The caradocian rocks of the north and west berwyn hills, north wales

Abstract

AbstractThe Caradoc of the Berwyn Hills is a thick sequence of marine mudstones and siltstones with interstratified sub‐aerial tuff formations. Near the top of the succession there is a sharp facies change to graptolitic shales. The sequence has been re‐mapped and two new formations (the Pen‐plaenau Siltstone Formation and Cwm Rhiwarth Siltstone Formation) and one new member are proposed. Assemblage zones can be traced round the outcrop in the lower part of the sequence (Soudleyan) but higher in the succession (Longvillian) there are differences between the faunal assemblages of the north and west. The northern faunas are referred to Dalmanella community which is believed to have occupied a shallower environment than the Nicolella community to the west. In spite of the presumed environmental control of the faunas, the sequence can be fairly precisely correlated with the classic sections in Shropshire and Bala.The Caradoc sedimentary sequence represents a mainly shallow sub‐tidal environment with periodic episodes of local emergence. The area deepened during the Longvillian, particularly in the west. There followed a period of non‐deposition with probably some erosion at the base of the Nod Glas which itself represents a period of reduced sediment.There is a well‐known stratigraphic break at the base of the Ashgill which rests on rocks of Onnian age and on different horizons of the Longvillian.