Sequence stratigraphic analysis of the Aptian-Albian Lower Greensand in southern England

Sequence stratigraphic analysis of the Aptian-Albian Lower Greensand in southern England A. H. Ruffell* Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, UK

and G. D. Wach Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK

Received 5 May 1990; revised 22 August 1990; accepted 22 September 1990 Facies analysis and its use in sequence stratigraphy is described using the Lower Greensand of southern England as an example, and the resulting interpretations are outlined. This work forms an independent test of sequence stratigraphic techniques and the Exxon 'cycle chart'. Sequence stratigraphy is a very powerful tool in the analysis of problematic successions, although some modification is made here in emphasizing the use of the transgressive surface. Changing palaeoenvironmental conditions, and the effect that palaeoceanography has on the recognition of sequence boundaries, are also discussed. Broad agreement is found between these results and the cycle chart, although changing tidal conditions alter the criteria available for analysis of the Aptian-Albian Lower Greensand succession. Comparison between this study and sequence stratigraphic analyses conducted in the more condensed type sections of England and France shows that further sequence boundaries are recognizable. Keywords: Lower Greensand, southern UK; sequence stratigraphy; facies analysis

Introduction Given the existence of an ammonite biostratigraphy in many areas of the Lower Greensand outcrop, the age of the major unconformity bounded units is well constrained, and the group is thus an ideal succession with which to test both the 'Exxon chart' (Haq et al., 1988) and sequence stratigraphic methodology (Posamentier et al., 1988) against conventional facies analysis. Use of the Aptian-Albian eustatic models produced both by Haq et al. (1988) and Cooper (1977) is valid because southern England was not utilized in the construction of the Aptian portion of the Haq et al. (1988) chart, and was used in only 1 of 17 localities by Cooper (1977). Although the Lower Greensand of southern England can be considered as 'virgin' territory for comparisons with the cycle chart of Haq et al. (1988), some degree of similarity can be expected, because the Exxon workers utilized outcrop sections in southern France and Mexico for the Aptian, and the Boulonnais for the Albian. Such areas were proximal to southern England, or shared a common seaway on the margins of the rifted Atlantic Ocean. The Lower Greensand Group (Aptian-Albian) of southern England contains a variety of facies representative of coastal plain, lagoonal and shelf sediments. These deposits represent a major marine transgression ending a 40 million year period of predominantly non-marine deposition characterized by the Wealden facies. The Lower Greensand Group occurs in the Mesozoic *Present address: Department of Geology, Queen's University of Belfast, Belfast BT7 INN, UK 0264-8172/91/030341-13 ©1991 Butterworth-Heinernann Ltd

Wessex Basin (Whittaker, 1985) of southern England, being the most fully developed in both time and thickness on the Isle of Wight in the Channel sub-basin, and in the Weald itself. Thin, condensed deposits of the Lower Aptian (Casey, 1961) and transgressive deposits of the Upper Aptian occur in the basin margins to the north and west of the main Wessex Basin in a series of minor basins from Cambridge to Oxford and thence to Swindon and Shaftesbury (Figure 1). Similar facies to those developed in the Lower Greensand Group can be found in adjacent areas such as the Paris Basin, and in many of the ofshore basins to the west of the UK, such as the Celtic Seas, Western Approaches Trough, Porcupine Basin (west of Ireland) and ShetlandFaroes Trough. Owing to the predominance of bioturbated shelf facies in the Lower Greensand, gross vertical and lateral facies variations can be difficult to discern in outcrop. However, the formation subdivisions of the group demonstrate mappable variations which serve to divide the group into the depositional sequences utilized in this study (see under Methodology). The limited areal extent of some of these sequences and the internal facies variations are some of the reasons why stratigraphic terminology varies between the Isle of Wight, the Weald and the northern outcrops. A simplified correlation of such divisions is shown in Figure 2. As a result of its fossil content and accessibility, the Lower Greensand Group has attracted a long and complex history of research, details of which can be found in Casey (1961), Middlemiss (1975) and Ruffell (1989a). The work of Casey (1961) on the

Marine and Petroleum Geology, 1991, Vol 8, August

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Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach description given under Results. Greater credence can generally be given to the sequence boundaries found throughout the basin than to those identifiable at only a few localities. This methodology has been used on a basin-wide 1 I o scale, in a similar manner to some of the seismic based studies of Bally (1987), and thus extends methods comparing a single section or small area to eustatic Potion models (such as Hesselbo et al., 1990), as areally restricted breaks in the succession can be given less CLophiH status than widespread unconformities (Figure 4). The field description of Lower Greensand sequences (bounded by unconformities or correlative conformities) has been compared to the results gained by the construction of two sediment distribution diagrams (chronostratigraphic charts or Wheeler diagrams after Wheeler, 1954) showing the temporal Sevenoaks Hog's Reigofe Redhi[l Noidsfone and spatial distribution of sediments and the variation Book ISurre ) in the type of sediments dated (i.e. condensed remani6 V/esfern Weold n or in situ sediment). Wheeler diagrams sensu stricto IHompshwe) show facies variations that have not been included in Figures 4 and 5 as much of the Lower Greensand occurs Sussex as shallow shelf deposits. The exceptions, such as lagoonal facies in the Mid-Aptian sediments of Isle Maidstone, are well known historically, and will be described in detail elsewhere (Ruffell et al., in Figure 1 Locality map showing outcrop successions discussed preparation). in text Upwore .....

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palaeontology of the group remains the most important published work; his recognition of two major stratigraphical breaks in the Early Aptian (fissicostatus zone) and Middle Aptian (martinioides-nuO~eldiensis zones) formed the basis for Bridges' (1982) summary of the sedimentology of the group. Bridges (1982) highlighted a Late Aptian transgressive phase, which is also recognized here in the jacobi zone.

Methodology The major outcrop and cored borehole sections in southern England were logged and calibrated (where possible) to pre-existing biostratigraphical schemes (Casey, 1961; Bristow et al,, 1987) and the major unconformities were identified by biostratigraphical gaps. These biostratigraphic gaps often coincide with abrupt lithological changes observable in the field. A detailed vertical facies analysis of each unconformity bounded sediment package was undertaken and is summarized in the Results section. Certain sections warranted the use of, or reference to, different analytical techniques to gain more information on problematic lithologies and sequences. Such techniques include palaeoecology (Ruffell, 1988), sedimentology (Allen, 1982; Bridges, 1982), geochemistry (Ruffell, 1990) and clay mineralogical analysis (Jeans et al., 1982). Facies changes within sedimentary packages can therefore be defined, and the subtler sequence stratigraphic patterns suggested to exist by van Wagoner et al. (1988) in the form of parasequence stacking patterns can also be tested. The fine details of the variable parasequences observable in the Lower Greensand outcrops of southern England cannot be easily summarized in a paper of this length. All the relevant lithological logs are included in Ruffell (1989a), and the patterns of each log are utilized in the 342

Results This study shows that the major sequence boundaries of the Haq et al. (1988) chart can be recognized in southern England, although doubts over the biostratigraphical control in the Aptian-Albian boundary beds allows a variety of sequence stratigraphic interpretations. In addition, facies changes in the form of varying tidal influences tend to mask sequence boundaries which are suggested to occur from the positioning of biostratigraphical gaps in localities on the margins of the depositional basin (Figures 4 and 5). These data indicate the existence of three sequence boundaries not shown on the Exxon cycle chart.

Barremian-Aptian Although not part of the Lower Greensand Group, sequence stratigraphic analysis of the Aptian of southern England must begin with an analysis of the Wealden Group below. An examination of this group and its transition into the Lower Greensand must be performed almost entirely on the Isle of Wight, where the only continuous and well exposed sections occur. Most other localities of the Wealden Group away from the Isle of Wight are isolated and temporary exposures (Ruffell, 1989b), or are borehole sections such as the Warlingham Borehole (Worssam and Ivimey-Cook, 1971). On the Isle of Wight, the vertical transition from the fluvial sediments of the Wessex Formation (Stewart, 1978) within the Wealden Group into lagoonal and interdistributary bay deposits of the Vectis Formation (Stewart et al., 1991) is abrupt, and marked by a transgressive surface. Here, lagoonal sediments with brackish and marine fauna overlie mixed fluvial and playa lake sediments (Figures 3 and 7). This facies change is dated as Late Barremian from the

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Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach palynomorphs (Hughes, 1958) and magnetoL o w e r Aptian stratigraphy (Kerth and Hailwood, 1988). Little work has been carried out on Lower Aptian sea The distinctive Barnes High Sandstone Member level changes in southern England; the next (Stewart, 1978) occurs 6-10 m above the base of the transgressive horizon known to exist is the well Vectis Formation, and records the progradation of documented nutfieldiensis zone deposits (Kirkaldy, arenaceous sediments into a predominantly 1937; Casey, 1961; Middlemiss, 1962; Cooper, 1977; argillaceous depositional setting. It could thus Bridges, 1982; Hesselbo et al., 1990). Casey (1961) represent a minor downwards shift in facies, marking a realized that this must be preceded by a regression (his minor sequence boundary at the base of this member 'retrenchment') in the martinioides zone. (Ruffell, 1989a) or within the lower beds. The Below this horizon a good deal of evidence exists for overlying Vectis Formation is an argillaceous lagoonal relative sea level changes throughout southern unit approximately 70 m thick (maximum), containing England. Casey (1961) suggested that there is a many minor upwards-fining cycles averaging 70 cm transgressive surface within the Atherfield Clay, at the thick (Stewart, 1978; Ruffell, 1988). The palaeoecology horizon represented by the callidiscus subzone. This of the Vectis Formation records a gradual transition evidence is mainly based on records of borings in the into marine conditions preceding the Perna Beds of the Kent Coalfield, although these were thought to show Lower Greensand (Arkell, 1947; Casey, 1961; Ruffell, callidiscus subzone deposits unconformably overlying 1988). The Perna Beds Member of Simpson (1985) at the Weald Clay; some doubt has been cast on this the type locality (Atherfield, Isle of Wight) comprises a interpretation by Simpson (1985) who is cited by thin bone bed (10-40 cm) overlain by 1.5 m of shelly Hesselbo et al. (1990). The present study has found that sandstone. From facies considerations this appears to a widespread 'mid-Atherfield Clay unconformity' can be the acme in the increasing salinities observed be found in the cored boreholes of the South Downs throughout the Vectis Formation beneath. The lack of (Bristow et al., 1987). A similar break is observable a regressive facies or fauna (Ruffell, 1988), time gap in the Warlingham Borehole (Worssam and (Kerth and Hailwood, 1988) or truncation (Casey, Ivimey-Cooke, 1971; Ruffell and Batten, 1990). From 1961) associated with the Perna Beds belies the fact the detailed biostratigraphy of Casey (1961) this break that this major transgressive surface throughout is suggested to be at the same horizon as the Punfield southern England is the largest facies change recorded Marine Band exposed at Swanage, Dorset (Arkell, in the time slot covered on the chart of Haq et al. (1988) 1947), and the distinctive Crackers sands of the Isle of by a Type 1 sequence boundary, a point made by Wight (Simpson, 1985; see Figure 3). Hesselbo et al. (1990). This event is independently The Crackers Member of the Isle of Wight consists of recorded in the North Sea by Rawson and Riley (1982). calcareous cemented concretionary horizons in an 8 m The possibilities therefore exist that (1) the thick upwards-coarsening sand body dividing the conventional facies indicators are not proving to be Upper and Lower Lobster Beds of the Atherfield Clay reliable in aiding the identification of this sequence Formation (Simpson, 1985). This may represent a boundary, or (2) the sequence boundary does not exist. regressive sand, as evidenced by the influx of estuarine The latter can be effectively discounted from a bivalves common to the Punfield Marine Band, also consideration of the palaeontological work of Casey seen some 24 km to the west. On the Isle of Wight, the (1961), the sedimentological analysis of Bridges (1982) Crackers sands are abruptly overlain by blue clays of and the sequence stratigraphic analysis of the the Upper Lobster Beds, marking a return to fully Folkestone area by Hesselbo et al. (1990). The marine Atherfield Clay-style deposition. This horizon sequence stratigraphic analysis of the Vectis Formation forms the best candidate transgressive surface observed to Lower Greensand transition avoids these issues by at outcrop with which to correlate the onlapping interpreting the Perna Beds as a transgressive surface surface of the Atherfield Clay in the South Downs and resting directly on the sequence boundary, here only Kent Coalfield. Remani6 ammonites of this age are also represented by a ravinement surface recorded in the recorded "from the north of the London Platform, bone bed. The Type 1 sequence boundary shown at this indicating the widespread nature of the transgression level on the Haq et al. (1988) chart would, according to (Figure 4). the Exxon Group, have exposed the entire shelf to A number of transgressive-regressive cycles are erosion, the only evidence of which is found here as a evident in the Lower Aptian Ferruginous Sands bone bed. Such bone-rich horizons are common in the Formation of the Isle of Wight (Figure 3) and Hythe Vectis Formation beneath and, using the analogue of Beds of the Weald (Figure 4; Sussex, Surrey, East Norris (1986) on the Pliocene Purisma Formation of Kent). These are intimately associated with the California, these could form by deep water winnowing. development of firmgrounds on the non-depositional There are few other localities in southern England surfaces found at the culmination of where this horizon is observed: beyond the UK coarsening-upwards cycles, as well as influxes of a widespread non-sequence occurs at the smectitic argillaceous sediment, most spectacularly Barremian-Aptian boundary. Elsewhere in rare, developed as the Fuller's earths of the South Downs complete sections, such as the Lower Saxony Basin of (Young and Morgan, 1981). The transgressive horizons Germany, complex depositional changes are recorded recognized from outcrop analysis in the relatively that could be ascribed to a sea level fall (Ruffell and complete sedimentary successions of the Isle of Wight Batten, 1990). A lack of ammonites beneath the Perna and Weald can be correlated with subzonal ammonite Beds in southern England makes the quantification of ages documented from remani6 faunas found at the the time gap difficult. If the magnetostratigraphy base of the Upper Aptian along the South Downs (Kerth and Hailwood, 1988) is correct, very little time (Casey, 1961) and in Cambridgeshire and Norfolk is represented by the Perna Beds erosion surface. (Figures 4 and 5). Marine and Petroleum Geology, 1991, Vol 8, August

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Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach In the Isle of Wight the interpretation of around Maidstone (Boughton Beds) is usually no shallowing-upwards cycles (Ruffeli, 1989a; Wach and younger than the martinioides zone, most deposits Ruffell, 1989) suggests that the pattern and hierarchy of lacking good biostratigraphical evidence. In the Isle of cycles varies in relation to relative sea level (Figure 4). Wight, the most rapid regression is interpreted to occur Coarse horizons with abundant plant debris are in the topmost subzone (buxtorfi), based on the suggestive of proximity to source, whereas argillaceous assumption of Casey (1961) for the age of this estuarine beds with ammonites indicate removal from source. member (Figure 3). Here the Foliated Clay and Sands The predominance of each lithofacies has been taken to (First Sandrock on Figure 3) record the first indicate relative low- and highstands of the sea. The development of a facies found extensively in the main deepening events are identified as the Aptian-Albian transition beds above, typical of the Australiceras gigas bed of the grandis subzone, the Sandrock. This is a nearshore, shallow shelf or Lower Crioceras Beds (transitoria subzone) and the estuarine sand with rare molluscan indicators of a Walpen Clay and Sand (meyendorffi subzone). In the regressive phase (Ruffell, 1989a) and tidal sedimentary Weald, transgressions in the meyendorffi and earlier structures (Wach, in preparation). grandis subzones resulted in the deposition of smectitic The only indications of a sub- or intra-nu~eldiensis mudstones (commonly with the belemnite (Figure 4) unconformity that can be dated are at Neohibolites). These horizons onlap structural 'highs' Folkestone, around Maidstone and in the Fuller's earth of Atherfield or Weald Clay, with hiatus concretions at pits of Redhill (Nutfield itself). As these localities are the base. The prediction from facies analysis of described in detail by Ruffell et al. (in preparation), transgressive phases is therefore confirmed by only a summary is given here and their stratigraphic comparison with results derived from the plotting of relationships are shown in Figure 6. time-distribution diagrams of derived and in situ The abrupt junction between the Hythe and ammonites (Casey, 1961; and Figure 4). Sandgate Beds in the Mid-Aptian Lower Greensand of Folkestone has been described by Fitton (1836), Casey (1961), Hesselbo et al. (1990) and Ruffell (1990). Casey Upper Aptian (1961) demonstrated that the basal conglomerate of the The regressive nature of the martinioides zone has been Sandgate Beds included a variety of clasts, some of discussed in the preceding section. Detailed facies which included ammonites of a late martinioides age analysis in the Isle of Wight and around Maidstone (buxtorfi subzone). The occurrence of these ammonites suggests that the zone contains a series of transgressive is unique in southern England, and led Casey (1961) to and regressive cycles (Ruffell, 1989a) shown by the suggest that the basal Sandgate Beds at Folkestone alternation between fully marine (ammonite-bearing) were of martinioides zone age. Elsewhere in southern beds and those bearing abundant plant debris and England, the Sandgate Beds were presumed by Casey terrestrial reptile bones. The martinioides zone begins (1961) to be nutfieldiensis zone deposits. New evidence with a minor transgressive phase in Kent [base of the on the ranges of Parahoplitid and Cheloniceratid Boughton Beds of Casey (1961) ] and a regressive phase ammonites has been collected by H. Owen and M. on the Isle of Wight (base Upper Crioceras Beds Delamette (personal communication), which casts nodules). A transgression occurs in the debile subzone, doubt on the correlation of the nu~eldiensis zone although a lack of dating and exposure makes deposits with the Aptian type sections in southern interpretation hazardous beyond the Isle of Wight, France, and supports the event correlation presented where complex phosphatic nodules occur at the base of here. This indicates that the martinioides zone nodules the Walpen and Ladder Sands (gracile subzone), a at Folkestone can be correlated with the Fuller's earth member typified by 7 - 8 coarsening-upwards cycles. bearing Lower Greensands at Redhill and with the The dating of the influx of tidal sands in the Western Foliated Clay and Sands (Group XII) on the Isle of Weald (Upper Hythe Beds) and of lagoonal deposits Wight. DORKING

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Figure 6 Ribbon diagram of the Mid-Aptian to Early Albian Lower Greensand outcrops and shallow boreholes around the Weald antiform of southern England (see Figure I for localities). The position of Fuller's earths basins and truncation at the base of the Bargate Beds is critical to sequence stratigraphic correlation with the martinioides-nutfieldiensis deposits of the Isle of Wight

348 Marine and Petroleum Geology, 1991, Vol 8, August

Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach margins by erosion; in the ensuing transgressive clays The development of lagoonal sediments in the and silts of the farnhamia band (Casey, 1961) were martinioides zone Hythe Beds around Maidstone has deposited (Figure 5). been well documented from the exceptional fossils Falling sea levels reduced accommodation space contained therein, such as the type Iguanodon (Vail et al., 1984) in Early Albian times and it is only (Mantell, 1834) and unusual plant and vertebrate within areas subsiding more rapidly than the sea remains (Bensted, 1860; 1862). The development of level fall that a clear record of the tardefurcatalagoonal sediments up to 10 m below the Sandgate mammillatum zone depositional changes can be found. Beds suggests that a correlative conformity to the One such place is east Kent, where the Folkestone unconformity seen at Folkestone may be present. To Beds deposition was a little more continuous and sands the east of Maidstone the lagoonal sediments overlying were preserved during the erosive phases that removed marine Sandgate Beds pass into Fuller's earths similar sediment elsewhere in the Weald. The succession of to those recorded 40 km to the west at Redhill (Figure phosphate nodule beds seen at Folkestone can be 6). analysed (Ruffell, 1990), and a detailed sequence The relative sea level appears to have been rising in stratigraphy developed for the area (Hesselbo et al., the late martinioides to early nu~eldiensis zones. The 1990). Such horizons were subsequently removed or input and preservation of some transgressive deposits condensed into a single bed in other areas, e.g. the Iron has been ascribed to tectonism (Casey, 1961; Lake and Grit of Sussex (Anderson, 1986). In the Leighton Shephard-Thorn, 1985), although the combined effects Buzzard area the passage from the Silty Beds to Coarse of sea level rise and accelerated subsidence can increase Red Sands occurred at this time (Casey, 1961). the available space in which to deposit the 40-50 m of The main biostratigraphical evidence concerning the Fuller's earths and pebbly detritus (Bargate Beds) timing and extent of breaks in sedimentation observed in isolated basins around the Weald (Figure throughout the basin is included in Figures 4 and 5. This 6). The tectonic influence is best demonstrated by the is combined with a summary of the major depositional influx of coarse detritus in th.e Bargate Beds of the changes and the possible positioning of sequence Western Weald, which unconformably overlie the boundaries in Figure 7. Fuller's earth beds (Ruffell, 1989a). The Bargate Beds or their equivalents overstep the Hythe Beds, Atherfield Clay and Weald Clay across structural highs within the Weald outcrop. In the subsurface north of Summary of sequence stratigraphy Kent the same pattern occurs, the Sandgate Beds The outcrop description of unconformity-correlative coming to rest (locally) on the Palaeozoic floor of the conformity bounded sequences is limited to a few London Platform (Kirkaldy, 1933), having transgressed studies (Kidwell, 1984; Plint, 1988; Haq et al., 1988; all the Mesozoic formations subcropping below van Wagoner et al., 1990). These studies derive much (Whittaker, 1985). of their data from conventional facies analysis, which is Immediately succeeding the basal nuq~eldiensis used to subdivide the sections into sequences and, in unconformity many areas underwent rapid subsidence. the case of the Exxon Group (Haq et al., 1988; van Deposition was initially similar to that of the Lower Wagoner et al., 1988; Posamentier et al., 1988), focus Aptian, with coarsening-upwards cycles typical of shelf on changing parasequence stacking patterns. conditions away from strong tidal influence. In the The three sequence boundaries of the topmost part of the nutfieldiensis zone (Casey's Barremian-Albian of southern England show good cunningtoni subzone), the interaction of shelf processes correlation with the Exxon cycle chart (Haq et al., characteristic of the Lower Aptian coarsening-upwards 1988). There is some doubt as to the positioning of a cycles and Upper Aptian tidal sand deposition occurs Type 1 sequence boundary at the horizon of the Perna [Pulborough Sandrock, Seend Ironsand, Group XIV Beds (112 Ma sequence boundary), whereas there is on the Isle of Wight (Casey, 1961)]. very good agreement with the top martinioides break at The latest Aptian jacobi zone transgression caused Folkestone and regressions at Maidstone and in the Isle the overstep of tidal sands deposited in a nearshore or of Wight with the 109.5 Ma sequence boundary (Figure estuarine environment across many areas of the Wessex 8). The Exxon 107.5 Ma sequence boundary is Basin. The unfossiliferous and rather uniform nature of probably expressed in southern England as the end of these sands has resulted in this transgression being Folkestone Beds deposition. On the cycle chart this is neglected in the literature, compared to the milletoides subzone in age, whereas at Folkestone the nutfieldiensis zone deposits below (Ruffell and Wignall, suggested horizon is regularis subzone. 1990). Many hydrocarbon exploration boreholes drilled This match between independent facies analysis and on Lower Cretaceous highs in the Wessex Basin the Exxon chart could be held as a vindication of the penetrate thin, white sands of Folkestone latter document's accuracy. However, it is not Beds-Woburn Sands type, of latest Aptian to earliest altogether surprising considering that the Albian age. Outcrop facies analysis predicts that such Aptian-AIbian portion of the chart had its origins in sediments are largely the product of the jacobi zone the type sections of southern France and the transgression. Boulonnais, and utilized the ammonite zonations of southern England (Casey, 1961; Owen, 1984) with the Albian Lower Greensand- Gault transition inherent gaps that they possess. Tidal sand deposition continued in most areas of The combination of facies analysis and sediment southern England until the end of the Aptian, when a distribution diagrams employed in this study shows the brief but significant hiatus at the base of the Albian existence of two maximum flooding surfaces within the tardefurcata zone interrupted Folkestone Beds Lower Aptian not shown on the cycle chart. The 111 deposition. This break is manifested in the basin maximum flooding surface of the cycle chart is probably Marine and Petroleum Geology, 1991, Vol 8, August

349

Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach Sequence stratigraphy

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Figure 7 Sequence stratigraphic summary of the uppermost Wealden and Lower Greensand Groups in the Wessex Basin. (MFS) Maximum flooding surface; (SB) sequence boundary. Changing sedimentary environments and tidal influence shown for comparison 350

M a r i n e a n d P e t r o l e u m G e o l o g y , 1991, Vol 8, A u g u s t

Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach

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recorded in the Mid-Atherfield Clay break discussed above, although the identifcation of this surface in the Isle of Wight section, at or around the Crackers sands, is problematic. Above this horizon three sequence boundaries can be picked in the sediments depicted by the chart as 'highstand systems tract' (Figure 8). These are of too great a magnitude to be classed as fourth order cycles (parasequences). In the Upper Aptian, the nutfieldiensis transgression (Casey, 1961; Bridges, 1982) probably represents the transgressive surface overlying the 109.5 sequence boundary, and is well known historically as the Bargate Beds, the base Group XIII pebble bed at Blackgang on the Isle of Wight, and the Faringdon Sponge Gravels in Berkshire. The middle of the nutfieldiensis zone (base cunningtoni subzone) may form a minor sequence boundary as deposits of this age mark a return to estuarine deposition (Pulborough Sandrock and Seend Ironsand, Ruffell, 1989a) from shelf conditions. It may be this erosive phase that reworked earlier nutfieldiensis Fuller's earths at Calne in Wiltshire (Hesselbo et al., 1990; Ruffell and Wignall, 1990). The succeeding transgressive surface to this sequence boundary is the jacobi transgression discussed above.

Applicability of sequence stratigraphic models The Exxon model of sequence stratigraphy is a very convenient method of organizing facies data and splitting up a sedimentary succession into 'packages' of sediment. The positioning of sequence boundaries in

the Lower Greensand is made very much easier in the Upper Aptian by the deposition of relatively coarse-grained tidal sands. The presence of tidal sands allows more direct comparison with documented examples of sequences and sequence boundaries. Galloway (1989), in his critique of sequence stratigraphy, questions the use of regressive surfaces as the boundaries between related 'sequences'. He preferred to use the transgressive surface as the boundary between genetic sequences as they are more easily dated and correlated. This is very true for the Upper Aptian Lower Greensand in southern England, where regressive sequence boundaries are poorly dated and lie between strongly transgressive and easily recognizable surfaces. Haq et al. (1987) suggest that at the point of maximum flooding on the shelf (the downlap surface), sediment supply to the basins will be reduced and sediment starvation (the condensed section) will occur, while sediment accumulates near to the shore. Einsele (1985, p. 68), in his alternative view of the role that sea level plays in controlling deposition, implied that in times of transgression (marine onlap), the same process occurs in basins with abundant arenaccous sediment supply. In areas of slow or argillaceous sedimentation Einsele (1985) believes that a sea level rise results in the reworking and condensation of the shoreline strata, with thicker stratal successions continuing to accumulate offshore. The conflict in these two models can be resolved via a consideration of the concept of accommodation (Vail et

Marine and Petroleum Geology, 1991,Vol 8, August 351

Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach al., 1984), i.e. the space made for the accumulation of Conclusions sediments by sea level rise and subsidence. In most There are a number of conclusions to be drawn from theoretical situations the subsidence factor is taken to this study. The first is that the depositional style of a be constant (except across the basin margin) but, along basin (e.g. epeiric with little tidal influence or more with the tectonic development of the basin, it can open marine with major tidal influences) can seriously explain the relative applicability of the Exxon, affect the criteria available to be utilized in sequence Galloway (1989) and Einsele (1985) models. In the stratigraphy. In this study the epeiric sea shelf epeiric sea settings of the German Jurassic where sediments of the Lower Aptian manifest breaks in Einsele (1985) developed his model, accommodation is deposition by changing sediment cyclicity and the achieved by slow subsidence rates, with slow sediment preservation of transgressive nodule beds. The tidal input. In the Tertiary strata of the Gulf Coast (passive deposits of the Upper Aptian and Early Albian of continental margin) where Exxon developed their southern England suggest that changes in estuarine and models, sediment supplies and thus mechanical shelf sandwave development record high and lowstands subsidence due to loading are much greater. of the sea (Figure 7). The complicating effects of In the Lower Aptian of southern England (and to the changing oceanographic and climatic influence are same extent the Lower Cretaceous of the North Sea; rarely considered in the methodology of the Exxon Ruffell, 1989a), the Einsele (1985) model is readily workers. applicable, as much of the area occupied an epeiric sea A second conclusion of this study is that taking characterized by modest tides. In this sedimentary differing oceanographic influences into account, the environment the Exxon models can be applied but with sequence stratigraphy of the Aptian-Albian deposits some difficulty (see earlier). The opposite is true of the of southern England shows good correlation with the Upper Aptian, where the parasequence stacking Exxon cycle chart (Haq et al., 1988) in that the three patterns of van Wagoner et al. (1988) are found to be Exxon sequence boundaries all coincide with major diagnostic of systems tracts and sequence boundaries. facies changes in basin depocentres and bioThis analysis suggests that the deposits of the stratigraphic gaps in the basin margins. On detailed Mid-Aptian, where the two sedimentation styles examination the characteristics of each horizon are interfinger, was a time of change from an enclosed shown to be highly complex, in that a variety of basin analogous to an epeiric sea (i.e. the Hythe Beds depositional scenarios can be drawn for the earliest of the Weald), with minimal tidal influence, to an open Aptian Perna Beds, and that tectonic 'enhancement' basin with higher subsidence and more continual (Hesselbo et al., 1990) may have occurred in the sediment supply under tidal influence (Figure 7). Mid-Aptian. A third conclusion to this study is that further Summary sequence boundaries are thought to exist in the Aptian-Albian than are at present recognized on the What characterizes the Upper Aptian in southern Exxon cycle chart (Figure 8). These have hitherto been England as a whole is that the most widespread and undetected by either the Exxon studies (Haq et al., hence 'transgressive' deposits are, from facies analysis, 1988) or by Hesselbo et al. (1990) due to the condensed the shallowest water deposits or most 'regressive' of nature of deposition in the type sections of southern facies, showing the greatest tidal influence and France and east Kent. containing an estuarine fauna (Ruffell, 1989a). This demonstrates how transgression and regression do not necessarily equate to episodes of deepening or Acknowledgements shallowing. Thus, in the shelf seas of the The research was supported by grants from British Aptian-Albian of southern England, transgression Petroleum, NERC (A.H.R.) at Birmingham rarely resulted in actual deepening, which may well University and ORS (G.D.W.) at Oxford University, have occurred in the deep marine areas of the North under the direction of A. Hallam and H. G. Reading. Sea (Rawson and Riley, 1982) and Atlantic Ocean. In Many of our ideas have resulted from discussions with this situation the concept of accommodation (Vail et S. Hesselbo (Oxford), J. Hardenbol (Exxon) and B. al., 1984) can be readily applied, wherein sea level rise Vining (Esso). keeps up with subsidence and maintains shallow water environments. The variation in facies throughout the AptianAlbian (and thus the nature of sequence stratigraphic References criteria) can be explained in terms of the depositional Allen, J. R. L. (1982) Mud drapes in sand-wave deposits: proximity to tidal (oceanographic) influence. The a physical model with application to the Folkestone Beds marine clays of the Atherfield Clay can be regarded as (early Cretaceous, SE England) Phil. Trans. Roy. Soc. London having experienced limited oceanographic influence. A306, 291-345 Anderson, I. D. (1986) The Gault Clay-Folkestone Beds junction The Hythe Beds and Ferruginous Sands record an in West Sussex, Southeast England Proc. GeoL Assoc. 97, increase in tidal influence, as well as an increased 45- 58 abundance of phosphates and glauconites. The MidArkell, W. J. (1947) The geology of the country around Aptian sediments of the martinioides and nutfieldiensis Weymouth, Swanage, Corfe and Lulworth Mem. Geol. Surv. zones show the interaction of deposition under still UK, 386pp Bally, A. W. (1987) Atlas of Seismic Stratigraphy. AAPG Studies restricted oceanographic influence and the maximum in Geology No. 27, AAPG development of tidal deposition represented by the Bensted, W. H. (1860) On the Kentish Ragstone as exhibited in Late Aptian to Early Albian tidal sandwave deposits the Iguanodon Quarry at Maidstone Proc. Geol, Assoc. 1, 57 of the Woburn Sands-Folkestone Beds-Sandrock Bensted, W. H. (1862) Notes on the geology of Maidstone Geologist 5, 294, 334, 378, 447 Formation (Figure 7). 352

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Sequence stratigraphy of the Aptian-Albian Lower Greensand: A. H. Ruffell and G. D. Wach Bridges, P. J. (1982) Sedimentology of a tidal sea: the Lower Greensand of southern England. In: Offshore Tidal Sands (Ed. A. H. Stride) Chapman & Hall, London, pp183-189 Bristow, C. R., Morter, A. A. and Wilkinson, I. (1987) The stratigraphy and palaeontology of the Lower Greensand of the Hoes Farm Borehole, near Petworth, Sussex Proc. GeoL Assoc. 98, 217-227 Casey, R. (1961) The stratigraphical palaeontology of the Lower Greensand Palaeontology 3, 487-622 Cooper, M. R. (1977) Eustasy during the Cretaceous: its implications and importance Palaeogeog. Palaeoclim. PalaeoecoL 22, 1-60 Einsele, G. (1985) Responses of sediments to sea-level changes in different subsiding storm-dominated marginal and epeiric basins. In: Sedimentary and Evolutionary Cycles (Eds. V. Bayer and A. Seilacher) Springer-Verlag, Berlin, pp.68-112 Fitton, W. H. (1836) Observations on some of the strata between the Chalk and the Oxford Oolite in the South-east of England Trans. GeoL Soc. London 4, 103-290 Galloway, W. E. (1989) Genetic sequence stratigraphy in basin analysis. Architecture and genesis of flooding surfacebounded depositional units Bull. Am. Assoc. Petrol GeoL 73, 1125-1142 Haq, B. U., Hardenbol, J. and Vail, P. R. (1987) Chronology of fluctuating sea-levels since the Triassic (250 million years ago to the present) Science 235, 1156-1167 Haq, B. U., Hardenbol, J. and Vail, P. R. (1988) Mesozoic and Cenozoic chronostratigraphy and cycles of sea level change. In: Sea-level Changes: An Integrated Approach (Ed. C. W. Wilgus) SEPM Spec. PubL 42, pp. 183-197 Hesselbo, S. P., Coe, A. L. and Jenkyns, H. C. (1990) Recognition of depositional sequences from outcrop: an example from the Aptian and Albian of the Wessex Basin J. GeoL Soc. London 197, 549-559 Hesselbo, S. P., Coe, A. L., Batten, D. J. and Wach, G. D. (1990) Stratigraphical relations of the Lower Greensand (Lower Cretaceous) of the Calne area, Wiltshire Proc. GeoL Assoc.,

101,265-278 Hughes, N. F. (1958) Palaeontological evidence for the age of the English Wealden GeoL Mag. 95, 41-49 Jeans, C. V., Merriman, R. J., Mitchell, J. G. and Bland, D. J. (1982) Volcanic clays in the Cretaceous of southern England and Northern Ireland Clay Min. 17, 105-156 Kerth, M. and Hailwood, E. A. (1988) Magnetostratigraphy of the Lower Cretaceous Vectis Formation (Wealden Group) on the Isle of Wight, southern England J. GeoL Soc. London 145, 351-360 Kidwell, S. M. (1984) Outcrop features and origins of basin margin unconformities in the Lower Chesapeake Group (Miocene) Atlantic Coastal Plain. In: Inter-regional Unconformities and Hydrocarbon Accumulations (Ed. J. S. Schlee) AAPG Mere. No. 36, pp.37-58 Kirkaldy, J. F. (1933) The Sandgate Beds of the Western Weald Proc. GeoL Assoc. 34, 270-311 Kirkaldy, J. F. (1937) The overstep of the Sandgate Beds in the Eastern Weald Q. J. GeoL Soc. London 93, 94-126 Lake, R. D. and Shephard-Thorn, E. R. (1985) The stratigraphy and geological structure of the Hog's Back, Surrey and adjoining areas Proc. GeoL Assoc. 96, 7-21 Mantell, G. (1834) Discovery of the bones of the Iguanodon in a quarry of the Kentish Rag (a limestone belonging to the Lower Greensand Formation) near Maidstone, Kent Ed. New Phil. J. 17, 200 Middlemiss, F. A. (1962) Brachiopod ecology and Lower Greensand palaeogeography Palaeontology 5, 253-267 Middlemiss, F. A. (1975) Studies in the sedimentation of the Lower Greensand of the Weald, 1875-1975: a review and commentary Proc. Geol. Assoc. 86, 457-473 Norris, R. D. (1986) Taphonomic gradients in shelf fossil assemblages: Pliocene Purisima Formation, California Palaios 1,256-270 Owen, H. G. (1984) The AIbian stage: province chronology and ammonite zonation Cret. Res. 5, 329-344 Plint, A. G. (1988) Global eustasy and the Eocene sequence in the Hampshire Basin, England Basin Res. 1, 11-22

Posamentier, H. W., Jervey, M. T. and Vail, P. R. (1988) Eustatic controls on clastic deposition: conceptual framework. In: Sea-level Changes: An Integrated Approach (Ed. C. K. Wilgus) SEPM Spec. Pub. No. 42, 109-124 Rawson, P. F. and Riley, L. A. (1982) Latest Jurassic-Early Cretaceous events and the 'Late Cimmerian Unconformity' in North Sea area Bull. Am. Assoc. Petrol GeoL 66, 2628-2648 Robaszynski, F. and Amedro, F. (1986) The Cretaceous of the Boulonnais (France) and a comparison with the Cretaceous of Kent (United Kingdom) Proc. GeoL Assoc. 97, 171-208 Ruffell, A. H. (1988) Palaeoecology and event stratigraphy of the Wealden-Lower Greensand transition in the Isle of Wight, southern England Proc. GeoL Assoc. 99, 133-141 Ruffell, A. H. (1989a) Facies analysis of the Aptian-Albian Lower Greensand in southern England PhD Thesis, University of Birmingham Ruffell, A. H. (1989b) The Weald Clay-Atherfield Clay junction at Brook, Surrey Proc. GeoL Assoc. 100, 409-411 Ruffell, A. H. (1990) Mineralogy and petrography of the Sulphur Band phosphates at Folkestone, Kent Proc. GeoL Assoc. 101, 79-84 Ruffell, A. H. and Batten, D. J. (1990) The Barremian arid phase in NW Europe Palaeogeog. Palaeoclim. PalaeoecoL, 80, 197-212 Ruffell, A. H. and Wignall, P. B. (1990) Depositional trends in the Upper Jurassic-Lower Cretaceous of the northern margin of the Wessex Basin Proc. Geol. Assoc., 101,279-288 Ruffell, A. H. and Batten, D. J. (1991) Palaeoenvironmental analysis of two German boreholes in the Lower Saxony Basin (Ed. E. Kemper), Geologische Jahrb. Spec. PubL (in press) Ruffell, A. H., Wach, G. D. and Hesselbo, S. P. Facies analysis of an unconformity: an example from the mid-Aptian Lower Greensand of southern England, in preparation Simpson, M. I. (1985) The stratigraphy of the Atherfield Clay Formation (Lower Aptian; Lower Cretaceous) at the type and other localities in southern England Proc. GeoL Assoc. 96, 23-45 Stewart, D. J. (1978) The sedimentology of the Wealden Group of the Isle of Wight PhD Thesis, Portsmouth Polytechnic Stewart, D. J., Ruffell, A. H., Wach, G. D. and Goldring, R. (1991) Lagoonal sedimentation and fluctuating salinities in the Vectis Formation (Wealden Group, Lower Cretaceous) of the Isle of Wight, southern England Sedim. GeoL, in press Vail, P. R., Hardenbol, J. and Todd, R. G. (1984) Jurassic unconformities, chronostratigraphy and sea-level changes from seismic stratigraphy and biostratigraphy. In: Interregional Unconformities and Hydrocarbon Accumulations (Ed. J. S. Schlee) AAPG Mere. No. 36, pp.129-144 van Wagoner, J. C., Posamentier, H. W., Mitchum, R. M., Sarg, J. F., Loutit, T. S. and Hardenbol, J. (1988) An overview of the fundamentals of sequence stratigraphy and key definitions. In: Sea-level Changes: An Integrated Approach (Ed. C. K. Wilgus) SEPM Spec. PubL No. 42, pp.39-45 van Wagoner, J. C., Mitchum, R. M., Campian, K. M. and Rahmanian, V. D. (1990) (Eds.) Siliciclastic Sequence Stratigraphy in Well-logs, Cores and Outcrops: Concepts for the High-resolution Correlation of Time and Facies. AAPG Methods in Exploration Series 7 Wach, G. D. and Ruffell, A. H. (1989) Firmground facies and faunas in the Ferruginous Sands Formation (Aptian) Isle of Wight Abstracts British Sedimentological Research Group 1989 Meeting, Leeds, UK Wach, G. D. Estuarine sedimentation in the Lower Greensand Group of the Isle of Wight (southern England), in preparation Wheeler, H. E. (1958) Time stratigraphy Bull. Am. Assoc. Pet. GeoL 42, 1047-1063 Whittaker, A. (1985) Atlas of Onshore Sedimentary Basins in England and Wales Blackie, Glasgow Worssam, B. C. and Ivimey-Cook, H. C. (1971) The stratigraphy of the geological survey borehole at Warlingham, Surrey Bull. GeoL Surv. Gt. Br. 36, 111 pp Young, B. and Morgan, D. J. (1981) The Aptian Lower Greensand Fuller's earth beds of Bognor Common, West Sussex Proc. GeoL Assoc. 92, 33-37

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