Geology of the Dinantian Outlier near Corwen, North Wales

Exposures in the Hafod-y-calch quarry, sited within the isolated outlier of Dinantian rocks near Corwen, provide a critical constraint on Dinantian tectonic and palaeogeographical understanding.  The presence of two of the main marker beds within the Dinantian succession in North Wales – the Main Shale and Coral Bed – permits precise correlation with sequences in Flintshire and at Llangollen, and allows lateral thickness and facies variations to be quantified.  The boundaries of cyclic sequences exposed in the quarry are marked by features that testify to synsedimentary emergence, karstic dissolution and soil forming processes, and can be correlated with comparable levels in outcrops further east.  The section demonstrates the early onset of Minera Formation facies deposition within a region of enhanced subsidence to the north of the Bala lineament.  Within the context of the outlier, the section provides evidence to show that late Dinantian (Brigantian) carbonate platform facies were deposited in narrow gulfs located along the major tectonic lineaments of North Wales, and which extended far into the contemporary hinterland of older rocks.  The section provides critical evidence to show that Dinantian rocks were deposited across a much wider area of North Wales than hitherto suggested.  It is recommended that the northern Hafod-y-calch quarry site is included within the GCR site and that remedial work is undertaken to re-expose and improve access to critical parts of the section.  The southern Hafod-y-calch quarry is very overgrown and further work is required to clear and re-expose the rock section present. However, the southern quarry has significant potential to provide lithological and stratigraphical information to underpin further the palaeogeographical importance of the outlier sequence; accordingly it is recommended that it too should be included within the GCR site.  Urgent consideration should also be given to the conservation of the only exposures in the lower part of the outlier sequence, at both Plas-isaf and Plas-uchaf. It is recommended that these exposures should also be included within the GCR site.