bluuebunny/crossref_metadata_2025 huggingface dataset:
Lower Permian Rotliegend reservoir of the Southern North Sea gas province
Authors: Glennie K. W. , Provan D. M. J.
Published: April 2008

Rotliegend aeolian sandstone of Early Permian age forms the main producing horizon in the Southern North Sea gas province. Currently some 30–40 fields have been identified in a broad zone measuring 450 by 100 km stretching from the English coast, through the UK and Dutch sectors of the North Sea and on to The Netherlands and West Germany. North Sea field sizes range between 10–300 Bcm (0.3–10.5 TCF). Ultimate recoverable reserves for the province are estimated at some 4250 Bcm (150 TCF). The Rotliegend is underlain by Carboniferous coals and carbonaceous shales, which form the source rock, while Late Permian Zechstein evaporites form the seal. The aeolian sandstones occur interbedded with wadi sandstones and sabkha siltstones. Dune sandstones with large-scale cross bedding generally form the best reservoirs, though even here reservoir quality is highly variable. Core porosities range between 5 and 30% and permeabilities from <1 to 3000 mD. Reservoir impairment through the effects of burial diagenesis (compaction, pressure solution, clay mineral transformations and cementation) is evident in many fields. In others, the occurrence of natural fractures serves to enhance production. As a result, individual well productivities show wide variation.
A regional assessment of the intra-Carboniferous play of Northern England
Authors: Fraser A. J. , Nash D. F. , Steele R. P. , Ebdon C. C. , Fraser A. J.
Published: April 2008

Hydrocarbons have been produced from the Carboniferous of the East Midlands since the 1920s. Some 30 discoveries have been made to date and although field sizes are generally small, the relatively low costs have made exploration and development an attractive commercial proposition. Many of the factors which contribute to success in the East Midlands occur in other basins in Northern England but have not produced significant hydrocarbon accumulations. This paper examines the key attributes of the intra-Carboniferous play and through an integrated regional geological framework contrasts the successful East Midlands oil province with the other Carboniferous basins of Northern England.
West Siberian oil and gas superprovince
Authors: Nesterov I. I. , Salvamanov F. K. , Kontorovich A. E. , Kulakhmetov N. K. , Surkov V. S. , Trofimuk A. A. , Shpilman V. I.
Published: April 2008

The West Siberian oil and gas superprovince is an area of more than 2.5 × 10
6
km
2
. The province basin is filled with Cenozoic, Cretaceous and Jurassic platform sediments and, in the north and west also with Triassic and Palaeozoic rocks. The volume of sedimentary platform deposits is more than 9 × 10
6
km
3
which includes 0.75 × 10
6
km
3
of Triassic deposits and 1.3 × 10
6
km
3
of Palaeozoic deposits. The composition of the reservoir rock is terrigenous and it is inferred that Palaeozoic deposits can be represented by carbonates. Sediments accumulated in marine conditions are predominant in the region. Oil and gas pools occur both in marine and continental deposits. The total amount of discovered oil and gas in various plays is distributed as follows: 43% of pools are associated with regional caprocks, 40% with zonal caprocks and 17% with local caprocks.

The largest gas pools are mainly accumulated under regional caprocks and oil pools occur under zonal caprocks. There are no known major gas and oil pools under local caprocks. In future, discoveries of large hydrocarbon accumulations under zonal caps and an increase in discoveries of local reservoirs are confidently anticipated.

Regional studies, basin evaluations and calculations confirm the West Siberian basin to be a superprovince according to its known hydrocarbon potential. However, the regional evaluations do not meet current commercial requirements of the petroleum exploration industry, the main task of which is substantiation of potential reserves in concrete regional, zonal and local layers, ranging in thickness from 10 to 20 m instead of thick formations. Such needs call for development of modern methods such as remote-sensing to assist in oil-and-gas potential predictive exploration. Such methods must later be further developed to identify thicker oil-and-gas reservoirs of several square kilometres in size with less than 5–10 m bed thickness.

The West Siberia basin is being developed very rapidly. It took only eight years from the first small gas field discovery (1953) to the first significant oil production. Since the first oil pipeline installation in 1964 an average of 848 × 10
3
m
3
per day of oil and gas has been produced, and in 1987 production had reached 2.38 × 10
6
m
3
per day.
A palaeomagnetic study of two Precambrian-Cambrian dyke swarms from the Armorican Massif
Authors: Taylor G. K.
Published: April 2008

The palaeomagnetic analysis of two dolerite dyke swarms, metamorphosed during the late stages of the Cadomian Orogeny, is reported. The Vazon dyke swarm of Guernsey, Channel Islands, has yielded a characteristic component of magnetization (D = 199°, I = 48°, a95 = 12°, n = 8 dykes) which is dual polarity, while six other sampled dykes record intermediate field directions. The magnetization is interpreted as being acquired shortly after dyke emplacement and the corresponding palaeomagnetic pole (lat, 9°S; long, 340°E) is in good agreement with previously determined Cambrian poles from Armorica. An emplacement age for the dykes of 560–540 Ma is therefore suggested on the basis of the radiometric ages assigned to these previously published results. The Dahouet dyke swarm of the Baie de St Brieuc was also sampled and has yielded two remanence directions (A and B) common to most sites. The A remanence (D = 198°, I = 2°, a95 = 7°, n = 7 dykes) yields a pole (lat, 37°S; long, 334°) which is interpreted as being an overprint acquired during the Variscan Orogeny and is consistent with Carboniferous results. The B remanence (D = 202°, I = 34°, A95 = 8°, N = 6 dykes) and its corresponding pole (lat, 20°S; long, 335°E) is more ambiguous in its interpretation as it lies between the known Cambrian APW path and a diffuse area of poles associated with overprinting during the Variscan orogeny. A tentative interpretation of this remanence as being Cambrian in origin is consistent with the suggestion that both dyke swarms were emplaced relatively closely in time, as suggested by their similar geological settings.
Cadomian tectonics in northern Brittany
Authors: Brun J. P. , Balé P.
Published: April 2008

Major Cadomian structures of northern Brittany, France, are reviewed from the Léon region in the west to the Mancellian region in the east. The whole region is dominated by a NE trending structural grain. From the NW to the SE three main tectonic units have been distinguished: (a) the Trégor volcanic and plutonic belt; (b) the Baie de St Brieuc metamorphic-thrust belt and (c) the Guingamp-St Malo high temperature belt. Principal strain trajectories, finite strain measurements, shear criteria and geochronological data are used to establish a kinematic model of Cadomian deformation. The three units have been thrust over each other and towards the SW at 590–580 Ma. An extrapolation at a regional scale is made using available geophysical data (gravimetry, magnetism and SWAT seismic lines). A new model for oblique thrusting toward the SW along an ENE trending plate boundary is proposed.
Cadomian strike-slip tectonics in NE Brittany
Authors: Treloar P. J. , Strachan R. A.
Published: April 2008

The Precambrian basement of NE Brittany contains a number of NNE-trending migmatite belts of Cadomian (
c.
540 Ma) age. These are heterogeneously reworked by subvertical, ductile shear zones which occur both within individual migmatite belts, and along their boundaries with low-grade Brioverian metasediments. The shear zones are characterized by blastomylonitic fabrics, penetrative sub-horizontal or gently-plunging stretching lineations, and shear criteria which indicate a consistent subhorizontal, sinistral strike slip sense of movement. The migmatite belts, and the greenschist facies Brioverian metasediments imbricated with them along the strike-slip zones, form the St Malo Terrane deformed at
c.
540 Ma. This terrane is bounded to the NW and SE by two lithologically and metamorphically distinct terranes, the St Brieuc and Mancellian terranes respectively. Boundaries between the three terranes are inferred to be crustal scale sinistral strike-slip zones. The St Brieuc Terrane was also deformed and metamorphosed in a transpressive setting either prior to or during amalgamation with the St Malo Terrane at
c
. 540 Ma. The Cadomian belt of northern Brittany is interpreted as a sinistrally transpressive strike-slip belt related to deformation in the hanging-wall of an oblique subduction zone located to the north, and across which there is likely to have been substantial lateral displacement.
Late Precambrian foliated plutonic complexes of the Channel Islands and La Hague: early Cadomian plutonism
Authors: Power G. M. , Brewer T. S. , Brown M. , Gibbons W.
Published: April 2008

The foliated granitic rocks of the Channel Islands and La Hague are predominantly quartz dioritic in composition with some granodiorite and minor granite. Their variably developed foliation is of post-solidification origin. Greenschist facies metamorphism affects some rocks as does patchy very low grade metamorphism. Field relations show that the granites are younger than the pre-Icartian metasedimentary gneisses and older than the post-Cadomian igneous complexes but there are no contacts with the Brioverian Supergroup. Isotopic age data are very unsatisfactory but suggest a possible age span of 700–580 Ma.

168 geochemical analyses show that the rocks are calc-alkaline, the majority are metaluminous with Na
2
O greater than K
2
O and that they have the characteristics of normal volcanic arc granites. Trace element plots show that there are differences in the suites of analyses from Guernsey, Sark, Alderney and La Hague reflecting minor differences in their source materials. REE plots for 34 rocks all show strong fractionation of LREEs regardless of rock composition and most show HREE depletions. These patterns are interpreted as originating in the source and suggest an origin by high pressure melting of predominantly basaltic compositions.

The evidence supports the model of Dupret
et al.
(1990) for a major period of Cadomian crustal growth by progressive cratonization of a basin and arc system but extension of the model on a more regional basis is difficult because of the uncertain status of neighbouring terranes.
The Northern Igneous Complex of Guernsey, Channel Islands
Authors: Topley C. G. , Brown M. , D’Lemos R. S. , Power G. M. , Roach R. A.
Published: April 2008

The Northern Igneous Complex of Guernsey consists of four intrusive bodies, the St Peter Port Gabbro, the Bordeaux Diorite Complex, the Cobo Granite and the L’Ancresse Granodiorite. Although these bodies can be shown to be emplaced in the order given, evidence suggests that a small time span separated the intrusive events, which occurred around 500 Ma ago. The geochemistry of the complex is calc-alkaline, compositions from olivine gabbro to granite being represented. The granitic rocks have compositions typical of volcanic arc granites. The St Peter Port Gabbro is a layered body, with layering present on three different scales. The layering and the geochemical variation can be explained by invoking magmatic processes. The Bordeaux Diorite Complex consists of a range of lithologies from meladiorite to granodiorite, the magmas of which are shown to have been contemporaneously mobile, although magma mingling as opposed to mixing has occurred. Despite their contemporaneity, the different magmas of the Bordeaux Diorite Complex are not related directly to each other by fractionation. The Cobo Granite is homogenous but has complex relationships with the Diorite Complex. At the northern margin of the Granite a zone of granite/diorite magma mixing occurs. Details of the process involve local chilling, breaking of the chilled carapace followed by comingling and partial mixing. Geochemistry reflects the mixing process. The L’Ancresse Granodiorite is lithologically and geochemically homogeneous. Its western contact with the Diorite Complex is faulted, but the eastern contact again shows a variety of granodioritic and dioritic lithologies which have led others to invoke processes of magma mixing and metasomatism. It is argued here, however, that these lithologies are part of the variation inherent in the Bordeaux Diorites, rather than having been produced during the emplacement of the Granodiorite.
The post-tectonic Cadomian plutonic complex of La Hague, Manche, N. France
Authors: Power G. M. , Brewer T. S. , D’Lemos R. S.
Published: April 2008

In the La Hague region, NW of Cherbourg, France, the post-tectonic Cadomian Northern Granites Complex consists of seven rock units which range in composition from diorites to trondhjemite and K-feldspar granite. With the notable exception of the Ecuty Granite, the units tend to become mineralogically more evolved with decreasing age. They are calc-alkaline and have the characteristics of the products of volcanic arc magmatism. In detail, their geochemical differences support magma formation from several different sources.

The Ecuty Granite gives a Rb-Sr whole rock isochron age of 527 ± 4 Ma, the St Germain Granite, 515 ± 12 Ma and the Cap de la Hague Granodiorite, 477 ± 8 Ma. Thus this igneous activity spanned at least 50 Ma of Cambro-Ordovician time. This, together with evidence of plutonic sequences of similar character and age in Guernsey and Jersey supports the view that post-tectonic Cadomian activity continued well into lower Palaeozoic times in the northern part of the Armorican Massif.
The geochemical character of late Cadomian extensional magmatism in Jersey, Channel Islands
Authors: Lees G. J.
Published: April 2008

An episode of magmatism, which may represent the last igneous phase attributable to the Cadomian orogenic cycle, is found in southern Jersey, Channel Islands. A zone, some 3 km wide, of intensive dyke injection, the Jersey Main Dyke Swarm, cuts across post-orogenic calc-alkaline plutons, though showing a close temporal link with their emplacement.

The nature of the dyke swarm magmatism is dominantly but not exclusively basic. A clear acid—basic bimodality is present with very little intermediate material. Although subjected to varying degrees of alteration under low-grade metamorphic conditions, the primary nature of the rocks is often preserved. All those having a basaltic composition are olivine-normative, but show a high-K calc-alkaline character. Fairly high LREE enrichments are present. MORB-normalized spidergrams confirm that the magmatism was of a mature calc-alkaline type, similar to that generated at active continental margins such as the Andes.

The presence of the swarm indicates a period of late post-orogenic crustal extension sub-parallel to the NE-SW strike of the proposed Cadomian active continental margin in the Armorican Massif. It could represent injection of the last remnants of magma from an already decayed subduction zone system into a newly cratonized environment.
Basement-cover relationships of a reactivated Cadomian mylonite zone: Rosslare Complex, SE Ireland
Authors: Murphy F. C.
Published: April 2008

Recurrent ductile and brittle displacements characterize the steep boundary mylonite zones that enclose Proterozoic gneisses of the Rosslare Complex in SE Ireland. The northern exposed limit of the Complex is a wide displacement zone containing greenschist facies mylonites derived from two protoliths: (a) basement gneisses and (b) supracrustal metasediments. Important sinistral transcurrent movements produced these mylonites probably during the late Precambrian. Docking of the Precambrian ‘Monian’ sediments to the Rosslare basement may indicate remobilization of a basement/cover relationship. The transpression, uplifted and eroded the mylonites into early Palaeozoic (?Cambrian and Arenig) overstep sediments. Renewed dip-slip displacement, possibly during the Ordovician, was distributed through the mylonites and deformed the sedimentary cover rocks. Finally, the initially wide displacement zone along the northern boundary of the Complex narrowed to discrete brittle faults during subsequent Palaeozoic reactivations. Following pre-Arenig ductile sinistral shear along the probable southern margin of the Complex, each bounding fault strand had divergent histories which involved decoupling of an anastomosing transpressive system.
Precambrian successions in SW Iberia: their relationship to ‘Cadomian’ orogenic events
Authors: Quesada Cecilio
Published: April 2008

Despite extensive reworking in Devonian and Carboniferous times, two major deformational events taking place in late Precambrian times may be recognized in the SW Iberian Massif. These have been collectively referred to as ‘Cadomian’ or ‘Pan-African’. The Precambrian successions may be subdivided into pre-orogenic and syn-orogenic groups on the basis of their tectonostratigraphic evolution with respect to the Cadomian orogeny. The pre-orogenic successions (roughly mid-upper Riphean) represent different parts of a miogeoclinal wedge, the basement of which is not known as yet. Polyphase deformation and variable degrees of metamorphism accompanied the first Cadomian event.

The syn-orogenic successions (roughly upper Riphean-Vendian) were laid down during and after the first Cadomian event and were in turn deformed and slightly metamorphosed by the second single-phased deformation. These correspond to two distinct types: calc-alkaline igneous successions and flysch-like deposits, that can be interpreted respectively as an orogenic continental arc suite and a foreland basin fill, on the basis of their geochemical and sedimentological characteristics.
Comparative geology and mineralogy of the southeastern United States and Togo phosphorites
Authors: Van Kauwenbergh S. J. , McClellan G. H
Published: April 2008

The physical and mineralogical characteristics of the southeastern United States and Togo phosphorite deposits are the result of both primary depositional factors and secondary diagenetic processes. The North Carolina phosphorites appear to be ‘unaltered’ or are in the first stages of alteration. Both the Florida and Togo phosphorites have resulted in the removal of mineral species (primarily carbonates), systematic decarbonatization of the carbonate-fluorapatite, the development of iron and aluminium phosphates, and clay mineral alteration profiles.
Microbial mediation in phosphatogenesis: new data from the Cretaceous phosphatic chalks of northern France
Authors: Lamboy Michel
Published: April 2008

Detailed petrographic study of the Upper Cretaceous phosphatic chalks of northern France, particularly by scanning electron microscopy, shows that phosphate occurs predominantly as phosphatized bacterial remains. Shiny phosphatic crusts on hardgrounds and anisotropic phosphate coatings on grains are interpretated as phosphate mineralized microbial (‘microstromatolitic’) structures. Phosphatic remains of ovoid bacteria and of microbial colonies with botryoidal surfaces have been observed in chambers of foraminiferans and bryozoans, and within other bioclasts, and occur as pore linings in peloids, coprolites and lenses of phosphatic chalk incorporated into hardgrounds. The microbial community which formed the crusts and grain coatings developed by surface accretion, whereas the other community grew solely in the intragranular porosity of grains and in the extragranular porosity of semilithified sediments. Analysis of these two types of microbial growth, which have also been documented from other phosphorite deposits, permits a better but as yet incomplete understanding of the mechanisms of phosphatization.
Pebbly phosphate and granular phosphorite (Late Cretaceous, southern Israel) and their bearing on phosphatization processes
Authors: Lewy Zeev
Published: April 2008

Phosphate nodules, including common mineralized internal moulds of macrofauna, are found only at certain levels in the chalky, locally bituminous Ghareb Formation (uppermost Campanian-Maastrichtian) of southern Israel. The microstructure of these nodules suggests that they consist of marine sediment cemented by a microbially-mediated apatite, precipitated locally in protected microenvironments, e.g., within partly closed burrows and shells, or within the upper sediment layer. The globular microstructures commonly recognized in the phosphate cements of the pebbly phosphates are regarded as mineralized cells of endobenthic micro-organisms. The global occurrence of pebbly phosphate in diverse lithofacies throughout the geological column indicates the tolerance of these globular micro-organisms to a wide range of ecological conditions.

The grains in the Upper Cretaceous phosphorites of the Negev (southern Israel) consist of bone fragments, phosphatized faecal pellets and ovular grains of different types. The commonest variety of ovular grain consists of microcrystalline phosphate (microphosphate) containing relics of globular microstructures, either forming the whole grain, coating a foraminifera or bone fragment, or cementing small clasts. The latter types resemble the pebbly phosphates and can be regarded as micronodules. The matrix of these granular phosphorites is mainly micritic, and in places argillaceous. In some Negev phosphorites the ovular grains and their matrix consist mainly of loosely packed tubules. These tubular microstructures resemble mineralized filaments of microphytes which form microbial mats on modern shallow seafloors lying within the photic zone. The origin of these granular phosphorites is discussed, and a comparison with the pebbly phosphates indicates that different ecological controls and microbial mediators resulted in the different kinds of phosphate bodies.

Since most of the phosphate matter can be related to microbial activity, it is doubtful whether direct replacement of solid carbonate by phosphate contributes significantly to phosphogenesis in sediments. It is suggested, however, that in many deposits phosphate-precipitating micro-organisms have penetrated and mineralized the borings of algae and other endolithic groups, or have filled dissolution cavities within lithified carbonates.
Geological significance of carbonate substitution in apatites: Israeli phosphorites as an example
Authors: Nathan Y. , Soudry D. , Avigour A.
Published: April 2008

The extent of carbonate for phosphate substitution in francolite (carbonate-fluorapatite) was measured in 177 selected phosphorite samples of Santonian to Eocene age from Israel. The results show that unaltered apatites from different stratigraphic positions have different CO
2
concentrations while those from the same stratigraphic level have a relatively narrow range of concentrations. As a rule phosphorite samples from rocks with a low P
2
O
5
content have a higher carbonate substitution than those from rocks with a high P
2
O
5
content. Apatites from the carbonate-rich Santonian-Campanian Menuha Formation contain on average 5.5% CO
2
, while the average content of the apatites from the phosphatic Campanian-Maastrichtian Mishash Formation is 3.7%. These changes are probably a reflection of the early diagenetic environment and, more specifically, are a function of the varying composition of the interstitial waters.

Epigenesis and/or weathering bring about a significant decrease in the CO
2
concentrations causing an overprint which masks the above picture. Therefore, it is possible to use a CO
2
value for geological interpretation only after making a petrographic analysis, in order to distinguish between early diagenesis and late epigenetic processes.
Depositional sequences of the Duwi, Sibâîya and Phosphate Formations, Egypt: phosphogenesis and glauconitization in a Late Cretaceous epeiric sea
Authors: Glenn Craig R.
Published: April 2008

Sedimentary facies, depositional environments and stratigraphic relationships of the Upper Cretaceous Duwi, Sibâîya and Phosphate Formations in Egypt are summarized in order to shed light on the origins and interrelationships between phosphorites and their commonly associated facies. The distribution of phosphorites, glauconitic sands, organic carbon-rich shales, cherts and porcelanites, and bioclastic and fine-grained limestones are correlated in both space and time across the central phosphorite provenance of the country. The correlations are linked by means of sequence stratigraphic analysis and are tied to global eustatic sea-level curves.

Two main depositional realms are inferred from the Campanian lithofacies assemblages: (1) a deeper-water hemipelagic environment accompanying maximum transgression dominated by deposition of phosphorites, organic carbon-rich shales and biosiliceous sediments, which, after maximum flooding, shoals upwards into (2) a progradational stage accompanying sea-level fall, during which oyster banks with brackish back-reef sediments (Eastern Desert) and deltaic sediments (upper Quseir Variegated Shale, Nile Valley) dominated eastern portions of the phosphorite belt, while greensands were reworked seaward from earlier inner shelf sediments in the west (Western Desert). These depositional realms encompass (1) transgressive to lower highstand systems tracts and (2) upper highstand to lowstand progradational wedge system tracts, respectively. The reoccurrence of lower Maastrichtian phosphatic shale, marl and associated phosphorite lithofacies at the top of the sequence indicates repetition of the sedimentary cycle.

Both phosphorites and glauconitic greensands appear to be the result of current winnowing and reworking of authigenic grains from previously deposited sediments. The resulting deposits range in scale from thin lag layers, to amalgamated beds, to giant sand waves up to 10 m thick. In the Egyptian setting, the common factors linking massive phosphorite and glauconite occurrences may have been the co-occurrence of humid climate, deep lateritic weathering in hinterlands, anoxic bottom waters on the inner shelf, and high fluvial discharge rates of phosphorus and iron. Superimposed on these factors were influences of cross-shelf currents necessary to concentrate authigenic precipitates, and a sea level change, which acted to markedly reduce the diluting effects of siliciclastic detritus during rapid sea level rise, as well as vertically stack ‘authigenic facies’ during sea level fall.
Clay minerals and phosphorite genesis in the Upper Cretaceous of the northern Siberian Platform
Authors: Zanin Yu. N. , Zverev K. V. , Solotchina E. P.
Published: April 2008

Nodular phosphorites are common in the Turonian to Maastrichtian deposits (sands, silts and clays) of the Yenisei Mouth Depression in the north of the Siberian Platform. Only the silty clays of Campanian age are non-phosphatic. Study of the fine-grained fraction (<2 µm) of these deposits demonstrates that the phosphorite-bearing parts of the succession contain similar clay mineral assemblages. These consist of kaolinite and chlorite, with a small but constant admixture of gibbsite, and in some cases a small amount of montmorillonite. In the non-phosphatic rocks of the Campanian, clay minerals consist only of montmorillonite and chlorite. Kaolinite and gibbsite are completely absent. It is argued that the clay mineralogy and other data suggest that the formation of the phosphorites was linked to the removal of phosphorus from continental areas during periods of intense weathering in a humid subtropical climate.
Phosphogenic events and skeletal preservation across the Precambrian-Cambrian boundary interval
Authors: Brasier M. D.
Published: April 2008

Refined biostratigraphic data from India and new data from Iran confirm that several major phosphogenic events took place within the Precambrian-Cambrian boundary interval across southern and central Asia. The palaeoceanographic setting of these event is briefly reviewed with reference to changes in the biosphere, where it is shown to have coincided with a widespread radiation of skeletons of diverse mineralogy (siliceous, apatitic, aragonitic, calcitic). Although a connection between palaeoceanographic changes and biomineralization cannot yet be demonstrated, the possibility that phosphogenesis enhanced the fossil record at this point is tested.

Preliminary criteria are put forward and illustrated for the recognition of primary phosphatic skeletons, secondarily phosphatized organic remains and secondarily phosphatized carbonate skeletons. This latter category comprises a major proportion of phosphatic small shelly fossils in the Chinese and Iranian sequences. Three kinds of phosphatic taphofacies are then distinguished for early skeletal biotas: residual phosphatic, phosphatized carbonate and phosphatized organic facies (i.e. lägerstatten conditions). In several phosphogenic regions, the skeletal fossil record begins with residual phosphatic assemblages, followed by phosphatized carbonate taphofacies. Such diagenesis played an important role in enhancing the fossil record at this time because the first skeletons were thin (5–40 μm) and vulnerable to neomorphism in dolomitic or clastic facies. Phosphatized organic remains appeared locally during the major phosphogenic events, perhaps associated with more anoxic seafloor conditions that supressed the shelly calcareous benthos.
Sedimentary and sea level changes during glacial cycles and their control on glacimarine facies architecture
Authors: Boulton G. S.
Published: April 2008

In part 1 a simple sedimentation model is presented in which subglacial icecontact, proglacial ice-contact, inner and outer proximal, and distal glacimarine zones are distinguished. In these, facies associations are controlled by: (a) distance from the glacier and the diffusion of glacially-derived water masses and their suspended sedimentary (including berg-transported) debris into nearby oceanic waters; (b) the form and depth of the sea bed, and its position in relation to the continental margin (which help determine water mass character and tidal, wave-driven and geostrophic currents and the stability of sea-bottom sediments).

In part 2 a model of eustatic/isostatic sea level change in the vicinity of a generic ice sheet through a whole glacial cycle is developed. The sedimentation model from part 1 is linked to the sea level model to predict patterns of marine sedimentation near to an ice sheet through a glacial cycle, in inner shelf, outer shelf and continental slope environments.

Comparisons are made between theoretical models and observed patterns of glacimarine facies architecture produced during the last glaciation of Britain, continental Europe and Spitsbergen. The phase relationship between local ice sheet advance and retreat and that of the Laurentide ice sheet, the principal determinant of global eustatic change, is a major determinant of large scale sedimentation patterns.
Modelling rates of sedimentation from icebergs
Authors: Dowdeswell Julian A. , Murray Tavi
Published: April 2008

Icebergs transport debris from ice masses into the glacimarine environment, but iceberg production, debris content and sedimentation rate will vary with glacier dynamics and the nature of marine waters. A model incorporating glaciological and oceanographic parameters calculates the flux of debris calved in icebergs, iceberg melt rates and sedimentation rates with distance from calving tidewater ice fronts. The flux of basal debris calved through a 1 m width of tidewater margin is (
h
d
c
/100)
v
c
, where
h
d
and
c
are basal debris-rich ice thickness and sediment content (%), and
v
c
is iceberg calving rate. Iceberg basal melting for any horizontal interval (d
x
) away from the calving glacier is dy = (
M
b
d
x
)/
v
b
, where
M
b
is iceberg melt rate and
v
b
is horizontal iceberg velocity. Sediment release for any
x
increment depends on iceberg debris concentration and previous berg melt history. Model parameters vary with the nature of climatic conditions, on a continuum from cold polar environments (e.g. High Canadian Arctic, Antarctic Peninsula), through sub-polar environments (e.g. Baffin Island, Svalbard) to more temperate environments (e.g. SE Alaska). These parameters include: glacier flow velocity, glacier thermal regime, basal debris-rich ice thickness, and marine water temperature. Model results suggest that iceberg sedimentation rates are lowest for high polar settings, and most rapid in warm maritime and some sub-polar areas. However, icebergs in cold waters transport debris to significantly greater distances from the parent glacier. The model provides calculations of order of magnitude rates of iceberg sedimentation in varying glacimarine settings.
Fossilization potential of Arctic fjord and continental shelf benthic macrofaunas
Authors: Aitken Alec E.
Published: April 2008

The organic remains of many organisms do not survive the post-mortem period. This study was undertaken to identify patterns of preservation in arctic marine macrofaunas with the goal of aiding Quaternary researchers in the interpretation of fossiliferous marine sediments. The probable fate of 310 genera of marine invertebrates which inhabit the fjords and continental shelf in the eastern Canadian Arctic was investigated. The preservation potential of marine invertebrates was assessed in three ways: by morphology (presence of macroscopic hardparts); by the previous existence of a fossil record (data for genera only taken from the
Treatise on Invertebrate Paleontology
); and by the previous record in Quaternary marine sediments on Baffin Island. Considering the arctic benthos as a whole, up to 860f the genera have not been preserved and are not likely to be preserved.

The fossil record preferentially preserves infaunal suspension feeding and epifaunal browsing organisms. Infaunal suspension feeding bivalves frequently contribute more than 500f the macroscopic hardparts in Quaternary fossil assemblages. The lack of preservation of soft-bodied carnivorous and deposit feeding organisms suggests that little understanding of community trophic dynamics based on the study of trophic group composition is likely to be derived from Quaternary fossil assemblages. Bivalves and gastropods inhabiting fjords and nearshore waters exhibit the greatest preservation potential of all the arctic marine benthos. The excellent preservation potential of arctic marine molluscs offers palaeoecologists the potential to reconstruct ancient marine biocoenoses in glacimarine environments.
Iceberg scours in the geological record: examples from glacial Lake Agassiz
Authors: Woodworth-Lynas C. M. T. , Guigné J. Y.
Published: April 2008

Observation of sub-surface deformation structures beneath modern oceanic iceberg scours is impossible, but Quaternary scours exposed above sea or lake level on land provide this opportunity. Well developed iceberg scours which are clearly visible on the former bed of glacial Lake Agassiz in southeastern Manitoba were chosen for detailed studies of sub-scour deformation.

Partial burial of the scour troughs and the absence of bioturbation of the clay sediments have preserved the surface morphology and sub-scour deformation structures in pristine condition. Shallow-angle faults and sub-horizontal thrust faults are developed below the scour trough, the former extending to at least 3 m beneath the deepest part of the scour incision surface. These faults formed in response to vertical and horizontal loading of the lake bed by the encroaching keel. High-angle normal faults developed outside the scour margin represent gravity collapse of sediment blocks that were piled up on the scour berms as a result of lateral displacement of material away from the axis of the scour. Definition of deeply penetrating faults has significant engineering implications for the burial depths required to protect oil and gas production pipelines in areas of active scouring today.

Remnant pods and lenses of tightly folded laminated sediments, in places carrying a well developed fracture cleavage, are all that remain of original primary layering in the severely reworked sub-scour sediments. Based on our findings, criteria for the recognition of scours and of ice keel turbates exposed in argillaceous sediments are proposed.
The Pleistocene—Holocene transition in Southwestern Sweden and the recognition of deglaciation effects in adjacent seas
Authors: Lord A. R.
Published: April 2008

Detailed investigation of Late Quaternary sequences in southwest Sweden via shallow cored boreholes provides an excellent record of sedimentological patterns and the biological history of the area during regional deglaciation. Climatic amelioration and consequent deglaciation led to the Baltic region being dominated by the Baltic Ice Lake, a major physiographic feature which is estimated to have become undammed at
c.
10 200 years BP, discharging some 10
4
km
3
of freshwater south and west into the Skagerrak and North Sea Basin. Such an event should be easily traced in the marine environment, but away from the immediate onshore area convincing records have yet to be found. The freshwater influx affected the upper part of the water column in the North Sea Basin and should be detectable via an isotopic signature, but good quality cores with planktonic organisms have not yet provided such a record. The effects of Baltic Ice Lake discharge into the North Sea Basin may well be overprinted by contemporary major oceanographic changes in the northeast Atlantic, especially the initiation of the Norwegian Current, and by meltwater input from adjacent land areas.
Sedimentation patterns and facies geometries on a temperate glacially-influenced continental shelf: the Yakataga Formation, Middleton Island, Alaska
Authors: Eyles Carolyn H. , Lagoe Martin B.
Published: April 2008

The 5 km thick Yakataga Formation of the Gulf of Alaska provides the longest and most accessible record of late Cenozoic glaciation in the world. The formation consists of interbedded marine and glacimarine sediments of late Miocene to Holocene age and is well exposed on offshore islands and in the coastal mountain ranges as a result of the convergent margin setting of the Gulf. About 1.3 km of early Pleistocene Yakataga Formation sediments are exposed on extensive intertidal platforms around Middleton Island which is sited close to the modern continental shelf edge. Variably graded gravels and sands at the base of the Middleton Island succession were deposited by sediment gravity flows and represent the fill of a large submarine channel. The remainder of the succession is dominated by thick (up to 100 m) planar tabular units of fossiliferous diamict containing a series of striated boulder pavements and shell-rich beds (coquinas). Diamict facies formed predominantly by the settling out of fines from suspended sediment plumes together with ice-rafting of coarser debris; foraminiferal biofacies data indicate palaeobathymetries of between 20 and 250 m. Boulder pavements are lag surfaces which developed in relatively shallow water and were subsequently faceted and striated by a grounding ice sheet extending to the continental shelf edge; coquinas record episodes of current winnowing in shallow water (20–50 m) but also contain ice-rafted debris. Extensive mud units formed in relatively deep water during periods of reduced ice volume and are similar to ‘mud blankets’ accumulating on the modern shelf. The Yakataga Formation on Middleton Island contains a rich record of changing palaeobathymetry and ice volumes in the Gulf of Alaska.
Control of fluids on deformation of rocks
Authors: Carter N. L. , Kronenberg A. K. , Ross J. V. , Wiltschko D. V.
Published: April 2008

Fluids of many compositions, concentrations and pressures, are ubiquitous throughout the continental lithosphere, exerting strong control on the deformation properties and processes of rocks both by mechanical means and by complex chemical rock-fluid interactions. Fluids of meteoric and juvenile origin, released by compaction, dehydration reactions, melting, and degassing, commonly during large-scale tectonic events, flow by means of thermal convection, advection (infiltration), and surface and intracrystalline diffusion. These fluids transport mass for distances ranging from the grain scale to hundreds of kilometres; fracture zones provide favourable conduits for flow. Abnormal pore pressures, recorded at all metamorphic grades, develop intermittently during syntectonic deformation, enhancing fluid infiltration by promoting increased porosity and permeability, hydraulic fracturing and severe grain size reductions. The infiltrating fluids enhance hydrolytic weakening, several grain boundary mechanisms, and reaction kinetics in a feedback manner so that strain is commonly localized into semibrittle and ductile shear zones. Large-scale detachments may take place along these shear zones at virtually any depth below the uppermost few kilometres, which, when combined with softening resulting from depth-dependent petrological and geochemical segregations, form a rheological stratigraphy. The rheology of the lithosphere through time has been governed by a combination of bulk rock flow and localized deformation is shear zones, both of which have been aided or controlled by pervasive dynamic rock-fluid interactions.
Conditions for fault-valve behaviour
Authors: Sibson Richard H.
Published: April 2008

Evidence for fluid pressures well in excess of hydrostatic during crustal deformation is provided by direct measurements of pressure levels in active tectonic regions and by hydrothermal vein systems in ancient orogenic belts. Faults that act as impermeable seals except immediately postfailure, when they become highly permeable channelways for fluid discharge, may behave as fluid-pressure-activated
valves
wherever they transect a suprahydrostatic fluid pressure gradient. Such
fault-valve
behaviour, causing abrupt fluctuations in fluid pressure linked to the earthquake cycle, is particularly likely for faults that remain active while unfavourably oriented for frictional reactivation in prevailing stress fields. The most extreme fault-valve action is likely to be associated with high-angle reverse faults. Valve activity may be especially prevalent near the base of the seismogenic zone when unstable frictional faulting gives way to aseismic shearing with increasing depth. At such structural levels, cyclic variations in the style of deformation may accompany fluid pressure cycling.

Fault-valve activity plays an important role in the development of much fault-hosted mineralization, and may also assist the migration of hydrocarbon fluids in some structural settings.
Textures, deformation mechanisms, and the role of fluids in the cataclastic deformation of granitic rocks
Authors: Evans James P.
Published: April 2008

Optical and scanning electron microscopy were used to examine the textures and infer deformation mechanisms in rocks deformed at upper crustal conditions in the Geesaman oblique-slip fault, southeastern Arizona, and the Washakie thrust fault, northwestern Wyoming. Feldspar in slightly and moderately deformed rocks in both faults deformed by formation of extension and shear fractures along cleavage planes, and quartz grains deformed by brittle fracture. Fracture density is greater in feldspar than in quartz. Intergranular fractures and faults developed by the linking of intragranular fractured feldspars, which in turn resulted in the development of foliated cataclasites in the Washakie fault and phyllonites in the Geesaman fault. Clays and mica developed by the syntectonic alteration of feldspar, and with increasing mica or clay content quartz grains deformed by fracture in a progressively softer matrix. Fracturing and healing of fractures in quartz resulted in irregularly-shaped, elongate pods of quartz in a mica matrix in the Geesaman fault and in a clay and feldspar fragment matrix in the Washakie fault. The syntectonic alteration of feldspars, the presence of iron oxides in the faults, and late-stage quartz veins attest to the flow of water during deformation. The fault zones developed by early fracture in feldspar which gave way to cataclastic flow in feldspars and subsequent development of clay-rich cataclasites at low temperature and of phyllonites at high temperatures. Most of the slip in mature cataclasites was localized by slip and cataclasis of clays. Slip in the phyllonites was localized by plastic deformation of micas. The brittle to ductile transition in the rocks was the result of fracture and subsequent cataclasitic flow in feldspars, followed by deformation of clay or mica, and for the most part excluded the plastic deformation of quartz.
The calculation of bulk rheologies of structured materials and its application to brittle failure in shear
Authors: Casey Martin , Wust Gilles
Published: April 2008

A finite element method is presented which allows the calculation of the bulk rheological properties of a material consisting of a regular array of heterogeneous sub-regions. The method is used to calculate the elastic properties of rocks with aligned cracks containing air or fluid. It is found that the cracked material has anisotropic properties. A compressible fluid filling makes the bulk rock less stiff. The stiffness perpendicular to the long axis of the cracks is reduced more strongly than that parallel to the cracks. It is shown that a concentration of aligned cracks in an inclined planar zone could result in an instability leading to shear failure.
Damage development during rupture of heterogeneous brittle materials: a numerical study
Authors: Cox S. J. D. , Paterson L.
Published: April 2008

We have used a finite element method with an iterative algorithm to simulate rupture development in complex brittle materials such as rocks. A simple set of continuous distributions of properties has been used, and only tensile failure of elements allowed. Wider strength distribution leads to more damage and less abrupt mechanical breakdown characteristics. Variations in strength are found to have a greater effect on the geometry of breakdown than variations in elastic properties. The material behaviour simulated here gives a basic explanation for the extent to which natural fractures in the earth, such as joints and dykes, have a non-planar form.