A symposium on Nordic Quaternary research in 1986

Different topics of Quaternary geology were dealt with at a symposium arranged in 1986 in Århus, Denmark: (1) reviews on recent Quaternary research in the Nordic countries, (2) studies of stratigraphy and geological processes in the Fennoscandian Border Zone, which demonstrate that the Quaternary deposition pattern is highly influenced by tectonic movements, and (3) Quaternary research as it can be applied to geotechnical investigations in the search for raw materials, in connection with pollution, and with conservation.     

Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub-basin and the North Sea Central Graben

The oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub-basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%_o to +3.00%_oδ¹³C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near-surface drilled sections fall in the range from —0.50%_o to — 2.00%_oδ¹⁸O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub-basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea. The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature-induced isotopic re-equilibration in the water-rock system. In these sequences the oxygen isotope values range from ? 1.50%_o to — 7.50%_oδ¹⁸O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure-dissolution, recrystallization, reprecipitation and ion-exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re-equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore-water controlled isotopic re-equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re-equilibration process increasingly becomes influenced by the rock-introduced isotopic change in the composition of the formation water.