Early to Mid-Cretaceous Tectonics and Unconformities of the Wessex Basin (Southern England)

Home , Carstone Formation, Geology of the North Sea, Tectonic uplift

Journal of the Geological Society, London, Vol. 149, 1992, pp. 443454, 12 figs. Printed in Northern Ireland

Early to mid-Cretaceous tectonics and unconformities of the Wessex Basin (southern England)

A. H. RUFFELL Department of Geology, Queen’s University, Belfast, Northern Ireland BT7 INN, UK

Abstract: Sediment distribution patterns, time-subsidence plots, seismic data and outcropanalysis of the Lower Cretaceous (?Ryazanian-Albian) of the Wessex Basin, southern England, suggest that a number of unconformablehorizons exist and that there is no single ‘late Cimmerian unconformity’ surface. A tectonic change, from areally restricted deposition in the Weald and Channel sub-basins to extensive sedimentation across the whole of southern England, began in the mid-Aptian. This change ended with transgression of basement ‘highs’ in the early Albian of the Dorset area,and later Albian of the Devon and London Platform basin margin areas: its onset can be linked to the Austrian tectonic phase of Europe. The onset of marine deposition in the Aptian is coincident with periodic transgression of the margins of the Wessex Basin throughout the Aptian-Albian. Mid-late Aptian and early/mid-Albian transgressions were preceded by periods of erosion. The Aptian-Albian Lower Greensand is characterized by transgressive- regressive phases, suggesting a short-term controlling process (third-order eustatic changes or intra-plate mechanisms). Tectonic changes in the mid-Aptian include uplift followed by widespread subsidence. This change coincides with a time of plate reorganization in the European and North Atlantic areas.

The Mesozoic Wessex Basin of southern England is a structur- (Drummond 1970; Stoneley 1982). Thus structures such as the ally complexbasin (Whittaker 1985), wherea predominant ‘mid-Dorset Swell’ (Drummond 1970,1982) may also be re- east-west pattern of extensional faults is cut by a seriesof lated to basement faults. northwest-southeast oriented faults (Karner et al.1987). These Chadwick (1 985) described the mid-Cretaceous develop- structurescontrolled the accumulation and preservation of ment of the Wessex Basinwith referenceto the ‘late Cimmerian thickMesozoic sedimentary successions within the basin Unconformity’, and produced a map of the older Jurassic and (Chadwick 1986). Major faults commonly form the marginsof Cretaceous formations that subcrop beneath the unconformity the upstanding basement blocks that surround much of the surface. In certain places within the Weald sub-basin(see Figs 1 basin,as well as intra-basinal highs (e.g. the Isleof Wight & 3) no obvious mid-Cretaceous unconformityis apparent, yet Monocline; Fig. 1). Inversion of the Mesozoic basins occurred onthe basin margins a number of discreteunconformities in the latest Mesozoic (late Cretaceous) to mid-Cenozoic occur.Chadwick (1985) took the oldest Cretaceous deposit (?Oligocene) interval, when these formerly upstanding Meso- above the unconformity surface (the oldest commonly being zoic massifs and highs became the sites of accumulation of early Aptian, the youngest late Albian), as his datum. The thick successions of Cenozoic clastic detritus (Karner et al. unconformity was also represented as a widely variable time 1987). gap by Karner et al.(1987), this being explainedby differential Numerous authors have published analyses (based on out- erosion priorto transgression of the basin margins. However, a crop, borehole and seismic data) of the Mesozoic-Cenozoic number of Aptian-Albian unconformities are known to occur development of the Wessex Basin, andsurrounding areas. above the lowest horizon mapped by Chadwick (1985) as the Theseinclude Stoneley (1982), Chadwick (1985,1986), ‘late Cimmerian Unconformity’ (Hesselboet al. 1990a; Ruffell Sellwood et al. (1986) and Karner et al.(1987). Previous evalu- & Wach 1991), and further unconformable horizonsbelow are ations of thetectonic evolution of the WessexBasin have demonstrated here. placed great importance on the early to mid-Cretaceousinter- val, and the development of the ‘late Cimmerian Unconfor- Methodology mity’ (Chadwick 1985; Karner et al. 1987). This surface of unconformity was linked by Karner etal. (1987) to the How best may the numberof unconformities, which may exist, post-Hercynian tectonic development of the northwest Euro- be definedwithin the basin, andhow may theybe characterized? pean region, and specifically to the ‘Austrian’ (Glennie 1986) The methods of study are outlined below as: (I) sediment dis- tectonic movements.The stratigraphyof the Lower Cretaceous tribution patterns from boreholes and isopach mapping; (ii) sediments is shown in Fig. 2, with a summaryof known tectonic time - subsidence plots and sediment accumulation (and pre- events from Glennie (1986). servation) patterns from outcrop and borehole sections; (iii) Stoneley (1982), Chadwick (1985) and Karner et al. (1987) seismic data; (iv) outcrop analysis. all give accounts of Mesozoic extensional faulting within the Wessex Basin, and subsequent compressionand basin inversion Sediment distribution patterns duringthe late Cretaceous-earlyCenozoic. Such compres- sional structures (e.g. the Purbeck-Isle of Wight monocline) The formal group-status divisionsof the Lower Cretaceous of are offset in an en echelon mannerwhich was relatedby Drum- southern England (sensu Rawson et al. 1978; Stewart 1981; mond (1970, 1982) to strike-slip motion of underlying base- Simpson 1985) are the smallest scale at which isopach maps ment faults. Mid-Cretaceous sediments in Dorset were sub- can be made, as borehole logs from hydrocarbon and water jected to minor inversion along northwest-southeast trends explorationrarely allow more detailed correlation. Gross 443

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P I l 1O"W 0" O"

55"N rz,

TROUGH

SEA 50"N WESTERN APPROACHES TROUGH PARIS

BAY OF Area of Map C l BISCAY ; I I I I C 2"W 1 "W 0" l"E

LONDON PLATFORM

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Fig. 1. Location maps. (a) Location of WEALD WINCHESTER 0 SUB-BASIN study area, with other basin successions mentioned in text. (b) Aptian-Albian MID-DORSET (Lower Greensand) outcrops in south SWELL eastern England and France with outcrops (1-15) utilized in sequence 0 BOREHOLE stratigraphic study of Aptian-Albian W WlLMlNGHAM A ARRETON sediments (Fig. 12). (c) Tectonic features V lNGN \ of the Cretaceous of the Wessex Basin. 0 25 50 km Lines of seismic sections: A, Fig. 9; B, Fig. 10; C, Fig. 11. Major boreholes l I I mentioned in text.

facies changes occurwithin all the groups studied (Casey 1961; (Fig. 3). A single transgression cannot explain the complex Owen 1975; Allen 1989 and references therein),making distribution of mid-Cretaceous sediments on themargins of the attemptsat decompaction difficult, as different lithologies Wessex Basin, as erosion occurred prior to each transgressive follow different compaction trends. The spacing of boreholes phase (e.g. Hesselbo et al. 1990b). In boreholes sunk to the within the basin is not dense enoughto allow mapping of large- north of the Isle of Wight Monocline (Figs 1 & 4), which was an scale facies changes, so only present-day (compacted) thick- area of positive relief in the early to mid-Cretaceous, the base- nesses are discussed in this section. These maps demonstrate Gault Carstone Formationis the first deposit to be preserved. that the basin depocentre remained around the core of the This rests unconformably on Jurassic strata (Fig. S), and the present-day Weald anticline(Fig. 3) for much of the time base of this formation might be taken as the 'late Cimmerian' or represented by the Wealden (?RyazanianNalanginianto 'Austrian' unconformity. In contrast, on the crest of the Lon- BarremiadearlyAptian) and Lower Greensand (Atherfield don Platform the first transgressive deposit is the Upper Gault Clay and Hythe Beds Formations: early Aptian). Inthe mid- to Clay, whilst on the flanks of the platform in the northern mar- late Aptian, deposition commenced on formerly upstanding gins of the Wessex Basin (Fig. 1) the first transgressive deposit fault blocks within and on massifs surrounding the Wessex preserved varies from early Aptian to early Albian in age. The Basin. These deposits are patchily preserved as the Sandgate, thin sedimentarycover on such areas gives some indication (e.g. Folkestone and CarstoneBeds of the Lower Greensand and the horizons of phosphatic nodules containing remanie Lower Gault Clay (Owen 1975). The Upper Gault Clay is the ammonites) of previous, now eroded transgressive phases. By first deposit to cover all topographic highs around the basin determining the age of these remanie deposits, and tracing their

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REGIONAL EVENTS REGIONAL ERA GROUP FORMATION APTUN - EARLY Ma STAGE TETHYS ATLANTICTETHYS PERIOD CHANNELWEAL1 CHANNEL WEALD ALBIANAMMONITE ZONES UPPER GREENSAN UPPER GAULT. UPPER GREENSAND mammrllalum LOWER GAULT y CARSTONE ' 0 / MID- SANDRO~K FOLKESTONE ATLANTIC 2 BEDS lardeturcata 50 SPREADINGALPINE OROGENY ERRUGINOUSSANDGATE SANDS HYTHE SANDS PLATEAU ATHERFIELDCLAY lamb! EASALTS -

W CLOSURE m+ UPPER VECTIS WEALD Wlkld,e"S#S loo SEA.FLOOR z5 yI CLAY SPREADING 2 ROTATION NEWFOUNDLAND c > OFlsERlA -IBERIA d manlomdes OQ AUSTRIAN 9 > LOWER ClMMERlAN - WEALDWESSEX 150 TECTONICS bowerbad! ~~~~$~~ CLAY Fig. 2. Tectonic history of the North ------Atlantic and Tethys Ocean (from TUNERIDGE deshayesr WELLS SAND Glennie 1986), and stratigraphy of the RIFTING IN

Lower-mid-Cretaceous of the Wessex ATLANTIC Basin. Aptian 'Austrian' tectonics (Zeigler 1982) are highlighted, and the _---_ GREENLAND Aptian succession and biostratigraphy is SEA shown (from Casey 1961).

WEALDEN ATHERFIELD CLAY a HYTHE BEDS

Fig. 3. Present-day isopach maps (in metres) of Lower Cretaceous Formations in southern England. Wealden = Ryazanian-BarremiadAptian; Atherfield Clay and Hythe Beds = Lower/mid Aptian; Sandgate and Folkestone Beds = mid-Aptian-early Albian; Gault Clay = early Albian. Data from Whittaker (1985); Sellwood et al. (1986); boreholes and outcrops SANDGATE 8 FOLKESTONE BEDS GAULT CLAY utilized in this study (Figs 1 & 10).

correlative horizons into the depositional basin, it is possible Dorset-Devon coast). Discerning the effects of the 'Austrian' to infer the age of intervening unconformities. The depositional tectonic phase (and any other phases that might exist) is central effects of these transgressive-regressive phases in the depo- to the present study. centresof the Weald and Channelsub-basins are complex Time-subsidence plots for the Wealden Group of the (Ruffell & Wach 1991). Wessex Basin are poorly constrained due to the lack of a re- fined biostratigraphy, or easily correlatedhorizons (Allen 1989). Thus the almostlinear plots for the Ryazanian- Subsidence plots and rates sediment accumulation of Barremian (Karner et al. 1987; this study, Fig. 6) could be Karner et al. (1987) used time-subsidence plots for the Meso- interpreted as eithercontinuous sedimentation within the zoicCenozoic sediments of the Wessex Basin to infer a late depositionalcentre of the basin orpoor biostratigraphic Jurassic-earlyCretaceous 'late CimmerianUnconformity' resolution. As outcrop techniques have not identified any and distinguished this from an Aptian-Albian unconformity major unconfonnities within the Wealden Group of the Weald related to the 'Austrian' phase of tectonics of Zeigler (1981, sub-basin, continuous deposition might be preferred. Con- 1982). In many basin margin settings (Fig. 1) these two events versely, seismic imaging of the Wealden Group (see below) are not separable and a single unconformity is found between indicates that extensive non-depositional surfaces exist, and mid-Cretaceous and Triassic to Upper Jurassic strata (on the that these may correspond to sequence boundaries developed

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ATHERFIELD CLAY

Fig. 4. Isopach plots of the mid-Cretaceous sediments of the Isle of Wight. Data from outcrops (Casey 1961; Simpson 1985; Wach & Ruffell 1990), and boreholes shown.

WINCHESTER KINGSCLEREWINCHESTER FARINGWN W WEALD A8 R BA

Fig. 5. Cross-sections across the Weald Basin. (A) Mid-Dorset Swell (see Fig. 1 and inset for location) to London Platform. (B)Channel Basin (Isle of Wight) to the London Platform. Key as for (A). Borehole data from the national Geosciences Data Centre (Keyworth).

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98111-l WILMINGHAM 99112-1 WILMINGHAM 98111-l

~ DATUM ON BASE CHALK - ISLE OF WlGHT (ARRETON) R UPPER GREENSAND RICHMOND/. /* h' 60- C. !

U)WER WWER GREENSAND GREENSAND I --- '1 WEALDEN 27921

OXFORD CLAY

Icl l .

thickness (cumulative In metres) Fig. 7. Example of well-log correlations along the Purbeck-Isle of Fig. 6. Cumulative sediment thickness plot of midJurassic to Wight monocline, showing different unconformity surfaces. Note Tertiary successions from four areas of the Wessex Basin margins the apparent absence of the Lower Greensand in 98/11-1. Data and depocentre (locations on Fig. 1). Vertical columns are stage courtesy of British Petroleum (98/1 I-]), Clyde Petroleum names (Bathonian to Oligocene) and age (Ma). Thickness and facies (Wilmingham) and Texas Gas (99/12-1). Borehole locations on variations during the Cretaceous are discussed in text. Fig. 1.

in the low-gradient braidplainflagoonal environments of the group. The same cannot be said of the Channel sub-basin in deposition. Periods of slow sedimentation within the basin can Dorset where careful facies analysis (Hesselbo & Allen 1991) further be related to early Aptian transgressive phases discer- hasrevealed the existence of two closely spaced unconfor- nible from the ages of nodule beds preserved on the basin mities. Seismic evidencealso suggests that correlativecon- margins (see below). formitiesoccur within the Weald andChannel sub-basins. Tracing such surfaces into the basin marginsis difficult due to later (Aptian-Albian) erosion, and only occasional borehole

records show evidenceof unconformable horizons overlainby EAST SURREY- KENT transgressive beds of Wealden age (Fig. 7). By inference there- ,. SUSSEX ISLE OF fore,the unconformities overlain by Wealdensediments in WIGHT. boreholes 98/11-1 and 99/12-1 must pre-date the mid-Aptian- Albian unconformities above. Biostratigraphic resolution greatly increases in the marine strata of theAptian Lower Greensand (Casey 1961). The Aptian-Albian was a period of considerable tectonic activity, with the development of a widespread unconformity that has been correlated with tectonic movementsin the Alps (Karneret al. 1987). Cretaceous time-subsidence plots from all areas of southern England confirm a change in subsidence patterns in the Aptian-Albian (Fig. 6), and uplift of the London Platform is evident from early Cretaceous times onward, a tectonic im- print on sedimentation previously recognized by Allen (1975) and Sladen & Batten (1984). The widespread Aptian-Albian unconformity developed in the Wessex Basin deserves further analysis in order todetermine whethera single phase of erosion (preceded by uplift?) was responsible,and toassess the relative roles of tectonism and eustasy in its formation. Aptian sedi- thickness uncomoacled ment accumulation rates (Fig. 8) reveala contrast between Fig. 8. Lower Greensand cumulative thicknesses per Aptian undulating patterns in basin-margin settings and smooth pat- ammonite zone in four areas of the Wessex Basin. Stratigraphy is terns in the basin centre. These suggest that phases of basin- shown in Fig. I. Steep lines indicate condensation, and may be margin footwall uplift, or eustatic sea-level fall were marked related to transgressive beds (Fig. 12). Vertical lines indicate no by periods of non-deposition. Closerto basin depocentres such sedimentation and may represent times of exposure in the basin phases were marked by continued and sometimes increased margins. Original thickness data utilized from Casey (1961).

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B- B- 0 1 km Seismic data Sdulh North 1 l In excess of 20 000 km of seismic data has been acquired (onshore) by oil companies involved in commercial exploration over the study area (Fig. 1). Accounts of regional basin structure derived from the studyof such data can be found in Whit- taker (1985), Penn et al. (1987) and Karner& Lake (1987). The poorly exposed Wealden Group strata of the basin display HASTINGS characteristic and useful seismic stratigraphic patterns. Such BEDS analysis is difficult in the thin (0.1 to 0.5 seconds one way time)

Lower Greensand and GaulUUpper Greensand Groups,which PURBECK - show few surfaces of downlap or truncation. In these strata PORTLAND

unconformitiescan be betterdiscerned through outcrop KlMMERlDGE analysis. Descriptionof all the seismic stratigraphic patternsof CLAY the WealdenGroup is beyond the scopeof this paper: the main , J points can be summarized by reference to the typical seismic characters shown in Figs 9 & 10. Fig. 10. Line drawing of a seismic line in the Weald Basin The seismic reflections found in the Hastings Beds of the (location B on Fig. l), showing truncation of reflections above the Weald sub-basin commonly show sigmoidal patterns. Inclined Purbeck Anhydrites, and inclined surfaces within the Hastings reflections with downlap also occur (Fig. 10) and thus pack- Beds. ages of sediment containing inclined reflections, bounded by parallel surfaces can be defined. Dating of these packages can beproblematic as correlation to boreholesections is often made across high areasdrilled for hydrocarbontests, where the minor sandstone and limestone beds well-known as marker Lower Cretaceous stratigraphic recordis often incomplete. An horizons in the Weald Clay (Worssam 1978). Two explana- example of such a welltie is given by Sellwood et al. (1987), tions can be offered, either (as they cut reflections) they are and similar ties were used in this study from wells at Albury sequence boundaries, or they reflect intermittent cessations in andDetention (Fig. 1). As no ostracode-or palynomorph- synsedimentary fault activity. based biostratigraphyexists for such boreholes, the ageof these In a regional context, the Hastings Beds form an areally- Wealden seismic packages is, as yet unknown. The work of restricted package of sediment in the core of the now-inverted Hesselbo & Allen (199 1) and observation of Wealden uncon- Wealdanticline (Fig. 11). This is overlain by successively formities from boreholes demonstratethat although unconfor- moreextensive Weald Clay, Lower Greensand, Gault Clay mities are rarely recognizedat outcrop orin boreholes, they are and Upper Greensand strata, recording overall transgression likely to exist within the group.Two seismic stratigraphic inter- of the basin margins. These higher units also possess more pretations of the Hastings Beds packages can be envisaged: continuous andgenerally strongerseismic reflections, enabling firstly, that they represent the sequences recognizedat outcrop, detailed structural mapping(see below). Truncation of Weald and that truncationis absent or is not imaged; or secondly, that Clay reflections and synsedimentary faults occurs anat horizon theyrepresent four systems tracts in theWealden Group equivalent to the base of the Lower Greensand in the Weald (Fig. 10). It may be that the low-gradient braidplain environ- Basin. This is likely to be the base of the Perna Beds (&wicos- ment represented by the Lower Cretaceous Wealden deposits tutus Zone: see Casey 1961) or possibly a younger horizonin the of the Wessex Basin (Allen 1959, 1967) resulted in unconfor- overlying Atherfield Clay Formation (Casey 1961; Bristow et mities being poorly developed. Further analysis of Hastings al., 1987). The Perna Beds are around 1 metrethick in the Beds unconformities might bestbe achieved by careful outcrop Weald depocentre, and thus not thick enough giveto any seis- facies analysis. mic response. The beds above and below do possess a relative Seismic reflections from the Weald Clay (stratigraphy in acoustic impedance contrast, and can thus be tied to borehole Fig. 2) are less easily defined,being dominated by low- sections (Fig. 1: Winchester,Humbly Grove, Albury). The amplitude reflections in some areas, and numerous small-scale pattern of truncation at the baseof the Lower Greensandis very faults in others (Fig. 9). The larger faults havebeen mapped at similar to that observed within the Weald Clay below, and surface (Kirkaldy 1975, fig. 2), and were recognized by Topley may explain the outcrop records of the Perna Beds unconfor- (1875) asgenerally southward-throwing, paralleled and mably overlying the Weald Clay (Holmes 1959; Casey 1961; draped by folds with steep northern limbs. Strong, laterally Ruffell 1989). Allen (1989) suggested that only a minor time continuous reflections truncatethese synsedimentary faults, gap may occur at the Weald Clay-Lower Greensand junction which from an indirect outcrop correlation seem likely to be withinthe Channel and Weald sub-basindepocentres. The

NORTH SOlJiH

Fig. 9. Geo-seismic data from the

Weald sub-basin: line drawing of a , portion of a seismic line across the -M. ---- Portsdowq area, passing north into the -. - basin depocentre (location A on Fig. l), W- ~ ~~ ~ ~ ~~ ~ ~~ I showing synsedimentary faults in the Weald Clay. HASTINGS BEK

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Fig. 11. Geoseismic representation of the Weald and Channel sub-basins of the Wessex Basin. Data derived from this study, Penn et al. (1987) and seismic data held by Kelt Petroleum. Location C on Fig. 1.

base Lower Greensand reflections are similar to intra-Weald UpperGreensand and Chalk on formerly upstanding fault Clay reflectors foundbelow, in that theycross-cut synsedi- blocks suggests that this is not a case of simple progressive mentary faults. In summary this major environmental change onlap of the basin margins: these groups represent deep and fromnon-marine to fully marineenvironments appears no shallow shelf along with variable-depth epeiric seaway depo- different on seismic data from event beds beneath. sits. As such, greatsea-level fluctuations are inferred (Hancock On seismic lines shot across the marginsof the Weald sub- 1969; Owen 1971;Robaszynski & Amedro 1986), and progress- basin, reflections thought from borehole records to represent ive thickening onto formerly upstanding areas occurs in all the late Aptian and early Albian Lower Greensand can be groups, and in many parts of the basin-margins (Whittaker observed unconformably overlying beds varying in age from 1985). Drummond (1970; 1982)also identified the early Albian early Cretaceous through to Palaeozoic (Fig. 5). Thus while (Carstone)as a timeof earlyinversion through strike-slip the early Aptian transgression is restricted to the Weald and motion on northwest-southeast oriented basement faults. ChannelBasins, the mid- to lateAptian Lower Greensand onlaps the basin margins to form a more areally extensive sediment distribution, reflected in the isopach maps (Fig. 3). Outcrop analysis The structure of the Lower Cretaceous Weald sub-basin in The more remote methods of basin analysis employed so far southernEngland is dominated by theLondon Platform, (boreholedatdsubsidence curves; sediment accumulation Portsdown Swell and Isleof Wight-Purbeckmonocline. rates and distribution patterns and seismic data) pinpoint key Figure 1 1 shows a representationof this structure, derived from horizons that requirefurther examination throughout the this work (Figs 4 & 5), seismic data held by Kelt Petroleum, basin. These will not necessarily correspond to well-exposed and the published work of Penn et al. (1987). The Portsdown or well-documented ('classic') sections. Thisstudy, together Swell differs in appearance (on seismic data) from the margin with the previous workof Chadwick (1985,1986)and Karneret of the London Platform or the Isle of Wight monocline, the al. (1987) hasspotlighted the Aptian-Albian as a time of Wealden strata1 reflections convergingacross the structure, important tectonic changes within the basin. Fortunatelythis is rather than being truncatedby the Lower Greensandand Gault also a time when biostratigraphic resolution is better than in reflections above. In contrast, the Palaeozoic rocksof the Lon- the preceding Wealden sediments. don Platform rise from a depthof around 3000 m in the Weald The 'late Cimmerian unconformity' is a term that has been sub-basin,via a series of faults to subcropbeneath Lower used in descriptions of the study area (Fyfeet al. 1981; Zeigler Greensand and Gault Clay below London (Middlemiss 1962; 1982; Chadwick 1985; Karner et al. 1987). The erroneoususe of Owen 197 1).The main bounding faultof the western endof the this termhas been highlighted in theNorth Sea,Eastern London Platform (the Hog'sBack fault) in Surrey appearsvery England, and to a minor extent southern Englandby Rawson similar from outcrop and seismic studies to the sub-monocline & Riley (1982). The main depositional change (termination of fault of the Isle of Wight (Smalley & Westbrook 1982; Lake & Kimmeridge Clay deposition in the Ryazanian) was shown to Shephard-Thorn 1985). In both cases, thick Lower Cretaceous be the result of a facies change from organic-rich to organic- sediments are preserved to the southof the faults, and virtually poor mudrocks, condensed deposition across highs, and tec- none to the north.Inversion of the basinsto the south (Channel tonic changes(in subsidence) that continued into the Albianby and Weald sub-basins) culminated in the latest Cretaceous. movement on fault blocks throughout the study area. Support The first deposits preservedon former upstanding areas(which for the latter contention canbe found in the effects of tectonics later became the Hampshire and London Tertiary Basins) are in theParis Basin continuinginto the Cenomanian lateAptian to earlyAlbian. Thus from seismic dataand (Robasyzinski & Amedro 1986), resulting in numerous similar isopach maps, it can be argued that the change in sediment unconformity horizons throughout the early-mid-Cretaceous preservation that preceded complete basin inversion began in of the WessedChannel Basins. In southern England generally the mid-Cretaceous withonlap of the basin margins. This con- 'late Cimmerian' tectonics havebeen linked to the uplift of the clusion is supported by astudy of sedimentdistributions London Platform and the onset of paralic siliciclastic deposi- aroundthe Isle of Wightmonocline. Data from outcrop tion in the Wealden Group (Allen 1975). The transition from sections and boreholes (Fig. 4) shows that the early Albian lateJurassic marine mudrocks into marinehon-marine Carstone is the first sediment to be preserved north of the carbonatesand non-marine siliciclastic earlyCretaceous monocline, in what was to become the Cenozoic Hampshire Wealden strata is commonly gradational (Hallam 1984; Mor- Basin (Whittaker 1985). The thickening of Carstone, Gault, ter 1984), and thus little evidence exists in support of a 'base

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Cretaceous’unconformity in the Weald sub-basin. Such an margins (deshayesi and bowerbanki zones). These transgressive unconformity would be most apparent in thedepositional beds are separated by time gaps that correspond to sequence basin margins, not now preserved due tomid-Cretaceous uplift boundaries (Ruffell & Wach 1991): these can be dated by and erosion of such areas, linked to a second phase of tectonic observing the correspondingfacies change (the correlative con- movements (the ‘Austrian’ phase of Zeigler 1981, 1982). formity) in the basin depocentre (in thecentre of the Weald-Surrey and Sussex, and on the Isle of Wight). This analysis has been carried out by Ruffell & Wach (1991), who Pre-Aptian events identified downward shifts in coastal onlap in the highly bio- Seismic stratigraphic analysis of the Wealden Group strata in turbated Aptian Lower Greensands with inputs of coarser sand, the Wessex Basin indicates that sequence boundaries exist, but often with estuarine facies and biofacies associations. that they are subtle as a result of mid-Cretaceous erosionof the The mid-Aptian unconformity is well known by the trans- depositional basin margins, where significant erosional uncon- gressive deposits that overlie it (the nutjieldiensis transgression formities would have been expected. Outcrop recognition of ofCasey 1961; Bridges 1982). The age of the preceding sequence sequence boundaries is made difficult by poor exposure and boundary was suggested by Hesselbo et al. (1990~) tobe late problems of correlation. In Dorset,two early Cretaceous rnartinioides, although as stated above ammonite evidence for sequence boundaries have been elucidated from facies analysis this only exists at Folkestone in Kent. The nutfieldiensis Zone by Hesselbo & Allen (1991), and tentatively correlated with sediments that overlie the unconformity surface in the Weald sections on the Sussex Coast. The excellent exposures of the sub-basin and on the margin of the London Platform (Casey Wealden and Lower Greensand strata on the Isle of Wight 1961; Middlemiss 1962, 1973) can include pebble beds (e.g. the (Fig. 1) show a transitionfrom non-marine (Wessex Formation Mill PointConglomerate at Folkestone in Kent), fuller’s of Stewart 1978) to lagoonal (Vectis Formation) sediments to earths (Ruffell & Wignall 1990) and shell-rich accumulations fully marine Lower Greensand. Again dating is problematical like the Faringdon Sponge Gravels in Berkshire (Krantz 1972). (Allen 1989), and is based largely on the magneto-stratigraphy These distinctive facies make it possible to trace the nutfieldien- of Kerth & Hailwood (1988). The transgressive surface rep- sis Zonetransgression throughout the subsurface of the Weald resenting the Wessex-Vectis Formation boundary most likely (Fig. 5) except where latest Aptian and early Albian erosion represents the transgression of a lagoonal depositional system has removed earlier material. This is especially true of areas of across non-marine deposits,and may coincide with a sequence Wiltshire and Dorset where erosion preceding the jacobi Zone boundary. This major environmental change from non-marine transgression reworked and removed earlier material braidplain and meander-belt sands to lagoons marks the initi- (Hesselbo et al. 1990b). ation ofa complete cycle of increasing marine influence, Abundant derived Upper Jurassic ammonites and reworked punctuated by a brief lowstand, and culminating in marine Jurassic and Palaeozoic lithologies in the nutjieldiensis Zone inundation in the early Aptian (Lower Greensand). deposits fringing the London Platform have been cited as evidence for mid-Aptian uplift of the platform (Kirkaldy 1937; Casey 1961; Middlemiss 1962). Similar derived material is Aptian-Albian events found in the transgressive early Albian Carstone throughout Seismic and borehole data, andtheir integration into sediment much of the basin,and similar uplift or tectonic enhancement is distribution maps, all indicate that the Aptian-Albian was a invoked to explain the influx of derived material here and not in time of changingsubsidence patterns. This confirms the studies the jacobiZone transgressive deposits below. The progressive of Chadwick (1985) and Karner etal. (1987); the’latter authors onlap of mid-Cretaceous sediments onto thebasin margins was attributing this tectonic changeto the‘Austrian’ tectonic phase. used by Hancock (1969) as evidence of continued Albian sea A key element in understanding the evolution of the Wessex level rise, although other explanations are possible (e.g. post- Basin is the lateral variationin age of the Lower Greensand and rift thermal subsidence: sensuDewey 1982; Leeder & Gaw- Gault Clay. This can be related to cycles of sedimentation in thorpe 1987). the complete succession of the Isle of Wight. Local tectonic effects can be screened out using this regional approach. For example, the Folkestone (Kent) area studied by Hesselbo et al. Discussion (1990~)is the only locality in the basin where buxtorfi Subzone Changes in rates of subsidence and areas of sediment accumu- (martinioides Zone: mid-Aptian) ammonitesare preserved; and lation/preservationpinpoint the mid-Aptian as a timeof the topof the Folkestone Beds is one zone younger than at all changing subsidence patterns. Renewed erosion of massifs sur- other localities in the basin (mammillatum Superzone of Owen rounding thebasin suggests uplift of the margins of the Wessex 1988). Thus a single area is rarely ever representative of the rest Basin, followed by transgression. Outcrop studies of the angu- of the basin. lar discordance commonly found at the base of the mid-Aptian The basin-wide age distribution of sediments (Fig. 12) in- (martinioides to nutjieldiensis Zones), and the large volume of dicates that zones represented by reduced sedimentation in the derived materialfound in the overlying transgressive beds, depocentres of the Weald and Channel basins (Fig. 8) corre- support thiscontention (Arkell 1939; Middlemiss 1962). spond to theages of transgressive nodule horizons on the basin However, this study has foundno evidence for similar tectonic

Fig. 12. Lithologies and age distributions of the Aptian-Albian sediments throughout southern and eastern England (locations on Fig. 1). Where no sediment or remani6 is shown beneath the basal unconformity, the sediments rest on earlier Cretaceous or Upper Jurassic strata (the ‘Austrian’ Unconformity/tectonic phase of Karner et al. 1987). Sequence stratigraphy from this study, based on age distributions and facies changeskediment cyclicity in the Isle of Wight (after Ruffell &L Wach 1991). TST, transgressive systems tract; SB, sequence boundary; HST, highstand systems tract; SMW, shelf margin wedge; MFS, maximum flooding surface; arrows indicate transgressive surfaces. PB, Perna Beds; CC, Chale Clay; ULB, Upper Lobster Beds; IV to XV, Member notation.

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activity in theearly jacobi Zoneas suggested by Kirkaldy data the corresponding reflection truncates Wealden Group (1933). The widespread nature of the early jacobi Zone trans- faults. The age of the Perna Beds is approximately the same as gression suggeststhat it is related to more widespread (possibly the 112 Ma sequence boundary of the Haq et al. (1988) cycle global) events (Kemper 1973; Cooper 1977). chart, as demonstratedby Hesselbo et al. (1990). However, no In the Channel Basin, the mid-Aptianwas a time of minor time gap is thought to be represented by the basal Perna Beds variations in subsidence and sedimentaccumulation rates. erosion surface (Kerth & Hailwood 1988), and the environ- Derived detritus is common in the ‘nutjieldiensis pebble bed’ mental transition from the uppermost Weald Clay Groupsedi- (Casey 1961; Wach & Ruffell 1990), and uplift, similar to that ments is gradual (Allen 1989). suggested for the London Platform, may have occurred north (3) A mid-Aptian(martinioides Zone) unconformityis found of the Isle of Wight monocline, where Aptian sediments are throughout the basin, and thedistinctive overlying transgress- rarelypreserved. Instead the early Albian Carstone rests ive sediments of the nutjieldiensis Zone are some of the first directly onthe Upper Jurassic; the angular discordance mid-Cretaceous sediments to be preserved on the tilted and between the two, and the volume of derived detritus in the eroded JurassicPalaeozoic strata of the London Platform. Carstone could also be taken as indicative of uplift. (4)Evidence of the late Aptian(jacobi Zone) unconformity The progressively increasingoutcrop areasof nutjieldiensis, is found in the erosive reworking of earlier nutjieldiensis sedi- jacobi, early Albian (mammillatum Zone) and mid-Albian (in- ments in the basin margins and on intra-basinal highs. jatum Zone) sediments compares closely with onlap patterns (5) The early Albian Carstone Formation is a laterally ex- observedin thethermal relaxation phase of rifted basins tensive transgressive deposit consistingof abundant reworked (Steckler & Watts 1978;Dewey 1982). Thus the episodes of Upper Jurassic detritus. The base is erosive on earlier Albian tectonic uplift described, are superimposed onan overall Lower Greensandin the Wessex Basin, and on Upper Jurassic transgression of the basin marginsthat occurred throughout the strata northof the Isle ofWight Monoclineand on the margins mid-Cretaceous. of the London Platform. Aptian tectonic activity is well documented in areas adja- (6) The first sediments preserved on the very crest of the cent to the Wessex Basin, and twophases (early and mid- London Platform are of mid-Albian age, these transgressive Aptian) of tectonic movement wereidentified by Ziegler (1981, deposits overlying a dentatus Zone unconformity. 1982, 1987) during his ‘Austrian’ phase. Eustaticsea level falls The early Aptian, mid-Aptian and early Albian unconfor- are recorded at the same time by Cooper(1977) and Haqet al. mities can each be related to specific tectonic events in the (1988). Tectonicevents include early Aptian rifting in the North Atlantic:minor unconformities within the Wealden North Sea (Rawson & Riley, fig. 2), WesternApproaches Group; Lower Greensand and Gault are mostlikely the result Trough and Celtic Seas. Thefirst opening of the Bay of Biscay of short-term sea level changes. during the separationof Iberia and Armorica is also dated as early Aptian (Montadert et al. 1979), concomitant with sea I am indebted to I. Emslie (Conoco), D. Ellis (Sun), B. James (Kelt), floorspreading in theSouth Atlantic north of theWalvis D. Bowler (Shell), J.Donato (Goal), A.ChadwicWS. Holloway Ridge. By contrast, subsidence greatly increasedon the Goban (BGS) and C. Todd (Occidental) for their help. S. Hesselbo and A. Spur in mid-Aptian times (Dingle& Scrutton 1979; Masson & Hallam provided invaluable criticism of an early version of this work. Roberts 1981). A second phaseof Alpine orogenesisis recorded Thanks also to P. Allen, P. Rawson and G. Wach for their continued at the same time as the end of Tethyan sea floor spreading support. This work was carried out under the tenure of a NERC studentship and a BP International Fellowship. (Zeigler 1987). Croker & Shannon (1987) record mid-Aptian uplift of the Porcupine Trough, but opening of the Rockall Trough was later, in mid-Albian times. In summary the mid- References Aptian tectonics widely recognized in the European and North ALLEN,P. 1959. The Wealden environment: AngleParis Basin. Philosophical Atlantic region also affected theWessex Basin, yet the equally Transactions of the Royal Society. London, B242, 283-346. widespread early Aptian tectonics have left little record. Con- -1967. Origin of the Hastingsfacies in north-western Europe. Proceedings of versely, the late Aptian uplift and subsequent transgression the Geologists’ Association, 78, 27-105. evident in the Wessex Basin has been recognized only in the -1975. Wealden of the Weald: a new model. 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The characteristicsof each are Petworth, Sussex. Proceedings of the Geologists’ Association, 98, 217-227. summarized below. CASEY,R. 1961. Thestratigraphical palaeontology of the Lower Greensand. (1) BasaVearly Wealdenunconformities have been Palaeontology, 3, 487422. identified from (a) outcrop studies in Dorset (Channel sub- CHADWCK,R. A. 1985. End Jurassic-early Cretaceous sedimentation and sub- basin) by Hesselbo & Allen (1991); (b) borehole records (also sidence (late Portlandian to Barremian), and the lateCimmerian unconformity. In: WHITTAKER,A. (ed.) Atlas of onshore sedimentary basins in England in theChannel Basin, e.g.98/11-1 of Fig. 1); and (c) from and Wales. British Geological Survey, Blackie, London, 52-56. reflection terminations on seismic data over different parts of -1986. Extension tectonics in the Wessex Basin, southern England.Journalof the basin. the Geological Society, London, 143, 465488. 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ZEIGLBR,P. A. 1981. Evolution of sedimentary basins in North-west Europe. In: Petroleum Mij BV, TheHague, Elsevier, Amsterdam. ILLING,L. V. & HOESON,G. D. (eds) Pefroleum Geology of the Continental -1987. Evolution of the Arctic-North Atlantic borderlands. In: BROOKS,J. & Shelfof North-west Europe. Institute of Petroleum, Heydm, London. GLENNIE,K. (eds) Petroleum Geology of North West Europe. Graham & -1982. GeologicalAtlas ofWestern and CentralEurope. ShellInternational Trotman, London, 1201-1204.

Received 24 July 1991; revised typescript accepted 4 October 1991.

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