Upper Oligocene to ?Lower Miocene in Hjøllund borehole
Based on analyses of benthic foraminifera, pyritised diatoms and Sr isotopes in the Hjøllund borehole (56º0.5’28.97’’N, 09º23’12.00E, Map 1) we recorded a 22 m-thick unit with Upper Oligocene sediments (227-205 m) and 14 m with Upper Oligocene to Lower Miocene deposits (205-191 m). The units were investigated with eleven ditch-cutting samples at 1 to 5 m intervals. Between 205 and 191 m there are no samples (Fig. 1). 14 samples from 191 to 18 m were also investigated. In this part the interval between the samples varies from 1 to 70 m. All these samples are barren of benthic foraminifera and planktonic index fossils, and no mollusc fragments for Sr isotope analyses were available. Consequently, this part of the borehole is not described and not included in Fig. 1. The base of the Upper Oligocene was not seen in the borehole. This borehole was also investigated palynologically and lithologically by Dybkjær & Rasmussen (2009), and the classification into lithological units is according to these authors.
Upper Oligocene (227-205 m, Branden Clay and Brejning Formation)
Benthic foraminifera of the Turrilina alsatica assemblage, Pararotalia canui assemblage and pyritised diatoms of the Diatom sp. 3 assemblage, together with a number of Sr isotope ages date this unit to Late Oligocene (Fig. 1). According to King (1989) the LAD of P. canui is close to the Oligocene/Miocene boundary. In addition to the nominate species, the lower part of the Turrilina alsatica assemblage also includes R. arnei and A. guerichi guerichi. The benthic foraminiferal assemblages are correlated with Zone NSB 8 of King (1989) and the diatom assemblage is correlated with Subzone NSP 9c of King (1989, North Sea). The fossil assemblage corresponds to the palynological Deflandrea phosphoritica Zone of Dybkjær & Piasecki (2008, 2010) recorded in the same borehole by Dybkjær & Rasmussen (2009).
Upper Oligocene-Lower Miocene (205-191 m, Vejle Fjord Formation and Billund Formation)
The sample at 191 m is the only one recovered from this succession (Fig. 1). The sample contains a sparse calcareous benthic foraminiferal fauna including Ehrenbergina serrata. According to King (1989) E. serrata is recorded from the Oligocne to lower part of Middle Miocene in the North Sea area. Two Sr isotope ages from this sample indicate an age close to the Oligocene/Miocene boundary. The benthic foraminiferal assemblage can probably be correlated with the upper part of Subzone NSB 8c or Lower part of Zone NSB 9 of King (1989). Dybkjær & Rasmussen (2009) recorded the Chiropteridium galea Zone of Dybkjær & Piasecki (2008, 2010) in the sample at 191 m, which is of earliest Miocene age according to these authors.
Sr isotope stratigraphy
Ten samples based on mollusc fragments from nine depths and two samples based on foraminiferal tests were analysed for Sr isotopes. All the analyses gave ages within the Late Oligocene with 26.9 Ma as the oldest age in the lower part (222 m), and 23.9 Ma as the youngest age from the uppermost sample (191 m, Table 1, Fig. 1). The Oligocene/Miocene boundary is at 23.8 Ma on the time scale of Berggren et al. (1995).
Hjøllund borehole, Denmark
|Litho. Unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Analysed fossils|
|Billund Fm||191 m||0.708266||0.000010||23.85||One mollusc fragment|
|Billund Fm||191 m||0.708218||0.000009||24.61||One mollusc fragment|
|Brejning Fm||205 m||0.708180||0,000008||25.21||Two small mollusc fragments|
|Brejning Fm||207 m||0.708218||0.000008||24.61||30 tests of Trifarina gracilis var. A|
|Brejning Fm||207 m||0.708202||0.000009||24.84||One mollusc fragment|
|Brejning Fm||209 m||0.708227||0.000008||24.48||30 tests of Trifarina gracilis var. A|
|Brejning Fm||209 m||0.000009||0.000009||25.08||One mollusc fragment|
|Brejning Fm||212 m||0.708153||0.000009||25.72||One mollusc fragment|
|Brejning Fm||215 m||0.708197||0.000009||24.92||One mollusc fragment|
|Brejning Fm||220 m||0.708135||0.000008||26.10||Two mollusc fragments|
|Brejning Fm||221 m||0.708101||0.000009||26.93||One mollusc fragment|
|Branden Clay||222 m||0.708217||0.000008||24.63||One mollusc fragment|
Table 1: Strontium isotope data from the Hjøllund borehole in Denmark. The samples were analysed at the University of Bergen. Sr ratios were corrected to NIST 987 = 0.710248. The numerical ages were derived from the SIS Look-up Table Version 3:10/99 of Howard & McArthur (1997). NIST = National Institute for Standard and Technology.
The samples in the Branden Clay and Brejning Formation consist of sand, silt and clay with common lignite coal in parts of sections. Mollusc fragments are also common in some samples. In the Vejle Fjord and Billund formations sand is dominant. The sand fraction is mainly medium to fine grained, and quartz is the dominant mineral (Fig. 1).
Berggren, W. A., Kent, D. V, Swisher, C. C., III & Aubry, M.- P., 1995: A Revised Cenozoic Geochronology and Chronostratigraphy. In Berggren, W. A. et al. (eds.): Geochronology Time Scale and Global Stratigraphic Correlation. Society for Sedimentary Geology Special Pulication 54, 129-212.
Dybkjær, K. & Rasmussen, E. S., 2009: Palynologisk datering og stratigrafi i boringen DGU nr. 86.2118, Hjøllund, Region Midtjylland. Danmarks og Grønlands geologiske Undersøgelse Rapport 2009/70, 34pp.
Dybkjær, K. & Piasecki, S., 2008: A new Neogene biostratigraphy for Denmark. Geological Survey of Denmark and Greenland Bulletin 15, 29-32.
Dybkjær, K. & Piasecki, S., 2010: Neogene dinocyst zonation in the eastern North Sea Basin, Denmark. Review of Palaeobotany and Palynology 161, 1-29.
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King, C., 1989: Cenozoic of the North Sea. In Jenkins, D. G. and Murray, J. W. (eds.), Stratigraphical Atlas of Fossils Foraminifera, 418-489. Ellis Horwood Ltd., Chichester.