bluuebunny/crossref_metadata_2025 huggingface dataset:
Comments on the pattern of post-rift subsidence in the Central and Northern North Sea Basin
Authors: Joy Aidan M.
Published: April 2008

Burial history analyses suggest that the post-rift subsidence history of the Central and Northern North Sea Basin is poorly explained by published models of thermal subsidence. The last rifting episode took place during the Late Jurassic. The early post-rift phase (earliest Cretaceous to Danian) was characterized by slow subsidence, with much of the sedimentation accommodated by the infilling of previously developed rift bathymetry. At this time the shoulders of the rift were supported. During the earliest Cretaceous to Danian, the Central and Northern North Sea Basin was situated in a continental interior, and its morphology and subsidence style resembled that of many modern intra-continental rifts. Sub-sequently, rates of subsidence increased markedly; this acceleration in subsidence was particularly pronounced in the region of the rift shoulders. The timing of the accelration was approximately mid-Palaeocene, the time at which the Norwegian-Greenland Sea opened immediately to the north of the North Sea Basin. It is suggested that there are differences between the subsidence rates and styles of extensional basins in intra-continental and continental margin settings which are not predicted by published models.
Permeability variation within aeolian sandstone: a case study using core cut sub-parallel to slipface bedding, the Auk Field, Central North Sea, UK
Authors: Prosser D. J. , Maskall R.
Published: April 2008

A combination of sedimentary dip, structural dip and borehole orientation resulted in a core recovered from the Auk Field being cut sub-parallel to aeolian slipface deposits. Individual 2 inch (5 cm) thick grainflow deposits may be followed for up to 16 ft (4.88 m) along core. A conventional core-plug permeability data-set indicated the importance of a deleterious effect of fine-grained ‘pin-stripe’ laminae which separate individual grainflows. Using a probe-permeameter, over 1000 permeability readings were taken from 27 ft (8.23 m) of the core, mostly on a 1 inch (2.5 cm) spaced grid, in order to study small-scale variations and controls of permeability. The readings were taken from five grainflow laminae and associated wind-ripple deposits in a single slipface deposit, and from wind-ripple laminae from an interdune setting.

Individual grainflow laminae sometimes show coarsening-upward patterns of grain-size variation which can be ascribed to dispersive forces acting during sedimentation. In terms of permeability distribution, individual grainflows may be distinct from their immediate neighbours. Geostatistical analyses indicate permeability correlation lengths across grain-flow laminae equal to lamina spacings, and a within grainflow correlation length of approximately 1.5 ft (46 cm). Similar tests within millimetre-scale laminated wind-ripple deposits indicate appreciably smaller correlation distances. These results can be translated into effective sampling strategies of one reading per grainflow deposit for a traverse perpendicular to sedimentary dip, and one reading every 6–12 inch (15cm–30cm) for a traverse along grainflow lamination. Wind-ripple laminae cannot feasibly be sampled in a truly representative fashion by standard core-plugs.

Petrographic analyses indicate that the principle microscopic controls upon permeability are grain-sorting, the development of authigenic cements and the detrital plus authigenic clay content.
The tectonic implications of contrasting granite magmatism between the Kohistan island arc and the Nanga Parbat-Haramosh Massif, Pakistan Himalaya
Authors: George Mark T. , Harris Nigel B. W. , Butler Robert W. H.
Published: April 2008

In northern Pakistan, an integrated structural and geochemical study across the contact between the Cretaceous-Tertiary Kohistan island arc and Indian crust of the Nanga Parbat-Haramosh massif (NPHM) has identified strongly contrasting source regions contributing to post-50 Ma magmatism, which constrains the tectonic evolution of the region. Nd-model ages of Tertiary intrusives increase dramatically across the contact from
c.
700 Ma in the Kohistan terrane to
c.
2500 Ma in the NPHM.

Following collision between the Indian continent and the Kohistan island arc terrane at
c.
50 Ma, biotite granite sheets (Confluence granites) were emplaced into the Kohistan batholith at 50–30 Ma, followed by muscovite granite sheets (Parri granites) at
c.
26 Ma. These are geochemically distinct, with the Confluence granites comprising a range of granitic compositions with high Sr and Ba abundances and the Parri granites forming granite sheets enriched in Rb. Undeformed granites in both suites have (
87
Sr/
86
Sr)
i
in the range 0.7045–0.7054 and ε
Nd
(
T
) of + 0.1 to +2.7, suggesting that both groups may be derived from juvenile arc sources. Detailed structural studies verify that the Kohistan granite sheets postdate the initial collision of the Kohistan terrane with the Indian continent, but are preor syntectonic with respect to continued underthrusting of the Indian continent below the Kohistan arc. These results, together with isotopic evidence from the undeformed granite sheets, suggest that significant underthrusting of northern Kohistan by the Indian continental crust could not have occurred until after 26 Ma.

Along the western margin of the NPHM, deformed granite sheets show a marked increase in (
87
Sr/
86
Sr)
i
(0.7075–0.7784), with decreased ε
Nd
(
T
) (−13 to −26). These trends are thought to be due to a combination of sub-solidus fluid infiltration and assimilation of crustal material, with fluids or material derived from the adjacent, isotopically evolved NPHM crust.

The NPHM (Indian continent) has been intruded by a series of tourmaline leucogranite dykes and plutons at 2–12 Ma. These intrusives have trace-element signatures consistent with generation by vapour-absent melting of a pelitic source. Sr-Nd isotope systematics indicate derivation from metasediments such as are currently exposed in the basement rocks of the massif, although melting at the present exposure level is generally precluded by subsolidus metamorphic grades in the country rock. Extreme heterogeneities in radiogenic Sr((
87
Sr/
86
Sr)
i
> 0.9) are observed in leucogranite dykes from the Liachar thrust zone, probably indicating subsolidus fluid infiltration. In general the NPHM leucogranites result from rapid exhumation of metasediments characterised by unusually high heat productivity.
Geochemistry of the continental basalts within the Tethyan Himalaya of Lahul-Spiti and SE Zanskar, northwest India
Authors: Vannay J. C. , Spring L.
Published: April 2008

The intrusive continental basalts of the Tethyan Himalaya are not necessarily comagmatic feeder dykes or the hypabyssal equivalents of the middle Permian Panjal Traps flows associated with the Permo-Mesozoic rifting of the Tethys, as is generally considered. A comparative study of basic rocks from SE Zanskar, NW Spiti and Upper Lahul shows the coexistence of geochemically different basalts in this region. The Panjal Traps of SE Zanskar and NW Spiti are tholeiitic continental flood basalts (CFB). They originated from an ‘enriched’ P-MORB-type magma which underwent a limited evolution by fractional crystallization and probable crustal contamination. In the adjacent Upper Lahul area, the Baralacha La Dyke Swarm shows a close spatial association with Lower Carboniferous transtensional synsedimentary faults. These dykes consist of tholeiitic to alkalic basalts, belonging to a comagmatic suite that evolved mainly by fractional crystallization. Geochemical data indicate that the Panjal Traps and the dykes are not comagmatically related. These characteristics imply either the coexistence, in the studied area, of contemporaneous but chemically different magmas or temporally unrelated magmatic events. The Baralacha La Dyke Swarm could be associated with a Lower Carboniferous transtensional tectonic event, related to an early phase of Permian rifting of Neo-Tethys.
Braided rivers: perspectives and problems
Authors: Bristow C. S. , Best J. L.
Published: April 2008

Progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems. Although many advances have been made within recent years in interpreting the mechanics of flow, transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed. In particular, several areas demand attention: the mechanisms of braid bar initiation; confluence-diffluence dynamics, the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales. This paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration.
Sedimentology of the Rough Rock: a Carboniferous braided river sheet sandstone in northern England
Authors: Bristow C. S.
Published: April 2008

The Rough Rock is a coarse-grained fluvial sheet sandstone which outcrops across northern and central England. It is interpreted as a braided river sandstone on the basis of sedimentary structures, internal erosion and growth surfaces. This study reveals that the Rough Rock represents more than a single phase of deposition and that internal erosion surfaces may indicate anabranch avulsions or more significant river avulsions. Detailed outcrop profiles are presented to try and distinguish the styles of deposition and the nature of internal bounding surfaces and channel geometry. The Rough Rock Formation contains a very large number of first and third order bedding contacts, which represent the migration of dunes and channels respectively, there are relatively few macroforms and bars preserved. The lack of second order surfaces within many outcrops could indicate that bars in the Rough Rock rivers were very large and therefore cannot be resolved at the outcrop scale. Similarly, the rivers which deposited the Rough Rock may have had such high width: depth ratios that the dip of second order bounding surfaces are too low to be identified. The large number of third order bounding surfaces indicates channel stacking which may have occurred in two ways: (1) relocation of rivers in the same area over time, or, (2) anabranch avulsion within a braided river within a hierarchy of channels. The problems associated with differentiating between these two mechanisms are discussed.
Basaltic-volcano systems
Authors: Walker George P. L.
Published: April 2008

This review and personal account of basaltic volcanism concentrates on the geological aspects and physical controls, unlike most others which concentrate on geochemistry. It serves as an update to reviews by Wentworth & Macdonald 20 to 40 years ago. The concept of volcanic systems is developed, a system including the plumbing, intrusions, and other accoutrements of volcanism, as well as the volcanic edifice. Five types of systems are described, namely lava-shield volcanoes, stratovolcanoes, flood-basalt fields, monogenetic-volcano fields, and central volcanoes (having silicic volcanics as well as basaltic). It is postulated that the system-type depends on (a) the magma-input rate and (b) the frequency with which magma-batches enter the system (or modulation frequency). These controls determine whether a hot pathway is maintained from the source to the high-level magma chamber, and hence if eruptions are concentrated in the central-vent system. Reviews are given on many aspects: the different kinds of rift zones that volcanoes exhibit, and their controls; the mostly small intrusions, including coherent-intrusion complexes, that occur in and under the volcanic edifices; the great sill swarms that are an alternative to flood basalts; the origin of the craters and calderas of basaltic volcanoes; high-level magma chambers, their locations, and cumulate prisms; the positive Bouguer gravity anomalies that are attributed to cumulates and intrusion complexes; the structures of basaltic lava flows; the characteristics of basaltic explosive volcanism and the consequences of participation by non-volcanic water; the products of underwater volcanism; the origin and significance of joints, formed either by contraction or expansion, in volcanic rocks; and the distribution of vesicles in basaltic rocks. A fairly extensive bibliography is included.
The development of a late Cretaceous microplate suture zone in SW Cyprus
Authors: Malpas J. , Calon T. , Squires G.
Published: April 2008

Recent detailed mapping and volcanic geochemistry of rocks exposed in erosional windows through post-Cretaceous cover rocks in SW Cyprus have provided evidence for the development of a model for late Cretaceous microplate collision in the eastern Mediterranean. Two major juxtaposed terranes are recognized: the older Mamonia Complex consisting of Triassic ocean floor, accompanying seamounts and a series of continental rise prism sediments, and the younger Troodos Complex consisting of ophiolitic rocks formed in a supra-subduction zone environment, and modified in part by transform fault tectonics and magmatism. The two complexes were juxtaposed along an intricate suture zone, the evolution of which displays two main phases of contractional deformation of late Cretaceous age. These are a primary north- and northwest-directed thrusting, and a secondary backthrusting episode towards the west and southwest, that in places reorganized the initial structural stacking sequence. Extensional faulting of Troodos Complex rocks, that took place on the ocean floor prior to juxtaposition, clearly influenced the orientation of the later thin-skinned delamination. Although there is some evidence for strike-slip faulting, this process does not appear as extensive as proposed by previous workers.
Re-evaluation of inclined intrusive sheets and dykes in the Cuillins volcano, Isle of Skye
Authors: Walker George P. L.
Published: April 2008

This paper presents new quantitative data on the mafic dykes and classic centrally inclined sheets (cone-sheets) of the Cuillin Hills. Most dykes were intruded into the NW and SE rift zones of the Cuillins volcano. The dyke-widths, and the downrift rate of increase of widths, are almost identical with those of Koolau dykes in Oahu (Hawaii). The dilation vector (direction of opening) is roughly normal to the plane of the inclined sheets, showing that the sheets are simple extensional intrusions like the dykes. Sheet thicknesses are nearly identical with dyke-widths, reinforcing this conclusion. Sheets cut dykes, and dykes cut sheets, showing that dyke or sheet injections alternated. Sheets are concentrated in the NW and SE sectors of the Cuillins where they comprise a coherent complex having intrusion-intensities of 40–807fb3e69creferentially injected into the rift zones. A mechanism is proposed in which magma batches were injected as bladed dykes or bladed sheets as the σ
3
stress direction oscillated between vertical and horizontal; this was in a tectonic setting where, because of slow extension, magma excursions could not all be accommodated as dykes. Injection of sheets resulted in a vertical thickening of the volcanic edifice by
c.
1.5 km about the same as the amount of widening by dykes.
Geochemistry of aliphatic-rich coals in the Cooper Basin, Australia and Taranaki Basin, New Zealand: implications for the occurrence of potentially oil-generative coals
Authors: Curry David J. , Emmett John K. , Hunt John W.
Published: April 2008

Although the concentration of long-chain aliphatic constituents is a primary determinant of the oil generation potential of coals, the factors which govern their occurrence in different coals are poorly understood. In this study, Permian coals from the Cooper Basin, Australia, and the Eocene coals from the Taranaki Basin, New Zealand, were compared to determine these factors.

The Taranaki Basin coals were deposited in temperate, fluvial-deltaic environments. HI values range from 236–365. Extracts have high pristane/phytane ratios and variable abundances of oleanane and other non-hopanoid terpanes. The extracts and pyrolysates contain high relative concentrations of aliphatic groups >
n
-C
20
. These data imply that much of this aliphatic carbon is derived directly from higher plant material.

The Cooper Basin coals were deposited in high latitude bogs and contain 40–70 0nertinite. The coals have been severely degraded. Pristane/phytane ratios are low (2.15–6), but HIs are moderate (up to 243 mg g
−1
OC). The extracts and pyrolysates both contain high relative concentrations of aliphatic groups; however, the distributions are different from higher plant-derived material. These data imply the bulk of the aliphatic carbon in these coals is derived from microbial biomass (both bacterial and fungal degradation products and algal input).

These results show that long-chain aliphatic groups in coals can be derived directly from the higher plant material, from microbial activity in the depositional environment, or from a combination of the two.
Crustal stress, faulting and fluid flow
Authors: Sibson Richard H.
Published: April 2008

Differential stress exerts both static and dynamic effects on rock-mass permeability, modulating fluid flow in the Earth’s crust. Static stress fields impose a permeability anisotropy from stress-controlled features such as faults, extension fractures, and stylolites which, depending on the tectonic regime, may enhance, or counteract existing anisotropic permeability in layered rock sequences. Textural evidence from hydrothermal veins suggests, however, that fluid flow in fault-related fracture systems generally occurs episodically and that dynamic stress cycling effects are widespread. In the vicinity of active faults that undergo intermittent rupturing, permeability and fluid flux may be tied to the earthquake cycle through a range of mechanisms, leading to complex interactions between stress cycling, the creation and destruction of permeability, and fluid flow. Mechanisms for fluid redistribution include: (1) various forms of
dilatancy
(localized to the fault zone or extending through the surrounding rock mass) related to changes in shear stress and/or mean stress that occur during the
fault loading
cycle; (2) localized
post-seismic redistribution around rupture irregularities
, especially dilational jogs and bends which act as
suction pumps
; and (3) post-seismic discharge of fluids from overpressured portions of the crust through
fault-valve
action when ruptures breach impermeable barriers. All of these processes may be involved in fluid redistribution around active faults, but they operate to varying extents at different crustal levels, and in different tectonic regimes.
Controls on two-phase fluid flow in heterogeneous sandstones
Authors: Ringrose P.S. , Corbett P.W.M.
Published: April 2008

The effective flow behaviour of two immiscible fluids in a permeable medium is a function of fluid/fluid as well as fluid/matrix interactions. These are described by relative permeability functions, which are strongly influenced by the spatial distribution of permeability in sedimentary media, given specified fluid and pore surface properties. For typical subsurface flow rates and patterns of rock heterogeneity in hydrocarbon reservoirs, capillary forces can result in significant amounts of trapping and bypassing of the non-wetting phase. Different types of sedimentary media (e.g. laminated, crossbedded and pervasively faulted strata) result in different, quantifiable, degrees of trapping. Examples of typical water-wet oil/water systems illustrate how the amount of trapped oil can vary between around 40% and 65% depending on the type of heterogeneity. Furthermore, the layered nature of most sedimentary strata means that the relative permeability of two immiscible phases is highly anisotropic. These findings have important implications for reservoir engineering and studies of oil migration.
Organic ligand distribution and speciation in sedimentary basin brines, diagenetic fluids and related ore solutions
Authors: Giordano Thomas H. , Kharaka Yousif K.
Published: April 2008

The nature, distribution, and interactions of dissolved organic ligands in the shallower zones of sedimentary basins are highly variable and poorly understood. Better understood is the chemistry of dissolved aqueous carboxylic acid species in the deeper regions of sedimentary basins. In most oil-field brines, acetate is generally the dominant organic ligand and is followed in level of concentration by longer chained aliphatic monocarboxylic acid anions. The total concentration of dicarboxylic acid anions is probably lower than previously reported and likely less than 500 mg l
−1
with succinate and glutarate being the dominant species. The highest concentrations of organic acid anions are present in formation waters at 80–120° C. Based on the available field and laboratory evidence, the concentrations of organic anions are controlled mainly by the rates of their generation from kerogen and their destruction thermally or by bacteria. An extensive compilation of experimentally determined stability constants and other thermodynamic data for a large number of non-humic organic ligands is available for temperatures near 25° C. However, corresponding high-temperature data are available for only a few species of interest; most notably, acetate complexes of some rock- and ore-forming metals and protonated species of several carboxylate ligands. Simulations of speciation in oil-field brines show that significant amounts of Ca, Mg, Fe, and Al can be complexed by carboxylate ligands, in the pH range 4–6, if ligand concentrations are on the order of 10–100 mg l
−1
. Calculated speciation in model ore fluids for Mississippi Valley-type deposits and red-bed related base metal deposits show that optimum conditions for Pb and Zn transport by organic ligand complexation are oxidized ore fluids with total reduced inorganic sulphide concentration less than 10
−9
molal.
Modern and fossil mangroves and mangals: their climatic and biogeographic variability
Authors: Plaziat Jean-Claude
Published: April 2008

The variability of mangrove environments (mangals) has generally been underestimated, as has their distribution from temperate to subequatorial climates and their histories in the Atlantic and Indo-Pacific biogeographic provinces. Subtropical, arid and equatorial mangals require different palaeoecological approaches.
In situ
fossilization is paradoxically very rare, and has often been misinterpreted, but current reworked shells, and human shell gathering (middens) provide valuable records of ancient mangals. The identification of ancient mangroves depends on the preservation of pollen or wood while the upward branching root system of
Avicennia
is another reliable indicator. The associated molluscan fauna may also be diagnostic, particularly if encrusting oysters are preserved with imprints of mangrove roots and bark. Several case studies are briefly reviewed and discussed: Holocene buried mangals of Abu Dhabi, Holocene shell middens of the Middle East, Miocene Japanese and European mangals documented by molluscs and pollen, and the Paleocene-Eocene mangals of western Europe. Problems concerning the evolution of the mangal biota and the differences between their distribution in ancient and modern climates forms the basis for discussion of the palaeogeographic and climatic significance of the palaeomangals reviewed in this paper.
Modelling Late Jurassic Milankovitch climate variations
Authors: Valdes P. J. , Sellwood B. W. , Price G. D.
Published: April 2008

Although largely circumstantial in character, evidence for orbitally-forced (Milankovitch) climate changes in Jurassic microrhythmic successions, such as those of the Lias and Kimmeridgian, is becoming more persuasive. We present here the results of experiments, using a general circulation model, testing ways in which orbitally-induced variations in solar energy might be translated into a Jurassic climate response. In particular, we address the problem of the 100 ka (eccentricity-forced) cycle. This is generally considered to have only a small direct effect on solar input. It would be expected to have little impact on an ice-free Earth (commonly assumed for the Jurassic). Nonetheless, this weak signal is claimed to have been recognized in many Jurassic successions. Our results simulate, for the Late Jurassic, the possible effects at the ‘minimum’ and ‘maximum’ extremes of seasonal forcing (i.e. comparable with those affecting the Earth at 115 and 9 ka
bp
, respectively). Model predictions are critically evaluated against the geological database. At times of ‘minimum seasonal forcing’ there is a significant expansion in the area of the Northern hemisphere monsoon. In the tropics, changes in precipitation predominate over changes in temperature, whereas at high southern latitudes there are very large seasonal variations in temperature, and heavy winter snows. During these times the model is close to predicting a modest, but significant, Jurassic ice-cap in the Antarctic. Ice build-up is particular larly likely over uplands. Such an ice-cap disappears at times of ‘maximum seasonal forcing’. Waxing and waning of ice may thus provide the elusive mechanism for metre-scale sea-level changes. It is argued that apparently similar microrhythms (e.g. limestone-shale) might be the sedimentary response to different climatic signals in different climate zones, cause and effect being exactly the opposite in some circumstances.
Hydrocarbon habitat and classification of inverted rift basins
Authors: Macgregor Duncan S.
Published: April 2008

This shortened paper presents a brief analysis of the relationship between hydrocarbon distribution and the intensity of inversion in rift basins. For this purpose, three intergradational classes of rift were identified according to the intensity and areal extent of inversion-related uplift. Simple rifts, showing no significant inversion, are characterised by high success rates and dispersed hydrocarbon distributions. Locally inverted rifts, such as those showing restricted areas of uplift along faults, also show a high degree of petroleum retention and success, with the key success factor often being the relationship between timing of trap formation and petroleum charge. Regionally inverted rifts show lower success rates, failures being often attributable to a redistribution of hydrocarbons during inversion, with losses having occurred through biodegradation, surface erosion or seepage on surface penetrating faults. Both types of inverted rift frequently show a concentration of reserves in one large field.
Applications of Thermal History Reconstruction in inverted basins
Authors: Green Paul F. , Duddy Ian R. , Bray Richard J.
Published: April 2008

Studies of inverted basins based solely on the preserved section provide only partial insight into the thermal and tectonic history of basin development. Equally important is that part of the history represented by section removed by erosion during inversion. Thermal History Reconstruction, involving application of Apatite Fission Track Analysis (AFTA™) and Vitrinite Reflectance (VR) to define the timing and magnitude of key erosional and/or thermal episodes, and integration of this information with data from the preserved section, provides a more complete description. Case histories from the UK, New Zealand and Australia are presented to demonstrate this approach. Much of northern and eastern England, the Irish Sea and the Southern North Sea was affected by heating apparently due largely to additional burial, followed by early Tertiary cooling due to regional uplift and erosion, synchronous with recognized basin inversion events. Heating associated with this episode caused generation of hydrocarbons throughout the region. Areas conventionally regarded as not inverted appear to have been subjected to km-scale uplift and erosion, which was not restricted to classic ‘inversion axes’. Failure to allow for these effects can lead to a significant underestimation of regional maturity trends. Thermal History Reconstruction is also essential in areas that have undergone more than one episode of inversion. For example in Inner Moray Firth well 12/16—1 maximum palaeotemperatures in the Carboniferous section were reached prior to Hercynian inversion and the chances of preserving any hydrocarbons generated from Carboniferous or older source rocks appear poor. In southeastern Australia, early Cretaceous rift basins underwent mid-Cretaceous inversion at a time of high palaeogeothermal gradients (
c.
60°C km
−1
), and basement margins were also inverted at that time. Again, early generation implies that the chances of preservation of hydrocarbons are small. By integrating stratigraphic and structural relationships within the preserved section with data on the palaeo-thermal history of a section derived from direct measurements of palaeotemperature profiles, the complete history of hydrocarbon generation may be reconstructed with confidence.
Regional Tertiary Exhumation in and around the United Kingdom
Authors: Hillis Richard R.
Published: April 2008

Sonic velocities in the Upper and Middle Chalk, the Bunter Sandstone and the Bunter Shale were used independently to quantify apparent exhumation (amount of missing section) in the UK Southern North Sea (SNS). Apparent exhumation results derived from these units are statistically similar. The consistency of results from chalks, shales, and sandstones in the UK SNS, and in other areas (Inner Moray Firth and Celtic Sea/South-Western Approaches), suggests that, at a formation and regional scale, burial-depth is the primary control on velocity (and porosity) in these units, and validates the use of lithologies other than shale in maximum burial-depth determination. The consistency of apparent exhumation results from units of Early Triassic to Late Cretaceous age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that maximum Mesozoic-Cenozoic burial-depth was attained prior to Tertiary exhumation.

The analysis of sonic velocities, apatite fission tracks, vitrinite reflectance, normalized drilling rate and clay mineral diagenesis all suggest that there was a regional component of approximately 1 km of exhumation during Tertiary times, which affected structurally uninverted areas in and around the UK. The widely-recognized inversion-related exhumation is superimposed on regional Tertiary exhumation. Regional Tertiary exhumation need not have been contemporaneous with structural inversion. Further refinement beyond Tertiary age is not possible from the sonic velocity data. Regional exhumation, however, is most likely to be associated with regional Palaeocene or Oligocene/Miocene unconformities.

The evidence for regional exhumation implies that there was a pre-exhumational phase of burial (during which the eroded sedimentary rocks were deposited). Exhumation and prior burial associated with the regional Tertiary event need to be incorporated in aspects of basin analysis such as sediment decompaction and diagenetic and maturation modelling, and can, for example, account for the maturity of the Carboniferous of the onshore East Midlands Shelf, and the Lias exposed on the north coast of Somerset. The regional, Tertiary tectonic uplift associated with exhumation must have had a thick-skinned origin, and, in areas where there is no evidence for significant Tertiary igneous activity, a decoupled, two-layer model of lithospheric compression is invoked to account for the subsidence/uplift patterns of both inverted and uninverted areas.
The application of fission track analysis to the dating of barren sequences: examples from red beds in Scotland and Thailand
Authors: Carter Andrew , Bristow Charles S. , Hurford Anthony J.
Published: April 2008

In continental red-beds facies, which often contain little or no organic material for biostratigraphic dating, fission track (FT) analysis of detrital zircon crystals represents one method by which a maximum stratigraphic age may be determined. In addition, FT analysis of detrital apatite provides a means of palaeotemperature estimation, especially in areas where there is insufficient, or unsuitable, organic matter for vitrinite reflectance. Two examples of the application of FT analysis to the solution of stratigraphic problems in redbed sequences are described. In Scotland, the age of the Tongue Outlier is disputed, previous studies having suggested either a Devonian or Permo-Triassic age. Comparison of FT data from both detrital apatite and zircon indicates a Permo-Triassic age. In Thailand, the 3 km thick continental clastic Khorat Group has a well-defined lithostratigraphy but is poorly constrained biostratigraphically. The lower Nan Phong Formation has a Rhaetic age whilst the Khok Kruat Formation at the top of the group has been assigned an Aptian age. In the absence of any biostratigraphic control, the intermediate Phu Kradung and Phra Wihan Formations have been assumed to be of Middle and Upper Jurassic age. FT data presented here for detrital zircons indicate a Lower Cretaceous age for Phra Wihan Formation of the Khorat Group.
Early Tertiary evolution and sequence stratigraphy of the Faeroe-Shetland Basin: implications for hydrocarbon prospectivity
Authors: Ebdon C. C. , Granger P. J. , Johnson H. D. , Evans A. M.
Published: April 2008

Eight seismically resolvable packages have been identified in the Early Tertiary succession of the Faeroe-Shetland Basin. These packages are calibrated with the microfaunal and microfloral biostratigraphy, thus allowing correlation between wells and comparison with other basins. Mapping of the facies distribution within each package has allowed a detailed understanding of the evolution of the southwestern part of the basin, including the Quad 204 acreage. The development of the area can be related to the plate tectonic evolution of the North Atlantic and has highlighted implications for hydrocarbon exploration. In the earliest part of the Palaeocene (Sequences T10–T32) the position of the shelf/slope break was controlled by the underlying end Cretaceous fault induced topography. The depositional character of the submarine fans which accumulated in the basin is aggradational, collecting in basin floor deeps and progressively onlapping basin floor highs. Reservoir quality is initially controlled by depositional facies but is, essentially, excellent. Basinal muds, particularly regionally extensive maximum flooding surfaces, are believed to provide effective seals. Sequences T36–T40 are progradational in character, the position of the shelf/slope break advancing significantly beyond the positions of underlying Cretaceous fault scarps. This change in depositional character is related to thermal doming and rifting in the North Atlantic. The results of this uplift in the Faeroe-Shetland Basin also instigated a change in sediment provenance, volcanism and the deposition of extensive base-of-slope fan systems. Reservoir effectiveness remains excellent in these fan systems and potential seals are provided by downlapping, progradational shelf/slope systems. Sequences T45 and T50 represent flooding of the Palaeocene shelf systems as the thermal dome collapsed and rifting progressed. The overall control on Early Tertiary reservoir effectiveness in the Faeroe-Shetland Basin is depth of burial. In this part of the basin the Early Tertiary reservoir sands are generally buried at depths less than 2.5 km below sea-bed and reservoir quality remains excellent (porosity > 25% and permeability > 100 Md). The challenge to the oil industry is to understand the charge of this attractive hydrocarbon play.
The formation of passive margins: constraints from the crustal structure and segmentation of the deep Galicia margin, Spain
Authors: Boillot G. , Beslier M. O. , Krawczyk C. M. , Rappin D. , Reston T. J.
Published: April 2008

The crustal structure of the Mesozoic deep Galicia margin and adjacent ocean-continent boundary (OCB) was investigated by seismic reflection (including pre-stack depth migration and attenuation of seismic waves with time). The seismic data were calibrated using numerous geological samples recovered by drilling and/or by diving with submersible.

The N-S trending margin and OCB are divided in two distinct segments by NE-SW synrift transverse faults locally reactivated and inverted by Cenozoic tectonics. The transverse faulting and OCB segmentation result from crustal stretching probably in a NE-SW direction during the rifting stage of the margin in early Cretaceous times. The Cenozoic tectonics are related to Iberia-Eurasia convergence in Palaeogene times (Pyrenean event). In both segments of the deep margin, the seismic crust is made of four horizontal layers: (1) two sedimentary layers corresponding to post- and syn-rift sequences, where velocity ranges from 1.9 to 3.5 km s
−1
, and where the
Q
factor is low, the two sedimentary layers being separated by a strong reflector marking the break-up unconformity; (2) a faulted layer, where velocity ranges from 4.0 to 5.2 km s
−1
, and where the
Q
factor is high. This layer corresponds to the margin tilted blocks, where continental basement and lithified pre-rift sediments were sampled; (3) the lower seismic crust, where the velocity (7 km s
−1
and more) and the
Q
factor are the highest. This layer, probably made of partly serpentinized peridotite, is roofed by a strong S-S’ seismic reflector, and resting on a scattering, poorly reflective Moho.

A composite model, based both on analogue modelling of lithosphere stretching and on available structural data, accounts for the present structure of the margin and OCB. Stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels (upper crust and uppermost mantle) and by simple shear in the ductile levels (lower crust and upper lithospheric mantle). Two main conjugate shear zones may account for the observations and seismic data: one (SZ1), located in the lower ductile continental crust, is synthetic to the tilting sense of the margin crustal blocks; another (SZ2), located in the ductile mantle, accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis (now the OCB). The S-S′ reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn-rift hydrothermal activity. It is probably related to both shear zones SZ1 and SZ2. The seismic Moho is lower within the lithosphere, at the fresh-serpentinized peridotite boundary.
Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland
Authors: Hitchen K. , Stoker M. S. , Evans D. , Beddoe-Stephens B.
Published: April 2008

Red-bed sequences of known or inferred Permian and Triassic age are preserved in numerous offshore basins to the north and west of Scotland. These developed following the Late Carboniferous Variscan Orogeny, as compressive uplift was superseded by a tensional stress regime causing the fragmentation of the Variscan foldbelt and its northern foreland. Basin formation persisted through the Permian and Triassic periods, and the sediment infills reflect syn- and post-depositional fault activity. Syndepositional basins take the form of deep, fault-bounded half-grabens whose development was controlled by the extensional reactivation of earlier (Precambrian/Caledonian) thrusts and transfer faults. These basins accumulated wedge-shaped packages of sediment dipping into major basin-bounding faults. In contrast, post-depositional basins are characterized by a parallel-bedded infill which appears to reflect local preservation as a result of later faulting. These basins may be Late or post-Triassic in age. The Permo-Triassic rocks were deposited during a period of major regression, with fluviatile, lacustrine and aeolian environments predominating. Principal lithologies include sandstones, that are partly conglomeratic, intercalated with mudstones and claystones. The localized development of carbonates and evaporites indicates some marine influence. Palaeontological data are commonly sparse throughout the succession and generally cannot readily differentiate the two systems. There is limited evidence for both Early Permian and Late Triassic volcanism. The Early Permian volcanism was ‘within-plate’ and ‘continental’ in character, and was probably related to the first phase of post-Variscan continental disintegration. The Late Triassic volcanism indicates a subsequent episode of synrift igneous activity west of Britain and Ireland.
A gravity map of Ireland and surrounding waters
Authors: Readman P. W. , O’Reilly B. M. , Edwards J. W. F. , Sankey M. J.
Published: April 2008

A Bouguer gravity map of Ireland and the surrounding shallow seas has been produced in the form of a vertically illuminated shaded relief image. This method of presentation accentuates subtle trends in the gravity field which reflect complex density variations present in the crust beneath Ireland. These can be correlated with well known geological features in the onshore region, such as Caledonian granites and major faults. The large normal fault systems which bound offshore Mesozoic basins are clearly indicated by strong gravity lineaments with a similar underlying Caledonian trend. In many cases these can be traced onshore to known Caledonian faults. Transverse Caledonian and Variscan tectonic trends are also highlighted by the method.
Tertiary uplift and inversion history in the North Celtic Sea Basin and its influence on source rock maturity
Authors: Murdoch L. M. , Musgrove F. W. , Perry J. S.
Published: April 2008

Tertiary uplift and basin inversion have been widely recognized in the area of the North Celtic Sea Basin offshore Ireland, but the magnitude and timing of the various events have not been well documented. An understanding of these events is important because recent basin history generally has a major influence on hydrocarbon source rock maturity levels. A multi-disciplinary approach was used to investigate the timing and nature of uplift in the North Celtic Sea Basin, involving seismic data interpretation, well log correlation, vitrinite reflectance measurement, apatite fission track analysis and a Cretaceous Chalk Group isopach and velocity study.

Both regional uplift and compressive inversion can be interpreted to have occurred, based on missing geological section, compressive reactivation of earlier normal faults and basin-centred uplift. Two Tertiary erosion events are recognized: a regional uplift in the Palaeocene, and compressive inversion characterized by basin doming and fault reversal, probably associated with an Oligo-Miocene phase of the Alpine Orogeny. Separating these was a period of basinal deposition in the Eocene to Oligocene. A map of the magnitude of erosion generated as a result of the two events shows that the amount of uplift is quite variable. In the northeast part of the basin, where the later event was most severe, uplift was as much as 1100 m. Reconstruction of the basin geometry before inversion, together with thermal modelling at selected well locations, suggests that Upper Jurassic source rocks reached maturity for oil generation only in the central portion of the basin, whereas Lower Jurassic source rocks entered the gas window in much of the basin prior to Tertiary uplift.
Sandstone diapirism and clastic intrusion in the Tertiary submarine fans of the Bruce-Beryl Embayment, Quadrant 9, UKCS
Authors: Dixon R. J. , Schofield K. , Anderton R. , Reynolds A. D. , Alexander R. W. S. , Williams M. C. , Davies K. G.
Published: April 2008

Submarine fans of Late Palaeocene and Early Eocene age form important hydrocarbon reservoirs in the Bruce-Beryl Embayment, northern North Sea. The Early Eocene fans are the main reservoirs in the Forth-Gryphon oilfields and in the giant Frigg gasfield. Significant oil discoveries have also been made in Late Palaeocene fans. Forth and Gryphon lie on the flanks of the Crawford Anticline, a drape structure that developed during the Palaeocene above the crest of a Mesozoic tilted fault block. The Early Eocene fans pinchout against the flanks of the anticline implying continued growth of the structure throughout the Eocene. Growth was accompanied by the development of major gravity slides that detached in a sequence of altered, basaltic tephras at the base of the Eocene sequence. Seismic-scale, post-depositional deformation (sandstone diapirism and the intrusion of clastic sills and dykes) connected with this sliding dramatically modified the original depositional geometries of the fans. A detailed account of the deformation features, illustrated with core, wireline log and three-dimensional seismic data is presented together with a discussion of their exploration/appraisal significance.
The sedimentological record of a late Jurassic transgression: Rona Member (Kimmeridge Clay Formation equivalent), West Shetland Basin, UKCS
Authors: Verstralen Ivo , Hartley Adrian , Hurst Andrew
Published: April 2008

Late Jurassic sandstones form important potential reservoirs within the West Shetland Basin. These strata are thin (4–84 m) and broadly equivalent in age to the Kimmeridge Clay Formation. They were deposited predominantly in deep marine environments although subaerial and shallow marine facies are also present. The Rona Sandstone forms the lowermost of three sandstone units within the Upper Jurassic succession. Data from 23 exploration wells show, however, that it is a heterogeneous unit ranging from coarse-grained pebbly sandstones to mudstones and, because of this wide lithological variation, the Rona Member is considered a more appropriate name. The facies identified (cohesionless debris flow, high and low density turbidites, upper and lower shoreface) and their vertical transitions are considered typical of fan delta deposition, initially in a subaerial environment and later under progressively more marine conditions. Coarse-grained sediments (debris flow and high density turbidites) of the Rona Member are restricted to isolated topographic lows within Lewisianoid basement whereas finergrained sediments (low density turbidites) have a wide distribution across the basin and cover much of the Rona Ridge. Previous models have suggested that the coarse-grained sediments were derived from active fault scarps. However, to explain the facies distribution it is suggested that footwall uplift on the Rona and Judd Faults (Rona Ridge and associated basement highs) was responsible for the supply of coarse-grained material to isolated fan deltas. Transgression eventually resulted in: flooding of the basement highs, cut off of sediment supply and deposition of finer-grained sediments. Deposition of the Rona Member is interpreted to have been controlled by the gradual transgression of residual islands of Lewisianoid basement.