Coplow Quarry, Lancashire

[SD 751 432]

Introduction

The Coplow Quarry GCR site is a disused quarry lying 1.5 km NNE of Clitheroe town centre at [SD 751 432]. Exposures at Coplow include part of the earliest phase of 'knoll reef' development in the Clitheroe area, but much of the 'knoll' structure has been quarried away and the centre of the quarry has been filled. Nevertheless, parts of the original Waulsortian bank that made up the knoll (Figure 6.7), and the associated facies, are well exposed in the quarry walls. The quarry is historically important for its rich and well-preserved echinoderm faunas (Figure 6.10). Aspects of the site geology have been described by many workers, including Parkinson (1950b), Earp et al. (1961), Miller and Grayson (1972) and Arthurton et al. (1982).

Description

The rocks exposed in the quarry walls belong to the lower part of the Clitheroe Limestone Formation. The section includes the type section of the Coplow Limestone Member of Riley (1990a), the base of which is defined at the site (Figure 6.2). The oldest exposed rocks consist of alternating dark calcareous shales and impure limestones, Informally assigned to the Lower Coplow Shales by Miller and Grayson (1972). The regional dip in the quarry is to the southeast. On the northern face, the proportion of limestone increases upwards and there is a transition first to cross-stratified crinoidal beds and then into the pale carbonate mudstones of the Waulsortian facies (= Coplow Bank Beds of Miller and Grayson, 1972). This rapid transition was described in detail by Miller (1986). On the west wall of the quarry there are complex inter-tonguings of Waulsortian bank and crinoidal flank facies, while the same horizon on the east wall is dominated by bank facies.

Coplow Quarry is particularly well known for its extensive faunas which include examples of bryozoans, sponges, tabulate and rugose corals, brachiopods, bivalves, gastropods, echinoderms, nautiloids, goniatites, trilobites and ostracodes (Parkinson, 1926). Of special importance are the abundant, superbly preserved echinoderms, described in a long series of papers by Wright (1928, 1935, 1942, 1943, 1947, 1948, 19501960; see (Figure 6.10)). Useful summaries of the Coplow echinoderm faunas are also provided by Westhead (1967) and Donovan (1992).

Of the 20 genera and 60 crinoid species described by Wright (1950–1960), many have type specimens from Coplow (Figure 6.10)A and some have distinctive local names such as Taxocrinus coplowensis, Pleurocrinus coplowensis, Actinocrinites coplowensis, Gilbertsocrinus coplowensis and Pimlicocrinus clitheroensis. Whilst Actinocrinites is the most common and characteristic genus represented (Wright, 1928; Earp et al., 1961), the flexible crinoid Euryocrinus rofei is worthy of note on account of its rarity. Other uncommon elements of the fauna include the blastoids Orophocrinus pentangularis and O. versus (Wright, 1947, 1948) and the echinoids Archaeocidaris spp., Lovenechinus anglicus, Melonechinus etheridgii, M. keepingi, Palaechinus spp., Perischodomus biserialis and Pholidocidaris tenuis (Jackson, 1912; Hawkins in Parkinson, 1926).

Within the mud-bank there is a distinctive fauna characterized by the absence of corals, apart from Amplexus coralloides which is locally abundant, and by the occurrence of brachiopods not usually found outside this facies, such as plicated varieties of Pugnax acuminatus and P. sulcatus, Brachythyris pinguis and Spinier coplowensis. The last named is the most common brachiopod in the bank facies, the holotype being originally described from Coplow by Parkinson (1926).

The limestone-shale bedding plane surfaces on the northern face of the quarry offer excellent opportunities for detailed palaeoecological studies of the bedded facies, although much less is visible today than formerly. Complete in-situ cup-shaped fenestellid bryozoans, outstretched or coiled crinoid stems, often with complete or partly dissociated calices are abundant, and mats of well-preserved hyalosteliid sponges with rod bundles over 15 cm long also occur. Articulated athyrid and spiriferoid brachiopods also occur, with an abundance of delicate 'stick' bryozoans. Many limestones contain a rich microbiota including Girvanella, foraminifera and palaeoberesellids. Chondrites and thalassinoid burrows are also common.

Interpretation

The Coplow Limestone Member is the lowest limestone-dominant member of the Clitheroe Limestone Formation in the Clitheroe area. Its development records the earliest development of the Chadian Waulsortian mud-bank facies in the Craven Basin (Figure 6.2). The section at Coplow begins approximately 60 m above the base of the formation (Earp et al., 1961), with the Waulsortian facies itself originally being more than 100 m thick (Parkinson, 1950b; Earp et al., 1961). Parkinson (1950b) maintained that the depositional slope on the south-east side of his alleged mound was 29.5°. He assumed that all changes In dip and strike of the exposed beds were caused by lateral changes in the facies associated with 'mound' development. However, using the attitudes of geopetal sediments, Miller and Grayson (1972) showed that original slopes were nowhere greater than 10°. The Coplow Waulsortian facies is now generally regarded as an aggregate of gently lensing carbonate mud-banks (cf. Lees and Miller, 1995)

Earlier ideas for the development of the Waulsortian bank facies are discussed in The Knolls GCR site report (this chapter). The Coplow accumulation includes the Waulsortian phases B, C and D in common with other buildups of the Clitheroe area, suggesting deposition in water depths ranging from approximately 280 m to around 150 m (Lees and Miller, 1985).

The superb preservation of the faunas in the facies surrounding the Waulsortian mud-bank, with virtually undisturbed bryozoans and crinoids, testifies to the tranquil environments during deposition and perhaps to rapid postmortem burial. However, the presence of cross-stratified crinoidal limestones elsewhere in the associated beds points to the existence of traction currents at other times.

Conclusions

Coplow Quarry is one of the most important multiple-interest sites of Lower Carboniferous age in the Craven Basin, with a range of stratigraphical, palaeontological and sedimentological features of interest within its boundary. It provides the type section of the Coplow Limestone Member and, in addition, a rare opportunity to examine the transition from Waulsortian bank facies laterally into bedded strata. It is also the type locality for many fossils. Furthermore, bedding planes with exceptionally well-preserved faunas on the north face of the quarry, although not as clearly seen as when the quarry was being worked, offer an unparalleled opportunity to study the palaeoecology of the Chadian mud-bank and associated facies.

References