Transgressive welded ash-flow tuffs among the Ordovician sediments of NE Snowdonia, N. Wales

Francis, E. H.; Howells, M. F.

doi:10.1144/gsjgs.129.6.0621

Cited by 74 publications

(37 citation statements)

References 13 publications

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“…Brenchley 1969). The ash-flows of the Swch Gorge Tuff were assumed by Brenchley to be sub-aerial, but the work of Francis and Howells (1973) in Snowdonia has thrown doubt on such assumptions about ignimbrites. However the associated volcaniclastic sediments show evidence of a high energy, probably channel environment and the underlying and overlying sediments are shallow marine.…”

Section: C the Swch Gorge Tuff Formationmentioning

confidence: 99%

The caradocian rocks of the north and west berwyn hills, north wales

Brenchley

1978

Geological Journal

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The Caradoc of the Berwyn Hills is a thick sequence of marine mudstones and siltstones with interstratified sub-aerial tuff formations. Near the top of the succession there is a sharp facies change to graptolitic shales. The sequence has been remapped and two new formations (the Pen-plaenau Siltstone Formation and Cwm Rhiwarth Siltstone Formation) and one new member are proposed. Assemblage zones can be traced round the outcrop in the lower part of the sequence (Soudleyan) but higher in the succession (Longvillian) there are differences between the faunal assemblages of the north and west. The northern faunas are referred to Dulmunellu community which is believed to have occupied a shallower environment than the Nicolellu community to the west. In spite of the presumed environmental control of the faunas, the sequence can be fairly precisely correlated with the classic sections in Shropshire and Bala.The Caradoc sedimentary sequence represents a mainly shallow sub-tidal environment with periodic episodes of local emergence. The area deepened during the Longvillian, particularly in the west. There followed a period of nondeposition with probably some erosion at the base o f the Nod Glas which itself represents a period of reduced sediment.There is a well-known stratigraphic break at the base of the Ashgill which rests on rocks of Onnian age and on different horizons of the Longvillian.

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Section: C the Swch Gorge Tuff Formationmentioning

confidence: 99%

The caradocian rocks of the north and west berwyn hills, north wales

Brenchley

1978

Geological Journal

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“…The outcrop at Capel Curig was remapped by Williams (1922) who interpreted most of the volcanics of the formation as rhyolite lavas emplaced in a marine environment. Francis & Howells (1973) reinterpreted these rocks as welded tufts.…”

mentioning

confidence: 99%

The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment

Howells

Leveridge

Addison

et al. 1979

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A detailed correlation of the tufts of the Capel Curig Volcanic Formation is presented. The depositional environment of the sediments associated with the tufts varied from subaerial to submarine. Variation in the ash-flow tufts are related to this changing environment. Subaerially emplaced tufts are characterised by even bases, distinctive recrystallisation textures, the absence of reworked tops and sparse siliceous nodules. Tufts emplaced in a subaqueous environment show irregular undersurfaces with welding to the lower contact, reworked tops and common siliceous nodules.

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“…The uppermost formation, the Cape1 Curig Volcanic Formation consists of welded acid ash-flow tuffs and the two most extensive flows were erupted from subaerial centres in the north and transgressed southwards into a marine environment (Francis & Howells 1973;Howells et al 1979;Howells & Leveridge 1980;Howells et al 1985).…”

Section: The Llewelyn Volcanic Groupmentioning

confidence: 99%

“…To the NE, outside the caldera, a single complete non-welded ash-flow tuff, the lowermost member of the Lower Crafnant Volcanic Formation (Howells et al 1973) was emplaced and preserved within the marine sedimentary sequence. It travelled for at least 20 km from source.…”

Section: The Snowdon Volcanic Groupmentioning

confidence: 99%

Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales

Reedman

Colman

Campbell³

et al. 1985

JGS

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The Ordovician (Caradoc) volcanic rocks of NE Snowdonia constitute two major groups, the Llewelyn Volcanic Group and the Snowdon Volcanic Group, which accumulated predominantly in shallow-water marine conditions. The younger Snowdon Volcanic Group comprised a bimodal, basalt-rhyolite suite and included a major caldera-forming eruption of acidic ash-flow tuffs superseded by both Surtseyan and Strombolian basaltic volcanism. Rhyolite domes were intruded into the volcanic sequence in the vicinity of the caldera. The Snowdon Cu-Pb-Zn vein mineralization comprises five paragenetic mineral assemblages. The veins cut rocks deposited within and over the caldera and it is proposed that the dominant controls of mineralization were volcanogenic. Circulation in hydrothermal cells, involving both juvenile fluids and seawater, deposited the minerals at a late stage in the evolution of the caldera.

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Transgressive welded ash-flow tuffs among the Ordovician sediments of NE Snowdonia, N. Wales

Cited by 74 publications

References 13 publications

The caradocian rocks of the north and west berwyn hills, north wales

The caradocian rocks of the north and west berwyn hills, north wales

The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment

Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales

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