Aptian lithostratigraphy and biostratigraphy (foraminifera) of block 49 in the southern North Sea (U.K. sector)

Strata of Aptian age in block 49 of the southern North Sea (U.K.) are reviewed in terms of lithostratigraphy, wireline log character and foraminiferal fauna. In some boreholes, the basal part of the Middle Holland Shale Member is interpreted as Late Aptian in age while the Lower Holland Marl Member is interpreted as Late to Early Aptian in age. The foraminiferal biostratigraphy of the Aptian strata in the designated reference borehole, 49/25-1, for the Dutch lithostratigraphical scheme in the U.K. southern North Sea is discussed briefly.


INTRODUCTION
This article discusses the recognition of Aptian age strata in the southern North Sea Basin in terms of lithostratigraphy and foraminiferal biostratigraphy. In order to present a clear picture some repetition (almost verbatim) is necessary of statements made in a previous publication (Crittenden, 1984b). The overall aim of this article is to contribute to the at present meagre knowledge of U.K. Aptian micropalaeontology and stratigraphy.
Strata of Aptian age in boreholes in block 49 of the U.K. southern North Sea Basin (Fig. 1) can be recognised by distinct lithological and wireline log characteristics (low gamma ray log response and low interval transit times of the sonic log response). This permits broad correlation with other boreholes in the southern North Sea (Crittenden, 1982a), the central North Sea, the Netherlands (e.g. L5-1, N.A.M. & R.G.D., 1980) and with areas further afield such as Germany (e.g. Scheerhorn Oilfield, boreholes I46 and H1, Kemper, 1979) and Scandinavia (e.g. Dansk Nordso E-1, 1-1 and J-1, Rasmussen, 1978).
Lithostratigraphically, strata of Aptian agc comprise the whole of the Lower Holland Marl Member cquivalent of the Holland Formation (Rijnland Group) as defined by N.A.M. & R.G.D. (1980). and in some boreholes includes the basal portion of the Middle Holland Shale Member equivalent. The "basal sand" horizon of the Middle Holland Shale Member equivalent may be the lateral equivalent of the Holland Greensand Member (Fig. 2). These lithostratigraphical units can be dated in gross terms by the contained foraminiferal faunas (Crittenden, 1984a(Crittenden, , 1984b. Borehole 49/25-1 (Shell/Esso) is used to illustrate strata of Aptian age in the U.K. southern North Sea.

LITHOSTRATIGRAPHY
A synthesis of Early Cretaceous lithostratigraphical schemes for the southern North Sea Basin has been discussed previously (Crittenden, 1982a). A comparison of the Dutch scheme (N.A.M. & R. G.D., 1980) with the British scheme (Rhys, 1974) concluded that the Dutch scheme offered more scope an'd finesse for correlation purposes. It was considered pertinent therefore to designate borehole 49/25-1 as the reference borehole for the Dutch lithostratigraphical scheme in the British sector of the southern North Sea (Crittenden, 1982a). In addition, Crittenden (1982a) (Crittenden, 1982a).
careous claystoneklay limestone of the Lower Holland Marl Member. All of the aforementioned lithostratigraphical studies have aided the understanding of the geological development of the North Sea Basin during the Early Cretaceous. It is apparent that as more detailed attention, due to hydrocarbon exploration, has been paid to the Early Cretaceous strata of the North Sea Basin the various lithostratigraphical models proposed have progressed from the simplistic (e.g. Deegan & Scull, 1977) to the complex (N. A.M. & R.G.D., 1980). This is the inevitable consequence of the acquisition of more and more data.
From a comparison of the lithostratigraphical schemes available it can be concluded that different areas of the North Sea region during the Early Cretaceous were characterised by broadly similar patterns of sedimentation. These patterns are the results of similar genetic processes and environments of deposition (Ziegler, 1982).
The various basins and sub-basins within the North Sea region during the Early Cretaceous underwent the same, or very similar , stratigraphic development related to local tectonics (and salt movement) and regional tectonics (e.g. associated with the development of the North Atlantic). This concept is invaluable when assessing the possibility of hydrocarbon traps (particularly stratigraphic plays) in the Early Cretaceous strata of the whole of the North Sea Basin (i.e. source rock potential, reservoir potential, trapping and sealing potential and migration pathways).
The scheme of N.A.M. & R. G.D. (1980) and the Aptian strata of boreholes in block 49, in particular borehole 49/25-1, are discussed in this paper.

Middle Holland Shale Member
This member, predominantly of Albian age, consists of grey-brown and green, micaceous claystones which are glauconitic and calcareous. It has a lower lime content than the lower and upper members of the  (Fig. 3 ) . This member is well developed in borehole 49/25-1 of the study area (Fig. 4).
The coarser horizon at the base or toward the base of the unit, is depicted by the reduced gamma ray and increased sonic log responses (c. 2384m in L5-1) and  (1980) and which in L5-1 (2380-2384m) is interpreted as the basal sand/siltstone of the Middle Holland Shale Member. The unit termed the Lower Holland Marl Member in Vlieland Oost-1 may be the basal sand/ siltstone of the Middle Holland Shale Member and represent the product of the "basal Albian" transgression (note the double point spike of the sonic log). This hypothesis, however, has not been confirmed or refuted by microfaunal analysis (by the present author) of these two boreholes. It is not clear where the base of this member should be drawnat the lower sand/ siltstone horizon, if present, or beneath at the point where there is a pronounced log character change (lower gamma ray response and lower interval transit times). The latter case is taken by Crittenden (1982a). (1980) define this member in the West Netherlands Basin as a green, very glauconitic, fine grained sandstone to siltstone, present only along the southern margin of the basin, which rapidly "shales out" to the north. According to N.A.M. & R.G.D. (1980) this member is not present in borehole L5-1. However the presept author is of the opinion that the sandy/silty horizon at approximately 2380m-2384m is the lateral equivalent of the Holland Greensand Member. The same unit may be recognised in boreholes 49/25-1 at c.6425ft, 49/25-2 at c.6795ft and 49/24-12 at c.5060ft (see Fig. 7).  (1980) point out that the base of the Middle Holland Shale Member marks a pronounced transgression ("Albian transgression"). They include the thin coarse clastic beds denoting this transgression within the Middle Holland Shale Member rather than suggesting that they may be the lateral equivalent of the Holland Greensand Member. This same transgression may be seen in the Scheerhorn Oilfield (Fig. 6). In this oilfield the unconformity is pre-rzolani-jacobi Biozone (Aptian age) in borehole H-1 but is of pre-tardefurcutri Biowne (Early Albian age) in borehole 146.
The exact position of these lithostratigraphic units and boundaries poses a problem when attempting to integrate the biostratigraphic data with the lithostratigraphic data. This is, as previously mentioned, further complicated by the nature of the samples (ditch cuttings) and sample interval.

Lower Holland Marl Member
This member is a grey and red-brown marl/calcareous claystone/very argillaceous limestone, with intercalated bituminous claystone beds. This member, as recognised by Crittenden (1982a) is of Aptian age. The whole unit is characterised by a low gamma ray log response and low interval transit times which are displayed as a "belly" motif. This member is well developed in borehole 49/25-1 of the study area (Fig.   4). This member is coeval with the "tock facies" and "ewaldi kreide" of Helgoland, and the "fischsheifer" and "ewaldi marl" of northwest Germany and the "ewaldi beds" of Speeton, U.K. (Bartenstein & Kaever 1973;Kemper, 1973Kemper, , 1979.

BIOSTRATIGRAPHY
Aptian age dating by means of foraminifera is hindered by a number of factors. 1. Imprecise knowledge of both Aptian foraminifera1 (benthic and planktonic) faunas and the foraminifer-a1 sequence across the Albian'Aptian boundary (Kent & Dilley, 1968) in the adjacent onshore U.K. and northwest Europe (Crittenden, 1982b(Crittenden, , 1983a(Crittenden, , b,c, 1984a, 2. The geographical separation of the Albian and Aptian stratotypes in France (Rawson, 1983). 3. The locations of the Aptian stratotypes are in different faunal provinces. They fall within the tethyan realm in contrast to the boreahransitional realm situation of the Early Cretaceous southern North Sea. 4. The material studied from the boreholes in block 49 is cuttings samples which hinders accurate biostratigraphic study (problems of caving, sample interval and lag timesee Crittenden, 1979). For this study there were no conventional core o r sidewall core samples available for biostratigraphic analysis. A detailed discussion of the inherent disadvantages of borehole ditch cutting samples is considered unneccessary. A stratigrapher, to aquaint lilm herself fully with the problems, should ideally spend time at the wellsite calculating lag times and catching and preparing ditch cutting samples for analysis. Even so it is surprising how many micropalaeontologists do not appreciate how ditch cuttings are "caught" and thus cannot fully comprehend the disadvantages and drawbacks of using such samples for biostratigraphic analysis.

. A very important consideration is the state of flux
which exists in the erection of a foraminiferal biozonal scheme for the boreal Early Cretaceous strata of northwest Europe. This situation was recognised by Bartenstein & Oertli (1977) who acknowledged the difficulties of comparing the stages of the Early Cretaceous in northwest Europe, as defined by ammonites, with a microfaunal subdivision. The division of the northwest European Early Cretaceous biozonal scheme (foraminifera and ostracods) of Bartenstein & Bettenstaedt (1962) and the worldwide Early Cretaceous biozonal scheme (foraminifera) of Bartenstein (e.g. 1976a,b,c) are in need of revision* (Bartenstein, 1978a). The schemes are dominated by the use of benthonic foraminifera. The revised Aptian and Albian stages, in terms of their ammonite biozones, have been compared by Bartenstein (1978a) with the foraminiferal biozonation of Bartenstein & Bettenstaedt (1962). The ammonite biozone boundaries d o not coincide with extinctions, new appearances or major differences in the foraminiferal fauna (Bartenstein & Kaever, 1973). To overcome this difficulty a complete revision of the Early Cretaceous index foraminiferal species and faunas is needed from samples of a section calibrated for a control by index ammonities (Bartenstein, 1978a). Due to these factors it is considered improper to asume that, in all cases, foraminiferal age dating from ditch cuttings can provide accuracy to the limits of an ammonite biozone or subzone. However, the inability to provide an age in terms of an ammonite biozone or subzone must not be construed as a failing of foraminifera] biostratigraphy. Equally, the inability of ammonites to provide an age in terms of foraminiferal biozones or subzones must also not be construed as a failure of ammonite biostratigraphy. Kemper (1973) neatly sums up the problem "micropalaeontologists have created the problems through their biostratigraphical methodby giving ammonite bio zonal names to microfossil assemblages without finding the nominate ammonites, instead of proposing an autonomous classification".

Middle Holland Shale Member
This member is assumed by N.A.M. & R.G.D. (1980) to be Early Albian in age. This is essentially correct as seen in boreholes in block 49. However in the same boreholes studied in block 49, the foraminiferal fauna in the basal part of this member has an earliest Early AlbiadLate Aptian character (Crittenden 1984a,b). The AlbiaWAptian boundary therefore falls somewhere toward the base of this member. This earliest Albian to latest Aptian age, from a synthesis of all the studied boreholes in block 49, is suggested by the association (Hecht, 1938: with amendations per Bartenstein 1978a of the following key benthic species: Textuluria bettenstaedti Reuss of Eichenberg 1935, andGaudryina D16 of Hecht, 1938) The most reliable indication for a Late Aptian age is the co-occurrence of T. bettenstaedti, L. (S.) spinosa (top occurrence) and G. dividens, (Bartenstein & Oertli, 1977) but equally the total fauna could be of earliest Early Albian age (Kent & Dilley, 1968;Crittenden 1984a). The sample interval precludes precise biostratigraphic analysis of the basal part of this member in block 49 (see previous lithostratigraphical discussion).
The situation of the unconformities or unconformity at or near the AlbiaWAptian boundary is complex and in some wells difficult to assess. The base of the Middle Holland Shale Member may be Early Albian in age as illustrated by borehole I46 (see Fig. 6) or Late Aptian in age as illustrated by borehole H1 (see Fig. 6) and depends on the structural position of the borehole and degree of overlap. This paper assumes that the boundary of the Middle Holland Shale Member and Lower Holland Member is defined by the change to a consistent lower gamma ray response, i.e. the top of the "ewaldi Mergel" of Fig. 6.
An incoming downhole (in the basal Middle Holland Shale Member or top of the Lower Holland Marl Member depending on degree of hiatus) of abundant green coloured specimens of Hedbergella infracretacea (e.g. 49/24-1 and 49/24-12, 5060') together with L. (S.) spinosa and G. dividens is suggestive of the nolanijacobi Biozones of the Late Aptian (Bartenstein & Kaever, 1973).
In some boreholes (e.g. 49/20-2) in the Lower Holland Marl Member, there is an additional flood of red coloured Hedbergella infracretacea (Hedbergella sp. D11, Hecht 1938) beneath, again in association with L. (S.) spinosa and G. dividens (nutfieldensis Biozone). By comparison with Heligoland, this association is indicative of the earliest Late Aptian (Bartenstein & Kaever, 1973) and equated with the orange-red chalk of the upper ewaldi Kreide which is 2m thick in Helgoland (Kemper et al., 1974). This same flood of planktonics is noted by Bertram & Kemper (1982) in north-west Germany. In some boreholes in block 49, the strata beneath the "Albian unconformity" are Early Aptian in age and the distinctive flood horizon of H . infracretacea ( H . D11) is not observed (a result of erosion associated with a transgression or non-deposition on highs).
In addition to the occurrence of planktonic foraminifera in the Lower Holland Marl Member (see above), the abundance of Hedbergella aptiana Bartenstein 1965(= Hedbergella sp. D9, Hecht 1938 in the basal part of the Lower Holland Marl Member (49/24-12, 5100'), by comparison with Heligoland and northwest Germany, provides additional evidence of an Early Aptian age (Hecht, 1938;Bartenstein & Kaever, 1973).

BOREHOLE 49/25-1 -SYNOPSIS
The development of strata of Aptian age in borehole 49/25-1 is typical for block 49 of the southern North Sea. The Aptian "belly" of the Lower Holland Marl Member of the Holland Formation is quite distinctive (gamma ray log and sonic log) and reflects the increasing calcareous content of the sediments.

Late? to Early Aptian age (6450'-6520')
The top of this interval in this borehole is defined by the first downhole occurrence of a red stained benthic foraminiferal fauna and the topmost occurrence of infrucrefacea plexus are also present but are rare in number (6450'). This rare planktonic fauna, if in place, suggests an earliest Late Aptian age (Bartenstein & Kaever, 1973). The characteristic flood occurrence of red stained Hedbergellids was not observed in this borehole. Grey coloured specimens of H . aptiuna are also present (derived from the limestone), and in sample 6470-80' are common and occur in clumps. In sample 6490-6500' there is a flood occurrence of greylwhite coloured H. aptiana. This plonktonic fauna is characteristic of the Early Aptian i n northwest Germany (Bartenstein, 1965;Hecht. 103s: Bartenstein 62 Kaever, 1973;Bartenstein. 197Ha). Thc lxisal part of this interval contains C. apticwsis a n d 1.. (S.) frankei, both of which characterise thc Eiarly Aptian. The sonic log feature at 6520' probably reprcsciits ;in unconformity. This interval rests unconform~ihly upon ?Late to Early Barremian age sediments.

CONCLUSIONS
Aptian strata in the North Sea Basin and the northwest European region are widespread and of diverse facies. The stratigraphic subdivision of the Aptian strata onshore is based on the study of the microfaunal, macrofaunal and lithological sequences encountered. The foramhiferal faunas contained in ditch cuttings from boreholes drilled offshore in the southern North Sea Basin, by comparison with dated Aptian age sections onshore in northwest Europe, indicate an Aptian age and provide a biostratigraphic subdivision of the established lithostratigraphic sequence used in the southern North Sea Early Cretaceous strata.