The Structural Development of the 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 to the 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 seems to 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/4/543/4887636/gsjgs.139.4.0543.pdf by guest on 30 September 2021 5 44 R. Stoneley 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 commonly shown on maps as a continuous, moreor 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 possibly dextrally 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- A series 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

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