Dinoflagellate cyst stratigraphy and depositional history of Miocene and Lower Pliocene formations in northern Belgium (southern North Sea Basin)

The occurrence of organic-walled dinoflagellate cysts (dinocysts) in the Di est Formation, a largely decalcified succession with a poor fossil content, and in the adjacent strata of Lower Miocene and Lower Pliocene formations, allowed a biostratigraphic evaluation of these deposits and an assessment of the hiatus between the lithostratigraphic units. The Diest Formation was deposited during Tortonian - Messinian times. Dinocyst biozones defined in the North Sea region and the U.S.A. East Coast are recognised within the D iest Formation, although environmental factors seem to have influenced the presence of some key zonal species in the shallow-marine deposits of northe rn Belgium. The two members of the Diest Formation studied, i.e., the Desse l Sands and the Diest Sands, appear to be strongly diachronous. The depocen tre was located in the Campine area during the early Tortonian and shifted to the area north of Antwerp during late Tortonian to Messinian times. The age assessment provides a correlation of the sequence boundaries of Haq et al. (1987) at the top of the Diest Formation with SB 5.5.