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

INTRODUCTION Thick 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. OBSERVATIONS Three 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 . . .


INTRODUCTION
Thick sequences of clastic deposits of Late Cretaceous age (loo@ 1500m) are present offshore mid and north Norway. Dinoflagellate assemblages from these deposits have arctic affinities and are dominated by species of Chatangiella, Trithyrodiniwn and Isabelidinium. The First Appearance Datum (FAD) of Heterosphaeridiwn dificile 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. dificile, members of the Endoceratiurn dettmanniaeE. 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.

OBSERVATIONS
Three 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. di@cile. At some levels typical forms of H. dificile with discrete processes intergrade with morphotypes having shorter, wider, coalescing processes, probably related to species in the Cyclonephelium membraniphorum complex. As recognized here, H . dificile is restricted to forms similar to the holotype, with discrete processes. Species of Endoceratiurn are present consistently in the lower part of the H . dificile range zone. There is gradation between forms close to E. dettmanniae and E. ludbrookiae to reticulate forms similar to E. turneri.
Well 35/3-5 is located about 190km NNW of Bergen in the North Sea. H . dijicile occurs in sidewall cores down to 2896 m and is associated with E. dettmanniae and E. cf turneri in sidewall cores between 2733 and 2896 m. Well 7 1 19/7-1 is located 200 km N of Tromse in the Barents Sea. E. dettmanniae occurs from 2529.5m and co-occurs with H. dificile in sidewalls between 2529.5m and 2654.0m. The lower part of the H . dificile range zone in this well is also associated with Batioladinium jaegeri. Cauca parva occurs at 2654m. Well 6507/7-1 is in the mid-Norway exploration area about 275km NW of Trondheim. A thick Cenomanian-Turonian sequence is present. H. dificile occurs in sidewall and core samples down to 3506 m; E. dettmanniae and B. jaegeri occur in samples downhole from 3 13 I m and 3239 m, respectively. The planktonic foraminifera Hedbergella delrioensis is present from 3419 m and is common at 3464 m. Hedbergella planispira is present at 3479 m. These foraminifera are typically, but not exclusively associated with Cenomanian deposits.

DISCUSSION
Costa & Davey (1992) in a critical review of (mainly British) Cretaceous dinoflagellate data indicate mutually exclusive ranges for H. dificile and E. detrmanniae. The FAD of H . drficile is given as Early Turonian and the Last Occurrence Datum (LOD) of E. dettmanniae within the Late Cenomanian. The widespread occurrence of the H. dificile-E. dettmanniae assemblage in these and other wells on the Norwegian shelf is taken to imply that the assemblage is in situ and the records documented above are not therefore consistent with the accepted ranges of these species. A consideration of the actual data on the occurrence of H. dificile in English and French Turonian deposits, as documented by Tocher & Jarvis (1987), (1995, Jarvis er al. (1988), Foucher (1981 and others shows the distribution to be erratic and it is not a consistent marker for the Turonian of these areas. In Norwegian arctic deposits H. dipcile is common and regular throughout its range. These differences are thought to be due, at least in part, to dinoflagellate provincialism though other factors may also be involved. Outside of Western Europe, Chlonova (1996), commenting on provincialism in Late Cretaceous assemblages from Western Siberia, notes that they differ markedly from Western Europe and that there are variations in species abundance and apparent range from area to area, though no details are given. In Western Greenland (Nehr-Hansen, 1996, & pers. comm., 1996 the oldest relevant deposits investigated are of Coniacian or possibly Late Turonian age and the FAD of H. drficile has not been located. Norvick & Burger (1 976) documented assemblages from two Cenomanian sections from Bathurst Island, Northern Territory, Australia. These sections include H. drficile, Endoceratiwn spp., B. jaegeri and Litosphaeridium siphoniphorwn. The chronostratigraphy is well documented. Kennedy (1970) indicated a mid-Cenomanian age for ammonites from an adjacent cliff section and Burger (pers. comm. 1996) notes that an ammonite from the same fauna was recovered from the sample containing the lowermost record of H. dificile. Planktonic foraminifera indicate an Early to mid-Cenomanian age for the lower part of the cored sections (Norvick & Burger, 1976, p.5).

CONCLUSIONS
A concurrent range zone defined by the FAD of H. dificile and the LOD of E. dettmanniae is widely recognizable in wells from the Norwegian shelf. Evidence from the Australian stratotype suggests that this zone is of Early to mid-Cenomanian age and that the FAD of H. dificile in a continuous section should probably be considered as close to the Early to mid-Cenomanian boundary. This may be of some significance to the regional model as potential hydrocarbon reservoir horizons are present around the H. dificile FAD level.