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Introduction

Bruce Levell

Department of Earth Sciences, University of Oxford, Oxford, UK

Summary

This short introduction sets out the purpose and structure of the book. In broad terms the theme is one of diversity: diversity of approaches and of emphasis, reflecting the diverse range of applications and questions which can be addressed using the twin subjects of sedimentology and stratigraphy.

1.1 Motivation for This Book

In 1996, when the third edition of this book was published, the most obvious application of sedimentology was to the exploration and production of hydrocarbons. The insatiable hunger for raw energy, food (fertiliser), steel and concrete of a global population (then 5.5 billion and growing at 80 million people per year) was driving demand for cheap and plentiful energy and food. The energy demand was, at that time, met for 84% by oil, gas and coal. Sedimentology, in particular the combination of a well-developed facies analysis methodology and insights into sedimentary architecture gained by sequence stratigraphy, was proving its worth in discovering and producing this energy. A main focus of sedimentological studies was the prediction of the properties and geometries of reservoirs, hydrocarbon source rocks and seals. At that time, a large amount of innovative academic research was either directly funded by the hydrocarbon industry, or inspired by the problems of subsurface prediction. Around 1996, this included a dramatic expansion in the collection of large amounts of quantitative analogue data from outcrops, wells, and high-resolution 3D seismic imaging of large swathes of the Earth's sedimentary basins.

Fast-forward to 2025 and sedimentology as both a field of research and as a profession finds itself in a very different environment. Global net population is now 8 billion and its growth is in only slightly down, at 75 million a year. Surprisingly hydrocarbons still underpin the energy demand of this 60% larger population, accounting, still, for around 84% of total energy supply. That energy demand has of course increased markedly, from ca 400 to nearly 700 EJ a-1 and the consequence is known to us all - a greenhouse gas related climate crisis.

Sedimentology now no longer has a single predominant driver as it did in 1996. The subject has multiple applications and is being applied to a much wider range of issues, by a much more diverse set of practitioners. Sedimentology is now well embedded both as a tool and in its own right as an integral part of Earth System approaches and is taught in broader Earth Science or Environmental Science courses rather than 'Geology' degrees. Sedimentological understanding, or at least awareness, is used to contextualise a vast range of scientific endeavours as indicated below.

At the same time, the 'schools of sedimentology' described in the introduction to the 3rd edition (namely the Anglo-Dutch school and two American schools) are no longer recognisable. The subject has gone global, and, at the same time, the male dominance of 1996 has weakened considerably.

Ironically, the diversity of applications and questions to which sedimentology is now applied occurs in the context of an apparently widespread view outside the subject that classical sedimentology and stratigraphy have been 'done'. In other words, a working toolkit that can address the main questions exists and just needs to be applied. This view (expressed in a relative reduction in pure sedimentological research and teaching) is in a way a compliment to the success of facies analysis, but it is both unhelpful and wrong. The new applications of the subject are in fact doing rather a good job of exposing our lack of knowledge. For example: (i) the uncertainties and non-uniquenesses in determining causal explanations for the observed sedimentology and stratigraphy; (ii) the biases and gaps in the record that depositional and preservational processes inevitably introduce; (iii) the updating of classical facies models with new data from process measurements in a wider range of modern examples, such as sub-surface imaging, short-lived isotope dating methods and, probably, in the near future, artificial intelligence for unravelling concatenations of complex non-linear processes; (iv) the widespread integration of sedimentology with geochemical and palaeoecological data to broaden the scope of environmental reconstructions beyond the purely physical.

Direct applications of sedimentology are many and certainly much more diverse than in 1996, for example:

1. Predicting sediment architecture and properties for the subsurface extraction and storage of fluids (e.g. hydrocarbons, water (Taylor et al. 2022), carbon dioxide (Krevor et al. 2023) and hydrogen).
2. Exploration for sediment-hosted ores and brines (e.g. copper, cobalt (Hitzman et al. 2010), lead, zinc, lithium and placer rare earth element deposits (Balaram 2023)).
3. Geotechnical engineering, for example, ground models for infrastructure projects, roads, tunnels, dykes and dams (de Freitas 2021).
4. Understanding the formation and protection of the critical zone (soil science (Hou et al. 2020)).
5. Engineering coastal defence against rising relative sea levels (Vousdoukas et al. 2020; Singhvi et al. 2022).
6. Natural hazard prediction and understanding landscape evolution: for example, glacier, river and flood basin behaviour, slope stability and landslides in the context of increased temperatures, changes in precipitation and storminess (Ercilla et al. 2021; Syvitski et al. 2022).
7. Tracing the dispersal of pollutants (including microplastics (Harris 2020; Waldschläger et al. 2022)).

Still central to many of these applications is the predictive power of being able to reconstruct the 3D geometry of sedimentary bodies through interpreting their depositional environment.

It was these changes in understanding, context and applications that prompted the thinking behind this completely new 4th edition of Harold Reading's book.

1.2 Structure of the Book

We decided that the diversity of end-uses and therefore of approaches to sedimentology would be reflected in the way the different chapters of this book have been presented. Rather than force the subject matter into a one-size-fits-all straight-jacket, we have tried to allow the authors to express their interests and personalities in the way they present each chapter.

The 'tools of the trade', an understanding of facies analysis, facies models and the larger-scale factors, such as climate, tectonics and changing base level, that control the nature of the depositional and preservational processes, are the subjects of Chapters 2-4. Chapters 2 and 3 explain and illustrate the work-flows of, respectively, physical and biogenic facies analysis using case studies from clastic and carbonate successions. Chapter 4 addresses stratigraphy and deliberately takes a more philosophical approach highlighting some of the uncertainties and debates that are still being addressed in our understanding of how the Earth system has constructed the 3D sedimentary record from the bed-scale to the basin-scale. It emphasises the importance of gaps in that record, differential preservation and how sedimentary architecture is controlled by the balance between rates of sediment supply and available depositional space (accommodation space). It concludes by presenting the conceptual framework of sequence stratigraphy as a unifying work flow for the analysis of sedimentary architecture and as a basis for understanding the terminology and methodology used in subsequent chapters.

The bulk of the book (Chapters 5-15) shows how the sedimentary rock record may be read by interpreting the process or processes which gave rise to the sediments, and thereby the environments in which these processes operated. We also explain how reconstructing the attributes of these environments can help us place them in the predictive context of a dynamic stratigraphy (Miall 2016) and a sediment routing system (Allen 2017).

The goal of reconstruction of past environments on the basis of understanding modern process forms a common structure for all chapters and requires the following:

1. A thorough description of the rocks, either in the field or in core, with additional laboratory data obtained from samples collected to answer specific questions. Since time is limited, rock description is inevitably selective, emphasising some features, underplaying others and rejecting yet others as unimportant. A useful basic check list is: 'colour, texture, lithology, bed geometry, sedimentary structures and fossils'. The selection depends on the judgement, experience and purpose of the investigator. Judgement and experience take time to acquire and can only be gained by seeing lots of rocks. The absence of certain features is often as important as their presence. For example, the consistent absence of shallow-water features, rather than any positive evidence for great depth, led sedimentologists to infer that most turbidites were deposited in deep water. The utilisation of negative evidence requires a familiarity with a wide range of sedimentary rocks and environments. This text does not address the descriptive aspects in...