Palynological investigations of Cenomanian chalks and marls from England

Abstract

The palynological analysis of chalk and marl samples from Cenomanian sections sampled at high resolution has yielded largely diverse and abundant, well-preserved microplankton assemblages. Four sections have been investigated, three from the Dover-Folkestone Warren area, south-east England and the fourth from South Ferriby, South Humberside, north-east England. The dinoflagellate cysts present accord well with previously described Cenomanian microplankton assemblages from western Europe, and are assignable to the established Litosphaeridium siphonophorum Zone of Davey (1970). Full descriptions of all dinoflagellate cyst species found have been included. A new species, Litosphaeridium sp. A is believed to be indicative of latest Cenomanian to earliest Turonian in England. A new status is suggested for Pterodinium cingulatum polygonalis and Palaeohystrichophora pauciestosa is believed to be a junior synonym of P. infusorioides. Dinoflagellate cysts assemblages have been investigated with the purpose of reconstructing the palaeoenvironment of the Cenomanian Chalk Sea. To this aim, samples were prepared qualitatively and quantitatively and data produced from normal logging techniques were investigated using several statistical techniques. Finally, such palynological data were integrated in a multidiscipline geological framework in order to interpret the data further. The application of quantitative preparation techniques has shown that the introduction of an oxidation stage causes mechanical loss of a significant proportion of the palynological residue. A further outcome of such quantitative techniques is the recognition that marl samples consistently yield more abundant palynological residues than do marly chalk and chalk lithologies, based on counts of dinoflagellate cysts per gramme of sediment. Statistical techniques such as diversity indices, cluster analysis and fourier analysis (spectral analysis) were applied to palynological data sets. As a result it is suggested that the percentage presence of certain dinoflagellate cysts species is strongly dependent on lithology and thus allows recognition of the lithological rhythmicity. Other key dinoflagellate cysts species appear to be controlled by palaeotemperature. Palaeohystrichophora infusorioides is therefore recognised as a slightly cooler water cyst (23°C), while Cleistosphaeridium huguoniotii and Cyclonephelium distinctum both are interpreted as being slightly warmer water cysts (25°C). Cluster analysis has successfully identified the existence of two dinoflagellate cyst associations in the Middle Cenomanian, which are linked to the Mid-Cenomanian non-sequence of Carter and Hart (1977) as they occur immediately above and below the non-sequence. Statistical interpretation of the Late Cenomanian dinoflagellate cyst assemblage of South Ferriby confirms that it records stressed palaeoenvironmental conditions.