Walsworth-Bell, Edward Benedict; (2001) Jurassic calcareous nannofossils and environmental cycles. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Calcareous nannofossils are microscopic remains of the calcite-secreting nannoplankton, a marine phytoplankton group which originated in the Late Triassic (Bown & Young 1998a). The ecology of the extant representatives of this group is reasonably well known, and the vastly abundant, geographically extensive and stratigraphically continuous fossil record of these organisms thus provides much in the way of palaeoceanographic, as well as biostratigraphic, information. This thesis considers the distribution of nannofossils through three British Jurassic hemipelagic mudrock formations: the Black Ven Marls (Sinemurian), the Belemnite Marls (Pliensbachian) and the Oxford Clay (Callovian-Oxfordian). All three formations exhibit sedimentary cyclicity; the Belemnite Marls and the Oxford Clay are thought to record orbital (Milankovitch) periodicities (Weedon & Jenkyns 1999; Hudson & Martill 1994; Coe pers. comm. 1998), whilst the Black Ven Marls contains putative sequence cycles (Hesselbo & Jenkyns 1998). However, the processes linking environmental change and sedimentation in Jurassic epicontinental marine settings remain enigmatic, largely due to a paucity of detailed investigations. In this study high-resolution, quantitative nannofossil assemblage data were collected and subjected to various statistical analyses. Absolute abundance and diversity fluctuations are compared to lithological evidence, and interpreted in terms of palaeoenvironmental processes. Responses to palaeoenvironmental fluctuations allow suggestions to be made concerning the little-known ecological preferences of the early nannoplankton. The Belemnite Marls dataset is of sufficient length for time-series analysis (Weedon 1993, in prep.); the presence of regular cycles is detected, and located within the Milankovitch spectrum using Weedon & Jenkyns' (1999) cyclostratigraphic time-scale for the formation. In addition, the unusually high resolution of the nannofossil datasets allows the biostratigraphy of all three formations to be reconsidered. This study reaffirms the potential for nannofloral investigation of the cyclostratigraphic record (Young et al. 1994). It is the first to apply time-series analysis to Jurassic nannofossil cycles.

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