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

Published online by Cambridge University Press:  01 May 2009

A. J. Reedman ,
M. F Howells ,
G. Orton  and
S. D. G Campbell
A. J. Reedman
Affiliation:
British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth SY23 4BY, U.K.
M. F Howells
Affiliation:
British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth SY23 4BY, U.K.
G. Orton
Affiliation:
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, U.K.
S. D. G Campbell
Affiliation:
British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth SY23 4BY, U.K.
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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.

Type
Research Article
Information
Geological Magazine , Volume 124 , Issue 5 , September 1987 , pp. 427 - 439
Copyright
Copyright © Cambridge University Press 1987

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