bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemical Study of the Brioverian (late Proterozoic) Volcanic Rocks in the Northern Armorican Massif (France). Implications for Geodynamic Evolution During the Cadomian
Authors: Cabanis B. , Chantraine J. , Rabu D.
Published: April 2008

A major and trace element geochemical study has been made of the Brioverian volcanic rocks in the NW of Brittany. They are compared with recent and modern volcanic rocks to identify their magmatic characteristics and thus to relate them to different tectonic contexts and integrate them into an overall geodynamic interpretation.

Three complexes in different locations across the Cadomian orogenic belt have been studied:

The volcanic rocks in Lannion Bay are typical of calcalkaline rocks of frontal volcanic arcs associated with subduction.

The volcanic rocks of St Brieuc Bay are similar to tholeiitic within-arc or back-arc volcanic rocks, associated with internal extension at a convergent plate margin.

The Fresnaye Bay volcanic rocks, however, resemble those from extensional within-plate settings and are quite different from the other two volcanic associations.

These various volcanic settings are integrated into a general model of geodynamic evolution in the Armorican Massif during the Cadomian, to which features of the regional geology lend support.
Fluid flow in sedimentary basins: a geologist’s perspective
Authors: Chapman Richard E.
Published: April 2008

Water begins to flow from higher-energy to lower-energy positions as soon as sediment begins to accumulate in a new sedimentary basin, and flow continues during the sedimentary development of the basin. Water expelled by compaction from the compactible less-permeable beds displaces water in the less-compactible more-permeable beds. Compaction flow also causes subsidence that contributes to the sedimentary development of a basin.

The pattern of flow is determined by the stratigraphy. In transgressive sequences, it is downwards in mudstone to the basal permeable unit and then lateral towards the land of the time. In regressive sequences, it is upwards and downwards in mudstones towards the intercalated sandstones and then lateral in these towards the land. In compacting sequences of alternating sandstones and mudstones, the sandstones drain distinct isolated hydraulic units that are related to the stratigraphy.

When a sedimentary basin ceases to accumulate sediment and compaction has reached equilibrium, the flow patterns change and the main energy comes from elevation of the intake areas.

In rift basins the main source of compaction water is the thick mudstone in the post-unconformity-disconformity sequence. Abnormal pressures in the pre-unconformity-disconformity sequence indicate lack of flow in this part of the basin.
Fluid flow in the Chalk of England
Authors: Price Michael
Published: April 2008

The Chalk aquifer of England can be thought of as a multi-porosity medium. The matrix is a fine-grained limestone which generally has high porosity but small pore throats, so that its permeability is typically only 0.1 to 10 millidarcys (10
−4
to 10
−2
m day
−1
). A fairly uniform fracture system imparts a secondary permeability, which appears to be about 100 to 1000 mD (0.1 to 1 m day
−1
). Where the Chalk forms a major aquifer, most of the transmissivity results from the enlargement of fractures, by solution, to form a few highly permeable zones. Over much of the outcrop, weathering leads to the development of shallow permeable layers to form an additional component. Each of these permeability systems influences some aspect of subsurface water movement, with implications for resources, quality and construction.
The role of low-permeability rocks in regional flow
Authors: Alexander J. , Black J. H. , Brightman M. A.
Published: April 2008

The role of low-permeability rocks in determining fluid migration within sedimentary basins has been shown to be crucial for deriving the direction and rate of fluid movement. The magnitude of the flows depends on the geometry of the sedimentary basin and on the bulk hydrogeological properties of the various strata and their relative values. Measurements of head and pore-water compositions of argillaceous formations show separate evidence for the existence of cross-formational flow resulting in groundwaters of mixed origins. Numerical and analytical models have been developed and applied to some basinal systems in England. They have been used to assess the directions of groundwater flow likely to be encountered by repository site investigation programmes and to aid in the interpretation of groundwater head and chemistry data. Such flow models still require detailed validation. Our knowledge of the flow mechanism and the physical properties of mudrocks at depth is still elementary, but the indications are that the role of low-permeability rocks in controlling fluid migration has been much underestimated.
The flow of water and displacement of hydrocarbons in fractured chalk reservoirs
Authors: Brown David A.
Published: April 2008

The fractured chalk reservoirs of the southern part of the Central Graben do not exhibit the flow characteristics more usually encountered in continuous sandstone formations. The Danian and Maastrichtian age formations of the Ekofisk Field are typical of these fractured chalk reservoirs. The Ekofisk Field has been extensively studied to understand better the geology and controlling fluid flow mechanisms prior to commencing enhanced oil recovery projects. During waterflooding the imbibition of water from the fracture network into the rock matrix will control the displacement of hydrocarbons. Extensive laboratory studies have been carried out to quantify the imbibition characteristics of the Danian and Maastrichtian formations. Several parameters that control the amount of water imbibed have been found. A field water-injection pilot project has been implemented to verify the mechanisms and laboratory results and to prove numerical techniques suitable for the prediction of recoveries under a full-field waterflood.
Fluviatile deposits and morphology of alluvial surfaces as indicators of Quaternary environmental changes in the southern Negev, Israel
Authors: Grossman S. , Gerson R.
Published: April 2008

Quaternary environmental changes in the southern Negev are recorded in the alluvial deposits and the geometry of depositional surfaces in the Timna Valley. There are some distinct differences between the alluvial deposits of the Holocene to modern flood plains and earlier (Pleistocene) ones. The Holocene to modern alluvium is characterized by sand, fine- to medium-sized gravel and very coarse gravel in large gravel bars. The sediments indicate environments ranging between extremely arid and semi-arid dominated by floods of low to moderate magnitude but with occasional short large floods. The Pleistocene sediments include: (1) debris-flow deposits indicating talus activity under a moderately arid to semi-arid climate; (2) alternating fine-grained and gravelly layers reflecting floods of low to moderate magnitudes. The nature of the soils supports these conclusions. Thick and argillic Reg soils are typical of the Pleistocene surfaces whereas shallow loamy ones are characteristic of the Holocene terraces. The alluvial surfaces usually converge downstream. The intersection points migrate upstream, following stages in the recession of a major escarpment. Broad pedimented surfaces and bajadas are characteristic of the Pleistocene period, whereas long narrow terraces of gentle declivity were formed during the Holocene. The latter suggest depletion of the available debris and in the effectiveness of large floods.
Rates and modes of dust accretion and deposition in an arid region—the Negev, Israel
Authors: Gerson R. , Amit R.
Published: April 2008

Rates of dust accretion and deposition are dependent on the amount of available dust and the trap efficiency of a particular site. Several types of dust-trapping terrains are widespread in deserts: (1) Gravelly (Serir) surfaces that turn with time into Reg soils; (2) vegetated surfaces in the desert fringe that may turn into löessial terrains; (3) stabilized sand dunes; (4) playa surfaces. Loessial terrains exhibit a high rate of dust accretion during the late Pleistocene—0.07–0.15 mm a−1on the interfluves and ≤0.5 mm a−1along the flood plains. Gravelly surfaces usually trap about 0.1 mm a−1of dust initially but the rates decrease to several μm a−1due to plugging with dust and salts and may ultimately remain constant as a gravel-free B horizon develops. The amounts of imported dust, from both local and distant sources, have changed during the Quaternary due to climatic fluctuations. Roofless ancient building—most efficient dust traps—show that although large amounts of dust were available (much of it from local sources) during the late Holocene, there was not intensive dust accretion during this period due to increasing aridity and decreasing trap efficiency. Wash and gullying led to destruction of the once widespread efficient trapping terrains.
Complex star dunes and associated aeolian bedforms, Hopeman Sandstone (Permo-Triassic), Moray Firth Basin, Scotland
Authors: Clemmensen L. B.
Published: April 2008

The Hopeman Sandstone, which is exposed at the southern edge of the Moray Firth Basin, is composed of a number of aeolian sandstone units. These units, which have been mapped continuously for 4.5 km, represent part of an ancient sand sea in which complex star dunes alternated with sand-filled star-dune corridors. The palaeowind regime was complex with prevailing winds from the NNE, secondary winds from the SSE and subordinate winds from the NW. The complex star dunes are represented by two types of deposits: Type I—large-scale or giant-scale mainly trough-formed high- to medium-angle cross-bedding, which constitute the slip-face deposits of actively migrating star dunes; and Type II—bimodally dipping more wedge-shaped large-scale or giant-scale sets, which apparently formed on the opposed flanks of relatively stationary star arm segments. The exposed star arms trend in three main directions being roughly parallel with the palaeowind directions. The dune-dune corridors contain small- and medium-sized crescentic or reversing dunes which frequently overlap the star-dune flanks, interdune flats, and rare ephemeral streams. The palaeowind pattern inferred from the Hopeman Sandstone is very similar to that inferred from early Triassic reversing dune deposits in E Greenland, suggesting the existence of a regional Trade Wind system in the period near the Permo-Triassic boundary.
Aeolian sedimentation from the Galtymore Formation (Devonian), Ireland
Authors: Carruthers R. A.
Published: April 2008

The Galtymore Erg (Devonian) of the Galtee Mountains, southern Ireland can be reconstructed by the coupling of field relationships with current hypotheses and observations from other ancient aeolian sequences and modern ergs. Stratigraphical relationships show that the Galtymore Erg formed at the northern, fault-controlled, margin of the Munster Basin. Northern sequences, close to the basin margin, are dominated by northerly derived, proximal braided stream deposits. Two types of aeolian-dominant sequences occur within the Galtymore Formation. A marginal erg facies, characterized by simple crescentic dunes showing S-N palaeocurrents is interbedded with fluvial deposits. To the S an erg facies is developed which is characterized by the vertical repetition of cosets of large-scale cross-strata and thin horizontal deposits. The cosets are interpreted as representing the downwind migration and climbing of large dunes and interdune areas, respectively. The geometrical relationship of the hierarchy of bounding surfaces suggests that the dunes of the erg facies were compound crescentic in form, with enclosed interdune areas. The distribution of stratification types and foreset dip dispersion similarly suggest crescentic dunes. Estimates of compound dune wavelength and some dune and interdune dimensions can be made. Crescentic dunes (simple and compound) were moved by south-southeasterly palaeowinds towards the foot of alluvial fans.
Sand sheets of the eastern Sahara and ripple blankets on Mars
Authors: Breed C. S. , McCauley J. F. , Davis P. A.
Published: April 2008

Landsat
images of the eastern Sahara (North Africa) show widespread sandy plains covered by broad, gentle undulations (not dunes) that are interpreted as giant aeolian ripples. This regionally extensive bedform pattern embodies the entire upper unit of the Selima Sand Sheet. This unit, which we interpret as modern and presently active, contains very coarse sand, granules, and small pebbles, medium to very fine sand, and silt in horizontally laminated pairs that form a tabular deposit 1 cm to 10 m thick. It is commonly underlain by a stabilized sand sheet unit, and one or both units are disconformable on an erosion surface of truncated alluvium, mixed alluvial-colluvial-aeolian deposits, or bedrock. Sediment in the sand sheet is derived from alluvium, eroded mostly from sandstones in the Nubia Formation and widely distributed in dry, relict valleys. The upper surface of the sand sheet is a layer of pebbles and granules identical to coarse particles in the internal layers and in the truncated alluvial valley fill. Elongate, thin ripples composed of small pebbles are driven across this surface by prevailing north winds. Fine to medium sand is widely dispersed across the sand sheet from isolated dune fields. Silt particles are added seasonally by dusty winds (Khamsins) from the S. The Selima Sand Sheet may provide an analogue for some thin, horizontally-bedded aeolian sandstones. It offers an explanation for ripple blankets on Mars, where locally-derived coarse particles transported by surface creep and saltation are mixed with globally-derived fine particles deposited from dust storms.
Surficial deposits of Qatar Peninsula
Authors: Ashour M. M.
Published: April 2008

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Structure, composition and age of the Faeroe basalt plateau
Authors: Waagstein R.
Published: April 2008

The lower Tertiary Faeroe basalt plateau is centred around the Faeroe Islands in the northern part of the supposed Faeroe-Rockall microcontinent. It consists of subaerial lavas, divided into three formations. The lower formation is probably thickest (>3 km) in the southern or central part of the islands, whereas the middle and upper formations are probably thickest (> 2 km in total) just N of the Faeroes, close to the line of opening between the Faeroes and Greenland. The lower formation consists of tholeiites relatively low in TiO
2
. The lower part of the middle formation shows an upward progression from high-Ti olivine tholeiites to high-Ti tholeiites, whereas the entire upper and rest of the middle formations consists of a contrasting population of high-Ti tholeiites and low-Ti MORB-like olivine tholeiites to tholeiites. The MORB type is confined to the northern Faeroes, increasing in abundance upwards. Similar MORB-like basalts have been recovered from further SW along the microcontinent (Faeroe Bank, Bill Bailey Bank, Lousy Bank and the Rockall Plateau), being associated with high-Ti tholeiites near the Faeroes and away from the Tertiary line of opening (Faeroe and Bill Bailey Banks). In addition, transitional to mildly alkaline basalts have been recovered on the E side of the Faeroe Bank Channel and on Bill Bailey Bank.

Based on field revision of the magnetostratigraphy of the lower formation it is tentatively suggested that the lower formation was extruded during chrons C26R to C25N and the two higher formations during C24R. Accumulation rates seem to have varied systematically and to have been related to the abundance and type of sediments between the lavas, which may indicate that extrusive activity ceased in the Faeroes before the opening of the NE Atlantic in C24R, and explain the presence of conjugate strike-slip faults in the NW Faeroes.

The spatial distribution of the various basalt types fits a model of shifting axes of intraplate volcanicity, shifting from the Faeroes to E Greenland and in part back again.
The geochemistry and origin of the Faeroe-Shetland sill complex
Authors: Gibb F. G. F. , Kanaris-Sotiriou R.
Published: April 2008

Geochemical investigations of dolerite cores from four intrusions in the recently discovered Faeroe-Shetland sill complex have established that the sills are of transition (T) mid-ocean ridge basalt (MORB)-type composition. Some uncertainty surrounds the age of the complex, but there is no doubt that it is, at least in part, of Tertiary age. Comparisons with previously proposed models for the development of sill-sediment complexes during initial stages of seafloor spreading suggests that the Faeroe-Shetland sills may represent an intrusive episode associated with a spreading axis that eventually produced oceanic crust W of the Faeroes.
Basaltic magmatism of late Cretaceous and Palaeogene age recorded in wells NNE of the Shetlands
Authors: Fitch F. J. , Heard G. L. , Miller J. A.
Published: April 2008

Two wells drilled by Sovereign in 1984 NNE of the Shetlands (Wells 219/28-1 & 219/28-2) passed through numerous primary basic ashfall tuff horizons within a sequence of abundantly tuffaceous rocks representing the basal early Eocene ‘ash-marker’. Small amounts of volcanic debris can be recognized in adjacent strata. In one of the wells, two chemically distinct sills of olivine-dolerite were penetrated below the ‘ash-marker’ in late Cretaceous rocks. The probable magmatic sequence revealed by petrographic studies, K/Ar and
40
Ar/
39
Ar dating is:

(1)

The near-sediment/sea water interface intrusion of a dolerite sill into Santonian strata around 80 Ma in early Campanian times (Well 219/28-2).

(2)

A major episode of explosive volcanic eruptions depositing many primary basic ashfall tuffs in the basal early Eocene (both wells).

(3)

Minor further volcanic activity continuing throughout most of the early Eocene (both wells) with two slightly more significant eruptions in the late early Eocene and early mid-Eocene respectively.

(4)

The intrusion of a dolerite sill into Campanian strata close to 50 Ma in mid-Eocene times (Well 219/28-2).
Early Tertiary basalts and tuffaceous sandstones from the Hebrides Shelf and Wyville-Thomson Ridge, NE Atlantic
Authors: Stoker M. S. , Morton A. C. , Evans D. , Hughes M. J. , Harland R. , Graham D. K.
Published: April 2008

A sequence of Palaeocene basalts and late Palaeocene-early Eocene tuffaceous sandstones has been recovered from three boreholes on the northern Hebrides Shelf and Wyville-Thomson Ridge. The basalts, cored in boreholes 85/5B and 85/7 on the northern Hebrides Shelf, can be seismically correlated with the extensive N Atlantic lava sequence. The basalt in 85/5B displays major element chemistry typical of alkali-basalts but trace and rare-earth element (REE) patterns are typical of olivine-tholeiites. In contrast, the basalt in 85/7 displays geochemical characteristics typical of a depleted tholeiite. Despite their different compositions, the basalts have similar ratios of the highly incompatible elements La and Ta which may reflect different degrees of partial melting of similar source material. The tuffaceous sandstones, cored in borehole 85/2B on the Wyville-Thomson Ridge, were deposited in a warm, shallow marine environment contemporaneous with the reworking of nearby basaltic terrains.
Nummulites rockallensis
Hinte & Wong occurs abundantly throughout the sandstones; its only previous known occurrence is from latest Palaeocene-earliest Eocene sediments at DSDP Site 117A on Rockall Bank. Regional mapping indicates that the tuffaceous sandstones overlie the basalts, thus the latter can be assigned to the Palaeocene (or older). This is consistent with K-Ar dates of 57.3–63.1 Ma obtained from the basalts.
Time and duration of activity in the British Tertiary Igneous Province
Authors: Mussett A. E. , Dagley P. , Skelhorn R. R.
Published: April 2008

The time and duration of igneous activity in the separate component areas of the British Tertiary Igneous Province (BTIP) are investigated using radiometric dates that pass internal consistency tests, used in conjunction with palaeomagnetic polarities and the available stratigraphic information.

Reliable results are available only for the currently subaerial parts of the Province. It is found that activity occurred within the approximate interval 63–52 Ma, with most activity at about 59 Ma. Later activity was predominantly of acid magmas though basic rocks preponderate in the province as a whole. The types and span of igneous activity in the separate areas followed no common pattern but areas of more complex geology tend to have had a longer span of activity.

Magnetic polarities are predominantly reversed, with all the lavas having this polarity. This predominance is probably largely, but not entirely, due to reversed polarity intervals being longer than normal ones at this time, and with much activity occurring in a single reversed interval. The sequence of polarities found within the BTIP cannot be fully reconciled with the polarity timescales of either Harland
et al.
(1982) or Berggren
et al.
(1985).
Rb-Sr isotopic determinations and the timing of Tertiary central complex magmatism in NE Ireland
Authors: Meighan I. G. , McCormick A. G. , Gibson D. , Gamble J. A. , Graham I. J.
Published: April 2008

Full Rb-Sr isotopic data and whole-rock regressions are presented for each of the Mourne Mountains granites (G1–G5 inclusive) and a set of granophyres/microgranites from the Slieve Gullion central complex. The former were intruded at approximately 56 Ma (G1–G4 inclusive) and later (G5), and
may
all be younger than the major intrusions of the Slieve Gullion and Carlingford central complexes. The initial Sr isotope ratios are >0.7060 for all five Mourne granites, the Slieve Gullion granophyres/microgranites analyzed and the rhyolitic rocks of the Tardree area (Co. Antrim), and crustal involvement is indicated in their petrogenesis. In the E Mourne Centre the first (the least silicic and most Sr-rich) granite has a significantly higher initial ratio (0.7129 ± 2) than G2 (0.7109 ± 3) and G3 (0.7104 ± 6), and the isotopic data provide further support for the recent revision to the mapping of the first and second granites. Preliminary Rb-Sr isotopic data for a comprehensive suite of Tertiary rhyolites from NE Ireland, some separated by >50 km, define a linear array whose explanation is considered.
The record of early Tertiary N Atlantic volcanism in sediments of the North Sea Basin
Authors: Knox R. W. O’B. , Morton A. C.
Published: April 2008

Volcaniclastic deposits in sedimentary sequences of the North Sea Basin and adjacent areas indicate that two phases of early Palaeogene explosive volcanism took place in the north-eastern Atlantic region. The earlier, late Palaeocene (NP5-NP6) phase involved significant activity along a N-S trend that included both the British and Faeroe-Greenland Tertiary volcanic provinces. The later phase spanned the latest Palaeocene and early Eocene (NP9 to NP13), with much or all of the activity taking place in the Faeroe-Greenland Province. Early ashfalls of mixed basaltic to silicic compositions may have included contributions from the final phase of British volcanism, but were followed by a series of 200 or more tholeiitic ashfalls of Faeroe-Greenland provenance. These tholeiitic eruptions appear to have marked the onset of separation of Greenland from Europe in mid NP10 times. A subsequent return to pyroclastic activity of more variable compositions appears to have marked the re-establishment of stresses within the E Greenland crust that continued throughout the early Eocene (mid NP10 to end NP13). The mechanism of eruption of the tholeiitic ashes, which are equivalent to a magma volume of several thousand cubic kilometres, is uncertain, but they would appear to involve hydrovolcanic processes.
Large Australian lakes during the last 20 million years: sites for petroleum source rock or metal ore deposition, or both?
Authors: De Deckker P.
Published: April 2008

Three basic ‘regimes’ of large lakes are recognized: (1) deep lake, (2) lake with ephemeral (and shallow) water, and (3) dry lake. The characteristics of these three lake types are discussed with regard to the composition of their water and sediment chemistry, but also to their potential for the production and preservation of organic matter, as well as the adsorption and accumulation of metals. Large Australian lakes, some with a sedimentation record spanning the last 20 million years, are examined in line with the above concept. Because of the low sedimentation rates in those lakes and the present-day water chemistry predominantly leading to the precipitation of gypsum and halite (especially in the groundwater below the lakes), it appears that any organic matter that would have been produced in the lakes during their carbonate phase in Miocene time would have been consumed by bacterial activity mainly in the groundwater. It is therefore suggested that the search for sites of metalliferous accumulation may prove more rewarding than will exploration for oil within sediments deposited since the early Miocene. As a comparison with large lakes on other continents, suggestions are presented as to what would have happened if sedimentation rates were much greater and if the lakes were in proximity to volcanism.