Proceedings of the Ussher Society
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Proceedings of the Ussher Society Research into the geology and geomorphology of south-west England Volume 6 Part 3 1986 Edited by G.M Power The Ussher Society Objects: To promote research into the geology and geomorphology of south- west England and the surrounding marine areas; to hold Annual Conferences at various places in South West England where those engaged in this research can meet formally to hear original contributions and progress reports and informally to effect personal contacts; to publish, proceedings of such Conferences or any other work which the Officers of the Society may deem suitable. Officers: Chairman Dr. C.T. Scrutton Vice-Chairman Dr. E. B. Selwood Secretary Mr M.C. George Treasurer Mr R.C. Scrivener Editor Dr. G.M. Power Committee Members Dr G. Warrington Mr. C. R. Morey Mr. C.D.N. Tubb Mr. C. Cornford Mr D. Tucker Membership of the Ussher Society is open to all on written application to the Secretary and payment of the subscription due on January lst each year. Back numbers may be purchased from the Secretary to whom correspondence should be directed at the following address: Mr M. C. George, Department of Geology, University of Exeter, North Park Road, Exeter, Devon EX4 4QE Proceedings of the Ussher Society Volume 6 Part 3 1986 Edited by G.M. Power Crediton, 1986 © Ussher Society ISSN 0566-3954 1986 Typeset, printed and bound bv Phillips & Co., The Kyrtonia Press, 115 High Street, Crediton, Devon EXl73LG Set in Baskerville and Printed by Photolithography Proceedings of the Ussher Society Volume 6, Part 3, 1986 Papers D.L. DINELEY: Cornubian Quarter-century: Advances in the geology of south-west England 1960-1985. 275 A. WHITTAKER: The deep geological structure and evolution of southern Britain. (The Scott Simpson Lecture). 291 M.R. HOUSE: Are Jurassic sedimentary microrhythms due to orbital forcing? 299 C. MAYBURY and R. WHATLEY: A high diversity ostracod fauna of late Pliocene age from St. Erth, south Cornwall. 312 E.M. DURRANCE and C.M. BRISTOW: Kaolinisation and isostatic readjustment in south- west England. 318 R.S. D'LEMOS: Interaction between co-existing magmas: field evidence from the Cobo Granite and the Bordeaux Diorite Complex, north-west Guernsey, Channel Islands. 323 K.E. BEER and T.K. BALL: Tin and tungsten in pelitic rocks from S.W. England and their behaviour in contact zones of granites in mineralised areas. 330 U. KELM: Mineralogy and illite crystallinity of the pelitic Devonian and Carboniferous strata of north Devon and western Somerset. 338 MARY M. SHUFFLEBOTHAM: The Fort de la Latte Complex: late Pentevrian plutonism in Penthievre, Brittany. 344 S.C. GRIMMER and P.A. FLOYD: Geochemical features of Permian rift volcanism-a comparison of Cornubian and Oslo basic volcanics. 352 CHRIS CORNFORD: The Bristol Channel Graben: organic geochemical limits on subsidence and speculation on the origin of inversion. 360 G. WARRINGTON, A. WHITTAKER and R.C. SCRIVENER: The late Triassic succession in central and eastern Somerset. 368 G.J. LEES: The Jersey Main Dyke Swarm, Jersey, Channel Islands. 375 J.D. CORNWELL: The geological significance of some geophysical anomalies in western Somerset. 383 R.G. GREGORY, E.M. DURRANCE and C.R. MITCHELL: Soil gas surveying at Whitchurch Down, near Tavistock, Devon: an integrated approach to vein and fracture mapping. 389 G. van MARCKE de LUMMEN: Geochemical evolution of the Land's End granite (south-west England) in relation to its tin potential in the light of data from western marginal areas. 398 T.S. BREWER and G.M. POWER: The geochemistry of the foliated granitic rocks of Alderney, Channel Islands. 405 R.C. SCRIVENER, T.J. SHEPHERD and N. GARRIOCH: Ore genesis at Wheal Pendarves 412 and South Crofty Mine, Cornwall- a preliminary fluid inclusion study. Notes and Abstracts R. HIGGS: 'Lake Bude' (early Westphalian, S.W. England): storm dominated siliclastic shelf sedimentation in an equatorial lake. 417 E.B. SELWOOD and J.M. THOMAS: Observations on the Padstow Confrontation, north Cornwall (Abstract). 419 G. WARRINGTON: British Triassic palaeontology: supplement 10. 420 P.A. FLOYD and B.E. LEVERIDGE: Lithological and geochemical characteristics of some Gramscatho turbidites, south Cornwall (Abstract). 421 Invited Address to the Annual Conference of the Ussher Society, January 1986 Cornubian Quarter-century: Advances in the geology of south-west England, 1960-1985. D.L. DINELEY Dineley, D.L. 1986. Cornubian Quarter-century: Advances in the geology of south-west England, 1960-1985. Proceedings of the Ussher Society 6, 275-290. Amongst the numerous disciplines within the earth sciences that have made notable advances in south-west England during the last 25 years palaeontology, sedimentology, stratigraphy and structural studies have added most significantly to our understanding of the geological evolution of the region. Geochemistry and geophysics have greatly aided this work and have also furthered our understanding of the economic geology. Palaeontological advances have largely been in Palaeozoic biostratigraphy, especially by use of conodonts. Stratigraphy has become greatly refined and the origins of formations better understood in the context of an active Cornubian branch of the Rhenohercynian basin and of post-orogenic (New Red Sandstone and Mesozoic) basin-infilling. Cornubian terrain is now regarded as structurally dominated by "thin-skinned', tectonics in south and west Devon and throughout Cornwall, with numerous flat and thin nappes moving northwards while Devonian and Carboniferous sedimentation took place. In north Devon the structures are steeper and simple. Stratigraphic and structural continuity to depths greater than a few kilometres is doubtful. As in mainland Europe, no simple plate tectonic model of Variscan orogeny is indicated here but strong and persistent dextral fault movement may be significant support for a model involving dextral transcurrent fault movement. Professor D.L Dineley, Department of Geology University of Bristol, Bristol BS8 IR J, U.K. Introduction The advances of geological research in its broadest 3. The origins, modes and timing of emplacement of sense in Britain has been nowhere more marked or the Cornubian granites and associated igneous rocks. significant than in the south-west of England. In this 4. Post-orogenic basin formation and basin-infilling, the Ussher Society has played a central role as a focus continental and marine. for the presentation and discussion of results. 5. Post-Cretaceous uplift and diastrophism, and the Founded in 1960, it has prospered not only as a forum stripping of the Mesozoic cover. for meetings but also in its Proceedings, which have Important advances in both technology and metho- now become internationally recognised as a dology applied to the earth sciences during these last publication of merit. The membership of the Society twenty five years have included rapid chemical has never greatly exceeded 250 but it has included a analyses (X-ray etc.), geophysical data collecting, large number of young researchers in the universities computing, scanning and other forms of electron and polytechnics, together with senior academics, microscopy and remote sensing. The studies of industrial, amateur and government earth scientists. European earth scientists in other parts of the When the Regional Office of the Geological Survey Variscides and in similar terrains elsewhere have was opened in Exeter in 1968 a vigorously pursued provided models and ideas for comparative studies. programme of remapping the "One Inch" sheets of All of these contributed to several main fields of the south-west was soon to add a welcome impetus to achievement:- research into the Cornubian terrain. 1. Characterisation of igneous bodies in terms of geo- In the early days of the Ussher Society it was already chemistry, mineralogy, age and spatial relationships, clear that research topics would fall into place within origins and history. one or other of the following general themes:- 2. Parameters for the identification of specific sedimentary bodies (facies analysis) and the means of 1. The nature, origin and history of the basins in detailed correlation of strata by means of fossils which the Devonian and Carboniferous rocks were (especially micro-fossils). deposited. 3. The recognition of sequential changes during 2. The effects, causes and timing of earth-movements diagenesis and metamorphism. that gave rise to the structures within the Palaeozoic 4. Means of determining stress relationships: plastic rocks, and the relationships of these structures to the flow and brittle fracture structures, leading to "Variscan Front". structural models explaining complex outcrop Table of Contents patterns and relationships in Cornubian terrain. for both Palaeozoic and Mesozoic rocks. 5. Means of modelling deeper crustal structures. Conodonts and ammonoids have proved invaluable in 6. Development of overall tectonic models to explain the Palaeozoic, with ostracods and palynomorphs also (a) evolution of the Cornubian basin(s) and orogen; locally very important. Prior to about 1950 records of (b) post-orogenic movements and changes of level ammonoids in south-west England were few but since consistent with models developed for adjacent areas then House and others have greatly extended the (plate tectonics). record, recognising many of the European. and Several publications indicate the progress these especially the German ammonoid zones here (House studies have made over the years; notable are British 1963; House and Butcher