Photic zone palaeoenvironments of the Kimmeridge Clay Formation (Upper Jurassic, UK) suggested by calcareous nannoplankton palaeoecology Jackie A. Lees a, * , Paul R. Bown a , Jeremy R. Young b a Department of Earth Sciences, UCL, Gower Street, London, WC1E 6BT, UK b Palaeontology Department, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Accepted 12 September 2005 Abstract Nannoplankton abundance data has revealed significant and systematic population fluxes through all representative lithologies and cycle types of the Kimmeridge Clay Formation (KCF), and also through intervals in which lithology is visually homogeneous. The majority of KCF samples yielded nannofossil assemblages of low species richness (1 to 10) and marked unevenness, independent of preservational state. All samples were dominated by coccoliths of one family, the Watznaueriaceae. While assemblage unevenness is characteristic of nannofossil populations, such low species richness is atypical, particularly when compared with coeval assemblages of similar latitude. Such anomalously low nannofossil diversity may be explained as a response to nutrient-rich euphotic environments. We postulate that consistently high trophic conditions supported the eurytopic watznauer- iaceans but excluded most normal open-ocean taxa. The switching of dominance within this group most likely reflects different adaptations within an r-selected ecological strategy, related to nutrient concentration. Lowest diversities were recorded in mudstone/oil-shale lithologies where Watznaueria britannica is the dominant species, suggesting adaptation to the highest nutrient concentrations. Watznaueria barnesiae /fossacincta is particularly dominant in coccolith stone bands, where diversity is slightly higher, suggesting adaptation to lower high nutrient levels. Cyclagelosphaera margerelii appears to be the most extremely r-selected species, exploiting unusual (very high trophic/lowered sea-level?) conditions that excluded even W. britannica . D 2005 Elsevier B.V. All rights reserved. Keywords: Upper Jurassic; Kimmeridge Clay Formation; Watznaueriaceae; Palaeoenvironment; Palaeoecology; Calcareous nannofossils 1. Introduction The Kimmeridge Clay Formation (KCF) of Dorset, southern England is a 620-m-thick sequence represent- ing c. 4 myr (Kent and Gradstein, 1985) of essentially continuous Late Jurassic sedimentation. It predominant- ly comprises dark, organic-rich mudstones and oil- shales, and in the North Sea the KCF sensu lato forms the most important single hydrocarbon source- rock. The formation is interrupted by infrequent, but laterally persistent, pale limestone and dolostone beds, known as dstone bandsT. Some of these stone bands are finely laminated and are formed predominantly of coc- coliths and coccospheres, providing spectacular evi- dence of ancient coccolithophore blooms, which have attracted much interest (Noe ¨l, 1973; Gallois, 1976; Gallois and Medd, 1979; Young and Bown, 1991; Lees et al., 2004). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.09.026 * Corresponding author. E-mail address: j.lees@ucl.ac.uk (J.A. Lees). Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 110–134 www.elsevier.com/locate/palaeo