Windley, Dawn Elizabeth; (1995) Calcareous nannofossil applications in the study of cyclic sediments of the Cenomanian. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The main aim of this study was to examine changes in nannofloral composition through the chalk/marl sequences of the Cenomanian (mid-Cretaceous) of south-east England and southern France. Nannofloras are sensitive to physical and chemical changes in surface water masses, as seen in the present day oceans, and nannofloral distribution in the Cenomanian is believed to have been affected by such variations. Nine closely sampled rhythmic sections have been exammed from the Anglo- Paris Basin, southern England and the Vocontian Trough, southern France. The quantitative analysis of nannofossil assemblages has revealed consistent and significant cyclic variations in the relative abundance of a number of key nannofossil species. The clearest abundance signal is seen in Watznaueria harnesae, Biscutum constans, Zeugrhabdotus noeliae and Zeugrhabdotus erectus, all species which have been previously recognized as markers of high or low productivity. The variation in abundance of these species appears to be in phase, or closely related to major lithological changes. Noisy, anomalous signals are however, common, but can be explained by bioturbation effects, natural variation and/or sampling effects. Biscutum constans. Zeugrhabdotus noeliae and Zeugrhabdotus erectus show abundance maxima m the chalk lithologies, indicating increased surface water productivity. Watznaueria harnesae, although abundant through all of the sections, shows abundance maxima within the marl horizons, and is thought to reflect lower surface water fertility. A relative value for productivity has been calculated using a productivity index devised from the ratio of high productivity indicators to Watznaueria barnesae. Nannofossil variation has been interpreted in terms of productivity cycles, coincident with chalk/marl lithologies. High nannoplankton productivities may be interpreted as a result of increased surface water temperatures and/or nutrient availability. Nutrient variation is the most likely cause, with temperature fluctuations being a secondary influence. Such conditions are most likely a response to vigorous oceanic circulation and increased depth of mixing. Both high surface water temperatures and increased nutrient availability can be explained by climatic change related to Milankovitch cycles, directly controlled by orbital patterns. Orbital perturbations influence the Earth's insolation and seasonality triggering intense climatic change. Fluctuations in climate cause variation in ocean water masses, surface water temperature and nutrient supply and hence, variations in phytoplankton productivity. The nannofossil evidence is very similar from the Anglo-Paris Basin and the Vocontian Trough, despite the great difference in palaeolatitude, palaeogeographic setting and sedimentation rate. Surface water conditions were, therefore, essentially the same in both areas, although great differences m background terrigene input produces the sedimentological differences. The longevity of Milankovitch chalk/marl cyclicity are not proven here, but, nevertheless, the evidence of Gale (1989b) is sound and the combination of sedimentological, geochemical, macropalaeontological and micropalaeontological data provides a self-supporting hypothesis for Cenomanian chalk/marl deposition. The analysis of nannoplankton assemblages proves a basic tool in the investigation of cyclicity.

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