A Late Palaeocene–Early Eocene benthic foraminiferal record from Bovlstrup, Denmark, showing a remarkable agglutinated fauna

Abstract. The Bovlstrup well, Denmark, provides a detailed record of benthic foraminifera from the Upper Palaeocene and Lower Eocene deposits. The investigated interval spans four litho-units: an informal Grey Clay unit, the Holmehus Formation, the Ølst Formation and the Røsnæs Clay Formation (Danian?–Ypresian). Five interval zones based on benthic foraminifera have been established. Three of these zones (Zones 2, 3, and 4) contain exclusively agglutinated faunas. No foraminifera have previously been found in the Ølst Formation (Late Thanetian–Early Ypresian), but at Bovlstrup the formation contains a remarkable low-diversity agglutinated fauna (Zone 4). A programme of relatively dense sampling yielded information that may be lost in commercial oil well analysis. The five foraminiferal zones at Bovlstrup are correlated to established North Sea zonations, and the recognition of the faunas of Zones 3 and 4 leads to the conclusion that the zonation of King (In: Jenkins, D. G. & Murray, J. W. (Eds), Stratigraphical Atlas of Fossil Foraminifera, Ellis Horwood, 1989) can be refined.The benthic faunas indicate changes in the bottom environment both at the sea floor and within the overlying water mass. A transition from a calcareous fauna to an agglutinated fauna is interpreted as the result of a change from a neutral to a slightly acidic environment at the sea floor. There is a fluctuation in water depth through the studied section with a minimum water depth during the Thanetian and Early Ypresian. Volcanic ash layers in the Ølst Formation presumably resulted in low pH values, thereby causing the extreme low diversity of the benthic foraminiferal faunas.