bluuebunny/crossref_metadata_2025_split huggingface dataset:
Surficial deposits of Qatar Peninsula
Authors: Ashour M. M. 
Published: April 2008

Summary

The present work is the first step of a geomorphological mapping of the surface of Qatar. In this work a regional map of the surficial deposits at a scale of 1:200 000 was prepared on the basis of
Landsat
imagery of aerial photographs and topographic maps. The Qatar Peninsula has been exposed since the Miocene and consequently a variety of erosional and depositional landforms have been formed. Features of depositional origin (surficial deposits) can be classified into seven main groups according to their occurrence and textural properties, namely hamadas, sand accumulations, saline flats, coastal sediments, depression sediments, fluvial sediments and duricrusts. The deposits may be of a local or exogenous origin.
Hydrodynamic modelling of cementation patterns in modern reefs
Authors: Goldsmith Ian R. , King Peter 
Published: April 2008

Summary
Cementation patterns in modern reefs are inhomogeneous, even on the thin section scale. The concept of the microenvironment has been developed in order to explain this irregularity, this microenvironment being determined by both chemistry and permeability. Identification of the rate limiting step in carbonate cement growth has led to the suggestion that cementation patterns are controlled principally by the microenvironmental permeability. In order to test this hypothesis, modelling of the permeability characteristics of biogenic frameworks has been attempted. Representative matrix geometries were created mathematically and Poiseuille flow through these has produced cementation patterns similar to those observed in thin section. Visual comparison of the features of the real system and model results indicates that the conceptual model is realistic and permeability may indeed be the major control on cementation.
In the future, modelling of the matrix geometry will be controlled using the measured pore throat radius distribution of the rock. At that stage the model will be applicable not only to the prediction of cementation patterns but to many other problems involving fluid flow through porous media.
Some aspects of diagenetic sulphate-hydrocarbon redox reactions
Authors: Machel H. G. 
Published: April 2008

Summary

Sulphate-hydrocarbon redox-reactions occur at two specific diagenetic temperature/thermal maturity levels: less than about 75–85°C (0.2–0.3%
R
0
), and more than 100–140°C (> 1.5%
R
0
), respectively. In low-temperature/maturity environments these redox reactions take place only with the mediation of bacteria. In high-temperature/maturity environments these reactions take place thermochemically, and certain catalysts must interact in order to overcome the high activation energies and to sustain the reactions at geologically significant rates.

The reaction products and by-products may be identical for both temperature/maturity levels: altered and oxidized hydrocarbons (including bitumen), hydrogen sulphide, metal sulphides (including Mississippi Valley Type deposits), elemental sulphur, carbonates (mainly calcite and dolomite), and other minerals. An important by-product of these redox reactions may be porosity resulting from the dissolution of solid sulphates and/or the carbonate host rock. The net reaction is exothermic, and the released heat may generate a geothermal hot-spot in some cases.
Porosity reduction, microfabric and resultant lithification in UK uncemented sands
Authors: Palmer S. N. , Barton M. E. 
Published: April 2008

Summary

Studies of the extent of diagenetic change in matrix-free, uncemented, quartzose sands ranging in age from the Jurassic to the Recent in the UK have been carried out as part of a geotechnical research programme. All the sands studied are thought to have experienced only a relatively small depth of burial and the extent of diagenetic change is consequently small. Previous studies of the
in situ
fabric of sands in this category have been limited owing to the sampling difficulties created by their very friable nature. Careful sampling, however, has succeeded in obtaining undisturbed material and has facilitated studies of the porosity, microfabric and degree of lithification. Distinctive changes, progressive with age, include reduction of porosity, an increase in the numbers and complexity of grain contacts and an increasing degree of lithification. The cause of these diagenetic changes are discussed and it is concluded that the evidence strongly favours pressure solution of the detrital quartz grains as the dominant process.
Non-steady state microbiological diagenesis and the origin of concretions and nodular limestones
Authors: Raiswell R. 
Published: April 2008

Summary

Morphological, chemical, isotopic and textural studies of pyritiferous carbonate concretions from the Jet Rock (Lower Jurassic, England) are all consistent in indicating an origin by anaerobic methane oxidation. Concretionary growth occurred in a stratigraphically thin, sub-surface zone of uncompacted sediment, where methane (diffusing from below) was consumed to stimulate a late phase of locally intense sulphate reduction, which caused the precipitation of CaCO
3
and isotopically heavy FeS
2
. The carbon isotope composition of the concretionary carbonate was indistinguishable from that of sulphate reduction (using sedimentary organic matter), due to the simultaneous diffusion of isotopically light CH
4
and isotopically heavy dissolved carbonate from the underlying methanogenic zone. The alkalinity generated by anaerobic methane oxidation resulted in precipitation being restricted to a thin, vertically confined zone beneath the sediment/water interface. A pause in deposition then allowed continued precipitation in the form of concretionary growth. Different stages in the evolution of isolated concretions into nodular limestones depend on the magnitude of carbonate supersaturation in the zone of anaerobic methane oxidation and the duration of the pause in sedimentation. Vertically confined zones of carbonate precipitation may also be developed deeper in the sediment due to iron reduction, a further pause in deposition can then produce isotopically heavy carbonate concretions in the zone of methanogenesis.
Magnesite formation in recent playa lakes, Los Monegros, Spain
Authors: Pueyo Mur J. J. , Inglés Urpinell M. 
Published: April 2008

Summary

Early diagenetic magnesite is at present forming at a depth of some 20 cm in recent playa lakes in NE Spain. Magnesite formation can be considered as a result of two factors: (a) an increase in CO
2
activity caused by the decay of organic matter, and (b) the presence of post-halitic brines, strongly concentrated in magnesium, that form in summer. These two factors are mainly observed in the ephemeral salt-pan zone where the highest accumulation of magnesite is found.
Early diagenetic phosphate cements in a turbidite basin
Authors: Smith R. D. A. 
Published: April 2008

Summary
Early diagenetic phosphate cements of late Llandovery age developed in fine-grained turbidites deposited in the Welsh Basin. Bottom water conditions fluctuated between dysaerobic and anaerobic. Phosphate enrichment of layers close to the tops of fine-grained turbidites occurred very rapidly after turbidite deposition. Early phosphate cement precipitation caused lithification of these layers prior to significant sediment compaction. The cements consist of apatite crystallites (0.5–5 µm in diameter) which filled pore spaces and also grew displacively in the cleavages of detrital phyllosilicates. Two possible mechanisms for the localization of cements close to turbidite tops are (a) bacterial concentration of phosphate in near-surface layers of transition between oxic and sulphidic environments, and (b) adsorption of upward-diffusing dissolved phosphate from underlying anoxic porewaters by ferric and manganese oxyhydroxides present above levels of oxygen and nitrate exhaustion. Phosphate cements are absent from adjacent Lower Silurian shelf facies deposited in fully oxic bottom water environments, and are also absent from turbidites deposited in totally anoxic bottom waters. In the former case oxidation of labile organic matter close to the sediment-water interface prevented a source of dissolved phosphate from being buried, whereas in the latter case phosphate would have been free to diffuse out into bottom waters.
Mixing zone dolomitization of Devonian carbonates, Guangxi, South China
Authors: Xun Zhao , Fairchild Ian J. 
Published: April 2008

Summary
Regional facies relationships indicate that dolomitization around the Lower-Middle Devonian boundary in Guangxi region is largely restricted to platform margin facies. In the Liujin area dolostones are represented by the Tangding and succeeding Yingtang Formations. The latter comprises alternations of laminated fine-grained dolostones (type A dolomite) and massive, more coarsely-crystalline dolostones (type B dolomite). Type B is richer in marine fossils and other allochems, and increases in importance upwards. The Formation is bounded above by a disconformity marked by a haematitic marl, interpreted as a subaerial erosion surface, and is succeeded by limestones lacking dolomite.

Petrographic observations indicate that the two dolomite types represent the dolomitization of originally different sedimentary facies by a single dolomitization process. The dolomite is dominantly replacive, but also partly cementing. Variably negative
δ
13
C and
δ
18
O values (interpreted as original) and covariation of these parameters with Sr and Na points to dolomitization by mixed meteoric-marine fluids. Stratigraphic variation in geochemical data is inconsistent with a single episode of subaerial exposure and therefore points to repeated emergence.

Dominantly replacive dolostones can form by the mixing-zone mechanism given sufficiently stable conditions.
Six million year diagenetic history, North Coles Levee, San Joaquin Basin, California
Authors: Boles J. R. 
Published: April 2008

Summary

North Coles Levee is an uppermost Miocene turbidite sandstone reservoir in the San Joaquin Basin. The sands have undergone progressive burial and a complex history of siderite, dolomite and calcite cementation. Precipitation of calcite cement bands occurred between 40 and 80°C with the last cementation event corresponding to about 2.5 Ma
bp
. Between this time and the present, extensive dissolution of plagioclase and calcite resulted from influx of acid during kerogen maturation. Emplacement of 500 million barrels of oil closely followed or was contemporaneous with these events. Very late stage compaction effects including albitization of plagioclase and dolomite crystallization in crushed biotite have resulted from cement removal and/or fluid pressure drops during release of the gas cap.
Aspects of the diagenesis of the Sherwood Sandstones of the Wessex Basin and their influence on reservoir characteristics
Authors: Strong G. E. , Milodowski A. E. 
Published: April 2008

Summary
The Sherwood Sandstone Group sandstones of the Wessex Basin are important potential reservoirs for both hydrocarbons and geothermal brines. Analyses of conglomerate, sandstone and siltstone samples from outcrop and deep boreholes indicate a complex diagenetic history that has had important positive and negative effects on the reservoir properties. Over most of the basin, early diagenesis (eodiagenesis) is dominated by calcrete development but, towards the basin centre, non-ferroan dolomite associated with early evaporitic sulphate cements that appear to have been deposited in an inland sabkha or playa environment are important. Later diagenesis (mesodiagenesis) is characterized by selective framework grain dissolution of feldspars, and anhydrite cementation and subsequent dissolution—processes that have locally yielded significant secondary porosity. However, in some cases this secondary porosity has been destroyed by the precipitation of late manganiferous ferroan calcite and ferroan dolomite. Quartz cements are locally important. Rocks near the present-day outcrop have been affected by weathering processes (telodiagenesis) that have resulted in decalcification, and the precipitation of kaolinite, illite and iron hydroxides. The key factors determining the permeability of the sandstones are the original grain size and degree of sorting, the precipitation and subsequent removal of anhydrite, and the extent of early and late carbonate cementation, framework grain dissolution and overgrowth.
Palaeocene-Oligocene tectonics of NW Europe
Authors: Dewey J. F. , Windley B. F. 
Published: April 2008

Summary
During the Palaeogene in NW Europe, a Gallic subplate was very nearly created. This plate was almost entirely bordered by plate boundaries, although not of equivalent ages. To the N, the N Atlantic began to open about 56 Ma ago, the Alpine convergent boundary extended southwestwards to the Pyrenean subduction zone; on the NE side a N British Palaeocene/early Eocene fracture-dyke boundary appears to pass southeastwards across the North Sea into a Lutetian to Burdigalian rift system.
There is an age disparity between the fracturing and NW dyke intrusion in Britain and the fracturing and formation of the Rhine Graben. The last dykes in Britain (Ypresian) were intruded not long before the important subsidence of the Rhine Graben (earliest Lutetian), but their causes were different. The British dyke fractures propagated southeastwards via the Cleveland Dyke towards the North Sea, whereas the Rhine Graben propagated northwards towards the Zuider Zee depression. However, the structures from both sides converged in the Sole Pit Basin in the North Sea with the result that a continuous though not coevally-originated fracture system developed from NW Scotland to the Alps.
Origin of the Palaeogene Vøring Plateau volcanic sequence
Authors: Viereck L. G. , Taylor P. N. , Parson L. M. , Morton A. C. , Hertogen J. , Gibson I. L. ,   the ODP Leg 104 Scientific Party
Published: April 2008

Summary
During ODP Leg 104 a 900 m thick sequence of volcanic rocks was drilled at Site 642E on the Vøring Plateau. It was subdivided into an Upper and a Lower Series separated by 7 m of estuarine volcaniclastic and epiclastic rocks. The Upper Series comprises transitional-type mid-oceanic ridge tholeiites with chemical affinities to the Faeroes and E Greenland plateau basalts and to Recent basalts from transitional segments of the mid-Atlantic ridge such as Reykjanes Ridge. The magmas are interpreted as having been derived from a secondarily LIL-element enriched, primarily strongly depleted mantle source. Some flows and dykes show evidence of minor contamination by upper continental crustal rocks. Interlayered tuffs are more differentiated and ferrobasaltic, and are interpreted to have been derived from a LIL-element enriched plume-type mantle source. The Lower Series contains 13 peraluminous dacite flows representing melts of upper crustal metasedimentary rocks such as shales and greywackes. The dacites overlie five basaltic andesite flows formed by mixing of LIL-element depleted tholeiitic magma with upper crustal melts.
Late Cretaceous and early Tertiary extension and volcanism around the Faeroe Islands
Authors: Andersen M. S. 
Published: April 2008

Summary
In an area around the Faeroe Islands, seismic reflection profiles have been used to map the depth to the surface of the early Tertiary basalts and the distribution of three different types of reflector configurations within the basalts. Using this information, the early Tertiary physiography of the area has been deduced.
The evolution of the seaward-dipping reflector sequence N and NW of the Faeroe Islands is discussed. In models of simple rifting of a normal lithosphere, extension is accompanied by significant subsidence (probably more than 2 km) before a large quantity of magma is produced. It is suggested that in the study area, subsidence prior to magma generation was reduced or avoided due to an additional increase in the geothermal gradient above that predicted by rifting models.
Finally, the relation between extension, volcanism and subsidence before and after the early Tertiary opening of the NE Atlantic Ocean is discussed.
Petrochemistry and isotope geochemistry of early Palaeogene basalts forming the dipping reflector sequence SW of Rockall Plateau, NE Atlantic
Authors: Merriman R. J. , Taylor P. N. , Morton A. C. 
Published: April 2008

Summary
A transect across the passive margin SW of Rockall Plateau was drilled during DSDP Leg 81. Site 555 was located on continental crust of the Rockall Plateau. Sites 552 and 553 were located over the wedge of dipping reflectors, and Site 554 drilled the outer high at the transition from the thick wedge of dipping reflectors to normal oceanic crust. New trace-element and Pb-isotope data are presented which characterize three basalt types: (1) Dipping reflector basalts resemble depleted N-type MORBs, strongly depleted in light rare earth elements (LREEs) and other incompatible elements; they have been contaminated by U-depleted continental crust, or alternatively derived from depleted subcontinental lithosphere. (2) Outer high basalts are less strongly depleted N-type MORB and show no evidence of crustal contamination. (3) Site 552 basalts postdate the onset of subsidence of SW Rockall margin; they are relatively evolved N-type MORB derived from a source similar to that of the outer high basalts but show evidence of crustal contamination. Genesis of the dipping reflector basalts is best explained by a stretching/rifting model, not the ‘subaerial seafloor spreading’ model.
Tertiary volcanism in the Kangerdlugssuaq region, E Greenland: trace-element geochemistry of the Lower Basalts and tholeiitic dyke swarms
Authors: Gill R. C. O. , Nielsen T. F. D. , Brooks C. K. , Ingram G. A. 
Published: April 2008

Summary

Tholeiitic dyke swarms in the Kangerdlugssuaq area can be divided, like the Lower Basalts, into two broad categories: (1) an early tholeiitic ‘Picrite-ankaramite Series’ (PAS) with relatively high levels of incompatible elements; and (2) a less incompatible-element enriched ‘Tholeiitic Series’ (TS) to which the majority of the dykes belong. The latest of the tholeiitic dykes tend to have the lowest incompatible element contents. The PAS dyke samples have steep light-enriched rare earth patterns and strongly fractionated spidergrams ([Nb/Yb]
N
>20), with no conclusive elemental evidence of significant crustal contamination. TS dykes have less steep rare earth element (REE) patterns and spidergrams, the latter having negative Sr anomalies. Corresponding PAS and TS lavas have similar patterns, although the Nb/La, Ta/La and P/Nd ratios are significantly lower.

The PAS dyke samples are more enriched in highly incompatible elements than typical ocean island basalt (OIB) tholeiites. Derivation of these magmas from a sub-lithospheric OIB source, such as that presently contributing to Icelandic volcanism, is tenable only if the picritic magmas underwent subsequent enrichment by incorporation of incompatible elements from sub-continental lithosphere or from continental crust. It is difficult to derive the observed enrichment entirely from the continental crust. The alternative is to envisage an OIB-source jet (the precursor to that currently situated under Iceland) that penetrates upward into the sub-continental lithosphere, and supplies parental picritic magmas which, in passing to the surface, scavenge incompatible elements from enriched lithospheric mantle.
Later tholeiitic lavas and dykes tend to be characterized by less steep incompatible-element profiles, resulting in two late dykes with spidergrams identical to sub-aerial Icelandic tholeiites. This trend may be interpreted in terms of a decreasing contribution of highly incompatible elements from the progressively thinner lithosphere.
Age constraints on Atlantic evolution: timing of magmatic activity along the E Greenland continental margin
Authors: Noble R. H. , Macintyre R. M. , Brown P. E. 
Published: April 2008

Summary
The results of a comprehensive K-Ar investigation of 130 petrographically selected rock and mineral specimens from the E Greenland Tertiary Igenous Province are reported. The new age data refine the chronology of this region. They suggest the following conclusions: (1) the lavas were extruded between 54 and 57 Ma; (2) the pre-flexure dyke swarm was intruded between 51 and 53 Ma; (3) the postflexure dyke swarm was intruded between 49 and 51 Ma; and (4) plutonic activity occurred at 50 Ma (Kangerdlugssuaq, Kraemers Ø, Lilloise, etc.), 42–45 Ma (Borgtinderne, Kap Deichman, etc.), 36 Ma (Kialineq), 38 and 28 Ma (Kong Oscars Fjord, Myggbukta).
A palaeomagnetic study of lower Tertiary lavas in E Greenland and comparison with other lower Tertiary observations in the northern Atlantic
Authors: Tarling D. H. , Hailwood E. A. , Løvlie R. 
Published: April 2008

Summary
Rock samples from five separate profiles, totalling 2.3 km, in the E Greenland lower Tertiary plateau basalts exhibit well-defined magnetic vectors and stability to both alternating magnetic fields and thermal demagnetization. Stability is confirmed by baked-contact evidence and the preservation of a secular variation record. The remanences are all of reversed polarity and the age relationships suggest that these were acquired during chron C24R, immediately prior to the formation of oceanic magnetic anomaly 24. Radiometric age determinations from the N Atlantic Igneous Province suggest that the plateau basalts of E Greenland may be of similar age to those of the Faeroes, but differ in age from those of the British Tertiary Igneous Province. Palaeomagnetic pole positions for each province suggest an age similarity between the Faeroes and British plateau basalts, while their age relationship to the E Greenland basalts is not clear. The pole positions generally confirm the published continental reconstructions for this period. The simplest polarity solution is for the magnetization of the British and Faeroes basalts to be predominantly associated with chrons C24R and C26R, with only chron C24R being represented in E Greenland. However, this model is dependent on the radiometric constraints on the Palaeocene/Eocene boundary.