Biostratigraphy and palaeoenvironmental analysis of a Lower to Middle Jurassic succession on Anholt, Denmark

Abstract. Palynomorph and foraminiferal assemblages have been studied from the Upper Pliensbachian to Bathonian of a borehole section on the island of Anholt in the Kattegat, situated near the eastern margin of the Norwegian-Danish Basin. Palynomorphs were recorded throughout the succession and have been used for both biostratigraphical and palaeoenvironmental assessments. Foraminifera were recorded from only the lower part of the succession, where they proved useful for interpreting palaeoenvironments. Four palynomorph and three foraminiferal zones have been established.

The interval encompasses a palaeoenvironmental transition from a marine, inner shelf setting to mainly terrestrial conditions. The Upper Pliensbachian-Toarcian boundary marks the beginning of a major regression, which continued through the Toarcian and Aalenian. Hence, it took place significantly earlier at Anholt than in the centre of the Norwegian-Danish Basin, where a lowering of sea level did not occur until the late Toarcian.

The Jurassic succession on Anholt spans the Fjerritslev and Haldager Sand formations; biostratigraphical data indicate that the Lower-Middle Jurassic boundary is here located within the uppermost part of the Fjerritslev Formation. The Fjerritslev and Haldager Sand formational transition was previously considered to coincide with the Lower-Middle Jurassic boundary.