Relation between currents and the growth of Palaeocene reef‐mounds

Abstract

Listen

Translate article

The Lower Palaeocene (Danian) reef-mounds of southern Scandinavia are asymmetrical with steep SE flanks and more gently sloping NW flanks. Fragments of mainly arborescent bryozoans make up between 23 and 45% by weight of the sediment. The asymmetrical development of the mounds is a result of differences in the accumulation rate of bryozoans which was controlled by the growth and density of the fauna. Density was relatively high on the SE flank and low on the NW flank. The morphology of individual bryozoan species also changes across the mound. Colonics living on the summit and the SE flank were more robust than colonics living on the NW flank and in the basins. The patterns of morphological and distributional changes are compared with the current regime of the mound reconstructed on the basis of model experiments in a flume. The experiments indicate that the current velocity was highest on the summit and lowest in the basins. Velocities on the flanks were intermediate but considerably higher on the upcurrent SE flank than on the downcurrent NW flank. The robustness of the colonics is positively correlated with the estimated current velocities, whereas the density of the fauna was highest in areas of relatively moderate velocities. The changes in density of the bryozoans, and consequently the asymmetrical growth of the mounds, were controlled by differences in food availability. □Bryozoans, density variation, flume experiments, morphological variation, reef-mounds, water currents.