LITHOSTRATIGRAPHIC SERIES / SUBSERIES BENTHIC UNITS NORDLAND GROUP ELPHIDIUM PLEISTO- CENE EXCAVATUM - NONION CUM ASSEM. GYRIODINA SOLDANII

Transcription

1 WELL 36/1-2 DEPTH (mrkb) GAMMA RAY Unit: GAPI LITHOLOGY LITHOSTRATIGRAPHIC UNITS NORDLAND GROUP HORDALAND GROUP BALD. - SELE SERIES / SUBSERIES PLEISTO- CENE UPPER OLIGOCENE LOWER - UPPER OLIGOC. LOWER OLIGOCENE U. PALEOC. - L. EOCENE BENTHIC FORAMINIFERAL S ELPHIDIUM EXCAVATUM - NONION LABRADORI- CUM ASSEM. PLANKTONIC FOSSIL S GYRIODINA SOLDANII UNDEFINED GIRARDANA ASSEMBL. DIATOM SP. 3 ELPHIDIUM SUBNODOSUM ASTIGERINA GUERICHI GUERICHI R. AMPLEC- TENS ASS. NEOGLOBO- QUADRINA PACHYDERMA (SINISTRAL) UNDEFINED PALEOBATHYMETRY MIDDLE MIDDLE - INNER OUTER MIDDLE - INNER Sr ISOTOPE AGES FROM MOLLUSC TESTS (Ma) 18,9* 1,1* 28,5 26,3 26,1 26,7 26,1 27,0 26,3 26,4 27,2 25,2 26,8 27,8+28,4 27,1 27,1 28,8 28,4+28,9 28,4 28,4 27,5 27,7 28,6 29,1 30,5 32,7 31,5 30,5* 30,2* Sr ISOTOPE AGES FROM FORAMINIFERAL TESTS (Ma) 26,0 27,4 27,5 SAMPLES (metres) 570 DC 580 DC 590 DC 610 DC 620 DC 630 DC 640 DC 650 DC 660 DC 670 DC 680 DC 690 DC 700 DC 710 DC 720 DC 730 DC 740 DC 750 DC 760 DC 770 DC 780 DC 790 DC 800 DC 810 DC 820 DC 830 DC 840 DC 850 DC 860 DC 870 DC 880 DC 890 DC 900 DC 910 DC 920 DC 930 DC 940 DC 950 DC Sea floor = 252 metres below rig floor (mrkb) DC = Ditch cuttings gapi = American Petroleum Institute gamma ray units = Abundant molluscs or mollusc fragments = Ice-rafted, angular pebbles = Rounded or sub-angular pebbles * = Caved OD Fig. 1: Well summary figure for the Upper Paleocene-Lower Eocene to Pleistocene for well 36/1-2 including gamma ray log, lithology, lithostratigraphic units, series/subseries, benthic foraminiferal assemblages, planktonic fossil assemblages, paleobathymetry, strontium isotope ages and analysed samples.

2 Upper Paleocene-Lower Eocene to Pleistocene in well 36/1-2 Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 36/1-2 (61º63'00.25''N, 04º0'52.19''E, Map 1), we recorded 10 m with Upper Paleocene-Lower Eocene sediments, 100 m with Lower Oligocene deposits, 40 m with Lower-Upper Oligocene deposits, an 180 m-thick column with Upper Oligocene sediments and 50 m with Pleistocene deposits. The base of the Upper Paleocene-Lower Eocene and the top of the Pleistocene were not investigated. The units were investigated with 38 ditch-cutting samples at ten metre intervals except between 610 and 590 m (Fig. 1). Biostratigraphy Upper Paleocene-Lower Eocene ( m, Balder Group) Benthic agglutinated foraminifera of the Reticulophragmium amplectens assemblage give a Late Paleocene-Early Eocene age for this unit. In addition to the nominate species, the benthic foraminiferal fauna also includes Reticulophragmium sp. A and Psammosiphonella gr. discreta, and the assemblage is correlated the zones NSA 1 to NSA 3 of King (1989, North Sea). Lower Oligocene ( m, Hordaland Group) A large number of Sr isotope analyses, based on mollusc fragments, are the main factors in dating this unit to the Early Oligocene (Fig. 1). The recorded benthic foraminifera, including A. guerichi guerichi (most common), T. alsatica and T. gracilis, can only give a general Oligocene to Lower Miocene age. No planktonic foraminifera and diatom index forms are recorded in this unit. The foraminiferal fauna is correlated with Zone NSR 7A and Zone NSR 7B of Gradstein & Bäckström (1996) and probably Zone NSB 7 of King (1989) from the North Sea. However, according to King (1989) A. guerichi guerichi did not occur in the North Sea area before the early Late Oligocene. According to Gradstein & Bäckström (1996), however, A. guerichi guerichi occurred from the early part of Early Oligocene. Lower - Upper Oligocene ( m, Hordaland Group) The fossil assemblage in this unit is similar to the one from m. However, the Sr isotope analyses give Early and Late Oligocene ages close to the Early/Late Oligocene boundary (Fig. 1). Upper Oligocene ( m, Hordaland Group) Benthic foraminifera of the Elphidium subnobosum assemblage and Gyroidina soldanii girardana assemblage and pyritised diatoms of the Diatom sp. 3 assemblage together with a number of Sr isotope ages, give a Late Oligocene age for this unit (Fig. 1). The units are nearly barren of planktonic foraminifera. In addition to the nominate species, the benthic foraminiferal fauna also includes T. alsatica, G. subglobosa, T. gracilis, T. gracilis var. A and R. arnei. The Gyroidina soldanii girardana assemblage and the uppermost part of the Elphidium subnodosum assemblage contain a few specimens of R. bulimoides, but these may be reworked. The benthic foraminiferal assemblages are correlated with Zone NSB 8 of King (1989) and probably Zone NSR 8A of Gradstein & Bäckström (1996, North Sea). The diatom assemblage at m is correlated with Subzone NSP 9c of King (1989, North Sea). Pleistocene ( m, Nordland Group)

3 Benthic foraminifera of the Nonion labradoricum assemblage and planktonic foraminifera of the Neogloboquadrina pachyderma (sinistral) assemblage give a Pleistocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblage also contains Elphidium excavatum (common), Cassidulina reniforme, Virgulina loeblichi, Islandiella norcrossi, Bulimina marginata (common) and Islandiella islandica. A few specimens of T. alsatica are also recorded in some samples, and these are reworked from Oligocene to Early Miocene deposits. The planktonic foraminiferal fauna also includes a few specimens of N. pachyderma (dextral), Globigerina bulloides and Heterohelix sp. The latter is reworked from the Upper Cretaceous. The benthic foraminiferal fauna is correlated with Subzone 16x of King (1989, North Sea) and Zone NSR 13 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area). The planktonic foraminiferal fauna is correlated with the Neogloboquadrina pachyderma (sinistral) Zone of Spiegler & Jansen (1989). The encrusted form of N. pachyderma (sinistral) has its first frequent occurrence, in the Norwegian Sea, at 1.8 Ma (Spiegler & Jansen 1989). Sr isotope stratigraphy The part of the well where the biostratigraphical correlations indicated an Oligocene age, were analysed with 29 samples (27 depths) based on mollusc fragments and three samples based on calcareous foraminiferal tests. Most of the samples from m gave Early Oligocene ages, i.e. approximately Ma in the lower part of this interval and approximately 28.5 Ma in the upper part. Two samples from the immediately underlying Upper Paleocene-Lower Eocene unit gave similar ages, but these were probably based on caved tests. The samples from m gave ages from Ma (latest Early to earliest Late Oligocene). The samples from 760 m and above gave ages from 28.5 to 25.2 Ma (early Late Oligocene). The two uppermost samples gave considerably younger ages and are probably based on caved mollusc fragments (Table 1, Fig. 1). Well 36/1-2 Litho. Unit Sample Corrected 2S error Age (Ma Comments Analysed fossils (DC) 87/86 Sr 620 m undefined Caved One mollusc fragment 630 m Caved One mollusc fragment 640 m Caved One mollusc fragment 650 m One mollusc fragment 670 m One mollusc fragment m tests of E. subnodosum 680 m One mollusc fragment 690 m One mollusc fragment 700 m One mollusc fragment 710 m One mollusc fragment 720 m One mollusc fragment 730 m One mollusc fragment 740 m One mollusc fragment 750 m One mollusc fragment

4 760 m One mollusc fragment 800 m One mollusc fragment 800 m One mollusc fragment 810 m One mollusc fragment 820 m One mollusc fragment 830 m One mollusc fragment 840 m One mollusc fragment 840 m One mollusc fragment m tests of E. subnodosum, R.arnei, T. alsatica, A. guerichi guerichi 850 m One mollusc fragment 860 m One mollusc fragment 870 m One mollusc fragment 880 m tests of E. subnodosum, T. alsatica, A. guerichi guerichi 880 m One mollusc fragment 890 m One mollusc fragment 900 m One mollusc fragment 910 m One mollusc fragment 920 m One mollusc fragment 930 m Two mollusc fragments 940 m One mollusc fragment 950 m One mollusc fragment Table 1: Strontium isotope data from well 36/1-2. The samples were analysed at the University of Bergen. Sr ratios were corrected to NIST 987 = 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 Paleocene-Lower Eocene ( m, Balder/Sele Formation) The Upper Paleocene-Lower Eocene contains silty claystone (Fig. 1). Lower Oligocene and Lower-Upper Oligocene ( m, Hordaland Group) Coarse to medium sand dominates this part. Quartz dominates the sand fraction. The contents of glauconite and mica vary from minor to common. In the lower part of the unit, rounded, subrounded and subangular pebbles of mainly quartzite are common. This part is also quite rich in mollusc fragments (Fig. 1). Lower-Upper Oligocene and Upper Oligocene (lower part, m, Hordaland Group) Clay dominates this part, but sand (quartzose, glauconitic and biotitic) is also common in parts of section (Fig. 1). Upper Oligocene (upper main part, m, Hordaland Group)

5 The samples in this part are dominated by medium to coarse sand with minor silt and clay. The sand fraction is dominated by quartz with common glauconite and minor mica. Mollusc fragments are common in parts of section (Fig. 1). Pleistocene ( m, Nordland Group) The Pleistocene unit contains a clay-rich diamicton with sand, silt and common pebbles. The pebbles are mainly of crystalline rock (Fig. 1) and are interpreted to have been ice-rafted. References 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, Gradstein, F. & Bäckström, S., 1996: Cainozoic Biostratigraphy and Paleobathymetry, northern North Sea and Haltenbanken. Norsk Geologisk Tidsskrift 76, Howarth, R. J. & McArthur, J. M., 1997: Statistics for Strontium Isotope Stratigraphy: A Robust LOWESS Fit to Marine Sr-Isotope Curve for 0 to 206 Ma, with Look-up table for Derivation of Numeric Age. Journal of Geology 105, King, C., 1989: Cenozoic of the North Sea. In Jenkins, D. G. and Murray, J. W. (eds.), Stratigraphical Atlas of Fossils Foraminifera, Ellis Horwood Ltd., Chichester. Spiegler, D. & Jansen, E., 1989: Planktonic Foraminifer Biostratigraphy of Norwegian Sea Sediments: ODP Leg 104. In Eldholm, O., Thiede, J., Tayler, E., et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results 104: College Station, TX (Ocean Drilling Program),