bluuebunny/crossref_metadata_2025_split huggingface dataset:
The influence of particle size variations on the magnetic properties of sediments from the north‐eastern Irish Sea
Authors: OLDFIELD FRANK , YU LIZHONG 
Published: June 2006

ABSTRACTThis paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. Principal component ordinations of the results show a close link between magnetic property variations and particle size. The magnetic properties of a subset of samples, particles sized by a combination of sieving and pipette analysis, confirm that variations in ferrimagnetic (‘magnetite’) grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1–2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magnetic grains occur in the clay fraction but are present in declining concentrations in the coarser grades up to 4ø as an artefact of the pipette method. Most samples have a biomodal distribution of magnetic minerals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magnetization (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particle size and, more specifically, clay content, by means of ARM or ARM/x values. The most likely source for the uniform and almost exclusive stable single domain magnetite, which dominates the magnetic properties of the clays, is thought to be bacterial magnetosomes. The measurements as a whole do not appear to hold much promise for discriminating sediment source types.
Clay fabrics in relation to the burial history of shales
Authors: SINTUBIN MANUEL 
Published: June 2006

ABSTRACTQuantitative appraisal of compaction strain is essential for the study of the burial history of shales in sedimentary basins. The results of a preliminary fabric analysis of Westphalian and Zechstein shales in the Campine Basin (Belgium) show that clay fabric analysis, using an X‐ray pole figure goniometer, is suitable for this purpose.Clay fabrics, in the range studied, are independent of depth and therefore cannot be used as depth indicators. This suggests that in the early stages of the burial history a stable clay fabric has to develop, which will basically remain unchanged during the subsequent burial history.The degree of clay particle preferred orientation not only reflects the compaction strain, but is also determined by mineralogical parameters: the presence of non‐platy particles and the relative concentrations of the different clay minerals. This degree of preferred orientation furthermore determines the degree of fissility of the shales.These mineralogical factors limit the use of clay fabrics as truly quantitative strain markers. Their use as semi‐quantitative strain markers remains advantageous, mainly because of the common occurrence of clay fabrics in the geological record. Moreover, the relative ease of measurement and the possibility of distinguishing compaction from tectonic strains favour the use of clay fabrics in the quantitative strain analysis of argillaceous rocks.
Crystal fabrics and microbiota in large pisoliths from Laguna Pastos Grandes, Bolivia
Authors: JONES BRIAN , RENAUT ROBIN W. 
Published: June 2006

ABSTRACTLarge pisoliths from the Laguna Pastos Grandes playa in the Bolivian Altiplano have a wide diversity of cortical fabrics and variable mineralogical composition. The cortical laminae are composed of radial calcite bundles, spar calcite, micrite, amorphous silica, mixed micrite‐amorphous silica, quartz, gypsum and halite. Diatoms are common in the outer parts of some radial calcite laminae and amorphous silica laminae, but cyanobacterial filaments are rare. Although the organization of the cortical laminae is highly variable, some repetitive sequences of different laminae are present. Cavities in and between pisoliths contain micrite, detrital grains, calcite bundles and peloids morphologically similar to those found in marine reefs.The pisoliths grow in shallow ephemeral pools fed by hot springs. Radial bundles of calcite precipitate rapidly by degassing and photosynthetic removal of CO2 following spring snowmelt. Conditions for micrite precipitation are unclear, but there is evidence to suggest formation in partially stagnant waters, some of elevated salinity. Amorphous silica laminae precipitate mainly by evaporative concentration; quartz may precipitate from warm silica‐rich spring waters that remain below amorphous silica saturation. The evaporite minerals form during desiccation of the pools or from spray. The peloids in cavities are probably primary precipitates. Different types of laminae may form simultaneously in different pools because of the highly variable conditions across the playa.Lateral migration of spring locations through time has created a complex carbonate‐silica pavement. Large spherical pisoliths form in outflow channels near spring orifices and across discharge aprons where waters are several decimetres deep. With mineral precipitation, channels are filled and become shallow, producing discoid pisoliths and crusts. In shallow waters and on distal aprons only small pisolith gravels form. As spring pools fill with deposits, their locations shift laterally; new pisoliths form elsewhere or precipitation may recommence on older abandoned pisoliths.
Structural diversity of biogenic carbonate particles in microbial mats
Authors: GERDES G. , DUNAJTSCHIK‐PIEWAK K. , RIEGE H. , TAHER A. G. , KRUMBEIN W. E. , REINECK H.‐E. 
Published: June 2006

ABSTRACTNon‐skeletal carbonate particles in microbial mats were studied using thin sections and scanning electron microscopy. The microbial mats form biolaminated units (so‐called potential stromatolites) in salterns. This study emphasizes the coexistence of different particle forms and makes a genetic connection between the heterogeneity of the organic substrate built by bacteria and diatoms and their extracellular polymeric substances (EPS). Whereas allochthonous particles are scarce, Lanzarote microbial mats provide various autochthonous surfaces for the attachment of cells and EPS, including sheaths and capsules of cyanobacteria, frustules of diatoms, metabolic products such as gas bubbles, liquid globules and faecal pellets, as well as the carbonate precipitates themselves. Morphologically different carbonate precipitates are: (i) calcified organic clumps (peloids), (ii) particles composed of concentric aragonite and biofilm laminae (ooids and oncoids), (hi) isolated particles floating in gel‐supported mats and coated by rims of fibrous cement (cortoids), (iv) particles bound by cryptocrystalline matrices or cement, resulting in aggregate grains and (v) lobate cement which fills out spaces and pores and fixes the particles. Peloids are suggested to represent faecal pellets although microbial systems also generate cell clumps by non‐faecal processes. Ooid and oncoid constructions clearly record alternating processes of biofilm accumulation and aragonite encrustation. Further characteristic features of carbonate particles generated within a microbial mat are: (i) an irregular distribution ranging from isolated particles floating within the gel‐like matrix to closely packed particles, (ii) the amalgamation of different particle types (e.g. peloids and ooids) in aggregate grains, (iii) the heterogeneous nature of nuclei comprising bacterial clumps, intraclasts, individual cells, cell colonies and bubbles, (iv) the enrichment of remains, casts and imprints of cells within precipitates and (v) deformation (e.g. truncated cortices) of particles.
On experimental reflected density currents and the interpretation of certain turbidites
Authors: EDWARDS D. A. , LEEDER M. R. , BEST J. L. , PANTIN H. M. 
Published: June 2006

ABSTRACTExperimental results are reported concerning the nature of reflected flows generated when density currents are incident upon ramp‐type flow obstructions. The reflected flows are bores (moving hydraulic jumps that transport mass) with flow characteristics in common with either a group of solitary waves (weak Type A bores) or the original density current (strong Type C bores). Alternatively, the bore may have attributes in common with both of these end‐member forms (intermediate Type B bores). Bore strength is positively correlated with the ratio of reverse flow thickness to that of the residual tail of the forward flow. The largest values of this ratio occur when ‘proximal’reflections arrive at the steeper ramps. Measured particle paths in the bores indicate that natural examples will have the potential to transport and deposit sediment. Strong bores have velocity characteristics very similar to the original current and thus in nature the generated sequence of sedimentary structures will resemble those of the original depositing current. The train of solitary waves that make up a weak bore sequence exhibits a pulsating velocity profile at a point. Such flows may thus generate repeated sequences of structures separated by fine ‘drapes’that are distinguishable from the deposits of the original turbidity current. These conclusions are applied to examples of reflected turbidites described from the Palaeozoic to Quaternary sedimentary record.
Aerodynamic dislodgement of multiple‐size sand grains over time
Authors: LI LONG , MARTZ LAWRENCE W. 
Published: June 2006

ABSTRACTAn experimental study was undertaken in a large‐scale wind tunnel to investigate sand particle dislodgement by wind over time in the absence of grain‐bed collisions. Aerodynamic dislodgement was measured for six groups of sand particles under two known wind velocity profiles. The results show that the dislodgement rate for both fine and coarse particles decreases rapidly during the transition of the particle surface from a non‐wind‐worked condition to a wind‐worked condition, and that the dislodgement rate continues to decay under a wind‐worked condition even though the mean grain size of surface particles remains nearly the same. A previously developed theoretical method for calculating the number of particles left on the bed by wind was developed further. The derived method was used to calculate the time‐decay of the dislodgement rate and the length of time required for the dislodgement rate to reach an equilibrium. The length of time for dislodgement rate to reach an equilibrium in this study is of the order of 10–15 min. This not only provides further observation of the second, long stage of aeolian sediment transport system development reported previously but also indicates a potentially large variation in the time‐decay of transport rate under different conditions. The results indicate that the time‐decay of the particle dislodgement rate is related to sorting processes. Because of the artificial method of preparation of the grain surface and the wind velocity profiles, the results of this study should be applied with caution to natural conditions.
Sedimentation in a disequilibrium river‐dominated estuary: the Raša River Estuary (Adriatic Sea, Croatia)
Authors: SONDI IVAN , JURAČIĆ MLADEN , PRAVDIĆ VELIMIR 
Published: June 2006

ABSTRACTThis paper examines sediment transport, sedimentation and properties of suspended matter and sediments in the Raša River estuary, a small, rock‐bounded, microtidal, low‐wave‐energy karstic estuary in the north‐eastern Adriatic. The Raša River is characterized by large variation in water flow and variable load of mineral particles. More than 90% of this load is brought into the estuary as fine‐grained suspended matter, consisting of only 24–36% of carbonates, the rest being clays. Sedimentation occurs at the salt wedge, resulting in a prograding estuarine delta. Salt‐induced flocculation is the predominant process of sediment deposition.The Raša estuary is infilling with sediment, and classifies as a disequilibrium estuary. We propose a modification of Cooper's (1993) classification scheme to include river‐dominated, disequilibrium estuaries, with the Raša River as an example.
Ground‐penetrating radar and coring used to study the large‐scale structure of point‐bar deposits in three dimensions
Authors: BRIDGE JOHN S. , ALEXANDER JAN , COLLIER RICHARD E. LL. , GAWTHORPE ROB. L. , JARVIS JACK 
Published: June 2006

ABSTRACTDetailed three‐dimensional (3‐D) observations of sandy point‐bar deposits from the River South Esk in Scotland were made using very closely spaced (metres) vibracores and ground‐penetrating radar (GPR) profiles. In order to explain the origin of the observed patterns of deposition, use was made of previous studies of channel geometry, flow and sediment transport. In addition, the mode and nature of channel migration and point‐bar accretion were determined using published maps, aerial photographs and detailed topographic surveys. Point‐bar deposits accumulated in response to channel‐bend expansion and downstream migration, resulting in preservation of sequences that fine upwards and downstream. Lower‐bar deposits are mainly very‐coarse to coarse sands with medium‐scale trough cross‐strata overlying basal gravels: associated radar facies are generally low‐amplitude, relatively discontinuous inclined reflectors. Upper‐bar deposits are mainly fine to medium sands with medium‐ and small‐scale cross‐strata and vegetation‐rich layers: associated radar facies are generally moderate‐ to high‐amplitude, laterally continuous, inclined reflectors. Large‐scale inclined stratasets seen in GPR profiles resulted from episodic point‐bar accretion. Abrupt lateral changes in inclination of these stratasets, and preservation of distinct unit bars (bar heads, scroll bars), lower‐bar platforms and inner‐bank swale fills, record discrete episodes of erosion and deposition associated with floods with recurrence intervals of decades to centuries. Such detailed 3‐D description and interpretation of these large‐scale features of point‐bar deposits was only possible through the use of GPR profiles tied closely to cores, and through the availability of much previously collected information on channel geometry, water flow, sediment transport, erosion and deposition.
Ooidal ironstones and ferruginous microbialites: origin and relation to sequence stratigraphy (Aalenian and Bajocian, Swiss Jura mountains)
Authors: BURKHALTER RETO M. 
Published: June 2006

ABSTRACTAalenian and lower Bajocian rocks in the central and northern Swiss Jura mountains comprise a series of parasequences that mainly reflect a shallowing‐upward trend in a shallow, mixed carbonate/siliciclastic depositional environment. Within a parasequence, ooidal ironstones may occur at three specific types of horizons. These are: regressional discontinuities and transgressional discontinuities formed by sediment bypassing, and omissional discontinuities formed by starvation.Ooidal ironstones, which principally are autochthonous, accumulated during both sea‐level rises and falls in a relatively broad bathymetric and hydrodynamic spectrum. The key physical factor for ferruginous ooid genesis is non‐deposition.Ferruginous ooids and microbialites consist of goethite, chamosite and mixtures thereof, with subordinate amounts of apatite and silica. Ferruginous ooids grew stepwise on the sediment surface in an oxygenated marine environment. Ferruginous microbialites, being the product of benthic microbial communities, grew − partly in cavities − in aerated moderate‐ to high‐energy environments. Thus, chamosite evolved from a precursor substance stable under oxidizing conditions.The close mineralogical and micromorphological resemblance of ferruginous microbialites and ooids suggests a common biogenic origin. Structural rearrangement of a biologically accreted gel‐like precursor substance consisting of various amorphous hydroxides is considered a probable mode of mineral genesis in both ferruginous ooids and microbialites.
Dynamics of sediment‐laden underflows passing over a subaqueous sill: glacier‐fed Peyto Lake, Alberta, Canada
Authors: CHIKITA KAZUHISA A. , SMITH NORMAN D. , YONEMITSU NOBORU , PEREZ‐ARLUCEA MARTA 
Published: June 2006

ABSTRACTThe dynamic behaviour of sediment‐laden underflows was examined in Peyto Lake, Alberta, Canada, which contains a midlake sill 7 m high. Sediment‐laden underflows are driven by the downslope component of negative buoyant gravity multiplied by the current's thickness. Our measurements of wind, lake currents and water properties indicate that underflows pass over the sill due to the active storage of turbid suspension near the bottom in the deepest proximal region. Sill overflows occurred only when a hydrological threshold of the inflowing river was exceeded, causing quasicontinuous underflow and associated sedimentation in the distal region of the lake basin.
Ordovician low‐ to intermediate‐Mg calcite marine cements from Sweden: marine alteration and implications for oxygen isotopes in Ordovician seawater
Authors: TOBIN KENNETH J. , WALKER KENNETH R. 
Published: June 2006

ABSTRACTPetrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO3, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low‐ to intermediate‐Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO3 content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light‐brown turbid fibrous calcite has low MgCO3 (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ13C and δ18O values of all fibrous calcite form a tight field (δ13C=1·7 to 3·1‰ PDB, δ18O= − 2·6 to − 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ18O values are larger than adjacent meteoric or burial cements, which have δ18O δ − 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First‐generation equant and bladed calcite cements, which pre‐date fibrous calcite, are interpreted as unaltered, low‐Mg calcite marine cements based on δ13C and δ18O data (δ13C = 2·3 to 2·7‰ PDB, δ18O= − 2·8 to − 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first‐generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ18O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ18O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.
Ulster White Limestone Formation (Upper Cretaceous) of Northern Ireland: effects of basalt loading on chalk diagenesis
Authors: MALIVA R. G. , DICKSON J. A. D. 
Published: June 2006

ABSTRACTThe Ulster White Limestone Formation is an unusual chalk because it underwent a period of post‐depositional emergence and erosion, followed by burial under 1·5–2·0 km of Tertiary basalts. A high degree of pressure solution and cementation produced a well‐lithified limestone with low porosities (2·3–10·4%). The Ulster White Limestone shows no evidence of thermally induced textural alteration, except for thin (<0·5 m) pseudospar contact recrystallization zones adjoining basalt dykes. Whole‐rock δ18O values of samples not associated with basalt dykes range from ‐ 3·26%o to ‐ 6·50%o (PDB). The δ18O values of macropore cements range from ‐ 4·96%o to ‐ 11·52%o (mean=‐8·27%o). Modelling of the diagenesis of the Ulster White Limestone using trace element concentrations and carbon, oxygen and strontium isotopic ratios of whole rock and cement samples suggests a low water‐rock ratio and either marine or mixed marine‐meteoric pore water environment during the main episode of recrystallization. The maximum possible burial temperature was modelled to be ≃ 105°C.The diagenetic history of the Ulster White Limestone is similar to that of North Sea chalks that are at comparable burial depths as the Ulster White Limestone after basalt deposition. The geochemical data show no indication of hydrothermal alteration associated with the overlying basalts. The degree of alteration of fine‐grained limestones composed predominantly of low‐magnesium calcite, such as the Cretaceous/Tertiary chalks, appears to be controlled largely by the burial (effective stress) history of the limestone.
Preservation of planar laminae due to migration of low‐relief bed waves over aggrading upper‐stage plane beds: comparison of experimental data with theory
Authors: BRIDGE JOHN , BEST JIM 
Published: June 2006

ABSTRACTExperimental studies of the formation of planar laminae by migration of low‐relief bed waves over aggrading upper‐stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point. In general, the preserved laminae represent only a small proportion (generally less than 50%) of the height of the largest bed waves in the population. The theory developed by Paola & Borgman (1991), relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation, was adapted for aggrading conditions and shown to agree well with data. These results suggest that the theory can be used to estimate, from the distribution of thickness of planar laminae, either the distribution of bed wave heights, mean aggradation rate, mean bed wave length or mean bed wave celerity, provided the other parameters can be estimated.