The First Appearance Datum (FAD) of Heterosphaeridium difficile (Manum & Cookson), dinoflagellata, in clastic deposits offshore Norway

INTRODUCTIONThick sequences of clastic deposits of Late Cretaceous age (1000–1500 m) are present offshore mid and north Norway. Dinoflagellate assemblages from these deposits have arctic affinities and are dominated by species of Chatangiella, Trithyrodinium and Isabelidinium. The First Appearance Datum (FAD) of Heterosphaeridium difficile is an important correlation point within this sequence and is equated with a Turonian age in most current zonal schemes. Our observations on the relative ranges of H. difficile, members of the Endoceratium dettmanniae–E. ludbrookiae plexus and other key index species in well materials offshore mid and north Norway are not consistent with ranges documented from European stratotypes and suggest a Cenomanian age for the correlation point. This note is intended as a basis for discussion pending a more detailed account of these deposits.OBSERVATIONSThree released offshore wells with good control from sidewall and core samples were selected for study. All of the well sections produced rich typical middle Cretaceous dinocyst assemblages with common H. difficile. At some levels typical forms of H. difficile with discrete processes intergrade with morphotypes having shorter, wider, coalescing processes, probably related to species in the Cyclonephelium membraniphorum complex. As recognized here, H. difficile is restricted to forms similar to the holotype, with discrete processes. Species of Endoceratium are present consistently in the lower part of the H. difficile range zone. There is gradation between forms close to E. dettmanniae and E. ludbrookiae to reticulate forms similar to E. lurneri.Well 35/3–5 is located about 190 km . . .