Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Sequence stratigraphy in British geology STEPHEN P. HESSELBO l & D. NEIL PARKINSON 2 IDepartment of Earth Sciences, University of Oxford, Parks Road, Oxford OXI 3PR, UK 2Western Atlas Logging Services, 455 London Road, Isleworth, Middlesex TW7 5AB, UK In what has now become the standard model for we do not provide a comprehensive summary of the sequence stratigraphy, a depositional sequence terminology that has grown-up around sequence comprises an unconformity-bounded package of stratigraphy: judging by its sparse usage within the genetically related strata, whose internal geometries papers of this volume much of it is redundant are influenced largely by fluctuating sea level (Fig. anyway. The interested reader should refer to the 1). This model evolved from regional mapping reviews of Haq (1991), Vail et al. (1991), Mitchum (Sloss 1963), and was later developed from seismic- & Van Wagoner (1991), Posamentier et al. (1992) reflection profiles on passive continental margins and Posamentier & James (1993) for further details. (Payton 1977). It was further extended to a higher In essence, sequence stratigraphy is practised resolution using data from the surface and sub- through the recognition of key surfaces which surface in many geological settings (Mutti 1985; define the boundaries of packages of genetically Wilgus et al. 1988; Van Wagoner et al. 1990). Here related strata (systems tracts) and by the recognition Fig. 1. The standard sequence stratigraphical model for siliciclastic systems (based on Haq et al. 1988 and, Christie- Blick & Driscoll 1995) showing the different areas covered by papers in this volume. Wright (this volume) and Coe (this volume) discuss successions that were carbonate ramps, with an implied sequence stratigraphical character not dissimilar to siliciclastic deposits (see Schlager 1992; Tucker et al. 1993). A sequence boundary. (SB) is defined as an unconformity and its basinward correlative conformity. The lower of the sequence boundaries in this figure (type 1) is distinguished from the upper one (type 2) by its association with 'fan' sedimentation at the toe of slope, which has been related to rapid, high-magnitude, relative sea-level fall. The maximumflooding surface (MFS) corresponds to the time of greatest landward extent of the facies belts within a depositional sequence, and is characterized by detrital sediment starvation in basinward regions. The transgressive surface (TS) is a widespread horizon across which deep- water sediments abruptly overlie shallow water sediments, and it forms as the underlying progradational sedimentary succession is flooded by rising relative sea level. Systems tracts are defined between these key surfaces. For full discussion of this model, definition of systems tracts and their relation to relative and eustatic sea-level change the reader should refer to Haq et al. (1988), Posamentier & Vail (1988), Posamentier et al. (1988, 1990, 1992), Mitchum & Van Wagoner (1991), Vail et al. (1991) Jacquin et al. (1992) and Posamentier & James (1993). Useful critiques and alternative models are provided by Miall (1986), Galloway (1989), Carter et al. (1991), Fulthorpe (1991), Schlager (1991),Walker (1992) and Christie-Blick & Driscoll (1995), and references therein. As a substitute for depositional sequences as defined above, Galloway (1989) has proposed use of genetic stratigraphic sequences bounded by maximum flooding surfaces, and his approach has also been followed by many workers. From HESSELBO, S. P. & PARKINSON, D. N. (eds), 1996, Sequence Stratigraphy in British Geology, Geoloeical Society Special Publication No. 103. Do. 1-7. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 2 S.P. HESSELBO & D. N. PARKINSON of transgressive and regressive facies trends within Neogene those packages (Fig. 1). The model is hierarchical and, to some extent, independent of time or physi- Paleogene cal scale. I~i Knox The power of the sequence stratigraphical model Shanmugam et al. probably lies in its simplicity (see summary in Gale Christie-Blick & Driscoll 1995) but many of the 100- Cretaceous I1. Wonham& Elliott fundamental questions posed as a result of the early sequence stratigraphical work remain unanswered, ~~---- Coe for example the relative importance of eustasy Tyson versus more localized tectonic effects (e.g. Parkin- Jurassic son & Summerhayes 1985; Hubbard 1988; and I[~ Parkinson many others) or sediment supply (e.g. Galloway z00- Macquaker et al. 1989). Thus, the topic of sequence stratigraphy pro- Triassic vides much fertile ground for further theoretical and ~" empirical work, whether concerned with verifi- v & cation, refutation or modification of the standard < Permian sequence stratigraphical model, or whether con- cerned with the application of the model to help 30o- solve wider geological problems. In addressing the application of sequence stratig- Carboniferous I1: .ee0er.S,e a. raphy to British geology, this book focuses perforce Wright on diverse aspects of sequence stratigraphy that tend to cut across divisions based on depositional Devonian environment or age alone. There are two main 4o0- ~ Macquakeret aL themes that run through most of the papers in this Silurian volume. I -~ Woodcocket aL (1) Biostratigraphical control is commonly so good in much of the British area, that precise corre- Ordovician lations are possible within and between basins, and the geologist need not speculate whether the ele- 5OO- ments of a depositional sequence in one basin are precisely synchronous with those in another: this Cambrian can be established as fact or fiction (see for example Hesselbo & Jenkyns 1995, 1996; Coe 1995, this volume; Parkinson, this volume). For this reason, Precambrian - Glover& McKie many of the British outcrop sections serve as stan- 6OO l dards that can be compared to other areas around Fig. 2. Stratigraphical column showing the range of the world. This was recognized early on by Haq et geological ages covered by papers in this volume. The al. (1988) who derived much of their proposed time scale is based on Harland et al. (1990), Cande & global sea-level history from a sequence strati- Kent (I 992), and Gradstein et al. (1994). graphical analysis of British sections. (2) Many geologists have to work with the rocks they are given, rather than the rocks they would interpret by choice. Many, perhaps all, of the suc- intention to maximize the stratigraphical coverage cessions described within this volume may rightly in order to gauge the impact that sequence stratigra- be considered as difficult to interpret using the phy is having upon our understanding of the ge- simple (to some, simplistic) framework of the stan- ology of the British Isles. Nonetheless, the papers dard sequence stratigraphical model. However, as is fall more evenly into groups based largely on depo- demonstrated by several of the papers in this sitional setting, and it is by this criterion that we volume, sequence stratigraphical ideas can cast new briefly review the contents below: light on problematic facies and, in return, problem- atic facies can offer critical insights into the Non-marine and paralic sequences sequence stratigraphical model (see also, for example, Tucker 1991). Non-marine settings perhaps represent the most The studies in this volume concern both surface difficult of depositional systems for the application and subsurface geology and cover most parts of the of sequence stratigraphy. Not only are the problems British stratigraphical column (Fig. 2). It was our of correlation at their most extreme but also the Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 INTRODUCTION 3 simple fact of distance from sea means that strata peritidal carbonates in Early Carboniferous deposi- may be totally unaffected by sea-level change. The tional sequences of southwest Britain, and con- process by which the standard sequence strati- cludes that their character and spatial distribution graphical model has been constructed, a 'distilla- do not support the operation of high frequency, tion' from many ancient and modern case studies by moderate amplitude eustatic sea-level fluctuations inductive reasoning (Walker 1990, 1992; Christie- during that time (cf. Elrick & Read, 1991). Blick & Driscoll 1995), is not of itself adequate for the sedimentary systems with poor chronostrati- Shallow marine sequences graphical records such as the fluvial system. Hence, recent attempts to understand the controls of stratal By virtue of their commonly highly refined bio- geometries in non-marine settings have concen- stratigraphy, shallow-marine deposits offer the trated on forward modelling. Two different greatest chance of assessing the synchoneity or approaches to modelling fluvial systems are illus- otherwise of inferred relative sea-level cycles in trated by papers in this volume. In the first study, widely separated basins (e.g. de Graciansky et al. Burgess & Allen take a geometrical approach, 1996). However, although simpler to correlate, key using equilibrium profiles in computer-generated stratigraphical surfaces from these settings may be simulations to show that changes in the shape of the no easier to relate to specific relative sea-level fluvial profile may be a first-order control on changes than they are in any other setting. sequence architecture, irrespective of conditions of In studies using subsurface data, regional correla- sea-level change. In