Mound bedding in a sponge-rich Coniacian chalk, Bornholm, Denmark

The Coniacian Arnager Limestone is exposed on the island of Bornholm in the Baltic Sea. The formation is composed of marly silica-rich chalk with a very sparse shelly macrofauna. Impressions of siliceous spon­ges occur in great abundance but flint is virtually absent. The chalk is intensely bioturbated and only the latest and deepest burrows have left recognisable trace fossils. Mapping of the irregular bedding reveals that deposition took place in a complex system of low mud-mounds. Distance between top of mounds and adjacent lows is Cr8 m, the amplitude is up to 1-2 m, and maximum dip of the flanks is 5-10". The mounds seem to be circular to oval in plan view. The association of mounds and high density siliceous sponge fau­nas suggests that the sponges and perhaps the spicule mats of dead decaying sponges were effective in baffling and trapping the fine-grained chalk ooze and clay which was swept by slow bottom currents. The currents seem to have intermittently reached sufficient strength to cause scouring of mound flanks, and mound formation was thus probably dependant of the delicate balance between production of carbonate ooze, current velocity and persistency, and of environmental conditions favourable to dense growth of sili­ceous sponges. The Arnager Limestone mounds are geometrically similar to Upper Maastrichtian and Da­nian bryozoan chalk mounds of Denmark and Upper Turonian and Santonian chalk mounds of Nor­mandy. If the mounds were caused, in part, by sponge baffling they represent a previously undescribed type of mound from the Cretaceous of north-west Europe.