bluuebunny/crossref_metadata_2025 huggingface dataset:
Palaeobathymetric reconstruction on a gridded database: the northern North Atlantic and southern Greenland-Iceland-Norwegian sea
Authors: Wold C. N.
Published: April 2008

A computer program combining backstripping with plate tectonics has been used to reconstruct the palaeobathymetry of the region between the Charlie Gibbs and Jan Mayen Fracture Zones from the East Greenland margin to the European shelf for the past 50 Ma. Two different models were tested, one assuming that continental extension and rifting opened the Rockall Trough and Faeroe-Shetland Channel during the Cretaceous and that the area has subsided ever since. The second model assumes that the Rockall-Faeroe region rifted in the Cretaceous and was uniformly uplifted in the late Palaeocene due to heating of the lithosphere by the mantle plume of the Iceland Hotspot. The alternative assumptions about the thermal age of the lithosphere produce very different results with implications for the palaeoceanography and sedimentation in the region. The isostatic model presented here predicts crustal thicknesses along the Iceland-Faeroe Ridge and in the Rockall-Faeroe region that are in close agreement with estimates using seismic and gravity techniques.
Triassic correlation and stratigraphy in the South Central Graben, UK North Sea
Authors: Goldsmith P. J. , Rich B. , Standring J.
Published: April 2008

Triassic strata in the South Central Graben of the North Sea have yielded moderate to good recoveries of palynofloral assemblages and have sufficiently distinct petrophysical responses to enable the subdivision of the succession into between seven and eleven units. The Triassic succession of the Amoco well, 30/12b-3, is proposed as the type section for the Skagerrak Formation of the area. Despite lacking good biostratigraphic control, typical log character for the sequences is well displayed. The mainly Scythian (early Triassic) red mudstones of the Smith Bank Formation can locally be divided into three units, but generally only one is recognized. A new lithostratigraphic nomenclature for the middle to late Triassic Skagerrak Formation is proposed. Six members, with eight units, are recognized over UK Quadrants 29 and 30, and may possibly continue into the southern part of Quadrant 22 and into Norway Quadrant 7. The member names are partly derived from the Phillips fields in which they were first recognized. The basal Judy Sandstone Member is named after the Judy Field discovery well 30/7a-4A, and is overlain by the petrophysically distinct Julius Mudstone Member. The Joanne Sandstone Member is named after the Joanne Field well 30/7a-3, and is overlain by the Jonathan Mudstone Member, which can be further subdivided into upper, middle and lower units on log character. The uppermost members are the Josephine Sandstone Member, from the type well on the Josephine structure, 30/13-1 (Deegan & Scull 1977), which is overlain by the Joshua Mudstone Member. The upper members are rarely preserved, due to extensive early to middle Jurassic uplift and erosion.
Distinction between fault and salt control of Mesozoic sedimentation on the southern margin of the Mid-North Sea High
Authors: Griffiths P. A. , Allen M. R. , Craig J. , Fitches W. R. , Whittington R. J.
Published: April 2008

The North Dogger Fault Zone (NDFZ) is situated at the northern margin of the UK Southern North Sea Basin at the northern limit of the mobile Zechstein Supergroup. This fault zone underwent movement during late Triassic and probably early Jurassic times. The NDFZ is comparable with the southern part of the Dowsing Fault Zone (SDFZ), which was initiated during late Scythian times and continued to move until at least mid-Carnian times. The SDFZ is located at the southwestern edge of mobile salt. Both fault zones form part of the basin-bounding fault system, which is proposed to have been initiated as a response to the growth of salt swells in the centre of the Southern North Sea Basin. The fault formation is due to folding of the Triassic strata over the swells and overall shortening of the Triassic cover. The shortening was taken up as extension at the basin margins, corresponding to the edge of the mobile Zechstein salt.
The timing and direction of Permo-Triassic extension in southern Britain
Authors: Chadwick R. A. , Evans D. J.
Published: April 2008

The Permo-Triassic sedimentary basins of southern Britain form a roughly N-S-trending linear rift system which extends more than 400 km from the northern English Channel to the Irish Sea. The component basins of the rift system typically contain 1000–4000m of dominantly continental red-beds. The basin margins are clearly imaged by seismic reflection data, and typically comprise a major syndepositional normal fault or faults lying in the hanging-wall block of a reactivated basement thrust. In southern England, basin-controlling faults have a dominant E-W (Variscoid) trend, in the Midlands the faults have a dominant N-S (Malvernoid) trend and in northern England NE-SW (Caledonoid) basin fault trends are prominent. Thus the rift system can be structurally divided into three segments, dependent on the structural gain of the underlying basement. Because the orientations of the basin-margin faults were controlled by basement structures, they did not develop perpendicular to the extension direction and consequently suffered varying degrees of oblique-normal displacements. An analytical model is developed which relates the displacement components of a set of variably oriented faults to a single regional extension direction. Application of the model to the rift basin-margin faults indicates that faults trending N-S had displacements which most closely approximated to dip-slip. Conversely, faults trending E-W suffered strongly oblique-slip displacements. This is consistent with a cumulative Permo-Triassic extension vector oriented 060°–240°, roughly ENE-WSW. Analysis of thickness changes across the basin-margin faults, and backstripped burial history plots from the basin depocentres, indicate at least two phases of crustal extension. The first occurred in late Permian times, with a second major episode in early Triassic times. They were characterized by rapid fault-controlled basin subsidence and coarse clastic sedimentation. Each was followed by a period of more gradual, regional subsidence, depositional overlap of the basin margins and a tendency towards finer-grained sedimentation.
Diagenetic history and reservoir potential of Permo-Triassic sandstones in the Rathlin Basin
Authors: Fitzsimons Steven , Parnell John
Published: April 2008

Exploration prospects in the Rathlin Basin depend in part upon the reservoir potential of sandstones in the Permo-Triassic. The sandstones are the deposits of alluvial fan, channel and floodplain environments; aeolian deposits may occur in the basal (Permian) part of the succession. Scattered outcrops around the basin margin in Ireland and Scotland, and boreholes in the deep part of the basin onshore Northern Ireland, allow determination of diagenetic history and reservoir quality.

Cementation is dominated by very early pore-filling sulphate/carbonate, illite-smectite grain-coatings, and overgrowths of potassium feldspar. Other cements are haematite graincoatings, quartz overgrowths and kaolinite pore-fillings. The sulphates/carbonates occlude porosity, and the clays/feldspars reduce permeability by bridging pore throats. Even in the deepest parts of boreholes, porosities can be high owing to dissolution of cements and unstable grains. This secondary porosity has in some cases been occluded by kaolinite, particularly in shallow-buried marginal parts of the basin.

Thick, coal-bearing Carboniferous and gypsiferous Mercia Mudstone Group sections onshore suggest that adequate source and seal rocks exist below and above the sandstones, respectively, thus enhancing the exploration potential of the basin.
Shallow gas accumulation and migration in the western Irish Sea
Authors: Croker Peter F.
Published: April 2008

The Irish Sea has long been known to contain extensive areas of shallow gas accumulation, particularly in the area southwest of the Isle of Man. More limited occurrences of seabed features associated with shallow gas migration, such as columnar disturbances, seabed doming and pockmarks, have also been reported. Until now this gas was generally assumed to be of biogenic origin.

Recently acquired route survey data have provided new insights into the distribution of both gas accumulation and gas migration in this area, as well as identifying areas of active seepage. It is proposed that the gas originates as coalbed methane or thermogenic gas in the extensive Carboniferous deposits underlying the region. This has considerable significance for hydrocarbon exploration in the area.
Hydrocarbon prospectivity of the Central Irish Sea Basin with reference to Block 42/12, offshore Ireland
Authors: Maddox Steven J. , Blow Richard , Hardman Martin
Published: April 2008

The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration & Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized.

Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion.

Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data.

The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.
Tectono-stratigraphy of the North Celtic Sea Basin
Authors: Rowell Philip
Published: April 2008

The Celtic Sea, offshore southern Ireland, is underlain by several discrete rift basins, which are part of a larger set of such basins that developed marginal to the North Atlantic within the tectonic framework of Pangaean break-up and episodic opening of the North Atlantic Ocean. This paper documents the development of depositional systems within a rift basin setting by analysis of the sequence stratigraphic evolution of the North Celtic Sea Basin, the largest basin in the area, with particular emphasis on the influence of pre-existing structural fabric on that evolution. The Caledonian and Variscan orogenies each imprinted distinct structural grains on this area, and the intersection pattern of their respective tectonic trends provided the structural framework which influenced subsequent Triassic, Late Jurassic and Early Cretaceous phases of rifting. Detailed analysis of an extensive well and seismic database indicates that the mode of reactivation of pre-existing lines of structural weakness depended on the orientation of the principal extensional stress direction relative to these lineaments. Triassic and Late Jurassic northwest-southeast orientated extensional stress reactivated Caledonian lineaments as half-graben boundary faults and Variscan lineaments as accommodation zones and lateral transfer faults which offset the basin margin faults. In contrast, during the Early Cretaceous, extensional stress was orientated approximately north-south, oblique to the Caledonian trend. This resulted in transtensional pull-apart basin geometries dominated by reactivation of Variscan structures. The intrinsic relationship between plate tectonic setting, pre-existing structural fabric and the resulting structural style provides insight into the influence of tectonism on depositional system development in an evolving rift basin. During Late Jurassic and Early Cretaceous rifting, depositional system development was predominantly controlled by tectonism and partly conforms to empirically derived tectono-stratigraphic models of sequence development in non-marine rifts. Depositional environments changed from fluvial-dominant to lacustrine-dominant as each rifting episode evolved through its early to active rift stages. During post-rift subsidence, marine transgressions dominated depositional system development, overriding the influence of local tectonism. Spatially, pre-existing structural fabric and the style of structural deformation are the dominant influences on sediment input and drainage systems within the evolving rift basin. Recognition of the intrinsic structural fabric of a rift basin is one of the key factors in understanding and predicting stratigraphic distributions within this type of basin.
The definition of the Albian ‘A’ Sand reservoir fairway and aspects of associated gas accumulations in the North Celtic Sea Basin
Authors: Taber D. R. , Vickers M. K. , Winn R. D.
Published: April 2008

Log and core data collected from exploration, appraisal and development wells, together with a large quantity of seismic data collected over a 21 year period, have allowed the delineation of a fairway of reservoir quality Cretaceous Albian sandstone (‘A’ Sand) in the north central part of the North Celtic Sea Basin. The ‘A’ Sand is the primary reservoir at the Kinsale Head and Ballycotton gas fields. Burrow types and the presence of glauconite indicate that it was deposited in a marine setting, although the precise depositional environment remains controversial. Regional isopach and lithofacies mapping indicates that only limited development of reservoir quality sandstone exists outside the main ‘A’ Sand fairway, such as at the well 48/18-1 discovery approximately 13 miles west of Ballycotton gas field. The sands are believed to have been derived from the Irish Massif to the north and were transported along a shallow-water shelf. Some northward thinning of the sandstone is recognized in the area that was closer to the interpreted palaeoshoreline. Reservoir quality is controlled, in part, by distance from the source area, while thickness, and to some extent coarseness, appears to have been controlled by palaeobathymetry. The thickest ‘A’ Sand is in the Kinsale Head field area, a local depocentre within the passively subsiding North Celtic Sea Basin. Reservoir quality diminishes to the south and southwest of the Kinsale Head gas field. Topseal for the reservoir is provided by the Albian to Cenomanian Gault Claystone and the associated overlying Cenomanian claystones. Untested structural closures within this ‘A’ Sand fairway may require a similar stratigraphy and structural configuration to the existing fields to prove successful. The Kinsale Head and Ballycotton fields are the result of fortunate geological coincidence. Albian reservoir sand accumulation was followed by deposition of deeper-water clay facies for topseal and subsequent burial beneath the Chalk. The Cretaceous stratigraphic evolution of the basin was succeeded by trap-forming basin inversion in the Early Tertiary, with concomitant and subsequent vertical migration of gas from the deeply buried and very mature thick shales within the Liassic. Additional ‘A’ Sand gas accumulations may be restricted to the structural closures in the general area of the existing fields, where there is well developed claystone facies above the reservoir and where fault offsets are less than the associated topseal thickness.
Sedimentology of the Cretaceous Greensand, Quadrants 48 and 49, North Celtic Sea Basin: a progradational shoreface deposit
Authors: Hartley Adrian
Published: April 2008

The Aptian-Albian Greensand interval is the main reservoir unit in the North Celtic Sea Basin. The Greensand comprises three units: (1) the Lower Claystone, (2) the ‘A’ Sand, and (3) the Gault Claystone. Sedimentological and wireline log studies of the Lower Claystone and ‘A’ Sand units within Irish Quadrants 48 and 49 indicate an overall 35 to 50 m thick, coarsening-upward profile from inner shelf mudstones to middle and possibly upper shoreface fine-grained sandstones. Both the mudstones and sandstones contain glauconite, marine fauna and marine ichnofauna. The ‘A’ Sand can be sub-divided into three coarsening-upward packages which show a progressive upward increase in higher-energy facies and thickness.

Previous interpretations have suggested that the Greensand interval represents a series of tidal sand bars formed during transgression. However, the wide lateral extent and the pulsed, progradational nature of the coarsening-upward units indicates deposition in a (wave-dominated?) shoreface environment during a punctuated regression. The overlying Gault Claystone represents an abrupt return to open marine shelf conditions.

Thickness variations across the study area suggest that faults were active to the south of the basin during Lower Claystone deposition but that activity had largely ceased by ‘A’ Sand times. Facies and isopach maps indicate that shoreface sandstones prograded west, north and east from a source lying somewhere to the south of the present-day position of the Kinsale Head field. Sediment may have been supplied from the uplifted South Celtic Sea Basin/Pembrokeshire Ridge.
A study of the hydrocarbon distribution and Lower Cretaceous Greensand prospectivity in Blocks 48/15, 48/17, 48/18 and 48/19, North Celtic Sea Basin
Authors: Howell Tim J. , Griffiths Paul
Published: April 2008

Well 48/19-2 was drilled in the North Celtic Sea in 1992 to test the Lower Cretaceous Greensand fairway along trend from the Kinsale Head and Ballycotton gas fields. The mapping of the 48/19-2 prospect is discussed and the results of the well are detailed. The discovery of a heavy oil accumulation in the well along this gas trend led to a comprehensive multi-disciplinary reappraisal of the 48/15, 48/17, 48/18 and 48/19 area. The results of this study highlight the adequacy of available source rocks, and suggest a possible Portlandian-Purbeck origin for the oils found in boreholes 48/24-1, 48/24-2 and 48/28-1. Maturation modelling in the study area pointed to a principal pulse of hydrocarbon generation and expulsion during the Late Cretaceous to early Tertiary. It was concluded that the Kinsale Head, Ballycotton and 48/18-1 gas accumulations could have been trapped in closures created after Miocene inversion. Earlier traps, which were pre-Miocene inversion in age, could have received oil which later became subjected to incursions of meteoric water and suffered biodegradation.
The Cretaceous succession in the Porcupine Basin, offshore Ireland: facies distribution and hydrocarbon potential
Authors: Moore J. G. , Shannon P. M.
Published: April 2008

The Porcupine Basin lies on the continental shelf to the west of Ireland. It contains up to 4 km of Cretaceous sediments. A tectono/sequence stratigraphic and seismic facies study has divided the Cretaceous succession into three workable, genetically related, chronostratigraphic sequences (PK1, PK2 and PK3) separated by basinwide sequence boundaries. The PK1 sequence, of Valanginian-Late Aptian age, is dominated by by low-energy, mudstone-prone, marine shelf and basinal facies (transgressive and highstand systems tracts). Higher energy, fault-bounded clastic fans, sandstone-prone deltaic packages and turbidite/mass flow deposits (lowstand systems tract) are also present. The PK2 sequence, of Late Aptian-Cenomanian age, is dominated by low energy, mudstone-prone shelf and basinal deposits (transgressive and highstand systems tracts). Erosional channels associated with the Late Aptian relative sea-level fall are also present, as are turbidite mounds and deltaic packages (lowstand systems tract). The PK3 sequence, of Cenomanian-Danian age, is predominantly a chalk facies and represents low energy deposition in a southward-deepening sea. Some higher energy calciturbidites/slump masses occur along the basin margins, and reefal build-ups are also present. The higher depositional energy deposits offer the best potential for hydrocarbon accumulations.
Fault styles in the Porcupine Basin, offshore Ireland: tectonic and sedimentary controls
Authors: McCann T. , Shannon P. M. , Moore J. G.
Published: April 2008

Analysis of fault style, size and distribution in the Porcupine Basin has demonstrated the existence of a number of fault types. Syn-tectonic growth faults occur on scales ranging from basin-bounding faults to small intrabasinal faults and they range in age from Jurassic through to the end of the Eocene. Tectonic non-growth faults are most pronounced in the post-rift succession. They are generally small and did not significantly influence sedimentation patterns. They frequently occur in clusters and are commonly recorded at mid-Eocene to mid-Miocene horizons. Slump-associated faults occur in association with large-scale sediment failure structures which are most pronounced in the Upper Tertiary succession. Unlike the other fault types, which are extensional in nature and typically planar in shape, these faults can be either extensional or compressional, and frequently have a listric geometry. The interplay of fault types with sedimentation leads to the identification of a number of recognizable seismic facies.
The Erris and eastern Rockall Troughs: structural and sedimentological development
Authors: O’Reilly Brian M. , Hauser Franz , Jacob A. W. Brian , Shannon Pat M. , Makris Jannis , Vogt Ulrike
Published: April 2008

Wide-angle seismic reflection profiles across the western Irish shelf region have resolved the deep crustal structure of the shelf transition into the Rockall Trough. A three-layered sedimentary succession up to 5 km thick is determined within the Rockall Trough. The lowermost layer probably comprises a thin early Mesozoic syn-rift sediment package, while the upper two layers represent Cretaceous to Recent, mostly post-rift, sediments. The seismic response of the crust changes beneath the Erris Trough into the eastern Rockall Trough, as the crust thins from
c.
30 km onshore in Ireland to between 5 and 6 km in the deep water region of the Rockall Trough. However, the thinning of seismic layers in response to stretching is not uniform. Differential stretching of the crust can explain seismic observations of both crustal and upper mantle structure. Low upper mantle velocities may be due to partial serpentinization of the upper mantle lithosphere, facilitated by rheological hardening during lithospheric stretching. The model predicts the occurrence of starved deep marine sediments during the late Mesozoic and Cenozoic.
Tectonic controls on Upper Jurassic to Lower Cretaceous reservoir architecture in the Jeanne d’Arc Basin, with some comparisons from the Porcupine and Moray Firth Basins
Authors: Shannon P. M. , Williams B. P. J. , Sinclair I. K.
Published: April 2008

Seismic, wireline log and core data from three geographically separate basins in the North Atlantic provide an understanding of the regional dynamics of the basin system, and quantify the sedimentary geometry response to a composite rift episode. The response to the Late Cimmerian rift episode in the Jeanne d’Arc, Porcupine and Moray Firth Basins is broadly comparable, with an initial episode of clastic reservoir facies deposition associated with active growth faulting. This was followed by a mid-rift period where subsidence outstripped sedimentation, resulting in organic-rich source rocks and culminating in a mappable maximum flooding surface. During the final phase of the rift episode, the decrease in subsidence rate resulted in the return of reservoir-quality clastics. The reservoir-scale response to the initial rifting is illustrated in cored sequences from the Jeanne d’Arc Basin. These indicate the development of axial and lateral sandy and conglomeratic braided fluvial and alluvial fan systems, with periods of muddy lacustrine deposition. Pedogenic surfaces offer much potential for detailed correlation and the analysis of sandbody geometry. The onset of anoxic conditions conducive to source rock deposition during the mid-rift phase of Late Cimmerian tectonism, and the preferential development of reservoir facies at the base and top of syn-rift sediment fill, are characteristics of considerable importance to exploration of the North Atlantic borderland rift basins.
Anisotropic grain fabric: volcanic and laboratory analogues for turbidites
Authors: Hughes Simon R. , Alexander Jan , Druitt Tim H.
Published: April 2008

A variety of grain fabrics are commonly observed in coarse-grained turbidites that form potential reservoir rocks. Such fabrics may impart an anisotropy to permeability; however, their origin is poorly understood. Observations of analogous volcanic and laboratory deposits are used here to infer the origin and character of clast alignments. Experimental gravity currents were produced by releasing suspensions of silt-grade silicon carbide (10, 15 or 200y volume) with minor volumes of larger, lower-density perspex clasts into a tank of water. Fabric was measured in the experimental deposits and in the Quaternary Upper Laacher See Tephra, Germany. The experimental and pyroclastic deposits both had a clast fabric with grain long-axes predominantly transverse, but not necessarily perpendicular, to the mean flow direction. The perspex clasts in the laboratory currents were initially transported in suspension, and marks on the sediment surface indicated that these clasts rolled along the bed immediately prior to final deposition. The perspex clasts were buried at differing levels and at a range of distances from source in the laboratory deposits, having been deposited at different stages in the flow history. In the volcanic deposits the same fabric is persistent vertically throughout the thickness of the bed at each site, suggesting that progressive aggradation provided a surface on which the clasts rolled prior to deposition. It is proposed here that similar fabrics in high-density turbidites are analogous to those observed in these experimental and pyroclastic deposits, and therefore they may be predicted from palaeocurrent interpretations, or conversely used to assess palaeocurrent directions.
Sandstone megabeds from the Tertiary of the North Sea
Authors: Pauley J. C.
Published: April 2008

Three cored sandstone megabeds of Tertiary (late Palaeocene) age from two quadrants in the Central Graben of the North Sea are discussed. The megabeds are upwards-fining units 3–9 m thick, underlain by gravelly mudstones (1–1.4 m thick) which are interpreted as debrites. The megabeds are divided into five units (M1–M5) based on grain size and sedimentary structures, which are organized into a type vertical sequence. The basal unit M1 comprises a clast-supported mudclast conglomerate. Unit M2 comprises very coarse- to medium-grained, occasionally gravelly, sandstone which is horizontally laminated to cross-stratified. Unit M3 is the thickest unit (2–4 m) and is a laminated, upwards-fining, medium- to fine-grained sandstone with occasional cross-lamination. Unit M4 is a fine- to very fine-grained sandstone characterized by disturbed lamination and water-escape structures. Unit M5 is an upwards-fining silty sandstone to siltstone characterized by discontinuous, irregular, and locally overfolded laminae. The megabeds are interpreted as having been deposited by turbidity currents. There are some similarities to Bouma sequences and high-density turbidite sequences, but massive sandstone is notably absent. The megabeds are associated with mudstones and debrites and only occasional thin turbidites, and as such they are not part of an organized sequence of turbidites. The beds are notably much thicker than those above and below, and lithologies are distinct from those of the surrounding sediments. These features are comparable to those of ‘megaturbidites’ as defined by Bouma. The debrite which immediately underlies the megabeds is significant in that this relationship is also a common feature of megaturbidites. The debrite is thought to represent slope failure which immediately preceded megabed deposition. A hypothesis is presented of earthquake-induced catastrophic slope failure, debrite deposition and associated megaturbidite generation. Such sandstone megabeds are thought to be widespread sheet-like deposits. They are exploited as a hydrocarbon reservoir in two of the wells discussed.
The formation of carbonate cements in the Forth and Balmoral Fields, northern North Sea: a case for biodegradation, carbonate cementation and oil leakage during early burial
Authors: Watson Roseleen S. , Trewin Nigel H. , Fallick Anthony E.
Published: April 2008

The Tertiary reservoirs of the Forth and Balmoral Fields are sand-rich turbidites, which were deposited in a deep marine setting in the vicinity of the Fladen Ground Spur and East Shetland Platform. The Forth Field reservoir contains biodegraded oil and consists of Palaeocene and Eocene unconsolidated to semi-consolidated, well sorted, medium- to fine-grained sub-arkoses at a maximum present burial depth of 1.8 km. Pervasive ferroan and non-ferroan calcite cements dominate the Forth paragenetic sequence. Bitumen-filled inclusions within these cements indicate that oil emplacement and carbonate cementation occurred simultaneously, probably within the first 1000 m of burial. Carbonate δ
18
O data suggest that meteoric water from the East Shetland Platform flushed the reservoir, biodegraded the migrated oil and displaced the original sea water. Biodegradation of oil took place at the palaeo-oil-water contact, producing a complex sequence of laterally extensive non-ferroan and ferroan calcite concretions. Frequent oil leakage during shallow burial may have produced a large number of concretionary cement layers. The cemented layers have the potential to act as major barriers to vertical fluid movement and to significantly reduce the net to gross ratio of the reservoir. Pore-filling ankerite post-dates non-ferroan and ferroan calcite. Ankerite precipitation marks the final stages of bacterial influence within the sediment column.

The Balmoral Field is located
c.
100 km to the south of Forth. The reservoir consists of a series of poorly sorted subfeldspathic arenites with infrequent spherical carbonate concretions. These non-ferroan and ferroan calcitic concretions preferentially precipitated around localized accumulations of organic matter at < 500 m burial depth. Pore fluids were meteoric, and carbonate was provided by both bacterial oxidation and sulphate reduction of organic matter. Meteoric water is thought to have been derived from the East Shetland Platform to the north of Balmoral. Oil migrated into Balmoral during the Oligocene, post-dating meteoric flooding.

Laterally extensive carbonate-cemented sand horizons, formed in association with oil biodegradation, have the potential to compartmentalize a reservoir. The distribution of these cements within Tertiary deep water sandstone reservoirs adjacent to the East Shetland Platform is likely to be controlled by the relative timing of meteoric flushing and oil migration.
Tectonic evolution of greenstone belts
Authors: Shackleton R. M.
Published: April 2008

Granite-greenstone terrains represent the main process by which continental crust was formed in the Archaean; the same process has continued since, at a diminishing rate. Granite plutonism added a layer about ten kilometres thick under the greenstones. From Early Archaean to Late Proterozoic, there was a progressive change in the volcanics; komatiites decreased from the Early Archaean, while the proportion of andesites and felsic volcanics increased. Continent-derived sediments, rare or absent from the Early Archaean, increased through time.

Tectonic deformation in the greenstone belts is separable into pre-diapiric, diapiric and post-diapiric. The first deformations were translational, mainly flat thrusts, the second compressional without crustal shortening, the third involved crustal shortening. Only the diapiric structures are an essential element of greenstone belt evolution but the others are generally important. The pre-diapiric and diapiric structures are related to subduction; the post-diapiric structures may be accretionary or collisional. Granite-greenstone terrains show only minor crustal thickening. Some adjacent high-grade terrains were intensely deformed, and the crust greatly thickened, by continent-continent collision.
Archaean deformation patterns in Zimbabwe: true indicators of Tibetan-style crustal extrusion or not?
Authors: Treloar Peter J. , Blenkinsop Tom G.
Published: April 2008

Recent models of the structural evolution of the Zimbabwe Archaean craton have stressed the role of WSW-trending sinistral strike-slip shears in accommodating the WSW-directed extrusion of crust thickened in response to NNW-SSE compression and shortening. This relationship between crustal thickening and crustal extrusion is analogous to the structural response of Tibetan crust to the Tertiary collision of India and Asia. We have re-examined many of the structures of the Zimbabwe craton and find the extrusion model largely untenable. Instead, a conjugate set of ESE-striking dextral shears and NNE-striking sinistral shears have been identified, on both regional and outcrop scale, which apparently reflect part of the structural response of the Zimbabwe crust to NNW-SSE shortening resultant from collision of the craton with the Central zone of the Limpopo belt to the south. This collision, at about 2.58 Ga, post-dated the 2.68 Ga aged collision between the Central zone and the Kaapvaal craton which represents the Limpopo orogeny sensu stricto. The suture between the Zimbabwe craton and the Central Zone was re-activated at
c.
2.0 a as a major intracontinental dextral strike-slip shear zone. Partial melting of the lower crust at
c.
2.58 Ga accompanied thickening of the Zimbabwe Archaean crust following collision with the Central zone. The conjugate shears deform fabrics that date from that collision, are synchronous with, or post-date, granite emplacement and may have acted as conduits for transport of the Chilimanzi monzogranitic magmas from the lower to the upper crust. Contacts between greenstone belts and granites are commonly sheared. These marginal shear zones are often parallel to the regional conjugate shears, although local variations in stress trajectories imposed by pluton emplacement may result in them being non-parallel to the major shears.
The generation and assembly of an Archaean supercontinent: evidence from the Yilgarn craton, Western Australia
Authors: Myers John S.
Published: April 2008

The Yilgarn craton consists of granites, greenstones and granitic gneisses ranging from at least 3730 to 2550 Ma. These rocks can be divided into a number of tectonostratigraphic terranes that comprise distinct rock units with different geological histories. However all the terranes, regardless of their age and composition, show evidence of intense tectonic, volcanic, plutonic, and metamorphic activity between 2780 and 2630 Ma. This is interpreted as a major episode of plate tectonic activity which swept together and amalgamated a number of diverse crustal fragments (including volcanic arcs, back-arc basins and microcontinents) to form a super-continent, of which the Yilgarn craton is a fragment. The main features of the Narryer, Murchison and Kalgoorlie terranes are described as examples of the diversity of crustal components within the craton and the nature of the processes by which these components were combined to form the Yilgarn craton.
Late Archaean structure and gold mineralization in the Kadoma region of the Midlands greenstone belt, Zimbabwe
Authors: Herrington R. J.
Published: April 2008

Major Archaean greenstone gold deposits are located in distinctive structural settings, and Zimbabwe is a good example with over 90% of its gold production being derived from structurally controlled veisns, shear zones and probably epigenetic replacement BIF hosted deposits.

The structural framework of the Zimbabwe craton is dominated by the evolution of the
c.
2700 Ma Limpopo belt. Strongly developed, major shear zones were initiated at this time. These major shear zones focused strain, which in the Midlands greenstone belt, evolved from pure N-S compression, to lateral extrusion of the main Rhodesdale gneiss block westwards.

The structural features seen in the gold deposits of the Midlands greenstone belt are consistent with such a model of NNW-SSE compression linked to lateral extrusion of the Rhodesdale gneiss block westwards. Initiation of structures during NNW-SSE compression resulted in early folding such as the Kadoma anticline, together with initiation of the major mineralized shear systems as thrust faults. Simple shear evolved to transpressive shear with development of oblique dextral and sinistral mineralized shear systems. Rotation of the principal stress direction clockwise to an ENE-WSW orientation led to evolution of dominantly dextral transpressive shear on the major Lily, Munyati and Rhodesdale boundary shear systems, probably in response to the lateral extrusion of the Rhodesdale gneiss block westwards. This even is linked to waning gold mineralization and accounts for the major dextral offsets along the Lily fault and the Munyati shear close to Battlefields.
Palaeotidal modelling of continental shelves: marine implications of a land-bridge in the Strait of Dover during the Holocene and Middle Pleistocene
Authors: Scourse J. D. , Austin R. M.
Published: April 2008

Numerical models of the tide on the NW European continental shelf are useful in predicting basic tidal dynamics (amplitude, current, bed stress, mixing) from bathymetry, coastline configuration and a known ocean tide at the shelf edge. Using the
M
2
constituent of the ocean tide, a numerical model has been used to investigate the tidal regime of the English Channel/southern North Sea at successive stages during the Holocene transgression, and also during the maximal sea-levels of Middle Pleistocene temperate stages, by incorporating palaeogeographical interpretations of geological data from these time-slices. Prior to the breaching of the Strait of Dover in the early Holocene, the Southern Bight of the North Sea was a quiet, shallow embayment of low
M
2
tidal amplitude (<0.5 m); following breaching it attained the present state of vigorous tidal action with mean amplitudes of around 2 m. Changes in sediment transport paths and shelf-sea fronts are also indicated. Models of the Middle Pleistocene interglacial English Channel incorporate modified palaeobathymetries to compensate for tectonic uplift in the central Channel area over the past 400 000 years. These models indicate increased seasonal stratification in the Channel compared with the present mixed condition. This is in agreement with independent evidence from coccolith assemblages contained within sediments of the same age from the central Channel area.
Malacological evidence relating to the insularity of the British Isles during the Quaternary
Authors: Meijer T. , Preece R. C.
Published: April 2008

The marine molluscan faunas from different temperate stages of the Pleistocene of the North Sea vary enormously both in terms of species richness and in the diversity of their biogeographical composition. The marine assemblages from the Middle Tiglian, Eemian and Holocene have all yieldedc.100 species or more, including many with southern or ‘Lusitanian’ affinities. It is thought that during these stages the Strait of Dover was open, allowing entry of these southern taxa into the southern North Sea. Conversely, the temperate stages from the Late Tiglian up to and including the Holsteinian have yielded relatively impoverished faunas (no more than about 40 species) virtually lacking any of the southern elements. During these stages it would appear that the Strait of Dover was closed, so preventing the spread of marine molluscs into the North Sea. Examination of the history of fluvial molluscs (particularly prosobranchs and larger bivalves) on either side of the English Channel supports this interpretation. Fluvial provinciality is recognized during the stages when Britain is thought to have become an island. Non-provinciality, pointing to fluvial exchange, occurs during the other stages.
Insularity of the British Isles 250 000–30 000 years ago: the mammalian, including human, evidence
Authors: Sutcliffe Antony J.
Published: April 2008

During the Middle-Late Quaternary the British Isles have alternately been part of the Continent of Europe and separated from it by the sea. Comparison of mammalian faunas from sites on both sides of the present English Channel can contribute to the reconstruction of Britain’s island history. The most suitable chronological framework for this study is discussed. Attention is drawn to problems of applying the 1973 scheme of the Geological Society of London, which is used here only in conjunction with the more straightforward chronology of the deep-sea oxygen isotope record. Special attention is directed to events during the period of time
c.
250 000 to 30 000 years
bp
. The chalk land-bridge that had previously joined the British Isles to the Continent of Europe had by this time been breached, and any further changes in the land connection are likely to have been the result of eustatic changes of sea-level, associated with glacial-interglacial fluctuations of climate.

Whether the British Isles became isolated from the Continent during the high sea-level phase that occurred during isotope stage 7 is uncertain from the mammalian evidence. The occurrence of human artefacts and remains of pond tortoise
Emys orbicularis
, which is unlikely to have tolerated salt-water, in deposits representing the earliest part of this stage, suggests a land connection then or not very long previously. Human presence continues into isotope stage 6, characterized also by the arrival of steppe rodents from eastern Europe; evidence of a further episode of low sea-level.

During the Last Interglacial, oxygen isotope stage 5e, sea-level rose once more, isolating the British Isles from the Continent. Remarkably, humans, horses and the mollusc
Corbicula fluminalis
, so common previously, fail to reappear, a consequence, perhaps, of being cut off by rapidly rising sea-level at the end of stage 6. A very characteristic fauna, including hippopotamus, straight-tusked elephant, narrow-nosed rhinoceros, fallow deer and abundant carnivores, did nevertheless manage to become established throughout England and Wales at this time, some as far north as Durham.

Falling sea-level associated with the Devensian (isotope stages 5d-2) re-established the land connection. Hippopotamus had by this time disappeared and been replaced with a fauna including woolly mammoth, woolly rhinoceros and reindeer. Humans and horses returned once more. It was not until sea-level rose again during the Holocene that the British Isles became finally isolated from the Continent, as they are today.
Evidence for Pleistocene contact between the British Isles and the European Continent based on distributions of larger carnivores
Authors: Turner Alan
Published: April 2008

The generalized ecological tolerances of larger terrestrial carnivores, reflected in their wide distributions in fossil and living faunas, make them good potential indicators of the changing pattern of links between Britain and the continent of Europe over time. Extensive contact is indicated until the earlier part of the Middle Pleistocene, that is prior to the Anglian-Elsterian glaciation. The pattern after that is more complex, with evidence for some form of marine barrier by the time of the Hoxnian and for complete isolation during the Last Interglacial.
An overview of database analysis and design for geological systems
Authors: Rasmussen Ken
Published: April 2008

Techniques for database analysis and design are well established for the development of administrative systems. When the techniques are used to develop geological databases, however, a number of difficulties arise. This paper provides an introduction to the nature of these problems. The context for the discussion is first set by describing the standard techniques for database analysis and design. The difficulties that arise when the techniques are applied to geological data are then reviewed. The paper concludes with a discussion of some recent developments in database analysis and design which will assist designers of geological systems.
Improving the value of geological data: a standardized data model for industry
Authors: Saunders Michael R. , Shields John A. , Taylor Michael R.
Published: April 2008

The preservation of observed geological data in a standardized and digitally accessible form presents a challenge. Geological data objects range in scale from the macroscopic, e.g. outcrop and core, to the microscopic, e.g. drill-cuttings. Each change in scale has a direct effect on apparent heterogeneity, which may result in contradictory data, and the providers of the observations may have widely differing degrees of experience. A system has been developed for the definitive description and preservation of features in core and drill-cuttings using workstations. It provides the opportunity to extract definitive individual attributes from geological descriptions with a minimum of extra effort. These digitally stored data are represented graphically, textually and symbolically to construct geological logs. Geological data may be presented in combination with drilling and mud gas data, Measurement While Drilling (MWD) and wireline logging data, and off-site analyses, to produce composite formation evaluation logs, as well as being exported in customized file formats, thereby enhancing the value of all data through integrated information management.
Towards the creation of an international database of palaeontology
Authors: Lhotak M. , Boulter M. C.
Published: April 2008

The Plant Fossil Record (PFR) database is being built to include biological, geological, geographical and bibliographic information of all authoritative plant fossil citations throughout the world. The interactions between these have created standardized records from different sources and programs have been written to add and reorganize data. Techniques such as optical character recognition (OCR), e-mail and diversity interfaces are used. Interdisciplinary and cultural aspects of this ambitious project are being considered. The work is to help as many different specialists as possible to use palaeontological data and to relate these data to evolutionary and biostratigraphic problems as well as to climatic aspects of global change. The structure of the database comprises some original features. It is a temporal facility: each edit or addition is timed and dated so that its state at any particular time can be recalled. Geological time and the associated global palaeogeography are also built into the database giving a time dimension to record likely evolutionary trends.