The biostratigraphy, palaeoecology and taxonomy of upper cretaceous (coniacian to lower maastrichtian) ostracoda from the chalk of East Anglia

Abstract

The thesis is based on 205 sediment samples supplied by the British Geological Survey from the Trunch Borehole and various outcrop localities in Norfolk and Suffolk. At least 80,000 ostracod specimens were recovered, belonging to 151 species and 53 genera. These are fully illustrated and described in a large systematic taxonomy chapter. Seventeen species and I subspecies were described as new. A stepped pattern of origination and extinction occurs throughout the Upper Chalk of East Anglia and the Ostracoda thereby display high biostratigraphical potential. The large number of Lazarus taxa in the Santonian and Lower Campanian is a distinctive feature. An analysis of the percentage of filter feeding platycopid Ostracoda, as a measure of palaeoxygen levels, reveals large variations in the amounts of dissolved oxygen in the Upper Cretaceous of East Anglia. High percentages of platycopids equate to low oxygen and vice versa. This is confirmed by the fact that high levels of platycopids are always associated with low species diversity. Using this technique, it was shown that the Coniacian was a time of low to very low oxygen, except for the upper part (coranguinum Zone) which was better ventilated. The Santonian and Lower Campanian were low to very low in dissolved oxygen, while the Upper Campanian and Lower Maastrichtian had much higher levels of dissolved oxygen. The term "Kenoxic Event" can be used to describe such periods of lowered oxygen, when instar-brooding platycopids display a preferential survival potential compared to podocopids. Notwithstanding these general trends, oxygen levels appear to have fluctuated rapidly throughout the interval of study. These fluctuations are thought to be due to the Oxygen Minimum Zone and its migrations onto the continental shelf and subsequent retreat to the continental slope; low oxygen levels on the continental shelf being extremely deleterious to all other Ostracoda other than platycopids resulting in low diversity. High diversity and high oxygen are associated with the retreat of the OMZ onto the continental shelf. This in turn is related to sea level fluctuations and the technique shows good potential for use in sequence stratigraphy. A model showing the position of the Oxygen Minimum Zone during the Upper Cretaceous in East Anglia was produced from platycopid data generated by this study.

Date of Award	06 Feb 2002
Original language	English
Awarding Institution	Aberystwyth University
Supervisor	Robin Whatley (Supervisor)