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Journal of the Geological Society 1982, v.139; p543-554.
139. The early rifting and the change from overall compression may reflect plate tectonic events in the North Atlantic region. Mikkelsen.). Most of thezones of structuraldisturbance show evidence of activity through muchof the Mesozoic and Tertiaryand. followed by tectonic inversion and. by northwards compressive movement along the former normal faults. The structures spectacularly exposed in Mesozoic and Tertiary strata along the coast of Dorset have for long excited geologists. A similarstorycomes W . These disturbances are virtually confined to the area of the Mesozoic-Cenozoic basin. This partially restored the earlier separative displacement at depth.Phillips1964). probably associated with roll-over anticlines. suffered inversion (reversal of sense of vertical displacement) during the Cretaceous.lyellcollection. O n e hastheimpression through more or less E-W in the central and eastern of relativedisplacement.it has become important to try to reachanunderstanding of theirdeeper causes. geol.hasremained tion: normal. of rather rigid basement from predominantly ENE in the Western Approaches. Soc. and salt movements have all been invoked.g. Printed in Northern Ireland.Basindevelopmentstartedwith Permo-Triassicrifting. This regime had ceasedby the Aptian and the basin passed through a period of relative stability in the late Cretaceous. perhaps. This paper attempts to explore the tectonic significance of thesestructures. often Wight). The evidence is consistent with the suggestion that a change of tectonic regime dates The S Dorset-Isle of Wight belt of structural disturof thewidespreadlateCimmerian of fromthetime bance (Fig.pers. 543-554. With the present phase of petroleum exploration following the discovoilfield (Colter & Havard ery of theWytchFarm 1981. the date of actualinversion uplift seems to withassociatedanticlines.Theirinterpretationin a subject of speculadepth.hiatus-post-Neocomian (Wealden)andpriortothe ‘Greensand’ transgression which was diachronous tures. Arkell1947. The trends of the main structural lineaments vary the dips seldom exceed a few degrees. Vol.however. it dates from about theAptian (HCritier & Villemin1971). where the earlier regime came to an end with theAptianandinversionupliftdatesfromthe Senonian or.They are reviewed in thelight of publisheddatafromsouthDorset.theEnglishChannelandWestern of Approaches.tobefollowed after a depositional hiatus by relative stability in the late Cretaceous and the commencement of reversal at the end of the Cretaceous: seismic profiles have confirmed that such inversion was widespread along this and other lines of disturbance in southern England (P.Itshouldtherefore regarded as an interim review.separateregionsin which have varied from place to place. Sharp monoclinal flexures or faults. and satisfactory explanations havebeenreachedfortheobservedfeatures (e. blocksseparated bylines which formtheinter-relatedexpressions of a single Channel and southern England. in the Miocene.throughconsideration of their regional setting in the Wessex Basin and through the use of natural scale cross-sections extrapolated to depth.Downloaded from http://jgs. regional tectonic system. however. The precise timing of thisinversionisfrequentlyuncertain. pp. extending through central southern and southfrom the Aptian to the Cenomanian (sub-Aptian and easternEngland.00 @ 1982 The Geological Society .and isbelieved to havecontinuedthroughtheJurassicandearly Cretaceous with intermittent growth on deep-seated listric normal faults.org/ at University of Aberdeen on October 24. 5 figs.inthebetter known anddocumented cases. reverse and strike-slip faulting. Turonian (Hancock & Scholle Regional structure 1975. fromthesouthernNorthSeaandnorthernEurope (Kent 1978).comm. 1982. free folding of the sedimentary cover above basement. and across the Paris Basin (e. on theBray-Bouchy-Juvanz6 and RouenSennely faults of the Paris Basin. Stoneley SUMMARY: The MesozoicandCenozoicstructures of southernEnglandformpart of a system of disturbances which extendsacrosstheChannelintonorthernFrance. Ziegler 1981).g. to SE and eventually 0016-7649/82/0700-0543$02. They have been studied in considerable detail at outcrop. InDorset.and it maythusbesurmisedthattheyare directly related to the post-Hercynian evolution of the basin itself. It is based entirely on published and publicly available data: no access has been possible to privately owned and confidential seismic sections nor to some be recentboreholeinformation. Hinde 1980). Smith & sub-AlbianbreaksarebothpresentintheIsle Curry 1975). Jurassic-early Cretaceous movement in the one sense terminatedataboutthissametime. London. 2014 J. 1) forms a part of a wholeseries interconnected and more or less analogous linear fea. pending the release of these data. The structural development of the Wessex Basin R. and probably accentuated pre-existing roll-over anticlines: it was expressed in the surficial Upper Cretaceous-Palaeogene sequence as N-S crustal stretching to monoclinal flexuring.due to lack of critical control: closely spaced boreholes have shown that.
Stoneley .org/ at University of Aberdeen on October 24.Downloaded from http://jgs.lyellcollection. 2014 5 44 R.
along this belt. and to suspect therefore that the basin developA series of anticlinalfoldsparallelstheflexureat of Permo. and that it was lines of disturbance. Webby 1965). positioned to take advantage of the been related to further stages in the opening controlavailable. probaIthaslongbeenknownthat. This saltmay.Withinthis These directions cannot entirely follow Armoricanlatter offset. albeit somewhat tentative. and reflected in the alignment of the Cherbourg Peninsula dextrally again in Dorset where the Purbeck Monowithstructuralculminations in thecentralChannel cline appears to die out westwards to be replaced to the possibly extending into the Hampshire Basin.Whatever its explanation(consideredbelow). may suggest a Sea and to other Permo-Triassic basins of the British genetic relationship. a further short intermediate faultlflexure Hercynian trends. in the Tertiary Beer Syncline. 2) andthetrend is visible onsatellite The monoclinalflexure in theUpperCretaceous 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 commonlyshownonmapsasacontinuous. strike-slip movement in the Mesozoic (Ziegler A parallel line of minor faulting is present to the W in Plan view (Fig.accountforthe ward. Mottram & House 1956). noid) trend is also present at the western marginof the possiblydextrallybeneath the sea S of Christchurch Wessex Basin. 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. These folds again are mentmaybeprimarilyaconsequence Triassicriftingassociatedwiththeearlyattempted not continuous but may be limited axially to the extent formation of the northern Atlantic.g. the Cross-sectional view (Fig.the bly a prolongation of the combined Bray and Rouensouthern side moved downwards on normal faults in Sennely fault zones. The Wessex Basin of the adjacent arcuate segment of the flexure. 2). when continuous gentle present in the mid-Cretaceous (Taitt reported in Ardisplacement mayhavebeenconfined tothemajor kell 1947. including Purbeck Monocline overlap en echelon. which may have exercised some control as a rigid block. of The S DorsetbeingarrangedconcentricallyaroundtheCherbourg Isle of Wight belt Peninsula.that itsincurious small circular inlier of Middle Jurassic in the terpretation should rather be sought regionally. in the Chalk at Compton Valance. but it is clear that the traces are scalloped towards doubt the universal control of earlier orogenic structhe S. alof the flexure Each of theseenechelonsectors though it is parallel to long-standing dextral strike-slip consists of up to 5 arcuate segments.distances of up to 3 km to the S. (Smith & Curry 1975) and may have suffered sinistral 1981).moreor activeinDorsetinthelateCretaceous(Drummond less straight line. In addition to these.however. which nevertheless certainly appear is probablydevelopedontheRingsteadAnticlinetoprovideastronginfluence. Hallam 1971.It is emphasizedthatthesehave North Atlantic (e. with its three culminations at Sutton An indication of early strong vertical movements is Poyntz. a N-S (possibly Malversinistrally inthewestern half of the Isle of Wight. 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. towards the S. Britain appears to have become rather uniform in the late Jurassic (Hallam & Sellwood 1976).lyellcollection. Roberts 1974).TheNW(Charnoid) Picnic Inn Fault. Theco-existance of varioustrendsleadsoneto 2.g.incidentally. to in the Neogene (e.PoxwellandChaldonHerring. encountered in in the local scene and contrasting attempts to explain boreholes at Winterborne Kingston and Nettlecombe it(e. 2014 The structural development of the Wessex Basin 545 N-S in France: they give the impression. A prominent NW trending flexure. . way Fault. It is prominent appreciable thicknesses of halite. Theseearlyverticalmovementsbecamemutedin there is clear evidence that an anticline was already the late Triassic and Jurassic. Dingwall 1971). but that this sense was reversed the Isle of Wight and may extend farther at depth.andthe Five cross-sections have been constructed across the episodes of subsequent inversion. it seemstotheauthor. may perhaps have of the flexure in Dorset. Isles region. elementinparticular is apparentlyanomalous. subsidence in general in southern further developed in the Tertiary. in plan. 2) Dorset (Fig. Ridd 1973) have been put for(Fig. always concave faults in Cornubia (Dearman 1963. Arkell 1947. continues at least t o a point E of the mid-Cretaceous. This may thushavebeenrelatedoriginally to the North association. House 1961).g. N by theRidgeway-AbbotsburyFault. However. Arkell 1947. 3) erosionprecedingtheAptiantransgression. which is strongly suggeslight of the S Dorset-Isle of Wight belt as a tive of diapirism (Falcon & Kent 1950). 1980). it may be (suggested by sea-floor outcrops.org/ at University of Aberdeen on October 24. It is neither! It is offset en echelon 1970). This effect is slightly exaggerated in Fig. of which the nearest is in central Somerset The intriguing sharp anticline lying S of the Ridge(Whittaker 1975.Downloaded from http://jgs. whole. tures.
org/ at University of Aberdeen on October 24.Downloaded from http://jgs. Stoneley .lyellcollection. 2014 546 R.
although probaflexed in contrast to the earlier clean faulting. if at all.). although the Kimon meridge Clay appears in wells further to the N(Fig. 2014 The structural development of the Wessex Basin 547 been drawn on the basis of published information only the widespread occurrences of Kimmeridgian ammoandthat.Kimmeridgeandotheranticlineshaveat & Kent least some roll-over component. the major caused theUpperCretaceous-Tertiarybedstobe faults were active during the Jurassic.however. major faults that were active before the midThe limitedcontrolavailablefortheJurassicsugCretaceous: it is possible. Ballard Down Fault. Durdle Door with a further localized thickness increase adjacent to Thrust. However. sequencewasdepositedacrosstheentirearea. of course. Kent. 87 et seq. perhaps aided by gravi.).Hallam & SellFig.thezone of thickest in factimmediately S of Wealdenaccumulationwas o n Section C-C' (Fig.).thetheoreticalprofile of a listric the control at present available.Theyhavebeendrawnfollowingthe shape of acycloid. unlessit bly at variable rates (P.abIt is instructive to use these cross-sections to try to sence of the higher Jurassic beds N of the flexure. Nevertheless. 3). The fault planes must normal fault intersecting the free surface at 45" (Hose have been bent downwards near the surface. prior to The effects of the Tertiary flexuring on the attitude the Aptian. p.Poxwell(Taitt 1939) and Radipole (House 1961) might imply dips to bly local constituents. such a situation. There is no factual To the N. Mikkelsen.thicknessesbecomeconjectural. No explanation is at present Colter & Havard (1981) and confirmed by A.)-that they are reflections of thefault itself? There is little directevidencefrom localizedcompressionatthefoot of the flexureand which toanswertheseinter-relatedquestions. for the explanations of Arkell (1947) and Phillips (1964) example at Wytch Farm? (ii) Did these growth faults forthedetailedstructuresmappedatsurface(the develop a roll-over anticline on the downthrown side.particularlyin the MiddleJurassic. rather diagpositions of the extrapolated anticlinal axes at depth. the This raises two questions: (i) Was the Wealden deposmall scale sections included here are consistent with sited to itsfullthickness. continuationbeneaththeUpperCretaceousmight pers. Lulworth Crumple. these were listric normal faults of theearlierfaultsandtheimmediatelyadjacent downthrowing S: this is supported by theircuspate pre-Aptian beds cannot be determined accurately with tracesinplan.inDorset. on the sections.asopposedtomere stability while the represent a reversal of displacement along the same southern side subsided. the displaced to the S and hence that the major faults tend primary detritus of the Wealden is far travelled-from Cornubia (Arkell 1947). comm.inboreholesat formations. As this geometry of a listric growth fault in effect leads to the flexurediesout. comm. inviting & Dane5 1973): this appears to be consistent with the the development of splay thrusts in the Tertiary. Thus we may be led Purbeck Monocline nearly 4 km to the S: both appear topostulatethat.org/ at University of Aberdeen on October 24. therefore. frombottomtractioncurrents (e. etc. 2). and equally UpperJurassicandpossiblyevenMiddleJurassic there are insufficient data to attempt an accurate prebeds:within theareacovered by thecross-sections. E.): indeed.are & Kent1939). that at depth the Tertiary effect was merely to restore on some of the earlier normal offset. as noted alsoby ChaldonandPoxwell(Taitt entirely of Tertiary origin. E.consequently.asencountered.Downloaded from http://jgs. fracture sets. nites in the Aptian (P. sand (shown up by marked differences in oil impregnation and porosity) may be symptomatic of contemporThe flexure marked by the Ridgeway Fault dies out E of theChaldonHerringanticlinalculmination. 3) stronglysuggeststhattheanticlinalaxesare wood 1976. may be greater S of the flexure than to the N.geometrical not necessarily have expression at the sea-floor or in construction of the asymmetrical Weymouth and Purthefacies of thesedimentsprovideddeposition was beck Anticlines (Sections A-A' and D-D' respectively. Chapman 1973. however.though 'large boulders of Wealden sand in Wealden tational collapse. pers. 4: it could be matched of the world's contiThere is no reason to doubt that the complete Jurassic seismic profiles from a number nental shelves.thatthe faults at depth. to the N. that some minor gestsconsistentlythat the thicknesses of thevarious splays. thereSome general points may be made. The implication is that. comm. foreimplyabsolute uplift anderosionintheearly All writers are agreed that the Tertiary movements Cretaceous.alconsequent relief of pressure. There is little direct control on the attitude of the thePurbeckMonoclineand. Neogenereconstruction. The existence of a mid-Cretaceous anperhaps link with one of the faults known from the ticline on the Poxwell-Chaldon feature could well be WytchFarm oilfield (Colter & Havard1981)which. Indeed the very however. and .theUpperCretaceousrests justification for these interpretations. need the S between 45" and 60". to flatten with depth.forexample. Presumably.). be a function of the gross lithology. comm. show no post-Aptian displacement.A ary degradation of a nearby fault scarp' (P. W.g. envisaged is shown in Fig. although boreholes at Chaldon (Taitt Purbeck. An attempt to illustrate this has been made. Whitforthcoming for the fact that the reversal of movement taker (pers. ramatically. and it has been assumed On the southern side of the flexures/faults.Asketch of thesituation this is generally the Oxford Clay. pers. Despite some probareported in Arkell1947).inplaces.lyellcollection. it is overlappedenechelon by the development of such a structure. the full succession is present up to and including the Wealden. accounted for in terms of roll-over. Kent.
Stoneley \ \ E i E. 2014 R .Downloaded from http://jgs.lyellcollection.org/ at University of Aberdeen on October 24. n L l L 8 8 E .
lyellcollection. 2014 549 The structural development of the Wessex Basin J E i .org/ at University of Aberdeen on October 24.Downloaded from http://jgs.
org/ at University of Aberdeen on October 24. although itmay be difficult to explainthe 2km I FIG. pers. 2014 550 R.4. Level of Organic Metamorphism (LOM) valuesareestimated using the curves of Hood et al. assuming a geothermal gradient of 27 "C/hm. PALEOG. LIAS M.g. McKenzie 1978). 5.lyellcollection. in southern Dorset since the early Mesozoic. 1975) andcomparedwiththeactual values. estimate the magnitudes of the crustal displacements that have taken place. method for checking the order of magnitude involved of is offered by the thermal alteration due to burial organic matter in the sediments: assuming a constant KIMMERIDGE BAY WYTCH LlAS 200 Y. Diagram showing the generalized interpreted profile across south Dorset at the close of the Cretaceous. LATE CRET. R. Stoneley Sutton PoyntrPoxwellChaidon SOUTH Kimmeridge NORTH I I I I I I I UPPER CRETACEOUS I WEALOEN PURBECK JURASSIC LlAS PERMO-TRIASSIC HERCYNIAN BASEMENT FIG. Level the of Organic Metamorphism (LOM) can be predicted theoretically (Hood et al. . NEOG.) are consistent with the valuespredictedfromtheburialdepthsshownin Fig. This burial history is in general compatible with the concept of basin subsidence due to crustal stretching and thinning by listric normal faulting (e. Rahman. Comparative burial history graphs on either side of the Purbeck Monocline. The scaleof the vertical movements can be illustrated by comparative burial history graphs of sequences on either side of the Purbeck Monocline (Fig. Not to scale. 0 m". NEOG. Measurements at Imperial College of the LOM of the Kimmeridge Clay at outcrop (R. ? = thickness estimated. E. Kinghorn & M. APT. 5). 40 70- 80- 907 geothermal gradient.JUR.5 .Downloaded from http://jgs.CRET APT. 420 80 PALEOG. andoutcropdata).CRET. (1975). KIM. both vertically and laterally. comm. LATECRET.JUR. UIM. F. E. sufficient data for tentativeinterpretationsareavailablefromWytch Farm(Colter & Havard 1981) andKimmeridgeBay A potential (Brunstrom 1963.
8. Recently discovered oilfields in Britain.5km. 2. D. pear to coincide with a change from a stateof regional 3 . R.5 km in the Isle of Wight.and itmay be asmuchas 1. The evidence suggests strongly that and Rouen-Sennely faults to the glish Channelhasadextralstrike-slipcomponent. The Wytch Farm Oil . J. the author is deeply grateful: Sir Peter Kent.K. Kent 1977. Selley. It is notimpossiblethatthe S Dorset-Isle of Wight belt has undergone a double inversion. Int. S. The minimumhorizontalseparativemovementprior totheAptian may thushaveexceeded2. Petroleum Geology. Mem. Elsevier Sci. S. offset in the same sense. BRUNSTROM. V. 5. after References ARKELL. House. 6. A Concise Study. and ocean formaporaneous rifting elsewhere in the British region. One of these lines. Ziegler 1975).whichhowever The Consideration of thestructuraldevelopment of the positiveduringtheJurassic-earlyCretaceous. continued rifting through Permo-Triassic. pre-existing roll-over anticlines. 1947. q f the sinistraldisplacement whichhadachievedahorizontalseparation of the reversingtheearlier Mesozoic. explanations are unnecessary.org/ at University of Aberdeen on October 24. This regime was terminated by the Aptian. COLTER. the S Dorset-Isle of N-S crustalextensioninsouthernEngland-northern Wightbelt. Petrol. They appear to be correlative with plate moveandintonorthernFrance. J h A. h b l .for say the Jurassic.Theeventsrecorded by the S Dorset-Isle of cident with its present limits.5 km from the beginning of the Jurassic to the mid-Cretaceous. localized differential vertical movements in the mentsintheNorthAtlanticregion:strong riftingin the Permo-Trias.. The Sutton Poyntz-PoxwellChaldon anticline is believed to have been formed in this manner. Subsequent subsidence was accompanied by con. E. Imperial College. Sum. the net effect across the disturbance is S: Section E-E (Fig. in the sense of Kent(1978. they represent pre-Aptian listric normal growth faults. W. Amsterdam. and each in plan scalloped to one might suggest that the flexure prolonging the Bray the S: this disposition is inconsistent with deep-seated NW across the Enstrike-slip faulting. basin. R. Brown. 2014 The structural development of the Wessex Basin 55 1 500 mplus of uplift N of the flexureintheearly Cretaceous. order of 2. for which withroll-overanticlinesontheirdownthrownsides.5 km.(e.a similarestimatemaybereached by combiningthe burial history graphs with the cross-section-assuming that the fault has been drawn at the correct angle! of relative stabilityuntil thelatest whichaperiod Cretaceous was followed by a tendency to reverse the sense of the earlier displacements: this may have been effective by thebeginning of theCenozoic(Arkell 1947) but culminated in a Miocene crustal shortening of up to 1. have reviewed a draft of ACKNOWLEDGMENTS. 7. Dr V. Co. Sect. Swanage. 1973.W.It of overallcrustalcompressionmanconsists of a number of en echelon segments. G.Thetermination of rifting andtheonset of of abasin-widecrustal ness. Cong. Thesefaultswere in all probabilityassociated this paper and provided most helpful suggestions.Itsevolutionbeganwith initial strong. 3 ) still adownthrowtothe suggests that the present horizontal separation of an horizon in the Lias is approximately the same amount. 10.. CHAPMAN.whichformelements continental separation in the North Atlantic thus apfracture system. other. Dr R. This must be less than the cumulativepre-mid-Cretaceousextensionsince. extending through eastWightbeltaremirrored onother lineswithinthe ern England. Is it possible toestimatethehorizontaldisplacementsinvolved? Theamount of lateralshortening represented by the Tertiary flexure is equivalent to its verticalheight.Thegreatestknown thickness of Permo-Triassic in the area has been found Concluding remarks wasrelatively tothe N of thebelt.in Dorset the figure reaches the order of 1. whichwerewidespreadthroughsouth-easternEngland and northern Europe.tion and widening from the mid-Cretaceous onwards tinuing gentle faulting along persistent lines of weak. the Channel and Western Approaches.following The 4. 1963. geof.believedtoberelatedtocontemthe Jurassic and early Cretaceous. if strata1 stretching is ignored. Colter. The extent of the basin was approximately coin9.lyellcollection. U.CorfeandLulworth. coincidence is nevertheless not exact and more data Wessex Basin should take account of the following: are required to resolve the question. l . not all France.2 km in the vicinity of WytchFarm. The illustrations were drafted by Mr A. Arelatedeffect of reversedmovementcould have been to accentuate. Whittaker. R. & HAVARD. This reversal of sense of displacement represents the more or less contemporaneous inversion tectonics. R.hasbeenconsideredin thisreview. p. 11-21.toone ifested along lines of earlier weakness. during the early Cretaceous. 1981. C . expressed along the pre-existing lines of weakness as a monoclinal flexure.g. In this setting. which influenced the accumulation of sediment at least Professor M. 1. more exotic. The Geology of the Countryaround Weymouth. J. by compression. 317).Downloaded from http://jgs.
London. 87. Inst. 1975.. Geol.. of Scienceand Discussion MRS EVEA. D. Mag. 2014 552 R. Publ. R. Crustal Evolution in Northwestern Europe. 1 (2).Departmentof Geology. 1950.GUTJAHR. 187-204. A. Geol. D.(eds.org/ at University of Aberdeen on October 24. London. L. London SW7 2BP. & SCHOLLE. The structure andgeological SMITH.R. 494-503. 75. 81. The structures in theJurassicand FALCON. A. P. The Development of the Wytch Farm HINDE. 112. N. Petroleum Geology of the Continental Shelf of North-west Europe. W.WHITTAKER. Pans. London. Tectonique Profonde duBassinde Paris par 1’ExploraLondon.Inst. ~ Received 10 April 1981. 1963. 1961. Proc. M. 1975. Continental Margins. Middle Mesozoic sedimentaHerring Anticlines. Issue.Earth planet.1971.Herinterpretation of this the Jurassic (Cimmerian movements). B. Notes on an examination of the Poxwell Anticline. Oilfield. 309-24. (eds. Proc. & HOBSON.K. A postulated post-Hercynian riftvalroleum. Assoc.M. Poxwell and Chaldon North Atlantic.westSomerset. Geol.London. London. DEARMAN. Sci. Chalk of the North Isles. Grauity and Tectonics. J.G. HOUSE. 1971.) Petroleum and the I n : BURK. A. 429-41. Mesozoic to Early Cretaceous structures of the Eastern R. Some remarks on the development of sedimentary basins. A. A. Assoc. The structure of the Weymouth AntiA. 373-406. cline. J.C. The Geology of Continental Shelf of North-WestEurope. Barking. 221-38. Essex. In: DE JONG. Geol. 129-35.G. 5th Int. Lett. (eds). KENT. J. Mag. P. 1965. 84. 1975.1978. 343-60.Assoc. Geol. Sea. HALLAM. 1973. she had elongatednegativeBougueranomalyextendinginto concluded that the Permo-Triassic was deposited in a the English Channel along an axis on trend with that N-S basin. Proc. London.Mesozoicgeologyandopening of the RIDD. P. Structural development of the British 84. Chalk Rock of Dorset: Cretaceousrocks on theDorsetcoastbetweenWhite More evidence of salt? Geol. MOTTRAM. NotheandMupeBay. Geol. Rep. ROBERTS. New ZIEGLER. SOC. 3-20. In: WOODLAND. Philos. 1978. 21. geol.thecontinentalmargin. W.B. tion. 1. Bur. residual anomaly was that it is due to a N-S Permoof themaindensity Shehadmadeisopachmaps Triassic basin.25532. Geol. Proc.L. (eds). framework of North-Western Europe. 76. of Albian-CenomanianmovementsinWessex. 1976. 1975. R.Downloaded from http://jgs. V. of the IGS Wessexbasin. 72. ley system in southern Britain. 72. 76. & HOBSON. M. Wrench-faulting in Cornwalland South Devon. & SELLWOOD. The stratigraphicalstructure of the DevonianrocksintheBrendonHills. J. & CURRY. 134.GasEngineers. London. surroundingcontinentalshelf. 986-96. & DANES. SaltSymp.R. WEBBY. 1956. & HEACOCK. E.ImperialCollege Technology. 1971. 1-18. The Sutton Poyntz. 1974. Applied Sci.1981. calculated palaeogeographical reconstructions of Audley-Charles theirtheoreticalgravitational effect andremoved it (1970) and Ziegler (1981).Z. Dorset. In: WOODLAND. Assoc. P. Soc.West LEAKE. Petml. ROBERTSTONELEY. Hayden & Son. Mise en Evidence de la evolution of the English Channel. ( S i r .). Southern England: A reinterpretation in relation to tectonics in the British Area. Sci. Soc. London. London.andtheRockallPlateau. Evidence margin of the Poxwell Pericline. J. 1970. Britain and Adjacent Regions. E. 81. 173Sect. Great Basin.Thestructuralandstratigraphical MereFaultandtheVale of WardourAnticline. A279. Dorset nat. 76. (ed. HOSE. AppliedSci. 1978. Northern Ohio geol. Geology. F. PHILLIPS. Development of the Late -1980. HANCOCK. & VILLEMIN. D. 129-57. HENTIER.C. A.P. Geol. 79. 302-21.Stoneley Field. W. Contributions to the geology of the DINGWALL. TAIT~. tion PCtroliere. M. Essex. Spec. min. HOWELLasked whether the author had anomalymapreflectsthedensityvariationsinthe made an interpretation of the gravity anomalies in the pre-Jurassic section. In: ILLING. H. & KENT. Geol. 1964. Triassic salt depositsin southern England. 1977. & SCHOLTEN. P.L. 0. geology of a portion of theeasternEnglishChannel.H. A m . D.Hamburg. W. 1. E. G. Prince Consort Road. 39-60. Rech. Bull. Proc.P. MCKENZIE. Assoc. fromthetotalfield. 13148. E. Geology. Evolution of sedimentary basins in North-West . & Petroleum Geology of the Continental Shelf of North. N. R. 83.lyellcollection. 3-40.A. 1.A. 11-30. (ed. R.W. 1961. 1975. K.M. The Mesozoicdevelopment of aseismic continental margins. the €-W trend only becoming dominant in of theWorcestergraben.F.TheresultantresidualBouguer . In: ILLING. HOOD. 265-87.. Organicmetamorphismandthegeneration of pet. 1973. . D. Petroleum and the Continental Shelf of North-WestEurope. & KENT. Communication 1133.). V. 59.1980. London. Publ. & DRAKE. L.Fromaninterpretation The most striking featureon the map is a prominent gravity survey overlay map (Sheets 19 & 23). 679-714.. Geol. Geol.). Proc. Assoc. London. 1-24. V. (eds). D. 413-25.& HOUSE.B. Proc. The Mid-Dorset Swell. 175-9. Geol. The structure of the northern DRUMMOND.C.R. 1939. P. North SeaBasin history in the tectonic York. Mesozoic vertical movements in Britain and the Europe. Geol. Dorset.13749. A. F.40. Proc. In: B o r n . A. archaeol. 10. Assoc. D. geol. Hist. Soc. Springer-Verlag. Geol. Bull. 302-3. . Barking. Assoc.C.. This orientation agrees with the Triassic subdivisions from surface to base Jurassic.Y. 1-8. Trans. J . London. Hayden & Son. B. Mag. Proc. J . L. 71/8. G. John Wiley & Sons. R.
jumpingthebasinalgaps tensional-compressional model for the development of between the ‘terminals’ of the original NW-SE axes. structurally Jurassicsourcerocks. At least the of Taunton deep back-filled basins(e. most are postCretaceous mini-inversions with little or no Mesozoic history of earlier positive activity. that owe their present prominence to mid-Tertiary ‘over-printing’ of the more fundamental underlying trend. The author hadnotmadeanindependentinterpretation of the gravity data.SouthDevon..proto-Weymouth. DRUMMOND stated. 9. This periclinal in- . 2014 The structural development of the Wessex Basin The AUTHORthanked Mrs Howell for her comments and was delighted to hear of her interesting results: it is to be hoped that these will be published. p. It is now clear that simple tension-compression is no one (E-E‘) crossed theWytchFarm oilfield. N. The ‘occasional NW-SE structure’ dismissed by the author is in fact the sign of the real underlying strucof post-Palaeozoic tural control for the development Wessex. All the main deeper than the Permo-Trias which now contains the NW-SE lines on this map are the surface expressionof oil. and ?Kimmeridge Clay) may be mature. 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-DorsetSwell)thatcontinueseastwardsinto Colter & Havard (1981) and Hinde (1980). and the central mid-Devon-midIn reply. could reflect variations in the pre-Permian ‘basement’.org/ at University of Aberdeen on October 24. mentioned by Mrs Howell. 709). The actual ous examples. P. Possibly the N-S negative residual anomaly. The Mid-Dorset Swell (Drummond 1970) is of a wholecomplex of NW-SE justoneexample basement lines that controlled Mesozoic sedimentation of Wessex andthe andthestructuraldevelopment Wealden Basin. and had tried to incorporate them into Purbeck. The former presents reasons for believing Weald. Mid-Cretaceousreversal to andhehadattemptedtoshowthis o n hiscrossof transpressionbeganthequasi-diapiricinversion sections. In Wessex Basin is considerably more complex than im. 0. mechanisms and pathways of oil migration may well Enhanced E-W transpressional shearing initiated by of the be complex.g. Contrary to the hence their typical sigmoidal plan.MarshwoodDome. but it seemsthatconsiderablymoreseismicand boreholedata will berequired fully to resolvethe matter. referred to the Portsdown-South Downsline of thesouthern in the text. 553 author’s opening remarks. the strong E-W faults (e. In addition two major growth fault and fold strucdextral basement shears that are the secondary comtureswereshown which wouldhaveimpededany ponents of strong E-W sinistral shear couples such as northwardoilmigrationtotheWytchFarmfield. 502) suggested that oil these clay-filled basins to cause the slow growth of the generationcouldhavebegunasearly as theearly NW-SE ‘swells andbasins’thatdominatedUpper Cretaceous to the S of the Purbeck flexure.eitherto inDrummond 1970.theselines of E-Wpericlines arelatestage plied by theauthor’sre-evaluation of thestandard structural ‘short-circuiting’. these now very pronounced E-W thrust periclinal lines are not the most important ‘basement-controlled’ structures.protoassessments.mid-Devon. The structure of the parallel to the main shear motion (Reading 1980). Colter & Havard (p. Fig.Downloaded from http://jgs.effect.butitalsotooknote views expressed by Mikkelsen in the oral presentation referred to in the paper.etc. Cannington Park.canbetraced eastwardsbeneaththe Mesozoic cover. the South Devon-South Dorset-Isle of of migration of the oil to the Wytch Farm field? Wight lines to the S. that the oil in both the Bridport Sands and Triassic Post-Armorican t o Lower Cretaceous activity along reservoirs is derivedfrom S of thePurbeckflexure. proto-Brixton periclines are the more obvihis own theoretical reconstruction.g. Similarly. andpre-Miocenerestoredcross-sectionspresented. to the N prior to the late Cretaceous-Tertiary reversal. these Palaeozoic lines was a transtensional phase exwhere the possible source rocks (Lias. theVale Lias was indeed at a deeper level than the reservoirs Dene)toaccumulategreatthicknesses of sediment.BADHAMsaid:In the interesting recent the way that the author infers. and to the nearlyE-W rifting in the Permo-Triassic of Somerset. and a clear understanding must await the early-midTertiarytransformringopening availability of considerably more data. ?Oxford Clay pressedasnormalfaultingthatallowedverysharp. The sheared nature of these NW-SE lines has always been appreciated and can be seen in the Palaeozoic outcropsof Somerset (Webby 1965 a & b). but not in DR J. His interpretationthatthePermo-Triassic wasaffectedby E-W riftingwasbasedprimarily onthescattered of the boreholeinformation. The N-S trend had been referred to as one of those of the basin andthe influencing thedevelopment author’s feeling was that it was inherited from a considerablyolderperiodthanthe Mesozoic. to influence the development of the structures described. North Atlantic greatly accentuated clay diapiric inversion to produce strong E-W periclines approximately DR P. the strong E-W lines of the region. V. especiallyJurassic clays. of which themid-Dorset The author knew of no reason to disagree with these Swell.lyellcollection. From longer(andneverreally was) abletoexplainthe thesesectionsit would appear that at no timewere structuralpatternandstyle of Wessex(summarized N o r S. Cretaceoussedimentation. the Bristol Channel-Glastonbury-Warminster-Mere Would the author comment on the source and timing lines to the N.
Spec.Devonianrocks in the QuantockHills. Additional references AUDLEY-CHARLES. 4.Theauthordidnot believe. 321-43.Downloaded from http://jgs. it is difficult to account forthe S Dorset-Isle of Wightline of disturbance in terms of deep-seated strike-slip displacement. 1970. The stratigraphy and structure of the the BritishIsles. B.however.Soc. and has to a greater or lesser extent beenreactivatedsubsequently. geol. especially where Kimmeridge and Wealden ClayswerepreservedtogreaterthicknessessouthwardsdownthestructuralstaircaseintotheHampshire Basin. 49-89. Publ.org/ at University of Aberdeen on October 24. Assoc.lyellcollection. Sedimentol. . D. J . 1980. London. M.Stoneley version short-circuiting is most pronounced where the Mesozoic clay-filled basins arebestdevelopedimmediately S of the southerly downthrowingE-W basement lines. in view of the fact that they were based on a number of strong assertions for which the evidence was not given. systems. London. Q. int. 7-26. 1965B. G. G. Thus. Geol. 76.Triassicpalaeogeography of WEBBY. for reasons given in the paper. Assoc.thatitcanberelatedtoan E-W sinistralshearthathasrepeatedlyexercisedoverall structural control. stated that he was interested in In reply.thestructuraldevelopment of the Wessex Basin and southern England generally is a product of major E-W sinistral shear zones and complementary secondary NUT-SE dextral shears passing through alternate transtension-transpression phases conditioned by the ‘transform ring’ opening of the North Atlantic andnotthesimpletensional-‘Alpinecompressional’ tectonics as traditionally inferred. 2014 554 R. Characteristics of strike-slip fault Proc. felt unable to comment. 126. H. the AUTHOR the views expressed by Dr Drummond but. The importance of the NW-SE trend is not doubted: possibly it originated as a response to northwards Hercynian compression. READING. He was still of the opinion that. West Somerset.

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