THE STRATIGRAPHY OF THE MAWDDACH GROUP IN THE CAMBRIAN SUCCESSION OF NORTH WALES

Abstract

SUMMARY The Mawddach Group is a new name for the rocks of late St. David’s to Tremadoc age in the Harlech Dome, previously called Menevian Beds, Lingula Flags and Tremadoc Slates. It follows conformably the Harlech Grits Group and is unconformably overlain on the eastern side of the dome by the late Tremadoc Rhobell Volcanic Group and elsewhere by sedimentary and volcanic rocks of Arenig age. The Mawddach Group comprises four formations, from bottom upwards called the Clogau, Maentwrog, Ffestiniog Flags and Cwmhesgen (new name). They are formally defined here for the first time and the stratigraphical nomenclature used in this area is revised. The group consists of about two kilometres of predominantly grey argillaceous rocks interbedded at intervals with quartzose sandstone and siltstone. Sedimentary structures show that the group represents a complete cycle of basin formation and infilling. The topmost formation, which consists of the Dolgellau and Dol-cyn-afon (new name) members contains new evidence of volcanism in the late Cambrian. The biostratigraphy of the group is considered in detail. Many of the zones of the Scandinavian succession are recognisable in the present area, but the brachymetopa Zone appears to be absent and some other zones remain unproved; the Acerocare Zone, however, is here demonstrated for the first time in Great Britain.