Middle Eocene to Upper Pliocene in well 7316/5-1
Modified after Eidvin et al. (1998b).
Based on analyses of benthic foraminifera, dinoflagellate cysts and Sr isotopes a 108 m thick unit of Lower Oligocene sediments, 22 m of Lower Oligocene to Lower Miocene deposits and 12 m of Lower Miocene sediments were recorded in well 7316/5-1 (73º 31’11.982’’N, 16 º 25’59.698’’E, Map 2). 30 ditch cutting samples and six sidewall cores were available for the analyses (Fig. 1, Eidvin et al. 1998b). The Lower Oligocene rests unconformably on Middle Eocene sediments and the Lower Miocene is capped by Upper Pliocene deposits. See Eidvin et al. (1998b) for the sections from 1600 to 1120 m and 920 to567 m.
Well summary figure for well 7316/5-1
Middle Eocene (1120-1090 m, Sotbakken Group)
According to Eidvin et al. (1998b), most of the recorded foraminifera in this interval are either reworked or are long ranging stratigraphically. Exceptions to this is Nonionella spissa which is known from the Middle Eocene of Belgium (Kaasschieter 1961) and Spiroplectammina carinata which is known from Lower Eocene to Lower Oligocene in the North Sea (Charnock & Jones 1990).
Lower Oligocene (1090-982 m, Sotbakken Group)
Benthic calcareous benthic foraminifera of the Turrilina alsatica – Angulogerina tenuistriata assemblage and strontium-isotope ages obtained from 1015-987 m and 1070-1050 m, date this interval to the earliest Oligocene (Fig. 1, Table 1, Eidvin et al. 1998b).
No planktonic foraminifera were recorded. In addition to the nominate species, the benthic foraminiferal assemblage also contains Gyroidina soldanii girardana, Stilostomella adolphina, Eponides pygmeus, Bolivina cf. antique and telegdi, and is tentatively correlated to the NSB 7 Zone of King (1989) from the North Sea and Zone NSR 7A of Gradstein & Bäckström (1996) from the North Sea and Haltenbanken area (Eidvin et al. 1998b).
Lower Oligocene – Lower Miocene (982-960 m, Sotbakken Group)
The recovered in situ foraminifera and dinoflagellates indicate only an Early Oligocene to Early Miocene age for this interval (Eidvin et al. 1998b). The unit correspond to the lower part of the agglutinated foraminiferal Cyclammina placenta assemblage and the dinoflagellate unzoned interval d3 of Eidvin et al. (1998b). In addition to the nominate species, the foraminiferal fauna also contains other agglutinated foraminifera with long stratigraphical range including Bathysiphon spp., Ammodiscus sp., Haplophrogmoides sp. and Spiroplectammina sp. and is tentatively correlated to Zone NSA of King (1989).
Lower Miocene (960-948 m, Sotbakken Group)
The agglutinated foraminifera of the upper part of the Cyclammina placenta assemblage indicate only a general Oligocene to Early Miocene age for this interval. However the dinocyst Impagidinium patulum occurs in the samples from 950 and 948 m, and according to Manum et al. (1989) I. patulum does not occur in sediments older than Early Miocene in Leg 104, Site 643 on the Vøring Plateau in the Norwegian Sea. According to Powell (1992) the oldest occurrence of I. patulum coincides with the base of NN2. Spiniferites pseudofurcatus is noted in the samples from 955 to 950 m. This species is common in the Lower Miocene, but is also found in older deposits (Fig. 1, Eidvin et al. 1998b).
Upper Pliocene (948-920 m, Nordland Group)
No index fossils are recorded in this unit, but Late Pliocene forms are recorded immediately above (see Eidvin et al. 1998b). However the lithology indicates that this unit is also of Late Pliocene age (on the time scale of Berggren et al. 1995, see below).
Sr isotope stratigraphy
Eidvin et al. (1998b) executed Sr isotope analyses on tests of calcareous foraminifera from the interval 1070-1050 m and 1015-987 (two analyses). When using the look-up table of Howard & McArthur the corrected 87Sr/86Sr-ratios gave ages close to Oligocene/Eocene boundary (Table 1, Fig. 1).
|Litho. Unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Laboratory||Analysed fossils|
|Sotbakken Gr||1017-990 m||0.707799||0.000011||34.16||IFE||Calcareous benthic index foraminifera|
|Sotbakken Gr||1017-990 m||0.707803||0.000008||34.05||IFE||Calcareous benthic index foraminifera|
|Sotbakken Gr||1070-1050 m||0.707811||0.000038||33.84||IFE||Calcareous benthic index foraminifera|
Table 1: Strontium isotope data from lowermost Oligocene deposits in well 7316/5-1. IFE = Analysed at the Institute for energy technology (at Kjeller, Norway). 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 and McArthur (1997). NIST = National Institute for Standard and Technology. Modified from Eidvin et al. (1998b).
Middle Eocene to Lower Pliocene (1120-948 m, Sotbakken Group)
This part of the Sotbakken Group consists primarily of clay-stone, which is slightly silty (Fig. 1).
Upper Pliocene (948-920 m, Nordland Group)
Both the ditch cutting samples and the sidewall cores from this unit contain a clay-rich diamicton which is rich in sand, silt and pebbles. The pebbles are of both crystalline and sedimentary rocks (Fig. 1). All these pebbles are interpreted as ice-rafted. According to Fronval & Jansen (1996), on the Vøring Plateau (Norwegian Sea) there is a marked increase in the supply of ice-rafted material after about 2.78 Ma which reflects the expansion of the northern European glaciers. The maximum age for this unit is therefore assigned to be 2.78 Ma belonging mainly to Gelasian Age (see Eidvin et al. 1998b for more details).
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