Chapter 9 Millstone Grit of the Central Province

The Central Province refers to the complex of depositional basins lying between the Wales–Brabant Barrier to the south and the Askrigg Block to the north (Figure 9.1). It is the thickest development of Millstone Grit in Britain, with a maximum thickness of over 180 m near Burnley and Skipton, and outcrops over an area of more than 4000 m. Much of the strata is relatively resistant to erosion, resulting in rugged scenery most typically represented by the moorland and hills of the High Peak between Sheffield and Manchester; names such as Kinder Scout and Edale are as well known to the hiker and rock-climber as they are to the geologist. Such scenery is well suited to producing good outcrop, and has attracted many geologists to the area. Furthermore, fossils occur abundantly, particularly in the marine bands which have yielded biostratigraphically sensitive ammonoids and conodonts. The non-marine parts of the sequence also contain fossils, although, except for the palynomorphs, have been subject to relatively little investigation. The combination of good exposure and good biostratigraphical control has meant that it is the classic area for the study of the Namurian, and most of the Namurian stage boundary stratotypes have been designated here (see Chapter 2).

The strata are of little direct economic importance. At one time, the sandstones were used to produce mill-wheels (hence the name). This industry has now effectively disappeared, although the remains of discarded wheels can still be seen in parts of the Peaks. The sandstones are still being used as aggregates, paving and building-stones, albeit on a smaller scale than previously, and have produced some of the characteristic architecture of cities such as Sheffield, Leeds and Manchester. Perhaps its most important economic role today is in water-supply, the larger valleys in the Millstone Grit being particularly well suited to damming. The resulting reservoirs, such as Ladybower, Derwent and Woodhead, are the major suppliers of water to a number of the northern industrial cities.

History of research

A number of local scholars, such as Martin Lister and John Michell, took note of the geology of the Pennines in the 17th and 18th centuries (Edwards and Trotter, 1954). The term Millstone Grit itself seems to have been coined in the late 18th century by Whitehurst (1778) for a sandstone in the south of the area (probably what is now known as the Ashover Grit). As elsewhere in England, however, major progress did not occur until William Smith started mapping the area in the early 19th century. Smith's pupil John Farey published the first geological account of the Millstone Grit in 1811, noting for the first time that it occurred between the Carboniferous Limestone and Coal Measures. However, for many years the definitive statement was by Smith's nephew John Phillips, whose 1836 book remained the standard reference until the Geological Survey started their work some decades later.

The Survey's first detailed analysis of the Millstone Grit was by Hull and Green (1864), the results of which were incorporated into a series of memoirs (e.g. Hull and Green, 1866; Green et al., 1878). These works contained the first coherent attempts to subdivide the Millstone Grit, based on the four major grit-intervals, the lowest now known as the Kinderscout Grit, and the highest as the Rough Rock. In Derbyshire, these were underlain by what was called the Noredale Series', then thought to be equivalents of the strata of the same name in the Northern Province (see Chapter 11). As pointed out by Hind (1899), however, the true Yoredales of the Northern Province are significantly older than these beds in Derbyshire and so, following detailed stratigraphical work by Jackson (1923, 1925b, 1926, 1927), the currently used subdivisions of Edale Shales, Mam Tor Sandstones, Shale Grit and Grindslow Shales was introduced.

Up until the 1920s, much of the stratigraphical work on the Millstone Grit had a lithostratigraphical basis. However, it rapidly became clear that such work in isolation had serious limitations, due to problems in correlating the geographically impersistent grits. The turning point came with the first attempts at biostratigraphy using ammonoids by Bisat (1924, 1928). For the first time, it was possible to establish accurate correlations throughout the Central Province, and for the pattern of sandstone distribution to be seen against an independent stratigraphical framework. Further discoveries of ammonoids and their associated marine fossils continued to refine the biostratigraphy (Hudson, 1936, 1945; Bisat and Hudson, 1943;

Hudson and Cotton, 1943; Hodson, 1957), and allowed the Geological Survey to pursue their mapping programme armed with an accurate means of correlation (e.g. Bromehead et al., 1933; Edwards et al., 1950; Stephens et al., 1953; Eden et al., 1957; Earp et al., 1961; Smith et al., 1967; Evans et al., 1968; Stevenson and Gaunt, 1971). The results of the work are well summarized in the detailed stratigraphical scheme published by Ramsbottom (1966; see also Ramsbottom et al., 1978).

One of the most interesting results of the integration of the sandstone distribution with the ammonoid biostratigraphy has been the development of the so-called 'Mesothem Model', mainly by Ramsbottom (1977). This attempted to interpret the broad pattern of sedimentation observable in the Millstone Grit of the Central Province in terms of eustatic sea-level changes, controlled possibly by Milankovich cyclicity. The significance of this would be that the cyclicity should also be recognizable at a regional if not global level, with important consequences for long range stratigraphical correlations. The evidence in the Millstone Grit of the Central Province has been reviewed by Holdsworth and Collinson (1988). They found clear evidence of small-scale cyclicity, which they believe was produced by sea-level changes, but evidence of the type of the large-scale cyclicity in Ramsbottom's model was less certain. For instance, the mesothem boundaries did not always seem to coincide exactly with the faunal changes, as required by the Ramsbottom model. However, there does seem to be a larger-scale pattern of cyclicity than Holdsworth and Collinson's 'minor cycles' and, whether or not they are called 'mesothems', they need to be explained. Ramsbottom's controversial ideas have provided one of the most lively subjects for debate in Carboniferous stratigraphy, and the evidence from the Central Province Millstone Grit has played a central role.

The most recent developments in geological investigation of these strata have been in the field of sedimentology. Early investigations by Sorby (1859) and Gilligan (1920) looked at the petrography of the rocks in order to establish details such as provenance. However, real progress did not occur until facies analysis was applied to the problem, pioneered by Trotter (1951). The Kinderscout Grit and its underlying sediments have attracted some of the most intense interest (Allen, 1960; Walker, 1964b, 1966a, 1966b; Collinson, 1968, 1969, 1970; McCabe, 1975, 1977, 1978) and is now modelled as a turbidite-fronted delta. However, there has also been work on the Rough Rock (Shackleton, 1962; Heath, 1975; Miller, 1986; Bristow, 1988), the Chatsworth and Ashover grits (Mayhew, 1967a, 1967b) and the Haslingden Flags (Collinson and Banks, 1975; Bristow, 1988). A useful summary of the evidence is provided by Collinson (1988).

Lithostratigraphy

As pointed out above, establishing a coherent lithostratigraphical scheme for these strata has presented serious problems, largely because of the impersistence of many of the main sandstone bodies (see for instance (Figure 9.2)). Even at the level of group, there is not a uniform classification for use throughout the Central Province, particularly in the lower part of the Millstone Grit.

The scheme used in this chapter is based partly on that used by the British Geological Survey, as summarized by Edwards and Trotter (1954). Four subgroups are recognized as the main divisions (Edwards and Trotter recognized a further two subgroups in the Pendleian and Arnsbergian parts of the Millstone Grit, but this lies outside the remit of this volume). Within each subgroup, the main grits or sandstones are recognized as formations; except in the lower part of the sequence, the shales are not normally named. In some cases, the grits occur as two discrete leaves, and these are recognized as members. As far as the authors are aware, no formal stratotypes have been designated for these lithostratigraphical units.

Edale Shales Subgroup

Base defined: base of shales immediately above the Warley Wise Grit Formation in the Craven Basin; elsewhere at the base of Millstone Grit.

Characteristic facies: marine shales with subsidiary mainly turbiditic sandstones. Major formations: Edale Shales (Edale Basin), Sabden Shales (Craven Basin).

Comments: This is referred to by Edwards and Trotter as the Middleton Grit Group. However, this grit occurs only in the very north of the Central Province and is totally atypical of the lithologies found elsewhere in this interval. The Edale Shales are a far more typical development, and so its name has been adopted for the subgroup, as in Moore (1967).

Kinderscout Subgroup

Base defined: base of first major turbiditic sandstone above the Edale Shales Subgroup.

Characteristic facies: turbiditic shales and sand stones in the lower part, fluvio-deltaic sandstones in the upper part.

Major formations: Kinderscout Grit over much of province; Mam Tor Sandstone, Shale Grit and Grindslow Shale (Edale Basin); Longnor and Blackstone Edge sandstones (Staffordshire Basin); Cobden Sandstone and Todmorden Grit (Craven Basin).

Middle Grit Subgroup

Base defined: base of shale immediately above the Kinderscout Grit and its lateral equivalents.

Characteristic facies: shales, both marine and non-marine; feldspathic, often cross-bedded sandstones.

Major formations: Chatsworth and Ashover grits over much of province; Roaches Grit (Staffordshire Basin); Alum Crag, Revidge, Hazel Greave and Holcombe Brook grits (Craven Basin); Huddersfield White Rock (Huddersfield Basin).

Rough Rock Subgroup

Base defined: base of shale immediately above the Chatsworth Grit, Huddersfield White Rock and Holcombe Brook Grit formations.

Characteristic facies: mainly non-marine shales with some thin coals and seat earths, and only occasional marine intercalations; fine to medium-grained sandstones.

Major formations: Rough Rock over much of province; Haslingden Flags (Craven Basin).

Geological setting

The Central Province is the remains of a broad area of sag between the Wales–Brabant Barrier to the south and the Askrigg Block and Lake District High to the north (Figure 9.2). Based on seismic data, Lee (1988) has shown that in the early to middle Namurian it comprised of a set of separate fault-bounded basins, the result of lithospheric extension and thinning (see also Leeder and McMahon, 1988). The most important of these were those first established in the Dinantian, namely the Staffordshire Basin, the Widmerpool Gulf, the Edale Basin, the Huddersfield Basin and the Craven Basin (Figure 9.3). These basins had a marked influence on sediment distribution in the Kinderscoutian and before, but by the late Marsdenian sedimentary patterns had become more uniform over the Central Province, with the possible exception of the Craven Basin.

The Millstone Grit of this area may be seen as comprising two major facies associations. The first consists of mainly fine-grained (shale, siltstones) basinal deposits. Salinity levels within the basin appear to have varied; at some levels conditions were fully marine, resulting in the stratigraphically important marine bands, whilst at others conditions were non-marine, and sometimes even emergent. This variation has been seen in terms of cyclical eustatic changes (Holdsworth and CoHinson, 1988), but there is also a clear long-term reduction in marine influence, culminating in strata that are very similar to the Coal Measures in the topmost Millstone Grit. In summary, the lower part of the Millstone Grit can be taken to be mainly marine deposits with occasional non-marine intercalation, whilst the upper Millstone Grit is mainly non-marine with occasional marine intercalations.

Imposed on this background of basinal deposition, was the periodic introduction of coarser fluvio-deltaic deposits. These sandstones and grits were mainly derived from the Caledonian Highlands to the north, except in the southern part of the basin (Staffordshire Basin, Widmerpool Gulf) where they seem to have originated in part from the Wales–Brabant Barrier. This distinction in provenance can be recognized both in general palaeocurrent direction, and in differences in petrography; the sandstones from the south are more quartzitic, those from the north more feldspathic. A third possible source to the west or north-west, in the present-day Irish Sea, has been postulated for the Haslingden Flags of the Craven Basin but this requires further work (Collinson, 1988).

According to Collinson (1988) the coarser deposits of the Central Province Millstone Grit were formed in three distinct types of delta (Figure 9.4).

1. Turbidite-fronted deltas. These were the most important basin-fill deposits during most of the Namurian, and resulted in major sandstone bodies such as the Kinderscout Grit and the Roaches Grit. There is no close modern analogue of this type of delta.
2. Sheet deltas. This type of delta became more significant in the later Namurian, resulting in sandstone bodies such as the Chatsworth and Ashover grits, and probably reflected the shallowing of the basin. They have been compared with the Lafourche and St Bernard deltas in the Holocene Mississippi delta-complex.
3. Elongate deltas. This is only known in the Haslingden Flags, and results in discrete, elongate bodies of sandstone. There is a close comparison with the birdfoot-type deltas seen in the Mississippi of today.

GCR site coverage

This reflects the two main facies associations in the Millstone Grit: the basinal deposits with marine bands, and the deltaic deposits. The basinal deposits are represented by a sequence of sites showing the standard succession of marine bands, summarized by Ramsbottom et al. (1978). These may either be sites showing a sequence of bands or, in certain circumstances an exposure of one band which is particularly fossiliferous (the latter include the stage boundary stratotypes dealt with in Chapter 2). This coverage is summarized in (Figure 9.5).

Against this background of biostratigraphical sites, there are the sites showing the deltaic deposits. These sites may be further divided into groups according to the type of delta represented, as outlined in the previous section, and the provenance of the sandstone. They are as follows.

1. Turbidite-fronted deltas. This has concentrated on the Kinderscout delta, which is the most extensive and well studied of this type. The sites may be further subdivided.

1. Sites showing the progradation of the delta over the Central Province: Mam Tor, Alport Castles, Blackden Brook, Crimsworth Dean.
2. Sites showing characteristic sedimentological features of the delta-top deposits: Standedge Road Cutting, Derby Delf Quarry, Ladcastle and Den Quarries.
3. Sheet deltas. These are the various smaller-scale deltaic deposits in the Middle Grit Subgroup. This is best shown with the Chatsworth Grit at Hathersage Moor, but other examples can also be seen at Gib Tor, Rake Dike and Hodge Clough.
4. Elongate deltas. These are only represented in the two members of the Haslingden Flags, seen at Closebrow Quarry and Heys Britannia Quarry.
5. Deltaic deposits derived from the Wales–Brabant Barrier to the south. These are only well exposed in the Staffordshire Basin, such as at Pot Bank Quarry.

References