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

Treagus, Jack E. and Treagus, Susan H. and Woodcock, Nigel H. (2013) The significance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey. Geological Magazine, 150 (03). pp. 519-535. ISSN 0016-7568, ESSN: 1469-5081 DOI 10.1017/S0016756812000775

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Official URL: http://dx.doi.org/10.1017/S0016756812000775

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.

Item Type: Article
Additional Information: Copyright © Cambridge University Press 2012
Uncontrolled Keywords: 2013AREP; IA65;
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
Journal or Publication Title: Geological Magazine
Volume: 150
Page Range: pp. 519-535
Identification Number: 10.1017/S0016756812000775
Depositing User: Sarah Humbert
Date Deposited: 03 May 2013 16:22
Last Modified: 23 Jul 2013 10:06
URI: http://eprints.esc.cam.ac.uk/id/eprint/2768

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