bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fragmentation of granitic quartz in water
Authors: MOSS A. J. , WALKER P. H. , HUTKA J. 
Published: June 2006

ABSTRACTStudies of grain fragmentation in natural streams have the limitation that the full size range of the debris produced is virtually unobtainable. Experiments described here for grain fragmentation in a rotating drum permitted the study of all of the debris, and a fragmentation load technique was used to relate experimentally and naturally fragmented material. The present investigation has been focused on granitic quartz.Relatively gentle collective movement in water can cause significant fragmentation of coarse, nascent, granitic quartz grains. The debris produced by rotating in a drum a range of single sieve fractions, taken from gravel in the headwaters of a stream draining granite, had continuous size distributions down to (and probably beyond) 0·06 μm. Quartz was the dominant fragmentation product in all fractions down to 2 μm and present in finer fractions. When pebbles moved with sand in these experiments, breakage of the latter was greatly increased. In comparison with that of breakage, the effect of attrition on granitic quartz was negligible. At least a proportion of granitic quartz grains are subject to a fatigue effect as a result of impacts in water. Evidently they are thus progressively weakened prior to being broken. Size analysis of debris showed a significant break at 20 μ, suggesting some special production of quartz particles just below this size.Granitic quartz is criss‐crossed with partially healed cracks acquired before the zone of weathering is reached. The wholesale breakage that affects it, particularly in pebbly streams, is largely due to the reopening of these cracks. Progressive fragmentation of this material must eventually reach a stage wherein grains comprising single original crack‐bounded volume elements are produced. Such grains, lacking significant internal weaknesses, must strongly resist further breakage. Possibly the preferential production of quartz grains just below 20 μm in size may represent an accumulation of these single, crack‐bounded volume elements.
Carbonate cementation of some Pleistocene temperate marine sediments
Authors: WEST IAN M. 
Published: June 2006

ABSTRACTCarbonate cementation of some carbonate and quartz sands in three raised beaches of temperate origins was investigated. The carbonate of the cements was found to have been derived from the dissolution of skeletal debris. The sandstones, so produced, now possess only low‐magnesium calcite, but the original sediments, like adjacent modern beach and blown sands, probably contained low‐magnesium calcite, aragonite and some high‐magnesium calcite, all of skeletal origin. In meteoric water the dissolution has occurred of all carbonate within minute, tubulelike, volumes of sand. Concurrent deposition in adjacent volumes of sand of low‐magnesium calcite formed cements that are irregularly nodular or uneven on a small scale. Aragonite within the minute nodules has been replaced paramorphically by low‐magnesium calcite. Additional local carbonate cements were formed at later dates, around and within solution pipes.
The distribution of strontium in limestones on Barbuda, West Indies
Authors: WIGLEY PETER 
Published: June 2006

ABSTRACTThe Sr2+ content of Barbudan limestones is proportional to the aragonite content. The Sr2+ content of calcite which has replaced aragonite varies from 3000 p.p.m. in the youngest limestones to 50 p.p.m., average values are approximately 300 p.p.m.The (mSr2+/mCa2+)L ratios from well waters within the limestone fall within the predicted values assuming KSrC∼0·1. The high (mSr2+/mCa)2+L ratios expected by a lower value of KSrC do not occur.The build up of (mSr2+/mCa2+)L ratios seen in waters from Barbados is not seen in the well waters from Barbuda. The Sr2+ content of calcite is generally lower than is seen in Barbados, but similar to that seen in parts of the Miami Oolite. Diagenesis of aragonite in Barbudan limestones is probably occurring in a more open system than in Barbados.
Holocene sediments and magnetic stratigraphy from Lakes Zug and Zurich, Switzerland
Authors: THOMPSON R. , KELTS K. 
Published: June 2006

ABSTRACTPiston cores 7 m and 6.5 m long were collected from Lakes Zug and Zurich respectively. Radiocarbon age determinations, pollen assemblage zones and varve counts indicate dates of 5000–6000 years B.P. (Zug), and 12,000–13,000 years B.P. (Zurich) for the base of the cores. Declination of the magnetic remanence varies through 70° and inclination varies through 30°. Although the correlation with the geomagnetic secular variation curve covering the last 15,000 years as determined from Lake Windermere, England (Creer et al., 1972) is not precise, the results suggest that Lake Zug and Lake Zurich sediments have been recording the broad outline of past secular variations of the earth's magnetic field. Since intensity of magnetization and susceptibility correlate markedly with lithology, a detailed sediment stratigraphy is presented. The amount and texture of the detrital input appears to be a controlling factor for the natural magnetic remanence. Intensity varies from 90 μG in zones of organic, sulphide pigmentation and those with ultra‐fine laminations to 0.8 μG in impure lacustrine chalk. Susceptibility ranges from 9 μG/Oe in laminated, fine‐grained glacially derived muds to 0.5 μG/Oe in impure lacustrine chalk. Intensity of magnetization also varies systematically within individual turbidites with lowest values in the coarse‐grained, basal fractions. Slumped beds were identified on the basis of erratic anomalies in magnetic declination and inclination measurements.
Regularities of rare‐earth element distribution in the sedimentary shell and in the crust of the earth
Authors: RONOV A. B. , BALASHOV YU. A. , GIRIN YU. P. , BRATISHKO R. KH. , KAZAKOV G. A. 
Published: June 2006

ABSTRACTAfter summarizing the vast analytical material it was possible to establish the differences in rare‐earth element (REE) distribution in geosynclines and on platforms. It is shown that the heavier REE composition in sediments of geosynclines and the lighter one in sediments of platforms was initially created by processes of endogenic differentiation and then inherited by sedimentary series of these zones. On the basis of the calculation of the REE content in rocks of various shells of the Earth crust the increase of the role of heavy lanthanides into the depths of the crust is shown. The obtained estimate of REE abundance in the lithosphere led to the conclusion that the crust of our planet, despite the repeated reworking of its matter by sedimentary processes, metamorphism and granitization, has inherited features peculiar to products of tholeiitic magmatism—the most widely spread type of mantle fusion.
Lithofacies relations in the Late Quaternary Niger Delta complex
Authors: OOMKENS EPPO 
Published: June 2006

ABSTRACTA study of cores from thirty‐three coreholes drilled in various parts of the Niger delta has shown tidal channel sand to be the dominant lithofacies type in the uppermost 30 m of the deltaic complex. Below 30 m fluviatile sand becomes predominant. Coastal barrier sand is present in the uppermost 5 m of the present coastal belt, but chances for preservation of this lithofacies appear to be small.The Post‐Glacial deltaic sediments can be divided into three units.

Alluvial valley‐fill sands and conglomerates deposited during the strong Post‐Glacial sea level rise.
An onlapping complex of lower coastal plain deposits which contains a lower member of fine grained lagoonal and mangrove swamp deposits and an upper member of tidal channel and coastal barrier sands. This complex is thought to have been deposited during the strong Post‐Glacial rise in sea level and is locally as much as 25 m thick.
An offlapping complex of fluviomarine and coastal deposits which contains a lower member of marine clay and silt and an upper member of tidal channel and coastal barrier sand. The presence of this late Holocene complex indicates that deltaic progradation was resumed as soon as the rapid rise in sea level slowed down. The offlapping complex is locally as much as 35 m thick.
Measurements of sand transport by wind on a natural beach
Authors: SVASEK J. N. , TERWINDT J. H. J. 
Published: June 2006

ABSTRACTBagnold's (1954) and Kawamura's (1951) formulae may be used for the calculation of the sand movement on a natural beach, provided the shear stress velocity U* > 0·D4 m/s. Great discrepancies have been found between calculated and measured sand transport rates for U* < 0·D4 m/s, mainly because of the capillary forces acting on a wet beach.The measured critical shear velocity U*c at the beginning of sand movement on a clean dry beach agrees very well with that predicted by Bagnold's formula. On a dry beach where the sand grains are stuck together, U*c was found to be about 10% higher. On a wet beach U*c appeared to depend on the moisture content of the surface layer. Grain size is a determining parameter in the U*c‐moisture content relation.When the angle a between the wind direction at sea and the dune face is between 15° and 85° the streamlines of the wind will bend in the vicinity of the dune face. In consequence this may influence the direction of sediment movement.
Diagenetic history of a North Sea chalk
Authors: MAPSTONE N. B. 
Published: June 2006

ABSTRACTA study of the petrofabrics of Danian and uppermost Maastrichtian chalk from the North Sea was undertaken to investigate its particulate components and diagenetic history.Danian and Maastrichtian chalks are intensely mottled and burrowed globi‐gerinid lime mudstones. The Danian chalk matrix is composed of coccolith and thoracosphaerid debris, whereas the Maastrichtian chalk matrix contains mainly coccoliths. The lower part of the Danian is often argillaceous.Three modes of lithification are evident—a spot‐welding of adjacent grains (important in Danian chalk), selective overgrowths (prolific in Maastrichtian chalk), and a sparry calcite pore filling associated with Maastrichtian stylolitization.Not only does the scant cementation of chalk stem from an inadequate source of metastable calcium carbonate in the form of aragonite, but also indirectly in that extensive pressure‐solution is impeded by certain pore fluid compositions. Pressure‐solution can occur only at point contacts where a threshold linear pressure is exceeded and so allows an increase in calcite solubility. It is proposed that through the formation of spot‐welds an initial rigid intergranular framework is constructed in chalk relatively early during diagenesis. Subsequent increases in overburden eventually permit extensive stylolitization and the late diagenetic reprecipitation of a sparry calcite pore filling adjacent to stylolites. The time and genesis of selective overgrowths is less clear.
Sedimentology and petrology of Quaternary sediments from the Hellenic Trench, Mediterranean Ridge and the Nile Cone from D.S.D.P., Leg 13, cores
Authors: BARTOLINI C. , MALESANI P. G. , MANETTI P. , WEZEL FORESE C. 
Published: June 2006

ABSTRACTFifty‐three samples from D.S.D.P. Sites 127, 128 (Hellenic Trench), 130 (Mediterranean Ridge) and 131 (Nile Cone) ranging in size from clayey silt to sand were submitted to grain‐size and compositional analyses. The former confirmed the evidences obtained from the visual observation of the sedimentary features as to the presence of turbidites (Sites 127, 128, 131) and fluxoturbidites (Site 131). The cumulative frequency distributions of the latter are closely similar to those obtained from ancient deposits of the same type.The X‐ray quantitative analyses of the total samples showed that at both Site 127 and Site 128 calcite and phyllosilicates make up together, on average, about two‐thirds of the total composition. Quartz averages about 20%. Dolomite, plagioclase and K‐feldspars are present subordinately in this order; the former, observed in all samples, is both detrital and authigenic. In the Nile Cone quartz comprises, on average, about 70% of the composition. Phyllosilicates, plagio‐clases and K‐feldspars were observed in minor quantities.The four samples of the Mediterranean Ridge, although Nile derived, show a composition closer to the Hellenic Trench samples than to the Nile Cone. This is due to the strong control exerted by the grain size on the quantitative mineralogical composition. The microscopic analyses carried out on both light and heavy minerals of the sand fraction and the clay mineralogy clearly differentiate between the Hellenic Trench and all the Nile derived sediments (Sites 130 and 131) and exclude an intermixing of even the finest fractions. Each group is well clustered in the compositional diagrams. The Hellenic Trench sediments could be classified as lithic arenites (Crook, 1960) or greywackes (Malesani & Manetti, 1970) and the Nile Cone sediments as sublabile lithic feldspathic arenites or feldspathic subgrey‐wackes.The semi‐quantitative clay mineralogy showed that, on average, almost 50% of the Nile derived clay sized sediments is montmorillonite. Mixed layer minerals make up a relevant part of the remaining fraction. In contrast, the Hellenic Trench sediments show lower quantities of montmorillonite (about 10%) and prevailing percentages of illite and chlorite. Kaolinite is present only in the latter samples and not in the Nile Cone sediments.The lack of relevant changes in the composition of the Nile Cone sediments along the cored sections of the holes suggests that the present hydrography of the Nile River dates from at least the Lower Pleistocene.
Sealers Bay submarine fan complex, Oligocene, southern New Zealand
Authors: CARTER ROBERT M. , LINDQVIST JON K. 
Published: June 2006

ABSTRACTThe Oligocene Balleny Group of Chalky Island, southwestern Fiordland, comprises a typical continental margin sequence 900 m in thickness. Thin nearshore traction deposited sediments at the base are overlain by submarine canyon and fan lithofacies that were deposited by the full range of subaqueous mass‐transport processes. A steep‐walled channel within Balleny Group is interpreted as a fossil proximal fan‐channel. The sedimentary fill of the channel is texturally similar to sediments moving by slump‐creep in Recent submarine canyons and fan‐valleys. The field data presented indicate (1) that a small canyon complex at Sealers Bay was initially cut by subaqueous debris‐flows derived from an adjacent cliffed continental coast; (2) that transport within the upper parts of the canyon and fanchannel complex was primarily by inertia‐flow and slump‐creep; and (3) that these more proximal types of mass‐transport gave way gradationally and successively to fluxoturbidity and turbidity currents at locations further down‐slope, with consequent deposition of sediment in more distal fan‐channel and fan‐surface environments as fluxoturbidites and turbidites, with lesser contributions from inertia‐flows.
Measurements of density underflows from Walensee, Switzerland*
Authors: LAMBERT ANDRÉ M. , KELTS KERRY R. , MARSHALL NEIL F. 
Published: June 2006

ABSTRACTSpeed and direction of bottom currents induced by density underflow of two sediment‐laden rivers were measured by oceanographic current meters in the Walensee (= Lake of Walenstadt), Switzerland. The apparently shooting flow of currents (up to 30 cm/s in this study) is suggested as an explanation for laminations in turbidite sequences. The current speed apparently stabilizes on slopes around 2°; this angle seems to correspond to the critical slope where the flow of the measured currents becomes steady. Current direction is controlled by bottom topography and direction of river inflow. Reversal of current direction observed at two sites is probably due to the underflow‐induced backward motion of the overlying lake water. Underflow activity in Walensee is correlative with density peaks of the river water input. The currents are compared to Lake Mead (Southwestern U.S.) underflows and sporadic currents in some submarine canyons.
Cycles and subsidence: their relationship in different sedimentary and tectonic environments in the Scottish Carboniferous*
Authors: READ W. A. , DEAN J. M. 
Published: June 2006

ABSTRACTTen data sets have been collected from stratigraphical intervals in the Upper Carboniferous of Central Scotland. They represent six structural units and both distal and proximal deltaic environments, plus a meandering river environment. All show a definite tendency towards a linear relationship between the number of deltaic or fluvial cycles and the total thickness of strata in the interval. This relationship is, however, much stronger in sequences laid down in actively subsiding depositional basins than in areas where subsidence is thought to have been controlled by block faulting in the basement or in areas where there is no clearly defined pattern or subsidence. With respect to facies, the relationship is less close in successions that were frequently affected by widespread marine transgressions.The slopes of linear regression lines fitted to the data sets vary much more than had hitherto been supposed. Thus they are no longer thought to provide possible evidence of some ubiquitous underlying process, such as ductile flow in the upper mantle, which affected all the structural units equally. The line which represents proximal deltaic deposits of Westphalian A age in the Kincardine Basin slopes twice as steeply as any of the eight lines representing other dominantly deltaic successions and considerably more steeply than the line representing a succession of Namurian fluvial cycles.Second‐ and third‐degree polynomial regression lines were also fitted to each data set but these generally satisfy little more of the total variation than do the corresponding linear regression lines, and F‐test results indicate that the gains are not statistically significant. Eight of the second‐degree lines however share a common shape that suggests a general tendency for both deltaic and fluvial cycles to be somewhat thicker in the areas of greatest net subsidence.
Grain size variations in ignimbrites and implications for the transport of pyroclastic flows
Authors: SPARKS R. S. J. 
Published: June 2006

ABSTRACTIgnimbrite flow units commonly show reverse grading of large pumice clasts and normal grading of large lithic clasts. Ignimbrites show coarse‐tail grading, in which particles beneath a critical diameter, ranging from 64 to 2 mm, are ungraded. Above this size the larger the clast diameter the more pronounced the segregation. The grading is consistent with the theoretical settling rates of particles in a dispersion with a high particle concentration. Ignimbrite flow units show a reversely graded, fine grained basal layer which is attributed to the action of boundary forces during flow. Ignimbrites are commonly associated with cross‐stratified pyroclastic surge deposits and fine ash fall deposits formed in the same eruption. The fine ash fall deposit is depleted in crystals and is thought to be the deposit of the fine turbulent cloud observed making up the upper parts of nuées ardentes.Pyroclastic flows are postulated to be dense, poorly expanded partly fluidized debris flows. Only its fine grained components can be fluidized by gas. Pyroclastic flows are believed to behave as a dispersion of larger clasts in a medium of fluidized fines, which acts as a lubricant similar to water in mud‐flows. Poor sorting in ignimbrites is attributed to high particle concentrations not turbulence. Many pyroclastic flows may be laminar in their movement with apparent viscosities, deduced from the lateral grading of large lithic clasts, in the range 101−103 poise.
Petrography, chemistry and origin of early diagenetic concretions in the Lower Carboniferous of the Isle of Man
Authors: DICKSON J. A. D. , BARBER C. 
Published: June 2006

ABSTRACTThree silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz.The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore‐waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater.The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods.A four‐fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions.Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth.
Clastic wedges and patterned ground in the Late Ordovician—Early Silurian tillites of South Africa
Authors: DAILY B. , COOPER M. R. 
Published: June 2006

ABSTRACTThe widespread Late Ordovician to Early Silurian glaciations are represented in South Africa by glaciogenic sediments of the Pakhuis Formation. Previously unrecorded clastic dykes within the basal tillite of this formation on Table Mountain, Cape Town, are interpreted as casts of frost‐fissures filled directly with clastics (sand‐wedges or ground‐wedges) rather than filled subsequent to the melting of ice‐wedges (ice‐wedge casts). This suggests sub‐aerial deposition for the tillite of the Sneeukop Member and the presence of a cold, dry climate with steep temperature gradients following deposition of that tillite in the Table Mountain area. A peculiar three‐dimensional joint‐pattern shown by the Pakhuis Formation tillites is interpreted as a relict of a reticulate ice‐vein network that formed in sandy till in permafrost. Such networks have been documented from the modern environment, but so far their relicts have not been recorded in ancient sediments.