Dinoflagellate cyst distributions and the Albian–Cenomanian boundary (mid-Cretaceous) at Cordebugle, NW France and Lewes, southern England

The Albian–Cenomanian boundary successions at Livet Quarry, Cordebugle and Rodmell Cement Works, Lewes are described. Moderately abundant and diverse dinoflagellate cyst assemblages comprising 89 taxa are recorded and related to ammonite, foraminiferal and other faunal data from the two sites. The genus Ovoidinium forms a major component of cyst assemblages from the boundary intervals at both localities. Ovoidinium scabrosum (Cookson & Hughes) Davey is replaced by abundant Ovoidinium verrucosum verrucosum (Cookson & Hughes) Davey close to, and possibly at, the stage boundary, offering a potential dinoflagellate cyst marker for the base of the Cenomanian Stage. The published ranges of a number of species are extended. Six taxa are recorded for the first time from NW Europe: Apteodinium reticulatum Singh, Disphaeria macropyla Cookson & Eisenack, Nematosphaeropsis densiradiata (Cookson & Eisenack) Stover & Evitt and Pervosphaeridium cenomaniense (Norvick) Below occur in the high Upper Albian; Ovoidinium verrucosum (Cookson & Hughes) ostium (Davey) Lentin & Williams and Tanyosphaeridium salpinx Norvick are recorded from the lowest Lower Cenomanian. Increased cyst abundance and diversity at Lewes when compared with Cordebugle is related to the more basinal setting of the former locality.


INTRODUCTION
The Cenomanian Stage, at the base of the Upper Cretaceous Series, is marked throughout most of NW Europe by the appearance of p'elagic carbonates (chalks and marls), replacing dominantly siliciclastic sediments (sandstones and mudstones) of the Lower Cretaceous. This major lithological change was a consequence of the continuing rise in eustatic sea-level, that bega,n in the earliest Cretaceous and which, by the Early Cenomanian, had drowned most available siliciclastic source areas, to form a broad shallow epicontinental sea (Hancock & Kauffmann, 1979: Juignet, 1980H,ancock, 1990. The sharp lithological change which occurs regionally at the bottom of the Cenomanian has recently been confirmed as a major sequence boundary, and is generally associated with a small hiatus (AmCdro, 1992; Hart e t a / . , Juignet & Breton, 1992: Robaszynski et a/., 1992. This was probably caused by a minor regressive event (Cooper, 1977;Haq et al., 1987: Simmons et al., 1991AmCdro, 1992) or period of still-stand (Hancock, 1989). superimposed on the main mid-Cretaceous sea-level rise.
In this paper, we describe the dinoflagellate cyst distributions of samples taken across two Albian-Cenomanian boundary intervals, one from NW France and the other from southern England. Samples of l o g were processed using standard palynological acid digestion techniques, and the strew mounts examined under a light microscope. Slides and residues are stored in the reference collection of the Palynological Research Centre, Institute of Earth Studies, University of Wales, Aberystwyth. IJK. In all cases, sampleij were precisely located within existing detailed litho-and biostratigraphic frameworks, enabling new observations to be made on the ranges and assemblages of dinoflagellate cysts during the Early-Late Cretaceous transition.

CORDEBUGLE
Cordebugle, 10 kni SE of Lisieux, dkpartement of Calvados. NW France, is situated close to the western limit of the 'Normandy Basin' (Juignet, 1980;Juignet & Breton, 1992), a structurally and sedimentologically distinct area located on the western margin of the Cretaceous Anglo-Paris Basin (Fig. 1). The region lies less than 100km N E of the Armorican Massif, a significant local source of sediment through most of the Cenomanian which resulted in the accumulation of thick siliciclastic packages on the adjacent 'Maine Platform'. However, further to the northeast, coeval Normandy deposits are dominantly pelagic, if somewhat marginal in nature. Thick glauconitic sands (the Gaize and Glauconie de base) of Albian age are here overlain by cherty, locally sandy, glauconitic Cenomanian chalks with prominent hardgrounds at several levels.
Livet Quarry, situated to the west of Cordebugle village (Fig. l), is a large working sand pit which exposes more than 30 m of mid-Cretaceous (Aptian-Cenomanian) sediments resting unconformably on Upper Jurassic (Oxfordian) sands and clays. The quarry (CoordonCes Lambert of the Institut gCographique National de France: x = 455,40: y = 157,80) has been described previously by Juignet (1974). Thirteen samples (Crd. 1-13) were collected across the Albian-Cenomanian boundary (Figs 2, 3). All of the residues contained palynomorphs, and a total of 66 species and subspecies of dinoflagellate cysts have been recorded (representative specimens are illustrated in Plates 1, 2;  Appendix 1). Since many samples yielded <200 individuals, cyst abundances are reported as absolute numbers (Fig. 3).

Lithostratigraphy
Approximately 19 m of Aptian shallow-marine sands and gravels (Sables ferrugineux Formation) overlie the Cretaceous unconformity at Cordebugle. Above this, more than 15 m of glauconitic Albian-Cenomanian sediments. the Glauconie de base and Craie glauconieuse Formations, are exposed. Glauconie de base The Glauconie de base rests with a sharp contact on a thin (10 cm) iron-cemented pebbly sandstone at the top of the Sables ferrugineux. Our first sample (Crd. 1) was taken 30cm above this contact. The Glauconie de base (Figs 2, 3) comprise 7.3 m of extremely glauconitic sands, with coarse-grained lenses, interbedded with more argillaceous horizons. The sediments are heavily bioturbated at several levels, and yield macrofossils from the upper beds. A distinctive omission surface overlain by glauconitic sands and gravels (Crd. 2) containing small black phosphatic nodules occurs 1.8 m above the base. Abundant fauna, dominantly bivalves and brachiopods, occur at two levels: the lower (Crd. 5 ) is a 1.0 m thick bed containing calcareous nodules with sponges; the upper (Crd. 7) occurs at the summit (top 0.9 m) of the formation. A thick coarse-grained dark green glauconitic sand (Crd. 6) containing abundant crustacean burrows, Spongeliomorpha annulatum Kennedy, occurs between these two beds. Craie glauconieuse de St Jouin The base of the Craie glauconieusc de St Jouin is marked by a prominent omission surface (Juignct & Breton, 1992;Fig. 3) overlain by green to dark brown bioturbated glauconitic sands (Crd. 8) containing S. annulaturn and fragments of bivalve shells. The St Jouin Formation is accessible for approximately 6 m and comprises green and dark brown glauconitic sediments at the base. passing up into paler-coloured marly sediments above. Scattered. partly silicified, carbonate nodules are common in the middle of the sequence, below and immediately above a well-developed omission surface termed thc 'Livet' surface by Juignet (1 974). Above this, there is a dramatic change in lithology with glauconitic mark at the very base (Crd. 12) overlain ( Fig.  3) by creamy-brown marly chalks containing numerous closely spaced tabular and semi-tabular bands of large, irregular, cavernous grey cherts.

Biostratigraphy
Macrofossils records, particularly ammonites, provide the initial means o f constraining the age of the succession at Cordebugle. but further biostratigraphic refinement has been possible by incorporating our new palynological data. Glauconie de base The lowest macrofaunal records from the Glauconie de base are from 3.Sm above the bottom of the formation, where the ammonite Sharpeicerus laticlauiitrn (Sharpe) has been recorded (Juignet, 1974) in association with abundant bivalves, Lima sp., Chlarny sp., and brachiopods, Cyclothyris diformis (Valenciennes in Lamarck) and terebratulids. The occurrence . of Shurpeiceras clearly demonstrates that this bed is Lower Cenomanian (Wright & Kennedy, 1984. 1987a. Additional ammonite records from the overlying Craie glauconieuse (see below), indicate that this level must lie within the lowest Lower Cenomanian Neostlingocerus carcitanense Subzone of the Mantelliceras mantelli Zone. However. in the absence of definitive Albian taxa, macrofaunal records do not allow the Albian-Cenomanian boundary to be placed in the succession with any confidence. The top of the Glauconie de base contains large sponges, bivalves Spondvlus striatus (J. Sowerby), Gryphaeostrea canaliculata (J. Sowerby). lnocerarnirs sp., Chlamys sp.. Lima sp. and brachiopods, principally C-yclothyris diformis and tere bratulids.
Seven samples (Crd.  Stover & Evitt, whose first appearances elsewhere are taken to indicate strata of latest Albian (S. dispar ammonite Zone) age Fauconnier, 1979;Foucher, 1981;Costa & Davey. 1992 fig. 11) occur in our lowest samples from the Glauconie de base (Crd, 1 and 3 respectively). These species have not been recol-ded previously from sediments above the Upper Albian M'ortoniceras (Mortoniceras) inflaturn ammonite Zone (Davey 6 i Foucher, 1981;Costa & Davey. 1992 fig. 31 compared to the immediately overlying beds containing typical S . dispm Zone forms. No macrofauna have been recovered from this part of the sequence, so it is possible that the lowest beds of the Glauconie de base (ix. below the phosphatic nodule bed) are M . (Mortoniceras) influturn Zone. However, P. spinocristaturn (Crd. 3; PI. 2, fig. 11) is also recorded higher in the succession at Cordebugle and at Lewes (see below) together with assemblages of typical S. dispar Zone dinoflagellate cysts, indicating that the species must extend further up in the Upper Albian than recognized previously. Similarly, although L. conispinum is recorded only from our basal sample at Cordebugle, it was also recovered from the S. dispar Zone at Lewes. The age of the oldest beds of the Glauconie de base, therefore, remains ~ uncertain, but on balance, is most probably S. dispar Zone.
A more diverse phosphatised ammonite fauna has been recorded (Juignet, 1974) (Juignet., 1974) suggest that the base of the overlying mid-Lower Cenomanian M . saxbii Subzone lies above the exposed section at Cordebugle.
Several stratigraphically significant dinoflagellate cysts have been recorded from the Craie glauconieuse. Most importantly, the last appearances of Canningia torulosa Davey & Verdier and 0. verrucosum verrucosum (Cookson & Hughes) Davey occur within the lower beds (Crd. 10) of the Craic: glauconieuse, indicating that this part of the succession is basal Lower Cenomanian (Foucher, 1981;Costa & Davey, 1992). This conclusion is confirmed by the presence of a Schloenbachia-dominated ammonite assemblage (see above) at this level.

LEWES
Lewes lies 9 km north of the East Sussex coast of southern England. The Upper Cretaceous of this region is characterized by thick successions of basinal chalks ( Rawson et al., 1!978;Ntortimore & Pornerol, 1987), typical of the central Anglo-Paris Basin. The supply of coarse detritus was cut-off to the area in the latest Aptian, the Albian being represented by a thick succession of silts and clays (Lake et a/., 1987) passing up, via a thin development of glauconitic mark, into a pelagic carbonate sequence of rhythmically bedded niarls and limestones in the Cenomanian. No Upper Albian sands (Upper Greensand Formation) occur around Lewes, although this facies is well developed in coastal exposures at Eastbourne, 23 km to the southeast.
Rodmell Cement Works, 4 k m S E of Lewes, once a complex of three pits (Fig. 4), now almost entirely infilled, prior to 1991-2 exposed strata ranging from Upper Albian to Lower Turonian. The locality (also referred to in the literature as Beddingham Limeworks) was described previously by Gaster (1929), Kennedy (1969), Carter & Hart (1977, Wright & Kennedy (1984) and Lake et al. (1987). Borehole data from the quarry were presented by Price (1977) and Lake et a / . (1987). The Albian-Cenomanian boundary was exposed on the edge of a large flooded clay pit (Fig. 4, Pit 1; UK National Grid Reference: TQ 441 071). The Lower-Middle Cenomanian was best seen in an adjacent pit (Pit 2, TQ 438 067), 500 m to the south.
Five samples (Rod. 1-5) were collected across the Albian-Cenomanian boundary for palynological analysis; one additional sample (Rod. 6) was obtained from the Middle Cenomanian. Each preparation contained a diverse assemblage of dinoflagellate cysts, a total of 85 cyst taxa being recorded ( Fig. 5; Plates 1, 2; Appendix 1). Since all samples yielded several hundred individuals, cyst occurrences are reported as percentage abundances (Fig 5 ) , based on counts of 200 individuals per slide.

Lithostratigraphy
More than 25 m of section, including the uppermost beds of the Upper Gault Clay Formation (Upper Albian) and the lower beds of the Lower Chalk Group, Glauconitic Marl and overlying Chalk Marl Formations (Lower-Middle Cenomanian), were intermittently exposed in Rodmell Pits 1 and 2. Upper Gault Clay The summit of the Upper Gault Clay was exposed at Rodmell Pit 1 (Fig. 4), where it consisted of approximately 1 m of blue-grey bioturbated silty clay, passing up into 3 m of light brown silty calcareous and micaceous bioturbated clay. The silt content increased towards the top of the succession and occasional thin diagenetically laminated units were present. The top of the Upper Gault was marked by a sharply defined omission surface which was penetrated by numerous glauconitic sand-filled Thalassinoides burrows (Fig. 5 ) . The sediment within these burrows was identical to that which immediately overlay the omission surface, forming the basal facies of the Lower Chalk, Glauconitic Marl. The facies consisted of intensely bioturbated, light brown and green, friable, glauconitic marly sands. An unconformity of up to 15" has been observed by other workers (Wright  Fig. 9. Palaeoperidinium cretuceum Pocock, 1962: emend. Davey, 1970emend. Harding, 1990, Rod.   & Kennedy, 1984) between the Upper Gault and Glauconitic Marl. Glauconitic Marl Abundant, small (1 -3 cm), black phosphatic (:lasts and common bivalve shell fragments, including small Aucellinu, occurred up to 15cm above the base of the Glauconitic Ma,rl. The glauconite content decreased rapidly upwards and virtually disappeared in a more indurated limestone which occurred at the top of the unit (Kennedy, 1969), and marked the boundary with the overlying Chalk Marl. Macrofossils were poorly preserved in the basal Glauconitic Marl, but prominent Tha1assiv;oides and other burrows occurred throughout. Sparse ammonites and common lnocrrumus crippsi Mantell have been recorded at the summit o f the formation (Kennedy, 1969), which is approximately 1.5 m thick. Palynological samples (Fig. 5 , Rod. 1-5) were taken across the Upper Gault/Glauconitic Marl boundary from 4 m b e h w to 0.7 m above the contact. Chalk Marl Nearby, in Rodmell Pit 2, intermittent exposures of Chalk Marl occurred within a 2 0 m thick succession of interbedded greyish white limestones and medium to pale grey m a r k One small exposure near the summit of the succession contained abundant Orbirhynchia rnantelliana (J. de C. Sowerby) brachiopods and Sciponoceras heteromorph ammonites, with common I. crippsi and echinoid fragments These beds have been termed the upper 0. rnantelliana band (Lake et al., 1987; = 0. mantellianu band of Kennedy, 1969), and occur only a few metres below the boundary between the Chalk Marl and the overlying Grey Chalk Formation. A single sample was taken from this interval (Fig. 5, Rod. 6) for palynological analysis.

Biostratigraphy
Published macro-and microfossil records, principally ammonites and foraminifera, provide a means of determining the age of the succession at Rodmell. When combined with our new dinoflagellate cyst data, they enable a highly refined biostratigraphy to be developed.

Upper Gault Clay
No stratigraphically significant macrofossils have been recorded from the summit of the Upper Gault at Rodmell. However, the foraminiferal assemblage in these beds (Carter & Hart, 1977) includes large numbers of planktonic Globigerinelloides bentonensis (Morrow), indicative of the Upper Albian G . bentonensis Zone. Evidence of a stratigraphic gap at the Upper Gault/Glauconitic Marl contact is provided by the benthonic foraminiferal biostratigraphy (Carter & Hart, 1977: Lake et ul., 1987 which demonstrates that the uppermost Upper Albian (Zone 6a of Carter & Hart, 1977; Zone 9 of Price, 1977) is absent. This suggests that the S. &par ammonite Zone is incomplete, the top of the Gault at Rodmell probably falling within the lower part of the Mortoniceras (Durnouarites) perinflaturn Subzone (Price, 1977;Lake et al., 1987).
The bottom of the Glauconitic Marl marks the appearance of keeled planktonic foraminifera, particularly Rotalipora rrppmninica (Renz), definitive of the IJpper Albian-Lower Cenomanian R. appenninica Zone (UKP.l of Hart et ul., 1989). However, the basal Glauconitic Marl contains a benthonic foraminifera1 assemblage which indicates the upper part of the Lower Cenomanian Flourensina interrnedialArenobulirnina unglica Concurrent Range Zone (Zone 8 of Carter & Hart, 1977: UKB.2 of Hart et al., 1989, demonstrating that in addition to Zone 6a, Zone 7 (UKB.l) and the lower portion of Zone 8 (UKB.2) are also unrepresented. A comparable, and often greater, stratigraphic gap occurs throughout most of southern England (Carter & Hart, 1977) and northern France (Robaszynski et al., 1980;Ame'dro, 1983Ame'dro, , 1992Robaszynski & AmCdro, 1986), and is equivalent to several metres of sediment in the more complete borehole sections described from Folkestone, East Kent, and offshore in the English Channel (La Manche).
Samples from the Glauconitic Marl (Rod. 4, 5 ) contain a number of species whose first appearances lie Foucher, 1979: Fauconnier, 1979Costa & Davejr, 199;!)  Protoc~llipso~dinium spinocristutum Davey & Verdier, a species which is thought to last occur in the Upper Albian Mortoriiceras (Mortoniceras) influturn ammonite Zone, is recorded1 from the Upper Gault (S. dispar Zone) and a single specimen also occurred in our basal Glauconitic Marl sample (Rod. 4). However, the observed angular discordance, the associated sudden facies change from silty clays to coarse glauconitic sands, and the proven hiatus at the conlact, would suggest that this last specimen is most likely a remanit5 element in the assemblage. Similarly, the coincident first appearances of some rarer species might be a consequence of condensation at the contact.
Several species, whose last appearances are thought to occur within the lowest Cenomanian (Foucher, 1981), occur in the Glauconitic Marl at Rodmell. These include Ovoidinium scubrosum, 0 . verrircosiam v e r r i~c o~i~m , Palaeop8eridinr'um cretaceum Pocock; emend. Davey: emend. Harding (PI. 1, fig. 9), Pterodi,nium cingulatum (0.   . With one excepticvn ( P . cingulalum reticulatum), all of these taxa are absent in our sample from the upper part of the Chalk Marl ( Fig. S), which is consistent with them disappearing within the Lower Cenomanian. However, we have taken too few samples to place their last occurrences more precisely. Our records of P. cingulum reticulaturn in the Middle Cenomanian indicates a more extensive range for this species.
Ovoitliniurrr verrucosum ostium (PI. 2, fig. 4) and Tanyosphaeridium sulpinx, species which have not been recorded previously from the .Anglo-Paris Basin. both occur in our basail Glauconitic Marl sample (Rod. 4). This confirms our records of these taxa from the basal Craie glauconieuse (Lower Cenomanian M. mantefli Zone) at Cordebugle (see above) and suggests that they probably have a widespread occurrence within the Anglo-Paris Basin. Chalk Marl The upper 0 . mantellianu band constitutes the summit of the low Middle Cenomanian ammonite Acarithoceras rhhotornagense Zone, Turrilites costatus Subzone (Kennedy, 1969;Wright & Kennedy, 1984), and also coincides with the top of the planktonic foraminiferal Rotalipora reicheli Zone (UKP.2). This level lies within the benthonic foraminiferal P. cenomana Interval Zone (UKB.5 of Hart et al., 1989). at the summit of Zone Il(i) of Carter & Hart (1977; Arenohulimina unglicalPlectina cenomana Concurrent Range Zone); its top marks the so-called mid-Cenomanian non-sequence, above which there is a sudden and marked increase in the planktonic/benthonic ratio of the foraminiferal assemblages. The base of the mid-Middle Cenomanian ammonite 7'. ~C U G U S Subzone is taken at the top of the brachiopod band.
Stratigraphically significant species are rare in our single Chalk Marl sample (Rod. 6). However, the continued presence of Cribroperidinium exilicristatum, Endoceratiurn dettmanniae and Epelidosphaeridia spinosu (Cookson & Hughes) Davey, are consistent with this part of the sequence being no younger than Middle to early Late Cenomanian (Foucher, 1981;Jarvis et al., 1988;Costa & Davey, 1992).

DISCUSSION
Palynomorph assemblages recovered from Cordebugle are of moderate abundance and diversity (Fig. 3). However, in the upper part of the Glauconie de base and lowest Craie glauconieuse de St Jouin, these assemblages are dominated to a large extent by specimens of Ovoidinium (particularly 0 . verrucosum verrucosum) and bisaccate pollen grains. This dominance continues up to sample Crd. 10 (a level yielding common Schloenbachia ammonites), above which Ovoidinium disappears and assemblages consist predominantly of Cleistosphaeridiurn cluvulim (Davey) Below (Pl. 2, fig. 15), Circulodinium distincfurn (Deflandre & Cookson) Jansonius (PI. 1, fig. 13) and Epelidosphaeridia spinosa (Cookson & Hughes) Davey. This change is associated with declining glauconite and the appearance of silicified nodules in the sediment, but takes place below the major facies change to flinty mark which occurs somewhat higher, above the Livet Omission Surface (Fig. 3).
Dinoflagellate cyst assemblages from Lewes are more abundant and diverse than those from Cordebugle, but are similar in a number of respects. The genus Ovoidinium is again a major component throughout the uppermost Upper Albian-basal Lower Cenomanian, but particularly in the lowest Lower Cenomanian Glauconitic Marl where 0 . verrucosum verrucosurn appears in large numbers. At both localities Ovoidinium scabrosum is essentially replaced by 0. uerrucosum verrucosum close to, and most probably at, the Albian-Cenomanian boundary and Cleistosphaeridium clavulum is abundant in the uppermost beds yielding Ovoidinium. However, other elements of the flora, particularly Dapsilidinium amhiguum (Deflandre) Wheeler & Sarjeant (Pl. 2, fig. 13), Epelidosphaeridia spinosa, Kiokansium unitubercufati~m (Tasch) Stover & Evitt, Odontochitina operculata (0. Wetzel) Deflandre & Cookson, Oligosphaeridium complex and Spiniferites ramos~is ramoms (Ehrenberg) Mantell are also important components of assemblages recovered across the stage boundary at Lewes. These latter taxa occur, but do not represent major components of coeval assemblages at Cordebugle. Furthermore. terrestrially derived material represents a much greater proportion of the palynofacies at Cordebugle. These differences reflect the contrasting palaeogeographic settings of the two sites. Cordebugle was situated in a shallower-water sediment-starved marginal environment close to the western edge of the Anglo-Paris Basin. In contrast, Lewes was located in a deeper basinal setting far removed from continental influences, promoting increased phytoplankton productivity and diversity but reduced terrestrial input.