The occurrence of dinoflagellate cysts in calcareous/siliceous microfossil preparations from the Eocene of southeast England

Abstract. INTRODUCTION

During routine analysis of some calcareous and siliceous microbiotas from the London Clay Formation of southeast England (Wilkinson, 2004a, b), some extremely well-preserved, dinoflagellate cysts were observed. These are all large, chorate (spine-bearing) forms, the overwhelming majority of which are referable to Cordosphaeridium gracile (Eisenack, 1954) Davey & Williams, 1966 (Fig. 1). This observation confirms that palynomorphs can be extracted effectively from clay-rich samples using a combination of clay deflocculation and sieving, as described recently by Riding & Kyffin-Hughes (2004).

PROCESSING

The London Clay Formation from 25 localities in southeast England was processed for calcareous and siliceous microfossils (diatoms, foraminifera, ostracods and radiolaria) (Wilkinson, 2004a, b). The standard British Geological Survey (BGS) procedure for the extraction of calcareous/siliceous microfossils from poorly- or non-indurated sediments was used. Samples were disaggregated by soaking in a mixture of warm water and detergent and agitated periodically. The majority of the clay fraction was removed using a 72 μm sieve. The residue was then boiled in sodium hexametaphosphate [(NaPO₃)₆] before being sieved again and dried. The sieving will have removed the majority of the palynomorphs, except the larger dinoflagellate cysts. Virtually all pollen and spores and most dinoflagellate cysts of this age are less than 72 μm in at least one dimension.

MICROFOSSIL ASSEMBLAGES

Specimens of Cordosphaeridium gracile were recovered from three clay quarries. These are Brambledown Pit, Kent [TQ 967 715], Bull’s Lodge Pit, Chelmsford, Essex [TL 7470 0850] and Heckfordbridge Pit, Colchester, Essex [TL 9550 2260], although only two are discussed here . . .