bluuebunny/crossref_metadata_2025 huggingface dataset:
Soft lithosphere during periods of Archaean crustal growth or crustal reworking
Authors: Choukroune P. , Bouhallier H. , Arndt N. T.
Published: April 2008

Field observations, and an analysis of the strain field, have been carried out in Archaean terranes of the Hebei province in the Dharwar craton of India, the Sino-Korean craton in China, and the Man shield in the Ivory Coast.

Two broadly different situations can be recognized. In the Dharwar and Man Shields the deformation shows a range of characteristic features. The foliation trajectories outline dome-and-basin structures in which supracrustal rocks of greenstone belts invariably occupy the basins. The foliation trajectories are perturbed only by bands of transcurrent shearing. Variations in deformation directions depend essentially on the geometry of plutonic or migmatitic bodies within which the foliation invariably has a domal form. Changes in the intensity of deformation are essentially limited to boundaries between granitoids and supracrustal series. These changes are largely horizontal. Characteristics of the finite strain ellipsoid (the parameterk) vary extremely rapidly.

The second situation is manifested in the Hebei province where the Archaean crust has deformed without the preservation of domes and basins. Here the deformation is characterized by the presence of a foliation and a very strong vertical flattening which extends into catazonal domains. This deformation is homogeneous. To explain these observations it is proposed that this homogeneous deformation was superimposed on pre-existing dome-and-basin structure, obliterating the original geometry. The Hebei example represents a stage of deformation greater than that of the Ivory Coast, which in turn is greater than that in the India craton.

It is apparent that the Archaean crust has specific characteristics that controlled the manner in which it deformed. During accretion, or reworking, it was dominated by body forces related to differences in the densities of the two major Archaean lithological units: granitoid and greenstones. Deformation resulting from boundary forces then led to a second type of behaviour controlled by a vertical planar anisotropy induced during the first stage.

These characteristics are never reported in modern orogenic belts, which generally are marked by a uniform structural trend and vergence. It is proposed that Archean cratons completely lacked a rigid element during the accretion events responsible for their formation, that the thermal regime in these periods was quite unlike that of today, and that the continental crust formed or was reworked during episodes of enhanced mantle plume activity.
The past of a future syntaxis across the Zagros
Authors: Talbot C. J. , Alavi M.
Published: April 2008

Longitudinal components of the Zagros mountain chain change in character and width across N-S trending zones of strike-slip transfer faults lying between 51° and 54°E. To the northwest, a fold-thrust belt with consistent SW vergence has a width ofc.220 km in front of an imbricate beltc.160 km wide. To the southeast, an imbricate zonec.80 km wide is fronted by a gently tapering festoon of upright periclines that isc.350 km wide and punctured by over a hundred emergent salt diapirs.

Pre-Zagros stages of the transfer zones in the Zagros are preserved on the Arabian platform and the two most obvious bound what we call the incipient Qatar syntaxis. This is at an early stage of one of the many syntaxes that compartmentalize the Alpine-Himalayan mountain chain. We use structures in the Hormuz salt to map and gauge the tectonic pulse of basement blocks that jostled as ocean basins opened and closed diachronously like zip fasteners along the Tethyan margin of Gondwana. This incipient syntaxis was a lithospheric key that went up while others went down during the rifting and riffling, but not drifting, of the still-born Hormuz basin we call Proto-Tethys.
Development of salt-related structures in the Central North Sea: results from section balancing
Authors: Buchanan Peter G. , Bishop Daniel J. , Hood David N.
Published: April 2008

The geometric and kinematic evolution of salt-related structures in the Central North Sea has been studied using structural restoration and balancing techniques. Strain associated with fault displacements, compaction and isostasy has been systematically removed, thus producing a graphical representation of the temporal and spatial distribution of salt structures through time. The results clearly demonstrate that the Central North Sea can be sub-divided into three structural provinces based on the type of salt structure present: (1) the Central Graben which is characterized by large offset extensional faults with broad salt swells and tall salt diapirs, (2) the Eastern platform comprising the Norwegian-Danish Basin which is dominated by symmetrical salt swells that are typically welded in intervening lows and deeply buried, and (3) the Western platform which is characterized by salt swells and gravity-driven, thin-skinned extension. By restoring the deformation at both a local and a regional scale, it becomes apparent that the Permian salt has controlled the development of structures throughout the Mesozoic and Tertiary in the Central Graben region and the Western Platform, whereas on the Eastern Platform the salt was primarily active during the Triassic only. This has led to a variation in the evolution of structural styles across the Central North Sea. The restorations also suggest that salt dissolution has not been a significant feature during the development of diapirs in this area.
Turonian correlation and sequence stratigraphy of the Chalk in southern England
Authors: Gale Andrew S.
Published: April 2008

A stratigraphical framework for the Turonian Chalk of southern England is described using lithostratigraphy, marker beds, cyclostratigraphy and biostratigraphy. For the Early and Mid-Turonian succession a basin-wide resolution of correlation of about a metre can be achieved. Sequences are identified from various criteria, which include bed-by-bed correlation along transects from platform to basin to provide a precise picture of sediment geometry in Turonian Chalk. Hardgrounds develop towards the platform margins and it is possible to identify some as lithified and commonly glauconitized sequence boundaries. For example, the Ogbourne Hardground, which represents a major Mid-Turonian sea-level fall, rests erosively on highstand chalks. Transgressive surfaces are also represented by hardgrounds; these onlap onto the basin margins and commonly show complex sedimentary histories and phosphatization. Transgressive events are marked by brief positive excursions in δ
13
C, detectable in basinal chalks. During the major Mid-Turonian sea-level lowstand, a thick ‘shelf margin wedge’ developed in the basin. A total of three complete and two partial sequences are identified in the Turonian of southern England. These compare well in number and biostratigraphical position with sequences described from a Turonian succession in Tunisia, which is an order of magnitude thicker.
Spectral gamma ray characterization of high resolution sequence stratigraphy: examples from Upper Carboniferous fluvio-deltaic systems, County Clare, Ireland
Authors: Davies S. J. , Elliott T.
Published: April 2008

The application of high resolution sequence stratigraphy requires the ability to recognize key surfaces which record fluctuations in relative sea-level. In sub-surface studies, gamma ray logs have been used to identify maximum flooding surfaces, but their full potential has not been realized. Gamma ray profiles produced using a portable spectrometer on exposed Upper Carboniferous fluvio-deltaic deposits in western Ireland reveal that key surfaces and systems tracts can be characterized more comprehensively and recognized with greater confidence if spectral gamma ray data (K, U, Th and their respective ratios) are used in conjunction with traditional total count data. Maximum flooding surfaces can be distinguished from lesser flooding surfaces by a distinctive U peak (> 5 ppm) and low Th/U ratio (< 2.5). Erosional unconformities and their associated incised valley fills are characterized by consistently low total counts (40–50 cps) and high Th/K ratios (> 6). Laterally correlative interfluves are represented by distinctive palaeosols that can be clearly identified in spectral gamma ray data by their anomalously low K content (<0.4%) and exceptionally high Th/K ratio (> 17). Finally, the stacking pattern of parasequence sets can be identified using the trends of Th/K ratios from sandstones in successive parasequences. These results have widespread implications for the recognition of high resolution sequence stratigraphic signatures in the stratigraphic record, with particular reference to the subsurface analysis of fluvio-deltaic deposits.
Slip partitioning at convergent plate boundaries of SE Asia
Authors: McCaffrey Robert
Published: April 2008

The active tectonics of SE Asia can be characterized by the interactions of large, rigid plates separated by broad zones of deformation. The relative motion of these plates across their boundaries is often partitioned in the sense that the normal and shear components occur on different structures. Earthquake slip vectors and geological and geodetic measurements are used to infer the degree to which oblique convergence, which is ubiquitous in SE Asia, is partitioned. The active tectonics of Sumatra, the Himalayan thrust, the Philippines, the New Guinea fold-and-thrust belt, the Huon-Finisterre collision, and the San Cristobal trench can be understood in terms of upper plate deformation associated with oblique convergence. Western Java may also exhibit partitioning of oblique subduction. Structures accommodating normal and shear components of the motion are often very close. Arc-parallel strain rates are estimated for forearcs of the region. The arc-parallel deformation of forearcs of the SE Asia region demonstrates that plate convergence, whether normal to structure or not, is a three-dimensional process.
Geophysical evidence for local indentor tectonics in the Banda arc east of Timor
Authors: Snyder David B. , Milsom John , Prasetyo Hardi
Published: April 2008

Two deep seismic reflection profiles and gravity measurements collected across the Banda arc east of Timor help to characterize crustal and uppermost mantle structures in the region where arc-continent collision is thought to be furthest advanced. Reflectors beneath the Sahul Platform of the Australian shelf indicate geometries consistent with older extensional rift structures overprinted by more recent horizontal shortening. To the north, the negative Bouguer gravity feature associated with the southern parts of the accretionary complex is unusually broad and deep. Post-collisional sediments are thin where this feature is crossed by the seismic lines, implying that older sediments or the crust are anomalously thick. Still further north, the forearc basin is notably narrow near eastern Timor and contains few sediments, most undeformed. The backarc region to the north is remarkable for the presence of a N-S trending line of seamounts culminating in an active volcano, Gunung Api, which is situated 400 km above the Wadati-Benioff zone. Reflection profiles across and along the seamount chain indicate underplating at Moho depths and a fault which appears to have acted as a conduit for basalt eruptions at the surface. Collectively, these observations imply anomalously thick and bouyant crust beneath the Banda arc east of Timor, and suggest two possible causes. Either a local promontory in the irregular boundary of the Australian craton was underthrust beneath the volcanic arc and forearc to 50–70 km depths or a Palaeozoic basin similar to the nearby Bonaparte Basin was underthrust and its former crustal structure inverted and thickened to form the buoyant crust. The new seismic reflection data help to locate the anomalously thick crust implied by gravity anomalies by defining the leaky fracture zone containing Gunung Api. It is inferred that the fracture propagates ahead of the indenting wedge of underthrust, thickened Australian crust.
Role of pre-Tertiary fractures in formation and development of the Malay and Penyu basins
Authors: Ngah Khalid , Madon Mazlan , Tjia H. D.
Published: April 2008

Major faults in Sundaland trend NNW to NW, WNW, N and E. Some of the NNW to NW and N-striking faults across Mesozoic areas of the Malay Peninsula were active until mid-Eocene time. Small, fault-bounded Tertiary basins onshore may be pull-apart basins associated with such faults. Mainly from seismic data, NNW to NW, N and E-striking faults have been recognized in the pre-Tertiary basement of the Malay and Penyu basins off the east coast of the peninsula. These faults were reactivated before the Late Oligocene and during the Middle to Late Miocene. N-striking faults in pre-Tertiary areas are common throughout Sundaland. In the field, these faults are found to be the oldest (possibly Jurassic) regional fractures. The regional NNW-NW and WNW fractures are believed to have originated as strike-slip faults when the peninsula was subjected to late Mesozoic deformation. The onshore E-W faults were probably extensional fractures that developed as secondary structures associated with sinistral slip motions along NNW-NW faults. Upper Cretaceous dolerite dykes fill some of the E-W fractures. NW-striking basement faults of the Malay basin continue onshore SE Asia as the Three Pagodas fault zone. Initially these were sinistral basement wrench faults creating secondary E-W extensional fractures. In the Middle to Late Miocene the regional stress field changed, resulting in reversal of slip movement along major wrench faults and structural inversion of the sedimentary basins. This inversion is manifested as E-W anticlines located over half-graben. In the Penyu basin similarly striking half-graben probably developed in the same fashion. There, the NW-striking Rumbia fault divides the basin into two parts. Half-graben in the western part remained orientated E-W, but those in the eastern part became rotated clockwise by continued left-lateral slip along the Rumbia fault. After the Miocene the two basins continued to subside, developing an almost undisturbed blanket of post-Miocene sediments. Locally, residual stress caused some of the structures to grow.
Tectonic evolution of the Bantimala Complex, South Sulawesi, Indonesia
Authors: Wakita Koji , Sopaheluwakan Jan , Miyazaki Kazuhiro , Zulkarnain Iskandar , Munasri
Published: April 2008

The Bantimala Complex of South Sulawesi, Indonesia is an assemblage of northeast-dipping tectonically stacked slices. The slices consist mainly of high pressure metamorphic rocks, radiolarian chert, breccia, sandstone and shale, and melange. In order to understand the tectonic evolution of the Bantimala Complex, we have investigated the lithology, age, stratigraphy, structure and relationships of the components. The K-Ar ages of high P-low T metamorphic rocks suggest that an oceanic plate subducted beneath the Sundaland continent during the Late Jurassic or earliest Cretaceous. The subduction ceased during the Albian, and the high pressure schists were exhumed and eroded at the surface before and during the deposition of middle Cretaceous radiolarian chert. The exhumation of the schists was related to the collision of microcontinents derived from Gondwanaland. The Jurassic shallow marine sedimentary rocks in the Bantimala Complex are possibly remnant fragments of the collided microcontinent. Tectonic stacking of the Bantimala Complex was caused by Neogene subduction and collision of another continental fragment further to the east.
Recent seismicity of Mount Etna: implications for flank instability
Authors: Montalto A. , Vinciguerra S. , Menza S. , Patanè G.
Published: April 2008

This study considers seismic activity recorded beneath the low eastern flank of Etna during the period between April 1989 and December 1991 with the aim of placing some seismological constraints on the investigation of slope instability related to the gravitational sliding of this unsupported side of the volcano. The seismicity essentially consists of rather superficial (
z
< 10 km for more than 90% of cases), low-magnitude (usually
M
< 3) earthquakes. Foci of earthquakes are clustered on the main fault systems crossing the region, such as the Santa Tecla fault and the San Leonardello fault system, that have in recent times produced earthquakes of unusually high intensity covering the whole of the Etnean area. The broad picture of seismicity may be partially attributed to the accumulation of stress related to the seaward displacement of moving blocks in the vicinity of the Valle del Bove and adjacent regions in response to the storage and release of elastic energy associated with frequent episodes of magma intrusion. We propose that eastward sliding probably involves a complicated ensemble of moving blocks, resting on shallow independent décollements rather than on a single, widespread decoupling surface.
The Makkovik Province: extension of the Ketilidian Mobile Belt in mainland North America
Authors: Kerr Andrew , Ryan Bruce , Gower Charles F. , Wardle Richard J. , Kerr Andrew
Published: April 2008

The Makkovik Province of Labrador represents the extension of the Ketilidian Mobile Belt of south Greenland into mainland North America; it exhibits a threefold division into a foreland region, a fold-and-thrust belt, and an interior magmatic zone. The
Kaipokok Domain
is dominated by Archaean basement rocks that form an extension of the North Atlantic Craton, but Proterozoic reworking is recorded by the reorientation of a
c
. 2230 Ma dyke swarm. Supracrustal rocks, consisting of shallow-marine sedimentary rocks overlain by greywackes and mafic volcanic rocks, rest unconformably upon Archaean basement, but towards the interior of the belt the basal unconformity is eradicated by northwest-directed thrusting. In the
Aillik Domain
, highgrade supracrustal rocks of similar aspect to those of the Kaipokok Domain are separated from the basement by mylonite zones, in a thick-skinned fold-and-thrust belt (
Kaipokok Bay Structural Zone
), believed to record significant northwest-directed translation. The Aillik Domain also contains abundant felsic volcanic rocks that lack typical arc-like geochemical signatures. The
Cape Harrison Domain
is dominated by plutonic rocks, including suites of 1840 Ma, 1800 Ma, 1720 Ma and 1650 Ma age, but gneissic inliers apparently represent ‘juvenile’ Proterozoic crust. The dominant 1800-1720 Ma plutonic suites are late-orogenic to post-orogenic, siliceous, potassic granitoid rocks, which resemble Phanerozoic post-collisional suites, rather than subduction-related arc batholiths. Nd isotopic variations position the eastern edge of the North Atlantic Craton close to the boundary between the Aillik and Cape Harrison Domains. The structural evolution of the Makkovik Province records a shift from pre- 1800 Ma northwest-directed thrusting to post-1800 Ma tight upright folding, also northwest-verging. However, there is also evidence for earlier (pre- 1890 Ma) events in the Kaipokok Domain. Major unresolved problems include the timing of early sub-horizontal deformation (perhaps related to collisional events), the age relations and setting of supracrustal sequences, the location of suture zones, the absence of clear arc-like magmatic assemblages, and the nature and antiquity of the eastern juvenile crustal block.
Palaeoproterozoic oblique plate convergence in South Greenland: a reappraisal of the Ketilidian Orogen
Authors: Chadwick B. , Garde A. A.
Published: April 2008

New field and petrographic data are presented for a revised four-fold division of the Ketilidian Orogen into a Border Zone in the north, followed to the south by the Julianehåb batholith, the Psammite Zone and the Pelite Zone, as the basis for a new plate tectonic model. The Border Zone within the southern margin of the Archaean foreland contains shallow marine, volcano-sedimentary basins, granites and appinite dykes which are regarded as part of an incipient backarc. The wedge-shaped Julianehåb batholith comprises polyphase calc-alkaline granites, granodiorites, tonalites and hornblende diorites with swarms of appinite dykes. The batholith was emplaced during sinistral transpression as a result of oblique convergence between the Archaean continental plate to the north and an oceanic plate (no longer preserved) to the south. New Rb-Sr isotopic data from the southern part of the batholith confirm previous views that it consists of juvenile Palaeoproterozoic crust. The batholith is interpreted as the root of a volcanic arc that was the provenance of, and in part the basement to, polymict conglomerates, arkosic arenites, semi-pelites and pelites in the Psammite and Pelite Zones, deposited in intra-arc basins and the inner part of a forearc between the batholith and an ocean to the south. The sedimentary rocks and their subordinate intercalations of basalt and andesite were intensely migmatized and deformed in high temperature-low pressure conditions prior to the emplacement of broadly concordant sheets of granites and related rocks of the rapakivi suite. Appinite dykes were emplaced at various stages of the migmatization and deformation. We attribute the thermal effects which characterize the Ketilidian oblique convergent system to voluminous basaltic underplating in accord with recent work on the rapakivi suite and associated metamorphism.
Geochemical aspects of the aetiology of trace element related diseases
Authors: Mills C. F.
Published: April 2008

Deficiencies, excesses or imbalances in the supply of inorganic elements from dietary sources can have an important influence on animal and human health and susceptibility to disease. Many such situations arise from anomalies in the inorganic element composition of food chains. These, in turn, are frequently attributable to the composition of the geochemical environment as modified by the influence of soil composition and botanical or cultural variables upon the inorganic composition of the diet.

Appreciation of the epidemiological importance of such factors in the aetiology of nutritional diseases can contribute, very significantly, to their detection and effective control.

Although the agricultural relevance of geochemical data is firmly established and widely appreciated, investigation of its value in the context of anticipation and control of major human nutritional diseases in the third world has yet to be undertaken adequately.
Records of seasonal flux in Holocene laminated sediments, Gulf of California
Authors: Pike Jennifer , Kemp Alan E. S.
Published: April 2008

Holocene laminated sediments from Guaymas Basin (Gulf of California, Mexico) have, in the past, been described as annual couplets of sedimentation. Couplets comprise an alternation of diatom- and terrigenous-rich sediment reflecting winter and summer flux to the sediment respectively. New data from backscattered electron imagery (BSEI) show that a three-component pattern of sedimentation is preserved, comprising (1) a lithogenic lamina consisting of clays, silt and minor diatomaceous material, deposited during the summer and autumn wet season; (2) a mixed flora diatomaceous lamina deposited during the early winter collapse of the thermocline and onset of water column mixing; and (3) a near-monospecific flora lamina ofChaetocerosresting spores or, more rarely,Skeletonema costatum, representing deposition from, or at the end of, the spring coastal upwelling bloom. Recent sediment trap biogenic and terrigenous flux data also show three to four major flux events per year. Direct comparisons are possible between BSEI records and recent sediment trap flux data to facilitate high-resolution palaeoceanaographic and palaeoclimatic reconstructions throughout the Holocene.
A review of the petrology of harzburgites at Hess Deep and Garrett Deep: implications for mantle processes beneath segments of the East Pacific Rise
Authors: Edwards Stephen J. , Falloon Trevor J. , Malpas John , Pedersen Rolf B.
Published: April 2008

In recent years a unique set of samples of uppermost mantle at the mantle-crust transition zone have been collected from two different environments along the fast-spreading East Pacific Rise (EPR): a ‘normal’ spreading segment (represented by samples from Hess Deep) and the end of a spreading segment where the EPR meets the Garrett transform fault (represented by samples from Garrett Deep). A review of the petrology of harzburgites from the two sites demonstrates that these rocks were produced by partial melting of adiabatically upwelling mantle and, subsequently, at the top of the mantle, they were impregnated by reactive and crystallizing mid-ocean ridge basaltic (MORB) melts. Despite this similar history, non-impregnated harzburgites at Garrett Deep have a more fertile spinel chemistry than those at Hess Deep, which is consistent with reduced partial melting of shallow mantle as a transform fault is approached — the ‘transform fault effect’.

The extent of reaction between melt and harzburgite during the impregnation event suggests that melt arrived in the uppermost mantle in a highly reactive state because along the adiabatic path it had been highly channelled in spatially restricted conduits. This implies that mantle upwelling below the EPR was, and presumably still is, dominantly two dimensional (sheet-like). Within this framework, the chemical evolution of MORB melt below fast-spreading ridges will be significantly affected by melt-periodotite reaction only when melt reaches the uppermost mantle and mantle-crust transition zone, where along-axis transport of melt may also be important.

Although the harzburgites from Hess Deep and Garrett Deep formed and evolved beneath different parts of different first-order segments of the EPR, the petrology of these rocks presents the best analogue available for defining real variations in mantle processes along a single first-order ridge segment in a fast-spreading environment.
Barrovian regional metamorphism: where’s the heat?
Authors: Jamieson R. A. , Beaumont C. , Fullsack P. , Lee B.
Published: April 2008

Coupled thermal-mechanical models of convergent orogens offer a novel way to investigate the interactions between heat and tectonics that lead to regional metamorphism. In this study, the effects of different distributions of heat-producing material in the crust and upper mantle on crustal thermal histories and deformation fields are investigated. The models involve subduction-driven collision with moderate convergence and erosion rates. For models involving standard continental crust, where heat production is initially concentrated in the upper crust,
P-T-t
paths do not intersect the field of typical Barrovian
P-T
conditions. However, heat-producing material can be tectonically redistributed, for example, by subduction of crustal rocks to upper mantle depths, or by formation of thick accretionary wedges or continental margin sequences during convergence. Models that include a wedge of heat-producing material in the upper mantle generate high temperatures in the lower crust and upper mantle that lead to a change in orogenic style; radioactive heating of partially subducted crustal material on time scales of 10–30 Ma yields temperatures high enough for partial melting. However, crustal
P-T-t
paths are unlikely to intersect the Barrovian field unless erosion or convergence rates change. Models that include a crustal-scale region with moderate, uniform heat production, simulating a large accretionary wedge or tectonically thickened continental margin sequence, generate
P-T-t
paths that intersect the Barrovian field. However, as convergence proceeds, the heat-producing region is deformed, eroded, and reduced in volume, so that the model orogen begins to cool down after about 20 Ma. The model results provide an explanation for many first-order tectonic and metamorphic features of small orogens, including metamorphic styles ranging from blueschists to the Barrovian series to granulites, late-orogenic granitoid magmatism, and the crustal-scale tectonic features associated with regional metamorphic belts. We conclude that the thermal state of an orogen is controlled by the evolving competition between cooling by subduction and radioactive heating within the deforming orogen.
Modelling of sandbody connectivity in the Schooner Field
Authors: Mijnssen F. C. J.
Published: April 2008

The Schooner field will be one of the first stand-alone developments of a gas accumulation in the Carboniferous Barren Red Measures Group of the UK Sector of the Southern North Sea. It illustrates the potential of Barren Red Measures Group as a future play in this area.

The Schooner Field, located some 150 km (94 miles) east of the South Yorkshire coastline, is a NW-SE-oriented faulted dip closure. The reservoir consists predominantly of fluvial channel sandstones of the Barren Red Measures Group, with small amounts of gas located in the underlying Coal Measures Group. The sandstone channels are embedded in flood plain mudstones, which comprise 65–70% of the stratigraphic section. Because of the relatively low proportion of sandstone to mudstone, sandbody connectivity has been identified as the critical factor affecting recovery of gas.

In order to asses the impact of sandbody connectivity on recovery efficiency, reservoir modelling work was performed based on eight different geological models and three well design options. The factors that were varied in order to build the geological models included (1) presence of minor faults, (2) the width of the channels, and (3) channel orientation. Well design options considered included (1) wells with deviations between 10 and 63°, (2) subhorizontal wells drilled along the strike of the structure, and 3) sub horizontal wells drilled along the dip of the structure.

The results of the modelling work showed that sandbody orientation had the largest influence on connectivity, with connectivity being the highest when the channels were oriented along the strike of the structure. The width of the channels and the presence of minor faults had some impact on connectivity, while varying well design made little or no difference. Close inspection of the models did, however, show that the major faults compartmentalize the reservoir, resulting in large quantities of gas being by-passed if wells are not properly positioned. Consequently, more work is needed to evaluate the optimal location of the development wells.
Tectonic evolution of the Larne Basin
Authors: Shelton Rob
Published: April 2008

The Larne Basin lies in the Midland Valley terrane between Northern Ireland and SW Scotland. A proportion of the basin lies onshore Northern Ireland, with an offshore continuation into the North Channel seaway. The basin is orientated NE-SW, and the sedimentary basin fill thickens to the west. Extensional faults in the south of the basin dip generally to the east, those in the north to the northwest. Preserved sediments range in age from Carboniferous to Tertiary, but are predominantly Permo-Triassic. Integration of seismic interpretations, well data and outcrop fault and fracture measurements has revealed a number of Mesozoic and Cenozoic tectonic events. Late Carboniferous N-S compression was followed by a major phase of regional early Permian ENE-WSW/NE-SW extension. ENE-WSW extension followed in the late Triassic and early Jurassic with post-early Jurassic to late Cretaceous minor subsidence and uplift resulting in a major erosional unconformity between the Lower Lias and Upper Cretaceous Greensand and Chalk. Early Tertiary (Palaeocene) uplift and NE-SW extension led to extensive basalt extrusion, and later NW-SE to NNE-SSW extension and faulting resulted in the deposition of the Upper Oligocene Lough Neagh Clays in the neighbouring Lough Neagh Basin. The late Tertiary (Miocene) saw a phase of compression, leading to uplift and erosion. AFTA® analyses of Permian and Triassic samples indicate
c.
450 m of Tertiary uplift and erosion. Sonic velocity analysis of the Mercia Mudstone Group and Sherwood Sandstone Group suggests uplift of 0.5–1.1 km, possibly reflecting maximum burial at an earlier? Jurassic time.
The hydrocarbon potential of the Cheshire Basin
Authors: Mikkelsen P. W. , Floodpage J. B.
Published: April 2008

Permo-Triassic basins, both onshore and offshore UK, typically have good reservoir rocks and frequently good seals, but their continental depositional environment requires that source beds of a different age must be invoked if they are to be prospective. In the Dorset-English Channel Basin, the overlying Liassic shales provide an oil prone source, while in the Southern Gas Basin of the North Sea and the East Irish Sea, the underlying Carboniferous has generated large volumes of gas and some oil. Other Permo-Triassic basins, onshore, are generally devoid of hydrocarbons and this can in large part be explained by an absence of an adequate source rock.

The Cheshire Basin lies adjacent to the East Irish Sea Basin and contains a similar, thick Permo-Triassic sequence. The presence of outcropping Coal Measures on three sides of the Basin gives encouragement as to sourcing potential. However, the six exploration wells drilled in the Basin to date (four during the last ten years) have proved dry.

Review of recent seismic data, particularly mid-late 1980s vibroseis surveys, suggests that the Basin developed as a Permo-Triassic half-graben over a Hercynian inversion. Therefore, as one moves from the western edge towards the basin centre progressive erosion of Westphalian strata below the base Permian event may be observed, culminating in a likely Lower Palaeozoic subcrop in the southeastern part of the Basin. A large, faulted high in the centre of the Basin, in many ways similar to the Morecambe Field structure, was proven dry by the Burford-1 well, apparently due to an absence of an underlying source. Nevertheless, seismic data in the northern part of the Basin, supported by the Knutsford-1 penetration, suggest the presence of underlying Coal Measures. However, regional projections suggest that the thick, Dinantian-Namurian bituminous shales which appear to have sourced the East Irish Sea fields, are absent from much of the Basin.

A further difference between the Cheshire and East Irish Sea Basins is the presence, in the former, of the Tarporley ‘Siltstone’ Formation above the main reservoir objective of the Helsby Sandstone Formation. The former may be regarded as a likely non-sealing waste zone over much of the central and eastern parts of the Basin, but in the SW where the coarser Malpas Sandstone facies is developed, and in the north, it becomes a viable reservoir target in its own right, sealed by the Bollin Mudstone Formation and, ultimately, by the Northwich Halite.

It is therefore proposed that the successful exploration for hydrocarbons in the Basin is likely to depend on finding a combination of sub-cropping Coal Measures and reservoir-grade Tarporley Siltstone, sufficiently deeply buried to retain an effective Mercia Mudstone and preferably halite seal. These conditions appear to prevail in the NW part of the Basin, where the search for suitably large, closed structures is currently in progress.
Fluid migration history in the north Irish Sea-North Channel region
Authors: Parnell John
Published: April 2008

Basins in the north Irish Sea region have experienced numerous stages of fluid migration. Carboniferous and Permo-Triassic sandstones both experienced rapid cementation due to high sedimentation/burial rates. Both successions also experienced syn-sedimentary faulting which focused cement precipitation and created permeability barriers. Further fracture-based diagenesis occurred post-lithification, and accompanying hydrothermal activity associated with Tertiary intrusions. The extensional activity which helped to define Permo-Triassic basins in the Southern Uplands terrane also channelled mineralizing fluids in the same terrane. These fluids included hydrocarbons in at least two districts. Along the North Solway Fault Zone, basinal fluids from the Carboniferous Solway Basin interacted with metalliferous groundwaters to precipitate metalliferous bitumens. The Solway Carboniferous outcrop also contains intergranular oil residues.
The Douglas Oil Field
Authors: Yaliz A. M.
Published: April 2008

The Douglas Field, with an estimated STOIIP of 225 MMBBL and on stream in February 1996, is the first oil field to be developed in the offshore East Irish Sea basin. The field structure consists of three tilted fault blocks formed during extensional faulting in Triassic-early Jurassic times, and later readjusted by contractional movements during Tertiary inversion. The oil is trapped in the Triassic Helsby Sandstone Formation, which comprises moderate- to high-permeability aeolian and fluvial sandstones. The reservoir depth is shallow (2200 ft) with a maximum oil column of 330 ft. The sandstones can be divided into several laterally extensive units based on vertical facies variations. Although reservoir quality is principally controlled by primary depositional processes, a complex diagenetic history is recognized, with various phases of silica and carbonate cements. Authigenic clay minerals are not important, but bitumen is extensive in limited areas of the field, causing significant permeability reduction. The hydrocarbon filling history of the field was complex, with the occurrence of at least two phases of oil generation and migration. The Douglas Field contains a relatively ‘dead’ oil with a low GOR, thus 11 production and seven water injection wells are planned. Although the crude is classified as sweet it contains low levels of mercaptans and H
2
S, which will be removed during processing offshore. First oil is planned for December 1995 with peak oil production reaching 40000 BPD.
An overview of the hydrocarbon potential of the Spratly Islands archipelago and its implications for regional development
Authors: Blanche J. B. , Blanche J. D.
Published: April 2008

The hydrocarbon potential of the Spratly Islands archipelago is relatively unknown and can thus only be evaluated via analogues. This area of the South China Sea has been, and remains, a region of many disputes and conflicts which has further confined the availability of hard technical information, in particular seismic and well data, to the confidential vaults of the claimant states. In addition, the authors have been constrained by the area’s political sensitivity and the commercial confidentiality of technical data. The Spratly Islands are claimed, either in whole or in part, by the People’s Republic of China (PRC), the Republic of China (ROC/Taiwan), Vietnam, the Philippines, Brunei and Malaysia.

The archipelago is surrounded by oil-producing areas, i.e. the Nam Con Son (Wan’an Bei) basin of Vietnam, the East and West Natuna basins of Indonesia, the Northwest Palawan (Nansha Trough) basin of the Philippines, the productive Luconia Shelf (Zengmu Shelf) offshore Sarawak and the Brunei/North West Sabah basins which provide regional analogues. This frontier region may yield considerable reserves, the range of which varies from 1–2 billion barrels to 225 billion barrels of oil equivalent. It is anticipated that the Spratly Island archipelago and Dangerous Grounds area of the South China Sea will become the focus of exploration for hydrocarbons over the next decade, once the multi-national boundary disputes are resolved, hopefully by negotiation and peaceful means by the claimant states.

This paper presents an overview of the hydrocarbon potential of this area within the context of regional political relationships.
Investigation and management of pesticide pollution in the Lincolnshire Limestone aquifer in eastern England
Authors: Sweeney J. , Hart P. A. , McConvey P. J.
Published: April 2008

This case study describes pollution of the Lincolnshire Limestone aquifer, predominantly by the herbicide Mecoprop, to the north of Peterborough, Cambridgeshire. Key features of the case study are the occurrence of a series of former landfill sites located on the Lincolnshire Limestone aquifer outcrop, a nearby groundwater abstraction for public supply within the confined aquifer and the presence of a major geological fault and its associated control on groundwater movement. At the time of licensing, the fault was considered to hydraulically isolate the landfill sites from the abstraction.

A multiphase investigation, begun in 1990, was conducted by the National Rivers Authority and involved close liaison with the Cambridgeshire County Council, landowners, site operators and the Anglian Water Company. Investigations have focused on identifying and proving the source of pollution and determining its severity and extent. Remedial strategies and the implications of future change in hydraulic conditions have also been considered.

A medium-term risk management strategy for the area has been devised. This is in line with the UK Government’s principles for contaminated land management.
Sediment delivery to the Gulf of Alaska: source mechanisms along a glaciated transform margin
Authors: Dobson M. R. , O’Leary D. , Veart M.
Published: April 2008

Sediment delivery to the Gulf of Alaska occurs via four areally extensive deepwater fans, sourced from grounded tidewater glaciers. During periods of climatic cooling, glaciers cross a narrow shelf and discharge sediment down the continental slope. Because the coastal terrain is dominated by fjords and a narrow, high-relief Pacific watershed, deposition is dominated by channellized point-source fan accumulations, the volumes of which are primarily a function of climate. The sediment distribution is modified by a long-term tectonic translation of the Pacific plate to the north along the transform margin. As a result, the deep-water fans are gradually moved away from the climatically controlled point sources. Sets of abandoned channels record the effect of translation during the Plio-Pleistocene.
Large-scale debrites and submarine landslides on the Barra Fan, west of Britain
Authors: Holmes R. , Long D. , Dodd L. R.
Published: April 2008

The Barra Fan is a large Neogene to Pleistocene composite fan that has built out into the deep-water basin of the Rockall Trough west of Britain. The bulk of the landward depocentre of the Barra Fan takes the form of a relatively undisturbed shelf-margin wedge, in places more than 600 m thick, that is perched above the deep-water basin and, together with the Donegal Fan, partially engulfs the Hebrides Terrace Seamount. A continuous series of slope-front channels and gullies incise into the shelf-margin wedge of the northernmost Barra Fan and are the likely transport paths for small- to medium-scale debrites found on the middle and lower slope-front. In contrast, at least four large-scale submarine landslide events making up the Peach Slide appear to have translated the bulk of the slope-front of the northern Barra Fan to the northwest, so that debrites and hemipelagic sediments exhibit maximum composite thicknesses of approximately 450 m on the deep-water basin floor of the eastern Rockall Trough. The extent and volume of successive large-scale debrite units decrease with age. From an interpretation of a limited historical dataset it is thought that the ground accelerations from earthquakes have been sufficient to trigger slope-front instability and sediment transfer.
Groundwater recharge and pollution vulnerability of British aquifers: a critical overview
Authors: Foster S. S. D.
Published: April 2008

The concept of aquifer pollution vulnerability, and the way it has been developed and incorporated as a central element in British groundwater protection policies, is reviewed in detail. A related question is: do we understand enough about groundwater recharge mechanisms to assess pollution vulnerability realistically and to define necessary protection measures efficiently? Some key remaining areas of uncertainty are analysed in the light of recent research results.
Identification of the source and fate of nitrate contamination of the Jersey bedrock aquifer using stable nitrogen isotopes
Authors: Green A. R. , Feast N. A. , Hiscock K. M. , Dennis P. F.
Published: April 2008

Measured NO
3
−
concentrations in the Jersey bedrock aquifer exceeded 50 mg l
−1
in 67% of groundwaters sampled in June 1995. The δ
15
N values of this dissolved NO
3
−
showed a total range in composition from + 3.6‰ to + 18.4‰ and exhibited a general, island-wide increase from + 4‰ in the centre of the island to + 9‰ towards coastal districts. The majority of the island is intensively cultivated and most of the sites sampled have δ
15
N values of less than + 7‰ (mean = + 7.7‰,
n
= 40) indicative of an agricultural source for NO
3
−
. There is no apparent evidence for point-source contamination from septic tanks. The progressive seaward increase in δ
15
N is attributed to an increasing volume of deeper, denitrified groundwater mixing with shallow groundwater. Values of δ
15
N greater than + 10‰ are restricted to those coastal and valley locations that abstract, due to their low topographic elevation, the greatest proportion of deeper, denitrified groundwater. An alternative explanation is that sewer leakage enhances the observed δ
15
N values in southern urban areas, although this is considered unlikely to be occurring at present.
Groundwater recharge: an overview of estimation ‘problems’ and recent developments
Authors: Simmers I.
Published: April 2008

Since the mid-1980s there has been a relative explosion of recharge studies for (semi-)arid regions reported in the scientific literature. It is therefore relevant to assess what we now know and to offer guidance to the water resources development practitioner. This paper summarizes recurring recharge estimation ‘problems’ and reviews some recent advances in (inter alia) estimation techniques and recharge time/space variability.

The determination of recharge fluxes in (semi-)arid regions remains fraught with uncertainty; multiple tracer approaches appear to offer the best potential for reliable results in local studies which require ‘at-point’ information. It is also clear that sufficient advances have been made in recent years to show that the value of water balance and Darcian approaches should not be under-estimated. The frequently studied issues of localized recharge and spatial variability need not be a problem if concern is with regional estimates. The key for the practitioner is the project objective; this will dictate whether multiple ‘at-point’ or area-based estimation methods are appropriate. For the latter, a combination of reliable local data, remote sensing, GIS and geostatistical techniques offers considerable promise for a better understanding and determination of recharge over extended areas.