SCOTT SIMPSON LECTURE the GLASTONBURY SHEET – ASPECTS of ONGOING RESEARCH Steve Booth 1, Elaine Burt 2 and Keith Ambrose
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Abstracts ABSTRACTS OF OTHER PAPERS /POSTERS PRESENTED AT THE ANNUAL CONFERENCE , J ANUARY 2009 SCOTT SIMPSON LECTURE THE GLASTONBURY SHEET – ASPECTS OF ONGOING RESEARCH Steve Booth 1, Elaine Burt 2 and Keith Ambrose STRUCTURE OF THE CENTRAL AND WESTERN 1British Geological Survey, Keyworth, EUROPE VARISCIDES IN THE LIGHT OF Nottingham, NG12 5GG, U.K. GRAVIMETRIC , MAGNETIC AND SEISMIC DATA 2British Geological Survey, Columbus House, Tongwynlais, Cardiff, CF15 7NE, U.K. Jean-Bernard Edel EOST, UMR 7516 of CNRS-Strasbourg I University The recent ongoing mapping programme by the British Geological Survey in the Glastonbury district (Somerset) has provided a wealth of new information about the Triassic to About 65 % of the Palaeozoic basement is hidden by a Jurassic rocks that underlie the area and the younger overlying Mesozoic to recent sedimentary cover. In order to fill up the deposits. Research in this area is still ongoing, but some gaps we used gravity and magnetic maps, - transformed maps of preliminary results are presented, along with some ideas for the gravimetric and magnetic anomalies (derivatives, downward further study. Some areas where the research is currently and upward continuations, analytical signal) and, - measurements focussed are: (1) The Lias Group rocks have been drilled and of the densities, induced and remanent magnetizations on cored and together with the use of field data a new stratigraphy outcropping rocks and cores. An interpretative geological map has been introduced for the Charmouth Mudstone Formation. of the Variscides from south west England to the Polish Sudetes (2) NextMap data has been studied to give an overall is presented. 3D gravimetric and magnetic modelling impression of the complex faulting and structure in the area. constrained by the available deep reflection seismic sections (3) Re-mapping of the Burtle Beds (sands and gravels) has allow us to image in depth the structures shown on the map. shown that current exposures are not as widespread as Thanks to the specific geophysical character of the subduction previously mapped. A re-evaluation of the deposits is currently related rocks, the Devonian and Carboniferous sutures can be ongoing. traced. Strong magnetic anomalies show the extension of the Carboniferous Morvan – S. Vosges – Swabian Jura – Central Bohemia and the N. Vosges – Odenwald –Thuringia magmatic arcs. The magnetic anomalies of the Central Channel probably represent the western extension of the latter arc. The interpretative map underlines the importance of Carboniferous ANIMAL , VEGETABLE OR MINERAL ? wrenching which is associated with block rotations and FOSSIL MOULDS FROM WEMBURY POINT , facilitated the intrusion of numerous granitoids. Therefore, emplacement of the large magnetic body responsible for the SOUTH DEVON Paris Basin anomaly was associated with wrenching and Elaine Burt and Chris Ruse clockwise rotation of the Cadomian Armorican block. British Geological Survey, Columbus House, Tongwynlais, Cardiff, CF15 7NE, U.K. The rocks on the foreshore at Wembury Point, near Plymouth belong to the Early Devonian Dartmouth Group. The rocks comprise mainly silty mudstones and siltstones and are likely to be of lacustrine or fluvial mudflat origin. The succession usually yields only fish fragments and very sparse examples of Bellerophon, however at this location unusual fossil moulds in a rosette form are preserved. The preservation of the moulds is unusually good and can be seen to take a radial form. ‘Spokes’ reach outwards from a central ‘orifice’ and between some of these connecting ladder like structures are seen. Although the features resemble modern ‘jellyfish’ or plant like forms, no such structures have been recorded from the Devonian. It is therefore most likely that the features were formed as mineral growths in a continental evaporate sequence. 171 Abstracts ROMAN MINING ON EXMOOR : A LONG PLEISTOCENE SEQUENCE AND RECENT A GEOMORPHOLOGICAL APPROACH AT ANSTEY ’S ARCHAEOLOGICAL FINDS AT CHARD JUNCTION PIT COMBE , D ULVERTON IN THE AXE VALLEY Jenny Bennett 1, Tony Brown 2 and Edward Rhodes 3 A.G. Brown, L.S. Basell and P.S. Toms 1School of Geography, Archaeology and Earth Resources, School of Geography, University of Southampton, Highfield University of Exeter, Exeter, EX4 1RJ, U.K. Campus, Southampton, SO17 1BJ, U.K. 2School of Geography, University of Southampton, Highfield Campus, Southampton, SO17 1BJ, U.K. The Axe Valley on the borders of Devon, Dorset and Somerset has long been known for its rich Palaeolithic 3Department of Environmental and Geographical Sciences, archaeology. Likewise it has a long history of research by Manchester Metropolitan University, Quaternary geologists having been once regarded as an outflow Chester Street, Manchester, M1 5GD, U.K. from Glacial Lake Maw which was thought to have occupied the Somerset Levels. Whilst this theory has been discredited A survey of valley fills in south facing combes (headwater there remains a geological conundrum in the thickness valleys) along the south side of the Exmoor massif revealed an and form of the Quaternary deposits within the valley. The anomalously deep infill in one valley. This infill of up to 5 m discovery in July 2008 of two lower Palaeolithic bifaces at the depth had been gullied revealing a complex stratigraphy. Studies lower levels of the Chard Junction quarry (Hodge Ditch) has of the stratigraphy, clast orientation and shape suggested re-invigorated this research question. Previous work by the several accumulation episodes under different environmental PRoSWEB Project had obtained dates on the upper part of the conditions commencing under a periglacial climatic regime. complex fluvial sequence at Chard Junction and this, along with Later units included sandy silts which can be dated using recent research, has proven a remarkably long sequence of optically stimulated luminescence (OSL) of quartz grains. The deposits extending from the Middle Pleistocene (probably OSL dates indicate that the inter-gravel silts accumulated in two before MIS 12) to the Holocene (MIS 1). This sequence periods, the Romano-British period and the 16th-17th centuries contains many hiatuses and lateral complexities but is clearly AD. A survey of the very small valley catchment revealed a a strong terrestrial reflection of the global climate change linear trench of a type associated with early iron-mining. Given record and is probably the longest semi-continuous stacked the anomalously high volume of accumulated sediment from fluvial sequence in the British Isles. This paper will briefly such a small catchment and evidence of mining on the slope summarise the sedimentary geology of the site and the recent above the site, the geomorphic mechanism is almost certainly archaeological finds and illustrate their importance for studies the downslope transport of mining debris from the slope to the of the Palaeolithic in SW England. valley floor. This study suggests that the systematic survey of headwater valleys in metalliferous uplands may be one way of locating areas of early mining activity and that such deposits could provide a chronology of working and abandonment. THE VARISCAN : WHERE WE ARE IN SW E NGLAND Robin K. Shail 1 and Brian E. Leveridge 2 THE ENGLISH RIVIERA GEOPARK – PAST , 1Camborne School of Mines, College of Engineering, PRESENT AND FUTURE Mathematics and Physical Sciences, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, U.K. Mel Border 2British Geological Survey, Keyworth, Nottingham, NG12 5GG, U.K. English Riviera Geopark Coordinator, The Seashore Centre, Tanners Road, Goodrington, Paignton, Devon, TQ4 6LP, U.K. SW England is bounded, offshore to the south, by a In September 2007 Torbay received international recognition reactivated late Devonian suture with the Normannian Terrane for its rich geological and cultural heritage, when it became (Armorican affinities), and to the north and east by the Bristol one, of what is now, 57 areas around the world to be endorsed Channel-Bray Fault that brought about late Westphalian by UNESCO and welcomed into the network of European juxtaposition against unequivocal Avalonian lithosphere. and Global Geoparks. The Geopark territory has a rich and Its Upper Palaeozoic geological evolution provides important well-exposed geological heritage, from Devonian reefs to constraint on Variscan tectonic models due to its proximity Pleistocene bone-caves. This geology has strong links to the to the Rhenohercynian and/or Rheic suture(s). Further east history of the science and culture, not least as Torbay includes in mainland Europe, late Devonian convergence along the sites crucial to the initial characterisation of the Devonian active northern margin of the Saxothuringian Zone (Mid- Period. Now just over a year on, the status is wholeheartedly German Crystalline High) resulted in continental collision and supported by the local council and is recognised as a key tool progressive inversion of the Devonian (Rhenohercynian) to promote both community pride in the Bay and sustainable passive margin during the Carboniferous. The westwards economic regeneration. Most exciting has been the Geoparks correlation of the Rhenohercynian suture, across the Bristol ability to successfully drive forward a major funding application Channel-Bray Fault, to SW England appears justified. However, totalling £5 million under the UK government Seachange the relationship between the closure of the Rheic Ocean and funding programme. The unique combination of a superb the development and destruction