Jurassic dinocysts from the Warboys Borehole, Cambridgeshire, England

The stratigraphic distribution of dinocysts in sediments of Toarcian to early Oxfordian age from the Warboys Borehole, Cambridgeshire are described. Several forms have relatively restricted ranges and appear to be of stratigraphic value. Selected forms are illustrated.


INTRODUCTION
The IGS Warboys Borehole, Broughton, Cambridgeshire (TL 2903 7839) was drilled in early 1965 ( Fig. 1) as part of an investigation into geophysical anomolies in the western Fens, and proved the following sequence: An excellent account of the sediments studied is given in Chapters 11 to 14 of Sylvester-Bradley & Ford ( 1968). The strata accumulated in the 'Oxfordshire shallows', (a swell region, close to the western edge of the London landmass), in a shallow water, mainly marine sedimentary regime.
The ammonite zonation used in this account is the work of Callomon (unpublished data, 1966) who proved a complete Callovian zonal sequence; those zones sampled for this study are shown in Fig. 2. The zonal sequence from the Toarcian to the Bathonian is incomplete and may be explained by both non-deposition and erosion.
This study is part of a project aiming at a refined zonation of the British Jurassic using dinocysts. The assemblage slides and figured material is housed in the MPA and MPK collections respectively, of the Institute o f Geological Science, Leeds.

PALY NO LO GICAL ANALYSIS
Twenty-six samples were prepared for palynological study ( Table 1). The residues were found to be dominated by miospores, plant cuticle and wood debris. This dominance of land plant derived material strongly suggests that the sediments accumulated in a relatively nearshore environment.
Rich, well-preserved dinocyst assemblages were encountered in all but two of the samples. Many of the taxa recognised have relatively long ranges although several with more restricted ranges appear to be stratigraphically useful.  Alberti, 1961 is abundant at the Toarcian/Aalenian boundary, but has a total range of Pleinsbachian to Bathonian (Thusu, 1978). Nannoceratopsis ambonis Drugg, 1978 is very characteristic of the Aalenian in Britain; it has never been encountered in the Lias.

Nannoceratopsis gracilis
Two samples taken from the Grantham Formation (at 79.248m. and 82.296m.) proved to be barren of marine palynomorphs (dinocysts, acritarchs and tasmanitids). The palynomorphs are entirely terrestrially derived, indicating that the sediment accumulated in a nonmarine environment. (76.200-70.104m.) -A number of taxa make their first appearance in the Upper Estuarine 'Series ' (76.200m.). Several of these taxa have been recorded from the late Bajocian, which is not represented in this section. These include, Valensiella ovula (Deflandre, 1947) (Thusu, 1978;Sarjeant, 1978   14 ~-

Explanation of Plate 1
All specimens are x 700. "England Finder" co-ordinates follow the slide number for each specimen. Hupsiduulax rnargarethue Sarjeant, 1975 was found in the Upper Estuarine 'Series'. This is the first record o f this form, other than the typc material from thc Bathonian of the Isle o f Skye.
MIDDLElLATE .) -This interval is characterised by a great diversity of dinocysts and the appearance of significant taxa. Ctenidodinium continuum Gocht, 1970 is not found above the late Callovian, which accords with the findings of Woollam (1980), but not with Thusu (1978), who records this form from the Oxfordian as well as the Callovian. Energlynia acollaris (Dodekova, 1974) Sarjeant, 1978 appears to die out in the athleta Zone, proniae Subzone. This agrees with the work o f both Thusu (1978) and Woollam (1980).
The presence of Reutlingia gochtii Drugg, 1978 in the athleta Zone, spinosum Subzone, (33.528-35.052m.) constitutes its first published record other than the type material (it was originally described from the athleta zone in Germany and appears to be an excellent marker).  Raynaud (1978), Thusu (1978) and Woollam (1980).