Sedimentation and Tectonics in the Welsh Basin
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Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Journal of the Geological Society, London, Vol. 144, 1987, pp. 199-200. Printed in Northern Ireland Conference Report Sedimentation and tectonics in the Welsh Basin W. R. FITCHES & N. H. WOODCOCK’ Department of Geology, University College of Wales, Aberystwyth, Dyfed SY23 3DB, UK Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK Report of a joint meeting of the Tectonic Studies Group andthe Cornelius & A. Hartley as shallow marine to tidal facies British Sedimentological Research Group of the Geological Society preserved in fault-repeated slices. Early to Mid-Ordovician held at the University College of Wales, Aberystwyth on 10-12 April sedimentation is dominated by the Arenig transgression and 1986. Theorganizers were Dr W. R. Fitchesand Dr N. H. the subsequent basin deepening. The transgression was Woodcock. documented by C. Cornelius & A. Hartley in the Fishguard area, where a local deltaic facies is important, and by J.-J. Research into the Early Palaeozoic history of Wales is like Traynor in southern Dyfed, where local facies control by that history itself;eventful and rarelyquiescent. Recent pre-Arenig topography is especially evident. New biostrat- events havebeen the lively debateon the tectonics of igraphic correlation of the Arenig in North Wales by A. J. Anglesey provoked by A. J. Barber and M. D. Max, the Beckley showed thatthe transgression was strongly exemplary volcanological studiesin North Wales by the diachronous.Early to Mid-Arenigdeposition occurred in BGS Snowdonia unit, and the definition of Silurian world small fault-controlledbasins, with a majorLate Arenig stratotypes by members of the Ludlow Research Group. founderinginitiating more widespread basinal marine The stimulus for this conference was the recently increased conditions. R.Trythall et al. described oolitic ironstones activity in structuraland sedimentologicalresearch in deposited duringthe Mid-Ordovician continuation of this mainland Wales. Twenty-eight papers were given over the basinal history in North Wales. Iron was sourcedfrom a first 2 days, followed by a day of field trips to Snowdonia, nearbylandmass, possibly the Irish SeaLandmass tothe the Aberystwyth area,and the Wenlock of eastern northwest, and deposited under local tectoniccontrol on Mid-Wales. upthrown fault blocks in the basin. Precambrian eventsfeatured in threepapers. D. S. Late Ordoviciansedimentation and volcanism were Wood concentratedon the evidence forthe superficial intimatelylinked in North Wales. G. Orton showed how ‘soft-sediment’ origin of the Gwna melange of Anglesey and volcaniclastic sediment distribution and facies were strongly postulated a setting predating rather than accompanying the controlled by volcanotectonic uplift and subsidence. subduction system that produced the Anglesey blueschists. Palaeogeographic configurations in the shallow marine to Inthe later deformation history of Anglesey, D. Carter emergentsetting changed dramatically after each volcanic distinguished two main deformations. A Caledonian event episode. S. D. G. Campbell et al. showed how the siting of was preceded by one of pre-Arenigor probably these eruptions was itself influenced by basementfaults. pre-Arvonian age, with an unconformitypresent between Some faults are attributable to caldera formation and other the Monian and LowerPalaeozoic rocks. Precambrian volcanotectonic processes. LatestOrdovician post-volcanic sedimentation of the Longmyndian of Salop was described sedimentation in South Wales was described by J. M. by J. Pauley in terms of a progradational basin fill. This is Anketell. represented inupward order by basinal shale,turbidite, Early Silurian sedimentation was described mainly from deltaic, alluvial floodplain and sandy braidplain facies. The Mid-Wales. J. M. Anketell documented the latest major syncline in the Longmydian is confirmed, but the age Ordovician to Llandovery lithostratigraphy over a large area of this and subsequent faulting is a sinistral strike-slipsystem of central Dyfed and, with T. Kishimoto, focused on the is uncertain. sedimentology of one Lower Llandovery turbidite unit. A Cambriansedimentation and diagenesiswere covered steadydowncurrent and crosscurrent waning flowof first in twostudies of the Harlech Grits Group of North strength is apparent in the axially transported main system, Wales. C. Gfiths identified a transgressive sequence, with but a separate western system was supplied laterally from shallow marine sediments overlain by turbidites deposited in local fault scarps. N. H. Woodcock gave evidence for confinedfault-controlled basins. Somewell-sorted, cross syndepositionalfault control in the shallow marine beddedtops to turbidite bedsindicate traction current Llandoverysediments across the SE margin of the basin. reworking, thoughthe type of flowis problematical. M. The Wenlock turbidites inPowys weredetailed by A. J. Bennett described stratiform manganesemineralization in Dimberliie & N. H. Woodcock. An axially supplied high theturbidite units,considered to be syngenetically to efficiency system was laterally confined by a fault-controlled syndiagenetically formed in anoxic marine conditions. southeastern margin tothe basin, which the system first Lowergreenschist facies metamorphismhas formed new onlapped then offlapped.Thin storm-triggered events also Mn-bearingminerals without destroying sedimentary and reached the basin from the bordering shelf, whose carbonate diagenetic textures. The sedimentology of the Cambrian in facies were interpreted by K. Ratcliffe. The Much Wenlock the Fishguard area of South Wales was interpreted by C. LimestoneFormation apparently shows two sea-level 199 Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 200 REPORT CONFERENCE controlled cycles of increasing depositional energy, reflected Caledonian deformation in South and Mid-Wales is still in facies and morphologies of Cyanophyte algae. hard to correlatewith that in the North, despite Fitches’ and Late Silurian sedimentation began in the Powys area Campbell’s evidence that the two regions were not widely with a facies of laminated hemipelagites interbedded with separated. J. M. Anketell described the common pattern of thin silt-mud event deposits. The hemipelagites,common SE-verging folds and NW-dipping cleavage cut, in central throughoutthe Wenlock andinto the Ludlow,were Dyfed, by a fault belt with sinistral strike-slip components. interpreted by A. J. Dimberline & J. E. Tyler in terms of Traced across striketo the NW, the structuresbecome annual or seasonal alternation of phytoplankton and silt vertical at a major vergencedivide, the Llangranog supply, with deposition in poorly oxygenated conditions. J. lineament, and verge NW on the Ceredigion coast. J. Craig E. Tyler & N. H. Woodcock demonstratedthat the low showed that this lineament is characterized by major inward oxygen zone impinged on the outer part of the shelf as well dipping oblique-slip faults and oblique cleavage-transected as the basin. InEarly Ludlowtime theouter shelf also folds. He interpreted it as a positive flower structure later received abundantcarbonate silt and shell debrisduring disrupted by post-Caledonian extension. N. H.Woodcock events attributed to major storms. Major slumping occurred described major SE-verging structures in the Llandovery in this area in mid Ludlow time. It is also widespread in the area of east Dyfed, localized partly by underlying basement North WalesSilurian, fromwhere A. Maltman described fractures and partly by lenticularsand bodies. The major the microstructuralcharacteristics of the deformed sedi- folds here affect Lower OldRed Sandstone rocks, again ments. He was ableto match field structures with those suggesting a late age for Caledoniandeformation. The formedinexperiments on clays, andtherefore make NE-SW trending structures across Mid-Wales swing estimates of thewater content of the Silurian sediments towards E-W when traced westward. Onthe coast near during deformation. fishguard C. Cornelius & D. D. Hawkes have identified a Caledonian deformation in North Wales was described major N-dipping thrust system repeating large sections of mainly with referenceto the pre-Silurian rocks of the the stratigraphy, and cut later by minor dextral strike-slip mainland. I. Wilkinson reported straina determination faults. study on the Caradoc volcanics of Snowdonia. Local strain Post-Caledonianevents were mentioned inseveral heterogeneity was particularly apparent but the averaged contributions but were the focus of two. D. Wilson et al. regional finite strain is close to plane strain and indicates a showed how inheritedbasement fractures in SE Wales strongcomponent of simple shear in the deformation of controlledDevonian and Carboniferoussedimentary pat- North Wales. Fracturepattern studies of one group of terns. Dextral offsets on a Severn Estuary lineament were Snowdoniafolds were described by G. Jenkiis. He particularly important. R. J. Whittington used seismic highlighted methodologicalproblems, particularly conflict- profiles to constrain fault locations and histories in Cardigan ing patternsobtained from ground survey andLandsat Bay. Upper Jurassic/EarlyCretaceous andLate Tertiary imagery. A detailed Landsat study of part of western North movements were particularly important offshore but faults Wales was described by R. Maude. Herethe main may be substantially decoupled from those onland by N-S lineament sets could