bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geochemistry and petrogenesis of a nepheline syenite-carbonatite complex from the Sudan
Authors: Harris N. B. W. , Mohammed A. E. R. O. , Shaddad M. Z. 
Published: May 2009

SummaryJabal Dumbeir is a nepheline syenite-carbonatite complex of Cambrian age (550 ± 87 Ma). The earliest igneous event of the complex was the intrusion of a phlogopite-sodalite nepheline syenite (ditroite) which was derived from the partial melting of the upper mantle. K, CO2 and F-rich volatiles metasomatise both the ditroite and the gneissic country rock which results in an orthoclase-rich undersaturated syenite (orthoclasite). Carbonatitic breccias and sovite dykes were subsequently emplaced carrying high levels of F, REE, Y and Th with extremely high LREE/HREE ratios (CeN/YbN = 64 ± 8). Such carbonatiticmagmas may be derived by partial melting of an LREE-enriched upper mantle. Hydrothermal emplacement of fluorite-quartz veins represent the final phase of igneous activity and these carry high levels of REE, Y, Th and U. Their emplacement was strongly controlled by a system of NNE strike-slip faults which have been reactivated in recent times.
The Late Precambrian succession in north-west Oscar II Land, Spitsbergen
Authors: Waddams P. 
Published: May 2009

SummaryThis paper describes the late Precambrian rocks in north-west Oscar II Land, western Spitsbergen, with particular emphasis on an upper (Haaken Formation) and lower (Trondheimfjella Formation) diamictite sequence. It is argued that both diamictites are glacigenic, formed by ice-rafting over distal turbidite basins, and correspond to the two Varangian tillite horizons of northern Norway and elsewhere. This leads to a radical reinterpretation of the age of the Oscar II Land succession, much of which now appears to be Vendian. With the exception of the Trondheimfjella Formation, which records an early phase of deepening of the depositional basin, the lower half of the succession (St Jonsfjorden Group) comprises mainly shallow water shelf sediments. The upper half of the succession (Comfortlessbreen Group) was deposited within a basinward prograding turbidite fan system, with the Haaken Formation representing the distal component of the complex. A return to a mixed clastic-carbonate shelf environment is recorded by the overlying Palaeozoic rocks.
A cored Jurassic sequence from north Lincolnshire, England: stratigraphy, facies analysis and regional context
Authors: Bradshaw M. J. , Penney S. R. 
Published: May 2009

SummaryA virtually complete Jurassic sequence recently drilled in north Lincolnshire is considered in a regional context. Liassic thicknesses and facies are largely unexceptional. The Frodingham and Pecten Ironstones are relatively thin and sandy, but the Marlstone Rock Bed is thicker and more ferruginous than elsewhere in the region. The Inferior Oolite is exceptionally thick and exhibits strong deltaic influence; equivalents of the Gristhorpe Member and Scarborough Formation of Yorkshire may be present. The Great Oolite was deposited in coastal-lacustrine-deltaic environments, more proximal than to the south or east. Thin Kellaways Clay separates the argillaceous Cornbrash from the Kellaways Sand. The organic-rich Lower Oxford Clay is thin owing to a basal non-sequence whilst the Upper Oxford Clay is condensed and less organic than in the East Midlands. The regressive West Walton Beds are followed by very thick Ampthill and Kimmeridge Clay sections beneath the Spilsby Sandstone. Regional thickness and facies analysis records the behaviour of the Market Weighton Block and reveals the influence of the Pennine High as both a positive tectonic feature and an occasionally active sediment source.
Morphology and surface sedimentary features of point bars in Welsh gravel-bed rivers
Authors: Blacknell C. 
Published: May 2009

SummaryTwo examples of point bars in single-channel gravel-bed rivers in Wales illustrate some of the morphological and surface sedimentary features observed during a wider examination of 40 sites of recent (less than 100 years) channel sedimentation. The point bars consist of predominantly sandy gravel bar platforms overlain by discontinuous sand bodies. Cut bank exposures show generally massive sandy gravels abruptly underlying a silt/sand unit. Morphologically the bars are platformlike, with primary inner-attached, or secondary medial, chutes. This relief, together with the effects of seasonal vegetation, determines the locations of finer-grained (non-gravel) material. The frequency with which deposits produced by this type of channel sedimentation may be encountered in Quaternary and pre-Quaternary successions is briefly discussed.
Characteristics and significance of some humate-cemented sands (humicretes) at Cape Flattery, Queensland, Australia
Authors: Pye K. 
Published: May 2009

SummaryHumate-cemented sands (humicretes) are exposed at several locations along the foreshore and in gullies near Cape Flattery, North Queensland, Australia. Three broad types of humate accumulation are recognized: (a) pedogenetic; (b) groundwater; and (c) aquatic. Pedogenetic and groundwater humicretes usually have a grain-supported fabric in which quartz represents > 99% of the framework grains. Hand specimens of humicrete are brittle but disintegrate rapidly in 0.1 N-NaOH. Concentration of humate is considered to have occurred by vertically and laterally migrating groundwater, while induration has probably been caused by irreversible drying of the humic substances during periods of seasonal water-table lowering. Radiocarbon dating of an in situ root embedded in indurated humate near Cape Flattery has indicated a ‘background’ age of > 48000 C–14 years.
Morphology and dimensions of the young comendite lavas of La Primavera volcano, Mexico
Authors: Clough B. J. , Wright J. V. , Walker G. P. L. 
Published: May 2009

SummaryQuantitative data on the rhyolitic lavas of the Quaternary La Primavera volcano, Mexico, are presented. Lavas vary in form from domes and mesa lavas to short coulées. Average thickness is 100 m, average surface area is 2.7 km2 and aspect ratios vary from 1:7 to 1:25. Two petrological lava types are distinguished – porphyritic and aphyric. Porphyritic lavas occur on or within an inner ring fracture and are associated with an area of caldera lake deposits. Many features suggest that these porphyritic lavas were emplaced into the wet sediments or water of the caldera lake. Aphyric lavas are largely found to the south of this region and form large polygenetic structures extruded on dry land. The porphyritic lavas generally are smaller and have higher aspect ratios than the aphyric, which are attributed to their special environment of emplacement. Some porphyritic lavas extruded on dry land, outside the region occupied by the caldera lake, are, however, the thinnest on the volcano.
Ultramafic volcaniclastics of the Precambrian Dalma Volcanic Belt, Singhbhum, Eastern India
Authors: Gupta A. , Basu A. , Ghosh P. K. 
Published: May 2009

SummaryThe Dalma volcanic belt in the Precambrian Eastern Indian shield is composed of a thick sequence of volcani-sedimentary assemblage with a group of predominantly ultramafic volcaniclastics having komatiitic chemistry. A volcanic origin is suggested by textural relationships and mineralogical composition, together with the presence of devitrified glass shards; these rocks are now mostly chlorite-tremolite schist.
Late Precambrian diamictites of northeastern Svalbard
Authors: Hambrey M. J. 
Published: May 2009

SummaryIn northeastern Spitsbergen and western Nordaustlandet well preserved Late Precambrian diamictites have variously been interpreted as tillites or the product of tectonic activity. By focusing on the sedimentology of the diamictites in two key areas, this paper presents unequivocal evidence that the diamictites are indeed glacigenic, confirming the conclusions of previous field investigations. In particular dropstones in large numbers and striated stones are diagnostic of glacial transport, while other characteristics are strongly suggestive of glacial conditions.Stratigraphic studies indicate two discrete glacial episodes, both belonging to the Varangian epoch. The earlier resulted in a thin tillite sequence deposited in a shallow shelf-tidal-lacustrine environment. It is closely associated with stromatolitic dolostones. The later episode resulted in a tillite sequence a few hundred metres thick with two units in northeastern Spitsbergen separated and bounded by dolostone units. Deposition occurred in a shallow marine environment but occasional uplift led to terrestrial conditions and the deposition of fluvioglacial sediments and the development of periglacial phenomena.
Abundance and distribution of uranium and thorium in the northern units of the Leinster granite, Ireland
Authors: O'Connor P. J. , Hennessy J. , Bruck P. M. , Williams C. T. 
Published: May 2009

SummaryAbundance data, determined by epithermal neutron activation analysis, are reported for radioelements in the northern units of the Leinster granite. Compared to similar data for silicic igneous rocks and British Caledonian granites, the Leinster granite shows normal U and K contents but is low in Th. Inter-element correlations and fission-track results suggest that most Th and up to 40% of the U is bound in resistate accessory phases (zircon, sphene, apatite), but that the bulk of U is associated with late-stage alteration products and is located at interstitial labile sites within the host-rock matrix. Mobilization of U located at such sites in the Leinster granite by hydrothermal leaching could lead to redeposition and enrichment in marginal sediments of Old Red Sandstone or younger age.
Lower Cambrian echinoderm plates from Cowley, Shropshire, England
Authors: Donovan S. K. , Paul C. R. C. 
Published: May 2009

SummaryEchinoderm plates from the Protolenus Limestone are recognised as belonging to three groups; plates with folded surfaces, plates with bevelled edges and a third group. They are not comparable with other Lower Cambrian echinoderms and may be derived from a genus or genera of eocrinoid which is not known from complete specimens.
Late quaternary explosive silicic volcanism on St Lucia, West Indies
Authors: Wright J. V. , Roobol M. J. , Smith A. L. , Sparks R. S. J. , Brazier S. A. , Rose W. I. , Sigurdsson H. 
Published: May 2009

AbstractMany explosive eruptions of dacitic magmas have occurred on St Lucia during the late Quaternary. These have produced widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands, with co-eruptive air-fall and surge layers interbedded with palaeosols and epiclastic deposits. Vents in the highlands have not been located because of the dense tropical jungle but we suspect they are now plugged by lava domes surrounded by aprons of block and ash flow deposits. Young magmatically related dacitic lava domes have been extruded in the Qualibou depression. The pumice succession can be divided into older quartz-poor deposits forming the Choiseul Pumice and younger crystal-rich deposits with abundant large quartz which are called the Belfond Pumice. The Choiseul Pumice groups together scattered remnants of the products of many eruptions of different low-silica dacitic magma types. The Belfond Pumice is the product of several eruptions of a high-silica magma type and 14C ages have dated these between 20900 to 34200 years B.P.The pumice flow deposits occur as small-volume valley fills. A granulometric study of Belfond pumice flow deposits shows them to be strongly depleted in finer ash and vitric components. It is suggested that the narrow, winding and vegetated valleys on the island locally induced turbulence and the flows moved with large, highly fluidized and inflated heads, resulting in substantial loss of fine vitric ash. One ash flow deposit which is extremely rich in crystals and carbonized vegetation is highly depleted in fines and shows enhanced vitric losses. This flow may have been a much more violent ash hurricane or blast which surmounted topography ingesting large amounts of lush vegetation. Ignition of this released the large quantities of gas needed to elutriate most of the fines.A model is suggested for the recent volcanic activity on St Lucia in which separate batches of silicic magma, each having a distinctive petrological and chemical character, rose into high level chambers over a large area. Eruptions of volatile-rich magma led to highly explosive pumice-forming activity from vents in the central highlands. Degassed and more crystal-rich magma was extruded later from the same vents or in the attenuated flank of the Qualibou depression to from lava dome complexes.
Re-interpreting ‘segmented’ grain-size curves
Authors: Christiansen C. , Blæsild P. , Dalsgaard K. 
Published: May 2009

AbstractCumulative curves of grain-size against frequency often show segmented shapes. These shapes have been interpreted as resulting from a combination of normally distributed populations each of which should reflect a mode of transport. The present study deals with samples in motion as bedload, pure saltation load and suspended load. All of these exhibited segmented shapes on cumulative curves. An explanation of this is that the populations are better described as log-hyperbolic distributions rather than as a mixture of log-normal distributions. The log-hyperbolic distribution when plotted on probability paper has a shape which can be misinterpreted as consisting of segments.
Precambrian–Cambrian boundary candidate, Meishucun, Jinning, Yunnan, China
Authors: Yusheng Xing , Huilin Luo 
Published: May 2009

AbstractBeing located in Jinning County, Yunnan Province, approximately 103°E, 25°N, the Meishucun section has a long history of study, convenient accessibility, mild spring-like climate all the year round and good exposures of the relevant strata. In the particular area there occur monofacial transitional carbonate deposits of marine origin, yielding in great abundance both small shelly fossils and trace fossils, which are present in close association and in some cases on the same bedding surface. The sequence of biotic assemblages is clearly evident and can be used for wide intercontinental correlation.In the lower part of the Qiongzhusi Stage, stratigraphically higher, there appear the oldest trilobites, archaeostracods andBaltisphaeridiumcharacteristic of the Early Cambrian.During the last six years great progress has been made in the study of the particular section through lithostratigraphic, biostratigraphic, geochronological, magnetostratigraphic and seismostratigraphic approaches, thus making the Meischucun Stage and the Sinian–Cambrian boundary stratotype section in the Meishucun area a Chinese candidate for global stratotype section and point for the Precambrian–Cambrian boundary.
Global distribution of the earliest shelly metazoans
Authors: Zhiwen Jiang 
Published: May 2009

abstractThe appearance of the first abundant skeletal organisms in the earliest Cambrian was a quantum leap in the evolutionary history of life. It provided the foundations of the animal kingdom from Cambrian times onwards. This paper demonstrates that this evolutionary process resulted from a combination of environmental and biological factors. The author maintains that the appearance of the oldest shelly fossils is a reasonable criterion for defining the Precambrian–Cambrian boundary. This arises not only from the viewpoint of taking the Cambrian Period as the first period in the Palaeozoic Era, but is also a logical extension of regarding the history of life as being divisible into a series of developmental stages.
Mass flow deposits on a mid-Tertiary carbonate shelf, southern New Zealand
Authors: Hyden Fiona M. 
Published: May 2009

SummarySheet-like and channel-filling conglomerates and pebbly biomicrudites occur within a sequence of cross-stratified shallow-water bioclastic limestones, the Forest Hill Formation of Late Oligocene to Early Miocene age. The largest channel, 50 m wide, 9 m deep, and at least 1.2 km long, is infilled with four separate conglomerate units of pebble- to boulder-sized limestone lithoclasts in a sparse muddy matrix (biomicrudite). Both the conglomerates and associated pebbly biomicrudites possess features characteristic of high concentration mass flows - both of laminar flow (preferred clast orientation, inverse grading or massive bedding) and of turbulent flow (normal coarse-tail or distribution grading and stratification). Clast imbrication (a- or b-axes) is suggestive of gradual deposition. Some flows travelled far enough to allow both vertical and horizontal differentiation of the coarse sediment. Facies models which have been proposed for conglomerates of deep water (turbidite) association can also be applied to the small-scale neritic conglomerates. Formation and remobilisation of such coarse grained facies in a shelf setting is attributed to eustatic sea-level changes and/or to local tectonism.
The volcanological significance of deep-sea ash layers associated with ignimbrites
Authors: Sparks R. S. J. , Huang T. C. 
Published: May 2009

SummaryMany volcanic ash layers preserved in deep-sea sediments are the products of large magnitude ignimbrite eruptions. The characteristics of such co-ignimbrite ash-fall deposits are illustrated by two layers from the Eastern Mediterranean: the Minoan ash, Santorini, and the Campanian ash, Italy. These layers are divisible into a coarse lower unit and a fine upper unit in proximal cores. Both layers also show striking bimodal grain size distributions in more distal cores. The coarser mode decreases in median diameter with distance from source whereas the finer mode shows no lateral variation. These features are interpreted in terms of a model for ignimbrite formation by eruption column collapse. Comparable volumes of ignimbrite and associated air-fall ejecta are produced.
Reading segmented grain-size curves
Authors: Walton E. K. , Stephens W. E. , Shawa M. S. 
Published: May 2009

SummaryCumulative curves of grain-size against frequency often show segmented shapes which have been interpreted as the result of combination of normally distributed populations. The origin of the segments is explored by the examination of two classes of models, truncation and mixing. Similar curves may be generated by a variety of models. The segments rarely reflect precisely the original populations. Simulation of the tri-segmented curves commonly ascribed to combinations of suspension, saltation and traction populations shows that the characteristics of the contributing populations are not consistent with these three modes of transportation.
Early Miocene extrusives and shallow intrusives from Small Nggela, Solomon Islands
Authors: Plimer I. R. , Neef G. 
Published: May 2009

SummaryChemically similar altered gabbro, tholeiitic basalt, basaltic andesite and calc-alkaline rhyolite of Early Miocene age is present on Small Nggela, Solomon Islands. Large vesicles are invariably lacking from the autochthonous basaltic pillow lava which has narrow chilled margins, but clasts of allochthonous basalt present in coeval Early Miocene sediment are commonly vesicular. This suggests that the autochthonous lava was deposited in deep water (> 2.5 km) whereas the allochthonous lava was formed upslope in shallow water. A progressive increase in the tholeiite in Zr, Ti, K2O/(K2O+Na2O), Rb/Sr and K2O/Rb from N to S confirms that an Early Miocene southward-descending subduction zone, postulated by some authors, was present. Its recognition implies that the subduction zone did not migrate far during the Early Miocene.
A tholeiitic andesite flow unit among the Causeway Basalts of North Antrim in Northern Ireland
Authors: Francis P. E. , Lyle P. , Preston J. 
Published: May 2009

AbstractA tholeiitic andesite flow unit occurs in tholeiitic basalt lava in the Giant's Causeway region of North Antrim, Northern Ireland. It is the first example of an intermediate differentiate to be found among these quartz-normative basalts. Separate magma batches for the preceding and succeeding basalt formations are indicated by their Zr/P2O5 ratios, and by the differing fractionation trends shown by molecular proportion ratio plots. The tholeiitic andesite was probably extruded in a superheated condition with few crystal nuclei, and subsequent undercooling produced an unusual fasciculate/spherulitic texture in contrast to the very fine and even grain of the host basalt. A liquid–liquid interface between the flow units shows small-scale lava mixing.
Remote sensing of geological structure in temperate agricultural terrains
Authors: Drury S. A. 
Published: May 2009

AbstractAgricultural patterns dominate most remotely sensed images of lowland north Europe and camouflage shadow patterns related to topography and hence to underlying geological structure. Images of those parts of the near-infrared spectrum close to absorption features controlled by H–O–H bond stretching in plant cells and soil moisture largely remove this hindrance to interpretation, if they are acquired at an early stage in the growing season. The most easily available and most useful images of this type are those of band 5 from the Landsat Thematic Mapper. Spatial filtering techniques applied to digital images can enhance them for structural interpretation if filter matrices and viewing scales are chosen to match the dimensions of the most common elements of the landscape and the peak of visual acuity. Filters to enhance directional attributes of an agricultural scene give appealing results but contain spurious linear features that masquerade as faults.
Mylonite-dominated footwall geometry in a shear zone, central Pyrenees
Authors: Passchier C. W. 
Published: May 2009

AbstractA major ductile shear zone in the gneissic core of the Saint-Barthélemy Massif, central Pyrenees, is characterized by an asymmetric shear strain profile defined as mylonite-dominated footwall geometry. The shear zone is part of a low angle fault system in the massif which caused thinning of a sequence of lithologic units and isograds. The shear zone is interpreted as a low angle normal fault zone of probably Cretaceous age, predating Alpine crustal shortening in the central Pyrenees.
The Alpine Fault, New Zealand, near Milford Sound and to the southwest
Authors: Wellman H. W. 
Published: May 2009

AbstractFrom 70 km northeast of Milford Sound (44.14° S, 168.55° E) to the coast at Milford Sound (44.53° S, 167.83° E) the trace of the Alpine Fault is nowhere more than 1.2 km from the arc with a centre at 48.81° S, 173.20° E and a radius of 628 km (5.65° latitude). Earthquake epicentres indicate that the arc defines the position of the fault just off Fiordland's outer coast and then to near the north end of Puysegur Trench at 46.9° S, 165.25° E. Along the fault vertically to a depth of 10 km the Fiordland rocks are 0.6 Mg/m3 denser than those to the northwest. Displaced post-glacial topography at and near Milford Sound shows that the dip of the fault is southeast at 75 ± 5° and that the faulting is pure dextral at a rate of at least 46 ± 5 mm/yr.