Two dinoflagellate cyst marker horizons in the Bathonian of the Nettleton Bottom Borehole, Lincolnshire, England

The occurrences of large numbers (floods) of Meiourogonyaulax reticulata Dodekova, 1975 in the Blisworth Clay and of Pareodinia prolongata Sarjeant, 1959 in the Cornbrash of the Nettleton Bottom Borehole are reported and their biostratigraphical value discussed.


INTRODUCTION
The dinoflagellate cyst floras of the Nettleton Bottom Borehole ( Fig. 1) were investigated as part of a Department of Energy-sponsored project undertaken by the Institute of Geological Sciences on the detailed stratigraphical distribution of dinoflagellate cysts in the Jurassic System of Britain. A detailed account of the palynostratigraphy of the Nettleton Bottom Borehole is in preparation, though some palynological information on the Middle Jurassic section of the core (b,y Dr. J.P.G. Fenfon) is already available in a paper concerning the borehole by Bradshaw & Penney (1 982). A dinoflagellate cyst zlonal scheme for the English Jurassic, incorporating information from the borehole, has been produced (Woollam & Riding, 1983). This paper deals with the stratigraphically restricted floods of two dinoflagellate cyst species within the Bathonian of the Nettleton Bottom section. Meiourogonj'uulax reticulata  occurs abundantly within the Blisworth Clay (Bathonian) and Pureodinia prolongata Sarjeant, 1959 occurs in high proportions in the IBathonian portion of the Cornbrash (Fig. 2).  (Sarjeant, 1975) Stover & Evitt, 1978, Energlyniu acollaris ) Sarjeant, 1978 and Pareodiniu ceratophora Detlandre, 1947.

LOCATION AND DETAILS OF THE SElCTION
The pre-Cornbrash strata of the Middle Jurassic o f eastern England are largely marginal marine deposits, representing a regressive rCgime; sedimentological and macropalaeontological evidence suggests that the Blisworth Clay in north Lincolnshire was deposited under fluctuating nearshore, shallow-water lagoonal and freshwater conditions. It is possible that a sudden marine incursion o r transgression into the lagoon brought in the flood of M . reticulata. Alternatively, a rapid environmental change (such as reductions in salinity o r availability of nutrients) may have caused mass encystment of the dinoflagellates.
Hence, large numbers of M . reticulata with its characteristic reticulate autophragm may be a good marker for the base of the Blisworth Clay throughout the East Midlands region.

Riding
The Pareodinia prolongata flood Two samples at 321.98 and 320.97m were examined from the Cornbrash in the Nettleton Bottom Borehole (Fig. 2). The lowermost sample (no. MPA 10070) yielded a relatively restricted dinoflagellate cyst flora.
Pareodinia prolongata comprises some 35% of the dinoflagellate cyst assemblage in this sample.
P. prolongata is a very distinctive species, with an elongate apical horn and hyaline autophragm. It was described from the Upper Cornbrash (Macrocephalus Zone) of Yorkshire by Sarjeant in 1959, who stated (p. 336) "representatives of this species were quite abundant"; Riley & Fenton (1982, Fig . 8) also record large numbers of this species from the same ammonite zone. Its total range appears to be late Bathonian to the top of the Callovian.
The Cornbrash is a deposit that represents a major marine transgression, a pulse of which was probably responsible for the large number of P . prolongata in this formation. The work of  and Riley & Fenton (1982) suggests that this abundance is stratigraphically significant.

CONCLUSIONS
Horizons with large numbers (or floods) of Meiourogonyaufax reticulata at the base of' the Blisworth Clay and of Pareodinia prolongata in the Cornbrash form two distinct markers in the Bathonian of the Nettleton Bottom Borehole. Despite the fact that these abundances have been commented upon previously, their very restricted stratigraphical intervals and potential use as marker horizons have not. Bathonian palynofloras Explanation of Plate 1 Meiourogonyaulax reticulata  All light photomicrographs are x 800, the 'England Finder' co-ordinates follow the slide number.
Figs. 1, 2. Dorsal view, two focal levels showing the well-developed reticulate ornament of the autophragm, the paracingulum and the apical (type t x ) archaeopyle. The archaeopyle has operated, but the operculum is still adherent and is 'hinged' at the parasulcus. MPK 3847, MPA 10075/2, C58/3. Fig. 3. Slightly oblique dorsal view. The epicystal paratabulation is perceivable. The apical (type t x ) archaeopyle has operated, but the operculum has not detached. MPK 3848, MPA 10075/1, U3214. Figs. 4, 5. Dorsal view, two focal levels. Note the unequal distribution of the reticulation on the autophragm, which is well-developed on the hypocyst, but relatively sparse on the epicyst. The reticulate ornament largely obscures the hypocystal paratabulation. The archaeopyle has operated (note the schism separating the apical and precingular paraplates) however the operculum is 'hinged' at the parasulcus. MPK 3849, MPA 1007511, 548. Fig. 6. Dorsal view. Note the incised nature of the paracingulum, the poorly developed reticulation on the epicyst and the low, discontinuous parasutures. The archaeopyle has operated and the operculum is 'hinged' at the parasulcus. Apical paraplates 2' and 3' are partially obscured as the operculum has begun to fold downwards into the cyst.
MPK 3853, MPA 10075/2, S43l4. Explanation of Plate 2 Pareodinia prolongata  All light photomicrographs are x 470, the 'England Finder' co-ordinates follow the slide number.   Fig. 9. A slightly oblique right lateral view of a highly folded specimen. The archaeopyle has operated; at least two intercalary paraplates seem to have participated in archaeopyle formation (one of which is still adherent). MPK 3862, MPA 1007212, 05013.

Riding
are extremely monotonous and dinoflagellate cyst biostratigraphical events a r e few (Woollam & Riding, 1983), hence these floods may prove useful as marker horizons.
T h e exact areal extent of the floods is, at the moment unclear, the M . reticulata flood, for example, does not extend into southern England (based on investigations by the author and R. Woollam