Celtic Sea Basin Research Articles

Deep structure of southern Ireland: a new geological synthesis using BIRPS deep reflection profiling

The BIRPS deep seismic reflection data of WIRE (West of IREland) 1/1B from Galway Bay to the Celtic Sea are integrated with geological and other geophysical data to produce a synthesis of the fundamental tectonic character of the crust in and around southern Ireland. The Iapetus Suture forms a zone of north-dipping reflectors which projects up between the Shannon estuary and the Dingle peninsula and is overlain by a late Caledonian reflector with a shallow, south dip. Across the Iapetus Suture zone the crust thickens southward and the lower crust becomes less reflective. It is proposed that the Lower Palaeozoic volcano-sedimentary rocks which form the Leinster massif extend southwestward beneath the Variscan province and at least 30 km offshore to the WIRE lines. We argue that the weak south-dipping reflectors in the upper and middle crust south of the Iapetus Suture represent an early-Caledonian fault system linked to a mid-crustal detachment which formed during the south-dipping subduction event recorded onshore by the Ordovician volcano-sedimentary successions. These faults were selectively reactivated causing the predominant ENE trend of younger regional tectonic features in and around southern Ireland. The three onshore Upper Palaeozoic basins close east of the WIRE lines. We point out that the ‘Variscan Front’ is only a map line across the British Isles with no consistent tectonic character. Thus the south-dipping reflector below the north margin of the North Celtic Sea Basin on SWAT 4, generally referred to as the ‘Variscan Front’, is correlated by us with the northern boundary of the Rosslare complex, thus making it the northern tectonic boundary of the Caledonian basement block known as the Rosslare-Monian Terrane. Furthermore the north-dipping South Irish Sea Lineament on WINCH 4 can be correlated with the Menai Straits Lineament and could thus be the southern boundary of this terrane. The south-dipping reflector on SWAT 2, also previously called the ‘Variscan Front’ is correlated with the Johnston-Benton fault system in southwest Wales.

Basement tectonics and their relationship to Mesozoic megasequences in the Celtic Seas and Bristol Channel area

Abstract A synthesis of new and pre-existing data on the deep geological structure of the Celtic Sea and Bristol Channel basins shows similarities to published models of deep structure from the onshore Wessex Basin to the east. A series of Variscan thrust planes, reactivated in the Mesozoic as normal faults, underlie the study area. Beneath the Bristol Channel, one such southward-dipping thrust may be equated with the ‘Variscan Front’. This structure is offset by NW-SE orientated transcurrent faults, structures thought to be active from the late Palaeozoic to the present-day. Mesozoic stratigraphic successions from the North Celtic Sea Basin and South Celtic Sea; Bristol Channel, and northern Wessex Basins suggest uplift and erosion of the latter areas in the late Jurassic. This is contrasted with increased subsidence at the same time in the North Celtic Sea. This uplift may have been facilitated by reactivation of the ‘Variscan Front’ thrust as a thrust in the east of the area, and by similar thrusts beneath the Pembrokeshire Ridge to the west. This reactivation and uplift during North Atlantic opening is suggested to have controlled the preservation of Upper Jurassic-Lower Cretaceous sedimentary megasequences.

Late Triassic to Middle Jurassic stratigraphy, palaeogeography and tectonics west of the British Isles

Abstract The Upper Triassic to Middle Jurassic stratigraphy of the Atlantic margin basins west of the British Isles (including Celtic Sea, Porcupine, Slyne, Erris, Hebrides, and west of Shetland) is summarized in terms of six major genetic sequences, defined as packages of conformable strata separated by surfaces of abrupt major facies change. They indicate common tectonic histories and are interpreted as representing episodes of evolution comprising extensional phases with laterally variable sequences (1A Late Triassic to Early Sinemurian; 2A latest Toarcian to Late Bajocian), thermal and loading sag phases (1B mid-Sinemurian to earliest Toarcian; 2B Late Bajocian to Bathonian) and stabilization phases with sequences of reduced thicknesses and hiatuses (1C Toarcian; 2C Late Bathonian to Callovian). The basins west of Shetlands, north of the Wyville-Thomson Ridge, have a different history, especially in the Early Jurassic. Similarly, basins south of an extension of the Clare Lineament (south of Celtic Sea and Porcupine Basins) differ in having limited Middle Jurassic subsidence. These transverse elements divide the Atlantic basin system into local subsystems, each of which has a different tectonic history.

Structure of the Variscan belt beneath the British and Armorican overstep sequences

Interpretation of gravity and magnetic data from southern England and of deep seismic data from the English Channel, Celtic Sea, and Paris basin provides a new picture of the buried Hercynian belt between France and England. The main structure corresponds to a mobile belt of crustal thickening that developed between the Midlands block, a northern block that had detached from the Armorican block prior to Hercynian time, and the Armorican block. The frontal collision between the two blocks recorded during the late Carboniferous was responsible for a pronounced narrowing of the belt between the Isle of Wight and Normandy. During collision, structural wedges were ejected in the channel and possibly also in the Hercynian basement beneath the Paris basin.

Structural and Stratigraphic Evolution of the Celtic Sea Basins, Offshore Ireland

Structural and Stratigraphic Evolution of the Celtic Sea Basins, Offshore Ireland

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

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

Stratigraphy of the Triassic, Lower Jurassic and Middle Jurassic (Aalenian) from the Fastnet Basin, Offshore South-west Ireland

The stratigraphy of the Triassic, Lower Jurassic and Middle Jurassic (Aalenian) of the Fastnet Basin is described. The sequence has been divided using wireline log criteria. Six rock units are recognized in the Triassic and 12 in the Lower Jurassic and Middle Jurassic (Aalenian). None of these major rock units shows any significant diachroneity. Apparent breaks in the sequence due to unconformities or condensed sections have been identified within or bounding three of the rock units: the Liassic Limestone, Liassic Marl and Liassic Sandstone. Dating is provided by both microfaunas and microfloras extracted from ditch cuttings and sidewall cores. The ages of the Triassic rock units are mainly determined by correlation with dateable sequences in the North Celtic Sea Basin, whereas the Jurassic rock units yielded abundant short-ranging microfossils.

Deep crustal features of the Celtic Sea from complementary processing on the SWAT data

Complementary processing develop by Bano (1989), including automatic extraction of reflections and migration of deep seismic data, are applied to the SWAT (South West Approaches Traverse) deep seismic profiles in the Celtic Sea. The automatic extraction of reflection gives a more reliable definition of the basement under the deep Celtic Sea Basins. Using migration, we derive a more accurate geometry of these basins

The development of Irish offshore sedimentary basins

A variety of ensialic Mesozoic and Cenozoic sedimentary basins surround Ireland. Three main periods of basin development occurred. The first, in the Permo-Triassic, produced a series of small rift basins which nucleated along reactivated, generally Caledonian, structures. The second, in the Mid- and Late Jurassic, represents a period of crustal extension and rifting. The third phase reflects thermal subsidence of Cretaceous and Tertiary age, and coincides with seafloor spreading in the North Atlantic. The basins developed in response to general east-west extension. Simple extensional structures, accompanied by crustal thinning, dominated in the north-south-trending Porcupine Basin. The NE-SW-oriented basins in the Celtic Sea, Slyne-Erris-Donegal and Rockall areas developed in a transtensional setting until Early Cretaceous times. The Goban Spur basins, at the confluence of regional structural trends, and adjacent to the continent-ocean boundary, are dominated by transtensional structures. The effects of Early Tertiary Alpine compression are seen especially in the Celtic Sea basins. The basins, whilst having some broad geological similarities which reflect eustatic sea-level variations, differ in their detailed evolution. Each basin experienced unique facets of local structural control, tectonic history and facies distribution.

Seismic studies in the North Celtic Sea Basin: implications for basin development

Modelling of seismic refraction data from the North Celtic Sea Basin offshore Ireland demonstrates the presence of a NNW-trending basement flexure. Mesozoic sediments are 4 km thick on the west and 6 km thick on the east of the feature, which is Interpreted as a transfer fault zone lying along a major reactivated basement fracture. Bulk extension is greater in the lower lithosphere than in the upper crust, with the interface being a rheologically regulated mid-crustal detachment surface. The basin developed in response to the interplay between periodic disengagement along the detachment and selective strain transfer into the upper crust.

Ostracod colour change by thermal alteration, offshore Ireland and Western UK

Ostracod colour changes, from white to amber to red-brown to black, are principally interpreted to be the result of thermal alteration. The changes in colour are thought to be caused by progressive carbon fixation of hydrocarbons derived from the surrounding sediments which invaded the ostracod shell. Exceptions to this scheme include faunas from organically poor limestones which remain white irrespective of the amount of heating, and grey-brown or yellow-brown specimens coloured by the oxidation of diffuse pyrite. Thirty-four exploration wells have been examined from the Irish and Western UK sectors, and these have shown a progressive darkening of colours which is correlated with both burial depth and distance from igneous intrusions. The Fastnet Basin and southern region of the North Celtic Sea exhibit higher colour change/depth ratios to the Porcupine Basin and northern region of the North Celtic Sea Basin, implying greater palaeoheat flow in the former basins.

Crustal structure of the Celtic Sea and western approaches from gravity data and deep seismic profiles: Constraints on the formation of continental basins

A free‐air anomaly map of the Celtic Sea and the Western Approaches is presented including all available marine gravimetric data and Bouguer anomalies on land. N60° features are predominant in the area. They correspond to the N60° so‐called Caledonian grain which played a major role in the Variscan orogeny and in the formation of the Permo‐Triassic sedimentary basins. The fundamental new point which comes out from this gravimetric map is the existence of a closely spaced pattern of N105° features which we regard as late Carboniferous transpressional structures. In the eastern pan of the Celtic Sea, thrusts are in the same N105° direction. West of 6°W longitude, however, folds and thrusts trend preferentially in a N60° direction, following the older grain. There, the N105° lineaments may be regarded as dextral shear structures with a possible compressive component. The Hercynian shortening of the Celtic Sea domain, located between the Variscan Front and the Lizard suture, resulted from the displacement, along the N160° direction of the two adjacent rigid domains. North of the Variscan Front, the N105° and N135° features were possibly active in more recent times. Then, both the N60° and N105° trends controlled the formation of Permo‐Triassic basins, the N105° directions being transfer faults. Two‐dimensional gravity modelling of the SWAT profiles in the Celtic Sea shows that the base of the reflective lower crust may be associated with the Moho discontinuity everywhere in the Celtic Sea except below the Saint George's Channel basin. The lower crust does not change significantly in depth and thickness beneath the Permo‐Triassic basins. None of the available models simply satisfies such conditions. They all require at least a modification of the geometry of the lower crust after the formation of the Celtic Sea basins. Nevertheless, if the simple shear model can be ruled out mainly from structural arguments, the uniform stretching and to a lesser degree the thermomechanical models seem to be more relevant models. However, two additional conditions are necessary: (1) the preservation of crustal seismic features formed before the creation of basins must be explained and (2) a significant portion of the volume of the stretched lower crust (about 40% of this volume for the North Celtic Sea basin) must be added below or perhaps partly above the lower crust by lateral flow of the crust or at the expense of the upper mantle.

Deep structure of the Celtic Sea: a discussion on the formation of basins

The SWAT (South West Approaches Traverse) deep Seismic reflection profiles provide complete sections of the crust in the Celtic Sea area. Deep Mesozoic-Cenozoic basins are underlain by a non-reflective upper crust cut by Variscan thrusts and a reflective lower crust.One of these thrusts, the Variscan Front, has been considered to be a detachment fault, which could have initiated the development of the North Celtic Sea Basin during Mesozoic times. This hypothesis is inconsistent with the obliquity of the front with respect to the basin, the migration of the basin depocentres and the existence of normal faults that controlled the subsidence of the basin and intersect the front.In the absence of refraction data, we used two-dimensional gravimetric modelling to determine the depth of the Moho. This modelling shows that the gravimetric Moho is coincident (±3 km) with the bottom of the reflective lower crust.The resulting depth sections show a nearly flat Moho and a lower crust with a relatively constant thickness. The thinning coefficients of the whole continental crust and the stretching coefficients, obtained by comparison of theoretical and computed (backstripping method) subsidence curves, are significantly different. We discuss three hypotheses to explain this discrepancy: 1.(a) syn-rifting processes not taken into account in the stretching model;2.(b) previously thickened (Variscan) crust, departing from the initial state assumed by the model;3.(c) post-extensional modification of the crustal configuration resulting from rifting.Each of these hypotheses or a combination of them may correctly account for the data. However, the flat nature of the Moho suggests that at least the third one occurred.

Vitrinite reflectance data from the Kinsale Harbour–Old Head of Kinsale area, southern Ireland, and its bearing on the interpretation of the Munster Basin

Vitrinite reflectance of the Upper Devonian–Carboniferous succession in the Kinsale Harbour–Old Head of Kinsale area ranges from 4.06–5.21% ( m ) and 6.21–7.82% ( max ) which is indicative of meta-anthracite coal rank. No correlation is apparent between vitrinite reflectance and stratigraphic position through c. 2 km of section, suggesting that near-maximum temperatures were maintained until after Variscan folding. Low reflectance values of 0.62% and 0.30–0.35% ( m ) at the local base of the Jurassic succession in the North Celtic Sea Basin (Well 48/30-1) and onshore at Cloyne respectively, indicate a major discontinuity in the maturation profile in the region. Comparison with high reflectance gradients from Upper Palaeozoic successions in the Halen and Turnhout boreholes in Belgium and the Münsterland No. 1 borehole in West Germany suggests that a thick (5–7 km) cover of pre-Permo-Tnassic rocks must once have been present in the Munster Basin to account for the high maturation levels observed. Whether the cover was emplaced by Silesian sedimentation or by late Palaeozoic thrusting is unclear.

Basin development and basement structure of the Celtic Sea basins (SW Britain)

Basin development and basement structure of the Celtic Sea basins (SW Britain)

The tectonic evolution of the North Celtic Sea and Cardigan Bay basins with special reference to basin inversion

The morphology and structural evolution of the North Celtic Sea Basin (NCSB) and Cardigan Bay Basin (CBB) are, in large part, controlled by reactivation of basement features of Caledonian and possibly Variscan origin. From earliest Triassic until Maastrichtian times sedimentation was almost continuous, initially in a rapidly subsiding rift system and finally in a progressive passive sag phase. At the onset of the Alpine orogeny, both basins experienced spectacular inversion along readily identifiable axes. The inversion takes the form of a compressive reactivation and reversal of the pre-existing growth faults and a broad regional upwarping of the Cretaceous depocentre. The amount of inversion varies depending on the location within the basin but values in excess of 1000 m are possible. By utilising regional seismic data the timing of inversion can be estimated as occurring between Maastrichtian and Eocene times which correlates with the so called Laramide pulse of the Alpine orogeny.

The Lenticulina (Astacolus) humilis (Reuss, 1863) “group” and Lenticulina (Astacolus) atherfieldensis (new species) in the lower aptian of the isle of wight (U.K.)

The taxonomy and taxonomic affinities of the Lenticulina (Astacolus) humilius (Reuss, 1863) “group” in the Lower Cretaceous of north-west Europe are discussed with reference to the naming of a new species in the Lower Aptian in the Isle of Wight. Lenticulina (Astacolus) atherfieldensis sp. nov. is recorded from the type section of the Atherfield “Group” (Lower Aptian) on the Isle of Wight (map ref. SZ 790452) and from Lower Aptian strata in the Paris Basin and the Celtic Sea Basin.

Pursuit of Wealden models

Abstract: SUMMARY: Qualitative basinal models presented previously (Allen 1975, 1976) are developed. Outwash plain models for Weald-Wessex depict the freshwater arenaceous formations as low-angle watery coalescent fans (`fan-deltas’ auctt .), ‘moving lakes', flood-basins, static lakes, interfan depressions, etc. Five major rivers dominate, spreading out from fault-scarps on the basin margins. There are also minor streams, extra-basinal and intra-basinal in origin. During wet periods vigorous floods mobilize large areas of fine sand, silt and clay; during dry spells there are still large expanses of water around, especially in the distal facies, acting as floral and fauna1 refuges. Pedogenic processes frequently extend to the distal silts and clays. Gravel is scarce, especially in the Weald sub-basin, even at the fan-apexes, and suspension load is abundant. The component fans are therefore atypical and the model has resemblances to sand- and mud-dominated proglacial outwash plains, with ingredients from arid and sheetflood plains. Lake-lagoon-bay-estuary models describe the argillaceous formations. Shallow-water environments transgress the alluvial plains and salinities rise locally, but plant colonization and exposure of most lithofacies remain common. Floods sweep the basin from the same directions as before, but less frequently. They account for most of the shoreline aggradation in the lakes and lagoons. Therefore beach deposits are hardly differentiated except where wave erosion in the larger bodies attacks the higher parts of the fans. To what extent the Weald-Wessex models are applicable to the Western Approaches and Celtic Sea basins is uncertain. In the provincial model for S England the basin is bordered by tectonically active source-massifs and featured internally by horst and step-fault swells. Marginal upfaulting and high relief on the massifs generate the outwash plains; down-faulting and low relief generate the lake-lagoon-bay environments. Outcrops on the massifs are predominantly of sedimentary rocks underlying deep soils or exposed in deeply cut river courses. ‘Londinia’ exposes mostly Old Red Sandstone, Lower Carboniferous and Upper Jurassic; Cornubia-New Red Sandstone and Upper Jurassic; Armorica-Precambrian, New Red Sandstone and Jurassic (with Middle Jurassic igneous rocks or pyroclastics); Hibernia-Lower and Upper Palaeozoic. Contemporary volcanicity and associated mineralization affect Cornubia and probably the W Channel basin. The climate is warm, with periodic heavy rains producing leached upland soils supporting forests frequently swept by fires. Soft and intensely weathered rocks on the massifs account for the scarcity of gravel in the Wealden outwash. Widespread removal of Portlandian and Kimmeridgian enhances the sand and mud contents respectively; and, broadly, in that order. Salinities in the S England basin are controlled by the effectiveness of coastal protection against advances by the Boreal sea into the Londinia-Cornubia gap. During arenaceous phases (low salinities) the alluvial plain is well protected, aided by high inputs of freshwater from the elevated massifs. During muddy lake-lagoon-bay phases (higher salinities) the coastal ‘barrier’ leaks, or sometimes breaks, aggravated by reduced freshwater input from the sunken massifs. Common tectonic control of coastal barriers and massif relief is indicated. The final phase (Hauterivian-Barremian clays) records more serious breaches than any before. Sandy shoals of mixed marine and alluvial compositions invade the muddy sub-basins, tidal influences appear, and salinities locally rise higher than at any time since the start of the Cretaceous. Tectonic subsidence on this occasion was possibly aided by rising sealevel. The regional model for the S British Isles area comprises a framework of active massifs loosely linked by coastal deposits similarly protecting the intervening basins against the Boreal and Tethys seas. Three out of nine possible palaeogeographies are illustrated. In the first, Cornubia, Armorica and Hibernia join at the present continental shelf edge to form a massif bordering Tethys and from which the drainage in all three basins runs NE to the Boreal sea. The second depicts the Celtic and Approaches basins as draining SW into Tethys; S England drains to the Boreal sea as before. In the third, the three basins are silted-up gulfs formerly connecting the Tethys and Boreal seas. Each model is initiated by early Cretaceous elevation of the Jurassic seafloor and degraded massifs. Highest in the W, it presages the revival of true ocean spreading in the N arm of Tethys. Collapse by rifting, listric faulting, etc. near the present continental margin then ends the Wealden regime. This is thought to have begun earlier than sometimes proposed, viz. during the Hauterivian, when subsidence of the western massifs cut off the supply of bedload to wide areas, and the N and S seas both made notable advances. Control of Wealden sedimentation is therefore ultimately plate-tectonic. But, `Having burned my fingers so consumedly with the Wealden, I am fearful for you...' (Darwin, writing to Lyell on 25 November, 1860).

Olivine-dolerite intrusionsin the Fastnet Basin

Sills of olivine-microgabbro and olivine-dolerite 0.5–180 m thick intrude Lower Liassic (Hettangian–Sinemurian) sediments in the Fastnet Basin, a SW extension of the N Celtic Sea Basin, about Lat. 50°N, Long. 10°W. The evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill-complexes, and 3 possible igneous centres or plugs. A high-level sill may intrude Lower Cretaceous sediments, and one plug apparently deforms Cretaceous reflectors. K-Ar ages on separated biotites from a thick sill intruding Liassic sediments gave 170 ± 4 Ma (Bajocian). This sill contains 30% olivine (FO 70-73 ), 20% clinopyroxene (magnesian salite), 45% plagioclase (An 60-78 ), &, with accessory magnetite, ilmenite, edenitic hornblende, Ti-phlogopite, apatite, zircon, spinel (in olivine) and sulphides. Alteration to serpentine minerals, chlorite, sericitic mica and leucoxene is sporadic. The magmatism in the Fastnet province thus appears to have been active from the mid-Jurassic to the early Tertiary, related initially to the time of initial rifting between the European and American plates, and subsequently to their final separation between Greenland and Rockall and associated Thulean magmatism.

THE GEOLOGY AND HYDROCARBON HABITAT OF THE BRISTOL CHANNEL BASIN

The Bristol Channel Basin forms an early Mesozoic ((?Permo‐) Triassic‐Jurassic) basin development with a relatively thin cover of Cretaceous and Tertiary sediments. On the basis of structural trends and stratigraphy, the Basin can be divided into two sub‐basins: the ENE‐WSW trending Main Bristol Channel Basin and the E‐W trending East Bristol Channel Basin. In between them there is an Intermediate Area which incorporates features of the sub‐basins on either side. This subdivision appears to be the result of the presence of major NW‐SE basement faults which intersect the Bristol Channel area in several places.As a result of intermittent periods of tectonic activity three main stages of basin development can be recognised: (1) a (?Permo‐) Triassic‐ Middle Jurassic stage terminated by mid‐Kimmerian epeirogenetic movements, (2) an Upper Jurassic—Lower Cretaceous stage terminated by a Lower Cretaceous (late Berriasian‐pre‐Aptian) period of deformation, (3) an Upper Cretaceous—Tertiary stage.The bulk of the preserved sedimentary fill in the main Bristol Channel Basin consists of up to 11,000 ft of (?Permo‐) Triassic‐Jurassic sediments. The section is severely truncated by the overlying late Lower Cretaceoussediments or, in theirabsence, by Upper Cretaceous strata. Below the unconformity, the Upper Jurassic and, depending on their structural position, parts or the whole of the Middle and Lower Jurassic are missing.The East Bristol Channel Basin hasapreserved fill of some 7,500 ft of Triassic‐Jurassic sediments. The Middle and Upper Jurassic are completely preserved in the centre of this Eastern Basin, contrasting with the situation found in the main Basin.The basin fill of both sub‐basins consists largely of Triassic continental red‐beds, mainly silty claystones and evaporites and Jurassic marine calcareous siltstonesand claystones.Structures are essentiully the result of the Lower Cretaceous tectonic phase(s) and are therefore mainly confined to the Triassic—Jurassic—early Lower Cretaceous sections. A number of structural traps of interest to exploration have been delineated and (unsuccessfully) tested in the Main Bristol Channel and Intermediate Area.Evaluation of the currently available data indicates that the absence of significant hydrocarbon indications in these wells is probably due to insufficient and untimely hydrocarbon generation. The possible generation is thought to have occurred prior to the main phases(s) of structural deformation, during a period of temporarily increased heatflow (Middle Jurassic—Lower Cretaceous). An additional unfavourable aspect for hydrocarbon prospects is the lack of good quality reservoir developments.From a megateetonic point of view the Bristol Channel Basin formspart of a regional rift basin development which also includes the Celtic Sea and Western Approaches Basins. The different basins and blocks of this area can, according to their behaviour during the Mesozoic, be grouped as follows: (a) the East Bristol Channel, E. of the zone of majorNWSE faults (e.g. Sticklepath fault), (b) a Central Fault Block, comprising the Cornubian Platform, the Main Bristol Channel and Haig Fras Basins, (c) Flanking Basins, such as the Intermediate Area, the Celtic Sea and Western Approaches Basins.