bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones
Authors: HESSELBO S. P. , PALMER T. J. 
Published: June 2006

ABSTRACTThe Coinstone is a well known hiatus‐concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay‐hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea‐floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack‐lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV‐fluorescent and inclusion‐rich, and is similar to the first calcite generations seen in many other clay‐hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post‐exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today.The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.
Aragonite laminae in hot water travertine crusts, Rapolano Terme, Italy
Authors: GUO LI , RIDING ROBERT 
Published: June 2006

ABSTRACTSmall (5–30 μm) aggregates of aragonite needles occur in calcite crystal crusts of present day hot water slope travertines at Rapolano Terme in Tuscany, Italy. The aggregates are mainly concentrated in irregular, wispy and dark laminae which cross‐cut calcite crystal feathers to create a pervasive millimetre scale banded appearance in the deposit; they also occur less commonly scattered irregularly through the calcite layers. The aragonite needle aggregates are in the form of crosses, fascicles (sheaf shaped bundles, or dumbbell shaped), rosettes and spherulites. Locally, irregular masses of needles also occur. The fascicles, rosettes and spherulites have hollow centres which resemble microbial components (?fungal spores, bacterial colonies and pollen), suggesting that the aragonite crystals are biotically nucleated. The lamination is interpreted to reflect diurnal control. Stimulation of microbial activity during daylight concentrates cells in laminae and promotes aragonite calcification. Calcite feather crystals, although traversed by the aragonite aggregate laminae, have a clear appearance under the light microscope. They form more or less continuously through the diurnal cycle by abiotic precipitation. The constant association of aragonite with organic nuclei, irrespective of whether the latter are in laminae or scattered through the calcite layers, supports a biotic control on aragonite formation. Lamination in Pleistocene travertines is superficially similar to that in the present day deposits, but is diagenetically altered. In the Pleistocene deposits, the calcite feathers appear dark under the light microscope and the aragonite aggregates, where they are not altered to dark calcite, are dissolved, together with parts of the adjacent spar calcite, and therefore appear light coloured.
Sedimentology and depositional model for glaciolacustrine deposits in an ice‐dammed tributary valley, western Tasmania, Australia
Authors: FITZSIMONS SEAN J. 
Published: June 2006

ABSTRACTQuaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake‐bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium‐ to high‐relief topography.
An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates
Authors: HOWARD JEFFREY L. 
Published: June 2006

ABSTRACTThe feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south‐west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate‐prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δp and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade‐shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and Rw is observed, hence the abundance of disc‐shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.
Current crescents formed by marine algae (Valonia sp.): a new record of obstacle marks lacking preservation of obstacles
Authors: BHATTACHARYA ASOKKUMAR 
Published: June 2006

ABSTRACTObstacle marks, formed around non‐permanent tools, commonly occur on the beach faces of Lower Long Sand Island and Sagar Island, two tidal islands in the Ganga River estuary, India. The marine globose algae, Valonia sp., which bloom profusely in the waters of the Bay of Bengal during winter, are deposited on beach faces after being carried by wave swash, and then act as impermeable obstacles in the path of wave backwash. This creates around the globose algal bodies U‐shaped or crescentic marks 2–3 cm in width with arms 1–3 cm in length. The globose vesicles are soon broken up by external forces or by forces of surface tension, leaving crescentic marks around a circular filamentous impression. The crescents always open toward the direction of wave backwash. These structures may prove useful not only for interpreting the palaeoslope and palaeocurrent directions but also for recognizing the palaeoecology.
Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications
Authors: CORNÉE ANNIE , DICKMAN MIKE , BUSSON GEORGES 
Published: June 2006

ABSTRACTFormation of microlaminated sediments in solar salt works along the Mediterranean coast in southern France only occurs within a restricted salinity range of 60–150 gl−1. These salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria Microcoleus chthonoplastes interbedded with detrital laminae. Transplants of the cyanobacterial mat to a less saline zone (36–60 gl−1) indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of Cladophora and Enteromorpha. Neither the snails nor the Cladophora and Enteromorpha masses are tolerant of salinities above 60 gl−1, and therefore the Microcoleus mats are restricted to those areas of the solar salt works with these higher salinities.Analyses of salinity, conductivity, dissolved oxygen and pH in shallow salt pans (with salinities of 60–150 gl−1) established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis.Sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats. These mats are frequently drained of their overlying water, and thus desiccation cracks divide them into polygonal plates. The development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats. No correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed. This was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons. This process results in a ‘patchwork quilt’of young and old cyanobacterial mat polygons with an irregular microlamination pattern. The presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified (meromictic) water bodies. The sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis.
The hyperbolic shape triangle as a tool for discriminating populations of sediment samples of closely connected origin
Authors: HARTMANN DANIEL , CHRISTIANSEN CHRISTIAN 
Published: June 2006

ABSTRACTThis paper describes a new method for discriminating populations of sediment samples of closely connected origin. It provides a dynamical interpretation to grain size data. The descriptive modelling power of the hyperbolic distribution, the hyperbolic shape triangle and the population concept have been applied to several hundred beach sand samples from the Mediterranean coast of southern Israel. The investigated samples are from a 60 km stretch of beach, from Rafah to Ashqelon, and represent an autumn storm profile. The domain of variation of the hyperbolic shape parameters χ and ξ is a triangle referred to as the hyperbolic shape triangle. The distribution of the estimated (χ, ξ) shape positions in the hyperbolic shape triangle was studied graphically and statistically using the newly developed tools described here to present, evaluate and compare hyperbolic shape position data for analysed grain size populations. The graphical methods of contouring the scatter diagrams of the samples in the domain of the hyperbolic shape triangle and chi‐squared tests are demonstrated. They were performed on two sedimentary environments normal to the beach, namely inshore and mid‐swash zone. The qualitative and quantitative estimated changes in the populations (χ, ξ) shape positions in the shape triangle suggest that they are subjected mainly to ε erosion‐deposition processes. Their modes are located along one of the upper ‘hammock’curves in the shape triangle. The location and scale invariant hyperbolic shape parameter ρ indicates a change of the grain size distribution across the left part of the hyperbolic shape triangle, which is the domain of predominantly depositional processes. However, the mid‐swash zone sediments reflect that they are subjected more to depositional processes and the inshore sediments more to erosional ones.
Deposition and diagenesis of debris flows in Upper Ordovician limestones, Hadeland, Norway
Authors: BRAITHWAITE C. J. R. , HEATH RACHEL A. 
Published: June 2006

ABSTRACTThe Upper Ordovician rocks of Hadeland, Norway, form a sequence of thin bedded nodular limestones (wackestones) and shales, hosting five distinctive sedimentary breccia complexes. These breccias contain blocks of varying sizes and shapes in a wackestone and grainstone matrix. Blocks differ in lithology, and in their included biotas and cement sequences.The thin bedded limestones are interpreted as turbidites, deposited against a background of hemipelagic calcareous shales. The breccias occupy channels cut into this sequence. The lithologies and biotas of blocks in the breccias record deposition in differing sedimentary environments, whereas their cements are the results of contrasting diagenetic histories. Blocks were eroded from a diverse and mature carbonate platform, close to sea level, which probably lay 5–10 km east of Hadeland. The breccias are interpreted as debris flow deposits, transported as channellized flows. Following channel cutting events, perhaps triggered by sea level change, channels were characterized by deposition rather than erosion. Wackestones and grainstones associated with the breccias also reflect resedimentation, their less diverse biota suggesting local derivation on the slope. The reworking of calcarenaceous muds locally produced clean washed calcarenites (now grainstones).A fall in sea level resulted in emergence of the upper slope and erosion of the debris flow complex to form caverns and fissures. As sea level rose again crinoidal calcarenites, now grainstones, were deposited within these cavities.Cement sequences in blocks record early marine and burial conditions on the shelf, and also precipitation of new marine cements following downslope transport. Those cements in lithologies formed in situ document later shallowing, culminating in emergence. The localized dissolution of cements in both blocks and associated grainstones reflects the infiltration of ‘aggressive’meteoric waters through permeable channel deposits. A subsequent rise in sea level is recorded in the generation of an additional marine cement with final burial reflected in the deposition of blocky calcite. The debris flow deposits therefore maintained their distinctive character from deposition through diagenesis.
Avulsion mechanisms on the Okavango fan, Botswana: the control of a fluvial system by vegetation
Authors: McCARTHY T. S. , ELLERY W. N. , STANISTREET I. G. 
Published: June 2006

ABSTRACTA study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2 in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1 in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre‐existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally strong.
Pliocene subaqueous fans and Gilbert‐type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA
Authors: WHITE JAMES D. L. 
Published: June 2006

ABSTRACTMaar craters of the Mio‐Pliocene Hopi Buttes volcanic field of Arizona formed within a broad playa system, and accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater‐fed lakes. Ephemeral streams crossing the playa entered some of the lake‐filled craters, and built coarse grained Gilbert‐type deltas and subaqueous fans along the margins of these craters. The small, coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits. However, the local production of coarse grained volcanic sediment, low gradients in the local stream catchment, steep subaqueous relief and the small size of the receiving ‘basins’resulted in a unique combination of features. Cone‐shaped subaqueous fans initially formed at the mouths of incised feeder streams. The fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density‐modified grain flows. Gravelly Gilbert‐type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream. They are concave lakeward, mimicking the underlying crater wall topography. Complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds. They suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes. Bed‐specific carbonate alteration is common, and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate‐saturated lakewaters during between flood periods of high net evaporation.
Phosphatization of calcium carbonate in phosphorites: microstructure and importance
Authors: LAMBOY MICHEL 
Published: June 2006

ABSTRACTFabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with X‐ray dispersive microanalysis. Replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests; replacement proceeds at constant volume with excellent preservation of the original microtextures. In some deposits, replacement of carbonate by apatite is the main phosphogenic process. However, in general, the process seems to be far less important than might be believed purely on the basis of thin section observations. In many phosphorites, internal or external apatite moulds of bioclasts are common, including very small particles such as coccoliths in phosphatized chalks. Apatite precipitation was typically followed by carbonate dissolution. Later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain. In these examples direct replacement of carbonate by phosphate cannot be demonstrated.
Scanning electron microscope evidence for offshore‐onshore sand transport at Fire Island, New York, USA
Authors: WILLIAMS A. T. , MORGAN P. 
Published: June 2006

ABSTRACTBalancing the sediment budget of the Atlantic shoreline of Long Island has proved to be problematic because more sediment has been found to be moving westward in littoral drift than was estimated to be supplied by wave erosion of the Ronkonkoma terminal moraine at Montauk Point. There is a need for the existence of an additional, possibly offshore source. As part of a larger Atlantic shore study, 11 beach samples from Fire Island were compared with samples representing the known glacial source at Montauk Point, and also with four offshore samples to the south. Comparisons were based upon quartz sand grain surface texture patterns derived by scanning electron microscope checklist analysis. Initial evidence of an offshore‐onshore link was provided by qualitative results examining surface texture variability between samples, illustrated by photographic evidence and written notes. This evidence was confirmed consistently during quantitative analysis. In particular, canonical variate analysis grouped two offshore samples representing deposits of buried glacial to fluvioglacial lobes with beach samples from Fire Island in adjacent onshore locations or slightly downdrift. In addition, canonical variate analysis and factor analysis using the spssx package, together with cluster analysis using the ARTHUR package, depicted the main factors and texture variables largely responsible for sample discrimination. The significance of both the offshore‐onshore link and the associated factors influencing quartz grain surface texture was revealed by the fact that they are strongly related to each other and to the regional geomorphology.