Dolomitization in chalk from the North Sea central graben

SEM studies of the Upper Cretaceous/Tertiary North Sea chalk have revealed minor amounts of secondary dolomite, occurring as cement and minor small-scale neomorphism of some enveloped grains. The dolomite content varies considerably, but generally amounts to 2 to 8 weight percent. The dolomite consists of limpid crystals, 10 to 30 micrometers in diameter, occurring as widely scattered single rhombohedra, minor aggregates, and interlocking rhombohedra fringing styloliths. The microtextural pattern observed demonstrates considerable envelopment and partial replacement of the biogenic constituents of the chalk by dolomite. Minor amounts of cryptograined clay minerals, pyrite, and silica are also found incorporated within the dolomite crystals. X-ray diffraction, electron probe microanalysis, and SEM examination indicate a slightly calcium-rich dolomite composition. Strontium concentrations average 1600 ppm. The dolomite crystals are slightly depleted in ¹⁸ O and enriched in ¹³ C in comparison to the host sediment. It is believed that the dolomitization resulted from relatively early diagenetic processes in a largely closed system.--Modified journal abstract.