Characterising Pre-Vegetation Paralic Sedimentary Systems and Developing Improved Architectural Reservoir Models

Home , Antietam Formation, Palaeochannel, Retrogradation

Characterising pre-vegetation paralic sedimentary systems and developing improved architectural reservoir models

A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering

2019

Ginny-Marie Bradley

University of Manchester Faculty of Science and Engineering School of Earth and Environmental Sciences Basin Studies and Petroleum Geoscience/Engineering Funded by NERC CDT in Oil and Gas

Table of Contents List of figures ...... 5 List of tables ...... 8 List of abbreviations ...... 9 Abstract ...... 10 Declaration ...... 12 Copyright statement ...... 13 Acknowledgements...... 14 Status of papers ...... 15 Chapter 1: Introduction ...... 17 1.1 Aims of the thesis ...... 18 1.2 Rationale...... 18 1.3 The Tumblagooda Sandstone...... 21 1.4 The advancement to pre-vegetation paralic studies ...... 21 1.5 Key aims of the project ...... 22 1.6 Thesis structure ...... 22 1.7 Architectural Definition ...... 24 Chapter 2: Previous Work ...... 26 2.0 Introduction to braided fluvial systems...... 27 2.1 Pre-vegetation fluvial systems ...... 29 2.2 Pre-vegetation tidal systems ...... 33 2.3 Impact of vegetation ...... 34 2.4 Fluvio-paralic stratigraphy...... 37 2.5 The Tumblagooda Sandstone...... 40 2.5.1 Geological history ...... 40 2.5.2 Tumblagooda Sandstone ...... 41 2.5.3 Previous interpretations ...... 45 2.5.4 Fauna ...... 50 Chapter 3: Dataset and methodology...... 53 3.0 Introduction...... 54 3.1 Fieldwork ...... 54 3.1.1 Logging ...... 54 3.1.2 Sampling ...... 56 3.1.3 Photogrammetry...... 59 3.2 Data processing and analysis ...... 62 3.2.1 Facies Analysis ...... 62 3.2.2 Petrography ...... 62 3.2.3 Digital outcrop modelling ...... 65 3.2.4 Annotating models and obtaining channel size statistics ...... 66 3.2.5 Stochastic modelling ...... 67 3.3 Limitations ...... 69 Chapter 4: The applicability of modern tidal analogues to pre-land plant paralic depositional models...... 72 Abstract ...... 74 4.1 Introduction...... 75 4.2 Geological setting ...... 77

4.3 Previous work ...... 79 4.3.1 A mixed marine to non-marine interpretation ...... 79 4.3.2 Non-marine interpretation ...... 80 4.4 Methodology ...... 81 4.5 Lithofacies descriptions ...... 82 4.6 Facies associations...... 98 4.6.1 Facies Association 1 ...... 98 4.6.2 Facies Association 2 ...... 99 4.7 Depositional system ...... 101 4.7.1 Fluvial braidplain ...... 101 4.7.2 Tidal dominance ...... 102 4.7.3 Intertidal sandflat...... 103 4.7.4 Intertidal sand-bar to shallow subtidal ...... 105 4.7.5 Stacking patterns ...... 105 4.8 Discussion ...... 106 4.8.1 Interpretation of the depositional system...... 106 4.8.2 Significance of Heimdallia assemblages ...... 107 4.8.3 Early Palaeozoic depositional controls in a pre-plant world ...... 108 4.8.4 Application of modern analogues...... 116 4.9 Conclusions ...... 117 Chapter 5: Reassessing the sedimentology and architecture of pre-vegetation fluvial successions using large digital outcrop models...... 120 Abstract ...... 122 5.1 Introduction ...... 123 5.2 Background ...... 124 5.3 Geological Setting ...... 128 5.4 Methods...... 129 5.5 Results ...... 132 5.5.1 Lithofacies ...... 132 5.5.2 Facies associations (FA)...... 146 5.5.3 Architectural elements ...... 151 5.5.4 Sequence stratigraphy ...... 161 5.6 Depositional model ...... 168 5.7 Discussion ...... 171 5.7.1 Pre-vegetation models...... 171 5.7.2 The lack of mudstone...... 173 5.7.3 Bank stability ...... 174 5.7.4 Comparison of pre-vegetation and post-vegetation braided systems ...... 175 5.7.5 Re-evaluation of “sheet-braided” and revised models ...... 177 5.7.6 Applicability to subsurface reservoirs...... 177 5.8 Conclusions ...... 178 Chapter 6: Re-evaluating fluvial architecture of pre-vegetation reservoirs using large digital outcrop datasets: application to subsurface modelling ...... 180 Abstract ...... 182 6.1 Introduction ...... 183 6.2 Background ...... 185 6.3 Methods...... 187

6.4 Results ...... 188 6.4.1 Facies Associations ...... 189 6.4.2 Geobody geometries ...... 192 6.4.3 Petrography ...... 195 6.4.4 Modelling ...... 200 6.4.5 Stochastic modelling ...... 203 6.5 Discussion ...... 209 6.5.1 Comparing depositional systems...... 209 6.5.2 The revised model ...... 211 6.5.3 Implications...... 212 6.6 Conclusion ...... 213 Chapter 7: Synthesis and implications ...... 215 7.1 Pre-Vegetation tidal sedimentology ...... 217 7.1.1 Characteristics of pre-land plant tidal sandstones ...... 218 7.2 Fluvial architecture...... 219 7.3 Depositional model ...... 221 7.3.1 Facies associations ...... 222 7.3.2 Sequence stratigraphy ...... 224 7.4 Comparison with modern analogues ...... 225 7.5 The lack of mudstone ...... 229 7.6 Implications ...... 230 7.6.1 Implications for pre-vegetation successions ...... 230 7.6.2 Implications for hydrocarbon reservoir prediction ...... 231 Chapter 8: Conclusions and further work ...... 233 8.1 Concluding remarks...... 234 8.2 Uncertainties and recommendations ...... 235 References...... 237

Additional material supplied on a USB drive at the end of the thesis: • Appendix one: Summary of previously described pre-vegetation tidal successions • Appendix two: Paper for publication: Using UAVs to create geological digital outcrop models of steep sided cliffs • Appendix three: Digitised sedimentary logs • Appendix four: Petrography o Grain size and composition o Point counting summary o QEMSCAN analysis • Appendix five: Digital outcrop models o VRGS files – Facies and Connectivity models o Videos from VRGS – Facies and Connectivity models o Petrel files – Facies and Connectivity models o Photos of petrel models – Facies models

Total word count (Including references): 69,946

List of figures

Chapter 1: Introduction Page Figure 1.1 Graphic display of fluvial terms used in this thesis 25 Chapter 2: Literature Review Figure 2.1 Previously documented river channel morphology 28 Figure 2.2 Architectural facies models for braided river systems 28 Figure 2.3 Heterolithic tidal bedding 33 Figure 2.4 Flume model of non-vegetated vs vegetated overbanks 37 Figure 2.5 Paralic sequence stratigraphy 38 Figure 2.6 Development of a fan shaped fluvial body 39 Figure 2.7 Structure of the Southern Carnarvon Basin 41 Figure 2.8 Palaeoreconstruction of the Ordovician 42 Figure 2.9 Location map for the Kalbarri study area 43 Figure 2.10 Stratigraphic column for the Palaeozoic fill of the Southern 44 Carnarvon Basin Figure 2.11 Hocking’s (1991) environmental interpretation 48 Figure 2.12 Trewin (1993b) environmental interpretation 49 Figure 2.13 Evans et al. (2006) environmental interpretation 50 Chapter 3: Methods Figure 3.1 Auslog GeoGAMMA V2, outcrop gamma scintillometer 55 Figure 3.2 DJI Phantom 4 UAV Drone 61 Figure 3.3 Powers (1953) scale for grain shape and sphericity 63 Figure 3.4 Grain boundary contacts 63 Figure 3.5 Separating grains in iExplorer 64 Figure 3.6 Producing the digital outcrop model in Agisoft 66 Figure 3.7 Interpretations in VRGS 70 Chapter 4: The applicability of modern tidal analogues to pre-land plant paralic depositional models. Figure 4.1 Location map for the study area 78 Figure 4.2 Hocking (1991) and Trewin (1993b) interpretation for Facies 81 Association 2 Figure 4.3 Example log from Z-Bend 83

Figure 4.4 Heimdallia burrow trace and Tumblagoodichnus hunting 84 burrow ichnofacies Figure 4.5 Sedimentary structures found within ripple laminated facies 92 Figure 4.6 Diplichnites trackways 94 Figure 4.7 Palaeocurrent rose diagrams 94 Figure 4.8 Internal structures within cross-bedded sandstone 95 Figure 4.9 Sections of digital outcrop models showing FA1 and FA2 102 relationships Figure 4.10 Palaeoenvironmental conceptual model 104 Figure 4.11 Graph showing the number of tidal features identified in the 109 published literature Figure 4.12 The Bay of Fundy intertidal zone at low tide 117 Chapter 5: Reassessing the sedimentology and architecture of pre=vegetation fluvial successions using large digital outcrop models. Figure 5.1 Location map 128 Figure 5.2 Photograph of DJI Phantom 4 drone 131 Figure 5.3 Summary of geobody aspect ratios using palaeocurrent of 300° 131 and 340° to correct width Figure 5.4 Photopanel summarising geobody geometries 133 Figure 5.5 Summary sedimentary log showing facies 134 Figure 5.6 Thin section micrographs of fluvial facies 136 Figure 5.7 Outcrop images of trough cross-bedded sandstone facies 139 Figure 5.8 Palaeocurrent rose diagrams 140 Figure 5.9 VRGS panels of amalgamated scour-fill geobodies 140 Figure 5.10 Outcrop images from the coastal outcrops of trough cross- 141 bedded sandstone with bioturbation Figure 5.11 Outcrop images of other facies observed 145 Figure 5.12 VRGS images of Facies Association 1A 148 Figure 5.13 Outcrop images of Facies Association 1B 149 Figure 5.14 Outcrop images of Facies Association 1C 150 Figure 5.15 Graph of geobody statistics 153 Figure 5.16 Sketch and VRGS images of amalgamated geobodies 156

Figure 5.17 Sketch and VRGS images of isolated geobodies 159 Figure 5.18 Sketch and VRGS images of accretion geobodies 162 Figure 5.19 Summary correlation of outcrop gamma and logs interpreted 167 with sequence stratigraphy and facies associations Figure 5.20 Paleoenvironmental conceptual model 171 Figure 5.21 Graph comparing geobody statistics and literature studies 176 Chapter 6: Application of digital outcrop modelling of pre-vegetation fluvio-paralic systems to subsurface modelling Figure 6.1 Location map 186 Figure 6.2 Log correlation 191 Figure 6.3 Examples of geobodies in VRGS 192 Figure 6.4 Graph comparing geobody statistics and literature studies 194 Figure 6.5 Thin section micrographs 197 Figure 6.6 Conceptual model for sheet and channelised fluvial systems 201 Figure 6.7 Depositional environment block diagram 202 Figure 6.8 Example of the stochastic models 206 Figure 6.9 Stochastic models showing facies distributions 208 Chapter 7: Synthesis Figure 7.1: Graph of isolated vs amalgamated geobody aspect ratios 222

List of tables

Chapter 1: Introduction Page Table 1.1 Summary of fields producing from pre-vegetation paralic 20 sandstones Table 1.2 Summary of fluvial terms used in the thesis 25 Chapter 2: Literature review Table 2.1 Hocking’s (1991) facies associations 47 Chapter 3: Methods Table 3.1 Log locations 56 Table 3.2 Sample locations 57 Chapter 4: The applicability of modern tidal analogues to pre-land plant paralic depositional models. Table 4.1 Lithofacies descriptions 85 Table 4.2 Summary of literature on pre-vegetation tidally recorded 111 successions Chapter 5: Reassessing the sedimentology and architecture of pre=vegetation fluvial successions using large digital outcrop models. Table 5.1 Summary of geobody statistics 152 Chapter 6: Application of digital outcrop modelling of pre-vegetation fluvio-paralic systems to subsurface modelling Table 6.1 Summary of geobody statistics 193 Table 6.2 Results of point counting 199 Table 6.3 Facies proportions from stochastic models 207 Chapter 7: Synthesis Table 7.1 Summary of key tidal characteristics 218 Table 7.2 Comparison of pre- and post-vegetation characteristics 227

List of abbreviations

Bbbl Billion barrels of oil bbl Barrels of oil BS Bioturbated Sandstone CON Conglomeratic sandstone CS Contorted Sandstone FA Facies Association LAXB Low-Angle Cross-Bedded sandstone Lidar Light Detection and Ranging MMBOE Million Barrels of Oil Equivalent MS Massive Sandstone PL Planar Laminated sandstone PXB Planar Cross-Bedded sandstone QEMSCAN Quantitative Evaluation of Minerals by SCANning electron microscopy Q QEMSCAN RL Ripple Laminated sandstone SEM Scanning Electron Microscopy SFM Structure From Motion sPL Planar Laminated Siltstone tcf Trillion Cubic Feet TS Thin section TXB Trough Cross-Bedded sandstone TXL Trough Cross-Laminated sandstone UAV Unmanned Aerial Vehicle VRGS Virtual Reality Geological Studio

Abstract

In modern siliciclastic environments, terrestrial vegetation binds substrate, controls weathering and erosion rates, influences run-off, sediment supply, composition and has an important influence on depositional architecture. This study assesses pre- vegetation paralic systems by integrating traditional outcrop sedimentology with Unmanned Aerial Vehicle (UAV) technology. Results show that these ancient sedimentary systems have unique characteristics and highlight the need for tailored models for both fluvial and tidal environments. This thesis examines the Tumblagooda Sandstone of Western Australia to assess the sedimentary characteristics and geobody architectures of pre-vegetation paralic systems. Many statistical datasets exist for fluvial geobody dimensions, mainly derived from modern rivers or Mesozoic outcrops, where deposition was influenced by terrestrial vegetation. Previous authors have noted that pre-vegetation successions have a unique preserved sedimentology and suggested that the depositional architecture is dominated by sheet-braided systems. This comprises planar basal erosion surfaces and aspect ratios of greater than 20:1. Recent work has suggested that this may be an over simplification of the sedimentology and architectures. Detailed outcrop analysis and acquisition of high-resolution 3D digital outcrop models have been used to map the large-scale geometry of the preserved fluvial geobodies. Fluvial facies are dominated by two types of architectural elements: isolated and amalgamated sheets and lenticular shaped geobodies. The overall depositional environment has been identified as a channel-braided system which was dominated by repeated avulsion, reworking and cannibalisation of previously deposited sandstones. These amalgamated units are interpreted to have formed during periods of reduced accommodation space. The overlying isolated fluvial units are encased in tidal sandstones and are interpreted to record periods of transgression. The system does not only display sheet-braided architectures. Geobody have a wide range of architectures extending above and below the previously defined 20:1 aspect ratio and this environment preserves both sheet and wide ribbon geometries.

Geobody statistics extracted from the digital reconstructions have allowed the generation of improved conceptual models for pre-vegetation successions. Stochastic modelling compares the sheet model vs the channelised model to determine which better represents the fluvial geometries observed in the outcrop model. These systems are best modelled using object-based methods, as the sheet-like pixel-based model grossly overestimates fluvial facies and does not accurately reflect the geometries and distributions observed in the outcrop analogues. This study also analyses the interbedded marine units in the Tumblagooda Sandstone, interpreting them as tidal successions. Ancient paralic pre-vegetation mature sandstones typically lack mudstone preservation which is an important criterion for identifying tidal facies. Notably they lack structures such as mud-draping and flaser bedding, which may be related to limited delivery and preservation of muds to the shallow marine realm prior to the evolution of deeply rooted plants on land. There is a paucity of pre-Devonian tidal successions reported in literature, and this thesis summarises that this is possibly due to the lack of characteristic tidal indicators. In such cases, application of modern analogues to interpret the environment may lead to erroneous environment designation. This study identifies key criteria required for identification of tidal facies in the absence of preserved mudstone features prior to the evolution of deeply rooted plants. The use of the correct analogues to interpret depositional environment and during reservoir modelling has a profound effect on prediction of subsurface facies distribution, connectivity between wells, fluid flow rates and production strategy. Using the sheet model implies that facies are laterally continuous for 100’s km and inter-well connectivity would be likely. This would result in increased fluvial reservoir volume and reserves predictions. Sweep would be more efficient as homogeneous facies are less tortuous than their channelised counterparts. The channelised model implies that fluids would have to flow through complex bounding surfaces at all scales, from cross-bedding foresets to channel margins. Facies would also vary laterally and vertically within channels and fluid flow would be enhanced along higher permeability pathways along channel axis. Production rates may be lower, and the strategic placement of wells would be required to target connected isolated elements and gain effective sweep.

Declaration

No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning

i. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii. The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and tables (“Reproductions”), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=24420), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.library.manchester.ac.uk/about/regulations/) and in The University’s policy on Presentation of Theses.

Acknowledgements

The work contained in this thesis contains work conducted during a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil and Gas [grant number NEM00578X/1] and is fully funded by NERC whose support is gratefully acknowledged. Firstly, I would like to thank the University of Manchester for accepting my application for the PhD and to my supervisors Prof. Jonathan Redfern, Dr. David Hodgetts and Prof. Annette George (UWA) for the starting the project and allowing myself to explore new ideas and for the opportunity, their support and advice throughout this PhD. I am also grateful to Lorraine Wilson (UWA) for help with field logistics, Marine N’Gom and Jake Wallace for field assistance and to Dr. Roger Hocking and Dr. Arthur Mory (GSWA), for their help in the field and support throughout the project. Also, thanks to Prof. Grant Wach (Dalhousie) for assistance on the project and co-authoring a paper and Prof. Martin Gibling and Darragh O'Connor (Dalhousie) for a tour of Nova Scotia. To John Underhill, Anna Clark and Lorna Morrow who have put everything into setting up and organising the NERC CDT in Oil and Gas training scheme. I would like to thank the admin staff at the University of Manchester for logistical help while in the field. I would like to thank Peter Heath for his comments on drafts and emotional support throughout the PhD. Finally, to my parents who have supported me throughout my life and PhD, encouraged me to get the most out of my experiences and without them I could not have done it. To all the wonderful people I have met on the NERC CDT training scheme and my fellow PhD students at Manchester. I have been lucky enough to be able to take my project to Western Australia, Morocco, Canada and the French Alps. I have had the opportunity to attend several international conferences and network with experienced people along the way and without everybody involved this could not have happened and I am truly grateful to everyone.

Status of papers

This thesis is presented as papers intended for publication. This section outlines the papers, current status with regards to publication and clarifies the input of co-authors.

Chapter 4 – Research Paper 1 Title: The applicability of modern tidal analogues to pre-vegetation paralic depositional models. Authors and contributions: Ginny-Marie Bradley – main author, data collection, all data analysis and results Jonathan Redfern – second author, assisted with conclusions David Hodgetts – manuscript alterations Annette D. George – manuscript alterations Grant D. Wach – guided in the Bay of Fundy study Published within Sedimentology, online January 2018 doi: 10.1111/sed.12461

Chapter 5 – Research Paper 2 Title: Fluvial architecture of pre-vegetation systems: revisiting the modes by using large digital outcrop datasets. Authors and contributions: Ginny-Marie Bradley – main author, data collection, all data analysis and results David Hodgetts – data collection and software developer Jonathan Redfern – second author, manuscript edits Submitted for publication in Sedimentology.

Chapter 6 – Research Paper 3 Title: Re-evaluating fluvial architecture of pre-vegetation reservoirs using large digital outcrop datasets: application to subsurface modelling Authors and contributions: Ginny-Marie Bradley – main author, data collection, all data analysis and results Jonathan Redfern – second author, aided with sequence stratigraphic model

David Hodgetts – data collection and software developer, assisted with the method on building models Intended for submission within Marine and Petroleum Geology.

Additional paper from the study This paper does not form a part of the research chapters as it is a review on the methodology of use of UAVs in geoscience. Found in Appendix 2 Title: Using UAVs to create geological digital outcrop models of steep sided cliffs Authors and contributions: David Taylor – main author Ginny-Marie Bradley – main author David Hodgetts – data collection and VRGS software development Intended for submission in Journal of Photogrammetry and Remote Sensing.

Chapter 1: Introduction

This chapter outlines the aims and rationale of the thesis, with the background motivation presented with the key questions which this thesis addresses. An overview of the thesis structure is also presented. 1.1 Aims of the thesis This thesis aims to address the characteristics, sedimentology, geometry and the depositional conditions for the Tumblagooda Sandstone, Western Australia and uses this formation as a case study to assess the applicability of current facies models based on modern environments which are influenced by rooted vascular vegetation. The thesis also assesses the implications of this and the need to apply this to subsurface oil and gas reservoir models to improve predictability and reservoir production. The overall aim is to reassess the current knowledge on pre-vegetation paralic sedimentary systems and apply a large-scale dataset to quantify fluvial geobodies in outcrop to produce a guideline for reservoir modelling these very different sedimentary systems. 1.2 Rationale Fluvial reservoirs are important globally for hydrocarbon reserves and differences in depositional character create complex architectures, facies variability and internal heterogeneity (Corbett et al. 1998). Pre-vegetation paralic systems have been rarely studied quantitatively and are not well understood compared to post-vegetation fluvial systems which are comparable to modern day rivers. Pre-vegetation fluvial systems are typically studied in a qualitative sense with little quantification (e.g. Fedo & Cooper 1990; Hocking 1991; Eriksson et al. 1998; Amireh et al. 2001). Many published cases propose that the lack of vegetation had a profound effect on depositional architecture and facies characteristics which means that they are very different to that what we see in the modern day, however many studies only qualitatively suggest that they are dominantly sheet-like. Early studies in the late 1960s and 1970s noted there was a distinct difference in fluvial style from pre-vegetation successions to those we observe in the middle to late Palaeozoic, Mesozoic and the Cenozoic. Although this is only postulated from qualitative studies which suggested that processes would have occurred on different scales compared to present day therefore the use of modern

analogues would be problematic. More recent studies have reignited the research area and proposed that they may not be so different after all based on field observations (Santos et al. 2014; Hartley et al. 2015; Ielpi et al. 2017; Bradley et al. 2018). These studies have been purely qualitative. Tal & Paola (2007) discussed the effect that alfalfa had on the evolution of a fluvial system in a flume tank, from braided to a single thread meandering system. Little work has been carried out on the detailed sedimentological characterisation of pre-vegetation tidal sandstones and the quantification of pre- vegetation fluvial geobodies. A more detailed literature review is given in chapter 2 of this thesis. Significant volumes of the world’s hydrocarbon reserves are produced from lower Palaeozoic paralic reservoirs and with increasing demand globally. The hydrocarbon industry is exploring deeper plays and more challenging play concepts. It is critical to understand the architectures and stacking patterns of pre-vegetation sedimentary systems for petroleum systems analysis, reservoir prediction and understanding facies variations. Precambrian and lower Palaeozoic paralic sandstones hold more than 30 billion barrels of oil (Bbbl) and 25 trillion cubic feet (tcf) of gas globally (Table 1.1). Algeria is the largest producer of oil from Cambrian fluvial and shallow marine sandstones with an estimated 28 Bbbl recoverable reserves within the giant fields of Hassi Messaoud, Rhourde El Baguel and El Gassi (Bacheller & Peterson 1991; Macgregor 1998; Mitra & Leslie 2003; Boudries & Dizene 2008). Other fields producing from pre-vegetation paralic reservoirs have been recorded from India, the Middle East, Australia and Africa (Konert et al. 2000; Fox & Ahlbrandt 2002; Craig et al. 2010; Cozzi et al. 2012). Therefore, these systems hold significant economic potential and thus it is important to understand the sub-seismic sedimentation style, geometry and facies distribution of outcrop analogues to predict connectivity in the subsurface.

Field Age Reservoir Reserves Reference Hassi Messaoud, Cambrian Channel-fill and 25 Bbbl Bacheller & Algeria shallow-marine Peterson 1991 sandstones Macgregor 1998 Tarija Basin, Siliurian Channel 3.6 Bbbl USGS 2016 Argentina sandstones Rhourde el Cambrian Channel-fill and 3 Bbbl Macgregor 1998 Baguel field, shallow-marine Mitra & Leslie Algeria sandstones 2003 Boudries & Dizene 2008 Kalpingtag Silurian Braided delta 732 MMbbl Zhang et al. Formation, Tarim 2008 Basin, western China El Gassi, Algeria Cambrian Channel-fill and 300 MMbbl Macgregor 1998 shallow-marine sandstones Mereenie Field, Ordovician Intertidal and 186 MMbbl Havord 1993 Amadeus Basin, shallow marine 593 bcf Northern sandstone Territory, Australia Baltic Basin Cambrian Marine 100 MMbbl Haselton et al. Sandstones 1991 Brangulis et al. 1993 Ghaba salt basin, Upper Fluvial braid-plain 17.6 tcf Milson et al. Barik Sandstone Cambrian and shoreface 2008 Member, Oman to Lower sandstones Konert et al. Ordovician 2000

Akkas field, Iraq Ordovician Alluvial 5.6 tcf Aqrawi 1998 continental to Iraq Business shallow marine News 2018 Risha Sandstone Upper Shallow-marine 215 bcf Tamar‐Agha Member, Ordovician sandstones 2009 Risha Gasfield, NE Energy 365 Ltd Jordan 2017 Gulf of Suez Cambrian- Channel and Play not yet Alsharhan 2003 Ordovician shallow marine exploited sandstones

Table 1.1: A table summarising the fields, age and volume of hydrocarbons in place which have been found in lower-Palaeozoic paralic sandstones.

1.3 The Tumblagooda Sandstone The Tumblagooda Sandstone of the Southern Carnarvon Basin, Western Australia was selected as the main field location for this study with its excellent, laterally continuous exposures within the river gorges of the Murchison River and in the coastal cliffs south of Kalbarri town. It presents an excellent location to study continuous three- dimensional outcrops for geometries as the succession is largely undeformed by tectonics and is correlatable laterally and vertically in stratigraphy (Hocking 1991). The modern intertidal sediments in the Bay of Fundy, Canada were suggested by Wach et al. (2008) to be analogous to the deposits found within the intertidal parts of the Tumblagooda Sandstone. The extreme tidal range in the Bay of Fundy allows for excellent observations of large scale intertidal bedform morphology and distribution, therefore, this area served as a supplementary field location to document the features seen to compare them to the Tumblagooda Sandstone tidal facies. 1.4 The advancement to pre-vegetation paralic studies The study of pre-vegetation successions was initiated by Schumm (1968), who recognised that pre-vegetation fluvial systems were different to modern river systems and further developments by Cotter (1978) who noted a distinctly different fluvial geometry and coined the term “sheet-braided”. This term was then applied to pre- vegetation successions globally and implied high rates of run-off characterised by thin low-lying non-incisional fluvial facies. Until recently this was the understanding of pre- vegetation facies, however Santos et al. (2014) implied that pre-vegetation fluvial facies could be more complex. This study presents a case which notes that pre-vegetation successions are complex, with internal macroforms and complex interactions between channel scour margins and channel forms. The thesis questions whether “sheet- braided” is an appropriate term to use for pre-vegetation successions and reassesses the geometries of pre-vegetation models. This thesis presents a number of advancements made within the study area of pre-vegetation sedimentology, digital outcrop modelling and subsurface recognition. This project highlights the implications for analysis of the subsurface, developing on existing models, using a modern approach to look at a pre-vegetation paralic succession. The importance of an accurate conceptual model and geostatistical data is a key

component in analysing the subsurface for facies distribution, subsurface predictions, reservoir performance and interconnectivity of geobodies between sparse wells for volume estimates. This study presents a more complex case model, developed from existing understanding of pre- and post-vegetation fluvial deposits and integration of new field data. The thesis presents a predictive tool for the subsurface using a well-developed theory that amalgamated geobodies occur during periods of lowered base level and isolation of geobodies occurs during periods of transgression. This thesis then applies the sheet model vs the channel model and uses stochastic modelling to give examples of why the sheet model is inadequate and how this affects reservoir connectivity, and how production techniques would differ. 1.5 Key aims of the project To re-evaluate the sedimentology of the Tumblagooda Sandstone and study the architecture of the fluvial geobodies within the outcrops studied, key questions were posed at the beginning and throughout the project and these are as follows: 1. Can the depositional environment of the Tumblagooda Sandstone be constrained to shallow marine, continental aeolian or a combination? 2. Are tidal sedimentary systems different in the absence of plants to their modern counterparts? What are the key characteristics that define a tidal succession in the absence of typical fine-grained mudstone facies? 4. What is the architecture of the fluvial geobodies in the absence of plants? Are they sheet-braided or channelised? What are the depositional controls? 5. Can the digital outcrop models, sedimentology and depositional controls be used as a guide to predict the nature of the fluvial deposits in the subsurface? 6. What impact does this have on reservoir modelling and prediction? 1.6 Thesis structure To answer the questions posed the thesis is divided into a literature review, methods used and three research chapters. Chapter 2: A detailed literature review will be given, which highlights the contention in the published literature and the discussions previously made about pre- vegetation successions. This chapter also highlights gaps in research where this project

fits in. Detailed background studies are provided on the methods used and why we chose a novel approach of using UAV based photogrammetry method without prior knowledge of this method being successful. Chapter 3: A detailed methodology is presented for the data collected and conclusions made within the thesis.

To answer the questions posed in section 1.2 above the thesis is divided into three research papers in chapters 4, 5 and 6. Chapter 4: The applicability of modern tidal analogues to pre-land plant paralic depositional models. Bradley G-M., Redfern J., Hodgetts, D. George A. D. and Wach, G.D. Published in Sedimentology 2018 - Article DOI: 10.1111/sed.12461 This chapter addresses the contention in the current literature about the sedimentology and the environment of deposition of the Tumblagooda Sandstone (discussed in section 2.2.3). This paper also discusses the wider literature on pre- vegetation paralic successions, addressing if it is applicable to use modern tidal analogues to describe pre-vegetation tidal successions and gives a guide on how to identify tidal features in the absence of preserved mudstone deposits, addressing questions 1, 2 and 3. This chapter has been published in Sedimentology: Chapter 5: Fluvial architecture of pre-vegetation systems: revisiting the modes by using large digital outcrop datasets. Bradley G-M., Hodgetts, D. and Redfern J. Submitted to Journal of Sedimentary Research Addresses the characterisation of the fluvial geobody geometries identified within the digital outcrop models in VRGS. This paper suggests that these systems are more complex that initially thought and are not sheet-like. Geometries lead to the conclusion of two differing systems; an amalgamated highly truncated system which is constantly undergoing erosion and reworking, and a second system that is single storey and isolated, which rarely truncate. This chapter proposes that the fluvial system studied is channelised rather than sheet-like, with indications of a “normal” post vegetation

braiding style, with rare upper flow regime characteristics and laterally discontinuous fluvial bodies, addressing question 4. Chapter 6: Re-evaluating fluvial architecture of pre-vegetation reservoirs using large digital outcrop datasets: application to subsurface modelling Bradley G-M., Redfern J. and Hodgetts, D. Intended for submission in Marine and Petroleum Geology. Addresses questions 5 and 6 and discusses the wider implications for chapter 5. This takes them further in how to predict the geometries of these systems in the subsurface using sequence stratigraphy to identify amalgamated vs isolated characteristics rather than if the system is sheet-like or channelised. This is then applied to a conceptual model which is used to condition the models produced in the subsurface and the implications of modelling using the old sheet-like model vs the new channelised model are discussed. Chapter 7: will synthesise these findings and discusses a workflow for subsurface analysis about how to address the differences between pre- and post-vegetation successions. This chapter will conclude and state the final recommendations for further work. 1.7 Architectural Definition This study presents several terms surrounding fluvial geobody architecture. There has been debate in the literature surrounding the terminology that should be used for pre-vegetation fluvial geometries as they are significantly different systems to that of modern fluvial systems. Current terminology is based on modern and Mesozoic fluvial systems (Miall 1977; 1985) and does not consider the processes which were active in the time prior to the evolution of land plants. This thesis changes the definition of several terms which have been used to previously describe pre-vegetation fluvial architecture (Table 1.2, Fig. 1.1).

Term Definition in this thesis Channel The larger scale channel is bound by a 5th order bounding surface. Contains multiple internal scours and fill (Miall 1996). Braided A rapidly shifting low-sinuosity multi-channel system with or without the formation of point bars (Miall 1977). Channelised Overall morphology of the channel belt. Sheet Originally defined by Cotter (1978) as sheet-braided geometry being “genetic units” which have an aspect ratio of greater than 20:1. Sheet bodies – formed from mobile channel belts (Friend 1983). Rygel & Gibling 2006) suggested sheets had a W/T: 100-1000 Broad sheets 15-100 Narrow sheets A thin, narrow to medium width (Gibling 2006) geobody, with a non- incisional 5th order bounding surface (Rygel & Gibling 2006). Aspect ratios can be any size and are not restricted to greater than 20:1. It can be laterally extensive within the outcrop exposure but are seen to taper out. Scour-fill A lenticular shaped geobody. Multiple of these can be found within the channel system. The basal erosion surface is incisional. Isolated Single story channel bodies which do not interact with another fluvial body. Amalgamated Multistory and multilateral scours which are both succession and erosion dominated (Gibling 2006). The geobodies truncate against another fluvial geobody. Generally flat 6th order surface with multistory fill (Miall 1996; Rygel & Gibling 2006).

Table 1.2: Definition of architectural terms used within this thesis.

Figure 1.1: A graphic display of the fluvial terminology from Table 1.2 (after Bridge & Diemer 1983).

Chapter 2: Previous Work

Chapter 2: Previous work

This chapter presents and discusses the published literature surrounding pre- vegetation paralic systems globally. First to be presented will be a short background on braided river systems, followed by a discussion of pre-vegetation fluvial and tidal depositional systems. A short literature summary is given on the impact that the evolution of rooted plant systems had on paralic zones. After this the geological evolution of the Tumblagooda Sandstone as a case study will be presented, which highlights the conflicting literature surrounding the depositional environment and the challenges faced. Finally, the background on the methods that are used to gather data and make conclusions within this thesis will be presented. 2.0 Introduction to braided fluvial systems. There is an extensive body of work published on characterising fluvial system planforms and architectures which are based on observations of modern river morphologies (Fig 2.1). Braided end-member fluvial channel geometries have been described as having ribbon, sheet-like, and amalgamated multi-storey and multilateral channels, some displaying wide channel wings or levees, inter-bedded with flood plain or arranged as massive braidplains (Gibling 2006; Rygel & Gibling 2006). Braided fluvial deposits typically have a high aspect and net/gross ratio and are sandstone dominated with little overbank preservation (Martin 1993; Miall 1996; 2014; Gibling 2006). Braided rivers are characteristic of steep-gradient environments which are dominated by bed-load transport (Miall 2014). Four type sections were proposed for braided deposits which were characterised by sedimentary structures produced by high- energy currents, such as parallel laminations and trough cross-bedding which are gravel and sand dominated complex bar macroforms (Miall 1977). Geometries are documented as being composed of stacked broad, shallow channels filled with channel floor dune migration structures and macroforms formed in a highly mobile channel belt. Braided systems typically differ from meandering systems as they lack point bar dominance, have a higher degree of vertical aggradation and show a lower channel sinuosity (Miall 1985; 1996). Of the architectural element models proposed by Miall (1985), the most relevant to this study are the more confined outwash braidplain, low-

sinuosity rivers, ephemeral braidplain and the least confined sheet-flood elements (Fig. 2.2).

Figure 2.1: Classification of channel morphology based on modern rivers. Pre-vegetation rivers are described to be sheet like and unlikely to resemble modern fluvial planforms (After Blum et al. 2013).

Figure 2.2: Architectural element models modified from Miall (1985), showing the different elements and geometries formed by braided and ephemeral systems. A) Is the most channelised and denotes an outwash braidplain. B) Represents a low-sinuosity river with linguoid bars. C) Is a deep low-sinuosity river with macroforms. D) Depicts a distal braidplain which is typically ephemeral. E) Shows a sheet-flood fluvial plain which is subject to high-energy flashy discharge. The latter is the least confined and has been described to represent pre-vegetation fluvial facies (described in section 2.1.1).

It has been documented that in modern river systems, braiding is supressed, and meandering is promoted by the presence of vegetation to retain mud, a stable discharge and a low gradient basin style, spatial confinement, and possibly microbial matgrounds (Santos & Owen 2016; Ielpi et al. 2017; Ielpi 2018). This has been supported by documentation of increased mudstone, up to 1.4 orders of magnitude, during the proceeding time which has coincided with the evolution of vascular land plants (Davies & Gibling 2010; McMahon & Davies 2018). Many pre-vegetation rivers have been described as having a braided planform (Schumm 1968; Cotter 1978; Long 1978; Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Sønderholm & Tirsgaard, 1998; Eriksson et al. 2006) even in the distal reaches of the basin plain, whereas modern rivers typically adopt a single channel morphology, with the exception of braid deltas (Hartley et al. 2010). Therefore, it is likely that these planform and facies models, which are based on analogues which have vegetation present, are not applicable to rivers which were completely devoid of vegetation in the lower Palaeozoic. This has created an argument for improved facies models and classification of geobody geometries for pre-vegetation successions. 2.1 Pre-vegetation fluvial systems The absence of vegetation had a profound effect on the processes acting upon continental surfaces. They would have been subjected to increased physical weathering because of a dominance of high-energy fluvial currents (Schumm 1968; Dalrymple et al. 1985; MacNaughton et al. 1997; Long 1978; 2002, 2004; Gibling & Davies 2012). Chemical weathering would have been less aggressive in the absence of roots breaking down the rock surface and the absence of organic acids produced by plant degradation. Aeolian processes would have been enhanced, as a lack of vegetation cover to protect exposed sediment would have allowed wind currents to rework overbank areas (Dalrymple et al. 1985; Dott et al. 1986; Davies & Gibling 2010). A review of published literature highlights a commonality and the importance of compositionally mature sandstones in pre-Devonian paralic systems. These sandstones typically exhibit horizontally stratified and low-angle cross-bedded facies interbedded with monotonous sheets of trough cross-bedded sandstone (Bhattacharyya & Morad 1993; Simpson & Eriksson 1993; Hjellbakk 1997; Davies & Gibling 2010) indicative of

high-energy currents. This has been attributed to high rates of sediment delivery that would have resulted in channel aggradation and deposited thick successions of sand prone facies (Davies et al. 2011; Santos et al. 2014). Prior to the development of rooted systems, the different physical and chemical processes resulted in a reduction in the volume of clay generated in terrestrial environments, and continental overbank fine- grained mudstone-rich facies are generally absent (or not preserved) in the rock record. McMahon and Davies (2018) noted that from the time up to the Middle Ordovician to the early Carboniferous there was an increase of 1.75 orders of magnitude of recorded mudstone deposits, further supporting that the evolution of plants aided in the preservation of mudstone. It has been suggested that there would have been a reduced generation of continental mudstones prior to the evolution of rooted plants was partly a result of feldspars being more stable in terrestrial sediments as there was an absence of potassium absorption by vegetation (Basu 1981) and in part due to the reduced abiotic, microbial and fungal process which are absent without the generation of soils (McMahon & Davies 2018). In the rare cases that mudstones are identified, they are preserved as rip-up clasts and minor in-situ beds (Dalrymple et al. 1985). This suggests the preservation of mud-prone facies was inhibited which may be attributed to the effect of a lack of binding processes by root systems. This removed one of the main modern processes that confines and retains clay within the system. It has been suggested that overbank areas composed of loose sand were up to 20,000 times less stable than modern systems with up to 18% grass roots (Smith 1976). This key process would have inhibited the development of a meandering system (Schumm 1968; Long 1978; Eriksson et al. 1998; Santos et al. 2014). In the absence of plants fluvial systems would have been akin to arid modern flash-flood ephemeral systems, where laterally continuous sheets of episodic upper-flow regime discharge spread across the basin as an unconfined flow, removing all fine grained material from the system and transporting it offshore (McKee et al. 1967; Miall 1977; Tunbridge 1981; Fedo & Cooper 1990; Bhattacharyya & Morad 1993; MacNaughton et al. 1997; Eriksson et al. 1998; Long 2004, 2006; 2011; Lowe & Arnott 2016). The constant sediment reworking as flow conditions varied would have inhibited the development of laterally accreting bar-forms (Eriksson et al. 2006; Long 2006).

Studies of pre-vegetation fluvial systems document the dominance of bed-load processes which generated laterally extensive ephemeral sheets (Schumm 1968; Miall 1977; Bhattacharyya & Morad 1993; Long 2004, 2006, 2011). The lack of effective vegetation cover meant that water was not absorbed and would have allowed for an increase in surface run-off and sediment yield, and all river systems prior to the evolution of land plants would have had a sheet-braided planform (Schumm 1968; Cotter 1978). Cotter (1978) coined the term “sheet-braided” and applied this to “genetic units” which were architecturally different to “channel-braided” styles observed. The study identified that the key defining factor is a 20:1 aspect ratio, with sheet-braided styles being greater than this. These genetic units were described as being on various scales, typically decimetres thick and composed of trough cross-bedding and planar laminations (Davies et al. 2011). Although Cotter (1978) did not identify these as being channel bodies many later studies applied this to pre-vegetation fluvial successions, documented sheet-like fluvial facies composed of planar laminated and trough cross- bedded sandstones (e.g Fedo & Cooper 1990; Hocking 1991; Todd & Went 1991; MacNaughton et al. 1997; Eriksson et al. 1998). The term sheet-braided has been used to define a unique set of features only observed in pre-vegetation fluvial systems (Gibling et al. 2014). It is defined as a lateral persistent “genetic unit” with a planar basal erosion surface, with little incision and width to depth ratios of greater than 20:1 (Cotter 1978), Fuller (1985) suggested ratios of up to 1000:1 should be considered for fluvial facies. The sheet-braided system model produces stacked shallow tabular sandstone bodies because of repeated reworking and frequent channel avulsion, attributed to the lack of a stable levee or over-bank (Schumm 1968; Smith 1976; Cotter 1978; Sønderholm & Tirsgaard 1998; Santos et al. 2014). It is possible that channelisation may have occurred and rapid migration and enhanced channel avulsion rates, which in turn led to the widening of channels across the plain rather than incising down into the underlying sediment (Cotter 1978; Long 1978; Miall 1980, 1985, 1996; Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Davies & Gibling 2010). This model would imply that a relatively homogeneous system would have been deposited which is laterally extensive and has simple geometries and

limited facies variations. This study shows that these systems are not as simple as has been suggested and facies and geometries are much more complex. The “channel-braided” style formed due to increased recordings of concave upwards channelisation with smaller aspect ratios, which is interpreted to have a direct link to increased vegetation cover. Schumm (1960) observed that even small amounts of mud increase cohesion and enhance bank stability and so it has been suggested that with the evolution of root systems and the ability to retain mud within the system, the removal of fines was reduced, and bank stability increased significantly (Davies et al. 2011). This intern facilitates meandering, which has been shown to have increased in fluvial succession identified after the evolution of root systems (Davies & Gibling 2010; Davies et al. 2011). In modern systems the formation of either sheet or a channelised system is dependent in part on the climatic setting, whether flow is perennial or ephemeral, however, this may not be the case prior to effective vegetation cover during the lower Palaeozoic and Precambrian (Eriksson et al. 1998). In the absence of rooted plants, flash floods would have occurred globally, in arid and humid climates (Tirsgaard & Øxnevad 1998). More recent studies have identified the possibility that pre-vegetation fluvial systems may have a more complex architecture, possibly even having some meandering planform geometries (Hartley et al. 2015; Ielpi 2016; Santos & Owen 2016). Rare lateral accretion packages have been identified within pre-vegetation fluvial packages (Sweet 1988; Santos & Owen 2016; Santos et al. 2016), however they are small and are generally contained within predominantly sheet-like sandstone bodies and don’t represent meandering systems as sometimes suggested (Davies et al. 2017). Other authors have documented pre-vegetation fluvial deposits to preserve a wide range of alluvial styles, including sheet-floods, channel-braided, deeply channelised, perennial and meandering channel features (Sweet 1988; Nicholson 1993; Long 2006, 2011; Santos et al. 2014; Ielpi & Ghinassi 2015; Ielpi & Rainbird 2016; Ielpi et al. 2016, 2017). This gave the suggestion that flow had components of channelisation and was not dominated by widely accepted typical sheet-flood processes as these features are more dominant than initially thought (Ielpi 2016; Santos & Owen 2016; Ielpi & Ghinassi 2015;

Ielpi & Rainbird 2016; Ielpi et al. 2016, 2017). It may be that they are not characterised just by low-sinuosity fluvial facies especially if there is a low-gradient and stable discharge (Ielpi et al. 2017). This study will highlight how important it is to document these systems for subsurface reservoir facies prediction and modelling as this study documents more complex facies variations. 2.2 Pre-vegetation tidal systems The lack of vegetation would also have influenced the sediment delivered and deposited in the marine realm (Dott 2003). As previously discussed, roots bind substrate, effect weathering processes, and control the amount of mud-grade sediment delivered from the continent. Observed tidal deposits record typical heterolithic laminations which are classified based on the mud content of sedimentary structures, as lenticular, wavy and flaser bedding, with the former preserving more mud than the latter (Fig. 2.3). Other typical tidal indicators include mud-draped foresets, mud-lined burrows and beds of mudstone deposited on the intertidal flats. In pre-vegetation continental systems, less mud was generated on land and any mud in the system would have been transported by high-energy fluvial systems, which on entering the marine realm would have bypassed the littoral zone and been carried into the deeper marine basin (Long 1978, Fedo & Cooper 1990, Simpson & Eriksson 1990; Eriksson et al. 1995). This may explain the paucity of recorded tidal facies and sedimentary structures in pre- Devonian strata.

Figure 2.3: Photos are showing flaser wavy and lenticular bedding, with flaser having a higher sand proportion than lenticular bedding which has higher mud content. This structure is a common indicator of tidal processes (after Kentucky Geological Survey 2018).

Published literature suggests that fine-grained shallow marine sandstone facies deposited prior to the evolution of land plants are texturally and compositionally mature

and typically non-fossiliferous; however, they do preserve an abundance of ichnofauna such as Skolithos, Diplichnites and Cruziana in some cases (Long 1978; Fedo & Cooper 1990; Eriksson et al. 1995). There is a limited amount of published cases identifying preserved mudstone or heterolithic muddy facies within lower Palaeozoic tidal successions. Mud grade deposits are generally absent which is suggested to be due to the limited residence time in the shallow marine realm, as a result of high-energy fluvial influx (Long 1978, Fedo & Cooper 1990, Simpson & Eriksson 1990; Eriksson et al. 1995). In addition to affecting the generation of sedimentary structures, the reduced mud content also affects the identification of specific palaeoenvironmental ichnospecies. 2.3 Impact of vegetation The evolution of rooted plants and the effect that this had on the Earth’s surface processes are well documented and the early evolution of plants is believed to have had a profound impact on the terrestrial and marine ecosystems by changing weathering processes, influencing the retention of mud, stabilising of overbanks and allowing the formation of soils. Plant cover influenced run-off and subsequent depositional geometry and sedimentary structures as meandering systems are only abundantly identified within post-Devonian sedimentary sequences (Schumm 1968; Algeo & Scheckler 1998, Davies et al. 2011). The colonisation of land by multicellular plants (embryophytes) started around the middle Ordovician, at 470 Ma, with the evolution of fresh water algae (Kenrick & Crane 1997; Pires & Dolan 2012). Early vascular plants (sporophytes) evidenced by Cooksonia megafossils, first appeared in the rock record in mid-late Silurian rocks, around 425 Ma (Gensel 2008; Pires & Dolan 2012). Sporophytes lacked deeply rooted systems and did not substantially anchor into the sediment (Gensel et al. 2001) and it has been suggested that these organisms could not withstand high-energy fluvial flows and probably had little effect on fluvial geometry (Davies & Gibling 2010). Pervasive root structures attributed to vascular land plants are first described in Lower Devonian, in Lochkovian and Pragian-Emsian age rocks (Gensel et al. 2001; Hillier et al. 2008; Xue et al. 2016). The development of root systems broadly corresponds with the identification of large volumes of mudstone deposited in terrestrial systems and the first recorded meandering fluvial systems in the rock record (Davies & Gibling 2010). The evolution of plants is said to have led to a reduction in flash flood events by the increased

storage capacity of soils (Algeo & Scheckler 1998). Overbank and channel bar elements began to stabilise because of cohesion of sediment and an increase in fine-grained muds in the overbank setting (Cotter 1978; Smith 1998). Following the evolution of rooted vascular plants in the Devonian, a sudden increase in physical and chemical weathering is interpreted to have caused the generation and preservation of significant volumes of terrestrial mudstone deposits (Schumm 1968; Cotter 1978; Davies et al. 2011; Gibling & Davies 2012; McMahon & Davies 2018). Rooted vegetation increased infiltration and bedrock weathering and decreased surface runoff leading to the preservation of mudstone facies (Knighton 1998; Gibling & Davies 2012). Mud within the overbank is said to significantly increase cohesion and bank stability promoting meandering channel style (Schumm 1960; van Dijk et al. 2013). Modelling the interplay between modern vegetation and mud reveals that a higher mud concentration increases floodplain aggradation and reduces overbank flow frequency (Kleinhans 2018). A self-formed cohesive floodplain is said to sustain meandering even without the aid of vegetation (van Dijk et al. 2013). The experiments have shown that a meandering river can develop without having an initial cohesive bank suggesting a transition from a braided to a meandering river can occur in the presence of a cohesive overbank and with no vegetation present (van Dijk et al. 2013). Above ground rootless vegetation is said to have still introduced a baffling effect which could trap and deposit muddy sediment (Davies et al. 2011; McMahon & Davies 2018). Chemical weathering in the upland areas would have to produce significant quantities of mud to enable rootless vegetation to baffle and retain the mud (Kleinhans et al. 2018). Bushy vegetation is shown to confine river channels and increase sinuosity by capturing mud on the floodplain and persisting long enough to form a significant mud deposit that would not be washed away during flood flows (Kleinhans et al. 2018). Vegetation causes mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation mutually enhance floodplain formation, increasing stability and causing promoting channel self-organisation and the formation of a single- thread channel (Davies et al. 2011; Kleinhans et al. 2018). Flume experiments carried out by Tal & Paola (2007) demonstrated how rooted vegetation in inactive areas of an alluvial plain stabilises bars, which allows channels to self-organise and evolve from a

braided morphology to a single thread sinuous laterally migrating (meandering) channel (Fig. 2.4). Whereas, Braudrick et al. (2009) added low-density material behaving as fine sediment, which vegetation was able to retain and sustain single-thread meandering. Experiments have shown that rooted bank undercutting and retreat are of lesser importance for persistent meandering than the build-up of bars and prevention of chute cutoffs by vegetation on the bars (Braudrick et al. 2009; van de Lageweg et al. 2014; van Oorschot et al. 2016). Arguments have been made that rooting cannot have been very important in large rivers (on the scale of the Ganges) with depths greatly exceeding root length, the smaller systems commonly preserved in the rock record are more likely to have been influenced by small roots which could have had significant binding effects on Palaeozoic rivers (Kleinhans et al. 2018). Mudstones are rare in Archean – Cambrian alluvial successions with only 3% containing >10% fines, compared to 74% in the Silurian- Devonian (Davies & Gibling 2010). Mudstones become a major component of Devonian fluvial systems, suggesting that suspended sediment was becoming increasingly available or that vegetation promoted storage of fine-grained material, by baffling and reducing aeolian activity (Davies & Gibling 2010). This also coincided with the identification of low-energy meandering and anastomosing fluvial geometries preserved in the rock record (Davies & Gibling 2010). Therefore, due to the lack of significant volumes of preserved mudstone and the absence of evolved plants, channels are unlikely to have self-organised and preserved meandering deposits. These are rare until the late Devonian where around 40% of recorded alluvial successions are interpreted as meandering (Davies et al. 2011). Vegetation led to the evolution of single channel meandering river systems (Cotter 1978) and the disappearance of self-formed, coarse-grained, sheet-braided systems, dominated by trough cross-bedding (Davies et al. 2011). Therefore, it is interpreted that the evolution of rooted vegetation and the increased retention of mud via plant induced processes, was the turning point for fluvial style and so palaeoenvironmental models reflect this (Miall 1977). By the upper Devonian fluvial deposits contain fossil trees, calcrete deposits, intense root generated bioturbation, large lateral accretion structures, crevasse splays and floodplain architecture (Davies & Gibling 2010; Davies et al. 2011).

Figure 2.4: Plan view images (from Tal & Paola 2007). A braided morphology in picture A, self-organises into a sinuous single thread channel (D) with the growth of vegetation, in this case alpha grass.

2.4 Fluvio-paralic stratigraphy The morphological classification of river systems has been a matter of debate for decades and classification schemes have been devised based on processes (e.g. Schumm 1965), architectural elements (e.g. Miall 1985) and planform (e.g. Hartley et al. 2010). These schemes are based on modern examples which are inherently developed in the presence of rooted vegetation and so it is difficult to apply this to pre-vegetation successions. It has been made apparent that even applying these schemes to post- vegetation deposits is more complex than initially thought, as a river can have multiple planforms throughout its course and not preserve all of them. The application of sequence stratigraphy to alluvial successions is in debate (Bhattacharya 2011; Catuneanu et al. 2011), as application of relative sea-level into the continental realm is difficult and applying stacking patterns based on the “exxon sequence stratigraphic method” is difficult at the field scale. The preservation of incised valleys in the rock record has given evidence for paralic sequence stratigraphy for many post-vegetation paralic systems (Shanley & McCabe 1993; Holbrook 1996; Hampson et al. 1999; Holbrook et al. 2006). Incised valleys have been defined as a “fluvially eroded topographic low that is larger than a single channel form and is filled with fluvial, estuarine and marine deposits and

preserves an abrupt seaward shift in depositional facies across a mappable 6th order sequence boundary” (Boyd et al. 2006). Initial fills are generally characterised by low- accommodation space lowstand facies which consist of laterally and vertically amalgamated (usually braided) fluvial facies which have a high degree of reworking and cannibalisation (Shanley & McCabe 1993; Hampson et al. 1999). Transgressive fills which follow this preserve estuarine facies which are typically influenced by tidal, wave and fluvial processes (Boyd et al. 2006). Fluvial facies are typically isolated and consist of highly sinuous fluvial facies (Shanley & McCabe 1994). This is diagrammatically presented in figure 2.5. This understanding has not been previously applied to pre-vegetation successions and this study shows that it is an effective way of describing paralic facies even in the absence of plants and suggests that processes would have been like what has been previously observed in modern fluvial facies.

Figure 2.5: Paralic sequence stratigraphic model derived from Cretaceous fluvial facies in Utah (Shanley & McCabe 1994).

The morphology of a river in its distal reaches is said to be largely controlled by the energy of the fluvial system, sea-level, climate and tectonics (Shanley & McCabe 1994; Boyd 2006; Holbrook et al. 2006). Nichols and Fisher (2007) identified proximal to distal trends with more amalgamated pebbly bedload deposits in the more proximal reaches with poorly channelised sheet-like splays being preserved within mudstone in the distal zone (Fig. 2.6). Typically, down stream there is a decrease in channel size and energy progressing from a high-gradient braided system to a low-gradient sinuous

system (Hartley et al.2010). This model suggests that in the rock record distal deposits should be sheet sandstones and encased in mudstone which contrasts with what has been observed for pre-vegetation successions. However, Davidson et al. (2013) described a bifurcating braided DFS model, characterised using the Canning River, Alaska, as an analogue. The braided DFS is split into three zones where the proximal zone has a low sinuosity with complex bar-forms and well drained overbanks. The medial zone is characterised by a moderately sinuous braided planform, with complex bars and an amalgamated overbank made up of palaeochannel deposits. The distal zone is characterised by sinuous anabranches and a braided planform and composed of complex bars and an amalgamated overbank made up of palaeochannel deposits (Davidson et al. 2013). This DFS type has been observed within high-gradient areas with relatively small catchment areas, which contrasts what is described in pre-vegetation fluvial systems as they typically fill the entire basin and are described as large distributary systems.

Figure 2.6: The development of a fan-shaped body formed by repeated avulsion. Architectures are characteristic of proximal to distal zones with amalgamated interconnected bedload deposits characterising the proximal zone and isolated sheet sandstones encased in thick mudstone characterising the distal zone, terminating in a marine basin. (Nichols & Fisher 2007; later developed by Hartley et al. 2010; Davidson et al. 2013; Owen et al. 2017)

2.5 The Tumblagooda Sandstone 2.5.1 Geological history The Tumblagooda Sandstone was deposited in the Southern Carnarvon and Northern Perth basins (Fig. 2.7) during the Ordovician-Silurian Periods (Hocking 1981; Veevers 2004; Kettenagh et al. 2015, Markwitz et al. 2017). During this time Western Australia was part of the continent of Gondwana (Fig. 2.8), located at arid low latitudes during the Ordovician (Schmidt & Hamilton 1990; Li & Powell 2001; Veevers 2004). Australia was rotated 90° anticlockwise compared to today, connected to India to the south, with a subduction system to the north (Li & Powell 2001). The Southern Carnarvon Basin was a north-south orientated intracratonic basin which formed as a result of a failed rift (Li & Powell 2001; Veevers 2004; Mory & Hocking 2008). The onshore Southern Carnarvon Basin covers an area of 115,000 km2 and is subdivided into three, northwards trending horst and graben sub basins (Hocking et al. 1987; Hocking 2000): the Gascoyne Platform, the Merlinleigh and Byro Sub-basins (Mory et al. 2003) (Fig. 2.7). The Southern Carnarvon Basin was a major depocentre from the Cambrian to the Permian (Hocking 1994) and the Kalbarri Group is the only Ordovician-Silurian succession in the Southern Carnarvon and the Northern Perth Basin (Hocking 1991). Subsequent fill consists of Palaeozoic sedimentary rocks with a Mesozoic and Cenozoic cover (Hocking & Mory 2006).

Figure 2.7: A map showing the structure of the Northern and Southern Carnarvon basins (SCB). The SCB is sub-divided into the Gascoyne Platform, Merlinleigh and Byro Sub-basins. The Kalbarri field area is situated within the Gascoyne Platform which is fault bounded by the Wandagee -Yanrey Fault System to the northeast and the Ajana Fault System to the southeast. The basin itself is bounded to the east by the Darling Fault (Hocking et al. 1987; Hocking 1991, 1994; Mory et al. 2003).

2.5.2 Tumblagooda Sandstone The Tumblagooda Sandstone is a quartzose paralic sandstone succession exposed, at its type locality and the studied section, in the gorges of the lower Murchison River and the coastal cliffs of Kalbarri National park, near the town of Kalbarri, Western Australia (Fig. 2.9) (Hocking 1981; 1991). The formation is also exposed in the gorges of the Hutt River and at Pencell Pool further upstream in the Murchison River (Kettenagh et al. 2015).

Figure 2.8 Palaeo-reconstruction for the Ordovician Period (Modified from Scotese 2000), showing Australia located within Gondwana and the field study area of Kalbarri within low latitude regions.

The Tumblagooda Sandstone is the oldest formation in the Kalbarri Group (Fig. 2.10) and unconformably overlies Precambrian gneissic basement (Young 1986; Hocking 1985). Exposure has led to the erosion of the upper parts of the formation in some areas, however, in the north of the basin the formation is conformably overlain by shallow- marine dolomite, limestone and evaporite deposits of the Silurian Dirk Hartog Formation (Hocking 2000). Petroleum exploration and stratigraphic drilling boreholes record a preserved thickness of 2500 m of Tumblagooda Sandstone (Hocking 1979; Mory et al. 2003), however, in outcrop around Kalbarri, the succession is only 1300 m thick (Hocking 2000). The rocks exposed in the Murchison River are relatively undeformed with a gentle dip up to 5° to the northwest, with stratigraphy progressing from the southeast to the northwest within the river section. Two minor faults cut the stratigraphy (Fig. 2.9) with little displacement on the Fourways fault and around 50 m of displacement on the Mooliabaanya Pool fault at the northern tip of the river (Hocking 1991). The exposure at Ross Graham is laterally equivalent to that of Hawks Head. The fluvial facies exposed at the top of this section is also laterally equivalent to the fluvial exposures at the base of Z-Bend and Fourways. Z-Bend and Fourways are also laterally equivalent (Hocking 1991). The outcrop exposes both along channel and channel cross-section profiles making it an ideal place to study the three-dimensional nature of the system.

Figure 2.9: Map of the surface exposure of the Tumblagooda Sandstone within Kalbarri National Park highlighting the studied locations (modified from Hocking and Mory 2006; McNamara 2014). It also highlights the distribution of the four facies associations identified by Hocking (1991).

The formation consists of mature arenites with rare siltstones and conglomerates (Hocking 1979). Hocking (1991) identified five facies associations (units), four in the exposures around Kalbarri National Park (Fig. 2.8) and one at Pencell Pool. The sandstones have a high quartz content and contain only rare matrix (Hocking 1979). The provenance for the Tumblagooda Sandstone is still largely debated. The Northampton Complex to the east has been ruled out as a source area due to the lack of garnets present in the Tumblagooda Sandstone (Hocking 1991; Kettanah et al. 2015). Uplift of the Yilgarn Craton, to the southeast, is believed to be the most likely sediment source for the Tumblagooda Sandstone, shedding sediments (off the Darling Fault scarp) south and south-west into the Southern Carnarvon and Northern Perth basins (Hocking 1991; Veevers 2004). Other studies have suggested a number of other possible sediment sources, including distant areas such as the Yilgarn Craton, Pinjarra Orogen (Leeuwin Complex, Northampton Complex, Mullingarra Complex), Albany-Fraser Orogen (Fig. 2.7) and even as far as the East African-Antarctic Orogen (Kettanah et al. 2015). Markwitz et al. (2017) suggested a source from Archean rocks of the interior Australian craton and

Early Palaeozoic rocks from Greater India, based on zircon analysis. This contrasts with the north-westerly fluvial palaeocurrents which have been measured in the Tumblagooda Sandstone (Hocking 1991). There has been a debate for several decades about the exact age of the Tumblagooda Sandstone (Phillip 1969; Schmidt & Embleton 1990; Hocking 1991; McNamara & Trewin 1993; Markwitz et al. 2017; Allen & Trinajstic 2017). Based on stratigraphic superposition it is the oldest sedimentary succession encountered in the Southern Carnarvon Basin (Fig. 2.10) (Hocking 1991; 2000).

Figure 2.10: A stratigraphic column for the Gascoyne Platform and Merlinleigh Sub-basin (modified from Hocking 1991; Ghori 1999). The Tumblagooda Sandstone is at the base of the succession, approximately 1400m thick containing two megacycles, each start with fluvial sedimentation and progress to marine sedimentation. Thicknesses of each facies association taken from Kalbarri outcrop section (Hocking 1991).

Age constraints are limited due to the coarse clastic nature of the sediments, and the resultant lack of identified body fossils within the succession. The Tumblagooda

Sandstone was first described as being Cretaceous in age (Clarke & Teichert 1948), until later recognition of the Cretaceous unconformity at the top of the Tumblagooda Sandstone, determined that the formation was older. The Dirk Hartog Formation conformably overlies the Tumblagooda Sandstone in boreholes and contains an Early Silurian conodont assemblage (Phillip 1969), therefore, the Tumblagooda Sandstone must predate the Early Silurian. Palaeomagnetic studies carried out in the 1990’s suggested a late Ordovician to early Silurian age (Schmidt & Hamilton 1990; Schmidt & Embleton 1990). Work on the trace fossil assemblage has suggested Early Silurian age based on tetrapod trackways (McNamara 2014) but identification of paddle impressions formed by arthropods has suggested an age of middle Ordovician (Mory et al. 2003). A recent zircon study also suggested an age of middle Ordovician 466 +/- 22 Ma (Markwitz et al. 2017). More controversially, a late Silurian to Devonian age was suggested based on the presence of Heimdallia burrows and eurypterid-like trackways (Bradshaw 1981; Trewin & McNamara 1994). Although the age of these trace fossils is not confirmed, they are known in deposits as old as the Late Ordovician. Poorly preserved trilete spores and acritarchs were recovered from borehole samples, identified as Silurian age (Hocking 1991; McNamara & Trewin 1993), but it is possible they may have come from other parts of the borehole. A recent report identifies Devonian-aged fish fauna in a dry petroleum exploration well, Wendy 1 (Allen & Trinajstic 2017). The study notes that the stratigraphy in this part of the core is lithologically distinct from the Tumblagooda Sandstone type section and perhaps the stratigraphy in the core needs revising (Allen & Trinajstic 2017). In summary, the most-likely age for the Tumblagooda Sandstone is Middle Ordovician to Early Silurian.

2.5.3 Previous interpretations The environment of deposition for the Tumblagooda Sandstone has remained enigmatic for several years as it has previously interpreted as a mixed fluvial and shallow marine system (Hocking 1981; 1991) and as an entirely continental succession dominated by aeolian and fluvial processes (Trewin 1993a & 1993b; McNamara & Trewin 1993).

2.5.3.1 Mixed marine to non-marine interpretation Hocking (1991) interpreted five facies associations within the Tumblagooda Sandstone which are summarised in Table 2.1 and graphically depicted in Figure 2.11. Facies Association 1 is located at the base of the succession, dominated by fluvial facies and grades north-westerly into tidal sandstones of Facies Association 2. Facies Association 3 overlies this, dominated by coarser grained fluvial facies, which grades up stratigraphy in the coastal sections into Facies Association 4 which is again tidally dominated (Hocking 1991). Tumblagooda Sandstone was interpreted as a braided fluvial system which prograded rapidly north-westward from the Darling Fault at the basin margin, bringing sediments from the Yilgarn Craton (Fig. 2.11A). Alluvial fans would have been active at the basin margin and a flood-dominated braided fluvial system spread into the basin to a tidally influenced coastline in an arid climate (Fig. 2.11B). The braided system was a large, low-sinuosity, sand-dominated episodic system, controlled by flooding (Hocking 1991). Large mid-channel bars did not form perhaps because of the dominance of unidirectional migration of ripples within the channels. Flood reworking and migration appears to truncate cycles rather than cannibalise them, leading to stacked sandstone bodies (Hocking 1991). Classic delta successions are not reported within the Tumblagooda Sandstone, instead the fluvial system is said to have prograded directly over tidal deposits in “sheet-like blankets”. Reworking by tidal processes occurred when fluvial channels were infilled by brackish water allowing for bioturbation, indicating this being a low-relief delta with no pro-delta sequence (Hocking 1991).

Facies Description Interpretation Code FA1 440 m thick in Kalbarri section Fluvial deposit, suggesting Texturally mature, trough cross-bedded, deposition from a low-sinuosity medium– to coarse–grained, feldspathic sheet-braided river (Hocking sandstone interbedded with planar- 1981, 1991). bedded sandstone (Hocking 1979; 1981). Palaeocurrents indicate north- This facies is poorly sorted which westward flow (Hocking 1991). improves up stratigraphy. Pebbles can be found as lag deposits at the base of units and fining-upwards cycles are common (Hocking 1981). Palaeocurrents trend to the northwest throughout this facies. FA2 200 m thick in Kalbarri section Tidal to shallow marine setting Thinly-bedded, fine- to medium-grained (Hocking 1981). sandstone, which is commonly intensely bioturbated and ripple cross-laminated which in places grades into low-angle cross-stratification (Hocking 1979; 1981). Bioturbation is limited to arthropod tracks and Heimdallia burrows (Hocking 1991). Structures such as climbing ripples, flat topped ripples and setulfs, herringbone stratification, contorted bedding are also present (Hocking 1991). FA3 260 m thick in Kalbarri section Deposition within a low- Pronounced fining-upwards cycles of sinuosity braided river system, of coarse-grained sandstone. At the top of higher energy conditions than each cycle there is bioturbation and F.A 1. A marine influence is also siltstones present. Pebbles are more more apparent within this common than in FA1. association than F.A 1, indicating deposition close to the shoreline where fluvial and marine processes dominated (Hocking 1981) FA4 45 m thick in Kalbarri section It is interpreted as a high-energy Medium- to coarse-grained, poorly braided system with a high sorted, feldspathic to quartzose sediment load with common sandstone. Trough cross-stratification is marine incursions, enough to common with over-steepened cross-sets. support life (Hocking 1979). Skolithos tubes are common in this facies, which cut down through the cross-sets (Hocking 1991). FA5 Located east of the Northampton Block, Alluvial fan deposit (Hocking et FA5 consists of coarse- to very coarse- al. 1987; Hocking 2000). grained, poorly sorted and well-rounded

pebble and cobble conglomerate. Palaeocurrents are to the northwest.

Table 2.1: Table summarising the facies associations identified by Hocking (1991)

Figure 2.11: Hocking’s (1991) conceptual models for the depositional facies associations (FA). A) Shows the relationship between FA1 and FA2, showing a sheet-braided system that fills the entire basin. B) Shows the intertidal zone model for FA2. C) Shows the model for FA3 and the relationship to FA4 in the bottom corner. FA3 is interpreted to be a sheet-braided system which fills the entire basin. D) shows the depositional model for FA4.

2.5.3.2 Non-marine interpretation Trewin (1993A, B) proposed an alternative interpretation for FA2; that deposition occurred within a fluvial overbank environment comprising rippled sandsheets with development of only small dunes. Inactive areas of the braid-plain were suggested to act as the sediment source for interbedded aeolian sandsheets (Fig. 2.12). Trewin (1993b) recognised deflation surfaces with adhesion structures and blown-out ripples. Straight-crested dunes were interpreted to have migrated to the south-east within the fluvial overbank areas. The study identified low-angle dune foresets with inversely graded avalanche (pin-striped) cross-laminae with wind-formed ripple cross- lamination at the toesets. The lack of large-scale aeolian dune bedforms was attributed

to be due to short transport distances and limited deposition time (Trewin 1993a, b; Trewin & McNamara 1994).

Figure 2.12: Depositional model for FA2, with fluvial transport deflected to the SW along interdune areas. Inter-channel areas were covered by aeolian sand-sheets; deflation hollows between dunes are flooded to form small colonised lakes (Trewin 1993B).

Fluvial channels were interpreted to control dune orientation and channels were typically deflected down interdune corridors. Wave ripples were interpreted to have formed during times of flooding. Organisms were interpreted to have colonised flooded interdune ponds, because bioturbation often terminates at the base of dune toesets. Arthropod trackways occur on rippled surfaces and foreset laminae, which was interpreted as organisms traversing the overbank areas to feed on organisms which colonised the ponds (Trewin & McNamara 1994). 2.5.3.3 Distal delta interpretation Evans et al. (2006) completed a study focused on FA3 and FA4 using gamma logging techniques to demonstrate the heterogeneity in ancient fluvial and deltaic systems as an analogue for the subsurface. The high-resolution study enabled thinner beds to be resolved, revealing more detail on the internal architecture and sandstone body stacking relationships. This allowed for the further identification of three subunits within the coastal outcrops (Evans et al. 2006).

Association A identified a sheet-braided river environment with occasional multi- storey and multilateral units which are composed of very coarse-grained amalgamated trough cross-bedded sandstones. Association B was defined as an overbank to interdistributary setting dominated by centimetre scale thin beds of fine-grained sandstone. Association C describes a series of multi-storey stacked distributary channel sandstones and sheet sandstones interpreted as mouth bar deposits. These channels are interpreted to have been confined by sandy levées (Evans et al. 2006). Figure 2.13 shows the distal delta interpretation for FA3 and FA4.

Figure 2.13: Evans et al. (2006) interpretation for the depositional environment of FA3 and FA4. A) A summary of vertical facies relationships B) a summary of lateral facies relationships C) models for deposition.

2.5.4 Fauna The Tumblagooda Sandstone has abundant and varied ichnofauna with 27 different types of trace-fossils preserved (Trewin & McNamara 1994). These trace-fossils range from locomotion tracks and trails, hunting, feeding and dwelling traces. Common

non-marine and marine ichnofauna found within the Tumblagooda Sandstone are Diplichnites, Didymaulichnus, Beaconites, Diplocraterion, Heimdallia and Skolithos (McNamara & Trewin 1993; Trewin & McNamara 1994; McNamara 2014). The presence of these enigmatic ichnofacies has led to contention about the depositional environments of many pre-vegetation mature sandstone sequences.

2.5.4.1 Trace-fossils The whole outcrop succession has been divided into two ichnofacies by McNamara (2014): the Scoyenia Ichnofacies which contains the Heimdallia-Diplichnites Ichnofauna found in FA2 and the Skolithos Ichnofacies which contains the Skolithos- Diplocraterion Ichnofauna found in FA4. The fluvial facies of FA1 and FA3 have rare traces. The Heimdallia-Diplichnites Ichnofauna is reported by Trewin & McNamara (1994) to preserve arthropod tracks which show that these organisms were able to walk on land and across dune foresets and inhabit high-energy terrestrial environments. Heimdallia is interpreted to occur in dense networks which were formed in flooded interdune hollows (Trewin & McNamara 1994). Conversely to this, Bradshaw (1981) described Heimdallia burrows in the Taylor Group, southern Victoria Land, Antarctica and suggested that the trace is indicative of a shallow marine environment. The Skolithos-Diplocraterion Ichnofauna is most dominant at the top of the sandstone sequence in the high-energy coarse-grained fluvial to marine transition zone (Trewin & McNamara 1994). This ichnofacies is generally described in estuarine to marine environments and this study supports this interpretation and further develops this into a sequence stratigraphic model in chapter 6.

2.5.4.2 Body fossils Only one body fossil has been identified within the Tumblagooda Sandstone, that of a euthycarcinoid arthropod Kalbarria brimmellae sp. (McNamara & Trewin 1993). It has been interpreted as living in freshwater pools which lay on sandflats between flood events (McNamara & Trewin 1993; Trewin & McNamara, 1994). This gives evidence for some of the earliest life in the terrestrial environment which are thought to have moved from pool to pool, over wet sand, as it dried up in an attempt to get back into the river (McNamara & Trewin 1993).

This work has led to contention in the literature about the depositional environment proposed for FA2 as Hocking (1991) proposed a tidal affinity, whereas McNamara (2014) suggests an entirely freshwater, aeolian and fluvial environment. This study disagrees with Trewin & McNamara’s (1994) and McNamara’s (2014) interpretation that the Tumblagooda Sandstone was deposited in an entirely continental environment and the argument for this is presented in chapter 4. The reinterpretation of these ichnofacies may result in the need to reinterpret other pre- vegetation successions which use Trewin and McNamara’s interpretation as an analogue.

Chapter 3: Dataset and methodology

Chapter 3: Dataset and Methodology 3.0 Introduction The study is a multidisciplinary approach which integrates traditional field sedimentology and petrography with new technologies of UAV (Unmanned Autonomous Vehicle) photogrammetry and digital outcrop modelling. This chapter presents the methods used to gather, analyse and interpret data which has led to the conclusions made within the research and discussions chapters. 3.1 Fieldwork The five major access points to the Murchison River are tourist roads, other parts of the outcrop are only accessible by foot and camping is prohibited. The five tourist lookouts along the Murchison River (The Loop, Fourways, Z-Bend, Hawks Head and Ross Graham (Fig. 2.8)) and the coastal cliffs were selected due to ease of access and as in Hocking (1991) this provided near complete coverage of the stratigraphy. According to the mapping that was carried out by Hocking (1991) there is a small amount of overlap between the southerly outcrops (Ross Graham and Hawks Head) and the outcrops in the middle of the study area (Z-bend and Fourways). Ross Graham and Hawks Head (Fig. 2.8) expose the same level of stratigraphy as do Fourways and Z-Bend (Fig. 2.8), this provided two large sections to assess the lateral facies variability over several kilometres at similar points in stratigraphy. While in the field access to the outcrop surrounding the gauging station (Fig. 2.8) was granted and the study area was extended for a further 8 km to the south, adding more of FA1 to the study. The coastal cliffs were also studied from Red Bluff to Eagle Gorge (south of Red Bluff in Fig. 2.8) to assess the upper parts of the stratigraphy.

3.1.1 Logging Thirty-one 1D high-resolution sedimentary logs (based on Nichols 2009) were obtained on a 1:20 scale from the river gorge and coastal cliff outcrops to detail the texture and structure of the sandstones (Table 3.1). Logs were taken at 200 m spacing to give a high-resolution dataset and allow for lateral correlation. Palaeocurrent measurements were taken for ripple cross-lamination, trough cross-axis and primary current lineation directions. Samples were taken of each

representative facies, samples would be taken from areas that have had minimal weathering, but however, this was not always possible. Total gamma was measured tracking the same position as the sedimentary log. This was obtained to see any subtle changes in the mineralogy of the rocks and to compare what you would expect to see with the changing facies in well gamma data. The data was also used to tie sequence stratigraphic interpretations made in the logs to the total gamma counts. Gamma counts were obtained at 25 cm intervals using an Auslog GeoGAMMA V2 scintillometer (Fig. 3.1) connected to a hand-held computer. This type of scintillometer measures total gamma emitted by the rocks, as the rocks are arenites we expected relatively low counts throughout, but the readings would give a relative change throughout the stratigraphy. At each data point 5 readings were taken, and the minimum and maximum value removed, and the remaining three measurements were averaged (Evans et al. 2006). This was to reduce the effect of erroneous measurements. The results were plotted against measured height to give a pseudo gamma ray borehole log.

Figure 3.1: Auslog GeoGAMMA V2, outcrop gamma scintillometer (Auslog Pty Ltd 2006).

Name Locality Start Lat Start Long Start Z Length m GS Gauging Station 27.85626 114.54709 150 16.4 RG1 Ross Graham 27.81314 114.47491 134 18 RG2 Ross Graham 27.81647 114.47543 112 31.7 RG3 Ross Graham 27.8175 114.4764 127 30.1 RG4 Ross Graham 27.81755 114.47645 115 39 HH1 Hawks Head 27.7879 114.46954 120 34.55 HH2 Hawks Head 27.78557 114.4692 122 32 HH3 Hawks Head 27.79156 114.47092 111 37.3 ZB1 Z-Bend 27.65441 114.45673 67 54.2 ZB2 Z-Bend 27.65449 114.45898 71 54.2 ZB3 Z-Bend 27.6566 114.45591 63 30 FW1 Fourways 27.63145 114.47276 57 46 FW2 Fourways 27.63534 114.47313 56 85 L1 The Loop 27.55317 114.44505 3 94.5 L2 The Loop 27.55128 117.44725 42 42 L3 The Loop 27.56311 114.44221 141 21.7 L4 The Loop 27.55269 114.43271 118 29.7 RB1 Red Bluff 27.74453 114.1426 14 39.2 RB2 Red Bluff 27.7464 114.13823 37 28 RB3 Red Bluff 27.74897 114.13799 11 34.5 PA1 Pot Alley 27.76024 114.13371 13 41 PA2 Pot Alley 27.76099 114.13299 7 38 PA3 Pot Alley 27.76381 114.13268 8 37.5 RV1 Rainbow Valley 27.75502 114.13493 14 34.5 RV2 Rainbow Valley 27.75775 114.1343 13 46 MR1 Mushroom Rock 27.75295 114.13567 12 15.75 MR2 Mushroom Rock 27.75238 114.13708 12 17.5 EG1 Eagle Gorge 27.76556 114.1314 14 39 EG2 Eagle Gorge 27.7664 114.13044 8 27.5 EG3 Eagle Gorge 27.76788 114.12827 9 31.3

Table 3.1: Information for log measurements obtained in the field, Location map Fig.2.8.

3.1.2 Sampling A permit to take samples from the national park was obtained although it was limited to 100 5 cm x 5 cm samples. Samples had to be limited to representative facies in each log. Samples could not be taken in view of tourists, which limited where samples could be taken as we worked within the tourist outlook areas. Samples were of sufficient size to carry out petrographic analysis and to take QEMSCAN plugs. Each sample has an identity number, log number and location (Table 3.2). Samples and thin sections were

used in detailing the different grain size, sorting and composition of each facies in facies analysis.

sample Location Log Height Facies FA Qemscan number no. (m) or Thin section GBLN01 Loop L1 1.2 LAXB 2B TS GBLN02 Loop L1 3.7 BS 2A TS GBLN03 Loop L1 4.3 RL 2A TS GBLN04 Loop L1 8.2 TXB 2A TS GBLN05 Loop L1 19.8 TXB 1A TS GBLN06 Loop L1 25.6 LAXB 2B TS GBLN07 Loop L1 37.8 TXB 1A TS GBGS08 Gauging Station GS 3.6 TXB 1A TS GBGS09 Gauging Station GS 3.8 PL 1B TS GBGS10 Gauging Station GS 7.3 TXB 1A Q GBGS11 Gauging Station GS 8 TXB 1A TS GBGS12 Gauging Station GS 14.3 TXB 1A TS GBRG13 Ross Graham RG1 8.4 TXB 1A TS GBRG14 Ross Graham RG1 10 RL 2A TS GBHH16 Hawks Head HH1 12.8 TXB 1A TS GBHH17 Hawks Head HH1 13.4 RL 2A TS GBRB18 Red Bluff RB1 6.1 TXB 1A GBRB19 Red Bluff RB1 6.6 TXB 1A TS GBRB20 Red Bluff RB1 21.8 PL 1B GBRB20 Red Bluff RB1 35.8 TXB 1A TS GBRB21 Red Bluff RB1 37.6 PL 1B TS GBRVN21 Rainbow Valley RB1 33.8 TXB 1C TS GBLN25 Loop L1 91.8 LAXB 2B GBLN26 Loop L3 94.3 TXB 1A TS & Q GBLD26 Loop L3 0.8 RL 2A TS GBLD27 Loop L3 3.6 TXB 1A TS GBLD28 Loop L3 4.8 LAXB 2A TS GBLD29 Loop L3 9.9 LAXB 2A TS GBLD30 Loop L3 21.4 TXB 1A TS GBLW31 Loop L4 2.8 RL 2A TS GBLW32 Loop L4 3.5 LAXB 2A TS GBLW33 Loop L4 4.4 RL 2A TS GBLW34 Loop L4 4.8 TXB 1A TS GBLW35 Loop L4 6.9 TXB 2B TS GBLW36 Loop L4 29.6 TXB 1A TS GBPA1-37 Pot Alley PA1 6.8 TXB 1A TS GBEGN38 Eagle Gorge EG1 13.7 TXB 1A TS GBEGN39 Eagle Gorge EG1 14.8 TXB 1A TS

GBEGS40 Eagle Gorge EG2 23.2 TXL 1B TS GBRG50 Ross Graham RG3 0.2 PL 1A GBRG51 Ross Graham RG3 3 LAXB 2B GBRG52 Ross Graham RG3 3.6 TXB 2B Q GBRG53 Ross Graham RG3 5.4 BS 2A Q GBRG54 Ross Graham RG3 11.3 TXB 1A GBRG55 Ross Graham RG3 19.6 TXB 1A GBRG56 Ross Graham RG3 19.9 MS 1A GBRG57 Ross Graham RG3 25.8 TXL 1A GBRG58 Ross Graham RG3 26.2 TXB 1A GBFW59 Fourways FW1 1.7 TXB 1A GBFW60 Fourways FW1 5 TXB 1A Q GBFW61 Fourways FW1 7.2 PL 1A GBFW62 Fourways FW1 8.6 PL 1A Q GBFW63 Fourways FW1 10 CONTORTED 1A GBFW63 Fourways FW1 13 PXB 2B GBFW64 Fourways FW1 13.8 TXB 1A GBFW64 Fourways FW1 17.4 TXB 1A GBFW2-67 Fourways FW2 7.2 TXB 1A GBFW268 Fourways FW2 17 TXB 1A GBFW269 Fourways FW2 30 LAXB 2B GBFW270 Fourways FW2 32 TXB 1A GBFW271 Fourways FW2 34.1 RL 2A GBFW272 Fourways FW2 36.4 RL 2A GBFW273 Fourways FW2 41.2 BS 2A GBFW274 Fourways FW2 47 TXB 1A GBFW275 Fourways FW2 48.6 TXB 1A GBFW276 Fourways FW2 54.6 LAXB 2B GBFW277 Fourways FW2 56.6 TXB 1A GBRG278 Ross Graham RG2 14.7 LAXB 2B Q GBRG279 Ross Graham RG2 16 TXB 1A GBRG280 Ross Graham RG2 19.9 PL 1A GBRG281 Ross Graham RG2 24.8 TXB 1A GBRG482 Ross Graham RG4 29.8 TXB 1A GBRG483 Ross Graham RG4 35.8 RL 2A Q GBHH0284 Hawks Head HH2 2 BS 2A GBHH0285 Hawks Head HH2 2.6 BS 2A GBHH0286 Hawks Head HH2 4 LAXB 2B GBHH0387 Hawks Head HH3 1.2 RL 2A GBHH0388 Hawks Head HH3 1.5 BS 2A GBHH0389 Hawks Head HH3 3 BS 2A GBL290 Loop L2 0.9 MOTTLED 2A

Table 3.2: Location, log number, height, facies, interpretation and analysis carried out on each sample collected during the study. GBLN01-01 – Ginny Bradley, Log code, log number, sample number. BS – Bioturbated Sandstone, TXB – Trough Cross-Bedded Sandstone, RL – Ripple

Laminated Sandstone, LAXB – Low Angle Cross-Bedded Sandstone, PL – Planar Laminated Sandstone, PXB – Planar Cross-Bedded Sandstone, MS – Massive Sandstone. TS – Thin Section, Q – QEMSCAN.

3.1.3 Photogrammetry Outcrop studies are generally qualitative, such as 1D sedimentary logging of facies and basic outcrop measurements. Most outcrops are a 2D section and do not represent the 3D nature of depositional systems, and accurate reconstruction of channel sinuosity and geobody connectivity in 3D may be hindered (Pringle et al. 2006). Semi- quantitative analysis of outcrops comes with measuring of elements in the field and recording photo-panels. Photogrammetric techniques to construct photopanels of outcrops have been common practice for several decades. If taking photos from a fixed point by hand, issues including lens distortion, low resolution and access to remote cliff faces, which can be difficult creates oblique angles and data holes (Pringle et al. 2006). More recently, use of high-resolution spatial data has allowed fully quantitative analysis of a 3D point cloud, such as accurate measurements of the size and geometry of channels (Bemis et al. 2014). This rapidly developed with the use of lidar scanning equipment (Pringle et al. 2006; Hodgetts 2013) and UAV (Unmanned Aerial Vehicle) technology. Digital outcrop models give detailed data on the lateral variability of facies and geometries of geobodies. Up-scaled data enables a correlation to be made more accurately in 3D rather than between 1D well logs (Hodgetts 2013). The quantification and distribution of barriers to flow can be mapped, for example faults and cemented beds, to gain information geometry and how they will impede flow of hydrocarbons or water (Pringle et al. 2004; Buckley et al. 2006, 2010; Hodgetts 2009). This project has integrated traditional field sedimentology with interpretation of high-resolution 3D digital outcrop data obtained via UAV technology. A DJI Phantom 4® drone was piloted from a remote-control handset attached to a large screen tablet (Fig. 3.2). The UAV was piloted around the river gorge outcrops in a horizontal zig-zag pattern, this minimised battery usage flying up and down, at a horizontal speed of around 2 m/s and a distance of 10 m away from the outcrop. The camera was set to take a photo every 5 seconds to ensure sufficient overlap was obtained to later reconstruct the outcrop. Each flight was limited to 25 minutes due to the battery life, we used 5 batteries per flying session, however, to make drone campaigns more efficient, additional batteries

would be used, perhaps a total of 10. This would mean that one outcrop could have been completed in one day rather than returning to the same outcrop for several days. When back at base after the field day a low-resolution stitch was generated in Agisoft Photoscan® to assess if coverage was sufficient and there would be no data holes. If data gaps occurred, then the next day we could re-fly that area. Previous studies have applied the use of UAV based photogrammetry to assess the structural geology and identify unstable zones in a quarry (Salvini et al. 2015). Other studies have used UAV photogrammetry to reconstruct archaeological and cultural heritage sites and create digital surface models in 3D (Verhoeven 2011; Sona et al. 2014; Themistocleous et al. 2015). More recently the benefits of UAV technology are being seen and there has been an increased use within the field of geology. During this study the use of UAV drone technology has allowed accurate documentation of the geometries of geobodies from outcrops of the Tumblagooda Sandstone. Digital outcrop analysis is more precise than traditional field methods and reconstructs data accurately in three-dimensions, in a georeferenced real time position. It also allows for data to be collected in remote inaccessible locations, for example cliff faces. This allows for surfaces to be traced over a distance and give strike, dip and other orientation information (Xu et al. 2000). UAVs are cost effective and reduce time required collecting data in the field, they can also access areas that humans cannot in a non-invasive manner and document vast areas very quickly (Themistocleous et al. 2015). The lightweight compact nature of a UAV means that they are easily carried into the field, however, flight times are limited to roughly 20 minutes per battery and so several batteries will be needed for large survey areas. A near constant flight distance from the target object gives optimum results, and flight planning is essential to avoid data holes and maximise coverage of the target (Burns et al. 2015). Outcrop studies have been used in the study of petroleum reservoirs for many decades, Bryant et al. (2000) was the first to document how 3D digital outcrop geology could be applied to the petroleum industry by describing how modern 3D models could be used to analyse and control the modelling of internal architectures and geometries in the subsurface. Existing subsurface hydrocarbon reservoirs are typically modelled within a software package (e.g Petrel®), bringing together all aspects of data and

information from the field, such as information on the types of rocks, porosity and permeability and quantitative geometrical data derived from seismic surveys and well logs (Pringle et al. 2006; Buckley et al. 2006). Well data does not provide information on 3D geometries; therefore, outcrop studies are essential to tie the observations of the lateral variations and heterogeneities of depositional environments made in the field with subsurface geological data (Xu et al. 2000; Pringle et al. 2006; Hodgetts 2009). Over the recent couple of decades, lidar scanning of outcrops has become common practice in sedimentary studies and the technique was discussed for use in this project, however, the location of the outcrops and the topography inhibited the use of this method. UAV technology allowed access to hard-to-reach steep cliffs and more rapid data collection. While in the field for the first field season the Murchison River was always in flood and access to the other side of the river was not possible. Most of the outcrop exposure is on the accessible side of the river and lidar scanning would require access to the other river bank. The only way to access the other bank is to cross the river as there is no road access. The opposite river bank is a low-level sand bank; to generate a complete lidar scan access to higher levels would be needed in order to not have large data holes due to oblique scanning angles. The river gorges are also steep, while they are terraced, there is scree and boulders making carrying heavy lidar equipment dangerous. These conditions inhibited the initial idea of obtaining lidar data during the next campaign. It was concluded that taking a UAV to document the geometries of the sandstones was a more rapid and practical way of collecting data.

Figure 3.2: A DJI Phantom 4 UAV drone, piloted from a large screen tablet. Photos were being taken of the outcrop for use in reconstructing a digital outcrop model.

3.2 Data processing and analysis

3.2.1 Facies Analysis Analysis of facies was carried out on the log data and eleven facies were identified. These were systematically described based on the following properties: • Grain properties; sorting, size and sphericity • Sedimentary structures • Bed thickness and lateral continuity • Set thickness (if applicable) • Ichnofacies present • Occurrence These facies were then interpreted based on processes that formed the described features. Facies were grouped together and interpreted based on stacking patterns, architectural elements and geometry as facies associations which reflect parts of the depositional environment. After facies associations were determined a palaeoenvironmental model was drawn to graphically represent the environment of deposition (Miall 1980; 1985). These facies and facies associations are discussed in chapter 4 of this thesis. Palaeocurrent data was plotted using Stereo32® software. Palaeocurrent roses were plotted in 10° bins to graphically show trough cross-axis, ripple migration and primary current lineation directions.

3.2.2 Petrography A total of 36 samples were thin sectioned and mineral analysis was carried out. Point counting was carried out using PETROG software on the thin section samples. 300 counts were taken of each sample; this detailed the composition of a grain, major and minor axis length, Wentworth and phi classification, roundness and sphericity (Fig. 3.3) and grain boundary contact properties (Fig. 3.4). This data was used in differentiating facies.

Figure 3.3: Powers (1953) scale for high and low sphericity, very angular, angular, sub-angular, sub-rounded, rounded, well rounded grains.

Figure 3.4: Pictorial representations of the grain boundary contacts.

Eight samples, representative of each facies (detailed in table 3.2), were analysed using a QEMSCAN scanning electron microscope (SEM) instrument. This provides mineralogy, porosity, rock texture and calculates rock properties, by automatically collecting data from back scattered electron and x-ray diffraction (Rocktype LTD 2018). This method is said to remove biased data collected during point counting and mineral identification is not based on human observation (Allen et al. 2012). This project uses the QEMSCAN map to assess grain size, sorting and visual porosity and compares it with the point counting data. The data was used in characterising the differences between each facies in facies analysis to determine grain size, sorting and composition. Each sample was scanned at a resolution of 5 µm, 10 µm and 20 µm between each data point. Data analysis of grain size statistics and sample mineralogy was carried out on the most detailed dataset (5 µm) for each sample. For each sample, the tiles had to be stitched together using the “field stitch option” to align the tiles created during the scanning processes. When the field was correctly aligned the granulator was applied to remove the cement and only display the grains present (Fig. 3.5A). The touching particle

process was applied to separate grains (Fig. 3.5B), this was not always effective due to the grains being over-compacted or cemented with quartz overgrowths. In this case, the grains had to be separated manually (Fig. 3.5C and D). This data was used to plot the amount of grains in the sample for each grain size based on the Wentworth grain size scale. This technique proved difficult for samples that were heavily cemented with quartz overgrowths as this meant grain boundaries were sometimes not resolvable, this could give a bias towards larger grain sizes such as coarse sand rather than fine sand. Equally the software identified small clusters of isolated pixels as silt sized grains of quartz when they may have been errors in the original data or patches of remnant quartz grains giving a larger proportion of silt sized grains within each sample. To combat this, a minimum grain size of 5 pixels was given to filter the data which filtered out erroneous mud sized particles. Observed porosity measurements were averaged based on pixel properties for each of the three datasets for each sample. The amount of cement in each sample was incorporated into the background measurement and counted as “pore space”. This left just the grains of quartz and feldspar. This gave statistics on observed porosity relative to each sample.

Figure 3.5: Screenshots taken from iExplorer. A) Sample with granulator applied, just showing the grains within the sample. In this case the pink is quartz and the dominant component of the sample. B) Is with the touching grained process applied. The grains which are cemented together cannot always be split by the processor so has to be done manually. C) This is the grain cluster from the bottom of b. Manual separation of grains had to be carried out. D) is the result of the manual and automatic separation of grains. The software then uses the amount of pixels each grain has in order to plot data on grain size.

3.2.3 Digital outcrop modelling Agisoft Photoscan® is a software package that performs photogrammetric processing of images and generates a 3D model (Agisoft LLC 2017). The software uses structure from motion (SFM) methods to automatically generate three-dimensional point clouds from photos and overlies a photo-realistic texture (Verhoeven 2011; Agisoft LLC 2017). SFM accurately reconstructs a scene in 3D from a series of overlapping 2D images taken from a camera moving around an object by detecting feature points and monitoring the movement of those points throughout the images (Verhoeven 2011; Burns et al. 2015). Photoscan can work with ordered and unordered images, varying angles of acquisition and even videos (Verhoven 2011; Agisoft LLC 2017). The software generates highly accurate polygonal models which are highly detailed and georeferenced (Themistocleous et al. 2015; Agisoft LLC 2017). The photos are loaded into the software and an alignment is carried out, where SFM generates a sparse 3D point cloud (Verhoven 2011). Secondly a dense cloud is generated by using multiview reconstruction. Photoscan removes lens distortion and using the high-quality settings produces a more detailed model to create a pseudo 3D photo-realistic reconstruction. The quality of the subsequent 3D model is reliant on the quality of the primary photographic data collected. To quality check the model, after generation of the dense cloud (Fig. 3.6a) any erroneous points can be removed, this includes any water and sky that the software has picked up. If areas of the model are left which do not add data to the model, then this will unnecessarily add to the model size and reduce the detail within the texturing phase. A methodology paper has been co-authored during this study to document good practice for use of UAVs in geosciences (appendix 2). The software automatically creates a sparse and dense point cloud (Fig. 3.6A), a triangulated mesh and a photorealistic textured model (Fig. 3.6B). Initially when starting

the process of making the 3D models, the software that the meshes would be annotated in (VRGS) could only import one 4096 pixel texture. This was a problem as it meant that there was little detail in the models and the mesh was more detailed. VRGS was reprogrammed to allow for more textures to be imported, this meant more detail could be put into the textures. Each point cloud was divided into two or three chunks and the triangulation and texturing was done on the separate chunks. Each chunk had 16 x 4096 textures applied; this was the limit of the processing power of the computer as 32 textures caused the computer to crash. With 16 texture tiles a large amount of detail can be seen, for example cross-bedding and individual bedding planes. As the photos taken by the UAV had GPS co-ordinates tagged to them the models were in correct space and the exported model could be exported with UTM co-ordinates (only a feature available in the professional version), as a .ply file.

Figure 3.6: A) an example dense point cloud constructed in Agisoft Photoscan® from UAV photos. B) The point cloud with a mesh and texture applied to give a photorealistic model.

3.2.4 Annotating models and obtaining channel size statistics Understanding geobody dimensions is important conditioning data for reservoir modelling, Gibling (2006) presents a synthesis of outcrop data and Blum et al. (2013) presents a database of width and depth data for Quaternary river systems. This data is

then applied to object models to add dimensions to reservoir models. This data is not representative of channels that were formed in the absence of plants as many sections have been described as distinctly different to post-vegetation river systems. This study provides a detailed database of geostatistical data for width and depth information for pre-vegetation channels that can be applied to real subsurface pre-vegetation paralic reservoirs. VRGS is a 3D digital outcrop visualisation and interpretation tool that allows the user to interpret geobodies, faults, bedding and other geological features. It integrates field measurements such as logs, palaeocurrent data and structural readings (Hodgetts 2017). Once the .ply file is imported into VRGS, digitising can occur. Firstly, polylines were digitised to highlight key bedding planes and horizons, erosion features and smaller scale features such as accretion foresets (Fig. 3.7A). This study focused on the geometry of the fluvial bodies within the outcrop and once they were mapped out with polylines geobodies could be traced out (Fig. 3.7B). The average palaeocurrent rearing of 303° was applied to triangulate and accurately calculate the corrected geobody width. Statistics for the geobodies was exported and used as conditioning data for stochastic modelling. To test if the 40° variation observed in palaeocurrent readings would significantly affect the corrected width of geobodies, values of 300 and 340 were also applied and exported.

3.2.5 Stochastic modelling A subsurface reservoir model is a computer-based representation which integrates geological features such as geometries, facies and petrophysical parameters integrated with stochastic modelling techniques to populate the area between wells to predict spatial distribution (Pringle et al. 2006; Keogh et al. 2007). Conditioning data may be 1D well or outcrop log, 2D photo-panels, 2D/3D seismic interpretations, conceptual models and outcrop analogues. The integration of this data is key for the accuracy of the reservoir model (Pringle et al.2006; Pyrcz & Deutsch 2014). Data that is input into the model is typically highly detailed and so requires upscaling, to improve computational efficiency by sacrificing the detail (Pyrcz & Deutsch 2014; Ringrose & Bentley 2015). The models produced can go on to be used for visualisation of the reservoir, estimations of volume, simulation for fluid flow, well planning, seismic modelling, modelling for

enhanced oil recovery and CO2 storage (Ringrose & Bentley 2015). Models do not give true facies proportions and a quality control must be carried out in order to assess if the model data matches the input control data. It will never be known if the predicted geometries are correct, but application of a conceptual model is key to refining the model and producing a suite of possible outcomes (Ringrose & Bentley 2015). The implementation of conceptual models is important here to quality control the stochastic models. Fluvial deposits are highly variable in geometry and shape, internal fill and connectivity (Corbett et al. 2012) and so the integration of well data with outcrop geology data such as aspect ratio, stacking patterns and facies information is a key way of defining what type of reservoir modelling technique to use (Keijzer & Kortekaas 1990; Meilng et al. 1990; Corbett et al. 1998). An estimation of geobody dimensions and interconnectivity is often used to determine well spacing, production strategy and rate as well as to calculate the estimated recoverable reserves within the reservoir (Zheng et al. 1996; Corbett et al. 1998). Therefore, understanding the lateral extent of facies and geobodies (channels and sheets) is important, whether they are amalgamated and extensive or if they are isolated or laterally constrained.

3.2.5.1 Method Fluvial systems are commonly modelled using an object-based modelling approach, where a series of probability distribution functions are used to describe the geometry of geobodies (Hodgetts 2013). Channel width, thickness, amplitude, wavelength and flow direction are input into the model, with values taken directly in the field or from digital outcrop models. Although some subsurface data may be present in high-resolution seismic, with thickness from well data, spatial data analysis relies upon outcrop analogues. Most outcrops are only a 2D slice through the channel and therefore only record an apparent width of any channel, although true widths can be calculated using trigonometry if the palaeoflow direction is known (Hodgetts 2013). This approach is commonly known as plane projection method, where a plane is projected onto a flat surface along the direction of flow, giving corrected width estimations (Hodgetts 2013).

Mid-channel bars can also be modelled using an ellipse to represent the architectural element. Sequential simulations distribute facies proportions using a pixel-based approach based on the input data (Journel et al. 1998; Hodgetts 2013; Ringrose & Bentley 2015). Therefore, if the distribution is continuous, i.e a sheet like fluvial system, it may be more beneficial to use this method (Ringrose & Bentley 2015). The main limitation of this method is that architectures are not defined, which is useful when elements do not have geometries. Closely spaced wells are better modelled with this approach as the high density of data gives accurate facies proportions (Ringrose & Bentley 2015). Integrating field and digital outcrop data enabled a model to be proposed for the environment of deposition and a block diagram depicting this was constructed. This assisted with the next process of applying the data to model the subsurface. In Petrel® each digital outcrop model was imported separately as an ascii petrel points and attributes file from VRGS®. This is the mesh detailing the geobody facies identified. Modelling each of the five outcrops enabled the prediction of lateral and vertical variability within the system. A grid with zones was produced to model all or part of the digital outcrop. This study implements both the object and simulator methods to compare the accuracy of one method to the other for representing pe-vegetation fluvial successions. Channels and bars were modelled as objects using data from the geobody statistics to condition the models. After this, channels were modelled using the sequential simulation approach to compare the difference between modelling as defined channels opposed to a sheet geometry. 3.3 Limitations Field based facies analysis is generally qualitative and rarely quantitative, although numerical information can be included, such as bed dimensions and palaeocurrent direction. Measurements made in the field are often subject to human error and measuring outcrops that are terraced and not vertical can be difficult. Creating photopanels from pictures acquired in the field also does not give accurate reconstructions due to lens distortion and lack of georeferencing.

Digital outcrop models provide accurate width and depth values for channels which is used in conditioning data for stochastic modelling. The digital outcrop models rarely provide true sinuosity data, this has to be obtained in the field from palaeocurrent data as an aerial view in outcrop is rare. Computing power is the main limitation of many digital processes today, for example: the amount of textures that could be used in the outcrop model and also the size of the models produced in Petrel® were limited due to computing power. Agisoft Photoscan® is an excellent tool for rapid generation of 3D models, however the stitching process is not always accurate. Occasionally there are images that are not correctly tied and as a result the model could have erroneous points which require manual removal or several iterations in order remove these errors.

Figure 3.7: A) An example of the polyline interpretation in VRGS. B) An example of the geobody interpretation, each geobody is individually coloured in this view.

VRGS is a continual work in progress and during this study the software has developed hugely and rapidly. This study has required the use of multiple textures to

give high resolution and detail to the digital outcrop models to identify true geobody architecture. Due to initial low-resolution models, prior to the development of multi- texture compatibility inserting outcrop logs was an option to be able to give high resolution centimetre scale resolution to the models and to ground truth them. Geobody manipulation was an issue as very often nodes would attach to trees or other surfaces when annotating the model therefore a method of editing the geobodies was required. To create realistic figures which allowed the visualisation of the outcrop and the geobody, transparency and polyline visualisation was integrated into the software. To be able to export the required statistics for analysis of aspect ratio and z position, exporting of the geobody statistics had to be integrated as initially statistics were exporting with infinite values rather than the realistic outcrop values.

Chapter 4: The applicability of modern tidal analogues to pre-land plant paralic depositional models.

Bradley G-M.1*, Redfern J.1, Hodgetts, D.1 George A. D.2, Wach, G.D3

1University of Manchester, School of Earth and Environmental Sciences, Manchester, M13 9PL, UK *[email protected] 2University of Western Australia, School of Earth Sciences, Perth WA 6009 3Dalhousie University, Department of Earth Sciences, Halifax, NS, B3H 4R2, Canada

Published in Sedimentology 2018 - Article DOI: 10.1111/sed.12461

Abstract In modern siliciclastic environments terrestrial and aquatic vegetation binds substrate, controls weathering and erosion rates, and influences run-off, sediment supply and subsequent depositional architecture. This study assesses the applicability of modern depositional models that are impacted by vascular vegetation, as analogues for ancient pre-land plant systems. A review of pre-Devonian published literature demonstrates a paucity of described tidal successions; this is possibly due to the application of modern analogues for interpreting the record where there is a lack of tidal indicators. This paucity suggests a need for revised models of tidal deposition that consider the different environmental conditions prior to land plant evolution. This study examines the Ordovician–Silurian Tumblagooda Sandstone, which is exposed in the gorge of the Murchison River and coastal cliffs near Kalbarri, Western Australia. The Tumblagooda Sandstone comprises stacked sand-rich facies, with well-preserved bedforms and trace fossils. Previous interpretations of the depositional setting have proposed a mixed sheet-braided fluvial and intertidal flats or a continental setting dominated by fluvial and aeolian processes. An enigmatic element is the rarity of mud- rich facies preserved in the succession. Outcrop logging, facies and petrographic analysis record dominantly shallow water conditions with episodes of emergence. Abundant ichnotaxa indicate marine conditions and bi-directional flow structures are evidence for an intertidal and subtidal depositional environment. A macrotidal estuary setting is proposed, with evidence for tidal channels and repeated fluvial incursions. Physical and biogenic sedimentary structures are indicative of tidal conditions. The lack of clay and silt resulted in the absence of flaser or lenticular bedding. Instead cyclic deposition of thin beds and foreset bioturbation replaced mud drape deposits. Higher energy conditions prevailed in the absence of the binding activity of plants in the terrestrial and marine realm. This is suggestive of different weathering processes and a reduction in the preservation of some sedimentary features.

4.1 Introduction Terrestrial and aquatic vegetation has a significant effect on depositional processes and resulting characteristics of siliciclastic sediments in modern environments. Vegetation influences weathering and erosion rates, run-off and sediment supply, binds substrate and thus plays a role in the subsequent depositional architecture and sedimentary structures. It is well documented that the evolution of vascular plants had a significant effect on the Earth’s surface process and weathering style (e.g. Davies & Gibling 2010). An increase in physical and chemical weathering following land plant evolution in the mid Ordovician led to the generation and preservation of significant volumes of terrestrial mud, retained by plant roots (Schumm 1968; Cotter 1978; Gibling & Davies 2012). This mud may subsequently be transported by fluvial systems into the shallow and ultimately deep marine environment. The vegetated floodplain also has a control on the style and energy of modern fluvial systems, binding the channel bank and allowing the development of low-energy meandering and anastomosing rivers (Davies & Gibling 2010). It is critical to understand the detailed sedimentology of pre-vegetation sedimentary systems for petroleum systems analysis, reservoir prediction and understanding facies variations. An estimated 16,000 Million Barrels of Oil Equivalent (MMBOE) and 16 trillion cubic feet (tcf) are reported to be contained within lower Palaeozoic paralic reservoirs globally (Havord 1993; Boote et al. 1998; Millson et al. 2008; Tamar-Agha 2009). This demonstrates the importance of correct environmental interpretation and facies recognition in the absence of typical tidal indicators. Published literature highlights the importance of common texturally and compositionally mature fluvial and marine sandstones, which were deposited prior to the evolution of vascular land plants (e.g. Long 1978; Fedo & Cooper 1990; Eriksson et al. 1995). Cotter (1978) first noted that pre-Devonian fluvial sandstones were dominated by stacked sheet-braided sand bodies; laterally continuous units with planar basal erosional surfaces. This was attributed to frequent channel avulsion due to the lack of levée or over-bank deposits stabilised by plant roots. Such fluvial architecture is only observed in modern systems that have extreme climates non-conducive to vegetation.

Prior to the colonisation of land by vascular plants, the exposed continental surface was prone to increased physical weathering, resulting in more energetic fluvial systems (Schumm 1968; Dalrymple et al. 1985; MacNaughton et al. 1997; Long 1978; 2002, 2004; Gibling & Davies 2012). The different physicochemical processes active on the eroding land surface resulted in a reduced volume of clay generated, which also would have had a limited residence time in paralic environments due to the dominance of high-energy fluvial systems which frequently avulsed to rework any mud-grade sediment that was deposited. Any clay transported by high-energy fluvial systems entering the marine realm would typically bypass the littoral zone and be carried into the deep marine basin (Long 1978, Fedo & Cooper 1990, Simpson & Eriksson 1990; Eriksson et al. 1995). In addition to affecting the generation of sedimentary structures, the reduced clay content also affects the identification of specific palaeoenvironmental ichnospecies. This is precluded by limited preserved mudstone or heterolithic muddy facies, therefore, a limited abundance of ichnospecies has been identified in lower Palaeozoic published reports. A comprehensive literature review has revealed limited recordings of tidal environments globally. Sedimentary structures containing mud derived from continental weathering, for example over-bank fines, mud drapes, lenticular bedding and mud-lined burrows, are rarely documented in lower Palaeozoic sections. It is suggested that the lack of diagnostic sedimentary structures has resulted in an under- recognition of tidal conditions in the ancient rock record. Interpretation of lower Palaeozoic systems using depositional models and facies associations developed from modern analogues may be erroneous, because younger systems are influenced by plant activity. Lower Palaeozoic shallow marine facies are described as mature sandstones which commonly have high quartz content. They are interpreted as being deposited as high energy sedimentary structures which lack mud or typical trace fossil assemblages. These sandstones may have erroneously been interpreted as continental or arid aeolian environments but contain subtle features suggestive of tidal deposition that have been previously overlooked. The Ordovician–Silurian Tumblagooda Sandstone of the Southern Carnarvon Basin, Western Australia (Fig. 4.1), has been the subject of several sedimentological

studies, but remains enigmatic with conflicting interpretations of its depositional setting. The earliest detailed work was carried out by Hocking (1981; 1991), who interpreted intervals of trough cross-bedding overlain by ripple cross-laminated sandsheets and heavily bioturbated units as recording a transition from fluvial to intertidal conditions. Subsequently Trewin (1993A) and McNamara & Trewin (1993) re- interpreted some of the sedimentary features and overall facies patterns as an aeolian dominated continental environment. This latter interpretation indicated a continental origin for the abundant, exceptionally preserved arthropod trackways and Heimdallia burrows; this was significant because this was the earliest reported colonisation of land. The facies and faunal relationships described by Trewin (1993A, B) and McNamara & Trewin (1993) have been used as an analogue for similar interpretations of the earliest continental colonisations at this time (Wizevich 1996; Retallack 2001; Hagadorn et al. 2011). The findings of this study have corroborated much of these earlier works, but significantly it is suggested here that the environment of deposition was more tidally influenced. Recognising the unique assemblage of sedimentary facies that reflects the different physicochemical conditions active in the pre-land plant world suggests that the trace fossil assemblages probably do not record the earliest land colonisation. This study presents new sedimentological data from the outcrop sections of the Tumblagooda Sandstone. The new interpretation of the depositional processes and environments are based on an assessment that takes into consideration the impact that pre-vegetative conditions had on lithology, facies and sedimentary structures. This study analyses the critical controls on depositional style and the implications for interpretations of similar sedimentary packages. 4.2 Geological setting The Tumblagooda Sandstone was deposited in the Southern Carnarvon Basin which, at the time of deposition, was a shallow intracratonic basin on the margin of eastern Gondwana (Li & Powell 2001; Veevers et al. 2004) with emerged continental plains bordering a broad epicontinental sea (Eyles 1993). The age of the unit is not precisely defined, palaeomagnetic studies suggest that deposition occurred between the late Ordovician and early Silurian (Schmidt & Hamilton 1990; Schmidt & Embleton 1990). Previous studies of ichnotaxa initially suggested a similarity to the Devonian

Taylor Group (Trewin & McNamara 1994). Recent work by Markwitz et al. (2017) on detrital zircon ages also supports a middle Ordovician age for deposition of 466 ± 8 Ma, placing it firmly in the time prior to the evolution of rooted land plants and after the onset of evolution of the first embryophytes (terrestrial vegetation).

Figure 4.1: Map of the surface exposure of the Tumblagooda Sandstone within Kalbarri National Park highlighting the studied locations (modified from Hocking and Mory 2006; McNamara 2014). Distribution of the four facies associations (stratigraphic units) identified by Hocking (1991) also highlighted.

The Tumblagooda Sandstone is up to 2000 m thick and forms the lowest unit in the Kalbarri Group which unconformably overlies gneissic basement and records deposition during Early Palaeozoic rifting in East Gondwana (Markwitz et al. 2017). The formation is dominated by compositionally mature quartz arenite and subarkose sandstones (Kettanah et al. 2015) with abundant trough and tabular low-angle cross- bedding, planar lamination and well-preserved bioturbated intervals (Hocking 1991). The study area contains a composite section that is 1300 m thick and is well-exposed in the lower parts of the Murchison River in Kalbarri National Park and around the coastal cliffs south of Kalbarri (Fig. 4.1) (Hocking & Mory 2006). Other outcrops can be found in

the Hutt River to the south of Kalbarri and to the east at Pencell Pool (Kettanah et al. 2015). The Tumblagooda Sandstone dips gently (around 5°) to the north-west, therefore dips indicate up-stratal progression to the northwest. The fault at Fourways (Fig. 4.1) has no visible throw and the fault in the north of the study area shows around 50m of throw. They have minimal effect on stratigraphic distribution. At the time of deposition palaeoslope is suggested to have been to the northwest which is interpreted from fluvial palaeocurrents which flow north-westerly offshore (Hocking 1991). 4.3 Previous work

4.3.1 A mixed marine to non-marine interpretation Hocking (1981; 1991) defined five facies associations (FA1 to FA5) and interpreted the whole stratigraphic succession as comprising two fluvial to marine cycles. Hocking (1991) identified facies associations which are actually stratigraphic units as the central river section preserves both FA1 and FA2. The fluvial facies (FA1 and FA3) were described as having a sheet-braided architecture (a term initially introduced by Cotter, 1978) based on the lateral persistence of the fluvial beds with little to no development of channel forms resulting in planar basal erosion surfaces. Facies Associations 2 and 4 were interpreted to have been deposited within an intertidal environment with intermittent exposure (Fig. 4.2A). Evidence for exposure included adhesion structures, blown-out and eroded ripple marks (Hocking 1991). Herringbone cross-stratification and rare mud-draped structures were also reported, interpreted to record tidal currents. Symmetrical ripple marks were interpreted as a product of wave- formed currents produced by wind blowing over a sand sheet in shallow water. Thick intervals of weakly bioturbated sandstones were interpreted as constant reworking of tidal flat sands between channels (Hocking 1991). The more intensely bioturbated units were interpreted as the result of slow or discontinuous deposition within the upper intertidal zone. Well-preserved, steep-sided track imprints were made either subaerially on a damp, temporally exposed surface or subaqueously and later exposed. Deposition was infrequent, fluvial progradation was common, and ephemeral shallow channels drained the tidal flats. Stratigraphically higher in the section at The Loop (Fig. 4.1), the sandstones were interpreted as dominantly lower intertidal deposits with little fluvial influx, extensive

reworking under conditions of low sediment supply, and few run-off channels scouring the flats. Hawks Head and Ross Graham sections (Fig. 4.1) are dominated by bioturbated sandstone interpreted as deposition in the upper intertidal zone (Hocking 1991).

4.3.2 Non-marine interpretation Trewin (1993a) proposed an alternative interpretation, that FA2 was deposited within a fluvial overbank environment comprising rippled sandsheets (Fig. 4.2B). Inactive areas of the braid-plain were suggested to act as the sediment source for interbedded aeolian sandsheets. The lack of large-scale aeolian dune bedforms was interpreted to be due to short transport distances and limited deposition time. Straight crested dunes were interpreted to have migrated to the south-east within the fluvial overbank areas. Low-angle dune foresets with inversely graded avalanche (pin-striped) cross-laminae with wind-formed ripple cross-lamination at the toesets were described. In this current study, such features have not been identified. Trewin (1993b) recognised deflation surfaces with adhesion structures and blown out ripples. Fluvial channels were interpreted to control dune orientation and channels were typically deflected down interdune corridors. Wave ripples were interpreted to have formed during times of flooding. Organisms were interpreted to have colonised flooded interdune ponds, because bioturbation often terminates at the base of dune toesets. Arthropod trackways occur on rippled surfaces and foreset laminae, which was interpreted as organisms traversing the overbank areas to feed on organisms that colonised the ponds (Trewin & McNamara 1994).

Figure 4.2: A) shows Hocking’s (1991) intertidal environmental interpretation for FA2. B) Shows Trewin’s (1993b) aeolian dominated overbank interpretation for FA2.

4.4 Methodology This study interprets new sedimentological and reinterprets the palaeoenvironmental setting of the enigmatic FA2 of the Tumblagooda Sandstone (Hocking 1991; Trewin 1993A). Hocking (1991) interpreted the central river section as all FA2 (Fig 4.1) however, the rocks exposed here preserve both FA1 and FA2, therefore this study reinterprets the facies associations identified in the succession. High- resolution (centimetre-scale) sedimentological logging of 30 outcrop sections was undertaken in Kalbarri National Park (Fig. 4.1). Figure 4.3 gives an example log from Z- bend. Fourteen sedimentary logs were measured between the outcrops of Ross Graham and The Loop (Fig. 4.1) and were also sampled for petrographic examination. Facies are defined by texture, fabric, sedimentary structures and trace fossils. Palaeocurrent

measurements were obtained for trough cross-bedding axis, ripple cross-lamination and primary current lineation directions. 4.5 Lithofacies descriptions This study has identified eight facies based upon textures in sample and thin- section, biogenic and physical sedimentary structures observed in outcrop, summarised in Table 1. An example log from Z-bend section is shown in Fig. 4.3 to illustrate the facies described below. Throughout the whole succession there is a notable lack of mud grade material. The following facies descriptions are discussed in order of abundance importance to the environmental interpretation. Bioturbated sandstone Description: The bioturbated sandstone facies (BS) comprises poorly sorted medium to fine- grained sandstone which is often heavily bioturbated by the ichnofossil Heimdallia. Primary sedimentary structures are locally entirely destroyed but locally ripple cross- laminated or low-angle cross-bedded sandstone can be observed. Beds range from 0.01 to 1.4 m thick. Thicker beds are typically laterally continuous whereas thin beds taper out or are scoured and truncated. Thin beds occur every 5cm interbedded with ripple cross-laminated facies. Thick beds are more spaced out, interbedded with ripple cross- laminated facies. The hunting trace Tumblagoodichnus (McNamara 2014) plan view (Fig. 4.4A) and in profile, Tumblagoodichnus Hockingi, (Fig. 4.4B), in cross-section (Fig. 4.4C) is common throughout this facies. Interpretation: This facies is interpreted as linked to colonisation during possible sea-level rise and reduced sedimentation. Heimdallia has been attributed by a number of authors based on the associated sedimentary facies to be a burrowing trace that occurs within a marginal marine environment (Bradshaw & Webers 1988; Mángano et al. 2012). However, Trewin & McNamara (1994) and Wizevich (1996) suggested, based on facies interpretation, it could have formed in a freshwater continental environment. The interpretation here is that there is no support for this later palaeoecological interpretation. This facies implies that colonisation was ordered and possibly cyclical linked to tidal cycles as a response to lower energy conditions, allowing for colonisation.

Figure 4.3: An example log taken from the outcrop at Z-bend. It shows seven of the eight facies identified within FA2. Massive sandstone is rare within the sections and generally restricted to the fluvial element rather than the tidal element of the outcrops.

Figure 4.4: Heimdallia burrow trace and Tumblagoodichnus hunting burrow ichnofacies (Trewin and McNamara 1994). A and B) Tumblagoodichnus in plan view as an elongate tube typically with a lump of sediment at the head end formed by an arthropod ploughing into the sediment, both taken near the base of Hawks Head section. C) Tumblagoodichnus Hockingi in cross section, resembling soft sediment deformation, taken at The Loop. D) Heimdallia resembling a bird’s nest, consisting of horizontal and vertical burrows, taken at The Loop. E) Arrow shows a laterally discontinuous bioturbated unit in cross-section, eroded by a low-angle dune form, taken at the base of the West Loop section. Scales: A) Ruler is 15cm, B) scale is 13cm, C) lens cap is 52mm and D) grain size chart is 10cm.

(Next page) Table 4.1: Lithofacies description for the Tumblagooda Sandstone outcrops within the Murchison River Gorge sections (Fig. 4.1).

Facies Texture Structure Interpretation Bioturbated Poorly sorted, medium- to fine- Pervasive bioturbation of the ichnogenus Heimdallia has been Sandstone (BS) grained, bioturbated sandstone. Heimdallia has almost entirely overprinted primary attributed to a marginal Typically thin to medium bedded structures although rarely ripple cross-lamination marine environment (0.05-0.20 m thick), however beds and low-angle or trough cross-bedding is observed (Bradshaw and Webers 1988; range from 0.01-1.4 m. Thicker beds (Fig 3). Bioturbation consists of the hunting trace Mángano et al. 2012). (>0.1m) are typically laterally Tumblagoodichnus (McNamara 2014) in plan view continuous and can be traced for (Fig. 4A) and in profile, Tumblagoodichnus tens to hundreds of metres whereas Hockingi, (Fig. 4B), in cross-section (Fig. 4C). thinner beds may thin or be Tumblagoodichnus often occurs in the same bed truncated by shallow scours. or above the 3D nest-like feeding trace Heimdallia Bioturbation is more abundant (Fig. 4A, B, D and E). Bioturbation index ranges where BS is interbedded with RL from 2 to 5 based on the scheme proposed by and LAXB (described below). Droser and Bottjer (1986). Ripple cross- Dominant at the Loop section. Ripples are typically 1 cm in height. Symmetrical, Shallow water currents. laminated Consists of laterally extensive fine- double-crested and ladder ripples (Fig. 5A, B and Emergent structures indicate sandsheet (RL) grained ripple cross-laminated C) indicate bidirectional flow from wave or wind an environment that was sandstones Ichnofauna include: action. Asymmetrical bifurcated ripple marks (Fig. undergoing periodic subaerial

Cruziana, Diplichnites gouldi, 5D) show palaeocurrents are dominantly to exposure. Symmetrical Heimdallia, Didymaulichnus cf. lyelli, southeast and southwest (Fig. 6A). Rain drop and ripples indicate wave or wind Didymaulyponomos cf. rowei and adhesion structures, flat-topped, blown-out and action. Adhesion surfaces may Tumblagoodichnus hockingi (Trewin washed-out ripple marks (Fig. 5E, F, G, H, I, J and have formed with the aid of and McNamara 1994). K). Two examples of supercritical climbing ripples microbial cohesion of have been observed (Fig. 5L). Ripples are sediment but not considered commonly modified by arthropod drag traces (Fig. essential. 5M). Possible wrinkle marks (runzelmarken) have been identified throughout The Loop section (Fig. 5N). Arthropod trackways are also common (Fig. 7). Low-angle Well sorted, tabular cross-bedded Foresets are low angle, normally graded, sigmoidal Rippled and wrinkled algal cross-bedded fine to medium grained, medium- or asymptotic and locally preserve parasitic ripple surfaces are proposed to have sandstone thick bedded sandstones (typically marks and wrinkle textures which possibly indicate formed by slack tide (LAXB) 0.2-0.3 m thick and locally up to to algal colonisation. Possible mica draping of colonisation. In many cases 0.7m thick). Ripple cross- laminated foresets has also been observed locally. Coarse the crest of the dune tabular sandsheets up to 0.1 m thick are sand lenses throughout this facies and frequent cross-beds are exceptionally intercalated. Cross-bedded sets are preservation of coarse crests indicate that there preserved and later

not laterally continuous and are was sediment starvation occurring (Fig. 5O and P). sedimentation indicates the generally 5m wide. Some surfaces also feature blown-out ripples and hollows of the dunes are adhesion surfaces suggesting periodic emergence infilled followed by crest (Fig. 5J). Reactivation surfaces, scour surfaces, and reworking. This facies herringbone cross-bedding and are common (Fig. resembles Dalrymple’s (1984) 8A, B, C and D). sandwave description. High-angle Well sorted, rounded coarse- to Toesets are often asymptotic and foresets are The migration of 2D dune cross-bedded fine- grained sandstone, forming locally wrinkled and cemented as described for bedforms. Herringbone cross sandstone sets 0.2-1 m thick of high-angle LAXB. This also contains herringbone cross- - bedding indicates (PXB) cross- bedding. Pebbles and rip-up bedding. bidirectional flow. clasts of vein quartz are rare. Locally beds fine upwards. Trough cross- Well sorted, sub-rounded, fine- Trough cross-lamination occurs on a centimetre Ripples which often top TXB laminated grained sandstone. Trough cross- scale with a set size of 1-5cm in height indicating modification. Sandstone (TXL) foresets are normally graded. Trough cross- Trough cross-cosets between 0.06- Soft sediment deformation and dewatering Complex channel fills have bedded 0.5 m thick within medium to thick structures that disturb parts or entire beds are been observed throughout bedded (0.4-0.6 m) very coarse- to common. Foresets are normally graded and may the sections interbedded with

sandstone fine-grained sandstones. Locally display minor bioturbation on the foreset face. all other facies. Typically the (TXB) beds are up to 2 m thick. Beds are Trough cross-bed set sizes are small (0.06-0.15 m, channel-fills are up to 0.5 m both laterally continuous and STXB), medium (0.16-0.3 m, MTXB) and large (0.3- thick and are 3 to 4 m wide. lenticular and the base of the beds 1 m, LTXB). The tops are locally reworked into is typically scoured. Commonly trough cross-lamination or heavily bioturbated by contains intraformational rip-up Heimdallia. Palaeocurrents are dominant to the clasts and pebbles that are northwest, southeast and southwest (Fig. 6B). concentrated at the base and carried as bedload. Planar Well sorted, fine-grained sandstone These beds show primary current lineation to the Primary current lineation laminated with planar lamination. north-northwest and south-south east (Fig. 6C). indicates upper flow regime, Sandstone (PL) plane bed conditions. Massive Coarse grained, well sorted and no Structureless sandstone. Interpreted as Sandstone (MS) grading to the bed. Typically, thin hyperconcentrated flow in a bedded (0.05 m thick), one example channel. of this facies is 1.25 m thick. Beds are laterally continuous with pebbles located at the base.

Ripple cross-laminated sandstone Description: The ripple cross-laminated (RL) sandstone facies dominates The Loop section and is also observed at all the logged sections (Fig. 4.1). This facies consists of laterally extensive ripple cross-laminated fine-grained sandstones that are interbedded with thin sheets of intensely bioturbated sandstone, which have a bioturbation index of 5 based on Droser & Bottjer (1986). Symmetrical, double-crested and ladder-back ripples (Fig. 4.5A to C) are common, as well as asymmetrical bifurcated ripples (Fig. 4.5D), typically developed every 2 to 5 cm vertically. Frosting of grains is observed. Palaeocurrent measurements for current ripple migration directions have a predominant flow to the south-east and south-west (Fig. 4.7A). Rain drop imprints, algal mat surfaces, adhesion structures, flat-topped, blown-out and washed-out ripple marks (Fig. 4.5E to K) were observed. Two examples of supercritical climbing ripple cross-lamination, with an angle of climb that is greater than the angle of the stoss slope, have been recognised (Fig. 4.5L). One set of climbing ripples exhibits palaeocurrents to the north-west and the other to the south-east. Figure 4.5M shows ripples that have been modified by an arthropod drag trace. Ripples migrate from the south east to northwest, but dominant to the southwest as seen in Fig. 4.7A. Possible wrinkle marks (runzelmarken) have also been identified (Fig. 4.5F, G and N). These are thin crusts which exhibit a mottled, wrinkly surface which resembles the descriptions provided by Klein (1975a) and Gingras (2002) of wrinkle marks. These surfaces are common throughout the RL facies, as thin cemented laminations (less than 1 mm thick). Arthropod locomotion traces, Diplichnites (Fig. 4.6A) and digging traces, Tumblagoodichnus, are very abundant within and on the rippled sandsheets. Didymaulichnus burrows (Fig. 4.6B) and unnamed drag trackways (Fig. 4.6K) have also been identified at The Loop section. Thin Heimdallia-rich beds are interbedded with the rippled sandsheets. Foresets of low-angle dune facies are also commonly colonised every 10 to 20 cm. Body fossils are extremely rare within the Tumblagooda Sandstone, with only one euthycarcinoid specimen having been identified (McNamara & Trewin

1993). A very small fragment of a possible cyanobacteria found in this study is potentially extraformational and carried into these sediments via tidal currents. Interpretation: The presence of abundant ripples indicates shallow water conditions. Rain-drop and mottled surfaces, adhesion structures, flat-topped, blown-out and washed-out ripple marks are interpreted to indicate periodic emergence, typical of shallow marine intertidal conditions. Beds rich in burrowed surfaces, trackways and repeated Heimdallia beds suggest rhythmical sedimentation, interpreted to be due to slack tide colonisation. Super-critical climbing ripples migrating in opposing directions suggests a high sediment influx within an alternating current. Wrinkle marks are typically formed in intertidal and subtidal environments by oscillating currents where microbial cohesion binds the sediment (Klein, 1975B). Exposure surfaces are common and associated with this is the frosting of grains which would give evidence for aeolian source and modification of exposed sands. Davies et al. (2017) interpreted the surface in Fig. 4.5G as a shallow pond feature with ripples in the deepest part, a smooth pond margin and adhesion structures around the edges of the wet pond. However, primary current lineation has been observed in the area that has previously been described as the smooth pond margin. Figure 4.5H shows the same surface approximately 5 m north of Fig. 4.5G, where the ripples are being deformed into the flow, which is interpreted as a washout structure from surface run-off during low tide. This is important because previous interpretations (Davies et al. 2017) have suggested a subaerial to entirely continental origin for the formation of this structure. Deposition was under sub-marine conditions, and later subaerial exposure modified the ripples into adhesion structures and run-off structures dissected and washed out the ripples. Low-angle tabular cross-bedded sandstone Description: The low-angle cross-bedded (LAXB) facies is common throughout the study area but is more common in the central and southerly outcrops (Ross Graham, Hawks Head, Z-bend and Fourways) and becomes less dominant stratigraphically upward. It can be found interbedded with RL faces and bioturbated sandstone facies (BS) (Table 1) at The Loop. At the West Loop locality, the succession is nearly entirely composed of stacked

LAXB facies. The facies consists of low-angle, sometimes asymptotic, cross-bedded sandstone. The sandstones are medium- to fine-grained and well-sorted. Beds are typically around 0.3 m thick but can be up to 0.7 m thick. Foreset migration indicates palaeocurrents dominate to the south-west and south-east (Fig. 4.7C). Parasitic ripples are present locally, and have palaeocurrents in all directions, commonly oblique to the cross-bedding direction. Locally the internal structure of the subaqueous dune beds is complex, with scour surfaces (Fig. 4.8A), and reactivation surfaces suggesting reversal of flow (Fig. 4.8A and B). Three-dimensional exposure allows the observation of herringbone cross-bedding commonly throughout the sections, especially at Fourways and The Loop (Figs 4.1 and 4.8C). Flow reversal is common in beds which are closely spaced overlying each other (Fig. 8). Interpretation: The dimensions and complex internal structure of the LAXB facies (Fig. 4.8A), with common reversal of palaeocurrents, are interpreted to record intertidal and subtidal sand-bars (Allen 1980; Dalrymple 1984; Berné et al. 1988; Dalrymple et al. 1990; Dalrymple et al. 2012). Allen (1980) described similar sand-bars, associated with reversing tidal currents, which are a result of a dominant flow direction and subordinate alternate current. Dalrymple (1984) described low-angle lee faces and stoss slopes with superimposed ebb ripples, like those seen in the Tumblagooda Sandstone; they typically have ripples superimposed on them and are commonly found in the near shore and shallow marine tidal environment (Allen 1980; Dalrymple 1984; Dalrymple et al. 2012). Figure 4.8A and B resemble Fig. 20D and E of Dalrymple et al. (1990) with compound cross-bedding and reversed ebb ripple preservation within a sediment sand-bar. Bradshaw & Harmsen (2007) also describes Heimdallia burrows interbedded with low angle cross-bedded facies, as also seen in the Tumblagooda Sandstone, and concludes a shallow marginal marine environment for the Taylor Group.

Figure 4.5: Sedimentary structures found within the ripple laminated facies. Lens cap for scale (52mm) and scale bar is 13cm. Yellow arrows indicate North or way up, white arrows indicate transport direction. A) Symmetrical catenary in phase ripples at the top of The Loop. B) Double- crested ripples at Z-Bend. C) Ladder ripples at The Loop. D) Symmetrical and asymmetrical ripples at Fourways migrating in different directions. E) Rain dropped rippled surface in a fallen block at The Loop. F) Adhesion mottled bed at The Loop. G) Rippled surface which has been washed out producing primary current lineation and has a wind adhesion surface to the right at the The Loop. H) Washed out ripple surface, with dragged ripples into the flow. Primary current lineation can be seen on the right. Located along the same bed as G.

I) Flat topped modified ripples at The Loop. J) Wind-altered surface at the base of Z-Bend. K) Adhesion surface which Hocking (1991) identified as setulfs, taken at Z-Bend. L) Supercritical climbing ripples at the base of West Loop. M) Rippled surface which has been modified by an arthropod drag trace and washed out in a loose block at The Loop. N) Possible wrinkles (runzelmarken) bed at the Loop. O) Coarse crested ripples formed by sediment starving at The Loop. P) Cross-bedded unit with coarse sand lenses, taken at Fourways

Figure 4.6: A) Diplichnites trackways at Fourways (scale is 10cm) B) Diplichnites trackways identified within The Loop section (scale is 13cm).

Figure 4.7: Palaeocurrent rose diagrams A) Ripple migration directions from RL facies, B) FA1 fluvial trough cross-bedding axes from TXB facies. C) FA2 tidal trough cross-bedding axes from TXB facies.. D) Primary current lineations from PL facies. Data from this study and courtesy of Hocking (1991). Arrows and degrees indicate to the mean of the data. N is the number of data points in the rose. Maximum is the amount of measurements within the maximum peak.

Figure 4.8: Examples of internal structures within LAXB sandstone facies within the Tumblagooda Sandstone, showing scour surfaces caused by reactivation of sediment by returning flood currents. Flow reversal structures indicate to ebb and flood flow conditions.

Trough cross-bedded sandstone Description: The trough cross-bedded (TXB) facies is common in beds that are typically up to 1 m thick and locally up to 2 m thick. This facies generally fines upwards from coarse to

fine-grained sandstone with common pebble and rip-up clast lags locally transported as bedload. Foresets are normally graded and cosets vary from small (0.06 to 0.15 m, STXB), medium (0.16 to 0.3 m, MTXB) and large (0.3 to 1.0 m, LTXB). Palaeocurrents are recorded to the north-west, south-east and south-west (Fig. 4.7B and C). The base of this facies is typically erosional, and the tops of some beds are locally reworked by bioturbation. Soft sediment deformation and dewatering structures have commonly destratified the entire bed. Interpretation: This facies is interpreted as channel fill by sinuous crested dunes migrating under fluvial and tidal channel conditions. Channels have erosional basal surfaces and are from 1 to 6 m thick and laterally continuous but locally taper out. Basal surfaces suggest a mixture of sheet-like and channel-braided deposition, typical of channels that are not stabilised by vegetation in the over-bank area (Davies & Gibling 2010). Rip-up clasts and pebble lags support evidence for high-energy, turbulent flows. Beds which are interpreted as alluvial have palaeocurrents which dominate to the northwest with local variations in flow direction (Fig. 4.7B). Which contrasts with sub-marine, tidal channels which contain bioturbation and show palaeocurrents which preserve contrasting flow directions to the southwest (Fig. 4.7C). Bioturbation of this facies could possibly indicate marine incursions and reworking of fluvial sediments (Savage et al. 2013). High angle cross-bedded sandstone Description: The high angle cross-bedded sandstone facies (PXB) is composed of well-sorted coarse- to fine-grained sandstone that locally fines upward but this is not common. Pebbles and rip-clasts are rare. Beds are 0.2 to 1.0 m thick representing one coset. Toesets are typically asymptotic and locally foresets show wrinkling as described for LAXB. Foresets also show changing current directions with flow in alternate directions. This facies is commonly interbedded with RL and LAXB facies. Interpretation: This facies is interpreted as the migration of straight crested dunes which often display herringbone cross-bedding due to the presence of strong alternating currents. Planar laminated sandstone Description:

The planar laminated sandstone (PL) facies is composed of a well-sorted, fine- grained sandstone with pervasive planar lamination forming 0.2 m thick laterally continuous beds for up to 200 m. Primary current lineation is common on bedding planes with trends to the north/north-west and south/south-east (Fig. 4.7D). This facies is not common but is typically associated with TXB facies. Interpretation: This facies is interpreted as upper flow regime, plane bed conditions formed during flood conditions. This is supported by the presence of primary current lineations which are pervasive throughout the beds. The uncommon nature of this facies indicates to periodic flood events depositing the Tumblagooda Sandstone and not deposition from flood dominant conditions as previously suggested by authors (Hocking 1991), which was given to explain the nature of the unconfined sheet-like deposits (Cotter 1978; Fedo & Cooper 1990; Gibling & Davies 2012). Trough cross-laminated sandstone Description: The trough cross-laminated sandstone facies (TXL) is well-sorted, fine-grained sandstone facies which preserves trough cross-lamination with set sizes from 1 to 5 cm. This facies is not common and typically occurs at the top of TXB units. Interpretation: This facies is interpreted as modification of sandstones by shallow high-energy fluid flow, forming sinuous crested ripples. Massive sandstone Description: Massive sandstone facies (MS) is rare in the studied sections but where observed it is structureless coarse-grained and well-sorted, with sporadic pebbles. The beds are typically 0.1 m thick, but one example is 1.25 m thick and beds have no grading. This facies is typically found in scoured lenses within RL and LAXB facies and the very thick bed is associated with TXB facies. Interpretation: This facies is structureless with a scoured base suggesting that deposition occurred rapidly from an upper-flow regime hyperconcentated suspension deposition which inhibited the formation of bedforms (Nichols 2009).

4.6 Facies associations

4.6.1 Facies Association 1 FA1: is composed of dominantly stacked TXB facies, although locally facies PL dominates with primary current lineation preserved. Facies Association 1 is composed of a series of stacked fining-upward packages separated by erosional surfaces which are often overlain by pebbly lags. Trough cross-bedding sets are dominantly medium to thick (>0.3 m) and generally this association is coarser grained and more poorly sorted to the south. Lithofacies BS, MS and PXB are less common within this element, BS is locally found at the base of channel-fill facies, above a strong erosional surface, where MS and PXB locally comprise the whole channel-fill complex. These sandstones are typically compositionally mature but texturally immature. This association displays mixed sheet and lenticular geometries with palaeocurrent flow dominating to the north-west (Fig. 4.6B). Minor PL facies preserves primary current lineation which shows palaeocurrents oriented north-west/south-east (Fig. 4.6C). Lithofacies BS can also be observed at the top of facies TXB (Table 1). Interpretation: Facies Association 1 is interpreted as high-energy braided channel-fill deposits, with subordinate sheet-like channel sandstones. FA1 is dominant in the southern outcrops and become less dominant and thinner northwards towards The Loop (Fig. 4.1) which suggests a reduction in fluvial influx upward in stratigraphy. Intervals where amalgamated scour facies dominate suggest rapid avulsion due to bank instability. Sheet-like units which punctuate tidal facies, indicate rapid episodic flooding of the basin, as seen in modern ephemeral systems (Miall, 1980). The general lack of bioturbation suggests a fluvial origin for the channel fills; however, bioturbated channel bases are interpreted to have formed as a hiatus in fluvial influx and a minor marine incursion before fluvial conditions returned. Facies Association 1 provides evidence for active channel belt avulsion; typical of fluvial systems during the lower Palaeozoic, suggesting channel bank instability due to the lack of binding activity of plants (MacNaughton et al. 1997). High discharge conditions are interpreted to have resulted in sheet-like units, which do not show evidence of incision. This deposited fluvial sediments over broad intertidal areas as laterally persistent, erosively based fluvial sandstones (Schumm 1968; Smith 1976;

Cotter 1978; Long; 2011; Davies & Gibling 2010). Fluvial palaeocurrent is strongly unimodal to the northwest (Fig 4.7B and Hocking 1991) supporting the interpretation by Hocking (1991) of a palaeoslope direction to the north-west. 4.6.2 Facies Association 2 This study has been able to subdivide FA2 of Hocking (1991) into three sub-facies; a dominantly intertidal sandflat (FA2A) association, a shallow subtidal sandbar association (FA2B) and a tidal channel (FA2C) association. Facies association FA2a: comprises stacked series of facies RL, BS, PXB and LAXB beds. These are fine to medium-grained, well-sorted and rounded texturally and compositionally mature sandstones. This association is found in the southern outcrops and becomes more dominant further north of the study area at The Loop. This association is up to 80 m thick in measured sections (but much larger in total) with individual beds ranging from 0.1 to 1.7 m thick but are usually around 0.2 m thick. The association contains common rippled sandsheets, containing double crested, ladder and opposing supercritical climbing ripples, interbeded with cross-bedded and bioturbated sandstones. Thick and thin intervals of bioturbation range from 0.05 to 0.8 m thick and thick beds of low angle planar cross-bedding are also observed. Herringbone cross- bedding is common (Fig. 4.8C). Wind adhesion, ripple modification and rain-dropped surfaces are all commonly developed throughout The Loop section and more rarely in the southern outcrops. Interpretation: This facies association is interpreted to have been deposited within a shallow intertidal sandflat environment, supported by evidence for multi- directional currents, wave activity and a high sediment influx (e.g Dalrymple et al. 1990; Eriksson & Simpson 1990; Simpson & Eriksson 1991; Mángano & Buatois 2004). The sandflat was covered by shallow water and allowed the migration of small ripples with periods of emergence. Palaeocurrents from ripple marks are variable, but show dominance to the south-east, south-west and north-west. Bi-directional flow indicators such as herringbone cross-bedding are common, supporting a tidal interpretation. Thin bioturbated interbeds and bioturbated foresets are interpreted to indicate intermittent shallow water and may provide evidence for slack water or neap conditions. Typically, in modern tidal environments during slack water conditions, mud is deposited, developing lenticular or flaser bedding and mud drapes. No mudstones are observed in 99

the Tumblagooda sections, which is interpreted as due to the lack of mud delivered or preserved into the shallow marine environment. Sedimentary structures such as wind adhesion, ripple modification and rain-drop surfaces indicate an environment with intermittent exposure/emergence. Facies association FA2A: is dominant throughout FA2 but more common in the south of the studied area. It is composed of stacked fine to medium-grained sandstone of facies LAXB, PXB and BS with minor RL facies. Rhythmic stacking of facies LAXB–RL– LAXB–BS is common in many of the sections, with LAXB being in alternate directions. Parasitic ripples can be seen at the base of toesets and climbing dune foresets as well as on stoss slope lamina where preserved. Dune palaeocurrent flow was dominantly to the south-east and south-west. Interpretation: Hocking (1991) Bi-directional cross-bedding indicates alternating palaeocurrents; however, more commonly palaeocurrents dominate in the longshore (SW) and ebb tide (NW) direction, and the flood tide (SE) is represented by ripple laminated bottom sets. Dune palaeocurrents that flow dominantly to the south-west are interpreted as longshore currents. Parasitic ripples that indicate flow to the north-east and south-east are interpreted to have formed by tidal ebb and eddy currents. The dominance of cross-bedding readings to the northwest and south east further supports the palaeoslope was to the northwest and the hinterland was to the southeast. This facies association records deposition within an intertidal sand-bar to shallow subtidal environment (Dalrymple et al. 1990; Eriksson & Simpson 1990; Simpson & Eriksson 1991; Mángano & Buatois 2004; Davis 2012) where longshore currents dominated. Interbedded thick bioturbated facies are interpreted to have formed during periods of reduced deposition and/or during times of slack water conditions. Thin bioturbated foresets are also interpreted to record slack tide conditions where mud deposits would usually accumulate (Abouessa & Morad 2009). Hocking (1991) included this facies within an intertidal interpretation and made no further subdivisions. Facies association FA2C: consists of TXB, PL, MS and BS facies. This association is composed of medium to fine-grained, well-sorted sub-rounded mature sandstones. This association displays fining-upward trends indicating waning flow conditions and is generally laterally discontinuous. This facies association is dominated by sinuous crested bedforms. Sets often contain vein quartz pebbles and sandy rip up-clasts at the base,

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above an erosion surface. The clasts are interpreted to have been deposited as traction surfaces at the base, with some pebbles also entrained in the high-energy flow. Foresets are both low- and high-angled indicating changing flow conditions, and they migrate to the south-west and south-east (Fig. 4.6B). Interpretation: Hocking (1991) interpreted this facies as intercalations of FA1 within FA2 units. This study reclassifies the units as FA2C based on noted differences in sedimentology. Compositionally and texturally FA2C is more mature than FA1 and trough cross-sets are smaller within the FA2C. This indicates more abrasion and reworking, and the common opposing palaeocurrent directions (Fig 4.6B) suggest these are more likely to have been deposited within tidal channels. 4.7 Depositional system The overall depositional setting of the Tumblagooda Sandstone is interpreted as a tidally dominant, periodically emergent, shallow marine coastal system fed by adjacent sandy braided fluvial channels (Fig. 4.9A). Subdivisions of the intertidal environments are recognised, similar to environments within the modern Bay of Fundy (Dalrymple 1984; Dalrymple et al. 1990) and those described from the ancient Mount Guide Quartzite, discussed by Eriksson & Simpson (1990). Two detailed models have been proposed to explain the laterally coexisting sub-environments within FA2 (intertidal environment Fig. 4.9C and subtidal environment Fig. 4.9D). It is possible to preserve intertidal deposits with periods of flooding and minor transgression, which has possibly been documented by heavily bioturbated units. 4.7.1 Fluvial braidplain Fluvial facies are dominated by moderate to high energy thalweg fill as described in FA1. The sharp erosional contacts at the base of fluvial facies overlying tidal facies (Fig. 4.9A), indicates possible forced regression and rapid movement of the shoreline. This occurs seven times within the studied sections and may be a response to basin faulting. Tidal influence has not been identified within the fluvial facies except locally where channels are intensely bioturbated, indicating a pause in fluvial conditions and a return to marine periodically before fluvial conditions returned and eroded into the bioturbated fluvial units. Fluvial facies become less frequent stratigraphically upward (Fig 4.9B) indicating a reduced fluvial input with time.

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Figure 4.9: Sections of digital outcrop models from VRGS (Virtual Reality Geological Studio). A) Highlights of FA1 facies identified at Z-Bend outcrop. Sharp erosional basal contacts can be seen to overly FA2 deposits. B) Frequency of fluvial incursions reducing up stratigraphy to The Loop outcrop here. Amalgamated scour-fills are less common with increased proportion of isolated geobodies.

4.7.2 Tidal dominance Figure 4.10A summarises the environment proposed for FA2, a tidally dominant system which was fed by a fluvial system. Intertidal sandflat and shallow subtidal sand- bar associations are recorded where onshore and longshore currents dominate. This study recognises a dominance of bar forms in the southerly outcrops (Ross Graham to Z-bend), that are interpreted to record shallow subtidal conditions, with rare preserved evidence for emergence. In the north of the study area bars are interbedded with intertidal sandflat deposits which have evidence for intermittent subaerial exposure (like the parasequences discussed by Eriksson & Simpson 1990). The subtidal sand-bar

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environment was dominated by shallow bars up to 0.3 m thick with bioturbation formed during slack water conditions. The presence of thick Heimdallia bioturbated facies may indicate fluctuations in sea-level conditions as proposed by Savage et al. (2013), who suggested that they represent marine incursions during relative sea level rise. Periods of relative sea-level fall may have resulted in progradation of the shoreline and enhanced sub-aerial exposure and more extensive development of wind- blown ripples, adhesion and rain-dropped surfaces in the intertidal sandflat environment (Fig. 4.10B). At the base of Z-Bend, exposed bar tops show wind-blown modification (Fig. 4.5J) indicating very shallow conditions such that, at the lowest tide, sand-bars were exposed. Small tidal channels are also observed within the sand-bar environment as seen at the Bay of Fundy.

4.7.3 Intertidal sandflat This setting comprises FA2A and is interpreted to have been deposited within the frequently emergent intertidal sand-flat environment (Fig. 4.10C), where the water depths were likely to have been very shallow, allowing thin ripple cross-laminated sandsheets to form. Evidence for subaerial exposure surfaces throughout the succession indicates a dominantly intertidal setting. This zone would have been exposed during low tide and subject to aeolian reworking and the formation of adhesion and blown-out structures. Periods of minor transgression and reduced sediment input allowed burrowing organisms to colonise and microbial mats to form, which in turn brought in grazing predators such as arthropods that are suggested by intensely bioturbated facies. Gastropod trackways have also been observed. The sandflats were densely populated but exhibit a low diversity of organisms suggesting that the environment was stressed. Ripples frequently bifurcate which indicates wave influence on depositional conditions. Pulses of high-energy fluvial discharge are shown by deposition of unidirectional braided fluvial facies with north-westerly trending palaeocurrents. During periods of low discharge shallow water tidal conditions dominated, producing small migrating ripples.

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(Previous page) Figure 4.10: A) Model proposed for the deposition of the Tumblagooda Sandstone. FA1: The fluvial system avulsed frequently and would have reworked the intertidal and subtidal zones. FA2A was deposited within the intertidal sandflats where shallow water facies predominate with frequent subaerial exposure. FA2B was deposited in the subtidal sand-bar environment which was shallow and occasionally emergent. FA2C is shown by the tidal channels present within the subtidal environment. Logs are taken from the Gauging Station (FA1), The Loop and the West Loop sections. B) Progradation of the shoreline and a lowered sea level resulted in the exposure of the enhanced intertidal sandflat environment. Run-off and wind activity on the exposed surface would have reworked and/or removed ripples with adhesion of dry sand onto the wet emergent surfaces. Wind acting on a shallow sheet of water would most likely have generated wave ripples and shoreline conditions would promote the formation of interference ripples.

4.7.4 Intertidal sand-bar to shallow subtidal This setting comprises FA2B and FA2C and is interpreted as a shallow subtidal sand-bar environment (Fig. 4.10D) dominated by fine-grained straight to slightly sinuous sandy bars that have palaeocurrent indicators showing flow to the south-west and south-east within an intertidal sand-bar to shallow subtidal environment (Dalrymple et al. 1990). Common tidal channels cut through this area, which contains small pebbles and rip-up clasts, and indicate palaeocurrents to the south-east. During low tide, bars were reworked by ebb tidal currents to produce ripple laminated sand. Typical tidal couplets and mud drapes are absent within the Tumblagooda Sandstone; however, bioturbation patterns indicate that colonisation occurred during periods of reduced sedimentation that could possibly indicate the equivalent slack tide or neap tide period (Abouessa & Morad 2009). More sustained deepening events resulted in lower energy conditions and provided optimum sites for colonisation and allowed development of thick bioturbated intervals.

4.7.5 Stacking patterns Frequent intercalations of FA1 reflect either autocyclic processes on channel avulsion within a larger braided channel belt system, or higher ordered parasequence progradational events forcing the shoreline further into the basin. Facies Association 2 shows an overall shallowing upward trend from subtidal deposition in the south (Ross Graham and Hawks Head) to dominantly intertidal deposition in the north (The Loop). This records a gradual relative sea-level fall throughout the succession. Internal autocyclic processes are interpreted to be responsible for high-order deepening events reflected in development of thick intensely bioturbated Heimdallia beds (Savage et al. 2013).

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Figure 4.10: continued C): A zoom in of the intertidal sandflat sub-environment (FA2a) with logs showing the sedimentology at each point. Logs are taken from parts of The Loop section. D) A zoom in of the subtidal sand-bar sub-environment (FA2b) with logs. Palaeocurrents flow to the southwest on a shallow flat sand bank. Logs are taken from parts of The Loop, Ross Graham and Hawks Head sections.

4.8 Discussion

4.8.1 Interpretation of the depositional system Facies Association 2 (FA2) of the Tumblagooda Sandstone exposed in Kalbarri National Park contains a complex suite of non-marine to marine-influenced facies deposited in the latest Ordovician to earliest Silurian, prior to the evolution of vascular land plants. The sedimentary suite is characterised by sand-rich facies, with virtually no mud preserved. Extensive field logging and petrographic analysis in this study has allowed interpretation of the Tumblagooda Sandstone as deposition on a low relief, sandy coastal plain dominated by fluvial, intertidal sandflat (FA2A) and subtidal sand-

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bar (FA2B) conditions with frequent tidal channels (FA2C). At periods of low tide, emergent sandflats were reworked, generating ripple marks and adhesion surfaces. Straight-crested to slightly sinuous-crested bar forms migrated within the sand-bar environment. Periods of non-deposition allowed for colonisation of the substrate to form thick, laterally continuous and pervasively bioturbated sandstones. Shorter periods of non-deposition, i.e. slack water conditions, allowed time for the growth of thin microbial films (Fig. 4.5F). These film surfaces are associated with Diplichnites ichnofacies, and thus it is possible that these algal films were a food source. This study has allowed subdivision of FA2 into three sub-facies associations. The tidal channel deposits of FA2A punctuate tidal facies of FA2B. Intercalations of fluvial facies of Facies Association 1 (FA1) are interpreted to reflect rapid progradation of fluvial facies, because of enhanced run-off and increased energy of the system, associated with the lack of binding activity by plants in the continental realm at the time. Following avulsion and subsequent lateral migration (or retrogradation) of the fluvial system, shallow marine/tidal conditions were re- established in the basin.

4.8.2 Significance of Heimdallia assemblages Heimdallia burrows in the Tumblagooda Sandstone were described by Trewin & McNamara (1994) and McNamara (2014) to have formed in interdune ponds within a continental setting. This description was significant because it suggested that the trace fossil assemblage records the first evidence of colonisation onto the continent. The reinterpretation proposed in this study, based on sedimentary structures and facies context, is that the Heimdallia burrows and Diplichnites tracks preserved were generated in a marine-influenced setting, showing similarity to assemblages described from other Palaeozoic marine sequences (Bradshaw 1981; Bradshaw & Webers 1988; Buckman 2008; Savage et al. 2013). Many of the Heimdallia beds examined in this study have a mapped extent of up to 1 km, which is somewhat contradictory to the earlier interdune pond interpretation. There are no other reports of arthropods inhabiting non- marine ecospaces before the Devonian period (Bradshaw 1981; Buatois et al. 1998) which suggests that a coastal dune environment for the Tumblagooda Sandstone is unlikely, and further supports an intertidal affinity.

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Savage et al. (2013) working on comparable facies suites from the Devonian Sperm Bluff Formation (Taylor Group), southern Victoria Land, Antarctica, suggested that the presence of thick Heimdallia beds recorded periodic marine incursions from fluctuating sea level before an influx of terrestrial coarse material. It is likely that this is also the case within the Tumblagooda Sandstone, because the presence of thick intensely bioturbated beds appears to be common throughout the succession and may suggest deepening events and an increase in relative sea level. 4.8.3 Early Palaeozoic depositional controls in a pre-plant world Published examples of lower Palaeozoic tidal successions are summarised in Table 4.2 and Fig. 4.11. It has been noted that a commonality in half of the cases identified the successions is the lack of mud-draped and flaser or lenticular-bedded structures (e.g Barnes & Klein 1975; Harris & Eriksson 1990; Eriksson & Simpson 1990; Eriksson et al. 1995; Abbot & Sweet 2000). This contrasts with modern tidal environments, where mud-draped couplets are the norm for identification of tidal bundles (Visser 1980; Williams 1991; Kvale 2012). Twenty-one of the 40 examples in Table 2 have little (less than 2%) to no documented mudstone within the succession and thus they are typically described as clean quartz-rich sandstones/quartz arenites. Most of the formations have planar or trough cross-bedded sandstone facies indicating higher energy conditions. Figure 4.11 graphically shows the proportion of formations identified and the types of tidal features indicated within the literature. Less than half of the identified formations contain features such as lenticular-bedding, mud desiccation cracks, mud-drapes on foresets and herringbone cross-bedding. Bioturbation such as Skolithos burrows and exposure surfaces with rain-drop features and adhesion structures are also commonly described Where mud is noted, flaser-bedding is more commonly identified than lenticular- bedding indicating sand dominated environments. Mud drapes are also rarely reported. In many cases only laterally discontinuous mudstones have been identified, thick mudstone packages are rare (see Table 4.2). The defining feature in many of the cases to describe a tidal origin for the sandstones has been the presence of trace fossils, for example: Skolithos, Diplichnites and Cruziana (Table 4.2).

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Figure 4.11: A graph to show the number of tidal features identified in the published literature (see Table 2), such as flaser, wavy and lenticular bedding, mud desiccation cracks, mud draped foresets, bi-directional flow features such as herringbone cross-bedding, bioturbation such as Skolithos, and exposure surfaces such as rain-dropped sandstones and adhesion surfaces.

Mud-grade tidal features are very rare within the Tumblagooda Sandstone. This is interpreted to be due to the combined effect of limited mud delivery from the continent, due to lack of land plants that affected weathering characteristics. Also, the high energy hydrodynamics of the delivery system resulted in fine-grained deposits being transported offshore and not being retained within the coastal plain. Tidal processes were active during the lower Palaeozoic and plate tectonic reconstructions indicate broad areas of shallow marine conditions (Veevers 2004), suggesting that tidal deposits should be abundant in the ancient rock record. The limited reporting of tidal facies may indicate a lack of recognition rather than a lack of preservation in the rock record (Fedo & Cooper 1990). Structures may have been misinterpreted due to the lack of distinctive sedimentary structures, such as flaser and mud draped structures, which are common in modern tidal settings. The Tumblagooda Sandstone has remarkable similarities to many other lower Palaeozoic tidal systems – for example: the Upper Silurian Grampian Group, Australia (George 1994); the Roper Group, Australia (Abbot & Sweet 2000); the Upper Mount Guide Quartzite, Australia (Eriksson & Simpson 1990); the Lower Sandfjord Formation, 109

Norway (Levell 1980); the Zabriskie Quartzite, California (Fedo & Cooper 1990); and the Antietam Formation, Tennessee (Cudzil & Driese 1987). These successions have all been described as composed of quartz arenites, with limited mudstones. This is comparable with the rare siltstone and mudstone facies observed in the Tumblagooda Sandstone and an absence of marine macrofauna, in contrast with most modern intertidal systems. Thin bioturbated interbeds and bioturbated foresets are interpreted to record cyclic deposition and are interpreted to be evidence for slack tide or neap tide conditions, the equivalent time when mud would usually deposited in modern environments (Abouessa & Morad 2009). Other key sedimentary structures are still recorded, that strongly support an interpretation of tidal conditions, such as the presence of symmetrical, double-crested, ladder and supercritical climbing ripples that are typically interpreted to be formed by bidirectional currents, either by tide or wave activity. Sedimentary structures such as wind adhesion ripples, ripple modification and rain-drop impressions indicate intermittent emergence/exposure. Bi-directional flow indicators such as herringbone cross-bedding indicate periods of alternating palaeocurrents, and frequent bounding surfaces, disconformities and reactivation surfaces also point to conditions of changing flow. The paucity of mudstone deposits in the sections means that tidal bundles are not present and so an estimation of palaeotidal conditions was not able to be carried out. The lack of mud in FA2 is enigmatic and several reasons may be proposed to account for this. Terrestrial conditions at that time may have inhibited or restricted the generation of large volumes of mud from the hinterland, as a result of different weathering styles associated with the lack of terrestrial vegetation and lack of root binding and soil-forming processes (Davies & Gibling 2010).The lack of plant cover is also reported to have had an effect on mechanical erosion, reducing the binding of channel margins, resulting in increased lateral avulsion and dominance of high-energy sheet- flood style fluvial systems (e.g Schumm 1968; Cotter 1978; Davies & Gibling 2010). The increased energy input, with more frequent flood events, may also have resulted in reworking by fluvial processes. Exposed floodplains without vegetation would be

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Formation Age Features Observed Interpretation Main References Moodies Group, Barberton Archean 2, 3, 4, 5, 6, 8, 9 Mid-tidal flats Eriksson 1977 Greenstone Belt, South Africa Chaibasa Formation, India Early 3, 4, 5, 10 High subtidal to low intertidal Bhattacharya & Proterozoic Bandyopadhyay 1998 Uncompahgre group, Early 1, 2 thin, 5, 11 Tidal inner shelf Harris & Eriksson Colorado Proterozoic 1990 Magaliesberg Formation, Early 1 Low proportion of mud and Braid delta and tidal flats Eriksson et al. 1995 Transvaal Supergroup, Proterozoic Lack of fossils, 3, 2, 4 South Africa Ramgundam Sandstone, Middle 3, 10 Intertidal Chaudhuri & India Proterozoic Howard 1985 Roper Group, Northern Mesoprotero 1 Lack typical tidal features, Tidally dominated shoreline Abbot & Sweet Australia zoic 10 Unidirectional flow 2000 Upper Mount Guide Precambrian 1, 4, 6, 8, 9, 10, 11, 13 Subtidal-sandwave to tidal-flat Eriksson & Simpson Quartzite, Australia 1990 Witwatersrand Supergroup, Precambrian 1, 5, 8, 9, 10, 13 1. Tidal inlet environment developed Simpson & Eriksson South Africa along a mesotidal coastline 1991 2. Subtidal to intertidal setting developed along a macrotidal coastline Big Cottonwood Formation, Precambrian 1, 2, 3, 4, 5, 6, 8 Intertidal and subtidal with tidal channels Eriksson et al. 1981 Utah Tidal estuary The Palms and Pokegama, Precambrian 3, 9, 13 Tidal flats Chan et al. 1994 Quartzite, Minnesota Ehlers & Chan 1999 Mozaan Group, Pongola Precambrian 1, 2, 3, 5, 8, 9, 10, 13 Shallow marine to tidal flat environment with Ojakangas 1983 Supergroup, South Africa common fluvial influx 111

Lower Fine-Grained Precambrian 2, 3, 10 Tide dominant sub tidal and intertidal sand Brunn & Hobday Quartzite, Scotland bar environment 1976 Lyell Land Group, Greenland Precambrian 1, 2 20-30% 3, 4 Tidal flat Klein 1970 Lower Sandfjord Formation, Late 1, No fine-grained suspension 1. Fluvial deposit Tirsgaard 1993 Finnmark, North Norway Precambrian deposits, 6, 7, 8, 10, 11 2. Shallow marine environment Levell 1980 Stangenes Formation Late 3, 5, 8, 13 Shallow Subtidal and intertidal Baldwin & Johnson Tanafjord Group, Norway Precambrian 1977 Antietam Formation Lower 1, no fine-grained suspension Nearshore shallow marine sandstones Tidal- Cudzil & Driese Chilhowee Group, Virginia Cambrian deposits, 8 – slack water flat and tidally influenced shoreface 1987 And Tennessee phase, 9 10, 12 Skolithos and environment with no uppertidal mudflat Simpson 1991 Cruziana traces, 13 deposits Doulbasgaissa Formation Lower 1, 2, 7, 10 Tidal shallow marine Banks 1973 Norway Cambrian Hardeberga Formation Lower 2 (2-10cm thick), 8, 11, 12 Tidal shallow marine Hamberg 1991 Sweden Cambrian Middle Member, Wood Lower 1, 3, 4, 6, 8, 9, 12 Skolithos, 1. Tide-dominated near shore-shallow Klein 1975b Canyon Formation California Cambrian 13 marine origin Lobo & Osborne 2. Braided fluvial environment to a 1976 marine dominated environment Fedo & Cooper 1990 Zabriskie Quartzite California Early 1, 2 <2%, 3, 7, 8, 10, 11, 12 1. Mid to Low Tidal flat Klein 1975a Cambrian Skolithos, Planolites, 2. Nearshore marine and terrestrial and Fedo & Cooper Rusophycus Environments 1990 3. Ephemeral sheet flows Prave 1991 Campanario Formation Upper Lower 2- thin, 3, 5, 7, 8, 9, 12 - Macrotidal shallow-marine seaway within the Mangano & Argentina to Middle Cruziana problematica intertidal and large subtidal sandbar area Buatois 2004 Cambrian Palaeophycus, Bergaueria,

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Rusophycus leifeirikssoni, Syringomorpha and Skolithos Mt. Simon Formation Upper 1, 3, 4, 5, 6, 8, 11, 12 1. Transgressive nearshore marine Driese et al. 1981 Wisconsin U.S.A Cambrian Skolithos, Arenicolires and deposits to deep lower shoreface deposits Cruziana, 13 2. Tidal flat Potsdam Group, Quebec Cambrian 1, 2, 4, 11, 12 – Diplichnites, 1. Aeolian to Fluvial marginal marine Collette et al. 2009 13 2. Intertidal zone Hagadorn et al. 2010 Tapeats Sandstone Arizona Cambrian 1 An absence of clay, 6, 9, 11, 1. Braided fluvial environment Hereford 1977 12 2. intertidal environment with sand bars 3. Tidal channels Eriboll Sandstone, Scotland Cambrian 1, 6, 8, 9, 11, 12 Skolithus and Tide-dominated, shallow subtidal or intertidal Swett et al. 1971 Monocraterion Cabos Series, Spain Middle 1, 3 Subtidal and tidal flats Baldwin & Johnson Cambrian 1977 Jordan Sandstone Minnesota Upper 1, 5 (less than 5% of foresets Tidally dominated shallow marine Tape et al. 2003 Cambrian are draped), 6, 11, 12 Umm Ishrin Sandstone Late 2, 3, 8 Braidplain and tidal flat Makhlouf & Abed Formation, Jordan Cambrian 1991 Crozon Formation, France Cambro- 2, 3 10, 11 Tidal Lagoon Baldwin & Johnson Ordovician 1977 Monkman Quartzite, British 1, 2 <1%, 6, 7, 8, 9, 10, 11, 12 Intertidal zone which were locally protected. Jansa 1975 Columbia, Canada Skolithos Exposure was frequent. Tidal channels and sandbars developed locally Possible subtidal or beach deposits are also present

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Graafwater Formation, Cape Lower 1, 3, 4, 5, 7, 8 with coprolites, 1. Small tidal channel deposits and Rust 1977 Town South Africa Ordovician 9, 10, 12 Skolithos burrows lagoon mudstones Hobday & Tankard and arthropod trackways 2. Distal alluvial floodplain environment 1978 Braddy & Almond 1999 Peninsula Formation Cape Middle 1, 2 ~ 0.5%, 4, 5, 8, 9, 10, 12, Subtidal deposits, Tidal inlets, sand bars, Tankard & Hobday Town, South Africa Ordovician 13 channels and foreshore deposits within a 1977 tide-dominated back barrier system Hobday & Tankard 1978 St Peter Sandstone, Middle 1 clay poor, interbedded with 1. Lower foreshore to sand bank Amaral & Pryor Michigan Ordovician silty to argillaceous 2. Fluvial and aeolian dune dominated 1977 carbonates, 5 – Dolomite 3. Transition to a shallow marine Mazzullo & Ehrlich 0.5%, 6, 7, 8, 9, 11, 12 4. High energy paralic, intertidal 1983 Cruziana, Skolithos and 5. supratidal sand flat Dott et al. 1986 Diplocraterion Barnes et al. 1992 Nadon et al. 2000 Hawaz Formation, Libya Middle 2, 5, 7, 10, 12 Subtidal Abouessa & Morad Ordovician 2009 Eureka Quartzite, California Ordovician 1, 3, 4, 5 dolomitic, 7, 8, 9, Shallow subtidal, tide dominated conditions Klein 1975a 11, 12 with tidal channels Lower Bald Eagle Formation Upper 1, 2, 3, 4, 5, 6, 8, 9, 10 Marginal marine – intertidal zone Thompson 1975 Central Appalachians Ordovician Tumblagooda Sandstone, Ordovician- 1 lacks mudstone, 7, 8, 9, 12, 1. Intertidal sand flat Hocking 1991 Western Australia Lower 13 2. Aeolian dominated overbank Trewin 1993 a, b Silurian Trewin & McNamara 1994 McNamara 2014

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Grampians Group, Victoria Silurian 1, 5, 7, 8, 11, 12, 13 1. Fluviodeltaic environment with minor George 1994 tidal influence MacNaughton et 2. Aeolian dune field with ephemeral al. 2002 streams Gouramanis et al. 3. Mesotidal uppershoreface and 2003 foreshore Dunquin Group, Ireland Silurian 4, 5 less than 1mm thick, 12 Tidal flat and subtidal facies of a shoreface Sloan & Williams succession 1991 Knocknaveen and Toormore Silurian 1, 4, 6, 9, 12 Intertidal flats Phillips 1974 Group, Ireland

Table 4.2: A summary of literature on pre-vegetation tidally recorded successions. 40 sections have been identified up to the late Silurian and 20 of those identify the sandstones are super mature quartz arenites with less than 2% mudstone deposits. Summary code: 1 – mature quartz arenite sandstones 2 – mudstone beds present (continuous and discontinuous) 3 – flaser, wavy or lenticular bedding 4 – desiccation cracks 5 – mud drapes 6 – reactivation surfaces 7 – planar bedded 8 – ripple laminated 9 – herringbone or bi-directional flow 10 – trough cross-bedded 11 – planar cross-bedded 12 – bioturbated 13 – exposure surfaces 14 – hummocky cross-bedding

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subject to enhanced aeolian reworking, with much of the fine-grained material winnowed and transported away. The combined effect of high-energy flows entering the shallow marine environment, and the reduction in the amount of mud from the continental realm, may have resulted in the bypassing of any mud deposition in the shallow marine nearshore environment.

4.8.4 Application of modern analogues In modern subtidal environments the sediment substrate is submerged during both slack-water stages, allowing deposition of two mud-layers whereas in the intertidal zone only one mud layer is deposited due to emergence (Visser 1980; Davis 2012; Dykstra 2012). In searching for a modern analogue to the sand-rich systems observed in the early Palaeozoic and older, key criteria may be to examine very energetic systems, for example the Bay of Fundy, Canada (suggested by Wach et al. 2008). The modern Bay of Fundy, Canada, has the largest macrotidal range, up to 15 m during spring tides in the Minas Basin (Klein 1963). Although mud is present in the Fundy inlets, the axial parts of the bay and shorelines are swept by powerful tidal currents and little mud is preserved within the sedimentary structures, this is analogous to the Tumblagooda Sandstone. At low tide intertidal deposits are exposed and comprise sub-metre scale two-dimensional dunes which are both straight and sinuous crested and migrate in the flood tide direction (Fig. 4.12A). Interdune areas are filled with asymmetrical ripples which migrate in all directions, often oblique to the flood and ebb tide directions but they are rarely symmetrical. Ripples are also seen to climb the lee slope of the dune (Fig. 4.12B). Washout structures, small creeks and collapse structures are also found in the interdune areas (Fig. 4.12C). Ripples piggy-back on the stoss side of the dunes and migrate in the ebb tide direction. The intertidal dunes here are much smaller than observed in aeolian settings but also lack mud deposits usually associated with tidal environments, like the Tumblagooda Sandstone. Burrows are rare within the dunes, however the upper surfaces and the interdune deposits are frequently bioturbated. Dalrymple et al. (1990) discussed the internal structure of intertidal sand-bars and similar deposits are found within the Tumblagooda Sandstone sand-bars (Fig. 4.5A and B). The lack of mud identified in the Tumblagooda Sandstone may be due to the lack of sheltered

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areas and baffling by vegetation, which is common close to the coastline in the Bay of Fundy.

Figure 4.12: The Bay of Fundy intertidal zone at low tide. A) Flood tide direction is from right to left and other arrows indicate flow direction of exposed interference ripples. Interdune/stoss slope area here is submerged and ripples are being funnelled towards the camera in the flow. Person is 170cm tall. B) Submerged interdune area showing interference ripples and current ripples migrating in different directions to the main dunes and flood tide direction. C) Dune tops showing interference ripples and interdune areas dominated by fluvial currents forming braided rivers with bars and wash-out structures. All structures are oblique to the tidal flow directions.

4.9 Conclusions This study has re-assessed an early Palaeozoic ‘pre-vascular land plant’ sandstone-rich formation of lower Palaeozoic age in Western Australia. Despite the lack of characteristic fine-grained sedimentary features, the sedimentary structures, lithofacies and ichnotaxa are interpreted to record deposition in a marginal marine tidally influenced depositional setting. Facies analysis has revealed eight lithofacies based on grain size, sedimentary structures and textures within Facies Association 2

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(FA2) of Hocking (1991). The association has been further subdivided into three divisions: FA2A was deposited because of tidal channel activity; FA2B was deposited in an intertidal sandflat environment dominated by ripple cross-lamination, bioturbation, washout structures and subaerial exposure surfaces; and FA2C was deposited in a deeper intertidal to shallow subtidal environment which was dominated by low-angle dunes, bioturbation, occasional subaerial exposure, tidal channels and bidirectional flow. The flood tidal currents dominated. There is a lack of typical large scale aeolian dunes reported in lower Palaeozoic fine-grained sedimentary facies as seen in many aeolian systems, for example in the Upper Triassic aeolian sequences of Nova Scotia (Hubert & Mertz 1984). Interpretations of a terrestrial affinity for these systems, including the ichnofacies Heimdallia and Diplichnites trackways, are open to re-evaluation in the context of all the evidence provided in this study and was also suggested by Dott et al. (1986) and Buatois et al. (1998). The study also has significant implications for depositional and reservoir models for pre-vegetation lower Palaeozoic and older sections. In modern systems, erosion rates, sediment type (sand:mud ratio) and some sedimentary structures and geometries are influenced by terrestrial vegetation. The resultant impact on the characteristics of paralic successions needs to be considered when interpreting these older sequences. The understanding of the regional distribution, architecture and internal heterogeneity of ancient tidal systems is important for oil and gas reservoir prediction. If typical mud-rich tidal indicators, such as mud drapes and lenticular bedding, are missing then it is quite possible that the environment of deposition could be misinterpreted as arid continental sand-sheets or as shelfal deposits rather than a tidally influenced shoreline/estuary. This has implications for subsurface lateral and vertical facies predictions. A review of published tidal facies in early Palaeozoic and older rocks, suggests that they are under-represented. The lack of certain characteristic tidal sedimentary structures, in particular those associated with mud deposits in older ‘pre-vegetation’ rocks does not preclude them from being tidally influenced. The lack of mud preserved could be due to the increased effects of winnowing on overbank

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sediments, therefore the fine-grained component is removed from the system by wind reworking. It could also be due to the reduced production of mud within the hinterland due to the lack of rooted plants to absorb potassium feldspar and produce clay minerals coupled with weathering processes more generally. Lastly it could be a function of the high-energy fluvial system largely bypassing the deposition of mud sized particles and transporting them into the deeper basin.

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Chapter 5: Reassessing the sedimentology and architecture of pre- vegetation fluvial successions using large digital outcrop models.

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Chapter 5: Assessing the sedimentology and architecture of pre-vegetation fluvial successions using large digital outcrop models.

Bradley G-M.*, Redfern J., Hodgetts, D. University of Manchester, School of Earth and Environmental Sciences, Manchester,

M13 9PL, UK *[email protected]

Submitted for publication in Sedimentology.

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Abstract Using newly acquired 3D photogrammetry, derived from UAV imagery, has been integrated with detailed sedimentary logging over a large area to map the geometry and sedimentological character of pre-vegetation fluvial systems. Few studies have quantitatively documented the geometries of fluvial systems prior to the evolution of rooted plant systems. The colonisation of land by plants influenced weathering, the energy of the system and stability of channel banks, and many authors have compared them to modern ephemeral environments, dominated by upper-flow regime runoff. “Sheet-braided” characteristics have been attributed to many pre-vegetation successions. This study interprets a large digital dataset acquired to document and quantify the Ordovician-Silurian Tumblagooda Sandstone in Western Australia, that has also previously been described as having a “sheet- braided” channel architecture, with only rare channel features noted. However, using the data acquired during this study a more diverse range of geobody geometries has been observed. A quantitative database has been extracted that characterises over 1500 fluvial geobodies have been recorded, with lenticular and sheet-like scours preserved within overall tabular bedded sandstones. Fluvial facies are dominated by trough cross-bedded sandstone, with pebble and rip-up clast lags. Parallel lamination is uncommon within the study area, suggesting ephemeral sheet- like flood events are not the dominant process of deposition. Palaeocurrents derived from trough cross-bedding axes and primary current lineation trend to the northwest and accretion surfaces indicate downstream and lateral accretion. Results suggest a system dominated by amalgamated multi-storey and multilateral low-sinuosity, low- amplitude, poorly confined channel-braided architectures with downstream and lateral bar accretion. The lack of rooted plants colonising the overbanks is interpreted to have allowed repeated channel widening, under low base-level conditions, preserving shallow and wide amalgamated scours. Subordinate tabular isolated sheet sandstones are interpreted to have been the result of higher-energy fluvial events reaching further into the basin during times of increasing base-level. The geobodies identified within this study suggest a dominance of broad ribbon geobodies with both planar and lenticular erosion surfaces and subordinate

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deep incision. This supports previous observations that in the absence of deeply rooted plants to stabilise the overbank areas, river channels would preferentially spread laterally rather than incise deeply into the underlying sediment, preserving channel-fills which are wider than the original channel-form. The presence of early rootless vegetation possibly baffling mud and helping to partially stabilise channel banks is given as supporting evidence for channel stability to preserve scour structures. 5.1 Introduction The processes and architecture of fluvial systems deposited prior to the evolution of rooted land plants have been a topic of debate for decades (i.e Schumm 1968; Cotter 1978; Tal & Paola 2007; MacNaughton et al. 1997; Long 2004, 2011; Davies & Gibling 2010, Ielpi & Ghinassi 2015; Ielpi & Rainbird 2016; Ielpi et al. 2016, 2017; Santos et al. 2017; McMahon & Davies 2018; Kleinhans et al. 2018). Many studies have qualitatively described fluvial successions deposited in the absence of vascular land plants, describing the lithofacies and facies associations, however, few studies have documented and quantified the channel geometry and morphodynamics (i.e Fuller 1985; Long 2006; 2011; Marconato et al. 2013; Ielpi & Ghinassi 2015; Ielpi et al. 2016; Lowe & Arnott 2016). Studies are often limited by the extent of surface exposure and inadequate constraints on aspect ratio of channel bodies (Rygel & Gibling 2006). This study reconstructs the lithofacies distribution and three-dimensional sedimentary architecture of the Ordovician-Silurian Tumblagooda Sandstone of Western Australia, a pre-vegetation fluvial sandstone, exposed for 50 km along the gorges cut by the Murchison River, in Kalbarri National Park. UAV-based photogrammetry has been used to capture long portions of the outcrop to reconstruct a three-dimensional digital outcrop model. This has been integrated with 29 sedimentary logs to build five digital outcrop models that document the facies, architectural elements and geometry of geobodies identified within the fluvial units. This work has application in providing analogue data for subsurface studies. When correlating between widely spaced wells it is always difficult to predict three- dimensional architecture in the subsurface, and outcrop analogue studies are a key

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source of data to understand reservoir geometry and facies distribution (Pringle et al. 2006). Accurately documenting fluvial architecture of the outcrop geobodies is important, as ranges of geobody aspect ratios are used to condition data for reservoir prediction (Gibling 2006). 5.2 Background Schumm (1968) was the first to infer that pre-vegetation fluvial systems were dominated by bedload processes with increased erosion and sediment yield capacity due to the lack of sediment binding and retention in the absence of roots. The lack of effective vegetation cover was interpreted to reduce water absorption, allowing increased surface run-off and sediment yield. It was suggested that all river systems prior to the evolution of rooted land plants would have had a sheet-braided planform geometry (Schumm 1968; Cotter 1978). Cotter (1978) coined the term “sheet- braided” and applied this to “genetic units” which were architecturally different to “channel-braided” styles observed. The study identified the key defining factor is a 20:1 aspect ratio, with sheet-braided styles being greater than this and preservation of planar basal erosion surfaces without the development of incision (Cotter 1978; Miall 1980). Greater ratios of up to 1000:1 have also been suggested (Fuller 1985). These genetic units were on various scales, typically decimetres thick and composed of trough cross-bedding and planar laminated sandstones (Davies et al. 2011). Many later studies applied this to pre-vegetation fluvial successions, documented sheet- like fluvial facies composed of planar laminated and trough cross-bedded sandstones (e.g Fedo & Cooper 1990; Hocking 1991; Todd & Went 1991; MacNaughton et al. 1997; Eriksson et al. 1998). These systems have been likened to modern arid continental systems, where wide braided streams occupy the entire valley floor (Schumm 1968; Eriksson et al. 1998; Love & Williams, 2000; Gouramanis et al. 2003; Long 2004). Modern arid systems are typically ephemeral, with short-duration, high-discharge, where upper- flow regime sheet-flood events are dominant and result in poorly confined sheet-like channel belts (McKee et al. 1967; Tunbridge 1981; Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Lowe & Arnott 2016). These systems produce high rates of sediment yield, with high run-off removing all fine-grained

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material from the system (Schumm 1968; Miall 1985, 1996; Long 2006; Santos et al. 2014). Many described sections note no change in grain size throughout the channel (Long 2011) and a dominance of horizontally stratified and low-angle cross-bedded sandstones indicative of upper-flow regime conditions (e.g. Bhattacharyya & Morad 1993; Simpson & Eriksson 1993; Hjellbakk 1997; Davies & Gibling 2010). Lower Palaeozoic and Precambrian fluvial facies are widely described as having a “sheet- braided” architecture and channelised architectures are rarely documented until the development of vascular land plants (Cotter 1978; MacNaughton et al. 1997; Long 2004; Davies & Gibling 2010). Smith (1976) suggested that banks composed of sand and lacking in mud would have been 20,000 times less stable than those with 16-18% grass roots and the development of meandering fluvial systems would have been inhibited because of the lack of mud preserved (Schumm 1968; Long 1978; Eriksson et al. 1998; Santos et al. 2014). The development of fan-like wide and flat bedload braided sheet sands was explained by rapid channel avulsion and lateral migration rates and the widening of channels preferentially over incision as a response to limited bank stability (Cotter 1978; Long 1978; Miall 1980, 1985, 1996; Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Davies & Gibling 2010). Repeated reworking with limited accommodation space would have resulted in the preservation of low relief, broad, tabular, stacked sandstone-sheets (Schumm 1968; Smith 1976; Cotter 1978; Sønderholm & Tirsgaard 1998; Santos et al. 2014). The earliest multicellular plants (embryophytes) are recorded as freshwater algae in Middle Ordovician (470 Ma) rocks (Kenrick & Crane 1997; Pires & Dolan 2012). Vascular plants (sporophytes) such as Cooksonia megafossils, do not appear until the mid-late Silurian (425 Ma) and pervasive root structures are first described in Lower Devonian, Lochkovian and Pragian-Emsian age sediments (Kenrick & Crane 1997; Gensel et al. 2001; Gensel 2008; Hillier et al. 2008; Davies & Gibling 2010; Long 2011; Pires & Dolan 2012; Xue et al. 2016). Early land plants (prior to the Devonian) were limited to wetland environments and lacked deeply rooted systems and thus did not anchor into the sediment (Alego & Scheckler 1998; Gensel et al. 2001; Kennedy et al. 2012). The lack of roots would have resulted in reduced mud retention

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and binding of substrate, with high-energy fluvial flows more likely, causing rapid channel avulsion and sediment would be washed to the distal parts of the basin (Davies & Gibling 2010). Evolution of rooted vegetation is interpreted to have contributed to a fundamental change in fluvial style, with an increase in bank stability allowing the development of a stable muddy overbank area. Chemical weathering in the upland areas would have to produce significant quantities of mud to enable rootless vegetation to baffle and retain the mud (Kleinhans et al. 2018). Rooted vegetation increased infiltration and bedrock weathering and decreased surface runoff leading to the preservation of mudstone facies (Knighton 1998; Gibling & Davies 2012). Vegetation allows mud to be deposited and preserved closer to the river channel as a levee, showing that mud sedimentation and vegetation mutually enhance floodplain formation, increasing stability and causing promoting channel self-organisation and the formation of a single-thread channel (Davies et al. 2011; Kleinhans et al. 2018). Flume experiments carried out by Tal & Paola (2007) indicated that with the increase of roots in the inactive areas of an alluvial plain, channels self- organise from a braided morphology to a single thread, sinuous laterally migrating channel. Applied to pre-vegetation fluvial systems, this suggested that prior to the evolution of rooted systems all fluvial systems would have been braided in nature (Tal & Paola 2007). Schumm (1960) and van Dijk et al. (2013) suggested that mud within the overbank significantly increases cohesion and bank stability promoting meandering channel style. Modelling the interplay between modern vegetation and mud reveals that a higher mud concentration increases floodplain aggradation and reduces overbank flow frequency (Kleinhans et al. 2018). Above ground rootless vegetation may still have produced a baffling effect, which could trap and deposit muddy sediment and stabilise overbanks (Davies et al. 2011; McMahon & Davies 2018). Fine- grained over-bank and channel deposits are rarely documented in pre-vegetation fluvial facies and locally only preserved as intraformational rip-up clasts (Dalrymple et al. 1985; Trewin 1993; Davies et al. 2011). It is rare for meandering channel deposits to be recognised prior to the Devonian period, implying that with the

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evolution of roots the stability of the overbank area was significantly increased leading to a change to meandering channel-forms (Davies & Gibling 2010). Most fluvial facies models for pre-vegetation systems are based on bedload- dominated modern arid ephemeral systems, as they have sparse vegetation giving the closest comparison we have to an environment without rooted plants (Miall 1977; Bhattacharyya & Morad 1993; Long 2004, 2006, 2011). However, this only provides a partial analogue for a world with a complete absence of land plants, as the palaeoclimatic range of pre-vegetation ephemeral systems would have extended across tropical and humid climates (Tirsgaard & Øxnevad 1998; Went 2005). More recent studies (Hartley et al. 2015; Santos & Owen 2016; Ielpi 2016; Ielpi et al. 2016, 2017) have highlighted the possibility that meandering fluvial systems may have existed as far back as the Precambrian and that analogous modern arid unvegetated rivers do sometimes display rare meandering geometries with expansional point bars (Santos et al. 2017). However, they are small and generally contained within predominantly sheet-like sandstone bodies and do not have the extent of modern meandering systems as sometimes suggested (Davies et al. 2017). Although less common, some authors have documented pre-vegetation fluvial deposits that preserve a wide range of alluvial styles, including sheet-floods, channel- braided, deeply channelised, perennial and meandering channel features (Sweet 1988; Nicholson 1993; Long 2006, 2011; Santos et al. 2014; Ielpi & Ghinassi 2015; Ielpi & Rainbird 2016; Ielpi et al. 2016, 2017). This suggests that flow had components of channelisation and was not just dominated by sheet-braided processes (Santos et al. 2014; Ielpi 2016; Santos & Owen 2016; Ielpi & Ghinassi 2015; Ielpi & Rainbird 2016; Ielpi et al. 2016, 2017). It may be that they are not characterised just by low-sinuosity fluvial facies especially if there is a low-gradient and stable discharge (Ielpi et al. 2017), raising the possibility that many braided sandstones previously described as sheet-like could possibly be misinterpreted (Ielpi 2016; Santos & Owen 2016). Further work to examine in detail the architecture of such ancient systems is required, and this study offers a contribution to this debate.

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5.3 Geological Setting The Ordovician–Silurian Tumblagooda Sandstone in Western Australia (Fig. 5.1) is exposed within a large river gorge system in Kalbarri National Park, which offers a series of 3D sections, with up to 1000 m of lateral outcrop exposure. The bedding is undeformed and localities for study have been chosen that expose both channel cross-sections and along-channel sections to generate a 3D model of the system. The Tumblagooda Sandstone was deposited in the Southern Carnarvon Basin, an intracratonic basin situated within the continent of Gondwana around 10° north of the equator (Young 1986; Hocking 1991; Li & Powell 2001). It lies unconformably on Precambrian gneissic basement and is the oldest member of the Kalbarri Group (Young 1986; Hocking 1985). Trace fossil assemblages (McNamara & Trewin 1993; Trewin & McNamara 1994), palaeomagnetic dating (Schmidt & Embleton 1990; Schmidt & Hamilton 1990) and zircon dates (Markwitz et al. 2017) suggest an age of late Ordovician to Silurian.

Figure 5.1: Study and log locations within Kalbarri National Park, Western Australia. GS- Gauging station, RG- Ross Graham, HH- Hawks Head, ZB- Z-bend, L- The Loop, RB – Red Bluff, MR- Mushroom rock, EG- Eagle Gorge. Numbers denote log number. Right column displays the summary stratigraphy of the Southern Carnarvon Basin with the Tumblagooda Sandstone at the base, overlying gneiss basement (modified from Hocking 1991; Ghori 1999).

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The outcrops studied form a continuous 1500 m thick, well exposed section in the lower parts of the Murchison River in Kalbarri National Park and around the coastal cliffs south of Kalbarri (Fig. 5.1) (Hocking & Mory 2006). Hocking (1991) defined five facies associations (FA1-FA5) and described FA1 and FA3 as a low- sinuosity sheet-braided fluvial system. This was interpreted as evidence for episodic low-relief, moderate- to high-energy flood events within a fluvially dominated delta system. Both FA1 and FA3 are dominated by trough cross-bedded coarse- to fine- grained sandstones, although FA3 is more pebble rich. FA1 is found in the south of the section at the Gauging Station outcrops (Fig. 5.1), and is also interbedded with tidal FA2 deposits, from the Ross Graham to The Loop outcrops (Bradley et al. 2018). In common with other described pre-land plant fluvial deposits (Fedo & Cooper 1990; Miall 1996; Davies & Gibling 2010) the fluvial facies within the Tumblagooda Sandstone has previously been described as having “sheet-braided” architecture with only rare channel features previously noted by Hocking (1981, 1991), Trewin (1993) and Hocking & Mory (2006). 5.4 Methods Previous studies that employed traditional photogrammetry methods, using a standard digital camera, on outcrops of limited lateral extent, have inherent issues for statistical analysis, such as lens and edge distortion and effects of oblique angles of the outcrop and photographs which reduces accuracy (Pringle et al. 2006). Rapid development of land-based lidar scanning of outcrops improved accuracy, with scaled X, Y and Z data (Hodgetts et al. 2004; Bellian et al. 2005; Bonnaffe et al. 2007; Hodgetts 2013). However, limitations are still apparent with this technique, such as data gaps and oblique-angle measurement bias created from single scanning locations (Lato et al. 2010). Overhangs and topography may also create data shadows and so multiple scans are required to reduce measurement errors (Pringle et al. 2006; Bonnaffe et al. 2007). Lidar scans generate large datasets, requiring significant computing power to process (Wilson et al. 2011). Equipment is heavy which can pose an issue in remote field locations and topographically complex areas. This study has used UAV-based photogrammetry to capture a three- dimensional digital data for the outcrop. This method improves the resolution of

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digital outcrop models as closer access to the outcrop captures high-resolution detail, (cross-bedding foresets etc). It also combats issues with distortion due to oblique angles, as the camera mounted on the drone can be controlled so that it is always at 90° to the outcrop and the data is georeferenced with an internal GPS to give real- space X, Y and Z coordinates. The drone allows access to remote locations and to steep cliff tops which would otherwise be inaccessible to log using traditional methods. Digital images of the Tumblagooda Sandstone from the lower Murchison River gorges were obtained using a DJI Phantom 4® drone (Fig. 5.2). The drone was flown in a horizontal flight path at a horizontal speed of 2 m/s and 10 m distance from the outcrop. Photos were taken every 5 seconds. This method maximises the flight time of each battery which was limited to 20 minutes. Five 3D outcrop models were produced using Agisoft Photoscan® to reconstruct highly detailed photorealistic models of the stratigraphic succession from the base at Ross Graham, Hawks Head, Z-Bend, Fourways and The Loop (Fig. 5.1). These models were annotated in VRGS® (Virtual Reality Geological Studio) (Hodgetts et al. 2015) using the detailed texture and mesh to identify fluvial morphologies and extract geobody measurements. To correct for oblique outcrop exposure, true widths were calculated within the software, using trigonometry (Rygel & Gibling 2006; Fabuel-Perez et al. 2009) correcting the apparent measured widths using the mean palaeocurrent direction of 303° within VRGS. Palaeocurrent readings were obtained from 765 trough cross- bedding axis measurements during this study and supplemented with data from Hocking (1991). These readings were taken from multiple bed surface exposures of troughs where the terraced outcrop exposure allowed. Multiple readings were also taken within the same bed and the average was taken from the entire studied outcrop from Ross Graham to the coastal cliffs. A test was carried out to determine if the 40° variation in palaeocurrent readings (Fig. 4.7) would affect the true width of the geobody, and the correction was run again using a palaeocurrent of 300° and 340°. Figure 5.3 shows the data envelopes for the readings which shows that there is little variation and using the mean would not significantly affect the calculation of

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true width and the application to subsurface predictions, due to inherent subsurface inaccuracies.

Figure 5.2: DJI Phantom 4 UAV drone, used in the data collection of the outcrops in Kalbarri National Park to create digital outcrop models to analyse the fluvial architectures within the Tumblagooda Sandstone.

Figure 5.3: A summary of the data envelopes showing the range of geobody widths corrected using trigonometry for the range of palaeocurrents observed within FA1 deposits. The envelopes show that there is little variation in widths observed and therefore using the average palaeocurrent (303°) does not greatly affect the dimensions that would be preserved.

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5.5 Results The fluvial units within the Tumblagooda Sandstone were previously interpreted as deposition from a high-energy sheet-braided system, comprising laterally continuous sheets extending for several kilometres (Hocking 1981; Trewin 1993b; Hocking & Mory 2006; Mory & Hocking 2008). The overall geometry of the fluvial facies over a kilometre scale (minimum of 20 km) is tabular, with planar basal erosion surfaces that show lateral continuity and very little erosion and thickness changes (Fig. 5.4A). However, from field observations and following construction of the digital outcrop models, it became apparent that the fluvial system was not as simple as previously observed and a complex internal structure of the fluvial units was observed (Fig 5.4B, C, D). Over 1500 geobodies have been identified in the 5 outcrop models that show common internal heterogeneity, not consistent with the typical sheet-braided model. Analysis has identified six facies and five architectural elements Bradley et al. (2018) identified two facies associations (FA): FA1 is a channel- braided fluvial system and FA2 dominated by shallow marine tidal processes. This study details the sedimentology of FA1 and provides a more detailed description of the fluvial geometry and palaeoenvironment.

5.5.1 Lithofacies Seven facies have been identified and trough cross-bedded sandstone (TXB) is the dominant facies with subordinate planar cross-bedded sandstone (PXB), parallel laminated sandstone (PL), low-angle laminated sandstone (LAL), massive sandstone (MS) and trough cross-laminated sandstone (TXL) which are shown in a stratigraphic log (Fig. 5.5). Petrographic analysis has concluded that all facies are moderately to well sorted and compositionally mature with a dominance of quartz with less than 2% feldspar, rare lithic grains and 1% heavy mineral content. Fluvial sandstones are texturally immature with a dominance of angular to sub-angular clasts with occasional well-rounded quartz grains (Fig. 5.6). Feldspars are often degraded or completely eroded, enhancing secondary porosity. Some quartz grains have

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haematitic rims, preserving intergranular porosity whereas bleached sandstones are often quartz cemented occluding the primary porosity (Fig 5.6). contained within the fluvial units.

Figure 5.4: A) Photopanel from the Ross Graham section showing planar basal erosion surfaces and overall tabular geometry of fluvial units typical within pre-vegetation fluvial systems. Person is 1.7 m tall. B-D are images extracted from the digital outcrop models within VRGS which are annotated with interpreted geobodies. Each geobody is given a different colour. B) Shows a dominance of sheet-like geometries as the outcrop orientation is along channel axis at Fourways. C) Single 6m deep incisional geobody surrounded by amalgamated geobodies at Z-Bend. To the eastern margin is a 6 km long downstream accretion package which is interpreted to be local incised valley fill. D) Amalgamated geobodies at Z-Bend overlie the incised geobody in C.

Figure 5.5: (Overpage) A: A summary log showing the stacking patterns of the fluvial facies identified within Fourways outcrop. The figure shows the first 15 m of logged stratigraphy at the base of the succession and then a small section taken from 40 m within the log to demonstrate the nature of the rare massive sandstone facies. Stacked fining upwards sequences of coarse-

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to fine-grained sandstone contain trough and planar cross-bedding. Low-angle and planar laminations, massive and trough cross-laminations are subordinate. Rip-up clasts, pebble lags and soft sediment deformation is commonly observed. Deformation often disrupts foreset structure and locally contorts the whole bed. Bioturbation is also locally observed at the top of fluvial facies. B: A summary log showing the stacking patterns of the fluvial facies identified at Red Bluff (which is representative of the coastal cliffs section). The log shows a much coarser granule to pebble rich sandstones and frequent conglomerate beds. Each package is around 1 m which shows fining upwards and uncommon Skolithos burrows. The upper parts of the log show a fine-grained sandstone unit which is dominated by parallel laminations and ripple laminations. Overlying this is a thick unit of trough cross-bedded fluvial channel sandstones which become increasingly bioturbated by Skolithos burrows towards the top.

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For 5.5A For 5.5B B

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Figure 5.6: Thin section micrographs of fluvial trough cross-bedded (TXB) facies. A) A PPL image from the Gauging Station, PL-overbank facies showing haematite coating of grains. Scale is 0.5 mm. B) A PPL image from Ross Graham showing an abundance of quartz overgrowths within scour-fill TXB facies. Porosity is secondary from dissolution of feldspars and fractures. Scale is 0.5 mm. C) A XPL image from the Loop showing quartz grains cemented by clay within TXB scour-fill facies. Scale is 0.5 mm. D) Is a XPL image of a feldspar which has degraded to clay, This can be seen coating the grains in image C. Scale is 0.2 mm. All show an immature texture with a dominance of angular to sub-rounded quartz grains with minor rounded grains. The composition is mature and often feldspar grains are degraded or completely dissolved. Pores are often cemented by clay (C) or quartz overgrowths (B) where the sands are white in colour from bleaching. If sandstones are red in colour, hematite coats the grains and pore spaces are still intact (A).

Trough cross-bedded sandstone (TXB) Description This facies is the most abundant facies found within FA1 (Bradley et al. 2018) and is composed of moderate to well sorted, coarse- to fine-grained sandstone (Fig. 5.7A, B). Each bed is typically around 0.4 – 0.6 m (can be up to 2 m locally) thick with trough cross-cosets between 0.2 – 1 m thick. Terraced outcrops allowed the measurement of 3D exposures of trough axes which show flow dominantly to the northwest (Fig. 5.8A). Forsets are locally deformed and contorted (Fig. 5.7C). Angular intraformational rip-up clasts (Fig. 5.7D, E) of fine-grained sandstone are common

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with rounded quartz pebbles preserved as lag deposits (Fig. 5.7F). Locally the tops of TXB facies are bioturbated which occasionally destratifies the top 10 cm of the bed (Fig. 5.7G). The top of TXB facies locally preserves wind alteration preserves ripples, wind-blown and adhesion structures. The base (5th order bounding surface) of the facies is erosional, both planar and concave upward (Fig 5.9A). One large incisional channel-form has been identified at the base of the Z-Bend outcrop with a significantly terraced concave upwards erosional basal surface (Fig 5.9B). The basal surface abruptly erodes into tidal FA2 sandstones. The geobody has a preserved depth of 6 m and width of 70 m. This channel-form is filled with several trough cross- sets which coincide with the terraced erosion surface. There is also a 6 km long downstream accretion package preserved on its eastern margin (Fig 5.9C), which dips below surface at Fourways. Several channel-forms cross-cut the accretion package, which is also common in other accretion bodies identified in the succession (see section 5.5.3). At the coastal outcrops the trough cross-bedded sandstones are granule to pebble grade (Fig. 5.10A) at the base of the outcrops which fines upwards where at the top of the cliffs they are fine-grained with occasional pebbles (Fig. 5.10B). The upper parts of the coastal outcrops are heavily bioturbated by Skolithos, Diplocraterion and Daedalus ichnofacies (Fig. 5.10C) (Trewin & McNamara 1994). The lower parts of the coastal outcrops are bioturbated more uncommonly by Aulichnites ichnofacies (Fig. 5.10D) (Trewin & McNamara 1994). Other deeply incised channel- forms are identified at the Red Bluff look out in the coastal section where a 4 m deep channel-form is preserved among other incisional bodies (Fig. 5.10E). These are also intensely bioturbated by Skolithos. The presence of these incised channel-forms supports evidence that channel-braiding was more common and sheet-braiding was not the dominant process. Interpretation This facies has been interpreted as channel-fill from sinuous crested dunes migrating within a river system. Basal surfaces suggest incision was common and scouring was evident within the larger channel belt. Widening and flattening of the unconsolidated channel margins would have enhanced shallowing and thus the

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preservation of more planar basal surfaces (Davies & Gibling 2010). Rip-up clasts and pebbles give evidence for high-energy turbulent flows. Bioturbation of facies tops indicates marine incursions and reworking of fluvial sediments (Savage et al. 2013). Wind altered surfaces indicate periodic exposure of the sediments, allowing for reworking by aeolian processes. The channel-form identified at Z-Bend (Fig. 5.8) is noted as being significant providing evidence for channel incision. Above the surface of incision, a thick amalgamated scour sequence is preserved, which together with the significant amount of erosion, is interpreted as a significant boundary across the entire study area which can be traced from Ross Graham, 20 km north to Fourways. The nature of this surface is discussed in section 5.5.4. The coastal cliffs outcrops are significantly coarser grained at the base and become more fine-grained towards the top, with an increase in the intensity of bioturbation. This is interpreted as a shift from a more proximal environment to an increasingly distal environment with a marine influence.

Figure 5.7 overpage: Trough cross-bedded sandstone facies. A) multiple scours within 1 m thick bed at Z-Bend. Scale is 1 m. B) multiple troughs within a bed at the Gauging Station. Scale 15 cm long. C) Amalgamated scour-fills at Red-Bluff. Partial annotation is made to highlight key surfaces. Scale is 1 m. D) Contorted area within a TXB bed at Fourways. Scale is 1 m. E) Large rip-up clasts composed of fine-grained RL facies at Z-Bend. Scale is 15 cm. F) Atypical green fine-grained rip-up clast at Fourways. Scale is 10 cm. G) Pebble lag at Red Bluff. Lens cap is 52mm wide. H) Bioturbated tops of TXB facies at Hawks Head. Scale is 13 cm.

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Figure 5.8: Palaeocurrent rose diagrams A) TXB trough cross-axes directions measured from 765 troughs. B) PL primary current lineation directions. Data obtained from this study and data supplemented by Hocking (1991) taken from the whole succession, multiple readings also obtained from the same bed.

Figure 5.9: Images taken from digital outcrop models within VRGS (Virtual Reality Geological Studio) from the Z-Bend outcrop. A) Concave and planar 5th order bounding surfaces. B) a 6 m deep 70 m wide channel-form which erodes into tidal FA2a facies (Bradley et al. 2018). The basal erosion surface is highly terraced and can be traced laterally for over 6 km. C.1,2) A section of a 6 km long downstream accretion body which terminates against the channel-form in B. Several channel-forms cross-cut the accretion package (Pink geobodies above the green), interpreted as chute channels. C.2) an annotated version of C.1. Scales are 5m in height.

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Figure 5.10: A) Coarse grained sandstones with granule and pebble components, much more poorly sorted at the base of the coastal cliffs than the other TXB units observed. Scale is 15cm. B) Fine-grained sandstones preserved at the top of the coastal cliff exposures. Also preserving draping of finer grained sediment on foresets. Scale is 1 m. C) Extensive Skolithos burrows preserved in the upper parts of the coastal cliffs exposure. Scale is 15 cm. D and E) Daedalus ichnofacies preserved in the upper parts of the coastal cliffs exposure. F) Aulichnites ichnofacies preserved in the upper parts of the coastal cliffs exposure. G) Multi-storey and multilateral Scour-fills preserved in the upper parts of the coastal cliffs exposure where a rock fall has occurred. Annotations have been made to highlight some of the concave upwards erosional nature of the basal surfaces. Several scour-fills are observed around 1 m thick with a striking 4 m thick scour-fill preserved towards the top. All are bioturbated by Skolithos burrows.

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Planar cross-bedded sandstone (PXB) Description This facies is the second most abundantly preserved facies within FA1 and is composed of coarse- to fine-grained, well sorted cross-bedded sandstones (Fig. 5.11A). Beds are typically 0.2 – 1 m thick which locally fine upwards, containing locally preserved pebble lags. In one instance identified this facies preserves a layer of green silt-grade rip-up clasts above a significantly erosional basal surface, which is atypical of the Tumblagooda Sandstone (Fig. 5.11B). The basal surfaces are typically erosional or sharp. Foresets are often deformed possibly due to dewatering or slumping. Interpretation This facies is interpreted as channel-fill deposition during waning flow conditions from the migration of straight crested dunes under low- to moderate- energy conditions. Planar laminated (PL) and Low-angle laminated (LAL) sandstone Description These facies are uncommon within the river gorge succession and are gradationally interbedded within a 2 m section of the Fourways outcrop (Fig. 5.11C). Another example of PL has been identified at Ross Graham (at the same stratigraphic level as Fourways). The beds are less than 0.5 m thick and are uniformly fine-grained. Basal surfaces are sharp to erosional and are generally planar. Primary current lineation is preserved within PL facies, this was measured within the same and multiple beds and shows flow direction dominantly to the northwest – southeast (Fig. 5.8B) which supports the measurements taken from TXB. Planar laminated to low-angle laminated sandstone facies are more common within a 10 m thick section in the middle of the coastal cliffs outcrops, between Red Bluff and Eagle Gorge. These facies are fine- to very fine-grained sandstone and siltstone grade interbedded with TXL facies (described below). Beds are typically 0.2 to 1 m thick and laterally continuous. Interpretation This facies is interpreted to have formed from deposition under upper-flow regime conditions. This facies is uncommon, thus does not support previous

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interpretation that deposition occurred dominantly from flood events (Hocking 1991). The outcrops of this facies in the coastal cliffs are interpreted to be a result of deposition from rapid shallow overbank flows. Massive Sandstone (MS) Description This facies has only been identified at Fourways as a single 1.5 m thick bed of well sorted, coarse-grained, structureless sandstone which preserves limited amounts of pebbles at the base (Fig. 5.11D). The base is a prominent erosional and planar, which extends for several hundreds of meters, beyond the edge of the exposed outcrop at Fourways. Interpretation This facies is interpreted as deposition from a single supercritical flow event. Deposition occurred rapidly which inhibited the formation of bedforms (Nichols 2009). Trough cross-laminated sandstone (TXL) Description This facies is uncommon within the river section and preserves trough cross- laminated, well-sorted, fine-grained sandstone. The set sizes are 1 – 5 cm and occurs in thin beds, around 5 to 10 cm thick at the top of TXB facies. The facies is more common within the coastal outcrops interbedded with PL and LAL facies. Beds are typically 0.2 m thick and forests are often draped by finer grained material (Fig 5.11E, F). Burrowing is uncommon but where identified can only be classified as poorly preserved cylindrical burrows (Skolithos?). Interpretation This facies is interpreted as ripple modification by shallow high-energy flows forming sinuous crested ripples, possibly during overbank flows or modification of previously lain deposits. Ripple cross-laminated sandstone (RL) Description

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This facies is identified only within the coastal outcrops in association with FA1 (Bradley et al. (2018) describes its affinity with FA2). Beds are typically 0.2 to 0.3 m thick and are uniformly fine-grained, well-sorted and interbedded with PL, LAL and TXL facies. Bedding planes are more commonly sharp and locally erosional. Ripple cross-laminations are well preserved often with finer-grained material (mud?) draping the foresets (Fig. 5.11G, H). Bioturbation is dominated by Skolithos burrows which can penetrate through multiple beds. Interpretation This facies is interpreted as deposition from lower-energy overbank flows compared to the PL and TXL facies. Conditions of flow must have been variable to static to allow mud draping to occur. Chaotic Sandstone (CS) Description Beds of this facies are typically 10 cm to 20 cm thick, and are only observed interbedded with RL, PL and TXL within the coastal outcrops. This facies is composed of fine-to medium grained sandstone with occasional laminations of coarse to granule-sized clasts. Beds preserve convolute bedding, remnant cross-bedded or parallel laminated sandstone can be seen locally within the beds but has undergone complete disruption by slumping and soft sediment deformation (Fig. 5.11I, J). Loading structures are common showing complete cut off and isolation of sandstone within fine-grained overbank facies. Beds often preserve thin laminations of finer- grained material, possibly silt grade material. Interpretation Loading and water escape structures provide evidence for a water saturated environment in which sediment was deposited by higher energy flow rapidly over already saturated sediments. This facies is interpreted as possible crevasse or chute channels, which were deposited over water saturated overbank, interdistributary deposits.

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Figure 5.11: Examples of facies identified within the fluvial units. Photos have been annotated on the left to define the structures, the right has been left in order to see the un-annotated rocks. A) Planar cross-bedded sandstone at the Gauging Station outcrop. Scale is 15 cm. B) Atypical fine-grained rip up clast located at the base of an erosional contact at Fourways. Scale is 10 cm. C) planar laminated and low-angle laminated sandstone interbedded at Fourways. Scale is 10 cm. D) Massive sandstone at Fourways. Scale is 50 cm. E) Trough cross-laminated sandstone at Red Bluff. Troughs show fine grained component. Scale is 15 cm. F) Trough cross- laminated sandstone at Red Bluff showing draping. G) Ripple laminated sandstone at Red Bluff set size increases upwards. Scale is 15 cm. H) Ripple laminated sandstone at Red Bluff showing draping of fine-grained material. Scale is 5.2 cm. I) Convolute bedding facies, local preservation of PL or RL but generally the bed is completely disrupted by soft sediment deformation and slumping. Load structures are also common indicating to deposition in a water saturated environment. Scale is 15 cm. J) A closer photo of I showing the detail in the convolute bedding with thin laminations of finer grained material (possibly siltstone). Scale is 15 cm.

5.5.2 Facies associations (FA) 5.5.2.1 Facies Association 1A – Channel-braided fluvial facies This FA is composed of dominantly coarse to fine-grained TXB and PXB facies with subordinate PL, LAL, MS and TXL facies. FA1A consists of a series of fining upwards packages separated by erosional bounding surfaces. These surfaces are typically overlain by pebble and rip-up clast lags. Major erosion surfaces typically occur on a metre scale with individual trough cross-bedding sets from 0.3-1.5 m thick. The overall geometry of the fluvial units is tabular with internal scouring which preserves low amplitude lenticular scours. Trough cross-axes and primary current lineation indicate palaeocurrents dominantly to the northwest. Bioturbation is uncommon, only observed at the top of TXB facies, where it is interpreted to and indicate marine influx after fluvial deposition. A reversal in palaeocurrents is observed in a few channels, interpreted to record as tidal channels (Bradley et al. 2018). This FA is dominantly preserved as amalgamated scours within a tabular fluvial unit but can also be observed as isolated sheets which pinch out laterally and are interbedded with tidal FA2 facies (Fig. 5.12) (Bradley et al. 2018). The amalgamated units are more dominant in the south of the river outcrops (Fig. 5.12A) with increasing isolation north-westwards towards The Loop (Fig. 5.12B). The coastal outcrops are composed of entirely amalgamated scours which is only interrupted by a 10 m thick unit of FA1B (described below).

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FA1A is interpreted as high-energy deposition within a channel-braided system, interspersed with uncommon flood events. The increase in isolated fluvial units towards The Loop is interpreted to record a reduction in fluvial influx upward in stratigraphy until a significant boundary at the top of The Loop. Isolated sheets are interpreted to be a result of episodic higher energy influxes which allowed the fluvial system to prograde over the tidal flats into the basin. The lack of bioturbation within the fluvial facies in the river gorge sections indicates a dominantly alluvial origin for channel-fills, however, bioturbated tops suggest marine incursion, a precursor to the overlying tidal FA2 facies (Bradley et al. 2018).

5.5.2.2 Facies Association 1B – Interdistributary facies This FA is only found in the upper parts of the exposed coastal cliffs section. The sandstones in this FA are the finest grained sandstones in the successions, composed of PL, LAL, RL, TXL and CS facies, punctuated by thin beds of laterally discontinuous FA1A and C (Fig. 5.13). FA1B is 10 m thick and thickens and thins laterally as the overlying FA1C erodes down into it. Occasional FA1B deposits can be seen interbedded with FA1A directly below this thick unit and FA1C above the unit. The contact between FA1A and FA1B in the coastal outcrops is interpreted as gradational and is completely absent within the gorge sections. FA1B is interpreted as overbank interdistributary deposits which formed from high-energy shallow flows that preserved PL, LAL and TXL facies. Interbedded RL facies is likely to have been formed under lower-energy conditions, perhaps waning flows or overbank static water pools, that allowed deposition of mud drapes. Lenses of TXB indicate influx of FA1A fluvial channel facies, which could be high-energy crevasse deposits or branches from the main distributary channel. Bioturbation is rare within this FA and therefore an overbank interpretation is preferred with a reduced marine influence. This study supports the interpretation of this FA by Evans et al. (2007).

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Figure 5.12: Annotations made in VRGS detailing the fluvial geobodies identified. A) A section of the Z-Bend outcrop which shows more abundant FA1A interbedded with FA2. FA1A is dominated by amalgamated scour-fills which are bound by laterally continuous erosional bounding surfaces. B) A section of The Loop outcrop which shows the dominance of FA2 deposits interbedded with isolated FA1A geobodies. This reduction in preservation of FA1A has been interpreted as a reduction in fluvial influx upwards in stratigraphy until the thick amalgamated unit at the top of The Loop. Individual geobodies in B are colour coded to represent individual geobodies and divided into isolated and amalgamated geobodies.

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Figure 5.13: Field photos of FA1B. A) A close image showing thin beds of interbedded RL, PL and TXL facies. Scale is 15 cm. B) A distance image showing the contact with FA1C above the fine-grained more eroded FA1B. Scale is 2 m. C) Laterally discontinuous beds of FA1B between erosional contacts of FA1C. Person is 1.70 m tall. D) A Laterally continuous bed of FA1B interbedded with FA1C. Erosional contact can be seen level with persons head. Person is 1.8 m. E and F) A thin bed of FA1B truncated between FA1C deposits. Scale is 15 cm. G) A thin bed of draped fine-grained deposits of FA1B. Scale is 15 cm. H) Fine-grained deposits at the top of Red Bluff overlain by thin trough cross-bedded crevasse deposits. Scale is 15 cm.

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5.5.2.3 Facies Association 1C – Estuarine facies This FA is only observed at the top of the coastal cliffs section, where is it overlain by FA1B and Cretaceous carbonates (Hocking 1990). This FA erosively overlies FA1B within the coastal outcrops and is absent in the gorge sections. This FA consists of TXB sandstones that are better sorted and finer grained that those at the base of the coastal cliffs and contain abundant Skolithos, Diplocraterion and Daedalus ichnofacies. TXB sets are more than 1 m thick, which is significantly thicker than observed in the rest of the succession and often preserve eroded remnants of FA1B between the troughs. Amalgamation of 1 m deep scour-fill packages can be observed (Fig. 5.14). Several large incised channel-forms are identified in the Red Bluff section (Fig. 5.14B) indicating to possible bank stability allowing for incision, this is discussed later in the paper. This facies is interpreted to have an increasing marine influence upwards with the preservation of intensely burrowed beds which are traceable for several 10s km. Skolithos has often been attributed to brackish shallow marine environments and combined with the finer-grained sandstones and increase in trough cross-set size this association is interpreted as deposition in an estuarine environment. A B

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Figure 5.14: Field photos of FA1C. Both images show a high degree of amalgamated scour-fill which are intensely bioturbated by Skolithos burrows. A) shows a section of the coastal cliffs which is N-S orientated and B) Shows a section NE-SW orientated and in both scouring is evident. A 4 m deep channel-form is observed to erode into amalgamated scour-fills above a unit of FA1B.

5.5.3 Architectural elements Outcrops are commonly oblique to palaeoflow direction and in many cases geobodies appear to have a wider aspect ratio than the true width of a geobody, have apparent planar basal erosion surface and a uniform thickness and sheet-like appearance. When analysed in 3D digital models, it is possible to identify these “sheet-bodies” are channel-forms or scours, with a much narrower aspect ratios. These scours are contained within tabular sandstone units, a minimum lateral extent of 20 km has been observed for the fluvial unit which extends from Ross Graham to Fourways. Within the fluvial units observed five architectural elements have been identified within the digital outcrop models studied; these include amalgamated geobodies, isolated geobodies, downstream and lateral accretion bodies. 1579 geobodies can be characterised into two categories: Amalgamated and isolated geobodies. Table 5.1 summarises geobody statistics identified within the five outcrop models. Calculated true widths and depths of each geobody have been measured from the digital outcrop models using VRGS software, with correction made for dip and strike (method outlined in section 5.4), to generate true width calculations. Figure 5.15 graphically shows the geobody aspect ratios and shows the 20:1 relationship described by Cotter (1978).

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Geobody type Geobody True Width Aspect Interpretation thickness Range (m) Ratio Range Based on Hirst Range (m) X:1 (1991); Gibling (2006), Rygel & Gibling (2006). Lenticular 0.25 - 5.35 0.23 - 85.13 0.2 – 102.5 Lens shaped scour-fills Amalgamated Mean 0.8 Mean 8.07 Mean 10.5 within a mobile (N=765) channel belt. Amalgamated 0.25 - 4.27 5 - 424.5 1.4 - 259 Flow-parallel Sheet-like Mean 1.3 Mean 56.5 Mean 41 exposure or widened (N=573) scours due to increased bank instability. Isolated 0.23 – 2.38 0.80 - 24.8 1.2 - 36 Broad ribbons which lenticular Mean 0.7 Mean 7.24 Mean 10 are single storey with (N=127) incisional 5th order bounding surfaces. Symmetrical and asymmetrical cross- section Isolated Sheet- 0.22 - 3.4 6 – 326.7 4 - 260 Narrow sheets which like Mean 1.0 Mean 67.3 Mean 61.5 are single storey with (N=114) flat non-incisional 5th order bounding surfaces, generally symmetrical cross- section formed by poorly confined flows. Downstream 1-10.17 2.31-396.88 - Downstream Accretion Mean 68.26 Mean 2.56 accretion of midchannel bar Lateral 1.60-11.19 21.82-293.91 - Lateral accretion of Accretion Mean 3.59 Mean 92.26 bars, possibly side attached

Table 5.1: Geobody statistics obtained from digital outcrop models annotated in VRGS. Maximum geobody thickness range is given. True widths are calculated within VRGS using trigonometry from the apparent width and the mean palaeocurrent direction of 303°. The aspect ratio (or width-thickness ratio) is calculated using the calculated true width and depth. These values are a minimum value as truncation of geobodies has eroded tops and margins. Dimensions of downstream and lateral accretion geobodies are also documented.

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Figure 5.15: Graph of width vs thickness of the elements identified within this study (defined in Fig. 9). Star denotes the average width and thickness for all geobodies (29.8:1). The facies identified in table 5.2 are plotted in the colours defined in the graph. The black stipple lines represent the whole channel geobody which encompasses the scour elements. The solid lines define the 20:1 width/depth ratio proposed by Cotter (1978) and the lines at 0.2:1 and 260:1 show the width/thickness ratios defined by this data.

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Amalgamated geobodies These geobodies are observed throughout the studied successions but decrease in abundance from south to the north of the river gorge section, to The Loop, and are commonly observed in the coastal sections. These geobodies can be divided into lenticular or sheet-like. Amalgamation has resulted in the tops and sides of the geobody being truncated against other geobodies and so erosionally bound remnants are preserved. This is interpreted to be due to reworking and cannibalisation within a highly mobile channel belt. As such their original width and thickness is uncertain and so values given are a minimum. Fill consists of dominantly TXB facies with subordinate PL, LAL and MS facies and is attributed to FA1 and FA3. Pebbles and rip-up clasts are common at the base of the element. Measured trough cross-bedding axes show palaeoflow which ranges from 270° to 0° with an average of 303° (Fig. 5.8A). This is supported by primary current lineation which are measured to trend north-west to south-east (Fig. 5.8B). Lenticular amalgamated geobodies are characterised by scour-fills with an incisional 4th order basal erosion surface with moderate to high relief margins, which truncate and erode other geobodies. Packages of amalgamated scour-fills are bound by erosional 5th order surfaces which are generally planar erosive (Fig. 5.16). Scour- fills are typically single storey, but locally multi-storey fills have been observed. Scours range from 0.2 m to 85 m wide, average 8 m and have a maximum thickness of 6m (ranging from 0.1 to 5 m), however, most are typically less than 1 m thick. Width to thickness ratios range from 0.2:1 to 102:1. Generally, these geobodies are less than 1 m thick, however, a large 6 m deep incised channel sits at the base of an amalgamated scour sequence at the Z-Bend locality (Fig. 5.16). This out-sized geobody is 70 m wide and has a stepped basal erosion surface which cuts down into the underlying tidal sediments (Bradley et al. 2018). The preserved channel-form is composed of multiple sets of pebbly trough cross-bedded sandstone which preserve large rip-up clasts of the underlying sediment. This suggests stages of incision before being filled by multiple stages of cut and fill. A downstream accretion package is preserved on its eastern margin, which can be traced for 6 km before dipping below the outcrop surface. Using the scheme proposed by Gibling (2006) and Rygel &

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Gibling (2006) this element has been interpreted as narrow to broad ribbons, which are succession dominated with incisional 5th order surfaces and a flat 6th order bounding surfaces. Thin narrow geobodies are interpreted as erosional remnants from multiple stages of incision. In cross-section these scours are both asymmetrical and symmetrical. Sheet-like amalgamated geobodies are characterised by scour-fills which have a planar 5th order basal erosion surface which shows little incision into the underlying sediment, with a maximum thickness of 4 m, which is more typically less than 2 m thick. They are characterised by low-relief architecture with a planar basal erosion surface and only rarely displaying incisional features (Fig. 5.16). Sheet-like geobodies are broad, with a high aspect ratio of up to 259:1. Widths range from 2 m to 424 m wide with an average of 56 m. Width to thickness ratios range from to 3:1 to 449:1 and deposition is dominated by aggradation. Margins of the geobodies are truncated by other geobodies and represent only partial remnant values. Using the scheme proposed by Gibling (2006) and Rygel & Gibling (2006) this element has been interpreted as narrow sheets which show multi-storey stacking with flat non- incisional 5th order surfaces that have a generally symmetrical cross-section. Due to outcrop orientation, they are interpreted to often represent flow-parallel exposure. These multi-storey and multilateral stacked geobodies are interpreted as scour-fills forming composite channel-fills as erosion surfaces are often difficult to trace out laterally as they are truncated by other geobodies. Composite sections are separated by laterally extensive 5th order erosion surfaces, for a minimum of the outcrop length 1-2 km, are typically 1 - 3 m thick with an unknown lateral extent due to modern erosion. The 6th order bounding surface is frequently planar and can be seen to be laterally extensive for a minimum of 20 km. This forms the tabular fluvial units observed. Lateral and vertical amalgamated complexes are interpreted to have formed by constant reworking under unstable bank conditions and the preservation of wider bodies under low base-level conditions.

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Figure 5.16: Sections of the digital outcrop models from VRGS with annotations of amalgamated geobodies and detailed sketches. Bounding surfaces are annotated in colour with first order bounding surfaces no marked but identified as trough cross-lamina within second order surfaces which are reactivation and bedset bounding surfaces. Third order surfaces are identified as accretion packages which are more identified in more detail within figure 5.18. Fourth order surfaces are the basal surfaces of scour-fill geobodies which are commonly

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truncated and eroded by other geobodies. Fifth order surfaces are taken as the base of the channel sequences and 6th order surfaces are identified as sequence boundaries, better described in section 5.5.4.

Isolated Geobodies These geobodies are observed as single storey channel-fills consisting of TXB facies and are interbedded with tidal facies (Bradley et al. 2018). They are completely encapsulated in FA2 (see Bradley et al. 2018), with palaeocurrent directions of NW and they are not truncated by other fluvial geobodies. These geobodies become increasingly common in the river section northwards from Ross Graham to The Loop. This facies is rarely observed in the coastal outcrops and so is limited to FA1. These geobodies are typically 1 m thick and exhibit lenticular and sheet-like geometries. This element is laterally continuous but can be seen to taper out over several 10s of metres (Fig. 5.17). Isolated lenticular geobodies have a smaller aspect ratio than the sheet-like geobodies and preserve an incisional basal erosion surface (Fig. 5.17). They are typically less than 1m deep, ranging from 0.2 m thick to 5 m thick. Widths range from 0.8 m to 24.8 m and average 7 m wide. Width to thickness ratios range from 1.2:1 to 36:1. Using the scheme proposed by Gibling (2006) and Rygel & Gibling (2006) this element has been interpreted as broad single storey ribbons with incisional 5th order bounding surfaces that can be both symmetrical and asymmetrical in cross-section. Isolated sheet-like geobodies are thin units that are between 0.2 m and 3.5 m thick, laterally continuous with calculated widths from 6 to 327 m with a mean of 67 m. Locally they thin towards the margins, tapering out laterally. The fill is always single storey TXB facies. They have low-amplitude wings (Fig. 5.17) at the margin of the geobody. They have a varying aspect ratio (width: thickness), which ranges from 4:1 to 260:1. Using the scheme proposed by Gibling (2006) and Rygel & Gibling (2006) these geobodies are interpreted as narrow single storey sheets with flat non- incisional 5th order bounding surfaces that are generally symmetrical in cross-section. These isolated geobodies are interpreted to have formed from episodic higher-energy events which were able to pushout further into the basin and flow over the shallow tidal flats.

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Figure 5.17: Sections of the digital outcrop models from VRGS with annotations of isolated geobodies and detailed sketches. Bounding surfaces are annotated in colour with the same scheme as in figure 5.16. Isolated geobodies are not truncated and are encased in FA2 sediments. Two types of geometries are described: lenticular and sheet-like.

Accretion bodies Fifty-eight downstream and lateral accretion surfaces have been recognised and a summary of the key packages are shown in Figure 5.18. They have been identified based on the method proposed by Ielpi & Rainbird (2015) which compares average measured palaeoflow, in this case trough cross-axes with accretion foreset

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migration direction. The macroform packages are composed of coarse-grained trough cross-bedded sandstone with occasional pebbles. The overall geometry of the packages is wedge shaped where preservation allows and can be seen to abut next to channel-like geobodies. Often the macroforms are truncated by scour-fill geobodies and so only preserve erosional remnants. Downstream accretion macroform packages are the most common macroform observed. Packages are between 1 and 10 m thick and laterally extensive for 2 to 397 m where they are commonly truncated by scour-fill geobodies or limited by outcrop exposure. Packages show surfaces which build in a north-westerly direction in the same direction as fluvial flow (Fig. 5.8A). They are filled by trough cross-bedded sandstone 1st order surfaces. These 3rd order surfaces often show erosional and reactivation surfaces (Fig. 5.18). Several cross-cutting channel-forms are observed across this and other bars appear to be oblique to the palaeocurrent direction. Channel-forms are interpreted to have formed by chute-channels (Miall 1977). Stacking of downstream accretion packages is uncommon but is occasionally observed, generally this macroform is interbedded with amalgamated geobodies, rarely isolated geobodies. The largest downstream accretion package is identified at the base of the Z-Bend outcrop which is up to 10 m thick and is laterally traceable for over 6 km northwards to Fourways where it dips below surface. To the northwest a 6 m deep channel-form is identified (Fig. 5.16). Lateral accretion macroform packages are around 1 m thick (up to 11 m thick) and internal sandstones are trough cross-bedded with and show foresets which show aggradation and progradation to the southwest (Fig. 5.18), oblique to measured palaeoflow direction. The largest identified lateral accretion package is identified in the upper parts of the Ross Graham and Hawks Head outcrops which is the lateral equivalent of the channel-form and downstream accretion package of Z- bend/Fourways. This lateral accretion package is 264 m wide and up to 11 m in thickness before it is truncated by scour-fill geobodies and limited by outcrop exposure. The preservation of these macroforms indicates migration and aggradation of bars as described by Miall (1996) and Best et al. (2003). Lateral accretion could possibly indicate meandering conditions, however, Miall (1996) also

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discussed that lateral accretion is also observed in braided channels as side attached (transverse) bars develop. Other studies have suggested that the presence of subordinate lateral accretion surfaces indicates to lateral migration of longitudinal bars in a low-sinuosity system (Bristow 1987) rather than a high-sinuosity point bar succession. Scroll bars have not been identified but it is possible that erosion removed these or they did not form, therefore a meandering affinity cannot be concluded, and a braided system is preferred. From the data in previous studies and this study the original statement by Cotter (1978) that pre-vegetation fluvial geobodies are restricted to ratios of over 20:1 is not applicable to this dataset. Cotter (1978) studied road cutting outcrops of limited extent and the full width of the system was unobservable therefore, the study was an early start and not yet a fully developed assessment. All four elements identified in this study show a spread across the 20:1 line and this should not be the definition of pre-vegetation fluvial geometries and so this definition should be reconsidered. This data shows that there is no relationship between isolated elements and amalgamated elements, for example they cannot be differentiated using the aspect ratio as proposed by Cotter (1978). This is in support of other studies which have concluded the 20:1 is arbitrary and should not be considered a fixed defining point.

5.5.4 Sequence stratigraphy Three distinct cycles are observed within the Tumblagooda Sandstone succession in outcrop (Fig. 5.19). Amalgamated fluvial packages are bound by planar erosional 5th and 6th order bounding surfaces which can be mapped for considerable distances, in one case over 20 km. Each cycle starts with a sharp basal contact, above which preserves a thick unit of multi-storey and multilateral amalgamated scour-fills and bar macroforms. Following this a landward shift in facies is preserved in the form of tidal sandstones (documented in Bradley et al. 2018).

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Figure 5.18: Descriptions of geobody downstream and lateral accretion macroforms. These both start and terminate against a 5th order erosion surface. Downstream macroform shows foreset surfaces which dip in the direction of palaeoflow (to the northwest). Lateral accretion surfaces dip at an angle 90° to palaeoflow. Bounding surfaces are labelled according to Miall (2014), with the same scheme as in figure 5.16.

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Sequence 1 (S1) The base of the succession overlies gneiss basement and the major erosion surface which denotes onset of sedimentation into the basin is interpreted as the first sequence boundary (Fig. 5.19). Above this a 20 m thick package of FA1 consisting of amalgamated scour-fills is observed at the Gauging Station locality. Scour-fill fining-upward cycles are around 1 m thick consisting of pebbly TXB facies with minor PXB and TXL. One local discontinuous lens of fine-grained PL is observed at the top of TXB facies and interpreted as an eroded remnant of overbank deposits. The same PL bed can be seen as rip-up clasts being carried into the bed above. The multi-storey and multilateral amalgamated scour-fills identified here have been interpreted as being deposited under low base-level conditions (Shanley & McCabe 1994; MacNaughton et al. 1997; Martinsen et al. 1999; Holbrook et al. 2006). Above this a flooding surface is observed where sediments grade into intertidal FA2A deposits (Bradley et al. 2018) which are around 80m thick and are occasionally punctuated by isolated sheet-like FA1 units. At the base of the Ross Graham and Hawks Head outcrops these intertidal deposits grade into subtidal FA2B and FA2C deposits (Bradley et al. 2018). These sediments are dominated by RL, BS, and LAXB facies (Bradley et al. 2018). The gamma signature, seen on the left in Figure 5.19, shows a decrease in gamma count and a cleaning upwards trend within this unit. The increase in marine characteristics denote an increase in base-level to allow marine incursions. Conditions are still shallow as exposure surfaces are common within FA2A. Isolated FA1A units preserve a low-sinuosity isolated fluvial facies during a period of rising base-level (Fig. 5.19). In modern paralic models isolated transgressive channels typically preserve high-sinuosity features, for example, lateral accretion packages and mud-filled abandoned channel deposits (Shanley & McCabe 1994). This is interpreted to be a feature which is attributed to pre-vegetation times as the lack of roots and mudstone would have hindered the development of a meandering system (Eriksson et al. 2006; Long 2006). Units of FA1A are around 1 m thick and are interpreted to be a product of rapid avulsion or branching off the main channel belt preserving thin amalgamated scour fills. Isolated FA1a units are interpreted as single

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storey high-energy events which can flood the basin depositing TXB facies over FA2 units. Sequence 2 (S2) The preservation of another 20 m thick package of FA1 is observed at the top of Ross Graham and Hawks Head, which is correlated with the same package observed at the base of Z-Bend and Fourways outcrops. The boundary is interpreted as an erosive 6th order bounding surface that erodes a minimum of 6m into FA2 deposits. This boundary is also highlighted in a spike in gamma readings observable in Figure 5.19 and is interpreted as the second sequence boundary in the succession and a basinward shift in facies. Above this there is a 20 m thick package of amalgamated scour-fills which preserves common cannibalisation and reworking characteristic of low-base level conditions. As in S1, following low-base level conditions an increase in base-level is preserved in the return of marine conditions in a 180 m thick package of FA2a and FA2b deposits. These are again punctured by thin packages of amalgamated and isolated FA1 deposits which become increasingly less common upwards to The Loop. This is interpreted as a steady decrease in fluvial influx into this part of the basin and a landward shift in facies. These fluvial influxes are interpreted to be a product of rapid avulsion and episodic higher-energy flows which were able to prograde further into the basin. This deposited laterally continuous sheet-like sandstones which can be over 1 km wide (value constrained by outcrop extent) or as little as several 100s m wide. The gamma signature for this package shows little deviation and a generally uniform gamma count throughout indicating to aggregation and little environmental change. Sequence 3 (S3) Located in the upper parts of The Loop section there is a 20 m preserved thick package of FA1 fluvial facies which is correlated with the outcrops to the north of the study area, not studied in this study but by Hocking (1991), and with the outcrops in the coastal cliffs. The basal boundary is laterally extensive, highly erosive and is interpreted as a third 6th order sequence boundary (Fig. 5.19). Above this a minimum of 300 m of amalgamated FA1 deposits are preserved which are significantly coarser

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grained than the units observed in the previous sequences. Between The Loop and the coastal cliffs studied in this study Hocking (1991) documents 250 m of very coarse-grained sandstone to granule and pebble grade conglomerates which are similar to what is observed at the coastal cliffs. These sandstones fine upwards throughout the coastal section up to a FA1B unit. The gamma signature throughout this package shows an increase in gamma counts and shows evidence for the fining upwards nature of the system. Marine influence also increases throughout the coastal section with minor evidence for bioturbation in S3. in the upper parts of the coastal section sandstones grade into the fine-grained FA1B unit. This is identified as being interbedded with RL, PL TXL with minor bioturbation. This 10 m thick package thicken and thins and is occasionally punctuated with small lenses of TXB. The gamma character if this package shows a lower gamma count than the surrounding FA1A and FA1C units. The fine-grained unit is interpreted to record interdistributary deposits which preserve evidence of overbank flows, mud-draping and splay deposits. Erosively overlying this is a 5 to 10 m thick package of amalgamated scour- fills attributed to FA1C deposits which is evidenced by the abundance of bioturbation preserved throughout the unit. The set sizes here are significantly larger than previously observed and are much better sorted with uncommon pebbles and rip-up clasts. This unit is interpreted to show a landward shift in facies from a fully alluvial environment, FA1A, to interdistributary, FA1B, and estuarine conditions FA1C, with increasing in marine influence. Where these 6th order bounding surfaces sit within the boarder basin scale is unknown as the position in stratigraphy and its age is still yet to be finally constrained. It is likely to form the onset of basin fill during extension during a failed rifting phase (Hocking 1991). Above the Tumblagooda Sandstone, preserved in boreholes is a Silurian limestone and deeper marine shale sequence. Therefore, this part of the basin stratigraphy is interpreted as an overall deepening cycle which is abruptly terminated by Devonian fluvial red sandstones (Mory et al. 2003).

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Figure 5.19: Sequence stratigraphic model which integrates the outcrop gamma and logs. A Sequence stratigraphic model has been interpreted with the reinterpretation of Hocking’s (1991) FA1 – FA4. A higher resolution FA model has been proposed which ties in with amalgamated FA1 deposits preserving times of low base-level when amalgamation and canalisation was common. Increasing base-level preserves FA2 deposits interbedded with isolated FA1 deposits. Three sequence boundaries have been identified which preserve retrogressive cycles, these tie in with peaks observed in the outcrop gamma. A peak in the centre of the section has been interpreted as the maximum flooding surface, above which aggradation takes place preserving a thick succession of intertidal deposits with decreasing fluvial influx until the next observed sequence boundary.

The Tumblagooda Sandstone has been interpreted to be strongly controlled by base-level changes and availability of accommodation space. The presence of thick amalgamated scour-fill units indicates that the succession was likely to be within the medial to distal DFS zone of Davidson et al. (2013), where the overbank deposits are characterised by palaeochannel deposits. These are interpreted to follow a sequence boundary which then preserves a lowstand event (Holbrook 1996). This is atypical of the model proposed by Nichols and Fisher (2007) as amalgamated braided deposits are characterised as proximal to medial deposits and this succession is interpreted to have been deposited in a low-gradient setting (Hocking 1991). During times of increasing base-level isolated poorly confined fluvial units are interpreted to be the distal reaches of the fluvial system as characterised by Shanley and McCabe 1994; Nichols and Fisher (2007). These are encased in tidal sediments rather than mudstone deposits which has been noted as a characteristic of pre-vegetation successions as thick deposits of mudstone are rarely preserved (Davies et al. 2011). The Tumblagooda Sandstone highlights the difficulty of applying modern sequence stratigraphic analysis to a pre-vegetation deposit, although it has similarities, with deposition of valley-fill (Shanley & McCabe 1994) one model cannot be applied alone.

5.6 Depositional model The dominance of trough cross-bedded sandy fill records migrating sinuous crested dunes within a moderate-energy low-sinuosity fluvial system. Rare planar laminations show occasional upper-flow regime conditions and not a dominance of sheet-flood conditions. Sandy downstream and lateral accretion packages indicate to the formation of mid-channel and side attached bars. The presence of accretion

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macroforms suggests a more complex perennial fluvial style rather than typical ephemeral sheet-braided flash-flood style of sedimentation (Best et al. 2003). Scour- fills are shallow which has been a feature noted within many pre-vegetation successions and has been attributed to the lack of bank stability present to facilitate deep incision. Two methods were used to calculate sinuosity from 765 fluvial trough cross-bedding palaeocurrent readings which span the entire studied section (Le Roux 1994; Bridge et al. 2000). Both methods give a sinuosity of 1.3 which is typical of deposits which are braided (Miall 1977; 2006). Integrating this fluvial architecture data extracted from the digital outcrop models with high-resolution sedimentary logging, suggests a low-sinuosity braided system is the most appropriate description (Fig. 5.20). Based on the described facies and architectural elements the system is interpreted to be a perennial low-sinuosity, low-amplitude, braided fluvial channel belt system characterised by channel avulsion (MacNaughton et al. 1997). The most notable feature of this fluvial system is that mudstone and siltstone are not preserved within the fluvial facies. There is no evidence for fine-grained overbank preservation, and this is a significant characteristic noted in other pre-vegetation fluvial successions (Schumm 1968; Cotter 1978; Long 1978; MacNaughton et al. 1997; Eriksson et al. 1998; Davies and Gibling 2010). Many authors have suggested this reflects the lack of roots to retain mud within the system leading to a profound effect on the processes controlling sedimentation. If any mud were deposited as overbank, it is highly likely that it was eroded by subsequent avulsing channel bodies or reworked by tidal currents. Most of the fluvial bodies identified are truncated and overlain by erosively based fluvial geobodies. Mud production in the absence of plants was likely also reduced, with less soil-associated chemical weathering. It is possible that any detrital mud produced was not deposited in the continental realm but transported into the deeper basin (Fedo & Cooper 1990; Bhattacharyya & Morad 1993; MacNaughton et al. 1997; Eriksson et al. 1998; Long 2004, 2006; 2011; Davies & Gibling 2010; Lowe & Arnott 2016). The braided model (Fig. 5.20A) describes a highly mobile channel belt system that developed on a flat plain, analogous to the Canning River, Alaska (Davidson et

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al. 2013). Channelised high- and low-amplitude scour-fills are common, within the larger channel belt due to cannibalisation and repeated reworking. This process inherently preserved channels that have a high aspect ratio as they widen due to lateral migration (Mack & Leeder 1998; Larue & Hovadik 2006). The channel belt margin is not resolvable in outcrop due to weathering and erosion, but the large distributary system deposited thick, tabular fluvial units over a wide area. Internal scours caused by multiple stages of avulsion and incision are common. Downstream and lateral accretionary packages provide evidence for mid-channel and side- attached bars. It is likely that channels were poorly confined in the absence of roots but would have had some stability for the scours to be preserved. Previous authors have suggested that microbial activity could have provided stability, but recent studies have questioned this (McMahon et al. 2017). Flow conditions would have been constantly high-energy, preserving dominantly trough cross-bedding. Figure 5.20B shows a depiction of the isolated FA1A units which are preserved within tidal deposits of FA2. This is interpreted to have formed by higher energy events which were able to push further into the basin over tidal flats.

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Figure 5.20: A channel-braided model proposed as a part of a mobile channel belt which avulses and migrates across a plain. Individual scour-fills are preserved within the outcrop. B) as stratigraphy progresses upwards this process becomes more dominant as isolated higher- energy events flow further into the basin. This process deposits laterally continuous isolated sheets of trough cross-bedded sandstone.

5.7 Discussion

5.7.1 Pre-vegetation models Pre-vegetation fluvial facies have previously been described as having a sheet-braided architecture (Cotter 1978). This is typically assumed to mean sheet- flood dominated, unconfined upper-flow regime conditions. Models developed for these facies are commonly based on modern ephemeral systems typified by planar laminated sandstones. The architectural elements described in this study record subordinate sheet-flood facies, with a dominance of stacked channel forms recorded,

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displaying complex bar morphology. This would suggest a perennial highly mobile braided system which displays multilateral and multi-storey scouring. Defining the correct model to use has a strong influence on the prediction of facies distribution, lateral/vertical connectivity with importance for modelling subsurface reservoirs. This study has identified a complex mix of sheet and ribbon morphologies (Rygel & Gibling 2006). Davidson et al. (2013) proposed a distributive fluvial system (DFS) model for braided channel belts and describes them in arid, mid-latitude and cold desert climates. The distal zone (the toe of slope where the system interacts with a body of water) is characterised by braided anabranches and compound braid bars, which have cross-bar chutes and channels. This is likened to that observed within the fluvial facies of the Tumblagooda Sandstone, as amalgamation of fluvial channels is common and reworking of previously deposited channel-fills is observed, which would have been a part of a larger channel belt (Davidson et al. 2013). Based on the distribution of facies and architectural style, it would seem appropriate to use the braided DFS model as an analogue for the similar architectures observed within the Tumblagooda Sandstone. Typical paralic sequence stratigraphic models highlight that isolated elements usually preserve high-sinuosity facies (Shanley & McCabe 1994), or delta distributaries cut into mud, whereas the isolated elements preserved within the Tumblagooda Sandstone preserve low-sinuosity sandy fluvial facies cutting into sandy facies. This is interpreted to be a result of the lack of rooted plant systems which would enhance meandering if present and the lack of mud or peat that would tend to lock channels in place in modern systems (Tal & Paola 2007). The geobodies identified within this study suggest a dominance of broad ribbon geobodies with planar basal erosion surfaces and subordinate deep incision. This supports previous observations that in the absence of plants to stabilise the overbank areas, river channels would preferentially spread laterally in thin sheets rather than incise deeply into the underlying sediment, preserving channel-fills which are wider than the original channel-form (Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Davies & Gibling 2010). This could also be a product of a stable craton with low rates of accommodation, and a fluvial system that is avulsing on a plane, dominated by gradual aggradation and constant reworking, rather than

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incising into valleys. In the absence of roots, stabilisation of overbank areas is reported to have been due to stable discharge from a large catchment area fed by year-round discharge (Ielpi et al. 2017). This further supports the hypothesis that the amalgamated fluvial units were deposited under low-accommodation conditions. The observations in this study and several other pre-vegetation fluvial studies, suggests a dominance of trough cross-bedded sandstone with only minor parallel laminated sandstone. Evidence of complex bar-forms suggest an environment which was subject to perennial discharge rather than flash-flood style discharge variations as suggested by Eriksson & Simpson (1993) and Ielpi et al. (2017). Ielpi et al. (2017) described three outcrops with a complex suite of sedimentological structures and architectures, deposited in the absence of rooted land plants. These systems are more closely related to perennial braided rivers rather than ephemeral systems as previously proposed. 5.7.2 The lack of mudstone A notable feature of pre-vegetation fluvial deposits is the lack of thick beds of preserved mudstone (McMahon & Davies 2018). Nearly all published examples define compositionally mature sandstone successions almost completely dominated by high- to moderate-energy sandstone facies (e.g Fedo & Cooper 1990; Røe & Hermansen 2006). Modern climate very strongly influences soil and clay content directly via temperature, rainfall and indirectly via vegetation (Curtis 1990). Dry arid environments undergo less intense chemical weathering and therefore little clay is formed. Modern humid environments are characterised by more intensive weathering and subsequent dominance of clay minerals (Singer 1984). The climatic regime and the lack of deeply rooted plants would have had a strong control on the formation and preservation of mud within Precambrian and lower Palaeozoic basins. The lack of potassium absorption by roots meant that feldspars were more stable, therefore production of clay minerals would have been reduced (Basu 1981; Drever 1994). Plants produce organic acids and change the chemical properties of soil which increases physical weathering rates (Drever 1994). The sediment-binding activity of roots allows soils and feldspars to spend longer in the environment and react more completely. Water retention is increased which permits increased

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hydrolysis (Curtis 1990). Biological weathering is also enhanced by the penetration of roots into the rock (Schumm 1968; Gibling & Davies 2012). The lack of effective vegetation cover prior to the Devonian meant that water was not absorbed by roots and could flow over the land surface eroding the bedrock. The combination of high- energy flows and the lack of vegetation resulted in the development of shallow fluvial channels, which could rapidly avulse, preventing the deposition and/or preservation of mudstone facies (Sønderholm & Tirsgaard 1998). Frequent avulsion would have led to reworking of the alluvial plain and the removal of mud off-shore into the deeper basin (Long 1978; Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Davies & Gibling 2010). The lack of roots would have also inhibited the aggradation of overbank mudstone facies (Kleinhans et al. 2018).

5.7.3 Bank stability The Tumblagooda Sandstone is interpreted to have been deposited during the middle Ordovician to the early Silurian by zircon analysis (Markwitz et al. 2017). This places the formation after the evolution of vascular land plants but prior to the evolution of deeply rooted systems (Kenrick & Crane 1997; Gensel et al. 2001; Gensel 2008; Hillier et al. 2008; Davies & Gibling 2010; Long 2011; Pires & Dolan 2012; Xue et al. 2016). Recent studies (Davies et al. 2011; van Dijk et al. 2013; Kleinhans et al. 2018; McMahon & Davies 2018) have discussed how even in the absence of rooted vegetation, rootless vegetation would be able to baffle and retain the mud, increasing bank stability and developing a stable overbank area (Davies et al. 2011; McMahon & Davies 2018). Vegetation caused mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation mutually enhance floodplain formation (Davies et al. 2011; Kleinhans et al. 2018), causing enough stability to allow for incision and the formation of scour-fills rather than broad sheets that are observed within the Tumblagooda Sandstone and several other pre-rooted vegetation successions. In several thin-section samples clay rims and mud matrix is observed to originate from degraded feldspars. It may have acted to aid in the cohesion of sands and allow for the incision and preservation of lenticular scour- fill packages. Deeply incised channel-forms may have also eroded into semi-

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consolidated sediment allowing for increased bank stability even in the absence of root binding processes.

5.7.4 Comparison of pre-vegetation and post-vegetation braided systems A detailed literature study has revealed that there are only 12 well documented studies in peer-reviewed journals in which pre-vegetation fluvial systems have been described quantitatively and only 5 have been corrected for palaeoflow direction and reconstructed to calculated true width and depth relationships. Due to the oblique nature of many outcrops it is possible that the dimensions of many other previous studies that lack digital outcrop techniques, have a large range of uncertainty, and if cut obliquely, they could bias to apparently wider channels or sheet-like systems. Another issue posed is the limited outcrop extent of many studied exposures. This gives a bias towards smaller systems where larger values cannot be resolved and the true lateral extent cannot be observed (Ielpi & Rainbird 2016). Figure 5.21 shows a comparison in the data obtained during this study and previous studies on the width and depth of ancient and Quaternary fluvial channels (Gibling 2006; Blum et al. 2013). Statistical analysis of geobodies recorded from this study (Table 5.2) are comparable to those reported in other pre-vegetation successions, with width and thickness ratios of other studies varying from 7 to 2543, with most of the values lying between 10 and 100. Width and thickness ratios determined during this study range from 0.2 to 260. The values obtained during this study are two orders of magnitude smaller than the Quaternary channels (Blum et al. 2013) and three orders smaller than the braided channels summarised by Gibling (2006) but are around average for the other reported pre-vegetation successions. It is possible that this is due to several reasons, that the datasets used are more detailed and more accurate than previously used therefore the resolution of the data is higher. It could also be due to the nature of the system; if the system is undergoing increased reworking and cannibalisation due to the lack of bank stability, this would favour the preservation of smaller scour-fills in both the horizontal and vertical axis. A third possibility could be the geometry of the basin at the time of deposition, for example if the base-level is low then aggradation is limited, and reworking, avulsion

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and cannibalisation would be favoured. This contrasts with post-vegetation successions where overbank areas are more stable, allowing for more complete preservation of channels in an environment of increasing base-level. This data presents an argument that the term “sheet-braided” architecture can be applied to post-vegetation braided successions and is not a feature exclusive to pre-vegetation fluvial facies, therefore, it is concluded that no correlation can be made to braided systems being narrower after the evolution of rooted land plants and the term “sheet-braided” should not be used as a description of fluvial architecture. The term also has connotations that the system is dominated by sheet- flood events when many studies have proved that this is not the case (Sweet 1988; Nicholson 1993; Long 2006, 2011; Santos et al. 2014; Ielpi & Ghinassi 2015; Ielpi & Rainbird 2016; Ielpi et al. 2016, 2017). A detailed review by Gibling (2006) describes river types as having an aspect ratio of greater than 20:1, therefore, this value cannot be used as a definition between sheet-like and channelised elements and Cotter’s (1978) definition of sheet- braided prior to the evolution of a rooted plant is an unrealistic determining value to use to define pre-vegetation systems.

Figure 5.21: A graph comparing the statistics from this study on the geobody dimensions from the Tumblagooda Sandstone with the ancient examples of delta distributaries, meandering channels and braided rivers (after Gibling 2006) and with Quaternary examples of channel belts and fills (after Blum et al. 2013). The data obtained from this study show a greater distribution

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between the 0.1-100 m width range which is two orders of magnitude smaller than other reported dimensions. Dots show the average width and thickness published data on pre- vegetation fluvial units (Hjellbakk 1997; Eriksson et al. 2006; Long 2006; Sarkar et al. 2012; Ielpi and Rainbird 2015; Almeida et al 2016; Ielpi and Ghinassi 2016; Ielpi and Rainbird 2016; Ielpi et al 2016; Lowe and Arnott 2016; Ielpi et al. 2017).

5.7.5 Re-evaluation of “sheet-braided” and revised models Cotter (1978) suggested a defining factor for pre-vegetation fluvial systems was the lateral persistence of units, with a planar erosion surface. This became the familiar term “sheet-braided”, defined by observation of width to depth ratios of greater than 20:1. Many authors have cited this and identified a sheet-flood dominant style, and compared them to modern ephemeral systems, with shallow sheet-flood-dominated deposits (e.g Fedo & Cooper 1990; Hocking 1991; Todd & Went 1991; MacNaughton et al. 1997; Eriksson et al. 1998). This study suggests the sheet-braided facies model of Cotter (1978) is not universally applicable for pre-vegetation fluvial systems. The detailed analysis of braided architectures in pre-vegetation deposits using digital outcrop studies of large exposures also suggest that geometries are more complex than initially thought, supporting the work of Ielpi et al. (2017). The statistical data presented in this study suggests that the distinction between sheet-like and lenticular geobodies cannot be defined by an aspect ratio as previously suggested, as both sheet elements and lenticular elements cannot be defined by an aspect ratio of 20:1.

5.7.6 Applicability to subsurface reservoirs The data from the Tumblagooda Sandstone describes a pre-vegetation system that was neither sheet-braided nor sheet-flood dominant but contains a more complex suite of sedimentary structures more akin to perennial mobile braided channel belts. A review of published literature suggests this may be more common in many pre-vegetation fluvial systems. When applied to the subsurface, for mapping and reservoir prediction, it is important that the geometry of these systems is modelled correctly for fluid flow and reservoir prediction. It is important to understand the connectivity and distribution of facies to predict reservoirs away from a borehole. Therefore, understanding the nature of the system is important, and a sheet-braided model would herald different reservoir distribution than an

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amalgamated scour-fill model even in a reservoir which is dominated by sandstone facies such as the Tumblagooda sandstone. Digital outcrop data collection methods, i.e lidar and UAV photogrammetry offers the potential to examine a much larger scale of outcrop, potentially removing a bias towards smaller fluvial systems (Ielpi & Rainbird 2016; Ielpi et al. 2017). This more detailed data allows improved accuracy and generation of statistically meaningful datasets for subsurface prediction. 5.8 Conclusions Pre-vegetation fluvial facies have previously been compared to modern ephemeral systems, as a result of the absence of binding activity of root systems. This study of the Tumblagooda Sandstone finds parallel laminations and massive beds to be uncommon and questions the use of ephemeral systems observed in modern arid climates as analogues for pre-vegetation fluvial systems. The presence of channelised scours and a suite of accretion features are described, with statistical data extracted from the acquired large digital outcrop dataset. This system is interpreted to be more akin to modern perennial braided distributary systems, rather than ephemeral flood style events. This contains architectural elements including braid bars preserved as downstream and lateral accretion surfaces. Complex geobody geometries have also been highlighted with amalgamation, truncation and cannibalisation. Limited bank stability has been attributed to preserved mud matrix which is said to have been baffled and retained by rootless vegetation (Davies et al. 2011; McMahon & Davies 2018). Considering this, the term sheet-braided should be used with caution and not assumed that the system is dominated by ephemeral sheet-like events. Quantitative statistics show two types of scour-fills; amalgamated and isolated geobodies. Analysis of these types and sub-dividing them into sheet-like and lenticular shows that suggestions made about sheet-like channels having an aspect ratio of greater than 20:1 (Cotter 1978) is not feasible and the term should be reconsidered and not used as a defining feature of pre-vegetation fluvial bodies. This is also shown in other reported cases on braided fluvial geometries, both pre- and post-vegetation evolution.

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The lack of thick beds of preserved mudstone within pre-vegetation systems has been noted in previous and during this current study. This has been attributed to the lack of deeply rooted systems at the time of deposition which would otherwise retain mud within the roots, binding and leading to the aggradation of floodplain facies and the preservation of mudstone (Kleinhans et al. 2018).

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Chapter 6: Re-evaluating fluvial architecture of pre-vegetation reservoirs using large digital outcrop datasets: application to subsurface modelling

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Chapter 6: Application of digital outcrop modelling of pre-vegetation fluvio-paralic systems to subsurface modelling

Bradley G-M.*, Redfern J. and Hodgetts, D. University of Manchester, School of Earth and Environmental Sciences, Manchester, M13 9PL, UK *[email protected]

Intended for submission in Marine and Petroleum Geology.

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Abstract During time prior to the evolution of deeply rooted plants, fluvial overbanks were not stabilised by rooting, which limited ability to store and preserve mudstone facies that can baffle fluid flow. Many authors have compared them to modern ephemeral environments, dominated by upper-flow regime runoff and described “sheet- braided” architectures. This study suggests that in the absence of plants, many pre- vegetation fluvial systems formed channelised braided systems rather than sheet- braided and so were dominated by amalgamation, cannibalisation and reworking. This study examines the Ordovician-Silurian Tumblagooda Sandstone of Western Australia, exposed within extensive 3D outcrops along the Murchison River gorge system. Detailed sedimentary logging, integrated with the newly acquired 3D photogrammetry derived from UAV imagery, has been used to extract a quantitative database to characterise the architecture. Acquisition of a large digital dataset has allowed documentation and quantification of 1579 fluvial geobodies and the architecture of this pre-vegetation paralic sandstone outcrop and recognises a heterogeneous system dominated by sheet and ribbon geometries. Geobodies exhibit low- and high-relief lenticular architectures, interbedded with the more typical sheet geometries. Parallel lamination is rare within the study area, suggesting sheet-floods are not the dominant process. Complex accretion surfaces have been identified indicating upstream, downstream and lateral accretion macroforms. We recognise low-sinuosity, low amplitude, multilateral and multi-storey braided architectures with common bar accretion surfaces. This questions the previously reported dominance of laterally continuous sheet-braided architectures. The preserved architecture has been interpreted to reflect limited accommodation, which resulted in cannibalising and reworking of channels and the preservation of broad amalgamated bodies recording repeated episodes of cut and fill enhanced by bank instability due to the lack of plants. Two techniques are proposed to stochastically model pre-vegetation fluvial facies in the subsurface: a simulator based and an object-based approach. The simulator represents the sheet model which overestimates fluvial facies and the object model represents a channelised system which better reflects the outcrop analogue and

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fluvial proportions. They can be modelled as having a labyrinth and jigsaw puzzle geometry comprising a complex suite of scours with limited lateral extent. The facies are complex with tortuous pathways, which interact with scour margins, trough cross-bedded foresets and internal facies distributions. This results in Kh being more heterogeneous, which has a profound effect on reservoir facies prediction and potential reservoir production.

6.1 Introduction Predicting fluvial geometries in subsurface reservoirs is challenging and stochastic models rely on outcrop analogues to condition and provide quantitative data for the depositional system (Keijzer & Kortekaas 1990; Bryant et al. 2000; Xu et al. 2000; Pringle et al. 2004, 2006; Hodgetts 2009, 2013). It is important to accurately document the styles of fluvial sedimentation, environment and conditions of deposition to correctly predict subsurface petroleum reservoir facies and distribution. It is important to quantify and map the distribution of barriers to flow, for example faults, facies changes and cemented beds, to gain information geometry and how they will impede flow of hydrocarbons or water (Pringle et al. 2004; Buckley et al. 2006, 2010; Hodgetts 2009). Hydrocarbon recovery in clastic reservoirs is said to depend on how well understood the architecture of sand bodies is (Labourdette & Jones 2007). Accurate geomodelling of spatial distribution requires accurate statistical data to condition models to populate a grid block from a 1D well. This is done using conceptual models, photopanels, digital outcrop models and seismic data (Pringle et al. 2006; Keogh et al. 2007; Pyrcz & Deutsch 2014). Data that is input into the model is typically highly detailed and so requires upscaling, to improve computational efficiency by sacrificing the detail (Pyrcz & Deutsch 2014; Ringrose & Bentley 2015). The models produced can go on to be used for visualisation of the reservoir, estimations of volume, simulation for fluid flow, well planning, seismic modelling, modelling for enhanced oil recovery and CO2 storage (Ringrose & Bentley 2015). Models do not give true facies proportions and a quality control must be carried out to assess if the model data matches the input control data. It will never be known if the predicted geometries are correct, but application of a conceptual

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model is key to refining the model and producing a suite of possible outcomes (Ringrose & Bentley 2015). The implementation of conceptual models is important here to quality control the stochastic models. Many studies have concentrated on documenting the geometry, lateral extent and dimensions of modern fluvial channels, to build a statistical dataset of aspect ratios for channels which are then applied to ancient systems (Miall 2014). Fluvial units deposited in the presence of vegetated overbanks in modern systems are typically laterally discontinuous and contain heterogeneous facies, many of which are associated with large proportions of preserved mudstone. These are described using end member models as either braided, anabranching or meandering systems (Miall 2014). Pre-vegetation fluvial systems have significantly different sedimentary style, geometry and facies to their modern counterparts that developed in the presence of plant root systems (Schumm 1968; Dalrymple et al. 1985; MacNaughton et al. 1997; Long 1978; 2002, 2004; Gibling & Davies 2012). They have been previously described as being dominated by sheet-braided laterally continuous bodies, forming a homogeneous sheet-like package. However, recent studies have suggested that ancient fluvial systems have a more complex architecture and may not be as simple as previously suggested (Santos et al. 2014; Ielpi et al. 2017; Bradley et al. 2018). This study presents a database of geobody measurements for a pre- vegetation fluvial system determined using high-resolution 3D digital outcrop models. Previously interpreted as being dominated by sheet-braided sandstones (Hocking & Mory 2006; Mory & Hocking 2008), the volume, quality and extent of the new dataset allows recognition of a channel-braided fluvial system, supporting a more complex amalgamated scour-fill model. An object-based model is proposed for amalgamated low-accommodation and transgressive pre-vegetation paralic systems, which is compared against previous models that assumed a sheet-braided dominant system. This highlights how the use of alternative models can have a profound effect on the prediction of subsurface facies distribution, connectivity and reservoir volume.

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6.2 Background Pre-vegetation fluvial geometries have been described as dominated by “sheet-braided” facies. This was a term proposed by Cotter (1978) to define the geometry of genetic units deposited prior to the evolution of plant root systems, which is characterised by flat planar basal erosion surfaces and aspect ratios of greater than 20:1. Many authors have recognised sheet-braided geometries in the lower Palaeozoic and Precambrian rock record, which are commonly described as being dominated by bedload and sheet-flood style sedimentation with beds of planar-laminated and trough cross-bedded sandstones and a notable lack of mudstone over-bank deposits (Schumm 1968; Smith 1976; Cotter 1978; Fedo & Cooper 1990; MacNaughton et al. 1997; Eriksson et al. 1998; Davies & Gibling 2010). Meandering river systems are rare prior to the evolution of rooted plants and only become common in the Devonian, which has been attributed to the evolution of rooted plants and increased identification of mudstone deposits (Knighton 1998; Davies & Gibling 2010; Davies et al. 2011; McMahon & Davies 2018). Roots stabilise the overbank area, retain mud and allow river channels to incise (Smith 1976; Tal & Paola 2007; Davies et al. 2011). Many pre-vegetation fluvial systems have been likened to sparsely vegetated modern ephemeral fluvial systems and commonly described as compositionally mature. These sandstones typically exhibit horizontally stratified, low-angle cross- bedded and trough cross-bedded facies (Bhattacharyya & Morad 1993; Simpson & Eriksson 1993; Hjellbakk 1997; Davies & Gibling 2010) indicative of high-energy currents. This has been attributed to high rates of sediment delivery that would have resulted in channel aggradation and deposited thick successions of sand prone facies (Davies et al. 2011; Santos et al. 2014). This model has been challenged in recent works, Santos et al. (2014), Ielpi et al. (2017) and Bradley et al. (2018) which have suggested that fluvial systems prior to the evolution of land plants were not as simple as previously documented and that more complex geometries can be recognised. The study has been carried out on the Tumblagooda Sandstone, Western Australia (Fig. 6.1). The formation is a fluvial and shallow marine sandstone succession which was deposited in the Southern Carnarvon Basin during the

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Ordovician to Silurian (Hocking 1991, Markwitz et al. 2017; Bradley et al. 2018). The sandstones are exposed in the lower Murchison River gorges in Kalbarri National Park, where around 100 km (laterally) of undeformed stratigraphy gently dips (less than 5°) to the northwest. This provides excellent exposure to study pre-vegetation fluvial geometries in 3D and the interaction with the tidal environment.

Figure 6.1: Study and log locations within Kalbarri National Park, Western Australia. GS- Gauging station, RG- Ross Graham, HH- Hawks Head, ZB- Z-bend, L- The Loop, RB – Red Bluff, MR- Mushroom rock, EG- Eagle Gorge. Numbers denote log number. Purple stars denote log locations and numbers correspond to the log ID.

Reservoir simulations aim to integrate different scales of geology, from petrophysical information, facies, outcrop analogues and large-scale regional geology, to generate a geologically realistic model to optimise field development (Keogh et al. 2007; Pringle et al. 2006). Outcrop analogues are fundamental to modelling subsurface reservoirs and it is important to note geometries, stacking patterns and facies to successfully predict subsurface reservoir facies. End member braided and meandering fluvial systems are typically modelled as having a jigsaw puzzle or labyrinth geometry, whereas shallow marine sands and sheet-flood deposits are typically defined as layer cake (Weber & van Geuns 1990; Miall 2014). Layer cake (sheet-like) geometries consist of packages of laterally extensive sandstone, where thickness changes are typically gradual, lateral reservoir properties

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are consistent, but vertical layering provides barriers or baffles to flow. Based on this theory sequential simulator-based methodology would be a more appropriate way of modelling pre-vegetation fluvial facies in the subsurface to distribute facies proportions using a pixel-based methodology (Hodgetts 2013; Ringrose & Bentley 2015). The oversimplification of reservoir architectures and modelling as homogeneous sheets leads to errors in geometries, facies distributions and potential fluid flow pathways (Keogh et al. 2007). The main limitation of this method is that architectures are not defined, but when elements do not have discrete or complex geometries this is acceptable, such as the case in sheet-braided systems. This method is compared to an object-based approach which allows geobody dimensions from the digital outcrop models to be used in the prediction of reservoir distribution. Channelised fluvial environments are modelled as a series of stacked sand bodies which fit together with no impermeable layers between each unit (Weber & van Geuns 1990). These are often modelled using an object-based modelling methodology, where a series of probability distribution functions are used to describe the geometry of geobodies (Hodgetts 2013). This study uses input parameters derived from digital outcrop data, including facies associations or architectural elements that are defined with geobody width, thickness, amplitude, wavelength and flow direction, allowing geometries and proportions to be better defined and modelled (Pranter et al. 2008; Hodgetts 2013). The accuracy and use of these two methods are discussed in this paper, to assess how each model predicts facies arrangements in relation to the observed fluvial facies in the digital outcrop models. 6.3 Methods Traditional outcrop analysis was used to acquire high-resolution sedimentary logs, noting lithology, sedimentary structures and defining facies. At each locality 3 or 4 logs were taken at different points to generate a correlation to assess lateral facies variability. Outcrop total gamma counts were also obtained to correlate with the sedimentology of each log and to generate pseudo-well data. Samples were obtained, representative of each facies, and analysed petrographically to observe small scale heterogeneity within each facies. Three-hundred point counts were

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obtained for 28 thin section samples and 8 samples were scanned using QEMSCAN techniques in order to digitally obtain more reliable data on the samples. This was used to characterise each facies. Point counts were used to detail the grain type, size, sphericity, grain boundary type or if it was pore space. Digital outcrop models were generated using automated photogrammetry software Agisoft Photoscan®. Photos from a UAV (Unmanned Aerial Vehicle) were stitched together to create a photo-realistic 3D triangulated mesh. Using the software Virtual Reality Geological Studio (VRGS), fluvial geobody geometries were mapped and statistics were extracted. Geobody depth information was taken directly from VRGS as the maximum depth of the outlined geobody. Depth statistics are a minimum value as many of them are eroded or truncated. Geobody width information was corrected using palaeoflow measurements obtained from trough cross-bedding axes and extracted from VRGS to use as conditioning data within subsurface modelling processes. Two conceptual models were determined using previously published literature to detail the isolated sheet-like geobody model and details from this study to detail an amalgamated scour-fill model. The annotated mesh with details of geobody facies distribution was imported into Petrel® where stochastic reservoir models were created using data directly obtained from the outcrop to simulate modelling the subsurface. Two different techniques were used to model the subsurface; a sequential simulation-based approach to model the laterally extensive isolated sheet-like system and an object-based approach to detail the amalgamated model. Channel objects were used to represent the braided channel geometries. Pseudo wells were used to select data from the outcrop data to simulate the sparse data that wells provide. This allowed a comparison of each model to the outcrop analogue to determine the most appropriate way to model these systems. 6.4 Results Facies and facies associations were developed using twenty-nine detailed outcrop logs and correlations (Fig. 6.2) were made using the digital outcrop models and logs.

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6.4.1 Facies Associations Two facies associations have been identified within the Tumblagooda Sandstone, which can be sub-divided into six sub-categories. FA1A consists of coarse- to medium-grained trough cross-bedded fluvial sandstones with rare parallel- laminated and massive-sandstone facies. Beds are typically around 0.5 to 1 m thick and contain pebble and intraformational rip-up clast lags at the base. Scour-fills have been interpreted as both amalgamated and isolated exhibiting sheet-like and lenticular geometries. This is interpreted as recording moderate- to high-energy fluvial flows, within a channel-braided system. A gradational contact is seen into FA1B within the coastal outcrops. FA1B comprises fine-grained ripple laminated sandstones to siltstones which locally preserve mud-draping and minor bioturbation. Isolated coarse- to fine-grained channel-forms punctuate the ripple laminated sandstones, interpreted as overbank flows and crevasse deposits. The laterally continuous sandstones are interpreted as interdistributary deposits. Overlying this is FA1C, which comprises fine- to medium-grained trough cross- bedded sandstones which preserve set sizes around 1 m in height, significantly larger than the ones observed within FA1A. Scour-fills are amalgamated and FA1C preserves intensely bioturbated horizons, often at the top of beds. Bioturbation consists of Skolithos, Diplocraterion and Daedalus burrows. This is interpreted to record estuarine conditions with a marine influence on sedimentation. FA2 is located within the gorge sections and preserves tidal sedimentary packages that are typically fine-grained, well-sorted ripple-laminated, intensely bioturbated and planar cross-bedded sandstones. FA2a is interpreted to preserve intertidal sedimentation with a dominance of ripple laminated and bioturbated sandstones which often preserve exposure surfaces. FA2b is proposed as a high energy subtidal environment dominated by sinuous crested bar forms which preserve evidence for alternating currents. Both facies associations lack typical mud-grade tidal indicators observed in modern tidal environments (Bradley et al. 2018). This has been noted as a significant

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difference between pre- and post-vegetation alluvial and shallow marine successions. Three cycles are observed within the succession (Fig. 6.2), each starting with amalgamated scour-fill fluvial influxes into the basin which are characterised by thick deposits of amalgamated scour-fills which preserve sheet (Fig. 6.3A, B), deep incisional (Fig. 6.3C) and lenticular (Fig. 6.3D) geometries. Amalgamated geobodies show a high degree of reworking and cannibalisation leading to truncation and erosion of other geobodies. The amalgamated scour-fills are interpreted to have formed under low-accommodation space conditions (Shanley & McCabe 1994; Holbrook 1996; Hampson et al. 1999). Following this a gradual transgression from fluvial to tidal is shown by local reworking of the top of fluvial facies by organisms and may indicate marine flooding and an increase in base-level. As tidal conditions dominate, fluvial facies are preserved as isolated units within tidal sediments (Fig. 6.3E) which is interpreted as high-energy flows (Shanley & McCabe 1994; Nichols & Fisher 2007). A key differing feature of pre-vegetation successions is the uncommon documentation of meandering deposits which is attributed to the absence of rooted plant systems and reduced storage capacity of mud within the overbank (Knighton 1998; Davies & Gibling 2010; Davies et al. 2011; McMahon & Davies 2018). Occasional units of amalgamated channels and isolated sheets within the tidal facies are interpreted as being a result of avulsion events or short-lived high-energy progradation into the basin. Limited bank stability has been attributed to the presence of rootless vegetation which would have been present at the time of deposition. This would have allowed mud to be trapped and aid in bank cohesion.

Figure 6.2 overpage: A summary correlation of the facies associations interpreted from outcrop facies. Correlations from outcrop and digital outcrop models.

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Figure 6.3: Examples of geobodies identified in the digital outcrop models A) Planar basal erosion surface with amalgamated scour-fills above. B) Shows a dominance of sheet geometries due to the orientation of the outcrop being along channel axis at Fourways. C) Deep incisional channel -form surrounded by amalgamated scour-fills. D) Amalgamated scour-fills at Z-Bend. E) Shows a dominance of isolated geobodies at The Loop. They are not laterally continuous and can be mapped tapering out.

6.4.2 Geobody geometries 1579 Geobodies have been identified within the 5 digital outcrop models produced from the river gorge outcrops. The geobodies have been classified into two categories; isolated and amalgamated geobodies. This has been determined on the basis that sheet and lenticular geobodies cannot be differentiated and sequence stratigraphic analysis has highlighted that it is more important to document isolated and amalgamated units within the subsurface (Chapter 5…). Table 6.1 shows a summary of statistics generated from geobodies. The width has been corrected using trigonometry and the apparent width. Thickness has been taken as the maximum preserved thickness for each geobody.

The amalgamated geobodies are characterised by erosion and truncation therefore the value given is a minimum. They consist of both lenticular and sheet-

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like geometries with planar and incisional features. The thickness values range from 0.25 to 5.35 m and average 1 m. Calculated widths range between 0.23 and 424.5 m and average at 28.8 m. These geobodies are interpreted as scour-fills which have a broad ribbon geometry (Rygel & Gibling 2006).

Isolated geobodies are characterised by being poorly confined and preserved interbedded with tidal FA2 units. Geobody thicknesses range between 0.22 - 3.4 m and average 0.86 m. Calculated widths range between 1.2 – 326.7 m

Geobody type Geobody Geobody Aspect Geometry Interpretation Thickness Width Ratio Based on Gibling (2006), Range (m) Range (m) Range Rygel & Gibling (2006). X:1 Isolated 0.22-3.4 1.2-326.7 1.2-260 Mixed narrow sheets geobodies Ave: 0.86 Ave: 35.64 and broad ribbons. (N= 241) 15.2% Amalgamated 0.25-5.35 0.23-424.5 0.2-259 Broad ribbons preserved geobodies Ave: 1.02 Ave: 28.81 as scour-fill packages. (N=1338) Values are given as a 84.8% minimum as erosion and truncation has reduced the size of the preserved geobody.

Table 6.1: Geobody statistics obtained from digital outcrop models annotated in VRGS. Maximum and average geobody depth is given directly from geobodies, widths are calculated within VRGS using trigonometry from the apparent width and using the mean palaeocurrent direction of 303°. N refers to the number of geobodies in that type and the percentage refers to the total proportion of each geobody type.

Most of the geobodies identified are thin, having an average thickness of around 1 m (Table 6.1). Width range, however, spans four orders of magnitude extending above and below the previously proposed 20:1 aspect ratio. The statistics plotted on Figure 6.4, show that the geobodies have a wide range of aspect ratios. Cotter’s (1978) 20:1 aspect ratio has been plotted to demonstrate that there is no aspect ratio which can distinguish between sheet-like and lenticular geometries. Also plotted is data provided by Gibling (2006) and Blum et al. (2013) which compares the data provided by this study to published data on ancient meandering, braided and delta distributaries and to Quaternary examples of channel belts and channel fills.

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Plotted in black dots is data obtained from other pre-vegetation case studies. This suggests that the Tumblagooda Sandstone generally conforms with other published data (Fig. 6.4). It is noted that pre-vegetation successions have a smaller aspect ratio than that of post vegetation fluvial sections summarised in Gibling (2006) and Blum et al. (2013) (Fig. 6.4). This may be due to repeated reworking and cannibalisation of previously deposited units in the absence of rooted plant systems. Many authors have proposed that post-vegetation braided systems have aspect ratios larger than 20:1, displaying planar sheet-like architecture (Gibling 2006). The graph shows that the average width and thickness for 11 pre-vegetation case studies and the Tumblagooda Sandstone are over the 20:1 regression line. The data provided for modern systems and ancient cases also shows a large proportion of data over the 20:1 regression line and therefore using this as a defining factor for pre-vegetation case studies is inaccurate and should be used with caution.

Figure 6.4: A graph plotting the data obtained from digital outcrop models of the Tumblagooda Sandstone data range in red, average value given by the blue star, with reported data for ancient delta distributaries, meandering rivers and braided rivers (Gibling 2006) and modern channel belts and channel fills (Blum et al. 2013). Previously described pre-vegetation datasets have been plotted in the black circles (Hjellbakk 1997; Eriksson et al. 2006; Long 2006; Sarkar et al. 2012; Ielpi & Rainbird 2015, 2016; Almeida et al 2016; Ielpi & Ghinassi 2016; Ielpi et al 2016; Lowe & Arnott 2016; Ielpi et al. 2017)

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6.4.3 Petrography Sedimentary deposits are characterised on a hierarchical scale from preserved channel-fill such as lamina to units, spatial distribution, geometries and morphology (Miall 1985, 1996, 2014; Koltermann & Gorelick 1996; Robinson & McCabe 1997; Corbett et al. 1998, 2012; Bridge 2003; Rygel & Gibling 2006). Braided fluvial reservoirs are very heterogeneous with heterogeneity on various scales (Miall 1985; Corbett 2012, 2014). Complex reservoir architectures give uncertainty when characterising reservoir properties (Bowman et al. 1993; Corbett et al. 2012) and characterisation of this is critical to understanding the subsurface architecture and internal heterogeneity (Ambrose et al. 1991; Tyler & Finley 1991; Scheibe & Yabusaki 1998; Vassena et al. 2009; Ronayne et al. 2010). Geological heterogeneity on various scales can vary statistics in fluid flow models, cross flow between layers in communication within a reservoir is said to affect well test responses which is a result of internal heterogeneities within the layers (Corbett et al. 2012). These heterogeneities will affect the pressure distribution as well as the vertical and horizontal permeability within the reservoir (Corbett et al. 2012). Pre-vegetation braided systems are high net-gross and as this study has shown they are interbedded with complex channel-fill with multiple stages of scour and fill, accretion bodies and overbank deposits. Complex depositional variations such as multiple bounding surfaces and reworked lag deposits often lead to differences in cementation and fluid flow (Taylor & Ritts 2004; Corbett 2014). Analysing the subsurface in 3D is difficult as access prevents samples being obtained throughout the given petroleum field. To overcome this a study of petrography, outcrop gamma and facies has been undertaken. Table 6.2 summarises the results of point counting of 28 representative samples from the five main lithofacies observed in outcrop, and 8 samples which were studied using QEMSCAN analysis. Samples were obtained from representative facies in each log, sampling both amalgamated and isolated fluvial geobodies as well as tidal ripple lamination, bioturbated, parallel laminated and cross-bedded sandstone facies. Samples are dominantly subarkosic to quartz arenites which are very well to moderately sorted. Fluvial (FA1) sandstones are composed of coarse- to fine-grained

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sandstones which have an average Phi of 1.9 (medium-grained sandstone). Grains are angular to rounded monocrystalline and polycrystalline quartz with subordinate feldspars and zircon grains. Fluvial sandstones are generally more cemented than tidal sandstones, with quartz overgrowth cements (Fig. 6.5a) or clay derived from degraded feldspars (Fig. 6.5b). Observed porosity ranges from 0 to 16% within fluvial sandstones, which is made up from primary intergranular and secondary degraded grains (Fig. 6.5c). Tidal FA2 sandstones are range from very well sorted to poorly sorted but are dominantly well sorted. They are dominantly fine-grained with a mean Phi of 2.1 (fine-grained sandstone). Grains are rounded to well-rounded monocrystalline and polycrystalline quartz with subordinate feldspars and zircon grains. One sample contained an unidentified calcified fragment possibly organic in origin. Tidal sandstones are less cemented with haematite clay rims where red in colour and bleached where white in colour (Fig. 6.5d). Observed porosity ranges from 4-22% from primary intergranular and secondary degraded grains. Many samples also contain mud grains which are observed in Figure 6.5e and f. Observations were made in the field that there is a key difference between units which are red in colour and units which are bleached white. Generally finer grained and more poorly sorted facies (e.g. bioturbated, parallel laminated and ripple laminated sandstone) are red in colour and coarse-grained facies (trough cross- bedded and cross-bedded sandstone) are white in colour. Results from petrographic analysis show that in red coloured sandstones grains are coated with a hematite clay matrix and primary porosity is maintained. This is shown in Table 6.2 where the observed porosity is generally higher within the ripple laminated, parallel laminated and bioturbated sandstones. Due to diagenetic processes quartz cementation has occluded much of the primary porosity within the white coloured sandstones, and triple points are observed within the quartz overgrowths. This is typical within the fluvial facies, indicating that they would have been more porous and permeable before diagenetic fluids were able to pass through and remove the haematite clay rims. Degradation of feldspars has resulted in the production of clay and enhancement of secondary porosity from eroded grains. This can be seen within

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Table 6.2 where the observed porosity of trough cross-bedded sandstone is highly variable from 0 to 19%.

Figure 6.5: Thin section micrographs. A) abundant quartz cement in bleached sandstones. Scale is 0.5 mm. B) Clay rims coating grains. Scale is 0.2 mm. C) Remnant haematite rims left from eroded feldspar grains. Scale is 0.2 mm. D) Red and white laminations in the outcrop are highlighted here by haematite clay rims forming the red colouration and no rims in the bleached white in the white areas. Scale is 0.5 mm. E) and F) both showing clay grains observed within the samples. Scales are 0.2 mm.

Outcrop gamma shows an unexpected profile, within the fluvial units, gamma counts decrease upwards indicating to cleaning upwards cycles within fluvial units. This could show the basal channel lags present within fluvial units which has said to

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cause additional cementation, with the sand grains acting as a pseudo-matrix (Taylor & Ritts 2004; Corbett 2014). The gamma signal for the tidal units is low and generally does not change upwards in FA2. This shows that conditions of deposition are relatively constant throughout FA2. The data presented illustrates the complexity of facies and the different scales of heterogeneity that exists within this fluvio-tidal sedimentary succession. It shows that there are complex interactions across all scales of bounding surfaces, from laminations to bed bounding surfaces which would increase the tortuosity and complexity of fluid flow within the subsurface (Corbett et al. 2012; Corbett 2014) and affect well test response (Toro-Rivera et al. 1994). Reservoirs would contain barriers to flow with finer-grained FA1b deposits which separate more permeable FA1a and FA1c deposits, in the coastal sections. Thick deposits of cemented fluvial units would also act as a barrier to flow in this case study. In a reversed circumstance, fluvial sandstones must have been more porous and permeable to allow diagenetic fluids to preferentially bleach them, therefore it is possible that had diagenetic fluids not been present then the more porous units (fluvial in this case), would localise flow and petroleum would more likely be found in these units and possibly flow along channel axis. Isolated fluvial units also add complexity to flow within the tidal units as fluids may flow into the fluvial sandstones and cause ponding and localised accumulations, as flow is restricted through the tidal units. These isolated units are lens shaped which results in poor lateral continuity over the scale of the whole outcrop area and would lead to localised flow variations. The amalgamated geobodies form a thick laterally continuous unit which can be traced for 10s of kilometres exhibiting good lateral connectivity, within internal heterogeneities from scour-fills and accretion packages.

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Mean Architectural Mean Qtz Mean Other Mean FA Feld Mean Phi Porosity % Grains % element % % sorting % Amalgamated 1.9 0-16 84-100 1a 98.0 1.2 0.8 Well sorted fluvial Medium Mean 12 Mean 88 1.9 8-16 84- 92 Isolated fluvial 1a 99.0 0.2 0.8 Well sorted Medium Mean 11 Mean 89 Downstream 1a 99.4 0.6 0 Well sorted 0 100 accretion 2.2 11-22 78-89 Intertidal 2a 97.3 1.5 1.2 Fine Well sorted Mean 15 Mean 85

2.1 4-13 87-96 Subtidal 2b 95.5 3.3 1.3 Well sorted Fine Mean 9 Mean 91 Tidal Channel 2c 86.3 13.7 0.1 Well sorted 18 82

Table 6.2: The results from point counting thin sections and QEMSCAN analysis of plug samples, comparing architectural elements.

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6.4.4 Modelling Some previous models of pre-vegetation fluvial systems have suggested a dominance of sheet-braided deposition, extending over large areas of the basin. This study and recent studies (Santos & Owen 2016; Ielpi et al. 2017; Bradley et al. 2018) have suggested deposition from channelised fluvial processes in times prior to the evolution of rooted plant systems. To compare the two concepts, two models are proposed: one assuming a sheet-braided geometry (Fig. 6.6A) with sheet-flood structures and one assuming a channel-braided geometry with complex internal sedimentary structures and channel to channel interactions (Fig. 6.6B). This modelling process has assumed there would be no cementation of fluvial bodies (as seen in the Tumblagooda Sandstone) to observe the facies distribution predicted using the digital outcrop models. In the sheet-braided model, a well drilled at location A would intersect isolated and stacked homogeneous sheets, which would be characterised by laterally continuous thin beds of sandstone extending beyond the edges of the field. The internal structure would be relatively homogeneous; comprising massive, planar laminated or low angle cross-bedded stacked sandstones, with generally high Kh and reduced Kv. The second model is representative of the amalgamated fluvial systems based on observations from the Tumblagooda Sandstone (Fig. 6.6B). In this model Well B would intersect a heterogeneous channel-braided system similar to that seen in modern braided systems, with macroforms that accrete downstream and laterally. There would be a more complex arrangement of bedforms, including foresets, bounding surfaces and channel margins. Beds often taper out laterally and are truncated by other geobodies, giving a multi-storey and multilateral stacked system, which consists of internal scours, trough cross-bedding, parallel laminations and accretional foresets. Tortuosity would be predicted to be more variable and generally higher, with a lower Kh perpendicular to channel palaeocurrent and enhanced Kh parallel to the palaeocurrent. Higher permeability zones could be juxtaposed next to lower permeability zones. Fluid flow could be channelised and a complex interplay between higher permeability zones, bounding surfaces and channel margins would

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give variable Kv/Kh. Single storey isolated channels are laterally discontinuous and disconnected which, unlike in the sheet model, will strongly influence how the reservoir performs by laterally constraining fluid flow. A 3D model was built based on the statistical distribution and size of geobodies observed from outcrop data, shown in figure 6.7. This model represents an environment of deposition dominated by a high energy braided channel system, which deposited stacked trough cross-bedded sandstone units containing bar macroforms. This conceptual model was used in visually conditioning data for the subsurface modelling process as it is important to understand the 3D nature of the system.

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Figure 6.6: Two different models presented to indicate the difference in the sheet-braided where geobodies are sheet-like, extending for large distances, with nothing to constrain flow. In cross section this would result in stacked thin (typically less than 1 m thick) fluvial units. These units would be laterally continuous beyond the edges of typical oil and gas fields and internal facies would be relatively homogeneous. A second model presented in this study, based on observations from outcrop data suggests a more complex depositional system with thick stacked units of multi-storey and multi-lateral channels, interbedded with sheets and tidal sandstones. The channelised scours truncate against adjacent channels. Although overall the fluvial units are laterally continuous, the internal structure consists of channel scours, trough cross-bedded sandstone and accretion foresets. In plan view channels are confined. Single storey channels may exist if accommodation is high, during periods of transgression.

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Figure 6.7: A conceptual model for the depositional environment. FA1 is dominated by fluvial sedimentation with FA1a being fully fluvial, FA1b being formed in interdistributary areas and FA1c forming in the fluvial/tidal transition zone with abundant Skolithos ichnofacies. FA2 was formed in an intertidal and subtidal zone where energy was high, and macroforms dominated. Shallow areas away from the fluvial influx could form small scale ripple lamination. Bioturbation is a common feature in the tidal zone.

6.4.5 Stochastic modelling Digital outcrop data collection methods presented in this study have offered a highly accurate method of studying a much larger dataset and shown that sheet- braided geometries are not the dominant feature of this succession. Computer based reservoir simulations have been common practice in the oil industry for many decades and they rely on data from outcrop analogues. Fluvial reservoirs are often modelled to be simple laterally discontinuous channels (Deutseh & Tran 2002) buy they can vary from a single ribbon channel to complex multi-storey and multilateral channels, however it is often assumed that correlation from well to well is not possible (Meilng et al. 1990; Corbett et al. 1998; Gibling 2006). Reservoir models stochastically distribute channel facies to honour the conditioning data whether this is outcrop data or well data (Deutsch & Tran 2002). Using the existing sheet-braided model for pre-vegetation fluvial systems, units would be expected to be extensive with increased chance of well to well correlations. Using the outcrop control data collected from the Tumblagooda Sandstone and taking the conceptual models presented in figure 6.6, stochastic models of the subsurface were produced. The simulator-based method was implemented for the sheet-like situation and an object-based method for the channelised units. There are two possible scenarios presented: - A simple case where the system is modelled as a “layer cake” (Weber & van Genus 1990) where sheet-like fluvial facies alternate with the interbedded tidal facies. This used the sequential simulator method which is a pixel-based simulation which fills each cell sequentially (Journel et al. 1998).

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- A more complex model where the amalgamated fluvial zones are characterised by complex geobodies as jigsaw or labyrinth models by using object-based algorithms (Weber & van Genus 1990; Journal et al. 1998; Deutseh & Tran 2002).

During this study statistical data, outcrop models and conceptual models were used in building stochastic models within Petrel®. Figure 6.8 shows a summary of the outcomes from one of the four facies models produced using geobody width and depth information (Pringle et al. 2006) and the digital outcrop data as conditioning data. Initially to create amalgamated model, all the data provided from the digital outcrop model was used (Fig. 6.8A). Figure 6.8B & 6.8C both present outcomes using only a subset of data, selected using pseudo wells as conditioning data.

6.4.4.1 Object-based model Figure 6.8A shows results from the object-based model using the entire dataset. This method matches the conditioning data well, but the algorithm attempts to fit all the narrow and wide geobodies into the model. In doing this while honouring the conditioning data, the large channels are disproportionally represented, giving an outcome which is populated by many string-like narrow channels. This is not what was observed at outcrop. This method over-populates the cells with fluvial reservoir facies, giving a gross overestimation of fluvial volume within the reservoir. Figure 6.8B shows another object-based model, reflecting the multi-storey and multi-lateral nature of the system using conditioning data from pseudo-wells. This model is a better representation of the digital outcrop model and shows increased amount of tidal sedimentation in the zones where transgression would be dominant with the isolated fluvial facies. Amalgamated fluvial zones are more accurately modelled and reworking is represented. In plan view a braided system is apparent reflecting more accurately the conceptual models. This model suggests that facies between two wells would not be correlatable within the transgressive zone, whereas in the amalgamated fluvial zone lateral correlatability would be high.

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6.4.4.2 Simulator model Figure 6.8b shows results from a simulator model with data from pseudo- wells, which represents the sheet-like model. This was the previous assumption of the main geobody/facies in pre-vegetation fluvial geometries. A layer cake sheet-like reservoir consists of packages of laterally extensive sandstone. This model does not accurately reflect the digital outcrop data and all the cells are populated with reservoir facies, giving a geologically unrepresentative model, with little tidal sediment present. Accretion geobodies are generally under represented but in the case of the Z-Bend outcrop a large overestimation of the accretion element is modelled. This facies has a large proportion in this zone, however the algorithm does not confine the facies to one area such as an object model would, instead it is modelled across the whole zone. Production and recoverable reserves estimates may be overestimated with a sheet model as no complex geometries are assumed and stochastic modelling overpopulates cells even when the data is conditioned to the input data. In order to gain data on connectivity and facies distribution statistics, the model had to be simplified to the isolated, amalgamated and accretion elements (Fig. 6.9). Table 6.3 gives a summary of the data produced on the amalgamated vs isolated facies from the sheet model and the amalgamated model. The data shows that the sheet model gives up to a 20% higher proportion of fluvial facies compared to the amalgamated model. This is interpreted to be due to the proportion of isolated geobodies estimated. This can be seen in the Loop model as this model generally has a higher proportion of isolated elements. This has a reduced effect in the models which have more amalgamated facies, i.e Z-Bend and Fourways.

(Next page) Figure 6.8: The three modelling techniques (Petrel®). The models were populated using the mesh data from the digital outcrop model as conditioning data. Values for channel width and depths were obtained from Table 6.1. A) Shows an object-based stochastic model of The Loop outcrop using all the data from the mesh. This method matches the data well, however due to the high volume of data input it cannot populate the model with larger channels and only shows lots of small channels, creating a dense network of string-like channels. Most cells are populated with reservoir and there is no representation of tidal (background) sedimentation, which gives an over estimation of the amount of fluvial sediments within the succession compared to the observed outcrop. B and C) Present models using only selected data equivalent to intersected wells, derived from the mesh produced in VRGS. Where the

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outcrop is terraced another well was put in to capture the data, as though it was a vertical succession. B) Shows the simulator-based sheet-like model. This populates every cell with reservoir facies and does not reflect the outcrop data as no tidal sedimentation is considered. This would yield an over estimation in the amount of fluvial reservoir in the model. C) Shows the object-based stochastic model, which best represents the outcrop data and the proportion of tidal sedimentation. This is a better representation of the geometry of the fluvial facies and better reflects the outcrop data.

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Loop Loop Hawks Head Hawks Head lenticular sheet-like lenticular sheet-like geobodies geobodies geobodies geobodies % % % % Background 30.69 10.95 35.11 16.18 Amalgamated 27.61 11.06 34.76 39.6 Isolated 35.15 72.38 19.55 34.43 Accretion 6.55 5.6 10.58 9.79

Proportion of fluvial facies 69.31 89.04 64.89 83.82

Z-Bend Z-Bend Fourways Fourways lenticular sheet-like lenticular sheet-like geobodies geobodies geobodies geobodies % % % % Background 10.08 8.88 14.76 8.04 Amalgamated 65.8 60.61 42.76 39.87 Isolated 14.9 22.47 20.5 29.25 Accretion 9.22 8.05 21.98 22.84

Proportion of fluvial facies 89.92 91.13 85.24 91.96

Table 6.3: Data on facies proportions generated from stochastic models for isolated vs amalgamated bodies. The data shows that the sheet model estimates a higher facies proportion than the amalgamated model. The difference in this is greater at The Loop as there are higher proportions of isolated geobodies which are overestimated in the sheet model. In the models which contain a higher proportion of amalgamated facies the proportion of fluvial facies less, i.e the Z-Bend and Fourways model.

Figure 6.9 (Next page): Stochastic models generated for the isolated and amalgamated elements show that the channelised model (A and B) produces laterally continuous facies within the amalgamated channel units but the isolated elements are not present in both wells 1 and 2. The sheet model (C and D) produces laterally continuous facies which are present in both well 1 and 2. Models A-D are from Hawks Head model. E and F show how tortuous the channelised model is (from Fourways) and flow pathways would be complex.

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Reservoir connectivity is a fundamental control on reservoir productivity and so it is important to understand the connectivity of fluvial facies within the subsurface (Larue & Hovadik 2006), as wells which are widely spaced are difficult to correlate in many fluvial reservoirs. The sheet-braided model previously proposed implies that wells placed anywhere would encounter the same fluvial body at the same stratigraphic horizon and facies between wells would be simple, as previous authors have described deposits which are laterally extensive to spread across the basin plain with aspect ratios of up to 1000:1 (Schumm 1968; Miall 1977; Fuller 1985; Bhattacharyya & Morad 1993; Long 2004, 2006, 2011). Previous authors have noted that in the absence of plants, fluvial systems would have been akin to arid modern flash-flood ephemeral systems, where laterally continuous sheets of episodic upper- flow regime discharge spread across the basin as an unconfined flow, removing all fine grained material from the system and transporting it offshore (McKee et al. 1967; Miall 1977; Tunbridge 1981; Fedo & Cooper 1990; Bhattacharyya & Morad

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1993; MacNaughton et al. 1997; Eriksson et al. 1998; Long 2004, 2006; 2011; Lowe & Arnott 2016). This study observes a more complex system and figure 6.9 shows the facies distributions for the preferred amalgamated scenario vs the sheet-braided fluvial system. Connectivity was used in order to display the 3D geometry of the projected fluvial systems (Fig. 6.9B, D). The amalgamated model (Fig. 9A, B) has less fluvial facies than the sheet model (Fig. 6.9C, D). If a well was to penetrate this field in different places, the sheet and the amalgamated models would give very different production strategies, for example well 1 and 2 in Figure 6.9. In the sheet model the fluvial facies encountered in each well would be the same horizon and connected across the field, therefore production and water-flooding would be simple. In contrast to this the amalgamated model shows that well 1 only encounters fluvial facies in the isolated zones as they are not present in well 2 and this would result in a different production and water-flood strategy and zones of higher flow rate. Enhanced tortuosity can be seen in Figure 6.9e and f, as with increasing channelisation comes a more complex flow pathway than in the sheet model.

6.5 Discussion

6.5.1 Comparing depositional systems The simple classification of fluvial channels based on typical modern river morphologies cannot be applied to pre-vegetation successions and processes active at the time of deposition need to be considered. Fluvial architecture descriptions are based on a hierarchical system which has to describe the channel belt, channel morphology, overbank morphology and stacking patterns (Miall 2014). Previous studies of pre-vegetation fluvial geometries have purely described a simple sheet- flood system which was devoid of macroforms and channel morphology. Other studies have noted that it is important to document the point within the sea level curve when defining geobody dimensions for appropriate use in reservoir modelling (Bryant & Flint 1992). This study has presented a case where pre-vegetation braided fluvial facies have similarities to their modern low-accommodation space (lowstand?) counterparts (Shanley & McCabe 1994), with channelisation over a vast

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extent, development of accreting macroforms, and complex internal facies variations from the foreset level to the channel level (Shanley & McCabe 1994; Holbrook 1996; Hampson et al. 1999). However, they do also have differences due to the processes that were or were not active in the time prior to the evolution of rooted plant systems. It can be assumed that the amalgamated unit is laterally continuous as the amalgamated elements identified in this study, and other pre- and post-vegetation braided systems, are typically extensive and will likely span the width of the field, which is typical of the unstable nature of the overbanks. However, the processes active in the absence of plants need to be considered as pre-vegetation fluvial systems do have differences to their modern counterparts. Channels are generally shallower and wider with incisional features which are not as deep as vegetated systems. Widening has been attributed to increased reworking and amalgamation of channel facies. One important difference in pre-vegetation fluvial facies typically lack mudstone preservation therefore the system is sand rich, however studies have shown that the complex heterogeneities between channel margins and foresets can affect fluid flow (Jones et al. 1995; Corbett et al. 2012). This is unlike meandering systems which are characterised by isolated channel elements encased in mudstone and dominated by point-bar sediments. There is a notable lack of mudstone preserved within the Tumblagooda Sandstone and other pre-vegetation successions which has been attributed as a defining feature of pre-vegetation paralic sandstones (Bradley et al. 2018). Pre-vegetation systems have been proved to have a mixture of sheet and ribbon geometries within the same horizon, more lenticular geometries have been observed by using digital outcrop methods. Traditional outcrop analysis could mislead interpretation and give a bias towards sheet geometries if geobodies geometries are not corrected using palaeoflow direction. It is important to understand that the internal structures of the fluvial units are heterogeneous. Isolated elements are thin and laterally confined and are more likely to cause a compartmentalised reservoir.

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6.5.2 The revised model Previous attempts to define channel geometry of pre-vegetation fluvial successions describe simple sheet-braided geometries and do not define stacking patterns. Previous depositional models have suggested pre-vegetation fluvial facies were dominated by sheet-like beds that had a laterally extensive sheet-like geometry (Fig. 6.8B). Previous studies have typically been carried out on small outcrops which are difficult to correlate, which impedes the ability to define the lateral extent of the fluvial body. Traditional outcrop analysis methods often do not accurately record the geometry of the individual geobodies that have a very large lateral extent. This study has used large scale outcrops to re-define pre-vegetation fluvial geometries and described a channelised system which is dominated by scouring, reworking and cannibalisation. These lenticular geometries are interbedded with more sheet geometries. This study has identified key stacking features such as amalgamated and isolated geobodies as connectivity of sand bodies is crucial in exploiting hydrocarbon reserves. This high-resolution outcrop study has recorded a more complex fluvial system and suggests that in time prior to the evolution of land plants sheet-braided geometries were not dominant. Individual geobodies are thin and have a high aspect ratio similar to braided channels reported in the literature and in modern systems (Gibling 2006; Blum et al. 2013). When we observe a pre-vegetation fluvial outcrop it is possible that there are uncertainties about the geometries due to oblique outcrop exposures, but when digital outcrop models are used to map out fluvial geometries it is possible to conclude that these systems are channel-braided, and this gives a more accurate facies model for use in subsurface modelling. Facies proportions are inherently harder to control using simulator methods as there is no way of defining the aspect ratio of channels within the model, and so larger proportions of fluvial facies will be given by this method. Sheets by their nature are also larger and will produce a higher proportion of fluvial facies. This study therefore concludes that the sheet model is unlikely to accurately reflect the nature of pre-vegetation fluvial systems as the geology, sedimentology, geometry and proportions are all inaccurate when looked at in outcrop and modelled in the subsurface, plus when stochastically modelled the

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resultant model, even when trained to conditioning data does not resemble the outcrop analogue.

6.5.3 Implications Initial reconnaissance studies and log correlations of the Tumblagooda Sandstone identified laterally continuous thick tabular sheets of trough cross-bedded sandstone. Only when large-scale outcrops were studied, using drone photogrammetry, was it revealed that the fluvial units have both lenticular and sheet-like geometries. Other previously disputed successions would benefit from being revisited in light of modern thinking on pre-vegetation systems and re-studied using modern technology. This may result in the identification of more channelised successions and the idea that geometries were sheet-braided being discarded. The sheet-braided model implies that production rates are likely to be higher and if water injection is required then placing the injection well anywhere would be effective to sweep hydrocarbons to the production well. Whereas in a channelised model, fluid flow rates would be higher in the amalgamated channel zones and initial flow would come from the higher permeability elements within the channels (Corbett et al. 2012). Fluid flow would be more tortuous due to the interactions between channel margin and internal facies structure. Production rates would be lower and strategic placement of the injection well would be required as the fluvial bodies may be isolated and correlations between wells may not be accurate. Corbett et al. (2012) showed that hydrocarbon well test responses in heterogeneous fluvial systems are often dependent on the initial flow from higher permeability zones, whether the channel is recharged by effective 3D migration or if there is no vertical connectivity and channels are left depleted and there is intra-layer cross flow. This means that if the system is assumed to be sheet-like then reservoir performance may be significantly higher than if it is channelised as proposed in this study. The amalgamated channel bodies are more likely to be connected in multi- storey or multilateral units (Larue & Hovadik 2006). Wells drilled will encounter complex facies arrangements, inter-well connectivity would only be applicable in the amalgamated multi-storey units and not in the single storey isolated units (Mack & Leeder 1998; Larue & Hovadik 2006). Facies between wells would be complex with

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channel macroforms and complex bounding surfaces between channel scours. This study has shown that connectivity is a key factor in characterising the subsurface and understanding this is important for reservoir calculations, predictions, production and flooding. Tidal units identified between the isolated elements are finer-grained, more structured and bioturbation creates cemented and more poorly sorted horizons (Bradley et al. 2018). Other fine-grained sections identified as interdistributary deposits are laterally continuous and may compartmentalise the reservoir. This may result in several baffles to flow even though the whole succession is generally a “tank of sand”, the vertical and lateral internal facies variations would strongly control fluid flow with flow enhanced along channel axis. 6.6 Conclusion Previously, pre-vegetation successions have been described as dominated by sheet-braided architectures, with preservation of stacked thin beds of parallel laminated and trough cross-bedded sandstones. This suggests that the fluvial facies are relatively homogeneous and laterally extensive with implications for reservoir modelling. This study records a pre-vegetation system that displays a more complex architecture. Data presented in this study has shown that the sheet-braided (Cotter 1978) architecture, previously thought to dominate pre-vegetation fluvial facies, does not define true architectures and may possibly reflect limitations of the data available (limited extent studies) in previous work. This study records complex high aspect ratio amalgamated braided architectures with macroforms and isolated channel geobodies interbedded with sheet elements. It is proposed that these systems comprise braided channels with internal scours and complex bars. Complex interactions are observed between geobodies that eroded and truncated earlier geobodies and non-reservoir sediments during frequent reworking and avulsion from moderate energy flows. Pre-vegetation fluvial systems are more complex and develop complex accreting macroforms. Stochastic models were generated to compare the sheet and channelised architectures. Simulator- and object-based methods were used to assess the impact of the different architectures and the optimum modelling techniques that most

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closely resemble the digital outcrop model. From this it was concluded that a sheet- like model overestimated the amount of fluvial reservoir facies within the grid block. A channelised model gave the most comparable model to the digital outcrop model. In the latter, the isolated elements and tidal sediments were better represented, which would have a profound effect on the connectivity, performance, volume predictions and drilling strategy within the reservoir. The channelised model proposed in this study implies that wells drilled will encounter complex facies arrangements, inter-well connectivity would only be applicable in the amalgamated multi-storey units and not in the single storey isolated units. Facies between wells would be complex with macroforms and complex bounding surfaces between scours. This study has shown that connectivity is a key factor in characterising the subsurface and understanding this is important for reservoir calculations, predictions, production and flooding.

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Chapter 7: Synthesis and implications

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Chapter 7: Synthesis and implications The aim of the thesis was to re-evaluate the sedimentology of the Tumblagooda Sandstone considering recent advances in sedimentology that highlight the differences between pre- and post-vegetation paralic systems. The study focuses on two main facies: the characteristics of tidal deposits and the style and geometry of fluvial units. The research was carried out using a multidisciplinary approach, integrating traditional outcrop sedimentology with digital outcrop modelling. The sedimentary character of pre-vegetation paralic systems has been described in detail based on high resolution sedimentary logging of the key features and mapping the geometries of preserved fluvial geobodies. UAV photogrammetry was used in reconstructing digital outcrops in 3D and allows for the interpretation of over 1500 fluvial geobodies. The findings are discussed in relation to an interpretation previously made by Cotter (1978), as either sheet-braided or channel- braided systems. The results suggest a need to revisit some outcrops which have previously been assumed to have a sheet-braided character. A statistical dataset has been generated and stochastic modelling undertaken to compare a sheet-like vs a channel-braided model. The results can be applied to other lower Palaeozoic and Precambrian paralic successions in outcrop and the subsurface. The work presented in this thesis examines the importance of using the correct conceptual models for subsurface analysis, to characterise the facies variability, architecture and geobody connectivity for improved reservoir extent and volume prediction. Six key questions were presented at the beginning of the thesis and have been discussed throughout the research chapters 4, 5 and 6, these will be revisited and summarised in this chapter. A conceptual model is presented and discussed to highlight the wider implications this has for pre-vegetation successions and subsurface modelling.

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7.1 Pre-Vegetation tidal sedimentology At the beginning of the study it was identified that there was uncertainty and often a quite different published interpretation for the depositional environment of mature thinly bedded sandstones deposited prior to the evolution of deeply rooted land plants. One of those successions was the Tumblagooda Sandstone, Western Australia. The work by Hocking (1991) and Trewin (1993a, b) suggested quite different interpretations for the depositional environment, with evidence provided for both marine and continental deposition. One key observation was the lack of mud preserved in the Tumblagooda Sandstone, a common feature identified in many other pre-vegetation sandstone units. Twenty-one out of thirty-seven documented global examples of pre- vegetation paralic sandstones were found to have less than 2% preserved mudstone. Mudstones are rare in Archean – Cambrian alluvial successions with only 3% containing >10% fines, compared to 74% in the Silurian- Devonian (Davies & Gibling 2010). McMahon and Davies (2018) identified that from the time up to the Middle Ordovician to the early Carboniferous there was an increase of 1.75 orders of magnitude of recorded mudstone deposits. Modern tidal flats often contain a large proportion of mud-grade material which, during periods of static tide, falls out of suspension and deposits onto foresets creating the typical heterolithic bedding. Due to their lack of modern characterising features sand rich pre-vegetation deposits have frequently been described as interbedded with enigmatic ripple laminated sheet sandstones. An extensive literature study indicated that in pre-vegetation successions the description of mud-draped foresets, flaser and lenticular bedding and other indicative mudstone sedimentary structures was rare. The lack of mudstone is interpreted to be due to several possible combined causes: limited mud delivery from the continent due to different weathering processes, the lack of land plants available to retain mud within the continental environment, and high-energy hydrodynamics of the fluvial and tidal system. This is discussed in more detail in section 7.6.

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7.1.1 Characteristics of pre-land plant tidal sandstones Tidal processes were clearly still active prior to the evolution of rooted land plants. Plate tectonic reconstructions (Veevers 2004) show broad areas of shallow marine conditions where tidal deposits should be abundant in the rock record, but a review of published literature suggests (Table 4.2) they are severely under represented/reported compared to post-vegetation successions. One possible reason for this is that the absence of abundant mud in the paralic system and the lack of deposition of tidal indicators may lead to the mis - interpretation of shore-line deposits as continental deposits. However, even in the absence of mudstone, many typical tidally generated sedimentary features should be present. These are summarised and compared to modern tidal indicators in table 7.1. In isolation these features are often enigmatic, but when a number of key features are identified together, it is possible to deduce a tidal environment. It is still possible to apply modern analogues to paralic sedimentology, but there is need to take into consideration the impact the lack of plants would have had on the system and the apparent lack of mudstone preserved in many of these pre- Devonian paralic successions. Several globally disputed sections would benefit from being revisited with this new knowledge in mind. Pre-vegetation tidal features Post-vegetation tidal features Bioturbation Bioturbation Herringbone cross-bedding Herringbone cross-bedding Foresets highlighted by microbial films – * Mudstone draped foresets – tidal occasional reported rhythmites bundles - Exposure surfaces - windblown structures, rhythmites rain drops Exposure surfaces – * desiccation, Short wavelength and symmetrical ripples rain drops Ripple lamination Reactivation surfaces * Flaser, wavy and lenticular bedding Sandstone dominant Ripple lamination Rare body fossils, abundant trace fossils Reactivation surfaces No plants to cause rooting

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* Heterolithic facies – mud dominated Body fossils abundant Rootlets

Table 7.1: Comparison of sedimentary features in pre- and post- vegetation successions (refs for post-vegetation facies: Nio & Yang 1991; Kvale & Barnhill 1994; Greb & Archer 1995; Willis & Gabel 2001; Martinius et al. 2005; Dalrymple & Choi 2007; Nichols 2009). * notes where mudstone would usually be preserved. 7.2 Fluvial architecture Several conceptual models had previously been proposed for pre-vegetation fluvial geometries; these include sheet-braided geometries (Cotter 1978), upper flow-regime flood events, channel-braided, and meandering point-bars (Cotter 1978; Fedo and Cooper 1990; Bhattacharyya & Morad 1993; MacNaughton et al. 1997; Eriksson et al. 1998; Long 2004, 2006; 2011; Hartley et al. 2015; Ielpi 2016; Lowe & Arnott 2016; Santos & Owen 2016). This study has presented evidence for complex channel features within a pre-vegetation system, identifying amalgamated facies which preserve complex channel bar macroforms. Many pre-vegetation fluvial systems were formed because of sustained perennial flow (Best et al. 2003; Went

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2005) rather than flash-flood processes, which is confirmed by the recognition of subordinate preservation of parallel laminated facies. Chapter 5 presented data from a high-resolution digital outcrop models generated from newly acquired UAV data, which allowed identification of 1579 fluvial geobodies. They have been classified into two categories: sheet-like and lenticular channel-like geobodies (based on Cotter 1978). No clear defining aspect ratio for sheet or lenticular geometries could be observed and it is suggested that Cotter’s (1987) original 20:1 ratio used to define sheets from channels should be re- assessed. All architectures (sheet-like and lenticular geobodies) identified in this study show a spread across the 20:1 line and no clear relationship could be discerned based on aspect ratio to differentiate between sheet-like and lenticular elements. The data presented in this thesis presents an argument that the term “sheet- braided” architecture can be applied to post-vegetation braided successions and is not a feature exclusive to pre-vegetation fluvial facies. Post-vegetation braided systems preserve geobodies that often exceed the 20:1 ratio, displaying planar sheet- like architecture and high aspect ratios with mixed sheet and lenticular geometries (Gibling 2006). Based on this study of the Tumblagooda Sandstone, it is suggested that the geometry of pre-vegetation braided channels is not exclusively sheet-like, as this does not reflect the observed geobody geometry or preserved stacking patterns. Figure 7.1 graphically depicts the aspect ratios for amalgamated and isolated geobodies observed in this study and it can be noted that there is very little difference in the width and depth values. The lack of root systems reduced bank stability, allowed for rapid avulsion and reduced the amount of incision into the underlying sediments which hindered the development of steep sided channels. Connectivity of these geobodies will profoundly affect the way the subsurface reservoir behaves. There are similarities in planform, behaviour and depositional facies between pre- and post-vegetation paralic successions. Although processes active at the time need to be considered such as the style of run-off, palaeoclimate, weathering and erosion styles. Studies have shown that the identification of complex channel structures and tidal facies in pre-vegetation successions are not grain-size dependant

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(Hartley et al. 2015; Bradley et al. 2018; Ielpi 2018) and this work has shown that in the absence of plants, fluvial and tidal systems are comparable to modern analogues. In subsurface modelling it is important to document the stacking patterns as well as the fluvial geometries to assess the connectivity of geobodies. Two end members have been identified from the digital outcrop models: amalgamated and isolated geobodies (Fig. 6.4). Amalgamated geobodies are typically characterised by erosion driven multi-lateral and multi-storey scours which truncate other geobodies due to repeated reworking and cannibalisation preserving only remnants of the previously deposited geobody. The amalgamated geobodies observed in this study are interpreted to have formed during periods of low base-level (Shanley & McCabe 1994; Holbrook 1996; Hampson et al. 1999). Isolated geobodies do not truncate other fluvial bodies and are typically described as narrow sheets deposited during periods of increasing base-level. In modern systems isolated fluvial facies can occur within muddy megafans and delta distributaries but a key component of the Tumblagooda Sandstone is the lack of mudstone preserved and therefore this is ruled out. 7.3 Depositional model This study has presented an improved conceptual model which illustrates the difference between the previously documented a simple sheet-braided model and the more complex amalgamated model. Previous conceptual models have assumed processes were akin to modern arid ephemeral systems, with a dominance of flash flood events which deposited simple sheet-flood facies. They were distinctly different from river systems which deposited after the evolution of deeply penetrating root systems and so lack commonly observed modern analogue features (Schumm 1968; Cotter 1978; Fedo & Cooper 1990; Bhattacharyya & Morad 1993; MacNaughton et al. 1997; Eriksson et al. 1998; Tirsgaard & Øxnevad 1998; Long 2004, 2006, 2011; Davies & Gibling 2010).

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Figure 7.1: A graph showing the aspect ratio for geobodies identified from digital outcrop models of the Tumblagooda Sandstone. This graph plots the values for isolated geobodies vs amalgamated geobodies. There is little difference in the data distribution. Geobodies are typically narrow and wide and the differentiation between each is minimal.

There is contention surrounding the interpreted depositional environment of many mature clastic pre-vegetation sandsheet deposits. This was the case for the Tumblagooda Sandstone, where Hocking’s (1991) Facies Association 2 has been interpreted as both marine and continental. Petrographic composition is mature with a dominance of quartz with subordinate feldspars. Grains have an immature texture with generally sub-angular to sub-rounded, low sphericity grains.

7.3.1 Facies associations Based upon new detailed outcrop facies sedimentology, petrography and digital outcrop models, a new conceptual model is proposed which re-characterises the original four stratigraphic units (which Hocking (1991) called facies associations) into two facies associations, FA1 and FA2. Facies Association 1: FA1a is characterised by trough cross-bedded sandstone with subordinate parallel-laminated and massive sandstone facies. Pebble and rip-up clast lags are common throughout. This is interpreted as deposition from fluvial processes. FA1b is dominated by thin beds of fine-grained ripple laminated sandstones and siltstones, this is interpreted as interdistributary deposits. FA1c is composed of large scale (>1

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m) trough cross-sets which are heavily bioturbated with Skolithos ichnofacies. This is interpreted as a marine influence on fluvial processes within an estuarine environment. Facies Association 2: FA2 preserves a mixture of marine and non-marine facies such as ripple laminated, cross-bedded and heavily bioturbated sandstones. Secondary reworking structures include adhesion structures and wind-blown ripples. These sediments are interpreted to have formed in a dominantly intertidal and subtidal environment which was subject to periods of emergence. Thick intensely bioturbated facies are interpreted to be formed during periods of non-deposition during flooding and minor sea level rise as these beds are at irregular stratigraphic intervals throughout the succession and do not appear to be cyclic in any manner. Thin bioturbated units and cemented crusts formed during periods of slack-tide as thin microbial mats could form. Based on the data presented in this thesis the fluvial system is interpreted to have been a perennial low-sinuosity braided river, characterised by mixed low amplitude sheet and ribbon geometries (Rygel & Gibling 2006) which preserved both planar and lenticular geometries. This river system terminated within a tidally influenced shallow marine environment. The reliability of the sheet-braided model has been called into question, with the statistics of geobody aspect ratio collected in this study. In many previous studies it is not clear if corrections were made for apparent palaeoflow and oblique exposure angles. It is possible that the dimensions documented within previous studies, which lack digital outcrop data, have a large range of uncertainty and offer a bias to apparently wider geometries. This old model has been revised and replaced by a model which suggests facies are more complex and other important outcrops should be revisited with this new knowledge in mind. Aspect ratios are highly variable to the process of channel widening and cannibalisation, which appears to be enhanced in pre-vegetation systems. This study presents evidence for a new conceptual model which combines the sedimentology, sequence stratigraphy and the depositional environment identified for the Tumblagooda Sandstone (Fig. 6.7). A complex

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sedimentological history has been identified with deposition from a braided fluvial system, with mid-channel bars which migrated downstream and laterally. Chute channels would have existed between the barforms, and the main channel thalweg would be filled by sinuous crested dunes. The energy of the system would have been moderate to high, but flood conditions were not prevalent. During periods of transgression marine conditions prevailed with the deposition of tidal laminated sandstones and isolated fluvial facies.

7.3.2 Sequence stratigraphy This study has identified units of amalgamated geobodies and units of isolated channels within tidal deposits. Shanley & McCabe (1994) developed a simple sequence stratigraphic model for paralic systems observed in post-vegetation successions. This model has been applied as an analogue for the facies observed within the Tumblagooda Sandstone, where we observe low-accommodation amalgamated channel sandstones which are overlain by transgressive marine units which preserve isolated channel bodies (Shanley & McCabe 1993, 1994; Hollbrook 1996; Hampson et al. 1999). It is noted that there are three cycles identified within the Tumblagooda Sandstone which all initially preserve low base-level amalgamated fluvial sandstones. During this period fluvial facies are characterised by multi-storey and multi-lateral geobodies which were subjected to reworking and repeated cannibalisation. Sedimentation following this is usually characterised by isolated geobodies, encased within tidal sediments. This is interpreted to have been due to transgression of the system and inundation of marine waters (Shanley & McCabe 1994; Davies et al. 2010). These isolated sheet sandstones are characterised by low-sinuosity facies, which is atypical to previously recorded Mesozoic or modern paralic sequences (Shanley & McCabe 1994). This has been attributed to the lack of plants, which reduced bank stability and channel sinuosity. The palaeocurrents within these isolated geobodies are to the northwest, coherent with alluvial palaeocurrents, and they lack bioturbation expected from tidal channels. Thin units of amalgamated channel sandstones are interpreted to be a result of small-scale avulsion events. Davies et al. (2010) noted that low-gradient pre-vegetation systems that terminated

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in a marine environment allowed transgressions to advance rapidly for considerable distances inland. A small base-level fluctuation resulted in progradation of the fluvial system basinward over considerable distances creating a low-relief sequence boundary (Boyd et al. 2006). The limited biostratigraphic control for dating the Tumblagooda Sandstone in the broader Southern Carnarvon Basin, has hindered a full analysis of sequence stratigraphy in a broader basin context. 7.4 Comparison with modern analogues Modern analogues which are completely devoid of plants are difficult to find which means a true perspective of a world without plants is rare to find on Earth. Many authors have suggested that modern arid environments are the closest analogue we have and assumed that all pre-vegetation successions were formed by low-sinuosity sheet-floods. The lack of large-scale lateral accretion packages preserved within the Tumblagooda and many other studies implies a scarcity of point-bar deposits and that braiding was the dominant process. In modern cases, braided distributive fluvial systems are typically observed in high-gradient areas (Davidson et al. 2013), however in the absence of plants and with high-energy sustained flows it may have been possible for pre-vegetation rivers to exhibit braided planforms throughout the length of the river. The Canning River, Alaska has been suggested as a possible analogue for pre-vegetation systems. The Canning is a large distributary system which is braided in nature, even in the lower reaches of the plain where it enters the marine realm (Davidson et al. 2013). It is fed by glacial meltwater and is generally devoid of vegetation, but it is not totally barren. The intertidal dunes within the Bay of Fundy, Nova Scotia, were identified as an analogue for a depositional environment dominated by high-energy cross-bedded tidal sandstones, like those identified within FA2 of the Tumblagooda Sandstone. The Bay of Fundy is characterised by straight and sinuous crested dunes, between 0.3 and >1 m in height when exposed at low-tide, which typically have parasitic interference ripples preserving the less dominant ebb tide deposits. Interdune areas are typically ripple laminated and bioturbated. The bay is macrotidal (up to 15 m range) (Klein 1963) and these dunes are exposed during low tide allowing for aeolian processes to modify them. The dunes also lack typical mud deposits that would be expected in a

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tidally dominated environment despite the estuarine areas being very mud dominated. This is due to the large tidal range that is funnelled into the bay which creates high-energy flood tides and subordinate ebb tides. This is analogous to the high-energy sedimentary structures and the dominance of unimodal palaeocurrents observed within the Tumblagooda Sandstone outcrops. This study has suggested that pre-vegetation paralic systems share similarities with modern and post-vegetation successions. Analysis suggests that there was a dominance of braided planform styles during the time prior to the evolution of vascular rooted plants, and architectures were not limited to a sheet- braided style. The evolution of plants is recorded to have coincided with the identification of large volumes of preserved overbank mudstone and heterolithic strata within the rock record, but it may have been possible to preserve modern facies even in the absence of plants, such as large scroll bars (Hartley et al. 2015). Successions should be studied with this in mind, and the interpretation based upon grain-size variations (between coarse-grained “braided” channels and fine-grained “meandering channels”) should be approached with caution and other data should be obtained, such as palaeoflow direction, channel geometry and accretionary data (Ielpi 2018). A Summary of the comparison between pre- and post-vegetation systems is given in Table 7.2. It can be noted that pre-vegetation systems are comparable in some ways to post-vegetation fluvial successions, however, there are key differences, for example: braided systems are observed everywhere with little spatial and temporal differences, whereas in modern cases braided systems are generally observed in high gradient and dryland environments. Mudstone is rarely observed within pre-vegetation successions and the mud contained in reported tidal successions is variable and does appear to correlate with latitude (see appendix 1).

Table 7.2 (Next page): Comparison of pre-vegetation features observed with post-vegetation literature from end member cases (Bridge 2003; Gibling 2006; Miall 1996; Rygel & Gibling 2006; Nichols 2009; Davies & Gibling 2010; Hartley et al. 2010; Davidson et al. 2013; Miall 2014; Owen et al. 2017).

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Pre-vegetation fluvial features Post-vegetation fluvial features Style Braided Braided Delta Meandering Temperate (but Climate (typical) All Dry land, humid All climates found in most climates) Plants None Low High High High gradient and Gradient All Low Low desert lowland Discharge High Flashy - high Moderate to low Moderate to Low Stability Low Low High High Sinuosity Low Low Variable High Avulsion rate High due to reduced bank stability High Low Low Sand content High High Highly mixed Low Mud content in Variable – more sand continental Rare Low but present High dominant realm Mud content in (Low latitude) (Mid latitude) (High latitude) shallow marine Low Low Moderate Low Moderate High realm (see appendix 1 for studies) Low sinuosity Sandstone Mixed amalgamated and isolated Low sinuosity ammalgamated and Isolated point bars preservation low sinuosity Isolated bodies isolated Geometry Sheets and ribbons with gentle margins Sheets Ribbons Ribbons and sheets

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Based on this study of the Tumblagooda Sandstone: Average of all geobodies: 30:1 High Aspect ratio 2 - 245 7 – 940 Average amalgamated geobodies: 24:1 15 – 15000+ Average isolated geobodies: 34:1

Distribution

Braided throughout the system from proximal Mixed system most reported – braided proximal region depositing to distal even over a low-relief slope into marine amalgamated sandbodies and single thread sinuous in the distal waters region – deposits isolated sandbodies (after Davidson et al. 2013).

Depositional characteristics

Braided typically dominated by sheet-like deposits, meandering Amalgamated palaeo-channel overbank. characterised by isolated pointbar and crevasse deposits in mudstone. Isolated channels encased in tidal deposits. Overbank is muddy and vegetated in both cases.

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7.5 The lack of mudstone Uncertainties remain about why there is a lack of mudstone preserved within pre-vegetation paralic systems. It is known that terrigenous mud is formed by the chemical weathering of rocks and volcanic ash, which ultimately controls soil formation (Potter et al. 2005). Important controls on mud production within the continental realm are type of sediment source, climate, tectonics, sedimentation rate; residence time and vegetation cover (Potter et al. 2005). This study has discussed that the lack of documentation has been due to complex interplay of several combined factors: • Plants produce organic acids and change the chemical properties of soils which increase physical weathering rates (Drever 1994). Water retention is also increased which permits increased hydrolysis (Curtis 1990). Biological weathering is also enhanced by the penetration of roots into the rock (Schumm 1968; Gibling & Davies 2012). • Residence time plays a strong control on the amount of mud contained within the terrestrial environment. The sediment-binding activity of roots allows soils and feldspars to spend longer in the environment and react more completely. The lack of plants meant that K-feldspars were more stable in terrestrial sediments due to the absence of potassium absorption by roots (Basu 1981). If less clay was being produced, wind-blown aeolian sand grains would have been much more common in marine deposits (Potter et al. 2005). • Climate very strongly influences soil and clay content directly via temperature, rainfall and indirectly via vegetation (Curtis 1990). Dry arid environments undergo less intensive chemical weathering, therefore little clay is formed. Humid environments are characterised by more intensive weathering and subsequent dominance of clay minerals (Singer 1984). Wet highland environments are a major contributor of large volumes of mud into the basin, whereas arid highlands only contribute a small amount into arid basins and arid lowlands contribute negligible amounts of mud material (Potter et al. 2005).The climatic regime and the lack of vascular plants would have led to similar processes observed in modern arid climates occurring within both arid and humid climates in the time

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prior to the evolution of land plants (Tirsgaard & Øxnevad 1998; Went 2005). This would have had a strong control on the formation and preservation of mud within Precambrian and lower Palaeozoic basins. • The absence of deep-rooted systems would have enhanced reworking and erosion of overbank material by fluvial processes, as well as inactive processes which are observed today such as binding and retention of mud-grade sediments. The lack of plant cover is also reported to have affected mechanical erosion, reducing the binding of channel margins resulting in increased avulsion and dominance of high- energy fluvial systems (e.g. Schumm, 1968; Cotter, 1978; Davies & Gibling, 2010). • The increased energy input may have resulted in reworking by high-energy fluvial processes. Exposed floodplains without vegetation would be subject to enhanced aeolian reworking, with much of the fine-grained material winnowed and transported away. • The combined effect of high-energy flows entering the shallow marine environment, and the reduction in the amount of mud delivered and produced in the continental realm would have resulted in the bypassing of any mud-grade material into the deeper basin. It has been documented that high-energy environments, typically associated with high-energy fluvial influx do not always preserve tidalites (Davis 2012). • Relative sea level and basin subsidence has a profound effect on the influx and deposition of marine mud as during a lowstand mud will be forced further into the basin off the continental shelf and highstands typically transport mud further onto the continental shelf (Potter et al. 2005). During the Ordovician Period it is documented that Gondwana was partly glaciated, therefore there was a global lowstand possibly contributing to a reduced amount of mud on the continental shelf. The Tumblagooda Sandstone is likely to be a result of deposition during a generally low sea-level with minor transgressions which would limit mudstone preservation. 7.6 Implications

7.6.1 Implications for pre-vegetation successions Considering this study, disputed sections would benefit from being revisited with the new understanding of a possible channelised fluvial system and tidal

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deposition in the absence of mudstone deposits. These sections should be revisited using more accurate technologies and techniques, such as UAV photogrammetry or LiDAR, to accurately document the true architectures. This would eliminate any errors made in observations due to oblique outcrop exposures. The misinterpretation of sheet-like systems is interpreted to be due to many studies being carried out on small outcrops without 3D exposure which limit the ability to study the true extent of the fluvial geometry. Fluvial aspect ratios of braided systems are typically wide and smaller outcrops would give a bias towards a sheet-like system as channel margins would not be resolvable. This would require the study of larger outcrops to characterise the true geometry. Revisiting disputed tidal sections with current linking on pre-vegetation tidal sedimentology may result in the increase in documentation of paralic successions. The limited reporting of tidal facies may indicate a lack of recognition rather than a lack of preservation in the rock record. Structures may have been misinterpreted due to the lack of distinctive sedimentary structures, such as flaser and mud draped structures, which are common in modern tidal settings.

7.6.2 Implications for hydrocarbon reservoir prediction Digital outcrop data collection methods presented in this thesis have offered a highly accurate method of studying a much larger dataset and the thesis has demonstrated that sheet-braided geometries are not the dominant feature of this succession, the system is mixed sheet- and channel-braided with complex macroforms and bounding surfaces. This study has generated a dataset which is statistically more meaningful to apply to subsurface modelling. This study has several implications for hydrocarbon reservoirs, as reservoirs typically rely on one well and outcrop analogues to characterise and predict reservoir volumes, facies and connectivity. It is important to use the correct conceptual model and outcrop analogue for prediction of productivity, production and economics. Computer based reservoir simulation has been common practice in reservoir geology for several decades, but questions remain as to the best way to model a braided fluvial system. They can be modelled as either stacked sheet sands, which does not fully represent the lateral variability, and gives a more layered system, or as multiple low sinuosity channels which rework and stack vertically and horizontally 231

(Martin 1993). This differentiation has a profound effect on the predicted characteristics of the reservoir as correlations may or may not be made from well to well due to the heterogeneity of sand bodies (Weber & van Geuns 1990; Bridge & Tye 2000). Ultimately from the dataset produced in this thesis, the digital outcrop models and the improved conceptual models have successfully been used in subsurface reservoir prediction. Previous sheet-like models were stochastically modelled, and large areas of the model were overpopulated with fluvial facies which did not accurately resemble the outcrop analogue. This model would imply that well to well correlations would be expected, and wells placed anywhere within the field would encounter the same stratigraphic horizons. This would be detrimental to subsurface reservoir predictions, volume predictions and well planning.

The improved channelised model has presented a method which more accurately represents the outcrop analogue and the geometries of the deposits. The channelised model proposed in this study has shown that during amalgamated channel units well to well connectivity would be highly likely, but during periods of transgression, fluvial geobodies would be isolated and disconnected. This study has shown that documenting stacking patterns and connectivity is a key factor in characterising the subsurface and understanding this is important for reservoir calculations, predictions, production and flooding.

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Chapter 8: Conclusions and further work

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This chapter makes concluding remarks about the research presented in this thesis and presents the uncertainties that remain and recommends further work. 8.1 Concluding remarks The research in this thesis has provided a detailed reassessment of the sedimentology of a pre-vegetation paralic system, using the Tumblagooda Sandstone as a case study. The initial focus of the study was to re-characterise the architectures preserved within pre-vegetation fluvial settings and produce a geostatistical database of the fluvial geobodies, later input into a stochastic model to assess the impact of models and techniques used in subsurface reservoir prediction. The study has also re-interpreted the tidal sedimentary sequences, highlighting the absence of mudstone deposits due to the lack of deeply rooted land plants. It has been noted that there is a paucity of recognised tidal deposits prior to the evolution of land plants and this study presents criteria to recognise them in the absence of typical mud related tidal indicators. The study has highlighted the lack of mudstone preserved in pre-vegetation continental and paralic systems and the dominance of braided morphologies within lower Palaeozoic and Precambrian successions. This is attributed to the lack of deeply rooted plants which affected both the weathering processes, the retention of mud and the stability of channel banks within the continental realm. The integration of new UAV technology and digital outcrop modelling has enabled improved definition of geobody dimensions, allowing accurate quantification of pre-vegetation fluvial geometries. This has been used to develop improved conceptual models for pre-vegetation paralic environments. Digital outcrop models have led to the identification of channelised systems characterised by reworking and cannibalisation of previous channel deposits. This suggests that pre-vegetation fluvial architectures needed to be redefined from the traditionally held idea of a “sheet-braided” dominant system to one that is has complex bars and mixed sheet and ribbon architectures. Stochastic models have been generated using the geobody database and digital outcrop models data, comparing channelised systems with sheet-like systems. The channelised object model best represents the outcrop analogue data, whereas

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the sheet-like model over estimates connectivity within the reservoir, which would affect assessment of reservoir volume and production. 8.2 Uncertainties and recommendations The Tumblagooda Sandstone offers an excellent outcrop for such a study and gives an insight into large scale basin processes. To exploit the knowledge that has been gained in this study and apply it to a wider basin context several issues would need to be addressed which were faced during the study. Uncertainties remain about what could have stabilised the overbank areas in the absence of deeply rooted vascular plants. Suggestions have been made about microbial activity giving cohesion to sediments, however, McMahon et al. (2017) discussed that microbes had little influence on stabilising overbank areas as they are surficial features and do not penetrate deep into the substrate like root systems. Other suggestions have been made that the presence of a low-gradient plain, which was subject to stable discharge, a channel-braided or meandering style could be attained (Ielpi et al. 2017). The age of the Tumblagooda section and the provenance is poorly constrained, with some conflicting data from palaeontology, palaeomagnetics and zircon studies. The age of the stratigraphy could be better constrained, as it has been suggested to have been deposited between the Cambrian and the Silurian, with a possible unconformity across the top of the section within the river gorge outcrops. If the age of the section is better constrained with confidence, then a larger scale correlation could be made and the assessment of controls on formation could be discerned. Until this issue is resolved the larger scale sequence stratigraphy cannot be deciphered as important bounding surfaces cannot be given a hierarchy. A study to document the processes which confined and retained channel geometries would enable us to further understand the processes active at the time of deposition. This could possibly be done within modelling software or flume tank research. Discussions were made with Utrecht University during this study to use flume experiments to understand processes. Many previously documented pre-vegetation sections could be revisited utilising the updated understanding of tidal systems and channelised fluvial

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sedimentology using new technologies. It is important to continue to generate a database of pre-vegetation geobody dimensions, which would benefit subsurface modelling and understanding depositional processes. What proportion of other systems are sheets or channelised bodies when larger scale outcrops are studies with the ability to document them using UAV technology?

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