Mesozoic micropalaeontology of exploration well Elf 55/30–1 from the Fasnet Basin, offshore southwest Ireland

The geology, biostratigraphy and palaeoecology of exploration well Elf 55/30–1 in the Fastnet Basin are summarised. The biostratigraphical and ecological distribution of the foraminifera and Ostracoda from the late Triassic, the Lower Jurassic and the Lower Cretaceous are reviewed with reference to microfaunas elsewhere in Europe. Selected microfossil taxa are illustrated.


INTRODUCTION
This paper presents a biostratigraphical and palaeoecological description o f the foraminifera and Ostracoda trom part 01' the late Triassic, the Lower Jurassic and Lower Cretaceous of exploration well Elf 55130-1. Other studies of the Mesozoic microfaunas within the Celtic Sea area include that of Colin et ul. (1981), describing the Cretaceous and late Jurassic microfaunas from the Esso-Marathon wells within quadrants 47,48, 56 and 57; and a study discussing the 12 microfaunal and microfloral associations recognised in the Lower Cretaceous in the Fastnet Basin (Ainsworthet a/., 1985).

GEOLOGY
ElfS.5,'30-1 was the first o f ten wells drilled within thc Fastnet Basin, which is situated at the southwestern end ot' the North Celtic Sea Graben, approximately 140 km south o f lreland (Fig. I). T h e fault-bounded basin is elongate, incasuring 110 km long and 4 0 k m wide and trends northeast t o southwest (Naylor & Shannon, 1982). It was spudded on 18th April 1976 and was plugged and abandoned on 28th June 1976. T h e well wasdrilled in 130m of water and reached a total depth of 2800 m., terminating in Devonian rocks.
The geology of the Fastnet Basin has been described by Robinson el u l . (1981) and by Naylor & Shannon (1982). Early Triassic continental red beds uncontormably overlie the Devonian red beds and tuffs. These sediments are su,cceeded conformably by the Liassic Limestone sequence characteristic of the Lower Jurassic marine transgression. In the basal part of this limestone unit both littoral and non-marine microfaunas occur, indicating a transgressiveiregressive shoreline in this area during the Rhaetian and early Hettangian. A shallow marine environment became established in the late Hettangian and earliest Sinemurian. A marly shale o f Sinemurian age overlies this limestone, above which is a Sinemurian sandstone sequence thought to represent a delta front. These sandstones pass into low energy outer-shelf calcareous shales of Sinemurian -Pliensbachian age. The early Toarcian is represented by shallow marine shales with much lignite and gypsum, suggesting a close proximity t o land.
A major unconformity occurs between the Cretaceous and the Lower Jurassic which is a result of Cimmerian movements. The lowermost Cretaceous rocks consist of non-marine sandstones and shales, devoid of in situ microfaunas, typical of the ' Wealden' facies. These pass into the Barremian marine clays, sandstones and lignites containing moderately abundant marine fossils. This marine incursion continued t o deepen until the earliest Albian, where a regressive Greensand phase of Albian to early Cenomanian is initially represented by a marginal marine facies. and later an inner shelf facies. This is followed by the late Cretaceous Chalk marine transgression spanning the Cenomanian t o Campanian Stages.
T h e Tertiary sediments unconformably overlie the Cretaceous and consist o f a thick inner shelf limestone succession of Middle Eocene to Oligocene age. This passes into opcn marine Miocene t o Pliocene clays with a topinost unit of arenaceous sediments.

BIOSTRATIGRAPHY AND PALAEOECOLOGY
One hundred and eleven cuttings samples were examined, 14 from the Lower Cretaceous and lowermost Upper Cretaceous, and Y7 from the Lower Jurassic and part of the late Triassic. The samples were taken at either 5 m o r 10m intervals, which in some cases has led to assemblages from different stages becoming mixed, especially in the condensed Lower Cretaceous sequence.
The more important foraminifera1 and ostracod taxa from the Lower Cretaceous, the Lower Jurassic and late Triassic are listed below. The first occurrences mentioned represent the topmost occurrence of the microfossil in the well (Figs. 2, 3).  The monospecific Orbitolina fauna in the top of the Greensand appears to be from the same form group (equivalent to form group IV of Hofker, 1963, range late Albian to early Cenomanian) as those described from the Upper Greensand of Britain and France which Carter & Hart (1977) have shown to be of early Cenomanian age. This widespread fauna reflects the northward movement of the Tethyan province due to its climatic amelioration and associated transgression in the late Albian and Cenomanian (Price, 1967).

I R E L A N D
Ostracoda: Cytherella sp. Only fragmental evidence of Ostracoda were seen in the early Cenomanian cuttings.

H e c h t i c y t h e r e d e r o o i N e o c y t h e r e ( C j g o t t i s i
The overall assemblage resembles that described from the early ,4lbian of Rumania by Neagu (1965), and is also similar to the fauna recorded from the Gault Clay of Britain (Chapman, 1891(Chapman, -1898 but is more Tethyan in aspect. The main characteristic of this fauna is the lack of planktonic foraminifera and the moderate diversity. This is probably a reflection of deposition in inner shelf conditions. Ostracolda: Bairdoppilata pseudoseptentrionalis Mertens, 1956, Cytherella gr. C . ovata (Roemer, 1841) Cytheropteron arguturn Kaye, 1965 (Triebel, 1940), Rehacythereis sp. 1, Schuleridea sp. 1, Veenia? compressa Kaye, 1965b, V . florentinensis Damotte, 1961. Many of these species have been recovered by Colin er al. (1981) from the Albian and Cenomanianof the North Celtic Sea. The first definite Albian index species which occurred in this well was Veenia? cornpressa which was recokered immediately below an interval without returns (1020 m-1045 m). This fauna has also been described by Kaye (1965a) in southern England, Germany (Griindel, 1966) and northern France (Damotte, 1971).
The Aptian and Barremian sequences are extremeiy condensed. As the samples examined were taken only every 10m, there has been considerable mixing of assemblages, particularly in the sample from 1080-1090m which contains the Barremian index fossil Epistornina hechti with ostracods more typical of the Aptian to late Barremian (see below).
As mentioned above, due to the considerable mixing o f the fauna, many of these species extend downwards into the latest Barremian intervals, however, all of these species d o make their first downhole occurrences in the Aptian.
' Wealden' facies (109Om-1242 m) As in the rest of the Fastnet Basin, the sediments of the Wealden facies were devoid of in situ microfauna. This contrasts with the Celtic Sea Basin which, in the authors' experience, contains non-marine Ostracoda sufficient to zone the continental section of the Lower Cretaceous and the uppermost Jurassic. The facies in the two basins are similar, and there is n o obvious explanation for these differences.

Lower Jurassic and late Triassic
Below the barren clays and sandstones of the 'Wealden' facies, there is an abrupt change to the grey shales of the Lias. Much of the following fauna persisted through several stages. This may be partly due to caving, so only the first downhole occurrences and subsequent downhole abundances are mentioned here.
The foraminifera] faunas are similar to those described by Copestake & Johnson (in press) from the Mochras Borehole in North Wales. The zonation given below is based on the ranges of index species established for the Lower Jurassic of Britain (Copestake & Johnson, 1981, 1984 and Europe (Bartenstein & Brand, 1937, Norling, 1972 Bate & Coleman, 1975. The fauna recovered was very sparse, with only a single specimen of the above index microfossil being recorded. The paucity of fauna is probably due to the marginal marine conditions; close proximity to land is indicated by the occurrence of gypsum and lignite within the grey/ black shales.  Bartenstein, 1937, Textularia aeroplecta Tappan, 1955, Trochammina cf. T. canningensis Tappan, 1955. The late Pliensbachian yielded a rich fauna of typical Upper Lias foraminifera which is very similar to that described by Copestake (1974) and Johnson (1975) from the Mochras Borehole and also to that described from the British mainland (Barnard, 1956(Barnard, , 1957(Barnard, , 1959 and northwest Germany (Bartenstein & Brand, 1937).
The fauna is diverse and numerous at the top and bottom of this interval, but there is a section between 1400 m-l450m, in which microfossils are rare or absent. This appears to correspond with the sparse foraminifera1 faunas encountered in the early Pliensbachian of Britain (Copestake & Johnson, 1981). Sedimentation in this interval consists of dark grey shales with pyrite suggesting basin stagnation with anoxic bottom conditions. Ostracoda: Bairdia aff. B. carinata Drexler, 1958, Isohythocypris aff. I . elongala (Tate & Blake, 1876), Pseudomacrocypris sp. 1, Indet. Gen.

Explanation of Plate 1
Conversely, Reinholdella murgarita only ranges up to the early Sinemurian in Britain (Copestake & Johnson, 1981) and its first downhole occurrence at 1600m may therefore represent the top of the early Sinemurian. However, its range in France extends into the late Sinemurian and until its range for this area is better established, it cannot be used as an early Sinemurian marker.
The fauna is abundant and diverse and includes a variety of long ranging species not mentioned above, such as Eoguttulina spp. and Lenticulinu spp. ex gp. L . muensteri.
While abundance is low through this interval there is moderate diversity, with numerous new taxa. These include undescribed species of Kinkelinella (Ektyphocythere) and some undescribed genera. Many specimens of' Kinkelinella recovered from 55i30-1 show marked similarity to K . (E.) treibeli (Klinger & Neuweiler, 1959) recorded from the late Sinemurian of northern Europe.
The top of this interval is marked by the flood occurrence of the Reinholdella spp. and Oberhausella mesotriassica. A similar flood is reported from the Middle Hettangian (liasicus zone) in the Mochras Borehole (Copestake & Johnson, 1981) and it is probable that the two are equivalent. Their age is supported by the occurrence of Planularia nucleata which was recorded from the Hettangian (Barnard, 1949) in his study of the Lower Lias of Byfield although its total range is uncertain.
Below 1820m, the fauna is extremely sparse with occasional incursions of a fauna of a broadly Lower Lias aspect and few simple agglutinated species at 1930m. Both Ogmoconcha hagenowi and Ogmoconchella ellipsoidea were recovered in flood abundance between the intervals 1800-1810m, however, both these species abruptly disappeared below 1820 m (the top of the limestone). No microfauna was recorded until 1880m where a new fauna occurred, consisting mainly of Darwinula spp. and Limnocythere 'p., often in abundance, in-dicating brackish environments (Kozur & Oravecz-Schoffer, 1972). Although many species of Limnocythere recorded in Europe occur within the Rhaetic, the present authors' species may range into the earliest Hettangian.
Below this non-marine fauna, between 2045-2080m, a marine ostracod fauna was recovered, consisting of abundant Ogmoconchella aff. 0. ellipsoidea from a clay interval. The presence of this fauna within the otherwise non-marine limestones indicates an unstable marginal environment at the commencement of the Rhaetian marine transgression (Robinson et al., 1981, Naylor & Shannon, 1982. This fauna disappeared after 2080m and below this no microfaunas were recovered.

CONCLUSIONS
The marginal marine and marine sediments encountered in Elf 55/30-1 contained microfaunas which were used to provide a good biostratigraphical and palaeoecological zonation of the Lower Cretaceous, the Lower Jurassic and part of the late Triassic. The nonmarine ' Wealden' interval could not be subdivided due to the absence of in situ faunas.
The facies and microfaunas occurring in this well were closely comparable with those seen in the Mesozoic of Great Britain and France, although ranges of some of the microfossils are different.