bluuebunny/crossref_metadata_2025_split huggingface dataset:
A laboratory flume investigation of the formation of fossil stem fills
Authors: REX GILLIAN 
Published: June 2006

ABSTRACTThe mechanism of formation of fossil plant ‘pith casts’ has been investigated experimentally using a small laboratory flume tank. The extent to which filling occurs is dependent on stem length and diameter, and on the current velocity. Stem fills produced by sediment carried in suspension show a distinct structure of two wedges of sediment deposited at either end of the stem cavity. These wedges are deposited from small flow vortices which form when the flow around the stem separates, as the flow through the stem becomes restricted. Stems filled by bedload currents do not show such a uniform structure. The experimentally produced stem fills are comparable with fossil pith casts, e.g. Catamites. The formation of cortical casts of Stigmaria is not simply explained in terms of the processes operating in open‐ended stem segments. This investigation of the mechanics of filling of hollow plant organs, and the structure of the fill produced, offers a basis for interpreting the sedimentary environment in which the fossil fills have formed, and leads to a fuller understanding of the mode of formation of such fossils.
Major bounding surfaces in aeolian sandstones—a climatic model
Authors: TALBOT M. R. 
Published: June 2006

ABSTRACTLarge areas of fixed, vegetated dunes occur in the Sahel and southern margin of the Sahara. These dunes were active during Late Pleistocene times and have been stabilized as a result of a change to more humid, less windy conditions in the Holocene. The stabilized surfaces, which are in part of erosional origin, are of regional extent. Within aeolian sand sequences, such surfaces form the highest order bounding surfaces. Since regional bounding surfaces of this type occur in all the world's major deserts, it is suggested that they should also be present in some ancient aeolian sandstone sequences.
The hydraulic interpretation of turbidites from their grain sizes and sedimentary structures
Authors: KOMAR PAUL D. 
Published: June 2006

ABSTRACTPrevious attempts to deduce the flow parameters of turbidity currents from their deposited sediments have focused on applications of criteria for the suspension or autosuspension of the grains and on hydrodynamic interpretations of the characteristic Bouma sequence of sedimentary structures. There has been a considerable diversity of opinion, however, as to how the transport criteria relate to the observed deposited grain‐size distribution, and no attempt has been made to determine whether the separate analyses of deposited sediments and sedimentary structures agree as to the magnitudes of the evaluated flow parameters.Such analyses are performed on a turbidite from the Capistrano Formation (Miocene‐Pliocene) of California. This turbidite is normally graded from medium sand at its base to very fine sand and silt at the top, and has the complete sequence of Bouma structures. Only a small degree of cementation has occurred so that samples from the layer could be disaggregated and grain sizes determined both by sieving and sedimentation balance analyses.It was decided to employ the grain‐suspension criterion for the calculation of the flow conditions at the time of deposition, published experiments on the pipe‐flow of suspensions having demonstrated that this criterion is one of deposition versus non‐deposition of grains according to their settling velocities. The published work relating types of sedimentary structures to the sediment grain size and either the flow power or Shield's dimensionless stress is used to evaluate the flow parameters from the observed Bouma sequence. All methods employed yield estimates of the mean flow velocity and bed stress (force per unit bottom area).The evaluated flow parameters for the transition from a flat bed to ripples (Bouma B to C divisions) are nearly an order of magnitude greater than obtained from the grain‐suspension criterion where the calculations are based on the median grain size of the deposited sediments. Agreement results only if the calculations utilizing the suspension criterion are based on nearly the coarsest grains deposited at any instant, a procedure that is difficult to justify with the expected sediment deposition from a waning current. Inclusion of other factors, such as possible lags in sediment deposition or ripple formation beneath the decelerating flow, provide no firm explanation as to the cause of the discrepancy between the results based on the two approaches, and in nearly all cases their inclusion would increase the difference. Something is amiss with our procedures for the hydraulic interpretation of sediments which can be resolved only by further study.
The sandstone memory of the evolution of a Triassic volcanic arc in the Southern Alps, Italy
Authors: GARZANTI EDUARDO 
Published: June 2006

ABSTRACTThe erosion of a southern volcanic belt provided the bulk of the fine‐grained and moderately sorted siliciclastic detritus in the whole deltaic to lagoonal Carnian (Upper Triassic) sequence of the Bergamasc Alps. Volcanic input began in the Early Carnian (Calcare Metallifero Bergamasco) and became prominent in Early‐Middle (Val Sabbia Sandstone) and Late Carnian times (S. Giovanni Bianco Fm.), after an intervening period of diminished supply (Gorno Fm.).The mineralogical and chemical composition of the sandstones compares closely with that of siliciclastics derived from modern, Pacific‐type magmatic arcs and it testifies to an evolution of the source rocks from andesites and dacites during the deposition of the Val Sabbia Sandstone, towards rhyodacitic ignimbrites for the S. Giovanni Bianco Fm. The marked reduction in feldspars at the top of the sequence contrasts with the classical trend from Undissected to Dissected Magmatic Arc provenance and is ascribed to the progressive deepening of erosion into Middle Triassic felsic volcanics. An increase in undulatory and polycrystalline quartz grains in the Late Carnian shows that the erosion did reach deeper into the crystalline roots of the arc, although non‐volcanic detritus never exceeded 10%.After a terminal pyroclastic event in Middle‐Late Carnian times, suggested by the sudden appearance of pumiceous vitric clasts at the top of the Val Sabbia Sandstone, the mid‐Triassic orogenic magmatism ended and was followed by the tensional stage which led to the birth of Neotethys.
Sedimentation on an early Proterozoic continental margin under glacial influence: the Gowganda Formation (Huronian), Elliot Lake area, Ontario, Canada
Authors: MIALL ANDREW D. 
Published: June 2006

ABSTRACTEight continuous cores up to 150 m long and spaced an average of 200 m apart yield a detailed local insight into the composition and architecture of an ancient continental margin sequence, the Gowganda Formation (early Proterozoic: Huronian) near Elliot Lake, Ontario. Nearby outcrops of similar facies provide important supplementary data on sedimentary structures.Continental glaciers provided an abundant supply of coarse debris but, apart from rafting of debris by floating ice, played little or no part in Gowganda sedimentation. The basal 50 m of the Gowganda Formation in the drill‐hole area represents a continental slope depositional system. It consists mainly of gravelly and sandy sediment gravity flow deposits, interbedded with minor rain‐out units of diamictite, and argillite containing dropstones. Ten types of sediment gravity flow deposit are distinguished. An overlying submarine‐channel depositional system, 10–50m thick, consists of hemipelagic argillites containing dropstones and showing deformation structures. These are interbedded with well‐sorted channel‐fill sandstones. Submarine point bars 4·5 m thick (identified in nearby outcrops) demonstrate a meandering channel geometry. This channel‐fill sequence probably formed during a period of high sea‐level and reduced sediment supply, but the relationship to ice advance‐retreat cycles is unclear. The subsurface sequence is completed by a blanket of massive rain‐out diamictites up to 55 m thick, and a younger slope sequence of sediment gravity flow diamictites and sandstones.The stratigraphy is quite different in outcrop section 10 km to the west of the drill‐holes, suggesting the presence of major lateral facies changes and/or internal erosion surfaces within the Gowganda Formation. This complexity of stratigraphy and depositional processes is probably a feature of many ancient glacial units, and points to the advisability of not making climatic or tectonic interpretations from a few generalized or composite sections.
Erosion and deposition of fine‐grained sediments from the Bay of Fundy
Authors: AMOS CARL L. , MOSHER DAVID C. 
Published: June 2006

ABSTRACTSimulations of the erosion, transport and deposition of fine‐grained sediment, such as that of Greenberg & Amos and the Hydraulics Research Station, have illustrated a general lack of reliable field data. Consequently, some standard equations and constants used in modelling the sedimentation character of fine‐grained cohesive sediment were evaluated based on data from two field studies and a flume experiment with undisturbed sediment from the Bay of Fundy.Initial results showed that the resistance to erosion of intertidal fine‐grained sediment is controlled largely by the degree of subaerial exposure and the consequent dehydration and compaction. The sediment shear strength was high (4 kPa), but generally decreased seawards across the intertidal zone. The resistance of intertidal mud to erosion can be 80 times greater than sub‐tidal counterparts.The rate of sediment erosion varied as a complex function of the applied bottom shear stress. At stresses immediately above the critical, the erosion rate decreased asymptotically with time. At higher excess stresses, the erosion rate was linear with respect to time. Thus sediment erosion cannot be represented by a single coefficient.The Krone method of computing sedimentation rates of suspended material was shown, by comparisons with direct measurement, to overpredict by 29%. All variables used in his method were measured in the evaluation with the exception of the critical deposition stress (τd). The closest comparisons were obtained when τd was assigned a value of 0.1 N m−2 following Creutzberg & Postma.The in situ still‐water particle settling rate (Vo) was constant with respect to time (2.1 × 10−3 m s−1). However, the settling tube measures of settling rate, compared to in situ results, underpredicted particle settling by an order of magnitude (2.7 × 10−4 m s−1). The reason for this discrepancy is not apparent from our results.
The petrology and diagenesis of Middle Jurassic clastic sediments, Ravenscar Group, Yorkshire
Authors: KANTOROWICZ J. D. 
Published: June 2006

ABSTRACTThe petrology of non‐marine clastic sediments from the Middle Jurassic Ravenscar Group was investigated to identify and to assess the significance of possible controls on diagenesis. Diagenetic modifications took place in three broad regimes: within the depositional groundwater (eogenesis), during burial (mesogenesis), and during uplift and erosion (telogenesis).Eogenesis involved the initial interaction of the original sedimentary assemblage with its depositional pore waters. Eogenetic processes were influenced strongly by bacterial degradation of organic matter present in the finer grained sediments. This bacterial activity caused a lowering of pore water pH, and subsequently reduced Eh. Feldspar dissolution and alteration of muscovite to kaolinite occurred. Quartz overgrowths and vermiform kaolinite precipitated in sands with oxygenated and mildly acidic pore waters, whereas chlorite and quartz overgrowths formed in sands with anoxic and neutral pore waters. This caused complete reduction of porosity in both finer‐grained and texturally less‐mature floodplain‐facies sandstones. Channel‐facies sandstones were cemented only with a thin veneer of quartz overgrowths, creating a rigid but still porous quartzose framework. Bacterial ferric‐iron reduction throughout the water table subsequently established more ubiquitous conditions, raised pH, and caused rhombic siderite cementation within the remaining porosity.Mesogenetic modifications, of the mineralogy formed at the surface, occurred when pore water composition changed during burial. Bicarbonate saturated solutions migrated through the surviving porosity, precipitating replacive, poikilotopic ferroan calcite. Carbonate dissolution occurred subsequently. Carbonate dissolution was possibly caused by the same solutions which then precipitated dickite and further authigenic quartz.Telogenetic modifications took place during uplift and exposure. Groundwater reactions and subaerial exposure probably caused further carbonate and feldspar dissolution, as well as alteration of chlorite to interstratified chlorite vermiculite.It is concluded that the diagenesis of these non‐marine clastic sediments reflects various factors, the complex interrelationship of which precludes the identification of any single factor as wholly controlling their diagenesis. Eogenetic modifications involved the interaction of the sediments and their pore waters. As a result of the metastability of the mineral assemblage established at this time, these modifications can still be recognized. despite the sediments' subsequent history.
The application of cathodoluminescence to interpreting the diagenesis of an ancient calcrete profile
Authors: SOLOMON STEPHEN T. , WALKDEN GORDON M. 
Published: June 2006

ABSTRACTA laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone.The profile contains a unique series of early pore‐filling vadose cements which only occur down to 1 m below the palaeokarstic surface. Cathodoluminescence reveals that these cements pre‐date the late pore‐filling meteoric phreatic cements which occur throughout local Asbian lithologies. A spar cement stratigraphy has been established for the calcrete profile. Subaerial vadose cements comprise two generations of non‐luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. This needle‐fibre calcite represents the final stage of vadose cementation. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone.Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. The most extensive phase of dissolution occurred immediately after the precipitation of the non‐luminescent subaerial vadose cements. Several different textures have been recorded, each reflecting the morphology of a partially dissolved substrate. Dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures. These probably acted as fluid pathways in the calcrete during early subaerial diagenesis.Much of the non‐marine micrite in the calcrete profile appears as needle‐fibre calcite under cathodoluminescence. This acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near‐surface evaporation. Since needle‐fibre luminescence is strongly variable, these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters. Needle‐fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite, probably during the late stages of subaerial diagenesis. Two generations of subaerial micrite which define a ‘micrite stratigraphy’, have been distinguished under cathodoluminescence.Reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged, with many diagenetic processes acting simultaneously during a single phase of emergence.
A comparative study of the Rare Earth Element (REE) distributions within the Lower Cretaceous dolomites and limestones of Central Tunisia
Authors: TLIG SAĪMD , M'RABET ALI 
Published: June 2006

ABSTRACTREE distribution, strontium content and oxygen isotopic composition have been studied in the Lower Cretaceous dolomites and their interbedded and/or parental limestones of Central Tunisia. Dolomites, as indicated by previous sedimentologic, geochemical and isotopic study, are different in origin. Environments of dolomite formation include: evaporitic sabkha, deep phreatic, karst and lacustrine. With the exception of the sabkha environment, dolomitizing fluids presumably have been essentially meteoric. The comparison between REE distributions in dolomites and limestones clearly suggests that the general shapes of the REE patterns are preserved during dolomitization. Nevertheless, the total REE amounts are somewhat lowered in dolomites comparatively to parental limestones. Moreover, within the dolomites, the total REE contents are positively correlated to the Sr and δ18O contents; thus indicating that the decrease in REE, Sr, and δ18O contents is related to a decrease in the salinity of dolomitizing fluids. In addition, dolomites associated with emergence surfaces (karst) are the most impoverished in REE. Furthermore, they are relatively enriched in light REE, indicating a more or less important fractionation with the REE distributions. This enrichment has been related to the influence of the inorganic complexes which favours the greatest solubility of the heavy REE in basic solution with respect to the lightest members of the REE group.
Proximal to distal sedimentological variation in a linear turbidite trough: implications for the fan model
Authors: MACDONALD DAVID I. M. 
Published: June 2006

ABSTRACTDetermination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back‐arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse‐grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in Tband Tcdivisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end‐member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical.
Skin friction behind isolated hemispheres and the formation of obstacle marks
Authors: PAOLA CHRIS , GUST GISELHER , SOUTHARD JOHN B. 
Published: June 2006

ABSTRACTWe have measured the mean magnitude and direction, and the rms fluctuation intensity, of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long. Downstream of reattachment, the mean skin‐friction magnitude is about 20% greater than its free‐stream value along the centre‐line and comparably reduced to either side of it. The horizontal divergence of the skin‐friction vector field is positive (divergent) along the centre‐line and negative (convergent) to either side of it. Neither of these conditions favours development of a ridge longer than the separation length along the centre line. The development of ridges or tails many obstacle heights long, commonly observed in nature, requires considerable modification of the simple sediment‐free wakes we have studied.
The St George Group (Lower Ordovician) of western Newfoundland: tidal flat island model for carbonate sedimentation in shallow epeiric seas
Authors: PRATT BRIAN R. , JAMES NOEL P. 
Published: June 2006

ABSTRACTThe St George Group consists of peritidal carbonate rocks deposited on the continental shelf of North America bordering the ancient Iapetus Ocean. These Lower Ordovician rocks are similar to other lower Palaeozoic limestones and dolostones that accumulated in epeiric seas and veneer cratonic areas worldwide. A wide variety of facies in the St George is grouped into seven lithotopes, interpreted to represent supratidal, intertidal and shallow, high‐ and low‐energy subtidal environments. Rapid lateral facies changes can be observed in some field exposures, and demonstrated by correlation of closely spaced sections. The stratigraphic array of these lithotopes, although too irregular to be simplified into shallowing‐upward cycles, suggests that they were deposited as small tidal flat islands and banks. Shallow subtidal areas around islands generated sediment and permitted tidal exchange. Tidal flat islands were somewhat variable in character at any one time, and evolved with changing regional hydrographic conditions.The St George rocks suggest an alternative theory of carbonate sedimentation in large, shallow epeiric seas, namely as small islands and banks built by processes that operated in a tidal regime. Furthermore, this island model provides a framework for a mechanism of cyclic carbonate sedimentation, by which small‐scale, peritidal cycles represent tidal flat islands that accreted vertically and migrated laterally as local sediment supply from neighbouring subtidal areas waxed and waned during relatively constant subsidence.