structural devlopment.pdf

Journal of the Geological Society

doi: 10.1144/gsjgs.139.4.0543 1982, v.139; p543-554.Journal of the Geological Society

R. Stoneley The structural development of the Wessex Basin

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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 Mesozoic and Cenozoic structures of southern England form part of a system of disturbances which extends across the Channel into northern France. They are reviewed in the light of published data from south Dorset. Basin development started with Permo-Triassic rifting, and is believed to have continued through the Jurassic and early Cretaceous with intermittent growth on deep-seated listric normal faults, probably associated with roll-over anticlines. This regime had ceased by 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 Tertiary strata along the coast of Dorset have for long excited geologists. They have been studied in consid- erable detail at outcrop, and satisfactory explanations have been reached for the observed features (e.g. Arkell 1947; Phillips 1964). Their interpretation in depth, however, has remained a subject of specula- tion: normal, reverse and strike-slip faulting, free fold- ing of the sedimentary cover above basement, and salt movements have all been invoked. With the present phase of petroleum exploration following the discov- ery of the Wytch Farm oilfield (Colter & Havard 1981; Hinde 1980), however, it has become important to try to reach an understanding of their deeper causes.

This paper attempts to explore the tectonic signifi- cance of these structures, through consideration of their regional setting in the Wessex Basin and through the use of natural scale cross-sections extrapolated to depth. 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 recent borehole information. It should therefore be regarded as an interim review, pending the release of these data.

Regional structure

The S Dorset-Isle of Wight belt of structural disturbance (Fig. 1) forms a part of a whole series of interconnected and more or less analogous linear features, extending through central southern and south- eastern England, the English Channel and Western Approaches, and across the Paris Basin (e.g. Smith & Curry 1975). Sharp monoclinal flexures or faults, often with associated anticlines, separate regions in which the dips seldom exceed a few degrees.

One has the impression of rather rigid basement blocks separated by lines of relative displacement, which form the inter-related expressions of a single

regional tectonic system. These disturbances are virtually confined to the area of the Mesozoic-Cenozoic basin, and it may thus be surmised that they are directly related to the post-Hercynian evolution of the basin itself.

Most of the zones of structural disturbance show evidence of activity through much of the Mesozoic and Tertiary and, in the better known and documented cases, suffered inversion (reversal of sense of vertical displacement) during the Cretaceous. The precise timing of this inversion is frequently uncertain, due to lack of critical control: closely spaced boreholes have shown that, on the Bray-Bouchy-Juvanz6 and Rouen- Sennely faults of the Paris Basin, it dates from about the Aptian (HCritier & Villemin 1971). In Dorset, Jurassic-early Cretaceous movement in the one sense terminated at about this same time, to be followed 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. W. Mikkelsen, pers. comm.). A similar story comes from the southern North Sea and northern Europe (Kent 1978), where the earlier regime came to an end with the Aptian and inversion uplift dates from the Senonian or, perhaps, Turonian (Hancock & Scholle 1975; Ziegler 1981). The evidence is consistent with the suggestion that a change of tectonic regime dates from the time of the widespread late Cimmerian hiatus-post-Neocomian (Wealden) and prior to the ‘Greensand’ transgression which was diachronous from the Aptian to the Cenomanian (sub-Aptian and sub-Albian breaks are both present in the Isle of Wight); the date of actual inversion uplift seems to have varied from place to place.

The trends of the main structural lineaments vary from predominantly ENE in the Western Approaches, through more or less E-W in the central and eastern Channel and southern England, to SE and eventually

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The structural development of the Wessex Basin 545

N-S in France: they give the impression, in plan, of being arranged concentrically around the Cherbourg Peninsula, which may have exercised some control as a rigid block. A prominent NW trending flexure, probably a prolongation of the combined Bray and Rouen- Sennely fault zones, continues at least to a point E of the Isle of Wight and may extend farther at depth, to the Vale of Wardour line (Mottram 1961): it provides a cross-link between several components of the system (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 Dorset (Fig. 2) and the trend is visible on satellite images both there and in Somerset: there may also be evidence that a positive element with a NW trend was active in Dorset in the late Cretaceous (Drummond 1970). In addition to these, a N-S (possibly Malver- noid) trend is also present at the western margin of the Wessex Basin, in the Tertiary Beer Syncline; it may be reflected in the alignment of the Cherbourg Peninsula with structural culminations in the central Channel possibly extending into the Hampshire Basin.

These directions cannot entirely follow Armorican- Hercynian trends, which nevertheless certainly appear to provide a strong influence. The NW (Charnoid) element in particular is apparently anomalous, although it is parallel to long-standing dextral strike-slip faults in Cornubia (Dearman 1963; Webby 1965).

The co-existance of various trends leads one to doubt the universal control of earlier orogenic structures, and to suspect therefore that the basin development may be primarily a consequence of Permo- Triassic rifting associated with the early attempted formation of the northern Atlantic. The Wessex Basin may thus have been related originally to the North Sea and to other Permo-Triassic basins of the British Isles region, of which the nearest is in central Somerset (Whittaker 1975, 1980).

An indication of early strong vertical movements is provided by the presence of a localized E-W basin with about 1500 m of Permo-Triassic beds, including appreciable thicknesses of halite, encountered in boreholes at Winterborne Kingston and Nettlecombe (Fig. 2). This salt may, incidentally, account for the curious small circular inlier of Middle Jurassic in the Chalk at Compton Valance, which is strongly sugges- tive of diapirism (Falcon & Kent 1950).

These early vertical movements became muted in the late Triassic and Jurassic, when continuous gentle displacement may have been confined to the major lines of disturbance; subsidence in general in southern Britain appears to have become rather uniform in the late Jurassic (Hallam & Sellwood 1976). However, the erosion preceding the Aptian transgression, and the episodes of subsequent inversion, may perhaps have been related to further stages in the opening of the North Atlantic (e.g. Hallam 1971; Roberts 1974).

The S Dorset- Isle of Wight belt

It has long been known that, along this belt, the southern side moved downwards on normal faults in the mid-Cretaceous, but that this sense was reversed in the Neogene (e.g. Arkell 1947; House 1961). We may develop this general theme by considering the structures firstly in plan view and then in profile.

Plan view (Fig. 2) The monoclinal flexure in the Upper Cretaceous

and Tertiary beds of the Isle of Wight and Purbeck is commonly shown on maps as a continuous, more or less straight line. It is neither! It is offset en echelon sinistrally in the western half of the Isle of Wight, possibly dextrally beneath the sea S of Christchurch (suggested by sea-floor outcrops, Dingwall 1971), and dextrally again in Dorset where the Purbeck Mono- cline appears to die out westwards to be replaced to the N by the Ridgeway-Abbotsbury Fault. Within this latter offset, a further short intermediate faultlflexure is probably developed on the Ringstead Anticline- Picnic Inn Fault.

Each of these en echelon sectors of the flexure consists of up to 5 arcuate segments, always concave towards the S. This effect is slightly exaggerated in Fig. 2, but it is clear that the traces are scalloped towards the S.

A series of anticlinal folds parallels the flexure at distances of up to 3 km to the S. These folds again are not continuous but may be limited axially to the extent of the adjacent arcuate segment of the flexure. This association, albeit somewhat tentative, may suggest a genetic relationship.

The intriguing sharp anticline lying S of the Ridge- way Fault, with its three culminations at Sutton Poyntz, Poxwell and Chaldon Herring, is virtually restricted to the sector where the Ridgeway Fault and Purbeck Monocline overlap en echelon. It is prominent in the local scene and contrasting attempts to explain it (e.g. Arkell 1947; Ridd 1973) have been put for- ward; it seems to the author, however, that its interpretation should rather be sought regionally, in the light of the S Dorset-Isle of Wight belt as a whole. Whatever its explanation (considered below), there is clear evidence that an anticline was already present in the mid-Cretaceous (Taitt reported in Ar- kell 1947; Mottram & House 1956), and that it was further developed in the Tertiary.

Cross-sectional view (Fig. 3) Five cross-sections have been constructed across the

flexure in Dorset, positioned to take advantage of the control available. It is emphasized that these have



546 R. Stoneley




been drawn on the basis of published information only and that, in places, thicknesses become conjectural. Some general points may be made.

All writers are agreed that the Tertiary movements represent a reversal of displacement along the same major faults that were active before the mid- Cretaceous: it is possible, however, that some minor splays, as encountered, for example, in boreholes at Chaldon and Poxwell (Taitt & Kent 1939), may be entirely of Tertiary origin. No explanation is at present forthcoming for the fact that the reversal of movement caused the Upper Cretaceous-Tertiary beds to be flexed in contrast to the earlier clean faulting, unless it be a function of the gross lithology. Nevertheless, the small scale sections included here are consistent with the explanations of Arkell (1947) and Phillips (1964) for the detailed structures mapped at surface (the Lulworth Crumple, Ballard Down Fault, Durdle Door Thrust, fracture sets, etc.)-that they are reflections of localized compression at the foot of the flexure and consequent relief of pressure, perhaps aided by gravitational collapse.

The flexure marked by the Ridgeway Fault dies out E of the Chaldon Herring anticlinal culmination. A continuation beneath the Upper Cretaceous might perhaps link with one of the faults known from the Wytch Farm oilfield (Colter & Havard 1981) which, however, show no post-Aptian displacement. As this flexure dies out, it is overlapped en echelon by the Purbeck Monocline nearly 4 km to the S: both appear o n Section C-C' (Fig. 3).

There is little direct control on the attitude of the faults at depth, although boreholes at Chaldon (Taitt reported in Arkell 1947), Poxwell (Taitt & Kent 1939) and Radipole (House 1961) might imply dips to the S between 45" and 60". However, geometrical construction of the asymmetrical Weymouth and Pur- beck Anticlines (Sections A-A' and D-D' respectively, Fig. 3) strongly suggests that the anticlinal axes are displaced to the S and hence that the major faults tend to flatten with depth. The implication is that, prior to the Aptian, these were listric normal faults downthrowing S: this is supported by their cuspate traces in plan. They have been drawn following the shape of a cycloid, the theoretical profile of a listric normal fault intersecting the free surface at 45" (Hose & Dane5 1973): this appears to be consistent with the positions of the extrapolated anticlinal axes at depth.

On the southern side of the flexures/faults, the full succession is present up to and including the Wealden. To the N, however, the Upper Cretaceous rests on Upper Jurassic and possibly even Middle Jurassic beds: within the area covered by the cross-sections, this is generally the Oxford Clay, although the Kim- meridge Clay appears in wells further to the N (Fig. 2). There is no reason to doubt that the complete Jurassic sequence was deposited across the entire area; ab- sence of the higher Jurassic beds N of the flexure, and

the widespread occurrences of Kimmeridgian ammo- nites in the Aptian (P. E. Kent, pers. comm.), therefore imply absolute uplift and erosion in the early Cretaceous, as opposed to mere stability while the southern side subsided.

The limited control available for the Jurassic suggests consistently that the thicknesses of the various formations, particularly in the Middle Jurassic, are greater S of the flexure than to the N, as noted also by Colter & Havard (1981) and confirmed by A. Whit- taker (pers. comm.). Presumably, therefore, the major faults were active during the Jurassic, although probably at variable rates (P. W. Mikkelsen, pers. comm.). This raises two questions: ( i ) Was the Wealden deposited to its full thickness, if at all, to the N, for example at Wytch Farm? (ii) Did these growth faults develop a roll-over anticline on the downthrown side, with a further localized thickness increase adjacent to the fault itself? There is little direct evidence from which to answer these inter-related questions, although 'large boulders of Wealden sand in Wealden sand (shown up by marked differences in oil impregna- tion and porosity) may be symptomatic of contempor- ary degradation of a nearby fault scarp' (P. E. Kent, pers. comm.). The existence of a mid-Cretaceous anticline on the Poxwell-Chaldon feature could well be accounted for in terms of roll-over. Indeed the very geometry of a listric growth fault in effect leads to the development of such a structure. Thus we may be led to postulate that, in Dorset, the zone of thickest Wealden accumulation was in fact immediately S of the Purbeck Monocline and, consequently, that the Purbeck, Kimmeridge and other anticlines have at least some roll-over component. Despite some probably local constituents, such a situation, of course, need not necessarily have expression at the sea-floor or in the facies of the sediments provided deposition was from bottom traction currents (e.g. Hallam & Sell- wood 1976; Chapman 1973, p. 87 et seq.): indeed, the primary detritus of the Wealden is far travelled-from Cornubia (Arkell 1947).

The effects of the Tertiary flexuring on the attitude of the earlier faults and the immediately adjacent pre-Aptian beds cannot be determined accurately with the control at present available. The fault planes must have been bent downwards near the surface, inviting the development of splay thrusts in the Tertiary. An attempt to illustrate this has been made, rather diag- ramatically, on the sections, and it has been assumed that at depth the Tertiary effect was merely to restore some of the earlier normal offset. There is no factual justification for these interpretations, and equally there are insufficient data to attempt an accurate pre- Neogene reconstruction. A sketch of the situation envisaged is shown in Fig. 4: it could be matched on seismic profiles from a number of the world's continental shelves.

It is instructive to use these cross-sections to try to



E i

R. Stoneley

\ \

E, n

l L

L 8 8 E



The structural development of the Wessex Basin

J

E i

549



550 R. Stoneley

Sutton Poyntr- Poxwell-

SOUTH Kimmeridge Chaidon

I NORTH I 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 that have taken place, both vertically and laterally, in southern Dorset since the early Mesozoic. The scale of the vertical movements can be illustrated by comparative burial history graphs of sequences on either side of the Purbeck Monocline (Fig. 5); sufficient data for tentative interpretations are available from Wytch Farm (Colter & Havard 1981) and Kimmeridge Bay (Brunstrom 1963, and outcrop data). A potential method for checking the order of magnitude involved is offered by the thermal alteration due to burial of organic matter in the sediments: assuming a constant

geothermal gradient, the Level of Organic Metamorphism (LOM) can be predicted theoretically (Hood et al. 1975) and compared with the actual values. Measurements at Imperial College of the LOM of the Kimmeridge Clay at outcrop (R. R. F. Kinghorn & M. Rahman, pers. comm.) are consistent with the values predicted from the burial depths shown in Fig. 5.

This burial history is in general compatible with the concept of basin subsidence due to crustal stretching and thinning by listric normal faulting (e.g. McKenzie 1978), although it may be difficult to explain the

WYTCH FARM 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 2 k m II FIG. 5 . Comparative burial history graphs on either side of the Purbeck Monocline. ? = thickness estimated. Level of Organic Metamorphism (LOM) values are estimated using the curves of Hood et al. (1975), assuming a geothermal gradient of 27 "C/hm.



The structural development of the Wessex Basin 55 1

500 m plus of uplift N of the flexure in the early Cretaceous.

Is it possible to estimate the horizontal displacements involved? The amount of lateral shortening represented by the Tertiary flexure is equivalent to its vertical height, if strata1 stretching is ignored; in Dorset the figure reaches the order of 1.2 km in the vicinity of Wytch Farm, and it may be as much as 1.5 km in the Isle of Wight. This must be less than the cumulative pre-mid-Cretaceous extension since, for say the Jurassic, the net effect across the disturbance is still a downthrow to the S: Section E-E (Fig. 3) suggests that the present horizontal separation of an horizon in the Lias is approximately the same amount. The minimum horizontal separative movement prior to the Aptian may thus have exceeded 2.5 km; a similar estimate may be reached by combining the burial history graphs with the cross-section-assuming that the fault has been drawn at the correct angle!

Concluding remarks Consideration of the structural development of the Wessex Basin should take account of the following:

1. The extent of the basin was approximately coin- cident with its present limits, extending through eastern England, the Channel and Western Approaches, and into northern France. Its evolution began with strong, localized differential vertical movements in the Permo-Triassic, believed to be related to contemporaneous rifting elsewhere in the British region.

2. Subsequent subsidence was accompanied by con- tinuing gentle faulting along persistent lines of weakness, which form elements of a basin-wide crustal fracture system.

3. One of these lines, the S Dorset-Isle of Wight belt, has been considered in this review. It consists of a number of en echelon segments, not all offset in the same sense, and each in plan scalloped to the S: this disposition is inconsistent with deep-seated strike-slip faulting. The evidence suggests strongly that they represent pre-Aptian listric normal growth faults, which had achieved a horizontal separation of the order of 2.5 km from the beginning of the Jurassic to the mid-Cretaceous.

4. These faults were in all probability associated with roll-over anticlines on their downthrown sides, which influenced the accumulation of sediment at least during the early Cretaceous.

5. This regime was terminated by the Aptian, after

which a period of relative stability until the latest Cretaceous was followed by a tendency to reverse the sense of the earlier displacements: this may have been effective by the beginning of the Cenozoic (Arkell 1947) but culminated in a Miocene crustal shortening of up to 1.5 km, expressed along the pre-existing lines of weakness as a monoclinal flexure.

6. This reversal of sense of displacement represents the more or less contemporaneous inversion tectonics, which were widespread through south-eastern Eng- land and northern Europe.

7. A related effect of reversed movement could have been to accentuate, by compression, pre-existing roll-over anticlines. The Sutton Poyntz-Poxwell- Chaldon anticline is believed to have been formed in this manner; other, more exotic, explanations are un- necessary.

8. It is not impossible that the S Dorset-Isle of 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 to the N of the belt, which however was relatively positive during the Jurassic-early Cretaceous. The coincidence is nevertheless not exact and more data are required to resolve the question.

9. The events recorded by the S Dorset-Isle of Wight belt are mirrored on other lines within the basin. They appear to be correlative with plate movements in the North Atlantic region: strong initial rifting in the Permo-Trias, continued rifting through the Jurassic and early Cretaceous, and ocean formation and widening from the mid-Cretaceous onwards (e.g. Kent 1977; Ziegler 1975).

10. The termination of rifting and the onset of continental separation in the North Atlantic thus appear to coincide with a change from a state of regional N-S crustal extension in southern England-northern France, to one of overall crustal compression man- ifested along lines of earlier weakness. In this setting, one might suggest that the flexure prolonging the Bray and Rouen-Sennely faults to the NW across the En- glish Channel has a dextral strike-slip component, reversing the earlier sinistral displacement q f the Mesozoic.

ACKNOWLEDGMENTS. The following have reviewed a draft of this paper and provided most helpful suggestions, for which the author is deeply grateful: Sir Peter Kent, Dr V. S. Colter, Professor M. R. House, Dr R. C . Selley, J h A. Whittaker. The illustrations were drafted by Mr A. R. Brown, Imperial College.

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309-24.

373-406.

SOC. 175-9.

Received 10 April 1981. ROBERT STONELEY, Department of Geology, Imperial College of Science and

Technology, Prince Consort Road, London SW7 2BP.

Discussion MRS EVE A. HOWELL asked whether the author had made an interpretation of the gravity anomalies in the Wessex basin. From an interpretation of the IGS gravity survey overlay map (Sheets 19 & 23), she had concluded that the Permo-Triassic was deposited in a N-S basin, the €-W trend only becoming dominant in the Jurassic (Cimmerian movements).

She had made isopach maps of the main density subdivisions from surface to base Jurassic, calculated their theoretical gravitational effect and removed it from the total field. The resultant residual Bouguer

anomaly map reflects the density variations in the pre-Jurassic section.

The most striking feature on the map is a prominent elongated negative Bouguer anomaly extending into the English Channel along an axis on trend with that of the Worcester graben. Her interpretation of this residual anomaly was that it is due to a N-S Permo- Triassic basin. This orientation agrees with the Triassic palaeogeographical reconstructions of Audley-Charles (1970) and Ziegler (1981).




The AUTHOR thanked Mrs Howell for her comments and was delighted to hear of her interesting results: it is to be hoped that these will be published. The author had not made an independent interpretation of the gravity data.

The N-S trend had been referred to as one of those influencing the development of the basin and the author’s feeling was that it was inherited from a considerably older period than the Mesozoic. His interpretation that the Permo-Triassic was affected by E-W rifting was based primarily on the scattered borehole information, but it also took note of the views expressed by Mikkelsen in the oral presentation referred to in the paper, and to the nearly E-W rifting in the Permo-Triassic of Somerset. Possibly the N-S negative residual anomaly, mentioned by Mrs Howell, could reflect variations in the pre-Permian ‘basement’, but it seems that considerably more seismic and borehole data will be required fully to resolve the matter.

DR J. P. N. BADHAM said: In the interesting recent and pre-Miocene restored cross-sections presented, one (E-E‘) crossed the Wytch Farm oilfield. From these sections it would appear that at no time were Jurassic source rocks, either to N or S, structurally deeper than the Permo-Trias which now contains the oil. In addition two major growth fault and fold structures were shown which would have impeded any northward oil migration to the Wytch Farm field. Would the author comment on the source and timing of migration of the oil to the Wytch Farm field?

In reply, the AUTHOR said that the only sources of information on the Wytch Farm field are the papers by Colter & Havard (1981) and Hinde (1980), referred to in the text. The former presents reasons for believing that the oil in both the Bridport Sands and Triassic reservoirs is derived from S of the Purbeck flexure, where the possible source rocks (Lias, ?Oxford Clay and ?Kimmeridge Clay) may be mature. At least the Lias was indeed at a deeper level than the reservoirs to the N prior to the late Cretaceous-Tertiary reversal, and he had attempted to show this on his cross- sections. Colter & Havard (p. 502) suggested that oil generation could have begun as early as the early Cretaceous to the S of the Purbeck flexure.

The author knew of no reason to disagree with these assessments, and had tried to incorporate them into his own theoretical reconstruction. The actual mechanisms and pathways of oil migration may well be complex, and a clear understanding must await the availability of considerably more data.

DR P. V. 0. DRUMMOND stated. The structure of the Wessex Basin is considerably more complex than im- plied by the author’s re-evaluation of the standard tensional-compressional model for the development of the strong E-W lines of the region. Contrary to the

author’s opening remarks, these now very pronounced E-W thrust periclinal lines are not the most important ‘basement-controlled’ structures; most are post- Cretaceous 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 ‘occasional NW-SE structure’ dismissed by the author is in fact the sign of the real underlying structural control for the development of post-Palaeozoic Wessex. The Mid-Dorset Swell (Drummond 1970) is just one example of a whole complex of NW-SE basement lines that controlled Mesozoic sedimentation and the structural development of Wessex and the Wealden Basin. The sheared nature of these NW-SE lines has always been appreciated and can be seen in the Palaeozoic outcrops of Somerset (Webby 1965 a & b). Similarly, the strong E-W faults (e.g. Cannington Park, mid-Devon, South Devon, etc., can be traced eastwards beneath the Mesozoic cover, to influence the development of the structures described, but not in the way that the author infers.

It is now clear that simple tension-compression is no longer (and never really was) able to explain the structural pattern and style of Wessex (summarized in Drummond 1970, Fig. 9, p. 709). All the main NW-SE lines on this map are the surface expression of dextral basement shears that are the secondary components of strong E-W sinistral shear couples such as the Bristol Channel-Glastonbury-Warminster-Mere lines to the N, the South Devon-South Dorset-Isle of Wight lines to the S, and the central mid-Devon-mid- Dorset lines (so clearly demonstrated by the offset of the mid-Dorset Swell) that continues eastwards into the Portsdown-South Downs line of the southern Weald.

Post-Armorican to Lower Cretaceous activity along these Palaeozoic lines was a transtensional phase expressed as normal faulting that allowed very sharp, deep back-filled basins (e.g. the Vale of Taunton Dene) to accumulate great thicknesses of sediment, especially Jurassic clays. Mid-Cretaceous reversal to transpression began the quasi-diapiric inversion of these clay-filled basins to cause the slow growth of the NW-SE ‘swells and basins’ that dominated Upper Cretaceous sedimentation, of which the mid-Dorset Swell, Marshwood Dome, proto-Weymouth, proto- Purbeck, proto-Brixton periclines are the more obvi- ous examples.

Enhanced E-W transpressional shearing initiated by early-mid Tertiary transform ring opening of the North Atlantic greatly accentuated clay diapiric inversion to produce strong E-W periclines approximately parallel to the main shear motion (Reading 1980). In effect, these lines of E-W periclines are late stage structural ‘short-circuiting’, jumping the basinal gaps between the ‘terminals’ of the original NW-SE axes; hence their typical sigmoidal plan. This periclinal in-



554 R. Stoneley

version short-circuiting is most pronounced where the Mesozoic clay-filled basins are best developed immediately S of the southerly downthrowing E-W basement lines, especially where Kimmeridge and Wealden Clays were preserved to greater thicknesses south- wards down the structural staircase into the Hamp- shire Basin.

Thus, the structural development 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 al- ternate transtension-transpression phases conditioned by the ‘transform ring’ opening of the North Atlantic and not the simple tensional-‘Alpine compressional’ tectonics as traditionally inferred.

In reply, the AUTHOR stated that he was interested in the views expressed by Dr Drummond but, in view of the fact that they were based on a number of strong assertions for which the evidence was not given, felt unable to comment. He was still of the opinion that, for reasons given in the paper, it is difficult to account for the S Dorset-Isle of Wight line of disturbance in terms of deep-seated strike-slip displacement. The importance of the NW-SE trend is not doubted: possibly it originated as a response to northwards Hercy- nian compression, and has to a greater or lesser extent been reactivated subsequently. The author did not believe, however, that it can be related to an E-W sinistral shear that has repeatedly exercised overall structural control.

Additional references

AUDLEY-CHARLES, M. G. 1970. Triassic palaeogeography of WEBBY, B. D. 1965B. The stratigraphy and structure of the the British Isles. Q. J . geol. Soc. London, 126, 49-89. Devonian rocks in the Quantock Hills, 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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