bluuebunny/crossref_metadata_2025 huggingface dataset:
Deformation of polycrystalline salt in compression and in shear at 250–350°C
Authors: Franssen R. C. M. W. , Spiers C. J.
Published: April 2008

Dry synthetic polycrystalline salt (NaCl) has been deformed in uniaxial compression and in shear in order to gain insight into the influence of deformation mode on the development of crystallographic preferred orientation, microstructural evolution and mechanical behaviour (flow strength). The experiments were carried out between 250 and 350°C, at strain rates between 10
−5
and 10
−7
s
−1
. Under these conditions halite deforms by climb-controlled dislocation creep. In our samples, the presence of deformed grains containing cellular networks of subgrains, and the development of texture are consistent with such mechanisms. The weaker {110} <110> slip plane appears to align parallel to the shear plane and perpendicular to the uniaxial compression direction. In the sheared samples, a <111> maximum is observed parallel to the shear direction. The textures obtained agree favourably with texture simulations. When the mechanical behaviour seen in uniaxial compression and shear is compared in terms of equivalent stress and strain, as defined in the von Mises theory of isotropic plasticity, the uniaxially deformed samples appear to be stronger. From a detailed consideration of the data, it is inferred that the difference in flow strength is largely due to anisotropy resulting from texture development. The results suggest that the assumptions underlying the von Mises theory of isotropic plasticity are not applicable in the present case, and that the associated flow rule does not offer an accurate method of generalizing creep laws for salt under the conditions investigated.
The SEM/ECP technique applied on twinned quartz crystals
Authors: Olesen Niels Ø. , Schmidt Niels-Henrik
Published: April 2008

The SEM/electron channelling technique, ECP, is useful in fabric studies, since a three-dimensional lattice orientation determination is performed with a precision of
c
. 0.5° from areas a few microns in diameter. Computer simulation of the ECP image (a line diffraction pattern) facilitates analysis, particularly in the case of low-symmetry crystals such as alpha-quartz. The most important types of twinning in quartz are after the Dauphiné and Brazil laws. The latter combines a left- and a right-handed crystal (enantiomorphism), and it is shown that it is
not
possible to discriminate between these two crystals, since the twin plane coincides with mirror planes in the ECP sphere resulting in identical ECPs for the individual Brazil twins. In contrast, Dauphiné (and other) twins are easily identified because their twin planes do not coincide with mirror planes. A specification is given of the point groups which permit a complete lattice orientation determination, and those which involve a loss of information (orientational solubility).
The undrained shear behaviour of fine-grained sediments
Authors: Yassir N. A.
Published: April 2008

A series of high pressure triaxial tests were conducted on three normally consolidated mud volcano silty clays to explore some of the effects on the undrained shear behaviour of argillaceous sediments. The tests followed two different consolidation paths before undrained shear: isotropic, with equal all-around stresses and anisotropic, at a horizontal to vertical effective stress ratio of 0.6. The tests showed that shear stresses produce overpressuring in the fine-grained sediments, and that the behaviour is sensitive to stress path, stress magnitude and grain size. Other important effects on the behaviour are also thought to include natural porosities and cementation.
Transition between seismic and aseismic deformation in the upper crust
Authors: Gratier J. P. , Gamond J. F.
Published: April 2008

Displacement on faults is often accommodated by a succession of two mechanical processes: aseismic sliding mass transfer and seismic cataclastic events. Pressure solution is attested both by dissolution markers of asperities which prevent sliding on and around the fault zone, and by the mechanism of growth of the mineral fibres by aseismic crack-seal in cavities opened by sliding. The cataclastic process is attested by observations of broken and kinked fibres, and by observations of euhedral crystals in the cavities opened by fault sliding. Natural examples are given to recognize and balance the mass transfers. Finally, a model is proposed to explain seismic/aseismic transitions on a given fault based on a gradual reduction in the cataclastic strength of the fault during sliding by successive breaking of asperities, and on the principle of minimum work consumed during the slip since the energy needed either for pressure solution sliding or for the cataclastic event may vary differently with the progressive sliding. Depending on the limiting processes for pressure solution slip, stable aseismic or unstable slip with seismic/aseismic transition is predicted.
Extensional veins and shear joint development in a thrust-fold zone (Northern Apennines, Italy)
Authors: Sani Federico
Published: April 2008

The Northern Apennines thrust system consists of clastic foredeep deposits that have been progressively involved in thrusting towards the northeast from the Oligocene to the Recent. The outcroping geological units are progressively younger towards the foreland implying that sedimentation and deformation developed together. The Umbrian Sequence is the most external unit and has been affected by Late Miocene to recent thrusting. The Marnoso-arenacea Formation, a marly arenaceous turbiditic sequence, belongs to this Unit. This formation provides good examples of pre-thrust and thrust-related structures. An extensional calcite vein system with two sub-vertical and orthogonal sets having constant orientation, developed before thrusting. A later conjugate shear joint system which cross-cuts the extensional veins, were developed in the early stages of thrusting. Folds related to the thrusting (i.e footwall synclines or hangingwall anticlines) spatially reoriented the previously formed extensional vein and shear-joint systems.
Rotliegend sediment distribution: a result of late Carboniferous movements
Authors: Glennie K. W.
Published: April 2008

The distribution of Rotliegend desert sandstones in Northwest Europe was mainly the outcome of four geological events that originated in the Carboniferous or earlier.

(1)

Westphalian culmination of collision between north-moving Laurussia and the faster moving Gondwana resulted in the E-W trending Variscan Mountains, which were to be the main source of Rotliegend fluvial sediment over successor basins to the former back-arc foreland.

(2)

During final convergence between Laurussia and Gondwana, the western Variscan orocline probably acted as a wedge that put Laurussia into transtension and, utilizing old lines of weakness, the Iapetus Suture between the Scottish-Greenland and Norwegian Caledonides and the Tornquist Line, initiated NW-SE and conjugate NE-SW systems of grabens.

(3)

Late orogenic relative movement between Laurussia and Gondwana also caused strike-slip faulting, related rapid collapse of the Variscan Mountains and the extrusion of Lower Rotliegend volcanics through parts of the Variscan foreland. The end of volcanism was followed by thermal subsidence of the Permian basins, many of them at rates that exceeded those of sedimentation.

(4)

The northward drift of Laurussia carried NW Europe from the equatorial zone of Carboniferous coal deposition to the latitude of a northern hemisphere trade-wind desert, where it lay in the rain shadow of the Variscan Mountains.
A review of Jurassic chronostratigraphy and age indicators for the UK
Authors: Cox B. M.
Published: April 2008

The basis of the chronostratigraphic subdivision of the Jurassic System is the sequence of ammonite faunas. At present, the 11 Jurassic stages (representing approximately 70 million years) can be divided into 145 ammonite-based zones or subzones. Methods of subdivision and correlation by various microfossil groups have also been developed for practical and economic reasons. Dating by fossils (biostratigraphy) underpins the broader-based approachs of event and sequence stratigraphy. It also supplies the primary stratigraphic control in the development of a standard magnetic polarity time-scale (magnetostratigraphy) of global applicability. At present, the Jurassic System of the UK area is divided into 38 units (biozones and subzones) on the basis of dinoflagellate cysts and 23 on the basis of calcareous nannofossils. Coverage is less comprehensive using the benthonic groups (foraminifera and ostracoda) which are more affected by environmental controls. The Lower Jurassic is divided into 16 units on the basis of foraminifera, but higher in the System only local divisions can be recognised in the Bathonian and informal divisions in the Callovian and oldest Oxfordian. There is fuller but still incomplete coverage using ostracoda which have proved particularly useful in the non-marine, brackish and marginal marine sequences of the Bathonian and Portlandian. The recovery of radiolaria from Jurassic sediments in the UK area is a new and exciting development which allows division of Kimmeridgian and Portlandian strata in graben areas in the North Sea Basin.
Mesozoic magmatic activity in the North Sea Basin: implications for stretching history
Authors: Latin David M. , Dixon John E. , White Nicky , Fitton J. Godfrey
Published: April 2008

Recent developments in the theory underlying magma generation allow the volumes and compositions of melt generated during extension of the lithosphere to be used to make quantitative statements about basin evolution. In the Mesozoic North Sea Basin there is a well defined relationship between magma chemistry and the degree of lithospheric attenuation. The largest extents of melting (<2%) resulted in the alkali basalts of the Forties province which occur in an area where β (the stretching factor) may be as high as 2, and which erupted within 30 million years of the onset of extension. In off-axis areas of the rift the degrees of melting are smaller and melt compositions are more undersaturated and extreme (nephelinites and ultrapotassic rocks). In the Forties province the observed melt compositions may be reconciled with melting on the dry peridotite solidus at a normal (1280°C) potential temperature (
T
p
) assuming a relatively thin mechanical boundary layer (MBL) of 70 km prior to rifting. Alternatively, a
T
p
of 1380°C will produce the alkali basalts from dry periodotite at the observed extension factors with an initial MBL thickness of 100 km. A
T
p
as high as 1480°C, which would give rise to regional pre- and syn-rift uplift, can be ruled out because the observed extent of melting is too small, the amounts of extension too large, and the rift episode too short. Off-axis magmatism may be explained by melting within a MBL with an initial thickness of ∼100 km or more at a
T
p
of 1280°C. Models for extension of the lithosphere by simple shear along an initially planar detachment fault fail to account for either the existence or the location of North Sea magmatism, with or without elevated potential temperatures if the dry solidus governs melting. This conclusion is independent of the initial dip of the detachment and also holds when the initial dip in the lithospheric mantle is double that in the crust. The timing, nature, and location of magmatic activity can be used to provide information about the onset of rifting, the rates of extension, the amount of strain, and the thermal history of a basin. When linked with studies of faulting and subsidence this approach may be of some importance for hydrocarbon exploration.
Timing, nature and sedimentary result of Jurassic tectonism in the Outer Moray Firth
Authors: Boldy S. A. R. , Brealey S.
Published: April 2008

Jurassic sedimentation in the Outer Moray Firth took place under a changing tectonic regime, accompanied by regional transgression in the late Jurassic. An early phase of tectonism is recognised, accompanying the collapse of the Central North Sea Dome. This Mid-Cimmerian phase of tectonism is characterized by North-South, Viking trend, faults and reactivated Northeast-Southwest Caledonian faults. These predominantly Bathonian age faults controlled subsidence and deposition of the Middle Jurassic Rattray Volcanics Formation. The Rattray Volcanics are unconformably overlain by the Sgiath and Piper Formations, of Oxfordian to early Kimmeridgian age, deposits of a northward prograding delta that continued to be affected by movement on Mid-Cimmerian faults.

A dramatic change in tectonic regime occurred during the Kimmeridgian, possibly concurrent with the major Eudoxus Zone transgression, that heralded the onset of Kimmeridge Clay deposition. During this Late Cimmerian rift-phase, sedimentation took place under conditions of active extension controlled by northwest-southeast, Witch Ground Graben trend, faults. The Kimmeridge Clay Formation is a typical syn-rift sequence with sands deposited on the downthrown sides of rotational fault blocks, whose crests were commonly eroded. Studies of ammonites recovered from cored sequences have refined the interpretation of timing of tectonic and transgressive events and have also highlighted anomalies in correlation between ammonite and dinocyst zonation schemes.
Fault traps in the Northern North Sea
Authors: Spencer A. M. , Larsen V. B.
Published: April 2008

The 250 hydrocarbon finds in the northern North Sea have total resources of 8000 × 10
6
Sm
3
oil-equivalent and 70% are in fault block traps, all with Jurassic or older reservoirs. They can be classified into plays by reservoir age: pre-rift is pre-Jurassic and Lower-Middle Jurassic; syn-rift is Upper Jurassic. 40 finds have sufficient published information to allow analysis of the geological relationships, structural and stratigraphic, which have given rise to their faulted hydrocarbon traps. All of the pre-rift and some of the syn-rift finds are traps in footwall blocks. They form a series with respect to the amount of conformable versus unconformable cap rock. In many the up-dip seal is due to stratigraphic truncation of the reservoir below an unconformable cap rock: the hydrocarbon pool does not extend to the bounding fault. The amount of erosion on the fault blocks and the footwall uplift which occurred seem to be related to the magnitude of fault throw. Most of the other syn-rift finds are hanging wall traps with entirely conformable cap rocks. The Brae-trend finds are classic syn-rift traps, located in the hanging wall of a major fault, movement on which was responsible for the supply of the reservoir clastics. The traps in the Central Graben are the most varied: all are different, probably due to complications produced by the underlying Permian salt.
Occurrence and depositional environment of the Lower Cretaceous sands in the southern Witch Ground Graben
Authors: Bisewski Hans
Published: April 2008

Lower Cretaceous sands in the southern Witch Ground Graben are of Aptian-Albian age and are part of the Sola Formation. In the Witch Ground Graben the Sola Formation is generally argillaceous. However, in the southern Witch Ground Graben, thick sands are developed and the argillaceous Sola Formation is replaced, almost in its entirety, by the sandy Kopervik Formation, also called the ‘Bosun Sands’. The increased clastic input in the upper part of the Lower Cretaceous is associated with the regression in the Central North Sea as a result of the Austrian tectonism. The sands of the Kopervik Formation are derived from sources on locally exposed and rotated fault blocks and emergent terraces and are deposited as submarine fan or turbiditic sands.
Early Palaeogene tectonics and sedimentation in the Central North Sea
Authors: Milton N. J. , Bertram G. T. , Vann I. R.
Published: April 2008

The early Palaeogene deposits of the Central North Sea have been divided by Stewart into ten depositional units, on the basis of seismic stratigraphy. These are interpreted as the products of variations in relative sea level. The units may be traced from the shelf into the basin using wireline log markers correlated within a biostratigraphic framework. These are interpreted as the signature of transgressive maxima when basinal clastic supply was at a minimum. The attitude of coal or lignite beds within the shelfal areas can be used to demonstrate a period of Palaeogene net uplift, followed by tilting and sinking of the shelf-edge and basinal areas during the later depositional episodes. Other than by a mechanism of load-induced differential compaction around buried or partially buried Mesozoic features no significiant rejuvenation of Mesozoic faults can be demonstrated. The area of uplift extended at least as far as the western limit of the Beauly Formation.

The products of Stewart’s depositional episodes vary depending on the tectonic activity. During the uplift phase, thick massive basinal fans were deposited and little or no shelfal deposits are preserved. During the tilting/sinking phase thick progradational wedges were deposited, and basinal deposition was largely confined to shales with relatively small local sand systems.
Cenozoic subsidence and uplift in the North Sea region: Implications for mechanisms of basin formation
Authors: Morgan Richard K.
Published: April 2008

The scale on which flexural subsidence has taken place in the North Sea region from the Late Palaeocene is very much greater than the scale on which fault-controlled subsidence and flexural subsidence occurred during the Mesozoic. In addition, uplift of the Scottish Highlands occurred during the Cenozoic on a scale comparable with that of the subsidence in the North Sea region during this time.

The Moray Firth, Mesozoic to Tertiary basin is divided into two distinct areas as a consequence of its Tertiary subsidence history, the Inner Moray Firth being uplifted and eroded and the Outer Moray Firth undergoing apparently continuous subsidence. The geometries of the pre-erosional sequences in the Moray Firth area indicate a Cenozoic history of both subsidence and uplift, providing a link between the subsiding North Sea region and the uplifted Scottish Highlands. Using seismic mapping and cross sections constructed therefrom, across the Moray Firth and North Central Graben areas, the relative subsidence and uplift is displayed as part of a unified process of crustal flexuring.

It is suggested that the flexural subsidence and uplift in the North Sea region and Scottish Highlands during the Cenozoic is associated with the onset of sea-floor spreading of the North Atlantic to the west. Furthermore it is proposed that the magnitude of Cenozoic subsidence in the North Sea region does not relate to the magnitude of Mesozoic extension which preceded it, the two processes being driven by different mechanisms.
The kinematic evolution of the Coffee Soil Fault
Authors: Cartwright Joseph
Published: April 2008

This paper addresses the problem of the dating of motion on large, extensional faults, from the seismic expression of the sedimentary sequences deposited during and after the extensional deformation. A general discussion of the interpretational methods and pitfalls involved in a seismically based, kinematic analysis is made by reference to a case study of the Coffee Soil Fault. The Coffee Soil Fault forms the eastern boundary of the Central Graben of the North Sea Rift, and is located in the Danish Sector. From a comparison of the stratigraphy of the hanging wall and footwall blocks, it is suggested that rifting commenced in the Late Permian/Early Triassic. The rifting proceeded in two distinct phases, a non-rotational Triassic phase, resulting in parallel reflection configurations, and a rotational Mid-Late Jurassic phase, resulting in divergent reflection configurations. Basin-floor and fault-scarp relief that resulted from a Late Jurassic acceleration of motion across the fault was infilled in a parallel-onlap configuration by the post-rift Lower Cretaceous sequence. The case study is used to demonstrate that onlap of remnant fault scarps can easily be mistaken for syn-rift deposition, and that this can lead to considerable errors in the timing of the end of rifting.
Extensional structures and their tectonic inversion in the Western Alps
Authors: Coward M. P. , Gillcrist R. , Trudgill B.
Published: April 2008

During Alpine collision the western margin of Tethys was uplifted on large-scale thrust zones, to preserve and give excellent 3D exposures of Mesozoic fault geometries. The faults are particularly well exposed along the margins of the Belledonne and Pelvoux basement massifs, east of Grenoble. Jurassic normal faults were in places reworked and the basins partially inverted during Cretaceous to Miocene compression. They were subsequently locally reworked by an important phase of late Alpine extension.

Both listric and straight (rotational) normal faults occur in Variscan granitic basement and Mesozoic cover. Several orders of synthetic and antithetic faults can be mapped, recording strains which are probably related to bends or kinks in the surfaces of the master faults. The extension directions vary, apparently related to variations in anisotropy of the Variscan basement, in particular to the trends of Variscan mylonites. In some regions there was apparently simultaneous extension on two orthogonal fault orientations. On the northern margin of the Pelvoux Massif, late Alpine extensional faults rework both Variscan fabrics in the basement and Alpine mylonites in the cover, producing a large extensional duplex whose roof is a low-angle fault zone in schistose Lias. The late Alpine extension may be caused by gravitational collapse of the thickened crust, or by regional continental extension as part of the opening of the Gulf of Lyon and the Rhone—Bresse Basins.

Pulses of compressional tectonics occurred in the Middle Cretaceous, latest Cretaceous—Early Palaeogene, Eocene—Oligocene and Miocene. During each pulse the deformation occurred principally on foreland-propagating fold and thrust sequences but there are important break-back thrusts, e.g. along the Frontal Pennine Zone, where the thrusts slice across already-sheared and back-tilted rocks to give apparent extensional cut-off relationships. The Alpine compression reactivated Mesozoic faults in basement and syn-rift sediments, deforming the syn-rift graben fill by tight upright folds and back-thrusts. NW-trending Mesozoic lateral ramps gave the basin margin a tooth-like geometry, which on compression did not fit back into the sockets of the opposite margin, causing material to be squeezed out laterally as well as vertically. Such variations in thrust displacement, a form of continental escape similar to large scale indentation tectonics, were partially responsible for the development of the Alpine arc.
Description of brittle extensional features in chalk on the crest of a salt ridge (NW Germany)
Authors: Koestler Andreas G. , Ehrmann Werner U.
Published: April 2008

At Laegerdorf, northern Germany, several large quarries provide access to study extensional deformation features in three dimensions, in the chalk overburden of a salt diapir. Numerous individual faults and the intervening blocks have been analyzed with respect to detailed fault morphology and geometry of brittle structures within the blocks. The studied fault zones, with displacements in the range of several metres, display geometries which vary within well defined limits. The fault zones are mostly very narrow (some tens of centimetres), but can widen up to a maximum of 7 m. The fractures composing a fault zone have a dominant length below 3 m. A 3D reconstruction of one quarry shows the rhombohedral shape of the fault blocks with a block width of 100 to 150 metres and the long diagonal parallel to the salt-ridge axis. The block-internal deformation is characterized by fractures which seem to be interconnected in a network. The locality of Laegerdorf is comparable with that of the Albuskjell and Ekofisk hydrocarbon reservoirs of the central North Sea in terms both of the structural setting of the deformed chalk on top of a salt diapir and the characteristics of extensional features.
The displacement patterns associated with a reverse-reactivated, normal growth fault
Authors: Chapman T. J. , Meneilly A. W.
Published: April 2008

The displacement geometry of a seismically imaged reverse fault is investigated with the aid of displacement contour and horizon separation diagrams. The present downward decreasing reverse displacement resulted from the reverse reactivation of an originally normal growth fault.
Reliability and application of detrital opaque Fe-Ti oxide minerals in provenance determination
Authors: Basu Abhijit , Molinaroli Emanuela
Published: April 2008

A survey of detrital opaque Fe-Ti oxide minerals in Holocene sands derived exclusively from known source rocks in the Rocky Mountains, USA (semi-arid climate, high relief) and the Appalachian Mountains, USA (humid climate, low relief) shows that, despite obvious differences in weathering rates, the abundance of detrital ilmenite relative to parent rocks is similar in these two regions. Because ilmenite is very common in many crystalline rocks, ilmenite is considered to be a useful Fe-Ti oxide mineral for varietal studies. About 30% to 50% of detrital Fe-Ti oxide minerals are polymineralic grains with intergrowths of two or more phases. Discriminant function analysis indicates that morphological varieties of detrital Fe-Ti oxide minerals in combination with their chemical compositions are useful guides to provenance. An igneous or metamorphic source of 96% of the Holocene samples is correctly identified by this method.

The method is used to determine the relative contribution of igneous and metamorphic source rocks to the sandstones of the part of the Tertiary Renova Formation, which was deposited within a dissected magmatic arc in southwestern Montana, USA. A separate discriminant function is calculated using the chemical compositions of detrital ilmenite and morphological properties of detrital Fe-Ti oxide minerals in Rocky Mountain Holocene sands. Classification of detrital Fe-Ti oxide minerals in Renova sandstones using this function suggests that igneous and metamorphic rocks contributed 77% and 23% respectively to the Renova sandstones in this area.
Reworking of plant microfossils and sedimentary provenance
Authors: Batten D. J.
Published: April 2008

Reworked plant microfossils are commonly recovered from sediments along with palynomorphs regarded as in situ. Their potential for locating the source of terrigenous clastics is perhaps generally underestimated. Recognition of reworking may depend on taxonomic identification or differences in colour and preservation, and sometimes on variations in response to staining and fluorescence microscopy. This is demonstrable in some Mesozoic and Cenozoic sequences in NW Europe, with mixed assemblages suggesting derivation from deposits of more than one age and/or source area and geothermal history. Reworked palynomorphs can also indicate the presence of rocks or sediments in the vicinity of a site of deposition that have now been completely removed by erosion.
Palaeoweathering or diagenesis as the principal modifier of sandstone framework composition? A case study from some Triassic rift-valley redbeds of eastern North America
Authors: Velbel Michael A. , Saad Mounir K.
Published: April 2008

Previous studies to recognize a ‘palaeoweathering’ signature in the framework composition of ancient sandstones have been undertaken in study areas where ancient climates are inferred to have been more humid than the present climate. Unfortunately, in studies in which the ancient climate was more humid than the modern, reduction of modal unstable-grain abundance by ancient weathering cannot be distinguished from unstable-grain destruction by burial diagenesis. Reinterpretation of data from Triassic Chatham Group sandstones of the Deep River basin (North Carolina) and modern sands derived from similar source rocks shed new light on the matter, as the Triassic sediments were generated under more arid conditions than the modern wet climate. If diagenesis were the more important factor, the sandstones would have lower contents of unstable grains. If weathering were more important, the ancient sandstones would have higher contents of unstable grains. Chatham Group sandstones have higher unstable grain contents, indicating that the compositional signature imposed on the sandstones under the Triassic climate has survived without being obliterated by diagenesis. Only ‘long-term average’ palaeoclimatic conditions are preserved in sandstone detrital framework modes, and several specific criteria must be met before these conditions can be inferred from sandstone framework composition. The framework-petrographic signatures of palaeoclimate can be discerned only in unglaciated sequences from mature extensional tectonic settings on continental plates, only in sedimentary basins with mild late-diagenetic histories, and only for those modern-ancient pairs where the modern climate is more humid than the inferred ancient climate.
Redistribution of rare earth elements during diagenesis of turbidite/hemipelagite mudrock sequences of Llandovery age from central Wales
Authors: Milodowski A. E. , Zalasiewicz J. A.
Published: April 2008

Geochemical studies of recently mapped Upper Llandovery turbidites in central Wales indicate localized rare earth element (REE) mobilization and fractionation in mudrock-dominated sequences. Closely associated turbidite sandstones, turbidite mudstones and laminated anoxic hemipelagites have generally similar major elemental compositions but display strongly differentiated REE distributions. Shale-normalized REE distribution patterns show that anoxic hemipelagites are consistently enriched in the light and middle REE, with greatest enrichment occurring for La-Eu. This enrichment reflects a concentration of millimetre-scale zoned REE-rich monazite nodules in the hemipelagites. In contrast, the REE patterns of the associated turbidites mirror those of the anoxic hemipelagite, being relatively depleted in the light and middle REE, with Nd and Sm generally showing greatest differentiation. The relationship between the concentration of nodules and degree of enrichment of REE in anoxic hemipelagite and turbidite unit thickness imply upward migration of REE into the overlying hemipelagite. However, mass balance considerations suggest that the turbidite-hemipelagite couplets were not necessarily closed systems.

Within the turbidites, REE were initially held in unstable volcanogenic minerals and adsorbed on to clay minerals and iron-manganese hydroxides. The REE were liberated into the sediment porewaters during early diagenesis and fractionated in the migrating porewaters as a result of the greater solubility of heavy REE relative to light REE. Expulsion of porewaters from the turbidite sediment during burial carried REE in solution through the intervening hemipelagic layers where REE were precipitated, probably in the presence of organic matter, and possibly initially as the hydrous phosphate rhabdophane which later transformed to monazite. Fractionation of the REE during this process led to preferential enrichment of the light and medium REE within the hemipelagic sediment.

The use of REE as indicators of sediment provenance in the Llandovery turbidite-hemipelagite mudrock sequences of central Wales must be treated with caution since significant REE element migration and fractionation is shown to have occurred during their diagenesis. Similar effects might be expected in other sequences comprising interbedded organic-rich/organic-poor mudrock and detailed petrographic analyses should be undertaken to evaluate the possibility of diagenetic REE fractionation and redistribution in conjunction with the use of REE patterns or Sm-Nd isotopes in provenance studies of these rocks.
Selective alteration of arkose framework in Oligo-Miocene turbidites of the Northern Apennines foreland: impact on sedimentary provenance analysis
Authors: Valloni R. , Lazzari D. , Calzolari M. A.
Published: April 2008

Upper Oligocene-Middle Miocene turbidites of the Macigno and Cervarola Formations fill post-collisional elongate basins of the Northern Apennines foreland; they crop out axially for distances up to 300 km and reach a total thickness of 4000 m. Sandstones are arkosic with quartzose grains averaging 50% and fine-grained lithic clasts from 5 to 25%; K-feldspar and plagioclase abundances are similar, and rock fragments are dominated by igneous and metamorphic types. The Macigno sandstones reach a much higher diagenetic grade than the younger Cervarola sandstones. The latter (mature stage of diagenesis), have a clay mineral assemblage with ordered I/S interlayers and illite crystallinity (I.K.) averaging from 8.0 to 10.1, while in the Lower Unit of the Macigno Formation clay crystallinity averages 3.8 (Kübler’s epizone). The evolution of diagenetic processes is unusual because of the abundance of unstable grains. Physical compaction is the main cause of pore reduction. Post-depositional mineralogical changes are relevant in both formations; ignoring authigenic overgrowths and late calcite replacements, they commonly affect 10–20% of the framework grains.

Several mineralogical-textural changes influence provenance determinations, including: dissolution processes, which almost totally eliminate unstable species, such as amphibole and pyroxene, originally dominating the heavy mineral population; replacement processes, mainly represented by albite after plagioclase, and by clay or mica materials after unstable grains; intense deformation plus replacement of ductile or brittle grains with the authigenesis of chlorite, white mica and pseudomatrix.
Sourcelands for the Carboniferous Pennine river system: constraints from sedimentary evidence and U-Pb geochronology using zircon and monazite
Authors: Cliff R. A. , Drewery S. E. , Leeder M. R.
Published: April 2008

Petrographic and U-Pb isotopic data are presented for Carboniferous sandstones from the Pennine basin ranging from Chadian to Westphalian B in age. Variations in the petrography of these samples, particularly the greater abundance of feldspar in Namurian sandstones reported qualitatively by previous authors, are confirmed by modal analyses. The influx of coarse feldspathic sandstones into the Pennine and Southern North Sea basins in the early Namurian is attributed to a marked increase of sediment yields and discharge due to increased precipitation in the hinterlands. The same detrital heavy mineral species occur throughout the Carboniferous. Zircons are petrographically variable both in colour and degree of rounding. Two major sub-populations are recognized; pink-purple grains and colourless-brown grains. U-Pb isotopic analyses of multi-grain fractions of zircon from five of the samples show very similar age patterns; the sixth sample, a Yoredale sandstone appears to be distinct.

Single grain analyses of zircons were used to investigate one sample of late Namurian (Rough Rock) sandstone in detail. Concordant or slightly discordant results demonstrate the presence of: rounded brown grains > 3.5 Ga, rounded purple grains 2.8–3.2 Ga, euhedral purple grains ≥2.9 Ga and clear euhedral grains
c
. 0.41–0.43 Ga.

Monazites are shown to be very suitable for single grain analysis, yielding concordant results more often than zircon. Most of the monazite in the Rough Rock and a Westphalian sandstone are Caledonian (0.41–0.43 Ga) but two individual grains have ages of 0.947 and 2.70 Ga.

The isotopic results appear to exclude large areas of the North Atlantic region as potential sourcelands and no presently exposed rocks provide a close match with the inferred sourceland geology; rocks now under the northern North sea and/or between Scotland and Greenland are possible, as yet untested, sources.
Isotopic characteristics of Ordovician greywacke provenance in the Southern Uplands of Scotland
Authors: Evans Jane A. , Stone Philip , Floyd James D.
Published: April 2008

Interbedded Ordovician greywackes in the Southern Uplands are of markedly different composition. An integration of Sm-Nd isotope data, petrography of detrital clasts, and palaeocurrent flow analyses allows the likely provenance character and distribution to be deduced. The Portpatrick and Galdenoch formations contain detritus from volcanic provenances: andesite dominates the composition of Caradoc-Ashgill greywacke from the Portpatrick Formation which have εNd
445
(time of deposition) between −2.3 and −2.9, the Llandeilo-Caradoc Galdenoch Formation is richer in hornblende and has a slightly lower range of εNd
445
between −3.4 and −4.4. These values contrast with the more isotopically depleted signature of the Llandeilo-Caradoc Kirkcolm Formation greywackes which contain mainly quartz and feldspar clasts and have εNd
445
as low as −11.2. The source of the volcanic rocks, deduced from palaeocurrent analysis, lay on side of the depositional basin (south in terms of modern geography) and the isotopic composition of the volcanic rocks is consistent with a calc-alkaline arc or back-arc assemblage, founded on continental crust. A Proterozoic terrane, represented by detritus from the Kirkcolm Formation, lay on the opposite side of the depositional basin. Dalradian metamorphic rocks currently exposed to the north of the Southern Uplands display a less radiogenic signature than Kirkcolm Formation greywackes and seem unlikely to have acted as their source. This may support interpretations of the Southern Upland Fault as a locus of major sinistral strike-slip movement.
Sedimentary petrology and the archaeologist: the study of ancient ceramics
Authors: Gerrard Christopher M.
Published: April 2008

Ceramics are essentially modified sediments. This paper describes the objectives of the scientific study of ceramics and the range of appropriate methodologies which have been developed in recent years with special reference to ceramic petrology. The paper is illustrated by a number of examples taken from the author’s own work on prehistoric and medieval ceramics from Britain and Spain which demonstrate how the techniques of sedimentology have been modified to provide greater insight into ancient technologies and patterns of social and economic data.
Petrological and geochemical determination of provenance in the southern Welsh Basin
Authors: McCann T.
Published: April 2008

The provenance of Llandeilo to upper Llandovery sediments from the Welsh Basin has been investigated by petrographic and geochemical methods. Sandstone-dominated formations were deposited from high-concentration turbidity currents. Petrographic data suggest derivation from a non-collisional recycled or cratonic setting. This is broadly confirmed by analysis of microconglomerate lithic fragments and the major element geochemistry. Mudstone-dominated formations were deposited from dilute turbidity currents and as hemipelagic sediments. The mudstone geochemistry indicates a tectonic setting transitional between an active continental margin and a passive margin. This study demonstrates that neither set of analytical methods are individually adequate for provenance reconstruction and it is advisable to use a variety of techniques for greater confidence in interpretation.
Sandstones of arc and ophiolite provenance in backarc basin, Halmahera, eastern Indonesia
Authors: Nichols Gary , Kusnama  , Hall Robert
Published: April 2008

Analyses of sedimentary rocks from a late Neogene backarc basin on the island of Halmahera in eastern Indonesia show that the detrital mineral assemblages in the sandstones have distinctive characteristics. Quartz is extremely rare in the entire sequence of up to 4 km of basin sediments, clearly indicating that there has been no input from continental sources throughout the basin history. Sediments derived from two distinct provenance areas are recognized: sandstones dominated by volcanic rocks occur in the western half of the basin and in the eastern part there are black sands composed largely of ultrabasic debris interbedded with carbonate mudstones. These sandstone petrographies reflect the nature of the terrains which have bordered the Halmahera Basin throughout its history and other small basins formed in an ‘oceanic’ setting may show similar characteristics. Temporal changes in the volcaniclastic components are attributed to stages in the evolution of the adjacent volcanic arc and do not reflect the degree of dissection of an arc massif.
Nature and record of igneous activity in the Tonga arc, SW Pacific, deduced from the phase chemistry of derived detrital grains
Authors: Cawood Peter A.
Published: April 2008

Sedimentary rocks in dredge samples from the Tonga arc, SW Pacific, are simple two-component admixtures of volcanogenic detritus and bioclastic material. The volcanic detritus consists largely of lithic and plagioclase grains, but also includes, in decreasing order of abundance, significant minor amounts of clinopyroxene, orthopyroxene and pigeonite. In addition, amphibole, olivine and opaque oxide minerals are accessory phases and trace amounts of monocrystalline quartz occur in a few samples. Plagioclase grains have high An and low Or contents and fall largely in the bytownite field. Pyroxene grains show progressive Fe enrichment and low Ti contents. Volcanic glasses also show Fe enrichment and have low K
2
O contents. These features indicate derivation of the volcanic detritus from a low-K tholeiite source. The uniform compositional range and Oligocene to Holocene age range of the detritus is consistent with their derivation from the adjacent Tongan arc, and indicate that Pliocene rifting of the arc and associated back-arc basin formation did not affect the composition and affinities of the magma source.
Benthos communities in oxygen deficient shelf and upper slope areas of the Peruvian and Chilean Pacific coast, and changes caused by El Niño
Authors: Arntz Wolf E. , Tarazona Juan , Gallardo Victor A. , Flores Luis A. , Salzwedel Horst
Published: April 2008

Besides a well oxygenated shallow water area characterized by an extremely rich benthic life, most of the Peruvian and Chilean continental shelf and upper slope lies within an oxygen minimum zone (omz), the poor macrobenthic colonization of which is in striking contrast to the abundant pelagic life in the overlying waters. However, the eukaryotic benthos in this zone shares the seafloor with an important prokaryotic component of filamentous (‘sphagetti’) bacteria belonging mainly to the genus
Thioploca
. Under ‘normal’, non-Niño conditions, near-seafloor oxygen saturation values in this area rarely exceed 0.5 ml 1
−1
, and in some cases the bottoms are even anoxic. During strong El Niños, saturation periodically increases to >3.5 ml l
−1
, and higher oxygenation may persist for more than a year after the end of the event proper (which is characterized by increased water temperatures). In the shallow parts of the omz, the changes induced by El Niño cause drastic increases of macrobenthic density, biomass and diversity; immigration of benthos and fish species from (sub)tropical equatorial and oceanic areas; changes in trophic structure; and reduction of ‘spaghetti’ bacteria. In the case of the particularly strong 1982–83 El Niño, the communities of the shallower part of the omz had shifted back into their pre-Niño state about 13 months after the end of the warming event. In the deeper part of the omz, despite a similarly strong increase of oxygen saturation, a clearly positive development of macrobenthos during El Niño could not be observed. The reasons that may be responsible for this difference are discussed.
Late Permian anoxia in central East Greenland
Authors: Piasecki Stefan , Stemmerik Lars
Published: April 2008

The Upper Permian Foldvik Creek Group of the East Greenland Basin provides excellent possibilities for studying regional anoxia, because the basinal, anoxic facies of the Ravnefjeld Formation are well exposed throughout this 400 km long and more than 80 km wide basin.

The shales of the Ravnefjeld Formation are dominated by two units of laminated, organic-rich, calcareous mudshales. The absence of a burrowing or benthonic fauna, and the high content of organic matter and sulphur suggest these units were formed during a basin-wide anoxic episode. The character of the graded laminae indicates deposition from suspension. In contrast, thin bioturbated units appear to have been deposited during episodes of more oxygenated bottom waters. The TOC, HI and sulphur content are all lower in these bioturbated units.

The associated marine limestones of the Wegener Halvø Formation record a depositional history of transgressions and regressions that can be directly related to the laminated—bioturbated units of the Ravnefjeld Formation. The anoxic period can therefore be correlated with the maximum transgression of the basin, which induced rapid vertical growth of reefs. A stratified water column with anoxic bottom conditions and restricted circulation was apparently induced by the maximum flooding of the basin. The extensive vertical growth of carbonate platforms may have further restricted the circulation in the basin, producing a ‘silled basin’ effect.
Distribution, dynamics and palaeoecology of Kimmeridgian (Upper Jurassic) shelf anoxia in western Europe
Authors: Oschmann Wolfgang
Published: April 2008

Facies and related macrobenthic associations in the Kimmeridgian epeiric basin in Western Europe are arranged along an onshore-offshore gradient. It ranges from an intermediate-to-high-energy nearshore zone to a low-energy oxygen-controlled offshore environment.

Anoxia in the Kimmeridgian epeiric basin was not a permanent feature and cannot be described by the classical models which define aerobic, dysaerobic and anaerobic facies zones. These models have been proposed for deep (slope and basin) environments, in which the oxygen values at a certain depth remain in general constant. In contrast, anoxia in the Kimmeridgian is more dynamic and resembles modern anoxia occurring in eutrophic shelf and epeiric basins (e.g. the coastal areas of Mid-Atlantic Bight, North Sea and Baltic Sea) and exhibit remarkable annual oxygen fluctuations.

Therefore, in contrast to the classical models, a modified oxygen zonation for shelf environments is introduced, in order to explain the palaeoenvironment and dynamic alternation of Kimmeridgian anoxia. Both end members, the aerobic and the anaerobic environment, do occur as well. However, the short-term fluctuations prevent establishment of the dysaerobic zone. Instead, there is a seasonal anaerobic to anoxic environment, here defined as
poikiloaerobic zone
, which is colonized only by a few opportunists.
Integration of geological data into hydrodynamic analysis of hydrocarbon movement
Authors: Davis Richard W.
Published: April 2008

As a general rule, when hydrodynamic analysis is used in the course of exploration for petroleum, it is used as a means of testing individual geological structures or stratigraphical variations to see if they can serve as traps for hydrocarbons. Hydrodynamic analysis can be used to accomplish much more, especially in the search for subtle and unusual traps. It can be used to evaluate whole basins, not just restricted areas of one prospect. In addition, by utilizing all the geological information available, it is possible to use the analysis as a test of the consequences of the geological model upon petroleum migration as well as entrapment. It is possible to use the geological information to improve the accuracy of potentiometric surface data. It cannot be stressed too much that the value of the hydrodynamic evaluation is only as good as the accuracy and detail of the geological model.