The use of SR isotope stratigraphy to date the pleistocene sediments of the norwegian continental shelf – a review

Strontium (Sr) isotope data from cores and ditch-cutting samples from hydrocarbon wells from Pleistocene sediments from four areas on the Norwegian continental shelf have been compared with previously published biostratigraphic, lithostratigraphic, seismic and new micropaleontological and palynological data. The aim for this paper is also to provide a review of previous investigations of the use of Sr analysis on this margin, and to give a new assessment of some of the well samples by re-analysing these data and discuss the main limitations of the use of Sr isotopes to date Pleistocene sediments. It is important to obtain accurate dating of the Pleistocene sediments in order to constrain the age of the glacial events, indicate the age of neotectonic periods and perform basin modelling. However, obtaining a good chronostratigraphy of the thick Pleistocene successions has proved to be very difficult. Since the sediments have been deposited during a relatively short time, there are few recorded fossil events as first and last appearance datums. Sr isotope stratigraphy based on analyses of calcareous mollusc and microfossil tests (foraminifera and Bolboforma) have proven an effective dating method particularly for Oligocene and Miocene sections on the Norwegian continental shelf. Dating Pliocene and especially Pleistocene sediments by means of Sr isotope stratigraphy has proved more difficult. The Sr isotope curves for the Pliocene to Pleistocene, in general, have lower gradients than the Oligocene and the Miocene parts, and small errors in the⁸⁷Sr/⁸⁶Sr isotopic composition have a much larger impact when calculating ages from low-gradient parts of the Sr isotope seawater curves than from high-gradient parts. In addition, possible error sources comprise reworked fossil tests since a large portion of the marine Pleistocene deposits on the Norwegian continental shelf consists of redeposited older sediments originally deposited closer to the coast. These factors are probably the main reasons why the Sr isotope analyses of the investigated Pleistocene sections, with some exceptions, have not yielded very reliable ages.