Major folds affecting the Lower Old Red Sandstone Group at Lligwy, Anglesey, North Wales, and their regional significance

Abstract

The Old Red Sandstone on Anglesey, North Wales, presumed Lower Devonian in age, is folded and locally cleaved, but the intensity of this deformation has previously been understated. We describe two S-verging anticline-syncline pairs, one with a strongly overturned middle limb, their associated minor folds and an axial-planar cleavage. The intensity of the deformation calls into question a proposed link to Variscan fault displacements, and the angular unconformity below the Old Red Sandstone precludes the deformation being part of a continuous 'late Caledonian' phase. We consider this deformation of the Old Red Sandstone to be mid-Devonian, correlating with the Acadian phase in mainland Wales. It is predated by a Silurian shortening deformation on Anglesey that is possibly related to the closure of the Iapetus Ocean, absent in mainland Wales and the Lake District, but perhaps preserved also on the Isle of Man.

Late Silurian event stratigraphy and facies of South Wales and the Welsh Borderland, United Kingdom

“…These strata do not preclude the possibility that, throughout much of west and North Wales, red bed ORS facies of late Ludfordian age formed part of the sedimentary overburden that facilitated the development of the Acadian cleavage in underlying Early Palaeozoic mudrocks (Soper & Woodcock, 2003; Woodcock & Soper, 2006). The isolated ORS succession on Anglesey is normally listed as Pridoli to Early Devonian in age, but its deformation implies that it formed part of a once much thicker ORS succession (Treagus et al 2011). Perhaps, in common with the unconformable succession in south-west Wales, it too includes a Ludfordian component.…”

Structure of the Dingle Peninsula, SW Ireland: evidence for the nature and timing of Caledonian, Acadian and Variscan tectonics

“…The collision is generally considered to be ‘soft’ within the Iapetus suture zone of Britain and Ireland, with continuous late Silurian subsidence recorded in many places (Soper & Woodcock, 2003). There is evidence of Silurian deformation in South Mayo (Dewey & Strachan, 2003), in Anglesey (Treagus, Treagus & Woodcock, 2011) and in Rosslare (Murphy, 1990). Such an episode might also be recorded in the main metamorphism of the schists and gneisses preserved as clasts in the Inch Conglomerate (Fig.…”