bluuebunny/crossref_metadata_2025 huggingface dataset:
Transpressional inversion due to episodic rotation of extensional stresses in Jeanne d’Arc Basin, offshore Newfoundland
Authors: Sinclair Iain K.
Published: April 2008

Variations in structural architecture indicate that a ninety degree rotation of extensional stresses occurred between multiple Mesozoic rift phases, resulting in local structural inversion in the Jeanne d’Arc Basin on the Grand Banks of Newfoundland, offshore eastern Canada.

NW-SE-orientated extension caused dip-slip movement on NE-SW-trending en echelon faults during the Late Triassic to earliest Jurassic rift episode. These en echelon faults appear to have been separated by tilted basement relay ramps in accommodation zones rather than by cross-strike transfer faults.

NE-SW-orientated extension during the mid-Aptian to late Albian rift episode resulted in oblique-slip reactivation and linkage of the earlier normal faults. Transpressional stresses were generated by oblique-slip motion at restraining bends in Jeanne d’Arc Basin faults. Structural responses include: ‘pop-up’ blocks between upward-diverging faults; reverse faults separating overlapping ‘shingles’ of strata; forced folds in strata overlying deep-seated faults; and wrench-related folds trending oblique to through-going transfer fault zones. Salt and shale layers are seen to act as horizons of fault detachment, modifying the transmission of tectonic stress and strain from the basement into the sedimentary cover. The transpressional mechanism and direction of strike-slip motion are confirmed through recognition of associated transtensional structures located along fault strike at nearby releasing bends.

Transpressional structures in the Jeanne d’Arc Basin are recognizable over a wide range of scales from relatively large-scale reverse faults visible on seismic data to small-scale reverse faults seen in core.
Late Jurassic-Early Cretaceous inversion of the northern East Shetland Basin, northern North Sea
Authors: Thomas D. W. , Coward M. P.
Published: April 2008

Current research into the structural evolution of the East Shetland Basin, northern North Sea, indicates that a significant phase of structural inversion occurred during the Latest Jurassic-Early Cretaceous.

Structural effects of this tectonic phase include the pronounced uplift of pre- and syn-rift sequences along the western margin of the basin and the partial inversion of Mesozoic half-graben along intra-basinal NE-SW controlling faults. In addition, the localized occurrence of flower geometries and pop-ups along NE-SW trending faults suggests a component of strike-slip during the reactivation of these pre-existing structures.

Work presented stresses the importance in the role of pre-existing lineaments and the effects of superimposing subsequent non-coaxial extensional events upon them. Lineaments may be prominent reactivated basement shear zones or fault systems from earlier extensional phases.

Inversion will be analysed in detail with respect to two areas within the northern East Shetland Basin, the Tern sub-basin and the northern Penguin ridge. A regional tectonic model of northern North Sea rift evolution is presented and its implications to observed Latest Jurassic-Early Cretaceous structure discussed.
Inversion of a Lower Cretaceous extensional basin, south central Pyrenees, Spain
Authors: Bond R. M. G. , McClay K. R.
Published: April 2008

The structure of the Boixols thrust sheet is dominated by kilometric-scale E-W trending folds, the most important of which is the Sant Corneli-Boixols-Nargo fault-propagation, growth anticline at the leading edge of the thrust sheet. Five megasequences have been identified within the Boixols thrust sheet and these may be related to phases of extension, tectonic quiescence and phases of inversion. The Boixols thrust sheet incorporates the Lower Cretaceous Organyá basin which is bounded by a north-dipping extensional fault that underwent inversion in the Late Campanian-Maastrichtian and was incorporated into the Pyrenean south-vergent thrust system during the Eocene-Oligocene. The inversion history is recorded by the syn-inversion Arén growth sequence that is only preserved in the footwall syncline of the main fault-propagation growth fold. During inversion a short-cut thrust fault developed in the footwall to the Lower Cretaceous extensional fault. This now forms the Boixols thrust at surface. An inversion model for the Boixols thrust sheet and the Organyá basin is proposed and compared to analogue and numerical models of half-graben inversion. This model for inversion in the Boixols thrust sheet may be applied to other thrust structures in the Pyrenees and to other foreland fold and thrust belts.
Structural development and stratigraphy of the Kyokpo Pull-Apart Basin, South Korea and tectonic implications for inverted extensional basins
Authors: Lambiase Joseph. J. , Bosworth William P.
Published: April 2008

Field studies indicate that the Kyokpo Basin initially formed as a pull-apart during the Late Cretaceous Period. Syn-sedimentary normal and strike-slip faults suggest early transtensional movements. Fault-plane analysis indicates an approximately E-W extension direction for this early event. The basin filled with a succession of lacustrine deposits interbedded with abundant pyroclastic rocks and rare volcanic flows. Rapid initial subsidence is demonstrated by deep-water sediments that lie directly on pre-Late Cretaceous rocks. All exposed sedimentary facies were deposited in deep-water environments, indicating continued rapid subsidence during deposition of the entire sequence.

Laminated grey siltstones interbedded with thin, distal, sandy turbidites are the most abundant sediments. Channels filled with conglomeratic debris flow deposits (clasts as large as 1 m in diameter) cut into the siltstone beds. Massive and graded pebbly sandstone turbidites are also interbedded with siltstone. A thick accumulation of stacked, conglomeratic, debris flow deposits interbedded with sandstone turbidites forms a subaqueous fan complex at the southern part of the outcrop area.

Local compression caused up to 3 km of uplift and erosion following deposition of the Kyokpo strata. Thrust faults and small- and large-scale folds were formed by the W-E directed compression. Timing of the compression cannot be determined but probably correlates with a regional late Miocene transpressional event.
Inversion around the Bass Basin, SE Australia
Authors: Hill Kevin C. , Hill Kathy A. , Cooper Gareth T. , O’Sullivan Andrea J. , O’Sullivan Paul B. , Richardson M. Jane
Published: April 2008

Regional seismic reflection and fission track thermochronological studies of the Bass Basin area illustrate two types of inversion, both confined to the Bass failed rift. The first type involved 1–2 km of uplift, denudation and cooling of basement over more than 200 km along both margins of a failed rift, with lesser erosion within the rift basin. This occurred during renewed extension that bypassed the Bass Basin area leaving it as a failed rift. The uplift resulted from breaking of the lithosphere and may have been due, in part, to rebound following several kilometres of rapid sediment loading in the preceding 20 Ma. The second type comprised repeated structural inversion involving 1–3 km of uplift by compressional reactivation of extensional faults and along new reverse faults. These inversions occurred along the zone of maximum extension in a failed Mesozoic rift, such that Tasmania acted as a buttress to compression. The rifting and structural inversions are interpreted to have been controlled by a long-lived zone of weakness, perhaps overlying a Palaeozoic greenstone belt, with the maximum principal stress being roughly perpendicular to faults within the zone during inversion. Significantly, Miocene-Pliocene inversion resulted from the Australian craton being placed into compression following arc collision 3500 km to the north, emphasizing the importance of long distance transmission of compressional stresses through the lithosphere.
Development of structurally inverted basins: a case study from the West Coast, South Island, New Zealand
Authors: Bishop Daniel J. , Buchanan Peter G.
Published: April 2008

Cretaceous-Palaeogene basins in the West Coast of South Island, New Zealand, underwent inversion during the Neogene. Initial extension was oriented WNW-ESE and had a typical magnitude of
c.
5%, whilst subsequent shortening in the same direction was typically
c.
3–4%. This deformation produced a NNE-SSW trending basin and range province. Shortening was commonly partitioned into narrow zones along basin and range margins, leaving unreactivated extensional structures inbetween. Such margins are characterized by subparallel and en echelon Cretaceous — Palaeogene faults, which have been reactivated as predominantly reverse faults during the Neogene. Reverse faults propagated into the syn-inversion sedimentary sequence with increasingly shallow dip. Inversion caused local erosion of the original rift deposits. New back-thrusts, low-angle thrusts and folds developed during the late Neogene. Locally, incompetent Eocene mudstones deformed in a quasi-plastic manner, so that reverse faults at deeper structural levels became decoupled from thrust faults at shallower levels. Computer-aided section restoration has allowed validation of the interpretations, provided estimates of the timing and magnitude of deformation, and given a better understanding of the sequential development of inversion structures. Natural inversion structures show both contrasts and similarities to those produced in sand-box experiments.
Subdivision and correlation of monotonous sandstone sequences using high-resolution heavy mineral analysis, a case study: the Triassic of the Central Graben
Authors: Mange-Rajetzky Maria A.
Published: April 2008

The subdivision and correlation of monotonous siliciclastic sequences with poor biostratigraphic control and few lithostratigraphic markers has long been a problem. The need to find an efficient approach is even greater for subsurface sequences where information from cores and wireline logs has proved inadequate. In this short paper a novel approach, high-resolution heavy minerals analysis (HRHMA), is introduced, which is particularly suitable for the subdivision of problematical clastic sediments where other techniques are inconclusive. A full paper, which will present the technique in more detail, will be published shortly.
Clay mineral stratigraphy in Palaeozoic and Mesozoic red bed facies onshore and offshore UK
Authors: Jeans C. V.
Published: April 2008

Different methods of clay mineral correlation within sedimentary sequences provide varying degrees of stratigraphical resolution. These range from: (1) precise time correlation by the recognition of clay-forming events; (2) levels of general stratigraphical significance based upon changes in clay mineral assemblages reflecting widespread climatic, topographic or sea-level changes; (3) correlations of no stratigraphical significance where the distribution of clay minerals reflect patterns of detrital dispersal, pore-fluid flow in buried sediments, or metamorphism. The main use of clay mineral stratigraphy is to provide an alternative means of correlating effectively unfossiliferous sediments by tying them into biostratigraphically dated standard sequences in which the clay mineral variations are well established. Since 1980 it has been used in offshore UK to help correlation in the Old Red Sandstone (Devonian-Lower Carboniferous) of the North Sea and west of Shetland, and the New Red Sandstone (late Carboniferous-Triassic) of the Southwest Approaches, southern North Sea Basin and the central North Sea. Results of these studies are described and critically assessed.
Luminescence dating of Quaternary sediments
Authors: Rendell H. M.
Published: April 2008

Luminescence techniques have the potential to date Quaternary sediments from a range of depositional environments. Recent developments of optical dating complement thermoluminesscence dating techniques, extending their applicability. The age range of samples that can be dated shows a strong dependence on the material involved. Recent work on both dune sands and loess indicates that accurate dating to beyond 500 ka may be possible in certain circumstances. The age range limits for other material may be closer to 100 ka. Examples are given of luminescence dating of material from a range of sedimentary environments.
Sequence stratigraphic response to Aptian-Albian rifting in conjugate margin basins: a comparison of the Jeanne d’Arc Basin, offshore Newfoundland, and the Porcupine Basin, offshore Ireland
Authors: Sinclair Iain K.
Published: April 2008

Analysis of well and seismic data indicates that both the Jeanne d’Arc Basin on the Grand Banks of Newfoundland and the Porcupine Basin on the continental shelf off western Ireland were undergoing extension characterized by subsidence and rotation of fault blocks during mid-Aptian to late Albian times. Stratigraphic sequences in these conjugate basins developed in response to synchronous rift tectonism.

The base of each syn-rift sequence is defined by an angular unconformity which formed in response to regional uplift. This mid-Aptian rift-onset unconformity is buried beneath a depositional sequence characterized by retrogradational coastal sediments overlain by progradational coastal sediments in each basin. However, the lower retrogradational deposits dominate the syn-rift sequence in the Jeanne d’Arc Basin, while the upper progradational deposits dominate the syn-rift sequence in the northern Porcupine Basin. The relative dominance of sediments deposited along either transgressive or regressive coastlines was mainly determined by the local interplay of variable rates of rift-induced subsidence and sediment input.

The initiation of seafloor spreading between the Flemish Cap/Orphan Knoll and Goban Spur/Porcupine Bank continental margins near the end of Albian time resulted in the establishment of regional thermal subsidence, rapid coastal transgression and fully marine deposition above a locally-developed sequence-bounding unconformity.
Seismic investigation of the Faeroe basalts and their substratum
Authors: Kiørboe Liselotte , Petersen Steen Agerlin
Published: April 2008

Various seismic techniques have been used to investigate the subsurface in the area around the 2.2 km deep Lopra-1 well located on the Faeroe Islands. The techniques include vertical seismic profiling, walk-away vertical seismic profiling and refraction seismic profiling. Integration of these three different techniques, using a combined profiling method, makes it possible to overcome some of the problems arising from the large acoustic impedance contrasts and S-wave scattering which often exist in volcanic areas.

The three seismic profiles show distinct compressional (P) and shear (S) energy arrivals. The direct S-wave is generated at basalt interfaces at or just below the sea bed and interferes in part with groundroll in the refraction recordings. The low noise activity before first arrival indicates the very high quality of the VSP data. The VSP and the walk-away VSP recordings show clear evidence of reflected P-waves, while reflected P-waves are more difficult to recognize on the surface recordings.

The velocity through the basalt flows is shown to be dependent on the thickness of the basalt beds. Thick beds yield high velocities, while thin beds generate lower velocities. Velocity anisotropy is observed in the upper 700 m, probably resulting from cracks in the columnar basalt flows or from master joints.

The seismic examination of the three profiles shows excellent interbasaltic reflections. A very strong reflector with a negative reflection coefficient is observed at a depth of
c.
2.35 km and is interpreted to be the base of the basalt flows. Deeper reflections are interpreted as sills in the substratum of the basalt flows.
Seismic stratigraphy of the Bill Bailey and Lousy Bank area: implications for subsidence history
Authors: Vanneste K. , Henriet J.-P. , Posewang J. , Theilen F.
Published: April 2008

The present study discusses some results of a reconnaissance seismic survey carried out in 1988 in the area around the prominent Bill Bailey and Lousy Banks, located SW of the Faeroe Islands. The area is thought to be part of the Rockall-Faeroe microcontinent, which was flooded by Palaeocene plateau basalts. The seafloor topography is largely due to the present-day organization of the basalt surface. Important structural features exhibited by this basement are sub-surface diverging reflectors and a major fault.

Sediment accumulation is confined to the basinal area where four unconformities were identified. The lower boundary corresponds to the well-known reflector R4 of the North Atlantic. The other unconformities are proposed to correlate with hiatuses at the beginning and end of the middle Miocene, and at the end of the Pliocene, respectively. The sediment units are briefly described by their upper and lower boundary, seismic facies and thickness variations. About the lithology, however, little is so far known. The sediments are deformed by intraformational faults and a diapir-like structure. Both deformational styles are probably related to temporary overpressurisation in fine-grained sediments, but resulting from different causes.

Stratal geometric patterns indicate that initially the subsidence of the basalt surface was rather uniform, but became non-uniform after the Eocene/Oligocene boundary, differentiating the two banks from the surrounding basin.
The relationship between the distribution of Tertiary sediments, tectonic processes and deep-water circulation around the Faeroe Islands
Authors: Boldreel Lars Ole , Andersen Morten Sparre
Published: April 2008

Compressional structures, which were initiated in the Eocene, and thermal subsidence played an important role in the distribution of the post-Palaeocene sediments around the Faeroe Islands. Additionally, the deep-water currents passing through the Faeroe-Shetland Channel and the Faeroe Bank Channel contributed to the distribution of the sediments from Oligocene-Miocene to Recent. The relationship is illustrated by the outcrop of four pre-Upper Pliocene sedimentary sequences below a thin cover of Pleistocene sediments and the surface of the upper Palaeocene basalts in the study area, which consists of three basins — the Norwegian Sea, the Faeroe-Shetland Channel and the Faeroe Bank Channel, separated by compressional ridges. The axial part of the Faeroe-Shetland Channel is characterized by a fairly flat bottom topography caused by differential sedimentation and erosion by deep-water currents flowing against the topography. The flat channel floor narrows southwards to the tip of the Munkagrunnur Ridge. Due to tectonic structure, differential deposition and erosion, progressively older sediment units are exposed to the south. The channel floor stays narrow in the Faeroe Bank Channel. To the west of the Faeroe Channel Knoll, which is located within the central part of the deep Arctic bottom current, up to 2000 m of sediments are present. At the northern end of the Faeroe Bank Channel the basalt crops out as a result of non-deposition and erosion by the deep Arctic bottom current.
The influence of glacigenic sedimentation on slope-apron development on the continental margin off Northwest Britain
Authors: Stoker M. S.
Published: April 2008

The Hebrides and West Shetland shelves were extensively glaciated on several occasions during the mid- to late Pleistocene, with grounded ice locally reaching the shelf edge and depositing vast amounts of sediment directly on to the adjacent slopes. On the Hebrides Slope, the discrete, slope-front depocentre of the Sula Sgeir Fan has accumulated a thick (200 m) succession of interbedded debris-flow diamictons, turbidite sands and muds. This contrasts with a ‘between fan’ area to the south, where a thinner (< 100 m) sediment drape of glaciomarine ice-rafted and suspended sediment, deposited partly under the influence of alongslope currents, is preserved. On the West Shetland Slope, the slope-apron may consist of several coalescing wedges of sediment which form a laterally more extensive, albeit thinner (up to 100 m) succession of mass flow deposits. On both slopes the glacigenic deposits can be separated into discrete mass-flow packages bounded by thin, occasionally slope-wide, prograding clinoforms. Accumulation of these packages was episodic and related to specific rapid phases of downslope sedimentation, concomitant with maximum glaciation and glacio-eustatic lowstand. The thin clinoform units reflect intervals of reduced sediment supply to the slope. The ‘between fan’ area of the Hebrides Slope has been a region of relatively persistent sediment bypass.

Analagous deposits preserved on other mid to high-latitude continental margins, such as the East Greenland, Barents Sea and East Canadian margins, highlight the regional importance of glacigenic processes to late Cenozoic slope-apron development in the North Atlantic region. In particular, they demonstrate the influence that climate can have on the stratigraphical and sedimentological record.
Petrography and biostratigraphy of Palaeogene volcaniclastic sediments dredged from the Faeroes shelf
Authors: Waagstein Regin , Heilmann-Clausen Claus
Published: April 2008

A large number of dredge hauls made on the outer shelf and slope east of the Faeroe Islands have yielded Lower Tertiary sedimentary rocks (bothin situand glacially transported). The rocks have been examined petrographically and biostratigraphically (dinoflagellates), and they are all considered to be derived from the Faeroe shelf. They can be classified according to sediment type and chronostratigraphy in the following groups: (1) Lower Eocene basaltic tuffs (dominantly non-marine); (2) Lower and Middle Eocene tuffaceous limestones and phosphatic sediments; and (3) Lower and possibly Upper, Oligocene fine-grained feldspathic volcanic sandstones. Two dredge hauls of local till near the shelf edge west of the Faeroe Islands gave coarse to fine grained volcanic sandstones of Oligocene age.

The basaltic tuffs of the eastern shelf bear witness to a major phase of explosive volcanism succeeding the extrusion of the Faeroe lava plateau. Whole-rock chemical analyses suggest that the explosive magmas had a tholeiitic composition rich in iron and titanium. Some of these eruptions are likely to be represented among the tuffs of the Balder Formation in the North Sea. After the deposition of the tuffaceous sediments in a marine setting during the Early and Middle Eocene the central parts of the Faeroe basalt plateau were probably uplifted in the Bartonian or Priabonian. As a result of erosion of the basalt plateau and overlying sediments volcanic sandstones (often rich in reworked dinoflagellates) were deposited in a marine environment during the Early Oligocene.
Tertiary structuration and erosion of the Inner Moray Firth
Authors: Thomson K. , Hillis R. R.
Published: April 2008

Seismic profiles and field data show that the Inner Moray Firth (IMF) experienced significant structural modification during Early Tertiary times with the development of inversion, strike-slip and extensional oblique-slip geometries as well as uplift and erosion at a mid-late Danian unconformity. Seismic reflection profiles across the IMF also show progressively older stratigraphic subcrop towards the west. Analysis of sonic velocities and vitrinite reflectance demonstrate that up to 1.5 km of basin fill has been removed from the IMF. The height of the sequences above maximum burial depth (apparent erosion) is at a maximum in the northwestern part of the basin, where inversion geometries are found, and decreases to zero in the Outer Moray Firth. However, if post-erosional burial is taken into account, the actual amount of erosion during Early Tertiary exhumation (total erosion) is shown to be more evenly distributed, and of greater magnitude throughout the IMF. Incorporation of the effects of Tertiary erosion into analysis of basin development requires much greater post-rift burial than if Tertiary erosion is ignored. It seems most likely that the Early Tertiary deformation of the IMF occurred in response to NE Atlantic (Thulean) and Alpine events.

Late Cretaceous-Early Tertiary times saw the change of the predominant stress field of North West Europe from one of extension to one of NW-SE compression (Ziegler, P. A. 1987,
Tectonophysics
,
137
, 1–5 & 389–420). The present-day stress field of Western Europe is described by a NW-SE direction of maximum principal stress (Müller, B.
et al.
1992,
Journal of Geophysical Research
,
B97
, 11 783–11 803). The present-day-, and by inference palaeo-, intraplate stress field of Western Europe can be attributed to plate-driving forces acting on the boundaries of the Eurasian plate. On average, the orientation of present-day maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe (Müller, B.
et al.
1992,
Journal of Geophysical Research
,
B97
, 11 783–11 803). A combination of stresses associated with Alpine collision between Europe and Africa, and those associated with opening of the North Atlantic are considered responsible for the Late Cretaceous-Early Tertiary NW-SE compressional stress field of North West Europe.
Structural and tectonic setting of the Permo-Triassic basins of northwest Europe
Authors: Coward M. P.
Published: April 2008

The Permo-Triassic basins of NW Europe formed by (i) late-orogenic spreading of the Variscan mountain belt, eventually leading to ocean floor formation in the Mid-Atlantic, (ii) continental rifting propagating south from the Arctic and (iii) thermal subsidence following Carboniferous volcanic activity in the North Sea. Spreading values in the northernmost North Sea, west of Shetland and west and southwest of Britain, are relatively high but the sediments are dominantly continental; the Permo-Triassic rifting affected lithosphere thickened during Caledonian and Variscan orogenesis. However, subsidence associated with the Arctic rifting, together with global sea level changes, allowed marine incursions into the North Sea, producing the large Zechstein salt basins. Interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across Britain and the North Sea. This fault pattern formed the basic framework for subsequent Jurassic and Cretaceous basin formation.
Influence of basement on the geometry of Permo-Triassic basins in the northwest British Isles
Authors: Anderson T. B. , Parnell J. , Ruffell A. H.
Published: April 2008

The orientation and shape of Permo-Triassic basins in the northwest British Isles are strongly influenced by the structure of the underlying basement. In the Hebridean region, NE-SW-elongated basins are controlled by reactivated Caledonian thrusts. Further south, the variable orientation of elongate basins reflects the local fabric of the Dalradian basement. In the Midland Valley of Scotland and Northern Ireland, Permo-Triassic basins have a less regular shape, as they are sited on Carboniferous basins with a weakly pronounced structural fabric: the NNW-SSE-elongation of some outcrops reflects the effects of later (Tertiary) faulting along this trend. In the Southern Uplands lower Palaeozoic basement, bedding and cleavage are tectonically aligned to produce a strong ENE-WSW high angle anisotropy, cross-cut by complicated arrays of steeply inclined faults. Crustal stretching would have occurred normal or parallel to the anisotropy: Permo-Triassic basins, typically elongate NNW-SSE, clearly formed in response to contemporary ENE-WSW stretching, producing half-graben by dip-slip reactivation of existing Caledonian fractures.
New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance
Authors: Keeley M. L.
Published: April 2008

Reconnaissance apatite fission track analysis data from onshore southern Ireland are used to construct a thermal history for Late Palaeozoic rocks. The data show clear evidence of post-Variscan heating. The most significant thermal event took place during the Permo-Triassic, and resulted from not less than 1 km of regional subsidence. Late Palaeozoic rocks were again uplifted and exposed during the Early Jurassic. Significant deposition did not resume until the Late Cretaceous, and was followed by uplift and inversion emanating from the North Celtic Sea Basin. Repeated post-Variscan extension and compression of the Variscan basement must therefore have taken place, making it now difficult to differentiate
bona fide
Variscan deformation. However, the northern extent to both Permo-Triassic and Late Cretaceous sedimentation was probably controlled by extension along a line of major Variscan thrusts in south-central Ireland — the South Ireland Lineament.
The Variscan fold-thrust belt of southeast Ireland and its control on early Mesozoic extension and deposition: a method to predict the Sherwood Sandstone
Authors: Musgrove F. W. , Murdoch L. M. , Lenehan T.
Published: April 2008

The Sherwood Sandstone Group (SSG), of Early Triassic Scythian age, contains attractive fluvial and aeolian sandstone reservoirs in the basins between Ireland and Britain (Morecambe Bay gas field, 6.5TCF) and in southern England (Wytch Farm oil field, 300 MMBBL). While the SSG is very thick in the northern portions of the Mesozoic basin system, it is thinner and generally less well preserved in the southern portions of the Mesozoic basin system which are underlain by the Carboniferous aged Variscan fold-thrust belt. Previously it has been suggested that the SSG was deposited during a time of rifting, and consequently its variable presence was attributed to wells penetrating positions within fault blocks that were high and devoid of syn-rift deposition. Data examined in this study from the East Irish Sea Basin and from the South Celtic Sea Basin, however, suggest that the SSG was deposited in palaeotopographic lows, the positions of which were controlled by erosion around massifs and resistant thrust belt hangingwalls that formed the palaeotopography. Furthermore, the evidence presented suggests that rifting occurred during the deposition of the Upper and Middle Triassic Mercia Mudstone Group (MMG), which is often defined as a post-rift succession.

Successful prediction of the areas of SSG deposition depends on the ability to reconstruct the Early Triassic palaeotopography as it was controlled by the Carboniferous thrust belt morphology. A reconstruction of the Variscan thrust belt is presented, which is based on onshore surface geology, thrusts and transfer faults from published onshore mapping, published offshore thrust plane reflector mapping, gravity data showing lineaments and indications of granite plutons, and a set of observations from well and seismic data. Some thrusts are postulated by observations of reactivation during Mesozoic extension.

A set of predictive tools has been developed for identifying the presence of the SSG. Determination of the presence of the SSG on seismic reflection data is often difficult. Where the data quality is poor, we postulate that there is a better chance of finding the SSG where halites are found in the overlying MMG. This empirical relationship is believed to occur because rifts developed over SSG foreland and intermontane valleys were near sea-level, allowing marine incursions and salt precipitation. By contrast, rifts that developed in high topographic areas were above sea-level and not subject to marine incursion. Additionally, the degree of deformation seen in the pre-Triassic section on seismic data can be used to predict the position of the Variscan Front and thus the topographically low, sand-prone areas overlying the Carboniferous foreland basin. A palaeogeographic map is presented, incorporating the analysis of all of these datasets and utilizing a comparison to the topography of the exposed Ouachita Mountains of the southern USA.
Sedimentology and diagenesis of the Jurassic and Cretaceous of the North Celtic Sea and Fastnet Basins
Authors: Ewins Nuala P. , Shannon Patrick M.
Published: April 2008

The North Celtic Sea and Fastnet Basins both contain a relatively complete Jurassic and Cretaceous section. These sediments were examined in order to gain an insight into the nature and extent of sedimentological and diagenetic processes which affected the basins. Significantly variable depositional environments existed in the basins through time. The Early and Middle Jurassic were dominated by low-energy marine conditions, below storm wave-base, with higher-energy storm events locally depositing coarse-grained sediments. The Upper Jurassic succession contains sediments deposited in marine, marginal marine and continental environments, with significant quantities of coarse-grained material. Lower Cretaceous sediments were initially characteristic of marginal marine to continental conditions but increasing marine influence resulted in deposition of fully marine sediments. Three sequences were examined petrographically, concentrating on those considered to have potential for reservoir development. In general, porosity and permeability appear to be principally related to primary depositional factors, or to the control of these primary factors on the subsequent diagenetic evolution. Quartz cementation appears to have been the most destructive in terms of porosity occlusion, but locally, calcite cements are significant. Significant reservoir potential is considered to remain in the three sequences examined in the area, with good sandstone development postulated in undrilled areas.
Toarcian source rock potential in the North Celtic Sea Basin, offshore Ireland
Authors: Murphy N. J. , Sauer M. J. , Armstrong J. P.
Published: April 2008

Conditions favourable to the development and preservation of rich oil-prone source rocks are demonstrated for the uppermost Lower Jurassic stage, the Toarcian, in the Irish Sector of the North Celtic Sea Basin (and adjacent South Celtic Sea Basin), located offshore to the south of Ireland. An integrated approach, involving geological and geochemical data and techniques, has permitted the quantification of source rock potential. Wireline log responses in conjunction with existing geochemical analyses have defined net thickness, quality and richness for potential source beds. Up to 450 ft of very good to rich source rocks, capable of generating liquid hydrocarbons, are calculated for marine claystones of Early and Middle-Late Toarcian age. The thickest and best quality source rocks are predicted to occur in Toarcian depocentres, mostly situated along the axis of the North Celtic Sea Basin. One-dimensional burial history modelling suggests that substantial quantities of hydrocarbons were expelled from basinal areas towards the end of the Cretaceous.
The Helvick oil accumulation, Block 49/9, North Celtic Sea Basin
Authors: Caston V. N. D.
Published: April 2008

The Helvick oil accumulation in Block 49/9 is situated within the northern margin of the North Celtic Sea Basin. It was discovered by well 49/9-2, drilled in 1983, which tested a cumulative total of 9901 BOPD and 7.44 MMSCFD from four reservoir intervals. These consist of marine limestone of Bathonian age, and an overlying series of Middle to Upper Jurassic age sandstones deposited in a continental/fluviatile environment. The principal ‘Main Sand’ reservoir, which flowed at a rate of 6467 BOPD, lies close to the base of the non-marine sequence and was deposited by a high-energy, braided fluvial system. It is laterally extensive and may be correlated for a minimum east-west distance of 6.5 km. Oil in the Main Sand is 44.06° API with a GOR of 680 SCF/BBL; it is derived from a predominantly herbaceous, terrestrial source which is identified, on regional evidence, as being most likely of Liassic age. The oil-water contact has not been positively established. Calculations based upon RFT and DST pressure measurements give a range of possible oil-water contacts, and hence a corresponding range of oil-in-place values. These range from 8.4 to 12.3 MMBO, with a ‘most likely’ figure of 10.7 MMBO. Reservoir simulation studies indicate potential recoverable reserves in the Main Sand of 2.27–4.42 MMBO. An additional ‘most likely’ figure of
c.
3 MMBO oil-in-place may be present within the underlying Bathonian Limestone, of which
c.
0.6 MMBO may be recoverable. Further, as yet unquantified, reserves are present in the other Upper Jurassic oil-bearing sandstones.
Westline: a deep near-normal incidence reflection profile across the Rockall Trough
Authors: England Richard W.
Published: April 2008

Westline is a 450 km long, near-normal incidence, deep seismic reflection profile crossing the Rockall Trough. The profile was shot perpendicular to the edges of the trough in order to image its conjugate margins. An interpretation of the brute stack of the normal incidence data, partly constrained by existing near-normal incidence and wide angle-data, reveals that the trough is asymmetric, with a broad faulted margin on its western edge and a steep faulted margin where it meets the Irish Shelf. A sill complex extends the whole width of the trough within the 5 km thick sedimentary infill.
Basin development and petroleum prospectivity of the Rockall and Hatton region
Authors: Shannon P. M. , Jacob A. W. B. , Makris J. , O’Reilly B. , Hauser F. , Vogt U.
Published: April 2008

The Rockall region, offshore west of Ireland, contains a number of large, virtually unexplored, frontier sedimentary basins: Rockall Trough, Hatton Basin and Hatton Continental Margin. It is suggested that the sedimentary section, up to 6 km thick, is of Late Palaeozoic to Tertiary age. Normal incidence reflection seismic data from the region are typically of poor quality owing to the effects of shallow Tertiary sills. Recent wide-angle reflection seismic profiles have helped to resolve the basin geometry and structural setting. Backstripping (reverse modelling) and forward modelling of the wide-angle model are consistent with development in a complex multi-phase rift setting. Rift events are suggested in the Jurassic and Cretaceous, and possibly locally during the Permo-Triassic. The variation in structural style across the region is interpreted as resulting from the interplay of intraplate stresses, magmatic underplating, thermal doming and thermal subsidence adjacent to the continent/ocean margin, and seafloor spreading dynamics in the region to the west. The basins are predicted to contain a range of petroleum plays. Structural, stratigraphic and combination traps are indicated from a range of data. Reservoirs are predicted at several levels, with source rocks in the Jurassic and lowermost Cretaceous.
Characterization of deep marine clastic systems
Authors: Hartley Adrian , Prosser Jeremy
Published: April 2008

Sandstones deposited in deep marine environments form important hydrocarbon reservoirs in many basins throughout the world. However, despite the plethora of outcrop studies and the development of numerous submarine fan models and classification schemes, very few applied studies at a reservoir scale have been published.

This publication has arisen from the perceived needs of the academic and industrial communities to understand the controls on the architecture and geometry of deep marine clastic reservoirs. A number of areas of concern have been addressed: (1) Are conceptual models applicable to understanding sandstone body development and distribution at a reservoir scale? (2) Do we understand the processes that are active in the formation of deep marine clastic systems and the likely influence of these processes on reservoir quality? (3) How do we correlate and at what scale do correlation mechanisms work within and between deep marine clastic reservoirs? (4) How can we quantify heterogeneity and reservoir quality within these reservoirs?
Experimental study of subaqueous fan development
Authors: Ouchi Shunji , Ethridge Frank G. , James Edward W. , Schumm S. A.
Published: April 2008

Subaqueous fan-shaped depositional forms were developed during flume experiments, that were conducted to investigate processes of underwater deposition. The flume was filled with fresh water, and clay plus salt water was introduced as a density flow at the head of the underwater slope. A lobe-shaped sandbody (sand lobe) migrated downslope, as a liquified flow, when sand was mixed into the clay plus salt water flow. In a steep-slope experiment (37°) sand lobes were deposited at the toe of the slope and overlapped each other to form a fan-shaped body. Without sand, clay plus salt water flows of small discharge, which showed characteristics of laminar flow, tended to incise a channel on the sandy fan surface. The larger discharge flows, in which some turbulence developed, however, spread into the overlying water body and did not cut a channel. Clay moved further and was deposited on the basin-floor. Sand lobes occasionally migrated on to the basin-floor clays, especially when an incised channel developed on the fan surface. When sand lobes reached the basin-floor clay deposits, movement of the sand accelerated basinward, the sand lobe slid on the clay like the floating plug of a debris flow, and formed crescent-shaped mounds and hummocky topography. The accelerated movement of the distal portion of the sand lobes appeared to cause them to become detached from the main sandbody. With gentler slopes (18°), clay deposition occurred on the slope. Sand moved in the form of an elongate lobe, and the fan itself became more elongate than the one developed on the steeper slope. The elongate nature of the fan deposit on the low slope was apparently related to the higher efficiency of sand movement on the clay that settled on this slope.

Subaqueous fans, on both slopes, collapsed and generated relatively large subaqueous debris flows when the water in the flume was being drained. The failure started on the surface with a fluidized flow, and the entire fan slumped basinward. A reduction of hydrostatic pressure caused by the lowering of the water levels apparently generated a pore-pressure gradient in the deposits. The fan slopes became unstable and pore water probably flowed upward as the pressure gradient developed. This pore-water flow induced the fluidized flow on the surface and then the slump of the entire fan.

The experimental subaqueous fans cannot be compared quantitatively with natural fans because they are not scale models. The general similarity of process, however, might provide insights into depositional and erosional processes in natural systems.
Mounded shallow gas sands from the Quaternary of the North Sea: analogues for the formation of sand mounds in deep water Tertiary sediments?
Authors: Brooke Candace M. , Trimble Tim J. , Mackay Tom A.
Published: April 2008

Shallow gas-bearing sand mounds have been clearly defined by a recent three-dimensional seismic survey and are believed to result from large-scale remobilization of conventionally deposited sand. This concept has been invoked by recent workers to explain the formation of mounded sand geometries in Tertiary turbidite reservoirs of the Forth-Gryphon area northern North Sea. However, the idea has been slow to gain acceptance due to the unconventional mechanism and lack of analogues.

In the Fisher Bank Basin area of the central North Sea unusual mounded gas anomalies have been recognized in the shallow subsurface at depths between 450 and 550 m subsea, most commonly occurring above deeply buried highs. Originally mapped as linear features on two-dimensional seismic data, three-dimensional seismic has revealed a circular to ovoid mounded geometry. The identification of fault zones extending up to this mounded horizon from deep in the section in an area where Upper Jurassic source rocks are gas mature may suggest a possible mechanism for mound formation. We propose here that gas has migrated to shallow levels along fault conduits into a conventionally deposited sand, and that this has resulted in sediment liquification. The movement of gas results in entrainment of sand grains towards subtle highs in the upper surface. Post-depositional processes thus result in the creation of a mound by withdrawal of sand from the flank areas and redeposition towards the centre of the feature.
Permeability heterogeneity within massive Jurassic submarine fan sandstones from the Miller Field, northern North Sea, UK
Authors: Prosser D. J. , McKeever M. E. , Hogg A. J. C. , Hurst A.
Published: April 2008

Analysis of high density (5 cm sample spacing) probe permeameter data has been used to evaluate some of the basic controls on permeability heterogeneity in the Miller Field. Probe permeameter data generally show good correspondence with conventional core-plug permeability data gathered at 25 cm spacings, but record a higher frequency of permeability variation than can be resolved using conventional sampling strategies. Comparison of probe permeameter data gathered from four lithofacies recognizable in core has allowed assessment of their reservoir quality and quantification of their internal permeability heterogeneity. Geostatistical analysis of the permeability data using semivariograms suggests permeability correlation structures and periodicities are related to the frequency of sedimentary phenomena visible within core. In particular, a relationship exists between permeability correlation structure and average bedform or fining upward unit thickness, i.e. the vertical scale at which individual lithofacies packages are stacked. Geostatistical analysis of high sample-density probe permeameter data demonstrates the usefulness of permeability data gathered at more than the conventional 25 cm spacings used for core plugs. High density probe permeameter data can yield important measures of the representative geological ‘structures’ which could form the basis of geologically realistic two-phase flow upscaling.
Crustal growth, surface processes, and atmospheric evolution on the early Earth
Authors: Eriksson Kenneth A.
Published: April 2008

Evolution of the atmosphere between 3.9 and 1.8 Ga can be related to changes in physical, chemical and biological surface processes. Changes in surface processes, in turn, took place in response to growth of continents. Growth of stable continents favoured expansion of epeiric seas and resultant increase in oxygen productivity. Prior to 2.6 Ga, continents probably comprised less than 10% of the Earth’s crust and, as a consequence, physical sedimentation and volcanism, and chemical and biological sedimentation took place mainly in oceanic and island-arc settings. Following major crustal growth at 2.6 Ga, surface processes became dominated by widespread stromatolite growth and iron and manganese precipitation in vast epicontinental seas.

Prior to 2.6 Ga, oxygen was consumed mainly in ocean-crust alteration. Following growth of large continents, the ocean crust oxygen reservoir decreased in scale freeing oxygen for the second major reservoir, namely reduced iron and manganese. Only after oceanic sinks were utilized did oxygen escape to the atmosphere. This was taking place on a large scale by 1.8 Ga at which time the atmosphere had an oxidation state necessary for formation of continental red beds and palaeosols.