The Structural Development of the Wessex Basin

Home , Wessex Basin

J. geol. Soc. London, Vol. 139, 1982, pp. 543-554, 5 figs. Printed in Northern Ireland. The structural development of the Wessex Basin

R. Stoneley

SUMMARY: The Mesozoicand Cenozoic structures of southernEngland form part of a system of disturbances which extendsacross the Channel into northern France. They are reviewed in the light of published data from south Dorset. Basin development started with Permo-Triassicrifting, and isbelieved to havecontinued through the Jurassic and early Cretaceous with intermittent growth on deep-seated listric normal faults, probably associated with roll-over anticlines. This regime had ceasedby the Aptian and the basin passed through a period of relative stability in the late Cretaceous, followed by tectonic inversion and, in the Miocene, by northwards compressive movement along the former normal faults. This partially restored the earlier separative displacement at depth, and probably accentuated pre-existing roll-over anticlines: it was expressed in the surficial Upper Cretaceous-Palaeogene sequence as monoclinal flexuring. The early rifting and the change from overall N-S crustal stretching to compression may reflect plate tectonic events in the North Atlantic region.

The structures spectacularly exposed in Mesozoic and regional tectonic system. These disturbances are virtu- Tertiary strata along the coast of Dorset have for long ally confined tothe area of the Mesozoic-Cenozoic excited geologists. They have been studied in consid- basin,and it maythus be surmised that they are erable detail at outcrop, and satisfactory explanations directly related to the post-Hercynian evolution of the havebeen reached for the observed features (e.g. basin itself. Arkell1947; Phillips 1964). Their interpretation in Most of thezones of structuraldisturbance show depth,however, has remained a subject of specula- evidence of activity through muchof the Mesozoic and tion: normal, reverse and strike-slip faulting, free fold- Tertiaryand, in the better known anddocumented ing of the sedimentary cover above basement, and salt cases, suffered inversion (reversal of sense of vertical movements have all been invoked. With the present displacement) during the Cretaceous. The precise tim- phase of petroleum exploration following the discov- ing of thisinversion is frequently uncertain, due to ery of theWytch Farm oilfield (Colter & Havard lack of critical control: closely spaced boreholes have 1981; Hinde 1980), however,it has become important shown that, on theBray-Bouchy-Juvanz6 and Rouen- to try to reachan understanding of theirdeeper Sennely faults of the Paris Basin, it dates from about causes. theAptian (HCritier & Villemin1971). InDorset, This paper attempts to explore the tectonic signifi- Jurassic-early Cretaceous movement in the one sense cance of thesestructures, through consideration of terminatedat about this same time, to be followed their regional setting in the Wessex Basin and through after a depositional hiatus by relative stability in the the use of natural scale cross-sections extrapolated to late Cretaceous and the commencement of reversal at depth. It is based entirely on published and publicly the end of the Cretaceous: seismic profiles have con- available data: no access has been possible to privately firmed that such inversion was widespread along this owned and confidential seismic sections nor to some and other lines of disturbance in southern England (P. recentborehole information. It should therefore be W. Mikkelsen,pers. comm.). A similarstory comes regarded as an interim review, pending the release of fromthe southern North Sea and northern Europe these data. (Kent 1978), where the earlier regime came to an end with theAptian and inversion uplift dates from the Regional structure Senonian or, perhaps, Turonian (Hancock & Scholle 1975; Ziegler 1981). The evidence is consistent with The S Dorset-Isle of Wight belt of structural distur- the suggestion that a change of tectonic regime dates bance (Fig. 1) forms a part of a wholeseries of fromthe time of thewidespread late Cimmerian interconnected and more or less analogous linear fea- hiatus-post-Neocomian (Wealden)and prior to the tures, extending through central southern and south- ‘Greensand’transgression which was diachronous easternEngland, the English Channel and Western from the Aptian to the Cenomanian (sub-Aptian and Approaches, and across the Paris Basin (e.g. Smith & sub-Albianbreaks are both present in the Isle of Curry 1975). Sharp monoclinal flexures or faults, often Wight);the date of actualinversion uplift seemsto withassociated anticlines, separate regions in which have varied from place to place. the dips seldom exceed a few degrees. The trends of the main structural lineaments vary One hasthe impression of rather rigid basement from predominantly ENE in the Western Approaches, blocksseparated bylines of relativedisplacement, through more or less E-W in the central and eastern which formthe inter-related expressions of a single Channel and southern England, to SE and eventually 0016-7649/82/0700-0543$02.00 @ 1982 The Geological Society

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/4/543/4887636/gsjgs.139.4.0543.pdf by guest on 30 September 2021 The structural development of the Wessex Basin 545 N-S in France: they give the impression, in plan, of The S Dorset- beingarranged concentrically around the Cherbourg Isle of Wight belt Peninsula, which may have exercised some control as a rigid block. A prominent NW trending flexure, proba- Ithas long been known that, along this belt,the bly a prolongation of the combined Bray and Rouen- southern side moved downwards on normal faults in Sennely fault zones, continues at least to a point E of the mid-Cretaceous, but that this sense was reversed the Isle of Wight and may extend farther at depth, to in the Neogene (e.g. Arkell 1947; House 1961). We the Vale of Wardour line (Mottram 1961): it provides maydevelop this generaltheme by considering the a cross-link between several components of the system structures firstly in plan view and then in profile. (Smith & Curry 1975) and may have suffered sinistral strike-slip movement in the Mesozoic (Ziegler 1981). A parallel line of minor faulting is present to the W in Plan view (Fig. 2) Dorset (Fig. 2) andthe trend is visible onsatellite The monoclinalflexure in theUpper Cretaceous images both there and in Somerset: there may also be and Tertiary beds of the Isle of Wight and Purbeck is evidence that a positive element with a NW trend was commonlyshown on maps as a continuous, more or activein Dorset in the late Cretaceous (Drummond less straight line. It is neither! It is offset en echelon 1970). In addition to these, a N-S (possibly Malver- sinistrally inthe western half of the Isle of Wight, noid) trend is also present at the western marginof the possiblydextrally beneath the sea S of Christchurch Wessex Basin, in the Tertiary Beer Syncline; it may be (suggested by sea-floor outcrops, Dingwall 1971), and reflected in the alignment of the Cherbourg Peninsula dextrally again in Dorset where the Purbeck Mono- withstructural culminations in thecentral Channel cline appears to die out westwards to be replaced to the possibly extending into the Hampshire Basin. N by theRidgeway-Abbotsbury Fault. Within this These directions cannot entirely follow Armorican- latter offset, a further short intermediate faultlflexure Hercynian trends, which nevertheless certainly appear is probablydeveloped on the Ringstead Anticline- toprovide a strong influence. The NW (Charnoid) Picnic Inn Fault. elementin particular is apparentlyanomalous, al- Each of theseen echelon sectors of the flexure though it is parallel to long-standing dextral strike-slip consists of up to 5 arcuate segments, always concave faults in Cornubia (Dearman 1963; Webby 1965). towards the S. This effect is slightly exaggerated in Fig. Theco-existance of varioustrends leads one to 2, but it is clear that the traces are scalloped towards doubt the universal control of earlier orogenic struc- the S. tures, and to suspect therefore that the basin develop- Aseries of anticlinalfolds parallels the flexure at mentmay be primarily aconsequence of Permo- distances of up to 3 km to the S. These folds again are Triassicrifting associated with the early attempted not continuous but may be limited axially to the extent formation of the northern Atlantic. The Wessex Basin of the adjacent arcuate segment of the flexure. This may thushave been related originally to the North association, albeit somewhat tentative, may suggest a Sea and to other Permo-Triassic basins of the British genetic relationship. Isles region, of which the nearest is in central Somerset The intriguing sharp anticline lying S of the Ridge- (Whittaker 1975, 1980). way Fault,with its threeculminations atSutton An indication of early strong vertical movements is Poyntz,Poxwell and Chaldon Herring, is virtually provided by thepresence of alocalized E-W basin restricted to the sector where the Ridgeway Fault and with about 1500 m of Permo-Triassic beds, including Purbeck Monocline overlap en echelon. It is prominent appreciablethicknesses of halite,encountered in in the local scene and contrasting attempts to explain boreholes at Winterborne Kingston and Nettlecombe it(e.g. Arkell 1947; Ridd 1973) have been put for- (Fig. 2). This saltmay, incidentally, account for the ward; it seemsto the author, however, that itsin- curious small circular inlier of Middle Jurassic in the terpretation should rather be sought regionally, in the Chalk at Compton Valance, which is strongly sugges- light of the S Dorset-Isle of Wightbelt as a tive of diapirism (Falcon & Kent 1950). whole.Whatever its explanation(considered below), Theseearly vertical movements became muted in there is clearevidence that an anticline was already the late Triassic and Jurassic, when continuous gentle present in the mid-Cretaceous (Taitt reported in Ar- displacement mayhave been confined tothe major kell 1947; Mottram & House 1956), and that it was lines of disturbance; subsidence in general in southern further developed in the Tertiary. Britain appears to have become rather uniform in the late Jurassic (Hallam & Sellwood 1976). However, the Cross-sectional view (Fig. 3) erosionpreceding the Aptian transgression, and the episodes of subsequentinversion, may perhaps have Five cross-sections have been constructed across the been related to further stages in the opening of the flexure in Dorset, positioned to take advantage of the North Atlantic (e.g. Hallam 1971; Roberts 1974). controlavailable. It is emphasizedthat these have

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been drawn on the basis of published information only the widespread occurrences of Kimmeridgian ammo- andthat, in places, thicknesses become conjectural. nites in the Aptian (P. E. Kent, pers. comm.), there- Some general points may be made. foreimply absolute uplift anderosion in the early All writers are agreed that the Tertiary movements Cretaceous,as opposed to mere stability while the represent a reversal of displacement along the same southern side subsided. majorfaults that were active before the mid- The limitedcontrol available for the Jurassic sug- Cretaceous: it is possible, however, that some minor gestsconsistently that the thicknesses of thevarious splays,as encountered, for example, in boreholes at formations,particularly in the MiddleJurassic, are Chaldonand Poxwell (Taitt & Kent1939), may be greater S of the flexure than to the N, as noted alsoby entirely of Tertiary origin. No explanation is at present Colter & Havard (1981) and confirmed by A. Whit- forthcoming for the fact that the reversal of movement taker (pers. comm.). Presumably, therefore, the major caused theUpper Cretaceous-Tertiary beds to be faults were active during the Jurassic, although proba- flexed in contrast to the earlier clean faulting, unlessit bly at variable rates (P. W. Mikkelsen, pers. comm.). be a function of the gross lithology. Nevertheless, the This raises two questions: (i) Was the Wealden depo- small scale sections included here are consistent with sited to itsfull thickness, if at all, to the N, for the explanations of Arkell (1947) and Phillips (1964) example at Wytch Farm? (ii) Did these growth faults forthe detailed structures mapped at surface (the develop a roll-over anticline on the downthrown side, Lulworth Crumple, Ballard Down Fault, Durdle Door with a further localized thickness increase adjacent to Thrust, fracture sets, etc.)-that they are reflections of thefault itself? There is little directevidence from localizedcompression at the foot of the flexureand which toanswer these inter-related questions, al- consequent relief of pressure, perhaps aided by gravi- though 'large boulders of Wealden sand in Wealden tational collapse. sand (shown up by marked differences in oil impregna- The flexure marked by the Ridgeway Fault dies out tion and porosity) may be symptomatic of contempor- E of theChaldon Herring anticlinal culmination. A ary degradation of a nearby fault scarp' (P. E. Kent, continuationbeneath the Upper Cretaceous might pers. comm.). The existence of a mid-Cretaceous an- perhaps link with one of the faults known from the ticline on the Poxwell-Chaldon feature could well be WytchFarm oilfield (Colter & Havard1981) which, accounted for in terms of roll-over. Indeed the very however, show no post-Aptian displacement. As this geometry of a listric growth fault in effect leads to the flexuredies out, it is overlappeden echelon by the development of such a structure. Thus we may be led Purbeck Monocline nearly 4 km to the S: both appear topostulate that, in Dorset, the zone of thickest on Section C-C' (Fig. 3). Wealdenaccumulation was in factimmediately S of There is little direct control on the attitude of the thePurbeck Monocline and, consequently, that the faults at depth, although boreholes at Chaldon (Taitt Purbeck,Kimmeridge and other anticlines have at reported in Arkell1947), Poxwell (Taitt & Kent least some roll-over component. Despite some proba- 1939) and Radipole (House 1961) might imply dips to bly local constituents, such a situation, of course, need the S between 45" and 60". However,geometrical not necessarily have expression at the sea-floor or in construction of the asymmetrical Weymouth and Pur- thefacies of thesediments provided deposition was beck Anticlines (Sections A-A' and D-D' respectively, frombottom traction currents (e.g.Hallam & Sell- Fig. 3) stronglysuggests that the anticlinal axes are wood 1976; Chapman 1973, p. 87 et seq.): indeed, the displaced to the S and hence that the major faults tend primary detritus of the Wealden is far travelled-from to flatten with depth. The implication is that, prior to Cornubia (Arkell 1947). theAptian, these were listric normalfaults The effects of the Tertiary flexuring on the attitude downthrowing S: this is supported by theircuspate of theearlier faults and the immediately adjacent tracesin plan. They have been drawn following the pre-Aptian beds cannot be determined accurately with shape of acycloid, the theoretical profile of a listric the control at present available. The fault planes must normal fault intersecting the free surface at 45" (Hose have been bent downwards near the surface, inviting & Dane5 1973): this appears to be consistent with the the development of splay thrusts in the Tertiary. An positions of the extrapolated anticlinal axes at depth. attempt to illustrate this has been made, rather diag- On the southern side of the flexures/faults, the full ramatically, on the sections, and it has been assumed succession is present up to and including the Wealden. that at depth the Tertiary effect was merely to restore Tothe N,however, the Upper Cretaceous rests on some of the earlier normal offset. There is no factual UpperJurassic and possibly even Middle Jurassic justificationfor these interpretations, and equally beds:within thearea covered by thecross-sections, there are insufficient data to attempt an accurate pre- this is generally the Oxford Clay, although the Kim- Neogenereconstruction. A sketch of thesituation meridge Clay appears in wells further to the N(Fig. 2). envisaged is shown in Fig. 4: it could be matched on There is no reason to doubt that the complete Jurassic seismic profiles from a number of the world's conti- sequencewas deposited across the entire area; ab- nental shelves. sence of the higher Jurassic beds N of the flexure, and It is instructive to use these cross-sections to try to

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Sutton Poyntr- Poxwell- Kimmeridge Chaidon Kimmeridge SOUTH I NORTH I I I I

UPPER CRETACEOUS I I WEALOEN PURBECK

JURASSIC

LlAS PERMO-TRIASSIC

HERCYNIAN BASEMENT

FIG.4. Diagram showing the generalized interpreted profile across south Dorset at the close of the Cretaceous. Not to scale.

estimate the magnitudes of the crustal displacements geothermalgradient, Level the of Organic that have taken place, both vertically and laterally, in Metamorphism (LOM) can be predicted theoretically southern Dorset since the early Mesozoic. The scaleof (Hood et al. 1975) andcompared with the actual the vertical movements can be illustrated by compara- values. Measurements at Imperial College of the LOM tive burial history graphs of sequences on either side of the Kimmeridge Clay at outcrop (R. R. F. Kinghorn of the Purbeck Monocline (Fig. 5); sufficient data for & M. Rahman, pers. comm.) are consistent with the tentativeinterpretations are available from Wytch valuespredicted from the burial depths shown in Farm(Colter & Havard 1981) andKimmeridge Bay Fig. 5. (Brunstrom 1963, andoutcrop data). A potential This burial history is in general compatible with the method for checking the order of magnitude involved concept of basin subsidence due to crustal stretching is offered by the thermal alteration due to burial of and thinning by listric normal faulting (e.g. McKenzie organic matter in the sediments: assuming a constant 1978), although itmay be difficult to explainthe

WYTCH FARM WYTCH KIMMERIDGE BAY

LlAS Y.JUR. KIM. E.CRET. APT. LATE CRET. PALEOG. NEOG. LIAS M.JUR. UIM. E.CRET APT. LATECRET. PALEOG. NEOG.

200 420 80 40 0 m".

70-

80-

90-

7 2km FIG.5. Comparative burial history graphs on eitherII side of the Purbeck Monocline. ? = thickness estimated. Level of Organic Metamorphism (LOM) valuesare estimated using the curves of Hood et al. (1975), assuming a geothermal gradient of 27 "C/hm.

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500 mplus of uplift N of the flexurein the early whicha period of relative stabilityuntil thelatest Cretaceous. Cretaceous was followed by a tendency to reverse the Is it possible toestimate the horizontal displace- sense of the earlier displacements: this may have been mentsinvolved? Theamount of lateralshortening effective by thebeginning of theCenozoic (Arkell represented by the Tertiary flexure is equivalent to its 1947) but culminated in a Miocene crustal shortening verticalheight, if strata1 stretching is ignored;in of up to 1.5 km, expressed along the pre-existing lines Dorset the figure reaches the order of 1.2 km in the of weakness as a monoclinal flexure. vicinity of WytchFarm, and itmay be asmuch as 6. This reversal of sense of displacement represents 1.5 km in the Isle of Wight. This must be less than the the more or less contemporaneous inversion tectonics, cumulativepre-mid-Cretaceous extension since, for whichwere widespread through south-eastern Eng- say the Jurassic, the net effect across the disturbanceis land and northern Europe. still adownthrow to the S: Section E-E (Fig. 3) 7. Arelated effect of reversedmovement could suggests that the present horizontal separation of an have been to accentuate, by compression, pre-existing horizon in the Lias is approximately the same amount. roll-overanticlines. TheSutton Poyntz-Poxwell- The minimumhorizontal separative movement prior Chaldon anticline is believed to have been formed in tothe Aptian may thushave exceeded 2.5 km; a this manner; other, more exotic, explanations are un- similarestimate may be reached by combiningthe necessary. burial history graphs with the cross-section-assuming 8. It is notimpossible that the S Dorset-Isle of that the fault has been drawn at the correct angle! Wight belt has undergone a double inversion, in the sense of Kent(1978, p. 317).The greatest known thickness of Permo-Triassic in the area has been found Concluding remarks tothe N of thebelt, which however wasrelatively Consideration of thestructural development of the positiveduring the Jurassic-early Cretaceous. The Wessex Basin should take account of the following: coincidence is neverthelessnot exact and more data 1. The extent of the basin was approximately coin- are required to resolve the question. cident with its present limits, extending through east- 9.The events recorded by the S Dorset-Isle of ern England, the Channel and Western Approaches, Wightbelt are mirrored onother lineswithin the andinto northern France. Its evolution began with basin. They appear to be correlative with plate move- strong, localized differential vertical movements in the mentsin the North Atlantic region: strong initial Permo-Triassic,believed to be related to contem- riftingin thePermo-Trias, continued rifting through poraneous rifting elsewhere in the British region. the Jurassic and early Cretaceous, and ocean forma- 2. Subsequent subsidence was accompanied by con- tion and widening from the mid-Cretaceous onwards tinuing gentle faulting along persistent lines of weak- (e.g. Kent 1977; Ziegler 1975). ness,which form elements of abasin-wide crustal 10.The termination of rifting andthe onset of fracture system. continental separation in the North Atlantic thus ap- 3. One of theselines, the S Dorset-Isleof pear to coincide with a change from a stateof regional Wightbelt, has been considered in thisreview. It N-S crustalextension in southern England-northern consists of a number of en echelon segments, not all France,to one of overallcrustal compression man- offset in the same sense, and each in plan scalloped to ifested along lines of earlier weakness. In this setting, the S: this disposition is inconsistent with deep-seated one might suggest that the flexure prolonging the Bray strike-slip faulting. The evidence suggests strongly that and Rouen-Sennely faults to the NW across the En- they represent pre-Aptian listric normal growth faults, glish Channelhas adextral strike-slip component, whichhad achieved a horizontal separation of the reversingthe earlier sinistraldisplacement qf the order of 2.5 km from the beginning of the Jurassic to Mesozoic. the mid-Cretaceous. ACKNOWLEDGMENTS.following The have reviewed a draft of 4. Thesefaults were in all probabilityassociated this paper and provided most helpful suggestions, for which withroll-over anticlines on their downthrown sides, the author is deeply grateful: Sir Peter Kent, Dr V. S. Colter, which influenced the accumulation of sediment at least Professor M. R. House, Dr R. C. Selley, JhA. Whittaker. during the early Cretaceous. The illustrations were drafted by Mr A. R. Brown, Imperial 5. This regime was terminated by the Aptian, after College.

References

ARKELL,W. J. 1947. The Geology of the Country around Britain. Int. Petrol. Cong., Sect. l, 11-21. Weymouth, Swanage,Corfe and Lulworth. Mem. geof. CHAPMAN,R. E. 1973. Petroleum Geology, A Concise Study. Sum. U.K. Elsevier Sci. hbl. Co., Amsterdam. BRUNSTROM,R. G. W. 1963. Recently discovered oilfields in COLTER,V. S. & HAVARD,D. J. 1981. The Wytch Farm Oil

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Field, Dorset. In: ILLING,L. V. & HOBSON,D. G. (eds). Britain and Adjacent Regions. Geol. J. Spec. Issue, 10, Petroleum Geology of the Continental Shelf of North-west 309-24. Europe, 494-503. Hayden & Son, London. MCKENZIE,D. P. 1978. Some remarks on the development DEARMAN,W. R. 1963. Wrench-faulting in Cornwalland of sedimentary basins.Earth planet. Sci. Lett.40,25532. South Devon. Proc. Geol. Assoc. London, 83, 265-87. MOTTRAM,B. H. 1961. Contributions to the geology of the DINGWALL,R.G. 1971. The structural and stratigraphical MereFault and the Vale of WardourAnticline. Proc. geology of a portion of theeastern English Channel. Geol. Assoc. London, 72, 187-204. Rep. Inst. Geol. Sci. London, 71/8, 1-24. - & HOUSE, M.R. 1956. The structure of the northern DRUMMOND,P. V. 0. 1970. The Mid-Dorset Swell. Evidence margin of the Poxwell Pericline. Proc. Dorset nat. Hist. of Albian-Cenomanianmovements in Wessex. Proc. archaeol. Soc. 76, 129-35. Geol. Assoc. London, 81, 679-714. PHILLIPS,W. J. 1964. The structures in theJurassic and FALCON,N. L. & KENT, P. E. 1950. Chalk Rock of Dorset: Cretaceousrocks on theDorset coast between White More evidence of salt? Geol. Mag. 87, 302-3. Notheand Mupe Bay. Proc. Geol. Assoc. London, 75, HALLAM,A. 1971.Mesozoic geology and opening of the 373-406. North Atlantic. 1. Geol. 79, 129-57. RIDD,M. F. 1973. The Sutton Poyntz, Poxwell and Chaldon

~ & SELLWOOD, W. B. 1976. Middle Mesozoic sedimenta- Herring Anticlines, Southern England: A reinterpreta- tion in relation to tectonics in the British Area. J. Geol. tion. Proc. Geol. Assoc. London, 84, 1-8. 84, 302-21. ROBERTS,D. G. 1974. Structural development of the British HANCOCK,J. M. & SCHOLLE, P. A. 1975. Chalkof the North Isles,the continental margin, and the Rockall Plateau. Sea. In: WOODLAND,A. W. (ed.) Petroleum and the In: BURK,C. A. & DRAKE,C. L. (eds.). The Geology of Continental Shelf of North-WestEurope. 1, Geology, Continental Margins, 343-60. Springer-Verlag, N.Y. 413-25. Applied Sci. Publ., Barking, Essex. SMITH, A.J. & CURRY,D. 1975. The structure andgeological HENTIER,F. & VILLEMIN,J. 1971. Mise en Evidence de la evolution of the English Channel. Philos. Trans. R. Soc. Tectonique Profonde du Bassin de Paris par 1’Explora- London, A279, 3-20. tion PCtroliere. Bull. Bur. Rech. geol. min. Pans, (Sir. 21, TAIT~, A.H. & KENT, P. E. 1939. Notes on an examination Sect. 1 (2). 11-30. of the Poxwell Anticline, Dorset. Geol. Mag. 76, 173- HINDE,P. 1980. TheDevelopment of the Wytch Farm 81. Oilfield. Communication1133, Inst. Gas Engineers, WEBBY,B. D. 1965. The stratigraphicalstructure of the London. Devonianrocks in the Brendon Hills, west Somerset. HOOD,A., GUTJAHR, C. C. M. & HEACOCK,R. L. 1975. Proc. Geol. Assoc. London, 76, 39-60. Organicmetamorphism and the generation of pet- WHITTAKER, A. 1975. A postulated post-Hercynian riftval- roleum. Bull. Am. Assoc. Petml. Geol. 59, 986-96. ley system in southern Britain. Geol. Mag. 112, 13749. HOSE,R. K. & DANES,Z. F. 1973. Development of the Late -1980. Triassic salt depositsin southern England. 5th Int. Mesozoic to Early Cretaceous structures of the Eastern SaltSymp., Hamburg, 1978. Proc. Northern Ohio geol. Great Basin. In: DE JONG,K. A. & SCHOLTEN,R. (eds). SOC. 175-9. Grauity and Tectonics, 429-41. John Wiley & Sons, New ZIEGLER,P. A., 1975. North SeaBasin history in the tectonic York. framework of North-Western Europe. In: WOODLAND, HOUSE,M. R. 1961. The structure of the Weymouth Anti- A. W. (ed.). Petroleum and the Continental Shelf of cline. Proc. Geol.Assoc. London, 72, 221-38. North-WestEurope, 1, Geology, 13148. AppliedSci. KENT,P. E. 1977. The Mesozoicdevelopment of aseismic Publ., Barking, Essex. continental margins. J. geol. Soc. London, 134, 1-18. - 1981. Evolution of sedimentary basins in North-West - 1978. Mesozoic vertical movements in Britain and the Europe. In: ILLING,L. V. & HOBSON,G. D.(eds.). surroundingcontinental shelf. In: Born, D.R. & Petroleum Geology of the Continental Shelf of North- West LEAKE,B. E. (eds). Crustal Evolution in Northwestern Europe, 3-40. Hayden & Son, London.

Received 10 April 1981. ROBERTSTONELEY, Department of Geology,Imperial College of Scienceand Technology, Prince Consort Road, London SW7 2BP.

Discussion

MRS EVEA. HOWELLasked whether the author had anomalymap reflects the density variations in the made an interpretation of the gravity anomalies in the pre-Jurassic section. Wessexbasin. From an interpretation of the IGS The most striking featureon the map is a prominent gravity survey overlay map (Sheets 19 & 23), she had elongatednegative Bouguer anomaly extending into concluded that the Permo-Triassic was deposited in a the English Channel along an axis on trend with that N-S basin, the €-W trend only becoming dominant in of theWorcester graben. Her interpretation of this the Jurassic (Cimmerian movements). residual anomaly was that it is due to a N-S Permo- Shehad made isopach maps of themain density Triassic basin. This orientation agrees with the Triassic subdivisions from surface to base Jurassic, calculated palaeogeographical reconstructions of Audley-Charles theirtheoretical gravitational effect andremoved it (1970) and Ziegler (1981). fromthe total field. The resultant residual Bouguer

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/4/543/4887636/gsjgs.139.4.0543.pdf by guest on 30 September 2021 The structural development of the WessexBasin 553

The AUTHORthanked Mrs Howell for her comments author’s opening remarks, these now very pronounced and was delighted to hear of her interesting results: it E-W thrust periclinal lines are not the most important is to be hoped that thesewill be published. The author ‘basement-controlled’structures; most are post- hadnot made an independent interpretation of the Cretaceous mini-inversions with little or no Mesozoic gravity data. history of earlier positive activity, that owe their pres- The N-S trend had been referred to as one of those ent prominence to mid-Tertiary ‘over-printing’ of the influencing thedevelopment of the basin andthe more fundamental underlying trend. author’s feeling was that it was inherited from a con- The ‘occasional NW-SE structure’ dismissed by the siderablyolder period than the Mesozoic. His in- author is in fact the sign of the real underlying struc- terpretationthat the Permo-Triassic wasaffected by tural control for the development of post-Palaeozoic E-W riftingwas based primarily onthe scattered Wessex. The Mid-Dorset Swell (Drummond 1970) is boreholeinformation, but it also took note of the justone example of a wholecomplex of NW-SE views expressed by Mikkelsen in the oral presentation basement lines that controlledMesozoic sedimentation referred to in the paper, and to the nearlyE-W rifting andthe structural development of Wessex andthe in the Permo-Triassic of Somerset. Possibly the N-S Wealden Basin. The sheared nature of these NW-SE negative residual anomaly, mentioned by Mrs Howell, lines has always been appreciated and can be seen in could reflect variations in the pre-Permian ‘basement’, the Palaeozoic outcropsof Somerset (Webby 1965 a & but it seemsthat considerably more seismic and b). Similarly, the strong E-W faults (e.g. Cannington boreholedata will berequired fully to resolvethe Park,mid-Devon, South Devon, etc., can be traced matter. eastwardsbeneath the Mesozoic cover, to influence DR J. P. N.BADHAM said: In the interesting recent the developmentof the structures described, but not in the way that the author infers. andpre-Miocene restored cross-sections presented, one (E-E‘) crossed theWytch Farm oilfield. From It is now clear that simple tension-compression is no longer(and never really was) ableto explain the thesesections it would appear that at no timewere structuralpattern and style of Wessex(summarized Jurassicsource rocks, either to N or S, structurally inDrummond 1970, Fig. 9, p. 709). All the main deeper than the Permo-Trias which now contains the NW-SE lines oil. In addition two major growth fault and fold struc- on this mapare the surface expressionof tureswere shown which wouldhave impeded any dextral basement shears that are the secondary components strong E-W sinistral shear couples such as northwardoil migration to the Wytch Farm field. of the Bristol Would the author comment on the source and timing Channel-Glastonbury-Warminster-Mere lines to the N, the South Devon-South Dorset-Isle of of migration of the oil to the Wytch Farm field? Wight lines to the S, and the central mid-Devon-mid- In reply,the AUTHORsaid that the only sources of Dorset lines (so clearly demonstrated by the offset of information on the Wytch Farmfield are the papersby themid-Dorset Swell) that continues eastwards into Colter & Havard (1981) and Hinde (1980), referred to the Portsdown-South Downsline of thesouthern in the text. The former presents reasons for believing Weald. thatthe oil in boththe Bridport Sands and Triassic Post-Armorican to Lower Cretaceous activity along reservoirs is derivedfrom S of thePurbeck flexure, these Palaeozoic lines was a transtensional phase ex- where the possible source rocks (Lias, ?Oxford Clay pressedas normal faulting that allowed very sharp, and ?Kimmeridge Clay) may be mature. At least the deep back-filled basins(e.g. theVale of Taunton Lias was indeed at a deeper level than the reservoirs Dene)to accumulate great thicknesses of sediment, to the N prior to the late Cretaceous-Tertiary reversal, especiallyJurassic clays. Mid-Cretaceousreversal to andhe had attempted to show this on hiscross- transpressionbegan the quasi-diapiric inversion of sections. Colter & Havard (p. 502) suggested that oil these clay-filled basins to cause the slow growth of the generationcould have begun as early as theearly NW-SE ‘swells andbasins’ that dominated Upper Cretaceous to the S of the Purbeck flexure. Cretaceoussedimentation, of which themid-Dorset The author knewof no reason to disagree with these Swell,Marshwood Dome, proto-Weymouth, proto- assessments, and had tried to incorporate them into Purbeck, proto-Brixton periclines are the more obvi- hisown theoretical reconstruction. Theactual ous examples. mechanisms and pathways of oil migration may well Enhanced E-W transpressional shearing initiated by be complex, and a clear understanding must await the early-midTertiary transform ring opening of the availability of considerably more data. North Atlantic greatly accentuated clay diapiric inversion to produce strong E-W periclines approximately DR P. V. 0. DRUMMONDstated. The structure of the parallel to the main shear motion (Reading 1980). In Wessex Basin is considerably more complex than im- effect,these lines of E-Wpericlines arelate stage plied by theauthor’s re-evaluation of thestandard structural ‘short-circuiting’, jumpingthe basinal gaps tensional-compressional model for the development of between the ‘terminals’ of the original NW-SE axes; the strong E-W lines of the region. Contrary to the hence their typical sigmoidal plan. This periclinal in-

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/4/543/4887636/gsjgs.139.4.0543.pdf by guest on 30 September 2021 554 R. Stoneley version short-circuiting is most pronounced where the In reply, the AUTHORstated that he was interested in Mesozoic clay-filled basins arebest developed im- the views expressed by Dr Drummond but, in view of mediately S of the southerly downthrowingE-W base- the fact that they were based on a number of strong ment lines, especially where Kimmeridge and Wealden assertions for which the evidence was not given, felt Clayswere preserved to greater thicknesses south- unable to comment. He was still of the opinion that, wardsdown the structural staircase into the Hamp- for reasons given in the paper, it is difficult to account shire Basin. forthe S Dorset-Isle of Wightline of disturbance Thus,the structural development of the Wessex in terms of deep-seated strike-slip displacement. The Basin and southern England generally is a product of importance of the NW-SE trend is not doubted: possi- major E-W sinistral shear zones and complementary bly it originated as a response to northwards Hercy- secondary NUT-SE dextral shears passing through al- nian compression, and has to a greater or lesser extent ternate transtension-transpression phases conditioned beenreactivated subsequently. The author did not by the ‘transform ring’ opening of the North Atlantic believe,however, that it can be related to an E-W andnot the simple tensional-‘Alpine compressional’ sinistralshear that has repeatedly exercised overall tectonics as traditionally inferred. structural control.

Additional references

AUDLEY-CHARLES,M. G. 1970. Triassic palaeogeography of WEBBY,B. D. 1965B. The stratigraphy and structure of the the BritishIsles. Q. J. geol.Soc. London, 126, 49-89.Devonian rocks in the QuantockHills, West Somerset. READING,H. G. 1980.Characteristics of strike-slip fault Proc. Geol. Assoc. London, 76, 321-43. systems. Spec. Publ. int. Assoc. Sedimentol. 4, 7-26.

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