Middle to Upper Miocene in borehole 6403/5-GB1
Borehole 6403/5-GB1 (64º40’47.6’’N, 03º39’12.8’’E, see Map 1) is a cored geotechnical boring retrieved from the lower to middle part of the erosional domain of the Storegga Slide area. Approximately 30 m of pelagic sediments of the Kai Formation were recorded. The base and top of the Middle to Upper Miocene were not seen. The core was investigated with seven samples. The core depth is in metres below sea floor (mbsf, Fig. 1).
Well summary figure for well 6403/5-GB1
Middle Miocene (110.2-96 m, Kai Formation)
Bolboforma of the Bolboforma badenensis – Bolboforma reticulata assemblage date this interval to Mid Miocene (Fig. 1). In addition to the nominate species, the Bolboforma assemblage also contains B. compressispinosa in the upper part. The planktonic foraminiferal fauna includes N. atlantica (dextral), N. acostaensis, N. atlantica (sinistral) and Neogloboquadrina mayeri, and the benthic foraminiferal fauna includes M. communis, U. pygmea langeri, Uvigerina pygmea langenfeldensis, Ehrenbergina variabilis and Globocassidulina subglobosa. Spiegler & Müller (1992) and Müller & Spiegler (1993) described B. badenensis and B. reticulate from deposits with an age slightly older than 14 to 11.7 Ma from the North Atlantic and the Norwegian Sea.
Middle to Upper Miocene (98-96 m, Kai Formation)
Bolboforma of the Bolboforma compressibadenensis and Bolboforma pseudohystrix assemblage date this interval to the latest Mid Miocene - earliest Late Miocene (Fig. 1). In addition to the nominate species, the Bolboforma assemblage also contains B. clodiusi and Bolboforma voeringensis. The planktonic foraminiferal fauna includes N. atlantica (dextral), N. atlantica (sinistral) and G. bulloides and the benthic foraminiferal fauna includes M. communis, U. pygmea langeri, E. variabilis and G. subglobosa.
According to Qvale & Spiegler (1989), B. compressibadenensis, B. pseudohystrix and B. voeringensis were common before the LADs of B. badenensis and B. reticulata and sparse after these events. B. clodiusi occurred regularly both just before and just after the LADs of these species. According to Spiegler & Müller (1992) and Müller & Spiegler (1993), the LADs of B. badenensis and B. reticulata are approximately 11.7 Ma, indicating that the 98-96 m interval is mainly of latest Mid Miocene age.
Upper Miocene (96-90.9 m, Kai Formation)
Bolboforma of the Bolboforma clodiusi assemblage and benthic foraminifera of the uppermost part of the Martinottiella communis assemblage date this interval to earliest Late Miocene (Fig. 1). Bolboforma, planktonic and benthic foraminifera are very sparse in this interval and the microfossil assemblage is totally dominated by radiolaria and pyritised diatoms. No planktonic foraminifera occur, and B. clodiusi is the sole Bolboforma species recorded.
B. clodiusi occurred from latest Mid Miocene to earliest Late Miocene. Since no Bolboforma species with a range limited to the Middle Miocene are recorded in this interval, the Bolboforma clodiusi assemblage (96-91.6 m) is most likely of Late Miocene age. The uppermost part of the Martinottiella communis assemblage (91.6-90.9 m) is also most likely of Late Miocene age since M. communis is known from the Middle Miocene to Lower Pliocene of the Netherlands (Doppert 1980) and from the Miocene on the Norwegian continental shelf (Skarbø & Verdenius 1986).
Sr isotope stratigraphy
In this borehole we have performed Sr isotope analyses on calcareous benthic foraminiferal tests from three depths (five samples). The obtained 87Sr/86Sr ratios gave ages slightly older and slightly younger than the Mid/Late Miocene boundary (11.2 Ma according to Berggren et al. 1995) and consequently broadly support the biostratigraphical correlation. Four of the samples gave earliest Late Miocene ages in a part where the biostratigraphical correlation indicates a latest Mid Miocene age (Table 1, Fig. 1). However, this discrepancy is within the precision of the method.
|Litho. Unit||Sample (cores)||Corrected 87/86Sr||2S error||Age (Ma)||Analysed fossil species|
|Kai Fm||96 m||0.708872||0.000005||10.62||26 tests of N. atlantica (sinistral), N. atlantica (dextral), G. bulloides|
|Kai Fm||96 m||0.708869||0.000004||10.44||20 tests of Ehrenbergina variabilis, Cibicides dutemplei, Lagena spp., Fissurina spp., P. bulloides, Pullenia subcarinata, U. pygmea langeri|
|Kai Fm||98 m||0.708888||0.000005||10.04||37 tests of N. atlantica (sinistral), N. atlantica (dextral), G. bulloides|
|Kai Fm||105.2 m||0.708844||0.000004||11.57||22 tests of P. bulloides, G. subglobosa, Dentalina spp., Lagena spp., Fissurina spp., S. bulloides|
|Kai Fm||105.2 m||0.708868||0.000004||10.76||35 tests of N. atlantica (sinistral), N. atlantica (dextral), N. mayeri|
Table 1: Strontium isotope data from borehole 6403/5-GB1. 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 contain mainly pelagic sediments (biogenic ooze) with some mineralogical sand and silt (Fig. 1). No ice-rafted, coarse, minerogenic grains were discovered in borehole 6403/5- GB1 as in the Middle to Upper Miocene section in 6704/12-GB1.
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.
Doppert, J. W. C., 1980: Lithostratigraphy and biostratigraphy of marine Neogene deposits in the Netherlands. Mededelingen Rijks Geologische Dienst 32-16, 2, 3-79.
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, 441-456.
Müller, C. & Spiegler, D., 1993: Revision of the late/middle Miocene boundary on the Voering Plateau (ODP Leg 104). Newsletter on Stratigraphy, 28 (2/3), 171-178.
Quale, G. & Spiegler, D., 1989: The stratigraphic significance of Bolboforma (Algae, Chrypsophyta) in Leg 104 samples from the Vøring Plateau. 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), 487-495.
Skarbø, O. & Verdenius, J. G., 1986: Catalogue of microfossils, Quaternary - Tertiary. IKU Publication 113, 19 pp, 187 pl.
Spiegler, D. & Müller, C., 1992: Correlation of Bolboforma zonation and nannoplankton stratigraphy in the Neogene of the North Atlantic: DSDP sites 12-116, 49-408, 81-555 and 94-608. Marine Micropaleontology 20, 45-58.