Abstract
The Brassington Formation of the Pennines is the most extensive onshore Miocene succession in the UK. It is preserved as outliers in Lower Carboniferous Limestone. During the Cenozoic, central England underwent uplift, with erosion of post-Mississippian strata from the Pennine axis in the Peak District. The Brassington Formation is hence significant in reconstructing Cenozoic geological history. It is non-marine, derived from Triassic sandstone and of Mid–Late Miocene age. The c. 60 outliers occur in three clusters over c. 220 km2. They are remnants of a sedimentary prism at least 75 m thick and with a volume of c. 10 km3. Suffosion of bedrock, conditioned by aggressive precursor fluids of hypogenic origin, was the major control on subsidence. The calculated volume of the fills in the Bees Nest and Green Clay outliers is 3.7–5.0 × 106 m3. Referenced to a sub-Miocene surface at c. 450 m OD in pre-subsidence times, the original volume of this subsidence complex was c. 21.8 × 106 m3 and the aggregated volume of the Late Neogene White Peak palaeokarst was at least 0.66 km3. Subsidence was concomitant with Pliocene uplift of the Pennine Axis, suggesting suffosion accommodation of 3 × 106 m3 per km2 or 244 m3 per annum.
Over 60 outliers of the Brassington Formation are known from the counties of Derbyshire and Staffordshire, in central England (Fig. 1). This Miocene unit was extensively quarried for silica sand (Yorke 1954, 1960, 1961). The ‘Pocket Deposits’ of the Brassington Formation occur largely in the dolomitised Lower Carboniferous Peak Limestone Group in three separate areas termed the northern, southern and western clusters. The latter is a small area (c. 6 km2) in the Weaver Hills, Staffordshire (Figs 1 and 2; Chisholm et al. 1988; Pound et al. 2012a, fig. 1). By contrast, the former two are in Derbyshire, occupying a narrow 19 km NW–SE-trending zone across the White Peak from Parsley Hay in the NW to Carsington Pasture in the SE (Fig. 2; Yorke 1961). There is an apparent absence of outliers in a zone 4 km wide between the northernmost outlier of the southern cluster (Minninglow; SK 2023 5760) and the southernmost outlier of the northern cluster (Newhaven; SK 1661 6024). Similarly, the easternmost outliers in the Weaver Hills cluster (Huddale Farm and Sallymoor) are >9 km from any outlier in either the northern or the southern clusters. The subsurface aspect of these outliers protected them from erosion during the Quaternary glaciations. Their outcrops are largely continuous with the ground surface; only rarely is there any expression of them in soil and topography, albeit there is an association with acid-tolerant plants, such as gorse.
The location of the former Brassington Formation sedimentary prism in Derbyshire and Staffordshire (main map) and its position in the UK (inset map), depicting the three clusters of the ‘Pocket Deposits’. The northern cluster, blue; the southern cluster, pink; and the western cluster (the Weaver Hills), yellow.
The location of the principal outliers of the Brassington Formation on a geological map of parts of Derbyshire and Staffordshire, UK. Bees Nest, Green Clay and Kirkham pits are in the southern cluster and Kenslow Top Pit is within the northern cluster. The considerably smaller western (Weaver Hills) cluster lies in Staffordshire to the SW of the two Derbyshire clusters. Note the preponderance of Pocket Deposits associated with the dolomitised limestone of the southern Pennines. The figure has been annotated to discriminate between the northern and southern clusters and to show pits with blocks of Bowland Shale Formation and pits that have yielded plant fossils. Numbered pockets: 1 Custard Field Pit; 2 Blakemoor Pit; 3 Heathcote Pit; 4 Washmere Pit; 5 Low Moor; 6 Low Moor Farm West; 7 Low Moor Farm 2; 8 Low Moor Farm 3; 9 Hoe Grange Quarry; 10 Harborough Works; 11 Green Clay Pit; and 12 Spencer's Pit. The location of the village of Hulland Ward is shown for reference.
The Brassington Formation comprises the Kirkham, Bees Nest and Kenslow members (Fig. 3; Boulter et al. 1971). The original stratigraphy is generally well preserved, with minor structural distortions attributed to localized subsidence effects. Early publications on the Pocket Deposits include Brown (1867), Maw (1867), Howe (1897, 1918, 1920) and Scott (1927). More recent contributions on the Brassington Formation include Kent (1957), Ford & King (1968, 1969), Boulter (1969, 1971a, b), Boulter & Chaloner (1970), Ford (1972a, b), Walsh et al. (1972, 1980, 1999), Wilson (1979) and Yorke (1954, 1960, 1961). The type section is a formerly well exposed and structurally uncomplicated succession at Bees Nest Pit [SK 1818 6158], c. 1 km NE of Brassington village (Fig. 2). As with virtually all the outcrops of the Brassington Formation, exposure at this locality is attributable to historical silica sand exploitation (Boswell 1918). The quarrying of high-grade silica sand began in the late eighteenth century and the peak of extraction was during the 1940s and 1950s. This material was used in the manufacture of refractory bricks for lining furnaces and production ceased during the late 1970s. The sand from the Kirkham Member (Fig. 3) was especially suitable for this purpose due to the presence of kaolinitic clay pellicles. The silica sand pits are now largely overgrown. Few of the pit walls are still exposed; therefore, recent research has necessarily relied on manual excavation or the reinterpretation of existing information. Fortunately, there is a fine black and white photographic record of sand quarrying operations from the 1940s and 1950s by Yorke (1954, 1960, 1961). An account of the background and the history of research on the Brassington Formation was also given by Pound et al. (2012a, pp. 26–29).
The chronostratigraphical context of the Brassington Formation. The currently accepted age of the uppermost Kenslow Member is from Pound et al. (2012a) and Pound & Riding (2016) and the original age determination of Boulter et al. (1971) is indicated. The chronostratigraphical framework is taken from Gradstein et al. (2012).
Yorke (1954, 1960, 1961) interpreted the outliers as the infillings of solution ‘swallow-holes’ or ‘limestone canyons’ in the Peak Limestone Group, the fill being Permo-Triassic sands and gravels. Kent (1957) further developed this hypothesis and suggested that both the pockets and their sediment fills of weathered and bleached sands were of Triassic age. On the basis of major lithological contrast and fossil content, Boulter et al. (1971) named this unit the Brassington Formation and subdivided it into the Kirkham, Bees Nest and Kenslow members (Fig. 3). These subdivisions were named after the silica sand pits with the best exposed sections at that time. Furthermore, Boulter (1971a) established the uppermost member of the Brassington Formation (the Kenslow Member) as being of Neogene (Late Miocene–Early Pliocene) age. This stratigraphy gained general acceptance and was adopted by the British Geological Survey (BGS), e.g. Frost & Smart (1979), Aitkenhead et al. (1985), Chisholm et al. (1988) and McMillan et al. (2011). Subsequently, Pound et al. (2012a) and Pound & Riding (2016) have demonstrated that the Kenslow Member at Kenslow Top Pit is of Late Miocene (Tortonian) age. This contrasts with the Mid Miocene (Late Serravallian) age of the Kenslow Member of Bees Nest Pit (Pound & Riding 2016) and possibly indicates that the aforementioned stratigraphical classification may only apply to the southern cluster of outliers.
The Brassington Formation is unique in British stratigraphy (King 2006; King et al. 2016). Firstly, it is the most geographically extensive and volumetrically abundant Miocene unit in the UK (Boulter et al. 1971; Pound & Riding 2015). Furthermore, with the possible exception of the Lenham Formation in Kent, interpreted as a residual deposit by McMillan et al. (2011), it is the only UK lithostratigraphical unit known to be preserved exclusively in subsidence sinkholes. There are no known examples of Brassington Formation outliers that have not subsided. There would have been no specific reason to invoke the former existence of a sheet of Miocene sediments above much of the southern half of the White Peak if the Brassington Formation pockets had not been discovered.
The existence of the Brassington Formation provides insights into the tectonic setting of the southern Pennines and enables reconstruction of important aspects of their geomorphological and palaeogeographical evolution (Walsh et al. 1972). As well as reassessing the significance of this important unit, this contribution considers the principal causes of subsidence and uses a volumetric determination of the outlier sediments as a proxy for quantifying the accommodation process. Since Walsh et al. (1972) there have been two developments that contribute to the current hypothesis regarding the causes of subsidence. The first relates to the dolomitisation process. Replacive dolomitisation (Parsons 1922) was once attributed to downward-percolating Permo-Triassic fluids (Ford 2002). However, Schofield & Adams (1986) demonstrated that dolomitisation of the Woo Dale Limestone Formation in Woo Dale, Derbyshire was attributable to fluids from the SE rising up basement faults that define a half-graben. This has influenced the understanding of dolomitisation elsewhere in the platform. Schofield (1982) and Schofield & Adams (1986) also showed that dolomitisation occurred in more than one phase associated with intervening suspected hydraulic fracturing, the second phase being characterized by higher iron and manganese concentrations. The second contributory factor is the increasing recognition of the significance of hypogene processes associated with the palaeokarst and Mississippi Valley Type (MVT) mineralization, e.g. Ford (1989), Klimchouk & Ford (2000a, b), Klimchouk (2007) and Banks et al. (2015). Hypogene karst comprises two components: these are hydrogeological recharge of soluble formations from below and deep-seated sources of geochemically aggressive karst water. Both of these are independent of meteoric recharge in the immediate environment (Dublyansky 2014) and here we consider both these components in the context of the platform edge in Derbyshire.
New observations have also contributed to the revised conceptual model for the karst. These comprise stratigraphical influences of the mechanical properties and bedding continuity on doline architecture (Banks et al. 2009). Recently excavated temporary exposures on Carsington Pasture have exposed subdued pinnacle karst. Between the pinnacles are preserved pockets of dolomite sand, isovolumetric with the parent dolomite rock, which collapse when gently touched and are readily eroded in the dolomitised Bee Low Limestone Formation (Jones & Banks 2014).
The lithostratigraphy of the Brassington Formation
The Brassington Formation is a Mid to Late Miocene (Serravallian–Tortonian; Fig. 3) fining-upwards, internally-conformable, siliciclastic lithostratigraphical unit. It is dominated by unlithified fine- to medium-grained varicoloured quartz sands with kaolinite-rich clay, significant pebble beds and minor silt (Howe 1918, 1920; Ijtaba 1973; Chisholm et al. 1988). The sand grains are rounded to subangular and are coated with a thin pellicle of kaolinite; heavy minerals are relatively rare. The pebbles are rounded and largely of quartzite which frequently exhibits pressure-pitting (Wilson 1979). However, other lithotypes, such as chert, dolomite, jasper, Millstone Grit, tourmaline-bearing quartz and various igneous and metamorphic rocks, typical of those in Triassic conglomerates and referred to as ‘Bunter’ pebbles, are present. Only the very rare chert fragments are angular. Limestone clasts are virtually absent. The uppermost beds of the Brassington Formation, the Bees Nest and Kenslow members, are lacustrine/paludal clays and silts, with plant fossils present in the highest preserved beds.
The maximum thickness recorded of the Brassington Formation, over 70 m, is at Kenslow Top Pit (Aitkenhead et al. 1985); by contrast, the type section at Bees Nest Pit is 43 m thick (Boulter et al. 1971; Walsh et al. 1972). However, these measurements were taken across the limbs of the sag synclines, which have probably been attenuated by subsidence. True thicknesses are therefore probably appreciably higher in the axial zones of the sag synclines, which may more accurately reflect the variation in the pre-subsidence thicknesses of the Brassington Formation. Within the clusters of Pocket Deposits, the preservation of the Brassington Formation is sufficient to enable recognition of a consistent tripartite lithostratigraphy (the Kirkham, Bees Nest and Kenslow members). However, a complete assemblage, as seen in the type section, comprises seven elements (Table 1), but few of the pockets contain all seven of these components (Walsh et al. 1980). Generally, the oldest units are marginal to the pockets and the youngest are axial to the downsags (Walsh et al. 1972, 1980).
The seven lithostratigraphical units associated with the Brassington Formation arranged in ascending stratigraphical order
The succession of elements one to seven in Table 1 comprises residual components derived from the dolomitised limestone and overlain by the Neogene succession and subsequent Quaternary glacigenic sediments. This is probably not a true temporal sequence, as it is still unclear whether the chert-clay solution residue (2) formed prior to the subsidence of the foundered blocks of the Bowland Shale Formation (3), or accumulated during karst suffosion in post-Kenslow Member times (Walsh et al. 1972). If the latter, the chert-clay solution residue probably represents post-Tortonian karst suffosion development and the succession is hence 1, 3, 4–6, 2 and 7.
The derivation of the chert-clay solution residues (unit 2)
The cherty residues (Figs 4 and 5) occur in the margins of many of the pockets. In Bees Nest Pit they are up to 6 m thick and their distribution reflects the distribution of chert in the source rock. Chert is sparse in the Bee Low Limestone Formation (Table 2), but is widespread in the overlying Monsal Dale Limestone Formation, for example at the former Hoe Grange Quarry near Longcliffe [SK 2234 5595], 2 km NW of Bees Nest Pit (Fig. 2). Derivation from the Monsal Dale Limestone Formation is supported by the presence of Viséan (Brigantian) fossil casts in the chert clasts (Walsh et al. 1972, identifications by Dr J.E. Robinson [pers. comm.]). The chert clasts are invariably angular to subangular, suggesting that no significant lateral movement took place following the decalcification of the source rock (Aitkenhead et al. 1985; Chisholm et al. 1988). By implication, the Monsal Dale Limestone Formation formerly extended across the area of Bees Nest Pit, as is currently the situation at Chariot Sandpit, which lies within a 20 m thick outlier of the Monsal Dale Limestone Formation <1 km from the northeastern edge of the Bees Nest subsidence complex (Fig. 2). The Monsal Dale Limestone Formation was evidently thick enough to produce, upon dissolution, a layer of chert-clay solution residue at least 6 m thick. Chert-clay solution residues have also been proven in boreholes and on many of the pocket floors. Yorke (1961, figs 5.1, 30) regarded them as ‘scree’, but they have since been reinterpreted as dissolution residues, as exposed in many places in the sand pit walls. It is possible that decalcification was ongoing throughout the genesis of the Brassington Formation such that a number of stratigraphical configurations of the chert-clay residue and the Bowland Shale Formation blocks are possible (e.g. Fig. 4; Walsh et al. 1972, fig. 2a; Dalton et al. 1988; Ford 1989). Walsh et al. (1972) considered that the solution residue was formed by Late Neogene dissolution, but other scenarios could have occurred, e.g. accumulation by Pleistocene glaciofluvial processes or mass movement from an earlier erosion surface.
The chert-clay solution residue (unit 2) which underlies the main Brassington Formation (units 4–6) at the margins of the larger pockets. This image is of the west end of Bees Nest Pit, NE of Brassington village, facing west. The chert-clay solution residue is extensively slumped in the foreground; the steep quarry wall on the right hand side is in situ unit 2. Note the clear blocks of white chert which have weathered out near the base of the slumped material, some of which yield moulds of the large brachiopod genus Productus. At this locality, the chert-clay solution residue is bleached white/cream which indicates that the bleaching process that has affected most of the Kirkham Member sediments at most localities also locally affected the solution residues. This white/cream colour is somewhat atypical of unit 2, which is normally brown/buff in colour (Fig. 5). Photograph taken by PTW in 1970, when the quarry was working. The telegraph pole at top left provides an indication of scale.
The southern wall of Bees Nest Pit, NE of Brassington village, facing south. Unit 2, the chert-clay solution residue, is the brown, stony deposit at the back of the quarry face. In front of it are discontinuous, relatively thin, steeply inclined (towards the camera) masses of blue/lilac Bowland Shale Formation (unit 3). Hummocks of spoil occur on the flat surface beyond the quarry face. Note how the chert-clay solution residue has a very uneven base and is prone to slumping. The dolomite wall (unit 1, visible in the extreme left hand side of the image) and units 2 and 3 are variously inclined at very steep angles, approaching vertical, towards the camera position. The photograph was taken by PTW in 1970 following the hourglass-shape mud slide of unit 2 material in the right-hand side of the image. Note the other large mud slide formed of unit 2 in the left-hand side of this photograph. The person in the centre of the prominent exposure of the Kirkham Member, indicated by an asterisk, is for scale (c. 2 m).
The lithostratigraphy of the Lower Carboniferous (Mississippian; Tournaisian–Viséan) Peak Limestone Group of the Peak District in the Buxton–Matlock area
To date, no evidence has been presented to confirm whether the chert-clay residues are a consequence of pre-Serpukhovian, post-Kenslow Member or any other period of dissolution. Ford & Jones (2007) suggested that at least some of the solution residues pre-date Bowland Shale Formation deposition; however, this was not evidence-based and we interpret them as being largely post-Tortonian in age.
The derivation of the Bowland Shale Formation blocks (unit 3)
The Bowland Shale Formation, of Mississippian–Pennsylvanian (Serpukhovian to Early Bashkirian) age, comprises blue-black fissile mudstones with thin calcareous siltstones and represents the deposition of marine pelagic clays (Chisholm et al. 1988). There is both a stratigraphical hiatus and a structural discordance of the Bowland Shale Formation along the southern margin of the White Peak (Oakman 1984). These mudstones onlap the reef facies of the Bee Low Limestone Formation along the southern margin of the White Peak and, as indicated by the plant spores present in the solution subsidence blocks at Bees Nest Pit, finally buried the reef belt by the Serpukhovian (Arnsbergian) according to Walsh et al. (1972).
Allochthonous blocks of the Bowland Shale Formation are found in many Brassington Formation pockets, including Bees Nest, Green Clay, Kenslow Top, Kirkham's, Low Moor and Minninglow pits (Fig. 2; Yorke 1954, 1960, 1961; Walsh et al. 1972). The blocks (or rafts) of mudstone are seldom more than a few metres thick. Since they were of no commercial interest to the sand pit operators, there have been few descriptions of exposures of the contacts of the mudstone blocks within the enclosing sediments. The allochthonous blocks of the Bowland Shale Formation exhibit various weathering states. Several occurrences of lilac-coloured staining to otherwise fresh black mudstone blocks have been observed and this possibly represents pedogenesis in pre-Kirkham Member times (Fig. 6). Generally, the undersides of the mudstone blocks appear to have moved in conjunction with the upper levels of the chert-clay residues during subsidence; the junction between them is relatively sharp (Yorke 1961). These foundered blocks indicate that the Bowland Shale Formation once covered substantial areas, if not the whole of the White Peak, and hints strongly that nowhere was the basal sand of the Kirkham Member deposited on a Viséan Limestone substrate. Physical laboratory experiments by Walsh et al. (1972) indicated that the maximum stress to the outlier sediments, as they subsided with the karst suffosion, occurred at the margins of the sinkholes (Fig. 7). Therefore the mudstone masses may have acted as a shear zone between the Brassington Formation sediments above and the weathered dolomitised limestone below. Given the proven depth of subsidence, frictional strains against the sides of the sinkholes would cause dislocation of the mudstone, thus the mudstone masses are intermittent in distribution.
A large mass of Bowland Shale Formation (unit 3) in the SW corner wall of Bees Nest Pit, NE of Brassington village, facing ENE. This is the dark lithotype at the right-hand side. The tops and sides of the blocks of Bowland Shale Formation are typically heavily weathered at this locality and are stained lilac-purple in colour. The mudstone block is overlain by the much lighter coloured Kirkham Member of the Brassington Formation (unit 4) to the left with a sharp boundary marked by a trowel (for scale). This sharp boundary is considered to be the pre-subsidence surface of the lowermost Brassington Formation sediments (i.e. a sub-Miocene unconformity). Slumped, iron-stained chert-clay dissolution residues (unit 2) cover the Bowland Shale Formation block. In the background, the floor of the quarry and the NE walls largely comprise bleached Kirkham Member sands and gravel beds. Dark brown Quaternary glacial and periglacial deposits (unit 7) are exposed in the NE and SE corners of the quarry in the distance to the right. Photograph taken by PTW in 1970, when the quarry was being worked.
The two-dimensional subsidence simulation apparatus used by Walsh et al. (1972, p. 527, table 1) to simulate the emplacement of the Brassington Formation into the sinkholes. The test result that is shown attempted to replicate the form of the central part of the deepest subsidence at Bees Nest Pit. The three stratal layers represent the Bowland Shale Formation, the Kirkham Member sands and the sediments younger than the Kirkham Member. The vertical wooden boards with regular peg holes are a proxy for the Peak Limestone Group host rock. These boards were gradually lowered over c. 1 hour to simulate a steep-sided, narrow, U-shaped depression. This was test eight of eight by Walsh et al. (1972, plates 1–4, table 1). Test eight produced a tight syncline in the ‘Bowland Shale’ and ‘Brassington formations’, which neatly simulated what is observed in the central part of the main sag syncline at Bees Nest Pit. Note how the ‘Bowland Shale Formation’ has dissociated into distinct marginal blocks. The ‘pre-exploitation ground level’ here approximates to a horizontal line across the middle of the number eight affixed to the perspex front of the model.
The derivation of the Kirkham, Bees Nest and Kenslow members (units 4, 5 and 6)
The Kirkham Member sand and gravel beds are normally bleached white, but they are red-brown in subsurface exposures in mine galleries. Their lithological properties indicate that they are the erosion products of a southwards-retreating scarp of the Sherwood Sandstone Group (Triassic) to the south of the Brassington Formation Basin, at least in respect of the outliers in the southern cluster (Walsh et al. 1972, 1980, fig. 15). The Bees Nest and Kenslow members are fine-grained and hence much less sandy. These units were considered by Ijtaba (1973) to represent the erosion of deeply weathered Serpukhovian mudstones in the area previously occupied by the Triassic cover rocks, which were eroded to form the Kirkham Member. However, no reworked Carboniferous spores have been observed in the Bees Nest or Kenslow members.
The palaeontology and stratigraphy of the Brassington Formation and its revisions
The Brassington Formation has been variously interpreted as Carboniferous (Scott 1927), Permian (Yorke 1961) and Triassic (Boswell 1918; Kent 1957), but there was no biostratigraphical evidence for these assessments. Boulter & Chaloner (1970) and Boulter (1971a, b) were the first to confirm the post-Triassic/pre-Quaternary age. Boulter, in Walsh et al. (1999), dated the Kenslow Member as Late Miocene. The tripartite subdivision of the formation and its widespread distribution appears robust. However, Pound et al. (2012a) and Pound & Riding (2016) have established disparate biostratigraphical ages for the post-Bees Nest Member succession and it has become necessary to review the stratigraphy of the various profiles recorded in the Brassington Formation.
Pre-Kenslow Member successions have usually been attributed to the Late Miocene (Walsh et al. 1999), because none of the members of the Brassington Formation exhibit significant internal sedimentary breaks (Ijtaba 1973) and sedimentation rates of the Kirkham Member appear to have been high (Wilson 1979). Walsh et al. (1980) reported a bed of clay with plant macrofossils reminiscent of the Kenslow Member within the Kirkham Member at Heathcote Pit. However, this has yet to be rediscovered and the flora analysed. Clearly, the sands must be post-Serpukhovian and pre-Late Serravallian at the type section, there being no discernible breaks in the Brassington Formation stratigraphy (Ijtaba 1973).
Whilst no faunal remains have been found in the Brassington Formation, as in many coeval European paralic environments, there are abundant plant fossils in the Kenslow Member (Table 1; Boulter & Chaloner 1970; Boulter 1971a, b; Aitkenhead et al. 1985; Pound et al. 2012a; Pound & Riding 2016). Furthermore, macroscopic plant fossils may not be limited to the Kenslow Member of Bees Nest and Kenslow Top pits. There have also been reports of fossil plants from other Pocket Deposits (Ludford 1950; Ford 1972a, b; Walsh et al. 1972, 1980).
The assumption by Boulter (1971a, b) that the Kenslow Member at Bees Nest and Kenslow Top pits is coeval was revised by Pound et al. (2012a) and Pound & Riding (2016). The Kenslow Member at the type section is Mid Miocene (Serravallian), whereas the in situ Kenslow Member and a glacially rafted allochthonous unit at Kenslow Top Pit are Late Miocene (Tortonian) (Jones et al. 2016; Pound & Riding 2016). These revisions indicate a c. 5 myr age difference between KM1 and KM3 (Fig. 3). The findings of Pound & Riding (2016) have also established that there is an apparent time gap of c. 2 myr between the oldest datable sediments at Kenslow Top Pit (KM2) and the only datable sediments at Bees Nest Pit (KM1, Fig. 3). This indicates that the Kenslow Member of the southern cluster is the older of the two. Pound et al. (2012a) and Pound & Riding (2016) deduced that the flora of the Kenslow Member is indicative of a warm temperate palaeoenvironment, a finding that is consistent with other British and Irish Miocene deposits (Watts 1962; Evans 1990; Curry 1992; Head 1993; Walsh et al. 1996). Moreover, the two floras represent distinctly different palaeoecological niches. It is thus feasible that each of the clusters may represent separate and heterochronous sub-basins, possibly with different sediment provenance. However, the evidence does not preclude the possibility that there was broadly continuous deposition of the Brassington Formation in a single basin for >5 myr (Pound & Riding 2016) and these authors regarded the various developments of the Kenslow Member as being diachronous. In turn, this has implications for the understanding of the depositional setting of the two older members, with the possibility that they were deposited in separate sub-basins of different ages (Fig. 1). The presence of the raft of presumed Kenslow Member within a succession of glacial deposits in the southern wall of Kenslow Top Pit shows that deposition of this unit continued either there or nearby until the Late Tortonian (Jones et al. 2016). It now appears that the subsidence of the Brassington Formation into the karst suffosion cavities occurring as a single, albeit prolonged, event later than the Miocene, as suggested by Walsh et al. (1980), is probably an oversimplification of the true karst situation.
Although the elevation of the original depositional surface is indeterminable, palaeontological and sedimentological evidence suggest that the contemporary marine shorelines lay an appreciable distance away. Most reconstructions of the rate of the Late Neogene Pennine uplift assume that the depositional surface of the Brassington Formation depocentre was mostly, if not entirely, within 50 m of sea-level throughout deposition (Pound et al. 2012a), although there is no unequivocal evidence for this.
The distribution and morphology of the Brassington Formation pockets
Pocket Deposits of the Brassington Formation are dominantly present on the outcrop of the Lower Carboniferous Peak Limestone Group at altitudes of between 250 and 320 m OD (Table 3). In contrast, the closest Triassic outcrops to these outliers lie to the south around Hulland Ward (Fig. 2). Structure contours on the base of this outcrop are at a significantly lower altitude (180–210 m OD; Chisholm et al. 1988). Assuming subsidence of 150–250 m of the Brassington Formation in the southern cluster, this suggests a Late Neogene uplift of the Peak Limestone Group massif of c. 450 m (Walsh et al. 1980). Post-Miocene denudation has entirely destroyed this surface; its counterpart only now exists as a sub-Miocene unconformity where it is associated with the Bowland Shale Formation blocks in the margins of the sinkholes such as Bees Nest Pit (Fig. 6).
The elevation (m OD) of the rims of eight prominent clusters of Brassington Formation outliers
The area that encloses the outermost outliers of the Brassington Formation is c. 220 km2 (Walsh et al. 1980). Although less than the depth of subsidence where pockets are preserved, the entire landscape of the White Peak has been lowered by several decametres in post-Kenslow Member times. The depths of the sinkholes represent the interplay between uplift-driven subsidence and surface lowering or denudation. Walsh et al. (1972) contended that there is no sedimentological evidence that subsidence of the outliers began before the uppermost Brassington Formation was deposited. Using selected profiles, a broad estimate of the volume of the pre-subsidence Brassington Formation sedimentary prism can be calculated. The maximum vertical quarried depth was 49 m at Washmere Pit, Friden, in the Kirkham Member, but the base of the unit was not reached (Ford & King 1969). Similarly, at Bees Nest Pit the base of the Kirkham Member was recorded at 46 m below ground level, underlain by 6 m of ‘clay’ (possibly weathered Bowland Shale Formation) resting on the limestone bedrock (Yorke 1961, part III, fig. 6). These data, with measured profiles in several of the pits, indicate that if the Brassington Formation extended across the entire area and was not restricted to smaller basins, the volume of the Brassington Formation prism was at least 10 km3.
Areas of the White Peak apparently devoid of substantial pockets include those north of the River Wye, the land between the Rivers Dove and Manifold, and between Matlock and Wirksworth (Fig. 2). Outliers of the southern cluster are close to the edge of the limestone outcrop. By contrast those in the northern cluster, around Friden, are close to the centre of the limestone plateau such that proximity to the edge of the limestone massif cannot have been a major control on pocket genesis (Fig. 2). The southern cluster includes a dense, physically associated, complex of pockets centred on Bees Nest Pit. The elevations of the rims of the pockets are given in Table 3. The host rock is generally, but not exclusively, the Bee Low Limestone Formation of Viséan (Asbian) age (Table 2). In the Carsington Pasture area and at Chariot Pit, the Brassington Formation is partly accommodated within the overlying Monsal Dale Limestone Formation (Viséan/Brigantian) whereas several large pockets in the Hartington area have rims that are of Woo Dale Limestone Formation (Viséan/Holkerian) or are likely to be housed in a Woo Dale Limestone Formation foundation at depth. At least half the known pockets and most of the major subsidences are within the dolomitised zones of the Bee Low Limestone Formation (Ford & King 1969). The dolomitisation occurred during the Pennsylvanian (Hollis & Walkden 1996) and was probably an early effect of mineralization (Schofield & Adams 1986; Ford & Jones 2007; Frazer et al. 2014). Ford & King (1968, 1969) suggested that the higher permeability of the dolostone compared to the limestone may have been a major factor in the development of the pockets. However, there are significant areas of intense dolomitisation, such as those around Matlock Bath and Winster where the Monsal Dale Limestone and Eyam Limestone formations crop out and pockets are apparently absent. It is also plausible that the pockets in apparently non-dolomitised limestone are associated with subsurface zones of dolomitisation, particularly where they are associated with faulting.
The shapes of the sinkhole floors and walls have been determined wherever possible from archive photography, borehole data, field observations, passive seismic surveys and terrestrial laser scanning (Yorke 1961; Banks et al. 2015; Pound & Riding 2015; Raines et al. 2015). These data indicate that, in most cases, the walls of the sinkholes, especially in dolomitised limestones, are usually sub-vertical immediately below the rims (Fig. 8). For example, Yorke (1961) reported sheer sinkhole walls for the pits at Blakemoor, Harborough Works, Heathcote and Newhaven, which suggested to him that the sediments in Bees Nest Pit were deposited in a steep-sided canyon. The sub-vertical form of the sinkholes may also be a consequence of bedrock joint or fault guidance. Banks et al. (2009) found that there is stratigraphical guidance of the hydrogeology of the Peak Limestone Group whereby the more massive bedding of the Bee Low Limestone Formation is more susceptible to pervasive jointing than the more bedding-dominated Monsal Dale Limestone and Woo Dale Limestone formations. This has implications for rock mass behaviour, with the Monsal Dale Limestone and Woo Dale Limestone formations being more resistant to upward stoping. Borehole-derived cross-sections (Fig. 9) indicate that the sinkholes are normally flat-floored at depths of several decametres, although only four boreholes reached bedrock and we consider it likely that sinkhole floors are much more uneven than indicated in Figures 9 and 10.
A view of Kenslow Top Pit, Friden, taken at the SE corner of the pit looking towards the NE. The intermittent grey outcrops on the grassy slopes above the prominent outcrop of the Brassington Formation are the steep-sided dolomite wall rock (unit 1). This unit at Kenslow Top Pit comprises highly dolomitised Monsal Dale Limestone Formation. Below the prominent small scarp are well-exposed faces of the Kirkham Member (unit 4). The chert-clay solution residue (unit 2) is also present towards the base of the succession here, but is largely grassed over. The trees give an indication of scale. Photograph taken by PFJ in June 2015.
Five borehole-derived cross-sections through the Bees Nest subsidence complex. The lengths and locations of the sections are indicated in Figures 10 and 13. The vertical scale for all sections is in metres above OD (Newlyn). These sections were based on the shallow borehole survey undertaken by the Steetley Refractory Firebrick Company in 1963 (see text); all the detailed logs of the boreholes have been lost. It should be noted that only four boreholes penetrated the host rock, which is why the nature of the base of the Brassington Formation was largely depicted as being uncertain (i.e. as a broken line). Cross-sections (A–B) and (C–D) are oriented approximately north–south; cross-sections (E–F) and (G–H) are approximately west–east; and cross-section (J–K) is SSW–NNE. Cross-section (A–B) extends through the Green Clay Pit, which today is virtually entirely covered by vegetation and cross-sections (C–D) and (J–K) transect Bees Nest Pit. Cross-section (E–F) transects the Green Clay Pit from west to east, whilst section (G–H) passes between Green Clay and Bees Nest pits; both cross-sections reveal ‘hidden’ components of subsidence loci that appear never to have been fully exploited. In cross-section (A–B), the intersections with west–east cross-sections (E–F) and (G–H) and borehole Cu4, are illustrated. In cross-section (C–D), the intersections with cross-sections (E–F), (G–H) and the oblique (SSW–NNE) (J–K) and boreholes F39 and F40 are depicted. In cross-sections (E–F) and (G–H), the intersections with north–south cross-sections (A–B) and (C–D) are illustrated; borehole 6 is shown in (E–F). In cross-section (J and K), the intersection with cross-section (C and D) and borehole Cu1 is shown. Four of the five boreholes illustrated penetrated Peak Limestone Group bedrock (the Bee Low Limestone Formation); the several other boreholes which terminated within the Kirkham Member are not shown to maximize clarity. In cross-section (A–B), note the peak/ridge of Bee Low Limestone Formation based on borehole F16; the peak/ridge is also indicated in Figure 13. By contrast, the base of Green Clay Pit to the north of the peak/ridge is interpreted as being relatively flat. The prominent pebble bed close to the top of the Kirkham Member is present immediately north of cross-section (E–F). The north side of the Green Clay Pit pocket is interpreted as having a relatively steep side immediately south of point B. In cross-section (C–D), note the prominent sag syncline close to the southern margin and the relatively gentle pocket wall on the northern side south of point D. Cross-section (E–F) illustrates five sag synclines which are intercalated with four peaks/ridges of the Bee Low Limestone Formation. The latter are invoked on the basis of surface exposures and borehole F48 and are indicated in Figure 13. Note the pebble bed in the uppermost Kirkham Member and the Bees Nest and Kenslow members in the western area. Cross-sections (G–H) and (J–K) illustrate a relatively similar pocket morphology to each other. The prominent peak/ridge of Bee Low Limestone Formation in the central areas was established by boreholes H6, H7, H16 and H23 and the thick Kirkham Member succession in the east side of the outlier. The peak/ridge is also indicated in Figure 13.
A map of the historical expansion of silica sand extraction at Bees Nest Pit. This illustrates the alignment of the five cross-sections shown in Figure 9, selected boreholes and the remnants of dolomite pinnacles. This map was digitised from BGS field slips and OS maps, dates as shown.
Burial history, erosion surfaces and the Miocene sub-surface
The Mid Carboniferous carbonate platform was drowned at the end of the Viséan and then blanketed with prograding deltaic deposits in rift basins during the Late Carboniferous. The basins that surrounded the platform had a significant relief (e.g. the Widmerpool Gulf) and were rapidly infilled with sequences of turbidite sediments. The top of the Derbyshire Platform was buried to a depth of 2.5 km within 8.5 myr (Frazer et al. 2014). Basin inversion associated with the collision of Gondwana and Laurasia occurred at the end of the Carboniferous (Fraser & Gawthorpe 2003). From the Cretaceous onwards, Britain lay on the edge of the downwarping North Sea Basin, the evolution of which was influenced by Arctic to North Atlantic rifting; the latter was probably associated with underplating and periodic uplift. Westaway (2009, 2017) argued that the Quaternary uplift in northern England was coupled to subsidence of the southern North Sea. Superimposed on this is the uplift associated with glacio-isostasy. Rates of Quaternary uplift have been constrained by analyses of cave levels and river terraces (e.g. Westaway 2009). The river terraces of the White Peak have contributed to this analysis (Waters & Johnson 1958; Johnson 1969; Walsh et al. 1972; Westaway 2009). Furthermore, uplift would have been a likely trigger for the onset of the suffosion processes associated with the preservation of the Brassington Formation.
To determine the amount of subsidence of the Brassington Formation and the associated volume of the hypogene karst suffosion, it is necessary to evaluate the relative pre-subsidence elevation of the sub-Miocene surface. Whilst this remains unknown, there are four candidate surfaces described below and in Table 4. This analysis assumes that the sub-Miocene surface was a planar landscape feature and that one or other of the modern planar Pleistocene landscape elements (Fearnsides 1932) could be an exhumed sub-Brassington Formation surface:
a 320–325 m surface, which incorporates the present level of the majority of the rims of the pockets at 250–320 m OD. It includes most of the 11 known Brassington Formation outliers in the southern cluster (i.e. Bees Nest Pit area). Occupying a 400 m wide bench, the trapezoidal area is c. 475 ha. It lies within an extensive planar landscape that was traditionally termed the ‘1000 foot surface’ (Fearnsides 1932). The pockets generally have no surface topographical expression, in part due to a thin veneer of glacigenic sediments and/or head capping some of them. Marginal blocks of the Bowland Shale Formation in various pockets suggest that this is unlikely to represent the pre-subsidence base of the Brassington Formation. Furthermore, the sub-Bowland Shale Formation succession is unlikely to have been lower than the summit of the adjacent Harborough Rocks escarpment (379 m OD) to the north of Bees Nest Pit in Brassington Formation times (Fig. 11).
a supra-Harborough Rocks surface at 379 m OD. It is possible that the summit of Harborough Rocks at 379 m OD is a residual of an extensive planar surface, also of presumed Pleistocene age, which cut across the southern Pennines at this level. This surface incorporates Aleck Low (398 m), Blakelow Hill (367 m), Minninglow (372 m) and Slipper Low (368 m) (Fig. 2). Largely due to the evidence that militates against the 320–325 m surface, we do not consider the 379 m surface to be a likely analogue for the pre-subsidence base of the Brassington Formation.
a surface based on a reconstruction above the margins of the Brassington Formation pockets (Walsh et al. 1972, fig. 12). This assumes that the Bee Low Limestone Formation was conformably overlain by a normal thickness (60 m) of the Monsal Dale Limestone Formation and that there was an extensive cover (c. 20 m) of the Bowland Shale Formation. In this scenario, the thickness of the ‘missing’ succession in the pre-subsidence structure is c. 170 m (Fig. 12). By implication, the pre-subsidence structure at the Bees Nest Pit was at 450 m OD, i.e. 70 m above the present summit of Harborough Rocks. This elevation has an approximate correspondence to Bradwell Moor (471 m OD) and Eldon Hill (470 m OD), 25 km further north.
a higher elevation surface, based on a theory that the lowermost Triassic once capped the crest of the present summit heights (Waltham et al. 1997; Ford & Jones 2007). Support for this lies in the spacing of the structure contours on the base of the Triassic (Frost & Smart 1979). If a ‘missing’ Triassic component was once present over the Bees Nest complex, the elevation of the sub-Brassington Formation surface would reflect the remnant thickness prior to Brassington Formation deposition. Certainly, a Triassic component is present in some of the outliers in the Weaver Hills where remnants of both the Upper Triassic Denstone and Hollington formations have been identified (Chisholm et al. 1988). However, none of the outliers in the northern and southern clusters are known to have a foundered Triassic subsidence component and it is clear that, at Bees Nest at least, the sub-Kirkham Member surface was cut directly across the Bowland Shale Formation.
Volumetric analysis, including an assessment of the Pocket Deposits in the Bees Nest subsidence complex
The Late Neogene karst suffosion subsidence in the White Peak is clearly volumetrically significant. A conservative estimate is that the volume of the combined Bees Nest and Green Clay sinkholes is between 3.7 × 106 and 5.0 × 106 m3. However, compared with dissolutional void formation on a global scale, it is far from exceptional. Substantially larger caverns, dissolutional features, dolines and sinkholes occur on most continents (Table 5). The volume of dolomite/limestone removed to accommodate the Pocket Deposits has been quantified using historic borehole data in conjunction with the methodology outlined in Table 4.
A view of Bees Nest Pit, NE of Brassington village, taken from the west end of the quarry looking towards the ENE. The majority of the exposure is of the Kirkham Member (unit 4) which was being worked for brickmaking. In the background, the undulating area (part of Carsington Pasture) is part of the 320–325 m surface, beneath which lies a variable thickness of overburden (generally <2 m) comprising Quaternary deposits. In the left-hand view, a prominent west–east-trending sag syncline in the central northern side of the quarry is clearly visible in slightly oblique view. Note the prominent steeply-dipping (to the north), dark, variegated Bees Nest Member (unit 5), which forms part of the southern limb of the structure. The lighter, grey, plant-bearing clay of the Kenslow Member (unit 6) overlies the Bees Nest Member adjacent to the tripod shown. The Kenslow Member outcrop is marked with the lignite symbol in Boulter et al. (1971, fig. 1). The person standing on the quarry road for scale (c. 2 m) is standing on the outcrop of the pebble beds in the uppermost Kirkham Member (unit 4). Photograph taken by PTW in 1970 when the quarry was working.
A diagrammatic north–south transect from Harborough Rocks, north of Harborough Works, to immediately south of Bees Nest Pit. For the geographical context, see Figures 10 and 13. The sinkhole in the Bee Low Limestone Formation of the Peak Limestone Group (unit 1) with steep sides and an undulating base, which is filled by the Brassington Formation, is clearly illustrated. The chert-clay solution residue and the blocks of Bowland Shale Formation (units 2 and 3, respectively) are illustrated on the southern wall of Bees Nest Pit. The ‘scree’ bodies of Yorke (1961) in the central part of the base of Bees Nest Pit are now interpreted as chert-clay solution residue (unit 2). The six units extrapolated to the north and south of Bees Nest Pit are the interpreted pre-erosion/uplift Neogene geology of the area and the boundaries in Bees Nest Pit represent the observed geology. The sub-Brassington Formation, 381 and 305 m surfaces are illustrated at Bees Nest Pit. The Monsal Dale Limestone Formation of Brigantian age overlies the Bee Low Limestone Formation (Asbian), which is the host rock of the Brassington Formation at Bees Nest Pit (Table 2).
Methodology and calculated volumes of the Brassington Formation
Examples of karst volumes at 16 locations worldwide for comparison with the volume of the Brassington Formation outliers
Quarrying of high-purity silica sand ceased during the late 1970s, thereby ending around 100 years of exploitation (Yorke 1954, 1961). In 1963, the then owners of the Bees Nest Pit complex (the Steetley Refractory Firebrick Company) drilled over 40 shallow boreholes to assess the remaining reserves of sand. Four of these penetrated the Bee Low Limestone Formation floor of the sinkholes (Fig. 9).
The five cross-sections in Figure 9 were constructed from images of a ground model produced by the Steetley Refractory Firebrick Company. Unfortunately, the original sections cannot be found. In the absence of detailed reports, the sections in Figure 9 provide the best approximation of the ground conditions that were proved and have allowed retrospective calculation of the pre-quarrying volume of the Brassington Formation in this complex. As the majority of the boreholes terminated in the Kirkham Member, there is considerable uncertainty associated with these calculations. The figures have been modified to account for the presence of the Kirkham Member underlying the various sag synclines in Bees Nest Pit (Walsh et al. 1972, fig. 2B). The borehole data indicate that unexploited Brassington Formation is present in significant volumes to the NE and east of Bees Nest Pit. This is consistent with large fragments of fossil wood, possibly derived from the Kenslow Member, at SK 241 548 (PTW pers. obs., 1997) and historical records of sandpits on the western side of Carsington Pasture.
The area of the outliers over the entire Brassington Formation Basin is at least 220 km2 (Walsh et al. 1980). Although they are contiguous, Bees Nest and Green Clay pits were regarded as separate subsidence outliers by Yorke (1961). As c. 60 such pockets are known, these two accumulations represent 3.3% of the preserved Late Neogene of the Peak District. By extension, the total volume of all of the White Peak pockets is c. 0.66 km3. This assumes that c. 21.8 × 106 m3 represents two of the larger pockets (i.e. Bees Nest and Green Clay pits; Figs 10 and 13; Table 4). The overall total of c. 0.66 km3 is considered a reasonable approximation because, although the Bees Nest and Green Clay pockets are among the larger forms, there are probably many undiscovered pockets in the White Peak. The uncertainty in these values is reflected in the relative elevation of the ground surface at the time of initiation, as described above and shown in Figure 12. There is also uncertainty in the assumptions made regarding the shape and distribution of the pockets (Table 4; Fig. 13). Whilst sub-vertical wall rock has been noted immediately below the rims it is likely that, above this, lateral spreading of hypogene fluid capped by the overlying strata would have resulted in an inverted cone form to the affected area. This is represented in the calculation of vertical and inclined (70°) margins (Table 4). The physical laboratory experiments of Walsh et al. (1972) optimized their representation of field conditions when a 45° margin was adopted, again suggesting a conservative approach to the calculations.
A structure contour map illustrating the base of the subsidence landforms (pockets) in the Bees Nest subsidence complex, in the area of the Bees Nest and Green Clay pits, NE of Brassington village. The 305 m contour has been omitted for clarity. For the geographical context, see Figures 1, 2 and 10. The map is based on the shallow borehole survey undertaken by the Steetley Refractory Firebrick Company in 1963 (see text). The five cross-sections of Fig. 9 are indicated by thick black lines. The positions of the boreholes drilled along the five cross-sections are not illustrated in the interests of clarity; these largely only proved the Kirkham Member. The three areas depicted by dark shading are physical highs (‘dolomite pinnacles’) on the floor of the complex. These are surrounded by basins where the Brassington Formation (largely the Kirkham Member) is thickest. The two areas in light grey shading are permanent lakes/ponds. The contours of the base of the pockets at the NNW corner are somewhat discontinuous. This is due to a relative lack of information at the site of the ‘Old Pit’, one of the oldest Brassington Formation quarries. Harborough Works, north of Manystones Lane is built on the site of the old brickworks which produced bricks made from the Kirkham Member sand quarried from local pits. The cul-de-sac road Wirksworth Dale was closed to through traffic in the 1980s and was formerly known informally as Brassington Cart Lane.
Whereas these estimates are approximate, they clearly indicate that the intensity of the Pliocene karst suffosion processes leading to the preservation of the Brassington Formation in sinkholes was comparable with the interstratal karst dissolution associated with the collapse dolines in the Carboniferous Limestone of the north crop of the South Wales Coalfield (Thomas 1974). At least 90% of the accommodation volume became occupied by the Brassington Formation sediments. The c. 220 km2 depositional area of the Brassington Formation Basin represents around 3 × 106 m3 of suffosion per 1 km2. This is disregarding any Late Neogene cavities that were not filled with the Brassington Formation. Furthermore, if the volume of the original (pre-subsidence) Brassington Formation was c. 10 km3 (Walsh et al. 1980), then at least 7% of this began to subside later than the Mid Miocene (Serravallian) and the Late Miocene (Tortonian) in the southern and northern clusters, respectively. If the karst suffosion of the former high-level Brassington Formation prism was regional, it seems probable that no part of the subsurface of this depocentre was unaffected by subsidence. The Late Carboniferous palaeokarst associated with the MVT mineralization must have provided the space for the suffosion to begin, presumably in the large phreatic vein cavities known in parts of the Derbyshire karst (Ford 1989; Ford & Worley 2016). The Brassington Formation outliers in the White Peak therefore represent the most voluminous carbonate-hosted suffosion palaeokarst in the British Isles.
Genesis of the pockets: a new conceptual model
The karst associated with the pockets is of a different scale to that of the classic karst of the non-mineralized areas of the White Peak. For example, the dimensions of the pockets exceed that of known dolines. Furthermore, the pockets appear to be hydrologically isolated rather than integrated with the regional hydrogeology. This suggests that the karst processes are different from the dissolution driven by rainwater and carbon dioxide that characterizes most of the known karst of the Peak District. Ford & Gunn (2008) suggested that the Brassington area is primarily drained via the mineral vein fissures that are under-drained by Meerbrook Sough (Shepley 2007). This indicates that the Miocene drainage was also strongly guided by mineral veins. To date, however, a satisfactory explanation of how the Late Neogene karst suffosion subsidence mechanisms operated has been elusive. Key observations that have influenced our conceptual model for the Late Neogene that supposedly hosts hypogene karst suffosion Pocket Deposits include: (1) the nature of the hypogene palaeokarst associated with the Late Carboniferous mineralization of the Peak District platform edge; (2) physical (i.e. gravitational) collapse or suffosion of the hypogene karst forms; (3) stratigraphical influences on the hydrogeological and mechanical properties of the host rocks; (4) the association of epigene and hypogene karst forms on common structural pathways; (5) the presence of residual dolomite sand, a product of leaching and decalcification of dolomite rock, preserved remnants of which were encountered in the footings of the Carsington Pasture wind turbines; and (6) the observation that most of the marginal blocks of Bowland Shale Formation are largely unweathered, which suggests that the residues were produced by subrosion and are not palaeosaprolites. Both before and during suffosion subsidence, the Bowland Shale Formation is likely to have been an aquitard to vertical fluid flow, whether epigene through the overlying Brassington Formation or hypogene fluids rising via joint systems and porous carbonates in the Peak Limestone Group below. Additionally, the occlusion of downward flow paths is evidenced by the sporadic cementation of the lowermost sands of the Kirkham Member by iron compounds, for example at Green Clay Pit (Yorke 1961). However, at Bees Nest Pit and elsewhere, much of the lowermost Kirkham Member is friable soft sand, although some of the beds in the upper part of this unit are partially cemented. This suggests that lithification may be patchy and merely reflecting local environmental conditions. However, this also indicates the potential for locally acidic water to have contributed to dissolution.
The host rock strength and the absence of sediments onlapping the pocket margins, as well as the volume of the pockets, compared with the size of other dolines in the White Peak, indicate that these sinkholes are unlikely to be the products of catastrophic cave roof collapse (Walsh et al. 1972, 1980). Consequently, the evidence points to subrosional conditioning processes operating in a covered karst context with subsequent epigene development. Based on the results of physical modelling, Ijtaba (1973) and Walsh et al. (1972, 1980) suggested that the subsidence took place as block-by-block or grain-by-grain removal of the host rock to accommodate the Brassington Formation and any underlying Bowland Shale Formation (Fig. 7). The axes of maximum subsidence generally appear to coincide with the troughs of the sag synclines within the pockets (Fig. 11). As the Bowland Shale Formation foundered during early subsidence, the consequential dislocations in the mudstone then provided routes for descending meteoric water. This may have been triggered by the Late Neogene Pennine uplift and associated focusing of recharge along newly-incised drainage lines.
The pockets largely lie within an area of dolomitised limestone of suspected hypogenic origin that has further been subject to MVT mineralization (Aitkenhead et al. 1985; Schofield & Adams 1986; Frazer et al. 2014; Ford & Worley 2016). The dolomite is considered to be replacive and a product of diagenesis attributed to basinal (hydrothermal) dolomitisation and/or seawater dolomitisation (Parsons 1922; Ford & King 1969; Warren 2000; Frazer et al. 2014). The tectonic setting, associated with the platform edge and adjacent to the subsiding Widmerpool Gulf, is typical for this model (Banks et al. 2015). Subsequent phases of MVT fluid flow were commonly preceded by a phase of leaching associated with hypogene speleogenesis (Worley & Ford 1977; Worley 1978; Ford 1989; Quirk 1993; Ford & Worley 2016). Evidence from the distribution of the dolomitisation indicates that these early fluids were driven into this part of the platform at considerable depth and in a NE direction. They appear to have risen sub-vertically along faults and master joints, while spreading laterally along more permeable beds as in Golconda Mine (Ford & King 1969).
Karst systems function by recharge, storage, through-flow and discharge (Hobbs & Gunn 1998) and the morphology of karst networks in the Peak District was strongly influenced by the stratigraphical distribution of discontinuities, such as bedding or jointing in the host limestone (Banks et al. 2009). The classification of Palmer (2000) for cave planforms based on recharge mechanisms indicates that bedding-dominated formations, such as the Monsal Dale Limestone Formation, would normally exhibit branchwork cave systems in response to epigene processes. By contrast, formations dominated by massive bedding with a propensity for more pervasive jointing, such as the Bee Low Limestone Formation, are associated with permeability developed by dissolutional enlargement of sub-vertical faults and joints. These flow paths are also evident in the previously described pattern of dolomitisation and its subsequent leaching during early mineralization. For example the complexity and pervasiveness of joint control on the distribution of dolomite sand at Carsington Pasture appears to reflect structural guidance of fluid flow in the Bee Low Limestone Formation (Fig. 14; Banks et al. 2015). This contrasts with the bedding guidance and bridging properties of the Monsal Dale Limestone Formation and the fault and stylolite guidance in the Woo Dale Limestone Formation described by Banks et al. (2009). It is therefore plausible that, where present, the Monsal Dale Limestone Formation contributed to the confinement of the hypogene fluids and focused leaching on joints in the Bee Low Limestone Formation.
View (looking north) of an enlarged fissure in the highly dolomitised Bee Low Limestone Formation at Harborough Rocks, NE of Brassington village (Fig. 10). On the floor of the fissure and on the ground in front of the rucksack (which was included for scale), are large amounts of loose, buff-coloured dolomite sand which was produced by the decalcification and leaching of dolomite rock. Photograph taken by VJB in March 2016.
While the sinkholes appear to have been initiated by hypogene karst conditioning, their size and form reflect a combination of subsequent bedrock suffosion and epigene processes (Fig. 15), as evidenced at Carsington Pasture (Jones & Banks 2014). The Pocket Deposits in the Bee Low Limestone Formation are usually exposed at high levels in this unit and generally not far below the boundary with the overlying Monsal Dale Limestone Formation. This observation supports the hypothesis that the subsequent epigene karst is likely to reflect recharge flow paths focused via preferential fractures (faults or joints). The evidence presented supports a model of coalescing hypogene conditioning, which is particularly well developed in the Bee Low Limestone Formation. This is connected with subsequent epigene bedrock suffosion processes, possibly focused on local precursor voids in the Monsal Dale Limestone Formation where present. The connectivity between the two systems is represented by the network of dominant fractures that have facilitated both processes, thereby enabling slow suffosion of mechanically weak (‘rotten’) dolomitised bedrock (Fig. 7). Subsequent collapse facilitated the gravitational lowering of the Brassington Formation sediments into these leached dolomitised zones. This model suggests that the depth of the sinkholes was primarily controlled by the hypogene dolomitising fluid flow paths, whereas the diameters of the pockets were guided by the spacing of the dominant fractures, which focused meteoric water into the hypogene karst suffosion dolostones.
Conceptual model illustrating the four stages of the formation of the Pocket Deposits: (1) hypogene dolomitisation and hydrofracturing condition the bedrock for subsequent suffosion; (2) uplift and incipient breaching of the cap rocks focusing localized dissolution and initiating suffosion; (3) suffosion increases as weathering and erosion of the Bowland Shale Formation occurs; and (4) deposition of the Brassington Formation, foundering of the cap rocks and ongoing suffosion accommodating Brassington Formation sediments. The acronyms used are: BF, Brassington Formation; BSF, Bowland Shale Formation; MDLF, Monsal Dale Limestone Formation; BLLF, Bee Low Limestone Formation; Hp, Hopton Wood Limestone; Mix, Mixon Limestone; WdF, Widmerpool Formation; and WDLF, Woo Dale Limestone Formation.
Further suffosion is likely to have occurred as a response to glacial meltwater inputs during the Pleistocene and subsequent Holocene (and current) climatic conditions, albeit at a lower rate of subsidence. Epigenetic carbon dioxide-driven decalcification processes are likely to be ongoing. Additionally, there are a number of sources of iron (Kirkham and Bees Nest members) and pyrite (Bowland Shale Formation, clay wayboards in the Peak Limestone Group, Kenslow Member and mineral veins) that may be associated with ongoing sulphuric acid-driven decalcification.
Discussion
Recent palynological research has provided further insights regarding the depositional environment of the Brassington Formation. These data indicate that the sedimentation of the various clusters took place over a much longer period than was previously supposed (c. 5 myr). This interval is now established as being from earlier than Late Serravallian to post-Late Tortonian. Pound & Riding (2016) postulated diachroneity in the post-Bees Nest Member clays between the northern and southern clusters. Similar analysis has yet to be undertaken for the western cluster. The detailed lithostratigraphical resolution is limited by the current extent of the exposures. If this heterochroneity is real, it is likely to have been a consequence of the complexity of the suffosion process, with more complete preservation in the northern cluster. An alternative hypothesis is that, for unknown reasons, there was a northwards shift of the main depocentre during the Tortonian (Pound & Riding 2016), reflecting a migration of the paralic facies of the Kenslow Member and associated sediments. This requires interpretation in the context of the entire genetic sequence of the Brassington Formation. Walsh et al. (1980) demonstrated that palaeocurrent indicators in the southern cluster showed a marked and consistent flow into the basin from the south. However, evidence of flow directions in the northern cluster was unsatisfactory and no particular trend could be discerned. The hypothesis presented by Pound & Riding (2016) requires a hiatus between the Bees Nest and Kenslow members, for which there is no evidence in the southern cluster outliers.
Prior to Pound et al. (2012a) and Pound & Riding (2016), the Brassington Formation was regarded as a largely contiguous and widespread sheet of braided river plain sediments, deposited near sea-level and up to 75 m thick. Overconsolidation ratios determined for the Kenslow Member indicate that the uppermost Brassington Formation (estimated to be 25–35% of its total thickness) was totally removed by post-Tortonian erosion and is not preserved (Ijtaba 1973). The presence of a glacial raft of uppermost Kenslow Member, preserved in Kenslow Top Pit is evidence of this erosion (Jones et al. 2016). Given the probability of thickness reduction across the limbs of the sag synclines during subsidence and the non-preservation of the highest levels of the Kenslow Formation, it is now postulated that the pre-subsidence thickness probably exceeded 100 m in places.
A key question emanating from the revised stratigraphy of Pound & Riding (2016) is whether the Pocket Deposit clusters represent different facies. Hughes (1952) postulated that some of the Kirkham Member sediments around Brassington were derived from the east and NE, though he presented no evidence. Theoretically, it is possible that at least some of the Kenslow Member may have been derived from the Cheshire Basin to the west (Jones & Drew 2015). The Bees Nest Member at the type section is of an appreciably different facies and thickness to the supposed equivalents at Kenslow Top Pit (the sub-KM2 sediments). This unit is 7 m thick at Bees Nest Pit and 21 m at Kenslow Top Pit. In the southern cluster, the Kirkham Member is bleached white; by contrast, it is reddish brown in the NW part of Kenslow Top Pit. Whether this is because the latter was never subjected to the diagenetic bleaching process that has affected the Kirkham Member at most localities, or they are stratigraphically younger, is not understood. Furthermore, the grain size of the Kirkham Member sand is appreciably finer in the northern cluster than in the southern cluster, hence either the former is more distal in the Brassington prism drainage or it represents a change of sediment source.
The revised stratigraphy has also given rise to further consideration of the rate of subsidence. It is important to establish a chronological framework for the subsidence of the Brassington Formation. If the Bees Nest Pit succession is typical of the southern cluster of outliers and there were no hiatuses between the three members of the Brassington Formation, it follows that no subsidence occurred prior to the deposition of the Kenslow Member at this locality (Walsh et al. 1972). This suggests that, while the subsidence process began in the latest Miocene and continued into the Pleistocene, it was almost exclusively of Pliocene age. Drainage systems delivering sediment into the northern cluster, as represented by Kenslow Top Pit, were still operating until the Tortonian. This may have coincided with a period of basin inversion, erosion and Pennine uplift and the initiation of subsidence of a significant part of the Brassington Formation sedimentary prism. The latter was largely complete by the time of the placement of the glacial deposits in Kenslow Top Pit (Jones et al. 2016). A post-Mid-Miocene (Late Serravallian) to Quaternary (pre-Devensian) time constraint for regional subsidence is therefore indicated. We contend that the subsidence of the Brassington Formation is ongoing, albeit at a reduced rate, as evidenced by the preservation and down-warping of the glacial raft at Kenslow Top Pit (Jones et al. 2016). If subsidence began during the Late Miocene (latest Tortonian) and has continued until the present day, all known subsidence took place in c. 7.25 myr. Hence sinkhole development in the Peak Limestone Group in the Brassington Formation Basin occurred at a rate of c. 91 m3 a-1. If, however, subsidence took place only during the Pliocene (5.3–2.6 Ma), the corresponding figure would be 244 m3 a-1, or 0.006–0.017 mm a-1. This value is lower than current epigene dissolution rates of surface lowering that have been calculated for the White Peak, e.g. 0.083 mm a-1 by Pitty (1966) and 0.031 mm a-1 by Banks & Jones (2014). As suggested by Westaway (2017), it is also lower than the rate of post-Miocene uplift (0.03–0.06 mm a-1) calculated by Pound et al. (2012a).
The hypothesized hypogene MVT-related karst processes involve fluids that are suspected to have reached temperatures of between 120 and 200°C in the palaeocavities (Hollis & Walkden 2002). Therefore it is possible that these fluids have contributed to hydrothermal alteration of some of the clays that have been preserved in the pockets. For example, halloysite was documented by Ford (1963) and Ijtaba (1973) recorded allogenic gibbsite in the Kenslow Member at Bees Nest Pit. Epigene fluids appear to have leached iron compounds from the coarser sediments and concentrated them in palaeocavities present along some of the mineral veins in the White Peak orefield. This is indicated by the reddish-brown staining of the illuviated Kirkham Member sediments preserved in the mine galleries (Ford & Worley 2016).
Conclusions
Geological investigation of the Brassington Formation is increasingly difficult as many of the pockets have been quarried away and infilled, whilst remnant sand excavations are largely overgrown (Fig. 8). The remaining exposures are rare and it is extremely unlikely that any of the pockets will be re-excavated commercially in the future. Utilization of the 1963 Steetley Refractory Firebrick Company borehole data has enabled us to make a reasonable estimate of the size of the Bees Nest and Green Clay pockets. Extrapolating from this information, further crude estimates have been made of the total volume of carbonate host rocks removed to accommodate the subsiding Brassington Formation sediments across the entire depocentre (about 0.66 km3) and also the rate of removal of the foundation (91 × 106 m3 Ma-1).
We propose a new model for the mechanisms of pocket development. This involves both hypogenic and epigenic processes providing conditioning for suffosion, with the former dominant in the early stages of sinkhole development and associated with MVT mineralization fluid pathways. Suffosion of the bedrock, as suggested here, requires epigene flow and an existing void to accommodate the mobilized sediment, which implies an intimate relationship with cavernization associated with the mineralization. Isolated patches of sand indicative of the suffosion process have been recorded, for example in Breck Hollow and Golconda Mines (Ford & Jones 2007). It is likely that the pre-Pliocene suffosion processes were irregular in their distribution, reflecting the convergence of meteoric recharge flow paths with former zones of focused hypogene flow. The latter is associated with the dolomitisation that is also apparently related to the platform edge setting, thereby preserving outliers of the Kenslow Member in different places. This was not necessarily specific to the larger subsidence complexes in the northern and southern clusters, albeit no localized pockets of exclusively Kenslow Member clay have been identified outside the main subsidence complexes, with the possible exception of Hindlow Quarry. It is possible that pockets have been eroded from other locations by post-Tortonian erosion.
Given that the Miocene was a significantly warmer and wetter interval than the Pliocene (Pound et al. 2011, 2012b; Salzmann et al. 2013) and that our conceptual model is based on suffosion, it seems most likely that uplift commenced during the Miocene and triggered the suffosion process. This has implications for the rate of uplift of the Pennines, given that palynology indicates that the Kenslow Member was deposited close to sea-level (Boulter 1971a,b; Pound et al. 2012a; Pound & Riding 2016).
Earlier conclusions concerning the current elevation of the sub-Brassington Formation surface over the Bees Nest solution subsidence complex (around 450 m OD) are supported, although this surface is postulated to be considerably higher in the northern cluster. It is clear that some of the generally accepted tenets of the 1960s and 1970s research on the Brassington Formation remain valid, e.g. the lithostratigraphy and sedimentary architecture of the pockets and the gentle nature of the subsidence. Although new data are presented here, some issues still need to be reassessed: (i) the precise depth of the sinkholes remains elusive, albeit there is potential for future investigation using seismic techniques, e.g. Raines et al. (2015); (ii) the detailed nature of the dolomitisation and its role in conditioning the bedrock for suffosion processes; (iii) the relationship of the karst suffosion with the MVT mineralisation; (iv) the reconstruction of previous geographies and the form of the Brassington Basin(s) high above the present landscape, now forever removed; and (v) the exact nature of the contact between the ‘KM3 beds’ of Late Tortonian age and the supposed bleached sands in the eastern walls of the Kenslow Top Pit is critical to an understanding of the nature of the subsidence which has involved the Brassington Formation. Because it is so difficult to obtain new data concerning the Brassington Formation and the sinkholes it resides within, the answers to these questions in the near future may inevitably remain elusive.
Acknowledgements
The authors thank Stanley Bowyer, formerly of the Steetley Refractory Firebrick Company, for providing the cross-sections based on the boreholes drilled in the Brassington Formation in the Bees Nest Pit complex. Jayne Spencer (Spencer Bros. (1903) Brassington Limited) and Natural England kindly allowed us access to Bees Nest Pit. We are also extremely grateful to Janet Renwick, the owner of Kenslow Top Pit, for generously facilitating our fieldwork. All of us extend our sincere thanks to Andy Farrant, Cathy Hollis, Stephen Lokier and Tony Waltham for insightful, helpful and thorough reviews of early drafts of this manuscript. Our thanks are also extended to Henry Holbrook and Ian Longhurst (British Geological Survey) for drafting Figures 1, 2, 9, 12, 13 and 15. VJB and JBR publish with the approval of the Executive Director, British Geological Survey (NERC). The authors respectfully dedicate this paper to the memory of Trevor D. Ford (1925–2017), with whom we consulted on this topic and on many others relating to the geology of the Peak District.
Scientific editing by Stephen Lokier
Footnotes
Correction notice: The copyright line has been updated. The publisher apologises for this error.
- © 2018 NERC. British Geological Survey
References
A reassessment of the Brassington Formation (Miocene) of Derbyshire, UK and a review of related hypogene karst suffosion processes
- Article
- Abstract
- The lithostratigraphy of the Brassington Formation
- The derivation of the chert-clay solution residues (unit 2)
- The derivation of the Bowland Shale Formation blocks (unit 3)
- The derivation of the Kirkham, Bees Nest and Kenslow members (units 4, 5 and 6)
- The palaeontology and stratigraphy of the Brassington Formation and its revisions
- The distribution and morphology of the Brassington Formation pockets
- Burial history, erosion surfaces and the Miocene sub-surface
- Volumetric analysis, including an assessment of the Pocket Deposits in the Bees Nest subsidence complex
- Genesis of the pockets: a new conceptual model
- Discussion
- Conclusions
- Acknowledgements
- Footnotes
- References
- Figures & Data
- Info & Metrics