The calcareous concretions (cementsten) in the Fur Formation (Paleogene, Denmark): isotopic evidence of early diagenetic growth

Abstract

The Paleogene Fur Formation in northwestem Denmark is a c. 60 m thick clayey diatomite that comprises calcite concretions, the so-called cementsten. The diatomite is interbedded with 179 isochronous and recognizable layers of volcanic ash, which demonstrate that the concretions occur at six stratigraphic levels throughout the Fur Formation. The concretions formed through precipitation of calcite in the pore space within the diatomite or the ash layers. Carbon and oxygen isotope compositions were determined on the concretionary calcite. 613C values around -20%0 to -16%0 indicate that most (bi)carbonate was of bacterial ongin, formed through metabolization of organic matter by sulphate reduction. 6180 values of -1%0 to -3%0 indicate that calcite precipitated at temperatures corresponding to those measured in contemporaneous marine shell materiai. It is speculated that rare blooms of phytoplankton made the six stratigraphic levels the preferred sites of nucleation of the concretions.