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Foraminiferal isoleucine epimerization determinations from the Nar Valley Clay, Norfolk, UK: implications for Quaternary correlations in the southern North Sea basin
- J. D. SCOURSE, W. E. N. AUSTIN, H. P. SEJRUP, M. H. ANSARI
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- Journal:
- Geological Magazine / Volume 136 / Issue 5 / September 1999
- Published online by Cambridge University Press:
- 01 September 1999, pp. 543-560
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Fully temperate freshwater, brackish and marine sediments overlying Anglian till and glacilacustrine sediments in the Nar Valley area of northwest Norfolk, UK, have been attributed to the Middle Pleistocene Hoxnian temperate stage on palynological grounds, and basal peats associated with this sequence have been recently correlated with oxygen isotope stage 9 on the basis of a series of 230Th/238U dates (mean 317±14 ka). At Tottenhill these sediments (Nar Valley Freshwater Beds, Nar Valley Clay) underlie a deltaic complex attributed to the Wolstonian ice margin. The lithostratigraphical relations between the major formations in the Nar Valley, and the pollen stratigraphy of the fully temperate sequence, are very similar to the Pleistocene sequence in the Inner Silver Pit area of the southern North Sea, and correlation has been proposed between the successions described from these two localities. However, the Inner Silver Pit sequence has yielded aminostratigraphic data consistent with isotopic stage 11.
Benthic foraminiferal assemblages and foraminiferal amino-acid determinations have been investigated from the Nar Valley Clay in order to test further the palaeoenvironmental setting of the sequence and to help resolve the age of the sequence and correlation with the Inner Silver Pit interglacial. The foraminiferal assemblages support previous sedimentological and palaeontological evidence for a transgressive tendency within this sequence. Multiple isoleucine epimerization determinations on Ammonia beccarii and Aubignyna perlucida from five levels within the Nar Valley Clay give mean aIle/Ile ratios of 0.135 and 0.111, respectively. The A. beccarii ratios are much lower than mean aIle/Ile ratios on equivalent species from the interglacial sequence in the Inner Silver Pit (upper Sand Hole Formation), which are close to 0.2, and the two datasets fail to overlap at the 1σ level. The new aminostratigraphic ratios indicate correlation of the Nar Valley Clay with oxygen isotope stage 9, and therefore support the pre-existing 230Th/238U data.
These results suggest that two temperate stages of Hoxnian palynological affinity are present in the Quaternary record of East Anglia and the southern North Sea basin, a conclusion consistent with independent new U-series data from other Hoxnian sites in East Anglia. An alternative model in which the amino-acid ratios are explained as a function of different post-depositional thermal histories, related to length of cover by ice and water, is discussed but considered unlikely. The conclusions have important implications for the timing and number of glacial events in and around the southern North Sea basin, and help to resolve discrepancies in relative sea-level histories and biogeography in temperate sequences hitherto accommodated within a single stage.
Geochronology and palaeoenvironment of marine Quaternary deposits in Denmark: new evidence from northern Jutland
- H. P. SEJRUP, K. L. KNUDSEN
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- Journal:
- Geological Magazine / Volume 136 / Issue 5 / September 1999
- Published online by Cambridge University Press:
- 01 September 1999, pp. 561-578
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- Article
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The degree of isoleucine epimerization in the benthic foraminiferal species Elphidium excavatum and Bulimina marginata have been measured in four boreholes, penetrating marine interglacial beds, from northern Jutland. The results of these analyses are compiled with results obtained from other sites in Denmark and the North Sea region, and four aminozones (AZs) have been erected. AZ1 (aIle/Ile < 0.05) include the Late Weichselian and the Holocene part of the record. AZ2 (0.08–0.12) includes samples of last interglacial age. AZ3 (0.14–0.16) includes samples from sites that have been previously correlated with the Holsteinian, and marine isotope stage 7 age for this zone is suggested. An age close to 400±100 ka is assigned to AZ4 (0.21–0.26). AZ4 is recorded in corings at Skagen and Nørre Lyngby, and includes the here defined Skagerrak Interglacial, which is tentatively correlated with marine isotope stage 11.
The Skagerrak Interglacial sediments are characterized by boreal lusitanian benthic foraminiferal faunas evidencing strong input of Atlantic water to the North Sea. These faunas are replaced by assemblages indicating cooler conditions at least twice during this period. The sedimentation pattern in northern Jutland during this interglacial seems to be different from the sedimentation regimes in the same area at the classical Eemian sites and during the Holocene, which were characterized by large sediment input from the Jutland Current. This difference may be attributed to a change in the geometry of the sedimentary basin through the later part of the Quaternary, which is a result of repeated periods of intense erosion in the confluence area of the Norwegian Channel ice stream. A strong influx of Atlantic water to the North Sea during stage 11 has been suggested earlier, based on records from the Devil's Hole area. However, the correlation of this event to the classic biostratigraphic based schemes of northwest Europe is still problematic.