Journal of the Geological Society, London, Vol. 148, 1991, pp. 105-113, 10 figs. Printed in Northern Ireland

Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous, south

STEPHENP. HESSELBO & PHILIPA. ALLEN Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK

AbstrPd: The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and facies relationships, are interpreted as unconformities. The lower unconfor- mity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces aretreated as candidate sequenceboundaries (WBl-WB3), with the thin intervening ?lagoonal depositsrepresenting peaks of transgression.The pattern of oneminor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to theEarly Cretaceous sequence stratigraphy suggested in recent ‘global cycle charts’. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas.

A long-standing problem in British stratigraphy has beenthe phases of deposition within these sequences. definition andunderstanding of the junctionbetween the The twomost importantexposures of the transitional Lower Cretaceous Purbeck Beds and the Wealden Beds in strataare at Mupe Bay (SY 844797) andBacon Hole the WessexBasin (sensu Whittaker 1985) of southern (SY 841797) on the (Fig. 1). These are the England (Strahan 1898; Arkell 1947; Howitt 1964; Morter only localities in the WessexBasin where the transitional 1984; Lake & Shepherd-Thorn 1987). There is here a strata are well exposedand not extensively faulted.Both transition,from evaporitic and lagoonal to fluviatile localities have restricted access asthey lie within the sediments, which has long been interpreted as a regressive Lulworth firing range. The exposure at BaconHole has succession, modulated by minorfluctuations in salinity recently (1986) been enhanced by landslipping. (Arkell 1947; P. Allen & Keith 1965; P. Allen 1975, 1981; The sedimentary facies at these two localities represent a Morter 1984; Anderson 1985) but essentially conformable range of environments,from lagoonal through to fluvial (e.g. Arkell 1947; Hughes & Croxton 1973). In view of the (Fig. 2). Abasal fine sand(unit 1 in Fig. 2) provides a importantRyazanian and Valanginianerosive and con- definite link between the twoexposures, being traceable densedepisodes in themarine successions of eastern across the cliff-top. A massive green, red-mottled, mudstone England (Casey 1973; Rawson & Riley 1982), the apparent (unit 2) seen at Bacon Hole probably continues to the gully continuity in southern England is somewhat surprising and with cliff stairs at Mupe Bay. It is overlain at both localities seems to contradict the assertion of Haq et al. (1988) that by fine sandstones and mudstones(units 3-6). The lower this part of the Lower Cretaceous records two of the largest erosion surface at Mupe Bay (WB2 in Fig. 2) truncates the eustatic sea-level falls that occurred during the Mesozoic. stratification in the underlying sandstone and is overlain by In discussing the role of sea-level in influencing Wealden greymuds with laminationconcordant tothe erosion stratigraphy, P. Allen (1981, p. 378)modified his 1959 surface. This surface cannotbe recognized unambiguously at hypothesis onthe overwhelming dominance of eustatic Bacon Hole, possibly because of truncationbeneath the sea-level changeagainst other factors,partly on the basis overlying erosionsurface. At Mupe Bay theupper, that there was a lack of evidence for deep erosion during prominent, erosion surface (WB3) occurs at the horizon of supposed phases of eustatic sea-level fall. Asimilar point the well-known conglomeraticpalaeo-oilseep. WB3 prob- was made by Hallam (1984)with regard tothe onset of ably correlates with the erosion surface at Bacon Hole which Wealden deposition. In this paper we present evidence from separates unit 6 from 7. southDorset demonstrating the presence of twomajor Unit l (Fig. 2) has Seen chosenby most workers as being erosionsurfaces, whichwe interpretas unconformities. the first major sandstone in the Wealden Beds of this area, These are associated with the change to fluvially-dominated the base of the Wealden Beds being arbitrarily taken at a environments and we suggest that regressive continuity is in point 2.35m lowerwhere thin sandlaminae first appear fact illusory. In addition we present evidence, in the form of (Strahan 1898 p. 100; Arkell 1947). More recently P. Allen facies analysis, showing distinct transgressive and regressive (1975) andMorter (1984)have classified the uppermost 105

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82 83 84 85 t)l GAULT1U.G.S ALBIAN

1 West”Lu1worth I

80

WEALDEN Cove Bacon HAUTERIVIANBEDS Hole l km INA 4AL ANGlNlAN

U.RYAZANIAN BEDS

- .. approximate positionof the strata under consideration.

strata of the PurbeckBeds Durlston Formation with the interpretation,but other beds are also briefly described. Wealden Beds. In this paper we prefer to follow the purely The successions are summarized in Fig. 2. lithostratigraphic usage of past workers. Laminated purplelgrey mudstone with shelly limestone Geological setting The sedimentsunderlying unit 1 are grey topurple, The Mupe Bay and Bacon Holelocalities lie onthe moderately well laminated muds, with occasional fine sand southern(downthrow) side of majora E-W-striking intercalationstowards thetop. Limestones containing synsedimentary fault system, evident as the Purbeck-Isle of abundant Viviparus occur 7 m below unit 1 and lower in the Wight Disturbance (Fig. 1B; Colter & Havard 1981; sequence. Thetransition with the overlying sandstone is Stoneley 1982; Chadwick 1986; Karner et al. 1986; Penn et gradational. al. 1987; Selley & Stoneley 1987). The fault system forms the Thisfacies is interpreted as deposited in afresh to northern margin of the Central Channel Basin (Whittaker brackish lagoon onthe basis of bivalve associations, 1985). Subsidencehistories (Chadwick 1986; Karner et al. ostracode assemblages and palynofacies (Morter 1984; 1986) andstrata1 geometries (Stoneley 1982; Selley & Anderson 1985; Clements et al. 1987). Ostracodefaunas Stoneley 1987) show that the main extensional activity on have a progressively greater fresh-water aspect up through thefault system tookplace in earlyJurassic and late the top Purbeck Beds (Anderson 1985). Jurassic/earlyCretaceous times, and that the faults were probably active throughout deposition of the Purbeck and Cross-stratified and rippled fine white sandstone and Wealden Beds. brown mudstone Unit 1 MupeBay and Bacon Hole. The basalWealden Facies and environments sand,unit 1 (Figs 2 & 3), comprises coarsely and finely We concentrate on three key units (units 1, 6 & 7 at Mupe interlayered fine sand and mud with moderate amounts of Bay) whichallow theleast ambiguous environmental dark brown comminutedplant material. The sandsare

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Bacon Hole fluvianle 7

m3- L L 1 l l

l lagmnal/lacusnne WE33

1

l I jl

inner lagmnal shoreline '"a cm n lagmnal 5 basal Wedden, l -B! west side Mupe Bay Fig. 2. Sedimentary succession at Mupe Bay (SY 844797) and Bacon Hole (SY 841797). Arrows indicate palaeocurrent directions, north at top of page. Unit numbers do not necessarily imply correlation between the two localities.

white, quartzose,moderately poorly to moderately well sorted,sub-angular to angularand of low sphericity. The muds are medium-brown and often interlaminated with silt or very fine sand. Three main primary sedimentary structures are present: tabular cross bedding, climbing-ripple cross lamination, and mud-draped symmetrical ripples (Figs 3 & 4). Tabular cross-strata vary in their complexity. Cross-strata1 planes are picked out by finely comminuted plant debris (bed la) or the concentration of fine sediment as drapes in toe-sets (Fig. Fig. 3. Detailed log of the Mupe Bay succession. Basal Wealden in 3, bed le; Fig. 4B). Locally, small-scale cross-laminae dip in the sense of Strahan (1898) & Arkell (1947). the opposite direction to the larger scale cross-stratification (bed lc). Tabular cross-sets in higher parts of unit l at Mupe Bay are apparentlysimple with nodrapes. Trochoidal highly undulating and discontinuous contact between sand wave-ripple profiles (wavelength 50 mm, height 8 mm) are and. mud layers. Followed laterally to the base of the cliff present in several beds (lj, In & It), overlain by clay drapes the bed is seen to be highly deformed (Fig. 5), and horizons preferentially preserved in the troughs (Figs 3 & 4A). lr and1s are truncated againstmassa of deformed The correlativeunit at BaconHole (Fig. 2) is also sediment.This massis itself truncatedand overlain by tabularcross-bedded, particularly the lower half. The cross-stratified sands continuous with the lower half of It. cross-sets are stacked and have drapes of comminuted plant The toe-sets are extremely rich in dark brown comminuted materialand mud. AtMupe Bay, where the exposure is plant debrisand below this the sediments are highly better, symmetrical ripple-laminated sands overlie climbing- deformed. This cross-set is again truncated and overlain by ripple sets, and are themselves overlain by thin muddy units another clean cross-stratified sand,continuous with the (e.g. Fig. 3, li-lk and lp-lq). At all sections, the base of upperpart of bed It or with lu. Again, soft-sediment unit 1 is significantly coarserthan the remainder of the deformation is apparentinthe lower parts of the unit, and the upper beds are substantially cleaner. cross-stratified bed (Fig. 5). Soft-sediment deformation is a common feature of this Trace fossils, in the form of simple vertical cylinders with facies. Inunit 1 atMupe Bay bed lq shows anirregular structureless fills and larger vertical burrows with concave-

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an environment of background wave activity, which reworked the current-supplied sand. Any coastal environ- ment characterized by both wave andtidal processes is possible. Theubiquitous penecontemporaneous soft-sediment deformationand incision of thesubstrate by steep-sided asymmetrical scours or small channels, as seenin the middle beds of unit 1, is typical of an intertidal zone crossed by a network of highly sinuous tidal creeks. Such environments may have existed in theWealden in sheltered, possibly back-barrier positions attached to the mainland or to the bamer. In thisinstance a mainland-attachedsetting is preferred because of the steady decrease in salinity indicated by the ostracode faunasthrough the Durlston Beds (Anderson 1985) andon account of the overlying fluvial facies.

Other fine to medium sand units. Unit 3 at Bacon Hole and unit 5 at Mupe Bay are broadly similar to unit 1. Unit 3 at Bacon Hole comprises coarsely interlayered sandstone and mudstone with softsediment deformation. Boundaries between the sand and mud layers aresharp, but very irregularly undulating. Both the grey-green mudstones and Fig. 4. (A) Mud-draped trochiodal ripples from bed lj, Mupe Bay, the orange sands lack internal structure to the naked eye. A = 50 mm.(B) Drapes rich in plant matter on tabular cross-set, bed Isolated balls of sand occur within the muds. Sand layers at le, Mupe Bay. the base, and in the centre, of the bed, are cemented with ferruginous carbonate. The upper contact of unit 3 is sharp, up infills, are present in this facies at Mupe Bay. Between the lower contact is not exposed. 5 cm and 20 cm in apparent length, the simple burrows are Unit 5 at Mupe Bay (Figs 2 & 3) is composed of very fine generally less than 0.5 cm in width, and are present in beds to fine, moderately well sorted to well sorted, quartz sand.

lp to lu. The larger, slightly more complex burrows (about Abundant ' anddiscontinuous layers of grey-green lOcm in length and 1cm width) are present in ly and lz. irregularly-shaped mudchips occur. The whole unit is lightly Thesedimentary structures and facies suggest an oil-stained at Mupe Bay. The major sedimentary structures environment influenced by both waves and currents causing are low angle cross-stratification or horizontallamination a net unidirectional sediment transport. The closely spaced (found especially at the bases of beds 0.5-0.75 m thick), and fine sediment drapes in bedform toesets indicate flows with ripple lamination, locally climbing as cosets 15-20 mm thick, net unidirectional sediment transport but with periodic slack with migration directionstowards the north. Trace fossils water. Locally opposed cross-laminae on foresets (bed lc) are present in the form of ill-defined, downward-branching suggest that flow reversalstook place (separationzone structures, picked-out by a darker oil-stain. These extend to counter currents are unlikely because clay drapes indicate between 5 cm and 10 cm vertically and occur within, and at distinct and repeated slack waterperiods). The unidirec- the top of, ripple-laminated beds. Thin beds with evidence tional sediment transport is most likely to have taken place, of soft sedimentdeformation occur atthe bed junctions therefore,under the influence of tidalcurrents. The 5c/5d and 5dt5e.Here, rotated blocks of silty sandstone trochoidal ripple profiles are unambiguous evidenceof wave (about lOcm long) occur within planarlaminated lens- action which produced vortex ripples,followed by periods of shaped beds. less (or no) wave activity which allowed fines to fall-out Environments of deposition of these sandstones are from suspension. Using an extension of the method outlined regarded as similar to unit 1. They are similarly sandwiched in P. A. Allen (1984) and Diem (1985) it appears that these between probable fluvial and lagoonal sediments,but ripples could only have been produced in water depths of contrast in not showing diagnostic evidence of either wave less than 2 m. The climbing unidirectional cosets indicate or tidal activity. high rates of bedaggradation under flows carrying high Unit 6 at Bacon Hole is a whitetgrey, moderately well suspended fluxes. Suchsediment-charged currents entered moderatelyto poorly sorted, predominantly medium

l

Fig. 5. Sketch of soft-sediment defor- mation and channel cutting inbasal sand lm (unit 1) at Mupe Bay. Vertical burrows W wmmon in bed It.

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grained,quartz sand. Occasional clay bands are present. unit 6 at Mupe Bay (Fig. 3). Bed 6a (0-6 cm thick) has very One 5 cm thick band near the base is a very plant-rich clay smallmudchips at itsbase and a ripple laminated upper finely interlayered withvery fine sand, with common portion. These ripple cross-sets have symmetrical tops and horizontal tubular sand-filled burrows upto 1cm in unidirectional internallaminae dipping south. The sand diameter. A second clay band in this unit contains much less gradesediment is poorlysorted and the grains are plant matter and the sands from above are load-casted into subangular to subrounded and generally of high sphericity. it. Darkhorizontal streaks are also present in thesands; Local truncation of laminae is common, particularly beneath these are thin and laterallydiscontinuous. Dark especially plant-matterrich, dark coloured horizons and purple/black spots and streaks (0.5-1 cm long), which may often in theform of pull-apartstructures with truncation be bituminous, occur at one horizon in unit 6. Towards the againstlow-angle extensionalsynsedimentary microfaults. top of the unit primary sedimentary structures are lacking, Larger-scale soft-sediment deformation includes folding and but ill-defined dark brown horizontalstreaks (c. 2-5 cm thrusting of lamina-sets upto about 15 cmthick and thick) occur with a 10-50 cm spacing. homogenization of sediment masses. Starved ripples of very The sedimentary environment of this facies is uncertain, fine to fine sand,some with northwarddipping unidirec- but is probably in regressive continuity with the underlying tional internal laminae, are seen in units 4 and 6 at Mupe lagoonal sediments as the junction is not erosional. Bay. In asecondary category of soft-sedimentdeformation structures characteristic of this facies, are clean silt dykelets Mottled green mudstone (Fig. 6) in unit 6 at Mupe Bay and unit 4 in Bacon Hole. This facies is exposed only as unit 2 in Bacon Hole, but is These are generally less than 2 mm wide and up to 50 mm likely to be present in the gully where the path leads down long. They are generally vertical, taperingup and down, and to Mupe Bay. It comprises massive light greenmudstone ptygmatically folded, and are most common in the darker with abundantirregular blood-red mottles. Thereare no muds. Some evidently originatedfrom clean silt layers, distinctive sedimentary structures. At Bacon Hole the facies others have no obvious terminations at silty laminae in the grades up from the underlying fine sandstone. A moderately plane of section. Dykelets cross-cut all other soft-sediment drained fluvialflood plain environment of deposition is deformation structures and are most common in the top half suggested.Similar facies occur higherin the Wealden of unit 6 at Mupe Bay. succession wherethey are associatedwith fining-upward Trace fossils are completelyabsent. Large pieces of sandy channel-fills and show rootlet traces. wood(cross-sectional width of greaterthan 10cm) are locally common. Sparse carbonate concretions (c. 10-15 cm long) occur at the base of bed 6d at Mupe Bay, and small Laminated and deformed purplelgrey muhtone pyrite nodulesare locally developedaround wood This facies is best exposed in unit 6 at Mupe Bay, and is fragments. recognizable in unit 4, both at Mupe Bay and Bacon Hole The mudstone of unit 6 at Mupe Bay has coarse sand (Figs 2 & 6). The sediments are purple/grey, laminated dark laminae and lenses at its base and at about 50cm above. A muds and light silts, with distinctive lamina-sets which can subtle overall upward increase in silt through the unit is also betraced over several metres. Lamination in the muds is seen. defined by variations in plant-mattercontent from The generally fine sedimentgrade and primary predominance to complete absence. Laminae have sharp or unbioturbated delicate laminationindicates a low energy gradationalboundaries; noconsistent size grading is lagoonal or lacustrine environment of deposition. Occasi- evident. Occasional very coarse to coarse sand laminae or onal influxes of very coarse sand may have been supplied by very thin beds are also present, particularly at the base of river floods or storm washover. The preservedbedforms (symmetrical ripples with unidirectional cross-laminae) were produced by residual or subsequent wave-action. The sediments were subject to two or moreepisodes of liquidization (sensu J. R. L. Allen1984), possibly seis- mically induced,at different stages in lithification. Early soft sediment deformation may have been facilitated by the build-up of decaygases commonly observed in such environments.

Oil-stained conglomeratic sand and sandy conglomerate This facies comprises predominantly coarse to very coarse sand, poorly to moderately well sorted,interbedded with discontinuous undulating mud layers. Mudchips and wood fragmentsare very common.Pebbles to boulders of heavy-oil-cemented sandstone occur at itsbase; no other pebble lithologies have beenobserved. This facies is represented by beds 7 and 9 at Mupe Bay, but oil-cemented clasts are knownonly from the base of unit 7. The Fig. 6. Laminated and deformed mudstone, unit 6, Mupe Bay. One sand-sized grains are sub-rounded to sub-angularquartz. dykelet arrowed bottom right as an example. NNE-dipping cross-stratification is prominent indiscon-

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Undulatingferruginous carbonate concretionary layers 5-1Ocm thick arepresent at 30-501x1 intervals. Pyrite nodules (2-5cm diameter) are scattered throughout. This j facies occurs above the oil-stained conglomerate facies at 1 Mupe Bay, and is interpreted as fluvial, possiblya clay plug 1 in anabandoned channel or floodplain overbanksediments. I The lack of oxidized mottlesand abundance of plant I material and pyrite suggest prolonged water-logging. I The uppermostunit (7) at BaconHole isasimilar massive grey clay buthas no plantmatter nor internal structure.

Major erosion surfaces At Mupe Bay an erosion surface (WB2) is present between Fig. 7. Outcrop expression of erosion surfaces at Mupe Bay viewed units 5 and 6 (Figs 2, 7 & 8). This is very gently undulating from beach, erosion surfaces arrowed,thickness of outcrop about andtruncates structures in the underlying strata.It is 6 m. overlain by a thin bed of coarse sand which thickens up to 6 cm locally. In the E-W plane of section (Fig. 8) WB2 dips tinuoussets (10-20cm thick) with undulatingupper and less steeply than the overlying erosion surface (WE33) and is lower contacts and abundant logs showing roughly SE-NW truncated about halfway up the gully. Lamination in the long-axis orientations. overlyingmuds is continuousacross theoutcrop and Shapes of theoil-cemented sand pebbles, cobbles and concordant with WB2, and is apparently discordant with the boulders are unusualand distinctive; all arerounded to base of unit 5. If thebase of unit 5 is taken tobe an sub-rounded and many have embayed margins (Fig. 3). The originally horizontal surface, then it follows that WB2 is an largest is about a metrein length. Pyrite nodules and angular unconformity. occasional light brownmudchips occur in someboulders. Erosion surface WB3 at Mupe Bay separates unit 7 from The basal bed, with an undulatory upper surface, varies in the underlying strata. It truncates distinctive horizons in unit thickness from 25-80cm and is draped with several muddy 6 and surface WB2, cutting down at least two metres when layers separated by discontinuous and thin layers of sand; followed up the gully. The surface is sharp but irregularly irregularsteep erosion surfaces within thisbed have the undulating and overlain by the conglomeratic lower part of overlying cross-sets abutting onto them (Fig. 3). unit 7. The evidence for WB3 at Mupe Bay suggests erosion This facies is thoughtto represent a fluvial channel of compacted mudswhich were cohesive enoughnot to environment in which largeboulders of oil-cemented collapse laterally when the surface was cut.Whether the conglomerate were preserved as a remani6 deposit on the erosionsurface separates parallel strata or involves an channel floor. River flow velocities were not necessarily very angular discordance is not known (Fig. 9). high as the matrix is no coarser than sand grade (except for At Bacon Hole an erosion surface cuts across the sands the mud-chips) anddeposition of mudfrom suspension of units 5 and 6 at ahigh angle (Fig. 2). The surface is also occurred episodically. There may also have been a lack undulating in three dimensions and the underlying sands are of coarse sediment available at source. stained purple to a depth of about 70 cm, the colouration increasing in intensity towards the actual contact. Pockets of smallclay balls occur patchily down to 2cm from the Massive greyand green Jgrey clay surface. The purple colour picks out fracture-like structures Beds 8 and 10 at Mupe are greenish grey slightly micaceous in the sands immediately under the surface, some of which and silty clayswith no macroscopicinternal structure. have a core of cleanwhite sand. Thin purple or Comminutedplant debris is commonnear the base. orange-stained seams, approximately vertical with respect to

Q. 8. W-E outcrop in gully at Mupe Bay showing erosion surfaces; stipples denote strata1packages divided by erosion surfaces.

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A

1 0-1 iI m* l’\ Erosion (missing time) wB3 -- l’ oil-cementedsandstone , 1 km / Fig. 9. W-E cross-sectional interpretations of WB3 and relation- l ships of the oil-cemented bouldersto their source (localities as shown in Fig. 1A). (A) Angular truncation of underlying strata.(B) Channelled erosion surface between parallel strata. 1 l \ the erosionsurface, descend at least1.5 m intothe Erosion (missing time) m23 underlying sands. The clays above show no discolouration. / In the centre of the present day slump scarclays of unit 7 lie on those of unit 4, having cut down at least 5 m. Colouration of the underlyingsandstone strongly suggests thatthis may havebeen a surface of prolonged exposure. We correlate it with the most prominent of the two majorerosion surfaces at Mupe Bay (WB3), but recognize an element of uncertainty because we are unable to positionunambiguously the overlyingclay within the Wealden succession. Selley & Stoneley (1987) argued that oil migrated to the surface in south Dorset early during Cretaceous times. This view has received supportfrom geochemical studies (Cornford et al. 1988)indicating a greater source-maturity for the oil in the matrix than in the boulders of unit 7 at Mupe Bay. A possible provenancefor the boulders isa WB1 palaeo-oilseeppreserved in situ in the WealdenBeds at and StOswald’s Bay (C.Clayton pers. (at about comm. 1989). The current velocities necessary to move the this level) oil-cemented boulders are hydrodynamically incompatible (Walker 1975, p. 139) with the associated matrix. They may have beenseparated from their source through local collapse of a river bank or by another,more extensive, process of surface degradation. Two possible scenarios for 1 0-1 the relationship above and below WB3 are shown in Fig. 9. Fig. 10. Schematic summary of observed stratigraphy, inferred water depth and sequence stratigraphic interpretation. Discussion A summary of the relativewater-depth through time, WB3 (Fig. 10). Thesecan be interpreted as sequence indicated by the succession described above, is shown in Fig. boundaries (Mitchum et al. 1977; Van Wagoner et al. 1988), 10. Making the simplifying assumption of approximately reflecting reducedand negative rates of accommodation. constant rates of sediment supply through time, the water Figure 10 incorporates no assumptionsas to whetherthe depthcurve may betaken to parallelrelative sea-level. inferred drivingsea-level change was local, regional or Increases in sea level (relative to the sea bed) make space global in nature.Nevertheless, our interpretation of the available forsediment to accumulate,i.e. accommodation local sea-levelhistory bears a remarkableresemblance to (Posamentier et al. 1988). The level at which marginal (inner that shownby Haq et al. (1988) forthe Ryazanian to shorelineand fluviatile) facies appearabruptly is labelled Valanginian interval.This predicts a minor(conformable) WB1, and the major erosion surfaces are labelled WB2 & sequenceboundary closelyfollowed by two major

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(unconformable) sequence boundaries. Their Ryazanian to Conclusions Valanginian sea-level curvewas apparently constructed from Two major erosion surfaces are identified in the Wealden sections in southern France and Switzerland with no input Beds (Lower Cretaceous) of southDorset which has from the Dorset Coast. hitherto been taken to be a succession essentially unbroken Few data are available regarding the precise age of the by majorstratigraphic gaps. Regressive andtransgressive Mupe Bay basalWealden. Anderson (1985,Fig. 5) phases of sedimentation areevident, fluctuating between tentatively placed the base of the Wealden in Dorset at the lagoonal and fluvial depositional environments. The major top of his ‘Battle’ostracode faunicycle, whichwas the erosion surfaces are interpreted as unconformities, and the highest faunicycle recorded in Dorset. Therefore, the basal most prominent of the two is overlain by reworked boulders Wealden sand at Mupe Bay probably lies at the top of the from apenecontemporaneous oilseep. The two unconfor- Cypridea setina ostracodeZone (which has the‘Battle’ mities,and a levelclose to the regressive basal sand, are faunicycle atthe top) orthe base of the Cypridea candidatesequence boundaries. Whether the causative brevirostrata ostracodeZone. Rawson & Riley (1982) base-level changes were local, regional or global in extent tentatively considered the base of the Dorset Wealden to remains to be determined by accurate correlation. However, correspond to the borealstenomphalus ammonite Zone. The the candidate sequence boundaries areof ages close to those ‘129 Ma’ minor sequence boundary of Haq et al. (1988) is predicted by recent ‘global’ sea-level cycle charts and alsoindicated as occumng in the stenomphalus Zone, apparently of similar magnitudes, despite the evident major although it is not explicitly stated how this date was arrived synsedimentary fault activity in the Dorset area. at. The ages of WB2 and WB3 are not closely constrained by any ostracode data. On the basis of palynology, Hughes Thisstudy was supported by researcha grant from British & Croxton (1973) thoughtthat the middle of theDorset Petroleum which we gratefully acknowledge. We thank A. Coe, H. Wealden(the Coarse Quartz Grit) lay atabout the Jenkyns, J. Kennedy, A. Ruffelland G. Wachwho contributed useful discussion in the field; P. Allen, G. Plint, J. Kennedy and H. Valanginian/Hauterivianboundary, Hughesbut & Reading made valuable suggestions for improving the manuscript. McDougall (1990) now favourageanabout the D. Batten and J. Athersuch examined specimens for microfossils. Hauterivian/Barremianboundary. It is therefore possible H. Ivimey-Cook (BGS) kindly provided information concerning the thatthebasal Wealden beds in Dorsetareearly late F. W. Anderson’s unpublished results. J. York aided with the Valanginian. preparation of diagrams and R. MacAvoy helped print some of the Thus correlation of the unconformities with the major photographs. late Ryazanian and early Valanginian sequence boundaries of Haq et al. (1989)may be entertained. Considering the Dorset sections in isolation, and in view of the proximity of References these localities to a major syn-sedimentary fault system, a ALLEN, J.R. L. 1984. Sedimentary structures, their character andphysical substantial case could be made for a local tectonic origin for basis. Developments in Sedimentology, 30, 1-593, 1-633. thesesurfaces. Likewise someaspects of the succession ALLEN,P. 1959. The Wealden environment: Anglo-Paris basin. Philosophical Transactions of the Royal Society, London, B242,283-346. (viewed in isolation) could be accounted for by changes in - 1975.Wealden of the Weald: a newmodel. Proceedingsof the sediment supply or stream power. However, it is now clear Geologists’ Association, London, 86,389-438. thatcandidate sequence boundaries occur in parts of the - 1981.Pursuit of Wealdenmodels. Journalof the Geological Society, Dorset succession thathitherto would appearto have London, 138,375-405. contradictedthe supposed ‘global’ sequencestratigraphic - & &m, M.L. 1965. Carbn isotope ratios andpaleosalinities of Purbeck-Wealden carbonates. Narure, m,1278-80. scheme of Haq et al. (1988). ALLEN,P. A. 1984. Reconstruction of ancient sea conditions with an example Depth of erosion on a scale comparable to the surfaces from the Swiss Molasse. Marine Geology, 68, 455-473. describedhere for Dorset evidently did occur in the ANDERSON,F.W. 1985. Ostracod faunas in the Purbeckand Wealden of Wealden Beds elsewhere, 10 m being common in the Weald England. Journal of Micropalaeontology, 4, 1-68. ARKEU,W. J. 1947. Geology of thecountry around Weymouth, , (Allen 1975). One striking example is the ‘Haddocks Rough Corfe and Lulworth. Memoirs of the Geological Survey of Great Britain. Unit’ of theAshdown Beds at Fairlight in Sussex (Allen CASEY,R. 1973. The ammonite succession at the Jurassic-Cretaceous 1975,1981; Stewart 1981; Lake & Shephard-Thorn1987). boundary in eastern England. In; CASEY,R. & RAWSON,P. F., (eds) The This is an erosion surface with relief of about 10m, infilled Boreal Lower Cretaceous. Journal of Geology Special Issue, 5, 193-266. CHADWICK,R. A. 1986. Extension tectonics in the WessexBasin, southern with interbedded sands and muds which were deposited on England. Journal of the Geological Society, London, 143, 465-488. surfacesinclined at up to 19”. The unithas been inter- CLEMENTS,R. G., BATEN, D. J., LISTER,J. K. & MACLELLAN, A. M. 1987. preted tentatively by P. Allen (1975) and Stewart (1981) as SampleD66. 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Received 22 November 1989; revised typescript accepted 10 March 1990.

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