Proceedings of the Geologists’ Association 122 (2011) 537–539

Editorial

The Marine Devonian of Great Britain

Contents lists available at ScienceDirect

Proceedings of the Geologists’ Association

jo ur n al ho m ep ag e: www .e ls evier . c om / lo cat e/p g eo la

1. Introduction

This is the first volume of the Geological Conservation Review(GCR) of Great Britain to be published in the Proceedings of theGeologists’ Association (PGA), and it is a great pleasure to seethis important Earth Science publication within the pages of thislong established Geological journal (2011 is the 153rd year ofpublication of the PGA). In many ways the PGA is a natural homefor the GCR publications as it is an international journal that coversthe whole range of Earth Science, but has a tradition of outreachand interaction with society not typical of many other EarthScience journals. Indeed the Geologists’ Association has long beenconcerned with Geoscience discovery and the impact of EarthScience on society, and the GCR is the archive that provides the keyinformation about the critical Earth Science sites in the BritishIsles. These sites cover all aspects of the subject, from topics likethe Marine Devonian, which is the subject of this issue (Leveridge,2011a), to sites of geomorphological significance which mayprovide critical evidence by which we can evaluate topics such asflood risk, or sites of palaeontological significance which mayprovide clues for our understanding of the development of life onthis planet. In addition the GCR provides the critical informationnecessary for evaluating the legal status of sites designated as Sitesof Special Scientific Interest (SSSIs) in Great Britain and as such ithas a status not usually found in Earth Science documents. Thepublication of the GCR in the PGA gives the journal a new role andmeans that the journal has practical impact which is in line withthe impact requirements of our research councils and theobjectives of the Research Excellence Framework (REF), used toevaluate UK higher education.

The background to the GCR is presented by Ellis (2011) in apaper in the most recently published issue of the PGA, and theimportance of geoscience to the scientific community and naturalheritage is outlined in a Viewpoint paper by Prosser et al. (2011),also in the most recent issue of the PGA. Further discussion of thistopic is to be published in the PGA in Gordon et al. (in press) and isreflected in the Natural Environment White Paper on ‘The NaturalChoice: securing the value of nature’ which was Presented toParliament by the Secretary of State for Environment, Food andRural Affairs by Command of Her Majesty in June 2011. Thisdocument is concerned with the value of the natural environmentto society; and apart from putting a monetary value upon nature(including of course the physical landscape on which we live), itfocuses on the ways that our natural environment realises benefitsfor us all through improved health and well being andunderstanding and managing environmental responses to geo-morphological stresses and climate change. By this approach it is

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hoped that we will be able to protect our natural assets, moreeffectively develop a green economy, and increase the under-standing of the environment around us. While this may seem along way from the Marine Devonian of Great Britain (Leveridge,2011a), further thought clearly reveals that this is not the case: anunderstanding of the rocks upon which Britain is built, and anability to recognise them and their mineral, topographic and soilcharacteristics is essential if we are to achieve the aspirations ofthe White Paper, and the Marine Devonian rock of southwestBritain are no exception.

The intrinsic geoscientific value of the volume also resides withinthe degree of observational detail and interpretation on all aspects ofDevonian stratigraphy and the manner in which the descriptions of58 GCR sites are integrated within a framework of the UpperPalaeozoic tectonic evolution of South-West England. That refinedscientific understanding, with its models of passive marginevolution and Variscan convergence, has been largely developedduring a programme of detailed mapping and collaborative researchover some thirty years by the British Geological Survey. It isappropriate that this volume will be presented to its Editor (BrianLeveridge) at the Elsevier Sponsored, Geologists’ AssociationMeeting 2011 ‘Geoconservation for Science and Society: an Agendafor the 21st Century’ to be held on 9th September 2011 at theUniversity of Worcester (http://www.geologistsassociation.org.uk/conferences.html).

2. The nature of the Marine Devonian rocks of Great Britain

Devonian rocks outcrop extensively in northern and westernBritain. They are largely non-marine and assigned to the Old RedSandstone (ORS), but in South-West England the successions arepredominantly marine, hosting fossil faunas that were critical tothe Sedgwick and Murchison proposal in 1839 of a DevonianSystem (Sedgwick and Murchison, 1839). The variety of fossilspromoted international geological correlations, consequent accep-tance, and establishment of the system name based on theDevonshire ‘stratotype’. One of the major geological periods,lasting some 57 million years, it was marked by the burgeoning ofvascular plants, vertebrates and invertebrates. Despite theproliferation of life forms being curbed at intervals by globalextinction events, particularly towards the end of the period,marine life achieved its greatest Palaeozoic diversity.

Devonian Britain was located close to the equator near thesouthern margin of the ORS continent (Laurussia), which hadformed by the amalgamation of Laurentia, Baltica and Avaloniaduring the Caledonian Orogeny. Latest Silurian and Early Devoniansedimentary basins in Scotland, the Lake District and Wales were

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Editorial / Proceedings of the Geologists’ Association 122 (2011) 537–539538

infilled with alluvial, fluvial and lacustrine successions, the lattercontaining important early fossil fish. In England and Wales thesebasins underwent a brief episode of late Early- to early Mid-Devonian inversion and deformation during the Acadian Orogeny.

In contrast, in South-West England there is an essentiallyuninterrupted record of sedimentation throughout the DevonianPeriod from its earliest stage. Successions are closely similar tothose of the Rhenohercynian Zone of the mainland EuropeanVariscides. An apparent relative north-westward displacement ofthe province from that belt by a few hundred kilometres wasaccommodated along the Bristol Channel–Bray Fault in Carbonif-erous times.

The fifty eight GCR sites recorded in this volume (Leveridge,2011a) provide a detailed view of Marine Devonian stratigraphy,rocks, fossils and sequences and the environmental, sedimento-logical and ecological regimes that developed during the complextectonic evolution of Britain’s Variscan orogenic province. Struc-tural complexity, formerly thought to obscure stratigraphicalinterconnection, is now recognised as a function of basingeometries during inversion, and such understanding aidssignificantly the interpretation of palaeogeographical relation-ships between successions. Sites are located within a framework ofparallel sedimentary basins that formed during Devonian conti-nental rifting. The geology of sites through much of the provincedetails the evolution of a northern passive plate margin. To thesouth sites display, within the stratigraphies of major nappes, thefill of an oceanised basin sourced by an active plate margin. Therifting is currently considered to have been related to theformation of a marginal ocean basin to the north of the majorRheic Ocean.

The oldest observed basin fill on the passive margin is non-marine, but episodic rift-related thinning of the continentallithosphere ensured the subsequent predominance of marinedeposition. The distribution of half-graben and graben, thatdeveloped sequentially northwards, controlled basinal and highdepositional environments, and the extent of northerly-derivedsediment influx. Basinal successions are mudstone dominated,with deep-water pelagic faunas such as the ammonoids. Highs arecapped by platform and reef-related limestones rich in shallowerwater brachiopods, corals and stromatoporoids. Microfossils,particularly conodonts and palynomorphs, have greatly enhanceddetailed correlations between the successions of these differingenvironments. Rift-related mafic magmatism was widespread,particularly about the highs, and Devonian oceanic lithosphere,ophiolite, is preserved in the south within the Lizard Complex. Itspresence, together with much of the Gramscatho Basin infill,attests to convergence between the passive northern and activesouthern plate margins during the Late Devonian; together theyprovide a glimpse of processes operating in the final stages of RheicOcean closure.

This volume presents detailed geological descriptions of therocks and fossils of the best known sites of the British MarineDevonian which are located in South-West England and anexplanation of the processes that resulted in their formation.With apposite companion volumes of the series it forms a keyreference work for those with interest in the Devonian geology ofthe British Isles.

3. Structure of the volume

The volume is divided into four papers. The first (Leveridge andShail, 2011a) gives an overview of the topic, including a history ofMarine Devonian research, the Devonian stratigraphical frame-work and chronostratigraphy and Devonian biostratigraphy. It alsooutlines the global position of Earth’s plates and associateddynamics and the processes operating in response to these factors.

The three other chapters deal with particular regions and theirproperties and definitive features. The second paper is concernedwith the Gramscatho Basin in southern Cornwall (Leveridge andShail, 2011b). Its 10 sites are largely within Devonian thrust nappesuccessions comprising the erosional products associated withconvergence and the obduction of the Lizard ophiolite; the lastmajor ocean closure in Britain. The third paper (Leveridge, 2011b)deals with the Rhenohercynian passive margin, in central andnorth Cornwall and South Devon and includes 38 sites. Here,largely marine deposition in the sequentially formed Looe, SouthDevon and Tavy rift basins filled with a variety of basin and highsuccessions. The fourth paper (Whittaker and Leveridge, 2011)focuses upon the North Devon Basin deposits in the north of theprovince. They represent a more proximal passive margin shelfsuccession (10 sites), with interdigitating terrestrial and marinefacies reflecting the effects of extensional rifting of the margin tothe south.

The volume is also supported by a full list of Contents, as well asthere being a Contents for each paper.

4. Copyright

The BGS data are used with permission of the ExecutiveDirector, British Geological Survey (NERC). The copyright ofmaterials derived from the British Geological Survey’s work isvested in the Natural Environment Research Council (NERC). Nopart of these materials (maps, charts, plans, diagrams, graphs,cross-sections, figures, sketch-maps, tables, photographs) may bereproduced or transmitted in any form or by any means, or storedin a retrieval system of any nature, without written permission ofthe copyright holder, in advance.

To ensure that copyright infringements do not arise, permissionhas to be obtained from the copyright owner. In the case of the BGSmaps this includes both BGS and Ordnance Survey. Most BGS andOrdnance Survey maps make use of Ordnance Survey topography(Crown Copyright), and this is acknowledged on BGS maps.Reproduction of Ordnance Survey materials may be independentlypermitted by the licences issues by the Ordnance Survey to manyusers. Users who do not have an Ordnance Survey Licence toreproduce the topography must make their own arrangementswith the Ordnance Survey, Copyright Branch, Romsey Road,Southampton, SO9 4HD (Telephone: +44 0238 079 2913).

Permission to reproduce BGS materials must be sought inwriting from the Intellectual Property Rights Manager, BritishGeological Survey, Kingsley Dunham Centre, Keyworth, Notting-ham, NG12 5GG (Telephone: +44 0115 936 3331).

The National Grid is used on diagrams with the permission ofthe Controller of Her Majesty’s Stationary Office, � Crowncopyright licence no. GD 27254X/01/00.

Acknowledgments

Although acknowledgements are in each paper we wish toexpress our appreciation for the whole venture here. The GCR onthe Marine Devonian was commissioned by the Joint NatureConservation Committee as a volume in the Geological Conserva-tion Review Series. This work was supported by the GCR team ledby N. Ellis, L.P. Thomas the series editor, and the series subeditors.JS Publications reproduced linework for figures. B.E. Leveridgeacknowledges the help of the BGS in providing access to maps andthe SW England Records Archive, and particularly the support ofP.J. Strange, Head of Geology at the time of writing, and assistanceof the staff of the BGS Exeter Office. Contributions to the volume byB.E. Leveridge and A. Whittaker are published with the approval ofthe Executive Director of the British Geological Survey. The authorsthank the GA, and Elsevier, for facilitating publication.

Editorial / Proceedings of the Geologists’ Association 122 (2011) 537–539 539

References

Ellis, N., 2011. The Geological Conservation Review (GCR) in Great Britain – rationaleand methods. Proceedings of the Geologists’ Association 122, 353–362.

Gordon, J., Barron, H.F., Hansom, J.D., Thomas, M.F., in press. Engaging withgeodiversity – why it matters. Proceedings of the Geologists’ Association.

Leveridge, B.E. (Ed.), 2011a. The Marine Devonian of Great Britain. Proceedings ofthe Geologists’ Association, 122, 537–744.

Leveridge, B.E., 2011b. The Looe South Devon and Tavy basins: the Devonian passiverifted margin successions. Proceedings of the Geologists’ Association 122,616–717.

Leveridge, B.E., Shail, R.K., 2011a. The marine Devonian stratigraphy of Great Britain.Proceedings of the Geologists’ Association 122, 540–567.

Leveridge, B.E., Shail, 2011b. The Gramscatho Basin, south Cornwall, UK: Devonianactive margin successions. Proceedings of the Geologists’ Association 122,568–615.

Prosser, C.D., Bridgland, D.R., Brown, E.J., Larwood, J.G., 2011. Geoconservation forscience and society: challenges and opportunities. Proceedings of the Geolo-gists’ Association 122, 337–342.

Sedgwick, A., Murchison, R.I., 1839. Classification of the older stratified deposits ofDevonshire and Cornwall. Philosophical Magazine Series 14 (3), 241–260.

Whittaker, A., Leveridge, B.E., 2011. The North Devon Basin: a Devonian passivemargin shelf succession. Proceedings of the Geologists’ Association 122, 718–744.

James Rosea,b,*aDepartment of Geography, Royal Holloway,

University of London, Egham Surrey TW20 0EX, UKbBritish Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Brian E. LeveridgeBritish Geological Survey, Keyworth, Nottingham NG12 5GG, UK

*Corresponding author at: Department of Geography,Royal Holloway, University of London,

Egham Surrey, TW20 0EX, UKE-mail addresses: j.rose@rhul.ac.uk

jrose@dircon.co.uk (J. Rose).bel@bgs.ac.uk (B.E. Leveridge)

Available online 14 September 2011