Benthic faunal response to late Maastrichtian chalk-marl cyclicity at Rørdal, Denmark

Chalk-marl cyclicity is a common and characteristic feature of the Upper Cretaceous Chalk Group of NW Europe. The mainly small-sized benthic fauna from an upper Maastrichtian cyclic chalk-marl succession at Rørdal, Denmark, is analysed in order to interpret the composition and structure of the benthic fauna, and to test if faunal density, diversity, composition, and mode of life substrate were controlled by changes in lithology. The material comprises 21 bulk samples weighing 5 kg each, collected from all chalk and marl half-cycles. The samples were washed and sieved and yielded a total of 25,974 specimens. These are referred to at least 75 species and 15 bryozoan morphotypes and form the basis for the establishment of 10 guilds. The lithological cyclicity is not reflected by the distribution of individual species or by changes in average adult size, but there is generally a higher species diversity and density in chalk compared to marl. The chalk thus represents times of more favourable living conditions, higher environmental stability, and probably higher nutrient influx. No changes in guilds between marl and chalk are found. The variation in substrate character between the two lithologies was thus too small to have exerted any significant influence on the mode of life of the benthic fauna in spite of a difference in clay content, averaging 10%. Two different fossil associations are recognized, each covering one to two chalk-marl cycles and characterized by a distinct fauna with similar ecological requirements. If the chalk-marl couplets represent precession cycles within the Milankovitch band, then the change from one association to the other and back again may represent the 100,000 years eccentricity cycle based on a calculated sedimentation rate of 6 cm ka^{- 1}.