Azooxanthellate corals in the Late Maastrichtian - Early Paleocene of the Danish basin: bryozoan and coral mounds in a boreal shelf setting

Abstract

The Late Cretaceous-Danian of the northwest European shelf represents one of the largest and longest-lived cool-water carbonate shelves in the stratigraphic record. The palaeolatitude of the Danish basin was 45°N during that time. The heterozoan faunas are dominated by bryozoans, echinoids, molluscs, brachiopods, serpulids, and, to varying degree, by azooxanthellate corals. During the Late Maastrichtian, rare soft-substrate-dwelling solitary scleractinians occur, including Parasmilia cylindrical, P. excavata, Caryophyllia sp. as well as octocorals, especially Moltkia minuta. Contemporaneous bryozoan mound complexes below the photic zone, which provided hard substrates for the settlement of larvae, were not colonized by azooxanthellate corals. Neither environmental nor faunal changes among the corals across the Cretaceous-Tertiary (K/T) boundary were significant. After the K/T boundary, the first solitary corals (moulds of Parasmilia biseriata, P. cincta, Trochocyathus hemisphaericus, Caryophyllia sp.) and octocorals appeared in the Cerithium Limestone, which lies above the Fish Clay. Similar to their Late Maastrichtian counterparts, these corals formed level-bottom communities. The Early Danian post-Cerithium Limestone represents the peak of bryozoan mound development. Corals are present but rare. The Middle Danian is characterized by reduced bryozoan mound growth and by the mound-forming dendroid scleractinians Dendrophyllia candelabrum, Oculina becki and Faksephyllia faxoensis, which flourished predominantely in the vicinity of the Ringkøbing-Fyn High. Nine species of solitary scleractinians, stylasterinid hydrocorals, and octocorals contributed to reef building. Important criteria for the interpretation of cold and deep-water coral bioherms are (1) absence of algae, (2) low-diverse azooxanthellate coral communities, (3) dominance of dendroid growth forms in the corals, (4) surrounding pelagic sediment adjacent to the coral mounds, (5) occurrence of pelagic organisms (globigerinid foraminifers, coccoliths) in the lime mud, (6) breakdown of coral colonies predominantly by bioerosion instead of mechanical destruction waves, (7) mound- or bank-like morphology of the buildups and (8) occurrence at a high palaeolatitude. Mound morphology and growth direction were traced by variations in the abundance of colonial corals. Gross morphology of scleractinian corals, stylasterinid hydrocorals and octocorals suggests an azooxanthellate character of the reefbuilders: the scleractinians developed dendroid growth forms, while stylasterinids and octocorals formed fan-like colonies oriented perpendicular to the nutrient-rich currents. Strong bioerosion was responsible for the breakdown of the skeletons, and the resulting bioclasts formed the substrate for larvae. Modern azooxanthellate Oculina coral reefs along the shelf edge off central eastern Florida, USA show similarities in position, morphology, environment, water depth and current orientation with the coral mounds of the Paleocene and suggest a palaeodepth for the counterparts from the Danish basin of 100-300 m.