MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



19 



Subsequently, examination of asssemblages has been extended upwards into the 

 Barremian. In the chapters which follow, the results are given of studies by R. J. 

 Davey and the writer on the dinoflagellate cysts from five assemblages in the West 

 Heslerton No. 1 Borehole, as follows : 



Depth 



Character of Sediment 



Stage 



19 



25/50 metres 



Soft, medium dark grey (N4) to 



Upper Barremian 







live grey (SY4/1) clay, containing 



(Middle part) 







much pyrite. 





39 



00/25 metres 



Softish, medium (N5) to olive 



Middle Barremian 







grey (5Y5/1) clay, pyritic, with 



(Cement Beds) 







some shell fragments. 





42 



■50/75 metres 



Lithology as last. 



Lower Barremian (top) 



99 



•25/50 metres 



Hard, medium olive grey (5Y5/1) 

 siltstone, laminated and pyritic. 



Middle Hauterivian 



103 



•25/50 metres 



Well-laminated olive g re y 

 (5Y6/1) pyritic clay, rather 

 streaky, with shell fragments. 



Middle Hauterivian 



(The numbers in parentheses are the American Rock Colour Chart numbers.) 

 For further stratigraphical information, see forthcoming paper by Neale & Kaye. 



b. The Lower Chalk 



By R. J. DAVEY 



Fossil microplankton from the Upper Chalk (Senonian) of Great Britain were first 

 described in the mid-nineteenth century by a group of amateur microscopists, 

 namely Mantell, Reade, Deane, White, Bowerbank and Wilkinson. After these 

 initial studies no subsequent work was performed in Great Britain on Upper Creta- 

 ceous microplankton until 1964, when Cookson & Hughes published a paper on 

 microplankton from the Cambridge Greensand, of presumed basal Cenomanian age. 

 The work referred to in the following chapters, on microplankton assemblages from 

 the English Lower Chalk (Cenomanian) , is part of a larger study of the Cenomanian 

 assemblages throughout the world. 



Specimens quoted in this paper as being of Cenomanian age have all been obtained 

 from samples from H.M. Geological Survey Borehole at Fetcham Mill, Leatherhead, 

 Surrey (National Grid Reference, TQ. 158565). At this locality the Cenomanian is 

 197 feet thick and lies conformably on the Upper Greensand. The basal Ceno- 

 manian is a grey, glauconitic impure chalk containing a relatively high percentage of 

 clay minerals. The percentage of clay minerals progressively decreases towards the 

 top of the stage where the Cenomanian is a hard, almost pure, white chalk. A full 

 account of the stratigraphy appears in the Geological Survey Bulletin No. 23. 



