Oligosphaeridium junctum sp. nov. A Hauterivian dinoflagellate cyst from the North Sea

The new species Oligosphaeridium junctum is described and illustrated from sidewall core samples in UKCS Well 15/29b-4Z. This species is considered to be an extremely useful stratigraphic marker in Hauterivian aged marine sediments of the North Sea.


INTRODUCTION
Well 15.29b-4Z was drilled by Conoco (UK) Limited in the South Viking Graben, UKCS North Sea, in 1989 (text Fig. 1). Rich and diverse dinoflagellate cyst assemblages were recovered from sidewall core samples shot through the Lower Cretaceous interval, including a new species, introduced here as Oligosphaeridiwn junctum. This form is restricted to Hauterivian deposits, and occurs in large numbers in Lower Hauterivian strata. The authors have also observed this species from numerous wells in the UK and Norwegian North Sea and it has previously been described informally, as Oligosphaeridium sp. 1 by Heilmann-Clausen (1987) from the Danish North Sea. As such, this is considered to be a very useful stratigraphic marker species.

Material
Standard palynological preparation techniques were used to produce strew mount slides of the horizons to be studied. All palynological slides examined are housed in the Palynological Collection of the Industrial Palynology Unit, Centre for Palynological Studies, Sheffield university, England.  fig. 3. Derivation of name. Latin junctus, a connection; with reference to the distally linked processes of this species.

SYSTEMATIC PALYNOLOGY
Diagnosis. Chorate gonyaulacoid cyst with a subspherical body and plate centred tubiform processes on the apical, precingular, postcingular and antapical paraplates. Endophragm and periphragm more or less smooth and closely appressed, except where the periphragm separates to form the processes. Processes expanded distally, with complex terminations, comprising elongated, flexuose spines which thin to thread-like trabeculae 0 km 150

Explanation of Plate 2
All specimens displaying both broken and unbroken distal trabecula.  Comparison. Of species published to date 0. junctum is most similar to the type species, 0. complex (White) Davey & Williams, 1966, which lacks distal trabecula and has generally less complicated process terminations. Oligosphaeridium trabeculosum Singh, 1983 has narrower processes and thicker trabeculae, which do not link pre-and post-cingular processes. Furthermore, the hypocyst processes of 0. trabeculosum are longer than those on the epicyst, whilst there is no such distinction in 0. junctum. The authors feel that this new species belongs to the genus Oligosphaeridium rather than Rigaudella Below, 1982, even though the processes are ditally linked by trabecula. Rigaudella exhibits both solid and hollow processes and has an undifferentiated autophragm. Occurrence. Oligosphaeridium junctum is stratigraphically restricted to Hauterivian-aged sediments in the 15/29b-4Z well. it has also been recorded from Hauterivian strata of the Danish central trough by Heilmann-Clausen (1987). The present authors have also observed 0. junctum in the Hauterivian of numerous North Sea wells (see discussion below).

DISCUSSION
Oligosphaeridium junctum has been observed by the present authors from numerous wells in the central and northern North Sea. It has also been reported in many unpublished palynological reports by various biostratigraphic contractors and oil companies (usually under the informal name of Oligosphaeridium 'amplexwn'). In all published and unpublished cases, this taxon is restricted to sediments of Hauterivian age and is a very widespread, often numerous and extremely useful stratigraphic marker species. however, to the authors' knowledge 0. junctum has not been reported from outcrop material and so age interpretations are based on associated assemblages and stratigraphic relationship to previously published ranges of other dinoflagellate cysts. No information is available as to the relationship of 0. junctum to other microfossil groups.
Based on the above criteria it is possible to recognize three distinct biostratigraphic events within the overall range of 0. junctum; range top: late Hauterivian, top acme event: intra-early Hauterivian, range base: intra early Hauterivian.