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The significance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey

Published online by Cambridge University Press:  20 December 2012

JACK E. TREAGUS ,
SUSAN H. TREAGUS  and
NIGEL H. WOODCOCK
JACK E. TREAGUS
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, England
SUSAN H. TREAGUS*
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, England
NIGEL H. WOODCOCK
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England
*
†Author for correspondence: susan.h.treagus@manchester.ac.uk
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Abstract

The boundary between the Rhoscolyn and New Harbour formations on Holy Island, Anglesey, has been described as a high strain zone or as a thrust. The boundary is here described at four localities, with reference to the contrasting sedimentary and deformational character of the two formations. At one of these localities, Borth Wen, sandstones and conglomerates at the top of the Rhoscolyn Formation are followed, without any break, by tuffs and then mudstones of the New Harbour Formation. It is concluded that there is clear evidence of depositional continuity across the boundary here, and that both formations subsequently shared a common two-phase deformation. The first (D1) was manifestly different in intensity and scale in the two formations, whereas the second (D2) produced very similar structures in both. The other three localities provide continuity of sedimentary and tectonic features at this boundary in a traverse along the length of Holy Island, leading us to identify two previously unrecognized major D1 folds in addition to the Rhoscolyn Anticline. At one of these localities (Holyhead), we confirm the presence of Skolithos just below the boundary, supporting radiometric evidence for a lower Cambrian or later age for the Rhoscolyn Formation. A turbidite interpretation for both the Rhoscolyn and New Harbour formations best fits the available evidence. A deep-water depositional environment is still compatible with the sporadic presence of Skolithos burrows, but less so with reported observations of hummocky and swaley cross-stratification lower down the South Stack Group.

Keywords

North WalesMonian SupergroupRhoscolyn FormationNew Harbour Formationstructurefolds
Type
Original Articles
Information
Geological Magazine , Volume 150 , Issue 3 , May 2013 , pp. 519 - 535
Copyright
Copyright © Cambridge University Press 2012

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