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Grand Cayman, British West Indies Origin of Island Dolostone: Case Study of Cayman Formation (Miocene), Grand Cayman, British West Indies by Min Ren A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Earth and Atmospheric Sciences University of Alberta © Min Ren, 2017 ii ABSTRACT Grand Cayman is located on an isolated fault block that is part of the Cayman Ridge that defines the southern margin of the North American Plate. The exposed part of the Oligocene to Pleistocene carbonate succession that forms the island comprises the Bluff Group (Brac Formation, Cayman Formation, Pedro Castle Formation) and Ironshore Formation. The Cayman Formation (Miocene), which is up to ~140 m, is formed of dolostones and limestones. Analyses of numerous dolostone samples from numerous wells drilled on the eastern part of the island show that there are no obvious stratigraphic variations in the dolostones. In contrast, there are significant geographic variations in the petrography, dolomite stoichiometry, and stable isotope signatures of these dolostones. Thus, from a geographic perspective, the Cayman Formation can be divided into the concentrically arranged peripheral dolostone, transitional dolostone, interior dolostone, and interior (dolomitic) limestone zones. 87Sr/86Sr ratios from the dolostones indicate that they probably resulted from two major phases of dolomitization that occurred during the late Miocene to early Pliocene and late Pliocene to early Pleistocene. Dolomitization was mediated by seawater as it flowed from the coast inland. As it migrated inland the composition of the water progressively changed as it interacted with the host rock and mixed with meteoric water. These changes were responsible for the landward variations in the petrographic and geochemical signatures of the dolostones. This model for the origin of island dolostones stresses the geographic variability in the dolostones and dolomitizing fluids that are controlled by various intrinsic and extrinsic factors. Over the last 1 Ma, the Cayman Formation has experienced rapid and frequent iii changes in diagenetic environments because of the frequent and rapid glacio-eustatic changes in sea level. The diagenetic fabrics evident in the dolostones and limestones of the formation, however, do not record all of these diagenetic regimes. The Cayman Model for island dolomitization can be applied to many island dolostone succession found throughout world. The extent and distribution of the concentrically arranged zones vary from island to island because their development is controlled by many different intrinsic and extrinsic factors. The Cayman Model, highlights the complexity of the dolomitization processes, clearly illustrates that geographic and stratigraphic variations must be integrated into any proposed dolomitization model. This study suggests that dolomitization models should not be based on a single geographic location because the progressive lateral changes in the dolomitizing fluids and environmental conditions cannot be assessed. iv PREFACE This thesis is an original work by Min Ren under the supervision of Professor Brian Jones. The PhD thesis project started in September 2013. The research conducted for this thesis forms part of a research project, led by Professor B. Jones at the University of Alberta. The research project received funding support from the Natural Sciences and Engineering Research Council of Canada (grant No. ZA635) to Professor B. Jones. Chapters two and three of this thesis have been published as: Ren, M., Jones, B., 2017, “Spatial variations in the stoichiometry and geochemistry of Miocene dolomite from Grand Cayman: Implications for the origin of island dolostone”, Sedimentary Geology 348, 69-93. Ren, M., Jones, B., 2016, “Diagenesis in limestone-dolostone successions after 1 million years of rapid sea-level fluctuations: a case study from Grand Cayman, British West Indies”, Sedimentary Geology 342, 15–30. Chapter four of this thesis has been submitted as: Ren, M., Jones, B., “New insights into Cenozoic island dolostones: geometries, and spatial variations”, Sedimentary Geology. The initial theme of the thesis was outlined by Professor Brian Jones, and the concept of each chapter was developed through discussions between both of us. The thesis is based on samples that were collected by Dr. Jones and the database that has been assembled by Dr. Jones over the last thirty years and supplemented by data that I obtained during my research. I analyzed the compiled data and wrote the initial drafts of the manuscripts with input from both authors. Both authors discussed the results and edited the manuscripts. v To the memory of my Grandfather vi ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my supervisor Dr. Brian Jones for letting me fulfill my dream of being a student of his. I am thankful to him for introducing me to carbonate research. I am indebted to his encouragement, guidance, and patience, and the valuable things I learned from him. He pushed me further than I thought I could go. I am thankful to the members of my thesis committee, Drs. Murray Gingras, Nicholas Harris, Long Li, and Hairuo Qing (external) for their valuable guidance and encouragement. I would like to thank the Natural Sciences and Engineering Research Council of Canada for the funding provided to Dr. Brian Jones that helped the completion of this thesis. I express my thanks to the drilling crews from Industrial Services and Equipment Ltd., who drilled many of the wells, and numerous staff members from the Water Authority, Cayman Islands, who helped collect the samples used in this study. I wish to thank many people in the Department of Earth and Atmospheric Sciences at the University of Alberta: Diane Caird for running the XRD analyses, Martin Von Dollen and Mark Labbe for preparing the thin sections, Nathan Gerein for helping on the SEM, Dr. Robert Creaser for the strontium analyses, and David Chesterman and Lisa Budney for helping with my TAs. Thanks to the great carbonate research group – Josh, Rong, Ting, Megan, Erjun, and Simone for the delightful academic discussions and chitchats. Thank you to my fellow classmates Merilie and Yuhao for the help they offered in the classroom and in my research. Thank you to all my family members for their faith and love in me, especially my loving grandmother. Thank you to my dear friends – you have always been there for me and shaped up a better me. vii TABLE OF CONTENTS ABSTRACT .........................................................................................................................II PREFACE .......................................................................................................................... IV ACKNOWLEDGEMENTS ............................................................................................ VI LIST OF TABLES ...........................................................................................................XII LIST OF FIGURES ......................................................................................................XIII CHAPTER ONE INTRODUCTION ..............................................................................1 1. Introduction .....................................................................................................1 2. Geological setting ............................................................................................5 3. Previous study of dolostones from the Cayman Islands ................................9 3.1. The Brac Formation ...............................................................................12 3.2. The Cayman Formation .........................................................................12 3.3. The Pedro Castle Formation ..................................................................12 3.4. The Ironshore Formation .......................................................................13 4. Methods .........................................................................................................13 5. Thesis structure .............................................................................................16 References ..........................................................................................................18 CHAPTER TWO DOLOMITES OF THE CAYMAN FORMATION AND THE CAYMAN DOLOMITIZATION MODEL .................................................23 1. Introduction ...................................................................................................23 2. Geological setting ..........................................................................................24 3. Methods .........................................................................................................25 4. Results ...........................................................................................................30 4.1. Sedimentary facies .................................................................................30 4.2. Definition and distribution of the dolostone and imestonel ...................34 4.3. Distribution of calcite cements ..............................................................35 4.4. Dolomite petrography ............................................................................39 viii 4.5. Dolomite stoichiometry ........................................................................40 4.5.1. LCD-HCD – crystal scale ..........................................................40 4.5.2. LCD-HCD – local scale .............................................................42
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