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

Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth

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

Noel J. Murphy Petroleum Affairs Division, Department of Energy, Beggars Bush, Haddington Road, Dublin 4, Ireland

and Nigel R. Ainsworth* Applied Geology Unit, Department of Geology, Trinity College, Dublin 2, Ireland

Received 12 December 1989; revised 29 January 1991; accepted 12 March 1991

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.

Keywords: Fastnet Basin, offshore Ireland; stratigraphy; Triassic, Lower Jurassic and Middle Jurassic

Introduction

The Fastnet Basin situated at the south-western end of the North Celtic Sea Basin is a small, elongate, north-east-south-west trending basin, with dimensions of 110 km long by 35-40 km wide (Figure 1). It is bounded on three sides by shallow basement platform areas which probably consist of Palaeozoic rocks, but to the north-east it passes into the much wider North Celtic Sea Basin. The boundary between the two basins is defined as a diffuse north-west trending zone of deformation extending across blocks 56/16, 56/17 and 56/23 (Figure 2).

Previous studies within the Fastnet Basin began with Robinson et al. (1981), who described the broad stratigraphy, structure and basin development. The basin was further described by Naylor and Shannon (1982). The biostratigraphy of the Fastnet Basin has been extensively described in numerous publications; the more important with reference to the Triassic and Lower-Middle Jurassic microfaunas and microfloras are those by Ainsworth (1986; 1987; 1989a, b, c), Ainsworth and Horton (1986), Ainsworth et al. (1987, 1989) and Rutherford and Ainsworth (1989).

To date no lithostratigraphical subdivision of the Triassic-Middle Jurassic (Aalenian) interval has been undertaken. The purpose of this paper is to present a stratigraphical subdivision of the Triassic, Lower Jurassic and Middle Jurassic (Aalenian) sequences

*Present address: Paleo Services, Unit 15, Paramount Industrial Estate, Sandown Road, Watford WD2 4XA. UK

0264-8172/91/040417-13 ©1991 Butterworth-Heinemann Ltd

which is based on the integration of independent lithostratigraphical and biostratigraphical studies. It is intended that these results should form the basis for further sequence stratigraphic and structural work.

The correlation of gamma-ray and sonic wireline logs forms the framework of the study. These logs were selected as they are less affected by borehole conditions and the nature of pore fluids than other nuclear and electrical logging devices. They were also available for all of the wells in the study area. Inspection of the logs showed them to be of acceptable quality for correlation purposes and environmental corrections were considered unnecessary. The log scales are compatible except for two wells where different scaling was applied to the gamma-ray log. As this did not cause any significant problems with the correlation work, and as we did not have at our disposal computer-based aids for re-scaling, the existing scales were retained.

The logs were correlated in a chronostratigraphic sense. That is, small-scale log signatures recognized in lower energy facies were used to construct the correlation. It was assumed that these facies would be the most widely developed and that the correlation lines would approximate time lines. By so doing, higher energy carbonate and clastic facies would be confined and their correlation established. This work was supplemented by detailed and independent palaeontological and lithological investigations of cuttings, sidewall cores and conventional cores. Dipmeter and seismic data have only been referred to for differentiating between faults and unconformities. In general, seismic data in the area is of poor quality.

Marine and Petroleum Geology, 1991, Vol 8, November 417

Stratigraphy of the Fastnet Basin: N. J. Murphy and IV. R. Ainsworth

Figure 1 Geographical location of the Fastnet Basin

Most of the data were acquired prior to 1980 and as such suffer from poor penetration and poor resolution below the Upper Cretaceous Chalk. As a result, we do not consider that the wider use of seismic would make any significant contribution to this study.

In the Fastnet Basin, 11 -wells have been drilled, commencing with the Elf 55/30-1 well in April 1976 and finishing with the Occidental 64/1-1 well in August 1984 (Figure 3). All these wells have been plugged and abandoned, with the only oil shows occurring in the Sinemurian sandstones of the Cities Service 63/10-1 and Elf 64/2-1 wells. This study uses all 11 wells, namely BP 56/26-1, BP 56/26-2, Cities Service 63/4-1, Cities Service 63/10-1, Deminex 56/21-1, Deminex 56/21-2,

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The rock names used in this paper are for descriptive purposes only. They are applied to discrete wireline log correlation units and, although these units are generally lithologically distinctive, the unit boundaries do not necessarily coincide everywhere with facies boundaries. Therefore not all of the rock units defined warrant group, formation or member status. However, lithostratigraphic entities can be readily extracted from the present stratigraphic study and it is intended to use this work in the context of a formal lithostratigraphic nomenclature project which is currently planned by the Irish Depar tment of Energy. This will extend across both of the Celtic Sea Basins as a whole.

G e o l o g i c a l s e t t i n g

The structural configuration of the Fastnet Basin is well illustrated by reference to the isochrone map on the top Liassic Limestone level (Figure 2). At this level the basin outline is controlled both by normal faulting and by subcrop beneath the base Cretaceous unconformity. The dominant fault trend is north-east-south-west , but two prominent north-west-south-east striking transfer fault systems traverse and segment the basin. These fault zones are responsible for an apparent sinistral offsetting of major structural elements and can be spacially related to the occurrence of Middle Jurassic (Bajocian) basic igneous intrusives (Robinson et al., 1981: Caston et al., 1981).

418 Marine and Petroleum Geology, 1991, Vol 8, November

Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth The Fastnet Basin has been interpreted to form part

of a formerly coherent north-east trending Upper Triassic-Lower Jurassic rift system which probably formed as a result of tensional stress between Europe, Africa and North America (Masson and Miles, 1986). Triassic rifting in the North Atlantic area has been mentioned by several workers (Pegrum and Mounteney, 1978; Jansa et al., 1980; Zeigler, 1982). In the Fastnet Basin seismic data quality at the Triassic level is poor and evidence for fault-controlled subsidence is ambiguous. It is our experience in the Celtic Sea area that Triassic and Lower-Middle Jurassic sedimentation took place over a wider area than that occupied by the presently observed rift basins which are largely of Upper Jurassic and Lower Cretaceous origin. The existence of pre-Triassic topographic relief is suggested locally by the attenuation of the lower part of the Triassic, and while the present authors consider that partly fault-controlled Triassic depocentres did exist, their configuration is still largely conjectural.

Subsidence in the Fastnet area commenced early in the Triassic and was followed by a prolonged period of sedimentation which lasted at least until the Middle Jurassic (Aalenian). This was followed by a synrift sedimentary regime during the ?Middle Jurassic, Upper Jurassic and Lower Cretaceous. Unfortunately, in this basin the sedimentary record for this period is very poorly preserved and it is therefore difficult to precisely date rifting events. However, active rifting in the neighbouring North Celtic Sea Basin occurred early in the Upper Jurassic and again in the Lower Cretaceous (Petrie et al., 1989) and as this is consistent with the timing of rifting events in the Newfoundland and Iberian basins it is considered likely to be the case for the Fastnet Basin. Our stratigraphical analysis, supported by the interpretation of selected seismic profiles and dipmeter data, shows that over much of the area Lower Cretaceous sediments rest with angular unconformity on Aalenian or older section. Only locally are Jurassic sediments younger than Aalenian preserved, but where present they also exhibit a strongly unconformable relationship with older sediments. A phase of major uplift, erosion and faulting within the Middle Jurassic, possibly contemporaneous with the igneous activity in the area, has been interpreted (Robinson et al., 1981 ; Naylor and Shannon, 1982). This is largely dependent, however, on the recognition of post-unconformity Middle Jurassic clastic sediments in a single well (Ranger 63/8-1) and, as dating of these sediments is poorly constrained, the timing of this tectonism has yet to be established with certainty.

Stratigraphy A stratigraphic correlation between the 11 wells is shown in Foldouts 1 and 2. The line of section is shown in Figure 3 and was selected to best illustrate the correlation. A detailed wireline log correlation is shown in Foldout 1, illustrating all the wireline log units, together with the identification of missing or condensed section. In Foldout 2 the correlation units are grouped into distinguishable rock units, while the breaks m sequence are interpreted as faults or unconformities. The principal 'clean lithologies' are

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also illustrated in Foldout 2. A summary of the stratigraphy of the Fastnet Basin is shown in Figure 4. The use of the term unconformity refers to apparent breaks in the sequence determined by correlation which are usually identifiable in more than one well and which cannot be attributed to faulting based on the inspection of dipmeter and seismic data.


Stratigraphy of the Fastnet Basin: IV. J, Murphy and N. R. Ainsworth Unconformities recognized for the first time within the studied interval are characterized as having little or no discordance and are interpreted to represent disconformities or condensed sequences. Where no apparent breaks in the sequence were detected, the sequence is termed conformable.

Basement Upper Palaeozoic rocks form the economic basement ua of the Fastnet Basin and have been penetrated in two wells. The Elf 55/30-1 well bot tomed in Devonian (?Frasnian) redbed and tufts, while the Cities Service 63/4-1 well bottomed in Upper Tournasian (Lower -,ao Carboniferous) shelf limestones. Both sets of Upper Palaeozoic rocks are considerably indurated. In the Elf 55/30-1 and Elf 64/2-1 wells, Lower Carboniferous limestones have also been penetrated, 38 and 15 m, respectively. In the former, these rest unconformably upon the Devonian. In both instances the carbonates are similar to those occurring in the Cities Service 63/4-1 well (light grey to red, hard, crystalline limestones, becoming silicified or dolomitic) (Sevastopulo, personal communication, 1989), and both possess distinct log motifs (low, serrate gamma-ray and sonic velocity responses). All Upper Palaeozoic sequences are unconformably overlain by rocks which have a characteristic Triassic lithology and nowhere in the basin have definitely dated Permian strata been encountered. The limited palaeontological control available indicates that the Permian and also the lowermost Triassic are absent from the well record.

GR 3 100

Triassic Triassic strata occur in eight of the eleven wells, reaching a maximum thickness of 629 m in the Elf 64/2-1 well. The Triassic sequence consists of six lithological units ranging in age from the Upper Scythian through to the Rhaetian.

TR-I Triassic Sandstone Unit 1. Thickness 31 -89 m. The principal lithology is sandstone. The unit consists of r ed -b rown , orange, fine to medium grained, occasionally coarse to conglomeratic, poorly sorted, angular to subangular, well indurated, quartz sandstones, with argillaceous, haematitic, calcareous and dolomitic matrices. Reworked Carboniferous clasts occur throughout this unit (Sevastopulo, personal communication, 1989). Thin stringers of greyish red, r ed -b rown, firm to hard, subfissile shales and siltstones are also present. The sandstones are frequently argillaceous with poor (less than 10%) to good (10-20%) porosities, although very good (greater than 20%) porosities are locally present in the fine grained sandstones within the Elf 55/30-1 well.

Log motif and boundary definitions. The lower boundary is sharp, coinciding with the base of the sandstone as seen by the relative increase in both gamma-ray and sonic transit times above that of the underlying limestones of the Carboniferous (Figure 5). The upper boundary is usually gradational, although it can be sharp, and is placed at the base of the overlying claystone, coinciding with an increase in gamma-ray values. This unit has a cylindrical or blocky gamma-ray motif.

Distribution. Although only occurring in five wells in the Fastnet Basin, three of which have fully penetrated the sequence, it can also be recognized within the

At 140 4 I

i Figure5 Type log for the Triassic Sandstone Unit 1 (TR-1), Triassic Claystone Unit 1 (TR-2) and Triassic Sandstone Unit 2 (TR-3) from Elf 55/30-1

North Celtic Sea Basin. in the latter region it attains a maximum thickness of 123.5 m. In the Fastnet Basin, this unit is thickest in the north-east, in the vicinity of the Elf 64/2-1 well (89 m) (Table 1).

Age. In the Fasmet Basin, this unit is barren of any in situ microfossils. HoweVer, by inference from a well situated in the south-western part of the North Celtic Sea Basin, an age of Upper Scythian to Anisian can be assigned, with the dating provided by rare palynomorphs from the well report.

Depositional environment. The unit was deposited under continental conditions. The presence of poorly sorted, angular to subangular, fine to coarse grained sandstones is envisaged to reflect ephemeral flash flood fluviatile deposits. Reworking of the Palaeozoic basement is seen by the recovery of clasts of Carboniferous Limestone, which yield Lower Carboniferous calcareous benthonic foraminifera (Sevastopulo, personal communication, 1989). These clasts occur throughout this unit.

TR-2 7~iassic ('laystone Unit l. Thickness 65-133 m. The principle lithology is claystone. The unit consists of r ed -b rown , greyish red, greenish grey (the latter colour mainly as reduction spots), firm to hard, slightly calcareous or dolomitic claystones and silty claystones. The latter grade into argillaceous siltstones. Minor stringers of argillaceous limestone and very fine, argillaceous sandstone are also present. White, crystalline, occasionally fibrous anhydrite is rare throughout the unit.

Log motif and boundary definitions. The lower boundary is gradational, altfiough it can be sharp. It is placed at the base of the claystone, coinciding with an increase in gamma-ray wdues (Figure 5). The upper boundary is sharp and is set at the top occurrence of claystones before the incoming of the Triassic Sandstone Unit 2 (TR-3). The overall log motif is characterized by a high, serrate gamma-ray response.

Distribution. This unit occurs in six wells within the Fastnet Basin and can also be observed in the North Celtic Sea Basin. As with the preceding unit (TR-I) ,

420 Ma r i ne and Pe t ro leum Geo logy , 1991, Vol 8, N o v e m b e r

Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth T a b l e 1 Sed imen ta r y th ickness (m) o f the l i tho log ica l uni ts fo r each exp lo ra t ion we l l (exc lud ing th ickness o f igneous in t rus ions)

Tr iassic S a n d s t o n e Unit 1 TR-1 Triassic C l a y s t o n e Unit 1 TR-2 Tr iassic S a n d s t o n e Unit 2 TR-3 Triassic C l a y s t o n e Unit 2 TR-4 Rhaet ian Mar l Unit TR-5 R h a e t i a n L i m e s t o n e Unit TR-6 Basal Liassic C lays tone Unit J-1

- J-2 J-3

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J-5 J-6

--I~ J-7 Liassic Marl Unit J-8

J-9 Liassic S a n d s t o n e Unit J-10

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the thickest development within the Fastnet Basin occurs in the Elf 64/2-1 well (133 m), in the north-east of the basin (Table 1).

Age. Similar to the Triassic Sandstone Unit 1 (TR-1), this unit is barren of in situ microfossils in the Fastnet Basin. However, in the North Celtic Sea Basin, well reports have described very rare palynomorphs which date the unit as Anisian-Ladinian.

Depositional environment. The main lithologies of this sequence (red-brown and greyish red claystones and siltstones) suggest a range of distal continental depositional environments, most probably related to large-scale coastal plain sabkhas, ephemeral lakes and inland sabkhas. The rare limestones imply either shallow or marginal lacustrine deposits.

TR-3 Triassic Sandstone Unit 2. Thickness 32-66 m. The principle lithology is sandstone. This unit consists of white, pink, dark red-brown, very fine to medium grained, occasionally very coarse, subangular to well rounded, poorly sorted, some grains frosted, sandstones, some with argillaceous matrices. Minor red-brown, hard, slightly calcareous claystones and white, red-brown, hard, microcrystalline limestones are also present. The latter are always less than 10 m in thickness. Porosities within the sandstones are poor (less than 10%) to good (10-20%). Siltstone lithologies are present in the Elf 64/2-1 well, whereas sandstones are completely absent.

Log motif and boundary definitions. The lower boundary is sharp, coinciding with the lowest appearance of sandstones and/or siltstones, as shown by the decrease in gamma-ray responses (Figure 5). The upper boundary is also sharp and is placed at the highest occurrence of sandstone and siltstone before the incoming of the claystone. The whole unit is characterized by a generally low gamma-ray response, with serrate upward fining and upward coarsening gamma-ray motifs.

Distribution. This unit is recorded in six Fastnet wells

and also within the North Celtic Sea Basin. The thickest section penetrated occurs near the basin axis in the Deminex 56/21-2 well (66 m) (Table 1).

Age. Similar to the preceding units, this section is barren of in situ microfossils due to the unfavourable depositional environments. At present only one well report in the North Celtic Sea Basin has described a rare palynomorph assemblage dating the unit as Anisian-Ladinian.

Depositional environment. Continental deposition. The more poorly sorted, angular to subangular sandstones and siltstones are typically deposited in flash flood fluviatile environments, whereas those sandstones which possess the more rounded, often frosted, poorly compacted grains are more typical of aeolian deposits. The interbedded carbonates most probably represent marginal lacustrine or shallow lake deposits.

TR-4 Triassic Claystone Unit 2. Thickness 44-283 m. The principle lithologies are claystones and shales. The majority of this unit consists of orange-red, greyish red, dark red-brown, greenish grey (the latter occurring as reduction spots), soft to firm, blocky to subfissile, slightly calcareous to calcareous, dolomitic claystones and shales, which grade into siltstones. Thin interbeds of light to medium grey, hard, crystalline or argillaceous dolomite grading to limestone are also present. Rare white to red-brown, fine grained, angular to subangular sandstones occur throughout the unit. White, pink, cream, hard, brittle, crystalline, occasionally fibrous anhydrite commonly occurs as nodules, veins and fracture fillings.

Log motif and boundary definitions. The lower boundary is sharp and is placed at the base of the claystone, coinciding with an increase in gamma-ray response (Figure 6). The upper boundary is also sharp and conformable, and coincides with the highest occurrence of continuous red bed lithologies. This unit is characterized by a high, slightly serrate, gamma-ray


Stratigraphy of the Fastnet Basin: N. J. Murphy and IV. R. Ainsworth lnterbedded are thin stringers of white, grey-brown,

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Figure 6 Type log for the Triassic Claystone Unit 2 (TR-4), Rhaetian Marl Unit (TR-5) and Rhaetian Limestone Unit (TR-6) from Texaco 56/22-1

response. Near the base, there is a tendency for funnel shaped motifs.

Distribution. This unit is recorded in eight wells in the Fastnet Basin, although in the Cities Service 63/10-1 and Deminex 56/21-1 wells the sequence has not been fully penetrated. The unit also occurs in many wells in the North Celtic Sea Basin, where thick halite sequences are often present. In the Fastnet Basin the thickest section occurs near the basin axis in the Deminex 56/21-1 well (283 m) (Table 1).

Age. No in situ microfossils have been described from this unit in the Fastnet Basin. In the North Celtic Sea Basin, rare palynomorph assemblages from well reports suggest a broad age of ?Anisian to Norian. The upper part of the unit yields moderately common palynomorphs of Carnian-Norian age. The lower part of the unit, however, yields longer-ranging palynomorph assemblages.

Depositional environment. The predominant occurrence of greyish red and reddish brown shales, claystones and siitstones interbedded with evaporites is suggestive of deposition in a coastal sabkha or in a marginal lacustrine environment (Robinson et al., 1981; Naylor and Shannon, 1982).

TR-5 Rhaetian Marl Unit. Thickness 22-41 m. This unit can be subdivided into two parts. The lower part consists of an interbedded mudstone/shale -limestone sequence. The mudstones/shales are brown, grey, grey-green, occasionally red-brown, firm, subfissile to fissile, non-calcareous to calcareous.

argillaceous limestone, locally becoming dolomitic. The upper part consists of a similar interbedded sequence; however, the shales are darker grey to black, brown, grey-green, subfissile to fissile, locally calcareous. Anhydrite occurs throughout the unit as discrete crystals within the shale beds.

Log motif and boundary definitions. The lower boundary is sharp and coincides with the lowest occurrence of the interbedded green-grey and grey shales and limestones, accompanied by a sharp decrease in the gamma-ray responses and a more irregularly serrated sonic motif (Figure 6). The upper boundary is also sharp and is defined by the sharp decrease in gamma-ray and sonic values. The unit is characterized by an overall increase in gamma-ray values towards the top of the section, with a corresponding change from smooth to spiky gamma-ray motifs. Throughout this unit the sonic response shows a slightly more serrated motif, due to the interbedded limestones and shales.

Distribution. This unit occurs in eight wells in the Fastnet Basin; it can also be recognized in the North Celtic Sea Basin, Bristol Channel Basin and southern Britain. Throughout these latter regions the lower part of TR-5 is currently interpreted as laterally equivalent to the Mercia Mudstone Group's Blue Anchor Formation, with the upper part laterally equivalent to the Penarth Group's Westbury Formation, and the Cotham Member of the Lilstock Formation (Warrington et al., 198(I). In the Fastnet Basin, the thickest development is in the north-eastern area, in the Elf 64/2-1 well (41 m) (Table 1).

Age. This section has been dated by palynomorphs within the Fastnet Basin. These designate an age of Lower-Middle Rhaetian (Ainsworth et al., 1987; 1989; Rutherford and Ainsworth, 1989). Diagnostic palynomorph taxa include Rhaetogonyaulax rhaetica (Sargeant), Ovalipollis pseudoalatus (Thiergart) and Rhaetipollis germanicus Schulz. These palynomorph assemblages compare well with those described from the onshore UK (Warrington, 1978; 1982).

Depositional environment. The bulk of the Rhaetian Marl Unit was deposited in a lagoonal environment, with intermittent connections to shallow water open marine environment. Bisaccate pollen grains are common, suggesting a close proximity to land, whereas the limited diversity of the dinocyst component of the palynomorph assemblages reflects deposition in an environment of reduced salinity (Warrington, 1978). Desiccation of the region is suggested by the formation of evaporites, occurring as discrete crystals within the shales. The absence of any in situ microfauna is due to the unfavourable depositional environment during this time.

TR-6 Rhaetian Limestone Unit. Thickness 13-32.5 m. The principle lithology is limestone. This unit consists of an off-white, light grey, medium grey-brown, hard, occasionally vuggy, oolitic, argillaceous or microcrystalline limestone. In part it is dolomitic, and more rarely glauconitic. The limestone becomes argillaceous towards the top of the unit. Thin stringers of light grey to brown grey, slightly calcareous to calcareous shale and mudstones are also present throughout the unit.

Log motif and boundary definitions. The lower


Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth boundary is sharp, coinciding with the lowest occurrence of a thick limestone, marked by an abrupt decrease in both gamma-ray response and sonic velocity (Figure 6). The upper boundary is also sharp and is placed at the top of the argillaceous limestones, which grade to cleaner limestones with depth. The boundary is marked by a sharp increase in both sonic and gamma-ray responses coincident with a thin claystone bed at the base of the overlying Basal Liassic Claystone Unit (J-l). The unit is characterized by bulbous or blocky low gamma-ray and sonic motifs.

Distribution. This unit is observed in eight Fastnet wells. It is also present within the North Celtic Sea Basin, the South Celtic Basin, Bristol Channel Basin, South Wales and southern England (Warrington et al., 1980). Onshore UK, the lateral equivalent to the Rhaetian Limestone Unit is the Langport Member of the Penarth Group's Lilstock Formation (Warrington et al., 1980). In the Fastnet Basin, the thickest section was encountered in the Elf 55/30-1 well (32.5 m) (Table 1).

Age. In both the Fastnet Basin and North Celtic Sea Basin, the Rhaetian Limestone Unit (TR-6) yields common long-ranging miospores and acritarchs. Important miospore taxa include Acanthotriletes ovalis Nilson, A. varius Nilsson, Lunatisporites rhaeticus Schulz (Warrington), Ricciisporites tuberculatus Lundblad and Porcellispora longdonensis (Clark) Morbey. Dinocysts are rare. The absence of any microfaunas is due to the well cemented nature of the lithologies making fossil extraction impossible. However, onshore UK, the laterally equivalent Langport Member yields palynomorph assemblages of Upper Rhaetian age (Warrington, 1982).

Depositional environment. The unit is interpreted to represent deposition in a low energy, warm water, carbonate-rich, shallow marine environment. The rare glauconite grains, ooliths within the carbonate, and the recovery of palynomorph assemblages consisting of acanthomorph acritarchs implies short, intermittent higher energy environments (Ainsworth et al., 1989; Rutherford and Ainsworth, 1989).

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=

c <) Figure 7 Type log for the Basal Liassic Claystone Unit (J-l) and Liassic Limestone Unit (J-2-6) from Deminex 56/21-2

Jurassic Jurassic rocks occur in all 11 wells, reaching a maximum thickness of 1546.5 m in the BP 56/26-1 well. The Jurassic sequence has been divided into five lithological units, ranging in age from uppermost Rhaetian-iowermost Hettangian to Aalenian.

J-1 Basal Liassic Claystone Unit. Thickness 12-40 m. The principle lithologies are claystones and shales. This unit consists of light to dark grey, dark brown, black, hard, variable carbonaceous content, moderately calcareous, claystones and shales. Thin stringers of medium-dark brown argillaceous limestone and white, grey, fine grained, calcareous sandstones also occur. In the south-western part of the basin, the unit displays a higher content of limestone, especially in its lowest section.

Log motif and boundary definitions. The lower boundary is sharp, marked by a high gamma-ray response, coinciding with a thin claystone bed just above the Rhaetian Limestone Unit (TR-6) (Figure 7). The upper boundary is also sharp and is coincident with the highest occurrence of serrated gamma-ray and sonic responses, associated with the shales and claystones

before the onset of the mudrocks and limestones of the Liassic Limestone Unit (J-2-6). The log motif is characterized by high, serrate gamma-ray and sonic motifs within the shales and claystones, with gradations to lower, blocky gamma-ray and sonic responses in the cleaner limestone and sandstone lithologies. This latter response is best defined in the lower part of the section.

Distribution. This unit occurs in eight of the eleven Fastnet wells. It can also be recognized in the North Celtic Sea Basin, in similar thicknesses. In the Fastnet Basin, the thickest penetrated sequence occurs in the Deminex 56/21-2 well (40 m), situated near the basin axis.

Age. Uppermost Rhaetian-lowermost Hettangian. In both the Fastnet Basin and North Celtic Sea Basin this unit yields a microflora similar to that described from the Rhaetian Limestone Unit (TR-6) (Rutherford and Ainsworth, 1989). Two of the ostracod taxa, (Ogmoconchella serratostriata Ainsworth, Cytherella sp. Ainsworth) have not been described from onshore while O. ellipsoidea ranges from the uppermost Rhaetian to lowermost Sinemurian, and therefore cannot be used for reliable age determinations. An age


Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth of uppermost Rhaet ian- lowermost Hettangian must 85 therefore be assigned to this unit.

Depositional environment. The presence of an often abundant but near monospecific ostracod microfauna suggests a restricted quiet water, shallow marine environment. The occurrence of large numbers of miospores with rare dinocysts implies a nearshore environment (Ainsworth, 1989a, b; Ainsworth et al., 1989; Rutherford and Ainsworth, 1989). In the south-western part of the basin a carbonate facies is replaced uphole by the more characteristic claystone-shale sequence. The former is thought to have been deposited in a low energy, shallow water marine environment.

J - 2 - 6 Liassic Limestone Unit. Thickness 8.5-304 m. The succession is dominated by carbonates, interbedded with thinner shales and sandstones. The limestones vary considerably within each well and also throughout the Fastnet Basin. Three major limestone lithologies occur. Firstly, white, off-white, light to dark grey, moderately soft, argillaceous limestones; secondly, light to dark grey, dark brown, hard, micro- to cryptocrystalline limestones; and thirdly, white, light grey, light to dark brown, hard, oolitic and sparitic limestones. Whereas the first two lithologies occur throughout the basin, the oolitic and sparry limestones predominate along the western side of the basin. Interbedded with the carbonates are shales, siltstones, sandstones and dolomites. The dolomites are light tan, hard, fine to coarse grained, some cryptocrystalline. The shales are medium to dark grey, light green, b rown-grey , generally hard, subfissile, with variable calcareous content, often with carbonaceous debris, grading to siltstone. The sandstones are white, grey, green, pink, red, fine to medium grained, friable, moderately well sorted, angular to subangular, mainly with a calcareous cement. The Cities Service 63/4-1 well penetrated the most arenaceous section.

Log motifs and boundary definitions. The lower boundary is sharp, coinciding with the base of the thicker limestones above the Basal Liassic Claystone (J- 1 ), marked by a sharp decrease in the sonic velocities and gamma-ray responses (Figure 7). The upper boundary is also sharp. [t generally coincides with the top of the topmost development of the thick limestone beds coincident with the top of the low, blocky gamma-ray and sonic motifs. The Liassic Limestone Unit (J-2-6) is characterized by low, cylindrical or blocky and funnel shaped gamma-ray motifs, with thin high gamma-ray and sonic velocity responses in the mudrocks. All the constituent limestone beds show a high degree of correlatability throughout the Fastnet Basin and into the North Celtic Sea, both on wireline logs and micropalaeontological data.

Distribution. This unit occurs in ten wells in the Fastnet Basin, although it is only fully penetrated in eight wells. An 86 m thick doleritic sill is present in the Elf 64/2-1 well. The thickest section penetrated occurs near the basin axis (Deminex 56/21-2, 304 m) (Figure 8, Table 1). This unit has also been described from the North Celtic Sea Basin (Rutherford and Ainsworth, 1989).

Age. This sequence yields a common Hettangian ostracod microfauna consisting of Darwinula hettangiana Ainsworth, D. rara Ainsworth,

t 5 6 56~2--1 56/14--1 "¢~(t98* )

56/20--1

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0_____ 50Km i

Figure 8 Isopach map for the Liassic Limestone Unit (J-2-6) in the Fastnet Basin

Lutkevichinella hortonae Ainsworth, L. ~astigam Ainsworth and Klinglerella translucens (Blake). Palynomorphs are also common and include such notable taxa as Kraeuselisporites reissingeri (Harris) Morbey, Classopollis torosus (Reissinger) Balme and Gliscopollis meyeriana (Klaus) Venkatachala (Ainsworth, 1989a, c; Rutherford and Ainsworth, 1989).

Depositional environment. Three environments are represented; freshwater, brackish and shallow marine, interpreted to reflect a transgressing-regressing shoreline (associated with a very low relief coastal plain) due to the unstable environment at the commencement of Rhae t ian-Het tangian marine transgression (Robinson et al., 1981; Naylor and Shannon, 1982). Freshwater and brackish salinities are implied by the abundant ostracod faunas of a limited diversity, extracted from the interbedded mudrocks and soft argillaceous limestones rather than from the well cemented limestones. The presence of large numbers of the pollen Classopollis denotes arid and semi-arid conditions, with the parent plants occurring in lowlands bordering lagoons (Vakhrameev, 1970; Sladen and Batten, 1984). The limestones are interpreted to have been deposited in a shallow marine environment, with the oolitic units deposited in high energy, very shallow water, warm, carbonate-rich environments. In the upper part of the Liassic Limestone Unit, rare marine ostracod faunas, in association with echinoderm debris, likewise suggest shallow water, marine environments. This implies a slight deepening of the shelf during this time, with the loss of the non-marine ostracod faunas. The Cities Service 63/4-1 penetrated a more arenaceous section, reflecting proximity to a positive relief source area for the sands. The red coloration of some of the sands suggests either temporary subaerial conditions or erosion of the underlying red Triassic sandstones, with the sediments being transported into this region


/ f

I

\

d.

1 T

\ /E

? , .J M. ,IC

Z o

th ,C

GR z~t 0 120 140

i I

Stratigraphy of the Fastnet Basin: N. J. Mu/ 9hy and N. R. Ainsworth 55 56 5e/12-t

40

I - r a Z

Figure 9 Type log correlation for the Liassic Marl Unit (J-7-9) from Deminex 56//21-1

without loss of their coloration (Ainsworth, 1989a).

i3

J-7-9 Liassic Marl Unit. Thickness 41-119 m. This unit consists of a cyclic sequence of mudstones and shales capped by thin limestones. The mudstones and shales are light to dark grey, soft to firm, fissile to subfissile, slightly calcareous to very calcareous, grading to grey, brown, soft to firm, calcareous siltstones. The thin limestones, more prominent in the lower section, are white to grey, hard, microcrystalline to argillaceous, grading locally to dolomitic limestones. As with the Basal Liassic Ciaystone Unit (J-l) the carbonates are again more prevalent in the south-western part of the basin. A 12 m dolerite sill is present in the Occidental 64/1-1 well.

Log motif and boundary definitions. The lower boundary is sharp. The base is placed at the occurrence of calcareous mudrocks above the thickly bedded limestones, coincident with an increase in gamma-ray and sonic velocity responses (Figure 9). The upper boundary is sharp, coincident with the highest occurrence of marls and the cyclic log motif. In many wells there is an unconformity near to the top of the unit. The unit is characterized by the very distinctive cyclic log motif, with funnel shaped gamma-ray and sonic responses.

Distribution. The Liassic Marl Unit (J-7-9) is recorded in ten of the Fastnet wells, excluding the BP 56/26-1 well in which the section was not reached. It has also been described from the North Celtic Sea Basin (Rutherford and Ainsworth, 1989). In the Fastnet Basin the sequence is thickest near the basin axis (Deminex 56/21-2, 119 m) (Figure 10, Table 1).

Age. Uppermost Hettangian-lowermost Sine- murian. A sequence well dated throughout the region by the short-ranging ostracod faunas, for example, Ogmoconcha hagenowi Drexler, Ogomoconchella ellipsoidea Jones, Donzocythere cf. D. convergens Donze sensu Donze (Ainsworth, 1989a; Ainsworth et al., 1989; Rutherford and Ainsworth, 1989).

Depositional environment. These sediments are thought to have been deposited in a low energy, inner shelf environment, implied by the profuse microfaunas,

56/14-1 /~143.5) / / /

/ 56/20-1 o I (85)

l l l ;', 56121 1 w /

I 56/21-2 56/22-1

1 .... ~, .~r_ ~ ~ J seT'e-2 ~'4 6 l l l - i

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Contour Interval every 25 m

0 L 50Kin p

Figure 10 Isopach map for the Liassic Marl Unit (J-7-9) in the Fastnet Basin

dominated by only a few species, all of which were most likely opportunistic taxa. The presence of large numbers of foraminiferal taxa such as Involutina liassica (Jones, 1853) implies shallow marine conditions (Gazdzicki, 1975). Towards the upper part of the unit, the absence or impoverished nature of the microfaunas is suggestive of anaerobic and/or dysaerobic conditions due to a restriction in water circulation (Ainsworth, 1989a). It is also envisaged to reflect a slight deepening of the basin during this time.

J-lO Liassic Sandstone Unit. Thickness 159-424.5 m. This unit consists of three main lithologies, sandstone, siltstone and shale, with minor limestone and dolomite. The former consists of white, light brown to light grey, very fine to medium grained, unconsolidated to firm, generally moderately well sorted, subangular to subrounded sandstones, with silica or carbonate cements. Porosities of the sandstones are variable, ranging from poor (less than 10%) to very good (greater than 30%). Both siltstones and shales are medium to dark grey, hard, subfissile to fissile, slightly calcareous to very calcareous. The rare limestones are thin, dark grey to tan, hard, argillaceous or microcrystalline, while the dolomites are light-medium grey, tan, hard and microcrystalline. Carbonaceous debris occurs throughout the sandstones and siltstones. The sandstones are developed exclusively in the south-western half of the basin. In the three most northerly wells (Deminex 56/21-1, Deminex 56/21-2 and Texaco 56/22-1), the unit dominantly consists of mudrocks and thin carbonates. The former are also more consistently darker in colour than those occurring in the other wells in the Fastnet Basin. The facies distribution is illustrated in Figure 11. The sandstones pinch out in the vicinity of the structural high, which is associated with the fault zone traversing the central part of the Fastnet Basin.

Log motif and boundary definitions. The lower


Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth IS5 156 ,,~2-, boundary is sharp, defined by the more regular, even = ~6/14-, gamma-ray response, above that of the cyclic motifs of _ ~(0)

Liassic Marl Unit (J-7-9) (Figures 12 and 13). The -56/20-1 I +,0, upper boundary can be sharp or gradationai. It is

56/21 1 56~8-1 defined by the topmost occurrence of irregularly serrate I $ ~ ~ gamma-ray response in the northern and central

regions of the basin (Figure 12), and by the top occurrence of the low gamma-ray and sonic responses

/22-1 ~,0,- coinciding with the sandstone lithologies in the - southern region of the basin (Figure 13).

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and Figure11 Facies map sand isopach for the Liassic . . . . Sandstone Unit (J-10)

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- - I . . . . . . . 2 ; "

, ,

Figure12 Type log for the Liassic Sandstone Unit (J-10) Figure13 Type log for the Liassic Sandstone Unit (J-10) (argillaceous) from Deminex 56/2%1 (arenaceous) from Cities Service 63/10-1



contour Interval .v.ry 250 m

I

0 JOKIll 0

Figure 14 lsopach map for the Liassic Sandstone Unit (J-10) in the Fastnet Basin

Two distinct gamma-ray log motifs characterize this unit. Those wells with a mudrock dominated sequence possess a highly serrate high gamma-ray log motif, whereas the sandstone lithologies possess low gamma-ray, funnel and occasional cylindrical and bell shaped log responses.

Distribution. This unit occurs in all 11 Fastnet wells, although it is only partially penetrated in the BP 56/26-l well. The Liassic Sandstone Unit (J-10) is also recognized in the North Celtic Sea and Bristol Channel Basins. Within the Fastnet Basin the unit is thickest towards the south-west, while towards the north-east the depocentre seems to constrict, although sediment thicknesses increase again into the North Celtic Sea Basin (Figure 14).

An unconformity is frequently present in the lower part of this unit (lower Sinemurian) and this has a significant influence on sedimentary thickness. This unconformity is recognized in those wells situated close to the central Fastnet transfer fault zone, namely Elf 55/30-l, BP 56/26-2 and Elf 64/2-l, as well as in those wells situated close to the basin margins, namely Cities Service 63/4-l, Ranger 63/8-l and Texaco 56/22-l.

Age. Sinemurian. Dating has been provided by the common, short-ranging ostracod and foraminiferal faunas. Short-ranging lower Sinemurian microfaunas are slightly more common and include such taxa as Klinglerella retia Ainsworth, Liubimovella? frequens Donze, Ruchholzella frequens Ainsworth, Astacolus semireticulata (Fuchs) and Znvolutina liassica (Jones). Diagnostic upper Sinemurian taxa include Klinglerella herrigi Ainsworth, K. exiloreticulata Ainsworth, Ogmoconchella dancia Michelsen, Lophodentina sp. A Ainsworth, Berthelinella involutina subsp. A and Lingulina tenera praepupa (Norvang). In many wells in the North Celtic Sea Basin and also those wells located in the northern region of the Fastnet Basin, dating is much less precise due to the paucity of microfaunas

and/or the recovery of only long-ranging microfossil taxa (Ainsworth, 1989a; Ainsworth et al., 1989).

Depositional environment. Inner to outer sublittoral. The sandstones are interpreted to be delta fringe deposits with a sedimentary source area lying to the west, south-east and south-west of the basin (Robinson et al., 1981; Naylor and Shannon, 1982). A facies map is shown in Figure II. This clearly illustrates the north-eastern pinch out of the sands. The encountered microfaunas are rich and diverse in the central and southern regions dominated by benthic shallow water ostracod and foraminiferal taxa, implying good water circulation. In the more northerly wells the microfaunas are rarer, with the exception of influxes of the foraminiferal taxa Reinholdella and Involutina. The former can tolerate more extreme and deeper water environments. This suggests that this part of the basin was deeper and possessed a restricted water circulation, often with anaerobic and dysaerobic bottom water conditions (Ainsworth, 1989a; Ainsworth et al., 1989). The presence of large numbers of miospores and carbonaceous debris is indicative of a proximity to land, with the palynomorphs washed into a low energy environment.

J-11 -12 Liassic Shale. Thickness 53- 1310 m. This unit dominantly consists of argillaceous rocks. These consist of light grey to dark grey, black, dark brown, soft to firm, subfissile to fissile, slightly calcareous to very calcareous claystones, mudstones, shales and siltstones. Thin dolomite, limestone and sandstone stringers also occur within this unit. The dolomites are dark grey, brown, tan, argillaceous, to hard and microcrystalline. The limestones are medium grey, brown, argillaceous or microcrystalline, while the sandstones are white, yellow, light grey, very fine to fine grained, generally well sorted, angular to subangular, hard, generally with calcareous matrices. Carbonaceous debris is moderately common.

Log motifs and boundary definitions. The lower boundary is either sharp or gradational, defined by the more monotonous slightly serrate log responses and the appearance of thick argillaceous sequences above the sandstones in the central and southern regions of the Fastnet Basin (Figure 15). The upper boundary is also sharp and is placed at the appearance of sandstones belonging to either the ?Middle Jurassic, Upper Jurassic or Lower Cretaceous, coinciding with a more irregularly serrate log motif. The Liassic Shale (J-11-12) unit is characterized by monotonous, finely serrate gamma-ray and sonic velocity responses, making subdivision of this unit very difficult.

Distribution. The Liassic Shale (J-11-12) occurs in all 11 Fastnet wells. It is also present throughout the North Celtic Sea, South Celtic Sea and Bristol Channel Basins. The isopach map (Figure 16) shows this sequence to be thickest along the axial part of the Fastnet Basin (Table 1). Thinning towards the basin margins is influenced by the erosive effect of the major unconformity which bounds the top of the unit.

Age. The Liassic Shale (J-11-12) extends from the lower Pliensbachian through to the top of the Aalenian. This sequence can be dated by both calcareous microfaunas and dinocysts. The ostracod faunas provide the best stratigraphical tool for subdivision of the unit, due to their profusion in these rocks and also their shorter stratigraphical ranges, for example,



Figure 15 Type log for the Liassic Shale Unit (J-11-12) from Deminex 56/21-1

Ogmoconcha convexa Boomer, lsobythocypris tumida Ainsworth, Bairdia parva Ainsworth, Praeschuleridea arguta Ainsworth, Ektyphocythere bizoni Ainsworth and Bairdia? eirensis Ainsworth (Ainsworth 1986, 1987; Ainsworth et al., 1989).

Depositional environment. The Liassic Shale (J-l 1-12) was deposited in shallow marine littoral environments. In both the lower Pliensbachian and lower Toarcian microfaunas are rare, or consist of taxa

55/30-1 (231):Z

56/14-1 ~t~-(338.5 )

/ f 8-, i6/21-1

r / ,oO / 56121-2 56122-1

56126-2

6 4 \ 64/2-1 ) \ o .... 64/1-1

• 500

5 6 / 2 0 - 1 ~)-(29.5}

N A Contour Interval every 500 m l

E 0 50Kin

Figure 16 Isopach map for the Liassic Shale Unit (J-11-12) in the Fastnet Basin

which can tolerate deeper and more extreme environments, for example, the foraminiferal genus Reinholdella. The green algae, Tasmanites spp. occurs profusely in the lower Toarcian, likewise implying restricted low energy deeper environments. Rock lithologies are also generally darker and less calcareous, but organically rich, especially those of the lower Toarcian. This suggests that both the lower Pliensbachian and lower Toarcian were deposited in a deeper water environment (outer sublittoral) with periods of major restriction; anaerobic and dysaerobic bottom water conditions are envisaged to have been prevalent throughout these sequences. During the lower parts of the upper Pliensbachian and upper Toarcian, conditions ameliorated, suggested by the more common appearance of shallow marine benthonic foraminifera and ostracod microfaunas, and the lighter coloured and more calcareous lithologies. During the major part of the upper Pliensbachian and the upper Toarcian-Aalenian, shallow (inner sublittoral) low energy, well oxygenated marine conditions are envisaged to have occurred, often high in carbonate, denoted by the profuse and diverse microfaunas and high carbonate content of the constituent lithologies. Throughout the Ptiensbachian to Aalenian, proximity to land is suggested by the large numbers of miospores and carbonaceous debris.

Conclusions

An integrated palaeontological and lithostratigraphical analysis of the Triassic to Middle Jurassic (Aalenian) interval has provided a stratigraphy for the 11 wells drilled in the Fastnet Basinl The rock units are essentially defined by wireline log correlation and have proved laterally contemporaneous by the palaeontological work. This has allowed temporal and spatial constraints to be placed on the early sedimentary fill of the Fastnet Basin. None of the major stratigraphic units

428 Mar ine and Pet ro leum Geology, 1991, Vol 8, N o v e m b e r

Stratigraphy of the Fastnet Basin: N. J. Murphy and N. R. Ainsworth defined show any significant diachroneity.

Carboniferous sediments are indicated in four wells, while the absence of Permian and lowermost Triassic is apparent. The occurrence of potential reservoir sandstone lithologies in the lower part of the Triassic is shown to be largely predictable. The boundary between the Triassic and the Jurassic is conformable in all control wells. The position of this boundary has now been confined to a narrow interval and is significantly higher than was previously assumed. Shallow marine, inner shelf carbonate sediments are confined to the Rhaetian and Hettangian. They do not extend into the Sinemurian. Slightly deeper water conditions are suggested in the north-east as the North Celtic Sea Basin is approached. Regression during the Sinemurian is marked by deltaic sequences building mainly from the south-western end of the basin, which reached a maximum geographical spread during the upper Sinemurian. Fine grained transgressive shallow marine (inner to outer sublittoral) sediments were laid down during the Pliensbachian, Toarcian and Aalenian. The youngest pre-Upper Mesozoic unconformity section penetrated by any of the wells is Aalenian in age.

The distribution of the rock units suggests that the loci of Upper Mesozoic rifting had been established at the latest by Rhaetian times. Control for the older Triassic units is more limited; however, distribution patterns can be interpreted to have been similar to Rhaetian and post-Rhaetian sediments. Both the thicknesses and the facies of all the defined rock units suggest that they were deposited over a wider area than that contained within the presently identified basin limits. While we suspect that Lower Mesozoic rifting may have occurred, the location and extent of fault control on the original depositional sites remains conjectural.

The refinement of the stratigraphy of the studied interval has been made possible by direct dating of Jurassic rock units, with a reasonably inferred age determination for those of the Triassic based on correlatable datable sequences from the North Celtic Sea Basin. This refinement, together with the identification of apparent unconformities in the upper Hettangian and Sinemurian, should form the basis for further sequence stratigraphic and structural work in this area, and also facilitate further soundly based regional stratigraphic comparisons.

References

Ainsworth, N. R. (1986) Toarcian and Aalenian Ostracoda from the Fastnet Basin, offshore southwest Ireland Bull. GeoL Surv. Ireland 3, 277-336

Ainsworth, N. R. (1987) Pliensbachian Ostracoda from the Fastnet Basin, offshore southwest Ireland Bull. Geol. Surv. Ireland 4, 41-62

Ainsworth, N. R. (1989a) Rhaetian, Hettangian and Sinemurian Ostracoda from the Fastnet Basin, offshore southwest Ireland Bull. GeoL Surv. Ireland4, 107-150

Ainsworth, N. R. (1989b) Remarks on the nomenclature of two ostracod species from the Pliensbachian and upper Toarcian-Aalenian of the Fastnet Basin, offshore southwest Ireland Bull. GeoL Surv. Ireland4, 165-166

Ainsworth, N. R. (1989c) Systematic descriptions of two ostracod species. In: Northwest European Micropalaeon- tology and Palynology (Eds. D. J. Batten and M. C. Keen), Ellis Horwood, Chichester, pp. 56-65

Ainsworth, N. R. and Horton, N. F. (1986) Mesozoic micropalaeontology of exploration well Elf55/30-1 from the Fastnet Basin, offshore southwest Ireland J. MicropalaeontoL 5, 19-29

Ainsworth, N. R., O'Neill, M., Rutherford, M. M., Clayton, G., Horton, N. F. and Penney, R. A. (1987) Biostratigraphy of the Lower Cretaceous, Jurassic and uppermost Triassic of the North Celtic Sea and Fastnet Basins. In: Petroleum Geology of North West Europe (Eds. J. Brooks and K. Glennie), Graham and Trotman, London, pp. 611-622

Ainsworth, N. R., O'Neill, M. and Rutherford, M. M. (1989) Jurassic and Upper Triassic biostratigraphy of the North Celtic Sea and Fastnet Basins. In: Northwest European Micropalaeontology and Palynology (Eds. D. J. Batten and M. C. Keen), Ellis Horwood, Chichester, pp. 1-44

Caston, V. N. D., Dearnby, R., Harrison, R. K., Rundle, C. C. and Styles, M. T. (1981) Olivine-dolerite intrusions in the Fastnet Basin J. GeoL Soc, London 138, 31-46

Gazdzicki, A. (1975) Lower Liassic ('Gresten Beds') microfacies and microforaminiferas from the Tatra Mts Acta Geol Polonika 25, 385-398

Jansa, L. V., Bujak, J. P. and Williams, G. L. (1980) Upper Triassic salt deposits of the western North Atlantic Can. J. Earth Sci. 17, 547-559

Jones, T. R. (1853) On the Lias near Fretherne, near Newham and Purton, near Sharpness, with an account of some foraminifera discovered there Ann Mag Nat Hist Ser 2 12, 272-277

Masson, D. G. and Miles, P. R. (1986) Development and hydrocarbon potential of Mesozoic basins around margins of North Atlantic Am, Assoc. Petrol GeoL Bull. 70, 721-729

Naylor, D. and Shannon, P. M. (1982) Fastnet Basin. In: The Geology of Offshore Ireland and West Britain (Eds. D. Naylor and P. M. Shannon), Graham and Trotman, London, pp. 49-58

Pegrum, R. M. and Mounteney, N. (1978) Rift basins flanking North Atlantic Ocean and their relation to North Sea area Am, Assoc. Petrol GeoL Bull. 62, 419-441

Petrie, S. H., Brown, J. R., Granger, P. J. and Lovell, J. P. B. (1989) Mesozoic history of the Celtic Sea basins Am. Assoc. Petrol. GeoL Mere. 46, 433-444

Robinson, K. W., Shannon, P. M. and Young, D. G. G. (1981) The Fastnet Basin: an integrated analysis. In: The Geology of the Continental Shelf of North West Europe (Eds. L. V. Illing and G. D. Hobson), Heyden, London, pp. 444-454

Rutherford, M. M. and Ainsworth, N. R. (1989) Micropalaeon- tological and stratigraphical recognition of the Triassic/ Jurassic boundary in the North Celtic Sea and Fastnet Basins. In: Northwest European Micropalaeontology and Palynology (Eds. D. J. Batten and M. C. Keen), Ellis Horwood, Chichester, pp. 45-69

Sladen, C. P. and Batten, D. J. (1984) Source-area environments of late Jurassic and early Cretaceous sediments in southeast England Proc. GeoL Assoc. 95, 149-163

Vakhrameev, V. A. (1970) Range and palaeoecology of the Mesozoic Cheirolepidiacae. Paleont. J. 12-25

Warrington, G. (1978) Appendix 1. Palynology of the Keuper, Westbury and Cotham Beds and White Lias of the Withy- combe Farm borehole. In: Stratigraphy of the Withycombe Farm Borehole, Near Banbury, Oxfordshire (Ed. E. G. Poole), Bull, GeoL Surv. G.B. 57, 40-42

Warrington, G. (1982) Palynology of cores from the basal Lias and the Permian-Triassic sequence of the Winterborne Kingston Borehole, Dorset. In: Winterbourne Kingston Borehole, Dorset, England (Eds. G. H. Rhys, G. K. Lott and C. A. Calver), Rept. Inst. GeoL Sci. 81, 122-126

Warrington, G., Audley-Charles, M. G., Elliot, R. E., Evans, W. B., Ivimey-Cook, H. C., Kent, P. E., Robinson, P. L., Shotton, F. W. and Taylor, F. M. (1980) A correlation of Triassic rocks in the British Isles GeoL Soc. London Spec. Rep. 13, pp. 1-78

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Documents

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