Dolomite and dedolomitization in Danian bryozoan limestone from Fakse, Denmark

Abstract

The bryozoan limestones in the middle Danian (Lower Paleocene) carbonate rock complex of Fakse Quarry, Denmark, includes small intraformational bodies of dolomite that occur in three different ways: 1) completely dolomitized bryozoan limestones; 2) partially dolomitized bryozoan limestones in which the larger skeletons have resisted total dolomitization, and 3) concretionary dolomite. Furthermore, scattered secondary rhombohedral porosity is observed in bryozoan limestones indicating former occurrences of dolomite. The dolomites are calcian with an elemental composition of approximately (Ca.55, Mg_45)Co3• The Sr content averages 500 ppm Sr, whereas the Mn and Fe contents are relatively low, c. 200 ppm Mn and 2000-4000 ppm total Fe respectively. The oxygen and carbon isotope values (-2.3%. to -3.9%. 1'>180; + 1.9%. to +4.6%. 1'>13C) are close to the field of early diagenetic dolomite replacing marine limestones. The carbon isotopes are enriched by approximately 2%. in comparison to the host sediment, which suggests that dolomitization most likely took place in a zone of methanogenesis. Crystal chemistry and geochemistry indicate a common genetic origin for these dolomites. The dolomite formation was probably an early diagenetic event in which the sequence from occurrences of individual dolomite rhombohedra, concretionary dolomite and partially dolomitized bryozoan limestones, to completely dolomitized bryo­zoan limestones is interpreted to represent progressive dolomitization. Dedolomitization is most pronounced in the dolomite concretions and to lesser extent in the dolom­itized bryozoan limestone beds. The isotopes of the replacive calcite suggests that dedolomitization took place under the influence of meteoric water. The progressive dedolomitization appears to be governed by the access of meteoric water along cleavage traces spreading to the entire dolomite crystals. Therefore, cleavage traces probably control dedolomitization under suitable physicochemical conditions.