New species of dinoflagellate cysts from Maastrichtian-Danian chalks of the Danish North Sea

A palynological study of Danian - Maastrichtian chalks from the Dan oilfield (Danish North Sea) has revealed the presence of nine new species and subspecies of dinoflagellate cysts: Achomosphaera antleriformis sp. nov., Cassiculosphaeridia? tocheri sp. nov., Chlamydophorella? multifibrata sp. nov., Isabelidinium majae sp. nov., Hystrichosphaeropsis perforata sp. nov., Hystrichostrogylon borisii sp. nov., Leberidocysta? flagellichnia sp. nov., Spiniferites foveolatus sp. nov. and Spiniferites pseudofurcatus granulosus ssp. nov. Seven of the taxa are considered important biostratigraphic markers in the area.


INTRODUCTION
The Geological Survey of Denmark is, in cooperation with Maersk Olie og Gas AS, carrying out a multidisciplinary geological research project (the Dan Field Project), on improved mapping and characterization of chalk reservoirs and seal properties in the Dan oilfield in the North Sea. The Dan Field produces oil from Maastrichtian and Danian chalks, which form part of the Chalk Group (Deegan and Scull, 1977). An overview of the exploration and production geology of the Danish chalk play is presented in Andersen and Doyle (1990). The structure, stratigraphy and development history of the Dan Field is treated comprehensively in Jorgensen (1993). The Dan Field Project integrates seismic forward modelling and interpretation with sedimentology and biostratigraphy. The project includes the establishment of a high resolution bio-stratigraphical zonation in the reservoir chalks, based on dinoflagellate cysts (Schiraler and Wilson, 1992Wilson, , 1993. During the study several previously undescribed dinoflagellate taxa were encountered, nine of which are described here.
The locations of the wells are shown in Fig. 1. The samples were prepared using normal initial procedures for palynological preparation of chalk samples (cf. Wilson, 1971), except for oil-bearing chalks, which were prepared according to the method described in Nrargaard et al. (1991). Hydrofluoric acid treatment followed the maceration tankmethod described by Poulsen et al. (1991). The material was sieved through 18 pm filter cloth; oxidation with nitric acid as well as heavy liquid separation with ZnCl2 was carried out whenever needed. Treatment with potassium hydroxide was not undertaken as this preparation step may alterate the morphologic appearance of the peridinioid dino-flagellates 6% 6% 636 6%

Fig. 1
Top Chalk Structure map of the Dan Field showing the locations of the wells studied herein. (Schrank, 1988). The slides were mounted in glycerin jelly. Specimens were photographed with a Zeiss photomicroscope, located at the Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand. The abundance of dinoflagellate cysts in the chalks is low, between 5 and 200 specimens per gram pure chalk. A higher abundance is found in stylolite-bearing chalks: 8 -300 specimens per gram. Consequently stylolite-rich intervals were preferred during sampling of the cores. As the cysts are inert to most diagenetic processes, their relative enrichment in stylolites is thought to reflect concentration during the stylolite formation.
Schialer ME-0 and MFB-7. A synthesis of the biostratigraphy of M-lx, M-9x, M-lox and MFA-4, as well as a preliminary dating of ME-6 and MFB-7, based on dinoflagellate cysts, is outlined below. Well M-lx (studied core interval: 1806.9m -2003.8m) Based on the influx of calcareous benthonic and planktonic foraminifera and the occurrence of Globigerina pseudobulloides and Gavelinella bullata, Bagnall et al. (1971) reported Danian strata in the interval 1798.3m (core) -1838.3m (core). They dated the interval 1845.0m -2066.6m (core and cuttings) as Maastrichtian, based on the occurrence of Boliuinoides draco dmco, Reiissella cristata, Globotruncana contusa as well as the presence of other characteristic forms. The presence of G. contirsa was held to suggest a relatively complete Maastrichtian succession, although the absence of the latest Maastrichtian species Pseudotextularia elegans was noted (Bagnall et al., 1971, p. 7). In-house information (Stouge, 1990) suggests that the boundary between the Late and Early Maastrichtian is positioned at 1928.5m, and that the stratigraphic interval down to 2003.8m includes most of the Early Maastrichtian. The interval covered by the present study thus represents strata of Danian to Maastrichtian age, including at least some Early Maastrichtian.
Well M-9x (studied core interval: 1967.8m -2060.8m) Bailey (1980) reported the biostratigraphy of the well based on foraminifera: Globigerina daubjergensis, the Early Danian zonal marker of Bang (in Rasmussen, 1978), was encountered at 1972.4m (side wall core). The Maas-trichtian/Danian boundary was placed between 1996.8m (core) and 1998.0m (core), based on the first downhole appearance of the planctonic foraminiferid association of Globotruncana contusa, Pseudotextularia elegans and Racemiguembelina fructicosa. The boundary between the latest and early Late Maastrichtian was placed between 2018.7m (side wall core) and 2021.5m (side wall core), based on the lowest in situ occurrence of Pseudotextularia elegans at 2018.7m associated with the highest occurrence of Neoflabellina reticulata at 2021.5m, following the zonation of Koch (1977). The boundary between the Early and Late Maastrichtian was, with some reservation, positioned at 2063.5m (cuttings), based on the common downhole occurrence of Stensioeina pommerana from this depth and following Koch (1977) (in Bailey 1980. The interval studied in the present paper thus ranges from the Late Danian to approximately 9 feet above the boundary between the Late and Early Maastrichtian. Well M-lox (studied core interval: 1944.6m -2029.4m) Bailey and Jutson (1983) examined two core samples just below the contact between the Palaeocene clay and the underlying Chalk Group, and encountered nannofossils indicating the Subzone D10 of Perch-Nielsen (1979) at the depth 1932.26m. Bailey (1983) examined 12 core samples from the depth interval 1956.8m -1990.4m, and dated the interval 1956.8m -1962.9m as Danian, based on the total range of Globorotalia pseudobulloides and other significant taxa. The interval 1966The interval .0m -1984 represents the Late Maastrichtian based on the occurrence of Pseudotextularia elegans. The interval 1987.3m -1990.4m was, with some reservation, dated as Early Maastrichtian based on the common occurrence of calcisphaeres in the sample at 1987.3m. Four core samples from around the K/T boundary (1962.17m -1963.84m) were investigated for calcareous microfossils by Chamock et al. (1985). The results show that an unconformity probably occurs between the Early Palaeocene and the latest Maastrichtian in the well (Chamock et al., 1985). The interval below 1990.4m has not been investigated for calcareous microfossils. In summary the interval studied herein spans the Danian and Late Maastrichtian, and probably Early Maastrichtian. Well MFA-4 (studied core interval: 2366.2m -2414.7m) Chamock et al. (1985) investigated core samples from the interval 2366.2m -2408.3m. The interval 2366.2m -2400.0 is dated as Early Palaeocene based on the presence of several calcareous nannofossil and foraminifera1 taxa; the presence of specimens of Globotruncana contusa, although questionable to the authors, provides some evidence for the presence of uppermost Maastrichtian strata from 2402.7m (Charnock et al., 1985). Age diagnostic foraminifera were not recovered from deeper samples. Based on the occurrence of large Arkhangelskiella cymbiformis down to 2408.3m, uppermost Maastrichtian strata are indicated at least to this level (Charnock et al., 1985). The interval studied herein thus covers the Early Palaeocene to at least the latest Maastrichtian. Wells ME-6 and MFB-7 (studied core intervals: ME-6: 2073.9m -2083.6m, MFB-7 2246.7m -2436.9m) The studied sequences from these wells have been correlated with the four other wells and with chronostratigraphical units by the present author using dinoflagellate cysts. Only two samples yielded dinoflagellate cysts in well ME-6. Both contain abundant Palynodinium grallator, and consequently belong to the Palynodinium grallator Zone of Hansen (1977),  Fig. 6 Zsabelidinium majae sp. nov. DGU Cat. no. 1992-PS-6. Paratype. Ventral surface up, sectional focus. Fig. 7 Hystrichostrogylon borisii sp. nov. DGU Cat. no. 1992-PS7. Holotype. Lateral view, sectional focus. Fig. 8 Hystrichostrogylon borisii sp. nov. DGU Cat. no. 1992-PS-8. Paratype. Dorsal surface up, sectional focus. Fig. 9 Hystrichostrogylon borisii sp. nov. DGU Cat. no. 1992-PS-9. Paratype. Lateral view, sectional focus. Schider representing the latest Maastrichtian in Denmark. The studied interval from the MFB-7 well was correlated with the five other wells by the present author using qualitative and quantitative stratigraphic techniques, and is considered to be of Danian to late Early Maastrichtian age.
SYSTEMATIC PALYNOLOGY Dinoflagellate cyst taxa described prior to 1989 and mentioned below are fully referenced in Lentin and Williams (1989). All other taxa are referenced herein. All type specimens and other figured specimens are lodged in the palynological collection of the Geological Survey of Denmark (DGU), and have been assigned DGU catalogue numbers. The inferred chronostratigraphical distribution of the new taxa is summarized in Fig. 2. Division Pyrrhophyta Pascher, 1914Class Dinophyceae Fritsch, 1929Order Peridiniales Haeckel, 1894 Genus Achomosphaera Evitt 1963 Achomosphaera antleriformis sp. nov.  which open to the exterior through irregular subcircular holes. The height of the reticulation and the size of the lumina may vary slightly from specimen to specimen but are approximately constant on a specimen. Height of reticulation and diameter of lumina: 6 -12 pm. The archaeopyle is apical, type (tA), judged from its size and shape (Pl. 4, fig. 4), but is often absent or difficult to detect. Paracingulum and parasulcus are absent. Paratabulation is not indicated apart from inconsistent presence of archaeopyle.

Dimensions (in pm):
holotype range total diameter 92 78 (89) 96 height of ectophragm 9 6 (9) 12 Specimens measured. 13 Remarks. As archaeopyle relationship is somewhat problematic in this species and paratabulation is absent, it is with some hesitation that the new taxon is established in Class Dinophyceae. Alternatively the taxon could be a spore, a pollen grain, or an acritarch; however, in all the Maastrichtian samples examined during the present study, spores and pollen as well as acritarchs are very rare. The occasional presence of features that point to an apical archaeopyle on the new species (see P1. 4, figs 4,6,9,11), is currently taken as sufficient evidence by the present author for the location of the taxon in Class Dinophyceae. The new species differs from all other species of Cassiculosphaeridia in having a high and thick-walled reticulum.

Schierler
hemisphere. The ventral pericoel opens to the exterior through a large rounded midventral claustrum which approximately equals the radius of the endophragm in diameter. The periphragm is smooth, the endophragm is ornamented with granulae, verrucae or vermiculae. Holotype. DGU Cat. no. 1992-PS-7 (Pl. 1, fig. 7). Well M-lox, depth 1966.6m, Dan Field, Danish North Sea. Paratypes. DGU Cat. no. 1992 . 1, fig. 8) DGU Cat. no. 1992-PS-9 (Pl. 1, fig. 9) DGU Cat. no. 1992-PS-12 (Pl. 2., fig. 3 -lx: 1837-lx: .7m, M-9x: 1973-lx: .3m -2004-lx: .4m, M-loX: 1950-lx: .7 -2001. Latest Maastrichtian to Early Danian in the Dan Field, Danish North Sea. Description. Camocavate cyst. The periphragm and endophragm are subspherical; the periphragm is appressed to the endophragm on the dorsal hemisphere. Ventrally a pericoel separates the periphragm from the endophragm. The pericoel communicates to the exterior through a subcircular claustrum in the periphragm. The claustrum occupies a midventral position and is approximately equal in sue to the radius of the endophragm. The periphragm is smooth. Bi-and trifurcating processes with bifid terminations are present on the periphragm; their paratabular position was not determined. In areas where pen-and endophragm are separated, the length of the process stems is inversely proportional to the distance between the pen-and endophragm (Pl. 1, fig. 9; P1. 2, figs 2, 3). The periphragm is smooth. The endophragm is granulate, venucate or vermiculate (height of orna-mentation: 0.5 -1.5 pm). The size and type of the ornament are constant within a sample, but vary from one group of samples to another. The archaeopyle type is P(3) and the operculum is free. The paracingulum is often indicated by equatorial folds on the periphragm and by the presence of low septa between the longer processes. A parasulsus was not observed. Apart from paracingulum and archaeopyle the paratabulation is not clearly expressed. Genus Isabelidinium Lentin and Williams 1977 Isabelidinium majae sp. nov. (Pl. 1, figs 1-6, text-fig. 4) Derivation of name: Named after my daughter Maja. Diagnosis. A cavate peridinioid cyst with a smooth endoand periphragm without indications of paracingulum. The ambitus shows overall axial symmetry. The periarcheopyle is lati-deltaform. Holotype. DGU Cat. no. 1992-PS-1 (Pl. 1, fig. 1, text-fig. 4a). Description. Cavate peridinioid cyst. Cyst outline is almost bilateral symmetric in ambital view. The periphragm is smooth and has a broad-based rounded to acute apical horn and two acute to broad rounded antapical horns. The epicyst lacks shoulders, and has a concave margin. The area between the antapical horns has a moderately concave curvature. The endophragm is smooth and subcircular to oval in ambital view. The peri-and endophragm are often in contact laterally. The archaeopyle type is 1(2) and is latideltaform with Transversal Archaeopyle Index (TAI) c. 0.70. The paracingulum is not indicated. The parasulcus is often indicated by a longitudal fold in the periphragm. Apart from the archaeopyle and the parasulcus no other paratabulation is present.  fig. 5 ) Derivation of name. After the flagellar scar on the endophragm. Diagnosis. A small holocavate oval cyst with a narrow pericoel. The periphragm is smooth, and occasionally has folds which indicate parts of the paratabulation. The endophragm is granulate and has a small, kidney-shaped scar in the sulcal area. The archaeopyle is apical and the opercula are usually attached. Holotype. DGU Cat. no. 1992-PS-23 (PI. 3, figs 7,8, text-fig. 5 (1992, p. 121, pl. 27, figs 11-13, 7-9) closely resemble the new species in general appearance and size; Form X sp. 2 apparently only differs from Leberidocysta? flagellichnia sp. nov. in having a larger antapical pericoel, and may be conspecific with the new species. Form X sp. 1 differs further in having an alveolate endophragm. "Hexagonifera uerrucosa" of Wilson (1974) differs from Leberidocysta? flagellichniu sp. nov. in having a verrucate endophragm (and thereby resembling Form X sp. 1 of Marheinecke, 1992). The kidney-shaped sulcal ornament is interpreted as representing the relict of Camera lucida drawing of the ventral surface of the holotype. Scale bar = 20 pm. the flagella insertion point by the present author. It is with some hesitation that the new species is located in genus Leberidocysta as this genus is not thought to have attached opercula (Stover and Evitt, 1978 p. 60). However, the illustration of the type specimen of the genus, L. cklamydata, shows peri-and endoperculum in place (Cookson and Eisenack, 1962, P1. 7, fig. 2). This fact is considered here to justify the generic location of the new species in Leberidocysta.  Lentin and Williams 1975 has a perforate endophragm with perforations much smaller than the foveolation of the new species, judging from the illustrations of Corradini's holotype. Due to its generally penitabular arangement, its restriction to the endophragm, and its consistent occurrence within a population, the ornament is not considered a preservational artefact, but an original morphological feature. (Klumpp 1953) Sarjeant 1970 emend. Sarjeant 1981 Spiniferites pseudofurcatus granulosus ssp. nov.

DINOFLAGELLATE STRATIGRAPHY
The stratigraphic distribution of the new taxa is summarised in Fig. 6. The restricted stratigraphic occurrence of Isabelidinium majae sp. nov. and Hystrickospkaeropsis perforata sp. nov. suggests that they are important marker species for narrow stratigraphic intervals in the Late Maastrichtian. The first appearance datum of Hystrichostrogylon borisii sp. nov. is considered an important stratigraphic marker for the latest Maastrichtian (Schisler DGU Cat. no. 1992-PS-28 (Pl. 4, fig. 1) DGU Cat. no. 1992-PS-30 (Pl. 4, fig. 3)