The Knolls, Lancashire

[SD 789 435], [SD 779 433], [SD 773 434], [SD 770 434]–[SD 767 429]

Introduction

The Craven Basin has long been famed for its 'knolls' of fine-grained carbonate of early Carboniferous (early Chadian) age which have been exhumed by recent weathering to form more-or-less conical hills. The Knolls GCR site, centred 4 km ENE of Clitheroe, embraces a number of these knolls, running from Gerna Hill [SD 789 435] westwards to include Worsaw Hill [SD 779 433] and Warren Hill [SD 773 434] (Figure 6.7). It also includes Crow Hill [SD 768 429] and part of the adjacent road cutting on the A59 [SD 770 434]–[SD 767 429]. The structure, composition and origin of these knolls has been discussed by many workers, especially Parkinson (1926, 1935, 1944), Earp et al. (1961) and Miller and Grayson (1972). The knolls and associated limestone facies are now generally accepted as part of a Waulsortian carbonate mud-bank complex (Lees and Miller, 1995) In the Clitheroe Limestone Formation (Figure 6.2).

Description

The best exposures at this site are to be found in the road cutting (Figure 6.8). The northern part of the cutting, north of the junction with the road to Chatburn, is in the Peach Quarry Limestone Member (Figure 6.2) and the southern section, on the Worston side of the junction, bisects the western margin of the Crow Hill knoll. Both exposures are briefly described by Miller and Grayson (1972). The Peach Quarry Limestone Member consists of an alternation of a thickly bedded limestone facies and a thinly bedded shale-rich facies. The thickly bedded facies consists of crinoidal packstones and grainstones with reworked fragments of Girvanella, lithoclasts, foraminifera, molluscan fragments and other skeletal debris. Sedimentary structures in this facies include hummocky cross-stratification and small-scale scour features, together with evidence of grading and current reworking. The thinly bedded facies consists of wackestones with an abundant in-situ biota including Girvanella mats, dasydadaceans, collapsed Hyalostelia sponges, gastropods, ostracodes and small brachiopods. Shale units yield abundant and only partly dissociated crinoid remains (Amphoracrinus and platycrinitids being common, together with inadunates) and numerous echinoids. The large productoid Levitusia humerosa is common and characteristic of these early Chadian beds. A prominent mudstone near the middle of the section has an interesting fauna of large, complete, smooth-shelled ostracodes, small productoids with spines attached, and euomphalid gastropods.

At the eastern side of the field above the road, the upward transition from the Peach Quarry Limestone Member through crinoidal facies into the Waulsortian bank facies of Crow Hill is seen particularly well (Miller and Grayson, 1972) (Figure 6.8). In the A59 road cutting on the western side of Crow Hill the exposure is entirely in the fine-grained limestones of the Waulsortian bank facies with local patches of crinoidal material and cavities filled with coarse calcite cements clearly visible. Macrofossils are often found in pockets and include Pleuropugnoides pleurodon, Spirifer bisulcatus, S. striatus, S. coplowensis, Pugnax acuminatus, Entomoconchus scouleri, Conocardium sp. and Amplexus coralloides, together with abundant Fenestella and smooth spiriferoids, productoids and athyrids. Two further and unusual finds from this locality include the blastoid Mesoblastus and a nautiloid comparable to Epidomatoceras subsulcatum. A notable feature of this locality is the development of fissures, predominantly vertical, but sometimes turning sideways to open laterally. The fissure-fills include finely laminated micrites, sometimes incorporating marine fossil fragments.

The old quarries and natural exposures of the knolls are mostly highly weathered and lichen-covered. Different facies are present, but the scattered nature of the exposures means that they are difficult to relate to one another. On Gerna Hill there appears to be a single development of the Waulsortian bank facies, whereas to the south-west on Worsaw Hill there is a lower unit, the Coplow Bank Beds (= Coplow Lime stone Member of Riley, 1990a) and a higher, thicker development, the Salthill Bank Beds (= Bellmanpark Limestone Member of Riley, 1990a) (Figure 6.2). The oldest beds of^, Worsaw and Warren hills are seen in Piked Acre Wood where crinoidal facies can be followed westwards, passing laterally into typical Peach Quarry Limestone. Both Worsaw Hill and Gerna Hill display outward ('quaquaversar') dips, but these are often seen in bedded crinoidal limestones, at least some of which post-date bank formation. A boulder–breccia bed, relating to the erosion of Salthill Bank Beds, has been traced from the south-west flank of The Ridge [SD 771 432] to a small quarry at [SD 771 433] where it apparently lies directly on the Peach Quarry Limestone Member.

Parkinson (1926) and Earp et al. (1961) have listed faunas from the knolls. Fossils are all typical of the various Waulsortian facies seen and referred to elsewhere (see Salthill Quarry GCR site report, this chapter). In addition, Osmolska (1968) and Riley (1982a) have described trilobites from the knolls from the area, including Archegonus (Phillibolina) worsawensis from Worsaw Hill, and Miller (1973) recorded two new trilobites, Carbonocoryphe (Winterbergia) hahnorum and Namuropyge decora from a fissure-fill of the Limekiln Wood Limestone Member (Riley, 1990a), formerly the 'Salthill Cap Beds' of Miller and Grayson (1972), in a now-infilled quarry on the south-west corner of Crow Hill.

Interpretation

The origin of the knolls of the Craven Basin has been debated for many years. The site described here includes knolls originally regarded as Carboniferous reef structures by Tiddeman (1889, 1891). Further work on the composition and relationships between these structures by Parkinson (1926, 1950b, 1957, 1967) re-inforced the idea that each knoll was an isolated lenticular reef surrounded by mudstone. It was argued that the topography of the knolls suggested that the reefs stood high above the contemporary sea floor and, although there was no direct evidence of an organism able to construct a rigid framework, sheets of cavity-filling sparry calcite, sometimes known as 'Stromatactis', were thought to be evidence for a vanished binding agent for the carbonate mud. Tiddeman and Parkinson made much of the quaquaversal dips — the tendency for beds to dip outwards away from the knolls — as evidence for their isolated lenticular nature.

Earp et al. (1961) dismissed the idea of a skeletal reef origin and instead compared the knolls to limebanks of a similar age in Ireland and the United States. No algal or bryozoan frame-builders were found, nor any evidence that the knolls were wave-resistant structures. They concluded that knolls formed from lime-bank deposition and the differential compaction of such banks within an envelope of mudstone, and that the original knolls were subject to strong tectonic deformation, with the resulting knoll topography enhanced by glacial erosion.

Miller and Grayson (1972) were the first to describe in detail the facies that make up the Clitheroe Limestone Formation and their lateral relationships. They recognized that their bank facies (the knoll limestone of previous authors) formed a thick wedge in the Twiston to Worsaw area which projected thinner wedges to the south-west, separated by a wedge of 'interbank facies' (Peach Quarry Limestone). In addition, they demonstrated that the knolls form part of a limestone sheet that was uplifted and eroded before burial. The Worston Shales (= Hodder Mudstone Formation of Riley, 1990a) were not deposited penecontemporaneously with the banks, but post-date them, burying the eroded remnants.

The Clitheroe knolls arc regarded as 'Waulsortian' mud-banks similar to those that occur in Ireland, North America and at Waulsort (the type locality) in Belgium (Lees and Miller, 1985, 1995). They originated as local accumulations of carbonate mud, probably of microbial origin. The original geometry of the structures cannot be ascertained with complete certainty. Most were sheet-like, although some may have formed lenses raised above the sea floor. However, Lees and Miller (1995) suggested that rather than representing isolated lenses, the 'knolls' formed part of an eroded complex of overlapping carbonate banks and associated 'off-bank' deposits. Furthermore, Lees et al. (1985) demonstrated the presence of an ecological zonation in Waulsortian buildups, with four phases, A to D, deposited in successively shallower water, and at Crow Hill, for example, phases B, C and D have been recognized (Lees and Miller, 1985) suggesting deposition in water depths ranging from approximately 280 m to around 200 m.

After deposition, the Clitheroe knolls were subjected to fissuring and submarine erosion with the development of boulder beds. Renewed transgression in late Chadian times led to the deposition of crinoidal limestones draped over the eroded bank complex and giving rise to the quaquaversal dips. Further deepening led to the deposition of the Worston Shales which completed the burial of the knolls. Glacial and post-glacial erosion has picked out the knoll topography by selectively removing the soft Worston Shales to produce the spectacular line of conical hills that can be seen today.

Conclusions

This site is of great interest for the spectacular outcrop of an early Chadian Waulsortian mud-bank complex whose origin and diagenesis is the subject of ongoing debate. Furthermore, the road cutting along the A59 provides the best available exposure of the Peach Quarry Limestone Member. Waulsortian bank facies are exposed in the Crow Hill road cutting and other exposures are scattered across the hills of the site. Lateral facies relationships are complex and even small exposures can be vital in helping to piece together the history of these important structures.

References