Correlation of Miocene North Sea sequences with the Danish land area based on foraminifera

In the present study comparisons are made between the Miocene North Sea sequences and the Danish onshore formations based on foraminiferal biostratigraphy. Foraminiferal faunas of four onshore borings previously analyzed by Kristoffersen have been reviewed. Biozones NSB 9 NSB 13 of the offshore sequences were all found in the Danish Miocene formations, and the obtained results promise well for a future more detailed correlation between the North Sea and the Danish land area.


Introduction
A recent sequence stratigraphical study <tf the eastern North Sea region have revealed 21 sequences during the Cenozoic (Michelsen et al., in press). Seven of these sequences are located within the Miocene. The sequence boundaries and the maximum flooding surfaces in the Miocene interval are chronostratigraphically allocated primarily by means of foraminiferal biostratigraphy. The North Sea foraminiferal zonations of King (1989) has been applied for the Miocene.
In the present study we have re-examined the foraminiferal biostratigraphy of four onshore borings, in order to test the applicability of the offshore stratigraphy in the Miocene of Denmark, and to assess its reliability in future offshore stratigraphic investigations. As onshore seismic data and petrophysical logs are of varying quality we have chosen to correlate the North Sea sequences and the Danish onshore formations primarily on the basis of foraminiferal biostratigraphy. Emphasis has been placed on examining whether the revised version of King's (1983) North Sea foraminiferal zonation (King, 1989), can also be applied in onshore regions (of the North Sea basin) in the Miocene, and in particular, whether biozones NSB 9 to NSB 13 are present in the Danish Miocene formations. To reach this goal the foraminiferal faunas of four of the onshore borings previously analyzed by Kristoffersen (1972) were reviewed (for location see Fig. 1).

The borings
Selected details of the foraminiferal stratigraphy of four borings at H0ruphav, Alkrersig, Gram and Sred, Denmark ( Fig.1) are shown in Figs. 2-5. In order to ease correlation with the biostratigraphical North Sea zonation (King, 1989) the data are presented as events (highest occurrence). The sample interval is approximately half a meter. The borings are described in ascending stratigraphical order.

H0ruphav (Fig. 2):
Bolivina antiqua d'Orbigny and Plectofrondicularia seminuda (Reuss) were found in the H0ruphav boring (DGU ref. No. 170.381). These are index species of North Sea Benthic Zones (NSB) 8c and 9 (King, 1989), respectively (Late Oligocene and Early Miocene). Between the highest occurrences of these two species a fossil poor interval with no stratigraphical important species was encountered. This interval is difficult to correlate to any established zonation. Loxostomum sinuosum (Cushman), substitute marker of NSB 10, had its highest occurrence approximately two metres above the highest occurrence of P. seminuda. In the same interval Asterigerina staeschei ten Dam & Reinhold was also present. The coexistence of L. sinuosum and A. staeschei has previously been shown from the Arnum Formation (Kristoffersen, 1972). The presence of the planktic species Cassigerinella chipolensis (Cushman & Ponton) indicates correlation to the German local Hemmoorian stage of the Lower Miocene (Spiegler, 1986). The Amum Formation was previously correlated to the Hemmoorian (Rasmussen, 1966).
Loxostomurn sinuosum (substitute marker of NSB 10) had its highest occurrence in the lower half of the boring, while Bulirnina dingdenensis Batjes marked the top of the Amum Formation (lithostratigraphic subdivision by Rasmussen, 1961). Bulirnina dingdenensis is also a substitute marker of NSB 10 (King, 1989). The highest occurrence of L. sinuosurn below the highest occurrence of B. dingdenensis could consequently be useful in subdividing NSB 10 in the Danish onshore area. Slightly below the top of NSB 10 the planktic Globorotalia praescitula Blow, index species of North Sea Planktic Zone (NSP) 11 (King, 1989), had its highest occurrence together with C. chipolensis. This indicates correlation to the German local Hemmoorian stage. Asterigerina staeschei (index species of NSB 11) dominates the fauna in the sediments of the Middle Miocene Hodde Clay (lithostratigraphic subdivision by Rasmussen, 1961). NSB 12 may be present in this boring, but according to King (1989) Uvigerina semiornata saprophila von Daniels & Spiegler should be present in NSB 12. In Alkcersig only Uvigerina serniornata semiornata d'Orbigny is present, and NSB 12 is, therefore, marked by a question mark.

Gram (Fig. 4):
The lithostratigraphical allocation of the lowermost part of the Gram boring (DGU ref. No. 141.277) is uncertain. It may belong to the Arnum Formation or the lower part of the Hodde Formation. However, a quartz gravel zone is present in the interval from 36.5-37.4 m below surface (Rasmussen, 1966). This feature has previously been described in the lowermost part of the Hodde Formation (Rasmussen, 1961(Rasmussen, , 1966. None of the characteristic species normally seen in the Arnum Formation are present in the interval below the Hodde Clay. Slightly below the top of the Hodde Clay A. staeschei, index species of NSB 11, has its highest occurrence. Bolboforma reticulata von Daniels & Spiegler and Bolboforma badenensis Szczechura have also their highest occurrences in this interval. In deep sea sediments these species occur in nannoplankton Zones NN 5-6 (Muller & Spiegler, 1993). This harmonizes with the fact that NSB 11 was previously correlated with NN 5 (King, 1989). The Gram Clay is separated from the Hodde Clay by a Glauconite Clay, which is barren of foraminifera. The Gram Clay is, however, relatively rich in foraminifera. In the lower part of this formation the occurrence of Elphidium antoninum (d'Orbigny) indicates the presence of NSB 12b. Slightly above this event, Bolboforma clodiusi von Daniels & Spiegler (index species of NSP 13) has its highest occurrence. None of the index species of NSB 12c and NSP 14a are present in the boring, but the occurrence of Uvigerina pygmaea langenfeldensis von Daniels & Spiegler could indicate the possible presence of NSB 12c. In the range chart of King (1989, p. 439) U. p. langenfeldensis has a range across the boundary between NSB 12c and NSB 13a. This is the reason for the question mark and the inclined dashed line at the top of NSB 12c in Fig. 4. In the topmost sample Bolboforrna metzmacheri (Clodius) and Uvigerina pygrnaea langeri von Daniels & Spiegler are present. These are index species of NSP 14b and NSB 13a, respectively.

Seed (Fig. 5):
Bolboforma metzmacheri and U. p. langeri were also found in the "typical" Gram Clay in the boring at Seed (DGU ref. No. 167.445). However, a rather scattered occurrence of U. p. langenfeldensis in the same interval could indicated the possible presence of NSB 12c.
A clay occurs above the typical Gram Clay which "must undoubtedly be termed Gram Clay" (Rasmussen, 1966, p. 320). Previously this clay was mentioned as Seed Clay. There are no index species present in this interval, but a substitute marker of NSB 13b, Valvulineria mexicana grammensis Langer, is found, and the planktic foraminifera Neogloboquadrina acostaensis (Blow), index species of NSP 15a, is also present. According to King (1989) the top of NSP 15a should correlate with the top of NSB 13a. This is not the case in the Seed boring.

Correlation
Correlation of the Danish formations with the NSB /NSP zonation has changed through time (Fig. 6). The first attempt by King (1983) to correlate the Danish Upper Oligocene and Miocene formations was based on Larsen & Dinesen's (1959) and Kristoffersen's (1972) data. In 1989 King published another correlation of the Danish formations with the NSB zones. The revised version was based on additional, unpublished data and is, therefore, unfortunately lacking an explanation for the major changes: The Vejlefjord and Klintinghoved Formations are not separated and they include NSB 8c and part of NSB 9. The boundary between the Klintinghoved Formation and the Amum Formation is rather variable but it occurs within NSB 9. The base of the Hodde Formation was moved down to within NSB 10 and the hiatus between the Gram and Hodde Formations disappeared. The boundary between these two formations coincides with the boundary between NSB 11 and 12 (Fig. 6) in King's 1989 stratigraphy. The present review of borings previously analyzed by Kristoffersen (1972)

Conclusion
The comparison of the foraminifera in four Danish onshore borings with the established North Sea foraminiferal zonation by King (1983King ( , 1989 shows that it is possible to use this zonation in the onshore region. Biozones NSB 9 -NSB 13 were all found in Danish onshore borings. In the light of this it should be possible to identify the North Sea sequences in the outcrops and borings in onshore Denmark. These results promise well for future more detailed correlations between the North Sea and the Danish land area. Analysis of land profiles should yield more information with a higher resolution than obtainable in offshore oil drillings due to closer sampling interval. This might perhaps successively result in a more detailed North Sea stratigraphy.   (King, 1989) oia.