Stratigraphy of the Late Aptian Lower Greensand around Nutfield, Surrey

Stratigraphy of the Late Aptian Lower Greensand around Nutfield, Surrey R. W. O'B. Knox*, A. H. RuffeUt and D. E. Highley * KNOX, R. W. O ' B., RUFFELL, A . H. & HIGHLEY, D . E . 1998 . Stratigraphy of the Lat e Ap tian Lower Greensa nd aro und Nutfi eld , Surrey. Proceedings of the Geologists ' Association , 109 , 169-1 85 . A detail ed study of Lowe r Greensa nd de pos its ex posed in the fulle r 's ea rth wo rk ings o f Nutfield , Surrey, has y ielded ne w info rma tio n on the ir lithofac ies a nd strat igrap hy. Two infor ma l litho strati graph ic unit s , represe nting co ntras ting de positional reg imes, are pro posed for the Sa ndgate Format ion o f the Nu tfiel d area . These units ar e of member status but , pend ing a com pre hensive review of Upper Apti an litho stratigraphic nom enclatu re , they are provisionally assigned the informal term s Nutfield Bed s a nd Redhill Sands. Th e N utfield Bed s are characterized by muddy, bioturbat ed, calcareous sand sto nes , deposited in quiet-w ater e nv iro nme nts; they include ec onom icall y important beds of fuller 's earth , deposited as reworked vo lca nic ash . The overlying Redhill San ds co nsist of poorly consolidated , glauconitic sandstones , characterized by large-scale foresets of tidal ba r or ig in . The Nutfield Beds form a geograph ica lly restricted lens, up to 15 m thick, lying w ithi n a sy nde positiona l basin referred to as the ' Nutfield Ba sin ' in thi s study. T he underly ing cherty sandstones (c. 1- 2 m), which co ns tit ute the top of the Hythe Formation , show a si mi lar ly res tric te d dis trib ution . Th e Redh ill Sand s are also th ickest (c . 15 m) in the midd le of the basin , but extend beyond the limi ts of the Nutfi eld Bed s to the west. Rel atively rapid subside nce within the Nutfield Bas in co mbined w ith high sea le ve l sta nd during dep o sition of the Nu tfield Bed s pro d uce d co nditions fa vou rab le for the acc um ulation and subse q uent preserva tion o f vo lcanic as h (now full er 's ea rth) deposits . Five ca nd ida te seq uence boundaries ha ve been ide ntified in thi s study . SB 1 is placed a t a le ve l withi n the uppermost Hyth e . S B la is placed at the base ofa th in gla uco nitic, pe bbly sa nds tone bed that unde rlies the top -Hythe cherty sa nds to nes . Th e cherty sands to nes an d the Nutfie ld Beds are in terpreted as transg ressive and hig hs ta nd de posits with in the same seq uence . S BZ is drawn at the ba se o f the Redh ill Sa nds , an d SB 2a at the base of a unit of pebbl y, clay-ri ch sa ndsto ne with in the upp er part of the Redh ill Sands . SB3 is take n at the base of the Folk estone San d s , and is ma rked by an erosiona l surface wi th a relief of up to 4. 5 m . Bio strat igraphi c con tro l is not suff icie ntly precise to allow correlatio n of the seq ue nces with those identifie d in the coastal sec tions of Folkes tone .

' British Geological Surve y, Keyworth. Nottingha m NG l2 SGG . "School of Geosciences , The Queen :. University of Belfast. Belfast 8 77 INN .

1. INTRODUCTION

The Sandgate Formation (Sandgate Beds of earlier authors) of the Nut field area, east of Redhill, has been the principal source of fuller 's earth (bentonite) in the UK for many years , and has acco unted for some 65 % of total UK production since official records began in 1854 . Nevertheless , the only detailed stratigraphic acco unts of these sediments are those of Gossling ( 1929) , Dines & Edm unds ( 1933) and Kirkaldy (1947). These account s were based on observations made in relati vely shallow fuller's earth workings in which only a small part of the ove rall Sandgate success ion was exposed. Subsequently, as the workings progressed northward s, down dip, the fuller 's eart h was extracted from beneath an increasi ng thickn ess of overburden . The Lower Greensand was eventually expose d to a depth of 45 m in the deepest workings, pro viding a good opportunity to establish a detailed and complete stratigraphy for the Sandgate Formation of this area . Proceedings of the Geologists ' Association , 109 , 169-1 85.

Th is paper is based on a study, begun in 1986, to document sections in the three remaining fuller 's earth quarries , at Bletchingley [TQ 3275 5035], Glebe Farm [TQ 307 514] and Patteson Court [TQ 292 509] (Fig . I ) . At the time of study, the Glebe Farm and Patteson Court quarr ies were operational , while the quarry at Bletchingley was being used as a landfill site . Producti on from the Glebe Farm quarry ceased in 1989 and from the Patteson Court quarry in 1992; both ha ve now been filled and landscaped . Add itional stratigraphic informati on has been obtained fro m boreh ole rec ord s , mostl y acq uired thro ugh the operations of Laporte Earth s Ltd , in their exploratio n for fuller 's earth. The latter boreh oles are referred to by the BGS borehole registration number, which is based on the 1: 10 000 quarter sheets of the Ordn ance Survey. Thu s ' Bh. l fi' , 'Bh .18T , ' Bh .212' and ' Bh.306' refer respectively to boreholes TQ 25SEIl 6 [TQ 2895 5095 ], TQ 35SEIl 87 [TQ 2999 5 105], TQ 35SW/212 [TQ 3 115 5096] and TQ 35SW/306 [TQ 3375 5090]. Reference is also made to 00 16-7878 /98 $ 10'00 © 199 8 Geologists' Associatio n

170

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boreholes in the area of Tandridge Golf Course [TQ 381 516] and the BGS Moorhouse Borehole (TQ45SW/l18) [TQ 42685343]. The fuller's earth seams are associated with a distinctive and localized facies of grey, calcareous sandstone. This sandstone facies is known only from a limited section of the Lower Greensand outcrop, extending from Redhill in the west to Oxted in the east, a distance of some 12 km (see Fig. I). Because of the localized distribution of this facies association, its correct lithostratigraphic assignment was initially uncertain, but was eventually accepted as belonging to the Sandgate Beds (see discussion in Dines & Edmunds, 1933: 59).

The present study has confirmed the general succession of the Sandgate Formation as recorded by Gossling (1929) and Dines & Edmunds (1933), in which grey sandstones with seams of fuller's earth are overlain by beds consisting dominantly of relatively soft, glauconitic, 'loamy' sandstone. Documentation of these two constituent facies units and discussion of their possible significance in terms of regional Lower Greensand stratigraphy were the main aims of this study. The occurrence of fuller's earth deposits within the dominantly arenaceous succession of the Lower Greensand has led to considerable debate as to how these virtually monomineralic smectite-rich clay deposits might have

STR ATI GR A PHY OF THE LOW ER G REENSA N D, S U R R E Y

formed with in what appe ars to have been a relatively high energy, sublittoral environment. It is now firmly established that the fuller 's earth deposits resulted from the diageneti c alterat ion of accumulations of reworked volcanic ash (Jeans. Merrim an & Mitchell, 1977 ; Jeans, Merriman,

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17I

Mitchell & Bland , 1982), but the mechanisms of transport and localized accumulation of the original ash are not yet fully understood. An additional aim of this study was therefore to determine whether stratigraphi c analysis might explain the exceptional concentration of fuller's earth deposits in the Sandgate Formation of the Nutfield area.

2. PREVIOUS LITHOSTRATIGRAPHIC STUDIES A generalized lithostratigraphic successio n for the Lower Greensand of the Nutfield area is illustrated in Fig . 2. In this study the Hythe Beds, Sandgate Beds and Fol kestone Beds of previous authors are referred to as formations, follow ing Rawson ( 1982). The stratigraphy of the Hythe Formation between Reigate and Bletchin gley was first established in detail by Leighton (1895), who se five-fold division was adopted by Gossling (in Kirk aldy, 1947). These five divisions are shown in Fig. 2. Of particul ar relevance to the pres ent study was recognition in the uppermost part of the Hythe Formatio n of two lithostratigraphic units: the 'Top Hythe "Stone" and Chert Beds' (Bed 5) and the ' Upper Hythe Pebbl e Bed ' (Bed 4) . The latter rests on variably pebbly, fine to coarse sandstones of the ' Mid-Hythe Sand s' (Bed 3). The Sandgate Form ation of the Nutfield area co nsists of two dist inct lith ofac ies unit s: a lower unit of grey , glauconitic. ca lcareous, fossiliferous, fine-grained sandstone assoc iated with beds of fuller 's earth . and an upper unit of dark green-grey to brown , poorly consolidated, highly glauconitic. medium-grained sandstone . The lower unit has been referred to as ' Nutfield Fuller 's Earth Beds' and ' beds of Nutf ield type ' (e.g . Dines & Edmund s, 1933: 59) and the upper unit as the ' loamy Folkestone Beds' (Dines & Edmunds, 1933). In this study, these units are inform ally termed the Nutfield Beds and the Redhill Sand s, respectively. Formal member status within the Sandgate Formation may well be warranted , but any piecemeal formalization of the nomenclature would be inappropriate at this stage because of an ongoing review of regional correlation and stratigraphic nomenclature in the North Weald area . The filling in of the fuller's earth quarri es also presents a problem in identifying appropriate type sections for the Nutfield Beds and Redhill Sands . However, both units have been cored in a recently drilled borehol e which , though currently confidential, should become available for this purpo se. The Folkestone Formation consists for the most part of yellow to buff, poorly consolidated, fine to coarse , variably pebbl y sandstone . Glau conite is scarce or abse nt. Much of the successio n is cross-bedded .

3. HYTHE FORMATION Stud y of the Hythe Formation was restricted to the Bletchin gley quarry (Fig . 3), where about 4 m of med ium to coarse , pebbly, poorly cemented sandstones were expose d at the base of the section. Sets of tabular and trough cross stratified sandstone up to I m in thickn ess, alternate with

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174

R. W. O'B. KNOX ET AL.

units of horizontally bedded or poorly bedded, bioturbated sandstone, also around I m thick. Foreset dips were measurable for only two of the cross-bedded units, and indicated current flow towards the southeast. Sporadic siliceous concretions are present. These medium to coarse sandstones are overlain by two beds of relatively fine sandstone that contain scattered small phosphatic nodules. The lower bed (0.8 m) consists of upward-fining medium to fine sandstone, with local siliceous layers and concretions. Phosphate nodules are common in the upper part and scattered in the lower part. The upper bed (0.98 m) consists of fine sandstone, also with sporadic phosphate nodules. These two sandstone units are separated by a 15 ern layer of reddish weathering, fine sandstone with clay laminae that are particularly abundant in the upper part, Bedding in the two thicker sandstone beds has been partly destroyed by burrowing, but both display lowangle (c. lO-ISO) northward-dipping cross-bedding. In both beds, the phosphate nodules occur exclusively within burrow fills, which may have provided the diagenetic conditions necessary for nodule formation. The nodular sandstones are overlain by a 2-3 cm bed of poorly sorted, variably cherty, glauconite-rich sandstone, with quartz granules and abundant angular 'chips' of phosphate and larger, sub angular, phosphate pebbles (up to 1 ern) near the top. This pebbly layer is overlain by 1.3 m of variably cherty sandstones, with impersistent layers of dense, vitreous chert in the middle part. An impersistent layer of calcite-cemented sandstone, up to 15 em thick, occurs in shallow scours at the top of the cherty sandstones. This unit of variably cherty sandstone equates with the "Iop-Hythe "Stone" and Chert Beds' (Bed 5) of Gossling (see Kirkaldy, 1947). It is here referred to as the 'top-Hythe cherty sandstone' unit. The cherty sandstones are overlain by grey, muddy, calcareous sandstone of the Sandgate Formation (Nutfield Beds - see below). The section recorded at Bletchingley is broadly similar (though substantially thinner) to that given by Gossling (1930: 209) for the section at Mid Street, Nutfield [TQ 3050 5025], where he described about 3 m of cherty sandstone underlain by beds of 'sand and soft sandstone, with a characteristic white silty matrix, and pebbles of black and brown lydite, the lower part markedly glauconitic' (seen to c. 3.5 m). The overlying cherty sandstones were included within Gossling's composite section for the Hythe Beds (1929: 224), but in his stratigraphic synopsis (p. 215) were included in the Sandgate Beds. Kirkaldy (1947: 184-185) stated that Gossling originally grouped these beds with the Sandgate Beds, but later regarded them as constituting the topmost part of the Hythe Beds (see also Gossling, 1935).

4. SANDGATE FORMATION Nutfield Beds The Nutfield Beds consist mostly of grey, argillaceous, fossiliferous, glauconitic, generally calcareous, fine to

medium sandstones, with a maximum recorded thickness of 12.6 m (base not exposed) in the Patteson Court section. A total thickness of c.l5 m is present in the nearby Bh.16 (see Fig. 1). The sandstones include calcareous and siliceous concretions, with phosphatic nodules also present at some levels. Primary depositional structures are generally lacking as a result of intense bioturbation, but some sandstone units at Bletchingley have a fragmental appearance due to abundant 'flakes' of mudstone. These 'flakes' are generally 2-3 mm thick and of variable length. They appear to represent the remnants of originally more persistent clay layers that have been disrupted by burrowing organisms, indicating that the sediment may originally have consisted of rapidly alternating mud and sand layers comparable to facies recorded from present-day tidally influenced shelf areas. The angularity of the mudstone flakes indicates that they were partly lithified prior to their disruption by burrowing. In most sections, however, the only trace of primary bedding in the Nutfield Beds is provided by sporadic bedding planes, generally between 0.3 m and 2 m apart. Most bedding planes are traceable for several tens of metres in quarry faces; they are essentially planar, but commonly include broad, shallow scours that erode a few centimetres into the underlying bed. These bedding planes are interpreted as erosional surfaces formed during exceptionally powerful storms, and their preservation is attributed to relatively rapid sediment aggradation during the waning phase of storm activity. The fauna is rich and varied (Casey, 1961: 550), including ammonites, bivalves, gastropods, brachiopods and corals, as well as abundant sponge spicules at some levels. Pieces of coniferous wood are common. Interbedded with the grey sandstones are beds of fuller's earth, green-grey when fresh, yellow-brown when weathered. These range in thickness from a few millimetres to over five metres, with a maximum of 5.5 m recorded in borehole TQ25SE1l01, about 600 m south of the Patteson Court section. The number, thickness, and extent of fuller's earth seams varies markedly from section to section, such that correlation is often uncertain. Locally, however, the succession of seams is sufficiently distinctive to allow correlation over a few kilometres (see Fig. 5). The fuller's earth seams display an overall northward thinning, with the main seam thinning from over 5 m in the Patteson Court area to 0.6 m some 1.8 km to the north (borehole TQ25SEIl). The fuller's earth seams typically display sharp bases and tops, although the upper surface is often disturbed by burrowing to depths up to 0.6 m. Internally, the fuller's earth commonly appears structureless, but some weathered surfaces show delicate cross-lamination of wave-induced type, as reported by Middlemiss (1975: 465-466). These may be analogous to the reworked bentonites described by Goldring (1996, fig. 7) from the Lower Greensand of Baulking (Oxfordshire). Isolated wave-scours were observed eroding the top 10 cm of the middle seam in the Glebe Farm section. The lack of bioturbation associated with the eroded surfaces indicates that the scouring took

STRATIGRAPHY OF THE LOWER GREENSAND, SURREY

place shortly before or during deposition of the overlying sandstones in response to storm-induced reworking. Although the base of the fuller's earth seams is typically planar, Leighton (1895: 117) described one of the fuller's earth seams of the Tilburstow Hill area as being preserved within 'basin-shaped hollows', indicating accumulation on a previously scoured surface. Redhill Sands

The Redhill Sands consist for the most part of green-grey to brown-grey, poorly consolidated, glauconitic, slightly argillaceous, fine to medium sandstones, reaching a maximum recorded thickness of 14.6 m in the Patteson Court section. Two principal subdivisions may be recognized. In ascending order, these are: Unit I, consisting of soft, brown to olive-green glauconitic sandstones (the 'loamy' sandstones of previous authors) and Unit 2, consisting of dense, dark grey to black, pebbly, clay-rich sands. Three divisions are recognized within the Unit I glauconitic sandstones: Unit l a (up to 1.2 m thick), consisting of horizontally bedded sandstones, Unit Ib (up to II m thick), consisting of sandstones with distinctive, largescale inclined bedding, and Unit l c (up to 1.7 m thick) consisting of horizontally bedded sandstones. Unit l a is crudely bedded, with much of the stratification destroyed by bioturbation. In the Glebe Farm section, the sandstone is medium to fine grained, but in the Patteson Court section, a bed of medium to coarse sandstone with small pebbles occurs at the base, with local calcite cementation. A distinctive feature of these sandstones is the

175

presence of subhorizontal burrows, generally 3-5 mm in diameter, filled with clean white sand that contrasts strongly with the enclosing glauconitic sand, which is dark greengrey when fresh, brown when weathered. These burrows are clearly identical to those described from the Lower Greensand of Godalming by Middlemiss (1962) as 'burrows of Milford type'. Two lenses of fuller's earth were observed within Unit l a in the Glebe Farm section; they were associated with a single bedding surface within the sandstone and appear to represent the deposition of suspended ash within earlier-formed wave scours. The sandstones of Unit Ib display large-scale crossstratification inclined at about 20° to the northwest. The method of excavation provided little opportunity to study the bedforms in detail, but in quarry faces parallel to the foreset dip direction (Fig. 4A) some beds of parallellaminated sand could be traced from the top to the bottom of the unit, indicating that the cross-stratification represents the migration of a single large bedform. These sections also displayed sporadic foreset reactivation surfaces (see Allen, 1982), revealed by truncation of laminae and by change in foreset dip angle. Transverse sections (Fig. 4B) revealed broad, shallow scours, several metres across. These scours are likely to be related to the reactivation surfaces observed in flow-parallel section, and may represent the occasional migration of large linguoid bedforms down the foreset surfaces. Further complexity is provided by sporadic ripplesized and small dune-sized bedforms superimposed on, and directed obliquely down, the large-scale foresets. The bedding in the cross-stratified Unit Ib sandstones is defined both by colour banding, which results from an alternation of more and less glauconitic sand, and by

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176

R. W . O ·B . K NOX E T A I .•

variation in grain size , with layers of fine to medium sand being interspersed with spora dic layers of coarse . pebbl y sand . lmpersistent laminae and thin layers of pale grey mudstone occ ur between the sandstone layers in some parts of the sectio n. At Patteson Court, some of the coarse pebbl y layers are locally ce mented into calc itic co ncre tio ns , especially in the lower part of the section. Some of the co ncretions con tain abunda nt she ll debris. incl udi ng brachiopods. bivalves and ec hinoid s. Th roughout Unit lb . primary de positional structures are mostly well preser ved. with bioturbation being limited to the occ urrence in some layers of burrows with white sand fills , similar to those in Unit l a. Unit I c marks a return to horizont ally bedded , soft glauco nitic sandstones simila r to those of Unit l a, with conspicuous burrows filled with white sand . Pebbles , up to I em diameter, are locally present in a coarse , calcitecemente d sandstone at the base of the unit in the Patteson Court section, but are absent from the Glebe Farm section. Unit 2 reaches a maximum thickness of 2.55 01 in the Patteson Co urt section. where it consists of two beds. The lower bed (max . c. 1.1 01 ) consists of dark grey to dark gree n-grey . poo rly sorted. argillaceous, glauconite-rich sandsto ne with abundant granul es in the lower part giving a distinctly gritty texture . The sandstone includes scattered patches of black mudstone . which may represent burrow fills or remn ants of primary mudstone layers, and local burr ows with white sand fills. similar to those of Unit I . At the base, black clay-rich burrow fills extend 15 Col into the underlying Unit lc sandstone . The sandstones display faint lam ination , revealing horizontal beddin g or, in places, lowangle cro ss-bedding directed to the northwest. The lower bed is of variable thickne ss and locally absent. Thi s thickness variation is partly the result of scouring of the base . to a maximum depth of 45 ern, and partly the result of ero sio n beneath the upper bed. Th e upper bed (max. 1.7 m thick) consists of c. 25 em of black , glauconitic muddy sandstone with abundant granules passing upwards into homogeneous black clayey , sandy, glauco nitic siltstone . The lower, granule-rich sandstone is impersistent, resting in erosional hollows. The sediments of both beds are dark grey to black when fresh, but weather to a mid-brown colour. The upper part of Unit 2 was not exp osed in the Glebe Farm sectio n, but the lower part is more sandy than at Patte son Co urt, with a basal unit ( 10 Col) of characteristically 'gritty ' , granule-rich. muddy sandstone resting on a much reduced section (0.33 m) of Unit lc sandstones . Th e j unc tio n bet ween the Redh ill Sand stone the Folkestone Formation was not clea rly seen in the Glebe Farm quarry. However , in the Patteson Court section. the Redhill Sand s are sharply overlain by orange-brown , coarse pebbly sandstones, with an erosional relief of 2 .8 m. In the Glebe Farm section , the Redh ill Sands are locally underlain by a 0-7 Col thick bed of black . pyritic mudstone (Fig . 3). weathered in places to iron pan . The mudstone contai ns a sparse and restricted fauna of small bivalves and ap pea rs to have been laid dow n under dysaerobi c

co ndi tions. It displays a sharp , planar contact with the underlying sandstones (Nutfield Beds) and a similarly sharp , but de monstrably erosional co ntact wi th the overlying Redhill Sands, with a local relief of several cen timetres and with total remova l of the black mudstone unit at the eastern end of the quarry. Because of the restr icted occurrence of the black mudstone unit, its stra tigraphic ass ignment is uncert ain . Ho we ver. it is provisionally grouped with the Redhill Sands. beca use it displays a sharp contact with the fossiliferous sandstones of the Nut field Beds. and because it is similar in genera l facies type to the black siltstone at the top of Redhill Sands Unit 2. A similar, thin bed of limonitic mud stone was observe d at comparab le level in the upper , weathered part of the Bletch ingley section.

5. FOLKESTONE FORMATION The Folkestone Formation con sists of poorly ceme nted , sparsely glauconitic , fine to coar se sandstones . They are buff. yellow or brown in co lour. probably as a res ult of oxidation by circulating meteor ic gro undwa ter. The present study was limited to the lowest 20 01 or so. as exposed in the Patteson Court section. As indica ted above. the base of the Folkestone Formation is here strongly erosional, cutting dow n 2.8 01 into the Redhill Sands in the space of 20 m. In the Patteson Court section, the basal 4 .2 m of the Folkestone Formation are light brown and locally coarse grained and pebbly; they are trou gh cross-bedded, with cosets up to 2 01 thick at the base. The se brown sandstones co ntrast with those above , which wea ther to a pale yellow to buff co lour, are dom inantl y fine to medium grained and displ ay planar rat her than trou gh cro ss-beddi ng . Th e upward change fro m brown to yellow sands to nes is acco mpanied by a change in cross- bedding direc tion. with north westerly foreset dips giving way upwards to southerly foreset dips. A thin tran sitional unit is present in which opposing northward- and southeastward-dipping foresets occur in relatively thin cosets. The basal brown sandstones are here equated with the ' Basal Pebbly Sands' of Go ssling (1929: 233), although as pebbles are not conspicuous in the Pattes on Court section , they are here referred to simply as the 'B asal Sands ' . The overlying sand stones, including the 'transition unit' , are equated with the lower part of the 'Silver Sands' of Gossling . Th e base of the Folke stone Form ation was poorl y ex posed in the Glebe Farm quarry but it appears that the Basal Sands of the Patteson Court section are absent, with the Sandgate Formation being overlain directly by buffcoloured fine sandstones . Th ese sandstones are thinly bedd ed , with low-amplitude cross-stra tific atio n , and probably represent the transitional unit at the base of the Silver Sands in the Patteson Court section. A small section near the quarry entrance, estimated to be a few metres above the top of the Sandgate Formation , confirmed the presence of typical 'S ilver Sands ', with southeastward-dipping cros sbedd ing .

STRATIGRAPHY OF THE LOWER GREENSAND, SURREY

6. LITHOSTRATIGRAPHIC CONTEXT The stratigraphy of the two main quarry sections studied is summarized in Fig. 5, together with adjacent borehole sections. Local correlation within the Nutfield Beds is achieved on the basis of the thicker fuller's earth seams, and reveals sharp changes in thickness that are not reflected in the overlying Redhill Sands, which maintain a more or less constant thickness between the Glebe Farm and Patteson Court sections. A more extended correlation is shown in Fig. 6. Comparison of the stratigraphic data obtained in this study with that of previous workers reveals that the most significant discrepancy concerns the thickness of the Redhill Sands. This unit was clearly recognized in east Redhill by Gossling (1929) (fig. 33: E. of Redstone Hill), where he identified up to 7 m of 'buff to green silty FS. [fine sand], often with current bedding dipping westward'. Because of the relatively thin overburden in the fuller's

BtH 16

PATTESON COURT

II FOLKESTONE FM.

BtH 187

1----

GLEBE m

UNIT 2 - ---

l--L

UNIT

tc

:I REDHILL

UNIT 18

SANDS BtH 212

--

--

-

UNIT lA

-----

NUTFIELD

------- .> ..-----;

~===== .... ....

HYT~

FM.

~~;HE FM.

•

FULLER'S EARTH

.

0

1km ,

Fig. 5. Correlation between quarry sections and selected borehole sections in the Nutfield area. Borehole numbers refer to BGS borehole register: TQ25SEIl6, TQ25SE1l87. Correlation of fuller's earth seams basedon data from intervening boreholes.

177

earth workings at that time, no good sections were available further to the east. Dines & Edmunds (1933, fig. 9) adopted Gossling's records of the Redhill Sands in the Reigate and Redhill areas but, in tracing them eastwards, underestimated their thickness. Thus they are shown as being absent in the Copyhold Pit, west of Nutfield, and as reaching a maximum thickness of about 4 m in what were then the northernmost workings of the Patteson Court quarry. Casey (1961: 550) gives a maximum of 80 ft (24.5 m) for the entire Sandgate Formation of Nutfield. The present estimate of c. 30 m for the Patteson Court section is based on a measured thickness of 14.6 m for the Redhill Sands and an estimated thickness of c. 15 m for the Nutfield Beds, obtained from nearby Bh. 16. Figure 6 demonstrates that the Nutfield area lies in the central part of a synclinal structure, here termed the 'Nutfield Basin', that had a strong influence on the accumulation and preservation of the Sandgate Formation. This local thickening had been recognized by Gossling (1929: 256) and was attributed by Kirkaldy (1933: 264) to folding prior to deposition of the Folkestone Formation. Kirkaldy considered it likely that the basin was orientated approximately north-south, in common with other preFolkestone structural basins within the region. His compilation of regional thickness data (Kirkaldy, 1933, fig. 4) suggests that the Nutfield Basin did not influence thickness of the Hythe Formation. Accounts of the correlation of the Lower Greensand to the east and west of the Nutfield area (Fig. 6) are provided by Gossling (1929), Dines & Edmunds (1933) and Kirkaldy (1947) and form the basis for the following regional assessment of the lithostratigraphic data acquired in the course of this study. Hythe Formation In the uppermost part of the Hythe Formation, Gossling recognized two useful marker beds: a distinctive 'pebble' bed that apparently equates with the 'upper Hythe nodular sandstone unit' of this study and a unit equivalent to the 'top-Hythe cherty sandstone unit' . The upper Hythe nodular sandstone unit appears to correspond to what Gossling (1930: 209) described as a distinctive 'pebbly' sandstone, c. 1 m thick, that could be traced from Brockham to Godstone, a distance of some 15 km, and which was said to maintain its distinctive character throughout, though becoming cherty towards its western recorded limit (Gossling, 1929: 223). Its eastward extension as far as B1etchingley is confirmed by the present study. Gossling (1929: 220) described the 'pebbly' sandstone as being readily recognized by two striking characters: (1) the presence of a 'pure white or pale buff matrix, speckled with a little glauconite, which consists essentially of fine sand and silt and a little clay, and constitutes 70-75% of the mass'; and (2) the presence of 'a variable proportion of black and coloured pebbles, ordinary colourless quartz pebbles being conspicuous by their absence or comparative rarity'. This description is matched

-J 00

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Fig. 6. Stratigraphic relationships in the middle part of the Lower Greensa nd of the study area (see Fig . I for locations). Candidate sequence boundaries indicated by heav y lines and by SB anno tation. Note that the base of the Sandga te For mation is taken at the top of the upper Hythe cherty sandstone.

::.

r-

STRAT IGRAP HY OF T H E LOWER GREE NSAND , SU R REY

in Go ssli ngs acco mpa nying description s of sections at Park Lane , Reigate (1929 : 22 1) and on the ea stern outsk irts of Redhill ( 1929 : 225 ). Howe ve r, nodul ar sa nds tone s co nstitute only the uppermost part of the bed s referred to by subsequent author s as ' Bed 4 ' or the ' Upper Hythe Pebb le Bed ' , since this un it clearly incl udes several met res of pebbly, medi um to coarse sa ndsto nes that underli e the nodu lar sands tones . Th is is evident from the account of Kirkaldy (1947) , in which he describes sec tio ns of Bed 4 (Upper Hythe Pebbl e Bed) that are betwee n 5 .5 and 6 .7 m thick . Bed 4 wo uld thus appear to be equivalent to the ' Pebble Bcd ' of Le ight on ( 1895) , which is show n on his figs 1- 5 as being between 3 and 5 .5 m th ick. Leighton (189 5, figs 1- 6) showed the Pebb le Bed (Bed 4) as being prese nt in the west of the reg ion but abse nt east of Bletch ingley. In the context of the ob servations made at Bletch ingley in the present study, the appare nt absence of Bed 4 east of B1etchingley is open to two interp retations . Either Bed 4 is represe nted by the th in pebbly sandsto ne bed that over! ies the nodul ar sandstones or it is rep rese nted by the pebbly, medium to coarse, cross-bedded sandstones that unde rlie the nodular sa nds tones . Consideri ng the wes tward persisten ce of the nodular sands tone (as inferred fro m Gossling 's 1929 acco unt) and the relative thickness of the units co ncerned. the second alternative seems the more likel y. It shou ld also be noted that eve n in its typical devel opment , the Upper Hythe Pebble Bed is not a discrete conglomeratic unit but co nsists of pebbly layers within sandstones of variable grai n size, so that lateral passage into the facie s encoun tered at the base of the Bletch ingley qu arry does not entai l any subs tantia l change in lithology. Further to the east , near Gods tone , Batch elor & Ward (1990) recorded a 5 m section of Bed 4 in whic h pebbles were restricted to one-metre bed s at the top and base . True (vit reo us) cherts were repo rted by Kirkald y to be absent we st of Nutfie ld . This is confirmed by bore hole records , whic h indicate that chert is absent fro m Bh s 16 and 2 12, but presen t in Bh . 306 (see Figs I and 6) , indicating that the wes tward limit of cherts lies some 2 km east of Nut field . W here chert is absent, most boreho le records are insu fficientl y detailed for Bed 5 to be identified with certainty, but it is reasonable to ass ume that it is continuous betw een Nu tfield and Redhill (Fig . 6) . Accordi ng to Gossling (1929), the top-Hythe cherty san dstones are absent west of Redhill , the wes ternmost occ urre nce being in the eastern -o utskirts of Redhill (the ' E . of Redstone Hill' sec tio n) . Tho ugh fig ured (Gossling , 1929 , fig . 33) as occurri ng at the base of the Sandgate Formation ('silty greensand and layers of stone' ), the unit is ass igned in the text to the Hythe Bed s (p. 224: 'flaggy spicular cherty sandstone bands separated by glauco nitic silt' ). Further to the we st , in the Reigate area, Goss ling showed the Sandga te Formation resting directl y on the Up per Hyth e Pebble Bed (the ' nodular sa ndsto ne ' of thi s study ). Le ighton (1895, figs 1-6) showed a slightly different picture , with the cherts being absent on Redh ill Co mmon, but pre sent (c . 1- 5 m) at Bell Stree t, Reigate . It th us seems like ly that the westward

179

pinch -ou t of the cherty sandsto nes may be more complex than that shown in Fig . 6, whic h is based on Gossl ing 's sections . In his review of Gossling's unpu blished notes , Kirkal dy (1947: 186 ) described the cherty sands tones as thicke ning eas twa rds from abo ut 2 m at Redhill to abou t 10 m at Tiibu rstowhill Com mo n, so uth of Godsto ne . A thick section of cherty sa nds tones and cherts was also reported by Le ighton ( 1895 : 11 8).

Sandgate Formation From the accounts of Go ssling (1929) and D ines & Edmunds (1933 ) it app ea rs that Nutfield Beds are of more rest ricted distrib utio n than the Redh ill Sa nds . Typical Nut field Beds were reported to extend eas twa rds as far as Tilburstow Hill , so uth of Godstone, but are now kn own to extend into the Tandridg e area. Borehole records (he ld by BGS) from the Ox ted-Li mpsfield area are old and of poor quality, but appear to show gla uco nitic Redh ill San ds resting on sandstones of the Hyt he Formatio n. T his relationship is co nfirmed in the M oorh ouse Borehole , some 7 km eas t of Tilbu rstow Hill , where sandstones of the Hythe Formation are ove rlai n by glaucon itic, clay-rich Redhill Sands (Fig . 6). A com parable relations hip appears to occur in the west of the area, as illu strated by Go ssling ( 1929 , fig . 33) . T here . the typica l Nu tfie ld Bed s are shown as disappearing be tween the Ch art Lane and Park Lane sec tions in Reigate , aga in apparently through overstep by the Red hill Sands . A possible occ urren ce of 1.5 m of Nu tfield Beds at the base of the Park Lane Sa ndga te Formation sec tion is suggested by the fig ure annotation 'g lauconitic silt wit h thin lami nae of impure fuller 's earth ' (Gossling , 1929 , fig . 33) . Howe ver, the de scription in the text, of highly glauconi tic , 'yellowish clayey and silty fine sand .... stria ted wit h thin lenticular laminae of white clay ' leaves little do ubt that the se bed s sho uld be assigned to the Redhill Sands . It is interesting to note that the base of the San dgate Fo rma tion is here marked by a thin bed of ' ha rd red -brown rock co mposed of wavy lent icular laminae of clay or silt and fine sand , containing scattered quartz and lydite pebbl es , the whole being firmly ce mented by limonite'. Gossling stated that this bed had bee n obse rved as far away as Nut field , indicating that it may corre late wit h the bed of black mudston e observe d beneath the Redh ill Sands in the Glebe Far m section. Th e latter is hig hly pyritic , and simila rly altered to a limo nitic iron pan in places . A co mparable bed occ urs at the base of the Redh ill Sands in the Bletchingley section. Uni t I of the Redh ill Sa nds appears to occur througho ut the area, excep t in the far east (Moorhouse Borehole), where it is cut out by the m uddy , pebbly sand sto nes of Unit 2. Uni t 2 is traceable in boreholes and qu arry sections as far wes t as the Patteson Co urt quarry, where is cut out to the northwest by eros ion at the base of the Folkestone Beds . Th is erosion is clearly related to the development of the Fol keston e For mation Basal Sa nds . Th e same explanation

R. W. O'B. KNOX ET AL.

180

probably applies to the absence of Redhill Sands Unit 2 further west, where the Basal [Pebbly 1 Sands occur.

probable that the relatively thick pebbly sandstones recorded by Kirkaldy (1947) to the east of Nutfield represent a lithofacies change within the Silver Sands.

Folkestone Formation The observation that the distinctive Basal Sands are restricted to the western part of the Patteson Court section is compatible with the records by Gossling (1929) of his Basal Pebbly Sands in the Reigate-Redhill area. Kirkaldy (1947: 189) states that the basal pebbly facies extends eastwards as far as Moorhouse and more than doubles its thickness. The present study suggests, however, that the Basal Sands are absent in the Nutfield area. Furthermore, the preservation of the pebbly, muddy sands of Redhill Sands Unit 2 in the Moorhouse Borehole argues against any significant development of the Basal Sands in that area. It is therefore

Subzone

A comprehensive review of the biostratigraphy and regional correlation of the Lower Greensand was provided by Casey (1961). A tentative assignment of the Nutfield succession to Casey's (1961) biozonal scheme is shown in Fig. 7, and is discussed below. Also shown is the revised late Aptian ammonite zonal scheme that was published in outline by Ruffell & Owen (1995), but which has since been refuted by Casey (1996). In the context of this study, the significant difference between the two schemes is that whereas Casey recognized two subzones (Parahoplites cunningtoni and

Candidate

Casey 1961

RufTcll & Owen 1995 Zone

7. BIOSTRATIGRAPHIC CONTEXT

sequence

Lithostratigraphy (Rcdhill area) Zone

boundaries

Subzone Silver Sands

FOLKESTONE FORMATION (part)

rubricosus

jacobi

Basal Sands

SB3

jacobi

nodocostatum

(part)

nolani

nolani Unit 2

REDHILL SANDS

SB2a

I I ~

cunningtoni

Unit 1

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la

nutfieldiensis nutfieldiensis subarcticum ,/

martinioides

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SANDGATE FORMATION

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gracile

martinioides

debile

bowerbanki (part)

meyendorffi

SB2

SBI HYTHE FORMATION (part)

gracile

debile

bowerbanki (part)

meyendorffi

Mid Hythe Sands

Fig. 7. Stratigraphic summary for the Sandgate Beds of the Redhill-Nutfield area. Datum = base Silver Sands (Folkestone Formation). Zonal assignments for the lithostratigraphic units are based on those of Casey (1961). Equivalent assignments using the proposed scheme published by Ruffell & Owen (1995) are also shown. It should be noted that ammonites have been recovered only from the Nutfield Beds (subarctic 11m Subzone) and that the zonal assignment of other units is uncertain, as are the timing and duration of hiatuses. Candidate sequence boundaries are indicated by SB numbers.

ST R AT I G R APHY OF TH E LOWE R G R E E N S A N D, SU R R E Y

Tropaeum subarcticumi within a Parahoplites nutfieldiensis Zo ne . Ruffell & Owen recogni ze onl y an undi vided Parahoplites nutfieldiensis Subzone .

Hythe Formation The top-Hythe cherty sandstones were assigned by Casey (1961: 548) to the Cheloniceras martinioides Zone and the Upper Hythe Pebble Bed to the Tropaeum bowerbanki Zon e. Since the cherty sandstones ha ve yie lded no ammonite fauna, their assignment was based on correlation with the amm on ite-bearing cherts of West Kent (Casey. 1961: 550 ). Zonal assignment of Upper Hythe Pebble Bed was based on a reco rd of the Lowe r Aptian ge nus Cheloniceras . The absence of the cherty sandstones in the Redhill-Reigate area was ascribed by Casey (1961: 550) to a hiatus at the base of the Sandgate Form ation . The observations of this study indica te , howev er, that any significant hiatus most probably occurs below the topHythe cherty sandstones and that the local absence of the cherty sand stones is due to non-d eposition rather than erosion. In the Bletch ingley section, the hiatus would be placed no higher than the base of the thin pebble-bed that ove rlies the nodular sandstones . If the pebbl y sandstones at the base of the Bletch ingley section equate with the Upper Hythe Pebbl e . as proposed above , then the hiatus may be even lower in the section. at the base of the pebbl y sandstones. It may be noted that the Cheloni ceras recorded from the Upper Hythe Pebbl e Bed at Reigate could well have been winnowed from the underlying Mid-Hyth e Sands (firmly dated as bowerbanki Zone) . since evidence for a remanie fauna is provided by the additional occurrence of fragments of Oxfordian ammonites (Wells & Go ssling, 1947: 2 13). Th is interpretation would suggest that the Upper Hythe Pebble Bed and the basal pebbl y sandstones of this study are no older than the martinioides Zone. An even yo unger inutfieldiens is Zone) age is possible. but this would require that the cherty sandstones are yo unger than those of Kent. In the absence of positive evidence to the contrary. a martinioid es Zone age is proposed here .

Sandgate Formation Casey (196 1) recognized four lithostratigra phic units in the Sandgate Form ation of the northern Weald . He identified two unnamed units in the Nutfie ld area: a lower unit of cherty and glauconitic sandstone and lime stone with bands of fuller 's earth, and an upper unit of glauconitic 'l oamy' sandstones. Th ese are clearly equivalent to the Nutfield Beds and the Redh ill Sa nds . respectively. as defined here. Further west, beyond Dorking , the Sandgate Formation was divided into a lower unit of pebbly sandstone with ' bands and doggers of calcareous stone ' (Bargate Beds) and an upper unit of ferr uginous loam s (Puttenham Beds). Casey (1961) recorded the am monites Parah oplites nutfi eldiensis and Tropae um subarcticum from the ' fuller 's earth beds' (Nutfield Beds). on which basis he assigned these beds to the Tropaeum subarcticum Subzone of the

181

Parahoplites nu tfieldiensis Zon e. He co nsider ed the ' glauconitic loam y sandstones' (Redhill Sands . Unit I) to be equ ivalent to the Puttenham Beds on litho logical grounds, both consisting dom inantly of soft, clay-rich, ' loamy' glau conitic sandstones. Consequently, although the only amm onites record ed from this level occur in a single bed within the Puttenh am Beds of the western Weald, both the Puttenh am Beds and Redh ill Sand s were assigned to the Parahoplites cunningtoni Subzone of the nutfieldiensis Zo ne . These correlations were subsequently adopted by Rawson . Curry. Dilley, Hancock. Kenned y. Neale. Wood & Worssam ( 1978. fig. 3) in their review of the Cretaceous stratigraphy of the British Isles. The black muddy sandstones and siltstones of Redhill Sands Unit 2 were not reco rded by Casey (196 1), but their occurrence at the top of the Sand gate Formation suggest s equ ivalence with the Marehill Clay of Sussex , which is of comparable lithofacies. According to Casey 's schem e , this would place Unit 2 of the Redhill Sands in the Nolaniceras nolani Subzone of the Hypacanthopl ites ja cobi Zone .

Folkestone Formation In view of the clear evidence for a substantial hiatus at the base of the Folkestone Form ation . it seems likely that the basal beds of the Folkestone Form ation are no older than the Hypacanthoplites rubricosus Subzone of the j acobi Zone.

8. DEPO SITIONAL HISTORY Several new observations have been made during the course of this study that throw light on the depositional history of the Lower Greensand deposits of the Nutfie ld area . The medium to co arse, pebb ly sandstones (Hythe Format ion) exposed in the base of the B1etchingley section (Fig. 3) can be attrib uted to intermittent sedimentation under a tidal regime . Per iods of stronger current act ivity. represented by cross-bedded, peb bly medium to coarse sandstone, alterna ted with period s of relative quiescence , repr esented by horizontally bedd ed, bioturbated medium sandstone . Palaeo curr ent directions are consistent with those recorded by Narayan (1963. 1971) from the Hythe Beds. The overlying sandsto nes with phosphate nodules are here interpreted as rep resentin g a period of reduced sedimen tation rate, with acc umulation of hor izontally bedded fine sands separated by periods of non-deposition , burrowi ng and phosphate nodule formation. Following a break in sedimentation. represented by a pebbly, glauconite-rich sandstone bed. deposition resumed with the accumul ation of clean , spiculitic sand (top-Hythe cherty sandstones) on a sea-bed subject to more or less continuous but gentle tidal and/or wave-indu ced currents. The upward change from cherty sandstone to the mudd y sandstone of the Nutfield Beds is interpreted as repre sentin g an increased rate of net sedimentation during a period of relatively high sea-level stand, with low-ene rgy bottom conditions allowing accumulation of mud layers dur ing

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periods of reduced current or wave activity. The remnants of primary bedding seen in the Bletchingley section suggest that the deposit may have originally consisted of a finescale alternation of mud and sand, possibly reflecting deposition under tidal influence. Modification of the sediment surface by wave action is indicated by the presence of symmetrical ripples in some of the fuller's earth beds and by the presence of local storm scours. The conditions prevailing during deposition of the Nutfield Beds were clearly favourable for the accumulation of volcanic ash, now represented by fuller's earth. The Redhill Sands represent a return to somewhat higher energy conditions in the Nutfield area, but with periods of low current or wave activity allowing intermittent mud deposition. The large-scale cross-stratification in Unit Ibis interpreted as representing migration of a major, tidally driven sandwave system, comparable to the complex sandwave facies B3 of Nio (1976). Much of the large-scale cross-bedding is associated with broad erosional troughs, indicating that the sandwave consisted of a complex association of northwestward-migrating linguoid dunes. The sharp truncation of the cross-bedded unit by the horizontally bedded sandstones of Unit Ic, which are of similar lithology, probably represents reworking of the upper surface of the sandwave deposits during storm phases. The origin of the complex, erosionally based, succession of pebbly, muddy, and bioturbated sandstones of Unit 2 is uncertain. They probably represent the eroded remnants of a transgressive unit, with deposition having taken place under conditions of restricted bottom-water circulation and limited sand supply. These muddy sediments are essentially similar to those occurring at the base of and within the Sandrock of Isle of Wight and also to the Marehill Clay of Sussex (Casey, 1961). The muddy beds of the former have been interpreted as transgressive estuary fills (Wach, 1991). The cross-bedded sandstones of the Folkestone Formation mark a return to more continuous current activity, with substantial scouring of the underlying deposits being followed by the deposition of more or less mud-free sands. The southeastward cross-bedding direction recorded in the Silver Sands is consistent with the regional pattern established by Narayan (1963, 1971), but the northwestward cross-bedding direction recorded in the Basal Sands is anomalous. The upward switch in trend from northwestward to southeastward cross-bedding is comparable to that recorded by Johnson & Levell (1980, 1995) from the Woburn Sands, in which the cross-bedding shows an upward change from dominantly northwestward to dominantly southward and southwestward directions. These authors attributed the change in cross-bedding direction as reflecting a change from deposition under channelized flood-tide flow to deposition under less confined ebb-tide flow, rather than to any fundamental change in the tidal regime itself. A similar interpretation for the change in cross-bedding direction observed in this study is supported by associated change from channelized facies (Basal Sands) to non-channelized facies (Silver Sands).

9. SEQUENCE STRATIGRAPHY In a study of such restricted areal extent it is not possible to carry out a comprehensive sequence stratigraphic analysis. However, from observations made at outcrop (Fig. 3), coupled with inferred stratigraphic relationships (Figs 5 & 6), it is evident that the Lower Greensand succession encountered in this study consists of four major lithofacies units, separated by successive discontinuities. An additional discontinuity appears to be present within the Redhill Sands (base of Unit 2). All of these discontinuities appear to be erosional, but only the base of the Redhill Sands displays angular unconformity and can therefore be described with confidence as a sequence boundary. We have therefore placed emphasis on the identification of candidate sequence boundaries (following Hesselbo, Coe & Jenkyns, 1990), whose true significance may be revealed only in relation to a broader stratigraphic assessment. Five such candidate sequence boundaries have been identified. Three of these (SB I, SB2 and SB3) are identified with confidence; the other two (SB la and SB2a) are weaker candidates and, if correctly identified, probably represent sequence boundaries of relatively low order. These candidate sequence boundaries (see Figs 6 and 7) are discussed below in their lithostratigraphic context. Hythe Formation The uppermost Hythe Formation appears to display a more or less consistent stratigraphy across the .region, with a topmost division (Bed 5) of spiculitic, cherty sandstone underlain by beds of nodular, bioturbated sandstone. These beds appear to constitute part of a gross upward-fining succession since they are underlain at Bletchingley by medium to coarse, pebbly sandstones which are believed to be equivalent to the Upper Hythe Pebble Bed of the Reigate-Redhill area (see above, p. 179). Significant erosion at the base of the Upper Hythe Pebble Bed is evident from the occurrence of derived Oxfordian ammonites (Wells & Gossling, 1947: 213) and their common basal surface is therefore a strong candidate for a sequence boundary (SB1). This equates with the sequence boundary that was tentatively related by Ruffell & Wach (1991: 349) to the 109.5 Ma sequence boundary of Haq, Hardenbol & Vail (1988). In view of the lateral persistence of the nodular sandstones at the top of Bed 4 it seems unlikely that the overlying thin pebbly sandstone recorded at Bletchingely represents a substantial hiatus, but the presence of abundant glauconite suggests that it represents more than a local storm layer. It is therefore identified as a candidate sequence boundary (SBla). Nutfield Beds The base of the Nutfield Beds appears to be conformable on the cherty sandstones of Bed 5, there being no pebbly development recorded at the boundary. Shallow scours, filled with calcite-cemented sandstone, are eroded into the

STRATIGRAPHY OF THE LOWER GREENSAND, SURREY

top of the cherty sandstone unit, but almost certainly represent no more than storm wave action. The overlap of the cherty sandstones by the Nutfield Beds in the Redhil! area (see Fig. 6) is provisionally ascribed to restricted deposition of the spiculitic sediments rather than to subsequent erosion. The sharp upward decrease in grain size at the base of the Nutfield Beds, together with the extension of the area of deposition, is interpreted as reflecting deepening in response to continued sea-level rise. The base of the Nutfield Beds is therefore interpreted as a flooding surface, and the Nutfield Beds themselves as possible highstand deposits. The Nutfield Beds are thus interpreted as part of the same long-term transgressive succession as the underlying upper Hythe Formation.

Redhill Sands The base of the Redhil! Sands is a strong candidate for a sequence boundary (SB2), being marked by an abrupt change in facies and by probable truncation of the underlying Nutfield Beds towards the margins of the Nutfield Basin (see Fig. 6). The existence of a minor sequence boundary at this level was suggested by Ruffell & Wach (1991: 351). Interpretation of individual elements of the Redhil! Sands succession is less straightforward. The thin, but apparently widespread, mudstone at the base of Unit I is believed be a transgressive deposit, with the overlying sandstones of Units Ia and Ib representing the migration of a substantial submarine bar during the highstand phase. also of the overlap of Unit I by Unit 2 in the east of the area (Fig. 6: Moorhouse Borehole). The bar would have developed an upper surface close to wave-base, and the horizontally bedded sandstones at the top (Unit I c) are interpreted as the product of storm reworking. The erosional contact at the base of the poorly sorted, granule-rich muddy sandstone of Unit 2 has a local relief of c. 0.45 m in the Patteson Court section. If the Unit Ic sands originally had comparable thickness in the Patteson Court and Glebe Farm successions (as do Units la and I b), then the thinning of Unit Ic from a maximum of 1.7 m in the Patteson Court sections to 0.33 m in the Glebe Farm section implies more substantial erosion (1.37 m), which could be accounted for by a slight angular unconformity. An unconformity at this level could also account for the absence of Unit I in the eastern margin of the Nutfield Basin. The base of Unit 2 may therefore be considered as a candidate sequence boundary (SB2a).

Folkestone Formation The presence of a sequence boundary (SB3) at the base of the Folkestone Formation is clearly indicated by erosion of the underlying Redhill Sands. It is possible that the more restricted Basal Sands represent a sediment-starved lowstand phase, with the overlying Silver Sands representing regional aggradation during the transgressive and highstand phases. If so, the base of the thin transition

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unit that displays the switch from northwestward (floodcurrent) to southeastward (ebb-current) cross-bedding would represent a flooding surface. This sequence boundary was not recognized by Ruffell & Wach (1991: 349, 351) who considered that the base of the Silver Sands represented the transgressive surface to the intranutfieldiensis sequence boundary (SB2a of this study).

Sequence stratigraphic summary The Lower Greensand succession examined in this study provides evience for five candidate sequence boundaries. It is clear that the most profound and sustained facies changes are associated with SBI, SB2 and SB3, from which it may be assumed that these are of greater magnitude than SB Ia and SB2a. Unfortunately, because of the scarcity of precise biostratigraphic data, it is not possible to assess the relative timing of these boundaries. The most likely means of obtaining greater chronostratigraphic control on the observed sequences is through correlation with well dated successions in the same general area. For the Lower Greensand of the Weald, the best such successions occur in the Folkestone area, for which a sequence stratigraphic analysis has been carried out by Hesselbo et al. (1990). They identified a series of candidate sequence boundaries within the early Aptian to early Albian interval, based on the identification of surfaces of hiatus and condensation. One of these, LG2, led to erosion sediments of the bowerbanki Zone with the martinioides Zone being represented only by reduced or possibly completely reworked sediments of the uppermost subszone (buxtorffi Subzone). Significant renewed sedimentation did not take place until the nutfieldiensis Zone. This succession clearly has parallels with that of the Nutfield area, with similar ambiguity as to the precise timing of the main erosional event. LG2 may therefore be equated with SB I of this study or possibly an amalgamation ofLGl and LGla. Unfortunately, as pointed out by Hesselbo et al. (1990: 557): 'the succession at Folkestone gives a poor record of the nutfieldiensis-jacobi and forbesi-deshayesi Zone sediments. Evidently important changes in sedimentary facies occurred in these intervals, e.g. the fairly abrupt transition from Sandgate to Folkestone Beds. Thus the possibility remains for additional sequence boundaries in the Wessex Basin in these intervals, perhaps conformable, which are not indicated on the current global sea-level cycle chart' . The present study has borne out the prediction of additional sequence boundaries within the nutjieidiensisjacobi interval, and provides further testimony to the complexity of inferred relative sea-level change in the late Aptian of the Weald area.

10. CONCLUSIONS The Lower Greensand of the Nutfield region appears to provide one of the most complete successions in the

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northern Weald area, largely through the local expansion of the Sandgate Beds the Nutfield Basin. Detailed lithological logging and sedimentological analysis of the succession from the uppermost Hythe Formation to the base of the Folkestone has confirmed the existence of two lithostratigraphic units within the Sandgate Formation - the Nutfield Beds and the Redhill Sands. Important information has also been gained on the nature of the lithofacies boundaries. The base of the Folkestone Formation and the base of the Redhill Sands are strongly erosional and mark substantial, sustained changes in depositional setting. Both are identified as candidate sequence boundaries. The base of the Sandgate Formation appears to be conformable, and the Nutfield Beds are regarded as related to a sequence cycle whose base is defined by a sequence boundary within the upper part of the Hythe Beds, probably at the base of Upper Hythe Pebble Bed (Bed 4). Additional candidate sequence boundaries are identified within the uppermost Hythe Formation and within the Redhill Sands. Varying degrees of erosion associated with each of the candidate sequence boundaries has resulted in the variable preservation of the three lithostratigraphic units within the Sandgate Formation. The Nutfield Beds are confined to more central parts of the basin, while Redhill Sands Unit I alone is preserved on the western margin of the Nutfield Basin and Redhill Sands Unit 2 alone is preserved on the eatern margin. While not resolving any of the current uncertainties over regional correlation in the middle part of the of the Lower Greensand, the results of this study should place such discussions on a firmer footing, both in the provision of new descriptive data and in the identification of a series of

disconformable sedimentary packages within the major mapping units. An appreciation of this stratigraphic complexity is essential in interpreting facies variation in the Sandgate Formation succession elsewhere in the region, since what might appear to be lateral facies change within a single depositional unit may, in reality, reflect differential stratigraphic preservation within apparently continuous formations. Clarification of the facies relationships within the Sandgate Formation ofthe Nutfield area also allows a better understanding of what factors controlled the distribution of the fuller's earth deposits. These deposits are almost exclusively confined to the Nutfield Beds, with the thickest seams occurring within central parts of the Nutfield Basin. It would appear that the relatively low energy environments associated with deposition of the Nutfield Beds were conducive to the accumulation of volcanic ash with a minimum of detrital contamination, and that the greater subsidence in central parts of the Nutfield Basin favoured the accumulation of relatively thick ash deposits and their preservation from subsequent storm erosion.

ACKNOWLEDGEMENTS The authors are indebted to Laporte Earths Ltd for allowing access to their quarries and for the use of their borehole records. They also thank Roy Shephard-Thorn for his advice on Lower Greensand stratigraphy and Roger Bristow, Steven Hesselbo, Peter Rawson and Mike Sumbler for their constructive reviews of an early draft of this paper. R.W.O'B.K. and DJ.H. publish with the approval of the Director, British Geological Survey (NERC).

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Manuscript received 17 March 1997; revised typescript accepted 16 March 1998