Upper Oligocene to Middle Miocene in Rødding borehole

Based on analyses of benthic and planktonic foraminifera, Bolboforma and Sr isotopes in the Rødding borehole (55º22’30.18’’N, 09º04’46.25E, Map 1), we recorded six metres with Upper Oligocene sediments, a 203 m succession with Lower Miocene deposits, 5 m with Middle Miocene deposits and 14 m with Upper Miocene sediments. The units were investigated with 69 ditch-cutting samples from 1 to 8 m intervals. Between 160 and 138 m, 120 and 104 m and 64 to 46 m there are no samples (Figs. 1-3). The base of the Upper Oligocene is not seen in the borehole. Pleistocene sediments lie immediately above the uppermost Upper Miocene sample according to Dybkjær et al. (2009), who investigated the same borehole palynologically and lithologically. The classification into lithological units is according to these authors.

Well summary figure for Rødding borehole, fig 1

Well summary figure for Rødding borehole, fig 2

Well summary figure for Rødding borehole, fig 3

Biostratigraphy

Upper Oligocene (249-243 m, lower main part of Brejning Formation)

Benthic foraminifera of the Pararotalia canui assemblage 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 foraminiferal fauna also includes N. granosum. The foraminiferal assemblage is correlated with Zone NSB 8 of King (1989).
Dybkjær et al. (2009) recorded the LAD of D. phosphoritica at 246 m in the same borehole, which is close to the Oligocene/Miocene boundary according to Dybkjær & Piasecki (2008, 2010).

Lower Miocene (243-40 m, uppermost part of the Brejning Formation, Vejle Fjord Formation, Klintinghoved Formation, Bastrup Formation, Arnum Formation, Odderup Formation and lowermost Hodde Formation)

Benthic foraminifera of the Nonion sp. A assemblage, Plectofrondicularia seminuda assemblage, Astigerina guerichi staeschei assemblage (lower, main part) and Bolboforma of the Bolboforma spinosa/Bolboforma rotunda assemblage give an Early Miocene age to this unit (Figs. 1-3). In addition to the nominate species C. contraria and Bolivina cf. antiqua occur in the Nonion sp. A and Plectofrondicularia seminuda assemblages. T. gracilis, T. gracilis var. A and T. alsatica are present in the Astigerina guerichi staeschei assemblage. G. soldanii girardana is also recorded sporadically throughout this part, but is probably reworked from Upper Oligocene sediments.

According to King (1989) the LADs of Nonion sp. A, P. seminuda and Bolivina cf. antiqua are in the early Late Miocene. U. tenuipustulata, the top Miocene benthic foraminiferal indicator according to King (1983, 1989), is missing in the Rødding borehole (Denmark), probably due to environmental factors. However, A. guerichi stashei is known from the Lower Miocene and the lowermost Middle Miocene in the North Sea (King 1983, 1989). According to King (1983) and Eidvin & Rundberg (2007), the LADs of A. guerichi stashei and U. tenuipustulata are approximately coincident in some areas of the North Sea. The benthic foraminiferal assemblage in this unit can probably be correlated with Zones NSB 9 and Zone NSB 10 of King (1989).

In the samples at 127 and 129 m, we recorded several specimens of Bolboforma Spinosa/B. rotunda (Fig. 2). Spiegler & Müller (1992) and Spiegler (1999) described a B. spinosa Zone and a B. rotunda Zone from the Lower Miocene in the North Atlantic. The palynological investigation of Dybkjær et al. (2009) gives a Middle Miocene age for the part of the unit which is above 99 m (based on the LAD of the dinoflagellate cyst Cousteaudinium aubryae).

Middle Miocene (40-35 m, Hodde Formation )

Bolboforma of the Bolboforma badenensis-B. reticulata assemblage and benthic foraminifera of the Astigerina guerichi staeschei assemblage (uppermost part) and Ceratobulimina haueri assemblage (lowermost part) date this unit to Middle Miocene (Fig. 3). The benthic foraminiferal fauna also include B. elongata. Spiegler & Müller (1992) described a B. badenensis Zone and a B. reticulata Zone from the North Atlantic in deposits with an age of slightly more than 14 to 11.7 Ma. Doppert (1980) described C. haueri from the Lower to Upper Miocene in the Netherlands, and King (1989) reported B. elongata from Oligocene to lowermost Upper Miocene deposits in the North Sea area. The benthic foraminiferal fauna is correlated with FD-Zone of Doppert (1980) from the Netherlands and Zone NSB11, Zone NSB 12 and the lower part of Subzone NSB 13a of King (1989) from the North Sea.

Upper Miocene (35-21 m, uppermost Hodde Formation, Ørnhøj Formation and Gram Formation)

Bolboforma of the Bolboforma laevis assemblage and Bolboforma metzmacheri assemblage and benthic foraminifera of the Ceratobulimina haueri assemblage (upper main part) give a Late Miocene age to this unit. The benthic foraminiferal fauna also includes Cancris auriculus, S. bulloides, G. subglobosa, U. pygmea langenfeldensis and U. pygmea langeri. Spiegler & Müller (1992) and Müller & Spiegler (1993) have recorded a Bolboforma laevis/B. capsula Zone from the North Atlantic and a Bolboforma laevis Zone from the Vøring Plateau from deposits with an age of approximately 10.3-10.0 Ma. The same authors have recorded a B. metzmacheri Zone from sediments with an age of 10.0-8.7 Ma in the same areas. From deposits in the North Sea area, King (1989) has described C. auriculus from the Lower Miocene to Lower Pliocene (common to Upper Miocene), U. pygmea langenfeldensis from the Middle to Upper Miocene and U. pygmea langeri from the Upper Miocene. The benthic foraminiferal fauna is correlated with the upper part of Subzone NSB 13a and lower part of Subzone NSB 13b of King (1989).

Sr isotope stratigraphy (SIS)

Sixty-oner samples (from 50 depths) based on mollusc fragments and four samples based on foraminifera (from three depths) were analysed for Sr isotopes in the Rødding borehole (Figs. 1-3 and Table 1).

The lowermost sample was taken from the Brejning Formation, uppermost in the part where biostratigraphical correlations indicate a Late Oligocene age. The obtained ⁸⁷Sr/⁸⁶Sr ratios gave an age of 23.9 Ma close to the Oligocene/Miocene boundary (23.8 Ma).

Eleven samples were taken from the Brejning Formation and Vejle Formation (Fig. 1). The nine lowermost samples gave similar ages from 24.9 to 23.7 Ma, close to the Oligocene/Miocene boundary (23.8 Ma). The two uppermost samples gave Early Miocene ages of 21.3 and 21.3 Ma. These ages are in accordance with the ages obtained by the foraminiferal and palynological correlations within the precision of the SIS method. However, it is important to note that the palynological zonation used by Dybkjær et al. (2009) is based on the time scale of Gradstein et al. (2004) and that the SIS look-up table version 3:10/99 of Howard & McArthur (1997) is based on the time scale of Berggren et al. (1995).

The Klintinghoved Formation and the lower main part of the Bastrup Formation contain very few calcareous foraminifera and mollusc fragments. These units were given an Early Miocene age by the biostratigraphical correlations. One analysed mollusc fragment from 185 m (upper half of the Klintinghoved Formation) gave an age of 5.7 Ma which obviously indicates caved material. One mollusc fragment from 160 m (lower part of the Bastrup Formation) gave 25.0 Ma which indicates reworked material (Fig. 1).

Seven samples based on mollusc fragments were taken from the uppermost part of the Bastrup Formation and Lowermost Arnum Formation. These sections were given an Early Miocene age by the biostratigraphical correlations. Except for one obvious caved sample at 120 m (11.3 Ma), all the obtained 87Sr/86Sr ratios gave Early Miocene ages (20.5-18.5 Ma, Fig. 2) and support the biostratigraphical correlations.

Fourteen samples based on mollusc fragments and one sample based on calcareous foraminifera were taken from the upper part of the Arnum Formation, Odderup Formation and lowermost Hodde Formation. These parts were also given an Early Miocene age by the biostratigraphical correlation. Ten of the Sr analyses gave ages close to the Lower/Middle Miocene boundary (16.4 Ma), which support the biostratigrapical correlations. The samples at 100, 92 and 65 m gave late Mid Miocene and early Late Miocene ages and indicate caved material. The sample at 81 m gave an Early Oligocene age and indicates reworked material (Figs. 2 and 3).

Twenty-nine samples based on mollusc fragments and three samples based on calcareous foraminifera were taken from the closely sampled Hodde (uppermost main part), Ørnhøj and Gram formations. These units were given Mid and Late Miocene ages by the biostratigraphical correlation. Twelve of the obtained 87Sr/86Sr ratios gave late Early Miocene ages, eleven gave early Middle Miocene ages, and four samples gave late Middle Miocene ages (three close to the Middle/Late Miocene boundary, Fig. 3). The Bolboforma assemblages that we recorded in these sections are especially good index fossils, and enable us to correlate the shelfal fossil assemblages with short-range, Late and Middle Miocene, deep-ocean Bolboforma zones which are calibrated with nannoplankton and paleomagnetic data. Unfortunately, the Bolboforma are too few and too small to give sufficient calcium carbonate to any Sr sample, and consequently mollusc fragment and foraminiferal tests were used. It is unlikely that the Bolboforma are caved, since Pleistocene sediments lie immediately above. The ages based on the correlations of the Bolboforma assemblages are supported by the palynological correlations of Dybkjær et al. (2009). To give an extra control to the Sr analyses we analysed two samples from most levels, and they came out with similar results. Consequently, we do not believe that the discrepancy between the ages obtained by Sr analyses and the fossil correlation is due to any analytical error. The reason for the discrepancy is discussed in detail in Eidvin et al. (work in progress) without arriving on a clear conclution.

Rødding borehole, Denmark

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Comments	Analysed fossils
Gram Fm	21 m	0.708817	0.000008	12.98	See Fig. 1	One mollusc fragment
Gram Fm	21 m	0.708792	0.000008	14.56	See Fig. 1	One mollusc fragment
Gram Fm	22 m	0.708837	0.000009	11.90		One mollusc fragment
Gram Fm	23 m	0.708830	0.000009	12.31	See Fig. 1	One mollusc fragment
Gram Fm	23 m	0.708749	0.000008	15.90	See Fig. 1	One mollusc fragment
Gram Fm	24 m	0.708798	0.000009	14.21	See Fig. 1	Two mollusc fragments
Gram Fm	24 m	0.708851	0.000008	11.30		One mollusc fragment
Gram Fm	25 m	0.708782	0.000010	14.98	See Fig. 1	One mollusc fragment
Gram Fm	25 m	0.708800	0.000009	14.30	See Fig. 1	One mollusc fragment
Gram Fm	26 m	0.708768	0.000009	15.53	See Fig. 1	One mollusc fragment
Gram Fm	26 m	0.708811	0.000008	13.26	See Fig. 1	One mollusc fragment
Gram Fm	27 m	0.708805	0.000009	13.59	See Fig. 1	One mollusc fragment
Gram Fm	28 m	0.708733	0.000009	16.25	See Fig. 1	One mollusc fragment
Gram Fm	28 m	0.708713	0.000009	16.59	See Fig. 1	One mollusc fragment
Ørnhøj Fm	29 m	0.708706	0.000008	16.70	See Fig. 1	One mollusc fragment
Ørnhøj Fm	30 m	0.708624	0.000008	17.68	See Fig. 1	One mollusc fragment
Ørnhøj Fm	30 m	0.708655	0.000009	17.35	See Fig. 1	One mollusc fragment
Ørnhøj Fm	32 m	0.708633	0.000009	17.59	See Fig. 1	One mollusc fragment
Ørnhøj Fm	32 m	0.708766	0.000008	15.48	See Fig. 1	One mollusc fragment
Ørnhøj Fm	33 m	0.708742	0.000008	16.06	See Fig. 1	One mollusc fragment
Hodde Fm	34 m	0.708727	0.000009	16.36	See Fig. 1	One mollusc fragment
Hodde Fm	34 m	0.708713	0.000008	16.59	See Fig. 1	One mollusc fragment
Hodde Fm	35 m	0.708767	0.000009	15.45	See Fig. 1	One mollusc fragment
Hodde Fm	35 m	0.708707	0.000008	16.68	See Fig. 1	One mollusc fragment
Hodde Fm	36 m	0.708836	0.000009	11.94		One mollusc fragment
Hodde Fm	36 m	0.708672	0.000008	17.15	See Fig. 1	12 tests of Ceratobulimina contraria
Hodde Fm	37 m	0.708704	0.000009	16.73	See Fig. 1	One mollusc fragment
Hodde Fm	37 m	0.708715	0.000008	16.65	See Fig. 1	One mollusc fragment
Hodde Fm	38 m	0.708589	0.000009	18.08	See Fig. 1	One mollusc fragment
Hodde Fm	38 m	0.708715	0.000008	16.56	See Fig. 1	One mollusc fragment
Hodde Fm	39 m	0.708747	0.000008	15.97	See Fig. 1	20 tests of A. guerichi staeschei
Hodde Fm	39 m	0.708754	0.000008	15.79	See Fig. 1	24 tests of A. guerichi staeschei
Hodde Fm	40 m	0.708741	0.000008	16.08	See Fig. 1	20 tests of U. semiornata semiornata
Hodde Fm	40 m	0.708700	0.000008	16.79	See Fig. 1	One mollusc fragment
Odderup Fm	41 m	0.708731	0.000008	16.29	See Fig. 1	One mollusc fragment
Odderup Fm	64 m	0.708764	0.000008	15.54		One mollusc fragment
Odderup Fm	65 m	0.708860	0.000009	11.01	Caved	One mollusc fragment
Odderup Fm	69 m	0.708746	0.000009	15.97		One small gastropod
Odderup Fm	72 m	0.708708	0.000009	16.67		One small gastropod
Odderup Fm	78 m	0.708752	0.000008	15.83		One mollusc fragment
Odderup Fm	81 m	0.707898	0.000009	32.13	Reworked	One bryozo fragment
Arnum Fm	91 m	0.708718	0.000008	16.51		One mollusc fragment
Arnum Fm	92 m	0.708805	0.000008	13.59	Caved	One mollusc fragment
Arnum Fm	94 m	0.708746	0.000008	15.97		One mollusc fragment
Arnum Fm	99 m	0.708734	0.000008	16.23		One mollusc fragment
Arnum Fm	100 m	0.708846	0.000011	11.49	Caved	One mollusc fragment
Arnum Fm	120 m	0.708852	0.000009	11.27	Caved	One mollusc fragment
Arnum Fm	124 m	0.708440	0.000009	20.29		One mollusc fragment
Arnum Fm	127 m	0.708502	0.000009	19.27		One mollusc fragment
Arnum Fm	129 m	0.708527	0.000008	18.89		One mollusc fragment
Arnum Fm	132 m	0.708424	0.000008	20.53		One mollusc fragment
Arnum Fm	135 m	0.708558	0.000009	18.46		One mollusc fragment
Bastrup Fm	138 m	0.708484	0.000009	19.57		One mollusc fragment
Bastrup Fm	160 m	0.708194	0.000031	24.97		One mollusc fragment
Klintingh. Fm	185 m	0.709012	0.000008	5.65	Caved	One mollusc fragment
Vejlefjord Fm	216 m	0.708387	0.000009	21.13		One mollusc fragment
Vejlefjord Fm	218 m	0.708380	0.000009	21.26		One mollusc fragment
Vejlefjord Fm	220 m	0.708263	0.000009	23.90		One mollusc fragment
Vejlefjord Fm	222 m	0.708276	0.000008	23.67		One mollusc fragment
Vejlefjord Fm	225 m	0.708230	0.000009	24.44		One mollusc fragment
Vejlefjord Fm	228 m	0.708262	0.000009	23.92		One mollusc fragment
Vejlefjord Fm	230 m	0.708232	0.000009	24.41		One mollusc fragment
Vejlefjord Fm	236 m	0.708258	0.000008	23.99		Three small mollusc fragments
Vejlefjord Fm	238 m	0.708233	0.000008	24.39		One mollusc fragment
Brejning Fm	241 m	0.708199	0.000009	24.89		One mollusc fragment
Brejning Fm	243 m	0.708265	0.000009	23.86		One mollusc fragment

Table 1: Strontium isotope data from the Rødding 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.

Lithology

Upper Oligocene (249-243 m, lower, main part of the Brejning Formation)

This unit contains mainly sand consisting of glauconite grains. Grains of quartz, mica and pyrite are recorded in the upper part (Fig. 1).

Lower Miocene (243-41 m, uppermost Brejning, Vejlefjord, Klintinghoved, Bastrup, Arnum and Odderup formations)

This succession consists mainly of course to fine sand, with the medium fraction dominating. Quartz dominates the sand fraction with minor glauconite, mica and pyrite. Glauconite is quite common in a few samples, and mollusc fragments in others (Figs. 1-3).

Middle to Upper Miocene (41-21 m, Hodde, Ørnhøj and Gram formations)

This part of the borehole is dominated by clay, silt and glauconite sand. In the Hodde Formation minor quartz is present in the sand fraction (Fig. 3).

References

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