Celtic Sea Basin Research Articles

Sedimentology and diagenesis of the Jurassic and Cretaceous of the North Celtic Sea and Fastnet Basins

Abstract 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

Abstract 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.

Evolution of the Cockburn Basin: implications for the structural development of the Celtic Sea basins

Abstract The Cockburn Basin is one of a series of sub-parallel basins in the Celtic Sea-Western Approaches area to the south and southwest of Ireland. These basins share broadly similar structural and depositional histories, and are regarded as the remnants of a regionally extensive Triassic-Middle Jurassic basin which was dissected by Late Jurassic-Early Cretaceous tectonics. During the late Paleogene the basin was offset dextrally by 4–5 km across the Cockburn Fault, part of a major Tertiary wrench zone. Displacement on the Cockburn Fault is entirely Tertiary in age; however, the adjacent Fastnet segment of the Goban-Cockburn Fault Zone appears to have exerted a degree of control on the pre-Aptian development of the Fastnet Basin. At least one major basement structure, the Scilly Wrench Zone, appears to have entirely avoided reactivation during the post-Variscan development of the Western Approaches Trough. It would appear that reactivation of basement structures in the Celtic Sea-Western Approaches area has occurred in a patchy and unpredictable manner and, although pre-existing faults locally exert considerable influence, there is no fundamental basement control of the evolution of sedimentary basins in the region.

New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance

Abstract 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 Helvick oil accumulation, Block 49/9, North Celtic Sea Basin

Abstract 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.

Tectono-stratigraphy of the North Celtic Sea Basin

Abstract 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 Variscan fold-thrust belt of southeast Ireland and its control on early Mesozoic extension and deposition: a method to predict the Sherwood Sandstone

Abstract 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.

The definition of the Albian ‘A’ Sand reservoir fairway and aspects of associated gas accumulations in the North Celtic Sea Basin

Abstract 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.

Tertiary uplift and inversion history in the North Celtic Sea Basin and its influence on source rock maturity

Abstract Tertiary uplift and basin inversion have been widely recognized in the area of the North Celtic Sea Basin offshore Ireland, but the magnitude and timing of the various events have not been well documented. An understanding of these events is important because recent basin history generally has a major influence on hydrocarbon source rock maturity levels. A multi-disciplinary approach was used to investigate the timing and nature of uplift in the North Celtic Sea Basin, involving seismic data interpretation, well log correlation, vitrinite reflectance measurement, apatite fission track analysis and a Cretaceous Chalk Group isopach and velocity study. Both regional uplift and compressive inversion can be interpreted to have occurred, based on missing geological section, compressive reactivation of earlier normal faults and basin-centred uplift. Two Tertiary erosion events are recognized: a regional uplift in the Palaeocene, and compressive inversion characterized by basin doming and fault reversal, probably associated with an Oligo-Miocene phase of the Alpine Orogeny. Separating these was a period of basinal deposition in the Eocene to Oligocene. A map of the magnitude of erosion generated as a result of the two events shows that the amount of uplift is quite variable. In the northeast part of the basin, where the later event was most severe, uplift was as much as 1100 m. Reconstruction of the basin geometry before inversion, together with thermal modelling at selected well locations, suggests that Upper Jurassic source rocks reached maturity for oil generation only in the central portion of the basin, whereas Lower Jurassic source rocks entered the gas window in much of the basin prior to Tertiary uplift.

Sedimentology of the Cretaceous Greensand, Quadrants 48 and 49, North Celtic Sea Basin: a progradational shoreface deposit

Abstract 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

Abstract 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.

Quantification of Tertiary exhumation from sonic velocity data, Celtic Sea/South-Western Approaches

Abstract Sonic velocities from the Danian Chalk, the Upper Cretaceous Chalk, the Lower Cretaceous Greensand/Gault Clay, and the Triassic Mercia Mudstone were used to quantify apparent exhumation (i.e. amount of missing section) in the Celtic Sea/South-Western Approaches (southern Irish, south-western United Kingdom, and north-western French continental shelves). Analysis of all four of these stratigraphic units yield similar results. Since it is unlikely that sedimentological and/or diagenetic processes are responsible for similar amounts of overcompaction in the chalks, shaly sandstones and shales analysed, burial at depth greater than that currently observed is the most likely cause of overcompaction (anomalously high velocities). The use of lithologies other than shales in maximum burial-depth/exhumation studies is thus validated. The consistency of results from units of Triassic to Danian age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that the maximum Mesozoic-Cenozoic burial-depth in the Celtic Sea/South-Western Approaches was attained prior to Tertiary exhumation. The recognized inversion axes of the Brittany Basin, South-West Channel Basin and North Celtic Sea Basin are highs in apparent exhumation, however highs in apparent exhumation of approximately 1 km also occur in the uninverted St. Mary’s and Plymouth Bay Basins, and on the margins of the Armorican and Cornubian Platforms and the Pembroke Ridge. Tertiary exhumation was regional and not restricted to inversion axes, and thus had a thick-skinned origin. Heterogeneous lithospheric compression is the geodynamic model preferred to account for the regional exhumation of the Celtic Sea/South-Western Approaches. The identification of regional exhumation of approximately 1 km has important implications for modelling thermal maturation in the Celtic Sea/South-Western Approaches, with source rocks previously believed to be immature shown to be potentially mature for oil generation. When corrected for exhumation, the post-rift sequence in the Celtic Sea/South-Western Approaches area shows relatively little localization over the syn-rift Jurassic-Lower Cretaceous sequence, suggesting that during basin formation, the lower lithosphere, thermal re-equilibration of which controls post-rift subsidence, did not experience localized thinning on the scale of the syn-rift basins.

Correlation of Upper Jurassic-Lower Cretaceous continental sequences from the southern Biscay margin, northern Spain

Biostratigraphical data indicate that fresh-water carbonate deposits from the southern part of the the Cantabrian Basin, northern Spain are of earliest Cretaceous (Berriasian) age. Charophyte assemblages enable correlation with fresh-water carbonate rocks of the western Cameros Basin to the south. The charophytes also support correlation with Berriasian marginal marine carbonate sequences in the Jura Mountains, southern England and northern Germany, indicating that these continental carbonate rocks represent fresh-water equivalents of the 'Purbeckian' facies. The biostratigraphical correlation is enhanced by the parallel facies evolution observed in the Upper Jurassic-Berriasian of the Cameros and Cantabrian basins. Interpretation of these successions as non-marine depositional sequences permits member-scale correlation between the two basins. Further strong facies similarities in the overlying alluvial clastic rocks of the two areas suggest a revised correlation of the overlying Valanginian-Albian deposits. The two basins were connected at least intermittently during the Late Jurassic-Early Cretaceous; only during periods of higher sea-level did facies differences develop as a result of greater marine influence in the north. At other times, vast alluvial, lake and marsh systems extended over the region, leading to the deposition of genetically-related alluvial and lacustrine or palustrine deposits in these linked half-graben systems. In Cantabria and the Cameros Basin, strong lateral variations in facies, thickness and stratigraphy reflect erosion at the onset of Late Jurassic rifting, differential subsidence in the Early Cretaceous, and erosion beneath Aptian-Albian unconformities. Improved understanding of the successions described here may assist interpretation of similar Upper Jurassic-Lower Cretaceous sequences on the northern margin of the Biscay rift, such as those in the Wessex and North Celtic Sea basins.

Subsidence analysis in the Paris Basin: a key to Northwest European intracontinental basins?

AbstractDue to the frequently observed disparities between stretching amounts obtained from faults, crustal thickness and tectonic subsidence, the development of intracratonic basins cannot always be explained by a simple model of lithospheric extension. Basin evolution may then be regarded as the result of superimposed and successive processes.The Paris Basin is chosen as a type example for the discussion of European intracratonic basins. The tectonic subsidence, reconstructed using a standard method, is small (maximum of 1600 m, mean rate of 8.5 mm/ka). The long‐term linear or concave curves are interrupted by periods of short‐term acceleration and deceleration. Thus, tectonic subsidence is clearly discontinuous and five phases, with duration of 20–60 Myr, constitute the long‐term Meso‐Cenozoic Subsidence. The boundaries and pattern of each phase are identical all over the basin. Subsidence is interpreted as the result of several modifications of the lithospheric structure. The uniform stretching model can explain the first Triassic to early 1, iassic phase, the only one with a concave‐upward trend (deceleration). The Jurassic to Cretaceous subsidence could be explained by superposing (1) a long‐term component caused by lower crustal flow and/or underplating and (2) several short‐term accelerations (convex‐upward trend) related to compressive or transpressive forces. Geophysical control is insufficient to test the first postulate accurately and the generation of sufficiently high compressional stresses during the Jurassic‐Cretaceous is questionable for the second.Other Northwest European basins are compared with the Paris Basin. Although similar features can be observed, the overall image is not uniform: ‘Paris Basin’ (intracratonic) as well as ‘Celtic Sea Basin’ (passive margin) signatures, variable long‐term trends, lack of synchronism of subsidence phases. This picture necessitates different driving mechanisms for subsidence across Northwest Europe. The variable subsidence patterns and processes result from the ‘remote’ geodynamics of the Atlantic and Tethyan realms, combined with mainly ‘active’ processes similar to those proposed for the Paris Basin.

Late Mesozoic reactivation of Variscan faults in the North Celtic Sea Basin, Ireland

Three major ENE trending reactivated fault systems downthrow to the north-west in the north-western part of the North Celtic Sea Basin. They control the location of three Mesozoic half-grabens. The location of the faults is determined by the underlying basement structures. These are Variscan in age although they exploit older Caledonide trends. Comparison of the Variscan structures identified in the area with regional structures suggests a series of imbricate thrust features. These structures, reactivated during the Mesozoic, controlled both the location and depositional histories of the half-graben basins in the region.

Shelf Sheet-Sand Reservoir of the Lower Cretaceous Greensand, North Celtic Sea Basin, Offshore Ireland

Core and log data show that the marine, early to middle Albian A Sand of the Aptian to lower Cenomanian Greensand-Gault interval, North Celtic Sea Basin, offshore Ireland, was deposited as an approximately tabular sand body in shelf water depths. The A Sand is the major reservoir interval at Kinsale Head and Ballycotton gas fields. The reservoir sandstone is bioturbated, variably glauconitic, shell rich, and least muddy toward its provenance in a local area of the Irish massif and finer grained southeastward into the basin. Thickness and coarseness of the A Sand are related, in part, to distance from a narrow area of the paleoshore. Bathymetric control of sand thickness was superimposed on proximal-distal trends. Specifically, thick intervals are inferred to have be n deposited in shelf lows, and thin zones were deposited over bathymetric highs. The A Sand was not deposited as a ridge sand, and positive relief on the depositional sand body appears to have been minor. Deposition of the Greensand occurred during thermal subsidence of the North Celtic Sea Basin following Early Cretaceous rifting. Overall late Early Cretaceous to Late Cretaceous transgression was interrupted by progradation of the A Sand, probably caused by a relative drop in sea level (forced regression). The A Sand sheet consists of several units within the central depositional basin. Three slightly coarsening-upward units up to 15 m thick probably are the consequence of high-frequency fluctuations in sea level during progradation. The top of the A Sand consists of a several-m ter-thick, very glauconitic, muddy sandstone to sandy mudstone. The upper unit accumulated in deepening water following the maximum lowstand.

Lower Cretaceous seismic stratigraphy and fault movement in the Celtic Sea Basin, Ireland

The Celtic Sea region encompasses a series of basins and sub-basins lying off the south coast of Ireland, and extending into UK-designated areas (Fig. I). The major basins are the North and South Celtic Sea basins, which are separated by a long-standing basement high, the Fastnet Basin and the Cockburn Basin. A number of recent publications have outlined the regional geological development of the area (Naylor and Shannon 1982; Tucker and Arter 1987; van Hoorn 1987; Ziegier 1987; Shannon 1991a, b, c). The present study is confined to a small area located on the westernmost limb of the North Celtic Sea Basin, and north of the Fastnet Basin (Fig. I).

Apatite fission track data from southeast Ireland: implications for post-Variscan burial history

AbstractThe results of a reconnaissance apatite fission track study from southeast Ireland are compared with modelled ages and length distributions predicted using both traditional thermal histories and the thermal history indicated by the fission track age and length data. All samples have experienced total annealing during the Variscan orogeny. There is strong evidence for significant post-Variscan, pre-Mid Jurassic heating and cooling. The modelling also recognizes a third thermal event subsequent to 100 Ma. The implications of these results are examined in terms of the timing and extent of subsidence and inversion along the northern margin of the North Celtic Sea Basin.

Triassic prospectivity in the Celtic Sea, Ireland - a case history

The Celtic Sea region south of Ireland contains Mesozoic sedimentary basins with an ENE trend. The North Celtic Sea, St George's Channel and Fastnet basins lie north of the Pembrokeshirè-Labadie Bank ridge complex (herafter referred to as the Pembrokeshire Ridge), with the South Celtic Sea, Bristol Channel and Cockburn basins to the south (Fig. 1). The Channel and Western Approaches basins parallel the Celtic Sea basins and are separated from them by the Cornubian Massif. East of the massif the Channel Basin sweeps northwards onto the English mainland as the Wessex Basin.

Crustal extension, subsidence and inversion in NW Europe

Examination of a regional grid of deep and commercial seismic data extending from Cardigan Bay (offshore Wales) to the south of the Paris Basin (Aquitaine–Provence–French Alps) has demonstrated that the deep Mesozoic sedimentary basins along this transect are underlain by low-angle crustal detachments. Such features are interpreted to be Variscan thrusts, reactivated as normal faults during Mesozoic crustal extension; they rise to surface in basement rocks surrounding Mesozoic basins, and can be matched to outcrop structures in S. Wales, S. Ireland, Cornwall, Devon and Brittany. Inversion of the NW European Mesozoic basins occurred in the Cenozoic, but was preceded by minor inversion and uplift of surrounding massifs at various times in the Mesozoic. Such uplift can be demonstrated to have occurred via crustal shortening and re-thrusting along Variscan thrusts at depth. The uplift of basins above such thrusts is concomitant with continued subsidence of basins in the foreland, and can explain the juxtaposition of over-thick sedimentary successions against basins with a predominant unconformity of similar age. In the Celtic Sea area (offshore SE Eire), the North Celtic Sea Basin contains over 2000 m of late Jurassic–early Cretaceous sediments, whilst the South Celtic Sea Basin has virtually no late Jurassic or early Cretaceous preserved: a prominent Aptian–Albian unconformity overlies early–mid-Jurassic sediments. The two basins are separated by the Pembroke Ridge, an ancient ‘high’ where reactivated Variscan thrusts are thought to crop out (Coward and Trudgill 1989). The Carboniferous–Permian–Triassic succession of the Plymouth Bay Basin (Western Approaches Trough) is another example of an ‘overthickened’ sedimentary succession . . .