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Chapter 89. Ostracod zones and dispersion of Mesozoic fossils in the Scandinavian North Sea area

Chapter 89. Ostracod zones and dispersion of Mesozoic fossils in the Scandinavian North Sea area

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1270 0. B. CHRISTENSEN



STRUCTURAL

EFFECTS

The Scandinavian North Sea area is the northeastern part of the North Sea Basin from the

offshore areas of southern Norway, the entire area of Denmark, through southernmost Sweden

(Scania) and into the islands of Bornholm in the Baltic Sea. From the late Palaeozoic into the

Tertiary, the North Sea areas are characterised by sub-basins of various sizes, relative connections, and with a long-time structural development of complicated faulting events in contrast to

most other areas in the Atlantic Ocean Regions. The area was important for development of

Mesozoic ostracods (Bate, 1977). Strong compression and tension along the Fennoscandian

Sheild affected by the Uralian and the Alpin plate collisions and the initial opening of the North

Atlantic Ocean into the Arctic Ocean, seems to have had strong effects on the biological balance

of the area. Moderate dislocations with some pull-apart basin effects (Baartmann and Christensen,

1975; Christensen 1975) have not been accepted for Latest Jurassic and Lowermost Cretaceous.

Older or younger, minor, but strong structural events with block rotations and dislocations,

however, have been described (Hesledal and Hamar, 1983; Pegrum, 1983).

Permian-Triassic rifting of the North Atlantic intensified a long sea-way, present in the North

Sea area from the Jurassic and renewed by rifting and faulting in various ages. A connection into

the embryonal Artic Ocean is most natural. The older graben structures in the northern North

Sea were elevated in the Middle Jurassic and then subsided as part of a North Sea-Arctic Seaway,

which crosses and follows the trends of the ancient Caledonides. With the sea-floor spreading

sigmoidally developed in the Northern Atlantic during the Late Cretaceous, Tertiary, and

Quarternary, the north-south Arctic Seaway does not necessarily terminate in the North Sea area.

In the eastern North Sea area, the Ringkabing-Fyn High, also elevated during Middle Jurassic,

has a great local effect on the development of the basins and sub-basins in the Scandinavian North

Sea area (Text-fig. 1).



THENORTH

SEA-ARCTIC

SEAWAY

The marine connections between various North Sea sub-basins and other shelf areas and oceans

are generally important for biostratigraphical evaluations. During the Jurassic, Cretaceous and

in the Early Tertiary, the North Sea basins were areas of conversions or interactions between cold,

more or less underset slow moving sea currents from the north and relatively warmer sea water

from the south, diluted in various degrees by runoff. Besides the large epicontinental sea areas in

arctic Alaska, Canada, on the Barents Shelf, and in the Greenland and Norwegian shelf areas, the

structurally downwarped areas most probably followed the central trends of the ancient Caledonian

range into the North Sea, and not into the Fennoscandian Shield. Hydrographical consistency

of the fact that the seaway was rather pronounced and structurally dependant during the Mesozoic,

may explain some paradoxes of the Boreal seas evaluated and referred to by Hallam (1969, 1981,

1983), Gordon (1974) and others. The seaway was defined as epicontinental, but not necessarily

shallow in all parts. In Text-fig. 1, some Mesozoic reconstructions of marine connections between

the arctic areas, the North Sea, and more southern seas are outlined. Influx of polar or boreal water

with contemporary marine effects from connections to basins around the North Sea seems to control much of the depositional environment as well. The eustatic sea-level changes and the structural events initiate the more or less pronounced hydrographical effects.



1271



ISEAWAYS



STRATIGRAPHIC UNITS



I N.E.



NORTH SEA AREA



Cenomanian



rn

3



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Albian



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Aptian



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t

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Barremian



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Hauterivian



124*9



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E



f



Valanginian



z



13825



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Ryazanian



144+5

Volgian

(150)



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Kimmeridgian



5



156k6

Odordian

163215

Callovian

169+15



rn

0



Bathonian



r n J



22 X=



176k34

Bajocian



183234

187?34



Toarcian

193?28

Pliensbachian

198+_32



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Sinemurian



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@ groups



of ostracod zones



i n the t e x t .



TEXTFIG. l-outline of stratigraphic relations between sea level changes (Vail and Todd, 1981). basin develop

ments, and ostracod distribution referred to and numbered in the text.



1272 0.B. CHRISTENSEN



DISTRIBUTION

OF FOSSILS

Generally, the ostracods comprise a minor part of other calcareous fossils recognised in the

Scandinavian Mesozoic. They are most well-preserved in the Late Mesozoic deposits and in the

southern and eastern areas. Only a few sections with pure ostracod assemblages are found in the

Triassic and in Purbeck-Wealden beds. Marine and brackish assemblages dominate. Secondary

diagenetic effects on the distribution of calcareous microfossils are seldom greater than the effects

of poor life conditions in the northwestern areas. There calcareous ostracods are present, but

scarce, compared with other groups of fossils, but very poor compared with the southeastern

areas in the Scandinavian North Sea. Solution plays a role in many coarse sediments. In these

beds, diagenetic calcite is rather common. Further south and in the eastern areas, probably not so

deeply exposed, rich calcareous faunas are relatively common between beds devoid of calcareous

fossils. These faunas are mostly very well-preserved, the most important, established through

the Scandinavian Upper Jurassic and Lower Cretaceous are noted (refer to numbers in Text-fig. 1).

7 . Mandocythere harrisiana Zone

Cythereis luermannae hannoverana Zone

Cythereis folkestonensis Subzone

? Cythereis 1. luermanniae-Neocythere ventrocostata Zone

Saxocythere notera senilis Subzone

Protocythere albae-Dolocythereidea bosquetiana Zone

Protocythere albae-Dolocytheridea vinculum Zone

Zsocythereis fissicostis Subzone

Cythereis corrigenda Subzone

Protocythere nodigera Zone

? Unnamed Zone

Protocythere intermedia Zone

Protocythere triplicata Zone

Mandocythere frankei Zone

Protocythere hannoverana Zone

Cytherella valanginiana Subzone

Stravia crossata Subzone

Paranotacythere globosa Subzone

Cytheropterina Subzone

Galliaecytheridea teres Zone

8. Fabanella boloniensis-Pachycytheridea compacta Zone

Cypridea aha formosa Zone

Cypridea inversa-C. valdensisprecursor Zone

9. Macrodentina reticulata Zone

? Galliaecytherideapolita Zone

Galliaecytherideaspinosa Zone

? Unnamed Zone

Eocytheropteron aquitanum Zone

? Unnamed Zone

Mandelstamia maculata Zone

Mandelstamia horria'a Zone

? Unnkned Zone

10. Mandelstama inflata Zone

Dicrorygma brotzeni Zonule



Ostracod Zones and Dispersion of Mesozoic Fossils 1273

Galliaecytherideaformosa Zonule

Galliaecytherideaoertlii Zonule

11. Macrodentina steghausi Zone

Galliaecytheridea elongata Zone

Macrodentinaproclivis Zone

Galliaecytherideamandelst. kilenyi Subzone

Galliaecytherideadissimilis Zone

12.



Amphicythere semisulcata-Schuleridea tribeli Zonule



13. ? Nophrecythere crusiata oxfordiana Zone

? Lophocythere interrupta Zone



TRIASSIC

The Triassic deposits in Scandinavian are mostly devoid of fossils. Red, oxidised continental

clastics are most common. An epicontinental sea occupied large areas in central Europe during

most of the Triassic. On strong mature landscape plains, a legacy from the Permian Period, marine

and lacustrine transgressions may reach large areas in the Scandinavian North Sea, but charophytae recognized in many wells in Denmark and Scania (Sorgenfrei and Buch, 1964; Brotzen, 1950)

indicate lacustrine environments in most areas.

Middle Triassic ostracods (e.g. Triassinella pulcra, Speluncella teres) are recognised with marine

fossils, indicating that the Muschelkalk sea reached the southernmost Danish area (Christensen,

1972). Late Triassic ostracods (Darwinula spp., Christellocythere sp., Speluncella vulgaris, S.

piriformis) were collected in outcrops of red to greenish fine clastics in Bornholm and are relatively

common in many localities in North Germany and Poland.

An arctic epicontinental sea north of the northern North Sea was hardly connected with the

Central European sea through the North Sea grabens, where fine clastics most probably indicate

some large lacustrine environments. In the Norian, contemporary with a change to a humid climate,

the deposits in the southern areas from the Central Graben into Denmark became rich in fossils.

A cyclical development in the sedimentation has been interpretated as due to eustatic sea level

changes (Bertelsen, 1978). The initial major transgression, marked by oolitic beds in most areas in

Denmark (Christensen, 1972), is probably not marine, but interpreted as lacustrine based on

the ostracods, below the Rhombocythere penarthensis Zone and beds with Rhaetavicula contorta,

foraminifera etc.



TRIASSIC

OSTRACOD ZONES

The Late Triassic ostracod assemblage is useful for zonation. Closely connected with the general

climatic change, a transgression, probably lacustrine, reached most Danish onshore areas. Later,

with a sea connection, marine faunas invaded. The zonation in England proposed by Anderson

(1964) is mainly based on marine ostracods. A zonation from Northwest Germany by Will (1969)

published after his death, it most useful for the Danish area. Anderson seems to have included

species from Germany in Rhombocythere penarthensis. Using the work of Will, it is easy to evaluate

some of those, as well as a number of Anderson's mistaken synonyms for Triassic ostracods in

Denmark, illustrated by Christensen (1962). R . penarthensis is similar to Notocythere magna

maritina Will. It occurs in the N . elegans Zone of Will with most species of the R. penarthensis

group and with N. hechti Zongu Will of the R. ruegeri group (Plate 1-I), but also above in marine



1274 0. B. CHRISTENSEN



beds. Anderson established his new genus Rhombocythere on studies of a small number of English

and German species. Will used in his work the genus name Notocythere in a wide sense. The genera

Rhombocythere and Notocythere (parts) are closely related to the genus EmphasiaMa ndelstam, 1956,

but the genera and species Rhombocythere penarthensis Anderson and Notocythere hechti Will are

established, and some genera (Notocythere), species, and subspecies need to be re-established. A

biostratigraphy close to the intentions of Will and for the Danish formations, revised by Bertelsen

(1978) is useful in Denmark:

Late Norian (Rhaet-Keuper)

Member G3 of Gassum Formation with megaspores (e.g. Trieletes),

Lepidopteris ottonis Flora.

Member G2 of Gassum Formation with agglutinated foraminifera

(e.g. Ammodiscus parva) and marine molluscs (e.g. Rhaetavicula contorta) and R . pernarthensis.



Early Norian (Steinmergelkeuper)

Beds of Gassum Formation G2 and G1 and Vinding Formations with ostracods.

Rhombocythere penarthensis Zone (Gassum Formation).

Rhombocytheepernarthensis Subzone.

Rhombocythere wicheri-R. penarthensis Subzone. (P1.l fig.1)

2. Notocythere media Zone (Vinding Formation).

Rhombocythere ruegeri Subzone.

Notocythere media Subzone (Pl. 1 fig. 2).

1.



The R . pernarthensis Subzone is most probably brachyhaline to euhaline. The older Subzones

are probably of lacustrine origin. The Rhombocythere wicheri-R. penarthensis Subzone has been

identified far north of the northern North Sea.



JURASSIC

In a great variety of Jurassic environments, poor ostracod assemblages have been collected from

Late and Middle Jurassic in the Viking Graben, more well-preserved assemblages at the margin.

In the Central Graben, rich Late Jurassic and scarce Middle Jurassic ostracod assemblages are

found. Rather well-preserved Lower Jurassic Ogmoconcha and Kinkelinella faunas are common in

most southern and eastern Scandinavian North Sea areas, but are also present in the Viking Graben.

Scarce and poorly preserved Middle Jurassic ostracods are present in some western and southern

parts of the Scandinavian North Sea grabens. During the Middle Jurassic most of the Scandinavian

areas were uplifted and the Rinksbing-Fyn High was created as a permanent structure. Volcanic

activities in the Central North Sea and in Scania illustrate a trend of faulting activities north of

the Ringksbing-Fyn High. There, fine to coarse clastics often with coal beds but altogether very

poor in fossils, are sandwiched between the fine clastics of the marine Early and Late Jurassic.

During the Late Jurassic strong rifting activities continued, with subsidence of most axial parts

of the Central and Viking Grabens. Poor benthonic faunas in the deepest part and common occurTriassic and Jurassic samples from the Danish Sub-basin. Fig. 1 . Assemblage from Rhombocythere

wicheri-R. penarthensis Subzone with R. penarthensis.R. elegans, R. cf. R. wicheri. R. ruegeri longa, Notocythere

media tuberculata a.0. Fig. 2. Assemblage from Notocythere media Subzone with Limnocythere keuperiana,

Notocythere media Subzone with Limnocythere keuperiana. Notocythere media prima, Darwinula spp. a.0.

Fig. 3. Monotypic assemblage of Polydentina? quadricostata.



PLATE1-Some



1275



1276 0. B. CHRISTENSEN



rences in some shallow areas are more an effect of poorly oxygenated seawater than diagenesis.

Agglutinated foraminifera1 assemblages with a low diversity are common in some intervals. A huge

number of more or less pyritized radiolarians in the grabens and along the margins indicate anaerobic to dysaerobic bottom conditions, and a stratified water column with space for a high production of heterotrophic plankton. In the Danish Sub-basin, Late Jurassic radiolarians are uncommon.

Calcareous foraminifera occur in certain horizons among the agglutinated foraminifers. They

are most common in the shallowest areas, but there also, most intervals are scarce in foraminifera.

Ostracods are most common in samples from horizons which also contain mollusc fragments.



JURASSICOSTRACOD ZONES

An ostracod zonation for the Early Jurassic Fjerritslev Formation was established by Michelsen

(1975) for the Danish Sub-basin and for the Central Graben (Michelsen, 1978):

3. Ogmoconchella adenticulata-Nanacythere simplex Zone (Late Pliensbachian)

Grammannella apostolescui-Kinkelinella foveolata Subzone (Early Pliensbachian)

Progonoidea reticulata Subzone (Early Late Sinemurian)

Ogmoconchella danica Zone (Late Sinemurian-Early Pliensbachian)

Cristacyrhere betzi-C. crassireticulata Zone (Late Sinemurian)

6. Ogmoconchella aspinata Zone (Hettangian-Early Sinemurian)



4.

5.



In Scania the C. betzi-C. crassireticulata Zone and the Ogmoconchella danica Zone are recorded

by Sivhed (1980). Ammonites and foraminifera investigated in the Danish wells and in Scania

directly support the establishment and general use of these ostracod zones. The affinity between

the Danish Sub-basin and the North German Basin is very high for the Lower Jurassic ostracods,

except in the Early Sinemurian (Michelsen, 1975).

Marine ostracods (Camptocythere praecox, Kinkelinella aff. adunca. K. oblonga. Pseudomacrocypris subaequabilis) found along the Fennoscandian Border Zone (Christensen, 1972) from Scania

to the Danish Sub-basin indicate Toarcian-Aalenian in the upper part of the Fjerritslev Formation,

below the non-marine Haldager Formation, which is difficult to correlate.

From wells in southwestern Scania and in the Hano Bay east of Scania, Norling (1981) indicates marine ostracods from the Bathonian (Oligocythereis woodwardi, 0. fullonica), from the

Callovian (Lophocythere interrupta interrupta, Ostracod Nos. 4,5,6 Lutze), and from the CallovianOxfordian (Lophyocythere scabra, L. cruciata spp. oxfordiana, Paracypris sp. A Schmidt). These

may indicate that during the Middle Jurassic, Scania had a close marine connection with the Polish

and North German Basins, east and south of the Ringksbing-Fyn High. Marine Ostracods from

the Middle Jurassic have not been reported from the Danish Sub-basin, but in most parts of the

North Sea Grabens ostracods of Bathonian-Callovian age (Praeschuleridea quadrata, P. batei, P.

subtrigonia, Pleurocythere borealis, Lophocythere bradiana, Macrodentina bathonica and others)

can be collected at some horizons. Along the eastern margins of the North Sea-Arctic Seaway, in

the northern North Sea, deltaic and coastal deposits successively built up during the Middle Jurassic

and closed the Seaway for long intervals after the Toarcian. High deposition rates, and erosion with

re-deposition and diagenesis probably resulted in the reduced ostracod assemblage during the

Middle Jurassic in Scandinavia.

Oxfordian transgressive events with opening, rifting and successive subsidence of the North

of boreal ostracod samples from the Danish Sub-basin. Fig. 1. Assemblage from

Galliaecytheridea spinosa Zone with G. spinosa sensu lato, Mandelstamia n. sp., Polydentina rudis, P. cf. P.

wicheri, Aaleniella (pro Pyrocytherura) inornata, A. gracilis a.0. Fig. 2, Assemblage from Mandocytherefiankei

Zone with M . jiankei fiankei, Acrocythere hauteriiana. Rehacythereis senckenbergi a.0.



PLATE2-Examples



1278 0.B. CHRISTENSEN



Sea-Arctic Seaway renewed the hydrographical conditions in the North Sea basins. Changes to

boreal conditions reached the Danish Sub-basin successively from Late Oxfordian to Early Kimmeridgian. At least on one occasion during the Late Oxfordian and Lowermost Kimmeridgian, and

contemporary with the deposition of coarser clastics (Flyvbjerg Member), the Danish Sub-basin

was part of a Danish-Polish Trough and probably also connected with the North German Basin.

A Jurassic ostracod, probably Polydentina quadricostata (Pl. 1 fig. 3), known from the Early Kimmeridgian in Central Poland, occurs monotypically (Pl. 2) and is found centrally in the Subbasin

and in the western euhaline areas belonging to the G. dissimilis Zone, as well. In the Fennoscandian

Border Zone an Amphicythere semisulcata-Schuleridea triebeli Zonule occurs in Scania and northermost Denmark. It includes a few Galliaecytheridea dorsetensis. In the North German basin of

Denmark, the zonule is found below the G. dissimilis Zone. In the Early Kimmeridgian (G. dissimilis

and G. elongata Zones) an open marine multi-lateral connection was established.

During the Volgian, the Norwegian-Danish Basin seems to have been exposed to the west as the

ostracod assemblage is closely related to the English Mandelstamia and Galliaecytheridea assemblages. The Polish and German Basins were cut off from the deeper boreal sea. Evaporites indicate

a warm, perhaps a more arid climate. In Scania, on the Fennoscandian Border Zone, and in the

North German Basin immediately south of the Rinksbing-Fyn High, an ostracod assemblage of

Macrodentina retirugata and limnophile genera was found. Fresh to brackish water environments

continued during the Portlandian and Berriasian. In Scania and Bornholm, Cypridea assemblages

are common (Christensen, 1968,1974). In the Fennoscandian Border Zone, not far from the boreal

sea and below the island of Anholt in Kattegat, Cypridea assemblages also, seem to be present

(Baartmann and Bruun Christensen, 1975).

The author and Tom Kilenyi (1970) developed a proposal for a basic ostracod zonation for the

Northwest European Kimmeridgian based on the type section in Dorset in which a great deal

of the Danish Middle Volgian (Upper Kimmeridgian) could be included. Wilkinson quite understood our review and gave valuable new information in his recent paper (Wilkinson, 1983),

also based on a parallel ammonite zonation. In the Scandinavian Upper Jurassic a great

number of Galliaecytheridea and Mandelstamia species have been collected, but this unfortunately

did not include a parallel investigation of other fossil groups. Variation in reticulation and the

peripheral outline are important features for determinations in these genera. The species of Mandelstarnia and Galliaecytheridea are most common in open sea deposits in the North Sea (Pl. 2, fig.

1). Species of Macrodentina dominate in more restricted environments. Most of the species of

these three genera have been observed in the North Sea area. This indicates that they are most

indigenous to the boreal sea. Mandelstarnia and Galliaecytheridea (from G. denticula Sharapova)

are also characteristic genera for the boreal Early Cretaceous in the USSR (Andreev and

Mandelstam, 1971), as well.

The species of Mandelstamia vary in outline and are in some degree environmentally or regionally dependant. As an example, two smooth species occur in the Danish Sub-basin. One of

these is closely related to, perhaps identical with, Limnocythere? infata (Steghaus). I have used it

as an index for the Mandelstamia infata Zone at the Kimmeridgian-Volgian boundary above the

A . sernisulcata-S. trebeli Zone in the Fennoscandian Border Zone.

The specimens of Galliaecytheridea are in many cases difficult to define as species in samples

without specimens of both sexes larval individuals. Dorsal views are virtually a necessity for descriptions and illustrations of an acceptable standard.

Species of the thermophilic Cytherelloidea have been found reither in the Early nor the Late

Jurassic in the Norwegian-Danish Basin. Lack of this genus in the Early Jurassic is also

characteristic of the North German Basin, but in England it is relatively common.



Ostracod Zones and Dispersion of Mesozoic Fossils 1219



CRETACEOUS

A zonation of the Early Cretaceous in the Scandinavian North Sea follows that from England by

Neale (1978), Wilkinson and Morton (1983), and from Germany. It is characteristic of the boreal

ostracod assemblages, that they are so alike, but with differences, at least in the earliest ages, easy

to determine as slight environmental variations mostly related to hydrographical rather than

local structural factors.

Effects of an influx of boreal water on the ostracod assemblages are mostly secondary in the

Early Cretaceous. Influx of warm surface water quickly stabilized in the Valanginian with a high

eustatic sea-level and a relatively thick surface layer of warm seawater in which the various ostracod

assemblages developed. I have recognised Acrocythere hauteriviana further north in the North

Sea-Arctic Seaway than other species. In the Danish area there is no evidence for a specific seaway

through the Danish Sub-basin and into a Danish-Polish Trough in the Earliest Cretaceous. Across

the submerged central parts of the Ringkobing-Fyn High, an early connection between the northeast

German Basin and the Danish Sub-basin seems to have been established in the Valanginian.

Barremian limestones, rich in Bairdia, Pontocyprella, Bythocypris and related genera, from

higher levels in some North Sea areas and far north along Norway, is an example of a warm surface

influx of seawater in the North Sea during the Early Cretaceous. Hydrographically more stable

conditions were established in the Late Cretaceous.

The Late Cretaceous ostracod assemblages in the Scandinavian North Sea Area vary from

open sea conditions to mangrove in Scania, and are relatively rich in ostracods, but a few have

been studied more intensively. In Denmark the outcrops from the Campanian and Maastrichtian

have been studied by Jorgensen from various aspects. He has established (1979) two biozones for

ostracods in the homogeneous chalk sequence : Bythoceratina danica Zone for the uppermost Maastrichtian, and Bythoceratina umbonatoides Zone for the Lower and lowermost part of Upper

Maastrichtian.



CONCLUSION

The ostracod assemblages from the Mesozoic in the Scandinavian North Sea area have been

studied to varying degrees. The exploration for oil and gas in the area provoked intensive studies

of the Jurassic and Lower Cretaceous. During this geological period, strong faulting activities with

various basin configurations and many ostracod assemblages were developed in the area, dominated

by boreal fauna elements. Differences between the deepest parts of the basins (graben areas) and

“more” epicontinental areas (e.g. the Norwegian-Danish Basin) seem to be related to hydrographical and structural factors. In many intervals only agglutinated foraminifera are present and

recognising or establishing an ostracod zonation is difficult without zonal parallels in other fossil

groups for reference to other areas with ostracods in the boreal faunal province. Gulliaecytherideu

and Mandelstamia are typical boreal ostracods which migrated along the North Sea-Artic Seaway.

This Seaway is considered to have been of great importance in the hydrographical and geological

development in the North Sea basins.



1280 0. B. CHRISTENSEN



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Chapter 89. Ostracod zones and dispersion of Mesozoic fossils in the Scandinavian North Sea area

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