A proposed Albian To Lower Cenomanian nannofossil biozonation for England and the North Sea Basin

Analysis of abundant, diverse and well-preserved Albian nannofloral assemblages from onshore English sections has enabled a high-resolution nannofloral zonation to be developed with the construction of sixteen zones. The results are correlated with the ammonite zonation. Five new species, Acaenolithus viriosus sp. nov., Staurolithites rotatus sp. nov., Staurolithites canthus sp. nov., Bownia glabra sp. nov. and Calculites percernis sp. nov. are described.


INTRODIJCTION
Extensive sampling of Albian, ammonite-dated field localities from England (Fig. I ) together with additional localities in Germany, France and the southern North Sea Shell/Esso well, 49/25a-9, has provided an opportunity for a refined Albian nannofossil biozonation scheme to be developed.
Prior to 1987, calcareous nannofossil zonations o f the Albian generally had low resolution, e.g. Thierstein (l976), Sissingh (1977) and . Many of the marker species used in these zonations (see Fig. 18) are based on localities in Tethyan areas. These species are often rare, entirely absent or have different age ranges in the Boreal Realm. Jakubowski (1987) highlighted the immense potential of using calcareous nannofossils to date the Lower Cretaceous using released well sections mainly from the Moray Firth Basin. He erected six zones for the Albian using a combination of last appearance datums (LAD), first appearance datums (FAD) and semi-quantitative events. However, he did not have the benefit of macrofossil-dated material and hence his correlations with the ammonite zonation were speculative.
This study attempts to establish a scheme useful both for academic workers and for industrial purposes, in which LADS and acme datums are regularly used. A total of sixteen zones are defined for the Albian. This study establishes a link between macrofossil (MF) and nannofossil (NF) biostratigraphies, thus extending interdisciplinary correlations both geographically and biostratigraphically. The zonation was developed in conjunction with studies of North Sea wells and its applicability in this area has been proven, although detailed primary data (except for well 49/25a-9) cannot be documented here for commercial reasons.

Sample preparation
For light microscope examination, the samples were prepared by placing a small amount of sediment directly onto a microscope slide. A pipette was used to place a drop of distilled water onto the sample and smeared out into a thin layer by using a clean picking brush (size 101). The smeared sample was dried on a hot plate and a coverslip was attached using a permanent mounting medium. The picking brush is placed in 10% HCI to remove any remaining residue.

Counting technique
Samples were examined with a light microscope at a magnification of 1000X. A transect of thirty fields of view is taken with all specimens counted. Some species, e.g.
Watznaiieria bartiesue, are so profuse that only ten or, in some instances, five fields of view are counted. Its abundance is subsequently multiplied out to thirty fields of view.
The following relative abundance categories which are used extensively in industry are also utilized in the present study: rare : less than 1 specimen per 30 fields of view. occasional : 1-2 specimens per 30 fields of view. common : 3-10 specimens per 30 fields of view. abundant : 11-29 specimens per 30 fields of view. influx

YORKSHIRE (a) Speeton [TA 155 7551
The upper part of the Speeton Clay at Speeton (Fig. 12) and the overlying Red Chalk are almost devoid of ammonites. Lampliigh ( 1924) divided the clays into five subdivisions lettered A 1. 0 E in descending sequence using the more abundant and better preserved belemnites. The A Beds are of Albian age and were subdivided A1 to AS by Wright (1935).
The dark clays representing Bed AS contain the belemnite Neohibolites ewaldi and were originally correlated with the Aptian Sutterby Marl of Lincolnshire by Spath (1924). The upper part of these beds, however, contain a Lower Albian ostracod assemblage (Kaye, 1964). This age assignment is supported by the palynological flora which contains Kleithriasphaeridiurn sirnplicispinurn and lacks any Aptian markers (R. Davey, pers. cornm., 1993).
The 'Greensand Streak', Bed A4, and the basal part of Bed .43 are non-calcareous and barren of calcareous nannofossils. Kaye (1964) postulated a Middle Albian age for these beds. The overlying sediments, however, yield Lower Albi,an nannofloral assemblages (NF Zone NAL 3) similar to those found in the rnarnmillaturn M F Zone at Chamberlain's Barn, Bedfordshire. Brown clays were also found yielding the Middle Albian markers, Ceratolithinu cruxii and Crucicribrurn anglicurn (NF Zone NAL 4) associated with Harnites and Hoplites ammonites (A.S. Gale, pers. comm., 1996), but their exact level from within Bed A3 was not established. It should be noted that extensive slumping occurs at this level (P. Rawson, pers. comm., 1994). Due to slumping, Beds A2 and A1 were not sampled, although Euhoplites and Mortoniceras ammonites have previously been recovered from Bed A1 and indicate an Upper Albian age. The overlying Red Chalk (16.lm) is also regarded as Upper Albian. Further south, at South Ferriby, in proximity to the Market Weighton High the Red Chalk ( = Hunstanton Formation) is only 2.5 m thick and, in part, forms a highly condensed lateral equivalent of the Red Chalk Formation at Speeton and the offshore southern UK Sector of the North Sea.

(b) Heslerton No. 2 borehole [SE 9199 75891
This Heslerton No. 2 borehole can be viewed at BGS, Keyworth. Macrofossil data are limited from the interval studied in this borehole (P. Rawson, pers. comm., 1994). Nannofossil data (Fig. 11) suggests these sediments are equivalent to the top of Bed AS, Bed A4 and the base of Bed A3, as exposed at Speeton.

(c) South Ferriby, Humberside [SE 911 2041
The Carstone Formation at South Ferriby is considered to be of Lower Albian age based on the brachiopod fauna (Whitham, 1991). This formation, which consists of hard, iron-rich, pebbly sands, is non-calcareous and barren of nannofossils.
At Melton Bottoms [SE 970 2701, just north of South Ferriby, the basal red marls of the Hunstanton Formation are considered to be of Middle Albian age (Whitham, 1991) based on a single whorl fragment of the ammonite Dirnorphoplites cf. hilli Spath. The absence of Middle Albian NF zones at South Ferriby ( Fig. 13) is possible evidence of a diachronous base to the Red Chalk particularly over the Market Weighton High. Sediments equivalent to the auritus M F Subzone are absent. This stratigraphic break is also present at the Heslerton No. 2 borehole (D. Rutledge, pers. comm., 1994) and possibly at Speeton (pers. obs.).
The upper part of the Hunstanton Formation at South Ferriby is equivalent to the majority of the Red Chalk at Speeton.

3239)
The section at Mt. Risou (Fig. 15) exposes a thick succession of mark and minor limestones across the Albian/ Cenomanian boundary which have been accurately dated with ammonites (A.S. Gale, pers. comm., 1994). This is the only location in the present study where common/abundant Broinsonia enorrnis have been found in Upper Albian dated sediments. A hiatus is present at this level in southern Britain.
The NF Zone NCl nominate marker, Calculites anfractus is found at Mt. Risou. The F A D at this locality is stratigraphically higher than that seen further north in southern England, where C. anfractus is 'found associated with Bownia glabra and Gartnerago chiasta. It appears that C. anfractus, although not endemic to the Boreal Realm, as  Eyers (1992). Abundance counts from 30 fields of view. P re%rs to species present outside 30 fields of view. Marker bpecies in bold. Ammonite stratigraphy after Owen (pers. comm., 1994) and Eyers (1992). Abundance counts from 30 fields of view. P re%rs to species present outside 30 fields of view. Marker bpecies in bold. Ammonite stratigraphy after Owen (pers. comm., 1994) and Eyers (1992).   this study has proven, certainly preferred high latitudes. The isolated occurrence of C. anfractus within sample R9 may indicate an earlier cold water incursion from the north. This is supported by the presence of Seribiscuturn primitiuutn, a form characteristic of Boreal Realm Albian nannofloral assemblages (Crux, 1991 and pers. obs.) but extremely rare at the Mt. Risou section.

VOHRUM, NORTHWEST GERMANY Vo TK 25
HAMELERWALD No. 3626 (re: 3578800, h: 58 00 000) This locality (Fig. 16) exposes a succession of dark grey to black mudstones of Lowermost Albian to Uppermost Aptian age. A secondarily altered tuff is located at the Albian/Aptian boundary. The calcareous nannofossils of this section have been studied by Cepek (1982) and Mutterlose (1989). The assemblages are of low abundance and diversity.
A detailed analysis of the sequence is given by Kemper & Zimmerle (1978) and Kemper (1982).

ZONATION
The zonation outlined below was developed as a practical tool for subdividing the Albian to Lower Cenomanian of onshore sections, mainly from England and offshore material from throughout the North Sea Basin. Work was started with the aim of improving on the NF scheme of Jakubowski (1987) and constructing a scheme useful for both academic and industrial purposes. In this study, sixteen zones are defined for the Albian/Lower Cenomanian interval. They are correlated with the MF zones and compared with previous NF zonations in Fig. 18. A composite range chart of biostratigraphically useful species is presented in Fig. 19.    and Jakubowski (1987) as a zonal boundary. In the presmt study P. columnatu was, however, found to bc sporadic towards the base of its range in the uppermost Lower Albian at Speeton and Chamberlain's Barn. In the Tethyan Realm, however, the F A D of P. columnata, in the absence of other markers, is a useful approximation to the base of the Middle Albian. IJnder the light rnicroscope no differentiation could be made between P. spinosu and P. cJ stoveri.

Braloweria boletiformis Partial Range NF Zone (NAL 4) Definition: Interval from F A D of Crucicribrum anglicum (and Ceratolithina cruxii) to L A D of Braloweria holetiformis.
Age: Middle Albian, base of lyelli M F Subzone to top of niobe M F Subzone. Remarks: Axopodorhabdus alhianus has its F A D at the base of the intermedius M F Subzone both at Folkestone (Bed 11) and at Mundays Hill (pers. obs.) but the occurrences are extremely rare and sporadic. These records, however, support the identification of A . alhianus by AmCdro et a/. (1981) from coeval sediments at Boulonnais, France.
The F A D of A . ulhianus which was used by Cepek & Hay (1969), Thierstein (1976) and Roth (1978) as a zonal marker is not taken as a reliable datum in this study due to its extreme rarity at the base of its range. Records of A . alhianus below the Middle Albian (Perch-Nielsen, 1985) are probably due to contamination or misidentification. The presence of A . alhianus and Ceratolithinu hamata in the niobe MF Subzone  is possibly due to mis-sampling over the niobelcristutum stratigraphic break. Hayesites alhiensis is inconsistently present and usually rare in the sections studied. The rare and intermittent occurrence limits any biostratigraphical usefulness. It however, appears to be more consistently present in localities from Kent than further north in Cambridgeshire and Bedfordshire. Hayesites alhiensis was not identified from Yorkshire field sections or the North Sea Basin.
Braloweria boletiformis, however, is far more widespread than previously recognized. It is consistently present in the condensed Middle Albian successions of the North Sea Basin (pers. obs.) and may be recorded outside northwest Europe in future studies. This form has only previously been recorded from onshore material .
Bownia glabra Interval Range NF Zone (NAL 5) Definition: Interval from L A D of Braloweria boletiformis to F A D of Ceratolithina bicornuta. Age: Middle Albian, base subdelaruei M F Subzone to top meandrinus MF Subzone. Remarks: The Bownia glahra NF Zone yields no FADS in this study and the assemblages show only local changes in the relative abundance of some taxa. Sediments equivalent to NAL 5 are very restricted in occurrence due to the erosion of Middle Albian sediments during the cristatum M F Subzone (Owen, 1975). This NF biozone was not sampled at Folkestone due to the attenuated sequence present at this locality. Sediments equivalent to this N F zone are identified, however, within the Soham borehole. NAL S has not been identified in the North Sea Basin.
The LAD of a Repagulum parvidentatum influx is an easily recognized and consistent event occurring within the North Sea Basin. The datum was used by Jakubowski (1987) in the identification of his Repagulum parvidentatum Zone (NLK5).
This event. however, as recorded by Crux (1991) using abundance variations at Munday's Hill is unreliable in  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I I I 1 Fig. 19. (Continued.)   , is anomalous possibly due to mis-sampling over the niobelcristatum stratigraphic break. Sediments equivalent to this NF biozone appear to be very restricted due to a short but widespread period of erosion in the cristatum MF Subzone (Owen, 1975). Ceratolithina bicornuta is rare in offshore wells possibly as a result of the condensed nature or absence of NAL 6 equivalent sediments. Ceratolithina bicornuta has, however, been observed (pers. obs.) in the South Halibut Basin (North Sea).

Ceratolithina hamata Interval Range NF Zone (NAL 7)
Definition: Interval from LAD of Ceratolithina bicornuta to FAD of Tegulalithus tessellatus (and Gartnerago praeobliquum). Age: Upper Albian, base of cristatum MF Subzone to top varicosum MF Subzone. Remarks: Due to the absence of the Middle Albian lautus MF Zone over much of the English Albian, the first occurrences of Ceratolithina hamata and Axopodorhabdus albianus are often found at the base of the Late Albian in the cristatum MF Subzone e.g. at Munday's Hill.  considered the FAD of Owenia hilli as a potential biostratigraphical datum for the basal Upper Albian. In the present study, the FAD of 0. hilli is recorded earlier within the daviesi MF Subzone.
Hayesites albiensis is confined to Albian sediments below (except for a single identification within the Folkestone borehole) the FAD of Eiffellithus turriseiffelii in this study  and . Many authors including Roth & Thierstein (1972), Verbeek (1977) and Manivit et al. (1977) used H . albiensis as a zonal marker co-occurring with E. turriseiffelii. H. albiensis is considered an unreliable biostratigraphic marker in this study.
Braarudosphaera stenorhetha appears in this NF zone in the southern North Sea where it is associated with comrnonlabundant Braarudospharea primula and Braarudosphaera quinquecostata. The Braarudosphaera assemblage is particularly well developed in East Anglia, Yorkshire and in the southern North Sea. The commonlabundant occurrence of the Braarudosphaera group in the orbignyi to varicosum MF Subzones is considered a localized event possibly due to palaeoenvironmental conditions. Hill (1976) recorded a similar event (Braarudosphaera quinquecostata Acme Zone) associated with Eiffellithus turriseiffelii.
Lambert (1986) grouped the separate species of Braarudosphaera under B. africana, based on the discovery of entire coccoliths in Albian laminated mudstones from Cameroon. The different architectural forms have not all been grouped under B. africana in the present study since they appear to have distinct stratigraphic ranges.
Remarks: This NF zone contains a period of nannofloral diversification characterized by the rapid evolution of the Eiffellithaceae. This evolutionary lineage has previously been documented by many authors, e.g. Verbeek (1977) and Hill & Bralower (1987). A similar evolutionary trend exists between Staurolithites angustus and Staurolithites rotatus within this NF biozone, whereby the bars which were near parallel to the axes of the ellipse rotated to form large angles with the axes. The FAD of E. turriseiffelii has been used as a zonal marker event by many authors, e.g. Roth (1973), Thierstein (1976), Sissingh (1977), Taylor (1982) and Jakubowski (1 987).
The FAD of abundant Eiffellithus monechiae is an alternative marker for the base of NAL 11.

Radiolithus hollandicus Partial Range NF Zone (NAL 12)
Definition: Interval from FAD of Crucibiscutum hayi to LAD of Radiolithus hollandicus. Age: Upper Albian, rostratum MF Subzone. Remarks: Radiolithus hollandicus occurs as distinct influxes at disparate stratigraphic levels and can be missed in field samples, sidewall cores or core samples. Its LAD in ditch cuttings, however, is a recognized correlatable event in North Sea wells.
Eiffellithus monechiae is subordinate to E. turriseiffelii within this biozone.
This NF biozone equates in part to the Gartnerago praeobliquum NF Zone (NLK4) of Jakubowski (1987). This NF zone was defined as Middle to Upper Albian in age without any reference to ammonite-dated material.

13)
Definition: Interval from L.AD of Radiolithus hollandicus to F A D of common/abundant Broinsonia enormis. Age: Upper Albian, rostratum-perinflatum M F Subzones. Remarks: This stratigraphic level has been correlated to a transgression within the dispar M F Zone (Eyers, 1992). In England, this sea-level rise is expressed by monotonous, very pale grey clays, with little evidence of breaks in sedimentation, unlike the underlying Gault Clay which is usually darker in colour and punctuated by numerous stratigraphic breaks.
Thz nannoflora within this N F biozone exhibit a degree of provincialism.
Crucibiscutum hayi, Cribrosphaerella ehrenbergii, Percivalia fenestrata and Staurolithites rotatus are characteristic components of this NF zone in England and the southern North Sea but are rare in the central and northern North Sea.

Broinsonia enormis Partial Range NF Zone (NAC) Definition: Interval from F A D of common/abundant
Broinsonia enormis to F A D of Calculites unfractus. Age: Upper Albian, perinflatum M F Subzone -'Lowermost' Cenomanian, carcitanense MF Subzone (lower part). Remarks: [n southern England NAC has only been found to correlate with macrofossil dated sediments of 'lowermost' Cenomanian age. A regional non-sequence is present at the Albian/Cenomanian boundary in northern France and southern Britain. An expanded ammonite dated equivalent of this NF biozone (Upper Albian to Lower Cenomanian) is preserved in the Vocontian Trough (A. S. Gale, pers. comm., 1994). A thick development of N A C dated sediments are also present in the Red Chalk of Speeton (pers. obs.). It is plausible that at Speeton, unlike in southern Britain, a complete Albian to Cenomanian boundary succession is present. Unfortunately, ammonite data are extremely sparse and the Cenomanian/Albian boundary is difficult to accurately locate (H. G. Owen, pers. comm., 1994).
The nannoflora at Mt. Risou does not exhibit any marked assemblage changes over the Albian/Cenomanian boundary. The F A D of Gartnerago n a n u m is of potential importance but further work is required to prove its value as a boundary marker. Gartnerago chiasta is present in the Vocontian Trough, England and southern North Sea but does not appear to have migrated any further north.
The L A D of abundant Biscutum constans approximates to the top of NCl in the central North Sea. This event is, however, diachronous and found in younger sediments from onshore localities and in well sections from the North Viking Graben (Northern North Sea).

TAXONOMY
A detailed taxonomic section is not provided, although taxa referred to in this paper are listed alphabetically in Appendix 11. Taxonomic references not included in, the reference list can be found in Perch-Nielsen (1985). New species and combinations are detailed below.
Genus ACAENOLITHUS  Acaenolithus viriosus sp. nov. Dimensions: Mean length -8.9 pm, 50 specimens measured. Maximum length -11.8 pm. Smallest forms appear to grade into A. galloisii (mean length of 5.5 pm). Forms with a maximum length of greater than 8 p m are assigned to A.viriosus. Remarks: Acaenolithus uiriosus appears to be constructed similarly to A. galloisii when viewed under the light microscope. Acaenolithus uiriosus is, however, much larger. A siimilar form to A. uiriosus, although possessing a more elalborate rim structure, occurs in the Glauconitic Marl at Folkestone and is possibly synonymous to A . cenomanicus .
Genus BOWNIA Varol, 1994 Bownia glabra sp. nov.  fig. 4; P1. 2, figs 5-6. Derivation of name: Latin percernis, meaning easily visible. Diagnosis: This holococcolith is composed of a narrow rim, a broad wall consisting of a limited number of calcite blocks and a central pore. Under crossed-nicols and with the axes of the ellipse aligned with the nicols, extinction gyres lie on the principal axes of the ellipse. With the axes rotated 45 degrees to the nicols, the gyres do not intersect but form arches about the acute ends of the ellipse. Holotype: SMH-12-05 (Pl. 2, fig. 8). Type locality and horizon: Burwell, Cambridgeshire, Bed 16 (Late Albian). Dimensions: L: 3-5 pm (mean length 4.4pm), SO specimens measured. Remarks: This small holococcolith is abundant at certain horizons within the auritus MF Subzone in Cambridgeshire and Bedfordshire and in the dispar MF Zone of the Vocontian Trough, France (pers. obs.).
Genus CRUCIBISCUTUM  Crucibiscutum sp. 1 Remarks: A species of Crucibiscuturn with an asymmetrical cross.

Staurolithites rotatus sp. nov.
Derivation of name: Latin rotatus, meaning to turn around. Diagnosis: This form is elliptical with a narrow, single zeugoid wall. The large central opening is bridged by a spine-bearing cross which is often missing. Each bar of the cross consists of two parallel blocks. The bars, unlike in Staurolithites angustus, form a large angle with the long axis of the ellipse. Staurolithites rotatus exhibits a slight bifurcation at the ends of the cross. Holotype: SMH-16-30 ( PI. 2, fig. 19). Type locality and horizon: Folkestone, Kent, Bed XI11 (Late Albiandispar M F Zone) Dimensions: L: 6.5-9 p m (mean length 8.5 pm), 50 specimens measured. Remarks: An evolutionary trend is considered to exist between S. angustus through intermediate forms to S. rotatus where by a gradual rotation of the bars occurs, from almost parallel to the axes of the ellipse to a position midway between the axes of the ellipse. This gradual change has also been documented by Stover (1966) and Verbeek (1977), although they did not differentiate the two end members.

CONCLUSIONS
Extensive sampling of Albian to Lower Cenomanian, ammonite dated localities, mainly from England, has provided an opportunity to develop a high resolution nannofossil biozonation scheme. A total of sixteen zones are defined.
The Repagulum parvidentatum (NAL 1) and Acaenolithus viriosus (NAL 2) NF Zones are poorly represented in onshore English sections possibly due to unfavourable, high energy, shallow marine environments. Exceptions include Beds A5 to A3 at Speeton and the Chamberlain's Barn section, Bedfordshire, which confirms NAL 2 as Lower Albian in age. The Bownia glahra (NAL 5 ) and Cerutolithina bicornuta (NAL 6) NF Zones of Middle Albian age are poorly represented at English sections and in the North Sea Basin as a result of the regional Upper Albian cristatum M F erosive event.
A major unconformity is present at the Albian/Cenomanian boundary in southern England with the dispar MF Zone truncated to varying levels. A complete sequence across the boundary is present at Mt. Risou, southern France and possibly in the Red Chalk facies of Yorkshire and the southern North Sea Basin.