Pentre-Bach Quarries, Clwyd

[SJ 061 783]

Potential GCR site

Introduction

Lying on the west side of Moel Hiraddug, 1 km south of Dyserth, the Pentre-bach Quarries site [SJ 061 783] comprises two overgrown quarries known as 'Pentre-bach North Quarry' and 'Pentre-bach South Quarry' (Somerville et al., 1989). The importance of the site lies in the exposure of the Chadian Foel Formation and the base of the overlying Dyserth Quarry Limestone of Arundian age. Early descriptions of the site were by Strahan (1885) and Neaverson (1935). More recently, Warren et al. (1984) published a log of the succession and supplied palaeontological data, while Somerville et al. (1989) provided additional sedimentological and biostratigraphical information.

Description

Pentre-bach North Quarry is the type locality for the Foel Formation of Warren et al. (1984), although the base of the formation and its contact with the Basement Beds are not seen. The exposed section is in two parts, the lower consisting of about 7.5 m of the Foel Formation and the higher comprising 2 m of the Foel Formation and about 12 m of the overlying Dyserth Quarry Limestone. The two sections are separated by a gap of about 3 m. The missing strata are mostly represented in the exposure in Pentre-bach South Quarry, 120 m to the SSE, where a 7.5 m section of the middle part of the Foe! Formation is seen. Logs of the sections at the two quarries are shown in (Figure 8.13).

The Foel Formation consists of mostly dolomitic and silty carbonates interbedded with grey or green mudstones. Lithologies reported by Somerville et al. (1989) include peloidal packstones and intraclastic and bioclastic grainstones. The fauna is dominated by brachiopods, especially Composita cf. ficoidea. Corals occur particularly in one bed in the middle of the formation, called the 'Coral Bed' by Somerville et al. (1989). Among the corals recorded from this bed by Somerville et al. (1989) were the typical Chadian forms Caninia cornucopiae and Koninckophyllum cyathophylloides. These occurred in association with Carruthersella cf. compacta and Axophyllum cf. simplex — forms that are known to span the Chadian–Arundian stage boundary (Mitchell, 1989; Riley, 1993). Somerville et al. (1989) also recorded a foraminiferal assemblage with abundant endothyrids and tournayellids, but lacking archaediscids. Pentre-bach is also famous for its Viséan floras (e.g. Walton, 1928; Lacey, 1952a,b, 1962), the significance of which is discussed in a companion GCR volume on Palaeozoic palaeobotany by Cleal and Thomas (1995).

The base of the overlying Dyserth Quarry Limestone is taken below a thick, coarse-grained dolomite unit towards the top of the face in Pentre-bach North Quarry (Figure 8.13). On the whole, this unit is more massive and lacks the interbedded mudstones of the Foel Formation. The fauna is sparse, but includes Composita and Syringopora.

Interpretation

The lower part of the succession at Moel Hiraddug was included in the Lower Brown limestone of Morton (1870, 1878, 1886) and was regarded as D₁ by Neaverson (1930, 1943). In their new lithostratigraphy for the area, Warren et al. (1984) placed the succession at Pentre-bach in the Foel Formation and the lower part of the Ochr-y-foel limestone, which they continued to regard as being entirely Asbian in age (see Warren et al., 1984). New discoveries in the south Clwyd and Prestatyn areas reported by Somerville et al. (1986, 1989) and Davies et al. (1989) led to a revision of the stratigraphy, although the name 'Foel Formation' was retained (Figure 8.2). Somerville et al. (1989) cast doubt on some earlier fossil identifications from the Foel Formation, especially an uncertain record of Daviesiella llangollensis reported by Neaverson (1929). Discarding this identification, the macrofauna indicates a Chadian–Arundian age. The foraminiferal assemblages of the Foel Formation lacking archaediscids are regarded as being of late Chadian age and broadly comparable with those obtained from the Stone Gill Beds and Coldbeck Beds of Ravenstonedale (Somerville et al., 1989) (see Chapter 5). The Dyserth Quarry Limestone has a less diverse macrofauna than the Foel Formation, but a rich foraminiferal fauna which indicates an Arundian age (Somerville et al., 1989).

Somerville et al. (1989) also supplied a sedimentological interpretation of the Foel Formation. They regarded it as predominantly the deposit of a wave- and storm-influenced shallow subtidal environment. Detrital material was continually being supplied from an adjacent source area, accounting for the impure nature of the limestones and the interbedded terrigenous clastic mudstones. The occurrence of carbonate mudstones suggests that back-barrier lagoonal conditions developed from time to time.

Conclusions

For many years the Dinantian carbonate succession in North Wales was regarded as being entirely of Asbian and Brigantian age. Recent work, however, has shown that older Dinantian strata are present, at least in the outcrop east of the Vale of Clwyd and north of the Bryneglwys Fault (Figure 8.1) and (Figure 8.3)a. Pentre-bach Quarries provide the best exposures of the late Chadian Foel Formation in the Prestatyn region and are therefore critical localities for the understanding of the revised stratigraphy and palaeogeography of the North Wales Dinantian shelf area.

References