Biostratigraphy and palaeoenvironmental analysis of a Lower to Middle Jurassic succession on Anholt, Denmark

Palynomorph and foraminiferal assemblages have been studied from the Upper Pliensbachian to Bathonian of a borehole section on the island of Anholt in the Kattegat, situated near the eastern margin of the Norwegian-Danish Basin. Palynomorphs were recorded throughout the succession and have been used for both biostratigraphical and palaeoenvironmental assessments. Foraminifera were recorded from only the lower part of the succession, where they proved useful for interpreting palaeoenvironments. Four palynomorph and three foraminiferal zones have been established. The interval encompasses a palaeoenvironmental transition from a marine, inner shelf setting to mainly terrestrial conditions. The Upper Pliensbachian-Toarcian boundary marks the beginning of a major regression, which continued through the Toarcian and Aalenian. Hence, it took place significantly earlier at Anholt than in the centre of the Norwegian-Danish Basin, where a lowering of sea level did not occur until the late Toarcian. The Jurassic succession on Anholt spans the Fjerritslev and Haldager Sand formations; biostratigraphical data indicate that the Lower-Middle Jurassic boundary is here located within the uppermost part of the Fjerritslev Formation. The Fjerritslev and Haldager Sand formational transition was previously considered to coincide with the Lower-Middle Jurassic boundary.


INTRODUCTION
Jurassic strata were studied from a boring on the small Danish island of Anholt in the Kattegat (Fig. 1). This site is located near the eastern margin of the Norwegian-Danish Basin, which extended across most of Denmark into the present North Sea during the Early and Mid Jurassic. This depocentre continued into the Danish-Polish Trough to the south-east, and was delimited by the Fennoscandian Shield to the north and north-east and by the Ringk~bing-Fyn High to the south (Sorgenfrei & Buch, 1964;Michelsen, 1978;Liboriussen e f al., 1987). Our study focuses on the biostratigraphy and palaeoecology of the uppermost Lower Jurassic to Middle Jurassic section.
Previous investigations h a v e revealed a generally regressive phase in the uppermost Fjerritslev Formation. Deposition of open marine claystones occurred during the Pliensbachian and possibly early Toarcian, and of more restricted marine claystones d u r i n g the late Toarcian (Michelsen, 1989a). Prograding delta or braided river sediments of the Haldager Sand Formation were deposited during the Mid Jurassic in the north-east of the basin (Michelsen, 1978(Michelsen, , 1989aKoch, 1983). The purpose of this study is to provide a biostratigraphic subdivision of the succession u s i n g microfossils, a n d to e v a l u a t e t h e palaeoenvironmental setting at the basin margin.
The succession from 196 m to 306 m depth in the Anholt core, consists of marine claystons with numerous very thin silty a n d sandy storm layers (a few m m or cm thick) referred to the Fjerritslev Formation, and from 104 m to 196 m depth it comprises alternating non-marine sand and silty clay layers with some thin brown coals referred to the Haldager Sand Formation ( Fig. 2; Nielsen, 1992).

MATERIAL AND METHODS
The present study is based on both core material and ditch cuttings from a 306 m deep boring in the northwestern part of Anholt (Fig. 1). The boring penetrated about 104 m of Quaternary sediments before it reached deposits of Mid Jurassic age and continued into the upper part of the Lower Jurassic. The elevation of the drilling site is about 2 m above present-day sea level, and all of the depths noted are calculated below this altitude.
The material from c. 104 m to 230 m consists of both cuttings and core samples, while that from the lowest part of the boring (c. 230 m to 306 m) consists solely of core samples (Fig. 2). The lithological log (Fig. 2) was prepared by Ole Bjerrslev Nielsen (University of Aarhus). The total organic carbon content (TOC) of the sediments (Fig. 2) was analyzed a t the University of Aarhus (AU), and at the Geological Survey of D e n m a r k (DGU) O.B. Nielsen, Foraminif era and megaspores. The samples for the foraminiferal and megaspore analyses (100-500 g of sediment) were processed at AU using standard techniques for foraminifera (Meldgaard & Knudsen, 1979). All foraminifera and megaspores present in the 0.1-1.0 mm fraction were counted except for those samples which yielded more than 300 specimens. In these cases only 300 specimens were identified. Selected foraminiferal species are included in Fig. 4 as estimated numbers of specimens per 1000 g of sediment. The foraminifera are stored at AU and the megaspores at the DGU.

Palynological zonation
Palynomorphs were recorded from 306 m to 107 m (Fig. 3 ) and a total of 137 species recognised. Two assemblages and two acme zones are defined using the definitions of  comm 1992;B.J. Smidt, pers. comm. 1992. The spectral natural gamma ray log was measured by Korsbech & Gynther Nielsen (1990 a, b) (Fig. 2).
Eighty-three samples wereanalysed for their palynoflora, and 190 samples were examined for their foraminifera and megaspore content, although only 63 and 27 are illustrated on Figures 3 and 4 respectively. Representatives of other fossil groups were also encountered in the microfossil samples: juvenile bivalves and gastropods, ostracods, otoliths, scolecodonts, foraminiferal test linings, animal burrows, ammonites, and a part of a thoracic vertebra from a fish (Fig. 4). Except for megaspores, fossils were only found in 27 of these latter samples.
Palynomorphs. The samples from 306 m to 229 m depth were processed at DGU using the technique described by Poulsen et al. (1990). The samples from 229 m to 107 m depth were processed at the AU using the technique described by Dybkjaer (1988). Two techniques were used because the palynological analysis was divided between two separate laboratories. Where possible, 300 specimens were counted from each sample. A selection of the palynomorphs recorded is shown in Fig. 3. The palynomorph samples and slides from the lowermost interval 306 m to 229 m are stored at the DGU, whereas the samples from the upper interval 229 m to 107 m are housed Hedberg (1976). The boundary between two zones is placed in the interval between two sampling horizons (Fig. 3).
The distribution of foraminifera and other fossil groups in the Jurassic interval of the Anholt boring. The selected foraminifera1 species are given as an estimated number of specimens in 1000 g sediment. For other species groups: x= present, xx= frequent. All samples are from cores. Barren samples are marked with a short line in the column headed "Analysed samples".  fig. l l ) , Nroraistrickia pisthorpemis, Sestrosporites pscudoalveolatus, Gleichviidites coiispiciendus, a n d G . s e n o n i c u s , a n d specimens of Botryococcus have been found sporadically throughout (Fig.   3). The assemblages from the lower half of the zone contain occasional acritarchs, which disappear in the upper part. The palynoflora of the uppermost few metres, which represents the youngest Jurassic sediments in the boring, is characterized by a low diversity a n d low density assemblage. Reworked Ordovician, Silurian, Devonian, and Triassic palynomorphs occur sporadically throughout.

Foraminifera1 zonation
Fifteen foraminiferal species were identified in the samples examined (Fig. 4). These are concentrated in the interval 306 m to 231 m. This interval is divided into 4 foraminiferal assemblage zones (sensu Hedberg, 1976 (Fig. 4) The A m m o b a c u l i t e s a g g l u t i n a n s Assemblage Zone (FC) contains almost solely the nominate arenaceous species in relatively high numbers (Fig. 4). Just a few specimens of Arnmobaculites aff. alaskaensis a n d o n e speciemen of a T r o c k a m m i n a w e r e f o u n d in the uppermost two samples. The lower boundary of the zone is defined at the d i s a p p e a r a n c e of the species characterizing Zone FB a n d the a p p e a r a n c e of Ammobaculites agglutinans (at 287 m). Bivalves and Fig. 6. Comparison of the interpreted palaeoenvironment with the palaeoenvironment of the central Danish Subbasin, as described by Michelsen (1978Michelsen ( , 1989b. sampling horizons (Fig. 4). The locations of the samples are shown on Fig. 4.
All foraminiferal tests are filled with pyrite and several gastropods are occasionally present in small numbers. The Amrnobaculites v e t u s t u s Assemblage Zone (FD) is characterized by the sporadic occurrence of A. vetustus and Bulbobaculites sp. 1. Animal b u r r o w s filled w i t h pyrite a r e common. The lower calcareous specimens are preserved only as pyrite casts. It is possible, therefore, that some specimens may have been destroyed during diagenesis.
The lowermost Astacolus variarzs -Plaiiularia beierana Assemblage Zone (FA) contains only the Nodosaridae Astacolus varians and Planularia beierana (Fig. 4). Other fossil g r o u p s recorded w e r e gastropods, bivalves, a n d ammonites.
The overlying Haplophragmoides aff. p y g m a e u s -Birlbohaculites sp. 1 Assemblage Zone (FB) represents the most diverse Jurassic foraminiferal fauna in the boring and boundary of this zone isdefined at the disappearance of Ammobaculites agglutirians and the first occurrence of A . v e s t u s t u s . The u p p e r b o u n d a r y is defined at t h e disappearance of foraminifera at 264 m.
The interval above Zone FD is barren of foraminifera except for a single specimen of Eoguttulina liassica at 231 m below core top.
The abundance of Splieripollenites and Corollina, and the presence of lsckyosporites variegatus and Manumia delcourtii indicate a Toarcian age for Zone PB. The palynomorph assemblage allows a correlation with Zone C4 (Hobro-1 boring) of Bertelsen (1974), with the Spkeripollenites-Leptolepidites Zone in the Stenlille borings (Dybkjaer, 1991), and with Zone I in the Hasle Klinkerfabrik clay Pit, Bornholm, of Hoelstad (1985) (Figs 1,s). The composition of the megaspore assemblages recorded from this interval is comparable to that known from part of the Bag5 Formation on Bornholm (Koppelhus & Batten, 1992).
'The dominance of Peririopollenites elafoides a n d t h e sporadic t ii Y h a t u s , Nariiioceratopsis Xracilis, and N . senex in Zone PC (Fig. 4) suggest a correlation of that zone with the Perinopollenites clataidrs Zone, which has been allocated a p r e s u m e d Aalenian age (Dybkjaer, 1991), with Zone I1 of Hoelstad (1985), and with assemblages recorded in other parts of the Bag5 Formation from Bornholm (Koppelhus & Nielsen, in prep.) (Figs 1,5).
The dominance of Perinopollenites elatoides a n d the presence of Callialasporites t u r b n t u s , C. dampieri, C . r~ricro-cwlatus, C. minus, Densosporites scanicus, Neoraistrickia gristliorplierisis, Sestrosporites pseudoalveolatus, Gleickeniidites coiispiciendus, a n d G . senonicus indicate that Zone PD correlates with the Middle Jurassic Zone D in Hobro-1 of Bertelsen (1974) and with Zone 111 (Hasle Klinkerfabrik clay pit) of Hoelstad (1985) (Fig. 5 ) . The lack of dinoflagellate cysts and other stratigraphically significant fossil groups hampers certain chronostratigraphical determination of this zone ( a n d those b e n e a t h ) zones. The s p o r e / p o l l e n o c c u r r e n c e of C a 11 ia I a s p o r i t es stratigraphy of the Middle Jurassic in the area is not well established. Since the examined assemblages lack any species indicating a Late Jurassic age, the zone is, however, suggested to encompass the Bajocian to Bathonian interval.

PALAEOENVIRONMENTAL CONDITIONS
The dominance of N o d o s a r i d a e (foraminifera) a n d dinoflagellate cysts together with a low TOC content (below 1 Yo) suggest a marine shelf e n v i r o n m e n t w i t h welloxygenated bottom conditions for foraminiferal Zone FA (the lower part of palynomorph Zone PA) in the late Pliensbachian. The presence of ammonite fragments and fish remains also support this suggestion (Fig. 4). The low species diversity in the foraminiferal and dinoflagellate cyst assemblages does, however, suggest a more marginal marine aspect. It is, therefore, suggested that Zone FA was deposited u n d e r m a r i n e i n n e r shelf conditions, a n interpretation which accords with Jurassic foraminiferal facies t r e n d s d o c u m e n t e d b y G o r d o n (1970), Nagy (1985a,b), Copestake (1989), and Nagy et a/. (1990).
The low species diversity a n d the dominance of the arenaceous foraminiferal genera Bulbobaculites, Haplophragmoides, and Kutseuella in foraminiferal Zone FB ( t h e u p p e r p a r t of p a l y n o m o r p h Z o n e PA a n d the lowermost part of PB), combined with the presence of 0.5 to 1 % TOC (Fig. 2), implies a well-oxygenated, shallow water environment w i t h reduced salinities (Johnson, 1976; Lnfaldli & Nagy, 1980;Nagy et a/., 1984;Copestake, 1989; J. Nagy, pers. comm. 1992). The foraminifera Eoguttuliiia linssica and the scolecodonts (Fig. 4) also indicate a shallow water environment, possibly of reduced salinity (Brouwer, 1969;Kozur, 1972;Copestake, 1989;Courtinat ef a/., 1991). Furthermore, t h e upwards-decreasing frequency o f dinoflagellate cysts and increasing amounts of spores and pollen (Fig. 3) suggest a gradual decrease in distance from the shoreline t h r o u g h t h e interval. Hence, this zone represents a regression f r o m m a r i n e inner shelf environments of Zone FA to near lagoonal or deltaic conditions during the late Pliensbachian and early Toarcian. The presence of a m m o n i t e fragments a n d isolated specimens of the dinoflagellate cysts Luelinden spinosa and Mendicodiniuni reticulaturn, indicate, however, that the marine influence on the environment of deposition had not ceased entirely.
The presence of the planktonic algal genus Botryococcus, which is normally considered to be a freshwater indicator, but which may be tolerant of slightly brackish water (Traverse & Ginsburg, 1966;Tappan, 1980), and the lack of foraminifera (Figs 3, 4) in the upper part of the palynomorph Zone PB, indicate further regression leading to an inner deltaic or estuarine and non-marine conditions.
The presence of dinoflagellate cysts, especially Naiziiocrratopsis gracilis, and acritarchs in palynomorph Zone PC (Fig. 3) and the presence of a single foraminiferal specimen and a few specimens of ostracods and bivalves just below 230 m may indicate a brief brackish or marine influx into a mainly lagoonal or deltaic environment. Naiiiioceratopsis gracilis may have been tolerant of brackish, but not of freshwater environments (Hancock & Fisher, 1981;Riding, 1983). Although acritarchs are mainly marine they may occur relatively abundantly in shallow bays and lagoons in association with low diversity assemblages of dinoflagellate cysts, and sometimes even in freshwater environments (Erkmen & Sarjeant, 1980;Hancock & Fisher, 1981;Batten, 1982).
In the uppermost part of the Jurassic section at Anholt, Zone PD, the foraminifera and dinoflagellate cysts are lacking and acritrachs only present in the lower part and Botryococcus present throughout the zone. This indicate a delta plain environment with occasional brackish water influence in the lower part

D I S C U S S I O N AND C O N C L U S I O N S
The palynomorph assemblages from the Jurassic of the Anholt boring are similar to those described from other sites in the Norwegian-Danish Basin and on Bornholm (Fig.  5). This facilitates a close correlation of the Anholt section to other sections within the region. Similar assemblages have also been described, for example, from Germany (Schulz, 1967), Britain (Woollam & Riding, 1983;Riding et at. 1991), and East Greenland (Lund & Pedersen, 1985), suggesting a rather uniform flora over a large part of north-west Europe and Greenland during the Early -Mid Jurassic. This coincides with the relatively rather uniform climatic regime in the area during that time (Hallam, 1984).
Lower Jurassic foraminiferal faunas have been described from several localities within the Norwegian-Danish Basin and adjacent areas (Nerrvang, 1957;Bang, a,b, 1971Bang, , 1973Norling, 1970Norling, , 1972, but those from the Anholt succession are markedly dissimilar. These previously described faunas are from older Lower Jurassic deposits and from fully marine assemblages, as opposed to the mainly marginal marine associations at Anholt. Shallow water foraminiferal faunas comparable to those encountered in the Toarcian of Anholt have, however, been encountered previously in the Lower Jurassic Wilhelmerya Formation of Spitsbergen (J. Nagy, pers. comm. 1992).
A comparison of the sedimentological and palaeontological data from the Anholt section with those from previous investigations in the region indicates a discrepancy in the timing of the onset of the regression during the latest Early Jurassic. The main phase of sea level lowering at Anholt occurred around the upper Pliensbachian / Toarcian boundary (Fig. 6), whereas in the centre of the Danish Subbasin most of the Toarcian was characterized by an increase in marine influence corresponding to the global sea level rise described by Hallam (1988). This sea level rise resulted in the establishment of stagnant bottom conditions in the centre of the basin (Michelsen, 1989b), and shallowing of the water apparently did not take place here until the late Toarcian, when a lagoonal environment was established (Michelsen, 1989a, b).
The regression at Anholt whichoccurredas early as the latest Pliensbachian, and thus slightly earlier than the shallowing in the centre of the basin, may have been caused by the prograding of the delta plain which characterized the basin during the Mid Jurassic (Michelsen, 1978). This delta progradation may have reached the more marginal Anholt area earlier than the basin centre. The Toarcian regression at Anholt may, however, also be the result of local tectonic uplift triggered within the Fennoscandian Border Zone.
The boundary between the Fjerritslev Formation and the Haldager Sand Formation in the Norwegian-Danish Basin has previously been considered coincident with the Lower-Middle Jurassic boundary (Michelsen 1978(Michelsen , 1989a. More recently, though, Michelsen & Nielsen (1991) have indicated the possibility of an Aalenian age for the upper part of the Fjerritslev Formation in the Terne-1 boring (Fig.1). But they chose to place the Lower/ Middle Jurassic boundary between the Fjerritslev and Haldager Sand formations on the basis of lithostratigraphy. The biostratigraphical data from the Anholt boring, however, presents new information concerning the age of this formational boundary. The boundary between the Fjerritslev and Haldager Sand formations corresponds to the boundary between palynomorph Zones PC and PD in the boring (Fig. 5) (see Nielsen, 1992). Since the palynomorph assemblage of Zone Explanation of Plate 4 Megaspores. Scanning electron micrographs. Sample depth, stub number, and catalogue the DGU number in brackets. Magnification x 150.  Palynomorph and foraminifera1 taxa recorded from the Jurassic section penetrated by the Anholt borehole, with author attributions and dates.