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Biostratigraphic and Paleoenvironmental Interpretation of Late Triassic Sediments on the Exmouth Plateau, Northern Carnarvon Basin, NW Australia

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Biostratigraphic and Paleoenvironmental Interpretation of Late Triassic Sediments on the Exmouth Plateau, Northern Carnarvon Basin, NW Australia Biostratigraphic and paleoenvironmental interpretation of Late Triassic sediments on the Exmouth Plateau, Northern Carnarvon Basin, NW Australia Liam Gallagher (20182588) Supervisor: Dr. Daniel Peyrot Co-Supervisor: Dr. Neil Marshall This thesis is submitted to fulfil the requirements for Master of Science (Geology) by way of Thesis & Coursework Faculty of Science November 2019 Abstract The project addresses the palynological characterization of Upper Triassic sediments of the Mungaroo and Brigadier formations in the Northern Carnarvon Basin, offshore Western Australia. The strata of these formations host hydrocarbon reservoirs of great economic significance, providing the basis for several LNG projects such as Gorgon, Wheatstone, and Pluto. Late Triassic palynological assemblages are documented in numerous individual well studies across the Northern Carnarvon Basin, but detailed studies integrating palynological analyses in a regional context are scarce (Dolby & Balme, 1976; Bint & Helby, 1988; Backhouse & Balme, 2002; Backhouse et al., 2002; Marshall & Lang, 2013). The objectives of the study are to characterize the depositional settings present in the fluvio-deltaic Mungaroo and Brigadier formations; to reconstruct the paleovegetation changes through time; and to confirm the climatic hypotheses previously established for the considered interval. This research refines the existing biostratigraphy of the upper Minutosaccus crenulatus and Ashmoripollis reducta spore-pollen zones (Norian to Rhaetian age) on the Exmouth Plateau and documents the observed environmental and vegetation changes during this period. The project involved the analysis of organic-walled microfossils (termed palynomorphs) including, dinoflagellate cysts, acritarchs, freshwater algae and miospores from 51 core samples taken from the Chandon-2 and Geryon-2 wells drilled in the Northern Carnarvon Basin (NCB). The interval of investigation spans the upper M. crenulatus (C. stonei Subzone) and the A. reducta spore-pollen zones across 2 wells to contrast the palynological successions and depositional environments in contemporaneous sections. The analysis revealed highly diverse and well-preserved assemblages including 139 spore-pollen species, 38 dinoflagellate cyst species, 24 prasinophyte algae species and 14 acritarch species. The diverse microfloral assemblages recorded in the study were dominated by an abundance of the terrestrial pollen Falcisporites australis and Dictyophillidites harrisii spores, with marine influence interpreted to decrease with depth. Variation in the observed palynological assemblage content is considered a reflection of change in paleoenvironmental and climatic conditions, Nine biostratigraphic subzones were identified in the study with additional palynostratigraphic events, including marine microplankton acmes, that are correlatable between the studied wells (spanning ~100km). These events have the potential to be stratigraphically valuable, reflecting paleoecological changes that provide valuable insight into the architecture of depositional systems through time. The application of selected microfossil species as paleoecological and paleoenvironmental proxies was also examined using the Palynomorph EcoGroup (PEG) model to provide further insight into temporal floristic changes related to climate in the study area. Contents Abstract Acknowledgements ............................................................................................................................ 4 1. Introduction ................................................................................................................................ 5 1.1 Aims and Objectives .......................................................... Error! Bookmark not defined. 2. Geological setting and stratigraphy .......................................................................................... 8 2.1 The West Australian Super Basin and North West Shelf .................................................. 8 2.2 Tectonostratigraphic evolution of the Northern Carnarvon Basin ................................. 9 2.2.1 Pre-rift Phase .............................................................................................................. 10 2.2.2 Syn-rift Phase .............................................................................................................. 13 2.2.3 Post-rift Phase ............................................................................................................. 14 2.3 The Exmouth Plateau ........................................................................................................ 15 2.4 Late Triassic stratigraphy of the Northern Carnarvon Basin ......................................... 15 2.4.1 The Mungaroo Formation .......................................................................................... 15 2.4.2 The Brigadier Formation ........................................................................................... 16 2.5 Northern Carnarvon Basin Petroleum Systems .............................................................. 18 3. Palynology ................................................................................................................................ 21 3.1 Introduction ....................................................................................................................... 21 3.2 Previous palynological studies on the NWS .................................................................... 21 3.3 Depositional setting ........................................................................................................... 23 3.4 Sequence stratigraphy ....................................................................................................... 24 3.4.1 TR20 Play interval ...................................................................................................... 25 3.4.2 TR30 Play interval ...................................................................................................... 26 4. Materials and methods ............................................................................................................ 27 4.1 Materials ............................................................................................................................. 27 4.2 Methods .............................................................................................................................. 29 4.2.1 Palynological Sample Processing Procedure ........................................................... 29 4.2.2 Palynological analysis ................................................................................................ 30 4.2.3 Palaeoecology .............................................................................................................. 30 5. Results ....................................................................................................................................... 34 5.1 Palynology .......................................................................................................................... 34 5.1.1 Chandon-2 ................................................................................................................... 34 5.1.2 Geryon-2 ...................................................................................................................... 36 6. Discussion ................................................................................................................................. 39 6.1 Biostratigraphy .................................................................................................................. 39 6.1.1 Chandon-2 ................................................................................................................... 39 6.1.2 Geryon-2 ...................................................................................................................... 43 2 6.2 Paleoecology....................................................................................................................... 47 6.3 Sequence stratigraphy ....................................................................................................... 48 7. Conclusions ............................................................................................................................... 54 7.1 Further work ...................................................................................................................... 54 8. References ................................................................................................................................ 56 Appendix 1: Record of studied samples ......................................................................................... 64 Appendix 2: Record of identified palynomorphs in the study ..................................................... 66 Appendix 3: Plates ............................................................................................................................ 69 Appendix 4: Palynological distribution charts .............................................................................. 80 3 Acknowledgements I would like to thank my main supervisor Dr. Daniel Peyrot for sharing his knowledge, his patience and support throughout this project. Thank you to Prof. Annette George, for taking a chance and for her support. Thank you very much to Louise Heyworth for her guidance, supervision and assistance over the course of the project. Thank you to Neil Marshall for
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