have been concentrated, whereas in the hard limestones they are minimal or absent. Bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe. The features produced by pressure‐dissolution are dissolution seams and fitted fabric: there are no stylolites. The original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed. The vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent.It is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed. Thenceforth, mechanical compaction and then pressure‐dissolution were concentrated in the less cemented strata: these became the fissile limestones. Pressure‐dissolution acted late in the diagenetic history and provided only an insignificant part, if any, of the carbonate for cementation.It is concluded that the orientation of beds (couplets) is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously. Similar couplets in platform limestones of the Mississippian and Pennsylvanian of the U.S.A. extend over thousands of square kilometres.The signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected. The presumed order of events was (1) accumulation of carbonate sediment, terrigenous clay and organic matter, (2) hydrodynamic reworking and bioturbation. the latter finally overprinting the former, (3) selective cementation of the more carbonate‐rich strata yielding couplets, each consisting of a relatively well‐cemented stratum and a poorly cemented stratum, (4) mechanical compaction concentrated in the less cemented strata, (5) pressure‐dissolution concentrated in the same strata.
Middle and Late Quaternary depositional sequences and cycles in the eastern Mediterranean
Authors: MURAT A. , GOT H. 
Published: June 2006

ABSTRACTAlthough most of the Quaternary sediments of the eastern Mediterranean appear reworked, extensive areas of hemipelagic sediments occur, mainly in the central part of the basin (Mediterranean Ridge). The most obvious character of these muddy sediments is the numerous changes in colour. They result from the organic carbon content as well as from concentration and chemical form of metallic elements (Mn, Fe, Cu, Zn). The succession of coloured layers is arranged into sequences developed between two sharp contacts; a complete sequence involves five lithofacies, from an oxidized layer at the base to a reduced sapropel at the top, and displays progressive internal changes of colour from ochre, beige, greyish‐beige, grey and black layers: several sequence‐variants (top‐truncated, base‐truncated, recurrent) are identified. These sequences are a consequence of changes in oceanographic conditions, i.e. from oxidizing to reduced environments, resulting in different organic and mineral contents.Based on detailed studies of Quaternary sediments collected by coring, a synthesized standard succession representative of the ordered sequences in the eastern Mediterranean is proposed, from about 650 000 yr as deduced from the calculated rate of sedimentation of 1·7 mm 1000yr−1. This succession appears modulated by cyclic events: large‐scale periods (200 000 yr) of reduced and oxidized sequences correlate with major alternate phases of flooding and reduced flow of the Nile River during the Quaternary. Subsequent sequences of shorter time‐scale (20 000–50 000 yr) results from two factors: heaviest monsoons leading to an increase of the influx of the Nile River and global climatic changes (Eurasian ice sheet meltwater); these two factors act synchronously or independently along the studied time‐span.These interplaying factors result in periodic stages of increasingly restricted conditions in the basin and explain the complexity of the eastern Mediterranean succession, at least since 650 000 yr ago.
Mass transport in Eocene pelagic chalk on the northwestern edge of the Arabian platform, Shefela area, Israel
Authors: BUCHBINDER B. , BENJAMINI C. , MIMRAN Y. , GVIRTZMAN G. 
Published: June 2006

ABSTRACTGlides, rotational slumps, debris‐ and mud‐flows occur in the Eocene pelagic chalk of the Shefela region, Israel. The pelagic lithofacies are present from the beginning of the Eocene, but mass transport is initiated only in the latest early Eocene and the early middle Eocene. Slight steepening of the palaeoslope, coupled with increased sedimentation rate and early diagenetic changes lowering inherent strength, led to gravity‐induced mass transport in the poorly consolidated pelagic sediments.Instability of the sediment pile was enhanced by underconsolidation and development of spacious packing associated with a relatively high rate of deposition and low initial strength due to high content of siliceous micro‐biota.The palaeogeographic position indicated by the mass‐transport features is the steepened, distal part of the ramp‐like continental margin of the Arabian Platform. Distal steepening of the ramp as part of regional downwarping is coincident with reactivation of Syrian Arc folding, reflecting northward convergence of the Arabian Plate during the Eocene.
Mineralogy of nitrogenous guano on the Bounty Islands, SW Pacific Ocean
Authors: CULLEN DAVID J. 
Published: June 2006

ABSTRACTBy far the commonest consituents of insular phosphate deposits are calcium phosphates of the apatite series of minerals, especially varieties that contain structural carbonate in their crystal lattices, e.g. francolite and dahllite. This reflects the fact that the vast majority of described insular phosphate deposits occur in low latitudes, where they have formed, in a tropical environment, by metasomatic replacement of carbonate in coral substrates by phosphate derived either from avian excrement or from phosphorus‐enriched lagoonal/lacustrine waters.Thin, recently‐formed guano accumulations on islands of the subantarctic Bounty group differ radically in that they consist principally of struvite, a hydrated ammonium magnesium phosphate that is better known as a component of human/mammalian urinary stones and enteroliths, and of bat guano in sheltered speleothems. Apatite occurs only in an accessory role in the Bounty Islands guano. This contrast in mineralogy, and the somewhat anomalous survival of soft, unstable struvite in a totally exposed environment on the Bounty Islands, are attributed to climatic factors and to the nature of the granodioritic rock substrate.
Sedimentology and evolution of the Martinsburg Formation (Upper Ordovician) fine‐grained turbidite depositional system, central Appalachians
Authors: LASH GARY G. 
Published: June 2006

ABSTRACTThe Upper Ordovician Martinsburg Formation of eastern Pennsylvania consists of mudstone, siltstone, and sandstone turbidites that accumulated in a tectonically active foreland basin. The mudstone‐rich Bushkill Member, the stratigraphically lowest unit of the Martinsburg in this area, grades upward into approximately equal proportions of mudstone, siltstone, and sandstone of the Ramseyburg Member. Many of the turbidites of these units are arranged in small‐scale (1–9 m) fining‐upward sequences that are interpreted as reflecting the influence of external or allocyclic controls such as variations in the local rate of sea‐level rise and/or variations in the intensity of tectonic activity in shelf/nearshore or hinterland areas rather than more commonly cited autocyclic mechanisms. The thick (approximately 2000 m) Bushkill‐Ramseyburg coarsening‐upward sequence records progradation of a muddy turbidite depositional system along the axis of the foreland basin. Although this sequence accumulated during a Caradocian eustatic rise in sea‐level, sedimentation rates landward of the shoreline were apparently great enough to allow for long‐term seaward progradation of the shelf source. The paucity of depositional lobe‐like facies (coarsening‐upward sequences) in the Bushkill Member allows for tentative comparison of the progradational Bushkill‐Ramseyburg system with the active fan lobe of the Mississippi Fan.Progradation of the Bushkill‐Ramseyburg system ceased abruptly when mudstone turbidites and laminated black shale of the upper unit of the Martinsburg, the Pen Argyl Member, accumulated. The great thickness of some mudstone turbidite beds of the Pen Argyl Member is interpreted to record topographic confinement of the central Appalachian foreland basin, which may have helped to preclude continued progradation of the Bushkill‐Ramseyburg turbidite system.
Time‐sequence observations of wave‐formed sand ripples on an ocean shoreface
Authors: BOYD R. , FORBES D. L. , HEFFLER D. E. 
Published: June 2006

ABSTRACTAnalysis of an 18‐day time‐lapse film record of shoreface ripple development, with concurrent measurements of near‐bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behaviour. The study was conducted in approximately 10 m water depth, 1 km off Martinique Beach on the Atlantic coast of Nova Scotia (Canada), under low‐energy summer wave conditions. Significant wave‐height and peak period during the study averaged 0–7 m and 8 s, respectively, with extremes up to 1–7 m and 11 s during passage of three weak weather disturbances. Six mutually exclusive ripple types have been defined: (1) short‐wavelength regular ripples; (2) variable bifurcated ripples; (3) variable terminated ripples; (4) short‐crested ripples; (5) long‐wavelength regular ripples; and (6) chaotic ripples. Ripple wavelength ranged from 0–07 m to 0–24 m and displayed a strong Reynolds number dependence. Together with other published field data, the results suggest a lower limit of γ=0–06 m for the wavelength of wave ripples in ocean shoreface environments. Ripple orientation ranged through 38° and responded rapidly to changes in wave approach direction, but did not conform to the orientation of the adjacent shoreline. Ripples were observed to migrate both on‐ and off‐shore (with and against the wave advance direction) at rates up to ±0–1 m h‐1, associated with net flows other than wave‐induced onshore asymmetry and mass transport. Migration (mainly of ripple types 1 and 2) occurred during the peak of storm events, but showed no obvious correlation with measured near‐bottom flow magnitude or direction. Ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight‐crested rippie types 1 and 5 are present. Disequilibrium in orientation or dimensions is expressed by increasing sinuosity, bifurcation and crest termination in types 2,3,4 and 6.
The initiation of particle movement by wind
Authors: NICKLING W. G. 
Published: June 2006

ABSTRACTWhen air blows across the surface of dry, loose sand, a critical shear velocity (fluid threshold, ut), must be achieved to initiate motion. However, since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size, shape, sorting and packing of the surface sediment. In order to investigate the initiation of particle movement by wind a series of wind‐tunnel tests was carried out on a range of pre‐screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics. A sensitive laser‐monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements. Test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep (rolling) or in saltation (bouncing or hopping downwind). As velocity continues to rise, larger or less exposed grains may also be moved by fluid drag. On striking the surface saltating grains impart momentum to stationary grains. This impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure.As a result, there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities (the fluid threshold range) and setting in motion a rapidly increasing number of grains. Results of the tests showed that the progression from fluid to dynamic threshold, based on grain movement, can be characterized by a power function, the coefficients of which are directly related to the mean size and sorting characteristics of the sediment.