Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo

Late Wenlock sequence and bentonite stratigraphy in the Malvern,Suckley and Abberley Hills, England

David C. Ray a,b,⁎, Thomas D. Richards c, Carlton E. Brett d, Andrew Morton e,f, Abigail M. Brown g

a School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, UKb Neftex, 97 Jubilee Avenue, Milton Park, Abingdon, Oxfordshire, OX14 4RW, UKc Herefordshire and Worcestershire Earth Heritage Trust, Geological Records Centre, University of Worcester, Henwick Grove, Worcester, WR2 6AJ, UKd Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USAe HM Research Associates, 2 Clive Road, Balsall Common, Coventry, CV7 7DW, UKf CASP, University of Cambridge, 181a Huntingdon Road, Cambridge, CB3 0DH, UKg 17 Shrubbery Street, Kidderminster, Worcestershire, DY10 2QZ, UK

⁎ Corresponding author at: Neftex, 97 Jubilee Avenue,MilOX14 4RW, UK. Tel.: +44 7792 638 177; fax: +44 1235 4

E-mail address: daveray01@yahoo.com (D.C. Ray).

0031-0182/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.palaeo.2013.03.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 December 2012Received in revised form 26 March 2013Accepted 27 March 2013Available online 10 April 2013

Keywords:BentonitesMidland PlatformMuch Wenlock Limestone FormationSequence stratigraphySilurianWenlock Series

The Late Wenlock Series (Homerian Stage) of the central Midland Platform occupies an area stretching fromLedbury to the Malvern, Suckley and Abberley Hills. Based upon the establishment of a sequence stratigraphicframework for the Much Wenlock Limestone Formation, and the immediately under- and over-lyingCoalbrookdale and Lower Elton Formations, comparisons can now be made with key sections across thenorthern Midland Platform and beyond. These correlations have been strengthened by the determinationof apatite rare earth element (REE) geochemical signatures obtained from four volcanic ash layers (benton-ites) at Whitman's Hill Quarry (Herefordshire), which allow for comparisons with published coeval sectionsat Wren's Nest Hill (West Midlands) and Wenlock Edge (Shropshire), as well as with bentonites describedfrom the Island of Gotland (Sweden).Across the study area fifteen parasequences associated with two pronounced regressive episodes, separatedby a marked transgression, can be identified. The lithological responses to these relative sea-level changes arethe same as those reported from the West Midlands, including the threefold division of the Much WenlockLimestone Formation into Lower Quarried Limestone, Nodular Beds and Upper Quarried Limestone Members.Apatite REE geochemical signatures from Whitman's Hill Quarry identify three bentonites which probablyoriginated from a granodiorite magmatic source, while a fourth bentonite has a distinctively mafic composi-tion, more akin to that of a gabbro or syenite. This distinctively mafic bentonite is preserved on a markedflooding surface within the Nodular Beds Member and appears compositionally and stratigraphically equiv-alent to a bentonite at Wren's Nest Hill (West Midlands). Furthermore in both sections this bentonite is ofnotable thickness (120–200 mm) allowing for its identification in other sections across the region. Compar-isons with Gotland identify three closely spaced bentonites of a similar mafic composition to the bentonitedescribed from the Midland Platform. While similarities in stratigraphic position and composition do not,at present, allow for the identification of a single ash fall event covering both the Midland Platform andGotland, they are indicative of a shared source region, which may offer the possibility for future bentonitecorrelation between these regions.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Geological context

Whitman's Hill Quarry, Storridge, Herefordshire contains one of themost extensive Late Wenlock Series (Homerian Stage) successions incentral England. In 1999 the site was designated a Regionally ImportantGeological Site (RIGS) (now known as a Local Geological Site (LGS)) for

ton Park, Abingdon, Oxfordshire,43 629.

rights reserved.

its rock formations (Upper Coalbrookdale and Much Wenlock Lime-stone), abundantWenlock Series faunas and educational value. Further-more the site was the subject of a Geodiversity Discovery Venture,carried out by the Herefordshire and Worcestershire Earth HeritageTrust and funded by English Nature (now Natural England) throughthe Department for Environment, Food and Rural Affairs (Defra) Aggre-gates Levy Sustainability Fund (ALSF) Grant Scheme. As part of thisproject, investigations into nine prominent bentonite horizons (alsotermed K-bentonites or metabentonites by some authors) have beenundertaken in order to establish apatite rare earth element (REE)geochemical signatures. These data aid in the understanding of theirmagmatic origin and provide the potential for correlation with broadly

116 D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

coeval bentonites such as those described from the northern MidlandPlatform (Ray et al., 2011) and Baltica (Batchelor and Jeppsson, 1999;Cramer et al., 2012). In addition, a sequence stratigraphic frameworkfor the uppermost Coalbrookdale and the Much Wenlock LimestoneFormations has been established at Whitman's Hill and nearby UpperVinesend Farm quarries, thereby allowing for a comparison withGurney's Quarry near Ledbury, along the Malvern Hills (Park Woodquarries), the Suckley Hills (Bruff Business Park) and Abberley Hills(PennyHill, Fetterlocks Lane andWallhouse Plantation (north) quarries);a distance of approximately 30 km between the most southerly andnortherly sections. A comparison has also been made with the bentoniteand sequence stratigraphy developed for Wren's Nest Hill, Dudley, WestMidlands (Ray and Thomas, 2007; Ray et al., 2010, 2011); approximately47 km north–northeast of Whitman's Hill Quarry, but developed in asimilar mid-platform setting (Figs. 1, 2, 3). The Wren's Nest Hill sectionpresented herein has been extended from that in Ray et al. (2010) bythe addition of a subterranean section which allowed access to a moreextensive interval of the Lower Elton Formation.

In this paper, we present an integrated sequence stratigraphyand bentonite correlation study of the Homerian strata within amid-platform setting upon the Midland Platform. The purpose ofthis study is to (1) document the sedimentology and sequence stra-tigraphy in an area stretching from Ledbury to the Malvern, Suckleyand Abberley Hills, (2) to determine apatite REE geochemistry ofkey bentonite horizons as a means of establishing their likely mag-matic origin, (3) to integrate sequence stratigraphy and bentonitegeochemistry as a means of placing this classic Homerian successioninto its regional and global contexts.

Fig. 1. Locality map with the Much Wenlock Limestone Formation outcrop highlighted, as wGurney's Quarry (GQ) [SO 7173 3838] near Ledbury; Park Wood (Minor) [SO 7630 4435], PaHill [SO 7490 4830] quarries Malvern Hills; Bruff Business Park [SO 7365 5065] Suckley Hills(north) [SO 7510 6372] quarries Abberley Hills; Wren's Nest Hill (WNH) [SO 937 920] Dud

2. Stratigraphy

2.1. Biostratigraphy and carbon isotope stratigraphy

Within an area stretching from Ledbury to Abberley, age-diagnosticgraptolites have not been collected from the upper part of theCoalbrookdale or the Much Wenlock Limestone Formations. However,the overlying basal Lower Elton Formation of theMalvern Hills containsMonograptus varians indicating a Neodiversograptus nilssoni graptoliteBiozone age and synchronicity of this interval with a similar litho-stratigraphic level along Wenlock Edge (Shropshire) (Bassett, 1976).At Gurney's Quarry, a well preserved microflora assignable to theLeptobrachium longhopense acritarch Biozone has been reported fromthe Lower Elton Formation (Aldridge et al., 2000, p 379) immediatelyabove theMuchWenlock Limestone Formation. Furthermore, decliningcarbon isotope values across the Much Wenlock Limestone–LowerElton Formation boundary are also reported (Corfield et al., 1992) andmay be attributable to the declining carbon isotope values at the endof the Mulde positive Carbon Isotopic Excursion, as observed near theWenlock–Ludlow Series boundary at Ludlow, Wenlock Edge andDudley (Corfield et al., 1992; Cramer et al., 2012; Marshall et al., 2012).

2.2. Sequence stratigraphy and lithostratigraphy

The Much Wenlock Limestone Formation described from the areabetween Ledbury and Abberley was deposited in a mid-platformsetting on the gently subsiding eastern margin of the Welsh Basin,as part of a broad carbonate platform (Bassett, 1974; Bassett et al.,

ell as the likely distribution of the Lower Quarried Limestone Member. Key sections arerk Wood (Main) [SO 7644 4440], Upper Vinesend Farm [SO 7514 4760] and Whitman's; Penny Hill [SO 7517 6132], Fetterlocks Lane [SO 7525 6325] and Wallhouse Plantationley.

117D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

1992; Ratcliffe and Thomas, 1999). Based upon the sequence stratig-raphy developed for the northern Midland Platform at Wenlock Edgeand between Dudley and Walsall in the West Midlands (Ray andThomas, 2007; Ray and Butcher, 2010; Ray et al., 2010), it is clearthat the Late Wenlock Series contains numerous parasequences,along with the bounding surfaces and systems tracts that make up athird-order cycle of relative sea-level (RSL) change. Owing to a simi-lar subsidence history (Woodcock et al., 1996), some expression ofthe RSL changes identified in the northern Midland Platform shouldbe detectable within the study area. In order to detect the lithofaciesresponse to RSL change, attention has been paid to the identificationof parasequences, flooding surfaces, erosive surfaces, condensed sec-tions and indicators of palaeobathymetry such as faunal assemblages,sedimentary structures and lithology (see Brett et al., 1993). Havingestablished the lithological response to RSL change, features such asthe local occurrence of bentonites, distinctive lithologies associatedwith prominent flooding surfaces and the number, thickness andstacking patterns (retrogradational, aggradational or progradational)of parasequences have been used for both local and regional correla-tions. It should be noted that the parasequences described herein andin Ray and Thomas (2007) and Ray et al. (2010) are defined bytheir flooding surfaces, such that each broadly upward shallowingparasequence begins at a basal flooding surface and ends at the over-lying flooding surface. While such an arrangement allows for easyidentification at outcrop and within wire-line logs, the bed or bedsat the top of the parasequence and immediately below the overlyingflooding surface may in many cases reflect transgression rather thanthe maximum point of regression.

Having established correlation on sequence stratigraphic grounds,these correlations have additionally been enhanced by the compari-son of apatite geochemical signatures obtained from bentonitesat Whitman's Hill Quarry, Wren's Nest Hill (West Midlands) andWenlock Edge (Shropshire) (Ray et al., 2011), as well as with benton-ites described from Gotland (Sweden) (Batchelor and Jeppsson, 1999;Cramer et al., 2012).

The Much Wenlock Limestone Formation between Ledbury andAbberley has received little scientific attention since the descriptionsof Phipps and Reeve (1967) (for a review see Bassett, 1974). Thosedescriptions were based principally on outcrops situated in the southof the current study area, near Ledbury, and identified five principallithofacies types: calcareous mudstones, nodular limestones, bioclastic(coquina) limestones, pisolitic (oncolitic) limestones, and bioherms.With the exception of pisolitic limestones, which are most common atthe base of the formation, these lithofacies were reported as occurringin no particular order. This precluded the recognition of the threefolddivision of the Much Wenlock Limestone Formation into Lower andUpper limestones separated by a nodular limestone, as reported acrossthe West Midlands (Butler, 1939; Ray et al., 2010) and from the nearbyMayHill Inlier, Gloucestershire (c. 30 km south–southwest ofWhitman'sHill Quarry; Lawson, 1955). In addition, substantial variations in thick-ness (61–152 m) were also reported for the Much Wenlock LimestoneFormation, but were based on stratigraphically incomplete sections in atectonically complex area (Phipps and Reeve, 1967) meaning that accu-rate measurements of thickness were difficult to obtain.

The sections reported herein indicate that lateral and verticallithofacies variations are much less pronounced than previously en-visaged and that there is much similarity with the West Midlands, in-cluding the threefold division of the Much Wenlock LimestoneFormation into Lower Quarried Limestone, Nodular Beds and UpperQuarried Limestone Members (Dorning, 1983). Furthermore, theMuch Wenlock Limestone Formation is considerably thinner thanpreviously estimated, at approximately 37 m (based on the BruffBusiness Park section), although such a thickness may only be reliablefor the area from Whitman's Hill Quarry northwards owing to a lackof exposure in the southern Malvern Hills and around Ledbury. Final-ly, while lateral and vertical lithofacies do occur in a predictable

manner, reflecting that seen in the West Midlands, there is evidencefor an increase in depositional energy (shallowing of sea floor) to-wards the north of the study area, with the highest-energy andshallowest lithofacies developed in the Abberley Hills.

2.2.1. The Coalbrookdale FormationThe Coalbrookdale Formation is estimated to range in thickness

from 198 m immediately south of Whitman's Hill Quarry (SO 75 47)to 244 m near Suckley (SO 75 53), although a combination of poor ex-posure, faulting and foldingmakes accurate measurements of thicknessdifficult to obtain (Phipps and Reeve, 1967). The most significant expo-sure of the uppermost Coalbrookdale Formation (17.3 m) occurs inWhitman's Hill Quarry (Figs. 1, 2, 3a). At this location the successionconsists of blue-grey weathering silty mudstones, occasional nodularlimestones (carbonate mudstones) and thin (20–50 mm) limestonebeds (fine-grained crinoidal grainstones). In addition, three white toblue-grey, clay-rich bentonites (WH1 to WH3) are present and act asuseful stratigraphic markers. Overall, the succession broadly reflects agradual shallowing, via two parasequences, toward the base of theMuch Wenlock Limestone Formation.

The lower of the parasequences consists of siltymudstones containingcrinoid ossicles, brachiopod and trilobite fragments along with wholebrachiopods and occasional bivalves, gastropods, stromatoporoids, tri-lobites and solitary rugose corals. Fine grained crinoidal grainstonescontaining rolled stromatoporoids and nodular carbonate mudstoneswith localised wackestones and packstones increase in frequencytowards the top of the parasequence, indicating shallowing to with-in the lower limit of the storm wave base. A single thin (30 mm)blue-grey bentonite (WH1) occurs within 1.4 m of the top of thisparasequence.

The upper of the parasequences contains many of the same char-acteristics as described from the underlying parasequence. However,fragmentary and whole fossil material is somewhat rarer, andpyritised burrows occur sporadically throughout suggesting a slightlydeeper water setting. In addition, two prominent bentonites (WH2 andWH3) occur 6.1 m and 9.9 m below the base of the Much WenlockLimestone Formation.

Additional exposures of the uppermost Coalbrookdale Formationoccur at Bruff Business Park (6.14 m) and PennyHill Quarry (1.9 m), lo-cated 2.6 km and 13 km north of Whitman's Hill Quarry, respectively.Both of these sections aremore fossiliferous and limestone-rich sugges-tive of a shallower sea floor setting when compared with Whitman'sHill Quarry. At Bruff Business Park, the formation is highly fossiliferouscontaining brachiopods, corals, stromatoporoids and trilobites withinnodular limestones (mudstones andwackestones) and siltymudstones.In addition, a single prominent bentonite (150 mm thick) occurs 3.7 mbelow the base of the Much Wenlock Limestone Formation. This ben-tonite may correlate with bentonite WH3 at Whitman's Hill Quarryand WNH1 at Wren's Nest Hill. At Penny Hill Quarry the uppermost1.9 m of the formation contain silty mudstones with much nodularlimestone and multiple thin crinoidal grainstone beds (storm beds).

2.2.2. The Much Wenlock Limestone FormationBased upon the comparison herein, it is apparent that the succes-

sion between Ledbury and Abberley shares many similarities with theMuchWenlock Limestone Formation of theWest Midlands. Such sim-ilarities are particularly apparent at Crews Hill (SO 7342 5315) wherethe steep dip of the MuchWenlock Limestone Formation has resultedin two parallel sets of abandoned quarries, separated by a ridge. Here,like the abandoned quarries of the Dudley area, the quarry workingsrepresent the extraction of relatively pure limestones belonging toLower and Upper Quarried Limestone Members, while the ridge con-sists of the unwanted nodular limestones and silty mudstones of theNodular Beds Member. Accordingly the descriptions and interpreta-tions given below reflect the three-fold division of the Much Wenlock

118 D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

Limestone Formation in the West Midlands into Lower QuarriedLimestone, Nodular Beds and Upper Quarried Limestone Members.

2.2.2.1. Lower Quarried Limestone Member. The lower third of theMuchWenlock Limestone Formation, as developed within a mid-platformsetting, comprises shallow-water limestones immediately under andoverlain by deeper-water silty mudstones with nodular limestones.

Fig. 2. Correlation of the Much Wenlock Limestone Formation between Ledbury, Malvern,Ray et al. (2011), the radiometric date given forWNH15 is from Cramer et al. (2012), the grapet al. (1992). TheWren's Nest Hill sequence stratigraphy and sedimentary log are after Ray ebounding surfaces: SB/TS — sequence boundary and combined transgressive surface; ETSTLTST — late transgressive systems tract; MFS — maximum flooding surface; HST — highstanNote the bedding is interpreted as overturned at Penny Hill and Wallhouse Plantation (nor

These shallow-water limestones are referred to as the Lower QuarriedLimestone Member (9.8–16.2 m thick) in the West Midlands and thelower limestone division (c. 18 m thick) in the May Hill Inlier, Glouces-tershire. The sections described herein between Malvern and Abberleyclearly show a similar succession to that reported from the WestMidlands and the May Hill Inlier (Fig. 1). Here, the Lower QuarriedLimestone Member increases in thickness northwards from 4.3 m at

Suckley and Abberley Hills and Dudley. Bentonites WNH1 to WNH16 are described intolite occurrence is reported in Butler (1939) and carbon isotope data are from Corfieldt al., 2010, fig. 2, p. 126. Within the study area are the following systems tracts and their— early transgressive systems tract; SMSS — surface of maximum sediment starvation;d systems tract; SFR — surface of forced regression; FSST — falling stage systems tract.th) quarries (see Phipps and Reeve, 1969) and is shown here the right way up.

Fig. 2 (continued).

119D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

Whitman's Hill to 8.2 m at Bruff Business Park and 14.6 m at PennyHill;though faulting, irregular reef masses and poor exposure at Penny Hillmake accurate determination of thickness difficult. The lithology ofthe succession at Whitman's Hill is the most diverse and allows theclear identification of parasequences; elsewhere, fine crinoidalgrainstones and/or reefal lithofacies are dominant. Of particular noteare the reports of pisolitic limestones (Phipps and Reeve, 1967) andalgae coated shell fragments (Penn, 1971) within the lower part of theMuch Wenlock Limestone Formation at Whitman's Hill Quarry andPark Wood Quarry (Main); pisolitic limestones are also reported fromthe lower limestone at May Hill (Lawson, 1955). Upon inspectionthese “pisolites” are, in fact, oncoids like those reported from theLower Quarried Limestone Member of the West Midlands (Ratcliffe,1988; Ratcliffe and Thomas, 1999).

At Whitman's Hill Quarry, the lower 4.3 m of the Much WenlockLimestone Formation consists of shallowmarine limestones containingfive parasequences (PS1 to PS5) equivalent to the Lower Quarried Lime-stoneMember of theWestMidlands (Ray et al., 2010). The lowest of theparasequences (PS1) reflects a pronounced RSL fall that equates to thebasalMuchWenlock Limestone Formation sequence boundary. PS1 ini-tially consists of carbonate mudstones containing numerous large(260 mm across) tabulate corals (Favosites sp.); a single distinctivelylarge coral also occurs in a similar stratigraphic position at PennyHill Quarry. Above, packstones and increasingly coarse-grained (0.5–1.0 mm) crinoidal grainstones dominate and appear equivalent to finecrinoidal grainstones containing overturned corals observed at BruffBusiness Park. The top of PS1 terminates at a distinctively undulose bed-ding plane, suggestive of localised erosion and reworking. Directly

Fig. 3. Outcrops of the Late Wenlock Series from the Malvern to Abberley Hills. Arrows mark the position of prominent bentonites (a–e). (a) Whitman's Hill Quarry (looking south)exposing the uppermost Coalbrookdale Formation to Middle Much Wenlock Limestone Formation (Lower Quarried Limestone and Nodular Beds Members). Arrows mark the po-sitions of bentonites WH1 (near base of quarry face) to WH9 (black arrow near top of quarry face). Height of main quarry face is approximately 35 m. (b) Bentonite WH9 exposedhigh on the southern face of Whitman's Hill Quarry. The field notebook is 200 mm high and rests directly on top of WH9. (c) Bruff Business Park section looking north; the sectionshows the Lower Much Wenlock Limestone Formation (Lower Quarried Limestone and lower Nodular Beds Members). The black arrow shows the position of a thick bentonite,which is the probable equivalent of WH9. Height of quarry face is approximately 5 m. (d) Penny Hill Quarry (looking north) in 1937 exposing the majority of the Much WenlockLimestone Formation, along with two conspicuous notches corresponding to prominent bentonite horizons. The black arrow shows the position of a thick bentonite, which is theprobable equivalent of WH9. Note the bedding is overturned and is younging to the left (west). Height of main quarry face is approximately 23 m (image P207157, reproducedwith the permission of the British Geological Survey ©NERC. All rights reserved). (e) Penny Hill Quarry (looking north) showing the Lower Much Wenlock Limestone Formation(Lower Quarried Limestone and lowest Nodular Beds Members). The younger of the bentonites (left) corresponds to the older of the conspicuous notches exposed in 1937 (d).Note the bedding is overturned and is younging to the left (west). Height of quarry face is approximately 5 m.

120 D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

above this bedding plane, and marking the flooding surface of PS2, is alocally discontinuous rusty orange bentonite (WH4). A bentonite occursat a similar stratigraphic position at both Bruff Business Park and PennyHill Quarry suggesting that bentonite WH4 may be a useful regionalmarker within the Lower Quarried Limestone Member. Above benton-ite WH4 is a fine grained crinoidal grainstone overlain by 0.6 m ofoncolitic limestones with lenses of crinoidal grainstone. Both PS3 andPS4 show a successive rise in RSL. The associated flooding surfaces aremarked by thin, silty mudstone bands, above which are carbonate

mudstones with occasional lenses of crinoidal grainstones, packstonesand oncoids. PS5 indicates continued RSL rise and consists of prominentnodular carbonatemudstones occasionally containing veryfine-grainedcomminuted crinoid ossicles. At both Whitman's Hill and Park Wood(Main) quarries, small patch reefs are associated with PS4 and PS5;these appear to be the same reefs described in detail by Penn (1971).It is of note that all the reefs observed are of the coral/stromatoporoidtype with a secondary microbial contribution, rather than microbialreefs as described from the Lower Quarried Limestone Member of the

121D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

WestMidlands (Ratcliffe and Thomas, 1999). Reefs which can be tracedlaterally to bedded limestones occupy much of the Lower QuarriedLimestoneMember at PennyHill Quarry (PS1 to PS3). Above the highestobserved reefs are carbonate mudstones and occasional fine crinoidalgrainstones interbeddedwith siltymudstones. A transition into increas-ingly nodular limestones with much silty mudstone is not observed atPenny Hill Quarry, rather the loss of reefs and dominance of mediumbedded (40–50 mm) carbonate mudstones with silty mudstones indi-cates RSL rise.

2.2.2.2. Nodular Beds Member. Above the shallow-water oncoid-bearinglimestones of the Lower Quarried Limestone Member are nodular andbedded limestones separated by silty mudstones, indicating a markedRSL rise, maximum flooding and the onset of RSL fall. Nodular lime-stones and silty mudstones are reported between the lower andupper limestone divisions of the May Hill Inlier (Lawson, 1955) andwithin the West Midlands are attributed to the Nodular Beds Member(Dorning, 1983). At Bruff Business Park, the entirety of the NodularBeds Member (23.0 m) can be seen, with significant sections addition-ally found at Whitman's Hill Quarry, Upper Vinesend Farm Quarry andWallhouse Plantation (north) Quarry. Based upon comparisons be-tween quarries, the Nodular Beds Member is observed to consist offive parasequences reflecting a pronounced RSL rise (PS6), gradualRSL fall (PS7 to PS9) and an abrupt shallowing (PS10) into the overly-ing Upper Quarried Limestone Member.

AtWhitman's Hill Quarry, the base of the Nodular BedsMember andPS6 ismarked by a thin (30 mm) rusty-orange bentonite (WH5),whichappears equivalent to a 50 mm thick rusty-orange bentonite observedat Bruff Business Park and Penny Hill Quarry. AboveWH5 atWhitman'sHill Quarry are alternating carbonate mudstones and silty mudstones(0.82 m) of equal thickness, representing condensation and the trans-portation of fine carbonate material into a deeper-water setting. Theseare in turn overlain by silty mudstones containing carbonate mudstoneand occasional wackestone nodules that increase in frequency and sizetowards the top of the parasequence. The upper half of PS6 contains 3prominent bentonites (WH6, WH7 and WH8) allowing for correlationaround Whitman's Hill Quarry. It is noteworthy that 3 prominent ben-tonites are also present at this level across the West Midlands (WNH4,WNH5, WNH6 of Ray et al., 2011).

PS7 represents the onset of progradation within the Nodular BedsMember, being capped by several prominent carbonate mudstonebeds, rather than bands of nodules. A particularly distinctive feature ofthe Nodular Beds Member is associated with the flooding surface thatcaps PS7. Here, the thickest (120 mm) bentonite (WH9) in the forma-tion is overlain by an unusually thick (0.31 m) silty mudstone band(Fig. 3b). A similarly thick bentonite overlain by a thick silty mudstoneband occurs at Bruff Business Park (120 mm and 0.47 m) (Fig. 3c) andWren's Nest Hill (WNH7; 200 mm and 0.50 m). Within the AbberleyHills, a 200 mm thick bentonite has been previously reported (Bassett,1974) and is most likely equivalent to a 240–300 mm thick bentoniteat Wallhouse Plantation (north) Quarry and in Callow Farm Quarry(SO 7526 6182). At both sections a 100 mm thick bentonite is addition-ally developed approximately 0.4 m stratigraphically above the thickbentonite, but a thick silty mudstone band is not present, possiblyreflecting a higher-energy shallower-water depositional setting. AtPenny Hill Quarry, much of the Nodular Beds Member is no longeraccessible due to landfill activities. However a photograph from theBritish Geological Survey archive (Fig. 3d) shows two conspicuousnotches within the Nodular Beds Member, the stratigraphically lowestof which corresponds to the bentonite currently exposed at the baseof the Nodular Beds Member (Fig. 3e). Approximately 5 m above thisbentonite is a second more conspicuous notch, which likely corre-sponds to the thick bentonite seen elsewhere.

Above the thick bentonite, progradation continues with two indis-tinct upward shallowing cycles (PS8 and PS9). Within the AbberleyHills, intervals of bedded and nodular ferruginous crinoidal grainstones

are developed and appear representative of the “ferruginous crinoidalgrainstone lithofacies” of Ratcliffe and Thomas (1999). Elsewhere, nod-ular limestones, carbonate mudstones and wackestones interbeddedwith silty mudstones dominate. The marked difference in lithology be-tween the Abberley Hills and the rest of the study area, including theWest Midlands, indicates a higher-energy, shallower-water environ-ment developed within the Abberley Hills area. At Park Wood Quarry(Minor), a small reef is developed within nodular limestones and siltymudstones. However, the small and isolated nature of this quarrymakes the identification of its exact stratigraphic position difficult.

PS10 is widely traceable across the northern Midland Platform(Wenlock Edge and West Midlands) owing to its substantial thickness(9–13 m) and conspicuous upward shallowing (Ray et al., 2010). PS10can only be seen in its entirety at Bruff Business Park, where it is7.3 m thick, contains three prominent (60 mm)bentonites and displayslithological change from nodular carbonate mudstones to crinoidalgrainstones with occasional silty mudstone and finally coarse crinoidalgrainstones (stormbeds). The upper fifth of PS10 (basal Upper QuarriedLimestone Member) consists of massive, rubbly crinoidal grainstones.

2.2.2.3. Upper Quarried Limestone Member. Late Wenlock successionsacross the Midland Platform are typically associated with shallow-water limestones, most commonly storm deposited and winnowedcrinoidal grainstones. Across the West Midlands, massive crinoidalgrainstones belong to the Upper Quarried Limestone Member(Ray and Thomas, 2007), at the May Hill Inlier, oolitic lithofaciesoccur within the upper limestone division (Lawson, 1955), andalong Wenlock Edge crinoidal limestone and ferruginous crinoidalgrainstone lithofacies are developed (Ratcliffe and Thomas, 1999;Ray et al., 2010). Within the study area, the Upper Quarried Lime-stone Member can be observed in its entirety at Bruff Business Park(6.14 m), with additional informative sections at Fetterlocks LaneQuarry, Upper Vinesend Farm Quarry and Gurney's Quarry. At BruffBusiness Park, the base of the Upper Quarried Limestone Memberis taken at the base of a massive (1.47 m) rubbly, coarse-grainedcrinoidal grainstone package (upper fifth of PS10), while at UpperVinesend Farm Quarry the transition is gradational and within nodu-lar fine-grained crinoidal grainstones.

PS11 at both Upper Vinesend Farm and Bruff Business Park isinitially dominated by silty mudstones with limestone nodules andthin limestone beds (carbonate mudstone and fine-grained crinoidalgrainstone storm beds) (c. 2.5 m). At Upper Vinesend Farm, the pro-portion of silty mudstone decreases up-section being replaced byincreasingly thick (100–250 mm) crinoidal grainstone beds, typicallywith scoured bases and rippled tops. PS11 at Bruff Business Park doesnot show an upward thickening of limestone beds, rather the upper2.10 m consists of planar bedded limestones (wackestones to finegrainstones) of near equal thickness (20–60 mm) to the interveningsilty mudstone bands. Only the top 3.67 m of the member is exposedat Gurney's Quarry, where PS11 consists of fine-grained crinoidalgrainstones with wavy to irregular nodular bedding; as also observedat Fetterlocks Lane Quarry.

Variations in lithology and bed-form within PS11 are reported in de-tail from across the West Midlands and along Wenlock Edge (Ray et al.,2010). Such variations result fromminor differences in palaeobathymetryand the localised development of grainstone shoals during the initialonset of transgression into the overlying Lower Elton Formation andLudlow Series.Within theMalvern Hills, a sandy facies has been reportedfrom the now infilled Mathon Park Quarry (SO 761 453) (Phipps andReeve, 1967). This sandy facies may be the proximal equivalent of thedisproportionally silty-mudstone-rich upper third of the member atBruff Business Park.

2.2.3. Basal Lower Elton Formation (Ludlow Series)The transition into the overlying Lower Elton Formationmarks an in-

crease in the rate of transgression through PS12 to PS15 and has been

122 D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

investigated at Gurney's Quarry, Bruff Business Park and FetterlocksLane Quarry. These sections are characterised by the developmentof nodular limestoneswith increasing amounts of siltymudstone associ-ated with successive parasequences. Of the sections, Bruff Business Parkappears to have been associatedwith a slightly higher energy shallower-marine environment as indicated by an increased ratio of nodularlimestones to silty mudstones and the presence of occasional crinoidgrainstone storm beds.

An additional and previously undescribed subterranean section atWren's Nest Hill, Dudley (Step Shaft entrance tunnel SO 939 918) hasalso been investigated, allowing the details of the sequence stratigra-phy and sedimentology of a significant portion (9.08 m) of the LowerElton Formation to be described. The basal Lower Elton Formation ofthe Step Shaft is most similar to the nearby Sports Centre drill hole(SO 952 908), in that it is dominated by nodular limestones withincreasing amounts of silty mudstone associated with successiveparasequences (PS12 to PS13). Correlation of this interval is basedon three prominent closely spaced bentonites some 3–4 m abovethe base of the formation, which can be traced across the region(Ray et al., 2010). Above the upper of the three bentonites it is appar-ent that two additional parasequences are present (PS14 and PS15).PS14 is dominated by silty mudstones with limestone nodules andAtrypa-rich (brachiopod) bands towards its top. The flooding surfacewith PS15 is marked by a prominent (40 mm) grey-white bentonite,above which the section consists of silty mudstone with a furtherprominent (60 mm) bentonite. It is of note that the identification ofPS13 to PS15 within the West Midlands allows for correlation withthe equivalent parasequences along Wenlock Edge (Ray et al., 2010).

3. Bentonite mineralogy and geochemistry

3.1. Bentonite heavy mineral sampling and analytical methods

At Whitman's Hill Quarry, investigations into the heavy mineralcontent of bentonites WH1 to WH9 were carried out as part of theWhitman's Hill Geodiversity Discovery Venture. Approximately 2 kgof each bentonite was sampled in order to obtain heavy mineral res-idues. These samples were coarsely crushed and immersed in waterfor 3 days to initiate the disaggregation process. They were thencleaned using an ultrasonic probe and washed through a 63 μm sieveto clean the grains and remove the silt and clay fractions. The >63 μmfraction was placed in the heavy liquid bromoform (sp. gr. 2.85), withheavy mineral separation achieved by gravity-settling. The heavy min-eral residues were then examined under a petrographic microscope toidentify the heavymineral components and to gain an initial impressionof grain morphology (Table 1). In addition, representative grains ofapatite from WH9 and zircon from WH1 were picked from heavymineral residues under a polarising microscope. These samples werethen examined in a scanning electronmicroscope and qualitative chem-ical analyseswere obtained by energy-dispersive X-raymicroanalysis inorder to verify the composition of the mineral grains.

Table 1A qualitative evaluation of heavy mineral residues from bentonites at Whitman's Hill Quar

Bentonite Thickness Zircon Apat

WH9* 120 Scarce ComWH8 30 Absent AbseWH7* 20 Absent ComWH6* 40 Scarce ComWH5 30 Scarce ScarcWH4* 30 Absent ComWH3 60 Scarce AbseWH2 60 Common AbseWH1 30 Common Scarc

3.2. Bentonite heavy mineral content

Apart from authigenic phases (mostly pyrite and Fe hydroxides),the two main heavy mineral phases in the bentonites studied areapatite and zircon. These two phases vary in abundance betweenbentonites (Table 1). Zircon is abundant inWH1 andWH2, but is scarceto absent higher in the section. By contrast, apatite is scarce inWH1 andWH2, but is commonhigher in the section, and is especially abundant inWH9. Both apatite and zircon display euhedral or broken habits, withlittle evidence for rounding or abrasion. Such features appear to be con-sistentwith volcanicmaterial deposited through air-fall processes, rath-er than representing detrital clastic material.

WH3 contains a small proportion of garnet and tourmaline whoseoriginmay reflect either contamination by terrigenous clastics, or depo-sition as volcanogenic xenocrysts derived aswall-rock contaminants. Ofnote among the non-volcanic grains are biogenic phosphates, which arecommonly associated with episodes of sediment starvation typicallyresulting from transgression (Brett et al., 1998). The occurrence of phos-phatic grains within the Much Wenlock Limestone Formation suggestsa close link between bentonite preservation and sediment starvation,a relationship that is not unexpected within a shallow water carbonatesetting, where bentonite preservation should otherwise be low as aresult of sediment dilution.

3.3. Apatite REE analysis

Heavyminerals such as zircon and apatite are common constituentsin many Silurian bentonites (Pearce et al., 2003; Ray, 2007; Ray et al.,2011). The presence of these heavy minerals is significant in that theyoffer the potential for greatly improving stratigraphic precision via theapplication of radiometric dating and geochemical correlation tech-niques. Apatite geochemistry and, in particular, rare earth element(REE) content can be obtained by laser ablation inductively coupledmass spectrometry (LA-ICP-MS) and offers the opportunity forgeochemical correlation with coeval bentonites and the possibilityof evaluating the composition of the source magmas (Fleischerand Altschuler, 1986). When associated with unaltered primarymicrophenocrysts, the REEs and yttrium contained within apatitesare considered immobile under most upper crustal conditions andare unaffected byweathering (Shaw, 2003). Furthermore, the REE com-position typically reflectsmagmatic processes as there is no preferentialconcentration of light REEs (LREEs) relative to heavy REEs (HREEs)within the apatite structure (Watson and Green, 1981). Finally, whilethe apatite composition of the same bentonite along strike (Batchelorand Jeppsson, 1994; Batchelor et al., 1995) and individual apatiteswith-in the same bed (Samson et al., 1988) demonstrate a small spread incomposition, variations between individual beds can exceed 20%, there-by increasing the likelihood of identifying a uniquely diagnostic chemi-cal fingerprint for any given bentonite.

Apatite geochemistry was carried out using LA-ICP-MS. The laserbeam diameter was 30 μm and laser repetition rate 5 Hz. Helium gaswas the ablation medium and a carrier gas of helium plus argon

ry. * — LA-ICP-MS apatite analysis. Thickness in mm.

ite Diagenetic components Other components

mon Opaquesnt Opaques, carbonatemon Opaques, carbonatemon Opaques, carbonatee Opaques, carbonatemon Opaques, carbonate Biogenic phosphatent Opaques, carbonate Garnet, tourmalinent Opaques, carbonatee Opaques, carbonate

Fig. 4. Chondrite-normalised REE data for apatites extracted from bentonites WH4,WH6, WH7 and WH9. Normalising data taken from Evensen et al. (1978).

123D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

transported the sample to the ICP-MS. Time-resolved analysis data ac-quisition software was used with a total acquisition time of 120 s peranalysis, allowing about 60s for background followed by 60s for laserablation. Data reduction used GLITTER (LA-ICP-MS software package)and internal standards of Si for the glass standards and Ca for apatite(54 wt.% CaO). Signals were checked for contamination from otherphases or the mounting medium and the integration intervals adjustedaccordingly. NIST 610 glass was used for instrument calibration, withNIST 612 used as a secondary check standard. LA-ICP-MS was carriedout on 28 apatite grains from bentonites WH4 and WH6, 30 grainsfromWH7 and31grains fromWH9. The apatite grainswere handpickedand care was taken to avoid grains containing obvious inclusions orsurface contamination. A summary of the trace element and REE datafrom bentonites WH4, WH6, WH7 andWH9 is given in Table 2.

3.3.1. Apatite REE geochemistry and magmatic sourcesAn assessment of chondrite-normalised REE curves for bentonites

WH4, WH6, WH7 and WH9 indicates (Fig. 4) that the apatiteswere derived from a volcanic source, in that they lack a negative Ceanomaly and also lack the enrichment in heavy rare earth elements(HREEs) relative to light rare earth elements (LREEs) that is typicallycharacteristic of sedimentary apatite (Fleischer and Altschuler, 1986).The concentration of LREEs relative to HREEs can be determined by theoverall slope of the chondrite-normalised REE plot and is reflected in the(Ce/Yb)N values (Table 2) where higher values indicate relative LREEenrichment or HREE depletion. Based upon the chondrite-normalisedREE curves and (Ce/Yb)N values, WH4, WH6 and WH7 (6.4 to 7.5)are rather similar in composition, while WH9 is particularly distinctive

Table 2Rare earth element and trace element concentrations (ppm) and chemical discrimina-tors obtained from apatite crystals from bentonites WH4, WH6, WH7 and WH9 fromWhitman's Hill Quarry.

REE WH4 WH6 WH7 WH9

La 510.6 392.8 524.8 2264.2Ce 1581.8 1369.3 1766.7 5503.5Pr 232.8 219.1 283.7 730.5Nd 1135.6 1106.6 1353.5 3081.8Sm 300.4 298.3 343.8 579.9Eu 43.2 58.7 63.1 87.45Gd 305.5 294.9 320.8 431.2Tb 44.8 41.3 45.1 54.7Dy 241.7 214.3 234.4 280.8Ho 45.3 39.1 43.0 51.6Er 104.1 87.1 96.7 120.7Tm 11.8 9.8 11.0 14.8Yb 64.4 53.5 61.0 83.8Lu 8.0 6.9 7.6 10.7ΣREE 4629.8 4191.5 5155.1 13295.8

Trace element WH4 WH6 WH7 WH9

Mn 1131.4 833.5 738.6 1309.4Ni 91.8 65.4 143.9 106.8Rb 3.8 10.6 13.7 11.0Sr 427.1 509.4 606.2 1102.8Y 1155.5 982.3 1071.9 1321.5Zr 10.3 24.9 67.2 62.8Nb 0.5 2.0 11.4 3.7Ba 6.1 11.5 14.1 39.9Th 64.4 82.8 135 194.8U 5.0 4.2 6.7 19.0

Discriminators WH4 WH6 WH7 WH9

Eu/Eu* 0.44 0.60 0.58 0.53La + Ce + Pr% 50.2 47.3 50.0 63.9La/Nd 0.45 0.35 0.39 0.73(Ce/Yb)N 6.4 6.6 7.5 17.0Ce/Y 1.4 1.4 1.6 4.2La–Nd% 74.7 73.7 76.2 87.1Sm–Ho% 21.2 22.6 20.4 11.2Er–Lu% 4.1 3.7 3.4 1.7

(17.0) being enriched in LREE. An additional feature of the chondrite-normalised REE curves is a negative Eu anomaly (Eu/Eu⁎). Thenegative Eu anomaly broadly reflects magmatic composition in thatdivalent Eu ions within a reducing magma can act as a proxy forcalcium in plagioclase feldspar. Consequently, apatites derived from amagma in which calcic plagioclase has already crystallised should bereflected by a strong negative Eu anomaly. The bentonites describedherein have Eu/Eu⁎ values between 0.44 and 0.6, which is indicativeof significant plagioclase feldspar fractionation from an evolvedmagmatic source.

Theproportionsof LREEs (La–Nd) toHREEs (Er–Lu) can be comparedwith those from apatites of a known igneous affinity (Fleischer andAltschuler, 1986, table 1) allowing an assessment of the likely composi-tion of the bentonite sourcemagma (Fig. 5). Based upon the proportionsof LREEs toHREEs as a percentage of total REE concentration,WH4,WH6and WH7 were derived from magmas whose composition most closelyreflects that of granodiorite, while WH9 has a composition whichplots between the composition of gabbro and syenite. Furthermore, acomparisonwith the published geochemical signatures of 47 bentonitesfromwithin the Llandovery (Telychian) andWenlock series of England,Norway, Scotland and Sweden (Fig. 5) indicates thatWH9 has a compo-sition more typical of Telychian and Sheinwoodian bentonites, beingrelatively enriched in LREE when compared with WH4, WH6 andWH7 and most other Homerian bentonites. The change in compositionbetween Telychian/Sheinwoodian bentonites and younger Homerianbentonites may be indicative of evolving magmatic sources, possiblyresulting from the changes occurring as subduction gave way to conti-nental collision during the final stages of closure of the Iapetus Oceanand/or the Tornquist Sea (Trench and Torsvik, 1992; Williams et al.,1992; Niocaill, 2000; Cocks and Torsvik, 2005).

A further assessment of the likely composition of the bentonitesource magmas can be made by comparing the chemistry of all ofthe apatite grains analysed (rather than the average compositionalvalues) using the atomic proportions of La/Nd versus La + Ce + Pr%(Fleischer and Altschuler, 1986) (Fig. 6). Such a comparison shows aconsiderable degree of overlap in apatite compositions betweenWH4, WH6 and WH7. WH4 contains a broad continuum of apatitecompositions, which range from felsic to intermediate/mafic fields,while WH6 and WH7 are broadly felsic in composition with the ex-ception of a small separate cluster of four apatite grains from WH7,which plot in the intermediate/mafic field. The presence of felsicand intermediate/mafic clusters for WH7 suggests that this bentonitemay either be made up of two separate ash fall events, containtwo separate magmatic phases of apatite grains, or contain primaryvolcanic apatite alongside detrital apatite. WH9 shows very littlecompositional overlap with the other bentonites and has a

Fig. 5. LREE versus HREE discrimination diagram for apatite (modified from Ray et al., 2011). (a) Average values for selected igneous rock types are based on REE data after Fleischerand Altschuler (1986). Apatite data from Whitman's Hill Quarry are superimposed alongside Llandovery and Wenlock Series apatite data from England, Norway, Scotland, Sweden(Batchelor and Clarkson, 1993; Batchelor and Jeppsson, 1994; Batchelor et al., 1995; Batchelor and Jeppsson, 1999; Ray, 2007; Ray et al., 2011; Cramer et al., 2012). Circled dataindicates that the apatites are considered to have originated from the same bentonite horizon. (b) Location of selected Llandovery and Wenlock Series bentonites. Palaeogeographyafter Trench and Torsvik (1992). (c) Stratigraphic distribution of selected bentonites. Chronostratigraphy is taken from Melchin et al. (2012).

124 D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

Fig. 6. Rock classification based on apatite REE content, after Fleischer and Altschuler (1986,fig. 1). 1, Granite pegmatites; 2, gneiss/migmatite; 3, granite; 4, granodiorite; 5, gabbro;6, phosphorite; 7, kimberlite; 8, syenite; 9, alkali ultramafic; 10, carbonatite; 11, iron ores;12, ultramafic; 13, alkalic; 14, alkalic pegmatite. Ellipses indicate the range of apatite datadetermined from individual grains from bentonites WH4, WH6, WH7 andWH9.

125D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

composition ranging between intermediate/mafic and alkaline fields.The degree of host melt alkalinity can additionally be accessed via theCe/Y value of apatite (Roeder et al., 1987), with values above 7.7considered to represent highly alkaline source magmas. Ce/Y valuesfor WH4, WH6 and WH7 vary between 1.4 and 1.6, compared witha more alkaline value of 4.3 for WH9.

3.3.2. Bentonite correlationIn order for a correlation to be achieved between bentonite

horizons, they must be stratigraphically coeval and compositionallysimilar, but also compositionally distinct from other bentoniteswithin the same stratigraphic interval. The four Whitman's Hill Quarrybentonites are all from within the Lower to Middle Much WenlockLimestone Formation, which across the Midland Platform correspondsto the Late Cryptograptus lundgreni to Colonograptus ludensis graptolitebiozones (Cocks et al., 1992; Ratcliffe and Thomas, 1999; Ray et al.,2010). Accordingly, bentonites from the latest Coalbrookdale andMuch Wenlock Limestone Formations of Wren's Nest Hill, Dudley(WNH1, WNH2, WNH5, WNH7, WNH9, WNH10, WNH13, WNH15)(Ray et al., 2011) offer the greatest potential for correlation, being geo-graphically nearby (c. 47 kmnorth–northeast) and corresponding to anequivalent stratigraphic interval; note that bentonites WNH9 toWNH15 are likely to occur above the youngest of the Whitman's HillQuarry bentonites according to the correlation of parasequences herein(Fig. 2). Additional bentonites from the Much Wenlock LimestoneFormation originate from Harley Hill (SJ 609 004) and Coates (SO 604993) quarries (WE2 and WE6) along Wenlock Edge, Shropshire. Thesebentonites are considered equivalent to Wren's Nest Hill bentoniteWNH13 (Ray et al., 2011) and consequently may be younger than theWhitman's Hill Quarry bentonites. However, like bentonites WNH9to WNH15, they are sufficiently similar in age to the Whitman'sHill Quarry bentonites to merit comparison. Outside of the MidlandPlatform the Halla Formation on Gotland (Baltica) contains the onlyother record of apatite REE data from Homerian bentonites. A correla-tion between the Homerian of Gotland and Wren's Nest Hill has beenestablished on biostratigraphic, carbon isotopic and sequence strati-graphic grounds (Ray et al., 2010; Cramer et al., 2012) and indicatesthat the Grotlingbo bentonite (Hörsne Member) and bentonitesSW10, SW10a, SW11, SW12 and SW14 (Djupvik Member) are broadlyage equivalent to the Lower Quarried Limestone and Nodular Bedsmembers of the Much Wenlock Limestone Formation (Batchelor andJeppsson, 1999; Cramer et al., 2012). Furthermore the grain size of phe-nocrysts from Halla Formation bentonites indicates that they may have

been derived from distant volcanic centres up to 2000 km away, whichcould include areas proximal to theMidland Platform (Ray et al., 2011).

Using the atomic proportions of La/Nd versus La + Ce + Pr%, as de-rived from individual apatite grains contained within each of theWhitman's Hill Quarry bentonites, the compositional range of each ben-tonite can be assessed and compared with other Homerian bentonites.Based upon the considerable degree of overlap in apatite compositionsbetween WH4, WH6 and WH7 it is clear that these bentonites cannotbe chemically distinguished from each other or indeedmany other ben-tonites. At best the compositions ofWH4,WH6 andWH7 can be consid-ered as typical of Homerian bentonites on theMidland Platform (Fig. 6).However,WH9shows very little compositional overlapwithWH4,WH6and WH7 and has a composition atypical of the majority of MidlandPlatform bentonites. Comparisons of the proportions of LREEs (La–Nd)to HREEs (Er–Lu) also highlight WH9 as compositionally distinctamongMidland Platform bentonites (Fig. 5). OnlyWren's Nest Hill ben-tonite WNH7 has a composition similar to WH9, plotting within thecompositional range of WH9 when the two are compared using La/Ndversus La + Ce + Pr%. These two bentonites are also in close proximityto each other when compared using LREE versus HREE (Figs. 5, 6). Addi-tional supporting evidence for the correlation ofWH9 andWNH7 comesfrom the application of sequence stratigraphy, as both bentonites arepreserved on the flooding surface between PS7 and PS8, within themid-dle of theNodular BedsMember (Fig. 2). Furthermore, atWhitman's HillQuarry and Wren's Nest Hill WH9 and WNH7 are the thickest of thebentonites preserved (120 mm and 200 mm thick respectively) andare also overlain by an unusually thick silty mudstone horizon; suchfeatures are also evident at the Bruff Business Park section. Thus, basedupon apatite REE composition, sequence stratigraphy and bentonitethickness it appears highly likely that WH9 and WNH7 are representa-tives of the same ash fall event, and provide an important regional time-line between Whitman's Hill Quarry and the West Midlands.

A comparison between the Whitman's Hill Quarry bentonites andthose from the Halla Formation, Gotland indicates considerable po-tential for correlation. Comparisons using LREE versus HREE indicatethat bentonite SW10 has a composition broadly similar to the typicalHomerian Midland Platform bentonites, while the Grotlingbo, SW11,SW12 and SW14 bentonites are relatively more enriched in LREE andhave a composition more similar to that of WH9 and WNH7. SW10ahas a composition which most closely resembles that of a kimberliteor carbonatite which has no equivalent on the Midland Platform(Fig. 5). These comparisons are also confirmed using La/Nd versusLa + Ce + Pr% (Fig. 6). In particular the Grotlingbo, SW11, SW12and SW14 bentonites all plot around the periphery of the composi-tional range of WH9, with SW11 and SW12 being most similar tothe average composition of WH9 and WNH7. Based upon the correla-tion between Wren's Nest Hill and Gotland (Cramer et al., 2012),the Grotlingbo bentonite is probably age equivalent to the LowerQuarried Limestone Member, precluding correlation with WH9 andWNH7. However SW11, SW12 and SW14 all occur stratigraphicallyhigher within the Djupvik Member, which is approximately ageequivalent to the highstand systems tract within the Nodular BedsMember and WH9 and WNH7 (Calner et al., 2008; Cramer et al.,2012). While such similarities in stratigraphic position and composi-tion do not, at present, allow for the identification of a single ash fallevent covering both the Midland Platform and Gotland, they areindicative of a shared source region, which may offer the possibilityfor future bentonite correlation between these regions.

4. Discussion

4.1. Regional and global comparisons

Based upon the integrated sequence and bentonite stratigraphypresented herein, the Much Wenlock Limestone Formation in thearea between Ledbury and Abberley may now be correlated in detail

126 D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

with that of the West Midlands and the type Wenlock area alongWenlock Edge (Ray et al., 2010, 2011), thereby encompassing themajority of the northern and central regions of the Midland Platform,an area of approximately 2075 km2. Furthermore, the three-fold divi-sion of the Much Wenlock Limestone Formation into Lower QuarriedLimestone, Nodular Beds and Upper Quarried Limestone Members,established within the West Midlands (Dorning, 1983), can nowalso be applied with confidence to sections stretching from the ParkWood quarries (Malvern Hills) in the south, to the Abberley Hills inthe north.

Across the study area, two regressive episodes separated by amarked transgression can be identified, and their relative age inferredby a correlation with well dated sections within the West Midlandsand along Wenlock Edge (Ray et al., 2010, 2011; Cramer et al.,2012). From the Malvern Hills northwards to the West Midlands,the boundary between the Coalbrookdale andMuchWenlock LimestoneFormations is associated with a marked regression (Late Cryptograptuslundgreni Biozone), resulting in the onset of the deposition of the LowerQuarried LimestoneMember; a shallowmarine limestone,which is local-ly rich in oncoids (Ratcliffe, 1988; Ray et al., 2010). Thiswas followed by aperiod of slow transgression (early transgressive systems tract) resultingin a gradual RSL rise as inferred for the Lower Quarried LimestoneMember at Whitman's Hill Quarry. Within the Suckley and AbberleyHills and across the West Midlands this slow rate of transgressionappears to have been exceeded by carbonate production allowingshallower water carbonate environments to prograde, resulting ina RSL fall. Towards the end of Lower Quarried Limestone deposition,the rate of transgression increased (late transgressive systems tract)resulting in the replacement of shallow marine limestones with thedeeper-water nodular limestones and silty mudstones of the NodularBeds Member. Based upon comparisons with the northern MidlandPlatform the majority of the Lower Quarried Limestone Member andlower part of the Nodular Beds Member, including the maximumflooding surface, belongs to the Gothograptus nassa Biozone. Theremainder of the Nodular Beds Member consists of an initial phaseof weak progradation (highstand systems tract) characterised by an up-ward shallowing succession of bedded limestone and siltymudstones. Amarked regression (falling stage systems tract) then followed, resultingin the deposition of shallow marine crinoidal grainstones and locallyferruginous crinoidal grainstone (Abberley Hills) within the UpperNodular Beds and lower Upper Quarried Limestone Members. Regionalcorrelation indicates that much of the highstand and falling stage sys-tems tracts, as well as the upper sequence boundary, occur within theColonograptus ludensis Biozone. Stratigraphically above an initiallyslow rate transgression is associated with both massive and beddedcrinoidal grainstones prior to a marked deepening indicated by anabrupt transition into the silty mudstone dominated Lower EltonFormation and the Ludlow Series.

Elsewhere on the Midland Platform, the two regressive episodesseparated by a marked transgression appear equivalent to the MuchWenlock Limestone Formation of the May Hill Inlier, which is divisi-ble into a lower oncolite-rich limestone, an intermediate nodular in-terval and an upper crinoidal grainstone (Lawson, 1955; Ratcliffeand Thomas, 1999). Within platform marginal areas, such as alongWenlock Edge, a lower oncolite-rich limestone is absent, with LateCryptograptus lundgreni Biozone shallowing being instead associatedwith the development of nodular limestones and silty mudstones(Farley Member of the Coalbrookdale Formation) above the siltymudstones of the Apedale Member (Coalbrookdale Formation).Detailed correlation of parasequences along Wenlock Edge (Rayet al., 2010) indicate that the Gothograptus nassa Biozone maximumflooding occurs within an interval of sediment starvation in the low-est Much Wenlock Limestone Formation; as developed within thereef tract succession, near Much Wenlock. Above, gradual and thenmore rapid shallowing occurred (Colonograptus ludensis Biozone), be-fore the onset of transgression within the uppermost Much Wenlock

Limestone Formation. It is of note that comparisons with platformmarginal areas indicate that the younger of the two regressive epi-sodes is the most pronounced.

Outside the Midland Platform, two Homerian regressive episodesseparated by a marked transgression have been reported from Wales(Ray et al., 2010), the Baltic region (Calner et al., 2008; Cramer et al.,2012), southern Europe (Brett et al., 2007) and North America(Cramer et al., 2006; Lenz et al., 2006), thereby validating the eustaticnature of these events (Loydell, 1998; Johnson, 2006). Of the sectionsoutside the Midland Platform, those on Gotland provide the greatestdetail and demonstrate a remarkable degree of similarity with thesequence stratigraphic framework described for the Midland Platform.The details of the correlations between the Midland Platform andGotland are given in Ray et al. (2010) and Cramer et al. (2012),but in summary the two marked regressions of the Much WenlockLimestone Formation correspond with the Fröjel (Gannarve Member)and Klinteberg Formations, with the intervening early and late trans-gressive and highstand systems tracts corresponding to the Bara Oolite,Mulde Brick Clay, and DjupvikMembers of the Halla Formation, respec-tively. Additional similarities relate to the early transgressive systemstract (Lower Quarried Limestone and Bara Oolite Members), whichin both regions is characterised by a proliferation ofmicrobial (oncolite)limestones (Ratcliffe, 1988; Ratcliffe and Thomas, 1999; Ray et al., 2010;Calner et al., 2012). Such changes are associatedwith theMuldepositivecarbon isotopic excursion and extinction event, recognised as one of themost important bioevents of the Silurian (Calner et al., 2006; Calner,2008; Cramer et al., 2012). Based upon the integrated sequence andbentonite stratigraphy presented herein it is apparent that themicrobi-al (oncolite) limestones (Lower Quarried LimestoneMember), associat-ed with the Mulde Event, are far more widespread than previouslyrecognised and occupy much of the mid-platform area of the MidlandPlatform (Fig. 1).

Acknowledgements

The investigation into the prominent bentonite horizons atWhitman's Hill Quarry was funded by the Aggregates Levy Sustain-ability Fund, as part of the Whitman's Hill Geodiversity DiscoveryVenture. This venture was run, led and managed by the Earth HeritageTrust. Iain McDonald is acknowledged for carrying out LA-ICP-MS anal-ysis at the University of Cardiff. The authors thank John Roberts-Powellfor access to Bruff Business Park, Suckley. Dudley Museum and GrahamWorton are acknowledged for organising access to the Step Shaft Mineat Dudley. John Nicklin is thanked for organising access to key outcropsin the Abberley Hills. The authors thank both reviewers for theirhelpful comments. This paper is a contribution to the InternationalGeoscience Programme (IGCP) Project 591— The Early toMiddle Paleo-zoic Revolution.

References

Aldridge, R.J., Siveter, David J., Siveter, Derek J., Lane, D., Palmer, D.C., Woodcock, N.H.,2000. British Silurian stratigraphy, xviii. Peterborough: Geological ConservationReview Series, Joint Nature Conservation Committee (542 pp.).

Bassett, M.G., 1974. Review of the stratigraphy of the Wenlock Series in the WelshBorderland and South Wales. Palaeontology 17, 45–77.

Bassett, M.G., 1976. A critique of diachronism, community distributions and correlationof the Wenlock–Ludlow boundary. Lethaia 9, 207–218.

Bassett, M.G., Bluck, B.J., Cave, R., Holland, C.H., Lawson, J.D., 1992. Silurian. In: Cope,J.C.W., Ingham, J.K., Rawson, P.F. (Eds.), Atlas of Palaeogeography and Lithofacies:Geological Society, London, Memoirs, 13, pp. 37–56.

Batchelor, R.A., Clarkson, E.N.K., 1993. Geochemistry of a Silurian metabentoniteand associated apatite from the North Esk Inlier, Pentland Hills. Scottish Journalof Geology 29, 123–130.

Batchelor, R.A., Jeppsson, L., 1994. Late Llandovery bentonites from Gotland, Sweden, aschemostratigraphic markers. Journal of the Geological Society of London 151, 741–746.

Batchelor, R.A., Jeppsson, L., 1999. Wenlock metabentonites from Gotland, Sweden:geochemistry, sources and potential as chemostratigraphic markers. GeologicalMagazine 136 (6), 661–669.

Batchelor, R.A., Weir, A.J., Spjeldnæs, N., 1995. Geochemistry of Telychian metabentonitesfrom Vik, Ringerike District, Oslo Region. Norsk Geologisk Tidsskrift 75, 219–228.

127D.C. Ray et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 115–127

Brett, C.E., Boucot, A.J., Jones, B., 1993. Absolute depth limits of Silurian benthic assemblages.Lethaia 26, 25–40.

Brett, C.E., Baarli, B.G., Chowns, T., Cotter, E., Driese, S., Goodman, W., Johnson, M.E.,1998. Early Silurian condensed intervals, ironstones, and sequence stratigraphyin the Appalachian foreland basin. In: Landing, E. (Ed.), New York State MuseumBulletin: 491, pp. 89–143.

Brett, C.E., Ferretti, A., Histon, K., Schönlaub, H.-P., 2007. Eustasy and basin dynamics ofthe Silurian of the Carnic Alps (Austria). Acta Palaeontologica Sinica 46, 43–49.

Butler, A.J., 1939. The stratigraphy of theWenlock Limestone at Dudley. Quarterly Journalof the Geological Society of London 95, 34–74.

Calner, M., 2008. Silurian global events: at the tipping point of climate change.In: Elewa, AshrafM.T. (Ed.),Mass Extinctions. Springer-Verlag, Berlin andHeidelberg,pp. 21–58.

Calner, M., Ahlberg, P., Axheimer, N., Gustavsson, L., 2006. The first record ofOdontopleura ovata (Trilobita) from Scandinavia: part of a Middle Silurian inter-continental shelly benthos mass occurrence. GFF 128 (1), 33–37.

Calner, M., Eriksson, M.E., Clarkson, E.N.K., Jeppsson, L., 2008. An atypical intra-platformenvironment and biota from the Silurian of Gotland, Sweden. GFF 130 (2), 79–86.

Calner, M., Lehnert, O., Jeppsson, L., 2012. New chemostratigraphic data through theMulde Event interval (Silurian, Wenlock), Gotland, Sweden. GFF 134 (1), 65–67.

Cocks, L.R.M., Torsvik, T.H., 2005. Baltica from the Late Precambrian to mid-Palaeozoictimes: the gain and loss of a terrane's identity. Earth-Science Reviews 72, 39–66.

Cocks, L.R.M., Holland, C.H., Rickards, R.B., 1992. A Revised Correlation of Silurian Rocksin the British Isles: Geological Society of London, Special Report, 21 (32 pp.).

Corfield, R.M., Siveter, Derek J., Cartlidge, J.E., McKerrow, W.S., 1992. Carbon isotopeexcursions near the Wenlock–Ludlow (Silurian) boundary in the Anglo-Welsharea. Geology 20, 371–374.

Cramer, B.S., Kleffner, M.A., Saltzman, M.R., 2006. The Late Wenlock Mulde positivecarbon isotope excursion in North America. GFF 128, 85–90.

Cramer, B.D., Condon, D.J., Söderlund, U., Marshall, C., Worton, G.W., Thomas, A.T.,Calner, M., Ray, D.C., Perrier, V., Boomer, I., Patchett, P.J., Jeppsson, L., 2012. U–Pb(zircon) age constraints on the timing and duration of Wenlock (Silurian)paleocommunity collapse and recovery during the ‘Big Crisis’. Geological Societyof America Bulletin 124 (11/12), 1841–1857.

Dorning, K.J., 1983. Palynology and stratigraphy of the Much Wenlock LimestoneFormation of Dudley, Central England. Mercian Geologist 9, 31–40.

Evensen, N.M., Hamilton, P.J., O'Nions, R.K., 1978. Rare-earth abundances in chondritemeteorites. Geochimica et Cosmochimica Acta 42, 1199–1212.

Fleischer, M., Altschuler, Z.S., 1986. The lanthanides and yttrium in minerals of theapatite group — an analysis of the available data. Neues Jahrbuch MineralogischeMonatschefte 10, 467–480.

Johnson, M.E., 2006. Relationship of Silurian sea fluctuations to oceanic episodes andevents. GFF 128, 115–121.

Lawson, J.D., 1955. The geology of the May Hill Inlier. Quarterly Journal of the Geolog-ical Society 111 (1–4), 85–116.

Lenz, A.C., Noble, P.J., Masiak, M., Poulson, S.R., Kozlowska, A., 2006. The lundgreniextinction event: integration of paleontological and geochemical data from ArcticCanada. GFF 128 (2), 153–158.

Loydell, D.K., 1998. Early Silurian sea-level changes. Geological Magazine 135, 447–471.Marshall, C., Thomas, A.T., Boomer, I., Ray, D.C., 2012. High resolution δ13C stratigraphy

of the Homerian (Wenlock) of the English Midlands and Wenlock Edge. Bulletin ofGeosciences 87 (4), 669–679.

Melchin, M.J., Sadler, P.M., Cramer, B.D., 2012. Chapter 21: the Silurian Period. In:Gradstein, F.M., Ogg, J.G., Schmitz, M., Ogg, G. (Eds.), The Geologic Time Scale2012. Elsevier, New York, pp. 525–558.

Niocaill, C.M., 2000. A new Silurian palaeolatitude for eastern Avalonia and evidencefor crustal rotations in the Avalonian margin of southwestern Ireland. GeophysicalJournal International 141, 661–671.

Pearce, N.J.G., Thorogood, J., Mathews, R.E., Cave, R., Loydell, D.K., Evans, J.A.,Williams, R.A.,2003. Correlating Lower Palaeozoic bentonites using clay and apatite chemistry andisotopic methods: problems at high resolution. Abstracts of the New frontiers in theFourth Dimension: Generation, Calibration and Application of Geological Timescales. :Geological Association of Canada. Mt. Tremblant meeting, Quebec, Canada.

Penn, J.S.W., 1971. Bioherms in the Wenlock Limestone of the Malvern area (Herefordshire,England). Mémoires du Bureau de Recherches Géologiques et Minières 73, 129–137.

Phipps, C.B., Reeve, F.A.E., 1967. Stratigraphy and geological history of the Malvern,Abberley and Ledbury Hills. Geological Journal 5 (2), 339–368.

Phipps, C.B., Reeve, F.A.E., 1969. Structural geology of the Malvern, Abberley andLedbury hills. Quarterly Journal of the Geological Society 125, 1–37.

Ratcliffe, K.T., 1988. Oncoids as environmental indicators in theMuchWenlock LimestoneFormation of the English Midlands. Journal of the Geological Society of London 145,117–124.

Ratcliffe, K.T., Thomas, A.T., 1999. Carbonate depositional environments in the LateWenlock of England and Wales. Geological Magazine 136, 189–204.

Ray, D.C., 2007. The correlation of Lower Wenlock Series (Silurian) bentonites from theLower Hill Farm and Eastnor Park boreholes, Midland Platform, England. Proceedingsof the Geologists' Association 118 (2), 175–185.

Ray, D.C., Butcher, A., 2010. Sequence stratigraphy of the type Wenlock area (Silurian),England. Bollettino della Società Paleontologica Italiana 49 (1), 47–54.

Ray, D.C., Thomas, A.T., 2007. Carbonate depositional environments, sequence stratig-raphy and exceptional skeletal preservation in the Much Wenlock LimestoneFormation (Silurian) of Dudley, England. Palaeontology 50, 197–222.

Ray, D.C., Brett, C.E., Thomas, A.T., Collings, A.V.J., 2010. Late Wenlock sequence stratig-raphy in central England. Geological Magazine 147 (1), 123–144.

Ray, D.C., Collings, A.V.J., Worton, G.J., Jones, G., 2011. Upper Wenlock bentonites fromWren's Nest Hill, Dudley; comparisons with prominent bentonites along WenlockEdge, Shropshire, England. Geological Magazine 148 (4), 670–681.

Roeder, P.I., Macarthur, D., Ma, X.-P., Palmer, G.R., Mariano, A.N., 1987. Cathodoluminescenceand microprobe study of rare-earth elements in apatite. American Mineralogist 72,801–811.

Samson, S.D., Kyle, P.R., Alexander Jr., E.C., 1988. Correlation of North AmericanOrdovician bentonites by apatite chemistry. Geology 16, 444–447.

Shaw, G.H., 2003. Trace element chemistry of individual apatite phenocrysts as atool for fingerprinting altered volcanic ash beds: assessing interbed and intrabedvariation at local and regional scales. GSA Bulletin 115 (8), 933–942.

Trench, A., Torsvik, T.H., 1992. The closure of the Iapetus Ocean and Tornquist Sea: newpalaeomagnetic constraints. Journal of the Geological Society 149 (6), 867–870.

Watson, E.B., Green, T.H., 1981. Apatite/liquid partition coefficient for the rare earthelements and strontium. Earth and Planetary Science Letters 56, 405–421.

Williams, D.M., O'Connor, P.D., Menuge, J.F., 1992. Silurian turbidite provenanceand the closure of Iapetus. Journal of the Geological Society of London 149,349–357.

Woodcock, N.H., Butler, A.J., Davies, J.R., Waters, R.A., 1996. Sequence stratigraphicalanalysis of Late Ordovician and Early Silurian depositional systems in the WelshBasin: a critical assessment. Special Publication of the Geological Society of London103, 197–208.