M. F. Howells

British Geological Survey

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S. D. G. Campbell

British Geological Survey

A. J. Reedman

British Geological Survey

R. E. Bevins

Aberystwyth University

B. P. Kokelaar

University of Liverpool

William J. Fritz

City University of New York

Adrian W.A. Rushton

Natural History Museum

Martin Smith

University of Brighton

Allan C. Mann

Goldsmiths University of London

Robert Allen Roach

Idaho National Laboratory

E. H. Francis

University of Leeds

Cooperative Institutions

British Geological Survey

Institute of Geological Sciences

University of Cambridge

Instytut Nauk Geologicznych

United States Geological Survey

Institute of Geology of Ore Deposits Petrography Mineralogy and Geochemistry

Aberystwyth University

Dublin Institute For Advanced Studies

Durham University

Natural History Museum

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Cryptovents and allied structures in Carboniferous strata between Port Seton and Aberlady, East Lothian

Scottish Journal of Geology (1969)

M. F. Howells

Synopsis Cryptovolcanic and allied structures cut strata of Passage Group, Upper Limestone Group, and Lower Limestone Group age on the coast between Port Seton and Aberlady. Two structures in Gosford Bay and one on Port Seton beach are oval in form; others at Kilspindie and Port Seton reef are linear in character. One of the structures at Gosford Bay, together with those at Port Seton reef and Kilspindie are breccias with tuff matrices containing basaltic glass. No brecciation or tuff occurs in the structures on Port Seton beach and the other Gosford Bay occurrence; for these a volcanic genesis is inferred by association and by comparison with records elsewhere; they are assumed to occur at a stratigraphic level above the upper limit of tuff penetration. These structures provide the first evidence of volcanic acitivity younger than the Calciferous Sandstone Measures to the east of the Pentland Fault on the south side of the Firth of Forth.

Port (circuit theory)

10.1144/sjg05010001

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Citations (3)

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

Journal of the Geological Society (1985)

A. J. Reedman T. B. Colman S. D. G. Campbell M. F. Howells

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.

10.1144/gsjgs.142.5.0875

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Citations (26)

Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula : a discussion of the Tremadoc to Caradoc rocks lying between the Menai Straits and the Llanderfel Syncline, and including an appendix on Cambrian rocks. Version 2

Adrian W.A. Rushton M. F. Howells

Syncline

Peninsula

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Citations (1)

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

Journal of the Geological Society (1973)

E. H. Francis M. F. Howells

In the type area, and southwest of it, the Capel Curig Volcanic Formation comprises up to three ash-flow tuffs interbedded with marine sediments. The tuffs are generally massive and welded, but pass laterally and upwards into current bedded, ripple marked tuffs. They are believed to have been deposited in a subaqueous environment, after originating from subaerial eruptions farther north where the Formation is represented by an uninterrupted sequence of intensely welded ash-flows. The lower contacts of the submarine ash-flows contrast with subaerial ash-flows in being welded and by locally transgressing the underlying sediments at angles up to 90°, with minor apophyses resembling magmatic intrusions. Eutaxitic foliation, however, remains generally parallel to the regional dip. The sediments adjacent to the transgressive undersurfaces are disturbed and reconstituted. The irregular contacts, together with adjacent discrete pipe-like tuff bodies are interpreted as down-sags analogous to load casts but of unprecedentedly large scale, up to 100 m, formed by the liquefaction and yielding of the sediments during and after the emplacement of the tuffs.

Transgressive

10.1144/gsjgs.129.6.0621

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Citations (73)

The submarine eruption and emplacement of the Lower Rhyolitic Tuff Formation (Ordovician), N Wales

Journal of the Geological Society (1986)

M. F. Howells A. J. Reedman S. D. G. Campbell

The Lower Rhyolitic Tuff Formation (up to 600 m thick) represents an eruptive cycle of acidic ash-flow tuff which is stratigraphically associated with marine sediments and subaqueously emplaced basalt lavas. The formation comprises volcaniclastic and pyroclastic megabreccias and breccias, massive welded and non-welded acidic ash-flow tuffs, reworked tuffs and tuffities, siltstones, rhyolite intrusions and extrusions. Its basal contacts vary from conformable, to disconformable and unconformable. The inter-relationships of these variations to pre-, syn- and post-emplacement structures define a submarine, asymmetric downsag caldera. The main eruptive centre, coincident with the thickest accumulation of intracaldera tuffs, lies close to its north margin, on the north side of the Snowdon Massif. To the SW, the intracaldera tuffs thin progressively and much of the formation comprises tuffs reworked in the vicinity of a Caradocian shoreline. To the NE and E, outflow tuffs escaped into a deeper marine basin. Many of the features of the caldera are similar to those of subaerial calderas, and it is concluded that the enveloping sediments and lavas, and the character of the reworked tuffs, hold the key to the recognition of its submarine development. Subsequent resurgence resulted in only local and short-lived emergence of the intracaldera tuffs.

10.1144/gsjgs.143.3.0411

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Citations (36)

Marginal basin geology : volcanic and associated sedimentary and tectonic processes in modern and ancient marginal basins

B. P. Kokelaar M. F. Howells

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A shallow marine volcaniclastic facies model: an example from sedimentary rocks bounding the subaqueously welded Ordovician Garth Tuff, North Wales, U.K.

Sedimentary Geology (1991)

William J. Fritz M. F. Howells

Siltstone

Graded bedding

Lapilli

Conglomerate

10.1016/0037-0738(91)90041-b

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Citations (25)

Geological Tensions in an Idyllic Field

Metascience (2004)

James A. Secord M. F. Howells Gary Douglas Couples David Oldroyd

Philosophy of technology

10.1023/b:mesc.0000023262.37758.d2

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The Pitts Head Tuff Formation: a subaerial to submarine welded ash-flow tuff of Ordovician age, North Wales

Geological Magazine (1987)

A. J. Reedman M. F. Howells Geoff Orton S. D. G. Campbell

Abstract The Pitts Head Tuff Formation, of Ordovician (Caradoc) age, was emplaced as a thick ( c. 700 m) intracaldera sequence and two outflow units comprising welded acidic ash-flow tuff. The Pitts Head pyroclastic flows were erupted subaerially but the lower and most extensive of the outflows crossed a shoreface, and continued for several kilometres offshore. The flow entered the sea without disruption and, following deflation and tuff emplacement, displaced the shoreface several kilometres to the east and northeast. Post-eruption subsidence in the northeast resulted here in the rapid establishment of environments deeper than had previously existed. The lower outflow tuff is parataxitically to eutaxitically welded in both the subaerial and marine environments. The extremely regular plane-parallel welding foliation of the subaerial tuffs, however, contrasts with the locally highly deformed foliation of the tuff deposited beyond the shoreface. The deformed foliation, associated with irregular zones of intense siliceous nodule development, is ascribed to the upward streaming of water vapour generated at the tuff/sediment boundary. Elsewhere rheomorphism within the tuff was caused by instability resulting from emplacement on slopes related to faulting. Continued movement initiated extensive brecciation, detachment, and local gravity sliding of large rafts of tuff.

Subaerial

Outflow

10.1017/s0016756800017015

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Citations (27)

The Ordovician marginal basin of Wales

Geological Society London Special Publications (1984)

B. P. Kokelaar M. F. Howells R. E. Bevins Robert Allen Roach P. N. Dunkley

Summary The Lower Palaeozoic Welsh Basin was founded on immature continental crust. During late Precambrian-early Cambrian times, volcanism and sedimentation were influenced by NE-SW-trending faults which defined the NW and SE margins of the basin. During the Cambrian, marine sediments infilled a graben and at the end of the Tremadoc widespread tectonism was associated with an island-arc volcanic episode. In the Ordovician this subduction-related activity was succeeded by mainly tholeiitic volcanism related to back-arc extension, with the locus of arc volcanism sited further N, in the Lake District—Leinster Zone of the Caledonides. In Wales, the Ordovician volcanic activity shifted in time and space. In S Wales volcanism persisted from the middle Arenig through the Llanvirn. In N Wales the volcanism can be broadly divided into dominantly pre-Caradoc activity in southern Snowdonia and an intra-Caradoc episode in central and northern Snowdonia. In eastern Wales, including the Welsh Borderland, and in Llŷn, both episodes are represented. In all areas faults greatly influenced both volcanism and sedimentation. Intrusive activity was dominated by high-level emplacement of sills. Granite ( s.l. ) stocks are restricted to central and northern Snowdonia and Llŷn and many were coeval with extrusive volcanism. Volcanism in the basin was essentially bimodal with voluminous eruptions of tholeiitic basalts with ocean-floor affinities, and of rhyolites. Minor volumes of andesite to rhyodacite resulted from low-pressure fractional crystallization of the tholeiitic basalts. Available evidence suggests that the rhyolites resulted mainly from crustal fusion, although in some instances evolution by crystal fractionation from intermediate magma has been proposed. Calc-alkaline assemblages are petrographically distinct, of minor occurence and, contrary to previous conclusions, are relatively insignificant in the characterization of the tectonic environment of the basin. Throughout the basin, volcanism was generally succeeded by deposition of black muds and then turbidite-dominated sequences.

10.1144/gsl.sp.1984.016.01.19

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Citations (109)