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Taphonomy of the Fossils of the Herefordshire (Silurian) Konservat

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Taphonomy of the Fossils of the Herefordshire (Silurian) Konservat Taphonomy of the fossils of the Herefordshire (Silurian) Konservat-Lagerstätte Thesis submitted for the degree of Doctor of Philosophy At the University of Leicester By David Anthony Riley MGeol. (Leicester) FGS Department of Geology University of Leicester January 2012 ABSTRACT Preservation and taphonomy of the fossils of the Herefordshire (Silurian) Konservat-Lagerstätte David Anthony Riley The Herefordshire Lagerstätte represents a fully marine Silurian ecosystem, three dimensionally soft-bodied fossils, preserved by sparry calcite, which are recovered from carbonate concretions. Although, a taphonomic model exists it does not determine the pathways that led to their preservation, nor has it been tested on the other taxa recovered from the Lagerstätte. X-ray fluorescence (XRF) analysis indicates the sediment originated from an andesitic volcano from a destructive plate margin. Field relationships suggest that the bed was deposited from either a turbidity current or debris flow. Given the highly reactive nature of the volcanic ash, the cation exchanges that occur provide a mechanism for supersaturating the pore-fluid with carbonates. Examination of the four common taxa; Offacolus kingi (arthropod), Tanazios dokeron (arthropod), Kenostrychus clementsi (polychaete worm) and Acaenoplax hayae (mollusc), indicates a similar pathway of preservation. Energy-dispersive X-ray spectroscopy (EDX) identifies the only impurity within the fossil calcite is manganese; in comparison the concretion carbonates contain a variety of different cations. Therefore, the fossils and the concretions are the result of different processes, which were not coeval. Comparing the preservation Kenostrychus clementsi (polychaete) against experimental work indicates that preservation occurred within 6 days and rules out the occurrence of a void stage as previously thought. In addition, it also suggests that preservation was either instantaneous or there must have been an intermediate ―medusa‖ stage that preserved the tissue prior to templating. Electron microprobe analysis (EMPA) data indicate that the clay minerals that precipitated around the decaying animal were templating the organic matter. To precipitate the sparry calcite without the void stage, an hypothesis is proposed in which the sparry calcite nucleates and grows through the soft tissues. Isotopic data indicates that the concretions are not produced by organic matter decay. This is supported by the lack of correlation between the concretion and the fossil and the occurrence of barren concretions. Radial variation in the mineralogy, chemistry and isotopic ratios support the hypothesis that these concretions grew concentrically around a non-organic nucleus. These exceptional conditions may account for a preservation style that is so far unique to this single exposure. I ACKNOWLEDGMENTS ACKNOWLEDGMENTS I would like to thank my supervisors for their help and guidance, but particularly Sarah Gabbott and David Siveter, who persevered and helped me improve my writing style, but also to Mike Norry for the many discussions we had (geologically related or not), and to Derek Siveter for letting me use the Oxford University Museum of Natural History collections and providing me with the valuable samples I needed to complete this work. Thank you to Derek Briggs and Mark Sutton for their advice and comments, but also for seeing how fast I could catch concretions during fieldwork. Many thanks to the support and encouragement I had from my family: Mr Robert Riley, Mrs Maggie Riley, Mr Ian Phillips, and also to my late grandparents Mrs Catherine Phillips and Mr Anthony Phillips to whom this work is dedicated. Steve Grebby, Chris Willcox, Irfan Jan, Jawad Afzal, Dinah Smith, Mohibullah Kakar thank you for your help, advice and friendship, but also for electing me president when we made F32 an independent country. A big thank you to Sarah Lee and Alison Tasker who offered to read through my work and helped me improve my grammar. Thanks to Jan Zalasiewicz, Mark Williams, Andrea Snelling, Vince Williams and Carys Bennett. I would also like to acknowledge the Daniel Pidgeon Fund from The Geological Society of London for support with thin section preparation and the Timothy Jefferson Field Research Fund for supporting my field work at the exposure of the Lagerstätte. Rob Wilson, Nick Marsh, Rob Kelly, Colin Cunningham, Lyn Marvin, Kay Hawkins and all the administrative staff at the Geology Department at the University of Leicester, and the support staff and the Oxford University Museum of Natural History II ACKNOWLEDGMENTS are thanked for their help and support. I‘d also like to thank Prof. Warren Huff (Cincinnati) for extra data on the Welsh Borderland Bentonites, Phil Wilby (BGS) for advice and help with the CL imaging, Melanie Leng and Ian Millar (NIGL) for the isotope analysis. A further thank you to my colleagues and friends at Argos Fosse Park who made my part time job fun and interesting, and gave me the moral support and encouragement I sometimes needed, in particular I‘d like to thank Henry Bonner, Martin Perchard, Wendy Brown, Chris Payne, Shaun Tunaley, Sarah Nutt, Abby Fenn, Michelle Cook, Sam Clarkson, James Lockton, Andy Brewin, Mel Key and Chris Medhurst. The list is endless so we‘ll leave it there, but thank you to those I have not personally mentioned. III CONTENTS TABLE OF CONTENTS ABSTRACT ....................................................................................................................... I ACKNOWLEDGMENTS ............................................................................................... II TABLE OF CONTENTS ................................................................................................ IV LIST OF FIGURES ........................................................................................................ IX LIST OF TABLES ......................................................................................................... XII DEDICATION .............................................................................................................. XV INTRODUCTION ............................................................................................................ 1 AIMS AND OBJECTIVES OF THIS THESIS ............................................................ 2 MATERIAL .................................................................................................................. 4 CURRENT TAPHONOMIC MODEL FOR THE HEREFORDSHIRE LAGERSTÄTTE .......................................................................................................... 4 VOLCANISM AND THE FOSSIL RECORD ............................................................. 6 FOSSILS AND CONCRETIONS ................................................................................ 8 STRUCTURE OF THE THESIS .................................................................................. 9 NATURE, PROVENANCE, AND EMPLACEMENT OF THE HOST DEPOSIT OF THE HEREFORDSHIRE LAGERSTÄTTE (SILURIAN), UK .................................... 11 GEOLOGICAL SETTING ......................................................................................... 12 MATERIALS AND ANALYTICAL METHODS ..................................................... 16 COMPOSITION OF THE HOST BENTONITE ....................................................... 17 Petrology of the host bentonite ............................................................................... 17 XRF and geochemistry of the host bentonite .......................................................... 21 IV CONTENTS DISCUSSION ............................................................................................................. 25 Composition of the sedimentary deposit ................................................................. 25 Provenance and emplacement of the volcanic ash ................................................. 27 Significance of the host geochemistry to the taphonomy of the Herefordshire Lagerstätte .............................................................................................................. 34 THE TAPHONOMY OF THE SILURIAN HEREFORDSHIRE KONSERVAT LAGERSTÄTTE (SILURIAN): THE EVOLUTION OF A PRESERVATIONAL PATHWAY AS CONTROLLED BY EH-PH ............................................................... 36 GEOLOGICAL CONTEXT ....................................................................................... 38 MATERIALS AND METHODS ................................................................................ 40 RESULTS ................................................................................................................... 43 Clay Mineralisation ................................................................................................ 43 Calcite Mineralisation ............................................................................................ 45 Sparry Calcite Stable Isotope Analysis ................................................................... 48 Concretion Matrix ................................................................................................... 49 DISCUSSION ............................................................................................................. 50 Decay ...................................................................................................................... 50 Clay Mineralisation ................................................................................................ 51 Calcite Precipitation ..............................................................................................
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